Method for determining position by differential interferometry and differential interferometer for doing so

Abstract

A method for determining the position of a target reflector in a measurement range by differential optical interferometry includes several steps: a coherent source beam is generated using a single-frequency laser, where the coherent source beam is modulated between a first wavelength and a second wavelength; a light beam and a delayed beam are generated from the coherent source beam, where the delayed beam includes a delay relative to the light beam; the light beam is preferably arranged parallel to (and spatially separated from) the delayed beam; each of the light beam and the delayed beam is separated into a first portion and a second portion, where the second portion is arranged parallel to (and spatially separated from) the corresponding first portion; a first optical signal is generated by directing a first portion of one of the light beam and the delayed beam along a first path that includes the target reflector; and a second optical signal is generated by directing a second portion of one of the light beam and the delayed beam along a second path that includes a reference reflector, where the second path is arranged parallel to (and spatially separated from) the first path. A third optical signal is generated by directing a first portion of another one of the light beam and the delayed light beam along a third path. A fourth optical signal is generated by directing a second portion of another one of the light beam and the delayed light beam along a fourth path, the fourth path being positioned parallel to (and spatially separated from) the third path. A first interference signal is generated between the first optical signal and the third optical signal. A second interference signal is generated between the second optical signal and the fourth optical signal. The position of the target reflector is determined based on the first and second interference signals.

Description

[Technical Field]

[0001] The present invention relates to a method for determining the position of an object with high stability and precision by differential interferometry, and to a differential interferometer for doing so. [Background technology]

[0002] Differential interferometry can be used to measure the optical path length difference for a movable target relative to the optical path length for a reference by generating an interference signal from the interaction between light traveling a reference path and light traveling a target path. A common challenge with differential interferometry and differential interferometry is changes in optical path length caused by environmental factors such as temperature changes, as these changes can affect the length of the beam's path and the refractive index of the medium through which the beam travels, both of which affect and determine the optical path length. Another common challenge is providing adequate spatial resolution, preferably uniform spatial resolution, to determine the phase and direction of propagation.

[0003] Patent Document 1 describes a position detection device including a differential interferometer for generating an interference pattern that depends on the length of the measurement section and the detector, and obtains the detected interference pattern as a basis for generating a measurement signal. The position detection device further includes 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 variation over time. A disadvantage of this solution is that it may require a relatively wide dynamic range for the modulation input, which introduces noise into the measurement value. A further disadvantage of this solution is that it is complex to implement in systems in which the position is detected along multiple axes, especially when the same modulation depth is required along multiple axes. Furthermore, it is not possible to determine an accurate position in situations where the reference distance and the target distance may be equal, which may typically occur in free-space solutions.

[0004] Each of Patent Documents 2, 3 and 4 describes a differential interferometer that includes a complex and expensive optical system for modulating the phase of a reference signal to measure a 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] Chinese Patent Application Publication No. 112432602 [Patent Document 3] U.S. Patent Application Publication No. 2021 / 0199418 [Patent Document 4] Chinese Patent Application Publication No. 112857206 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, the 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 optical interferometry, in accordance with the appended claims. A coherent source beam can be generated using a single-frequency laser, where the coherent source beam is modulated between a first wavelength and a second wavelength. A light beam and a delayed light beam can be generated from the coherent source beam, where the delayed light beam includes a delay relative to the light beam. The light beam is preferably arranged parallel to (and spatially separated from) the delayed light beam. Each of the light beam and the delayed light beam is separated into a first portion and a second portion, where the second portion is arranged parallel to (and spatially separated from) the corresponding first portion. A first optical signal can be generated by directing a first portion of one of the light beam and the delayed light beam along a first path that includes the target reflector. The second optical signal can be generated by directing a second portion of one of the light beam and the delayed light beam along a second path that includes the reference reflector, the second path being parallel to (and spatially separated from) the first path. The third optical signal can be generated by directing a first portion of another one of the light beam and the delayed light beam along a third path. The fourth optical signal can be generated by directing a second portion of another one of the light beam and the delayed light beam along a fourth path, the fourth path being parallel to (and spatially separated from) the third path. A first interference signal can be generated between the first optical signal and the third optical signal. A second interference signal can be generated between the second optical signal and the fourth optical signal. The position of the target reflector can be determined based on the first interference signal and the second interference signal.

[0008] The method achieves the object of the present invention by generating two interference patterns: a first for a first optical signal (e.g., a target signal) and a second for a second optical signal (e.g., a first reference signal). Here, such an optical signal interferes with another (e.g., third and fourth) reference signal with an optical path length difference introduced by the delay between generating the light beam and the delayed light beam. This reduces the relative fluctuations in (phase) modulation depth even with fixed-frequency modulation of a coherent source beam, thus avoiding adjusting the modulation depth depending on the target position. This allows and / or simplifies reducing noise in the measurements. Furthermore, minimizing the fluctuations in modulation depth is advantageous, especially when position measurements are performed along multiple axes, since it improves the contrast of the interference between different signals.

[0009] Furthermore, by directing the respective portions of the light beam and delayed beam along the indicated optical paths with parallel, spatially separated portions, sensitivity to environmental effects (e.g., temperature, magnetic fields, shear stress) that may affect the optical path length of such paths is reduced, as such environmental effects affect both interference patterns to the same extent. Thus, any of these environmental effects will be compensated for when determining the location of the target reflector based on the first and second interference signals.

[0010] For example, the phase difference sign information can be resolved by modulating the wavelength of the coherent source light beam between two states, such as by modulating between a first (single) wavelength generated at a first time point and a second (single) wavelength generated at a second time point. The resolution of the path length difference can also be improved by leveling the resolution across the range of path length differences over the target's measurement range along the axis. Preferably, the wavelength is modulated 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 frequency of the coherent source light beam can be modulated by modulating a light source, such as a (single-frequency) laser of the light generating means, using a modulator, where the light generating means can comprise the modulator. The modulation frequency, at which the coherent source light beam is modulated between the first wavelength and the second single wavelength, can be selected / adapted based on the maximum speed of the target reflector.

[0011] Advantageously, the first, second, third and fourth optical signals are generated using a sensor head configured to implement single-pass interferometry, the advantage of which is that non-linearity errors due to misalignment of optical components introduced by the sensor head can be minimized.

[0012] Preferably, the light generating means comprises a low-cost light source. The light source is advantageously configured to emit light having a single wavelength at a time. Advantageously, the light source is modulated to change the single wavelength between two or more states, where each state corresponds to a different single wavelength. The light source may be configured to change between the two or more states by adapting a parameter of the light source, such as a (driving) current, a (driving) voltage, or a temperature. Advantageously, the light source is configured to have a linear correlation between the adaptation of the parameter and the change in the single wavelength. For example, the light source may comprise a tunable homodyne light source (e.g., a single-frequency laser), such as a semiconductor or diode laser, which typically provides a low-cost light source. Examples of advantageous light sources are distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers. Alternatively, for example, a vertical-cavity surface-emitting laser (VCSEL) may be used as an advantageous light source in embodiments configured for limited target distances.

[0013] To determine the position of the target reflector, the first and second sets of quadrature signals can be determined by demodulating the first and second interference signals, respectively, according to a demodulation scheme. As will be apparent to those skilled in the art, several modulation schemes may be suitable. For example, the first and second interference 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 interference signals using a low-pass filter and then demodulate the first and second interference signals at the modulation frequency.

[0014] The measurement range preferably includes all possible positions of the target reflector over its entire range of motion along the axis. The position of the target reflector can be determined within the measurement range to, for example, monitor the position of a stage attached to the target reflector.

[0015] The target reflector can 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 the tolerance for translation of the target reflector in a direction orthogonal to the incident light. This advantage is important for measuring the position of the stage, because translation 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 makes it possible to monitor the position of the target or stage along an orthogonal measurement range, since this allows for separation of position measurements along orthogonal axes.

[0016] The reference reflector may comprise any type of suitable reflector, such as a retroreflector (e.g., cube corner) or a plane mirror, etc. Preferably, the target reflector comprises a plane mirror, since a plane mirror can be translated relative to the interferometer in a direction orthogonal to the measurement direction without affecting the measurement, creating the possibility of isolating displacement measurements along orthogonally disposed axes.

[0017] The fifth, sixth, seventh, and eighth paths are defined by the (total) optical paths traveled by the light generated by the light source, forming the first, second, third, and fourth optical signals, respectively. Advantageously, the fifth and sixth paths or the seventh and eighth paths include delays. This ensures that the first and second interference signals are each generated from unbalanced respective signals (i.e., the unbalanced first and third optical signals, and the unbalanced second and fourth optical signals). Here, a first ratio between the optical path length of the fifth path and the optical path length of the seventh path, and a second ratio between the optical path length of the sixth path and the eighth path, are each either greater than 1 or less than 1. Such delays reduce relative variations in (phase) modulation depth even with fixed frequency modulation of the source, thus avoiding target position-dependent modulation of the modulation depth. This allows and / or simplifies reducing noise in the measurements, since reducing the relative variation in the measured distance reduces the required variation in the modulation input. Furthermore, especially when measurements are made in multiple (orthogonal) axes, minimizing the variation in modulation depth is advantageous, since this improves the interference contrast between different signals and therefore the signal strength of the demodulated signal for the measurement range of the target distance for such axes.

[0018] The light beam and the delayed light beam can be generated from a (single) coherent light source beam. This coherent light beam is preferably generated by a (single) light source to ensure that any changes affecting the light source (e.g., frequency switching between states) affect both beams simultaneously, without requiring any additional means, e.g., for synchronization. Advantageously, the delayed light beam is generated using a first light guide when it exits a first optical terminal of the first light guide. Optionally, the light beam is generated using a second light guide when it exits a second optical terminal of the second light guide.

[0019] The light beam and the delay beam are preferably arranged parallel (and spatially separated) so that external influences on the optical path downstream of each optical terminal equally affect the optical path length traveled by both the light beam and the delay beam. To achieve this, the first and second optical terminals are advantageously aligned parallel.

[0020] Generating the delayed light beam and the light beam preferably includes splitting a coherent source light beam into a first portion and a second portion, each of which is spatially separated. The coherent source light beam can be split using, for example, a beam splitter (e.g., a fiber-based beam splitter, a free-space beam splitter). The first portion can then be directed along a delay path (e.g., a first light guide) and the second portion can be directed along a path (e.g., a second light guide). Preferably, the optical path length of the delay path is longer than the optical path length of the path.

[0021] Preferably, the delay is caused by a difference in optical path length between the delay paths. The advantage of implementing the delay in this manner is that environmental effects, such as thermal expansion, equally affect the optical path lengths of the fifth and sixth paths or the seventh and eighth paths, and have a similar effect on the first delay between the first and third optical signals and the second and fourth optical signals. Therefore, determining the target's position is essentially unaffected by these environmental effects. Advantageously, one of both the fifth and sixth paths and one of both the seventh and eighth paths includes a delay path, and the other of both the fifth and sixth paths and the seventh and eighth paths includes a delay path. Preferably, both the fifth and sixth paths include a delay path, since both of these paths typically require a longer optical path length than the other paths to account for the target's measurement range. Therefore, the length of such a delay path may typically be shorter than the length of the delay path provided in the seventh and eighth paths.

[0022] Advantageously, for separated position measurements along different (orthogonal) axes, the delay corresponds to an optical path length (e.g., optical path length difference between the delay path and the path) that is longer than the measurement range of the target reflector to minimize relative variations in path length differences. Furthermore, because modulation depth is proportional to the optical path length difference between the interfering optical signals, variations in modulation depth between independent differential interferometers can also be minimized.

[0023] The delay can be configured to correspond to an optical path length 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 measurement range of the interferometer target reflector for a (fixed) frequency modulation amplitude of the light generating means (e.g., light source). This can be achieved when the delay is longer than the measurement range, for example, when the delay corresponds to an optical path length at least twice as long as the target measurement range. For example, the delay is selected so that the third ratio between the measurement range of the target reflector and the absolute value of the optical path length difference between the optical path length of the sixth path and the eighth path 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 substantially greater than 10 or substantially less than 0.1. In the event that the fifth and sixth paths include delays such that the light provided by the delays is used to generate the light of the first and second optical signals, the optical path length of the delays may be substantially an order of magnitude greater than the optical path length of the seventh or eighth path. Preferably, the optical path length of the delays is selected so that the amplitude of each of the quadrature signals in the first and second sets of quadrature signals does not exhibit zero crossings over the measurement range of the target reflector. Preferably, the optical path length of the delays is selected so that the amplitude of each of the quadrature signals in the first and second sets of quadrature signals is sufficiently large over the measurement range of the target reflector, resulting in a signal-to-noise ratio that does not limit the performance of the sensor. For example, the optical path length of the delays is selected so that the amplitude of each of the quadrature signals in the first and second sets of quadrature signals does not vary by more than a factor of four. Preferably, the optical path length of the delays is selected so that the amplitude of each of the quadrature signals in the first and second sets of quadrature signals does not vary by more than a factor of two.

[0024] Preferably, the light generating means comprises a light guide configured as a delay path. A delay path comprising a light guide has the advantage that it can be used to achieve large and identical path length differences between the first and third optical signals and between the second and fourth optical signals in a relatively small space. For example, if such a light guide is longer than the measurement range, the relative variation in 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 in modulation depth between independent differential interferometers can also be minimized.

[0025] The delay path may comprise a first light guide to provide a simple and compact means for delaying light. The first light guide may form a first optical terminal configured to generate a delayed light beam. Additionally or alternatively, the path may comprise a second light guide. The second light guide may comprise a second optical terminal configured to generate a light beam.

[0026] Preferably, the light is collimated along at least portions of the fifth, sixth, seventh and eighth paths. This is particularly advantageous for at least portions of each optical path that are free space.

[0027] A first beam splitter, such as a non-polarizing beam splitter (e.g., a lateral displacement beam splitter, a modified cube beam splitter), can be provided to separate each of the light beam and the delayed beam into a first portion and a second portion that are spatially separated from each other. The first beam splitter can be provided downstream of the light generating means, for example downstream of the first and second optical terminals. The first beam splitter can introduce a second delay between the first portion and a corresponding second portion. Preferably, the second delay introduced between the first and second portions of the light beam corresponds to the second delay introduced between the first and second portions of the delayed beam. Such a corresponding delay affects the first and second interference patterns equally and therefore does not affect position determination.

[0028] A second beam splitter can be provided to generate the first, second, third, and fourth optical signals. The second beam splitter can be provided downstream of the first beam splitter. For example, the first, second, third, and fourth paths can include a second beam splitter, such as a non-polarizing beam splitter or a polarizing beam splitter. The first and second paths may pass through the second beam splitter twice: once by light traveling toward a corresponding reflector; and once by light reflected by the corresponding reflector and traveling toward a detection means (e.g., a corresponding detector of the detection means).

[0029] In a preferred embodiment, the optical path length of the fourth path corresponds to the optical path length of the third path, so that environmental effects, such as causing thermal expansion, affect the third and fourth optical signals equally.

[0030] For example, a quarter wave plate (QWP) may be provided in the first and / or second paths, such as between the second beam splitter and the target reflector and / or between the second beam splitter and the reference reflector. This is particularly advantageous in embodiments where the second beam splitter comprises a polarizing beam splitter to reduce noise in the respective signals.

[0031] Polarizing means can be provided to generate the first and second interference signals. This is particularly advantageous in embodiments in which the second beam splitter comprises a polarizing beam splitter. The polarizing means can comprise a single (linear) polarizer to generate the first and second interference signals. Alternatively, the polarizing means can comprise first and second polarizers to generate the first and second interference signals, respectively. The first and second polarizers can be oriented to optimize the respective interference signals, for example, by improving contrast. For example, the polarizing means can be configured with an adjustable orientation for each polarizer to change the direction of polarization, so that the intensity ratio between the respective interfering signals can be adjusted to optimize (maximize) the interference contrast.

[0032] The detecting means may include a first detector and a second detector for detecting the first interference signal and the second interference signal, respectively, so that the position of the target reflector can be determined. To this end, a processing means (e.g., a processing unit) may be provided, where the detecting means (e.g., the first and second detectors) are connected to the processing means. The processing means may be configured to determine the position of the target reflector by calculating a difference between the first path and the second path, where the difference may be based on the first and second interference signals for a coherent source light beam modulated between a first wavelength and a second wavelength.

[0033] To determine the position of the target reflector, the quadrature phases can be determined, for example, by the processing means 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 may be 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.

[0034] In a preferred embodiment, the method further includes a calibration step for more accurately determining the target's position. The calibration step includes, for example, 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 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, if the detection means includes at least one analog-to-digital converter (ADC), the calibration step can be 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.

[0035] The first and second amplitudes may be determined by peak detection or regular 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 includes at least three consecutive zero crossings. Zero crossing detection has the advantage that the accuracy of amplitude determination is often higher than for sampling data for peak detection. For example, the first and second amplitudes may be determined based on a minimum value of the respective interference pattern (e.g., a point halfway between the first and second zero crossings of the set of zero crossings) and a maximum value of the respective interference pattern (e.g., a point halfway between the second and third zero crossings of the set of zero crossings). For faster-moving targets, it may be advantageous to determine the first and second amplitudes based on fitting a (periodic) function, such as a trigonometric function like a cosine function, to the respective interference signals. For example, the first and second amplitudes may be determined based on fitting first and second functions to the first and second interference signals, respectively.

[0036] However, in some situations, such zero crossings may not occur automatically. For example, zero crossings may not occur when the optical path length of the first path is nearly constant (e.g., varying by less than 1 / 16 or 1 / 8 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 pattern and changes in the optical path length of the first path (e.g., caused by movement of the target reflector along the measurement range). Such disturbances may include factors caused by, for example, particulate matter, condensation or dirt on optical components, or changes in the alignment of optical components, which affect, to some extent, light guiding in one of the respective optical paths, such as part of the fifth path (e.g., the first path). Additionally or alternatively, such disturbances may include factors caused by thermal drift of electronic circuits, such as the electronic circuits of the detection means, which affect signal detection and processing. In any of these situations, a zero crossing in the interference signal can be forced by adapting (e.g., modulating) the optical path length of one of the optical paths, such as the fifth path corresponding to the interference signal. For example, the optical path length of the first optical path can be adapted by moving the target reflector. Preferably, the optical path length is modulated to oscillate at an amplitude of at least half the amplitude of one of the first wavelength, the second wavelength, and the nominal values ​​of the first and second wavelengths, preferably at least half the amplitude of the longer of the first and second wavelengths. Advantageously, the delay is adapted (e.g., modulated) so that the difference in optical path length varies by at least an optical path length corresponding to one of the nominal values ​​of the first wavelength, the second wavelength, and the first and second wavelengths, preferably the longer of the first and second wavelengths. In a preferred embodiment, the optical path length difference introduced by the delay is adapted. The advantage of adapting the optical path length of the delay is that both interference patterns are affected to the same extent. Therefore, once the target reflector position is determined, the optical path length adaptation is automatically cancelled.Preferably, the optical path length difference introduced by the delay is modulated with an amplitude corresponding to at least half of the nominal value of the first wavelength, the second wavelength, and one of the first and second wavelengths, preferably at least half of the amplitude of the longer of the first and second wavelengths.

[0037] The delay can be adapted by adapting the optical path length of one of the paths and the delay path. Preferably, the delay is adapted by adapting the optical path length of the delay path, since a suitable change in optical path length is more easily achieved in the path having the longest optical path length.

[0038] The optical path length of (a portion of) a path (e.g., a delay path) can be adapted or modulated by changing at least one of the refractive index of (a portion of) the path and the length of (a portion of) the path. In an advantageous embodiment, the optical path length difference between the path and the delay path is configured to oscillate at an amplitude of at least half of the first wavelength, the second wavelength, and one of the nominal values ​​of the first and second wavelengths, preferably at least half of the longer of the first and second wavelengths. For example, when such paths include, for example, a Pockels cell, the optical path length can be changed by exposing one of the paths (e.g., the delay path) to a magnetic field (e.g., generated by an electromagnetic coil), exposing one of the paths (e.g., the delay path) to heat (e.g., generated by a heating element), or exposing one of the paths (e.g., the delay path) to a shear force (e.g., generated by an actuator). In a preferred embodiment, heat is applied to the delay path. For example, at least a portion of the delay path (e.g., the light guide) is exposed to a heating element configured to (alternately) heat the first light guide. In an advantageous embodiment, at least a portion of the first light guide is wrapped around the heating element.

[0039] The optical path length can be oscillated at an oscillation frequency, preferably at least 1 Hz, more preferably at least 10 Hz. The temperature setpoint for heating the lightguide can be selected so that the cooling rate corresponds to the selected oscillation frequency. For example, the oscillation frequency can be increased by increasing the difference between the nominal temperature of the lightguide and the ambient temperature of the lightguide. The ambient temperature of the lightguide can also be actively cooled to lower the ambient temperature of the lightguide, thereby increasing the upper limit of the oscillation frequency.

[0040] According to a second aspect of the present invention, the object is achieved by a differential spectroscopic interferometer according to the appended claims, which achieves the object of the invention in a manner similar to that according to the first aspect, wherein the relevant features apply equally mutatis mutandis to the differential spectroscopic interferometer and vice versa.

[0041] A differential spectroscopic interferometer for determining the position of a target reflector within a measurement range may include a light generating means. The light generating means may include a single-frequency laser and may be configured to generate a coherent source light beam modulated between a first wavelength and a second wavelength. The light generating means may further include first and second optical terminals. The light generating means may be configured to manipulate the coherent source light beam to generate a delayed light beam at the first terminal and a light beam at the second terminal, where the delayed light beam is delayed relative to the light beam. The differential spectroscopic interferometer may further include a sensor head disposed downstream of the first and second optical terminals. The delayed light beam may be arranged parallel to the light beam. The sensor head may include a first beam splitter and a second beam splitter disposed downstream of the first beam splitter. The first beam splitter may be configured to split each of the light beam and the delayed light beam into a first portion and a second portion. The second beam splitter may be configured to generate a first optical signal by directing a first portion of one of the light beam and the delayed beam along a first path that includes the target reflector. The second beam splitter may be further configured to generate a second optical signal by directing a second portion of one of the light beam and the delayed beam along a second path that includes the reference reflector. The second path may be disposed parallel to the first path. The second beam splitter may be further configured to generate a third optical signal by allowing passage of at least a portion of the first portion of the other one of the light beam and the delayed beam along a third path. The second beam splitter may be further configured to generate a fourth optical signal by allowing passage of at least a portion of the second portion of the other one of the light beam and the delayed beam along a fourth path. The fourth path may be disposed parallel to the third path. The sensor head may be further configured to generate a first interference signal between the first optical signal and the third optical signal, and to generate a second interference signal between the second optical signal and the fourth optical signal. The sensor head may further comprise detection means configured to detect the first and second interference signals, respectively.The differential interferometer may further comprise processing means configured to determine the position of the target reflector based on the first and second interference signals.

[0042] The differential spectroscopic interferometer may comprise a light generating means comprising a light source as described in the first aspect. The light source may comprise, for example, a tunable homodyne laser, which is typically a low-cost light source. The light source may be configured to modulate between a first state and a second state. The light source may comprise, for example, a laser configured to be modulated between a first state and a second state. In the first state (e.g., at a first time point), the light generating means may be configured to generate monochromatic light at a first wavelength. In the second state (e.g., at a second time point), the light generating means may be configured to generate monochromatic light at a second wavelength different from the first wavelength. Preferably, the light source generates a coherent source beam (e.g., a coherent, parallel source beam). As described in the first aspect, this may be used downstream to generate the light beam and the delayed light beam, for example, by splitting the coherent source beam using a first beam splitter.

[0043] The light-generating means may further comprise a modulator configured to modulate the wavelength of the coherent light source beam. Modulation of the wavelength between the two states may be achieved by modulating the light source. For example, the light-generating means may comprise a modulator as described in the first aspect. The modulator may be configured to modulate the coherent light source beam such that light of a first wavelength is emitted by the light-generating means in the first state and light of a second single wavelength is emitted by the light-generating means in the second state. Such a modulator may be configured to adapt a parameter of the light source, such as, for example, a (driving) current, a (driving) voltage, or a temperature.

[0044] The light generating means is configured to manipulate a coherent source beam to generate a delayed beam at a first terminal and a beam at a second terminal. To this end, the light generating means may be configured to split the coherent source beam into a first portion and a second portion, which may be used to generate the delayed beam and the beam, respectively. To split the coherent source beam, the light generating means may comprise a beam splitter as described in the first aspect. To generate the delayed beam and the beam, the light generating means may comprise a delay path and a light guiding means configured to provide the path, respectively. Preferably, the delay is generated by the delay path having an optical path length longer than the optical path length of the path. In an advantageous embodiment, the light generating means comprises a first light guiding body (provided downstream of the beam splitter) as described in the first aspect. The (downstream) end of the first light guiding body may be configured to form the first optical terminal of the light generating means. Additionally or alternatively, as described in the first aspect, the light generating means may comprise a second light guide (arranged downstream of the beam splitter). The (downstream) end of the second light guide may be configured to form a first optical terminal of the light generating means. An advantage of using a light guide instead of folded optics is that the light guide can provide a relatively long optical path in a compact manner, while typically being more robust. In a preferred embodiment, the beam splitter and at least parts of the first and second light guides are realized by a single fiber-based beam splitter.

[0045] The sensor head may include a first beam splitter, as described in the first aspect, for separating each of the light beam and the delayed light beam into a first portion and a second portion. The sensor head may include a second beam splitter, as described in the first aspect, for generating first, second, third, and fourth optical signals. The second beam splitter may be provided downstream of the first beam splitter. The sensor head may further include a quarter-wave plate, as described in the first aspect, for generating the first and second optical signals. The sensor head may further include polarizing means, as described in the first aspect, for generating the first and second interference signals. The sensor head may further include detecting means, as described in the first aspect, for detecting the first and second interference signals.

[0046] The differential interferometer may further comprise processing means for determining the position of the target reflector based on the first and second interference signals as described in the first aspect, and the processing means may be configured to demodulate each of the first and second interference signals according to a demodulation scheme.

[0047] In an advantageous embodiment, the differential interferometer can be configured to determine first and second amplitudes of the first and second interference signals, respectively, so as to control the detection of the first and second interference signals. For example, the sensor head (e.g., detection means) or processing means can be configured to determine the first and second amplitudes and control the detection of the first and second interference signals. Such amplitudes can be determined with relatively high accuracy, for example, in events based on detecting zero crossings with the first and second interference signals. For faster moving targets, it may be advantageous to determine the first and second amplitudes based on fitting a (periodic) function, such as a trigonometric function such as a cosine function, to the respective interference signals. For example, the first and second amplitudes can be determined based on fitting the first and second functions to the first and second interference signals, respectively.

[0048] The differential interferometer may further comprise an adapting means configured to adapt the difference in optical path length so that a zero crossing occurs in the first and second interference signals. Preferably, the adapting means is configured to adapt or modulate the optical path length difference by at least an optical path length 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 path length corresponding to the longer of the first and second wavelengths. Alternatively, the adapting means is configured to adapt and modulate the delay so that the optical path length difference oscillates with at least half the amplitude of one of the nominal values ​​of the first wavelength, the second wavelength, and the first and second wavelengths, preferably at least half the amplitude of the longer of the first and second wavelengths. Advantageously, the adapting means is configured to adapt the common delay with an oscillation frequency preferably of at least 1 Hz, more preferably of at least 10 Hz.

[0049] The optical path length difference can be adapted by changing at least one of the refractive index and the length of the path or delay path. The adapting means can therefore be configured to expose such path 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. For example, the delay path (e.g., the first light guide) can be at least partially exposed to (e.g., wrapped around) a heating element.

[0050] The differential interferometer according to the present invention is particularly advantageous in situations where multiple measurements need to be compared, such as situations where distance is determined along multiple axes. Such a system may, for example, comprise multiple differential interferometers as described herein. Preferably, each of the multiple differential interferometers is configured to measure distance along multiple axes. Advantageously, such a system comprises a common light source for generating a source beam, the common light source being configured upstream of the multiple differential interferometers. For example, an advantage of such a system comprising a (single) laser source (e.g., a modulated and line-locked laser source) as the common light source is that it is suitable for performing several independent position measurements, each having approximately the same (optimized) modulation depth.

[0051] As will be apparent to those skilled in the art, the various sections along different optical paths can be configured in free space or with a fiber-based solution, preferably with a fiber-based solution configured to maintain the polarization state, such polarization-maintaining fiber producing an interference signal that is insensitive to fiber deformations.

[0052] According to a third aspect of the present invention, the object is achieved by implementing a method according to the provisions mentioned in this paragraph, which method achieves the object of the present invention in a similar manner to the method and interferometer according to the first and second aspects, respectively, whereby, as will be clear to those skilled in the art, the relevant features may equally be applied mutatis mutandis to the method according to the third aspect. Clause 1: generating a coherent source beam using a single frequency laser, the coherent source beam being modulated between a first wavelength and a second wavelength; generating a delayed light beam and a light beam from a coherent source light beam, the light beam being positioned parallel to the delayed light beam; Separating each of the light beam and the delayed light beam into a first portion and a second portion, the second portion being disposed parallel to the corresponding first portion; generating a first optical signal by directing a first portion of one of the light beam and the delayed light beam along a first path that includes a target reflector; generating a second optical signal by directing a second portion of one of the light beam and the delayed light beam along a second path; generating a third optical signal by directing a first portion of another one of the light beam and the delayed light beam along a third path that includes the target reflector; generating a fourth optical signal by directing a second portion of another one of the light beam and the delayed light beam along a fourth path; a step in which a first portion of the second path overlaps with a first portion of the fourth path, a second portion of the second path overlaps with a portion of the third path, and a second portion of the fourth path overlaps with a portion of the first path; generating a first interference signal between the first optical signal and a third optical signal; generating a second interference signal between the second optical signal and the fourth optical signal; determining a position of the target based on the first and second interference signals, preferably the position is determined by determining quadrature phases by demodulating each of the first and second interference signals, and the optical path lengths of the delay paths are configured such that the amplitude of each demodulated first and second interference signal does not exhibit zero crossings over a measurement range of the target reflector, preferably such that the amplitude of each demodulated first and second interference signal does not vary by more than a factor of four, preferably by more than a factor of two; 1. A method for determining the position of a target reflector in a measurement range by optical interferometry, comprising: Clause 2: A method according to clause 1, wherein the step of generating the delayed beam and the light beam from the coherent source beam comprises the steps of separating the coherent source beam into a first portion and a second portion, respectively, directing the first portion along a delay path to generate the delayed beam, and directing the second portion along the path to generate the light beam, wherein the optical path length of the delay path is longer than the optical path length of the path. Clause 3: A method according to clause 1 or 2, comprising determining a first amplitude of the first interference signal and a second amplitude of the second interference signal, wherein determining the position of the target reflector is further based on the first and second amplitudes. Clause 4: A method according to clause 3, wherein the first and second amplitudes are determined by one of detecting zero crossings of the first and second interference signals and fitting first and second functions to the first and second interference signals, respectively. Clause 5: A method according to clause 3 or 4, wherein the delay corresponds to an optical path length difference, and the optical path length difference is modulated with a first wavelength, a second wavelength, and an amplitude corresponding to at least half of a nominal value of one of the first and second wavelengths, preferably at least half of the amplitude of the longer of the first and second wavelengths. Clause 6: The method according to clause 5, wherein the optical path length difference is modulated by changing at least one of the length and refractive index, preferably by exposure to at least one of heat, shear stress, and a magnetic field.

[0053] According to a fourth aspect of the present invention, the object is achieved by providing a differential spectroscopic interferometer according to the provisions mentioned in this paragraph, which achieves the object of the present invention in a manner similar to the methods and interferometers according to the first and second aspects, respectively, whereby, as will be clear to those skilled in the art, the relevant features may equally be applied mutatis mutandis to the differential spectroscopic interferometer according to the fourth aspect. Clause 7: A light generating means comprising a single frequency laser and configured to generate a coherent source light beam modulated between a first wavelength and a second wavelength, a light generating means further comprising first and second optical terminals, the light generating means configured to manipulate the coherent source light beam to generate a delayed light beam at the first optical terminal and a light beam at the second optical terminal, the delayed light beam being delayed relative to the light beam; a sensor head disposed downstream of the first and second optical terminals, the delay beam being disposed parallel to the beam; the sensor head includes a first beam splitter, a second beam splitter, a first detector, and a second detector; the first beam splitter is configured to split the delayed beam into a first portion and a second portion, direct the first portion of the delayed beam along a first path including the target reflector to a first detector to form a first optical signal, and direct the second portion of the delayed beam along a second path through the second beam splitter to a second detector to form a second optical signal; the second beam splitter is configured to split the light beam into a first portion and a second portion, direct the first portion of the light beam along a third path including the target reflector to a second detector to form a third optical signal, and direct the second portion of the light beam along a second path passing through the first beam splitter to the first detector to form a fourth optical signal; the sensor head is further configured to generate a first interference signal between the first optical signal and the third optical signal, and to generate a second interference signal between the second optical signal and the fourth optical signal; a sensor head including first and second detectors configured to detect the first and second interference signals, respectively; processing means configured to determine the position of the target reflector based on the first and second interference signals, preferably the processing means is configured to determine the position by determining first and second sets of quadrature signals by demodulating the first and second interference signals, respectively, and wherein the delayed light beam is delayed relative to the light beam by introducing an optical path length difference configured such that the amplitude of each quadrature signal of the first and second sets of quadrature signals does not exhibit zero crossings over a measurement range, preferably such that the amplitude of each quadrature signal of the first and second sets of quadrature signals does not vary by more than a factor of four, preferably by a factor of two; 1. A differential interferometer for determining the position of a target reflector in a measurement range, comprising: Clause 8: A differential optical interferometer according to clause 7, wherein the light generating means comprises a beam splitter provided downstream of the single frequency laser, the beam splitter configured to separate the coherent source light beam into a first portion and a second portion and direct the first and second portions to the first and second optical terminals, respectively. Clause 9: A differential optical interferometer according to clause 8, wherein the light generating means comprises a first light guide arranged downstream of the beam splitter and configured to guide the first portion to the first optical terminal. Preferably, the light generating means comprises a second light guide arranged downstream of the beam splitter and configured to guide the second portion to the second optical terminal. Preferably, the optical path length of the first light guide is longer than the optical path length of the second light guide. Clause 10: A differential optical interferometer according to any one of clauses 7 to 9, wherein each of the first and second beam splitters comprises a non-polarizing beam splitter. Clause 11: A differential interferometer according to any one of clauses 7 to 10, wherein one of the detection means and the processing means is configured to determine first and second amplitudes of the first and second interference signals, respectively. Clause 12: A differential optical interferometer according to clause 11, wherein the first and second amplitudes are determined by any one of detecting zero crossings of the first and second interference signals and fitting first and second functions to the first and second interference signals, respectively. Clause 13: A differential optical interferometer according to clause 11 or 12, further comprising an adaptation means, wherein the delay corresponds to the optical path length difference, and wherein the adaptation means (134) is configured to modulate the first wavelength, the second wavelength, and the optical path length difference with an amplitude corresponding to at least half of a nominal value of one of the first and second wavelengths, preferably at least half of the amplitude of the longer of the first and second wavelengths. Clause 14: A differential optical interferometer according to clause 13, wherein the optical path length difference is preferably modulated by changing at least one of the length and refractive index by exposure to at least one of heat, shear stress, and a magnetic field. Clause 15: A differential optical interferometer according to clause 13 or 14, wherein the light generating means comprises first and second light guides configured to generate a delayed light beam and a light beam respectively, the first light guide having a longer optical path length than the second light guide, and the light generating means further comprises adaptation means comprising one of a heating element, an actuator, and an electromagnetic coil configured to modulate the optical path length of any one of the first and second light guides.

[0054] According to a fifth aspect of the present invention, the object is achieved by providing a differential spectroscopic interferometer system comprising a plurality of differential spectroscopic interferometers according to the second and fourth aspects, each of the plurality of differential spectroscopic interferometers preferably sharing a common light generating means and configured to detect the position of a target reflector in a measurement volume oriented along different non-parallel axes.

[0055] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like or similar reference numerals describe like or similar features in each of the drawings. [Brief explanation of the drawings]

[0056] [Figure 1A] 1 is a schematic side view of an embodiment of a differential interferometer according to a second aspect of the invention, configured to perform the method according to the first aspect of the invention; [Figure 1B] 1 is a schematic top view of an embodiment of a differential interferometer according to a second aspect of the invention, configured to perform the method according to the first aspect of the invention; FIG. [Figure 2] 1 is a schematic diagram illustrating an embodiment of a differential interferometer according to a fourth aspect of the invention, configured to perform the method according to the third aspect of the invention; DETAILED DESCRIPTION OF THE INVENTION

[0057] 1A and 1B, an embodiment of a differential interferometer 100 according to a second aspect of the present invention suitable for carrying out the method according to the first aspect of the present invention comprises light generating means 101 configured to generate light beam 102 and delayed light beam 103, a sensor head 104 configured to generate a first interference signal and a second interference signal, and processing means 105 configured to determine the position of a target reflector 106 along a measurement range A. Optionally, differential interferometer 100 comprises adaptation means 134 configured to modulate the optical path length difference introduced between generating light beam 102 and generating delayed light beam 103.

[0058] The light generating means 101 comprises a laser source (single frequency laser) 107, a beam splitter 108 (not shown in FIG. 1B ), a first light conductor 109, a second light conductor 110, and a first optical terminal 111 and a second optical terminal 112. The laser source 107 provides light to the beam splitter 108, which separates the light into a first portion and a second portion. The beam splitter 108 is configured to guide the first portion to the first light conductor 109 and the second portion to the second light conductor 110. The first light conductor 109 is configured to provide a delay path, and the second light conductor 110 is configured to provide a path. To achieve the delay, the delay path is configured to have a longer optical path length than the path. The first light conductor 109 comprises the first optical terminal 111, which comprises a first collimator configured to collimate the generated delayed light beam 103. The second light guide 110 comprises a second optical terminal 112 that comprises a second collimator configured to collimate the generated light beam 102 .

[0059] The first light conductor 109 and the second light conductor 110 may each comprise one or more further beam splitters 113, 114 configured to provide one or more further light rays and one or more further delayed beams to one or more further sensor heads. Such further sensor heads may be configured, for example, to determine the position of the target reflector 106 or another target reflector in the measurement range along another axis, for example, the other axis being positioned orthogonal to the axis.

[0060] The sensor head 104 comprises a first beam splitter (lateral displacement beam splitter) 115, a reflector 116, a second beam splitter 117, and detection means 118. In the event that the second beam splitter 117 is configured as a polarizing beam splitter, the sensor head 104 may further comprise a first quarter-wave plate 119 and a second quarter-wave plate 120, as well as polarization means 121 comprising one or two polarizers.

[0061] In the illustrated embodiment, first beam splitter 115 is a non-polarizing transverse displacement beam splitter. This first beam splitter receives parallel-aligned beam 102 and delayed beam 103 and separates them into first portions 122, 124 and second portions 123, 125. Thus, there are four optical signals exiting the first beam splitter: first portion 124 and second portion 125 of the beam, and first portion 122 and second portion 123 of the delayed beam. First portion 122 and second portion 123 of the delayed beam travel along first path 127 and second path 128, respectively. First portion 124 and second portion 125 of the beam travel along third path 132 and fourth path 133, respectively.

[0062] In the embodiment shown, the first and second portions of the delayed beam enter a second beam splitter 117, which is a polarizing beam splitter. Thus, the first and second portions of light of the delayed beam in the first polarization state are directed to a first quarter-wave plate 119 and a second quarter-wave plate 120, respectively, and converted to circularly polarized light.

[0063] The circularly polarized light of the first and second portions of the delayed beam are directed to the target reflector 106 and the reference reflector 126, respectively. In the embodiment shown, the target reflector 106 and the reference reflector 126 are located remotely from the sensor head 104. In an alternative embodiment, the reference reflector 126 can be realized by the sensor head 104 itself, for example, attached to the second beam splitter 117 via a second quarter-wave plate 120.

[0064] The target reflector 106 and the reference reflector 126 reflect the first and second portions of the circularly polarized light back to the first and second quarter-wave plates, respectively. The first and second quarter-wave plates change the circularly polarized light to linearly polarized light in a second polarization state, which is orthogonal to the first polarization state. This light enters the second beam splitter 117 and is reflected by the beam splitter coating towards the polarizing means 121 to form the first and second optical signals.

[0065] The first and second portions 124, 125 of the light beam are directed by the reflector 116 to the second beam splitter 117. The second beam splitter 117 allows the first and second portions 124, 125 of the light beam having a first polarization state to pass through and directs this light to the polarizing means 121, which forms the third and fourth optical signals. The sensor head is configured so that the first portion of the light beam with the first polarization state (i.e., the third optical signal) and the first portion of the light of the delayed light beam with the second polarization state (i.e., the first optical signal) overlap and reach the polarizing means 121. The sensor head is further configured so that the second portion of the light beam with the first polarization state (i.e., the fourth optical signal) and the second portion of the light of the delayed light beam with the second polarization state (i.e., the second optical signal) overlap and reach the polarizing means 121.

[0066] The polarizing means 121 is configured to generate a first interference signal (e.g., by a first polarizer of the polarizing means) based on the first and third optical signals, and the polarizing means 121 is further configured to generate a second interference signal (e.g., by a second polarizer of the polarizing means) based on the second and fourth optical signals.

[0067] The detecting means 118 is configured to detect the first and second interference patterns, for example the detecting means comprises first and second detectors (e.g. photodiodes) that detect the first and second interference patterns, respectively.

[0068] The processing means 105 is connected to the detecting means 118 and is configured to determine the position of the target reflector 106 based on the first and second interference signals.

[0069] In the embodiment shown, delay beam 103 is used to generate the first and second optical signals, and beam 102 is used to generate the third and fourth optical signals, but it is also possible to switch beams 102 and 103.

[0070] In another embodiment, the sensor head comprises a first beam splitter, a reflector, and a second beam splitter. The second beam splitter is a non-polarizing beam splitter. Therefore, there is no need to provide either first and second quarter-wave plates or polarization means comprising one or two polarizers. The advantage is that this is cheaper because fewer optical components are required and the alignment requirements for the optical components are less stringent.

[0071] 2 , an embodiment of a differential interferometer 200 according to a fourth aspect of the present invention suitable for carrying out the method according to the third aspect of the present invention comprises light generating means 201 configured to generate light beam 202 and delayed light beam 203, a sensor head 204 configured to generate a first interference signal and a second interference signal, and processing means 205 configured to determine the position of a target reflector 206 along a measurement range B. Optionally, differential interferometer 200 comprises adaptation means 234 configured to modulate the optical path length difference introduced between generating light beam 202 and generating delayed light beam 203.

[0072] The light generating means 201 comprises a laser source 207, a beam splitter 208, a first light conductor 209, a second light conductor 210, a first optical terminal 211, and a second optical terminal 212. The laser source 207 provides light to the beam splitter 208, which splits the light into a first portion and a second portion. The beam splitter 208 is configured to direct the first portion to the first light conductor 209 and the second portion to the second light conductor 210. The first light conductor 209 is configured to provide a delay path, and the second light conductor 210 is configured to provide a path. To achieve the delay, the delay path is configured to have a longer optical path length than the path. The first light conductor 209 comprises the first optical terminal 211, which comprises a first collimator configured to collimate the generated delayed light beam 203. The second light guide 210 comprises a second optical terminal 212 that comprises a second collimator configured to collimate the generated light beam 202 .

[0073] The sensor head comprises a first beam splitter 215, a second beam splitter 217, and a detection means 218 comprising a first detector 229 and a second detector 230. The first beam splitter 215 and the second beam splitter 217 are both non-polarizing beam splitters. The first beam splitter 215 and the second beam splitter 217 receive the parallel aligned delayed beam 203 and beam 202.

[0074] The first beam splitter 215 separates the delayed beam 203 into a first portion 222 and a second portion 223. The first portion 222 of the delayed beam passes through the first beam splitter 215 and is directed to the target reflector 206. The target reflector 206 reflects the first portion of the delayed beam back towards the first beam splitter 215, and the beam splitter coating of the first beam splitter 215 directs (e.g., reflects) the first portion of the delayed beam towards a first detector 229 as a first optical signal.

[0075] A second portion 223 of the delayed beam 203 is directed to a second beam splitter 217. The second portion of the delayed beam passes through the second beam splitter 217 and is directed as a second optical signal towards a second detector 230.

[0076] The second beam splitter splits the beam into a first portion 224 and a second portion 225. The first portion 224 of the beam is directed to the first beam splitter 215. The first portion 224 of the delayed beam passes through the first beam splitter 215 and is directed to the first detector 229 as a fourth optical signal.

[0077] A second portion 225 of the light beam passes through the second beam splitter 217 and is directed to the target reflector 206. The target reflector 206 reflects the second portion of the light beam back towards the second beam splitter 217, and the beam splitter coating of the second beam splitter 217 directs (e.g., reflects) the second portion of the light beam towards the second detector 230 as a third optical signal.

[0078] The first interference signal is formed by the first optical signal and the third optical signal, and the second interference signal is formed by the second optical signal and the fourth optical signal. The first detector 229 and the second detector 230 detect the first interference signal and the second interference signal, respectively. The processing means 205 connected to the first detector 229 and the second detector 230 determines the position of the target reflector 206 based on the first interference signal and the second interference signal.

[0079] For noise reduction, both the first beam splitter 215 and the second beam splitter 217 can be configured as polarizing beam splitters. In such an embodiment, the differential interferometer 200 further comprises a quarter-wave plate 219 disposed between the first beam splitter 215 and the target reflector 206 and between the second beam splitter 217 and the target reflector 206. Furthermore, a half-wave plate 231 is disposed between the first beam splitter 215 and the second beam splitter 217, and the polarizing means 221 comprises a first polarizer disposed upstream of the first detector and a second polarizer upstream of the second detector. [Explanation of symbols]

[0080] 100 Differential Interferometer 101 Light generation means 102 Rays 103 Delayed Ray 104 Sensor head 105 Processing means 106 Target reflector 107 Laser Source (Single Frequency Laser) 108 Beam Splitter 109 First light guide 110 Second light guide 111 First Optical Terminal 112 Second Optical Terminal 113 Beam Splitter 114 Beam Splitter 115 First beam splitter (lateral displacement beam splitter) 116 Reflector 117 Second beam splitter 118 Detection Methods 119 First quarter-wave plate 120 Second quarter-wave plate 121 Polarization Means 122 First Part 123 Second Part 124 First Part 125 Second Part 126 Reference reflector 127 First Route 128 Second Route 132 Third Route 133 The Fourth Route 134 Adaptation Measures 200 Differential Interferometer 201 Light generation means 202 Ray of light 203 Delayed Ray 204 Sensor head 205 Processing means 206 Target reflector 207 Laser Source 208 Beam Splitter 209 First light guide 210 Second light guide 211 First Optical Terminal 212 Second Optical Terminal 215 First beam splitter 217 Second beam splitter 218 Detection means 219 Quarter Wave Plate 221 Polarization Means 222 First Part 223 Second Part 224 First Part 225 Second Part 229 First Detector 230 Second Detector 231 Half-wave plate 234 Adaptation Measures

Claims

1. 1. A method for determining the position of a target reflector in a measurement range (A) by differential optical interferometry, comprising: generating a coherent source beam using a single frequency laser (107), said coherent source beam being modulated between a first wavelength and a second wavelength; generating a light beam (102) and a delayed light beam (103) from the coherent source light beam, the delayed light beam (103) having a delay relative to the light beam (102), and the light beam (102) being positioned parallel to the delayed light beam (103); Separating each of the light beam (102) and the delayed light beam (103) into a first portion (122, 124) and a second portion (123, 125), wherein the second portions (123, 125) are arranged parallel to the corresponding first portions (122, 124); generating a first optical signal by directing the first portion of one of the light beam (102) and the delayed light beam (103) along a first path (127) that includes the target reflector (106); generating a second optical signal by directing the second portion of the one of the beam (102) and the delayed beam (103) along a second path (128) that includes a reference reflector (126), the second path (128) being positioned parallel to the first path (127); generating a third optical signal by directing the first portion of another one of the light beam (102) and the delayed light beam (103) along a third path (132); generating a fourth optical signal by directing the second portion of the other one of the light beam (102) and the delayed light beam (103) along a fourth path (133), the fourth path (133) being arranged parallel to the third path (132); generating a first interference signal between the first optical signal and the third optical signal; generating a second interference signal between the second optical signal and the fourth optical signal; determining a position of the target reflector (106) based on the first and second interference signals; A method comprising:

2. 2. The method of claim 1, wherein the position is determined by determining quadrature phases by demodulating each of the first and second interference signals, and wherein the delay corresponds to an optical path length difference configured such that the amplitude of each demodulated first and second interference signal does not exhibit zero crossings over the measurement range (A), preferably such that the amplitude of each demodulated first and second interference signal does not vary by more than a factor of four, preferably by a factor of two.

3. 3. The method of claim 1, wherein generating the light beam and the delayed light beam from the coherent source light beam comprises splitting the coherent source light beam into a first portion and a second portion, directing the first portion along a delay path, and directing the second portion along a path, wherein the delay is created by an optical path length of the delay path being longer than an optical path length of the path.

4. 4. The method of claim 1, wherein the step of generating the first and second interference signals comprises using polarizing means (121).

5. 5. The method of claim 1, wherein the step of splitting each of the light beam and the delayed beam into the first and second portions comprises the step of introducing a second delay between the first portion and the second portion corresponding to the first portion for both the light beam and the delayed beam, wherein the second delay introduced between the first and second portions by splitting the light beam corresponds to the second delay introduced between the first and second portions by splitting the delayed beam, and preferably a lateral displacement beam splitter (115) is used to split each of the light beam and the delayed beam, and preferably a non-polarizing lateral displacement beam splitter is used.

6. 6. The method of claim 1, further comprising determining a first amplitude of the first interference signal and a second amplitude of the second interference signal, wherein the determining the position of the target reflector is further based on the first and second amplitudes.

7. 7. The method of claim 6, wherein the first and second amplitudes are determined by one of detecting zero crossings of the first and second interfering signals and fitting first and second functions to the first and second interfering signals, respectively.

8. 8. The method of claim 6 or 7, wherein the delay corresponds to a difference in optical path length, and the difference in optical path length is modulated with an amplitude corresponding to at least half of the nominal values ​​of the first wavelength, the second wavelength, and one of the first and second wavelengths, preferably at least half of the amplitude of the longer of the first and second wavelengths.

9. 9. The method of claim 8, wherein the optical path length difference is modulated by changing at least one of length and refractive index, preferably by exposure to at least one of heat, shear stress, and a magnetic field.

10. a differential optical interferometer for determining the position of a target reflector within a measurement range (B), A light generating means (101) comprising a single frequency laser (107) and configured to generate a coherent source light beam modulated between a first wavelength and a second wavelength, a light generating means (101) further comprising first and second optical terminals (111, 112) configured to manipulate the coherent source light beam to generate a delayed light beam (103) at the first optical terminal (111) and a light beam (102) at the second optical terminal (112), the delayed light beam (103) being delayed relative to the light beam (102); a sensor head (104) disposed downstream of the first and second optical terminals (111, 112), the delayed light beam (103) being arranged parallel to the light beam (102); Equipped with The sensor head (104) comprises a first beam splitter (115) and a second beam splitter (117) provided downstream of the first beam splitter (115); the first beam splitter (115) is configured to split each of the light beam (102) and the delayed light beam (103) into a first portion and a second portion; The second beam splitter (117) is configured to generate a first optical signal by directing the first portion of one of the light beam (102) and the delayed light beam (103) along a first path (127) that includes the target reflector (106), and to generate a second optical signal by directing the second portion of the one of the light beam (102) and the delayed light beam (103) along a second path (128) that includes a reference reflector (126), the second path (128) being connected to the first path (127). and a fourth path (133) arranged parallel to the third path (132), configured to generate a third optical signal by allowing passage of at least a portion of the first portion of the other one of the light beam (102) and the delayed light beam (103) along a third path (132), and configured to generate a fourth optical signal by allowing passage of at least a portion of the second portion of the other one of the light beam (102) and the delayed light beam (103) along a fourth path (133), the fourth path (133) being arranged parallel to the third path (132); the sensor head (104) is configured to generate a first interference signal between the first optical signal and the third optical signal, and to generate a second interference signal between the second optical signal and the fourth optical signal; the sensor head (104) comprising a detection means (118) configured to detect the first and second interference signals; processing means (105) configured to determine the position of the target reflector (106) based on the first and second interference signals; A differential spectroscopic interferometer comprising:

11. 11. The differential interferometer of claim 10, wherein the processing means (105) is configured to determine the position by determining first and second sets of quadrature signals by demodulating the first and second interference signals, respectively, and wherein the delayed beam (103) is delayed with respect to the beam (102) by introducing an optical path length difference configured such that the amplitude of each quadrature signal of the first and second sets of quadrature signals does not exhibit zero crossings over the measurement range (B), preferably such that the amplitude of each quadrature signal of the first and second sets of quadrature signals does not vary by more than a factor of four, preferably by a factor of two.

12. 12. The differential spectroscopic interferometer of claim 10 or 11, wherein the light generating means (101) comprises a beam splitter (108) arranged downstream of the single-frequency laser (107), the beam splitter (108) configured to split the coherent source light beam into a first part and a second part suitable for directing to the first and second optical terminals (111, 112), respectively.

13. 13. The differential spectroscopic interferometer of claim 12, wherein the light generating means (101) comprises a first light guide (109) arranged downstream of the beam splitter (108) and configured to guide the first portion to the first optical terminal (111), and preferably the light generating means (101) comprises a second light guide (110) arranged downstream of the beam splitter (108) and configured to guide the second portion to the second optical terminal (112), and preferably the optical path length of the first light guide (109) is longer than the optical path length of the second light guide (110).

14. 14. A differential spectroscopic interferometer according to claim 13, wherein the first light guide (109) comprises a first polarization-maintaining fiber and preferably the second light guide (110) comprises a second polarization-maintaining fiber.

15. 15. A differential spectroscopic interferometer according to any one of claims 10 to 14, wherein the first beam splitter (115) comprises a lateral displacement beam splitter, preferably a non-polarizing lateral displacement beam splitter.

16. 16. A differential spectroscopic interferometer according to any one of claims 10 to 15, wherein the second beam splitter (117) comprises a second polarizing beam splitter, and preferably the first path (127) and the second path (128) comprise quarter-wave plates (119, 120).

17. 17. A differential interferometer according to any one of claims 10 to 16, wherein the sensor head (104) comprises polarizing means (121) configured to generate the first and second interference signals.

18. 18. A differential interferometer according to any one of claims 10 to 17, wherein one of the detection means (118) and the processing means (105) is configured to determine first and second amplitudes of the first and second interference signals, respectively.

19. 20. The differential interferometer of claim 18, wherein the first and second amplitudes are determined by one of detecting zero crossings of the first and second interference signals and fitting first and second functions to the first and second interference signals, respectively.

20. 20. A differential spectroscopic interferometer according to claim 18 or 19, further comprising an adaptation means (134), wherein the delay corresponds to a difference in optical path length, and wherein the adaptation means (134) is configured to modulate the optical path length difference with an amplitude corresponding to at least half of the first wavelength, the second wavelength, and one of the nominal values ​​of the first and second wavelengths, preferably at least half of the amplitude of the longer of the first and second wavelengths.

21. 21. The differential interferometer of claim 20, wherein the optical path length difference is modulated by changing at least one of length and refractive index, preferably by exposure to at least one of heat, shear stress, and magnetic field.

22. 22. The differential interferometer of claim 20 or 21, wherein the light generating means (101) comprises first and second light guides (109, 110) configured to generate the delayed light beam (103) and the light beam (102), respectively, wherein an optical path length of the first light guide (109) is longer than an optical path length of the second light guide (110), and the light generating means (101) further comprises the adapting means (134) comprising one of a heating element, an actuator, and an electromagnetic coil configured to modulate the optical path length of one of the first and second light guides (109, 110).

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