Abstract:
A light source having a gain chip, a retro reflecting prism and a first actuator is disclosed. The gain chip amplifies light passing therethrough. The retro reflecting prism is characterized by a pivot axis within the retro reflecting prism, an input light direction, an output light direction, and a diffraction grating that receives light emitted by the gain chip traveling in the input direction, returns a diffracted light beam to the gain chip along the input light direction and generates an output light beam. The first actuator causes the retro reflecting prism to rotate about the pivot axis in response to a control signal being coupled to the first actuator.

Description:
BACKGROUND 
     A tunable external cavity laser light source is typically constructed from a reflective cavity having a gain chip and a wavelength selective filter that suppresses light of a wavelength outside a narrow band of wavelengths. The light that is passed by the filter is amplified by the gain chip via stimulated emission. A portion of the light is removed from the cavity and becomes the output beam from the light source. For any given cavity length, the gain chip can lase in a number of different modes. Ideally, only one of these modes is selected by the wavelength filter. If the passband of the filter is too great, the light source can jump between modes of the cavity with corresponding jumps in wavelength and other properties of the output light beam. Accordingly, it is advantageous to provide a wavelength filter with as narrow a passband as possible, while providing a means to change the center frequency of the passband when the light source is tuned to a new frequency. 
     SUMMARY 
     The present invention includes a light source having a gain chip, a retro reflecting prism and a first actuator. The gain chip amplifies light passing therethrough. The retro reflecting prism is characterized by a pivot axis within the retro reflecting prism, an input light direction, an output light direction, and a diffraction grating that receives light emitted by the gain chip traveling in the input direction, returns a diffracted light beam to the gain chip along the input light direction and generates an output light beam. The first actuator causes the retro reflecting prism to rotate about the pivot axis in response to a control signal being coupled to the first actuator. 
     In one aspect of the invention, the light source includes a second actuator that causes the retro reflecting prism and the first actuator to move in a direction orthogonal to the output beam direction. 
     In another aspect of the invention, the output beam light direction is parallel to the input light direction. 
     In another aspect of the invention, the gain chip includes a reflecting surface that forms a cavity having one end on the reflecting surface and another end on the diffraction grating. 
     In another aspect of the invention, the light source includes an optical assembly that generates a collimated beam traveling in the input light direction from light leaving the gain chip and focuses light in the diffracted light beam into the gain chip. 
     In another aspect of the invention, the retro reflecting prism is constructed from a material having an index of refraction greater than 1. 
     In another aspect of the invention, the retro reflection prism is constructed from a material having an index of refraction equal to 1. In one exemplary embodiment, the material includes germanium. In another exemplary embodiment, the medium is chosen from the group consisting of Si, ZnSe, InP, and CdZnTe. 
     In another aspect of the invention, the retro reflecting prism is characterized by a planar diffraction grating, a planar entrance face through which the input beam passes, and a reflector at right angles to the planar diffraction grating, the pivot axis is parallel to a junction of the planar diffraction grating and the reflector and is displaced from the entrance face along a normal to the entrance face that passes through the junction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one configuration used in a prior art external cavity quantum cascade laser. 
         FIG. 2  illustrates a configuration for an external cavity quantum cascade laser according to one embodiment of the present invention. 
         FIG. 3  illustrates the shift in the output beam when the diffraction grating is rotated. 
         FIG. 4  illustrates an arrangement that removes the variability in the output beam direction as a function of the angle between the diffraction grating and the incoming light beam. 
         FIG. 5  illustrates the beam trajectories when the diffraction grating is on the surface of a high index of refraction prism. 
         FIG. 6  illustrates the center of rotation and the input and output beams for two different angles of rotation of a retro reflecting prism according to one embodiment of the present invention about the center of rotation. 
         FIG. 7  illustrates the displacement error as a function of rotation angle in one embodiment of the present invention. 
         FIG. 8  illustrates an external cavity quantum cascade laser according to one embodiment of the present invention. 
         FIG. 9  illustrates an external cavity quantum cascade laser according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1 , which illustrates one configuration used in a prior art external cavity quantum cascade laser. The laser cavity is defined by back facet  11  of gain chip  12  and the surface of diffraction grating  15 . In an ideal system, the angle of the diffraction grating relative to the light beam from the gain chip is chosen to ensure that the laser lases on a particular mode. The angle is set by an actuator  16  that rotates the grating around a pivot axis  17  that is chosen such that the wavelength of the diffracted light that returns to the gain chip and the length of the laser cavity are maintained to provide the desired output wavelength. Lens  14  collimates the light leaving front surface  13  of gain chip  12 . Lens  14  sets the diameter of the beam striking diffraction grating  15  and also sets the diameter of the output beam in this configuration. 
     The number of grating grooves that are illuminated by the beam on the surface of diffraction grating  15  sets the resolution of diffraction grating  15 , larger beam diameters providing better resolution for any given grating. In particular, at any given wavelength, the passband width is inversely proportional to the number of grating grooves that are illuminated by the incident beam. Hence, it is advantageous to illuminate as many grating grooves as possible. In principle, the number of grating grooves that are illuminated can be increased by increasing the diameter of lens  14 . However, providing a larger lens results in an economic penalty, since the lens cost also increases significantly with lens diameter. In addition, the physical size of diffraction grating  15  may also need to be increased, which further increases the cost of the light source. 
     The present invention is based on the observation that the number of lines that are illuminated on the diffraction grating can be increased by decreasing the line spacing on the diffraction grating. For a given wavelength and angle of the light beam on the diffraction grating, the spacing of the grooves on the diffraction grating is proportional to the wavelength of the incident light. If the wavelength of the light that strikes the diffraction grating grooves can be reduced, then the number of lines that are illuminated can be increased. The wavelength of the light striking the diffraction grating can be reduced while still maintaining the wavelength of the light that is processed by the gain chip and output from the light source by immersing the diffraction grating in a medium that has a high index of refraction. If the index of refraction of that medium is n, then the spacing of the lines on the diffraction grating can be reduced by a factor of n. In effect, the immersion medium shortens the incident light wavelength prior to the light striking the diffraction grating. Upon leaving the immersion medium, the wavelength is returned the original wavelength. 
     Refer now to  FIG. 2 , which illustrates a configuration for an external cavity quantum cascade laser according to one embodiment of the present invention. Laser  20  differs from laser  10  shown in  FIG. 1  in that diffraction grating  15  has been replaced by a diffraction grating  22  that is formed on the outer surface of a prism  21  constructed from a high index of refraction material. Diffraction grating  22  is formed by scribing lines into the surface of prism  21 . The lines have a spacing that is 1/n that of the lines on diffraction grating  15 , where n is the index of refraction of the prism material. 
     To tune the cavity to a different wavelength, diffraction grating  22  must be rotated relative to the direction of input beam  23 . Unfortunately, rotating diffraction grating  22  about pivot axis  17  results in the direction of output beam  24  changing, which presents challenges for any apparatus using laser  20  as a tunable light source. The manner in which the output beam shifts can be more easily understood with reference to  FIG. 3 , which illustrates the shift in the output beam when the diffraction grating is rotated. The two positions of diffraction grating  22  are shown at  22   a  and  22   b . Input beam  23  and the reflected first order refraction at the desired wavelength must traverse the same light path for the gain chip to amplify the light at the desired wavelength. The output beam from position  22   a  is shown at  31 , and that from position  22   b  is shown at  32 . The direction of each output beam is set by the angle of incidence of the input beam with respect to the normal to diffraction grating  22  in the corresponding position. Hence, the output beam direction shifts by an angle φ when diffraction grating  22  moves from position  22   a  to position  22   b . An apparatus that uses the output beam would need to compensate for the change of direction of the output beam as the laser is tuned. As noted above, such compensation presents significant challenges. 
     Refer now to  FIG. 4 , which illustrates an arrangement that removes the variability in the output beam direction as a function of the angle between the diffraction grating and the incoming light beam. In this embodiment, a mirror  36  rotates with diffraction grating  22  such that mirror  36  remains at 90 degrees to the plane of diffraction grating  22 . In this arrangement, the output beam remains along path  35  for both rotations of diffraction grating  22 . 
     The arrangements shown in  FIGS. 3 and 4  assume there is no interface between materials with different indices of refraction before the light strikes diffraction grating  22 . When a high index of refraction medium is included, the output beams are no longer coincident when the rotation angle of the gradient changes. Refer now to  FIG. 5 , which illustrates the beam trajectories when the diffraction grating is on the surface of a high index of refraction prism. To simplify the following discussion, a prism having a diffraction grating on one face and a reflecting surface on an intersecting face at right angles to the face having the diffraction grating will be referred to as a retro reflecting prism. The reflecting surface can be provided by a reflective coating or by using total internal reflection. Given that the preferred embodiments utilize a prism material having a large index of refraction, total internal reflection at the surface is the preferred embodiment. Retro reflecting prism  45  is constructed from a high index of refraction material in which a diffraction grating  46  has been formed on a first surface. A second surface  47  at right angles to diffraction grating  46  includes a mirror that alters the direction of the beam from the diffraction grating in a manner analogous to that described above. When a light beam enters or leaves the third surface shown at  48 , the light beam direction is altered unless the light beam enters or leaves at right angles to surface  48 . In the first rotation position, input beam  52  enters retro reflecting prism  45  at right angles to surface  48 , and hence, is not bent prior to striking diffraction grating  46 . The output beam  54  likewise strikes surface  48  at right angles, and hence, is also not bent. 
     In the second rotational position shown in dotted lines, input beam  52  is bent prior to striking diffraction grating  46  and also on leaving retro reflecting prism  45 . As a result, output beam  53  is displaced relative to output beam  54  by an amount ΔY. While the two output beams are parallel to one another, the displacement still presents challenges for a device that uses a laser with this form of diffraction grating. The displacement can be eliminated by translating the gradient vertically in addition to rotating the gradient. The ideal displacement is ΔY/2. If mode-hop-free tuning of the laser is required, tuning the cavity laser requires that the cavity length be changed in addition to the center wavelength of the reflection from the diffraction grating. Hence, the prism would need to be moved along the input beam path as well as vertically to that pass. While the prism could be moved in two dimensions and also rotated during tuning, such a mechanical system increases the cost of the laser. 
     One aspect of the present invention is based on the observation that there is an axis of rotation that passes through the prism that has the property that when the retro reflecting prism is rotated about this axis, the retro reflecting prism translates in a manner that substantially reduces the movement of the output beam as a function of the rotation angle for angles within a useful tuning range. Refer now to  FIG. 6 , which illustrates the center of rotation and the input and output beams for two different angles of rotation of a retro reflecting prism according to one embodiment of the present invention about the center of rotation. The diffraction grating on retro reflecting prism  70  is on surface  71 , which has a length h. To simplify the drawing, the diffraction grating grooves are not shown. The reflective surface is on side  72  of retro reflecting prism  70 . Retro reflecting prism  70  rotates about an axis through point (X c ,Y c ) which is perpendicular to the plane of the figure. Retro reflecting prism  70  is characterized by an angle of rotation, ⊖. The angle is defined to be 0 for the rotation at which input beam  75  is perpendicular to face  73  of retro reflecting prism  70 . In the example shown in  FIG. 6 , the XY coordinate system has its origin at the junction of surface  71  and side  72 . Denote the index of refraction of the medium outside of retro reflecting prism  70  by n 1  and the index of refraction of the material from which retro reflecting prism  70  is constructed by n 2 . In one aspect of the invention, Xc=(h/2)(1−n1/n2). For this value of Xc, the output beams will all be parallel to one another; however, there will be a small displacement error, ε. The magnitude of the displacement error, as a function of rotation angle, is shown in  FIG. 7  for n1=1 and n2=4. 
     Refer now to  FIG. 8 , which illustrates an external cavity quantum cascade laser according to one embodiment of the present invention. Laser  80  includes a gain chip  87  that amplifies the light in beam  84  whose diameter is set by lens  86 . The light is selectively filtered by retro reflecting prism  81  as described above. Actuator  83  causes retro reflecting prism  81  to rotate about rotational axis  82  which is perpendicular to the plane of the figure. An output beam  85  is provided by the mirrored surface of retro reflecting prism  81 . 
     As noted above, the arrangement shown in  FIG. 8  still has a small variation in the beam position as a function of the rotation angle. If the application requires rotation angles at which this variation is significant, an additional drive can be provided to correct for any remaining variation in position as a function of angle. Refer now to  FIG. 9 , which illustrates an external cavity quantum cascade laser according to another embodiment of the present invention. Those elements of laser  90  that serve functions analogous to corresponding elements shown in  FIG. 8  have been given the same numerical designations and will not be discussed further here. In laser  90 , retro reflecting prism  81  and actuator  83  are mounted on a stage  92  that moves relative to the direction of beam  84  and is driven by a second actuator  95 . A controller  96  controls both actuator  83  and actuator  95 . The controller can include a lookup table that stores the Y displacement values to be used with each angle of rotation of retro reflecting prism  81  to correct for any remaining displacement error. Controller  96  can also include a table of displacements to be used by actuator  83  in setting the rotation angle for any desired output wavelength. The desired output wavelength can be set by user input to controller  96 , either directly or indirectly through some other device that is connected to controller  96 . 
     The above-described embodiments utilize a high index of refraction medium for the retro reflecting prism. In one exemplary embodiment, the grooves are etched or machined on a Germanium crystal block. Germanium has an index of refraction of approximately 4 for light in the mid-infrared portion of the optical spectrum. However, any other IR transmissive material having an index of refraction greater than one could be utilized. For example. Si (silicon). ZnSe (zinc selenide), InP (indium phosphide), or CdZnTe (cadmium zinc telluride) could be utilized. As noted above, materials with large indices of refraction are preferred. However, any transmissive medium with an index of refraction greater than 1 will provide some improvement. In another exemplary embodiment, the retro reflecting prism medium has an index of refraction greater than 2. In one exemplary embodiment the retro reflecting prism medium has an index of refraction greater than 3. 
     Refer again to the arrangement shown in  FIG. 2 . If the material from which prism  21  is constructed exhibits chromatic dispersion, then the direction of the output beam can vary as a function of wavelength. In contrast, the embodiments shown in  FIG. 6  automatically correct for any such chromatic dispersion, since the output beam is always parallel to the input beam. Hence, any dispersion introduced by the input beam entering the prism at an angle other than 90 degrees to the surface is corrected when the output beam leaves the prism. 
     In the above-described embodiments, the material from which the retro-reflecting prism is constructed has an index of refraction greater than 1. However, embodiments in which the prism has an index of refraction equal to one, i.e., a grating with a reflecting surface attached at right angles to the plane of the grating, has advantages in situations in which an output beam that is displaced from the input beam, but parallel thereto and which remains stationary when the grating is rotated. To simplify the following discussion, such an arrangement will also be referred to as a retro-reflecting prism. 
     The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.