Abstract:
An optical filter, system, and method capable of detecting optical signals in an optical fiber trunk for purposes of diagnostics and control of a wavelength division multiplexing system using optical fiber network. The optical filter is described comprising a predetermined number N of cascaded Mach-Zender circuits having a predetermined transfer function with a predetermined scanning peak, the cascaded Mach-Zender circuits having an optical input and an optical output. In another embodiment, a system is provided which employs the optical filter in which a control circuit coupled to each of the Mach-Zender circuits to shift the scanning peak according to a predetermined criterion. The control circuit advantageously causes a phase shift in the Mach-Zender circuits, thereby allowing the optical filter to scan a predetermined frequency spectrum with a narrow pass band or scanning peak to determine the existence and/or the condition of optical signals within the predetermined frequency spectrum.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     TECHNICAL FIELD 
     The present invention is generally related to optical monitoring and, more particularly, is related to a system and method for optical monitoring of multiple channels in a wavelength division multiplexing communications system. 
     BACKGROUND OF THE INVENTION 
     Network management in a communications network often requires information relative to the operation of the network. In the case of fiber optic communications networks, it is often desirable to know the precise condition of the optical signals transmitted through an optical fiber to perform channel diagnostics and to control the operation of the network. For example, in an optical fiber used to transmit up to sixteen channels or optical signals, certain information about each optical signal as it is transmitted through the optical fiber is important to ensure that the underlying information of the signal is relayed accurately. 
     For example, the actual channel presence is an important factor. This is important, for example, to inform a network monitor that a discontinuity has been created that prevents a single channel or multiple channels from being transmitted through the optical fiber. In such a circumstance, a network monitor function may be implemented that routes optical signals around the faulty pathway until the discontinuity is remedied. 
     Also, the power of each channel is another important factor used to maintain channel equalization. Specifically, the power of the optical signal over a channel may be affected by many different factors inherent in the optical network. In the case of a single fiber, different channels may operate at different power levels a which is undesirable due to the periodical use of optical amplifiers in an optical network and due to other factors. Determining the actual power of the optical signals in an optical fiber for each channel is important in order to determine necessary adjustments in performing channel equalization functions. Other important information may include the noise density across the various channels in an optical fiber as well as any frequency shift experienced by a particular channel. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an optical filter, system, and method for detecting optical signals in an optical fiber trunk for purposes of diagnostics and control of a wavelength division multiplexing system using optical fiber network. According to one embodiment, an optical filter comprises a predetermined number N of cascaded Mach-Zender circuits, the optical filter having a predetermined transfer function with a predetermined scanning peak, the cascaded Mach-Zender circuits having an optical input and an optical output. In another embodiment, a system is provided which employs the aforementioned optical filter in which a control circuit is coupled to each of the Mach-Zender circuits that causes a phase shift in the Mach-Zender circuits, thereby allowing the optical filter to scan a predetermined frequency spectrum with the predetermined scanning peak to determine the existence and/or the condition of optical signals within the predetermined frequency spectrum. 
     In yet another embodiment, the present invention may be viewed as a method for monitoring optical signals in a wavelength division multiplexing system. This method comprises the steps of filtering an optical signal obtained from an optical fiber trunk in a wavelength division multiplexing system with a predetermined number N of cascaded Mach-Zender circuits having a predetermined transfer function with a predetermined scanning peak, the cascaded Mach-Zender circuits having an optical input and an optical output. The method further includes the steps of moving the scanning peak across a predetermined frequency band by adjusting the phase of an optical signal in a branch of each of the Mach-Zender circuits, and finally, measuring the optical signal at predetermined frequency points along the predetermined frequency band. 
     The present invention has numerous advantages, a few of which include, for example, an integrated optical filter and optical detector circuit which are without moving parts. In addition, the present invention is simple in design, user friendly, robust and reliable in operation, efficient in operation, and easily implemented for mass commercial production. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein as within the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a block diagram of an optical communications network according to an embodiment of the present invention; 
     FIG. 2 is a graph of the frequency spectrum of the optical signals transmitted through an optical fiber trunk of the optical communications network of FIG. 1; 
     FIG. 3A is a schematic of a Mach-Zender circuit similar to those employed in an optical detector circuit the optical communications network of FIG. 1; 
     FIG. 3B is a graph of the transfer function of the Mach-Zender circuit of FIG. 3A; 
     FIG. 4A is a schematic of an optical filter employed in the optical detector circuit in the optical communications network of FIG. 1; 
     FIG. 4B is a graph of the transfer function of the optical filter of FIG. 4A; 
     FIG. 4C is a graph of a phase shift of the transfer function for each Mach-Zender circuit of the optical filter of FIG. 4A; 
     FIG. 5 is a block diagram of an optical detector circuit employed in the optical communications network of FIG. 1; and 
     FIG. 6 is a graph of the frequency spectrum of the optical signals transmitted through an optical fiber trunk of the optical communications network of FIG. 1 with a superimposed peak from the transfer function of the optical filter of FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to FIG. 1, shown is an optical communications network  100  according to an embodiment of the present invention. The optical communications network  100  includes an optical fiber trunk  103  that is optically coupled between an optical multiplexer  106  and an optical demultiplexer  109 . The optical multiplexer  106  includes, for example, sixteen optical inputs  111  and the optical demultiplexer  109  includes, for example, sixteen optical outputs  113 , although it is possible that any number of optical inputs and outputs  111  and  113  may exist. Each optical input  111  may receive and each optical output  113  may transmit an optical signal  116  to and from an extended optical network (not shown) of which the optical communications network is a part or branch. 
     The optical fiber trunk  103  includes optical amplifiers  119  or the like which boost an optical signal as it is transmitted through the optical fiber trunk  103 . Imposed in the optical fiber trunk  103  is a tap  123  which may be an optical splitter, for example. The tap  123  is optically coupled to an optical detector circuit  126  used for network monitoring functions. The optical detector circuit  126  is in turn electrically coupled to network monitoring and control equipment  129 . 
     In the general operation of the optical communications network  100 , the optical multiplexer/demultiplexer  106 / 109  receives and transmits the optical signals  116  via the optical inputs and outputs  111  and  113  to and from predetermined destinations in the extended optical network. In other words, the optical signals  116  are transmitted through the optical fiber trunk  103  which is part of the route taken through the extended optical network. In fact, the extended optical network may comprise numerous optical communications networks  100  as well as other components. The tap  123  splits a portion of the optical signals  116  transmitted through the optical fiber trunk  103  and applies the split portion to the optical detector circuit  126 . The optical detector circuit  126  determines the state of the optical signals  116  as will be discussed hereinafter, and provides this information to the network monitoring and control equipment  129  which then will adjust and control the amplification and general transmission of the optical signals  116  accordingly. 
     FIG. 2 is a graph of the power across a predetermined frequency spectrum for the optical signals  116  transmitted through the optical fiber trunk  103 . When the optical signals  116  are transmitted through the optical fiber trunk  103 , they are assigned to one of multiple channels  133 . When an optical signal  116  is transmitted across a specific channel  133 , a peak  136  appears in the frequency spectrum at predetermined center frequencies which are denoted by their wavelengths λ 0 , λ 1 , λ 2 , . . . , λ 15  that are allotted for the channels  133 . Although only sixteen channels  133  which correspond to the number of inputs  111  and outputs  113  of the optical multiplexer and demultiplexer  106  and  109  are shown, it is understood that there may be any number of channels  133  limited by the optical transmission capabilities of the optical fiber trunk  103  and amplifiers. Each channel  133  is separated, for example, by approximately 0.1 Terahertz, although different frequency intervals may be employed as well. All of the channels  133  are contained within a predetermined frequency bandwidth W which may be, for example, approximately 191.05 THz to 193.65 THz as shown, although the predetermined frequency bandwidth W is not limited to this particular range of frequencies. The predetermined frequency bandwidth W generally defines a range of frequencies which are to be evaluated in order to monitor the transmission of the channels  133 . 
     Note that not all of the peaks  136  may appear at any given time indicating that the corresponding channel  133  is not being used. Also, the peaks  136  may drift from their allotted frequency location which may result in unacceptable frequency intervals. 
     Referring next to FIG. 3A, shown is a Mach-Zender circuit  200  which comprises a first branch  203  and a second branch  206  which are coupled at a first optical coupler  209  and a second optical coupler  213  as is known in the art. The first and second branches  203  and  206  each comprise a waveguide with an effective index of refraction n which, for example, may be approximately equal to 1.455, although the actual value of the effective index of refraction n is application specific. Between the first and second optical couplers  209  and  213 , the first branch  203  has a length L 1  and the second branch  206  has a length L 2 , thereby defining a length difference l=L 1 −L 2 . 
     The first branch  203  also includes, for example, a heater  216  which will alter the phase of an optical signal traveling through the first branch  203  in proportion to the temperature of the heater  216 . Although a heater  216  is shown, it may be possible that the first branch includes other phase shifting devices in order to alter the phase of an optical signal traveling therethrough, such as and including the use of an electric field or magnetic field, etc., as discussed in U.S. Pat. No. 5,502,781, issued to Li et al. 
     Accordingly, an optical signal  219  enters a port of either the first branch  203  or the second branch  206  of the Mach-Zender circuit  200  and exits out of a port of the branch which opposes the one entered. However, it may also be possible that the optical signal exits out of the same it branch entered depending upon any phase difference imposed by the heater  216  and the length difference l, as well as other factors known in the art. 
     With reference to FIG. 3B, shown is a graph of the transfer function T(υ) of the Mach-Zender circuit  200  (FIG. 3A) where            T        (   υ   )       =         (     t        (   υ   )       )     2     =         P   OUT          (   υ   )           P   IN          (   υ   )             ,                          
     where υ is the optical frequency of the optical signal  219  (FIG.  3 A). P IN (υ) and P OUT (υ) are defined as the power of the optical signal  219  (FIG. 3A) entering and leaving the Mach-Zender circuit  200 , respectively, and,            t        (   υ   )       =     cos        (       2      π                 n                 l                 υ     C     )         ,                          
     where n is the effective index of refraction of the waveguide of the first and second branches  203  (FIG. 3A) and  206  (FIG.  3 A), l is defined as the length difference between the first and second branches  203  and  206  of the Mach-Zender circuit, and C is the speed of light. The wide periodic peaks  223  of the transfer function may be shifted up and down the frequency spectrum by manipulating the heater  216  (FIG. 3A) or other phase shifting device. 
     Turning next to FIG. 4A, shown is an optical filter  300  according to an embodiment of the present invention. The optical filter comprises a predetermined number N of cascaded Mach-Zender circuits  200  (FIG. 3A) which are labeled MZ1, MZ2, MZ3, . . . , MZN. The cascaded Mach-Zender circuits  200  are coupled in series using single port connections as shown. An optical signal  303  enters a first Mach-Zender circuit MZ1 on one side of the optical filter  300  and exits out of the last Mach-Zender circuit MZN. For each i th  successive cascaded Mach-Zender circuit MZ1, MZ2, MZ3, . . . , MZN, the length difference l i  may increase, for example, according to the equation l i =1 1 ×2 i−1 , where i=1, 2, 3, . . . , N. Conversely, the length difference l i  may decrease, for example, where each i th  successive cascaded Mach-Zender circuit MZ1, MZ2, MZ3, . . . , MZN has a length difference l i  defined by the equation l i =1 N ×2 N−i , where i=1, 2, 3, . . . , N, in which the optical filter  300  includes Mach-Zender circuits MZ1, MZ2, MZ3, . . . , MZN placed in the reverse order. In addition, the Mach-Zender circuits MZ1, MZ2, MZ3, . . . , MZN may also be cascaded in any random order other than the consecutive arrangements described above. Also, the actual length differences l i  may be, for example, 50 um, 100 um, 200 um, 400 um, 800 um, 1600 um, and 3200 um, etc., although other length differences l i  may be employed. 
     FIG. 4B is a graph of the transfer function T C (υ) of the optical filter  300  (FIG.  4 A). The transfer function T C (υ) is defined by the equation              T   C          (   υ   )       =       [       cos        (       2   0        x     )       ×     cos        (       2   1        x     )       ×     cos        (       2   2        x     )       ×   …   ×     cos        (       2     N   -   1          x     )         ]     2       ,                  where                 x     =       2      π                 n                 υ                 l     C       ,                          
     n is the effective index of refraction of the waveguide of the first and second branches  203  (FIG. 4A) and  206  (FIG. 4A) of the Mach-Zender circuits  200 , υ is the optical frequency of the optical signal  303  (FIG.  4 A), C is the speed of light, l is the length difference of the smallest Mach-Zender circuit  200  (the smallest length difference), and, once again N is the total number of cascaded Mach-Zender circuits  200 , which may also be defined as the order of the optical filter  300 . Note that the transfer function T C (υ) applies to any optical filter  300  regardless of the particular order of the Mach-Zender circuits  200 . 
     The graph of the transfer function T C (υ) shows periodic peaks  306  which are separated by a predetermined frequency interval I which may determined according to the equation          C     2      n1       ,                          
     where C, n, and l are defined as above, and            2      π                 n                 υ1     C     =     π   .                            
     Note that by altering the phase using the phase shifting device such as heaters  216 , the peaks  306  can be caused to shift a predetermined shift distance  309 . The present invention advantageously includes a frequency interval I that is greater than or equal to the predetermined frequency bandwidth W (FIG. 2) defining the range of frequencies which are evaluated in order to monitor the transmission of the channels  133  (FIG.  2 ). 
     In addition, it is possible that multiple cascaded optical filters  300  be employed, which results in an overall transfer function T C (υ) of even sharper periodic peaks  306 . However, generally a signal power loss of 3 dB may occur with each cascaded optical filter  300 . Consequently, a tradeoff exists between the precision of the periodic peaks  306  achieved with a greater number of cascaded optical filters  300  and the efficiency associated with the overall cascaded filter. Also, a single optical filter  300  or a number of cascaded optical filters  300  can be doubled by placing a mirror at the output of the single optical filter  300  or cascaded optical filters  300  and reflecting an optical signal through the same optical filter(s)  300  twice. 
     FIG. 4C is a graph of the power applied to the heaters  216  causing a corresponding phase shift versus time for an exemplary 5 th  order optical filter  300  (FIG. 4A) with Mach-Zender circuits MZ1, MZ2, MZ3, MZ4, and MZ5 (FIG. 4A) where N is equal to 5. The maximum power applied to any one of the heaters  216  that is necessary to cause a phase shift of π may vary depending on the nature of the heaters  216  employed in the optical filter  300 . In general, the power applied to each heater  216  is cyclic in nature, where each power cycle has a “saw tooth” shape. The cycle period of the heater power cycle of each particular Mach-Zender circuit MZ1, MZ2, MZ3, MZ4, and MZ5 decreases with each successive cascaded Mach-Zender circuit MZ1, MZ2, MZ3, MZ4, and MZ5. For example, Mach-Zender circuits MZ1 through MZ5 have respective heat cycle periods of approximately 100 ms, 50 ms, 25 ms, 12.5 ms, and 6.25 ms. Thus, the periodic peaks  306  (FIG. 4B) of the transfer function T C (υ) (FIG. 4B) are preferably shifted by the predetermined shift distance  309  with the tolling of the longest heater cycle, which in the present case is the heater cycle of the Mach-Zender circuit MZ1. 
     FIG. 5 depicts the optical detector circuit  126  according to an embodiment of the present invention. The optical detector circuit  126  includes the optical filter  300 , a control circuit  400 , and a substrate temperature control  403 . The optical filter  300  is a fifth order filter as described with regard to FIG. 4A, including five Mach-Zender circuits MZ1, MZ2, MZ3, MZ4, MZ5, each Mach-Zender circuit having a heater  216 . It is understood, as discussed previously, that a greater number of Mach-Zender circuits may be employed than are shown. The optical detector circuit  126  includes an optical input  406  which is also the in input to the optical filter  300 . 
     The control circuit  400  includes an optical detector  409  with an optical input coupled to the output of the optical filter  300 . The optical detector  409  has an electrical output coupled to an analog input of an analog-to-digital (A/D) converter  413 . The A/D converter  413  includes a digital output which is also the digital output of the optical detector circuit  126  for coupling to the network monitoring and control equipment  129  (FIG.  1 ). The A/D converter  413  also includes a trigger input to receive a trigger signal which causes the A/D converter  413  to receive and convert an analog signal applied at the analog input of the A/D converter  413  into a digital signal. 
     The control circuit  400  further includes a signal generator  416  with several heater signal outputs that are coupled to a number of digital-to-analog (D/A) converters  423 . The signal generator  416  also has a trigger output which is coupled to the trigger input of the A/D converter  413 , as well as a clock input which is coupled to an output of a clock  426 . The D/A converters  423  each have an analog output coupled to a variable resistor  429  which in turn is coupled to the respective heaters  216  of the Mach-Zender circuits MZ1, MZ2, MZ3, MZ4, MZ5. 
     The optical detector circuit  126  may be, for example, an integrated optical-electrical circuit which ensures durability, reliability, and may be manufactured using micro-electronics techniques. Note, however, that the optical detector circuit  126  may comprise other non-integrated circuits and components. 
     The general operation of the optical detector circuit  126  is described as follows. The control circuit  400  is designed to take discrete measurements of the frequency spectrum of the optical signal  116  from the optical fiber trunk  103  during a scan across a predetermined frequency band within which the optical signals  116  are transmitted using the optical filter  300 . The optical signal  116  is received from the optical fiber trunk  103  and is applied to the optical input of the optical detector circuit  126 . The optical filter  300  filters the optical signal  116  according to the state of the heaters  216  which are controlled by the control circuit  400 . The state or temperature of the individual heaters  216  depends upon the output of the signal generator  416  which generates a heater power signal for each heater  216  which is a discrete or digital representation of the “saw tooth” power signals discussed with reference to FIG.  4 C. These discrete heater power signals may be stored, for example, in memory associated with the signal generator  416 . At any given time, corresponding samples of the discrete heater power signals are transmitted to each of the heaters  216  via the D/A converters  423  and the variable resistors  429  to control the state of the transfer function T C (υ) (FIG. 4B) of the optical filter  300 . By progressively changing the discrete heater power signals according to the graph of FIG. 4C, the transfer function T C (υ) (FIG. 4B) of the optical filter  300  is caused to shift in discrete steps across the channels  133  (FIG.  2 ). 
     The signal generator  416  moves to the next discrete heater power signal for each heater  216  upon receiving a timing pulse from the clock  426 . That is to say, the clock  426  is designed to deliver timing pulses to the signal generator  416  which cause the signal generator  416  to advance to the next signal generator output for each heater  216 . In this manner, the optical filter  300  scans across discrete frequencies of the predetermined frequency band W (FIG.  2 ). The timing pulses from the clock  426  are spaced apart in time to allow the heaters  216  to adjust to the new heater power levels and achieve a state of equilibrium. At a time between each timing pulse from the clock  426  when the heaters  216  are settled at the new heat level or have reached equilibrium, the signal generator  416  sends a trigger pulse to the A/D converter  413  which converts the analog signal received from the optical detector  409  into a digital value that is provided to the network monitoring and control equipment  129 . Note that the variable resistors  429  are tuned to achieve a predetermined specific impulse response by the heaters  216 , the predetermined specific impulse response being application specific. 
     The optical detector circuit  400  takes the discrete measurements of the channels  133  of the optical signals  116  until the last frequency is reached in the predetermined frequency band. At that time, the scan may be repeated either automatically or upon a specific enabling command received from the network monitoring and control equipment  129 . Note that the substrate temperature control  403  maintains the overall temperature of the entire optical detector circuit  126  as known in the art. 
     Finally, with reference to FIG. 6, shown is the predetermined frequency bandwidth W with a superimposed scanning peak  501  which is actually one of the periodic peaks  306  of the transfer function T C (υ) (FIG. 4B) of the optical filter  300 . The scanning peak  501  moves across the predetermined frequency bandwidth W in discrete frequencies  503  based on the heater powers applied at a given time. In this manner, the center frequency and amplitude of the peaks  136  corresponding to each channel are determined. Such information is then used in the control of the optical communications network  100  (FIG.  1 ). 
     Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.