This PDF file contains the front matter associated with SPIE Proceedings Volume 8601, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

There is a great interest in obtaining laser pulses with a high average power as well as high pulse energies. Continuously pulsed systems face many problems to satisfy those requirements, independent on the amplifier concept. While many applications such as electron beam characterization and free-electron-laser seeding need high pulse energies at high repetition rates, they only need those laser pulses for a certain amount of time. Therefore, it is not necessary to run a laser system with continuous pulses at those parameters and a so-called burst mode might be sufficient and even essential in such cases. We report on a CPA-laser system, based on a large pitch fiber as a main-amplifier delivering bursts containing ultra-short, highly-energetic pulses. The burst rate is set to 20Hz, while each burst contains 2000 pulses at a pulse-repetition-rate of 10MHz and with a pulse-duration of 700fs. Hence the duty cycle D is 0.4%. To achieve a homogeneous pulse energy level between 27μJ and 31μJ after the compression, the main amplifier is pumped with a very high power of 1.6kW in a burst-mode (D=10%). By using an acousto-optical modulator (AOM) after the main-amp fiber, the residual output before and after the burst is removed to suppress ASE and any underground-pulses around the amplified burst. The limitations that could be observed during this experiment were mainly due to mode instabilities, which were detectable even on a very short time scale of a few hundred μs using a high speed camera.

We demonstrate a 1480 nm cascaded Raman fiber laser with a new high efficiency architecture providing a record output power of 204 W. We achieve this through multiple Raman shifts of a high power 1117nm Yb-doped fiber laser in a single pass configuration mediated at all intermediate wavelengths using a seed source comprising a low power conventional 1480nm Raman laser. The conversion efficiency from 1117nm to 1480nm is ~65% (for a quantum limited efficiency of 75%). Enhancement in efficiency is achieved by elimination of excess optical loss present in the conventional cascaded Raman resonator based architecture.

Diffraction limited fiber amplifiers in a circular geometry are likely to be limited by nonlinearities to 2 kW for narrowband and 10-36 kW for broadband lasers. We have proposed a ribbon fiber geometry to allow scaling fiber lasers above these limits in which a high order ribbon mode is amplified and converted back to the fundamental mode in free space. Novel methods of illuminating a high order ribbon fiber mode are discussed and compared with modeling and experimental results showing high purity illumination, > 90%. A 10 kW single frequency ribbon fiber amplifier design is presented and BPM simulation results verify the approach.

We show by detailed numerical modeling that stimulated thermal Rayleigh scattering can account for the modal instability observed in high power fiber amplifiers. Our model illustrates how the instability threshold power can be maximized by eliminating amplitude and phase modulation of the signal seed light and the pump light and by careful injection of the signal seed light. We also illustrate the influence of photodarkening and mode specific loss.

A time-dependent analytical model is rigorously derived which shows that the thermally induced modal instability in high power rare-earth doped fiber amplifiers is fundamentally a two-wave mixing between fundamental and higher-order modes through a thermally-induced grating imprinted by beating between these modes. We show that previously postulated movement of this grating to phase-match the coupling between the modes naturally occurs due to a finite thermal-response time of a fiber. This theory is consistent with experimental observations in that it accurately predicts the onset-like threshold and temporal instabilities in the kilohertz-frequency range.

By dynamically varying the power content of the excited fiber modes of the main amplifier of a fiber-based MOPA system at high average output power levels, it was possible to mitigate mode instabilities to a large extent. In order to achieve the excitation variation, we used an acousto-optic deflector in front of the Yb-doped rod-type fiber. Therewith, it was possible to significantly increase both the average and the instantaneous minimum power content of the fundamental mode. This, consequently, led to a substantial improvement of the beam quality and pointing stability at power levels well beyond the threshold of mode instabilities.

The phenomenon of mode instabilities has quickly become the most limiting effect for a further scaling of the average power of fiber laser systems. It is important to get a detailed understanding of its physical origin in order to develop efficient mitigation strategies. In this work we present an analysis of the different physical processes that give rise to mode instabilities and reveal that thermally-induced non-adiabatic waveguide changes play a key role. With this insight in the physical processes underlying mode instabilities a semi-analytic formula is obtained and several mitigation guidelines will be presented and discussed.

XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber laser.

It has recently been demonstrated that Bi-doped glass optical fibers are a promising active laser medium. In this paper the luminescence and absorption spectra of various types of Bi-doped optical fibers are presented. The exact nature of the near IR emitting bismuth centers is still unclear, so some hypotheses and recent experimental results on this subject are discussed. Then the paper reviews the recent results regarding the development of Bi-doped fiber lasers and optical amplifiers for the 1150 to 1550 nm spectral region. In conclusion some problems to be solved to get an efficient laser medium on the base of Bi-doped fibers are identified.

Increasing power levels and novel applications are demanding from fibers performance capabilities that have, to date, not been realized. One such example arises from the nascent push towards the 10-kW power threshold for narrow linewidth fiber lasers designed for applications including coherently-phased laser arrays and spectroscopic lidars. It is well-known that Brillouin scattering still restricts continued power scaling in these systems, despite several recent advances in acoustic-wave Brillouin management. Accordingly, novel fibers possessing a Brillouin gain coefficient 10 dB or more less than previously demonstrated would be of great practical benefit if they comprise novel materials in simple geometries and are manufactured using industry-accepted methods. Introducing a new and effective approach to the management of Brillouin scattering, we present on all-glass optical fibers derived from silica-clad sapphire with alumina concentrations up to 55 mole percent; considerably greater than conventionally possible enabling the design of optical fiber possessing a series of essential properties. Markedly, a Brillouin gain coefficient of 3.1 × 10^-13 m/W was measured for a fiber with an average alumina concentration of 54 mole percent. This value is nearly 100 times lower than standard commercial single-mode fiber and is likely the lowest ever specified value. This reduction in Brillouin gain is enabled by a number of key material properties of the alumina-silica system, amazingly even leading to a predicted, but not yet demonstrated, composition with zero Brillouin gain. Optical fiber materials with these and other crucial properties will be discussed in the context high energy fiber laser systems.

Described herein, for the first time to the best of our knowledge, are results on optical fibers possessing significant compositional gradations along its length due to longitudinal control of the core glass composition. More specifically, MCVD-derived germanosilicate fibers were fabricated that exhibited a gradient of up to about 0.55 weight % GeO₂ per meter. These gradients are about 1900 times greater than previously reported fibers possessing longitudinal changes in composition. The refractive index difference is shown to change by about 0.001, representing a numerical aperture change of about 10%, over a fiber length of less than 20 m. The lowest attenuation measured from the present longitudinally-graded fiber (LGF) was 82 dB/km at a wavelength of 1550 nm, though this is shown to result from extrinsic process-induced factors and could be reduced with further optimization. The stimulated Brillouin scattering (SBS) spectrum from the LGF exhibited a 4.4 dB increase in the spectral width, and thus reduction in Brillouin gain, relative to a standard commercial single mode fiber, over a fiber length of only 17 m. The method employed is very straight-forward and provides for a wide variety of longitudinal refractive index and acoustic velocity profiles, as well as core shapes, which could be especially valuable for SBS suppression in high-energy laser systems. Next generation analogs, with longitudinally-graded compositional profiles that are very reasonable to fabricate, are shown computationally to be more effective at suppressing SBS than present alternatives, such as externally-applied temperature or strain gradients.

We analyze the modal properties of an 85μm core distributed mode filtering rod fiber using cross-correlated (C²) imaging. We evaluate suppression of higher-order modes (HOMs) under severely misaligned mode excitation and identify a single-mode regime where HOMs are suppressed by more than 20dB.

We use the correlation filter method for mode analysis and characterize a fiber coupling process modally resolved. In our experiment light is coupled from a single-mode fiber into a multi-mode fiber. During the coupling process the transverse position of the single-mode fiber towards the multi-mode fiber is varied and the actually guided mode content is measured simultaneously. This measurement process allows to maximize the power guided by the fundamental mode and, hence, to achieve most brilliant beam quality in real-time.

Degenerate spontaneous four wave mixing is studied for the rst time in a large mode area hybrid photonic crystal ber, where light con nement is achieved by combined index- and bandgap guiding. Four wave mixing products are generated on the edges of the bandgaps, which is veri ed by numerical and experimental results. Since the core mode is in resonance with cladding modes near the bandedges an unconventional measurement technique is used, in this work named nonlinear spatial mode imaging.

Excited state absorption (ESA) measurements performed on Yb-doped silica bers show the onset of a strong absorption band in the visible range. In this work, we perform experiments to investigate the possibility for ESA to be part of the induced optical losses (photodarkening) observed in Yb-doped ber lasers. Our results indicate that an ESA process, from the ²F_5/2 excited state manifold in the Yb³⁺ ion to the charge-transfer state with absorption bands in the UV range, may constitute a transfer route for pump- and laser photons in the near-infrared range.

The invention of the semiconductor saturable absorber mirror (SESAM) nearly 20 years ago was a major advancement for the development of ultrafast laser systems. Today, SESAMs have become key devices for modelocking of numerous laser types, including DPSSLs, fiber lasers, and semiconductor lasers. Semiconductors are ideally suited as saturable absorbers because they can cover a broad wavelength range and yield short recovery times, supporting the generation of picosecond to femtosecond pulse durations. The macroscopic nonlinear optical parameters for modelocking can be optimized over a wide range by the design of the mirror structure and the choice of the semiconductor absorber. Furthermore, their damage threshold can be controlled making them ideally suited for high-power levels. In this presentation, we will focus on recent advances in SESAMs for cutting-edge ultrafast lasers. In particular, we will focus on recent damage and lifetime investigations of SESAMs designed for high-power oscillators. We will present guidelines for robust SESAMs in a large range of saturation parameters, and give an outlook towards novel SESAM designs that will enable future kW-level ultrafast oscillators.

We present a selective mode filter inscribed with ultrashort pulses directly into a few mode large mode area (LMA) fiber. The mode filter consists of two refractive index modifications alongside the fiber core in the cladding. The refractive index modifications, which were of approximately the same order of magnitude as the refractive index difference between core and cladding have been inscribed by nonlinear absorption of femtosecond laser pulses (800 nm wavelength, 120 fs pulse duration). If light is guided in the core, it will interact with the inscribed modifications causing modes to be coupled out of the core. In order to characterize the mode filter, we used a femtosecond inscribed fiber Bragg grating (FBG), which acts as a wavelength and therefore mode selective element in the LMA fiber. Since each mode has different Bragg reflection wavelengths, an FBG in a multimode fiber will exhibit multiple Bragg reflection peaks. In our experiments, we first inscribed the FBG using the phase mask scanning technique. Then the mode filter was inscribed. The reflection spectrum of the FBG was measured in situ during the inscription process using a supercontinuum source. The reflectivities of the LP₀₁ and LP₁₁ modes show a dependency on the length of the mode filter. Two stages of the filter were obtained: one, in which the LP₁₁ mode was reduced by 60% and one where the LP₀₁ mode was reduced by 80%. The other mode respectively showed almost no losses. In conclusion, we could selectively filter either the fundamental or higher order modes.

With increasing output power of lasers, absorption in optical components grows larger and demands on heat withdrawal become challenging. We report on the fabrication of a Faraday isolator for high power fiber laser applications (P = 1 kW) at a wavelength of 1080 nm and operation at ambient conditions. We investigate direct bonding of Terbium Gallium Garnet to sapphire disks, to benefit from the good heat spreading properties (having a 6-fold higher thermal conductivity than TGG) at high transparency of the latter. Successful bonding was achieved by extensive cleaning of the plane and smooth surfaces prior to low pressure plasma activation. The surfaces to be bonded were then contacted in a vacuum environment at elevated temperature under axial load. Our measurements show that the bonded interface has no measurable influence on transmission properties and bonded samples are stable for laser output powers of at least 260 W. As compared to a single Terbium Gallium Garnet substrate, wavefront aberrations were significantly decreased by bonding sapphire disks to Terbium Gallium Garnet.

We investigate the temporal dynamics of Modal instabilities (MI) in ROD fiber amplifiers using a 100 μm core rod fiber in a single-pass amplifier configuration, and we achieve ~200W of extracted output power before the onset of MI. Above the MI threshold, we investigate the temporal dynamics of beam fluctuations in both the transient and chaotic regime. We identify a set of discrete frequencies in the transient regime and a white distribution of frequencies in the chaotic regime. We test three identical rods using a multiple ramp-up procedure, where each rod is tested in three test series and thermally annealed between each test series. We find that the MI threshold degrades as it is reached multiple times, but is recovered by thermal annealing. We also find that the test history of the rods affects the temporal dynamics.

Further power scaling of single frequency fiber lasers is of significant interests for many scientific and defense applications. It is currently limited by stimulated Brillouin scattering (SBS). In recent years, a variety of techniques have been investigated for the suppression of SBS in optical fibers. A notable example is to design transverse acoustic properties of optical fibers in order to minimize optical and acoustic mode overlap. It was pointed out recently that SBS suppression from such transverse acoustic tailoring is limited when considering the existence of acoustic leaky modes. We demonstrate, for the first time, a post-processing technique where hydrogen is diffused in to a fiber core and then locally and permanently bonded to core glass by a subsequent UV exposure. Large local acoustic property can be altered this way for significant SBS suppression. It is also possible to use this technique to implement precisely tailored acoustic properties along a fiber for more optimized SBS suppression in a fiber amplifier. Change in Brillouin Stokes frequency of ~320MHz at 1.064μm has been demonstrated using hydrogen, corresponding to a SBS suppression of ~8dB. Much higher SBS suppression is possible at higher hydrogen concentrations.

Linearly polarized wavelength stable single frequency ytterbium (Yb³⁺) doped fiber lasers below 1 μm, namely threelevel Yb³⁺ fiber lasers, are highly demanded for nonlinear wavelength conversion to generate coherent blue light or even deep ultraviolet coherent sources. We present performance of a 976 nm single-frequency core-pumped distributed Bragg reflector (DBR) fiber laser consisting of a 2-cm long highly ytterbium-doped phosphate fiber and a pair of silica fiber Bragg gratings (FBGs) and their use for frequency doubling experiment. The high reflection (HR > 99%) and partial reflection (PR = 60%) FBGs were cleaved very close to the index modulation region and directly spliced to a 2-cm-long highly Yb³⁺-doped phosphate fiber. Over 100 mW of linearly polarized output with a linewidth less than 2 kHz can be obtained when the launched pump power is about 450 mW. The efficiency of the 976 nm single-frequency fiber laser (the output power vs the launched pump power) is about 25%. The relative intensity noise was measured to be -110 dB/Hz at 1 MHz and the variation of the center wavelength is less than 0.0005 nm during a measurement period of 2.5 hours. This single-frequency fiber laser has an SNR of over 50 dB and there is no strong ASE or spurious lasing at long wavelengths even at the maximum pump power. This all-fiber single-frequency DBR laser with attractive features can be used for efficient blue and UV generation through nonlinear frequency conversion. Moreover, this high-performance 976 nm single-frequency fiber laser can be used as a single-frequency, low RIN pump laser for long wavelength Yb³⁺-, Er³⁺-, or Yb³⁺/Er³⁺-doped fiber lasers and amplifiers.

We present the performance of a single frequency, single-polarization holmium (Ho³⁺)-doped ZBLAN (ZrF₄-BaF₂-LaF₃- AlF₃-NaF) fiber laser at 1200 nm. This distributed Bragg reflector (DBR) fiber laser was developed by splicing a 22 mm long highly Ho³⁺-doped ZBLAN fiber to a pair of silica fiber Bragg gratings (FBG). The successful fusion splicing of silica fiber to ZBLAN fiber, with their very different melting temperatures, was accomplished by using NP Photonics proprietary splicing technique. The 3 mol% Ho³⁺-doped ZBLAN fiber had a core diameter of 6.5 μm and a cladding diameter of 125 μm. The threshold of this laser was seen to be about 260 mW, and when the pump power was 520 mW, the output power was about 10 mW. The efficiency of the 1200 nm single-frequency fiber laser, i.e. the ratio of the output power to the launched pump power, was about 3.8%. The linewidth of the 1200 nm single-frequency fiber laser was estimated to be about 100 kHz by comparing the measured frequency noise of the 1200 nm single-frequency fiber laser with that of 1 μm NP Photonics single-frequency fiber lasers whose linewidths have been measured to be in the 1- 10 kHz range. The relative intensity noise of this DBR all-fiber laser was measured to be < 110 dB/Hz at the relaxation oscillation peak and the polarization extinction ratio was measured to be > 19 dB. Due to its low phonon energy and long radiative lifetimes, rare-earth-doped ZBLAN allows various transitions that are typically terminated in silica glass, resulting in ultraviolet, visible, and infrared rare-earth doped ZBLAN lasers. Therefore, our results highlight the exciting prospect that the accessible wavelength range of single-frequency DBR fiber lasers can be expanded significantly by using rare-earth-doped ZBLAN fibers.

We present experimental and theoretical studies on the stimulated Brillouin scattering (SBS) threshold in fiber amplifiers seeded with a spectrally broadened single-frequency laser source. An electro-optic phase modulator is driven with various pseudo-random binary sequence (PRBS) patterns to highlight the unique characteristics of this linewidth broadening technique and its facility in SBS mitigation. Theoretical predictions show a variation in SBS suppression based on PRBS pattern and modulation frequency. These predictions are experimentally investigated in a kilowatt level monolithic fiber amplifier operating with near diffraction-limited beam quality. We also show Rayleigh scattering and other sources of back reflected light in phase modulated signals can seed the SBS process and significantly reduce the nonlinear threshold.

We describe a pulsed blue (485 nm) laser source based on frequency quadrupling a pulsed Tm fiber laser. Up to 1.2 W at 485 nm was generated with an M² of 1.3. At 10 kHz pulse repetition frequency, the output pulse at 485 nm was 65 ns FWHM resulting in an estimated peak power of 1.8 kW. We anticipate further improvements in power scaling with higher power Tm fiber lasers and improved conversion efficiency to the blue with optimized AR coatings and nonlinear optical crystals.

We have recently developed an industrial laser platform emitting in the non-conventional range around 976 nm. This 15 W continuous wave spatially single mode linearly polarized fiber laser can be forced to work in narrow line width or single frequency configuration. Its frequency doubling at 488 nm can be used to replace argon gas laser technology in many applications. We have studied the second harmonic generation of our source to verify its suitability with several industrial application needs in terms of efficiency, temporal stability and noise level.

Many industrial applications such as glass cutting, ceramic micro-machining or photovoltaic processes require high average and high peak power Picosecond pulses. The main limitation for the expansion of the picosecond market is the cost of high power picosecond laser sources, which is due to the complexity of the architecture used for picosecond pulse amplification, and the difficulty to keep an excellent beam quality at high average power. Amplification with fibers is a good technology to achieve high power in picosecond regime but, because of its tight confinement over long distances, light undergoes dramatic non linearities while propagating in fibers. One way to avoid strong non linearities is to increase fiber’s mode area. Nineteen missing holes fibers offering core diameter larger than 80μm have been used over the past few years [1-3] but it has been shown that mode instabilities occur at approximately 100W average output power in these fibers [4]. Recently a new fiber design has been introduced, in which HOMs are delocalized from the core to the clad, preventing from HOMs amplification [5]. In these so-called Large Pitch Fibers, threshold for mode instabilities is increased to 294W offering robust single-mode operation below this power level [6]. We have demonstrated a high power-high efficiency industrial picosecond source using single-mode Large Pitch rod-type fibers doped with Ytterbium. Large Pitch Rod type fibers can offer a unique combination of single-mode output with a very large mode area from 40 μm up to 100μm and very high gain. This enables to directly amplify a low power-low energy Mode Locked Fiber laser with a simple amplification architecture, achieving very high power together with singlemode output independent of power level or repetition rate.

Spectral Beam Combining (SBC) of fiber lasers provides a simple, robust architecture for power scaling lasers to high power. With appropriate designs, power scaling beyond the single fiber limit can be achieved while maintaining near diffraction limited beam quality and high efficiency. We present experimental results where we achieved > 3 kW at an M² = 1.35 and > 39% E-O efficiency by combining 12 individual fiber lasers into a single high brightness beam.

We demonstrate a passive coherent beam combination of two nanosecond amplifiers by using an all-optical feedback loop. An electro-optic amplitude modulator is utilized to tune the pulse width and the pulse repetition frequency of combined laser pulse. The positive correlation between the visibility of far-field coherent patterns and the pulse duty ratio is found. The range of tunable pulse repetition frequency is from 2.023 MHz to 6.069 MHz, and the range of tunable pulse width is from 10 ns to 50 ns. The maximum visibility is up to 85%. This approach presented here provides a promising way for power scaling of high power nanosecond fiber laser and maintaining beam quality simultaneously.

We present an experimental study on active coherent combining of five Yb (Ytterbium)-doped fiber laser amplifiers that employs multiplexed volume Bragg gratings (MVBGs), reporting a combining efficiency of 82% and near-diffraction limited beam quality at a combined input power of 380 W, and 70% combining efficiency with equal beam quality at 670 W input power.

The process of high harmonic generation allows for up-conversion of infrared laser light towards the EUV or soft X-ray region. If very short (few-cycle) laser pulses are employed and their carrier envelope phase (CEP) is well controlled the generation of so-called isolated attosecond pulses becomes feasible. Today’s few-cycle laser technology relies on Ti:Sapphire laser systems and hollow fiber based post-compression. The output power of such lasers is typically below 1 W and the repetition rate is limited to a few kilohertz due to thermo-optical limitations of the Ti:Sapphire amplifiers. In this contribution we present a different approach combining the advantages of fiber laser technology with nonlinear frequency conversion. A high power femtosecond fiber laser system serves as pump laser for an ultrabroadband optical parametric amplifier. As a result we are able to generate intense CEP-stable pulses with only two optical cycles duration at repetition rates up to 0.6 MHz. The excellent beam quality ensured by the fiber based pump laser enables focusing of these pulses to high intensities, thus, allowing for the generation of high harmonics and attosecond pulses at exceptionally high repetition rates. We will present the design of the laser system and discuss specific challenges such as the broadband signal generation, the temporal synchronization of the pump laser and the carrier envelop phase stabilization. In addition, experimental results on high repetition rate XUV continuum generation will be presented, demonstrating the feasibility of our approach.

We report on a linear polarized high power femtosecond fiber chirped-pulse amplification (FCPA) system operating at 360 W of average power with an excellent beam quality (M²=1.2). A mode locked fiber oscillator with a repetition rate of 250 MHz seeds the FCPA system. The 265 fs pulses are shortened in time employing the nonlinear compression technique. An unprecedented combination of average power, pulse duration and repetition rate is reached with an excellent beam quality by using a solid-core photonic-crystal fiber nonlinear compression stage. Thereby, the peak power of the fiber chirped pulsed amplification system is close to the self-focusing threshold of fused silica (case of linear polarization). In order to avoid self-focusing the threshold is increased by changing the polarization from linear to circular. Finally, the second order dispersion is compensated with a chirped-mirror compressor reaching shorter pulse durations. We achieve pulse shortening by more than one order of magnitude down to 23 fs pulses, compressed pulse energy of 0.9 μJ and a peak power of 34 MW at an average power level of 250 W. At this power level we measure an excellent beam quality (M²=1.3). This system is an ideal laser source for studying high field physics, e. g. driving enhanced cavities for high-repetition-rate high-harmonic generation.

Mode-locked mid-infrared (mid-IR) fiber lasers are of increasing interest due to their many potential applications in spectroscopic sensors, infrared countermeasures, laser surgery, and high-efficiency pump sources for nonlinear wavelength convertors. Er³⁺-doped ZBLAN (ZrF₄-BaF₂-LaF₃-AlF₃-NaF) fiber lasers, which can emit mid-IR light at 2.65-2.9 μm through the transition from the upper energy level ⁴I_11/2 to the lower laser level ⁴I_13/2, have attracted much attention because of their broad emission range, high optical efficiency, and the ready availability of diode pump lasers at the two absorption peaks of Er³⁺ ions (975 nm and 976 nm). In recent years, significant progress on high power Er³⁺- doped ZBLAN fiber lasers has been achieved and over 20 watt cw output at 2.8 μm has been demonstrated; however, there has been little progress on ultrafast mid-IR ZBLAN fiber lasers to date. We report a passively mode-locked Er³⁺- doped ZBLAN fiber laser in which a Fe²⁺:ZnSe crystal was used as the intracavity saturable absorber. Fe²⁺:ZnSe is an ideal material for mid-IR laser pulse generation because of its large saturable absorption cross-section and small saturation energy along with the excellent opto-mechanical (damage threshold ~2 J/cm²) and physical characteristics of the crystalline ZnSe host. A 1.6 m double-clad 8 mol% Er³⁺-doped ZBLAN fiber was used in our experiment. The fiber core has a diameter of 15 μm and a numerical aperture (NA) of 0.1. The inner circular cladding has a diameter of 125 μm and an NA of 0.5. Both continuous-wave and Q-switched mode-locking pulses at 2.8 μm were obtained. Continuous-wave mode locking operation with a pulse duration of 19 ps and an average power of 51 mW were achieved when a collimated beam traversed the Fe²⁺:ZnSe crystal. When the cavity was modified to provide a focused beam at the Fe²⁺:ZnSe crystal, Q-switched mode-locked operation with a pulse duration of 60 ps and an average power of 4.6 mW was achieved. More powerful and narrower pulses are expected if the dispersion of the cavity can be properly managed.

A growing number of applications are calling for compact laser sources operating in the mid-infrared spectral region. A review of our recent work on monolithic fiber lasers based either on the use of rare-earth fluoride fibers or on Raman gain in both fluoride and chalcogenide glass fibers is presented. Accordingly, an erbium-doped double clad fluoride glass all-fiber laser operating in the vicinity of 3 μm is shown. In addition, we present recent results on the first demonstrations of both fluoride and chalcogenide Raman fiber lasers operating at 2.23 μm and 3.34 μm, respectively. It is shown that based on this approach, monolithic fiber lasers could be developed to cover the whole 2-4 μm spectral band.

We realize a novel scheme for emitting narrow linewidth laser pulses in the 1100-1200 nm wavelength range. A narrowband CW laser at the desired wavelength is combined, together with a broadband pulsed laser at ~1080 nm, into a Large-Mode-Area Ytterbium fiber amplifier. The increase in peak intensity leads to efficient Raman conversion of energy from the amplified 1080 nm pulses into the mode seeded by the CW laser. To demonstrate the concept, a narroband CW diode at 1130 mm and a pulsed broadband laser at 1080 nm are combined to generate >15W pulses at 1130 nm with excellent beam quality and sub-100 pm linewidth, along with a demonstration of high efficiency nonlinear conversion into Yellow (565 nm).

We report on experimental studies of random lasing realized in optical fibers with the use of Brillouin amplification and Rayleigh backscattering employed as a distributed feedback instead of a cavity mirror. In our experiment 25-km-long high quality standard telecom single-mode fiber was employed for Rayleigh reflection uniformly distributed over all fiber length. We have observed a clear competition between a classical Brillouin scattering and Brillouin lasing. Presence of extended fluctuation-free fragments in the recorded oscilloscope traces highlights Stokes power statistics typical for laser radiation rather than for Brillouin process. The results of the experiments are in a perfect agreement with the model of Brillouin - Rayleigh cooperative process in long fibers.

Chalcogenide glass fibers offer broad transparency range up to the mid-infrared and high nonlinear coefficients making them excellent candidates for four wave mixing frequency conversion. However, the use of microstructured airchalcogenide fibers is mandatory to achieve phase-matching in such a fiber. Numerical modelling of the phase matching condition can be done using the simplified effective index model, initially developed and extensively used to design airsilica fibers. In this paper, we investigate the use of the effective index model in the case of microstructured As₂S₃ and As₂Se₃ fibers. One essential step in the method is to evaluate the core radius of a step-index fiber equivalent to the microstructured fiber. Using accurate reference results provided by finite-element computation, we compare the different formulae of the effective core radius proposed in the literature and validated for air-silica fibers. As expected, some discrepancies are observed, especially for the highest wavelengths. We propose new coefficients for these formulae so that the effective index method can be used for numerical modelling of propagation in air-chalcogenide fibers up to 5 μm wavelength. We derive a new formula providing both high accuracy of the effective core radius estimate whatever the microstucture geometry and wavelength, as well as uniqueness of its set of coefficients. This analysis reveals that the value of the effective core radius in the effective index model is only dependent on the microstructure geometry, not on the fiber material. Thus, it can be used for air-silica or air-chalcogenide fibers indifferently.

3C fiber technology advances the performance frontier of practical, high-pulse-energy fiber lasers by providing very large core fibers with the handling and packaging benefits associated with single mode fibers. First-generation fibers demonstrate scaling to > 240 W average power coincident with 100-kW peak power in 1-mJ, 10-ns pulses while maintaining single-mode beam quality, polarized output, and efficiencies > 70%. Peak powers over 0.5 MW with negligible spectral distortion can be achieved with sub ns, near-transform-limited pulses. In-development second-generation 3C Yb-fiber based on core sizes around 55 μm¹ have produced >8 mJ, 13 ns pulses with peak powers exceeding 600 kW.

We report a passively Q-switched all-fiber laser using a large mode area (LMA) Yb³⁺-doped fiber cladding-pumped at 915 nm and an unpumped single-mode Yb³⁺-doped fiber as the saturable absorber (SA). The saturable absorber and gain fibers were first coupled with a free-space telescope to better study the composite system, and then fusion spliced with fiber tapers to match the mode field diameters. ASE generated in the LMA gain fiber preferentially bleaches the SA fiber before depleting the gain, thereby causing the SA fiber to act as a passive saturable absorber. Using this scheme we first demonstrate a Q-switched oscillator with 40 μJ 79 ns pulses at 1026 nm using a free-space taper, and show that pulses can be generated from 1020 nm to 1040 nm. We scale the pulse energy to 0.40 mJ using an Yb³⁺-doped cladding pumped fiber amplifier. Experimental studies in which the saturable absorber length, pump times, and wavelengths are independently varied reveal the impact of these parameters on laser performance. Finally, we demonstrate 60 μJ 81 ns pulses at 1030 nm in an all fiber architecture using tapered mode field adaptors to match the mode filed diameters of the gain and SA fibers.

We demonstrate the amplification of a 1064nm pulse-programmable fiber laser with Large Pitch Rod-Type Fibers of various Mode field diameters from 50 to 70 μm. We have developed a high power fiber amplifier at 1064nm delivering up to 100W/1mJ at 15ns pulses and 30W/300μJ at 2ns with linearly polarized and diffraction limited output beam (M²<1.2). The specific seeder from ESI – Pyrophotonics Lasers used in the experiment allowed us to obtain tailored-pulse programmable on demand at the output from 2ns to 600ns for various repetition rates from 10 to 500 kHz. We could demonstrate square pulses or any other shapes (also multi-pulses) whatever the repetition rate or the pulse duration. We also performed frequency conversion with LBO crystals leading to 50W at 532nm and 25W at 355nm with a diffraction limited output. Similar experiments performed at 1032nm are also reported.

The origin of random highly peaked backward pulses arising from(in) pulsed master oscillator power amplifier (MOPA) fiber lasers systems is studied. These large amplitude short duration pulses when compared to the Rayleigh backscattering may lead to catastrophic failures. This paper presents a systematic study of the backscatter using different seed sources (externally and directly modulated) in a hardened MOPA setup and a delayed self-heterodyne RF domain spectrum analysis technique for its characterization. Fabry-Perot seeded non-polarization maintaining system is found to show hallmark Brillouin frequency shift. The frequency of occurrence strongly depends on the seed characteristics.

In 2004 ITT Exelis developed the Multifunctional Fiber Laser Lidar (MFLL) for measuring atmospheric CO₂. This lidar relies on high efficiency telecom laser components and Erbium Doped Fiber Amplifiers (EDFA’s) to implement a unique Continuous Wave (CW) Intensity Modulated (IM) differential absorption lidar measurement. This same approach has also been used to measure atmospheric O₂ by replacing the EDFA’s with fiber Raman amplifier technology. The use of all fiber coupled components results in a highly reliable, flexible and robust instrument. The general architecture of the MFLL, its implementation for greenhouse gas measurements, and as a pseudorandom noise encoded altimeter system is reviewed. Results from a 2011 flight campaign on the NASA DC-8 aircraft which included CO₂, O₂, and PN altimetry using a single receiver for all three measurements are also discussed. In addition, an introduction to a novel variation of this approach that will enable greenhouse gas monitoring from a geostationary orbit is given. This paper provides a general overview of a set of applications for fiber lasers in the area of active remote sensing that have been developed by Exelis over the past several years.

We present a special high spectral resolution lidar (HSRL) by using a novel tunable fiber based transmitter. The transmitter can produce 50μJ pulse energy at 1064nm and >25μJ pulse energy at 532nm with 10 kHz repetition rate, 5ns pulse width, respectively. A key advantage of the transmitter is the frequency-tunability. The laser can be tuned over the Iodine absorption lines from 1111 to 1104. The laser has a ~130MHz linewidth at 1064nm close to the transform limit linewidth ~ 88MHz for a pulse width of 5ns. Even though it was not frequency locked, the laser has very good frequency stability, which is on the order of ~200MHz over minutes. The beam quality M² is less than 1.5. All the preliminary transmitter parameters meet the basic requirements of a HSRL. The transmitter was implemented in UMBC’s lidar lab that includes a ceiling hatch to enable vertical propagation and viewing of transmitted laser beams into the atmosphere. The atmospheric measurement demonstrates good agreement of the signal to the model Rayleigh decay over the profile range with no significant deviations. Most importantly, these results show that the measurement successfully suppresses the Mie scattering from clouds while recovering the full molecular signal as expected.

Tm³⁺-doped fluoride (ZBLAN) fibers offer amplification and lasing in a wide variety of wavelength ranges, including 810 nm, 1480 nm, 1900 nm, and 2300 nm.¹ Amplification and lasing around 1480 nm through the ³H₄→³F₄ transition is of interest for extending the capacity of WDM transmission systems, as well as developing sources for pumping erbium-doped fiber and fiber Raman amplifiers. The ³H₄→³F₄ transition, however, poses a challenge due to its self terminating nature. As such, the ³F₄ level can be depleted either by colasing at 1900 nm² or by using upconversion pumping at 1064 nm. High-power 1480 nm Tm³⁺:ZBLAN fiber lasers with upconversion pumping at 1064 nm have been demonstrated.^3-6 Recent research has focused on improving further the power conversion efficiency as well as the development of monolithic fiber lasers, e.g., by incorporating fiber Bragg gratings (FBGs) directly within the Tm³⁺: ZBLAN fiber gain medium. Dual-wavelength and multi-wavelength sources can have many applications in instrumentation (e.g., component testing), LIDAR systems, and fiber optics sensing. There have been several reports of dual-wavelength Tm³⁺-doped fiber lasers. For example, Androz et al. demonstrated operation at 785 nm and 810 nm, corresponding to the ¹G₄→³H₅ and ³H₄→³H₆ transitions, respectively, with a Tm³⁺:ZBLAN fiber gain medium.⁷ Wang et al. obtained dual-wavelength lasing around 2 μm with a tunable wavelength spacing from 1 nm – 40 nm in a Tm³⁺:silica fiber laser.⁸ We realized oscillation at 805 nm and 810 nm through the ³H₄→³H₆ transition in a Tm³⁺:ZBLAN fiber laser; we also reported wavelength switching capability as well as bistable operation in both single cavity and cascaded cavity configurations.⁹ In this paper, we extend our work further and report a dual-wavelength Tm³⁺:ZBLAN fiber laser operating in the S-band. Wavelength spacings of 11 nm and as narrow as 0.6 nm are achieved in a linear cascaded cavity configuration with bidirectional upconversion pumping at 1064 nm.

There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an allsolid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~920μm² can be made with excellent higher order mode suppression.

Optical fiber refractive index profiles were measured across a 2.5 octave wavelength range (from 375 nm to 2100 nm) using a single phase-shifting interferometer. This spectral range is more than a factor of 2 larger than previously reported multi-wavelength interferometers, and includes the pump and amplification bands of Er-doped, Yb-doped, Er:Yb-doped, and Tm-doped fibers. The apparatus can measure the material dispersion in a spatially-resolved manner that permits more accurate prediction of the fiber’s waveguide properties. The measurement wavelength can be tailored to optimize the beneficial or deleterious effects of material dispersion, optical resolution, and interferometric phase ambiguities.

We introduce a new spectrogram approach for analyzing spatially- and spectrally-resolved interferometry (S²) data that overcomes the previously overlooked ambiguity between dispersion and distributed scattering. Traditionally, S² yields a one-dimensional spectrum of inter-modal group delays between higher-order-modes (HOMs) and the dominant fundamental mode. According to this interpretation, inter-modal group delay broadening is considered to be a location signature of HOM scattering; for example, a narrow peak in the spectrum is interpreted to be a discrete scattering event whereas a broad feature is interpreted to be distributed scattering along the fiber. Since the inter-modal dispersion is high for weakly guided HOMs, discrete scattering events will also manifest as broadened features. For the first time, we demonstrate a spectrogram approach to S² analysis in which the spectral interference data is analyzed over small staggered wavelength windows and the inter-modal group delay is plotted as a function of wavelength. In this new two-dimensional map the wavelength dependence of the inter-modal group delay produces an inclination of streaks traversing the spectrogram. This new perspective resolves the ambiguity as to whether group delay broadening is caused by fiber dispersion or distributed scattering that is inherent to the previous one-dimensional S² mapping. The spectrogram is a more accurate map of mode conversion along the fiber length and is essential for evaluating high power fibers and devices. Results for standard telecom single mode fiber and a large-mode-area fiber are presented.

By limiting the core diameter of 15 μm at maximum with NA=0.07±0.01 for the near-diffraction-limited output (V <3.6), we successfully generate the pulse with the peak power of 36 kW and the duration of 4.6 ns in FWHM at the repetition rate of 20 kHz. To the best of our knowledge, the signal pulse energy corresponding to 264 μJ is the highest to date in the diode-seeded 15-μm all-fiber MOPA system with the efficiency of 35%. The success in the energy/power scaling is attributed to the further raise of input energy for more extracted energy, the tradeoff between the Raman-limited signal energy and the amplifier slope efficiency for more signal energy ratio, and the proper adjustment of both pump wavelength and power for avoiding coat damage without forced cooling.

In this paper, we propose a scheme on precisely tunable L-band multi-wavelength fiber laser. This fiber laser has two main characteristics namely broad wavelength band, uniform power spectrum and precise electronic tunability. About 65 wavelengths output within ± 1.5dB power variation with 50GHz channel spacing in broad spectrum range can be obtained at room temperature. The measured optical signal noise ratio (OSNR) and line width of each wavelength are about 20dB and 345.5MHz respectively. Theses 65 wavelengths are able to be tuned simultaneously up or down in frequency domain with a tuning step ranging from 10 MHz to 14 GHz. The tuning resolution can potentially be as low as 1 Hz in our experiment.

We have experimentally investigated fundamental mode propagation in few-meter-long adiabatic step-index tapers with high numerical aperture, core diameter up to 117μm (V=38), and tapering ratio up to 18. We confirmed single fundamental mode guiding in tapers with uniform core index profile by several experiments. We observed an annular near field distribution and degraded beam quality for large output core diameters, found to occur due to intrinsic mechanical stress in the fibers. We expect that eliminating the stress would prevent the mode deformation and allow constructing single-mode, diffraction-limited tapered large-mode-area amplifiers with a good beam shape.

The modal instability (MI) threshold is estimated for four rod fiber designs by combining a semi-analytic model with the finite element method. The thermal load due to the quantum defect is calculated and used to numerically determine the mode distributions on which the expression for the onset of MIs is highly dependent. The relative intensity noise of the seed laser in an amplifier setup is used to seed the mode coupling between the fundamental and higher order mode, and lead to MI threshold values of 174 W – 348 W of extracted output power for the four rod fibers having core diameters in the range 53 μm – 95 μm.

A stable Q-switched laser is useful in the area of remote sensing, range finding, optical imaging, material processing, and fiber communications. With its excellent linear and nonlinear optical characteristics, graphene has been proven to be an attractive material to generate nanosecond, picosecond and femtosecond laser pulses. It has a lot of advantages, such as lower saturation intensity, larger saturable-absorption modulation depth, higher damage threshold, sub-picosecond recovery time and an ultrabroad wavelength-independent saturable-absorption range. In this paper, we demonstrate a graphene based Q-switched fiber laser. Graphene was deposited on the fiber interface by the optically driven deposition method. The thickness of the graphene can be controlled by changing depositing time. The compact Q-switched erbium-doped fiber laser based on graphene operated stably, and got Q-switched pulse sequences output with the repetition rate of 19KHz and the average power of 1.4mW when pump power is 40mW. Higher peak power, shorter pulse duration, and higher repetition rate could be achieved by adjusting the thickness of the graphene layer appropriately. Besides, the pulse duration and output power is proved to be a function of the pump power. The repetition rate of this fiber laser had a characteristic of monotonically increasing, near-linear with the changing of pump power. The stable Q-switching pulse output can be observed on the oscilloscope with differently specific repetition rate and pump power. Theory analysis of this fiber laser and further improvement methods is also studied combined with the experimental results.

High strength metal alloys and ceramics offer a huge potential for increased efficiency (e. g. in engine components for aerospace or components for gas turbines). However, mass application is still hampered by cost- and time-consuming end-machining due to long processing times and high tool wear. Laser-induced heating shortly before machining can reduce the material strength and improve machinability significantly. The Fraunhofer IPT has developed and successfully realized a new approach for laser-assisted milling with spindle and tool integrated, co-rotating optics. The novel optical system inside the tool consists of one deflection prism to position the laser spot in front of the cutting insert and one focusing lens. Using a fiber laser with high beam quality the laser spot diameter can be precisely adjusted to the chip size. A high dynamic adaption of the laser power signal according to the engagement condition of the cutting tool was realized in order not to irradiate already machined work piece material. During the tool engagement the laser power is controlled in proportion to the current material removal rate, which has to be calculated continuously. The needed geometric values are generated by a CAD/CAM program and converted into a laser power signal by a real-time controller. The developed milling tool with integrated optics and the algorithm for laser power control enable a multi-axis laser-assisted machining of complex parts.