## 4 Comparison of Simulation with Experimental Results  
The time-domain representation of the OCDMA USER-1 code is shown in Figure 3a. The experimentally obtained OCDMA auto-correlation that was recovered by the OCDMA decoder after OCDMA code travelled \(L=19.5\) km at room temperature \(T=20\)\({}^{\circ}\)C is shown in Figure 3b. The measured FWHM width was found to be 12 ps. This experiment was then repeated for \(T=50\)\({}^{\circ}\)C and the recovered OCDMA auto-correlation is shown in Figure 3c. We can see that the FWHM width has been broadened to 13 ps by this 30 \({}^{\circ}\)C increase in temperature. The corresponding results that are based on our simulations are shown in Figure 4. Figure 4a shows the simulated OCDMA auto-correlation envelope \(S_{\text{L=19.5;T=20}}\) that was obtained when the fibre temperature was \(T=20\)\({}^{\circ}\)C and the code propagation distance \(L=19.5\) km. The calculated FWHM value is 12 ps, which is identical to the value that was obtained in our experiment (see Figure 3b). Similarly, simulations of the recovered OCDMA auto-correlation envelope \(S_{\text{L=19.5;T=50}}\) for the temperature \(T=50\)\({}^{\circ}\)C is shown in Figure 4b. Here, the calculated FWHM value is 12.8 ps and it is in very good agreement with the experimentally observed 13 ps seen in Figure 3c. Figure 4: Simulations of the recovered OCDMA auto-correlation \(S_{\text{L,T}}\) envelope by the OCDMA decoder: **(a)** for \(L=19.5\) km and \(T=20\)\({}^{\circ}\)C the FWHM width \(=12\) ps; **(b)** for \(L=19.5\) km and \(T=50\)\({}^{\circ}\)C the FWHM width \(=12.8\) ps; and, **(c)** for \(L=40\) km and \(T=80\)\({}^{\circ}\)C the FWHM width \(=16.7\) ps. Figure 3: **(a) OCDMA encoder output showing a user-1 (1-\(\lambda_{2}\), 21-\(\lambda_{4}\), 24-\(\lambda_{1}\), 39-\(\lambda_{3}\)) 2D-WH/TS OCDMA code 400 ps long as seen by a bandwidth limited oscilloscope with 64 GHz optical sampling head; **(b)** Recovered OCDMA autocorrelation by the OCDMA decoder at the room temperature \(T=20\)\({}^{\circ}\)C (FWHM width \(=12\) ps); and, **(c)** at \(T=50\)\({}^{\circ}\)C (FWHM width \(=13\) ps), respectively. **  
To further emphasise the severity of detrimental effects of temperature induced fiber dispersion, Figure 4c shows the recovered OCDMA auto-correlation envelope \(S_{\text{L=40};\text{T=80}}\) simulated for a fiber length \(L\) = 40 km when exposed to the temperature \(T\) = 80 \({}^{\circ}\)C. The corresponding FWHM value is 16.7 ps. Our next step was to experimentally investigate a possibility of using the SOA to restore the OCDMA auto-correlation width that is affected by fibre temperature increases to its original value that was observed at room temperature, \(T\) = 20 \({}^{\circ}\)C. To conduct this investigation, we inserted an SOA (OPA-20-N-C, Kameian, Glasgow, UK) at the transmission site between point A and B (see Figure 1). The role of the SOA will be to control the incoming chirp of the code carriers passing through the SOA by changing the bias current, \(I\). This technique is known as a pre-chirping. Figure 5 shows the experimentally obtained OCDMA auto-correlations that were recovered by the OCDMA decoder when the OCDMA code first passed the biased SOA and then travelled \(L\) = 19.5 km in the transmission link that was exposed at temperature \(T\) = 50 \({}^{\circ}\)C. The OCDMA auto-correlations that are shown in Figure 5a-c were obtained for SOA bias currents \(I\) = 42 mA, 14 mA, and 8 mA, respectively, and resulted in respective FWHM values of 14 ps, 13 ps, and 12 ps. The loss/gain of SOA w.r.t. the various bias currents have been shown in Figure 6.  
Because transmitting low signal levels can affect the system's performance and bit error rate (BER), an EDFA-1 on the transmitter site (see Figure 1) was used to amplify the signal.