Dynamics in a compound cavity semiconductor laser induced by smallexternal-cavity-length change

For the short-range variations of the external cavity length, the dynamic behaviors of a semiconductor laser with optical feedback are investigated by the numerical simulations and experiments. Under the condition of a single-mode oscillation, the laser output power shows periodic undulation along the external cavity length whose period is equal to half of the optical wavelength. At weak to moderate external optical feedback, stable, periodic, quasi-periodic, and chaotic oscillations of the laser output power are observed within the periodic undulation. Experimental results of periodic and quasi-periodic oscillations obtained by the laser output power and the optical spectrum support the predictions from the numerical analysis based on the rate equations     

Frequency-stabilized mode-locked solid-state laser system forprecision range-Doppler imaging

We report measurements which show that an actively stabilized cw mode-locked Nd:YLF laser, in combination with a flashlamp-pumped Nd:glass amplifier, can achieve better than 1 cm resolution of distant rotating targets using range-Doppler imaging. To do this, we have produced trains of 50 ps mode-locked pulses with less than 25 kHz peak-to-peak optical frequency broadening and jitter of the laser modes. This frequency stability is achieved by active control of the oscillator cavity length using an external cavity as a reference. Cavity length stabilization can also reduce mode-locked laser timing jitter if the jitter is caused by cavity optical path length changes common to all laser modes. In our laser, however, the active optical-frequency-stabilization did not significantly improve laser pulse timing stability from the approximate 2-ps jitter levels achieved in our passively stabilized cavity. Analysis of the data indicates that a significant fraction of the timing jitter was due to laser cavity path length changes that varied from mode to mode     

Frequency noise reduction of visible InGaAlP laser diodes bydifferent optical feedback methods

The emission spectra of several commercial InGaAlP laser diodes operating in the visible range were investigated. Strong relaxation sidebands at frequency spacings up to 2 GHz were observed in the field spectra. By using optical feedback from an external high-finesse resonator, the laser line-width was reduced from a few hundred megahertz to less than one megahertz. However, only a small part of the laser power was found within the narrow bandwidth, whereas most of the power finds itself in relaxation sidebands a few gigahertz apart from the carrier. The frequency noise reduction was investigated by using an external cavity configuration. With a short cavity length the relaxation oscillation sidebands were completely suppressed, and the laser linewidth was reduced to a few megahertz. Additional strong reduction in the laser linewidth to roughly 50 kHz was obtained using a combination of the external cavity laser with optical feedback from an external high-finesse resonator     

Theory of optical parametric oscillation internal to the laser cavity

Since the fields inside a laser cavity are much higher than the external fields, an analysis of a parametric oscillator with the nonlinear crystal internal to the laser is performed. Using self-consistency equations as the starting point, the equations of motion of such an oscillator are derived. Depending on various cavity, pumping, and nonlinearity parameters, these lead to several types of oscillation with distinctly different operating characteristics: (1) efficient parametric oscillation similar to that of previous analyses; (2) inefficient parametric oscillation resulting from the fact that the nonlinear interaction drives the phases rather than the amplitudes of the signal, idler, and pump; and (3) a pulsing output from the oscillator with repetitive pulses of the signal and idler. A stability analysis of these various regions shows that they are mutually exclusive and can be experimentally chosen by changing the laser gain, the oscillator output coupling, or the strength of the nonlinear interaction. It is shown that the internal oscillator efficiency rapidly approaches the Manley-Rowe limit, as the available pump power becomes several times greater than that required for threshold. The efficiency of an external oscillator having a triply resonant optical cavity is found to be generally less than that of the corresponding internal oscillator.     

一种可降低Littman-Metcalf光栅外腔半导体激光器衍射损耗的结构设计

基于原有Littman-Metcalf型光栅外腔半导体激光器的工作原理,设计了一种可以降低衍射损耗的外腔结构。在Littman-Metcalf结构的基础上增加一个反射镜,将闪耀光栅二次衍射产生的零级衍射光反馈回半导体激光器本征腔。推导了新结构模型外腔损耗的表达式,通过等效腔的概念对两种结构激光器的外腔损耗、阈值电流、输出线宽以及输出功率进行了仿真分析。结果表明：将二次衍射产生的零级光反馈回有源区可有效降低Littman-Metcalf结构激光器的外腔损耗,提高了系统的耦合效率,从而降低阈值电流,提高了激光器的输出功率。同时,由于提高了外腔反射效率,该外腔结构进一步压窄激光器的输出线宽。对影响低损耗Littman-Metcalf外腔激光器输出线宽以及输出功率的因素(端面反射率、内外腔长、闪耀光栅衍射效率以及反射镜反射率等)也进行了仿真分析,为后期激光器制作提高了理论指导。     

外部光反馈对强耦合外腔半导体激光器特性影响的理论分析

本文从理论上分析了外部光反馈对强耦合外腔半导体激光器特性的影响，对相干外部光反馈和非相干外部光反馈两种情形的分析结果均表明强耦合外腔半导体激光器可将半导体激光器所能容忍的临界光反馈强度提高大约２０ｄＢ以上，同时得出了长外腔和强耦合有利于提高外腔半导体激光器抗反馈能力的结论。     

Characteristics of a semiconductor laser with external feedback

The theoretical analysis of the compound cavity of a semiconductor laser with external optical feedback is conducted. For large optical feedback, the output power from the laser and its oscillation frequency differ from those for small optical feedback. From the rate equations of the compound cavity, the conditions of the laser oscillation are derived in the presence of large optical feedback The dependencies of the output power and the laser oscillation frequency on the external-cavity length are investigated. Some new results involving laser oscillation depending on the external-cavity length are presented. The experimental results are compared with theoretical predictions. The dependence of the laser oscillation on the external-cavity length is qualitatively explained in the present model     

高功率266 nm全固态单频连续波激光器研究进展

高功率单频连续波266 nm激光在大容量信息存储、高分辨光谱监测及高精度紫外光刻等领域具有重要应用价值,近年来已成为国内外紫外激光领域的研究热点之一。文中首先综合比较了用于产生高功率266 nm紫外激光的非线性光学晶体基本性能,并根据主要的激光器频率锁定方法,重点分析了H?nsch-Couillaud (H-C)频率锁定和Pound-Drever-Hall (PDH)频率锁定方法的优缺点以及连续波单频266 nm激光器发展现状,介绍了本课题组最新研究成果,即基于H-C频率锁定方法实现了功率1.1 W的单频连续波266 nm紫外激光稳定输出。最后,针对进一步提升全固态单频连续波266 nm激光器性能亟需解决的问题和可能解决路径进行了简要分析和展望。     

Measurement of the linewidth enhancement factor of semiconductor lasers based on the optical feedback self-mixing effect

A new method for the measurement of the linewidth enhancement factor of semiconductor lasers is presented, based on the interferometric self-mixing effect. It is a fast and easy to perform method that does not require radio frequency nor optical spectrum measurements. A small fraction of the emitted light is backreflected into the laser cavity by a remote target driven by a sine waveform. The mixing of the returned and the lasing fields generates a modulation of the optical output power in the form of an interferometric waveform, with a shape that depends on the optical feedback strength and the linewidth enhancement factor /spl alpha/, according to the well-known Lang-Kobayashi theory. We show that the value of /spl alpha/ can be retrieved from a simple measurement of two characteristic time intervals of the interferometric waveform. Experimental results obtained on different laser diodes show an accuracy of /spl plusmn/6.5%.     

Noise properties of semiconductor lasers due to optical feedback

It has been recognized for some time that reflections back into the cavity of an injection laser from a mechanically unstable external mirror cause noise in the output of the laser (reflection noise). In this paper, noise measurements are presented for index-guided lasers as functions of the output power, the degree of optical isolation and external cavity length (5-15 cm), while current modulation (using 2-4 mA at 50-200 MHz) has been used to reduce the noise levels by as much as 20 dB. It is demonstrated that the mechanism of noise reduction by modulation is the frequency modulation of the laser's spectrum and its interaction with the modes of the external cavity. It is shown that using an optical isolator can give a relative intensity noise of -110 dB while using modulation in addition can reduce this to -125 dB; the bandwidth performance of optical storage systems is discussed in terms of these two schemes.     

Self-calibrated interferometric-intensity-based optical fibersensors

This paper presents self-calibrated interferometric-intensity-based optical fiber sensors, which combine for the first time fiber interferometry and intensity-based devices into a single sensor system. The sensor involves an extrinsic Fabry-Perot (FP) interferometric cavity. The broadband light returned from the FP cavity is split into two channels in such a way that one channel has a coherence length much longer than the doubled air-gap separation in the sensor so the FP generates effective interference, while the coherence length in the other channel is so short that no effective interference takes place. As a result, the optical signal in the channel with a long coherence length yields information about the FP cavity length while the signal in the other channel is proportional only to the source power, fiber attenuation, and other optical loss factors in the optical path. To eliminate fringe direction ambiguity and relative measurement limitations associated with interferometric sensors, the sensor is designed such that it is operated over the linear range between a valley and a peak of one interference fringe in the first channel. Moreover, the ratiometric signal-processing method is applied for the signals in the two channels to obtain self-calibrating measurement to compensate for all unwanted factors, including source power variations and fiber bending losses. Various pressure and temperature sensors based on the self-calibrated interferometric/intensity-based scheme are designed, fabricated, and tested. Experimental results show that a resolution as high as 0.02% of full scale can be obtained for both the pressure and temperature measurements     

基于激光回馈效应的应力测量系统研究

玻璃材料的内应力直接影响玻璃零件加工质量和光学器件使用寿命,在航空航天、精密光学系统,精密加工等领域受到高度重视,高灵敏度,大测量范围的应力检测技术已经成为当前的研究热点。本文提出一种基于激光回馈效应的应力测量方法。激光回馈系统由激光器和外部反射镜构成,待测样品放置在回馈外腔中。由于应力引起的双折射效应,带有应力的样品使外腔分裂为两个“物理长度”,不同的外腔长决定了不同偏振方向的回馈光相位,通过提取相位差信息,可获得应力的大小。从理论上分析了回馈系统中激光器的输出光在正交方向的相位与外腔应力双折射的关系；通过傅里叶变换的方式得到双折射外腔激光回馈系统光强调谐曲线的相位信息；最后,采用激光回馈系统对不同的飞机座舱有机玻璃样品内应力进行了测量,并给出测量结果。该方法具有结构简单、精度高的优势,并且具有应用于玻璃材料生产线、改进制备工艺的潜力。     

Multistability in grating-tuned external-cavity semiconductorlasers

In this paper, we discuss the multistable state in a semiconductor laser that has an external-cavity grating system for the first time (to the best of our knowledge). We analyzed onset conditions and stable solution conditions of the multistability and bistability, and observed open- and closed-loop multistabilities. The properties of the multistable loops depend upon whether the internal facet reflectivity of the laser diode is larger or smaller than the external grating cavity reflectivity. This phenomenon directly reflects the intrinsic properties of the coherent interactions between the internal facet and the external grating cavity in the semiconductor laser diode. We experimentally observed the optical pulse-width memory effect in the external grating-tuned semiconductor lasers. Based upon this setup and results, a multistable optical logic device can be built with a carefully designed compound cavity semiconductor laser device     

Linewidth determination from self-heterodyne measurements with subcoherence delay times

The behavior of the power spectrum of an external cavity semiconductor laser has been studied using a delayed self-heterodyne interferometric technique that uses delay times less than the laser's coherence time. Experimental results show that the resulting power spectrum is consistent with the theoretical model. However, there is evidence that additional frequency fluctuations are present that cause the delta function portion of the power spectrum to have a finite width.     

外腔长度对FBG-ECL输出特性的影响

根据光纤光栅外腔半导体激光器(FBG-ECL)的等效腔模型,讨论了外腔长度对FBG-ECL的调制特性、线宽以及强度噪声的影响.指出增大外腔长度可以使FBG-ECL具有很窄的线宽,非常适合应用在相干光通信系统中.同时发现,外腔长度的增大也将导致强度噪声、调制特性的恶化,外腔的选择对FBG-ECL应用在不同光通信系统中十分重要.     

Stable operation range for laser diodes with an integrated passivecavity in the presence of external optical feedback

It is shown that a laser diode with an integrated passive cavity yields higher tolerable levels of feedback from distant reflections than an equivalent long solitary laser diode. To ensure this high tolerance for external feedback, the internal feedback level in the integrated passive cavity must be of the order of about 10%. Then the laser diode remains stable up to external feedback levels in excess of 1% even if the internal reflection phase is not carefully adjusted. External reflections from a cleaved fiber end are then not sufficient to drive the laser into coherence collapse, if realistic coupling efficiencies between laser diode and fiber are taken into account. The low feedback sensitivity is explained by the combination of the long effective length of the laser and a high relaxation resonance frequency. Therefore, quantum-well laser diodes with a long length and a high relaxation resonance frequency are likewise expected to exhibit low feedback sensitivity     

High slope efficiency measured from a composite-resonator vertical-cavity laser

We report high differential slope of the light versus current (L-I) characteristic in excess of 400% external quantum efficiency from a monolithic dual resonator vertical-cavity surface-emitting laser. The additional optical cavity of the composite resonator can provide gain or loss to the distributed laser mode, depending on the bias conditions. We describe the factors contributing to the internal optical loss, and present a qualitative model for the L-I characteristic. With sufficient current injected into the top cavity, the composite-resonator vertical-cavity laser achieves over 6 W/A of differential slope efficiency from threshold to greater than 1 mW of output power, which may be applicable for analog optical data links.     

Noise properties of injection lasers due to reflected waves

This paper describes the theoretical investigation of the origin, qualitative, and quantitative properties of the low frequency noise appearing in the light output of the laser diode, which is strongly coupled to optical fibers. This kind of noise has caused serious problems for reliable optical communications, especially for analog-modulation systems. It is shown that there are two different phenomena which generate such noise. One of them is the double cavity state, and the other is the external light injection state. The cause of our noise considered in the double cavity state is the phase variation due to the variation of the equivalent length between the laser and the reflection point generated by mechanical vibrations. On the other hand, the cause in the external light injection state is the random generations of locking and unlocking states due to the frequency variation (or mode jumping) caused by the variation of the internal temperature of the laser diode. We conclude from our theory that an effective method to reduce such noise is to operate the laser diode at well above the threshold current. The complete elimination will be attained by use of the optical isolator inserted between the laser diode and the transmission lines.     

A 28 μm water-vapor laser interferometer for plasma diagnostics

A high-power CW 27.972 μm water-vapor laser has been developed for interferometric and polarimetric measurements of plasmas. The maximum output power and the power density in the cavity of the laser with 2.0 cm bore and 220 cm length reach 85 mW and 109 μW/cm³, respectively. An interferometer using this laser is constructed and its performance is examined as a study on polarimetric method of magnetic field measurement. Preliminary interferometric experiments are also carried out on a compact toroidal plasma with field-reversal configuration.     

光栅外腔提高VCSEL微波调制效率实验研究

实验研究了垂直腔面发射激光器(vertical cavity surface emitting laser,VCSEL)在闪耀 光栅作为外腔反馈时的直流和微波调制特性。VCSEL底部端面和外部闪耀光栅组成法布里- 珀罗(Fabry-Perot,FP)腔,由于闪耀光栅的反馈作用,可以提高VCSEL微波调制效率。在VCSEL仅有直流 电流驱动时,实验观察到了频率与FP腔长有关的边带功率,调节FP腔长,使FP腔谐振频率与 微波调制频率耦合,可在相同微波功率下得到2倍的有效边带功率。研究结果表明,闪耀光 栅外腔VCSEL可以在较宽的输入较小微波功率范围内得到较高的有效边带功率。本文研究将 为微波调制VCSEL及其他半导体激光器应用提供一定的参考意义。