Laser doppler velocimeter

An acousto-optic deflector system including first and second transducers for producing acoustic waves for interaction with an optic beam in an acousto-optic device. The transducers are driven by signals of differing frequencies (f, + Af, f,) in order to produce two deflected optical beams from an incident optical beam launched from a laser. The two deflected beams are detected by a common photodetector whose output is filtered whereby to separate a component which is dependent on the frequency difference. The phase of this component is the relative optical phase of the two deflected optical beams. By determining the phase of the component for different frequencies (f1, f, =fT etc.), the optical angle of incidence (θi) may be obtained. Alternatively, by frequency modulating both transducer drive signals and appropriately electronically processing the detector output, a signal whose amplitude varies with the optical angle of incidence can be obtained. This signal is a minimum when the optical angle of incidence is optimized. Optimization of the optical angle of incidence can be preformed automatically.

Acousto-optic deflector systems

Linearization of a closed-loop acousto-optic modulation system is maintained over a uniform closed-loop bandwidth that is independent of the output light intensity level through the use of a compensating square-root function generator which employs the third quadrant characteristic of a MOSFET. The MOSFET may be connected as the feedback impedance of an operational amplifier which provides the square-root gain control function for complementing the non-linearity of the acousto-optic modulator. By utilizing the third quadrant characteristics of the source-to-drain voltage relative to the drain current of the MOSFET, the square-root function generator has a transfer function which insures that a linear relationship between modulator output light intensity and input signal current can be obtained irrespective of light intensity. In accordance with a preferred embodiment of the present invention, the operational amplifier of the square-root function generator may be supplied with an offset voltage in order to insure that the desired square-root transfer function can be attained regardless of differences in the manufactured characteristics of the field effect transistors that may be used. Temperature compensation circuitry may also be adopted as part of the square-root gain control component of the system to counteract the temperature sensitivity of the MOSFET's square-root current-voltage characteristic.

Linearization technique for closed-loop acousto-optic modulators

A disclosed acoustic telemetry system includes a downhole acoustic telemetry module that generates an acoustic uplink signal such as a pulsed fluid flow or compressional waves in a tubing string wall. An optical waveguide transports an optical signal representing the acoustic uplink signal to the surface interface. A related telemetry method includes acquiring measurements downhole, transmitting the measurements in the form of an acoustic signal, and sensing the acoustic signal via an optical waveguide.

Fiberoptic systems and methods for acoustic telemetry

In a method of detecting the spatial position of a laser beam relative to a body locatable in the path of the laser beam by detecting an acoustic signal produced by the beam in the body, wherein the laser beam carries a power modulation signal and the body has a plurality of acoustic sensors for detecting the acoustic signal produced in the body by the laser beam, the method includes detecting in the outputs of the acoustic sensors signals corresponding to the power modulation of the incident laser beam and detecting any difference in phase between the detected signals. A processor may be provided for processing the outputs of the acoustic sensors, the processor including a sensor for sensing in the respective outputs signals corresponding to the power modulation of the output beam of the laser and a phase sensitive detector for detecting differences in phase between the detected signals. There may be at least three acoustic sensors whereby the spatial position of the laser beam may be detected in two dimensions.

Apparatus and a method for detecting the position of a laser beam

A beam of coherent monochromatic light from a laser interacts with an acoustic wave signal in a Debye-Sears interaction cell, thereby to phase modulate the beam with the acoustic wave signal and with intelligence information carried thereby. At a fixed distance along the beam path from the interaction cell the modulation of the light beam shall have been converted from phase to amplitude modulation and a slit positioned at such location admits both the undiffracted and the first-order diffracted components of the modulated beam to a detector where the beam is demodulated and the intelligence recovered. Increased efficiency is obtained by using a series of slits, rather than a single slit, disposed across the light beam at the aforesaid particular location. Where Bragg angle diffraction is achieved in an interaction cell, as distinguished from Debye-Sears diffraction, the emerging light beam exhibits both amplitude and phase modulation. If the system employs two acoustic wave signals interacting with a single light beam in the cell, with the pair of acoustic wave signals having the proper angular relation, the output of the interaction cell may be demodulated as to its phase modulation content by the same technique employed in demodulating the output of the Debye-Sears diffraction cell. This increases the efficiency of the system over what is otherwise obtained if demodulation is confined to the initial amplitude modulation of the modulated light beam.

Adrianus Korpel

Papers

Phase-modulated light detection