Current sensor using heterodyne detection

Based on the Faraday effect for measuring ac current, we describe a fiber-optic sensor that uses laser-diode intensity modulation to perform heterodyne signal detection. The sensor output at the carrier frequency is used as a reference signal to normalize the results. The sensing element consists of a few coils low-birefringence fibers between polarizers. We built the current sensor described above and tested its performance-sensitivity and noise-as functions of the angle between polarizers.     

Optical heterodyne detection of pulsed ultrasonic pressures

An optical heterodyne interferometer was used for measuring short ultrasonic pulses in fluids. The optical phase shift of the probe beam, proportional to the acoustic pressure integrated along the light path, was demodulated quantitatively by electronic processing. The detection sensitivities of 10(-3) (Pa/ radicalHz) in water and 10(-4 ) (Pa/ radicalHz) in air were deduced. The experimental results obtained in water as well as in air show that this optical heterodyne technique offers a very sensitive way of measuring ultrasonic pressures in the low megahertz range even for very low pressures.     

Optical heterodyne detection of random electromagnetic beams

Abstract

In this paper we present a general analysis for the optical heterodyne detection of random electromagnetic beams. To describe the ensemble of quasimonochromatic beams which are partially polarized and partially coherent, we use a recently developed matrix treatment. We derive an expression for the signal-to-noise ratio (SNR) in terms of the beam coherence polarization matrices of the beams on the detector surface. Numerical examples are given for the SNR variation in the case of partially polarized Gaussian Schell model beams and the optimum detection is discussed in terms of beam parameters of the local oscillator.     

Polarimetry of scattered light using heterodyne detection

Abstract

Heterodyne detection has been used to measure the polarization state of light back-scattered from various targets (including flame-sprayed aluminium, sandpaper and painted surfaces). The samples are illuminated with a linearly polarized single-frequency continuous-wave CO₂ laser operating at a wavelength of 10.6 μm. The back-scattered co-polarized and cross-polarized components are both coherently detected by beating with an optical local oscillator. This process allows the relative amplitudes and phases of the two components to be measured and hence the light's polarization state can be evaluated. When the target undergoes movement, the scattered light demonstrates the usual properties of dynamic speckle, and the technique allows observation of the time evolution of the polarization ellipse.     

High-resolution diffusing-wave spectroscopy using optimized heterodyne detection

We show experimentally and theoretically that the use of optimized heterodyne detection in a diffusing-wave spectroscopy experiment leads to the detection of much smaller intensity autocorrelations than with conventional (either homodyne or heterodyne) setups. This enhanced resolution may be useful for the study of longtime dynamics of multiple-scattering disordered systems.     

Stabilization by harmonic intensities for the output signal in heterodyne detection

We experimentally demonstrated that the technique to stabilize the output signal by harmonic intensities is useful in heterodyne detection with an incoherent light source such as a halogen lamp. The relation between the relative standard deviation of an output signal and the fluctuation of the light intensity is analyzed and simulated. Using the fundamental to sixth harmonics increases the stabilities of output signal approximately 3 times, and subtracting the relative standard deviation of the intensity of light source enhances the stabilities 49 times. The fluctuating phase that is due to the fluctuating frequency and temperature and its power spectrum density for an interferometer is also calculated with the Allan variance.     

Saturation effects in heterodyne detection with Geiger-mode InGaAs avalanche photodiode detector arrays

We report, to the best of our knowledge, the first demonstration of heterodyne detection of a glint target using an InGaAs avalanche photodiode detector (APD) array in the Geiger mode. Due to the finite number of pixels, all such photon-counting arrays necessarily suffer from saturation effects. At large photon fluxes, saturation of the APD degrades the Doppler frequency resolution and the signal-to-noise ratio (SNR). We derive analytical expressions for the Doppler resolution and SNR, taking saturation effects into account. The optimal local oscillator power can be obtained numerically from the SNR expression.     

Spatiotemporal filters in the detection of background modulation targets

The background modulation method has been proposed as a useful test of early visual mechanisms [Biol. Cybern. 37, 77 (1980); Biol. Cybern. 47, 173 (1983)]. The task involves measuring detection thresholds for a luminous spot (increment) drifting over a spatially or temporally modulated background. The study explores the nature of the detecting mechanism in terms of spatial and temporal filters for both spatial and temporal background modulations. In both cases we find that thresholds can be explained by spatial contrast cues generated by the moving spot and that their spatiotemporal characteristics suggest detection by magnocellular processes.     

Electro-optic heterodyne interferometer

We propose a new configuration for using a triangle-wave signal to drive the electro-optic modulator in an electro-optic heterodyne interferometer system. The new configuration is adapted to measure the phase retardation of a wave plate and the optical rotation angle of a chiral medium. By adding optic elements, the second-harmonic component amplitude of the interferometer photodetector output signal became proportional to the phase retardation or optical rotation angle of the samples being tested.     

Experimental comparison of heterodyne and direct detection for pulsed differential absorption CO2 lidar

A pulsed dual-wavelength dual-CO2-laser differential-absorption lidar (DIAL) system has been developed which permits simultaneous heterodyne and direct detection of the same lidar returns. This system has been used to make an experimental comparison of the SNRs and statistical and temporal characteristics of the DIAL returns from several topographic targets. These results were found to be in general agreement with theory and were used to quantify the relative merits of the two detection techniques. The measured parameter values were applied to an analytical treatment to predict system trade-offs for the remote sensing of atmospheric species, with application to both path-averaged and range-resolved measurements.     

Phase detection sensitivity enhancement of surface plasmon resonance sensor in a heterodyne interferometer system

We propose a method of rotating the analyzer in front of the photodetector in a heterodyne interferometer system to implement optical phase shift and detection sensitivity enhancement of surface plasmon resonance (SPR) sensors. When the analyzer is rotated to shift the phase curve to be near the phase jump point, the phase detection sensitivity of the SPR sensor can be greatly enhanced. Theoretical calculations of a prism-coupled SPR device were performed using this method. Experimental result using an electro-optic heterodyne interferometer is reported.     

Error analysis of a heterodyne submillimeter sounder for the detection of stratospheric trace gases

Heterodyne submillimeter detection techniques represent an important development in the field of remote sensing of atmospheric composition. The disclosure of this wavelength region by new low-noise detectors and multichannel high-resolution spectrometers leads to expectations of improved accuracy and vertical resolution of the vertical composition profiles derived from these measurements. Because of the low-noise levels of newly developed receivers, special care is required to ensure that fundamental limitations of the components used do not contribute to systematic errors exceeding the random errors. Operated in an upward-looking geometry, the sensitivity of the retrieval algorithm to noise and instrumental errors can be rather high, and hence instrumental limitations could induce large uncertainties in the derived atmospheric information. Instrumental uncertainties typical for a passive heterodyne sounder are quantified, and their effects on the accuracy of the derived vertical mixing ratio profiles are presented.     

Degree of polarization and heterodyne spectrum in coherent detection of partially polarized wave fields

We first study the state of polarization of the superposition of partially polarized, statistically independent optical fields with application in coherent detection systems with emphasis on heterodyne configurations and obtain the DOP of the mixture of wave fields. Next we obtain the available heterodyne power after photodetection. Finally we obtain the spectrum of the photocurrent process resulting from the coherent detection of partially polarized amplitude-stabilized fields.     

Performance limitations of a free-space optical communication satellite network owing to vibrations: heterodyne detection

Free-space optical communication between satellites in a distributed network can permit high data rates of communication between different places on Earth. To establish optical communication between any two satellites requires that the line of sight of their optics be aligned during the entire communication time. Because of the large distance between the satellites and the alignment accuracy required, the pointing from one satellite to another is complicated because of vibrations of the pointing system caused by two fundamental stochastic mechanisms: tracking noise created by the electro-optic tracker and vibrations derived from mechanical components. Vibration of the transmitter beam in the receiver plane causes a decrease in the received optical power. Vibrations of the receiver telescope relative to the received beam decrease the heterodyne mixing efficiency. These two factors increase the bit-error rate of a coherent detection network. We derive simple mathematical models of the network bit-error rate versus the system parameters and the transmitter and receiver vibration statistics. An example of a practical optical heterodyne free-space satellite optical communication network is presented. From this research it is clear that even low-amplitude vibration of the satellite-pointing systems dramatically decreases network performance.