腔内波片产生的Nd∶YAG激光纵模分裂规律研究

简要介绍了激光纵模分裂的基本原理，研究了腔内含有1个和2个1/4波片时产生Nd∶YAG激光纵模分裂的规律。实验结果表明：在1064nm Nd∶YAG激光的腔内放置一个1/4波片时，每一激光纵模分裂为2个正交的线偏振模，在波片表面垂直于激光光线的条件下，纵模分裂量（即频率差或波长差）恰好等于激光纵模间隔的一半；当在Nd∶YAG激光腔内沿垂直光线方向平行放置2个1/4波片时，也能产生纵模分裂现象，其纵模分裂量取决于2波片快（慢）轴之间的夹角。在0°～ 90°范围内调节角度，可使纵模分裂量在一个激光纵模间隔内线性调谐。实验结果与理论分析相吻合。     

电流调谐光纤光栅外腔分布布拉格反射激光器

报道一种新颖的波长可调谐光纤光栅外腔分布拉格反射激光器。把经过金属管封装的光纤面布拉格光栅作为激光二极管的外腔反射镜,得到的单模激光输出不仅线宽窄,边模抑制比高,而且,调节流过金属管的电流,可在5nm范围内的6个不同波长间转换。     

Mode competition and frequency locking

A theoretical study is presented of frequency locking to be observed in lasers with broad homogeneous emission lines, when the cavity losses are reduced, at a certain frequency ω₀, by placing inside the cavity a suitable element (e.g., a selectively reflecting “mirror”). Two kinds of locking effects are analyzed: i) When tuning the center frequency of the laser output ω_f towards ω₀, by means of a filter or a grating, the laser frequency becomes locked to ω₀, when |ω_f ? ω₀| reaches a critical value. ii) In the absence of frequency-selective elements, the laser oscillates only in a narrow spectral range centered at ω₀. Special emphasis is given to the role played by mode competition in these locking phenomena.     

Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity

The single mode pulsed dye laser is an attractive tool for many spectroscopic applications. Long cavity tunable dye lasers generally operate in multi-longitudinal modes within the bandwidth of gain profile. Single longitudinal mode oscillation can be obtained by either making the cavity short enough or introducing an additional loss mechanism, in which all modes but one have a gain less than their loss. A new technique to achieve single mode operation in a long cavity dye laser, based on Rhodamine 6G dye in ethanol and ethylene glycol solution, pumped by a high repetition rate copper vapor laser, is reported. This laser, which operates in three modes in grazing incidence grating configuration (cavity length of 16 cm), has been made to lase in single mode by increasing the loss in the resonator through beam walk-off.     

光纤环形谐振腔中饱和吸收体光栅的数值模拟和特性分析

利用耦合非线性微分方程组求解饱和吸收体光栅(SAG)的损耗谱,得到了在无谐振腔时SAG参数与光栅带宽、中心频率损耗、边模抑制比等光栅滤波特性的关系。考虑谐振腔的存在,对SAG纵模边模抑制比进行修正,数值分析了优化区范围及优化程度。结果表明,当满足饱和吸收体长度约为腔内光纤长度1/3时,可实现平坦的边模损耗谱,且在小于由注入功率决定的掺铒光纤长度内,纵模的边模抑制比较无谐振腔时提高了0.3～0.6dB。采用扫频法初步验证了达到平坦边模损耗谱的条件。     

Single Mode Er-Doped π-Phase-Shifted Distributed Feedback Fibre Grating Laser

A novel method incorporating the shielded method and the post-processing method has been proposed to fabricate the π-phase-shifted fibre grating. Then an Er-doped π-phase-shifted distributed feedback fibre grating laser has been fabricated using the grating. The laser threshold is 20mW. When pumped with 90mW light at 980nm, the laser gives an output of 1.1mW. Its signal-to-noise ratio is better than 60dB. It is demonstrated that the laser is single mode operation by means of a Fabry--Perot scanning interferometer.     

Dual wavelength dye laser incorporating distributed feedback

A dual wavelength dye laser has been constructed in which one wavelength is selected by distributed feedback and the other by a conventional external cavity tuned by a grazing incidence/Littrow grating pair. Both wavelengths are emitted collinearly and simultaneously, show the same temporal profile (10 ns pulse), have comparable linewidths (0.1/0.2 Å), are of comparable energies (0.5 mJ), and are independently tunable. The laser exhibits the low amplified spontaneous emission properties normally associated with distributed feedback lasers. Gain competition effects have been examined.     

Dual-wavelength distributed Bragg reflector semiconductor laser based on composite resonant cavity

We report a monolithic integrated dual-wavelength laser diode based on a distributed Bragg reflector (DBR) composite resonant cavity. The device consists of three sections, a DBR grating section, a passive phase section, and an active gain section. The gain section facet is cleaved to work as a laser cavity mirror. The other laser mirror is the DBR grating, which also functions as a wavelength filter and can control the number of wavelengths involved in the laser action. The reflection bandwidth of the DBR grating is fabricated to have an appropriate value to make the device work at the dual-wavelength lasing state. We adopt the quantum well intermixing (QWI) technique to provide low-absorption loss grating and passive phase section in the fabrication process. By tuning the injection currents on the DBR and the gain sections, the device can generate 0.596 nm-spaced dual-wavelength lasing at room temperature.     

高谱分辨X光能谱诊断技术

 采用国内首次研制出的2 000线/mm的自支撑透射光栅配上背照射软X光CCD(charge coupled device)组成了高谱分辨透射光栅谱仪。通过实验标定和理论模型计算相结合得到了高线对透射光栅的绝对衍射效率；同时建立了透射光栅谱仪测谱解谱方法，编制了相应的解谱程序。在“神光”激光装置上利用该谱仪通过激光打靶实验获得了金腔靶注入口发射的X光能谱定量实验结果，实验结果表明，该谱仪测谱范围在高能区达到6 000eV,谱分辨达到0.1nm，能够清晰地分辨金等离子体M带三峰分布X光谱结构。     

Dual-wavelength distributed Bragg reflector semiconductor laser based on a composite resonant cavity

We report a monolithic integrated dual-wavelength laser diode based on a distributed Bragg reflector (DBR) composite resonant cavity. The device consists of three sections, a DBR grating section, a passive phase section, and an active gain section. The gain section facet is cleaved to work as a laser cavity mirror. The other laser mirror is the DBR grating, which also functions as a wavelength filter and can control the number of wavelengths involved in the laser action. The reflection bandwidth of the DBR grating is fabricated to have an appropriate value to make the device work at the dual-wavelength lasing state. We adopt the quantum well intermixing (QWI) technique to provide low-absorption loss grating and passive phase section in the fabrication process. By tuning the injection currents on the DBR and the gain sections, the device can generate 0.596 nm-spaced dual-wavelength lasing at room temperature.     

基于液晶/聚合物光栅的高转化效率有机半导体激光器

采用有机半导体发光材料聚[2-甲氧基-5-(2-乙基己氧基)-1,4-苯乙炔]作为增益介质,低官能度光敏单体制备的液晶/聚合物光栅作为外部反馈谐振腔,制备出参数可独立控制的分离式结构的有机半导体激光器.液晶/聚合物光栅中液晶分子的取向影响光栅折射率调制量,从而影响光栅的反馈能力,最终影响激光器出射激光的性能.通过研究发现决定液晶分子取向的主要有两种与光栅周期有关的作用力,利用这一原理制备不同周期的光栅,光栅周期小于450 nm时,相分离出的液晶分子取向由光栅矢量方向变为光栅沟槽方向,此时光栅的折射率调制量增加,光反馈能力增强.采用周期为395 nm的液晶/聚合物光栅制备二级布拉格散射的有机半导体激光器,相较于大周期光栅(593 nm)制备的激光器,激光阈值由0.70μJ/pulse降低至0.18μJ/pulse,转化效率由2.5%提高到6.4%,且出射激光垂直于基板表面发射,有利于后续的处理及应用.     

Dual-wavelength external cavity laser with a sampled grating formed in a silica PLC waveguide for terahertz beat signal generation

We propose and demonstrate a dual-wavelength external cavity laser (ECL). In our design, a Fabry–Pérot laser diode (FP-LD) is hybrid-integrated with a sampled Bragg grating written in a silica planar lightwave circuit (PLC). The grating selects two laser wavelengths that share the same laser cavity. The dual-wavelength oscillation with a side mode suppression ratio >32 dB has been demonstrated experimentally. Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission corresponding to beat frequency of 1 THz. Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1 THz beat signal. PACS 42.79.Dj; 42.60.By; 42.55.Px     

强反馈光纤光栅外腔半导体激光器

在理论上对强外腔反馈情形的半导体激光器线宽压窄效应进行了分析，对消反膜剩余反射率，外腔反射率，外腔腔长对线宽压缩的影响进行了研究，在实验上采用光纤光栅作为反馈元件，与一端镀有消反膜的１．５μｍ波段的常规多纵模交导体激光器耦合，构成强反馈光纤光栅外腔半导体激光器，得到单频窄线宽的激光输出，静态下边模抑制比大于３０ｄＢ，线宽小于１２０ｋＨｚ。     

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process.     

A device based on a grazing-incidence grating for tuning the frequency of a single-mode dye laser

Numerical simulation methods have been used to study and optimize the mechanism based on grazing incidence of the radiation on a diffraction grating for scanning the frequency of a single-mode dye laser. The tuning mechanism uses a glass wedge (an optical reducer) placed in the laser cavity. It is shown that when this cavity is used it is possible by a special choice of parameters to avoid longitudinal mode hopping as the laser frequency is scanned over a broad range. Zh. Tekh. Fiz. 67, 53–57 (March 1997)     

Design of a single-mode directly modulated orbital angular momentum laser

We propose a design of single-mode orbital angular momentum(OAM) beam laser with high direct-modulation bandwidth. It is a microcylinder/microring cavity interacted with two types of second-order gratings: the complex top grating containing the real part and the imaginary part modulations and the side grating. The side grating etched on the periphery of the microcylinder/microring cavity can select a whispering gallery mode with a specific azimuthal mode number, while the complex top grating can scatter the lasing mode with travelling-wave pattern vertically. With the cooperation of the gratings, the laser works with a single mode and emits radially polarized OAM beams. With an asymmetrical pad metal on the top of the cavity, the OAM on-chip laser can firstly be directly modulated with electrical pumping. Due to the small active volume, the laser with low threshold current is predicted to have a high direct modulation bandwidth about 29 GHz with the bias current of ten times the threshold from the simulation. The semiconductor OAM laser can be rather easily realized at different wavelengths such as the O band, C band, and L band.     

二极管泵浦激光器中泵浦光分布对激光器模式影响程度的计算方法

在激光器中,通常泵浦光呈不均匀分布,由此引起介质的增益也相应地出现不均匀分布,导致振荡光通过增益区时其分布发生变化。提出一种计算不均匀的泵浦光分布对于激光器内振荡光模式的影响程度,可以近似获得振荡光对原来场分布偏离的百分比的方法。在此基础上,对准连续运转的三向侧面泵浦激光器进行研究,分析不均匀泵浦光分布对振荡光场影响的规律。结果表明:当泵浦功率增加或谐振腔内损耗增加时,振荡光通过增益区时对原来分布的偏离程度也同时增加,呈单调变化关系;泵浦光不均匀分布对振荡光场的影响,与其沿振荡光轴向上的分布无关,仅仅取决于其在与振荡光垂直方向上的不均匀分布程度。通过计算还获得了此类效应的数量级,在2 000 W泵浦功率条件下,这种影响仅使振荡光偏离理想分布不到4%,而泵浦功率达到6 000 W时这种偏离则高达20%以上。     

Self-mixing interference effects in orthogonally polarized dual-frequency Nd:YAG lasers at different feedback levels

The self-mixing interference effects are investigated in birefringence dual-frequency Nd:YAG lasers at different optical feedback levels. For weak feedback level, both of the intensities of the two modes are sinusoidally modulated by external cavity length with a feedback period of half laser wavelength. While these two modes can only coexist in a part of feedback period at moderate feedback level, i.e., one mode reaches its maximum intensity, the other one extinguishes. At high feedback level, complete polarization switching (one mode oscillating, the other one extinguishing) between two polarized modes is observed, and every switching corresponds to quarter-wavelength variation of external cavity length. Furthermore, two feedback schemes, i.e., isotropic optical feedback (IOF) and polarized optical feedback (POF) are taken into account. The theoretical analysis based on a model of two-mode-coupled laser is presented, which is in good agreement with experimental results. Our results can advance the research of self-mixing interferometer of orthogonally polarized dual-frequency lasers.     

光梳主动滤波放大实现锶原子光钟二级冷却光源

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.     

External-cavity quantum cascade lasers with fast wavelength scanning

Fast wavelength scanning of an external-cavity quantum cascade laser (EC-QCL) has been developed using a modified Littrow-type cavity configuration. Scan rates up to 5 kHz and tuning ranges up to 7 cm⁻¹ have been realized using folded cavity arrangement with a fast piezoactuated intracavity mirror to vary the effective diffraction grating angle. High-resolution molecular spectroscopy, limited primarily by the effective laser linewidth, has been demonstrated in pulsed and CW mode using direct absorption spectroscopy of ammonia and ethylene in the 10-μm wavelength region. A high-resolution broadband spectral scan employing mode-hop-free tuning of a set of discrete laser modes within the fast frequency scan has been developed. The method has been demonstrated by performing high-resolution broadband spectrum of ethylene under atmospheric and reduced pressures.