Beam Quality Factor M2 Research Articles

Single-frequency 45-mJ pulses from a MOPA system using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier.

We demonstrate a master oscillator power amplifier (MOPA) system that emits single-frequency high-energy optical pulses at 1540 nm using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier. A double under-cladding and a 50-µm-thick core structure are employed for the planar waveguide amplifier to increase the output energy without degrading the beam quality. A pulse energy of 45.2 mJ with a peak power of 27 kW is generated at a pulse repetition rate of 150 Hz with a pulse duration of 1.7 µs. Moreover, the beam quality factor M2 of the output beam is 1.84 at the highest pulse energy thanks to its waveguide structure.

A Novel α-BaTeMo2O9 Acousto-Optic Switch for Generating Stable 639 nm Pulsed Laser

In this paper, an acousto-optic (AO) Q-switch based on α-BaTeMo2O9 (α-BTM) crystal is designed and further applied to generate a laser pulse at 639 nm for the first time. The α-BTM AO Q-switch demonstrates a large diffraction angle of 0.93° and a high diffraction efficiency of 85% at 639 nm. In the experiment, a maximum AO Q-switched output power of 362 mW is achieved at a repetition rate of 30 kHz, under a maximum absorbed pump power of 3.60 W, corresponding to a slope efficiency of 15.2%. With transmittance of T = 3%, the shortest Q-switching pulse width of 54.7 ns is generated at a repetition rate of 1 kHz. Meanwhile, the beam quality factor M2 of the above laser is measured, presenting the magnitude of 1.14 at both x and y directions. Our findings indicate that α-BTM AO Q-switch could act as an excellent switching device at 639 nm which may help to explore potential applications in the visible field.

Nanosecond pulsed narrow-linewidth all-fiber source for ZGP-OPO pumping

We demonstrate a pulsed three-stage master oscillator power amplifier (MOPA) at a wavelength of 2047 nm and its application for mid-IR generation via pumping a ZnGeP2 (ZGP) optical parametric oscillator (OPO). A maximum MOPA output power of 19.8 W is reached while achieving a diffraction limited beam quality M2 <1.1. By applying the MOPA setup to pump a ZGP crystal in a linear OPO cavity, a combined signal and idler mid-IR output power of 8.1 W is reached. For the highest mid-IR output power, a mid-IR conversion efficiency of 44 % and beam quality factors M2 of 2.2 and 2.0 are determined for signal and idler, respectively.

2 µm cylindrical vector beam generation from a c-cut Tm:CaYAlO4 crystal resonator.

Different from the traditional ideal column symmetry cavities, we directly generated the cylindrical vector pulsed beams in the folded six-mirror cavity by employing a c-cut Tm:CaYAlO4 (Tm:CYA) crystal and SESAM. By adjusting the distance between the curved cavity mirror (M4) and the SESAM, both the radially polarized beam and azimuthally polarized beam are generated around 1962 nm and the two vectorial modes can be freely switched in the resonator. Further increased the pump power to 7 W, the stable radially polarized Q-switched mode-locked (QML) cylindrical vector beams were also obtained with an output power of 55 mW, the sub-pulse repetition rate of 120.42 MHz, pulse duration of ∼0.5 ns and the beam quality factor M2 of ∼2.9. To our knowledge, this is the first report of radially and azimuthally polarized beams in the 2 µm wavelength solid-state resonator.

Realization of thermally-near-unstable resonator thin-disk laser and compensation of air wedge effect

This paper reports on a Yb: YAG thin-disk laser based on a thermally-near-unstable resonator. The cavity length of the resonator is 3 m, which is 3.5 m less than the cavity length of a stable resonator, simplifying the oscillator structure and reducing the optical loss of the resonator cavity. The disk laser based on a thermally-near-unstable resonator achieves a laser output power of 38.3 W with beam quality approaching the diffraction limit. The laser center wavelength is 1.03 μm. The resonator becomes misaligned during the increase in pump power due to air convection, reducing laser output power. In this paper, one end mirror of the thermally-near-unstable resonator is replaced with a retro-reflector to compensate for the misalignment of the resonator. The laser output power of the resonator with retro-reflector increased to 81 W. The beam qualityM2 factors in the × and y directions are 1.03 and 1.14, respectively. retro-reflector significantly improves the operational stability of the laser.

High beam quality yellow laser at 588 nm by an intracavity frequency-doubled composite Nd:YVO4 Raman laser.

High beam quality 588 nm radiation was realized based on a frequency-doubled crystalline Raman laser. The bonding crystal of YVO4/Nd:YVO4/YVO4 was used as the laser gain medium, which can accelerate the thermal diffusion. The intracavity Raman conversion and the second harmonic generation were realized by a YVO4 crystal and an LBO crystal, respectively. Under an incident pump power of 49.2 W and a pulse repetition frequency of 50 kHz, the 588 nm power of 2.85 W was obtained with a pulse duration of 3 ns, corresponding to a diode-to-yellow laser conversion efficiency of 5.75% and a slope efficiency of 7.6%. Meanwhile, a single pulse's pulse energy and peak power were 57 µJ and 19 kW, respectively. The severe thermal effects of the self-Raman structure were overcome in the V-shaped cavity, which has excellent mode matching, and combined with the self-cleaning effect of `Raman scattering, the beam quality factor M2 was effectively improved, which was measured optimally to be Mx 2 = 1.207, and My 2 = 1.200, with the incident pump power being 49.2 W.

Infrared tunable 5%MgO:PPLN OPO pumped by a 1-kHz sub-nanosecond microchip laser

We built and studied a singly resonant optical parametric oscillator using a 5%MgO:PPLN partial cylinder pumped by a sub-nanosecond microlaser emitting 1064 nm at a repetition rate of 1kHz. It is continuously tunable from 1410 nm up to 4330 nm by rotating the cylinder and a total energy of several micro Joules is emitted with a beam quality factor M² lower than 3.

LD End-Pumped Nd:YAG InnoSlab Laser at 1319 nm

The 1319 nm lasers have important research value and application prospects in optical communications, biomedicine and nonlinear frequency conversion. Currently, there are few reports of high power 1319 nm continuous lasers with high beam quality. We have demonstrated a high output power, high beam quality 1319 nm continuous-wave laser by laser diode end-pumped Nd:YAG slab with a stable-unstable hybrid resonator. With a pumping power of 477 W, an output of 96.8 W was obtained with a slope efficiency of 26.5% and stability of 0.27%. A single wavelength laser operation at 1318.7 nm was demonstrated. At an output of 91.1 W, the beam quality factors M2 in stable and unstable directions were 2.95 and 1.88, respectively.

Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser

We fabricate a pair of fiber Bragg gratings (FBGs) by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators. The bandwidth of high-reflection (HR) and low-reflection (LR) FBG is ∼1.6 nm and 0.3 nm, respectively. The reflection of the HR-FBG is higher than 99%, and that of the LR-FBG is about 10%. A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs, a record output power of 5027 W located in the signal core is achieved with a slope efficiency of ∼82.1%, and the beam quality factor M2 is measured to be ∼1.6 at the maximum power. The FBGs are simply fixed on a water cooling plate without a special package, and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76°C/kW and 1°C/kW, respectively. Our research provides an effective solution for robust high-power all-fiber laser oscillators.

High-stability and high-beam-quality single-frequency nanosecond 589 nm laser based on sum-frequency generation

A high repetition rate, high peak power, narrow linewidth nanosecond 589 nm laser based on sum-frequency generation (SFG) of 1064 nm and 1319 nm laser is demonstrated. A 1064 nm single-frequency distributed-feedback laser diode (DFB-LD) externally modulated by an acousto-optic modulator (AOM) and amplified by a fiber-solid hybrid amplifier with three-stage fiber amplifier and six-stage solid-state end-pump Nd:YVO4/Nd:YAG amplifier, a high beam quality laser output with pulse energy of 20 mJ at repetition rate of 500 Hz is achieved, which pulse duration and waveform are adjustable and editable. The ramp–hold–fire detection and dual piezoelectric ceramic (PZT) feedback compensation are employed in the 1319 nm oscillator to achieve high stability and low jitter laser. Then the 1319 nm laser pulse is amplified to 12.2 mJ by three-stage end-pumped Nd:YAG amplifier, with pulse duration of 70 ns. By sum-frequency of the 1064 nm and 1319 nm lasers, finally, 589.1595 nm laser with pulse energy of 10.4 mJ and pulse duration of 62 ns is generated efficiently, the laser beam quality factor M2 is less than 1.3. With the ability of switching frequencies quickly and the characteristic of compatible frequency-stabilized, this laser is believed to be a very potential solution for sodium Doppler lidar.

178-W picosecond green laser with active beam-pointing stabilization

Picosecond lasers with high average power and high beam quality have been widely used for precision processing and space exploration. In this study, we report a high-power picosecond green laser using a multistage Yb-doped rod-shaped photonic crystal fiber as an amplifier combined with a beam combination. The single amplification module achieves a 1,030 nm laser output of 146.8 W, and the maximum second harmonic generation (SHG) power is 92 W with a frequency conversion efficiency of 63.5%. The combined beam of the two SHGs resulted in a final output of 178 W with a repetition frequency of 24.07 MHz, pulse width of 50.1 ps, and beam quality factor of M2 = 1.16. Furthermore, an adaptive filter control method of a two-axis fast-steering mirror was applied to suppress the beam jitter to up to 45 Hz.

Electro-optically Q-switching performance of diode-pumped Ho:GdVO4 laser at 2.05 μm

We proposed a Ho:GdVO4 laser operating at 2.05 µm in-band directly pumped by the 1.94 μm fiber-coupled laser diode, which electro-optically Q-switched by an LGS(La3Ga5SiO14) crystal. Adopting 1.0 at.%-doped Ho:GdVO4 crystal as the gain medium, a maximum output power of 6.5 W at 2047.9 nm with the absorbed pump power of 28 W was obtained in the continuous-wave mode, corresponding to a slope efficiency of 37.6 %. At a pulse repetition frequency of 1 kHz, the highest pulse energy of 3.25 mJ with a pulse width of 4.2 ns was achieved, corresponding to a peak power of 773.8 kW. In addition, the beam quality factor M2 was measured to be 1.9 at the maximum output level by the 90/10 knife-edge method. TEM00 propagation has been verified by a camera. To the best of our knowledge, this is the first demonstration of high peak power electro-optically Q-switched Ho:GdVO4 laser directly pumped by the 1.94 μm laser diode.

Resonators with a continuously variable output coupling rate to enhance output performance of Yb:YAG thin-disk lasers.

Beam quality and average output power are two long-sought parameters of continuous-wave lasers. The operating characteristics of a Yb:YAG thin-disk laser based on a 72-pass pump module using output couplers with continuously variable coupling rates are reported. When the pump power is 450 W, the average power of more than 210 W is obtained, and the corresponding optical-optical efficiency is about 46.67%. The output beam is near diffraction limited with M2=1.20 and 1.18 on the horizontal and vertical directions respectively. The thin-disk laser maintains the beam quality near diffraction limited within the whole pump power range by continuously changing the coupling rate of the output coupler (the combination of thin film polarizer and quarter-wave plate) according to the incident pump power. The experimental results show that by continuously changing the coupling rate, combined with the dynamic change of thermal lens effect, the laser beam quality factor M2 can be reduced from 2.0 to 1.2, and the beam quality can be improved by about 40%. The improvement of and increase of average output power confirm that resonators with continuously variable coupling rate are a promising method for enhancing output performance of thin-disk lasers.

175 W average power from a single-core rod fiber-based chirped-pulse-amplification system.

We report on a fiber-based chirped-pulse-amplification laser system with bulk transmission grating compression to a pulse duration of 357 fs, average power of 175 W, and pulse energy of 233μ J. The compressed pulse train has a beam quality factor M2 of 1.21. The power amplifier is based on a state-of-the-art single-mode photonic crystal rod-type ytterbium-doped fiber operating at 248 W of average power and a repetition rate of 750 kHz. The long-term stability of the laser system has been tested continuously for more than 4000 hours and shows no sign of transverse mode instability.

Pulse-quality metric for nonstationary partially coherent fields.

This paper generalizes a pulse-quality metric referred to as P2, i.e., the time analogue of Siegman's beam quality factor M2, to include pulsed (nonstationary) random fields of any state of coherence. The analysis begins with the derivation of a general P2 relation, which we then specialize to the important cases of coherent and Schell-model pulsed beams. As examples, we derive the P2 for two stochastic sources: (1) a cosine Gaussian-correlated Schell-model pulsed beam and (2) a nonuniformly correlated pulsed beam. For both of these sources, we generate (in simulation) random instances of each and compare the simulated (Monte Carlo) P2, i.e., computed directly from its definition, to the theoretical quantity. The agreement is excellent, thereby validating our P2 analysis.

Graphene Passively Q-Switched Nd:YAG Laser by 885 nm Laser Diode Resonant Pumping

A graphene passively Q-switched Nd:YAG laser experienced resonant pumping by an 885 nm laser diode (LD), as demonstrated in this paper. In the continuous-wave operation, the maximum average output power was up to 1.8 W with the absorbed pump power being 11.7 W, and the slope efficiency was 51.2%. In the Q-switching operation, the maximum average output power was up to 639 mW with a pulse width of 2.06 μs at the repetition frequency of 102.7 kHz, while the slope efficiency and the beam quality factor M2 were 25.3% and 1.25, respectively.

3.3 kW narrow linewidth FBG-based MOPA configuration fiber laser with near-diffraction-limited beam quality

This paper demonstrates a narrow-linewidth fiber laser with near-diffraction-limited beam quality and high Raman suppression ratio by utilizing Master Oscillator Power-Amplifier structure based on narrow-bandwidth fiber Bragg gratings. The output power reaches 3.31 kW with a slope efficiency of 85.0 %. At the maximum output power, the 3 dB, 10 dB, and 20 dB linewidths are 0.40 nm, 1.20 nm, and 3.09 nm, respectively. The beam quality M2 factor is 1.32 and the Raman suppression ratio reaches 26.3 dB. A pair of narrow-bandwidth fiber gratings were used in the oscillator to compress the linewidth of the seed laser. We used backward-pumping configuration and plum-coiled gain fiber in the amplification stage to suppress the Raman effect and select transverse mode by fiber-bending. Finally, a compact all-fiber laser was obtained with narrow-linewidth, high beam quality, and effective suppression of the Raman effect. To the best of our knowledge, this is the highest output power for a sub-nanometer-linewidth laser using narrow-bandwidth fiber gratings and none-wavelength-locked laser diodes, which achieves near-diffraction-limit beam quality. This provides a solution for high-power narrow-linewidth laser sources with high beam quality.

Research on 946 nm LG01 mode laser considering thermal effect

Laguerre–Gaussian (LG) beams have important application prospects in Frontier Fields such as optical micromanipulation, optical communication, and super-resolution microscopy. In this letter, the thermal effect generated by Nd:YAG crystal that is end-pumped by the annular-beam is taken into account in the analysis of the LG01 mode beam generation. The 946 nm LG01 mode laser is obtained by using annular-beam end-pumped Nd:YAG crystal shaped by custom mirror M01. When the incident pump power is 19.9 W, the maximum output power is 1.35 W, the optical-optical conversion efficiency is 6.8%, the slope efficiency is 10.9%, and the beam quality factor M2 in X and Y directions is 2.22 and 2.28 respectively. The handedness of LG01 mode laser remains stable in the whole pump range. The theoretical model established considering thermal effects can guide the design of LG mode lasers, and the 946 nm LG01 mode laser obtained expands the new wavelength of vortex light and increases its potential new application direction.

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.

Study on linewidth compression at 3.8 μm with multiple F-P etalon pumped by compound intra-cavity optical parametric oscillator

A compact, multiple F-P etalon and narrow linewidth intra-cavity single-resonant mid-infrared optical parametric oscillator based on MgO:PPLN is reported, which is composed of a 1064 nm gain cavity and a mid-infrared frequency conversion cavity. The narrow linewidth at 3.8 μm can be obtained indirectly through the linewidth compression at the fundamental wavelength and the signal wavelength. The linewidth at 3.8 μm is 0.73 nm without adjacent mode when the fundamental wavelength is acted by the single F-P etalon with the thickness of 2.5 mm and the signal wavelength is acted by the dual F-P etalon with the thickness of 2.5 mm and 0.5 mm. Meanwhile, the output power of 1.06 W is obtained, corresponding to the conversion efficiency of 2.41%, the pulse width of 11.17 ns is obtained synchronously, and the beam quality factor M2 at 3.8 μm are Mx2 = 1.92 and My2 = 1.87, respectively. It provides a method for intra-cavity pumped mid-infrared linewidth compression.