Threshold analysis for a THz-wave parametric oscillator with a quasi-phase-matching scheme in lithium niobate

The analytical expressions of the parametric gain for the terahertz wave (THz-wave) and the Stokes wave of THz-wave parametric oscillator (TPO) with a quasi-phase-matching scheme in periodically poled LiNbO₃ (PPLN) are deduced. The pump threshold expression of the TPO with a quasi-phase-matching scheme in PPLN is derived based on the gain expression of the Stokes wave. The expression is theoretically analyzed to find the relationship between the pump threshold and the parameters of the resonant cavity.     

High-power tunable terahertz generation from a surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals

A high-powered tunable terahertz wave (THz-wave) has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO) pumped by a multi-longitudinal-mode Q-switched Nd:YAG laser. The effective parametric gain length was enlarged by employing two MgO:LiNbO₃ crystals. The tunable THz-wave radiation from 0.8 to 2.8 THz was realized via varying phase-matching angle between the pump wave and the Stokes wave. The maximum THz-wave radiation was 173.9 nJ/pulse at 1.7 THz as the pump energy was 82 mJ, corresponding to an energy conversion efficiency of about 2.12 × 10⁻⁶ and a photon conversion efficiency of about 0.035%. The first-order, the second-order and the third-order Stokes waves were observed during the experiments.     

High-power terahertz radiation from surface-emitted THz-wave parametric oscillator

We report a pulsed surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals pumped by a multi-longitudinal mode Q-switched Nd:YAG laser.Through varying the phase matching angle,the tunable THzwave output from 0.79 THz to 2.84 THz is realized.The maximum THz-wave output was 193.2 nJ/pulse at 1.84 THz as the pump power density was 212.5 MW/cm 2,corresponding to the energy conversion efficiency of 2.42×10-6 and the photon conversion efficiency of about 0.037%.When the pump power density changed from 123 MW/cm 2 to 148 MW/cm 2 and 164 MW/cm 2,the maximum output of the THz-wave moved to the high frequency band.We give a reasonable explanation for this phenomenon.     

小型化外腔可调谐THz参量振荡器

基于LiNbO3晶体垂直表面输出技术,设计了一台小型化外腔THz参量振荡器。利用小型化灯1064 nm脉冲激光器泵浦MgO:LiNbO3,通过优化设计三波非共线相位匹配的光学参量振荡腔结构,实现THz垂直晶体表面输出,减少LiNbO3晶体对THz波的吸收,提高了THz波输出光束质量。当在泵浦光能量为128 mJ、重复频率为10 Hz时,获得THz波的调谐范围为0.69~3.01 THz,在1.6 THz处获得THz波最大平均功率为10.8 W,脉冲宽度为10 ns,对应THz波能量转换效率为8.4310-6。     

Terahertz-wave parametric oscillator with a misalignment-resistant tuning cavity

We demonstrate a terahertz-wave parametric oscillator (TPO) with a corner-cube resonator consisting of a corner-cube prism (CCP) and a flat mirror. By using the cavity configuration proposed in this Letter, the generation of tunable monochromatic terahertz (THz) waves can be achieved just by rotating the flat mirror instead of rotating the TPO cavity relative to the pump beam. The THz-wave output intensity and pulse width can be controlled periodically by rotating the CCP around the cavity axis. The TPO stability against cavity misalignment is significantly improved by at least 1 to 2 orders of magnitude compared with the conventional plane-parallel resonator configuration.     

Widely Frequency-Tunable Terahertz Wave Generation and Spectroscopic Application

The THz-wave generator based on the resonance of phonon-polaritons in GaP enabled THz spectral measurements of organic molecules like DNA/RNA-related molecules in wide frequency range (0.6–5.8 THz). High spectral purity of the generated THz-wave enables even the detection of defect structures of organic compounds, and will give a new tool for molecular sciences. We demonstrated THz imaging of a liver cancer. The THz-wave generator can be made small-size, light-weight system.     

基于钽酸锂晶体的太赫兹波参量振荡器运转特性的研究

基于晶格振动模受激电磁耦子散射过程的基本原理,对由LiTaO₃（LT）晶体组成的太赫兹波参量振荡器（TPO）的输出调谐特性、增益和吸收损耗特性,以及基于硅棱镜阵列耦合装置的THz波输出特性等方面进行了详细的理论研究和分析.研究结果表明,基于LiTaO₃晶体A₁对称性晶格振动的特点以及自身优异的非线性光学特性,通过利用短波长抽运光、适当提高抽运能量以及缩短TPO谐振腔腔长等方法,完全可以实现LT-TPO的高性能运转,证明了LiTaO₃晶体是一种性能优良的TPO工作介质.理论计算结果及方法为以后的LT-TPO实验工作提供了详实的理论依据和实验指导. 关键词： 非线性光学 THz辐射 3晶体')" href="#">LiTaO₃晶体 电磁耦子     

周期结构GaAs晶体ps脉冲差频产生窄带THz辐射的研究

计算模拟了在ps脉冲抽运周期结构GaAs晶体差频产生窄带THz波过程中,对于不同波长、不同脉宽的抽运光,周期结构GaAs晶体的走离长度和最佳周期长度的变化,并对计算结果进行了理论分析.研究了周期结构GaAs晶体的畴数对THz波光谱的影响.根据GaAs晶体温度色散公式,计算并分析了晶体最佳周期长度的温度调谐特性,研究了温度变化对THz波光谱的影响,提出了通过温度调谐实现在周期结构GaAs晶体中产生宽调谐、窄带宽THz波的新方法,该计算结果为下一步的实验研究提供了理论基础. 关键词： ps脉冲 差频产生窄带THz波 周期结构GaAs晶体 温度调谐     

Compact and widely tunable terahertz source based on a dual-wavelength intracavity optical parametric oscillation

In this paper, a continuously tunable terahertz (THz) source is obtained using a compact intracavity pumped dual-wavelength optical parametric oscillation operating around 2.1 μm as difference-frequency generation pump source. The tuning range of the THz-wave frequency covers from 0.147 THz to 3.651 THz. Based on the collinear difference-frequency generation in the GaSe crystal, the experiment result shows that our schematic is a good option to construct a compact and portable terahertz source with widely tunable range.     

Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

It was shown that the periodically poled LiNbO₃-waveguide with period of poling λ≈λ/n_g (λ is the wavelength of emitted THz-wave, n_g is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (∼6°) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at λ=150 μm is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region. Received: 16 May 2001 / Revised version: 13 August 2001 / Published online: 2 November 2001     

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.     

Terahertz-wave generation with periodically inverted gallium arsenide

We overview methods of THz-wave generation using frequency down-conversion in GaAs with periodically-inverted crystalline orientation. First, we compare different nonlinear-optical materials suitable for THz generation, analyze THz generation process in quasi-phase-matched crystals and consider theoretical limits of optical-to-THz conversion. Then, we review single-pass optical rectification experiments with femtosecond pump pulses, performed in periodically-inverted GaAs, where monochromatic THz output tunable in the range 0.9–3.0 THz was produced. Finally, we describe a novel approach to create a compact highly efficient tunable (0.5–3.5 THz) room temperature monochromatic THz source, based on the concept of intracavity THz generation via resonantly-enhanced difference frequency mixing. This approach allowed generating of 1 mW of average THz power, potentially scalable to 10–100 mW.     

Threshold Analysis of a THz-Wave Parametric Oscillator

The parametric gain of a terahertz wave parametric oscillator （TPO） is analyzed. Meanwhile the expression of TPO threshold pump intensity is derived and theoretically analyzed with different factors. The effective interaction length between the pump wave and Stokes wave is calculated, and particular attention is paid to the coupling efficiency of the pump wave and Stokes wave. Such an analysis is useful for the experiments of TPO.     

Suppression of pump-induced frequency noise in fiber-laser frequency combs leading to sub-radian f ceo phase excursions

We discuss the origins and the suppression of the large frequency jitter on the carrier-envelope offset frequency (f_ceo) of fiber-laser frequency combs. While this frequency noise appears most prominently on f_ceo, its effects are felt across the frequency comb and it is a potential limiting factor in applications of fiber-laser frequency combs. Here we show that its origin lies in the white amplitude noise of the pump laser output. We dramatically reduce this noise by operating the pump laser in a lower-noise state, i.e. at higher pump current, and by more aggressively feeding back to the pump current with an optimal feedback network. We demonstrate instrument-limited f_ceo linewidths and an integrated f_ceo phase jitter of 1 rad. PACS 42.55.Wd; 42.62.Eh; 42.60.Lh; 06.30.Ft     

Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal

Widely tunable terahertz (THz)-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. To our knowledge, this is the first report of THz-wave generation by BNA DFG. Large, high-quality single crystals of BNA (phi 8 mm x 30 mm) were grown using the vertical Bridgman method. The nonlinear optical (NLO) coefficient d(33) of the BNA crystal is approximately 234 pm/V, which is the largest value reported for any yellow NLO material. The collinear phase-matching condition of the type-0 configuration is satisfied using a 0.7-1 microm band pump wavelength. We generated THz waves using an organic BNA crystal; the generation range is 0.1-15 THz.     

Enhancement of terahertz-wave output from LiNbO(3) optical parametric oscillators by cryogenic cooling

In recent years widely tunable terahertz- (THz-) wave generation from LiNbO(3) optical parametric oscillators (OPO's) has been successfully demonstrated by use of the prism output-coupler method. However, there remains a problem of large absorption loss for generated terahertz waves inside the crystal, so we investigated the cryogenic characteristics of the OPO. We achieved 125-times-higher THz-wave output and 32% reduction of the generation threshold by cooling the crystal to 78 K. This scheme also provides direct loss measurement at THz frequency, and we found that the THz-wave enhancement mechanism is improvement of the gain as well as the reduction of the absorption coefficient.     

Surface-emitted terahertz-wave generation by ridged periodically poled lithium niobate and enhancement by mixing of two terahertz waves

Surface-emitted terahertz- (THz-) wave generation by difference-frequency mixing with ridge-shaped periodically poled lithium niobate (PPLN) was demonstrated. The PPLN had a ridge height of 300 microm, a thickness of 20 microm, and an interaction length of 35 mm. The ridge behaves as a slab waveguide for optical pump beams. The PPLN gives rise to THz waves in opposite directions, perpendicular to the pump-beam direction. Reflecting the THz wave on one side and overlapping it with the THz wave on the other side increased the total THz-wave intensity approximately 2.7 times compared with that without reflection and mixing.     

Optimization of the UTC-PD Epitaxy for Photomixing at 340?GHz

We present a method to optimize the epitaxial layer structure of an InGaAs/InP uni-traveling-carrier photo-diode (UTC-PD) for continuous THz-wave generation. The design approach used is general in that it can be applied for any target frequency while this study focuses on 340 GHz. The photodiode epitaxy is modeled and optimized using a TCAD software implementing the hydrodynamic semiconductor equations. This physical device model was found to be in good agreement with reported experimental results. It is shown that the UTC-PD can generate ~1 mW at 340 GHz by choosing the optimum absorption layer and collection layer thicknesses.     

Efficient generation of orange light in a quasi-periodically poled LiTaO3 crystal

We present an approach to generating a tunable orange laser from 0.601 to 0.604 μm based on a quasi-periodically poled superlattice in LiTaO₃ and a Q-switched 1.064 μm Nd:YVO₄ laser as pump. The orange laser was generated in a cavity by a parametric process cascaded by a frequency mixing with a maximum output of 310 mW using 15 W pump power.     

Generation of 840 mW of red light by frequency doubling a diode-pumped 1342 nm Nd:YVO4 laser with periodically-poled LiTaO3

We report an efficient generation of red light in a periodically-poled LiTaO₃ (PPLT) crystal by extracavity single-pass frequency doubling of a diode-pumped, Q-switched Nd:YVO₄ laser at 1342 nm. The sample used in the experiment is 20 mm in length and 14.77 μm in period. An average power of 840 mW of the 671 nm red light is obtained with a 808 nm pump of 12.3 W, the overall optical-to-optical efficiency being 6.8%. The measured effective nonlinear coefficient of the sample is ∼3.8 pm/V. The high conversion efficiency and output power demonstrate that the periodically-poled crystal serving as a frequency conversion device may be used in practice to construct an all-solid-state red laser based on an extracavity single-pass quasi-continuous scheme. Received: 17 September 2001 / Revised version: 23 January 2002 / Published online: 8 May 2002