Abstract:
The deep penetration of optical transmission from the very edges of the network with optical access networks to the very core with routing data within data centers before transmission has resulted in competing demands for increased functionality, reduced cost, enhanced manufacturability, and reduced footprint. At the same time monitoring and fault detection with prior art optical time domain reflectometry systems have not kept up to the demands of these networks and systems as they are expensive test equipment based solutions. It would be beneficial to provide embedded OTDR functionality within each transmitter, receiver or transceiver deployed within the network allowing every link to be monitored continuously. It would be further beneficial for such embedded OTDRs to meet the demands for lower cost, high volumes, and smaller footprints with enhanced manufacturability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application 61/940,568 filed Feb. 17, 2014 entitled “Methods and Systems relating to Embedded Optical Time Domain Reflectometry”, the entire contents of both patent applications being included by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to optical time domain reflectometry (OTDR) and more particularly to embedding OTDR functionality into transmitters, receivers and transceivers. 
       BACKGROUND OF THE INVENTION 
       [0003]    The deep penetration of optical fiber into the access networks requires an unparalleled massive deployment of the optical interface equipment that drives the traffic to and from users. For example, optical transceivers, which receive downstream signals on one wavelength and send upstream signals on another wavelength, both wavelengths sharing the same optical fiber, have to be deployed at every optical line terminal (OLT)/optical network unit (ONU). Therefore, cost efficiency, volume scalability in manufacturing of such components are major issues together with maintaining a small footprint within either the OLT/ONU or even set-top boxes of subscribers. 
         [0004]    At the same time these access networks are typically distributing data to/from the Internet which comprises today an estimated 100 billion plus web pages on over 100 million websites as well as streaming audiovisual content from service providers and server hosted systems. Accordingly by 2016 with almost two billion users accessing this data cloud, including a growing amount of high bandwidth video, then user traffic is expected to exceed 100 exabytes per month, over 100,000,000 terabytes per month, or over 42,000 gigabytes per second. However, peak demand will be considerably higher with projections of over 600 million users streaming Internet high-definition video simultaneously at peak times. 
         [0005]    All of this data will flow to and from users via these access networks but be sourced from data centers and accordingly additional traffic flows at significant capacity will exist between data centers and within data centers. Data centers are filled with tall racks of electronics surrounded by cable racks where data is typically stored on big, fast hard drives where in servers take requests and move the data using fast switches which access the right hard drives. At the same time as applications such as cloud computing increase computing platforms are no longer stand alone systems but homogenous interconnected computing infrastructures hosted in massive data centers known as warehouse scale computers (WSC) which provide ubiquitous interconnected platforms as a shared resource for many distributed services with requirements that are different to the traditional racks/servers of data centers. Accordingly, as with the access networks there is demand for cost efficiency and volume scalability in manufacturing of such components which will only increase as the goal to move data as fast as possible with the lowest latency, lowest cost, smallest footprint, and lowest power consumption migrates the optoelectronic interfaces to the server blades themselves. 
         [0006]    Accordingly, there is an ongoing drive to not only reduce costs but also improve connection reliability to subscribers, enterprises, etc. with diagnostics and fault detection techniques. Today, typically, such diagnostics and fault detection techniques are applied after notification of an issue unless onerous service level agreements (SLAs) exist that financially punish a service provider in the event of outage. Amongst the diagnostic and fault detection techniques employed on optical fiber networks is optical time domain reflectometry (OTDR, a term also used to describe the test instrument providing the measurement). Within OTDR testing a series of optical pulses are injected into the optical fiber under test and extracted from the same end of the optical fiber, wherein the reflected light extracted is either that scattered inherently by the optical fiber (Rayleigh backscatter) or reflected back from points along the fiber where there are refractive index discontinuities. The former provide essentially a sloping trace on the OTDR as the Rayleigh backscatter reduces with the attenuation of the optical signal propagating down the fiber with step-wise reductions reflecting optical elements such as optical attenuators, optical isolators, etc. along the optical link whereas step-wise peaks represent discrete points of higher reflectivity, e.g. an optical fiber break wherein the optical signal now encounters a transition from the silica refractive index of the optical fiber to, typically, air. Accordingly, the strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length (time). Shorter pulses allow for increased resolution in the location of the features within the OTDR results. 
         [0007]    However, such OTDR equipment is normally a separate element within a rack of optoelectronic transceivers and is typically connected to a link to be monitored and/or tested via an optical switch so that a single OTDR can be used upon multiple channels. This is because typically OTDR devices are typically expensive test instruments providing advanced functionality and analysis algorithms, such as depicted by first to fourth OTDRs  100 A to  100 D in  FIG. 1  or have been re-packaged for rack mounting such as fifth to seventh OTDRs  100 E to  100 G in  FIG. 1 . Fifth OTDR  100 E incorporates a 1×24 optical switch which can be extended by adding additional optical switches to the outputs of the optical switch within the fifth OTDR  100 E. 
         [0008]    However, it would be evident that where an OTDR is shared across a large number of optical fibers the actual testing uptime per fiber reduces linearly with the number of fibers so that with high numbers of averaged measurements to achieve a high dynamic range and measurement times of 1-3 minutes each fiber may only be evaluated every few hours. Accordingly, it would be beneficial to provide embedded OTDR functionality within each transmitter, receiver or transceiver deployed within the network allowing every link to be monitored continuously. However, as noted above the demand is for lower cost, high volumes, and smaller footprints for optical components within network applications. Embodiments of the invention address such requirements. 
         [0009]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to mitigate limitations in the prior art relating to relates to relates to optical time domain reflectometry (OTDR) and more particularly to embedding OTDR functionality into transmitters, receivers and transceivers. 
         [0011]    In accordance with an embodiment of the invention there is provided a device
   a first optical emitter emitting at first predetermined wavelength;   a first optical receiver for receiving optical signals at the first predetermined wavelength;   a wavelength division multiplexer having a first port relating to optical signals at the first predetermined wavelength, a second port relating to optical signals at the first predetermined wavelength; and a third port relating to optical signals at both the first and second predetermined wavelengths; and   an optical sub-assembly coupled between the first optical emitter and the first port of the wavelength division multiplexer comprising a fourth port for receiving optical signals from the first optical emitter and coupling them to the first port of the wavelength division multiplexer and a fifth port for coupling optical signals from the wavelength division multiplexer to the first optical receiver.   
 
         [0016]    In accordance with an embodiment of the invention there is provided a device comprising:
   a first optical emitter emitting at a first predetermined wavelength;   a wavelength division multiplexer having a first port relating to optical signals at the first predetermined wavelength, a second port relating to optical signals at a second predetermined wavelength; and a third port relating to optical signals at both the first and second predetermined wavelengths through which optical signals are coupled to and from an optical network coupled to the device;   a first optical component coupled to the second port for either receiving optical signals at the second predetermined wavelength or generating optical signals at the second predetermined wavelength; and   an optical sub-assembly disposed between the first optical emitter and the first port of the wavelength division multiplexer for coupling the optical signals from the first optical emitter to the first port of the wavelength division multiplexer and for coupling received signals at the first predetermined wavelength from the first port of the wavelength division multiplexer to fourth and fifth ports wherein the optical signals coupled to the fourth and fifth ports are orthogonally polarized with respect to each other and the optical sub-assembly isolates the first optical emitter from received optical signals at the device at the first predetermined wavelength.   
 
         [0021]    In accordance with an embodiment of the invention there is provided a device comprising:
   a first optical emitter emitting at a first predetermined wavelength;   a first optical component for either receiving optical signals at a second predetermined wavelength or generating optical signals at the second predetermined wavelength;   a second optical component for either receiving optical signals at a third predetermined wavelength or generating optical signals at the third predetermined wavelength;   a first wavelength division multiplexer having a first port relating to optical signals at the first predetermined wavelength, a second port relating to optical signals at the second predetermined wavelength, a third port relating to optical signals at the third predetermined wavelength, and a fourth port relating to optical signals at the first, second and third predetermined wavelengths through which optical signals are coupled to and from an optical network coupled to the device;   an optical sub-assembly disposed between the first optical emitter and the first port of the wavelength division multiplexer for coupling the optical signals from the first optical emitter to the first port of the wavelength division multiplexer and for coupling received signals at the first predetermined wavelength from the first port of the wavelength division multiplexer to fifth and sixth ports wherein the optical signals coupled to the fifth and sixth ports are orthogonally polarized with respect to each other and the optical sub-assembly isolates the first optical emitter from received optical signals at the device at the first predetermined wavelength.   
 
         [0027]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0029]      FIG. 1  depicts OTDR devices currently deployed within network environment as both discrete units and rack mounted units; 
           [0030]      FIG. 2A  depicts an OTDR deployment according to the prior art with a separate OTDR to a transmitter; 
           [0031]      FIGS. 2B and 2C  depict prior art embodiments of embedded OTDR functionality within receiver and transmitter components exploiting a passive splitter; 
           [0032]      FIGS. 3A and 3B  depict schematics of embedded OTDR functionality within receiver and transmitter components exploiting embodiments of the invention; 
           [0033]      FIGS. 4A and 4B  depict schematics of embedded OTDR functionality within receiver and transmitter components exploiting embodiments of the invention; 
           [0034]      FIGS. 5A and 5B  depict schematics of embedded OTDR functionality within a receiver component according to an embodiment of the invention; 
           [0035]      FIGS. 6A and 6B  depict schematics of embedded OTDR functionality within receiver components according to embodiments of the invention; and 
           [0036]      FIGS. 7A and 7B  depict schematics of embedded OTDR functionality within diplexer components according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The present invention is directed to optical time domain reflectometry (OTDR) and more particularly to embedding OTDR functionality into transmitters, receivers and transceivers. 
         [0038]    The ensuing description provides exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
         [0039]    Referring to  FIG. 2A  there is depicted an OTDR deployment according to the prior art with a separate OTDR  120  to a transmitter  110 . Accordingly, as depicted the transmitter  110  contains a laser diode (LD) operating at a first wavelength, λ 1 , which is coupled to a wavelength division multiplexer (WDM)  140  before being transmitted to a receiver  130  comprising at least a photodiode (PD)  135  via first and second optical components  150  and  160  respectively and optical fiber  170  within which there is depicted a break  180 . First and second optical components  150  and  160  respectively may comprise optical splitters, WDMs, optical circulators, optical amplifiers, optical isolators, and optical attenuators for example. The OTDR  120  is also coupled to the WDM  140  and comprises a LD  125 A operating at a second wavelength, λ 2  hereinafter LD λ 2    125 A, and an avalanche photodiode (APD)  125 B sensitive to λ 2  hereinafter APD λ 2    125 B. The APD λ 2    125 B and LD λ 2    125 A are coupled via third optical component  125 C, e.g. a passive coupler or for lower insertion loss and optical circulator. 
         [0040]    Now referring to  FIGS. 2B and 2C  there are prior art embodiments of embedded OTDR functionality within receiver and transmitter components exploiting a passive splitter. Referring first to  FIG. 2B  the Receiver with Embedded OTDR (Rx-EOTDR)  200 A is depicted comprising APD λ 2    125 B and LD λ 2    125 A which are coupled via a beamsplitter  210 A to a WDM filter  230  and therein the optical fiber  240  which interfaces the Rx-EOTDR  200 A to the network. Also coupled to the WDM filter  230  is APD λ 1    220 . The WDM filter  230  transmits optical signals at λ 2  and reflects those at λ 1 . Accordingly, the optical signal from LD λ 2    125 A is coupled straight-through to the optical fiber  240  and the optical network. Reflected optical signals at λ 2  are similarly coupled straight through the WDM filter  230 . However, optical signals at λ 1  in contrast coupled from the optical fiber  240  are reflected from the WDM filter  230  to the APD λ 1    220 . 
         [0041]    Referring first to  FIG. 2C  the Transmitter with Embedded OTDR (Tx-EOTDR)  200 B is depicted comprising APD λ 2    125 B and LD λ 2    125 A which are coupled via a beamsplitter  210  to a WDM filter  230  and therein the optical fiber  240  which interfaces the Tx-EOTDR  200 B to the network. Also coupled to the WDM filter  230  is APD λ 1    220 . The WDM filter  230  transmits optical signals at λ 2  and reflects those at λ 1 . Accordingly, the optical signal from LD λ 2    125 A is coupled straight-through to the optical fiber  240  and the optical network. Reflected optical signals at λ 2  are similarly coupled straight through the WDM filter  230 . However, optical signals at λ 1  in contrast coupled from the laser diode, LD λ 1    250 , are coupled via an optical isolator  260  and WDM filter  230  to the optical fiber  240 . Any optical signals at λ 1  reflected from the optical network to which the Tx-EOTDR  200 B is connected would be coupled via the WDM filter  230  to the optical isolator  260 . As an optical isolator  260  is a non-reciprocal optical device exploiting the Faraday rotation effect within magneto-optical materials such that optical signals propagating in one direction are transmitted with low insertion loss whilst those in the reverse direction suffer a high insertion loss. Accordingly, an optical isolator  260  is commonly employed in conjunction with high performance optical laser diode emitters such as external cavity lasers (ECL) and distributed feedback (DFB) lasers with narrow linewidth and high sidelobe suppression for wavelength division multiplexed links and/or networks. 
         [0042]    However, the use of a beamsplitter  210  to couple the optical pulses from the LD λ 2    125 A to the WDM filter  230  and therein the optical fiber  240  and subsequently from the optical fiber  240  to the APD λ 2    125 B incurs a 3 dB insertion loss penalty in each direction. Accordingly, the overall reduction in the optical dynamic range between the peak optical signal from the LD λ 2    125 A to the noise floor of the APD λ 2    125 B is 6 dB. Accordingly, it would be beneficial to replace the beamsplitter  210  with an optical sub-assembly that reduces the insertion loss allowing the dynamic range of the embedded OTDR to be increased. 
         [0043]    Now referring to  FIGS. 3A and 3B  there are depicted first and second schematics of embedded OTDR functionality within receiver and transmitter components exploiting embodiments of the invention. Referring to  FIG. 3A  there is depicted an Rx-EOTDR  300 A comprising a WDM filter  360  which couples optical signals from the optical fiber  305  at λ 1  to the photodiode, APD λ 1    330 . The EOTDR comprises the optical pulse source, LD λ 2    310 , optical sub-assembly  3000 , first and second APDs being APD λ 2    320 A and APD λ 2    320 B respectively, and electronic circuit  330 . As depicted the optical pulse source LD λ 2    310  emits in a highly linear polarization state, shown as transverse electric (TE), wherein this optical signal propagates with low attenuation through first polarization beamsplitter  340 , a generalized Faraday rotator  370 , and second polarization beamsplitter  350  before being coupled through the WDM filter  360  to the optical fiber  305 . 
         [0044]    Optical signals at λ 2  reflected and/or backscattered from the optical fiber  305  and subsequent network are of undefined polarization state and after coupling back through WDM filter  360  impinge upon the second polarization beamsplitter  350  wherein that portion of the optical signal in transverse magnetic (TM) is coupled to second APD, APD λ 2    320 B. That portion of the optical signal in TE polarization is coupled back through the second polarization beamsplitter  350  to Generalized Faraday rotator  370  wherein its polarization is rotated 90° to TM such that it reflects from the first polarization beamsplitter  340  to first APD, APD λ 2    320 A. Accordingly, the optical sub-assembly  3000  separates the return signal from the optical path between optical fiber  305  to LD λ 2    310  and couples it to first and second APDs APD λ 2    320 A and APD λ 2    320 B. The electrical signals from first and second APDs APD λ 2    320 A and APD λ 2    320 B are coupled to the electronic circuit  330  wherein they are processed and coupled to external control and decision elements of the optical network control layer. 
         [0045]    It would be evident to one skilled in the art that in order to achieve the indicated 90° rotation that the generalized Faraday rotator  370  would comprise a Faraday rotating element which provides 45° polarization rotation coupled with a half waveplate (λ/2-plate). Alternatively, in the instance that the generalized Faraday rotator  370  only comprises a Faraday rotating element that this would provide 45° downstream from the LD λ 2    310  to the optical fiber  405  and 45° upstream. Accordingly, in this alternate embodiment of the invention the second polarization beamsplitter  350  would be rotated 45° whilst the depictions for polarization states would similarly have to be adjusted. 
         [0046]      FIG. 3B  depicts a Tx-EOTDR  300 B according to an embodiment of the invention exploiting the optical configuration of the Rx-EOTDR  300 A with optical sub-assembly  3000 , LD λ 2    310 , first and second APDs APD λ 2    320 A and APD λ 2    320 B, and electronic circuit  330  in conjunction with WDM filter  360 . Since Tx-EOTDR  300 B is a transmitter the WDM filter  360  receives the optical signal emitted from laser source, LD λ 1    390  via optical isolator  380 . 
         [0047]    Now referring to  FIGS. 4A and 4B  there are depicted first and second schematics of embedded OTDR functionality within receiver and transmitter components exploiting embodiments of the invention. Referring to  FIG. 4A  there is depicted an Rx-EOTDR  400 A comprising a WDM filter  460  which couples optical signals from the optical fiber  405  optical signals from the optical fiber  405  at λ 1  to the photodiode, APD λ 1    430 . The EOTDR comprises the optical pulse source, LD λ 2    410 , optical sub-assembly  4000 , APD λ 2    420 , and first and second mirrors  450  and  455  respectively. As depicted the optical pulse source LD λ 2    410  emits in a highly linear polarization state, shown as transverse electric (TE), wherein this optical signal propagates with low attenuation through first polarization beamsplitter  440 A, Generalized Faraday rotator  470 , and second polarization beamsplitter  440 B before being coupled through the WDM filter  460  to the optical fiber  405 . 
         [0048]    Optical signals at λ 2  reflected and/or backscattered from the optical fiber  405  and subsequent network are of undefined polarization state and after coupling back through WDM filter  460  impinge upon the second polarization beamsplitter  440 B wherein that portion of the optical signal in transverse magnetic (TM) is coupled to the APD λ 2    420 . That portion of the optical signal in TE polarization is coupled back through the second polarization beamsplitter  440 B to Generalized Faraday rotator  470  wherein its polarization is rotated 90° to TM such that it reflects from the first polarization beamsplitter  440 A. This signal is then coupled via first and second mirrors  450  and  455  to the APD λ 2    420 . Accordingly, the optical sub-assembly  4000  separates the return signal from the optical path between optical fiber  405  to LD λ 2    410  and couples it to APD λ 2    420 . The electrical signal from APD λ 2    420  is coupled to external control and decision elements of the optical network control layer. 
         [0049]    It would be evident to one skilled in the art that in order to achieve the indicated 90° rotation that the generalized Faraday rotator  470  would comprise a Faraday rotating element which provides 45° polarization rotation coupled with a half waveplate (λ/2-plate). Alternatively, in the instance that the generalized Faraday rotator  470  only comprises a Faraday rotating element that this would provide 45° downstream from the LD λ 2    410  to the optical fiber  405  and 45° upstream. Accordingly, in this alternate embodiment of the invention the second polarization beamsplitter  440 B would be rotated 45° whilst the depictions for polarization states would similarly have to be adjusted. 
         [0050]      FIG. 4B  depicts a Tx-EOTDR  400 B according to an embodiment of the invention exploiting the optical configuration of the Rx-EOTDR  400 A with optical sub-assembly  4000 , LD λ 2    410 , APD λ 2    420 , and first and second mirrors  450  and  455  respectively in conjunction with WDM filter  460 . Since Tx-EOTDR  300 B is a transmitter the WDM filter  460  now receives the optical signal emitted from laser source, LD λ 1    480  via optical isolator  470 . 
         [0051]    Now referring to  FIGS. 5A and 5B  there are depicted first and second schematics of embedded OTDR functionality within a receiver component according to an embodiment of the invention. The receiver component in common with embodiments of the invention described supra wherein an optical signal from an optical fiber (not identified for clarity) is coupled via WDM filter  560  to APD λ 1    520 . In this instance the optical signal is shown being collimated by first lens  570 A from the optical fiber and focused by second lens  570 B onto the APD λ 1    520 . Such collimating and focusing lenses may be employed within the preceding embodiments depicted in  FIGS. 3A to 4B  respectively but if employed have been omitted for clarity. In respect of  FIG. 5A  the optical path for the EOTDR transmit pulse is depicted. As depicted the optical signal from a laser source, LD λ 2    510 , is coupled via third lens  540  before being coupled to isolator  550  comprising first and second birefringent elements  550 A and  550 C respectively together with Faraday rotator element  550 B. Accordingly, the optical signal is propagated through the isolator  550  and coupled through WDM filter  560  and first lens  570 A to the optical fiber. Disposed either side of LD λ 2    510  are first and second APDs APD λ 1    530 A and APD λ 1    530 B respectively but these play no part in the transmission action of the EOTDR. 
         [0052]    Now referring to  FIG. 5B  the return path of the EOTDR pulse is depicted. Accordingly, optical signals at λ 2  reflected and/or backscattered from the optical fiber are coupled back through first lens  570 A and WDM filter  560  to the isolator  550 . However, now due to the 90° rotation of their polarization within the Faraday rotator element  550 B the combined effect of this together with the first and second birefringent elements  550 A and  550 C respectively leads to the optical paths of the TE and TM polarizations being focused by the third lens  540  onto the first and second APDs APD λ 1    530 A and APD λ 1    530 B respectively rather than back to the LD λ 2    510 . The electrical signals from the first and second APDs APD λ 1    530 A and APD λ 1    530 B respectively are coupled to electronic circuit  580  and therein a processed signal is provided to external control circuits. 
         [0053]    Now referring to  FIGS. 6A and 6B  there are depicted first and second variant schematics  600 A and  600 B respectively of embedded OTDR (EOTDR) functionality within an optical receiver component such as depicted supra in respect of  FIGS. 5A and 5B  respectively. According in each of first and second variant schematics  600 A and  600 B signals at a first wavelength or wavelength range λ 1  would be coupled to APD λ 1    520 . The EOTDR in transmit exploits LD λ 2    510  and the returned signals at λ 2  are now coupled via free space optics to the receiver APD, APD λ 2    610 . Accordingly, first and second collimating lenses  620 A and  620 B collimate the reverse direction polarization signals focused by the third lens  540 . The polarization coupled via first collimating lens  620 A is reflected by mirror  630  before being coupled via polarizing beamsplitter  640  to APD λ 2    610 . The polarization coupled via second collimating lens  620 B is coupled via polarizing beamsplitter  640  to APD λ 2    610 . As the polarizations are orthogonal they are both coupled via the polarizing beamsplitter  640  to APD λ 2    610  with low loss. 
         [0054]    In second variant schematic  600 B the two orthogonal polarizations in the return signal path at λ 2  are coupled to first and second optical fibers  650 A and  650 B respectively which are then combined in combiner  660  before being coupled to APD λ 2    610 . Combiner  660  may for example be a singlemode to multimode coupler where the first and second optical fibers  650 A and  650 B are singlemode. Referring to  FIGS. 7A and 7B  there are depicted first and second schematics  700 A and  700 B of embedded OTDR functionality within diplexer components according to embodiments of the invention. As depicted first schematic  700 A depicts a diplexer operating with upstream receiver operating at λ 1 , downstream transmitter at λ 3  and EOTDR at λ 2  wherein the EOTDR exploits an optical configuration similar to that depicted in  FIGS. 4A and 4B  respectively. Second schematic  700 B depicts a diplexer operating with upstream receiver operating at λ 3  downstream transmitter at λ 1 , and EOTDR at λ 2  wherein the EOTDR exploits an optical configuration similar to that depicted in  FIGS. 5A through 6B  respectively. 
         [0055]    Within the embodiments of the invention free space solutions provide for low loss coupling of the optical pulse source from the optical transmitter, receiver, or transceiver with embedded OTDR and for low loss coupling of the reflected and/or backscattered OTDR signal to one or more optical detectors thereby removing the insertion loss penalty of the prior art beamsplitter solutions. It would be evident to one skilled in the art that the different optical designs presented may be implemented as free space discrete components, e.g. with packaged optoelectronic components within a housing, or as a micro-bench for example exploiting silicon micromachining and opto-electronic die placement via flip-chip bonding etc. for example. 
         [0056]    Embodiments of the invention such as depicted in  FIG. 6B  for example with an micro-bench implementation may replace the first and second optical fibers  650 A and  650 B respectively together with combiner  660  may be implemented with optical waveguides, e.g. silica, silicon, or silicon oxynitride for example. Similarly, first to third lenses  540 ,  570 A and  570 B respectively may be implemented using ball lenses 
         [0057]    Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
         [0058]    The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
         [0059]    Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.