Abstract:
Certain embodiments of the present invention provide a monitoring system configured to monitor output wavelengths, power, and channel switching of tunable lasers employed in an optical transmitter and provide feedback to those lasers, e.g., to lock on the wavelengths corresponding to optical channels in the transmitter. The monitoring system has a monitoring switch fabric, such as an optical waveguide grating (AWG), and one or more photodetector arrays coupled to the transmitter. Optical channels in the monitoring AWG may be offset relative to the optical channels in the transmitter and shaped to allow more sensitive monitoring of, e.g., wavelength drifting of the tunable lasers. The monitoring system may track the lasers in a non-disruptive continuous manner while data is transmitted through the transmitter.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to optical communication equipment.  
           [0003]    2. Description of the Related Art  
           [0004]    Tunable lasers are used to generate optical carrier signals that can be modulated with data for transmission over fiber-optic networks. Optical components in such networks (e.g., optical switches) may include numerous tunable lasers. As a result of aging, output characteristics in a laser, such as an output wavelength at selected hardware settings, may change (drift) significantly over the laser&#39;s lifetime. To compensate for the drift, each laser is typically monitored and calibrated, preferably in a cost effective, non-disruptive, and dependable manner.  
           [0005]    [0005]FIG. 1 shows a representative system  100  for transmitting data using an optical switch  102 . Switch  102  is a 3×3 switch comprising a 3×3 arrayed waveguide grating (AWG)  104 , three line cards  106  coupled to input ports of AWG  104 , and three receivers  130  coupled to output ports of AWG  104 . Each line card  106  comprises a tunable laser  110  and a modulator  120 . Laser  110  feeds an optical carrier signal into modulator  120 . Modulator  120  modulates the carrier signal with data to produce a data-modulated output signal of the respective line card  106 . Each line card  106  can be configured to send its output signal to any chosen receiver  130  by setting the wavelength of laser  110  to the value corresponding to the desired output port of AWG  104 .  
           [0006]    System  100  further comprises one or more optical wavelength monitors (OWM)  112 , typically one OWM  112  per line card  106 . Each OWM  112  is configured to receive a small portion of laser output, analyze it, and generate a feedback signal. Using the feedback signal, laser  110  of the respective line card  106  can adjust its output to lock on a desired wavelength.  
           [0007]    [0007]FIG. 2 illustrates one typical prior art implementation of OWM  112 . OWM  112  comprises a plurality of optical wavelength lockers (OWL)  202 , analog-to-digital (A/D) converters  204 , a processor  206 , and digital-to-analog (D/A) converters  208 . The number of OWLs  202  in OWM  112  usually corresponds to the number of optical channels in switch  102 . Different OWLs  202  are configured to different wavelengths corresponding to the respective optical channels in switch  102 . The output of each OWL  202  is converted into a digital signal by A/D converters  204 , processed by processor  206 , and converted back into analog form by D/A converters  208  to produce a feedback signal applied to the corresponding laser  110 .  
           [0008]    OWLs are well known in the art and may be, for example, Santec OWL-10 or OWL-20 available from Santec Corporation of Japan. OWLs are fixed wavelength devices with a relatively narrow capture range of, e.g., 0.25 nm. Therefore, to monitor an N×N optical switch having N lasers, each of which can tune to N different wavelengths, one needs a total of N 2  OWLs. With large optical switches, e.g., having 100 channels, such a system becomes large and expensive to implement.  
         SUMMARY OF THE INVENTION  
         [0009]    Certain embodiments of the present invention provide a compact monitoring system for an optical system, such as optical switch  102  of FIG. 1. The monitoring system is configured to monitor output wavelengths, power, and channel switching of tunable lasers employed in the switch and provide feedback to said lasers, e.g., to lock on the wavelengths corresponding to optical channels in the switch. The monitoring system has a monitoring switch fabric, e.g., an optical waveguide grating (AWG), and one or more photodetector arrays coupled to the switch. Optical channels in the monitoring AWG may be offset relative to the optical channels in the switch and shaped to allow more sensitive monitoring of, e.g., wavelength drifting of the tunable lasers. The monitoring system may track the lasers in a non-disruptive continuous manner while data is transmitted through the switch.  
           [0010]    According to one embodiment, the present invention is an apparatus for monitoring an optical system configured to route optical signals generated by one or more lasers, the apparatus comprising: (a) a first optical switch fabric (OSF) having input ports and output ports and configured to route optical signals from its input ports to its output ports; and (b) a first photo-sensing device configured to monitor optical signals output from the first OSF, wherein: the optical signals routed by the first OSF are a portion of the optical signals generated by the one or more lasers; and the first OSF is configured to enable determination of wavelengths of the optical signals generated by the one or more lasers based on signals generated by the first photo-sensing device.  
           [0011]    According to another embodiment, the present invention is a method for monitoring an optical system configured to route optical signals generated by one or more lasers, the method comprising the steps of: (A) generating signals using a first photo-sensing device configured to monitor optical signals output from a first optical switch fabric (OSF) having input ports and output ports and configured to route optical signals from its input ports to its output ports, wherein the optical signals routed by the first OSF are a portion of the optical signals generated by the one or more lasers; and (B) determining wavelengths of the optical signals generated by the one or more lasers using the signals generated by the first photo-sensing device.  
           [0012]    According to yet another embodiment, the present invention is an apparatus, comprising: (a) a data optical switch fabric (OSF) having input ports and output ports and configured to route optical signals generated by one or more lasers from its input ports to its output ports; (b) a monitoring OSF having input ports and output ports and configured to route optical signals from its input ports to its output ports; and (c) a first photo-sensing device configured to monitor optical signals output from the monitoring OSF, wherein: the optical signals routed by the monitoring OSF are a portion of the optical signals generated by the one or more lasers; and the monitoring OSF is configured to enable determination of wavelengths of the optical signals generated by the one or more lasers and routed by the data OSF based on signals generated by the first photo-sensing device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:  
         [0014]    [0014]FIG. 1 shows a prior art system for transmitting data using an optical switch;  
         [0015]    [0015]FIG. 2 illustrates a prior art optical wavelength monitor that can be used in the system of FIG. 1;  
         [0016]    [0016]FIG. 3 shows a system for transmitting data and monitoring lasers in line cards according to one embodiment of the present invention;  
         [0017]    [0017]FIG. 4 illustrates how the passbands in the monitoring AWG used in the system of FIG. 3 may be configured according to one embodiment of the present invention;  
         [0018]    [0018]FIG. 5 illustrates the amount of crosstalk between different channels of the monitoring AWG used in the system of FIG. 3 in one embodiment of the present invention; and  
         [0019]    [0019]FIG. 6 shows a method of monitoring that may be used in the system of FIG. 3 according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.  
         [0021]    [0021]FIG. 3 shows a system  300  for transmitting data and monitoring lasers in line cards according to one embodiment of the present invention. System  300  comprises an optical switch  302  and a laser monitor  312 . Switch  302  is an N×N switch comprising an N×N data AWG  304 , N line cards  306  coupled to input ports of data AWG  304 , and N receivers  330  coupled to output ports of data AWG  304 . In one embodiment, line cards  306  may be similar to line cards  106  of system  100 . Line cards  306  are configured to generate data modulated signals using tunable lasers (e.g., similar to lasers  110 ) and modulators (e.g., similar to modulators  120 ). Each line card  306  can be configured to send its output signal to any chosen receiver  330  by using an output wavelength (λ 1  ε{λ 1 , . . . , λ N }) corresponding to a desired output port of data AWG  304 . Switch  302  further comprises a scheduler  332  configured to control output wavelengths of line cards  306  to manage data traffic through data AWG  304 , e.g., to avoid instances of two different line cards simultaneously sending data to the same receiver  330 . Scheduler  332  may be configured to communicate with line cards  306  through data AWG  304  or through a separate control bus (not shown).  
         [0022]    Monitor  312  comprises an N×N monitoring AWG  314 , a signal processor  316 , and one or more photodetector arrays  318 . In one embodiment, monitor  312  may have three arrays  318 A-C configured to monitor optical signals in system  300  as shown in FIG. 3. More specifically, array  318 A measures optical power of optical signals output from line cards  306 ; array  318 B measures optical power of optical signals after monitoring AWG  314 ; and array  318 C measures optical power of optical signals after data AWG  304 . In different embodiments, monitor  312  may have a different number of differently configured arrays  318 . In addition, different photo-sensing devices may be used in place of arrays  318 .  
         [0023]    Outputs of photodetectors in arrays  318 A-C are connected to processor  316 . Processor  316  is configured to analyze said outputs and generate feedback signals (not shown in FIG. 3) to line cards  306 , e.g., using scheduler  332 . Based on said feedback signals, line cards  306  may adjust their output wavelengths corresponding to individual optical channels in switch  302 , e.g., to compensate for possible wavelength drifting.  
         [0024]    In one embodiment, arrays  318 A-C comprise linear photodetectors, each of which generates a photocurrent (I) proportional to the power (P) of light impinging on that photodetector according to the following equation:  
         P k =c k I k   (1)  
         [0025]    where index k represents the k-th photodetector in the array and c k  is a proportionality constant. In different embodiments, non-linear detectors may be used.  
         [0026]    Each input port of monitoring AWG  314  is configured to receive a small portion of the optical output from the corresponding line card  306  using an optical tap. Light tapped to the i-th input port of monitoring AWG  314  from the corresponding line card  306  is monitored by the i-th detector in array  318 A. Monitoring AWG  314  routes the light received from each line card to array  318 B, which is coupled to the output ports of monitoring AWG  314 . In general, each photodetector in array  318 B receives a portion of that light. However for most photodetectors, that portion is very small. For a given signal input at a particular input port of monitoring AWG  314 , only several (e.g., two or three) photodetectors in array  318 B connected to the output ports of monitoring AWG  314  with the pass bands approximately matching the wavelength of light might receive a measurable portion of that light.  
         [0027]    When only one line card is transmitting, the photocurrent (I j ) generated by the j-th detector in array  318 B depends on (A) the wavelength (λ i ) and power (P i ) of light generated by the i-th line card  306  and (B) the passband function (g ij ) characterizing the throughput from the i-th input port to the j-th output port in monitoring AWG  314 . Said dependence may be expressed by Equation (2) as follows:  
           I   j   =g   ij ( P   i ,λ 1 )  (2)  
         [0028]    In one embodiment, passbands of monitoring AWG  314  may be Gaussian-shaped. In other embodiments, differently shaped passbands may be used. For linear photodetectors in arrays  318  and Gaussian-shaped passbands, Equation (2) may be expressed as follows:  
               I   j     =       d     i                 j            c   i          I   i          exp        (     -         (       λ   i     -     Λ     i                 j         )     2       2                   Δ     i                 j     2           )                 (   3   )                               
 
         [0029]    where Λ ij  is the center wavelength of the passband corresponding to the i-th input and j-th output ports of monitoring AWG  314 ; Δ ij  is the width (equal to approximately 42.5% of the full-width at half-maximum) of that passband; d ij  is the combined on-resonance loss coefficient; and I i  and c i  are the photocurrent and proportionality constant, respectively, of the i-th photodetector in array  318 A.  
         [0030]    As already mentioned, only several photodetectors in array  318 B will typically produce measurable photocurrents corresponding to the output of an i-th line card. In one embodiment of the present invention, the wavelength of that output may be determined using the output channel (designated herein as jmax-th channel) of monitoring AWG  314  corresponding to the largest photocurrent (I jmax ) in the measurable photocurrents. The wavelength is determined using Equation (4) as follows:  
               λ   i     =       Λ     i                 j                 max       ±       Δ     i                 j                 max              2                   ln        (         d     i                 j                 max            c   i          I   i         I     j                 max         )                       (   4   )                               
 
         [0031]    The “±” ambiguity in Equation (4) represents the fact that the same value of photocurrent, I jmax , may be produced with λ i  being either on the left or right shoulder of the corresponding passband.  
         [0032]    In one embodiment, the correct sign in Equation (4) is determined using the second largest photocurrent in the measurable photocurrents. For example, if the second largest photocurrent is detected at the (jmax−1)-th output port, then the minus sign is selected. Likewise, if the second largest photocurrent is detected at the (jmax+1)-th output port, then the plus sign is selected. In another embodiment, the correct sign in Equation (4) is determined using wavelength dithering. For example, the i-th line card may be configured to change its output wavelength by a small amount around λ i  while I jmax  is monitored. If an increase in wavelength corresponds to an increase in I jmax  then the minus sign is selected. Similarly, if an increase in wavelength corresponds to a decrease in I jmax , then the plus sign is selected.  
         [0033]    In one embodiment of the present invention, line cards  306  may employ relatively stable lasers whose output power is stabilized around predetermined values (i.e., Pi 0  for the i-th line card) within a relatively narrow range (e.g., ±1%) regardless of the output wavelength. In that case, photodetectors  318 A may be optionally removed from system  300 . Consequently, the wavelength is determined using a modified Equation (4), wherein the term c i I i  is replaced with P i0 .  
         [0034]    [0034]FIG. 4 illustrates how the passbands in monitoring AWG  314  may be configured according to one embodiment of the present invention. The top and bottom panels in FIG. 4 show transmission curves corresponding to different channels of AWGs  314  and  304 , respectively. Channel spacing (Δλ ch ) is chosen to be approximately the same for both AWGs. However, the passbands in monitoring AWG  314  are shifted by Δλ offset  relative to those in data AWG  304 . Passband widths (Δ ij ) for different channels in monitoring AWG  314  and data AWG  304  are uniform and equal to Δ m  and Δ d , respectively, where Δ m  may or may not equal Δ d .  
         [0035]    As can be seen in FIG. 4, Δλ offset  causes a resonance wavelength in data AWG  304  (point P 3  in FIG. 4) to be located on a shoulder of the respective passband in monitoring AWG  314 . Due to this feature, relatively small changes in laser wavelength around P 3  are converted to relatively large changes in the photocurrent generated by the corresponding photodetector in array  318 B. In a preferred implementation, P 3  corresponds to a point of the steepest slope (e.g., point P 1  in FIG. 4) on the respective passband of monitoring AWG  314 . For Gaussian-shaped passbands, Δλ offset  may be selected using Equation (5) as follows:  
               Δ                   λ   offset       =       Δ   m     =         FWHM   m       2          2                 ln                 2           ≈     0.425                   FWHM   m                   (   5   )                               
 
         [0036]    where FWHM m  is the full-width at half-maximum for a passband in monitoring AWG  314 . The amplitude ratio (ΔA offset ) between the passband peak (point P 0  in FIG. 4) and point P 1  is given by Equation (6):  
         Δ A   offset   ={square root}{square root over (e)}= 2.17  dB   (6)  
         [0037]    In one embodiment of the present invention, a suitable value for FWHM m  may be selected based on the following considerations. On one hand, it is desirable to have narrow passbands so that relatively small laser wavelength variations result in relatively large photocurrent variations. On the other hand, it is desirable for monitor  312  to detect any laser wavelength within the wavelength range of monitoring AWG  314 , which favors relatively wide, overlapping passbands. For example, at the point of lowest transmission (e.g., point P 2  in FIG. 4) between two passbands, there should be enough transmission to ensure reliable detection. The following choice of FWHM m  may balance these two conflicting requirements:  
               FWHM   m     ≈     Δ                   λ   ch              ln                 2         ln                   R     d                 yn         -     ln                 Δ                 P                     (   7   )                               
 
         [0038]    where R dyn  is the dynamic range of photodetectors in array  318 B and ΔP is the power variation at photodetectors in array  318 B due to (i) variation in transmission losses corresponding to different combinations of input and output ports of monitoring AWG  314  and (ii) variation in the output power of different line cards  306 . ΔP may be calculated, e.g., using the following expression:  
         Δ                 P     =       max     i   ,   j              (       d   ij          c   i          I   i       )     /       min     i   ,   j            (       d   ij          c   i          I   i       )                                 
 
         [0039]    Using the example illustrated by FIG. 4, R dyn  may be selected as follows:  
         R dyn ≧ΔA m ΔA offset ΔP  (8)  
         [0040]    where ΔA m  is the difference in transmission between points P 1  and P 2  in a passband of monitoring AWG  314 .  
         [0041]    [0041]FIG. 5 illustrates the amount of crosstalk between different channels of monitoring AWG  314  as a function of various parameters of system  300  in one embodiment of the present invention. More specifically, for Gaussian-shaped passbands, the amount of crosstalk (C x ) between the j-th and (j−1)-th output ports of monitoring AWG  314  can be expressed as follows:  
               C   x     =       exp        (         -     1   2            (     1   -   p     )            Δ                   λ   ch         Δ   m         -   1     )       2             (   9   )                               
 
         [0042]    where p is a shift factor defined by the following equation:  
             p   =         Λ     i                 j       -     Δ                   λ   offset       -     λ   i         Δ                   λ   ch                 (   10   )                               
 
         [0043]    Combining Equations (7) and (9), one can obtain the following expression for the crosstalk:  
               C   x     ≈     exp        (       -     1   2              (         (     1   -   p     )            8        (     ln          R     dy                 n         Δ                 P         )           -   1     )     2       )               (   11   )                               
 
         [0044]    [0044]FIG. 5 illustrates Equation (11) for three representative shift factors (i.e., p=0.0; 0.1; and 0.2). More specifically, line 1 corresponds to an ideal unmodulated laser source and p=0.0; line 2 corresponds to a finite line-width unmodulated laser source and p=0.0; line 3 corresponds to a finite line-width modulated laser source and p=0.0; line 4 corresponds to a finite line-width modulated laser source and p=0.1; and line 5 corresponds to a finite line-width modulated laser source and p=0.2.  
         [0045]    Using the analysis presented above, one can design monitor  312  for system  300  to correspond to selected (e.g., customer) specifications. For example, the following table gives a representative set of parameters that may be used in one implementation of system  300 .  
                                                                         #   Parameter   Value                                        1   Bit Rate (Gb/s)   40           2   Channel Spacing Δλ ch  (GHz)   100           3   Crosstalk C x  (dB)   ≦−30           4   Power variation ΔP (dB)   ≦3           5   Dynamic Range R dyn  (dB)   ≧23           6   FWHM m  (GHz)   40           7   Δλ offset  (GHz)   17                      
 
         [0046]    Lines 1-4 in the table show representative specifications for switch  302  of system  300 . Lines 5-7 in the table show representative parameters for monitor  312  of system  300  derived based on the specifications in lines 1-4 and Equations (5), (7), (8), and (11). Depending on the particular implementation and system requirements, parameters different from those given in the table may be used in different embodiments of system  300 .  
         [0047]    [0047]FIG. 6 shows a method  600  of monitoring switch  302  using monitor  312  according to one embodiment of the present invention. In step  602  of method  600 , a mode of monitoring is selected, e.g., using signal processor  316  and scheduler  332 . Two exemplary modes of monitoring, e.g., full calibration mode and tracking mode, are illustrated in FIG. 6. The full calibration mode is a service mode, during which transmission of data through switch  302  is interrupted, whereas the tracking mode is a non-disruptive continuous monitoring mode performed in the background while user data is transmitted through switch  302 . In different implementations of method  600 , additional and/or different modes of monitoring may be utilized.  
         [0048]    If the full calibration mode is selected in step  602 , then system  300  proceeds to step  604 , wherein one selected line card  306  is activated while the remaining line cards are turned off. In step  606 , the active line card is tuned to a selected laser channel. In step  608 , the wavelength corresponding to that laser channel is measured, e.g., using arrays  318 A-B, Equation (4), and at least one of the above-described procedures for determining the correct sign in that equation. In step  612 , the active line card is tuned to a different laser channel. In step  614 , the switching speed for switching that line card between those two channels is determined, e.g., by using array  318 C and the time required for the signal to appear at the output port of data AWG  304  corresponding to the new channel. Steps  608 - 614  may be repeated until all laser channels of the selected line card  306  are characterized. After that, a similar procedure (e.g., steps  604  through  614 ) may be performed for a different line card.  
         [0049]    In situations where lasers in line cards  306  are relatively stable (e.g., undergo small wavelength drifting relatively far from a mode hop), the tracking mode may be selected in step  602 . Switch  302  is usually operated using scheduler  332  so that only one line card  306  sends data to a specific receiver  330 . Therefore, up to N different laser channels corresponding to N different line cards may be active simultaneously. Those channels may be monitored without disrupting the operation of switch  302 . The wavelengths of all lasers can be computed from the sets of photocurrents, taking next-neighbor crosstalk into account.  
         [0050]    For example, in step  616 , the wavelengths of the currently active laser channels are determined, e.g., using a scheduling table of switch  302 , arrays  318 A-B, Equation (4), and the correct sign in that equation, e.g., as obtained in the last full calibration. In one implementation of method  600 , step  616  may include the following representative steps:  
         [0051]    Step  616 A: determining the wavelength of the channel that suffers the least amount of crosstalk (one possible choice of such channel may be the rightmost channel in FIG. 4);  
         [0052]    Step  616 B: estimating the amount of crosstalk that the measured channel induces in the nearest neighbor(s);  
         [0053]    Step  616 C: determining the wavelength(s) of the nearest neighbor channel(s) while subtracting the crosstalk estimated in step  616 B from the corresponding photocurrents detected by array  318 B; and  
         [0054]    Step  616 D: repeating steps  616 B-C until all active channels are characterized.  
         [0055]    As switch  302  keeps changing corresponding combinations of line cards  306  and receivers  330  during data transmission, potentially all N 2  laser channels may eventually be characterized during step  616 . Using the results of step  616 , feedback signals may be generated and provided to line cards  306  in step  618 , e.g., using signal processor  316  and scheduler  332 , to compensate for wavelength drifting.  
         [0056]    If one of the lasers in line cards  306  approaches or undergoes a mode hop, recalibration of the system may be performed, e.g., using steps  604 - 614  as described above. After such recalibration, the tracking mode may resume. System recalibrations may be scheduled using a fixed time schedule and/or using laser mode stability data (e.g., the occurrence of mode hops) gathered during the tracking mode.  
         [0057]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the claims.  
         [0058]    For example, data AWG  304  and monitoring AWG  314  may be implemented using non-uniform passband widths and/or different channel spacing. Furthermore, those AWGs may be implemented on a single wafer or two different wafers. In the latter case, the AWGs may be maintained at different temperatures, e.g., to enable temperature tuning of Δλ offset . Photodetector arrays  318  may be based on any suitable light-sensitive device, such as, for example, a photodiode, a phototransistor, a photogate, photo-conductor, a charge-coupled device, a charge-transfer device, or a charge-injection device. Similarly, as used in this specification, the term “light” refers to any suitable electromagnetic radiation in any wavelength that may be used in an optical transmission system, such as system  300 . Although the invention has been described for a system employing AWGs, those skilled in the art can appreciate that the invention can be equally applied to systems employing other types of optical switch fabrics.  
         [0059]    Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.