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] The present invention relates to an optical system. In one aspect, the present invention relates to an optical amplifier, and more particularly, but not limited to, a multipass optical amplifier.

Background Art

[0002] In a laser system where a high gain of an optical beam is required, one solution is to provide a plurality of amplifiers that sequentially amplify the optical beam. An alternative solution is to use a multipass amplifier configured such that the optical beam passes through the same gain medium multiple times. The latter solution enables a reduction in the number, size, and weight of the components of the laser system compared to using a plurality of amplifiers.

[0003] FIG. 1A shows a simple arrangement of a prior art multipass amplifier 100. A beam source 101 is arranged to direct a beam 102 through a gain medium 103. A mirror arrangement 104 on the output side of the gain medium 103 is arranged to reflect the amplified beam 102 at an angle from the beam 102 back into the medium 103, whereby the reflected beam 105 takes a path back through the medium 103 that is different from the input beam 102. This solution has several problems. First, it is necessary to space the beam source 101 at a relatively large distance from the gain medium 103 in order to provide sufficient space to accommodate the next component of the optical assembly, which will receive the reflected beam 105 once it exits 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 at a relatively large distance to provide the necessary separation at the beam angle between the input beam 102 and the reflected beam 105, and thus also need to be spaced at a relatively large distance from the gain medium 103. This results in the optical assembly being physically long. A further problem is that the gain medium 103 is not used efficiently, meaning that a larger pump source is required, which also increases the heat load within the entire amplification medium.

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

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

[0006] The present invention was conceived to provide a compact-sized optical amplifier that does not suffer from the leakage problem of the design of FIG. 1B. SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, an optical system is provided, the optical system comprising a gain medium having a first side and a second side, and a pumping mechanism, both of which are configured to amplify first and second coherent optical beams traveling between the first and second sides through the gain medium, wherein the first and second coherent optical beams travel along different respective first and second paths that are neither parallel nor coaxial when 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 changing the spacing between the first and second optical beams, the optical spacing means comprising a prism having facets, the prism being positioned around (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, and the prism and the facets being configured to be mainly transmissive to one of the first and second optical beams and to redirect the other of the first and second optical beams by internal total reflection.

[0008] The prism can be arranged within the paths of both the first and second beams and can thus be positioned very close to, optionally in physical contact with, the gain medium. The optical spacing means can be configured to increase the separation distance between the beams, enabling a plurality of optical components, such as beam sources and further optical components, on the same side of the gain medium to be positioned much closer to the gain medium, thus enabling a reduction in the length of the optical system.

[0009] Each beam can be associated with the following respectively: a first passing 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 passing point where the beam enters the optical spacing means on its way towards the gain medium or exits the optical spacing means after already passing through the gain medium.

[0010] The optical spacing means is preferably configured such that the physical spacing between the second passing points is greater than the physical spacing between the first passing points and / or the divergence angle between the first and second beams at the second passing point is greater than the divergence angle at the first passing point. The former is desirable, for example, when it is desirable for the beams to be parallel when passing through the second passing 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 the first surface of the gain medium and where the first and second optical beams travel in opposite directions through the first side of the gain medium. In one embodiment, the present invention is directed to a multipass optical amplifier. That being said, the inventors recognize that the present invention may have a broader range of applications. For example: For amplifying two similar wavelength beams on a single pass through the gain medium and extracting them after amplification, In an optical amplifier where one of the first and second beams is used to optically pump a laser gain material, and In the case where one of these beams is used to excite a species on a facet of the gain medium and the other beam is the result of optical fluorescence arising from the excited chemical species.

[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, and the optical spacing means is arranged to cause the first and second beams to have substantially parallel paths when passing through the second side of the optical spacing means. Providing parallel paths may allow for further reduction in the separation of a plurality of optical components on the same side of the gain medium.

[0013] The optical spacing means may comprise a second prism. The second prism is arranged in the path of either the first and second beams such that it deflects the beam. The second prism may function to correct the refraction of the transmitted beam as a result of passing through the facet such that it exits the optical spacing means on a path substantially parallel to the path by which it entered the optical spacing means.

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

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

[0016] An optical system including a gain medium may be arranged such that first and second coherent optical beams enter or exit the gain medium through the second side. In that case, the reflector is arranged on the second side of the gain medium and may reflect the first optical beam after it exits the second side of the gain medium and return it into the gain medium about the 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 can be such that when the first and second optical beams enter or exit the second side of the gain medium, they are neither parallel nor coaxial. In that case, the optical system can comprise further optical spacing means, which are arranged to receive the first beam output from the gain medium, direct the first beam towards a reflector, receive the second beam from the reflector and direct it back to the gain medium.

[0018] The further optical spacing means can comprise a further prism having further facets, the further prism and further facets being mainly transparent 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 with optical spacing means on both sides of the gain medium allows for a compactification of the optical system at both ends.

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

[0020] The pumping mechanism can comprise a laser diode pump.

[0021] The optical system can comprise a heat sink arranged in direct contact with the third side of the gain medium to extract heat from the gain medium through the third side by conduction cooling, the third side extending between the first side and the second side. The heat exchanger can be adhered or clamped to the gain medium and attached to a suitable cooling plate. It is beneficial for the heat exchanger to have a coefficient of thermal expansion similar to that of the material of the gain medium.

[0022] The pumping mechanism can 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 applications, although not all, the facets of the prism are preferably as transmissive as possible for one of the beams and minimize the optical losses in that beam. Thus, the facet can have an optical transmissivity of 90% or more for that beam.

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

[0025] Next, the present invention will be described by way of example with reference to the following drawings.

Brief Description of the Drawings

[0026]

Figure 1A

Figure 1B

Figure 2

Best Mode for Carrying Out the Invention

[0027] FIG. 2 shows a multipass optical amplifier 1. The amplifier 1 includes a first beam spacer 2, a gain medium 3, a laser pump 4 for pumping the gain medium 3, a heat sink 5, and a reflector assembly 6 composed 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. Each of the third and fourth sides extends between the first side 3A and the second side 3B and faces in opposite directions to each other.

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

[0030] The first beam spacer 2 is disposed around the first side 3A of the gain medium 3. The reflector assembly 6 is disposed around the 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 disposed in direct contact with the fourth side 3D of the gain medium 3 in order to extract heat from the gain medium 3 through the fourth side 3D. As a result, both the injection of pump radiation into the gain medium 3 and the extraction of heat from the gain medium 3 occur in a direction orthogonal to the general direction of travel of the optical beam amplified through the gain medium 3. This arrangement minimizes the thermal lensing effect in the gain material.

[0033] A coherent beam of light from the seed source 9 passes through the first beam spacer 2, enters the gain medium 3 through the first surface 3AA, and follows a first path A (solid line) that passes through the gain medium 3 along a first zigzag path. The amplified beam exits the gain medium 3 through the second surface 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 amplification. Taking different zigzag paths through the gain medium 3, the return beam exits the gain medium 3 through the first surface 3AA and then passes back 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 immediately before entering the first beam spacer 2.

[0034] Importantly, the paths A and B of each forward and return beam diverge (or at least are not parallel or coaxial) when exiting the gain medium through the first surface 3AA and the second surface 3BB.

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

[0036] The first beam spacer 2 is composed of a first optical prism 20 and a second optical prism 21. Each prism 20, 21 is composed of a single integral piece of a material that is highly transparent, for example, higher than 90%, with respect to the wavelengths of the forward 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 towards the gain medium 3. The fourth facet 20D has an outer surface facing in a direction opposite to the first facet 20A, i.e., facing away from the gain medium 3, and extends parallel to the first facet 20A.

[0038] The second prism 21 comprises a first facet 21A and a second facet 21B. The first facet 21A has an outer surface facing towards the second facet 20B of the first prism 20 and extending parallel to the second facet 20B. The second facet 21B of the second prism 21 faces in a direction opposite 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 such that the first facet 20A is perpendicular to the forward beam and the fourth facet 20D is perpendicular to the return beam. The second prism 21 is oriented such that the second facet 21B is perpendicular to the path A of the forward beam.

[0040] The second facet 20B of the first prism 20 is configured as follows: a) Extending at an angle with respect to the path A of the forward beam, allowing the forward beam to pass through it with minimal reflection (transmission rate higher than 90%) and enter the first prism 20, and b) Extending at an angle with respect to the path B of the return beam to cause total internal reflection (TIR) of the return beam.

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

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

[0043] The return beam emerging from the first surface 3AA of the gain medium 3 enters the first prism 20 through the first surface 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 TIR again 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 to be angled with respect to the return beam such that the third facet 20C redirects the return beam exiting the first prism 20 in a path parallel to the forward 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 of the prisms 70, 71 is composed of, for example, a single integral piece of a material that is highly transparent, for example, higher than 90%, with respect 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 in a mirror image with respect to 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 has an outer surface facing in a direction opposite to that of the first facet 70A, that is, facing away from the gain medium 3, and extends parallel to the first facet 70A.

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

[0047] The third prism 70 is oriented such that the first facet 70A is perpendicular to the path A of the forward beam and the fourth facet 70D is perpendicular to the path B of the backward beam. Similarly, the fourth prism 71 is oriented such that the second facet 71B is perpendicular to the forward beam.

[0048] The third prism 70 is configured such that the second facet 70B is as follows: a) Extending at an angle to the path A of the forward beam, allowing the forward beam to pass through it with minimum reflection (transmission higher than 90%) and enter the third prism 70, and b) Extending at an angle to the path B of the return beam to cause total internal reflection (TIR) of the return beam.

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

[0050] The forward beam exits from the second surface 3BB of the gain medium 3 and travels towards the reflector assembly 6. The forward beam enters the third prism 70 through the first facet 70A, exits the third prism 70 through the second facet 70B, and is refracted when it exits the third prism 70. Then, the refracted forward beam enters the fourth prism 71 through the facet 71A. Since the facets 70B and 71A are parallel, the path B of the forward beam is refracted back in a direction parallel to the direction before it enters the second facet 70B of the third prism 70. The forward beam exits the fourth prism 71 through the second facet 71B and travels towards the reflector 8.

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

[0052] The return beam B enters the third prism 70 through the fourth facet 70D, undergoes total internal reflection when it hits the third facet 70C, and is reflected towards the second facet 70B. When the return beam B hits the second facet 70B, it undergoes a second total internal reflection, which redirects it to exit the third prism 70 through the first facet 70A and enter the gain medium 3.

[0053] With the above arrangement, when the forward beam and the backward beam pass through the respective second facet 21B and fourth facet 20D, the physical distance between the forward beam and the backward beam is significantly greater than their distance when they pass through the first facet 20A of the first prism 20. This enables both the seed source 9 and the further optical element 10 arranged to receive the backward beam after it exits the first beam spacer 2 to be positioned closer to the gain medium 3 compared to an arrangement that relies solely on the divergence between the forward beam and the backward beam as a result of their non-parallel paths emerging from the gain medium 3.

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

[0055] The further optical element 10 may comprise, for example, any one or more of a reflector, a lens, a waveplate, and a polarizer for guiding and / or modifying the beam.

[0056] As will be understood by those skilled in the art, all the surfaces of the prisms 20, 21, 70, 71 of the first and second spacers 2, 7 may be slightly angled from the ideal in order to minimize the retroreflective path.

[0057] The first optical spacer 2 may, for example, not comprise the second optical prism 21 if it is not necessary to parallelize the paths of the forward beam and the backward beam. The same applies to the second optical spacer 7.

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

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

[0060] Rather than arranging the third facets 20C and 70C of the first and third prisms 20, 70 so as to reflect the return beam through TIR, these facets can be coated so as to reflect the return beam.

[0061] Rather than a corner cube reflector, an image rotation reflector can be used. Either is preferable as it reflects light so as to return it along a parallel path. This makes it possible to keep the dimensions of the amplifier more compact. That being said, alternatively, other reflector means, for example, a plurality of separate mirrors, can be used instead.

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

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

[0064] The first and second prisms can be manufactured from materials having different refractive indices from the third and fourth prisms. The first and second prisms can be of materials having different refractive indices. Similarly, although less preferably, the third and fourth prisms can be of materials having different refractive indices.

[0065] The zigzag path slab gain medium of the described embodiment is trapezoidal, but other prism forms, for example, where the first side and the second side are parallel, can be used. In that case, the prisms 70, 71 of the second beam spacer 7 may not be arranged mirror-image to the prisms 20, 21 of the first beam spacer 2.

Claims

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 between the first and second sides through the gain medium, wherein the optical system is configured such that, in use, the first and second coherent optical beams travel along different respective first and second paths, whereby the first and second optical beams are neither parallel nor coaxial with each other when they enter or exit the gain medium through the first side of the gain medium, comprising optical spacing means for changing the spacing between the first and second optical beams, wherein the optical spacing means comprises a prism having facets, the prism being positioned around 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 substantially transmissive to one of the first and second optical beams, configured to redirect the other of the first and second optical beams by internal total reflection characterized in that.

2. 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 a path that is substantially parallel to each other when they pass through the second side of the optical spacing means. The optical system according to claim 1.

3. The first and second optical beams travel along their respective first and second paths in opposite directions through the first side of the gain medium. The optical system according to claim 1 or 2.

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 includes a reflector on the first side of the gain medium, and the reflector is adapted to reflect the first beam after it exits the optical spacer and return it as the second beam towards the optical spacing means, and the optical spacing means is adapted to direct the second beam received from the reflector to return through the first side of the gain medium. The optical system according to claim 3.

5. The first beam travels into the gain medium through the first side of the gain medium, where the optical system includes reflector means on the second side of the gain medium, and the reflector means is adapted to reflect the first optical beam after it exits the second side of the gain medium and cause it to travel back through the gain medium along the second path as the second optical beam. The optical system according to claim 3 or 4.

6. The first and second paths through the gain medium are such that when the first and second optical beams enter or exit the second side of the gain medium, they are neither parallel nor coaxial. The reflector means includes 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 direct the second beam to return from the reflector towards the gain medium. The further optical spacing means includes a further prism having further facets, and the further prism and further facets are arranged in the first and second paths and are mainly transmissive to one of the first and second optical beams, and are adapted to redirect the other of the first and second optical beams by total internal reflection. The optical system according to claim 5.

7. The reflector, and / or, where applicable, the further reflector includes a corner cube and / or an image-rotating reflector. The optical system according to any one of claims 4 to 6.

8. The laser gain medium has a zigzag slab geometry. The optical system according to any one of claims 1 to 7.

9. A multipass optical amplifier comprising the optical system according to any one of claims 3 to 8.

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