Abstract:
A laser diode apparatus including a diode laser, optics efficiently collimate the diode laser beam, and a narrow band reflector to provide optical feedback for wavelength stabilization of the diode laser in an extended cavity configuration. The extended cavity laser diode assembly has a low reflectivity coating applied to the front facet, and a narrow-band reflectivity engineered to optimize the output power from the diode laser, leading to power penalty-free operation of the extended cavity laser diode assembly as compared to a free-running diode laser. The extended cavity laser diode assembly can equally applied to a plurality of laser diodes, with either a single or a plurality of optical feedback devices forming the extended cavity configuration.

Description:
STATEMENT REGARDING FEDERAL FUNDING 
     This invention was made with government support under contract no. FA9451-08-D-0218-0002 awarded by the U.S. Department of Defense. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the field of the present invention relates to wavelength stabilized laser diodes and laser diode modules. 
     2. Background 
     Lasers have enjoyed wide usage in different industries for years. Diode lasers in particular offer high electrical to optical efficiency, high output power, and reliable performance. For example, compact modules may be constructed that house a plurality of semiconductor diode lasers, either in the form of a bar of such diode lasers or singly and separate from each other. Laser light emitted from the diode lasers can then be fiber-coupled using miniature optics and the fiber-coupled light can in turn be used for various applications, such as directly pumping larger solid-state lasers or high power fiber amplifiers. However, the lasing wavelength and spectral width are current and temperature dependent, and are often too broad for the narrow absorption line of certain materials. The use of diode lasers to pump these materials requires methods to narrow the spectral linewidth of the device, and lock the output wavelength to a predetermined value. 
     Many methods of optical feedback have been developed to narrow and lock the lasing spectrum of laser diodes. By including frequency selective feedback techniques and suppressing the Fabry-Perot modes of the cleaved laser facets, the diode can be forced to laser at a designed wavelength, as opposed to lasing at the peak of the gain bandwidth on one of the closely spaced Fabry-Perot modes. These frequency selective feedback techniques can be fabricated or incorporated either inside the cavity, as is the case for distributed Bragg reflector (DBR) or distributed feedback (DFB) lasers, or external to the cavity, as is the case for external cavity lasers (ECLS) fabricated in a Littrow, Littman, volume Bragg gratings, or fiber Bragg gratings. In each of these cases, optical feedback is used to lock and narrow the spectrum of laser diodes. 
     Ideally, wavelength stabilized devices would have the same power and efficiency characteristics as unlocked devices, and would be able to operate over a wide temperature range. Unfortunately, the feedback mechanisms used in stabilize diode laser wavelength introduce optical loss, reducing the output power and operating efficiency of the device. Additionally, variations in the device temperature can cause a broadening and shift of the peak of the optical gain in the semiconductor device, causing the device to lase on the parasitic Fabry-Perot modes, as opposed to the design wavelength. Despite the need by industry of an external wavelength locking semiconductor diode laser apparatus that may operate substantially power penalty-free, no such device has been created. 
     SUMMARY OF THE INVENTION 
     The present invention provides an innovation that satisfies the aforementioned need and allows extended cavity locked high power laser diode apparatuses to exhibit a low power penalty. Thus, according to one aspect of the present invention, a laser apparatus includes a semiconductor diode laser optically coupled to a wavelength selective feedback component and thereby forming an extended cavity laser, the extended cavity laser having less than a 2% reduction in slope efficiency over the diode laser without unlocked devices, the low power penalty being achieved by collimating lenses in fast and slow axes providing low-loss optical feedback directly to diode laser gain region, optimizing the anti-reflection coated exit facet of the diode laser to substantially reduce the mirror loss of the Fabry-Perot modes in the diode laser cavity so as to effectively increase the laser threshold for these FP modes, providing anti-reflective coated optics in the optical path with substantially reduced broad-band optical reflectivity so as to eliminate the possibility of parasitic Fabry-Perot modes in the extended laser cavity, ensuring that the optical feedback is efficiently coupled back into the laser cavity, and optimizing the reflectivity of the frequency selective optical feedback component for output power and efficiency of the extended cavity laser. 
     The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 2  is a perspective view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 3  is a perspective view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 4  is a chart depicting power penalty advantages according to an aspect of the present invention. 
         FIG. 5  is a top view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 6  is a top view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 7  is a top view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 8  is a perspective view of one embodiment of an extended cavity frequency-locked diode laser bar apparatus according to an aspect of the present invention. 
         FIG. 9  is a perspective view of another embodiment of an extended cavity frequency-locked diode laser bar apparatus according to an aspect of the present invention. 
         FIG. 10  is a perspective view of one embodiment of an extended cavity frequency-locked diode laser apparatus using a plurality of single-emitter diode lasers according to an aspect of the present invention. 
         FIG. 11  is a top view of another embodiment of an extended cavity frequency-locked diode laser apparatus according to an aspect of the present invention. 
         FIG. 12  is a chart showing a wavelength range of a locked and unlocked diode lasers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a schematic is shown of one embodiment of an extended cavity wavelength-locked semiconductor diode laser apparatus  10  of the present invention. The laser apparatus  10  includes a semiconductor laser diode  12  that has an internal Fabry-Perot optical resonator region  14  interposed between semiconductor layers  16  and bound at opposite ends by first and second facets  18 ,  20 . With the resonator region  14  doped with laser dopants and the facets  18 ,  20  configured with particular reflectivities, laser operation can be enabled therein. Typically first facet  18  is operative as a high reflector, reflecting a maximum amount of light back into the resonant region  14  for further amplification. The second facet  20  is typically an exit facet operating as an output coupler to allow some light loss from the optical cavity for subsequent modification or use in various applications. 
     A frequency selective optical feedback component  22  is disposed in the path  24  of or otherwise optically coupled to the beam of light  26  emitted from the exit facet  20 . The frequency selective optical feedback component  22  is typically in the form of a narrow-band reflector that includes an input surface  28  and an output surface  30 . Suitable narrow-band reflectors include, for example, volumetric Bragg or holographic gratings. The feedback component  22  is operative to selectively reflect a narrower frequency range back towards the semiconductor resonator cavity  14  so that the light resonating therein is similarly locked to the narrower frequency reflected by the component  22 . By spacing apart the feedback component  22  from the exit facet  20  of the diode laser  12  and optically coupling the component  22  with the facet  20 , an extended cavity  32  is formed that can provide laser operation based on cavity  32 . As seen in  FIG. 12 , the use of feedback component  22  significantly narrows the range of wavelengths emitted from the laser cavity. Broader curve  100  shows a typical wavelength range provided by a diode laser operating without a frequency selective optical feedback component  22  whereas narrower curve  101  shows a curve of wavelength range provided by frequency selective optical feedback component  22 . 
     Referring now to  FIG. 2 , another embodiment of an extended cavity frequency-locked diode laser apparatus  34  of the present invention is shown in perspective. Since the beam of light  26  emitted from exit facet  20  tends to diverge quickly, collimation optics  36  are disposed in the beam path  24  to redirect the beam along a straight path. A fast axis collimation optic  38  is usually disposed in the beam path  24  closer to the exit facet  20  due to the corresponding faster divergence rate. A slow axis collimation optic  40  is disposed further down the path  24  and receives and collimates the relatively slower diverging slow axis of the beam  26 . Beam shaping optics  42  are disposed in the beam path  24  in intermediate relation to the collimation optics  36  and frequency selective optical feedback component  22 . Many different possible implementations of beam shaping optics  42  may be used to manipulate beam  26  and beam path  24  for various output requirements. For example, step mirrors, beam twisting optics, and prisms may be used each of which in combination or alone being a non-exhaustive enumeration of such optical shaping configurations. 
     When using extended feedback, such as with component  22 , to spectrally narrow and lock high power laser diodes, such as diode lasers  12 , very high levels of optical feedback provided by, for example, grating reflectivity, VHG reflectivity, a fiber Bragg grating, etc., are used. The high levels of optical feedback thereby provided suppress undesired Fabry-Perot modes of the extended cavity, but have the unintended consequence of forming a laser cavity that reduces the output power and laser efficiency, leading to a power penalty typically greater than 5% and with a corresponding slope efficiency drop. This approach also leads to poor selectivity between the extended cavity and the Fabry-Perot modes thereof, leading to the unintended consequence of poor temperature locking range in addition to the aforementioned power and efficiency penalty. 
     In order to achieve wavelength stabilized laser diode performance that can operate with minimal power penalty over a broad temperature range, parasitic Fabry-Perot modes should be suppressed and the narrow-band feedback reflectivity of the extended cavity reflector in the extended cavity configuration should be matched to the optimal reflectivity of the laser diode operating in an internal cavity or free-standing configuration. The parasitic Fabry-Perot modes are effectively suppressed by making the threshold gain for the cavity defined by the narrow-band reflector substantially smaller than the threshold gain of the cavity defined by the Fabry-Perot modes of the diode laser. The laser gain equation used for different laser cavities can be defined as: 
                     γ   th     =         ∝   i     ⁢       +     1     2   ⁢           ⁢   L         ⁢     ln   ⁡     (     2       R   Back     ⁢     R   Front         )           =       ∝   i     ⁢     +     ∝   mirror                   (     Eq   ⁢           ⁢   1     )               
where γ th  is the threshold gain, α i  is the intrinsic material loss, L is the laser cavity length, R Back , and R Front  are back and front mirror reflectivities, respectively, and α mirror  is the mirror loss. Typical optimal reflectivity values for a 1.5 mm cavity length diode are in the 5%-9% range, corresponding to a mirror loss of 10 and 8 cm −1 , respectively.
 
     For an extended cavity configuration  32 , the front facet reflectivity of the laser diode cavity  14  should be substantially reduced, leading to a very high mirror loss. For instance, reducing the facet reflectivity to &lt;0.1% increases the mirror reflectivity loss, α mirror , of the laser diode cavity  14  to over 23 cm −1 . As the peak value of the optical gain bandwidth changes as a function of drive current and temperature, the reduction in front facet reflectivity is determined by balancing the desired operating temperature range while maintaining wavelength stabilization and by the physical limits on reduction of front facet reflectivity. Thus, optimizing the reflectivity of the feedback element  22  based on the principles described herein provides an extended cavity laser  32  suitable for various applications and with minimal power penalty compared to operation of the laser diode  12 . 
     Also, the broad-band optical feedback from other optics in the optical path  24  between the diode  12  and the narrow-band reflector  22  should be substantially reduced. For example, referring to another embodiment of an extended cavity frequency-locked diode laser apparatus  34  of  FIG. 3 , several examples of competing cavities  44  are shown that typically resonate Fabry-Perot modes and provide undesirable feedback. Additionally, the reflectivity of the narrow-band reflector  22  should be configured to match the mirror loss, α mirror , of the optimal mirror loss of the laser diode  12 . Incorporating the principles described herein, the apparatus  34  as well as other embodiments within the scope of the present invention can be made to lase at the desired single wavelength determined by the extended cavity  32  with little to no power penalty, such as, for example, less than a 3% efficiency drop, and with lasing on competing Fabry-Perot modes substantially reduced. One effective way to incorporate the principles herein is the application of anti-reflective coating  43  at the exit facet  20  of the laser diode. Additional loss from broadband reflections, such as parasitic Fabry-Perot modes resonated by competing cavities  44  and defined by optical components situated in the optical path, can be diminished with application of anti-reflective coating  43  to the surfaces of respective components, such as the fast and slow axis collimation optics  36  and beam-shaping optics  42 , if present. Low reflectivity anti-reflective coating is particularly suited for coverage of bulk optics, such as collimation and beam shaping optics, because they are typically made from homogenous materials. The application of coatings  43  should reduce reflectivity values of optical surfaces to below 0.5% or otherwise as low as possible so as to provide better selectivity between desired feedback and broadband parasitic feedback. 
     The output power of a high power laser diode, such as diode  12 , is largely determined by the differential slope efficiency thereof, that is, the efficiency of the laser diode in coupling generated photons out of the laser cavity: 
                     η   d     =       η   i     ⁢       α   mirror         α   mirror     +     α   i                   (     Eq   .           ⁢   2     )               
where η d  is the differential slope efficiency, η i  is the intrinsic efficiency, α mirror  is the mirror loss and α i  is the intrinsic loss. Conventional frequency-locked extended cavities use mirror reflectivity values for the internal diode cavity that are too high, resulting in mirror loss values that are very low, causing the ratio of mirror loss of the internal diode cavity to total loss of the frequency-locked extended cavity to be low, reducing the differential slope efficiency of the extended cavity, the output power, and the diode efficiency. By configuring the narrow-band feedback reflectivity to be the same as the optimized reflectivity of the diode laser without the frequency selective optical feedback component, the slope efficiency power penalty is minimized.
 
     The power penalty is further exacerbated by imperfect feedback into the laser diode cavity. For example, in the embodiment of an extended cavity frequency-locked diode laser apparatus as shown in  FIG. 5 , reflected light that is not coupled into the laser diode resonator  14  becomes a further source of loss  46 . Example marginal rays  48   a ,  48   b  are emitted from diode resonator region  14  and are directed to respective fast and slow axis collimators  38 ,  40  before being reflected by frequency selective feedback element  22 . The reflected marginal rays  48   a ,  48   b  propagate back towards diode cavity  14 . However, due to misalignment and other imperfections in the optical feedback path, example marginal ray  48   b  fails to be properly coupled into the cavity  14  and becomes a source of loss  46 . Also, conventionally, frequency locking elements  22 , such as VBGs, have been incorporated into fast axis collimation lenses, or placed in the optical path of a diode that is collimated in the fast axis but not in the slow axis. Such configurations lead to additional optical scattering loss, further reducing the power and efficiency of the locked laser diode. By placing the feedback device  22  normal to the path of the collimated light so that light reflected thereby is directed back into the laser resonator  14 , the optical loss  46  can be significantly reduced and lead to a reduction in power penalty for spectrally locked laser diodes or laser diode arrays. 
     In  FIG. 4  a typical power penalty curve  50  is shown for a conventional laser diode module locked with an extended cavity frequency locking element. The curve  50  can be compared against a typical power curve  52  for a corresponding unlocked module. The typical externally locked power curve experiences significant power reduction at higher currents and for normal operation. In the embodiments of the apparatus of the present invention, low to zero power penalties are achieved, as exhibited by typical power curve  54 . The lack of penalty allows the curve  54  to closely match the curve  52  of an unlocked module, even at high power operation. While  FIG. 4  shows relatively high power ranges, power ranges within the scope of the present invention may vary significantly. Powers at or above 0.5 W are typical for single or few diode laser apparatuses while lower or higher powers outputs are possible as well. 
     A corresponding low to zero power penalty is achievable with single diode, diode bar, and other laser diode and laser diode module configurations utilizing one or more frequency selective optical feedback elements  22 . Referring now to  FIGS. 6 and 7 , in the illustrated embodiments of the extended cavity frequency-locked diode laser apparatus, diode lasers  12  are shown with corresponding collimation optics  38 ,  40  and some frequency selective optical feedback elements  22  alternatives to volume Bragg gratings hereinbefore described. One alternative includes a Littrow-type configuration, shown in  FIG. 6 , which includes a diffraction grating  56  disposed in the optical path at a predetermined angle. The first-order diffracted beam  58  provides optical feedback via reflection back into cavity  14  while an output beam  60  is also diffracted by the grating  56  at a frequency tuned by the angle of the grating  56  in relation to the feedback path. In another example shown in  FIG. 7 , a Littman-Metcalf configuration is used which includes a diffraction grating  62  disposed at an angle in relation to the optical feedback path of the extended cavity laser and a mirror  64  disposed in relation to the grating  62  for reflecting diffracted light back towards the grating  62 . An output beam  66  is provided by the combination of diffraction and reflection. Other configurations are possible as well, including the optical coupling of diode laser to a fiber Bragg grating, etc. 
     Laser diode bar based embodiments of the apparatus in accordance with the present invention are shown in  FIGS. 8 and 9 . As shown in  FIG. 8 , a corresponding diode laser apparatus  68  includes one or more diode laser bars  70  in optical communication with collimation optics  72 , beam shaping optics  74 , one or more frequency selective optical feedback elements  76 . Diode laser bars  70  typically include several diode laser emitters  78  formed so as to be laterally situated with respect to each other and arranged to emit laser beams  80  in a common parallel direction from the respective exit facets  82  of the emitters  78 . Frequency-selective optical feedback component  76  can be a single unit or separate units configured to lock the same or different frequencies. As shown in  FIG. 9 , a diode laser apparatus  84  includes a diode laser bar  70  and collimation optics  72  and operates similarly to diode laser apparatus  68 . However, apparatus  84  uses beam shaping optics  74  to combine beams  80  and direct the paths thereof to a single frequency selective optical feedback component  86  that locks each of the beam-directed diode emitters  78 . It should be understood that a plurality of bars may be used instead of a single bar as shown in  FIGS. 8 and 9 . Similarly, in other aspects of the various figures herein, where a single object is shown, a plurality of objects may be used as well. 
     Referring now to  FIG. 10 , in an another exemplary embodiment of the apparatus of the present invention an diode module apparatus  88  is shown that includes two diode lasers  12  disposed relative to each other in a module housing  90  and arranged to emit in a parallel direction. Housing  90  shown in cut-away may be made from different materials though heat conductive materials are well-suited to allow for effective heat dissipation during laser operation. In other embodiments more than two diode lasers  12  may be used. The diode lasers emit beams  26  along beam paths  24 . The beams  26  are collimated with collimation optics  36  and directed to and through beam shaping optics  42 . In some embodiments the beams  26  are stacked in the fast axis for subsequent application, such as coupling into an optical fiber or pumping a solid state block. In other embodiments the collimation optics  36  and beam shaping optics  42  share optical components. In still other embodiments the separate diode lasers are arranged to emit in a direction other than parallel. A frequency selective optical feedback component  22  receives the combined beams and partially reflects the beams  26  back to their respective diode lasers  12 . In some embodiments a plurality of frequency selective optical feedback components  22  are optically coupled with respective diode lasers  12  instead of using only one frequency selective optical feedback component  22 . 
     As shown in  FIG. 5 , frequency selective optical feedback components  22  in the form of a volume Bragg gratings typically have a parallel periodic layering  92  of dielectric and periodic refractive index change associated therewith. In some embodiments of the apparatus of the present invention, such as the one shown in  FIG. 11 , the VBG is characterized by a periodic layering  94  that is at an angle α with respect to the reflective input and output surfaces  28 ,  30 . By writing the input surface at an off-angle α, broadband optical feedback in the extended cavity laser can be further reduced. In this way, a designed misalignment allows feedback of undesirable frequencies to be scattered as loss beams  96  away from a return optical path  98  to the diode gain region  14 . 
     It is thought that the present invention and many of the attendant advantages thereof will be understood from the foregoing description and it will be apparent that various changes may be made in the parts thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the forms hereinbefore described being merely exemplary embodiments thereof.