Abstract:
A system for measuring a shape of a target object includes a photonic integrated circuit and a light detector. The photonic integrated circuit includes a phase shifter configured to change a phase difference between a first portion of light and a second portion of light within the phase shifter, and an output element configured to output the light from the phase shifter directly toward the target object. The output element includes a first output waveguide configured to act as a first point source; and a second output waveguide configured to act as a second point source. The light detector is positioned to receive reflected light from the target object.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/216,230, filed May 14, 2009, and entitled “Micro-chip fringe projectors used for 3D imaging system;” U.S. Provisional Application No. 61/269,654, filed Jun. 26, 2009, and entitled “Micro-chip fringe projectors comprising multiple phase shifters for 3D imaging system;” and U.S. Provisional Application No. 61/270,821, filed Jul. 14, 2009, and entitled “Micro-chip fringe projector using delay line interferometric phase shifter for 3D imaging system,” the contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Non-contact shape measurement of objects is of great interest in many areas of technology, medicine, and art. In industrial applications, accurately examining the shape of machine parts and tolerances is of great importance. Recently, dentists have used non-contact shape measurement devices to image and document the three-dimensional (3D) shape of the teeth. Documentation of antique artifacts is another application for 3D shape measurement systems. 
     Many devices and methods have been developed for precision of 3D measurements. Among these, methods based on structured light projectors have attracted the most attention. Structured light techniques are considered to be one of the most effective, reliable, and robust optical non-contact methods for 3D surface height measurement. A common structured light projector is a fringe projector that is composed of lines with different intensities. The fringes can be generated by digital light processing (DLP), by a slide inserted in a light projector, or by two coherent light spots located a short distance apart (such as in Young&#39;s double slit experiment). Fringe projection based on Young&#39;s double slit experiment has the advantage of having infinite depth of focus with infinite spatial resolution, making it attractive for 3D imaging systems. 
     Several devices, methods and algorithms have been developed as the bases of 3D shape measurements systems using fringes. For example, fringe phases are shifted and imaged by an imaging sensor, such as a CCD or CMOS camera, and the phase on the surface is measured. This method is known as phase shifting interferometry (PSI). 
     A 2π ambiguity exists in phase determination using PSI methods and additional efforts are needed to resolve this ambiguity. Methods for resolving phase ambiguity resulting from PSI are known as phase unwrapping methods. Over the years several methods and algorithms have been developed to solve phase unwrapping problems. For instance, the use of more than one fringe projector, each with a different fringe frequency, can unambiguously resolve the phase wrapping problem. 
     SUMMARY 
     In a general aspect, a system for measuring a shape of a target object includes a photonic integrated circuit and a light detector. The photonic integrated circuit includes a phase shifter configured to accept input light and to emit a plurality of portions of light, each portion of light having a different phase, and an output element configured to output the light emitted from the phase shifter directly toward the target object. The output element includes a first output waveguide configured to act as a first point source; and a second output waveguide configured to act as a second point source. The light detector is positioned to receive reflected light from the target object. 
     Embodiments may include one or more of the following. The phase shifter includes an input splitter configured to divide the input light into at least a first portion of light and a second portion of light; a first phase shifter waveguide configured to receive the first portion of the light and to change the phase of the first portion of the light; and a second phase shifter waveguide configured to receive a second portion of the light. The first output waveguide is configured to output the first portion of the light and the second output waveguide is configured to output the second portion of the light. The first portion of the light and the second portion of the light are coherent. 
     The photonic integrated circuit comprises a plurality of phase shifters, each phase shifter configured to accept input light and to emit a plurality of portions of light, each portion of light having a different phase. 
     The output element is configured to output the light emitted from each of the plurality of phase shifters. The photonic integrated circuit further comprises a combiner configured to combine the light emitted from the plurality of phase shifters and to provide the combined light to the output element. 
     The photonic integrated circuit further comprises a plurality of output elements, each output element configured to output the light emitted from at least one of the plurality of phase shifters. That is, in some examples, multiple phase shifters may be associated with each output element. A separation between the first output waveguide and the second output waveguide is the same for each output element. A separation between the first output waveguide and the second output waveguide is different for each output element. 
     The plurality of first output waveguides are positioned in a first region of the photonic integrated circuit and the plurality of second output waveguides are positioned in a second region of the photonic integrated circuit. 
     Each of the plurality of phase shifters is associated with at least one of light at a different wavelength and light of a different polarization. 
     The photonic integrated circuit further includes an output delivering system configured to deliver the light emitted from the phase shifter to the output element, the output delivering system comprising at least one of an integrated circuit based optical beam splitter, an integrated circuit based optical combiner, an integrated circuit based optical beam attenuator, and an integrated circuit based on-off switch. 
     The photonic integrated circuit includes a first integrated circuit including the phase shifter; and a second integrated circuit including the output element. The system further includes an output delivering system configured to deliver the light emitted from of the phase shifter to the output element, the output delivering system comprising at least one of an optical lens, an optical beam splitter, an optical combiner, an optical beam attenuator, and an on-off switch. 
     The photonic integrated circuit further includes a light source configured to provide coherent input light to the phase shifter. The light source includes at least one of a laser, a coherent light emitting diode (LED), and a superluminescent LED. The photonic integrated circuit further comprises an input delivering system configured to deliver light from the light source to the phase shifter, the input delivering system comprising at least one of an integrated circuit based optical beam splitter, an integrated circuit based optical combiner, an integrated circuit based optical beam attenuator, and an integrated circuit based on-off switch. 
     The system further includes a light source configured to provide coherent input light to the photonic integrated circuit. The light source includes at least one of a laser, a coherent LED, and a superluminescent LED. The system further includes an input delivering system configured to deliver light from the light source to the phase shifter, the input delivering system comprising at least one of an optical lens, an optical beam splitter, an optical combiner, an optical beam attenuator, and an on-off switch. In some examples, the input delivering system may be located on the photonic integrated circuit, off the photonic integrated circuit, or partially on and partially off the photonic integrated circuit. 
     The phase shifter is configured to change the phase of at least one of the plurality of portions of light by at least one of an electro-optic effect, a thermo-optic effect, and an acoustic-optic effect. 
     The system further includes a processor configured to determine a shape of at least a portion of the target object on the basis of the detected reflected light. 
     In another general aspect, a method for determining a shape of a target object includes receiving input light into a phase shifter fabricated on a photonic integrated circuit; emitting a plurality of portions of light from the phase shifter, each portion of light having a different phase; receiving the plurality of portions of light emitted from the phase shifter an output element fabricated on the photonic integrated circuit; outputting the light from the output element directly toward the target object; and detecting light reflected from the target object. 
     Embodiments may include one or more of the following. 
     Receiving the input light into the phase shifter includes directing a first portion of the input light into a first phase shifter waveguide; and directing a second portion of the input light into a second phase shifter waveguide. The first phase shifter waveguide is configured to change the phase of the first portion of the input light relative to the phase of the second portion of the input light. 
     The method further includes applying an electric power to at least a part of the phase shifter. 
     Receiving input light into the phase shifter includes receiving input light into at least some of a plurality of phase shifters fabricated on the photonic integrated circuit. Receiving input light into the phase shifter includes selecting at least one of the plurality of phase shifters to change the phase of a first portion of light emitted by the selected phase shifter relative to a second portion of the light emitted by the selected phase shifter. 
     Receiving the plurality of portions of light emitted from the phase shifter into the output element includes receiving the light emitted from at least some of the plurality of phase shifters into the output element. Receiving the plurality of portions of light emitted from the phase shifter into the output element includes receiving the light emitted from each of at least some of the plurality of phase shifters into a corresponding one of a plurality of output elements. 
     Receiving input light into the plurality of phase shifters includes receiving input light having at least one of a different wavelength and a different polarization into each of the plurality of phase shifters. 
     The method further includes controlling at least some of the plurality of phase shifters to generate a plurality of consecutive phase changes in the received input light. In some instances, at least some of the plurality of phase shifters are static phase shifters and the method further includes controlling at least some of the plurality of phase shifters to generate a predetermined phase change in the received input light. The method further includes determining a shape of at least a portion of the target object on the basis of the detected light. 
     Among other advantages, the systems and methods described herein are able to shift the phase of light using a system-on-chip (SoC) solution. Using a micro-chip based fringe projector, PSI techniques together with standard phase unwrapping algorithms can be performed. Using an image sensor, the fringes from PSI can be captured and phases can be unwrapped to render 3D shape measurements of objects. 
     Fringes with infinite depth of focus can be generated, enabling high precision shape measurements. The fringe generation system is stable, not complex, and relatively inexpensive. Furthermore, little to no calibration is needed. 
     Fringe projection at multiple phase settings can be simplified by the use of a plurality of phase shifters on a single chip because phase feedback control is allowed to control only the stability of the setpoint with no concern to ramping processes. 
     Other features and advantages of the invention are apparent from the following description and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A-1D  are block diagrams of a 3D imaging system. 
         FIG. 2  is a block diagram of a 3D imaging system. 
         FIG. 3  is a schematic diagram of a fringe projector. 
         FIGS. 4A and 4B  are schematic diagrams of waveguides. 
         FIGS. 5A and 5B  are schematic diagrams of heating configurations for waveguides. 
         FIG. 6  is a schematic diagram of a delay line fringe projector. 
         FIG. 7  is a block diagram of a 3D imaging system. 
         FIG. 8  is a flow chart for 3D imaging. 
         FIG. 9  is a block diagram of a fringe projector including multiple phase shifters. 
         FIGS. 10A-10D  are schematic diagrams of light source distributors. 
         FIGS. 11A-11F  are schematic diagrams of light path combiners. 
         FIG. 12  is a schematic diagram of a fringe projector chip including multiple phase shifters. 
         FIG. 13  is a schematic diagram of a fringe projector chip including multiple phase shifters. 
         FIG. 14  is a schematic diagram of a fringe projector chip including multiple phase shifters. 
         FIG. 15  is a schematic diagram of a fringe projector chip for fringe projection of multiple wavelengths. 
         FIG. 16  is a schematic diagram of a fringe projector chip for fringe projection at multiple polarizations. 
         FIG. 17  is a schematic diagram of a fringe projector chip including multiple phase shifters. 
         FIG. 18  is a schematic diagram of a pair of integrated circuits for fringe projection using multiple phase shifters. 
     
    
    
     DETAILED DESCRIPTION 
     1 Three-Dimensional Shape Measurement 
     Referring to  FIG. 1A , a three-dimensional (3D) shape measurement apparatus  100  is used to measure the 3D surface shape or surface point coordinates of a target object  105 . Measurement apparatus  100  employs a Young&#39;s coherent light pattern projector  101  which projects a mathematically known interference light pattern or fringe pattern  106  onto the surface of object  105 . Due to the 3D nature of the contour of the surface of object  105 , fringe pattern  106  is distorted according to the local curvature of the object when observed by an imaging sensor  102 . In order to observe the distortion of the fringe pattern, the angle between the pattern projector  101  and the object  105  is different from the angle between the imaging sensor  102  and the object. A control and process unit  104  is used to control and synchronize the light pattern or fringe pattern generated by light pattern projector  101  with the detection by imaging sensor  102 . Control and process unit  104  is also used to collect and process the distorted fringe data received by imaging sensor  102 , for instance by applying a mathematical model to calculate the 3D coordinates (x, y, z) of any point (e.g., point O 1 ) on the surface of object  105 . 
     In some embodiments, light pattern projector  101  includes a single pair of point sources S 1  and S 2 . In the context of this description, a point source is a light source for which the size of the light intensity at the location of the light source is small compared to the distance between the light pattern projector  101  and the object  105 . For instance, the size (e.g., the full width at half max, FWHM) of the light intensity at the point source is about 0.1-10 μm (or 0.1λ-10λ, where λ is the wavelength of the light), and/or at least 100 times less than the distance between light pattern projector  101  and object  105 . Point sources S 1  and S 2  are separated by a fixed distance a, which is, for instance, in the range of 5-500 μm (or 5λ-500λ), or any other value that is relevant to the measurement criteria of a particular application. 
     Point source S 1  generates a first beam  111  of light and point source S 2  generates a second beam  112  of light. The two beams  111 ,  112  are mutually coherent. In some cases, the light intensity at S 1  and S 2  is equal. As was shown by Tomas Young&#39;s double slit experiment, any two coherent beams (e.g., beams  111  and  112 ) of light from two point sources (S 1  and S 2 ) interfere with each other at any given point within their divergent angle of light radiation (e.g., point O 1  on the surface of object  105 ) due to the wave nature of the light. Thus, light pattern projector  101 , which includes the point sources S 1  and S 2 , produces an interference light pattern or fringe pattern  106  on the surface of object  105 . Fringe pattern  106  has an infinite depth of focus and closely follows the Young&#39;s interference formula to infinity with almost no distortion. 
     Without losing generality, the origin of the coordinate system for measurement apparatus  100  can be set at the mid-point S 0  of the two point sources S 1 , S 2 . Thus, the point source S 1  has the coordinates (−a/2, 0, 0) and the point source S 2  has the coordinates (a/2, 0, 0), where a is the separation between S 1  and S 2 . At an arbitrary point on the surface of object  105 , the fringe phase φ, which is the phase difference between the two beams  111 ,  112 , is a function of the light wavelength λ, the separation distance a between point sources, and the 3D coordinates (x, y, z) of point O 1 . According to the Young&#39;s interference principle, the fringe phase is given as 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                       
                         
                           
                             x 
                             2 
                           
                           + 
                           
                             y 
                             2 
                           
                           + 
                           
                             z 
                             2 
                           
                         
                       
                     
                     ⁢ 
                     
                       a 
                       λ 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The profile of the light intensity I of the interfering beams  111 ,  112  on the surface of object  105  is detected by imaging sensor and used to calculate the coordinates (x, y, z) of point O 1 . Imaging sensor  102  includes, for instance, a CCD or CMOS imager  121  and a lens system  122 . The origin of the CCD or CMOS imager  121  is at C 0 , and the normal direction vector of the imager  121  is c. The scattered light from arbitrary point O 1  on the surface of object  105  is collected by lens  122  and projected on to the surface of imager  121  at a point P 1  with imager local coordinates (x′, y′). The intensity of the scattered light and the local coordinates (x′, y′) are recorded by imager  121 . Via calibration of the imager  121  and its lens system  122 , any distortion induced by lens  122  and/or imager  121  can be corrected. Thus, the corrected intensity recorded by imager  121  can be used to calculate the fringe phase at point O 1  on object  105 , and the local coordinates (x′, y′) can be used to calculate a linear projection  113  from point O 1  to point P 1 . 
     Since both light pattern projector  101  and imaging sensor  102  are rigidly constructed, and the distance vector b, which is drawn between the midpoint S 0  of point sources S 1  and S 2  to the center of the imager or the origin C 0  of the local coordinate system of the imager, is known (e.g., by calibration prior to measurement), the coordinates (x, y, z) of point O 1  on the surface of object  105  can also be calculated, provided the fringe phase φ at point O 1 , the imager normal direction vector c, and the projection vector  113  are also known. 
     Referring to  FIG. 1B , an equal-phase plane  107  is a curved plane on which each point has the same fringe phase value φ, which is caused by the light path difference between beam  111  from source S 1  and beam  112  from source S 2 . Since the point O 1  is the intersection of the equal-phase plane  107  and the projection vector  113 , the coordinates (x, y, z) of point O 1  are uniquely determined by knowing the shape of equal-phase plane  107  according to Equation (1), the projection vector  113 , the distance vector d, and the imager normal direction vector c. 
     Referring to  FIG. 1C , the fringe phase φ at point O 1  on the surface of object  105  can be calculated by knowing the intensity profile of the interfering light. In one embodiment, a phase shifting interferometry (PSI) method is used. In this case, the light intensity that is produced by pattern projector  101  at any point O 1 , with coordinates (x, y, z), on the surface of object  105 , is given as
 
 I ( x,y,z )= I ′( x,y,z )+ I ″( x,y,z )cos(φ( x,y,z )+θ),  (2)
 
where I′ is the background intensity, I″ is half of the peak-to-valley intensity modulation of fringes  106 , φ is the fringe phase, and φ is the initial phase difference between point sources S 1  and S 2 . To solve the three unknowns in Equation (2) (I′, I″, and φ), at least three phase shifts of initial phase θ are used. For instance, assuming that there are three initial phases used (θ 1 =−2π/3, θ 2 =0, and θ 3 =2π/3), then the fringe phase φ can be calculated as follows:
 
                     φ   =       tan     -   1       ⁡     (       3     ⁢         I   1     -     I   3           2   ⁢     I   2       -     I   1     -     I   3           )         ,           (   3   )               
where I 1 , I 2 , and I 3  are the intensities resulting from initial phases θ 1 , θ 2 , and θ 3 , respectively.
 
     The fringe phase calculated from Equation (3) has 2π ambiguity, and a discontinuity occurs every time φ changes by 2π. Fringe phase unwrapping is used to eliminate this ambiguity. Exemplary methods of fringe phase unwrapping include path integration phase unwrapping and spatial coherent phase unwrapping. 
     Referring to  FIG. 1D , in one embodiment, a multi-fringe-wavelength beating strategy is used for phase unwrapping by employing fringe projections with two or more fringe wavelengths. For instance, two different phase maps φ 1  and φ 2  with two different fringe wavelengths λ 1  and λ 2 , respectively, are superimposed. The superposition produces the phase-correct subtraction of these two functions, yielding the desired beat function Δφ with fringe wavelength λb: 
                     λ   b     =           λ   1     ⁢     λ   2           λ   1     -     λ   2         .             (   4   )                 FIG. 1D  shows two phase maps with initial fringe wavelengths selected to obtain an unambiguous beating function. To unwrap a large number of phase periods to cover the whole measurement field of view and/or to reduce unwrapping error, phase maps with three or more fringe wavelengths can also be used.
 
     According to Equation (1), the fringe wavelength is inversely proportional to the separation distance between the two point sources and proportional to the wavelength of the light. For a single pair Young&#39;s double-point interference light pattern projector, different fringe wavelengths can be generated using multiple light wavelengths from the same point sources. Alternatively, fringe wavelength can be changed by using multiple fringe projectors with a different separation distance between the point sources in each pair. 
     Referring to  FIG. 2 , in one embodiment of a shape measurement apparatus  200 , two or more pattern projectors  101 ,  101 ′ are used both to facilitate fringe phase unwrapping by using the multiple-fringe-wavelength scheme shown in  FIG. 1D  and, in some cases, to perform additional measurements to improve the accuracy of the system. Pattern generators  101 ,  101 ′ each project a fringe pattern  206  onto the surface of object  105 . The fringe pattern  206  is distorted when observed by imaging sensor  102 . A control and process unit  204  is used to control and synchronize the fringe pattern generated by pattern projectors  101 ,  101 ′ with the detection by imaging sensor  102  and to collect and process the distorted fringe data received by imaging sensor  102 . 
     As discussed above, pattern projector  101  includes point sources S 1  and S 2  separated by distance a. The point sources generate first and second mutually coherent beams  111 ,  112  of light that are incident on the surface of object  105  at point O 1 . The distance vector b represents the separation between the midpoint S 0  between point sources S 1 , S 2  and the center of imager  102 . Similarly, pattern projector  101 ′ includes point sources S 1 ′ and S 2 ′ separated by distance a 2 . Point sources S 1 ′ and S 2 ′ generate first and second mutually coherent beams  211 ,  212  of light that are also incident at point O 1  on the surface of object  105 . A distance vector b 2  represents the separation between the midpoint S 0 ′ between point sources S 1 ′, S 2 ′ and the center of imager  102 . A linear projection  213  can be calculated from point O 1  to point P 1  in imager  102 . 
     With two fringe wavelength pattern projectors, two triangulation measurement systems are formed with imaging sensor  102 , one system involving pattern projector  101  and a second system involving pattern projector  101 ′. Thus, a second set of 3D measurement data is generated and can be used to improve the accuracy and/or resolution of the measurements of shape measurement apparatus  200 , in addition to unwrapping fringe phase by multiple-fringe-wavelength scheme. 
     2 3D Imaging System 
     Referring to  FIG. 7 , a 3D imaging system  700  includes a fringe projector  702 , such as the fringe projector described above. A base  703  rigidly connects fringe projector  702  to a camera  701 . The base is made of a metal, such as aluminum or Invar®, or another material, such as carbon fiber. 
     Fringe projector  702  projects fringes  705  onto an object  704 . The fringes are also shown in an image  706 . Fringe projector  702  induces at least three different phase shifts (e.g., 0, 120, and 240 degrees) to enable phase shift interferometry (PSI) measurements. Camera  701  captures images of the fringes for each phase shift and saves the images for later calculations of the shape of object  704  by a data analysis module (not shown). 
     3D imaging system  700  is based on triangulation. That is, the phase of the fringes  705  on the surface of the object  704  is proportional to the angle between the fringe projector  702  and a point on the object  704 . Thus, measurement of the phase of the fringes is equivalent to determining the angle between the base  703  and a beam of light at a point on the object. Second and third triangulation angles can be determined by knowing the pixel on the camera sensor, which may be a CCD or a CMOS image sensor. Extensive calibration is preferably performed on the camera in order to correct the triangulation angles. 
     Referring to  FIG. 8 , in a 3D imaging system, the phase value of the fringe projector is established (e.g., by setting the voltage or current to an active waveguide in the fringe generator, as discussed below; step  801 ). The generated fringe phase is measured and adjusted if necessary (step  802 ). The phase value is measured (step  803 ) and compared to the desired value (step  804 ). If the phase value is incorrect, precise adjustments are performed to bring the measured value into agreement with the desired value (step  802 ). If the phase value is correct, images of the fringes are captured by the camera for multiple fringe phases produced by the projector (step  806 ). If necessary, the camera is calibrated (step  813 ) and the image is recalculated using the new camera calibration (step  808 ). The above steps are then repeated for a different fringe frequency or light wavelength (step  810 ). 
     Phase unwrapping algorithms are performed to determine the absolute phase of the fringes (step  812 ) and the triangulation angle on the camera side is calculated using the camera calibration (step  814 ). Using the results, the shape of the object can be determined (step  816 ). In some cases, another set of measurements may be performed using light with a different polarization (step  818 ), after which the shape of the object is recalculated, compared with the previously determined shape, and used to correct for any errors in the measurement (step  820 ). 
     3 On-Chip Fringe Projector 
     In general, in a microchip-based fringe projector, the pair of point sources (i.e., point sources S 1  and S 2  of  FIG. 1A ) is formed by the end facets of two optical waveguides fabricated on a wafer that is processed by micro-fabrication technology. Additionally, in general, a fringe phase shifter in a microchip-based fringe projector is also fabricated on a wafer that is processed by micro-fabrication technology. 
     Referring to  FIG. 3 , a fringe projector chip  310  includes a phase shifter  300  (including at least 3 dB splitter  303  and active waveguides  306  and  307 ) capable of generating fringes with continuously variable phase and infinite depth of focus. Fringe projector chip  310  is also able to product various fringe frequencies spaced at a different distance between two output waveguides  308  and  309 . 
     Chip  310  is formed from a wafer processed by micro-fabrication technology. For instance, the micro-fabrication technologies may include fabrication processes such as material epitaxy growth or deposition, photolithography, etching, doping, implantation, oxidation, metal deposition, sputtering, and other processes. Available micro-fabrication technology platforms include platforms such as complementary metal-oxide-silicon (CMOS) technology, BiCMOS technology, SiGe technology, GaAs/GaN/InP III-V semiconductor technology, and glass/quartz/crystal/polymer micro-fabrication technology platforms. The wafer may be, for instance, a Si wafer, a silicon-on-insulator (SOI) wafer, a silicon-on-quartz (SOQ) wafer, a GaAs wafer, an InP wafer, a GaN wafer, a sapphire wafer, a quartz wafer, a LiNbO 3  wafer, or any other type of wafer suitable for micro-fabrication. 
     3.1 Active Phase Shifter 
     Fringe projector chip  310  includes a light-to-waveguide coupler  301  that receives incident coherent light from a light source (not shown). The incident coherent light is provided by a laser, such as a semiconductor diode laser, a solid state laser, a gas laser, or a fiber laser, or a coherent LED (light emitting diode) or SLED (superluminescent light emitting diode). The wavelength of the incident light may be any wavelength that is relevant to the desired 3D shape measurement application, and may be any of visible light, infrared light, or ultraviolet light. The light passes through an input optical waveguide  302  and into 3 dB beam splitter  303 . In some embodiments, beam splitter  303  is another type of beam splitter, as dictated by design considerations. Beam splitter  303  divides the light into two beams, each with the same phase, polarization, and amplitude. Beam splitter  303  may be, for instance, a Y splitter, a multimode interference (MMI) splitter, or a directional coupler splitter. The facet of chip  310  on which the input to waveguide  302  resides may be coated with an anti-reflection (AR) coating to reduce reflection. 
     The first beam of light travels along a passive waveguide  304 , passes through active waveguide  306 , and travels along passive output waveguide  308 . The second beam of light travels along a passive waveguide  305 , passes through active waveguide  307 , and travels along passive output waveguide  309 . The light from output waveguides  308  and  309 , which are separated by a distance d, is emitted directly from chip  310  and travels through free space (or another environment) to a target object. The wafer facet from which the light is emitted may be coated with an AR coating to reduce reflection. The two waveguide outputs thus form the two point sources used for the Young&#39;s double-point fringe projector described above. 
     Each active waveguide  306 ,  307  applies a controllable phase shift to the light. In some embodiments, only a single arm generates a phase shift. For instance, only one active waveguide may be used while the other active waveguide acts as a passive waveguide. Alternatively, one active waveguide (e.g., active waveguide  307 ) is eliminated and replaced by an additional passive waveguide. In other embodiments, in a differential arm phase shift scheme, each active waveguide  306 ,  307  is used to generate a different phase shift for each light beam. 
     The phase changes in active waveguides  306 ,  307  are due to the refractive index changes of one or both active waveguides. The refractive index changes are induced by the application of controllable electric power into one (in the single arm phase shift embodiments) or both (in the differential arm phase shift embodiments) active waveguides. For instance, in a single arm phase shift scheme, an electric power is applied to active waveguide  306 , creating a refractive index change in waveguide  306  relative to the refractive index of waveguide  307  (which, in this case, acts as a passive waveguide). Thus, when a first light beam travels along the top arm of phase shifter  300  and a second light beam travels along the bottom arm, the two light beams are output from passive waveguides  308 ,  309 , respectively, with the same amplitude, the same polarization, and with their coherence retained, but with a different phase due to the difference in refractive index between active waveguides  306  and  307 . Because the distance d between the output waveguides  308  and  309  can be precisely controlled during the fabrication of phase shifter  300 , allowing a desired spatial fringe frequency to be produced, the two coherent but phase shifted light beams can be used in phase shift interferometry (PSI) and fringe projection technologies. 
     In some instances, the on-chip light-to-waveguide coupler  301  is not present. However, a well-designed coupler  301  reduces the loss of laser power upon receipt of the incident light. 
     The optical waveguides in chip  310  may be any type of waveguide, such as a channel waveguide, ridge waveguide, rib waveguide, buried waveguide, a slot waveguide, or a photonic crystal waveguide. For instance, referring to  FIG. 4A , an exemplary channel waveguide  401  is shown; a dashed line  402  shows the profile of a light beam confined within channel waveguide  401 . Alternatively, referring to  FIG. 4B , an exemplary ridge waveguide  403  is shown; a dashed line  404  shows the profile of a light beam confined within ridge waveguide  403 . 
     The core of channel waveguide  401  or ridge waveguide  403  is formed of a material that has a refractive index higher than that of the surrounding cladding materials. The waveguide core may be formed of any dielectric material that is suitable for the particular wavelength and application. The waveguide core may be crystalline or non-crystalline. For instance, the waveguide core may be formed of Si; amorphous Si; Ge; any III-V or II-VI semiconductor such as GaAs, InP, GaP, InP, InAs, GaN, AlN, InN, or any combination thereof; Si x N y , Si x O y ; Si x ON y ; Ti x O y ; or LiNbO 3  or LiTaO 3  crystals. Alternatively, the waveguide core may be made of an organic thin film polymer prepared by the mixing of nonlinear optical chromophore molecules. The waveguide core material may also be any combination of applicable dielectric materials or any combination of composition of such materials. In other embodiments, the waveguides are formed by intentional impurity doping rather than by geometric shaping. 
     The waveguide cladding may be air or any dielectric material that is suitable for the particular wavelength and application. The cladding may be crystalline or non-crystalline. For instance, the cladding may be formed of GaAs, InP, GaP, InP, InAs, GaN, AlN, InN, or any combination thereof; Si x N y , Si x O y ; Si x ON y ; Ti x O y ; LiNbO 3  or LiTaO 3  crystals; or organic thin film polymers. The cladding material may also be any combination of applicable dielectric materials or any combination of composition of such materials. 
     The specific refractive index for the waveguide core and cladding material is chosen based on the specific wavelength and application of phase shifter  300 . In general, the core has a higher refractive index than the cladding. In some embodiments, the refractive index difference between core and cladding is greater than about 0.1. For instance, a waveguide can be formed with a core of Si 3 N 4 , which has an index of about 2 in the red color visible light wavelength; and an SiO 2  cladding, which has an index of about 1.45 in the red color visible light wavelength. A single mode channel waveguide formed with these materials can be quite small (e.g. in the range of about 200-400 nm) for visible light wavelengths. However, the single mode size of the waveguide can be, for instance, between about 100 nm-10 μm, or any other size that is applicable to a specific application. The waveguides can be either single mode or multimode waveguides. 
     Active waveguides  306 ,  307  may be formed from any type of material that experiences an electric power induced refractive index shift, directly or indirectly. After the application of an electric power to the active waveguide, the refractive index shift may be caused by, for instance, an electro-optic effect, in which the refractive index of the material is changed by the application of an electric field; a thermo-optic effect, in which the refractive index of the material is changed due to a temperature change induced by the application of the electric power; or an acoustic-optic effect. 
     In one embodiment, an electric powered thermal phase shifter is used to locally heat the active waveguide, causing a temperature change in the active waveguide. Due to the thermo-optic effect, the change in temperature induces a change in refractive index. Local heating of one arm of a fringe projector will create an index difference between the two arms of the fringe projector, resulting in a phase difference between the light output from each of the two arms. The heater used in a thermal phase shifter may be any type of resistance heater, such as a metal heater, a silicide heater, or a doped semiconductor heater, and is operated by applying an electric voltage or current. 
     Referring to  FIG. 5A , in one instance, an active channel waveguide  502  embedded in a cladding material (not shown) is heated by a metal strip  503 . Upon the application of a voltage, the metal strip  503  acts as a local heater, elevating the temperature of channel waveguide  502  and inducing an index change in the waveguide. As another example, referring to  FIG. 5B , two electrical contacts  506 ,  507  are positioned on both sides of a ridge waveguide  505  such that a voltage can be applied across the ridge waveguide. When the ridge waveguide is made from a semiconductor material and is properly doped, the waveguide acts as a resistive heater. 
     Referring again to  FIG. 3 , the active waveguides  306 ,  307  may be formed from any material that is relevant to the particular application of phase shifter  300  and may be formed from the same material or different material from other waveguides on chip  310 . For good phase shifting and control performance, materials exhibiting a strong electro-optic effect, such as LiNbO 3 , GaAs, or GaN; thermo-optic effect, such as Si; or acoustic-optic effect, such as LiNbO 3 , may be used. 
     In some embodiments, chip  310  may be in contact with a temperature control system such as a thermal electrical cooler (TEC), which acts as a global heat sink, stabilizing the temperature of chip  310 . Active waveguides  306 ,  307  may create localized hot spots that can be stabilized by such a global heat sink. 
     3.2 Delay line phase shifter 
     Referring to  FIG. 6 , in one embodiment, a fringe projector chip  610  includes a delay-line phase shifter  600 . Fringe projector chip  610  includes a light-to-waveguide coupler  601  that receives incident coherent light from a light source, such as a laser (not shown). The light passes through an input optical waveguide  602  and into a 3 dB beam splitter  603 . Beam splitter  603  divides the light into two beams, each with the same phase, polarization, and amplitude. The first beam of light travels along a first passive optical waveguide  604 ; the second beam of light travels along a second passive optical waveguide  605 . Optical waveguides  604  and  605  have different lengths. 
     While traveling along waveguides  604 ,  605 , each beam experiences a different phase change due to the difference in length of the two waveguides. For instance, for a length L 1  of waveguide  604 , a length L 2  of waveguide  605 , a free space wavelength of the light of λ 0 , a chip temperature of T, and an effective index of the two waveguides of n, the fractional wave numbers that the beams travel along waveguides  604 ,  605  after beam splitter  603  are, respectively: 
                       m   1     =       nL   1       λ   0         ⁢     
     ⁢       m   2     =       nL   2       λ   0         ⁢     
     ⁢         Δ   ⁢           ⁢   m     =         m   1     -     m   2       =         n   ⁡     (       L   1     -     L   2       )         λ   0       =       n   ⁢           ⁢   Δ   ⁢           ⁢   L       λ   0             ,             (   5   )               
where m 1  and m 2  are real numbers. Without losing generality, it is safe to assume that Δm is an integer number at the current temperature T, meaning that the phase difference between the two light beams is zero due to the periodic nature of the light wave.
 
     Now, assume that the chip temperature is changed to T′. Due to the thermo-optic effect, the effective refractive index of the two waveguide arms is changed to n′. Thus, the wave numbers of the two light beams after beam splitter  603  become, respectively, 
     
       
         
           
             
               
                 
                   
                     
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     Since ΔL is fixed by the micro-fabrication process and λ 0  is fixed by the light source, the phase difference between the two light beams is fully controlled by Δn, which is in turn caused by the thermo-optic effect. Thus, when light travels along waveguides  604  and  605 , the two beams of light emerge from chip  600  as coherent light with the same amplitude and the same polarization but with different phase due to this refractive index difference. 
     4 Multiple Phase Shifters 
     In some embodiments, multiple phase shifters are located on each chip, with an on-off switch associated with each phase shifter. In some cases, each phase shifter has a preset phase shift (e.g., 0, 120, and 240 degrees). If a single phase shifter were used, rapidly and accurately switching that phase shifter from one stable phase setting to the next would pose a challenge for the phase feedback control system, which would control both the stability of the phase setpoint and the ramping from one setting to the next. With each phase shifter dedicated to one single phase shift, the complexity of the phase feedback control system can be reduced, as the feedback controls only the stability of the setpoint. 
     Alternatively, each of the multiple phase shifters is associated with a different wavelength of light. Having a dedicated phase shifter for each wavelength facilitates measurement, reduces error, and eases post-measurement phase unwrapping calculations. 
     Referring to  FIG. 9 , a fringe projector  900  including multiple phase shifters P 1  . . . Pn receives light from multiple coherent light sources L 1  . . . Lm (function block I; m≧1). The coherent light sources can be any type of laser, such as a semiconductor diode laser, a solid state laser, a gas laser, and a fiber laser, or any type of light emitting diode or superluminescent light emitting diode. The wavelength of light can be any wavelength that is relevant to the desired application of fringe projector  900 , such as visible light, infrared light, or ultraviolet light. In some embodiments, function block I resides on the same chip as fringe projector  900 , such as in a fully integrated fringe projector chip. In other embodiments, function block I is constructed on a separate micro-chip. Alternatively, function block I may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. 
     The coherent light from the light sources takes paths I 1  . . . Im (function block II) into the fringe projector  900 . The wavelength, intensity, phase, and/or polarization of light in each input path is independent from that in the other input paths. Each light input path may have an on-off optical switch IS 1  . . . ISm to allow or prohibit the light from passing through the path. In addition or instead of an on-off switch, each path may have a variable optical attenuator (VOA) to adjust the intensity of the light in the path in a continuous range from the maximum light intensity (equal to the output intensity of the light source) to zero. In some embodiments, function block II resides on the same chip as fringe projector  900 ; in other embodiments, function block II is constructed on a separate microchip. Alternatively, function block II may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. The on-off switches and/or VOAs may be any of a variety of devices. For instance, when the input light paths are built by free space lens optics, a step motor driven optical shutter may be used as an on-off switch. As another example, when the input light paths are formed on a semiconductor wafer, a VOA using a PIN semiconductor diode may be used. If the input light paths include on-off switches and/or VOAs, function block II is considered to be an active function block, in which external control is used to operate components of the function block. 
     Function block III includes a light source distributor (D), which delivers light from any one of the input light paths I 1  . . . Im to one or more fringe projector phase shifter light paths. That is, function block III connects m input light paths, where m≧1, and n phase shifter light paths, where n≧m. In some embodiments, function block III resides on the same chip as fringe projector  900 ; in other embodiments, function block III is constructed on a separate microchip. Alternatively, function block III may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. 
     Referring to  FIG. 10A , in one embodiment of function block III, a distributor  1000  simultaneously divides the input light  1002  into one or more outputs  1004   a ,  1004   b ,  1004   c , generally each with equal intensity. Distributor  1000  is generally a passive component that does not require external control. For instance, referring to  FIG. 10B , when used with waveguide optics, one example of a distributor  1000  is a multimode interference (MMI) type 1-to-3 splitter  1006  used to divide light from one input waveguide  1008  into three output waveguides  1010   a ,  1010   b ,  1010   c  simultaneously. 
     Referring to  FIG. 10C , in a second embodiment of function block III, the input light  1002  can be directed into only one of multiple outputs  1004   a ,  1004   b ,  1004   c  at a time by a distributor  1012 . Distributor  1012  is an active component controlled by an external control (not shown) and does not deliver light to multiple paths simultaneously. For instance, referring to  FIG. 10D , in free space optics, a step motor driven optical mirror  1016  is rotated to reflect input light  1002  to one of the three output lenses  1018   a ,  1018   b ,  1018   c  (in this case, to lens  1018   b ), and thus to deliver the light to the corresponding light path. 
     Referring again to  FIG. 9 , the distributed light arrives at function block IV, which may include one or more of on/off switches S 1  . . . Sn and/or a VOA array. Function block IV turns the light on or off, in the case of switches, or adjusts the intensity of the light, in the case of VOAs. In some embodiments, function block IV resides on the same chip as fringe projector  900 ; in other embodiments, function block IV is constructed on a separate microchip. Alternatively, function block IV may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. The on-off switches and/or VOAs may be any of a variety of devices. For instance, when the input light paths are built by free space lens optics, a step motor driven optical shutter may be used as an on-off switch. As another example, when the input light paths are formed on a semiconductor wafer, a VOA using a PIN semiconductor diode may be used. Because an on-off switch and/or a VOA require external control, function block IV is an active function block. 
     Each light path then proceeds to the phase shifter array P 1  . . . Pn (function block V). Each phase shifter has a structure equivalent to the phase shifter described in  FIG. 3 . Any one of the phase shifters P 1  . . . Pn can be used as a static phase shifter (whose phase is fixed at some value) or a as dynamic phase shifter (whose phase is ramped to and stabilize at phase values consecutively according to measurement criteria, for example to achieve a sequential PSI measurement). Function block V is formed on a fringe projector micro-chip and is an active function block. 
     After passing the phase shifter array, the light proceeds to function block VI, which may include post-phase shifter on-off switches S 1 ′ . . . Sn′ and/or VOAs. In some embodiments, function block VI resides on the same chip as fringe projector  900 ; in other embodiments, function block VI is constructed on a separate microchip. Alternatively, function block VI may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. The on-off switches and/or VOAs may be any of a variety of devices, as described above in conjunction with function block VI. Each phase shifter in function block V has two output light paths; the on-off switch and/or VOA corresponding to each phase shifter may be used for either or both light paths. Function block VI is an active function block. 
     Function block VII combines and/or redirects light paths coming from one or more of n phase shifters to k fringe projector output paths, where k≦n. In some embodiments, function block VII resides on the same chip as fringe projector  900 ; in other embodiments, function block VII is constructed on a separate microchip. Alternatively, function block VII may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. 
     Referring to  FIG. 11A , in one embodiment of function block VII, a passive combiner  1100  simultaneously combines one or more input light paths  1102   a ,  1102   b ,  1102   c  into a single output light path  1104 . For instance, referring to  FIG. 11B , for waveguide optics, an MMI type 3-to-1 wavelength multiplexer  1106  combines light at three different wavelengths from three input waveguides  1108   a ,  1108   b ,  1108   c  into a single output waveguide  1110 . Combiner  1100  is generally used to combine light that has different wavelengths or polarizations. 
     Referring to  FIG. 11C , in another embodiment of function block VII, an active combiner  1112  transfers light from only one of the input light paths  1102   a ,  1102   b ,  1102   c  into the output light path  1104 . Combiner  1112  does not simultaneously accept light from multiple input light paths. In some cases, light may be present on all of the input light paths; in other cases, light is present on only some of the input light paths. Combiner  1112  is controlled by external control, which selects the input light path. For instance, referring to  FIG. 11D , for free space optics, a step motor driven optical mirror  1116  is rotated to reflect one of the input light beams (in this case, light beam  1102   b ) to an output lens  1118  and thus to deliver that light beam to the output light path. 
     Referring to  FIG. 11E , in a further embodiment of function block VII, a passive combiner  1120  transfers light from only one of the input light paths  1102   a ,  1102   b ,  1102   c  into the output light path  1104 . Combiner  1120  does not simultaneously accept light from multiple input light paths. Combiner  1120  can only function when on-off switches are used in one or more previous function blocks such that light exists in only one of the input light paths. For instance, referring to  FIG. 11F , a 3-to-1 waveguide lens  1122  focuses light from any one of three input waveguides  1124   a ,  1124   b ,  1124   c  into a single output waveguide  1126 . The specific input light that is passed into the output waveguide is determined by the on-off switch state in previous function blocks. 
     Referring again to  FIG. 9 , in function block VIII, the output light paths OS 1  . . . OSk (k≧1) output coherent light path pairs leaving the fringe projector  900 . The output light paths maintain a set separation for each fringe projection light path pair, which determines the spatial frequency of the fringe projection. Function block VIII may include on-off switches and/or VOAs for each fringe projection light path pair. In some embodiments, function block VIII resides on the same chip as fringe projector  900 ; in other embodiments, function block VIII is constructed on a separate microchip. Alternatively, function block VIII may be built as a free space optical system that may or may not include optical lenses, or may be built as a fiber optical system. The on-off switches and/or VOAs may be any of a variety of devices, as described above in conjunction with function block VI. If on-off switches and/or VOAs are used, function block VIII is an active function block. 
     4.1 Multiple Phase Shifters for Fringe Projection 
     Referring to  FIG. 12 , in one embodiment, a fringe projector chip  1200  generates three phase projections  1201   a ,  1201   b ,  1201   c  with different fringe frequencies. A laser diode light source  1202  is attached to a single mode optical fiber  1204 . The light in optical fiber  1204  is divided equally by a fiber-based 1-to-3 splitter  1206  into three phase shifter channels  1208   a ,  1208   b ,  1208   c . On each phase shifter channel, there is a fiber-based on-off switch  1210   a ,  1210   b ,  1210   c  to turn on or off the light passing through the channel. 
     The three phase shifter channels are coupled into fringe projector chip  1200  through on-chip fiber-to-waveguide couplers  1212   a ,  1212   b ,  1212   c , which reduce optical loss. On the fringe projector chip, there are three 3 dB splitters  1214   a ,  1214   b ,  1214   c , each of which splits the incoming light into two paths. Each pair of incoming light paths is received by a phase shifter active waveguide  1216   a ,  1216   b ,  1216   c . The phase shifted light is output from each phase shifter through an output waveguide pair  1218   a ,  1218   b ,  1218   c . The two waveguides in each output waveguide pair are separated by a different distance dn (where n=1, 2, 3, and d 1 ≠d 2 ≠d 3 ). That is, fringe projector chip  1200  includes three output waveguide pairs in parallel that output light on the chip edge and thus are able to produce three fringe patterns  1220   a ,  1220   b ,  1220   c , each pattern with a different fringe frequency. This configuration is equivalent to having three fringe pattern projectors capable of dynamic phase shifting located on a single chip  1200 . Thus, fringe projector chip  1200  can be used as three separate and independent fringe projectors for 3D measurement and also to unwrap the fringe phases as described above with reference to  FIG. 1D . 
     Referring still to  FIG. 12 , in another embodiment, fringe projector chip  1200  can also be configured such that the two waveguides in each output waveguide pair  1218   a ,  1218   b ,  1218   c  are separated by the same distance d (where d 1 =d 2 =d 3 =d). That is, fringe projector chip  1200  includes three output waveguide pairs in parallel that output light on the chip edge and produce three fringe patterns  1220   a ,  1220   b ,  1220   c , each pattern with the same fringe frequency. 
     In this embodiment, the three phase shifter active waveguides  1216   a ,  1216   b ,  1216   c  are each initially ramped to and stabilized at a different phase value. For instance, phase shifter active waveguide  1216   a  may produce a phase difference Δθ 1  of 0 degrees between the light in one output waveguide and the light in the second output waveguide. Phase shifter active waveguide  1216   b  may produce a phase difference Δθ 2  of 120 degrees, and phase shifter active waveguides  1216   c  may produce a phase difference Δθ 3  of 240 degrees. Fiber-based on-off switches  1210   a ,  1210   b ,  1210   c  can be alternately turned on or off as appropriate to allow only one output waveguide pair at a time to generate a fringe pattern. 
     The use of three separate phase shifters, each phase shifter dedicated to generating a particular phase difference, has a number of advantages over using a single phase shifter to consecutively generate several phase differences. With dedicated phase shifters, the need to ramp and stabilize a single phase shifter at each phase difference with high speed and high precision is eliminated. For instance, in order to satisfy a particular measurement criterion, a single phase shifter may be asked to ramp from a 0 degree phase difference to a 120 degree phase difference within tens of milliseconds and to settle at the 120 degree phase difference with a precision of ±0.1 degree. The feedback control to achieve such a goal may be complex. In contrast, by using multiple phase shifters, there is no need to ramp between consecutive phase differences; the generation of consecutive phase differences is performed by the on-off switches. In this case, the feedback control can focus on the simpler task of maintaining the appropriate precision. Thus, high speed and high precision control are achievable. 
     Referring to  FIG. 13 , in another embodiment, a fringe projector chip  1300  allows three static phase projections. Three pairs of output waveguides ( 1326   a  and  1326   b ,  1328   a  and  1328   b , and  1330   a  and  1330   b ) are interleaved such that the separation among the fringes output from each pair is minimized, thus reducing either measurement error or the complexity of post-measurement processing to account for this separation. 
     In this embodiment, a laser diode light source  1304  is coupled onto an on-chip laser-to-waveguide coupler  1306  by a free space optical lens  1308 . Once on fringe projector chip  1300 , the light is divided into three phase shifter channels  1310   a ,  1310   b ,  1310   c  by a waveguide-based 1-to-3 splitter  1311 . Each phase shifter channel has a Mach-Zehnder interferometry (MZI) type waveguide on-off switch  1312   a ,  1312   b ,  1312   c  to turn on or off light passing along that channel. 
     The light along each path is split into two arms by a 3 dB splitter  1313   a ,  1313   b ,  1313   c . In order to reducing the separation among the pairs of output waveguides, the two arms of each phase shifter are separated such that a first arm  1314   a ,  1316   a ,  1318   a  of each phase shifter is routed to the left side of chip  1300  and a second arm  1314   b ,  1316   b ,  1318   b  is routed to the right side of the chip. Output waveguide pairs  1326   a  and  1326   b ,  1328   a  and  1328   b , and  1330   a  and  1330   b  are interleaved such that the first waveguide  1326   a ,  1328   a ,  1330   a  of each pair outputs light on the left side of chip  1300  and the second waveguide  1326   b ,  1328   b ,  1330   b  outputs light on the right side of the chip. The distance between the first waveguide and the second waveguide of each pair is d (where d 1 =d 2 =d 3 =d). 
     In routing each arm of the phase shifters to a different side of the chip, waveguide crossings between passive waveguides of the phase shifters are used. An exemplary waveguide crossing  1320  between a first single mode waveguide input path  1322   a  and a second single mode waveguide input path  1322   b  gradually tapers the two input paths into wider multimode waveguides in order to reduce crossing losses and crosstalk. Once crossed, the waveguide is tapered back into single mode waveguide output paths  1324   a ,  1324   b.    
     Phase shifter active waveguide pair  1314  generates a phase difference of Δθ 1 , phase shifter active waveguide pair  1316  generates a phase difference of Δθ 2 , and phase shifter active waveguide pair  1318  generates a phase difference of Δθ 3 . Because of the layout of the output waveguides, fringe patterns  1326  generated by each phase shifter are projected onto approximately the same position on the target object. MZI on-off switches  1312   a ,  1312   b ,  1312   c  can be alternately turned on or off as appropriate to allow only one output waveguide pair at a time to generate a fringe pattern. 
     Referring to  FIG. 14 , in an alternative embodiment of a fringe projector chip  1400 , three static phase projections  1401  can be generated using a single output waveguide pair  1402   a ,  1402   b . By using only a single output waveguide pair, no offset between fringe patterns of different phase is observed, reducing measurement error and simplifying post-measurement data analysis. 
     Fringe projector chip  1400  is an example of a fully integrated fringe projector chip that includes an on-chip laser diode  1404 . Light from laser diode  1404  is split by an on-chip 1-to-3 splitter  1406  into three phase shifter paths  1408   a ,  1408   b ,  1408   c . There is a VOA  1410   a ,  1410   b ,  1410   c  along each phase shifter path to adjust the intensity of light along the path, for instance to compensate unequal light intensities due to fabrication variations in the light paths. VOAs  1410   a ,  1410   b ,  1410   c  can also be used as on-off switches. Each light path includes a 3 dB splitter  1412   a ,  1412   b ,  1412   c  to split the light along that path into two paths, which are received by phase shifter active waveguide pairs  1414   a  and  1414   b ,  1416   a  and  1416   b , and  1418   a  and  1418   b . Waveguide crossings  1420  allow for the light paths to cross each other. After passing through the phase shifters, the light is combined by output path combiners  1422   a ,  1422   b  into the single output waveguide pair  1402   a ,  1402   b . Combiners  1422   a ,  1422   b  are, for instance, the combiner shown in  FIGS. 11E and 11F . The distance between the two output waveguides is d. 
     Phase shifter active waveguide pair  1414  generates a phase difference of Δθ 1 , phase shifter active waveguide pair  1416  generates a phase difference of Δθ 2 , and phase shifter active waveguide pair  1418  generates a phase difference of Δθ 3 . Because of the layout of the output waveguides, fringe patterns  1426  generated by each phase shifter are projected onto the same position. VOAs  1410   a ,  1410   b ,  1410   c  can be alternately turned on or off as appropriate to allow only one output waveguide pair at a time to generate a fringe pattern. 
     4.2 Other Embodiments of Multiple Phase Shifters 
     Referring to  FIG. 15 , a multi-wavelength fringe projector chip  1500  allows multi-wavelength fringe projections  1501  using a single output waveguide pair  1502   a ,  1502   b . Three laser sources  1504   a ,  1504   b ,  1504   c  generate light at three different wavelengths λ 1 , λ 2 , λ 3 , respectively. An off-chip optical shutter  1506   a ,  1506   b ,  1506   c  turns each optical path on or off. The light from each path is coupled onto fringe projector chip  1500  by a free space optical lens  1508   a ,  1508   b ,  1508   c.    
     On fringe projector chip  1500 , each of the three input light paths is received by an on-chip laser-to-waveguide coupler array  1510   a ,  1510   b ,  1510   c  and split into a pair by 3 dB splitters  1512   a ,  1512   b ,  1512   c . The pairs of optical paths are grouped as described above, with a first arm  1514   a ,  1516   a ,  1518   a  of a phase shifter sent to the left side of chip  1500  and a second arm  1514   b ,  1516   b ,  1518   b  of the phase shifter sent to the right side of chip  1500 . Waveguide crosses  1520  are positioned as needed. After passing through the phase shifters, the light is combined by output path combiners  1522   a ,  1522   b  into the single output waveguide pair  1502   a ,  1502   b . Combiners  1522   a ,  1522   b  are, for instance, the combiner shown in  FIGS. 11E and 11F . The distance between the two output waveguides is d. A fringe pattern  1524  is generated. 
     Phase shifters comprising active waveguide pairs  1514 ,  1516 ,  1518  are dynamic phase shifters rather than static phase shifters, which ramp to and stabilize at phase values consecutively according to measurement criteria. The shutters  1506   a ,  1506   b ,  1506   c  that control the on-off state of each wavelength path can be turned on or off simultaneously or alternately, allowing multi-fringe-frequency (i.e., multi-fringe-wavelength) phase shift interferometry. As described in the previous paragraphs, the use of multi-light-frequency with the same separation of the output waveguide pairs is inherently multi-fringe-frequency in nature. Thus the unwrapping scheme described in  FIG. 1D  can also be used to facilitate the measurement. 
     Referring to  FIG. 16 , a multi-polarization fringe projector chip  1600  allows multi-polarization fringe projections  1602  using a single output waveguide pair  1604   a ,  1604   b . Light from a laser diode light source  1606  is split by a fiber polarization splitter  1608  into a first input path  1610   a  for TE polarized light and a second input path  1610   b  for TM polarized light. That is, the two input paths have the same wavelength but different polarization. Input light paths  1610   a ,  1610   b  are coupled onto chip  1600  by on-chip fiber-to-waveguide couplers  1612   a ,  1612   b . The intensity and on-off status of each path is controlled by an on-chip VOA  1614   a ,  1614   b . The two input paths are split by a 3 dB splitter  1616   a ,  1616   b , separated and grouped as described above using a waveguide cross  1620 , and passed through phase shifter active waveguide pairs  1618   a ,  1618   b  and  1621   a ,  1621   b . After passing through the phase shifters, the light is combined by output path combiners  1622   a ,  1622   b  into the single output waveguide pair  1604   a ,  1604   b . The distance between the two output waveguides is d. A fringe pattern  1624  is generated. 
     Phase shifters comprising the active waveguide pairs  1618 ,  1620  are dynamic phase shifters that ramp to and stabilize at phase values consecutively according to measurement criteria. The VOAs  1614   a ,  1614   b  that control the on-off state of each wavelength path can be turned on or off alternately, allowing multi-polarization phase shift interferometry. 
     Referring to  FIG. 17 , a fringe projector chip  1700  allows multi-fringe-frequency fringe projections  1702   a ,  1702   b ,  1702   c  using groups of closely spaced output waveguides. Light from a laser diode light source  1706  is coupled onto fringe projector chip  1700  by a coupling lens  1708  and an on-chip laser-to-waveguide coupler  1710 . The input light is split into three paths  1712   a ,  1712   b ,  1712   c  by a 1-to-3 splitter  1711 . Each path  1712   a ,  1712   b ,  1712   c  is then divided into two by a 3 dB splitter  1714   a ,  1714   b ,  1714   c . The divided input paths are separated and grouped as described above using waveguide crosses  1716  and passed through phase shifters active waveguide pairs  1718   a ,  1718   b ;  1720   a ,  1720   b ; and  1722   a ,  1722   b.    
     The output light from phase shifter active waveguide pair  1718   a ,  1718   b  is received by an output waveguide pair  1724   a ,  1724   b ; the output light from phase shifter active waveguide pair  1720   a ,  1720   b  is received by an output waveguide pair  1726   a ,  1726   b ; and the output light from phase shifter active waveguide pair  1722   a ,  1722   b  is received by an output waveguide pair  1728   a ,  1728   b . The output waveguide pairs have different separation distances: the separation between waveguides  1724   a  and  1724   b  is d 1 , the separation between waveguides  1726   a  and  1726   b  is d 2 , and the separation between waveguides  1726   a  and  1726   b  is d 3 . These different separations correspond to three fringe pattern frequencies. Fringe patterns  1732   a ,  1732   b ,  1732   c  are generated. 
     Off-chip optical shutters  1730  are used to alternately control the on-off state of each fringe-frequency waveguide pair, allowing multi-fringe-frequency phase shift interferometry. 
     In general, the multiple phase shifters described above may be combined in any fashion to allow multi-phase, multi-light-wavelength, multi-light-polarization, and/or multi-fringe-frequency phase shift interferometry. For instance, referring to  FIG. 18 , in one embodiment, a fringe projector system  1800  allows three static phase projections  1801   a  and  1801   b  at two different input light wavelengths. Two chips are used in fringe projector system  1800 : a fringe projector chip  1802  and a combiner chip  1804 . 
     Two laser diode light sources  1806   a ,  1806   b  generate light at two different wavelengths λ 1 , λ 2 . Fiber switches  1808   a ,  1808   b  turn each optical path on or off. The light from each path is carried on an optical fiber  1810   a ,  1810   b  and coupled onto fringe projector chip  1802  by an on-chip fiber-to-waveguide coupler  1812   a ,  1812   b . The light in each path is split into three paths by a 1-to-3 splitter  1814   a ,  1814   b . Each path is then split into an array of 3 dB splitters  1816   a - 1816   f . The pairs of optical paths are grouped as described above. A first arm  1818   a ,  1820   a ,  1822   a  of a static phase shifter for light at wavelength λ 1  is sent to first region of chip  1802  and a second arm  1818   b ,  1820   b ,  1822   b  of a static phase shifter for light at wavelength λ 1  is sent to a second region of chip  1802 . A first arm  1824   a ,  1826   a ,  1828   a  for light at wavelength λ 2  is sent to a third region of the chip and a second arm  1824   b ,  1826   b ,  1828   b  for light at wavelength λ 2  is sent to a fourth region of the chip. Waveguide crosses  1830   a  and  1830   b  are positioned as needed. 
     After passing through the phase shifters, the light is output from fringe projector chip  1802  by output waveguide pairs  1832   a ,  1832   b ;  1834   a ,  1834   b ; and  1836   a ,  1836   b  at wavelength λ 1  and output waveguide pairs  1838   a ,  1838   b ;  1840   a ,  1840   b ; and  1842   a ,  1842   b  at wavelength λ 2 . Light in each output waveguide pair may have a different static phase difference Δθ 1 , Δθ 2 , Δθ 3 . 
     An optical shutter array  1844  is positioned between fringe projector chip  1802  and combiner chip  1804  to turn on or turn off each waveguide pair according to measurement criteria. The light is then received onto combiner chip  1804 , where the light at wavelength λ 1  is first received by waveguides  1845 ,  1847 , and  1849 , and then combined via output path 3-to-1 combiners  1846   a ,  1846   b  and passed into an output waveguide pair  1850   a  and  1850   b ; and light at wavelength λ 2  is first received by waveguides  1851 ,  1853 , and  1855 , and then combined via output path 3-to-1 combiners  1848   a ,  1848   b  and passed into another output waveguide pair  1852   a  and  1852   b . Output waveguides  1850   a  and  1850   b  are separated by a distance d 1 ; and output waveguides  1852   a  and  1852   b  are separated by a distance d 2 . Fringe patterns  1854   a  and  1854   b  are generated at two wavelengths, λ 1  and λ 2 . The configuration in  FIG. 18  enables multi-fringe-phase and multi-light-wavelength fringe phase shift interferometry in a compact chip-based system. 
     It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.