Master Oscillator Power Amplifier Laser Research Articles

2 J, 10 Hz, 1047 nm Nd:YLF master oscillator power amplifier laser with near-diffraction-limited beam quality.

We report on a joule-level, 1047 nm Nd:YLF master oscillator power amplifier laser with near-diffraction-limited beam quality. The master oscillator and two-stage two-pass amplifier were all-flashlamp-pumped. The laser produced 2 J output pulses with 3 ns duration at 10 Hz pulse repetition rate. High beam quality was achieved by using a liquid stimulated Brillouin scattering mirror and additional astigmatism compensation.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Abstract Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot. At present, research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser. However, the suppression of spectral broadening, caused by self-phase modulation or four-wave mixing, is still a challenging attribute to the close distance between the broadened laser and signal laser. If using a traditional fiber grating with only one stopband to suppress the spectral broadening, the signal power will be stripped simultaneously. Confronting this challenge, we propose a novel method based on phase-shifted long-period fiber grating (PS-LPFG) to suppress spectral broadening in a high-power fiber master oscillator power amplifier (MOPA) laser system in this paper. A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique. The resonant wavelength of the fabricated PS-LPFG is 1080 nm, the full width at half maximum of the passband is 5.48 nm, and stopband extinction exceeds 90%. To evaluate the performance of the PS-LPFG, the grating is inserted into the seed of a kilowatt-level continuous-wave MOPA system. Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%, providing an effective and flexible way for optimizing the output linewidth of high-power fiber MOPA laser systems.

3 mJ All-Fiber MOPA With a Short-Length Highly Er3+-Doped Phosphate Fiber

A very large mode-area 2 wt% erbium-doped double-clad phosphate fiber with a core diameter of 60 μm and core numerical aperture of 0.03 was fabricated and used for the last stage amplifier of an all-fiber pulsed laser master-oscillator power amplifier (MOPA) at 1550 nm. The Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> double-clad phosphate fiber has a cladding-core ratio of 2.17 and a V number of 3.6 at 1550 nm. 3 mJ pulses with 21.5 ns duration were obtained with Gaussian-like spatial beam profile with only a 75 cm long phosphate gain fiber.

500 Hz, 47.1 mJ, sub-nanosecond MOPA laser system

A high-pulse-energy, sub-nanosecond master oscillator power amplifier (MOPA) laser system at 1.06 μm was demonstrated. The thermal effect in laser diode-side-pumped module was studied numerically and experimentally. The master oscillator was an electro-optically Q-switched laser with a pulse width of ~730 ps. Using an end-pumped preamplifier and two-stage side-pumped amplifiers, laser pulses with a pulse energy of 47.1 mJ were obtained at a repetition rate of 500 Hz, corresponding to a peak power of ~64.5 MW. The coefficient of variation of the pulse train was ~4.9%.

Yb/Ce Codoped Aluminosilicate Fiber With High Laser Stability for Multi-kW Level Laser

Further power scaling and stable laser performance were demonstrated in the Yb/Ce codoped aluminosilicate fiber fabricated through low-temperature chelate gas phase deposition technique. The molar ratio of Ce/Yb was designed and optimized to be 0.58 for low background loss, effective photodarkening suppression, and no additional thermal load. The background loss of this active fiber was 4.7 dB/km and its photodarkening loss at equilibrium was as low as 3.9 dB/m at 633 nm. Benefiting from low-temperature deposition technique, the fiber showed uniform core composition devoid of clustering and central `dip' of refractive index profile and 0.19 mol% Yb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was homogeneously dissolved into the fiber core plus with 0.41 mol% Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , 0.11 mol% Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and 0.32 mol% SiF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> . Based on a master oscillator power amplifier laser setup, 5.04 kW laser output at 1079.80 nm was achieved with a slope efficiency of 81.1%. Stabilized at 5kW-level laser for over 60 minutes, the output power presented almost no power degradation, directly confirming a noticeable photodarkening mitigation.

High-power narrow-band mode-locked sodium laser via double-stage sum-frequency generation

A high-power, narrow-band, mode-locked sodium D2a laser with a high sum-frequency efficiency is demonstrated by mixing actively mode-locked Nd:YAG 1064 and 1319 nm lasers in two LiB3O5 (LBO) crystals. The mode-locked lasers at 1064 nm (23.02 W output) and 1319 nm (17.98 W output) are generated from two diode, side-pumped, Nd:YAG, master oscillator power amplifier laser systems. A macro-micro pulse sodium beacon laser with a 30.2 W output power, 0.3 GHz (0.3 pm) linewidth, and a beam quality given by M2 = 1.23 is obtained with a conversion efficiency of 73.6%. Compared with a single LBO crystal, the sum-frequency efficiency is enhanced by 14.5%. A laser coupled amplitude equation model, including the acousto-optic mode-locker, is applied for theoretical and numerical analyses of the sum-frequency conversion efficiency. The experimental results agree well with the theoretical calculations.

30 W single-frequency continuous-wave master oscillator power amplifier laser at 1342 nm.

We present a power-scalable high-power single-frequency continuous-wave 1342 nm master oscillator power amplifier (MOPA) system that consists of a polarized single-frequency 1342 nm LD seed laser, a Raman fiber preamplifier, and a three-stage ${\rm Nd}:{{\rm YVO}_4}$Nd:YVO4 power amplifier. The single-frequency output power of 30 W at 1342 nm is achieved with the beam quality factors ${{\rm M}^{2\:}} = {1}.{26}$M2=1.26, and the power stability for 1 h is better than ${\pm }\;{0}.{5}\% $±0.5%.

High-efficiency and high-power single-frequency fiber laser at 1.6 μm based on cascaded energy-transfer pumping

In this paper, a technique combining the cascaded energy-transfer pumping (CEP) method and master-oscillator power-amplifier (MOPA) configuration is proposed for power scaling of 1.6-μm-band single-frequency fiber lasers (SFFLs), where the Er 3 + ion has a limited gain. The CEP technique is fulfilled by coupling a primary signal light at 1.6 μm and a C-band auxiliary laser. The numerical model of the fiber amplifier with the CEP technique reveals that the energy transfer process involves the pump competition and the in-band particle transition between the signal and auxiliary lights. Moreover, for the signal emission, the population density in the upper level is enhanced, and the effective population inversion is achieved thanks to the CEP. A single-frequency MOPA laser at 1603 nm with an output power of 52.6 W and an improved slope efficiency of 30.4% is experimentally obtained through the CEP technique. Besides, a laser linewidth of 5.2 kHz and a signal-to-auxiliary laser ratio of 60.7 dB are obtained at the maximum output power. The proposed technique is anticipated to be promising for increasing the slope efficiency and power scaling for fiber lasers operating at L band.

A 51.3 W, sub-kHz-linewidth linearly polarized all-fiber laser at 1560 nm

We demonstrate a continuous-wave high output power and narrow linewidth all-fiber master oscillator power amplifier laser at 1560 nm. The laser output power is 51.3 W with 36.38% optical-to-optical conversion efficiency and a linewidth of 0.865 kHz. The measured signal-to-noise ratio is 49 dB, and the polarization extinction ratio is greater than 20 dB. The laser can be used for high-efficiency nonlinear frequency conversion.

High-beam-quality sealed-off laser system oscillating in middle infrared spectral range on strontium atomic transitions

High-beam-quality laser oscillation is obtained at two Sr+ and several Sr atom lines in the middle infrared spectral region with average output power of about 1 W in a sealed-off laser tube. Using the same laser tube construction and enhancing the active volume in bore and length, average output power of about 3 W is also achieved in a sealed-off regime. Based on these laser tubes, master oscillator–power amplifier laser system is also developed and investigated.

24 mJ Cr+4:forsterite four-stage master-oscillator power-amplifier laser system for high resolution mid-infrared spectroscopy.

We present the design of a Cr:forsterite based single-frequency master-oscillator power-amplifier laser system delivering much higher output energy compared to previous literature reports. The system has four amplifying stages with two-pass configuration each, thus enabling the generation of 24 mJ output energy in the spectral region around 1262 nm. It is demonstrated that the presented Cr:forsterite amplifier preserves high spectral and pulse quality, allowing a straightforward energy scaling. This laser system is a promising tool for tunable nonlinear down-conversion to the mid-infrared spectral range and will be a key building block in a system for high-resolution muonic hydrogen spectroscopy in the 6.8 μm range.

Development of pulse-profile-controllable 2-μm wavelength single-frequency all-fiber laser system operating at 10-Hz repetition rate

A compact single-frequency master oscillator power amplifier laser system composed of three-stage thulium-doped fiber amplifiers was developed. At a repetition rate of 10 Hz, &gt;100-μJ pulse energy at 2050.5-nm wavelength, with ∼431-ns pulse width, was achieved successfully. The pulse profile could be actively controlled by adjusting the drive signal of an acoustic-optical modulator. This all-fiber laser system could be utilized as a seeder laser for a solid-state power amplifier system.

Direct Amplification of High Energy Pulsed Laser in Fiber-Single Crystal Fiber with High Average Power

A laser master oscillator power amplifier (MOPA) system consisting of a fiber amplifier and a two-stage Yb:YAG single crystal fiber (SCF) is experimentally studied. The nonlinear stimulated Raman scattering (SRS) is avoided by limiting the output power of the fiber preamplifier to 600 mW. Due to the benefit from the low nonlinearity and high amplification gain of the SCF, a laser pulse duration of 16.95 ps and a high average power of 41.7 W at a repetition rate of 250 kHz are obtained by using a two-stage polarization controlled double-pass amplification of Yb:YAG SCF, corresponding to an output energy of 166.8 μJ and a peak power of 9.84 MW, respectively. The polarization controlled SCF amplification scheme achieved a gain as high as more than 69 times. During the amplification, the spectra gain narrowing effect and the polarization controlled four-pass amplification setup are also studied. The laser spectrum is narrowed from over 10 nm to less than 3 nm, and the pulse width is also compressed to hundreds of femtosecond by dechirping the laser pulse. This compact-sized, cost-effective laser source can be used in laser micromachining, or as the seeder source for generating much higher power and energy laser for scientific research. For some applications which need femtosecond laser, this laser source can also be compressed to femtosecond regime.

Low-noise single-frequency 50 W fiber laser operating at 1013 nm

We have developed a 50 W all-fiber, single-frequency (SF), low-noise master oscillator power amplifier laser operating at 1013 nm, with an efficiency of 67%. The output signal shows an optical signal-to-noise ratio of 50 dB and excellent noise properties. These are to date and to the best of our knowledge, the highest reported figures of merit for a SF laser at this wavelength. The relative intensity noise of the laser is analyzed and is shown to follow the theoretical behavior for fiber amplifier.

High-power sampled-grating-based master oscillator power amplifier system with 23.5 nm wavelength tuning around 970 nm.

Tunable high-power diode lasers are key components in various established and emerging applications. In this work, we present a compact hybrid master oscillator power amplifier (MOPA) laser system. The system utilizes a tunable GaAs-based sampled-grating (SG) distributed Bragg reflector (DBR) laser as the master oscillator (MO), which emits around a wavelength of 970nm in a single longitudinal mode with a spectral width below 20pm. The SG-DBR laser consists of two SGs, each of which can be thermally tuned with microheaters. By tuning one of the two SGs, a discrete wavelength tuning of 21.1nm can be obtained with a Vernier mode spacing of about 2.3nm. By tuning both SGs, 23.5nm of quasi-continuous tuning is obtained, with a mode spacing of about 115pm. The coupling of the beam emitted by the MO into a tapered power amplifier provides an amplified output power in the watt range having a nearly diffraction-limited beam with a propagation factor of M1/e22=1.6. The combination of high power and wide wavelength tuning in a compact system makes this light source ideal for, among other things, nonlinear frequency conversion.

Pump-gain integrated functional laser fiber towards 10 kW-level high-power applications

To incorporate sufficient pump energy from multi-hundred watt-level 976 nm commercial laser diodes, a new kind of (8 + 1)-type pump-gain integrated functional laser ((8 + 1)-PIFL) fiber was fabricated by our newly optimized special fiber assembly and coating techniques for the first time. This (8 + 1)-PIFL fiber allowed for 10.66 kW aggregated pump power at 976 nm from six counter-pump ports in a master oscillator power amplifier laser setup and stably presented 8.72 kW laser output at 1079.4 nm with a high optical-to-optical efficiency of 81.0%. This is the highest power of the fiber amplifiers pumped by commercial laser diodes that has been reported so far, and only limited by the available pump power. Up to this power level, pure output spectrum with narrow 3 dB bandwidth Δλ3 dB of 2.76 nm and without any sign of stimulated Raman scattering and amplified spontaneous emission was obtained. Neither roll over of output power nor obvious thermal issue was observed with increased pump power, indirectly justifying strong photodarkening and transverse mode instability suppression. With another ten pump ports to collect more pump power, a further power scaling towards 10 kW or above can be expected.

Multi-kW Yb-doped aluminophosphosilicate fiber

For directly characterizing photodarkening effects, we demonstrated long-term laser stability of a 5kW-level fiber amplifier based on a Yb-doped aluminophosphosilicate fiber with a 30µm-core and 600µm-clad. The molar ratio of Al3+/P5+ was designed to be 1.1 ± 0.05 for remarkable photodarkening suppression, suitable numerical aperture, and central dip mitigation of refractive index profile. A modified chemical vapor deposition system combined with an all-gas-phase chelate precursor doping technique was applied to fabricate this fiber, doped with 0.15mol% Yb2O3, 1.7mol% Al2O3, and 1.4mol% P2O5. Tested at a master oscillator power amplifier laser setup, the 18m-long 30/600 Yb-doped aluminophosphosilicate fiber allowed for 6.14kW aggregated pump power at 976nm, and then showed a 5.19kW laser output at 1064.4nm with a high optical-to-optical efficiency of 85.2%. Up to this power level, an output spectrum without any sign of stimulated Raman scattering and amplified spontaneous emission was obtained. The fiber amplifier setup was kept at a 5.16kW output for over 600 minutes without power degradation, justifying a remarkable suppression of photodarkening. The results indicated that the all-gas-phase chelate precursor doping technique is highly competitive for low-photodarkening Yb-doped aluminophosphosilicate fiber fabrication towards a 5kW-level commercial high-power laser.

High brightness narrow-linewidth microsecond pulse green laser by frequency doubling of a master oscillator power amplifier Nd:YAG laser

We demonstrated a high brightness narrow-linewidth microsecond (μs) pulse green laser from second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) laser system. The oscillator was a quasi-continuous wave pulsed diode-side-pumped 1064 nm Nd:YAG laser with an average output power of 45.2 W and a beam quality factor of M2 = 1.21, operating at a pulse repetition frequency of 500 Hz with a ∼160 μs of pulse duration. After a two-stage amplifier, the fundamental output power of MOPA system was increased up to 198 W. Using a type-I lithium triborate nonlinear crystal, the maximum SHG output power of 78.4 W at 532 nm was obtained with a linewidth of 0.54 pm and a beam quality factor M2 less than 1.61. The brightness of the green light was as high as 1.08 × 1010 W/cm2 sr. This is, to the best of our knowledge, the highest brightness solid state light source for the narrow-linewidth μs pulse green lasers at 532 nm.

30/900 Yb-doped Aluminophosphosilicate Fiber Presenting 6.85-kW Laser Output Pumped With Commercial 976-nm Laser Diodes

To incorporate sufficient pumping power, a large-scale Yb-doped aluminophosphosilicate (Al2O3 -P2O5-SiO2, APS) fiber with 30- μ m core and 900- μ m inner clad in diameter, i.e., a 30/900 Yb-APS fiber, was theoretically designed and experimentally fabricated by using modified chemical vapor deposition system combining with chelate precursor doping technique. Analyzed by an electron probe microanalyzer, the dopant concentration of Yb2O3, Al2O 3, and P2O5 was estimated to be 0.14 mol%, 1.1 mol%, and 1.4 mol%, respectively. Based on an all-fiberized master oscillator power-amplifier laser setup directly forward pumped by commercial 976-nm laser diodes, 6.85-kW laser output at 1079.79 nm was achieved with an optical-to-optical efficiency of 67.1%, and the beam factor of $M^{2}$ was 2.38. The results indicate that large-scale Yb-APS fiber is quite competitive for high-power fiber laser development.

Femtosecond laser inscribed cladding waveguide lasers in Nd:LiYF4 crystals

Depressed circular cladding, buried waveguides were fabricated in Nd:LiYF4 crystals with an ultrafast Yb-doped fiber master-oscillator power amplifier laser. Waveguides were optimized by varying the laser writing conditions, such as pulse energy, focus depth, femtosecond laser polarization and scanning velocity. Under optical pump at 799 nm, cladding waveguides showed continuous-wave laser oscillation at 1047 nm. Single- and multi-transverse modes waveguide laser were realized by varying the waveguide diameter. The maximum output power in the 40 μm waveguide is ∼195 mW with a slope efficiency of 34.3%. The waveguide lasers with hexagonal and cubic cladding geometry were also realized.