U.S. patent number 5,723,856 [Application Number 08/510,064] was granted by the patent office on 1998-03-03 for opto-electronic oscillator having a positive feedback with an open loop gain greater than one.
This patent grant is currently assigned to California Institute of Technology. Invention is credited to Lutfollah Maleki, Xiaotian Steve Yao.
United States Patent |
5,723,856 |
Yao , et al. |
March 3, 1998 |
Opto-electronic oscillator having a positive feedback with an open
loop gain greater than one
Abstract
An electro-optical oscillator includes an electro-optical
modulator having an electrical input port that accepts an
electrical control signal and an optical output port. The
electro-optical modulator is operable to generate at the optical
output port an optical signal that oscillates at a frequency
related to the electrical control signal. The oscillator also
includes a photodetector that converts a portion of the optical
signal from the optical output port of the electro-optical
modulator to an electrical signal and provides the electrical
signal to the electrical input port of the electro-optical
modulator as the electrical control signal. An open loop gain of a
feedback loop including the optical output port, the photodetector
and the electrical input port is greater than one.
Inventors: |
Yao; Xiaotian Steve (Diamond
Bar, CA), Maleki; Lutfollah (San Marino, CA) |
Assignee: |
California Institute of
Technology (Pasadena, CA)
|
Family
ID: |
24029229 |
Appl.
No.: |
08/510,064 |
Filed: |
August 1, 1995 |
Current U.S.
Class: |
250/227.11;
250/205; 372/18 |
Current CPC
Class: |
G02F
1/0123 (20130101); G02F 2203/56 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); H01S 003/098 () |
Field of
Search: |
;250/205,214.1,214R,227.11,227.12,227.17,227.18,227.21
;359/315,316,317,318,319,320,181,187,184 ;372/12,18 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
A Neyer and E. Voges, Hybrid Electro-Optical Multivibrator
Operating By Finite Feedback Delay, Jan. 21, 1982, Electronics
Letters. .
H.M. Gibs, F.A. Hopf, D.L. Kaplan, M.W. Derstine, R.L. Shoemaker,
Periodic Oscillations and Chaos in Optical Bistability: Possible
Guided-Wave All-Optical Square-Wave Oscillators, 1981, SPIE col.
317. .
A. Neyer and E. Voges, High-Frequency Electro-Optic Using an
Integrated Interferometer, Jan. 1, 1982, Appl. Phys. Lett. 40(1).
.
A. Neyer and E. Voges, NonLinear Electrooptic Oscillator Using an
Integrated Interferometer, May 1, 1981 Optics Communications vol.
37, No. 3. .
A. Neyer and E. Voges, Dynamics of Electrooptic Bistable Devices
with Delayed Feedback, Dec. 1982, IEEE Journal of Quantum
Electronics, vol. QE-18, No. 12. .
H.F. Schlaak and R.Th. Kersten, Integrated Optical Oscillators and
Their Applications to Optical Communication Systems, Optics
Communications vol. 36, No. 3, Feb. 1981. .
Tahito Aida and Peter Davis, Oscillation Modes of Laser Diode
Pumped Hybrid Bistable with Large Delay and Application to
Dynamical Memory, Mar. 1992, IEEE Journal of Quantum Electronics,
vol. 28, No. 3. .
X. Steve Yao and Lute Maleki, Optoelectronic Microwave Oscillator,
Aug. 1996, J. Opt. Soc. Am. B/Vol. 13, No. 8. .
X. Steve Yao and George Lutes, A High-Speed Photonic Clock and
Carrier Recovery Device, May 1996, IEEE Photonics Technology
Letters, vol. 8, No. 5. .
X. Steve Yao and Lute Maleki, Converting Light Into Spectrally Pure
Microwave Oscillation, Apr. 1, 1996, Optics Letters vol. 21, No. 7.
.
X. Steve Yao and Lute Maleki, Optoelectronic Oscillator for
Photonic Systems, Apr. 7, 1996, IEEE Journal of Quantum
Electronics, vol. 32, No. 7. .
X.S. Yao and L. Maleki, High Frequency Optical Subcarrier
Generator, Apr. 21, 1994, Electronics Letters Online No.: 19941033.
.
X.S. Yao et al., "High Frequency Optical Subcarrier Generator,"
Electronics Letters, vol. 30, No. 18, Sep. 1, 1994, pp.
1525-1526..
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Lee; John R.
Attorney, Agent or Firm: Fish & Richardson P.C.
Claims
What is claimed is:
1. An opto-electronic oscillator comprising:
an electric-optical modulator having an electrical input port that
accepts an electrical control signal and an optical output port,
wherein the electric-optical modulator is operable to generate at
the optical output port an optical signal being modulated at an
oscillation frequency related to the electrical control signal,
a photodetector operable to convert a portion of the optical signal
from the optical output port of the electro-optical modulator to an
electrical signal and to provide the electrical signal to the
electrical input port of the electro-optical modulator as the
electrical control signal, the electrical signal from the
photodetector oscillating at the same oscillation frequency at
which the optical signal at the optical output port is modulated,
and
an active feedback loop connecting the optical output port, the
photodetector and the electrical input port, said feedback loop
having a positive feedback with an open loop gain which is greater
than one.
2. The electro-optical oscillator of claim 1, wherein the
opto-electronic modulator further comprises an electrical bias port
and the frequency at which the optical output signal oscillates is
affected by a voltage applied to the bias port.
3. The electro-optical oscillator of claim 1, further comprising an
optical fiber having a length longer than a predetermined length to
effect a delay time in the active feedback loop, thereby causing
the optical signal at the optical output port to have a spectral
linewidth at the oscillation frequency below a desired linewidth
having a first relation with the delay time and a phase noise below
a desired noise level having a second relation with the delay time,
the opto-electronic oscillator operable to receive the portion of
the optical output signal and to transport said portion to the
photodetector.
4. The electro-optical oscillator of claim 3, further comprising a
fiber stretcher coupled to the optical fiber, wherein the fiber
stretcher includes an electrical input port and is operable to
modify a length of the optical fiber in response to a signal
applied to the electrical input port.
5. The electro-optical oscillator of claim 4, wherein the
opto-electronic oscillator is configured so that the output signal
oscillates at a higher frequency and a lower frequency, the
oscillator further comprising a second feedback loop operable to
combine a low frequency portion of the output signal with a low
frequency reference signal to produce a combined electrical signal,
and the combined electrical signal being applied to the electrical
input port of the fiber stretcher to affect the length of the
optical fiber.
6. The electro-optical oscillator of claim 5, wherein the
oscillator produces an electrical-output signal at an electrical
output that oscillates at the same frequency as the optical output
signal, and wherein the feedback loop includes:
a low pass filter connected to the electrical output and operable
to produce a filtered electrical signal from which a high frequency
component of the electrical output signal has been removed,
an amplifier connected to an output of the low pass filter and
operable to produce an amplified filtered electrical signal,
a combiner having a first input port connected to an output of the
amplifier, a second input port for receiving the low frequency
reference signal, and an output port for producing the combined
electrical signal.
7. The electro-optical oscillator of claim 1, further comprising an
RF coupler operable to receive the electrical signal produced by
the photodetector and to produce an electrical output signal from
the received electrical signal.
8. The electro-optical oscillator of claim 7, further comprising an
optical coupler operable to combine an external optical control
signal with the portion of the optical output signal to produce a
combined optical signal, and wherein the photodetector is operable
to convert the combined optical signal to an electrical signal and
to supply the electrical signal to the electrical input port of the
electro-optical modulator as the electrical control signal.
9. The electro-optical oscillator of claim 8, wherein the RF
coupler is further operable to combine an external electrical
control signal with the electrical signal produced by the
photodetector to produce a combined electrical signal, to supply
the combined electrical signal as the electrical output signal, and
to supply the combined electrical signal to the electrical input
port of the electro-optical modulator as the electrical control
signal.
10. The electro-optical oscillator of claim 9, wherein the
electro-optical modulator further comprises an electrical bias port
and the frequency at which the optical output signal oscillates is
affected by a voltage applied to the bias port.
11. The electro-optical oscillator of claim 10, further comprising
an optical fiber operable to receive the portion of the optical
output signal and to transport it to the photodetector.
12. The electro-optical oscillator of claim 11, further comprising
a fiber stretcher coupled to the optical fiber, wherein the fiber
stretcher includes an electrical input port and is operable to
modify a length of the optical fiber in response to a signal
applied to the electrical input port.
13. The electro-optical oscillator of claim 12, further comprising
an RF amplifier operable to amplify the electrical signal produced
by the photodetector to produce an amplified electrical signal, and
to supply the amplified electrical signal to the RF coupler.
14. The electro-optical oscillator of claim 12, further comprising
a band pass filter disposed in the active feedback loop between the
photodetector and the electrical input port of the
electrical-optical modulator, operable to effect a filtered
electrical signal from the combined electrical signal produced by
the RF coupler and to supply the filtered electrical signal to the
electrical input port of the electro-optical modulator as the
electrical control signal, the filtered electrical signal having a
characteristic in frequency domain that is related to the band pass
filter.
15. The electro-optical oscillator of claim 8, further
comprising:
a remote source of an electrical reference signal,
a laser diode operable to convert the electrical reference signal
to an optical reference signal, and
an optical fiber operable to supply the optical reference signal to
the optical coupler as the external optical control signal.
16. The electro-optical oscillator of claim 15, wherein:
the remote source is operable to produce the electrical reference
signal at a first frequency and at a power sufficient to cause the
laser diode to produce harmonics of the first frequency in the
optical reference signal, and
the electro-optical oscillator is configured to oscillate in a
frequency range that includes a second frequency that is an integer
multiple of the first frequency so that the optical reference
signal causes the output signal to oscillate at the second
frequency.
17. The electro-optical oscillator of claim 8, wherein:
the external optical control signal comprises a stream of digital
data at a clock rate having a first frequency, and
the electro-optical oscillator is configured to oscillate in a
frequency range that includes the first frequency so that the
external optical control signal causes the output signal to
oscillate at the first frequency and thereby causes the
electro-optical oscillator to extract a clock signal from the
stream of data.
18. A data recovery system including the electro-optical oscillator
of claim 17, and further comprising:
an optical delay line operable to delay the stream of digital data
for a time sufficient to permit the electro-optical oscillator to
extract the clock signal from the stream of data,
an optical-to-electrical converter operable to convert the delayed
stream of digital data to an electrical data stream, and
a data recovery circuit operable to receive the electrical output
signal of the electro-optical oscillator and the electrical data
stream and to produce digital data therefrom.
19. The electro-optical oscillator of claim 8, wherein:
the external optical control signal comprises a carrier signal
having a first frequency and noise, and
the electro-optical oscillator is configured to oscillate in a
frequency range that includes the first frequency so that the
external optical control signal causes the output signal to
oscillate at the first frequency and thereby causes the
electro-optical oscillator to extract the clock signal from the
noise.
20. The electro-optical oscillator of claim 7, further comprising a
combining circuit operable to combine the electrical output with an
electrical reference signal to produce a combined electrical
control signal and to supply the combined electrical control signal
to a control port to control the oscillation frequency of the
output signal to implement a phase locked loop.
21. The electro-optical oscillator of claim 20, wherein the
electro-optical modulator further comprises an electrical bias port
and the frequency at which the optical output signal oscillates is
affected by a voltage applied to the bias port, and wherein the
electrical bias port is the control port to which the combined
electrical control signal is supplied.
22. The electro-optical oscillator of claim 20, further
comprising:
an optical fiber operable to receive the portion of the optical
output signal and to transport it to the photodetector, and
a fiber stretcher coupled to the optical fiber, wherein the fiber
stretcher includes an electrical input port and is operable to
modify a length of the optical fiber in response to a signal
applied to the electrical input port, and wherein the electrical
input port is the control port to which the combined electrical
control signal is supplied.
23. The electro-optical oscillator of claim 20, further comprising
an electrical reference source operable to produce the electrical
reference signal.
24. The electro-optical oscillator of claim 20, further
comprising:
a fiber delay line for producing a delayed version of the optical
output signal of the electro-optical modulator, and
a second photodetector operable to convert the delayed version of
the optical output signal to an electrical signal and to supply the
electrical signal to the combining circuit as the electrical
reference signal.
25. A data downlink system including the electro-optical oscillator
of claim 20, and further comprising:
a second electro-optical modulator operable to receive the optical
output signal of the electro-optical oscillator and to modulate the
optical output signal using an electrical control signal from a
downlink receiver to produce a modulated optical signal, and
a low speed photodetector operable to extract the electrical
control signal from the modulated optical signal.
26. The electro-optical oscillator of claim 1, further comprising
an RF coupler operable to receive the electrical signal produced by
the photodetector, to combine an external electrical control signal
with the electrical signal produced by the photodetector to produce
a combined electrical signal, and to supply the combined electrical
signal to the electrical input port of the electro-optical
modulator as the electrical control signal.
27. The electro-optical oscillator of claim 26, further
comprising:
a fiber delay line for producing a delayed version of the optical
output signal of the electro-optical modulator, and
a second photodetector operable to convert the delayed version of
the optical output signal to a delayed electrical signal and to
supply the delayed electrical signal to the RF coupler as the
external electrical control signal.
28. The electro-optical oscillator of claim 27, wherein a length of
the fiber delay line is greater than one kilometer.
29. The electro-optical oscillator of claim 1, further comprising
an optical coupler operable to combine an external optical control
signal with the portion of the optical output signal to produce a
combined optical signal, and wherein the photodetector is operable
to convert the combined optical signal to an electrical signal and
to supply the electrical signal to the electrical input port of the
electro-optical modulator as the electrical control signal.
30. The electro-optical oscillator of claim 1, further comprising a
laser source for supplying a laser signal to the electro-optical
modulator, wherein the laser source, the electro-optical modulator
and the photodetector are all implemented on a single integrated
circuit substrate.
31. The opto-electronic oscillator of claim 1, wherein the active
feedback loop comprises an optical delay line.
32. The opto-electronic oscillator of claim 1, wherein the active
feedback loop comprises an RF delay line.
33. A method of generating an oscillatory optical signal comprising
the steps of:
modulating an optical signal with an electrical control signal in
an electro-optical modulator to produce a modulated optical output
signal having a frequency related to the electrical control
signal,
forming an active feedback loop having a positive feedback with an
open loop gain greater than one to effect the oscillation,
converting a portion of the modulated optical output signal to an
electrical signal that propagates in the feedback loop, and
supplying the electrical signal produced in the converting step to
the electro-optical modulator through the feedback loop as the
electrical control signal.
34. A photonic device, comprising:
an electro-optical modulator having an input port for signals of a
predetermined form, and having an output port for signals of a
second predetermined form, one of said first and second
predetermined forms being an electrical oscillating signal at an
oscillating frequency and the other of said first and second
predetermined forms being an optical oscillating signal that is
modulated at said oscillating frequency; and
a converter operable to convert said first predetermined form
signal to said second predetermined form signal; and
an element operable to conduct said first predetermined form signal
to said converter and to conduct said second predetermined form
signal to said input port, to form a feedback loop which uses both
said electrical and optical signal and synchronizes to both, said
feedback loop having a positive feedback with an open loop gain
greater than one.
35. A system as in claim 34, further comprising optical and
electrical injection ports, respectively allowing injection of
optical and electrical injection signals, and to which said output
signal is injection locked.
36. A system as in claim 34 further comprising an element operable
to delay an output signal of said modulator by a time which is
effective to lock said output to a past state.
37. A system as in claim 36 wherein said first predetermined output
is optical, and wherein said element is a fiber delay line of at
least 1 kilometer in length.
38. An opto-electronic oscillator as in claim 14, wherein the RF
coupler includes an electrical output port for exporting an
electrical signal, and an electrical input port for injecting an
external electrical signal into the active feedback loop and
enabling the RF coupler to combine the electrical signal from the
photodetector and the external electrical signal, the external
electrical signal oscillating at an injection frequency which is a
subharmonic of the oscillating frequency of the active feedback
loop, thereby producing a signal gain and frequency multiplication
in an electrical output signal at the electrical output port with
respect to the injected external electrical signal.
39. An opto-electronic oscillator as in claim 12, wherein the RF
coupler includes an electrical output port for exporting an
electrical signal, and an electrical input port for injecting an
external electrical signal into the active feedback loop and
enabling the RF coupler to combine the electrical signal from the
photodetector and the external electrical signal, the
opto-electronic oscillator further comprising:
an electrical signal generator connected to the electrical input
port in the RF coupler, operating to produce a periodic signal
having a signal period in frequency domain that is substantially
equal to the mode spacing of the oscillation in the active feedback
loop or a multiplication thereof, the periodic signal being
injected at the input port as the external electrical signal;
and
mode locking means for forcing different modes oscillate with a
certain phase relation with respect to one another in a way that is
determined by the injected periodic signal, producing a periodic
pulsed signal in at least one of optical form and electrical
form.
40. A method as in claim 33, further comprising:
filtering the electrical signal in the active feedback loop at a
center frequency with a predetermined bandwidth with a band pass
filter; and
effecting a single-mode oscillation by the filtering.
41. A method as in claim 33, further comprising:
producing a phase delay in the electrical control signal with the
active feedback loop; and
increasing the phase delay larger than a predetermined delay value
to effect a delay time in the active feedback loop to cause the
optical signal to have a spectral linewidth at the oscillation
frequency below a desired linewidth having a first relation with
the delay time and a phase noise below a desired noise level having
a second relation with the delay time.
Description
ORIGIN OF INVENTION
The invention described herein was made in the performance of work
under a NASA contract, and is subject to the provisions of Public
Law 96-517 (35 USC 202) in which the Contractor has elected to
retain title.
BACKGROUND AND SUMMARY OF THE INVENTION
Radio frequency ("RF") oscillators, especially voltage controlled
oscillators ("VCOs"), are essential to RF communication,
broadcasting and receiving systems. RF oscillators often generate,
track, clean, amplify and distribute RF carriers. In a phase locked
loop configuration, VCOs can also be used for clock recovery,
carrier recovery, signal modulation and demodulation, and frequency
synthesis.
A photonic RF system embeds photonic (i.e., optical) technology
into a traditional RF system. In particular, a photonic RF system
uses optical waves as carriers to transport RF signals through
optical media (e.g., optical fiber) to remote locations. RF signal
processing functions, such as signal mixing, antenna beam steering
and signal filtering, can be accomplished optically. Optical
technology offers the advantages of low loss, light weight, high
frequency, high security, remoting capability, and immunity to
electromagnetic interference, and therefore is desirable in many RF
systems.
The inventors realized that traditional RF oscillators cannot meet
all of the requirements of photonic RF systems. One reason is
because photonic RF systems involve RF signals in both the optical
domain and the electrical domain. The inventors realized that an
ideal oscillator for a photonic RF system should be able to
generate both optical and electrical RF signals. In addition, such
a system should allow synchronizing or controlling the oscillator
using both electrical and optical references or signals.
Presently, an optical high frequency RF signal is usually generated
by modulating a diode laser or an electro-optical ("E/O") modulator
using a high frequency stable electrical signal from a local
oscillator ("LO"). Such a local oscillator signal is generally
obtained by multiplying a low frequency reference, (e.g., a signal
produced by a quartz oscillator or a Hydrogen maser) to a required
high frequency (e.g., 32 GHz) with several stages of multipliers
and amplifiers. Consequently, the resulting system is bulky,
complicated, inefficient, and costly.
An alternative way of generating photonic RF carriers is by mixing
two lasers with different optical frequencies. However, the
bandwidth of the resulting signal, which is limited by the spectral
width of the lasers, is wide (typically greater than tens of
kilohertz). In addition, drift of the optical frequencies of the
two lasers causes the frequency stability of the beat signal to
become poor. While the frequency stability of the beat signal can
be stabilized by phase locking the signal to an external stable
reference having the same frequency, this would defeat the purpose
of using lasers to generate a high frequency (i.e., a high
frequency LO signal would still be required).
The invention features a novel opto-electronic oscillator ("O/E")
for photonic RF systems. The oscillator, which may also be referred
to as a photonic oscillator, is capable of generating stable
signals at frequencies up to 70 GHz in both the electrical domain
and the optical domain. The oscillator also functions as a voltage
controlled oscillator with optical and electrical outputs. It can
be used to make a phase locked loop (PLL) and to perform all
functions performed for RF systems by a traditional PLL for
photonic systems. It has optical and electrical inputs, and can be
synchronized to a remote reference source by optical or electrical
injection locking. It can also be self phase-locked and self
injection-locked to generate a high stability photonic RF
reference. Applications of the opto-electronic oscillator include
high frequency reference regeneration and distribution, high gain
frequency multiplication, comb frequency and square wave
generation, and clock recovery. The oscillator is inherently
unidirectional, immune to back reflections in the loop, and
therefore generically stable.
Essentially, an opto-electronic oscillator includes an
opto-electronic modulator that is controlled by a feedback loop
between an optical output and an electrical control port of the
modulator. The feedback loop preferably includes an optical fiber
that delivers the optical output to a photodetector. The
photodetector converts the optical signal to the electrical signal
supplied to the control port. The loop also includes an amplifier
and a filter, and an RF coupler that permits the insertion of an
external electrical control signal and the extraction of an
electrical output signal. An optical coupler positioned before the
photodetector permits the insertion of an external optical control
signal. The oscillation frequency of the opto-electronic oscillator
is controlled by the external control signals and by signals
applied to a bias port of the E/O modulator and to a fiber
stretcher. The fiber stretcher is connected to the fiber in the
feedback loop.
When the opto-electronic oscillator is used for clock recovery or
carrier regeneration, it provides a number of advantages over
traditional techniques. First, it operates at speeds of up to 70
GHz, and is limited only by the speeds of the photodetector and the
E/O modulator of the oscillator. In addition, the amplitude of the
recovered signal (clock or carrier) is constant, and is independent
of the input power of the signal to be recovered. This is
particularly important in clock recovery for time division
multiplexed systems, because the clock component contained in the
received data stream may vary with time and sender. Also, the
recovered signal may be accessed both optically and electrically,
which provides easy interfacing with complex systems.
The inventors recognized that since clock recovery and carrier
regeneration with an opto-electronic oscillator is based on
injection locking, its acquisition time is much shorter than that
of a recovery device based on a phase locked loop. Fast acquisition
is important for high speed communications such as burst mode
communications. In addition, the tracking range is on the order of
a few percent of the clock frequency, compared to less than 100 Hz
for a device based on a phase locked loop. This, of course, means
that the device does not have to be precisely tuned to match the
incoming data rate.
The opto-electronic oscillator can be tuned over a broad spectral
range, such as many tens of GHz, by changing the filter in the
feedback loop, and may be fine tuned by changing the loop delay (by
stretching the fiber) or the bias point of the electro-optical
modulator. This makes the device flexible in accommodating
different systems, designs and signal conditions.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1A is a block diagram of an opto-electronic oscillator.
FIG. 1B is a symbolic representation of the opto-electronic
oscillator of FIG. 1A.
FIGS. 2A and 2B are graphs of spectra of a free running
opto-electronic oscillator oscillating at 9.22 GHz (FIG. 2A) and
100 MHz (FIG. 2B).
FIGS. 3A and 3B are graphs of oscillation amplitude as a function
of open loop gain, where FIG. 3A provides theoretical curves and
FIG. 3B presents experimental data.
FIG. 4A is a graph of an opto-electronic oscillator's frequency
relative to the bias voltage of an opto-electronic modulator of the
oscillator.
FIG. 4B is a graph of an opto-electronic oscillator's oscillation
power relative to bias voltage.
FIG. 4C is a graph of an opto-electronic oscillator's measured
phase noise as a function of frequency offset.
FIG. 4D is a graph of an opto-electronic oscillator's measured
phase noise as a function of loop delay time, extrapolated from the
curves of FIG. 4A.
FIG. 4E is a graph of experimental results of the phase noise of an
opto-electronic oscillator at frequencies 100 MHz, 300 MHz, 700
MHz, and 800 MHz.
FIG. 5A is a graph of phase noise of an opto-electronic oscillator
that was injection-locked to a maser.
FIG. 5B is a graph of locking range as a function of the
square-root of the injection power.
FIG. 6A is a block diagram of the opto-electronic oscillator of
FIG. 2 in a self-injection locking configuration.
FIG. 6B is a graph of the frequency response of the configuration
of FIG. 6A.
FIGS. 7A and 7B are block diagrams of the opto-electronic
oscillator configured as a phase locked loop, with
FIG. 7A illustrating phase locking to a reference source and
FIG. 7B illustrating self-phase locking.
FIG. 8 is a block diagram of the opto-electronic oscillator of FIG.
2 when configured for photonic down conversion.
FIG. 9 is a block diagram of the opto-electronic oscillator of FIG.
2 configured for reference regeneration and distribution.
FIGS. 10A and 10B are block diagrams of the opto-electronic
oscillator of FIG. 2 configured for frequency multiplication, with
FIG. 10A illustrating frequency multiplication using nonlinearity
of the opto-electronic oscillator, and FIG. 10B illustrating
frequency multiplication using a laser diode's nonlinearity.
FIGS. 11A and 11B are graphs of comb frequencies (FIG. 11A) and
square waves (FIG. 11B) produced by an opto-electronic
oscillator.
FIG. 12 is a block diagram of the opto-electronic oscillator of
FIG. 2 in a loop-length stabilization configuration.
FIGS. 13A and 13B are graphs of frequency responses of an
opto-electronic oscillator.
FIGS. 14A and 14B are block diagrams of the opto-electronic
oscillator of FIG. 2 configured for clock (FIG. 14A) and carrier
(FIG. 14B) recovery.
FIG. 15 is a block diagram of a clock recovery, synchronization and
signal recovery system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated in FIG. 1A, an opto-electronic ("O/E") oscillator 10
includes an E/O modulator 12. Light from an output port 14 of the
E/O modulator 12 is detected by a photodetector 16 that converts
the light to an electrical signal. The electrical signal is
supplied to an RF amplifier 18 that amplifies the signal. After
passing through an RF coupler 20 and a bandpass filter 22, the
amplified signal is supplied to an electrical input port 24 of the
E/O modulator 12. The RF amplifier 18 increases the open loop gain
of the system, and the bandpass filter 22 removes unwanted
oscillation modes and harmonic signals of the modulation. The
amplifier and filter, which are not critical to the generation of
RF oscillations of the system operation, are each optional
components for generating the RF oscillations.
Proper biasing of the E/O modulator 12 is critical for proper
operation of the opto-electronic oscillator 10. The biasing point
of modulator 12 determines whether the oscillator is bistable,
oscillatory or chaotic when the bandpass filter 22 of FIG. 1A is
removed from the active feedback loop. Biasing is carried out
through bias port 26.
If the E/O modulator 12 is properly biased through bias port 26,
and the open loop gain of the feedback loop between output port 14
and RF driving port 24 is properly chosen, self oscillation is
sustained. Both optical and electrical processes are involved in
the oscillation as described herein; hence, an optical subcarrier
and an electrical signal are generated simultaneously. The optical
subcarrier is generated at an optical output 32 of the E/O
modulator. The electrical signal is generated at an electrical
output 34 of the RF coupler 20.
The opto-electronic oscillator 10 includes an optical variable
delay line 35 and/or an RF variable delay line 37. Delay lines 35,
37 permit large changes to the loop delay of opto-electronic
oscillator 10 and thereby permit the oscillation frequency of
opto-electronic oscillator 10 to be adjusted over a large range of
frequencies, such as a one hundred megahertz or a few gigahertz.
The variable delay lines may be manually or electrically
adjusted.
The oscillation frequency of opto-electronic oscillator 10 is
fine-tuned through a bias port 26 and a loop length adjustment port
36. Bias port 26 biases the E/O modulator 12. Loop length
adjustment port 36 is connected to a fiber stretcher 38 for
controlling the loop length and thereby controlling the loop
delay.
The above-described opto-electronic oscillator can be summarized by
the simple functional block diagram shown in FIG. 1B. The
oscillator 10 is a six-port device, with an optical injection port
28, an electrical injection port 30, an optical output port 32, an
electrical output port 34, and two voltage controlling ports. Bias
port 26 biases the E/O modulator 12. Loop length adjustment port 36
is connected to a fiber stretcher 38 for controlling the loop
length and thereby controlling the loop delay. Because delay lines
35 and 37 typically are not adjusted during operation, they are not
illustrated as ports in the functional block diagram of FIG. 1B.
However, they may be adjusted during operation in some
applications.
As will be explained below, the two injection ports 28, 30 operate
to injection lock the opto-electronic oscillator 10 to a reference
source either optically or electrically. The two output ports 32,
34 provide an RF carrier in both an optical form (port 32) and an
electrical form (port 34). Finally, the two controlling ports 26,
36 operate to tune the oscillation frequency and voltage-control
the frequency of opto-electronic oscillator 10. The six ports
collectively simplify interfacing of the oscillator to a photonic
RF system.
E/O modulator 12 is preferably of the well known Mach-Zehnder type.
Alternately, it can be a directional coupler modulator, an
electro-absorption modulator, or a quantum well modulator.
Modulator 12 generates optical outputs at ports 14 and 32 based on
an optical input from an optical source, preferably a pump laser
40. The light from the pump laser 40 is separated into two beams
42, 44 that are directed to wave guides 46, 48 on opposite sides of
the modulator 12. The beams 42, 44 are guided in the wave guides
46, 48. The beams then recombine to produce the optical outputs 49
of the modulator.
If the two beams travel paths of different lengths, the beams will
be out of phase with each other when they are recombined. The
resulting signal will hence have a beat frequency that varies with
the phase difference between the beams. The voltages applied to
ports 24 and 26 change the lengths of the paths along which the
beams 42, 44 travel. Because the phase difference between the beams
varies with the difference in the path lengths, the frequency of
the output signal of the E/O modulator 12 is controlled by
controlling the voltages applied to ports 24 and 26. The voltage
applied to the bias port 26 establishes the initial phase
difference between the beams and thereby establishes a proper
bias.
One important feature of this modulator is the interconnection
between the optical and electrical systems in the feedback loop.
The optical and electrical signals are both used in feedback: hence
both are inherently locked together. If either signal changes, that
change is accommodated by the feedback loop.
The opto-electronic oscillator used for obtaining the measurements
shown in FIGS. 2A and 2B was constructed with different E/O
modulators for the modulator 12 and a diode-pumped YAG laser at a
wavelength of 1320 nm to generate optical subcarriers as high as
9.2 GHz. FIG. 2A shows the RF signal 50 generated at 9.2 GHz and
FIG. 2B shows the signal 52 generated at 100 MHz. In both cases,
the opto-electronic oscillators are free running and no effort was
made to reduce the noise of the oscillators. For comparison, a
signal 54 from a HP8656A signal generator is also shown in FIG. 2B.
The spectral purity of signal 52 is dramatically better than that
of signal 54.
In the embodiment producing the results of FIG. 2A, the
Mach-Zehnder modulator has a bandwidth of 8 GHz and a halfwave
voltage V.sub..pi. of approximately 17 V. The photodetector has a
bandwidth of 12 GHz and a responsivity of approximately 0.35 A/W.
The amplifier has an electrical power gain of 50 dB, a bandwidth of
5 GHz centered around 8 GHz, and output dB compression of 20 dBm.
The input and output impedances of all electrical components are 50
.OMEGA.. The loop length is approximately 9 meters. In other
embodiments, the modulator could be, for example, a directional
coupler modulator or an electro-absorption modulator.
As is well known, ambient noise will cause an oscillator to begin
oscillating if the gain of a feedback loop of the oscillator is
sufficient to amplify the feedback signal of the oscillator
relative to the output signal thereof (i.e., when the gain is
greater than one). Thus, to start oscillation from noise, the open
loop gain of the opto-electronic oscillator 10 must be larger than
one. The open loop gain of the opto-electronic oscillator is simply
the RF power gain of an externally-modulated photonic link and is
given by: ##EQU1## where G.sub.open is the open loop voltage gain,
I.sub.ph is the photocurrent in the receiver, R.sub.L is the load
resistance of the receiver, R.sub.m is the input impedance of the
modulator, V.sub..pi. is the half-wave voltage of the modulator,
and G.sub.amp is the voltage gain of the amplifier following the
photoreceiver. The modulator 12 in Equation 1 is assumed to be a
Mach-Zehnder modulator that is biased at quadrature. The
oscillation condition of the opto-electronic oscillator is thus
G.sub.open .gtoreq.1. For R.sub.L =R.sub.m =R, this condition
becomes:
It is important to note that the amplifier in the loop is not a
necessary condition for oscillation. If I.sub.ph
R.gtoreq.V.sub..pi. /.pi., no amplifier is needed (G.sub.amp =1).
The optical power from the pump laser actually supplies the
necessary energy for the opto-electronic oscillator. This is
significant because it means that, when the amplifier is
eliminated, the opto-electronic oscillator may be powered remotely
using an optical fiber. In addition, elimination of the amplifier
in the loop would also eliminate any noise associated with the
amplifier, and would result in a more stable oscillator. For a
modulator with a V.sub..pi. of 3.14 volts and an impedance R of
50.OMEGA., a photocurrent of 20 mA is required to sustain the
photonic oscillation without an amplifier. This corresponds to an
optical power of 25 mW, taking the responsivity .rho. of the
photodetector to be 0.8 A/W.
The optical power from the output port that forms the loop is
related to an applied voltage by:
where .alpha. is the fractional insertion loss of the modulator,
V.sub.B is the bias voltage, and .eta. relates to the extinction
ratio of the modulator by (1+.eta.) (1-.eta.). If P(t) has a
positive slope as a function of driving voltage V(t), the modulator
is said to be positively biased, otherwise it is negatively biased.
Consequently, when V.sub.B =0, the modulator is biased at negative
quadrature, while when V.sub.B =V.sub..pi., the modulator is biased
at positive quadrature. Note that in most externally modulated
photonic links, the E/O modulator can be biased at either positive
or negative quadrature without affecting its performance. However,
as will be seen next, the biasing polarity will have an important
effect on the operation of the opto-electronic oscillator 10.
The applied voltage V(t) of the opto-electronic oscillator is the
photovoltage after the filter in the loop: V(t)=.rho.RG.sub.amp
P(t), where .rho. is the responsivity of the detector, R is the
loop impedance, and G.sub.amp is the amplifier voltage gain. The
recurrence relation of the oscillating signal in the loop can be
easily obtained from Equation 3 to be:
where .tau. is the loop delay time, and u.sub.O, u.sub.B, and u(t)
are the normalized photovoltage, normalized bias voltage, and
normalized oscillation voltage respectively. They are defined
as:
In Equation 5, I.sub.ph =.alpha..rho.P.sub.O /2. Equation 4 simply
relates the oscillation voltage at t with that of an earlier time,
t-.tau..
The oscillating voltage at steady state should repeat itself after
a round trip in the loop, that is, u(t)=u(t-.tau.). This is the
self-consistent condition. When a filter in the loop permits only
one frequency (.omega..sub.O) to oscillate, the solution to
Equation 4 has the form:
where a is the normalized oscillation amplitude and .phi..sub.O is
the phase. Note that in expressing Equation 8 this way, the
observation point of the field is chosen to be right after the
filter. Substituting Equation 8 in Equation 4 and using the
self-consistent condition produces:
Expanding the right hand side of Equation 9 using the Bessel
function and picking out the term with the fundamental frequency
component results in:
Since the oscillation amplitude V.sub.OSC of the oscillator is
usually much less than V.sub..pi., the following discussion is
restricted to the case in which J.sub.1 (.pi.a).gtoreq.0 or
.alpha.=V.sub.OSC /V.sub..pi. .gtoreq.1.21. With this restriction,
Equation 10 results in the following relations for determining the
oscillation amplitude and frequency: ##EQU2##
where f.sub.O =.omega..sub.O /2.pi. and k is an integer. It is
interesting to note from Equation 12 that the oscillation frequency
depends on the biasing polarity of the modulator. For negative
biasing (cos .pi.u.sub.B >0), the fundamental frequency is
1/(2.tau.), while for positive biasing (cos .pi.u.sub.B <0), the
fundamental frequency is doubled to 1/.tau..
G.sub.open in Equation 11 is the open loop voltage gain of the
oscillator and is defined as ##EQU3##
For the ideal case in which .eta.=1 and .vertline.cos .pi.u.sub.B
.vertline.=1, G.sub.open equals the value expected from Equation 1.
The amplitude of the oscillation can be obtained by graphically
solving Equation 11. The result is shown in FIG. 3A.
Equation 9 can also be solved by expanding its right side into a
Taylor series. The resulting normalized oscillation amplitude is:
##EQU4##
It is clear from Equation 14 that the threshold condition for
oscillation is G.sub.open .gtoreq.1 or I.sub.ph
RG.eta..vertline.cos u.sub.B .vertline..gtoreq.V.sub..pi. /.pi..
The oscillation frequency obtained using this procedure is the same
as is produced by Equation 12.
FIG. 3A shows the graphically-obtained normalized oscillation
amplitude from Equation 11 (56), Equation 14a (58) and Equation 14b
(60), respectively. Comparing the three theoretical curves one can
see that for G.sub.open .ltoreq.1.57, the 3rd order expansion
result is a good approximation. For G.sub.open .ltoreq.3.14, the
fifth order expansion result is a good approximation. FIG. 3B shows
the experimental data 62 and is in good agreement with the
theoretical results of Equation 14a (58).
As described above, the oscillation frequency of the
opto-electronic oscillator 10 can be tuned by changing the loop
length using a piezo-electric stretcher 38. The frequency change,
.DELTA.f, is given by .DELTA.f=-f.sub.O .DELTA.L/L, where L is the
loop length, .DELTA.L is the loop length change, and f.sub.O is the
nominal oscillation frequency. However, the tuning sensitivity
(Hz/volt) is expected to be small.
The oscillation frequency can also be tuned by changing the bias
voltage of the E/O modulator 12. FIG. 4A shows that the frequency
detuning 64 of the oscillator 10 is linearly proportional to the
bias voltage, with a slope of 38.8 kHz/volt. The output power 66 of
the oscillator 10 remains relatively unchanged in a wide voltage
range, as shown in FIG. 4B. This result is significant because it
provides a simple way to tune the oscillation frequency with high
sensitivity and is instrumental for implementing a phase locked
loop (PLL) using the opto-electronic oscillator 10.
The noise properties of the O/E oscillator 10 have also been
analyzed. The RF spectral density of the O/E oscillator 10 obtained
from the analysis is: ##EQU5## where .tau. is the total group delay
of the loop, .function.' is the frequency offset from the
oscillation frequency, and .delta. is the input noise to signal
ration of the oscillator, which is defined as:
In Equation 16, .rho..sub.N is the equivalent input noise density
injected into the oscillator from the input port of the amplifier,
which includes the contributions of thermal noise, shot noise, and
the laser's relative intensity noise. In addition, P.sub.OSC is the
total oscillating power measured after the amplifier and it relates
to the oscillation amplitude V.sub.OSC in Equation 11 and Equation
14 by P.sub.OSC =V.sup.2.sub.OSC /(2R) .
As can be seen from Equation 15, the spectral density of the
oscillating mode is a Lorentzian function of frequency. The full
width at half maximum (FWHM) .DELTA..function..sub.FWHM of the
function is: ##EQU6##
From Equation 17 one can see that .DELTA..function..sub.FWHM is
inversely proportional to the square of loop delay time and
linearly proportional to the input noise to signal ratio .delta..
For a typical .delta. of 10.sup.-16 /Hz and loop delay of 100 ns
(20 m), the resulting spectral width is sub-millihertz. The
fractional power contained in .DELTA..function..sub.FWHM is
.DELTA..function..sub.FWHM S.sub.RF (0)=64%.
From Equation 15 it can be seen that: ##EQU7##
The single side band phase noise of a oscillation at a frequency
offset .function.' from the center frequency is equal to the
spectral density at .function.' if .function.' is much larger than
the line width of the oscillation. Therefore, it is evident from
Equation 18b that the phase noise of the photonic oscillator
decreases quadratically with the frequency offset .function.'. For
a fixed .function.', the phase noise decreases quadratically with
the loop delay time. The larger the .tau., the smaller the phase
noise.
Equation 15 also indicates that the oscillator's phase noise is
independent of the oscillation frequency. This result is
significant because it allows the generation of high frequency and
low phase noise signals using the O/E oscillator 10. The phase
noise of a signal generated using other methods generally increases
linearly with the frequency.
FIG. 4C is the measured phase noise as a function of the frequency
offset .function.' plotted in a log scale. Clearly, the phase noise
has a 20 per decade dependency with the frequency offset
.function.', which agrees well with the theoretical prediction of
Equation 18b. FIG. 4D is the measured phase noise as a function of
loop delay time, extrapolated from the curves of FIG. 4C. Again,
the experimental data agrees well with the theoretical prediction.
FIG. 4E shows the experimental results of the phase noise of an O/E
oscillator 10 at frequencies 100 MHz, 300 MHz, 700 MHz, and 800
MHz. The phase noise curves overlap with one another. The results
indicate that the phase noise is essentially independent of the
osillation frequency of the photonic oscillator.
Injection locking is a commonly-used technique for synchronizing an
oscillator with a reference frequency. The opto-electronic
oscillator 10 can be injection locked by either an optical signal
28 or an electrical signal 30 as shown in FIGS. 1A and 1B. Optical
injection locking of an oscillator is important to allow remote
synchronization. This function is critical for high frequency RF
systems that require many oscillators locked to a single master, as
in a phased-array radar. Optical injection locking also allows the
locking oscillator to be electrically isolated from the locked
oscillator, which eliminates the need for impedance matching
between the oscillators. Importantly, the present system allows
locking to either optical or electrical signals.
FIG. 5A shows the experimental results of injection locking the
opto-electronic oscillator 10 with a maser reference at 100 MHz
through the electrical injection port 30. The data shown in the
curves are smoothed and a peak corresponding to the 60 Hz AC noise
is taken out. Similar results are expected for optical injection
since the optical injection signal 30 will first be converted to an
electrical signal by internal photodetector 16 before affecting the
E/O modulator 12.
As shown in FIG. 5A, an injection power of -5 dBm results in the
phase noise 68 of the opto-electronic oscillator 10 that is almost
identical to that of the injected maser signal 70. Note that the
output RF power of the opto-electronic oscillator is 13 dBm,
resulting in a gain of 18 dB. As the injection power decreases, the
phase noise of the opto-electronic oscillator increases somewhat.
However, the output RF power remains the same so that the gain,
effectively, is increased. The opto-electronic oscillator 10 has
been injection locked experimentally to a maser reference with an
injection power as low as -50 dBm.
FIG. 5B shows an experimentally produced locking range 72 as a
function of injection power. As expected, the locking range is
linearly proportional to the square-root of the injection power.
This agrees well with the well-known Adler's injection locking
theory.
Although injection locking is an effective way of synchronizing and
stabilizing oscillators, it requires the oscillator to be supplied
with a low noise and high stability source. However, at high
frequencies, producing such a source is itself a difficult
task.
Another aspect of the present invention uses self-injection
locking. This technique stabilizes the frequency and phase of the
opto-electronic oscillator 10 to facilitate using the oscillator 10
as the frequency reference. This is illustrated in FIG. 6A.
The self-injection locking sends a small portion of the output
optical signal from the O/E oscillator through a fiber delay line
74 that is typically at least one kilometer long and may be many
kilometers long. A photodetector 76 converts the output from the
fiber delay line 74 to an electric signal and feeds the electrical
signal back to the RF driving port 30 of the E/O modulator 12. Note
that the open-loop gain of this feedback loop should be kept well
below unity to prevent self-oscillation. Basically, self-injection
locking injects a delayed replica of the O/E oscillator's output 32
back to the oscillator 10 and forces the oscillator 10 to lock to
its "past". This tends to prevent the oscillator 10 from changing
its frequency and phase, and thereby reduces or eliminates
frequency and phase fluctuations that may be caused by noise
sources such as temperature fluctuations, acoustic perturbations
and amplifier noise. The frequency stability of the oscillator 10
is proportional to the length fluctuation .DELTA.L/L of the fiber
delay line, which can be known and controlled very precisely.
FIG. 6B provides experimental results 78 showing the effectiveness
of the self-injection technique (peak 80) in reducing the frequency
noise of the O/E oscillator 10 relative to a free-running O/E
oscillator (peak 82). The length of the delay line 74 used in the
experiment is 12 km and the feedback injection RF power is -19.23
dBm. It is evident that self-injection locking greatly reduces the
frequency fluctuations of the O/E oscillator.
Even further noise reduction can be obtained by reducing the length
fluctuation of the fiber delay line 74 by temperature control of
the environment and isolation from acoustic vibrations.
Because the opto-electronic oscillator is also a voltage controlled
oscillator (VCO), it can be synchronized to a reference source 84
via a phase locked loop as shown in FIG. 7A. A summing junction 86
combines the reference 84 and the electrical output 34 of the
opto-electronic oscillator 10, and supplies the result to the loop
length adjustment terminal 36 or the bias terminal 26 after passing
the result through a loop filter 88.
One unique property of the opto-electronic oscillator is its
optical output 32. This optical output permits stabilization
through a self-phase locked loop, as shown in FIG. 7B. In a similar
way to the self-injection locking described above, a self-phase
locked loop forces the oscillator to be locked to its past and
prevents fluctuation. The optical output 32 of the opto-electronic
oscillator is passed through a long fiber delay line 90. A
photodetector 92 monitors the output of the fiber delay line and
produces an electrical signal corresponding to the output. This
electrical signal is supplied to a summing junction 94 that sums
the signal with the electrical output 34 of the opto-electronic
oscillator and supplies the result to one of ports 26, 36 after
passing it through a loop filter 88.
There have been previous suggestions to use a delay line to
stabilize an oscillator. However, effective stabilization of an
optical oscillator was considered impractical. The reason is
exemplified by systems like that proposed by R. Logan et al.
("Stabilization of Oscillator Phase Using a Fiber Optic
Delay-Line", Proceedings of the 45th Annual Symposium on Frequency
Control (IEEE Ultrasonic Ferroelectric and Frequency Control
Society), May 29-31, Los opto-electronic Angeles, Calif., 1991).
Logan proposed a fiber optic delay line to stabilize a traditional
VCO. The fiber optic delay line included a laser transmitter to
convert the electrical output of the VCO into an optical signal
that was then transmitted through a few kilometers of fiber. The
opto-electronic oscillator 10 automatically produces an optical
output. The inventors recognized that this makes it ideally suited
for use with the fiber delay line technique of self-stabilization.
No electrical to optical signal conversion is necessary.
Consequently, the device is simple, low loss, and relatively
inexpensive.
As mentioned earlier, the opto-electronic oscillator 10 is a
photonic VCO with both optical and electrical outputs. It can
perform for photonic RF systems all functions that a VCO is capable
of performing for RF systems. These functions include generating,
tracking, cleaning, amplifying, and distributing RF carriers. In a
phase locked loop configuration, the photonic VCO can also be used
for clock recovery, carrier recovery, signal modulation and
demodulation, and frequency synthesis.
The opto-electronic oscillator 10 can also be used for photonic
signal up/down conversion, as shown in FIG. 8. Such an application
requires a stable optical RF local oscillation (LO) signal (i.e.,
an optical signal modulated at a RF frequency). The
self-stabilization technique discussed above with reference to FIG.
7B configures the opto-electronic oscillator to produce the
required stable optical RF LO signal (f.sub.O). This signal is
transmitted through an optical fiber 96 to an E/O modulator 98 that
modulates the signal based on a signal (f.sub.O +I.sub.F) received
from a downlink 100. The modulated signal is transmitted through an
optical fiber 102 to a low speed photodetector 104 that produces an
electrical signal corresponding to the baseband signal (IF).
The opto-electronic oscillator can be injection locked by a remote
optical signal to carry out high frequency RF carrier regeneration,
amplification, and distribution, as shown in FIG. 9. Laser diode
106 converts an electrical reference signal 107 from a remote
source 108 to an optical signal 109. Optical signal 109 is
transmitted by an optical fiber 110 to the optical input port 28 of
opto-electronic oscillator 10. The oscillator is injection-locked
to optical signal 109 so that the outputs of opto-electronic
oscillator 10 oscillate at the frequency of, and in phase with,
optical signal 109. The electrical output 34 of the opto-electronic
oscillator 10 is supplied to a local user while the optical output
32 of the oscillator is redistributed to remote terminals. This
capability is important in large photonic RF systems.
The injection locking property of the opto-electronic oscillator
can also be used for high gain frequency multiplication. A first
approach is shown in FIG. 10A. A signal supplied to the electrical
input 30 of the oscillator has a frequency f.sub.O that is a
subharmonic of the frequency at which the oscillator is configured
to operate (e.g., 3f.sub.O). The oscillator is injection-locked to
the signal supplied to the electrical input 30. Due to nonlinearity
of the E/O modulator 12 (FIG. 1A), the oscillator produces output
signals at ports 32, 34 having frequencies equal to the operating
frequency of the oscillator (i.e., 3f.sub.O). This is called
subharmonic injection locking. Phase-locking of an oscillator
operating at 300 MHz to a 100 MHz reference of 4 dBm has been
achieved. The output of the oscillator is 15 dBm, resulting in a
gain of 11 dB and a frequency multiplication factor of 3.
In a second approach, as illustrated in FIG. 10B, the nonlinearity
of a laser diode 114 is used to achieve frequency multiplication.
If the laser diode 114 is biased properly and is driven hard
enough, its output will contain many harmonics of the driving
signal at frequency f.sub.O supplied by a driving source 116. The
opto-electronic oscillator 10 is tuned to operate at a nominal
frequency close to the nth harmonic (nf.sub.O) of the signal
driving the laser diode 114. The output of the laser diode 114 is
supplied to the optical injection port 28 of the oscillator. This
locks the electrical and optical outputs of the oscillator to the
nth harmonic (nf.sub.O). This approach offers remote frequency
multiplication capability and may be useful for many photonic RF
systems.
The opto-electronic oscillator can also be used to generate
frequency combs and square waves, as shown in FIGS. 11A and 11B.
For this application, the opto-electronic oscillator 10 is
configured for multimode operation by eliminating the filter 22. A
sinusoidal signal with a frequency equal to the mode spacing or a
multiple of the mode spacing of the oscillator is injected into the
oscillator. Just like laser mode-locking, this injected signal
forces all modes to oscillate in phase. Consequently, as shown in
FIG. 11A, the oscillator produces a comb of frequencies 118 that
are in phase. In the time domain, as shown in FIG. 11B, the output
signal corresponds to a square wave 120. When the oscillating modes
of the oscillator are not mode-locked, the phase of each mode
fluctuates independently, and the output of the oscillator is
chaotic in the time domain.
As noted above, further noise reduction and frequency stabilization
can be obtained by placing the fiber delay in a temperature
controlled environment and isolating it from acoustic
vibrations.
Another approach to frequency stabilization is illustrated in FIG.
12. In general, with reference to FIG. 1A, the frequency response
of the open loop gain of an oscillator 10 is determined by the
frequency responses of the modulator 12, photodetector 16,
amplifier 18 and filter 22. By choosing the proper components, the
open loop response 122 shown in FIG. 13A can be obtained. The
resulting oscillator 124 will oscillate in two modes, as shown in
FIG. 13B, where one of the modes 126 is at a low frequency (e.g.,
less than 100 MHz) and the other mode 128 is at a high frequency
(e.g., 32 GHz). The oscillator is designed to generate the high
frequency oscillation subcarrier.
FIG. 12 shows the electrical output 34 being supplied to a low pass
filter 130 that removes the high frequency component thereof and
provides the low frequency component to an amplifier 132. A mixer
134 combines the output of the amplifier with a reference signal
136 operating at the frequency of the low frequency component. The
result of the mixing operation is supplied to the loop length
adjustment port 36 of the opto-electronic oscillator 124. Since
.DELTA.f/f=-.DELTA.L/L, the frequency stability of the reference
signal is translated to the relative loop length stability. When
the loop length is stabilized, the high frequency oscillation will
also be stabilized with the stability of the reference signal.
Since a stable low frequency reference signal is relatively easy to
generate, this method of frequency stabilization may be more
attractive and practical than the injection-locking technique in
some circumstances.
High speed fiber optic communication systems must have the ability
to recover a clock from incoming random data. The injection locking
technique described above allows the opto-electronic oscillator 10
to be used for clock and carrier recovery.
FIG. 14A shows incoming random data 138 having a transmission rate
R being injected into an opto-electronic oscillator 10 through
either the optical input port 28 or the electrical input port 30.
The free running opto-electronic oscillator 10 is tuned to
oscillate in a frequency range that includes the clock frequency
(R) of the incoming data. The oscillator 10 quickly phase locks to
the clock frequency of the injected data stream while rejecting
other frequency components (harmonics and subharmonics) associated
with the data. Consequently, the output of the opto-electronic
oscillator 10 is a continuous periodic wave 140 synchronized with
the incoming data (i.e., the output is the recovered clock).
Clock recovery from both return-to-zero and non-return-to-zero data
has been demonstrated with excellent results at 100 Mb/s and at 5
Gb/s. Data rates up to 70 Gb/s can also be recovered using the
injection locking technique with an opto-electronic oscillator
operating at 70 GHz. By contrast, current electronic clock recovery
techniques are unable to recover clocks at even half this data
rate. Another important feature of the opto-electronic oscillator
technique is that the clock can be recovered directly from data as
the data exits a fiber optic transmission line, without any need
for optical to electrical conversion. In addition, the recovered
clock signal has both optical and electrical forms and is easy to
interface with a fiber optic communication system.
FIG. 14B shows another approach which is similar to clock recovery.
The opto-electronic oscillator can also recover a carrier buried in
noise. A "spoiled" carrier 142 is injected into an opto-electronic
oscillator that has a free running frequency in a range that
includes the carrier and an output power level N dB higher than the
carrier (where N>>1). The injected carrier locks the
opto-electronic oscillator with the carrier and results in an
equivalent carrier gain of N dB in the output signal at ports 32
and 34. Because the open loop gain of the opto-electronic
oscillator is only n dB (where n<<N), the noise of the input
is only amplified by n dB. Hence, the signal to noise ratio of the
carrier is increased by (N-n) dB.
FIG. 15 illustrates the flow of a clock recovery, synchronization
and signal recovery system based on the recovery technique
described above. An optical carrier 144 including high data rate
information arrives from a remote location, and is split into two
parts. One of these parts is injected into an opto-electronic
oscillator 10 configured as a clock regenerator of FIG. 14A. The
other part is delayed in an optical delay line 146 that delays the
received signal long enough for the oscillator to lock. This
prevents data bits from being lost from the leading edge of the
data stream. After passing through the delay line 146, the signal
is applied to an optical-to-electrical converter 148 that produces
an electrical signal corresponding to the delayed optical signal.
This signal, along with the clock signal from the electrical output
34 of the opto-electronic oscillator, is supplied to a data
recovery circuit 150 that recovers the digital data from the
delayed signal. The clock signal from the optical output 32 of the
oscillator can be transmitted over optical fiber of up to several
kilometers in length for use by other devices. This eliminates any
need for multiple clock recovery devices.
Other contemplated uses of the opto-electronic oscillator include
cellular telephone antenna remoting, satellite earth station
antenna remoting, and as both an optical and an electrical high
frequency oscillator for signal up and down conversions. A
mode-locked opto-electronic oscillator can be used as both an
electrical and an optical frequency comb generator, where stable
frequency comb generation is critical in dense optical and
electrical frequency multiplexing systems.
An important advantage of the opto-electronic oscillator 10 is that
it can be integrated on a single integrated circuit chip. All of
the key components of the device, such as the laser 40, the
amplifier 18, the E/O modulator 12, and the photodetector 16 can be
based on GaAs technology and can be fabricated on a common
substrate.
Other embodiments are within the following claims.
* * * * *