Abstract:
A method and apparatus for the detection of optical channels in an optical signal and their reliable discrimination against optical amplifier noise includes splitting an input optical signal into two replicas separated by a delay and adjusting the delay such that a periodicity of the optical filter substantially matches the channel spacing of the input optical signal, such that the optical channels within the input optical signal are directed to a first output and noise within the input optical signal is divided between the first output and a second output. The method and apparatus further include determining a difference in power between the first output and the second output wherein the difference in power is an indication of the presence or absence of optical channels within the input optical signal.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of wavelength-division multiplexed (WDM) optical networks and, more specifically, to the detection of optical channels in the presence of optical amplifier noise. 
   BACKGROUND OF THE INVENTION 
   Unambiguous detection of signal channels in optically amplified systems is necessary for performance monitoring, and to initiate corrective action in case of system failures. Reliable distinction of signal channels from optical amplifier noise is particularly critical for fast suppression of amplifier transients if the signal channels of a system are lost due to a fiber cut or some other failure. In addition, the continuous monitoring of optical amplifier performance to detect and localize degradations or faults before they affect service provides significant savings in the operation costs of optically amplified networks. 
   Currently, optical performance monitoring is accomplished using various sophisticated detectors, such as optical spectrum analyzers (OSAs) and optical channel monitors (OMONs). OSAs and OMONs may be used to detect changes of optical signal-to-noise ratios, either quantitatively by exact calibration, or by trend analysis over time. However, even in their most rudimentary and scaled-down form, these OSA and OMON devices are bulky and relatively expensive. 
   SUMMARY OF THE INVENTION 
   The present invention advantageously provides a novel method and apparatus for the detection of optical signal channels and their reliable discrimination against optical amplifier noise, i.e. amplified spontaneous emission (ASE). 
   In one embodiment of the present invention, a method for the detection of optical signal channels in optical signals includes splitting, using an optical filter, an input optical signal into two replicas separated by a delay and adjusting the delay such that a periodicity of the optical filter substantially matches the channel spacing of the input optical signal, such that the optical channels within the input optical signal are directed to a first output and noise within the input optical signal is divided between the first output and a second output. The method further includes determining a difference in power between the first output and the second output wherein the difference in power is an indication of the presence or absence of optical channels within the input optical signal. According to one embodiment of the present invention, if the difference in power between the first output and the second output is substantially zero, there are substantially no optical channels present in the input optical signal. Conversely, if the difference in power between the first output and the second output is substantially greater than zero, there is at least one optical channel present in the input optical signal. 
   In an alternate embodiment of the present invention, an apparatus for the detection of optical signal channels in optical signals includes an optical filter for splitting an input optical signal into two replicas separated by a delay, and a phase controller for adjusting the delay such that a periodicity of the optical filter substantially matches the channel spacing of the input optical signal, such that the optical channels within the input optical signal are directed to a first output and noise within the input optical signal is divided between the first output and a second output. The apparatus further includes a detection means for determining a difference in power between the first output and the second output, where the difference in power is an indication of the presence or absence of optical channels with the input optical signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high level block diagram of one embodiment of a channel detection device in accordance with the present invention; 
       FIG. 2   a  graphically depicts an exemplary optical spectrum of the complimentary outputs of the channel detection device of  FIG. 1  for the case when an input signal does not contain any optical signal channels; 
       FIG. 2   b  graphically depicts an exemplary optical spectrum of the complimentary outputs of the channel detection device of  FIG. 1  for the case when an input signal does contain a number of optical signal channels; and 
       FIG. 3  depicts a high level block diagram of an alternate embodiment of a channel detection device in accordance with the present invention. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention advantageously provides a relatively low cost method and apparatus for the detection of optical channels. Although embodiments of the present invention will be described with respect to the implementation of a Mach-Zehnder interferometer, it will be appreciated by those skilled in the art and informed by the teachings of the present invention, that various other embodiments of the present invention may be developed implementing various other components for accomplishing the splitting and delaying of input optical signals in accordance with the present invention, such as an optical filter with a periodic transfer function. 
     FIG. 1  depicts a high level block diagram of one embodiment of a low-cost channel detection device in accordance with the present invention. The channel detection device  100  of  FIG. 1  comprises an optical filter (illustratively an asymetric Mach-Zehnder Interferometer)  110  having a phase control device (illustratively a phase shifter)  120  in one arm, a constructive output port  130 , a destructive output port  140 , and a differential photodetector  150  comprised of a matched (balanced) pair of photodiodes  152 ,  154 . The differential photodetector  150  may further include an optional differential pre-amplifier (not shown). That is, in alternate embodiments of the present invention, a channel detection device, in accordance with the present invention, includes a differential pre-amplifier. Although the channel detection device  100  of  FIG. 1  is depicted as comprising an interferometer, other optical components comprising substantially similar functionality, such as an optical filter with a periodic transfer function, may be implemented in a channel detection device in accordance with the present invention. 
   In the channel detection device  100  of  FIG. 1 , an optical signal enters the Mach-Zehnder interferometer  110 , which splits the optical signal into two, and recombines the two split optical signals with a time delay that determines the periodicity of the filter transmission, which is designed to match the WDM channel spacing. For example, a 20 ps delay in the asymmetric Mach-Zehnder interferometer  110  produces a transfer function with 50 GHz periodicity, suitable to detect channels on a 50 GHz ITU grid. 
   The optical phase between the recombined optical signals is controlled by the phase shifter  120 . The phase delay imparted to the optical signal allows the shifting of the transfer function relative to the International Telecommunications Union (ITU) grid. If the transfer function of the optical signal is aligned with an ITU grid, the optical channels of the optical signal will be directed to the constructive output port  130 . In contrast, amplified spontaneous emission (ASE) noise is always split between the constructive output port  130  and the destructive output port  140 . 
   The outputs of the constructive output port  130  and the destructive output port  140  are communicated to the differential photodetector  150 . The differential photodetector  150  functionally subtracts the two output signals and determines a resultant difference in optical power of the signal from the constructive output port  130  and the destructive output port  140 . The difference between the powers of the constructive output port  130  and the destructive output port  140  may be characterized according to equation one (1), which follows:
 
 I   DIFF   =∫dλkv×P   constr (λ v )−∫ dv×P   destr (λ v )  (1) 
 
wherein P constr (λv) is the spectral power density as a function of wavelength λ from the constructive output port  130 , P destr (λv) is the spectral power density as a function of wavelength λ from the destructive output port  140 , and the integrals with respect to wavelength λ extend over the full optical bandwidth of the WDM system.
 
   The measurement of the difference in optical power between the constructive output port  130  and the destructive output port  140  results in a large signal if optical channels are present. In contrast, a difference of substantially zero indicates that no optical channels are present in an input optical signal, and as such is an indication that only noise is present in the input optical signal. This measurement provides a reliable indication of the presence of optical channels, largely independent of the total optical power. Although in the channel detection device  100  of  FIG. 1  a differential photodetector  150  is implemented to determine a difference between the spectral power of the outputs  130 ,  140 , it will be appreciated by one skilled in the art informed by the teachings of the present invention, that other means for determining the difference in output powers may be implemented in accordance with the present invention. For example, the output signals from each output  130 ,  140  may be communicated to respective detectors, and the outputs of the respective detectors may then be subsequently compared or subtracted to determine a difference. 
   For example, in one embodiment of the present invention, the channel detection device  100  of  FIG. 1  is used to detect the presence or absence of optical signal channels on the output of an optical amplifier operating in high saturation, such that the total output power of the optical amplifier is independent of the number of input optical channels. As such, in the case of no input optical signal, all of the output power from the amplifier is ASE noise. Because ASE noise is always equally split between the constructive output port  130  and the destructive output port  140  of a channel detection device of the present invention, the resultant difference between the two output ports measured by the differential photodetector  150  in such a case would be substantially zero. As such, the substantially zero difference measured by the differential photodetector  150  would indicate that substantially only ASE noise is present in the measured output of the optical amplifier operating in high saturation. 
     FIG. 2   a  graphically depicts an exemplary optical spectrum of the complimentary outputs  130 ,  140  of the channel detection device  100  of  FIG. 1  for the case when an input signal does not contain any optical signal channels. The graph of  FIG. 2   a  plots a comparison of the optical spectra of the constructive output port  130  and the destructive output port  140  in the vertical axis versus wavelength in the horizontal axis. As evident in  FIG. 2   a , when an input signal to the channel detection device  100  does not contain any optical signal channels, the outputs of the constructive output port  130  and the destructive output port  140  are substantially equal in power. As such a resulting difference is substantially zero. 
     FIG. 2   b  graphically depicts an exemplary optical spectrum of the complimentary outputs  130 ,  140  of the channel detection device  100  of  FIG. 1  for the case when an input signal does contain a number of optical signal channels. The graph of  FIG. 2   b  plots a comparison of the optical powers of the constructive output port  130  and the destructive output port  140  in the vertical axis versus wavelength in the horizontal axis. As evident in  FIG. 2   b , when an input signal to the channel detection device  100  does contain optical signal channels, the outputs of the constructive output port  130  and the destructive output port  140  have a relatively large difference in power. As such a difference measurement results in a relatively large signal. 
   To optimize the results of a channel detection device in accordance with the present invention, the position of the maxima/minima of the Mach-Zehnder interferometer transfer function needs to be adjusted to obtain the greatest differences between the constructive output port and the destructive output port. 
     FIG. 3  depicts a high level block diagram of an alternate embodiment of a channel detection device in accordance with the present invention. The channel detection device  300  of  FIG. 3  comprises an optical filter (illustratively a Mach-Zehnder Interferometer)  310  having a phase control device (illustratively a phase shifter)  320  in one arm, a constructive output port  330 , a destructive output port  340 , and a differential photodetector  350 . The differential photodetector  350  comprises a matched (balanced) pair of photodiodes  352 ,  354 . The differential photodetector  350  may further include an optional differential pre-amplifier (not shown). That is, in alternate embodiments of the present invention, a channel detection device, in accordance with the present invention, comprises a differential pre-amplifier. 
   The channel detection device  300  of  FIG. 3  further comprises a feedback phase control circuit  400 . The phase control circuit  400  illustratively comprises an oscillator  410 , an RF mixer  420 , a low-pass filter (LPF)  430 , a feedback signal path  440 , and a bias controller  450  for the phase shifter  320 . Feedback circuits such as the feedback phase control circuit  400  of  FIG. 3  are well-known to those skilled in the art and as such will not be described in detail herein. 
   The functionality of the channel detection device  300  of  FIG. 3  is substantially similar to that of the channel detection device  100  of FIG.  1 . That is, an optical signal enters the Mach-Zehnder interferometer  310 , which generates two replicas of the optical signal separated by a phase delay. The phase delay between the replicas is controlled by the phase shifter  320 . The phase delay imparted to the optical signal allows the shifting of the transfer function relative to an International Telecommunications Union (ITU) grid. Again, if the transfer function of the optical signal is aligned with an ITU grid, the optical channels of the optical signal will be directed to the constructive output port  330 . In contrast, amplified spontaneous emission (ASE) noise is always equally split between the constructive output port  130  and the destructive output port  340 . 
   The outputs of the constructive output port  330  and the destructive output port  340  are communicated to the differential photodetector  350 . The differential photodetector  350  functionally subtracts the two output signals and determines a resultant difference in optical power of the signal from the constructive output port  330  and the destructive output port  340 . 
   As describe above, a measurement of the difference in optical power between the constructive output port  330  and the destructive output port  340  results in a large signal if optical channels are present. In contrast, a difference of substantially zero indicates that no optical channels are present in an input optical signal, and as such is an indication that only noise is present in the input optical signal. Although in the channel detection device  300  of  FIG. 3  a differential photodetector  350  is implemented to determine a difference between the spectral power of the outputs  330 ,  340 , it will be appreciated by one skilled in the art informed by the teachings of the present invention, that other means for determining the difference in output powers may be implemented in accordance with the present invention. For example, the output signals from each output  330 ,  340  may be communicated to respective detectors, and the outputs of the respective detectors may then be subsequently compared or subtracted to determine a difference. 
   Furthermore, in the channel detection device  300  of  FIG. 3 , a portion of the optical signal from the differential photodetector  350  is directed to the phase control circuit  400 . The optical signal from the comparison circuit  350  is received by the RF mixer RF mixer  420 . The signal from the RF mixer is then communicated to the LPF  430 . The output of the LPF  430  is communicated as a feedback signal to the bias controller  450  via the feedback signal path  440 . The oscillator  410  provides a small dithering to the bias signal from the bias controller  450  to the phase shifter  320 . 
   The dithering from the oscillator  410  and the relatively slow feedback signal from the LPF  430  provided to the bias control signal of the bias controller  450  are implemented to optimize the bias setting of the phase delay in the Mach-Zehnder Interferometer  310 . The optimized bias setting better aligns the optical transfer function of the Mach-Zehnder Interferometer  310  with a channel ITU grid while the differential signal is monitored to indicate loss of channels to maximize the differential signal. As such the fast detection of channel loss in an input optical signal may be achieved. 
   While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.