Patent Publication Number: US-2015077759-A1

Title: Compact, Slope Sensitive Optical Probe

Description:
CROSS REFERENCE 
     This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 61/859,944, filed Jul. 30, 2013, which is hereby incorporated by reference in its entirety. 
    
    
     This invention was made with government support under Grant Nos. N68936-12-C-0092, N68936-12-00018 and N68936-12-00036 awarded by the United States Navy. The government has certain rights in this invention. 
    
    
     FIELD 
     The present disclosure relates to an optical probe system, and in particular, an optical probe system that is slope sensitive. 
     BACKGROUND 
     A sketch of the operating principle of a known optical probe is shown in  FIG. 1 . In essence, a light source (LED, laser diode, or fiber delivered laser) is split equally at a beamsplitter where one beam travels to the target/part, while the other beam reflects from a known, tilted reference surface. When the two beams recombine at the beamsplitter, a line sensor can be used to detect an interferogram that contains tilt fringes. The parallelism of the part/target and reference surface must be sufficient to have tilt fringes while not being too parallel so less than a fringe is imaged. Fringes shown on a line sensor can be analyzed in the Fourier Domain. The peak amplitude can be used to determine the nominal frequency of the tilt fringes and the phase can be determined from the Fourier analysis. When the part/target slope changes slightly, then the peak location of the amplitude in the Fourier domain shifts but the phase remains constant at the location of the peak amplitude in frequency. When the distance between the part/target changes, then the relative phase of the signal changes, which can be detected via Fourier analysis techniques. This can be modeled as well, showing that for resolution on the line sensor, high accuracy can be obtained. 
     Referring to the known optical system of  FIG. 1 , light from an optical source  2  is collimated using a lens  3  and sent to a beamsplitter  4 . Part of the light is split at the beamsplitter  4  and the reflected part makes up the reference arm beam  13 . The reference arm beam reflects from a reference surface  5  and then transmits through the beamsplitter  4  to make up part of the interference signal  15 . The initially transmitted beam through the beamsplitter  4  is the measurement arm beam  14  and reflects from the measurement surface  6 . The measurement surface&#39;s normal vector has a slight tilt with respect to the propagation direction of the measurement arm beam. The reflected beam from the measurement surface reflects at the beamsplitter  4  and makes up the other part of the interference signal  15 . The interference signal  15  is imaged onto a detector  8  using imaging optics  7 . The image detected by the detector  8  is sent to a processing unit  12  to determine signal attributes based on the recorded image. When the light source  2  has a long coherence length, the image detected  9  shows fringes over the full aperture. When the light source  2  has a short coherence length, the image detected  10  only shows fringes in part of the image based on the coherence length of the light source  2  and the relative positions between the measurement surface  5  and the reference surface  6 . The optical path lengths between the measurement and reference arms in the interferometer must be matched to within the coherence length of the source for sufficient interference. When a single line of the images is analyzed  11 , the measured signals have a series of fringes with amplitude and phase dependent on the light source  2  and optical path difference between the reference arm beam  13  and the measurement arm beam  14 . 
     SUMMARY 
     In accordance with one aspect illustrated herein, there is provided an optical probe system including a fiber collimator; an optical fiber capable of transmitting light from an optical source to the fiber collimator, the fiber collimator capable of splitting the transmitted light into first and second collimated light beams; and a beamsplitter capable of splitting the first collimated light beam into a reference arm beam and a measurement arm beam, wherein the reference arm beam includes light split from the first collimated light beam which is initially reflected from the beamsplitter to a reference surface and reflected from the reference surface back to the beamsplitter where part of the reference arm beam is transmitted, and wherein the measurement arm beam includes light split from the first collimated light beam which is initially transmitted through the beamsplitter to a sample surface, reflected from the sample surface to the beamsplitter then reflected by the beamsplitter where the reflected measurement arm beam interferes with the transmitted reference arm beam to form an interference signal, wherein an offset distance from the beamsplitter to the sample surface is such that the total optical paths of the measurement arm beam and reference arm beam are nominally equal and the interference signal is imaged into an optical fiber bundle and transmitted along an optical fiber where the nominal fringe pattern of the interference signal is retained. 
     In accordance with another aspect illustrated herein, there is provided a surface metrology system including a coordinate measuring machine having an optical probe system including a fiber collimator; an optical fiber capable of transmitting light from an optical source to the fiber collimator, the fiber collimator capable of splitting the transmitted light into first and second collimated light beams; and a beamsplitter capable of splitting the first collimated light beam into a reference arm beam and a measurement arm beam, wherein the reference arm beam includes light split from the first collimated light beam which is initially reflected from the beamsplitter to a reference surface and reflected from the reference surface back to the beamsplitter where part of the reference arm beam is transmitted, and wherein the measurement arm beam includes light split from the first collimated light beam which is initially transmitted through the beamsplitter to a sample surface, reflected from the sample surface to the beamsplitter then reflected by the beamsplitter where the reflected measurement arm beam interferes with the transmitted reference arm beam to form an interference signal, wherein an offset distance from the beamsplitter to the sample surface is such that the total optical paths of the measurement arm beam and reference arm beam are nominally equal and the interference signal is imaged into an optical fiber bundle and transmitted along an optical fiber where the nominal fringe pattern of the interference signal is retained; a detection system including a second beamsplitter where part of the light from the interference signal is reflected to an array detector which images the fiber interference signal resulting in a recorded array interference and part of the light from the interference signal is transmitted; and a third beamsplitter where part of the transmitted interference signal light from the second beamsplitter is reflected and imaged onto a first line sensor and part of the transmitted interference signal light from the second beamsplitter is transmitted and imaged onto a second line sensor, wherein the first line sensor records a line image from the fiber interference image and the second line sensor records an orthogonal line image from the fiber interference image where the orthogonality is with respect to the line image; and a processing unit capable of determining the frequency and phase of the images from the recorded array interference, line image, and orthogonal line image. 
     In accordance with another aspect illustrated herein, there is provided a dual surface metrology system including a coordinate measuring machine having an optical probe system including a fiber collimator; an optical fiber capable of transmitting light from an optical source to the fiber collimator, the fiber collimator capable of splitting the transmitted light into first and second collimated light beams; and a beamsplitter capable of splitting the first collimated light beam into a reference arm beam and a measurement arm beam, wherein the reference arm beam includes light split from the first collimated light beam which is initially reflected from the beamsplitter to a reference surface and reflected from the reference surface back to the beamsplitter where part of the reference arm beam is transmitted, and wherein the measurement arm beam includes light split from the first collimated light beam which is initially transmitted through the beamsplitter to a sample surface, reflected from the sample surface to the beamsplitter then reflected by the beamsplitter where the reflected measurement arm beam interferes with the transmitted reference arm beam to form an interference signal, wherein an offset distance from the beamsplitter to the sample surface is such that the total optical paths of the measurement arm beam and reference arm beam are nominally equal and the interference signal is imaged into an optical fiber bundle and transmitted along an optical fiber where the nominal fringe pattern of the interference signal is retained, wherein the optical source includes a first optical fiber transmitted light source and a second optical fiber transmitted light source, where one of the wavelengths of the first and second light sources is transparent to the sample and the first optical fiber and second optical fiber are combined prior to being sent to the fiber collimator through the optical fiber; and wherein the reference surface includes a dichroic mirror having a thickness and refractive index nominally equal to the sample thickness and refractive index, that reflects light with wavelengths nominally equal to the first optical fiber transmitted light source and transmits light with wavelengths nominally equal to the second optical fiber transmitted light source, such that a front surface interference beam and back surface interference beam are imaged into the optical fiber bundle; a detection system including a second fiber collimator capable of collimating the front surface interference beam and the back surface interference beam of the optical fiber bundle; a dichroic beamsplitter capable of reflecting the back surface interference beam and transmitting the front surface interference beam; a second beamsplitter which splits the front surface interference beam transmitted through the dichroic beamsplitter into a reflected beam and a transmitted beam, a first array detector which images the reflected beam from the second beamsplitter; a third beamsplitter which splits the transmitted beam from the second beamsplitter into a reflected beam and a transmitted beam; a front surface line sensor which images the reflected beam from the third beamsplitter; an orthogonal front surface line sensor which images the transmitted beam through the third beamsplitter, wherein the orthogonal front surface line sensor is orthogonal with respect to the front surface line sensor; a fourth beamsplitter which splits the back surface interference beam reflected from the dichroic beamsplitter into a reflected beam and a transmitted beam; a second array detector which images the transmitted beam from the fourth beamsplitter; a fifth beamsplitter which splits the reflected beam from the fourth beamsplitter into a reflected beam and a transmitted beam; a back surface line sensor which images the reflected beam from the fifth beamsplitter; and an orthogonal back surface line sensor which images the transmitted beam through the fifth beamsplitter, wherein the orthogonal back surface line sensor is orthogonal with respect to the back surface line sensor and the front surface line sensor is aligned parallel with the back surface line sensor; and a processing unit capable of determining the frequency and phase of the images from the recorded signals from the first array detector, second array detector, front surface line sensor, orthogonal front surface line sensor, back surface line sensor, and orthogonal back surface line sensor. 
     These and other aspects of the subject matter illustrated herein will become apparent upon a review of the following detailed description and the claims appended thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a prior art optical system for measuring displacement and tilt of a target; 
         FIG. 2  is a schematic of an embodiment illustrated herein of an optical system for measuring displacement and tilt of a target utilizing an optical fiber bundle; 
         FIG. 3  is a schematic of an embodiment illustrated herein of a detection system for use in the optical system of Figure; 
         FIG. 4  is a schematic of an embodiment illustrated herein of an optical system where the measurement arm beam is focused on to the back surface of the sample optic through the front surface of the sample optic; 
         FIG. 5  is a schematic of an embodiment illustrated herein of a dual surface optical probing system; 
         FIG. 6  is a schematic of an embodiment illustrated herein of a splitting and detection system for use in the optical system of  FIG. 5 ; 
         FIG. 7  is a schematic of an embodiment illustrated herein of a dual surface metrology system; 
         FIG. 8  is the first image in a series of movie images that was taken directly from the camera of overall tilt fringes generated from Example 1; 
         FIG. 9  is the raw signal taken from the image of  FIG. 8  using only a single horizontal line from the center; 
         FIG. 10  is the spatial Fourier domain magnitude signal showing the raw and processed generated in Example 1; 
         FIG. 11  is a graph showing the phase at the spatial frequency number generated in Example 1; and 
         FIG. 12  is a diagram of the angle relative to the reference surface. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to an optical sensor system having the source fiber-delivered and the detector fiber-coupled for analyzing carrier fringes using a line sensor to measure displacement and tilt. 
     An example of an embodiment that achieves this is shown in  FIG. 2 . The sensor is composed of a fiber coupled light source that transmits light through an optical fiber to the interferometer. The interference signal is transmitted through an optical fiber bundle. The light from this optical fiber bundle is then collimated and split where it is imaged on several detectors. Preferably, one detector is a CMOS or CCD detector that gives an overall image of the interference fringes. Preferably, the other two detectors are high speed line sensors that are oriented orthogonally from each other. These line sensors are processed at high speeds to determine the displacement and angle from the phase and frequency, respectively. 
     The signal from the line sensor (such as a truncated CMOS image) is a N-point array where N is the number of pixels across the sensor and spatial position is determined based on the period spacing between pixels. This can range from a few micrometers to 10&#39;s of micrometers depending on the sensor. The relative spacing of the fringes is known, provided the diameter of the fiber bundle is known. A Fourier analysis can be performed continuously on the tilt fringes to determine slight changes in spatial frequency and phase, which enables determining displacement and tilt of the target surface. When the number of fringes changes, the frequency of the primary peak in the Fourier doman shifts. The angle relative to the reference surface is related to the number of fringes by the following relationships: 
     
       
         
           
             
               
                 
                   
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     where F is the number of tip or tilt fringes in the current frame, F, is the initial number of fringes, λ is the nominal wavelength of the laser source, L is the width of the image in the fibers, and Δθ is the change in relative tip or tilt, as shown in  FIG. 12 . This assumes the light imaged on the line sensors fills the sensor completely. As the measurement surface is displaced, the phase at a fixed point in the frequency domain changes. The displacement is related to phase angle through 
     
       
         
           
             
               
                 
                   
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     where θ is the current phase angle, and θ i ; is the initial phase angle. Because the phase changes between 0 and 2π, the data should be unwrapped before this calculation can be made correctly. This angle is in radians and therefore it is divided by 2π rad (as seen in Equation 1.3) to achieve units of length. The measured displacement should be the same when calculated from both orthogonal line samples. 
     The operating tilt range is governed by a simple principle that says at least one complete tilt fringe should be present across the fiber array because the analysis is Fourier-based. To maintain Nyquist sampling criteria, at least 2 points per fringe are used. Thus, based on the number of fibers in the fiber bundle and the line sensor pixel pitch, the tilt range of the sensor can be determined in accordance with methods known in the art. In practice and simulations, several fringes across the line sensor are used to more accurately employ Fourier analysis techniques. Signal processing techniques such as zero padding, windowing, and parabolic curve fitting can be used to enhance the displacement and angle resolution by helping interpolate in the Fourier Domain. 
     Referring to an embodiment of an optical probe system  20  of  FIG. 2 , light from a fiber coupled optical source  21  is transmitted along an optical fiber  22  that is typically single mode. The light from the optical fiber  22  is collimated with a fiber collimator  23  and sent to the beamsplitter  16 . The initially reflected light from the beamsplitter is the reference arm beam  17  which reflects from a reference surface  18 . The reflected light then travels back to the beamsplitter  16  where part of the reference arm beam is transmitted. The initially transmitted light from the beamsplitter  16  is the measurement arm beam  19 . The measurement arm beam  19  reflects from the surface of the sample  45  where it is reflected by the beamsplitter and interferes with the reference arm beam  17 . The offset distance from the beamsplitter  16  to the surface of the sample optic  45  is such that the total optical paths of the measurement arm beam  19  and reference arm beam  17  are nominally equal. The interference signal  25  is imaged into an optical fiber bundle  27  using imaging optics  33  and/or a coupling lens  26 . The interference signal  25  is transmitted into the fiber bundle and transmitted along the fiber where it retains the same nominal fringe pattern  28  although it may rotate based on the orientation of the optical fiber bundle  27 . The optical fiber bundle  27  is sent to a detection system  29  where the detected signals are processed in processing unit  36 . 
     Referring to  FIG. 3  is shown an embodiment of the detection system  29  of  FIG. 2  where the fiber interference image  28  in the optical fiber bundle  27  is collimated using a fiber collimator  30 . The collimated light is sent to a first beamsplitter  31  where part of the light is reflected to an array detector  34 , such as a CCD or CMOS array. The array detector  34  images the fiber interference image  28  resulting in a recorded array interference  35 . The initially transmitted beam from the first beamsplitter  31  is sent to a second beamsplitter  32  where part of the light is reflected and imaged on to a first line sensor  37  and part of the light is transmitted and imaged on to a second line sensor  40 . The first line sensor records a line image  38  from the fiber interference image  28 . The second line sensor records an orthogonal line image  41  from the fiber interference image  28  where the orthogonality is with respect to the line image  38 . The recorded array interference  35 , line image  38 , and orthogonal line image  41  are sent to a processing unit  36  where the frequency and phase of the images can be determined using known techniques. 
     Referring to an embodiment of an optical probe system  50  of  FIG. 4 , the measurement arm beam  19  reflects from the back surface of the sample optic  45  through the front surface of the sample optic. The sample optic  45  should be at least partially transparent to the wavelength of light from the fiber coupled optical source  21 . The offset distance from the beamsplitter  16  to the back surface of the sample optic  45  is such that the total optical paths of the measurement arm beam  19  and reference arm beam  17  are nominally equal. The light source  21  is chosen to be nominally transmissive given the material properties of the sample optic  45 . 
     Referring to an embodiment of a dual surface optical probing system  60  of  FIG. 5 , included is a first fiber light source  21  and a second fiber light source  61  where one of the wavelengths of the sources is transparent to the sample optic  45 . In  FIG. 5 , the second fiber light source  61  is depicted as the one transparent to the sample optic  45 . The first fiber light source  21  is transmitted through a first optical fiber  22  and the second fiber light source  61  is also transmitted through a second optical fiber  62 . The first optical fiber  22  and second optical fiber  62  are combined using a 2×1 coupler  63  prior to being sent to the fiber probing system  80 . A fiber collimator  23  collimates the first optical beam  64  and the second optical beam  65  from the 2×1 coupler  63 . The first optical beam  64  travels to a beamsplitter  16  where the first reference arm beam  17  reflects from the beamsplitter  16  and travels to a dichroic mirror  66  that reflects light with wavelengths nominally equal to the first fiber optical source  21  and transmits light with wavelengths nominally equal to the second fiber optical source  61 . The first reference arm beam  17  reflects from the dichroic minor  66  and transmits through the beamsplitter  16 . The initially transmitted first optical beam  64  from the beamsplitter  16  is the first measurement arm beam  19  that reflects from the front surface of the sample optic  45 . The first measurement arm beam  19  reflects from the front surface and then is reflected at the beamsplitter  14  where it interferes with the first reference arm beam  17 , creating the front surface interference beam  70 . 
     The second optical beam  65  travels to a beamsplitter  16  where the second reference arm beam  67  reflects from the beamsplitter  16  and travels to a dichroic mirror  66  that reflects light with wavelengths nominally equal to the first fiber optical source  21  and transmits light with wavelengths nominally equal to the second fiber optical source  61 . The second reference arm beam  67  reflects from the back of the dichroic mirror  66  whose thickness and refractive index is nominally equal to the sample optic  45  thickness and refractive index and transmits through the beamsplitter  16 . The initially transmitted second optical beam  65  from the beamsplitter  16  is the second measurement arm beam  68  that reflects from the back surface of the sample optic  45 . The second measurement arm beam  68  reflects from the back surface and then is reflected at the beamsplitter  16  where it interferes with the first reference arm beam  67 , creating the back surface interference beam  69 . The front surface interference beam  70  and back surface interference beam  69  are imaged into an optical fiber bundle  27  using at least one of imaging optics  13  and a fiber coupler  26 . The fiber bundle is sent to the splitting-and-detection-system  71  where the detected signals are sent to a processing unit  36 . 
     Referring to an embodiment of a splitting and detection system  71  of  FIG. 6  where the optical fiber bundle  27  has the front surface detection beam  73  and the back surface detection beam  74  collimated using a fiber collimator  30 . The front surface detection beam  73  and the back surface detection beam  74  are both sent to a dichroic beamsplitter  72  where the back surface detection beam  74  reflects and the front surface detection beam  73  transmits. The front surface detection beam  73  transmits through the dichroic beamsplitter  72  where the beam is split by a first beamsplitter  31 . The reflected beam from the first beamsplitter is imaged on to a first array detector  34 . The initially transmitted beam through the first beamsplitter  31  is sent to a second beamsplitter where the beam is split again. The reflected beam from the second beamsplitter is imaged on to a front surface line sensor  37  and the initially transmitted beam from the second beamsplitter is imaged on to an orthogonal front surface line sensor  40 . The orthogonal front surface line sensor  40  is orthogonal with respect to the front surface line senor  37 . 
     The back surface detection beam  74  reflects at the dichroic beamsplitter  72  where it is split by a third beamsplitter  75 . The transmitted beam from the third beamsplitter  75  is imaged on to a second array detector  76 . The initially reflected beam through the third beamsplitter  75  is sent to a fourth beamsplitter  77  where it is split again. The reflected beam from the fourth beamsplitter  77  is imaged on to a back surface line sensor  78  and the initially transmitted beam from the fourth beamsplitter  77  is imaged on to an orthogonal back surface line sensor  79 . The orthogonal back surface line sensor  79  is orthogonal with respect to the back surface line senor  78 . The front surface line sensor  37  is typically aligned to be parallel with the back surface line sensor  78 . 
     Signals from the first array detector  34 , second array detector  76 , front surface line sensor  37 , orthogonal front surface line sensor  40 , back surface line sensor  78 , and orthogonal back surface line sensor  79  are sent to a processing unit  36 . 
     Referring to an embodiment of a dual surface metrology system  90  of  FIG. 7 , including the first fiber optical source  21 , first optical fiber  22 , second fiber optical source  61 , second optical fiber  62 , 2×1 fiber coupler  63 , fiber probing system  80 , optical fiber bundle  27 , splitting and detection system  71 , and processing unit  36 . The fiber probing system  80  is mounted on computer controlled stages  92  which are mounted on a machine base  91 . The sample optic  45  is mounted on sample computer controlled stages  94 , which are mounted to the same machine base  91 . The first measurement arm beam  19  and second measurement arm beam  68  are nominally focused on to the front surface and back surface, respectively, of the sample optic  45 . The signals from the processing unit  12  are sent the machine controller  93  that controls the computer controlled stages  92  and sample computer controlled stages  94 . Based on the signals processed and recorded in the processing unit  36 , the positions of the computer controlled stages  92  and sample computer controlled stages  94  are adjusted to ensure the fiber probing system  80  is nominally normal to the sample optic  45  and the first measurement arm beam is in focus at the sample optic  45  front surface. 
     Example 1—The following example was conducted in accordance with the present invention. Quasi-monochromatic light with a wavelength of nominally about  646  nm from a fiber coupled laser source  21  was delivered via the fiber  22  to the optical probe system  20 , as depicted in  FIG. 1 . An approximately 25 mm aspheric lens  23  was used to collimate the light into the beamsplitter  16 . The light reflecting from the beamsplitter  16  was sent to the reference arm beam  17 , which reflects from the stationary minor  18 , whose position, tip, and tilt can be changed as desired. The light transmitting from the beamsplitter  16  is the measurement arm beam. The sample  45  used was a second mirror, also with position, tip, and tilt control. Further, the sample mirror (the sample analog) was on a stage that can be positioned remotely by sending an electrical signal to a piezoelectric device. Both beams  17 ,  19  reflect from their respective minors and interfere at the beamsplitter  16 . The information depicted in  FIGS. 8-11  was generated without using the imaging system  33  or coupling lens  26  shown in  FIG. 1 , as no magnification of the signal was needed. Rather, the interference signal  25  was directly transmitted through the fiber bundle  27 . The detection system  29  was simplified from that shown in  FIG. 3 , to a fiber collimator  30  and another lens to image onto an area detector  34 . The signal from the area detector  34  was sent to the processing unit  36 , which was a computer in this case. A series of images were then acquired in a video form, which were then post-processed to select only a single line of pixels and determine the spatial frequency and phase, as shown in  FIGS. 8-11 . 
       FIG. 8  represents the first image in a series of images taken as a movie depicting the signal generating the tilt fringes from Example 1. The overall tilt fringes are apparent but there are other smaller features shown due to the fiber bundle. The outer edge of the fiber bundle is about 1.1 mm in diameter. 
       FIG. 9  is the raw signal taken from  FIG. 8  using only a single horizontal line from the center. There are several overall transitions (signals of interest) superimposed on a bunch of noise due to the fiber bundle (which is removed for processing of the signals). 
       FIG. 10  is the spatial Fourier domain magnitude the signal generated by Example 1. The raw signal without any processing and the processed signal are shown using standard techniques. The y-axis is scaled but this does not affect the measurement. The raw data is more jagged and has a peak somewhere around 12, but it is not well defined. The processed data, however, has been smoothed and upsampled. While not readily apparent from the figure, the processed data is very smooth around the peak and has many more points to help define the actual peak. The peak detection algorithm further interpolates this data to establish a well defined spatial frequency number. The location in the spatial frequency determines the angle of the mirror. 
       FIG. 11  is a graph from Example 1, showing that once the spatial frequency number is determined, the phase at that spatial frequency point is taken. This is a plot of the unprocessed raw data and the processed data after interpolation, filtering, and unwrapping. The point corresponding to the spatial frequency from the previous  FIG. 10  is the point of interest. 
     In addition to using a long coherence source, suitable sources further include a white light source to have a short window where the optical path length between the reference minor and measurement mirror produce interference fringes. This has the added benefit of measuring the absolute distance, rather than just the relative distance. When this method is employed, the absolute distance between the optical probe and the target is determined by the peak location of the correlogram. As the absolute distance between the optical probe and the target changes, the peak location of the correlogram shifts, which location can be detected and used for feedback control. This feedback control can be used to ensure that the optical probe maintains a constant distance from the target. 
     The present invention has advantages over exiting optical probing technologies because it can inherently sense two degrees of freedom and is readily adaptable for three degree of freedom sensing. These added degrees of freedom means the probe can be aligned with a known surface normality, improving the accuracy of the measurement over other optical probes. 
     The present technology has advantages over existing technologies, such as capacitive sensors, because the present technology has a similar cost and the potential for 100× greater displacement ranges to be measured. Also, the target can be much smaller than typical capacitive sensors. The present technology has advantages over eddy current/inductive sensors because it can get a much higher resolution and is not influenced by stray magnetic fields. Additionally, it has better drift than eddy current/inductive sensors. It is better than linescales because it can work on-axis rather than perpendicular to the axis and the standoff distance is much greater. Also, a glass scale is not needed. It is better than displacement interferometers because the potential production cost is significantly less (−20×) and it can be fiber-fed. It is better than other optical sensors because it does not require an expensive laser source or a known scan of the laser source, which is a significant cost driver for these sensors. The present technology enables the light source to be fiber delivered and the signals generated to be fiber detected. This enables systems to be lighter in weight, more compact, and not heat sensitive as compared with existing systems. Additionally, the ability to fiber detect the signals allows the signals to be split into several different channels which can then be used for different types of sensing. 
     One novel and distinct feature of the present technology is that it can measure displacement and tilt of an object, thus it is inherently a 2-axis sensor. Additionally, there is the potential to modify the sensor to measure three axes (displacement, tip, and tilt). It can be adapted for silicon based devices, further shrinking the overall size, enhancing the scalability, enabling mass production, and has the potential to open up applications in biomedical fields. 
     The present technology solves a significant problem of sensor range, resolution, and bandwidth while limiting the overall cost. Typically, most sensors can achieve only two specifications but not the third. If a sensor can achieve all three, then the cost of the sensor is generally very high. Thus, it makes it impractical to use except in specific circumstances where those specifications impact the overall functioning system. For other optical sensors that may have this range, the bandwidth is typically too slow and then resolution is insufficient for many applications. This is because those sensors are built on technologies that require complex sources that scan in wavelength, frequency, or phase. However, the present sensor uses a passive architecture, which means it needs fewer components and can be made relatively cheaply while maintaining nanometer resolution with 10&#39;s of millimeters of range. Currently, there are no prior sensors which meet this capability. 
     The present technology has application in measuring optical surfaces, specifically freeform optical surfaces when used in conjunction with a coordinate measuring machine, such as a 5 axis coordinate measuring machine. The combination of measurements with the present technology and a coordinate measuring machine allows for measuring surfaces where the shape is only nominally known. When the present technology is aligned and accurately measuring position and orientation relative to the surface, the system is then repositioned using the coordinate measuring machine while using the measurement signals to ensure the relative position and orientation is maintained at a constant level. The coordinate measuring machine&#39;s trajectory is then used to determine the surface&#39;s topography. 
     One novel feature of the technology when used in this configuration is the ability to measure both the front and back surface of the same optic, provided the two surfaces are parallel to within the measurement range of the invention. This enables simultaneous surface metrology, which reduces the measurement time. Alternatively, the front surface can be measured and then subsequently the back surface. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.