Laser Diode Coupler

There are inherent problems with coupling a linear array of laser diodes with a solid state laser rod due to the geometry involved, and the properties of a typical Nd:Yag laser rod. Typically, a laser rod is side pumped by a linear array of laser diodes aligned in a straight line along the height dimension and oriented at the periphery thereof. As the laser diodes are energized, light enters through the side of the rod and the energy is absorbed by the atoms in the rod and excited or pumped to a higher energy level, thereby creating the lasing effect. Additional power output can be obtained from the rod by increasing the number of linear arrays mounted along the periphery thereof. However, these prior art arrangements have produced generally less than satisfactory results for several reasons.

One of the problems is that for smaller diameter rods, the optical thickness of the rod is minimized, thereby decreasing the absorption length or light path for the light emitted from the diodes. This results in only a small amount of pump energy being deposited within the lasing mode volume in the rod. A secondary effect caused by the poor rod absorption is the increased sensitivity to laser diode junction temperature and resulting wavelength mismatch experienced in the smaller diameter rods. Wavelength mismatch is the difference between the laser diode wavelength and the laser rod pump absorption bands. As the laser diode wavelength output varies with respect to temperature, performance can be seriously affected should the laser diode temperature not be carefully controlled to produce an output at the wavelength required to optimize absorption in the rod.

Several attempts have been made to improve the typical prior art arrangement to increase the coupling between the pump and the rod. One of these has been the addition of a sleeve over the rod, with a reflective coating being applied to the periphery of the sleeve except for a polished transparent flat or slit against which is mounted the linear laser diode array. In this configuration, laser diode light is directed through the sleeve, into the rod, through the center of the rod to the far periphery of the sleeve, and then reflected back through the center of the rod a second time. While this arrangement does offer some improvement, it still suffers from poor coupling efficiency in that nearly all of the pump energy is not concentrated in the laser rod nor in the laser mode volume within the rod. Furthermore, the only diode light that is reflected back through the rod must originate along a path through the center of the rod, and the optical design of this configuration does not permit most of the pump light to pass near the center of the laser rod.

To solve these and other problems in the prior art, the inventors herein have succeeded in developing a laser diode pump coupler which dramatically increases the efficiency of coupling between a linear array of laser diodes and a laser rod. The laser diode pump coupler of the present invention includes a cylindrical sleeve which slides over the laser rod, the sleeve having a reflective coating around its periphery with one or more transparent flats against which is positioned a pair of optical elements. The sleeve may either be hollow or solid, and if hollow, filled with transparent coolant fluid. If solid, the gap between the rod and sleeve is filled with an index matching compound, such as RTV.

The first of these two optical elements is a cylindrical focus lens for focusing the light being emitted by the linear array, and the second of these is a retro-reflector, the retro-reflector being mounted adjacent to the lens. Each lens focuses the laser diode beam from the linear array through the rod in an off-center alignment. By doing so, the beam impinges on the opposite wall of the cylindrical sleeve and bounces back through the laser rod in another off-center alignment to a point on the cylindrical sleeve's periphery spaced from its point of origin. As the sleeve is cylindrical, these reflections continue to traverse the rod in different paths. Thus, the beam continues to reflect back and forth through the laser rod thereby "walking" its way around the periphery of the sleeve.

Depending upon how many sets of focal lenses, retro-reflectors, and linear arrays are used, the beam will bounce through a corresponding angular sector of the sleeve and rod but eventually impinge upon a retro-reflector. The retro-reflector is designed to reverse the "walking" of the beam so that it substantially retraces its various paths through the laser rod until the pump light escapes through the focal lens at its point of origin. As can be appreciated, this pump coupler design has the advantage of theoretically multiplying the effective optical thickness (i.e. diameter) of the laser rod by 18-20 times to thereby concomitantly increase the pump light absorption length of the laser rod. Furthermore, because of the off-center focus, the pump coupler concentrates and focuses the pump energy within the laser mode within the laser rod resulting in maximum energy utilization between the laser mode and pump light. By increasing the effective optical thickness of the laser rod, the laser diode wavelength mismatch and temperature sensitivity is dramatically reduced, thereby providing a much wider range of wavelengths which can be selected for use as a laser pump, and also a wider range of temperatures over which the laser diodes may be operated with very small changes in laser output power.

Because of the very nature of the invention, the pump coupler design can be easily modified to increase the number of laser diodes either around the laser rod or along the laser rod by respectively adding more linear laser diode arrays or more diodes per array. In an alternative embodiment, offset reflective cylindrical elements may be utilized in place of the refractive cylindrical lens elements to collect and focus the laser diode light into the sleeve and through the laser rod. This provides flexibility and opportunities for design choice heretofore unavailable in the prior art. As their preferred embodiment, the inventors herein utilize three linear arrays with three associated sets of cylindrical focus lenses and cylindrical retro-reflector elements bonded to three flats located 120.degree. apart on the cylindrical sleeve. With this arrangement, there are three sets of beams which are independently "walking" their way around the periphery of the sleeve and which impinge upon and energize different portions of the laser rod.

FIG. 1 is a representational view of the prior art with a solid state laser rod side pumped by a linear array of laser diodes;

FIG. 2 is a representational view of a solid state laser rod with cylindrical sleeve side pumped with a linear array of laser diodes through its central axis;

FIG. 3 is a top view of a solid state laser rod with cylindrical sleeve and three sets of focus and retro-reflector lenses as utilized in the present invention;

FIG. 4 is a side view of the invention partially broken away to further detail the lenses, sleeve, and rod;

FIG. 5 is a top view of the invention with a central ray traced therethrough to depict the path thereof for one of the laser diode sources;

FIG. 6 is a cross-sectional view of an alternative embodiment utilizing offset cylindrical reflectors for the primary focal element; and

FIG. 7 is a cross-sectional view of an alternative embodiment utilizing externally mounted cylindrical retro-reflectors in addition to the off-set cylindrical reflectors as shown in FIG. 5.

As shown in FIG. 1, the prior art typically included a solid state laser rod 20, the rod having an area of principal lasing mode 22 therein which was side pumped by a linear laser diode array 24 comprised of a plurality of laser diodes 26. Each laser diode had a typical laser beam shown representationally as 28 in FIG. 1 with a beam elevation of approximately plus and minus 70.degree. off-center and an azimuth of plus and minus 15.degree.. Thus, a large portion of the energy from each diode 26 contained within the array 24 did not impinge upon the lasing mode 22, thereby resulting in minimal power output.

A step towards improving the pump coupling is shown in FIG. 2 wherein a solid state laser rod 30 is surrounded by a cylindrical sleeve 32, sleeve 32 having a reflective coating 34 along its periphery and at both ends thereof to reflect light back through the cylindrical sleeve 32. A polished transparent slit or flat 36 is formed along the periphery of sleeve 32 and provides an access for the laser diode array 38. With the laser diode array 38 aligned with the center line of each of the laser diodes 40 through the central axis of the laser rod 30, most of the light impinging upon the solid state laser rod 30 traverses the rod at least twice, as a portion of it bounces back from the opposite sidewall of sleeve 32.

As shown in FIGS. 3 and 4, the present invention includes a solid state laser rod 42 surrounded by a cylindrical sleeve 44 with a reflective coating applied to its periphery and with three transparent slits or flats 46 positioned 120.degree. apart around the periphery of the cylindrical sleeve 44. Sleeve 44 may be either hollow and filled with transparent liquid coolant, or solid. At each of these transparent flats 46 is mounted a cylindrical focus lens 48 and a cylindrical retro-reflector 50. As shown in the drawing, each of the focus lens 48 is off axis from a radius drawn from the center of the laser rod 42 to the focus lens 48. Each cylindrical focus lens 48 has an associated linear laser diode array 52 aligned therewith such that its beam is focused through an off axis portion of the laser rod 42. This is best shown in FIG. 5.

As shown in FIG. 5, the beam or central ray 54 is focused by focus lens 48 through the laser rod 42 in an off axis path. When it impinges upon the far periphery of the cylindrical sleeve 44, it is reflected back through a different path, again off axis, through laser rod 42 until it impinges upon the periphery of cylindrical sleeve 44 at a point spaced apart from the focus lens 48 but adjacent thereto. These last two points are labeled 6 and 2 in FIG. 5. The central ray 54 is then reflected in a similar manner to point 7 on the far peripheral wall of cylindrical sleeve 44, then back to point 3 adjacent focus lens 48, then to point 8, point 4, point 9, point 5, and then point 10 where the central ray impacts retro-flector 50. Retro-reflector 50 has a reflective coating applied thereto and is shaped such that it returns the central ray 54 back along its same path to return to its point of origin, or point 5. Because of the cylindrical shape of sleeve 44, central ray 54 will then reflect back along its original path where it will finally escape sleeve 44 through focus lens 48. Thus, the focus lens at the six o'clock position in FIG. 5 works in tandem with the retro-reflector at the ten o'clock position to bounce the energy from a single linear diode array such that it passes through the laser rod 42 a total of twenty times. This significantly improves the coupling between each linear diode array and the laser rod 42. While not described in detail, it should be understood that each of the other focus lenses 48 has an associated linear laser diode array and works in tandem with a retro-reflector to similarly bounce the energy therefrom along a similar path which traverses the laser rod 42 a total of twenty times. Although three sets of focal elements are shown in the preferred embodiment, it should be understood that a greater or lesser number may be used depending upon the desired operating characteristics, geometry chosen, and other parameters as is well known in the art to optimize the laser performance.

An alternative embodiment is shown in FIG. 6 wherein an offset cylindrical reflector 56 is used to focus the light from the laser diode array source 52 instead of the refractive focus lens 48 shown in the first embodiment. A further modification is shown in FIG. 6 in which an externally mounted cylindrical retro-reflective lens 58 is used in place of the retro-reflective lens element 50 of the first embodiment. Functionally, these second and third alternative embodiments operate exactly as does the first embodiment, the only difference being in the choice of optics utilized to focus the laser light from the source, and to reflect the focused light back through its original path.

There are various changes and modifications which may be made to the invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of the disclosure, and it is intended that the invention be limited only by the scope of the claims appended hereto.