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     

Phase-modulating optical coherence domain reflectometry bysynthesis of coherence function

Phase-modulating optical coherence domain reflectometry by synthesis of the coherence function is proposed as a promising method for diagnosing optical waveguide devices and systems, and basic experiments are carried out. In this method, a delta-function-like optical coherence function is synthesised by the direct frequency modulation of a laser diode, and scanned by the phase modulation of a reference lightwave in an interferometer. A spatial resolution of ~1.8 mm is achieved with ~18 cm measuring range in basic experiments     

Microwave signal generation using an optical self-heterodyne technique

A new technique for mixing optical waves to generate microwave-frequency signals using an injection laser diode and a fibre-optic interferometer is reported. A beat signal is produced by interfering light from the laser with light from the same laser emitted earlier at a different frequency. The interferometer consists of an evanescent-field fibre coupler joined to a length of single-mode optical fibre. The laser is tuned by pulsed or bipolar current waveforms superimposed on a DC bias current. Feedback from an external cavity stabilises the laser frequency and reduces its linewidth. Efficient mixing at difference frequencies from 250 MHz to 2 GHz is obtained, with spectral widths of less than 1% of the centre frequency.     

基于等光频细分重采样的调频干涉测距方法

为了解决调频连续波(FMCW)激光器调制非线性导致的测量信号频谱展宽降低激光干涉测距精度的问题, 采用一种基于等光频细分重采样的调频干涉测距方法, 进行了理论分析和实验验证, 获得了双光路测距系统对不同位置目标信号等光频细分重采样后的波形数据, 并进行了频谱分析。结果表明, 通过等光频细分重采样的方法, 使用细分后的时钟信号点对距离大于辅助干涉光路光程差的目标测量信号进行重采样, 消除了激光器的调制非线性的影响, 并且避免了采样点数不足引起信号失真的问题; 在4.3m测量范围内, 等光频细分重采样测距系统与激光干涉仪相比最大残余误差不超过±18.46μm, 最大测量标准差为23.39μm; 该方法使用的辅助干涉光路光程差很短, 受环境的影响较小, 可以获得稳定的时钟信号, 并且可以减少双光路FMCW测距系统的体积与成本。该研究为长距离、高精度调频连续波测量提供了实用参考。     

Fiber ring laser with flint glass fiber and its sensor applications

A new fiber ring laser has been proposed which is composed of a laser diode amplifier, a flint glass fiber as a sensing element, and a Faraday cell as a bias element, and applied to a frequency-domain sensor in which nonreciprocal effects such as current or rotation can be measured from a frequency change in the beat signal. A sign of current or rotation direction can be determined and a lock-in phenomenon is avoided owing to the bias element     

Optical homodyne frequency domain reflectometer using an externalcavity semiconductor laser

An external cavity traveling-wave semiconductor ring laser with narrow linewidth is used as a light source for research in frequency domain reflectometry. The optical frequency of the laser is linearly chirped by an intra-cavity phase modulator. The time-delayed reflection signal is mixed with a reference signal to produce a microwave frequency that indicates the position of the reflection. For optical fiber measurement, a spatial resolution of 30 m and a one-way dynamic range of 28 dB for Rayleigh backscattered light have been achieved     

FMCW反射计检测距离影响因素分析

频率调制连续波（FMCW）反射计以其高动态范围和窄的空间分辨率优势,在光网络检测、集成光路诊断和光纤传感等领域有着广泛的应用前景。在介绍FMCW反射计基本原理的基础上,分析了光源扫描重复频率、光源功率与FMCW反射计检测距离之间的关系,并探讨了光源相干长度和相位噪声对检测距离的影响。理论分析表明,当待测光纤的长度接近光源的相干长度时,中频信号和相位噪声之间的信噪比会急剧下降,所以FMCW反射计要使用高相干性的光源和一定的相位噪声补偿方法才能应用于长距离的光纤检测。     

基于锁相放大器和LabVIEW的激光功率测量系统

为了降低激光噪声对光学实验的影响,采用了锁相放大器配合光学斩波器进行降噪。斩波器将激光调制成某一特定的频率,并把该频率传送至锁相放大器作为参考信号,锁相放大器再利用相干检测的方式提取出有用信号。为了减轻实验过程中操作员的劳动,使用LabVIEW平台开发了一套自动化软件系统,实现了实验操作和数据记录的自动化,极大地简化了操作流程。结果表明,与直接使用功率计测量激光强度的方法相比,该方法测量噪声从9.1%降低到4.7%;与人工记录数据相比,使用自动化软件系统节约了一半以上的实验时间。     

Narrow linewidth fiber laser for 100-km optical frequency domain reflectometry

A diode-pumped single-frequency piezoelectrically tuned fiber laser with narrow spectral linewidth has been used as a light source in applications for long-range coherent frequency-domain reflectometry. Frequency-modulated continuous-wave (FMCW) measurements of Rayleigh back-scattering and Fresnel reflection from a 95-km-long fiber have been demonstrated without the use of an optical amplifier. This is, to our knowledge, the longest distance measurement with FMCW. The high sensitivity and dynamic range of the long-range backscattering measurements benefit from the extremely long coherence length of the narrow linewidth fiber laser, which has been estimated to be 210 km in air.     

光调制解调器的设计与实现

研制了一种光调制解调器,该光调制器是由高频DFB激光二极管组件与APC电路、ATC电路组成,光纤传输线为5km光纤;光解调器是由高速跨阻放大器的PD组件与宽带低噪声放大器组成.系统实现了宽带微波信号的高线性、低失真、远距离光纤传输.     

Optical coherence domain reflectometry by synthesis of coherencefunction with nonlinearity compensation in frequency modulation of alaser diode

Optical coherence domain reflectometry (OCDR) is proposed by synthesis of the coherence function for high-spatial-resolution diagnoses of optical systems and waveguide devices. By changing the output frequency of a laser diode using its direct frequency modulation characteristic, the coherence function having the shape of a delta function is synthesized. In the basic experiments, the reflectivity distribution is measured successfully by sweeping the position of the peak. Nonlinearity compensation in the direct frequency modulation is experimentally demonstrated in order to improve the resolution. A three-electrode DFB laser diode with wide-frequency tunable range is introduced as the light source. A spatial resolution of 2-4 mm in air is demonstrated. The measurement range flexibility of this method is confirmed in the experiments     

以半导体激光器线性频率调制为基础的距离测量

给出一种以频率调制连续波(FMCW)技术为基础的进行无接触、无累加误差和较大动态范围距离测量方法.由可调谐半导体激光器产生一个啁啾信号,通过测量拍信号频率计算距离.用商用GaAlAs激光器组成的测量系统的动态范围为1m,测距精度达10-3.     

32通道光纤阵列式高灵敏飞秒激光光谱分析仪

针对飞秒激光光谱学研究中对微弱光信号的多通 道光谱分析的需求,提出了一种32通道光纤阵列- 锁相放大光谱分析仪(OSA)设计方案。32通道光纤阵列用于传输线型分布光谱信号,经光电 探测器(PD)转换的电信号 由32通道同步数字锁相放大器进行处理。基于现场可编程门阵列(FPGA)设计的32通道同步 数字锁相放 大器,实现了各路独立的微弱光谱信号同时检测功能。针对飞秒激光光谱测量中重复频率为 1kHz的强干 扰,设计1kHz陷波器,提高信号测量的动态范围。设计的OSA的测量光谱范围 为400～1100nm,通 道光谱间隔为4nm。实验结果表明:32通道光纤阵列OSA的通道隔离度大于80dB,输入光噪声密 度为37.9fW/Hz,各通道极限检测灵敏度可达 18.9fW,动态储备达到120dB。应用于飞秒时间分辨荧光 光谱测量,对超荧光干扰抑制效果明显,同时极大地提高了测量灵敏度并缩短动力学测量时 间至6min。 本文实现的OSA具有灵敏度高、体积小和性价比高等优点,适用于对微弱光谱 信号的高灵敏与快速检测分析。     

Laser detection by electronic instead of optical heterodyne using a two-frequency laser

A laser ranging system using a two-frequency laser is demonstrated, and the lidar-radar concept is introduced. A laser beam carrying 100 MHz radio frequency is obtained by a monolithic nonplanar ring single-frequency oscillator and an acousto-optical modulator, which is used as the light source of the two-frequency detecting experimental setup. With the optical transmitting and collecting system, the displacement information of a target mounted on a motorized translation stage is achieved. In signal processing, the displacement is obtained by calculating the phase difference between the reference and detection signals executed by a radio-frequency lock-in amplifier. The ranging system turns the optical heterodyne into an electronic demodulation, and the repetition error is less than 3%. The system takes advantage of the signal processing technologies of radar, and meanwhile maintains the advantages of laser detection.     

基于受激增益与自发衰减的激发－探测显微技术

基于激发-探测显微技术研究受激增益与自发衰减现象。采用脉冲二极管激光器（pu=635 nm作为激发光束）与锁模掺钛蓝宝石激光器（pr=780 nm作为探测光束）。受激增益部分,发射光束在频率f1被调制,利用光电二极管作为探测器（PDA 36A,Thorlabs）,探测光被相应地解调来提取透射方向的信号;自发损耗部分,探测光束在频率f2被调制,从探测荧光由光电倍增管以反射模式解调自发损耗信号。所有情形下使用高性能锁模放大器（HF2LI,Zurich Instruments）。锁模放大器的输出信号接着输入扫描单元的A/D通道用于图像重构。扫描速率设为频率500 Hz,与锁模放大器1.99 ms的时间常数相匹配。由解调荧光信号获取背景大大减少的荧光寿命和光学部分属于散粒噪声的图像。另外,此技术改善信噪比,提高类似多光子显微镜的穿透深度,无须昂贵的飞秒激光器。     

Simultaneous Amplification of Terahertz Difference Frequencies by an Injection-Seeded Semiconductor Laser Amplifier at 850 nm

Two-frequency operation of an 850 nm semiconductor optical amplifier was achieved by simultaneously injection seeding it with two diode lasers. The two frequencies could be independently amplified without strong interference when they were separated by more than 10 GHz, and the spectral purity was preserved by the amplification process. At frequency differences below 10 GHz, unbalanced two-frequency output was observed, which can be explained by a two-mode interaction driven by the refractive index modulation at the beat frequency. The laser system is suitable for the difference-frequency generation of coherent terahertz radiation in ultra-fast photoconductors or nonlinear optical media.     

多类型调频连续波激光雷达测距原理与仿真分析

依据调频连续波激光雷达测距原理,详细阐述了三种不同类型的调频连续波激光测距雷达的基本过程,并对其测距过程进行了推导;搭建了锯齿波、三角波和正弦波三种不同类型激光测距原理验证平台,设计了调制信号产生模块,拍频仿真模块,并测定拍频后的频率值;分析了三种类型调频连续波激光雷达的适用条件和范围,为后续系统设计和硬件选型提供了参考依据。     

Analog CMOS design for optical coherence tomography signal detection and processing.

A CMOS circuit was designed and fabricated for optical coherence tomography (OCT) signal detection and processing. The circuit includes a photoreceiver, differential gain stage and lock-in amplifier based demodulator. The photoreceiver consists of a CMOS photodetector and low noise differential transimpedance amplifier which converts the optical interference signal into a voltage. The differential gain stage further amplifies the signal. The in-phase and quadrature channels of the lock-in amplifier each include an analog mixer and switched-capacitor low-pass filter with an external mixer reference signal. The interferogram envelope and phase can be extracted with this configuration, enabling Doppler OCT measurements. A sensitivity of -80 dB is achieved with faithful reproduction of the interferometric signal envelope. A sample image of finger tip is presented.     

Limitations and noise in interferometric systems using frequency ramped single-mode diode lasers

Frequency ramped diode laser sensing and measurement systems suffer from a variety of limitations and noise sources. Nonlinearities in the frequency ramp produce unwanted sidebands in the frequency spectra of the system output and make accurate distance determination difficult in the frequency domain. Thermally induced drifts in the laser frequency prohibit long-term sensitive phase measurements even with a reference interferometer. It is shown that phase noise due to the fundamental linewidth of the diode laser and not bias current noise determines the noise floor of most FMCW systems in the regimes away from (1/f) noise. Time domain techniques suffer from low resolution because only a few data points can be taken during each frequency ramp and thus achieve poor averaging of the phase noise. The signal to noise ratio (SNR) of frequency ramped systems is shown to be lower (10-30 dB) than the theoretical prediction for an unmodulated heterodyne system, which was substantiated by showing that the minimum detectable phase is somewhat higher than that predicted by the idealized model.     

DFB semiconductor diode laser frequency stabilization employingelectronic feedback and Bragg grating Fabry-Perot interferometer

A method for stabilizing the center frequency drift of a diode laser is described. The method employs electronic feedback of the frequency error signal and utilizes an in-fiber Bragg grating Fabry-Perot interferometer as a frequency discriminator. The measured standard deviation of the frequency drift of the heterodyne beat signal of two lasers was less than 16 MHz in a time of 2.5 h