When scaling CW single-mode fiber amplifiers to high power, the first nonlinear limitation that appears for narrowlinewidth seed lasers is stimulated Brillouin scattering (SBS). We present a dynamical simulation of Brillouin scattering in a Yb-doped fiber amplifier that numerically solves the differential equations in z and t describing the laser, Stokes and pump waves, the inversion, and the density fluctuations that seed the scattering process. We compare the model to experimental data, and show that a linearly chirped seed laser is an efficient form of SBS suppression; especially for long delivery fibers. The frequency chirp decreases the interaction length by chirping through the Brillouin resonance in a time that is short compared to the fiber transit time. The seed has a highly linear chirp of 1014 – 1016 Hz/s at 1064 nm which preserves a well-defined phase relationship in time. This method of SBS suppression retains a long effective coherence length for purposes of coherent combining, while at high chirps appears to the SBS as a large linewidth, increasing the threshold. An increase in fiber length increases the laser bandwidth as seen by the SBS, leading to a fiberlength- independent SBS threshold.
When seeding a high power fiber amplifier with a frequency-chirped seed, the backward Brillouin scattering can be kept at the spontaneous level because the coherent laser/Stokes interaction is interrupted. Operating a conventional vertical cavity surface-emitting diode laser in an optoelectronic feedback loop can yield a linear frequency chirp of ~1016 Hz/s at a constant output power. The simple and deterministic variation of phase with time preserves temporal coherence, in the sense that it is straightforward to coherently combine multiple amplifiers despite a large length mismatch. The seed bandwidth as seen by the counter-propagating SBS is large, and also increases linearly with fiber length, resulting in a nearly-length-independent SBS threshold. Experimental results at the 600W level will be presented. The impact of a chirped seed on multimode instability is also addressed theoretically.
We report a monolithic specialized high stimulated Brillouin scattering (SBS) threshold fiber laser/amplifier in the C and
L band based on highly co-doped phosphate glass fibers. This represents an important new development for coherent
LIDAR and remote sensing applications. By using single mode polarization-maintaining large core highly Er/Yb codoped
phosphate fibers in the power amplifier stages, we have achieved the highest peak power of 2.02 kW at 1530 nm
for 105 ns pulses with transform-limited linewidth, and with a corresponding pulse energy of about 0.212 mJ. The
achieved high-energy pulses were frequency doubled by using a commercial periodically poled lithium niobate (PPLN)
crystal, and the highest SHG peak power of 271 W has been achieved for the SHG pulses at 765 nm that can be used for
oxygen coherent remote sensing. In the L band, more than 80 μJ fiber laser pulses at 1572 nm with 1-2 μs pulse width
and transform-limited linewidth have been achieved by using a monolithic fiber laser system in MOPA configuration,
which can be used for CO2 coherent remote sensing.
We report an all fiber-based single-frequency Q-switched 2 μm pulsed laser based on highly Tm-doped germanate fiber
by using a piezo to induce stress in fiber laser cavity. The pulse width of this Q-switched fiber laser can be tuned from 10's
ns to sub-μs. The repetition rate can be tuned from 100 Hz to 100's kHz. The average power is ~ mW-level, peak power wattlevel,
and pulse energy 30-75 nJ without any amplifier. Moreover, this transform-limited fiber laser pulses has been scaled up
to 220 μJ by using a newly developed SM PM highly Tm-doped germanate fiber 25/250μm for transform-limited 80 ns pulses
at repetition rate 20 kHz. This narrow linewidth high energy MOPA-based pulsed fiber laser can be used for LIDAR and laser
remote sensing.
We report coherent monolithic THz generation in GaP QPM bonded structures based on difference-frequency generation
(DFG) using two pulsed fiber lasers in the C-band. We observed that the QPM-GaP crystals effectively increase the THz
generation power and efficiency with increasing periods of QPM structures. The azimuthal dependence of the THz
generation for the GaP QPM bonded structure has been measured when the polarization directions of the two pump
beams are orthogonal and parallel, respectively. Moreover, we observed the external cavity enhanced THz DFG when
we put the QPM-GaP crystal in an external ring cavity, for the first time. The THz cavity enhancement factor of ~250
has been achieved compared with the single-pass THz DFG. The maximum THz average power can reach 339 μW,
corresponding to a power conversion efficiency of 2.43×10-4 and quantum efficiency of 3.16%.
We have implemented monolithic narrow linewidth single-mode single-frequency pulsed fiber lasers in master oscillator
and power amplifier (MOPA) configuration based on highly Er/Yb co-doped phosphate fiber with core size of 25 μm.
The narrow linewidth pulsed fiber laser seed has been achieved by directly modulating single-frequency CW fiber laser.
An arbitrary waveform generator (AWG) was used to pre-shape the pulse shapes in order to avoid the pulse distortion
and the dynamic gain saturation in the cascade fiber amplifiers. Based on the newly developed large core single-mode
highly Er/Yb co-doped phosphate fiber in the power amplifier stage, the peak power of single-mode pulses can be scaled
to more than 100 kW with transform-limited linewidth and diffraction-limited beam quality. These high power narrow
linewidth single-mode fiber laser pulses have been successfully used to generate coherent THz waves based on
difference-frequency generation (DFG) in GaSe crystal. The single-pass generated THz peak power can reach 0.12 W.
Moreover, we have observed the external cavity enhancement of DFG THz generation by using ZnGeP2 for the first
time, and implemented a high spectral resolution THz spectrometer based on the developed fiber-based tunable narrow
linewidth THz source.
We report a monolithic high SBS-threshold pulsed fiber laser in MOPA for longer nanosecond pulses with transformlimited
linewidth. By using a single mode polarization-maintaining large core 25 μm highly Er/Yb co-doped phosphate
fiber in the power amplifier stage, we have achieved the highest peak power of 1.2 kW at 1530 nm for 105 ns pulses
with transform-limited linewidth, and with a corresponding pulse energy of about 0.126 mJ. The achieved high-energy
pulses were frequency doubled by using a commercial periodically poled lithium niobate (PPLN) crystal, and the highest
SHG peak power of 271 W has been achieved for the SHG pulses at 765 nm.
Compact, efficient, narrow linewidth, fiber based THz sources have been achieved by using the monolithic high power single-frequency pulsed fiber lasers in MOPA based on difference-frequency generation (DFG) in nonlinear optical crystals. We have observed the external cavity enhancement of DFG THz generation by using ZnGeP2 for the first time, and implemented a high spectral resolution THz spectrometer based on the developed fiber-based tunable narrow linewidth THz source.
We report a new pulsed, narrow linewidth, single-mode, polarization maintaining (PM) all-fiber laser source in master
oscillator and power amplifier (MOPA) configuration that can operate over the C-band. The single-frequency pulsed
fiber laser seed was achieved by actively Q-switching a fiber laser using a piezo, with a wide pulse duration tuning range
of 7.5 ns - 1.24 μs. We use single-mode PM large core highly Er/Yb
co-doped phosphate glass fiber (LC-EYPhF) in the
power amplifier stage of MOPA to achieve 54 μJ/pulse for 153-ns pulses at 1538 nm with repetition rate of 20 kHz and
an estimated linewidth of ~ 5 MHz.
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