Abstract:
Two-port optical retro-reflectors with high isolation and high return loss are described. Such retro-reflectors are designed to increase the number of optical filtering using one or more filters uniquely disposed to increase the isolation and return loss.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefits of U.S. Provisional Application No. 62/122,224, filed Oct. 16, 2014, and entitled “High Isolation and High Return Loss 2-Port Optical Retro-Reflector”, which is hereby incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The invention is generally related to the area of optical communications. In particular, the invention is related to high isolation and high return loss 2-Port optical retro-reflector. 
         [0004]    The Background of Related Art 
         [0005]    In optical network, it is important to have OTDR (optical time-domain reflectometer) to monitor the system operation to detect any possible breakdown or issue in the network. Previously, people have broadly discussed and deployed OTDR monitoring in optical network such as US patent 2012/0134663. At the same time, optical network nowadays has been rapidly developed into smart network with more functional layers and complicated multiple dimensional configurations, and thus monitoring and feedback function of the OTDR signal implementation are ever increasingly demanded. 
         [0006]    With multiple layer of network configurations, high isolation and high return loss requirement is inevitably important since each layer is supposed to superimpose the signals one by another and thus any unwanted returned signal shall be degraded and deteriorate the single-to-noise ratio. Furthermore, Next Generation PON (NGPON) is pushing the last mile into individual homes and migrates into much higher speed for optical network such as FTTX (fiber to the x), the premium grade retro-reflector which can provide OTDR function for the multiple layer smart network plays an important role and is in urgent demand. 
         [0007]    The present invention disclosure teaches unique devices with high isolation for the data signal that has transmitted through and high return loss in retro-reflected OTDR signal so as to meet the premium grade retro-reflector requirement in multiple layer smart network. 
       SUMMARY OF THE INVENTION 
       [0008]    This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. 
         [0009]    In general, the present invention is related to two-port optical retro-reflectors with high isolation and high return loss. According to one aspect of the present invention, the device is designed to increase the number of optical filtering using one or more filters uniquely disposed to increase the isolation and return loss. 
         [0010]    The present invention may be implemented as an individual device, a method and part of system. According to one embodiment, the present invention is an two-port optical retro-reflector comprising: a first port for receiving an incoming signal including a first signal and a second signal; a second port; a first filter designed to transmit the first signal and reflect the second signal; a reflector; and a lens directing the incoming signal to the first filter that transmits the first signal and reflects the second signal, wherein the transmitted first signal impinges upon the reflector to cause a reflected first signal to go through the first filter again before the reflected first signal goes to the second port, and the reflected second signal is coupled to the first port. Depending on implementation, the reflector may be a mirror or a second filter. The first and second filters are identical in optical characteristics. 
         [0011]    According to another embodiment, the present invention is a two-port optical retro-reflector comprising: a first port for receiving an incoming signal including a first signal and a second signal; a second port; a first filter designed to transmit the first signal and reflect the second signal; a lens directing the incoming signal to the first filter that transmits the first signal and reflects the second signal, wherein the transmitted first signal is coupled to the second port, and the reflected second signal is coupled to a device including: a single fiber pigtail; a lens; and a second filter, wherein the reflected second signal is impinged upon the second filter via the single fiber pigtail and the lens. 
         [0012]    Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0014]      FIG. 1  shows a configuration in which a retro-reflector is coupled between two connectors; 
           [0015]      FIG. 2  shows an exemplary embodiment according to one embodiment of the present invention; 
           [0016]      FIG. 3A  shows a design with an incoming optical signal of full band going through a com port fiber, a data signal is reflected from a filter, an OTDR signal goes through the filter and is then reflected by a mirror; 
           [0017]      FIG. 3B  shows an improvement over  FIG. 2A  with the change of the mirror to a thin film filter; 
           [0018]      FIG. 3C  shows an improvement over  FIG. 3A  that improves the data port isolation by using a quad fiber pigtail in the small tubular device; 
           [0019]      FIG. 3D  shows an improvement over  FIG. 3C  by replacing the mirror with the filter  312 . The light path of passing through is exactly same as the path in  FIG. 3C ; 
           [0020]      FIG. 4A  shows a device made of two mini tubular devices, where the two mini tubular devices are spliced together to achieve the high isolation in data port and high return loss in retro reflected com port; 
           [0021]      FIG. 4B  shows an improvement over  FIG. 4A  by adding a second filter in series to another filter to make double filtering; 
           [0022]      FIG. 4C  shows an improvement over  FIG. 4A  by replacing a mirror with a thin film filter; 
           [0023]      FIG. 4D  shows an improvement over  FIG. 4B  by replacing the mirror with a thin film filter; 
           [0024]      FIG. 5A - FIG. 5D  show the use of a quad fiber pigtail respectively in each of the designs in  FIG. 4A - FIG. 4D ; and 
           [0025]      FIG. 6  shows a cassette design encapsulating all the parts shown in the previous figures. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will become obvious to those skilled in the art that the present invention may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the present invention. 
         [0027]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
         [0028]    Embodiments of the present invention are discussed herein with reference to  FIGS. 2-10 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
         [0029]    To provide a solution for high isolation and high return loss, a 2-port optical retro-reflector is described herein. Referring now to the drawings, in which like numerals refer to like parts throughout the several views.  FIG. 1  shows a configuration  100  in which a retro-reflector  102  is coupled between two connectors  104  and  106 , where the connector  104  couples an incoming light signal  108  to an input of the retro-reflector  102  and an output of the retro-reflector  102  is coupled to the connector  106 . In operation, when some or all of the incoming signal  108  is transmitted through in the retro-reflector  102 , the returned or reflected signal  112  is minimized by the unique designs of the retro-reflector  102 . 
         [0030]      FIG. 2  shows an exemplary embodiment according to one embodiment  200  of the present invention. The device includes two ports  201  and  203 , wherein the port  201  is also referred to as a com port or com port fiber while the port  203  is referred to as a data port or data port fiber. In operation, an incoming signal (light) is coupled to the com port and impinged upon a lens  210  via a dual fiber pigtail  208 . The incoming light then hits an optical filter  202  (e.g., a thin film filter). The optical filter  202  is designed to pass signals at certain wavelengths while reflecting others. The transmitted or passed signal is reflected by a reflector  212 . Depending on implementation, the reflector  212  may be a mirror or another optical filter. The passed signal is caused to pass through the filter  202  again, thus increasing the isolation. The twice filtered signed by the filter  202  is led to the data port  203 . Meanwhile, the reflected signal by the filter  202  is led to a designated port. As a two-port device, the reflected signal is led to the corn port  201 . As described below, the passed signal corning out from the data port  203  is referred to as a data signal while the reflected signal is referred to as an OTDR signal. 
         [0031]    It is general known in the industry, the thin film filter coating intrinsic reflection isolation can only provide 20 dB and thus the pass-through isolation is only 40 dB in  FIG. 2 . To increase isolation and return loss, multiple reflections and multiple pass-through in a mini tubular structure are used to achieve the requirement. It should be noted in the description herein that an OTDR signal may also be referred to as a retro reflect signal and a data signal may also be referred to as a pass-through signal. Depending on implementation, an actual device may be very versatile with various wavelength combinations that may be realized by different thin film coatings. For example, the incoming signal carries both OTDR signal (1630-1670 nm) and data signal (1260-1618 nm), the data signal is supposed to pass through the device  200  while the OTDR is supposed to be reflected back to the incoming corn port thereof. Without implying any limitations, depending on the filter, the data signal can be a reflected signal while an OTDR signal may also be a passed signal. 
         [0032]    Referring now to  FIG. 3A , it shows a design with an incoming optical signal of full band (e.g., 1260-1670 nm) going through a corn port fiber and a data signal (e.g., 1260-1618 nm) is reflected from a filter  302 , an OTDR signal (e.g., 1630-1670 nm) goes through the filter  302  and is then reflected by a mirror. The reflected OTDR signal is then going through the filter  302  again back into the corn fiber as a retro signal. With this configuration, it is estimated that it can achieve a high return loss 80 dB for the data signal in retro-reflected OTDR 80 dB but the data port isolation is only 20 dB. 
         [0033]      FIG. 3B  shows an improvement over  FIG. 2A  with the change of the mirror to a thin film filter, the return loss of data signal retro reflected back to the com port can be improved to 100 dB since the data signal is passed through the filter  302  twice and reflected by the filter  312  once while the data port signal isolation remains 20 dB for this embodiment. Thus this embodiment has 20 dB data port isolation and 100 dB data signal return loss in the retro com port. 
         [0034]      FIG. 3C  shows an improvement over  FIG. 3A  that improves the data port isolation by using a quad fiber pigtail in the small tubular device. The light path to pass through is exactly same as  FIG. 3B , but the reflected data signal from the filter  302  is caused to go back to another fiber of the quad fiber pigtail  308  and is reflected from the filter  302  as second reflection, thus the data port isolation is enhanced by this double reflection (each reflection has 20 dB isolation) and the final isolation for this design for data port is 40 dB. Thus the design in  FIG. 3C  has 40 dB data port isolation and 80 dB data signal return loss in retro com port. 
         [0035]      FIG. 3D  shows an improvement over  FIG. 3C  by replacing the mirror with the filter  312 . The light path of passing through is exactly same as the path in  FIG. 3C . The light path of reflecting is exactly same as the path in  FIG. 2B . Thus the design in  FIG. 3D  has 40 dB data port isolation and 100 dB data signal return loss in retro com port. 
         [0036]    Referring now to  FIG. 4A , it shows a device made of two mini tubular devices  401  and  403 . The two mini tubular devices  401  and  403  are spliced together to achieve the high isolation in data port and high return loss in retro reflected com port. As shown in  FIG. 4A , the data signal goes through filter  302  and enters the data port with isolation of 40 dB, the OTDR signal reflected on filter  302  and comes out from the dual fiber on com port side and then enter the single fiber tubular device  303 . This OTDR signal is reflected by the mirror and goes back to filter  302  one more time and eventually retro reflected back to the com port. With such configuration, this embodiment has 40 dB data port isolation and 40 dB data signal return loss in retro com port. 
         [0037]      FIG. 4B  shows an improvement over  FIG. 4A  by adding a second filter  305  in series to the filter  302  to make double filtering, the data signal passes through the filter  302  twice to enhance the isolation while the OTDR signal path goes exactly same as  FIG. 4A . Thus the design in  FIG. 4B  has 80 dB data port isolation and 40 dB data signal return loss in the retro reflected com port. 
         [0038]      FIG. 4C  shows an improvement over  FIG. 4A  by replacing the mirror with a thin film filter  302  then the OTDR signal will be enhance by 20 dB additional return loss for data signal while the data signal will pass through the filter  302  exactly same as in  FIG. 4A . Thus the design in  FIG. 4C  has 40 dB data port isolation and 60 dB data signal return loss in the retro reflected com port. 
         [0039]      FIG. 4D  shows an improvement over  FIG. 4B  by replacing the mirror with a thin film filter  302  then the OTDR signal will be enhanced by 20 dB additional return loss for the data signal while the data signal passes through the filter  302  twice as exactly as in  FIG. 4B . Thus this design has 80 dB data port isolation and 60 dB data signal return loss in retro reflected com port. 
         [0040]      FIG. 5A  - FIG. 5D  show the use of a quad fiber pigtail respectively in each of the designs in  FIG. 4A - FIG. 4D . As explained above, the introduction of such a quad fiber pigtail is to increase the isolation, resulting in 40 dB data port isolation and 80 dB data signal return loss in the retro reflected com port in  FIG. 5A , 80 dB data port isolation and 80 dB data signal return loss in the retro reflected com port in  FIG. 5B , 40 dB data port isolation and 100 dB data signal return loss in the retro reflected com port in FIG.  5 C, and 80 dB data port isolation and 100 dB data signal return loss in the retro reflected corn port in  FIG. 5D . 
         [0041]    For completeness,  FIG. 6  shows a cassette design encapsulating all the parts shown in the previous figures. In one embodiment, all the parts are packaged in a small ruggedized cassette with a 2-mm jacket protected cable for various tough environment deployment. 
         [0042]    The present invention has been described in sufficient details with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. For example, the variable neutral density filter may be replaced by another device that can strengthen an optical signal. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.