GB2590352A

GB2590352A - Compact laser spectroscopy

Info

Publication number: GB2590352A
Application number: GB1916446.6A
Authority: GB
Inventors: Hackermuller Lucia; Cooper Nathan; Coles Laurence
Original assignee: Added Scient Ltd; University of Nottingham
Current assignee: Added Scient Ltd; University of Nottingham
Priority date: 2019-11-12
Filing date: 2019-11-12
Publication date: 2021-06-30
Also published as: GB201916446D0

Abstract

A device (100) for performing saturated spectroscopy and laser frequency locking comprises a frame (105). A vapour cell (141) and a mirror ( I 5 I) are supported by the frame (105) . A first optical coupler ( 111) is supported by the frame (I 05) and is configured to couple light from a first laser onto a first optical path ( IOI). A second optical coupler (112) supported by the frame (105) and configured to couple light from a second laser onto a second optical path ( 102). The first optical path (101) is combined with the second optical path (102), and is directed through the vapour cell (141) into a first pass to saturate absorption in a region of the vapour cell ( 14 I). The combined optical path (101, 102) reflects from the mirror (151) and makes a second pass through the region of the vapour cell (141) and subsequently exits the vapour cell (141). The combined optical path (101,102) is split on exiting the vapour cell (141) into a first component corresponding with light from the first optical coupler ( 111) and a second component corresponding with light from the second optical coupler (112). The first component is directed to a first photosensor ( 121) and a second component to the second photosensor (122).

Description

COMPACT LASER SPECTROSCOPY

The present invention relates to laser spectroscopy apparatus for stabilisation of lasers, and to methods applicable to laser spectroscopy laser frequency stabilisation.

Laser spectroscopy of atomic samples in vapour cells (e.g. saturation laser spectroscopy') is an important technique for many applications including sensing and metrology. Another important application is in the production of spectroscopic signals used to stabilise lasers. Such frequency stabilised lasers may be used in a magneto-optical trap (MOT), which is an essential component for many quantum technologies based on ultra-cold atoms MOTs arc dev cos that usc the technique of a laser cooling in conjunction with magnetic fields to cool and trap atoms or molecules down to very low temperatures.

IS Typically, this involves addressing several atomic transitions using several highly stable laser frequencies. Laser spectroscopy may be carried out in such experiments to provide spectroscopic signals required to stabilise the laser frequencies to correspond with particular atomic transitions. Presently, large custom-built experimental set-ups are used to provide the spectroscopic signals. Existing devices for laser spectroscopy are bulky and expensive.

The present invention aims to at least ameliorate at least some of these problems.

According to a first aspect of the invention, there is provided a device for performing saturated laser spectroscopy and laser frequency locking, comprising: a frame, a vapour cell supported by the frame; a mirror supported by the frame adjacent to the vapour cell; a first optical coupler supported by the frame and configured to couple light from a first laser onto a first optical path within the frame; a second optical coupler supported by the frame and configured to couple light from a second laser onto a second optical path within the frame; and Preston. Daryl W. "Doppler-free saturated absorption Laser spectroscopy." American Journal of Physics 64.11 (1996): 1432-1436.

a first photosensor and a second photosensor; wherein the device is configured to: combine the first optical path with the second optical path, direct the combined optical path through the vapour cell in a first pass to saturate absorption in a region of the vapour cell, the combined optical path reflecting from the mirror and making a second pass through the region of the vapour cell and subsequently exiting the vapour cell; split the combined optical path exiting the vapour cell into a first component corresponding with light from the first optical coupler and a second component corresponding with light from the second optical coupler; direct the first component to the first photosensor and the second component to the second photosensor.

The device may further comprise: a first beam splitter, a second beam splitter and a third beam splitter; wherein: the first beam splitter is configured to combine the first and second optical path, the second beam splitter is configured to direct a portion of the combined first and second optical path into the vapour cell and to direct a portion of light exiting the vapour cell toward the third beam splitter; the third beam splitter is a polarising beam splitter, configured to split the light exiting the vapour cell into the first component and the second component; and the first photosensor is configured to receive the first component and the second photosensor is configured to receive the second component.

The first beam splitter may be a polarising beam splitter, configured to cause the first laser in the combined optical path to have a different polarisation than the second laser in the combined optical path.

The device may further comprise: a third optical coupler supported by the frame and configured to couple light from a third laser onto a third optical path within the frame; a third photosensor; wherein the device is configured to: combine a portion of light from the first optical coupler with the third optical path; direct the combined portion of light from the first optical coupler and the third optical path to the third photosensor, the third photosensor thereby operable to produce a beat signal for offset stabilisation of the third laser relative to the first laser.

The device may further comprise: a fourth beam splitter configured to split the portion of light from the first optical coupler from the first optical path; and a fifth beam splitter configured to combine the portion of light from the first optical coupler with the third optical path.

The path length of the third optical path from the third optical coupler to the third may be less than 50mm.

According to a second aspect, there is provided a device for performing saturated laser spectroscopy and laser frequency locking, comprising: a frame; a vapour cell supported by the frame; a mirror supported by the frame adjacent to the vapour cell; a first optical coupler supported by the frame and configured to couple light from a first laser onto a first optical path within the frame; a second optical coupler supported by the frame and configured to couple light from a second laser onto a second optical path within the frame; a first photosensor and a second photosensor; wherein the spectrometer is configured to: direct the first optical path through the vapour cell in a first pass to saturate absorption in a region of the vapour cell, the first optical path reflecting from the mirror, making a second pass through the region of the vapour cell and subsequently exiting the vapour cell; direct the first optical path to the first photosensor, the first photosensor thereby operable to produce a saturated laser spectroscopy signal for laser frequency 35 locking direct a portion of light from the first optical coupler onto the second optical path and subsequently direct the combined light from both the first and second optical couplers to the second photosensor, the second photosensor thereby operable to produce a beat signal for offset locking of laser frequency.

The device may further comprise a first beam splitter and a second beam splitter; wherein: the first beam splitter is configured to direct a portion of the light from the first optical coupler into the vapour cell and to direct a portion of light exiting the vapour cell to the first photosensor, the second beam splitter is configured to combine a portion of the light from the first optical coupler with light from the second optical coupler and to direct the combined light from the first and second optical couplers to the second photodetector.

The device may further comprise a third beam splitter configured to combine a portion of the light from the first optical coupler with light from the third optical coupler and to direct the combined light from the first and third optical couplers to the third photodetector Each of the beam splitters (first, second, third, etc) may be cube beam splitters.

The frame may comprises a respective recess for housing and aligning each of the beam splitters.

Each of the beam splitters may be configured to have a push fit with its respective recess.

Each recess may comprise at least one sidewall with a non-contacting central region that is configured to avoid contact with the respective beam splitter during fitting of the respective beam splitter into the recess, and at least one non-central contacting sidewall region that contacts with and aligns the respective beam splitter.

Each recess may comprise non-contacting sidewall corners configured to provide relief between the beam splitter and the respective recess when the beam splitter is disposed in the recess.

The device may further comprise an adhesive material retaining each beam splitter in its respective recess.

The frame may comprise a vapour cell recess for housing and aligning the vapour cell, and a mirror recess for housing and aligning the mirror.

The frame may comprise a first through hole in a perimeter wall for receiving and aligning the first optical coupler and a second through hole in a perimeter wall for receiving and aligning the second optical coupler.

At least one of the recesses and/or through holes may provide a non-adjustable alignment arrangement.

Each of the recesses may comprise a through hole with an axis perpendicular to a plane defined by the first and second optical path. The device may further comprise a plate with a flat reference surface. The frame may be secured to the reference surface, and each of the first, second and third beam splitters may be in contact with the reference surface A path length of the first optical path from the first optical coupler to the first photosensor may be less than 300mm.

The frame may comprise a unitary body.

The frame may comprise a polymeric material, and/or the frame may be a product of an additive manufacturing process.

According to a third aspect, there is provided a method of producing a device for frequency stabilisation of at least two lasers, comprising: forming a unitary frame by an additive manufacturing process, the frame comprising: a plurality of recesses for housing and aligning respective optical components of the device; fitting each of: beam splitters, a vapour cell and a mirror into respective recesses in the frame, wherein each of the beam splitters, vapour cell and mirror are aligned by contact with sidewalls of the respective recess; securing a first and second photodiode to the frame; disposing a first optical coupler and a second optical coupler in respective apertures in the frame perimeter, the first optical coupler configured to couple light from a first laser onto a first optical path within the frame, and the second optical coupler configured to coupled light from the second laser onto a second optical path within the frame.

Disposing the first optical coupler and the second optical coupler in their respective apertures may comprise aligning the first optical coupler to maximise a signal at the first photosensor, and aligning the second optical coupler to maximise a signal at the second photosensor.

According to a fourth aspect, there is provided a method of producing signals for laser stabilisation using a device according to the first aspect or the second aspect.

Each feature of each aspect may be combined with the features of any other embodiment, including optional features. The method of the third aspect may be used to produce a device according to either the first or second aspect, including any of the optional features thereof.

Non-limiting example embodiments will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic of a device for producing two spectroscopic laser stabilisation signals and one offset locking signal; Figure 2 is a detail view of a recess of the device of Figure 1; Figure 3 is a graph showing a typical spectroscopy signal obtained from the device of Figure 1; Figure 4 is an example beat signal for offset locking produced by a device according to an embodiment; Figure 5 is an schematic of a device for producing two spectroscopy stabilisation signals from the same vapour cell and two offset locking signals; and Figure 6 is a schematic of a device for producing one spectroscopy stabilisation signal and two offset locking signals.

Referring to Figure 1, a device 100 is shown for stabilising three laser frequencies. The device 100 comprises a frame 105; first, second and third optical coupler Ill, 112, 113; first, second and third photosensor 121, 122, 123; first, second, third, fourth and fifth beam splitters 131, 132, 133, 134, 135; vapour cell 141; mirror 151 and lens 161.

The device 100 of figure 1 is configured to stabilise three lasers. The device 100 is configured to provide a saturated laser spectroscopy signal for a first and second laser from a single vapour cell, and to provide an offset locking signal from a beat signal produced by combining the third laser with the first laser. In other embodiments (as will be described more fully below), the device may be configured to provide signals for stabilising two or three or more lasers in either of these two ways. For example, a device may be configured to perform saturation laser spectroscopy on a first laser, and to provide offset locking signals for a second and third laser from the first laser.

S

The frame 105 is substantially planar (with a flat lower face and a flat upper face) and comprises recesses 106 for housing and aligning each of the optical components of the device 100. The recesses are separated by walls and optical paths between each recess are provided by through holes 174 in the appropriate sidewalls of the frame 105.

The frame 105 may be formed by an additive manufacturing process, for example from a polymer material (e.g. by additive layer manufacturing of a thermosetting polymer such as ABS). Additive manufacturing enables the frame 105 to be produced economically and in a single unitary body, which may be difficult or expensive to achieve with conventional subtractive machining techniques. The frame 105 in this embodiment is combined with a plate (not shown) that provides a datum face at the bottom of the recesses. Each component placed in a recess may also be in contact with the plate to accurately locate it in the out-of-plane direction.

Each of the recesses 106 for the beam splitters 131-135 is substantially identical in this embodiment, but this is not essential to the invention. The shape and geometry of the recess 106 will correspond with the component to be housed and aligned by the respective recess 106, in this embodiment, each of the first to fifth beam splitters 131-135 are cube splitters having the same shape and size Figure 2 shows the recess 106 that houses and aligns the fourth beam splitter 134 in more detail (the recesses for the other beam splitters are substantially similar). The recess 106 is a substantially square through hole in the frame 105 with an axis normal to the plane of the frame 105. Each corner of the recess 106 has a circular relief feature 171. This makes the design more suitable for additive manufacturing, because it relaxes the precision that would otherwise be required in the corner regions of the recesses 106 to avoid interference with the relevant beam splitter 131-135.

Each sidewall of the recesses 106 for the beam splitters 131-135 further comprises a non-contacting central region 173 that is configured not to come into contact with the respective beam splitter 131-135 during insertion or removal or the beam splitter 131135 from the recess 106. This avoids any potential for scratching or contamination of optical surfaces of the beam splitters 131-135 which may interact with light beams in the device 100.

A non-contacting region 173 may be provided for each through hole 174 between recesses. Each sidewall of the recess 106 further comprises at least one contacting region 172, which may conveniently be located to either side of the central non- :, contacting region 173 (although a single contacting region on each sidewall is also adequate to locate and align a part in such a recess 106, since three points of contact generally being sufficient to locate a part).

The frame 105 further comprises a recess for housing and aligning the vapour cell 141, the mirror 151 and the lens 161. Through holes in sidcwalls of the frame 105 are provided to support and align each of the first, second and third optical coupler 111, 112, 113 and the first, second and third photosensors 121, 122, 123.

Each of the optical components (e.g. beam splitter, vapour cell, photosensor, optical coupler etc) located in a recess or through hole may be secured by adhesive, for example at the corners of the component or recess 106.

The beam splitters 131-135 may each be cube beam splitters. With the exception of the mirror 151, the optical path of the lasers within the device is exclusively defined by the beam splitters 131-135. This makes optical alignment and layout very straightforward and cost effective, because the beam splitters 131-135 are simply slotted into fixed alignment slots. The beam splitters and other components may be laid out on a grid pattern, which simplifies the design and the alignment.

The optical path of the first laser 101 within the device starts at the first optical coupler III. The first laser 101 is split at the fourth beam splitter 134 into a first optical path (for producing a saturation laser spectroscopy signal at the first photosensor) and the remaining portion of the first laser 101 is directed to the third photosensor 123 in combination with the third laser 103 for producing a beat signal for offset locking of the third laser 103 to the first laser 101.

The first optical path is directed by the fourth beam splitter 134 to the first beam splitter 131, which combines the first optical path with a second optical path of the second laser 102 (thereby producing a combined optical path). Preferably, the first beam splitter 131 is a polarising beam splitter, which results in the first laser in 101 in the combined optical path having a different polarisation state that the second laser 102 in the combined optical path. This facilitates separation of the components of the combined optical path after passing through the vapour cell 141 into a component corresponding with the first laser 101 and a component corresponding with the second laser 102.

In other embodiments, the first and second lasers may already have different polarisation states, and the first beam splitter 131 in this case may be a non-polarising beam splitter.

The combined optical path is directed into the vapour cell 141 via the second beam splitter 132. The combined optical path passes through the vapour cell 141, and is reflected back along the same path by the mirror 151. The vapour cell 141 may comprise a rubidium vapour cell, and may have a length of between 25 and 100mm, for example 50mm. The mirror 151 may comprise a dielectric mirror. The mirror 151 may have a reflectance of at least 98% over a range of wavelengths from 700nm to 1300nm.

The first pass through the vapour cell 141 serves as a pump beam (saturating absorption in a region of the vapour cell) and the second pass serves as a probe beam (for producing the saturated laser spectroscopy signal). On exiting the vapour cell, the combined optical path is directed by the second beam splitter 132 to the third beam splitter 133.

The third beam splitter 133 is a polarising beam splitter, and separates the first laser 101 from the second laser 102. The first laser 101 is directed by the third beam splitter 133 to the first photosensor 121, and the second laser 102 is directed by the third beam splitter 133 to the second photoscnsor 122.

The first and second photosensors 121, 122 may be photodiodes, and may comprise a sensor area of at least 0.5mm diameter, preferably of lmm in diameter. A relatively large sensor area in combination with a very short path length results in a relaxed alignment tolerance for the various beam splitters and the first and second optical coupler.

The applicant has found that relatively low cost additive manufacturing techniques provide sufficient precision in the frame to align the beam splitters well, with the result that the only optical alignment required is of the first and second optical couplers 111, 112. The angular alignment tolerance for the first and second optical couplers 111, 112 is relatively relaxed, and may be at least 3 milliradians (for example, 4 or 5 milliradians). This level of angular alignment can be achieved by hand adjustment of the optical couplers 111, 112 to optimise signal strength at the photosensors 121, 122 before gluing the optical couplers II I, 112 in place.

The total path length of the first optical path of the first laser 101 from the optical coupler 111 to the first photosensor 121 may be less than 300mm, for example about 240mm. The total path length of the second optical path (of the second laser 102 from the second coupler 112 to the second photoscnsor 122) may also be less than 300mm, for example about 190mm.

The remaining portion of the first laser 101 that is not sent along the first optical path by the fourth beam splitter 134 is combined with the third laser 103 from the third light coupler 113 at the fifth beam splitter 135. The combined first laser 101 and third laser 103 are communicated via the lens 161 to the third photosensor 123. The frame 105 comprises a rail or tube along which the lens 161 can be slid in order to adjust the position of the focal plane of the lens 161 relative to the third photosensor 123. Once the appropriate position has been found, the lens 161 is fixed into place (e.g. using glue). The lens 161 may comprise an aspheric lens (e.g. with a focal length of llmm and a NA of 0.25).

The lens 161 converts beam angle variations reaching the lens 161 to positional variations at the third photosensor 123. The nominal spot size for the lens 161 at focus will typically be very small, on the order of a few microns. If the lens 161 is arranged with its focal plane coincident with the third photosensor 133, the angular tolerance for beam entering the lens 161 would be very stringent (e.g. significantly less than 1 milliradian). For coherence at the detector, the angular tolerance is given by the optical wavelength divided by the spot size, which may be very large (e.g. half a radian). However, this is the requirement on the light actually hitting the third photosensor 123. The stringency of the requirement on the light emitted from the first and third fibre coupler 111, 113 is increased by a factor of the ratio of the beam path length to the lens focal length.

The spot size can be increased (for instance up to the initial beam size at the optical :s coupler) by deliberately placing the third photosensor 123 outside the focal plane of the lens 161. An appropriate choice of focal length and spot size can therefore balance the stringency of the coherence and position based requirements in order to minimise the overall sensitivity of the system to beam misalignment (taking account that the angular tolerance for the first optical coupler 111 will be the lower of that defined by the saturation spectroscopy path and the offset locking path).

The applicant has found that a focal length of 0.1 to 0.3 times the average path length for the offset locking path is a good compromise. For the specific path lengths mentioned below (of 60mm and 40mm) a lens focal length of between 9mm and 13mm (e.g. I lmns) has been found to be preferable. The spot diameter at the third photodiode is preferably between 20 and 80 micrometers, or more preferably between 30 and 50 micrometers (e.g. 40 micrometers). The angular tolerance may thereby be at least 3 milliradians (e.g. within 50% of' that defined for the saturation spectroscopy beam paths, or more preferably within 30%).

The path length for both the first laser 101 and the third laser 103 for offset locking (i.e. respectively from the first coupler 111 to the third photosensor 123 and from the third coupler 113 to the third photosensor 123) are preferably kept as short as possible, to enable a beam spot to be produced that is phase coherent at the third photodiode so as to produce the beat signal without the need for very precise alignment e.g. of the first and third beam coupler. The optical path length from the first coupler to the third photosensor may be less than 100mm or less than 75mm, for example 60mm. The optical path length from the third coupler 113 to the third photosensor 123 may be less than 100mm or less than 50mm, for example 40mm.

The photosensors for spectrometry (in the embodiment of figure I, the first and second photosensors 121, 122) may be standard photodiodes (e.g with a lOns rise time). The photosensor for offset locking (in the embodiment of figure 1, the third photosensor 123) may be a fast photodiode (e.g. with a rise time of 50ps or less), to facilitate production of the beat signal required for offset locking.

Figure 3 shows an example spectroscopy signal 201 obtained from the second photosensor 122 (a signal from the first photosensor 121 would look similar) according to the example of figure 1. The F=2 transitions for Rb85 and Rb87 are clearly present in the signal, as is the F=3 transition for Rb85. A suitable controller (such as are already known in the art, for example as taught by Udem2) may be used to lock a laser to either of the example transitions shown based on this sort of signal. Other vapours (e.g. other alkali metal vapours) may be used in the vapour cell (rubidium is not essential), and other transitions may also be used.

Figure 4 shows an example beat signal 203 obtained from the third photosensor. This signal can be used to lock a laser (e.g. as taught by Puentes3) at a selected offset from the first laser (which in turn can be locked to an atomic transition via saturation spectroscopy).

The embodiment described above is both compact, with the frame 106 having external dimensions of only 112mm by 66mm by 13mm, but provides signals for stabilisation of three lasers, two of which are spectroscopy signals that enable locking to a specific atomic transition, and one of which has an adjustable offset. Such a system is a significant advance, and enables more compact and lower cost systems comprising magneto optical traps (among other things).

Thanks to the fixed and straightforward alignment of all the optical dements, and the very short path lengths, the device provides high stability against environmental disturbances, such as vibration or temperature fluctuations. Traditional (adjustable) optical mounts are replaced by a unitary structure that can be produced very economically by additive manufacturing.

Although stabilisation of three lasers may be particularly useful in some contexts, some applications may require only two frequency stabilised lasers. In this context, T. Udem, J. Reichert, T. W. Haensch and NI.Kourogi:"Absolute optical frequency measurement of the cesium D2 line-, PRA 62, 031801 (2000).

G. Puentes, "Laser frequency offset locking scheme for high-field imaging of cold atoms" Appl. Phys. B 107. 11-16 (2012) either the second optical path (stabilising the second laser by saturation spectroscopy) or the third optical path (stabilising the third laser by offset locking) may be omitted An embodiment without the offset locking can be achieved by omission (from the design of figure 1) of the fourth beam splitter 134, fifth beam splitter 135, lens 161, third photosensor 123 and third optical coupler 113. It is thereby straightforward to make the device 100 smaller (since the column of components to the left hand side of the vapour cell 141 have been omitted).

An embodiment with additional offset locking can be achieved by addition of a further column of components, as shown in figure 5 (i.e. the dotted box being the additional column). A further portion of the first laser can combined with a fourth laser from a fourth optical coupler 114 at a sixth beam splitter 136. The combined fourth laser and first laser light can then be combined to produce a beat signal at a fourth photodiode 124, via a second lens 162 (as described with reference to the offset locking of the third laser). The other features of this embodiment are as described with reference to figure 1 (e.g. the frame and the optical paths for stabilisation of the first, second and third beams work as described with reference to that embodiment).

In some embodiments, a single saturated laser spectroscopy signal may be produced, with at least one further laser (e.g. 1, 2 or more) lasers stabilised by offset locking with the laser that is stabilised by spectroscopy.

Figure 6 illustrates an example embodiment in which one laser is stabilised by spectroscopy, and two lasers are stabilised by offset locking with the spectroscopy stabilised laser. The frame in this example is not shown in figure 6 for simplicity, but the way that the elements are aligned and supported in this embodiment is as described with reference to figure 1 This example comprises first, second and third optical couplers 111, 112, 113; first second and third beam splitters 131, 132, 133, first second and third photodetectors 121, 122, 123; a vapour cell 141 and a mirror 151.

The first laser is directed from the first optical coupler 111 on a first pass through the vapour cell 141 via the first beam splitter 131, and is reflected by the mirror 151 to return in a second pass along the same path through the vapour cell 141. The first beam splitter 131 directs the light leaving the vapour cell 141 from the second pass to the first photosensor, which produces a saturated laser spectroscopy signal for locking the first laser.

The second and third beam splitter 132, 133 are each arranged to split a portion of the light from the first optical coupler 111 before it reaches the first beam splitter 131. The second and third laser are respectively directed from the second and third optical couplers 112, 113 to the second and third beam splitters 132, 133 for combination with the first laser at the respective beam splitters 132, 133. The combined first laser and second laser are directed via the first lens 161 to the second photosensor 122. The combined first and third laser are directed via the second lens 162 to the third photoscnsor 123. Both the second and third photosensor 122, 123 thereby produce beat signals for offset locking of the second and third lasers.

This embodiment does not require polarisation splitting, and may be also be compact, in common with the embodiment of figure 1. Although it may require two fast photosensors, it also requires fewer optical components than the example of figure 1.

An arrangement for stabilising two lasers can be produced by omitting the third optical coupler 113, third beam splitter 133 second lens 162 and third photosensor 123.

Although specific example embodiments have been described, other variations are possible, and the scope of the invention should be determined with reference to the appended claims.

Claims

CLAIMSA device for performing saturated laser spectroscopy and laser frequency locking, comprising: a frame; a vapour cell supported by the frame; a mirror supported by the frame adjacent to the vapour cell; a first optical coupler supported by the frame and configured to couple light from a first laser onto a first optical path within the frame; a second optical coupler supported by the frame and configured to couple light from a second laser onto a second optical path within the frame; and a first photosensor and a second photosensor; wherein the device is configured to: combine the first optical path with the second optical path, IS direct the combined optical path through the vapour cell in a first pass to saturate absorption in a region of the vapour cell, the combined optical path reflecting from the mirror and making a second pass through the region of the vapour cell and subsequently exiting the vapour cell; split the combined optical path exiting the vapour cell into a first component corresponding with light from the first optical coupler and a second component corresponding with light from the second optical coupler; direct the first component to the first photosensor and the second component to the second photosensor.
2. The device of claim 1, further comprising: a first beam splitter, a second beam splitter and a third beam splitter; wherein: the first beam splitter s configured to combine the first and second optical path; the second beam splitter is configured to direct a portion of the combined first and second optical path into the vapour cell and to direct a portion of light exiting the vapour cell toward the third beam splitter; the third beam splitter is a polarising beam splitter, configured to split the light exiting the vapour cell into the first component and the second component; the first photosensor is configured to receive the first component and the second photosensor is configured to receive the second component.
3. The device of claim 2, wherein the first beam splitter is a polarising beam splitter, configured to cause the first laser in the combined optical path to have a different polarisation than the second laser in the combined optical path.
The device of any preceding claim, further comprising: a third optical coupler supported by the frame and configured to couple light from a third laser onto a third optical path within the frame, a third photosensor; wherein the device is configured to: combine a portion of light from the first optical coupler with the third optical path; direct the combined portion of light from the first optical coupler and the third optical path to the third photosensor, the third photosensor thereby operable to produce a beat signal for offset stabilisation of the third laser relative to the first laser.
The device of claim 4, further comprising: a fourth beam splitter configured to split the portion of light from the first optical coupler from the first optical path; and a fifth beam splitter configured to combine the portion of light from the first optical coupler with the third optical path.
6. The device claim 4 or 5, wherein the path length of the third optical path from the third optical coupler to the third photosensor is less than 50mm.
7. A device for performing saturated laser spectroscopy and laser frequency locking, comprising: a frame; a vapour cell supported by the frame; a mirror supported by the frame adjacent to the vapour cell; a first optical coupler supported by the frame and configured to couple light from a first laser onto a first optical path within the frame, a second optical coupler supported by the frame and configured to couple light from a second laser onto a second optical path within the frame, a first photosensor and a second photosensor; wherein the spectrometer is configured to: direct the first optical path through the vapour cell in a first pass to saturate absorption in a region of the vapour cell, the first optical path reflecting from the mirror, making a second pass through the region of the vapour cell and subsequently exiting the vapour cell; direct the first optical path to the first photosensor, the first photosensor thereby operable to produce a saturated laser spectroscopy signal for laser frequency locking; direct a portion of light from the first optical coupler onto the second optical path and subsequently direct the combined light from both the first and second optical couplers to the second photosensor, the second photosensor thereby operable to produce a beat signal for offset locking of laser frequency.
8. The device of claim 7, further comprising: a first beam splitter, and a second beam splitter and a third beam splitter; wherein: the first beam splitter is configured to direct a portion of the light from the first optical coupler into the vapour cell and to direct a portion of light exiting the vapour cell to the first photosensor; the second beam splitter is configured to combine a portion of the light from the first optical coupler with light from the second optical coupler and to direct the combined light from the first and second optical couplers to the second photodetector The device of claim 7 or 8, further comprising: a third optical coupler supported by the frame and configured to couple light from a third laser onto a third optical path within the frame; a third photosensor; wherein the device is configured to: combine a portion of light from the first optical coupler with the third optical path; direct the combined portion of light from the first optical coupler and the third optical path to the third photosensor, the third photosensor thereby operable to produce a beat signal for offset stabilisation of the third laser relative to the first laser.10. The device of claim 9, further comprising a third beam splitter configured to combine a portion of the light from the first optical coupler with light from the third optical coupler and to direct the combined light from the first and third optical couplers to the third photodetector.11. The device of any preceding claim wherein each of the beam splitters are cube beam splitters.12. The device of any preceding claim, wherein the frame comprises a respective recess for housing and aligning each of the beam splitters.13 The device of claim 12, wherein each of the beam splitters is configured to have a push fit with its respective recess.14. The device of claim 12 or 13, wherein each recess comprises at least one sidewall with a non-contacting central region that is configured to avoid contact with the respective beam splitter during fitting of the respective beam splitter into the recess, and at least one non-central contacting sidewall region that contacts with and aligns the respective beam splitter.IS. The device of claim 14, wherein each recess comprises non-contacting sidewall corners configured to provide relief between the beam splitter and the respective recess when the beam splitter is disposed in the recess.16. The device of any of claims 12 to 15, further comprising an adhesive material retaining each beam splitter in its respective recess.17. The device of any preceding claim, wherein the frame comprises a vapour cell recess for housing and aligning the vapour cell, and a mirror recess for housing and aligning the mirror.18. The device of any preceding claim, wherein the frame comprises a first through hole in a perimeter wall for receiving and aligning the first optical coupler and a second through hole in a perimeter wall for receiving and aligning the second optical coupler.19. The device of any of claims 12 to 18, wherein at least one of the recesses and/or through holes provides a non-adjustable alignment arrangement.20. The device of any of claims 12 to 19, wherein each of the recesses comprises a through hole with an axis perpendicular to a plane defined by the first and second optical path, and the device further comprises a plate with a flat reference surface, wherein the frame is secured to the reference surface, and each of the first second and third beam splitters are in contact with the reference surface.21. The device of any preceding claim, wherein a path length of the first optical path from the first optical coupler to the first photosensor is less than 300mm.22. The device of any preceding claim, wherein the frame comprises a unitary body.23. The device of any preceding claim, wherein the frame comprises a polymeric material, and/or the frame is a product of an additive manufacturing process.24. A method of producing a device for frequency stabilisation of at least two lasers comprising: forming a unitary frame by an additive manufacturing process, the frame comprising: a plurality of recesses for housing and aligning respective optical components of the device: fitting each of: beam splitters, a vapour cell and a mirror into respective recesses in the frame, wherein each of the beam splitters, vapour cell and mirror are aligned by contact with sidewalls of the respective recess; securing a first and second photodiodc to the frame; disposing a first optical coupler and a second optical coupler in respective apertures in the frame perimeter, the first optical coupler configured to couple light from a first laser onto a first optical path within the frame, and the second optical coupler configured to coupled light from the second laser onto a second optical path within the frame.25. The method of claim 24, wherein disposing the first optical coupler and the second optical coupler in their respective apertures comprises aligning the first optical coupler to maximise a signal at the first photosensor, and aligning the second optical coupler to maximise a signal at the second photosensor.