Measuring vibration amplitudes in the picometer range using moving light gratings in photoconductive GaAs:Cr

Speckle Measurement for Small In-Plane Vibration Using GaAs

In this study, the measurement characteristics of speckles based on the photoinduced electromotive force (photo-emf) effect for high-frequency, small-amplitude, and in-plane vibration were theoretically and experimentally studied. The relevant theoretical models were utilized. A GaAs crystal was used as the photo-emf detector for experimental research, as well as to study the influence of the amplitude and frequency of the vibration, the imaging magnification of the measuring system, and the average speckle size of the measuring light on the first harmonic of the induced photocurrent in the experiments. The correctness of the supplemented theoretical model was verified, and a theoretical and experimental basis was provided for the feasibility of using GaAs to measure in-plane vibrations with nanoscale amplitudes.

Non-steady-state photo-EMF in β-Ga2O3 crystals at λ = 457 nm

The non-steady-state photoelectromotive force is excited in a monoclinic gallium oxide crystal at wavelength λ = 457 nm. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved without application of any electric field to the sample. The dependencies of the signal amplitude on the frequency of phase modulation, light intensity, spatial frequency and light polarization are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal's photoconductivity, responsivity and diffusion length of electrons are estimated for the chosen light wavelength and compared with the ones for other wide-bandgap crystals.

Non-steady-state photo-EMF interferometer for detection of mechanical oscillations in transparent scattering objects

The operation of an adaptive non-steady-state photo-electromagnetic field (EMF) sensor is studied in an interferometric arrangement including a diffuse scattering object-fiber optic plate. The mechanical oscillations of this plate induce the strains and stresses of the medium, which modulates the phase of the propagating light wave across the plate. The resonant frequencies of the mechanical system and the distribution of the phase modulation amplitude across the plate's surface are measured. The minimal detectable stress amplitude is estimated.

Visualization of oscillatory stresses in transparent media using a β-Ga2O3 adaptive detector

The phase-modulated optical signal produced by light propagation through stressed transparent media is detected using the non-steady-state photoelectromotive force technique. The mechanical system, including the glass plate and piezoelectric transducer, demonstrates resonant behavior in the vicinity of 100 kHz. The measured distribution of the optical phase is a bell-shaped surface for these frequencies. The simulation of the piezo-optic response shows qualitative coincidence with the experimental data and provides maps of the stress field. The characterization of the β-Ga₂O₃ adaptive detector is performed for the light wavelength λ=532  nm.

Space-and-time current spectroscopy of a β-Ga₂O₃ crystal

We report the excitation of the non-steady-state photoelectro-motive force in a monoclinic gallium oxide crystal. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved under the laser illumination with wavelength λ = 532 nm. The induced ac current is studied for the cases of zero and non-zero external electric fields, which imply the non-resonant and resonant mechanisms of space-charge recording. The dependencies of the signal amplitude versus the frequency of phase modulation, light intensity, spatial frequency, light polarization and value of the external dc electric field are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a weak difference of the transport parameters and a pronounced polarization dependence of the signal. The photoconductivity and diffusion length of electrons are estimated for the chosen light wavelength.

Self-adaptive vibrometry with CMOS-LCOS digital holography

A self-adaptive interferometer based on digital holography is here reported for applications involving measurements of very small amplitude vibrations. The two-beam coupling gain is optimized through an electronic feedback, while the dynamic character of the hologram allows reaching a high sensitivity of the interferometric measurements even in unstable environments and with strongly distorted wave-fronts. The frequency bandwidth of the adaptive interferometer and its spatial resolution are determined, respectively, by the maximum frame rate and the pixel size of the camera and of the spatial light modulator used to build the digital holographic setup.

Continual mechanical vibration trajectory tracking based on electro-optical heterodyne interferometry

Vibration is one of the confused problems in many fields. To give a comprehensive analysis of vibration, an electro-optical heterodyne interferometry with temporal intensity analysis method that can track the trajectory of the vibration dynamically has been built in this paper. The carrier frequency is introduced by the electrically controlled electro-optical frequency shifter. The trajectory is obtained by using temporal evolution of the light intensity in heterodyne interferometry. The instantaneous displacement of the vibration is extracted with spectral analysis technique. No target mirror and moving parts are required in our self-developed system. The principle and system configuration are described. The simulations and the preliminary experiments have been performed and the results show that this trajectory tracking system is high-efficiency, low-cost, jamproof, robust, precise and simple.

Sagnac interferometer with adaptive nonlinear detection

We present a Sagnac interferometer that uses a nonlinear adaptive medium for detection. The nonlinearity of the medium being characterized by a finite response time, the detection has a finite frequency bandwidth determining two operational regimes: at low variation rate of the rotation speed the interferometer operates in the slow-light regime, providing a direct measurement of the angular acceleration. For high variation rate of the rotation speed the amplitude of the alternating component of the Sagnac phase shift is instead detected, with the continuous and low-frequency components filtered out by the adaptive nonlinear process in the medium.

Picometer detection by adaptive holographic interferometry in a liquid-crystal light valve

The large dispersive properties and the narrow frequency bandwidth of the two-wave mixing in a liquid-crystal light valve is used to realize an adaptive holographic interferometer in the Raman-Nath regime. We report experimental observation of picometer periodic displacements and estimate the theoretical signal-to-noise ratio and the minimum quantum-noise-limited detectable displacement.

Photoconductive arrays for monitoring motion of spatial optical intensity patterns

We describe a photodetector array based on photoconductance-monitoring by four-point probing. This detection scheme is aimed specifically at detecting changes within a speckle or microscopic fringes within a larger nonuniform optical intensity distribution. One specific application is the detection of lateral displacements of these speckles or fringes, for example, in laser light reflected from optically rough vibrating surfaces. With a prototype, we have detected subnanometer surface displacements interferometrically. We also demonstrate speckle-based, noninterferometric detection of a guitar body's vibrations at a standoff distance of 6 m with nanowatt power. We observe and explain the prototype's limited frequency response by considering space-charge effects. This detection scheme is most useful in low-power, low-frequency applications.

Displacement measurement that uses transient photoelectromotive force effects in CdTe:V with frequency-modulated lasers

A new method of displacement measurement that uses the transient photoelectromotive force effects that arise in semiconductors illuminated by two frequency-modulated lasers is proposed and demonstrated experimentally. A height resolution of 0.85 microm was achieved experimentally; theoretical analysis charts the path toward eventual improvement of this resolution.

Enhanced responsivity of non-steady-state photoinduced electromotive force sensors using asymmetric interdigitated contacts

The responsivity at a constant detection area of non-steady-state photoinduced electromotive force (photo-emf) detectors is improved by a factor equal to the number of contact pairs contained in asymmetric interdigitated surface contacts. The polar nature of photo-emf current generation requires contact asymmetry in which one increases the total signal by blocking the illumination between alternate contact pairs, in distinct contrast to the behavior of conventional interdigitated contacts fabricated upon isotropic photoconductors.

Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well diodes

Oscillatory mode coupling between two coherent laser beams is produced when an interference pattern moves against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200cm(-1)during two-wave mixing using moving gratings.

Transient photocurrents in photoconductive semiconductors generated by step-phase modulations

Transient photocurrents generated in photoconductive semiconductors by two coherently interfering optical fields in two-wave mixing experiments were observed when the signal beam was phase modulated by a step signal. Expressions describing the behavior of the transient photocurrent were derived and verified experimentally. Such transient photocurrents are insensitive to the static carrier phase differences between the two optical fields and can be used to detect sudden phase variations between two interfering optical fields.

Photocurrents in photoconductive semiconductors generated by a moving space-charge field

The behavior of photocurrents generated in the semiconductors InP:Fe, GaAs:Cr, and undoped GaAs inoptical two-wave mixing experiments as a function of the frequency difference between the two waves is characterized and verified experimentally. The sign, the concentration, and the ratio gammaR/micro of the dominant charge carriers and the effective concentration of trapping centers can be found from this dependence.

Sensitivity of non-steady-state photoelectromotive force-based adaptive photodetectors and characterization techniques

The light-to-electricity conversion efficiency of the non-steady-state photoelectromotive force effect and its threshold sensitivity for the detection of phase-modulated optical signals and space-charge electric fields are evaluated. It is shown that for the optimal conditions of operation (the carrier spatial frequency is equal to the inverse diffusion length of the photocarriers, the detected frequency is higher than the cutoff frequency of the electromotive force signal, and the load resistance is higher than the resistance of the sample), the generation-recombination noise is approximately equal to the thermal noise of the sample resistance. In this case the threshold sensitivity of the adaptive photodetector without an external dc bias is independent of the parameters of the crystal used and can be only 4√2 times lower than that caused by the generation-recombination noise in a conventional photoresistor. Unlike in photodiodes and photoresistors, the output noise caused by laser intensity fluctuations is of the multiplicative type in the adaptive photodetectors.