A polarization-maintaining ytterbium-doped tapered double-cladding fiber was made from an aluminophosphosilicate glass preform, with core/cladding diameters of 17/125 μm and 56/400 µm at the small and large ends of the flared section, respectively. Amplifier gain exceeding 50 dB and average output power beyond 200 W were simultaneously achieved after the 1032-nm seed laser, while amplified spontaneous emission was measured ⪅ 1% of total output power. This fiber also yields an optical efficiency close to 90%, near diffraction-limited output with M2 ⪅ 1.2 and polarization extinction ratio ⪆ 18 dB. The newly developed fiber holds the potential to combine two successive amplifier stages in a single device, with foreseen benefits for ultrafast fiber amplifiers and laser harmonics generation.
An ytterbium-doped large mode area polarization-maintaining fiber with core/cladding diameters of 35/250 µm was fabricated from modified chemical vapor deposition technique and solution doping process. High cladding absorption and low photodarkening were achieved from aluminophosphosilicate core glass with optimal molar composition. The fiber was tested as a power amplifier using a 1064-nm narrow-linewidth laser oscillator with 34 ps pulse duration and 120 MHz pulse repetition frequency. The slope efficiency was seen to exceed 80% while the average output power was scaled beyond 420 W, before the onset of transverse mode instability. The fabricated fiber also yields near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio.
KEYWORDS: Transverse mode instability, Cladding, Spectral density, Design and modelling, Polarization maintaining fibers, Doping, Fiber characterization, High power fiber amplifiers, High power fiber lasers
The optimization of large-mode-area fiber design for the amplification of narrow-linewidth content or short pulses, susceptible to nonlinear effects, while reaching average powers exceeding the kW level is not a simple endeavor. The rapidly decreasing TMI-threshold with increasing core size leaves very little room in the 20 to 30 μm core diameter space for improved performance with respect to nonlinear effects while still delivering significant average power. We present results on a 29 μm core, polarization-maintaining LMA fiber, with a 400 μm cladding for high average power scaling. A carefully designed depressed-clad surrounds the core and enhances the bending losses for the Higher-Order Modes (HOM). Even when the fiber is loosely coiled (25 cm diameter), the filtering is very efficient which is advantageous for spreading out the fiber heat load and minimizing the effective area reduction resulting from the coiling-induced mode deformation. The fiber has been tested in a CW MOPA configuration, seeded with a longitudinally single-mode source emitting at 1064 nm, phase-modulated for Stimulated Brillouin Scattering (SBS) mitigation. The resulting slope efficiency has been measured at 88%, the PER was in the 12-15 dB range. The main feature of this fiber is its highly efficient HOM filtering capability, consequently one can maintain single mode-like operation up to the TMI threshold (slightly below 1 kW) without significant beam quality (BQ) degradation. As soon as coupling occurs between the fundamental mode and the first higher-order mode through the thermo-optic long-period grating, the energy is shed away and is coupled out in the fiber cladding.
A short-pulse Yb-doped fiber laser based on a master oscillator and power amplifier scheme is reported to yield an average power exceeding 500 W and pulse energy over 1 mJ. The final amplifier stage features a polarization-maintaining, large mode area tapered fiber with core/cladding diameters of 35/250 μm and 56/400 μm at each end of the flared section. The latter yields excellent optical conversion efficiency, near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio. The threshold for the onset of stimulated Raman scattering was further investigated using a pulsed seeder with ps-ns digitally programmable waveforms. Besides, no indication for transverse mode instability could be observed below the stimulated Raman scattering threshold, as beam quality M2 was measured < 1.3 and no fluctuations were further detected from photodiode time-traces of near-field laser beam samples.
A short-pulse Yb-doped fiber laser is reported to yield more than 50 W of UV light after single-pass second- and thirdharmonic generation using LBO crystals. The NIR laser generates ns-mJ pulses with average power exceeding 200 W after multipart fiber amplifier stages. The polarization-maintaining LMA tapered fiber used as the last amplifier stage features near diffraction-limited output, narrow linewidth and high polarization extinction ratio. Conversion efficiency from NIR to UV is found near 30%, with output pulse energy as high as 260 μJ for an oscillator pulse repetition frequency of 200 kHz. Generated UV light is seen close to diffraction limit, with M2 factor measured < 1.3 both along and perpendicular to walk-off axis.
Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Suppression of higher-order modes is enhanced compared with large mode area stepindex fibers owing to the depressed-index inner cladding and confined doping. Average power and pulse energy exceeding respectively 90 W and 50 μJ were achieved after multipart fiber amplifier stages, with final-stage amplifier gain larger than 40 dB. Pulse compression using a chirped volume Bragg grating later yields durations as short as 1 ps and peak powers exceeding 10 MW, with near diffraction-limited output. Pulse-on-demand and burst modes are straightforward, given the master oscillator/power amplifier scheme, with pulse generation first initiated from direct current modulation of a seed laser diode while subsequent external phase modulation and spectral selection yield pulse trains/bursts following digitized arbitrary waveforms. The proposed scheme is considered to be relevant for laser materials processing.
Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Single-stage amplifier gain larger than 43 dB is achieved, with energy of seed pulses (35 ps, 200 kHz) boosted up to 50 μJ at the amplifier output. Spectral broadening induced by self-phase modulation is shown to take place advantageously along the larger end of the counter-pumped active tapered fiber, where the mode area scales beyond 1000 μm2. Pulse durations as short as 1 ps and peak powers exceeding 16 MW are demonstrated thereafter using a chirped volume Bragg grating as a dispersive compressor. Efficient suppression of higher-order modes in the large mode area tapered fiber yields diffraction-limited output (M2 < 1.2) for optimal pulse compression.
A polarization-maintaining Yb-doped large mode area fiber with depressed-index inner cladding layer and confinement of rare-earth dopants has been drawn as a long tapered fiber. The larger end features a core/clad diameter of 56/400 μm and core NA ~ 0.07, thus leading to an effective mode area over 1000 μm2. The fiber was tested up to 100 W average power, with near diffraction-limited output as the beam quality M2 was measured < 1.2. As effective single-mode guidance is enforced in the first section due to enhanced bending loss, subsequent adiabatic transition of the mode field in the taper section preserves single-mode amplification towards the larger end of the fiber.
Long range land surveillance is a critical need in numerous military and civilian security applications, such as threat detection, terrain mapping and disaster prevention. A key technology for land surveillance, synthetic aperture radar (SAR) continues to provide high resolution radar images in all weather conditions from remote distances. State of the art SAR systems based on dual-use satellites are capable of providing ground resolutions of one meter; while their airborne counterparts obtain resolutions of 10 cm. Certain land surveillance applications such as subsidence monitoring, landslide hazard prediction and tactical target tracking could benefit from improved resolution. The ultimate limitation to the achievable resolution of any imaging system is its wavelength. State-of-the-art SAR systems are approaching this limit. The natural extension to improve resolution is to thus decrease the wavelength, i.e. design a synthetic aperture system in a different wavelength regime. One such system offering the potential for vastly improved resolution is Synthetic Aperture Ladar (SAL). This system operates at infrared wavelengths, ten thousand times smaller radar wavelengths. This paper presents a SAL platform based on the INO Master Oscillator with Programmable Amplitude Waveform (MOPAW) laser that has a wavelength sweep of Δλ=1.22 nm, a pulse repetition rate up to 1 kHz and up to 200 μJ per pulse. The results for SAL 2D imagery at a range of 10 m are presented, indicating a reflectance sensibility of 8 %, ground-range and azimuth resolution of 1.7 mm and 0.84 mm respectively.
Large mode area fibers with depressed-index cladding layer and confinement of rare-earth dopants can provide effective suppression of high-order modes. A polarization-maintaining Yb-doped double-clad fiber with 35/250 μm core/clad diameter has been fabricated from conventional methods according to this design. The fiber which has an effective mode area close to 500 μm2 yields near diffraction-limited output with beam quality factor M2 close to 1.1 when tested as a power amplifier with a coherent seed light source. Beam pointing measurements provide further evidence for near single-mode behavior as the pointing fluctuations are shown to be negligible once the fiber is coiled to a given diameter.
Nanosecond pulsed fiber laser sources have found multiple usages in material processing applications. Their reliability,
flexibility, low cost, high average power and high beam quality are the reasons for their commercial success. With
appropriate means, nanosecond fiber lasers based on a MOPA configuration can emit pulses with tailored shapes. This
feature greatly increases the flexibility of the laser as it allows the emission of pulses of adjustable duration, complex
pulse shapes such as bursts of short pulses, gain saturation-compensated shapes, chair-like shapes, or any other
variations. Pulse shaping can have a significant impact on the ablation rate, the surface quality of the processed sample and different materials have been shown to respond differently to those pulse shape variations (the thermal conductivity of the material being a key parameter). Pulse shaping is also very valuable for optimizing the pulse energy from a system as it allows pre-compensation for the pulse distortion caused by gain saturation which tends to narrow the pulse duration, increase the peak power and associated SRS sensitivity. Through pulse shaping, one can achieve pulse energies that are significantly higher than the saturation energy of the amplifier while mitigating the detrimental effects of SRS. It is however important to consider the impact of pulse shape and pulse duration on the SBS threshold of an optical system. We have performed numerous SBS threshold measurements for pulses of varying duration and of varying levels of precompensation for gain saturation. We have demonstrated that pulse shapes with effective pulse duration of 40 ns have the lowest SBS threshold.
In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare
the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape.
The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz,
grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 40 ns. The laser output central
wavelength was at 1064 nm and the output beam M2 value was below 1.15. Micro-milling experiments were performed
on silicon for two levels of energy per burst and with different burst amplitude profiles. We show that the maximum
material removal efficiency and the surface quality can be increased by more than 25% when using bursts of picosecond
pulses with respect to nanosecond pulses with similar energy per pulse. Effect of shaping the burst envelope of the
picosecond laser on the maximum material removal efficiency is also presented.
Increasing the ablation efficiency of picosecond laser sources can be performed by bunching pulses in bursts1 and benefit
from heat accumulation effects2-5 in the target. Pulsed fiber lasers are well suited for such a regime of operation, as the
single pulse energy in a fiber is limited by the onset of nonlinear effects (SPM, SRS). Increasing the number of pulses to
form a burst of pulses allows for average power scaling of picosecond fiber lasers. We are presenting in this paper a
high-power fiber laser emitting arbitrarily-shaped bursts of picosecond pulses at 20 W of average output power. Burst
duration can be varied from 2.5 ns to 80 ns. The burst repetition rate is externally triggered and can be varied from 100
kHz to 1 MHz. The single pulse duration is 60 ps and the repetition rate within a burst is 1.8 GHz. The output beam is
linearly polarized (PER > 20 dB) and its M2 value is smaller than 1.15. The laser source has a tunable central wavelength
around 1064 nm and a spectral linewidth compatible with frequency conversion. Conversion efficiency higher than 60%
has been obtained at 10 W of 1064-nm output power.
A 225-μJ polarization maintaining ytterbium-doped large-mode-area multiclad fiber was designed and fabricated with an
effective mode area of 450 μm2 and a photodarkening maximum excess loss of ~1 dB/m at 1064 nm. The fiber index
profile is based on a depressed-clad to obtain a diffraction-limited output. Optimization for low photodarkening and high
conversion efficiency while maintaining a good control on the core's refractive index profile has been achieved by
adjusting the ytterbium/phosphorus/aluminum concentrations in the fiber core. Concentration ratios of
phosphorus/aluminum from 0.12 to 1.25 were experimentally investigated in terms of photodarkening rate and excess
loss. Within this range, the photodarkening excess loss was observed to decrease by a factor of 8. The large-mode-area
fiber was used in a 10-ns pulse amplifier at 1064 nm with a repetition rate of 100 kHz and 0.5-nm bandwidth. The
diffraction-limited output has a measured M2 value of 1.04 when the fiber is coiled to a diameter of 12 cm. The fiber
amplifier slope efficiency is 70% with a polarization extinction ratio greater than 23 dB. It is shown how the
phosphorus/aluminum ratio reduces photodarkening, and how a depressed-clad design improves higher-order mode
filtering for reliable, efficient, and compact ytterbium-doped fiber amplifiers.
The relations between dopant concentrations (phosphorus and aluminum) and photodarkening rate, excess loss, and
activation energies in ytterbium-doped silica fibers are experimentally investigated. It is shown that increasing the
concentration of phosphorus from 0.2 to 2.5 mol% in phosphorus/aluminum codoped fiber cores decreases the
photodarkening excess loss by a factor of 8 and the photodarkening rate by a factor of 10. Moreover, the effective
number of ytterbium ions involved in the photodarkening process increases from 4 to more than 6 for tested
phosphorus/aluminum concentration ratios varying from 0.1 to 1 respectively. In contrast, increasing the aluminum
concentration from 2 to 5 mol% for a fixed phosphorus concentration of 0.2 mol% has negligible effect on the initial
photodarkening rate or the effective number of ytterbium ions involved in the process, but still decreases the
photodarkening excess loss by a factor of 5. Those results suggest photodarkening activation energies of 5.2 eV for
ytterbium/aluminum-codoped silica fibers and more than 7.8 eV for ytterbium/phosphorus/aluminum-codoped silica
fibers. The net improvement in photodegradation of fiber amplifiers based on such phosphorus and aluminum codoping
is measured experimentally and numerically simulated. The output power loss of 1064-nm ytterbium-doped LMA fiber
amplifiers with phosphorus/aluminum ratios of 0.1 and 0.6 is reduced after 10 000 hours from 17% to less than 2%,
respectively. Better understanding of the effects of phosphorus and aluminum on photodarkening will help to design
reliable and efficient ytterbium-doped fiber amplifiers.
We present here the architecture of an all-fiber, high-power FCPA source emitting at 1550 nm. This system generates
sub-300 fs pulses at a repetition rate of 22 MHz and with an average output power of 1.5 W after pulse compression. The
power amplifier consists of a polarization-maintaining Er:Yb doped LMA fiber which results in a beam quality factor M2
< 1.2. The seed laser pulses are stretched to 240 ps using dispersion-shifted fiber before being amplified and compressed
using a bulk compressor based on a diffraction grating pair. The output power of the source is not limited by the onset of
detrimental nonlinear effects such as self-phase modulation or stimulated Raman scattering since the accumulated
nonlinear phase-shift in the power amplifier is well below π rad. Maximum output power is rather limited by the
available pump power; a likely five-fold increase, given actual state-of-the-art technology, would thus yield a laser
source that may serve as a substitute for widespread solid-state lasers in various fields such as laser machining,
biophotonics and nonlinear optics.
Photodarkening and photobleaching processes affect the level of photodegradation of Yb-doped fibers. Characterization
and modeling of each process is crucial to understand how to optimize the operating conditions of fiber amplifiers and
lasers to obtain acceptable output power degradation. We show that photobleaching is a key factor in the modeling and
simulation of a 10-ns pulsed Yb-doped LMA fiber amplifier. Each parameter of the model was separately determined
from induced excess loss measurements under selective pump and wavelength excitations. The model was used to
simulate accurately the measured fiber amplifier degradation. Optimized fiber length and gain were calculated to
improve the output power stability over time and increase the fiber lifetime. Furthermore, eight fibers have been
fabricated with various Yb, Al, and P content using the MCVD process to optimize the core composition. The level of
photodarkening in each fiber was evaluated by measuring separately rate coefficient and excess loss. It was found that all
fibers followed a similar inversion-dependent rate while the maximum excess loss was dependent on the ratios [Al]/[Yb]
and [P]/[Yb]. The proposed model allows for rapid evaluation and optimization of fiber parameters and operation
conditions to assist Yb-doped laser system design in achieving the desired performance with low photodegradation.
The work presented in this paper had two main objectives. The first objective was to develop a very stable nanosecond
infrared pulsed fiber laser oscillator platform offering a straightforward and accurate control over the pulse
characteristics in the time domain. The second objective was to deliver what we call "high quality photons", which
means delivering pulses with high energy and excellent beam quality and narrow spectral linewidth, all at the same time
and with very good stability. Oscillators with such attributes find applications in material processing fields, for example
in memory repair, photovoltaic cell processing or micro-milling, to name just a few. In order to achieve the first
objective, an embedded digital platform using high-speed electronics was developed. Using this platform and a
computer, pulse shapes have been programmed straightforwardly in the non-volatile memory of the instrument, with an
amplitude resolution of 10 bits and a time resolution of 2.5 ns. Optical pulses having tailored temporal profiles, with rise
times around 1 ns and pulse energy stability levels better than ± 3% at 3σ, have been generated at high repetition rates (>
100 kHz) at a wavelength of 1064 nm. Achieving the second objective required amplifying the low power master
oscillator signal (10-100 mW) to output power levels in the range of 1 to 50 W. A multi-clad, polarization maintaining,
Yb-doped large mode area fiber was specially designed to allow for the amplification of high peak power optical pulses,
while keeping control over the nonlinear effects and preserving an excellent beam quality. Optical pulses with tailored
shapes and pulse energy levels in excess of 140 μJ have been produced for pulse durations in the range of 10 to 80 ns,
with 86% of the power emitted in a 0.5-nm bandwidth. The linearly polarized beam M2 parameter was smaller than 1.1,
with both the astigmatism and the asymmetry below 15%. The pulse energy stability was better than ± 3% at 3σ. We
conclude with a discussion about some of the applications of the developed platform.
Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber
lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of
single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no
comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi-empirical
approach seems at the moment the best way to gain a better understanding of the growth behavior of photo-induced
losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation
describing the activation and deactivation of color centers involving stretched exponential functions has been developed.
For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and
three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been
characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential
functions has been measured separately from the others. The model has then been used to simulate the power decay from
a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior
predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is
general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will
be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb-doped
fiber-based devices affected by photodegradation.
We present new results on supercontinuum generation obtained with a high-power Er-doped femtosecond fiber laser. Our results cover many different types of optical fibers: silica, dispersion-shifted fibers, doped fibers, etc. We have obtained supercontinua covering a wide spectrum from the visible to the mid-infrared ( >2μm ). We also identified third harmonic generation phenomena and we present experimental results that may exhibit the signature of two-photon absorption in an Yb-doped fiber.
We report on the experimental demonstration of a fiber-based chirped-pulse amplification system using a dispersion
shifted fiber as stretcher, an electro-optic modulator for frequency division, two erbium-doped fiber amplifiers (a single
mode and a large mode area) and a grating pair pulse compressor. We obtained 500-nJ pulses at a repetition rate of 1
MHz. Pulse duration was under 500 fs, which is in part due to the fine control of the beam polarization maintained
throughout the system.
We report new results on the generation and characterization of picosecond pulses from a self-mode-locked
semiconductor diode laser. The active medium (InGaAs, 830-870 nm) is a semiconductor optical amplifier whose facets are
cut at angle and AR coated. The amplifier is inserted in a three-minor ring cavity. Mode locking is purely passive; it takes
place for specific alignment conditions. Trains of counterpropagating pulses are produced, with pulse duration varying from
1 .2 to 2 ps. The spectra of the counterpropagatmg pulses do not fully overlap; their central wavelengths differ by a few nm.
The pulse repetition rate has been varied from 0.3 to 3 GHz. The pulses have been compressed to less than 500-fs duration
with a grating pair. We discuss some of the potential physical mechanisms that could be involved in the dynamics of the
mode-locked regime. Hysteresis in the LI curve has been observed. To characterize the pulses, we introduce the idea of a
Pulse Quality Factor, where the pulse duration and spectral width are calculated from the second-order moments of the
measured intensity autocorrelation and power spectral density.
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