# Lossless WDM PON Photonic Integrated Receivers Including SOAs
## 4 Results
### Experimental Results  
The experimental setup for validation of the developed software model is shown in Figure 10. The 25 Gb/s NRZ signal for the Ch.u.t. was generated by exploiting an Mach-Zehnder modulator (MZM) fed with a continuous wave (CW) laser at 1552 nm and driven by a bit pattern generator (BPG) with a 2\({}^{31}\)-1 PRBS. A second MZM was fed by 16 CW lasers spaced by 200 GHz and driven by a second bit pattern generator (BPG) with a 25 Gb/s \({}^{231}\)-1 PRBS to generate the interfering channels. The channel allocation on the SOA gain curve is shown in Figure 11.  
Ch.u.t. and interfering channels were coupled together and to emulate the effect of 32 channels, we reduced the power of the Ch.u.t. by 3 dB with respect to the interfering channels. The worst case for XGM has been considered here as the interfering channels are all modulated with the same data stream. Two identical SOAs have been used in the experiment to implement the polarization-diversity scheme with twin coupled SOAs.  
Figure 8: Simulated eye diagrams for the Ch.u.t. (**left**) and its 10 dB stronger side channel (**right**) for the case of input power of \(-16\) dBm and \(-6\) dBm respectively.  
Figure 9: Simulated output data pattern and eye diagram for the Ch.u.t. with very high input power (\(-4\) dBm per channel).  
To emulate the worst case polarization condition, the optical signal is fed to the first SOA only (with optimized polarization) while the second SOA works in the absence of input power and emits ASE noise only. Both SOAs are biased at 500 mA. Total input power for SOA1 is set to \(-5\) dBm (corresponding to \(-20\) dBm for Ch.u.t.). After the 3 dB coupler, the Ch.u.t. power is \(-8\) dBm, far above the receiver sensitivity of \(-16\) dBm of the reference architecture. A 1 nm optical band-pass filter (OBPF) was used to extract the Ch.u.t. for eye-diagram evaluation and bit-error-rate (BER) measurements. At the receiver, a 30 GHz photo receiver was used, followed by a 45 GHz oscilloscope and a 25 Gb/s error detector. Unfortunately, due to the high insertion loss (6 dB) of the OBPF used in the experiment and the receiver sensitivity of the available photoreceiver (\(-10\) dBm), the thermal noise was dominating the ASE noise of the SOAs and hiding other possible impairments. This required the use of a pre-amplified receiver to accurately evaluate the impact of the SOAs in absence of the receiver thermal noise.  
Figure 10 reports the eye diagrams of the Ch.u.t. in back-to-back, after SOA1 and after coupling with SOA2 cases. BER has been evaluated by varying the optical power at the input of the pre-amplified receiver.  
To highlight the impact of the two SOAs, both cases of a single SOA and twin SOAs have been considered. Experimental BER results are shown in Figure 12. The curves show that at BER = \(1\times 10^{-9}\), there is only a 2 dB penalty for the twin SOAs scheme.  
Figure 11: Channels (ch) used for the experiment and SOA gain profile in the wavelength region of interest.  
Figure 10: Experimental setup for BER measurements with twin parallel SOAs; eye diagrams of the received signals are shown for the back-to-back case as well as after a single SOA and after coupling with the twin parallel SOAs. BPG: bit pattern generator, OBPF: optical band-pass filter, VOA: variable optical attenuator.  
In order to examine the device's nonlinear behavior performance in terms of cross gain modulation (XGM), we injected two signals to the SOA with \(-20\) dBm (Ch.u.t at 1552 nm) and \(+5\) dBm (\(3\times 0.8\) nm from Ch.u.t) optical input power, respectively. Both signals were modulated to produce a 25 Gb/s 211-1 PRBS sequence. The results of the simulated and measured signal of the channel under test are depicted in Figure 13. Since the available SOA is linear with high saturation input power of \(+6\) dBm, within this optical power range, the eye diagram does not exhibit a significant nonlinear effect.