Optically pumped far infrared lasers

The far-infrared (FIR) region of the electromagnetic spectrum is commonly thought of as the wavelength region ≈ 30μm-≈2 mm. Thus, the FIR wavelength region is located between the more familiar areas of microwaves and optics. Primarily due to the lack of FIR sources and detectors, the FIR region is difficult to access and therefore relatively unexplored and unused. The FIR source problem is presently under attack from neighbouring disciplines; from the microwave side by extending the frequency operating range of classical electron tube oscillators (e.g. backward wave oscillators) and semiconductor devices (e.g. IMPATT and quantum well oscillators) and from the optical side primarily by optically pumped molecular gas lasers.The FIR technology evolution accelerated in the mid 60's with the discovery of the discharge pumped hydrogen cyanide laser, lasing at a handful of lines located at about 330μm wavelength. However, the most important step towards a useful coherent FIR source was the discovery of the optically pumped FIR laser in 1970. In optically pumped FIR lasers a molecular gas (e.g. methyl fluoride methyl alcohol, formic acid) is pumped by an external laser, usually a carbon dioxide laser. The FIR laser transitions typically takes place between adjacent rotational levels in an excited vibrational state. Today, optically pumped FIR lasers cover the full FIR region by more than one thousand discrete laser lines observed in hundreds of FIR laser media. FIR output powers on the order of 1–100 mW are available from a vast number of laser transitions.Despite the rapid development of semiconductor FIR oscillators the optically pumped FIR laser is still the only practical unit that bridge the full frequency-gap between microwaves and optics. The fact that FIR lasers are considered as local oscillators in space born applications, indicate that FIR laser technology has matured considerably.This survey paper discusses optically pumped FIR lasers from the engineer's point of view: principles of operation, design and characteristics.     

Optically (solar) pumped oxygen-iodine lasers

We present the results of theoretical and experimental studies demonstrating the possibility of developing an oxygen-iodine laser (OIL) with direct optical pumping of molecular oxygen involving inter-molecular interaction with charge transfer from donor molecule (buffer gas) to acceptor molecule (oxygen). This interaction lifts degeneracy of the lower energy states of molecular oxygen and increases its absorption cross section in the visible spectral region and the UV Herzberg band, where high quantum yield of singlet oxygen is achieved (QY ～ 1 and QY ～ 2, respectively) at the same time. A pulse-periodic optical pump sources with pulse energy of ～50 kJ, pulse duration of ～25 μs, and repetition rate of ～10 Hz, which are synchronized with the mechanism of singlet oxygen generation, are developed. This allows implementation of a pulse-periodic oxygen-iodine laser with an efficiency of ～25%, optical efficiency of ～40%, and parameter L/T ～ 1/1.5, where T is the thermal energy released in the laser active medium upon generation of energy L. It is demonstrated that, under direct solar pumping of molecular oxygen, the efficiency parameter of the OIL can reach L/T ～ 1/0.8 in a wide range of scaling factors.     

高脉冲重复频率调Q Tm,Ho:GdVO4激光器

报道了一个高效率连续波和调Q高重频两种运行方式的Tm,HoGdVO4激光器.Tm,HoGdVO4晶体尺寸4 mm×4 mm×7 mm,a轴通光,液氮制冷到100 K,由发射中心波长为793 nm的光纤耦合激光二极管端面泵浦.Tm,HoGdVO4激光连续波输出功率4.0 W,光光转换效率26%.声光调Q条件下输出平均高功率3.9 W,脉冲重复频率10 kHz,脉冲宽度50 ns. 通过减小声光Q开关的开启时间,激光脉冲宽度由50 ns减小至23 ns.在10 kHz重频下,测量最大脉冲能量0.39 mJ , 峰值功率7.8 kW.     

二极管泵浦腔内OPO高重复频率2 μm激光器

概述了2μm激光器的发展状况及应用背景, 通过对2 μm激光两种实现方式的比较,最终采用了可调谐方式间接获得2 μm激光的方案.以1.06 μm激光为泵浦源,采用二极管泵浦腔内OPO的方案能有效提高泵浦峰值功率密度,有利于实现高重复频率2 μm激光输出.利用KTP晶体在特定切割角度下实现波长简并输出,合理设计OPO腔,可以有效提高2 μm激光的输出效率.同时进行了激光输出实验,实现了功率11.2 W的2 μm波长激光输出.     

An efficient cavity for optically pumped terahertz lasers

The efficiency of a simply designed cavity for optically pumped pulsed terahertz lasers is studied experimentally. A coated Ge and a crystal quartz act as the input and output windows, respectively. The thickness of the Ge window is designed according to etalon effects to maximize terahertz reflectivity. NH₃ is filled in the cavity as the active medium. When NH₃ is pumped by the 10P(32) line of a TEA CO₂ laser, intense 151.5 μm terahertz radiation is emitted. As high as 19.6 mJ terahertz radiation is extracted from 1.57 J pump energy. The corresponding photon conversion efficiency reaches 35.3%.     

200 Hz高重复频率SBS脉宽压缩实验研究

受激布里渊散射(SBS)脉宽压缩是实现高峰值功率、短脉冲激光输出的重要途径之一,然而,目前SBS脉宽压缩仅限于1~10 Hz低重复频率激光器,限制了高重频短脉冲激光器在激光雷达、空间碎片探测以及目标成像等领域的应用。基于此,开展了高重复频率下的SBS脉宽压缩实验研究。设计搭建了高重复频率的主振荡放大激光器,开展了SBS二次级联脉宽压缩和SBS振荡放大双池脉宽压缩实验。通过SBS二次级联压缩实现了脉冲宽度从~32 ns压缩到~1.9 ns,脉宽压缩比达16倍;而通过SBS振荡放大双池结构实现了脉冲宽度从~4 ns压缩到376 ps,脉宽压缩比达10倍。实验结果表明,采用该超净封闭型SBS相位共轭镜,在Stokes光输出能量达50 m J时,无光学击穿现象,实现了在200 Hz高重复频率下的SBS脉宽压缩。     

Optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well (QW) active region design for terahertz (THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature. The fast longitudinal optical (LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state, and more importantly, the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures. The influences of temperature and pump intensity on gain and electron densities are investigated. Based on our simulations, we predict that with a sufficiently high pump intensity, a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.     

Optically pumped distributed feedback thin film waveguide lasers with multiwavelength and polarized emissions

Zirconia titania organically modified silicate (ZrO₂-TiO₂-ORMOSIL) thin film waveguides of thickness from 0.4 to 7.0 μm were synthesized using low temperature sol–gel method. Narrow linewidth distributed feedback (DFB) lasing was demonstrated in rhodamine 6G-doped ZrO₂-TiO₂-ORMOSIL waveguides. Simultaneous tuning of multiple-output wavelengths was achieved in the dye-doped waveguides by varying the period of the gain modulation generated by a nanosecond Nd:YAG laser at 532 nm. As many as eight separate output wavelengths were observed for a planar ZrO₂-TiO₂-ORMOSIL waveguide of thickness 7.0-μm. The output polarizations of the DFB waveguide lasers can be tuned by varying the polarization of the crossing pump beams. TE and TM optical waves belonging to the same propagation mode were generated by crossing two polarized pump beams, resulting in an effective double of the number of output wavelengths. Continuous tuning of the polarized laser outputs was also achieved by varying the crossing angle.     

Optically pumped GaAs waveguide lasers with a fundamental 0.11 μ corrugation feedback

Surface corrugations with a period of 0.115 μ were ion-milled on GaAs dielectric waveguides. Laser action was observed under optical pumping. Single mode as well as multimode oscillation was obtained under different pumping conditions.     

Optically pumped lasers between high-lying states in the sodium dimer

More than 25 laser lines around 2.5 m from theC ¹ _u state to (3)¹ _g ⁺ and 7 laser lines around 0.9 m from the high-lying¹ _u state to (2)¹ _g ⁺ in the sodium dimer were detected using a nitrogen laser as the pumping source. Most of the optically pumped laser lines have been identified.Projects Supported by the Science Fund of the Chinese Academy of Sciences     

Limiting pulse repetition rate in copper vapor lasers

Two groups of processes which limit the rate-power characteristics of copper vapor lasers are examined. The first of these is related to the inadequate relaxation rate of the metastable states and accordingly their high prepulse concentration, which degrades the lasing pulse parameters. The second group of processes is initiated by the high prepulse electron concentration. It is shown that if the effect of the prepulse electrons is neutralized in pulse-periodic copper vapor lasers, then lasing starts up again in the next pulse within 1 μs after the preceding pulse stops. It is concluded that the rate of deexcitation of the metastable states is so high that it has no effect whatever on the lasing parameters in real gas-discharge lasers based on copper vapor or copper bromide vapor, and the prospects are opened up for attaining a lasing power of ∼10 kW/m with electron-beam pumping. Zh. Tekh. Fiz. 67, 54–60 (May 1997)     

On the pulse width of synchronously pumped lasers

The pulse width of a synchronously pumped laser chiefly depends on the available cavity bandwidth and the pump-pulse duration. A functional form of this dependence was suggested in the literature. We present an alternate relation which is supported by analytical and numerical results, as well as by experimental results, obtained by us and by others.     

Advantages of plasma harmonic generation using high pulse repetition rate lasers: Review of recent studies

The studies of coherence properties of the harmonics generating in laser-produced plasmas, the analysis of the optical nonlinearities of deoxyribonucleic acid components, the resonance enhancement of harmonic in the cases of excitation of indium plasma by multi- and few-cycle pulses, and the application of nanoparticle-based emitters of harmonics using high-pulse repetition rate lasers are reviewed. The analysis of various aspects of plasma harmonic generation at the conditions of optimal excitation of the ablated targets irradiating by 1 kHz lasers is presented. The growth of plasma harmonic conversion efficiency, single harmonic emission, nonlinear spectroscopy of complex organic components, as well as high coherency of harmonic radiation show the advantages of using plasma harmonic technique for optimization of the sources of coherent extreme ultraviolet radiation and for the material science studies. These studies allowed a significant growth of the average power of harmonics compared with the case of 10 Hz lasers.     

Optically pumped quantum M_x-M_R magnetometer with high oscillating magnetic field

A dip of the transverse component of the magnetic moment as a function of resonance frequency-detuning will emerge when the intensity of oscillating magnetic field exceeds a specific value, which is demonstrated theoretically and experimentally. The linewidth of magnetic-resonance signal can be much smaller than when the Mx–MR magnetometer operates on condition that the intensity of oscillating magnetic field is smaller than this specific value, and the Mx–MR magnetometer can possess a much higher signal-to-noise ratio. The experimental result shows that the sensitivity of Mx–MR magnetometer can be improved by an order of magnitude under this condition.     

Optically levitated nanosphere with high trapping frequency

正The optically levitated object is intrinsically isolated from the thermal bath compared with the opto-mechanical oscillator connected to the thermal environment via the cantilever [1,2].Thus the limitation of the thermalization and decoherence introduced by the cantilever is cancelled. The Q-factor of the system is predicted to approach 10~(12) and the system is ex-     

Optically pumped InGaN/GaN MQW lift-off lasers grown on silicon substrates

Chemical etching and removal of the silicon substrate was used for the creation of optically pumped lift-off InGaN/GaN multiple quantum well (MQW) lasers from heterostructures grown on silicon substrate by MOVPE. Luminescence and laser properties of these heterostructures on silicon substrates as well as those of MQWs lifted-off from their substrate by chemical etching were investigated. The lowest value of the lasing threshold of the lift-off lasers at room temperature was about 205 kW/cm² for a laser wavelength of 463 nm and about 360 kW/cm² for a wavelength of 475 nm. It was shown theoretically that the reduction of internal losses, caused by the absence of absorption in the substrate (resulting from its removal) is most significant for the high order modes having lower values of mirror losses and can lead to a 50% reduction of the threshold (or material gain in InGaN necessary to achieve the threshold).     

Spectral hole burning for pulse repetition frequency analysis

We demonstrate an optical processor based on spectral hole burning (SHB) that maps the carrier frequency into the time domain and the pulse repetition frequency (PRF) into the spatial domain by illuminating an SHB crystal with a signal beam that is scanned by a tilting mirror across a slice of the crystal. This time-to-space mapping makes it possible to measure signal envelopes with a resolution of . A signal with a pulsed envelope engraves a vertical absorption grating with a spatial periodicity given by the product of the PRF and the scan velocity. Reading the grating, which the crystal stores for up to T₁, with a collimated beam yields orders diffracted at angles proportional to the PRF, which are Fourier-transformed to produce spots displaced from the DC position by distances proportional to the PRF. Increasing the PRF increases the grating periodicity, causing the diffracted spots to move away from the DC position.     

高功率重复频率Marx型脉冲功率源小型化技术研究进展

传统的高功率重复频率脉冲功率源通常以低电压储能、升压、高压脉冲形成线、输出的顺序工作。因而系统至少包括低压储能和高压脉冲形成线两个储能环节,同时高压脉冲形成线的体积随着电压的升高快速增长。针对这些问题,课题组提出了一种高功率重复频率Marx型脉冲功率源小型化研究的设计思路和实现方式,并开展了相关技术研究。主要介绍了课题组在关键技术上取得的重要进展,包括高储能密度的储能/脉冲成形一体化技术、低抖动重复频率气体开关技术、低抖动高能触发技术、紧凑型Marx高压串叠技术等一系列关键技术。同时介绍了课题组研制的几种典型紧凑结构重复频率Marx型脉冲功率装置：同轴结构快Marx发生器、基于薄膜介质线的脉冲功率源、模块化低阻抗紧凑型Marx发生器、20 GW高功率重复频率脉冲驱动源。通过探讨关键技术研究及其发展现状,为未来脉冲功率源小型化研究的发展和应用方向提供参考。     

Subpicosecond pulse generation in synchronously pumped and hybrid ring dye lasers

Subpicosecond pulse generation has been examined in synchronously pumped mode-locked ring dye laser systems. These include hybrid and composite absorber/gain media arrangements as well as a simple synchronous cavity. The shortest pulses recorded were 0.3 ps for the hybrid system, and this has been shown to be critically dependent on the positioning of the absorber jet in the centre of the cavity to better than 50 m. Stable operation for subpicosecond pulse generation has been achieved in the ring configuration with greater wavelength tunability and higher average power conversion efficiency than with conventional cavity arrangements.