Characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We present results of the characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO₄)₂ using a z-scan technique over the 800–1600 nm wavelength range. Based on our experimental and theoretical results we conclude that Yb:KGW crystal is a good candidate for efficient Kerr-lens mode locking. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

300 kHz femtosecond Yb:KGW regenerative amplifier using an acousto–optic Q-switch

We report a diode-pumped high-repetition-rate chirped-pulse Yb:KGW regenerative amplifier delivering tens of microjoules pulse energy at repetition rates of up to 300 kHz. High repetition rates are obtained by using an acousto-optic modulator as switching element in the regenerative amplifier cavity. The output pulses were compressed down to 600 fs pulse duration. Even shorter pulses in the 100 fs range are obtained using an amplifier configuration without chirped pulse amplification. PACS 42.55.Xi; 42.60.Da; 42.65 Re     

Laser performance and thermal lensing in high-power diode-pumped Yb:KGW with athermal orientation

A comparative, experimental study of the high-power diode-pumped laser performance and thermal lensing properties between standard b-cut Yb:KGW and Yb:KGW cut along an athermal direction is presented. The results show that thermal lens properties in both the b-cut and the athermal direction-cut crystals are determined by anisotropic thermal expansion in Yb:KGW. Thermal gradients due to the pump beam cause thermal lensing even in the athermal direction-cut geometry. The thermal lens is much weaker and less astigmatic in the athermal direction-cut crystal, for the same absorbed power. These properties allow generation of better-quality laser beams with the athermal direction-cut crystal as compared to the b-cut crystal. PACS 42.55.Xi; 42.60.Jf; 42.70.Hj     

Transient grating measurements of refractive-index changes in intensively pumped Yb-doped laser crystals

Mechanisms of transient light-induced refractive-index changes in Yb-doped crystals Yb:KYW, Yb:KGW and Yb:YVO₄ were studied. Relative amplitudes and lifetimes of the electronic component of phase gratings caused by the polarizability difference of the excited and unexcited Yb³⁺ ions and the thermal component arising from thermalization of the pump energy were measured. Polarizability difference values of the ground ² F _7/2 and the excited metastable ² F _5/2 levels at the testing wavelength of 633 nm were determined for all crystals. The thermal diffusivity of the samples was estimated for selected directions of the thermal gradient. An athermal direction, along which the thermal component of refraction disappears, was found in Yb:KYW crystal under specific experimental conditions. PACS 42.70.Hj; 42.65.An; 66.30.Xj     

Stable, efficient diode-pumped femtosecond Yb:KGW laser through optimization of energy density on SESAM

An efficient high-power diode-pumped femtosecond Yb:KGW laser is repored. Through optimization of energy density by semiconductor saturable absorber mirror, output power achieved 2.4 W with pulse duration of 350 fs and repetition rate of 53 MHz at a pump power of 12.5 W, corresponding to an optical-to-optical efficiency of 19.2%. We believe that it is the highest optical-to-optical efficiency for single-diode-pumped bulk Yb:KGW femtosecond lasers to date.     

Polarization-tunable Yb:KGW laser based on internal conical refraction

Using internal conical refraction, we demonstrate an Yb:KGW laser where the polarization state can be arbitrarily altered without any additional cavity components. The extinction ratio can also be altered between 1:1 and 40:1. The maximum output power achieved was 8.6 W at a slope efficiency of 60.5% with respect to incident pump power. This equals the power performance of a standard Yb:KGW laser used for reference.     

Yellow nanosecond sum-frequency generating optical parametric oscillator using periodically poled LiNbO<Subscript>3</Subscript>

Nanosecond yellow light has been generated through simultaneously phase matched sum-frequency generation and optical parametric oscillation in a periodically poled LiNbO₃ crystal. 300 mW of yellow light at a wavelength of 586 nm has been generated from 1.3 W of laser power from a Q-switched Yb:YAG laser operating at 1031 nm. The conversion efficiency of the device is 23%. PACS 42.65.Ky; 42.65.Yj     

Diffraction grating fabrication in lithium niobate and KDP crystals with femtosecond laser pulses

We report studies of the modifications induced inside lithium niobate and KDP crystals by Yb:KGW ultrafast laser pulses, having 300-fs pulse duration and operating at 100-kHz repetition rate. By focusing the laser beam with a 0.42 numerical-aperture objective, we have recorded homogeneous volume Bragg gratings in the bulk of the niobate crystal that showed excellent diffraction efficiencies, reaching up to 87%, and remained permanent after thermal annealing for one hour at 150°C. The refractive index modification level was found to be 0.002 in lithium niobate. The results show that lithium niobate is a very promising crystal candidate for microphotonics applications. On the contrary, in the KDP crystal no smoothly modified refractive index zones were created.     

Spectroscopy and femtosecond laser performance of Yb3+ :Gd0.64Y0.36VO4 crystal

We report for the first time on the efficient continuous-wave and mode-locked laser operation of a diode-pumped Yb:Gd_0.64Y_0.36VO₄ laser. cw output power of 855 mW with a slope efficiency of 45% with respect to the absorbed pump power was demonstrated for the 2.1%-doped crystal. In mode-locked regime with SESAM as passive shutter pulses as short as 100 fs were obtained. PACS 42.55.Xi; 42.65.Re; 42.70.Hj     

Nd:KGW/KGW crystal: efficient medium for continuous-wave intracavity Raman generation

Diode pumped continuous-wave intracavity Raman conversion in a composite Nd:KGW/KGW crystal has been investigated. The use of a Nd:KGW/KGW crystal instead of the ordinary Nd:KGW crystal has made it possible to considerably decrease intracavity losses. A Raman threshold corresponding to a diode laser power of 230 mW and a quantum efficiency of conversion of diode laser radiation to the Stokes radiation of 22% have been obtained. PACS 42.55.Rz; 42.55.Xi; 42.55.Ye     

Second Harmonic Generation in Microstructured Barium Titanate

We investigate the second harmonic generation under femtosecond pulse-periodic laser radiation in barium titanate in the form of ceramics, in pores of a globular photonic crystal, and in a water colloidal suspension. We measure the dependence of the second harmonic radiation intensity on the incident laser power. Excitation of the second harmonic was carried out by powerful (10⁸ W) pulses of a solid-state Yb:KGW laser (wavelength, 1,026 nm) operating at 200 kHz. We estimate the efficiency of the second harmonic generation in various microstructured phases of barium titanate and show that the threshold of plasma formation in a suspension of barium titanate microparticles in water is substantially higher than in ceramics and in the ferroelectric photonic crystal. The second-harmonicgeneration power can be significantly increased in a water suspension of barium titanate microparticles.     

7.6-W diode-pumped femtosecond Yb:KGW laser

We have demonstrated a high power diode-pumped mode-locked femtosecond Yb:KGW laser with semiconductor saturable absorber mirror(SESAM). By using an output coupler with 10% transmittance, the laser delivered 160-fs pulses with average output power of 7.6 W at a repetition rate of 78 MHz, corresponding to pulse energy of 97 nJ and peak power of 606 kW.     

Er3+/Yb3+:KG(WO4)2的熔盐提拉法生长及光谱性能

采用熔盐提拉法生长了尺寸达30 mm×25 mm×15 mm的Er3+/yb3+KGd(WO4)2透明晶体,从中切出5 mm×5 mm×3mm大小的激光器件,在室温下测试了吸收光谱,可以观察到晶体存在366,380,489,522,651,811,980 nm等吸收峰,并对之进行了能级归属.同时在980 nm激光激发下测试了荧光光谱,可以发现在1 024,1 535 nm处存在较宽的强发射峰,其半高宽分别为60和36 nm.研究结果表明,Er3+/yb3+KG(WO4)2适合于InGaAs LD(980 nm)泵浦,有可能产生1 024 nm波段及人眼安全的1 535 nm波段的可调谐激光.     

Development of a highly stable Yb:YAG thin disk pulsed green laser for high power Ti:sapphire based amplifier at 100 kHz repetition rate

We have developed a pulsed Yb:YAG thin disk green laser for pumping a 100 kHz and 10 W femtosecond laser. We optimized numerically and experimentally the open duration of the acousto-optic (AO) Q-switcher and the internal SHG method, generating stable output energy at a 100 kHz repetition rate. We demonstrated a 515 nm output power of over 80 W with a pulse-to-pulse stability of less than 1% RMS with an LD power to green power conversion efficiency of 24%. With the developed green laser, we obtained 16 W of a Ti:sapphire laser in a cavity dumped mode. PACS 42.55.Xi; 42.60.Gd; 42.60.By; 42.65.-k; 42.65.Re     

Twinning and “natural quasi-phase matching” in Yb:YAB

Defects obtained during crystal growth of many laser and nonlinear crystals limit their commercial potential, as is the case of the yttrium aluminium borate family. Typically, in Yb:YAB, birefringent phase-matching is used to obtain self frequency doubled output from a fundamental laser operating near 1 μm. Many crystals however show signs of twin inversion defects during crystal growth resulting in the reversal of the sign of the nonlinear coefficient and hence can result in quasi-phase-matched-like structures. In this report we examine the second harmonic conversion characteristics of a 3.8 mm thick birefringently phase matched Yb:YAB crystal which is then examined in more detail by slicing into four thinner sections each approximately 400 μm thick. Etching of crystal surfaces reveals clear boundaries between reversed domains, which under illumination with a laser at 1.064 μm shows a cancellation of the nonlinear conversion along the interface of the two regions. PACS 42.55.-f; 42.65.Ky; 42.70.Mp     

Laser performance of Yb:GdCa<Subscript>4</Subscript>O(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> compared to Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> under diode-bar pumping

A study of Yb:GdCOB under diode-bar pumping has been performed and the results are compared, experimentally and theoretically, with Yb:KGW in the same cavity. An output power of 7.3 W and a slope efficiency of 57.4% were obtained. Self-frequency-doubling experiments are also discussed.     

Feasibility analysis of radiation balanced laser

An end-pumped slab laser model is analyzed to evaluate the feasibility of radiation balanced laser (RBL) when the laser crystal is chosen to be Yb:KGW. The ratio between fluorescence and stimulated photon number is first introduced to help estimate the output power of RBL. Through the analysis it turned out to be difficult to build high power RBL, but the idea of radiation balance can be used to reduce the waste heat for all laser design.     

Efficient Yb:KGW lasers end-pumped by high-power diode bars

The laser performance of Yb:KGW under end-pumping with high-power diode bars has been investigated. In one configuration, the pump is delivered through an optical fiber yielding a symmetric but unpolarized pump spot. In the second configuration, the pump is delivered through an optical lens system yielding a polarized but asymmetric pump spot. Both configurations resulted in a maximum optical to optical efficiency of 47% and a maximum power of 12.4 W has been generated with the fiber coupled pump source. The output power was limited by crystal fracture, which occurred at an estimated temperature gradient of 1.2×10⁵ K/m. Thermal lensing and laser tuning characteristics are also investigated. PACS 42.55.Xi; 42.60.Lh     

Harmonically pump a femtosecond optical parametric oscillator to 1.13 GHz by a femtosecond 515 nm laser

We demonstrate a harmonically pumped femtosecond optical parametric oscillator(OPO)laser using a frequency-doubled mode-locked Yb:KGW laser at a repetition rate of 75.5 MHz as the pump laser.Based on a bismuth borate nonlinear crystal,repetition rates up to 1.13 GHz are realized,which is 15 times that of the pump laser.The signal wavelength is tunable from 700 nm to 887 nm.The maximum power of the signal is 207 m W at the central wavelength of 750 nm and the shortest pulse duration is 117 fs at 780 nm.The beam quality(M^2 factor)in the horizontal and vertical directions of the output beam are 1.077 and 1.141,respectively.     

Yb∶KGW激光晶体无热方向的计算与分析

针对各向异性Yb∶KGW激光晶体有可能存在热透镜效应为零的传播方向（即无热方向）,利用Biswal等人2篇文献和Pujol等人对Yb∶KGW晶体热膨胀系数的测量结果,分别采用最小二乘法和二阶张量旋转计算出对应晶体的热膨胀张量,据此计算出2篇文献中所述晶体的无热方向。通过对比分析发现：尽管晶体热膨胀张量存在一定的个体差异,但是该晶体普遍只存在2个无热方向,即当光为m偏振时,在p-g平面内与p轴成正负二十多度的2个方向,而不是Biswal等人给出的4个无热方向。另外,利用得到的热膨胀张量计算出某特定方向的热光特性与瑞典的Hellstrom等人报道的激光器热透镜实验结果相一致。