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

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 and horizontal component at port-2.

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 delivered at port-2.

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

A Novel Method of Developing Frequency Encoded

Different Optical Logic Processors Using Semiconductor Optical Amplifier 53

The scheme of conversion of all optical decimal data to frequency encoded binary data works based on the principle of frequency conversion by polarization switches (PSW) and it is explained with the help of Fig.2(a) [Garai S.K.,2010,2011a]. The optical circuit comprises two polarization switches PSW1 and PSW2. Major part of the output beam of PSW1 is applied as the input pump beam of PSW2 and the rest part is coupled with the output beam of PSW2. The probe beam X1 of PSW1 is of frequency ν1 and the probe beam of PSW2 is X2 of frequency ν2.

Fig. 2(a). Optical circuit for converting decimal to frequency encoded binary data

Decimal Number Binary Number in terms of

Table 1. Decimal numbers and their corresponding frequency encoded binary numbers

In the absence of input pump beam 'A', the PSW1 will be in ON state which in turn will suppress PSW2. The least fraction of the output beam of PSW1 of frequency ν1 will appear at the output. In the presence of the input beam A, the PSW1 will be in OFF state which in turn

'0' and '1' 'ν1'

and 'ν2'

**ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>1</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup> ν<sup>2</sup>**

$$\boldsymbol{\rho} = \boldsymbol{\rho}^{\rm TE} - \boldsymbol{\rho}^{\rm TM} = \frac{1}{2} \left[ \frac{\boldsymbol{a}^{\rm TE} \boldsymbol{\Gamma}^{\rm TE} \mathbf{g}^{\rm TE}}{\boldsymbol{v}\_{\rm g}^{\rm TE}} - \frac{\boldsymbol{a}^{\rm TM} \boldsymbol{\Gamma}^{\rm TM} \mathbf{g}^{\rm TM}}{\boldsymbol{v}\_{\rm g}^{\rm TM}} \right] \boldsymbol{L} \tag{3}$$

where L is the length of SOA, *TE <sup>g</sup> v* is the group velocity of the envelop of the optical electric field for TE mode, *TE* is the confinement factor, *TE g* is the real gain function , *TE* is the phase modulation parameter and *TE int* is the modal loss. All the parameters corresponding to superscript TM and TE represent the parameters for TM mode and TE mode of propagation respectively. At the PBS, the two modes coherently combine. If the phase difference is an odd multiple of **,** the angle of rotation of the beam after combination of TE and TM mode (having almost same amplitude) at output end of SOA is */* 2 and then at the output port-2 no beam will appear. In this case the output from port-2 will be suppressed i.e. switched off. Here the induced phase difference is controlled by the power of input pump beam as well as choosing the suitable parameters and length of SOA (intensity > 0.4 mW) [Garai S.K.,2010,2011a].

Thus in the absence of pump beam, probe beam will appear at port-2 (ON-state) and in the presence of the pump beam of specific intensity, the probe beam will be suppressed in port-2(OFF-state). Obviously the state of port-1 will be complementary with respect to port-2 i.e., in the presence of the pump beam, power will develop at port-1.

Fig. 1(c). SOA acting as a polarization switch
