# Investigation of 2D-WH/TS OCDMA Code Stability in Systems with SOA-Based Device
## 2 Impact of SOA-Based Devices Deployed in Fiber Link on Multi-Wavelength Picosecond Code Carriers
### Investigation of 2D-WH/TS OCDMA Code Carriers' Distortion under Different SOA Driving Conditions
The SOA-induced redshift on individual code carriers \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), and \(\lambda_{4}\) is shown in Figure 4b-d, respectively. Figure 4f shows how the entire code spectrum is affected by the SOA biased at 250 mA/24 dB gain (i.e., under the shortest gain recovery time \(\sim\)30 ps (see Figure 2)). Figure 2: SOA’s gain recovery time against the bias current. In the case of 7 mA/6 dB gain, all four code carriers exhibit slight linewidth and spectral shape variations after passing the SOA and the 18.5 km long chromatic-dispersion compensated fiber link. In the case of 80 mA/12 dB gain, the carriers' spectra are affected by self-phase modulation (SPM). Spectral peaks shift towards the longer wavelengths (redshift), leaving a noticeable 'trace' in the original spectral pulse position. The amount of shifting is mainly dominated by the SOA gain  
Figure 4: Experimental demonstration of code carriers’ wavelength redshift observed on the optical spectrum analyser: (**a**) code carriers at the input of the SOA; (**b**) effect of SOA on code carrier \(\lambda_{1}\) at an SOA current of 7 mA/6 dB gain, 80 mA/12 dB gain, and 250 mA/24 dB gain, respectively; (**c**) similarly for \(\lambda_{2}\); (**d**) for \(\lambda_{3}\); (**e**) for \(\lambda_{4}\); (**f**) illustration of redshift on all four wavelength code carriers for an SOA current of 250 mA/24 dB gain. Figure 3: (**a**) Optical spectrum of the 2D-WH/TS OCDMA code based on four \(\lambda_{1}\) to \(\lambda_{4}\) multi-wavelength picosecond code carries before entering the SOA biased at \(\sim\)175 mA; (**b**) after passing the SOA followed by dispersion compensated fiber link (DCF) and \(\lambda_{1}\)–\(\lambda_{4}\) decoder. VOA—variable optical attenuator, OFL—optical fiber lin. and carrier density changes, causing noticeable variations in the refractive index [12]. Consequently, an SPM-induced frequency chirp is imposed on all spectral pulses as they pass the SOA [20]. In the third case of 250 mA/24 dB gain, the SOA is in its highly nonlinear regime and the pulse spectrum shifts further to the red side in addition to one or more new peaks on the blue side. These observations agree with [20]. The achieved redshift was \(\sim\)100 GHz/0.8 nm. As explained above, a matched OCDMA decoder and encoder pair are required to accurately recover the user's data and produce an OCDMA autocorrelation peak. Based on the results in Figure 4f, this would not be possible. It can be seen that the 100 GHz spectral redshift affected all four wavelength code carriers: \(\lambda_{1}\) became \(\lambda_{2}\), \(\lambda_{2}\rightarrow\lambda_{3}\), \(\lambda_{3}\rightarrow\lambda_{4}\), and \(\lambda_{4}\rightarrow\lambda_{5}\). However, the matching 100 GHz lambda DMUX in the OCDMA decoder (see Figure 1) was designed to handle only (\(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\)), thus it cannot recognize the newly 'created' wavelength \(\lambda_{5}\) resulting from the \(\lambda_{4}\rightarrow\lambda_{5}\) wavelength redshift. As a consequence, the number of wavelengths forming the OCDMA autocorrelation peak at the decoder output will be reduced from four to three. Furthermore, a slightly unequal amount of redshift imposed on individual wavelength code carriers is also observed. This is a result of the fact that the SOA gain is not flat and peaks at around 1550 nm [15]. Thus, multi wavelength code carriers passing the SOA experience slightly different saturation rates. This leads to a slightly different SPM, and thus amount of redshift. This was confirmed by the measurements shown in Figure 5.  
The measured amount of the code carriers' redshift inflicted by the SOA on individual code carriers can be also found theoretically. The nonlinear effect induced by the SOA can be simulated by adopting the model described in [21]. A great deal of consistency (see Figure 5) was found between the simulated and measured results.