Spectral characteristics of an all solid-state frequency-shiftedfeedback laser

A laser cavity closed via the first-order diffracted light of an acoustooptic modulator is referred to as a frequency-shifted feedback laser (FSF laser). This laser exhibits outstanding features in its oscillation spectrum. We previously demonstrated the first oscillation of an all solid-state FSF laser, and it turned out experimentally that the oscillation spectrum consists of a continuously chirped comb of frequency components. Furthermore, we have been investigating oscillation dynamics both experimentally and analytically for a better understanding of FSF laser. Here experimental results and analyses on the spectral characteristics of the laser are discussed     

Comb generation bandwidth for frequency-shifted feedbacksemiconductor lasers

An analytical expression is obtained for the comb frequency bandwidth in frequency-shifted feedback (FSF) semiconductor lasers. In contrast to previous theoretical predictions, it is found that the achievable bandwidth and a figure of merit, defined for maximizing the product of the bandwidth and output power for the FSF laser, are mainly determined by the spectral width of the gain. It is thus expected that semiconductor lasers, with their high net gain and ease of operation, will offer significant advantages over other laser systems for the application of FSF     

啁啾脉冲高斯光束在自由空间的传输

基于瑞利-索末菲衍射积分,使用复解析信号法推导出了啁啾脉冲高斯光束在自由空间中的传输方程及其傅里叶谱,给出了远场的光场和空间光强的解析式,研究了啁啾参数C对脉冲光束传输的影响。结果表明,当啁啾参数C较小时,随啁啾参数增加,其轴上光谱蓝移增加C2倍,其轴上谱线宽度增加(1 C2)1/2倍。随衍射角增大,轴外光谱红移比无啁啾参数时快。脉冲宽度较小时,啁啾参数增大,轴上光强增大,横向光强分布越集中于传输轴附近;脉冲宽度较大时,啁啾参数增大对横向光强的影响减小。啁啾参数的正负号不影响横向光强分布和光谱分布。     

基于Mach-Zehnder干涉仪的扫描限幅法用于啁啾脉冲频谱整形

为了补偿啁啾脉冲放大(CPA)中的光谱增益窄 化效应,需要对种子激光脉冲进行频谱整形。基 于内置KTN电光晶体的Mach-Zehnder干涉仪,提出了一种用于啁啾激光脉冲频谱整形的扫描 限幅方法。从 理论上分析了KTN电光晶体上加载的静态电压和动态电压对扫描限幅装置透射谱函数的影响 ,并讨论了电 压相对于频率变化的时延和电压自身的抖动对脉冲频谱整形效果的影响。数值结果表明:根 据所需要的频 谱分布可以逆算得到加载到电光晶体上的电压,从而随时间控制啁啾脉冲各频率成分的透射 率,实现啁啾 脉冲频谱的整形;采用所提出的扫描限幅方法,对中心波长800nm, 啁啾参数等于21230,对应带宽为20nm的一阶高斯型脉冲进行频谱整形,得到了与所需频谱分布几乎一致的近平顶 形状的频谱分布;电压相对于 频率变化的时延和电压自身的抖动越大,整形后的脉冲频谱分布与所需频谱相差越远。为了 获得与所需频 谱一致的整形频谱分布,需要尽量保证电压与频率在时间上的匹配,以及尽量减小电压的抖 动。     

Optical frequency domain ranging by a frequency-shifted feedbacklaser

This paper describes the theoretical and experimental study of a new technique for optical frequency domain ranging (OFDR) by a frequency-shifted feedback (FSF) laser. In conventional OFDR, a frequency chirped single-mode laser is used as a light source to convert a distance into a beat frequency, and a tradeoff exists between measurement range and resolution. The FSF laser output consists of periodically generated chirped frequency components whose chirp rate is faster than 100 PHz/s (P=10¹⁵), By use of the FSF laser, the tradeoff is removed and long-distance high-resolution OFDR is realized In the experiment, a distance of 18.5 km was measured with a resolution of 20 mm     

高斯光束的光谱传输特性分析

为了研究光束的光谱传输特性,根据广义惠更斯-菲涅耳原理,推导了高斯光束光谱经自由空间和湍流大气传输的表达式,并以中心频率为1.78×1015rad/s、谱宽为8.39×1013rad/s的高斯光束为例进行了数值分析。结果表明,高斯光束传输过程中,z轴附近光谱存在少量蓝移,离轴距离增大到约为0.5w(z)时,光谱由蓝移变为红移,湍流效应对离轴较远点的谱移现象有比较明显的抑制作用,而增大源光谱的带宽,则会使谱移现象更显著;计算1.78×1015rad/s和1.75×1015rad/s两束单色光传输5000m时传输截面上的光强分布发现,对于某个横向位置,源光谱中心频率激光的强度会比某个其它频率激光的强度小,产生传输截面上的谱移现象,谱移现象随各参量的变化规律可由高斯光束的束腰变化规律和湍流效应对光束的展宽效应进行解释。     

中红外大能量高功率周期量级光学参量啁啾脉冲放大的发展及应用（特邀）

近十年来,超强超短脉冲是激光光学发展的一个重要趋势。尤其是在中红外（MIR）波段,由于中红外波长具有更大的有质动力并且其光谱范围几乎包含了所有分子“指纹”共振峰,这使得中红外激光的研究在强场物理、中红外光谱学、材料加工以及生物医学研究等领域中至关重要。目前已经有许多比较成熟的激光技术可以对脉冲进行整形、放大,例如差频（DFG）、啁啾脉冲放大（CPA）、光学参量放大技术（OPA）以及光学参量啁啾脉冲放大（OPCPA）等。利用OPCPA技术具有的高放大增益、高信噪比、宽增益带宽的优点在高非线性系数的非线性晶体中进行脉冲放大已经成为当前获取超强超短中红外脉冲的主要手段之一。文中总结了利用OPCPA技术在2~20 μm波长范围内产生和放大MIR少周期脉冲的研究进展,并对其在强场物理、分子频谱探测以及生物医学方面的应用进行了简要的阐述。     

FM-laser operation of the Nd:YAG laser

The FM-laser or frequency-sweeping mode of laser oscillation has been demonstrated in a Nd :YAG 1.06-μ laser with an intracavity LiNbO3phase modulator. The experimental results are in excellent agreement with the theoretical expressionDelta= (DeltaOmega/Deltanu) (delta/pi)where δ= peak single-pass phase retardation in the modulator,DeltaOmega= axial mode spacing,Deltanu=modulator detuning, and Δ=resulting FM index of the laser output. Modulation indices as large asDelta approx 230rad have been obtained, in which case the instantaneous laser frequency is sweeping over a full spectral range of2Delta cdot f_{m} approx 120GHz (≈ 4 cm^-1) at a repetition frequencyf_{m} approx 260MHz, with a time-bandwidth product per periodapprox 2Delta approx 460. The coherently mode-locked spectral bandwidth thus obtained in the FM-laser case is very much wider than can be achieved in the pulsed mode-locked case with the same Nd:YAG laser. Some possible ways of using this broad-band coherent FM spectrum are suggested.     

全固态频移反馈激光器的谱宽稳定特性

从速率方程出发,把激光介质的增益带宽以环程频移量为单位划分为若干频带并进行移频处理,仿真分析了基于Nd∶YVO4介质的频移反馈(FSF)激光器在连续输出情况下的特性,给出了在改变抽运功率水平、声光调制器(AOM)衍射效率、环程频移量、介质增益带宽等条件下频移反馈激光器的输出谱宽、频谱位移等的仿真计算结果。实验中,采用端面抽运驻波腔结构并以声光调制器作为腔内移频器,构建了基于Nd∶YVO4介质的全固态频移反馈激光器,实验研究验证了理论计算的结果,并得到了1 W以上的连续激光输出。研究结果表明频移反馈激光器的输出频谱在不同抽运水平下具有谱宽基本稳定的特点。     

Simple dynamic model of all-optical gain-clamped erbium-doped fiberamplifiers

A simple dynamic model of laser-gain clamped erbium-doped fiber amplifiers (EDFAs) using ordinary differential equations is presented. The model not only provides a fast means of calculating EDFA gain dynamics, but also shows explicitly how the laser relaxation oscillation frequency and damping constant are affected by design parameters through analytical expressions. It is shown that the dynamic power excursion caused by relaxation oscillations can be reduced by increasing the oscillation frequency. The simplified model has two limitations: first, the amplified spontaneous emission (ASE) spectrum is not fully resolved and the noise figure can not be studied; second, the effect of spectral hole burning, which also causes gain excursion, can not be modeled. The first limitation can be removed by ASE-resolved modeling, where the concept of average inversion is also utilized to save computing time     

Broad-band wavelength-tunable, single frequency, and single polarization bismuth oxide-based erbium-doped fiber laser

We demonstrate a broad-band wavelength-tunable Bismuth Oxide-based Erbium-doped fiber (Bi-EDF) laser covering both the conventional wavelength band and the long wavelength band. It features single frequency and single polarization oscillation; the former is obtained by making the cavity length short and incorporating an intracavity narrow-band fiber Fabry-Pe/spl acute/rot filter, and the latter is achieved with a sigma laser configuration using nonpolarization maintaining fibers. Tuning over 90 nm (1520-1610 nm) is achieved by utilizing the broad gain bandwidth of Bi-EDF amplifier.     

Nonlinear dynamics of a laser diode with optical feedback systemssubject to modulation

The nonlinear dynamic behavior of a direct frequency-modulated diode laser with strong optical feedback is examined and compared to a laser diode subject to electro-optically modulated, strong optical feedback. Direct modulation is achieved by sinusoidal modulation of the diode laser injection current. Electro-optic modulation is achieved by applying a sinusoidal voltage to an intracavity phase modulating element. The output state (characterized by the output power versus time, the intensity noise spectrum and the optical frequency spectrum) for both types of modulation is dependent on the ratio of the modulation frequency to the external cavity resonant frequency, and the modulation power. A number of distinct states are observed: conventional amplitude modulation (with FM spectra); multimode, low-noise amplitude modulation; multimode, high-noise amplitude modulation; periodic limit-cycle operation; quasi-periodicity; chaos; low-frequency fluctuations; and mode-locking. There are significant differences between the direct and electro-optic frequency-modulation cases. The onset of the dynamic instability is characterized as a noisy period-one oscillation for direct modulation and a low-frequency fluctuation for intracavity electro-optic modulation. Phase portraits produced experimentally with the use of a digital phosphor oscilloscope are shown to agree well with those constructed from output power versus time data. This represents an experimental method for examining the dynamics phase portraits in real-time     

基于腔内光放大器结构的光纤激光器噪声抑制方法

设计了一种将半导体光放大器耦 合于光纤激光器腔内的结构,以抑制光纤激光器的相对强 度噪声。基于半导体光放大器的增益饱和效应,将光纤 激光器在弛豫振荡峰处的相对强度噪声抑制了30 dB。更进 一步地,通过腔内的放大器结构使得放大器内部的光学 强度达到一个稳定态,最终使得激光器的频率噪声没有因 光放大器的加入而产生劣化。     

A continuous tunable source of coherent UV radiation

We describe a continuous tunable ultraviolet radiation source, operating in the spectral range 290-315 nm by intracavity frequency doubling of a rhodamine 6G dye laser. Maximum intracavity second-harmonic power of 1.1 mW was obtained at 296 nm with a bandwidth of 0.1 nm.     

Time and Spectrum Resolved Model for Quasi-Three-Level Gain-Switched Lasers

We derive and compute a model to describe the spectral dynamic behavior of quasi-three-level laser resonators operated in gain-switched conditions. This model, based on rate equations, predicts both intensity and spectrum evolutions of the laser output. It is specifically applied to a Yb:YAG laser resonator, and the effects of intracavity losses on the emitted wavelength are also investigated. The phenomena forecasted by our model are in very good agreement with our experiments     

宽带啁啾镜的制备及其损伤特性研究

基于损伤特性、应力特性较好的电子束蒸发技术,制备了Ta2O5/SiO2和HfO2/SiO2两种带宽不同、电场分布不同的啁啾镜,利用中心波长为1 030 nm,脉宽为350 fs的激光脉冲对两种样品进行了损伤测试。实验结果表明:材料特性及电场分布是影响啁啾镜损伤阈值的主要因素,通过膜系设计降低电场分布可以制备带宽更宽、损伤阈值与HfO2/SiO2啁啾镜相近的Ta2O5/SiO2啁啾镜。利用基于导带电子数密度的理论模型对此现象进行了解释,同时对啁啾镜的损伤形貌进行了观测、分析。     

平顶高斯模式宽带激光通过光阑的光强分布

为了研究横向分布为平顶高斯模式的宽带激光通过硬边光阑系统的光强分布特性,采用衍射积分公式推导了平顶高斯模式宽带激光的每一频率分量在光阑系统中的传输公式,再通过傅里叶变换得到时间域的传输公式,详细分析了光阑和频带宽度对强度分布的影响。结果表明,近场光强在截断参量较大时,仍保持一定的平顶分布特性,但是随带宽的增加强度分布变窄;截断参量较小时强度呈高斯分布,并且带宽的变化对强度分布没有太大影响;然而在远场时截断参量较小情况下的光强分布也明显表现出随带宽增加而变窄的现象,但随着截断参量增大,这一变化趋势变缓。这些结果对平顶高斯模式宽带激光的应用具有重要参考价值。     

色散管理光纤锁模激光器在近零色散域的非线性优化

目前,飞秒激光脉冲因脉冲宽度窄和峰值功率高的特点被广泛运用在多种领域中。其中,色散管理光纤锁模激光器因其特有的腔内呼吸机制使输出的激光脉冲能量更高,光谱更宽、脉宽更窄。使用啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器能够实现真正的全光纤结构,提升激光器的紧凑性和稳定性,因此基于啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器具有更加实际的应用意义。采用数值模拟的方法,研究了基于啁啾布拉格光纤光栅进行色散管理的掺镱光纤锁模激光器中单模光纤在腔内的不同分布对脉冲动力学过程和输出脉冲参数的影响。系统分析了谐振腔内净色散值不同时,腔内单模光纤的分布对脉冲在腔内的动力学过程的影响。模拟结果表明,在腔内净色散值为负时,啁啾布拉格光纤光栅与增益光纤间的单模光纤越短,光纤激光器维持稳定单脉冲运行的最大泵浦强度更高且输出光谱更宽,从而能够获得脉宽更窄的去啁啾脉冲;腔内净色散值越接近零时,啁啾布拉格光纤光栅与增益光纤间的单模光纤长度对输出脉冲参数作用的影响越显著;腔内净色散值为正时,单模光纤在腔内的分布对输出脉冲影响逐渐减弱,优化单模光纤分布提升锁模激光器性能并不明显。最后,提出了一种通过改变单模光纤在腔内的分布来提高激光器输出性能的优化方法。     

Theory of FM laser oscillation

The paper presents a detailed analysis of FM laser oscillation which includes the effect of arbitrary atomic lineshape, saturation, and mode pulling. Such oscillation is achieved by means of an intracavity phase perturbation, and is a parametric oscillation wherein the laser modes oscillate with FM phases and nearly Bessel function amplitudes. One principal idea is that of the competition between different FM oscillations. The effect of the intracavity phase perturbation is to associate a set of sidebands with each of the previously free-running laser modes. While the free-running laser modes experienced their gain from essentially independent atomic populations, the competing FM oscillations to a large extent see the same atomic population; and in cases of interest the strongest of these oscillations is able to quench the competing weaker oscillations and establish the desired steady state condition. Results of the analysis include the following: threshold and power output, amplitudes and phases of all sidebands, frequency pulling of the entire oscillation, time domain behavior, distortion, super-mode conversion efficiency, and effect of mirror motion. Results of numerical application of the theory to a number of specific cases are given.     

Improved semiconductor-laser dynamics from induced population pulsation

This paper investigates theoretically the modification of dynamical properties in a semiconductor laser by a strong injected signal. It is found that enhanced relaxation oscillations are governed by the pulsations of the intracavity field and population at frequencies determined by the injected field and cavity resonances. Furthermore, the bandwidth enhancement is associated with the undamping of the injection-induced relaxation oscillation and strong population pulsation effects. There are two limitations to the modulation-bandwidth enhancement: Overdamping of relaxation oscillation and degradation of flat response at low frequencies. The injected-laser rate-equations used in the investigation reproduce the relevant aspects of modulation-bandwidth enhancement found in the experiment on injection-locked vertical-cavity surface-emitting lasers.