All optical analog-to-digital converter employing an improved stabilized optical interferometer

An apparatus for stabilizing optical interferometers utilized in an all analog to digital converter using an additional optical signal that differs in wavelength by a factor of two from the wavelength used for the analog to digital conversion is disclosed. The optical interferometers have an optical path length that is tunable. They develop a first interference pattern from the additional optical signal when the optical path length is a prescribed value. The interferometers develop a second interference pattern from the additional optical signal when the optical path length is not the prescribed value. Optoelectronic detectors are responsive to the optical interference pattern generated by the additional optical signal and develop electronic feedback signals when the first interference patterns are not present. Accordingly, feedback circuits produce the optical path length adjustment drive signals, which serve to change the optical path lengths until each interferometer reaches the prescribed value, thereby tuning the optical interferometers. Each tuned interferometer then generates the appropriate optical output from the original optical signal that is made to very in wavelength in accordance to the amplitude of an analog signal so as to produce the appropriate data bit in a corresponding digital word.

CROSS-REFERENCE TO RELATED APPLICATIONS
 This application is related to four commonly assigned applications
 entitled: "Apparatus And Method Employing An Additional Optical Signal For
 Stabilizing An Optical Interferometer", TRW Docket No. 11-0976, having
 inventor Donald Heflinger, filed concurrently with this application, Ser.
 No. 09/336,248; "Apparatus And Method For Tuning An Optical
 Interferometer", TRW Docket No. 11-0975, having inventors Donald
 Heflinger, Jeffrey Bauch and Todd Humes, filed on Jan. 26, 1999, Ser. No.
 09/236,981; "All Optical Analog To Digital Converter", TRW Docket No.
 11-0918, having inventor Donald Heflinger; filed on Jun. 17, 1998, Ser.
 No. 09/089,844; and "Active Multimode Optical Signal Splitter", TRW Docket
 No. 11-0837, having inventor Charles Zmudzinski, filed on May 30, 1997,
 Ser. No. 08/866,656.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention generally relates to an all optical analog-to-digital
 converter, and more particularly to such an apparatus that employs an
 optical interferometer that uses two optical signals that differ in
 wavelength by a factor of two.
 2. Description of the Prior Art
 It is often desirable to convert an analog amplitude varying signal to a
 digital set of values which corresponds to various voltages in the analog
 waveform to generate a corresponding digital signal. Conventional
 approaches generally rely on iterative and/or comparative techniques for
 determining a digital signal based on an analog waveform voltage. In
 particular, a common conventional approach compares the actual voltage of
 the analog amplitude varying signal to a comparison voltage which is
 generated from a digital word. Various digital words are utilized to
 create comparison voltages which are then rapidly compared to the actual
 voltage to determine whether the comparison voltages are greater or less
 than, in an instant of time, the analog amplitude varying signal. Through
 a continuous iterative comparison process, a digital word which
 corresponds to the actual voltage of the analog amplitude varying signal
 is generated. The digital word is recorded for that instant of time and
 the same iterative and/or comparative process is repeated for subsequent
 instants of time corresponding to the analog signal. This conventional
 approach is limited in speed by the iterative comparison process.
 An approach for performing analog-to-digital conversion that eliminates
 this time consuming iterative comparison process is described in U.S.
 patent application Ser. No. 09/089,844, filed Jun. 17, 1998, entitled "All
 Optical Analog-To-Digital Converter", and assigned to the same Assignee as
 the present invention. In this approach, an analog signal is converted to
 an optical signal that varies in wavelength in accordance with the
 amplitude of the analog signal. This optical signal is then analyzed in
 parallel by an array of optical interferometers that have optical path
 length differences that correspond to the weighting factor of each digital
 bit in the digital word that is used to represent the analog voltage at a
 particular instant in time. The individual optical interferometers
 simultaneously deliver, via interference the appropriate optical output
 from each of their two outputs so that the optical levels, when detected
 by photodetection, will generate, in parallel, the appropriate digital
 bits that make up the digital word that corresponds to the analog voltage
 at that particular instant in time.
 The process for creating the appropriate state of interference in each of
 the optical interferometers requires that the optical path length
 difference in each optical interferometer be maintained to within a
 fraction of a wavelength of the light being interfered. Optical
 interferometers made using optical fiber or silica waveguide are not
 stable devices. They are particularly susceptible to uncontrollable
 conditions, such as temperature variations. As the temperature proximate
 the optical interferometer changes, the path length of the optical fibers
 or silica waveguide making up its legs likewise change. This results in a
 change in the interference pattern created by the optical interferometer.
 To compensate, the optical interferometer must be tuned continuously.
 An apparatus and method for tuning an optical interferometer is known in
 the art. An example of such an apparatus is described in an article by
 Eric A. Swanson, Jeffrey C. Livas and Roy S. Bondurant, entitled "High
 Sensitivity Optically Preamplified Direct Detection DPSK Receiver With
 Active Delay-Line Stabilization," in IEEE Photonics Technology Letters,
 Vol. 6, No. 2, February 1994. This article describes an optical
 communication system that modulates digital information onto transmitted
 light using differential phase shift keying (DPSK) and then demodulates
 this information using an actively tuned unbalanced Mach-Zehnder optical
 interferometer that is tuned using an apparatus and a method known in the
 art. The unbalanced Mach-Zehnder optical interferometer has an additional
 optical path length in one leg that provides a propagation delay duration
 of one data bit. The imbalance in the Mach-Zehnder optical interferometer
 enables light in one data bit to be optically interfered with light in the
 data bit immediately following this data bit. The relative state of
 optical phase between these two DPSK data bits determines in which of the
 two output legs of the interferometer light is produced provided that the
 unbalanced Mach-Zehnder optical interferometer is properly tuned within a
 fraction of a wavelength of the light. Light produced from one leg
 constitutes digital "ones" while light produced in the other leg
 constitutes digital "zeros" in the transmitted digital information signal.
 The apparatus described in the article includes a laser and a phase
 modulator for producing an optical DPSK signal at a preselected
 wavelength, a tunable unbalanced Mach-Zehnder optical interferometer, a
 dual balanced detector and a feedback electronic circuit coupling the
 signal developed across one detector of the balanced detector to one leg
 of the Mach-Zehnder interferometer. Two different approaches are described
 for tuning the optical path length in the unbalanced Mach-Zehnder optical
 interferometer. In the first approach, the interferometer is made of
 optical fiber and one leg of the interferometer is wrapped around a
 piezoelectric transducer (PZT) that enables an electronic signal to
 stretch the fiber, thereby increasing the optical path length. In the
 second approach, the interferometer comprises a silica integrated optical
 waveguide with an integral thermal heater that enables an electronic
 signal to increase the temperature of one leg of the interferometer,
 thereby increasing the optical path length. To tune the Mach-Zehnder
 interferometer a small electronic dither signal is applied to the actively
 tuned optical path length to provide a feedback signal for the electronic
 controller. This enables proper adjustment of the optical path length.
 Electronic synchronous detection techniques on this dither signal are used
 to provide the appropriate corrections to the optical path length,
 enabling the error in tuning to be below an acceptable level. This same
 tuning approach can be utilized to tune the optical interferometer in the
 all optical analog-to-digital converter, but there are some adverse
 consequences.
 The prior art approaches for actively tuning an optical interferometer have
 several disadvantages. First, they introduce an undesired optical
 intensity dither on top of the original optical signal that is intended to
 be extracted. This dither arising from the intentional dither of the
 optical path length is actually a source of noise that degrades the
 fidelity of the original signal. In the case of the all optical
 analog-to-digital converter, this dither leads to significant errors in
 the determination of the digital word since each data bit in this word can
 be affected by the dither. Second, the approach using the heater to
 perform the dither and tuning is restricted to relatively low frequencies
 of dither due to the relatively large thermal time constant of the heater.
 Third, the approaches introduce a small dithering variation in the
 interference state delivered at the output of the Mach-Zehnder
 interferometer. This precludes the use of the interferometer in
 applications where an absolute quiet state of interference must be
 maintained such as is the case in the all optical analog-to-digital
 converter.
 What is needed, therefore, is an improved all optical analog-to-digital
 converter that employs an improved optical interferometer tuning approach
 which utilizes an additional optical signal for tuning without introducing
 any dither in its optical path length.
 SUMMARY OF THE INVENTION
 The preceding and other shortcomings of the prior art are addressed and
 overcome by the present invention which provides generally, in a first
 aspect, an apparatus for converting an analog electrical signal into a
 digital electrical signal including a converter which converts the analog
 signal into an optical signal which varies in wavelength in accordance
 with the amplitude of the analog signal, a splitter which applies the
 optical signal over a preselected number of light paths and a plurality of
 optical interferometers, each connected to the light paths, each having an
 optical light path that is tunable, and using an additional optical signal
 to actively tune and stabilize the optical interferometers that have a
 wavelength that differs by a factor of two from the average wavelength of
 the light that varies in wavelength in accordance with the amplitude of
 the analog signal.
 Briefly, the interferometers comprise a dithering signal generator, means
 for generating an additional optical signal having a wavelength that
 differs from the original wavelength on which the optical interferometer
 is to act by a multiple of two, means for applying the dithering signal to
 the additional optical signal so as to slightly vary the wavelength about
 the multiple of two, the optical interferometer being responsive to the
 additional and original optical signals and a path length adjustment drive
 signal and being operative to develop a first interference pattern when
 the optical path length is a prescribed value and being operative to
 develop a second interference pattern when the optical path length is
 changed to develop an output signal at the additional wavelength, detector
 means responsive to the optical interference pattern and being operative
 to develop an electronic feedback signal when the first interference
 pattern is not present, and a feedback loop that responds to the dithering
 signal and the electronic feedback signal and produces the optical path
 length adjustment drive signal. The optical path length adjustment drive
 signal serves to tune the optical path length until it reaches the
 prescribed value, thereby producing the first interference pattern and
 stabilizing the optical interferometer for use by the original-wavelength
 light. Having utilized the additional dithered optical signal to stabilize
 and actively tune each optical interferometer, the digital conversion
 process can be performed using the same optical interferometers and the
 optical signal that varies in wavelength in accordance with the amplitude
 of the analog signal. Each interferometer generates two complementary
 output signals from this optical signal that varies in wavelength in
 accordance with the amplitude of the analog signal. A pair of detectors
 connected to each of the interferometers generates an electrical digital
 bit in response to the two complementary output signals wherein each of
 the digital bits are combined to form a parallel digital word. Each
 optical interferometer has an optical path length that provides the
 correct weighting factor to correspond to the particular digital bit in
 the digital word in accordance with the analog-to-digital conversion
 process described in U.S. patent application Ser. No. 09/089,844, filed
 Jun. 17, 1998, entitled "All Optical Analog-To-Digital Converter", and
 assigned to the same Assignee as the present invention.
 In another aspect, the present invention provides a system having a
 plurality of optical interferometers for converting an analog optical
 signal into a parallel digital word and that uses an additional optical
 signal for tuning and stabilizing the plurality of optical interferometers
 by selecting the wavelength of the additional optical signal to be twice
 or one-half the wavelength of the original light detected.
 The foregoing and additional features and advantages of this invention will
 become apparent from the detailed description and accompanying drawing
 figures below. In the figures and the written description, numerals
 indicate the various elements of the invention, like numerals referring to
 like elements throughout both the drawing figures and the written
 description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention provides a fully optical analog-to-digital converter
 which is automatic and independent of iterative techniques, thus providing
 for an expedited conversion rate. As will be described in detail
 subsequently, the fully optical analog-to-digital converter initially
 converts an electrical analog signal, such as a signal in the radio
 frequency (RF) range, into an optical signal which varies in wavelength in
 accordance with the amplitude of the analog signal. The amplitude of the
 optical signal remains constant as the wavelength is varied over a range
 which corresponds to the analog amplitude of the analog signal. The
 wavelength variation on the optical signal is utilized to generate a
 corresponding parallel digital word. The fully optical analog-to-digital
 converter employs a plurality of optical interferometers that are tunable
 and stabilized using an additional optical signal that has a wavelength
 that differs by a factor of two from the average of the optical signal
 which varies in wavelength in accordance with the analog signal. The
 present invention will be described by first describing an embodiment of
 an improved stabilized optical interferometer used in the optical
 analog-to-digital converter.
 As illustrated in the diagram of FIG. 1, an apparatus for stabilizing an
 optical interferometer, generally designated by the numeral 10 is shown.
 Briefly, the apparatus 10 employs an optical signal, such as the optical
 signal which varies in wavelength in accordance with the analog signal,
 that enters optical input 12, a dithering signal generator 14, a source 16
 for a stabilizing optical signal, an optical interferometer 20 having
 optical outputs 28 and 36, a tuning leg 30, a wavelength demultiplexer 40
 on output 36 with outputs 45 and 46, and output 45 feeding a photodetector
 22 in a feedback circuit 24 that includes a synchronous detection lock-in
 amplifier 26 for supplying an electrical signal to buffer amplifier 54
 that operates a tuning leg heater 56.
 The optical signal entering input 12 preferably is provided by a laser
 diode that supplies an optical signal having a wavelength that varies on
 average about a value of 1.5 .mu.m. This light has its wavelength vary a
 small amount around 1.5 .mu.m in accordance with the amplitude of the
 analog signal that is to be extracted by the tuned interference state of
 the optical interferometer.
 The dithering signal generator 14 is a conventional audio frequency
 electronic sine wave generator that is capable of providing an electronic
 dithering signal to both the optical source 16 and the synchronous
 detection lock-in amplifier 26 simultaneously. By providing the same
 dithering signal to optical source 16 and the lock-in amplifier it is
 possible to detect both the amplitude and the phase of a very small
 amplitude dithering signal in the presence of many other stronger signals.
 The stabilizing optical signal source 16 generally comprises a laser diode
 that emits a beam of light that is stable in wavelength and is a multiple
 of a factor of two of the original light entering input 12. The wavelength
 of this signal is preferably 0.75 .mu.m, which is a multiple of a factor
 of two, and more particularly, a multiple of one-half times the wavelength
 of the original optical signal entering input 12. This choice of
 wavelength is readily accommodated by available laser diodes since InGaAsP
 lasers emit at 1.5 .mu.m and GaAlAs lasers emit at 0.75 .mu.m. In response
 to current supplied by the dithering generator 14 on conductor 34, the
 laser diode 16 is driven with a current modulation that varies in
 accordance with the dithering signal. Because the wavelength of a laser
 diode depends on both the temperature, which is generated by resistive
 heating stemming from the drive current, and the carrier density within
 the lasing junction, which is directly taken from the drive current, the
 wavelength of the laser diode is a very sensitive function of the level of
 the drive current. Thus, the small modulation in the drive current from
 the addition of the dithering signal causes the wavelength of the laser
 diode to vary a small amount around 0.75 .mu.m in accordance with the
 dithering signal. This provides the required means of delivering an
 optical signal on the input 18 comprising an optical fiber, which has a
 dithering wavelength that varies in accordance with the dithering signal
 about a value that is a factor of two different from the wavelength of the
 light entering input 12.
 The optical interferometer 20 has two inputs 12 and 18, two optical legs or
 paths 30 and 32, and two output ports 28 and 36. It is preferably a
 Mach-Zehnder interferometer made of silica waveguide material as is
 commercially available from Photonic Integration Research, Inc., Columbus,
 Ohio, Model FDM-10G-1.5-M-PM. It includes an integrated thermal heater to
 enable tuning of its optical leg or path 30. However, the interferometer
 can be made using many materials including optical fiber, free space, or
 silica waveguide. The optical interferometer splits light into the two
 separate optical legs 30 and 32 and then recombines the light
 interferometrically to create optical outputs that can present
 constructive and destructive interference. The wavelength of light and the
 relative optical path lengths of the two legs 30 and 32 determine the
 particular state of interference that takes place when the light is
 combined. The state of interference determines if the output presents
 constructive interference, in which case there is an optical intensity
 output, or destructive interference, in which case there is an absence of
 optical intensity output. When one of the two optical path lengths within
 the interferometer is made to be adjustable, the state of interference can
 be continuously varied between the constructive and destructive
 interference states as long as the wavelength is held constant. This
 enables the optical interferometer to be set to a particular state for a
 given wavelength of light. When the wavelength is changed, the state of
 interference can be maintained by appropriately tuning the relative
 optical path length of the interferometer. Otherwise the state of
 interference will change as the wavelength is changed.
 In this patent the phrase "optical path length" is characterized and
 mathematically defined as the product of the physical propagation distance
 and the associated refractive index of the medium through which the light
 propagates. It should be recognized that adjusting the optical path length
 can be accomplished by adjusting the physical propagation distance or
 adjusting the index of refraction of the medium through which the light
 propagates in the interferometer. The physical propagation distance can be
 adjusted by stretching a fiber with a piezoelectric transducer, thermally
 expanding the length with heat, or positioning a mirror within the
 interferometer via a piezoelectric piston. Adjusting the index of
 refraction could be accomplished by heating the medium or by an electro
 optic affect created via an applied electric field. In the preferred
 embodiment, the optical path length is changed by applying heat to
 thermally expand the segment of silica optical waveguide comprising leg 30
 of the optical interferometer. Thermal heating can be used to perform the
 optical path length tuning in this invention despite the relatively long
 thermal time constants associated with this approach because only slow
 tuning adjustments are needed and dithering of the optical path is not
 required.
 This apparatus utilizes the particular optical interference case where the
 optical path lengths are not changed when the wavelength is changed.
 Starting with an interference state where constructive interference occurs
 at one output port and destructive interference occurs at the other output
 port of the interferometer, a slight change in the wavelength is produced
 by the optical source 16, in this case by the dithering signal, where
 there is no immediate adjustment made to the optical path lengths in the
 interferometer within a time scale on the order of the dithering signal
 frequency. The destructive interference output occurs on output 36 where
 there is a demultiplexer 40 that will direct the light from optical source
 16 to output 45, in a manner that will be described later, so that this
 light is directed to the photodetector or photodiode 22 for this example.
 This slight wavelength change in optical source 16 causes an increase in
 optical intensity every time the wavelength is pulled slightly from the
 preselected destructive interference pattern state. In the course of one
 sinusoidal dithering signal swing, the wavelength is slightly too short
 for half the cycle and slightly too long for the other half of the cycle.
 This leads to two cycles of increased intensity in the optical intensity
 of the destructive interference output directed to photodetector 22 for
 every complete cycle of dithering signal.
 The electronic feedback circuit 24 comprises the photodetector 22 and a
 biasing resistor 48 coupled to a positive DC supply voltage V. The anode
 of the photodiode 22 is electrically connected via conductor 52 to the
 lock-in amplifier 26.
 The synchronous detection lock-in amplifier 26 is available commercially
 from several manufacturers, such as Stanford Research Systems, Inc. in
 Sunnyvale Calif. The amplifiers synchronously detects a small electronic
 signal, complete with phase detection, by using a reference signal having
 the same frequency and delivers a steady state output voltage with a value
 corresponding to the amplitude swing of the detected signal and a voltage
 polarity that reflects the phase of the detected signal in relation to the
 original reference signal. The output voltage delivered by the commercial
 amplifiers is directly suitable as an electronic drive adjustment signal
 as will be subsequently described. This electronic drive adjustment
 voltage can be considered to be an error signal that indicates how far
 out-of-tune the optical interferometer is and the direction in which this
 error correction should be applied to return the optical interferometer to
 the preselected interference state.
 As is well known, the photodetector generates a photocurrent in response to
 optical intensity. Hence, the electronic feedback signal in feedback
 circuit 24 generated by the photocurrent in photodetector 22 corresponds
 to a signal that varies with twice the frequency of the dithering signal.
 Thus the electronic feedback signal conveyed by the conductor 52 to the
 synchronous detection lock-in amplifier 26 comprises the second harmonic
 of the dithering frequency. This signal has twice the frequency of the
 dithering signal frequency. The synchronous lock-in amplifier 26 only
 amplifies signals that have the same frequency as the dithering signal
 frequency conveyed to it. Thus, the second harmonic contribution is not
 amplified by the amplifier 26. This is the normal properly tuned
 preselected interference state for the optical interferometer, and under
 these conditions there is no electronic drive adjustment signal generated
 to cause a correction in the stabilization or tuning of the optical
 interferometer.
 When the interferometer is slightly out of tune, there will be a
 contribution of the fundamental component of the dithering signal
 frequency present in the electronic feedback signal that is detected by
 the lock-in amplifier 26.
 An electronic optical path controller 54, preferably a buffer amplifier,
 translates the electronic drive adjustment signal to the appropriate drive
 signal necessary to electronically tune the optical path length of leg 30.
 In the embodiment where the optical interferometer 20 comprises a silica
 waveguide with an integrated heater, the voltage signal is converted to a
 current to drive a resistive heater element 56. In the embodiment where
 the optical interferometer uses an optical fiber and a PZT to stretch the
 fiber, an amplifier is employed to drive the PZT and create the necessary
 optical path length change to tune the optical interferometer.
 It should be recognized that the stabilizing optical signal supplied
 through optical fiber 18 and the original optical signal having an average
 wavelength of 1.5 .mu.m that enters through input 12 interfere
 independently within the optical interferometer 20. The interferometer is
 tuned so as to allow the wavelength varying optical signal at 1.5.mu.m to
 interfere and deliver light out of the output ports 28 and 36, which for
 the case of output 36 the demultiplexer conveys to output 46. The
 stabilizing optical signal from source 16 that appears in output 36 is
 directed by the demultiplexer 40 to output 45 where it is detected by
 photodetector 22. The only aspect that is necessary is that interference,
 such as destructive interference as used in the example of the stabilizing
 optical signal occurs in the output port 36 so it can be directed by
 demultiplexer 40 to output 45. The state of interference delivered by the
 1.5 .mu.m light in output port 28 and output 46 can vary as the wavelength
 of this light is varied. The 1.5 .mu.m light delivered by the two
 interferometer outputs 28 and 36, where the demultiplexer 40 delivers this
 light to output 46, is then detected by photodetectors 50 and 51 and used
 to determine the digital bit for the digital word. The photodetectors 50
 and 51 are high speed photodetectors.
 Still with reference to FIG. 1, the light in the output 36 is directed into
 the wavelength demultiplexer 40 which serves as a wavelength selective
 filter. This demultiplexer 40 comprises an optical circulator 42 and a
 retro reflector Bragg grating 44 characterized as having a period that is
 a first order diffractive retroreflector for 0.75 .mu.m light. The Bragg
 grating 44 serves to pass the 1.5 .mu.m light and to diffract just the
 0.75 .mu.m light back into the direction from which it came. More
 particularly, the Bragg grating 44 has a period that equals one-half times
 0.75 .mu.m divided by 1.46, which is the effective index of refraction of
 the fiber. This period of roughly 0.25 .mu.m is half the size of the
 period needed to diffractively retroreflect 1.5 .mu.m light, and will not
 effect the 1.5 .mu.m light. Grating periods of this type can be made in
 optical fiber by holographic exposure using UV laser light as is known in
 the art. It is important that the length of this grating in the fiber is
 appropriate so that the retroreflection generally will occur equally for
 all the wavelengths about which the 0.75 .mu.m source is dithered. Light
 delivered by the interferometer at 1.5 .mu.m will pass through the
 circulator 42 and the Bragg grating 44 to the output 46 without
 retroreflection because the period of the Bragg grating is too small to
 diffract this light. The length of this path is made identical to that
 leading to the output 28 for recovery of the digital information. The
 light at a wavelength of 0.75 .mu.m delivered by output 36 is also
 conveyed through the optical circulator 42, where it is retroreflected by
 diffraction from Bragg grating 44 and returned into the circulator. In a
 manner well known in the art, the circulator 42 directs this light via the
 Faraday rotation effect to its other output 45 which directs this light
 onto photodetector 22.
 In operation, the source 16 provides light dithering about a wavelength of
 0.75 .mu.m that is used to stabilize the optical interferometer. Because
 it is selected to be a multiple of two (i.e. one-half is a multiple of two
 in that it is 2.sup.-1) of the 1.5 .mu.m wavelength that enters input 12,
 the interference state established by the interferometer at a wavelength
 of 0.75 .mu.m is the same as that established for the average wavelength
 of the 1.5 .mu.m light that enters input 12. This operation requires the
 light entering input 12 to vary about a wavelength that on average remains
 stable with time so that the optical source 16 can be dithered about a
 stable fixed wavelength that is a multiple of two of the average of this
 light entering input 12. Moreover, the stabilizing source 16 enables the
 interferometer to be stabilized even though this light entering input 12
 is varied in wavelength with the amplitude information of the analog
 signal that is to be converted to a digital signal.
 The operation just described used, as an example, the destructive state of
 interference of the 0.75 .mu.m light to output 36. The dither in the
 wavelength applied to the 0.75 .mu.m light by dither generator 14 will
 generate the same frequency components in the output electrical signal of
 photodetector 22 as a function of interferometer 20 tuning as was
 previously described for the example for destructive interference in the
 case of constructive interference also. In the case of constructive
 interference, the slight wavelength change in the 0.75 .mu.m light causes
 a decrease, as opposed to an increase, in the light intensity directed to
 the photodetector. Thus, for each cycle in the dithering signal, the
 wavelength changes slightly to cause two decreases in intensity, which
 again yields an intensity variation at twice the frequency of the dither
 frequency when the interferometer is properly tuned. Slightly out-of-tuned
 interferometer states will cause a contribution of the fundamental dither
 frequency in the electronic feedback signal that is detected by the
 lock-in amplifier in the same manner as previously described. Thus, the
 stabilizing optical signal can be used with either constructive or
 destructive interference in this invention.
 In the embodiment shown in FIG. 1, it is convenient to use the constructive
 state of interference so that the dither signal at a wavelength of 0.75
 .mu.m is applied on its own independent interferometer input such that the
 0.75 .mu.m light is separated from the output 28 and thus directed to the
 output 36 where the demultiplexer 40 directs the light to output 45 and
 thus photodetector 22 receives the constructive interference. Hence, the
 light delivered by the 0.75 .mu.m source is made to deliver constructive
 interference at the output 36. When the wavelength demultiplexer 40
 directs this light to the photodetector 22, the photodetector 22 develops
 the photocurrent signal for use in the feedback circuit 24 for tuning and
 stabilizing the interferometer 20 in the manner described relative to the
 embodiment of FIG. 1. The outputs 28 and 46 deliver 1.5 .mu.m light with
 an interference state that depends upon the particular characteristics of
 the 1.5 .mu.m light. In this way, the wavelength demultiplexer 40
 separates the interference patterns generated by the dithered optical
 source 16 from the complimentary outputs generated by the wavelength
 varying source delivered by input 12.
 This structure enables the relative optical path length of an optical
 interferometer to be actively maintained, even when it is not possible to
 utilize the original light detected by the interferometer for
 stabilization because the wavelength of this light is varied and
 deliberately not held to a constant wavelength. Accordingly the unbalanced
 Mach-Zehnder interferometer is stabilized.
 It is important that the optical path length of the two output ports 28 and
 46 via the demultiplexer 40 on output 36 be identical in the all optical
 analog-to-digital converter. Since the outputs are complementary, there is
 light delivered in the one port exactly when there is no light delivered
 by the other port. If the optical path lengths in each output port are
 different, the two signals would be delivered at different times, thus
 degrading the digital signal determination. Thus, high speed
 photodetectors 50 and 51 must be used to detect the 1.5 .mu.m optical
 signal. To gain access to output port 36 with photodetector 22 to detect
 the 0.75 .mu.m stabilization optical signal, a special optical filter is
 used in output 36 that splits off the 0.75 .mu.m light so it is directed
 to photodetector 22 without interrupting the output path required by the
 1.5 .mu.m light used to recover the digital path bit from the
 photodetectors 50 and 51 to create the digital word. This optical filter
 has the special property that it can split light that differs by a factor
 of two in wavelength. Most optical filters and wavelength division
 multiplexers will not separate light that differs in wavelength by a
 factor of two. An example of an optical filter that can separate light
 that differs in wavelength by a factor of two is a Bragg grating fiber
 filter used in retro reflective diffraction in conjunction with an optical
 circulator.
 With reference to FIG. 2 an all optical analog-to-digital converter system
 employing improved stabilized optical interferometers in accordance with
 the present invention is illustrated and generally designated by the
 numeral 100. The all optical analog-to-digital converter system is similar
 to the system disclosed in copending U.S. patent application Ser. No.
 09/089,844, filed Jun. 17, 1998, entitled "All Optical Analog-To-Digital
 Converter", and assigned to the same Assignee as the present invention. As
 shown the system 100 comprises an analog signal amplitude to optical
 wavelength converter 112 for developing a first optical signal, a
 dithering generator 14, a source for a second optical signal 16 whose
 wavelength is a multiple of one-half of the wavelength (i.e. a multiple of
 two) of the wavelength of the optical signal developed by the converter
 112, optical splitters 120 and 122, interferometers 124, feedback circuits
 26, 54 and 56, wavelength division multiplexers 40, and an optoelectronic
 circuit 128.
 Many of the parts of the system 100 are identical in construction to like
 parts in the apparatus 10 illustrated in FIG. 1 described above, and
 accordingly, there have been applied to each part of the system in FIG. 2
 a reference numeral corresponding to the reference numeral that was
 applied to the like part of the apparatus described above and shown in
 FIG. 1. In FIG. 2 electrical wires or conductors are shown as dashed lines
 and optical fibers are shown as solid lines.
 The fundamental difference between the apparatus 10 of FIG. 1 is that the
 system 100 stabilizing technique is applied to a plurality of optical
 interferometers and that the system is an all optical analog-to-digital
 converter that converts an analog signal into an optical signal that
 varies in wavelength in accordance with the input analog signal. It
 utilizes the wavelength variation to generate a corresponding parallel
 digital word.
 In particular, the amplitude to optical wavelength converter serves to
 convert the analog signal 129 into an optical signal having a nominal 1.5
 .mu.m wavelength, but which varies in wavelength about the nominal
 wavelength in accordance with the amplitude of the analog signal.
 Typically, the analog signal is in the radio frequency (RF) range. The
 amplitude to optical wavelength converter preferably comprises a laser
 diode, although fiber lasers, optical fiber amplifiers, and other solid
 state amplifier system may be employed. Generally, a laser diode which is
 driven by a varying amplitude analog signal will deliver a longer output
 wavelength as the amplitude of the signal increases.
 The wavelength varying optical signal generated by the converter is applied
 to the splitter 120 which splits the optical signal into a preselected
 number of light paths 130. Each path 130 is applied to a perspective leg
 140 of the interferometer 124 and the interference state delivered by
 interferometers 124 is utilized to create the digital bits D.sub.0,
 D.sub.1, D.sub.2 and D.sub.3 in the final parallel digital word. The
 number of paths 130 is dependent on the desired level of resolution. For
 example, as illustrated in FIG. 2, for 4-bit resolution (16 signal
 levels), the wavelength varying optical signal is split into four light
 paths 130. The splitter 120 is preferably an active multimode signal
 splitter, such as the splitter disclosed in U.S. patent application Ser.
 No. 08/866,656, filed May 30, 1997, entitled "Active Multimode Optical
 Signal Splitter", and assigned to the same Assignee as the present
 invention. The splitter 120, in addition to splitting the optical signal,
 maintains an approximately equal intensity of each optical signal along
 each light path. Alternatively, the optical splitter could be a
 conventional splitter or a fiber optic star coupler comprising a group of
 optical fibers which have their cladding layer removed prior to being
 twisted together. This allows the light in one fiber to evanescently
 couple equally into all the other fibers, thereby allowing the wavelength
 varying optical signal to be split.
 Referring still to FIG. 2, the two output signals from each interferometer
 present an optical intensity that varies in a complementary sinusoidal
 variation between the two outputs when the wavelength of the input light
 is varied such that when these outputs are applied to the optoelectronic
 circuit 128, and more particularly to the dual detectors 150 and 151, they
 are directly converted to an electronic digital bit. The dual detector is
 preferably a balanced detector including high speed photodiodes, although
 it should be recognized that the optoelectronic circuit may have other
 configurations. The circuit 128 also includes a limiting electronic
 amplifier 152. The limiting electronic amplifier 152 converts the signal
 to a digital "0" or "1" depending on the state of interference delivered
 by the interferometer which in turn depends on the wavelength.
 When light is applied to photodiode 151, photocurrent proportional to the
 intensity of the light is conducted in a direction into the input of the
 limiting electronic amplifier. When light is applied to photodiode 150,
 photocurrent proportional to the intensity of the light is conducted in a
 direction out of the input of the limiting electronic amplifier. Thus,
 depending on which output is delivering greater light intensity and hence
 which photocurrent is greater, current either moves in or out of the input
 of the limiting amplifier. If current moves into the input, the limiting
 amplifier delivers a digital "one" bit, if it moves out of the input, the
 limiting amplifier delivers a digital "zero" bit. In this way, the state
 of optical interference established by the input wavelength creates a
 digital bit. Thus, for the entire first part of the complementary
 sinusoidal variation cycle, where more light is delivered from one of the
 two complementary ports, the electrical output corresponds to a "1". For
 the entire second portion of the cycle, where there is more light in the
 other complementary output, the output corresponds to a "0".
 As an example of operation, consider a portion of an analog signal 129
 where the amplitude ramps up from the minimum amplitude to the maximum
 amplitude. This particular analog signal is converted to an optical signal
 where the wavelength of light is swept through the range of wavelengths
 corresponding to the amplitude of the analog signal. As this light changes
 wavelength the output of each interferometer is a complementary sinusoidal
 variation in the intensity partitioned between the two output signals. The
 interferometer for the most significant bit (MSB) D.sub.3 delivers just
 one cycle of variation in intensity. The interferometer for the next most
 significant bit D.sub.2 experiences two complete cycles because the
 optical path length difference is made to be twice as large as that for
 the most significant bit. The next most significant bit D.sub.1
 experiences four cycles because the path length difference is four times
 as large, and finally the least significant bit D.sub.0 (LSB) experiences
 eight cycles because the path length difference is eight times as large.
 It should be noted that the pattern in the optical length difference
 increases as a power of two for successively less significant digital
 bits. The most significant bit (MSB) will only go through one
 complementary sinusoidal cycle change for the entire analog amplitude
 domain. For 4-bit resolution, the least significant bit (LSB), the fourth
 bit, will go through eight complementary sinusoidal cycles of variation in
 intensity for the entire analog amplitude domain. Thus, the optical path
 length difference determines the weighting that specifies which digital
 bit in the digital word is generated.
 A single stabilizing optical signal 16 is used that has a wavelength that
 is a multiple of two of the average value of wavelength varying signal
 created from the analog signal. In particular, this wavelength can be
 one-half of 1.5 .mu.m or 0.75 .mu.m. This stabilizing optical signal 16 is
 then dithered in wavelength a small amount about this multiple of two
 value by dither generator 14, just as in the embodiment of FIG. 1. This
 dithered stabilizing light is then optically split by the splitter 122
 into a path for each optical interferometer 124 via the same means as was
 previously described to split the wavelength varying optical signal.
 The optical interferometer 124 has two optical inputs 138 and 140 and two
 output ports 142 and 144. The stabilizing optical signal at a wavelength
 of 0.75 nm is supplied to the input 138 and the original optical signal
 having a wavelength that varies about 1.5 nm is applied to the second
 input 140. The optical interferometer is tuned so that the constructive
 interference of the 0.75 .mu.m light takes place in the output 144 with
 the demultiplexer that utilizes the wavelength selective filter as
 previously described. The interference that occurs from the 1.5 .mu.m
 light at the two outputs 142 and 144 reflects the information that will
 determine if the data bit is a zero or a one. In this invention the only
 aspect that is necessary is that the constructive interference of the
 stabilizing optical signal occurs in the output 144 and this is achieved
 by active stabilization through feedback of the detected 0.75 .mu.m light.
 The interferometers may be monolithic frequency division multiplexers (FDM)
 manufactured by Photonic Integration and Research, Inc. of Columbus, Ohio,
 under a typical model number like FDM-10G-1.5, which corresponds to an
 unbalanced interferometer that delivers one cycle of intensity variation
 when the light wavelength at 1.5 .mu.m is changed by 0.08 nm (a frequency
 change of 10 GHz). Although the interferometers are illustrated in FIG. 2
 as separate units, they may be configured monolithically on the same
 wafer. The interferometers may also be made from fiber optic cable and
 fiber optic couplers through the use of either polarization maintaining
 fiber or polarization controllers.
 It is important that the interferometers 124 in this all optical analog to
 digital converter remain tuned to create the particular path differences.
 The particular path differences determines how the wavelength varying
 signal interferes and hence delivers more light in one output than the
 other which ultimately determines if the digital bits from the
 interferometers are a one or a zero. The invention serves to
 simultaneously stabilize all the optical interferometers in the all
 optical analog-to-digital converter.
 A wavelength division demultiplexer that utilizes a fiber optic filter 126
 is placed in one output of each interferometer. These filters utilize a
 Bragg grating with a period suited for retro reflecting, via optical
 diffraction, the 0.75 .mu.m signals after the light has passed through one
 input of an optical circulator. This Bragg grating passes without
 diffractive retro reflection, the 1.5 .mu.m optical signals to deliver
 this light at output ports 147. The 0.75 .mu.m optical feedback signal is
 sent back into the optical circulator by the retro reflection form the
 Bragg grating and is delivered out the additional output ports 148 to the
 photodetectors 22. The feedback signals are sensed by the photodetectors
 22 and they develop electronic is feedback signals that are used to
 individually adjust the optical path length of the leg 30 of each of the
 interferometers 124. The feedback circuit operates in the manner described
 with reference to FIG. 1.
 In operation, the source 16 of light at a wavelength of 0.75 .mu.m is used
 to develop the electronic feedback and hence the optical path length drive
 adjustment signal to stabilize the optical interferometer. Because it is
 selected to be a multiple of two (i.e. one-half the average wavelength
 from the 1.5 .mu.m source 12, the interference state established by the
 interferometer is the same at that established for the average 1.5 .mu.m
 wavelength. That is, there will be constructive interference in one leg
 and destructive interference in the other leg for both the average 1.5
 .mu.m wavelength and the stabilizing light source when the interferometer
 is properly tuned. In this invention, the stabilizing source 16 enables
 the interferometer to be stabilized even though the wavelength sensed by
 the interferometer is deliberately made to vary in accordance with the
 amplitude of the analog signal to be digitized.
 In yet another embodiment, a system 200 for stabilizing optical
 interferometers in an all optical analog-to-digital converter made in
 accordance with the present invention is illustrated in FIG. 3. Many parts
 of the system 200 are identical in construction to like parts of the
 system 10 illustrated in FIG. 1 and the apparatus 100 illustrated in FIG.
 2, and accordingly, there has been applied to each part of the system 200
 a reference numeral corresponding to the reference numeral that was
 applied to the like part of the apparatus described above. In FIG. 3
 electrical wires or conductors are shown as dashed lines and optical
 fibers are shown as solid lines.
 The fundamental differences between the system and the apparatus
 illustrated in FIG. 2 is that the system includes wavelength division
 multiplexers in the input and output circuits, and like the embodiment of
 FIG. 2 is an all optical analog-to-digital converter that converts an
 analog signal into an optical signal that varies in wavelength in
 accordance with the analog input optical signal and uses the wavelength
 variation to generate a corresponding parallel digital word.
 The wavelength division multiplexer 202 that combines the two optical
 signals again utilizes a circulator 204 and a Bragg grating retro
 reflector 206 to combine light signals that differ in wavelength by a
 factor of two. The light at 1.5 .mu.m is passed by the grating retro
 reflector 206 which has a period set to retro reflect light with a
 wavelength of 0.75 .mu.m. This period is the same as the period used in
 the grating 126 of the demultiplexers 146. The 1.5 .mu.m light then passes
 into the circulator 204 and is delivered out to the input of the splitter
 134. The 0.75 .mu.m is sent into the other input of the circulator 204
 where it is delivered into the Bragg grating retro reflector 206. When the
 0.75 .mu.m light enters the Bragg grating retro reflector 206, it becomes
 retro reflected back into the circulator 204 where it gets delivered along
 with the 1.5 .mu.m light into the splitter 134.
 This embodiment again uses a single wavelength dithered source 16 to
 provide the light for generating individual feedback signals that are used
 to stabilize each optical interferometer 124. The constructive
 interference of the stabilizing source can be made to occur in either
 output leg of the interferometers, but in the preferred embodiment
 constructive interference is made to occur in outputs 144 so that the
 wavelength demultiplexers 146 can deliver this stabilizing light to
 outputs 148. It should be recognized that these demultiplexers 146 can be
 placed in either outputs 142 or 144 since the optical interferometers 124
 can be tuned to present constructive interference in either output.
 It should also be recognized that additional wavelength demultiplexers
 identical in construction to 146, but not shown, can be placed in the
 other interferometer outputs 142 to act as filters to remove the small
 amount of 0.75 .mu.m light that develops in the destructive interference
 output during the wavelength dither of the 0.75 .mu.m optical stabilizing
 source 16. By removing this small amount of dithered light from the
 destructive interference output 142, the accurate determination of the
 digital bit from the interference state of the 1.5 .mu.m light is further
 improved. The demultiplexer used in this application as a filter, will
 convey the 0.75 .mu.m light to a separate unused output thereby removing
 it from the interferometer output 142 used for determining the digital
 bit. In a similar way, these demultiplexers can also be used as 0.75 .mu.m
 light filters in the embodiment shown in FIG. 2 by placing additional
 demultiplexers in interferometer outputs 142.
 In this embodiment, the optical splitter 134 that divides the light to each
 interferometer must act on both the 1.5 .mu.m light and the 0.75 .mu.m
 light simultaneously. This will preclude the use of a semiconductor active
 signal splitter since the wavelength spread of these two signals is too
 great to be accommodated by known semiconductor materials. Also, the
 stabilizing light and the wavelength varying light both enter the
 interferometer on the same input. The function is exactly as described
 relative to the embodiment in FIG. 2.
 Obviously, many modifications and variations of the present invention are
 possible in view of the above teachings. Thus, it is to be understood
 that, within the scope of the appended claims, the invention may be
 practiced otherwise than as specifically described above.
 Addendum
 1. ANALOG-TO-DIGITAL CONVERTER EMPLOYING AN IMPROVED STABILIZED OPTICAL
 INTERFEROMETER