Method for determining the location of a target by interferometry and device for doing so

Abstract

A method for determining the position of a target reflector in a measurement range by differential interferometry comprises the following steps: First, second, and third coherent light beams are generated at a first instant at a first wavelength and at a second instant at a second wavelength different from the first wavelength; a first reference signal is generated by directing the first coherent light beam along a first optical path having a first optical distance; a measurement signal is generated by directing the second coherent light beam along a second optical path having a second optical distance and including the target reflector; and a second reference signal is generated by directing the third coherent light beam along a third optical path having a third optical distance, the third optical distance being different from the first and second optical distances. A common delay is provided between generating the second reference signal and the first reference signal, and between generating the second reference signal and the measurement signal. A first interference signal is generated from a first portion of the first reference signal and a first portion of the second reference signal at a first time instant and a second time instant. A second interference signal is generated from a second portion of the measurement signal and a second reference signal at a first time instant and a second time instant. A first amplitude and a second amplitude of the first interference signal and the second interference signal are determined, respectively. A first signal is generated based on the first interference signal and the first amplitude. A second signal is generated based on the second interference signal and the second amplitude. The position of the target can be determined by calculating a difference between the first optical distance and the second optical distance based on the first signal and the second signal at both the first time instant and the second time instant.

Description

[Technical Field]

[0001] The present invention relates to a method for stable and high precision interferometry for determining the position of a target by interferometry, and a device for doing so. [Background technology]

[0002] Interferometers may be used to measure the difference in optical distance to a movable target relative to the optical distance to a reference by generating an interference signal from the interaction of light passing through a reference optical path with light passing through a target optical path. A common problem with interferometry and interferometers is the change in optical distance caused by environmental factors such as temperature changes, as this can affect the path length of the light beam and the refractive index of the medium through which the light beam passes, both of which affect and determine the optical distance. Another common problem is providing adequate spatial resolution, preferably uniform spatial resolution, to determine the phase and direction of movement.

[0003] U.S. Patent No. 8,570,529 describes a position detection device comprising an interferometer for generating interference fringes dependent on the length of a measurement section and a detector for interpreting the detected interference fringes as a basis for generating a measurement signal. The position detection device further comprises a source for generating a wave field in the measurement section, a wave field variation device for varying the wavelength of the wave field over time, and an evaluation circuit for evaluating the measurement signal based on the temporal variation. A disadvantage of this approach is that it may require a relatively wide dynamic range of the modulation input, which introduces noise into the measurement results. A further disadvantage of this approach is that it is complex to implement in systems where position is detected along multiple axes, especially when the same modulation depth is desired along multiple axes. Furthermore, it does not enable accurate position determination in situations where the reference distance and the target distance may be equal, which typically occurs in free-space approaches.

[0004] CN112432602, US Patent Application Publication No. 2021 / 0199418, and CN112857206 each describe an interferometer with a complex and expensive optical device for modulating the phase of a reference signal to measure target distance with a stable modulation depth. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 8,570,529 [Patent Document 2] CN112432602 [Patent Document 3] U.S. Patent Application Publication No. 2021 / 0199418 [Patent Document 4] CN112857206 Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to overcome at least one, and preferably all, of the disadvantages associated with the prior art. [Means for solving the problem]

[0007] According to a first aspect of the present invention, this object is achieved by providing a method for determining the position of a target reflector in a measurement range (e.g., along an axis) by differential interferometry, as set forth in the appended claims. The method comprises generating first, second, and third coherent light beams at a first time point (e.g., a first instant) at a first single wavelength and at a second time point (e.g., a second instant) at a second single wavelength different from the first wavelength. The first reference signal may be generated by directing the first coherent light beam along a first optical path having a first optical distance. The measurement signal may be generated by directing the second coherent light beam along a second optical path having a second optical distance, the second optical path comprising a (movable) target. The second reference signal may be generated by directing the third coherent light beam along a third optical path having a third optical distance, the third optical distance being different from the first and second optical distances. A common delay may be provided between generating the second reference signal and generating the first reference signal, and between generating the second reference signal and generating the measurement signal. The first interference signal may be generated from a first portion of the first reference signal and the second reference signal at the first time point and the second time point. The second interference signal may be generated from a second portion of the measurement signal and the second reference signal at the first time point and the second time point. A first amplitude of the first interference signal may be determined. A second amplitude of the second interference signal may be determined. The first signal may be generated based on the first interference signal and the first amplitude. The second signal may be generated based on the second interference signal and the second amplitude. The target position may be determined by calculating a difference between a first optical distance and a second optical distance based on the first signal and the second signal at both the first time point and the second time point.

[0008] This method achieves the objectives of the present invention by generating two interference fringes: a first fringe between a first reference signal and a second reference signal; and a second fringe between a measurement signal and the second reference signal. The second reference signal is generated by directing a third coherent light beam along a third optical path having a third optical distance, the third optical distance being different from the first and second optical distances. This reduces the relative fluctuation of the (phase) modulation depth, even with constant frequency modulation of the light generating means, and thus eliminates the need to adjust the modulation depth according to the target position. This allows for and / or simplifies measurement noise reduction. Furthermore, minimizing the fluctuation of the modulation depth is advantageous, especially when measurements are performed along multiple axes, because it improves the contrast of the interference between different signals.

[0009] By modulating the wavelength between two states, for example, between a first (single) wavelength generated at a first time point and a second (single) wavelength generated at a second time point, the sign information of the phase difference can be resolved. This can also improve the resolution of the path length difference by equalizing the resolution over the entire range of path length differences across the measurement range of the target along the axis. Preferably, the wavelength is modulated between the two states according to a sinusoidal wave. This can simplify signal generation and / or processing. However, any other transition of the laser wavelength is also possible. The modulation of the wavelength between two states (e.g., the first state and the second state) can be achieved by frequency modulation of a light generating means comprising a light source, for example, a (single-frequency) laser. Such frequency modulation to generate a first single wavelength at a first time point and a second single wavelength at a second time point can be performed at a modulation frequency, e.g., the frequency of modulation between the first state corresponding to the first single wavelength and the second state corresponding to the second single wavelength. A modulator can be provided to modulate the light source. Such a modulator may be configured to adapt a parameter of the light source, such as, for example, the (drive) current, the (drive) voltage, or the temperature.

[0010] Preferably, a low-cost light source is used. A suitable light source is configured to emit light having a single wavelength at a time and capable of changing the single wavelength between two or more states. Preferably, the light source generates light having a single wavelength at a time and is capable of changing the single wavelength between two or more states, each state corresponding to a single wavelength having a different wavelength. The light source can be configured to change between two or more states by adapting a parameter of the light source, such as current, voltage, or temperature, using, for example, a modulator. Advantageously, the light source is configured to have a linear correlation between the adaptation of the parameter and the change in the single wavelength. The light source may comprise, for example, a tunable homodyne laser source, such as a semiconductor laser or a diode laser, which typically results in a low-cost light source. Examples of advantageous light sources are distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers. In certain embodiments where the target distance is limited, for example, a vertical-cavity surface-emitting laser (VCSEL) can be used as an advantageous light source.

[0011] The first interference signal may be generated by interference between the first reference signal and a first portion of the second reference signal. The second interference signal may be generated by interference between the measurement signal and a second portion of the second reference signal. Both reference signals are generated at a first time point and a second time point and detected using detection means. Such detection means may, for example, comprise a first detector for detecting the first interference signal and a second detector for detecting the second interference signal. The first detector and the second detector may, for example, each comprise a photodiode for detecting the corresponding interference signal.

[0012] To determine the target's location, the first and second sets of orthogonal signals can be determined by demodulating the first and second interfering signals, respectively, according to a demodulation scheme. As will be apparent to those skilled in the art, several modulation schemes are suitable. For example, the first and second interfering signals can be demodulated at a first frequency corresponding to the modulation frequency and a second frequency corresponding to twice the modulation frequency. Another example is to filter the first and second interfering signals using a low-pass filter and then demodulate the first and second interfering signals at the modulation frequency.

[0013] Preferably, the measurement range comprises all possible positions of the target reflector over the entire range of motion of the target reflector along the axis, the position of which can be determined within the measurement range, for example to monitor the position of a platform attached to the target reflector.

[0014] The target reflector may comprise any type of suitable reflector, such as a cube corner, a retroreflector, or a plane mirror. Preferably, the target reflector comprises a plane mirror to increase tolerance for movement of the target reflector in a direction orthogonal to the incident light. For measuring the position of the stage, this advantage is crucial, as movement in a direction orthogonal to the measurement direction does not affect the interference signal. Therefore, the determination of the position of the target reflector along the measurement range is not affected. The main advantage is that this provides independent position measurements along orthogonal axes, allowing for monitoring of the target or stage position along mutually orthogonal measurement ranges.

[0015] The interference reflector may comprise any type of suitable reflector, such as a retroreflector (e.g., cube corner) or a plane mirror. Preferably, the target reflector comprises a plane mirror, as a plane mirror can be moved relative to the interferometer in a direction orthogonal to the measurement direction without affecting the measurement, creating the possibility of measuring independent displacements along orthogonally disposed axes.

[0016] Preferably, a common delay is provided between generating the second reference signal and generating the first reference signal, and between generating the second reference signal and generating the measurement signal. This ensures that the first interference signal and the second interference signal are each generated from unbalanced signals (i.e., unbalanced measurement signal and second reference signal, and unbalanced first reference signal and second reference signal), and the first ratio between the first optical distance and the third optical distance and the second ratio between the second optical distance and the third optical distance are greater than or less than 1, respectively. Such a common delay reduces the relative fluctuation of the (phase) modulation depth even with constant frequency modulation of the source, thus eliminating the need to adjust the modulation depth according to the target position. This allows for and / or simplifies the measurement results, since reducing the relative fluctuation of the measured distance can reduce the required fluctuation in the modulation input. Furthermore, minimizing modulation depth variation is advantageous, particularly in situations where measurements are made along multiple (orthogonal) axes, as this may improve the interference contrast between the respective signals and therefore the signal strength of the demodulated signal relative to the target measurement range (e.g., target distance range) for such axes.

[0017] Advantageously, the common delay can be configured to correspond to an optical distance difference such that the amplitude of each of the demodulated first and second interference signals does not exhibit a zero crossing due to changes in the phase modulation depth of either the first or second interference signal across the interferometer target's measurement range relative to the (constant) frequency modulation amplitude of the light source. This can be achieved when the delay corresponds to an optical distance longer than the target's measurement range, e.g., at least twice the target's measurement range. For example, the common delay is selected so that a third ratio between the target's measurement range and the absolute value of the difference between the second and third optical distances is greater than 2, preferably greater than 2.1, and preferably greater than 2.2. This can typically be achieved when both the first and second ratios are either sufficiently greater than 10 or sufficiently less than 0.1. When the common delay forms part of both the first and second optical paths, the optical distance of the common delay can be substantially orders of magnitude longer than the third optical distance.

[0018] Preferably, the optical path length of the common delay is selected so that the amplitude of each orthogonal signal of the first set and the second set of orthogonal signals does not exhibit zero crossings over the measurement range of the target reflector. Preferably, the optical path length of the common delay is selected so that the amplitude of each orthogonal signal of the first set and the second set of orthogonal signals is sufficiently large over the measurement range of the target so that a signal-to-noise ratio that does not limit the performance of the sensor can be achieved. For example, the optical length of the delay is selected so that the amplitude of each orthogonal signal of the first set and the second set of orthogonal signals does not vary by more than a factor of four. Preferably, the optical length of the delay is selected so that the amplitude of each orthogonal signal of the first set and the second set of orthogonal signals does not vary by more than a factor of two.

[0019] Preferably, the common delay is provided by a common delay path. The advantage of providing a common delay path is that environmental influences, such as thermal expansion, equally affect the first delay between generating the second reference signal and generating the first reference signal, and the second delay between generating the second reference signal and generating the measurement signal. Therefore, the determination of the target position is essentially unaffected by these environmental influences. Advantageously, one of the third optical path and both the first and second optical paths (i.e., the third optical path or both the first and second optical paths) includes a delay path that provides a common delay. Such a delay path may include a light guide to provide a simple and compact means for introducing a delay into the signal. Optionally, the third optical path includes such a delay path. Preferably, both the first and second optical paths (i.e., the third optical path, or both the first and second optical paths) include a delay path, since both of these paths are usually longer than the third optical path to begin with. Therefore, the length of such a delay path may typically be smaller in alternative embodiments in which the third optical path comprises a delay path.

[0020] Along the coincident or overlapping portions of the first and second optical paths, the first coherent light beam may comprise light of a first polarization direction, and the second coherent light beam may comprise light of a second polarization direction different from the first polarization direction. Typically, the first polarization direction is orthogonal to the second polarization direction. Along each optical path, the polarization direction of the corresponding light beam may be changed.

[0021] Preferably, the coherent light beams corresponding to the first and second optical paths are collimated along at least a portion of the corresponding optical path. This is particularly beneficial for portions of the respective optical paths related to free-space interferometry. Preferably, the first and second coherent light beams are generated from a single coherent light source, since changes in the light source (e.g., modulation of frequency between states) affect both coherent light beams simultaneously, without the need for any additional means, for example, for synchronization. Generating the first and second coherent light beams may comprise splitting light generated by the light source. For the same reason, it is preferred that an additional third coherent light beam is generated from a single coherent light source. Generating the third coherent light beam may comprise splitting light generated by the light source.

[0022] Preferably, splitting the light to generate the first and second coherent light beams comprises splitting light generated by the light source according to a first and second polarization state, respectively. The first and second portions of the second reference signal may be generated by splitting the third coherent light beam according to a third and fourth polarization state, respectively. Preferably, the first and second polarization states correspond to the third and fourth polarization states, respectively.

[0023] Generating the first reference signal and the measurement signal may comprise directing the first and second coherent light beams along (parts of) coincident optical paths. Preferably, an optical assembly similar to that used in a Michelson interferometer is used to generate the first reference signal and the measurement signal. Advantageously, the common delay is provided external to the optical assembly to reduce the size and number of parts of the interferometer.

[0024] In a preferred embodiment, both the first and second optical paths pass through a polarizing beam splitter, the polarizing beam splitter being configured to split the first and second optical paths, and a beam splitter may be used to rejoin the first and second optical paths following the non-coincident portions of the first and second optical paths.

[0025] In a preferred embodiment, the method further comprises a calibration step for more accurately determining the target position. The calibration step, for example, comprises determining a first amplitude of the first interference signal and a second amplitude of the second interference signal. A first signal (e.g., an electrical signal) can be generated based on the first interference signal and the first amplitude. A second signal (e.g., an electrical signal) can be generated based on the second interference signal and the second amplitude. For example, the determined first and second amplitudes can be used to adapt the sensitivities of the detection means used to generate the first and second signals for the first and second interference signals, respectively. For example, the first amplitude can be used to adapt the sensitivity of a first detector of the detection means, and the second amplitude can be used to adapt the sensitivity of a second detector of the detection means. Preferably, each sensitivity is adapted so that the measurement range of the detection means is optimally (e.g., fully) used. For example, the detection means comprises at least one analog-to-digital converter (ADC), and the calibration step is used to adapt the sensitivity of the ADC so that the range of the corresponding interference signal corresponds to the measurement range of the ADC.

[0026] The first and second amplitudes can be determined by peak detection or by ordinary sampling of the interference signals. Preferably, the first and second amplitudes are determined based on a set of zero crossings for the first and second interference signals, respectively, where the set of zero crossings preferably comprises at least three consecutive zero crossings. An advantage of zero crossing detection is that the accuracy of amplitude determination is often higher than sampling data for peak detection. For example, the first and second amplitudes can be determined based on a minimum value of each interference fringe (e.g., reaching a midpoint between the first and second zero crossings of the set of zero crossings) and a maximum value of each interference fringe (e.g., reaching a midpoint between the second and third zero crossings of the set of zero crossings). For faster-moving targets, it can be beneficial to determine the first and second amplitudes based on fitting a (periodic) function, e.g., a trigonometric function such as a cosine function, to the respective interference signals. For example, the first amplitude and the second amplitude may be determined based on fitting a first function and a second function to the first interfering signal and the second interfering signal, respectively.

[0027] However, in some situations (e.g., when the target reflector is not moving), such zero crossings may not occur automatically. For example, zero crossings may not occur when the optical distance of the second optical path is substantially constant (e.g., varies by less than one-sixteenth or one-eighth of a wavelength), or when the target reflector is stationary along the measurement range. Without zero crossings, it is impossible to distinguish between disturbances affecting the interference fringes and changes in the optical distance of the second optical path (e.g., caused by movement of the target reflector along the measurement range). Such disturbances may comprise factors that affect to some extent the guidance of one of the light beams in the respective optical paths, such as those caused by particulate matter, condensation or contamination of optical components, or changes in the alignment of optical components. Additionally or alternatively, such disturbances may comprise factors affecting signal detection and processing, such as those caused by thermal drift of electronic components, such as the electronic components of the detection means. In any such situation, a zero crossing in the interference signal can be forced by adapting (e.g., modulating) the optical distance of one of the optical paths corresponding to the interference signal. For example, the optical distance of the first optical path can be changed by moving the target reflector. Preferably, the optical distance is modulated to oscillate with an amplitude of at least half of one of the first wavelength, the second wavelength, and the nominal values ​​of the first and second wavelengths, preferably at least half of the amplitude of the larger of the first and second wavelengths. Advantageously, the common delay is adapted (e.g., modulated) so that the difference in optical distance varies by at least an optical distance corresponding to one of the first wavelength, the second wavelength, and the nominal values ​​of the first and second wavelengths, preferably the larger of the first and second wavelengths. In a preferred embodiment, the difference in optical distance introduced by the common delay is adapted. The advantage of adapting the optical distance of the delay is that both interference fringes are affected to the same extent. Therefore, the adaptation of the optical distance is automatically cancelled when determining the position of the target reflector.Preferably, the optical distance difference caused by the delay is modulated with an amplitude of at least half of the first wavelength, the second wavelength, and one of the nominal values ​​of the first wavelength and the second wavelength, preferably at least half of the greater of the first wavelength and the second wavelength.

[0028] The common delay may be adapted by adapting the optical distance of one of the third optical path and the matching optical path, regardless of which of them comprises the common delay path. Preferably, the common delay is adapted by adapting the optical distance of the common delay path, since appropriate changes in optical distance are more easily achieved in the path having the longest optical distance.

[0029] The optical distance of (a portion of) a path (e.g., a delay path) can be adapted or modulated by varying at least one of the refractive index in (a portion of) the path and the length of (a portion of) the path. In certain useful embodiments, the optical distance of the delay path is configured to oscillate with an amplitude of at least half of one of the first wavelength, the second wavelength, and the nominal value of the first wavelength and the second wavelength, preferably at least half of the amplitude of the larger of the first wavelength and the second wavelength. For example, the optical distance can be changed by exposing one of the paths (e.g., delay path) to a magnetic field (e.g., generated by an electromagnetic coil), for example when such path comprises a Pockels cell, exposing one of the paths to heat (e.g., generated by a heating element), or exposing one of the paths to a shear force (e.g., generated by an actuator). In certain preferred embodiments, heat is applied to the delay. For example, at least a portion of the delay path (e.g., a light guide) is exposed to a heating element configured to (alternatively) heat the light guide. In some beneficial embodiments, at least a portion of the light guide is wrapped around the heating element.

[0030] The optical path may be oscillated at an oscillation frequency that is preferably at least 1 Hz, more preferably at least 10 Hz. The temperature set point for heating the lightguide may be selected so that the cooling rate corresponds to the selected oscillation frequency. For example, the oscillation frequency may be increased by increasing the difference between the nominal temperature of the lightguide and the temperature of the lightguide's environment. The lightguide's environment may also be actively lowered to raise the upper limit of the oscillation frequency by lowering the lightguide's environmental temperature.

[0031] According to a second aspect of the invention, the object is achieved by a device according to the appended claims, which device achieves the object of the invention in a similar manner to the method according to the invention, and the relevant features apply equally mutatis mutandis to the device, or conversely to the method according to the first aspect.

[0032] A device for determining the position of a target in a measurement range by interferometry comprises light generating means, optical means, detection means and processing means.

[0033] The light generating means comprises a light source that can be configured to be modulated between a first state and a second state. In the first state, the light generating means can be configured to generate light at a first single wavelength. In the second state, the light generating means can be configured to generate light at a second single wavelength different from the first wavelength.

[0034] The optical means may be provided downstream of the light generating means. The optical means may be configured to generate a first coherent reference signal, a coherent measurement signal, and a second coherent reference signal. The optical means preferably includes a first optical path, a second optical path, and a third optical path. The first optical path may have a first optical distance and be configured to generate the first coherent reference signal by directing light generated by the light source (217). The second optical path may have a second optical distance, include a movable target, and be configured to generate the coherent measurement signal by directing light generated by the light source. The third optical path has a third optical distance different from the first and second optical distances. The third optical path may be configured to generate the second coherent measurement signal by directing light generated by the light source. The optical means may further comprise a delay path that introduces a common delay between generating the second coherent reference signal and generating the first coherent reference signal, and between generating the second coherent reference signal and generating the coherent measurement signal.

[0035] The detecting means may be provided downstream of the optical means. The detecting means may be configured to generate a first interference signal between the first coherent reference signal and a first portion of the second coherent reference signal. The detecting means may be configured to generate a second interference signal between the coherent measurement signal and a second portion of the second coherent reference signal. The detecting means may comprise a first detector configured to detect the first interference signal and determine a first amplitude of the first interference. The detecting means may be configured to generate a first signal based on the first interference signal and the first amplitude. The detecting means may comprise a second detector configured to detect the second interference signal and a second amplitude of the second interference. The detecting means may be configured to generate a second signal based on the second interference signal and the second amplitude. The first and second detectors may each comprise, for example, a photodiode for detecting the corresponding interference signal.

[0036] The processing means may be configured to determine the position of the target by calculating a difference between the first optical distance and the second optical distance based on the first signal and the second signal in both the first state and the second state. The processing means may, for example, comprise a processing unit for determining the position of the target.

[0037] In certain preferred embodiments, the device further comprises a light guide to reduce the size and need for folded optics, e.g., the third light path comprises a light guide, such that at least a portion of the third light path is provided by the light guide.

[0038] Advantageously, the first and second optical paths comprise a coincident portion where a portion of the first and second optical paths coincide, and in the coincident portion the first coherent light beam has a first polarization direction and the second coherent light beam has a second polarization direction different from (e.g., orthogonal to) the first polarization direction. An advantage of such coincident paths is that environmental influences (e.g., temperature) affect the first and second optical paths equally along the coincident path, while still allowing the first and second coherent light beams to be separated based on polarization state or direction.

[0039] The device preferably comprises a light source for generating a source light beam, and a first optical element configured to generate a first coherent light beam and a second coherent light beam. The light source may be configured to collimate the source light beam.

[0040] The first optical element may include a polarizing beam splitter configured to interact with the source light beam and generate the first light beam and the second light beam.

[0041] The first optical element comprises an optical assembly configured to (redundantly) direct light at least once (as part of the first optical path) to and from a reference reflector (e.g., a [flat] mirror, a retroreflector) to generate a first reference signal, and to (redundantly) direct light at least once (as part of the second optical path) to and from a target reflector (e.g., a [flat] mirror, a retroreflector) to generate a measurement signal. This can be achieved, for example, using one of a single-pass optical assembly (e.g., similar to part of the optical assembly of a single-pass interferometer) and a dual-pass optical assembly (e.g., similar to part of the optical assembly of a dual-pass interferometer), for example, including a retroreflector. Preferably, the first optical element comprises a dual-pass optical assembly, because the reflective target of the interferometer need only be used to fold the light beam toward the retroreflector. As a result, the target mirror can be a flat mirror, which has less stringent tolerances to rotation. For measurements of stage position, this advantage is crucial, since the plane mirror can be moved in a direction perpendicular to the measurement direction without affecting the measurement, creating the possibility of measuring orthogonal displacements that are independent of each other. The less stringent tolerance to mirror rotation translates into the acceptance of rotation of the stage being measured.

[0042] Typically, a dual-path interferometer comprises a dual-path optical assembly including at least two reflectors (e.g., mirrors) configured orthogonal to one another, in that the planes formed by each of the at least two reflectors are configured orthogonal to one another and face adjacent sides of a polarizing beam splitter, and each of the two reflectors is configured to reflect a different one of the first and second light beams toward the polarizing beam splitter. Furthermore, the optical path between the polarizing beam splitter and the corresponding reflector may include a means for switching the polarization direction between a first and a second orientation of polarization. Such a means may, for example, include a quarter-wave plate configured to interact with the corresponding first and second light beams before and after reflection by the corresponding one of the two reflectors.

[0043] Advantageously, the device according to the present invention further comprises a beam splitter configured to interact with the source beam to generate a third beam, which allows a single light source to be used to determine the position of the target. Examples of suitable beam splitters include fiber-based beam splitters and free-space beam splitters.

[0044] In one embodiment according to the invention, the means advantageously comprises a second optical element downstream of the first optical element configured to generate interference between the first reference signal and the second reference signal and a second interference pattern, the second optical element comprising a first output connected to the first detector and a second output connected to the second detector.

[0045] The second optical element preferably comprises a further polarizing beam splitter configured to split the first optical path and the second optical path, a first polarizer downstream of the further polarizing beam splitter configured to generate a first interference signal, and a second polarizer downstream of the further polarizing beam splitter configured to generate a second interference signal.

[0046] The device according to the invention is particularly useful in situations where multiple measurements need to be compared, such as when the position is determined along multiple axes. Such a system may, for example, comprise multiple devices as described herein. Preferably, each of the multiple devices is configured to measure the position along multiple axes. Advantageously, such a system comprises a common light source for generating a source light beam, the common light source being configured upstream of the multiple devices. The advantage of such a system, for example comprising a single laser source (e.g., a modulated line-locked laser source) as the common light source, is that it is suitable for performing multiple independent position measurements, each with approximately the same (optimized) modulation depth.

[0047] As will be apparent to those skilled in the art, the various sections along the different optical paths can be configured with free-space or fiber-based solutions. Preferably, the fiber used in a fiber-based solution is configured to maintain the polarization state, and such polarization-maintaining fiber is used to make the measurements less sensitive to fiber deformations.

[0048] The device may include a light generating means comprising a light source configured to switch between a first state and a second state, e.g., a laser configured to be modulated between the first state and the second state. In the first state (e.g., at a first time point), the light generating means may be configured to generate light at a first single wavelength. In the second state (e.g., at a second time point), the light generating means may be configured to generate light at a second single wavelength different from the first wavelength. Preferably, the light source generates a coherent (collimated) light beam, which can be used downstream to generate first, second, and third (collimated) coherent light beams, e.g., by splitting the coherent light beam using multiple beam splitters. The light source may, for example, comprise a tunable homodyne laser, which is typically a low-cost light source. Switching the wavelength between the two states may be achieved by frequency modulation of the light generating means, e.g., the light source. For example, the light-generating means may comprise a modulator configured to modulate the frequency of the light-generating means such that in a first state light is generated at a first single wavelength and in a second state light is generated at a second single wavelength. Such a modulator may be configured to adapt a parameter of the light source, such as current, voltage, or temperature, for example.

[0049] The device may include an optical means (e.g., a first optical element) downstream of the light generating means, further configured to generate a first (coherent) reference signal, a (coherent) measurement signal, and a second (coherent) reference signal. The optical means may include a first optical path, a second optical path, and a third optical path. The first optical path may be configured to generate the first reference signal by directing light (e.g., a first reference beam) generated by the light generating means. The second optical path may include a movable target and may be configured to generate the measurement signal by directing light (e.g., a measurement / target beam) generated by the light generating means. The third optical path may be configured to generate the second reference signal by directing light (e.g., a second reference beam) generated by the light generating means, the third optical distance being different from the first optical distance and the second optical distance.

[0050] Advantageously, the optical means further comprises a delay path providing a common delay between generating the second reference signal and generating the first reference signal, and between generating the second reference signal and generating the measurement signal. Such a delay path is advantageously provided upstream of the optical assembly. Preferably, the optical means comprises a light guide configured as the delay path providing the common delay. Using a light guide as the common delay has the advantage that the light guide can be used to provide large, identical path length differences between the first reference signal and the second reference signal, and between the measurement signal and the second reference beam, in a relatively small space. If such a light guide is much longer than the measurement range of the movable target, the relative variation of the path length differences for the independent measurement axes is minimized. Furthermore, since the modulation depth is proportional to the path length difference between the interfering beams, the variation of the modulation depth between independent devices can also be minimized.

[0051] For example, the third optical path comprises a delay path in a portion of the third optical path that does not coincide with the first and second optical paths. Advantageously, a common optical path shared between the first and second optical paths but that does not coincide with the third optical path comprises a light guide, whereby at least portions of the first and second optical paths comprise a delay path provided by the light guide. An advantage of the latter example is that the light guide can be shorter than in the former example, since the first and second optical distances will typically already be longer due to the optical paths between the reference and target mirrors, respectively.

[0052] The device may further comprise a detector, e.g., comprising a second optical element, disposed downstream of the optical assembly. The detector may be configured to generate a first interference signal between the first reference signal and a first portion of the second reference signal. The detector may further be configured to generate a second interference signal between the measurement signal and a second portion of the second reference signal. The detector may further be configured to detect the first interference signal and the second interference signal. To this end, the detector may comprise a first detector and a second detector, respectively.

[0053] The device may further comprise processing means configured to determine a position of the target, which may be determined by calculating a difference between the first optical distance and the second optical distance, which difference may be based on the first interference signal and the second interference signal in both the first state and the second state.

[0054] To determine the target's position, the processing means may be configured to determine the quadrature by demodulating each of the first and second interference signals according to a demodulation scheme. As will be apparent to those skilled in the art, several modulation schemes are suitable. For example, the first and second interference signals may be demodulated at a first frequency corresponding to the modulation frequency and a second frequency corresponding to twice the modulation frequency. Another example is to filter the first and second interference signals using a low-pass filter and demodulate the first and second interference signals at the modulation frequency. The modulation frequency is the frequency at which the light generating means is modulated or switched between a first state and a second state.

[0055] The optical means may include a first common optical path where the first optical path, the second optical path, and the third optical path overlap. The first common optical path may include a first end including an optical pickup optically connected to the light generating means. The first common optical path may include a second end downstream of the first end, which may be configured to split the third optical path. For example, the second end may include a first beam splitter configured to split the third optical path from the first optical path and the second optical path. Preferably, a light guide is provided downstream of the first common optical path, such that at least a portion of the third optical path downstream of the first common optical path includes or is provided by the light guide. The first common optical path has the advantage of allowing a single light source to be used to generate the first reference signal, the measurement signal, and the second reference signal. Furthermore, it eliminates differences in path length along the overlapping portions of the respective optical paths.

[0056] Preferably, the optical means comprises a second common optical path (downstream of the first common optical path), where the first and second optical paths overlap (e.g., coincide). The second common optical path and the third common optical path do not coincide. The second common optical path may comprise a first end optically connected to the first beam splitter. The second common optical path may comprise a second end downstream of the first end. The second end may be configured to separate the first and second optical paths, for example, to split the second optical path from the first optical path. Preferably, the second end comprises a second beam splitter (e.g., forming part of the first optical element) configured to split the second optical path, such as a first polarizing beam splitter (e.g., a polarizing beam splitter of the first optical element). Advantageously, the second common optical path comprises a further light guide. The second common optical path has the advantage of allowing a single light source to be used to generate the first reference signal and the measurement signal, and it also eliminates the path length difference along the overlapping portions of the respective optical paths.

[0057] The optical means preferably comprises an optical assembly similar to that used in a Michelson interferometer in that it is configured to (redundantly) direct light to and from a target reflector (e.g., a [flat] mirror, a retroreflector) and to and from a reference reflector (e.g., a [flat] mirror, a retroreflector). Such an optical assembly configured to (redundantly) direct light to and from a target reflector and to and from a reference reflector without itself causing interference may be referred to as a Michelson-like optical assembly in this disclosure. The first and second optical paths may be configured to traverse the optical assembly. The optical assembly may be configured as one of a single-pass (mirror) interferometer and a dual-pass (mirror) optical assembly. An advantage of a double-pass optical assembly is that the movable target may be a reflective target mirror used only to fold the light beam toward the retroreflector. As a result, the target mirror may be a flat mirror, which has less stringent rotation tolerances. For measurements of stage position, this advantage is crucial, since the plane mirror can be moved in a direction perpendicular to the measurement direction without affecting the measurement, creating the possibility of measuring orthogonal displacements that are independent of each other. Less stringent tolerance to mirror rotation leads to the acceptance of rotation of the stage being measured. Another advantage is that equal amounts of glass length for both beams or optical paths results in high thermal stability.

[0058] For example, the optical assembly may include a polarizing beam splitter, a retroreflector, a target mirror, a reference mirror, a first quarter-wave plate, and a second quarter-wave plate. The first side of the polarizing beam splitter may be configured to receive the first coherent reference signal and the coherent measurement signal. The second side of the polarizing beam splitter may be configured to face the reference mirror, and the first quarter-wave plate may be inserted between the polarizing beam splitter and the reference mirror. The third side of the polarizing beam splitter may be configured to face the target mirror, and the second wave plate may be inserted between the polarizing beam splitter and the target mirror. The fourth side of the polarizing beam splitter may be configured to face the retroreflector. The first side, the second side, the third side, and the fourth side may form adjacent sides of the polarizing beam splitter. For example, the first side may be located opposite one of the second side and the third side, and optionally parallel to it. The fourth side and the other of the second and third sides may both be disposed adjacent to, and optionally perpendicular to, the first side, and the other of the second and third sides may be disposed adjacent to, and optionally perpendicular to, one of the second and third sides.

[0059] The second beam splitter may be disposed within the optical assembly and form an integral part of the interferometer. Alternatively, the second beam splitter may be configured upstream of the optical assembly. In the latter embodiment, the first and second optical paths may be spatially separated upstream of the optical assembly and thus may traverse the optical assembly without overlapping.

[0060] In some embodiments, the optical means may comprise a third beam splitter configured to generate the first and second portions of the second reference signal. If the first and second optical paths are spatially separated upstream of the optical assembly, the third beam splitter may comprise a third polarizing beam splitter. Such a third beam splitter and the second beam splitter may be part of or form a single beam splitting means.

[0061] The detection means preferably comprises a fourth beam splitter downstream of the optical means or optical assembly configured to generate the first and second interference signals. The fourth beam splitter may comprise a second polarizing beam splitter.

[0062] The detection means may further comprise a first (linear) polarizer upstream of a first detector configured to generate a first interference signal. The detection means may further comprise a second (linear) polarizer upstream of a second detector configured to generate a second interference signal. Such polarizers offer the advantage that they can be rotated to control the intensity ratio between the respective signals in order to optimize for maximum interference contrast.

[0063] The device may comprise a further optical means or a further detection means. The further optical means and the further detection means may be configured to measure the position of the target along a direction different from that of the optical means and the detection means. Such a device may comprise an additional beam splitter arranged to share the light generating means between the optical means and the further optical means. Such a beam splitter may be provided between the light generating means and both the optical means and the further optical means.

[0064] Advantageously, the light generating means of the device comprises a single light source for generating light in the first state and in the second state. Preferably, the light source comprises a laser.

[0065] In certain advantageous embodiments, the device may be configured to determine first and second amplitudes of the first and second interference signals, respectively, so that detection of the first and second interference signals may be controlled. For example, the sensor head (e.g., the detection means or the processing means) may be configured to determine the first and second amplitudes and to control detection of the first and second interference signals. Such amplitudes may be determined with relatively high accuracy, for example, based on detecting zero crossings of the first and second interference signals.

[0066] The device may further comprise an adapting means configured to adapt the optical distance difference so that a zero crossing occurs in the first interference signal and the second interference signal. Preferably, the adapting means is configured to adapt or modulate the optical distance difference by at least an optical distance corresponding to one of the first wavelength, the second wavelength, and the nominal values ​​of the first and second wavelengths, preferably by at least an optical distance corresponding to the larger of the first and second wavelengths. Alternatively, the adapting means is configured to adapt and modulate the delay so that the optical distance difference oscillates with an amplitude of at least half of one of the first wavelength, the second wavelength, and the nominal values ​​of the first and second wavelengths, preferably at least half of the larger of the first and second wavelengths. Beneficially, the adapting means is configured to adapt the common delay at an oscillating frequency of preferably at least 1 Hz, more preferably at least 10 Hz.

[0067] The difference in optical distance can be adapted by changing at least one of the refractive index and the length of the path or delay path. Accordingly, the adapting means can be configured to expose such paths to at least one of heat, shear force, or magnetic field. Examples of such adapting means include a heating element, an actuator, and an electromagnetic coil, respectively. For example, the common delay path (e.g., a light guide) can be at least partially exposed to (e.g., wrapped around) a heating element.

[0068] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like reference numerals refer to like features, and in which: [Brief explanation of the drawings]

[0069] [Figure 1] 1 is a general schematic diagram of one embodiment according to the present invention; [Figure 2] 1 is a schematic diagram of a first embodiment according to the present invention; [Figure 3] FIG. 2 is a schematic diagram of a second embodiment according to the present invention. [Figure 4] FIG. 4 is a schematic diagram of a third embodiment according to the present invention. [Figure 5A] FIG. 10 is a schematic diagram of a fourth embodiment according to the present invention. [Figure 5B] FIG. 10 is a schematic diagram of a fourth embodiment according to the present invention. [Figure 6] 3 is a schematic diagram of a measured signal between a first state and a second state; DETAILED DESCRIPTION OF THE INVENTION

[0070] 1 , an embodiment of an interferometer 100 for determining the position of a target reflector along a measurement range A according to the present invention may comprise a single light source 101 (e.g., a laser) that provides light to a splitter 102 (e.g., a fiber coupler) that is configured to split light and direct a first portion comprising a first coherent light beam and a second coherent light beam along corresponding portions of a first and second optical path, e.g., via a first fiber 103, towards a sensor head 105, and direct a second portion comprising a third light beam towards a second fiber 104 configured as a delay line and forming a third optical path. The second fiber 104 is exposed in part to an adapting means 136 that is configured to vary the optical length of the second fiber 104.

[0071] The sensor head 105 splits the light from the first fiber 103 and directs a first coherent beam along a portion of the first optical path that is not coincident with the second optical path to generate a first reference signal 106, and directs a second coherent beam along a portion of the second optical path that is not coincident with the first optical path to generate a measurement signal 108. The first reference beam travels back and forth along a portion of the first optical path between the sensor head 105 and a reflective reference 107 (e.g., a mirror), and the measurement beam travels back and forth between the sensor head 105 and a movable reflective target 109 (e.g., a mirror).

[0072] The second fiber 104 directs the third light beam to a further beam splitter 114, which splits the third light beam and directs a first portion 110 of the second reference signal towards a first detector 112 and a second portion 111 of the second reference signal towards a second detector 113.

[0073] The first reference signal 106 and the measurement signal 108 are generated after the corresponding one of the first coherent light beam and the second coherent light beam travels back and forth between the sensor head 105 and the corresponding reflecting surfaces 107, 109, exits the sensor head, and is directed towards the first detector 112 and the second detector 113, respectively.

[0074] The first detector 112 is configured to generate and measure a first interference signal between the first reference signal 106 and a first portion of the second reference signal 110. The second detector 113 is configured to generate and measure a second interference signal between the measurement signal 108 and a second portion of the second reference signal 111.

[0075] Referring to FIG. 2 , an embodiment of a device 201 for determining the position of a target reflector along a measurement range B according to the present invention comprises a sensor head including a first optical assembly 202 based on a Michelson interferometer and a second optical assembly 203. In the first optical assembly, a light beam 204 is split using a first polarizing beam splitter (PBS) 205. The first PBS 205 may be provided, for example, in a cube-shaped configuration. Such a PBS as the first PBS 205 may include a beam splitter coating, for example, configured diagonally from one end of the cube-shaped PBS to another end. The light beam 204 may be directed toward a first side 208 of the first PBS 205. A first reference light beam 206 having a first polarization state exiting the first PBS 205 is directed toward a reference reflector 207 (e.g., a reference mirror such as an internal reference mirror) provided on a second side 209 of the first PBS 205. A target beam 210 having a second polarization state exiting the first PBS 205 is directed toward a reflective target 211 on a third side 212 of the first PBS 205. A double-pass configuration may be created by providing a first quarter-wave plate (QWP) 213 on the second side 209, a second QWP 214 on the third side 212, and a retroreflector (RR) 215 on a fourth side 216 of the first PBS 205. In such an embodiment, the first reference beam 206 and the target beam 210 pass through the first QWP 213 and the second QWP 214 twice, respectively, thereby switching their polarization states between a first state and a second state.

[0076] A light source 217 (e.g., a diode) configured to oscillate at a single frequency over time is provided. The light source is switchable between a first state having a first wavelength and a second state having a second wavelength. The light source 217 provides a light beam 204, e.g., a collimated light beam generated by a laser, to a first common optical path 234. The light beam 204 may travel in free space or may pass at least partially through a fiber having, e.g., an angled-polished fiber end, with a collimator provided downstream of the angled-polished fiber end. The light beam 204 comprises light having multiple polarization states, e.g., light having a first polarization state (e.g., P-polarized light) and light having a second polarization state (e.g., S-polarized light). A portion of the light beam is guided along a second common optical path 235 and directed toward an off-center position on a first side 208 of the first PBS 205. The first PBS 205 splits the light beam into a first light beam 206 (e.g., a first reference light beam) having a first polarization state (e.g., P-polarized light) and a second light beam 210 (e.g., a target light beam) having a second polarization state (e.g., S-polarized light).

[0077] The first reference beam 206 travels along the first optical path 231 and is separated from the portion of the light bundle guided along the second common optical path 235 by interaction with the first PBS 205 (e.g., a beam splitter coating disposed within the first PBS), which is configured to reflect light of a first polarization state. The first QWP 213 changes the first linear polarization state of the reference beam 206 to a circular polarization state. The reflective first reference reflector 207 then changes the propagation direction of the reference beam 206 without changing the polarization rotation, which changes the handedness of the circular polarization. Successive passages through the first QWP 213 change the polarization state from circularly polarized light to light having a second linear polarization state. The reference beam 206 in the second polarization state is transmitted through the first PBS 205 and reflected by the RR 215. The reference beam 206 reflected by the RR 215 retains its second polarization state and is transmitted back to the first PBS 205, entering the first PBS 205 at a location offset from where it exited the first PBS 205 immediately before being reflected by the RR 215. The reference beam 206 is then transmitted through the first PBS 205 toward the reflective first target 207. The first QWP 213 converts the second polarization state to a circular polarization, and reflection at the reflective reference reflector 207 changes handedness, and successive passes through the first QWP 213 convert the polarization state from this circular polarization to a first linear polarization state. This light with the first polarization state is then reflected by the first PBS 205 (e.g., a beamsplitter coating on the first PBS) and directed to the second optical assembly 203, which is located at another off-center location on the first side 208 of the first PBS 205, adjacent to the off-center location.

[0078] The target light beam 210 travels along a second optical path 232 and is separated from the portion of the light bundle guided along a second common optical path 235 by interaction with the first PBS 205 (e.g., a beam splitter coating on the first PBS) configured to transmit light of the second polarization state. The same principle is now used as for the first reference light beam 206 to create a double-pass configuration for this target light beam. The second QWP 214 changes the second linear polarization state of the target light beam to a circular polarization state. The reflective target 211 then changes the propagation direction of the target light beam without changing the polarization rotation, so as to change the handedness of the circular polarization. Successive passes through the second QWP 214 change the polarization state of the light from the circular polarization state to the first linear polarization state. The light of the first polarization state is reflected by the beam splitter coating of the first PBS 205 toward the RR 215, which then reflects the target light beam toward the first PBS 205 while retaining its first polarization state. The target light beam is then reflected by the beamsplitter coating of the first PBS 205 toward the reflective target 211. The second QWP 214 converts the first polarization state of the target light beam to circular polarization, and reflection at the reflective target 211 changes handedness, converting this circular polarization state to a second linear polarization state through successive passes through the second QWP 214. This light with the second polarization state is then transmitted by the first PBS 205 and directed to the second optical assembly 203, which is located at another off-center position on the first side 208 of the first PBS 205 adjacent to the off-center position.

[0079] An advantage of such a first optical assembly 202 providing a double-pass configuration is that the reflective target mirror 211 is used only to fold the light beam back towards the retroreflector. As a result, the target mirror can be a flat mirror with less stringent tolerances to rotation. For measurements of the stage position, this advantage is crucial, as the flat mirror can move in a direction orthogonal to the measurement direction without affecting the measurement, creating the possibility of measuring orthogonal displacements that are independent of each other. The less stringent tolerance to rotation of the mirror translates into the acceptance of rotation of the stage being measured.

[0080] Another advantage is that equal amounts of glass length for both beams or paths results in high thermal stability: both the first reference beam 206 and the target beam 210 travel equal distances through the first PBS 205 and their respective QWPs 213, 214, and 215, thereby compensating for any thermal expansion or refractive index changes in the glass of these components.

[0081] The detection means 230 comprises a second optical assembly 203 comprising a second PBS 219, which may have, for example, a cube-shaped configuration similar to the first PBS 205. The first reference beam 206 and the target beam 210 are directed collinearly towards a first side 218 of the second PBS 219 and are separated from each other by a beam splitter coating of the second PBS 219. A second collimated reference beam 220, which may have been split into a bundle of coherent laser light generated by laser 217 using a beam splitter 228 (e.g., a fiber coupler) and traveled along a third optical path 233 configured to provide a delay 229, e.g., comprising a delay line (e.g., a fiber), is directed to a second side 221 of the second PBS 219, so that it strikes the beam splitter coating of the second PBS 219 at the same location as both the first reference beam 206 and the target beam 210, but on the opposite side of the beam splitter coating of the second PBS 219. An embodiment comprising a fiber-based delay may comprise an angled-polished fiber end and a collimator configured to direct the second collimated reference beam 220 to the second side 221 of the second PBS 219. The delay 229 is exposed in part to an adaptor 236 configured to vary the optical length of the delay 229.

[0082] A first portion of the second reference beam 220 having the second polarization state is transmitted by the second PBS 219 and travels with the first reference beam 206, which has the first polarization state and is reflected by the second PBS 219 toward a first sensor 225 (e.g., a first detector) located on a third side 224 of the second PBS 219 adjacent to the first side 218. A second portion of the second reference beam 220 having the first polarization state is reflected by the second PBS 219 and travels with the target beam 210, which has the second polarization state and is transmitted by the second PBS 219 toward a second sensor 223 (e.g., a second detector) located on a fourth side 222 of the second PBS 219 opposite the first side 218.

[0083] The second PBS 219 further includes a first polarizer 227 and a second polarizer 226 disposed on a third side 224 and a fourth side 222 of the second PBS, respectively. The first polarizer 226 and the second polarizer 227 may be linear polarizers. The first reference beam 206 and a first portion of the second reference beam 220 are passed through the first polarizer 227 to generate interference between the first reference beam 206 and a first portion of the second interference beam 220, which can be detected by a first sensor 225. The second portion of the target beam 210 and the second reference beam 220 are passed through the second polarizer 226 to generate interference between the target beam 210 and a second portion of the second reference beam 220, which can be detected by a second sensor 223. Optionally, such a (linear) polarizer can be rotated to control the intensity ratio between the respective beams to optimize for maximum interference contrast. The first sensor 223 and / or the second sensor 225 may include a photodiode as a detector for detecting the interference signal. Additionally, the first sensor 223 and / or the second sensor 225 may include one or more fiber optic pickups for picking up the interference signal and relaying it to the corresponding detector. In the second optical assembly 203, the optical distances of both the first reference beam 206 and the target beam 210 coincide where their paths overlap. After the paths of the first reference beam 206 and the target beam 210 are split by the beam splitter coating of the second PBS, the optical distances of the target beam 210 and the second portion of the second reference beam 220 coincide, and the optical distances of the first reference beam 210 and the first portion of the second reference beam 220 coincide. As a result, any path length deviations caused by environmental factors, such as temperature changes, will affect the first and second optical paths equally, the first and third optical paths, and / or the second and third optical paths equally, and can therefore be compensated for directly or measured and compensated for by the processing means.

[0084] An advantage of providing a delay line to create the second reference beam 220 is that it allows for long, identical path differences between the first reference beam 206 and the second reference beam 220, and between the target beam 210 and the second reference beam 220. If such a delay line is much longer than the measurement range, the relative variation of the path length difference for the independent measurement axes is minimized.

[0085] Furthermore, because modulation depth is proportional to the path length difference between interfering beams, variation in modulation depth between independent devices can also be minimized. In this setup, one laser source (e.g., a modulated line-locked laser source) can be used to provide input for several independent position measurements, each with approximately the same optimal modulation depth. This is ideally suited, for example, to systems with devices for measuring relative position along multiple axes.

[0086] The sensor head shown in Figure 2 can be adapted to make differential measurements by removing the reflective reference 207 and folding both measurement paths, for example by introducing a mirror, and aligning them to be parallel before passing through the quarter-wave plate.

[0087] 3, an embodiment of a device 301 for determining the position of a target reflector along a measurement range C according to the present invention may also comprise a primarily fiber-based approach. For example, light from a light source 302 is split by a first splitter, such as a first fiber coupler 303, configured to split the light and direct a first portion, for example, via a first fiber 304, to a sensor head 305. The first splitter is configured to direct a second portion towards a second fiber 308, for example, further configured as a delay line, thereby providing a second reference beam. The second fiber 308 is in part exposed to an adaptation means 336 configured to vary the optical length of the second fiber 308.

[0088] The sensor head 305 is configured to split the first portion into a first reference beam 306 having a first polarization state and a target beam 307 having a second polarization state. To this end, the sensor head 305 may include a reflective polarizer 312 for splitting the first portion into the first interference beam 306 and the target beam 307. The sensor head 305 may further include a second fiber coupler 309, a fiber end 310 (e.g., an angle-polished fiber end), and a collimator 311. The first reference beam 306 is reflected by the reflective polarizer 312, which thereby acts as a reference reflector, and subsequently enters the second fiber coupler 309. The target beam 307 passes through the reflective polarizer 312, is reflected by a reflective target 313, and subsequently enters the second fiber coupler 309.

[0089] The second fiber coupler 309 passes a portion of the reflected first reference beam 306 and a portion of the reflected target beam 312 to a first fiber-based polarizing beam splitter 314, for example, via a third fiber 315. The first fiber-based polarizing beam splitter 314 splits the first reference beam and the target beam according to their polarization states and directs the first reference beam (e.g., in a first polarization state) to a first sensor 316 (e.g., a first detector) and the target beam (e.g., in a second polarization state) to a second sensor 317 (e.g., a second detector), for example, via a fourth fiber 318 and a fifth fiber 319, respectively.

[0090] The second fiber 308 is configured to direct the second reference beam toward a second fiber-based polarizing beam splitter 320. The second fiber-based polarizing beam splitter 320 may be configured to split the second reference beam into a first portion 323 comprising light having a first polarization state (e.g., a P polarization state) and a second portion 324 comprising light having a second polarization state (e.g., an S polarization state). The first portion 323 is coupled to one of the first sensor and the second sensor, and the second portion 324 is coupled to the other of the first sensor and the second sensor. For example, this is done so that the first portion 323 of the second reference beam is combined with one of the first reference beam 306 and the target beam 307 (e.g., the target beam) having the same polarization state as the first portion 323, and so that the second portion 324 of the second reference beam is combined with the other of the first reference beam 306 and the target beam 307 (e.g., the first reference beam) having the same polarization state as the second portion 324.

[0091] The first sensor 316 and the second sensor 317 include a third fiber coupler 321 and a fourth fiber coupler 322, respectively. The fourth fiber coupler 322 may be configured to combine one of the first reference beam 306 and the target beam 307 with a first portion 323. The third fiber coupler 321 may be configured to combine the other of the first reference beam 306 and the target beam 307 with a second portion 324. The third fiber coupler 321 and the fourth fiber coupler 322 may be configured to generate an interference signal between corresponding beams combined by the respective fiber couplers.

[0092] 4, an embodiment of a device 401 for determining the position of a target reflector along a measurement range D according to the present invention may also comprise a partially fiber-based and partially free-space approach. For example, light from a light source 402 is split by a first splitter, such as a first fiber coupler 403, configured to split the light and direct a first portion, for example, via a first fiber 404, towards a first fiber-based polarizing beam splitter 405. The first fiber-based polarizing beam splitter 405 is configured to split the first portion into a first reference beam having a first polarization state (e.g., a P polarization state) and a target beam having a second polarization state (e.g., an S polarization state), with the first reference beam being directed, for example, by a second fiber 406, towards a first sensor 410 (first detector), and the target beam being directed, for example, by a third fiber 407, towards a sensor head 411 configured as a free-space optical assembly.

[0093] The first splitter is further configured to direct the second portion toward a fourth fiber 408, configured, for example, as a delay line, thereby providing a second reference beam. The fourth fiber 408 is partially exposed to an adapting means 436 configured to change the optical length of the fourth fiber 408. The second reference beam may be directed by the fourth fiber 408 toward a second fiber-based polarizing beam splitter 409. The second fiber-based polarizing beam splitter 409 may be configured to split the second reference beam into a first portion comprising light having a third polarization state (e.g., a P polarization state) and a second portion comprising light having a fourth polarization state (e.g., an S polarization state). The first portion is coupled to one of the first sensor 410 and the sensor head 411, and the second portion is coupled to the other of the first sensor 410 and the sensor head 411. For example, a first portion of the second reference beam may be combined with one of the first reference beam and the target beam (e.g., the target beam) having the same polarization state as the first portion. For example, the first portion may be directed toward the first sensor 410 via the fifth fiber 412, and the second portion may be directed toward the sensor head 411 via the sixth fiber 413. For that purpose, the light passing through the second fiber 406 and the light passing through the fifth fiber 412 may be combined by the second fiber coupler 425.

[0094] The sensor head 411 includes a first input 414, a second input 415, an input / output toward a reflective target 416, and an output toward a second sensor 417 (e.g., a second detector) configured to detect interference between the first and second inputs. The first input 414 is configured to receive a target beam, and the second input 415 is configured to receive one of a first portion and a second portion of a second reference beam, preferably the one having the same polarization state as the target beam (e.g., the second portion). For example, the first input 414 and the second input 415 of the free-space optical assembly each include fiber ends 418, 419 (e.g., angle-polished fiber ends) and collimators 420, 421. The optical assembly may further include a PBS 422, a QWP 423 configured between the PBS 422 and the reflective target 416, and a polarizer 424 configured between the PBS 422 and the second sensor 417.

[0095] 5A and 5B, a preferred embodiment of a device 500 for determining the position of a target reflector along a measurement range E according to the present invention comprises a light generating means 501 configured to subsequently generate coherent light at a first wavelength and a second wavelength. The light generated by the light generating means 501 travels along three optical paths, preferably each having a distinct optical length, towards a detecting means 502 for detecting interference fringes. The first optical path may provide a first reference signal, the second optical path may provide a measurement signal, and the third optical path may provide a second reference signal. The detecting means 502 comprises two detectors 503, 503′, one of which is configured to determine a first interference signal between the first reference signal and a second reference signal, and the other of which is configured to determine a second interference signal between the measurement signal and the second reference signal. Compared to the example of Figure 2, the second PBS 219 of the detection means itself is configured to split the second reference signal into a first part and a second part, and in the example of Figure 5, the beam splitting means 504 of the optical means (forming the first PBS 505 and the third PBS 506) is configured to generate the first part and the second part of the second reference signal.

[0096] The coherent light generated by the light generating means 501 travels along a first common optical path 508, which may comprise a first optical path, a second optical path, and a third optical path, towards a first beam splitter 507 (e.g., a fiber coupler). The first beam splitter 507 may then split the optical path into a second common optical path 509, which may comprise a first optical path, a second optical path, and a third optical path 510. This embodiment may be constructed in free space, but may also be at least partially fiber-based.

[0097] In at least such fiber-based approaches, the first common optical path 508, the second common optical path 509, and / or the third common optical path 510 may move along a light guide. Such a light guide of the third optical path 509 or the second common optical path 510 may comprise a delay path or line 511 to create a common delay between the third optical path 510 and the first optical path 512 on the one hand, and between the third optical path 510 and the second optical path 513 on the other hand. The delay path 511 is in part exposed to an adaptation means 536 configured to vary the optical length of the delay path 511. The light guides of the second common light path 509 and the third light path 510 may each include a fiber end 514, 514′ (e.g., an angled polished fiber end) and a collimator 515, 515′ configured to provide two collimated light beams, one of which includes a first coherent light beam and a second coherent light beam, and the other of which includes a third coherent light beam.

[0098] The two collimated beams travel toward the beam splitting means 504. The beam splitting means 504 may be configured to split each of the collimated beams into two parallel, spatially separated beams, with the dashed and solid lines representing light traveling in different planes separated along a direction orthogonal to the image plane. FIG. 5B shows a top view of a suitable beam splitting means 504. The beam splitting means 504 (e.g., a modified cube beamsplitter or a Wollaston prism) may be configured to form a first PBS 505, with the first and second optical paths split. The first PBS 505 of the beam splitting means 504 may be configured to generate a first beam and a second beam, with light of the first and second polarization states traveling along the first and second optical paths, respectively, toward the optical assembly 516. The beam splitting means 504 may be further configured to form a third PBS 506, where the third optical path is split into two corresponding third optical paths 517, 517'. One of the two corresponding third optical paths 517, 517' may be configured to generate a first portion of a second coherent reference signal. The other of the two corresponding third optical paths 517, 517' may be configured to generate a second portion of the second coherent reference signal. The first and second portions may comprise light having a third and fourth polarization states, respectively. Preferably, the third and fourth polarization states correspond to the first and second polarization states, respectively.

[0099] The spatially separated, parallel first and second light beams are directed toward corresponding first and second off-center locations on a first side 518 of a PBS (e.g., a fourth PBS) of the optical assembly 516. A mirror 519 may be provided to direct the first and second light beams toward the optical assembly 516, which includes a movable reflective target 522. The optical assembly functions in a manner similar to the optical assembly 202 of the example shown in FIG. 2, except that the first and second light paths do not overlap in the interferometer 516 of the example shown in FIG. 5. The RR 520 in the example of FIG. 5 may be, for example, a cube-corner retroreflector or a cat-eye retroreflector, thereby changing the plane through which the light beam travels from entering the RR 520 to exiting the RR 520. After exiting the optical assembly, both the first and second light beams enter the detection means 502.

[0100] The detection means 502 comprises a third beam splitter 521. Preferably, the third beam splitter is a polarizing beam splitter (e.g., a second PBS), but may also be a non-polarizing beam splitter. The first optical path 512 and the second optical path 513 enter a first side of the third beam splitter 521 (at their respective non-overlapping positions), and two corresponding third optical paths 517, 517′ both enter a second side of the third beam splitter 521 (at their respective non-overlapping positions) adjacent to the first side. The third beam splitter 521 may be configured such that the first optical path 512 and one of the two corresponding third optical paths 517, 517′ can overlap downstream of the third beam splitter 521 and exit the third beam splitter on the third side. The third beam splitter may be further configured to allow the second optical path 513 and the other of the two corresponding third optical paths 517, 517′ to overlap downstream of the third beam splitter 521 and exit the third beam splitter on a fourth side. A linear polarizer may be provided between the third beam splitter and each of the two detectors 503, 503′, the linear polarizer configured to generate a first interference signal and a second interference signal.

[0101] Referring to FIG. 6, the following wave functions relate to two signals, for example a measurement signal and a first interference signal, or a first reference signal and a second reference signal:

[0102]

number

[0103] where λ is the wavelength of the laser and Δx is the difference in path length between the two signals. As the two signals travel along their respective paths, the total wave function is

[0104]

number

[0105] The interference between these two signals is

[0106]

number

[0107] With reference to this equation, the difference in travel distance between the two signals results in a phase difference between the two signals, which causes interference. The phase difference between the two signals is given by the difference in travel distance Δx. However, the resulting interference does not convey information about the sign of the phase difference. By varying the laser wavelength between two states, each with a different wavelength, the phase also passes between these two states. This can be used to resolve the sign information of the phase difference and / or to improve the resolution of Δx by equalizing the resolution over the entire range of Δx. The phase that can be measured is shown in Figure 6A. Each of the different arrows 601a-601h indicates a different phase. A change in wavelength results in a change in phase, which is shown in Figure 6B. The length of the arrow is related to the modulation depth of the vibration. This modulation depth is

[0108]

number

[0109] where δΦ is the modulation depth in radians, δλ is the change in wavelength of the source laser between the two different phases, λ is the central wavelength of the source laser, Δx is the path length difference between the two interference signals, and n medium is the refractive index of the medium.

[0110] The device and method are c The target distance can be measured using the formula cos(x), where I is the intensity of the interference signal as a function of the target distance x, I is the offset of the interference signal, and I c is the contrast of the interference signal. The offset and contrast are determined by the amount of light entering the detection means and the electronic components of the detection means, such as the photodiode and the electronic circuitry comprising, for example, an ADC. If the offset and contrast were determined solely by the photodiode and the electronic circuitry, the offset would always be greater than the contrast. This is because the range of the signal 0 <I0-I c <I(x)<I0+I c <2×I0 This range can be shifted by the circuit to optimize the generation of the signal by the ADC. In one useful embodiment, I0-I c From I0+I c The range up to corresponds to the full range of the ADC. During execution of the method and operation of the device, this range can be continuously adjusted to achieve an optimum signal.

[0111] A method for determining this range is to detect the zero crossings of the signal, I(x) = I0. However, during initialization, or when the interferometer target is not moving, the range may shift so that it is different from the expected range (e.g.,

[0112]

number

[0113] ,

[0114]

number

[0115] ), and therefore the zero crossing may never occur. For example, this

[0116]

number

[0117] In such cases, a naive algorithm for detecting zero crossings cannot recover. Examples of situations in which this can occur are when the target rotates during initialization, thus reducing the target signal strength, or when the target is stationary during operation and the electrons undergo thermal drift.

[0118] This can be solved by creating a continuous movement of the optical path corresponding to the interference signal, preferably a movement that does not disturb the determination of the target's position, such that zero crossings occur. This can be achieved, for example, by intermittently (thermally) heating at least a portion of the light guide forming the delay path. This creates an oscillation of the optical distance of the delay path due to thermal expansion and refractive index fluctuations of the light guide. The oscillation of the optical distance is automatically canceled when determining the target's position because this delay path creates a delay that is incorporated into both the first and second interference signals. Such an embodiment can ensure that the interference signal always includes the zero crossings, so that the range of the interference signal can be determined based on the maximum and minimum values ​​of the interference signal. A simple algorithm based on detecting the zero crossings can then be used to optimize the signal, for example, by taking into account the thermal drift that interferometers typically experience. [Explanation of symbols]

[0119] 100 Interferometer 101 Light source 102 Splitter 103 First Fiber 104 Second Fiber 105 Sensor head 106 First Reference Signal 107 Reflective surface 108 Measurement Signal 109 Reflective surface 110 First Part 111 Second Reference Signal 112 First Detector 113 Second Detector 114 Beam Splitter 136 Adaptation Measures 201 devices 202 First Optical Assembly 203 Second Optical Assembly 204 Beam of Light 205 First PBS 206 First reference ray 207 Reference reflector 208 First Side 209 Second Side 210 Target Ray 211 Reflector, target reflector 212 Third Side 213 First QWP 214 Second QWP 215 Retroreflector 216 The Fourth Side 217 Light source 218 First Side 219 Second PBS 220 Reference Beam 221 Second Side 222 The Fourth Side 223 Second Sensor 224 The Third Side 225 First Sensor 226 Second Polarizer 227 First Polarizer 228 Beam Splitter 229 Delay 230 Detection means 231 First optical path 232 Second optical path 233 Third Path 234 First shared path 235 Second common path 236 Adaptation Measures 301 Devices 302 Light source 303 First Fiber Coupler 304 First Fiber 305 Sensor Head 306 First reference beam 307 Target Ray 308 Second Fiber 309 Second Fiber Coupler 310 Fiber end 311 Collimator 312 Reflective Polarizer 313 Target 314 Polarizing Beam Splitter 315 Third Fiber 316 First Sensor 317 Second Sensor 318 Fourth Fiber 319 Fifth Fiber 320 Polarizing Beam Splitter 321 Third Fiber Coupler 322 Fourth Fiber Coupler 323 First Part 324 Second Part 336 Adaptation Measures 402 Light source 403 First Fiber Coupler 404 First Fiber 405 Polarizing Beam Splitter 406 Second Fiber 407 Third Fiber 408 Fourth Fiber 409 Polarizing Beam Splitter 410 First Sensor 411 Sensor head 412 Fifth Fiber 413 The Sixth Fiber 414 First Input 415 Second Input 416 Target 417 Second Sensor 418 Fiber End 419 Fiber End 420 Collimator 421 Collimator 422 PBS 423 QWP 424 Polarizer 425 Second Fiber Coupler 436 Adaptation Measures 500 devices 501 Light generation means 502 Detection means 503 detector 504 Beam Splitting Means 505 First PBS 506 Third PBS 507 First beam splitter 508 First shared path 509 Second common path 510 Third common path 511 Delayed Route 512 First optical path 513 Second optical path 514 Fiber End 515 Collimator 516 Optical Assembly 517 Third Path 518 First Side 519 Mirror 520RR 521 Third Beam Splitter 522 Target 536 Adaptation Measures

Claims

1. 1. A method for determining the position by interferometry of a target reflector along a measurement range (B), comprising: generating, by modulation, first, second, and third coherent beams of light at a first single wavelength at a first time point and at a second single wavelength different from the first single wavelength at a second time point; generating a first reference signal by directing the first coherent light beam (206) along a first optical path (231), the first optical path having a first optical length; generating a measurement signal by directing the second coherent light beam (210) along a second optical path (232) comprising the target reflector (211), the second optical path having a second optical length; generating a second reference signal by directing the third coherent light beam (220) along a third optical path (233), the third optical path (233) having a third optical distance different from the first optical distance and the second optical distance; providing a common delay (229) between generating the second reference signal and generating the first reference signal, and between generating the second reference signal and generating the measurement signal; generating a first interference signal from first portions of the first reference signal and the second reference signal at the first time point and the second time point; generating the second interference signal from the measurement signal and a second portion of the second interference signal at the first time point and the second time point; determining a first amplitude of the first interfering signal and a second amplitude of the second interfering signal; generating a first signal based on the first interference signal and the first amplitude; generating a second signal based on the second interference signal and the second amplitude; determining a position of the target reflector (211) by calculating a difference between the first optical distance and the second optical distance based on the first signal and the second signal at both the first time point and the second time point; A method comprising:

2. 2. The method of claim 1 , wherein the first amplitude and the second amplitude are determined by one of detecting zero crossings of the first interfering signal and the second interfering signal and fitting first and second functions to the first and second interfering signals, respectively.

3. 3. The method of claim 1 or 2, wherein the common delay corresponds to a difference in optical distance, and the difference in optical distance is modulated with an amplitude corresponding to at least half of one of the first wavelength, the second wavelength, and a nominal value of the first wavelength and the second wavelength, preferably at least half of the amplitude of the larger of the first wavelength and the second wavelength.

4. 4. The method of claim 3, wherein the optical path difference oscillates at an oscillation frequency lower than 10 Hz, preferably at most 1 Hz.

5. 5. The method of claim 3 or 4, wherein the optical distance difference is modulated by changing at least one of the length and the refractive index, preferably the optical distance difference is modulated by exposing to at least one of heat, shear force, and magnetic field.

6. 6. The method of claim 1, wherein the position is determined by determining a first set and a second set of quadrature signals by demodulating the first interference signal and the second interference signal, respectively, and wherein the common delay (229) has an optical path length configured such that the amplitude of each quadrature signal of the first set and the second set of quadrature signals does not exhibit zero crossings over the measurement range (B).

7. 7. The method according to claim 1, wherein the step of generating the first coherent light beam (206) and the second coherent light beam (210) comprises a step of splitting light generated by a light source (217), and preferably the step of generating the third coherent light beam (220) comprises a step of splitting light generated by the light source (217).

8. 8. The method of claim 7, wherein splitting light to generate the first coherent light beam (206) and the second coherent light beam (210) comprises splitting light generated by the light source (217) according to a first polarization state and a second polarization state, respectively, and preferably wherein the first and second portions of the second interference signal comprise splitting the third coherent light beam (220) according to a third polarization state and a fourth polarization state, respectively, and preferably wherein the first and second polarization states correspond to the third and fourth polarization states, respectively.

9. 9. The method of claim 1, wherein generating the first reference signal and the measurement signal comprises directing the first coherent light beam (206) and the second coherent light beam (210) at least in part along coincident optical paths.

10. 10. The method according to any one of claims 1 to 9, wherein an optical assembly (202) configured to direct the first coherent light beam (206) to and from a reference reflector (207) and to direct the second coherent light beam (210) to and from a target reflector (211) is used to generate the first reference signal and the measurement signal, respectively, and preferably the common delay (229) is provided external to the optical assembly (202).

11. The optical assembly (202) comprises a polarizing beam splitter (205), a retroreflector (215), the target reflector (211), the reference reflector (207), a first quarter-wave plate (213), and a second quarter-wave plate (214), and the optical assembly (202) is configured such that a first side (208) of the polarizing beam splitter (205) receives the first coherent light beam (206) and the second coherent light beam (210), and a second side (209), a third side (212), and a fourth side (213) of the polarizing beam splitter (205) receive the first coherent light beam (206) and the second coherent light beam (210).

11. The method of claim 10, wherein the first quarter-wave plate (213) and the second quarter-wave plate (214) are configured to face the reference reflector (207), the target reflector (211), and the retroreflector (215), respectively, and are inserted between the polarizing beam splitter (205) and the reference reflector (207) and between the polarizing beam splitter (205) and the target reflector (211), respectively, and preferably the target reflector (211) comprises a target mirror and the reference reflector (207) comprises a reference mirror.

12. A device for determining the position of a target reflector (211) along a measurement range (B) by interferometry, comprising: a light generating means comprising a light source (217) configured to be modulated between a first state and a second state, wherein in the first state the light generating means is configured to generate light at a first single wavelength and in the second state the light generating means is configured to generate light at a second single wavelength different from the first single wavelength; optical means downstream of the light generating means configured to generate a first coherent reference signal, a coherent measurement signal, and a second coherent reference signal, a first optical path (231) configured to generate the first coherent reference signal by directing light generated by the light source (217), the first optical path having a first optical length; a second optical path (232) comprising a movable target reflector (211) and configured to generate the coherent measurement signal by directing light generated by the light source (217), the second optical path having a second optical length; a third optical path (233) configured to generate the second coherent reference signal by directing light generated by the light source (217), the third optical path having a third optical distance different from the first optical distance and the second optical distance; Equipped with the optical means further comprising a delay path that provides a common delay (229) between generating the second coherent reference signal and generating the first coherent reference signal, and between generating the second coherent reference signal and generating the coherent measurement signal; Optical means; detection means (230) downstream of the optical means configured to generate a first interference signal between the first coherent reference signal and a first portion of the second coherent reference signal and to generate a second interference signal between the coherent measurement signal and a second portion of the second coherent reference signal, the detection means (230) comprising: a first detector (225) configured to detect the first interference signal and determine a first amplitude of the first interference signal; and a second detector (223) configured to detect the second interference signal and determine a second amplitude of the second interference signal, configured to generate a first signal based on the first interference signal and the first amplitude, and configured to generate a second signal based on the second interference signal and the second amplitude; processing means configured to determine the position of the target reflector (211) by calculating a difference between the first optical distance and the second optical distance based on the first signal and the second signal in both the first state and the second state; A device comprising:

13. 13. The device of claim 12, wherein the device is configured to determine the first amplitude and the second amplitude based on one of detecting zero crossings of the first interfering signal and the second interfering signal and fitting first and second functions to the first interfering signal and the second interfering signal, respectively.

14. 14. The device according to claim 12 or 13, further comprising adaptation means (236) configured to modulate the difference in optical distance of the common delay with an amplitude corresponding to at least half of one of the first wavelength, the second wavelength and a nominal value of the first wavelength and the second wavelength, preferably at least half of the amplitude of the larger of the first wavelength and the second wavelength.

15. 15. The device according to claim 14, wherein the adaptation means (236) are configured to adapt the common delay at an oscillation frequency lower than 10 Hz, preferably at most 1 Hz.

16. 16. The device of claim 14 or 15, wherein the adapting means (236) comprises one of a heating element, an actuator, and an electromagnetic coil.

17. 17. The device of claim 12, wherein the processing means is configured to determine the position by determining a first set and a second set of quadrature signals by demodulating the first interference signal and the second interference signal, respectively, and wherein the common delay (229) has an optical path length configured such that the amplitude of each quadrature signal of the first set and the second set of quadrature signals does not exhibit zero crossings over the measurement range (B).

18. 18. The device of any one of claims 12 to 17, wherein the optical means further comprises a light guide configured as the delay path providing the common delay (229).

19. 19. The device of claim 12, wherein the optical means comprises a first common optical path (234) where the first optical path, the second optical path, and the third optical path overlap, the first common optical path comprising a first end comprising an optical pickup optically connected to the light generating means, and a second end downstream of the first end comprising a first beam splitter (228), the first beam splitter (228) configured to split the third optical path (233), preferably a portion of the third optical path (233) provided downstream of the first common optical path comprising a light guide configured as the delay path providing the common delay (229).

20. 20. The device of claim 19, wherein the optical means comprises: a second common optical path (235), along which the first optical path (206) and the second optical path (210) overlap; and a second beam splitter, the second common optical path (235) having a first end optically connected to the first beam splitter (228) and a second end downstream of the first end optically connected to the second beam splitter, the second beam splitter configured to split the second optical path (206), preferably the second common optical path (206) comprising a further light guide.

21. 21. The device of claim 20, wherein the second beam splitter comprises a first polarizing beam splitter.

22. The optical means comprises an optical assembly (202) comprising a polarizing beam splitter (205), a retroreflector (215), the target reflector (211), a reference reflector (207), a first quarter-wave plate (213), and a second quarter-wave plate (214), wherein the optical assembly (202) is configured such that a first side (208) of the polarizing beam splitter (205) receives the first coherent light beam (206) and the second coherent light beam (210), and a second side (209), a third side (212), and a fourth side (214) of the polarizing beam splitter (205) receive the first coherent light beam (206) and the second coherent light beam (210).

22. The device of claim 12, wherein the first and second quarter-wave plates (213, 214) are configured such that their respective sides (216) face the reference reflector (207), the target reflector (211), and the retroreflector (215), respectively, and the first and second quarter-wave plates (213, 214) are inserted between the polarizing beam splitter (205) and the reference reflector (207) and between the polarizing beam splitter (205) and the target reflector (211), respectively, and preferably the target reflector comprises a target mirror and the reference reflector comprises a reference mirror.

23. 23. A device according to claim 22 in combination with claim 20 or 21, wherein the second beam splitter is arranged within the optical assembly and forms part of the optical assembly, preferably the first polarizing beam splitter forms the polarizing beam splitter (205).

24. 23. The device of claim 22 in combination with claim 20 or 21, wherein the second beam splitter is positioned upstream of the optical assembly and configured such that the first optical path and the second optical path traverse the optical assembly without overlapping.

25. 25. The device of claim 12, wherein the optical means comprises a third beam splitter configured to generate the first and second portions of the second coherent reference signal.

26. 26. The device of claim 25 in combination with claim 24, wherein the third beam splitter comprises a third polarizing beam splitter, preferably the second beam splitter and the third beam splitter form a single beam splitting means.

27. 27. The device of any one of claims 12 to 26, wherein the detection means comprises a fourth beam splitter (219) downstream of the optical means configured to generate the first interference signal and the second interference signal, preferably wherein the fourth beam splitter comprises a second polarizing beam splitter.

28. 28. A device according to any one of claims 12 to 27, wherein the light generating means comprises a single light source for generating light in the first state and in the second state, preferably said light source comprising a laser.

29. 29. The device of claim 12, wherein the detection means comprises a first polarizer (227) upstream of the first detector (225) configured to generate the first interference signal, and a second polarizer (226) upstream of the second detector (223) configured to generate the second interference signal.

30. 30. The device of any one of claims 12 to 29, comprising further optical means and further detection means, said further optical means and further detection means configured to measure the position of the target along a different direction to said optical means and said detection means.

31. 31. The device of claim 30, comprising an additional beam splitter arranged to share light generated by the light generating means between the optical means and the further optical means, the additional beam splitter being provided between the light generating means and both the optical means and the further optical means.

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