Abstract:
A multi-stripe laser diode chip is integrated with a beam combiner on a single optical bench, and is thus, applicable to placement in a single pigtailed module. Specifically, multiple beams emitted from the chip stripes are spatially merged using a birefringent material and then coupled into an optical fiber. The use of the birefringent material provides an efficient solution for generating the merged beam, which can be coupled into a single optical fiber.

Description:
BACKGROUND OF THE INVENTION 
     In several contexts, it is desirable to couple the optical energy from several, such as two, semiconductor laser diodes into a single optical fiber. 
     This combining capability is relevant to wavelength division multiplexing (WDM) applications where the laser diodes operate at different wavelengths and are modulated in response to different information signals, but couple into the same fiber. 
     The combining capability is also relevant in pump or other applications where power is the primary metric. In pump applications, the light from laser diodes is used to optically-pump rare-earth doped fiber or alternatively regular fiber, in a Raman pumping scheme. Multi-laser pump modules are attractive because there are limitations in the power that can be produced by a single high-power laser diode. As electrical current is increased in these lasers, the typical failure mode is catastrophic optical damage (COD) at the facet, especially in shorter wavelength laser devices such as 980 nanometer (nm) pump lasers, or excessive current densities in the ridge. Using multiple laser modules enables powers that are greater than could be generated by a single laser module. 
     Despite advantages, there are few commercial, integrated examples of laser systems that attempt to couple the optical energy from multiple lasers into a fiber, especially the single mode fiber that is used in most optical communication systems today. The explanation for this is the cost to manufacture—1) combining the optical energy from two lasers with bulk optics, such as beam splitter cubes, lenses, and mirrors, can more than double the module costs; and 2) the two laser diode chips double the semiconductor material costs. When these factors are accounted, the cost per Watt of the coupled power or multiplexed scheme is higher than a single laser coupled to the same fiber. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a laser system utilizing multiplexed laser diodes. It uses a beam combiner that is integrated with the laser diodes on a single optical bench, and is thus, applicable to placement in a single pigtailed module. Specifically, multiple beams emitted from laser diodes are spatially merged using a birefringent material and then coupled into an optical fiber. The use of the birefringent material provides an feasible solution for generating the merged beam. The solution is most applicable, especially from a cost standpoint, to a configuration where the laser diodes are combined on a single chip with multiple, two, stripes or ridges. 
     In general, according to one aspect, the invention features a laser system. The system comprises an optical bench and a laser device, which is connected to the optical bench and emits multiple beams. A birefringent material, also connected to the optical bench, spatially merges the beams from the laser system. The merged beam is then coupled into an optical fiber. 
     In one embodiment, the laser device comprises a single chip. The chip, however, has multiple stripes or ridges, such that it is capable of generating the multiple beams. In the present embodiment, the laser diode chip has only two stripes to generate two beams. 
     One advantage associated with using semiconductor laser chips having multiple stripes arises from efficient utilization of the underlying semiconductor wafer material. Such dual or multi-stripe laser chips are less expensive per Watt of power generation capacity because the multi-stripe chip is not much larger in terms of wafer area than a single stripe chip because substantial wafer material area is lost to the scribe lanes required to separate chips during fabrication. With a multi-stripe laser chip, this lost material can be amortized over a larger number of the light-generating stripes. 
     In order for the birefringent material to perform the spatial merging function, the beams must have different polarizations with respect to each other when passing through the material, specifically linearly orthogonal polarization. Typically, when generated by one, multistripe chip, the beams will have similar polarizations with respect to the optical bench. As a result, provisions must typically be made for rotating at least some of the beams. 
     In the present embodiment, a polarization rotator is connected to the optical bench. It rotates a polarization of some of the multiple beams. As a result, the beams have different polarizations with respect to each other at the birefringent material. 
     In one implementation, the polarization rotator is a plate, such as a half-wave plate. In another implementation, the polarization rotator comprises a sub-wavelength period grating. 
     In the present embodiment, collimation optics is installed on the bench optically between the laser device and the birefringent material to counteract the beam divergence typical in semiconductor lasers. 
     Focusing optics is also preferably used, optically after the birefringent material, to focus the merged beam exiting from the birefringent material onto the end of the optical fiber pigtail. 
     In general, according to another aspect, the invention can also be characterized as a process for beam combination in a multi-beam laser system. As such, the invention comprises generating multiple beams from a multi-stripe laser chip. At least some of the beams have their polarizations subsequently rotated to enable spatial merging using a birefringent material. The merged beam is then coupled into an optical fiber. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a top plan, schematic view illustrating a laser system with a multi-stripe diode chip and beam combiner; 
     FIG. 2 is a top plan, schematic view of another embodiment of the inventive laser system using a sub-wavelength grating to perform beam rotation; 
     FIG. 3 is a perspective view of the laser system, illustrating its integration on a single optical bench in a module; 
     FIG. 4 is a schematic top plan view of a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a laser system  100  with a multi-stripe diode laser chip  120  and beam combiner  102 , which has been constructed according to the principles of the present invention. 
     Specifically, the light is generated by a semiconductor diode chip  120 . The chip has multiple stripes  110 A,  110 B or ridge waveguides fabricated in it. Each stripe produces a, typically diverging, beam  150 A,  150 B. 
     In the preferred embodiment, the chip  120  has a highly reflective rear facet  122 , i.e., providing greater than 90% power reflectivity. Other configurations, however, are possible such as a Fabry-Perot configuration in which the rear facet  122  is anti-reflection coated and a grating or other cavity device is provided behind the rear facet  122 . Alternatively, such in other configurations, the grating is located in front of the chip, before or after the combiner. 
     The chip preferably has a low reflectivity front facet  124 . Typically, in pump applications, the front facet power reflectivities of 2-10% are used. 
     To address beam divergence, an optical collimation structure  205  is used to counteract the divergence and improve the collimation of beams  150 A,  150 B. A single cylindrical lens, for example, could be used. In the preferred embodiment, however, the collimation optical structure  205  comprises oval or circular microlenses  210 A,  210 B that fully collimate the beams  150 A,  150 B. Preferably, these are lenses formed on a single substrate of the optical structure  205  utilizing mass-transport processes as described in U.S. Pat. No. 5,618,474, which is incorporated herein by this reference in its entirety. 
     The formation of the two microlenses on a single substrate utilizing a photolithographically-controlled process is an important enabling technology and aspect of the invention. The transverse spacing between the stripes, and thus the beams, is known since lithographic processes form the stripes during fabrication of the chip  120 . This center-to-center ridge spacing is then used as the center-to-center spacing in the lithographic formation of the microlenses  210 A and  210 B. Thus, lens spacing is not a variable in the installation process of the optical collimation structure  205 . 
     The two beams  150 A,  150 B are typically polarized parallel to each other as they emerge from the ridges  110 A,  110 B of the chip  120 . In the preferred embodiment, a polarization rotator is also provided in the optical path to rotate a polarization of some of the multiple beams such that the beams have different polarizations with respect to each other. 
     The preferred implementation of the polarization rotator is a half-wave plate, which has been installed in one of the paths for the beams  150 A,  150 B. In the illustrated implementation, the half-wave plate  320  is installed in the path of beam  150 A. This rotates the plane of polarization of beam  150 A by 90°. 
     A plain substrate  310  or no substrate is placed in the path of the second beam  150 B. The plane of polarization of beam  150 B is thus unchanged. The phase delay of the two beams is equalized if the plain substrate is used. 
     One challenge to this embodiment is a fabrication of a thin half-wave plate that can be inserted into only one beam at a location that is optically upstream of the birefringent plate or material. Preferably, the plate has very low loss and has a sharp edge, i.e., the edge of the plate has a high optical quality. In the preferred embodiment, the half-wave plate  320  is constructed from quartz crystal in the zero order. The plate is thus about 60-90 micrometers thick for 980 nm radiation. 
     The beams  150 A,  150 B enter the birefringent material  500 . In the illustrated implementation, the nonrotated beam  150 B passes, without deviation, through the birefringent material  500 , while the rotated beam  150 A propagates at an angle (α). Various bifringent materials provides the necessary merging, such as rutile and calcite, for example. 
     The length or thickness of the birefringent material is selected so that the two beams overlap at the second face  502  of the plate  500 . When the two constituent beams exit from the block of birefringent material  500 , they are redirected to be parallel to the input direction to thereby form a merged beam  520 . 
     The merged beam  520  emerging from the birefringent material  500  comprises the two collinear beams with orthogonal polarizations with respect to each other. 
     The merged beam  520  is focused by optical focusing structure onto the core at the end  710  of an optical fiber  700 . In the preferred embodiment, the optical focusing structure comprises a discrete circular or oval lens  600  formed by the mass-transport process. In alternative embodiments, fiber lens systems are fabricated on the end  710  of the fiber  700 , in place of a cleaved end-surface, to improve coupling efficiency either in addition to or in place of the discrete lens. In still other embodiments, the curvature of the collimation lens  210 A,  210 B are selected to generate a focussed beam at the end of the fiber. 
     The optical fiber  700  is preferably single mode optical fiber. While a many solutions exist for coupling light into the core of relatively-large multimode fiber, coupling into single mode fiber requires the precise focusing attainable with the present illustrated embodiment due to the much smaller core cross-section. 
     The polarization characteristics of the merged beam make the present invention particularly relevant to Raman pumping applications, which require a non-polarized beam. In the present case, the merged beam  520  has balanced orthogonal states of polarization. 
     FIG. 2 illustrates an alternative configuration in which a sub-wavelength period grating  209 , instead of a half-wave plate, functions to rotate the polarization of one of the beams  150 A,  150 B. In the preferred embodiment, the grating is etched, or otherwise formed, onto a side of the substrate of the optical collimation structure  205 . Such gratings, as described in  Applied Physics Letters  42 (6), Mar. 15, 1983, page 492, do not diffract the light, but instead, operate as a homogenous birefringent material. 
     In still other embodiments the polarization rotation is performed by liquid crystal, preferably in photopolymerizable polymer utilizing photoalignment. 
     FIG. 3 illustrates the integration of the second embodiment of the laser system  100  on a single optical bench  105  in a laser system module  108 . Specifically, the multi-stripe laser chip  120  is attached to a submount  115 , which provides temperature control and electrical connections. 
     The collimation optical structure-subwavelength grating combination  205  is held on a first frame or mount  207 , which is attached to the optical bench  105  via bonding process such as solder bonding, or laser welding. In the preferred embodiment, eutectic solder bonding is used. Although in other systems, the substrate of the structure  205  is bonded directly to the optical bench. The birefringent plate  500  is supported on a second mount  505 , or alternatively is bonded directly to the bench  105 . The focusing lens substrate  600  is supported on a third mount  605  or bench bonded. Finally, the optical fiber  700  is also secured to the optical bench  105  via an alignment and mounting structure  705 . 
     Of course, in the case of the first embodiment, the half-wave plate  320  is added between the optical structure  205  and bifringent material  500 . In one implementation, the half-wave plate is attached to one of the frames illustrated in FIG. 3, such as frame  207  or  505 . In other implementations, another frame is used to support the plate. 
     FIG. 4 shows a third embodiment of the present invention. Rather than a rectangular bifringent plate as shown in FIGS. 1 and 2, this embodiment utilizes a wedge-shaped plate of bifringent material  500 A to spatially merge the converging beams from the laser chip  120 . Such converging beams are generated by the illustrated non-parallel stripes  110 A,  110 B or tilted facets, in other chip configurations. 
     The following is the assembly/alignment sequence for the system  100  illustrated in FIG.  3 : 
     First, the dual stripe laser  120  is installed on submount  115  and then submount/laser is installed onto bench  105 . Contemporaneously, the collimation optical structure  205 , focusing structure  605 , and the fiber alignment structure  705 , without fiber  700 , are installed on the bench  105 . 
     Next, wire-bonding is performed to laser  122 . 
     A large multi-mode fiber is then placed in fiber alignment structure  705  and connected to a detector and the laser  122  is energized. 
     While monitoring the strength of the signal received by the detector, the relative positions of the laser  122  and collimation and focusing structures  205 ,  605  are adjusted to maximize signal coupled into fiber. 
     When the signal maximum is detected, the multimode fiber is replaced with single mode fiber pigtail  700  and the half-wave plate  320  if used is installed. 
     Finally, the position of collimation and focusing structures  205 ,  605  is readjusted to maximize coupling into fiber  700 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.