# Patent

## Title
Integrated performance monitoring, performance maintenance, and failure detection for photonic regenerators

## 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.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
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
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.
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].
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
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
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:
FIG. 1 is a schematic representation of a photodetector integrated semiconductor optical amplifier—Mach-Zehnder interferometer circuit device with circuit monitoring;
FIG. 2 is a graph showing the photocurrent as a function of input power and preamplifier drive current density on SOA1;
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;
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
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
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 SOA1. The probe (local laser signal) enters the chip at SOA2 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 PD1 and SOA5. The pre-amplified pump changes the phase of the probe signal in SOA5, 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 SOA6 and PD2. PD1 and PD2 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 SOA1 and SOA6.
When the input power to a P2R device changes, the phase change induced on the MZI arm SOA5 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 PD1. 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 SOA2 fails, both the photocurrent monitors PD1 and PD2 are affected. Therefore, by observing the photocurrents in different arms of the P2R device, different classes of input failure can be distinguished.
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.
Table 1 shows the current density in the input stage and interferometric arms (SOA5 and SOA6) of the device for different values of input power, along with the photocurrents measured in PD1 and PD2. These relate to a preferred embodiment of P2R devices with monolithically integrated photodetectors.
FIG. 2 shows the photocurrent as a function of input power for different preamplifier drive current densities in SOA1. This graph was obtained by measuring an actual device in operation.
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 PD1 changes from 1096 μA to 1077 μA. Using a linear interpolation of PD1_new with respect to the input power, the new input pump power was calculated as follows:

PD1new=Y×PD1−8.5+(1−Y)PD1−5.5 

Where PD1new is the new value of PD1 current (i.e. 1077 μA) and PD1 −8.5 and PD1 −5.5 are the values of PD1 current corresponding to −8.5 dBm and −5.5 dBm, respectively. Inserting the value for PD1New, one can solve for Y. Then the new input power, Pnew can be calculated from:

Pnew=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 SOA1 and SOA6 in manner well known to those skilled in the art.

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 SOA1 and SOA6) 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/cm2)
(kA/cm2)
(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









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.
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


    1. T. Durhuus et al., “All optical wavelength conversion by semiconductor optical amplifiers”, J. Lightwave Technology, Vol. 14, No 6, pp. 942-954, June '1996
    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'02, pp. 17-22

## Claims

1. A method for monitoring the performance of an integrated photonic regenerator device, including a first semiconductor optical amplifier (SOA) receiving an input signal and providing an amplified output signal and a second SOA receiving as an input at least a portion of the amplified output signal, the method comprising:
sensing photocurrent in the second SOA;
utilizing the value of the current as a measure of the performance photonic regenerator device; and
utilizing the value of the current to control a bias current applied to the first SOA.

2. A method for monitoring the performance of an integrated photonic regenerator device, including a first semiconductor optical amplifier (SOA) receiving an input signal and providing an amplified output signal and a second SOA receiving as an input at least a portion of the amplified output signal, the method comprising sensing photocurrent in the second SOA and utilizing the value of the current as a measure of the performance photonic regenerator device,
wherein the value of the current is processed to obtain an estimate of the input power to the first SOA, and
wherein the value of the current is processed in conjunction with a bias current applied to the first SOA.

3. A method for monitoring the performance of an integrated photonic regenerator device, including a first semiconductor optical amplifier (SOA) receiving an input signal and providing an amplified output signal and a second SOA receiving as an input at least a portion of the amplified output signal, the method comprising sensing photocurrent in the second SOA and utilizing the value of the current as a measure of the performance photonic regenerator device,
wherein the value of the current is processed to obtain an estimate of the input power to the first SOA,
and wherein the method further comprises utilizing the estimate of the input power to control a bias current applied to the first SOA.

4. A method for monitoring the performance of an integrated photonic regenerator device, including a first semiconductor optical amplifier (SOA) receiving an input signal and providing an amplified output signal and a second SOA receiving as an input at least a portion of the amplified output signal, the method comprising sensing photocurrent in the second SOA and utilizing the value of the current as a measure of the performance photonic regenerator device,
wherein the method is performed with a device receiving a probe input and having a third SOA receiving a portion of the probe input, the method further sensing photocurrent in the third SOA and utilizing the value of that current together with the value of the second SOA photocurrent as a measure of the performance of the photonic regenerator device.

5. The method of claim 4 further comprising utilizing the value of photocurrent in the third SOA together with the value of the second SOA photocurrent to control a bias current applied to the first SOA.

6. The method of claim 5 wherein the value of the second SOA photocurrent is processed to obtain an estimate of the input power to the first SOA.

7. The method of claim 6 utilized in a device having a fourth SOA, the bias current to at least one of the first and fourth SOAs being controlled by values of the second and third SOA photocurrents which are controlled so as to maintain a predetermined relationship between a value of one of the bias currents and the estimate of the input power to the first SOA.

8. The method of claim 4 utilized in a device having a fourth SOA and further comprising utilizing the value of photocurrent in the third SOA together with the value of the second SOA photocurrent to control a bias current applied to the fourth SOA.

9. The method of claim 4 wherein the value of the second SOA photocurrent is processed to obtain an estimate of the input power to the first SOA.

10. The method of claim 9 utilized in a device having a fourth SOA, the bias current to at least one of the first and fourth SOAs being controlled by values of the second and third SOA photocurrents which are controlled so as to maintain a predetermined relationship between a value of one of the bias currents and the estimate of the input power to the first SOA.