Optical Gates for High-Speed Optical Processing Using Semiconductor-Based Amplifiers

In modern electronics, optical amplifiers play a significant role in many crucial activities ranging from boosting optical signals to long-distance optical networking. They are a critical part of any circuit’s performance by generating high-power optical signals compared to the low-power input they receive. As a result, optical amplifiers can be used not only in the electronics industry, but also in the defense, automobile, farming, and other industries that require high processing speed and efficiency.

To perform any function based on optical processing, it is essential to implement logic gates through which control functions of the amplifiers can be achieved, as depicted in Fig 1.

Fig 1: Schematic of optically controlled gate used for applications based on different set parameters.

All optical gates are realized by optical nonlinearities in both glass and semiconductor materials. A gate used to modulate a CW signal or a pulse train can function as a wavelength converter, or part of an optical regenerator, respectively. Depending on the transfer function of these gates, inverted or in-phase output signals result.

GATE Operations during Conversion of Optical Transmission

Firstly, the experiment was done using XGM gates. XGM gates are optically controlled optical gates that modulate a CW signal (probe) at the desired output wavelength. For this, WDM networks with all-optical cross-connect were a requirement needed for the efficient transport of information. Furthermore, wavelength conversion–translation within the network or at its interfaces was also necessary for efficient dynamic transport reconfiguration. Using converters, assigning wavelengths on a link-by-link or a subnetwork basis was possible, thereby relaxing the requirements for wavelength precision throughout the whole network.

Fig 2: MZI and MI configurations of SOA optical gates.

For the second type of gate operation, a test was considered on the XPM gate. In these gates, the optical input signal controls the phase difference between the interferometer arms through the relation between the carrier density and the refractive index in the SOAs (cross-phase modulation, XPM) For stable operation, the XPM converters must be integrated. An early realization of a two-port Mach–Zehnder structure was based on a back-to-back coupling of the Y-lasers made by Alcatel SEL. Following these early versions of XPM gates, an impressive activity on monolithic integration of interferometric gates has taken place, making these gates one of the test grounds for the monolithic integration of active optical elements. These schematics have been depicted above in Fig 2.

 The differential-control configuration has also been used to demonstrate wavelength conversion at an impressive bit rate of 168 Gb/s, using nonlinear loop mirrors with SOAs. It could also be observed that the trailing edges become steep when the differential scheme is applied.  Gates operated in the differential configuration have successfully been used for time demultiplexing from 160 to 10 Gb/s. Due to this,  the interferometric SOA gates are a crucial component for the evolution of all-optical regenerators and simple signal-processing elements.

FOUR-WAVE MIXING Method In Semiconductor Optical Amplifiers

The FWM process is inherently fast and the gates have the advantage that many WDM channels can be handled simultaneously. There has already been well-established work on wavelength conversion with FWM in SOAs published in 1989. The output signal wavelength depends on both the pump (λp) and the input signal (λi) wavelengths. Therefore, the pump must be tunable even for converters with a fixed output wavelength. As the pumping scheme is relatively complex, there is a good chance that FWM gates will only be used at bit rates above 100 Gb/s. Figure 3 shows the FWM technique in semiconductor optical amplifiers.

Fig 3: Four-wave mixing gate Schematics

In such cases, time demultiplexing could be seen ranging from 100 to 10 Gb/s with FWM gates, with a clock extraction of 6.3 GHz from an incoming signal of 400 Gb/s.

Fully Optical 3-R Regenerators and Logical Gates

When an all-optical network is set up, regenerators play a crucial role as there may be different transmission paths through the system networks. As explained above, the interferometric gates are the ideal choice for optical regeneration due to their 2-R regeneration owing to their non-linear function. Fig 1 demonstrates how regeneration can be possible by making use of signal at the input to the optical clock pulses, for a more refined application of the interferometer, which is further explained in Fig 4.

Fig 4: Improved optical regeneration using the Mach–Zehnder interferometer

Full regeneration can be achieved by using the input signal to gate extracted optical-clock pulses. The optical clock can be extracted optically for combinations with electronic clock recovery units. SOA-based gates can gain data rates for more than 100 Gb/s, which makes it achievable to realize fast optical regenerator units.

Another technique is by using all-optical logic gates for different devices’ optical paths in a network. Here, The XOR gate can be considered the perfect setup since it is the basic and important building block for a number of optical network functions. The function of the XPM gate used as an XOR gate is shown in Fig. 5. High-speed XOR gates based on loop and UNI configurations were taken into review, while on the other hand, integrated versions based on XPM gates were also studied.

Fig 5: Michelson interferometer used as an XOR gate

The above figure 5, shows the Michelson interferometer used as an XOR gate. During the observation, it was noticed that the data signals that were launched, gave rise to a phase modulation Φ of the propagating C-W light in the amplifier. The output of this amplifier leads to the logical XOR of the two inputs of data signals given, with the capability to reach approximately 20 Gb/s.

Conclusion

Semiconductor-based optical amplifiers are critical elements in any signal processing circuit to ensure an efficient and coherent transfer of signals between two points. The gates used (XOM and XGM) were operable in wavelength conversion applications and OTDM demultiplexers at bit rates exceeding 100 Gb/s throughout the experiment.

Even though research on optical amplifiers is still in its inception, this thesis continues to explore ways to improve upon current signal processing capabilities. Further research and development of key components and devices are crucial to attaining bit rates beyond 100 Gb/s and even beyond.