Abstract:
For wavelength division multiplexing (WDM) communications, apparatus and methods are provided for performing tone-based optical channel monitoring that is less sensitive to stimulated Raman scattering (SRS). In tone-based optical channel monitoring, in which WDM channels are modulated with one or more tones, detecting and measuring the tone power is commonly used as a measure of signal power in each channel. In WDM systems with long fiber spans and high signal powers, however, SRS tends to transfer energy from shorter wavelengths to longer wavelengths, whereby the tones are no longer accurately indicative of the signal power of the individual wavelength channels. Apparatus and methods are provided which reduce the effect of SRS by filtering the monitored WDM signals into sub-bands and detecting each sub-band independently.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optical communications, and more specifically to apparatus and methods relating to tone-based optical channel monitoring. 
     BACKGROUND INFORMATION 
     For tone-based channel monitoring in a wavelength division multiplexing (WDM) communications system, each wavelength is modulated with one or more tones that are specific to the wavelength. U.S. Pat. No. 7,054,556 to Wan et al. describes a scheme in which channels in an optical WDM system are each modulated by two or more alternating dither tones so that at any instant, each channel is modulated by at least one dither tone. As described therein, channel monitoring by detecting the dither tones makes use of a Fast Fourier Transform (FFT) process which can decode and measure dither tones encoded on the WDM channels. 
     In carrying out such non-intrusive real-time channel monitoring, a tone decode subsystem (also referred to as a wavelength tracker) is used. In a wavelength tracker, the optical WDM signal is typically converted to an electrical signal using a photodiode. The tones in the converted electrical signal and their respective levels provide information regarding which wavelength channels are present in the WDM signal as well as the optical power levels of the channels present. This information is critical to the WDM system for channel monitoring and power management. For example, wavelength channels whose power levels are monitored to be below (above) the desired level can be boosted (attenuated), e.g., via gain equalization in an optical amplification stage, in order to achieve good transmission performance of all the channels. 
     To monitor the powers of the wavelength channels in a WDM system, the operation of a conventional wavelength tracker detector is based on the simultaneous measurement of the powers of the different frequency tones that are assigned to different WDM channels. Stimulated Raman scattering (SRS) during optical fiber transmission, however, has the effect of transferring energy from shorter wavelength channels to longer wavelength channels, especially when the powers of these channels are high and/or the transmission distance is long. Such energy transfer is also called SRS-induced channel crosstalk which causes an appreciable portion of the frequency tone that is originally assigned to a given wavelength channel to be transferred to other WDM channels. This makes a conventional wavelength tracker inaccurate in reporting the power level of each channel and subsequently causes power tilt and degradation in optical signal-to-noise ratio (OSNR), thereby negatively affecting system performance. In some cases, the inaccuracy may become so large that the wavelength tracker will incorrectly report the presence or absence of a wavelength channel. 
     It is thus desired to improve the tolerance of tone-based wavelength tracking to SRS so that accurate optical channel monitoring can be achieved even with high signal power and system reach. 
     There is no known solution to the above-described problem. A possible approach towards remedying this problem is to estimate the tone transfers among the WDM channels in each fiber span, and calibrate the measured power for each frequency tone to reflect the actual power of the wavelength channel to which the tone frequency is assigned. This approach, however, requires the knowledge of the channels transmitted in each fiber span such as the locations and input powers of the channels, the tone components currently carried in each channel, and the fiber nonlinear and loss coefficients. The tone transfers in the fiber span then need to be computed, which is computationally intensive. 
     Moreover, the powers of the frequency tones impressed on a given channel due to the SRS need to be recorded and this information passed with the channel for further computation at the next wavelength tracker. This becomes impractical to realize in transparent WDM systems using reconfigurable optical add/drop multiplexers (ROADMs), where a wavelength channel can be added, dropped, or re-routed on demand. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to apparatus and methods that substantially increase the accuracy of tone-based optical channel monitoring in the presence of SRS-induced crosstalk. Based on the understanding that the majority of the SRS-induced crosstalk occurs between channels that are far apart, an exemplary embodiment of the present invention uses a WDM filter to separate the WDM channels into at least two groups, e.g., a short-wavelength group and a long-wavelength group, and measures the tone powers for each of the channel groups individually. Since the wavelength range in each channel group is substantially reduced and the worst-case SRS-induced crosstalk roughly scales quadratically with the wavelength range, the tolerance of the wavelength tracker to SRS-induced crosstalk is much improved. 
     The present invention thereby provides a cost-effective wavelength tracker technology that can be used for WDM systems with extended reach and increased signal power. 
     The aforementioned and other features and aspects of the present invention are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary wavelength tracker system wherein all channels of a tapped optical WDM signal are electrically converted using a single photodiode. 
         FIG. 2A  is a block diagram of an exemplary embodiment of a wavelength tracker system wherein a tapped optical WDM signal is filtered into red and blue bands, with each band electrically converted by a respective photodiode; and  FIG. 2B  shows the transmittance characteristics of a red/blue filter device for use in the exemplary system of  FIG. 2A . 
         FIG. 3  is a block diagram of a further exemplary embodiment of a wavelength tracker system wherein a tapped optical WDM signal is filtered into four bands, with each band electrically converted by a respective photodiode. 
         FIG. 4  is a block diagram of yet a further exemplary embodiment of a wavelength tracker system wherein a tapped optical WDM signal is filtered into multiple bands, with each band electrically converted by a time-shared photodiode. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an exemplary wavelength tracker system  100 . The system comprises an optical tap  101  which taps a small portion (e.g., ≦10% by power) of an optical WDM signal on an optical path  10  to be monitored. The optical tapped signal is converted to an electrical signal by an optical detector  102 , such as a photodiode, a PIN detector, or the like. In an exemplary embodiment, a relatively low-speed (˜1 MHz) photodiode can be used for this purpose. 
     The electrical signal is provided to a tone detector  120 . In the tone detector  120 , the electrical signal is provided to a variable gain amplifier (VGA)  121  whose gain is controlled by a digital signal processing (DSP) unit  110 , described in greater detail below. The output of the VGA  121  is coupled to a low-pass filter (LPF)  122  and a high-pass filter (HPF)  123 . The output of the HPF  123  is coupled to an amplifier  125  whose output is coupled to a high-speed analog-to-digital converter (ADC)  127 . The output of the LPF  122  is coupled to a low-speed ADC  128 . The outputs of the ADCs  127  and  128  are monitored by the DSP unit  110 . 
     The LPF  122  and low-speed ADC  128  provide a representation of the DC level of the WDM signal to the DSP unit  110 , and the HPF  123 , amplifier  125 , and high-speed ADC  127  provide a representation of any signals within the frequency band of any dither tones that may have been imposed on channels of the WDM signal. The DC level can be used to estimate the average total power of the WDM signal. In an exemplary embodiment, the tones may be within a band of frequencies from 782 kHz to 814 kHz. Accordingly, the HPF  123  and the high-speed ADC  127  are designed to pass and convert signals in that band. As examples, the cut-off frequencies of the LPF  122  and the HPF  123  can be about 1 kHz and 2 kHz, respectively. 
     The DSP unit  110  comprises a processor  111  coupled to a memory  112  containing Fast Fourier Transform (FFT) data. The processor  111  may be implemented, for example, with a microprocessor, a digital signal processor, or the like. 
     In operation, the processor  111  controls the gain of the amplifier  121  in accordance with the low-frequency signal provided by the ADC  128  such that the signal power levels at the inputs of high-speed ADC  127  and low-speed ADC  128  are optimized with respect to the dynamic ranges of the ADCs. The processor  111  operates in known manner using the memory  112  to perform FFT processing of the digital signals provided by the ADCs  127  and  128  to detect any dither tone modulation of the optical WDM signal on the optical path  10 . This determines the respective dither tones, and hence optical WDM channel identification, thereby providing an indication of which channels are present in the optical path  10 . Additionally, the DSP unit  110  can measure the levels of the detected tones, thereby providing an indication of the respective optical signal power level of each channel in the WDM signal on optical path  10 . 
       FIG. 2A  is a block diagram of an exemplary embodiment of a wavelength tracker system  200  in accordance with the present invention for monitoring a WDM signal. The system comprises an optical tap  201  which taps a small portion of the WDM signal to be monitored. In this embodiment, the WDM signal is in the C-band, ranging in wavelength from 1,529 nm to 1,562 nm, although the present invention is not limited to any particular band and may also be used in applications with more than one band. 
     A red-blue WDM filter  220  separates the tapped signal into two groups, a blue-band group ranging from 1,529 nm to 1,543 nm, and a red-band group ranging from 1,547 nm to 1,562 nm. The two groups are then simultaneously detected by respective detectors  221  and  222 , followed by respective tone detectors  223  and  224 , similar to that described above. 
     Based on the outputs of the tone detectors  223  and  224 , a digital signal processing (DSP) unit  210  calculates the power of each WDM channel that is present in the blue-band group and each WDM channel that is present in the red-band group. 
     By separating the WDM channels into the red-band and blue-band groups, and detecting the groups separately, it is expected that the wavelength tracker  200  will have an approximately 6 dB improvement in tolerance to stimulated Raman scattering (SRS) over the wavelength tracker  100  described above. The 6 dB improvement in tolerance to SRS means that a four times greater signal power or a four times longer transmission distance can be allowed for the same monitoring accuracy. 
     The red/blue WDM filter  220  can be implemented using a variety of conventional, off-the-shelf optical filtering devices, such as for example, a device with one input port that takes in the WDM signal and two output ports, a “blue” output that outputs the so-called “blue” band with a 1-dB passband between 1,529 nm and 1,543 nm, and a “red” output that outputs the so-called “red” band with a 1-dB passband between 1,547 nm and 1,562 nm. The transmittance characteristics of such a device are illustrated in  FIG. 2B , with  250  representing the transmittance of the blue output and  260  representing the transmittance of the red output. The 1-dB passband  255  of the blue output is defined as the region where the transmittance  250  is within 1-dB from the peak transmittance, and similarly the 1-dB passband  265  of the red output is defined as the region where the transmittance  260  is within 1-dB from the peak transmittance. This leaves an “intermediate” band  275  between the red and blue bands, i.e., 1,543-1,547 nm that is partially attenuated and passed to different degrees by the red and blue output ports. The intermediate band  275  may also carry WDM channels, and it is desirable that the powers of these channels be monitored as well. In common red/blue WDM filters, the transmittance of the red passband and that of the blue passband are complementary (due to energy conservation). Thus, the channel power of a WDM channel in the intermediate band in the 1,543 nm-1,547 nm range can be obtained by summing up the powers measured from the red-band output and the blue-band output for the tone frequency corresponding to the WDM channel. This summation can be carried out by the DSP unit  210 . 
       FIG. 3  is a block diagram of a further exemplary embodiment of a wavelength tracker system  300  in which the red and blue bands are further split into four wavelength sub-bands by respective WDM filters  321  and  322 . The filters  321  and  322  are preferably wavelength non-skipping filters. 
     In the exemplary embodiment, the four wavelength sub-bands are 1,529 nm to 1,536 nm, 1,536 nm to 1,543 nm, 1,547 nm to 1,554.5 nm, and 1,554.5 nm to 1,562 nm. The four wavelength groups are then simultaneously detected by respective PIN detectors  323 - 326 , followed by respective tone-detection circuitry  327 - 330 , similar to that described above. 
     Note that in this embodiment, the WDM filters  321  and  322  can be implemented with conventional, off-the-shelf optical filtering devices so as to provide no gap between the sub-bands within each of the red and blue bands. This is due to the narrower bandwidths of the sub-bands. The intermediate band between the red and blue bands, i.e., 1,543 nm-1,547 nm, can be handled as described above. 
     A digital signal processing (DSP) unit  310  calculates the power of each wavelength channel in each of the wavelength sub-bands and controls the gains of the tone detectors  327 - 330 , in similar manner to the DSP unit  210  described above. The expected improvement in SRS tolerance of the wavelength tracker  300  over the wavelength tracker  100  described above is approximately 12 dB. 
     In further exemplary embodiments (not shown) the four wavelength sub-bands of the embodiment of  FIG. 3  can be further sub-divided to eight sub-bands, which in turn can be further sub-divided to 16 sub-bands, and so on, to provide even better SRS tolerance. 
       FIG. 4  is a block diagram of yet a further exemplary embodiment of a wavelength tracker system  400  in which instead of using multiple PIN detectors and multiple tone detectors, one PIN detector  422  and one tone detector  423  are used to sequentially process each of a plurality of N (≧2) wavelength sub-bands split by a 1-by-N WDM filter  420 . A 1-by-N optical switch  421  is used to connect one of the N wavelength sub-bands to the PIN detector  422  at a given time. This can be done under the control of a DSP unit  410 . The DSP unit  410  operates as described above to determine the channel powers for the wavelength sub-band currently selected by the 1-by-N optical switch  421 . Once that is done, the DSP unit  410  can control the 1-by-N optical switch  421  to select another wavelength sub-band, and so on, until each of the N wavelength sub-bands has been processed. 
     It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.