At functional level, the AOWC architecture is based on three main modules acquired from external providers: a Tunable All-optical Signal Regenerator module (TASR), a Voltage-Controlled optical band-pass Filter (VCF), and an Universal Network Controller module (UNC).

Fig. 2. Asymmetric Sagnac Loop schematic

This regenerator module (the biggest subsystem in Figure 1) is able to perform a 2R regeneration, resulting in a CD and PMD mitigation, and also an all-optical wavelength conversion across the entire C-band on the signal received, with a bit-rate, bit-shape and protocol transparency to 12,5Gbps. In the TASR the wavelength conversion is done utilising cross gain modulation (XGM) in a Semiconductor Optical Amplifier (SOA) which is incorporated in an Asymmetric Sagnac fiber Loop (ASL) or Sagnac interferometer. The ASL regenerates the incident signal’s bit-pattern, reduces its noise, increases the extinction ratio, opens the eye-pattern and improves its bits-error rate (BER). Figure 2 shows the schematic for an ASL, where Pin is the input signal at the original wavelength, Pcw is the continuous wave signal at the converted wavelength, PC1 and PC2 are polarization controllers, C1 and C2 are optical couplers, and F is the spectral bandpass filter used to block the undesired signal.

In addition, the TASR module also has an EDFA amplifier inside for optical input signal (received through the blue fiber shown in Figure 3), a C-band tunable laser for the creation of the new wavelength (with 50GHz channel spacing), different points for optical monitoring and a Variable Optical Attenuator (VOA) placed just before the orange fiber (that is connected to the input of an external bandpass filter). From this orange fiber it’s possible to obtain the incoming data signal multiplexed in the incoming wavelength (blue fiber) and also in the new wavelength generated by the internal tunable laser. Moreover, the output of the external bandpass filter is connected to the grey fiber shown in Figure 3; so there is only one wavelength available at this point and it is possible to select the desired one by tuning the external filter. Then, the power of this resulting signal is monitored and, finally, this signal is sent towards the red fiber which is the output optical port of TASR module and represents the global output of AOWC.

This subsystem, shown in gold in Figure 1, consists of a bandpass optical filter, tunable across the whole C-band and designed for 50GHz DWDM channel spacing. The filter is based on the LVF (Linear Variable Filter) standard, comprised of alternating reflective and cavity layers. Within the module there is also a small two-phase DC stepper motor, used for tuning the desired central wavelength by means of the reception of logic sequences of bits from the pins of the module connected to both motor coils. Furthermore, there is a potentiometer within this VCF module, which is useful to determine the position of the multi-cavity filter and also to define the range of travel between a minimum and a maximum wavelength. With this potentiometer it is possible to obtain a reading of an analog voltage that indicates, according to a table provided by the manufacturer, the central wavelength tuned at all times.

This module, placed on the right side of Figure 1 above the Ethernet jack, is based on a NS7520 microcontroller with a 32-bit 55MHz ARM7TDMI core and μClinux operating system. It has a 16Mbytes SDRAM and 8MB Flash memory in which to save the binary (within the directory tree of the OS) corresponding to the software that manages and monitors the other two main modules of the equipment.

The module also has the following ports and interfaces that are used in the designed AOWC: one 10/100 Ethernet interface used to communicate with the remote process which will manage the whole equipment, two 8-bit GPIO general purpose ports available for digital inputs and outputs, and two serial channels which are multiplexed with the corresponding 16 pins of GPIO ports and which are configurable to UART, HDLC and SPI modes.

Within AOWC, the functions of UNC module are:

• Monitor and configure the TASR module through a RS-232 bus, according to the operation mode specified by the remote process.
• Generate the logical sequence of bits from one of the GPIO ports towards the driver of the stepper motor of the VCF module, in order to move it and tune the bandpass filter.
• Read the positions of the tunable filter by means of the reception, from the other GPIO port, of the 8-bit coded value corresponding to the A/D conversion of the reading of the analog voltage provided by the potentiometer located within the VCF module.
• Receive the request frames that arrive from the remote process through Ethernet interface, process them according to the specified operation mode and send the corresponding response frames to this process.

Figure 3 represents the interconnection of these main modules within the designed PCB:

Fig. 3. Interconnection of main modules

As shown in the figure, the interconnection between TASR and VCF modules is just optical and is done through fiber cords.

On one hand, the UNC module is connected to the driver used for the stepper motor of the VCF module through one GPIO port. In this way the driver can receive, in parallel and in the correct sense, the sequence of logical values that is needed for tuning the new central wavelength of the bandpass filter, according to the table given in Figure 3. The driver, which is connected directly to the stepper motor coils, generates the switching of currents corresponding to the generated sequence of logical values. On the other hand, the pin of the potentiometer within the VCF module used to monitor the position of the bandpass filter is connected to the input of an 8-bit ADC. Thus, the UNC module receives the coded value of the bandpass filter position through the other GPIO port.

With regard to the communication between TASR and UNC modules, they are connected through an internal RS-232 bus. During normal operation, that is to say, when the equipment is governed by the UNC module which receives the request frames from the remote process, this serial bus is operating permanently. Otherwise, the internal RS-232 bus can be destroyed as long as necessary and instead a new RS-232 bus can be established between the TASR module and an external PC running a GUI, in order to monitor and configure the TASR module manually.

For normal operation, the equipment can operate in two modes: either 2R regeneration on the optical input signal or 2R regeneration adding an all-optical wavelength conversion (AOWC) of the incoming signal. The mode of operation will be indicated in one of the fields of the received frame from the remote process. The summary of performance for each mode of operation would be:

• 2R operation mode: UNC module configures TASR module parameters with their appropriate values, but keeping the internal tunable laser off. On the other side starts to generate the corresponding logical sequences so that the bandpass filter inside the VCF module is going to be tuned to the same wavelength as the channel of the input signal. After the execution of each step in the motor inside the VCF module, the UNC module reads the new position reached in order to check whether the filter tuning has been completed. This tuning process consists of two phases with different stages: a fast approach to a central wavelength value very close to the desired one within an interest margin, and a fine tuning of the central wavelength by means of an implicit power study of samples contained within that range. Thus, after the completion of all the configuration stages in the equipment, the input signal received by the blue fiber is regenerated in the TASR module and then is sent towards the bandpass filter inside the VCF module through the orange fiber. The bandpass filter in this case allows it to pass and returns it to the TASR module through the grey fiber. Then, the regenerated signal is monitored and finally is sent towards the output of the equipment through the red fiber.

• 2R+AOWC operation mode: TASR parameters are configured by UNC module with the appropriate values for this mode, and in this case the internal laser is tuned to the desired new output channel. On the other side, the UNC module performs the same tuning process for the bandpass filter, but in this case the tuned central wavelength corresponds to the channel to which the internal laser has been tuned. Thus, after finishing all the configuration stages, the input signal received by the blue fiber is regenerated in the TASR module, and a copy of this signal is created inside the ASL but multiplexed to the new wavelength. Both signals are sent to the VCF module through the orange fiber, and the bandpass filter in this case eliminates the signal multiplexed to the original wavelength and allows the new wavelength to pass. Then, the new signal is returned to the TASR module and finally is sent to the output of the equipment.

Some interfaces are available in both front and rear panels of the equipment in order to be controlled and managed by an operator or a remote process.

Figure 4 shows the appearance of this panel. On the left, the LC-LC duplex optical adapter represents the optical interface (Input and Output) between the AOWC and other equipment. The Input port is internally connected to the blue fiber of TASR module, and the Output port is internally connected to the red one. In the figure, different LEDs can be found: some of them useful to indicate the correct state of the converters inside the power subsystem, another LED indicates that TASR module is ready to work, another one represents the link signal and activity of Ethernet interface, and there are also some LEDs that inform about possible alarms related to the TASR module. There are also three pushbuttons on the front panel: the first one is used to enable or disable optical components within TASR module, the second one represents the global reset of the equipment, and by means of the last pushbutton it is possible to choose between the normal operation (in which the internal RS-232 bus is connected between TASR and VCF modules) or the use of external RS-232 bus between TASR module and an external PC. On the right of this third pushbutton, there is a DB9 connector in order to connect the external PC through the external RS-232 bus. Finally, a RJ45 connector is placed on the right side of the panel for the Ethernet communication between UNC module and the remote process that will manage the equipment.

Fig. 4. AOWC front panel

Figure 5 shows that the equipment is directly pluggable to standard 220V/50Hz line voltage through the switched and fused power entry module placed on the left side of this panel. In the central area there are two SC/APC optical adapters that are internally connected to the Input and Output of the bandpass filter within the VCF module. On the right of those adapters there is another LC-LC duplex optical adapter which is internally connected to the orange fiber and to the grey one of the TASR module. Then, according to the block diagram of the equipment shown above in Figure 3, the normal configuration is given through the external connection, by means of fiber cords connections, one side between the Input adapter of VCF module and the Pre-filter port of TASR module, and secondly between the Output adapter of VCF module and the Post-filter port of TASR module. Otherwise, it is possible to override the VCF Filter module and cause a bypass in place by means of a fiber cord connection between Pre and Post-filter ports of TASR module, and finally another external filter can be connected to the equipment overriding the internal one.

Fig. 5. AOWC rear panel

The UNC module of AOWC has been programmed so that the whole equipment can operate and be integrated in the CTTC ADRENALINE testbed®, as a new Optical Networking System that incorporates new functionalities within OXC nodes whose architecture is shown in Figure 6. Nevertheless, the AOWC has the flexibility to be integrated within other DWDM Optical Networks with different kinds of remote processes which behave like hardware controllers. This is achieved by reprogramming the UNC module according to the most appropriate protocol for communicating with the remote process.

Fig. 6. OXC nodes

Fig. 7. UI messages during initialization

Within the framework of the CTTC ADRENALINE testbed®, a proprietary protocol has been defined and implemented for the communication between the AOWC and the remote Optical Link Resource Manager (OLRM) process located inside the control plane of ADRENALINE testbed ®. In this protocol the AOWC always plays the role of Server, listening for requests that arrive from OLRM process, which plays the role of Client. The OLRM, once has sent the request frame to AOWC, keeps waiting for a response frame that has to arrive from the AOWC, indicating whether the request has been performed successfully, or any event has occurred in the system or any error was contained in the own request frame. Finally, a User Interface is accessible through a TCP connection to the appropriate port of AOWC, which shows informative messages about the operation of the equipment; Figure 7 shows an example of messages generated during initialization process after power-up.