# 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
### Description of Experimental Setup  
The experimental setup for the above investigation is shown in Figure 1. Here, an optical clock of wavelength of 1547 nm, generated by an erbium doped fiber mode locked laser (FMLL) manufactured by PriTel, Inc. (Naperville, IL, USA), has a linewidth of 1.4 nm. The optical clock is driven by an RF synthesizer (Agilent E8257D) at 2.5 GHz. The optical clock pulses then enter an optical supercontinuum (OS) generator, which consists of a 25 dBm erbium doped fiber amplifier (EDFA) and an approximately 1 km long dispersion decreasing fiber, thus producing a 3.2 nm wide optical supercontinuum in the spectral region from 1550 to 1553.2 nm. The 2D-WH/TS code is then generated by an encoder comprising of a \(1\times 4\) DWDM de-multiplexer (DMUX), four adjustable fiber optical delay lines, and a \(4\times 1\) DWDM multiplexer (MUX). Both MUX and DMUX have a channel spacing of 100 GHz (i.e., 0.8 nm). The resulting pulse-width at the MUX output is 7 ps at Full Width at Half Maximum (FWHM). The DMUX spectrally slices the optical supercontinuum into four wavelengths (to be used as code carriers), \(\lambda_{1}\) = 1550.12 nm, \(\lambda_{2}\) = 1550.92 nm, \(\lambda_{3}\) = 1551.72 nm, and \(\lambda_{4}\) = 1552.52 nm. The wavelength separation corresponds to a 100 GHz channel spacing (ITU channels CH34 to CH31). After being appropriately delayed by optical delay lines, the individual code carriers are then recombined by a MUX to form the desired 2D-WH/TS code. The generated code then passes through a 16 km long SMF-28 optical fiber followed by a 2.5 km long matched chromatic dispersion compensating fiber (DCF) module, thus forming an 18.5 km chromatic-dispersion compensated (CDC) fiber link. The SMF-28 fiber has its group velocity dispersion parameter (GVD) \(\beta_{2}=-20\) ps\({}^{2}\)/km. The 2D-WH/TS OCDMA encoder's delay lines, D1, D2, D3, and D4 (see Figure 1), were set to generate 1, 6, 11, and 16 chip delays, respectively. An optical spectrum analyzer (OSA) (Agilent 86146B) with 0.06 nm resolution was used in all measurements.  
It should be noted that a 1 chip delay is defined as Tbit /N, where Tbit is a bit period and N is the number of chips. Here, Tbit/N = 400/53, i.e., \(\sim\)7.5 ps/chip (\(\sim\)133 GHz chip rate). The encoder thus generates a 2D-WH/TS code (1-\(\lambda_{1}\), 6-\(\lambda_{2}\), 11-\(\lambda_{3}\), 16-\(\lambda_{4}\)) having a code carriers' temporal separation of 30 ps \(<\) t (where t is the gain recovery time of the SOA). This means that the SOA-based device does not have enough time for full gain recovery. Similarly, a matched decoder was set to (16-\(\lambda_{1}\), 11-\(\lambda_{2}\), 6-\(\lambda_{3}\), 1-\(\lambda_{4}\)). These settings will guarantee the arrival of all four wavelengths code carriers to arrive at the OCDMA decoder's output at the same time. This enables the formation of an OCDMA autocorrelation peak of the code weight w = 4.  
The impact of the SOA under different driving conditions on code carriers will be investigated next.