Compensation for the thermal effect in the second-harmonic generation of a Q-switched nanosecond-kilohertz Nd:YVO4 laser

We report the quantitative relationship between the phase-matching condition and the surrounding temperature in a critical phase-matching second-harmonic generation of a Q-switched Nd:YVO₄ laser with a high repetition rate. The thermal effect can be effectively compensated for by appropriately changing the phase-matching angle with respect to the surrounding temperature of a nonlinear optical crystal. A stable wavelength conversion from the 1064 nm fundamental to the 532 nm second harmonic in a type-I critical phase-matching lithium triborate crystal is experimentally achieved and the corresponding conversion efficiency up to 70% is obtained.     

Continuous-wave high-power Nd:YAG-KNbO3 laser at 473 nm

A continuous-wave high-power Nd:YAG laser operating on the ⁴F_3/2→⁴I_9/2 transition at 946 nm and intracavity frequency-doubled to 473 nm by a KNbO₃ nonlinear crystal at room temperature is reported. The Nd:YAG laser outputs a randomly polarized beam of 3.8 W maximum power (38% optical-to-optical efficiency and 44% slope efficiency with respect to the absorbed pump power) at the 946 nm fundamental wavelength. Intracavity frequency-doubling with a 2.0-mm thick KNbO₃ crystal in a linear resonator yielded 159-mW single-ended blue-output with 4.8% optical-to-optical conversion efficiency versus the absorbed pump power. The 473-nm maximum power of 418 mW with 11.6% optical-to-optical conversion efficiency in absorbed power was obtained from a V-type resonator; the overall optical-to-optical conversion efficiency was 6.7%, while the conversion of the available infrared power reached 50%.     

High power continues wave laser diode array end pumped Nd:YAG/LBO red lasers operated at 669 nm and 659 nm

In this paper, we present using Nd:YAG as the laser media, and Brewster plate as a polarizer device to obtain the linear polarization output at the fundamental wavelength. Using LBO (LiB₃O₅) crystal as non-linear crystal to obtain intra-cavity frequency doubled red laser at 659 and 669 nm and sum frequency mixing at 665 nm. Using a thin fused silica plate as an etalon to obtain the single wavelength operation. At total pump power of 30 W at 808 nm, the output powers at two wavelengths are about 4.1 and 4.5 W, respectively, only one fundamental wavelength light is resonating. The corresponding optical to optical conversion efficiency from pump light to second harmonic generation are 13.7 and 15.0%, respectively.     

准晶非线性光子晶体中二次谐波波长和温度调谐的研究

在八重准周期极化的铌酸锂非线性光子晶体中,通过调节基频光波长实现了多个波长的同时共线准相位匹配倍频,最高转换效率达36%.同时,测量了准晶非线性光子晶体中二次谐波转换效率随晶体温度以及入射波长的变化,结果表明二次谐波在长波处具有更宽的温度以及波长调谐带宽.该项研究对于准周期非线性光子晶体在实际工作中的运用具有重要的指导意义. 关键词： 准周期 非线性光子晶体 温度调谐 波长调谐     

Frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals

We report on frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals (LiIO₃). The conversion coefficient and the matching angle were measured as a function of the fundamental wavelength in the range 586.3–624 nm. For Type I phasematching the matching angle _m=90° for noncritical matching was found at 586.3 nm. At this wavelength the conversion coefficient reaches the maximum value which was observed to be as high as 1.6×10^–2 W^–1 for a LiIO₃ crystal of 1 cm length. From the dependence of the matching angle on the wavelength the refractive induces were determined for the ordinary as well as for the extraordinary ray in the wavelength range from 293 nm up to 355 nm. The influence of focussing on the conversion coefficient is discussed.     

400 mW and 16.5% wavelength conversion efficiency at 488 nm using a diode laser and a PPLN crystal in single pass configuration

Continuous wave power of more than 400 mW at 488 nm has been generated by frequency doubling of 2.45 W at 976 nm obtained from a distributed Bragg reflector tapered diode laser. This results in a wavelength conversion efficiency of 16.5% and an electrical-to-optical efficiency of more than 4.5%. We used a 50 mm long periodically poled MgO:LiNbO₃ bulk crystal in single-pass configuration for the second harmonic generation. This is to the author’s knowledge the highest output power and the highest wavelength conversion efficiency at 488 nm generated by a monolithic semiconductor laser device in single pass configuration with a bulk crystal. A deviation from the quadratic dependency of the frequency doubling is explained by the decrease of the beam quality of the fundamental wave.     

Generation of Tunable Amplitude-Squeezed Lights

Frequency doubling of a Ti:sapphire laser with a KNbO₃ crystal has been used to generate wavelength tunable amplitude-squeezed lights in the second harmonic mode. The crystal is placed in an enhancement cavity designed to resonate only the fundamental mode, and the phase matching condition is achieved by changing the crystal temperature. Noise reduction of 1.8–2.0 dB below the shot noise level is observed between 427.28 and 433.76 nm.     

KNbO_3高效倍频产生427～470nm蓝紫光

厚1.5 mm的KNbO_ 3晶体在室温下角度相位匹配倍频脉冲参量激光获得427～470nm蓝紫光,能量转换效率超过40％,输出单脉冲蓝紫光的峰功率达60kW.对相位匹配曲线、倍频效率与基波光强的关系和相应匹配角允差作了计算和讨论.     

Third harmonic generation of CO2 laser radiation in AgGaSe2 crystal

Generation of third harmonic of CO₂ laser radiation has been obtained in a type-II, ϑ=57° cut 9 mm thick AgGaSe₂ crystal for the first time by sum-frequency-mixing of the fundamental with its second harmonic, the latter being obtained using another type-I, ϑ=55° cut 11 mm thick AgGaSe₂ crystal. The energy conversion efficiencies obtained for second harmonic and third harmonic generations are 6.3% and 2.4% respectively with the input fundamental pump power density of 5.9 MW/cm² only. The wavelength of the fundamental CO₂ laser radiation used for the generation of harmonics is 10.6 μm, P(20) line. A compact TEA CO₂ laser source has been built in the laboratory.     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.     

Tunable nonlinear beam shaping by non‐collinear interactions

A method is proposed for nonlinear beam shaping, employing a non‐collinear quasi phase‐matched interaction in a crystal whose nonlinear coefficient is encoded by a computer generated hologram pattern. In this method the same axis is used for both satisfying the phase‐matching requirements and encoding the holographic information, the result is a single shaped beam in the generated frequency. This allows to shape beams in one‐dimension using a very simple method to fabricate patterned nonlinear crystals and to shape beams in two‐dimensions with high conversion efficiency. The one‐dimensional case is experimentally demonstrated by converting a fundamental Gaussian beam into Hermite‐Gaussian beams at the second harmonic in a KTiOPO₄ crystal. The two‐dimensional case is demonstrated by generating Hermite‐Gaussian and Laguerre‐Gaussian beams in a stoichiometric lithium tantalate crystal. The suggested scheme enables broad wavelength tuning by simply tilting the crystal.     

Non-critical phase-matched second harmonic generation in Gd<Subscript>x</Subscript>Y<Subscript>1-x</Subscript>COB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut Gd_xY_1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd_0.28Y_0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser. PACS 42.25.Lc; 42.65.Ky; 42.70.Mp; 42.79.Nv     

啁啾补偿的折返点匹配二倍频

提出了一种啁啾补偿群速色散的倍频方法,通过向入射基频光引入合适的初始啁啾,让其与色散相互作用,以实现对基频光脉冲宽度的主动控制,提高转换效率。研究结果表明,这种方法能显著地提高倍频转换效率,以氘含量12.6%(摩尔百分比)的KD*P晶体对脉宽30fs,中心波长为1.053μm的宽带基频光的折返点匹配宽带二倍频过程为例,当基频光转换极限位置处于晶体中心时,能取得最佳的啁啾补偿效果,转换效率可提高近22%。进一步研究了转换效率和转换带宽与晶体长度的关系。     

Highly efficient wavelength-tunable anti-Stokes signal conversion by phase-matched four-wave mixing in the second-order mode of photonic crystal fiber

Using the photonic crystal fiber (PCF) with zero dispersion wavelengths of the fundamental mode and the second-order mode at 985 nm and 885 nm designed and fabricated in our lab, the anti-Stokes signals from 586.5 to 558 nm are efficiently generated in the second-order mode. When the pump working wavelength λ ₀ increases from 830 to 880 nm and the input average power P _in reduces from 43 to 25 mW, the output power of anti-Stokes signal increases 1.76 times, the power ratio of anti-Stokes signal at 558 nm to the residual pump component at 880 nm is estimated as 5:1, and the maximal conversion efficiency P _as/P _p0 can be up to 36%. The possible reasons for the difference from theoretical results are discussed. The combined effects of the interval between the pump working wavelength and zero dispersion wavelength and the input power on the signal conversion process are analyzed.     

High efficiency second-harmonic generation in KNbO3 crystals

Second-harmonic generation in KNbO₃ crystals has been studied with CW and pulsed Al_xGa_1?xAs and dye lasers. High conversion efficiencies were obtained by noncritical Type I phase-matching at room temperature for the wavelength λ_PM = (860±2) nm using the nonlinear optical coefficient d₃₂ = 21 pm/V. The measured peak power was P_2ω=13.6 μW for a fundamental power P_ω=21 mW with a crystal of l = 8.97 mm in length. The phase-matching wavelength in the orthorhombic phase of KNbO₃ can be temperature tuned between 838 nm (for T = -36°C) and 950 nm (for T = +180°C).     

Noncritically phase-matched fourth harmonic generation of Nd:glass lasers in partially deuterated KDP crystals

Noncritically phase-matched (NCPM) fourth harmonic generation (FHG) of Nd:glass laser radiation in partially deuterated dihydrogen phosphate (KD*P) crystals has been demonstrated. At an Nd:glass laser wavelength of 1053.0 nm, NCPM FHG is achieved in 70% deuterated KD*P at a crystal temperature of 18.5±0.1 °C. Tuning the fundamental laser wavelength from 1052.9 to 1053.2 nm, FHG in KD*P is NCPM by changing the crystal temperature from 17.9 °C to 20.5 °C. When driven with 2.4 J of second harmonic radiation in a 3 ns flat-top pulse, corresponding to 1 GW/cm(2) 2ω drive intensity, 1.9 J of fourth harmonic radiation was generated in a 6 mm long KD*P crystal, yielding a second to fourth harmonic energy conversion efficiency of 79%.     

液晶填充碲酸盐光子晶体光纤偏振旋转器

提出一种液晶填充碲酸盐玻璃的柚子型光子晶体光纤偏振旋转器,利用全矢量有限元法,对液晶填充碲酸盐玻璃的柚子型光子晶体光纤的偏振旋转特性进行数值模拟,并分析了光纤结构参数、环境温度、工作波长等对光纤偏振旋转特性的影响.研究结果表明:此种偏振旋转器具有较高的旋转效率、较低的工作串扰和较短的旋转长度,在工作波长为1.55μm、偏振角度为45°时,其值分别达到99.947%、-32.84dB和197μm;另外,随着光纤薄壁厚度的增加,旋转长度随之升高,随着工作波长的变大,旋转长度随之降低,随着温度的增加,旋转长度随之升高.这种新型的光子晶体光纤为偏振旋转器的研发提供了参考.     

Efficient cw violet-light generation in a ring cavity with a periodically poled KTP

In this work, we experimentally demonstrate an efficient cw second harmonic generation (SHG) at 780 nm wavelength with a first-order type-I phase matching periodically poled KTP (PPKTP) crystal in a ring cavity, the wavelength corresponds to the D₂ line of Rb atom transition. The fundamental laser used is a grating-stabilized external cavity diode laser and its frequency is precisely locked to Rb atom transition frequency using the saturated absorption technique. About maximal 6.9 mW UV radiation of 390 nm with a net conversion of 9.5% at an input mode-matched power of 73 mW is generated with one crystal, and about maximal 8.8 mW with the net conversion of 12% is obtained with another crystal; the powers in stable operation are about 1.7 mW and 3.4 mW, respectively. This is, to our best knowledge, the first SHG experiment at 780 nm wavelength with the PPKTP in a ring cavity.     

Broadband frequency conversion of laser radiation in ZnGeP2 crystal

The phase matching conditions in the ZnGeP₂ crystal are calculated and analyzed from the viewpoint of broadband frequency conversion of mid-IR radiation. The frequency conversion of Cr:ZnS laser radiation in the ZnGeP₂ crystal is calculated. The results show the possibility of developing a broadband laser source whose spectrum spans a wavelength range of 3.2–9.5 µm.     

四倍频磷酸二氘钾晶体

生长了不同氘含量的磷酸二氘钾(DKDP)晶体,切割角度统一沿Ⅰ类非临界相位匹配方向,即与晶体z轴成90与x轴成45,分别在1 064 nm和1 053 nm两种基频波长下进行了四倍频实验,通过测定氘含量与相位匹配角的联系,确定出能够实现非临界四倍频的DKDP晶体的最佳氘含量。实验发现在1 064 nm的基频波长下通过调节DKDP晶体的氘含量无法实现室温的非临界相位匹配,而在1 053 nm基频波长下实现室温的非临界相位匹配的DKDP晶体最佳氘含量为85%左右