阅读说明：本技术 通过偏振光纤连接到源/检测器的部分相干范围传感器笔 (Partially coherent range sensor pen connected to source/detector by polarized fiber ) 是由 D·B·凯 于 2020-01-28 设计创作，主要内容包括：一种用于光学测量系统的探头,包括探头主体,该探头主体被布置成可调节地安装在用于光学地测量测试物体的测量机中。光学耦合在探头主体内的偏振光纤将具有跨越一系列波长的瞬时或顺序建立的带宽的源光束传输到探头主题,并且还将测量光束从探头主体朝向检测器传输。提供了可调节光束操纵器,用于沿着参考臂角度地重新分布参考光束。(A probe for an optical measurement system comprises a probe body arranged to be adjustably mounted in a measuring machine for optically measuring a test object. A polarizing fiber optically coupled within the probe body transmits a source beam having an instantaneously or sequentially established bandwidth spanning a range of wavelengths to the probe body, and also transmits a measurement beam from the probe body toward a detector. An adjustable beam manipulator is provided for angularly redistributing the reference beam along the reference arm.)

1. A method of enhancing interferometric contrast in an optical measurement system having an interferometer probe connected to both a light source and a detector by an external polarizing fiber, the method comprising:

a beam splitter that directs a collimated source beam having an instantaneously or sequentially established bandwidth spanning a range of wavelengths into the interferometer probe;

dividing the collimated source beam at the beam splitter into:

an object beam directed along the object arm through an object objective in the interferometer probe to an object focus proximate the test object, an

A reference beam directed along a reference arm to a reference reflector within the interferometer probe;

angularly redistributing the reference beam along the reference arm;

combining, at a beam splitter, an object beam reflected from the test object and an angularly redistributed reference beam reflected from a reference reflector into a measurement beam; and

focusing a measuring beam towards an end of a polarizing fiber having an acceptance cone that limits an angular distribution of the measuring beam received for further propagation along the polarizing fiber towards a detector;

wherein the angular redistribution of the reference beam comprises adjusting the angular redistribution of the reference beam to confine a reference beam portion of the focused measurement beam, the reference beam portion being received through the cone of acceptance of the polarizing fiber for further propagation towards the detector.

2. The method of claim 1, further comprising:

comparing the respective intensities of the reflected object beam and the reflected reference beam within the measurement beam; and

the reference beam portion of the measurement beam received through the cone of acceptance of the polarizing fiber is limited to more closely balance the intensities of the reflected object beam portion and the reference beam portion of the measurement beam propagating along the polarizing fiber.

3. The method of claim 2, wherein the excluded portion comprises a discontinuous portion of the reference beam.

4. The method of claim 1, wherein the dividing comprises directing the reference beam along the reference arm through a reference objective lens within the interferometer probe to a reference focal point proximate to the reference reflector.

5. The method of claim 4, wherein the angular redistribution of the reference beam comprises defocusing the reference beam on the reference reflector.

6. The method of claim 5, wherein the reference beam is defocused by translating the reference reflector relative to the reference objective along the common optical axis and the optical path length adjustment to maintain a relative optical path length between the reference arm and the object arm.

7. The method of claim 6, wherein the optical path length adjustment is effected by relatively translating the object objective with respect to the beam splitter.

8. The method of claim 4, wherein the angular redistribution of the reference beam comprises pivoting the reference reflector about an axis passing through the reference focal point.

9. The method of claim 2, wherein the comparing comprises measuring contrast between phase modulations of different wavelengths in the detector.

10. A method as in claim 1, further comprising transmitting a source beam from the light source along a polarizing fiber to a collimating lens for directing the collimated source beam to a beam splitter within the interferometer probe.

11. The method of claim 10, wherein the light source is a first light source for emitting invisible light, and further comprising:

transmitting the visible light from the second light source to the beam splitter through the collimating lens along the polarizing fiber; and

the visible light from the beam splitter is directed along the object arm through the object objective to a visible focal spot on the test object.

12. A probe for an optical measurement system, comprising:

a probe body arranged to be adjustably mounted in a measuring machine for optically measuring a test object;

at least one polarizing fiber optically coupled within the probe body for transmitting a source beam having an instantaneously or sequentially established bandwidth spanning a range of wavelengths to the probe body and for transmitting a measurement beam from the probe body towards the detector;

at least one collimator/coupler, a beam splitter, an object objective, a reference lens, and a reflector mounted within the probe body;

the at least one collimator/coupler is arranged for collimating a source light beam emitted from the at least one polarizing fiber;

a beam splitter arranged to divide the collimated source beam into both an object beam directed along the object arm through the object objective to an object focus near the test object and a reference beam directed along the reference arm to the reference reflector;

an adjustable beam manipulator for angularly redistributing the reference beam along the reference arm;

the beam splitter is further arranged for combining an object beam reflected from the test object and an angularly redistributed reference beam reflected from the reference reflector into a measurement beam;

the at least one collimator/coupler is arranged for focusing the measuring beam towards at least one polarizing fiber having an acceptance cone of the at least one polarizing fiber, the acceptance cone limiting an angular distribution of the measuring beam received for further propagation along the polarizing fiber towards the detector; and

the adjustable beam manipulator is arranged for adjusting the angular redistribution of the reference beam to limit a reference beam portion of the focused measurement beam, which is received through the acceptance cone of the polarizing fiber for further propagation towards the detector.

13. The probe of claim 12, wherein a reference objective is also mounted in the probe body, and the reference beam is directed along the reference arm through the reference objective to a reference focal point proximate to the reference reflector.

14. The probe of claim 13, wherein an adjustable beam manipulator provides for defocusing the reference beam differently on the reference reflector.

15. The probe of claim 14, wherein the adjustable beam manipulator comprises a first linear actuator for translating the reference reflector relative to the reference objective along the common optical axis and a second linear actuator for relatively adjusting the relative optical path length between the reference arm and the object arm to compensate for variations in the optical path length associated with the translation of the reference reflector.

16. The probe of claim 13, wherein the adjustable beam manipulator comprises a tilt adjuster for pivoting the reference reflector about an axis passing through the reference focus.

17. The probe of claim 16, wherein the at least one polarizing fiber optically coupled within the probe body is a single fiber provided for transmitting both the source beam to the probe body and the measurement beam from the probe body.

18. The probe of claim 16, wherein each of the excluded portions includes a discontinuous portion of the reference beam.

Technical Field

In the field of optical metrology, the optical probe of a coordinate measuring machine is typically moved over a test object to obtain a point-by-point height measurement of the test object. The optics are typically divided into the probe and another part of the machine.

Background

By measuring the rate at which the interferometric phase varies with the number of waves, point-by-point measurements of the relative optical path length displacement can be made over a series (range of) such optical displacements. For example, a spatially coherent source beam consisting of multiple wavelengths, i.e., a low temporal coherence beam, may be divided by a beam splitter into an object beam reflected from a test object and a reference beam reflected from a reference reflector. The reflected light from both the test object and the reference reflector is recombined at the beam splitter into a measurement beam and refocused within a detector, such as a spectrometer, which records the interference intensities of the different spectral components of the returned measurement beam. The relative optical displacement between different measurement points can be determined based on an approximately linear relationship between (a) the rate of change in interference phase with change in beam frequency, referred to as modulation frequency, and (b) the optical path length difference between the object beam and the reference beam.

Since the information is collected on a point-by-point basis, single mode optical fibers can be used to convey light along portions of the object and reference arm, as well as to and from the light source and detector. However, bending motions that induce stress-induced (induce) birefringence in single-mode fibers can produce optical path length variations that reduce interference amplitude and measurement accuracy. Fiber optic cables attached to articulating optical probes are susceptible to such interference, especially when they use separate transmit and receive optical fibers.

Disclosure of Invention

Certain embodiments provide for enhancing interferometric phase contrast in an optical measurement system having an interferometer probe connected to both a light source and a detector by an external polarizing fiber. According to one method, a collimated source beam having an instantaneously or sequentially established bandwidth spanning a range of wavelengths is directed to a beam splitter within an interferometer probe where the collimated source beam is divided into (a) an object beam directed along an object arm through an object objective lens within the interferometer probe to an object focus on a test object, and (b) a reference beam directed along a reference arm to a reference reflector within the interferometer probe, the reference beam being angularly redistributed along the reference arm. The object beam reflected from the test object and the angularly redistributed reference beam reflected from the reference reflector are recombined at the beam splitter into a measurement beam. The measuring beam is focused towards the end of the polarizing fiber having an acceptance cone that limits the angular distribution of the measuring beam received for further propagation along the polarizing fiber towards the detector. The angular redistribution of the reference beam includes adjusting the angular redistribution of the reference beam to confine a reference beam portion of the focused measurement beam that is received through the cone of acceptance of the polarizing fiber for further propagation toward the detector.

For adjustment, the respective intensities of the reflected object beam and the reflected reference beam in the measurement beam may be compared, and the reference beam portion of the measurement beam received through the acceptance cone of the polarizing fiber may be limited to more closely balance the intensities of the reflected object beam portion and the reference beam portion of the measurement beam propagating along the polarizing fiber. For example, the comparison may be made by measuring the contrast between phase modulations of different wavelengths in the detector. The excluded portion may comprise a continuous (contiguous) or discontinuous portion of the reference beam.

At the beam splitter, the reference beam may be directed along a reference arm through a reference objective lens within the interferometer probe to a reference focal point on a reference reflector. The angular redistribution of the reference beam may include defocusing the reference beam on the reflector. The reference beam may be defocused by translating the reference reflector along the common optical axis relative to the reference objective and the optical path length adjustment to maintain the relative optical path length between the reference arm and the object arm. Alternatively, the reference beam may be angularly redistributed by pivoting the reference reflector about an axis passing through the reference focal point.

Preferably, the source beam is transmitted by the same polarizing fiber to a collimating lens for directing the collimated source beam to a beam splitter within the interferometer probe. Since the preferred multi-wavelength light source of the current art typically emits invisible light, a second light source can be used to emit visible light that is visible on the test object. Visible light can be transmitted along the polarizing fiber to the collimating lens, through the beam splitter, and along the object arm through the object objective to a focal spot on the test object. Thus, the focal position of the object beam can be seen on the test object for setup and monitoring purposes.

The reference objective may also be mounted within the probe body such that the reference beam propagates through the reference objective along the reference arm to a reference focal point proximate the reference reflector. To spatially exclude a variable portion of the reference beam from entering the polarizing fiber, the adjustable beam manipulator may be arranged to defocus the reference beam differently on the reference reflector. For example, the adjustable beam manipulator may include a first linear adjuster for translating the reference reflector relative to the reference objective along the common optical axis and a second linear adjuster for relatively adjusting the relative optical path length between the reference arm and the object arm to compensate for variations in the optical path length associated with the translation of the reference reflector. Alternatively, the adjustable beam manipulator may comprise a tilt actuator or other tilt adjuster for pivoting the reference reflector about an axis passing through the reference focus. Alternatively, the modulator may block a portion of the reference beam.

Preferably, the polarizing fiber optically coupled within the probe body is a single fiber that provides for transmitting both the source beam to and the measurement beam from the probe body. The excluded portion of the reference beam may include a discontinuous portion of the reference beam. For example, during an initial calibration of the measuring machine based on the expected reflectivity of the test object, during a subsequent recalibration of the measuring machine for the same or other purposes, between measurements of the test objects, including between test objects having different expected reflectivities, or during measurements of individual test objects, the relative intensity of the reference beam portion of the measuring beam may be adjusted relative to the intensity of the object beam portion of the measuring beam to maintain a desired level of interference contrast within the detector.

Drawings

FIG. 1 is a schematic side view of a multi-axis measuring machine having a vertically displaceable slide mechanism.

FIG. 2 is a diagram of an optical measurement system for the measuring machine of FIG. 1 in which the object and reference arms of the interferometer are mounted in the probe head and connected to both the light source and the detector by external polarizing fibers, with a reference beam manipulator associated with the reference arm for more closely balancing the intensities of the object and reference beams directed to the detector.

FIGS. 3A and 3B graphically depict two different measurement outputs of an interferometer in which the intensity varies as a function of wavenumber at different modulation frequencies associated with different optical path lengths of the object relative to the reference beam.

FIG. 4 graphically depicts a calculated output of a processor for identifying a modulation frequency based on an output of a detector.

FIG. 5 depicts the effect of a beam manipulator according to the arrangement of FIG. 2, wherein a portion of the reference beam is expanded outside the acceptance cone of the polarizing fiber.

FIG. 6 is an enlarged view of an alternative probe in which the reference arm is folded and a different type of beam manipulator is characterised by (is fed for) more closely balancing the intensity of the object and reference beams directed to the detector.

FIG. 7 depicts the effect of a beam manipulator according to the arrangement of FIG. 6, wherein a portion of the reference beam is excluded from the acceptance cone of the polarizing fiber.

FIG. 8 is an enlarged view of an alternative reference arm incorporating another different type of beam manipulator.

Fig. 9 depicts the effect of the beam manipulator according to the arrangement of fig. 8, wherein a portion of the reference beam is intercepted (interleaved) and diffracted from reaching the acceptance cone of the polarizing fiber.

FIG. 10 is an enlarged view of an alternative reference arm simplified for use with the beam manipulator of FIG. 8.

FIG. 11 is a diagram of a similar optical measurement system in which a visible light source is incorporated into the system for illuminating a focal spot on the test object.

FIG. 12 graphically depicts the calculated output of a processor for identifying modulation frequencies using a "polarization maintaining" single mode fiber.

FIG. 13 graphically depicts the modulation frequency of FIG. 12 when the object beam is blocked and cannot return to the detector, illustrating the return reference beam and two fixed delays.

Detailed Description

The multi-axis machine 10 depicted in FIG. 1 in one of many possible configurations of an optical measuring machine includes an X-Y stage 14 for translating a test object 18 horizontally along the X and Y coordinate axes and a slide mechanism 16 for translating an interferometer probe 20 vertically along the Z coordinate axis. The x-y stage 14 is supported on a machine base 22. The slide mechanism 16 is supported in a slide support 26 carried on the post 24. The interferometer probe 20 is carried on an articulated arm 28 that is both pivotable about a horizontal axis and rotatable with a pivot about the Z coordinate axis, although it may be fixedly mounted to the Z coordinate axis. Other portions of the probe optics, not visible, including one or more light sources and detectors, or other devices that support the metering functions of the machine 10, may be housed in a slide support 26, with the slide mechanism 16 being translatable within the slide support 26. Relative motion between interferometer probe 20 and test object 18 is measured along or about various axes to monitor the relative position of the interferometer probe with respect to test object 18 within a common coordinate system. In addition to the interferometric probe 20, the multi-axis machine may include a vision system mounted to the Z coordinate axis and other probes.

The measuring machine 10 may be arranged with other combinations of rotational and translational axes for relatively moving one or the other of the test object 18 and the interferometer probe 20. Preferably, to gather information about the test object 18, such as an empirical description of the test object profile, the relative motion provides for maintaining an optical focus 30 of light emitted by the optical elements of the probe 20 proximate the test object 18 at a series of different locations on the test object 18 at an orientation effective to collect light from the test object 18 through specular or diffuse reflection by the same optical elements of the probe 20.

An optical arrangement for measuring a test object 18 with an optical measuring system for a measuring machine 10 is shown in fig. 2. A light source 32, such as a superluminescent diode, housed in a machine housing, such as the sliding support 26, provides for transmitting high spatial coherence but low temporal coherence (i.e., light containing a range of wavelengths within a continuum of wavelengths) to the interferometer probe 20, typically through a single mode fiber 34 (although it may be a polarizing fiber). Preferably, within the same housing (such as the sliding support 26), the detector 36 is arranged for receiving light returned from the interferometer probe 20 through a generally single mode optical fibre 38 (although it could be a polarising optical fibre), although it could be a polarising optical fibre. A processor 40 for processing information from the detector 36 is preferably located outside the housing for communication with a user interface (not shown).

A fiber coupler 42, which may be a 50%/50% coupler, connects the single mode fibers 34 and 38 to a common external polarizing fiber 44 for transmitting high spatial coherent, low temporal coherent light to and from the interferometer probe 20. As schematically shown, the polarizing fiber 44 has an extra length to accommodate movement of the (acomod) interferometer probe 20 relative to the sliding support 26.

The polarizing fiber 44 is configured such that the stress created by bending the polarizing fiber 44 has an insignificant effect on the SNR and measurement accuracy of the probe 20. Polarizing fiber 44 reduces the throughput (throughput) from source 32 to probe 20 by approximately 50%, but can be increased by selecting a source with a greater radiation output. Thus, the source beam 48 (shown in dashed lines) is linearly polarized, rather than unpolarized.

Fig. 4 illustrates the signal from a remote probe system, where the abscissa is proportional to the distance between the test object 18 and the probe 20, and the ordinate is proportional to the signal strength. Movement of the probe 20 in the Z-axis (away from the test object 18) will cause the peak of the signal to shift toward the right hand direction. However, if a single mode fiber is used instead of the polarizing fiber 44 and the single mode fiber is strongly bent, the peak level of the signal will decrease and a small shift in the peak level (approximately between 0 and 1.5 um) may occur. These effects are reduced to insignificant levels using the polarizing fiber 44 of the measuring machine 10.

However, a "polarization-maintaining/maintaining" single-mode fiber would not have the same benefits as the polarization fiber 44 of the measuring machine 10. As shown in fig. 12, multiple peaks occur because the "polarization maintaining" single mode fiber distributes an unpolarized light beam (e.g., a light beam from the light source 32 arriving through the single mode fiber) into orthogonal components, and then propagates the orthogonal components (e.g., fast and slow) having different refractive indices along the fast and slow axes of the "polarization maintaining" single mode fiber. Thus, for a relatively unpolarized beam propagating from the light source 32 to the probe 20 and then back again to the detector 36 through an external "polarization maintaining" single mode fiber, a fixed delay in the wavefront (wavefront) occurs.

Polarizing fiber 44 is a special-purpose fiber that propagates in only one polarization direction without significant loss, thereby polarizing light propagating through polarizing fiber 44. This form of single polarization transmission has several benefits over single mode or polarization maintaining fibers. Although the polarization maintaining fiber maintains a polarization direction aligned with the birefringence axis, crosstalk (cross talk) may occur because the polarization maintaining fiber is capable of guiding any polarization direction. Single mode fibers may be stressed to induce birefringence, which causes the single mode fibers to behave much like wave plates (wave plates). Although the polarization axis can be manipulated in this case, the single mode fiber does not polarize light.

In contrast, polarizing fiber 44 includes only one polarization direction; all other directions are attenuated. As a result, the polarizing fiber 44 will polarize the light guided through it, resulting in excellent suppression of other polarization directions.

FIG. 13 illustrates the detection signal of the system of FIG. 2 passing through a "polarization maintaining/sustaining" single mode fiber when the object beam is blocked. If the object beam 60 is blocked so that it does not combine with the reference beam 70, the reference beam 70 is returned to the detector of the measuring machine 10. The two peaks observed in FIG. 13 are caused by the separation of the unpolarized source beam 48 into (break into) orthogonal components by a "polarization-maintaining/sustaining" single-mode fiber and propagating them with different indices of refraction; thus, a fixed delay is built (built) into the beam that propagates from the source 32 to the probe 20 and returns to the detector 36 of the measuring machine 10 through a "polarization maintaining/maintaining" single mode fiber.

The following paragraphs demonstrate why multiple peaks occur in fig. 12 and 13 without attempting to determine intensity, where OP = optical path length:

Intensity=[(2Fast+2Slow)]²for forward and backward passage through a "polarization maintaining" single mode fiber.

·[(2Fast + 2Slow)]² = [e^{-i2π/λ[(2Fast OP)} + e^{-i2π/λ(2Slow OP)]}] [e^{i2π/λ[(2Fast OP)} + e^{i2π/λ(2Slow OP)]}]

= {2 + 2cos[(2π/λ)(2FastOP-2SlowOP)]}

= 2{1 + cos(2π/λ)(2δ)}

Wherein FastOP = fast axis optical path for a single pass,

SlowOP = slow axis optical path for a single pass, and

δ = (FastOP-SlowOP) single mode fiber-single pass through "polarization maintaining".

As shown in fig. 13, the second highest peak on the right is due to the above-mentioned interference. If there is a small amount of depolarization (depolarisation) in the probe 20 of the measuring machine 10, or crosstalk in a polarization maintaining/preserving single mode fiber, then a portion of the fast axis can be converted to the slow axis, and vice versa, with the following additional disturbances:

· [(Fast out + Slow return)]²

· [(Slow out + Fast return)]²

these disturbances are approximately equal to 4{1+ cos (2 π/λ) (δ) }, which results in the highest peak on the left, as shown in FIG. 13. As shown in fig. 12 and provided in the table below, the offset Δ is fixed in the partially coherent interferometer by adding an additional combination in the object beam that contains the offset Δ of the object path from the reference path.

Wherein:

s = "polarization maintaining" single mode fiber-slow optical path in single pass.

F = "polarization maintaining" single mode fiber-fast optical path in single pass.

SS = "polarization maintaining" slow path out and slow path back in single mode fiber.

SF = "polarization maintaining" slow path out and fast path back in single mode fiber.

FS = "polarization maintaining" fast path out and slow path back in single mode fiber.

FF = "polarization maintaining" fast path out and fast path back in single mode fiber.

ref. = reference path in interferometer pen

obj = object path in the interferometer pen.

Because of the additional interfering peaks in the signal caused by the inherent properties of a "polarization-maintaining" single-mode fiber, a "polarization-maintaining" single-mode fiber does not provide the same benefits as the polarization fiber 44 of the measuring machine 10.

Within interferometer probe 20 having a probe body 20a that schematically coincides with the representative dashed outline of interferometer probe 20, light is directed to a Linnik type interferometer, although other interference arrangements may be used. In the arrangement shown, light emitted as a source beam 48 (shown in phantom) from the end 46 of the polarizing fiber 44 is collected and collimated by a collimator/condenser lens 50, the collimator/condenser lens 50 being aligned with a non-polarizing beam splitter 52. At the partially reflective surface 54 of the non-polarizing beam splitter 52, the source beam 48 is divided into an object beam 56 (shown in phantom) that is transmitted through the partially reflective surface 54 and a reference beam 58 (shown in phantom) that is reflected by the partially reflective surface 54. Object beam 56 propagates along object arm 60 through object objective 62 in probe body 20a to an object focus 64 outside probe body 20a near test object 18. Reference beam 58 propagates along reference arm 70 through a reference objective 72 in probe body 20a to a reference focal point 74 proximate a reference reflector 76, which reference reflector 76 may be in the form of a flat mirror also in probe body 20 a. Preferably, all three lenses 50, 62 and 72 are achromatic, low dispersion lenses for matching the focusing effect of the different wavelengths within the source, object, reference and measurement beams 48, 56, 58 and 80.

Specular or diffuse reflections of the object beam 56 from the test object 18 are collected and re-collimated by the object objective 62 on the way back to the beam splitter 52 (on route). Similarly, the reflection from reference reflector 76 is collected and re-collimated by reference objective 72 on the way back to beam splitter 52. At the beam splitter 52, at least a portion of the return object beam 56 transmitted through the partially reflective surface 54 and at least a portion of the return reference beam 58 reflected from the partially reflective surface 54 are recombined on a return path to the collimator/condenser lens 50 into a common measurement beam 80 (shown overlapping the source beam 48). Because the reflectivity of test object 18 is generally less than the reflectivity of reference reflector 76, beam splitter 52 is preferably arranged to transmit light more efficiently through partially reflective surface 54, and is arranged to reflect light from partially reflective surface 54 less efficiently. The collimator/condenser lens 50 focuses the measurement beam 80, which contains portions of the object and reference beams 56 and 58, back into the polarizing fiber 44 for transmission to the detector 36. The end 46 of the polarizing fiber 44 receives the measuring beam 80 through the volume of the acceptance cone, which is typically related to the refractive index of the fiber core and cladding (cladding).

Within the detector 36 arranged as a spectrometer, the measuring beam 80 may be re-collimated and reflected from the diffraction grating in a series of spectrally dispersed orientations, and the dispersed orientation of the measuring beam 80 may be focused along a linear array of photodiodes or Charge Coupled Devices (CCDs). Each different frequency (as the inverse of wavelength) of the object beam 56 portion from the measuring beam 80 interferes with the corresponding frequency of the reference beam 58 portion of the measuring beam 80 at a different focal position along the array. The intensity of the light focused along the array, representing the modulo-2 pi phase difference between the object and reference beam 56, 58 portions of the measuring beam 80, is modulated at a detectable frequency, referred to as the modulation frequency, which varies within the Nyquist interval (due to pixel sampling) proportional to the optical path length difference between the object and reference beam 56, 58 portions of the measuring beam 80. Since the intensity information is collected by a discrete number of pixels, the distinguishable frequency ranges typically from zero up to half the number of pixels involved in the measurement.

Fig. 3A and 3B graphically illustrate (graph) two different examples of intensity variations captured along a linear array of pixels, and along which focal point locations of different frequencies (wavenumbers) are dispersed. The change in intensity corresponding to the change in interference phase is substantially periodic at a measurable frequency called the modulation frequency. As the optical path length difference between the object and reference beam 56, 58 portions of the measuring beam 80 increases from zero (i.e., the zero position), the modulation frequency increases proportionally within the measured Nyquist interval. For example, the frequency of the modulation depicted in FIG. 3A appears to be higher than the frequency of the modulation depicted in FIG. 3B, thereby demonstrating a larger optical path length difference between the object and reference beam 56, 58 portions of the measurement beam 80 in the measurement captured by the detector 36 as shown in FIG. 3A as compared to the measurement of the optical path length difference captured by the detector 36 as shown in FIG. 3B. Fig. 4 shows the calculated modulation frequency as a frequency spike 86 within the depicted measured range, as may be graphically output from processor 40.

Within the processor 40, the calculated modulation frequency may also be converted to a height above the surface of the test object 18. To collect data at a series of points on test object 18 within a common coordinate system, the relative motion between probe 20 and test object 18 is monitored to track the position of focal point 64 of probe 20 in space. During setup, the optical path length difference between the object and reference beams 56, 58 considered at the ideal focus position is set at a given modulation frequency. During measurement, deviations from a given modulation frequency, which are interpreted as surface height variations, may be added or subtracted to the relative position of the measured probe focal point 64 to provide a finer measurement of the position of the measurement point on the test object 18 within the depth of focus of the objective 62.

Since deviation from a given modulation frequency is also a measure of deviation from an ideal focus position, deviation from a given modulation frequency may also be used to maintain focus within a usable range. In other words, the relative position of the probe 20 may be corrected by shifting the probe 20 along the Z-axis to position the ideal focal point closer to the surface of the test object 18 and at a modulation frequency closer to the given modulation frequency. The focus correction in turn maintains the probe within both the expected measured Nyquist interval and the depth of focus of the objective lens 62.

The accuracy with which the modulation frequency can be determined is based in part on the contrast with which the interferometric phase modulation is expressed. Since the intensity is related to the square of the amplitude of the waveform, the highest contrast of interferometric phase modulation occurs when the relative intensities of the object and reference beam 56, 58 portions of the measuring beam 80 are equal. The intensity of the return object beam 56 component of the measuring beam 80 depends on the reflectivity of the test object 18 at the point of measurement, which can vary considerably between test objects or between different parts of the same test object.

To more closely balance the intensity of the reflected object beam 56 and the reflected reference beam 58, various embodiments provide for adjustably excluding a portion of the reference beam 58 from being focused within the cone of acceptance of the polarizing fiber 44 over a series (a progression of) differently sized portions. Different sized portions of the reference beam 58 may be blocked or otherwise excluded from reaching the cone of acceptance of the polarizing fiber 44 to adjust the intensity of the reference beam 58 according to the nominal reflectivity from the test object 18.

For example, as shown in fig. 2, reference reflector 76 may be connected to an adjustable beam manipulator in the form of a linear adjuster 82, such as an adjustment-screw-drive mechanism, for displacing reference reflector 76 along the optical axis of objective lens 72 to variably defocus objective lens 72. As a further part of the manipulator, a second linear actuator 84, which may be in the form of a threaded barrel (threaded barrel), displaces the objective lens 62 by a related amount to compensate for the optical path length difference between the object arm 60 and the reference arm 70 associated with the translation of the reference reflector 76. The resulting displacement of the object focus 64 may be accommodated by recalibrating the object focus position relative to the coordinate positions defined by the other machine axes. Alternatively, the objective lens 72 of the reference arm 70 may be similarly translated with the reference reflector 76 to compensate for the optical path length difference (impart) resulting from the translation of the reference reflector 76. The combined translation of objective lens 72 and reference reflector 76 eliminates the need for recalibration for variations in the position of object focus 64. Instead of moving reference reflector 76, objective lens 72 may similarly be translated along its optical axis to variably defocus reference beam 58 on reference reflector 76 without changing the optical path length of reference arm 70. For example, the objective lens 72 may be mounted in a threaded barrel as part of a similar linear actuator to more closely match the intensity of the return reference beam 58 to the nominal intensity of the return object beam 56.

Defocusing the objective lens 72 of the reference arm 70 introduces different amounts of wavefront curvature into the reflected reference beam 58, which extends the focal volume of the measurement beam beyond the acceptance cone of the polarizing fiber 44. Increasing defocus excludes a larger portion of reflected reference beam 58. This adjustment provides a simple and symmetric way of adjusting the intensity of the reflected reference beam 58 for countering erratic effects from disturbances such as thermal drift. To determine the desired amount of defocus, the interference contrast can be measured in the detector 36 by measuring the overall intensity change and the returned object beam intensity 56, and the amount of defocus can be adjusted to better optimize the measured intensity change.

As shown in FIG. 5, when refocused by the collimator/condenser lens 72, the reference beam 58 portion of the measurement beam 80 encompasses a range of larger 111 or smaller 106 angles (depending on the direction of defocus) about the optical axis 98 and results in a larger spot size at the end 46 of the polarizing fiber 44 such that at least some of the converging elements of the reference beam 58 portion of the measurement beam 80 are oriented outside the acceptance cone of the polarizing fiber 44.

For example, as shown in FIG. 5, the amount of light that can enter the polarizing fiber 44 is contained in an acceptance cone 100 shown in dashed lines. The cross-section of the polarizing fiber 44 shows the core 102 of the polarizing fiber 44 with the surrounding cladding 104 exposed. Another cone 106, shown in solid lines, represents the reference beam 58 portion of the measuring beam 80 that experiences (subject to) being defocused by the reference reflector 76 on a path that converges before the end 46 of the polarizing fiber 44. While the cone 106 still converges in a symmetrical manner about the optical axis 98 of the collimator/condenser lens 50, the cone 106 has angular elements distributed outside the cone of acceptance 100. As a result, the relative intensity of the reference beam 58 portion of the measuring beam 80 relative to the object beam 56 portion of the measuring beam 80 is reduced.

Fig. 6 depicts an alternative interferometer probe 90 in a more compact configuration. Most of the components are identical and are denoted by the same reference numerals. However, a reflector 92, such as a flat mirror, is added to the reference arm 94 to fold the reference arm 94 into a more compact configuration. Although reference reflector 76 may still be arranged for translation in a more compact configuration, reference reflector 76 is shown in fig. 6 as being mounted on a tilt actuator 96, tilt actuator 96 pivoting reference reflector 76 about an axis passing through reference focal point 74. For example, the reference reflector 76 may be arranged in the form of a flat mirror that is tiltable about a fixed axis located on the reflecting surface of the mirror. The mirror may be supported on, for example, a gimbal, semi-cylindrical bearing, or flexible joint, and may be tilted manually, such as by a screw-type tilt adjuster, or automatically, such as by a piezoelectric actuator.

Tilting reference reflector 76 about focal point 74 does not change the nominal optical path length of reference arm 94 relative to the optical path length of object arm 60 or require any recalibration associated with the displacement of the position of object focal point 64. The objective lens 72 re-collimates the tilted reflected reference beam 58 in a laterally offset position, e.g., no longer centered on the optical axis 98. When refocused by the collimator/condenser lens 50, the reference beam 58 portion of the measuring beam 80 contains an asymmetric angular distribution about the optical axis 98 such that at least some of the angular elements of the reference beam 58 portion of the measuring beam 80 are removed from the cone of acceptance of the polarizing fiber 44.

For example, as shown in FIG. 7, a cone 112 shown in solid lines (as opposed to the acceptance cone 100 shown in dashed lines) represents the portion of the reference beam 58 of the measuring beam 80 that is subject to being tilted about the focal point 74 by the reference reflector 76 on a path converging toward the end 46 of the polarizing fiber 44. While the cone 112 still converges along the optical axis 98 of the collimator/condenser lens 50 toward the core 102 of the polarizing fiber 44, the cone 112 has angular elements that are asymmetrically distributed about the optical axis 98. Thus, the angular portion of 112 that would have been received within the taper 100 of the polarizing fiber 44 is removed. As a result, the relative intensity of the reference beam 58 portion of the measuring beam 80 relative to the object beam 56 portion of the measuring beam 80 is reduced.

Based on the shape and diffuse characteristics of test object 18, the object beam 56 portion of measuring beam 80 may be subject to similar rejection, but an adjustable beam manipulator (such as linear adjuster 82 or tilt adjuster 96) may separately adjust the intensity of the reference beam 58 portion of measuring beam 80 to more closely match the nominal intensity of the object beam 56 portion of measuring beam 80.

While some asymmetry or other diverting element of the reference beam 58 will be physically excluded by the finite acceptance cone 100 of the polarizing fiber 44, reference exclusion may also occur through other limiting apertures of the optical components before the acceptance cone 100. For example, the elements of the reference beam 58 may be tilted outside the collection range of the objective lens 72, or vignetted by the collimator/condenser lens 50 in its collimated form. In either case, the exclusion is associated with elements of the reference beam 58 that reach outside of the cone of acceptance 100 of the polarizing fiber 44.

The propagating elements of the reference beam 58 may also be eliminated, otherwise the reference beam 58 would reach into the cone of acceptance 100 of the polarizing fiber 44, as shown and described in the embodiment of fig. 8, for example. Fig. 8 shows an enlarged reference arm 116 similar to the folded reference arm shown in fig. 6, wherein corresponding optical components share the same reference numerals. However, instead of adjusting the reference reflector 76 linearly or angularly for directing a portion of the volume of the reference beam 58 portion of the measuring beam 80 outside the volume of the cone of acceptance 100 of the polarizing fiber 44, the reference arm 116 includes an adjustable aperture stop (stop) 118, such as an adjustable aperture, for blocking a portion of the reference beam 58 that would otherwise reach the cone of acceptance 100 of the polarizing fiber 44. Here, a portion of reference beam 58 is blocked or even does not reach reference reflector 76, and diffraction amplifies the spot size at entrance 46 of polarizing fiber 44. The surviving (surviving) portion 120 of the reference beam 58 is shown in thin dashed lines, as compared to the dashed line depiction of the original reference beam 58. As described, for example, with respect to the tilt adjuster 96, the adjustable aperture stop 118 may be manually or automatically adjusted to intercept different sized portions of the reference beam 58 over continuous or discontinuous areas. Since the same useful information (i.e., the phase of each wavelength) is contained in the wavefront extending through the reference beam 58, any portion of the reference beam 58 can be blocked to balance its intensity with the intensity of the object beam 56 portion of the measuring beam 80 and thereby enhance the interference contrast.

As shown in FIG. 9, instead of filling the acceptance cone 100 of the polarizing fiber 44 with the size originally launched from the polarizing fiber 44, the portion of the survivor reference beam 120 of the measurement beam 80 as cropped by the adjustable aperture stop 118 converges in the form of a cone 122 that underfills the acceptance cone 100 of the polarizing fiber 44 and diffracts to a larger spot at 46. Thus, a portion of the original reference beam 58 portion of the measuring beam 80 that would otherwise be within the acceptance cone 100 of the polarizing fiber 44 is lost. The aperture size controlled by the adjustable aperture stop 118 can be adjusted to relatively adjust the intensity of the reference beam 58 portion of the measuring beam 80 entering the polarizing fiber 44.

Instead of blocking and diffracting light by radially reducing the aperture size, any one or more portions of the transverse area of the reference beam 58 may be blocked. For example, the adjustable aperture stop 118 may be configured in the form of a louver, wherein one or more vanes are angularly displaced so as to block more or less light. Furthermore, by blocking and diffracting various combinations of portions of the reference beam 58 or by directing portions of the reference beam 58 out of the cone of acceptance 100, portions of the reference beam 58 over a series of differently sized portions can be excluded from being focused within the cone of acceptance 100 of the polarizing fiber 44 that would otherwise reach the cone of acceptance 100. Assuming that the intensity of the reference beam 58 portion of the measuring beam initially matches the given intensity of the object beam 56 portion of the reference beam 80, the relative intensity of the reflected object beam 56 can be monitored during the course of the measurement to determine whether more or less defocus, tilt, or increased or decreased aperture size is required to rebalance the intensity of the object beam of the measuring beam 80 and the reference beam 56 and 58 portions.

FIG. 10 shows an enlarged reference arm 124 similar to folded reference arm 116 shown in FIG. 8, wherein corresponding optical components share the same reference numerals, for use in interferometer probe 20. In contrast to the reference arm 116 of fig. 8, the reference arm 124 does not comprise a reference objective for focusing the reference beam 58. Instead, a reference reflector 126, shown in the form of a retro-reflector (retro-reflector), such as a corner cube (comer cube), retro-reflects the collimated reference beam 58. However, similar to the embodiment of FIG. 8, adjustable aperture stop 118 provides for similarly blocking and diffracting a portion of reference beam 58 that would otherwise reach cone of acceptance 100 of polarizing fiber 44.

The embodiment of fig. 10 eliminates the objective lens and does not require readjustment of the relative optical path lengths of the object and reference arms 60, 124 or realignment of the shifted object focus 64. Although shown between beam splitter 52 and reflector 92, adjustable aperture stop 118 may be positioned anywhere along reference arm 124, including at or near reference reflector 126, and may be arranged to block and diffract any one or more portions of reference beam 58.

The light source 32 for powering the optical profiler (profiler) may be a superluminescent diode for generating light in a continuous wavelength, typically in the infrared spectrum. Preferably, the gain ripple (gain ripple) over the operating spectrum is low and the bandwidth is matched together with the operating bandwidth of the detector 36. A disadvantage of using light within the infrared spectrum is that the light is not visible and therefore does not produce a visible focal spot on the test object 18, which may be useful during setup and use to allow the operator to see where the measurement is being made and whether the measurement point is in focus.

Fig. 11 shows a similar optical measurement system in which a visible light source 130, such as a conventional laser diode, is optically coupled to the non-visible light source of the measurement system, which is still indicated by reference numeral 32 for better comparison with other embodiments. Light output from the visible light source 130, as conveyed by single mode fiber 132, is combined at fiber coupler 136 with light output from the invisible light source 32, as conveyed by single mode fiber 134, and further propagates together along single mode fiber 138. The fiber coupler 136 may be arranged to compensate for power differences between the two sources, in particular for keeping more invisible light intended for measurement and for transmitting only the amount of visible light needed to produce the desired visible focal spot. For example, the fiber coupler 136 may be arranged as a wavelength division multiplexing coupler. Thereafter, the combined visible and non-visible light is transmitted along single mode fiber 138 through fiber coupler 142 to polarizing fiber 144, polarizing fiber 144 corresponding to polarizing fiber 44 for transmitting light to and from interferometer probe 20. Fiber coupler 142 also couples polarizing fiber 144 to single mode fiber 140 for transmitting light from interferometer probe 20 to detector 36. Single mode fibers 132, 134, 138 and 140 may also be polarizing fibers.

Within interferometer probe 20, the visible light follows the path of transmission, resulting in a visible focal spot 146 on test object 18. That is, visible light from the visible light source 130 is transmitted along the polarizing fiber 144 through the collimating/condensing lens 50 to the beam splitter 52, and is directed from the beam splitter 52 along the object arm 60 through the objective lens 62 to form the visible focal spot 146.

Instead of generating an instantaneous bandwidth, the light source 32 may establish a similar bandwidth by generating a series of different wavelengths over the desired bandwidth. The detector 36 may be simplified, such as in the form of a simple photodetector, with a single interference phase produced by each wavelength for a given measurement point on the test object 18.

Those skilled in the art will appreciate that the reference acceptance cone and the converging beam cone are idealized forms, and that the actual size of the end of the polarizing fiber and the wave properties of the light itself render the cones (renderers) an approximation of the overall light interaction in question. Furthermore, those of skill in the art will appreciate that substitutions, changes, modifications, additions and different combinations of the elements disclosed in the example embodiments may be made in accordance with the general teachings of the invention, and are intended to be encompassed by the following claims.

It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

The present disclosure contemplates that many changes and modifications may be made. Thus, while improved forms have been shown and described, and many alternatives discussed, it will be readily appreciated by those skilled in the art that various additional changes and modifications may be made without departing from the scope of the invention, as defined and differentiated by the following claims.