High power single-frequency 1112†nm laser by an insertable Nd:YAG/YAG bonded monolithic planar ring oscillator.

A high power single-frequency operation at 1112 nm with novel insertable monolithic planar ring oscillator based on a Nd:YAG/YAG bonded crystal is proposed. In a proof-of-principle experiment, a finely designed coating on the output surface is carried out to ensure single-wavelength oscillation at 1112 nm, together with a half-wave plate and a Tb3Ga5O12 crystal inserted in the open space of the bonded block to realize the unidirectional operation with power scalability. Consequently, the single-frequency laser delivers an output power of 3.9 W at 1112.3 nm with a slope efficiency of 58.6% and an optical-to-optical efficiency of 17.7%. The power fluctuation is measured to be within ± 0.26% over 20 min, and the laser linewidth is estimated to be 4.15 MHz (Δλ = 0.017 pm).

Multilevel synergically controlling wavefront correction of a high-power slab laser system.

We present a multilevel synergically controlling wavefront correction method that can apply in a slab laser system. To fully utilize the response frequency and the stroke of actuators of the single deformable mirror (DM), we design a set of multilevel wavefront correction devices to reduce the root-mean square of wavefront aberration before the DM. As the wavefront of slab geometry solid-state lasers mainly consists of fourth and longitudinally distributed aberration, such as 5th, 9th, and 14th orders of Legendre polynomials. We design a precompensating level of the aberration with a slow-drift mirror, fast-steer mirror, one-dimensional adjustable slab-aberration compensator, and beam-shaping system to reduce these orders of wavefront aberration with low spatial resolution and large stroke. As the controlling bandwidth of different devices is diverse, the coupling oscillation between the precompensating level and adaptive optics (AO) level occurs, then we develop the multilevel synergically control to address the coupling. With the precompensating level, the experimental result shows the residual wavefront aberration of the slab laser is compensated well by the AO level effectively within the compensating capability. We clean up a 9.8 kW slab laser system with the beam quality ß of far-field focus spots improved from 17.71 to 2.24 times the diffraction limit.

1.9†µJ external-cavity dumped ultra-broad-area semiconductor nanosecond laser.

An external-cavity dumped nanosecond (ns) ultra-broad-area laser diode (UBALD) at around 966 nm with high pulse energy is demonstrated. A 1 mm UBALD is used to produce high output power and high pulse energy. A Pockels cell (PC) combines with two polarization beam splitters (PBSs) and is employed to cavity-dump a UBALD operating at 10 kHz repetition rate. At a pump current of 23 A, 11.4 ns pulses with a maximum pulse energy of ≈1.9 µJ and a maximum peak power of ≈166 W are achieved. The beam quality factor is measured to be M x 2=19.5 in the slow axis direction and M y 2=2.17 in the fast axis direction. Moreover, maximum average output power stability is confirmed, with a power fluctuation of less than 0.8% rms over 60 min. To the best of our knowledge, this is the first high-energy external-cavity dumped demonstration from an UBALD.

Sub-pm linewidth, high pulse energy, high beam quality microsecond-pulse Ti:sapphire laser at 766.699†nm.

In this letter, a sub-pm linewidth, high pulse energy and high beam quality microsecond-pulse 766.699 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At an incident pump energy of 824 mJ, the maximum output energy of 132.5 mJ at 766.699 nm with linewidth of 0.66 pm and a pulse width of 100 µs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 766.699 nm with pulse width of hundred micro-seconds for a Ti:sapphire laser. The beam quality factor M2 is measured to be 1.21. It could be precisely tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm. The wavelength stability is measured to be less than ±0.7 pm over 30 min. The sub-pm linewidth, high pulse energy and high beam quality Ti:sapphire laser at 766.699 nm can be used to create a polychromatic laser guide star together with a home-made 589 nm laser in the mesospheric sodium and potassium layer for the tip-tilt correction resulting in the near-diffraction limited imagery on a large telescope.

Compact 200 W level linearly polarized microsecond-pulse Nd:YAG oscillator with nearly diffraction-limited beam quality.

A compact 200 W level diode-side-pumped microsecond (µs) pulse linearly polarized rod Nd:YAG laser oscillator was demonstrated with nearly diffraction-limited beam quality. The oscillator was based on a thermally near-unstable cavity design with two concave lenses in the cavity to enlarge the volume of the fundamental mode, leading to improvement of the laser efficiency and beam quality. Consequently, a record-high average power of 222 W was obtained at a repetition rate of 400 Hz with a 180 µs pulse width, corresponding to an optical-to-optical (o-o) conversion efficiency of 37%. The average beam quality factor was measured to be M2=1.32, resulting in a brightness value as high as of 11.25GW/sr⋅cm2. To the best of our knowledge, this represented the highest average power, the highest o-o efficiency, and the highest brightness for a µs pulse 1064 nm rod Nd:YAG laser oscillator.

QCW surface-pumped cryogenically cooled single-slab laser with 1 kW level at 946 nm.

We present a kilowatt-level quasi-continuous-wave (QCW) cryogenically cooled 946-nm slab laser oscillator for the first time, to the best of our knowledge. The laser system is based on a double-face-pumped large-size single-slab Nd:YAG design, delivering a record-high average power of 1.06 kW without additional amplification. This laser oscillator operates at repetition rate of 400 Hz with a pulse duration of 175 µs, resulting in a single pulse energy of 2.65 J. To the best of our knowledge, these results represent the highest output power and pulse energy for any all-solid-state 946-nm laser ever reported to date. Our scheme paves a new path for the development of the compact high-power solid-state 946-nm laser.

1.53 W all-solid-state nanosecond pulsed mid-infrared laser at 6.45 µm.

A compact and robust all-solid-state mid-infrared (MIR) laser at 6.45 µm with high average output power and near-Gaussian beam quality is demonstrated. A maximum output power of 1.53 W with a pulse width of approximately 42 ns at 10 kHz is achieved using a ZnGeP2 (ZGP) optical parametric oscillator (OPO). This is the highest average power at 6.45 µm of any all-solid-state laser to the best of our knowledge. The average beam quality factor is measured to be M2 = 1.19. Moreover, high output power stability is confirmed, with a power fluctuation of less than 1.35% rms over 2 h, and the laser can run efficiently for more than 500 h in total. Using this 6.45 µm pulse as a radiation source, ablation of animal brain tissue is tested. Furthermore, the collateral damage effect is theoretically analyzed for the first time, to the best of our knowledge, and the results indicate that this MIR laser has excellent ablation ability, making it a potential replacement for free electron lasers.

Primary aberration optimization for double-plane symmetric beam shaping systems using a pair of curved reference surfaces.

The geometric aberration of centered refracting double-plane symmetric optical systems (DPSOS) is investigated. For DPSOS with different defocus values in the tangential plane and the sagittal plane (astigmatic wavefront), a pair of curved reference surfaces which vanishes the quadratic terms of the optical path difference (OPD) between a general ray and a reference ray are deduced. With the curved reference surfaces, the primary (fourth-order) wave aberration function for DPSOS is calculated and analyzed, which can be used for beam shaping designs with astigmatic input wavefront, such as slab lasers and semiconductor lasers. Further, the proposed curved reference surfaces can be applied to analyze the aberrations of general DPSOS.

Compact integrated aberration-compensating module for a 9 kW Nd:YAG slab laser based on an off-axis stable-unstable resonator.

An integrated aberration-compensating module (IACM), consisting mainly of an adjustable slab-aberration compensator, a one-dimensional Shack-Hartmann wavefront sensor, and a data processor, which meet the urgent requirements of correcting the specific wavefront aberrations of a slab laser based on an off-axis stable-unstable resonator, is designed and experimentally demonstrated. Benefits include compactness, robustness, simplicity, automation, and cost-effectiveness. The particular wavefront aberrations of the 9 kW level quasi-continuous-wave Nd:YAG slab laser, which have characteristics of asymmetry, large amplitude and gradient, high spatial frequency, and low temporal frequency, were measured and theoretically analyzed. In the experiment, the wavefront aberrations of the slab laser were corrected by the IACM. At the average output power of 9 kW, the diffraction-limited factor ß was improved from 20.3 times diffraction limit (DL) to 3.6 times DL. The peak-to-valley and root-mean-square values of aberrations were reduced from 9.6 to 0.85 µm and from 2.86 to 0.18 µm within five iterations of the IACM, respectively. Moreover, The IACM is capable of maintaining the compensating surface figure after power-off.

Simplified analytical method for an anamorphic refractive shaping system of laser beams with a large aspect ratio.

For reshaping aperture size and correcting low-order aberration of laser beams with large aspect ratios, a simplified analytical method is proposed to design an anamorphic refractive shaping system, which is composed of double-plane symmetric lenses. The simplified method enables performing a global study of aberrations via calculating the analytical primary wave aberration function under paraxial approximation. The aberration balance is analyzed with a three-lens laser collimating system and a compact four-lens laser expanding system. Lens bending and conic surfaces are introduced to decrease ray errors. Through the simplified analytical method, anamorphic refractive shaping systems for laser beams with large aspect ratios can be adequately analyzed and conveniently designed.

2.55 W continuous-wave 378 nm laser by intracavity frequency doubling of a diode-pumped Alexandrite laser.

A high-power continuous-wave (CW) ultraviolet (UV) laser at 378 nm from an intracavity frequency-doubled Alexandrite laser has been demonstrated with 638 nm fiber-coupled laser diodes as the pump source. A maximum output power of 2.55 W was obtained, which is the highest power for CW frequency-doubled Alexandrite lasers, to the best of our knowledge, corresponding to the optical-to-optical conversion efficiency of 7.9% from 638 nm pump laser to 378 nm UV laser. The beam quality factors M2 were measured to be 2.19 and 2.47 in x and y directions at UV output power of 1 W, respectively.

Excellent performance of a cryogenic Nd:YAlO3 laser with low wavefront distortion based on zero thermal expansion.

High-power solid-state lasers with good beam quality are attracting great attention on account of their important applications in industry and military. However, the thermal effects generated in the laser host materials seriously limit power scaling and degrade the beam quality. Thermal lensing and thermally induced wavefront deformation are the main causes of the beam quality deterioration. Here we investigate the performance of a zero thermal expansion (ZTE) solid-state laser gain material. In a proof-of-principle experiment, an ${a}$-cut rod ${\rm Nd}\!:\!{{\rm YAlO}_3}$ (Nd:YAP) perovskite crystal is chosen to be the gain medium for ZTE around 180 K. The laser performance spanning the temperature range from 80 to 290 K is studied. The maximum output power and minimum threshold pump power were obtained at a temperature of 180 K. Moreover, the measured thermal focal power and peak-to-valley value of the wavefront distortion also reach a minimum at this temperature, an additional benefit from the crystal's ZTE coefficient. We envisage that these results will open a new route towards the development of high-power and high-beam-quality lasers through the use of ZTE gain materials.

22.9 W CW single-frequency laser at 671 nm by frequency doubling of Nd:YVO4 laser.

A stable, 22.9 W, 671 nm single-frequency laser using a type II noncritically phase-matched external-cavity frequency doubling is demonstrated. The output power of the fundamental laser is 32.1 W; the corresponding conversion efficiency of frequency doubling from 1342 to 671 nm is calculated to be 71.3%. The M2 factors are measured to be 1.10 and 1.08 in the x and y directions, respectively. To the best of our knowledge, 22.9 W is the highest power obtained for a 671 nm single-frequency laser.

High-power repetition rate- and pulse width-tunable 589†nm versatile laser for adaptive optical systems.

Compact high-power yellow laser is a critical part for sodium beacon adaptive optical systems. A narrow-linewidth quasi-continuous-wave (QCW) solid-state 589 nm laser with high-power and high beam quality simultaneously is investigated here, operating in hundreds-microsecond pulse duration with a tunable repetition rate of 400 to 1 kHz, which is flexible to allow the telescope to move in observing direction. The laser source is based on employing sum-frequency generation between 1319 and 1064 nm QCW Nd:YAG amplifiers. For a 100 µs pulse duration and 400 Hz repetition rate, the yellow laser provides a highest output power of 86.1 W with beam quality M2 = 1.37. The central wavelength can be precisely tuned to sodium-D2a line at 589.159 nm with a ∼440 MHz linewidth. This is the maximum power-reported for all-solid-state sodium guide star laser demonstrated to date. The result represents a key step toward solving the requirement of multi-conjugate adaptive optics for large adaptive optical telescopes.

Void-free bonding for a large slab laser crystal.

A void-free bonding technique was demonstrated for a large slab Nd: YAG crystal with a bonding surface dimension of ∼160mm×70mm. By using the novel fluxless oxide layer removal technology, the indium-oxide barrier problem was resolved. With the help of electrochemical-polished indium solder and a plasma-cleaned heat sink, the solderability of the indium was enhanced; in particular, the contact angle of the solder was improved from 51° to 31°. With the largest-bonding-size slab, a single-slab laser created a maximum output power of 7.3 kW under an absorbed pump power of 12.8 kW, corresponding to an optical to optical efficiency of 57% and a slope conversion of 67.8%. By detecting the wavefront of the interferometer before and after bonding, the RMS of wavefront was 0.192λ and 0.434λ (λ=633nm), respectively. To the best of our knowledge, this is the largest void-free bonding size for a laser slab and the highest output power achieved from a single-slab crystal laser oscillator.

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-repetition-rate 100 W level sodium beacon laser for a multi-conjugate adaptive optics system.

A 100 W level kilohertz repetition-rate microsecond (µs)-pulse all-solid-state sodium beacon laser at 589 nm is demonstrated for the first time, to the best of our knowledge, via combining two independent µs-pulsed lasers. Each beamlet is generated by the sum-frequency mixing of pulsed 1064 and 1319 nm lasers in a lithium triborate (LBO) crystal, which operate at 500 Hz pulse repetition frequency with 61 W $p$p-polarized and 53 W $s$s-polarized output, respectively. An incoherent sequence combining technology of polarized laser beams is employed to add the two beamlets. The average power of the combined beam is up to 107.5 W with a combining efficiency of 94.3%. The combined beam has a 1 kHz repetition rate with ${\sim}{120}\;\unicode{x00B5} {\rm s}$∼120µs pulse duration and beam quality ${M^2} = {1.41}$M2=1.41. The central wavelength with a linewidth of ${\sim}{0.3}\;{\rm GHz}$∼0.3GHz is locked to a sodium ${{\rm D}_{2a}}$D2a absorption line. To the best of our knowledge, this is a record-high power operating at kilohertz for µs-pulsed solid-state sodium beacon lasers.

Pulse duration adjusted by changing the cavity length with a multi-pass cavity in a Q-switched Nd:YAG laser.

We report a compact, long nanosecond (ns) pulse duration stretched laser source by a multi-pass cavity (MPC). Based on the combination of the MPC and pump power, a high-power high beam quality 1064 nm Q-switched Nd:YAG laser with a pulse duration adjustable over the range of 160-1000 ns was obtained at a pulse repetition frequency of 10 kHz for the first time, to the best of our knowledge. At a typical pulse width of 560 ns, an average output power of 10.6 W was successfully achieved. The beam quality factor M2 was measured to be 1.45 with a good Gaussian mode.

Watt level laser source for a polychromatic laser guide stars: double resonant fluorescence from 3S1/2-3P3/2-3D5/2 transition of sodium atoms.

The polychromatic laser guide star (PLGS) is one of the solutions proposed to measure the differential atmospheric tip-tilt. A watts-level microsecond pulse all solid state laser source with two wavelengths at 589 and 819.7 nm are developed to perform a proof-of-concept on-sky test for what is believed to be the first time. By sum-frequency of 1319 and 1064 nm, a 44 W maximum average output power at 589.159 nm is generated with the pulse width of ~90 µs at 500 Hz, the linewidth of 0.46 pm, and the beam quality of M2 = 1.50. Meanwhile, a 2.4 W average output power is achieved operating at 819.710 nm with the pulse width of ~25 µs at 500 Hz, the linewidth of 0.8 pm, and beam quality factor of M2 = 1.20, which is end-pumped by a frequency-doubled 1064 nm Nd:YAG laser. Moreover, double resonant fluorescence in sodium cell with two step excitation of sodium atom from 3S1/2 to 3D5/2 via 3P3/2 level is observed clearly by tuning the wavelength of 589 and 819.7 nm beams. In the proof-of-principle experiment, it is preliminarily verified that this laser system is expected to be applied to the sky experiment.

Picosecond slab regenerative amplifier using a large fundamental mode stable-unstable hybrid cavity.

Slab gain media with large aspect ratios were difficult to be adopted in ultrafast regenerative amplifiers (RAs) due to the obstacle of mode matching with the seed beam. We proposed that an unstable cavity could be employed to solve this difficulty by taking the advantage of its large fundamental mode volume. In this way, an Nd:YVO4 slab-based picosecond RA has been successfully demonstrated using a stable-unstable hybrid cavity. The maximum average output power of 10.5 W was achieved at the repetition rate of 10 kHz. The beam quality factor M2 was measured to be 1.54 in the stable direction and 2.26 in the unstable direction.