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We claim:
1. An optical system incorporating a feedback control loop for controlling
an optical output wavelength of an optical output beam to match an
incoming frequency and wavelength of an incoming optical signal having an
incoming signal wavelength comprising:
signal comparison means for comparing said incoming optical signal with a
loop optical signal to form an rf beat signal;
a voltage controlled oscillator for generating an rf control signal, having
an rf control frequency, that controls said loop optical signal in
frequency;
coarse frequency control means, connected to said signal comparison means,
for generating a coarse tuning control signal by applying said rf beat
signal to an acousto-optic modulator that deflects a beam from a local
optical source to strike one of a plurality of optical detectors disposed
transversely from a local source axis, whereby that one of said plurality
of optical detectors receiving deflected radiation passes a signal to a
voltage switch that generates a tuning voltage having one of a
predetermined set of tuning voltage values and applies said tuning voltage
as said coarse tuning control signal to a first input of said voltage
controlled oscillator;
fine tuning control means, connected to said signal comparison means and to
said voltage switch, for generating and applying a fine tuning control
signal to a second input of said voltage controlled oscillator, including
a loop filter for filtering said beat frequency to generate therefrom said
fine tuning control signal and applying said fine tuning control signal to
a second input of said voltage controlled oscillator after a relative
delay after said tuning voltage;
an output frequency shifting unit, connected to an output of said voltage
controlled oscillator and to a local optical source, for frequency
modulating a loop optical beam, having a loop frequency, passing
therethrough to generate an optical output beam having said output
frequency; and
optical feedback means disposed to intercept said optical output beam for
transporting a fraction of said optical output beam to said signal
comparison means, whereby said loop operates to drive said output
wavelength to match said incoming signal wavelength.
2. A system according to claim 1, in which said signal comparison means
comprises an optical mixer responsive to said output optical beam and to
said incoming optical beam that generates a first rf beat signal and an rf
mixer responsive to said first rf beat signal and to an offset rf signal
source to generate said rf beat signal having a predetermined offset
value.
3. A system according to claim 1, in which said coarse frequency control
means contains latching means for maintaining said coarse tuning control
signal at an initial value as said feedback control loop converges to a
final value, whereby said coarse tuning voltage maintains said initial
value adapted to compensate for a frequency difference between said
incoming optical signal and said local optical source.
4. A system according to claim 1, in which said local optical source
transmits a local beam having a local frequency to an acousto-optical unit
that shifts said local frequency by an amount equal to said rf control
frequency and selects a single sideband of a frequency-shifted local beam
as said optical output beam.
5. A system incorporating a feedback control loop for controlling an output
wavelength of an electromagnetic output beam to match an incoming
frequency and wavelength of an incoming electromagnetic signal having an
incoming signal wavelength comprising:
signal comparison means for comparing said incoming electromagnetic signal
with a loop electromagnetic signal to form an rf beat signal;
a voltage controlled oscillator for generating an rf control signal, having
an rf control frequency, that controls said loop electromagnetic signal in
frequency;
coarse frequency control means, connected to said signal comparison means,
for generating a coarse tuning control signal by applying said rf beat
signal to an acousto-optic modulator that deflects a beam from a local
electromagnetic source to strike one of a plurality of optical detectors
disposed transversely from a local source axis, whereby that one of said
plurality of optical detectors receiving deflected radiation passes a
signal to a voltage switch that generates a tuning voltage having one of a
predetermined set of tuning voltage values and applies said tuning voltage
as said coarse tuning control signal to a first input of said voltage
controlled oscillator;
fine tuning control means, connected to said signal comparison means and to
said voltage switch, for generating and applying a fine tuning control
signal to a second input of said voltage controlled oscillator, including
a loop filter for filtering said beat frequency to generate therefrom said
fine tuning control signal and applying said fine tuning control signal to
a second input of said voltage controlled oscillator after a relative
delay after said tuning voltage;
an output frequency shifting unit, connected to an output of said voltage
controlled oscillator and to a local electromagnetic source, for frequency
modulating a loop electromagnetic beam, having a loop frequency, passing
therethrough to generate an electromagnetic output beam having said output
frequency; and
electromagnetic feedback means disposed to intercept said electromagnetic
output beam for transporting a fraction of said electromagnetic output
beam to said signal comparison means, whereby said loop operates to drive
said output wavelength to match said incoming signal wavelength.
6. A system according to claim 5, in which said coarse frequency control
means contains latching means for maintaining said coarse tuning control
signal at an initial value as said feedback control loop converges to a
final value, whereby said coarse tuning voltage maintains said initial
value adapted to compensate for a frequency difference between said
incoming optical signal and said local optical source.
7. A system according to claim 5, in which said local optical source
transmits a local beam having a local frequency to an acousto-optical unit
that shifts said local frequency by an amount equal to said rf control
frequency and selects a single sideband of a frequency-shifted local beam
as said optical output beam.
8. A system according to claim 5, in which said local electromagnetic
source operates in the microwave portion of the electromagnetic spectrum.
9. A system according to claim 8, in which said coarse frequency control
means contains latching means for maintaining said coarse tuning control
signal at an initial value as said feedback control loop converges to a
final value, whereby said coarse tuning voltage maintains said initial
value adapted to compensate for a frequency difference between said
incoming optical signal and said local optical source.
10. A system according to claim 8, in which said local optical source
transmits a local beam having a local frequency to an acousto-optical unit
that shifts said local frequency by an amount equal to said rf control
frequency and selects a single sideband of a frequency-shifted local beam
as said optical output beam. |
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Claims |
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Description |
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CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the subject matter disclosed and claimed in
copending U.S. Ser. No. 07/829800 entitled Lidar Countermeasure by Dennis
W. Davis, William F. Conley and Donald J. Link filed on even date herewith
and assigned to the same assignee herein incorporated by reference.
TECHNICAL FIELD
The field of the invention is that of controlling the frequency of an
optical source to match the frequency of another source.
BACKGROUND ART
Laser warning receivers and high data rate optical communication systems
require local oscillators which can rapidly track the frequency of
received radiation. Situations occur where significant uncertainty exists
regarding the optical carrier frequency of the anticipated signal. Such
uncertainty can result from doppler shifts in the radiation transmitted
from satellites, missiles, etc., or in the case of uncooperative
transmitters, transmitters tuned to differing operating frequencies. A
common approach to tracking the frequency of received radiation employs a
phase-locked-loop. Recent optical implementations of this widely used
electronic concept, such as A. Scholtz, W. Leeb, R. Flatscher, and H.
Philips, "Realization of a 10.mu.m Homodyne Receiver," Journal of
Lightwave Tech., Vol. LT-5, No. 4, Apr. 1987 and S. Lowney and D.
Marquis., "Frequency Acquisition and Tracking for Optical Heterodyne
Communication Systems," Journal of Lightwave Tech., Vol. LT-5, No. 4,
Apr. 1987., exploit an optical voltage controlled oscillator (OVCO), to
provide frequency tunable laser output in a feedback control loop with
various kinds of rf frequency discrimination to compare the OVCO frequency
with that of the received radiation. These implementations achieve
frequency lock in times on the order of hundreds of microseconds. The
presently disclosed approach can acquire frequency lock at least two
orders of magnitude faster with a reduced amount of hardware.
DISCLOSURE OF INVENTION
The invention relates to a system for controlling an optical frequency to
match an input frequency, in which a beat frequency passes into two
control loops, both of which affect the output frequency. A fast control
loop uses an optical technique for quickly generating a coarse contact
signal, while a slower control loop uses electronic components to generate
a more accurate control signal. Both loops are input to a VCO that
frequency shifts the output of a stable optical source. Both loops merge
in a voltage controlled oscillator that produces an output frequency that
shifts the frequency of a local oscillator.
Other features and advantages will be apparent from the specification and
claims and from the accompanying drawings which illustrate an embodiment
of the invention.
BRIEF DESCRIPTION OF DRAWINGS
The sole FIGURE illustrates schematically an embodiment of the invention.
BEST MODE OF CARRYING OUT THE INVENTION
In FIG. 1, a partially schematic, partially functional diagram of the
presently disclosed invention, received optical radiation entering on beam
110 at the upper left, is detected by heterodyne means on a mercury
cadmium telluride photodetector 112. The loop radiation or loop optical
signal from a local oscillator is shown fed from below along beam 122. The
output radio-frequency beat signal having a frequency which is the
difference between that of the received radiation and shifted output
radiation is fed from detector 112 to amplifier 115 and further to an rf
mixer 135. An offset oscillator 130 operates at a frequency high enough to
ensure that the frequency of the output from this second mixer 135 is at a
convenient frequency (e.g., not at baseband given the aforementioned
received radiation frequency uncertainty). A portion of this mixer output
is input to a Bragg cell spectrum analyzer denoted generally by the
numeral 200 with a bandwidth of a gigahertz and a spectral resolution or
"bin width" of several hundred kilohertz. The output from mixer 135 sets
up a temporary grating in electro-optic crystal 220 that depicts a portion
of the beam from laser 210 to one of a set of detectors 250. These
detectors are sized and placed to give the desired frequency resolution,
which for this application is on the order of several hundred kilohertz.
Energy appearing in one of the frequency bins of the analyzer will cause a
corresponding tuning voltage to be connected to a voltage controlled
oscillator (VCO) 400 through a FET switch bank 260.
This VCO is tunable over an rf band on the order of 1 gigahertz, which is
large enough to accommodate the offset oscillator and uncertainty in the
received radiation frequency 4. The frequency of the local waveguide laser
510 in source 500 is shifted by the VCO frequency using an external single
sideband frequency shifter 525 which comprises a thin (10-25 microns)
electro-optic crystal such as GaAs contained between two microwave
stripline electrodes to form a TW waveguide configuration as illustrated
in Izutsu et al, "Integrated Optical SSB Modulator/Frequency Shifter",
IEEE Journal of Quantum Electronics, Vol. QE-17, No. 11, Nov. 1981. The
output of the frequency shifter is now coarsely tuned on the first pass
through the loop to within several hundred kilohertz of the received
radiation within a time period of a few tens of nanoseconds.
FET switch bank 260 composes a set of resisters 261, 261', etc., controlled
by units 263, 263', etc. Units 263 illustratively compose a flip-flop or
other latching unit controlling a current path connecting resistor 261 to
ground. One of units 263 will be energized on the first pass through the
loop, after which the connection between analyzer 200 and switch bank 260
is inhibited by conventional circuits not shown, so that the signal on
line 402 does not change as the signal out of mixer 135 reflects loop
convergence.
The voltage comparator 265 of the upper right, having incorporated a time
delay to allow the coarse tuning of the laser to transpire, now causes a
FET switch 267 to close a fine tuning loop with the second mixer output
connected to a "fast lock" loop filter 300. The output of the loop filter
provides a fine tuning error voltage to the VCO that operates over a time
scale on the order of a microsecond, compared with prior art time scales
of the order of a millisecond.
Referring now to analyzer 200, the operation is that of deflection of a
beam from laser 210 by acousto-optic cell 220. Lenses 212, 214 and 230
operate in conventional fashion to expand the beam and focus it onto the
set of optical detectors 250. When the system first receives incident
radiation. one of detectors 250 will be excited, depending on the
frequency difference between the incoming beam and the beam from loop
laser 510. Each of detectors 250 controls a different transistor 262,
etc., to apply a voltage to VCO 400. The resistors 261, etc., will be set
to give a set of different voltages. For CO.sub.2 radiation and
conventional values of acousto-optic crystals, the bandwidth of the
spectrum analyzer will be about 1 gigahertz and the resolution will be
about several hundred kilohertz. Those skilled in the art will readily be
able to adapt this embodiment for other wavelength ranges or their own
resolution applications. In applications using other than CO2 lasers, the
only changes to be made in the present configuration involve use of a
different optical heterodyne detector, fixed frequency local laser and
corresponding frequency shifter. For sources at shorter wavelengths, each
of these components is available.
VCO 400 is a transmission line VCO such as that shown in V. Manassewitsch,
"Frequency Synthesizers--Theory and Design", John Wiley & Sons, 1976, in
which a microstrip delay line having a delay of one quarter of the
free-running wavelength of the oscillator is used.
An outer loop having a smaller dynamic range and slower response time is
controlled by comparator 265 and transistor 267, so that the outer loop
will only open after the inner loop has had time to engage the VCO. Loop
filter 300, operating as described in B. Glance, "New Phase-Lock Circuit
Providing Very Fast Acquisition Time," IEEE Trans. on Microwave Theory and
Techniques, Vol. MTT-33, No. 9, Sept. 1985, uses diodes 314 and 316 to
bypass resistor 312 and create large loop filter gain for large amplitude
error signals by shorting resistor 312. Op amps 320 and 330 with their
associated components operate conventionally as a phase lock loop filter
to generate a slowly varying output signal that depends on the rf beat
frequency of the loop.
Preferably, the components are sized that the output of filter 300 will
move the frequency of beam 122 by one bin of Bragg analyzer 200, so that
filter 300 functions as a vernier to adjust the frequency within one
"bin-width" of analyzer 200.
In source 500, laser 510 generates a stable reference beam that is
frequency-shifted to match the input. As described above, the output
signal from VCO 400 is applied on line 408 to crystal 522, from which M.
Izutsu, S. Shikama, and T. Sueta, "Integrated Optical SSB
Modulator/Frequency Shifter" IEEE Journal of Quantum Electronics, Vol.
OE-1 7, No. 11, Nov. 1981 and "Design Studies of 10 Micron Laser Radar
Modulators," United Technologies Research Center Final Technical Report
R87-927477. One single sideband is selected to travel on path 122 back to
mixer 112. The remainder of the output beam travels as beam 120 for
whatever purpose is required.
Those skilled in the art will readily appreciate that the invention can be
applied to many different lasers, such as dye lasers and diode lasers. The
incoming radiation need not be travelling in free space, but may be in an
optical fiber. The range of possible output wavelengths may be expanded by
placing two or more output lasers 500, 500', etc., in parallel, each tuned
to a different fixed wavelength. The invention could also be applied to
microwave values with the substitution of a microwave transmitter for
output laser 500 and the elimination of detector 112 and 115. The feedback
is then directed into mixer 135. Offset oscillator 130 will not be
necessary for all applications.
It should be understood that the invention is not limited to the particular
embodiments shown and described herein, but that various changes and
modifications may be made without departing from the spirit and scope of
this novel concept as defined by the following claims.
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