Optical analog-to-digital converter

An optical analog-to-digital converter (ADC) is disclosed which converts an input optical analog signal to an output optical digital signal at a sampling rate defined by a sampling optical signal. Each bit of the digital representation is separately determined using an optical waveguide interferometer and an optical thresholding element. The interferometer uses the optical analog signal and the sampling optical signal to generate a sinusoidally-varying output signal using cross-phase-modulation (XPM) or a photocurrent generated from the optical analog signal. The sinusoidally-varying output signal is then digitized by the thresholding element, which includes a saturable absorber or at least one semiconductor optical amplifier, to form the optical digital signal which can be output either in parallel or serially.

FIELD OF THE INVENTION

The present invention relates in general to optical analog-to-digital converters (ADCs), and in particular to an optical ADC which can be formed as a photonic integrated circuit (PIC) and which can convert an optical analog input signal into an optical digital output signal.

BACKGROUND OF THE INVENTION

Analog-to-digital converters (ADCs) have traditionally been fabricated using electronic integrated circuits. The function of an ADC is to repeatedly sample a time-varying analog waveform, generally at fixed time intervals, and to generate a digital representation of the analog waveform with a certain level of precision. The precision of electronic ADCs, however, is currently limited to an effective number of bits (ENOB) of 8.5 at 2 GHz RF bandwidth, and decreases with increasing frequency.

Optical ADCs promise to overcome the limitations of electronic ADCs and to provide an improved speed and resolution for the digitization of analog waveforms at very high frequencies greater than about 10 GHz. There are many different and varied types of optical ADCs known to the art which have been summarized in a recent review article by G. C. Valley entitled “Photonic Analog-to-Digital Converters” published in Optics Express, vol. 15, paper no. 75372, 5 Mar. 2007. Various types of optical ADCs are also disclosed in the following U.S. Pat. Nos. 4,058,722; 4,928,007; 6,292,119; and 6,326,910; and in U.S. Statutory Invention Registration No. H353.

The present invention provides an advance in the art of optical ADCs by providing an optical ADC which can be made to be all-optical in that it can receive an optical analog input signal and convert this signal into an optical digital output signal.

The optical ADC of the present invention can be formed as a photonic integrated circuit (PIC) which provides each bit of the optical digital signal spatially separated so that each bit of the optical digital signal can be directed into a different optical fiber for parallel data transmission. Alternately, an optical waveguide combiner can be provided in the optical ADC or as a separate PIC for use with the optical ADC to generate a serial optical output by interleaving the various bits of the optical digital signal.

These and other advantages of the present invention will become evident to those skilled in the art.

SUMMARY OF THE INVENTION

The present invention relates to an optical analog-to-digital converter (ADC) which converts an optical analog signal to an optical digital signal which is a digital representation of the optical analog signal. The optical ADC comprises a plurality of channels, with each channel receiving the optical analog signal and a sampling optical signal and generating therefrom a bit of the optical digital signal. The plurality of channels includes a first channel providing a most significant bit (MSB), a last channel providing a least significant bit (LSB), and a plurality of intermediate channels providing bits of lessening significance between the MSB and the LSB. The plurality of channels receiving the optical analog signal and the sampling optical signal can comprise, for example, four channels to provide an 8-bit digital representation of the optical analog signal.

Each channel can be formed on a common semiconductor substrate and includes an optical waveguide interferometer and an optical thresholding element. The optical waveguide interferometer has a pair of waveguide arms into which the sampling optical signal is split and directed, and can be formed as a Mach-Zehnder interferometer. The optical analog signal is directed into only one waveguide arm (also referred to herein as a phase delay arm) to produce a phase delay of the sampling optical signal in that waveguide arm. The phase delay is proportional to the intensity of the optical analog signal and also to the length of the phase delay arm. The optical waveguide interferometer then generates an optical output signal by combining the sampling optical signal from the pair of waveguide arms (i.e. the sampling optical signal with the phase delay from the phase delay arm, and the sampling optical signal from the other waveguide arm without the phase delay). This produces a sinusoidal modulation of the optical output signal which depends upon the intensity of the optical analog signal. The optical thresholding element then receives the optical output signal from the optical waveguide interferometer and generates one of the bits of the optical digital signal from the optical output signal.

The optical ADC can be formed on a semiconductor substrate which comprises a III-V compound semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP).

The optical ADC can also include a plurality of optical waveguides formed on the semiconductor substrate to conduct the optical analog signal and the sampling optical signal from input ports located on one edge of the semiconductor substrate to the plurality of channels, and to conduct each bit of the optical digital signal from one of the optical thresholding elements to a different output port located on another edge of the semiconductor substrate. The plurality of optical waveguides can form an optical waveguide splitter to split the optical analog signal into a plurality of portions prior to being received into the various channels of the optical ADC. The plurality of optical waveguides can also form another waveguide splitter to split the sampling optical signal into a plurality of portions which can be received into the plurality of channels of the optical ADC. Certain of the optical waveguides can also include semiconductor optical amplifiers (SOAs), as needed, to amplify the optical analog signal, or the sampling optical signal, or both.

The optical analog signal can be provided by a semiconductor laser; and the sampling optical signal can be provided by another semiconductor laser. The semiconductor lasers will generally be located off the semiconductor substrate, although in certain embodiments of the present invention, at least one of the semiconductor lasers can be located on the semiconductor substrate with the Optical ADC.

The optical waveguide interferometer in each of the plurality of channels of the optical ADC can have a different length in certain embodiments of the present invention. In these embodiments, the lengths of the optical waveguide interferometer, which is generally the length of each waveguide arm therein, can be given by L, 2L, 4L, . . . 2(N−1)L where N is a number of bits of the optical digital signal, and L is the length of the optical waveguide interferometer in the channel providing the MSB of the optical digital signal, and 2(N−1)L is the length of the optical waveguide interferometer in the channel providing the LSB of the optical digital signal. The ordering of the various optical waveguide interferometers on the semiconductor substrate can be from the MSB to the LSB, although those skilled in the art will understand that any other ordering arrangement can be used since each bit of the optical digital signal can be separately output to a different optical fiber to provide a parallel optical output, or alternately directed into an optical waveguide combiner which rearrange the various bits of the optical digital signal, as needed, to generate a serial optical output.

The optical thresholding element can comprise an asymmetric active Mach-Zehnder interferometer which includes an SOA which provides a different optically-induced phase shift in each arm thereof to provide a step-like optical transfer characteristic. This step-like optical transfer characteristic conditions the optical output signal which is input into the optical thresholding element and thereby produces the optical digital signal at an output side of the optical thresholding element. Alternately, the optical thresholding element can comprise a saturable absorber (SA).

The present invention also relates to an ADC for converting an optical analog signal to an optical digital signal which comprises a plurality of channels each receiving the optical analog signal and a sampling optical signal and providing a bit of the optical digital signal, with the plurality of channels including a first channel providing a most significant bit (MSB), a last channel providing a least significant bit (LSB), and a plurality of intermediate channels providing bits of lessening significance between the MSB and the LSB. Each channel can be formed on a common semiconductor substrate and includes a waveguide photodetector to convert the optical analog signal into an photocurrent signal; an optical waveguide interferometer to generate an optical output signal from inputs of the photocurrent signal and the sampling optical signal, and an optical thresholding element to generate one of the bits of the optical digital signal from the optical output signal from the optical waveguide interferometer. The optical ADC can be formed as a photonic integrated circuit (PIC) on the semiconductor substrate which can comprise a III-V compound semiconductor material such as indium phosphide (InP) or gallium arsenide (GaAs).

The optical waveguide interferometer, which can be a Mach-Zehnder interferometer, has a pair of waveguide arms, with the sampling optical signal being split and directed into each waveguide arm, and with the photocurrent signal being provided to an electrode located proximate to only one of the pair of waveguide arms (i.e. on the phase delay arm). The photocurrent signal, which flows through the phase delay arm to a resistor connected to ground electrical potential, produces a phase delay in the sampling optical signal in the phase delay arm. The optical waveguide interferometer then recombines the sampling optical signals from each waveguide arm to generate an optical output signal which contains information that can be used to construct a particular bit of the optical digital signal by feeding the optical output signal into the optical thresholding element.

A plurality of optical waveguides can be formed on the semiconductor substrate to conduct the optical analog signal and the sampling optical signal from input ports located on one edge of the semiconductor substrate to the plurality of channels, and to conduct each bit of the optical digital signal from one of the optical thresholding elements to a different output port located on another edge of the semiconductor substrate. The waveguides connected to the input ports can include a pair of optical waveguide splitters, with one of the optical waveguide splitters being used to split the optical analog signal into a plurality of portions prior to being received into the plurality of channels, and with the other optical waveguide splitter being similarly used to split the sampling optical signal into a plurality of portions before this signal is received into the plurality of channels. The plurality of channels receiving the optical analog signal and the sampling optical signal can comprise, for example, four channels to provide four bits of resolution for the optical digital signal produced by the optical ADC.

The optical analog signal can be provided by a semiconductor laser; and the sampling optical signal can be provided by another semiconductor laser. Each semiconductor laser can comprise, for example, a distributed feedback (DFB) laser, or a vertical-cavity surface-emitting laser (VCSEL). Each semiconductor laser can be located adjacent to the optical ADC, located remotely from the optical ADC (e.g. using optical fibers to couple the optical signals from the lasers to the optical ADC), or in some cases located on the same semiconductor substrate as the optical ADC.

The optical waveguide interferometer for each channel of the optical ADC can have substantially the same length when each photodetector produces a different photocurrent signal (e.g. in a ratio 1:2:4: . . . :2N where N is a number of bits of the optical digital signal). Alternately, the optical waveguide interferometer for each channel of the optical ADC can have a different length given by L, 2L, 4L, . . . 2(N−1)L where N is the number of bits of the optical digital signal, and L is the length of the optical waveguide interferometer in the channel providing the MSB of the optical digital signal, and 2(N−1)L is the length of the optical waveguide interferometer in the channel providing the LSB of the optical digital signal.

The optical thresholding element, which is used to convert the optical output signal for each channel of the optical ADC into one of the bits of the optical digital signal, can comprise an asymmetric active Mach-Zehnder interferometer having an SOA located in each waveguide arm thereof. The asymmetric active Mach-Zehnder interferometer provides a different optically-induced phase shift in each waveguide arm thereof to provide a step-like optical transfer characteristic which conditions the optical output signal and thereby produces the optical digital signal. Alternately, the optical thresholding element can comprise a saturable absorber.

The present invention further relates to an optical ADC which is formed on a semiconductor substrate as a photonic integrated circuit (PIC). The optical ADC has a pair of optical input ports on the semiconductor substrate to receive an optical analog signal and a sampling optical signal, with the sampling optical signal defining a sampling rate at which the optical analog signal is to be converted into an optical digital signal. A pair of optical waveguide splitters is provided on the semiconductor substrate, with one of the optical waveguide splitters receiving the optical analog signal and splitting this signal into a number N portions where N is equal to a number of bits of the optical digital signal, and with the other optical waveguide splitter receiving the sampling optical signal and splitting the sampling optical signal into N portions. A plurality of waveguide photodetectors are also provided on the semiconductor substrate to receive the N portions of the optical analog signal to generate therefrom N photocurrent signals. The semiconductor substrate also includes a plurality of optical waveguide interferometers, with each optical waveguide interferometer having a pair of waveguide arms which are interconnected at each end of that optical waveguide interferometer. Each optical waveguide interferometer receives one of the N portions of the sampling optical signal and one of the N photocurrent signals and uses these signals to generate an optical output signal which contains information to form one bit of the optical digital signal. A plurality of optical thresholding elements are located on the semiconductor substrate, with each optical thresholding element receiving the optical output signal from one of the plurality of optical waveguide interferometers and generating therefrom one of the bits of the optical digital signal.

A plurality of optical output ports can also be provided on the semiconductor substrate, with each optical output port providing an optical output of one of the bits of the optical digital signal. In some cases, an optical waveguide combiner can be used to receive each bit of the optical digital signal and to generate therefrom a serial optical output.

The optical ADC can also optionally include a plurality of semiconductor optical amplifiers located between the optical waveguide splitter for the optical analog signal and the plurality of waveguide photodetectors. The semiconductor optical amplifiers are useful to amplify the N portions of the optical analog signal prior to generating the N photocurrent signals.

The semiconductor substrate can comprise a III-V compound semiconductor substrate (e.g. a GaAs or InP substrate). The sampling optical signal can be in a wavelength range of 0.8-2.0 microns; and the optical analog signal can also be in this same wavelength range.

The different lengths of each optical waveguide interferometer in some embodiments of the optical ADC of the present invention can be given by L, 2L, 4L, . . . 2(N−1)L where N is the number of bits of the optical digital signal, and L is the length of the optical waveguide interferometer which provides a most significant bit (MSB) of the optical digital signal, and 2(N−1)L is the length of the optical waveguide interferometer which provides a least significant bit (LSB) of the optical digital signal. In other embodiments of the optical ADC of the present invention, the length of each optical waveguide interferometer can be substantially the same when each photodetector produces a different photocurrent signal (e.g. in a ratio 1:2:4: . . . :2N where N is the number of bits of the optical digital signal).

Each optical thresholding element can comprise an asymmetric active Mach-Zehnder interferometer having a semiconductor optical amplifier which provides a different optically-induced phase shift in each waveguide arm thereof to provide a step-like optical transfer characteristic and thereby condition the optical output signal to produce the optical digital signal.

Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, there is shown a schematic plan view of a first example of the optical analog-to-digital converter (ADC)10of the present invention. The optical ADC10, which can be formed as a photonic integrated circuit (PIC) on a common III-V compound semiconductor substrate12such as a gallium arsenide (GaAs) substrate or an indium phosphide (InP) substrate, is useful to generate an optical digital signal14output which is a digital representation of an optical analog signal100which is input to the optical ADC10. The optical analog signal100can be provided, for example, by an optical fiber or a semiconductor laser120and can have a radio-frequency (rf) bandwidth from 100 MHz to 10 GHz or more.

The optical analog signal100is sampled and converted to the optical digital signal14at a sampling rate which is defined by a sampling optical signal110which acts as a clock for the analog-to-digital conversion. The sampling optical signal110can be provided from another optical fiber, or from another semiconductor laser120′ which can be modulated or pulsed (e.g. mode-lock pulsed) at a predetermined sampling rate which can be, for example, 1-50 Gigasamples per second (GSPS). The exact sampling rate will depend upon the frequency of the optical analog signal100which is being digitized by the optical ADC10, and on a desired effective number of bits (ENOB) for the optical digital signal14.

The optical ADC10comprises a plurality of channels16which are formed on the common semiconductor substrate12as shown inFIG. 1, with the various channels16being labelled as “A,” “B,” “C,” and “D.” In general, the optical ADC10can include N channels16where N is the number of bits being generated for the optical digital signal14. Each channel16receives the optical analog signal100and a sampling optical signal110at a pair of optical input ports18located on one edge of the semiconductor substrate12and uses these signals100and110to generate a different bit of the optical digital signal14. Each bit of the optical digital signal14can be provided to a separate output port20which can be located on another edge of the semiconductor substrate12as shown inFIG. 1. Each channel16of the optical ADC10comprises an optical waveguide interferometer22, which can be either a Mach-Zehnder interferometer as shown inFIG. 1or a Michelson interferometer, and also comprises an optical thresholding element24.

The optical waveguide interferometer22in each channel16has a pair of waveguide arms26which receive the sampling optical signal110after this signal110is split into a number N of portions corresponding to the number of channels16in the optical ADC10. In the example ofFIG. 1, N=4 to provide nominally 4-bits of resolution, although in other embodiments of the present invention, N can range, for example, from 2 to 12 or more, depending upon the precision required for conversion of the optical analog signal100to a digital format and the exact rf bandwidth of the optical analog signal100.

For the optical ADC10InFIG. 1, the sampling optical signal110is split into four portions using an optical splitter28. The optical splitter28includes a plurality of single-mode waveguides30and can be formed using branching waveguide Y-junctions32as shown inFIG. 1. In other embodiments of the present invention, a 1×4 multimode interference splitter (also termed a MMI splitter) can be substituted for the optical splitter28inFIG. 1(seeFIG. 4).

A 1×2 MMI splitter34is used in the interferometer22inFIG. 1to further split the sampling optical signal110into an additional two portions, with each portion of the sampling optical signal110being directed into one of the waveguide arms26of the interferometer22. The MMI splitter34comprises a single-mode input waveguide, a laterally-multimoded waveguide section36, and two single-mode output waveguides which form the arms26of the interferometer22.

MMI splitters are well known in the art and need not be discussed in detail herein. See, for example, an article by L. B. Soldano et al., entitled “Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications,” in theJournal of Lightwave Technology, vol. 13, pp. 615-627, April 1995.

InFIG. 1, each interferometer22also includes a 2×1 MMI combiner38to recombine the sampling optical signal110from each waveguide arm26at an output end of that interferometer22. The 2×1 MMI combiner38can be formed as a mirror image of the 1×2 MMI splitter34, with the two arms26of the interferometer22providing inputs to a laterally-multimoded waveguide section36, which, in turn, is connected to a single-mode waveguide44at an output of the 2×1 MMI combiner38. In other embodiments of the present invention, branching waveguide Y-junctions32can be substituted for the 1×2 MMI splitter34and the 2×1 MMI combiner38to form each Mach-Zehnder interferometer22(seeFIG. 4).

InFIG. 1, another optical splitter28′ is similarly used to split up the optical analog signal100into the same number N portions as the sampling optical signal110. To route each portion of the optical analog signal100, the single-mode waveguides30at the outputs of the two optical splitters28and28′ can cross over each other or can intersect directly with each other in a waveguide crossing40as shown inFIG. 1. Such waveguide crossings40can have a relatively low loss and low cross-talk which can be further reduced by slightly tapering each single-mode optical waveguide at the location of the waveguide crossing40.

Each portion of the optical analog signal100can be optionally amplified after the optical splitter28′. This can be done using a semiconductor optical amplifier which can be located between the optical splitter28′ and each optical waveguide interferometer22(seeFIG. 3). Similarly, each portion of the sampling optical signal110can be optionally amplified after the optical splitter28.

After being split, each portion of the optical analog signal100is fed into one waveguide arm26(i.e. the phase delay arm) of each interferometer22using a converging waveguide Y-junction42. In the phase delay arm26, which contains both the sampling optical signal110and the optical analog signal100, the optical analog signal100produces a phase delay of the sampling optical signal110; whereas no phase delay is generated in the other arm26of the interferometer22in which only the sampling optical signal110is present. This phase delay of the sampling optical signal110is due to a cross-phase-modulation (XPM) effect which arises from an optical nonlinearity in the phase delay arm26due to the presence of the optical analog signal100.

The cross-phase-modulation effect, which occurs in a nonlinear Kerr-effect optical medium such as a III-V compound semiconductor material (e.g. InP, InGaAsP or GaAs), produces a change Δn in the refractive index n of the nonlinear optical medium which is given by:
Δn(λ2)=2n2I(λ1)
where λ2is the wavelength of the sampling optical signal110, n2is a nonlinear coefficient of the III-V compound material forming a waveguide core of each waveguide arm26, I is the intensity of the optical analog signal100in the phase delay arm26, and λ1is the wavelength of the optical analog signal100. The cross-phase-modulation effect allows the intensity of the optical analog signal100to generate a phase delay Δφ of the sampling optical signal110which accumulates with an increasing interaction length l where the signals110and100spatially overlap in the waveguide arm26. At the end of the waveguide arm26, the phase delay Δφ is:
Δφ=Δn·l
When the sampling optical signal110from the phase delay arm26is recombined in the 2×1 MMI combiner38with the sampling optical signal110from the other arm26in which no phase delay occurs due to the absence of the optical analog signal100in this arm26, this produces a sinusoidal-intensity-modulated optical output signal in the output waveguide44from each interferometer22. This optical output signal is modulated at the same frequency as the sampling optical signal110and has the same wavelength (i.e. λ2).

The optical analog signal100is preferably not present in the output waveguide44and is also preferably not reflected back into the phase delay arm26by the 2×1 MMI combiner38. If needed, an optical filter can be provided in the output waveguide44to remove or suppress the optical analog signal100. This can be done, for example, by using a waveguide core which transmits at λ2and is absorptive at λ1, or by providing a waveguide ring drop filter in the optical ADC10to remove the optical analog signal100from the waveguide44in each channel16.

The lengths of the waveguide arms26in the different channels16of the optical ADC10in the example ofFIG. 1can be selected to be approximately equal to L, 2L, 4L and 8L to provide interaction lengths l which are in this same ratio (i.e. l, 2l, 4l and 8l) so that each interferometer22can be used to generate a different bit of the optical digital signal14. In other embodiments of the present invention where more than four channels16are used in the optical ADC10to provide additional bits for the optical digital signal14, the lengths of the interferometers22can be given by L, 2L, 4L, . . . 2(N−1)L where N is the number of bits of the optical digital signal14. In general, the lengths of the waveguide arms26for the various interferometers22will range from about 100 μm up to a few millimeters depending upon the number of channels16in the optical ADC10. The width of the waveguide arms26and the various other waveguides in the optical ADC10can be, for example, 3-4 μm and will generally be selected to provide a fundamental mode propagation of the light (i.e. the optical signals100and110) therein.

In the example ofFIG. 1, a most significant bit (MSB) of the optical digital signal14is provided by channel “A” and a least significant bit (LSB) of the optical digital signal14is provided by channel “D.” Each bit of the optical digital signal14inFIG. 1, which is indicated by a horizontal arrow exiting the substrate12, is spatially separated so that it can be detected with a separate photodetector (not shown), or so that it can be directed into a separate optical fiber (not shown) to provide for parallel optical data transmission. When optical fibers are used for the parallel optical data transmission, they can be in the form of individual fibers or an optical fiber ribbon cable.

FIGS. 2A-2Dschematically illustrate the sinusoidally varying intensity of the optical output signal in the output waveguides44of the four channels of the optical ADC10ofFIG. 1prior to being digitized by the thresholding elements24. InFIGS. 2A-2D, the optical output signals for the four channels are labelled “A” B” “C” and “D” and correspond to the same channels inFIG. 1. The horizontal dashed line inFIGS. 2A-2Dindicates a threshold level of the optical thresholding element24above which the optical output signal is preferably converted to a logical “1” state by the optical thresholding element24, and below which the optical output signal is preferably converted to a logical “0” state.

InFIGS. 2A-4D, the vertical dot-dash lines are provided to illustrate the optical digital signal14which will be generated by the optical ADC10for particular intensity levels of the input optical analog signal100corresponding to the locations of the vertical dot-dash lines. The digitized output which is generated by each channel of the optical ADC10is also indicated by the logical “1” and “0” states where the vertical dot-dash lines intersect with the sinusoidally varying curves for the optical output signals. Thus, for an optical analog signal intensity corresponding to the left-most vertical dot-dash line, the generated optical digital signal14after the thresholding element24will have a 4-bit binary representation “1101” and the generated optical digital signal14for the optical analog signal intensity corresponding to the right-most vertical dot-dash line will be “0110.” The sampling rate at which the optical digital signal14is generated in the optical ADC10is determined by the sampling rate of the sampling optical signal110which can be, for example, up to about 50 GSPS.

Returning toFIG. 1, the optical thresholding element24used to convert the optical output signal from each interferometer22to being either high (i.e. a logical “1” state) or low (i.e. a logical “0” state) can comprise a saturable absorber (SA) or a semiconductor optical amplifier (SOA) formed from a III-V compound semiconductor such as InP, InGaAsP or GaAs. The SA and SOA both have an optical transmission characteristic which is nonlinear with the intensity of input light. Both the SA and SOA also have essentially the same structure which comprises a semiconductor optical waveguide containing a semiconductor p-n or p-i-n junction which will be described in more detail hereinafter.

Operation of the SA or SOA will, in general, depend upon the exact biasing conditions used. In a SA, a relatively low bias (e.g. near zero Volts) or a negative bias (i.e. a reverse bias) is used so that light signals below a saturation threshold of the SA will be absorbed to provide a relatively low output level corresponding to the logical “0” state, while light signals above the saturation threshold will be transmitted with little absorption to provide a relatively high output level corresponding to the logical “1” state.

In an SOA, forward biasing conditions are used. However, when the SOA is forward biased below a gain threshold level where optical amplification occurs, the SOA will behave much like a SA by absorbing low-level light signals while being saturated to transmit high-level light signals. When the SOA is operated above the threshold level for optical amplification, the SOA will amplify both low-level light signals and high-level light signals with the high-level light signals possibly saturating the optical gain of the SOA. This latter mode of operation is generally not used for the optical thresholding device24although it is useful elsewhere in the optical ADC10when amplification of light signals is needed.

The saturation threshold level of the SA or SOA can be set by the particular doping levels used for the p-n or p-i-n junction, and also by the bias conditions of the SA or SOA. The saturation threshold level of the SA or SOA can also be controlled by using the width, length or optical confinement factor of the thresholding element24as variables. When the thresholding element24has a width greater than that of the optical waveguides44as shown inFIG. 1, a tapered waveguide section can be provided on either side of the thresholding element24to laterally expand or contract the mode of the light for coupling into and out of the thresholding element.

In the example ofFIG. 1, the threshold level can be set to be an average value of the expected output optical signal intensity from each interferometer22as shown inFIGS. 2A-2D. The threshold level for each optical thresholding element24can be set independently to compensate for differences in the output optical signal intensities for each channel16of the optical ADC10due to the different lengths of the interferometers22.

Each optical thresholding element24operates to produce a digital train of pulses for a particular bit of the optical digital signal14at the frequency and wavelength of the sampling optical signal110which serves as a clock input to the optical ADC10for conversion of the input optical analog signal100to a digital representation.

FIG. 3schematically illustrates a second example of the optical ADC10of the present invention. In this example, each channel16of the optical ADC10can be formed with a Mach-Zehnder interferometer22and an optical thresholding element24as previously described. The thresholding elements24can each be formed as previously described with reference toFIG. 1with a width that is the same or larger than that of the waveguides44, and with a length that can be in the range of 0.1-1 millimeters.

Each interferometer22in the example ofFIG. 3can be formed by coupling together a 1×2 MMI splitter34and a 2×1 MMI combiner38. An electrode46is provided over the phase delay arm26of each interferometer22to allow a phase delay to be electrically generated only in this arm26. This electrically-generated phase delay of the sampling optical signal110in the phase delay arm26of each interferometer22produces a sinusoidal-intensity-modulated optical output signal for each interferometer22which is similar to that previously described with reference toFIGS. 2A-2D.

To provide an electrical current signal for the electrodes46, which is proportional to the optical analog signal100, the optical analog signal100can be amplified with a semiconductor optical amplifier (SOA)48and then can be detected with a waveguide photodetector50. The wavelength of the optical analog signal100need not be different from the wavelength of the sampling optical signal110for this device10since the optical analog signal100is completely absorbed in the waveguide photodetector50to generate a photocurrent signal. The photocurrent signal flows through to the electrode46extending over the phase delay arm26of each interferometer22to a resistor52which is located at an opposite end of the electrode46; and this generates a voltage on the electrode46which reverse biases a semiconductor p-n or p-i-n junction in the phase delay arm26therebeneath to produce the phase delay of the sampling optical signal110. Each SOA48and waveguide photodetector50can be up to a few hundred microns long.

In other embodiments of the present invention, the optical ADC10can be formed with each interferometer22having the same length. This can be done, for example, by fabricating the waveguide photodetectors50to each have a different effective light-to-current conversion efficiencies which are matched to the strength of the bit being generated in each channel16of the optical ADC10. This allows the generation of photocurrents which can be in the ratios 1:2:4: . . . :2N where N is the number of bits being generated by the optical ADC10. Thus, for example, the effective light-to-current conversion efficiency for the photodetector50in channel “B” can be twice that of channel “A” to provide a phase delay for channel “B” which is twice the phase delay for channel “A.” Similarly, the effective light-to-current conversion efficiency for the photodetector50in channel “C” can be twice that of channel “B;” and the effective light-to-current conversion efficiency for the photodetector50in channel “D” can be twice that of channel “C.” In this example, the effective light-to-current conversion efficiency for each photodetector50can be adjusted by using different lengths for the photodetectors50in each channel16(e.g. shortening certain of the photodetectors50so that the light100is not completely absorbed in these photodetectors50and escapes out of one end of the photodetectors50), or by changing the light absorption within the photodetector50using a quantum-well intermixing process as described hereinafter.

As another example, the interferometers22and waveguide photodetectors50can be formed identically for each channel16, and the optical gain of the SOAs48associated with each photodiode50can be scaled (e.g. using the length of the SOAs48) to provide a factor of two increase in amplification of the optical analog signal100for each successive channel16which, in turn, will produce a factor of two increase in the photocurrent generated by each successive photodetector50to provide the required phase delay for each channel16.

FIG. 4schematically illustrates a third example of the optical ADC10of the present invention. In this example of the optical ADC10, a 1×4 MMI splitter54is used to split the sampling optical signal110for the various Mach-Zehnder interferometers22. The 1×4 MMI splitter54can be formed similarly to the 1×2 splitters inFIGS. 1 and 3by using a single-mode input waveguide56and a laterally-multimoded waveguide section36which is designed for coupling into four single-mode output waveguides58. The output waveguides58can be routed to the various interferometers22as shown inFIG. 4with each output waveguide58preferably having about the same length.

InFIG. 4, the optical analog signal100can be input into another 1×4 MMI splitter54′ and split into the same number of portions (e.g. 4) as the sampling optical signal110. The split optical analog signals100can be directed through the output waveguides58to SOAs48and therefrom into waveguide photodetectors50. Each photodetector50generates a photocurrent signal which is conducted through wiring60to an electrode46terminated by a resistor52, with the electrode46being located on a phase delay arm26of each interferometer22. The photocurrent signal flowing through the electrode46and resistor52to ground produces a reverse-bias voltage across the III-V compound semiconductor material in the phase delay arm26. This produces a phase delay of the sampling optical signal110in the phase delay arm26which is proportional to the intensity of the optical analog signal100. When the signals110in the two arms26of each interferometer22are recombined, the result is a sinusoidal-intensity-modulated optical output signal for each interferometer22which is similar to that previously described with reference toFIGS. 2A-2D. The sinusoidal-intensity-modulated optical output signals are then routed through single-mode waveguides44to an optical thresholding element24where these signals are converted into the optical digital output signal14.

In the example ofFIG. 4, each interferometer22can be formed using a pair of oppositely-directed waveguide Y-junctions32connected together with single-mode waveguides which form the two waveguide arms26. Although the interferometers22inFIG. 4are shown with lengths given by L, 2L, 4L and 8L, in other embodiments of the present invention, the lengths of each interferometer22can be the same, with a photocurrent from the photodetectors50for the various channels scaled in the ratios 1:2:4:8 to provide the required phase delays for each interferometer22. Alternately, the optical gain from the SOAs48associated with each photodetector can be scaled in the ratios 1:2:4:8 with each photodetector50then being formed identically but providing a different photocurrents due to a different intensity of the amplified optical analog signal100received by that photodetector50.

In the example ofFIG. 4, each optical thresholding element24comprises an asymmetric active Mach-Zehnder interferometer formed from a pair of waveguide Y-junctions32arranged back-to-back about a pair of SOAs48of different widths. In other embodiments of the present invention, a 1×2 MMI splitter34and a 2×1 MMI combiner38as shown inFIGS. 1 and 3can be substituted for the Y-junctions32in forming each thresholding element24.

In each thresholding element24in the optical ADC10ofFIG. 4, the two SOAs48with different widths produce different optically-induced phase shifts, and this results in a step-like optical transfer characteristic which can be used to condition the optical output signal from the interferometer22to produce the optical digital signal14. One of the SOAs48can have a narrow width which is about the same as the waveguide44(e.g. 3-4 μm), and the other SOA48can have a wide width which can be several times the width of the waveguide44(e.g. 10-20 μm). The wide SOAs48can be formed using flared waveguides as shown inFIG. 4, or alternately can be located in the laterally-multimoded waveguide section36of a 1×1 MMI coupler.

The SOAs48in each optical thresholding element24can have the same unsaturated optical gain, but the optically-induced phase shift responses for the two SOAs48in each thresholding element24will be different due to different current densities in the two SOAs48resulting from their different widths. These different optically-induced phase shift responses will cause the output of the thresholding element24to switch between a low level (i.e. a logical “0” state) and a high level (i.e. a logical “1” state) at some critical input light intensity. Thus, light which is input into the thresholding elements24and SOAs48at a relatively low optical power level will be amplified about the same in each arm of the interferometer with about the same optically-induced phase shift and thus will be cancelled out via destructive interference at an output side of the thresholding elements24. This will produce a low optical digital signal14corresponding to the logical “0” state. On the other hand, light which is input into the thresholding elements24and SOAs48above the critical input light intensity will be amplified with a different optically-induced phase shift in each SOA48so that constructive interference will occur at the output of each thresholding element24. This will produce a high optical digital signal14corresponding to the logical “1” state.

The various SOAs48inFIGS. 3 and 4can be electrically activated with a dc bias. This can be done using one or more external power supplies which can be connected to the SOAs48using additional wiring60formed on the substrate12. This additional wiring60for the SOAs48can be connected to contact pads62located on the substrate12. An additional contact pad62′ can be electrically connected to a lower electrode semiconductor layer formed on the substrate12, or to the substrate12itself when the substrate12is electrically conducting, to provide a common (i.e. ground) electrical connection for the phase delay arms22, SOAs48, photodetectors50and resistors52. The wiring60connecting the waveguide photodetectors50to the electrodes46on the phase delay arms26and the electrodes46and resistors52can be designed for low-impedance (e.g. 50Ω), high-speed operation.

The various examples of the optical ADC10of the present invention can all be fabricated with a quantum-well intermixing process as described hereinafter using a plurality of III-V compound semiconductor layers epitaxially grown upon the substrate12. The quantum-well intermixing process allows the fabrication of many different PIC elements to be formed on a common semiconductor substrate12much like integrated circuit fabrication while allowing the various elements including the waveguides, interferometers, SOAs, photodetectors and resistors to be individually optimized. This quantum-well intermixing process will be described hereinafter with reference toFIGS. 5A-5Dwhich show schematic cross-section views along the section line1-1inFIG. 1during various steps in the manufacture of the optical ADC10.

FIG. 5Ashows a schematic cross-section view of the plurality of III-V compound semiconductor layers which can be initially epitaxially grown on the substrate12in preparation for fabricating the optical ADC10. The III-V compound semiconductor layers can comprise, for example, indium phosphide (InP) and indium gallium arsenide phosphide (InGaAsP) and indium gallium arsenide (InGaAs) when the substrate12comprises InP. Alternately, the III-V compound semiconductor layers can comprise gallium arsenide (GaAs) and either aluminum gallium arsenide (AlGaAs) or InGaAsP when the substrate12comprises GaAs. The following discussion will describe fabrication of the optical ADC10using InP, InGaAsP and InGaAs, but those skilled in the art will understand that the various process steps described hereinafter can be applied with minor modifications to an optical ADC10formed from GaAs and AlGaAs, or any other III-V compound semiconductor materials.

InFIG. 5A, the substrate12can comprise a Fe-doped InP substrate12upon which are epitaxially grown by metal-organic chemical vapor deposition (MOCVD) in order the following layers: an InP buffer layer (not shown), an n-type InGaAs lower contact layer64; a lower cladding layer66of n-type-doped InP which can be 1-2 μm thick; a lower waveguide layer68of InGaAsP which is n-type doped and about 0.11 μm thick with a composition selected to provide an energy bandgap λg=1.1 μm; an undoped (i.e. not intentionally doped) MQW region70which is about 0.11 μm thick and comprises a plurality of alternating quantum well (QW) layers72and barrier layers74of InGaAsP each about 8 nanometers (nm) thick, with the quantum well layers72having an energy bandgap λg in the range of 1.3-1.7 μm, and with the barrier layers74having an energy bandgap λg=1.1 μm; a upper waveguide layer76of p-type-doped InGaAsP about 0.11 μm thick with λg=1.1 μm; an undoped InP etch stop layer78about 15 nm thick; an undoped InGaAsP etch stop layer80about 20 nm thick with λg=1.3 μm; and an undoped InP implant buffer layer82about 0.45 μm thick.

An implant mask (e.g. comprising silicon nitride about 0.5 μm thick) can then be provided over the substrate12and III-V compound semiconductor layers with openings at locations wherein phosphorous ions are to be implanted into the InP implant buffer layer82for use in selectively disordering the MQW region70. The locations where the waveguide photodetectors50and the SOAs48are to be formed will generally not have a disordered MQW region70since the MQW region70is epitaxially grown to optimize the performance of the photodetectors50and SOAs48. The phosphorous ions can be implanted into the layer82at an ion energy of about 100 keV and a dose of about 5×1014cm−2with the substrate12being at a temperature of about 200° C. The implanted phosphorous ions produce vacancies in the InP implant buffer layer82.

A rapid thermal annealing step can then be used to drive the vacancies into the MQW region70to intermix the QW layers72and the buffer layers74at the interfaces therebetween. This intermixing produces a blue-shift the energy bandgap in the MQW region70. The rapid thermal annealing step can be performed at a temperature in the range of 630-700° C. and can be timed for a time interval from one minute up to a few tens of minutes to provide a predetermined energy bandgap for the MQW region70which will depend upon the exact elements of the optical ADC10being formed. To form the waveguide arms26in each interferometer22, a first rapid thermal annealing step can be used to provide a few tens of nanometer blue-shift in the energy bandgap of the MQW region70to reduce an absorption loss therein. The blue-shift in the energy bandgap of the MQW region70can be determined using a laser-excited room-temperature photoluminescence spectroscopy measurement.

After the first rapid thermal annealing step, the InP implant buffer layer82can be removed above the phase delay arm26while leaving the layer82in place over the other waveguide arm26of each interferometer22. This can be done using a wet etching step to etch away the layer82, with the wet etching being terminated upon reaching the InGaAsP etch stop layer80. Removal of the InP implant buffer layer82above the phase delay arm26prevents any further blue-shift in the MQW region70at this location since it removes the source of vacancies necessary for quantum-well intermixing.

A second rapid thermal annealing step can then be performed at about the same temperature for up to a few minutes (e.g. 2-3 minutes) to provide further intermixing of the QW and barrier layers72and74to produce an additional few tens of nanometers blue-shift the energy bandgap of the MQW region70in the remaining regions where the InP implant buffer layer82is still present. This additional blue-shift in the energy bandgap of the MQW region70further reduces the absorption loss in the various waveguides forming the optical ADC10.

After the second rapid thermal annealing step is performed, the remaining InP implant buffer layer82and the InGaAsP etch stop layer80can be removed from the substrate12by wet etching. This is schematically illustrated in the cross-section view ofFIG. 5B.

A blanket MOCVD regrowth can then be performed to epitaxially grow an upper cladding layer84of p-type-doped InP which can be, for example, 2.35 μm thick followed by a cap layer86of p-type-doped InGaAs about 0.2 μm thick. This is shown inFIG. 5C. The p-type-doped upper waveguide layer76and InP upper cladding layer84in combination with the n-type-doped lower cladding layer66and waveguide layer68form a semiconductor p-i-n junction about the MQW region70when the region70is left undoped (i.e. not intentionally doped). This semiconductor p-i-n junction is needed for electrically-activated elements in the optical ADC including the phase delay arm26, the thresholding elements24, the SOAs48and the waveguide photodetectors50.

An etch mask (not shown) can be provided over the substrate12and photolithographically patterned for use in etching down through the InGaAs cap layer86and the InP upper cladding layer84as shown inFIG. 5D. This defines the lateral dimensions of the various waveguides26,30,44and56inFIGS. 1,3and4which can be 3-4 μm wide. Etching down to the InGaAs lower contact layer64can also be performed in preparation for forming the contact pad62′.

Layers of silicon nitride and benzocyclobutene (BCB) can then be deposited over the substrate12and patterned to provide openings where the various electrodes, resistors52, wiring60and contact pads62and62′ are to be formed. The silicon nitride layer can be about 0.1-0.2 μm thick. The BCB layer can be about the same thickness (e.g. 2-3 μm) as the InP upper cladding layer84and can be used to planarize the substrate12. This is useful to reduce the capacitance of the electrical wiring60for high-speed operation. The resistors52can be deposited as thin-film metal resistors (e.g. comprising tantalum nitride or nichrome). A Ti/Pt/Au metallization can then be deposited and patterned by lift-off to form the electrodes46, wiring60and contact pads62and62′ and electrical connections to the optical thresholding elements24, SOAs48and resistors52.

Although the various examples of the optical ADC10described herein with reference toFIGS. 1,3and4provide a parallel output of each bit of the optical digital signal14, in other embodiments of the optical ADC10an optical waveguide combiner90can be used to convert the parallel output of the optical ADC10into a serial output. Since each bit of the optical digital signal14comprises a series of logical “1” state and logical “0” state pulses at a clock frequency determined by the sampling optical signal110, these pulses for each bit of the optical digital signal14can be interleaved and combined to form a train of N-bit digital words for the serial output. To properly time the pulses for each bit of the optical digital signal14so that they can be interleaved and combined, delay lines can be used. These delay lines are formed from different-length optical waveguides92located on the common semiconductor substrate12after the thresholding elements24as shown inFIG. 6, or alternately can be located on a separate substrate94as shown inFIG. 7for butt-coupling to the optical ADC10. The exact difference in length of the optical waveguides92for adjacent bits of the optical digital signal14will depend upon the sampling rate and can be, for example, 1-20 millimeters (mm). For example when the sampling rate is 10 Gigabits per second (Gb/s) with each pulse being 100 picoseconds (ps), the difference in length of the each adjacent pair of the optical waveguides92can be 10-20 mm; and when the sampling rate is 60 Gb/s with 12 ps pulses, the length difference can be 1-2 mm.

The various bits of the optical digital signal14are delayed in time by the waveguides92which can be appropriately sized using waveguide bends so that each bit generated by one of the channels16can be interlaced in time and combined into a single output waveguide96. Combining of the various bits from each channel16into the single output waveguide96can be performed using a plurality of converging waveguide Y-junctions98as shown inFIGS. 6 and 7, or alternately with a N×1 MMI combiner which can be formed in a manner similar to the 1×4 MMI splitter54inFIG. 4. Each waveguide92and96of the optical waveguide combiner90can be formed as single-mode waveguides using the quantum-well intermixing fabrication process previously described with reference toFIGS. 5A-5D.

The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.