Abstract:
A birefringent wedge is used to multiplex beams of the same or similar wavelength. The multiplexing system has a compact construction and does not require beam splitters or right angle prisms. The birefringent wedge may also be used to polarization demultiplex an incoming multiplexed communication beam. The invention may be used to increase the data-carrying capacity of optical fiber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of optical devices and systems. More particularly, the invention relates to a system for multiplexing polarized signals for simultaneous transmission through a communication fiber. The invention also relates to a system for demultiplexing a beam into polarized components. 
     2. Description of the Related Art 
     In the prior art, optical fiber capacity has been increased by multiplexing signals of different wavelengths. The technique is known as wavelength division multiplexing (WDM). A problem with wavelength division multiplexing is that the propagation rate of light is wavelength dependent. Consequently, the different propagation rates of different signals must be taken into account to accurately extract all transmitted information. Wavelength division multiplexing is also limited by the number of different wavelength signals that can be accommodated in a single mode fiber under commercial conditions. 
     There is a need in the art for a convenient system for multiplexing signals of the same or similar wavelength. 
     SUMMARY OF THE INVENTION 
     The disadvantages of the prior art are overcome to a great extent by the present invention, in which signals having substantially the same wavelength but different polarizations are multiplexed into a single beam. The multiplexed beam may be transmitted through a single mode fiber without interference between the two signals. The signals may be polarization demultiplexed at a downstream location. 
     In one aspect of the invention, a compact multiplexing system is provided with first and second sources (such as pump lasers) for producing first and second linearly polarized signal beams, and a birefringent device for combining the two beams into a single multiplexed beam. 
     In a preferred embodiment of the invention, the birefringent device is in the form of a wedge, with first and second planar surfaces arranged at an angle to each other. The first and second surfaces are not parallel to each other. The polarized beams are incident on the first surface. The multiplexed beam exits through the second surface. Thus, the multiplexing system is arranged to align and linearly walk the polarized beams together such that they are parallel and coincident with each other as they exit the birefringent wedge. 
     The birefringent wedge may be formed of a suitable uniaxial crystal material, such as rutile. The birefringent wedge may be a modified walkoff plate or prism. 
     In another aspect of the invention, a single graded index lens is used to collimate the signal beams and launch them into the birefringent wedge. In a preferred embodiment, the lens causes the signal beams to slightly diverge from each other, such that they are differentially refracted into a parallel condition by the birefringent wedge. 
     In a preferred embodiment of the invention, the multiplexed beam is focused into a communication fiber by a second graded index lens. The birefringent lens may be sandwiched between the two graded index lenses to form a compact, uncomplicated apparatus. An advantage of the invention is that it can be constructed as a compact package, without using any beam splitters or right angle prisms. 
     The present invention also relates to a communication system with a multiplexer for launching a polarization multiplexed signal into a single mode fiber, and a downstream demultiplexer for separating polarization dependent signals from the multiplexed signal. In a preferred embodiment of the invention, the demultiplexer may be formed of essentially the same components as those of the multiplexer, but arranged in reverse. 
     A retarder mechanism may be provided to align the polarized components of the multiplexed signal. The retarder mechanism may be controlled to maximize the strength (intensity) of the demultiplexed signals. The retarder mechanism may be, for example, a set of paddles that are actively tuned to provide a desired amount of retardance. 
     The present invention also relates to a method of multiplexing optical signals. Thus, in one aspect of the invention, linearly polarized signal beams are independently generated by separately modulating respective lasers. A birefringent wedge combines the beams into a single multiplexed beam. The multiplexed beam is propagated through a single mode fiber, and polarization demultiplexed at a downstream location. 
     The present invention permits a single optical fiber, or optical capillary, to transmit, without interference or interaction, two beams of light that are of the same or similar wavelength. Thus, the present invention may be used to increase fiber data capacity by at least two fold, over known wavelength division multiplexing/demultiplexing techniques. The invention may also be used to provide pump laser diversity in optical amplifiers. 
     These and other features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a polarization multiplexer constructed in accordance with the present invention. 
     FIG. 2 is a schematic diagram of a polarization demultiplexer constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, where like reference numerals designate like elements, there is shown in FIG. 1 a multiplexing system  10  constructed in accordance with a preferred embodiment of the invention. The system  10  takes orthogonally polarized light beams  12 ,  14 , multiplexes them into a single beam  16 , and launches the multiplexed beam  16  into a communication fiber  18 . As the multiplexed beam  16  propagates through the fiber  18 , the polarized components  12 ,  14  do not interact or become scrambled. Consequently, the components  12 ,  14  may be polarization demultiplexed at a downstream location. 
     The polarized beams  12 ,  14  may be produced by respective pump lasers  20 ,  22 . The lasers  20 ,  22  are modulated independently of each other. The beams  12 ,  14  may have substantially the same wavelength, such that they would interfere with each other if they had the same polarization. In the illustrated embodiment, the wavelength of each beam  12 ,  14  is about 1550 nanometers. The beams  12 ,  14  are linearly polarized by the lasers  20 ,  22 . The polarization of the first beam  12  is preferably orthogonal to that of the second beam  14 . The orthogonal polarization states are designated in the drawings by the symbols “+” and “o. ” 
     The pump lasers  20 ,  22  are connected to the multiplexing system  10  by polarization maintaining fibers  24 ,  26 . The system  10  has a capillary  28  for receiving the ends of the fibers  24 ,  26 . In the illustrated embodiment, the capillary  28  is a two fiber termination device (DFT). 
     The multiplexing system  10  also has a first lens  30 , a birefringent wedge  32 , and a second lens  34 . The first lens  30  collimates the polarized beams  12 ,  14  and transmits them toward the wedge  32 . The birefringent wedge  32  walks the beams  12 ,  14  together, to form the single multiplexed beam  16 , as discussed in more detail below. The second lens  34  transmits the multiplexed beam  16  into a single fiber termination (SFT)  36  coupled to the end of the communication fiber  18 . An advantage of the illustrated embodiment is that only a single lens  30  is required between the capillary  28  and the wedge  32 , and only a single lens  34  is required downstream of the wedge  32 . Moreover, the invention may be advantageously constructed without any epoxy in the optical path. 
     The first lens  30  preferably has a graded index (GRIN). The ends of the input fibers  24 ,  26  (at the capillary  28 ) are preferably beveled by about 8 degrees to match the contour of the first lens  30 . The polarized beams  12 ,  14  are skewed with respect to each other as they emerge from the first lens  30 . In the illustrated embodiment, between the first lens  30  and the wedge  32 , the beams  12 ,  14  diverge away from each other by an angle of about 3.6 degrees. 
     The birefringent wedge  32  may be formed of rutile (a positive uniaxial crystal) or another suitable material. In the illustrated embodiment, the indices of refraction of the rutile material at 1550 manometers are n c =2.710 and n o =2.454, and Δn=10.4%. Other suitable materials, especially birefringent materials having refraction indices of about 2.5 and Δn of about 10%, may be used to construct the wedge  32 . The present invention is not limited to the illustrated embodiment. 
     The wedge  32  has a first planar surface  38 , a second planar surface  50 , and a crystal axis  52 . The surfaces  38 ,  50  are not coplanar. The first surface  38  forms an angle α of about 18 degrees with respect to the second surface  50 . The crystal axis  52  is oriented at 45+/−2 degrees with respect to the second surface  50 . 
     In operation, the diverging beams  12 ,  14  are differentially refracted toward each other as they enter the birefringent wedge  32 . The beams  12 ,  14  have different polarizations and therefore see different refractive indices (ordinary and extraordinary) in the wedge  32 . The angle α between the surfaces  38 ,  50  of the wedge  32  is such that the beams  12 ,  14  become substantially parallel to each other as they exit the wedge  32  at the second surface  50 . In addition, the length  54  of the wedge  32  is such that the beams  12 ,  14  walk onto each other and become substantially coincident, forming the single multiplexed beam  16 . 
     The second lens  34  may have a graded index. The output lens  34  focuses the multiplexed beam  16  into the single fiber termination  36 . The end of the communication fiber  18  is beveled to match the surface of the second lens  34 . The communication fiber  18  may be a single mode silica based fiber. 
     In the illustrated embodiment, the distance  56  from the first lens  30  to the first surface  38  of the wedge  32  is about 2 millimeters. The distance  58  from the second wedge surface  50  to the second lens  34  is also about 2 millimeters. The length  54  of the wedge  32  is a function of the distance  56  from the first lens  30  to the first wedge surface  38 . In the illustrated embodiment, the length  54  of the birefringent wedge  32  is 1.3+/−0.1 millimeters. 
     It has been found that attenuation loss caused by the birefringent wedge  32  can be reduced by controlling the wedge angle α. In the illustrated embodiment, if the angle α is maintained within +/−0.13 degrees of the desired angle (18 degrees), the wedge  32  may cause a loss of 0.5 decibel. Where the angle α is more closely maintained within a tolerance of +/−0.08 degrees, the wedge  32  may cause a loss of 0.2 decibel. Where the angle α is maintained within +/−0.06 degrees, the wedge  32  may cause a loss of no more than about 0.1 decibel. 
     A demultiplexing system  70  constructed in accordance with the present invention is shown in FIG.  2 . The demultiplexing system  70  may be used to demultiplex an incoming beam  16  into orthogonally polarized component beams  12 ,  14 . The demultiplexing system  70  employs essentially the same optical devices discussed above in connection with FIG.  1 . The arrangement and relative positions of the devices may be the same in both systems  10 ,  70  except that the devices are arranged in reverse in the demultiplexing system  70 . 
     In operation, an incoming beam  16  is collimated by the second lens  34  and transmitted through the second surface  50  of the birefringent wedge  32 . The wedge  32  separates the beam  16  into orthogonally polarized beams  12 ,  14 . The signal beams  12 ,  14  are converged by the first lens  30  into respective fibers  72 ,  74 . The strength of the signal in the first signal fiber  72  is monitored by a monitoring device  76 . The monitoring device  76  is operatively connected by a feedback mechanism  78  to an adjustable retarding mechanism  80 . The retarding mechanism  80  is located in series with the incoming fiber  18 . The retarding mechanism  80  rotates one of the polarized signals  12  with respect to the other  14  to maximize the strength of the signal  12  at the monitoring device  76 . 
     The retarding mechanism  80  may be, for example, an FPC-1 fiber polarization controller (retarding device) marketed by Fiber Control Industries. Alternatively, the retarding mechanism  80  may be a polarization controller of the type shown in U.S. Pat. No. 5,659,412 (Hakki). 
     The demultiplexing system  70  may be used to demultiplex a beam  16  created by the multiplexing system  10  of FIG.  1 . The present invention should not be limited, however, to the use of the demultiplexing system  70  in combination with the multiplexing system  10 . The multiplexed beam  16  may be demultiplexed by other systems, such as systems which employ beam splitters and/or dichroic materials. 
     The above descriptions and drawings are only illustrative of preferred embodiments which achieve the features and advantages of the present invention, and it is not intended that the present invention be limited thereto. Any modification of the present invention which comes within the spirit and scope of the following claims is considered part of the present invention.