U.S. patent application number 10/727473 was filed with the patent office on 2007-05-10 for wavefront sensor using hybrid optical/electronic heterodyne techniques.
Invention is credited to Stephen John Brosnan, Mark Ernest Weber.
Application Number | 20070103695 10/727473 |
Document ID | / |
Family ID | 35510364 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070103695 |
Kind Code |
A1 |
Brosnan; Stephen John ; et
al. |
May 10, 2007 |
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.
Inventors: |
Brosnan; Stephen John; (San
Pedro, CA) ; Weber; Mark Ernest; (Hawthorne,
CA) |
Correspondence
Address: |
PATENT ADMINISTRATOR;KATTEN MUCHIN ROSENMAN LLP
1025 THOMAS JEFFERSON STREET, N.W.
EAST LOBBY: SUITE 700
WASHINGTON
DC
20007-5201
US
|
Family ID: |
35510364 |
Appl. No.: |
10/727473 |
Filed: |
December 4, 2003 |
Current U.S.
Class: |
356/489 |
Current CPC
Class: |
G01J 9/04 20130101 |
Class at
Publication: |
356/489 |
International
Class: |
G01B 9/02 20060101
G01B009/02 |
Claims
1. An optical wavefront sensor comprising: an optical subsystem for
optically heterodyning an optical test signal and an optical
reference signal to generate an optically heterodyned signal; a
photodetector for converting said optically heterodyned signal to
an electronic heterodyned signal; an electronic subsystem for
electonically heterodyning said electronic heterodyned signal and
an electronic reference signal and generating a resultant signal; a
pulse counter for counting said resultant signal; a control circuit
for generating control signals for controlling said pulse counter;
and a first clock signal for clocking said pulse counter.
2. The optical wavefront sensor as recited in claim 1, wherein said
optical subsystem includes a beam splitter for optically combining
said optical test signal with an optical reference signal.
3. The optical wavefront sensor as recited in claim 2, wherein said
optical subsystem includes an optical frequency shifter for
frequency shifting said optical reference signal.
4. The optical wavefront sensor as recited in claim 3, wherein said
optical frequency shifter is an electro-acoustical device driven by
an RF drive which in turn is driven by a said clock having a
frequency f.sub.1.
5. The optical wavefront sensor as recited in claim 4, wherein said
electro-acoustical device is a Bragg cell.
6. The optical wavefront sensor as recited in claim 1, wherein said
control circuit includes a second clock having a frequency f.sub.2
and a mixer for mixing said first clock signal f.sub.1 and said
second clock signal f.sub.2.
7. The optical wavefront sensor as recited in claim 6, wherein said
second clock f.sub.2 signal is offset from said first clock signal
by a value between 100 KHz and 1 MHz.
8. The optical wavefront sensor as recited in claim 7, wherein the
low frequency output signal f.sub.1-f.sub.2 from said mixer is used
as a reference signal.
9. The optical wavefront sensor as recited in claim 1, wherein said
pulse counter has a preload input to enable compensation values to
be preloaded therein.
10. A method for measuring the phase of an optical test signal
relative to a reference signal, the method comprising the steps of:
(a) heterodyning the optical test signal with an optical reference
signal to develop an optical heterodyned signal; (b) directing said
optically heterodyned signal to a photodetector to generate a
heterodyned signal having a test frequency equal to the beat
frequency between the optical test signal and the optical reference
signal and a phase equal to the optical test signal; (c)
heterodyning said heterodyned signal with which an electronic
reference signal to generate an electronic heterodyned signal; and
(d) measuring the phase difference between said electronic
reference signal and said electronic heterodyned signal and
generating a signal representative of the difference
therebetween.
11. The method as recited in claim 10, further including the step
of squaring up said electronic heterodyned signal to develop
pulses.
12. The method as recited in claim 11, wherein step (d) comprises
counting said pulses by way of a pulse counter.
13. The method as recited in claim 12, further including the step
(e) for generating stop and start signals to enable said pulse
counter.
14. The method as recited in claim 13, wherein step (a) includes
optically shifting an optical reference signal by way of an
electro-acoustical device.
15. The method as recited in claim 14, wherein said step of
optically shifting includes providing a first clock having a
frequency f.sub.1 and driving said electro-acoustical device at
said first frequency f.sub.1.
16. The method as recited in claim 15, wherein step (e) comprises
generating a start signal by mixing said first clock signal having
a frequency f.sub.1 with said electronic reference signal having a
frequency f.sub.2.
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 known which predistort the
wavefront to compensate for atmospheric aberrations so that maximum
laser power is delivered at the targets.
[0005] Examples of wavefront sensors are disclosed in commonly
owned U.S. Pat. Nos. 6,229,616 and 6,366,356. These wavefront
sensors are based upon optical heterodyning a reference optical
signal with an optical test signal. More particularly, an
electro-acoustical device, such as a Bragg cell, is used to
frequency upshift an optical reference signal. The optical test
signal and frequency upshifted optical reference signal are then
optically combined, which results in optical heterodyning of the
two optical signals. The resulting optically heterodyned signal has
a frequency equivalent to the beat frequency of the two signals,
the RF signal driving the Bragg cell, normally in tens of MHz. The
optically heterodyned signal is subsequently directed to a detector
which converts the optical signal to an electronic signal having
the same phase as the optical test signal. The electronic
heterodyned signal is then used to develop a compensation signal to
compensate for phase distortion in the original optical test
signal. More particularly, the output of the photodetector is a
sinusoidal output with a phase equivalent to the original optical
phase. A heterodyne signal processor is used to convert the
sinusoidal waveform into a plurality of pulse trains whose duty
cycles are proportional to the sampled optical phase. These pulse
trains are electronically integrated by a low pass filter in order
to develop a DC voltage that is proportional to the duty cycle and
to the phase of the optical test signal.
[0006] 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. In addition, known acoustical
optical devices, such as Bragg cells, normally frequency shift at
frequencies in the tens of MHz. 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. Thus, there is a need for a optical wavefront sensor which
is faster than known optical wavefront sensors while virtually
eliminating electronic jitter.
SUMMARY OF THE INVENTION
[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.
DESCRIPTION OF THE DRAWINGS
[0008] These and other advantages of the present invention will be
readily understood with reference to the following specification
and attached drawing wherein:
[0009] FIG. 1A is a block diagram of a hybrid optical/electronic
wavefront sensor in accordance with the present invention.
[0010] FIG. 1B is a block diagram of an alternate embodiment of the
wavefront sensor illustrated in FIG. 1A.
[0011] FIG. 2 is a timing diagram of the various signals available
in the wavefront sensor illustrated in FIG. 1B.
DETAILED DESCRIPTION
[0012] The present invention relates to an optical wavefront sensor
which utilizes both optical and electronic heterodyning in order to
provide relatively accurate and efficient measurements of the phase
front of an optical waveform. Known optical wavefront sensors, such
as those disclosed in U.S. Pat. Nos. 6,229,616 and 6,366,356, rely
on integration of a pulse representative of an optically
heterodyned signal in order to generate a signal representative of
the optical phase of the optical test signal. Such integration
slows the process down considerably. Moreover, as discussed above,
a heterodyne signal processor is used to convert the output
waveform from the forward detectors to a pulse train whose duty
cycle is proportional to the sampled optical phase. The electronic
jitter used for edge detection of these signals is on the order of
a 1 nanosecond which can be a substantial source of phase
measurement noise relative to the high speed devices used for edge
detection in these applications. The present invention utilizes
electronic as well as optical heterodyning which eliminates
integration altogether, while at the same time enables lower cost
and slower components to be used while improving the speed of the
sensor output signal about 250 times while improving the accuracy
substantially.
[0013] Referring to FIG. 1A, the optical wavefront sensor in
accordance with the present invention is generally identified with
the reference numeral 20. An important aspect of the wavefront
sensor 20 is that it employs both optical heterodyning and
electronic heterodyning. In the first stage of the wavefront
sensor, an optical test signal, identified with the reference
numeral 22, is heterodyned with an optical reference beam 24. The
optical reference beam 24 may be a beam of coherent light or an
optical signal at a frequency .gamma.. The optical reference signal
24 is shifted by an optical frequency shifter 26. The optical
frequency shifter 26 may be, for example, an electro-acoustical
device, such as a Bragg cell. Such Bragg cells are driven by an RF
signal which excites a crystal within the Bragg cell to create a
sound wave. An RF driver 28, for example at frequency of f.sub.1,
of 40 MHz, may be used to drive the Bragg cell. The sound wave
generated within the electro-acoustical device 26 changes the index
of refraction of the crystal so that two beams emerge from the
electro-acoustical device. One of the beams is unchanged in both
path and frequency while the other reflects off the sound wave and
is shifted in frequency by the frequency of the sound wave. For a
40 MHz drive signal, the optical reference signal 24 is shifted by
40 MHz. In addition to Bragg cells, the frequency shifter 26 may
optionally be an optical modulator, such as a Mach-Zehnder
modulator followed by a narrow pass band optical filter to extract
the shifted side band light. The frequency upshifted beam,
identified with the reference numeral 30, is available at the
output of the optical phase shifter 26.
[0014] The frequency upshifted beam 30 is then optically
heterodyned with an optical test signal 22. The optical
heterodyning may be accomplished by way of a beam splitter 32,
configured such that the light from the frequency upshifted beam 30
and the optical test signal 22 have approximately the same
intensity. In particular, the optical test signal 22 is usually
much brighter than the frequency of the frequency upshifted beam
30. As such, a relatively low split ratio beam splitter 32 is
selected so that both beams 22 and 30 have generally the same
intensity and thus interfere more strongly. Optical interference
between the frequency upshifted beam 30 and the optical test beam
22 heterodynes the two optical beams 22 and 30 resulting in a
signal 34 having a beat frequency representative of the RF
modulation frequency of the drive signal 28 and a phase that
corresponds to the state of the optical phase of the optical test
signal 22. The heterodyned optical signal 34 is applied to a
photodetector 36. The photodetector 36 generates an electronic
sinusoidal signal having a frequency equal to the beat frequency
between the optical test signal 22 and the upshifted reference
signal 30 and a phase corresponding to the phase of the optical
test signal 22. The output of the photodetector 36 is applied to a
conventional DC blocking filter, such as a series capacitor filter
38.
[0015] In accordance with an important aspect of the invention, the
electronic output of the photodetector 36 is electronically
heterodyned with a clock signal 40 having a frequency f.sub.2. In
particular, the clock signal 40 is mixed with the output of the
photodetector 36 by way of a mixer 42. The low frequency product of
the mixer 42 is then filtered by a conventional bandpass filter 44
and squared up by way of a comparator 46 and applied to a pulse
counter 48.
[0016] The pulse counter 48 is under the control of an electronic
reference signal. The electronic reference signal is generated by
mixing the clock signal from a clock 27 having a frequency f.sub.1,
used to drive the RF driver 28, with the clock signal f.sub.2 from
the clock 40. The second clock signal 40 is offset in frequency
from the first clock signal 27 by, for example, 100 kHz to 1 MHz.
The output of the low frequency product output of the second mixer
50 (i.e. f.sub.1-f.sub.2) is then filtered by a conventional
bandpass filter 52 and squared up by comparator 54 to form a
reference signal that is applied to the pulse counter 48. The first
clock signal 27 is squared up by way of a comparator 56 and used as
the clock signal for the pulse counter 48.
[0017] The leading edge of the pulse of the output signal from the
mixers 42 and 50 serve as control signals to stop and start the
pulse counter 48, respectively. More particularly, the output of
the clock 27 is squared up by a comparator 56 and used as a clock
input for the pulse counter 48. The optical phase is measured by
counting the pulses at the clock input while the pulse counter 48
is enabled. The reference signal (REF), available at the output of
the comparator 54, is used as the start for the pulse counter 48.
The signal (Signal), available at the output of the comparator 46,
is used to disable the pulse counter 48. Since the difference in
the phase between the mixer 42 and the mixer 50 is directly related
to the difference in the phase between the optical test signal 22
and the frequency shifted RF drive signal 30, the pulse counter
count signal will be linearly related to the measured optical phase
and the electronic reference phase.
[0018] The sensor output signal may be converted to analog form by
way of a digital-to-analog converter 58 and used to drive an
optical phase modulator 60. Such optical phase modulators are known
in the art. A suitable optical phase modulator is an electro-optic
device, such as a lithium niobate waveguide. With such a device, a
voltage applied to the top of the waveguide causes a refractive
index change of the medium within the waveguide. The optical path
of the emitted wave is changed by the waveguide length times the
change in the refractive index. The phase change is the path change
divided by the wavelength. Such devices are available at Eospace
Inc. (www.eospace.com/phase-modulator.htm).
[0019] In order to pre-compensate for atmospheric aberrations, a
phase offset may be preloaded into the pulse counter 48, for
example, by a master controller 62 which may be a simple
microprocessor. The phase offset is simply the difference between
the electronic reference signal, available at the output of the
clock 40, and the optical test signal, available at the output of
the filter 38. The optical phase can be set to any desired phase
shift from the reference edge of the clock 40 by specifying the
difference in count between the electronic reference signal and the
optical test signal.
[0020] FIG. 1B is an alternate embodiment of the wavefront sensor
illustrated in FIG. 1A and is generally identified with the
reference numeral 70. The wavefront sensor 70 is similar to the
wavefront sensor 20 illustrated in FIG. 1A with the exception of
the two clocks f.sub.1 and f.sub.2. Otherwise, like devices are
identified with like reference numerals. In this embodiment, the
two clocks 27 and 40 are locked together. For example, the clocks
27 and 40 may be synthesized from a master clock source 72 of a
much higher frequency, for example, 256 MHz. The same master clock
source 72 may be used as a clock source for the pulse counter 48,
which results in even higher accuracy or higher speed.
[0021] A timing diagram for the various signals is illustrated in
FIGS. 2A-2D. FIG. 2A illustrates the clock pulses applied to the
pulse counter 48. FIG. 2B illustrates the reference signal (REF)
which is used to start counting while the reference signal is high,
the pulse counter 48 counts clock pulses. FIG. 2C illustrates the
signal (Signal) used to stop counting of the pulse counter 48. This
signal is active high and disables the pulse counter 48 when it
goes high. FIG. 2D illustrates the clock pulses counted.
[0022] There are many advantages of the optical wavefront sensors
illustrated in FIGS. 1A and 1B in accordance with the present
invention. First, the optical rate for the front sensor is 250
times faster than other known wavefront sensors, such as disclosed
in U.S. Pat. No. 6,243,168. Additionally, the wavefront sensor 20
is relatively more accurate. In particular, due to the relatively
slower edge detection circuits possible, the 1 nanosecond jitter is
no longer a factor providing improvement in the accuracy by a
factor of 40 to 50 percent. In addition, the wavefront sensor
provides a relatively low-cost solution relative to other known
sensors. In particular, the implementation of beam steering of a
fiber amplifier array requires temporal displacement of square
waveforms, normally accomplished with relatively high-speed ICs.
Wavefront sensor here allows much slower and therefore less costly
component.
[0023] Obviously, many modifications and variations of the present
invention are possible in appended claims, the invention may be
practiced otherwise than is specifically described above.
* * * * *