Abstract:
An interchangeable insertable diode laser and lens frame is described in which a pair of lenses is disposed in the frame with a diode laser disposed between the lenses. The laser light from one of the facets is collimated by one of the lenses, such that when the frame is inserted into a housing and coupled to an external cavity in which a diffraction grating is disposed, the grating disperses the light and dispersed light is focused on the facet causing the diode laser to generate single frequency radiation.

Description:
GOVERNMENT FUNDING 
     The Government has rights in this invention pursuant to Contract Number F19628-85-C-0002 awarded by the Department of the Air Force. 
    
    
     RELATED APPLICATIONS 
     This is a continuation-in-part of co-pending application U.S. Ser. No. 07/712,185 filed Jun. 7, 1991 which is a continuation of U.S. Ser. No. 07/341,028 filed Apr. 20, 1989, now U.S. Pat. No. 5,050,179. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to lasers and, more specifically, to semiconductor or diode lasers operated in an external cavity. 
     As noted in the parent application, Ser. No. 07/341,028,now U.S. Pat. No. 5,050,179, numerous advantages accrue to operation of semiconductor lasers in an external cavity as contrasted to solitary diode laser operation. In external cavity-controlled semiconductor lasers, a diode laser with opposed anti-reflective (AR) coated first and second facets is mounted between a pair of AR coated lenses. The facets extend in a plane perpendicular to the plane of the diode gain region extending along the length of the diode laser. One of the lenses collects the laser radiation from the first facet and collimates it onto a frequency tuning element, such as a diffraction grating, where the incident beam is dispersed and reflected colinear with the incident beam and re-imaged onto the first diode facet. Radiation from the second facet is collimated onto an output coupler formed by a dielectric coated, partially reflecting plane mirror. 
     Selection and tunability of the emission wavelength of the diode laser is achieved by varying the tilt of the diffraction grating to select an appropriate single frequency within the bandwidth of the diode laser to feed back energy to lock the diode laser at the selected frequency. This is in contrast to single diode laser operation which is amenable to reversible frequency control only by externally induced variation of band gap energy and refraction index (through changes in temperature or pressure for example) or by variation of injection current. 
     Stable, single-mode laser operation in an external cavity necessitates a high degree of thermal and mechanical stability in the cavity. Furthermore, the lens which collimates the laser radiation onto the diffraction grating must be accurately aligned with the facet to avoid mode instabilities. The linear alignment precision requirement is in the order of one wavelength. Alignment is a time-consuming, tedious procedure requiring expensive calibrating equipment. Of necessity, therefore, this alignment process is presently performed by the cavity manufacturer before the cavity is sold to the consumer. 
     The spectral range of the diffraction grating is in the order of 400 nm, whereas the spectral range of most diodes lasers is only about 50 nm. Therefore, when output wavelengths are required beyond the range of a given diode, as a practical matter, it is usually necessary to purchase another external cavity package with a diode operating in the desired wavelength range to avoid the aforementioned tedious realignment. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, diode and lens frame assemblies are provided in which a first facet of the diode laser is pre-aligned with a first of two lenses, i.e., the one which collimates the light onto the tuning element. The frames are easily insertable into an external cavity housing. Each frame assembly is interchangeable with another, except that each diode laser within a given frame assembly is pre-aligned with a lens and covers a different wavelength range of, for example, 40-50 nm, within the spectral range of the tuning element. Each frame assembly consists of a diode lasers supported between a first and second lens. The frame assembly consists of a rigid walled member which extends from a planar support. Opposed sides of the member are provided with a circular opening with an axis which extends parallel to the support plane. The lenses are mounted in these holes with the diode laser disposed between the lenses. 
     A housing is provided with an opening for receiving the frame assembly. The housing opening is precisely dimensioned, such that the frame securely fits within the opening. 
     The tuning element is preferably in the form of a diffraction grating which is located opposite the lenses when the frame assembly is inserted in the housing. Other tuning techniques known in the art can also be used, including: tilted etalons, adjustable gap etalons, electro optically (EO) tuned double etalons, and mechanical or EO tuned Lyot filters. The etalon gap spacing may be varied using a piezo-electric crystal. 
     In one embodiment, the first facet of the diode laser is AR coated and the other facet serves as the output coupler of the external cavity, while the second lens serves as an optical collimator. In an alternate embodiment, both lenses are part of the external cavity and both diode laser facets are AR coated and the second lens forms an output coupler to an output mirror. By AR coating, both facets of the diode, the residual reflectivity is reduced to a minimum and the tuning characteristics and other operating characteristics are controlled (dominated) by the external cavity. This is particularly important for continuous tuning and mode-locked operation. 
     In either case, light from the grating is focused back on the first facet and the light is dispersed perpendicular to the plane of the gain region of the diode laser. The gain region is usually much wider than its length and the height is typically about 1 micrometer, therefore vertical alignment is not critical. In the azimuthal direction, the grating may be easily aligned by rotation so that the reflected light image falls onto the gain region. Thus, if the original alignment between the diode laser and the collimating lens is precise enough to provide a collimated beam, each frame assembly may be removed and replaced without significant difficulty in re-alignment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric schematic drawing of a first embodiment of an external cavity operated diode laser system. 
     FIG. 2 is a schematic sectional view of the frame assembly of the system of FIG. 1 shown disposed opposite a diffraction grating. 
     FIG. 3 is a schematic perspective view of a diode laser. 
     FIG. 4 is an isometric schematic drawing of an alternate embodiment of the invention. 
     FIG. 5 is a schematic drawing of another alternate embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing and FIGS. 1-3, the invention will now be described in detail in connection therewith. An external cavity for a diode laser is shown in FIG. 1 to comprise, in general, a diode and lens frame assembly 10, a frame housing 32 and an extension housing 38. 
     Frame assembly 10, as shown in FIG. 2, consists of a four-sided frame 11 which extends from a planar support member 18 in a direction transverse the plane of the support. 
     A coaxial connector 20 is mounted on the planar member 18. Power from a source (not shown) is coupled to a diode laser 16 over wire 22 to provide operating current for the diode laser 16, which is mounted on a copper heat sink 19 fastened to an inner side wall of the frame 11 by fasteners (not shown). An access opening 74 is provided in a side wall 11a to facilitate adjustment of the location of diode laser 16 with respect to the lenses 14 and 12. 
     A pair of lenses 14 and 12 are disposed in bores provided in opposite side walls 11a and 11b of frame 11. The axis of these bores extends transverse the plane of member 18. 
     Lenses 14 and 12 may be spherical, cylindrical, aspheric or a combination of both, depending upon the cavity design. The facets 16a and 16b of the diode laser, 16 are placed at the focus of the two lenses, respectively. In the embodiment of FIG. 2, lens 12 is AR coated and forms part of the external cavity, while lens 14 serves as an optical output collimator. Facet 16a of diode 16 serves as the output coupler (with an appropriate reflectivity) for the external cavity, while facet 16b is AR coated. 
     Housing 32 is provided with a chamber 44 for receiving frame assembly 10, which is inserted into chamber 44 with the lenses 14 and 12 aligned with corresponding opposed openings 13 formed in housing 32. (Note: only one opening 13 can be seen in FIG. 1.) 
     The chamber 44 is dimensioned such that frame 11 fits within the chamber with negligible clearance and is retained in place by four bolts 42 provided about the periphery of member 18 and extending through bores 52 and 53 in member 18 and housing 32, respectively. 
     Housing 32 is adapted to mate with a second housing 38 and is secured thereto by four bolts 40 which extend through bores 46 and 48 in housing 32 and 38, respectively. Housing 38 is provided with a circular opening 78 into a cavity in which a diffraction grating 18 is provided. Dispersed diode light 70, reflected from the grating 18, is directed by lens 12 back onto the facet 16b. The grating 18 is positioned with respect to the lens, such that diode light directed by lens 12 is dispersed perpendicular to a plane extending through the plane of the gain region 16c (see FIG. 3). 
     Optionally, a lens 60 (as shown in dotted lines in FIG. 2), having a focal length longer than lens 12, may be provided between lens 12 and grating 18 to provide a more precise control of the focus on the diode laser facet 16b. Lens 60 is adapted for longitudinal movement along the length of the optical axis B of the cavity. As lens 60 is moved longitudinally along the optical axis, the focus of the light reflected back from grating 18 moves also; thereby providing a more precise focus on the facet than a single lens alone would provide. 
     Grating 18 is mounted on grating holder 37 by means of set screws 35 located on four sides of the grating. Holder 37 is suspended in two directions from the back plate 39 of housing 38 by means of springs 94 and 92 located in bores 96 and 98 provided in back plate 39 and holder 37 leaving a gap 91 which allows the grating to be tilted in elevation by rotation of screw 26 which bears upon a sapphire bearing pad 95 inserted on the back of holder 37. 
     Optionally, screw 26 may be coupled by mechanical linkage (not shown) to a computer controlled step-motor or servo-mechanism for remote control of the output frequency of the diode laser 16. 
     Preferably, the housing and other structural items of the system are formed of low thermal coefficient of expansion materials, such as, Super Invar metal. As previously noted, other tuning elements may be used in place of grating 18. 
     An optional quarter wave plate, not shown, may be provided between lens 12 and grating 18 to adjust the polarization of light falling on grating 18 to obtain maximum reflectivity from grating 18. 
     Referring now to FIG. 4, an alternate embodiment of the external cavity controlled semiconductor laser system of the invention will now be described in connection therewith. Note that like parts in FIG. 4 retain the same numeral as in FIG. 1 with a prime added thereto. In this embodiment the frame 10&#39; is cylindrical in shape and is formed with a circular cover plate 18&#39;. Cylindrical wall 11&#39; extends longitudinally from cover plate 18&#39;. A diode laser 16&#39; (shown in dotted lines) is mounted on heat sink 19&#39; and is supported within a bore provided in the cylindrical wall 11&#39;. Lenses 14&#39; and 12&#39; are coaxially aligned and mounted on opposite ends of frame 10&#39; and focussed on opposite facets of diode laser 16&#39;. A coaxial connector 20&#39; is coupled by lead 22&#39; to diode laser 16&#39; for supplying power to the laser. 
     A slot or groove 102 extends along the length of cylindrical wall 11&#39; and is adapted to mate with a corresponding tongue 104 on an inner wall of a circular opening 44&#39; formed in external cavity housing 106. Housing 106 contains a diffraction grating 18&#39; which is tiltably mounted on holder 37&#39; adjacent back plate 39&#39; as in FIG. 2. 
     In the embodiment of FIG. 4, interchangeable frame structures 10&#39; with pre-aligned lenses and diode laser covering different frequency ranges may be conveniently inserted into housing 106 by sliding the cylindrical wall along tongue or guide rail 104 and aligned with tuning element 18&#39;. Once inserted the frame is affixed by removable pins or other equivalent fastening techniques (not shown). 
     The frame assembly 10 may be provided with an additional coaxial connector 120 as shown in FIG. 4 and FIG. 5 for coupling DC voltage to diode laser 16 in addition to, or as an alternative to, the RF voltage supplied via coaxial connector 20. Note that like items in FIGS. 5 and 2 contain the same reference numeral. The RF voltage supplied at the input to connector 20 is coupled to one side of capacitor 112 of bias-Tee 130 and passes through the capacitor to mode-lock or provide high speed pulsing for laser 16. A DC bias voltage may also be applied to diode 16 via connector 120 and is coupled through inductor 110 to either provide a DC threshold voltage for pulsed operation of laser 16 by the RF voltage on connector 20 or for CW operation absent RF voltage on connector 20. 
     EQUIVALENTS 
     Those skilled in the art will recognize, or be able to ascertain, using no more than routing experimentation, many equivalents to the specific embodiments of the invention described herein. 
     These and all other equivalents are intended to be encompassed by the following claims.