Polarization multiplexer, demultiplexer, and method

[0002] 1. Field of the Invention

[0003] 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.

[0004] 2. Description of the Related Art

[0005] 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.

[0006] There is a need in the art for a convenient system for multiplexing signals of the same or similar wavelength.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] These and other features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the invention.

[0018] FIG. 1 is a schematic diagram of a polarization multiplexer constructed in accordance with the present invention.

[0019] FIG. 2 is a schematic diagram of a polarization demultiplexer constructed in accordance with the present invention.

[0020] 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.

[0021] 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. ”

[0022] 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).

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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).

[0034] 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.

[0035] 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.