Patent Publication Number: US-2005135814-A1

Title: Apparatus and method for simulating a length of optical fiber

Description:
FIELD OF THE INVENTION  
      The invention relates to the field of testing of optical systems that incorporate substantial lengths of optical fiber and more specifically to the field of devices and methods for simulating a length of fiber of optical along with variable optical properties associated therewith.  
     BACKGROUND OF THE INVENTION  
      Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM) are recent developments of classic WDM (Wavelength Division Multiplexing) systems that allow for increased optical data carrying capacity for a single mode fiber. DWDM systems support a very densely packed series of optical signals in which each optical signal has a characteristicspectral width.. The optical signals corresponding to conventional CWDM systems use a few widely spaced wavelength bands. For both CWDM and DWDM, this requires the development of wavelength multiplexers, with laser sources tuned to specific ITU channel wavelengths, in which wavelength channels are typically spaced at either 100 GHz or 50 Ghz. Of course, other wavelength channel spacings of 12.5 GHz, 25 GHz, 50 GHz, 100 GHz and 200 GHz are also attainable.  
      High bit rate optical networks like 10 Gb/sec and the emerging 40 Gb/sec require precise tolerance optical components and systems that are able to accommodate optical effects of the fibers, such as attenuation and chromatic dispersion, and physical impairments such as splices and connectors. As the data rate increases above 2.5 Gb/second, the effects of attenuation, chromatic dispersion, especially for DWDM channels, and degraded Bit Error Rate, as optical pulses merge and create Inter-Symbol Interference (ISI), become more problematic.  
      The need for a fiber optic physical layer simulator is apparent as the single mode fiber characteristics of attenuation, chromatic dispersion (CD), Non-Linear effects, such as Amplified Spontaneous Emissions (ASE) and Four Wave Mixing (FWM) and polarization mode dispersion (PMD) cause issues in optical networks. These effects are now impacting the deployment of high bitrate optical systems, such as those that offer data rates of 10 GB/sec and above. This is especially a significant issue for WDM and DWDM optical systems currently being developed for the minimum attenuation C band with a 1550 nm center wavelength. DWDM systems offer the possibility of high data rates in the hundreds of Terabits/sec by using the inherent wide bandwidth and optical wavelength division techniques now available.  
      As the demand for capacity grows, due to an increase in Internet traffic coupled with the high cost of installing new fiber, DWDM techniques currently being developed are for use for propagating and receiving optical signals on this previously deployed “dark” fiber, at the C band. This requires new compensation techniques to be developed for transmitting and receiving of optical signals using this existing fiber. Newer fibers being deployed have improved performance, over the previously installed optical fiber, in selected areas, however the effects still adversely affect the performance of the optical network. For WDM and DWDM systems, each wavelength typically requires unique CD and PMD tuning, or compensation, at high bit rates and for various optical fiber lengths. Additionally, several types of fibers are typically used in a real world fiber optic link, each having different characteristics which reduce one adverse physical parameter at the expense of some other. Additionally Non-Linear effects such as ASE, arising from the use of EDFAs and Four Wave Mixing effects, have significant effects on optical systems that propagate multiple wavelengths.  
      Due to the high cost of using tremendous lengths of any of the various types of optical fibers in testing systems, a need exists to be able to simulate different fiber types in a single test system, as well as being able to independently control Attenuation, CD and PMD for each simulated fiber length. This allows for the development of the next generation OC-192 and OC-768 optical networks over existing deployed single mode fiber as well as newly developed and deployed fiber. WDM and DWDM optical system developers appreciate the ability to “tune” or compensate their systems to match the fiber link, often at specific wavelengths for CD. In the past this has been manually adjusted in the field for hardwired optical links. For next generation networks, dynamic CD and PMD compensation is required. This requires optical system designers to be able to adjust CD and PMD independently and dynamically as the network topology changes. However, in order to be able to build fiber optic devices that are adjustable for CD and PMD, optical testing thereof is required in an environment the as closely as possible emulates actual optical links in the optical networks.  
      Typically, optical networks are simulated using very long spools of optical fiber, with lengths in the hundreds of kilometers. Unfortunately, these spools are very difficult to handle and often do not provide for a sufficient emulation of an actual optical link in the optical network.  
      It would be beneficial to provide a simple apparatus that conveniently simulates a fiber optic link, such as that found in an optical network. Further it would be beneficial if such a device were highly configurable to simulate a wide variety of fiber types and fiber lengths. It is therefore an object of the invention to provide an optical testing apparatus for simulating of various fiber types and for simulating various optical characteristics that are typically associated therewith.  
     SUMMARY OF THE INVENTION  
      In accordance with the invention there is provided a test apparatus for receiving of an optical input signal at one of a plurality of different wavelengths comprising: a variable optical attenuator for providing optical attenuation to an optical signal propagating from an input port to an output port thereof in response to a first control signal; a first variable chromatic dispersion device for imparting a positive dispersion on an optical signal propagating from an input port to an output port thereof in response to a second control signal; a second variable chromatic dispersion device for imparting a negative dispersion on an optical signal propagating from an input port to an output port thereof in response to a third control signal; a jumper for coupling at least one of the output ports to at least one of the input ports; and, a microcontroller having an input port for receiving an external control signal and for providing the first, second and third control signals in dependence thereon in order to control all three devices in a coordinated fashion, where the test apparatus supports at least a first wavelength within a first optical channel and a second wavelength within a second optical channel.  
      In accordance with the invention there is provided a test apparatus for imparting an optical impairment on a first optical signal, comprising: an optical input port for receiving the first optical input signal; an optical output port for providing an optical output signal corresponding to the first optical signal additionally comprising the optical impairment; an optical path formed between the optical input port and the optical output port; a variable optical attenuator disposed in the optical path for providing optical attenuation to an optical signal propagating therethrough in response to a first control signal; a first variable chromatic dispersion device disposed in the optical path for imparting a positive dispersion on an optical signal propagating therethrough in response to a second control signal; a second variable chromatic dispersion device disposed in the optical path for imparting a negative dispersion on an optical signal propagating therethrough in response to a third control signal; and, a microcontroller having an input port for receiving an external control signal and for providing the first, second and third control signals in dependence thereon.  
      In accordance with the invention there is provided a method of creating an impairment in an optical signal using an electronic control device, comprising: adjusting an optical attenuation of the optical signal; adjusting a positive chromatic dispersion of the optical signal; adjusting a negative chromatic dispersion of the optical signal; and, providing the optical signal with the impairment comprising the optical attenuation, the positive chromatic dispersion and the negative chromatic dispersion, the optical impairments controlled by the electronic control device.  
      In accordance with the invention there is provided a method of optically simulating an optical network link, comprising: propagating of an optical signal along an optical path; providing a plurality of sets of first, second and third data; receiving of an input signal for selecting one of a plurality of sets of first, second and third data; generating a first control signal in dependence upon the first set of data; attenuating of the optical signal propagating along the optical path in dependence upon the first control signal; generating a second control signal in dependence upon the second set of data; varying a first chromatic dispersion of the optical signal propagating along the optical path in dependence upon the second control signal; generating a third control signal in dependence upon the third set of data; and, varying a second chromatic dispersion of the optical signal propagating along the optical path in dependence upon the third control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:  
       FIG. 1  illustrates an optical fiber simulator (OFS) according to a first embodiment of the invention;  
       FIG. 2  illustrates support for a plurality of different single-mode fiber types within limits of attenuation and chromatic dispersion for a selected channel for the OFS;  
       FIG. 3  illustrates the OFS for use in a laboratory computer controlled optical physical layer impairment simulator for use in testing of enterprise and metro optical network and C band optical systems;  
       FIG. 4  illustrates an OFS in accordance with a second embodiment of the invention, where the OFS in accordance with the second embodiment offers the introduction of an optical polarization impairment into an optical signal propagating through the OFS; and,  
       FIG. 5  illustrates operating steps for the OFS in accordance with the embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION  
       FIG. 1  illustrates an optical fiber simulator (OFS)  100  according to a first embodiment of the invention. The OFS is comprised of a housing  101  having first through sixth optical ports  1001  through  1006 . A first variable optical attenuator (VOA)  102  is disposed between the first and second optical ports,  1001  and  1002 . A first optical circulator (OC)  103  has its first port connected to the third optical port  1003  and its third port connected to the fourth optical port. A second port of the first optical circulator  103  is connected to a first tuneable dispersion compensator (TDC)  104 . Connected to the fifth optical port  1005  is a first port of a second OC  105 , with a third port thereof connected to the sixth optical port  1006 . A second port of the second OC  105  is connected to a second TDC  106 . The third optical port  1003 , the first circulator  103 , the first TDC  104  and the fourth optical port form a first variable chromatic dispersion device. The fifth optical port  1005 , the second circulator  105 , the second TDC  106  and the sixth optical port  1006  form a second variable chromatic dispersion device.  
      An electronic control device, in the form of a microcontroller  107  is disposed within the housing  101  in order to provide a first control signal to a control port of the first VOA  102 , a second control signal to a control port of the first variable chromatic dispersion device  104 , and to provide a third control signal to a control port of the second variable chromatic dispersion device. An external control signal for controlling of the microcontroller  107  is provided through an OFS control port  100   a.  A memory circuit  108  is provided as part of the microcontroller  107  for storing a plurality of sets of first, second, and third data that encodes for first, second, and third control signals in relation to the external control signal. Preferably a lookup table (LUT)  109  is stored within the memory circuit  108  for storing the various combinations of a formed within that is used to store the various combinations of first, second, and third data in relation to the OFS control signal.  
      In use, the OFS  100  is used for creating an optical impairment associated with a single optical channel from a plurality of supported optical channels in an optical communications network. The optical channel is selectable through the OFS control port  100   a.  An optical input signal is provided to the optical port  1001  and low loss optical jumpers  110   a  and  110   b  couple ports  1002  to  1003  and  1004  to  1005 , respectively. An optical output signal having the optical impairment created thereon is provided from optical port  1006 . Utilizing two jumpers and providing the optical input signal to port  1001  and receiving the output signal with the optical impairment created thereon is the preferably mode of operation of the OPS  100 . Of course, other optical configurations of the OFS  100  are also possible.  
      In order to select between the different supported types of optical fibers, the memory circuit  108  and LUT  109  are used for storing data representative of optical properties of chromatic dispersion and attenuation that are associated with each of the supported types of optical fibers. Referring to  FIG. 2 , the OFS  100  is used to support a plurality of different SMF types,  202  and  203 , within the limits  201  of the attenuation and chromatic dispersion ranges for a selected channel. The OFS  100  provides for optical attenuations up to 60 dB with a CD of approximately −1500 ps/nm to +1500 ps/nm.  
      The OFS  100  in the first embodiment of the invention provides several modes of operation. It provides for optical fiber distance (km), optical fiber attenuation (dB) and adjustment of Chromatic Dispersion (CD) (ps/nm). For example, the LUT  109  has a provision for storing of first, second and third data for emulating G.652 type fibers, SMF-28, SMF-28e, ALLWave®, G.655 type fibers, LEAF®, MetroCor®, TrueWave® RS. Of course, support for any type of SMF fiber is possible within the limits of the attenuation and chromatic dispersion ranges for a selected channel. Up to 60 dB of wideband optical attenuation and chromatic dispersion of at least −1500 ps/nm to +1500 ps/nm range per selected channel allows for SMF-28, or G.652 standard, fiber spans of over 95 km to be supported. Preferably CD is adjustable in three ranges, −1000 ps/nm to +1000 ps/nm, −1500 ps/nm to −500 ps/nm and from +500 ps/nm to +1500 ps/nm.  
      The OFS  100  allows for user control of optical attenuation and of CD for a selected optical channel over wide dispersion and attenuation ranges. Attenuation and CD are combined into a single optical path to optically simulate individual ITU channel characteristics for different fibers like SMF-28 or LEAF®. Additionally, by adding additional optical components between ports  1002  and  1003 , and  1004  and  1005 , splices and other optical impairments are optionally simulated.  
       FIG. 3  illustrates the OFS  100  for use in a laboratory computer controlled optical physical layer impairment simulator for use in testing of enterprise and metro optical network and C band optical systems. As shown in  FIG. 3 , an optical data traffic and analysis system  301  is optically coupled to an optical device under test  302 , which is further optically coupled to port  1001  of the OFS  100 . Port  1006  of the OFS  100  is optically connected to an optical switch  303 , which is optically coupled back to the optical data traffic and analysis system  301 . A computer  304  is used to control the optical data traffic and analysis system  301 , optical switch  303  and the OFS  100 .  
      The OFS  100  advantageously reduces testing time and replaces reels of spliced fiber in the optical path. The OFS  100  is used for emulating long haul fiber links up to 100 km between EDFA&#39;s and provides an inexpensive alternative to actual optical network field testing. Through the OFS control port  100   a,  the computer  304  provides data signals to the VOA  102  and TDC  104  in order to simulate different fiber types, and allows for individual control, or in combination, attenuation and chromatic dispersion. Furthermore, it allows for emulating selected ITU channel dispersion and attenuation characteristics. Software selection by either one of ITU channel and center wavelength allows for optical channel by optical channel dispersion testing in CWDM and DWDM C band systems. The channel spacing is either 50 GHz or 100 GHz, in dependence upon user requirements. Of course, other exemplary wavelength channel spacings of 12.5 GHz, 25 GHz, and 200 GHz are also attainable.  
      Advantageously, the OFS  100  provides precise control over optical attenuation and chromatic dispersion. The OFS  100  chassis is rack mountable and is provide with front mounted optical connectors that function as the six optical ports,  1001  to  1006 . The microcontroller  107  utilizes its internal processor for controlling the OFSs optical characteristics. The microcontroller  107  is connected to each of the optical components,  102 ,  104  and  106 , using a thermally and mechanically isolated path in order to not interfere with the optical signals propagating between the optical components. Optionally, several OFSs are connected together to facilitate increased channel density or wider band testing. Advantageously, the OFS  100  is for operating at any one of a plurality of optical channels in telecommunication bands that are known to those of skill in the art. Exemplary telecommunication bands are O, E, S, C, and L bands.  
       FIG. 4  illustrates an OFS  400  in accordance with a second embodiment of the invention. The OFS  400  offers similar functionality to that of the OFS  100 , however it is also supports the introduction of an optical polarization impairment in addition to attenuation and CD. The OFS  400  is comprised of a housing  401  having first through eighth optical ports  4001  through  4008 . A first variable optical attenuator (VOA)  402  is optically disposed between the first and second optical ports,  4001  and  4002 . A first optical circulator (OC)  403  has its first port coupled to the third optical port  4003  and its third port coupled to the fourth optical port  4004 . A second port of the first optical circulator  403  is coupled to a first tuneable dispersion compensator (TDC)  404 . Coupled to the fifth optical port  4005  is a first port of a second OC  405 , with a third port thereof coupled to the sixth optical port  4006 . A second port of the second OC  405  is coupled to a second TDC  406 . An input port of a polarization controller  411  is coupled to the seventh optical port  4007  and an optical output port thereof is coupled to an input port of a variable optical delay component  412 . An output port of the variable optical delay component is coupled to at least one of a polarization scrambler and polarization monitor  413 . An output port of the at least one of a polarization scrambler and polarization monitor  413  is coupled to the eighth optical port  4008 . Of course, optionally multiple polarization mode dispersion optical devices are also coupled in series with the polarization mode dispersion optical device in order to simulate high order polarization effects.  
      The third optical port  4003 , the first circulator  403 , the first TDC  404  and the fourth optical port form a first variable chromatic dispersion device. The fifth optical port  4005 , the second circulator  405 , the second TDC  406  and the sixth optical port  4006  form a second variable chromatic dispersion device. The seventh optical port  4007 , polarization controller  411 , optical delay line  412 , the at least one of a polarization scrambler and polarization monitor  413  and eighth optical port  4008  form a polarization mode dispersion optical device for imparting a polarization mode dispersion optical impairment on an optical signal propagating from ports  4007  to  4008 .  
      An electronic control device, in the form of a microcontroller  407  is disposed within the housing  401  in order to provide a first control signal to a control port of the first VOA  402 , a second control signal to a control port of the first variable chromatic dispersion device, and to provide a third control signal to a control port of the second variable chromatic dispersion device. A fourth control signal is provided to the polarization mode dispersion optical device for controlling of the polarization controller  411 , the variable optical delay component  411  and the at least one of a polarization scrambler and polarization monitor  412 . The polarization monitor optionally provides a feedback signal to the microcontroller  407  in order to provide feedback relating to the change in the optical polarization of light realized by the polarization controller  411 .  
      An external control signal for controlling of the microcontroller  407  is provided through an OFS control port  400   a.  A memory circuit  408  is provided as part of the microcontroller  407  for storing various combinations of first, second, third and fourth data that encodes for the first, second, third and fourth control signals in relation to the external control signal. Preferably a lookup table (LUT)  409  is implemented within the memory circuit  408  for storing the various data coding for the first, second, third and fourth control signals in relation to the external control signal.  
      In use, the OFS  400  is used for creating an optical impairment for a single optical channel from a plurality of optical channels for simulating of an optical communications network. Of course, the polarization mode dispersion optical device—disposed between ports  4007  and  4008  is a broadband optical device that affects all WDM channels propagating therethrough simultaneously. The single optical channel is selected using the OFS control port  400   a.  An optical input signal is provided to the optical port  4001  and low loss optical jumpers  410   a  through and  410   c  connect ports  4002  and  4003 ,  4004  and  4005 , and  4006  and  4007 , respectively. An optical output signal having the optical impairment created thereon is provided from optical port  4008 . Utilizing three jumpers and providing the optical input signal to port  4001  and receiving the output signal from port  4008  is the preferable mode of operation for the OPS  400 . Of course, other optical configurations of the OFS  400  are also possible, where additional optical devices are inserted between optical ports  4002  and  4003 ,  4004  and  4005 , and  4006  and  4007 .  
      The OFS  400  is utilized in a similar manner to the OFS  100 . Computer control of the OFS  400  provides for an automated testing system that is capable of testing of a plurality of optical channels, such as the testing system illustrated in  FIG. 3 . Of course, because the OFS  400  includes polarization varying components, a more accurate representation of an actual optical link of an optical network is attained. With the means for varying the optical attenuation, positive and negative CD, as well as polarization varying capabilities, the OFS  400  advantageously offers a more accurate representation of an actual fiber optic link.  
       FIG. 5  illustrates the operating steps for the OFS  100  and the OFS  400  for an optical signal propagating therethrough. Referring to step  501 , adjusting an optical attenuation of the optical signal is performed. In step  502 , a positive chromatic dispersion of the optical signal is adjusted. Adjusting a negative chromatic dispersion of the optical signal is performed in step  503 . Referring to the OFS  100 , in step  504   a,  providing the optical signal with the impairments comprising the optical attenuation, the positive chromatic dispersion and the negative chromatic dispersion, where the electronic control device  107  controls the optical impairments. Referring to the OFS  400 , in step  504   b  adjusting a polarization mode dispersion of the optical signal is performed. Thereafter in step  504   c,  providing the optical signal with the impairments comprising the optical attenuation, the positive chromatic dispersion, the negative chromatic dispersion and the polarization mode dispersion, where the electronic control device  407  controls the optical impairments.  
      The embodiments of the invention advantageously allow for dramatic time and cost savings to be realized in automated testing of optical components or of an optical system test environment. Through interfacing with the OFS,  100  and  400 , via the OFS control port,  100   a  and  400   a,  automated testing scripts in execution on a computer  304  ( FIG. 3 ) reduce optical device testing costs in optical device production testing.  
      The use of external jumpers in conjunction with the embodiments of the invention is to enhance the flexibility of the embodiments of the invention. It will be apparent to those of skill in the art of optical design that the various optical components are optionally optically coupled together in, for example, a fixed manner by splicing their optical fibers together to form configurations optically equivalent to either of the first and second embodiments of the invention.  
      Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.