Study on Frequency Coherence Properties of Light Beams

This paper presents a new concept of frequency coherence in the frequency-time domain to describe the field correlations between two lightwaves with different frequencies. The coherence properties of the modulated beams from lightwave sources with different spectral widths and the modes of Fabry-Perot (FP) laser are investigated. It is shown that the lightwave and its corresponding sidebands produced by the optical intensity modulation are perfectly coherent. The measured linewidth of the beat signal is narrow and almost identical no matter how wide the spectral width of the beam is. The frequency spacing of the adjacent FP modes is beyond the operation frequency range of the measurement instruments. In our experiment, optical heterodyne technique is used to investigate the frequency coherence of the modes of FP laser by means of the frequency shift induced by the optical intensity modulation. Experiments show that the FP modes are partially coherent and the mode spacing is relatively fixed even when the wavelength changes with ambient temperature, bias current and other factors. Therefore, it is possible to generate stable and narrow-linewidth signals at frequencies corresponding to several mode intervals of the laser.     

短相干长度全固态多纵模绿光激光器的研究

本文从理论上分析了多纵模激光器的相干长度随光程差的改变而呈现周期性的特点,利用计算机对激光器多纵模输出时的相干特性进行模拟,分析了相干长度与纵模数之间的天系,并且通过实验进行了验证,理论分析与实验结果一致.采用拉长谐振腔腔长的方法得到了短相干长度的全固态绿光激光器.     

Spectral analysis of optical mixing measurements

A general rigorous theory of optical heterodyne and homodyne measurements is presented. The power spectrum of the photocurrent resulting from two uncorrelated optical beams mixing on a photodetector is derived. In particular, a rigorous analysis is presented for the delayed self-homodyne method which is used to characterize laser source linewidth by a Mach-Zehnder interferometer with a delay exceeding the source coherence length. Existing treatments are generalized to address non-Lorentzian laser sources of arbitrary lineshape. The analysis is further generalized to cover the case of modulated nonstationary sources. An example of the application of this theory is given. It is shown how the theory may be used to interpret an experimental result obtained using the gated delayed self-homodyne technique for characterizing the frequency chirp of laser sources under modulation     

激光外差干涉中声光器件非互易特性研究

针对声光器件存在的非互易现象导致光强衰减影响衍射效率并降低激光外差干涉效率的问题,进行了一种基于声光驱动100MHz时声光器件非互易效应对激光外差干涉影响的研究。以光束衍射实验为主体,结合光阑、TeOa晶体对声光器件非互易值进行测量,利用光功率计获得测量过程中的器件非互易数值,在不同驱动频率下分析衍射效率分布情况;并利...     

Polarization-independent transmission on a single mode fiber usingcoherence modulation of light

A polarization-independent transmission link on a single mode fiber at 1300-nm wavelength is demonstrated using coherence multiplexing. The use of a source with a very short coherence length (about 200 μm) allows an easy implementation of the suitable optical delays (some millimeters) using LiNbO₃ modulators matched to fiber interferometers. Experiments were conducted with transmission of analog signals and baseband TV signals on a 2500-m-long single mode fiber. The polarization-independence of the method is obtained at the cost of relatively high losses. The high losses are introduced by the interferometers that are required at both ends of the link (theoretical losses of -3 dB/interferometer). They are physically inherent in the coding and detection process, which requires optical delays greater than the coherence length, and hence cannot be decreased     

Electrooptic modulation of multilongitudinal mode laser diodes: Demonstration at 850 nm with simultaneous data transmission by coherence multiplexing

An electrooptic modulation method based on signal coding through optical delays larger than the source coherence length is described. The method is illustrated with the simultaneous transmission of several signals by path-difference multiplexing. In this method, the signals are coded as a sequence of optical delays introduced by a series of electrooptic modulators working as delay lines. Decoding is carried out by temporal correlation performed by another sequence of optical delays introduced by birefringent plates, and matched to those used at the emission process. The permissible delays are shown to be ruled by the source coherence length and by the number of laser modes.     

采用MOPA结构的光学外差干涉激光器

为了满足激光干涉成像,尤其是傅里叶望远镜成像对差频稳定的高功率光学外差干涉模式的需要,提出基于主振功放（MOPA）结构的光学外差干涉激光器的概念,并通过实验验证可行性。从原理上指出现有产生光学外差干涉模式方法的局限性,同时给出基于MOPA结构的光学外差干涉激光器的主要优点。指出基于MOPA结构的激光器可能存在明显影响相干性和产生光频漂移等的限制并设计实验验证。结果表明:功率放大过程对相干性没有明显影响,经过单级功放仍能保持线宽小于0.1 GHz（根据实测相干长度计算线宽约30 MHz）,功放过程和倍频过程对光频漂移无影响,实测频漂小于10 Hz,与声光移频器的频率稳定性相吻合,故推知频漂完全由移频器引起。     

Comparison of optical processing techniques for optical microwavesignal generation

Recently, there have been several proposals on using the higher RF harmonics of detected pulses from mode-locked semiconductor lasers as a source of microwave and millimeter waves. This paper compares the performance of three optical techniques of signal processing that have been proposed to select a higher harmonic of a mode-locked laser, by using extensive numerical simulations. We show that techniques using delays and splitters are insensitive to the coherence properties of the source, but can introduce amplitude patterning if pulses overlap when recombined. We see that techniques relying on optical filtering to select optical modes require extremely high-Q filters and, thus, are extremely sensitive to tuning. A Fabry-Perot interferometer (FPI) is the optimum fitter method in terms of power efficiency for low harmonics, but using two separate bandpass filters can give comparable efficiency when selecting higher harmonics. We also show that gain-switched lasers are unsuitable as sources when used with narrow-band optical filtering techniques because of their low pulse-to-pulse optical coherence     

一种测量液体旋光特性的共光路外差干涉技术

为了提高旋光特性检测的灵敏度,设计搭建了一套基于塞曼效应的共光路外差干涉仪应用于旋光液体的旋光特性检测.采用塞曼效应激光器作为光源能发出两束正交线偏光,简化了实验装置的复杂性;且在系统中利用平衡检测与差动放大器,可降低激光器本身与外部环境所造成的大部分噪声,从而提高检测灵敏度.结果表明,对(0～50)mg/dL的低浓度葡萄糖水溶液的旋光特性测量,其检测灵敏度可达2.3×10-5(°).这种结构简单的干涉仪可应用于固态、液体或气态旋光物质的旋光特性分析中.     

Effect of laser-diode power change on optical heterodyneinterferometry

A dynamic interferometer with a frequency-ramped laser diode (LD) has been constructed that is based on an optical heterodyne technique. Effect of LD power change on the optical heterodyne interferometry has been investigated theoretically and experimentally. A general formulation of the interferometry has been presented in which the demodulation scheme is analyzed in the frequency domain by Fourier-transforming a beat signal. Phase errors caused by the LD-power variation alter sinusoidally with the same period as the interference beat signal. The error can be eliminated by using an amplitude-stabilized, frequency-modulated laser diode source in the interferometer. The stabilization of the LD amplitude can be achieved by a feedback system with a superluminescent diode as an external light-power controller     

高灵敏度的多波长多光束干涉仪

作者用多纵模He-Ne激光照明多光束Fabry-Perot或Fizeau干涉仪,在一个自由光谱范围内,形成与不同波长对应的子条纹,相邻条纹间的间隔代表的波长数依赖于He-Ne激光腔的光学长度npL与F-P腔长nd之比r=nd/npL.当r是整数时,不同波长对应的相邻干涉级次相互重叠,条纹间隔为λ/2且强度最大;当r是分数时,r=N/M,N,M为互质的整数,相邻条纹之间间隔为λ/2M.不难做到M=10,条纹间隔为K/20.由于多光束干涉条纹细而锐,有利于读数精度提高,可以测量λ/500的程差变化,不需内插就可以给出空间分布的足够信息.这对光学元件的高精度面形检验及低密度流场显示有实际的应用前景.给出了应用的若干例子.     

Coherence multiplexing using a parallel array of electrooptic modulators and multimode semiconductor lasers

The paper presents a rather new approach of electrooptic modulation of light suitable for coherence multiplexingNtransmission channels. A parallel array ofNelectrooptic multiplexers powered by multimode semiconductor lasers is used for imprinting the signals on light simultaneously. It is shown that the electrooptic multiplexers have to generate optical time delays whose optimal values are ruled by the sequence1/2N cdot delta f, 1/N cdot delta f, 3/2N cdot delta f, etc., wheredelta fis the intermodal separation of the laser longitudinal modes. The performance of the system is discussed in terms of crosstalk, number of possible channels, and signal power. The demonstration of the method is reported in the area of optical communications with the transmission of base-band video signals.     

Modulated single-longitudinal mode semiconductor laser and fiber transmission characteristics at 1.55 µm

A laser light injection technique was studied to realize a semiconductor laser transmitter oscillating in a 1.55 μm single-longitudinal mode. When -15 dBm optical power was injected into the directly modulated laser, no dispersion degradation was observed in the error rate characteristics after transmitting through 44.3 km single-mode fibers at 100 Mbits/s. Effective gain coefficientg-alpha, measured by the light injection method, was 45 cm^-1near threshold. As this gain was sufficiently saturated at the -15 dBm injection power, undesired longitudinal modes in the modulated laser were suppressed.     

光纤相移点衍射干涉仪关键技术

为了实现光学系统波像差的高精度检测,引入了改进的光纤相移点衍射干涉仪,介绍了其工作原理,并对干涉仪的关键部件包括激光光源及光纤的参数进行了选择和分析。经测试,激光光源功率稳定性约为1%(10 min),光斑尺寸在实现最佳耦合效率允许范围内,光束位置稳定度约为6 um,相干长度为1 cm 左右,都在测试精度允许范围内;选择了纤芯直径为3.5 um 的单模不保偏光纤,对光纤端面镀半反半透金属膜,实现了较高的条纹对比度和光能利用率,并设计了波前参考源,方便了光纤端面的抛光、镀膜及装卡。最后,利用选择的部件搭建了光纤相移点衍射干涉仪实验装置,为最终能够实现光学系统波像差的高精度检测提供了前期的准备。     

基于受激布里渊散射的自调Q掺铒光纤激光器简化模型

文中分析了在980nm低功率连续泵浦源泵浦的条件下基于瑞利(RS)受激布里渊散射(SBS)效应的掺铒光纤激光器的自调Q过程。根据泵浦功率与重复频率、有效单模光纤长度与输出功率、有效单模光纤长度与脉冲宽度(FWHM)以及光纤干涉环分光比与输出功率等关系,通过仿真得知单模光纤长度为10m和光纤干涉环分光比为70:30时的输出功率最大,得到了最佳有效单模光纤长度和最佳光纤干涉环分光比,优化了激光器结构参数,并建立了自调Q掺铒光纤激光器的简化模型。     

High-Power Distributed Feedback Laser Diodes Emitting at 820nm

通过将二级光栅直接刻在脊形波导AlGaInAs/AlGaAs DFB激光器的无铝光波导层上,实现了波长约为820nm,单面功率为30mW的单纵模激光器.由于采用无铝光栅,保证了二次外延质量,从而得到较好的器件性能.激光器的阈值电流为57mA,斜率效率约为0.32mW/mA.     

激射波长为820nm的大功率分布反馈激光器

通过将二级光栅直接刻在脊形波导AlGaInAs/AlGaAs DFB激光器的无铝光波导层上,实现了波长约为820nm,单面功率为30mW的单纵模激光器.由于采用无铝光栅,保证了二次外延质量,从而得到较好的器件性能.激光器的阈值电流为57mA,斜率效率约为0.32mW/mA.     

Modal discrimination in leaky-mode (antiguided) arrays [diodelasers]

Presents on the fabrication of a ten-element antiguided (leaky-mode) diode laser array that operates in an in-phase mode with a diffraction-limited far-field pattern at drive currents up to 8.7 times the threshold current (pulsed) and an output power of 400 mW (both facets). This device evidences good discrimination against the out-of-phase leaky mode without the benefit of a `Talbot' spatial filter, relying instead on mode-sensitive interelement losses in an overlying GaAs layer. This device has been studied using two numerical models. Results from the more realistic of the two models predict in-phase-mode operation for certain restricted ranges of waveguide dielectric constant, in agreement with experiment. This behavior is shown to result from resonances between the waveguide modes and the modes of the active region. These results suggest that improved yields would result from a different type of leaky-mode array employing smaller, rectangularly shaped waveguides     

相干分辨外差干涉法的光学系统表面检测

提出了一人用相干分辨的外差干涉仪进行光学系统表面检测的方法，利用相干长度短的光源和位置探测器。该方法具有表面选择性，高分辨率和测量时间短等特点。     

Characterization of the array modes of high-power gain-guided GaAssingle-quantum-well laser arrays

The characteristics of gain-guided GaAs single-quantum-well laser arrays, using proton bombardment to confine the current flow to 11 stripes on 9-μm centers, are described. The longitudinal-mode spectrum, the spectrally resolved far-field, and the near-field of this laser array were studied experimentally up to three times threshold current. The laser arrays are found to emit at ~10 longitudinal modes corresponding to the Fabry-Perot modes of the laser. The longitudinal spectrum is current dependent. Near threshold current, the spectrum consists of a number of longitudinal modes where only the fundamental mode of the array laser. At larger current, each of the longitudinal modes is broadened and the spectrum of each longitudinal mode consists of many individual emission lines of the array modes. For each longitudinal mode, the lowest order array mode emits at the longest wavelength and the highest order array mode emits at the shortest wavelength. The wavelength separation between neighboring array modes increases with increasing mode order. Fourteen array modes are found to emit simultaneously at three times threshold current. The experimental results are in good agreement with direct array-mode analysis based on the finite-difference method