Special features of the distributed feedback lasers based on gyrotropic cubic crystals

The oscillation conditions were determined for the distributed feedback lasers based on gyrotropic cubic crystals. The frequencies of clockwise (counterclockwise) polarized longitudinal eigenmodes decrease (increase) by ω_ρ=ρc/n (ρ is the specific optical rotation of the crystal, n is the index of refraction, c is the speed of light in vacuum) relative to the Bragg frequency ω₀=πc/Λn (Λ is the phase grating period). The gain threshold is higher for the clockwise than for the counterclockwise polarized waves.     

Structural monitoring by use of a Brillouin distributed sensor

The testing of a fiber-optic distributed-strain sensor attached to a simple structural member is reported. A Brillouin scattering-based sensor system was used to measure both tensile and compressive strains along the length of a cantilever beam subjected to various loads. The sensing fiber was attached to the beam in such a way that some sections experienced uniform strain, whereas others were subjected to a nonuniform strain distribution. A spatial resolution of 0.4 m was used, and a measurement precision of approximately +/-50 microepsilon was achieved.     

腔面镀膜分布反馈量子级联激光器

报道了后腔面蒸镀高反射率镀膜工艺及其对分布反馈量子级联激光器性能影响的研究结果.与没有腔面镀膜的器件相比,采用了腔面镀膜工艺的器件,室温下的阈值电流密度降低了20%,前腔面出光峰值功率提高了50%,斜率效率提高了44%.通过对比镀膜和未镀膜器件的闻值电流密度,估算出器件的波导损耗约为7.25cm-1.     

Transportable automated ammonia sensor based on a pulsed thermoelectrically cooled quantum-cascade distributed feedback laser

A compact ammonia sensor based on a 10-microm single-frequency, thermoelectrically cooled, pulsed quantum-cascade laser with an embedded distributed feedback structure has been developed. To measure NH3 concentrations, we scanned the laser over two absorption lines of its fundamental v2 band. A sensitivity of better than 0.3 parts per million was achieved with just a 1-m optical path length. The sensor is computer controlled and automated to monitor NH3 concentrations continuously for extended periods of time and to store data in the computer memory.     

分布反馈量子级联激光器的光栅制备

利用全息曝光方法制备了分布反馈量子级联激光器的光栅掩模,选择和发展了恰当的用于InGaAs/InP材料的光栅腐蚀优化工艺,得到腐蚀规律,讨论了腐蚀机制。在量子级联激光器的InGaAs/InP层上制备光栅得到分布反馈量子级联激光器,其单模特性较好,信噪比大于30dB。     

Near-infrared distributed feedback solgel lasers by intensity modulation and polarization modulation

Near-infrared distributed feedback (DFB) laser actions of Oxazine 725 dye in zirconia thin films and in silica bulks were investigated. Intensity modulation and polarization modulation were used to generate the DFB lasing. Wideband tuning of the output wavelength was achieved by varying the period of the modulation generated by a nanosecond Nd:YAG laser at 532 nm. Tuning ranges were 716-778 nm and 724-813 nm for the thin film lasers and the bulk lasers, respectively. The laser output showed different polarization characteristics and threshold energy variation when the feedback mechanism was changed from intensity modulation to polarization modulation.     

Active synchronization and carrier phase locking of two separate mode-locked femtosecond lasers

Two independent mode-locked femtosecond lasers are synchronized to an unprecedented precision. The rms timing jitter between the lasers is 4.3 fs observed within a 160 Hz bandwidth over tens of seconds, or 26 fs within a 50 kHz bandwidth. Novel multi-stage phase-locked loops help to preserve this ultrahigh timing resolution while setting on arbitrary delay between the two pulse trains (0–5 ns). Under such synchronization, phase locking between the carrier frequencies of the two femtosecond lasers has been achieved. It is also demonstrated that the same level of synchronization can be achieved with two lasers at different repetition frequencies.     

Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering

A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency (<500 MHz) region with excellent long-term stability has been demonstrated for distributed measurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouin fiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shifted local oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scattering with high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distributed measurements of both temperature and strain over long fiber has been developed utilizing advanced digital signal processing techniques.     

Stabilization of electro-optic modulator bias voltage drift using a lock-in amplifier and a proportional-integral-derivative controller in a distributed Brillouin sensor system

In a distributed Brillouin sensor system, it is crucial to keep the pulse energy uniform for a constant signal-to-noise ratio. This means that the variable dc leakage (pulse base) for the electro-optic modulator (EOM) must be locked. We examine two different methods of locking the EOM bias voltage and look at the advantages and disadvantages of each locking method. It is found that the two locking methods, one based on a lock-in amplifier and the other using proportional-integral-derivative control, both have applications in which they excel at locking the pulse base.     

Frequency switching multiwavelength Brillouin Raman fibre laser based on feedback power adjustment technique

A simple switchable multiwavelength Brillouin–Raman fibre laser (BRFL) was demonstrated. The laser was arranged in a half-open cavity configuration including a physical mirror device with an adjustable reflectivity at one side of the laser cavity. The impact of the feedback power adjustment on frequency switching was carried out by comparing the peak power difference between odd- and even-order Stokes lines. Up to 468 flat-amplitude lines with a 10?GHz frequency spacing and average optical signal to noise ratio (OSNR) of 33 dB were observed with mirror reflectivity values of ～15% up to 60% at a 1534?nm Brillouin pump power of 7?dBm and 0.9?W Raman pump power of 0.9?W. – Under the same pumping conditions, setting the mirror reflectivity at its OFF state (where reflectivity is nearly 0%) allows for up to 242 lines with 20?GHz spacing to be realized, with ONSR values of ～35?dB.     

Polarimetric distributed feedback fiber laser sensor for simultaneous strain and temperature measurements

We report the application of a dual polarization distributed feedback (DFB) fiber laser as a strain and temperature sensor. By measurement of the absolute wavelength of one polarization as well as the polarization beat frequency, strain and temperature were determined simultaneously. The sensor has an accuracy of +3 microepsilon and +/-0.04 degrees C. Self-heating of the DFB fiber laser as a function of pump power was measured with this sensor.     

Distributed Brillouin fiber sensor for detecting pipeline buckling in an energy pipe under internal pressure

A distributed Brillouin fiber sensor has been employed to detect localized pipe-wall buckling in an energy pipe by measuring the longitudinal and hoop strain distributions along the outer surface of the pipe for the first time. The locations of the localized pipe-wall buckling are found and distinguished using their corresponding strain-load data. The formation of the buckling process for the compression and tension characters is studied in the longitudinal and hoop directions. For the pipe with internal pressure, concentric load, and bending load, a localized pipe-wall buckling takes place away from the middle of the pipe on the compressive side and a strain peak with an overall buckling occurs on the tensile side according to the longitudinal strain distributions along the pipe. Different strains on two neutral lines are also observed in the hoop strain distribution, which should be caused by the pipe weld joint.     

Dependence of coupling coefficient and lasing wavelength on injection current in pure gain-coupled distributed feedback semiconductor lasers

A theoretical analysis of purely gain-coupled distributed feedback lasers (PGC-DFB) with antireflection facets is given. The effects of longitudinal non-uniformity of photon and carrier density above a threshold circumstance are considered. In addition, the influence of carrier-induced refractive index change on the coupling coefficient of the PGC-DFB structure is assumed. It is shown that the coupling coefficient in this structure varies with injected current and it gets a real part or an index coupled term. As a result the normalized coupling coefficient, κL, becomes a complex number. So, above the threshold condition, the PGC-DFB laser operates like a complex coupled one. Variation of the oscillation wavelength and the threshold gain of the PGC-DFB laser in terms of current is analyzed too. Numerical analysis shows that this structure has wavelength tunability with respect to current. The theoretical model is based on the self-consistent solution of coupled wave equations and the carrier rate equation by the transfer matrix method.     

DC sensor based on magnetic potential self-balance and feedback compensation

A new type of DC current sensor for 10 000 A, which integrates the advantages of DC measurement devices based on the open-loop and closed-loop principle, is presented. One magnetic core carrying two windings is designed as a magnetic potential self-balance loop. In order to detect and compensate the remaining ampere-turns except magnetic potential self-balanced, two feedback compensation detection magnetic cores are designed as a feedback compensation loop. Its operation principle and characteristics, including stability, error analysis and dynamic response, are analysed in detail. Finally, its validity is verified by experiment and the results show that the measuring accuracy of the proposed DC sensor can be enhanced up to 0.13% whereas the accuracy of a conventional DC sensor is only 0.5%.     

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The laser's total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.     

High-resolution position sensor based on two sensing elements

A high-resolution optical position sensor based on two sensing elements is described. The important system nonlinearities and error sources are identified, and correction methods are presented. An autozeroing scheme is employed to obtain an accurate center point and to reduce the effects of temperature variations and component tolerances on the overall system. The predicted results are verified experimentally using commercial parts     

激光器自动锁频方法研究综述

介绍了激光自动锁频的意义与目的,阐述了实现激光自动锁频的算法和常用的光学频率参考,分析了不同算法和频率参考的区别和优劣,探讨了在自动领域下实现激光锁频需要解决的问题和方法,指出了有待加强的领域,提出了部分仍待解决的困难,得出了当前激光自动锁频领域的发展方向和改良趋势,为激光器自动锁频的后续研究提供了参考。     

Influence of ZnO nanoparticles on Coumarin-503 and Coumarin-540 dye mixture for energy transfer distributed feedback dye lasers

Abstract

Using organic dyes Coumarin-503 (C503) and Coumarin-540 (C540) as donor and acceptor dyes, respectively, and Nd-YAG as pumping source (355 nm), an energy transfer-distributed feedback dye laser (ETDFDL) was constructed and its characteristics studied. Theoretical studies such as critical transfer radius (R_o), critical concentration (C_o), and half quenching concentration (C_1/2) were carried out using the absorption and fluorescence spectra of donor and acceptor dyes. On varying the input pump energy to the nanoparticle-incorporated ETDFDL and keeping the acceptor and donor dye concentrations constant, the lasing output obtained was found to be higher than that without the use of nanoparticles. This enhancement was due to the size, shape, and coupling between nanoparticles with the dye mixture. Tunability in the range of 435–553 nm was obtained for both donor (C503) and acceptor (C540) DFDL as a function of the angle of interfering beams of the pump laser.     

基于分布式控制的即插即用机械臂系统

提出了将即插即用技术与嵌入分布式控制系统应用于小型通用机械臂系统的方法,详细介绍了基于此方法研制的Rinar机械臂系统。该机械臂由个人计算机(PC)通过通用串口总线(USB)接口控制,支持热插拔,内部采用PID算法进行运动控制。实验显示,Rinar能快速响应用户指令,运动性能良好,证明了通过USB接口与嵌入式分布式控制机构对机械臂进行控制的可行性和有效性。