Intensity dependent residual amplitude modulation in electro-optic phase modulators

Residual amplitude modulation (RAM) mechanisms in electro-optic phase modulators are detrimental in applications that require high purity phase modulation of the incident laser beam. While the origins of RAM are not fully understood, measurements have revealed that it depends on the beam properties of the laser as well as the properties of the medium. Here we present experimental and theoretical results that demonstrate, for the first time, the dependence of RAM production in electro-optic phase modulators on beam intensity. The results show an order of magnitude increase in the level of RAM, around 10 dB, with a fifteenfold enhancement in the input intensity from 12 to 190 mW/mm2. We show that this intensity dependent RAM is photorefractive in origin.     

Localization of an object in tissue with phase modulation spectroscopy: Reduction of the effects of the relative amplitude and phase on amplitude cancellation

Abstract

In this paper, we discuss the effects of various factors on the localization of an object with amplitude cancellation phase modulation spectroscopy (PMS) by an analytical method. The factors include the relative modulation phase and the amplitude between two sources, the absorption coefficient and the size of the localized absorber. A localization method that can reduce the effects of the relative modulation amplitude and phase between two sources on measurement is presented. In the new method, two measurements are carried out, where modulation signals of two sources in the second measurement are exchanged relative to those in the first measurement. The principle and the feasibility of the method are shown theoretically. Primary experimental results are presented.     

High-resolution absorption measurements by use of two-tone frequency-modulation spectroscopy with diode lasers

The capability of two-tone frequency-modulation spectroscopy (TTFMS) in deriving spectral line-shape information was investigated. Two oxygen A-band transitions at 760 nm were selected, and the Voigt profile and two different collisionally narrowed line profiles were employed in their analysis. By means of a least-squares fitting procedure, we obtained accurate information regarding transition strengths and pressure-induced broadening, shift, and narrowing coefficients. Both TTFMS and direct absorption line shapes were modeled with deviations as small as 0.3% over a wide pressure range by use of the collisionally narrowed line profiles. Line parameters measured with TTFMS showed excellent agreement with the parameters measured with direct absorption. The experimental technique used constant-current fast-wavelength scanning, which improved measurement accuracy.     

Long-path monitoring of NO2 with a 635 nm diode laser using frequency-modulation spectroscopy

In situ monitoring of traffic-generated nitrogen dioxide (NO2) emissions using long-path absorption spectroscopy is reported. High-sensitivity detection of NO2 is achieved by employing two-tone frequency-modulation spectroscopy at a visible absorption band using a tunable high-power diode laser operated around 635 nm. A real-time absorption spectrometer is accomplished by repetitively applying a rectangular current pulse to the diode-laser operating current, allowing detection of isolated NO2 absorption lines. A detection limit of 10 microg/m3 for NO2 at atmospheric pressure using a 160 m absorption path is demonstrated. Continuous monitoring of NO2 over a road intersection at peak traffic is performed.     

Improved method of calibration of working standards of the amplitude modulation index

A new method of indirect calibration of working standards of amplitude modulation of the 1st class based on the K2-34 setup is described. Calibration is carried out with the aid of working standards of power, voltage, and nonlinear distorition factor adopted from other calibration schemes. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 53–56, November, 1997.     

Effect of modulation amplitude on low-field microwave absorption in hafnium-doped Y-Ba-Cu-O

Low-field differential microwave absorption has been studied for hafnium-doped Y-Ba-Cu-O at the first harmonic of modulation field. The effect of modulation amplitude on peak height, peak position and the area under hysteresis curve has been discussed.     

Thermal modulation spectroscopy of biological films

Thermal modulation spectroscopic techniques have been developed to study the optical properties of copper complexes of spermine and spermidine. Both pure and mixed solutions of the complexes were used to prepare films on special substrate heaters. Measurement techniques were developed to obtain approximately quantitative estimates of the composition of the mixed films from the modulation spectra. It is concluded that modulation spectroscopy is an appropriate tool for obtaining quantitative information on biologically important compounds. The spectra at 77 K and 300 K were significantly different.     

Method of measuring the envelope harmonic factor of frequency converters in amplitude modulation meters

Translated from Izmeritel'naya Tekhnika, No. 4, pp. 40–42, April, 1988.     

基于最优模型残差的指纹方向校正法

指纹的方向信息是指纹图像处理中非常重要的信息,指纹方向场的获取以及校正直接影响指纹自动识别的性能。本文提出了一种基于指纹方向场数学模型的方向校正方法,根据指纹中的奇异点性质和分布,建立起与对应指纹方向场的最优方向场模型;然后,根据模型与实际指纹方向场的残差对指纹方向进行校正。由于算法利用指纹整体的拓扑信息对指纹噪声方向进行校正,具有很强的抗干扰能力,较适合劣质指纹图像处理。     

Phase retrieval of complex optical fields by binary amplitude modulation

A method to retrieve the complex optical fields from the intensity information recorded in only one plane is discussed. The wavefront to be reconstructed is modulated by using a random binary amplitude mask. Wavefronts with a rather large phase depth can be accurately retrieved by using several masks in combination with iterative Fresnel algorithms and without any prior information. The influences of the contrast of the mask and the dynamic range of the detector on the accuracy of the phase retrieval are analyzed.     

Complex amplitude modulation by use of liquid-crystal spatial light modulators

We propose to study the conditions for implementation of complex amplitude modulation on standard liquid-crystal spatial light modulators when illuminated by polarized light. The spatial light modulators are used in a conventional configuration, i.e., the voltage is applied parallel to the wave-front propagation direction. The most commonly used liquid-crystal materials are compared and their usefulness in some general applications discussed. Their specificities with respect to different modulation types and application requirements are briefly described. Typical characteristics such as response time, modulation range, and wavelength insensitivity are also discussed. Finally, as an illustration, a first attempt at nomenclature is proposed for the case of a linearly polarized light.     

On the correction of residual stress measurements obtained using the centre-hole method

Significant errors in interpreting the results of the centre-hole technique may arise where test conditions deviate from a uniform uniaxial low-level stress field. This paper demonstrates that where the stress field varies linearly with depth, the as-measured stress represents the stress at one-quarter hole depth. In cases where surface preparation for strain gauging has induced residual stress or where stress levels are sufficiently high to induce plasticity during hole forming, simple formulae are proposed to correct for these effects.     

Advances in the application of modulation spectroscopy to vertical cavity structures

Modulated reflectance is a straightforward technique in which the reflectivity spectrum of a semiconductor is perturbed by an external periodic influence—most usefully a chopped laser beam, i.e. photo-modulated reflectance (PR). Since samples need no special mounting, can be studied in air at room-temperature and can be full-sized pre-fabrication wafers, PR is truly non-destructive. In contrast to conventional photo-luminescence, PR spectra are often replete with sharp, detailed derivative-like features arising from ground-state, and other possible higher-energy optical transitions within a sample, from which can be extracted material parameters crucial to successful and efficient device operation, such as compositions, layer thicknesses, built-in electric fields and band line-ups. The paper discusses the advances made by our group in the application and interpretation of modulation spectroscopy of vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes, which so far have defied analysis by the more conventional non-destructive techniques.