Spectral Beam Combining (SBC) of fiber lasers provides a simple, robust architecture for high brightness power scaling beyond the limit of a single fiber. We review recent progress in power scaling and describe what we believe is the highest power SBC fiber demonstration and largest number of fiber lasers combined to date. Here we report results on a fiber SBC system where we achieved > 30 kW by combining 96 individual fiber lasers into a single high brightness beam with a beam quality of M2 = 1.6 x 1.8. The potential for further power scaling at the system level is highlighted with examples of beam combinable fiber laser power scaling.
We report on two types of modal instabilities observed in high power Yb amplifiers based on Large Mode Area Fibers. The first is observed to occur at a Threshold Power, which we refer to as Threshold Power Modal Instabilities (TPMI). The modal instability is observed as a decrease in beam quality or reduced core light output as higher order modes leak into the fiber cladding. In PM 25/400 fiber amplifiers, we observe the threshold for the modal instability to vary depending on pump wavelength detuning, with the onset occurring at approximately 15 W/m peak heat load. In PM 20/400 and 25/400 fiber amplifiers without stress rods or other polarization control, we can achieve 1 kW output, limited by available pump power, without modal instabilities. The second type of modal instability is observed for certain cases where the fiber initially operates without any sign of MI but then degrades over an extended operating time, leading to a similar behavior as the TPMI. We refer to the second class as Fiber Degradation Modal Instabilities (FDMI). For these degraded fibers, we observe that fiber performance is unchanged below the critical power for modal instabilities. Experiments on degraded fiber show a wavelength dependent permanent change in the degraded fiber with a memory of the original operating wavelength.
Spectral Beam Combining (SBC) of fiber lasers provides a simple, robust architecture for power scaling lasers to high power. With appropriate designs, power scaling beyond the single fiber limit can be achieved while maintaining near diffraction limited beam quality and high efficiency. We present experimental results where we achieved > 3 kW at an M2 = 1.35 and > 39% E-O efficiency by combining 12 individual fiber lasers into a single high brightness beam.
We describe a pulsed blue (485 nm) laser source based on frequency quadrupling a pulsed Tm fiber laser. Up to 1.2 W at 485 nm was generated with an M2 of 1.3. At 10 kHz pulse repetition frequency, the output pulse at 485 nm was 65 ns FWHM resulting in an estimated peak power of 1.8 kW. We anticipate further improvements in power scaling with higher power Tm fiber lasers and improved conversion efficiency to the blue with optimized AR coatings and nonlinear optical crystals.
Spectrally Beam Combined fiber laser provide a superior architecture for power scaling laser to high power. We present
experimental results where we achieved < 3 kW, M2 = 1.35 with < 39% E-O efficiency by combining 12 individual fiber
lasers into a single high brightness beam.
We describe a pulsed green and ultraviolet laser source based on a frequency converted photonic crystal fiber (PCF)
amplifier. The flexible-format all-fiber polarization maintaining (PM) seed source consists of cascaded ytterbium doped
amplifiers seeded by a directly modulated 1064 nm laser diode. The seed generates pulses fixed at 3.5 ns duration from
20 kHz to 10 MHz. Up to 2.9 W of 532 nm (green) and 1 W of 355 nm (UV) output were obtained in tests from 20-100
kHz using LBO crystals for both the doubling and tripling conversion process. Scaling the pulse energies demonstrated
at low repetition rates, to the high repetition rate operation possible with the flexible format seed source, shows the
potential for high power, high repetition rate pulsed green and UV output for advanced applications.
We report on the development of a multi-channel all-fiber based laser system for
LADAR applications. Two design pathways for the multi-channel laser system were
investigated: (i) multiple externally triggered single channel MOPA (SCMOPA) systems
and (ii) a single master-oscillator/multiple power-amplifier (SMOMPA) system. We
designed and tested a single channel MOPA system that consists of a seed source, two
single mode amplifiers and a power amplifier. With this system we were able to achieve
up to 195&mgr;J/pulse at 6kHz with a pulse duration of 3ns. For evaluating the SMOMPA
approach we built a bread board based on the single channel results with one master
oscillator and 4 power amplifiers. With this breadboard we achieved pulse energies from
each of the four power amplifiers of 120±4&mgr;J at 6kHz and 78-90&mgr;J at 18kHz. The
temporal line-shapes emitted from each power amplifier were identical within the signal-to-noise level of the temporal traces and have a FWHM=2.2ns.
In recent years fiber pulsed fiber lasers have began to challenge diode pumped solid state lasers in
performance. In particular double-clad fiber lasers and amplifiers with mJ energies and near diffraction
limited beam quality are gaining respect for applications such as materials processing, laser radar and
remote sensing. Frequency conversion of single-polarization fiber lasers further increases the application
space to such as underwater communications, underwater imaging, semiconductor processing and gas
Yb fiber lasers have to date produced several mJ pulse energy and several MW peak power but, largely
due to materials issues Er based fiber laser systems underperforms in comparison. Relevant technologies
All-fiber contained laser systems play a key role, in the development of rugged, compact, and highly
efficient eye-safe laser sources that can generate high peak and average powers and short (<5 ns)
pulses. Application of such laser systems include spectroscopy, LIDAR, free-space
communications, materials processing and nonlinear optics.
In this paper we present further improvement on a novel high power all-fiber-based master
oscillator power amplifier (MOPA) laser system operating in the C-band with <5 ns pulses and a
repetition rate range of 6kHz − 200kHz. The system was optimized for performance of repetition
rates between 6kHz and 18kHz. With this system, pulse energies of 322 μJ with a peak power of
170kW and an average power of 1.9W were generated using a custom designed Er:Yb co-doped
double-clad fiber. The spectral output of the amplified pulses shows no spectral broadening due to
Four-Wave-Mixing or Stimulated Raman scattering for pulse energies with less than 260μJ.
Additionally, a beam quality M2=2.1+/-0.1 was achieved. The physical performance parameters of
the all-fiber laser system make it very suitable for a variety of applications. The performance of the
MOPA system and the experimental data are presented and discussed. To our knowledge the
combination of the presented pulses energies, peak power, average power are the highest ever
recorded in an all fiber system.
We present a novel high power all-fiber-based master oscillator power amplifier (MOPA) laser system operating in the C-band (1.5 mm) with pulses <5ns and a repetition rate range of 200 kHz. This system generates >4 Watts of average power and a maximum pulse energy of 20 mJ and peak power of 5 kW at 200 kHz using custom designed Er:Yb co-doped double-clad fibers. This system was also operated at reduced repetition rates of 6 kHz and pulse energy of 165 mJ was generated with a peak power of 28 kW. By shortening the seed pulses a peak power of up to 33.9 kW with a pulse energy of 73 mJ was achieved at 20 kHz. A beam quality of M2=1.2 was achieved, which makes this system very suitable for scanning lidar applications.
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.
The propagation of high power narrowband laser pulses in multimode fibers and the limitations due to SBS are presented. An injection seeded pulsed Nd:YAG laser operating at 10 Hz was used to pump undoped step index fibers to determine the SBS threshold under various conditions. Measurements on 50μm core diameter fibers with various fiber lengths and pulse durations at 1064 nm were performed and simulated with a computer code. The code considers the time dependent coupling between the pump wave, the Stokes wave, and the acoustic wave. The experimental results are in good agreement with the numerical predictions. Our results quantify the limitations of high power narrowband pulse transmission in multimode fibers.