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.     

Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation

We demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030 nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.     

Mid-infrared tunable dual-wavelength generation based on a quasi-phase-matched optical parametric oscillator

We report an efficient optical parametric oscillator (OPO) of dual idler wave output based on periodically poled MgO:LiNbO₃ with a periodically-phase-reversed grating structure, which is pumped by a Q-switched 1.064 μm laser with a repetition rate of 50 kHz. 0.98 W of dual idler-waves at 3.824 μm and 3.731 μm is achieved at room temperature, leading to a 12.9% conversion efficiency. The crystal temperature tuning provides output tunability of the dual idler wavelengths. In addition, the sum frequency generation of the dual signal waves is simultaneously observed in the OPO cavity.     

Angle-tuned signal-resonated optical parametric oscillator based on periodically poled lithium niobate

We demonstrate an angle-tuned signal-resonated optical parametric oscillator (OPO) with periodically poled lithium niobate (PPLN) pumped by a diode-pumped Nd:YVO4 laser. 1499.8 - 1506.6 nm of signal wavelength is achieved at 140℃ by rotating a 29-μm period PPLN from 0° - 10.22° in the x-y plane while keeping the pump wave vertical to the resonator mirrors. Two pairs of the signal and idler waves of the same wavelengths can be achieved symmetrically for each pair of angles of rotation with same absolute value and opposite sign. Theoretical analyses on angle-tuned PPLN-OPO with pump wave vertical to the resonator mirrors are presented and in good agreement with our experimental results. It is also found that all interacting waves in the cavity (not inside the crystal) are always collineax for PPLN-OPO with the pump wave vertical to the resonator mirrors while phase-matching is noncollinear within the crystal.     

Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs

We generated 1 mW of average output power at 2.8 THz (bandwidth of approximately 300 GHz) in a diffraction-limited beam by placing a 6-mm-long quasi-phase-matched GaAs crystal inside the cavity of a synchronously pumped optical parametric oscillator (OPO). The OPO used type-II-phase-matched periodically poled lithium niobate as a gain medium and was pumped by a mode-locked laser at 1064 nm, with a 7 ps pulse duration, 50 MHz repetition rate, and 10 W average output power. The terahertz radiation was generated by difference frequency mixing between the signal and idler waves of the near-degenerate doubly resonant OPO.     

Dual-wavelength, two-crystal, continuous-wave optical parametric oscillator

We report a cw optical parametric oscillator (OPO) in a novel architecture comprising two nonlinear crystals in a single cavity, providing two independently tunable pairs of signal and idler wavelengths. Based on a singly resonant oscillator design, the device permits access to arbitrary signal and idler wavelength combinations within the parametric gain bandwidth and reflectivity of the OPO cavity mirrors. Using two identical 30 mm long MgO:sPPLT crystals in a compact four-mirror ring resonator pumped at 532 nm, we generate two pairs of signal and idler wavelengths with arbitrary tuning across 850-1430 nm, and demonstrate a frequency separation in the resonant signal waves down to 0.55 THz. Moreover, near wavelength-matched condition, coherent energy coupling between the resonant signal waves, results in reduced operation threshold and increased output power. A total output power >2.8 W with peak-to-peak power stability of 16% over 2 h is obtained.     

Low-threshold, self-frequency-stabilized AgGaS(2) continuous-wave subharmonic optical parametric oscillator

We report the demonstration of a cw AgGaS(2) optical parametric oscillator (OPO). The subharmonic (3omega ? 2omega + omega) OPO is configured as a doubly resonant oscillator with weak pump enhancement. The temperature-tuned, noncritically phase-matched crystal is pumped by a diode laser at lambda(p) approximately 845 nm . Oscillation at lambda(s) approximately 1267 nm and lambda(i) approximately 2535 nm is observed at an input threshold power of 60 mW. Crystal thermal loading induces a robust passive self-frequency stabilization of any single-axial-mode pair to the OPO cavity resonance. The conversion efficiency is limited by thermal effects to 2% for a 200-mW pump input.     

12 W mid-infrared output,singly resonant,continuous-wave optical parametric oscillator pumped by a Yb<Superscript>3+</Superscript>-doped fiber amplifier

We report a continuous-wave singly resonant optical parametric oscillator (SRO) with more than 12 W of idler power at 3414 nm when it was operated at 30°C. The SRO was directly pumped by a single-frequency, ytterbium-doped fiber laser with 49 W linear polarization pump powers, and based on 50 mm long periodically poled MgO:LiNbO₃ crystal (PPMgLN) in two-mirror linear cavity with 30.5 μm grating period. It’s pump power at threshold was 5.4 W. The slope-efficiency and quantum-limited performance reached 26 and 79.2%, respectively. The beam polarization matched the e → e + e interaction in crystal. The idler waves were temperature tuned in the range of 3654 to 3811 nm and 3248 to 3414 nm based on two 50 mm long PPMgLN with 29.5 and 30.5 μm grating period. To the best of our knowledge, this is the highest continuouswave mid-IR output obtained for a fiber laser pumped optical parametric oscillator (OPO).     

Tunable optical parametric oscillator based on a KTP crystal,pumped by a pulsed Ti<Superscript>3+</Superscript>:Al<Subscript>2</Subscript>O<Subscript>3</Subscript> laser

We present the characteristics of an optical parametric oscillator based on a KTP crystal, pumped with noncritical phase matching by a pulsed Ti³⁺:Al₂O₃ laser, tunable in the range 677–970 nm. Tunable generation of signal and idler waves is obtained in the ranges 1030–1390 nm and 2690–3050 nm respectively. The efficiency of conversion of the pump to the signal wave is ≈23%, which for pulses of duration ≈8 nsec ensures an energy in the range 1.0–11.5 mJ. The width of the emission spectrum for the signal wave is within the range 0.8–1.8 nm and is predominantly determined by the linewidth of the Ti³⁺:Al₂O₃ pump laser. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 351–356, May–June, 2007.     

Generation of pulsed squeezed light in a mode-locked optical parametric oscillator

The generation of pulsed squeezed light using an optical parametric oscillator (OPO) is discussed. This mode-locked optical parametric oscillator consists of a nonlinear crystal in a cavity which is resonant for both signal and idler waves and which is synchronously pumped by the second-harmonic of an acousto-optically mode-locked cw Nd: YAG laser. The fundamental wavelength of the pump laser provides local oscillator pulses for balanced homodyne detection of squeezed vacuum pulses emitted by the oscillator when operated below oscillation threshold. Photocurrent noise reduction to 30% below the classical shot-noise limit is observed, corresponding to squeezing of the field to a level approximately a factor of two below the mean square vacuum noise.     

Cascaded continuous-wave singly resonant optical parametric oscillator pumped by a single-frequency fiber laser

We present a cascaded continuous-wave singly resonant optical parametric oscillator (SRO) delivering idler output in mid-IR and terahertz frequency range. The SRO was pumped by an ytterbium-doped fiber laser with 27 W linear polarization pump powers, and based on periodically poled MgO:LiNbO₃ crystal (PPMgLN) in two-mirror linear cavity. The PPMgLN is 50 mm long with 29.5 μm period. The idler power output at 3811 nm was obtained 2.6 W. The additional spectral components that have been attributed to cascaded optical parametric processes are described at increasing pump levels. Besides the initial signal component at about 1476.8 nm, further generated wavelengths with frequency shifts about 47 cm^?1, 94 cm^?1 and 104 cm^?1 were observed. It was speculated that the idler waves lie in the terahertz (THz) domain from the observed results.     

Prospects for increasing average power of optical parametric chirped pulse amplifiers via multi-beam pumping

We demonstrate optical parametric amplification of broadband chirped pulses in BBO crystal pumped by several intersecting beams extracted from independent laser amplifiers. The energy of amplified signal ranged from 0.23 to 0.72 mJ depending on a number of pump beams used as well as pumping configuration employed. The conversion efficiency dependence on intersection angles of pump beams is revealed and modeling of interplay of interacting waves is presented.     

High-efficiency Nd:YVO4 laser emission under direct pumping at 880 nm

We report a high-efficiency Nd:YVO₄ laser pumped by an all-solid-state Q-switched Ti:Sapphire laser at 880 nm in this paper. Output power at 1064 nm with different-doped Nd:YVO₄ crystals of 0.4-, 1.0- and 3.0-at.% under the 880 nm pumping was measured, respectively. Comparative results obtained by the traditional pumping at 808 nm into the highly absorbing ⁴F_5/2 level were presented, showing that the slope efficiency and the threshold with respect to the absorbed pump power of the 1.0-at.% Nd:YVO₄ laser under the 880 nm pumping was 17.5% higher and 11.5% lower than those of 808 nm pumping. In a 4-mm-thick, 1.0-at.% Nd:YVO₄ crystal, a high slope efficiency of 75% was achieved under the 880 nm pumping, with an optical-to-optical conversion efficiency of 52.4%.     

A narrowband optical parametric oscillator tunable over 6.8 THz through degeneracy with a transversely-chirped volume Bragg grating

An efficient nanosecond optical parametric oscillator (OPO) with output energies of 0.75 mJ using a periodically poled KTiOPO₄ crystal pumped at 532 nm and generating narrowband output continuously tunable over the range of 6.8 THz, between 1053 nm and 1075 nm, is demonstrated by employing a transversely-chirped volume Bragg grating. The tunable reflectivity spectrum of the chirped volume Bragg grating allowed a smooth transition between singly-resonant and doubly-resonant operation of the OPO without cavity rearrangement. This gave a unique possibility to experimentally verify theoretical predictions regarding the efficiency of type-I and type-0 phase matched degenerate OPOs pumped by multimode Q-switched lasers.     

LD端面抽运Nd:YLF/KTA连续光参量振荡激光

采用非临界相位匹配切割,尺寸5mm×5mm×20mm的KTA作为非线性光学晶体,进行了基于半导体激光端面抽运Nd:YLF/KTA的内腔式连续光学参量振荡激光研究,获得了中红外3.5μm波段的连续激光输出。为了提高连续光参量振荡腔内信号光的功率密度,降低激光输出阈值,采用对信号光高反射的单谐振腔结构进行激光实验。在8.35W的抽运功率下,分别获得了335mW和110mW的3440nm和1505nm的激光输出,对应的总转换效率达到了5.6%。该实验研究表明半导体激光端面抽运的内腔式KTA连续光学参量振荡也能获得高效的中红外激光输出。     

宽波段连续调谐全固态CW PPMgLN光学参量振荡器

利用半导体激光泵浦输出1064 nm波长的全固态连续Nd:YVO4激光器作为泵浦源,采用周期调谐和温度调谐组合调谐技术,对基于掺氧化镁周期性极化铌酸锂晶体（MgO:LiNbO3, PPMgLN）准相位匹配（QPM）的全固态连续波（CW）光学参量振荡器（OPO）宽波段无分立连续调谐输出特性进行研究。实验采用连续工作模式和外腔结构,基于多周期PPMgLN晶体的30.2,30.4和30.6 m周期,在改变晶体的极化周期的基础上,同时在30～100 ℃范围内调节晶体工作温度。实验结果表明:CW PPMgLN OPO的泵浦阈值仅为0.22 W;不同极化周期需要的温度调谐范围不同;信号光在1 559.8～1 597.2 nm近红外波段和闲频光在3 187.3~3 347.3 nm中红外波段连续调谐输出。实现了外腔式全固态CW OPO在信号光和闲频光波段的无分立连续调谐输出。     

用短谐振腔结构优化THz电磁波参量振荡器的输出特性

以MgO：LiNbO₃为非线性光学介质，通过采用一种高性能腔反射镜实现了一种85mm短腔长的法布里-珀罗式的光学参量振荡器，产生THz电磁波的实验结果.这种短腔长THz参量振荡器比传统的160mm腔长的振荡阈值降低了22.3％；峰值能量提高了170％；频率调谐范围从0.5—2.4THz提高到0.8—3.1THz.还报道了一种基于平面金属丝网的法布里-珀罗干涉仪测量THz波线宽的实验结果. 关键词： THz电磁波 OPO 非线性光学 3')" href="#">LiNbO₃     

Highly efficient Nd:GdVO4/BiBO laser at 456 nm under direct 880 nm diode laser pumping

The continuous-wave high-efficiency laser emission of Nd:GdVO₄ at the second-harmonic of 456 nm obtained by intracavity frequency doubling with an BiB₃O₆(BiBO) nonlinear crystal is investigated under pumping by diode laser at 880 nm into emitting level ⁴F_3/2. About 3.8 W at 456 nm with M² = 1.4 was obtained from a 5 mm-thick 0.4 at.% Nd:GdVO₄ laser medium and a 12 mm-long BiBO nonlinear crystal in a Z-type cavity for 13.9 W absorbed pump power. An optical-to-optical efficiency with respect to the absorbed pump power was 0.274. Comparative results obtained for the pump with diode laser at 808 nm, into the highly-absorbing ⁴F_5/2 level, are given in order to prove the advantages of the 880 nm wavelength pumping.     

90% pump depletion and good beam quality in a pulse-injection-seeded nanosecond optical parametric oscillator

We measured 90% pump depletion in a singly resonant image-rotating nanosecond optical parametric oscillator that was pulse-injection seeded by a self-generated signal pulse. The oscillator was pumped by an 8 ns duration single-frequency 532 nm pulse from an injection-seeded Q-switched Nd:YAG laser and resonated an 803 nm signal. The pump and pulsed-seed beams had flat-topped spatial fluence profiles with diameters of approximately 6 mm, giving a cavity Fresnel number at 803 nm approaching 400. The beam cleanup effects of the image-rotating cavity produce a far-field signal spatial fluence profile with approximately 60% of its energy falling within the diffraction-limited spot size.     

Widely tunable broadband terahertz radiation generation using a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile crystal via difference frequency generation

Widely tunable terahertz (THz) waves were successfully generated from 0.5 to 10 THz via difference frequency generation (DFG) in a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile (OH1) crystal. Potassium titanium oxide phosphate optical parametric oscillator pumped by nanosecond Q-switched Nd:YAG laser was used to generate two waves, which were then used to irradiate OH1 crystal. The maximum energy of the generated THz wave was about 461 pJ/pulse. We investigated the dependency of generated THz energy to the excitation pump power density and OH1 crystal thickness. In addition, we compared the THz energy generated by OH1 crystal to 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) crystal using DFG, and we achieved 560 times higher energy using OH1 crystal than DAST crystal at around 1.1 THz.