Abstract:
A fiber collimator device includes polarization controlling optics to simplify the manufacture of fiber collimators that use polarization maintaining fiber. In particular, the use of the polarization control optics in a fiber collimator reduces the need to align the polarization axis of the polarization maintaining fiber, thus reducing the time spent on fabricating the device. This is useful where the collimation device includes a polarization mode combiner.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed generally to fiber optic devices, and more particularly to single and dual-fiber collimators utilizing polarization-maintaining fibers.  
         BACKGROUND  
         [0002]    In the field of fiber optic communications, information is transmitted optically over a network of single-mode or multi-mode fibers. Many of the switching and splitting functions in these networks are accomplished in free space, where the light may exit the fiber and interact with active and/or passive optical components. In some instances, it may be necessary to collimate the optical beam exiting the fiber for efficient interaction with the external components. Also, the transmitter unit in fiber optic communication systems is typically a semiconductor laser diode, which has a linearly polarized output which may be coupled to a polarization-maintaining fiber.  
           [0003]    When preparing a break in an optical communication link, it is common to insert the exposed fiber optic into an optical ferrule for protection of the delicate glass fiber. There are applications where it may be desirable to have two or more such fibers in the same ferrule transmitting and/or receiving at different optical wavelengths, or at the same wavelength.  
           [0004]    Given the above, there is a need for an optical collimating device incorporating a multi-port ferrule which can accommodate an input optical signal carried by a polarization maintaining fiber.  
         SUMMARY OF THE INVENTION  
         [0005]    Generally, the present invention relates to a fiber collimator device that includes polarization controlling optics to simplify the manufacture of fiber collimators that use polarization maintaining fiber. In particular, the use of the polarization control optics in a fiber collimator reduces the need to align the polarization axis of the polarization maintaining fiber, thus reducing the time spent on fabricating the device.  
           [0006]    In particular, one embodiment of the invention is directed to a fiber collimator unit having a first focusing element having an optical axis and a first focal length. A first optical fiber is optically coupled to a first side of the first focusing element, and is disposed at a first transverse distance from the optical axis so that light from the first optical fiber propagates on a second side of the first focusing element as a substantially collimated beam at a first angle to the optical axis. A first polarization rotator is disposed on the second side of the first focusing element. The polarization rotator deviates the polarization state of the substantially collimated beam. There is an optical element that substantially redirects light at a first pre-determined polarization state and substantially transmits light at a second pre-determined polarization state orthogonal to the first polarization state.  
           [0007]    Another embodiment of the invention is directed to a method of aligning light in a fiber optic device. The method includes transmitting a first polarized light from a first port disposed towards a first end of the device through a polarization rotator and adjusting the polarization of the first output light to a selected state by rotating the polarization rotator. The method also includes reflecting light in the selected polarization state back so that the reflected light propagates through the polarization rotator to a second port disposed towards the first end of the device.  
           [0008]    Another embodiment of the invention is directed to an optical system that has an optical transmitter producing output light, an optical receiver receiving at least a portion of the output light, and an optical fiber link coupling between the optical transmitter and the optical receiver. At least two light sources have outputs combined in fiber device having a first focusing element having an optical axis and a first focal length. A first optical fiber is optically coupled to a first side of the first focusing element. The first optical fiber is disposed at a first transverse distance from the optical axis so that light from the first optical fiber propagates on a second side of the first focusing element as a substantially collimated beam at a first angle to the optical axis. A first polarization rotator is disposed on the second side of the first focusing element, the polarization rotator deviating the polarization state of the substantially collimated beam. An optical element substantially reflects light at a first pre-determined polarization state and substantially transmits light at a second pre-determined polarization state orthogonal to the first polarization state. An output form the fiber device is coupled into the fiber link.  
           [0009]    The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 schematically illustrates a dual-fiber collimator unit;  
         [0012]    [0012]FIG. 2 schematically illustrates another embodiment of a dual-fiber collimator unit; and  
         [0013]    [0013]FIG. 3 schematically illustrates an embodiment of an optical communications system according to an embodiment of the invention.  
     
    
       [0014]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0015]    The present invention is applicable to fiber optic devices and is believed to be particularly useful with fiber optic devices that use single and dual-fiber collimators utilizing polarization-maintaining fibers.  
         [0016]    A dual-fiber collimator (DFC) assembly  100  is an important building block for optical add/drop multiplexers, monitor arrays, and hybrid assemblies. A typical design for a DFC  100  is shown in FIG. 1, which schematically illustrates a dual port, filter-based optical device. The device may be part of a multiplexer/demultiplexer, add/drop filter, power tap, or the like. The dual-fiber collimator  100  includes a first lens  102  and dual-fiber ferrule  104 . Two fibers  106  and  108  are held in the ferrule  104 , with their ends  106   a  and  108   a  positioned at a distance from the lens  102  equal to about the focal length of the lens  102 . The ferrule end  104   a , and the fiber ends  106   a  and  108   a  may be polished at a small angle to prevent reflections feeding to other elements. Fiber  106  may be a polarization maintaining (PM) fiber with a linearly polarized output beam  110 , wherein the polarization vector of the beam  110  may project in an arbitrary direction in a plane orthogonal to the direction of propagation.  
         [0017]    In the illustrated embodiment, a first linearly polarized light beam  110 , from the first fiber  106 , passes through the lens  102  and is collimated. However, since the beam  110  is not positioned on the lens axis  112 , the collimated beam  114  propagates at an angle, θ1, relative to the axis  112 . For typical systems, the value of θ1 may be around 2°, depending on such factors as the focal length of the lens  102  and the separation between the two fibers  106  and  108 .  
         [0018]    The collimated beam  114   a  is incident on a polarization rotator  124 . The polarization rotator  124  may be a retardation waveplate, or other type of optical device that rotates polarization. Here, the term optical waveplate is used to cover optical devices that rotate the polarization of the light, including retardation waveplates and Faraday rotators. The optical waveplate  124  may vary the polarization state of the output beam  114   b  to any desired linear state, for example by rotating the optical waveplate  124  about the optical axis  112 . The optical waveplate  124  may be adjusted such that the output beam  114   b  is in a desired polarization state when incident on the optical element  116 . The optical element  116  may be a single or multiple element optical device designed to substantially reflect light at a pre-determined linear polarization state and to transmit the orthogonal linear polarization as a transmitted beam  122 . The optical element  116  may substantially reflect the beam  114   b  as a collimated reflected beam  118 . The reflected beam  118  re-transits the optical waveplate  124  and emerges as beam  120 , still linearly polarized but, typically, no longer parallel to the input polarization vector of beam  118 . Beam  120  is redirected as beam  126  and focused by lens  102  into the second fiber  108 . The second fiber may be a single mode fiber insensitive to the polarization state of beam  126 .  
         [0019]    An advantage of the embodiment of DFC illustrated in FIG. 1 is that the polarization axis of the polarizing maintaining fiber  106  need not be aligned precisely to deliver light in the desired polarization state for reflection and/or transmission at the optical element  116 . Such alignment is labor intensive and, therefore, increases the cost of the device. Instead, the waveplate  124  is used to control the polarization of the light incident on the element  116 . Rotating the waveplate  124  to achieve the desired polarization state is simpler than rotating the orientation of the polarization maintaining fiber  106  in the ferrule  104 . Furthermore, the polarization of the reflected beam  120  relative to the second fiber  108  may be unimportant, particularly where the second fiber  108  is not a polarization maintaining fiber, and so the polarizing effects of the second passage of the light through the waveplate  124 , towards the second fiber  108 , may be unimportant.  
         [0020]    Another particular embodiment  200  of the invention is illustrated in FIG. 2, which shows a single fiber collimator (SFC) coupling light into one arm, fiber  108 , of a dual fiber collimator (DFC). The DFC contains elements substantially identical to those depicted in FIG. 1, and light is coupled from input fiber  106  to output fiber  108  in the manner outlined previously. Light passing from the optical element  116  is denoted, in this case, as beam  218 . The portion of light beam  114   b  that is reflected by the element  116  forms a component of the beam  218 .  
         [0021]    The single fiber collimator includes a lens  202  and a single-fiber ferrule  204 . The SFC has an optical axis  212  that is set at an angle θ2 relative to optical axis  112  of the DFC. The angle θ2 may be chosen such that the light transmitted through the element  116  is colinear with the portion of beam  114   b  reflected from the element  116 , and forms another component of optical beam  218 . Fiber  206  is held in ferrule  204 , with its end  206   a  positioned at a distance from the lens  202  equal to about the focal length of the lens  202 . The ferrule end  204   a  and the fiber end  206   a  may be polished at a small angle to prevent reflections feeding to other elements. Fiber  206  may be a polarization maintaining fiber with a linearly polarized output beam  210 , wherein the polarization vector of the beam  210  may project in an arbitrary direction in a plane orthogonal to the direction of propagation. In the illustrated embodiment, a linearly polarized light beam  210  from fiber  206  passes through lens  202  and is collimated. The collimated beam  214   a  is incident on a polarization rotator  224 , also referred to as an optical waveplate. Optical waveplate  224  may vary the polarization state of the output beam  214   b  to any desired linear state by a technique such as rotating the optical waveplate  224  about the axis  212 . The optical waveplate may be adjusted such that the output beam  214   b  is in the appropriate polarization state to be substantially transmitted by optical element  116 . The optical beam  214   c  exits optical element  116  substantially collimated and combines with beam  118  and traverses substantially the same pathway as beams  118 , 120  and  126  and is directed by lens  102  and focused into fiber  108 .  
         [0022]    An advantage of the this embodiment is that the polarization axis of the polarization maintaining fiber  206  need not be aligned relative to the optical element  116 . Instead, the optical waveplate  224  may rotated to align the polarization of the light beam  120  to a desired state, for example a polarization state that is transmitted through the optical element  116  to the second fiber  108 .  
         [0023]    One particular application of the present invention is in polarization mode combining polarized beams from two different sources, for example to combine the output from two lasers. An optical system  300  that uses a polarization mode combiner is schematically illustrated in FIG. 3. A DWDM transmitter  302  directs a DWDM signal having m channels through a fiber communications link  304  to a DWDM receiver  306 .  
         [0024]    In this particular embodiment of DWDM transmitter  302 , a number of light sources  308   a ,  308   b -  308   m  generate light at different wavelengths, λa, λb . . . λm, corresponding to the different optical channels. One or more of the light sources  308   a - 308   m  may include two light generators, for example lasers  308   m ′ and  308   m ″, whose output is polarization combined in a polarization mode combiner  316 . The wavelengths of the light generators  308   m ′ and  308   m ″ need not be the same. The light output from the light sources  308   a - 308   m  is combined in a DWDM combiner unit  310 , or multiplexer (MUX) unit to produce a DWDM output  312  propagating along the fiber link  304 .  
         [0025]    Light sources  308   a - 308   m  are typically laser sources whose output is externally modulated, although they may also be modulated laser sources, or the like. It will be appreciated that the DWDM transmitter  302  may be configured in many different ways to produce the DWDM output signal. For example, the MUX unit  310  may include an interleaver to interleave the outputs from different multiplexers. Furthermore, the DWDM transmitter  302  may be equipped with any suitable number of light sources for generating the required number of optical channels. For example, there may be twenty, forty or eighty optical channels, or more. The DWDM transmitter  302  may also be redundantly equipped with additional light sources to replace failed light sources.  
         [0026]    Upon reaching the DWDM receiver  306 , the DWDM signal is passed through a demultiplexer unit (DMUX)  330 , which separates the multiplexed signal into individual channels that are directed to respective detectors  332   a ,  332   b - 332   m.    
         [0027]    The fiber link  304  may include one or more fiber amplifier units  314 , for example rare earth-doped fiber amplifiers, Raman fiber amplifiers or a combination of rare earth-doped and Raman fiber amplifiers. The pump light may be introduced to the fiber amplifier  314  from a pump unit having two pump lasers  315   a  and  315   b . The outputs from these two lasers  315   a  and  315   b  may be polarization combined in a polarization mode combiner  316  and the combined pump beam coupled into the fiber link  304  via a coupler  318 . The wavelengths of the two pump lasers  315   a  and  315   b  may be the same, or may be different.  
         [0028]    The fiber link  304  may include one or more DWDM channel monitors  326  for monitoring the power in each of the channels propagating along the link  304 . Typically, a fraction of the light propagating along the fiber link  304  is coupled out by a tap coupler  324  and directed to the DWDM channel monitor  326 . The fiber link  304  may also include one or more gain flattening filters  325 , typically positioned after an amplifier unit  314 , to make the power spectrum of different channels flat. The channel monitor  326  may optionally direct channel power profile information to the gain flattening filter. The gain flattening filter  325  may, in response to the information received from the channel monitor  326 , alter the amount of attenuation of different channels in order to maintain a flat channel power profile, or a channel power profile having a desired profile.  
         [0029]    The fiber link  304  may include one or more optical add/drop multiplexers (OADM)  342  for directing one or more channels to a local fiber system  344 . The local loop  344  may also direct information back to the OADM  342  for propagating along the fiber link  304  to the DWDM receiver  306 . It will be appreciated that the information directed from the local fiber system  344  to the OADM  342  need not be at the same wavelength as the information directed to the local loop  344  from the OADM  342 . Furthermore, it will be appreciated that the OADM  342  may direct more than one channel to, and may receive more than one channel from, the local system  344 . The amount of light being added to the fiber link  304  from the local system  344  may be monitored by a channel monitor to ensure that the light in the channel(s) being added to the fiber link has an amplitude similar to that of the existing channels.  
         [0030]    As noted above, the present invention is applicable to fiber optic devices and is believed to be particularly useful in fiber optic devices that have polarized inputs. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification.