Apparatus and method for an electronically tuned, wavelength-dependent optical detector

An electronically tuned, wavelength-dependent optical detector is provided. The electronically tuned, wavelength-dependent optical detector is a modified metal-semiconductor-metal photodetector including a comb-like metal electrode at a common voltage and metal electrodes each supplied with a control voltage by a voltage means. The wavelength to be detected in an optical input illuminating the detector is selected based on the set of control voltages applied to the metal electrodes. In another embodiment of the invention, the wavelength to be detected with the electronically tuned, wavelength-dependent optical detector is also selected using a standing wave generator, such as an interferometer, to produce a spatially varying light intensity on the surface of the electronically tuned, wavelength-dependent optical detector. Electronic wavelength demultiplexing is also provided. Design flexibility can be obtained by providing two or more optical patterns at a detector surface, each pattern having a different wavelength dependence.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method for an electronically tuned, wavelength-dependent optical detector.

BACKGROUND OF THE INVENTION

Wavelength-dependent optical detectors are essential optical components that are incorporated in a myriad of applications including spectrometers, optical interconnects and optical communications systems.

An existing wavelength-dependent optical detector is the so-called metal-semiconductor-metal (MSM) photodetector. In this device, an interdigitated pair of metal electrodes is deposited on a surface of a semiconductor. Light illuminating the MSM device is absorbed in the semiconductor producing charge carriers that drift to the neighboring metal electrodes when a voltage is applied to the metal electrodes. The resulting light-induced current is amplified and detected by an amplifier. The wavelength-dependence of the MSM device is partially determined by the absorption characteristics of the semiconductor in the MSM device. GaAs is used as the semiconductor for MSM devices in the 800 nm wavelength range. InAlAs deposited on InGaAs is used as the semiconductor for MSM devices in the 1600 nm wavelength range. The prior art teaches that the wavelength-dependence of the MSM device can be further selected by creating a standing wave on the MSM detector and fabricating the MSM device such that metal electrodes have a particular spacing, for example, a quarter of the wavelength of light to be detected.

While such MSM devices have been successfully employed in a variety of applications, a principal limitation of the MSM device is that the wavelength-dependence cannot be dynamically tuned. It is manifest that this is also the case for other optical detectors that are not wavelength-dependent, such as photodiodes and photomultiplier tubes. Prior art solutions to this technical challenge include external means for dynamically tuning the wavelength of light detected. Solutions include monochromators, interferometers, multiplexers/demultiplexers, spatial optical filters, spectral optical filters (including cavity resonators) and diffraction gratings. For example, see U.S. Pat. Nos. 6,583,900, 6,594,410 and 6,597,841. However, the speed with which the selected wavelength can be changed in these approaches is limited when the dynamic tuning is based on mechanical motion, such as that associated with a stepper motor or thermal expansion. This is also the case when the dynamic tuning is based on the propagation of waves (for example, sound) in a medium, such as in an acousto-optic modulator or a dynamic diffraction grating. The response time for dynamic tuning of the wavelength-dependence of the existing optical detectors in conjunction with such external means is substantially longer than a microsecond and is typically hundreds to thousands of microseconds. A PIN detector with multiple quantum wells can be dynamically tuned with a fast response time; however, such devices only have a coarse tuning capability over a small range of wavelengths and require a large biasing voltage. These limitations in the dynamic tuning of the wavelength dependence of existing optical detectors are particularly problematic in existing or proposed optical communications systems based on Wavelength Division Multiplexing (WDM).

In optical communications systems based on WDM, a combination of time dependent multiplexing (interleaved packets of information), frequency dependent multiplexing (information communicated using multiple, different wavelengths) and/or spread spectrum (wideband) encoding techniques such as code division multiple access are used. Systems include coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). Recent proposals include 80 channels utilizing a wavelength range centered around 1550 nm (193,300 GHz) with a channel spacing of approximately 0.4 nm (50 GHz) and optical packets of information spaced on time scales on the order of nanoseconds. Future systems will employ more channels (smaller channel spacing) and packets of information spaced on shorter time scales.

To be useful in detecting packets of information based on wavelength in a WDM system, it is highly desirable to be able to switch the wavelength dependence of the optical detector on times scales on the order of or less than the length of the optical packets of information. This necessitates response times for dynamic tuning of the wavelength dependence of the optical detector of a few nanoseconds or less. Response times of this order are well beyond the capability of most of the existing solutions. The alternative, involving a plurality of wavelength-dependent optical detectors with slow dynamic tuning response times, would be expensive and difficult to manufacture and maintain. Each wavelength in the optical system would require a separate detector, the related electronics for amplifying detected signals, as well as a fixed optical filter capable of resolving the small band of wavelengths corresponding to the channel spacing. For example, see U.S. Pat. Nos. 5,546,209, 5,910,851, 6,307,660 and 6,556,321.

As a consequence, there is a need for a wavelength-dependent optical detector that can be dynamically tuned with a response time less than a few nanoseconds for WDM applications, and more generally with a response time less than a microsecond for other applications. It would also be advantageous if the wavelength-dependent optical detector could be dynamically tuned to resolve the narrow channel spacing in WDM systems yet have a wide tuning range. Furthermore, it would be advantageous if such a wavelength-dependent optical detector with fast dynamic tuning were electronically controlled using a low voltage thereby allowing ease of integration with other components.

Objects and Advantages

In view of the above, it is a primary object of the present invention to provide an apparatus and method for a wavelength-dependent optical detector that can be dynamically tuned over a wide range with a response time of less than a few nanoseconds. More specifically, it is an object of the present invention to provide an electronically tuned, wavelength-dependent optical detector.

These and numerous other objects and advantages of the present invention will become apparent upon reading the following description.

SUMMARY

The objects and advantages of the present invention are secured by an apparatus and method for an electronically tuned, wavelength-dependent optical detector. The electronically tuned, wavelength-dependent optical detector is a modified MSM photodetector. In the modified MSM device, a comb-like metal electrode, comprising at least five, substantially parallel arms with a fixed spacing from each other and having a common voltage, is deposited on a surface of a semiconductor. At least four metal electrodes, interdigitated with the comb-like metal electrode are also deposited on the surface of the semiconductor. Each of the metal electrodes is connected to a voltage means that applies a control voltage to each metal electrode. By applying a set of control voltages to the metal electrodes using the voltage means, a wavelength to be detected in a stream of light illuminating the modified MSM device is selected.

In one embodiment of the invention, the comb-like metal electrode in the modified MSM device is connected to an amplifier.

In another embodiment, an opaque coating is deposited on parts of the surface of the modified MSM device thereby grouping the arms of the comb-like metal electrode and the metal electrodes into pairs.

In another embodiment, the semiconductor in the modified MSM device is selected based on the wavelengths to be detected. GaAs is used for MSM devices in the 800 nm wavelength range. InAlAs deposited on InGaAs is used for MSM devices in the 1600 nm wavelength range.

In another embodiment, a plurality of modified MSM devices are used in an optical system where a stream of light comprised of multiple wavelengths is at least partially spatially segregated using a dispersion device.

In another embodiment, a standing wave generator is used to produce a spatially varying light intensity of the surface of the modified MSM device. By appropriately positioning the modified MSM device relative to the varying light intensity and applying a set of voltages to the metal electrodes using the voltage means, the wavelength to be detected is selected.

In another embodiment, the standing wave generator is an interferometer, and position of fringes in the spatially varying light intensity on the electronically tuned, wavelength-dependent optical detector is adjusted by varying the optical path-length difference in the interferometer. In addition, the wavelength spacing of the detected channels is adjusted by changing the optical path-length difference.

In another embodiment, the standing wave generator is an interferometer that interferes two beams separated by an angle on the MSM device, and the relative phase of the fringes in the spatially varying light intensity and the channel spacing is adjusted by varying the optical path-length difference in the interferometer.

In yet another embodiment, the standing wave generator is an interferometer that interferes two beams separated by an angle on the MSM device, and period of the fringes in the spatially varying light intensity is adjusted by varying the angle of incidence of the interfered beams.

Another embodiment of the invention provides multiple outputs from a single detector for electronic wavelength demultiplexing. The multiple outputs are combined with electrically adjustable weights to provide one or more demultiplexer output channels which are electrically tunable.

Still other embodiments of the invention provide enhanced design flexibility by detecting two or more optical patterns having different wavelength dependences. Some of these embodiments can be understood in terms of Fourier series. A particularly simple example of such an embodiment is a detector having one wavelength dependent pattern (e.g., a fringe pattern) and one wavelength independent pattern (e.g., a simple beam spot).

A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A side view of an embodiment of the invention is illustrated inFIG. 1. A stream of light110comprising at least one wavelength illuminates an electronically tuned, wavelength-dependent optical detector apparatus100. The apparatus100is a modified MSM photodetector. The apparatus100has a comb-like metal electrode114, comprising at least five, substantially parallel arms with a fixed spacing from each other and maintained at a common voltage. The parallel arms of metal electrode114are deposited on a surface of a semiconductor112. At least four metal electrodes118,120,122and124, interdigitated with the comb-like metal electrode114are also deposited on the surface of the semiconductor112. Each of the metal electrodes118,120,122and124is connected by an electrical connection128to a voltage means130, such as a power supply, that applies a control voltage to each metal electrode118,120,122and124. In an alternative embodiment, the voltage means130is flip-chip bonded to the semiconductor112. The stream of light110is absorbed in the semiconductor112producing charge carriers that drift to the neighboring metal electrodes118,120,122or124or the arms in the comb-like metal electrode114when the control voltage is applied to each of the metal electrode118,120,122and124.

FIG. 2illustrates a top view of the apparatus100, with the comb-like metal electrode114deposited on the surface of the semiconductor112. The comb-like metal electrode114has a common connection point116. At least four metal electrodes118,120,122and124, interdigitated with the comb-like metal electrode114, are deposited on the surface of the semiconductor112. The metal electrodes118,120,122and124are connected by the electrical connection128to the voltage means130.

FIG. 3illustrates a top view of alternative embodiments of apparatus150. In this embodiment, an opaque coating126is deposited on the surface of the semiconductor112such that the arms in the comb-like metal electrode114are paired with one of the metal electrodes118,120,122and124. Another alternative embodiment has an amplifier134connected132to the common connection point116of the comb-like metal electrode114. In a preferred embodiment, the amplifier134is a trans-impedance amplifier. In yet another alternative embodiment, the amplifier134is flip-chip bonded to the semiconductor substrate112.

FIG. 4illustrates a side view of alternative embodiments of apparatus160. One alternative embodiment has an optional base layer136, an optional intermediate layer138and an optional top layer140deposited in layers located above the surface of the semiconductor112and below the layer containing the comb-like metal electrode114and the metal electrodes118,120,122and124. For wavelengths in the stream of light110less than 860 nm, a preferred embodiment has a semi-insulating GaAs semiconductor112, a substantially 0.3 micron thick GaAs base layer136as a buffer, a substantially 0.3 micron AlGaAs intermediate layer138comprising relative proportions of substantially 85% aluminum and 15% gallium in the AlGaAs compound and being substantially 1 micron thick, and an undoped GaAs top layer140that functions as the active layer in the apparatus160absorbing the stream of light110. The intermediate layer138in this embodiment serves two functions: it acts as an etch stop for etching through the semiconductor112, thereby allowing flip-chip bonding of electronics such as the voltage means130and the amplifier134to the apparatus160, and it acts as a barrier to keep carriers in the top layer140thereby improving temporal response of the apparatus160when stream of light110is absorbed in the top layer140.

When the intermediate layer138is used as an etch stop layer, after flip-chip bonding of electronics such as the voltage means130and the amplifier means134, the entire substrate layer112can be chemically removed using a selective etch or etches well known to those skilled in the art, with the etch substantially stopping when layer138is reached. In this case, stream of light110can impinge on the intermediate layer138from the bottom, which may be convenient since the flip-chip bonded electronics might otherwise get in the way of the stream of light110when the stream of light110is incident from above as shown inFIG. 4.

For wavelengths in the stream of light110substantially larger than 850 nm but less than 1650 nm, such as wavelength in the range 1200–1600 nm commonly used in telecommunications, a preferred embodiment has a semi-insulating InP semiconductor112, an InP base layer136as a buffer, an undoped InGaAs intermediate layer138(comprising substantially 47% In and substantially 53% Ga and As) that functions as the active layer in the apparatus160absorbing the stream of light110, and a thin InAlAs top layer140that increases the Schottky barrier height in the apparatus160and thereby reduces the leakage current that flows even when the stream of light110does not illuminate the apparatus160.

Those skilled in the art will recognize that the details of the wafer structure can be modified for other applications of the invention in spectroscopy, optical interconnects, optical sensing and optical detection since the range of wavelength that can be detected with the apparatus160are confined to the absorption range of the semiconductor in the active layer of the apparatus160.

As described thus far, the apparatus160cannot distinguish between two wavelengths so long as they are within the absorption range of the semiconductor in the active layer of the apparatus160. For example, an apparatus with a GaAs active layer cannot distinguish 850 nm from 840 nm. To further distinguish the wavelength in the stream of light110to be detected in this invention, rapid, electronic tuning of the apparatus160occurs by varying the control voltage applied by the voltage means130to each of the metal electrodes118,120,122and124. Temporal response of the apparatus160to a change in the control voltage is determined by resistance-capacitance (RC) time constant. For the apparatus160with the arms in the comb-like metal electrode114and the metal electrodes118,120,122and124having tens of micron length113(inFIGS. 2 and 3) and with micron spacing115(inFIGS. 2 and 3) between the arms in the comb-like metal electrode114and the metal electrodes118,120,122and124, and width117of comb-like metal electrode114and the metal electrodes118,120,122and124, capacitance of the apparatus160is less than several hundred fF. For the MSM device100inFIG. 2with 0.8 micron electrode spacing115and width117and with interdigitated pattern covering total area of 100 micron length (not shown) by 52 micron width (not shown), the capacitance is theoretically 167.25 fF. For the MSM device100inFIG. 2with 0.8 micron electrode spacing115and width117and with the interdigitated pattern covering total area of 40 micron length (not shown) by 26.4 micron width (not shown), the capacitance is theoretically 33.5 fF. Referring back toFIG. 4, in conjunction with low resistance of the apparatus160, the temporal response of the apparatus160to change in the control voltage is less than a nanosecond. The rapid, electronic tuning of the apparatus160in this invention is further described below.

FIG. 8schematically illustrates the electronic tuning of the wavelength in this invention. Here a spatially varying light intensity400corresponding to a wavelength in the beam of light110is shown on the surface of the apparatus100with a fringe period between adjacent maxima410(or adjacent minima412) preferably equal to the device width. For example, in apparatus100ofFIG. 2, the device width is the distance between the centers of the two outermost tines of comb electrode116. Note that it is also important that the apparatus100be appropriately positioned relative to the spatially varying light intensity. In this example, the locations of the maxima410and the minima412in the spatially varying light intensity400are aligned with the locations of the comb-like metal electrode114. If a negative control voltage is applied by the voltage means130to the metal electrodes118and124, and a positive control voltage is applied by the voltage means130to the metal electrodes120and122, the wavelength in the stream of light110that gives rise to the spatially varying light intensity400inFIG. 8will give rise to zero net current and thus will not be detected. In contrast, the same control voltages will allow a neighboring wavelength with maxima414and minima416, shown by the dashed line inFIG. 9, to produce a net current and therefore to be detected.

In principle, due to its symmetric structure, the current-voltage characteristic of the ideal electronically tuned, wavelength-dependent optical detector apparatus100has positive/negative symmetry with respect to the control voltage applied by the voltage means130to the metal electrodes118,120,122and124. In practice, variations in fabrication may necessitate different voltages when wavelength-dependent optical detector apparatus100is illuminated by stream of light110with fringe intensity variation400.

The detection of multiple wavelengths with a single apparatus100is enabled by this invention by increasing the number of metal electrodes118,120,122and124and the corresponding arms in the comb-like metal electrode114in the apparatus100. The detection of multiple wavelengths with a single apparatus100is further enabled by appropriately positioning the apparatus relative to the spatially varying light intensity on the surface of the apparatus100, i.e., by selecting the appropriate spatial phase relation, and by applying the appropriate control voltage to each metal electrode118,120,122and124.

FIG. 10illustrates in an exemplary embodiment how a standing wave generator such as an interferometer500is used to produce the spatially varying light intensity and to select relative phase of this spatially varying light intensity on wavelength dependent optical detector504. A beam of light502is divided into two beams of light506and508by a beam splitter510. Beam of light508is reflected off of mirror512. Beams of light506and508pass through a lens514and are incident on the wavelength dependent optical detector504with an incident angle516relative to the normal518of the wavelength dependent optical detector504. Beams of light506and508interfere due to a relative difference520in their optical path lengths. Such a path length difference520results in a time delay between the two beams506and508because of the additional time taken for light to propagate through the additional distance520in the path of beam of light508. As a result of this delay and the incident angle516relative to the normal to the wavelength dependent optical detector504, when the wavelength changes, the spatially varying light intensity substantially moves, corresponding to a change of the relative phase of the spatially varying light intensity. There is also a change in period of the spatially varying light intensity, although for small wavelength changes the variation in position of the spatially varying light intensity (and thus the relative phase) is more important for the operation of the wavelength-dependent optical detector504. Additional means for forming such spatially varying light intensities on the surface of detectors with multiple elements are discussed in D. A. B. Miller, “Laser Tuners and Wavelength-Sensitive Detectors Based on Absorbers in Standing Waves,” IEEE Journal of Quantum Electronics, 30, 732–749 (1994), which is hereby incorporated by reference.

The dependence of the spacing between fringes in the spatially varying light intensity on the incident angle516of the two interfered beams of light506and508can be conceptually understood in terms of a plane wave with a fixed wavelength λ incident at an angle Ω relative to the normal of a flat mirror. The effective period of the standing wave pattern projected on the mirror is λ/sin(Ω). Since sin(Ω) is always less than one, if beams of light506and508are incident at incident angle516(equal to Ω) with respect to the wavelength dependent optical detector504, the fringe width in the interference pattern, given by λ/(2 sin(Ω)), is increased compared to half a wavelength λ/2.

The theory behind the wavelength-dependent optical detector in this invention is described below for two illustrative examples.

Suppose we want the electronically tuned, wavelength-dependent optical detector in this invention to distinguish between two different wavelengths. As described above, the apparatus100with the comb-like metal electrode114and the four metal electrodes118,120,122and124is required. This is the minimum modified MSM device section size. Take the wavelength in the spatially varying light intensity aligned with the metal electrodes118,120,122and124, such as that corresponding to the dashed curve414inFIG. 9, to be 860 nm. This is the wavelength to be detected. Metal electrodes120and122at the positive portions of the cycle corresponding to this wavelength have positive control voltage. Metal electrodes118and124at the negative portions of the cycle corresponding to this wavelength have negative control voltage. The relative phase between spatially varying intensity pattern for this wavelength and spatially varying intensity pattern for the neighboring wavelength that is not to be detected, such as the solid curve inFIGS. 8 and 9, is taken to be π/2. Furthermore, the neighboring wavelength that is not to be detected is taken to be 860.24 nm. For an arbitrary wavelength λ, constructive superposition at each metal electrode118,120,122and124requires
mλ=(m+φ/2π)860.
Taking m=896 as an illustrative example and rearranging we have
φ=2π[896(λ−860)/860].
The integrated intensity I over one period (0 to 2π) is I=I1−I2, where I1∝∫(1+sin(θ−φ)) dθ evaluated between 0 and π and I2∝∫(1+sin(θ−φ))dθ evaluated between π and 2π. After some math, we find that
I∝4 cos(φ)=4 cos(2π[896(λ−860)/860]).
For λ=860 nm, I=1. For λ=860.24 nm, I=0. This value of I corresponds to φ=π/2, the phase shift between the two spatially varying light intensity patterns of the two wavelengths that we wished to discriminate between. Since the spatially varying light intensity pattern corresponding to the wavelength we wanted to detect (860 nm) is aligned with the metal electrodes118,120,122and124, this result is tantamount to saying that the phase shift of the spatially varying light intensity pattern that we do not want to detect, i.e., the one corresponding to a wavelength of 860.24 nm, relative to the metal electrodes118,120,122and124is π/2. This, in turn, indicates the relative placement and biasing of the metal electrodes118,120,122and124in the apparatus100relative to the spatially varying light intensity corresponding to this wavelength. If we now wish to use the same detector to detect light of wavelength 860.24 nm and not detect light of wavelength 860 nm, we merely need to change the biasing of the fingers. Biasing metal electrodes118and120positively and metal electrodes122and124negatively will cause wavelength 860.24 nm to be detected and wavelength 860 nm not to be detected. This change in biasing corresponds to shifting the biasing pattern by a phase of π/2. Hence merely changing the biasing of the electrodes in the detector changes the wavelengths that it will detect and those that it will not detect.

To discriminate between 4 wavelengths (for example, 860 nm, 860.24 nm, 860.48 nm and 860.72 nm), a second minimum modified MSM device section is required. Thus, there are now 8 metal electrodes, where each of the MSM device sections has electrodes such as118,120,122and124. One of the minimum modified MSM device sections is placed in the interference pattern between the beams such that the relative phase between the spatially varying intensity patterns of two adjacent wavelengths is π/4. The second minimum modified MSM device section is placed in the interference pattern between the beams such that the relative phase between the spatially varying intensity patterns of two adjacent wavelengths is 3π/4. Repeating the previous calculation with m=448 we find that
I∝ cos(2π[448(λ−860)/860])+cos(2π[1344(λ−860)/860]).
For λ=860 nm, I=2. For λ=860.24 nm, I=0. For λ=860.48 nm, I=0. For λ=860.72 nm, I=0. Once again, the arguments of the cosine functions at the wavelengths that are not to be detected correspond to the relative phases between the spatially varying light intensities (in this example, multiples of π/4 and 3π/4). Once again, these phases indicate the relative placement and biasing of the metal electrodes, such as118,120,122and124, in the apparatus100relative to the spatially varying light intensities corresponding to these wavelengths.

This invention can be generalized to discriminate between an arbitrarily large number of wavelengths. As such, this invention enables a wide tuning range. The results are summarized in Table I.

FIG. 5shows an illustration of an embodiment of an optical system200that uses this invention. A stream of light210comprised of a plurality of wavelengths and containing information is at least partially spatially segregated by a dispersion device212into three optical beams214,216and218each containing at least a wavelength and possibly a range of wavelengths. Suitable dispersion devices include, among others, a prism, a diffraction grating or an arrayed waveguide grating. The optical beams214,216and218are collimated by a lens220and illuminated onto electronically tuned, wavelength-dependent optical detectors224,226and228, each of which is the same as the apparatus160as shown inFIG. 4. The optical detectors224,226,228are each connected230,232and234to voltage means236,238and240. In an alternative embodiment, the voltage means236,238and240are incorporated in the optical detectors224,226,228. While this embodiment could use a standard photodetector that is neither wavelength-dependent nor dynamically tunable, such as those known in the prior art, the electronically tuned, wavelength-dependent optical detector of this invention still offers an advantage. In particular, the electronically tuned, wavelength-dependent optical detectors224,226and228are able to detect the range of wavelengths in each beam214,216and218, thereby reducing the number of optical detectors224,226and228in the optical system200. In addition, in this example the dispersion device212provides coarse dispersion of the stream of light210and the optical detectors224,226and228provide fine wavelength resolution. Standing wave generators222,225and227are inserted between the lens220and optical detectors224,226and228thereby producing a spatially varying light intensity on optical detectors224,226and228. Interferometer500is suitable for standing wave generators222,225and227. As described in the theory behind this invention described above, by appropriately positioning the spatially varying light intensity on optical detectors224,226and228such that the metal electrodes118,120,122and124have the appropriate spatial phase relationship with the spatially varying light intensity and by applying the appropriate control voltage to each of the metal electrodes118,120,122and124, particular wavelengths in the stream of light210can be selected for detection.

The interferometer500used as standing wave generators222,225and227is also used to adjust the channel spacing. In an interferometer, a larger the optical path-length difference in the arms of the interferometer will result in a different phase for the fringes in the resultant interference pattern for a given wavelength since φ=2πnΔd/λ, where φ is the phase difference between the light in the arms of the interferometer that gives rise to the interference, n is the index of refraction, λ is the wavelength and Δd is the path-length difference. As described in the theory behind this invention described above, for discrimination between two wavelengths λ1and λ2a relative phase of π/2 is desired. In this case, the phase difference φ is

2·π·n·Δ⁢⁢dλ1-2·π·n·Δ⁢⁢dλ2=π2.
Dividing both sides of this equation by 2π yields

This embodiment of this invention is further illustrated inFIG. 6, which shows a schematic of the experimental setup300used to perform measurements with an electronically tuned, wavelength-dependent optical detector346of this invention. A stream of light312is produced by a light source310. A tunable Ti-Sapphire laser is suitable. The light312passes through a chopper314, to enable sensitive lock-in detection, and lenses316and318, which determine the size of the interference pattern on the optical detector346. The light312passes through an attenuator320and is directed along two paths in the Michelson interferometer by a 50/50 beam splitter322. The two paths have path lengths324and328. As noted above, the path length difference Δd, which is twice the difference of path lengths324and328, partially determines the phase difference φ and the channel spacing. After reflecting off of mirrors326and330, the light passes through a beam splitter332, a focal lens344and illuminates optical detector346. While not shown inFIG. 6, two beams in the stream of light312illuminate the optical detector346at an angle, as shown inFIG. 10, producing an interference pattern (not shown) on the optical detector346. Referring back toFIG. 6, the angle between the two beams is controlled by tilting mirrors326and330. In practice, the tilt, the path length difference Δd and wavelength λ1are fixed. As wavelength λ2varies, the relative phase is varied. The optical detector346is connected348to a voltage means350. As disclosed in this invention, the position of the optical detector346relative to the fringes in the interference pattern is important. This is determined in this experimental setup300using an LED342to produce light341that is focused by a lens340, passes through beam splitters334and332, the focal lens344and illuminates the optical detector346. The light reflected from the optical detector346passes through the focal lens344, through the beam splitters332and334, and is focused by a lens336onto a CCD camera338.

An electronically tuned, wavelength-dependent optical detector346of this invention has been fabricated where the spacing115and the width117of the comb-like metal electrode114and metal electrodes118,120,122and124are both 0.8 micron. The interdigitated pattern covers a 40 micron by 13.6 micron area.FIG. 7shows the results of measurements performed using the optical detector346to distinguish two neighboring wavelengths. Positive control voltage of 2.1 V was applied to the metal electrodes120and122in the optical detector346by the voltage means350and negative control voltage of −1.25 V was applied to the metal electrodes118and124in the optical detector346by the voltage means350as illustrated schematically inFIGS. 8 and 9. The optical detector346is sensitive to the wavelength labeled “On” at 807.54 nm and is insensitive to the wavelength labeled “Off” at 808.33 nm. The theoretical sinusoidal response of the optical detector346is also shown. The demonstrated channel spacing of 0.76 nm is sufficient to enable a 365 GHz channel spacing in a WDM system. Additional experiments were able to resolve a channel spacing of 50 GHz. The channel spacing capability of the experimental setup300is limited by the line width and discontinuous tuning of the light source310not the capabilities of the optical detector346.

FIG. 11shows an electronic wavelength division demultiplexer according to an embodiment of the invention, and is best appreciated by comparison to the embodiment ofFIG. 2. OnFIG. 2, voltage source130provides individual control voltages to electrodes118,120,122, and124for setting the wavelength response of the detector output (i.e., the current from electrode116). OnFIG. 11, a modified wavelength dependent detector1100according to the invention is shown, which is similar to the detector ofFIG. 2except that the roles of input and output are reversed. More specifically, electrodes118,120,122, and124provide corresponding individual outputs (e.g., currents)1102,1104,1106, and1108, while electrode116provides the bias voltage for detector1100. Thus when detector1100is illuminated with optical radiation having a spatial pattern, outputs1102–1108effectively sample this pattern. If the spatial pattern is wavelength dependent, then so is the distribution among outputs1102–1108. Equivalently, outputs1102–1108have different wavelength responses. Such wavelength dependence of multiple outputs can be exploited to create an electronic wavelength division demultiplexer, as shown onFIG. 11.

More specifically, outputs1102–1108are connected to a first amplifier array1110and also to a second amplifier array1120. Note that intersecting lines without a filled circle on top of the intersection point are not electrically connected onFIG. 11. In amplifier array1110, gains A1–A4are applied to inputs1102–1108respectively. Similarly, in amplifier array1120, gains B1–B4are applied to inputs1102–1108respectively. The outputs from amplifier arrays1110and1120are received by summing junctions1130and1140to provide outputs1150and1160respectively. By setting gains A1–A4appropriately, the wavelength dependence of output1150can be controlled. Similarly, setting the gains B1–B4determines the wavelength dependence of output1160. Thus outputs1150and1160are generally weighted sums of the inputs1102–1108. For example, one approach for setting these gains is to use values limited to +1 and −1, and in this case, the gains to use can be determined by analogy to the examples ofFIGS. 8 and 9, where regions of positive bias and negative bias are defined.

Suppose gains A1–A4are set such that output1150has a maximum value at λ1and has a minimum value close to zero at λ2, and gains B1–B4are set such that output1160has its maximum and minimum values at λ2and λ1respectively. Then the embodiment ofFIG. 11acts as a wavelength division multiplexer, since “channel 1” at λ1appears only at output1150, while “channel 2” at λ2appears only at output1160. Note that this demultiplexing function is mainly performed electronically, since the only optical element before detector1100is an interferometer (not shown) for creating a wavelength-dependent spatial pattern on detector1100. Such electronic wavelength demultiplexing advantageously provides a remarkable degree of flexibility. For example, the channels that outputs1150and1160are sensitive to can be altered by electronically setting the gains A1–A4and B1–B4. These two outputs can be electronically tuned to widely separated wavelengths, or to nearby wavelengths, or even to the same wavelength (i.e., a non-blocking broadcast capability is inherent in this approach). Since tuning is performed entirely electronically, it can be performed much more rapidly than in wavelength demultiplexers that are tuned by altering an optical configuration. Although only four outputs and two output channels are shown onFIG. 11for simplicity, the invention can be practiced with any number of outputs and output channels.

FIG. 12shows an embodiment of the invention where two different interference patterns are formed on a detector. More specifically, an optical input1202is provided to an interferometer1206and to an interferometer1208(e.g., by using a beam splitter1204as shown). Any other method of splitting optical input1202is also suitable for practicing the invention. Interferometers1206and1208have path length differences Δd1and Δd2respectively, which differ. A detector1210has separate photodetection regions1212and1214. Preferably, detector1210is a monolithic semiconductor structure including integrated regions1212and1214. An interference pattern formed by interferometer1206is incident on region1212, and an interference pattern formed by interferometer1208is incident on region1214. Photodetection regions1212and1214provide a response which depends on the spatial pattern of incident radiation. For example, photodetection regions1212and1214can be MSM detectors as inFIG. 2orFIG. 11, or can be other kinds of pattern-dependent detectors (e.g., PIN detectors). For MSM detectors illuminated with an interference pattern having fringes as discussed above, the output y(λ) for wavelengths λ near a center wavelength λ0can be expressed as

y⁡(λ)=A⁢⁢cos⁡(2⁢π⁢⁢n⁢⁢Δ⁢⁢d⁡(λ-λ0)λ02),
where Δd is the interferometer path length difference, n is the index of refraction and A is the amplitude. The outputs from regions1212and1214are scaled by amplifiers1216and1218respectively, and are then combined in a summing junction1220to provide a combined output z(λ) at1222. For detector outputs as given above, the combined output z(λ) is given by

The embodiment ofFIG. 12thus provides a detector having an output z(λ) with a wavelength dependence that can be adjusted by altering the parameters C1, C2, Δd1, and Δd2. Such flexibility can be exploited by a designer to provide a detector having a response z(λ) that approximates an arbitrary desired response f(λ) over a certain wavelength range (e.g., λa<λ<λb). For example, C1, C2, Δd1, and Δd2can be determined by numerically minimizing

∫λaλb⁢(f⁡(λ)-z⁡(λ))2⁢⁢ⅆλ.
Algorithms for such determination are known in the art. An alternative approach for designing z(λ) which provides additional insight is based on the similarity of the expression for z(λ) to a finite Fourier series. In particular, if Δd2is set to an integer multiple of Δd1, then z(λ) is a finite Fourier series (without a DC or wavelength independent term).

This Fourier series approach for designing detector response enables simple approximate design of desired detector spectral responses. For example, suppose a square-wave detector response is desired having a 50% duty cycle. Since the first four Fourier coefficients for such a square wave are {1/2, 2/π, 0, −2/3π}, the corresponding detector design has Δd2=3Δd1, and C2/C1=−⅓, where the DC term (i.e., the wavelength independent term) is neglected for now. Since this desired square wave has no second harmonic component, the two terms included in the approximation z(λ) are the fundamental term and the third harmonic term.

AlthoughFIG. 12shows two interferometers1206and1208providing interference patterns to regions1212and1214, interferometers1206and1208can be replaced by a single device or assembly (e.g., a MEMS device or assembly) that performs the same function (i.e., providing interference patterns at regions1212and1214having different wavelength dependences). The invention can also be practiced with more than 2 separate photodetection regions, each illuminated with a pattern having a different wavelength dependence. In this manner, the number of terms in the response z(λ) can be increased, making it possible for z(λ) to more closely approximate a desired wavelength response f(λ). For example, adding a DC term (or a fifth harmonic term) to the above square wave Fourier series example will increase the accuracy of the approximation.

FIG. 13shown an embodiment of the invention similar to the embodiment ofFIG. 12, except that amplifiers1216and1218onFIG. 12are not present and optical attenuators (or amplifiers)1302and1304are added. The function of attenuators1302and1304onFIG. 13is to set the magnitudes of coefficients C1and C2in z(λ). Since optical attenuation cannot provide the signs of C1and C2, these signs are preferably set by appropriately biasing regions1212and1214. For example, if regions1212and1214are MSM detectors as discussed above, then the signs of the outputs can be changed by biasing (e.g., by positively biasing the negative part of the interference pattern, and negatively biasing the positive part of the interference pattern, or vice versa). Alternatively, controllable inverters (not shown) can be placed between region1212and summing junction1220and between region1214and summing junction1220in order to set the signs of C1and C2. Thus the relative values of C1and C2in z(λ) can be set electronically (as inFIG. 12) and/or optically (as inFIG. 13). AlthoughFIG. 13shows attenuators1302and1304disposed after the corresponding interferometers, the invention can also be practiced with attenuators1302and1304disposed before the corresponding interferometers. As in the embodiment ofFIG. 12, one or more interferometers can be used to provide two or more interference patterns having different wavelength dependences.

FIG. 14shows an embodiment of the invention similar to the embodiment ofFIG. 12, except that interferometer1208onFIG. 12is removed. A detector1410includes photodetection regions1412and1414. Photodetection region1412has a response that depends on the spatial pattern of incident radiation. Photodetection region1414need not provide a pattern-dependent response, and can therefore be any conventional detector region. In this embodiment, assuming region1412is an MSM detector as discussed above, the detector output z0(λ) at1222is given by

z0⁡(λ)=A⁡[C0+C1⁢cos⁡(2⁢π⁢⁢n⁢⁢Δ⁢⁢d1⁡(λ-λ0)λ02)],
where C0is determined by amplifier1218. This response can be regarded as a two-term Fourier series including the DC and fundamental terms. The invention can also be practiced with more than two terms in z0(λ), as indicated above in connection withFIG. 12.

FIG. 15shows an embodiment of the invention similar to the embodiment ofFIG. 13, except that interferometer1208onFIG. 13is removed. Here the magnitude of C0is determined optically by attenuator1304, and its sign is set as indicated in connection withFIG. 13. Here also, the number of terms in z0(λ) can also be greater than two.

The embodiments ofFIGS. 12–15can all be regarded as examples of a wavelength dependent detector having an optical subassembly including one or more interferometers. The optical subassembly provides two or more patterns to corresponding detector regions, where each pattern has a different dependence on wavelength. For example, onFIGS. 12 and 13, the two patterns are interference fringe patterns having different wavelength periods. OnFIGS. 14 and 15, one pattern is an interference pattern having a periodic wavelength dependence, and the other pattern is a simple beam spot pattern having substantially no dependence on wavelength. One or more of the detector regions has a pattern-dependent response. Outputs from each detector region are combined according to weights to form a combined detector output. The weights can be set optically and/or electronically. For example, a weight such as C1can be determined by any combination of the split ratio of beam splitter1204, loss in interferometer1206, optical loss (or gain) in optical attenuator (or optical amplifier)1302, and electronic gain (or loss) in electronic amplifier1216. Electronic demultiplexing as shown inFIG. 11is also applicable to the embodiments ofFIGS. 12–15. For example, each photodetection region (such as1212,1214,1412, and1414) can provide two or more region suboutputs having different wavelength responses. One or more summing junctions (analogous to1130and1140onFIG. 11) can receive the weighted region suboutputs and provide corresponding junction outputs each having a wavelength response that is individually adjustable by changing the weights.

The electronically tuned, wavelength-dependent optical detector in this invention has numerous advantages with respect to wavelength-dependent optical detectors in the prior art. It is capable of rapid tuning for multiple wavelengths with sub-nanosecond switching time. Electronic tuning is at low voltages (unlike devices with an external micro-machined optical filter) allowing easy integration with CMOS electronics. The wavelength-dependence is substantially insensitive to temperature variations (unlike devices with an external cavity-based optical filters). The channel spacing can be adjusted dynamically. The wavelength-dependence of the apparatus100is integrated into the apparatus100, obviating the need for an external optical filter. And, finally, by adding additional metal electrodes118,120,122and124and arms to the comb-like metal electrode114the electronically tuned, wavelength-dependent optical detector is capable of a wide tuning range limited only by the absorption characteristics of the semiconductor in the active layer of the apparatus100.

In view of the above, it will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.