Wavefront sensor using hybrid optical/electronic heterodyne techniques

Abstract

A hybrid optical / electronic wavefront sensor includes an electro-acoustical device used to upshift an optical reference signal. An optical test signal and the frequency upshifted optical reference signal are optically heterodyned to create a signal having a frequency equivalent to the beat frequency of the two signals, for example, the RF driving frequency of the Bragg cell. The optically heterodyned signal is then converted by way of a detector to an electronic signal having the same phase as the optical test signal. The output of the detector is a sinusoidal signal having the same phase as the phase of the optical test signal. This signal is filtered by way of an AC filter and mixed with a second clock signal, for example, a clock signal that is offset in frequency from the electro-acoustical drive signal by a frequency, for example, between 100 kHz and 1 MHz. These two signals are mixed by way of a mixer. The low frequency product of the mixer is passed by way of a filter and converted to a square wave by way of a comparator. The output of the comparator is applied to a simple pulse counter and used to disable the pulse counter. The pulse counter counts the clock pulses while it is enabled and is linearly related to the difference in phase between the optical test signal and the frequency upshifted signal.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to an optical wavefront sensor and more particularly to an optical wavefront sensor which incorporates optical and electronic heterodyning to enable high accuracy and high speed phase measurements to be made, relative to known optical wavefront sensors. [0003] 2. Description of the Prior Art [0004] Wavefront sensors are known to be used to correct for distortions in optical beams caused by, for example, atmospheric aberrations. In particular, such wavefront sensors are known to be used with high power laser weapon systems, for example, as disclosed in commonly owned U.S. Pat. No. 5,198,607. The effectiveness of such laser weapon systems depends on many factors including the power of the laser at the target. Atmospheric aberrations are known to cause distortion of the wavefront of high powered laser beams and thus reduce the power and effectiveness of such weapons. As such, systems are k...

AI Technical Summary

Benefits of technology

[0007] Briefly the present invention relates to a hybrid optical / electronic wavefront sensor. The hybrid wavefront sensor includes an electro-acoustical device, such as a Bragg cell, that is used to upshift an optical reference signal. An optical test signal and the frequency upshifted optical reference signal are optically heterodyned to create a signal having a frequency equivalent to the beat frequency of the two signals, for example, the RF driving frequency of the Bragg cell. The optically heterodyned signal is then converted by way of a detector to an electronic signal having the same phase as the optical test signal. The output of the detector is a sinusoidal signal having the same phase as the phase of the optical test signal. This signal is filtered by way of an AC filter and mixed with a second clock signal, for example, a clock signal that is offset in frequency from the electro-acoustical drive signal by a frequency, for example, between 100 kHz and 1 MHz. These two signals are mixed by way of a mixer. The low frequency product of the mixer is passed by way of a filter and converted to a square wave by way of a comparator. The output of the comparator is applied to a simple pulse counter and used to disable the pulse counter. An electronic reference signal is formed by mixing the two RF signals, filtering the output, and squaring up the output by way of another comparator. The reference signal is used to start the pulse counter. A clock signal for the pulse counter is developed by squaring up the RF driving signal applied to the electro-acoustical device by way of a comparator. The pulse counter counts the clock pulses while it is enabled. The pulse count is linearly related to the difference in phase between the optical test signal and the frequency upshifted signal. The hybrid optical / electronic hybrid wavefront sensor in accordance with the present invention is about 250 times faster than known wavefront sensors and provides relatively more accurate phase measurement in spite of the 1 nanosecond jitter inherent in the electronic edge detection circuits.

Problems solved by technology

Atmospheric aberrations are known to cause distortion of the wavefront of high powered laser beams and thus reduce the power and effectiveness of such weapons.

There are several problems with such known optical heterodyne wavefront sensors.

First, such wavefront sensors are relatively slow due to the need to integrate the pulse trains from the optical heterodyne processors.

However at these frequencies, the electronic jitter of approximately 1 nanosecond of the devices used for edge detection can be a source of substantial phase measurement noise.

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