Optical System

Abstract

An optical amplifier system comprising a gain medium and a pump mechanism configured to amplify first and second optical beams as they pass through the gain medium. The optical amplifier system is arranged such that the first and second beams having the same wavelength take different paths through the gain medium, whereby the beams are neither parallel nor coaxial when they exit and / or enter through the first side of the gain medium. The system comprises a prism having a facet positioned on the first side of the gain medium in the path of both beams. The facet is oriented such that it causes total internal reflection of one of these beams within the prism, but is transmissive with respect to the other. In this way, the paths of the two beams can be altered relative to each other, enabling the beams to be brought closer together or separated from each other. The present invention relates in particular to a multipass amplifier resulting from guiding the first and second beams to pass through the gain medium a plurality of times as a single beam.

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention relates to optical systems. In one aspect, the present invention relates to optical amplifiers, particularly but not exclusively to multi-pass optical amplifiers. [Background technology]

[0002] In laser systems where high gain of the optical beam is required, one solution is to provide multiple amplifiers that sequentially amplify the optical beam. An alternative solution is to use a multi-pass amplifier, in which the optical beam passes through the same gain medium multiple times. This latter solution allows for a reduction in the number, size, and weight of components in the laser system compared to using multiple amplifiers.

[0003] 1A shows a simple arrangement of a prior art multi-pass amplifier 100. A beam source 101 is positioned to direct a beam 102 through a gain medium 103. A mirror arrangement 104 at the output of the gain medium 103 is positioned to reflect the amplified beam 102 back into the medium 103 at an angle from the beam 102, causing the reflected beam 105 to take a different path back through the medium 103 than the input beam 102. This solution has several problems. First, the beam source 101 must be spaced a relatively large distance from the gain medium 103 to provide enough space to accommodate the next component in the optical assembly, which will receive the reflected beam 105 once it has output from the gain medium 103. Second, if multiple mirrors are required to redirect the input beam 102 back into the gain medium 103, they also need to be spaced a relatively large distance apart to provide the necessary separation in beam angle between the input beam 102 and the reflected beam 105, and therefore also need to be spaced a relatively large distance apart from the gain medium 103. This results in a physically long optical assembly. An additional problem is that the gain medium 103 is not used efficiently, meaning a larger pump source is required, which also increases the heat load within the entire gain medium.

[0004] An alternative arrangement is shown in FIG. 1B. Input beam 102 passes through polarizer 107 (also used as a beam splitter) to provide linearly polarized input beam 102A, then through λ / 4 waveplate 106, which outputs circularly polarized input beam 102B, which enters gain medium 103. Mirror 104 is positioned so that reflected beam 105 follows the same path as input beam 102B back through gain medium 103. As a result of the reflection, reflected beam 105 has the opposite circular polarization as circularly polarized input beam 102B. λ / 4 waveplate 106 converts reflected beam 105 into linearly polarized reflected beam 105A, which has a polarization orthogonal to linearly polarized input beam 102A. Polarizer 107 separates amplified output beam 105B from input beam 102A. While this arrangement allows for a more compact design compared to the arrangement of FIG. 1A, over time the polarizer 106 tends to leak amplified light back towards the beam source 101, which can damage or interfere with the beam source 101.

[0005] Kane et al., 62-dB-Gain Multiple-Pass Slab Geometry Nd:YAG Amplifier; Optics Letters, 11, Issue 4, pp. 216-218, 1986, shows a multi-pass device with an array of mirrors arranged to circulate the beam through the gain medium so that the beam travels in the same direction through the gain medium each time. This design is relatively large compared to the designs of FIGS. 1A and 1B.

[0006] The present invention has been devised to provide an optical amplifier of compact size that does not suffer from the leakage problems of the design of FIG. 1B. Summary of the Invention

[0007] According to a first aspect of the present invention, there is provided an optical system comprising: a gain medium having a first side and a second side; and a pump mechanism, both configured to amplify first and second coherent optical beams traveling through the gain medium between the first and second sides; the first and second coherent optical beams traveling different respective first and second paths which are neither parallel nor coaxial as the first and second optical beams enter or exit the gain medium through the first side of the gain medium; and optical spacing means for varying the spacing between the first and second optical beams, wherein the optical spacing means comprises a prism having facets positioned about the first side of the gain medium such that the facets are in the first and second paths of the respective first and second coherent optical beams, the prism and facets being configured to be primarily transmissive to one of the first and second optical beams and to redirect the other of the first and second optical beams by total internal reflection.

[0008] Since the prism may be positioned in the path of both the first and second beams, it may be positioned very close to the gain medium, optionally in physical contact with the gain medium. The optical spacing means may be configured to increase the separation distance between the beams, allowing multiple optical components on the same side of the gain medium, e.g., a beam source and further optics, to be positioned much closer to the gain medium, thus allowing a reduction in the length of the optical system.

[0009] Each beam may be associated with a respective: a first pass point where the beam enters the optical spacing means from the gain medium or exits the optical spacing means towards the gain medium; and A second pass where the beam enters the optical spacing means on its way to the gain medium, or exits the optical spacing means having already passed through the gain medium.

[0010] The optical spacing means is preferably configured so that the physical spacing between the second way points is greater than the physical spacing between the first way points and / or so that the divergence angle between the first and second beams at the second way point is greater than the divergence angle at the first way point, the former being desirable if, for example, it is desired that the beams are parallel when passing through the second way point.

[0011] As a result, the present invention provides advantages in both applications where the first and second beams travel in the same direction through a first surface of a gain medium, and where the first and second optical beams travel in opposite directions through a first side of a gain medium. In one embodiment, the present invention is directed to multi-pass optical amplifiers. However, the inventors recognize that the present invention may have broader applications. For example: For amplifying two similar wavelength beams on a single pass through a gain medium and extracting them after amplification; in an optical amplifier, wherein one of the first and second beams is used to optically pump a laser gain material; and One of these beams is used to excite a chemical species on the facet of the gain medium, and the other beam is the result of optical fluorescence resulting from the excited chemical.

[0012] In one arrangement, the optical spacing means may have a first side facing towards the gain medium and a second side facing away from the gain medium, the optical spacing means being arranged to cause the first and second beams to have substantially parallel paths as they pass through the second side of the optical spacing means. Providing parallel paths may allow for further reduction in separation of multiple optical components on the same side of the gain medium.

[0013] The optical spacing means may comprise a second prism disposed in the path of either the first or second beam through the facet to redirect the beam, and the second prism may function to correct refraction of the transmitted beam as a result of transmission through the facet, e.g., so that it exits the optical spacing means on a path substantially parallel to the path on which it entered the optical spacing means.

[0014] The optical system may include a reflector on the first side of the gain medium, the reflector adapted to reflect the first beam after it exits the optical spacing means back toward the optical spacing means as a second beam, and the optical spacing means adapted to direct the second beam received from the reflector back through the first side of the gain medium. The reflector may include, for example, a corner cube and / or an image rotator.

[0015] This arrangement allows the reflector to be positioned relatively closer to the gain medium compared to the prior art embodiment of FIG. 1A.

[0016] The optical system including the gain medium may be arranged so that the first and second coherent optical beams enter or exit the gain medium through a second side. In that case, a reflector may be arranged on the second side of the gain medium to reflect the first optical beam after it exits the second side of the gain medium back into the gain medium about a second path. In this arrangement, the optical spacer means functions to increase the separation between the input beam (first beam) and the amplified output beam (second beam).

[0017] The first and second paths through the gain medium may be such that the first and second optical beams are neither parallel nor coaxial when they enter or exit the second side of the gain medium, in which case the optical system may comprise further optical spacing means arranged to receive the first beam output from the gain medium and direct the first beam to the reflector, and to receive the second beam from the reflector and direct it back to the gain medium.

[0018] The further optical spacing means may comprise a further prism having a further facet, the further prism and the further facet being primarily transmissive to one of the first and second optical beams and adapted to redirect the other of the first and second optical beams by total internal reflection. This arrangement of optical spacing means on either side of the gain medium allows for compactness of the optical system at both ends.

[0019] The laser gain medium may have a zig-zag slab geometry such that the first and second paths through the gain medium are zig-zag paths.

[0020] The pump mechanism may comprise a laser diode pump.

[0021] The optical system may include a heat sink disposed directly adjacent a third side of the gain medium, the third side extending between the first and second sides, for extracting heat from the gain medium through the third side by conductive cooling. The heat exchanger may be glued or clamped to the gain medium and attached to a suitable cooling plate. Beneficially, the heat exchanger has a thermal expansion coefficient similar to that of the material of the gain medium.

[0022] The pump mechanism may be adapted to inject pump radiation into the gain medium through a fourth side of the heat sink, the fourth side extending between the first side and the second side.

[0023] For most, if not all, applications, it is preferable for the prism facets to be as transmissive as possible to one of the beams to minimize optical losses in that beam, so the facets may be 90% or more optically transmissive to that beam.

[0024] The first and second optical beams may be of the same optical wavelength.

[0025] The invention will now be described by way of example with reference to the following drawings, in which: [Brief explanation of the drawings]

[0026] [Figure 1A] FIG. 1A is a simplified schematic diagram of a prior art multi-pass optical amplifier. [Figure 1B] FIG. 1B is a simplified schematic diagram of an alternative prior art multi-pass optical amplifier. [Figure 2] FIG. 2 is a simplified schematic diagram of a multi-pass optical amplifier. DETAILED DESCRIPTION OF THE INVENTION

[0027] 2 shows a multi-pass optical amplifier 1. Amplifier 1 comprises a first beam spacer 2, a gain medium 3, a laser pump 4 that pumps gain medium 3, a heat sink 5, and a reflector assembly 6 consisting of a second beam spacer 7 and a reflector 8.

[0028] The gain medium 3 has a first side 3A, a second side 3B, a third side 3C, and a fourth side 3D. The first side 3A and the second side 3B face in opposite directions. The third and fourth sides extend between the first side 3A and the second side 3B and face in opposite directions.

[0029] The gain medium 3 has a first face 3AA on a first side 3A and a second face 3BB on a second side 3B, the first face 3AA and the second face 3BB facing in opposite directions.

[0030] A first beam spacer 2 is disposed around a first side 3A of the gain medium 3. A reflector assembly 6 is disposed around a second side 3B of the gain medium 3.

[0031] A laser pump 4, which may comprise a laser diode pump, is adapted to inject pump radiation into the gain medium 3 through the third side 3C.

[0032] The heat sink 5 is positioned directly against the fourth side 3D of the gain medium 3 for extracting heat from the gain medium 3 through the fourth side 3D. As a result, both the pump radiation into the gain medium 3 and the heat extraction from the gain medium 3 occur in a direction orthogonal to the general direction of travel of the optical beam being amplified through the gain medium 3. This arrangement minimizes thermal lensing within the gain material.

[0033] A coherent beam of light from a seed source 9 passes through the first beam spacer 2, enters the gain medium 3 through the first face 3AA, and provides a forward beam that follows a first path A (solid line) through the gain medium 3 along a first zigzag path. The amplified beam exits the gain medium 3 through the second face 3BB and is redirected by the reflector assembly 6 as a return beam along a second path B (dashed line) back into the gain medium for a second round of amplification. Taking a different zigzag path through the gain medium 3, the return beam exits the gain medium 3 through the first face 3AA and then returns through the first beam spacer 2. Upon exiting the first beam spacer 2, the second path B of the return beam is parallel to the first path A of the forward beam just before entering the first beam spacer 2.

[0034] Importantly, the paths A and B of the respective outbound and inbound beams diverge (or at least are not parallel or coaxial) as they exit the gain medium through the first and second faces 3AA, 3BB.

[0035] The seed source 9 comprises a laser oscillator and, optionally, one or more further optical components for directing and / or modifying the beam, such as reflectors, lenses, wave plates and polarizers.

[0036] The first beam spacer 2 is comprised of a first optical prism 20 and a second optical prism 21. Each prism 20, 21 is comprised of a single, integral piece of material that is highly transparent, for example, greater than 90%, to the wavelengths of the outgoing and return beams. The first and second prisms are made of substantially the same material.

[0037] The first prism 20 defines a first facet 20A, a second facet 20B, a third facet 20C, and a fourth facet 20D. The first facet 20A has an outer surface facing toward the gain medium 3. The fourth facet 20D extends parallel to the first facet 20A, with its outer surface facing in the opposite direction from the first facet 20A, i.e., away from the gain medium 3.

[0038] The second prism 21 includes a first facet 21A and a second facet 21B. The first facet 21A has an outer surface that faces toward and extends parallel to the second facet 20B of the first prism 20. The second facet 21B of the second prism 21 faces in the opposite direction to the first facet 21A and is parallel to the first facet 20A and the fourth facet 20D of the first prism 20.

[0039] The first prism 20 is oriented so that the first facet 20A is perpendicular to the outgoing beam and the fourth facet 20D is perpendicular to the return beam. The second prism 21 is oriented so that the second facet 21B is perpendicular to the path A of the outgoing beam.

[0040] The first prism 20 has a second facet 20B configured as follows: a) extends at an angle to the path A of the outgoing beam, allowing the outgoing beam to pass through it and enter the first prism 20 with minimal reflection (greater than 90% transmission); and b) Extends at an angle to the path B of the return beam so as to cause total internal reflection (TIR) ​​of the return beam.

[0041] The required angle of facet 20B will depend in part on the material of the prism.

[0042] The outgoing beam travels from beam source 9, passes through second facet 21B, enters second prism 21, and exits through first facet 21A. The angle of facet 21A refracts path A of the outgoing beam in a first direction as it exits second prism 21. The refracted beam enters first prism 20 through second facet 20B. Due to the parallel nature of facets 21A, 20B, the refracted outgoing beam is refracted in the opposite direction by the same angle as it enters first prism 20. The outgoing beam exits first prism 20 through first facet 20A and toward gain medium 3.

[0043] The return beam exiting the first face 3AA of the gain medium 3 enters the first prism 20 through the first face 20A. Due to the angle of incidence with the first facet 20A, the return beam is diffracted. The diffracted beam passes through the first prism 20 until it reaches the second facet 20B, where it undergoes TIR and is redirected towards the third facet 20C. At the third facet 20C, the second beam undergoes another TIR so that it is redirected towards the fourth facet 20D. The second beam exits the first prism 20 through the fourth facet 20D. The first prism 20 is configured such that the third facet 20C is angled with respect to the return beam so that it redirects the return beam exiting the first prism 20 along a path parallel to the outgoing beam entering the second prism 21.

[0044] The second beam spacer 7 is composed of a third optical prism 70 and a fourth optical prism 71. Each prism 70, 71 is composed of a single, integral piece of material that is highly transparent, for example, greater than 90%, to the wavelengths of the first beam A and the second beam B. The third and fourth prisms are made of substantially the same material. The prisms 70, 71 of the second beam spacer 7 are arranged as mirror images of the prisms 20, 21 of the first beam spacer 2.

[0045] The third prism 70 defines a first facet 70A, a second facet 70B, a third facet 70C, and a fourth facet 70D. The first facet 70A has an outer surface facing toward the gain medium 3. The fourth facet 70D extends parallel to the first facet 70A, with its outer surface facing in the opposite direction from the first facet 70A, i.e., away from the gain medium 3.

[0046] The fourth prism 71 includes a first facet 71A and a second facet 71B. The first facet 71A has an outer surface that faces toward and extends parallel to the second facet 70B of the third prism 70. The second facet 71B of the fourth prism 71 faces in the opposite direction to the first facet 71A and is parallel to the first facet 70A and fourth facet 70D of the third prism 70.

[0047] The third prism 70 is oriented so that its first facet 70A is perpendicular to the outbound beam path A and its fourth facet 70D is perpendicular to the return beam path B. Similarly, the fourth prism 71 is oriented so that its second facet 71B is perpendicular to the outbound beam.

[0048] The third prism 70 has a second facet 70B configured as follows: a) extends at an angle to the path A of the outgoing beam, allowing the outgoing beam to pass through it and enter the third prism 70 with minimal reflection (greater than 90% transmission); and b) Extends at an angle to the path B of the return beam to cause total internal reflection (TIR) ​​of the return beam.

[0049] Again, the required angle of facet 70B will depend in part on the material of third prism 70.

[0050] The outgoing beam exits the second face 3BB of the gain medium 3 and travels toward the reflector assembly 6. The outgoing beam enters the third prism 70 through the first facet 70A, exits the third prism 70 through the second facet 70B, and is refracted as it exits the third prism 70. The refracted outgoing beam then enters the fourth prism 71 through facet 71A. Because facets 70B and 71A are parallel, path B of the outgoing beam is refracted back in a direction parallel to its direction before entering the second facet 70B of the third prism 70. The outgoing beam exits the fourth prism 71 through the second facet 71B and travels toward the reflector 8.

[0051] The outgoing beam is reflected by reflector 8, which in this example is a corner cube, redirecting the outgoing beam back towards gain medium 3 on a path parallel to the outgoing beam as the return beam.

[0052] Return beam B enters third prism 70 through fourth facet 70D and undergoes total internal reflection upon striking third facet 70C, where it is reflected towards second facet 70B. Return beam B undergoes a second total internal reflection upon striking second facet 70B, where it is redirected out of third prism 70 through first facet 70A and into gain medium 3.

[0053] With the above-described arrangement, the physical spacing between the outbound and return beams as they pass through the respective second and fourth facets 21B and 20D is significantly greater than the spacing between them as they pass through the first facet 20A of the first prism 20. This allows both the seed source 9 and the further optical element 10 arranged to receive the return beam after it exits the first beam spacer 2 to be located closer to the gain medium 3 compared to arrangements that rely solely on divergence between the outbound and return beams as a result of their non-parallel paths out of the gain medium 3.

[0054] For similar reasons, the second beam spacer 7 allows the corner cube reflector 8 to be located closer to the gain medium 3 .

[0055] The further optical elements 10 may comprise, for example, any one or more of reflectors, lenses, wave plates and polarizers for directing and / or modifying the beam.

[0056] It will be appreciated that, as is well known to those skilled in the art, all surfaces of the prisms 20, 21, 70, 71 of the first and second spacers 2, 7 may be angled slightly from ideal to minimize the retroreflection path.

[0057] The first optical spacer 2 may not comprise the second optical prism 21, for example, if parallelism of the paths of the outgoing and return beams is not required. The same applies to the second optical spacer 7.

[0058] The configuration of the first optical prism 20, and in particular the arrangement of the third and fourth facets 20C and 20D, may differ from that described. For example, the fourth facet 20D may not be parallel to the first facet 20A, or alternatively, the first optical prism 20 may be adapted so that the return beam exits the third facet 20C, for example at an angle perpendicular to the outgoing beam, rather than through the fourth facet 20D.

[0059] The same applies mutatis mutandis to the second prism 21 .

[0060] The third facets 20C and 70C of the first and third prisms 20, 70 may be coated to reflect the return beam, rather than positioning these facets to reflect the return beam through TIR.

[0061] An image-rotating reflector may be used rather than a corner cube reflector. Both are preferred as they reflect light back along a parallel path. This allows the amplifier dimensions to be kept more compact. Alternatively, however, other reflector means, such as multiple separate mirrors, may be used instead.

[0062] In a further arrangement in which the reflector assembly 6 comprises a plurality of separate mirrors rather than a single reflective element, the third prism 70 of the second optical spacer 7 may be adapted to receive the return beam, for example, through the third facet 70C rather than the fourth facet 70D.

[0063] In another variation, the reflector assembly 6 may be omitted and instead the gain medium 3 may be configured to reflect the outgoing beam from the second face 3BB, for example through TIR or by coating the second face 3BB.

[0064] The first and second prisms may be made from a material with a different refractive index than the third and fourth prisms. The first and second prisms may be made from a material with a different refractive index. Similarly, although less preferred, the third and fourth prisms may be made from a material with a different refractive index.

[0065] Although the zigzag path slab gain medium in the described embodiment is trapezoidal, other prism shapes may be used, for example, where the first and second sides are parallel, in which case the prisms 70, 71 of the second beam spacer 7 may not be positioned as mirror images relative to the prisms 20, 21 of the first beam spacer 2. The following is a summary of the claims as originally filed: [C1] 1. An optical system comprising: a gain medium having a first side and a second side; and a pump mechanism, together configured to amplify first and second coherent optical beams as they travel through the gain medium between the first and second sides; the optical system is configured such that, in use, the first and second coherent optical beams travel different respective first and second paths, whereby the first and second optical beams are neither parallel nor coaxial with each other as they enter or exit the gain medium through the first side of the gain medium; optical spacing means for varying the spacing between the first and second optical beams; the optical spacing means comprising a prism having a facet, the prism positioned around the periphery of the first side of the gain medium such that the facet is in the first and second paths of the respective first and second coherent optical beams, the prism and facet being: is primarily transmissive to one of the first and second optical beams; Redirecting the other of the first and second optical beams by total internal reflection. An optical system characterized by being configured as follows. [C2] The optical system of claim 1, wherein the optical spacing means has a first side facing towards the gain medium and a second side facing away from the gain medium, and the optical spacing means is arranged to cause the first and second beams to have paths that are substantially parallel to each other when passing through the second side of the optical spacing means. [C3] The optical system of any one of C1 and C2, wherein the first and second optical beams travel their respective first and second paths in opposite directions through the first side of the gain medium. [C4] the optical spacing means is positioned to receive the first beam after it exits the first side of the gain medium; the optical system comprises a reflector on the first side of the gain medium, the reflector adapted to reflect the first beam after it exits the optical spacer back towards the optical spacing means as the second beam, and the optical spacing means adapted to direct the second beam received from the reflector back through the first side of the gain medium. The optical system described in C3. [C5] 5. The optical system of claim 3 or 4, wherein the first beam travels through the first side of the gain medium and into the gain medium, wherein the optical system comprises reflector means on a second side of the gain medium, the reflector means adapted to reflect the first optical beam after it exits the second side of the gain medium to travel the second path back through the gain medium as the second optical beam. [C6] the first and second paths through the gain medium are such that the first and second optical beams are neither parallel nor coaxial when entering or exiting the second side of the gain medium; the reflector means comprises a further optical spacing means and a further reflector; the reflector is adapted to reflect the first beam to provide the second beam, and the further optical spacing means is arranged to direct the first beam output from the gain medium towards the reflector and to direct the second beam from the reflector back towards the gain medium; the further optical spacing means comprises a further prism having a further facet, the further prism and the further facet being disposed in the first and second paths; and is primarily transmissive to one of the first and second optical beams; Redirecting the other of the first and second optical beams by total internal reflection. 10. The optical system of claim 5, wherein the optical system is adapted to: [C7] The optical system of any one of C4 to C6, wherein the reflector and / or, if applicable, the further reflector comprises a corner cube and / or an image-rotating reflector. [C8] The optical system of any one of C1 to C7, wherein the laser gain medium has a zigzag slab geometry. [C9] A multi-path optical amplifier comprising the optical system according to any one of C3 to C8.

Claims

1. 1. An optical system comprising: a gain medium having a first side and a second side; and a pump mechanism, both configured to amplify first and second coherent optical beams as they travel through the gain medium between the first and second sides; the optical system is configured such that, in use, the first and second coherent optical beams travel different respective first and second paths, whereby the first and second optical beams are neither parallel nor coaxial with each other when they enter the gain medium through the first side of the gain medium and when they exit the gain medium; optical spacing means for varying the spacing between the first and second optical beams; the optical spacing means comprising a prism having facets positioned around the periphery of the first side of the gain medium such that the facets are in the first and second paths of the respective first and second coherent optical beams, the prism and facets being arranged such that a different angle of incidence of the first beam on the facet compared to the second beam on the facet causes the prism and facet to: is primarily transmissive to one of the first and second optical beams; redirecting the other of the first and second optical beams by total internal reflection An optical system characterized by being configured as follows.

2. 10. The optical system of claim 1, wherein the optical spacing means has a first side facing toward the gain medium and a second side facing away from the gain medium, the optical spacing means being positioned to cause the first and second beams to have paths that are substantially parallel to each other as they pass through the second side of the optical spacing means.

3. 3. The optical system of claim 1, wherein the first and second optical beams travel their respective first and second paths in opposite directions through the first side of the gain medium.

4. the optical spacing means is positioned to receive the first beam after it exits the first side of the gain medium; the optical system comprises a reflector on the first side of the gain medium, the reflector adapted to reflect the first beam after it exits the optical spacing means back towards the optical spacing means as the second beam, and the optical spacing means adapted to direct the second beam received from the reflector back through the first side of the gain medium.

4. The optical system of claim 3.

5. 4. The optical system of claim 3, wherein the first beam travels through the first side of the gain medium and into the gain medium, and wherein the optical system comprises reflector means on a second side of the gain medium, the reflector means adapted to reflect the first optical beam after it exits the second side of the gain medium to travel the second path back through the gain medium as the second optical beam.

6. the first and second paths through the gain medium are such that the first and second optical beams are neither parallel nor coaxial when entering the second side of the gain medium and when exiting the second side; the reflector means comprises a further optical spacing means and a further reflector; the reflector is adapted to reflect the first beam to provide the second beam, and the further optical spacing means is arranged to direct the first beam output from the gain medium towards the reflector and to direct the second beam from the reflector back towards the gain medium; the further optical spacing means comprises a further prism having a further facet, the further prism and the further facet being disposed in the first and second paths; and is primarily transmissive to one of the first and second optical beams; redirecting the other of the first and second optical beams by total internal reflection 6. The optical system of claim 5, adapted to:

7. The optical system of claim 4 , wherein the reflector comprises a corner cube and / or an image-rotating reflector.

8. The optical system of claim 1 , wherein the laser gain medium has a zigzag slab geometry.

9. A multi-pass optical amplifier comprising the optical system of claim 3.

10. 1. An optical system comprising: a gain medium having a first side and a second side; and a pump mechanism, together configured to amplify first and second coherent light beams as they travel through the gain medium between the first and second sides; the optical system is configured such that, in use, the first and second coherent light beams travel different respective first and second paths, whereby the first and second light beams are neither parallel nor coaxial with each other as they enter or exit the gain medium through the first side of the gain medium; an optical spacing means for varying the spacing between the first and second light beams, the optical spacing means comprising a prism having facets positioned around the periphery of the first side of the gain medium such that the facets are within the first and second paths of the respective first and second coherent light beams, the prism and facets being primarily transparent to one of the first and second light beams; redirecting the other of the first and second light beams by total internal reflection wherein the optical spacing means has a first side facing toward the gain medium and a second side facing away from the gain medium, and the optical spacing means is arranged to cause the first and second beams to have paths that are substantially parallel to each other when passing through the second side of the optical spacing means.

11. 1. An optical system comprising: a gain medium having a first side and a second side; and a pump mechanism, together configured to amplify first and second coherent light beams as they travel through the gain medium between the first and second sides; the optical system is configured such that, in use, the first and second coherent light beams travel different respective first and second paths, whereby the first and second light beams are neither parallel nor coaxial with each other when they enter the gain medium through the first side of the gain medium and when they exit the gain medium; an optical spacing means for varying the spacing between the first and second light beams, the optical spacing means comprising a prism having facets positioned around the periphery of the first side of the gain medium such that the facets are within the first and second paths of the respective first and second coherent light beams, the prism and facets being primarily transparent to one of the first and second light beams; redirecting the other of the first and second light beams by total internal reflection wherein the first and second optical beams travel in opposite directions through the first side of the gain medium about their respective first and second paths, the first beam traveling through the first side of the gain medium and into the gain medium, and wherein the optical system comprises reflector means on the second side of the gain medium, the reflector means adapted to reflect the first optical beam after it has passed out of the second side of the gain medium to travel back through the gain medium about the second path as the second optical beam; the first and second paths through the gain medium are such that the first and second light beams are neither parallel nor coaxial when entering nor exiting the second side of the gain medium; the reflector means comprises a further optical spacing means and a further reflector; the reflector is adapted to reflect the first beam to provide the second beam, and the further optical spacing means is arranged to direct the first beam output from the gain medium towards the reflector and to direct the second beam from the reflector back towards the gain medium; the further optical spacing means comprises a further prism having a further facet, the further prism and the further facet being disposed in the first and second paths; and primarily transparent to one of the first and second light beams; redirecting the other of the first and second light beams by total internal reflection characterized in that it is adapted to Optical system.

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