基于KTP键合晶体的Hansch-Couillaud双波长外腔频率锁定机理

基于KTP键合晶体采用Hansch-Couillaud频率锁定技术实现了双波长外腔同时共振,理论和实验上分别研究了基于键合KTP晶体的HC频率锁定方案. 研究表明,与采用单KTP晶体的结果相比,采用键合KTP晶体进行HC锁频时,能将激光频率分别锁定到e₁光或e₂光的共振峰值. 实验中将环形腔腔模频率锁定到938nm激光器的输出频率上,1583nm激光器的输出频率锁定到环形腔腔模频率上,从而实现了三者之间的相位关联锁定. 关键词： 键合KTP晶体 Hansch-Couillaud锁频 双波长外腔共振     

Doubly Q-switched intracavity-frequency-doubled c-cut Nd:GdVO4/KTP green laser with AO and GaAs

By simultaneously using both active and passive Q-switches in the same cavity, a diode-pumped doubly Q-switched intracavity-frequency-doubled c-cut Nd:GdVO₄/KTP green laser with acoustic-optic (AO) modulator and GaAs semiconductor saturable absorber is realized. A comparison between c-cut and a-cut Nd:GdVO₄ crystals shows that the doubly Q-switched c-cut Nd:GdVO₄/KTP green laser can generate narrower pulse and higher peak power when the incident pump power is higher than 4.4 W. In addition, the doubly Q-switched c-cut Nd:GdVO₄/KTP green laser can generate more symmetric and shorter pulse in comparison with singly AO- or GaAs-Q-switched laser. The coupled rate equations are used to simulate the process of these lasers.     

New approaches for the fabrication of photonic structures of nonlinear optical materials

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO₄ crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO₄ crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO₄ (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented.     

Pyroelectric properties of high-resistant KTiOPO4 crystals in the temperature range 4.2–300 K

The results of measuring the pyroelectric coefficient γ _s ^σ of nominally perfect KTiOPO₄ (KTP) crystals grown from solution in a melt with a potassium to phosphorus ratio of ～2 are presented. The γ _s ^σ (T) dependence is monotonic in the range from 4.2 to 250 K. Deviations from a linear dependence are observed beginning from 250 K, which is considered to be due to interstitial-potassium transport in the KTP crystal field. The spontaneous polarization of unclamped KTP samples is estimated from the results of the measurements. In terms of the crystal-physics approach, it is shown that the main contribution to a polar state of KTP is made by the dipole moments of two nonequivalent mesoscopic tetrahedra forming two sublattices that are polarized in opposite directions and bound by Ti(1) ions.     

Theoretical and experimental study of a diode-pumped doubly Q-switched intracavity-frequency-doubling Nd:LuVO4/KTP green laser with acoustic-optic (AO) modulator and GaAs saturable absorber

By considering the Gaussian transversal and longitudinal distributions of the intracavity photon density as well as the walk-off effect of KTP crystal, the coupled rate equations of the doubly Q-switched intracavity-frequency-doubling Nd:LuVO₄/KTP green laser with acoustic-optic (AO) modulator and GaAs saturable absorber are given. These equations are solved numerically and the doubly Q-switched green laser characteristics, such as the pulse width and the pulse symmetry, have been obtained. In the experiment, a diode-pumped doubly Q-switched intracavity-frequency-doubling Nd:LuVO₄/KTP green laser with AO and GaAs is presented. This doubly Q-switched green laser can generate the almost absolutely symmetric pulse profile with the shorter pulse width and the higher power than the singly Q-switched green laser. The experimental results are in agreement with the theoretically numerical calculations.     

Comparative analysis and identification of low-symmetry paramagnetic centers: Cr3+ in KTiOPO4

A full comparative analysis of the data sets for low-symmetry Cr³⁺ centers in KTiOPO₄ (KTP) crystals reported by various authors is carried out using our recently developed computer program COMPARE. The novel method introduces the quantitative parametersS _A andS _D to describe the closeness of the orientation of any two-axis systems and the principal values of any two tensors, respectively. The method helps to identify the pairs of data sets, comprising the principal values as well as the orientations of the principal axes of the second-rank spin Hamiltonian terms, which may likely belong to the same physical center but have been ascribed to distinct centers. This paper provides a comprehensive analysis of the available EPR data for various Cr³⁺ centers in KTP. Our inter-comparison and/or intra-comparison of various data sets on Cr³⁺:KTP system reveals several inconsistencies in the previous results, including some ill-defined data sets. The numerical results do not confirm fully the equivalencies/correlation assumed by other authors.     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG1.0642μm及Nd:YA1O₃1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb⁵⁺的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb⁵⁺的引入使Nd:YAG1.0642μm激光和Nd:YA1O₃倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ=88.32°、Φ=0°,非常接近于90°非临界相位匹配方向.     

LD-pumped actively Q-switched Nd:GdVO4/KTP red laser

LD-pumped actively Q-switched intracavity frequency-doubled Nd:GdVO₄/KTP red laser has been introduced. A maximum red average output power of 0.78 W at 671 nm was obtained at pulse-repetition frequency (PRF) of 15 kHz and the optical conversion efficiency was 6.5%. At the incident pump power of 12 W, the shortest laser pulse occurred at PRF of 15 kHz. Its full-width at half-maximum and the highest peak power were measured to be 35.8 ns and 3.38 kW. The largest single pulse energy of 56.3 μJ was achieved at PRF of 10 kHz. The influence factors on the Q-switched Nd:GdVO₄/KTP red laser have been discussed.     

Continuous-wave Nd:YVO4/KTiOPO4 green laser at 542 nm under diode pumping into the emitting level

We report a green laser at 542 nm generation by intracavity frequency doubling of a continuous wave (CW) laser operation of a 1086 nm Nd:YVO₄ laser under 880 nm diode pumping into the emitting level ⁴ F _3/2. A KTiOPO₄ (KTP) crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation of the laser. At an incident pump power of 14.5 W, as high as 1.33 W of CW output power at 542 nm is achieved. The optical-to-optical conversion efficiency is up to 9.2%, and the fluctuation of the green output power was better than 3.8% in the given 30 min.     

KTP(001)晶体分光性能研究

根据同步辐射光源对软X射线分光晶体的性能要求,分析了软X射线能区常用分光晶体的性能优劣,指出对于晶格常数值大的分光晶体,KTP(KTiOPO₄)(011)是该能区比较理想的分光晶体. 同时提出了一种利用同步辐射光源测量晶体衍射效率的实验方法,指出光源的发散度与晶体的衍射效率密切相关. 测量了KTP(011)晶体的晶格常数,给出了KTP(011)晶体的实测衍射效率. 关键词： 同步辐射 KTP(011)晶体 衍射效率 光源发散度     

Efficient Nd:YAG pumped mid-IR laser based on cascaded KTP and ZGP optical parametric oscillators and a ZGP parametric amplifier

Efficient conversion into the mid-IR of a low pulse-energy (2.5 mJ) Nd:YAG laser is achieved by cascaded KTiOPO₄ (KTP) and ZnGeP₂ (ZGP) optical parametric oscillators followed by a ZGP optical parametric amplifier. The first stage 2.13 μm degenerate KTP OPO uses four KTP crystals in a walk-off compensated geometry and an elliptical pump beam focal geometry to produce up to 2.2 W from 6.3 W incident. The 2.13 μm e-ray pumps a Type-I ZGP OPO, which produces 0.5 W of light in the 3.8-4.8 μm spectral region that in turn is amplified by a 2.13 μm o-ray pumped optical parametric amplifier generating 0.84 W with an M² of <2.     

Laser-diode end-pumped intracavity frequency doubling semiconductor saturable absorber mirror passively mode-locked picosecond lasers

A diode pumped Nd:YVO₄ semiconductor saturable absorber mirror (SESAM) passive mode-locked intracavity frequency doubled laser was studied. A type II phase matching KTiOPO₄(KTP) crystal and a type I phase matching LiB₃O₅ (LBO) frequency doubling crystal were respectively inserted in the cavity. With a pump power of 4.5 W, a 90 mW output of frequency doubled beam and 29.7 ps pulse duration were achieved with frequency doubled by KTP. With the same pump power a 140 mW output of frequency doubled beam and 3.67 ps pulse duration were achieved with LBO.     

Increased temperature bandwidth of second harmonic generator using two KTiOPO4 crystals cut at different angles

We report a second harmonic generator (SHG), whose temperature full-width of half-maximum (FWHM) bandwidth is significantly increased by replacing a single long type-II phase-matched KTiOPO₄ (KTP) crystal with two shorter crystals, which are cut at slightly different phase-matching angles. The total length of the two crystals is the same as that of the longer ones. The experimental results show that the measured temperature FWHM bandwidth of the SHG is significantly increased from 11.8 °C in the single 7 × 7 × 10 mm³ KTP crystal to 60.2 °C when the single KTP is replaced with two of 7 × 7 × 5 mm³ KTP crystals whose phase-matching angle are 1.0° apart. Such a SHG allows stable output when it is operated in the environment of very rough temperature condition.     

Second Stokes 1129 nm generation in gray-trace resistance KTP intracavity driven by a diode-pumped Q-switched Nd:YVO4 laser

A gray-trace resistance KTP (GTR-KTP) second Stokes Raman laser intracavity driven by a diode-pumped acousto-optic Q-switched Nd:YVO₄ laser was first demonstrated in this paper. With an incident pump power of 9.5 W, the intracavity GTR-KTP Raman laser, operating at the repetition rate of 20 kHz, produced the maximum average output power of 860 mW at 1129 nm. The minimum pulse width obtained in this GTR-KTP Raman laser was 10.8 ns. When the GTR-KTP was substituted with a common KTP, a lower average output power of 720 mW and longer pulse width of 15.9 ns were obtained in the common KTP Raman laser under the same pump condition and cavity setup as the GTR-KTP Raman laser. Experimental results indicated that the decreased absorption at the fundamental and Stokes wave in GTR-KTP was beneficial to improve the stimulated Raman scattering performance.     

Second-harmonic power of KTiOPO4 with double refraction

Second harmonic (SH) power of the type-II phase matching in KTiOPO₄ (KTP) is calculated for the fundamental beams of TEM₀₀ mode within the near field approximation (parallel Gaussian beam) taking the beam walk-off effect among the two fundamental and the SH beams into account. The slit intensity and the total SH power are calculated as functions of the beam radius and the crystal length. Two approximations are made for the SH power calculation at tight beam focusing condition. The measurements of the SH power in KTP are carried out for three crystals of 6, 3, and 1.5 mm thickness. The effective nonlinear coefficient for Nd: YAG incident radiation is estimated to be 4.3 pm/V with 10% uncertainty, taking the enhancement factor due to the multimode structure of the laser beam into account.     

Numerical solutions of diode-pumped dual-loss-modulated Q-switched and mode-locked green laser with acousto-optic modulator and Cr: YAG saturable absorber

By considering the influence of the acousto-optic (AO) modulator and the nonlinear loss due to harmonic conversion in the intra-cavity frequency-doubling laser, a developed rate equation model for diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) Nd:LuVO₄/KTP green laser with acousto-optic (AO) modulator and Cr⁴⁺:YAG saturable absorber is presented. With this developed model, the pulse width, single-pulse energy and the other dual-loss-modulated QML green laser characteristics are numerically simulated. The simulation results show that the turnoff time is of great importance for the pulse energy and pulse width of the Q-switched pulse. A diode-pumped dual-loss-modulated QML Nd:LuVO₄/KTP laser is constructed to successfully demonstrate the numerical simulation application.     

Emission properties of laser ablation of SnO2: Sb transparent conducting film and KTiOPO4 crystal

Optical emission spectra of Nd:YAG laser ablation of KTiOPO₄ (KTP) crystal and SnO₂:Sb transparent conducting thin film were recorded and analyzed in vacuum and in air. The integral intensities of spectral lines from laser-ablated KTP crystal were obtained as functions of distance from the target surface and laser power density in vacuum and in air. The ambient gas effects on pulsed laser ablation of target were discussed. We also performed laser ablation of SnO₂:Sb transparent conducting thin film in air and the electron temperature and full-width at half-maximum (FWHM) of atomic and ionic spectral lines in the plasma were quantified using Boltzmann plot method and Lorentzian fit, respectively. Integral intensities of atomic and ionic Sn spectral lines were also obtained as functions of distance from the target surface and laser irradiance. The intensity ratio of ionic and atomic Sn spectral lines as a function of laser power density was got which gives some information about the variation of ionization ratio with laser irradiance in the plasma produced by high-power laser.     

熔盐生长KTiOPO4晶体的层绵成因与克服方法

本文用光学显微术观察了在多磷酸钾盐熔盐中生长的KTiOPO₄(KTP)晶体的表面形态和内部疵病,并借助扫描电子显微镜和能谱分析得到了横贯层绵结构的体过饱和度变化规律。基于上述结果,阐明了在不适当的生长条件下,由于晶面上熔质分布高度不均匀而形成宏观包藏——层绵的原因与过程。当熔液浓度位于稳定生长所需的临界过饱和度区域内,即各处σ均大于该区的下限(～1.3)时,重复长出了无层绵透明的KTP晶体。 关键词：     

New hole configurations in X-irradiated KTiOPO4 Crystals

Electron paramagnetic resonance has been used to study the structure and thermostability of oxygen hole centers produced in KTiOPO₄ (KTP) crystals X-irradiated at 77 K. During the annealing of KTP crystals above 160 K the redistribution of charges took place. Four hole centers were observed at 40 K after the heating of the X-irradiated KTP crystal at different temperatures. The intensity of the hole center 1 decreased and new hole configurations (center 2 and center 3) appeared in the crystals. Theg-matrices were obtained from the angular dependencies of EPR spectra. The principalg-values for center 1 were 2.0040, 2.0209, and 2.0437. The principalg-values for centers 2 and 3 were 2.0053, 2.0204, 2.0431 and 2.0035, 2.0183, 2.0628, respectively. The transformation of Ti³⁺ spectra indicated that the trivalent titanium ions were involved into the recombination process. Subsequent annealing at temperatures above 220 K led to the formation of a new hole center 4 (g _a =2.0213,g _b =2.0236,g _c =2.0370).     

High-power diode-pumped nonlinear mirror mode-locked Nd:YVO4 laser with periodically-poled KTP

We demonstrate a high-power nonlinear mirror (NLM) mode-locked Nd:YVO₄ laser with a periodically poled KTP (PPKTP). With a 10-mm-long PPKTP crystal, 5.6 W of average power with 20-ps of pulse duration was generated at 18-W of pump power. Compared with conventional type-II KTP crystal with the same length, the stability against the Q-switched mode-locking (QML) is significantly increased with PPKTP in NLM laser; and the pulse duration was also considerably reduced. Received: 21 July 2000 / Revised version: 30 August 2000 / Published online: 10 January 2001