Abstract:
A polarization dependent isolator includes a Faraday element, a linear polarizer positioned at a first end of the Faraday element to polarize light entering the first end of the Faraday element, and a single polarization fiber positioned at a second end of the Faraday element to receive light emerging from the second end of the Faraday element. A laser module includes a semiconductor laser diode, a Faraday element positioned adjacent the semiconductor laser diode, a linear polarizer positioned at a first end of the Faraday element nearest to the semiconductor laser diode to polarizer light passing from the laser diode to the first end of the Faraday element, and a single polarization fiber positioned at a second end of the Faraday element furthest from the semiconductor laser diode to receive light emerging from the second end of the Faraday element, wherein the single polarization fiber also serves as coupling output fiber for the laser module.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to optical isolators and specifically to design and assembly of polarization dependent isolators. This application claims priority to Provisional Application No. 60/639,707, filed on Dec. 28, 2004, entitled HYBIRD FIBER POLARIZATION DEPENDENT ISOLATOR, AND LASER MODULE INCORPOATING THE SAME.  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical transmitter and transponder systems use polarization dependent isolators (PDIs) to immunize lasers from return beams since such return beams are known to destabilize oscillation of lasers.  
         [0003]      FIG. 1  shows a typical single stage PDI  100 . For high optical isolation, two or more of the single stage PDI  100  can be cascaded in series. The PDI  100  includes a Faraday element  102 , typically made of Yttrium-Iron-Garnet (YIG) or Terbium-Gallium-Garnet (TGG). The Faraday element  102  is positioned between an input polarizer  104  and an output polarizer (also known as analyzer)  106 . The polarization axis of the output polarizer  106  is set at 45° relative to the polarization axis of the input polarizer  104 . A permanent magnet  108 , typically made of a rare-earth metal, applies a magnetic field to the Faraday element  102 , making the Faraday element  102  optically active. The direction of the magnetic field is represented by arrow  108   a.    
         [0004]     Input beam  110 , moving in the forward direction, is linearly polarized in the input polarizer  104 . The linearly polarized beam passes through the Faraday element  102 , where the magnetic field applied by the permanent magnet  108  acts in concert with the Faraday element  102  to rotate the polarization plane of the beam by 45°, allowing the beam to then pass through the output polarizer  106 , as indicated at  112 . Any return beam is first polarized at 45° by the output polarizer  106 . Since the Faraday effect is non-reciprocal, the return beam is rotated an additional 45° upon passing through the Faraday element  102 , and then blocked by the input polarizer  104 .  
         [0005]     The polarizers  104 ,  106  are typically polarizing glass plates, polarizing prisms, and the like. To ensure desired characteristics of the PDI  100 , the polarizers  104 ,  106  must be accurately aligned in a plane perpendicular to an optical axis of the Faraday element  102  and the appropriate angle, in this case 45°, must be formed between the polarizers  104 ,  106 . Once the polarizers  104 ,  106  are aligned with the Faraday element  102 , the PDI components are individually fixed in place using techniques such as soldering, gluing, or welding. To maintain the appropriate angle between the polarizers  104 ,  106 , fixing of the PDI components in place must be highly precise. This makes assembly of the PDI somewhat labor intensive.  
         [0006]     Various solutions have been proposed to make it easier to assemble a PDI. For example, U.S. Pat. No. 5,757,538 (Siroki et al.) proposes forming wire grid polarizers, i.e., unidirectional gratings of thin silver films, on opposite surfaces of a garnet film at the appropriate angle and working the garnet film into a chip that then serves as a Faraday element. This avoids the need to individually fix the polarizers and Faraday element in place. The Faraday element is placed within a permanent magnet and used as a PDI. U.S. Pat. No. 6,813,077 (Borrelli et al.) discloses a method of forming wire grid polarizers on a garnet material and a wire grid structure that suppresses reflection of rejected polarization.  
         [0007]     In addition to finding easier ways to assemble the PDI, it is also desirable to miniaturize the PDI, thereby allowing a laser module incorporating the PDI to be made small.  
       SUMMARY OF THE INVENTION  
       [0008]     In one aspect, the invention relates to a PDI which comprises a Faraday element, an input polarizer positioned at an input end of the Faraday element to polarize an input beam entering the input end of the Faraday element, and a single polarization fiber positioned at an output end of the Faraday element to receive an output beam emerging from the output end of the Faraday element.  
         [0009]     In another aspect, the invention relates to a polarization dependent isolator which comprises a first isolator unit, a second isolator unit cascaded in series with the first isolator unit, and a single polarization fiber positioned adjacent the second isolator unit to receive a beam emerging from the second isolator unit, wherein each of the isolator units comprises an input polarizer positioned at an input end of a Faraday element to polarize an input beam entering the input end of the Faraday element.  
         [0010]     In yet another aspect, the invention relates to a laser module which comprises a laser diode, a Faraday element positioned adjacent the laser diode, an input polarizer positioned at an input end of the Faraday element nearest to the laser diode to polarize light passing from the laser diode to the input end of the Faraday element, and a single polarization fiber positioned at an output end of the Faraday element furthest from the laser diode to receive light emerging from the output end of the Faraday element, wherein the single polarization fiber also serves as coupling output fiber for the laser module.  
         [0011]     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic of a prior art PDI.  
         [0013]      FIG. 2A  is a schematic of a single stage PDI according to one embodiment of the invention.  
         [0014]      FIG. 2B  is a schematic of a single stage PDI according to one embodiment of the invention.  
         [0015]      FIG. 3  is a cross-section of a single polarization fiber.  
         [0016]      FIG. 4  shows typical cutoff wavelengths for two polarization modes of a single polarization fiber designed to operate at a nominal wavelength of 1550 nm.  
         [0017]      FIGS. 5A and 5B  are schematics of laser modules incorporating a PDI according to one embodiment of the invention.  
         [0018]      FIG. 6A  is a schematic of a double stage PDI according to one embodiment of the invention.  
         [0019]      FIG. 6B  is a schematic of a double stage PDI according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail in order to not unnecessarily obscure the invention. The features and advantages of the invention may be better understood with reference to the drawings and discussions that follow.  
         [0021]     Embodiments of the invention provide a polarization dependent isolator (PDI) which has fewer number of assembly steps in comparison to conventional PDIs. The PDI enables a laser module to be produced with fewer components. In particular, the PDI uses a single polarization fiber instead of the conventional analyzer or output polarizer. When the PDI is incorporated in a laser module, the single polarization fiber doubles up as the coupling output fiber of the laser module. In one embodiment, the PDI has an insertion loss ≦0.5 dB. In one embodiment, the PDI has an isolation ≧40 dB. PDIs according to embodiments of the invention may be designed to operate at nominal wavelengths in a range from 800 to 1900 nm. PDIs of the invention may be cascaded in series for high optical isolation applications.  
         [0022]      FIG. 2A  illustrates a single stage PDI  200  according to one embodiment of the invention. The PDI  200  includes a Faraday element  202  made of a magneto-optical garnet, such as rare-earth iron garnet, e.g., yttrium iron garnet (YIG), bismuth-substituted iron garnet, e.g., bismuth-substituted yttrium iron garnet, and rare-earth gallium garnet, e.g., terbium gallium garnet (TGG). YIG is typically used at wavelengths in a range from 1100 to 2100 nm. TGG is typically used at wavelengths in a range from 500 to 1100 nm. The Faraday element  202  can be of the latching or non-latching type. In the illustration, the Faraday element  202  is of the non-latching type and is disposed within a permanent magnet  204 . A Faraday element of the latching type may be operated without a bias magnet. Typically, the permanent magnet  204  is a rare-earth magnet, e.g., Sm—Co type rare-earth magnet. The permanent magnet  204  applies a magnetic field to the Faraday element  202 , allowing the Faraday element  202  to become optically active. When a polarized light passes through the Faraday element  202  in a direction  204   a  of the magnetic field, the polarization plane of the light is rotated. The amount of rotation depends on the field strength and the distance the light travels through the Faraday element  202 . In one embodiment, the permanent magnet  204  and the Faraday element  202  are designed such that the polarization plane of a polarized light passing through the Faraday element  202  is rotated by approximately 45°.  
         [0023]     An input polarizer  206  is formed on an input end  208  of the Faraday element  202 . In one embodiment, the input polarizer  206  is a linear polarizer. In one embodiment, the polarization axis of the input polarizer  206  is at 0° relative to the polarization axis of the input beam  209 . That is, the polarization axis of the input beam  209  and the polarization axis of the input polarizer  206  are aligned to a maximum transmission. The polarization axis is referred to as the direction of the electric-field vector {right arrow over (E)}(r,t), where r is the radial distance in spherical coordinates (in meter) and t is the time (in seconds). The input polarizer  206  may be a dichroic polarizer, such as one sold under the trade name Polarcor® glass polarizer. Alternatively, the input polarizer  206  may be a wire grid polarizer. The wire grid polarizer may be formed directly on the input end  208  of the Faraday element  202 . U.S. Pat. No. 6,813,077 (Borrelli et al.) describes a method of forming a wire grid polarizer directly on a garnet material. In a forward direction, the input polarizer  206  polarizes the input beam  209  prior to the input beam entering the Faraday element  202 .  
         [0024]     A single polarization fiber  210  is positioned adjacent an output end  212  of the Faraday element  202 . The single polarization fiber  210  is positioned to receive beam  213  emerging from the Faraday element  202 . Where the input beam  209  is collimated, a focusing lens ( 215  in  FIG. 2B ) is preferably inserted between the Faraday element  202  and the single polarization fiber  210  to focus beam  213  into the single polarization fiber  210 . The single polarization fiber  210  propagates only one of two orthogonally polarized polarizations while suppressing the other polarization by increasing its transmission loss. The polarization axis of the single polarization fiber  210  is set at 45° relative to the polarization axis of the input polarizer  206 . In comparison to the conventional PDI, the single polarization fiber  210  replaces the output polarizer or analyzer. When the PDI  200  is incorporated in a laser module, the single polarization fiber  210  doubles up as the coupling output fiber, thereby reducing the number of components in the laser module.  
         [0025]     Any suitable single polarization fiber may be used in the invention. A suitable example of a single polarization fiber is described in U.S. application Ser. No. 10/864,732, the disclosure of which is incorporated herein by reference.  FIG. 3  shows a cross-section  300  of the single polarization fiber disclosed in U.S. application Ser. No. 10/864,732. The cross-section  300  shows an elongated core  302  with two air holes  304 ,  306  placed next to the core  302 . In one embodiment, the elongated core  302  is elliptical and the air holes  304 ,  306  are placed along the minor axis of the ellipse. The aspect ratio of the core  302  is typically between 1.5 and 8, preferably greater than 1.5, more preferably between 2 and 5. The air holes  304 ,  306  and core  304  are surrounded by cladding  308 . The cladding  308  has a higher refractive index than the core  304 . The core  304  may be made of germania-doped silica, and the cladding  308  may be made of fluorine-doped silica. The polarization axis  310  is shown at 45° relative to the polarization axis P of the input polarizer ( 206  in  FIGS. 2A and 2B ).  
         [0026]     For a single polarization fiber having the cross-section  310 , the air holes  304 ,  306  create differential cutoff wavelengths for the two polarization modes, i.e., the attenuated and the transmitted modes. This differential cutoff makes single polarization propagation possible.  FIG. 4  shows typical cutoff wavelengths for the two polarization modes of a single polarization fiber designed for a nominal wavelength of 1550 nm. The polarization bandwidth is around 60 nm. The polarization bandwidth is the difference in wavelength measured as &gt;5 dB of loss on the attenuated polarization and &lt;1 dB of loss on the transmitted polarization. The polarization bandwidth can be tuned by changing the fiber parameters. Single polarization fibers having polarization bandwidth in a range from 18 to 100 nm are useful in the invention.  
         [0027]     A laser module incorporating a PDI of the invention is suitable for use in optical transmission and transponder systems, such as DWDM (Dense Wavelength Division Multiplexing), SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy, and ATM (Asynchronous Transfer Mode) systems. Also, it could be used in fiber optic sensors (such as fiber optic gyroscopes and current sensors), in optical interferometers and measurements systems.  
         [0028]      FIG. 5A  illustrates a laser module  500  incorporating the PDI  200  (also shown in  FIG. 2A ). The laser module  500  includes a laser diode  502 , e.g., a distributed feedback (DFB) laser or a Fabry-Pérot laser. The laser module  500  includes a lens  504  which focuses a beam  505  generated by the laser  502  on the input polarizer  206  of the PDI  200 . The focused beam  506  passes through the input polarizer  206 , where it is linearly polarized, and then through the Faraday element  202 , where it is rotated 45°. The beam  507  emerging from the Faraday element  202  is coupled into the single polarization fiber  210  of the PDI  200 . In an alternate embodiment, as shown in  FIG. 5B , the beam  506  entering the input polarizer  206  is a collimated beam, and the lens  215  (also shown in  FIG. 2B ) improves coupling efficiency between the Faraday element  202  and the single polarization fiber  210  by focusing the beam  507  emerging from the Faraday element  202  into the single polarization fiber  210 . In either of the embodiments illustrated in  FIGS. 5A and 5B , any return beam from the single polarization fiber  210  is rotated an additional 45° by the Faraday element  202  and prevented from reaching the laser  502  by the input polarizer  206 .  
         [0029]     Returning to  FIGS. 2A and 2B , the performance of the PDI  200  can be optimized by adjusting the linear and angular position of the single polarization fiber  210  relative to the Faraday element  202  such that insertion loss is minimized and isolation is maximized. In one embodiment, the PDI has an insertion loss ≦0.5 dB. In one embodiment, the PDI has an isolation ≧40 dB. PDIs according to embodiments of the invention having isolation ≧40 dB for nominal wavelengths of 1310 nm and 1550 nm have been designed. Using the appropriate and optimized materials and components (polarizers, Faraday elements, and single polarization fibers), PDIs according to embodiments of the invention having isolation ≧40 dB for nominal wavelengths other than 1310 and 1510 nm, generally, in a range from 800 nm to 1900 nm, can also be designed. PDIs according to embodiments of the invention can be cascaded in series for high optical isolation. Examples of double stage PDIs according to embodiments of the invention will now be described.  
         [0030]      FIG. 6A  shows a double stage PDI  600  according to one embodiment of the invention. The PDI  600  includes two half-isolator units  602 ,  604 . The isolator unit  602  includes an input polarizer  602   a , a Faraday element  602   b , and a permanent magnet  602   c  (which may be omitted if the Faraday element  602   b  is of the latching type). The isolator unit  604  includes an input polarizer  604   a , a Faraday element  604   b , and a permanent magnet  604   c  (which may be omitted if the Faraday element  604   b  is of the latching type). A single polarization fiber  608  is positioned adjacent the isolator unit  604  to receive beam  610  emerging from the Faraday element  604   b . When the input beam  606  is a collimated beam, a focusing lens  612  is preferably inserted between the Faraday element  604   b  and the single polarization fiber  608  to focus the beam  610  into the single polarization fiber  608 .  
         [0031]     In one embodiment, the polarization axis of the input polarizer  602   a  is at 0° relative to the polarization axis of the input beam  606 , the polarization axis of the input polarizer  604   a  is at 45° relative to the polarization axis of the input polarizer  602   a , and the polarization axis of the single polarization fiber  608  is at 90° relative to the polarization axis of the input polarizer  602   a . In the forward direction, the input beam  606  passes through the input polarizer  602   a , where it is linearly polarized at 0°, and then through the Faraday element  602   b , where it is rotated 45°, and then through the input polarizer  604   a , where it is linearly polarized at 45°, and then through the Faraday element  604   b , where it is rotated an additional 45° so that it can be coupled into the single polarization fiber  608 . Any return beam from the single polarization fiber  608  is rotated an additional 45° by the Faraday element  604   b  and then blocked by the input polarizer  604   a . Any return beam escaping the input polarizer  604   a  (i.e., any return beam at 45° after rotation by the Faraday element  604   b ) is rotated an additional 45° by the Faraday element  602   b  and then blocked by the input polarizer  602   a.    
         [0032]      FIG. 6B  shows a double-stage PDI  620  according to another embodiment of the invention. The PDI  620  includes a full-isolator unit  622  and a half-isolator unit  624 . The full isolator unit  622  includes an input polarizer  622   a , a Faraday element  622   b , an output polarizer  622   c , and a permanent magnet  622   d  (which may be omitted if the Faraday element  622   b  is of the latching type). The input polarizer  622   a  and output polarizer  622   c  are formed on opposite sides of the Faraday element  622   b . In one embodiment, the polarization axis of the output polarizer  622   c  is at 45° relative to the polarization axis of the input polarizer  622   a , and the polarization axis of the input polarizer  622   a  is at 0° relative to the polarization axis of the input beam  626 . The half-isolator unit  624  includes an input polarizer  624   a , a Faraday element  624   b , and a permanent magnet  624   c  (which may be omitted if the Faraday element  624   b  is of the latching type). The input polarizer  624   a  is in opposing relation to the output polarizer  622   c , and the polarization axis of the input polarizer  624   a  is aligned with the polarization axis of the output polarizer  622   c.    
         [0033]     The PDI  620  also includes a single polarization fiber  628  positioned adjacent the half-isolator unit  624  to receive beam  630  emerging from the Faraday element  624   b . Where the input beam  626  is a collimated beam, a focusing lens  632  is preferably inserted between the Faraday element  624   b  and the single polarization fiber  628  to focus the beam  630  into the single polarization fiber  628 . In one embodiment, the polarization axis of the single polarization fiber  628  is at 90° relative to the polarization axis of the input polarizer  622   a , and the PDI  620  operates similarly to the PDI ( 600  in  FIG. 6A ) described above.  
         [0034]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.