Homodyne Demodulation Research Articles

Lissajous figures in the application of micro-vibration measurement

Based on Michelson interferometer and phase generated carrier (PGC) homodyne demodulation technique, an optical interferometer system is built, and a novel method using the central angles of Lissajous figures to measure micro-vibration displacement is proposed. The Lissajous figures are obtained by synthesizing two interferometric signals that their vibration directions are orthogonal and their intensities are equal. Through theoretical analysis, software simulation and experimental test, it is concluded that the central angles of Lissajous figures have a linear relationship with the phase modulation coefficients and the micro-vibration displacements when the phase modulation coefficients are small. So the micro-vibration displacements can be directly obtained by measuring the central angles of Lissajous figures. The method is simple and convenient. The experimental results indicate that the micro-vibration displacements measuring range can reach 5–775 nm, with a resolution of about 5 nm.

Modified phase-generated carrier demodulation compensated for the propagation delay of the fiber

In general applications, the output signals of the fiber-optic interferometric sensor (FOIS) need some demodulation techniques to linearly demodulate the sensing phase signal. The common signal demodulation circuit of the FOIS is passive homodyne demodulation using phase-generated carrier [(PGC) demodulation]. Preliminary analysis of the research demonstrates that a variation in the output demodulated signal is related to the length of the fiber, and the output demodulated signal will approach zero at some certain lengths of the fiber. To improve the performance of the FOIS, it is necessary to develop the modified PGC demodulation to compensate for the propagation delay of the fiber.

Wide-Band RF Receiver Using the “Five-Port” Technology

Many experiments have been proposed in order to demonstrate that the system performs a homodyne demodulation . The large bandwith of a five-port circuit is its main advantage. However, this kind of system must be calibrated for each frequency in order to carry out the inphase and quadrature demodulation. The subject of this paper is to show a new calibration method. This technique uses the training sequence received at each received burst of data to calibrate the five-port system. A method to synchronize the receiver with the training sequence of each burst is presented. We also study the effect of adjacent channels and cochannel interferer on the calibration. Results on quaternary phase-shift keying demodulation are presented.

AN OPTIC-FIBER INTERFEROMETER FOR THE DYNAMIC CHARACTERIZATION OF MICROSTRUCTURES

This article describes a measurement system designed to evaluate the dynamic characteristics of micromechanical structures. The system is based on Active Homodyne demodulation M-Z optic-fiber interferometer and can measure out-of-plane movement with a sensitivity of 0.1 nm. To validate the system, vibration measurements have been carried on a typical MEMS structure — the sensitive elements of a kind of uncooled IR imaging system, an array of micromachined bimaterial cantilevers (micron dimensions) — and the results are compared with the ones from FEM model analysis and another measurement method which can be known as spectrum plane optic filtering. The three results agree with each other. The first natural frequencies of the structures are determined from data obtained by vibrating the structures using a PZT excitation system. The mode shapes and modal damping will be carried out soon after a scanning system is completed.

Single-beam diode-laser technique for optical path-length measurements

A simple single-beam technique employing radio-frequency modulation of a tunable diode laser with homodyne demodulation is demonstrated as a means of measuring optical path lengths. This technique offers a straightforward method for determining path lengths traversed through optical multipass cells or performing optical range-finding over short (i.e., tens of meters) standoff distances. The radio-frequency phase-sensitive nature of the technique permits narrow-band detection and high signal-to-noise ratios, even when range-finding measurements are made with range resolutions of «1 m. This compares favorably with traditional short-pulse, wide-bandwidth optical range finders.

Homodyne detection in magnetic resonance imaging

Magnetic detection of complex images in magnetic resonance imaging (MRI) is immune to the effects of incidental phase variations, although in some applications information is lost or images are degraded. It is suggested that synchronous detection or demodulation can be used in MRI systems in place of magnitude detection to provide complete suppression of undesired quadrature components, to preserve polarity and phase information, and to eliminate the biases and reduction in signal-to-noise ratio (SNR) and contrast in low SNR images. The incidental phase variations in an image are removed through the use of a homodyne demodulation reference, which is derived from the image or the object itself. Synchronous homodyne detection has been applied to the detection of low SNR images, the reconstruction of partial k-space images, the simultaneous detection of water and lipid signals in quadrature, and the preservation of polarity in inversion-recovery images.

Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier

A method of homodyne demodulation using a phase generated carrier is described and experimentally demonstrated. The method has a large dynamic range, good linearity, and is capable of detecting phase shifts in the microradian range. The detection scheme obviates the phase tracker resetting problem encountered in active homodyne detection schemes. Two methods of producing the carrier are presented, one employing a piezoelectric stretcher, the other using current induced frequency modulation of the diode laser source. These two methods are compared. The origins of the noise limiting the system are briefly discussed.

Homodyne demodulation scheme for fiber optic sensors using phase generated carrier

A method of homodyne demodulation using a phase generated carrier is described and experimentally demonstrated. The method has a large dynamic range, good linearity, and is capable of detecting phase shifts in the microradian range. The detection scheme obviates the phase tracker resetting problem encountered in active homodyne detection schemes. Two methods of producing the carrier are presented, one employing a piezoelectric stretcher, the other using current induced frequency modulation of the diode laser source. These two methods are compared. The origins of the noise limiting the system are briefly discussed.