**2.2 Function of an add/drop multiplexer**

Input optical data signals may be of different frequencies and these data signals can be directed through separate paths using add/drop multiplexer (ADM) [Yu S., et.al.,2005; Jiang Y., et.al,2010]. The function of an 'ADM' is to separate a particular channel without interference from adjacent channels. This can be achieved by using an integrated 'SOA' with a tunable filter with it. The filter can be tuned at different frequencies by changing the bias current of SOA. The selected channel is reflected by the filter, amplified a second time by the Multi Quantum Well (MQW) section and extracted to a drop port by means of a circulator. The remaining channels pass through the filter section without any drop. In Fig.1(b) the frequency 'ν2' is extracted from incoming signals of frequencies ν 1, ν 2,ν 3 ..... ν N using drop multiplexer and also added to the next step by means of add multiplexer using another circulator.

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

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

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

Input optical data signals may be of different frequencies and these data signals can be directed through separate paths using add/drop multiplexer (ADM) [Yu S., et.al.,2005; Jiang Y., et.al,2010]. The function of an 'ADM' is to separate a particular channel without interference from adjacent channels. This can be achieved by using an integrated 'SOA' with a tunable filter with it. The filter can be tuned at different frequencies by changing the bias current of SOA. The selected channel is reflected by the filter, amplified a second time by the Multi Quantum Well (MQW) section and extracted to a drop port by means of a circulator. The remaining channels pass through the filter section without any drop. In Fig.1(b) the frequency 'ν2' is extracted from incoming signals of frequencies ν 1, ν 2,ν 3 ..... ν N using drop multiplexer and also added to the next step by means of add multiplexer using another

but weak in intensity and, it is coupled with the pump beams in SOA. The

frequency

output end of an SOA.

circulator.

**2.2 Function of an add/drop multiplexer** 

scheme is shown in Fig.1(a).

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