Abstract:
A unique sensor is used to detect a transmission impairment that may have affected incoming optical channel signals. The sensor, more specifically, selects a group of the incoming channel signals and generates a first power signal, P 0 , over the selected group of signals and generates a second power signals, P 1 , over a weighted version of the selected group of channel signals. The sensor then generates, as a function of the first and second power signals, P 0  and P 1 , a signal indicative of whether the particular transmission impairment affected the levels of individual ones of the incoming channel signals. If so, then control apparatus offsets the impairment accordingly.

Description:
FIELD OF THE INVENTION 
   The invention relates to a sensor responsive to a dense band of signals transported over an optical transmission system. 
   BACKGROUND OF THE INVENTION 
   As is well known, when a number of optical channels are propagating over an optical fiber, so-called stimulated Raman scattering (gain) may cause an optical channel to interact with a channel of a longer wavelength. Such interaction causes the power in the shorter wavelength channel to decrease and power in the longer wavelength channel to increase. In effect, the power in the shorter wavelength channels is “pumped” into the longer wavelength channels. The most pronounced effects occur when the channels are separated by about 15 THz. When an appreciable number of channels are transmitted over an optical fiber with a high level of power per channel, then the effect tilts the power divergence between the channels significantly to the channels of longer wavelengths. 
   The effect of Raman scattering increases when more than one band of optical channels are transported over an optical fiber, e.g., C and L bands. In that instance, the effect is approximately linear with channel separation, and may be determined by summing the contribution provided by each of the channels. If the different bands of channels are produced by different sources, then the possibility arises in which an entire band of channels may be suddenly lost or present based on whether the corresponding source has suddenly failed or come on line. This problem would be manifested by a sudden change in the spectra of the other bands, which may significantly increase the error rate of those bands. Consequently, the affected bands need to be adjusted immediately, e.g., within microseconds, to changes in average signal level and tilt. 
   The prior art uses an optical spectrum analyzer to generate the information needed to make the above adjustments. 
   However, what is needed is a sensor that quickly analyzes a band of channels to quickly detect changes in power level due to Raman scattering/pumping whenever the number of channels in another band of channels changes. 
   SUMMARY OF THE INVENTION 
   We have recognized that the effect of Raman scattering may be determined very quickly by determining the ratio between the total power and a real-time weighted total power. 
   More specifically, a sensor processes a group of incoming channel signals to generate a first signal, P 0 , that is indicative of the total power across the group channel signals, and a second signal, P 1 , that is indicative of the total power across the group of channels after the group of channel signals has been subjected to a predetermined weighting function. The system then offsets, as a function of the first and second signals, any Raman scattering that may be affecting the channels signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  illustrates signals in different bands and is useful in defining different terms discussed below; 
       FIG. 2  illustrates in block diagram form an optical system in which the principles of the invention may be practiced; 
       FIG. 3  is broad block diagram of the sensor of  FIG. 2 ; and 
       FIG. 4  illustrates an alternative way of generating weighted signals, in accordance with an aspect of the invention. 
     GENERAL DESCRIPTION 
     We have recognized that, in accordance with various aspects of the invention, that the effect that an arbitrary band of channels may have on another channel due to Raman scattering may be simulated by a single channel having an effective power of P E  and an effective wavelength of λ E . 
     If all of the signal bands are within a particular bandwidth, e.g., within the range of 13 THz to 16 THz, then the Raman interaction between any two channels may be described approximately by the following relationship:
 
 P   R   =γ•I   L   I   S (λ L −λ S )  (1) 
 
where γ is the coefficient of the Raman Interaction, I L  and I S  (averaged over all polarizations) are the intensities of the longest and shortest wavelength channels, and λ L  and λ S  are the wavelengths. The effect of j channels in a single band on a single channel having a wavelength of λ L  may be determined by summing each such effect as follows: 
               P   R     =     γ   ⁢           ⁢       ∑   j             ⁢           ⁢       I   L     ⁢           ⁢     I   L     ⁢           ⁢     (       λ   L     -     λ   j       )                   (   2   )             
 
Recognizing that equation (2) may be separated into two sums, then: 
               P   R     =         γ   ·     I   L       ⁢           ⁢     λ   L     ⁢           ⁢       ∑   j             ⁢           ⁢     I   j         -       γ   ·           ⁢     I   L       ⁢           ⁢       ∑   j             ⁢           ⁢       I   j     ⁢           ⁢     λ   j                     (   3   )             
 
where the first sum is the total power in the band, P 0 .
 
     Referring to  FIG. 1  which shows signals in different bands, we define Δλ j =λ j −λ min  and Δλ B =λ max −λ min  so that equation (3) may be rewritten as follows: 
               P   R     =         γ   ·     I   L       ⁢           ⁢     (       λ   L     -     λ   min       )     ⁢           ⁢     P   0       -       γ   ·     I   L       ⁢           ⁢   Δ   ⁢           ⁢     λ   B     ⁢           ⁢       ∑   j             ⁢           ⁢         I   j     ⁢           ⁢   Δ   ⁢           ⁢     λ   j         Δ   ⁢           ⁢     λ   B                       (   4   )             
 
Note that the above summation is the sum of the powers in the band weighted linearly by the distance from the beginning of the band. Other than P 0 , the remaining terms are constants. Therefore, equation (4) may be rewritten as follows:
 
 P   R   =C   0   P   0   +C   1   P   1   (5) 
 
where P 1  is the weighted sum. The full effect of Raman pumping may then be obtained by apparatus which provides P 0  and P 1  directly in real time.
 
   

   DETAILED DESCRIPTION 
   An illustrative optical transmission system embodying the principles of the invention is shown in simplified form in FIG.  2 . The optical system, more particularly, includes head-end node  100  having, inter alia, a plurality of laser transmitters (XMTR)  110 - 1  through  110 -n, multiplexer  115  and optical amplifier  120 . Each of the transmitters generates an information bearing optical signal and supplies the signal to a respective input of multiplexer  115 . The optical signals, λ l  through λ n , so generated may constitute two different bands of optical signals/channels such that signals λ l  and λ n  would respectively have the longest and shortest wavelengths of the signals in the two different bands. Multiplexer  115  multiplexes the signals to an output extending to optical amplifier (OA), which amplifies and outputs the multiplexed signals to optical path segment  130  extending to a next downstream node. A number of downstream/intermediate nodes may be disposed along optical transmission path  130  as represented by the dashed portions of segments  130 . Node  200  represents each such intermediate node. Thus, the following discussion of node  200  equally pertains to each of the other similarly arranged nodes. 
   Node  200 , includes, inter alia, optical amplifier  210  that amplifies an optical signal received via path  130  and outputs the amplified signal via splitter  215  to other processing equipment, e.g., a demultiplexer, signal translation units, add/drop apparatus, etc., as represented by the dashed line  230  in node  200 . Optical signal splitter  215  supplies a small portion of the amplified signal to sensor  220  and supplies the remainder of the amplified signal to the other equipment for further processing. Sensor  220  processes its portion of the amplified signal to determine if the signal had been tilted as a result of Raman scattering occurring along the transmission path  130 . Sensor  220  supplies the results of its determination to control circuit  225 , which then directs optical amplifier  235  to tilt the signal that it receives at its input in an opposite direction to offset the effect of the Raman scattering, if needed. 
   Sensor  220 , shown in more detail in  FIG. 3  includes, inter alia, band pass filter  10  which is tuned to one of the bands of signals received via path  221 . Assuming that filter  10  is tuned to the L-band of signals, then those signals pass through filter  10 , while signals of different bands/wavelengths are rejected. Splitter  15  splits the signal emerging from filter  10  into two signals, respectively supplying substantially equal portions of the split signal to total power detector  40 - 1  via path  17  and to port  20 - 1  of conventional optical signal circulator  20  via path  16 . As is well-know a signal received at a circulator port is circulated in a particular direction, e.g., counterclockwise, and outputted at a next port. For example, a signal received at port  20 - 1  is circulated to a next port, e.g., port  20 - 2 , and outputted at that port; a signal received at port  20 - 2  is similarly circulated to a next port, e.g., port  20 - 3 , and outputted at that port, and so on. Thus, the L-band signal received at port  20 - 1  is circulated to and outputted at port  20 - 2 , where it is presented via path  31  to section  32 - 1  of conventional Dragone router  30 . Section  32 - 1  of Dragone router  30 , in a conventional manner, demultiplexes the signal that it receives via path  31  and outputs the component signals forming the band of signals to respective output ports extending to section  32 - 2  of Dragone router  30 . Section  32 - 2  of Dragone router outputs the demultiplexed signals, λ 1  through λ n  of the filtered band of signal, to respective inputs of Variable Reflection Filter (VRF)  35 . VRF  35  reflects an optical signal that it receives at one of its inputs proportional to the wavelength of the signal. Thus, the level of reflection provided by filter  35  linearly increases across a band of signal, from the longest wavelength, λ l , to the shortest wavelength, λ n , such that the former signal is reflected the most while the latter signal is reflected the least. For example, the reflectivity might be R(λ)=(λ−λ min )/(λ max −λ min ), which ranges from 0 (for the shortest wavelength) to 1 (for the longest wavelength). In this way, the signals forming the band are linearly weighted proportional to their respective wavelengths. The reflected, weighted signals are returned to Dragone section  32 - 2 , which then routes the weighted signals to Dragone section  32 - 1 . The latter section then multiplexes the weighted signals onto path  31  extending to port  20 - 2  of circulator  20 . As pointed out above, signals received at port  20 - 2  are circulated to and outputted at port  20 - 3  of circulator  20 , where the multiplexed weighted signal is presented to weighted power detector  40 - 2 . Weighted power detector  40 - 2 , in a conventional manner, detects the level of power in the signal that it receives and outputs a signal, P 1 , indicative thereof to amplifier  45 - 2 . (Detector  40 - 2  may do this using a conventional light detector that outputs a signal having a power level proportional to the level of the light signal that it receives at its input.) Similarly, total power detector  40 - 1  detects the level of power in the (unweighted) signal that it receives and outputs a signal, P 0 , indicative thereof to amplifier  45 - 1 . Amplifier  45 - 1  multiples the signal P 0  by a constant C 0  (represented by the value of resistor R 1 ) to form the sought after signal C 0 P 0 . Similarly, amplifier  45 - 2  multiplies the signal P 1  by a constant C 1  (represented by resistor R 2 ) to form the other sough-after signal C 1 P 1 . Summing amplifier  50  sums the outputs of amplifiers  45 - 1  and  45 - 2  to combine signals C 0 P 0  and C 1 P 1  as a linear weighted sum to form above-defined signal P R . The latter signal is then supplied to controller  225 , which, as mentioned above, adjusts the tilt of the signal being amplified by amplifier  235  to correct for the effect of Raman scattering, if needed. 
   In accordance an aspect of the invention, the values of resistors R 1  and R 2  are calibrated for a given installation at the factory using a signal comprising all of the intended signals in the band, e.g., the L band, and then using just half of those signals. More specifically, the calibration maybe done using wavelengths of λ max  and λ min . For λ max , P 0  is set to equal P 1 , and for λ min , P 1  is set to 0. To determine the effective power, P E , and wavelength, λ E , P E =P 0  and λ E =λ min +Δλ B P 1 /P 0 . 
   In an alternative embodiment of the invention, a variable loss device in combination with a reflector may be used in place of variable reflection filter (VRF)  35 , as shown in FIG.  4 . Specifically, the amount of loss inserted in each path of the demultiplexed signals is proportional to the wavelength of the signal. That is, the most loss is inserted in the path of the signal having the shortest wavelength and most loss is inserted in the path of the signal having the longest wavelength. The signals are then reflected/returned to Dragone section  32 - 2  by an optical reflector as shown. In this way the signals are weighted according to the amount of loss that they encounter on their way to the reflector and on their return to Dragone section  32 - 2 . 
   (Note that for a Dragone router having a sufficiently large free-spectral range (FSR), the intensities in section  32 - 2  are uniform across all channels. Also note, that for a smaller FSR, the intensities may be approximated by a Gaussian function. As such, a Dragone router having a large FSR is preferable over a Dragone router having a smaller FSR. However, if a router of the latter type is used, then R(λ) will need to include the Gaussian Shaping factor. 
   Further note, that other wavelength dependent effects may be handled using other R(λ) functions in the reflector. For example, a polarization dependent sensor may be implemented by placing a polarization splitter between filter  10  and splitter  15  ( FIG. 3 ) and duplicating the circuitry that follows splitter  15  so that sensor values may be obtained for each polarization.) 
   It will thus be appreciated that, although the invention illustrated herein is described in the context of a specific illustrative embodiment, those skilled in the art will be able to devise numerous alternative arrangements which, although, not explicitly shown or described herein, nevertheless, embody the principles of the invention and are within its spirit and scope. For example, the inventive sensor may be used to deal with transmission impairments other than Raman scattering.