Abstract:
A particular focusing arrangement in a dual fiber collimator improves the transmission efficiency and ease of alignment of the collimator. In particular, the collimator includes two optical fibers mounted in a dual-fiber ferrule. The fiber end-faces are co-planar with the angle-polished end-face of the ferrule and displaced along the optical axis of the ferrule. The collimator unit also includes a first focusing element centered along the optical axis of the ferrule and positioned such that its back front focal point is between the two end faces of the fibers. An advantage provided by this arrangement is that by changing the fiber polish angle and polish azimuth, relative to a line connecting the fiber centers, superior coupling from one fiber to the other can be achieved, even with different working distances and at various wavelengths.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed generally to fiber optic communications systems, and more particularly to techniques to compensate for multiple wavelengths and working distances in dual-fiber components in such systems.  
         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, in some cases the optical fibers carry multiple channels at different optical wavelengths and the collimating optics may need to accommodate multiple wavelengths simultaneously.  
           [0003]    When preparing a break in an optical communication link, it is common practice 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 simultaneously accommodate multiple optical wavelengths.  
         SUMMARY OF THE INVENTION  
         [0005]    Generally, the present invention is directed to improving the transmission efficiency and ease of alignment of multiple wavelength dual-fiber collimator devices.  
           [0006]    The invention is directed to a collimator unit that includes two optical fibers mounted in a dual-fiber ferrule, the fiber end-faces are co-planar with the angle-polished end-face of the ferrule and displaced along the optical axis of the ferrule. The collimator unit also includes a first focusing element centered along the optical axis of the ferrule and positioned such that its front focal point is between the two fibers end-face.  
           [0007]    In one aspect of the invention, the input signal to the collimator unit enters through the fiber furthest from the collimator lens. The output of the collimator lens is partially reflected by an optical filter placed at the back focus of the collimator lens and focused back through the collimator lens into the output fiber mounted adjacent to the input fiber in the dual-fiber ferrule. In this embodiment, the input fiber end-face is positioned slightly outside the front focal plane of the collimating lens, and therefore the optical beam emerging from the collimator lens towards the filter is nearly collimated but slightly converging. An advantage provided by this arrangement is that by changing the fiber polish angle and polish azimuth, relative to a line connecting the fiber centers, superior coupling from one fiber to the other can be achieved, even with different working distances and at various wavelengths.  
           [0008]    One particular embodiment of the invention is directed to a dual fiber collimator unit that includes a dual-fiber ferrule having an angled end face and having a ferrule axis. First and second optical fibers are mounted in the ferrule with respective first and second fiber ends approximately coplanar with the angled end face. The first fiber end corresponds to a first position along the ferrule axis and the second fiber end corresponds to a second position along the ferrule axis different from the first position. A focusing unit having an optical axis coincident with the ferrule axis is set at a working distance between i) the focusing unit and ii) a working point along the ferrule axis between the first and second positions that is substantially equal to the front focal length of the focusing unit.  
           [0009]    Another embodiment of the invention is directed to an optical system that has an optical transmitter producing output light in a signal wavelength band, 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. The optical fiber link includes a fiber optic device having a dual fiber collimator that has a dual-fiber ferrule having an angled end face and a ferrule axis. First and second fibers are mounted in the ferrule with respective first and second fiber ends approximately coplanar with the angled end face. The first fiber end corresponds to a first position along the ferrule axis and the second fiber end corresponding to a second position along the ferrule axis different from the first position. A focusing unit has an optical axis coincident with the ferrule axis. A working distance between i) the focusing unit and ii) a working point along the ferrule axis between the first and second positions is substantially equal to the front focal length of the focusing unit.  
           [0010]    Another embodiment of the invention is directed to a method of controlling light in a dual fiber collimator unit. The method includes propagating the light diverging from a first fiber, having a first fiber end, towards a focusing unit. The light from the first fiber is focused with the focusing unit so as to produce a convergent beam. The convergent beam is reflected and is focused with the focusing unit to a second fiber having a second fiber end disposed closer to focusing unit than the first fiber end.  
           [0011]    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  
       [0012]    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:  
         [0013]    [0013]FIG. 1 schematically illustrates an embodiment of a three-port fiber optic filter unit according to the present invention.  
         [0014]    [0014]FIG. 2 schematically illustrates an embodiment of an optical fiber unit according to the present invention.  
         [0015]    [0015]FIG. 3 schematically illustrates another embodiment of an optical fiber unit according to the present invention.  
         [0016]    [0016]FIG. 4 schematically illustrates an embodiment of an optical fiber unit according to the present invention. 
     
    
       [0017]    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  
       [0018]    The present invention is applicable to fiber optic communications systems, and is believed to be particularly suited to techniques to compensate for multiple wavelengths and working distances in dual-fiber components in such systems.  
         [0019]    A dual-fiber collimator (DFC) assembly is an important building block for optical add/drop multiplexers, monitor arrays, and hybrid assemblies. An embodiment of a DFC  100  is shown in FIG. 1. The dual-fiber collimator  100  includes a first lens  102  and a dual-fiber ferrule  104 . Two fibers  106  and  108  are held in the ferrule  104 , with their ends  106   a  and  108   a  coplanar with the end-face  104   a  of the ferrule. The ferrule end  104   a,  and the fiber ends  106   a  and  108   a  may be angle-polished, thereby offsetting the fiber end-faces  106   a  and  108   a  along the optical axis  112  of the ferrule. The centerlines of the fibers  106  and  108  are also displaced transverse to the optical axis  112 . A condensing lens  102 , or condensing optical system, is centered along the optical axis  112  of the ferrule and is displaced along the optical axis  112  such that its front focus, f f , lies between the ends of the optical fibers  106   a  and  108   a.  In this configuration, a first light beam  110 , from the first fiber  106 , passes through the lens  102  and is nearly collimated. However, since the end-face of fiber  106   a  is typically positioned at a distance greater than the front focal length of the condensing lens  102 , the optical beam  114  exiting the condensing lens  102  converges at a small angle. This condition of a nearly collimated beam which is either converging to a focus or diverging at a small angle is hereafter referred to a substantially collimated beam.  
         [0020]    Typical optical communication devices utilize fibers have a 125 μm diameter and, when packaged in a dual-fiber ferrule, may be end-face polished nominally at 8 degrees. The convergence angle for such a device when utilizing a condensing lens with a focal length around 2 millimeters is around 0.2 degrees. Also, since the beam  110  is not positioned on the lens axis  112 , the substantially collimated beam  114  propagates at an angle, θ, relative to the optical axis  112 . For the device mentioned above with the fiber cores spaced center-to-center approximately 125 μm, the value of θ may be approximately 2 degrees.  
         [0021]    The substantially collimated beam  114  is incident upon a filter  116 ,or other optical element, placed at a distance roughly equal to the back focal length, f b , of the lens  102 . The filter  116  reflects a portion of the beam  114  as a reflected beam  118 , and transmits the remainder of the beam  114  as a transmitted beam  122 . The reflected beam  118  is reflected to the first lens  102  which focuses the beam  120  to the second fiber  108   
         [0022]    The transmitted beam  122  passes through the filter  116  and exits through the back surface  116   b  of the filter  116 . The wedge angle of the back surface  116   b  may be selected so that the exiting beam  124  propagates substantially parallel to the optical axis  112 . The transmitted beam  122  exits the DFC unit  100  as a substantially collimated beam  124  nearly parallel to the optical axis  112  and may be coupled to other free space bulk optic components. This is described further in U.S. application Ser. No. 09/999,891, incorporated herein by reference. The wedge angle of the of the back surface  116   b  need not be selected so that the beam  124  propagates parallel to the axis  112 .  
         [0023]    Note that the decollimation of the beams  114  and  118  does not significantly affect the coupling of the fiber  106  to the fiber  108 , because the front focal plane of the lens  102  typically falls midway between the ends of the fibers  106   a  and  108   a.  One can generally achieve the same coupling efficiency with this system as with a similar system in which both fiber ends  106   a  and  108   a  lie in the front focal plane of the lens  102 .  
         [0024]    The decollimation of the beams  114  and  124  can increase the coupling efficiency from fiber  106  to a fiber in a receiver module that is coupled to exiting beam  124 . The polish angle and orientation of the ferrule face  104   a  are useful parameters when designing such a receiver system, and can be used to increase the coupling efficiency for a range of wavelengths and working distances (i.e. distances between the dual-fiber collimator  100  and a receiver module).  
         [0025]    The element  116  may be a filter having a multilayer dielectric filter coating, typically on the first surface  116   a,  with the second surface  116   b  having an anti-reflection coating. The filter  116  may be, for example, a wavelength dependent beamsplitter. This is useful for multiplexing/demultiplexing, or adding or dropping channels in a WDM (wavelength division multiplexed) or DWDM (dense wavelength division multiplexed) optical communications system. The filter  116  may also split off a fraction of the incident light over the entire wavelength band of interest, for example to make a power measurement. The filter  116  may also perform other functions.  
         [0026]    Another embodiment of a dual-fiber collimator device  200  is schematically illustrated in FIG. 2. The device includes components similar to those in device  100 . However, device  200  may also be utilized as part of an optical amplifier where the input signal in fiber  108  may be used to amplify the main signal carried in fiber  106 . In this mode, the communications input signal enters the device via fiber  106 , where fiber  106  may include a section of an erbium-doped fiber amplifier. The input optical signal in fiber  108 , hereafter referred to as the pump beam λ p , is typically at a shorter wavelength/higher energy than the communications signal λ s  in fiber  106 .  
         [0027]    In the illustrated embodiment, a signal light beam  210 , from the first fiber  106 , passes through the lens  102  and is collimated. However, since the beam  210  is not positioned on the lens axis  212 , the collimated beam  214  propagates at a downward angle relative to the axis  212 . For typical systems, the downward angle 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 . The collimated beam  214  is incident on the filter  116 , which is placed at a distance approximately equal to the back focal length, f b , of the lens  102 . The filter  116  in this case may be a dichroic edge filter which transmits wavelengths in the range of the communications signal and reflects shorter wavelengths in the range of the pump signal. The communications main signal  222  passes through the filter  116  and is refracted on the back surface  116   b  of filter  116 . The signal  224  may propagate parallel to the axis  212 . The transmitted beam  224  exits the DFC device  200  and may be coupled to other free space bulk optical components.  
         [0028]    The pump light beam  230  exits the fiber  108  and is collimated and redirected by lens  102 . The output beam  232  of the collimating lens is incident upon filter  116  which reflects the pump beam  232 . The reflected pump beam  234  is focused by lens  202  and the focused pump beam  236  is coupled into fiber  106 . The coupled pump beam  238  is counter-propagating in the signal fiber  106  and in the region where fiber  106  is erbium-doped for amplification, the counter-propagating pump signal  238  may be used to amplify the main communications signal.  
         [0029]    Another embodiment  300  of the invention is illustrated in FIG. 3, which shows a DFC  301  coupling light to a Single Fiber Collimator SFC  330 . The DFC  301  includes two fibers  306  and  308  held in a dual-fiber ferrule  304 . The ferrule end  304   a  and the fiber ends  306   a  and  308   a  may be polished at a small angle to prevent reflections feeding to other elements.  
         [0030]    Beam  310  from the first fiber  306  diverges towards the lens  302 . The lens  302  may be any suitable type of lens, such as a spherical or aspherical lens, having at least one curved surface, or may be a gradient index (GRIN) lens. Beam  314  propagating from the lens  302  is substantially collimated and, since the first fiber  306  is positioned at a distance from the axis  312  of lens  302 , beam  314  propagates at an angle relative to the axis  312 .  
         [0031]    Beam  314  is incident on optical element  316 , which reflects light as beam  318  to the lens  302  which redirects and focuses the beam  320  to the second fiber  308 . The optical element  316  is wedged at an angle, for example greater than around 20°, and perhaps around 40°, so that refraction of the transmitted beam  322  directs the beam  322  along a direction substantially parallel to the optical axis  312  of the first lens  302 , to the SFC  330 . The transmitted beam  322  is focused by a lens  332  into a third fiber  334  held in a single fiber ferrule  336 . In this embodiment, the axis  337  of the SFC  330  is substantially parallel to the axis  312  of the DFC  301 , and the axis  337  of the SFC  330  is translated by a small amount off to the side from the axis  312  of the DFC  301 . This device  300  is further described in U.S. patent application Ser. No. 09/999,891.  
         [0032]    The optical element  316  may be, for example, a filter that reflects light in a selected wavelength range. The filter  316  may have a reflective coating on a first surface  316   a  and an antireflective coating on a second surface  316   b.  Such an optical element  316  may permit the device to operate as a multiplexer (MUX) or, a demultiplexer (DMUX), or an optical add-drop multiplexer (OADM). In an example illustrating the operation of a MUX, light at one wavelength, or wavelength range, may enter the device through the second fiber  308 , and be reflected by the optical element  316  towards the first fiber  306 . Light at another wavelength, or wavelength range, may enter the device through the third fiber  334  and be transmitted to the first fiber  306  through the optical element  316 . Thus, the output from the first fiber  306  is a combination of the light entering the device from both the second and third fibers  308  and  334 .  
         [0033]    In an example illustrating the operation of a DMUX, light having components at two different wavelengths, or wavelength ranges, may enter the device through the first fiber  306 . Light at one of the wavelengths or wavelength ranges is reflected by the optical element  316  towards the second fiber  308  while light at the other wavelength or wavelength range is transmitted to the third fiber  334 .  
         [0034]    The light entering the device may, instead of comprising two wavelengths or wavelength ranges, include several different wavelengths to form a multiple channel optical communications signal. The optical element may be set to reflect light in one or more particular channels, and transmit light in the other channels. Therefore, depending on the direction of the light entering the device and the range of wavelengths over which the optical element  316  is reflective, the device may drop one or more channels from the multiple channel signal or may add one or more channels to the multiple channel signal.  
         [0035]    Optionally, another element  340  may be included in the device. For example, the other element  340  may be an optical isolator, a circulator or a filter element, a polarizer or an attenuator.  
         [0036]    In an example of a device  300  as illustrated in FIG. 3, the fibers  306  and  308  may have a diameter of around 125 μm and are set in the dual-fiber ferrule  304  at a center-to-center spacing of 125 μm. The lenses  302  and  332  may be aspherical lenses having a focal length in the range 1.5-2.5 mm, and so θ3 has a value of approximately 1.5°-2.5°. The optical element  316  may be based on a glass substrate, for example BK7 or B270 glass, and have a wedge angle of around 2°-5°. It is to be understood that the values for the various components provided in this paragraph are provided for illustrative purposes only, and are not intended to limit the invention in any way. The arrangement illustrated in FIG. 3 may also be adapted for use in higher-level modules that use multiple dual-fiber or multiple-fiber collimator assemblies. Specifically, a parallel transmitted beam may permit a narrower acquisition range for further alignments.  
         [0037]    A schematic of an embodiment of an optical communications system  400  that includes the present invention is presented in FIG. 4. A DWDM transmitter  402  directs a DWDM signal having m channels through a fiber communications link  404  to a DWDM receiver  406 .  
         [0038]    In this particular embodiment of DWDM transmitter  402 , a number of light sources  408   a,    408   b - 408   m  generate light at different wavelengths, λa, λb . . . λm, corresponding to the different optical channels. The light output from the light sources  408   a - 408   m  is combined in a DWDM combiner unit  410 , or multiplexer (MUX) unit to produce a DWDM output  412  propagating along the fiber link  404 .  
         [0039]    Light sources  408   a - 408   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  402  may be configured in many different ways to produce the DWDM output signal. For example, the MUX unit  410  may include an interleaver to interleave the outputs from different multiplexers. Furthermore, the DWDM transmitter  402  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  402  may also be redundantly equipped with additional light sources to replace failed light sources.  
         [0040]    Upon reaching the DWDM receiver  406 , the DWDM signal is passed through a demultiplexer unit (DMUX)  430 , which separates the multiplexed signal into individual channels that are directed to respective detectors  432   a,    432   b - 432   m.    
         [0041]    The fiber link  404  may include one or more fiber amplifier units  414 , 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  414  from a pump laser unit  416  via a dual-fiber collimator (DFC) to single fiber unit  424 , similar to the device described in FIG. 2. The amplifier unit  414  may be pumped with counter-propagating light, as shown, and/or with co-propagating light.  
         [0042]    The fiber link  404  may include one or more optical add/drop multiplexers (OADM)  442  for directing one or more channels to a local fiber system  444 . The local loop  444  may also direct information back to the OADM  442  for propagating along the fiber link  404  to the DWDM receiver  406 . It will be appreciated that the information directed from the local fiber system  444  to the OADM  442  need not be at the same wavelength as the information directed to the local loop  444  from the OADM  442 . Furthermore, it will be appreciated that the OADM  442  may direct more than one channel to, and may receive more than one channel from, the local system  444 . The amount of light being added to the fiber link  404  from the local system  444  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.  
         [0043]    As noted above, the present invention is applicable to fiber optic devices and is believed to be particularly useful in fiber optic devices that use one or more dual-fiber collimator units. 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.