This PDF file contains the front matter associated with SPIE Proceedings Volume 7580, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

A rapidly tunable, electronically controlled, pulse duration adjustable, arbitrarily programmable wavelength, picosecond mode-locked fiber laser is presented. The laser is tunable over 80 nm with sweeping frequency over 10 million wavelengths per second. The user can select from a preset linear sweep in either wavelength or optical frequency (kspace) or a custom (user-defined) sweep. Pulse duration is adjustable over tens of picoseconds with nearly Fourier limited linewidth. The laser can be harmonically mode-locked over 1 GHz. The average power is again fully programmable and is at least 50 mW, Watt level is possible with a high power amplifier. The output is a single mode polarization maintaining fiber. The laser possesses several external triggers, such as one trigger per optical pulse, one delayed trigger per optical pulse to synchronize with the experiments, one at the beginning when the laser is ready to sweep to start the data acquisition and one for each consecutive sweep, and finally one trigger for each wavelength change. Such a laser is so versatile that it can be used for medical imaging, material machining and nonlinear optics. It proves also a valuable research tool since all the parameters are adjustable.

We report the performance of an actively Q-switched Tm fiber laser system. The laser was stabilized to sub-nanometer spectral width using each of two feedback elements: a blazed reflection grating and a volume Bragg grating. Maximum pulse energy using the reflection grating was 325 μJ pulses at 1992 nm (< 200 pm width) with a 125 ns duration at a 20 kHz repetition rate. Maximum pulse energy using the volume Bragg grating was 225 μJ pulses at 2052 nm (<200 pm width) with a 200 ns duration also at 20 kHz. We also report the laser's performance as an ablation source for LIBS experiments on copper.

An air-cooled, light-weight, fiber-based, high power green laser has been prototyped. The system consists of an all-fibercoupled IR pump laser at 1064 nm and a frequency-conversion module in a compact and flexible configuration. The IR laser operates in QCW mode, with 10 MHz pulse repetition frequency and 3-5 ns pulse width, to generate sufficient peak power for frequency doubling in the converter module. The IR laser can produce more than 200 W in a linearlypolarized diffraction-limited output beam with high spectral brightness for frequency conversion. The converter module has an input telescope and an oven with a nonlinear crystal to efficiently convert the 1064-nm IR fiber laser output to 532-nm green output. The IR laser and conversion module are connected via a stainless-steel protected delivery fiber for optical beam delivery and an electrical cable harness for electrical power delivery and system control. The beam quality of the 532 nm output remains near diffraction-limited, with M²<1.4. Up to 101 W of 532 nm output was demonstrated and multi-hour runs were characterized at 75 W output. The weights of the IR laser package and doubler are 69 lbs and 14 lbs respectively. An overview of the system and full characterization results will be presented. Such compact, highbrightness green laser sources are expected to enable various scientific, defense and industrial applications.

We demonstrate an Ytterbium-doped fiber laser system generating high energy pulses at the non-conventional wavelength of 977 nm. An actively Q-switched master fiber oscillator delivers 1.2 W of average power in 12 ns pulses at 82 kHz of repetition rate. This pulsed fiber source is then amplified in an ultra-large core photonic crystal fiber amplifier up to 71 W. Deducing the fraction of power contained in interpulse ASE, we obtained 0.7 mJ pulses at 977 nm, resulting in a pulse peak-power of >55 kW. To the best to our knowledge, this system delivers the highest performances ever demonstrated in this spectral window.

We report on the performance of an Yb-doped, 100μm-core, rod-type photonic crystal fiber (PCF) used as the power amplifier in an actively controlled master-oscillator/power-amplifier optical (MOPA) source. Such PCF is the largestcore fiber to date to exhibit concurrent single-mode beam quality and robust linear polarization in its output. From this MOPA, we obtained peak power in excess of 1.35MW with excellent spectral brightness.

We report the realisation of a high power, picosecond pulse source at 530 nm pumped by an all-fiber, single mode, single polarisation, Yb-doped MOPA. The pump MOPA comprised of a gain switched seed source generating 20 ps pulse source at a repetition frequency of 910 MHz followed by three amplification stages. Output power in excess of 100 W was obtained at 85% slope efficiency with respect to launched pump power at 975 nm. A 15mm long LBO crystal was used to frequency double the single mode, single polarisation output of the fiber MOPA. To satisfy the phase matching condition, the internal temperature of the LBO crystal was maintained at 1550C. Frequency doubled power in excess of 55 W was obtained at 56% optical-to-optical conversion efficiency. Output power at 530 nm started to roll-off after 50 W due to self-phase modulation (SPM) assisted spectral broadening of the fundamental light within the final stage amplifier. Measured spectral bandwidth of the frequency doubled signal remained at ~0.4 nm with the increase in fundamental power even though that of the fundamental increased steadily with output power and reached to a value of 0.9 nm at 100 W output power.

We have recently shown that the photodarkening (PD) resistivity in Yb/Al-doped fibers can be greatly improved by adding cerium (CE) to the core glass composition. We are now further investigating the laser performance and logn term stability of Yb/Ce/Al fibers in high inversion applications such as 980nm lasers and amplifiers. Our objective is to study the limitations of Yb/Ce/Al fibers and elaborate on their potential of becoming the next generation of fibers for high power fiber lasers and applifiers.

We report on the characterization of photodarkening (PD) kinetics at Yb-doped fiber samples, inducing the PD loss by core-pumping at 975 nm with respect to different fiber temperatures in the range of 77 to 770 K. The thermal dependency of important PD parameters is presented. Additionally, we introduce a phenomenological model to include thermal and recovery effects in the description of the PD loss evolution and to improve the understanding of the PD process.

The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm region has been well documented to date but little has been presented on the reliability of this technology. Early experiments highlighted that photodarkening can be a significant concern when Tm-doped silica fibers are exposed to high intensity blue light. We present a discussion of the processes responsible for the production of blue light in Tmdoped fibers pumped at 790nm and how fiber composition influences these processes. Through optimization of fiber composition we have demonstrated highly efficient lasers exhibiting less than 1% output power degradation per thousand hours.

We study thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers. Post-irradiation heating of a photodarkened fiber is shown to result in further increase the loss which is attributed to both a permanent increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. The permanent heat-induced increase of loss is believed to indicate presence of an intermediate energy state in the NIRpho tochemical mechanism for photodarkening. Further, we apply the demarcation energy curve approach to derive the thermal activation energy of the induced defects. For the studied commercial 20-μm-core-diameter LMA fiber, the energy distribution consists of a single peak, located at 1.3 eV with a FWHM of 0.31 eV.

We have demonstrated 23 Watts total output power from a CW OPO. We believe this to be the highest total power output yet demonstrated from this class of device. The OPO was based on MgO-doped PPLN and was pumped by a 30 Watt fiber laser. The external efficiency was 77 % and the corresponding pump depletion was 83%. Output of 16.8 Watts at 1.5 microns and 6.2 Watts at 3.6 microns was demonstrated. We have also demonstrated simultaneous output of 10 Watts at 1.96 microns and 8 Watts at 2.34 microns in the same device.

Guided mode resonance filters (GMRF) were used to spectrally-stabilize and line-narrow the output spectrum from Tm fiber lasers operating in the 2 μm wavelength regime. The GMRFs were placed in the output path of an amplified spontaneous emission (ASE) light source and the transmitted light was measured as a notch in the spectrum on resonance. The GMRFs were characterized to determine their peak reflectivity, resonance wavelength, and spectral linewidth of each element. These measurements showed various resonance wavelengths and linewidths varying from 0.50-1.5 nm depending on the individual GMRF parameters. Using GMRFs as feedback elements in Tm fiber laser oscillators resulted in output powers up to 10 W and slope efficiencies of 30-45% with respect to launched 790 nm pump power. In order to scale to higher powers and maintain narrow linewidths, a master oscillator power amplifier (MOPA) setup was employed with a GMRF stabilized master oscillator. In addition to the laser and amplifier characteristics, thermal and damage testing of the GMRFs is reported.

Red (631 nm), green (532 nm), and blue (448 nm) continuous-wave (CW) lasers have been developed by Evans & Sutherland (E&S). These multi-watt RGB lasers are used as light sources in E&S' laser projector (ESLP), which delivers ultrahigh-resolution content (8192 × 4096 pixels) to large-surface-area venues (e.g., planetariums, simulators, visualization centers, etc.). Efficient visible wavelength generation is obtained by coupling single-frequency nearinfrared (NIR) beams into free-space enhancement cavities containing critically phase-matched lithium triborate (LBO) crystals. The NIR energy is produced by a master-oscillator-power-amplifier (MOPA) system which is fiber-based, thus yielding Gaussian beams which are near-ideal for efficient fundamental-to-harmonic conversion. Both polarizationmaintaining (PM) fibers and non-PM fibers have been employed with non-PM fiber systems requiring polarization sensing and control. Green laser light is produced by a second-harmonic generation (SHG) process with a 1064 nm fundamental. Red laser light is produced by a sum-frequency mixing (SFM) process with 1064 nm and 1550 nm as fundamentals. Blue laser light is produced by an SFM process with 1064 nm and 775 nm as fundamentals, where 775 nm is first produced by an SHG process with a 1550 nm fundamental. All resulting visible lasers are single-axialfrequency with FWHM bandwidths less than 400 kHz, and are spatially pure with M² values less than 1.05. At least 18 W of CW optical power has been generated at all three visible wavelengths, with available NIR amplifier power as the primary limiting factor.

Highly reliable DUV light sources are required for semiconductor applications such as a photomask inspection. The mask inspection for the advanced devices requires the UV lightning wavelength beyond 200 nm. By use of dual fiber lasers as fundamental light sources and the multi-wavelength conversion we have constructed a light source of 198nm with more than 100 mW. The first laser is Yb doped fiber laser with the wavelength of 1064 nm; the second is Er doped fiber laser with 1560 nm. To obtain the robustness and to simplify the configuration, the fundamental lights are run in the pulsed operation and all wavelength conversions are made in single-pass scheme. The PRFs of more than 2 MHz are chosen as an alternative of a CW light source; such a high PRF light is equivalent to CW light for inspection cameras. The light source is operated described as follows. Automatic weekly maintenance within an hour is done if it is required; automatic monthly maintenance within 4 hours is done on fixed date per month; manufacturer's maintenance is done every 6 month. Now this 198 nm light sources are equipped in the leading edge photomask inspection machines.

We report on the demonstration of over 140W at 1030 nm with pulse duration down to 10 ns and M²<1.3 from a very simple Q-switched architecture based on rod type fiber laser. These very high peak and average power lead to over 53W at 515 nm and 19W at 343 nm. We have also obtained diffraction limited beams with an output power exceeding 240W at 1030 nm and 120W at 515 nm in a very simple MOPA configuration. Due to the very high gain in these fibers, we can keep pulse durations below 20 ns up to 500 kHz in a purely Q-switched system.

Optical coherence tomography (OCT) imaging at 1060 nm region proved to be a successful alternative in ophthalmology not only for resolving intraretinal layers, but also for enabling sufficient penetration to monitor the sub-retinal vasculature in the choroids when compared to most commonly used OCT imaging systems at 800 nm region. To encourage further clinical research at this particular wavelength, we have developed a compact fiber optic source based on amplified spontaneous emission (ASE) centered at ~1060 nm with ~70 nm spectral bandwidth at full-width half maximum (FWHM) and output power >20 mW. Our approach is based on a combination of slightly shifted ASE emission spectra from a combination of two rare-earth doped fibers (Ytterbium and Neodymium). Spectral shaping and power optimization have been achieved using in-fiber filtering solutions. We have tested the performances of the source in an OCT system optimized for this wavelength.

The ability to tune the Bragg wavelength of a fibre-Bragg grating (FBG) in an all-fibre laser can offer added functionality such as laser wavelength tunability, polarization selectivity,1 and Q-switching.2 Compared to current techniques which rely on mechanically straining the FBG to achieve Bragg-wavelength tunability, an all-optical technique for tuning an FBG offers potentially faster switching speeds and a more robust and simple cavity. All-optical tuning of the Bragg wavelength of an FBG has been demonstrated previously by resonant optical pumping; however this technique has only been applied to passive systems for switching applications.³ In this work, we have further investigated this optical-tuning process, experimentally identifying three time-scale regimes, and optimised it for application to active systems. Furthermore, we constructed an erbium all-fibre laser cavity consisting of an outputcoupler FBG and an optically-tunable, high-reflector FBG. The cavity pumping and the optical tuning of the FBG were kept independent. By repetitively tuning the high-reflector FBG on- and off-resonance with the output-coupler FBG, we actively Q-switched the erbium fibre laser at repetition rates up to 35 kHz, limited only by our diode driver. We show that grating tuning at >300 kHz is possible with the existing embodiment, and discuss further potential to operate at MHz rates.

Next generation semiconductor, photovoltaic and display devices require precise, highly reliable, and cost-effective laser processing system solutions capable of high average power output at pulse repetition frequencies substantially exceeding 100 KHz. Emerging 45 nm and 32 nm node logic devices contain increasingly complex stacks of difficult to process materials, such as copper and low K dielectrics, demanding precise laser micromachining with minimum heat affected zones and melt effects. Laser memory repair of 5x nm node DRAM memory devices requires fine control of the fuse stack removal to avoid damage to adjacent circuitry. Significant improvements to patterning processes essential to the production of thin film photovoltaic devices are necessary to drive increased device efficiency and reductions in module prices. Similar challenges are faced in devising laser process solutions for crystalline silicon photovoltaic and advanced display devices. Robust picosecond laser architectures, including diode-pumped solid state master oscillator power amplifier or regenerative amplifier, fiber master oscillator power amplifier and fiber-bulk hybrids are broadly employed in investigations to identify solutions for these important industrial thin film laser processing applications. The prospects for broader adoption of industrial picosecond laser processing systems will be addressed along with related implications for advanced picosecond laser and laser system design requirements.

Commercial Fluorescence Lifetime Spectrometers have long suffered from the lack of a simple, compact and relatively inexpensive broad spectral band light source that can be flexibly employed for both quasi-steady state and time resolved measurements (using Time Correlated Single Photon Counting [TCSPC]). This paper reports the integration of an optically pumped photonic crystal fibre, supercontinuum source¹ (Fianium model SC400PP) as a light source in Fluorescence Lifetime Spectrometers (Edinburgh Instruments FLS920 and Lifespec II), with single photon counting detectors (micro-channel plate photomultiplier and a near-infrared photomultiplier) covering the UV to NIR range. An innovative method of spectral selection of the supercontinuum source involving wedge interference filters is also discussed.

We present a compact femtosecond fiber laser at 1.6 μm based on chirped-pulse amplification in an erbiumdoped large-mode-area fiber. In erbium-doped fibers, low gain at 1.6 μm increases the fiber length needed to achieve amplification, which enhances nonlinear phase accumulation. In this work, 12 m of a 20-μm-diameter core highly-doped fiber was used to reduce nonlinearities. Nonlinear compensation of the stretcher/compressor dispersion mismatch results in the generation of 605 fs pulses at 300 kHz with an energy of 1.5 μJ. This is, to our knowledge, the first sub-picosecond laser reaching the μJ level at 1.6 μm in erbium-doped fibers.

Recent work has shown that stable, highly-chirped pulses can be generated by all-normal-dispersion fiber lasers. Pulses in the ~100-ps range but with bandwidth to support ~500-fs pulses are produced, with energies of tens of nanojoules and at repetition rates of 1 MHz and below. The chirped pulses can be compressed to near the transform limit. Lasers that produce pulses with such giant chirp should greatly simplify chirped-pulse fiber amplifiers. After a brief review of fiber lasers based on dissipative solitons, recent developments will be summarized.

We present a high peak and average power laser system with ultrashort pulses at high repetition rates. Pulse shortening and peak power enhancement of a state-of-the-art fiber laser system is achieved by utilizing nonlinearity, namely selfphase modulation and subsequent compression in a chirped mirror compressor. The nonlinear interaction is achieved by propagation in a noble gas filled hollow core fiber with an inner diameter of 200 μm and a length of 0.5 m. A total second dispersion of -7000 fs² is applied by a chirped mirror compressor resulting in ultrashort pulses of 71 fs duration. This is achieved by coupling 400 MW, 800 fs pulses from the CPA system to the Xenon filled hollow core fiber. The average power at the output of the compressor is measured to be 10 W at 50 kHz repetition rate resulting in 200 μJ pulse energy. Hence, the compressed pulses have a peak power of more than 2 GW. Consequently, the pulses of the CPA system are shortened by a factor of ten and the peak power is enhanced by a factor of 5. In addition this approach offers further peak and average power scalability.

We present a theoretical description of the generation of ultra-short, high-energy pulses in two laser cavities driven by periodic spectral filtering or dispersion management. Critical in driving the intra-cavity dynamics is the nontrivial phase profiles generated and their periodic modification from either spectral filtering or dispersion management. For laser cavities with a spectral filter, the theory gives a simple geometrical description of the intra-cavity dynamics and provides a simple and efficient method for optimizing the laser cavity performance. In the dispersion managed cavity, analysis shows the generated self-similar behavior to be governed by the porous media equation with a rapidly-varying, mean-zero diffusion coefficient whose solution is the well-known Barenblatt similarity solution with parabolic profile.

In this work we present a new method for peak-power scaling in nonlinear CPA-systems. By clipping the tails of the spectrum we demonstrate pulse quality enhancement and an increase of peak-power at the output of the CPA-system. A theoretical model allows us to determine an optimal ratio between the spectral clipping bandwidth and the pulse bandwidth at a certain B-integral. Additionally, a simple redesign of a grating based stretcher unit, applying our new spectral clipping technique, would significantly increase the output peak-power of such nonlinear CPA-systems by a factor up to six due to the higher stretching ratio.

This paper was presented at the SPIE conference indicated above and has been withdrawn from publication at the request of the authors.

To further scale the peak-power of state-of-the-art fiber CPA-systems, a careful optimization of the spectral as well as temporal dynamics is required. The wavelength dependence of the small-signal gain, as well as the saturation of the amplifier, strongly affect the signal bandwidth. For unsaturated amplifiers only a spectral optimization is required. It can be shown that both the spectral center and the width of the input spectrum strongly affect the output bandwidth. An optimization regarding these two parameters will be given. Design guidelines are presented. We develop a simple yet efficient model to simulate the impact of saturation in broadband Ytterbium-doped fiber CPA-systems. Using this model, we reveal that significant peak-power scaling up to 10 GW of current fiber CPA-systems is possible.

We report recent advances in the development of fibers for the delivery and generation of both single-mode and heavily multimode laser beams as well as recent progress in fibers for supercontinuum generation in spectral regimes spanning the visible to mid-IR.

Non-axially symmetric micro-structured fibers including photonic crystal fibers and leakage channel fibers have been investigated as possible means to enable mode field area scaling while retaining low-loss fundamental mode propagation. In this paper we report on the theoretical and experimental investigation of a photonic crystal fiber designed for single-polarization operation in a coiled configuration incorporating a resonant higher-order mode suppressing structure within the cladding. We performed spatially and spectrally resolved (S2) imaging of the output of a 18.6 meter length of fiber when seeded with a broadband ASE source. We observed no trace of any localized or distributed scattering to higher order modes in the spatially resolved Fourier transformed output spectra indicating robust single-transverse-mode output.

A Large-Mode Area (LMA) single-mode Ytterbium-doped fiber amplifier with distributed narrow passband filtering is demonstrated. The fiber passband is ~ 40nm wide and centered at 1070nm for efficient filtering of both short- and longwavelength Amplified Spontaneous Emission (ASE) as well as Stimulated Raman Scattering (SRS). The fiber design provides Higher-Order Mode (HOM) suppression and is polarization maintaining. It has a mode field diameter of 27μm and exhibits a slope efficiency of >60%.

Ytterbium-doped photonic-bandgap fiber sources operating at the long-wavelength edge of the ytterbium gain band are being investigated for high power amplification. Artificial shaping of the gain spectrum by the characteristic distributed filtering effect of the photonic bandgap enables power scaling free of amplified spontaneous emission. As high as 167 W power and 16 dB saturated gain at 1178 nm have been demonstrated. Single-pass frequency doubling to 14.5W 589nm light was also demonstrated with 34% conversion efficiency.

Watt-level Bi fiber lasers have been demonstrated at 1280, 1330, 1340, 1360 and 1480 nm with the maximum output power of up to 10W and with the efficiency of up to 50% for the first time. The bismuth-doped phosphogermanosilicate fiber amplifiers operating within the wavelength range 1300-1500 nm have been developed. The net gain of more than 20 dB at the wavelengths of 1320 and 1440 nm under the 200-300 mW pump power was obtained. The 3 dB bandwidth of the amplifiers was larger than 30 nm, the noise figure being 4-6 dB.

Fourier transform spectroscopy and interference microscopy are combined to provide the world's first multi-wavelength optical fiber refractive index profile (RIP) measurements. The RIP and its spectral dependence are obtained with submicron spatial resolution across an octave stretching from about 500 nm to the 1 micron operating band of Yb-doped fiber lasers and amplifiers. In contrast to commercial Refracted Near Field (RNF) technology, which measures at a cleave, the technique described here measures transversely through the side of an uncleaved fiber, enabling measurements of axial fiber RIP variations found in fiber gratings, physical tapers, and fusion splices.

Power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities. Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of the fibers. Both fibers exhibited a mode field diameter comparable to a 40μm step index fiber with 0.07 NA. The fibers were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to the fibers.

We report the theoretical and experimental study of the properties of an active tapered double clad fiber (T-DCF). Optimization of the most important parameters of T-DCF such as tapering ratio, longitudinal profile, core/clad ratio and absorption have been considered. Using optimized T-DCF design we have demonstrated a high-power (750W) and highly efficient (80%) ytterbium fiber laser.

Prospects of fabrication of solid-core photonic bandgap fibers with a large mode area (LMA) are discussed. Properties of solid-core photonic bandgap fibers with a small ratio of the cladding element diameter d to the distance Λ between neighboring cladding elements are studied. The range of fiber parameters at which the fiber is single-mode over the fundamental band gap is found.

The availability of fiber combiners has been essential to the wide deployment of robust fiber lasers in the market. Some standard parts have emerged in larger volume, but new designs involving both different processes and different fiber configurations are being proposed to the market, offering novel or improved specifications. We will present some of those new designs involving true signal fiber feed-throughs and better return loss and isolation properties. The new fabrication process also allows more latitude in selecting the number of input pump fibers and is independent of the signal fiber's internal structure.

A 7+1 to 1 pump/signal combiner with single-mode (SM) polarization maintaining (PM) 15 μm mode-field-diameter (MFD) signal feed-through is demonstrated. The combiner is designed for pulse amplification in an active Yb-doped airclad fiber operated in backward pumped configuration. Signal coupling through the device is realized by a microstructured taper element allowing single-mode guidance and constant MFD at a taper ratio of 3.4.

In this contribution we present a simple and robust pulse shaping device based on coherent pulse stacking. The device is based on fiber Bragg gratings written in a polarisation maintaining step index fiber and a fiber optical circulator. Up to four pulse replicas are reflected by fiber Bragg gratings and interfere at the output of the device. Temperature control allows tuning of the relative pulse amplitudes and phases of the pulse replicas. We experimentally demonstrated 235 ps and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse durations up to several nanoseconds.

We report on the performance of a monolithic 6+1X1 fiber pump signal multiplexer for use in fiber amplifiers. The key component of this coupler design is an etched taper that transforms the low-numerical aperture large diameter pump radiation into a high numerical aperture small diamter format for injection into the pump cladding of an air-clad fiber while maintaining a constant refractive index profile in the core for efficient signal coupling. This taper was then fused onto the 6+1 fiber bundle at the large end and to the air-clad large mode area polarization maintaining photonic crystal fiber at the small end. We employed 6 pump delivery fibers in a 200/220/0.22 core/clad/NA format and a 25/250 polarization maintaining step index signal delivery fiber for the bundle. The large end of the taper had a cladding diameter of 650 μm while the small end had a cladding diameter of 300 μm to match the pump cladding diameter of the PCF which was 314 μm. The core within the taper had a constant diameter of 40 μm and NA of 0.07 achieved through a step index profile. The mode field diameter of the PCF was 54 μm. Signal coupling efficiency at 1550 nm was measured to be 90% with a polarization extinction ratio > 20dB while pump coupling efficiency was measured to be 87% at 1532nm. The low pump coupling efficiency was found to be due to pump delivery fibers that had a numerical aperture of 0.24, higher than the specification of 0.22. A simple calculation shows that with 0.22 NA pump fibers, the pump coupling efficiency would increase to 94%.

We demonstrate electrical tunability of a fiber laser using a liquid crystal photonic bandgap fiber. Tuning of the laser is achieved by combining the wavelength filtering effect of a liquid crystal photonic bandgap fiber device with an ytterbium-doped photonic crystal fiber. We fabricate an all-spliced laser cavity based on a liquid crystal photonic bandgap fiber mounted on a silicon assembly, a pump/signal combiner with single-mode signal feed-through and an ytterbium-doped photonic crystal fiber. The laser cavity produces a single-mode output and is tuned in the range 1040- 1065 nm by applying an electric field to the silicon assembly.

We present an all-fiber side pump combiner for high power fiber lasers and amplifiers. It consists of a capillary with decreasing wall thickness fused around the active fiber in a way that it becomes an additional cladding layer of it. The pump fibers are spliced at the edge of the capillary with the thickest wall. This provides enough room to allocate around 12 pump fibers (100μm diameter) in a standard 400μm double-clad fiber. This side pump combiner offers several advantages such as the fact that it does not interrupt the active fiber core at any point, thus allowing for truly monolithic all fiber lasers and amplifiers. Additionally, the pump light is coupled into the active double-clad fiber all along the combiner's body (~ 1-2 cm long), which avoids the concentration of heat in a very small area, resulting in high pump power handling capabilities. If the taper angle of the capillary wall is low enough a high coupling efficiency (< 95%) is possible. Using this structure we have achieved a maximum combined pump power of 86 Watt from 7 pump diodes. This, we believe, is the highest combined pump power reported so far from a single lumped side pump combiner.

A volume Bragg grating is used in two different configurations to control the output spectrum of a thulium doped silica fiber laser. When used in a direct feedback configuration on the end of a bidirectionally pumped resonator, a power of up to 159 W with 54% slope efficiency is produced with a narrow output spectrum centered at 2052.5 nm with <1.5 nm full-width at 10 dB down from spectral peak. Maximum laser linewidth is limited by the VBG reflectivity width. The VBG based laser is compared to a laser resonator based on a standard HR mirror and is able to maintain stable spectrally narrow operation while the HR mirror laser has a wide and varied spectral output over 20-30 nm. Both lasers have similar slope efficiency, threshold and power performance with any difference attributed to lack of AR coatings on the VBG. In a second cavity, the VBG is used in a tunable configuration by rotating the VBG away from normal incidence. Tuning range was found to be >100 nm from 1947 nm to 2052.5 nm with output powers as high as 48 W and up to 52% slope efficiency. Tuning range is determined by VBG center wavelength on the long wavelength end and by the VBG aperture size on the short end. In both system configurations, M² is maintained at less than 1.2 at all power levels and long term operating stability at full power is demonstrated.

This paper discusses the transverse acoustic index design of Yb-doped large mode area (LMA) LP01 optical fibers that provide ~10 dB of SBS threshold suppression relative to conventional LMA fibers with homogeneous elastic properties and equivalent optical effective areas Aeff. SBS suppression is achieved with a ramp-like negative acoustic lens structure that refracts the electrostrictive density fluctuations away from the optical mode thereby reducing the acousto-optic interaction that generates the stimulated light scattering. The fundamentals of the SBS process and its mitigation are briefly reviewed. Two figures-of-merit (FsOM) are identified to quantify the SBS suppression capability; the SBS reflectivity RSBS and the SBS threshold power Pth. An initial design of an SBS suppressing Yb-doped double clad fiber is incorporated in the power amplifier stage of a 200 W cw singlefrequency (SF) four stage master-oscillator power-amplifier (MOPA). The MOPA is also exercised as a pulsed amplifier and is utilized to measure the SBS reflectivities and thresholds in passive (i.e. un-pumped) fibers with ~100 ns pulses exhibiting peak powers up to ~250 W. In separate experiments, the SBS suppressing fibers are incorporated into the final stage of the pulsed four stage MOPA and the SBS reflectivities of the active gain fibers are measured. Pulsed SF MOPAs with peak pulsed power outputs ~900 W are demonstrated and confirm the kilowatt SF performance capability of these SBS suppressing fibers.

We report on the high power amplification of narrow linewidth laser radiation with close to diffraction limited beam quality using a large mode area photonic crystal fiber amplifier. The observation of threshold-like higher order mode amplification by transverse spatial-hole burning at the highest power level is reported. The measured M² stays below 1.3 but increases at the critical power level, where the fundamental mode turns into the next higher order mode. At the maximum power of 1.2 kW a linewidth of <80 pm limited by self-phase modulation is obtained.

We report on experimental and theoretical investigations of single frequency high power PCF amplifiers. A model describing the interplay among laser gain, thermals effects, and SBS was developed to study the power limitations of single frequency amplifiers in general, and PCF amplifiers in particular. A distributed noise term was used to initiate the SBS process with the Stokes light spanning multi-frequency channels. The use of thermal and acoustic gradients in conjunction was considered and indicated marginal improvement. In the set of experiments, slope efficiencies as high as 77% were obtained with a maximum output of 427 W. The linewidth was measured and yielded values that were less than 10 KHz. A pump-probe measurement of the Brillouin gain spectrum revealed secondary peaks lying at the highfrequency side. Measurements conducted on a novel PCF, specifically designed to utilize thermal and acoustic gradients in conjunction, showed the existence of two primary gain peaks.

We report on the development of a high power monolithic CW fiber oscillator with an output power of 215 W in a 20μm core diameter few-mode Large Mode Area fiber (LMA). The key parameters for stable operation are reviewed. With these optimizations the root mean square of the output power fluctuations can be reduced to less than 0.5 % on a timescale of 20 s, which represents an improvement of more than a factor 5 over a non-optimized fiber laser. With a real-time measurement of the mode content of the fiber laser it can be shown that the few-mode nature of LMA fibers is the main factor for the residual instability of our optimized fiber laser. The root of the problem is that Fiber Bragg Gratings (FBGs) written in multimode fibers exhibit a multi-peak reflexion spectrum in which each resonance corresponds to a different transversal mode. This reflectivity spectrum stimulates multimode laser operation, which results in power and pointing instabilities due to gain competition between the different transversal modes . To stabilize the temporal and spatial behavior of the laser output, we propose an innovative passive in-fiber transversal mode filter based on modified FBG-Fabry Perot structure. This structure provides different reflectivities to the different transversal modes according to the transversal distribution of their intensity profile. Furthermore, this structure can be completely written into the active fiber using fs-laser pulses. Moreover, this concept scales very well with the fiber core diameter, which implies that there is no performance loss in fibers with even larger cores. In consequence this structure is inherently power scalable and can, therefore, be used in kW-level fiber laser systems.

We analyze theoretically limitations on brightness enhancement of a multimode pump beam into a diffraction-limited Stokes beam in efficient cladding-pumped fiber Raman amplifiers. Firstly, the power-scaling of the 1^st Stokes (hence the brightness enhancement) is limited by the generation of the 2^nd order Stokes. Thus using a spectral waveguide filter such as a W-type fiber core, it is possible to improve this limit to nearly five times that of a normal fiber without spectral filter. Secondly, we analyze limits set by glass damage, propagation loss, and pump-signal pulse walk-off in the multimode fiber. We show that a well-designed fiber with a propagation loss of 3.5 dB/km allows for a pump-to-signal brightness improvement of over 1000 times for pulses longer than 40 ns and up to 3500 times in the cw regime.

We report on the generation of 830 W compressed average power at 78 MHz pulse repetition frequency and 640 fs pulse duration. We discuss further power scaling including the issue of transversal spatial hole burning. Therefore, we describe a low-nonlinearity fiber design capable of producing fundamental mode radiation at ultra high average powers from short length (range of 1m) and large mode field diameter (>50μm) fibers. In conventional large mode area fiber most of the core is typically uniformly doped. As a consequence gain factors for the fundamental mode and the next higher order modes are comparable. Furthermore, the fundamental mode extracts inversion only in the central part of the core according to its intensity profile, leading at high pump and signal power levels to high and unused inversion density with a strong overlap with higher order transversal modes. In experiments this leads to a threshold-like onset of mode instability, originating from mode competition. Finally, this effect avoids further power scaling. The presented fiber features an optimized doping profile to prefer the amplification of the fundamental mode. In addition non-extracted inversion is minimized avoiding the issue of transversal spatial hole burning. As a consequence ultrafast fiber laser systems with novel performance are in reach, i.e. systems delivering simultaneously >1GW peak power and >1kW average power. In a first iteration a ROD-type fiber with 60μm MFD and 1.7m length was used in a CPA system to produce pump power limited 355 W of average power at 1 MHz.

We report a 100 W continuous-wave cladding-pumped fiber Raman laser operating at 1120 nm. The fiber Raman laser consists of an 85 m long germanium-doped double-clad fiber in a 4% - 100% linear cavity, which is end-pumped by a multimode ytterbium-doped fiber laser source at 1064 nm. The Raman laser has a slope efficiency of 71% with respect to launched pump power. The laser output M² is measured to be ~ 1.6 at 80 W of output power.

A novel method is presented for beam shaping far field intensity distributions using coherently combined fiber arrays. Traditionally, coherent arrays have been composed of linearly polarized elements having their polarization vector along a common axis. In this novel method, the fibers are arranged uniformly on the perimeter of a circle, and the linearly polarized beams are oriented with their polarization vectors arranged in a cylindrical fashion such that each subsequent vector is rotated by 2π/N where N is the number of elements on the circle. The elements each have the same Gaussian intensity distribution and power. The ensemble yields a far field intensity pattern that is a good approximation to a cylindrical vector (CV) beam which is characterized by a nonuniform polarization distribution and a null in the center of the beam. These synthetically created CV beams, or discrete cylindrical vector (DCV) beams, can be represented in a closed form solution to predict the far field intensity distributions. This solution is shown to agree with experimental results where several values of N, the number of elements, were tested. In addition, some more complex geometries such as nested geometries, fractal geometries, and some nonuniform geometries have been simulated, all of which also have a central null in the beam and have a nonuniform polarization distribution. These results are in contrast to linearly polarized beams, where the intensity peaks on axis, and from traditional cylindrical vector beams, which are generated by a single laser cavity.

Beams from three frequency stabilized master oscillator power amplifier (MOPA) thulium fiber laser systems were spectrally beam combined using a metal diffraction grating. Two of the laser oscillators were stabilized with guided mode resonances filters while the third was stabilized using a gold-coated diffraction grating. Each system was capable of producing a minimum of 40 W output powers with slope efficiencies between 50-60 %. The three lasers undergoing combination were operating at wavelengths of 1984.3, 2002.1, and 2011.9 nm with spectral linewidths between 250-400 pm. Beam combining was accomplished by spatially overlapping the spectrally separated beams on a water-cooled gold-coated diffraction grating with 600 lines/mm. Beam quality measurements were completed using M² measurements at multiple power levels of the combined beam. Power levels of 49 W were achieved before thermal heating of the metal diffraction grating cause degradation in beam quality. The combining grating was ~66% efficient for the unpolarized light corresponding to a total optical-to-optical efficiency of 33% with respect to launched pump power.

Beam combining with tapered array of single mode fibers is presented. A mutual incoherent CW power launch into the entries introduces intensity addition of which brightness can be utmost conserved. The best limit for the combined output beam quality is given by the brightness conservation law. An experiment where three and seven input CW signals are combined supports this claim. The respective M² values were close to the best predicted limit, thus it is found to have good agreement with the brightness limit. The power transfer is higher than 90%.

We introduce a novel technique of coherently locking fiber lasers using volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass as a passive multiplexer between channels. A two-channel coherently-locked Ybdoped fiber laser system with a narrow linewidth (~2.5 pm) and linear polarization (PER >20 dB) is demonstrated at a level of ~ 4 W (limited by pump). Scaling of this technique to coherently lock multiple (>2) fiber laser channels is discussed.

In this contribution we introduce a simple scheme to spectrally combine four single beams using three low-cost dielectric interference filters as combining elements. 25 ns pulses from four independent and actively Q-switched fiber seed-sources are amplified in a single stage fiber-amplifier. Temporally and spatially combined 208 W of average power and 6.3 mJ of pulse energy are obtained out of a fiber-based laser system. A detailed observation of beam quality as well as the thermal behavior of the combining elements have been carried out and reveal mutual dependency. The deterioration of beam quality can be led back to thermal induced wave-front distortions on the part of the interference filters. This effect as well as other influences on M² will be discussed and compared to the competing combining approach with dielectric gratings.

A tabletop kW-level spectral beam combining (SBC) system using volume Bragg gratings (VBGs) recorded in photothermo- refractive (PTR) glass was presented at the last meeting [^1]. Diffraction efficiency of VBGs close to 100% was demonstrated. However, when using VBGs for spectral beam combining, it is important to ensure high diffraction efficiency for the diffracted beam and low diffraction efficiency for the transmitted beams simultaneously. The unique, unmatched properties of VBGs allow spectral beam combining achieving this condition at wavelengths with less than 0.25 nm separation. We present modeling of reflecting VBGs for high power SBC that takes into account laser spectral bandwidth, beam divergence, PTR-glass scattering losses, and grating non-uniformity. A method for optimization of VBG parameters for high-efficiency SBC with an arbitrary number of channels is developed. Another important aspect of spectral beam combiner design is maintaining high diffraction efficiency as the temperature of beam-combining VBGs changes during operation due to absorption of high power radiation. A new technique of thermal tuning of large aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide range of laser power, is developed. Finally, these tools are used to demonstrate a robust and portable 5-channel SBC system with near diffraction limited spectrally-combined output beam.

We report on the phase locking of a fibre bundle laser based on a single frequency oscillator coupled into four fibre amplifiers to provide a coherent beam of over 600 W. The oscillator was phase modulated to a width of up to 2 GHz to increase the threshold for stimulated Brillouin scattering and then a fraction split off and frequency shifted to form a reference beam. The oscillator output was amplified by end-pumped fibre amplifiers based on 20 μm core Yb doped fibre to provide a power of up to 260 W per channel. The beams combined to form a coherent output with phase errors of a twentieth of a wave, unaffected by the spectral broadening.

In this paper, I will present the design analysis of a novel concept that may be used to generate a diffraction-limited beam from an aperture so that as much as 450 kW of laser power can be efficiently deposited on a diffraction-limited spot at a range. The laser beam will be comprised of many closely spaced wavelength channels as in a DWDM. The technique relies on the ability of an angular dispersion amplifier to multiplex a large number of high power narrow frequency lasers, wavelengths of which may be as close as 0.4 nm.

We demonstrate an all-fiber femtosecond master oscillator / power amplifier operating at the central wavelength of 1033 nm, based on Yb-doped fiber as gain medium, and two different kinds of photonic crystal fibers for dispersion control and stabilization. An all-solid (AS) polarization maintaining (PM) photonic bandgap fiber (PBG) is used in the cavity of the master oscillator for dispersion compensation and stabilization of modelocking. The final compression of an chirped-pulse-amplified laser signal is performed in a hollow PM PCF, yielding final fiber-delivered pulse energy of around 7 nJ, and pulse duration of around 297 fs. The self-stabilization mechanism of the oscillator, based on the optical nonlinearities in an AS PCF, results in excellent environmental and operational stability of our laser. Stable self-starting fundamental modelocking is maintained for at least 4 days of operation. During the 6-hour long observation of the output power, power fluctuations of less than 0.38 % are measured. Stable fundamental modelocking is maintained during temperature sweeps in the range 10 - 40 °C.

The relations between dopant concentrations (phosphorus and aluminum) and photodarkening rate, excess loss, and activation energies in ytterbium-doped silica fibers are experimentally investigated. It is shown that increasing the concentration of phosphorus from 0.2 to 2.5 mol% in phosphorus/aluminum codoped fiber cores decreases the photodarkening excess loss by a factor of 8 and the photodarkening rate by a factor of 10. Moreover, the effective number of ytterbium ions involved in the photodarkening process increases from 4 to more than 6 for tested phosphorus/aluminum concentration ratios varying from 0.1 to 1 respectively. In contrast, increasing the aluminum concentration from 2 to 5 mol% for a fixed phosphorus concentration of 0.2 mol% has negligible effect on the initial photodarkening rate or the effective number of ytterbium ions involved in the process, but still decreases the photodarkening excess loss by a factor of 5. Those results suggest photodarkening activation energies of 5.2 eV for ytterbium/aluminum-codoped silica fibers and more than 7.8 eV for ytterbium/phosphorus/aluminum-codoped silica fibers. The net improvement in photodegradation of fiber amplifiers based on such phosphorus and aluminum codoping is measured experimentally and numerically simulated. The output power loss of 1064-nm ytterbium-doped LMA fiber amplifiers with phosphorus/aluminum ratios of 0.1 and 0.6 is reduced after 10 000 hours from 17% to less than 2%, respectively. Better understanding of the effects of phosphorus and aluminum on photodarkening will help to design reliable and efficient ytterbium-doped fiber amplifiers.

We consider experimentally and theoretically a refined parameter space near the transition to multi-pulse modelocking. Near the transition, the onset of instability is initiated by a Hopf (periodic) bifurcation. As cavity energy is increased, the band of unstable, oscillatory modes generates a chaotic behavior between single- and multi-pulse operation. Both theory and experiment are in good qualitative agreement and they suggest that the phenomenon is of a universal nature in mode-locked lasers at the onset of multi-pulsing from N to N + 1 pulses per round trip. This is the first theoretical and experimental characterization of the transition behavior, made possible by a highly refined tuning of the gain pump level.

An iterative method is developed to characterize the mode-locking dynamics in a laser cavity mode-locked with a combination of waveplates and a passive polarizer. The method explicitly accounts for an arbitrary alignment of the fast- and slow-axes of the fiber with the waveplates and polarizer, fiber birefringence and saturating gain dynamics. The general averaging scheme results in the cubic-quintic complex Ginzburg-Landau equation (CQGLE), and an extensive comparison shows the agreement between the full model and the CQGLE and allows for a characterization of the stability and operating regimes of the laser cavity.

Coherent laser beam combining potentially provides an opportunity to achieve extremely high brightness of the output beam, permitting high on-target power density. All passive phasing techniques are limited by existence of optical path differences of individual fiber amplifiers. Cold-cavity theory predicts fast decrease in efficiency of coherent fiber laser beam combining with number of lasers. Experiments demonstrated in such systems that high degree of phasing takes place for laser array of up to 16 lasers. Origins of this illusory contradiction will be analyzed in the paper. Effects of laser wavelength self-adjustment and non-linearity of gain will be discussed.

The averaged mode-locking dynamics in a multi-mode fiber is studied. The transverse mode structures of the electric field are determined from a linear eigenvalue problem, and the co-propagation of the corresponding mode envelopes is governed by a system of coupled Ginzburg-Landau equations (CGLEs) which accounts explicitly for bandwidth-limited saturable gain as well as saturable absorption. Simulations show that stable and robust modelocked pulses with high energy can be produced. The maximum pulse energy is simulated as a function of the linear coupling and coiling loss. The present work provides for an excellent tool for characterizing mode-locking performance.

This report for the first time presents the results of experimental investigations into femto- and picosecond all-positive dispersion wavelength-tuneable Yb-doped fibre laser with efficient intra-cavity and extra-cavity Raman conversion of radiation in the range of 1070-1300 nm. We demonstrate smooth spectral detuning of radiation Stokes components within ranges 1130-1174, 1190-1235, and 1255-1300 nm generated when the fundamental harmonic of the laser was tuned within the 1075-1120-nm range. The average output power of the laser radiation at different Stokes components reached up to 250 mW.

In this work we propose and demonstrate a single mode fiber design that alleviates photo darkening. The fiber design is based on a reduced signal mode to gain material overlap which is found to reduce the induced losses of PD. For the fiber, saturated photo darkening operation is observed after 1500 hours operation with less than 7 % reduction in slope efficiency from 350 W output power in Yb/Al co-doped material. Power scalability up to 5 kW of the RMO fiber design is theoretically predicted.

Most Pulsed Fiber Lasers (FLs) are built on a Master Oscillator - Power Amplifier (MOPA) architecture, as this configuration has the advantage, among others, of exploiting direct modulation of the diode laser seed (the MO) to reach high repetition rates and high peak-power pulsed operation. To enhance the FL global performance and reliability, high power single-lateral-mode 1064 nm diodes with outstanding long-term behavior are needed. The reliability of these devices at high power has been a challenge for years, due to the high built-in strain in the Quantum Well (QW). In this paper, we present excellent reliability results obtained, in both cw and pulsed conditions, on the latest generation of 1064 nm single-lateral-mode diodes developed at 3S PHOTONICS. Aging tests in cw conditions prove the intrinsic robustness of the diode even at very high junction temperatures, while specific tests in pulsed operation at 45 °C heat-sink temperature, and high repetition rates of several hundred kHz, confirm the stability of the devices in accelerated conditions directly derived from real applications. Both free-running and wavelength stabilized (by means of a Fiber Bragg Grating (FBG)) packaged devices show very stable performances under pulsed conditions. Reliable operation at higher average power than currently commercially available diode lasers seeds is demonstrated.

We performed a linearized stability analysis and preliminary simulations of passive phasing in a CW operating ring-geometry fiber laser array coupled in an external cavity with a single-mode feedback fiber that functions as spatial filter. A two-element array with path length error is predicted to have a dynamically stable stationary operating state at the compputer operating wavelength.

Beam quality analysis of mutual off-centered fibers is presented. The relevant scenario is where a single mode input fiber is injecting optical radiation into a few-mode output fiber. Both are step-index, single clad fibers. Assuming the source's coherence time is short enough, the major finding is that the output beam quality is the highest when the injecting fiber is at some offset with respect to the center of the receiving core. A Theoretical analysis and an experiment are in good agreement with each other. These results may have practical consequences in beam-coupling schemes and in applications emphasizing the importance of beam quality.

Different generation modes of all-positive-dispersion all-fibre Yb laser mode-locked due to effect of non-linear polarization evolution are investigated. For the first time we realized in the same laser both generation of single picoseconds pulse train and a newly observed lasing regime where generated are picosecond wave-packets, each being a train of femtosecond sub-pulses. Using both experimental results and numerical modeling we discuss in detail the mechanisms of laser mode-locking and switching of generation regimes and show a strong dependence of output laser characteristics on configuration of polarization controllers. A good qualitative agreement between experimental results and numerical modeling is demonstrated.

The development of the supercontinuum spectrum in the quasi-CW regime is studied analytically, numerically and experimentally. An interpretation in terms of localized periodic structures known as "Akhmediev Breathers" is proposed. Theory, numerical simulation and experiment are in excellent agreement. We also briefly consider the role of breather collisions in the presence of higher order dispersion and show that they lead to the formation of very large amplitude localized structures that may be analogous to the infamous oceanic rogue waves.

A single-mode fiber with a linear or sinusoidal variation in the group-velocity dispersion is designed and fabricated. The nonreciprocal effects and pulse compression due to the longitudinal oscillations of the fiber dispersion are experimentally demonstrated. The periodic modulation of the dispersion can be used to control precisely the pulse dynamics.

Ever demanding network implementations brought new requirements to be addressed to offer cost effective and power efficient solutions with smaller footprints. This general trend together with the constant need to improve L-band optical amplification efficiency account for the renewed interest on highly doped Erbium fibers. Erbium doped fiber amplifiers (EDFAs) performance degradation with Er³⁺ concentration increase has extensively been studied1 and is attributed to additional losses due to energy transfers between neighbouring ions. Experimental observations have been interpreted by the homogeneous up-conversion (HUC) and pair-induced quenching (PIQ) models, which account for pump power penalty and unsaturable absorption respectively. For a given Er3+ concentration, studies have also showed that both fiber manufacturing process and core matrix composition have a strong impact on quenching parameters. In 2009, we introduced a new doping concept involving Al₂O₃Er nanoparticles (NP) in a MCVD-compatible process showing improved performances in terms of erbium homogeneity along the fiber length for standard doping levels.² In this paper, we address our most recent work on concentration quenching encountered in both standard and NP Erbium doped fibers.

We report a scheme for controlling pulse width in a robust self-starting mode-locked ytterbium fiber laser using a semiconductor saturable absorber mirror (SESAM). We demonstrate that the pulse width in a mode-locked laser made of all-normal-dispersive fiber can be adjusted by changing ump power to the laser or by adjusting the axial position of the SESAM with respect to a focusing beam. We have obtained optical pulse width of 7.4 ps and the adjustable range was 2 ps without dispersion compensators in the all-normal-dispersive cavity and provides a high reliability of turn-key operation. We have explained that the principle of position dependent pulse width change in a mode-locked laser with a SESAM and verified with numerical simulations.

We demonstrate the fabrication of a multi-mode (MM) to 61 port single-mode (SM) splitter or "Photonic Lantern". Low port count Photonic Lanterns were first described by Leon-Saval et al. (2005). These are based on a photonic crystal fiber type design, with air-holes defining the multi-mode fiber (MMF) cladding. Our fabricated Photonic Lanterns are solid all-glass versions, with the MMF defined by a low-index tube surrounding the single-mode fibers (SMFs). We show experimentally that these devices can be used to achieve efficient and reversible coupling between a MMF and 61 SMFs, when perfectly matched launch conditions into the MMF are ensured. The total coupling loss from a 100 μm core diameter MM section to the ensemble of 61 SMFs and back to another 100 μm core MM section is measured to be as low as 0.76 dB. This demonstrates the feasibility of using the Photonic Lanterns within the field of astrophotonics for coupling MM star-light to an ensemble of SM fibers in order to perform fiber Bragg grating based spectral filtering.

We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system, the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing threshold can be enhanced.

We report on a novel concept for monolithic pump combining technology to integrate efficiently multi pump fiber channel into a double clad ytterbium doped fiber. The proposed structure consists of a dichromatically coated planar convex lens spliced to an Ytterbium-doped double-clad photonic crystal fiber surrounded by multiple pump fibers. The lens is also used as a protecting end cap where the laser beam expands before exiting the surface. The pump fibers are also attached in this lens circularly surrounding fibers. The lens images these pump fibers end facets into the pump core, where the lens surface is coated by a dichroic mirror (reflective for 980 nm, transmissive for >1030 nm). The allglass structure, assembled by laser splicing, makes the system stable, efficient and suitable for high power operation. We selected 5 channels as testing channels among 14 pump channels (200 μm, NA=0.12) in order to confirm reliability of the system. The coupled pump power efficiency into the 500 μm core with NA=0.5 was over 80% and typical slope efficiency of the laser output is over 70%. Theoretical analysis was discussed in order to get optimized parameters and scaling this type of coupler to higher average powers is considered. With the monolithic pump combining technologies, we confirmed that the proposed device has a potential application not only in kW range high power fiber lasers but also compact photonic devices.

We present an all-silica fiber-based module with anomalous dispersion below 800nm. The fiber module is based on propagation in a higher-order-mode (HOM), and mode conversion is achieved using UV inscribed broadband longperiod gratings. The large normal material dispersion in silica in the near infrared is compensated by anomalous waveguide dispersion of the HOM resulting in a total HOM dispersion of +112.7ps/(nm•km) (β₂ = -0.0355ps²/m) at 770nm. The dispersion has been calculated from the preform index profile and measured with a white light interferometer. The operation bandwidth is ~20nm with an insertion loss of ~1.5dB. The multipath interference noise is less than -27dB in the operation bandwidth. Nearly linear pulse propagation can be obtained for pulse energies up to 65pJ at 75fs pulse duration. This power regime is interesting for e.g. medical two-photon fluorescence imaging. The proposed anomalous dispersion module is demonstrated in a 3.6m long femtosecond fiber delivery application to deliver 110fs pulses directly from the output of a Ti:Sapphire femtosecond laser without the need for pre-chirping.

We present a systematic study on the inhibition of stimulated Raman scattering by lumped spectral filters both in passive optical transport fibers and in fiber amplifiers. This study reveals the parameters that have the strongest influence on the suppression of the Raman scattering (such as the attenuation at the Raman wavelength and the insertion losses at the signal wavelength). These parameters have to be optimized in order to achieve the desired Raman inhibition and/or to minimize the loss in amplifier efficiency. The study is concluded with realistic predictions on the use of spectral filtering elements for Raman scattering inhibition in real-world high power fiber amplifiers. Thus, using for example 10 lumped spectral filters with 20 dB effective Raman attenuation and less than 0.25 dB insertion losses, a maximum Raman threshold increase by a factor of 3 is expected. In this context, long period gratings are proposed as promising filtering elements for Raman inhibition in high power fiber amplifiers. In order to experimentally verify the theoretical predictions and the suitability of long period gratings, a fiber amplifier consisting of 2 m active Ytterbium doped fiber was built. Three long period gratings were consecutively inserted at different positions along the fiber, and the Raman threshold was determined for each situation. It is shown that, with three long period gratings, the Raman threshold (defined as the 20 dB ratio of Raman to signal output power) was increased by about 60%, which offers a good agreement with the theoretical predictions.

We experimentally demonstrate phase-shaping in fiber CPA-systems, providing pulse-energies at the mJ-level. The applied method is based on an analytical model describing the impact of SPM in CPA-systems. Using this phase-shaping technique nearly transform limited pulses are produced at B-integrals up to 10 rad. Compared to a nonlinear CPAsystem with the best performance being achieved by adjusting the compressor, operation of the same system using the phase-shaping method permits peak-power enhancement by a factor better than 2.

We present experimental verification of a novel technique to suppress stimulated Brillouin scattering (SBS) in single frequency fiber amplifiers. This technique relies on seeding with a combination of broadband and single frequency laser beams to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber configuration was used. Broadband 1045 nm light and single frequency 1064 nm light were coupled into an Yb-doped gain fiber. With appropriate selection of seed power ratio, we were able to generate an output signal predominantly comprised of 1064 nm light while simultaneously suppressing the back-scattered Stokes light. The slope efficiency for the two-tone amplifier was approximately 78%; slightly below that of a single-tone amplifier. The SBS threshold for the former, on the other hand, was appreciably higher than that of the latter which is in excellent agreement with the theory. In preliminary implementation of this technique at high power, we generated close to 100 W without encountering the SBS threshold. Finally, we show numerically that due to a favorable thermal gradient much higher powers can be obtained.

Based on a depressed-clad index profile design, a PM Yb-doped large mode area (LMA) fiber with an effective mode area of 450 μm² was designed and fabricated. The fiber was used to amplify 10-ns pulses at 1064 nm with a repetition rate of 100 kHz, and an output energy higher than 200 μJ was obtained, within a bandwidth of 0.5 nm. The fiber was coiled on a 12-cm diameter mandrel to obtain a single-mode output having a measured M² value of 1.04. The output polarization extinction ratio was higher than 20 dB. The photodarkening excess loss of the Ytterbium//Phosphorus Aluminum co-doped fiber was measured to be a factor 5 lower than that of a reference Ytterbium/Aluminum co-doped fiber. It is shown how a depressed-clad index profile design can improve higher-order mode filtering while keeping the coiling diameter practical for compact fiber amplifier packaging.

We study the temperature of an Yb-doped large-mode-area (LMA) fiber during an accelerated photodarkening experiment. In these measurements, photodarkening is optically induced by IR irradiation (i.e. 915 nm) while the fiber temperature is measured by a thermal camera. Fiber temperature is observed to exceed 120 °C under conditions of 10.5 W of pump power and unforced air cooling. We show evidences that this temperature increase is caused by the lost pump power due to photodarkening. A thermal model is used to explain the fiber temperature in terms of pump power absorbed by photodarkening-induced defects. Furthermore, the effect of temperature on the rate of photodarkening and saturation of the losses is studied. Both the photodarkening saturation level and the photodarkening rate are observed to show significant temperature dependence that result on a variation of the photodarkening rate ion dependency. The use of an air cooling system and low inversion measurements is shown to reduce the ion dependency from 7 to 4.5.

We report numerically and experimentally analysis of optic fiber Sagnac interferometer and fine adjustment of cavity loss by the use of the FOLM with a hi-bi fiber in the loop. Changes in transmittance profile amplitude and wavelength shift are caused by the whirl effect in the connectors of a coupler with ports output connected to a birefringent fiber. Also the experimental demonstration of dual wavelength operation of a fiber laser through fine adjustment of cavity loss, using a Fiber Optical Loop Mirror (FOLM) with a high-birefringence fiber in the loop. The reflection and transmission of the FOLM presents a sinusoidal wavelength dependence which can be shifted by controlling the temperature of the hibi fiber. A temperature change of the hi-bi fiber by 0.1°C causes a measurable change in the ratio between the reflectance for the wavelengths R(λ₁)/R(λ₂). Using this adjustment be able to change the generation mode from single wavelength to stable dual wavelength generation with equal powers for λ₁ and λ₂ or to stable dual wavelength generation with unequal powers at λ₁ and λ₂. The change of the ratio between the FOLM reflection R(λ1)/R(λ2) allows the investigation of tolerance of dual wavelength generation on the ratio between cavity loss. Was found that for the switch from a single wavelength emission at λ₁ to single wavelength emission at λ₂ the ratio R(λ₁)/R(λ₂) has to be changed by the order of magnitude of 10^-2. This value shows the tolerance of the dual wavelength laser to the cavity loss adjustment.

To our knowledge, we report the first Fourier domain modelocked laser (FDML) constructed using optical parameter amplifier (OPA) in conjunction with an erbium-doped fiber amplifier (EDFA), centered at ~1556nm. We utilized a onepump OPA and a C-band EDFA in a series configuration with a polygon-grating wavelength filter to generate a hybrid FDML spectrum. Results demonstrate a substantially higher output power, better spectral shape and significantly more stable bandwidth than individual configurations. We believe this technique has the potential to enable several amplifiers to complement individual deficiencies resulting in improved spectral shapes and power generation for imaging applications such as optical coherence tomography (OCT).

In this study, a cascaded Raman fiber laser in Fourier domain mode lock operation (FDML) is presented. This laser utilizes a Ytterbium doped twin core pump laser source at 1109 nm. The pump light is directed to a cascaded Raman cavity, which consists of multiple cascaded fiber Bragg grating pairs and 3.86 km of dispersion compensation fiber, which provides Raman gain. The output wavelength of a cascaded Raman laser is determined by the Stoke's shift (≈ 60 to 70 nm in optical fiber) and the pump laser wavelength. The power build up in the cascaded Raman cavity and shift to higher Stoke's orders produce multiple spectral peaks. At higher Stoke's orders, the overlapping Raman peaks create broad bandwidth gain with relatively large gain ripples. FDML operation using a polygon-based tunable filter helps to suppress the ripples. The overall laser in linear operation has a bandwidth of 316 nm with a center wavelength of 1445 nm. An output optical power was measured to be (> 10 mW). On the other hand, the sweeping bandwidth was 35 nm with an output power in the micro-watt range. The utilization of broadband tunable lasers are important in applications such as swept-source optical coherence tomography for use in biomedical imaging.

We demonstrate high efficiency and wide bandwidth gain in a Ytterbium doped fiber amplifier. The highpowered amplifier has potential applications for use with a swept-source fiber ring laser in multi-channel optical coherence tomography (OCT) system. The ring cavity design includes a 976nm pumped dual core Yb doped fiber as the gain medium, where a rotating polygon mirror is used as a wavelength-sweeping filter for this source. The amplified spontaneous emission (ASE) had a spectral bandwidth of 1037-1145nm at -60dBm, where a tunable lasing bandwidth of the ring cavity ranged from 1057-1115nm. The highest output power, for both the ASE and lasing spectrum, with this configuration was ~200mW, however it is possible to have a larger bandwidth and a larger output power. Higher power, in the wattage range is achievable if free space components are employed. Pumped with 976nm light at 1.27W, the use of this novel dual core Yb doped fiber as an amplifier has been successfully demonstrated, as it provided a small signal gain of 29.6 dB at 1085nm, where the gain medium was successfully saturated during operation. This is important for the spectral shaping requirements of OCT to improve image quality. The gain was demonstrated for several different wavelengths and for several pumping powers at a 1085nm wavelength. Fourier domain mode locked operation (FDML) was achieved with a bandwidth of 15nm and a sweep rate of 51.4kHz. This laser source offers a low-cost, high power alternative for biomedical imaging with multi-channel optical coherence tomography.

For the first time, the self-mode-locking regime was obtained in an all-fiber erbium laser with a cavity length of more than 1 km and a normal net cavity dispersion as large as 217 ps². To construct this laser, only commercial telecommunication fibers and conventional fiber-optic elements were used. An original linear-ring cavity design with polarization instability compensation provides high reliability of the self-mode-locking regime and good frequency stability. The laser emits short (~5 ns) pulses with a record high energy (564.3 nJ) at a very low repetition rate (82.4 kHz) and a moderate pump power of 450 mW. Since no output saturation and no wave-breaking effects were observed at the available pump power, the pulse energy can be further augmented by increasing the pump power or (and) by lengthening the cavity. Such a laser can be used in LIDAR systems, telecommunications, and industry.

Diffraction-limited sources around 976 nm are attractive to build sources around 488 nm, or to pump rare-earth-doped materials. Continuous wave efficient lasers have been obtained with single-mode fibres, in which we demonstrate pulsed laser diode amplification with gains higher than 30 dB. Theoretical studies can predict the amplifier performances versus the input signal characteristics and show the pulse width influence on excited state population. We used a classical timespace two-level model that we solved thanks to a Finite-Difference Time-Domain method. These simulations taking into account both time and space evolutions and a parasitic laser effect describe correctly the experimental results.

At the Photonics West 2008 we presented our rare earth doped fused bulk silica for fiber laser applications [1]. This approach overcame the typical geometrical limitations of other well known production methods for rare earth doped silica materials. Our unique production technique is based on the sintering of Yb-doped granulates of high-purity SiO2 particles. We have processed our Yb-doped bulk silica rods into ultra large mode area (XLMA) multi-mode double cladding laser fibers with an active core diameter in the range of 40 μm to 100 μm (depending on the core doping level). In the XLMA fiber the active core is surrounded by a so-called 2D- or 4D-shaped pure silica pump cladding (with diameter between 850 μm and 900 μm) and an F-doped outer silica cladding with an outer diameter of 1000 μm. We have investigated the refractive index and the intrinsic stress profiles of different XLMA laser fibers and their preforms to visualize interface effects. The fiber cross section designs, the quality of all interfaces and the material composition are important factors for the laser fiber performance. The laser properties of these fibers have been investigated in detail. In addition, the preparation of the fiber end-face is important to reduce heat effects and we have developed concepts to mitigate such thermal load at the fiber end face.

An erbium-doped fiber laser cavity design is demonstrated to produce radially and azimuthally polarized beams. A c-cut calcite crystal is set within a three-lens telescope system in the laser cavity. Due to the birefringence of the crystal, radial and azimuthal polarizations are focused to different foci. The ray behavior through the crystal is discussed in details with birefringent ray tracing. By translating a collimation lens, one can select either the radial or the azimuthal polarization to lase in the cavity. The maximum output power obtained is about 140 mW for both polarizations.