Yong Bo

High-power and high-quality, green-beam generation by employing a thermally near-unstable resonator design

We have obtained green-beam quality of M2 = 6.2 at an average output power of 120 W by intracavity frequency doubling of a diode-side-pumped, Q-switched Nd:YAG rod laser with a repetition rate of 10 kHz and an optical-to-optical conversion efficiency of 15.2%. To achieve high-beam quality at high average power, the laser employs a thermally near-unstable resonator design with two-rod birefringence compensation in an L-shaped flat-flat cavity. The output power fluctuation of the green laser remains less than 0.9% in 4 h.

Efficient improvement of laser beam quality by coherent combining in an improved Michelson cavity

The efficient improvement of laser beam quality is demonstrated by coherent combining of two Nd:YVO4 lasers in an improved Michelson cavity. When the two lasers are separately operated with beam qualities of M(x)2 = 1.74 and M(y)2 = 1.34 for laser 1 and M(x)2 = 1.53 and M(y)2 = 1.39 for laser 2, by coherent combining of the two lasers, a single beam output exceeding 2 W with a nearly perfect TEM00 mode (M(x)2 = 1.08, M(y)2 = 1.04) is obtained. Moreover, it can withstand environmental perturbations with no change in beam quality or output power and has the potential for scaling to much higher output power with high beam quality by coherent addition of multiple lasers in this configuration.

Diode-pumped passively dual-wavelength Q-switched Nd:GYSGG laser using graphene oxide as the saturable absorber.

The performance of a diode end-pumped passively Q-switched dual-wavelength Nd:GYSGG laser operating at 1057.28 and 1060.65 nm with graphene oxide as the saturable absorber was demonstrated. The maximum dual-wavelength average output power of 521 mW was achieved under the absorbed pump power of 5.4 W, corresponding to the optical-to-optical conversion and slope efficiency of 9.8% and 21%, respectively. The minimum pulse width was 115 ns with a pulse repetition rate of 338 kHz.

High-power 880-nm diode-directly-pumped passively mode-locked Nd:YVO 4 laser at 1342 nm with a semiconductor saturable absorber mirror

A high-power 880-nm diode-directly-pumped passively mode-locked 1342 nm Nd:YVO₄ laser was demonstrated with a semiconductor saturable absorber mirror (SESAM). The laser mode radii in the laser crystal and on the SESAM were optimized carefully using the ABCD matrix formalism. An average output power of 2.3 W was obtained with a repetition rate of 76 MHz and a pulse width of 29.2 ps under an absorbed pump power of 12.1 W, corresponding to an optical-optical efficiency of 19.0% and a slope efficiency of 23.9%, respectively.

Stable operation of 4 mW nanoseconds radiation at 177.3 nm by Second Harmonic Generation in KBe 2 BO 3 F 2 Crystals

We report the generation of nanoseconds radiation at 177.3 nm with a maximum average power of 34.7 mW by second harmonic generation (SHG) in a 2.06-mm thick KBe(2)BO(3)F(2) (KBBF) crystal pumped with a homemade 4.2 W nanoseconds Nd: YAG laser at 355 nm operating at 10 KHz and 49 ns, which corresponds to an energy efficiency of approximately 0.826%. To our knowledge, it is the highest power generated at 177.3 nm. The dependence of phase matching angle of KBBF on temperature is presented for the first time. We also present the details on the measures for stable operation of a 4 mW nanosecond output at 177.3 nm with a lower pumping power in a thinner 1.37-mm KBBF crystal, and the stable output power is improved by about 20 times compared with previous results.

Generation of 4.3-W coherent blue light by frequency-tripling of a side-pumped Nd:YAG laser in LBO crystals.

High average power nanosecond pulsed light is obtained using a compact all solid state Nd:YAG laser. The laser is operated in Q-switched mode at the 1.3 micron fundamental and internally frequency tripled using two LiB3O5 (LBO) crystals. 4.3 W average power at 440 nm was demonstrated at 3.5 kHz and a pulse width of 150+/-10 ns (FWHM). The beam quality of M2 value is 5+/-1 in both dimensions. The short-term average power stability of the light source is better than 5.6%. Spectral selecting is proposed to increase production efficiency at 440 nm.

Comparison of laser induced thermal fracture between polycrystalline ceramic and crystal Nd:YAG

Continuous wave 808 nm pump laser-induced thermal damage of polycrystalline transparent ceramic and crystalline Nd:YAG materials was investigated both experimentally and theoretically. The measured temperature agrees well with the theoretical simulation, and the maximum hoop stresses occur on the incident facet of the end-pumped rod at about √2 times of the pump beam radius w0, where the temperature gradient is the highest and the damage occurs first at this location. The fracture-limited laser intensity of ceramics was experimentally measured to be 6.4±0.6  kW/cm2, nearly 64% higher than that of the crystals (3.9±0.3  kW/cm2). The deduced thermal fracture stress for ceramic was 386±50  MPa, which is 64% higher than that of the crystals (235±16  MPa).

Photon Return On-Sky Test of Pulsed Sodium Laser Guide Star with D-2b Repumping

Sodium laser guide star (LGS) system has become one of the critical components in modern astronomical adaptive optics system (AOS), especially for the next-generation extremely large telescopes, such as the Thirty Meter Telescope and the European Extremely Large Telescope. Since the wavefront detection performance of AOS is directly related to the brightness of LGS, it is important for AOS to maximize its photon generation efficiency by all means. Sodium D-2b line repumping is such a technique that can greatly increase the returned photons for either sodium continuous wave (CW) laser or pulsed laser. This technique has been studied theoretically and field tested with a 20 W CW laser by European Southern Observatory team. However, field test results of a 20 W class pulsed laser with D-2b repumping have not been reported yet. In this paper, our latest field test results with theoretical comparison of D-2b repumping with a 20 W quasi-continuous wave (QCW) pulsed laser will be presented. With a linearly polarized beam, approximate 40% photon return enhancement was achieved when 10% of laser power was detuned to D-2b line, which agreed well with results from a rate equation-based Monte Carlo photon return simulation program. Both experiment and simulation results indicate that with a higher laser intensity projected at the sodium layer, the D-2b repumping will be more effective.

Yellow-green 52.3 W laser at 556 nm based on frequency doubling of a diode side-pumped Q-switched Nd:YAG laser

We demonstrate a high-power 556nm yellow-green laser generated by intracavity frequency doubling of a diode side-pumped Nd:YAG laser at 1112nm. A symmetrical L-shaped flat-flat cavity was employed to implement efficient operation of the low-gain 1112nm transition and to achieve good power scalability. The coatings of the cavity mirrors were carefully designed to optimize the performance of the laser, and a 92W continuous wave laser output at 1112nm was achieved when the pumping power of the laser diodes reached 960W. By intracavity frequency doubling of the fundamental laser in a lithium triborate crystal, the maximum power of the frequency-doubled output at 556nm was found to be as high as 52.3W with a pulse repetition frequency of 10kHz. This corresponds to an optical-to-optical conversion efficiency of about 5.4%.

2.14 mW deep-ultraviolet laser at 165 nm by eighth-harmonic generation of a 1319 nm Nd:YAG laser in KBBF

We report a nanosecond (ns) 165 nm deep-ultraviolet (DUV) laser with a maximum average power of 2.14 mW by eighth-harmonic generation (EHG) of a homemade ns 1319 nm Nd:YAG laser. The EHG was performed by cascaded second-harmonic generation (SHG) through successive stages of two LiB3O5 crystals and one KBe2BO3F2 crystal. The generated radiation at 165 nm is the shortest DUV wavelength ever generated through SHG method and the highest average power of all solid state lasers below 170 nm to our knowledge. Such a ns DUV laser at 165 nm is of great interest for photoemission spectroscopy and Raman spectroscopy.