We present an efficient way to remove unwanted Amplified Stimulated Emission (ASE) in high-power fiber lasers and amplifiers using intracavity Chirped Tilted Fiber Bragg Grating (CTFBG) filters. The grating is written with tilted fringes so that the unwanted ASE is reflected into the fiber cladding where it is no longer amplified. Depending on the desired emission wavelength and active fiber, one or several filters are spliced within the active fiber to suppress ASE before it reaches a detrimental power. Numerical simulations clearly show that adding the filters allows amplification in configurations that would just be impossible due to the onset of ASE. The filter bandwidth and extinction, and the maximum allowed active fiber length between each filter are also computed depending on the core/cladding diameter ratio of the active fiber used and the targeted emission wavelength. As an example, a fiber laser at 1018 nm is assembled in a 20/400μm core/cladding diameter ytterbium fiber that is cladding pumped at 976 nm. Two CTFBGs with 20 dB attenuation from 1025 nm to 1070 nm are spliced within the 6-meter-long ytterbium fiber. 432 W of laser emission at 1018 nm is efficiently achieved at 77% slope efficiency with respect to the absorbed pump power. The extinction between the 1018 nm signal and the ASE is greater than 50 dB. Removing the ASE filters from the cavity clearly leads to only self-pulsation of the ASE between 1030 nm and 1050 nm, no generation of 1018 nm light was possible. The measured thermal slope of the filters shows scalability above the kW level. Demonstration at 1908 nm with a 25/400 core/cladding diameter thulium doped fiber is also done. Tests were done to inscribe the CTFBG directly in an ytterbium fiber for simpler implementation and avoid additional splicing.
In this paper we study the pulse quality degradation of an Ytterbium-based fiber laser in which B-integral ranges from 5 to 20 rad. A tunable chirped-fiber-Bragg-grating stretcher is optimized in term of reflectivity and phase profiles thank to a multivariable optimization algorithm. Parabolic and Sech2 spectral profiles are both studied in order to evaluate their robustness to non-linear degradation. Phase and reflectivity optimization can provide insight to design tailored specific pulse stretchers for CPA systems.
An optical coherent receiver for the down conversion of radio frequency (RF) signals from 10-18 GHz to 2 GHz is presented. Light from a distributed feedback semiconductor laser is split between two lithium niobate Mach-Zehnder modulators driven either by a tunable local oscillator (LO) tone or a RF signal coming, for example, from a receiving antenna. The modulated light signals are combined with an optical coupler and filtered by two fiber Bragg gratings (FBG) that select one optical sideband from each signal. Detection of the filtered light by a balanced photo-detector produces an electrical signal at an intermediate frequency equal to the beat difference between the RF and LO frequencies.
Most current RF photonic systems are made from individually packaged devices that are interconnected with fiber-optic cables. In order to reduce size and weight and make the coherent receiver suitable for use in smaller airborne and mobile platforms, optical and opto-electronic components are packaged within a common enclosure where light routing is performed by micro-optics. A printed circuit board (PCB) is included within the module. It comprises a micro-processor to control and monitor the laser, the FBGs and thermo-electric coolers to ensure a robust operation over time and fluctuating environmental conditions. The module including the PCB, laser, modulators, optics, optical filters and balanced detector has a size of 89 x 64 x 32 mm3.
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.
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.
We demonstrate the usefulness of INO's pulse-shaping fiber laser platform to rapidly develop complex laser micromachining
processes. The versatility of such laser sources allows for straightforward control of the emitting energy
envelop on the nanosecond timescale to create multi-amplitude level pulses and/or multi-pulse regimes. The pulses are
amplified in an amplifier chain in a MOPA configuration that delivers output energy per pulse up to 60 μJ at 1064 nm at
a repetition rate of 200 kHz with excellent beam quality (M2 < 1.1) and narrow line widths suitable for efficient
frequency conversion. Also, their pulse-on-demand and pulse-to-pulse shape selection capability at high repetition rates
makes those agile laser sources suitable for the implementation of high-throughput complex laser processing. Micro-milling
experiments were carried out on two metals, aluminum and stainless steel, having very different thermal
properties. For aluminum, our results show that the material removal efficiency depends strongly on the pulse shape,
especially near the ablation threshold, and can be maximized to develop efficient laser micro-milling processes. But, the
material removal efficiency is not always correlated with a good surface quality. However, the roughness of the milled
surface can be improved by removing a few layers of material using another type of pulse shape. The agility of INO's
fiber laser enables the implementation of a fast laser process including two steps employing different pulse
characteristics for maximizing the material removal rate and obtaining a good surface quality at the same time. A
comparison of material removal efficiency with stainless steel, well known to be difficult to mill on the micron scale, is
also presented.
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 report on recent advances in laser material processing using a novel pulsed fiber laser platform providing pulse
shape agility at the nanosecond time scale and at high repetition rates. The pulse shapes can be programmed with a time
resolution of 2.5 ns and with an amplitude resolution of 10 bits. Depending on the desired laser performances, the pulses
are generated either by directly modulating the drive current of a seed laser diode or by modulating the output of a seed
laser diode operated in CW with electro-optic modulators. The pulses are amplified in an amplifier chain in a MOPA
configuration. Advanced polarization maintaining LMA fiber designs enable output energy per pulse up to 60 μJ at 1064
nm at a repetition rate of 200 kHz with excellent beam quality (M2< 1.1) and narrow line widths suitable for efficient
frequency conversion. Micro-milling experiments were carried out with stainless steel, in which processing
microstructures of a few tens of microns in size usually represents a challenge, and aluminum, whose thermal
conductivity is about 20 times higher than stainless steel. The results obtained with two metals having very different
thermal properties using different pulse shapes with durations varying between 3 ns and 80 ns demonstrate the benefits
of using lasers offering flexible pulse durations and controllable pulse intensity profiles for rapidly optimizing a process
in different applications while using the same laser with respect to conventional methods based on pulsed laser with
fixed pulse shapes. Numerous applications are envisioned in a near future, like the micromachining of multi-layered
structures, in particular when working with the harmonics of the laser.
We report on recent advances in the development of a 1064 nm pulsed master oscillator fiber power amplifier (MOFPA) with integrated modulators enabling programmable temporal pulse shapes and its employment in a tandem photonic amplifier. The MOFPA amplifier chain is seeded by a laser diode operated in the CW regime, yielding very stable spectral characteristics that are independent of the pulse repetition rate and pulse shape. The use of 3 GHz integrated LiNbO3 electro-optic modulators in conjunction with high speed digital electronics results in an excellent pulse shaping capability, a fine pulse amplitude stability and high repetition rate operation (100 kHz-1MHz) with fast rise times (<1ns). Energy per pulse of 8-10 μJ with good beam quality characteristics are obtained using advanced large mode area (LMA) fiber designs in the final power amplifier stage. The output is linearly polarized with a spectral bandwidth of < 0.1 nm. When employed in a tandem amplifier configuration, in which the MOFPA output is input to a single-stage, single-pass Nd:YVO4 amplifier pumped by a single 30 W fiber-coupled 808 nm diode, a 600 mW average power at 100 KHz signal input from the MOFPA was amplified to 6 W with faithful amplification of the input temporal pulse profile while achieving excellent beam quality (M2<1.1) and pulse amplitude stability (< ±3%, 3σ). A model of tandem amplifier performance shows good agreement with experimental results and indicates prospective performance of advanced tandem photonic amplifier configurations.
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