GB2507721A

GB2507721A - Optical cell comprising a telescope

Info

Publication number: GB2507721A
Application number: GB201217406A
Authority: GB
Inventors: Andrew Borthwick; Mcrae Ian; Gerald Byrne
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2012-09-28
Filing date: 2012-09-28
Publication date: 2014-05-14
Also published as: GB201217406D0; WO2014049358A1

Abstract

An optical cell 5 comprising at least one telescope 10, where the cell 5 is configured such that light makes at least one transit through the telescope 10 in a forward direction and a reverse direction before exiting from the cell 5. The telescope 10 is preferably a folded/reflector telescope 10, comprising a first retro-reflector 15 and at least one positively optically powered surface 20 and the optical cell 5 preferably comprises at least one counter reflector 30, wherein the reflectors define a cavity between them, at least one input 40 for allowing light into the cavity and at least one output 45 for light to exit the cavity. The cell is optionally configured to support helical modes by providing a tangential component to the input beam angle. The cell may, but is not limited to, be a resonator or cavity.

Description

Optical Cell The present invenfion 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 ampfifiers, interferometers, spectrometers, delay fines and the fike. 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 fight within the cell for a number of transits of the cell before the light exts the cefl though an output mechanism.

Such optical cells are useful in many applications for achieving vey 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 proded in the cell, and the confined fight 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 thaL 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 Apedure' Journal of the Optical Society of America, 32 (5:285' Th.e White cell comprises three spherical concave mirrors having the same radius of curvature. One of the mirrors (Ml) 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 (Ml and M2) oppose the third mirror (M3). In particular, the mirrors (Ml and M2) and are configured to reflect fight 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 Ml, 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 opdcal ce is the Herriott cell, as described in "0 Herriett of. at FoIded Optical Delay LThas' Appl/ed Optics 4 (8J:8&3891°. This ce comprises a pair of opposed spherical mirrors, wherein at least one aperture is provided in one or both of the mirrors in order to aow input and output light beams to respectively enter and leave the cavity defined between the opposed rn!rrors. The number of transits made by a Ught beam between the opposed mirrors may be controed by adjusting the mirror's focal length, separation and input angle of the beam.

However, the prior art cefl configurations may suffer from a variety of problems, such as tight tolerances on the alignment of the opflcal elements, excesve losses on reflection and/or transmission losses miting the total number of transits, poor fill factors of the total ceH 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 coD, It is at east one object of at least one embodiment of the present invention to eliminate or mitigate at east 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 c&l.

The optical ccii may be configured such that light makes a plurality of transits through the teiescope 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 coD 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 SE may comprise at east one counter reflector, wherein the first retroreflector and the at east one counter reflector are configured to define a cavity or chamber therebetween.

According to a second aspect of the present invention is an optical cefi comprising: a folded t&escope, the folded telescope comprising: a first retroreflector; and at east one positive opticMy powered surface; the opticai ce further comprising: at least one counter reflector; wherein ihe first retroreflector and at least one counter reflector are configured to define a cavity or chamber therebeeen; and at least one input for allowing Eight to be input to the cavity; and at least one output for aflowing Hght to exit from the cavity.

The optical ceU may comprise or be comprised in an optical cefi described above in relation to the first aspect and/or may indMduay and separably or in combination comprise one or more feature described in relation thereto.

The folded telescope may comprise at east one positive opticatiy powered surface, such as a convex surface. The at east one positive opticatiy powered surface may be provided proximate. adjacent, on or comprised in the retroreflector, The at least one positive opticatiy powered surface may be provided on or be formed by the input face of the retroreflector, for example, the positive opticaUy powered surface may be integral with the first retroreflector, The input face of the first retroreflector may be a face of the retrorefiector that receives light from the input and/or counter reflector.

As is known in the art, a retro-refiector reflects Ught back at the same angle but opposite direction to incident light over a range of incident angles, l.a the radiation is reflected back along a direction of travel that is paraflel but opposite to that of the incident radiation over a range of incident angles. in a particular example, the first retrorefiector may comprise a corner cube retrorefiector. The retrorefiector may be arranged to be offaxis, Le. the beams are not received or reflected along the axis of symmetry of the retroreflector, for at least some and optionally each transit of the ceti by the light beam.

The first retroreflector 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 retroreflector and/or counter reflector may be provided with an antirefiective coating.

The optical cell may be configured to receive at least one oThaxis opflcal beam at the at east 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 opticay powered surface, such as a concave surface, or optionafly a positive opticay powered surface, such as a convex surface. The input surface may he a surface that expands or diverses an incident beam before it reaches the first retroreflector and/or at least one positive opticaUy powered surface. For example, if the input surface is a positive opticay powered surface, then the focal length of the positive opticay powered surface may be suitably less than the distance between the inDut(s) and the first retro-refiector 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 east one of the inputs and/or at east one of the outputs may be formed in the counter reflector. The at east one counter reflector may comprise a retroreflector, such as a corner cube. For example, in the specific example where the counter reflector is a Garner cube retroreflectcr, 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 retroreflector, e.g. via the positive optically powered surface, In this way, the optical beam may be reflected back by the first retroref!ector towards the counter reflector, i.e. it is folded back on itself. The positive opticay powered surface may act to converge the beam.

The opticai 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 cotically powered surface is such that the retroreflected / 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 s used, the retroreflected beam a aways paraUe to the hput beam, for at east a range of nput anges. In this way, any error due to misagnmont that occurs in a pass or transit through the cefi may be undone by the next (reverse) pass or transit, which may effectively result in a selfaUgning 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 1⁄4 a wavelength.

The optic& ce may he configured to support odd and even modes, defined by the number of transions of the ceH. The optical ce may be configured to support hec modes by provithng a tangential component to the input beam angle. The optical ceU may he configured to support symmetrical modes, which may aow the beam to be inddent on an output, which may be after a pluraty of passes or transits through the celL Use of higher order modes, that is path lengths greater than 1⁄4 a wavelength, may result in at least one focal point ri the optical cell, In such cases, the optical ce may be configured such that the focal point(s) are provided toward, proximate or adjacent the first retrc>reflector and away from the counter reflector. It will be appreciated that use of higher order modes may result in a more compact resonator but may require the formation of focal pons, which may be disadvantageous in certain appcations, such as those invoMng explosive environments, At least one of the inputs and/or outputs may be provided on a centrene / 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 (eg. a first output). The additional reflector(s) may be configured to reflect light received from the first retroreflector at a corresponding (first) output to reenter the ceU via at east one other (second) input, which may optionaUy be an off-axis input, Ic. an input not provided on the optical axis or axis of symmetry of the optical caD. Alternatively, the at least one other (second) input may be an on-axis input, ag providing a second output/input on another side of the caD, such: as an opposite side to the first output.

OøtionaDy. at least one of the outputs (such as a second output) may be provided at or in the first retro-refiector, such as on an optical axis or axis of symmetry of the optical cell, ag at an apex of the first retro-reflector, These output(s) may comprise a flat or collimated output.

In this way, at east one output may be separated from the inputs, e.g. to show improved separation of a hot light source at an input from heat sensitive detectors at an output.

A light source for providing Ught to the input may comprise an annidar ring Ught source or be adapted to illuminate an annular ring and/or the input may be configured to receive Ught from an annular ring light source.

The telescope may be configured to be convergent Ito have residual convergence.

The optical ceU may comprise a pump reflector. The pump reflector may comprise a retro-reflector such as a carrier cube. The pump reflector may oppose, face and/or be configured to receive ght from at least one of the outputs of the folded telescope. For example, the pump reflector may he provided on an oppo&te side of the counter reflector to the first retro reflector. The opficai cefi may be adapted such that the input beam is provided or providabie at a specff!c 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 operabe as a virtual positivey powered ens sandwiched between the pump reflector and the first retrorellector.

Equally, this could he achieved with an afocal telescope and a spedfical positive optically powered surtce between the pump reflector and the afocal telescope. Such an opficEd cell may be operable to set up a heilcal mode, such that the output beam received at the output may be spaflally rotated r&ative 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. Ic. the pump reflector.

The t&escope may be athermahsed, for example, by using ethermalised mountings.

At east one and preferably each optical component, such as the first retrore.flector and/or the counter reflector and/or the pump reflector and/or the input I output surface and/or the positive optically powered surface, may he achromatSd, for example by comprising a doublet component such as a component formed from at east 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 optica 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 east one input of the optical cell; and/or a receiver or detector configured to receive light from at least one output of the optics ceil.

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

The optical ceil may comprise a sample chamber between the first retroreflector and the counter reflector, such that a sample may be receivable within the sample chamber, The optical system may comprise or he comprised in a laser, The optical system may comprise or be comprised in an optical ampilfier. The optical coil may comprise or be configured to receive an opUcal gain medium between the first retroreflector and the counter reflector.

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

It wiil be appreciated that features analogous to those described in relation to any of the above aspects may be indMdually and seperably appilcable 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 lectures described above in relation to a method are also contemplated as failing within the scope of the present invention,

Brief Description of the Drawings

The invention wiil be described herein with respect to the foilowing 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 cM of Figure 1; Figure 4 shows the agnment tolerances of the optical cell of Figure 1; Figure 5 shows an opUcal cell according to an embodiment of the present invention; Figure 6 shows a schematic end view of the optical cM of FigureS; Figure 7 shows the location of the internal beams on a cross section of an optical cM according to an embodiment of the present invention; Figure 8 shows a schematic end view of an optical ceU according to an embodiment of the present invention; and Figure 9 shows an optical cM according to an embodiment of the present invention.

Figure 10 shows a cross section of a possible mode structure within a hybrid ceO,

Detailed Description of the Drawings

Figure 1 shows an optical cell 5 that is configured to confine light within the ceO 5 for a number of transits of the ceO 5 before Sting. The optical ceO 5 comprises a folded telescope arrangement 10, wherein the folded telescope 10 comprises a primary retro-reflector 15 and a positve optically powered surface 20, which in this case comprises a convex surface formed on an input face 25 of the primary retro-refiector 15 A counter reflector 30 is provided opposing the primary retro-refiector 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 epex 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 I output surface 35 is usable as both an input 40 and an output 45 of the optical ceO 5. One or more faces of the primary retro-reflector 15 and the counter reflector 30 are optionaOy provided with anti-reflective coatings in order to improve the optical efficiency of the cell 5. The optical cell 5 is also advantageously athermased, for example, by mounting the prh'nary retro**reflector 15 and the counter reflector 30 on a mount that is formed of materials having compmentary thermal expansion co-efficients. In add Won the optical components of the ce 5, such as the primary reLroreflector 15 and/or counter reflector 30, are achromatised, for example, by constructing S the components from doublets formed from materials having comphmenlary indices of refraction.

A Ught source (not shown) is provided and configured such that a light beam 50 from the llght source is incident on the input of the optical cell. Opfionally, the incident ght beam 50 can be provided oWaxis, e.g. the incident beam is provided obUquely to the opUcal axis / axis of symmetry of the optical cell. Providing on off-axis or obllque incident beam can minimise cUpping at field angles due to the optical cell being configured so as to avoid an initi& reflection to the inputioutput 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 retroreflector 15. Parameters of the optical ccli 5 such as focal length of the input/output surläce 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 refiector 30 by total internal reflection back towards the positive optical element 20 and the primary retroreflector 15 The light 50 is then reflected back and forth between the counter reflector 30 and the primary retro-reector 15. passing through the positive optically powered surface 15 with each pass across the ceU 5. It wfll 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 beamwil be collimated after an odd or even number of passes. As such, the optical celi S 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 retroreflector 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 rnisalignments during a given pass through the cell 5 are undone by a subsequent pass through the cell 5.

The beam 50 is coUimated arid to field angle reduced by the passes back and forth though the optic cell 5. The optical ce 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 / tranefte of the cell S 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 retroreflector 15. The mathematical design rules for perbdic focusing waveguides are described in r Herriott ef aL "Offaxis Paths in Spherical Mirror Intertdrometera' April 19841 VoL 3. No, 4 IA pp/led Optics".

In the arrangement shown in Figure 1, a centre llne or optical axis 55 of the optical cell 5 is unused. In alternate embodiments of the optical cefi 5', such as when the optica cell 5 is being used in a aser; opUcal ampllfier or spectrometer, the centrehne or optical axis 55 may be used as part of the pump or diagnostic port couphng optics. In particular, this arrangement may be used for colhmating 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 avallable length. The beam 50 is coupled / reflected to the centreline 55 and thereby to a further onaxis 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 onaxis output may be provided in the primary retroreflector 15. AfternaUvely, the supplementary reflector 60 may be configured to reflect Ught 50 at the first output 45 to an offaxis output (not shown), It wlll be appreciated that the optical ceUs 5 of the present invention can be configured to support a primary mode and/cr 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-reflectcr than the counter reflector.

Furthentiore. in the above conguration, 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 dng, thereby exciting multiple modes in parallel, wherein [he 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 ft is dispced tangentially around the primary retrorefiector 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 ceU 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 retroreflectors, 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 teescope 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 bean, 50 is provided at a specThc tangential angle, a helical mode is set up, wherein the beam 50 will be processed around the retroreflector 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. [his is illusLrated 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 "0 Herriott at a!. "Offaxis Paths in Spherical Mirror lnterferornoters Apr!! 1964 / Vol. 3, No. 4/Applied Opt/cC The theory of thin lens equivalence of a de4ocussed telescope is described in "0 C F/anna et a!.

"1 élescopic resonators for largevolume TEM00mode operahon' Optical and Quantum Electron/cs 13 (1981) 49&507". Together, one skilled in the art could select the telescope pump reflector separation, telescope delOcus and input angle to achieve the desired helical mode path.

The opc ccli 5, 5', 5" can be advantageously used in a wide variety of appcafions, 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 form a laser cavity. with a gain medium provided between the primary retroreflector 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 optic& celi 5.

5', 5" and the resuWng output beam emttted at the one or more outputs 45. This arrangement can advantageously provide a long path length cavity, which can be of particular benefit in applicaflons that require fast switching. The planar ceH arrangement of the present invention and large path length to volume ratio are particularly beneficial in applicaUons that require a compact resonator.

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/tran&ts through the gain medium provided by use of the optical cell 5, 5'S" can be beneficial in providing improved amplification and/or a smalier amplifier. In this case, the input signal is provided to the one or more inputs 40 of the optical ce and the ampilfied output signal is emitted from the one or more outputs 45. nfl

The optical cell 5, 5', 5" can also be beneficially used in analyticS 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 retrorefiector 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 east one output 45 alter having been reflected between the primary retroreflector 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 he provided between the output 45 of the optical cell 5, 5', 5" and the detector, In these cases, the slit can advantageously he 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 retroreflector 15 and the counter reflector 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' (eg. in the form of a flat or collimating surface) on the apex of the primary retry-reflector 15. ri this way, for example, hot equipment such as the Ught source, may be located away from heat sensitive equipment such as detectors. Alternativeiy, the separation of the input 40 and output 45< can be beneficial in certain appilcations. for example. that require a specEtk 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 misallgnment than conventional White and Herriott cells. in particular, the reflectors 15, 30, of the opficai cells 5, 5<, 5" may be subject to misallgnments in rotation and translation with limited or no effect on the output of the cell 5, 5', 5", as any defects introduced by such misallgnments during a pass through the cell are undone during the following (reverse) pass through the cell.

Furthermore, the retroreflector based folded telescope optical cell 5, 5, 5" of the present invention advantageously provides a larçe number of trants/passes of the cefi 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 refradilve optics, such as those based on total internal reflection, rather than relying on conventional mirrors.

the cell 5, 6', 5" does not use dellcate reflective coatings and has ess inherent losses This means that the optical cell 5, 5, 5" of the present invention can provide a longer llfetime 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 contempted.

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 simHar arrangement that comprises a Galllean 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 s optcay equvalerd to that shown n Figure 3. Far exampe, the pump reflector 65 and/or the counter reflector 30 may he repced by a mftror, such as a curved mfrror.

Smary, although he exampe described above comprises a poaftive opdcally powered surface 20 n the form of a convex face of a corner cube primary retroreIiector 15, ft wifi be apprecated that other arrangements may also be used. For exampe, the posWve opUcay powered surface 20 need not be ntegra wfth the primary retroreflector 15 and may be provided separataly, for exampe, as one or more separate enses.

Furthermore, although the primary retrorefiector 15, the counter reflector 30 and the pump retlector 65 advantageou&y comprise corner cube retroreflectors, ft wil' be apprecated that one or more of these reflectors may be repaced by efternaflve reflector arrangements, such as crossed porros, mrrors, or the ke.

h addfton, although a specific exarnpki described above has an nput 40 and an output 45 provided on a common negaUvaly powered opfical surface 35, 1 wifi be appredated that thS negafivaly powered opfica surface 35 may he rapaced by a posdvaly powered surface having a focal ength sStaby shorter than the od ength n order to produce a thverging beam at the positive opficaUy powered surface 20. Similarly, the opbc& coil 5, 5', 5" may comprise more than one input 40 and/or output 45, 45' and one or more nput 40 may be provided n a dftërent surface to one or more output 45, 45.

Furthermore, whst the optical ces of the present nvenUon are described in relation to theft use wdn ight and ight beams, it wifi be appreciated that they may also be configured to be operabe with other forms of radiation, for exampe, electromagnetic radiation having wavalengths faffing ouiith the visible region.

Therefore, it wil be appreciated that the above specific description a provided by way of exampe only and that the scope of (he invention is defined by the calms.