**2.1 Frequency conversion exploiting Nonlinear Polarization Rotation (NPR) of the probe beam in SOA**

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

A Novel Method of Developing Frequency Encoded

Fig. 1(b). SOA based add/drop multiplexer

**2.3 Action of polarization-switch (PSW)** 

and horizontal component at port-2.

modes [Dorren H.J.S.,et-al.,2003] given by

delivered at port-2.

Different Optical Logic Processors Using Semiconductor Optical Amplifier 51

The principle of polarization SOA–gain saturation property may be exploited to design the nonlinear polarization switching (PSW) [Dorren H.J.S.,et-al.,2003; Garai S.K., Mukhopadhyay S.(2010); Garai S.K.,2011a]. The scheme of the polarization switching is shown in Fig.1(c). It is consisting of two laser sources having different frequencies, three polarization controllers, one strained bulk SOA, one polarization beam splitter (PBS), an attenuator and a power meter. The probe beam is a CW laser of frequency ν1 whereas the pump laser beam is a highly intense beam of frequency ν2. The state of polarization of the probe beam, pump beam and output beam of SOA are controlled by polarization controllers PC1, PC2 and PC3 respectively. The probe beam is fed to one input terminal of SOA via an attenuator so that the input probe beam power injected to the SOA be very low (-15 dBm) and it confirms the operation of SOA in the linear regime under the action of probe beam alone. The orientation of linearly polarized probe beam is adjusted by PC1 in such a way that the polarization direction of the input probe beam be approximately 450 to the orientation of SOA layer. The output beam of SOA is combined by means of polarization beam splitter (PBS). The PBS is used to split the SOA output into horizontal (H) and vertical polarization component (V). The vertical component of SOA output is obtained at port-1

In the absence of pump beam, the optical field of linearly polarized probe beam may be decomposed into a transverse electric field (TE) and transverse magnetic field (TM) components. These two modes propagate through SOA independently and amplify by the biasing current in SOA. The biasing current is set to such a value (162 mA) that the maximum gain is obtained for TE and TM modes which are almost equal. Under this situation the state of polarization of output beam of SOA is oriented in such a way by PC3 that the beam at the output port-1 becomes zero (it is measured by power meter) i.e. vertical component (V) of the output beam of SOA is absent and obviously maximum power is

SOA have the property of polarization dependent gain saturation. Therefore, in the presence of highly intense pump beam the polarization dependent gain saturation character give rise to different refractive index change for TE and TM. Under gain saturation condition the output of port-2 will be a function of saturation-induced phase difference between two

At first the SOA is to be biased with suitable current and also the power level of input pump beams 'A' and 'B' are to be adjusted properly. 'X' is the linearly polarized probe beam of frequency but weak in intensity and, it is coupled with the pump beams in SOA. The scheme is shown in Fig.1(a).

Fig. 1(a). Frequency conversion by SOP of probe beam

In the absence of both the input pump beams 'A' and 'B', the polarizer is adjusted in such a way that the pass axis of the polarization beam splitter (PBS) is crossed with respect to SOP of the linearly polarized probe beam(X). For this setting no light is obtained at output end (D). One input pump beam (A/B) alone does not change the SOP of the probe beam dramatically and no light will pass through the PBS. When both the input pump beams 'A', 'B' are present, the state of polarization of the probe beam will change drastically and as result a considerable amount of light of frequency ν will pass through the PBS and will appear the output end 'D'. It is to be noted that only one pump beam of intensity equal to the sum of the intensity of both the pump beams A and B can also rotate the state of polarization of the probe beam in SOA and therefore with the help of the control beam (pump beam) of such intensity it is possible to transmit the probe beam from input end to output end of an SOA.
