Widely phase-matched tunable difference-frequency generation in periodically poled LiNbO3 crystal

We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO₃ (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 μm was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 μm and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at Λ = 22.5 μm. A large frequency scan of 128 cm⁻¹ (about 78 cm⁻¹ above 1 μW) near 4.2 μm was achieved. The whole absorption spectrum of the P and R branches of the ν₃ band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.     

Widely tunable mid-IR difference-frequency generation based on fiber lasers

A wide tuning technique for mid-IR difference-frequency generation (DFG) with uniform grating periodically poled LiNbO(3) (PPLN) is presented. Based on the dispersion property of the PPLN, the quasi-phase matching (QPM) band for the pump can evolve to two separate bands, and the spacing between them can be increased with the decrease of the crystal temperature. Two such separate QPM bands can be used for increasing the idler tuning range when the crystal temperature is set to adapt the pump tuning. With the technique, an idler tuning range of 690nm is experimentally achieved with fiber laser fundamental lights.     

Review of recent efforts on efficient generation of monochromatic THz pulses based on difference-frequency generation

We review our recent efforts on power scaling of THz pulses generated from several nonlinear-optical crystals. By using a high-resistivity GaP crystal, we have significally increased the output peak power to as high as 722 W. By stacking three GaP wafers, we have further increased the highest output peak power to 2.36 kW. On the other hand, by using CO₂ laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO₃ crystals based on a backward configuration. The highest output power obtained by us so far is 10.7 μW.     

基于GaSe和ZnGeP2晶体差频产生可调谐太赫兹辐射的理论研究

基于非线性光学频率变换理论,采用已报道的利用非线性光学差频方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,计算模拟出在不同相位匹配条件下,GaSe和ZnGeP2晶体差频的相位匹配角、走离角、允许角和有效非线性系数,并对计算结果进行了分析比较,总结出对应输出不同太赫兹波长的最佳相位匹配方式.计算结果为利用非线性晶体差频产生可调谐太赫兹辐射的实验研究提供深入和全面的理论基础.     

基于GaSe和ZnGeP2晶体差频产生可调谐太赫兹辐射的理论研究

基于非线性光学频率变换理论,采用已报道的利用非线性光学差频方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,计算模拟出在不同相位匹配条件下,GaSe和ZnGeP₂晶体差频的相位匹配角、走离角、允许角和有效非线性系数,并对计算结果进行了分析比较,总结出对应输出不同太赫兹波长的最佳相位匹配方式.计算结果为利用非线性晶体差频产生可调谐太赫兹辐射的实验研究提供深入和全面的理论基础. 关键词： 太赫兹波 GaSe晶体 2晶体')" href="#">ZnGeP₂晶体 差频     

AgGaSe_2晶体差频产生可调谐太赫兹波的理论研究

基于单轴晶体中三波互作用的共线相位匹配技术,采用CO2激光器泵浦非线性晶体AgGaSe2,数值计算出在oee类和oeo类相位匹配条件下,差频产生太赫兹波的相位匹配角、走离角、允许角和有效非线性系数。计算结果表明:两类相位匹配太赫兹波可调谐的范围都非常宽,而且走离角都在1°以下,允许角足够大。在绝大部分太赫兹波区域,oee类匹配有效非线性系数远大于oeo类匹配,实验中适合采用oee类相位匹配方式。     

Widely tunable fourth harmonic generation of a Ti:sapphire laser based on RBBF crystal

Fourth harmonic generation of a tunable Ti:sapphire laser with a repetition rate of 80?MHz and pulse duration ??150 fs has been realized in a single pass with an RbBe₂BO₃F₂ crystal for the first time. A high average power was obtained which was tunable throughout a wide range from 180 to 232.5?nm. The output power over the whole range exceeded 2?mW, the highest being 43.3?mW at 202.5?nm. Moreover, deep-ultraviolet output with powers above 75?mW was obtained at 193.5 and 200?nm under a stronger focusing condition. This is the first attempt at the generation of tunable deep-ultraviolet light without using a KBBF crystal, which is very difficult to grow due to its strong layering tendency, and our attempt may provide new opportunities for the development of all-solid-state deep-ultraviolet coherent light sources.     

Broadly tunable single-frequency cw mid-infrared source with milliwatt-level output based on difference-frequency generation in orientation-patterned GaAs

A narrow-linewidth mid-IR source based on difference-frequency generation of an amplified 1.5 microm diode laser and a cw Tm-doped fiber laser in orientation-patterned (OP) GaAs has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 7.6-8.2 microm range with an output power of 0.5 mW. The measured characteristics of the OP-GaAs sample demonstrate a high quality of the material.     

基于双通构型的和频、差频可变波长路由方案及其应用

提出了双通构型的基于“和频”和“差频”级联二阶非线性效应的可变波长路由方案,通过调节两个抽运源的频率,可以从一个WDM(wavelength division multiplexing)链路中取出任意一个信道,然后将其转换为任意一个波长并插入到另一个WDM链路中去.实现了可调谐波长转换和光波长路由的结合.通过小信号分析,给出了相位匹配情况下,输入信号光在波导中功率变化的表达式和输出转换光功率变化的表达式,并给出了提高系统效率的对两个抽运光功率的优化方案.数值计算表明,只要信道间隔大于02nm(25GHz 关键词： 光波长路由 光波长转换 光交换 PPLN     

基于双通构型的和频、差频可变波长路由方案及其应用

提出了双通构型的基于“和频”和“差频”级联二阶非线性效应的可变波长路由方案，通过调节两个抽运源的频率，可以从一个WDM(wavelength division multiplexing)链路中取出任意一个信道，然后将其转换为任意一个波长并插入到另一个WDM链路中去.实现了可调谐波长转换和光波长路由的结合.通过小信号分析，给出了相位匹配情况下，输入信号光在波导中功率变化的表达式和输出转换光功率变化的表达式，并给出了提高系统效率的对两个抽运光功率的优化方案.数值计算表明，只要信道间隔大于02nm(25GHz     

A tunable XUV monochromatic light source based on the time preserving grating selection of high-order harmonic generation

We accomplish a laboratory facility for producing a femtosecond XUV coherent monochromatic radiation with a broad tunable spectral range of 20 eV-75 eV. It is based on spectral selected single-order harmonics from intense laser driven high harmonic generation in gas phase. The time preserving for the selected harmonic radiation is achieved by a Czerny-Turner type monochromator designed with a conical diffraction grating mount for minimizing the time broadening caused by grating diffraction and keeping a relatively high diffraction efficiency. Our measurement shows that the photon flux of the 23-order harmonic(H23) centered at 35.7 eV is 1×10~9 photons/s approximately with a resolving power E/?E ≈ 36.This source provides an ultrashort tunable monochromatic XUV beam for ultrafast studies of electronic and structural dynamics in a large variety of matters.     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

A tunable dual-band THz absorber based on graphene sheet and ribbons

A tunable terahertz dual band absorber consisted of graphene ribbon arrays and a graphene sheet with near-unity absorption is proposed and studied in this letter. With the coupling enhancement of graphene ribbon and graphene sheet, a perfect dual band absorber with absorption over 99% at 3.67 THz and 5.88 THz is achieved. Moreover, the principal of the coupling enhancement is analyzed. By optimizing the parameters of the structure, not only the absorption can be enhanced, but also the center of absorbing frequency could be shifted. Another approach to adjust the absorber–tuning the chemical potential of graphene which is more convenient and timely is investigated. Furthermore, the sensing of the refractive index is emulated by changing the refractive index of the medium on the top. Results show a broad application prospect of the absorber proposed in this paper.     

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%.     

光子学太赫兹源研究进展

为了进一步开发、利用高功率、高效率、可调谐且在室温下稳定运转的太赫兹(THz)源,介绍、分析了国内外利用光子技术产生THz波的研究进展,包括THz气体激光器、空气等离子体THz源、光电导天线以及基于非线性光学效应的光学整流、光学差频、参量振荡等THz源,并指出了光子学THz源未来发展所面临的困难和需要解决的关键技术。     

Widely tunable, narrow-linewidth, subnanosecond pulse generation in an electronically tuned Ti:sapphire laser

Widely tunable subnanosecond Ti:sapphire laser radiation pumped with a cw Q -switched laser-diode-pumped Nd:YAG laser has been demonstrated in a simple laser system with a configuration of variable cavity length. Laser wavelengths can be continuously tuned by adjustment of the rf of an intracavity acousto-optic tunable filter with a computer through the whole range of the laser gain. During tuning of the whole spectral range, there is no need to realign any optics in the laser, except for moving the mirror to track the change of the rf. The peak powers of the output pulses at a pump level of 300 mW are comparable with those of conventional tunable picosecond Ti:sapphire lasers at a cw pump level of almost 10 W.     

Trace-gas sensor based on mid-IR difference-frequency generation in PPLN with saturated output power

The development and characterization of a compact pulsed mid-IR laser source for sensitive on-line trace-gas analyses in the 3–4 μm wavelength range is reported. The source is based on an advantageous difference-frequency mixing configuration in periodically poled LiNbO₃ (PPLN) with a cw external-cavity diode laser (ECDL; 810–830 nm) for broad and accurate tunability and a diode-pumped passively Q-switched Nd:YAG laser (1064 nm) for high mid-IR peak power. With 5 mW cw pump and 4.7 kW signal peak power incident on a 19 mm long PPLN crystal, a maximum of 360 μW mid-IR peak power was generated. The narrowband (∼150 MHz) radiation was saturated by a factor of 15 compared with the nonsaturated case due to depletion of the pump laser radiation. This results in a very high amplitude stability of the generated mid-IR power and thus in a high detection sensitivity. A minimum detectable absorption coefficient of 2.8×10⁻⁸ cm⁻¹ was achieved in combination with a 36.2 m multipass cell in an averaging time of 20 s, as demonstrated by on-line analyses of formaldehyde traces near 3.53 μm.     

Ambient formaldehyde detection with a laser spectrometer based on difference-frequency generation in PPLN

A laser spectrometer based on difference-frequency generation in periodically poled LiNbO₃ (PPLN) has been used to quantify atmospheric formaldehyde with a detection limit of 0.32 parts per billion in a given volume (ppbV) using specifically developed data-processing techniques. With state-of-the-art fiber-coupled diode-laser pump sources at 1083 nm and 1561 nm, difference-frequency radiation has been generated in the 3.53-μm (2832-cm^-1) spectral region. Formaldehyde in ambient air in the 1- to 10-ppb V range has been detected continuously for nine and five days at two separate field sites in the Greater Houston area operated by the Texas Natural Resource Conservation Commission (TNRCC) and the Houston Regional Monitoring Corporation (HRM). The acquired spectroscopic data are compared with results obtained by a well-established wet-chemical o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) technique. Received: 8 November 2000 / Revised version: 30 January 2001 / Published online: 21 March 2001     

A tunable diode-laser spectrometer for the MIR region near 7.2 μm applying difference-frequency generation in AgGaSe2

2 and two diode lasers as pump sources are presented. A single-mode Fabry–Pérot-type tunable diode laser (TDL) and an external-cavity diode laser (ECL) were combined to generate radiation in the mid-infrared region near 7.2 μm. With a TDL at a wavelength of approximately 1290 nm and an ECL emitting between 1504 and 1589 nm it was possible to carry out spectroscopic experiments concerning SO₂ at five different phasematching points between 1350 and 1400 cm^-1 by fixing the wavelength of one pump laser and tuning the wavelength of the other. With an input power of 8 mW for the single-mode Fabry–Pérot-type diode laser and 6 mW for the external-cavity laser an output power of about 10 nW was generated. Using the tuning capabilities of the external-cavity laser a spectral region up to 5 cm^-1 could be covered within one scan. Measurements of SO₂ absorption lines at low pressure demonstrate the high-resolution features of the spectrometer. Moreover, these data provide new direct experimental phasematching data for the rarely investigated spectral region at 7.2 μm. Received: 27 October 1997/Revised version: 8 May 1998