WO2021163143A1

WO2021163143A1 - Interferometer systems and methods thereof

Info

Publication number: WO2021163143A1
Application number: PCT/US2021/017394
Authority: WO
Inventors: James F. Munro
Original assignee: Optipro Systems, LLC
Priority date: 2020-02-10
Filing date: 2021-02-10
Publication date: 2021-08-19

Abstract

An interferometer system includes a measurement optical arm, a reference optical arm and an interferometer processing system. The measurement optical arm comprises a measurement optical element positioned in a housing to receive a portion of input light, direct chromatic measurement light from the portion of the input light towards a target, receive chromatic measurement light from the target, and direct detected measurement light from the received chromatic measurement light towards an interferometer processing system. The reference optical arm comprises a prismatic optical element positioned in the housing to receive and direct reference light from the another portion of the input light to the interferometer processing system. The interferometer processing system is coupled to the measurement and reference optical arms to receive the detected measurement light and the reference light. The interferometer processing system is configured to determine at least one measured property of the target when the detected measurement reference lights are received.

Description

INTERFEROME TER SYSTEMS AND METHODS THEREOF

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial

No. 62/972,546, filed February 10, 2020, which is hereby incorporated by reference in its entirety.

FIELD

[0002] The technology generally relates to systems and methods for measuring a property of a test sample, for measuring the displacement of a surface with high accuracy over a long measurement range, and for measuring displacement and depth of artifacts within a test sample volume.

BACKGROUND

[0003] Areal surface interferometry, including areal phase-measuring interferometry, has been used to measure the shape or form of optical surfaces for several decades. While generally quite fast and accurate, prior areal surface interferometry suffers from errors - such as retrace errors - and also introduces unexpected costs and complexities in the surface metrology process.

[0004] For example, areal interferometers often depend on test spheres and null correctors, and an error in their fabrication can result in later errors in the surface topography measurement results. Indeed, the infamous surface errors in the primary mirror of the Hubble Space Telescope have been traced to problems with a null corrector. Since that time NASA - and associated manufacturers of large optics - have been seeking non-areal yet non-contact approaches for high-precision surface metrology. Generally, these approaches entail the use of an optical probe that measures displacement of a surface at a given location, and the probe is then scanned across the surface of interest to generate a complete surface profile.

[0005] One such prior art optical probe is the chromatic probe 10 as shown Figure 1. As seen in Figure 1, a broadband light source 12 produces light that is routed to chromatic probe body 20 through a source fiber optic 14, a fiber coupler 16, and a probe fiber optic 18. Light then exits the probe fiber optic 18 in chromatic probe body 20 and enters a collimating lens 24 which collimates the light. The collimated light then enters a chromatic lens 26 which causes the collimated light to become focused in a spectrally dispersed manner as chromatic light 28.

[0006] Note that the focal position of the chromatic light 28 on optical axis 22 is a function of wavelength, with shorter wavelengths generally coming to a focus closer to chromatic probe body 20 than the longer wavelengths. A test surface 90 of unknown displacement relative to probe body 20 is positioned within the focal field of chromatic light 28 such that one of the wavelengths within chromatic light 28 is well-focused on test surface 90 at measurement spot 30.

[0007] Next, a portion of the chromatic light 28 is reflected from the test surface 90, re enters the chromatic lens 26, and then is re-collimated as it exits the chromatic lens 26. The re collimated light exiting from the chromatic lens 26 re-enters the collimating lens 24 which then focuses the light reflected from the test surface 90 onto the aperture of the probe fiber optic 18 where a substantial portion of the light passes through the aperture and enters the probe fiber optic 18. The same light then propagates through the probe fiber optic 18 to the fiber coupler 16 where approximately half of the light is coupled into an output fiber optic 32 through which the light is coupled to an input of spectrograph 34. The spectrograph 34 spectrally disperses the light and presents an image of the dispersed light on the image sensor of camera 36. The image sensor of the camera 36 captures the spectral image and transmits the spectral image to a digital processor 40 as an electronic signal through camera output line 38. The digital processor 40 analyzes the electronic spectral image to determine the displacement of the surface 90 and outputs the displacement information to a user through the chromatic probe output 42.

[0008] Chromatic probes, such as chromatic probe 10, have been available in the market for several years, and are fast and relatively inexpensive. However, because the spectral image within the spectrograph 34 and captured by camera 36 and then processed by digital processor 40 is a simple Gaussian curve with only one inflection point and low-slope tails, the processing will necessarily lead to a poor and inadequate determination of the central wavelength of light at the spectrograph 34. An inadequate determination of the central wavelength of light will lead to a poor and inadequate estimate of the displacement. Typically, these instruments have displacement measurement accuracies on the order of 100 nanometers at best. Furthermore, these instruments do not perform well when the test surface is polished and tilted at a high angle relative to chromatic light as very little light will be reflected back from test surface 90 to chromatic lens 26, and the subsequent spectral image captured by camera 36 will be dark and non-processable by digital processor 40, and the displacement measurement will fail.

[0009] An alternate displacement measuring device consisting of a spectral interferometric probe 50 is shown in Figure 2. The spectral interferometric probe 50 has a broadband light source 52 that emits light that is subsequently collimated by collimating lens 54 which then enters a beamsplitter 56. The beamsplitter 56 reflects a portion of the collimated light beam whereupon it enters a chromatic lens 68 that focuses the light in such a way that its focal position along an optical axis 58 varies with wavelength. This converging chromatic light then strikes a second beamsplitter 61 which further divides the beam into a reference beam, shown reflecting to the left in Figure 2 towards a reference mirror 64, and a test beam shown propagating downward through the second beamsplitter 61 towards test surface 90. Note that the chromatic lens 68, the second beamsplitter 61, and reference mirror 64 are located within the measurement head 60, which in turn is coupled to a linear piezo-electric transducer stage 62.

The linear piezo-electric transducer stage 62 can cause the measurement head 60, and its internal constituents, to move along optical axis 58 closer to or further away from the test surface 90. Light reflected from reference mirror 64 and test surface 90 (at measurement spot 66) both reflect back to the second beamsplitter 61 and then both re-enter chromatic lens 68 which then re-collimates the two light beams.

[0010] The two re-collimated light beams then pass through the beamsplitter 56, enter focusing lens 70, and then enter the spectrograph 72 through a small aperture at the focal point of the focusing lens 70. The two re-collimated beams then form a spectral interference pattern on the image sensor of camera 74 associated with the spectrograph 72, where the resulting spectral interference fringe pattern is quite similar to the wavelet illustrated in Figure 7. The resulting spectral interference pattern has several inflection points and high-slope regions for improved downstream processing and fitting by digital processor 76.

[0011] Under these conditions, the displacement can be found quite accurately, to less than a nanometer, and is a particular strength of spectral interferometric probe 50. Another strength is that if the measurement test light reflected from the test surface 90 is weak (perhaps because the test surface 90 is highly polished and tilted) then the interferometric gain present in the interference pattern provides a means of intensifying the weak optical signal so that it is of sufficient brightness to be image-able by spectrograph 72 and to be processed by digital processor 76.

[0012] Unfortunately, a serious drawback of spectral interferometric probe 50 is that the optical path lengths of the reference arm and the measurement arm in this example are substantially equal in order to obtain interference fringes at the image sensor of camera 74.

Since the optical path length, or equivalently the displacement, associated with the test surface 90 is unknown, then the position of the reference mirror in this example is scanned, or equivalently, the reference arm is not scanned and instead the whole measurement head 60 is scanned by virtue of PZT 62 until a scanning position is found that produces the desired interference fringes. This scanning process requires a significant amount of time and limits the measurement throughput rate of spectral interferometer probe 50 to about 100 displacement measurement per second.

SUMMARY

[0013] An example of an interferometer system for measuring a property of a test sample has a broadband “white light” laser source, a beamsplitter, a prismatic optical element, a dispersive focusing element, an output focusing element, a fiber optic, a spectrometer, camera or other image sensor, and a digital processing system. The beamsplitter is positioned to divide the broadband beam produced by the laser source into a measurement beam and a reference beam. The prismatic optical element is positioned to receive and cause back-reflected reference light from the reference beam to become incident on the beamsplitter which reflects the back-reflected reference light onto an output focusing element. The output focusing element causes back- reflected reference light to come to a focus at the entrance aperture of a fiber optic.

[0014] The dispersive focusing element is positioned in a path of and the measurement beam passes through resulting in chromatically dispersed focused light which comes to a focus at or on a test sample in which the longitudinal focus position varies with wavelength. The test sample back-scatters or back-reflects a portion of the chromatically dispersed focused light back through the dispersive focusing element which substantially collimates the back-scattered or back-reflected light whereupon it is incident on the beamsplitter. The beam splitter reflects the back-scattered or back-reflected light onto the output focusing element which causes the back- scattered or back-reflected light to also come to a focus at the entrance aperture of the fiber optic. [0015] The fiber optic transmits both the back-scattered reference light and the back- scattered and back-reflected measurement light to the spectrometer. The spectrometer disperses and spectrally and interferometrically combines the back- scattered reference light and the back- scattered or back-reflected measurement light on an image sensor within the spectrometer whereupon an interference pattern is subsequently analyzed by a digital processor system to determine a property of the test sample.

[0016] Another example of an interferometer system includes a measurement optical arm, a reference optical arm, and an interferometer processing system. The measurement optical arm comprises a measurement optical element positioned in an interferometer housing to receive a portion of input source light, direct chromatic measurement light from the portion of the input source light towards a target, receive chromatic measurement light from the target, and direct detected measurement light from the received chromatic measurement light towards an interferometer processing system. The reference optical arm comprises a prismatic optical element positioned in the interferometer housing to receive another portion of the input source light and direct reference light from the another portion of the input source light to the interferometer processing system. The interferometer processing system is coupled to the measurement optical arm and the reference optical arm to receive the detected measurement light and the reference light. The interferometer processing system is configured to determine at least one measured property of the target when the detected measurement light and the reference light are received from the measurement optical arm and the reference optical arm.

[0017] A method for making an interferometer system includes positioning a measurement optical arm comprises a measurement optical element in an interferometer housing to receive a portion of input source light, direct chromatic measurement light from the portion of the input source light towards a target, receive chromatic measurement light from the target, and direct detected measurement light from the received chromatic measurement light towards an interferometer processing system. A reference optical arm comprises a prismatic optical element positioned in the interferometer housing to receive and direct reference light from another portion of the input source light to the interferometer processing system. The interferometer processing system is coupled to the measurement optical arm and the reference optical arm to receive the detected measurement light and the reference light. The interferometer processing system is configured to determine at least one measured property of the target when the detected measurement light and the reference light are received from the measurement optical arm and the reference optical arm.

[0018] Accordingly, examples of the claimed technology provide a number of advantages including providing a displacement measuring system that has the fast displacement measuring rate of a chromatic probe, the high accuracy of a spectral interferometric probe, and the ability of a spectral interferometer probe to measure displacement of uncooperative surfaces. With examples of this technology, the use of an positioning of a reference prism or other reference prismatic element in a reference path fulfills at least three requirements for the successful operation of the interferometer to accomplish at least the functionality noted above, although other types and/or numbers of requirements may be satisfied. By way of example, the reference prism or other reference prismatic element: first has to be and is able to route substantially all wavelengths of light, such as 400nm to 800nm for example, that can be interfered with light from a measurement arm around a beamsplitter; second the optical path length of the light routed through the reference prism or other reference prismatic element of the reference arm has to be and is within a few millimeters of the optical path length traversed by the measurement arm light of the same wavelength; and third the reference prism or other reference prismatic optical element of the reference arm must and can accomplish the first and second requirements with minimal attenuation of the reference light passing through the reference arm.

BRIEF DESCRIPTION OF THE DRAWINGS [0019] Figure 1 is a block diagram of a prior art confocal chromatic displacement measuring system;

[0020] Figure 2 is a block diagram of a prior art confocal chromatic spectral interferometric displacement measuring system;

[0021] Figure 3 is a block diagram of an example of a spectral interferometer system with an exemplary single test surface;

[0022] Figure 4 is a diagram of an example of a spectral interferometer in the spectral interferometer system shown in Figure 3; [0023] Figure 5A is an enlarged view of a diagram of an example of the spectral interferometer shown in Figure 4;

[0024] Figure 5B is an enlarged view of another diagram of the example of the spectral interferometer shown in Figure 4 illustrating an optical path difference; [0025] Figure 6 is a spectral plot of an example of light output by a supercontinuum laser;

[0026] Figure 7 is a spectral plot of an example of an interference pattern at the image sensor of spectral interferometer shown in Figure 4 where a test sample is an optical surface;

[0027] Figure 8 is a diagram of the example of the spectral interferometer system shown in Figure 3 where another exemplary test sample has two test surfaces;

[0028] Figure 9 is a diagram of the example of the spectral interferometer system shown in Figure 3 where yet another exemplary test sample is a volume contained in a cuvette; and

[0029] Figure 10 is a spectral plot of an example of an interference pattern at the image sensor of the spectral interferometer system shown in Figure 3 where a test sample has two surfaces.

DETAILED DESCRIPTION

[0030] An interferometer system 100 in accordance with examples of the claimed technology is illustrated in Figure 3-5B. In this example, the interferometer system 100 includes a laser driver 102, a fiber laser 104, a filter assembly 106, an interferometer 150, an output fiber 114, a spectrograph 116, a camera 118 or other imaging device, a digital processor device 120 and an interferometer system output 122, although the interferometer system 100 may have other types and/or numbers of other components and/or other elements in other configurations. The claimed technology provides a number of advantages including providing a displacement measurement system that has the fast displacement measurement rate of a chromatic probe, the high accuracy of a spectral interferometer probe, and the ability of a spectral interferometer probe to measure displacement of uncooperative surfaces. [0031] Referring more specifically to Figure 3, a light source comprising the laser driver

102 which has an output coupled to a fiber laser 104 whose light output is coupled to an input of filter assembly 106, although other types of light sources with other systems, devices, components and/or elements in other configurations may be used. In this example, the laser driver 102 and the fiber laser 104 together comprise a broadband light source whose output is through a small-diameter aperture at the end of fiber laser 104, although other types of light sources may be used. In one example, the laser driver 102 and the fiber laser 104 are a so-called white-light laser, more technically known as a supercontinuum laser, although other types of white light or broadband light sources, such as those that utilize LEDs or incandescence by way of example, can be used.

[0032] In this example, the requirements and characteristics of the light source are: (1) that the output light pass through a small-diameter output aperture; (2) that as much optical flux passes through the output aperture as possible; and (3) that the output photon flux is broad-band, although other types and/or numbers of requirements and/or characteristics of the light source may be used in other examples. For example, typically the fiber laser 104 is a single-mode fiber and has a core diameter - and exit aperture diameter - of less than lOpm, and in some example advantageously less than or equal to 5pm. As will be seen later, smaller apertures generally yield better displacement-measuring performance. The optical flux exiting fiber laser 104 in this example should be as great as possible, being at least 100pW/nm, or in some examples advantageously at least 200pW/nm, or in other examples advantageously greater than lmW/nm. Finally, in this example the spectrum of the light exiting the laser driver 102 or other light source should be broadband, and also of a wavelength range that the downstream image sensor of camera 118 is responsive to. In particular, in this example light in the range of 450nm to 650nm is advantageous, while light from 400nm to 800nm is even more advantageous, although in other examples other ranges may be used.

[0033] One of ordinary skill in the art will appreciate that the laws of etendue generally restrict the ability of a light source to output relatively large spectral flux values through a small aperture. However, in this example the laser driver 102 and the fiber laser 104 comprising a supercontinuum laser can economically meet these flux emission values with a small (e.g., 5pm diameter) aperture as well as the desired wavelength range. By way of example, Figure 6 shows the spectral output of a typical supercontinuum laser can exceed lOOpW/nm over the entire 400nm to 800nm spectral range. Note that the spectral output of the supercontinuum laser shown in Figure 6 also includes significant amounts of light above 800nm. Unfortunately these wavelengths of light are generally not needed or used by examples of the claimed technology, but instead, if not removed, can propagate into interferometer 150 and be absorbed by components or surfaces inside interferometer 150 thereby generating heat and internal thermal gradients. Since these internal thermal gradients can cause poor displacement measuring performance, it is desirable to filter these longer wavelengths or otherwise prevent them from entering interferometer 150.

[0034] The filter assembly 106 has an input coupled to an output of the fiber laser 104 and an output coupled to a source fiber 108, although other systems, devices, components and/or elements in other configurations may be used. The filter assembly 106 has provisions for filtering the unwanted wavelengths from the light output from fiber laser 104, although other types of filters may be used. In other examples, the filter assembly 106 may also have provisions for filtering unwanted polarizations from the light output from the fiber laser 104 and also ensuring that the polarization passing through the filter assembly 106 and into the source fiber 108 is of a known polarization state and orientation, although the filter assembly may have other types and/or number of provisions. Such polarization filtering is necessary in this example because the quality of the optical interference occurring within the interferometer 100, i.e., the contrast of the resulting interference fringes at the image sensor of camera 118, is a strong function of the polarization states of the two interfering light beams.

[0035] The source fiber 108 is used to couple the filtered light output by the filter assembly 106 to an input of interferometer 150, which in this example is an input to a source arm of the interferometer 150, although other systems, devices, components and/or elements in other configurations may be used. The source fiber 108 is in some examples a single-mode fiber, having a core diameter less than 10pm, or in other example less than 5pm, and transmits all wavelengths of light that are used by interferometer 150, such as 400nm to 800nm, to the interferometer with minimal attenuation. Additionally, since the light output by filter assembly 106 can be polarized, the source fiber 108 has polarization-preserving or polarization- maintaining properties. Further, since the laser driver 102 and the fiber laser 104 are heat generating, in this example the laser driver 102 and the fiber laser 104 are placed a sufficient distance from interferometer 150 so the performance of interferometer 150 is not affected by this heat generating source. In this example the length of the source fiber 108 is at least one meter to provide the sufficient distance, or in other examples at least two meters, provided the length does not significantly attenuate any of the wavelengths transmitted by source fiber 108.

[0036] The interferometer 150 is a device for creating spectral interfering beams of light.

In this example, the interferometer 150 outputs chromatic light 110 that is used as part of an interferometric process for determining a displacement of a test surface 90 at a measurement spot 112

[0037] Referring to Figure 4, 5A, and 5B, a more detailed view of an example of the interferometer 150 of the interferometer system 100 from Figure 3 is illustrated. The interferometer 150 may comprise a housing 151 in which is placed a source right angle parabolic mirror (RAPM) 160, a beamsplitter 170, a measurement arm chromatic lens 180, an output RAPM 190, and a reference arm prism 200 or other reference prismatic optical element, although the interferometer 150 may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations. In this example, the interferometer 150 has a source arm, a reference arm, an output arm, and a measurement arm, although the interferometer may comprise other types and/or numbers of arms, components and/or other elements in other configurations.

[0038] The source arm of the interferometer is where light is introduced into the interferometer 150. The source arm of interferometer 150 comprises the source RAPM 160 which is coupled or otherwise positioned with respect to and between an output end or aperture of the source fiber 108 and the beamsplitter 170, although the source arm may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations.

[0039] In this example, the source RAPM 160 functions to collimate the diverging source light 152 and reflect the collimated light into a direction that causes it to be incident on the input surfaces 170A and 170B of the beamsplitter 170. Since the collimation occurs over a broad spectral range, in this particular example a reflective optic (which has no dispersion) is preferred over a refractive optical element. Further, the source RAPM 160 in this example is located at some distance from the exit aperture of the source fiber 108 so that the diffracting light exiting the exit aperture of the source fiber 108 has expanded enough to substantially fill the reflecting surface of the source RAPM 160. In this example, the distance from the exit aperture of source fiber 108 to the source RAPM 160 to substantially fill the reflecting surface of the source RAPM 160 can be between 25mm and 300mm, with 100mm being a typical distance, although in other examples other distances may be used. For the light reflected from source RAPM 160 to be collimated, the exit aperture of the source fiber 108 in this example is located at the focal point of the source RAPM 160, meaning the focal length of the source RAPM 160 is between 25mm and 300mm, with 100mm being a typical focal length, although again in other examples other distances may be used. The diameter of the source RAPM 160 can be between 5mm and 50mm, although the source RAPM 160 can have other dimensions in other examples.

In this example, the operative parabolic surface of the source RAPM 160 can be made reflective by the use of a reflective coating, the coating being metallic or dielectric, or the substrate of source RAPM 160 can be a reflective metal, such as aluminum, that is polished to the correct optical prescription. If the reflector of source RAPM 160 is a reflective metal, such as aluminum, silver, or gold, then in this example the reflector of the source RAPM 160 can be over-coated with a protective layer of SiO or Si02. In this example, the reflectance of the reflective surface of the source RAPM 160 is at least 90% over the spectral band of interest, or, in some examples greater than 95%, although other percentages may be used in other examples. The substrate of the source RAPM 160 can be a metal such as aluminum or steel, or a non- metallic material such as glass, or even a specialized low-CTE material such as Zerodur or Invar or other material having a CTE of less than lppm per degree K, although other types and/or numbers of materials may be used in other examples.

[0040] The beamsplitter 170 is a six-sided optical object that is substantially shaped like a rectangular cuboid, although other types and/or numbers of beamsplitters and/or other prisms in other shapes may be used in other examples. In this example, four of the six beamsplitter surfaces, 170A, 170B, 170C, 170D, are substantially planar, highly polished, and specularly reflective. Additionally in this example, the remaining two sides (substantially square and parallel to a plane of the viewing perspective shown in in Figures 4, 5A and 5B) are generally unused, but can be planar as well, although having a surface texture that prevents them from being specularly reflective (in this example, the two unused sides are generally coated with a light-absorptive material). Two of the reflective sides, the surfaces 170A and 170B, face the source RAPM 160 and are used to reflect collimated light from the source RAPM 160 into the reference arm (via surface 170A) and into the measurement arm (via surface 170B). The two other reflective sides, surfaces 170C and 170D, face the output RAPM 190 and are used to reflect collimated light from the reference arm to the output RAPM 190 (via surface 170C) and from the measurement arm to the output RAPM 190 (via surface 170D).

[0041] The substrate of beamsplitter 170 can be a metal, such as aluminum or steel, or a non-metallic material, such as glass, or even a low CTE material such as Zerodur or Invar, although other types and/or numbers of materials may be used in other examples. The four reflective surfaces, 170A, 170B, 170C, and 170D, can be between 2mm and 20mm across and between 2mm and 20mm in length, and can be polished to a flatness better than 0. lpm peak-to- valley. Additionally, the intersection between two adjacent surfaces (e g., between surface 170A and 170B) can be dead-sharp, or at least having an edge radius of less than 10 pm to minimize stray light and maximize the optical utilization of the adjoining surfaces. The four reflective surfaces, 170A, 170B, 170C, and 170D, of the beamsplitter 170 can be made reflective by the use of a reflective coating, the coating being metallic or dielectric, or the substrate of beamsplitter 170 can be a reflective metal, such as aluminum, that is highly polished. If the reflector of beamsplitter 170 is a reflective metal, such as aluminum, silver, or gold, then in this example the reflector of beamsplitter 170 can be over-coated with a protective layer of SiO or Si02. In this example, the reflectance of the four reflective surfaces 170A, 170B, 170C, and 170D, of the beamsplitter 170 is at least 90% over the spectral band of interest, or, in some other examples greater than 95%. The central beamsplitter 170 is shared among the source arm, reference arm, measurement arm and output arm, and its reflective surfaces are used to reflect light into and out of the four arms as described herein.

[0042] The reference arm of the interferometer 150 has optics that produce a relatively known reference optical signal. The reference arm of the interferometer 150 may comprise the reference prism 200 which is spaced from the beamsplitter 170 along an optical axis that intersects the reference prism 200 and the beamsplitter 170, although the reference arm may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations.

[0043] The reference prism 200 is a right-angle optical prism oriented with the hypotenuse 200E of the reference prism 200 facing the beamsplitter 170 and substantially orthogonal to the optical axis 154, and further that the first short side 200A of the reference prism 200 is substantially parallel to the input surface 170A of the beamsplitter 170 and that the second short side 200C is substantially parallel to the output surface 170C of the beamsplitter 170, although other configurations of the reference prism 200 and its orientation with the beamsplitter 170 is possible in other examples as well. The first short side 200A of the reference prism 200 can be coated with a reflective coating, such as a metallic coating such as aluminum, silver, or gold, for example, or a dielectric coating, or in other example the first short side 200A of the reference prism 200 can be uncoated. If the first short side 200A of the reference prism 200 is uncoated, for example, then the reflection of light from the first short side 200A can occur with total internal reflection optical phenomenon. The second short side 200C of the reference prism 200 can be coated with a reflective coating, such as a metallic coating such as aluminum, silver, or gold, for example, or a dielectric coating, or in other example the second short side 200C of the reference prism 200 can be uncoated. If the second short side 200C of the reference prism 200 is uncoated, for example, then the reflection of light from the second short side 200C can occur with total internal reflection optical phenomenon for example. The hypotenuse 200E of the reference prism 200 can be left uncoated, or in an example can be coated with an anti- reflective coating to maximize the amount of light transmitted through hypotenuse 200E, and more particularly the entrance section 200B and the exit section 200D comprising the hypotenuse 200E.

[0044] The reference prism 200 can be comprised of optical glass that is substantially transparent to the wavelengths of reference light passing through it. Examples of optical glass that reference prism can be comprised of include fused silica and BK7, for example, although other glass types are possible as well.

[0045] In this example, the reference prism 200 provides a reference path of optical light around the beamsplitter 170 and fulfills at least three requirements for the successful operation of the interferometer 150, although other types and/or numbers of requirements may be used for successful operation of the interferometer 150. First, in this example the reference prism 200 has to be and is able to route substantially all wavelengths of light, such as 400nm to 800nm for example, that can be interfered with light from the measurement arm around the beamsplitter 170; second the optical path length of the light routed through the reference prism 200 of the reference arm has to be and is within a few millimeters of the optical path length traversed by the measurement arm light of the same wavelength; and third the reference prism 200 of the reference arm must and can accomplish the first and second requirements with minimal attenuation of the reference light passing through the reference arm. Accordingly, in this example by fulfilling at least these three requirements the reference prism 200 or other reference optical prismatic element helps to enable the interferometer 150 to accomplish, for example, fast displacement measuring, high accuracy, and measurement of displacement of uncooperative surfaces.

[0046] The measurement arm of the interferometer 150 has optics that produce a relatively unknown measurement arm optical signal carrying information about an unknown property of a test sample, such as a test surface 90 or other test object by way of example only. The measurement arm may comprise the measurement arm chromatic lens 180 which is spaced from the beamsplitter 170 along the optical axis in an opposing direction from the reference prism 200, although the measurement arm may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations. The test sample, such as the exemplary test surface 90, is external to interferometer 150 and during measurement is intersected by the optical axis.

[0047] In this example the measurement arm chromatic lens 180 is a spectrally dispersive optical element that causes collimated measurement input light 176 incident upon the measurement arm chromatic lens 180 to be transmitted through the measurement arm chromatic lens 180 in such a way that the light comes to a sharp focus substantially on the optical axis 154. Being refractive, longer wavelengths are generally brought to a sharp focus further from the measurement arm chromatic lens 180 than the shorter wavelengths. The distance between the focal points at a longer wavelength (such as 800nm) and a shorter wavelength (such as 400nm) can be between 0.01mm and 100mm, and in this example it is highly desirable that the relationship between focal distance and wavelength is a substantially linear relationship. The measurement arm chromatic lens 180 can be a singlet lens element, such as a meniscus lens, and can have a prescription wherein a surface is planar, spherical, or aspherical, and be concave or convex if non-planar, although the measurement arm chromatic lens 180 can comprise other types and/or numbers of lens elements with other configurations, such as six or more lens elements, being typically positioned substantially centered on optical axis 154 by way of example. The measurement arm chromatic lens 180 can also be a free-form lens, although in this example the measurement arm chromatic lens 180 is not free-form and has rotational symmetry about optical axis 154. [0048] One example of the measurement arm chromatic lens 180 results in a 1mm chromatic working range (i.e., maximum to minimum measurement range) at a distance of 13.5mm from the output surface of chromatic lens 180. In this example, the chromatic lens 180 is a singlet lens in which the input surface is aspherical with a base radius of curvature of 10.274mm and a -0.356105 conic constant, the output surface is spherical with a 35.419mm radius of curvature, a center thickness of 3.5mm, and is comprising S-TIH3 glass from Ohara Inc., (Kanagawa, Japan). Alternately, instead of the chromatic lens 180, the spectrally dispersive optical element in the measurement arm of interferometer 150 can be a diffractive optical element (DOE), such as a holographic optical element (HOE) or even a computer generated holographic optical element.

[0049] The output arm of the interferometer 150 carries the combined reference arm optical signal and measurement arm optical signal downstream to an interferometer processing system comprising the spectrograph 116, the camera 118, and the digital processor device 120, although the interferometer processing system may comprises other types and/or numbers of other systems, devices, components and/or other elements in other configurations. The output arm of the interferometer 150 comprises the output RAPM 190 which is coupled or otherwise positioned with respect to and between the beamsplitter 170 and an input end or aperture of the output fiber 114, although the output arm may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations.

[0050] In this example, the output RAPM 190 functions to focus the collimated output light incident on the RAPM 190 from the beamsplitter 170 and reflect the resulting converging output light 192 into a direction that causes it to be incident on the entrance aperture of the output fiber 114. Since the focusing occurs over a broad spectral range, in this example a reflective optic (which has no dispersion) is preferred over a refractive optical element. Additionally, the output RAPM 190 in this example is located at a distance from the entrance aperture of the output fiber 114 so that the focusing can occur over a distance, such as 50mm by way of example only. Further, for mechanical symmetry (which can mitigate spurious mechanical component positional movements due to thermal gradients) the distance from the output RAPM 190 to the entrance aperture of the output fiber 114 is in some examples the same as that distance from the exit aperture of the source fiber 108 to the source RAPM 160. The distance from the output RAPM 190 to the entrance aperture of the output fiber 114 can be between 25mm and 300mm, with 100mm being a typical distance, although in other examples other distances may be used. For the light reflected from the output RAPM 190 to be focused onto the entrance aperture of the output fiber 114, the entrance aperture of the output fiber 114 in this example is located at the focal point of the output RAPM 190, meaning the focal length of the output RAPM 190 is between 25mm and 300mm, with 100mm being a typical focal length, although again in other examples other distances may be used. The diameter of the output RAPM 190 can be between 5mm and 50mm, although the output RAPM 190 can have other dimensions in other examples.

[0051] In this example, the operative parabolic surface of the output RAPM 190 can be made reflective by the use of a reflective coating, the coating being metallic or dielectric, or the substrate of the output RAPM 190 can be a reflective metal, such as aluminum, that is polished to the correct optical prescription. If the reflector of the output RAPM 190 is a reflective metal, such as aluminum, silver, or gold, then in this example the reflector of the output RAPM 190 can be over-coated with a protective layer of SiO or Si02. In this example, the reflectance of the reflective surface of the output RAPM 190 is at least 90% over the spectral band of interest, or, in some examples advantageously greater than 95%, although other percentages may be used in other examples. The substrate of the output RAPM 190 can be a metal such as aluminum or steel, or a non-metallic material such as glass, or even a specialized low-CTE material such as Zerodur or Invar or other material having a CTE of less than lppm per degree K, although other types and/or numbers of materials may be used in other examples.

[0052] The housing 151 in this example is a mechanical component which houses or encloses, and onto which are mounted (either directly or through additional mechanical coupling, mounting, and/or positional adjustment components) the output end of the source fiber 108, the source RAPM 160, the beamsplitter 170, the reference prism 200, the measurement arm chromatic lens 180, the output RAPM 190, and the input end of the output fiber optic 114, although other mounting configurations with other types and/or numbers of elements may be used. The housing 151 is made of a material having a low CTE (coefficient of thermal expansion) so that one or more of the output end of the source fiber 108, the source RAPM 160, the beamsplitter 170, the reference prism 200, the measurement arm chromatic lens 180, the output RAPM 190, and the input end of the output fiber optic 114 do not move relative to one another as the ambient temperature varies which can in turn cause erroneous displacement measurements. In this example, the housing 151 can be made of Invar, a metal alloy having an exceptionally low CTE, or a glass or glass/ceramic such as Zerodur or ULE which also have low CTE’s, although other types and/or numbers of materials may be used. Additionally, the housing 151 in this example completely encloses the source RAPM 160, the beamsplitter 170, the reference prism 200, the measurement arm chromatic lens 180, and the output RAPM 190, while leaving open the optical aperture 181 associated with the measurement arm chromatic lens 180, and may be optically opaque so that stray ambient light does not affect the performance of interferometer system 100 and cause erroneous displacement measurements. Further, an interior surface of the housing 151 may be painted or otherwise coated with a light absorbing material - with light-trapping surface properties - such as a heavily textured black paint, to absorb any stray light that may inadvertently enter into the housing 151 from the outside, or inadvertently generated from inside the housing 151.

[0053] Referring to FIGS. 3-5B, in this example the test surface 90 is the surface whose displacement, or distance, from the interferometer 150, or a reference point on the interferometer 150, such as the apex of the lower surface of the measurement arm chromatic lens 180, is to be measured, although other types of surfaces may be measured. The interferometer system 100 is generally capable of measuring the displacement at only one location on the test surface 90, such as at measurement spot 112, at a time, and therefore in order to measure the topography of the test surface 90 the interferometer 150 in this example must be translated across test surface 90 in at least one, but in some example two, axes. Note that if the maximum to minimum displacement of the topography of test surface 90 exceeds the working measurement range of the interferometer system 100 then the interferometer 150 will have to translate in the vertical direction as well during the scanning to accommodate the wide variations in displacement.

[0054] By way of example only, the test surface 90 can be a surface of a relatively small object, having a measurement width as small as 1mm, or a surface of a relatively large object having a width as large as ten meters, although surfaces with other dimensions can be measured. The test surface 90 can be highly polished, such as a telescope mirror, or have a texture, although the test surface 90 can have other surface characteristics in other examples. The test surface 90 can be a metallic surface, such as aluminum, gold, silver, or silicon, or a non-metallic surface such as glass or even polymer, although other types and/or numbers of materials can be used for the test surface 90. The test surface 90 can be smooth and free of discontinuities or other abrupt changes in elevation, or it can have discontinuities, either of which can be readily measured by examples of the claimed technology.

[0055] The test surface 90 can be substantially perpendicular to the optical axis 154 that extends through the interferometer 150, or the test surface 90 can be tilted with respect to optical axis 154 up to 60 degrees (in any direction about optical axis 154) or even up to 80 degrees in other examples. As such, the light back-reflected or back-scattered from test surface 90 back into the interferometer 150 through the measurement arm chromatic lens 180, such as the diverging chromatic test light 186, can be either diffusely or specularly reflected from test surface 90, and can be between 0.000001% and 99.99% of the converging chromatic test light 182 directed onto test surface 90.

[0056] In this example, the output fiber 114 is used to couple the light signal output by the interferometer 150 to an input of spectrograph 116 of the interferometer processing system, although manners for outputting the reference arm optical signal and measurement arm optical signal or other signals may be used. The output fiber 114 is in this example is a single-mode fiber, having a core diameter less than 10pm, or in other examples less than 5pm, and transmits all wavelengths of light that are used by the interferometer 150, such as from 400nm to 800nm, to the spectrograph 116 with minimal attenuation, although optical fibers with other characteristics may be used. Additionally, since the light utilized and output by the interferometer 150 can be polarized, the output fiber 114 in this example has polarization preserving or polarization-maintaining properties. Further, in this example the spectrograph 116 and accompanying camera 118 that are heat generating are placed a sufficient distance from the interferometer 150 so the performance of the interferometer 150 is not affected by these adjacent heat sources. In this example, the length of output fiber 114 to provide sufficient distance is at least one meter, or in some examples at least two meters, provided the length does not significantly attenuate any of the wavelengths of light output by the interferometer 150 which in this example is the reference arm optical signal and measurement arm optical signal.

[0057] The spectrograph 116 is an optical instrument that is used to spectrally disperse an optical signal, such as the reference arm optical signal and the measurement arm optical signal by way of example, into a spectrum of wavelengths such that the constituent wavelengths, which are generally unknown but are desired to be known, within the optical signal can be analyzed. Spectra produced by the spectrograph 116 is coupled to an input of an image sensor of the camera 118 that captures imagery of the spectra produced by the spectrograph 116, although other types of imaging devices may be used. That is, the output of the spectrograph 116 is an optical signal, such as the reference arm optical signal and the measurement arm optical signal by way of example, being presented as intensity as a function of wavelength, and an image of this optical signal is subsequently presented to the camera 118 which captures the image, converts the image to an electronic format, and transmits the electronically formatted spectral image to the digital processor device 120 for processing. The spectrograph 116 nominally has the same spectral bandwidth, or free spectral range, or chromatic range, of the interferometer 150, such as the 400nm to 800nm spectral range example cited earlier. The spectral resolution of the spectrograph 116 in this example is fine enough that the individual interference fringes within the wavelet interferogram of Figure 7 can be resolved. Therefore, the resolution of spectrograph 116 can be better than 100pm (picometers), or in other examples less than 50pm, or in yet other examples better than 20pm. One such spectrograph that meets these requirements is the Hornet Hyperfme Spectrometer from LightMachinery Inc., Ottawa, Ontario, Canada.

[0058] In this example, the camera 118 captures an image of the optical signal or spectrum created by the spectrograph 116 and converts the image to an electronic format. In this example, the camera 118 is a line camera, wherein the image sensor of the camera 118 includes a row of pixels arranged linearly, and onto which a spectral image is projected by the spectrograph 116. In such a case, there can be between 256 and 16,384 pixels in the image sensor whose length can be up to 100mm, and the imaging frame rate can be up to 200,000 captured images per second.

[0059] In other examples, the spectrograph 116 and the camera 118 may be operative with two-dimensional spectral images. In this two-dimensional example, the camera 118 can have an image sensor whose pixels are arranged in a two-dimensional array wherein the pixel count can be from 640 x 480 pixels up to 10,000 x 5000 pixels, the size of the image sensor can be from 3.2mm x 2.4mm up to 50mm x 25mm, and the frame rate can be between one image / second up to 50,000 images / second. The camera 118 in this example is a monochrome camera (as opposed to color) and has a gray-scale bit depth of from 8 bits up to 20 bits, although other types of cameras may be used. The output of the camera 118 is coupled to an input of the digital processor device 120. [0060] The digital processor device 120 may include one or more processors, a memory, and/or a communication interface, which are coupled together by a bus or other communication link, although the digital processor device 120 can include other types and/or numbers of elements in other configurations and also other types of processing systems may be used. The processor(s) of the digital processor device 120 may execute programmed instructions stored in the memory for the any number of the functions described and illustrated herein. The processor(s) of the digital processor device 120 may include one or more CPUs or general purpose processors with one or more processing cores, for example, although other types of processor(s) can also be used although the digital processor device 120 may comprise other types and/or numbers of systems, devices, components and/or elements in other configurations.

[0061] The memory of the digital processor device 120 stores these programmed instructions for one or more aspects of the present technology as described and illustrated herein, such as for generating spectral content values of light output by the interferometer 150 and for determining displacement or some other property of a test sample, such as test surface 90 or a test object as described and illustrated herein for execution by the digital processor unit 120 by way of example, although some or all of the programmed instructions could be stored elsewhere. A variety of different types of memory storage devices, such as a random access memory (RAM), read only memory (ROM), hard disk, solid state drives, flash memory, or other computer readable medium which is read from and written to by a magnetic, optical, or other reading and writing system coupled to the processor(s), can be used for the memory.

[0062] By way of example only, the digital processor device 120 also could be a conventional microprocessor with an external memory or the digital processor device 120 can be a microcontroller with all memory located onboard. In another example, the digital processor device 120 could be a digital signal processor (DSP) integrated circuit, which is a microcomputer that has been optimized for digital signal processing applications, including centroid computations, regression, and curve-fitting. In yet another example, the digital processor device 120 could be a graphical processing unit (GPU) integrated circuit, which is a microcomputer that has been optimized for parallel-processing applications. The digital processor device 120 could be as simple as a sixteen-bit integer device for low-cost applications or the digital processor device 120 can be a thirty-two bit or sixty-four bit or higher floating-point device or system for higher performance when cost is not an issue. Also, by way of example only, the digital processor device 120 could be an FPGA (Field-programmable gate array) or a CPLD (complex programmable logic device) which are attractive for use in examples of this technology owing to their compact and cost-effective hardware implementations.

[0063] Examples of one or more portions of the claimed technology as illustrated and described by way of the examples herein may also be embodied as one or more non-transitory computer readable media having instructions stored thereon for one or more aspects of the present technology, such as the memory of the digital processor device 120. The instructions in some examples include executable code that, when executed by one or more processors, such as the processor(s) of the digital processor device 120, cause the one or more processors to carry out steps necessary to implement the methods of the examples of this technology that are described and illustrated herein.

[0064] In this example, the interferometer system output 122 is an electronic signal line that couples an output of digital processor device 120 to an input of a downstream electronic device, such as a client computer or a display (not shown). As such the interferometer system output 122 is typically a serial bus such as USB or SPI bus, for inter-computer communications, or HDMI in the case where the downstream electronic device is a display. The data communicated through interferometer system output 122 bus can be the displacement measured by the interferometer system 100, as well as other data, such as meta-data, about the displacement measurement process.

[0065] An example of a method for measuring displacement with the interferometer system 100 will be described with reference to Figures 3-5B. Referring to Figure 3, a light source comprising the laser driver 102 and the fiber laser 104 outputs broadband light from the fiber laser 104 which then enters the filter assembly 106.

[0066] In this example the filter assembly 106 then removes unwanted wavelengths from the broadband light, such as those from 800nm to 2400nm, although the filter assembly 106 could provide other types of filtering. The filter assembly 106 can also remove unwanted polarizations from the source light, such that the light, for example, that exits the filter assembly 106 is linearly polarized by way of example. The filtered light that exits the filter assembly 106 is transmitted through the source fiber 108 to an input port of the source arm associated with the interferometer 150. [0067] Referring now to Figures 4, 5A, and 5B, filtered light that exits the source fiber

108 is shown as diverging source light 152, whose divergence is due primarily to the light being diffracted as it exits from the 5 pm diameter aperture of single mode source fiber 108. The diverging source light 152 is incident on the source RAPM 160 which collimates the light and also reflects the collimated light 90 degrees such that the collimated reflected light becomes incident on two mirrored sides of the beamsplitter 170, namely input surface 170A and input surface 170B in this example, although other configurations could be used in other examples.

[0068] As shown in greater detail in Figure 5A, the upper portion of the reflected collimated light beam is the upper source beam 162, which reflects from the input surface 170A into the input reference light 172. The input reference light 172 is incident on, and is transmitted through, the entrance section 200B of the hypotenuse 200E of the reference prism 200, whereupon the light is reflected from the first short side 200A and directed onto and reflected from the second short side 200C. Light that is reflected from the second short side 200C is incident on the exit section 200D of the hypotenuse 200E of the reference prism 200 whereupon the light is transmitted through the exit section 200D and forms the output reference light 174. Note that the output reference light 174 can be substantially parallel to the input reference light 172, and in an example are parallel to the axis 154. Note further that all wavelengths of light present in the input reference light 172, in this example, are present in the output reference light 174. Further, in this example, both the input reference light 172 and the output reference light 174 are substantially collimated.

[0069] The output reference light 174 is reflected from output surface 170C into reflected output reference light 194, which is still collimated, which is then incident on the output RAPM 190. The output RAPM 190 then both causes the collimated reflected output reference light 194 to come to a focus, and reflects the reflected output reference light 194 into a direction such that the reflected light, converging output light 192, comes to a sharp focus at the entrance aperture of the output fiber 114.

[0070] Note that all of the wavelengths of light present in the light output by the filter assembly 106 are also present in the light entering output fiber 114 from the reference arm of interferometer 150 and is a one feature of examples of the claimed technology provided by the reference prism 200. Further, this broadband output reference light that enters the output fiber 114 propagates through the output fiber 114 to the spectrograph 116. [0071] The spectrograph 116 spectrally disperses all the wavelengths of the broadband reference light and projects the spectrum onto the image sensor of the camera 118. This reference light is then available to produce interference fringes on the image sensor of the camera 118 with any dispersed light from the measurement arm that is concurrently projected onto the image sensor of the camera 118.

[0072] As mentioned earlier, the diverging source light 152 is incident on the source

RAPM 160 which collimates the light and also reflects the collimated light 90 degrees such that the collimated reflected light becomes incident on two mirrored surfaces of the beamsplitter 170, namely the input surface 170A and the input surface 170B. As shown in greater detail in Figure 5 A, the lower portion of the reflected collimated light beam is the lower source beam 164, which reflects from the input surface 170B into the input measurement light 176 such that the input measurement light 176 is incident on the measurement arm chromatic lens 180. The measurement arm chromatic lens 180 then causes the input measurement light 176 to come to a sharp focus along optical axis 154 in the measurement space through the converging chromatic measurement light 182. Note, however that since measurement arm chromatic lens 180 is designed to be highly dispersive, the sharp focus for each wavelength occurs at a different position on the optical axis 154 at or near the test surface 90. For example, longer wavelength light, such as light having a wavelength of 800nm, is shown coming to a focus at the long wavelength focal position 185 while shorter wavelength light, such as light having a wavelength of 400nm, is shown coming to a focus at the short wavelength focal position 183.

[0073] The test surface 90 will reflect or back-scatter a portion (i.e., that portion that was not absorbed or transmitted) all light incident upon it, including the light that is brought to a sharp focus along the optical axis 154 at measurement spot 112. A portion of light from measurement spot 112 is back-reflected as diverging chromatic measurement light 186 which subsequently becomes incident on the measurement arm chromatic lens 180. The measurement arm chromatic lens 180 then collimates and effectively de-achromatizes the diverging chromatic measurement light 186 and outputs the output measurement light 178 which then becomes incident on the mirrored output surface 170D of the beamsplitter 170. The output measurement light 178 is reflected from the mirrored output surface 170D into the reflected output measurement light 196, which is still collimated, which is then incident on the output RAPM 190. [0074] The output RAPM 190 then causes the collimated reflected output measurement light 196 to come to a focus, and reflects the reflected output measurement light 196 into a direction such that the reflected light, part of converging output light 192, comes to a sharp focus at the entrance aperture of the output fiber 114. Note, however, unlike the reference arm light, the light entering output fiber 114 from the measurement arm is nearly monochromatic because in this example (1) only a narrow band of wavelengths of converging chromatic light 182 is in focus at measurement spot 112 on test surface 90, where the center value of the wavelengths is a direct function of the displacement of test surface 90 along the optical axis 154; and (2) of all the light back-reflected from test surface 90, whether in focus or not at the displacement of the test surface 90, only that light originating at the measurement spot 112 will be in sharp focus at the entrance aperture of and actually enter the output fiber 114.

[0075] Between these two focus mechanisms the optical bandwidth of the measurement light actually entering output fiber 114 is on the order of a few nanometers. This measurement arm wavelength selectivity is also one of the advantages of examples of the claimed technology. In this example, the narrower the spectral bandwidth of the measurement light entering the output fiber 114, the narrower the envelope of the spectral fringe wavelet (see for example Figure 7) and the better the localization and determination of the parameters of the wavelet, and the better the determination of the displacement of test surface 90 or some other property of a test object. The narrowband output measurement arm light that enters the output fiber 114 propagates through the output fiber 114 to the spectrograph 116.

[0076] The spectrograph 116 spectrally disperses the narrowband measurement light and projects the spectrum onto the image sensor of the camera 118. This measurement light interferes and produce interference fringes on the image sensor of the camera 118 with the dispersed reference arm optical light from the reference arm that is concurrently projected onto the image sensor Note, in this example another requirement for interference to occur is that the two interfering beams in this example are of substantially the same polarization, which occurs in examples of the claimed technology by virtue of the polarizing filter in filter assembly 106 and the polarization-preserving properties of the source fiber 108 and the output fiber 114.

[0077] In this example, another requirement to produce high contrast interference fringes is that the two interfering beams are coherent with one another, which means that the optical path difference (OPD) between the lengths of the propagation paths of the two interfering beams (namely the measurement arm path and the reference arm path) is less than the coherence length of the light being interfered.

[0078] As shown in Figure 5B, in this example the OPD is the optical path length of the reference arm (OPLR) minus the optical path length of the measurement arm (OPLM) within the interferometer. Note that the path lengths are the same in the source arm and in the output arm since the paths are the same. In this example, the OPLR is the optical distance from the centerline 155 to the apex 217 of the reference prism 200 and the OPLM is the optical distance from the centerline 155 to the measurement spot 112 wherein OPLM can be further subdivided into the sum of a constant optical path length (OPLK) that extends from centerline 155 to an arbitrary reference point, such as the vertex of measurement chromatic lens 180 and D. The displacement D to be determined by interferometer system 100 can extend from the arbitrary reference point at the terminus of OPLK to the elevation of test surface 90 at measurement spot 112, where the centerline 155 is that line that passes through the center of the RAPM 190 as well as the center of the beamsplitter 170. For interference to occur:

OPD = I OPLM — OPLR \ < Coherence length Equation 1

[0079] The coherence length is a function of the spectral bandwidth of the light beams that are interfering, which in examples of the claimed technology is dictated by the spectral resolution of the spectrograph 116. The formula for coherence length is:

Equation 2

[0080] where l is the center wavelength of the interfering light and BW is the spectral bandwidth of the interfering light. The coherence length, l, and BW all have units of microns.

As an example, if l= 0.5pm, if the resolution of the spectrograph 116 is O.OOOlpm (100pm), then the coherence length is 1103pm, which means the maximum displacement measurement range is approximately ±l.lmm from its mid-point or 2.2mm total.

[0081] As mentioned above, in this example the reference arm optical signal comprising the reflected output reference light 194 and the measurement arm optical signal comprising the reflected output measurement light 196 are both focused and reflected onto the entrance aperture of output fiber 114 which transmits these two light signals to the spectrograph 116 which spectrally disperses them and projects them onto the image sensor of the camera 118 where they interfere and form an interference signal, or spectral interferogram, on the image sensor of the camera 118. Equation 3, below, symbolically Equation 3

[0082] describes the spectral interferogram, where IT(l) is the total intensity of brightness at a given wavelength, IR is a substantially constant term due to the light back- reflected from the reference prism 200 (i.e., output reference light 174), AM represents the amplitude intensity of the light reflected from the test surface (i.e., diverging chromatic measurement light 186) the third (Gaussian) term is the spectral envelope of a non-interfering portion of light back-reflected from the test surface (i.e., diverging chromatic measurement light 214), and the final term represents the interference between the reference arm and measurement arm light signals. Equation 3 is illustrated graphically as the wavelet function in Figure 7. The product of IR times AM within the square root of Equation 3 represents interferometric gain, especially if IR is much greater than AM (which is usually the case when test surface 90 is uncooperative) which in turn greatly increases the amplitude of the cosine with the result that the fringes have a larger amplitude on the image sensor of the camera 118. Note that larger signal amplitudes on the image sensor of camera 118 can also translate to faster camera 118 shutter speeds and in turn can lead to faster displacement measurement rates of examples of the claimed technology. Nonetheless, the primary quantity of interest in Equation 3 is OPD, which includes the displacement quantity, D, to be determined. Continuing from Equation 1:

OPD — OPLR — ( OPLK + D ) Equation 4 and after rearranging terms:

D = OPD — OPLR + OPLK . Equation 5

[0083] Since OPLK is a constant, OPD is determined during the processing operations executed by digital processor device 120, and the relationship between OPLR and OPLM is known a priori by way of a calibration process, then the displacement D can be found by a determination of OPD in Equation 3. [0084] After the digital processor device 120 determines the displacement D from the spectral image data electronically communicated to the digital processor device 120 from an output of camera 118, the digital processor device 120 can then transmit displacement data to a user or a remote computing device through the interferometer system output 122. [0085] Returning to Equation 3, if the reference arm was absent and the reflected output reference light 194 did not exist, then there would be no interference occurring at the image sensor of the camera 118 and the curve of Figure 7 would be a simple Gaussian curve according to Equation 6:

Equation 6 [0086] in which case the displacement is found by determining the location of the peak of the Gaussian curve at Xc, which is generally accurate to a few microns of displacement error (this is essentially the configuration of the prior art discussed in connection with Figure 1). However, with the addition of the cosine term of Equation 3 caused by the reference arm signal, the wavelet waveform of Figure 7 can be further processed by a more sophisticated algorithm, such as a least squares fitting algorithm by way of example, executing within digital processor device 120 to find the value of the displacement to less than a nanometer, or even better, which is a another benefit of examples of the claimed technology. Further, if the displacement measuring range, defined by the distance between the long wavelength focal position 185 and the short wavelength focal position 183 along the optical axis 154 is greater than 1mm, and the displacement measurement performance (performance defined to be either accuracy and/or repeatability) is one nanometer, then the ratio of range to performance of examples of the claimed technology can be greater than or equal to 1,000,000.

[0087] A few examples of other variations of the claimed technology are illustrated and described below. For example, as shown in Figure 8, instead of there being a single test surface, such as test surface 90, there can be multiple test surfaces, such as upper test surface 290 and lower test surface 292 separated by a medium 294 having a refractive index and a thickness “t”.

[0088] When two such test surfaces are present, the resulting spectral interferogram will be as illustrated in Figure 10 and will comprise two wavelet patterns which are processed by the digital processor device 120 to determine their respective OPD’s and displacement. Note that the refractive index of the medium 294 is taken into account during the wavelet processing for the lower test surface 292 as the chromatic light 110 passes through medium 294 in order to reach the lower test surface 292. Alternately, if the refractive index of medium 294 is unknown and the thickness “t” of medium 294 is known, the refractive index of medium 294 can then be determined with great precision with interferometer system 100.

[0089] Additionally, instead of there being just one or two surfaces, there can be three or more surfaces in the sample volume of the interferometer 150 along the optical axis 154. For example, as shown in Figure 9, the interferometer 150 is used to measure the displacement of various objects within a test medium 298 within a cuvette 296, although in other example test medium 298 does not reside within a cuvette 296. Further, if the cuvette 296 along with a medium 298 within the cuvette 296 is translationally scanned laterally within the chromatic light 110, or in some examples interferometer 150 is translationally scanned across the cuvette 296, then when a multitude of displacement measurements are made by the interferometer system 100 of the objects within medium 298 a complete and detailed three-dimensional image of the objects within medium 298 can be produced.

[0090] Example of objects or artifacts that can be measured or 3D-imaged by the examples of the claimed technology that can be in the medium 298 of the cuvette 296 can include biological samples, such as larval fish, algae, plankton, and other single and multicellular organisms by way of example. The medium 298 can be a solid, a liquid, or gaseous, and if the medium 298 is a liquid can be organic or aqueous and if the medium 298 is gaseous can be air, partially evacuated air, or a noble gas by way of example. Alternately, the cuvette 296 can be dispensed with, and the medium 298 can be a solid article of manufacture in which case the object within the medium 298 can be an objectionable crack, bubble, defect, or inclusion, and the three-dimensional scanning process amounts to a non-destructive (subsurface) test of an article of manufacture. Lastly, the test object can be a sample of tissue, such as human tissue by way of example, in which the interferometer system 100 is used, again in scanning mode, to generate a detailed three-dimensional image of the sub-surface structure and organelles below the surface of the tissue sample.

[0091] Other benefits of examples of the claimed technology are that the longitudinal depth of field, the lateral resolution, and the longitudinal accuracy are not coupled to one another. In this example, the depth of field is determined by the distance from the longitudinal position of the short wavelength focal position 183 to the longitudinal position of the long wavelength focal position 185, the lateral resolution is determined by the width of measurement spot 112 (for each wavelength) of chromatic light 110, and the longitudinal accuracy is determined by the accuracy performance of the processing of Equation 3. [0092] Another benefit of examples of the claimed technology is the ability to process weak optical signals, such as occur when the test surface 90 is highly sloped and non-diffusive (i.e., most of the converging chromatic measurement light 182 is reflected by the test surface 90 away from the chromatic lens 180), absorptive, or otherwise uncooperative; when a 3D test object has weak optical reflections from surfaces or objects deep within the 3D test object; or when the displacement is relatively large. In each case - and others - the diverging chromatic measurement light 186 can include as few as only a couple dozen photons per measurement time yet because of the interference gain term of Equation 3 the spectral interference pattern can still be discernable and processable and yield accurate displacement measurements.

[0093] Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations, such as arrows in the diagrams therefore, is not intended to limit the claimed processes to any order or direction of travel of signals or other data and/or information except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.

Claims

CLAIMS What is claimed is:

1. An interferometer system comprising: a measurement optical arm comprising a measurement optical element positioned in an interferometer housing to receive a portion of input source light, direct chromatic measurement light from the portion of the input source light towards a target, receive chromatic measurement light from the target, and direct detected measurement light from the received chromatic measurement light towards an interferometer processing system; a reference optical arm comprising a prismatic optical element positioned in the interferometer housing to receive and direct reference light from another portion of the input source light to the interferometer processing system; and the interferometer processing system is coupled to the measurement optical arm and the reference optical arm to receive the detected measurement light and the reference light, wherein the interferometer processing system is configured to determine at least one measured property of the target when the detected measurement light and the reference light are received from the measurement optical arm and the reference optical arm.

2. The system as set forth in claim 1 wherein the interferometer processing system further comprises: a spectrograph with an image sensor coupled to receive the detected measurement light and the reference light and generate spectral image data; and a digital processing device coupled to the spectrograph with the image sensor, wherein the digital processing system comprises a memory comprising programmed instructions stored thereon and one or more processors configured to be capable of executing the stored programmed instructions to: determine at least one measured property of the target from the generated spectral image data; and provide the determined at least measured property of the target.

3. The system as set forth in claim 1 wherein the measurement optical element further comprises a measurement chromatic lens.

4. The system as set forth in claim 3 further comprising: a reflective prism structure positioned between the measurement optical arm and the reference optical arm; wherein the reflective prism structure comprises measurement mirrored surfaces and reference mirrored surfaces; wherein an axis extends between the measurement mirrored surfaces and between the reference mirrored surfaces; wherein one of the measurement mirrored surfaces on one side of the axis is positioned to direct the portion of the input source light towards the measurement chromatic lens and wherein another one of the measurement mirrored surfaces on an opposing side of the axis is positioned to direct the detected measurement light received towards the interferometer processing system; and wherein one of the reference mirrored surfaces on another side of the axis is positioned to direct the another portion of the input source light towards the reference arm prism and wherein another one of the reference mirrored surfaces on an opposing side of the axis is positioned to direct the reference light received from the reference arm prism towards the interferometer processing system.

5. The system as set forth in claim 4 further comprising: an input optical arm positioned in the interferometer system to direct the input source light towards the one of the measurement mirrored surfaces and the one of the reference mirrored surfaces on one side of the axis; and an output optical arm positioned in the interferometer system to direct the detected measurement light from the another one of the measurement mirrored surfaces and the reference light from another one of the reference mirrored surfaces towards the interferometer processing system.

6. The system as set forth in claim 5 wherein: the input optical arm comprises an input right angle parabolic mirror; the output optical arm comprises an output right angle parabolic mirror; and the reflective prism structure comprises a beamsplitter.

7. The system as set forth in claim 5 further comprising: a light source coupled to direct the input source light towards the input optical arm, wherein the light source comprises a laser driver coupled to a fiber laser comprising a single mode fiber with a core diameter and exit aperture diameter of less than or equal to lOpm.

8. The system as set forth in claim 7 wherein the light source further comprises: a filter assembly is coupled in the light source to filter at least one property of the source light from the laser, wherein the at least one property comprises at least one of one or more wavelengths or one or more polarizations.

9. A method for making an interferometer system, the method comprising: positioning a measurement optical arm comprising a measurement optical element in an interferometer housing to receive a portion of input source light, direct chromatic measurement light from the portion of the input source light towards a target, receive chromatic measurement light from the target, and direct detected measurement light from the received chromatic measurement light towards an interferometer processing system; positioning a reference optical arm comprising a prismatic optical element in the interferometer housing to receive and direct reference light from another portion of the input source light to the interferometer processing system; and coupling the interferometer processing system to the measurement optical arm and the reference optical arm to receive the detected measurement light and the reference light, wherein the interferometer processing system is configured to determine at least one measured property of the target when the detected measurement light and the reference light are received from the measurement optical arm and the reference optical arm.

10. The method as set forth in claim 9 wherein the coupling the interferometer processing system to the measurement optical arm and the reference optical arm further comprises: coupling a spectrograph with an image sensor to receive the detected measurement light and the reference light, wherein the spectrograph and the image sensor are configured to generate spectral image data; and coupling a digital processing device to the spectrograph with the image sensor, wherein the digital processing system comprises a memory comprising programmed instructions stored thereon and one or more processors configured to be capable of executing the stored programmed instructions to: determine at least one measured property of the target from the generated spectral image data; and provide the determined at least measured property of the target.

11. The method as set forth in claim 9 wherein the measurement optical element further comprises a measurement chromatic lens.

12. The method as set forth in claim 11 further comprising: positioning a reflective prism structure between the measurement optical arm and the reference optical arm; wherein the reflective prism structure comprises measurement mirrored surfaces and reference mirrored surfaces; wherein an axis extends between the measurement mirrored surfaces and between the reference mirrored surfaces; wherein one of the measurement mirrored surfaces on one side of the axis is positioned to direct the portion of the input source light towards the measurement chromatic lens and wherein another one of the measurement mirrored surfaces on an opposing side of the axis is positioned to direct the detected measurement light received towards the interferometer processing system; and wherein one of the reference mirrored surfaces on another side of the axis is positioned to direct the another portion of the input source light towards the reference arm prism and wherein another one of the reference mirrored surfaces on an opposing side of the axis is positioned to direct the reference light received from the reference arm prism towards the interferometer processing system.

13. The method as set forth in claim 12 further comprising: positioning an input optical arm in the interferometer system to direct the input source light towards the one of the measurement mirrored surfaces and the one of the reference mirrored surfaces on one side of the axis; and positioning an output optical arm in the interferometer system to direct the detected measurement light from the another one of the measurement mirrored surfaces and the reference light from another one of the reference mirrored surfaces towards the interferometer processing system.

14. The method as set forth in claim 13 wherein: the input optical arm comprises an input right angle parabolic mirror; the output optical arm comprises an output right angle parabolic mirror; and the reflective prism structure comprises a beamsplitter.

15. The method as set forth in claim 13 further comprising: coupling a light source to direct the input source light towards the input optical arm, wherein the light source comprises a laser driver coupled to a fiber laser comprising a single mode fiber with a core diameter and exit aperture diameter of less than or equal to 10pm.

16. The method as set forth in claim 15 wherein the coupling the light source further comprises: coupling a fdter assembly in the light source to filter at least one property of the source light from the laser, wherein the at least one property comprises at least one of one or more wavelengths or one or more polarizations.