Abstract:
An optical pumping module for use in optical amplifiers in fiber-optic communication. The pump module integrates laser diodes and a polarization beam combiner (PBC). The laser diodes and PBC may be attached to a substrate. The laser diodes may be formed onto the substrate either monolithically or by attaching discrete components to the substrate. The PBC may include isolation to prevent reflections back into the laser diodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application No. 60/395,413, filed Jul. 13, 2002, entitled Optical Pump Module; which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The invention relates to the field of optical amplifiers. More specifically, the invention relates to systems and methods for combining optical pumping sources for use in optical amplifiers. 
     2. The Relevant Technology 
     In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber-optic cable as can occur with electronic signals in wire-based systems. Light signals also can be conducted over greater distances without the signal loss typically associated with electronic signals on wire-based systems. 
     While signal loss in a fiber-optic cable is less than that in wire-based systems, there is nonetheless some signal loss over the long transmission distances that light signals are transmitted. To compensate for the signal loss, optical amplifiers are used. Two common optical amplifiers are Raman amplifiers and Erbium Doped Fiber Amplifiers (EDFAs). Both of these amplifiers use characteristics of fiber-optic cables to amplify light signals. 
     The amplifier pumps light on the fiber-optic cable that is at a different frequency than the light signal that is to be amplified. Energy from the light that is pumped on to the fiber-optic cable is transferred to the light signal due to the characteristics of the fiber-optic cable. Optical amplifiers use optical pumps to generate the light that is pumped into the fiber-optic cable. Optical pumps, however, are expensive. Although the price of a low-power optical pump is relatively low, high-power optical pumps are substantially higher priced. 
     When light is pumped into an optical system, the gain provided by the pumped light is dependent on the polarization of the light that is pumped into the optical system. If the polarization of the pumped light source fluctuates, the gain may fluctuate. To achieve consistent gain, a pumping source that provides a beam that is the combination of two orthogonally polarized beams with equal power in each beam is desired. 
     For this reason, polarization beam combiners (PBCs) are widely used in Raman amplifiers and EDFAs. They provide a simple way to combine two optical pumping sources that have perpendicular polarization directions and equal power into a single beam. They also provide even polarization distribution in the combined pumping beam in two orthogonal directions to minimize gain that is dependent on the pumping beam&#39;s state of polarization. 
     Commercially available PBCs are made using two different approaches: micro-optics and fused fiber. A micro-optic PBC, shown in  FIG. 1A , is a PBC associated with fiber coupling devices, such as collimators including an optical lens and a fiber pigtail. A PBC can be made in several different ways, including Wollaston, Nicol, Rochon, Glan-Thompson, or Glan-Taylor prisms, using thin-film coatings on Right Angle Prisms (RAPs), or a single piece of birefringent crystal. A PBC made of high quality birefringence material tends to have better optical performance, such as a higher polarization extinction ratio, than dielectric-coating-based devices, yielding lower combining loss and higher power handling. Typical birefringent materials include Calcite, YVO4, Rutile, LiNbO 3  and other single crystalline materials. A fused fiber PBC is simply a polarization maintaining (PM) fiber 2×2 fused fiber bi-conic coupler as shown in  FIG. 1B , for example. A fused fiber PBC has a simple structure as well as low loss at the center wavelength and potential for low cost. 
     Both of these devices have limitations in practical applications. Multiple-wavelength pumping is often used to obtain wide and flat optical gains in the light signal bandwidth. Typically, two or three different wavelengths are used by the pump. These wavelengths fall about 20 nm apart and can cover a complete light signal bandwidth of at least 60 nm. To serve this wide bandwidth requirement, devices used for the pumping module should have flat performance response over the wavelength range. However, fused fiber devices show a 0.4 dB combining loss variance over a 60 nm wavelength bandwidth. Furthermore, it can be difficult to get equal combining efficiency for each input beam. 
     Another concern is controlling optical back reflections into the pumps. Most Raman pumps include multiple pump lasers. It is necessary to control the optical back reflection in order to stabilize the output of each laser and protect these lasers from being damaged. One common way to reduce back reflection is to employ optical isolators. The isolators are typically either in-line isolators, which can be fiber spliced into the optical path, or free space isolators used inside the pump laser module. 
     Many commercially available pump laser modules typically use fiber Bragg gratings (FBGs) to stabilize the pumping wavelengths. The FBGs are configured to reflect different wavelengths of light at different points in the optical path to compensate for the different speeds at which different frequencies of light travel in a fiber-optic cable. Free space isolators would block the reflections if placed in between the laser and the FBG, and thus cannot be used in such applications. Laser pumps thus require multiple external in-line isolators, which increases the cost and size of the pump. 
     Accordingly, it is desirable to integrate as many of the components of a pump as possible. This would serve to reduce component count and improve manufacturability, yielding improved optical performance in a smaller and less expensive module. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect of the invention, two individual pump lasers are incorporated within an integrated pump module having high output optical power. The integration allows reduced component count and manufacturing time, a smaller size, and a lower cost. The module includes at least two active devices, such as laser diodes. Other active devices, such as photo diodes, may also be included within the same subassembly. The active devices may be fabricated on a single substrate monolithically or fabricated separately and then bonded to a common substrate. 
     In another aspect of the invention, the assembly includes a miniature optical beam combiner. The combiner redirects the beams of each laser into a single output beam. The combiner may also have an isolation function that enhances stability by preventing optical energy entering the module from falling on the active areas of the lasers. 
     In another aspect of the invention, active components may be integrated with passive components in the same subassembly. In comparison to discrete approaches, this provides a simple structure and uses fewer components, yielding higher optical performance, a smaller footprint, and lower cost. 
     These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1A  illustrates a prior art micro-optic polarization beam combiner; 
         FIG. 1B  illustrates a prior art fused fiber polarization beam combiner; 
         FIG. 2A  illustrates an integrated optical pump module; 
         FIG. 2B  illustrates active components in an optical pump; 
         FIGS. 3A  illustrates a perspective views of one embodiment of an optical birefringent wedge used in the construction of a combiner; 
         FIG. 3B  illustrates a perspective views of one embodiment of an optical birefringent wedge used in the construction of a combiner; 
         FIGS. 4A  illustrates a perspective view of a Rochon prism; 
         FIG. 4B  illustrates a perspective view of an assembly of wedges and a Faraday rotator, sometimes referred to as the core of the combiner; and 
         FIG. 5  illustrates an embodiment of the core, lenses, and optical fibers in a combiner. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a combiner that can be used to combine/split light while providing optical isolation in a backward path. As previously discussed, it is difficult to use optical isolators in pump lasers because many lasers use Fiber Bragg Gratings (FBGs) to stabilize pumping wavelengths. The present invention provides high combining efficiency of two or more optical pumping sources that have substantially equal polarizations in orthogonal directions while providing optical isolation in a backward path. 
       FIG. 2A  shows a diagram of the active and passive components in the optical path of one embodiment of an optical pump module  200  embodying aspects of the invention. The active components are the laser diodes  202  and  214  and the photo diodes  220  and  222 . The passive components are the optical lenses  206 ,  212  and  218  and the optical combiner  208 . 
     The optical paths are as follows. Laser diode  202  produces a first pumping laser beam  204  in response to a drive current. The first pumping laser beam  204  is linearly polarized at some state of polarization (SOP). The SOP may be controlled using conventional techniques, such as through the manner of manufacture of the laser diode  202  or by using a wave plate. The first pumping laser beam  204  is collimated by a lens  206  and directed into a combiner  208 . The combiner  208  refracts the first pumping laser beam  204  in a manner that will be described in more detail below, which is then focused and output to an output port  210  from the module  200  by a lens  212 . The output port  210  that may be a single fiber pigtail or a standard fiber receptacle or plug such as small form factor, small form factor pluggable, or GBIC receptacles and/or plugs, and the like. 
     A laser diode  214  produces a second pumping laser beam  216 . The second pumping laser beam  216  is a linearly polarized output beam polarized at an SOP that is perpendicular to the SOP of first pumping laser beam  204 . The second pumping laser beam  216  is collimated by a lens  218  and directed into the combiner  208 . The second pumping laser beam  216  is deflected by the combiner  208  into the same path as the first pumping laser beam. The second pumping laser beam  216  is focused into the output port  210  by the lens  212 . The first and second pumping laser beams  204 ,  216  are thereby combined and output. 
     The devices shown in  FIG. 2A  may be included in the same optical subassembly, realizing space savings and improved performance. The subassembly may be a standard package, such as the various TO packages or an FCA package. Alternatively, the active and passive components may be realized in separate packages, and later appropriately assembled as part of an optical pump that may be used in an optical amplifier. With this approach, the components could be fiber spliced together or aligned in a free space arrangement. 
     Other approaches may be employed to combine laser beams into a single beam and other optical elements may be added to increase performance or functionality. For example, depending on the configuration of the laser diodes  202 ,  214 , the lenses  206  and  218  may be realized as a single lens. Photo diodes such as the photodiodes  220  and  222  shown in  FIGS. 2A and 2B , may also be included in the pump module  200  to monitor the output power of the laser diodes  202  and  214  in a conventional manner. 
     The combiner  208  may be a micro-optic PBC or it may be an isolating PBC (iPBC) in accordance with the present invention. An iPBC is a micro-optic device that has all the functionality of a conventional PBC in the forward optical path (laser-to-output) but has a large transmission loss in the backward optical path (output-to-laser). In other words, light traveling through the iPBC in the backward direction is diverted away from active optical devices. As will be explained in more detail below, the iPBC uses a Faraday rotator in one embodiment to change the SOP of light such that light traveling in the backward optical path has a large transmission loss. Considerations in selecting a combiner include laser wavelength and need for isolation. In some cases, a PBC may be preferable to an iPBC for shorter wavelengths due to the lowered efficiency caused by the Faraday rotator. 
     An embodiment of the active components of the integrated pump module  200  is shown in  FIG. 2B  as an active part  250 . Four active components are shown: two laser diodes  202  and  214  and two photo diodes  220  and  222 . The exemplary active part  250  can be fabricated in many ways. One approach is to fabricate the four diodes  202 ,  214 ,  220  and  222  on the same substrate  252  monolithically. Because the operation of all the components depends on common physical principles, they can be designed and fabricated using the same material and processes. This offers one or more advantages, such as a simplified fabrication process, increased reliability from having fewer individual bonding joints, less cost by using fewer bonding and packing processes, and a small device footprint. 
     Another approach is to fabricate one or more diodes separately, and then bond them to the same substrate  252  or carrier. This offers the advantage of being able to take advantage of conventionally manufactured optical components available at low cost. 
     The configuration of the optical components should be chosen to match the requirements of the combiner  208 . For example, the dimension d between the first and second pumping laser beams  204  and  216  and the emitting angles θ of the laser diodes  202  and  214  should be optimized to maximize the coupling efficiency to the combiner  208 . In addition, some combiners may have polarization requirements. In the example in which an iPBC is used, the two light sources are linearly polarized in perpendicular directions. The polarization state of a light source can be determined in many ways. In the case of vertical-cavity surface-emitting laser (VCSEL) diodes, for example, methods to select single polarization operation can include use of an external reflector, suitable etching of the VCSEL&#39;s surface or its optical aperture, or the use of subwavelength transmission gratings. Polarization requirements of the combined output beam for a particular application may also affect the selection and configuration of combiner and light sources. 
     Referring to  FIGS. 3A ,  3 B,  4 A,  4 B and  5 , a combiner core  504  is constructed using a Polarization Beam Combiner (PBC) that includes, for example, Wollaston, Rochon, Glan-Thompson or Glan-Taylor prisms  302 ,  304 , or even thin film cubes. These types of prisms are well known and widely used in optics, laser optics, and medical optics. All conventional PBCs are optically isotropic meaning that light travels the same in both directions. 
     The core may also include a Faraday rotator  402  that in one embodiment of the invention is a yttrium iron garnet (YIG) crystal. The Faraday rotator  402  may be latching magnetic material or non-latching magnetic material. For a non-latching material, an external magnet may be used to apply a magnetic filed while a latching material does not need an external magnetic field. In one example, when the bi-directional communications transceiver is intended to be used in an environment with stray magnetic fields, a non latching material design may be preferable as the external magnets will be better able to control the polarization changes of light traveling within the combiner core. This is true because the external magnets exert a stronger magnetic field on the light than the stray magnetic influences. If designs using a latching material were placed in an environment containing stray magnetic fields, the stray magnetic fields may cause a polarization shift in the light traveling in the combiner core. 
     The combiner may use two optical birefringent wedges  302 ,  304 . The wedge  302  and wedge  304  are shown in  FIG. 3A  and  FIG. 3B , respectively. Putting these wedges  302 ,  304  side by side creates a prism similar to the Rochon prism shown in  FIG. 4A . To cause this prism to be optically non-reciprocal (meaning that light travels differently in forward and backward directions), a Faraday rotator  402  is inserted in between the two wedges  302 ,  304  as shown in  FIG. 4B . 
     The ray traces  501 ,  502  and  503  in the combiner  500  are shown in  FIG. 5  for both forward and backward directions. A first linearly polarized beam is shown as a ray trace  501  from a first light source  506 . The first linearly polarized beam from first light source  506  is polarized to be parallel to the optical axis of the first wedge  304 . The first linearly polarized beam is focused by a lens  505  into the first wedge  304 . The first wedge  304  in this example has an optical axis of 90°, but other configurations may work as well. The light source  506  may be any conventional source, such as an integrated laser diode, i.e laser diode  202 , or the end of a fiber. When the light source is the end of a fiber, the fiber may be a polarization maintaining (PM) fiber. The fiber should have a high extinction ratio to obtain high beam combining efficiency. For example, an extinction ratio higher than 20 dB is preferable in some embodiments of the invention. 
     A second beam from the second light source  508 , shown as ray trace  502 , is also linearly polarized, but in a perpendicular direction to that of the first linearly polarized beam from the first light source (i.e., perpendicular to the optical axis of the first wedge  304 ). The second linearly polarized beam from the second light source  508  is focused by the lens  505  into the first wedge  304 . As with the first light source  506 , the second light source  508  may be a laser diode or PM fiber. If a PM fiber is used for the second light source  508 , the PM fiber should be as perpendicular to the PM fiber used as the first light source  506  as possible such that the SOP of the first beam is perpendicular to the SOP of the second beam. In other words, the optical slow (fast) axes of the PM fibers should be perpendicular to each other. If the PM fibers are not perpendicular, power may be lost. This loss may be expressed by the equation:
 
Loss=−10 log  10 (Cos  2 β)  (1)
         where β is the amount of misalignment from perpendicular. Notably, this equation also holds true for loss for whatever light source is used. The light emitted by the sources should be polarized such that they are as close to perpendicular as possible with respect to each other.       

     The first and second light sources  506  and  508  are configured such that the two linearly polarized beams come to the first wedge  304  with an angle γ:
 
γ=2 ·arc sin[( n   o   −n   e )·tan θ]  (2)
 
     (where θ is the wedge angle, and n o , n e  are the refractive indices for the ordinary beam and the extraordinary beam in the two birefringent wedges  302  and  304 ). After exiting the first wedge  304 , the beams pass through a Faraday rotator  402 . The exemplary Faraday rotator  402  shown in  FIG. 5  has a rotation angle of 45° in the forward direction. Other rotators may be used in other configurations. The beams are refracted by the second optical wedge  302  into the same path, shown as ray trace  503 . The beams are then focused into an output fiber  510  by a lens  512 . 
     Light entering the combiner, which is an iPBC in this embodiment, from the output fiber  510  takes a different path through the combiner core  504 . Depending on its polarization, light traveling in the backward path may take one of two different paths, but will exit the first wedge  304  in a direction perpendicular to the face of the wedge  304 , as shown. The backward paths do not pass through the ray traces  501  or  502 . Light can be prevented from returning to the two light sources  506  and  508  by a suitable optical configuration. Thus, any optical reflections along the output fiber  510  are isolated from the two laser diodes  202  and  214 , protecting them and stabilizing their output power. 
     The foregoing example exhibits one embodiment of the invention in which two linearly polarized beams are combined to form a pumping beam. The pumping beam preferably has substantially equal polarizations in two orthogonal directions. 
     Generally, the physical construction of a combiner such as the one described above is accomplished by attaching discrete optical components onto a substrate material such as glass or polysilicon. The attachment can be accomplished by using epoxy or some other type of glue. 
     A device constructed according to the foregoing principles may be used to improve the pumping efficiency of EDFAs, Raman amplifiers, and the like. It may also be used for other applications such as polarization division multiplexing. Due to its performance, compact size, and integrated functionality, it allows the realization of compact, high-performance optical pump modules at less cost. 
     Multiple combiners can be utilized to pump with more than one wavelength of light. Multiple wavelength pumping is useful to obtain wide and flat optical gains in the signal band. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.