**1. Introduction**

46 Selected Topics on Optical Amplifiers in Present Scenario

Yang X., Weng Q. and Hu W. (2010). High-speed, all-optical XOR gates using

*Optoelectron. China*, vol. 3, no. 3, pp.245–252, ISSN 1674-4128.

semiconductor optical amplifiers in ultrafast nonlinear interferometers. *Front.* 

To implement different digital processors in optical domain, encoding and decoding of optical data are the prime issues. Till now several encoding/decoding techniques have been reported for representing the optical information. In this connection spatial encoding [Toyohiko Y., 1986], intensity encoding [Mukhopadhyay S.,et-al., 2004], polarization encoding [Awwal A.A.S., et-al., 1990; Zaghloul Y.A., et-al., 2006, 2011], phase encoding [Chakraborty B., et-al., 2009] etc. may be mentioned. But these coding processes have some inherent problems. In spatial encoding, two specific pixcells, each having two different types of opaque and transparent sub-cells distribution are encoded either as '1' and '0'states respectively in 2-D plane. Here input signal bits are generated by electro-optic/electronic switching (with suitable nonlinear materials) which limits the speed of processing. Again in pixels based operation, interference and diffraction effect may change the expected result of the image pattern at the output end which leads to bit error problem. Moreover, as output result is obtained using decoding mask, and the encoding and decoding technologies not being the same, therefore it is not possible to design sequential or combinational logic circuit using spatial encoding technique. In intensity encoding, presence of optical signal or the intensity of a signal greater than that of a specific reference intensity have been encoded as '1' state and absence of signal or the intensity of a signal lower than that of a specific reference intensity have been encoded as '0' state. But for long distance communication, intensity of optical signal may fall and dropdown below the reference level and for which the '1' state may be treated as '0'state of the signal which can also lead to the bit error problem. In most of the cases the all-optical logic gates are implemented by non-linear materials extending its 2nd order of nonlinearity. This material sends the light passing through it in different channels if the intensity of light varies. So the change of a prefixed value of intensity creates some major problems in channel selection and therefore this intensity based encoding principle is problematic. In intensity based refractive index variation technology, small fluctuation of intensity of the input beams may collapse the total set up. In polarization encoding, one specific state of polarization of the optical beam is encoded as '0' state and another specific orthogonal state of polarization is treated as '1'

A Novel Method of Developing Frequency Encoded

in section-5 and conclusion is drawn in section-6.

**probe beam in SOA** 

**2. Some important functions of SOA as the elements of optical processor 2.1 Frequency conversion exploiting Nonlinear Polarization Rotation (NPR) of the** 

rotation in terms of intensity difference. The mechanism is explained below.

One of the important properties of SOA is non- linear polarization rotation of the probe beam due to optically induced nonlinear refractive index in a bulk SOA by highly intense pump beams [Guo L.Q., Connelly M.J.(2005),(2006),(2007); Dutta N.K. et.al.,2006, Liu Y., et.al.,2003, Fu S. et.al.,2007 ]. During the interaction of the intense pump beam with probe beam in nonlinear SOA, the intense pump beam can modify the optical properties of the SOA which, in turn modify the intensity of probe beam as well as its SOP. If a linearly polarized light is coupled in a SOA, after leaving the SOA its SOP will change. A polarization beam splitter (PBS) at the output end can detect the nonlinear polarization

Different Optical Logic Processors Using Semiconductor Optical Amplifier 49

gates[Connelly M.J.(2002); Dutta N.K. et.al.,2006, Asghari M.,et-al.,1997; Soto.H.,et.al.,1999; Guo L.Q., Connelly M.J.(2007) ]. The wavelength conversion by XGM is accompanied by large chirp and low extinction ratio with restricted speed (limited by the carrier recovery time) up to 40Gbit/s and even up to 100Gbit/ with some degradation. On the other hand the XPM schemes enable wavelength conversion with lower signal powers, reduced chirp, enhanced extinction ratios and ultra fast speed of switching. The wavelength conversion by FWM is very promising one due to its independent modulation format as well as dispersion compensation property with ultra speed However, it has polarization sensitive low wavelength conversion efficiency. Wavelength conversion based on cross polarization modulation (XPolM) is another promising approach. In XPolM process, nonlinear polarization rotation (NPR), an optically induced birefringence and dichroism property of an SOA have been exploited for wavelength conversion and it has drawn the attention of scientists and technologists [Guo L.Q., Connelly M.J.(2005,2006); Lacey J.P.R., et.al.,1998] Very recently all-optical wavelength (both up and down) conversion has realized exploiting non-linear properties of SOA [Guo L.Q., Connelly M.J.(2008)]. In this chapter the author has presented a novel method of developing all optical logic processor exploiting frequency conversion and switching character of SOA. The author has organized the chapter in brief based on his established works and actually it is a review one with some modifications. The Chapter covers (a) a method of generating all-optical decimal data to frequency encoded binary data (b) a method of developing all optical frequency encoded binary logic gates such as AND, OR, NAND, NOR, EX-OR and finally (c) an all optical memory unit, and all of these are the integral part of the all-optical logic processors and these are developed exploiting different attractive features of SOA. The author has exploited here the principle of nonlinear rotation of the state of polarization (SOP) of the probe beam in semiconductor optical amplifier for the frequency conversion as well as for the switching purpose and this type of switching is so called polarization switching (PSW). The chapter is organized as follow: In section-2, the author has presented the basic principle of frequency conversion using nonlinear polarization rotation (NPR) of the probe beam in SOA, principle of channel (frequency) routing by optical add/drop multiplexer and the action of polarization switch made of SOA. Section-3 covers the method of all-optical decimal to frequency encoded binary data generation. Method of developing frequency encoded alloptical logic units are presented in section-4. An all optical binary memory unit is presented

state. Again, the state of polarization may change for several causes which can also lead to the bit error in information processing. In phase encoding, one specific phase of the optical beams is encoded as '0' state and another specific phase is treated as 1 state. But it is very difficult to maintain the constant phase relationship throughout the optical signal processing, specially, beyond the coherent length. Similarly the other coding norms may extend some other limitations in wide range data processing.

In contrast to the above mentioned encoding, the author has established the frequency encoded technique to represent the Boolean logic states. It is known that if '1' and '0' logic states are encoded by two different frequencies in optical domain, then one may ensure about the state of a signal during data transmission. If '0' state is encoded by the frequency 'ν1'and the '1'state by the other frequency 'ν2' then 'ν1' and 'ν2' will normally remain unaltered throughout the transmission of data. The frequency encoded technique offers so many advantages [Garai S.K., Mukhopadhyay S.(2009),2010; Garai S.K.(2010); Garai S.K.(2011),2011a,2011b]. The prime beauty of the frequency encoding is that, frequency is the fundamental property of the wave and it can preserve its identity irrespective of the absorption, reflection, transmission during its propagation throughout the communicating media. This is the most potential advantage of the frequency encoding technique over other conventional encoding techniques. In addition, frequency encoding in optical domain uses the spectrum of a broadband optical source and can accommodate a large pool of subscribers. Moreover different signals are characterized by different specific frequencies in optical domain and if one signal of specific frequency can be encoded to represent a specific state of information, then using different signals of different frequency, other different states can be encoded. Thus a larger number of states of information can be accommodated which can propagate through the same channel i.e., through the same optical fiber without interference or cross-talk. Again using frequency encoding it is easier to represent multi-bit states of information which are very useful for conducting multivalued logic operations using wavelength conversion properties of different high speed nonlinear optical switches such as semiconductor optical amplifier, periodically poled lithium niobate (LiNbO3) waveguide, Chalkogenide glass etc. Since the information is frequency encoded, therefore the coded signal is very useful for optical wavelength division multiplexing (WDM), frequency division multiplexing (FDM) and combination of WDM and time division multiplexing (TDM) in interconnection of telecommunication networks.

Basic building blocks required to implement the frequency encoded optical logic processors are the Frequency Router Unit (F.R.U) and Frequency Converter Unit (F.C.U) and SOA is found to be the very promising in this aspect. A rapid growth in the optical fibre communication was noticed over the last thirty years exploiting the enormous bandwidth property and many other characters of optical fibre. The massive advancement of optical technology has been made possible because of several reasons. In this regard it can be specially mentioned that Semiconductor Optical Amplifier (SOA) is a promising optical device that help a lot for the acceleration of advancing the network systems in communication. SOAs are highly nonlinear in an optical gain range. This is due to the consequence of a large number of free carriers confined in a small active region and it affects the gain as well as refractive index within the active region. The SOA nonlinear properties such as cross gain modulation (XGM), cross phase modulation (XPM), four-wave mixing (FWM) have been studied several times and are applied to implement wavelength conversion, optical division multiplexing-demultiplexing, clock recovery, and optical logic

state. Again, the state of polarization may change for several causes which can also lead to the bit error in information processing. In phase encoding, one specific phase of the optical beams is encoded as '0' state and another specific phase is treated as 1 state. But it is very difficult to maintain the constant phase relationship throughout the optical signal processing, specially, beyond the coherent length. Similarly the other coding norms may

In contrast to the above mentioned encoding, the author has established the frequency encoded technique to represent the Boolean logic states. It is known that if '1' and '0' logic states are encoded by two different frequencies in optical domain, then one may ensure about the state of a signal during data transmission. If '0' state is encoded by the frequency 'ν1'and the '1'state by the other frequency 'ν2' then 'ν1' and 'ν2' will normally remain unaltered throughout the transmission of data. The frequency encoded technique offers so many advantages [Garai S.K., Mukhopadhyay S.(2009),2010; Garai S.K.(2010); Garai S.K.(2011),2011a,2011b]. The prime beauty of the frequency encoding is that, frequency is the fundamental property of the wave and it can preserve its identity irrespective of the absorption, reflection, transmission during its propagation throughout the communicating media. This is the most potential advantage of the frequency encoding technique over other conventional encoding techniques. In addition, frequency encoding in optical domain uses the spectrum of a broadband optical source and can accommodate a large pool of subscribers. Moreover different signals are characterized by different specific frequencies in optical domain and if one signal of specific frequency can be encoded to represent a specific state of information, then using different signals of different frequency, other different states can be encoded. Thus a larger number of states of information can be accommodated which can propagate through the same channel i.e., through the same optical fiber without interference or cross-talk. Again using frequency encoding it is easier to represent multi-bit states of information which are very useful for conducting multivalued logic operations using wavelength conversion properties of different high speed nonlinear optical switches such as semiconductor optical amplifier, periodically poled lithium niobate (LiNbO3) waveguide, Chalkogenide glass etc. Since the information is frequency encoded, therefore the coded signal is very useful for optical wavelength division multiplexing (WDM), frequency division multiplexing (FDM) and combination of WDM and time division

extend some other limitations in wide range data processing.

multiplexing (TDM) in interconnection of telecommunication networks.

Basic building blocks required to implement the frequency encoded optical logic processors are the Frequency Router Unit (F.R.U) and Frequency Converter Unit (F.C.U) and SOA is found to be the very promising in this aspect. A rapid growth in the optical fibre communication was noticed over the last thirty years exploiting the enormous bandwidth property and many other characters of optical fibre. The massive advancement of optical technology has been made possible because of several reasons. In this regard it can be specially mentioned that Semiconductor Optical Amplifier (SOA) is a promising optical device that help a lot for the acceleration of advancing the network systems in communication. SOAs are highly nonlinear in an optical gain range. This is due to the consequence of a large number of free carriers confined in a small active region and it affects the gain as well as refractive index within the active region. The SOA nonlinear properties such as cross gain modulation (XGM), cross phase modulation (XPM), four-wave mixing (FWM) have been studied several times and are applied to implement wavelength conversion, optical division multiplexing-demultiplexing, clock recovery, and optical logic gates[Connelly M.J.(2002); Dutta N.K. et.al.,2006, Asghari M.,et-al.,1997; Soto.H.,et.al.,1999; Guo L.Q., Connelly M.J.(2007) ]. The wavelength conversion by XGM is accompanied by large chirp and low extinction ratio with restricted speed (limited by the carrier recovery time) up to 40Gbit/s and even up to 100Gbit/ with some degradation. On the other hand the XPM schemes enable wavelength conversion with lower signal powers, reduced chirp, enhanced extinction ratios and ultra fast speed of switching. The wavelength conversion by FWM is very promising one due to its independent modulation format as well as dispersion compensation property with ultra speed However, it has polarization sensitive low wavelength conversion efficiency. Wavelength conversion based on cross polarization modulation (XPolM) is another promising approach. In XPolM process, nonlinear polarization rotation (NPR), an optically induced birefringence and dichroism property of an SOA have been exploited for wavelength conversion and it has drawn the attention of scientists and technologists [Guo L.Q., Connelly M.J.(2005,2006); Lacey J.P.R., et.al.,1998] Very recently all-optical wavelength (both up and down) conversion has realized exploiting non-linear properties of SOA [Guo L.Q., Connelly M.J.(2008)]. In this chapter the author has presented a novel method of developing all optical logic processor exploiting frequency conversion and switching character of SOA. The author has organized the chapter in brief based on his established works and actually it is a review one with some modifications.

The Chapter covers (a) a method of generating all-optical decimal data to frequency encoded binary data (b) a method of developing all optical frequency encoded binary logic gates such as AND, OR, NAND, NOR, EX-OR and finally (c) an all optical memory unit, and all of these are the integral part of the all-optical logic processors and these are developed exploiting different attractive features of SOA. The author has exploited here the principle of nonlinear rotation of the state of polarization (SOP) of the probe beam in semiconductor optical amplifier for the frequency conversion as well as for the switching purpose and this type of switching is so called polarization switching (PSW). The chapter is organized as follow: In section-2, the author has presented the basic principle of frequency conversion using nonlinear polarization rotation (NPR) of the probe beam in SOA, principle of channel (frequency) routing by optical add/drop multiplexer and the action of polarization switch made of SOA. Section-3 covers the method of all-optical decimal to frequency encoded binary data generation. Method of developing frequency encoded alloptical logic units are presented in section-4. An all optical binary memory unit is presented in section-5 and conclusion is drawn in section-6.
