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.
We present a simple way to achieve and optimize hundreds of kW peak power pulsed output using a monolithic amplifier chain based on solid core double cladding fiber tightly packaged. A fiber pigtailed current driven diode is used to produce nanosecond pulses at 1064 nm. We present how to optimize the use of Fabry-Perot versus DFB type diode along with the proper wavelength locking using a fiber Bragg grating. The optimization of the two pre-amplifiers with respect to the pump wavelength and Yb inversions is presented. We explain how to manage ASE using core and cladding pumping and by using single pass and double pass amplifier. ASE rejection within the Yb fiber itself and with the use of bandpass filter is discussed. Maximizing the amplifier conversion efficiency with regards to the fiber parameters, glass matrix and signal wavelength is described in details. We present how to achieve high peak power at the power amplifier stage using large core/cladding diameter ratio highly doped Yb fibers pumped at 975 nm. The effect of pump bleaching on the effective Yb fiber length is analyzed carefully. We demonstrate that counter-pumping brings little advantage in very short length amplifier. Dealing with the self-pulsation limit of stimulated Brillouin scattering is presented with the adjustment of the seed pulsewidth and linewidth. Future prospects for doubling the output peak power are discussed.
Pulseshaping is important in high energy pulsed fiber MOPA system to mitigate non-linear effects and optimize the
processing of different materials. However, pulseshaping is greatly limited by the spectral features of the semiconductor
seed source commonly used as the master oscillator. Through the appropriate design of an external fiber Bragg grating
(FBG) and adequate current modulation, the spectrum of the fiber-coupled seed laser was broadened to suppress
stimulated Brillouin scattering occurring in the amplifier chain and the central emission wavelength and bandwidth were
controlled. Pulseshaping is also quickly limited by the saturation energy and doping level of standard aluminosilicate
ytterbium doped fibers used in the power amplifier even with large core diameter. Co-doping the fiber with phosphorus
greatly increases the saturation energy of the system, which gives smoother pulseshape and significantly lower
stimulated Raman scattering (SRS). It is shown that going from 1060 nm to longer emission wavelength at 1090 nm with
this fiber increases further the pulseshaping capabilities and reduces SRS. The phosphorus codoping also allows higher
ytterbium doping level without photo-degradation, which decreases nonlinear effects generation during the amplification
while giving more flexible pump wavelength choice and efficiency.
A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding
fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to eliminate higher order modes. We describe here multimode (MM) high NA ytterbium doped fibers used in single mode output high power laser/amplifier configuration. Efficient single mode amplification is realized in the multimode doped fiber by matching the fundamental mode of the doped fiber to the LP01 mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP01 to the higher order modes. We show that negligible mode coupling is realized in the doped fiber, which ensures a stable power output over external perturbation without the use of tapers. Fundamental mode operation is maintained at all time without coiling through the use of FBG written in a single mode fiber. We show that such fiber is inherently more photosensitive and easier to splice than LMA fiber. We demonstrate an efficient 75W singlemode CW fiber laser using this configuration and predict that the power scaling to the kW level can be achieved, the design being more practical and resistant to photodarkening compared to conventional low NA LMA fiber.
We designed a high output power double cladding erbium-ytterbium fibre amplifier that showed no amplified
spontaneous emission (ASE) at 1.0 &mgr;m using a quasi singlemode fibre. The reduction of the amplified stimulated
emission (ASE) at 1.0 &mgr;m was found to be the combination of fibre design and temperature effect in the core. A 10W
output double cladding Er-Yb amplifier with a core/cladding fibre diameter of 10/125 &mgr;m was realized with a seed signal
of 1.4 W at 1563 nm and with counter-propagating pump power of 35 W at 976 nm without any significant ASE
generation at 1.0 &mgr;m. The fibre also exhibits singlemode behaviour with M2 <1.1 and a high slope efficiency of 30%.
The fibre was designed to minimize ASE at 1.0 &mgr;m by heavily doping the fibre and using the appropriate ratio between
Yb3+ and Er3+ ions. By incorporating into our model the core temperature increase coming from the quantum defect of
the Er-Yb system, we can also predict a raise in the absorption cross-section of the ytterbium ions around 1060 nm
yielding to an increase of the 1 &mgr;m ASE threshold from 14 W to 35 W pump power, which allowed us to reach a 10 W
output power at 1563 nm instead of 5 W normally predicted by the theory. These results show potential power scaling of
the output power or double cladding erbium ytterbium amplifier using quasi singlemode core erbium ytterbium fibre
avoiding the need of large core dimension that degrades the beam quality.
A single cladding ytterbium doped fibre amplifier pumped at 980 nm that exhibits negligible amount of photodarkening
over a long period of time is demonstrated. The output power as a function of time decreased by a very small factor
compared to standard single mode ytterbium fibres. To achieve this photodarkening resistant amplifier, a special
ytterbium doped fibre has been developed. Codoping with aluminium or other rare-earth such as erbium is shown to
decrease the multi-excitation of ytterbium clusters and thus lower photodarkening. Photodarkening was characterized by
comparing the amount of excess loss created by core pumping single cladding fibres at high intensity at 980 nm.
Photodarkening was found to be directly proportional to the excitation of the ytterbium ions by comparing different
pumping scheme and pump wavelength. Core pumping of a single cladding ytterbium doped fibre amplifier at 980 nm
represents the worst case scenario for photodarkening. Engineering ytterbium fibres for low photodarkening is therefore
critical in pulsed amplification where short length of fibre with high doping level is required as demonstrated with 6 &mgr;m
core ytterbium fibre amplifier pumped in the core or in the cladding. Photodarkening was correlated to clustering from
cooperative luminescence measurement at 500 nm produced by ytterbium clusters that would emit UV radiation under
strong pumping.
We developed a model that accurately predicts the performances of high power double cladding erbium ytterbium fibre
amplifiers. We experimentally validate the model in co- and counter-propagation configuration for different pump
wavelengths and fibre lengths. By adjusting the ytterbium to erbium cross-relaxation rate with a simple amplifier
experiment, we obtained a complete agreement between the model predictions and the experimental data regarding the
output signal. We measured that the McCumber relationship considerably overestimates the ytterbium emission crosssection;
with a correction of this parameter, we obtained an excellent prediction of the ytterbium spontaneous emission at
1.0 μm. The model is valid for high power singlemode amplifiers as we obtained a full agreement for a 4W output power amplifier from a seed signal of 8 mW at 1556nm. The output was diffraction limited with a measured M2 parameter of
1.03 without doing any mode selection from a slightly multimode fibre with a core diameter of 10 μm and a numerical
aperture of 0.18.
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