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.
A new scheme for suppression of Stimulated Brillouin Scattering (SBS) in high power fiber amplifiers is proposed where the fiber core diameter varies along the fiber length. The fiber has an ultra-large core diameter at most locations to suppress SBS, while at certain locations the core is relatively small to reduce bending sensitivity. A numerical model based on SBS rate equations is used to compare the SBS threshold of a uniform fiber to that of a fiber with variations in physical properties vs position. The model takes into account SBS gain dependence on temperature distributions as well as inhomogeneous spectral broadening due to NA variations. The modeling results show that the SBS threshold can be increased significantly when the seed power, fiber length, pump configuration, and fiber parameters are optimized. As a result, a single frequency fiber amplifier can generate up to 1 kW output power without the onset of SBS. Furthermore, the nonuniform fiber allows a high power amplifier to be packaged in a much more compact enclosure than an ultra-low NA fiber based amplifier.
We report on progress toward power scaling Yb fiber lasers beyond kW levels by an efficient and versatile architecture that maintains near diffraction limited beam quality. For this work, power scaling is performed at two distinct levels. The first utilizes a diffraction grating to spectrally beam combine (SBC) the output from several master-oscillator, poweramplifier (MOPA) fiber lasers with a goal of producing high quality combined beams with > 1 kW of power. The second involves scaling individual MOPA outputs to > 200 W, thereby reducing the number of lasers required for SBC. As a first step toward reaching these goals, we have developed Yb fiber MOPAs producing up to 208 W of polarized, narrow band, and near diffraction limited output and have demonstrated two-channel fiber laser SBC with a power combining efficiency of 93%, a combined beam power of 258 W, and a dispersed axis M2 of 1.06. These results represent a significant advance in high brightness, spectrally beam combined laser systems.
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.
We report a 120 W, linearly polarized, single-mode Yb-doped fiber laser operating at 10 MHz pulse repetition rate. To produce narrow linewidth and high peak power, a fiber oscillator power amplifier (FOPA) configuration is used. With an Yb-doped large mode area (LMA) fiber, the FOPA has generated up to 2.4 kW peak power and less than 20 pm linewidth without the onset of nonlinear effects at 5 ns pulse duration. 120 W average output power corresponding to a slope efficiency of 71% with respect to the launched power has been generated. The beam quality is 1.1 times diffraction-limited and the polarization extinction ratio is better than 95%. No linewidth broadening beyond the 20 pm instrumental resolution or nonlinear effects are observed at 120 W output power. The diffraction-limited beam quality from the FOPA allows us to use LBO crystals to achieve efficient second harmonic generation (SHG) without “gray tracking” problems. The frequency doubling of 110 W FOPA output has generated 60 W, near-diffraction-limited, linearly polarized green output. With two LBO crystals at noncritical phase-matching, a maximum of 54.5% doubling efficiency has been demonstrated. The overall electrical efficiency to green output is 10%.
We report up to 7.2 W, 800 nm wide supercontinuum generation in a photonic crystal fiber. The pump laser is an Yb-doped fiber oscillator/amplifier generating up to 120 W output at 5~20 MHz repetition rate, with pulse duration of 5~100 ns. The experiment has demonstrated ~60% conversion efficiency for supercontinuum generation using nanosecond lasers. The maximum power is limited by the thermal lensing from the optical components.
We have demonstrated spectral beam combining of two high power fiber lasers and obtained more than 40 W output power from the system. The system consists of two 30 W fiber lasers, a diffraction grating and a resonator. Both fiber lasers have broadband fiber Bragg gratings on the rear sides and share the diffraction grating and output coupler on the output sides. The wavelengths of the fiber lasers are determined by the optical dispersion provided by the grating and the collimating lens, as well as the fiber spacing. A model that analyzes dependence of laser line-width on beam quality of an SBC system is given in this paper. We also model a novel configuration that can significantly improve beam quality. The experimental results have shown that control of fiber laser line-width is the key to achieving high power SBC lasers. In addition, a new approach using three gratings is proposed and has been proven by the experiment. The new approach consists of three gratings, in which one grating is used by the SBC cavity to lock wavelengths while the other two combine the collimated beams without beam quality reduction. This approach has successfully improved beam quality M2 from 11 to 2.0.
Numerical analysis for the high-power double-clad fiber lasers is presented and experimental results using different microscope objectives for focusing into a Nd-doped rectangular double-clad fiber also performed. The numerical analysis includes dependence of output power on output mirror reflectivity, absorbed pump power, loss, and fiber length and pump power distribution for the cases of one-end and two-end pumps with 20 dB/km loss. Calculated conversion efficiencies are 76.36%, 69.73%, and 63.84% for lossless, two-end pump, and one-end pump fiber lasers, respectively. Slope efficiencies from absorbed pump power/output powers measured using microscope objectives are 16.8%/182 mW, 53.8%/351 mW, 24.9%/1240 mW, and 13.9%/ 649 mW for magnifications of 5x, 10x, 20x, and 40x, respectively.
Simultaneous multiple wavelength cw laser operations were achieved in two types of composite laser rods composed of Nd3+:YAG and Nd3+:YLF crystals, which were laser diode (LD) pumped within a single virtual-point-source cavity. Up to 30 W total output from wavelength of both 1064 nm and 1047 nm was obtained under 150 W LD input power, among which about 25% was from 1047 nm wavelength. Different bonding methods were compared which shows that the use of an optical adhesive is effective and presents no deterioration at low and middle power level. Simultaneous multiple wavelength operation at 1064-nm and 1053 nm was also studied.
In this paper, we report analytical and experimental studies on the characteristics of a high-brightness LD end-pumped Nd:YVO4/KTP laser. A simple model was developed to optimize the cavity parameters and estimate the green output power of intracavity frequency doubled lasers. Using a 1 W high brightness laser diode as the pump source, high efficiency operation was realized based on optimized cavity design. The second-harmonic output power at 532 nm was measured to be 286.5 mW at an incident pump power of 881.4 mW, corresponding to an optical to optical efficiency of 32.5%.
The absorption characteristics of circular, offset, and rectangular double-clad fibers are investigated with simple and comprehensible 2D models. In the model, the pump modes in the first-cladding are divided into absorbable and unabsorbable modes. For circular fiber, a concept of absorbable power ratio is defined to characterize the effective absorption. For offset and rectangular fibers, almost all pump modes are absorbable, it is possible to achieve very high absorption efficiency. Such 2D models have been verified by 3D ray tracing simulations. The influences of offset distance for offset fibers, as well as cross sectional shapes for rectangular one on absorption are also discussed. The propagation losses of pump light in rectangular double-clad fibers are characterized both experimentally and theoretically. The measures to reduce pump loss in use of rectangular double-clad fibers are proposed. At last, the two ends of an Nd-doped rectangular double-clad fiber are bundled together to achieve efficient matching with a 10 W fiber coupled LD. With such a configuration, about 5.4 W pump light is launched into the first-cladding and 2.8 W laser output is obtained. A slope efficiency of 58 percent with respect to launched pump power has been achieved.
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