Abstract:
A method and apparatus for a photoinduced electromotive force sensor. The sensor has an active substrate formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective charge grating and pairs of electrodes disposed on the active substrate, where the sensor is configured to reduce the photovoltaic effect caused by the incident light in the vicinity of the electrodes. The shape or composition of the electrodes may be selected to reduce the photovoltaic effect or the electrodes may be disposed on the substrate to average out the photovoltaic effect arising from each one of the electrodes.

Description:
CROSS REFERENCE TO RELATED PATENTS &amp; APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application 60/658,956 filed Mar. 4, 2005, the disclosure of which is hereby incorporated wherein by reference. 
     The present application may have subject matter similar to that disclosed in U.S. Pat. No. 6,342,721 B1, granted Jan. 29, 2002 to Nolte et al., and United States Patent Application Publication No. 2003/0151102 A1, published Aug. 14, 2002. The contents of U.S. Pat. No. 6,342,721 B1 and United States Patent Application Publication No. 2003/0151102 A1 are incorporated herein by reference in their entireties. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No. F33615-95-2-5245 awarded by DARPA. The Government may have certain rights in this invention. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to non-steady-state photo-induced electromotive force (EMF) sensors and, more specifically, to an apparatus and method for improving the performance of photo-EMF sensors by minimizing or eliminating spurious noise sources. 
     2. Description of the Related Art 
     A non-steady-state photo-induced electromotive force (photo-EMF) device can generate time-varying photocurrents in response to a corresponding lateral and rapid shift of an optical pattern across its surface. The optical pattern may have, for example, the form of a moving optical fringe pattern or a moving speckle pattern. Conventional photodetectors, on the other hand, respond only to changes in the incident light level, and not to the lateral motion of an optical pattern. For example, if there are several fringes incident on the surface of a conventional photodetector, there may not be any response to a moving pattern, since as each bright fringe leaves the active region of the conventional photodetector, another bright fringe may enter the active region at the same time from the opposite side of the active region. 
     A photo-EMF sensor resembles a conventional semiconductor detector; however, to provide the pattern response characteristics described above, it can develop an internal lateral electric field that stores the spatial intensity pattern of an incident optical beam.  FIG. 1  shows a conventional photo-EMF sensor  100  comprising a detection substrate  110  and a pair of surface electrodes  120 . To provide the photo-EMF function, the detection substrate  110  is typically formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective space charge grating.  FIG. 1  also shows an intensity pattern  150  of an incident laser beam  152  with the resulting lateral internal space charge field  160  developed in the material between the electrodes  120 . The detection substrate  110  may comprise gallium arsenide and the surface electrodes  120  may comprise a titanium/gold alloy. 
     One application of a photo-EMF sensor is in a velocity, or differential, sensing interferometer system, in which the photo-EMF sensor acts as a fringe processing unit. See, for example, the system described by Pepper et al. in U.S. Pat. No. 5,909,279, “Ultrasonic Sensor Using Short Coherence Length Optical Source, and Operating Method,” issued Jun. 1, 1999. In such a system, interference fringes are directed onto the surface of a photo-EMF sensor. Photo-generated carriers are then produced which diffuse away from regions of intense optical radiation. These carriers become trapped and formed a periodic charge pattern and the corresponding space-charged field. In the absence of any change in the interference pattern, the space charge field is static and no net current is produced from the photo-EMF detector. However, if the interference pattern changes at a sufficiently high rate, the space charge field can not track the changes, and a net current is produced by the photo-EMF sensor. 
     Hence, a key feature of a photo-EMF sensor is that it senses rapid motion of an optical pattern across its surface by generating a photo-EMF current in response to the rapidly moving pattern. As noted above, the pattern may be a set of interference fringes resulting from the interference of two light beams, and the rapid motion of the pattern may be the result of one of the beams experiencing a dynamic phase shift. In a laser-ultrasound probing system, the dynamic phase shift may be the result of motion in a probed surface. In a laser communication receiver, the motion may be due to phase-modulation encoded onto a light beam. The photo-EMF sensor is adaptive, since static and slowly varying changes, such as those due to beam wander, vibrations, thermal effects, turbulence, are adaptively tracked. The photo-EMF sensor will produce no current for such changes, as long as the space-charge field formation time is faster than the changes. Thus, the photo-EMF sensor can adaptively compensate for the effects due to these changes. This adaptive compensation capability makes the photo-EMF sensor extremely attractive for multiple applications, since the sensor detects high-bandwidth information, while suppressing low bandwidth noise. 
     Photo-EMF sensors known in the art may comprise a plurality of interlaced electrode pairs, e.g., interdigitated contacts. See, for example, U.S. Pat. No. 6,342,721, issued Jan. 29, 2002 to Nolte et al. See also Nolte et al., “Enhanced Responsivity of Non-Steady-State Photoinduced Electromotive Force Sensors Using Asymmetric Interdigitated Contacts,” Optics Letters, vol. 24, no. 5, March 1999, pp. 342-344. These references disclose the use of multiple contacts with alternating wide and narrow active-area spacings, with the currents from the wide-spaced active area regions summed, while the narrow areas are optically blocked or rendered insensitive. The use of multiple contacts helps improve the response of the photo-EMF sensor, while using narrow-active areas that are optically blocked or rendered insensitive helps decrease the back action current that may arise from proximate electrodes. 
     However, in photo-EMF sensors known in the art, even those using multiple electrodes, the photo-EMF sensor may not always track out (or, adapt to) all the undesirable noise, owing to photovoltaic (i.e., band bending) effects at or near the electrodes. Those skilled in the art recognize that a band-bending region exists as a small region near a metal contact on semiconducting material, which has an electric field that emerges from the contact and goes into an active area. This band-bending region arises because the Fermi energy of the metal pins at a surface energy different than a Fermi level of the semiconducting material. The electric field may exist over a limited distance from the contact, which may be in the range of several microns from the contact. The band-bending region and the photovoltaic effect are well known to those skilled in the art of conventional semiconductors and photodetectors. 
     However, in photo-EMF sensors known in the art, the response of the photo-EMF sensor to an optical pattern that travels through the band-bending region is different than the response of the sensor as the optical pattern travels through other portions of the active region or regions of the sensor. That is, as the optical pattern moves through the electric field gradient due to the photovoltaic effect, a measurable current is generated, even at low frequencies of the lateral motion of the pattern. This effect may result in large-voltage output level variations from the presence of dynamic and spatial amplitude changes in the optical intensity near the electrodes, whose time scales can range from DC (a fixed pattern feature near an electrode) to the upper limit of the detection bandwidth. As such, these noise sources are not adaptively compensated. This can result in significant saturation of subsequent high-gain amplifiers, as well as a degradation in the system dynamic range. Simply AC coupling the sensor to the amplifiers will not eliminate this problem, especially in the case of signals whose bandwidths are similar to the photovoltaic induced noise. 
     SUMMARY 
     Embodiments of the present invention provide a method and apparatus for reducing the noise effects caused by an interaction of the optical pattern incident on the surface of a photo-EMF sensor and the electrodes used to couple current from the sensor. Embodiments of the present invention may use selected materials and/or selected shapes for the electrodes to reduce the photovoltaic effect arising from the electrodes. Preferably, the Fermi energy of the metal of the electrodes roughly matches up with the Fermi level of the semiconducting material in the vicinity of the electrodes. Embodiments of the present invention may use spatial and/or temporal averaging of the photovoltaic effect arising from each electrode to result in an overall lessening of the photovoltaic effect. The spatial and/or temporal averaging may be accomplished by choosing desired shapes, sizes and/or spacing for electrodes. The spatial and/or temporal averaging may also be accomplished by patterning or rotating the light incident on the sensor. 
     One embodiment of the present invention comprises a photoinduced electromotive force sensor receiving incident light, where the sensor comprises: an active substrate formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective charge grating, and at least one pair of electrodes disposed on the active substrate, where the photo-EMF sensor is configured to reduce the photovoltaic effect caused by the incident light in the vicinity of the electrodes. 
     Another embodiment of the present invention comprises a method of making a photoinduced electromotive force sensor comprising the steps of providing an active substrate; fabricating at least one pair of electrodes on the substrate; and configuring the sensor to reduce the photovoltaic effect caused by incident light in the vicinity of the electrodes. 
     Still another embodiment of the present invention comprises a photoinduced electromotive force sensor apparatus comprising; an active substrate formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective charge grating; and means for sensing a time-varying photocurrent in response to a shift of an optical pattern across a surface of the active substrate, where the means for sensing is configured to reduce the photovoltaic effect caused by light incident on the surface of said active substrate. 
     Still another embodiment of the present invention comprises a photoinduced electromotive force sensor comprising: a substrate formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective space charge grating; and a plurality of interlaced electrode pairs disposed over the substrate, each electrode pair including two parallel electrodes defining an active area therebetween with an active area width for each electrode pair, one electrode of each pair being disposed between an adjacent pair of electrodes and proximate one electrode of the adjacent pair; wherein the active area width for each electrode pair varies from the active area width for an adjacent electrode pair. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (prior art) shows a conventional photo-EMF sensor. 
         FIG. 2  shows the frequency response provided by various embodiments of the present invention. 
         FIG. 3  shows an embodiment of the present invention using selective ion implantation near the electrodes of a photo-EMF sensor. 
         FIG. 4A  (prior art) shows a prior art photo-EMF sensor having interference pattern lines parallel to the electrodes of the sensor. 
         FIG. 4B  shows a photo-EMF sensor according to an embodiment of the present invention having interference pattern lines rotated in relation to the electrodes of the sensor. 
         FIG. 5  shows a photo-EMF sensor according to an embodiment of the present invention having jagged electrodes. 
         FIG. 6  shows a photo-EMF sensor according to an embodiment of the present invention having irregular interference pattern lines. 
         FIG. 7A  (prior art) shows a photo-EMF sensor with two pairs of electrodes with equal spacing between the electrodes. 
         FIG. 7B  (prior art) shows a photo-EMF sensor with three pairs of electrodes with equal spacing between the electrodes. 
         FIG. 8  shows a photo-EMF sensor according to an embodiment of the present invention with staggered spacing between the electrodes. 
         FIG. 9  illustrates an interferometer system in which a distorting apparatus is used to distort light incident on a photo-EMF sensor. 
         FIGS. 10A-10D  illustrate various embodiments of the present invention in which sets of electrodes are laid out in an array to provide a photo-EMF sensor. 
         FIG. 11  illustrates an embodiment of the present invention in which proton implantation is used beneath the metal contacts of a photo-EMF sensor. 
         FIGS. 11A-11C  show process steps used to fabricate the photo-EMF sensor depicted in  FIG. 11 . 
         FIG. 12  illustrates an embodiment of the present invention in which an ion-implanted n-type region is disposed beneath the metal contacts of a photo-EMF sensor. 
         FIG. 12A  shows a close-up view of the regions around the ion-implanted n-type region shown in  FIG. 12 . 
         FIG. 13  illustrates an embodiment of the present invention in which the metal contacts of a photo-EMF sensor are disposed in an alternative configuration. 
         FIG. 14  illustrates an embodiment of the present invention in which a mask is used to direct signal light onto a photo-EMF sensor in regions away from the depletion regions around the contacts. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Further, the dimensions of electrodes and other elements shown in the accompanying drawings may be exaggerated to more clearly show details. The present invention should not be construed as being limited to the dimensional relations shown in the drawings, nor should the individual elements shown in the drawings be construed to be limited to the dimensions shown. 
     As discussed above, a significant source of noise or non-adaptive signals in a photo-EMF sensor may result from the photovoltaic effects at or near the electrodes used in the sensor. Embodiments of the present invention reduce the photovoltaic effect as described below. 
     A first approach for reducing the photovoltaic effect according to embodiments of the present invention is to select an appropriate contact material that provides a smaller band-bending (i.e., depletion) region. Some metals or metal alloys are known in the art to provide smaller band-bending regions, but these metals or metal alloys may also reduce the sensitivity of a photo-EMF sensor in which these metals or metal alloys are used in the electrodes. Hence, a tradeoff may be required between the reduction of the nonadaptive component caused by the photovoltaic effect and the overall sensitivity of the sensor. The tradeoff may depend upon the frequencies which are to be detected by the sensor, since the nonadaptive component primarily appears at low frequencies and sensitivity of the sensor generally rolls off at higher frequencies. 
     Conventional photo-EMF sensors typically use Titanium/Gold (Ti/Au) electrodes, since Ti/Au contacts are typical in the art of ohmic contacts used in the semiconductor industry. However, contacts comprising Ti/Au generally create a relatively large band-bending region that results in a relatively large non-adaptive photo-EMF noise signal. This non-adaptive problem is especially pronounced at low frequencies. See, for example,  FIG. 2  which shows the frequency response (see triangles labeled as  201 ) of a conventional photo-EMF sensor using Ti/Au contacts. Specifically, note that the frequency response  201  is essentially flat at lower frequencies (less than 1000 Hz). 
     A first embodiment according to the present invention is provided by forming the electrodes of the photo-EMF sensor using a gold-germanium (Au/Ge) eutectic, which is then annealed, or forming the electrodes using platinum (Pt). Au/Ge is known to produce more ohmic behavior and Au/Ge contacts will have a smaller depletion region with less band bending than the Ti/Au contacts used with prior art photo-EMF sensors. Pt demonstrates ohmic behavior similar to that of Au/Ge. Therefore, in the photo-EMF sensor  100  depicted in  FIG. 1 , the surface electrodes  120  may comprise a Au/Ge eutectic or Pt, rather than a gold/titanium alloy. Note, however, that the formation of the electrodes using a AuGe eutectic or Pt must be compatible with the sensor materials and fabrication. Photo-EMF sensor electrodes formed in this manner have been shown to significantly minimize the non-adaptive component in the sensor output. Returning to  FIG. 2 , the measured frequency response of a photo-EMF sensor using Au/Ge electrodes is shown by squares designated as element  202 . Note that the photo-EMF device having conventional electrodes has an undesirable (nonadaptive) “noise floor” at low frequencies (see triangles labeled  201 ), whereas the device with Au/Ge electrodes shows a desired linear relationship between frequency and voltage output, essentially free of spurious nonadaptive response. Other embodiments of the present invention may use other materials for contacts to provide a smaller band-bending region. 
     As indicated above, the use of materials for contacts that provide smaller band-bending regions may also result in decreased sensitivity for the photo-EMF sensor. For example, the use of either Au/Ge or Pt ohmic contacts may result in a photo-EMF sensor that has a sensitivity that is 2 to 5 times less than that of a sensor using Ti/Au contacts. Hence, a photo-EMF sensor using Au/Ge or Pt ohmic contacts may be less desirable than one using Ti/Au contacts, since it may be less sensitive to a given fringe motion amplitude. However, if minimizing the nonadaptive component is desired, a photo-EMF sensor according to the present invention using Au/Ge, Pt, or similar material for the ohmic contacts may be more desirable than one using Ti/Au contacts. Hence, according to some embodiments of the present invention, selection of an appropriate material for the photo-EMF sensor contacts provides for sensors with the desired response in regard to the nonadaptive components. 
     As discussed above, according to some embodiments of the present invention, one approach to reducing the photovoltaic effect due to the contacts is by selection of appropriate material for the contacts. According to other embodiments of the present invention, another approach is to modify the semiconducting material in a region around the contacts to decrease the band bending around the contacts. For example, another embodiment according to the present invention uses selective ion implantation in a small region, preferably 1-2 microns, near the electrodes in the active area of a photo-EMF sensor.  FIG. 3  depicts a photo-EMF sensor  300  comprising a substrate  310  and electrodes  320 . As shown in  FIG. 3 , the photo-EMF sensor  300  further comprises implantation regions  321  located in the vicinity of the electrodes. Localized ion implantation modifies the band bending near the electrodes  320 , which minimizes the non-adaptive noise source. 
       FIG. 11  shows another embodiment of a photo-EMF sensor  330  according to the present invention. In  FIG. 11 , proton-implanted contact regions  335  in a substrate  331  are disposed beneath metal contacts  333 .  FIGS. 11A-11C  show the steps that may be used to fabricate the sensor  330  depicted in  FIG. 11 .  FIG. 11A  shows the deposition of thin layers of gold  334  (a few monolayers) above the contact regions  335 .  FIG. 11B  shows the implantation of the contact regions with protons  336  that pass through the thin layers of gold  334 . This helps to pin the Fermi level of the substrate in the contact regions at the Fermi level of the gold. Finally, as shown in  FIG. 11C , thick layers of gold  337  are applied on top of the thin layers  334  to form the metal contacts  333 . Formation of the metal contacts  333  in this manner should help to reduce band-bending. 
     The non-adaptive signals of a photo-EMF sensor may also be reduced by greater contact homogeneity. Greater contact homogeneity should make the non-adaptive signal less susceptible to speckle.  FIG. 12  shows an embodiment of a photo-EMF sensor according to the present invention that provides more homogeneous contacts. As shown in  FIG. 12 , the photo-EMF sensor  340  comprises a substrate  331 , contacts  333  and contact regions  345 . The contact regions  345  may be formed in a fashion similar to that described above for  FIG. 11 . Thin layers of the contact material are disposed on the substrate  345  and the contact regions  345  are ion implanted through the thin layers with doping ions. After ion implanting, thick layers of the contact material are applied to the thin layers to form the contacts  333 . The contact regions  345  preferably comprise ion-implanted n-type regions (preferably implanted Si in GaAs or implanted P in Si). 
     The band-bending of the sensor  340  depicted in  FIG. 12  may be worse than the other embodiments described above, but the strength of the nonadaptive photovoltaic effect depends on the volume of the depletion (band-bending) region.  FIG. 12A  shows the depletion region  347  around one of the contacts  333  of the sensor  340  depicted in  FIG. 12 .  FIG. 12A  shows that the volumes of the depletion region  347  may be small, even though the amount of band-bending provided in the depletion region may be quite large. As shown in  FIG. 12A , the contact  333  also provides a shadow region  349 , which shields much of the depletion region  347  from light incident on the sensor  340 . Hence, the only contribution to the non-adaptive signal from the sensor  340  would be from light incident on that small portion of the depletion region  347  not under the metal contact  333 . 
       FIGS. 3 ,  11  and  12  show photo-EMF sensors with the contacts on top of the substrate in a planar arrangement. However, other embodiments according to the present invention may use other contact geometries. For example,  FIG. 13  shows an embodiment of a photo-EMF sensor  350  according to the present invention in which contacts  353  are disposed to extend across the depth of the substrate  331 . As similarly described above, implanted regions  355  (proton or doped), disposed between the substrate  331  and the metal contacts  353 , may be used as described above. Those skilled in the art will understand that contact geometries other than those specifically described and depicted may be used in accordance with the present invention. 
     The undesirable non-adaptive noise is maximized under conditions where the electrodes of the photo-EMF sensor “see” the same spatial intensity feature along their lengths. The maximization of the noise effect occurs because the photovoltaic-induced voltage level results from a given intensity at or near the electrodes, which can either be fixed in time, or dynamically varying. Given that the optical pattern incident on a photo-EMF sensor is typically in the form of an interference pattern, lines of uniform intensity (constructive interference) are typically parallel to the surface electrodes.  FIG. 4A  depicts interference pattern fringes  190  parallel to the surface electrodes  120  on the active substrate  110  of the photo-EMF sensor  100 . Given this symmetric situation, the same intensity may be present parallel to and along the entire length of the electrode, thus maximizing the non-adaptive noise effect. 
     Therefore, if the electrodes and the optical pattern are configured such that the symmetry described above is avoided, the non-adaptive noise effect should be reduced. That is, if the photovoltaic effect can be spatially or temporally “averaged out,” the non-adaptive noise effect should be minimized. Discussed below are several approaches according to embodiments of the present invention that may be used alone or combined to achieve the desired minimization of the non-adaptive noise effect. 
     In some embodiments according to the present invention, it is preferable that the intensity of the light in the vicinity of the electrodes be non-uniform to provide for the desired spatial averaging of the photovoltaic effect. Note that a speckle light pattern provides for non-uniform intensity, since the speckle spots are randomly distributed near the electrodes, resulting in a spatially averaging effect. However, photo-EMF sensors, as described above, are often deployed to detect motion in parallel interference fringe patterns. 
     The interference pattern fringes may be rotated in relation to the parallel electrodes of a photo-EMF sensor to vary the intensity along the surface electrodes.  FIG. 4B  depicts rotated interference pattern fringes  191  incident on the active substrate  110 . Since the interference pattern fringes  191  are not parallel to the surface electrodes  120 , the light intensity varies along the surface electrodes  120 , resulting in reduced non-adaptive noise. However, rotation of the interference pattern fringes such that they are not parallel to the surface electrodes  120  will reduce the current output of the photo-EMF sensor, and thus result in a decreased signal-to-noise ratio. Further, tilting the interference pattern fringes may result in some periodicity to the noise, since the fringe pattern may be periodic. 
     Another embodiment of a photo-EMF sensor, according to the present invention, uses serrated or jagged electrodes. By fabricating the electrodes so that they are jagged, different locations along the electrode will experience different intensity levels (in the case of a uniform optical fringe parallel to the electrode). Therefore, the photovoltaic effect will be spatially averaged out by the serrated pattern on the electrode.  FIG. 5  shows a photo-EMF sensor  500  comprising an active substrate  510  and a pair of serrated electrodes  520 .  FIG. 5  also depicts lines of parallel interference fringes  590 , showing that the serration of the electrodes  520  provides that different locations of the electrodes will experience different optical intensity levels. Preferably, the electrodes  520  are fabricated such that the amplitudes of the jagged edges are greater than the optical fringe spacing, so that the desired averaging is achieved. Returning to  FIG. 2 , the results of measurements of a photo-EMF sensor output using electrodes with jagged features are shown by the circle elements  203 . Again, a near linear response is shown to occur over the frequencies of interest. 
     As discussed above, providing non-uniform intensity near the electrodes reduces the non-adaptive noise. In an alternative embodiment of the present invention, the optical fringe pattern itself is intentionally made irregular or jagged (spatially) so as to achieve a similar effect as the serrated electrode pattern discussed above. Since the photovoltaic effect results from a local field gradient, the spatial averaging (i.e., elimination) of this spurious source can be either accomplished by fabricating a jagged pattern on an electrode edge to provide the desired voltage averaging (as above) for an incident parallel optical fringe or by modifying the optical fringe lines of constructive interference as to be jagged, so that illumination of this jagged optical line in the presence of a straight and parallel electrode can realize the same effect of averaging. 
       FIG. 6  shows an embodiment of a photo-EMF sensor  600  according to the present invention receiving an irregular optical fringe pattern  690 . In  FIG. 6 , a pair of electrodes  620  is disposed on an active substrate  610 . As discussed above, the optical fringe pattern  690  is made irregular, so that the intensity of the optical fringe pattern  690  varies across the electrodes  620 . 
     The interference pattern incident on a photo-EMF sensor may be obtained by illuminating the sensor with two laser beams: a reference beam and a probe beam, which has the desired time dependent phase shifting signal to be sensed. The reference beam is typically a plane wave (no phase distortions), while the probe beam is typically distorted. In an interferometer system, the distortion may be due to having been scattered from a rough-cut work-piece. In a communication system, the distortion may be due to propagation through a turbulent atmosphere. Therefore, in the case of minor phase aberrations, the optical interference pattern is basically parallel with some small spatial features. 
     In a system in which the photo-EMF sensor is illuminated by the interference pattern created by two beams, either one beam, the other, or both may be intentionally aberrated so that, at any time, there will be bright as well as dark regions that strike the electrodes of the photo-EMF sensor. In such a case, the electrodes of the photo-EMF sensors may be straight electrodes, because the irregular nature of the interference pattern provides the desired averaging of the nonadaptive component.  FIG. 9  illustrates an interferometer system in which a distorting apparatus  950  is used to distort a reference beam  910  to create a distorted reference beam  915 . The distorted reference beam  915  typically arrives at a probe beam  920  at an angle from 1 to 10 degrees. The beams  915 ,  920  then combine to create an interference pattern  940  that illuminates the photo-EMF sensor  100 . In alternative embodiments, the distorting apparatus  950  may be placed in the path of the probe beam  920  or both beams  910 ,  920 . 
     The distorting apparatus  950  may comprise a fixed spatial phase mask. The mask may be in the form of a computer generated glass plate, a predistorted mirror surface, a randomly distorted chemically etched plate, or other such apparatus or methods known in the art. Alternatively, the distorting apparatus  950  may comprise a multi-mode fiber that provides a highly speckled interference pattern. The predistortion of either or both beams should result in an irregular interference pattern at the sensor. Therefore, even in the presence of a set of perfectly straight and parallel electrodes, a spatial averaging of the photovoltaic-induced voltage levels will occur along the length of the electrodes, resulting in the desired elimination of the non-adaptive noise term. 
     The embodiment of the present invention using predistorted optical fringe patterns may be preferred over the serrated electrode discussed above, since high electric fields may occur at the tips of the jagged structure, which may be undesirable. Also, since the electrode pattern is fixed, changes in the jagged features cannot be easily made, if there are materials variations, etc. Also, the jagged electrode pattern may not be as mechanically stable (e.g., liftoff may occur) as a symmetric, parallel structure. 
     As discussed above, the non-adaptive signal from a photo-EMF sensor arises due to the presence of interference fringes in the depletion (i.e., band-bending) regions near the contacts. Therefore, the non-adaptive signal may be reduced by eliminating the presence of the interference fringes in the depletion regions.  FIG. 14  depicts an embodiment of a photo-EMF sensor according to the present invention, in which a light beam carrying a signal is limited to an area of the photo-EMF sensor away from the depletion regions. As shown in  FIG. 14 , the sensor comprises a substrate  110  and contacts  120 . However, imaging optics (i.e., a mask) are used to limit the exposure of a signal carrying beam to a portion  980  of the substrate that is spaced away from the contacts  120  by the diffusion length  975 , i.e., the width of the band-bending regions. A reference beam still illuminates the entire sensor as shown by the reference illumination area  970 . The use of a mask for the signal beam to provide that the signal beam only illuminates a portion  980  of the substrate and the provision of a reference illumination area  970  as shown in  FIG. 14  provides that there is no shadowing of any portion of the device (resulting in high-resistance regions), while also providing that the interference fringes occur away from the depletion regions. Of course, this approach may also be used with other embodiments of photo-EMF sensors according to the present invention described above and below. 
     Photo-EMF sensor having asymmetric interdigitated contacts (AIDC&#39;s) are known in the art. See, for example, U.S. Pat. No. 6,342,721 discussed above. Photo-EMF sensors with AIDCs have alternating wide and narrow active-area spacings, with the currents from the wide-spaced active area regions summed while the narrow areas are covered by an optically opaque or reflective layer to prevent backaction photo-emf.  FIGS. 7A and 7B  depict two-pair and three-pair electrode embodiments, respectively, of photo-EMF sensors using the AIDC design. In both  FIG. 7A  and  FIG. 7B , wide surface electrodes  720 ,  722  are electrically connected to narrow surface electrodes  721 ,  723  on an active substrate  710 . An opaque layer  740  is used to block the illumination between the narrow surface electrodes to prevent backaction photo-EMF. In the two-pair electrode sensor depicted in  FIG. 7A , the active region spacing has a width of W/2, where W is the width of the active region of a single pair photo-EMF sensor providing the same amount of photo emf current. In the three-pair sensor depicted in  FIG. 7B , the width of the active regions between the electrodes is W/3. 
     In photo-EMF sensors using the AIDC design, the function of the AIDC electrode pattern is to provide photo-emf current summation at each fringe of a multi-fringe pattern, whose spacing is typically on the order of a carrier diffusion length (about 60 μm for GaAs). Generally, photo-EMF sensors fabricated using multiple electrode pairs will demonstrate an improvement in performance roughly equal to the number of electrode pairs used. For example, a 32 electrode pair photo-EMF sensor has demonstrated a factor of 32 times improvement in the detector responsivity over a single pair sensor. However, such AIDC photo-EMF sensors have also demonstrated the same “non adaptive” noise source as has been shown in single electrode pair devices. The spurious signal (noise source) tends to be prominent when the optical fringe pattern matches the spatial period (or a spatial harmonic thereof). Thus, when a Moire-like interference pattern begins to appear, the spurious signal becomes dominant. 
     Reduction of the non-adaptive noise in photo-EMF sensors using the AIDC design may be obtained by using the techniques described above. That is, the AIDC electrodes may be fabricated from materials selected to reduce the photovoltaic effect and/or the material around the electrodes may be treated to reduce the photovoltaic effect. The AIDC electrodes may also be serrated and/or the optical fringe pattern may be serrated. However, finely spaced electrode patterns may be difficult to serrate sufficiently (physically or optically) for the desired averaging effect to take place. 
     In another embodiment of the present invention, the electrode spacings on the active substrate in a photo-EMF sensor having AIDC electrodes are staggered. That is, even though all the electrodes are straight and parallel, the interdigitated electrodes are formed so that the spacing of the active regions is not the same for all electrode pairs. This may be achieved, for example, by varying the widths of the active regions between the electrodes, by varying the gaps of the back-action regions, or by varying the width of the electrodes themselves. The staggering of the active regions can either be simply random (over a given range of widths), chirped, made Gaussian, or using a well-defined sequence (a Fibbinochi set, as an example).  FIG. 8  shows a photo-EMF sensor  800 , with an incident interference fringe pattern  890 , comprising a pair of wide surface electrodes  820 ,  822 , a plurality of narrow surface electrodes  821 ,  823  and an active substrate  810  with active regions  815  having random widths, w 1 , w 2 , w 3 , w 4 , w 5 . 
     Alternative embodiments of the present invention comprise an array of short-length, yet parallel AIDC electrodes are that arranged in sets of rows and columns, so that a reasonable electrode aspect ratio (length to width) can be maintained, as well as for graceful degradation, in the case of an electrically open or shorted AIDC column.  FIGS. 10A-10D  illustrate various embodiments of a sensor  1000  comprising sets of AIDC electrodes  1050  arranged in a two-dimensional array.  FIG. 10A  illustrates a sensor  1000  with sets of AIDC electrodes  1050 , which may be spaced internally in a varying pattern as described above, that are laid out in a regular pattern.  FIG. 10B  illustrates a sensor  1000  where each AIDC electrode set  1050  may be spatially staggered with respect to adjacent AIDC electrode sets  1050 . This combined set of staggered electrodes (rows and columns) will tend to provide additional spatial averaging across the entire area of the sensor. Of course, the electrode sets  1050  may also only be staggered in a row-wise sense or a column-wise sense. 
     Alternative embodiments of the present invention may also comprise sets of short length but parallel AIDC electrodes where the orientation of the electrodes in each set are rotated with respect to the electrodes in another set.  FIG. 10C  illustrates a sensor  1000  with sets of AIDC electrodes  1050 , which may be spaced internally in a varying pattern, that are laid out in a regular pattern, but the adjacent sets are rotated 90 degrees with respect to each other. Finally,  FIG. 10D  illustrates a sensor  1000  with sets of AIDC electrodes  1050  that are tilted differently with respect to each other (over small angles, for example, ˜10°), either within a AIDC column and/or across the AIDC rows, to achieve yet additional spatial averaging of the photovoltaic-induced voltage level. In sensors using the arrays as discussed above, a small loss of the desired signal may result due to geometrical projection effects of the desired fringe motion relative to the AIDC grating vector direction. 
     In a preferred embodiment of the present invention, randomly spaced AIDC electrodes may be used in a photo-EMF sensor, where the electrodes comprise Ti/Au material. In such a sensor, the aperiodic electrode spacing would tend to spatially average out the nonadaptive component, while the use of Ti/Au for the electrodes would provide increased sensitivity for the sensor. 
     From the foregoing description, it will be apparent that the present invention has a number of advantages, some of which have been described herein, and others of which are inherent in the embodiments of the invention described herein. Also, it will be understood that modifications can be made to the apparatus and method described above without departing from the teachings of subject matter described herein. As such, the invention is not to be limited to the described embodiments except as required by the appended claims.