FIELD

The present disclosure relates to an optical system comprising a linear actuator arrangement and an optical apparatus and a method for use in sensing a position of a movable reflector of the optical apparatus and, in particular though not exclusively, for use in sensing a position of a movable reflector of an interferometer such as a Michelson interferometer for Fourier Transform Infrared Spectroscopy (FTIR).

BACKGROUND

Typical Michelson interferometers that are utilised in FTIR spectrometers comprise a beamsplitter and two mirrors. One mirror is placed at the transmitted light port and one at the reflected light port of the interferometer. When a broadband infrared light beam hits the beamsplitter, 50% of it may be transmitted and 50% of it may be reflected to the corresponding ports of the interferometer. This creates two beams. These beams are reflected back by the mirrors, recombined by the beamsplitter and interfere at the output of the interferometer. By moving one mirror relative to the other one, a phase difference is induced between the two beams that causes modulation of the light at the output of the interferometer, with a modulation frequency proportional to the speed of the moving mirror(s) and inversely proportional to the wavelength of the light. The moving mirror may be actuated by a voice coil actuator due to its frictionless and noiseless properties, and may be mounted relative to the other components of the Michelson interferometer via a low friction bearing arrangement for reciprocal motion of the moving mirror.

In addition to the infrared beam, a second light source in the form of a laser, usually a helium-neon (HeNe) laser, is used to generate a laser beam and to introduce the laser beam into a path of the beam of infrared light so as to keep track of the position of the moving mirror. Because of the narrow spectral linewidth of the HeNe laser, the modulation of the laser light at the output of the interferometer is of sinusoid form. By measuring the zero crossings of the HeNe laser modulated signal, the position of the moving mirror can be calculated with very high accuracy. This in turn allows the averaging of consecutive scans in order to achieve measurements with high signal-to-noise ratios.

However, the introduction of the laser beam into the path of the beam of infrared light has some undesirable side effects. For example, introduction of the laser beam into the path of the beam of infrared light may require that a portion of the beamsplitter is dedicated for splitting the laser beam and cannot be used to split the beam of infrared light, thereby limiting the cross-section of the beam of infrared light that can be measured and effectively reducing the sensitivity of the FTIR spectrometer.

It is also known to use flat mirrors in an FTIR spectrometer based on a Michelson interferometer. However, flat mirrors are sensitive to tilt misalignment which can reduce the accuracy with which the position of the moving flat mirror can be detected when using an off- axis laser beam. Therefore, it is known to use a dynamic alignment mechanism on the fixed mirror of the interferometer to correct the tilt error of the moving mirror in real time. This may require the laser beam to be expanded by a beam expander and detected by a quadrature detector. Alternatively, this may require the laser beam to be split into different beams and to detect the different beams using different detectors. However, these arrangements can block a significant amount of the infrared beam resulting in a loss of infrared power which can be detrimental when performing sensitive high-resolution measurements at low infrared power levels.

Another type of infrared light loss in the interferometer can be caused by the usage of an off-axis parabolic mirror defining a hole on-axis in order to allow the laser light to enter the infrared interferometer. However, the area on the off-axis parabolic mirror where the laser light enters the interferometer cannot reflect any infrared light, also resulting in a loss of infrared power.

In addition, due to the fact that the infrared light occurs in a different spectral range when compared to the HeNe or similar lasers in the visible part to the spectrum (approx. 400- 700nm), multiple coatings are needed on the beamsplitter of the interferometer, in order to handle the visible laser and infrared light beams. This in turn complicates the design of the beamsplitter. The design of the beamsplitter becomes more challenging when the interferometer is designed to operate in the far infrared spectral region (approx. 25000- 2500000 nm) where most common beamsplitters are very thin standalone films of Mylar that cannot incorporate coatings for the laser beam. In order to overcome this limitation, it is known to dedicate separate regions of the beamsplitter to the splitting of the infrared light and the splitting of the laser light. Since the region of the beamsplitter that splits the laser light cannot split the infrared light, yet another loss occurs.

A method proposed to address this problem is the use of oversized interferometer mirrors while locating the laser beamsplitter on top and/or below the infrared light beamsplitter. The disadvantage of this configuration is the difficult alignment of the two beamsplitters while sited on the same holder. In addition, off-the-shelf low friction bearing assemblies which incorporate a glass tube and graphite bushings currently cannot accommodate optics larger than 38.1 mm (1.5”) in diameter. Furthermore, successful attempts have been realised where the flat mirrors are replaced by retroreflectors in order to remove the need of dynamic alignment of the optics as well as to improve the robustness of the interferometer by using optics that are not sensitive to tilt misalignment. However, when corner cube retroreflectors are used in the interferometer instead of flat mirrors, the laser beam is laterally displaced not only by the beamsplitter, but also by the retroreflectors. This requires the interferometer retroreflectors to be realigned when the beamsplitter is changed.

A solution for monitoring the position of the movable mirror of a FTIR spectrometer is to create a laser interferometer by arranging a movable mirror for a laser beam of the laser interferometer back-to-back with the movable mirror of the infrared interferometer such that the movable mirror of the laser interferometer and the movable mirror of the FTIR spectrometer are each offset relative to a motion axis of the movable mirrors to allow the beam of infrared light to be incident on the movable mirror of the FTIR spectrometer whilst also allowing the laser beam to be incident on the movable mirror of the laser interferometer. However, use of such an arrangement may result in tilt misalignment of the movable mirrors relative to the motion axis of the movable mirrors which can reduce the accuracy with which the position of the movable mirror of the FTIR spectrometer can be detected.

SUMMARY

According to an aspect of the present disclosure there is provided an optical system, comprising: an optical apparatus including a first movable reflector; a laser interferometer for use in sensing a position of the first movable reflector, the laser interferometer including a second movable reflector; and a linear actuator arrangement including a static portion and a movable portion, wherein the first and second movable reflectors face in different directions and are mounted co-axially on the movable portion along an axis of the linear actuator arrangement, and wherein the linear actuator arrangement is configured to move the movable portion and the first and second movable reflectors together along the axis relative to the static portion.

Use of a second movable reflector which is mounted co-axially on the movable portion of the linear actuator arrangement for sensing a position of the first movable reflector may reduce tilt misalignment of the first and second movable reflectors and may improve the accuracy with which the position of the first movable reflector can be detected using the laser interferometer. Use of a second movable reflector which is mounted co-axially on the movable portion of the linear actuator arrangement for sensing a position of the first movable reflector may also avoid any requirement for frequent alignment of optics of the optical apparatus, may avoid any need to use a beamsplitter in the optical apparatus with multiple coatings, and/or may avoid any need to use oversized optics in the optical apparatus.

Optionally, the linear actuator arrangement comprises a voice coil actuator.

Optionally, the linear actuator arrangement comprises a solenoid, an electric motor or the like.

Optionally, the linear actuator arrangement is configured to move the movable portion and the first and second movable reflectors together reciprocally along the axis relative to the static portion.

Optionally, the optical apparatus comprises an interferometer such as a Michelson interferometer.

Optionally, the optical apparatus is configured to operate at an infra-red wavelength or wavelengths, for example at a near infra-red wavelength or wavelengths.

Optionally, the optical apparatus is configured for Fourier transform spectroscopy.

Optionally, the optical apparatus comprises a Fourier transform infra-red (FTIR) spectrometer.

Optionally, the laser interferometer comprises a Michelson interferometer.

Optionally, the laser interferometer comprises a laser, a beamsplitter, a fixed reflector, and a photodetector, wherein, in use, the beamsplitter splits a laser beam emitted by the laser source into a measurement laser beam and a reference laser beam, the measurement laser beam is reflected by the second movable reflector, the reference laser beam is reflected by the fixed reflector and the beamsplitter combines the reflected measurement laser beam and the reflected reference laser beam at the photodetector so as to generate a modulated laser beam at the photodetector which varies in intensity according to the position of the second movable reflector. Optionally, the photodetector is configured to generate an electrical signal which is representative of the intensity of the modulated laser beam.

Optionally, the optical system comprises a processing resource which is configured to receive the generated electrical signal from the photodetector and to determine a position of the second movable reflector, and therefore also a position of the first movable reflector, from the generated electrical signal.

The interference of the reflected measurement laser beam and the reflected reference laser beam may be a sinusoidal pattern. The processing resource may be configured to accurately measure the zero crossings of the sinusoidal pattern and to determine the position of the second movable reflector, and therefore also the position of the first movable reflector, with extreme accuracy and precision. This may improve the reproducibility of measurements performed using the optical apparatus. For example, where the optical apparatus comprises a FTIR spectrometer, this may improve measurement reproducibility. This may in turn enable an improvement in the signal-to-noise ratio of a measured spectrum obtained by averaging the spectra measured from a plurality of scans using the FTIR spectrometer.

Optionally, the laser is monochromatic.

Optionally, the laser is configured to operate continuous wave. In that way, the motion of the first and second movable reflectors can be monitored throughout the movement of the first and second movable reflectors.

Optionally, the laser comprises a helium-neon (HeNe) laser.

There is no requirement for the laser interferometer to share any of the optics of the optical apparatus. Consequently, the laser interferometer may be used to track the motion of the first movable reflector of the optical apparatus without compromising the design of the optics of the optical apparatus thereby alleviating the afore-mentioned problems associated with prior art optical apparatus such as prior art FTIR spectrometers of frequent alignment of the optics of the prior art optical apparatus, or the need to use beamsplitters with multiple coatings and/or the need to use oversized optics.

The alignment of the laser interferometer is not altered when the alignment of the optical apparatus is changed due to e.g. change of a beamsplitter of the optical apparatus, since, as mentioned earlier, the laser interferometer and the optical apparatus do not share optical components.

Advantageously, such an optical system may reduce the losses of the optical apparatus thereby increasing the sensitivity of measurements made using the optical apparatus since the second movable reflector, the fixed reflector, the beamsplitter and the photodetector of the laser interferometer are not located in the beam path of the optical apparatus, allowing the usage of the full aperture of the optics of the optical apparatus.

In addition, use of such an optical system means that the measurement laser beam may be aligned on, or close to, the axis of the linear actuator arrangement, minimising sine errors that would otherwise occur when the measurement laser beam is aligned parallel to, but offset from, the axis of the linear actuator arrangement by a distance required to allow a beam of light to be incident on the first movable reflector whilst also allowing a laser beam of the laser interferometer to be incident on the second movable reflector of the laser interferometer.

Optionally, the first and second movable reflectors are separated axially along the axis of the linear actuator arrangement.

Optionally, at least a part of the movable portion of the linear actuator arrangement extends between the first and second movable reflectors. Optionally, the static portion of the linear actuator arrangement comprises a hollow core defining a passageway therethrough.

Optionally, the hollow core comprises a hollow magnetic core.

Optionally, the passageway extends along the axis of the linear actuator arrangement.

Optionally, the passageway is configured to allow a laser beam of the laser interferometer to propagate through the passageway to and from the second movable reflector.

Optionally, the static portion of the linear actuator arrangement comprises an opening at one end, wherein the opening provides access to the passageway. This may allow the laser beam of the laser interferometer to enter the static portion of the linear actuator arrangement and propagate through the passageway to and from the second movable reflector.

Optionally, the second movable reflector is mounted within the linear actuator arrangement.

Optionally, the movable portion of the linear actuator arrangement comprises a tubular member configured to extend around the core.

Optionally, the tubular member comprises a tubular coil member.

Optionally, the movable portion of the linear actuator arrangement is movable along the axis of the linear actuator arrangement relative to the static portion of the linear actuator arrangement across a range of positions and wherein the first and second movable reflectors remain separated axially from the core for any position within the range of positions.

Optionally, the movable portion of the linear actuator arrangement further comprises a cap across a first end of the tubular member.

Optionally, the first movable reflector is attached to a first side of the cap.

Optionally, the movable portion of the linear actuator arrangement comprises an extension member arranged co-axially with the tubular member, wherein the first movable reflector is mounted on a first end of the extension member and the extension member extends from the tubular member.

Optionally, the extension member is tubular.

Optionally, the tubular member and the extension member are unitary.

Optionally, the tubular member and the extension member are formed separately and a second end of the extension member is then attached to the first end of the tubular member.

Optionally, the static portion of the linear actuator arrangement comprises a linear actuator housing and a body, wherein the linear actuator housing is fixed relative to the body. Optionally, the linear actuator arrangement comprises a bearing arrangement which is configured to enable motion of the movable portion of the linear actuator arrangement relative to the static portion of the linear actuator arrangement along the axis.

Optionally, the bearing arrangement comprises an outer tube fixed relative to the body and one or more annular bushings mounted on an outer surface of the extension member, wherein the one or more bushings and the outer tube are configured so that an outer surface of each bushing engages an inner surface of the outer tube for the reduction of friction therebetween during motion of the extension member inside the outer tube along the axis.

Optionally, one or more of the bushings comprises, or is formed from, graphite and/or wherein the outer tube comprises, or is formed from, glass.

Optionally, the extension member comprises one or more lift magnets and one or more skates, races or tracks, wherein each lift magnet is slidable axially along a corresponding skate, race or track.

Optionally, the static portion of the linear actuator arrangement comprises a guide bar secured to the body and extending along the outer tube externally to the outer tube, wherein the one or more lift magnets and the guide bar are configured so that each lift magnet exerts a corresponding attractive force on the extension member towards the guide bar.

Optionally, a position and/or orientation of the guide bar relative to the body is adjustable, for example by means of one or more screws.

Optionally, the bearing arrangement comprises an outer tube fixed relative to the body, wherein the outer tube has one or more air holes formed through a wall thereof for the delivery of pressurized air into an annulus between an inner surface of the outer tube and an outer surface of the extension member so as to reduce friction therebetween during motion of the extension member inside the outer tube along the axis of the linear actuator arrangement.

Optionally, the bearing arrangement comprises one or more flexure bearings or flexure springs which moveably attach the extension member to the body so as to enable motion of the movable portion of the linear actuator arrangement relative to the body along the axis of the linear actuator arrangement.

Optionally, the second movable reflector is attached to a second side of the cap so that light can propagate through the passageway defined by the core to the second movable reflector and the second movable reflector can reflect the light back through the passageway.

Optionally, the movable portion of the linear actuator arrangement comprises a core such as a magnetic core mounted on a shaft and the static portion of the linear actuator arrangement comprises a tubular member such as a tubular coil member configured to extend around the core. Optionally, the linear actuator arrangement further comprises a housing and one or more bearings which are configured to enable movement of the shaft relative to the housing along the axis of the linear actuator arrangement.

Optionally, the first movable reflector is connected to a first end of the shaft and the second movable reflector is connected to a second end of the shaft.

Optionally, the first and second movable reflectors face in opposite directions.

Optionally, the first movable reflector defines a first optical axis and the second movable reflector defines a second optical axis.

Optionally, the first and second optical axes are parallel.

Optionally, the first movable reflector has a greater dimension transverse to the axis than the second movable reflector.

Optionally, the first movable reflector has a greater diameter transverse to the axis than the second movable reflector.

Optionally, the first movable reflector comprises a retroreflector such as a solid retroreflector prism or a hollow corner retroreflector.

Optionally, the first movable reflector comprises a movable mirror such as a movable plane mirror.

Optionally, the second movable reflector comprises a retroreflector such as a solid retroreflector prism or a hollow corner retroreflector.

Optionally, the second movable reflector comprises a movable mirror such as a movable plane mirror.

According to an aspect of the present disclosure there is provided a linear actuator arrangement for use in sensing a position of a movable reflector of an optical apparatus such as a FTIR spectrometer, the linear actuator arrangement comprising: a static portion; and a movable portion including a first attachment region or site for attaching a rear side of the movable reflector of the optical apparatus and a second attachment region or site for attaching a rear side of a movable reflector of a laser interferometer, the first and second attachment regions or sites facing in different directions and being arranged co-axially along an axis of the linear actuator arrangement, wherein the linear actuator arrangement is configured to move the movable portion along the axis relative to the static portion.

Optionally, the linear actuator arrangement comprises a voice coil actuator.

Optionally, the linear actuator arrangement comprises a solenoid, an electric motor or the like. Optionally, the linear actuator arrangement is configured to move the movable portion reciprocally along the axis relative to the static portion.

Optionally, the first and second attachment regions or sites are separated axially along the axis.

Optionally, at least a part of the movable portion of the linear actuator arrangement extends between the first and second attachment regions or sites.

Optionally, the static portion of the linear actuator arrangement comprises a hollow core defining a passageway therethrough.

Optionally, the hollow core comprises a hollow magnetic core.

Optionally, the passageway extends along the axis of the linear actuator arrangement.

Optionally, the passageway is configured to allow a laser beam of the laser interferometer to propagate through the passageway to and from the second attachment region or site.

Optionally, the static portion of the linear actuator arrangement comprises an opening at one end, wherein the opening provides access to the passageway. This may allow the laser beam of the laser interferometer to enter the static portion of the linear actuator arrangement and propagate through the passageway to and from the second attachment region or site.

Optionally, the movable portion of the linear actuator arrangement comprises a tubular member configured to extend around the core.

Optionally, the tubular member comprises a tubular coil member.

Optionally, the movable portion of the linear actuator arrangement is movable along the axis relative to the static portion of the linear actuator arrangement across a range of positions and wherein the first and second attachment regions or sites remain separated axially from the core for any position within the range of positions.

Optionally, the movable portion of the linear actuator arrangement further comprises a cap across a first end of the tubular member.

Optionally, the first attachment region or site is located on a first side of the cap.

Optionally, the movable portion of the linear actuator arrangement comprises an extension member arranged co-axially with the tubular member, wherein the first attachment region or site is located at a first end of the extension member and the extension member extends from the tubular member.

Optionally, the extension member is tubular.

Optionally, the tubular member and the extension member are unitary.

Optionally, the tubular member and the extension member are formed separately and the second end of the extension member is then attached to a first end of the tubular member. Optionally, the static portion of the linear actuator arrangement comprises a linear actuator housing and a body, wherein the linear actuator housing is fixed relative to the body.

Optionally, the linear actuator arrangement comprises a bearing arrangement which is configured to enable motion of the movable portion of the linear actuator arrangement relative to the static portion of the linear actuator arrangement along the axis.

Optionally, the bearing arrangement comprises an outer tube fixed relative to the body and one or more annular bushings mounted on an outer surface of the extension member, wherein the one or more bushings and the outer tube are configured so that an outer surface of each bushing engages an inner surface of the outer tube for the reduction of friction therebetween during motion of the extension member inside the outer tube along the axis of the linear actuator arrangement.

Optionally, the one or more of the bushings comprises, or is formed from, graphite Optionally, the outer tube comprises, or is formed from, glass.

Optionally, the extension member comprises one or more lift magnets and one or more skates, races or tracks, wherein each lift magnet is slidable axially along a corresponding skate, race or track.

Optionally, the static portion of the linear actuator arrangement comprises a guide bar secured to the body and extending along the outer tube externally to the outer tube, and wherein the one or more lift magnets and the guide bar are configured so that each lift magnet exerts a corresponding attractive force on the extension member towards the guide bar.

Optionally, a position and/or orientation of the guide bar relative to the body is adjustable, for example by means of one or more screws.

Optionally, the bearing arrangement comprises an outer tube fixed relative to the body, wherein the outer tube has one or more air holes formed through a wall thereof for the delivery of pressurized air into an annulus between an inner surface of the outer tube and an outer surface of the extension member so as to reduce friction therebetween during motion of the extension member inside the outer tube along the axis of the linear actuator arrangement.

Optionally, the bearing arrangement comprises one or more flexure bearings or flexure springs which moveably attach the extension member to the body so as to enable motion of the movable portion of the linear actuator arrangement relative to the body along the axis of the linear actuator arrangement.

Optionally, the second attachment region or site is located on a second side of the cap so that light can propagate through the passageway defined by the core towards and away from the second attachment region or site.

Optionally, the movable portion of the linear actuator arrangement comprises a core such as a magnetic core mounted on a shaft and the static portion of the linear actuator arrangement comprises a tubular member such as a tubular coil member configured to extend around the core.

Optionally, the linear actuator arrangement further comprises a housing and one or more bearings which are configured to enable movement of the shaft relative to the housing reciprocally along the axis of the linear actuator arrangement.

Optionally, the first attachment region or site is located at a first end of the shaft and the second attachment region or site is located at a second end of the shaft.

Optionally, the first and second attachment regions or sites face in opposite directions.

Optionally, the first attachment region or site defines a first axis and the second attachment region or site defines a second axis.

Optionally, the first and second axes are parallel.

Optionally, the movable reflector of the optical apparatus has a greater dimension transverse to the axis than the movable reflector of the laser interferometer, and the first and second attachment regions or sites are configured accordingly.

Optionally, the movable reflector of the optical apparatus has a greater diameter transverse to the axis than the movable reflector of the laser interferometer and the first and second attachment regions or sites are configured accordingly.

Optionally, the movable reflector of the optical apparatus comprises a retroreflector such as a solid retroreflector prism or a hollow corner retroreflector, and the first attachment region or site is configured accordingly.

Optionally, the movable reflector of the optical apparatus comprises a mirror such as a plane mirror, and the first attachment region or site is configured accordingly.

Optionally, the movable reflector of the laser interferometer comprises a retroreflector such as a solid retroreflector prism or a hollow corner retroreflector, and the second attachment region or site is configured accordingly.

Optionally, the movable reflector of the laser interferometer comprises a mirror such as a plane mirror, and the second attachment region or site is configured accordingly.

Optionally, the linear actuator arrangement comprises the movable reflector of the optical apparatus, wherein the rear side of the movable reflector of the optical apparatus is attached to the first attachment region or site.

Optionally, the linear actuator arrangement comprises the movable reflector of the laser interferometer, wherein the rear side of the movable reflector of the laser interferometer is attached to the second attachment region or site.

Optionally, the first and second movable reflectors face in opposite directions.

Optionally, the first movable reflector defines a first optical axis and the second movable reflector defines a second optical axis. Optionally, the first and second optical axes are parallel.

According to an aspect of the present disclosure there is provided a laser interferometer, comprising the linear actuator arrangement as described above.

Optionally, the laser interferometer comprises a Michelson interferometer.

Optionally, the laser interferometer comprises a laser, a beamsplitter, a fixed reflector, and a photodetector, wherein, in use, the beamsplitter splits a laser beam emitted by the laser source into a measurement laser beam and a reference laser beam, the measurement laser beam is reflected by the movable reflector, the reference laser beam is reflected by the fixed reflector and the beamsplitter combines the reflected measurement laser beam and the reflected reference laser beam at the photodetector so as to generate a modulated laser beam at the photodetector which varies in intensity according to the position of the second movable reflector.

Optionally, the photodetector is configured to generate an electrical signal which is representative of the intensity of the modulated laser beam.

Optionally, the laser interferometer comprises a processing resource which is configured to receive the generated electrical signal from the photodetector and to determine a position of the movable reflector from the generated electrical signal.

The interference of the reflected measurement laser beam and the reflected reference laser beam may be a sinusoidal pattern. The processing resource may be configured to accurately measure the zero crossings of the sinusoidal pattern and to determine the position of the movable reflector with extreme accuracy and precision.

Optionally, the laser is monochromatic.

Optionally, the laser is configured to operate continuous wave. In that way, the motion of the movable reflector of the laser interferometer, and therefore also the movable reflector of the optical apparatus, can be monitored throughout the movement of the movable reflector of the laser interferometer and the movable reflector of the optical apparatus.

Optionally, the laser comprises a HeNe laser.

Optionally, the laser interferometer comprises beam steering optics to allow alignment of the laser beam with the axis of the linear actuator arrangement.

Optionally, the beam steering optics comprise at least a pair of flat mirrors to allow alignment of the laser beam with the axis of the linear actuator arrangement.

Optionally, the beamsplitter comprises a beamsplitting cube. Use of a beamsplitting cube may avoid any lateral displacement between the measurement and reference laser beams that would otherwise occur if a plate beamsplitter were used.

Optionally, the beamsplitter comprises a plate beamsplitter. Optionally, the movable reflector of the laser interferometer comprises a solid retroreflector prism. Such a movable reflector may be advantageous because solid retroreflector prisms may be of a size and/or shape which enables it to be inserted into and/or mounted within a tubular member such as a tubular coil member of a voice coil actuator.

Optionally, the movable reflector of the laser interferometer comprises a hollow corner retroreflector.

According to an aspect of the present disclosure there is provided an optical apparatus, comprising the linear actuator arrangement as described above.

Optionally, the optical apparatus comprises an interferometer such as a Michelson interferometer.

Optionally, the optical apparatus is configured to operate at an infra-red wavelength or wavelengths, for example at a near infra-red wavelength or wavelengths.

Optionally, the optical apparatus is configured for Fourier transform spectroscopy.

Optionally, the optical apparatus comprises a Fourier transform infra-red (FTIR) spectrometer.

According to an aspect of the present disclosure there is provided a method for sensing a position of a first movable reflector of an optical apparatus such as a FTIR spectrometer, the method comprising: using a laser interferometer to sense a position of the first movable reflector, wherein the laser interferometer includes a second movable reflector mounted on a movable portion of a linear actuator arrangement, which linear actuator arrangement includes the movable portion and a static portion, wherein the first and second movable reflectors face in different directions and are mounted on the movable portion co-axially along an axis of the linear actuator arrangement, and wherein the linear actuator arrangement is configured to move the movable portion and the first and second movable reflectors together reciprocally along the axis relative to the static portion.

It should be understood that any one or more of the features of any one of the foregoing aspects of the present disclosure may be combined with any one or more of the features of any of the other foregoing aspects of the present disclosure.

BRIEF DESCIRPTION OF THE DRAWINGS

A linear actuator arrangement, a laser interferometer, a FTIR spectrometer and an optical system will now be described by way of non-limiting example only with reference to the drawings of which: FIG. 1 is a plan view of an optical system including a FTIR spectrometer;

FIG. 2 is a plan view of a laser interferometer and a linear stage assembly of the optical system of FIG. 1 for use in sensing a position of a movable reflector of the FTIR spectrometer;

FIG. 3 is a perspective view including a partial section of a voice coil actuator of the linear stage assembly shown in FIG. 2 showing a movable retroreflector prism of the laser interferometer;

FIG. 4 is a perspective view of a movable optics assembly of the linear stage assembly shown in FIG. 2 showing a movable retroreflector of the FTIR spectrometer mounted at one end of the movable optics assembly;

FIG. 5 is a perspective view of the linear stage assembly shown in FIG. 2;

FIG. 6 is a perspective view of a partial longitudinal section of the linear stage assembly shown in FIGS. 2 and 5;

FIG. 7 is a plot of the modulation of optical intensity at a photodetector of the laser interferometer shown in FIG. 2 highlighting the basic features used to determine a position of the movable reflector of the laser interferometer;

FIG. 8 is a perspective view of a first alternative linear stage assembly showing a movable retroreflector of the FTIR spectrometer mounted at one end of a movable optics assembly; and

FIG. 9 is a perspective view of part of a second alternative linear stage assembly.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is made initially to FIG. 3, which illustrates a linear actuator in the form of a hollow voice coil actuator 26 for use in sensing a position of a movable reflector of an optical apparatus such as a FTIR spectrometer (not shown in FIG. 3). The voice coil actuator 26 comprises an adapter ring 27 for attachment of the voice coil actuator 26 to an intended location as will be described in more detail below. Within the adaptor ring 27, the housing 30 is held. Coaxially to the housing 30, a stack of permanent ring magnets 31 is affixed. A tubular voice coil 29 is movable within the gap that exists between the housing 30 and the stack of ring magnets 31 along an axis of the voice coil actuator 26. The tubular voice coil 29 includes a cap located at one end of the tubular voice coil 29. A movable retroreflector in the form of a corner cube prism 28 is axially mounted on a rear side of the cap, for example using an adhesive such as an epoxy. The voice coil 29 may be less than 38.1 mm in diameter. The movable retroreflector 28 is lightweight and small enough to fit into the voice coil actuator 26 as shown in FIG. 3.

FIG. 2 shows a laser interferometer which includes a laser in the form of a HeNe laser 10, a primary steering mirror assembly 11 , a secondary steering mirror assembly 13, a cube beamsplitter assembly 25, a fixed retroreflector 32, the movable retroreflector 28, and a position reference photodetector 16. As will be described in more detail below, the movable retroreflector 28 is provided as part of a linear stage assembly 9 which includes the voice coil actuator 26.

A HeNe laser may be used when the laser interferometer is used to sense the position of a movable reflector of a FTIR spectrometer with high resolution requirements (< 0.5 cm-1). In general, the laser beam diameter needs to be smaller than the diameter of the corner cube prism 28 as well as the diameter of the hollow magnetic core 31 , preferably in the range of 0.5 to 2 mm to ease alignment and avoid beam clipping. The photodetector 16 needs to be fast enough to detect the modulated signal of the laser and have a comparable diameter with that of the laser beam.

In use, the laser 10 emits a coherent beam 12 which is directed towards the cube beamsplitter assembly 25 via the primary steering mirror assembly 11 and the secondary steering mirror assembly 13. When the coherent beam 12 meets the beamsplitter assembly 25, the beamsplitter assembly 25 splits the coherent beam 12 into two beams, the measurement or tracking beam 14 and the reference beam 17. The reference beam travels towards the fixed retroreflector 32 and is returned towards the beamsplitter assembly 25. The measurement or tracking beam 14 is directed towards the movable retroreflector 28 and the voice coil actuator 26.

When the voice coil 29 is actuated, the voice coil 29 translates linearly in the direction J1 , as marked in FIGS. 2 and 3 depending on the direction of the current flow through the winding of the voice coil 29. As the tracking laser beam 14 enters the voice coil actuator 26 via an aperture in the housing 30 and passes through the stack of ring magnets 31 , the tracking laser beam 14 hits the retroreflector 28 and is returned towards the direction of its entry. Upon the return of the reference beam 17 and the tracking beam 14 to the beamsplitter assembly 25, the reference beam 17 and tracking beam 14 are recombined into a new coherent beam 15 and interfere. Due to the path length difference between the reference beam 17 and the tracking beam 14, beam 15 demonstrates fringing, which in turn causes the amount of light reaching the position reference photodetector 16 to fluctuate in a sinusoidal pattern. The zero crossings of the sinusoidal pattern can repeatedly determine the position of the actuator 26 with high precision and accuracy. FIG. 7 shows such an amplitude plot 20. The horizontal axis 56 represents the total distance travelled by the retroreflector 28. The vertical axis 53 represents the amplitude of light reaching the photodetector 16. Here the values are given at a relative scale from 0 to 1. The value 0 represents no light reaching the photodetector 16 as result of destructive interference and total cancellation of the beams 14 and 17 while value 1 represents the maximum amount of light reaching the photodetector 16 as a result of constructive interference of the beams 14 and 17. Points 55 are minima and maxima of the sinusoidal function, between which the A wavelength 57 of the laser can be subdivided into smaller displacement distances 54, enabling the tracking of the retroreflector 28.

Referring now to FIG. 4, there is shown a movable optics assembly 41 which includes the movable retroreflector 28 of the laser interferometer (not shown in FIG. 4), a movable retroreflector 39 of an optical apparatus which may, for example, be an interferometer and/or a FTIR spectrometer and a movable portion of a linear actuator arrangement, which movable portion includes the movable voice coil 29 of the voice coil actuator 26. The movable portion of the linear actuator arrangement further includes an extension member in the form of a hollow tube 40, a back graphite bushing 33, a lower lift magnet 34 and the lower lift magnet skate 35. The back graphite bushing 33, the lower lift magnet 34 and the lower lift magnet skate 35 are attached to the hollow tube 40. The movable portion of the linear actuator arrangement further includes a front graphite bushing 38, an upper lift magnet 37 and an upper lift magnet skate 36. The front graphite bushing 38, the upper lift magnet 37 and the upper lift magnet skate 36 are attached towards a front of the hollow tube 40. The movable retroreflector 39 of the optical apparatus is attached co-axially to one end of the hollow tube 40.

The moving optics assembly 41 is part of the larger linear stage assembly 9 as shown in FIGS. 5 and 6. The linear stage assembly 9 comprises the moving optics assembly 41 and a static portion of the linear actuator arrangement, which static portion includes a front foot 45, a main body 44, and a back foot 43. The static portion further includes a glass tube 46 attached within the main body 44. Above the glass tube 46, a cover 47 is affixed. Through the cover 47, two adjustment screws 48, 49 suspend a guide bar 42. The voice coil actuator 26 is mounted co-axially with the moving optics assembly 41. The static portion of the linear actuator arrangement further includes the adaptor ring 27, the housing 30, and the stack of ring magnets 31 of the voice coil actuator 26. The adaptor ring 27 connects the housing 30 of the voice coil actuator 26 to the glass tube 46 so that the adaptor ring 27, the housing 30 of the glass tube 46 are all fixed relative to the main body 44. The interaction of the various features of the linear stage assembly 9 is shown in FIG. 6. The moving optics assembly 41 is lifted by the lower lift magnet 34 and the upper lift magnet 37 towards the guide bar 42. The adjustment screws 48, 49 allow a position of the guide bar 42 to be adjusted in such a way so as to balance the weight of the voice coil 29 and retroreflector 28 and the weight of the moving optics assembly 41. In addition, the interaction between the guide bar 42 and the lower lift magnet 34 and the upper lift magnet 37 stabilises the moving optics assembly 41 by preventing rotation of the moving optics assembly 41 about the axis of linear actuator arrangement along which the moving optics assembly 41 moves reciprocally as defined by the direction J1 shown in FIGS. 2, 3 and 6.

FIG. 1 shows an optical system comprising the laser interferometer, the linear stage assembly 9 and an optical apparatus in the form of a FTIR spectrometer. The optical system comprises a base plate 1. The FTIR spectrometer comprises a heat source 2 attached to the base plate 1 . The FTIR spectrometer comprises an off-axis parabolic (GAP) mirror assembly 3 attached to the base plate 1 in front of the heat source 2. The FTIR spectrometer comprises another OAP mirror assembly 4 attached to the base plate 1 in axial alignment with the OAP mirror assembly 3. The FTIR spectrometer comprises yet another OAP mirror assembly 6 attached to the base plate 1. The FTIR spectrometer comprises a circular aperture 50 attached to the base plate 1 and located between the OAP mirror assembly 4 and the OAP mirror assembly 6. The FTIR spectrometer comprises a beamsplitter 7 attached to the base plate 1 and located axially with respect to the OAP mirror assembly 6. The FTIR spectrometer further comprises a fixed retroreflector assembly 8 attached to the base plate 1 and located at a position which is located above the beamsplitter 7 in the plan view of FIG. 1 . The linear stage assembly 9 is attached to the base plate 1 at a position shown to the left of the beamsplitter 7 in the plan view of FIG. 1. As previously described, the linear stage assembly 9 includes a movable retroreflector 39 of the FTIR spectrometer. The FTIR spectrometer comprises an OAP mirror assembly 24 attached to the base plate 1 at a position shown below the beamsplitter 7 in the plan view of FIG. 1. The FTIR spectrometer comprises another OAP mirror assembly 21 attached to the base plate 1 at a position opposite to the OAP mirror assembly 24. The FTIR spectrometer comprises an OAP assembly 19 attached to the base plate 1 at a position located axially with respect to the OAP mirror assembly 21. The FTIR spectrometer comprises a detector assembly 22 attached to the base plate 1 at a position opposite to the OAP assembly 19.

In use, the heat source 2 emits rays 5 that travel from OAP assembly 3 to OAP assembly 4 via the aperture 50 towards the OAP assembly 6 towards the beamsplitter 7. At the beamsplitter 7, the beam is split into beam 51 and beam 52. After beam 51 is reflected from the fixed retroreflector of the fixed retroreflector assembly 8 and beam 52 is reflected from the movable retroreflector 39 of the linear stage assembly 9, the reflected beams are recombined to form beam 23, which demonstrates fringing due to interference as the assembly 41 moves in the direction J1. As beam 23 travels from OAP assembly 24 to OAP assembly 21 and OAP assembly 19, it reaches the detector 22. The position of the movable optics assembly 41 within the linear stage assembly 9 is tracked via the interference of the tracking laser beam 14 and reference laser beam 17 when recombined via the beamsplitter assembly 25 into laser beam 15 which then is directed towards the position reference photodetector 16.

Referring now to FIG. 8, there is shown a first alternative linear stage assembly 55. Here the voice coil actuator 26 is attached to a frame 58. The connecting tube 40 is attached at one end to the voice coil actuator 26 while it is suspended between a back flexure spring 59 and a front flexure spring 60. The movable retroreflector 39 is attached co-axially to one end of the connecting tube 40. The activation of the voice coil 29 of the voice coil actuator 26 forces the connecting tube 40 forward. The flexure mechanism constructed by the two flexure springs 59, 60 allows only one degree of freedom resulting in reciprocal motion in the direction J1 as shown in FIG. 8. The flexure springs 59, 60 can be constructed as discrete items or additively manufactured as part of the frame 58.

Referring now to FIG. 9, there is shown part of a second alternative linear stage assembly which comprises a voice coil actuator 126, a movable reflector of the FTIR spectrometer in the form of a first movable plane mirror 139, and a movable reflector of the laser interferometer in the form of a second movable plane mirror 128. The voice coil actuator 126 comprises a static voice coil 129, a housing 130, an axially movable permanent magnet 131 mounted on an axially movable shaft 170 and flexure springs or flexure bearings 133, 138 at either end of the housing 130, which bearings 133, 138 support the movable shaft 170 whilst enabling axial motion of the movable permanent magnet 131 and the movable shaft 170 relative to the housing 130 and the static voice coil 129. The first movable plane mirror 139 is attached to a first end of the movable shaft 170 and the second movable plane mirror 128 is attached to a second end of the movable shaft 170. In use, an alternating electrical current is driven through the static voice coil 129 causing the movable permanent magnet 131 and the movable shaft 170 to reciprocate axially relative to the housing 130 and the static voice coil 129 along an axis of the voice coil actuator 126. In a variant of the second alternative linear stage assembly of FIG. 9, one or both of the movable plane mirrors 139, 128 may be replaced by a retroreflector such as a solid retroreflector prism or a hollow corner retroreflector.

In a third alternative linear stage assembly (not shown), a low friction solution can be realised by replacing the outer glass tube 46 of the linear stage assembly 9 described with reference to FIGS. 3 to 6 with an outer tube having the ability to deliver pressurised air between an outer surface of the hollow tube 40, an inner surface of the outer tube, and the graphite bushings 33, 38 to provide an air bearing assembly.

One of ordinary skill in the art will also understand that various modifications are possible to any of the systems and methods described above. For example, rather than using a HeNe laser, the laser interferometer may use a diode laser with a smaller coherence length for a spectrometer with low resolution requirements, preferably in the visible region (400- 700nm of wavelength) to aid alignment.

A plate beamsplitter could be used as the beamsplitter of the laser interferometer in place of the cube beamsplitter but a cube beamsplitter is preferred due minimal lateral displacements effects on the laser beam which can aid alignment and initial placement of the fixed optics.

In some of the embodiments described above, the fixed and movable reflectors are solid prism retroreflectors. In a variant of these embodiments, one or more of the retroreflectors may be replaced by a hollow retroreflector or by a mirror such as a plane mirror.

Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives to the described embodiments in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiment, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. In particular, one of ordinary skill in the art will understand that one or more of the features of the embodiments of the present disclosure described above with reference to the drawings may produce effects or provide advantages when used in isolation from one or more of the other features of the embodiments of the present disclosure and that different combinations of the features are possible other than the specific combinations of the features of the embodiments of the present disclosure described above.

The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

Use of the term "comprising" when used in relation to a feature of an embodiment of the present disclosure does not exclude other features or steps. Use of the term "a" or "an" when used in relation to a feature of an embodiment of the present disclosure does not exclude the possibility that the embodiment may include a plurality of such features.

The drawings are not necessarily to scale, nor or all items shown in full detail. Furthermore, at least some of the features may be shown exaggerated or may be omitted altogether to aid clarity.

The use of reference signs in the claims should not be construed as limiting the scope of the claims.