Chapter 3: Nonlinear Optical Thresholding Using Two-Photon Absorption
3.4 Two-Channel OCDMA System using TPA-Based Detection
To determine the feasibility of a TPA-based optical thresholder and detector in an OCDMA system, a simple two-channel back-to-back OCDMA system was investigated in [17]. This OCDMA system employed a temporal phase coding scheme using fibre Bragg gratings (FBGs). The decoded signal was detected using either a standard linear detector or a TPA- based detector with the output electrical signal from both devices then being analysed.
3.4.1 Experimental Setup
The experimental setup of the two-channel OCDMA system is shown in Figure 3.9. Optical pulses with a duration of 28 ps were generated at repetition rate of 10 GHz at a wavelength of 1561 nm. This operating wavelength corresponds to the operational wavelength of the FBGs used for the encoding/decoding of the optical signal. This optical pulse train was gated down to 155 MHz and encoded with a PRBS data signal before being split by a 50 : 50 optical coupler. The first arm of the OCDMA encoding stage consists of a 150 m reel of single-mode fibre (SMF), an EDFA, an optical circulator and a super-structured fibre Bragg grating (SSFBG) optical encoder. The SMF was used to ensure that the two copies of the signal that propagate through the first and second arms of the encoding stage are uncorre- lated and are not coherently interfering when recombined. The encoding/decoding process was achieved using SSFBGs, with the SSFBG encoders applying two 31-chip, 40 Gchip/s quaternary phase shift keyed codes, similar to those presented in [18]. The second arm of the encoding stage consists of an optical delay line (ODL), a second EDFA, an optical cir- culator and a second SSFBG for channel two. The ODL allows the relative timing between the two codes to be adjusted at the output of the transmitter. The two encoded signals were then combined before entering the decoding stage. The decoder stage used in the experi- ment consisted of an EDFA and a single SSFBG, with the SSFBG designed to match that which is used to encode data for channel one. Once decoded, the signal is then detected using either linear detection or TPA-based detection.
The optical receiver consists of two detection subsystems, a nonlinear thresholder and de- tector and a linear detector. The output signals from these detectors are passed to a digital communications analyzer (DCA) which allowed the production of optical eye diagrams.
Figure 3.9: Experimental setup of a two-channel OCDMA system employing both linear and TPA-based detection.
The output of the optical decoder enters an inline power meter/attenuator which allowed the average power of the optical signal to be constantly monitored. The nonlinear thresh- olding and detection subsystem consisted of an EDFA, polarization controller, a 1.3 µm laser diode (InGaAsP, bandwidth ∼1 GHz) acting as a TPA detector and a low noise RF amplifier. The linear detector used was the optical input of the DCA. To match the band- widths of the two detection schemes, the outputs from both detection schemes were passed through a low-pass filter with a bandwidth of 155 MHz prior to entering the DCA. The bandwidth of the DCA was 622 MHz.
3.4.2 Experimental Results
The TPA-based device used was characterised in terms of the average photocurrent gener- ated as a function of incident peak power using a femtosecond laser pulse source generating pulses at a repetition rate of 100 MHz with pulse widths of 5 ps at a wavelength of 1550 nm. The resultant plot is shown in Figure 3.10 (a) for both the experimental characterisation data, given by the squares () in the figure, and for the TPA simulation model, given by the dashed line, which is discussed in more detail in section 3.5.1. From this plot it can be seen that the TPA-based device starts to exhibit a nonlinear response at incident peak powers greater than ∼ 1 W.
Figure 3.10 (b) and (c) show the traces obtained for the back-to-back two-channel OCDMA system using standard linear detection and TPA-based nonlinear detection, respectively. In Figure 3.10 (b) the overlaid traces for each transmitting channel are shown. The large re- sponse corresponds to the correctly decoded pulse while the second response is that of the interfering channel generated by the unmatched encoder/decoder pair. In comparison, Fig-
Figure 3.10: Experimental results for the two-channel OCDMA system (a) Plot of pho- tocurrent as a function of optical peak power for the TPA-based detector (b) Received eye diagram using a linear detector (c) Received eye diagram using a TPA-based detector.
ure 3.10 (c) shows only the correctly decoded pulse while suppressing any noise generated by the interfering channel. Using the linear detector, the extinction ratio between the de- coded pulse and the MAI was 5.3 dB. This value was improved to ∼ 10 dB using the TPA detector, an improvement of ∼ 5 dB. From these results, the performance improvement gained through the use of a TPA-based detector is clearly visible, with a significant level of suppression of MAI achieved when compared to standard detection methods.