This PDF file contains the front matter associated with SPIE Proceedings Volume 9344, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

Fiber lasers are in the process of revolutionizing modern manufacturing. Further power scaling is still much desired to increase throughput and to break new frontiers in science and defense. It has become very clear now that highly single-mode fibers with large effective mode areas are required to overcome both nonlinear effects and mode instability [1-3]. We have been studying all-solid photonic bandgap fibers (AS-PBF), which have open and highly dispersive cladding, making them ideal for higher-order-mode controls in large-mode-area fibers. I will review our recent progress in this area and, especially in ytterbium-doped AS-PBF lasers and amplifiers.

Polarizing optical fibers are important components for building compact fiber lasers with linearly polarized laser output. Conventional single-mode optical fibers with birefringence can only preserve the polarization when the incident beam is launched properly. Recent reports demonstrate that the birefringence in photonic bandgap fibers (PBFs) can provide single-polarization operation near the edge of transmission band by shifting the transmission band for the light with orthogonal polarizations. Here, we demonstrate a 50μm core Yb-doped polarizing photonic bandgap fiber (PBF) for single-polarization operation throughout the entire transmission band from 1010nm to 1170nm with a polarization extinction ratio (PER) of >5dB/m, which is >15dB/m near the short wavelength edge of the transmission band. The polarizing effect is due to the differential polarization transmission loss presented in this fiber, which is benefited from the fiber birefringence of 3.2x10^-4, obtained by incorporating low-index boron-doped rods on either side of the core. The achievement is based on the fact that light at fast axis has lower effective mode index which is closer to the modes in the photonic cladding and thus to be easily coupled into cladding. A 2.6m long straight fiber was tested in a laser configuration without any polarizers to achieve single polarized laser output with a PER value of 21dB at 1026nm lasing wavelength.

Large mode area double-cladding Photonic Crystal Fibers (PCFs) with reduced cladding symmetry can provide effective suppression of high-order modes even under a significant heat load. In this work the single mode properties of different PCFs with reduced cladding symmetry are numerically compared by means of a full-vector modal solver based on the finite-element method, capable of taking into account the effects of the fiber heating. The maximum coupled pump power allowing single-mode propagation has been evaluated for each design, providing useful guidelines for further development of this kind of fibers.

Cylindrically symmetric hybrid fiber proposed by our group recently allow one to achieve high anomalous dispersion (13-100 ps/nm/km) in the 1 μm spectral range. Refractive index of such fibers consists of one or two high index ring layer surrounding the core and depressed layer placed outside the ring layer. The drawback of the realized designs was presence of modes, localized in high-index rings, which tails nevertheless spread over the hybrid fiber core. Excitation of the operating hybrid mode LP₀₃ in the two high-index ring design (or LP₀₂ in the one high-index layer design) was accompanied by weak excitation (10-20% of total propagating power) of the modes, located in the high-index rings, that has non-zero electric field at fiber axis (mainly LP₀₂ mode for two high-index layers design and LP₀₁ for one high-index layer design).

In the present work we have experimentally demonstrated possibility to suppress all the propagating modes except the operating hybrid one (LP₀₃ for two high-index layer design). The main idea is based on introduction a thin ring layer with an extremely high optical loss to the position of electrical field minimum of the operating hybrid mode. Analysis of the mode composition carried out by off-center hybrid mode excitation confirms the asymptotically single-mode propagation regime.

Ytterbium-doped large mode area all-solid photonic bandgap fiber amplifiers were used to demonstrate <400 W of output power at 1064 nm. In an initial set of experiments, a fiber with a core diameter of ~50 μm, and a calculated effective area of 1450 μm² in a straight fiber, was used to generate approximately 600 W. In this case, the input seed was modulated using a sinusoidal format at a frequency of 400 MHz. The output, however, was multimode as the fiber design did not allow for single-mode operation at this wavelength. A second fiber was then fabricated to operate predominantly in single mode at 1064 nm by having the seed closer to the short wavelength edge of the bandgap. This fiber was used to demonstrate 400 W of single-frequency output with excellent beam quality. As the signal power exceeded 450 W, there was significant degradation in the beam quality due to the modal instability. Nevertheless, to the best of our knowledge, the power scaling results obtained in this work far exceed results from prior state of the art all-solid photonic bandgap fiber lasers.

In this work we present results on the amplification of narrow-linewidth nanosecond pulses in a recently developed ytterbium-free erbium-doped large mode area fiber cladding pumped at 980 nm. At first, a co-propagating all fiber amplifier scheme was used. Using 1 m of active fiber length in such configuration a peak power of 4 kW limited by instabilities caused by stimulated Brillouin scattering (SBS) was achieved. To the best of our knowledge this is the highest peak power ever reported for single-frequency single mode silica-based fiber sources near 1550 nm. Then all fiber counter-pumped configuration with 6 m of active fiber was used and peak power of 900 W was obtained. In this case peak power was limited by SBS generation in a passive fiber at the laser output. Slope electrical-to-optical conversion efficiency of 8 % demonstrated in such scheme is a record value for kW-level single mode single-frequency Er-doped nanosecond fiber lasers.

In scope of the ESA funded “High stability Laser” activity, a single-mode and single-frequency fiber power amplifier with 500 mW output power at 1064 nm wavelength has been developed. It is part of an elegant breadboard (EBB) which consists additionally of an ultra-stable Fabry-Perot reference for frequency stabilization. The monolithic fiber amplifier is seeded by a non-planar ring oscillator (NPRO) with a linewidth below 10 kHz. The amplifier is stabilized in power via pump diode modulation and achieves a RIN performance of < 0.01/sqrt(Hz) in the range from 10^-3 Hz to 10 Hz and a polarization extinction ratio of >30 dB.

High-efficiency fiber-based extreme ultraviolet driver in the alignment-free configuration has been experimentally achieved with a maximum intensity of 6.4×10¹⁰ W/cm² on target at a repetition rate of 20 kHz. The output EUV signal within 10~20 nm in wavelength was confirmed with a Si/Zr-coated x-ray photodiode by varying numbers of Be and Al foil filters. The measured spectral range is consistent with that obtained by the weighted oscillator strengths of Sn⁸+ to Sn⁺¹³ ions using an one-dimensional hydrodynamic code coupled with the ionization model of collisional-radiative equilibrium. The driver is based on a 1064-nm nanosecond coiled ytterbium all-fiber laser system in diode-seeded master oscillator power amplification. With an overall optical efficiency up to 56%, it can deliver a 1.16-mJ, 117-kW, 6.1-ns laser pulses with a FWHM linewidth of 10 nm and beam propagation factor of M²~1.55. The full advantages of using fiber laser for a movable LPP EUV metrology source are revealed.

We present the experimental and theoretical analysis of a mode modulator in a few mode LMA fiber. The mode modulator consists of a section with a modified refractive index alongside the fiber core in the cladding, disturbing the guidance of the modes in the core. The extent of excitation of these disturbed modes depends on the overlap of the excited undisturbed and disturbed modes. At the end of the modulator, undisturbed modes will be excited again in the fiber core, in dependency of the spatial field distribution of the disturbed modes at the boundary. In the mode modulator disturbed higher order modes lead to modal interference, causing a dependency of the spatial distribution of the light in the mode modulator on the propagation length of the disturbed modes. Hence, the modal output field depends on the length of the mode modulator. For the experiments, the mode modulator was inscribed directly into the LMA fiber with ultrashort laser pulses. During the inscription process the modal content at the end of the fiber was measured using a computer generated hologram as a correlation filter. In dependency of the length of the modulator strong oscillations between the content of the fundamental and the higher order modes are observable. In the case of an initially excited fundamental mode, its content could be reduced to below 5%, whereat the content of the LP11 modes was up to 90%. While measurement and simulation show qualitative agreement, differences are caused by inhomogeneities of the refractive index modifications.

A novel approach for stripping cladding light from double clad fibers is demonstrated. This is achieved by index matching the cladding of the fiber with a glass capillary collapsed onto the fiber, allowing the cladding modes to expand in a larger volume of the capillary before they are dissipated through a high-index heat sink material into a metal package. We minimize the signal quality degradation by using a lower melting point capillary glass. We demonstrate a device with 100W cladding power removed with 99% (20dB) extinction. Continuous operation for an hour without any power degradation is demonstrated.

Fiber lasers provide the foundation to combine an excellent beam quality in single mode operation with a robust and highly efficient design. Based on fiber-integrated configurations, they are employed in many different applications ranging from industry over research to medical technology. However, there is lots of potential to approach even new fields of applications e.g. in spectroscopy based on tunable systems with an adjustable emission wavelength. We present a novel tuning concept for pulsed fiber-integrated laser systems using an array of fiber Bragg gratings (FBGs) as discrete spectral filter. Based on stacking many standard FBGs, the bandwidth and filter properties are easy to scale by increasing the number of gratings allowing huge tuning ranges as well as tailored tuning characteristics. In this work, we demonstrate the potential of this electrically controlled tuning concept. Using an Ytterbium (Yb)-doped fiber laser, we investigate the general tuning characteristics. With variable pulse durations in the nanosecond regime, we demonstrate high signal contrast (~45 dB), excellent wavelength stability and narrow linewidth (<15 GHz). In order to highlight the great spectral freedom, a tuning range of 74 nm in the Yb band is realized which, to the best of our knowledge, is the largest bandwidth reported based on a monolithic filter design.

A new transversal pumping scheme of fiber lasers based on the optimized manufacturing of an array of large scale ridge waveguides in fused silica is presented. Moreover their application as directional couplers interacting with a double clad optical active fiber for laser application is discussed. Conventional broad area emitters without slow axis collimation (SAC) can be used to couple light (wavelength λp = 976 nm) into the waveguide array.

We demonstrate the use of a pulsed superluminescent diode (SLD) through direct current injection modulation as seeding source in a master oscillator power amplifier (MOPA) configuration when compared to a Fabry-Perot (FP) laser diode in the same system. The performance limitations imposed by the use of the Fabry-Perot lasers, caused by the backward high peak power pulses triggered due to stimulated Brillouin scattering (SBS) are not observed in the case of the SLD. Compared to conventional Fabry–Perot laser diodes, the SLD provides a smooth and broad output spectrum which is independent of the input pulse parameters. Moreover, the spectrum can be sliced and tailored to the application. Thus, free SBS operation is shown when using the SLD seeder in the same system, allowing for a significant increase on the extractable power and energy.

In this work the latest progress in the understanding of mode instabilities is reviewed. Particular emphasis is put on the recently established influence of photodarkening on the mode instability threshold and its behavior. It is shown, for example, that even degradations of the output power in the order of a few percent can lead to very significant reductions of the mode instability threshold. Moreover, our analysis shows that photodarkening also alters the expected behavior of the mode instability threshold with respect to the signal wavelength and the seed power. Thus photodarkening is revealed as one of the main effects shaping the behavior of the mode instability threshold observed in experiments.

We show how signal bandwidth affects the gain of stimulated thermal Rayleigh scattering (STRS) which is responsible for a transverse mode instability in fiber amplifiers. The gain is reduced if the coherence time of the signal is less than the group-velocity-induced walk off between the two interacting modes, usually LP₀₁ and LP₁₁. We derive expressions for the bandwidth required to suppress gain for short pulses, for periodically chirped continuous waves, and for general periodically modulated cases.

We propose a novel fiber design single-trench fiber (STF) for large mode area fiber laser and amplifier. Fiber offers cylindrical symmetry and also offer higher refractive index of core compared to cladding. This avoids the need of stack and draw process and refractive index compensation of core doped with index raising rare earth and co-dopants ions, which are an indispensable condition in most of other fiber designs. That is why, this fiber design can be fabricated with conventional modified chemical vapour deposition process in conjunction with solution doping process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Fiber offers very high loss (>10dB/m) and low power fraction in core (<50%) to the higher order modes for low loss of fundamental mode (<0.1dB/m) and high power fraction in core (>80%) thanks to the resonant coupling between core and ring modes. We fabricated a preform for 30μm core STF using MCVD process in conjunction with solution doping process in a single step, without using any micro-structuration and pixilation of core. Experiments ensure the robust single mode behaviour irrespective of launching condition of input beam.

Furthermore, this fiber used in three stages MOPA provides 80kW peak pulses with repetition rate of 500 kHz, average power up to 10W, with M² ~ 1.14. Moreover, all solid structure ensures easy cleaving and splicing. In nutshell, an ultra-low cost, monolithic, compact, and an effective single mode fiber amplifier device can be achieved using single-trench fiber.

We proposed and investigated a novel tapered ytterbium-doped fiber design. The tapered fiber has length of 2.1 m, single-mode thin end and thick end with core/clad diameter of about 50/430 µm. Master-oscillator power-amplifier monolithic fiber scheme have been developed using this fiber and 5 ps duration, 28 nm spectral width and 0.5 MW peak power pulses was achieved at the output. FROG measurements reveal linear chirp that allow one to expect decompression of these pulses down to sub-100 fs duration.

We report on the development of large-core Yb-doped fibers with up to 100 μm core diameter and present first experimental results for high peak power amplification. The material for core and pump cladding was fabricated by Powder Sinter Technology. Using a high Al concentration we achieved a numerical aperture (NA) of 0.21 of the pump cladding and a core NA below 0.1. The rod-type fiber exhibits high pump absorption. Using a 0.55 m short fiber sample as the main amplifier in a 3-stage ns pulsed fiber Master Oscillator Power Amplifier system we achieved 3 ns output pulses with 360 kW peak power and 2 mJ pulse energy. We observed suppressed Stimulated Raman Scattering with respect to the signal pulses, which offers the possibility of further power scaling of such fiber amplifier systems.

Raman fiber laser is an efficient way to expand the spectral coverage of fiber lasers. In recent years, output power of Raman fiber laser has been scaled quickly. There is a great potential in further power scaling, technical innovations, and scientific applications. An integrated ytterbium-Raman fiber amplifier architecture was proposed, which allows power scaling of Raman fiber laser to over kilowatt and more. Hundred watt level single frequency Raman fiber amplifier was achieved, which allows the generation of high power sodium guide star laser. New scheme of cladding pumped Raman fiber laser is studied in order to improve the brightness enhancement. Furthermore, possible application in spectral beam combing is discussed.

We report the optical system design of a novel speckle-free ultrafast Red-Green-Blue (RGB) source based on angularly multiplexed simultaneous second harmonic generation from the efficiently generated Stokes and anti-Stokes pulses from a commercially available photonic crystal fiber (PCF) with two zero dispersion wavelengths (TZDW). We describe the optimized configuration of the TZDW fiber source which supports excitations of dual narrow-band pulses with peak wavelengths at 850 nm, 1260 nm and spectral bandwidths of 23 nm, 26 nm, respectively within 12 cm of commercially available TZDW PCF. The conversion efficiencies are as high as 44% and 33% from the pump source (a custom-built Yb:fiber master-oscillator-power-amplifier). As a result of the nonlinear dynamics of propagation, the dual pulses preserve their ultrashort pulse width (with measured autocorrelation traces of 200 fs and 227 fs,) which eliminates the need for dispersion compensation before harmonic generation. With proper optical design of the free-space harmonic generation system, we achieve milli-Watt power level red, green and blue pulses at 630 nm, 517 nm and 425 nm. Having much broader spectral bandwidths compared to picosecond RGB laser sources, the source is inherently speckle-free due to the ultra-short coherence length (<37 μm) while still maintaining an excellent color rendering capability with >99.4% excitation purities of the three primaries, leading to the coverage of 192% NTSC color gamut (CIE 1976). The reported RGB source features a very simple system geometry, its potential for power scaling is discussed with currently available technologies.

In this paper we demonstrate a 51.5 W supercontinuum spanning 405 nm to 2660 nm with endlessly single mode output, and a blue enhanced 33.6 W supercontinuum spanning 380 nm to 2575 nm. We also show results combining 7 SC laser outputs, resulting in 26 W visible power. This method of combining fiber laser outputs can be scaled up, creating a pathway to kW level supercontinuum fiber lasers with greater than 95 W in the visible part of the spectrum.

We report on an ultraviolet-enhanced supercontinuum generation in a uniform photonic crystal fiber pumped by a giant-chirped mode-locked Yb-doped fiber laser. We find experimentally that the initial pluses with giant chirp leads more initial energy transferred to the dispersive waves in visible and ultraviolet wavelength. An extremely wide optical spectrum spanning from 370 nm to beyond 2400 nm with a broad 3 dB spectral bandwidth of 367 nm (from 431 nm to 798 nm) is obtained. Over 36% (350 mW) of the total output power locates in the visible and ultraviolet regime between 370 nm and 850 nm with a maximum spectral power density of 1.6 mW/nm at 550 nm. In addition, a blue-enhanced supercontinuum generation pumped by a giant-chirped SESAM mode-locked ytterbium-doped fiber laser is studied. An extremely wide optical spectrum spanning from 380 nm to 2400 nm with total power of 3 W is obtained.

We report high power all fiber mid-infrared (mid-IR) supercontinuum (SC) generation in a single-mode ZBLAN (ZrF_4- BaF_2-LaF_3-AlF_3-NaF) fiber with up to 21.8 W average output power from 1.9 to beyond 3.8 μm pumped by amplified picosecond pulses from a master oscillator power amplifier (MOPA) based on small-core single-mode thulium-doped fiber (TDF) with injected seed pulse width of 24 ps and repetition of 93.6 MHz at 1963 nm. The optical-optical conversion efficiency from the 793 nm pump laser of the last stage thulium-doped fiber amplifier (TDFA) to mid-IR SC output is 17%. It is, to the best of our knowledge, the highest average power mid-IR SC generation in a ZBLAN fiber to date. In addition, a noise-like fiber oscillator based on a nonlinear loop mirror (NOLM) with wavepacket width of ~1.4 ns and repetition rate of 3.36 MHz at 1966 nm is also used as a seed of the MOPA for mid-IR SC generation in the ZBLAN fiber. At last, a mid-IR SC from 1.9 to beyond 3.6 μm with average output power of 14.3W, which is limited by injected noise-like pulses power, is generated. The optical-optical conversion efficiency from the 793 nm pump laser of the last stage TDFA to mid-IR SC output is 14.9%. This proves the amplified noise-like pulses are also appropriate for high power mid-IR SC generation in the ZBLAN fiber.

We present a power-scalable approach for yellow laser-light generation based on standard Ytterbium (Yb) doped fibers. To force the cavity to lase at 1154 nm, far above the gain-maximum, measures must be taken to fulfill lasing condition and to suppress competing amplified spontaneous emission (ASE) in the high-gain region. To prove the principle we built a fiber-laser cavity and a fiber-amplifier both at 1154 nm. In between cavity and amplifier we suppressed the ASE by 70 dB using a fiber Bragg grating (FBG) based filter. Finally we demonstrated efficient single pass frequency doubling to 577 nm with a periodically poled lithium niobate crystal (PPLN). With our linearly polarized 1154 nm master oscillator power fiber amplifier (MOFA) system we achieved slope efficiencies of more than 15 % inside the cavity and 24 % with the fiber-amplifier. The frequency doubling followed the predicted optimal efficiency achievable with a PPLN crystal. So far we generated 1.5 W at 1154nm and 90 mW at 577 nm. Our MOFA approach for generation of 1154 nm laser radiation is power-scalable by using multi-stage amplifiers and large mode-area fibers and is therefore very promising for building a high power yellow laser-light source of several tens of Watt.

Nanoparticle (NP) doping is a new technique for making erbium-doped fibers (EDFs); the Er ions are surrounded by a cage of aluminum and oxygen ions, substantially reducing Er³⁺ ion-ion energy exchange and its deleterious effects on laser performance. Er-Al-doped NPs have been synthesized and doped in-situ into the silica soot of the preform core. We report the first known measurements of NP-doped EDFs in a resonantly-core pumped master oscillator-power amplifier (MOPA) configuration; the optical-to-optical slope efficiency was 80.4%, which we believe is a record for this type of fiber.

Very high temperature operation of a rare earth doped fibre laser is investigated. An all-glass ytterbium-doped fibre laser is demonstrated, operating at heat sink temperatures ranging from 25°C to 400°C and output powers of > 40 W. Over this temperature range only a small reduction in laser output power was observed. The emission spectrum of this source was also measured for heat sink temperatures up to 400°C. We discuss the advantages of operating at such extended temperatures such as weight savings from reduced heat sink size, extended long wavelength operation and suppression of detrimental non-linear effects.

With the use of fiber lasers pervading diverse applications and environmental conditions, the long-term reliability of low index (LI) polymer coated double-clad (DC) fibers used for this purpose is significant. Mechanical reliability standards for 125um fibers are well established by Telcordia GR-20-CORE requirements and some work1 has been published investigating optical reliability of DC fibers with specially engineered coatings with respect to accelerated temperature and humidity aging. While these are helpful in providing a figure of merit to the optical reliability of LI fluoroacrylate coatings, it becomes important to decouple the effects of temperature and humidity in order to understand the underlying degradation mechanisms of LI polymers in various storage and operating environments. This paper identifies the effects of temperature and humidity on the spectral attenuation of DC fibers and presents a reliability model capable of predicting lifetimes under prolonged exposure to typical temperature and humidity conditions experienced during storage and operation of fiber lasers.

We demonstrate for the first time both spatial and temporal multiplexing in a scalable amplification scheme of ultrashort pulses. Using a division into two amplification channels and four pulse replicas high recombination efficiencies have been achieved at output energies far beyond the single-emitter damage threshold.

Over the last decade, the performance of femtosecond fiber laser systems has been rapidly improved. However, further improvements might be held back due to different physical limitations such as nonlinearities or optically induced damage. We demonstrate that with the coherent combination of four parallel fiber amplifiers record pulse energies and peak-powers of 5.7 mJ and 22 GW, respectively, could be achieved. These values could be realized with a chirped-pulse-amplification (CPA) laser system running at a repetition rate of 40 kHz and delivering a compressed average power of 230 W. A high combination efficiency of 89% was achieved demonstrating the scalability of the combining approach to a larger number of channels.

Femtosecond fiber chirped pulse amplifiers have numerous advantages, but are limited in energy because of the small interaction area with the fiber core. In this contribution, we create two orthogonally-polarized stretched pulse replicas in the time domain, following the divided-pulse amplification (DPA) principle. This beam is subsequently separated into two counter-propagating beams in a Sagnac interferometer to finally generate four pulse replicas. These pulses are amplified in two state-of-the-art large mode area rod-type fiber amplifiers in series, before final coherent combination and compression.

Because the stretched-pulse duration is of the order of hundreds of picoseconds, the DPA delay is induced using a freespace interferometer with reasonable arm lengths of few tens of centimeters. The use of a common interferometer to divide and recombine temporal pulse replicas, together with the Sagnac geometry, results in an identical optical path for all four replicas. Therefore, the whole spatio-temporal combining architecture is passive, avoiding the need for active electronic stabilization systems. Because we only use two temporal replicas, the system is immune to differential saturation levels or B-integrals between successive pulses: this is compensated by controlling the amplitude of both pulses at the input of the amplifying setup.

This setup allows the generation of 1 mJ, 300 fs compressed pulses at 50 kHz repetition rate, corresponding to 50 W output average power, with a combining efficiency above 90% at all power levels.

The active phase stabilization of spatially and temporally combined ultrashort pulses is investigated theoretically and experimentally. Particularly, considering a combining scheme applying 2 amplifier channels and 4 divided-pulse replicas a bistable behavior is observed. The reason is mutual influence of the optical error signals that is intrinsic to temporal polarization beam combining. A successful mitigation strategy is proposed and is analyzed theoretically and experimentally.

We present an Erbium:Ytterbium codoped fiber-amplifer system based on Divided-Pulses-Amplification (DPA) for ultrashort pulses. The output from a saturable-absorber mode-locked polarization-maintaining (PM) fiber oscillator is amplified in a PM normal-dispersion Erbium-doped fiber. After this stage the pulses are positively chirped and have a duration of 2.0 ps at an average power of 93 mW. A stack of 5 birefringent Yttrium-Vanadate crystals divides these pulses 32 times. We amplify these pulses using a double-clad Erbium:Ytterbium codoped fiber pumped through a multimode fiber combiner. The pulses double pass the amplifier and recombine in the crystals using non-reciprocal polarization 90◦ rotation by a Faraday rotating mirror. Pulses with a duration of 144 fs are obtained after separation from the input beam using a polarizing beam splitter cube. These pulses have an average power of 1.85 W at a repetition rate of 80 MHz. The generation of femtosecond pulses directly from the amplifier was enabled by a positively chirped seed pulse, normally dispersive Yttrium-Vanadate crystals, and anomalously dispersive amplifier fibers. Efficient frequency doubling to 780 nm with an average power of 725 mW and a pulse duration of 156 fs is demonstrated. In summary we show a DPA setup that enables the generation of femtosecond pulses at watt-level at 1560 nm without the need for further external dechirping and demonstrate a good pulse quality by efficient frequency doubling. Due to the use of PM fiber components and a Faraday rotator the setup is environmentally stable.

High power combiners are essential for practical fiber lasers, recent developments in pump technology has increased the available brightness and power of pumps significantly, enabling multi kW lasers and pushing combiner designs to new limits. I will present the challenges, measurements and some solutions to these issues. Traditional calculations for combiners underestimate the issues associated with the ‘tails’ of the pump NA distribution, losses in fully filled combiners increase rapidly as pump NA blooms, and subsequent heating effects dominate the combiner’s power handling. Acrylate coated pump fibers are reaching their limits and devices and measurements on double clad pump combiners with losses <0.05dB, will be presented enabling multi kW operation, The use of triple clad fibers in the gain section will discussed as a solution for multi kW applications. Results on ultra-low background loss FBG’s will be presented, along with developed measurement techniques.

A robust, alignment-free monolithic 2.1 kW single-mode continuous wave fiber laser, operating at 1083 nm is demonstrated. The laser is pumped with commercial fiber pigtailed multimode diodes through all-fiber pump-signal power combiners in a MOPA architecture. The oscillator was formed with high reflector and output coupler fiber Bragg gratings written in 11/200 μm (mode field/cladding diameter) single-mode fiber. The gain medium was a 19m OFS commercial 11/200 μm double clad Yb-doped fiber (DCY). Pump light was coupled to the oscillator using two 11/200 μm pump-signal power combiners (PSC). A total of 20 commercially available 58W pump diodes at 915 nm were used to generate 800W of signal, as measured before the amplifier. The Raman power after the oscillator was more than 60 dB below the signal power. The amplifier was built using 13 m of 14/200 µm DCY and two (18+1)x1 PSC combiners with more than 95% pump and signal light transmission. A total of 2 kW of power was used to bi-directionally pump the amplifier. The output was measured after 3 m 14/200 μm fiber, and 10 m 100/360 μm delivery cable. Total signal output power was 2.1 kW, corresponding to an amplifier slope efficiency of 77%. The Raman power is more than 30 dB below the signal power. At maximum power, no modal instabilities, thermal effects, nor power rollover were observed. With higher power pumps, it is predicted that a power level of 2.6 kW can be achieved with the Raman level below 20 dB.

We demonstrate a quasi-continuous wave laser amplifier with a peak output power of 6.8 kW pumped by an industrial thin-disk laser. A high slope efficiency of 84 % has been obtained within a duty cycle of 10 % at a signal wavelength of 1071 nm. For cw-pumping we measured a maximum output power of 985W. The amplifier fiber had a step index profile with a core diameter of 45μm and a pump core diameter of 120μm. A signal to ASE peak ratio of 48 dB could be determined.

Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.

We demonstrate a simple and compact Holmium-doped fiber (HDF) femtosecond oscillator with 35 MHz repetition rate. The oscillator is pumped by a Tm-doped fiber laser. A dispersion compensating fiber was utilized to set the net-intracavity dispersion to zero at 2.06 μm. The output pulses had an maximum average power of 40 mW and 160 fs FWHM pulse duration. To further shorten the pulse duration, we implemented a nonlinear compressor consisting of a solid core highly nonlinear fiber (HNLF) for SPM broadening and single mode fiber (SMF) for pulse compression. Sub-100 fs pulses were achieved.

We have demonstrated a high-power all-polarization-maintaining thulium-doped all fiber picosecond pulsed master oscillator power amplifier. The thulium-doped all fiber oscillator was mode locked by a semiconductor saturable absorber mirror to generate average output power of 100 mW at a repetition rate of 611.5 MHz in a short linear cavity. The first PM thulium-doped fiber preamplifier produced 4.5 W average output power for 17 W incident pump power, and the pulse width was measured to be 18 ps. In the second PM thulium-doped fiber preamplifier and the final PM thulium-doped fiber power amplifier, a segment of 4.5 m LMA PM thulium-doped double-clad fiber were used as the gain medium. Both the thulium-doped active fiber has a core diameter of 25 μm, a core NA of 0.09, inner cladding diameter of 400 μm and a NA of 0.46. The second PM fiber preamplifier produced 73 W average output power for 135 W incident pump power. In the final PM fiber power amplifier, the maximum average output power was up to 203 W at the available pump power of 300 W, the slope efficiency for the final PM fiber power amplifier was 50.7%. The PER at the highest average output power was measured to be <15 dB. The pulse width was 15 ps and the central wavelength was 1985 nm, which corresponds to peak power 22 kW. To the best of our knowledge, this is the highest average output power ever reported for an all fiber ultra-short-pulsed laser at 2 μm wavelength region.

We investigate challenges for scaling the output peak power of thulium-doped fiber chirped-pulse amplification systems (FCPA) to and beyond the GW-level. A major limitation for reaching high peak powers in the 2 μm regime is the presence of strong water vapor absorption features that cause detrimental propagation effects in the spatial and the temporal domain. Based on the investigation and understanding of these effects mitigation strategies have been developed, that have been one of the keys to demonstrate a new record pulse peak power of more than 200 MW from a thulium-based ultrafast fiber laser. Future experiments can ultimately lead to a further increase of pulse peak power way beyond the GW-level.

We demonstrate high average power all-fiber single-polarization thulium(Tm)-doped nanosecond laser based on all-fiber master-oscillator power-amplifier (MOPA).The seed source employed is a directly modulated discrete-mode (DM) diode at 2 μm with pulse width of 15.8 ns and tunable repetition rate range of 400 kHz to 4 MHz. The pulse duration reduced to 6 ns as the gain reshaping during amplification. The MOPA yields 160 W maximum average powers at 4 MHz repetition with 6 ns pulse width, with respect to pulse energy of 40 μJ and peak-power of 6.7 kW. The slope efficiency for the last stage of PM LMA Tm-doped fiber amplifier was 48.8% with respect to launched pump power. The polarization extinction ratio (PER) of system measured at maximum output power is beyond 15 dB. The beam quality factor M² was measured to be 1.5 at average output power of 160 W. To the best of our knowledge, this is the highest average power 2 μm nanosecond pulses laser from laser diode seeded all-fiber MOPA systems to date.

A way to increase the efficiency of Thulium-doped fiber systems and simultaneously prevent the generation of heat by pumping the excited state around 1460nm has been recently proposed by the authors. In this contribution we show that a Thulium-doped fiber amplifier can lase around 1460nm while simultaneously amplifying signals around ~2μm. Such an operation results in considerably higher amplification efficiencies and in lower operating temperatures without the need for an external pump around 1460nm.

Ultra-large mode area thulium-doped photonic crystal fibers (Tm:PCF) have enabled the highest peak powers in 2 micron fiber laser systems to date. However, Tm:PCFs are limited by slope efficiencies of <50% when pumped with 790 nm laser diodes. A well-known alternative is pumping at 1550 nm with erbium/ytterbium-doped fiber (Er/Yb:fiber) lasers for efficiencies approaching ~70%. However, these 1550 nm pump lasers are also relatively inefficient themselves. A recently demonstrated and more attractive approach to enable slope efficiencies over 90% in thuliumdoped step-index fibers is resonant pumping (or in-band pumping). This utilizes a high power thulium fiber laser operating at a shorter wavelength as the pump. In this manuscript, we describe an initial demonstration of resonant pumping in Tm:PCF. While the extracted power was still in the exponential regime due to pump power limitations, slope efficiencies in excess of ~64 have been observed, and there is still room for improvement. These initial results show promise for applying resonant pumping in Tm:PCF to improve efficiencies and facilitate high power scaling in ultralarge mode area systems.

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like OPOs. However, a phase modulator operating at the frequency-converted wavelength is needed, which is non standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition correlating, not only the wave vectors of the coupled waves, but also their phases. This paper demonstrates that, using this phase correlation for indirect control of the phase, coherent combining of optical frequency converters is feasible using standard all-fibered electro-optic modulators.

For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonic-generator (SHG) combination: i) combining 2 SHG of 1.55-μm erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-μm ytterbium-doped fiber amplifiers in LBO crystals generating 532-nm beams. Excellent CBC efficiency is achieved on the harmonic waves in both these experiments, with λ/20 and λ/30 residual phase error respectively.

In the second experiment, I/Q phase detection is added on fundamental and harmonic waves to measure their phase variations simultaneously. These measurements confirm the theoretical expectations and formulae of correlation between the phases of the fundamental and harmonic waves. Unexpectedly, in both experiments, when harmonic waves are phase-locked, a residual phase difference remains between the fundamen tal waves. Measurements of the spectrum of these residual phase differences locate them above 50 Hz, revealing that they most probably originate in fast-varying optical path differences induced by turbulence and acoustic-waves on the experimental breadboard.

Coherent beam combining of fiber amplifiers provides an attractive mean of reaching high power laser. In an interferometric phase measurement the beams issued for each fiber combined are imaged onto a sensor and interfere with a reference plane wave. This registration of interference patterns on a camera allows the measurement of the exact phase error of each fiber beam in a single shot. Therefore, this method is a promising candidate toward very large number of combined fibers. Based on this technique, several architectures can be proposed to coherently combine a high number of fibers. The first one based on digital holography transfers directly the image of the camera to spatial light modulator (SLM). The generated hologram is used to compensate the phase errors induced by the amplifiers. This architecture has therefore a collective phase measurement and correction. Unlike previous digital holography technique, the probe beams measuring the phase errors between the fibers are co-propagating with the phase-locked signal beams. This architecture is compatible with the use of multi-stage isolated amplifying fibers. In that case, only 20 pixels per fiber on the SLM are needed to obtain a residual phase shift error below λ/10rms. The second proposed architecture calculates the correction applied to each fiber channel by tracking the relative position of the interference finges. In this case, a phase modulator is placed on each channel. In that configuration, only 8 pixels per fiber on the camera is required for a stable close loop operation with a residual phase error of λ/20rms, which demonstrates the scalability of this concept.

White noise phase modulation (WNS) and pseudo-random binary sequence phase modulation (PRBS) are effective techniques for mitigation of nonlinear effects such as stimulated Brillouin scattering (SBS); thereby paving the way for higher power narrow linewidth fiber amplifiers. However, detailed studies comparing both coherent beam combination and the SBS suppression of these phase modulation schemes have not been reported. In this study an active fiber cutback experiment is performed comparing the enhancement factor of a PRBS and WNS broadened seed as a function of linewidth and fiber length. Furthermore, two WNS and PRBS modulated fiber lasers are coherently combined to measure and compare the fringe visibility and coherence length as a function of optical path length difference. Notably, the discrete frequency comb of PRBS modulation provides a beam combining re-coherence effect where the lasers periodically come back into phase. Significantly, this may reduce path length matching complexity in coherently combined fiber laser systems.

Mode conversion at the output of a higher-order mode fiber amplifier is proposed and demonstrated with an axicon for the first time. M² of 1.25 is achieved for 82% conversion efficiency.

Increasing the dimensions of a waveguide provides the simplest means of reducing detrimental nonlinear effects, but such systems are inherently multi-mode, reducing the brightness of the system. Furthermore, using rectangular dimensions allows for improved heat extraction, as well as uniform temperature profile within the core. We propose a method of using the angular acceptance of a transmitting Bragg grating (TBG) to filter the fundamental mode of a fiber laser resonator, and as a means to increase the brightness of multi-mode fiber laser. Numerical modeling is used to calculate the diffraction losses needed to suppress the higher order modes in a laser system with saturable gain. The model is tested by constructing an external cavity resonator using an ytterbium doped ribbon fiber with core dimensions of 107.8μm by 8.3μm as the active medium. We show that the TBG increases the beam quality of the system from M² = 11.3 to M² = 1.45, while reducing the slope efficiency from 76% to 53%, overall increasing the brightness by 5.1 times.

The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber-laser systems with diffraction limited beam quality. In this contribution wavelength shifting of the seed signal has been experimentally investigated in order to mitigate mode instabilities. Against the expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal wavelength. This finding implies that the quantum defect is not the sole significant source for thermal heating in the fiber.

We present an Er-doped fiber mode-locked laser based on an evanescent field interaction with the Sb2Te3 topological insulator. The saturable absorber (SA) consist of a bulk piece of Sb2Te3 material placed on the side-polished fiber in the presence of UV curable polymer. The measured SA optical parameters like: linear absorption, modulation depth and non-saturable loses were of 50%, 6% and 43%, respectively. The SA was spliced into the ring laser cavities characterized by the all-anomalous, all-normal and balanced dispersion. Such laser resonators allowed for optical solitons, dissipative solitons and Gaussian pulses generation with 3dB bandwidth of 8.5 nm, 37 nm and 17 nm, respectively.

Chirally-Coupled-Core (3C) fiber structure can preserve a single mode quality and even a linear polarization for a large core size. A principal advantage of fiber laser is its compatibility with monolithic integration and robust system. But so far, devices such as a combiner using the 3C fibers have not been reported. Here we report the first demonstration of such monolithic amplifier structure which contains an active fiber and a combiner based on 3C fibers. A single-stage amplifier is seeded by an EO Q-switched micro-laser and pumped by two high power fiber pigtailed 976-nm laser diodes via an in-house fabricated (2 + 1) × 1 pump signal combiner. The active fiber is based on a 3-m-long, 3C Yb-doped fiber (33 μm/250 μm core/cladding diameter with 0.06/0.46 NA). The amplifier demonstrates scaling up to 30W average power and 150 kW peak power in 0.3mJ, 2ns pulses. The beam profiles and beam qualities were characterized as its output power was varied up to 30W. The beam profile was maintained at a high beam quality of around M²=1.2. The spectral properties of the 3C fiber were also characterized as its output peak power was varied.

In this study, a polarization maintaining (PM) all-fiber laser oscillator passively mode locked at 1.03 μm is presented. The mode locking is achieved by nonlinear polarization evolution occurring along a long span of standard PM fiber (26 m) spliced between an off-axis polarizer and a Faraday rotator mirror. The influence of the total chromatic dispersion and intra-cavity spectral filtering on pulsed operation is studied. Two experimental configurations have been tested. The first configuration is an all normal dispersion cavity using a looped fibered circulator combined to a 1.5 nm filter used as an end cavity mirror. The second configuration used highly reflective chirped Fiber Bragg Grating (FBG) exhibiting different bandwidths (0.7 nm, 1.1 nm and 1.83 nm). The chromatic dispersion induced is +7.2 ps/nm for each FBG. Stable single-pulse mode locked operation has been demonstrated for each configuration. The study highlights however different mode-locking operations according to the intra-cavity spectral filtering and total chromatic dispersion of the cavity. For the first configuration, pulse duration is about 7 ps. According to the optical spectrum which has a FWHM of 2.2 nm, pulses may be compressed to subpicosecond durations with the help of a suited compressor like bulk gratings. Shortest pulses of 2.2 ps have been obtained at a repetition rate of 3.3 MHz with the second experimental configuration. To our knowledge, this is the smallest pulse duration delivered by a fully fibered mode locked laser operating at a repetition rate lower than 10 MHz without any external pulse compressor.

Fiber laser instabilities may significantly influence fiber laser performance. Recently, we have shown that the self-Qswitching instability mode can evolve from the so-called self-induced laser line sweeping (SLLS). The SLLS effect is characterized by quasiperiodic laser line drift in a relatively broad wavelength interval. The SLLS is caused by standing-wave in the cavity. Such regular standing waves series create also significant reflection grating through resonant index change of the rare-earth doped core. We show analysis of the transient gratings spectra, including the effect of superimposed gratings. The numerical model of the fiber laser accounts for the effect of the fiber coiling and twisting on the pump absorption along the active fiber.

We demonstrate a pulsed green fiber laser using highly efficient frequency conversion in a robust single-pass configuration of a narrow Linewidth source based on specialty LMA fibers for enabling high pulse energies and peak power. The ability to reach short pulses that are selectable from 1 to 20ns, peak powers up to 20kW, and pulse energies up to 200 μJ in the 532nm wavelength regime, while maintaining excellent beam quality with high repetition rate capabilities, enables new opportunities for next-generation material processing applications. We have demonstrated a variety of processes with high throughput and quality, including glass cutting, wafer cutting, PCB cutting, and ceramic scribing.

We investigate the measurement of bend losses in few-mode large-mode-area (LMA) fibers. The influence of the light source spectral characteristics, modal power content and cladding light on the measurement accuracy and precision is studied experimentally. Monte-Carlo simulations are performed to understand the distribution of the variations. This study provides practical guidelines for bend loss measurements.

We present an effective method to realize continuously one-dimensional steering of coherently combined beam in the field-of-view of PALCOPA. To achieve this purpose, besides the linear phase profiles to steer the incident lasers, extra phase modulations should be applied to them. These phase offsets depend on both the assigned deflection angle of combined beam and the parameters of beam combining system. Using the Fraunhofer propagation principle, we derive the analytical expressions of the far-field intensity distribution of the combined beam. The analytical functions demonstrate the validity of the proposed method. Finally, we evaluate the proposed technique through computer simulations and experiments, by considering three main indicators of the combined beam, i.e. deflection accuracy, mainlobe width and combining gain.

We demonstrate a fiber laser design capable of generating shaped pulses with up to 1mJ pulse energy while keeping near diffraction limited beam quality. By controlling both the temporal and spectral characteristics of the seeder in a MOPA configuration we were able to achieve long pulses in the 100’s nanosecond range without the limiting effect of gain depletion and without reaching the detrimental SBS threshold. With this configuration we demonstrated 1mJ rectangular pulses which prove highly efficient for micromachining applications over equally energetic higher peak power gain saturated pulses.

The present work summarises the results of studies of an all-fibre figure-eight mode-locked Yb fibre master oscillator based on non-linear amplifying loop mirror with the average output power exceeding 1 W. We show that the studied resonator configuration can generate clusters of 200-femtosecond pulses at repetition rate of 25 MHz with the cluster envelope duration of around 20 ps. We demonstrate pulse properties of two typical mode-locked generation regimes with widely different output pulse spectra and discuss specific features of pulses exiting the laser cavity via two fibre ports. We also provide analytical estimations and numerical modelling which agree well with experimental measurements.

In applications involving lasers with high peak intensities, such as optical amplification or pulse delivery through an optical fiber, self-phase modulation is an unwanted phenomenon which affects the spectrum, phase, and temporal profile of laser pulses. Here we report on the use of binary phase shaping for mitigation of self-phase modulation. We provide theoretical simulation and estimated efficiency of the mitigation supported by experimental results using both chirped and binary phase shaped pulses.

In this work, we demonstrate a high-pulse energy, fiber-based chirped pulse amplification (CPA) setup utilizing Er- and Er/Yb-doped fibers, operating at 1555 nm central wavelength. The integrated pulse-picker allows to reduce the repetition frequency down to the kHz-range, which enables generation of sub-picosecond pulses with energies above 2 μJ and pulse peak power exceeding 1 MW. The system utilizes an Er/Yb co-doped large mode area fiber in the final amplification stage. Thanks to the used mode-field adaptors, the setup is almost fully fiberized, except the bulk grating pulse compressor.

We study self-injection locking of DFB laser through ring fiber optic resonator in the different regimes. Experimentally measured transmitted and reflected from fiber cavity powers at undercoupled, critically coupled, and overcoupled regimes are properly coincide with the calculated values. Significant reduction of locked laser linewidth was observed. For the free running laser, the full-width at half-maximum linewidth was equal to 2.55 MHz assuming that line shape is Lorentzian. Meanwhile the self-injection locked DFB laser demonstrates linewidth equal to 2.5 KHz for undercoupled and critically coupled regimes and about two times more for overcoupled one.

An ytterbium based all fiber, all normal dispersion fiber oscillator with integrated SESAM can have several operation modes like mode-locked, Q-switched and noise-like. To know and to control the quality of the mode-locking is essential for the application of such laser oscillators, otherwise the whole laser setup can be damaged or the expected operation characteristics of the oscillator driven systems cannot be achieved. Usually the two-photon signal generated by the short pulses is used to indicate the mode locked operation, however such detection can be misleading in certain cases and not always able to predict the forthcoming degradation or vanishing of mode locking. The characterization method that we propose uses only the radio frequency spectrum of the oscillator output and can identify the different operation regimes of our laser setup. The optical spectra measured simultaneously with the RF signals proves the reliability of our method. With this kind of characterization stable mode locking can be initiated and maintained during the laser operation. The method combined with the ability to align the polarization states automatically in the laser cavity leads to the possibility to record a polarization map where the stability domains can be identified and classified. With such map the region where the mode locking is self starting and maintainable with minimal polarization alignment can be selected. The developed oscillator reported here with its compact setup and self alignment ability can be a reliable source with long term error free operation without the need of expensive monitoring tools.

A phase-resolved system based on swept source optical coherence tomography (SS-OCT) has to incorporate a phase-stabilized wavelength-swept light source. The phase variation is induced by fluctuation of a beginning swept frequency. The conventional phase-sensitive SS-OCTs use a fiber Bragg grating (FBG) in order to avoid A-scan trigger fluctuations. However this method does not always solve the trigger fluctuation problem. In actively mode-locked ring lasers (AMLLs), the beginning swept frequency fluctuates by abrupt frequency change between the end of a sweep and the beginning of the subsequent one. To overcome this issue, we proposes a new phase stabilization method. By employing the method with an auxiliary reference configuration, the sweeping phase has successfully stabilized because the timing jitter is calculated by interference signals from the auxiliary reference path. In this research, we have proposed the phase stabilization method that has nanometer sensitivity with millisecond response. In addition, the method has successfully suppressed the depth dependence of phase instability.

Although the Master Oscillator Power-Amplifier (MOPA) based fiber laser has received much attention for laser marking process due to its large tunabilty of pulse duration (from 10ns to 1ms), repetition rate (100Hz to 500kHz), high peak power and extraordinary heat dissipating capability, the output pulse deformation due to the saturation effect of fiber amplifier is detrimental for many applications. We proposed and demonstrated that, by utilizing Genetic algorithm (GA) based optimization technique, the input pulse profile from the master oscillator (current-driven laser diode) could be conveniently optimized to achieve targeted output pulse shape according to real parameters' constraints. In this work, an Yb-doped high power fiber amplifier is considered and a 200ns square shaped pulse profile is the optimization target. Since the input pulse with longer leading edge and shorter trailing edge can compensate the saturation effect, linear, quadratic and cubic polynomial functions are used to describe the input pulse with limited number of unknowns(<5). Coefficients of the polynomial functions are the optimization objects. With reasonable cost and hardware limitations, the cubic input pulse with 4 coefficients is found to be the best as the output amplified pulse can achieve excellent flatness within the square shape. Considering the bandwidth constraint of practical electronics, we examined high-frequency component cut-off effect of input pulses and found that the optimized cubic input pulses with 300MHz bandwidth is still quite acceptable to satisfy the requirement for the amplified output pulse and it is feasible to establish such a pulse generator in real applications.

We present a time dependent computer model for modal instabilities (MI) in high power fiber amplifiers based on beam propagation method. Three regimes of temporal dynamics that are characteristic of the transfer of energy between the fundamental mode and higher order mode are captured and applied to predicting the threshold of these instabilities in absence of any frequency offset between the interfering modes. Simulation results show an increase of the instabilities threshold by a factor of approximately %30 in the case of bi-directional pump scheme with respect to the forward pump configuration. Furthermore, we estimated the MI threshold applying a coupled-mode model of thermally induced instabilities which also takes account of gain saturation to its first order approximation, and obtained reasonably good agreement with respect to our beam propagation simulation results.

In our contribution we report novel mode field adapter incorporated inside bundled tapered pump and signal combiner. Pump and signal combiners are crucial component of contemporary double clad high power fiber lasers. Proposed combiner allows simultaneous matching to single mode core on input and output. We used advanced optimization techniques to match the combiner to a single mode core simultaneously on input and output and to minimalize losses of the combiner signal branch. We designed two arrangements of combiners’ mode field adapters. Our numerical simulations estimates losses in signal branches of optimized combiners of 0.23 dB for the first design and 0.16 dB for the second design for SMF-28 input fiber and SMF-28 matched output double clad fiber for the wavelength of 2000 nm. The splice losses of the actual combiner are expected to be even lower thanks to dopant diffusion during the splicing process.

The concentration of trace sample can be determined by, and therefore an indicator of, the absorption property in fiber-loop ring-down spectroscopy. The aim of this study was to investigate the use of an ultrashort-pulse laser as a source of the fiber-loop ring-down spectroscopy and its applicability for loss-compensated fiber loops. A composite-type carbon-nanotube saturable absorber mode-locked fiber laser was built to be used as a source of experimental setup. The simulation has been done to check dispersion effects on ring-down signals, and it was found that the initial pulse duration affected ring-down information. To avoid this limitation, we just used the chirped pulses without any compensation process in the experiment. A loss-compensated fiber loop was constructed to measure the concentration of water in water/acetone mixture samples. The measurements were shown to lead to an accurate determination of the ring-down time. The result indicates that it is possible to measure the sample-induced absorption with the ultrashort-pulse laser source and loss-compensated fiber loop to determine the concentration of the sample.

This paper presents a method to obtain spectral ring-down information simultaneously via double-looped Mach-Zehnder interferometry. The symmetric split-step Fourier method was used to simulate the pulse propagation through the fiber loop to check fiber-induced effects on the pulse. The loop was built with a segment of fiber, output coupler, and fiber Bragg grating. The fiber Bragg grating was used as an artificial sample. We found that the pulse spectrum is not distorted as the pulse propagates the fiber loop. We have also demonstrated the possibility to extract the ring-down signals from spectrum variation via simulation. This approach can be applied to sample detection which requires high sensitivity and high spectral resolution.

The temperature dependence of the beating frequencies in multi-longitudinal mode hybrid semiconductor-fiber based ring lasers is studied theoretically and experimentally. The variation of the beating frequency with temperature is found to be smaller for larger cavity length and lower beating order. Measured frequency variation as low as -0.24 Hz/°C is obtained for cavity length of 2.7 km. The stability of the frequency is evaluated using the Allan variance technique. The measurement is carried out for different beating frequency orders. The lowest order beating frequency has about 20x better long-term frequency stability than the beating frequency of the 100th order.

Generation of a single-longitudinal mode (SLM) in bidirectional ring lasers has direct impact on the laser linewidth and dynamic range of operation, when used in rotation sensing applications. Besides, operating at a specific wavelength helps in optimizing the performance of the system components. In this work, we report a novel method for generating SLM in bidirectional SOA-fiber ring laser using mechanically tunable Fabry-Perot filter with 1-nm bandwidth. The method is based on gain starvation by tuning the central wavelength of the filter in the blue edge of the gain-wavelength response. By adjusting the SOA driving current, the oscillation condition is satisfied mainly for single mode and bidirectional operation can be achieved simultaneously. The SLM operation is verified by monitoring the beating signal between the modes on an RF spectrum analyzer. Using an SOA with a small-signal gain of 20 dB at 300 mA pumping current and a gain bandwidth of 100 nm centered around 1490 nm; the central wavelength of the ring laser could be tuned from 1440 nm to 1480 nm with a side-mode suppression ratio of 25 dB.

We demonstrate a single mode fiber based master oscillator power amplifier (MOPA) with a single polarization and a fully monolithic design. We have built a passive mode-locked polarization maintaining Yb doped fiber as the master oscillator contains a semiconductor saturable absorber mirror and a chirped fiber Bragg grating for the dispersion management. The net intracavity dispersion was managed to be slightly anomalous. The oscillator generates the 150 fs (sech₂) pulses at the center wavelength of 1065 nm, and the repetition rate of 42 MHz. The oscillator output was amplified to 1.4 W from 80 mW in the single stage fiber amplifier which results in pulse shape distortion. The pulse shaping with a band pass filter and a compressor was applied to the amplified pulses. The shaping pulses have the pulse width of 90 fs and the pulse energy of 16 nJ.

Developments in the field of high-power fiber lasers require extensive knowledge of the fiber structure parameters in various operating states. In an effort to gather such knowledge, this paper presents the results of dispersion characterization measurements using ytterbium-doped large mode area double-clad fibers implemented in the cavity of a high power fiber laser during operation. The laser cavity is built into one arm of a Mach-Zehnder interferometer. Using dichroic mirrors the radiation of a supercontinuum light source is coupled into the fiber sample. Various types of large mode-area fibers were examined under different operating conditions of the laser setup. Group-velocity dispersion characteristics of two large mode-area double-clad fiber amplifiers with various launching laser power levels were analyzed. The dispersion parameters for different fiber designs and various doping levels are investigated over a broad spectral range of about 1.3 μm. The experiment utilizes a supercontinuum source developed within this laboratory, as well as a Mach-Zehnder interferometer with a dual-channel spectral-detection system sensitive to wavelengths from 0.4 μm to 1.7 μm. Temporally resolved spectrograms recorded at distinct delay positions enable the detection of interference fringes for the equalization-wavelength. By applying a Sellmeier polynomial fit to the wavelength dependent differential group-delay function, the group-velocity dispersion has been derived.

We study a passively mode-locked double-clad Erbium-Ytterbium fiber ring laser producing noise-like pulse through nonlinear polarization evolution and polarization selection. Single noise-like pulsing is only observed at moderate pump power. As pump power is increased, and through polarization controllers adjustments, harmonic mode-locking of growing order were successively appearing. For pump powers close to the damage threshold of the setup, we reach harmonic orders beyond 1200 and repetition frequencies in excess of a quarter of a GHz. Finally, these experimental results could be useful in the quest for higher pulse energies and higher repetition rates in passively mode-locked fiber lasers.

In a master oscillator fiber amplifier (MOPA) system, we have experimentally studied nonlinear phenomena originating in the amplifier Yb-fiber during amplification of Q-switched pulse. During amplification, the Yb-doped active fiber generated backscattered pulses followed by amplified pulse breaking and high peak power short pulse generation. The backscattered pulses were delayed maximum by 30ns and seed pulse of width 108ns was broadened to 140ns at 20KHz repetition rate due to different scattering effects in a 8m long Yb-doped double clad fiber. At higher pump power, the output pulse showed unstable pulse envelops and eventually broke into multiple short pulses. A broadband output was observed from 1050 to 1650nm. This unwanted pulse breaking was observed which caused catastrophic damage to the MOPA system occasionally.

We propose a novel method to multiply the pulse repetition rate of actively mode-locked fiber laser (AMLL) by spectrum-slicing and time-division-multiplexing for photonic analog-to-digital converters (PADC). Self-phase modulation (SPM) is employed to broaden the spectrum in order to obtain more parallel channels. The SPM induces additional phase jitter that degrades PADC’s performance. We theoretically and experimentally show phase jitter can be effectively reduced by precise control of the tunable true-time delay array. Sampling clock of 40 GHz or higher with low phase jitter is achieved based on 10-GHz AMLL, which can be used to realize high performance PADC.

We report an all-fiber dual-wavelength actively Q-switched pulsed laser. The linear cavity is limited by two fiber Bragg gratings with central wavelengths of 1538.3 nm and 1548 nm respectively in one side and Sagnac interferometer (SI) in the other side. The pump power is coupled into the doped fiber through a beam combiner. The SI used to equalize generated laser lines competition by cavity losses adjustment is formed by high birefringence (Hi-Bi) fiber in the loop. The SI spectrum wavelength displacement adjustment is performed by temperature variations applied on the Hi-Bi fiber loop with a temperature controller. The Q-switch operation is achieved at pulse repetition rate of 120 kHz with average pulsed output power of 404 mW and pulse duration of 470 ns.

Cladding-pumped Raman fiber amplifiers (RFA) have been proposed as gain media to achieve power scaling. It is well-known that the onset of the modal instability (MI) phenomenon is a limiting factor for achieving higher output powers in Yb-doped fiber amplifiers with good beam quality. In this paper, we present an analytical approach to the investigation of the MI phenomenon in high-power, cladding-pumped RFAs. By utilizing the conservation of the number of photons and the conservation of energy in the absence of loss, the nonlinear equations for the propagation of the pump power and the total signal power can be decoupled from one another. Decoupling lead to exact solutions for the pump power and transverse modes signal powers. Further we investigate various MI suppression techniques including increasing the seed power and gain-tailored design.