Abstract:
Monitoring of the input power is performed on-chip and is used to monitor and maintain performance, detect failure and trigger network protection strategies. An optical power-monitoring technique uses a photodetector monolithically integrated with the semiconductor optical amplifier—Mach-Zehnder interferometer circuit to monitor the P2R device and keep the output stable while the input power varies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present patent application claims priority from U.S. Provisional Patent Application No. 60/675,257, filed Apr. 27, 2005, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Noise and attenuation in long-haul optical line systems result in the deterioration of the transmitted signal, both as to its amplitude as well as its shape. Consequently, one of the fundamental requirements of nodal equipment in optical networks is the capability to regenerate and reshape the optical pulses. These functions are known as P2R, for photonic regeneration and reshaping, which is a low-cost alternative to opto-electronic transponders at network nodes that do not require data access. P2R devices can be bit-rate and data-format insensitive, which is a key advantage in system design for multi-protocol transport systems, and a signal-quality monitoring methodology that is insensitive to bit-rate and data-format is essential for implementing P2R devices in optical networks.  
         [0003]     One well-known method for implementing P2R devices uses cross phase modulation in semiconductor optical amplifier—Mach-Zehnder interferometer (SOA-MZI) photonic integrated circuits in the Indium Phosphide materials system [1]. The ability to integrate multiple active and passive elements, operating in the C- and L-band of the optical spectrum on a single chip, is a significant advantage of this device technology. Commercial P2R devices with gains of over 10 db, which operate at up to 10 GHz over the entire C-band, have been demonstrated [2]. In addition, advanced P2R device architectures capable of achieving speeds of up to 40 GHz have been demonstrated [3].  
         [0004]     While past work has focused on demonstrating the feasibility of P2R technology, the present invention makes P2R devices system-ready. In an optical network, power variations arising out of transients or as a precursor to failure can cause signal degradations that can propagate throughout the network. In order to arrest this propagation, optical regenerators within the network must either possess a large input power dynamic range or be able to monitor input power variations to adjust the operating set points of the regenerator.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with the present invention, monitoring of the input power with on-chip, monitoring is used to monitor and maintain performance, detect failure and trigger network protection strategies. An optical power-monitoring technique using a photodetector monolithically integrated with the semiconductor optical amplifier—Mach-Zehnder interferometer circuit to monitor the P2R device and keep the output power and signal quality stable while the input power varies. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The forgoing brief description, and further objects features and advantages of the present invention will be understood more completely from the following detailed description of a presently preferred, but nonetheless exemplary, embodiment, with reference being had to the accompanying drawings in which:  
         [0007]      FIG. 1  is a schematic representation of a photodetector integrated semiconductor optical amplifier—Mach-Zehnder interferometer circuit device with circuit monitoring;  
         [0008]      FIG. 2  is a graph showing the photocurrent as a function of input power and preamplifier drive current density on SOA 1 ;  
         [0009]      FIG. 3  is an output eye diagram at 10 Gb/s for a presently preferred P2R device, derived for an input power of −8.5 dBm;  
         [0010]      FIG. 4  is a degraded output eye diagram as in  FIG. 3 , with the input power reduced from −8.5 dBm to −6.8 dBm; and  
         [0011]      FIG. 5  is a restored output eye diagram as in  FIG. 4 , which is obtained after the drive set-points of the device are adjusted in accordance with Table 1.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]      FIG. 1  is a schematic representation of a photodetector integrated semiconductor optical amplifier—Mach-Zehnder interferometer circuit device  10  with circuit monitoring. The transmission degraded pump signal is pre-amplified at SOA 1 . The probe (local laser signal) enters the chip at SOA 2  and is held in destructive interference in the circuit device  10 . The probe is then split between an upper branch  12  and a lower branch  14 . The upper branch  12  being combined with the pre-amplified pump and then split between PD 1  and SOA 5 . The pre-amplified pump changes the phase of the probe signal in SOA 5 , thereby gating the probe signal as a function of the amplified pump signal. Regeneration and wavelength conversion are achieved because cross phase modulation transfers the data in the pump to the probe. The probe signal in lower branch  14  is split between SOA 6  and PD 2 . PD 1  and PD 2  are sensing points at which the combined pump and probe power, and probe power, respectively, may be sensed. Using the sensed optical power, an external electronic control circuitry adjusts the bias currents applied to SOA 1  and SOA 6 .  
         [0013]     When the input power to a P2R device changes, the phase change induced on the MZI arm SOA 5  changes. This leads to a change in the output characteristics of the probe signal. In the disclosed embodiment, the change in the input optical power also causes a change in the photocurrent in the detector PD 1 . Using this photocurrent, and the drive current to the pre-amplifier, the input power to the P2R can be computed. Once the input power is known, the various SOA drive currents of the device are set to pre-computed values for that input power. Variations beyond the operational range of the device are detected as network level power failures and used to trigger restoration. In addition, if the local laser or SOA 2  fails, both the photocurrent monitors PD 1  and PD 2  are affected. Therefore, by observing the photocurrents in different arms of the P2R device, different classes of input failure can be distinguished.  
         [0014]     Besides maintaining the output stable, the present invention has the advantage that, since use is made of an on-chip integrated monitor, devices in accordance with the present invention are more compact than alternatives that use discrete components, such as power taps, to achieve the same effect. In addition, using the residual polarization sensitivity of the gain medium it is possible to observe and compensate for other input properties, such as polarization, which cannot be inferred readily through discrete components.  
         [0015]     Table 1 shows the current density in the input stage and interferometric arms (SOA 5  and SOA 6 ) of the device for different values of input power, along with the photocurrents measured in PD 1  and PD 2 . These relate to a preferred embodiment of P2R devices with monolithically integrated photodetectors.  
         [0016]      FIG. 2  shows the photocurrent as a function of input power for different preamplifier drive current densities in SOA 1 . This graph was obtained by measuring an actual device in operation.  
         [0017]      FIG. 3  shows an output eye diagram at 10 Gb/s for a presently preferred P2R device, derived for an input power of −8.5 dBm. This diagram shows output power as a function of time for superposed positive and negative pulses between logical 1 and logical 0 signal levels. The photocurrent monitoring technique of the present invention is capable of maintaining the output performance not only at the characterized points, but also everywhere in-between. This is demonstrated by changing the input probe power to the device from −8.5 dBm to −6.8 dBm, resulting in the degraded eye (Note the reduction in “Eye S/N”) shown in  FIG. 4 . As a result, the photocurrent in PD 1  changes from 1096 μA to 1077 μA. Using a linear interpolation of PD 1 _new with respect to the input power, the new input pump power was calculated as follows: 
   PD 1 new   =Y×PD 1 −8.5 +(1 −Y )  PD   −5.5    
 Where PD 1   new  is the new value of PD 1  current (i.e. 1077 μA) and PD 1  −8.5 and PD 1  −5.5 are the values of PD 1  current corresponding to −8.5 dBm and −5.5 dBm, respectively. Inserting the value for PD 1   New , one can solve for Y. Then the new input power, P new  can be calculated from: 
   P   new   =Y×P   −8.5 +(1 −Y )  P   −5.5    
 where P −8.5  and P −5.5  are the power levels at −8.5 and −5.5 dBm, respectively. This results in the input being at a power level corresponding to −6.8 dBm. Those skilled in the art will appreciate that this interpolation would normally be done on a computer at the site which has been programmed with the values in Table 1 and  FIG. 2  and which controls the bias currents provided to SOA 1  and SOA 6  in manner well known to those skilled in the art. 
 
         [0018]     Using the set point information in Table 1, the drive set-points of the semiconductor optical amplifier—Mach-Zehnder interferometer circuit  10  (the bias currents to SOA 1  and SOA 6 ) were adjusted to obtain the restored eye diagram shown in  FIG. 5 . The improved “Eye S/N should be noted. At the same time, the output power remained unchanged at −4.4 dBm, thereby maintaining the quality of the output signal from the semiconductor optical amplifier—Mach-Zehnder interferometer circuit  10 . Subsequently, the new input pump power was measured to be exactly −6.8 dBm. This measurement was repeated for a number of set-points between −5.5 dBm and −10.5 dBm with similar results.  
                                                           TABLE 1                           SOA Drive Current Densities and Photocurrents       for Different Input Power Levels                SOA 1   SOA 6   PD1   PD2       Input   Current   current   Photo-   Photo-       Power   Density   Density   current   current       (dBm)   (kA/cm 2 )   (kA/cm 2 )   (μA)   (μA)                    −10.5   9.5   11.7   1144   626       −9.5   7.8   11.61   1101   626       −8.5   6.85   11.53   1096   626       −5.5   4.86   11.34   1057   625       −2.5   3.67   11.21   1032   625       +0.5   3   11.08   1018   621                  
 
         [0019]     It is a feature of one aspect of the present invention that an integrated performance monitoring, maintenance, and restoration triggering mechanism for P2R devices, in additional to being compact and economical, is capable-of maintaining P2R performance over a wide range of input power, and promptly isolating and reporting failure beyond the operational range of the device.  
         [0020]     Although a preferred embodiment of the invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.  
       REFERENCES  
       [0000]    
       
          1. T. Durhuus et al., “All optical wavelength conversion by semiconductor optical amplifiers”, J. Lightwave Technology, Vol. 14, No 6, pp. 942-954, June &#39;96  
          2. G. Lakshminarayana et al., “A new architecture for counter-propagation based photonic regeneration and re-shaping”, OFC 2005  
          3. P. Guerber et al., “Ultimate performance of SOA-based interferometer as decision element in 40 Gbit/s all-optical regenerator”, OFC&#39;02, pp. 17-22