# 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 2D-WH/TS OCDMA code carriers' distortion was explored for SOA bias currents of 7 mA, 80 mA, and 250 mA, which provide a gain of 6 dB, 12 dB, and 24 dB, respectively. For this investigation, we used a Kamelian small gain SOA (OPA-20-N-C-FA) with a max gain of 24 dB, saturation output of 9 dBm, and polarisation dependency of 0.6 dBm [15]. The ASE power level was \(-\)26 dBm, a slightly  
Figure 1: Schematic diagram of the experimental setup with illustration of two-dimensional wavelength-hopping time-spreading (2D-WH/TS) code carriers in frequency (FD, top) and time (TD, bottom) domain, respectively. TD—time domain, FD—frequency domain, ps FML—picosecond fiber mode locked laser, OS—optical supercontinuum, DMUX—de-multiplexer, MUX—multiplexer, SOA—semiconductor optical amplifier, DCF—chromatic dispersion compensating fiber. higher value when compared with its counterparts made by Thorlabs and InPhenix Inc. (Livermore, CA, USA) when biased at 500 mA [16; 17]. At the start, the SOA gain recovery time was measured using the technique described in [18]. The results are shown in Figure 2. The shortest measured value 28 ps corresponds to a 250 mA driving current. The code integrity was first observed using an optical spectrum analyzer by comparing the code spectrum before and after traveling through a chromatic dispersion compensating fibre link with an SOA device driven at 175 mA (see Figure 3). This current level was chosen arbitrarily to illustrate a code carriers' redshift by comparing Figure 3a,b. For ease of comparison, the measurements obtained were plotted relative to the signal strength at the SOA input, i.e., a variable optical attenuator (VOA) was used to compensate for the SOA gain. This way, both graphs in Figure 3 can use an identical Y-scale. Figure 3a depicts four wavelength code carriers with 30 ps temporal separation before entering the SOA. The optical average power and carriers' peak power was 4 mW and 57 mW, respectively. Figure 3b shows the impact of the SOA on the code integrity--the code carriers are clearly affected by a 0.73 nm wavelength redshift. A carriers' height variation is also observed. In WDM systems, the effect of non-uniformity of the gain during a stream of pulses passing the SOA is resolved using a Lyot filter [19]. Contrary to the WDM and its regular bit rate, in 2D-WH/TS OCDMA systems, the 2D-WH/TS OCDMA code formation leads to an uneven temporal multi-wavelength code carrier's separation. Using this solution is thus not feasible. Further analyses of Figure 3b reveal that the induced wavelength redshift has led to a code carrier 'loss'. This is explained by inspection of Figure 3. Here, a 2D-WH/TS code composed of four wavelength code carriers is shown before entering the SOA (biased at 175 mA), after the SOA, and then again after passing a \(\lambda_{1}\)-\(\lambda_{4}\) decoder. After passing the \(\lambda_{1}\)-\(\lambda_{4}\) decoder with an unmatched channel spacing of 0.8 nm, the code exits distorted and with three code carries only because of the code carriers' redshift of \(\sim\)0.73 nm. The observed height differences among code carriers results in part from a non-ideal SOA gain flatness in the 1550 nm region. The impact of the SOA on individual wavelength code carriers will now be investigated under three different SOA driving conditions: 7 mA, 80 mA, and 250 mA, representing an SOA gain of +6 dB, +12 dB, and +24 dB, respectively. The results obtained are shown in Figure 4. For reference, Figure 4a shows all four wavelength code carriers at the input of the SOA. To graph the results obtained using an identical Y-axis scale for different SOA gain levels, a VOA was placed after the SOA, but before the OSA. The measurements obtained are shown in red, green, and yellow and represent SOA driving currents of 7 mA, 80 mA, and 250 mA, respectively.