Optics is considered a potential candidate for the realization of logic devices, digital optical systems for communication, and computation exploiting its super-fast speed. Optical logic gates also can act on the basis of frequency conversion process of some nonlinear materials. Further, in this chapter, the authors have mentioned the dibit representation technique for reducing bit error problem at the input and output terminals of all optical digital logic circuits and a control input for selecting particular logic operation. Here the authors have proposed frequency encoded all optical dibit-based integrated AND and OR logic gates with control input, where a single circuit acts as both AND logic gate and OR logic gate using the optical switches like reflected semiconductor optical amplifier and add/drop multiplexer.
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For tremendous swift information processing (Ghosh et al., 2012), photon can be treated as better information carrier than electron. Many all optical logic gates (Dey & Mukhopadhyay, 2017), flip-flops (Gayen et al., 2015), bi-stable multivibrators, latches etc. are developed with optics (Amiri et al., 2015). Semiconductor optical amplifier (SOA) (Sarkar et al, 2017) is established as a very promising optical device for conducting many all- optical (Sarkar & Mukhopadhyay, 2017) logical operations (Dutta & Mukhopadhyay, All optical …., 2011). SOA is an optoelectronic device that under suitable operating conditions can amplify an input light signal (Cakmak et al., 2017). The active region in the device imparts gain to an input signal. Three radiative mechanisms are possible in the semiconductor. Incident light of photon of adequate energy can excite a carrier from the valance band to the conduction band in stimulated absorption. This type of process is a lossy process as the incident photon is extinguished. Now, if a photon with appropriate energy is incident on the semiconductor, it causes stimulated recombination between a conduction band carrier and a valance band hole. After this process the carrier loses its energy in the form of a photon. This new stimulated photon will be identical in all aspects to the previous inducing photon with identical phase (Chandra & Mukhopadhyay, 2012), frequency and direction that means a coherent interaction happens. Now, both the incident photon and stimulated photon can go up to additional stimulated transitions. For the sufficiently high injected current a population inversion is formed when the carrier population in the conduction band exceeds population in the valance band. For this time the likelihood of stimulated emission is greater than stimulated absorption. So semiconductor will show the optical gain. In the spontaneous emission process, there is a non-zero probability per unit time that a carrier of conduction band will spontaneously recombine with a hole of valance band and thus produce a photon (Wang & Chen, 2015) with random phase and direction. The spontaneous emissions of photons have a wide range of frequencies. These spontaneously emitted photons are basically a noise and also take part in reducing the carrier population available for the optical gain. This type of spontaneous emission of photons is a direct consequence of the amplification process.
SOA can be used as a super-fast non-linear medium. When photo-excitation increases, absorption reduces and becomes negative for high excitation levels. This causes the changing in refractive index. Slow response associated with carrier recombination is one of the most inconvenient features of the SOA for their ultra-fast signal processing (Dey & Mukhopadhyay, Implementation …., 2018).
For implementing (Dey & Mukhopadhyay, A method of generation …., 2018) the all optical logic and arithmetic devices different types of encoding principle like, spatial encoding (Saha et al., 2017), polarization encoding, frequency encoding (Dutta & Mukhopadhyay, Alternating …., 2011), intensity encoding and phase encoding (Hu et al., 2017) are needed. As the frequency of light beam (Sarkar et al, 2016) remains unaffected and constant after reflection, refraction, absorption etc, the frequency encoding principle (Ghosh et al., 2018) is the most suitable one among all other encoding principles (Zhu et al., 2016).