基于DAVLL技术的半导体激光器稳频装置设计

分析了二向色性原子蒸气激光频率锁定(dichroic atomic vapor laser lock,DAVLL)技术稳定激光器频率的原理,并采用DFB894.6nm半导体激光器和Cs原子气室搭建稳频实验装置。实验测量了不同磁场条件下的DAVLL光谱,发现Cs原子D1线的DAVLL光谱零值点处的斜率随磁场强度增加而增大,但谱线零点斜率不随磁场变化。根据半导体激光器锁频原理设计制作了驱动电路,测试结果表明,该稳频装置的短时频率稳定度达16 MHz。     

Note: Demonstration of an external-cavity diode laser system immune to current and temperature fluctuations

We demonstrate an external-cavity laser system using an anti-reflection coated laser diode as gain medium with about 60?nm fluorescence spectrum, and a Rb Faraday anomalous dispersion optical filter (FADOF) as frequency-selecting element with a transmission bandwidth of 1.3?GHz. With 6.4% optical feedback, a single stable longitudinal mode is obtained with a linewidth of 69?kHz. The wavelength of this laser is operating within the center of the highest transmission peak of FADOF over a diode current range from 55?mA to 142?mA and a diode temperature range from 15?°C to 35?°C, thus it is immune to the fluctuations of current and temperature.     

纳米精度干涉测量中半导体激光器的光频调制方法

根据半导体激光器光注入调制理论和实验,基于不同光注入条件下的频率调制特性,提出了三种半导体激光器光频调制的具体方案。通过使用给出的光强反馈及电路控制方法,能使半导体激光器在其光频受调制期间,其输出光强的变化的范围大幅度减小。这些半导体激光器光频调制的方法用于干涉测量系统中,能极大地减小光强波动对测量精度的影响,可实现纳米级的高精度激光干涉测量。     

Extended tuning of an injection-locked diode laser

We have investigated the application of an electronic feedback technique recently reported by Repasky et al. [Appl. Opt. 45, 9013 (2006)] to an injection-locked semiconductor diode laser. We find that without electronic feedback, the injection-locked slave laser will only follow the master for less than 1 GHz, but once the electronic feedback is applied, the slave laser is capable of following for more than 20 GHz, corresponding to the full scan range of the master laser.     

An oscillator circuit to produce a radio-frequency discharge and application to metastable helium saturated absorption spectroscopy

We present a rf gas discharge apparatus which provides an atomic frequency reference for laser manipulation of metastable helium. We discuss the biasing and operation of a Colpitts oscillator in which the discharge coil is part of the oscillator circuit. Radiofrequency radiation is reduced by placing the entire oscillator in a metal enclosure.     

Phase-shifting interferometer using a frequency-tunable diode laser calibrated by an optical frequency comb

We present a phase-shifting interferometer based on a frequency-tunable diode laser calibrated by an optical frequency comb and the Carre? algorithm. By use of the frequency control strategies of locking the diode laser to different comb modes and scanning the repetition rate, an arbitrary single optical frequency synthesizer is obtained. The relative laser frequency uncertainty is 5.7 × 10(-12) for 1 s averaging time with tracing to an Rb clock and accurate phase steps are achieved by optical frequency tuning. The surface topography of a standard sphere is measured by this phase-shifting interferometer based on a flat reference. The phase measurement repeatability is λ∕200. With this technique, phase measurement uncertainties from the laser frequency and phase steps are negligible.     

Creation of arbitrary spectra with an acousto-optic modulator and an injection-locked diode laser

We use a double-passed acousto-optic modulator (AOM), driven by an arbitrary waveform generator to produce multiple frequency components for a laser with arbitrary frequency spacings. A programmed sequence containing various sections of radio-frequency sinusoidal signal at different frequency is applied to drive the AOM. The diffracted light is used to injection-lock a diode laser. The combined techniques allow us to generate the multi-line spectra for the diode laser with arbitrary frequency spacings in the range of 100 MHz at a relatively high output power of 80 mW and a small power variation of 2%. Such a light source can be used in the application for laser cooling of molecules.     

柱状楔形微透镜光纤与半导体激光器耦合效率研究

为了提高半导体激光器与光纤耦合的耦合效率和失配容忍度,提出了利用带柱状楔形微透镜的多模光纤与大功率单片式宽发射域半导体激光器进行耦合的新技术。采用光线追迹的方法,进行数值模拟。通过对光纤微透镜结构参数进行优化,得到了高达99%的理论耦合效率。同时,还在理论上讨论了耦合时在光线传输方向上的失配容忍度。在实验中,得到了最高为87.06%的耦合效率。     

Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance

The linewidth of a distributed-feedback (DFB) diode laser at 1156 nm, of which free-running linewidth was 3 MHz, was reduced to 15 kHz using an all-fiber interferometer with 5-m-long path imbalance. Optical power loss and bandwidth limitation were negligible with this short optical fiber patch cord. This result was achieved without acoustic and vibration isolations, and the frequency lock could be maintained over weeks. In addition to its simplicity, compactness, robustness, and cost-effectiveness, this technique can be applied at any wavelength owing to the availability of DFB diode lasers and fiber-optic components.     

Wave-number stability of a laser diode mounted in a closed cycle helium refrigerator

The wave-number stability of a laser diode mounted in a closed cycle helium refrigerator was examined using a CO(2) laser heterodyne spectrometer. The set current supplied by a commercial laser diode supply appeared stable enough over the time periods normally used. Temperature control was achieved through both passive and active loop control. A stage of open loop control of the cold tip allowed the closed loop control of diode heat sink to be operated at higher gains than conventionally used. Long-term stability of the center of the line shape function was better than 10 MHz per hour and the 1-min average was better than +/-5 MHz. Vibration of the diode mount determined the instrument line shape in spite of steps taken to eliminate the vibrations. The line shape had a half-width of 10 MHz with broad shoulders out to 50 MHz.     

Closed-loop wavelength stabilization of an optical parametric oscillator as a front end of a high-power iodine laser chain

We present a complex stabilization and control system for a commercially available optical parametric oscillator. The system is able to stabilize the oscillator's output wavelength at a narrow spectral line of atomic iodine with subpicometer precision, allowing utilization of this solid-state parametric oscillator as a front end of a high-power photodissociation laser chain formed by iodine gas amplifiers. In such setup, a precise wavelength matching between the front end and the amplifier chain is necessary due to extremely narrow spectral lines of the gaseous iodine (approximately 20 pm). The system is based on a personal computer, a heated iodine cell, and a few other low-cost components. It automatically identifies the proper peak within the iodine absorption spectrum, and then keeps the oscillator tuned to this peak with high precision and reliability. The use of the solid-state oscillator as the front end allows us to use the whole iodine laser system as a pump laser for the optical parametric chirped pulse amplification, as it enables precise time synchronization with a signal Ti:sapphire laser.     

低噪声半导体激光器驱动电源的研制

根据半导体激光器的工作特性,研制了一台低噪声、高稳定度半导体激光器的驱动电流源。该电流源特点具有慢启动电路和过流保护电路,提高了半导体激光器抗冲击能力和工作稳定性。     

Digital feedback stabilization of a single-axial-mode CO2 TEA laser

We have achieved long-term stability in producing high-power single-axial-mode CO2 laser pulses with a hybrid oscillator (TEA laser + low-pressure section) by using a novel digital feedback system that electronically adjusts cavity length.     

Low-cost method for stabilization of a CO2 laser for use in far infrared laser pumping

A simple and inexpensive technique for stabilization of a CO2 laser used to optically pump a far infrared waveguide laser is discussed. The technique uses the fact that the output power of the CO2 laser varies as the output frequency is swept through the Doppler profile of the emission line. Stabilization is obtained by adjusting the mirror separation and forcing the output power to remain constant at the level corresponding to the desired pump frequency.     

New approach to cathodoluminescence studies in application to InGaN/GaN laser diode degradation

Cathodoluminescence (CL) studies are widely applied in semi-conductor science and technology. However, for structures with a p-n junction the CL spatial distribution can be strongly affected by internal current flows of the electron beam induced current generated within the structure. This influence is the investigated in application to CL studies of degradation in aged laser diodes with InGaN multiquantum wells.     

The installation to study the prebreakdown stage of a gas discharge by laser probing

An installation to study a pulsed gas discharge in a diode with a single metal tip by laser probing has been devised. The installation represents a generator comprised of cable sections connected to a five-channel laser-triggered switch. The synchronization of the probing laser beam with the voltage pulse is not worse than ~1 ns. The moment of probing is determined by the optical delay and can be varied independently within the range of 10–20 ns by changing the charging voltage of the cables. The pressure in the discharge chamber can be varied from 10^–5 Torr to atmospheric pressure. Copper or molybdenum wires with a diameter of 10 μm or greater were used as cathode pins. Three independent optical record channels with time delays of 2 and 13 ns relative to the first channel allow one to simultaneously obtain interferograms and shadow and schlieren images of the discharge gap with spatial and time resolutions of 20 μm and 70 ps, respectively, with the exposure time of each frame being equal to a laser pulse duration of 70 ps. Using the devised installation, qualitative and quantitative data on the prebreakdown processes that occur in a tip-type diode have been obtained within a wide range of pressures.     

激光加工用半导体激光器的光束变换

考虑不同的激光加工方式对激光功率密度和激光光斑尺寸的要求不同,研究了如何通过调整光路设计实现各种尺寸的聚焦光斑输出,使半导体激光器满足不同激光加工方式的需求.利用ZEMAX光学设计软件模拟半导体激光光路,包括光束整形、准直、聚焦等光束变换方式,实现了多种尺寸的光斑输出.实验中采用16个bar叠加而成的980 nm半导体激光叠阵,阈值电流为6.4A,最大工作电流为84.8A,最大输出功率为1 280 W,总的电-光转换效率为58.9％.准直后快轴的发散角小于4 mrad,慢轴的发散角小于20 mrad.通过实验对该激光叠阵进行光束整形和扩束准直、聚焦,最终实现了功率为1 031 W的激光输出,聚焦镜焦距为300 mm时的聚焦光斑尺寸达1.2 mm×1.5 mm,功率密度达3.8×104 W/cm2,可以用于金属的表面重熔、合金化、熔覆和热导型焊接.     

高功率半导体激光器列阵封装引入应变的测量

考虑高功率半导体激光器列阵在封装过程中引入的封装应变会影响激光器的功率、波长和可靠性,对激光器封装应变的测量进行了研究。基于激光器输出光的偏振度变化可反映激光器有源区中量子阱的带隙变化,采用电致发光谱法推导了高功率半导体激光器输出的偏振度值与有源区应变值的关系。对800nmGaAsP/GaInP高功率半导体激光器列阵有源区的应变进行了测量,测量结果与有限元模拟计算结果吻合较好。与理论计算出的有源区固有应变的对比结果显示,激光器芯片在封装过程中受到铜热沉的压缩,会将封装应变引入到有源区中,并且激光器中间的封装应变大于边缘的封装应变。另外,激光器有源区的应变起伏比较明显,认为这是由于采用电镀方法制备的铟焊接层中存在缺陷。测量得到的最大封装应变为1.370×10-3,缺陷密度为40.8%。得到的结果表明,激光器偏振度的测量能够正确反映激光器的缺陷和封装应变值,进而可以有效衡量激光器封装质量的好坏。     

Measurement of temperature of laser cooled atoms by one-dimensional expansion in a magneto-optical trap

We discuss a simple time of flight technique for measurement of temperature of a cold cloud in a magneto-optical trap (MOT). The technique is based on spatiotemporal fluorescence imaging of the cloud that is allowed to undergo one-dimensional expansion in the presence of the orthogonal two-dimensional configuration of laser beams by temporal modulation of a pair of counterpropagating trapping beams in the MOT. We show that, in the time scale 0< or =t<5 ms, the expansion of the cloud is ballistic and the temperature can be extracted from the time variation of the rms size of the cloud in the expansion direction. The reliability of the technique has been established by comparing the results with release and recapture method, and also by fitting them to the known temperature scaling law.     

Surface and moisture characteristics of jute using a D.C. glow discharge argon plasma

A D.C. glow discharge plasma source was employed to characterize changes in water absorbance and surface morphology of natural jute fibers. The low temperature plasma removed moisture from the fibers and significantly modified surface properties. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used for characterization. Open source software was used for the processing of the scanning electron micrographs. The changes in the macromolecular structure and the crystallinity were characterized by X-ray diffraction. Thermogravimetric analysis demonstrated moisture removal from the fibers following plasma treatment.