Thermal-birefringence-induced depolarization in a 450 W Ho:YAG MOPA system.

We demonstrated an efficient, high-power Ho:YAG master-oscillator power amplifier (MOPA) system and investigated its thermal-birefringence-induced depolarization. The maximum output power was 450 W with a depolarized power of 32.1 W and depolarization of 0.071 via three power amplifiers. To our knowledge, this is the highest average power generated from a Ho:YAG MOPA system. In theory, a simplified model was built to calculate the depolarization in the amplifier, and the theoretical results agreed with the actual value well. Moreover, the overall optical-to-optical efficiency of the MOPA system was near 60%, and the beam quality M2 factors of s-polarized laser were measured to be ∼ 1.8 at 400 W. In pulse operation, the per pulse energy was ∼ 11 mJ at the pulse repetition frequency of 40 kHz with the corresponding peak power of 220 kW.

113 W Ho:YLF oscillator with good beam quality efficiently pumped by a Tm:YAP laser.

We demonstrate a linearly polarized Tm:YAP slab laser pumped by fiber-coupled laser diodes. The maximum output power is 202 W at 1937.5 nm with a slope efficiency of 47.4% and an optical-to-optical efficiency of 35.6%. The beam quality M2 factors are 10.1 and 8.33 in x and y directions, respectively. Using the Tm:YAP laser as the pump source, the maximum power of the Ho:YLF oscillator is 113 W at 2063.3 nm, corresponding to an optical-to-optical efficiency of 55.9%. In addition, the beam quality factors of the Ho:YLF laser are ∼1.5 at maximum power.

161 W middle infrared ZnGeP2 MOPA system pumped by 300 W-class Ho:YAG MOPA system.

We demonstrated a high-power Q-switched two-stage Ho:YAG master-oscillator power-amplifier (MOPA) system dual-end pumped by Tm:YLF lasers. A new method was introduced by rotating and swapping spatial axial directions of pump beams to improve the beam quality of the Ho:YAG oscillator and first-stage amplifier. Two parallel second-stage Ho:YAG amplifiers were employed to output high power. A total maximum average output power of 332 W at 2091 nm with pulse repetition frequency of 20 kHz was achieved. Then a ZnGeP2 MOPA system was demonstrated using the Ho:YAG MOPA as the pump source. A maximum average output power of 161 W at 3-5 µm was obtained with 290 W incident Ho pump power, corresponding to beam quality factors M2 of 3.42 and 3.83 for horizontal and vertical directions, respectively.

Tunable twisted-mode Ho:YAG laser at continuous-wave and pulsed operation.

A single-longitudinal-mode Ho:YAG laser with a twisted-mode cavity under the continuous-wave and pulsed operation was demonstrated. The maximum continuous-wave single-longitudinal-mode output power of 0.76 W was obtained at the wavelength of 2097.46 nm, corresponding to a slope efficiency of 28.9%. The output wavelength was tuned from 2096.94 nm to 2098.48 nm by utilizing a 0.5-mm-thick etalon. Furthermore, the single-frequency pulsed operation of the twisted-mode Ho:YAG laser was realized by the electro-optic switch. For the 2 kHz pulse repetition frequency, the single-frequency pulse energy reached 0.2 mJ with the pulse width of 116.5 ns. The beam quality factors M2 of the pulsed twisted-mode Ho:YAG laser in the x and y directions were 1.15 and 1.10, respectively.

570†MHz harmonic mode-locking in an all polarization-maintaining Ho-doped fiber laser.

In this paper, we demonstrate a 570.0 MHz harmonically mode-locked all-polarization-maintaining Ho-doped fiber laser based on semiconductor saturable absorbed mirror. Firstly, the laser operates in the 15.4 MHz fundamental mode-locked soliton regime, emitting 2051.5 nm, 1.62 ps soliton pulse without Kelly sidebands. And then, the stable 37th-order harmonic mode-locked soliton with maximum repetition rate up to 570.0 MHz at 2053nm is generated. Moreover, colorful soliton rain behaviors are also discussed.

11 µm, high beam quality idler-resonant CdSe optical parametric oscillator with continuous-wave injection-seeded at 2.58 µm.

We report an idler-resonant, continuous-wave (CW) seed injected, optical parametric oscillator (OPO) based on cadmium selenide (CdSe). The CdSe OPO was pumped by a 2.09 µm ns-pulsed laser and injection-seeded by a 2.58 µm CW laser. The idler-resonant oscillator was designed to maximize the optical-to-optical conversion efficiency and optimize the beam quality. The injected seed laser was designed to reduce the pump threshold. With this setup, the average idler output power of 802 mW was obtained corresponding to a pulse energy of 0.8 mJ at the wavelength of 11.01 µm and linewidth (FWHM) of 0.6 cm-1, optical-to-optical conversion efficiency of 4.4%, quantum conversion efficiency of 23.3%, beam quality of M2x = 1.23, M2y = 1.12, and pulse width of 21 ns. In addition, by turning the angle of the CdSe, wavelength tuning of 10.55-11.98 µm was achieved.

Continuously tunable high-power single-longitudinal-mode Ho:YLF laser around the P12 CO2 absorption line.

A continuously tuned single-longitudinal-mode (SLM) Ho:YLF laser around the P12 CO2 absorption line was demonstrated. The continuous tuning range of 5.75 pm within one longitudinal mode spacing of the Ho:YLF resonator was realized by using a novel intra-cavity wedge prism device fixed on the piezoelectric transducer (PZT) as the cavity length controller. High SLM power of 11.3 W was obtained from a Ho:YLF amplifier with three crystals at the pump power of 31.8 W and master oscillator power of 323 mW, corresponding to a gain of 15.44 dB and an optical-to-optical conversion efficiency of 34.5%. The beam quality factors M2 of the SLM Ho:YLF amplifier in the x and y directions were estimated to be 1.04 and 1.05, respectively.

Research on performance improvement technology of a BaGa4Se7 mid-infrared optical parametric oscillator.

In this Letter, we demonstrate an average output power of 5.12 W at 3-5 µm from a type-I phase-matching BaGa4Se7 (BGSe) optical parametric oscillator (OPO), which is pumped by a 2090 nm Q-switched Ho:YAG laser with pulse repetition frequency of 1 kHz. At maximum output level, the corresponding slope efficiency and optical-to-optical conversion efficiency are 30.0% and 18.3%, respectively. Moreover, under ring cavity conditions, the BGSe OPO produced a 3.04 W mid-infrared laser with high beam quality factors M2 of 1.47 in the horizontal direction and 1.51 in the vertical direction. Besides, the wavelength-tuning curve for type-I BGSe was also investigated, corresponding to an idler wavelength-tuning range of 4.5-5.3 µm, and the signal light wavelength was 4.5 to 4.1 µm.

1 W, 10.1 µm, CdSe optical parametric oscillator with continuous-wave seed injection.

We demonstrated a 2.1 µm nanosecond laser pumped, 2.6 µm continuous-wave (CW) seed injected, cadmium selenide (CdSe) signal singly resonant optical parametric oscillator (OPO). A maximum average power of 1.05 W was obtained corresponding to a pulse energy of 1.05 mJ at the idler wavelength of 10.1 µm and optical-to-optical conversion efficiency of 4.69%, beam quality of $M_x^2={2.25}$Mx2=2.25, $M_y^2={2.12}$My2=2.12 and pulse width of 24.4 ns. To the best of our knowledge, this is the first time to achieve 10-12 µm laser with watt-level average power using OPO technology.

High-beam-quality 2.1 µm pumped mid-infrared type-II phase-matching BaGa4Se7 optical parametric oscillator with a ZnGeP2 amplifier.

We firstly demonstrated a tunable 2.1 µm pumped mid-infrared type-II phase-matching BaGa4Se7 (BGSe) optical parametric oscillator (OPO). A signal tuning range of 3.82-3.99 µm was achieved. With a signal light produced by the BGSe OPO serving as seed, the ZnGeP2 (ZGP) optical parametric amplifier (OPA) was first introduced. A 4.35 W mid-infrared laser was obtained in the novel type-II BGSe OPO with a ZGP OPA system, which was pumped by a 24 W 1 kHz Q-switched Ho:YAG laser. The beam quality factor M2 of the whole system is ∼2.3, which is over a 50% promotion on the previous M2 results of single BGSe OPO.

3.5 W long-wave infrared ZnGeP2 optical parametric oscillator at 9.8 µm.

We demonstrated a high-power long-wave infrared optical parametric oscillator at 9.8 µm based on a type-I phase-matching ${{\rm ZnGeP}_2}$ZnGeP2 crystal. By using a ${Q}$Q-switched 2091 nm Ho:YAG laser with pulse repetition frequency of 10 kHz as the pump source, the maximum average output power of 3.51 W at 9.8 µm was achieved with incident pump power of 90 W, corresponding to a slope efficiency of 4.81% and conversion efficiency at maximum pump power of 3.9%. The pulse width of 19.6 ns and linewidth of 142 nm were obtained at maximum output level. In addition, the beam quality factor ${M^2}$M2 was measured to be ${\sim}{2.2}$∼2.2.

Measurement of optical homogeneity of ZnGeP2 crystal using a 2.02 µm single-longitudinal-mode Tm:LuAG ring laser.

We demonstrate a single-longitudinal-mode Tm:LuAG unidirectional ring laser based on the Faraday effect and its application in measuring the optical homogeneity of ZnGeP2 (ZGP) crystal. The maximum single-longitudinal-mode power was 511 mW at 2022.4 nm, with a slope efficiency of 9.7%, and the M2 factors were 1.13 and 1.23 in the x and y directions, respectively. The laser was s polarized with a polarization degree of 30 dB. Also, the optical homogeneity of ZGP crystal was measured by the Mach-Zehnder interference method, which adopts the single-longitudinal-mode laser as the monochrome source. According to the interference fringes, the quantitative calculation or qualitative analysis of the optical homogeneity of ZGP crystal was achieved, which could quickly and conveniently judge the quality of ZGP crystal, and provided high-quality crystals for high-power mid-infrared lasers. To our knowledge, it is the first report of the optical homogeneity measurement of ZGP crystal by a 2 µm single-longitudinal-mode laser.

Broadband second-harmonic and sum-frequency generation with a long-wave infrared laser in AgGaGe5Se12.

Using an 8 µm long-wave infrared laser as the fundamental wave, we achieved second-harmonic generation (SHG) and sum-frequency generation simultaneously in AgGaGe5Se12 and obtained a 4 µm laser output. Among them, SHG was achieved in the 173 nm spectral range of the fundamental wave, which was consistent with theoretical calculations. The average power of the obtained 4 µm laser was 41 mW, corresponding to an optical-to-optical conversion efficiency of 3.2%. The measured temperature acceptance bandwidth (LδT) (FWHM) was 50 K·cm; the angular acceptance bandwidth (Lδθ) (FWHM) was 13.3 mrad·cm; and the average absorption coefficient in the wavelength range of 0.86-11.30 µm was 0.07cm-1. In addition, the spectral acceptance bandwidth (Lδλ) of fundamental wave in AgGaGe5Se12 SHG and the spectral gain bandwidth of frequency downconversion in AgGaGe5Se12 were calculated. In view of the small absorption coefficient, the large temperature acceptance bandwidth, and the large spectral gain bandwidth, we conclude that AgGaGe5Se12 is a suitable nonlinear crystal for high-power short/mid/long-wave infrared lasers and frequency conversions of nanosecond-femtosecond infrared lasers. These results are conducive to the further development of AgGaGe5Se12 lasers.

Single-longitudinal-mode Ho:YVO4 MOPA system with a passively Q-switched unidirectional ring oscillator.

A 2052.96 nm single-longitudinal-mode pulsed Ho:YVO4 MOPA system was demonstrated for the first time. The pulsed Ho:YVO4 MOPA system consisted of a unidirectional ring passively Q-switched oscillator and a single-pass amplifier. By inserting an isolator, a half-wave plate and a Cr2+:ZnS plate into the ring Ho:YVO4 oscillator cavity, the single-longitudinal-mode pulsed laser was achieved with an average output power of 1.02 W with pulse width of 910 ns and pulse repetition frequency (PRF) of 67 kHz. Using the residual (non-absorbed) pump power of the oscillator as pump, the single-pass pulsed Ho:YVO4 amplifier obtained an average output power of 1.67 W. The total optical-to-optical efficiency of the pulsed Ho:YVO4 MOPA system was 14.3%. Single-longitudinal-mode pulsed MOPA system based on isolator and Cr2+:ZnS around 2 µm has not been reported yet to the best of our knowledge.

Active Q-switching operation of slab Ho:SYSO laser wing-pumped by fiber coupled laser diodes.

In this paper, we demonstrate the continuous-wave and acousto-optical Q-switched performance of diode-pumped slab Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at 2.1 µm for the first time. Two 1.91-µm laser diodes were used to pump the Ho:SYSO crystal. With a wing-pumping structure, at absorbed pump power of 44.7 W, the continuous wave slab Ho:SYSO laser produced 20.7 W maximum output power at 2097.9 nm, resulting in a slope efficiency of 53.1% with respect to the absorbed pump power. In the Q-switched regime, the slab Ho:SYSO laser produced up to 3.4 mJ pulse energy with 20 ns minimum pulse width at pulse repetition frequency of 5 kHz, corresponding to a peak power of 170 kW.

Efficient Ho:(Sc0.5Y0.5)2SiO5 laser at 2.1 µm in-band pumped by Tm fiber laser.

In this paper, we demonstrate a new 2.1-µm Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at room temperature. The absorption and emission spectra of Ho:SYSO crystal were studied at temperature of 300 K. The strongest absorption peak of 5I7 level of Ho ions in SYSO crystal is located at 1943 nm with cross section of 0.79 × 10-20 cm2. The maximum emission cross section of 1.74 × 10-20 cm2 is located at 2032 nm. A 1940.3-nm narrow-linewidth Tm fiber was used to pump the Ho:SYSO crystal. At absorbed pump power of 20.4 W, the continuous wave Ho:SYSO laser yielded 10.3 W maximum output power at 2097.67 nm and 54.7% slope efficiency respect with absorbed pump power. In addition, we have estimated the beam quality factor (M2) of Ho:SYSO laser to be 1.7 near maximum output level.

Dual-wavelength injection-seeded Q-switched Ho:YLF laser for CO2 differential absorption lidar application.

A dual-wavelength injection-seeded Q-switched Ho:YLF laser for CO2 differential absorption lidar pumped by the thulium-doped fiber laser was demonstrated. The single-frequency Ho:YLF seed laser was achieved by a pair of highly anti-misalignment corner cubes. The wavelength of an online seed laser was corrected to the P12 CO2 absorption line at 2064.414 nm by using a CO2 absorption cell. At the pulse repetition frequency of 100 Hz, the single-frequency pulsed energy was 16.1 mJ with the pulse width of 221.3 ns after a single-pass amplifier. The full width at half-maximum of a single-frequency pulsed spectrum was about 3.87 MHz, and the fluctuation of center frequency was 2.8 MHz in 30 min.

High repetition rate 102 W middle infrared ZnGeP2 master oscillator power amplifier system with thermal lens compensation.

We demonstrate a 102 W middle infrared ZnGeP2 (ZGP) optical parametric amplifier (OPA) pumped by a 2097-nm Q-switched Ho:YAG laser at a pulse repetition frequency of 10 kHz. The seed middle infrared laser was produced by a ZGP optical parametric oscillator. Its average power was 28.4 W pumped by a 50 W 2097-nm laser. By thermal lens compensation, the beam factor M2 reduced from 3.1 to 2.1. When the incident Ho pump power was 120 W, the middle infrared ZGP OPA yielded the maximum average output power of 102 W and slope efficiency of 61.7%. The overall optical conversion efficiency of 60% from Ho to middle infrared was obtained for the whole middle infrared laser system. In addition, at the maximum average output power, the beam quality factors of the middle infrared ZGP OPA were measured to be about 2.7 and 2.8 for horizontal and vertical directions, respectively.

110.4 mJ, 1 kHz repetition rate, Ho:YAG master oscillator power amplifier.

We reported a high-energy, high-pulse-repetition-frequency Ho:YAG master oscillator and power amplifier (MOPA), which was resonantly dual-end pumped by Tm:YLF lasers at room temperature. At a pulse repetition frequency of 1 kHz, the Ho:YAG MOPA laser system produced a maximum pulse energy of 110.4 mJ with a 28 ns pulse width, corresponding to a peak power of approximately 3.94 MW. The output wavelength of the Ho:YAG MOPA laser system was 2090.9 nm. In addition, a beam quality factor M2 of about 1.7 was achieved at maximum output level.

High-repetition-rate laser ultrasonic generation in carbon-fiber-reinforced plastics excited by a 3.2-3.4 µm ZGP master oscillator power amplifier system.

In this paper, we demonstrate 1-kHz-repetition-rate laser ultrasound in carbon-fiber-reinforced plastics (CFRPs). A ZGP master oscillator and power amplifier (MOPA) system was used to generate high-repetition-rate laser radiation in the spectral range 3.2-3.4 µm. At the output wavelength of 3.4 µm, the maximum output energy of the ZGP MOPA system is 5.62 mJ with a pulse width of 24 ns, corresponding to a peak power of approximately 233.9 kW. The ultrasound was generated by the laser converted from 3.2 to 3.4 µm in the graphite/epoxy composite. The maximum ultrasonic signal amplitude in the CFRP sample was 36.2 mV in the condition of thermoelastic excitation at 3.4 µm. Ablation occurred in the CFRP sample when the energy fluence was over 90 mJ/cm2. Compared in different samples, laser-ultrasound generation was influenced by the wavelength of the laser.