Digital holography measurement system

Abstract

To provide technology that pertains to precision measurement and, more specifically, to a precision workpiece surface measurement device and system.SOLUTION: A digital holography metrology system is provided including a heterodyne light source, an interferometric optical arrangement and a sensor arrangement. The heterodyne light source provides combined laser beams of different corresponding frequencies and wavelengths (e.g., for which each combined beam may include a corresponding wavelength laser beam and a corresponding frequency shifted laser beam, which may be orthogonally polarized). The interferometric optical arrangement utilizes the combined beams for providing an output for imaging a workpiece, for which the output includes interference beams. The sensor arrangement includes dichroic components which separate the interference beams to be directed to respective time of flight sensors. The outputs from the time of flight sensors are utilized to determine measurements (e.g., the outputs of the time of flight sensors may be utilized to determine a measurement distance to a surface point on a workpiece, etc.).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to precision measurement, and more specifically, to precision workpiece surface measurement devices and systems. [Background technology]

[0002] Quality control of objects (e.g., workpieces) containing specific surface profiles (e.g., generated by molding and / or machining) is becoming increasingly demanding in terms of throughput, measurement resolution, and accuracy. Such workpieces may include, but are not limited to, optical components, electrical components, and small mechanical components. Ideally, such workpieces should be measured and inspected to ensure proper dimensions, functionality, etc. However, in some applications, measurement tolerances at the submicron or subnanometer level may be required to verify that the workpiece surface has the desired characteristics. [Overview of the Initiative]

[0003] Various precision measurement systems can be used for measuring and inspecting the surface of a workpiece. For example, in some cases, such measurement systems utilize digital holography (e.g., a camera can be used to acquire holograms, and these holograms can be processed to determine measurements, etc.). A crucial part of such systems and / or other equivalent measurement systems is the light source used to illuminate the workpiece being measured (e.g., an inaccurate or unstable light source can result in inaccurate measurements, etc.). The signal-to-noise ratio of such systems is also an important factor (e.g., a low ratio can result in inaccurate measurements). It is desirable to have a configuration that allows for improvement or other enhancement of such measurement systems (e.g., for measuring and inspecting the surface of a workpiece, etc.).

[0004] This summary is provided to introduce, in a simplified form, some of the concepts that will be further described below in "Modes for Carrying Out the Invention." This summary is not intended to identify any important features of the claimed subject matter, nor is it intended to be used to help determine the scope of the claimed subject matter.

[0005] A digital holography measurement system is provided, comprising a heterodyne light source, an interference optics mechanism, and a sensor mechanism. The heterodyne light source includes a multi-wavelength light source and an acousto-optic modulator. The multi-wavelength light source (e.g., a multi-wavelength laser light source) provides at least a first-wavelength laser beam of a first frequency and a second-wavelength laser beam of a second frequency different from the first frequency. The acousto-optic modulator is configured to receive the first-wavelength laser beam, further generate a first-frequency-shifted laser beam corresponding to this first-wavelength laser beam, and combine this first-frequency-shifted laser beam and the first-wavelength laser beam to produce a first-composite beam. The acousto-optic modulator is also configured to receive the second-wavelength laser beam, further generate a second-frequency-shifted laser beam corresponding to this second-wavelength laser beam, and combine this second-frequency-shifted laser beam and the second-wavelength laser beam to produce a second-composite beam (for example, the second-composite beam and the first-composite beam can be part of the total composite beam from the heterodyne light source).

[0006] The interferometric optics mechanism receives a composite beam from a heterodyne light source and uses the composite beam to provide an output for imaging a workpiece. In various embodiments, the output of the interferometric optics mechanism includes at least a first interference beam generated based on a first composite beam from a heterodyne light source and a second interference beam generated based on a second composite beam from a heterodyne light source. The sensor mechanism receives the output from the interferometric optics mechanism and includes at least first and second TOF (Time of Flight) sensors and a first dichroic component. The first dichroic component (dichroic filter) is configured to separate the first interference beam from the second interference beam. The first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. In various embodiments, the digital holography measurement system further includes a processing unit. This processing unit receives outputs from the first and second TOF sensors and uses these outputs to determine at least one measurement distance to at least one surface point on the workpiece.

[0007] In various embodiments, the multi-wavelength light source further provides a third-wavelength laser beam at a third frequency different from the first and second frequencies. In such embodiments, the acousto-optic modulator may further be configured to receive the third-wavelength laser beam and to generate a corresponding third-frequency-shifted laser beam, synthesized with this third-wavelength laser beam, as a third combined beam. The output of the interference optics mechanism may further include a corresponding third interference beam generated based on the third combined beam from the heterodyne light source. The sensor mechanism may further include a third TOF sensor and a second dichroic component. The second dichroic component is configured to separate the second interference beam from the third interference beam. The second interference beam is directed to be received by the second TOF sensor, and the third interference beam is directed to be received by the third TOF sensor.

[0008] In various embodiments, the interference optics mechanism includes a beam splitter, a reference plane, and an imaging lens section including an imaging lens. The beam splitter receives a combined beam from a heterodyne light source and splits these combined beams into a first portion directed toward the workpiece surface and a second portion directed toward the reference plane. The reflected first and second portions are received by the beam splitter, combined as an interference beam, and directed toward the sensor mechanism via the imaging lens.

[0009] In various embodiments, the digital holography measurement system further includes a timer. The timer provides signals (such as timing signals like clock signals) to operate the acousto-optic modulator to the heterodyne light source, as well as to the TOF sensor. In various embodiments, the timer can be included as part of one of the TOF sensors.

[0010] In various embodiments, the digital holography measurement system is configured to operate in digital holography mode and amplitude-modulated continuous wave ("AMCW") mode. In digital holography mode, a timer is coupled to operate an acousto-optic modulator that generates first and second combined beams, and a processing unit receives outputs from first and second TOF sensors and uses these outputs to determine the first digital holography mode measurement distance (e.g., to a first surface point on a workpiece). In AMCW mode, the acousto-optic modulator does not generate first and second combined beams, and the processing unit uses the output of at least one of the TOF sensors to determine the first AMCW mode measurement distance (e.g., to a first surface point on a workpiece) by homodyne detection. In various embodiments, at least a portion of the first AMCW mode measurement distance and the first digital holography mode measurement distance are combined to determine the combined measurement distance (e.g., to a first surface point on a workpiece).

[0011] In various embodiments, the digital holography non-ambiguity range (e.g., absolute measurement range) of the digital holography mode is greater than the potential distance error of the AMCW mode, and the first AMCW mode measurement distance is an integer multiple of the digital holography non-ambiguity range. This integer multiple is combined with the first digital holography mode measurement distance to determine the composite measurement distance to a first surface point on the workpiece. In various embodiments, the non-ambiguity range of the AMCW mode is at least 50 times greater than the non-ambiguity range of the digital holography mode. In various embodiments, the AMCW mode has a non-ambiguity range greater than 500 millimeters, and the digital holography mode (e.g., when using first, second, and third wavelength laser beams from a light source) has a non-ambiguity range greater than 5 millimeters.

[0012] In various embodiments, the difference between the first wavelength of the first-wavelength laser beam and the second wavelength of the second-wavelength laser beam is greater than 2 percent of the first wavelength. In various embodiments, the difference between the first wavelength of the first-wavelength laser beam and the second wavelength of the second-wavelength laser beam is greater than 10 nanometers.

[0013] A method for operating a digital holography measurement system is provided. This method includes operating a heterodyne light source and a sensor mechanism of the digital holography measurement system. Operating the heterodyne light source includes operating a multi-wavelength light source and an acousto-optic modulator. The multi-wavelength light source is operated to provide at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency. The acousto-optic modulator is operated to receive the first wavelength laser beam, further generate a first frequency-shifted laser beam corresponding to this first wavelength laser beam, and combine this first frequency-shifted laser beam and the first wavelength laser beam to produce a first combined beam. The acousto-optic modulator is also operated to receive the second wavelength laser beam, further generate a second frequency-shifted laser beam corresponding to this second wavelength laser beam, and combine this second frequency-shifted laser beam and the second wavelength laser beam to produce a second combined beam. An interferometric optics mechanism receives a composite beam from a heterodyne light source and uses the composite beam to provide an output for imaging a workpiece. The output of the interferometric optics mechanism includes at least a first interference beam generated based on a first composite beam from the heterodyne light source and a second interference beam generated based on a second composite beam from the heterodyne light source. A sensor mechanism is operated to receive an output from the interferometric optics mechanism. The sensor mechanism includes at least first and second TOF sensors and a first dichroic component. The first dichroic component is configured to separate the first interference beam from the second interference beam. The first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. In various embodiments, the method further includes receiving outputs from the first and second TOF sensors and using these outputs to determine at least one measurement distance to at least one surface point on the workpiece.

[0014] In various embodiments, the method further includes operating a multi-wavelength light source to also provide a third wavelength laser beam at a third frequency different from the first and second frequencies. In such embodiments, the acousto-optic modulator can be operated to also receive the third wavelength laser beam, and to generate a third frequency-shifted laser beam corresponding to this third wavelength laser beam, and to combine this third frequency-shifted laser beam and the third wavelength laser beam to produce a third combined beam. The output of the interference optics mechanism may further include a third interference beam generated based on the third combined beam from the heterodyne light source. The sensor mechanism may further include a third TOF sensor and a second dichroic component. The second dichroic component may be configured to separate the second interference beam from the third interference beam. The second interference beam is directed to be received by the second TOF sensor, and the third interference beam is directed to be received by the third TOF sensor.

[0015] In various embodiments, the method may further include operating a timer to provide a heterodyne light source with a signal (e.g., a timing signal such as a clock signal) to operate an acousto-optic modulator, and also providing a signal to a TOF sensor. In various embodiments, the method may further include operating the system in digital holography mode and amplitude-modulated continuous wave ("AMCW") mode. In digital holography mode, a timer is coupled to operate an acousto-optic modulator that generates first and second combined beams, and a processing unit receives outputs from first and second TOF sensors and uses these outputs to determine the first digital holography mode measurement distance to a first surface point on the workpiece. In AMCW mode, the acousto-optic modulator does not generate first and second combined beams, and the processing unit uses the output of at least one of the TOF sensors to determine the first AMCW mode measurement distance to a first surface point on the workpiece by homodyne detection. In various embodiments, the combined measurement distance to a first surface point on the workpiece is determined by combining at least a portion of the first AMCW mode measurement distance and the first digital holography mode measurement distance. In various embodiments, the digital holography unambiguity range of the digital holography mode is greater than the potential distance error of the AMCW mode, and the first AMCW mode measurement distance is an integer multiple of the digital holography unambiguity range. This integer multiple is combined with the first digital holography mode measurement distance to determine the combined measurement distance to a first surface point on the workpiece.

[0016] In various embodiments, a sensor mechanism is provided for use in a digital holography measurement system. The sensor mechanism includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component. The sensor mechanism can receive an output from an interference optics mechanism. The first dichroic component is configured to separate a first interference beam from a second interference beam. The first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. In various embodiments, the sensor mechanism may further include a third TOF sensor and a second dichroic component. The second dichroic component is configured to separate a second interference beam from a third interference beam. The second interference beam is directed to be received by the second TOF sensor, and the third interference beam is directed to be received by the third TOF sensor. [Brief explanation of the drawing]

[0017] [Figure 1] This is a block diagram showing various components of a measurement system, including a heterodyne light source. [Figure 2] This is a block diagram of a digital holography measurement system including a heterodyne light source, similar to that shown in Figure 1. [Figure 3A] Figure 2 shows some of the operating principles of a digital holography measurement system. [Figure 3B] Figure 2 shows some of the operating principles of a digital holography measurement system. [Figure 3C] Figure 2 shows some of the operating principles of a digital holography measurement system. [Figure 4] This is a diagram of a lookup table used for phase unwrapping as part of the operation of a digital holography measurement system. [Figure 5] Figure 4 is a graph showing some of the values ​​from the lookup table. [Figure 6A] This is a diagram showing different operation modes of a digital holography measurement system. [Figure 6B] This is a diagram showing different operation modes of a digital holography measurement system. [Figure 7] This is a diagram of a heterodyne light source including an imaging system having a single imaging lens and a Wollaston prism. [Figure 8] This is a diagram of a heterodyne light source including an imaging system having two imaging lenses and a Wollaston prism. [Figure 9] This is a diagram of a heterodyne light source including a light-receiving lens and a birefringent beam displacer. [Figure 10] This is a diagram of a heterodyne light source including a light-receiving prism and a birefringent beam displacer. [Figure 11] This is a diagram of a heterodyne light source including a spatial filtering configuration and a collimation lens. [Figure 12] This is a flow diagram showing an exemplary embodiment of a routine for operating a heterodyne light source. [Figure 13] This is a flow diagram showing an exemplary embodiment of a routine for operating a digital holography measurement system including a heterodyne light source. [Modes for Carrying Out the Invention]

[0018] Figure 1 is a block diagram showing various components of an embodiment of the measurement system 100. As shown in Figure 1, the measurement system 100 includes a heterodyne light source 110, an optical mechanism 120, a sensor mechanism 130, a timer 140, and a processing unit 150. In various embodiments, the heterodyne light source 110 may include a light source, an acousto-optic modulator, and a light source optical mechanism (as detailed below with reference to examples in Figures 7 to 11, for example). Briefly, the acousto-optic modulator may be configured to receive one or more input wavelength laser beams from the light source and to further generate one or more corresponding frequency-shifted laser beams (which may be orthogonally polarized, for example). The light source optical mechanism receives the orthogonally polarized laser beams from the acousto-optic modulator, combines them, and outputs a corresponding combined laser beam.

[0019] The optical mechanism 120 (which may be, for example, a measurement optical mechanism) receives a composite output beam from the heterodyne light source 110. The optical mechanism 120 uses the composite beam as part of the measurement process (e.g., guides it) (for example, one or more beams may be guided toward the surface 192 of the workpiece 190 as part of a process to measure / determine at least one measurement distance to at least one surface point on the workpiece surface 192). As an example, in some embodiments (detailed below with reference to, for example, Figure 2), the measurement optical mechanism 120 may be an interferometric optical mechanism that receives a composite beam from the heterodyne light source 110 and uses the composite beam to provide an output for imaging the workpiece surface 192 (e.g., relative to an image plane) as part of the measurement process.

[0020] The sensor mechanism 130 includes one or more sensors to receive outputs from the measuring optical mechanism 120, detect these outputs, and provide corresponding output signals. The timer 140 provides timing signals (e.g., clock signals) used to control the timing of both the acousto-optic modulator of the heterodyne light source 110 and the sensors of the sensor mechanism 130. The processing unit 150 (including, for example, one or more processors 152) receives outputs from the sensor mechanism 130 and uses these outputs (for example, to determine the measurement distance to a surface point on the surface 192 of the workpiece 190).

[0021] It will be acknowledged to those skilled in the art that the processing unit 150 (e.g., including or implemented in a computing system) and / or other processing or control systems described or available together with the elements and methods described herein can generally be implemented using any suitable computing system or device, including distributed or networked computing environments. Such a system or device may include one or more general-purpose or special-purpose processors (e.g., non-custom or custom devices) that run software to implement the functions described herein. The software may be stored in memory such as random-access memory (RAM), read-only memory (ROM), flash memory, or a combination of such components. The software may also be stored in one or more storage devices such as optical-based disks, flash memory devices, or any other type of non-volatile storage medium for storing data. The software may include one or more program modules, including routines, programs, objects, components, data structures, etc., that perform a specific task or implement a specific abstract data type. In a distributed computing environment, the functionality of the program modules may be combined or distributed across multiple computing systems or devices in either a wired or wireless configuration and accessed via service calls.

[0022] Figure 2 is a block diagram of a digital holography measurement system 200, including a heterodyne light source 210, which can perform several functions similar to those of the measurement system in Figure 1 in various embodiments. Unless otherwise specifically stated below, it will be acknowledged that some numbered components 2XX in Figure 2 correspond to and / or perform similar functions to the corresponding similarly numbered components 1XX in Figure 1, and can be understood by analogy. This numbering scheme for showing elements of similar design and / or function (i.e., unless specifically illustrated and / or described) also applies to other figures in this specification. As shown in Figure 2, the digital holography measurement system 200 includes a heterodyne light source 210, an interference optics mechanism 220, a sensor mechanism 230, a timer 240, and a processing unit 250. In various embodiments, the digital holography measurement system 200 can operate as a heterodyne imaging interferometer.

[0023] In various embodiments, the heterodyne light source 210 includes a multi-wavelength light source coupled to an acousto-optic modulator (e.g., a multi-wavelength laser light source for providing laser beams of different wavelengths) (for example, several examples are detailed below with reference to Figures 7 to 11). In one embodiment, the multi-wavelength laser light source can provide a first wavelength laser beam with a first frequency v1. The acousto-optic modulator receives the first wavelength laser beam, generates a first frequency-shifted laser beam exhibiting a frequency v1+Δv corresponding to the first wavelength laser beam, and combines the first frequency-shifted laser beam and the first wavelength laser beam to generate a first combined beam. Similarly, the multi-wavelength laser light source can provide a second wavelength laser beam with a second frequency v2 different from the first frequency (i.e., the corresponding second wavelength is different from the first wavelength). The acousto-optic modulator receives a second wavelength laser beam, generates a second frequency-shifted laser beam exhibiting a frequency v2+Δv corresponding to the second wavelength laser beam, and combines the second frequency-shifted laser beam with the second wavelength laser beam to generate a second combined beam. Similarly, a multi-wavelength laser source can provide a third wavelength laser beam with a third frequency v3 different from the first and second frequencies (i.e., the corresponding third wavelength is different from the first and second wavelengths). The acousto-optic modulator receives a third wavelength laser beam, generates a third frequency-shifted laser beam exhibiting a frequency v2+Δv corresponding to the third wavelength laser beam, and combines the third frequency-shifted laser beam with the third wavelength laser beam to generate a third combined beam. In various embodiments, it is also possible for the multi-wavelength laser source to provide additional wavelength laser beams, and for the acousto-optic modulator to generate corresponding additional frequency-shifted laser beams. In various embodiments, different signs may be used interchangeably to represent / correspond to the frequencies of the laser beams (e.g., v, f, ω, etc.).

[0024] Timer 240 is coupled to the heterodyne light source 210 (e.g., via a driver for the heterodyne light source to the acousto-optic modulator) to provide a reference clock signal. A modulation frequency Δv is generated based on this reference clock signal. In various embodiments, the clock signal is provided at a modulation frequency Δv (e.g., in a range of 40 MHz to 100 MHz in a particular embodiment). In various embodiments, the distributed clock signal may be equivalent to the frequency of the acoustic wave of the acousto-optic modulator and also equivalent to the frequency shift Δv of the light beam (e.g., by adding one acoustic phonon to the photon frequency). In various alternative embodiments, the distributed clock signal can be any other frequency that is multiplied / divided by an additional electronic component to a desired target frequency.

[0025] The interferometric optics mechanism 220 (which may also be referred to as the measuring optics mechanism) includes a beam splitter 221, quarter-wave plates 223 and 226, a reference mirror 224 (which may also be referred to herein as the reference plane 224), an imaging lens section 227 (which may include an imaging lens such as a camera lens), and a polarizer 228. In various embodiments, the interferometric optics mechanism 220 forms and / or operates as a dual-path interferometer. The imaging lens section 227 (which may include multiple lenses in some embodiments and may be in a telecentric configuration) is provided to image the surface 192 of the workpiece 190 (which is the object of measurement and / or other inspection, etc.) with respect to the image plane. In various embodiments, the image plane may be located in front of, on, or behind the sensors of the sensor mechanism 230 (e.g., sensor TOF-1, sensor TOF-2, etc.) (for example, in various embodiments, these sensors may be spatially arranged such that the optical path length from the imaging lens unit 227 to each sensor is approximately the same). As detailed below, in various embodiments, each sensor TOF can detect an optical beat signal of a different frequency Δv and demodulate it pixel by pixel. In various embodiments, one depth frame may consist of four phase measurements. The recorded hologram can be numerically propagated to the image plane, where a height map (e.g., showing measurements of surface points on the magnified workpiece surface 192 of the workpiece 190) is extracted (e.g., after phase unwrapping).

[0026] The beam splitter 221 is optically aligned with the reference mirror 224 along axis 222. The beam splitter 221 is also optically aligned with the workpiece surface 192 along axis 225, which is substantially perpendicular to axis 222. A quarter-wave plate 226 is provided between the beam splitter 221 and the workpiece surface 192, and a quarter-wave plate 223 is provided between the beam splitter 221 and the reference mirror 224. The imaging lens 227 and polarizer 228 are provided between the beam splitter 221 and the sensor mechanism 230 along axis 225. The imaging lens 227 and polarizer 228 are optically aligned between the beam splitter 221 and the sensor mechanism 230.

[0027] The workpiece 190 to be inspected / measured is positioned relative to the digital holography measurement system 200 such that the workpiece surface 192 is imaged by the imaging lens unit 227. In certain embodiments, the workpiece surface 192 may be imaged on the sensors of the sensor mechanism 230 (e.g., sensor TOF-1, sensor TOF-2, etc.), but this is not a requirement for the digital holography measurement system (for example, in various embodiments, the workpiece surface 192, including each surface point, may be imaged on an image plane in front of or behind the sensor TOF of the sensor mechanism 230).

[0028] In digital holographic measurement systems such as those disclosed herein, whenever a sensor TOF is positioned relative to the image plane, the measurement of the laser beam by the sensor TOF can be numerically propagated to the image plane to determine the values ​​that will occur on the image plane and can be used in the measurement process described herein. As an example of such numerical propagation, if the amplitude and phase of the laser beam are measured / determined on the measurement plane (e.g., sensor TOF), then since it is known how the amplitude and phase of the laser beam will vary with time and distance, the corresponding values ​​that will occur on the image plane can be numerically propagated (e.g., calculated) (for example, the relative position / distance on the image plane can be determined according to the characteristics of the imaging lens, etc.). Such features can be particularly useful for measuring specific workpieces / workpiece elements (for example, in some systems, the height variation of the workpiece surface is large, so the entire workpiece surface may not fall within the single measurement range of the system, but a digital holographic measurement system such as those disclosed herein can alternatively perform numerical propagation on the image plane to obtain / determine the values ​​necessary for measurement).

[0029] In the example in Figure 2, each frequency-shifted laser beam is acquired by acousto-optic modulation of the corresponding wavelength laser beam (i.e., in the heterodyne light source 210). However, it will be acknowledged that in other embodiments, each frequency-shifted laser beam can be acquired using other techniques. For example, for each pair of wavelength / frequency-shifted laser beams, the laser can be a Zeeman laser with a laser medium in a strong axial magnetic field, since it generates two laser beams with different wavelengths / frequencies. According to the terminology used herein, laser beams with different wavelengths / frequencies may be called separate laser beams even if they share the same beam path. In some examples, such a combination of laser beams may be called a composite beam, such as when the laser beams share a beam path and / or co-propagate.

[0030] During operation, the timer 240 provides a clock or reference signal to the sensor mechanism 230 and the driver of the heterodyne light source 210. The acousto-optic modulator of the heterodyne light source 210 is driven by the driver at a modulation frequency Δv. As described above, the multi-wavelength laser light source of the heterodyne light source 210 generates laser beams of different frequencies. For example, as described above, the multi-wavelength laser light source can provide a first wavelength laser beam of a first frequency v1. The acousto-optic modulator receives the input first wavelength laser beam and generates / produces a corresponding first frequency-shifted laser beam of frequency v1+Δv that propagates along the first wavelength laser beam. Thus, with respect to the first wavelength laser beam, the output of the heterodyne light source 210 is a first composite beam. As detailed below, the first composite beam can be detected by the sensor mechanism 230, and its envelope is modulated at a different frequency Δv.

[0031] Similarly, the multi-wavelength laser source of the heterodyne light source 210 can provide a second-wavelength laser beam with a second frequency v2. The acousto-optic modulator receives the input second-wavelength laser beam and generates a corresponding second-frequency-shifted laser beam with frequency v2 + Δv that propagates along the second-wavelength laser beam. The corresponding output of the heterodyne light source 210 is the second combined beam. Similarly, the multi-wavelength laser source can provide a third-wavelength laser beam with a third frequency v3. The acousto-optic modulator receives the input third-wavelength laser beam and generates a corresponding third-frequency-shifted laser beam with frequency v3 + Δv that propagates along the third-wavelength laser beam. The corresponding output of the heterodyne light source 210 is the third combined beam. Thus, it can be acknowledged that the total output of the heterodyne light source 210 may include a total combined beam (e.g., including the first, second, and third combined beams, etc.). In various embodiments, the multi-wavelength laser source of the heterodyne light source 210 may provide additional wavelength laser beams, and the acousto-optic modulator may generate corresponding additional frequency-shifted laser beams to form the corresponding additional composite beams, which can then be included in the total composite beam as the output from the heterodyne light source 210.

[0032] In various embodiments, the multi-wavelength laser light source of the heterodyne light source 210 may, in some examples, include individual laser light sources to provide laser beams of different wavelengths. In various embodiments, it may be desirable that such individual laser light sources (e.g., laser diodes) have low temperature sensitivity. In various embodiments using individual laser light sources (i.e., laser light sources having different wavelengths), the wavelengths of each laser light source may drift in the same direction due to temperature changes, etc., and the ratio between wavelengths may remain relatively stable, thus potentially allowing for relatively stable measurement accuracy in the system. In various embodiments, the temperature sensitivity of each individual laser light source can be configured to match the corresponding wavelength, and the drift of the wavelength ratio due to temperature can be reduced or eliminated. When configuring the heterodyne light source 210 as disclosed herein, in various embodiments, individual laser light sources (which may have different values ​​due to, for example, manufacturing tolerances) can be tested and a combination of laser light sources that gives the desired ratio between wavelengths can be selected / used.

[0033] Accordingly, various embodiments may include different laser light sources that provide desired combinations of wavelength ratios (for example, one specific embodiment provides wavelengths such as 633 nm, 687 nm, and 767 nm). In one embodiment, the first laser light source included in the multi-wavelength laser light source may provide a first wavelength laser beam (for example, one specific embodiment has a nominal wavelength of 633 nm). Accordingly, the heterodyne light source 210 provides a first wavelength λ of vertical polarization as described above. S1 A first wavelength laser beam having (e.g., 633 nm) and the wavelength λ of horizontal polarization FS1 A first combined beam can be generated, which includes a first frequency-shifted laser beam (each linearly polarized) having the following characteristics. The timer 240 generates a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator of the heterodyne light source 210. As a result, a beat signal (e.g., 40 MHz) is detected by the first combined beam.

[0034] Similarly, a second laser light source included in the multi-wavelength laser light source of the heterodyne light source 210 can provide a second wavelength laser beam (for example, in one specific embodiment, the nominal wavelength is 687 nm). Accordingly, the heterodyne light source 210 provides a first wavelength λ of vertical polarization as described above. S2 A second wavelength laser beam having (e.g., 687 nm) and the wavelength λ of horizontal polarization FS2 A second composite beam can be generated, including a second frequency-shifted laser beam having λ. Similarly, a third laser source included in a multi-wavelength laser source can provide a third wavelength laser beam (for example, in one specific embodiment, the nominal wavelength is 767 nm). Accordingly, the heterodyne light source 210, as described above, provides a third wavelength λ of vertical polarization. S A third wavelength laser beam having 3 (e.g., 767 nm) and the wavelength λ of horizontal polarization FS3 A third composite beam can be generated, including a third frequency-shifted laser beam having the same properties. As described above, the timer 240 can generate a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator of the heterodyne light source 210. As a result, each of the second and third composite beams generates a beat signal that is detected at a modulation frequency (e.g., 40 MHz).

[0035] As described above, during the operation of the digital holography measurement system 200, the heterodyne light source 210 outputs a fully combined beam, including the combined beam, to the interference optics mechanism 220. As detailed below, the interference optics mechanism 220 uses the combined beam to provide an output for imaging the surface 192 of the workpiece 190 (for example, relative to the image plane). In one embodiment where three wavelengths are used, for example as detailed below, the output of the interference optics mechanism 220 may include first, second, and third interference beams. In various embodiments, the first interference beam is generated based on a first combined beam from the heterodyne light source, the second interference beam is generated based on a second combined beam from the heterodyne light source, and the third interference beam is generated based on a third combined beam from the heterodyne light source.

[0036] Furthermore, in the above example, when the first combined beam is incident on the beam splitter 221 (e.g., polarizing beam splitter) of the interference optics mechanism 220, a first reference beam having frequency v1 and vertical polarization propagates along axis 222 toward the reference mirror 224, and a first signal beam having frequency v1+Δv and horizontal polarization propagates along axis 225 to illuminate a portion of the workpiece surface 192. Next, the first reflected reference beam (i.e., from the reference mirror 224) and the first reflected signal beam (i.e., from the workpiece surface 192) are combined by the beam splitter 221 to form a first reflected combined beam that propagates along axis 225 toward the sensor mechanism 230 via the imaging lens section 227. A quarter-wave plate 226 is used to convert the first incident beam of frequency v1+Δv from horizontal polarization to circular polarization, and to convert the first reflected signal beam from circular polarization to vertical polarization. Similarly, the quarter-wave plate 223 is used to convert the first incident beam at frequency v1 from vertically polarized to circularly polarized, and to convert the first reflected reference beam from circularly polarized to horizontally polarized. As a result, the polarization of the first reflected signal beam and the first reflected reference beam after passing through the first and second quarter-wave plates, respectively, becomes orthogonal, thereby preventing interference.

[0037] As detailed below, the workpiece surface 192 is imaged onto the image plane via the imaging lens 227. A polarizer 228 is used to control the polarization of the two first reflected beams, allowing them to form a first interference beam. A first dichroic component 231-1 (e.g., a dichroic filter used to separate individual wavelengths) operates to reflect the first interference beam IB-1 toward the sensor TOF-1 of the sensor mechanism 230 (e.g., a TOF sensor, which in various embodiments may be a TOF camera, etc.). As detailed below, the first dichroic component 231-1 operates to transmit / pass other interference beams of different wavelengths along the axis 225, which are reflected toward other sensors of the sensor mechanism 230 by other dichroic components. In various embodiments, each of the dichroic components defined herein may include one or more optical components capable of splitting a light beam into two beams of different wavelengths. As described above, the workpiece surface 192 is imaged onto the image plane via the imaging lens unit 227 (for example, the image plane may be in front of the sensor TOF-1, on the sensor TOF-1, or behind the sensor TOF-1).

[0038] When the second combined beam is incident on the beam splitter 221, a second reference beam with frequency v2 and vertical polarization propagates along axis 222 toward the reference mirror 224, and a second signal beam with frequency v2+Δv and horizontal polarization propagates along axis 225 to illuminate a portion of the workpiece surface 192. Next, the second reflected reference beam and the second reflected signal beam are combined by the beam splitter 221 to form a second reflected combined beam that propagates along axis 225 toward the sensor mechanism 230 via the imaging lens unit 227. A quarter-wave plate 226 is used to convert the second incident beam with frequency v2+Δv from horizontal polarization to circular polarization, and to convert the second reflected signal beam from circular polarization to vertical polarization. Similarly, a quarter-wave plate 223 is used to convert the second incident beam with frequency v2 from vertical polarization to circular polarization, and to convert the second reflected reference beam from circular polarization to horizontal polarization. As a result, the polarization of the second reflected signal beam and the second reflected reference beam after passing through the first and second quarter-wave plates, respectively, becomes orthogonal, thereby preventing interference.

[0039] As detailed below, the workpiece surface 192 is imaged onto the image plane via the imaging lens unit 227. A polarizer 228 is used to control the polarization of the two second reflected beams, allowing them to form a second interference beam. The second dichroic component 231-2 operates to reflect the second interference beam IB-2 toward the sensor TOF-2 of the sensor mechanism 230. The second dichroic component 231-2 also operates to transmit / pass other interference beams of different wavelengths along the axis 225, which are reflected toward one or more other sensors of the sensor mechanism 230 by one or more other dichroic components. This is similar to how the first dichroic component 231-1 reflects the first interference beam IB-1 while simultaneously transmitting / passing the second interference beam IB-2. As described above, the workpiece surface 192 is imaged onto the image plane via the imaging lens unit 227 (for example, the image plane may be in front of the sensor TOF-2, on the sensor TOF-2, or behind the sensor TOF-2).

[0040] It can be acknowledged that a similar process can be performed for additional composite beams in the system. For example, similar to the process described above, there exists a final composite beam (e.g., numbered as the Nth composite beam), and correspondingly, frequency v N and a final reference beam having vertical polarization, and frequency v N A final signal beam with +Δv and horizontal polarization may exist. The corresponding final reflection reference beam and final reflection signal beam form a final reflection composite beam, from which a final interference beam can be formed. As detailed below, the workpiece surface 192 can be imaged into the image plane via the imaging lens unit 227. The final dichroic component 231-N operates to reflect the final interference beam IB-N towards the sensor TOF-N. As described above, the workpiece surface 192 is imaged into the image plane via the imaging lens unit 227 (for example, the image plane may be in front of the sensor TOF-N, on the sensor TOF-N, or behind the sensor TOF-N).

[0041] In embodiments where N=3, the final combined beam is a third combined beam, or in embodiments where N is greater than 3, a third combined beam may exist in the system. In either case, correspondingly, a third reference beam with frequency v3 and vertical polarization and a third signal beam with frequency v3+Δv and horizontal polarization may exist. The corresponding third reflected reference beam and third reflected signal beam form a third reflected combined beam, from which a third interference beam can be formed. The third dichroic component 231-3 may operate to reflect the third interference beam IB-3 toward the sensor TOF-3 (i.e., sensor TOF-N in the case of N=3). The use of dichroic components may enable the use of multiple wavelength laser beams in the heterodyne light source 210. These beams are combined in a common beam path, then split by the dichroic component and received by the sensor TOF (e.g., as part of the associated measurement function).

[0042] More specifically, as described above, the first dichroic component 231-1 is configured to separate the first interference beam IB-1 from the second and third interference beams IB-2 and IB-3. The first interference beam IB-1 is directed (e.g., by the first dichroic component 231-1) and received by the first sensor TOF-1 (e.g., a TOF sensor). The second dichroic component 231-2 is configured to separate the second interference beam IB-2 from the third interference beam IB-3. The second interference beam IB-2 is directed (e.g., by the second dichroic component 231-2) and received by the second sensor TOF-2 (e.g., a TOF sensor). The third interference beam IB-3 is directed (e.g., by the third dichroic component 231-3) and received by the third sensor TOF-3 (e.g., a TOF sensor).

[0043] As described above, the total output of the heterodyne light source 210 can be a total combined beam (e.g., as in the above example where N=3, which may include the first, second, and third combined beams). According to the operation described above, when the total combined beam from the heterodyne light source 210 enters the beam splitter 221, the combined reference beam including the first, second, and third reference beams each having frequencies v1, v2, and v3, and vertical polarization propagates along the axis 222 toward the reference mirror 224 (e.g., the beam combination at the reference mirror 224 is Σv i ), which in this example indicates that the combined reference beam includes reference beams having frequencies v1, v2, and v3. Further, according to the operation described above, when the total combined beam from the heterodyne light source 210 enters the beam splitter 221, the combined signal beam including the first, second, and third signal beams each having frequencies v1+Δv, v2+Δv, and v3+Δv, and horizontal polarization propagates along the axis 225 to illuminate a portion of the workpiece surface 192 (e.g., the beam combination at the workpiece surface 192 is Σv iThis can be expressed as +Δv, and in this example, it indicates that the combined signal beam includes signal beams having frequencies v1+Δv, v2+Δv, and v3+Δv.

[0044] In various embodiments, axis 225 can be defined as the measurement axis of the digital holography measurement system 200. The measurement distance may correspond to the distance from the digital holography measurement system 200 to a surface point on the workpiece surface 192 (e.g., from a designated component or reference point of the digital holography measurement system 200, such as the quarter-wave plate 226 or its vicinity, or the end of the case housing the interference optics mechanism 220 or its vicinity). In various embodiments, additionally or alternatively, axis 225 can be defined as the z-axis of the digital holography measurement system 200. The measurement distance may be defined as the z-distance or according to the z-height. In various embodiments, various z-heights of surface points on the workpiece surface 192 (e.g., as part of the entire surface profile or otherwise) can be related to a reference point and / or related to each other according to the relative z-height, for example, a reference point on the workpiece surface that may be specified to have a z-height of z=0. In various embodiments, the z-height of each surface point can be determined according to the measurement distance / z-distance from the digital holography measurement system 200 to the surface point, or according to such a distance.

[0045] In configurations using dichroic components as disclosed herein, in various embodiments, it may be desirable to sufficiently separate the wavelengths between beams so that the beams can be effectively separated by wavelength selectivity. In contrast, some conventional systems use beams with very close wavelengths (e.g., separation of less than 1 nm to achieve a combined wavelength to achieve a long absolute measurement range, etc.). Alternatively, in the configurations disclosed herein, it may be advantageous to use a combination of wavelengths that are relatively well separated so that the beams can be effectively separated by wavelength selectivity (e.g., separation of at least 2% or 5% of the shortest wavelength value, or separation of at least 10 nm or 20 nm). One specific example of a wavelength combination that satisfies such requirements is λ1=633 nm, λ2=687 nm, and λ3=767 nm (e.g., 2% of the shortest wavelength is less than 13 nm, 5% is less than 32 nm, and these are smaller than the separation between the shortest and longest wavelengths, or between any of the wavelengths in this example, and the same applies to the separation of 10 nm or 20 nm).

[0046] For each sensor TOF (e.g., TOF-1, TOF-2...TOF-N) of the sensor mechanism 230, the sensor TOF can be a TOF sensor configured in various embodiments to determine the phase relationship between the modulated signal received at each pixel and the interference beam. The signal output by the sensor TOF compares the phase of the modulation intensity of the optical signal located at each pixel with an electrical reference signal of the modulation frequency derived from the signal from the timer 240 (which may function as a common clock source, for example). In various embodiments, the sensor TOF outputs an orthogonal signal (IQ), from which the phase can be calculated. In various embodiments, such calculations can be performed by the sensor itself (e.g., on a chip / processor on the sensor TOF) or by an associated processor / chip (e.g., provided in the sensor electronics 255 of the processing unit 250 or the host computer 258, etc.). Generally, the processing unit 250 receives output from the sensor TOF (e.g., TOF-1, TOF-2...TOF-N) of the sensor mechanism 230 and uses this output (for example, to determine the measurement distance to a surface point on the surface 192 of the workpiece 190).

[0047] In various embodiments, the timer 240 is part of at least one of the sensor TOFs (e.g., sensor TOF-1) or can receive a timing signal from it. In such embodiments, the phase and frequency of the acousto-optic modulator of the heterodyne light source 210 can be controlled substantially by the sensor TOF (i.e., by the timer of the sensor TOF). In various embodiments, each sensor TOF may perform four phase measurements shifted by 90 degrees within a single depth frame to measure amplitude and phase. Each pixel on each sensor TOF can be a photonic mixer device that combines detection and demodulation. In various embodiments, the sensor TOF may be sensitive only to the difference frequency from the measurement signal and the reference arm (i.e., including the reference mirror 224). The sum frequency term can be averaged by correlated double sampling and removed along with the DC term.

[0048] In various embodiments, the digital holography measurement system 200 operates as a phase-shift holography system that measures the phase difference between a workpiece wave and a reference wave on a pixel-by-pixel basis (e.g., from each reflected signal beam and a reflected reference beam). While some conventional systems require a computational step to calculate the hologram from at least three interferograms, according to the principles disclosed herein, the calculation can be performed at least in near real-time (e.g., at the sensor TOF or in the processing unit, etc.). In various embodiments, the measured hologram is numerically propagated to the image plane. On the image plane, phase information from all wavelengths is synthesized using a phase unwrapping routine to reconstruct a surface profile over a relatively long unambiguous range (e.g., absolute measurement range).

[0049] Regarding sensor TOF, TOF sensors such as TOF cameras in various embodiments have extremely good phase resolution, which can be used to achieve a large non-ambiguous range (NAR) for system measurements (e.g., for determining the absolute measurement distance to a surface point on a workpiece). Generally, the better the resolution of the sensor TOF, the longer the non-ambiguous range that can be achieved (e.g., as a decodeable path). As defined herein, a TOF sensor is any sensor or configuration configured to detect values ​​indicating the amplitude and phase of a laser beam. Another important factor with respect to the achievable non-ambiguous range is the wavelength ratio used. Therefore, in various embodiments, it is desirable to determine / use a wavelength ratio that yields a signal that can be unwrapped / decoded in a given system, enabling the achievement of a large non-ambiguous range.

[0050] Figures 3A to 3C illustrate some of the operating principles of a digital holography measurement system 300, similar to the digital holography measurement system 200 in Figure 2. To simplify the explanation of the relevant operating principles, Figures 3A to 3C show only specific parts of the digital holography measurement system 300. Unless otherwise specifically stated below, it will be acknowledged that some numbered components 3XX in Figures 3A to 3C correspond to and / or operate similarly to the corresponding numbered components 1XX or 2XX in Figure 1 or Figure 2, and can be understood by analogy. As shown in Figure 3A, the output of the heterodyne light source 310 is received by the interference optics mechanism 320. Correspondingly, the output of the interference optics mechanism 320 is received by the sensor mechanism 330.

[0051] Similar to the operation described above in relation to Figure 2, the heterodyne light source 310 can provide an output consisting of a collimated beam having multiple fundamental frequencies of a single polarization state and all frequency shift components of orthogonal polarization states (for example, provided as a fully combined collimated beam including a first combined beam, a second combined beam, a third combined beam, etc.). When the first combined beam is incident on the beam splitter 321 (e.g., a polarizing beam splitter) of the interference optics mechanism 320, a first signal beam having frequency f1 propagates to illuminate a portion of the workpiece surface 192, while a first reference beam having orthogonal polarization and frequency f1+Δf propagates toward the reference surface 324 (e.g., a reference mirror). In this example, the beams used for the signal beam and the reference beam are different from the beams used in the example described above in relation to Figure 2, but it will be recognized that any configuration can be used in various embodiments. Next, the first reflected signal beam and the first reflected reference beam are combined by the beam splitter 321 to form a first reflected combined beam that propagates toward the sensor mechanism 330.

[0052] When the second combined beam is incident on the beam splitter 321, a second signal beam with frequency f2 propagates, illuminating a portion of the workpiece surface 192, while a second reference beam with orthogonal polarization and frequency f2+Δf propagates toward the reference surface 324. The second reflected signal beam and the second reflected reference beam are then combined by the beam splitter 321 to form a second reflected combined beam that propagates toward the sensor mechanism 330. A similar process is performed when a third combined beam is incident on the beam splitter 321, etc. In various embodiments, this type of process can be performed to measure a reference point RP on the workpiece surface 192 (which may be specified as, for example, z=0 or other z heights), and then to measure other surface points SP on the workpiece surface. This will be described in detail below in relation to Figure 3B.

[0053] Similar to the process described above, the entire output of the heterodyne light source 310 can be a fully combined beam (for example, in the N=3 example, it may include the first, second, and third combined beams). According to the operation described above, when the fully combined beam from the heterodyne light source 310 is incident on the beam splitter 321, the combined signal beam, including the first, second, and third signal beams having frequencies f1, f2, and f3 and perpendicular polarization, propagates along the axis to illuminate a portion of the workpiece surface 192 (for example, the beam combination on the workpiece surface 192 is Σf i This can be expressed as follows, and in this example, it is shown that the combined signal beam includes signal beams having frequencies f1, f2, and f3). Furthermore, according to the operation described above, when the total combined beam from the heterodyne light source 310 is incident on the beam splitter 321, a combined reference beam including first, second, and third reference beams having frequencies f1+Δf, f2+Δf, and f3+Δf, as well as horizontal polarization, propagates along the axis toward the reference plane 324 (for example, the beam combination at the reference plane 324 is Σf i This can be expressed as +Δf, and in this example, it indicates that the composite reference beam includes reference beams having frequencies f1+Δf, f2+Δf, and f3+Δf.

[0054] Figure 3B shows various exemplary signals related to the process described above, including a first exemplary signal set 380-1 and a second exemplary signal set 380-2. The first exemplary signal set 380-1 includes a first reference beam signal RB1 (e.g., having frequency f1 + Δf), a first signal beam signal SB1 (e.g., having frequency f1), a first composite beam signal CB1 (e.g., modulated with beat frequency Δf), and a first composite beam phase shift signal CBPS1 (e.g., modulated with beat frequency Δ and having an additional phase shift as a result of a path length difference 2Δz). In various embodiments, the first composite beam signal CB1 can be obtained from a measurement of a reference point on the workpiece surface (e.g., reference point RP in Figure 3A). The first composite beam phase shift signal CBPS1 can be obtained from a subsequent measurement of a different surface point on the workpiece surface (e.g., surface point SP in Figure 3A). As shown in Figure 3B, the difference between signals CB1 and CBPS1 corresponds to the first phase shift Φ1 = (f1)(2Δz) / c, where c is the speed of light.

[0055] The second exemplary signal set 380-2 includes a second reference beam signal RB2 (e.g., having frequency f2 + Δf), a second signal beam signal SB2 (e.g., having frequency f2), a second combined beam signal CB2 (e.g., modulated with beat frequency Δf), and a second combined beam phase shift signal CBPS2 (e.g., modulated with beat frequency Δ and having an additional phase shift as a result of a path length difference of 2Δz). In various embodiments, the second combined beam signal CB2 can be obtained from a measurement of a reference point on the workpiece surface (e.g., reference point RP in Figure 3A). The second combined beam phase shift signal CBPS2 can be obtained from a subsequent measurement of a surface point on the workpiece surface (e.g., surface point SP in Figure 3A). As shown in Figure 3B, the difference between signals CB2 and CBPS2 corresponds to a second phase shift Φ2 = (f2)(2Δz) / c, where c is the speed of light. It will be acknowledged that a similar process can be performed using a third combined beam, etc., to determine a third phase shift Φ3.

[0056] Figure 3C shows some components of the digital holography measurement system 300 related to the phase shift processing shown in Figure 3B. As shown in Figure 3C, the sensor mechanism 330 includes sensors TOF-1, TOF-2, and TOF-3, which include pixel arrays 335-1, 335-2, and 335-3, respectively. In each of the pixel arrays, pixels 336-1, 336-2, and 336-3 are shown as being used to measure a specific surface point on the workpiece surface 192 (e.g., surface point SP shown in Figure 3A). In various embodiments, the processing unit 350 can receive data regarding phase shifts Φ1, Φ2, Φ3, etc. (e.g., those described above) (e.g., by the sensor electronic equipment 355). The processing unit 350 (e.g., utilizing the computer 358) can then perform phase unwrapping (e.g., to determine the value of Δz based on the phase shifts Φ1, Φ2, Φ3, etc.). In various embodiments, determining the value of Δz corresponds to determining the measured distance to a surface point (e.g., surface point SP) on the surface 192 of the workpiece 190. A simplified example of performing phase unwrapping (e.g., to determine Δz) using a lookup table is detailed below in relation to Figures 4 and 5.

[0057] Figure 4 shows a lookup table 400 used for phase unwrapping as part of the operation of a digital holography measurement system. In the example in Figure 4, the first column corresponds to a first sensor TOF-1 that receives a signal corresponding to a first wavelength laser beam having a first wavelength λ1 (e.g., λ1 = 500 nm). The first column contains values ​​corresponding to various possible phases Φ1 (for example, phases Φ1, Φ2, and Φ3 in Figure 4 may correspond to the phase shifts described above in relation to Figures 3A to 3C). In Figure 4, the second column corresponds to a second sensor TOF-2 that receives a signal corresponding to a second wavelength laser beam having a second wavelength λ2 (e.g., λ2 = 800 nm). The second column contains values ​​corresponding to various possible phases Φ2. The third column corresponds to a third sensor TOF-3 that receives a signal corresponding to a third wavelength laser beam having a third wavelength λ3 (e.g., λ3 = 1000 nm). The third column contains values ​​corresponding to various possible phases Φ3. As a concrete example using the lookup table 400, if the phases Φ1, Φ2, and Φ3 have values ​​corresponding to 216 degrees, 90 degrees, and 288 degrees, respectively, then such values ​​can be considered to correspond to a surface height Δz of the measured surface point SP (for example, relative to the height of the reference point RP on the workpiece surface shown in Figure 3A) of Δz = 0.9 microns. The values ​​in the fourth column of table 400 are shown in 0.1 micron increments over a range of surface heights from Δz = 0 to Δz = 2.0 microns (a fully unambiguous range of 2.0 microns). This relationship is further illustrated in Figure 5, which is detailed below.

[0058] Figure 5 is a graph showing some values ​​from the lookup table in Figure 4. Figure 5 includes three graphs 500A, 500B, and 500C, with the cycle of the phase value shown on the Y axis of each graph, and the values ​​plotted against a surface height scale included along the X axis at the top of graph 500A (the phase values ​​corresponding to each surface height, shown in 0.1 micron increments, are also shown on the X axis at the bottom of each graph). As shown in Figure 5, graph 500A shows the values ​​of the first phase Φ1 corresponding to a laser beam with a first wavelength λ1 (i.e., λ1 = 500 nm), and these values ​​represent an 8-period cycle. Graph 500B shows the values ​​of the second phase Φ2 corresponding to a laser beam with a second wavelength λ2 (i.e., λ2 = 800 nm), and these values ​​represent a 5-period cycle. Graph 500C shows the values ​​of the third phase Φ3 corresponding to a laser beam with a third wavelength λ3 (i.e., λ3 = 1000 nm), and these values ​​represent a cycle of 4 periods.

[0059] As shown in Figure 5, the value of the surface height Δz depends not only on each phase value but also on which period each phase value is included in. For example, with a surface height Δz = 0.9 microns as described above, the phase Φ1 = 216 degrees, which is included in the fourth period of the corresponding waveform; the phase Φ2 = 90 degrees, which is included in the third period of the corresponding waveform; and the phase Φ3 = 288 degrees, which is included in the second period of the corresponding waveform. Therefore, the phase unwrapping process determines a unique surface height Δz corresponding to a unique combination of these phase values ​​(i.e., according to the graph in Figure 5, each of these phase values ​​is included in a particular period).

[0060] In the examples shown in Figures 4 and 5, wavelengths λ1=500nm, λ2=800nm, and λ3=1000nm are selected to simplify the explanation of the operating principle as described above, resulting in a de-obfuscated range of 2.0 microns in this example. Other wavelength combinations may be selected and used according to the principles disclosed herein (for example, this may result in a longer de-obfuscated range). For example, a combination of wavelengths λ1=633nm, λ2=687nm, and λ3=767nm, which can be achieved using some commercially available laser sources, may result in a de-obfuscated range of approximately 8.82 mm in some embodiments. In various embodiments, the size of the corresponding Δz step may be selected / used based on how precise the phase measurement can be (e.g., with respect to the signal-to-noise ratio, etc.).

[0061] Various types of phase unwrapping routines can be used in various embodiments (for example, to determine the surface height Δz value based on a unique combination of phase values). For example, a machine learning process may be used for phase unwrapping instead of, or in addition to, the lookup tables described above in relation to Figures 4 and 5. As an example of such phase unwrapping using machine learning, a K-nearest neighbor regressor (KNN) can be trained on machine learning data. The effectiveness of such a process is determined by training the KNN regressor on clean, noise-free data and then demonstrating phase unwrapping performed on relatively noisy data using it. Such tests have confirmed that these processes (including the use of machine learning for phase unwrapping, such as KNN models) provide high accuracy in determining measurements.

[0062] Digital holographic measurement systems such as those disclosed herein (e.g., those using heterodyne detection with TOF cameras such as sensors) can be found to offer various advantages over some conventional measurement systems. The heterodyne detection used enables a high signal-to-noise ratio in holography, which can be used to expand the non-ambiguous range (NAR) and improve the accuracy of the measurements performed. Phase unwrapping routines that do not rely on composite wavelengths are used to enable long non-ambiguous ranges (e.g., absolute measurement ranges). Wavelength multiplexing is performed (e.g., by using dichroic components, which are relatively inexpensive and readily available, to separate signal beams with distinct wavelengths). By using wavelength multiplexing, it is possible to capture all phases of all wavelengths relatively simultaneously within a single depth frame using multiple TOF cameras, thereby increasing environmental robustness.

[0063] This rapid acquisition of measurement data helps ensure that movement between the workpiece and the system is minimal or nonexistent while the measurement data is being captured (for example, in conventional systems, images and / or data are captured individually or sequentially, which takes longer, and the workpiece may move relative to the system due to, for example, vibration of the workpiece, accidental movement, or normal forward movement along the conveyor). Direct measurement of the phase eliminates the need for phase lookup routines, reducing the computational workload compared to conventional systems. In some embodiments, the configurations disclosed herein can achieve performance similar to existing multi-wavelength systems using six different wavelengths using only three different wavelengths, thus potentially lowering the cost for similar performance. In various embodiments, configurations like those disclosed herein can be used for applications such as high-throughput inline measurement sampling / measurement.

[0064] Figures 6A and 6B show different operating modes of the digital holography measurement system 600 as disclosed herein. Unless otherwise specifically stated below, the digital holography measurement system 600 is similar to the digital holography measurement systems 200 and 300 in Figures 2 and 3A to 3C. To simplify the explanation of the relevant operating principles of the different operating modes, Figures 6A and 6B show only specific parts of the digital holography measurement system 600. As shown in Figures 6A and 6B, the digital holography measurement system 600 includes a heterodyne light source 610, an interference optics mechanism 620, a sensor mechanism 630, a timer 640, and a processing unit 650. The heterodyne light source 610 includes a multi-wavelength laser light source 611 and an acousto-optic modulator (AOM) 612 (similar components are described, for example, as included in the heterodyne light source 210 in Figure 2). One difference between the digital holography measurement system 200 in Figure 2 and the digital holography measurement system 600 is that, as detailed below, the digital holography measurement system 600 includes a switch 619 (e.g., included in the heterodyne light source 610 or others) for switching between different operating modes.

[0065] Figure 6A shows the digital holography measurement system 600 operating in amplitude-modulated continuous wave (AMCW) mode 601A, and Figure 6B shows the digital holography measurement system 600 operating in digital holography mode 601B (for example, using switch 619 as part of switching between these modes). As detailed below, AMCW mode 601A can be characterized as providing measurements on a relatively coarse scale (for example, performing a specific TOF measurement operation using one or more of the sensor TOFs), while digital holography mode 601B (for example, operating as described above in relation to Figures 2 and 3A to 3C) can be characterized as providing measurements on a relatively fine scale. Combining measurements in these two modes can provide high-precision measurements over a relatively large unambiguity range.

[0066] As shown in Figure 6A, in AMCW mode 601A, switch 619 couples timer 640 (e.g., providing a clock signal) to the multi-wavelength laser light source 611 (e.g., coupled to the current source of the multi-wavelength laser light source 611) to modulate the intensity of the light. Acousto-optic modulator 612 does not receive a clock signal from timer 640 and remains off in various embodiments. Thus, light from the multi-wavelength laser light source 611 passes through acousto-optic modulator 612 (e.g., as zero-order light). According to the operation of this example, it can be seen that there is no reference beam directed toward the reference mirror 624 by beam splitter 621. The zero-order light that has passed through acousto-optic modulator 612 passes through beam splitter 621, illuminating the surface 192 of workpiece 190, which is imaged by optical mechanism 620 according to the reflected light (e.g., directed toward sensor mechanism 630 by beam splitter 621). One or more of the sensor TOF sensors of the sensor mechanism 630 (see, for example, Figure 2) measure the distance / surface height of a surface point on the workpiece surface 192 by homodyne detection. A relatively coarse measurement range can be achieved by one or more of the sensor TOF sensors operating in this manner (for example, in one specific embodiment, an unambiguous range of about 1.5 meters is included, with a potential distance error of about 0.5% at such a distance, corresponding to about 7.5 mm).

[0067] Therefore, this operation of AMCW mode 601A effectively provides coarse-scale measurements. Digital holography mode 601B in Figure 6B can be used to provide fine-scale measurements that can resolve the potential distance error of the coarse-scale measurement in Figure 6A, such as an unambiguous range larger than the potential distance error (e.g., an unambiguous range larger than 7.5 mm in this example). In digital holography mode 601B, switch 619 couples timer 640 to acousto-optic modulator 612, which is turned on. The digital holography measurement system 600 operates as described above in relation to Figures 2 and 3A to 3C. In various embodiments, the digital holography measurement system 600 can alternate between modes 601A and 601B frame by frame to achieve the desired measurement.

[0068] As described above, in one specific embodiment of the configuration in Figure 2 (i.e., the configuration in Figure 6B operates similarly), a de-obfuscated range of approximately 8.82 mm can be obtained in several embodiments by utilizing a combination of wavelengths λ1=633 nm, λ2=687 nm, and λ3=767 nm, which can be achieved using several commercially available laser sources. This is larger than the potential error of 7.5 mm in the coarse-scale measurement in the example described above for AMCW mode 601A. Therefore, by combining the measurements of the two modes in these examples, a total de-obfuscated range of approximately 1.5 meters can be achieved with submicron accuracy. It will be acknowledged that by using the two modes 601A and 601B, the total de-obfuscated range can be extended (to, for example, 1.5 meters) without the need to add an additional laser source. In one specific example, additional laser sources may be required to similarly expand the un-blurred range using only digital holography modes such as mode 601B (for example, adding three additional laser sources for a total of six laser sources with wavelengths λ1, λ2, λ3, λ4, λ5, and λ6). Compared to such alternative configurations, using just two modes 601A and 601B, as shown in Figures 6A and 6B, results in a less complex and lower-cost system (for example, using only three laser sources / wavelengths λ1, λ2, λ3 instead of six, and performing measurements from these).

[0069] In various embodiments, different techniques can be used to combine measurements in AMCW mode 601A and digital holography mode 601B. As described above, in various embodiments, AMCW mode 601A provides surface height data at a coarse scale and may, in some cases, be used primarily to resolve unwrapping ambiguity beyond the original holographic range. In one specific embodiment, α = the unambiguous range of the measurement in digital holography mode 601B, z_AMCW = the surface height measurement in AMCW mode 601A, and z_Holo = the holographic z-height measurement in digital holography mode 601B. The term l = lower limit(floor)(z_AMCW / α) gives an integer multiple of the digital holographic unambiguous range of digital holography mode 601B (for example, it is desirable that the measured height falls within the unambiguous range of AMCW mode 601A). As described above, in one specific embodiment, a modulation frequency of 100 MHz may correspond to an AMCW unambiguous range of 1.5 meters. The digital holographic unambiguity range may be larger than the potential distance error (for example, in this example the unambiguity range is larger than 7.5 mm). The final z height can be determined by Δz = I·α + z_Holo. In one example of determining the combined measurement distance to a first surface point on the workpiece according to this formula, the first AMCW mode measurement distance gives an integer multiple of the digital holographic unambiguity range. This integer multiple is combined with the first digital holographic mode measurement distance to determine the combined measurement distance to the first surface point on the workpiece.

[0070] Figures 7 to 11 show various embodiments of heterodyne light sources (which may be used, for example, as heterodyne light source 110 in Figure 1 and / or heterodyne light source 210 in Figure 2). In various embodiments, each of the configurations in Figures 7 to 10 can be characterized as an in-line optical mechanism for copropagating and cross-polarized beams (from an acousto-optic modulator, etc., for heterodyne interferometry). More specifically, each of these embodiments is characterized as an in-line optical mechanism aimed at combining orthogonal-polarized output beams (e.g., from an acousto-optic modulator) into a single beam used as the output of a heterodyne light source. In the in-line optical mechanisms disclosed herein, it is shown that the orthogonal-polarized beams each travel through the same set of components of the light source optical mechanism, and the birefringent optical element section (i.e., including birefringent optical elements) combines the orthogonal-polarized output beams into a single beam as part of a single combined beam.

[0071] It will be acknowledged that the embodiments described below may have advantages over several alternative embodiments. For example, in one alternative embodiment, fiber coupling can be used (e.g., when the output of the acousto-optic modulator can be focused into an optical fiber such as a polarization-maintaining optical fiber by a lens). However, such a configuration may have high coupling losses due to a combination of in-focal-plane beam offset causing clipping, beam size / mode field diameter mismatch, and / or a limited fiber input numerical aperture (NA). In another alternative embodiment, two polarization beam splitters and reflectors may be used. The first beam from the acousto-optic modulator is directed upward by the first beam splitter, reflected by the first reflector to the second reflector, which reflects the first beam to the second beam splitter, which guides the first beam forward. The second beam from the acousto-optic modulator travels straight through the first and second beam splitters, copropagating with the first beam and exiting the configuration. However, such configurations may have mismatched beam parameters due to the non-identical path lengths (i.e., those of the first and second beams) and the lack of a common beam path (i.e., the beams traveling through different optical components). As a result, robustness may be lacking. The embodiments described below in association with Figures 7 to 11 will be found to have several advantages over these configurations.

[0072] Figure 7 shows a heterodyne light source 710, which includes a laser light source 711, an acousto-optic modulator 712, a driver 713, and a light source optical mechanism 718 (i.e., including a receiving optical element section 714p and a birefringent optical element section 715p). The receiving optical element section 714p includes an imaging system having a single imaging lens 714. The birefringent optical element section 715p includes a Wollaston prism 715.

[0073] During operation, timers (e.g., Timer 140, Timer 240, etc.) can provide timing signals (e.g., clock / reference signals) to the driver 713. The acousto-optic modulator 712 is then driven by the driver 713 at a modulation frequency Δω. The laser light source 711 can provide a first wavelength laser beam with a first frequency ω1 (i.e., having a corresponding first wavelength). The acousto-optic modulator 712 (e.g., a shear wave AOM) receives the input first wavelength laser beam and generates / produces a corresponding first frequency-shifted laser beam with frequency ω1 + Δω that is orthogonally polarized with respect to this input first wavelength laser beam. The objective plane in the acousto-optic modulator 712 is imaged into the Wollaston prism 715 by the imaging lens 714 at a magnification M that matches both the separation angle α of the acousto-optic modulator 712 and the separation angle β of the Wollaston prism 715, where M = tan(α) / tan(β). According to the operating characteristics of the Wollaston prism 715, the output is a first composite beam including a first combined wavelength laser beam with a first frequency ω1 and a first orthogonally polarized frequency-shifted laser beam with a frequency ω1 + Δω.

[0074] Accordingly, the components of the heterodyne light source 710 (including, for example, the imaging lens 714 and the Wollaston prism 715) can be characterized as an in-line optical mechanism for copropagating and cross-polarized beams. More specifically, these components are characterized as an in-line optical mechanism intended to combine orthogonally polarized output beams into a single beam output from the heterodyne light source. As shown in Figure 7, the output of the heterodyne light source 710 can be provided to / received by a measurement optical mechanism MOA (e.g., measurement optical mechanisms 120, 220, etc.) for use in measurement operations (e.g., to determine at least one measurement distance to at least one surface point on a workpiece based on a measurement process utilizing a first combined beam).

[0075] Figure 8 is a diagram of a heterodyne light source 810, which includes a laser light source 811, an acousto-optic modulator 812, a driver 813, and a light source optical mechanism 818 (i.e., including a receiving optical element section 814p and a birefringent optical element section 815p). The receiving optical element section 814p includes an imaging system having two imaging lenses 814A and 814B. The birefringent optical element section 815p includes a Wollaston prism 815. Unless otherwise specifically stated below, the heterodyne light source 810, as well as the heterodyne light sources 910, 1010, and 1110 in Figures 9, 10, and 11, are understood to operate similarly to the heterodyne light source 710 in Figure 7. The main difference between the heterodyne light source 810 in Figure 8 and the heterodyne light source 710 in Figure 7, which has an imaging system with a single imaging lens 714, is that the heterodyne light source 810 in Figure 8 includes an imaging system with multiple lenses 814A and 814B. This allows us to impart desirable specific operating characteristics to several application examples, as will be detailed below.

[0076] In embodiments where the laser light source 811 may be a multi-wavelength laser light source, the diffraction angle α of the acousto-optic modulator 812 may be wavelength-dependent. Therefore, in configurations like that shown in Figure 8, in some embodiments, beams of different wavelengths may not copropagate (for example, this may cause beam clipping later in the configuration). To address this problem, the imaging system (i.e., including lens 814A and Figure 814B) can be configured as a chromatic imaging system, with the magnification wavelength-dependent to match the diffraction angle α of the acousto-optic modulator 812 (i.e., at each wavelength) (for example, so that laser beams of different wavelengths copropagate and / or so that beam clipping does not occur later in the configuration by other means). More specifically, according to this configuration, the objective plane in the acousto-optic modulator 812 is imaged at magnification M in the Wollaston prism 815 by the chromatic imaging system (i.e., including imaging lens 814A and Figure 814B). The magnification M varies with each wavelength and matches both the separation angle α(ω) of the acousto-optic modulator 812 and the separation angle β(ω) of the Wollaston prism 815 at each wavelength. Here, M = tan(α(ω)) / tan(β(ω)).

[0077] Figure 9 shows a heterodyne light source 910, which includes a laser light source 911, an acousto-optic modulator 912, a driver 913, and a light source optical mechanism 918 (i.e., including a receiving optical element section 914p and a birefringent optical element section 915p). The receiving optical element section 914p includes a receiving lens 914. The birefringent optical element section 915p includes a birefringent beam displacement 915.

[0078] As shown in Figure 7 and similar to the operation described above, the acousto-optic modulator 912 (e.g., shear wave AOM) receives the input first wavelength laser beam and generates / produces a corresponding first frequency-shifted laser beam that is orthogonally polarized to this input first wavelength laser beam. The input lens 914 is focused within the acousto-optic modulator 912 (e.g., at focal length f) and images at infinity (i.e., the individual beams are collimated). According to the operating characteristics of the birefringent beam displacer 915, the first wavelength laser beam passes through, but the first frequency-shifted laser beam (i.e., orthogonally polarized) is substantially shifted and output together with the first wavelength laser beam. Thus, the output of the birefringent beam displacer 915 is a first composite beam containing the combined first wavelength laser beam and the orthogonally polarized first frequency-shifted laser beam.

[0079] Accordingly, the components of the heterodyne light source 910 (including, for example, the imaging lens 914 and the birefringent beam displacement 915) can be characterized as an in-line optical mechanism for copropagating and cross-polarized beams. More specifically, these components are characterized as an in-line optical mechanism aimed at combining orthogonally polarized output beams into a single beam output from the heterodyne light source. As shown in Figure 9, the output of the heterodyne light source 910 can be provided to / received by a measurement optical mechanism MOA (e.g., measurement optical mechanisms 120, 220, etc.) for use in measurement operations (e.g., to determine at least one measurement distance to at least one surface point on a workpiece based on a measurement process utilizing a first combined beam).

[0080] Figure 10 is a diagram of a heterodyne light source 1010, which includes a laser light source 1011, an acousto-optic modulator 1012, a driver 1013, and a light source optical mechanism 1018 (i.e., including a receiving optical element section 1014p and a birefringent optical element section 1015p). The receiving optical element section 1014p includes a receiving prism 1014. The birefringent optical element section 1015p includes a birefringent beam displacement 1015. Unless otherwise specifically stated below, the heterodyne light source 1010 is understood to operate similarly to the heterodyne light source 910 in Figure 9. The main difference between the heterodyne light source 910 in Figure 9, which has a receiving lens 914, and the heterodyne light source 1010 in Figure 10 is the alternative inclusion of a receiving prism 1014, which allows the beams to be combined without imaging. In various embodiments, the receiving prism 1014 can have its prism vertex angle (for example, the number of vertices in some embodiments) matched to the separation angle of the acousto-optic modulator 1012 and one or more input wavelength laser beams.

[0081] As described above, the embodiments in Figures 7 to 10 have several similar operational features. In each configuration, the light source optical mechanism (e.g., light source optical mechanisms 718, 818, 918, or 1018) receives at least a first wavelength laser beam and a first frequency-shifted laser beam from an acoustic-optic modulator, combines them, and outputs a corresponding first combined beam. In each configuration, the light source optical mechanism includes a receiving optical element section (e.g., each lens 714, 814A, and 814B, 914, and prism 1014, including at least one receiving optical element) and a birefringent optical element section (e.g., each Wollaston prism 715 and 815, and each birefringent beam displacement 915 and 1015, including at least one birefringent optical element). In other embodiments, alternative birefringent optical elements (e.g., Rochon prisms) may be used. In each configuration, the receiving optical element is configured to receive a first wavelength laser beam and a first frequency-shifted laser beam, and to guide these beams along the optical path toward the birefringent optical element. The birefringent optical element is configured to receive the first wavelength laser beam and the first frequency-shifted laser beam, and to combine these beams to output a corresponding first combined beam.

[0082] Each of the birefringent optical elements in the configurations of Figures 7 to 10 (including, for example, the Wollaston prisms 715 and 815 and the birefringent beam displacers 915 and 1015) can be characterized in various embodiments as being used in the opposite direction to some other uses of such components. More specifically, such birefringent optical elements are typically used to receive beams and / or copropagating beams along the optical path on one side, and then to split these beams and provide them as outputs. Unlike such conventional uses, the configurations of Figures 7 to 10 use each birefringent optical element to receive various beams, and then combine these beams to output as part of a single combined beam (including, for example, a copropagating beam). Thus, as described above, each of the embodiments of Figures 7 to 10 is configured as an in-line optical mechanism aimed at combining orthogonally polarized output beams into a single beam output from a heterodyne light source. As shown in Figures 7 to 10, the output of each heterodyne light source can be provided to / received by a measurement optical mechanism MOA (e.g., measurement optical mechanisms 120, 220, etc.) for use in measurement operations (e.g., to determine at least one measurement distance to at least one surface point on a workpiece based on a measurement process utilizing a composite beam).

[0083] Figure 11 is a diagram of a heterodyne light source 1110, which includes a laser light source 1111, an acousto-optic modulator 1112, a driver 1113, and a light source optical mechanism 1118 (i.e., including a spatial filtering configuration 1116 and a collimation lens 1117). In various embodiments, the laser light source 1111 can be a multi-wavelength laser light source (e.g., providing laser beams of first, second, third, etc. wavelengths), and the acousto-optic modulator 1112 can accordingly generate a composite beam as described herein. In various embodiments, the composite beam from the acousto-optic modulator 1112 (e.g., including first, second, third, etc. composite beams) passes through the spatial filtering configuration 1116 and the collimation lens 1117 on its path to the measuring optical mechanism MOA. According to the law of conservation of momentum, different wavelengths can be diffracted to different angles by the operation of the acousto-optic modulator 1112. In one embodiment, the spatial filtering configuration 1116 may include a microscope objective lens 1116A and a pinhole filter element 1116B. The composite beam from the acousto-optic modulator 1112 is imaged by the objective lens 1116A onto the pinhole filter element 1116B (including, for example, a 10 μm pinhole) so that diffracted and non-diffracted beams of different wavelengths overlap, and the light emitted from the pinhole filter element 1116B becomes substantially a point source for beaming into the collimation lens 1117 (for example, there is a usable overlap area UA as shown in Figure 11). After the composite beam (including, for example, first, second, third, etc. composite beams) has passed through the spatial filtering configuration 1116, the collimation lens 1117 operates to collimate these composite beams.

[0084] In general, multi-wavelength laser sources can be used in various configurations of the embodiments shown in Figures 7 to 11 (for example, as described above in relation to Figures 8 and 11 in particular, but they can be used in other configurations as well). The following description of the operation of the multi-wavelength laser source will be made in relation to the multi-wavelength laser source 1111 in Figure 11, but it will be acknowledged that this description can be applied to any of the configurations in Figures 7 to 10 when a multi-wavelength laser source is used. In one embodiment, the multi-wavelength laser source 1111 can provide a first-wavelength laser beam with a first frequency ω1 (i.e., having a corresponding first wavelength). The acousto-optic modulator 1112 receives the first-wavelength laser beam and generates a corresponding first-frequency-shifted laser beam with a frequency ω1 + Δω. Similarly, the multi-wavelength laser source 1111 can provide a second-wavelength laser beam with a second frequency ω2 (i.e., having a corresponding second wavelength). The acousto-optic modulator 1112 receives the second-wavelength laser beam and generates a corresponding second-frequency-shifted laser beam with a frequency ω2 + Δω. Similarly, the multi-wavelength laser source 1111 can provide a third-wavelength laser beam with a third frequency ω3 (i.e., having a corresponding third wavelength). The acousto-optic modulator 1112 receives the third-wavelength laser beam and generates a corresponding third-frequency-shifted laser beam with frequency ω3 + Δω. In various embodiments, the multi-wavelength laser source 1111 may provide additional or fewer wavelength laser beams (for example, the acousto-optic modulator 1112 generates additional or fewer frequency-shifted laser beams accordingly). In general, in the operation of the acousto-optic modulator 1112, it will be acknowledged that the phase shift Δω of the acoustic wave generates the same amount of optical phase shift Δω for all beams, regardless of wavelength. In such operation, in various embodiments, it may be desirable that the driver 1113 be a phase-fixed radio frequency (RF) driver.

[0085] As described above, a timer is coupled to the driver 1113 of the acousto-optic modulator 1112 to provide a base reference clock signal for generating the modulation frequency Δω in the driver 1113. In one embodiment, the clock signal is provided at the modulation frequency Δω (e.g., in the range of 40 MHz to 100 MHz). The distributed clock signal may be equivalent to the frequency of the acoustic wave of the acousto-optic modulator 1112 and also equivalent to the frequency shift Δω of the light beam (e.g., by adding one acoustic phonon to the photon frequency). In various alternative embodiments, the distributed clock signal may be any other frequency and may be multiplied / divided by an additional electronic component to a desired target frequency.

[0086] In this example, each frequency-shifted laser beam is acquired by acousto-optic modulation of the corresponding wavelength laser beam. Some advantages of this configuration include the ability to provide multiple wavelengths, all having the same frequency shift and being phase-fixed, and the overall configuration being relatively simple and low-cost (compared to some conventional configurations, for example). In various alternative configurations, each frequency-shifted laser beam can be acquired using other methods. For example, for each light source / frequency-shifted laser beam pair, the laser can be a Zeeman laser with the laser medium in a strong axial magnetic field, since it generates two beams with different frequencies. In various other alternative embodiments, it is also possible to use an electro-optic modulator instead of an acousto-optic modulator, or a combination of multiple two-wavelength light sources (e.g., lasers operating at two wavelengths simultaneously).

[0087] In various embodiments, the multi-wavelength laser light source 1111 may, in some configurations, include separate laser light sources to provide laser beams of different wavelengths. For example, it may include different laser light sources that provide a desired combination of wavelength ratios (for example, in one specific embodiment, wavelengths such as 633 nm, 687 nm, and 767 nm are provided). For example, in one embodiment, the first laser light source included in the multi-wavelength laser light source 1111 may provide a first wavelength laser beam (for example, in one specific embodiment, the nominal wavelength is 633 nm). Accordingly, the heterodyne light source 1110 provides a first wavelength λ of vertical polarization as described above. S1 A first wavelength laser beam having (e.g., 633 nm) and the wavelength λ of horizontal polarization FS1 A first combined beam can be generated, which includes a first frequency-shifted laser beam (each linearly polarized) having the following characteristics. The timer generates a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator 1112 (e.g., driver 1113). As a result, the first combined beam detects a beat signal (e.g., 40 MHz).

[0088] Similarly, a second laser light source included in the multi-wavelength laser light source 1111 can provide a second wavelength laser beam (for example, in one specific embodiment, the nominal wavelength is 687 nm). Accordingly, the heterodyne light source 1110 provides a first wavelength λ of vertical polarization as described above. S2 A second wavelength laser beam having (e.g., 687 nm) and the wavelength λ of horizontal polarization FS2 A second composite beam can be generated, including a second frequency-shifted laser beam (each linearly polarized) having . Similarly, a third laser source included in the multi-wavelength laser source 1111 can provide a third wavelength laser beam (for example, in one specific embodiment, the nominal wavelength is 767 nm). Accordingly, the heterodyne light source 1110 can provide a third wavelength λ of vertical polarization as described above. S3 A third-wavelength laser beam having (e.g., 767 nm) and the wavelength λ of horizontal polarization FS3A third composite beam can be generated, including a third frequency-shifted laser beam (each linearly polarized) having the same characteristics. As described above, the timer can generate a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator 1112 (e.g., the driver 1113). As a result, each of the second and third composite beams generates a detectable beat signal with a modulation frequency (e.g., 40 MHz). In various embodiments, it may be desirable for the individual laser sources used to have narrow linewidths (e.g., due to having long coherence tapes).

[0089] Figure 12 is a flowchart illustrating an exemplary embodiment of routine 1200 for operating a heterodyne light source. Block 1210 operates the first light source of the heterodyne light source to provide at least a first wavelength laser beam. Block 1220 operates the acousto-optic modulator of the heterodyne light source to receive the first wavelength laser beam and generate a corresponding first frequency-shifted laser beam (e.g., having polarization orthogonal to the polarization of the first wavelength laser beam). In various embodiments, a light source optics mechanism receives and combines the first wavelength laser beam and the first frequency-shifted laser beam from the acousto-optic modulator and outputs a corresponding first combined beam. The light source optics mechanism includes a receiving optical element and a birefringent optical element. The receiving optical element is configured to receive the first wavelength laser beam and the first frequency-shifted laser beam and direct these beams along the optical path toward the birefringent optical element. The birefringent optical element is configured to receive the first wavelength laser beam and the first frequency-shifted laser beam, combine these beams and output a corresponding first combined beam (e.g., to a measuring optical mechanism). In block 1230, at least one measurement distance to at least one surface point on the workpiece is determined based on a measurement process using a first composite beam.

[0090] Figure 13 is a flowchart illustrating an exemplary embodiment of routine 1300 for operating a digital holography measurement system including a heterodyne light source. Block 1310 operates the multi-wavelength light source (e.g., multi-wavelength laser light source) of the heterodyne light source to provide at least a first-wavelength laser beam and a second-wavelength laser beam. Block 1320 operates the acousto-optic modulator of the heterodyne light source to receive the first-wavelength laser beam, generate a first-frequency-shifted laser beam corresponding to this first-wavelength laser beam, and combine this first-frequency-shifted laser beam and the first-wavelength laser beam to produce a first composite beam, and further to receive the second-wavelength laser beam, generate a second-frequency-shifted laser beam corresponding to this second-wavelength laser beam, and combine this second-frequency-shifted laser beam and the second-wavelength laser beam to produce a second composite beam. The interferometric optics mechanism receives the composite beam from the heterodyne light source and uses the composite beam to provide an output for imaging a workpiece. This output includes at least a first interference beam generated based on a first combined beam from a heterodyne light source and a second interference beam generated based on a second combined beam from a heterodyne light source.

[0091] In block 1330, a sensor mechanism is operated to receive an output from an interference optics mechanism. The sensor mechanism includes at least first and second TOF sensors and a first dichroic component. The first dichroic component is configured to separate a first interference beam from a second interference beam. The first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. In block 1340, outputs are received from the first and second TOF sensors and used to determine at least one measurement distance to at least one surface point on the workpiece.

[0092] Preferred embodiments of the Disclosure have been illustrated and described, but numerous variations in the configuration and sequence of operation of the illustrated and described elements will be apparent to those skilled in the art based on this Disclosure. The principles disclosed herein can be carried out using various alternative forms. Furthermore, it is possible to provide other embodiments by combining the various embodiments described above. All U.S. patents and U.S. patent applications referenced herein are incorporated in their entirety by reference. The embodiments described above can be modified as necessary to provide further embodiments using the concepts of these various patents and applications.

[0093] In light of the foregoing, these and other modifications may be made to the examples. In general, the terms used in the following claims should not be construed as limiting the claims to any specific examples disclosed herein and in the claims, but rather as encompassing all possible examples in addition to the entire scope of equivalents entitled to such claims.

Claims

1. A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first composite beam from the heterodyne light source and a second interference beam generated based on the second composite beam from the heterodyne light source, A sensor mechanism that receives the output from the interference optical mechanism and includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. Equipped with, The multi-wavelength light source further provides a third wavelength laser beam having a third frequency different from the first and second frequencies. The acousto-optic modulator is further capable of receiving the third wavelength laser beam, generating a third frequency-shifted laser beam corresponding to the third wavelength laser beam, and combining the third frequency-shifted laser beam and the third wavelength laser beam to generate a third composite beam. The output of the interference optics mechanism further includes a third interference beam generated based on the third composite beam from the heterodyne light source, A digital holography measurement system comprising a sensor mechanism further including a third TOF sensor and a second dichroic component, wherein the second dichroic component is configured to separate the second interference beam from the third interference beam, the second interference beam is directed to be received by the second TOF sensor, and the third interference beam is directed to be received by the third TOF sensor.

2. The digital holography measurement system according to claim 1, further comprising a processing unit that receives outputs from the first TOF sensor and the second TOF sensor and uses the outputs to determine at least one measurement distance to at least one surface point of the workpiece.

3. A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first composite beam from the heterodyne light source and a second interference beam generated based on the second composite beam from the heterodyne light source, A sensor mechanism that receives the output from the interference optical mechanism and includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. Equipped with, The interference optical mechanism includes a beam splitter, a reference plane, and an imaging lens section including an imaging lens. The beam splitter receives the first combined beam and the second combined beam from the heterodyne light source. The first composite beam and the second composite beam are divided into a first portion directed toward the surface of the workpiece and a second portion directed toward the reference plane. A digital holography measurement system that receives a first reflected portion and a second reflected portion, combines them as a first interference beam and a second interference beam, and directs the first interference beam and the second interference beam toward the sensor mechanism via the imaging lens.

4. A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first composite beam from the heterodyne light source and a second interference beam generated based on the second composite beam from the heterodyne light source, A sensor mechanism that receives the output from the interference optical mechanism and includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. Equipped with, It also has a timer, A digital holography measurement system comprising a timer that provides a signal to the heterodyne light source to operate the acoustic-optic modulator, and also provides signals to the first TOF sensor and the second TOF sensor.

5. The digital holography measurement system according to claim 4, wherein the timer is included as part of one of the first TOF sensor and the second TOF sensor.

6. While operating in digital holography mode, the timer is coupled to operate the acousto-optic modulator that generates the first composite beam and the second composite beam, and the processing unit receives outputs from the first TOF sensor and the second TOF sensor, and uses these outputs to determine the first digital holography mode measurement distance to the first surface point of the workpiece. While operating in amplitude-modulated continuous wave ("AMCW") mode, the acousto-optic modulator does not generate the first composite beam and the second composite beam, and the processing unit uses the output of at least one of the first TOF sensor and the second TOF sensor to determine the first AMCW mode measurement distance to the first surface point of the workpiece by homodyne detection, and combines at least a portion of the first AMCW mode measurement distance and the first digital holography mode measurement distance to determine the composite measurement distance to the first surface point of the workpiece. The digital holography measurement system according to claim 4, which is capable of doing so.

7. The digital holography measurement system according to claim 6, wherein the digital holography unambiguity range of the digital holography mode is greater than the potential distance error of the AMCW mode, the first AMCW mode measurement distance is an integer multiple of the digital holography unambiguity range, and the integer multiple is combined with the first digital holography mode measurement distance to determine the composite measurement distance to the first surface point of the workpiece.

8. The digital holography measurement system according to claim 6, wherein the unambiguous range of the AMCW mode is at least 50 times larger than the unambiguous range of the digital holography mode.

9. The digital holography measurement system according to claim 6, wherein the AMCW mode has an unambiguous range greater than 500 millimeters, and the digital holography mode has an unambiguous range greater than 5 millimeters when using the first wavelength laser beam and the second wavelength laser beam from the heterodyne light source and a third wavelength laser beam having a third frequency different from the first and second frequencies.

10. A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first composite beam from the heterodyne light source and a second interference beam generated based on the second composite beam from the heterodyne light source, A sensor mechanism that receives the output from the interference optical mechanism and includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. Equipped with, A digital holography measurement system in which the difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 2 percent of the first wavelength.

11. A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first composite beam from the heterodyne light source and a second interference beam generated based on the second composite beam from the heterodyne light source, A sensor mechanism that receives the output from the interference optical mechanism and includes at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. Equipped with, A digital holography measurement system in which the difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 10 nanometers.

12. A method for operating a digital holography measurement system, The multi-wavelength light source is operated to provide at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. The acousto-optic modulator is operated to receive the second wavelength laser beam, generate a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and combine the second frequency-shifted laser beam and the second wavelength laser beam to generate a second combined beam. This includes operating a heterodyne light source, The interferometry optics mechanism receives the first combined beam and the second combined beam from the heterodyne light source, and uses the first combined beam and the second combined beam to provide an output for imaging the workpiece. The output is, A first interference beam generated based on the first composite beam from the heterodyne light source, The system includes at least a second interference beam generated based on the second composite beam from the heterodyne light source, The method further includes operating a sensor mechanism to receive the output from the interference optical mechanism, the sensor mechanism comprising at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. The multi-wavelength light source is operated to further provide a third wavelength laser beam having a third frequency different from the first and second frequencies, The present invention further includes receiving the third wavelength laser beam, generating a third frequency-shifted laser beam corresponding to the third wavelength laser beam, and operating the acousto-optic modulator to combine the third frequency-shifted laser beam and the third wavelength laser beam to generate a third combined beam, The output of the interference optics mechanism further includes a third interference beam generated based on the third composite beam from the heterodyne light source, The sensor mechanism further includes a third TOF sensor and a second dichroic component, the second dichroic component configured to separate the second interference beam from the third interference beam, the second interference beam being directed to be received by the second TOF sensor, and the third interference beam being directed to be received by the third TOF sensor, method.

13. The method according to claim 12, further comprising receiving outputs from the first TOF sensor and the second TOF sensor, and using the outputs to determine at least one measuring distance to at least one surface point of the workpiece.

14. A method for operating a digital holography measurement system, The multi-wavelength light source is operated to provide at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. The acousto-optic modulator is operated to receive the second wavelength laser beam, generate a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and combine the second frequency-shifted laser beam and the second wavelength laser beam to generate a second combined beam. This includes operating a heterodyne light source, The interferometry optics mechanism receives the first combined beam and the second combined beam from the heterodyne light source, and uses the first combined beam and the second combined beam to provide an output for imaging the workpiece. The output is, A first interference beam generated based on the first composite beam from the heterodyne light source, The system includes at least a second interference beam generated based on the second composite beam from the heterodyne light source, The method further includes operating a sensor mechanism to receive the output from the interference optical mechanism, the sensor mechanism comprising at least a first TOF sensor, a second TOF sensor, and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam is directed to be received by the first TOF sensor, and the second interference beam is directed to be received by the second TOF sensor. A signal for operating the acousto-optic modulator is provided to the heterodyne light source. The first TOF sensor and the second TOF sensor are provided with signals. A method that further includes operating a timer in this manner.

15. While operating in digital holography mode, the timer is coupled to operate the acousto-optic modulator that generates the first composite beam and the second composite beam, and the processing unit receives outputs from the first TOF sensor and the second TOF sensor, and uses these outputs to determine the first digital holography mode measurement distance to the first surface point of the workpiece, While operating in amplitude-modulated continuous wave ("AMCW") mode, the acousto-optic modulator does not generate the first and second combined beams, and the processing unit uses the output of at least one of the first TOF sensor and the second TOF sensor to determine the first AMCW mode measurement distance to the first surface point of the workpiece by homodyne detection, and determines the combined measurement distance to the first surface point of the workpiece by combining at least a portion of the first AMCW mode measurement distance and the first digital holography mode measurement distance. The method according to claim 14, further comprising:

16. The method according to claim 15, wherein the digital holography unambiguity range of the digital holography mode is greater than the potential distance error of the AMCW mode, the first AMCW mode measurement distance is an integer multiple of the digital holography unambiguity range, and the integer multiple is combined with the first digital holography mode measurement distance to determine the composite measurement distance to the first surface point of the workpiece.

17. A sensor mechanism for use in a digital holography measurement system, wherein the digital holography measurement system is A multi-wavelength light source for providing at least a first wavelength laser beam of a first frequency and a second wavelength laser beam of a second frequency different from the first frequency, The first wavelength laser beam is received, a first frequency-shifted laser beam corresponding to the first wavelength laser beam is generated, and the first frequency-shifted laser beam and the first wavelength laser beam are combined to generate a first combined beam. An acoustic-optic modulator capable of receiving the second wavelength laser beam, generating a second frequency-shifted laser beam corresponding to the second wavelength laser beam, and generating a second combined beam by combining the second frequency-shifted laser beam and the second wavelength laser beam, A heterodyne light source including, An interference optics mechanism that receives a first composite beam and a second composite beam from the heterodyne light source and uses the first composite beam and the second composite beam to provide an output for imaging a workpiece, The output comprises an interference optics mechanism including at least a first interference beam generated based on the first combined beam from the heterodyne light source and a second interference beam generated based on the second combined beam from the heterodyne light source. The aforementioned sensor mechanism is It includes a first TOF sensor, a second TOF sensor, and a first dichroic component, The first dichroic component receives the output from the interference optical mechanism and is configured to separate the first interference beam from the second interference beam, with the first interference beam directed to be received by the first TOF sensor and the second interference beam directed to be received by the second TOF sensor. The multi-wavelength light source further provides a third wavelength laser beam having a third frequency different from the first and second frequencies. The acousto-optic modulator is further capable of receiving the third wavelength laser beam and generating a corresponding third frequency-shifted laser beam, which is a composite beam formed by combining the third wavelength laser beam with the modulator. The output of the interference optics mechanism further includes a third interference beam generated based on the third composite beam from the heterodyne light source, The sensor mechanism further includes: A third TOF sensor, A second dichroic component is provided, A sensor mechanism comprising a second dichroic component configured to separate the second interference beam from the third interference beam, the second interference beam being directed to be received by the second TOF sensor, and the third interference beam being directed to be received by the third TOF sensor.

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