WO2014049358A1

WO2014049358A1 - Optical cell

Info

Publication number: WO2014049358A1
Application number: PCT/GB2013/052513
Authority: WO
Inventors: Andrew Borthwick; Ian Mcrae; Gerald Byrne
Original assignee: Thales Holdings Uk Plc
Priority date: 2012-09-28
Filing date: 2013-09-26
Publication date: 2014-04-03
Also published as: GB2507721A; GB201217406D0

Abstract

An optical cell and systems such as lasers, optical amplifiers, interferometers, spectrometers, delay lines comprising said optical cell are described. The optical cell comprises at least one telescope and is configured such that light provided to the optical cell makes at least one transit through the telescope in both a first or forward direction and a second or reverse direction opposite to the first or forward direction before exiting from the cell. In embodiments, the telescope is a folded telescope (10), the folded telescope comprising a first retro-reflector (15), and at least one positive optically powered surface (20); and the optical cell further comprises at least one counter reflector (30), wherein the first retro-reflector and the at least one counter reflector are configured to define a cavity or chamber therebetween; at least one input (40) for allowing light to be input to the cavity; and at least one output (45) for allowing light to exit from the cavity.

Description

Optical Cell

The present invention relates to an optical cell, such as but not limited to a resonator or cavity. For example, the optical cell may be for use in optical apparatus such as lasers, optical amplifiers, interferometers, spectrometers, delay lines and the like. The present invention also comprises apparatus comprising an optical cell and methods of operating an optical cell.

Background

Optical cells, such as resonators and optical cavities, confine light within the cell for a number of transits of the cell before the light exits the cell though an output mechanism. Such optical cells are useful in many applications for achieving very long path lengths through a medium in the cell. For example, in lasers, an optical gain medium may be placed in the optical cell and the pump light confined in the cell may make a number of transits through the gain medium, pumping the gain medium in the process. Similarly, in optical amplifiers, a gain medium may be provided in the cell, and the confined light may make a number of passes through the gain medium, resulting in an increase in intensity of light at the output of the amplifier relative to the light provided at an input. In various spectroscopic methods and apparatus, multiple transits of a sample that has been placed in the optical cell by a light beam may advantageously improve an output signal.

While a basic optical cell arrangement comprises opposing mirrors, various improved cell designs have been proposed.

One example is the White cell, as described in "J U White, "Long Optical Paths of Large Aperture", Journal of the Optical Society of America, 32 (5):285". The White cell comprises three spherical concave mirrors having the same radius of curvature. One of the mirrors (M1 ) is configured to receive light from an input or source and another of the mirrors (M2) is configured to reflect light to an output. Both of these mirrors (M1 and M2) oppose the third mirror (M3). In particular, the mirrors (M1 and M2) and are configured to reflect light to, and receive light reflected from, the third mirror (M3). In this way, light from the input or source is reflected back and forth between the three mirrors M1 , M2 and M3 before being eventually being reflected to the output. Some adjustment of the number of transits may be achieved by rotating one or more of the mirrors. Another example of an optical cell is the Herriott cell, as described in "D Herriott et. al. "Folded Optical Delay Lines", Applied Optics 4 (8):883-89Γ. This cell comprises a pair of opposed spherical mirrors, wherein at least one aperture is provided in one or both of the mirrors in order to allow input and output light beams to respectively enter and leave the cavity defined between the opposed mirrors. The number of transits made by a light beam between the opposed mirrors may be controlled by adjusting the mirror's focal length, separation and input angle of the beam.

However, the prior art cell configurations may suffer from a variety of problems, such as tight tolerances on the alignment of the optical elements, excessive losses on reflection and/or transmission losses limiting the total number of transits, poor fill factors of the total cell volume, delicate optical components (particularly for certain wavelengths, such as UV) and the formation of focal points within the cell (which may be disadvantageous in explosive environments).

It is at least one object of at least one embodiment of the present invention to provide an improved optical cell. It is at least one object of at least one embodiment of the present invention to eliminate or mitigate at least one problem with the prior art. Statements of Invention

According to a first aspect of the present invention is an optical cell comprising:

at least one telescope;

wherein the optical cell is configured such that light provided to the optical cell makes at least one transit through the telescope in both a first or forward direction and a second or reverse direction opposite to the first or forward direction before exiting from the cell.

The optical cell may be configured such that light makes a plurality of transits through the telescope in both the first or forward direction and the second or reverse direction before exiting from the cell.

The telescope may comprise a folded telescope. The optical cell may comprise a first retro- reflector. The telescope may comprise at least one positive optically powered surface. The optical cell may be provided with one or more inputs and one or more outputs. The optical cell may comprise at least one counter reflector, wherein the first retro-reflector and the at least one counter reflector are configured to define a cavity or chamber therebetween. According to a second aspect of the present invention is an optical cell comprising:

a folded telescope, the folded telescope comprising:

a first retro-reflector; and

at least one positive optically powered surface;

the optical cell further comprising:

at least one counter reflector;

wherein the first retro-reflector and at least one counter reflector are configured to define a cavity or chamber therebetween; and

at least one input for allowing light to be input to the cavity; and at least one output for allowing light to exit from the cavity.

The optical cell may comprise or be comprised in an optical cell described above in relation to the first aspect and/or may individually and separably or in combination comprise one or more feature described in relation thereto. The folded telescope may comprise at least one positive optically powered surface, such as a convex surface. The at least one positive optically powered surface may be provided proximate, adjacent, on or comprised in the retro-reflector. The at least one positive optically powered surface may be provided on or be formed by the input face of the retro-reflector, for example, the positive optically powered surface may be integral with the first retro-reflector. The input face of the first retro-reflector may be a face of the retro-reflector that receives light from the input and/or counter reflector.

As is known in the art, a retro-reflector reflects light back at the same angle but opposite direction to incident light over a range of incident angles, i.e. the radiation is reflected back along a direction of travel that is parallel but opposite to that of the incident radiation over a range of incident angles. In a particular example, the first retro-reflector may comprise a corner cube retro-reflector. The retro-reflector may be arranged to be off-axis, i.e. the beams are not received or reflected along the axis of symmetry of the retro-reflector, for at least some and optionally each transit of the cell by the light beam.

The first retro-reflector and/or the counter reflector may comprise refractive optical elements, e.g. they may rely on total internal reflection at one or more internal surfaces rather than reflection from a mirrored surface. One or more faces of the first retro-reflector and/or counter reflector may be provided with an anti-reflective coating.

The optical cell may be configured to receive at least one off-axis optical beam at the at least one input. The input(s) and/or output(s) may be provided with an input and/or output surface, wherein the input and/or output surface may preferably comprise a negative optically powered surface, such as a concave surface, or optionally a positive optically powered surface, such as a convex surface. The input surface may be a surface that expands or diverges an incident beam before it reaches the first retro-reflector and/or at least one positive optically powered surface. For example, if the input surface is a positive optically powered surface, then the focal length of the positive optically powered surface may be suitably less than the distance between the input(s) and the first retro-reflector such that it achieves the required divergence/expansion of the beam. At least one input and at least one output may be integral, i.e. the same structure functions as both an input and an output. At least one of the inputs and/or at least one of the outputs may be formed in the counter reflector. The at least one counter reflector may comprise a retro-reflector, such as a corner cube. For example, in the specific example where the counter reflector is a corner cube retro-reflector, the apex of the corner cube may be replaced by or formed into the optically powered input and/or output surface, such as a concave or convex surface, which may be useable as both an input and an output.

The optical cell may be configured such that the optical beam from the input is incident on the first retro-reflector, e.g. via the positive optically powered surface. In this way, the optical beam may be reflected back by the first retro-reflector towards the counter reflector, i.e. it is folded back on itself. The positive optically powered surface may act to converge the beam.

The optical cell may be configured such that the initial divergence of the beam by the input surface and/or the convergence of the beam by the positive optically powered surface is such that the retro-reflected / folded beam is reflected to a point on the counter reflector other than the input and/or the output for at least an initial transit of the cell. In other words, the beam may initially miss the input(s) and output(s) and may then be further reflected by the counter reflector back towards the positive optically powered surface and the first retro- reflector.

This reverse pass or transit back from the counter reflector passes through the same optical components as the previous pass or transit but in reverse. In addition, since a retro- reflector is used, the retro-reflected beam is always parallel to the input beam, for at least a range of input angles. In this way, any error due to misalignment that occurs in a pass or transit through the cell may be undone by the next (reverse) pass or transit, which may effectively result in a self-aligning system for symmetrical modes.

The optical cell may be configured to support a primary mode and/or higher order modes, where a primary mode describes a sinusoidal mode structure of path length ½ a wavelength. The optical cell may be configured to support odd and even modes, defined by the number of transitions of the cell. The optical cell may be configured to support helical modes by providing a tangential component to the input beam angle. The optical cell may be configured to support symmetrical modes, which may allow the beam to be incident on an output, which may be after a plurality of passes or transits through the cell. Use of higher order modes, that is path lengths greater than ½ a wavelength, may result in at least one focal point in the optical cell. In such cases, the optical cell may be configured such that the focal point(s) are provided toward, proximate or adjacent the first retro-reflector and away from the counter reflector. It will be appreciated that use of higher order modes may result in a more compact cell but may require the formation of focal points, which may be disadvantageous in certain applications, such as those involving explosive environments. At least one of the inputs and/or outputs may be provided on a centreline / axis of symmetry of the optical cell. At least one additional reflector, such as a roof prism or corner cube reflector, may be provided at one or more of the outputs (e.g. a first output). The additional reflector(s) may be configured to reflect light received from the first retro-reflector at a corresponding (first) output to re-enter the cell via at least one other (second) input, which may optionally be an off-axis input, i.e. an input not provided on the optical axis or axis of symmetry of the optical cell. Alternatively, the at least one other (second) input may be an on-axis input, e.g. providing a second output/input on another side of the cell, such as an opposite side to the first output. Optionally, at least one of the outputs (such as a second output) may be provided at or in the first retro-reflector, such as on an optical axis or axis of symmetry of the optical cell, e.g. at an apex of the first retro-reflector. These output(s) may comprise a flat or collimated output. In this way, at least one output may be separated from the inputs, e.g. to allow improved separation of a hot light source at an input from heat sensitive detectors at an output. A light source for providing light to the input may comprise an annular ring light source or be adapted to illuminate an annular ring and/or the input may be configured to receive light from an annular ring light source. The telescope may be configured to be convergent / to have residual convergence.

The optical cell may comprise a pump reflector. The pump reflector may comprise a retro- reflector, such as a corner cube. The pump reflector may oppose, face and/or be configured to receive light from at least one of the outputs of the folded telescope. For example, the pump reflector may be provided on an opposite side of the counter reflector to the first retro- reflector. The optical cell may be adapted such that the input beam is provided or providable at a specific tangential angle to the input(s) and/or the centreline or axis of symmetry of the cell. In this way, the residual convergence of the telescope may be operable as a virtual positively powered lens sandwiched between the pump reflector and the first retro-reflector. Equally, this could be achieved with an afocal telescope and a specifical positive optically powered surface between the pump reflector and the afocal telescope. Such an optical cell may be operable to set up a helical mode, such that the output beam received at the output may be spatially rotated relative to the input beam, which may self index multiple serially connected modes within the telescope. This may advantageously provide an increased number of passes and/or improved fill factor, which may be achieved with the provision of only one extra optical component, i.e. the pump reflector.

The telescope may be athermalised, for example, by using athermalised mountings. At least one and preferably each optical component, such as the first retro-reflector and/or the counter reflector and/or the pump reflector and/or the input / output surface and/or the positive optically powered surface, may be achromatised, for example by comprising a doublet component such as a component formed from at least two parts, wherein at least one of the parts has an optical property such as index of refraction, that is complimentary to the optical property of at least one other part of the optical component so as to result in an achromatised optical component.

According to a third aspect of the present invention is an optical system comprising:

an optical cell according to the first and/or second aspect;

a light source configured to provide light to at least one input of the optical cell; and/or a receiver or detector configured to receive light from at least one output of the optical cell.

The optical system may comprise or be comprised in a spectrometer. The spectrometer may comprise a slit positioned between an output of the optical cell and the detector or receiver. The slit may be provided in a plane relative to an optical mode of the optical cell least sensitive to a preferential misalignment. For example, if the slit were square to the axis of symmetry of the cell, the optical system may minimise the effects of changes in separation between the first retro-reflector and/or the counter reflector and/or the pump reflector.

The optical cell may comprise a sample chamber between the first retro-reflector and the counter reflector, such that a sample may be receivable within the sample chamber.

The optical system may comprise or be comprised in a laser. The optical system may comprise or be comprised in an optical amplifier. The optical cell may comprise or be configured to receive an optical gain medium between the first retro-reflector and the counter reflector.

According to a fourth aspect of the present invention is a method of operating an optical cell of the first and/or second aspects and/or a system according to the third aspect.

It will be appreciated that features analogous to those described in relation to any of the above aspects may be individually and seperably applicable to any of the other aspects, Method features corresponding to use of any features described above in relation to an apparatus and/or apparatus features configured to implement any features described above in relation to a method are also contemplated as falling within the scope of the present invention. Brief Description of the Drawings

The invention will be described herein with respect to the following drawings: Figure 1 shows an optical cell according to an embodiment the present invention;

Figure 2 shows an end view of the optical cell of Figure 1 ; Figure 3 shows an equivalent optical system to the optical cell of Figure 1 ; Figure 4 shows the alignment tolerances of the optical cell of Figure 1 ; Figure 5 shows an optical cell according to an embodiment of the present invention; Figure 6 shows a schematic end view of the optical cell of Figure 5;

Figure 7 shows the location of the internal beams on a cross section of an optical cell according to an embodiment of the present invention;

Figure 8 shows a schematic end view of an optical cell according to an embodiment of the present invention; and Figure 9 shows an optical cell according to an embodiment of the present invention. Figure 10 shows a cross section of a possible mode structure within a hybrid cell. Detailed Description of the Drawings

Figure 1 shows an optical cell 5 that is configured to confine light within the cell 5 for a number of transits of the cell 5 before exiting. The optical cell 5 comprises a folded telescope arrangement 10, wherein the folded telescope 10 comprises a primary retro- reflector 15 and a positive optically powered surface 20, which in this case comprises a convex surface formed on an input face 25 of the primary retro-reflector 15

A counter reflector 30 is provided opposing the primary retro-reflector 15. In this embodiment, the primary retro-reflector 15 and the counter reflector 30 are advantageously both formed from corner cube retro-reflectors. The counter reflector 30 is arranged to receive light reflected from and to reflect light to the primary retro-reflector 15.

The optical cell 5 has a combined input / output surface 35 formed by replacing the apex of the counter reflector 30 with a negatively powered optical surface, in the form of a concave optical surface, as shown in Figures 1 and 2. The input / output surface 35 is usable as both an input 40 and an output 45 of the optical cell 5. One or more faces of the primary retro- reflector 15 and the counter reflector 30 are optionally provided with anti-reflective coatings in order to improve the optical efficiency of the cell 5. The optical cell 5 is also advantageously athermalised, for example, by mounting the primary retro-reflector 15 and the counter reflector 30 on a mount that is formed of materials having complimentary thermal expansion co-efficients. In addition the optical components of the cell 5, such as the primary retro-reflector 15 and/or counter reflector 30, are achromatised, for example, by constructing the components from doublets formed from materials having complimentary indices of refraction.

A light source (not shown) is provided and configured such that a light beam 50 from the light source is incident on the input of the optical cell. Optionally, the incident light beam 50 can be provided off-axis, e.g. the incident beam is provided obliquely to the optical axis / axis of symmetry of the optical cell. Providing on off-axis or oblique incident beam can minimise clipping of field angles due to the optical cell being configured so as to avoid an initial reflection to the input/output surface. The incident light beam 50 is expanded by the negatively powered optical surface 35, causing the light beam 50 to diverge. The beam 50 is then partially converged by the positive optically powered surface 20 and folded back on itself by being retro-reflected by the primary retro-reflector 15. Parameters of the optical cell 5 such as focal length of the input/output surface 35 of the counter reflector 30 and separation thereof are selected to achieve the desired number of passes. By suitable choice of the cell magnification and input offset radius, the folded beam initially entirely misses the input/output surface 35 of the counter reflector 30 and is instead reflected by the counter reflector 30 by total internal reflection back towards the positive optical element 20 and the primary retro-reflector 15 The light 50 is then reflected back and forth between the counter reflector 30 and the primary retro-reflector 15, passing through the positive optically powered surface 15 with each pass across the cell 5. It will be appreciated that this constitutes a periodic focusing waveguide capable of sustaining multiple modes, presuming that such modes do not escape via the input surface. If the optical parameters of focal length and separation are chosen appropriately then the beam will be collimated after an odd or even number of passes. As such, the optical cell 5 is the equivalent of a series of back to back telescopes, as illustrated in Figure 3. In this way, any error caused by misalignment in the cell 5 during one pass is perfectly undone by passing through the same optical elements in reverse on the next pass. In other words, if the primary retro-reflector 15 is tilted out of alignment relative to the counter reflector 30 or is laterally translated relative to the counter reflector 30, as shown in Figure 4, then the errors induced by these misalignments during a given pass through the cell 5 are undone by a subsequent pass through the cell 5. The beam 50 is collimated and the field angle reduced by the passes back and forth though the optical cell 5. The optical cell 5 is configured such that, after a number of passes through the cell 5, the beam 50 is reflected to the output 45 whereupon it can pass out from the cell 5, for example, to a detector (not shown). The number of passes / transits of the cell 5 before the light is incident on an output 45 can be determined by the cell geometry and parameters such as the focal length of the input surface 35, the focal length of the positive optically powered surface 20 and the separation of the counter reflector 30 and primary retro-reflector 15. The mathematical design rules for periodic focusing waveguides are described in "D Herriott et al, "Off-axis Paths in Spherical Mirror Interferometers", April 1964 / Vol.3, No. 4 / Applied Optics".

In the arrangement shown in Figure 1 , a centre line or optical axis 55 of the optical cell 5 is unused. In alternate embodiments of the optical cell 5', such as when the optical cell 5' is being used in a laser, optical amplifier or spectrometer, the centreline or optical axis 55 may be used as part of the pump or diagnostic port coupling optics. In particular, this arrangement may be used for collimating optics to reduce the field angle of the pump. By using this centre line of the optical cell 5', an additional pass and benign tolerances may be obtained by virtue of the available length. The beam 50 is coupled / reflected to the centreline 55 and thereby to a further on-axis input or output 45' provided opposite the first output 45 by providing a suitable supplementary reflector 60, such as a roof reflector or corner cube, at the first input 40 or output 40, as shown in Figures 5 and 6. For example, the further on-axis output may be provided in the primary retro-reflector 15. Alternatively, the supplementary reflector 60 may be configured to reflect light 50 at the first output 45 to an off-axis output (not shown).

It will be appreciated that the optical cells 5 of the present invention can be configured to support a primary mode and/or higher order modes. The supported modes must be symmetrical in order to allow the beam to return to the input / output. Use of higher order modes can lead to a more compact cell, but need a focal point to be formed. Advantageously, the optical cell can be configured such that the focal point is formed closer to the primary retro-reflector than the counter reflector.

Furthermore, in the above configuration, the optical cell only fills one plane of the retro- reflector / counter reflector, as shown in Figure 7. As such, the number of passes of the cell/path length of the beam and/or the fill factor of the cell may be increased by utilising more of the available space. One option for doing this is to provide a light source in the form of at least part of an annular ring, thereby exciting multiple modes in parallel, wherein the excited modes would be rotated relative to each other.

Another option would be to provide one or more suitable supplementary reflectors 60' at the output 45" of the cell, as shown by Figure 8, wherein the supplementary reflectors 60' are arranged to reflect the output of at least one mode such that it is displaced tangentially around the primary retro-reflector 15, counter reflector 30 and/or output 45". Examples of suitable supplementary reflectors 60' include roof or corner cube reflectors.

An additional or alternate option would be to create a hybrid cell 5", as shown in Figure 9. In this case, a pump reflector 65 is provided opposed to the output 45 of the counter reflector 30. Examples of suitable pump reflectors 65 include retro-reflectors, such as a corner cubes. Although not as beneficial, it will be appreciated that conventional mirrored reflectors, such as concave mirrors, could alternatively be used as the pump reflector 65. In this embodiment, the telescope 10 is configured so as to provide a residual convergence such that the residual convergence acts as a positively powered lens provided between the primary retro-reflector 15 and the pump reflector 30. This would appear optically equivalent to an unfolded Herriot cell with an additional perfect telescope. Therefore, in this case, if the input beam 50 is provided at a specific tangential angle, a helical mode is set up, wherein the beam 50 will be processed around the retro-reflector 15, thereby, utilising more of the available space within the optical cell 5" and/or permitting a larger number of passes for the volume used. This is illustrated in a cross section of a possible mode structure within a cell as shown in Figure 10. It will be appreciated that a similar effect is possible with an afocal telescope and an additional convergent lens between the telescopic cell and the pump reflector.

The design rules for Herriott cell operation are described in "D Herriott et al, "Off-axis Paths in Spherical Mirror Interferometers", April 1964 / Vol.3, No. 4 / Applied Optics". The theory of thin lens equivalence of a de-focussed telescope is described in "D C Hanna et al, "Telescopic resonators for large-volume TEM₀₀-mode operation", Optical and Quantum Electronics 13 (1981) 493-507". Together, one skilled in the art could select the telescope - pump reflector separation, telescope de-focus and input angle to achieve the desired helical mode path. The optical cell 5, 5', 5" can be advantageously used in a wide variety of applications. For example, the optical cell 5, 5', 5"can be advantageously used in lasers, optical amplifiers, interferometers, spectrometers, and the like. In a laser system, the optical cell 5, 5', 5" can be used to pump a laser cavity, with a gain medium provided between the primary retro-reflector 15 and the counter reflector 30. In this case, light from a pump light source is provided at one of more inputs 40 to the optical cell 5, 5', 5" and the resulting output beam emitted at the one or more outputs 45. The large path length to volume ratio are particularly beneficial in applications that require a compact solution.

The provision of a gain medium between the primary retro-reflector 15 and the counter reflector 30 can also be used to provide an optical amplifier, where the large number of passes/transits through the gain medium provided by use of the optical cell 5, 5', 5" can be beneficial in providing improved amplification and/or a smaller amplifier. In this case, the input signal is provided to the one or more inputs 40 of the optical cell and the amplified output signal is emitted from the one or more outputs 45.

The optical cell 5, 5', 5" can also be beneficially used in analytical instruments that would benefit from multiple passages through a sample, such as interferometers, spectrometers and the like. In this case, for example, a sample may be placed within the optical cell 5, 5', 5" between the primary retro-reflector 15 and the counter reflector 30. A beam 50 of suitable radiation is provided to the at least one input and a detector system can be arranged to receive radiation from the at least one output 45 after having been reflected between the primary retro-reflector 15 and the counter reflector 30 and/or the pump reflector 65 and thereby through the sample a number of times. Some analytical instruments require a slit to be provided between the output 45 of the optical cell 5, 5', 5" and the detector. In these cases, the slit can advantageously be provided square to the plane of a mode supported by the optical cell 5, 5', 5". In this way, the analytical instrument can be made less tolerant to errors due to the relative separation of the primary retro-reflector 15 and the counter reflector 30 and/or pump reflector 65.

If the embodiment of the optical cell 5' shown in Figures 5 and 6 is used, the input 40 is separated from the output 45' by folding the light beam 50 back through the centreline 55 of the cell 5' and providing an input 40 or output 45' (e.g. in the form of a flat or collimating surface) on the apex of the primary retro-reflector 15. In this way, for example, hot equipment such as the light source, may be located away from heat sensitive equipment such as detectors. Alternatively, the separation of the input 40 and output 45' can be beneficial in certain applications, for example, that require a specific shape of optical cell or use of space around the cell. By providing an optical cell 5, 5', 5" that comprises a folded telescope 10 having a retro- reflector 15 such that the beam 50 passes through the telescope 10 in both forward and reverse directions, the optical cell 5, 5', 5" is considerably more robust and less sensitive to misalignment than conventional White and Herriott cells. In particular, the reflectors 15, 30, 65 of the optical cells 5, 5', 5" may be subject to misalignments in rotation and translation with limited or no effect on the output of the cell 5, 5', 5", as any defects introduced by such misalignments during a pass through the cell are undone during the following (reverse) pass through the cell.

Furthermore, the retro-reflector based folded telescope optical cell 5, 5', 5" of the present invention advantageously provides a large number of transits/passes of the cell 5, 5', 5" in a compact geometry, allowing improvements in the number of cell transits/passes and/or cell size relative to traditional White and Herriott cells.

Furthermore, since embodiments of the invention described above use refractive optics, such as those based on total internal reflection, rather than relying on conventional mirrors, the cell 5, 5', 5" does not use delicate reflective coatings and has less inherent losses. This means that the optical cell 5, 5', 5" of the present invention can provide a longer lifetime and be operable in more aggressive environments that prior art systems based on conventional mirrors. Low losses confer on the cell a higher optimal number of passes, enabling more compact cells for a desired path length.

It will be appreciated that although an advantageous example of the invention is described above, variations to the above example are contemplated. For example, although the optical cell 5, 5', 5" described above provides an optical equivalent of back to back telescopes wherein the optical beam sequentially passes through a telescope 10 in a forward and reverse direction using a folded telescope arrangement based on refractive optics, it will be appreciated that a similar arrangement that comprises a Galilean telescope and/or conventional optics instead of one or more of the refractive optical elements described above can be used to produce an equivalent system, such as a system that is optically equivalent to that shown in Figure 3. For example, the pump reflector 65 and/or the counter reflector 30 may be replaced by a mirror, such as a curved mirror. Similarly, although the example described above comprises a positive optically powered surface 20 in the form of a convex face of a corner cube primary retro-reflector 15, it will be appreciated that other arrangements may also be used. For example, the positive optically powered surface 20 need not be integral with the primary retro-reflector 15 and may be provided separately, for example, as one or more separate lenses.

Furthermore, although the primary retro-reflector 15, the counter reflector 30 and the pump reflector 65 advantageously comprise corner cube retro -reflectors, it will be appreciated that one or more of these reflectors may be replaced by alternative reflector arrangements, such as crossed porros, mirrors, or the like.

In addition, although a specific example described above has an input 40 and an output 45 provided on a common negatively powered optical surface 35, it will be appreciated that this negatively powered optical surface 35 may be replaced by a positively powered surface having a focal length suitably shorter than the cell length in order to produce a diverging beam at the positive optically powered surface 20. Similarly, the optical cell 5, 5', 5" may comprise more than one input 40 and/or output 45, 45' and one or more input 40 may be provided in a different surface to one or more output 45, 45'.

Furthermore, whilst the optical cells of the present invention are described in relation to their use with light and light beams, it will be appreciated that they may also be configured to be operable with other forms of radiation, for example, electromagnetic radiation having wavelengths falling outwith the visible region.

Therefore, it will be appreciated that the above specific description is provided by way of example only and that the scope of the invention is defined by the claims.

Claims

1 . An optical cell comprising:

at least one telescope;

2. An optical cell according to claim 1 , wherein:

the telescope is a folded telescope, the folded telescope comprising: a first retro-reflector; and

at least one positive optically powered surface;

the optical cell further comprising:

at least one counter reflector;

3. An optical cell according to claim 2, wherein the at least one positive optically powered surface is provided proximate, adjacent, on or comprised in the retro- reflector.

4. An optical cell according to claim 2 or claim 3, wherein the first retro-reflector and/or the at least one counter reflector comprises a corner cube retro-reflector.

5. An optical cell according to any of claims 2 to 4, wherein the first retro- reflector is arranged to be off-axis, such that beams of radiation are not received or reflected along the axis of symmetry of the first retro-reflector, for at least some transits of the cell by a light beam.

6. An optical cell according to any of claims 2 to 5, wherein the first retro- reflector and/or the counter reflector comprise refractive optical elements.

7. An optical cell according to any of claims 2 to 6, wherein the optical cell is configured to receive at least one off-axis optical beam at the at least one input.

8. An optical cell according to any of claims 2 to 7, wherein the input(s) and/or output(s) are provided with an input and/or output surface, wherein the input and/or output surface comprises a negative optically powered surface or a positive optically powered surface

9. An optical cell according to any of claims 2 to 8, wherein at least one input and at least one output are integral.

10. An optical cell according to any of claims 2 to 9, wherein at least one of the inputs and/or at least one of the outputs are formed in the counter reflector.

1 1 . An optical cell according to any of claims 2 to 10, wherein the optical cell is configured such that the initial divergence of the beam by the input surface and/or the convergence of the beam by the positive optically powered surface is such that the retro-reflected / folded beam is reflected to a point on the counter reflector other than the input and/or the output for at least an initial transit of the cell.

12. An optical cell according to any of claims 2 to 1 1 , wherein a reverse pass or transit back from the counter reflector passes through the same optical components as the previous pass or transit but in reverse.

13. An optical cell according to any of claims 2 to 12, wherein the optical cell is configured to support a primary mode and/or higher order modes, where the primary mode describes a sinusoidal mode structure of path length ½ a wavelength.

14. An optical cell according to any preceding claim, wherein the optical cell is configured to support helical modes by providing a tangential component to the input beam angle.

15. An optical cell according to any of claims 2 to 14, wherein the optical cell is configured to support symmetrical modes that allow the beam to be incident on an output after a plurality of passes or transits through the cell.

16. An optical cell according to claim 13, wherein the cell is configured such that use of higher order modes results in at least one focal point in the optical cell such that the focal point(s) are provided toward, proximate or adjacent the first retro- reflector.

17. An optical cell according to any of claims 2 to 16, wherein at least one of the inputs and/or outputs are provided on a centreline / axis of symmetry of the optical cell.

18. An optical cell according to any of claims 2 to 17, wherein at least one additional reflector is provided at one or more of the outputs, wherein the additional reflector(s) are configured to reflect light received from the first retro-reflector at the corresponding output to re-enter the cell via at least one other input.

19. An optical cell according to claim 18, wherein at least one of the outputs are provided at or in the first retro-reflector.

20. An optical cell according to any of claims 2 to 19, wherein the telescope is configured to be convergent / to have residual convergence.

21 . An optical cell according to any of claims 2 to 20, wherein the optical cell comprises a pump reflector that opposes, faces and/or is configured to receive light from at least one of the outputs

22. An optical cell according to any of claims 2 to 21 , wherein the optical cell comprises a positive optical surface between the pump reflector and the at least one input.

23. An optical system comprising:

an optical cell according to any preceding claim;

a light source configured to provide light to at least one input of the optical cell; and/or

a receiver or detector configured to receive light from at least one output of the optical cell.

24. The optical system according to claim 23, wherein the optical system comprises or is comprised in a spectrometer.

25. The optical system of claim 23 or claim 24, wherein the optical cell comprises a sample chamber between the first retro-reflector and the counter reflector, such that a sample may be receivable within the sample chamber.

26. The optical system of claim 23, wherein the optical system comprises or is comprised in a laser or optical amplifier, wherein the optical cell comprises or is configured to pump an optical gain medium between the first retro-reflector and the counter reflector.

27. An optical cell substantially as shown in the drawings and/or described in the description.