WO2007103848A1

WO2007103848A1 - Compact laser cavity extender and associated method

Info

Publication number: WO2007103848A1
Application number: PCT/US2007/063263
Authority: WO
Inventors: Michael Meredith Albert; Marc Spencer Adams
Original assignee: Northrop Grumman Corporation
Priority date: 2006-03-03
Filing date: 2007-03-05
Publication date: 2007-09-13

Abstract

A cavity extender (10) for a laser resonator includes a first prism. The first prism defines a longitudinal axis (14) and includes a plurality of axial faces (12). The axial faces are disposed such that a laser beam entering one of the axial faces of the first prism at an angle (30) with respect to the longitudinal axis (14) is reflected from a plurality of axial faces to form a helically shaped optical path (36). The helically shaped optical path (36) advances about the longitudinal end (16) of the first prism toward a second longitudinal end (18) of the first prism.

Description

COMPACT LASER CAVITY EXTENDER AND ASSOCIATED METHOD

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to laser systems, and more particularly, to a laser resonator of a laser system.

BACKGROUND OF THE INVENTION

Laser systems are widely used in portable applications, including military applications. For example, laser rangefmders and target designators may be carried by soldiers, vehicles, or aircraft to be used during combat operations. Accordingly, the weight and volume of such systems are desirably minimized to ensure minimal impact on both the soldiers and design considerations for the vehicles or aircraft.

The physical dimensions of a laser resonator have an impact upon the performance of the laser resonator since spatial properties of an output beam of the laser resonator depend strongly thereupon. For a given beam diameter, which is often determined by physical dimensions of an active gain medium, divergence of the output beam is largely determined by losses experienced by competing spatial modes. In general, an amount of loss exhibited by a transverse laser mode is related to a Fresnel number, given by an equation L²/4λD. Higher order modes experience increasing loss as the Fresnel number decreases. Typical lasers operating in a single transverse mode exhibit Fresnel numbers near unity, while multi-mode lasers often have Fresnel numbers greater than 10. One means often used to improve beam quality of a laser system therefore includes increasing a length of the laser resonator, although such an increase in length may cut against the weight and mobility of the laser resonator.

In Q-switched lasers, for example, a long laser resonator responds more slowly to changes in operating conditions such as input power, intracavity losses, or outcoupling than a short laser resonator. Thus, duration of an output pulse of a Q-switched laser is directly related to the length of the laser resonator. In such a Q-switched laser, beam-induced optical damage may occur in response to high peak optical powers in a cavity of the laser resonator. High peak optical powers may be achieved when high desired output pulse energy occurs for a short pulse duration. Such short pulse duration would more likely be required for a laser resonator having a short length. Accordingly, increasing the length of the laser resonator may prevent beam-induced optical damage of the laser resonator. Increasing the length of the laser resonator may also minimize intracavity fluence and match optical receiver electronic bandwidths.

As stated above, improvement of beam quality, prevention of damage to the laser resonator and other benefits may be achieved by increasing the length of the laser resonator to increase the optical path of the beam. Accordingly, techniques have been developed to increase the optical path of the beam. One such method includes linearly extending the laser resonator. However, increasing the length of the laser resonator typically introduces a disadvantage of increasing the physical length of the laser system. Consequently, weight and volume of the laser system are increased. Thus, methods for increasing the optical path of the beam while minimizing the length of the laser resonator have been sought. Examples of such methods include folding the optical path of the beam using mirrors or prisms. In this regard, multiple legs of the optical path may be achieved in a given space by use of multiple mirrors or prisms, thereby greatly increasing the optical path length of the beam. However, use of multiple mirrors or prisms results in increased cost, limited reductions in weight and volume of the laser system, and introduces a number of alignment issues. Thus, a need exists for a device capable of increasing the optical path length of the beam without increasing the cost, weight and volume of the laser system.

BRIEF SUMMARY OF THE INVENTION

A cavity extender is therefore provided that uses at least one prism to increase a path length of a laser beam through a laser resonator. Since the path length is increased while employing a minimal number of components, a laser system may be constructed with reduced weight, volume and/or cost relative to conventional laser systems.

In one exemplary embodiment, a cavity extender for a laser resonator includes a first prism. The first prism defines a longitudinal axis and includes a plurality of axial faces. The axial faces are disposed such that a laser beam entering one of the axial faces of the first prism at an angle with respect to the longitudinal axis is reflected from respective ones of the axial faces to form a helically shaped optical path. The helically shaped optical path advances about the longitudinal axis in a first direction extending from a first longitudinal end of the first prism toward a second longitudinal end of the first prism.

In another exemplary embodiment, a cavity extender for a laser resonator includes a first prism. The first prism defines a longitudinal axis and includes a body and a return portion. The body includes first, second, third and fourth axial faces. The axial faces are disposed such that a laser beam entering one of the axial faces of the first prism at an angle with respect to the longitudinal axis is reflected from a plurality of the axial faces to form a helically shaped optical path. The helically shaped optical path advances about the longitudinal axis in a first direction extending from a first longitudinal end of the first prism toward a second longitudinal end of the first prism. In addition to the cavity extender as described above, other aspects of the present invention are directed to corresponding methods for extending a path length of a laser beam in a laser resonator. In one exemplary embodiment, the method includes directing the laser beam into a prism at an angle with respect to a longitudinal axis of the prism, reflecting the laser beam from respective axial faces of the prism to form a helically shaped optical path which advances about the longitudinal axis of the prism, and outputting the laser beam from the prism.

Embodiments of the invention provide an increased path length of a laser beam using a minimal number of components. As a result, cost, weight and volume of a laser resonator, and consequently a laser system, may be reduced while improving beam quality and reducing susceptibility to laser system component damage. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: Fig. 1 is a perspective view of a cavity extender of a laser resonator according to an exemplary embodiment of the invention;

Fig. 2 is a perspective view of a cavity extender including a return facet according to an exemplary embodiment of the invention;

Fig. 3 is a sectional view of a cavity extender of a laser resonator according to an exemplary embodiment of the invention;

Fig. 4 is a sectional view of a cavity extender of a laser resonator according to another exemplary embodiment of the invention;

Fig. 5 is a sectional view of a cavity extender of a laser resonator according to still another exemplary embodiment of the invention; Fig. 6 is a sectional view of a cavity extender of a laser resonator according to yet another exemplary embodiment of the invention;

Fig. 7 is a perspective view of a cavity extender according to an exemplary embodiment of the invention;

Fig. 8 is a top view of a first prism of Figure 7; Fig. 9 is a side view of the first prism of Figure 7;

Fig. 10 is an alternative side view of the first prism of Figure 7;

Fig. 11 is a top view of a second prism of Figure 7;

Fig. 12 is a side view of the second prism of Figure 7;

Fig. 13 is an alternative side view of the second prism of Figure 7; Fig. 14 is a perspective view of a cavity extender according to an exemplary embodiment of the invention;

Fig. 15 is a top view of a second prism of Figure 14;

Fig. 16 is a side view of the second prism of Figure 14;

Fig. 17 is an alternative side view of the second prism of Figure 14; Fig. 18 is perspective view of a cavity extender according to another exemplary embodiment of the invention; Fig. 19 is a graph showing polarization defect for a "straight through" delay coil according to an exemplary embodiment of the invention;

Fig. 20 is a graph showing polarization defect for a delay coil having one degree of misalignment about a y-axis of the delay coil according to an exemplary embodiment of the invention;

Fig. 21 is a graph showing polarization defect for a delay coil having one degree of misalignment about a z-axis of the delay coil according to an exemplary embodiment of the invention;

Fig. 22 is a graph showing polarization defect for a delay coil having one degree of misalignment about an x-axis of the delay coil according to an exemplary embodiment of the invention; and

Fig. 23 is a block diagram showing a method for extending a path length of a laser beam in a laser resonator according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout.

Figure 1 is a perspective view of a cavity extender 10 of a laser resonator in accordance with an exemplary embodiment of the invention. The cavity extender 10 has a prism shape which includes a plurality of axial faces 12. Although a rectangular prism shape is shown in Figure 1 , it should be noted that any suitable shape is envisioned. Examples of cross sections of some suitable shapes are shown in Figures 3 through 6 below. Each of the axial faces 12 is disposed to lie in a plane that is substantially parallel to a longitudinal axis 14 of the cavity extender 10. The cavity extender 10 also includes a first longitudinal end 16 and a second longitudinal end 18 which is disposed opposite to the first longitudinal end 16 with respect to the axial faces 12. The cavity extender 10 may be constructed from a transparent dielectric material having total internal reflection. Alternatively, the cavity extender 10 may be constructed such that the cavity extender 10 is a hollow structure and each of the surfaces defining the cavity extender 10 is a reflective surface.

A laser beam 20 is passed through the cavity extender 10 with an input beam 22 being incident upon the cavity extender 10 and an output beam 24 being emitted therefrom. The input beam 22 is optically communicated into the cavity extender 10 such that the laser beam 20 forms a first angle 30 with respect to the longitudinal axis 14. The laser beam 20 is reflected from each of the axial faces 12 at reflection points 25 due to the total internal reflection within the cavity extender 10. As the laser beam 20 advances toward one of the axial faces 12, a path of travel through the cavity extender 10 includes a component of motion in a first direction 32 that extends from the first longitudinal end 16 toward the second longitudinal end 18. In response to the laser beam 20 advancing toward one of the axial faces 12 at the first angle 30, the laser beam 20 is reflected from the one of the axial faces 12 at a second angle that is complementary to the first angle 30. Thus, following a plurality of reflections of the laser beam 20 a helically shaped path 36 or spiral shaped path is formed by the laser beam 20 through the cavity extender 10. In response to entry of the input beam 22 through one of the axial faces 12 proximate to the first longitudinal end 16, the laser beam 20 traces the helically shaped path 36 which advances about the longitudinal axis 14 in the first direction 32. Thus, the laser beam 20 advances in the first direction 32 and the output beam 24 eventually exits the cavity extender 10 as described below through one of the axial faces 12 proximate to the second longitudinal end 18. In the exemplary embodiment of Figure 1, a cross section view of the helically shaped path has a shape of a square. However, other shapes are possible as shown in Figures 3 through 6. Accordingly, a long optical path length is achieved in a relatively short element. In order to further increase the optical path length without increasing the physical length of the cavity, the optical path may be doubled back upon itself. In this regard, Figure 2 shows a perspective view of a cavity extender 10 including a return facet 38. The return facet 38 is disposed, for example, proximate to the second longitudinal end 18 of one of the axial faces 12. The return facet 38 reflects the laser beam 20 such that the laser beam 20 shifts from advancing in the first direction 32 to advancing in a second direction 39 that is opposite to the first direction 32. The return facet 38 may be formed by truncation of a portion of the one of the axial faces 12. In other words, a portion of one of the axial faces 12 proximate to the second longitudinal end 18 may be slanted toward the longitudinal axis 14. The truncated portion may have a shape of a triangular prism having a hypotenuse at least as long as a diameter of the laser beam 20. In response to reflection at the return facet 38, the laser beam 20 advances in the second direction 39 about the longitudinal axis 14 to trace the helically shaped path 36. Thus, the laser beam 20 advances in the first direction 32 until it is reflected by the return facet 38. Following reflection by the return facet 38, the laser beam 20 advances in the second direction 39 toward the first longitudinal end 16. Accordingly, the output beam 24 may be discharged at a same longitudinal end from which the input beam 22 was launched.

Figures 3 through 6 each show a sectional view of a cavity extender of a laser resonator, in accordance with exemplary embodiments of the invention. Figure 3 shows a cross section of a regular pentagonal prism 40. Arrows 42 show a cross section of a spiral shaped path of the laser beam 20 responsive to the regular pentagonal prism 40 understanding that the beam path does not lie entirely in the cross sectional plane but, instead, advances along the longitudinal axis. Figure 4 shows a cross section of a regular octagonal prism 44. Arrows 46 show a cross section of a spiral shaped path of the laser beam 20 responsive to the regular octagonal prism 40. Figure 5 shows a cross section of a square prism having truncated apexes 48. In this regard, the apexes 48 of an otherwise square prism may be truncated or, alternatively, never formed without any effect upon the signal path since the laser beam 20 does not propagate through the apexes in any event. Thus, the size of the prism may be reduced and/or material costs may be reduced. Arrows 50 show a cross section of a spiral shaped path of the laser beam 20 responsive to the square prism having truncated apexes 48. Figure 6 shows a cross section of a hexagonal prism 52 with two axial faces having greater widths than remaining axial faces. Arrows 54 show a cross section of a spiral shaped path of the laser beam 20 responsive to the hexagonal prism 52.

Figure 7 is a perspective view of a cavity extender 60 according to an exemplary embodiment of the invention. Figure 7 shows a cavity extender in which the input and output beams of the laser beam 20 lie collinear. Figure 8 shows a top view of a first prism 62 of the cavity extender 60. Figure 9 shows a side view of the first prism 62. Figure 10 shows an alternative side view of the first prism 62. Referring now to Figures 7 through 10, the cavity extender 60 includes the first prism 62, a second prism 64 and a third prism 66. In this exemplary embodiment, one of the second and third prisms 64 and 66 may function as a coupling prism to launch a laser beam into the first prism 62. In other words, the laser beam 20 is provided to the first prism 62 via optical coupling with the second prism 64 or the third prism 66. Additionally, the other of the second and third prisms 64 and 66 may function as a coupling prism to output the laser beam from the first prism 62 via optical coupling with the second prism 64 or the third prism 66. It should be noted that in an exemplary embodiment in which the cavity extender 60 is made of dielectric material, the second and third prisms 64 and 66 (i.e., input and output prisms) are required.

The first prism 62 includes a body 68 and a return portion 69. The body 68 includes a first axial face 70, a second axial face 72, a third axial face 74, and a fourth axial face 76. The first and third axial faces 70 and 74 extend substantially parallel to each other. The second and fourth axial faces 72 and 76 extend substantially parallel to each other and substantially perpendicular to the first and third axial faces 70 and 74. The body 68 further includes a first longitudinal end face 78 that lies in a plane substantially perpendicular to each of the first, second, third and fourth axial faces 70, 72, 74 and 76. The first longitudinal end face 78 is disposed proximate to a first longitudinal end 80 of the first prism 62. The first, second, third and fourth axial faces 70, 72, 74 and 76 each extend in a first direction 82 toward a second longitudinal end face 84 which is disposed at a second longitudinal end 86 of the first prism 62. The second longitudinal end face 84 is substantially parallel to the first longitudinal end face 78. In an exemplary embodiment, a first vertex 88 is disposed between the first and second axial faces 70 and 72. The first vertex 88 is shorter than a second vertex 90, which is disposed between the second and third axial faces 72 and 74. The second vertex is shorter than both a third vertex 92, which is disposed between the third and fourth axial faces 74 and 76, and a fourth vertex 94, which is disposed between the first and fourth axial faces 70 and 76. The third and fourth vertexes 92 and 94 are equal in length.

The return portion 69 is disposed proximate to the second longitudinal end 86 of the first prism 62. The return portion 69 includes the second longitudinal end face 84 and a first face 96, a second face 98, a third face 100 and a fourth face 102. The first, second, third and fourth faces 96, 98, 100 and 102 are disposed between the second longitudinal end face 84 and corresponding first, second, third and fourth axial faces 70, 72, 74 and 76. The first face 96 extends from the first axial face 70 to the second longitudinal end face 84 and lies in a plane with the first axial face 70. The first face 96 is substantially rectangular in shape except that a vertex between the first face 96 and the first axial face 70 is longer than a vertex between the second longitudinal end face 84 and the first face 96 and a vertex between the first face 96 and the second face 98 is slanted toward the fourth face 102. The fourth face 102 extends from the fourth axial face 76 and lies in a plane with the fourth axial face 76. The fourth face 102 is substantially a mirror image of the first face 96.

Each of the second and third faces 98 and 100 is slanted inwardly. In other words, the second and third faces 98 and 100 are each slanted toward a longitudinal axis 104 of the first prism 62. The second and third faces 98 and 100 cooperate to function as a return facet. In other words, in response to a laser beam 20 being launched proximate to the first longitudinal end 80 of the first prism 62, the laser beam 20 is reflected from the first, second, third and fourth axial faces 70, 72, 74 and 76 to advance in the first direction 82 in the helically shaped path 36. In response to reflection of the laser beam 20 from the second and third faces 98 and 100, the laser beam is directed to advance in a second direction 108, which is opposite to the first direction 82, in the helically shaped path 36 toward the first longitudinal end 80. A vertex between the second face 98 and the second axial face 72 is longer than a vertex between the second face 98 and the second longitudinal end face 84. The vertex between the second face 98 and the second axial face 72 is slanted away from the second longitudinal end face 84 as the vertex between the second face 98 and the second axial face 72 extends from the third face 100 toward the first face 96. The vertex between the second face 98 and the second longitudinal end face 84 slants toward the fourth face 102 as the vertex between the second face 98 and the second longitudinal end face 84 extends from the third face 100 toward the first face 96. A vertex between the third face 100 and the third axial face 74 is longer than a vertex between the third face 100 and the second longitudinal end face 84. The vertex between the third face 100 and the third axial face 74 is slanted away from the second longitudinal end face 84 as the vertex between the third face 100 and the third axial face 74 extends from the fourth face 102 toward the second face 98. The vertex between the third face 100 and the second longitudinal end face 84 slants toward the first face 96 as the vertex between the third face 98 and the second longitudinal end face 84 extends from the second face 98 toward the fourth face 102.

In an exemplary embodiment, a distance between the first and second longitudinal end faces 78 and 84 is about 16 mm. A distance between the first and third axial faces 70 and 74 and a distance between the second and fourth axial faces 72 and 76 is about 15 mm. Both the second and third faces 98 and 100 slant toward the longitudinal axis 104 at about an 8.38 degree angle with respect to a plane of the second and third axial faces 72 and 74, respectively. The first and third axial faces 70 and 74 are parallel to each other to less than about 10 arc- seconds. The second and fourth axial faces 72 and 76 are parallel to each other to less than about 10 arc-seconds and perpendicular to the first and third axial faces 70 and 74 to less than about 10 arc-seconds. The first and second longitudinal end faces 78 and 84 are perpendicular to the first, second, third and fourth axial faces 70, 72, 74 and 76 to less than about 1 arc-minute. The first and second longitudinal end faces 78 and 84 are rough ground. All other faces are polished to a scratch/dig ratio of about 20/10. All surfaces are flat to less than about 1/10 wave at 633 nm over an aperture excluding 0.5 mm near any vertex. Additionally, it should be noted that other sizes, angles and finishes may be implemented if desired.

Figure 11 is a top view of the second prism 64 of the cavity extender 60 of Figure 7. Figure 12 is a side view of a side face of the second prism 64. Figure 13 is a side view of another side face of the second prism 64. The second prism 64 will now be described with reference to Figures 7 and 11 through 13. It should be noted that the third prism 66 is structured substantially identically to the second prism 64 except that the third prism 66 is a mirror image of the second prism 64. The second prism 64 includes a first radial face 110 and a second radial face 112. The first and second radial faces 110 and 112 are each substantially shaped as a triangle. In an exemplary embodiment, the first and second radial faces 110 and 112 are each substantially shaped as a right isosceles triangle. The first and second radial faces 110 and 112 are disposed substantially parallel to each other. The first radial face 110 is larger than the second radial face 112. The second prism 64 includes a first side face 114, a second side face 116 and a third side face 118. The first side face 114 corresponds to a hypotenuse of the right isosceles triangle. The first and second radial faces 110 and 112 are disposed to face each other such that a first vertex 120 disposed between the first radial face 110 and the first side face 114 lies coplanar with a second vertex 122 disposed between the first side face 114 and the second radial face 112 in a plane substantially perpendicular to both the first and second radial faces 110 and 112. A third vertex 124 disposed between the first radial face 110 and the second side face 116 lies coplanar with a fourth vertex 126 disposed between the second side face 116 and the second radial face 112 in a plane substantially perpendicular to both the first and second radial faces 110 and 112. A fifth vertex 128 disposed between the first radial face 110 and the third side face 118 lies coplanar with a sixth vertex 129 disposed between the third side face 118 and the second radial face 112 in a plane slanted to form an acute angle with respect to the first radial face 110 and an obtuse angle with respect to the second radial face 112. In an exemplary embodiment, a distance between the first and second radial faces 110 and 112 is about 3 mm, a length of the second and third side faces 116 and 118 is about 10.6 mm, and an angle between the third side face 118 and the first radial face 110 is about 66 degrees. However, other sizes and angles are also possible. It should be noted that although the first and second radial faces 110 and 112 are shown to have a right isosceles triangular shape, other shapes are also possible. In an exemplary embodiment, as shown in Figure 7, the cavity extender 60 includes the first prism 62 having the second prism 64 disposed in optical communication with the second axial face 72 and the third prism 66 disposed in optical communication with the third axial face 74. The first side face 114 of the second prism 64 is disposed proximate to a portion of the second axial face 72 that is proximate to the first longitudinal end face 78 such that the second radial face 112 lies in a plane with the first longitudinal end face 78. Additionally, the cavity extender 60 includes the third prism 66 having an identical shape to that of the second prism 64, except that the third prism 66 is substantially a mirror image of the second prism 64, being disposed proximate to the third axial face 74 at a portion of the second axial face 72 that is proximate to the first longitudinal end face 78. Thus, in response to the laser beam 20 being launched into the cavity extender 60 at an initial angle of 90 degrees with respect to the longitudinal axis 104 via one of the second and third prisms 64 and 66, it is assured that the laser beam 20 will exit the cavity extender 60 at an angle parallel to the initial angle via the other of the second and third prisms 64 and 66. In other words, input and output beams are parallel regardless of orientation of the cavity extender 60. Thus, if the cavity extender 60 is made from a laser gain medium and pumped with an external source, the cavity extender 60 may produce a compact laser.

Figure 14 shows a perspective view of a cavity extender 60' according to another exemplary embodiment of the present invention. Figure 15 shows a top view of a second prism 64' of Figure 14. Figure 16 shows a side view of the second prism 64' of Figure 14 and Figure 17 shows an alternative side view of the second prism 64' of Figure 14. The cavity extender 60' will now be described with reference to Figures 14 through 17. The cavity extender 60' of Figure 14 includes a first prism 62' that is substantially similar to the first prism 62 of Figures 7 through 10 except that a return portion 69' includes only one slanted facet or return facet 130. The cavity extender 60' of Figure 14 further includes a second prism 64' that is substantially similar to the second prism 64 of Figures 7 and 11 through 13 except that a second side face 116' of the second prism 64' is slanted with respect to first and second radial faces 110' and 112' to form an acute angle with respect to the first radial face 110' and an obtuse angle with respect to the second radial face 112'. Additionally, a portion of both second and third side faces 116' and 118' which contacts a first side face 114' is truncated.

In an exemplary embodiment, the second prism 64' is disposed in optical communication with the first prism 64'. The first side face 114' is disposed proximate to an axial face that is at an opposite side of the first prism 64' with respect to the return facet 130. In such a configuration, the second prism 64' provides both input and output coupling for the cavity extender 60' via the second and third side faces 116' and 118'. A distance between first and second radial faces 110' and 112' is about 8 mm. An angle formed between the second and third side faces 116' and 118' and the first radial face 110' is about 58 degrees. A length of the first side face 114' is about 12.5 mm. Each of the second and third side faces 116' and 118' includes an antireflection coating for less than about 0.5% reflection S and P polarization of 1064 nm light at about a 32 degree angle of incidence. Additionally, each of the second and third side faces 116' and 118' includes a clear aperture 134 having about a 6 mm diameter. A center of the clear aperture 134 is disposed about 3.15 mm from a vertex between the second and third side faces 116' and 118' and the second radial face 112' and about 5 mm from an edge of the second and third side faces 116' and 118' formed by truncation. The first side face 114' includes a substantially oval shaped clear aperture 136 that has about a 6.2 mm minor axis and about an 11.5 mm major axis. A center of the substantially oval shaped clear aperture 136 is disposed about 3.2 mm from a vertex between the first side face 114' and the first radial face 110'.

Figure 18 shows a perspective view of a cavity extender 140 according to another exemplary embodiment of the invention. The cavity extender 140 includes a first axial face, 142, a second axial face 144, a third axial face 146, a fourth axial face 148, a fifth axial face 150 and a sixth axial face 152. The first and fourth axial faces 142 and 148 are disposed substantially parallel to each other and face each other. The second and fifth axial faces 144 and 150 are substantially parallel to each other and the third and sixth axial faces 146 and 152 are substantially parallel to each other. The first and fourth axial faces 142 and 148 are longer than the second, third, fifth and sixth axial faces 144, 146, 150 and 152, which are all equal in length. The first, second, third, fourth, fifth and sixth axial faces 142, 144, 146, 148, 150 and 152 are disposed proximate to each other to define a hexagonal prism shape having two elongated sides. A first longitudinal end face 154 and a second longitudinal end face 156 are disposed substantially perpendicular to the first, second, third, fourth, fifth and sixth axial faces 142, 144, 146, 148, 150 and 152 at opposite ends of the cavity extender 140, respectively. In response to the laser beam 20 being launched into the cavity extender 140 through one of the first, second, third, fourth, fifth and sixth axial faces 142, 144, 146, 148, 150 and 152 at a position proximate to the first longitudinal end face 154, the laser beam is reflected at the second, third, fifth and sixth axial faces 144, 146, 150 and 152 sequentially to form the helically shaped optical pattern 36 advancing in a direction from the first longitudinal end face 154 toward the second longitudinal end face 156. In response to reflection at a portion of the cavity extender 140 proximate to the second longitudinal end face 156 back toward the first longitudinal end face 154, the laser beam 20 advances toward the first longitudinal end face 154 in the helically shaped optical pattern 36. In an exemplary embodiment, the first and fourth axial faces 142 and 148 each have a length of about 15 mm and are spaced apart by a distance of about 4 mm. A distance between the first and second longitudinal end faces 154 and 156 is about 15 mm. Thus, for example, in response to a beam having up to a 2 mm diameter being introduced into the cavity extender 140, an optical path length of about 560 mm may be achieved.

As stated above, the cavity extender 10 or delay coil may be constructed from a transparent dielectric material having total internal reflection. Internal angles of the cavity extender 10 may be selected such that a net effect on polarization of the laser beam is minimized. Figures 19 through 22 show a polarization defect of the cavity extender 10 given various coil angles. As such, Figures 19 through 22 show mispolarization as a function of internal angle and showing a selected point of zero mispolarization 185. Figure 19 shows polarization defect for a "straight through" delay coil having one, two, three or four loops. Figure 20 shows polarization defect for a delay coil having one degree of misalignment about a y-axis of the delay coil for one, two, three or four loops. Figure 21 shows polarization defect for a delay coil having one degree of misalignment about a z-axis of the delay coil for one, two, three or four loops. Figure 22 shows polarization defect for a delay coil having one degree of misalignment about an x-axis of the delay coil for one, two, three or four loops.

Figure 23 shows a block diagram of a method for extending a path length of a laser beam in a laser resonator. The method includes directing the laser beam into a first prism at block 200, reflecting the laser beam from respective ones of the axial faces at block 210, and outputting the laser beam from the first prism at a respective axial face at block 220. The first prism includes a plurality of axial faces disposed along a longitudinal axis of the first prism. In order to direct the laser beam into the first prism, the laser beam is directed into one of the axial faces at an angle with respect to the longitudinal axis. Reflecting the laser beam from respective ones of the axial faces 220 forms a helically shaped optical path which advances about the longitudinal axis in a first direction. The first direction extends from a first longitudinal end of the first prism to a second longitudinal end of the first prism. The method may further include reflecting the laser beam at the second longitudinal end of the first prism such that the laser beam advances in the helical shaped optical path about the longitudinal axis in a second direction that is substantially opposite to the first direction.

Accordingly, embodiments of the present invention provide for input and output beams to be coplanar or collinear to simplify laser design considerations. Furthermore, an optical path length of a laser beam can be substantially increased without substantially increasing the size or weight of the laser.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. For example, while embodiments of the invention may be described in terms of a portable laser system, systems and methods of embodiments of the present invention can be used in any laser system in which improved beam quality is desirable. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

THAT WHICH IS CLAIMED:

1. A cavity extender for a laser resonator comprising: a first prism defining a longitudinal axis and including a plurality of axial faces, wherein the axial faces are disposed such that a laser beam entering one of the axial faces of the first prism at an angle with respect to the longitudinal axis is reflected from respective ones of the axial faces to form a helically shaped optical path which advances about the longitudinal axis in a first direction extending from a first longitudinal end of the first prism toward a second longitudinal end of the first prism.

2. The cavity extender of claim 1 , wherein the first prism comprises a transparent dielectric material.

3. The cavity extender of claim 1 , wherein the first prism comprises a hollow structure defined by reflective surfaces.

4. The cavity extender of claim 1 , wherein the axial faces are structured such that an input angle of the laser beam and an output angle of the laser beam are substantially parallel to each other.

5. The cavity extender of claim 1 , wherein the laser beam enters the first prism via a respective axial face proximate the first longitudinal end, and wherein the first prism includes a return portion proximate the second longitudinal end.

6. The cavity extender of claim 5, wherein the return portion is disposed to reflect the laser beam such that the laser beam advances in the helically shaped optical path about the longitudinal axis in a second direction that is substantially opposite to the first direction.

7. The cavity extender of claim 6, wherein the return portion comprises a reflecting facet.

8. The cavity extender of claim 6, further comprising a second prism in optical communication with the first prism to provide an input path and an output path for the laser beam.

9. The cavity extender of claim 1 , further comprising a second prism in optical communication with the first prism to provide one of an input path for the laser beam to the first prism and an output path for the laser beam from the first prism.

10. The cavity extender of claim 1 , wherein the first prism includes at least three axial faces.

11. The cavity extender of claim 1 , wherein the axial faces are structured such that the laser beam is reflected from less than all of the axial faces.

12. A cavity extender for a laser resonator comprising: a first prism defining a longitudinal axis and including a body and a return portion, the body including first, second, third and fourth axial faces, wherein the axial faces are disposed such that a laser beam entering one of the axial faces of the first prism at an angle with respect to the longitudinal axis is reflected from a plurality of the axial faces to form a helical shaped optical path advancing about the longitudinal axis in a first direction extending from a first longitudinal end of the first prism to a second longitudinal end of the first prism.

13. The cavity extender of claim 12, wherein the first to fourth axial faces are disposed such that: the first axial face is substantially parallel to the third axial face; the second axial face is substantially parallel to the fourth axial face; the first and third axial faces are disposed substantially perpendicular to the second and fourth axial faces; a vertex between the first and second axial faces is shorter than a vertex between the second and third axial faces; a vertex between the second an third axial faces is shorter than a vertex between the third and fourth axial faces; and a vertex between the first and fourth axial faces is substantially equal in length to the vertex between the third and fourth axial faces.

14. The cavity extender of claim 13, wherein the return portion includes first, second, third and fourth faces and the first face extends from the first axial face to form an obtuse angle with respect to the first axial face, the second face extends from the second axial face to form an obtuse angle with respect to the second axial face, the third face extends from the third axial face coplanar with the third axial face, the fourth face extends from the fourth axial face coplanar with the fourth axial face, and ends of a vertex of each of the first, second, third and fourth faces are connected to form a second longitudinal end face of the first prism that is substantially parallel to a first longitudinal end face of the first prism.

15. The cavity extender of claim 14, further comprising a second prism in optical communication with the first prism to provide at least one of an input path and an output path for the laser beam.

16. The cavity extender of claim 15, wherein the second prism is a substantially triangular prism including first and second radial faces each comprising a right triangle, the first and second radial faces each including first, second and third vertices, the vertices of the first radial face being connected to corresponding vertices of the second radial face by corresponding first, second and third side faces, the first radial face being larger than the second radial face, and the first radial face being disposed substantially parallel to the second radial face such that the first vertices of each of the first and second radial faces lie in a plane that is substantially perpendicular to the first and second radial faces and the second vertices of each of the first and second radial faces lie in a plane that is substantially perpendicular to the first and second radial faces and the third vertices of each of the first and second radial faces extend substantially parallel to each other in a plane that forms an acute angle with respect to the first radial face and an obtuse angle with respect to the second radial face.

17. The cavity extender of claim 16, wherein one of the first and second side faces of the second prism is in contact with one of the second and third axial faces of the first prism such that the second radial face is disposed coplanar with the first longitudinal end face of the first prism.

18. The cavity extender of claim 12, wherein each of the axial faces is identical in size and shape, the return portion includes first, second, third and fourth faces and the first face extends from the first axial face to form an obtuse angle with respect to the first axial face, the second face extends from the second axial face coplanar with the second axial face, the third face extends from the third axial face coplanar with the third axial face, the fourth face extends from the fourth axial face coplanar with the fourth axial face, and ends of a vertex of each of the faces are connected to form a second longitudinal end face of the first prism that is substantially parallel to a first longitudinal end face of the first prism.

19. The cavity extender of claim 18, wherein the second prism is a substantially triangular prism including first and second radial faces each comprising a right triangle, the first and second radial faces each including first, second and third vertices, the vertices of the first radial face being connected to corresponding vertices of the second radial face by corresponding first, second and third side faces, the first radial face being larger than the second radial face, the first radial face being disposed substantially parallel to the second radial face such that the first vertices of each of the first and second radial faces lie in a plane that is substantially perpendicular to the first and second radial faces and the second vertices of each of the first and second radial faces extend substantially parallel to each other in a plane that forms an acute angle with respect to the first radial face and an obtuse angle with respect to the second radial face and the third vertices of each of the first and second radial faces extend substantially parallel to each other in a plane that forms an acute angle with respect to the first radial face and an obtuse angle with respect to the second radial face, and the second and third side faces each being truncated at a portion of the second and third side faces which contacts the first side face.

20. The cavity extender of claim 19, wherein the first side face of the second prism contacts the third axial face of the first prism proximate the first longitudinal end of the first prism.

21. A method for extending a path length of a laser beam in a laser resonator, the method comprising: directing the laser beam into a first prism including a plurality of axial faces disposed along a longitudinal axis of the first prism, wherein the directing the laser beam comprises directing the laser beam to enter one of the axial faces at an angle with respect to the longitudinal axis; reflecting the laser beam from respective ones of the axial faces to form a helically shaped optical path which advances about the longitudinal axis in a first direction extending from a first longitudinal end of the first prism to a second longitudinal end of the first prism; and outputting the laser beam from the first prism at a respective axial face.

22. The method of claim 21 , further comprising: reflecting the laser beam at the second longitudinal end of the first prism such that the laser beam advances in the helical shaped optical path about the longitudinal axis in a second direction that is substantially opposite to the first direction.

23. The method of claim 21 , wherein a second prism is in optical communication with one of the axial faces of the first prism, and wherein the directing the laser beam into the first prism further comprises optically coupling the laser beam from the second prism to the first prism.

24. The method of claim 21 , wherein a second prism is in optical communication with one of the axial faces of the first prism, and wherein the outputting the laser beam from the first prism comprises optically coupling the laser beam from the first prism to the second prism.