We report a MOPA (master oscillator - power amplifier) pulsed optical fiber source emitting high-brightness radiation (M² = 1.65) in the “eye-safe” 1.55 μm region. A high pulse energy of 1.15 mJ was reached at low repetition rates while the maximum average output power was 2.2 W at a wavelength of 1562 nm.

Here we report, for the first time to our knowledge, a cladding-pumped passively Q-switched Er-Yb codoped fiber laser with Cr²⁺:ZnSe and Co²⁺:MgAl₂O₄ as saturable absorbers. The maximum average output power for both crystals was 1.4 W, with typical pulse energy was 20 μJ corresponding to 60 W peak power. The pulse duration could be varied between 370 - 700 ns and repetition rate between 20 and 85 kHz by adjusting the pump power.

In this paper we report on the performance of a modular single mode pulsed fiber laser system operating in the C-band. With off-the-shelf telecom components and specialty-designed electronics, 3 kW peak power can be generated in a short (1 ns) pulse at 10 kHz at 1545 nm; however, the onset of nonlinear optical effects (SRS, FWM, and SPM) is observed at a 1kW peak power level. Using highly doped erbium fibers, peak powers up to 13kW and pulse energies of up to 20μJ have been generated with a pulse duration range of 0.6-5 nsec, repetition rates between 3kHz to 1 MHz, and at a wavelength of 1545.3nm and 1567.5 nm before the onset of nonlinear effects became noticeable. Therefore, with the use of highly doped erbium fiber, the onset of nonlinear effects can be increased by an order of magnitude. For narrowband amplification, stimulated Brillouin scattering (SBS) is the limiting nonlinear process. In this regime we recorded the onset of SBS at 8μJ/pulse with a duration of 2.5 nsec. Depending on the pulse shape and pulse duration, self phase modulation (SPM) was also observed, which spectrally broadens the output centered at the signal wavelength; however, the spectral broadening due to SPM is only minor compared to SRS and FWM. It was also demonstrated that pulse steepening is minimized with an appropriate seed waveform. A 3 ns, shaped, input pulse nearly maintained its pulse duration after amplification. Without pulse shaping, the pulse shortened to 1.1 ns.

Various military lidar applications such as underwater mine detection, obstacle avoidance, IRCM, and 3 D lidar incorporate high repetition rate solid-state lasers to accomplish the mission. The recent advances and demonstrations in high power Ytterbium (Yb) fiber lasers/amplifiers make the fiber media a viable alternative to bulk lasers for these applications. The fiber laser geometry maximizes the pump absorption and mode matching for overall high efficiency, (factor-of-two over bulk laser sources) while minimizing thermal effects. In this presentation we will show experimental and modeling results on various master oscillator Yb doped polarization maintaining (PM) fiber amplifiers being developed for high repetition rate applications. We have demonstrated >20 W of average output power with M² <1.3, repetition rates up to 75 kHz and pulse widths ranging from <1 ns to 250 ns. Results of a pulsed PM MOFA efficiently pumping a Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator (OPO) and KTP doubler will also be presented.

Gain and output power as a function of pump power has been calculated for short Er/Yb doped single mode fibers for various fiber parameters. The calculation shows (i) small signal gain increases with increasing Er concentration, (ii) long fiber lengths provide both higher gain and higher output, (iii) calculated gain is larger for higher Er emission cross section, and, (iv) >25 dB gain is feasible for 5 cm long fiber for Er and Yb concentrations of 3 X 10²⁶ m^-3 and 1.2 X 10²⁷ m^-3.

The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction limited, linearly polarized beam by a self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO3). Up to 11.6W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency.

A CW Nd:YAG master oscillator - fiber power amplifier (MOPFA) with fiber based SBS phase conjugate mirror is reported. A two-pass amplifier configuration is employed to compensate beam distortions in the multi-mode diode pumped Yb-doped fiber amplifier in conjunction with a fiber phase conjugator. The compensation of distortions is observed with ~30% of the total reflected power being of diffraction limited quality. Possibilities for improving the beam quality and power scaling in this system is proposed.

A 978 nm Yb-doped jacketed-air-clad fiber MOPA generates 18 mW of power at 488.7 nm when single-pass frequency-doubled in periodically poled KTP at room temperature. The tunable fiber laser - fiber amplifier MOPA provided 2.7 W of output power at 978 nm.

We report for the first time, more than 400 mW of output power at 1056.1nm from a distributed feedback (DFB) fiber laser. The DFB fiber laser comprises a simple π-phase-shifted Bragg grating written into a photosensitive ytterbium-doped fiber. The laser operates with a single longitudinal mode at a wavelength defined by the phase shift and the grating period. Without any internal polarisation selection mechanism, the cavity supports orthogonal polarisation modes, which operate simultaneously. The DFB fiber laser was pumped by a 976nm amplified spontaneous emission (ASE) source based on a ytterbium doped jacketed air clad (JAC) fiber pumped by a 915nm multimode laser diode source. An output of 400mW at 1056.1nm was obtained from the output port while 70mW was obtained from the other port, when pumped with 1.5W of 976nm radiation. The total output from the DFB fiber laser was approximately linear with increasing pump power and the overall performance was limited by the available pump power. The spectral characteristics and signal to noise ratio remained similar over the pump power range. The output of the DFB was in single-mode fiber (ie. M²~1).

Ytterbium only and erbium-ytterbium co-doped phosphate glass Double Clad (DC) cladding pumped Large Mode Area (LMA) core fibers are manufactured at Kigre by the “rod-in-tube” method. The ytterbium and erbium doping concentration levels in phosphate glass are as much as two orders of magnitude higher than the doping concentrations found in fused silica fiber manufactured by Modified Chemical Vapor Deposition (MCVD) method. The background loss of the fiber’s core and cladding measured ~ 0.01 dB/per cm at 1310nm. The measured absorption coefficient at 974 nm is 0.3 dB/per cm for the ytterbium-erbium co-doped fiber. Greater than 4.6 Watts CW laser output was demonstrated from the Er:Yb:glass fiber at 1535nm with a 37.4% slop efficiency and 36.3% optical efficiency. The Yb:glass fiber produced a maximum output power of 1.6 Watts in a 14cm gain length.

We have demonstrated spectral beam combining of two high power fiber lasers and obtained more than 40 W output power from the system. The system consists of two 30 W fiber lasers, a diffraction grating and a resonator. Both fiber lasers have broadband fiber Bragg gratings on the rear sides and share the diffraction grating and output coupler on the output sides. The wavelengths of the fiber lasers are determined by the optical dispersion provided by the grating and the collimating lens, as well as the fiber spacing. A model that analyzes dependence of laser line-width on beam quality of an SBC system is given in this paper. We also model a novel configuration that can significantly improve beam quality. The experimental results have shown that control of fiber laser line-width is the key to achieving high power SBC lasers. In addition, a new approach using three gratings is proposed and has been proven by the experiment. The new approach consists of three gratings, in which one grating is used by the SBC cavity to lock wavelengths while the other two combine the collimated beams without beam quality reduction. This approach has successfully improved beam quality M² from 11 to 2.0.

Self-organized coherence between fiber lasers has been reported both via all-fiber 2x2 directional coupler trees and in spatially multi-core fibers. We have taken this a major step forward, coupling together a number of independent fiber lasers to obtain a spatially and spectrally coherent far field, with no active length, polarization, or amplitude control. The near field output comes from a spatial array rather than from a single fiber, making this approach scalable to extremely high power.

We have developed and demonstrated a fiber amplifier based architecture capable of high peak power pulsed operation that is scalable to high average powers ⪆10 kW. Our approach uses a mode-locked master oscillator to provide the short pulse waveform and an array of fiber amplifiers to provide high efficiency power amplification using a coherently phased, wavelength multiplexed approach. The mode-locked oscillator input is decomposed into its individual modes using a grating, the individual wavelengths are then amplified in a CW format in the fiber amplifier array, recombined with a second grating, and phased to reproduce a high peak power, short pulsewidth, mode-locked output train. We report the initial demonstration of this architecture with an array of 4 fiber amplifiers. 4 modes of the master oscillator output were selected and amplified from 1 mW to 1 Watt each, followed by recombination and phasing. The phased output pulse train effectively reproduced the master oscillator pulse shape, providing a mode-locked output train of 50 psec pulses. The phase control is performed in parallel for each element of the amplifier array and is directly scalable to high average powers by increasing the power of the individual amplifiers and increasing the size of the amplifier array.

Since the observation in June 2000 of a high-brightness beam emitted from a 7-core Yb-doped phase-locked fiber laser array, much progress has been made to advance this laser technology by (1) increasing the number of the Yb-doped cores from 7 to 19, (2) establishing a power-scaling model for fabricating a multicore fiber laser containing 127 cores, that can potentially yield an output power greater than 10 KW, (3) developing a demountable side-pumping coupler to allow injection of the diode laser pump directly into the fiber along its length, (4) improving the beam quality with appropriate choice of the fiber parameters, and (5) enhancing the Q-switched laser energy utilizing multicore fiber laser arrays. This paper will present details on these developments.

High-efficient volume Bragg gratings (VBG) in inorganic photo-thermo-refractive (PTR) glass were recently reported for the use in high-power laser systems. Both transmission and reflection gratings have shown diffraction efficiency greater than 95% from visible to near IR spectra in a wide range of spatial frequencies. Those gratings have exhibited perfect thermal, optical and mechanical stability. Spectral beam combining (SBC) using PTR Bragg grating with efficiency more than 92% for two 100 W Yb-fiber-laser beams with the 11 nm wavelength separation between them is reported. The paper presents results of modeling and experimental study of a beam combiner for high-power lasers with the only passive PTR grating component in it. Two laser beams illuminate a thick Bragg grating which has only two symmetric resonant angles providing total diffraction of a beam with a certain wavelength. Incidence angle for all transmitting beams should correspond to the Bragg angle for the diffracted beam. Transmitting beams are not diffracted by grating if spectral sift corresponds to zeros in a spectral selectivity curve, and propagate in the same direction as a diffracted beam. It is shown the efficient trade-off between grating period and refractive index modulation allows modeling of high-efficient combining setup for each of arbitrary chosen grating thickness. Comparison between calculation results and experimental data is given.

We have developed and demonstrated a large flattened mode (LFM) optical fiber, which raises the threshold for non-linear interactions in the fiber core by a factor of 2.5 over conventional large mode area fiber amplifiers. The LFM fiber works by incorporating a raised index ring around the outer edge of the fiber core, which serves to flatten the fundamental fiber mode from a Bessel function to a top hat function. This increases the effective area of the core intersected by the mode by a factor of 2.5 without increasing the physical size of the core. This is because the core is uniformly illuminated by the LFM mode rather than having most of the light confined to the center of the core. We present experimental and theoretical results relating to this fiber and its design.

The advent of double clad fiber technology has made high power lasers and amplifiers possible. However, the scalability of output powers can be limited by amplified spontaneous emission and nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using low numerical apertures (NAs), large-mode areas (LMAs), novel index profiles and high dopant concentrations. This paper discusses advances made in design and fabrication of highly efficient, large-mode area double clad fibers. Experimental and modeling results pertaining to changes in mode area, resultant power density and nonlinear threshold with changing core size are discussed. In addition, the mechanical reliability of the LMA fibers is discussed.

We used coupled-mode theory in an Yb-doped multimode fiber amplifier to compute the effects of gain saturation, nonlinear index, and fiber curvature on the evolution of the field. A positive nonlinear index results in power transfer to lower-order modes, usually the fundamental LP₁₀ mode, and for negative nonlinear index the reverse is predicted. The nonlinear interaction between modes breaks the core’s cylindrical symmetry, resulting in recombination of degenerate LP mode pairs into super-modes: consisting of an expected in- and anti-phase pair, but also a quadrature of super-mode that reflects an increase of “information” capacity associated with nonlinearity. Convergence to all three super-modes was observed in our simulations, but the last more often. We also present observed evidence of mode phasing in experiments with two fiber amplifiers.

Most recently the output power of fiber lasers with diffraction limited beam quality has been significantly increased. Further power scaling is usually limited by damage of the fiber end facets, thermo-optical problems or nonlinear effects. Microstructuring the fiber adds several preferable features to the fiber to overcome these restrictions. We review the advantages of rare-earth-doped photonic crystal fibers for power scaling of fiber lasers to the multi kW range with excellent beam quality.

We have analysed different 1D and 2D arrays of evanescently coupled cores within a fibre laser structure. The supermodes (phase-locked modes) have been calculated using coupled mode theory. We show that without a Talbot mirror, the out-of-phase supermode has the lowest threshold. Supermode selection is obtained using a Talbot cavity. A threshold analysis is carried out and it is shown than the in-phase supermode can be selected for a densely packed array of cores. 2D core structures are much more effective than 1D core structures for in-phase supermode selection. The influence of parameters like the strength of the evanescent coupling constant or the core-to-core detunings of propagation constant on the dynamical stability of the supermodes is investigated. We give figures of the minimum bend radius for phase locking. We show that large multicore structures can potentially be bent tighter than the equivalent single large core fibre laser.

In this work a new model based on the work with new double-clad Yb-doped photonic crystal fibers is presented. The model describes the effect of a coupling between core modes and cladding modes, causing a loss of power out of the core. The model agrees well with an experimental observed asymmetry in the output powers from the two ends of a pumped fiber due to pump depletion and the core-cladding coupling. A method to estimate the core-cladding coupling is included. The magnitude of the coupling depends on the geometry of the fiber. If an asymmetry is introduced between the coupling for the two orthogonal linear polarization directions, an polarization creating mechanism is predicted. This feature is investigated for a fiber where the asymmetric loss has been implemented by manufacturing the fiber to have an asymmetric transverse geometry.

Air-clad photonic crystal fibers hold promise to bring the single mode power levels past the 1kW limit through the utilization of extremely high numerical apertures, large mode field diameters and short fiber lengths. Here we discuss design, fabrication and handling issues of such fibers.

Side-pumping of double clad photonic crystal fibers is experimentally demonstrated. Optical access to the multimode cladding is obtained by collapsing the airholes over a short length of fiber while leaving the inner single mode core undisturbed. Coupling efficiencies above 90% are obtained. A side-pumped Yb fiber laser with a slope efficiency of 81% is demonstrated using this method.

We review current work on fiber laser systems at Corning. In particular, we describe design and performance of all-glass double-clad laser fibers, broad-area laser pumps, and pump coupling optics. We discuss our approaches using single-polarization fiber and low-nonlinearity photonic band gap fiber as technologies for developing the next generation of high-power fiber lasers.

Fiber lasers products have been developed at JDS Uniphase with up to 25 Watt cw output power and diffraction limited beam. Similar fiber lasers have been demonstrated with over 100 Watt cw output power. The fiber laser is based on an all fiber optic cavity with no free alignments or possibility for contamination resulting in a reliable laser cavity. A distributed pump architecture based on an array of 3-5 Watt fiber coupled pumps provides redundancy for reliability. The unpolarized, fiber delivered, compact and direct modulated fiber laser sources are ideal for a range of applications including material processing, marking and reprographics. Moreover the pump source has applications in material processing as well. The advantages of the fiber laser are illustrated in marking system.

High Power Fiber Lasers (HPFLs) and High Power Fiber Amplifiers (HPFAs) promise a number of benefits in terms of their high optical efficiency, degree of integration, beam quality, reliability, spatial compactness and thermal management. These benefits are driving the rapid adoption of HPFLs in an increasingly wide range of applications and power levels ranging from a few Watts, in for example analytical applications, to high-power >1kW materials processing (machining and welding) applications. This paper describes SPI’s innovative technologies, HPFL products and their performance capabilities. The paper highlights key aspects of the design basis and provides an overview of the applications space in both the industrial and aerospace domains. Single-fiber CW lasers delivering 1kW output power at 1080nm have been demonstrated and are being commercialized for aerospace and industrial applications with wall-plug efficiencies in the range 20 to 25%, and with beam parameter products in the range 0.5 to 100 mm.mrad (corresponding to M² = 1.5 to 300) tailored to application requirements. At power levels in the 1 - 200 W range, SPI’s proprietary cladding-pumping technology, GTWave^TM, has been employed to produce completely fiber-integrated systems using single-emitter broad-stripe multimode pump diodes. This modular construction enables an agile and flexible approach to the configuration of a range of fiber laser / amplifier systems for operation in the 1080nm and 1550nm wavelength ranges. Reliability modeling is applied to determine Systems martins such that performance specifications are robustly met throughout the designed product lifetime. An extensive Qualification and Reliability-proving programme is underway to qualify the technology building blocks that are utilized for the fiber laser cavity, pump modules, pump-driver systems and thermo-mechanical management. In addition to the CW products, pulsed fiber lasers with pulse energies exceeding 1mJ with peak pulse powers of up to 50kW have been developed and are being commercialized. In all cases reducing the total “cost of ownership” for customers and end users is our primary objective.

We compare the frequency detuning properties of the optical pulses generated from erbium-doped fiber lasers (EDFL’s) by using harmonic mode-locking and regenerative amplification techniques. The frequency detuning range of regeneratively amplified pulse (17.78 kHz) is wider than that of harmonic mode-locked pulses (7 kHz). The regeneratively amplidied EDFL pulse has a smaller pulsewidth (22 ps), a higher peak power (40.7 mW), a lower phase noise (-107 dBc/Hz at offset frequency of 100 kHz), and a lower timing jitter (0.33 ps). This is attributed to that the characteristic of the gain-switched optical pulse is remained under regenerative amplification operation. Our harmonic mode-locked erbium-doped fiber laser has a lower phase noise (-100 dBc/Hz @ offset 1 kHz; -105 dBc/Hz @ 10 kHz) than that ever reported in a regeneratively harmonic mode-locked fiber ring laser.

For emerging real world applications the availability of high repetition rate and high energy ultrashort pulse laser systems is of significant importance. Ytterbium-doped fiber laser systems have established themselves as a very attractive gain medium for pulsed amplification. We discuss the feasibility of high average power (>100 W) and high energy (~100 μJ) femtosecond fiber CPA systems. Novel fiber geometries based on microstructured large-core air-clad fibers are introduced, which allow for a significant performance scaling. Furthermore, an all-fiber CPA system including an air-core photonic band-gap fiber compressor is presented. This approach opens the avenue to a completely fiber integrated high performance short pulse laser system.

Recent advances in femtosecond fiber lasers are described. Self-similar evolution of parabolic pulses in a modelocked laser can be exploited to substantially increase the pulse energy and peak power that can be achieved without wave-breaking. Experimentally, pulse energies as high as 10 nJ and peak powers as high as 80 kW are obtained from Yb fiber lasers operating in the wave-breaking-free regime.

We present the initial results of entraining colloidal quantum dots emitting at wavelengths from 0.5um through 1.2um, in various micro-structured optical fibers. Conventional and non-conventional, micro-structured optical fibers fabricated at Virginia Tech’s Fiber & ElectroOptics Research Center (FEORC) have been combined with semiconductor, colloidal quantum dots fabricated by the VT Advanced Biomedical Center (VTabc). The results are presented primarily in the form of visual verification and analysis of entrainment phenomena, for a cross-section of colloidal dot and micro-structured fiber forms. Unique optical, electro-optical and material properties resulting from the combinations are visibly suggested in the results. Core/clad/free space propagation properties and effects of emitted and absorbed light fields are observed to be dependent on the structure, aspect ratio and materials of the fibers as well as the properties of the colloidal quantum dots. Basic spectral data on representative free-space materials will be presented in the current paper. The presentation will explore in passing, the research options available to such quantum dot-fiber combinations, including advanced sensors, sources and filters.

We demonstrate a high power erbium-ytterbium co-doped large-core fiber laser with narrow linewidth, an M² value of 1.7 and a broad tuning range. The fiber was cladding-pumped by a diode stack emitting at 975 nm. The laser had a linewidth around 0.16 nm and was tuned from 1533 nm to 1566 nm by compression-tuning a fiber Bragg grating. Output powers in excess of 30 W were obtained over the entire laser tuning range which was limited by the low gain at wavelengths shorter than 1533 nm and by the grating fabrication wavelength at 1566 nm. The laser slope efficiency was ~30% and the threshold ~3.3 W. Our results underline the capability for efficient, broad-band, high-power operation of large-core Er-Yb doped fibers and demonstrate compatibility with telecom components like standard single-mode fibers and fiber Bragg gratings.

We describe the experimental study of phase locking of a four element phased array of fibres, in which the output brightness of the bundle is enhanced by phase locking of the individual elements, and steered by controlling the phase of each channel.

We describe a model of a Yb:glass fibre amplifier incorporating amplified spontaneous emission (ASE). The model is able to predict the output spectrum and power of spectral components in a fibre amplifier, and identifies the optimum configuration for efficiently extracting pump power at a given signal wavelength.

Fiber lasers have shown extraordinary progress in power level, reaching the kilowatt range. These results were achieved with large mode area fibers pumped with high power laser diodes coupled with bulk-optics. To enable the commercial development of these high power fiber lasers, we have demonstrated several All-Fiber components, which replace the bulk-optic interface in the present laser configurations. These components include multimode fused fiber bundle combiners with or without signal fiber feed-through, Bragg gratings and mode field adaptors. The multimode fibers are used to couple several fiber pigtailed pump diodes to a double-clad fiber. Such combiners may contain a signal fiber to provide an input or output for the core modes of the double-clad fiber. Mode field adaptors perform fundamental mode matching between different core fibers. Bragg gratings are used as reflectors for the laser cavity. These components exhibit low-loss and high power handling of 200 Watts has been demonstrated. They enable the design of true high power single-mode All-Fiber lasers that will be small, rugged and reliable.

We have developed a single-frequency thulium doped silica fiber laser with a Distributed FeedBack (DFB) cavity operating at a wavelength of 1735 nm. The laser cavity is 5 cm long formed by a UV-written Bragg grating with a phase shift and is pumped by a Ti:Sapphire laser at 790 nm. The laser operates in a single-polarization mode and is tunable over a few nanometers. To the best of our knowledge, this represents the first short cavity, single frequency fiber laser using thulium as the amplifying medium. The lasing wavelength is among the lowest demonstrated in a thulium-doped fiber laser and it falls in an attractive near-to-mid infrared wavelength region only offered by few sources. Single-frequency DFB fiber lasers are compact and stable optical sources, which offer low-noise coherent output with ultra-narrow-linewidth. Typical applications for DFB fiber lasers are as sources for coherent sensing, spectroscopy and several high-end applications. Using optical fiber doped with erbium and/or ytterbium these sources provide emission within the wavelength bands of 980 - 1200 nm and 1525 - 1620 nm. A thulium doped DFB laser opens a new broad wavelength range from 1.7 μm - 2.0 μm, depending on co-dopants. This wavelength range is especially interesting for use in gas sensors, frequency mixing and as a source for eye-safe LIDAR applications.