Laser frequency stabilization using a balanced bi-polarimeter

We report on a Doppler-free polarization spectroscopy based technique of laser frequency stabilization using a balanced bi-polarimeter set-up. Two linearly polarized weak laser beams are used to probe birefringence induced by two oppositely circularly polarized strong pump beams in a vapour cell. Subtraction of balanced polarimeter signals obtained from the two probe beams results in a background-free dispersion-like reference signal without frequency modulation. The dispersion-like signal corresponding to the closed transition 5 ² S _1/2 (F=2) →5 ² P _3/2 (F^′=3) of ⁸⁷Rb was used for frequency locking of a diode laser. The frequency fluctuations and the drift were measured to be less than 0.25 MHz and 0.02 MHz, respectively, for an observation period of more than 10 hours. PACS 42.62.Fi; 42.55.Px; 32.30.-r     

混沌信号相关法测量电介质传输线的断点及阻抗失配

本文通过实验验证了一种基于宽带混沌信号测量电介质传输线的断点和阻抗失配的方法.该方法利用光反馈半导体激光器产生的混沌信号作为探测信号,通过相关法对传输线进行断点定位、阻抗失配测量以及在线检测.实验结果表明,断点的定位偏差小于1%,阻抗的测量值在额定范围内,对于频率为2 MHz的矩形脉冲信号的干扰容限为48 dB. 关键词： 混沌 故障定位 半导体激光器 时域反射法     

Numerical analysis of complex impedance and microwave absorption of metamaterials composed of split cut wires on grounded dielectric substrate

The microwave absorption of metamaterials composed of split cut wire (SCW) on grounded dielectric substrate has been investigated on the basis of equivalent transmission line circuit. S-parameters (S ₁₁ and S ₂₁) and input impedance are numerically simulated with variations of the thickness and dielectric loss of the substrate and the geometry of the SCW. Magnetic resonance resulting from antiparallel currents between SCW and ground plane was observed at the frequency of minimum reflection loss. The simulated resonance frequency and reflection loss can be explained well on the basis of the circuit theory of an LC resonator. Analysis of the input impedance of the high impedance surface has shown that perfect absorption can be obtained at the optimized impedance-matching condition, which is dependent on SCW width, thickness and the dielectric loss of the substrate. Better insight into the absorption mechanism of metamaterial absorbers can be attained through the parametric analysis on complex impedance of SCW and its relationship with reflection loss.     

Absolute optical frequency measurement of saturated absorption lines in Rb near 422 nm

We present absolute optical frequency measurements of the 5s² S _1/2–6p² P _1/2 hyperfine resolved transitions in both ⁸⁵Rb and ⁸⁷Rb near 422 nm. The frequency of each transition was measured by stabilizing a narrow-linewidth extended cavity diode laser to the transition under study and by measuring that frequency with a femtosecond laser frequency comb. A frequency-doubled 844 nm laser was used as a frequency link to connect the 422 nm probe laser to the near infrared part of the comb. The resulting uncertainties of <70 kHz in the Rb transition frequencies represent a four-order of magnitude improvement over previously published results. The frequencies reported in this paper are one of the most accurate series of measurements made in the violet region of the spectrum. PACS 06.30.Ft; 33.20.Kf; 43.62.Fi     

Experimental determination of the thermal lens parameters in a broad area semiconductor laser amplifier

Due to the build-up of temperature gradients along their width, semiconductor laser diodes tend to be affected by thermal lensing effects. We propose a simple and easy-to-implement experiment in order to determine the thermal lens coefficient in a broad area semiconductor laser amplifier during operation. The results obtained are compared to simulations of the temperature distribution in the laser structure. In order to further validate our method, we compare the measured M ² value of a free running broad area laser diode with the calculated M ² of such a laser under the influence of a thermal lens as predicted by diffraction theory of first-order optical resonators. The experimental results are seen to be in good agreement with the calculations.     

Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser

Continuously tunable ultraviolet laser radiation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW AlGaAs laser diode injected by a low-power AlGaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radiation was used for sub-Doppler spectroscopy of the Ar I 4s ³ P ⁰ ₀–4p ¹ P ₁ transition at 795 nm; the second-harmonic radiation was used for spectroscopy of the Ca II 4² S _1/2–4² P _1/2 transition at 397 nm.     

基于二次谐波特性的DFB激光器稳频新方法研究

窄线宽稳频激光器在工业生产控制中具有广泛的应用,但自由运转的半导体激光器的频率漂移限制了激光器的使用。为稳定半导体激光器的频率,提出了一种基于二次谐波吸收特性来实现窄线宽二极管激光器的稳频新方法,利用1.396 μm的DFB二极管激光器测量水汽的二次谐波信号来实现激光的稳频,实验结果表明在100 h内激光器输出波长漂移有效的抑制在±0.16 pm范围内,激光稳频后,其吸收峰的位置不随环境温度的变化而漂移。该方法具有简单、可靠等优点,对二极管激光频率的稳定具有广阔的应用前景。     

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/N ratio ranging from 100 to 7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.     

Quantum conductance through a uniform scatterer in a narrow-wire semiconductor

The quantum conductance for electrons scattering from a uniform scatterer in a narrow-wire semiconductor is calculated. Instead of getting the conductance directly from the calculation of transmission coefficient, we calculate the reflection coefficient instead. The transmission coefficient is then calculated by using the conservation law, T=I−R. This alternative method can avoid the instability of the conductance obtained by including more evanescent modes for a finite-range scatterer in a narrow-wire semiconductor. This method is applied to a semi-infinite strip potential barrier and a rectangular potential barrier in a narrow wire. The quantum stepwise conductance is obtained in both cases. For a repulsive rectangular potential barrier, there are oscillations in each stepwise conductance. For an attractive rectangular potential barrier, there exist multiple quasi-bound states below the sub-band energies which can cause the drop of the quantum conductance. The effect of the continuum quasi-bound states diminishes as the energy of the incident electron increases, but the influence of the discrete quasi-bound states still persists.     

Far infrared optical properties of amorphous semiconductors As?S systems

Far infrared reflection, Raman, and Brillouin spectra of amorphous semiconductor As–S systems are measured. From the reflection spectra, the optical constants are calculatured by the Kramers Kronig analysis. The existence of some intermediate range interaction among molecular units AsS₃ is suggested. From the analysis of the Boson peak at low frequency region in Raman scattering, the correlation length is obtained to be ca. 6 Å. From the absorption coefficient in the low frequency region about 3% fluctuation of charge is suggested for As₂S₃ by schlömann theory.     

Frequency Bandwidth Estimation of TO Packaging Techniques for Laser Modules

Two simple methods for estimating the potential modulation bandwidth of TO packaging technique are presented. The first method is based upon the comparison of the measured frequency responses of the laser diodes and the TO laser modules, and the second is from the equivalent circuit for the test fixture, the TO header, the submount and the bonding wire. It is shown that the TO packaging techniques used in the experiments can potentially achieve a frequency bandwidth of over 10.5 GHz, and the two proposed methods give similar results.     

Analysis of characteristics of long-wavelength semiconductor laser diode amplifiers

The effects of amplifier facet reflectivities, cavity length, input signal frequency, amplifier cavity resonant mode, modulation frequency and bias current on various semiconductor laser diode amplifier characteristics have been investigated for both 1.3m- and 1.55m-wavelength amplifiers. The analysis is based on a parabolic relationship between the amplifier peak-gain coefficient and injected carrier density, which improves the accuracy and range of applicability.     

基于飞秒光频梳的双频He-Ne激光器频率测量

利用光纤飞秒光频梳和外腔可调谐半导体激光器, 建立了一套双频He-Ne激光器频率测量系统. 选用铷钟作为系统的频率基准, 通过将外腔半导体激光锁定至光频梳使得其频率溯源至铷钟, 再利用外腔可调谐半导体激光与双频He-Ne激光器输出的正交偏振激光拍频, 同时测量两路正交偏振激光频率. 将可调谐半导体激光器锁定至光频梳第1894449个梳齿, 其绝对频率为473612190000.0±2.7 kHz, 相对不确定度为5.7×10^-12. 对商品双频He-Ne激光器进行频率测量实验, 双频He-Ne激光器水平方向偏振激光频率均值为473612229934 kHz, 竖直方向偏振激光频率均值为473612232111 kHz, 平均时间为1024 s的相对Allan标准差为5.2×10^-11, 频差均值为2.177 MHz, 标准偏差为2 kHz.     

Method to measure acoustic impedance and reflection coefficient.

A frequency-domain based system for measuring acoustic impedance and reflection coefficient is described. The calibration procedure uses a least-mean-squares approximation to the Thevenin parameters describing the source and receiver characteristics in which the data measured on closed, cylindrical tubes are matched to a viscothermal tube model. The system is intended for use in acoustical measurement in human ear canals, in which the cross-sectional area of the ear canal at the point of insertion is imprecisely known. This area is acoustically estimated from the impedance data, and the reflection coefficient is calculated in terms of this area and the impedance data. Measurements on a variety of closed tubes show the method is accurate over the frequency range investigated (less than 10.7 kHz). The time-domain reflection function is evaluated by transforming the reflection coefficient from the frequency domain, but the finite bandwidth of the measured data limits the accuracy of time-domain response measurements. The method is well suited for frequency-domain measurements in human ear canals.     

A fiber bragg grating acceleration sensor interrogated by a DFB laser diode

A fiber Bragg grating (FBG) acceleration sensor interrogated by a DFB laser diode is demonstrated; the sensor is fabricated by packaging a FBG on to a cantilever; the interrogation scheme takes advantage of the intensity modulation of the narrow spectral bandwidth light of a distributed feedback laser diode, when the reflection spectrum curve of a FBG moves due to the strain which is applied on the sensor. The sensor’s response to the frequency and acceleration is measured by the experiment, and the factors which have an impact on the sensor’s sensitivity are also discussed.     

硅的间接跃迁双光子吸收系数谱

利用皮秒Nd:YAG脉冲激光器作为激发光源,测量出光子能量介于1.36 μm (0.912 eV)—1.80 μm (0.689 eV)之间的硅间接跃迁双光子吸收系数谱.尽管此波段范围内的激光光子能量小于硅间接带隙,但当激光辐照在硅基光电二极管受光面时,在二极管两电极端仍然探测到了显著的脉冲光伏信号.光伏信号峰值强度与入射光强呈二次幂函数关系,表明其是双光子吸收过程.采用pn结等效结电容充放电模型,将光伏响应信号峰值与入射光强相关联,从中提取出硅的间接跃迁双光子吸收系数,改变入射波长得到系数谱.研究表明:     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

The effect of spatial filtering on non-linear distortions of laser diodes

It is demonstrated for the first time that spatial filtering of partially reflected irradiance of light from a directly modulated semiconductor laser diode produces a weak but appreciable amount of harmonic distortions. The laser diode discussed here is assembled to be misaligned in such a way that there is significant reflection of light from the submount attached to the heat sink. This geometry of laser and submount gives rise to interfered light intensities of simultaneous AM- and FM-modulated outputs in the far-field region. Numerical calculations are presented for a variety of laser structures and FM-modulation parameters. The second-order harmonic distortion is found to disclose multiple peaks with respect to the aperture radius of a receiving optical system.     

Effect of surface scattering of charge carriers on the high-frequency conductivity of a thin cylindrical semiconductor wire

The problem of high-frequency conductivity of a thin cylindrical semiconductor wire has been solved by the kinetic method. The diffuse-specular mechanism of reflection of charge carriers from the inner surface of the wire has been considered. Calculations have been performed for an n-type (p-type) nondegenerate semiconductor with a standard spherically symmetric energy band. The ratio between the cross-section radius of the wire and the mean free path of charge carriers is assumed to be arbitrary.     

光注入提高半导体激光器混沌载波发射机的带宽

外光反馈下的半导体激光器可视为混沌载波发射机.数值研究发现，外部强光注入可以显著提高混沌载波发射机的带宽，带宽提高的程度在一定范围内与注入光的强度成正比.当外部光注入系数k_inj=0.39时，混沌载波的带宽由无光注入时的2.7GHz增大到14.5GHz，提高了5倍多.研究还发现，在相同的注入强度条件下，当注入光的频率比半导体激光器的中心频率高2—4GHz时，可实现最大限度的带宽增强.此外，适当提高半导体激光器的偏置电流也可以在一定程度上提高其产生的混沌载波的带宽. 关键词： 半导体激光器 混沌 带宽