## 3 Experiment Setup and Results
### 160 Gb/s Wavelength Division Multiplexing Operation  
Figure 13 shows the experimental setup that was used to record the eye diagrams and to evaluate the BER performance of the four WDM channels. To demonstrate the WDM operation, four laser sources emitting continuous-wave (CW) light at wavelengths of 1552 nm, 1555.2 nm, 1558.4 nm and 1561.6 nm set with an optical power of 10 dBm/channel. The wavelengths of the CW signal were chosen such that they were aligned with the transmission power peaks of the four channel optical demultiplexer (Figure 12b). Figure 11: Normalized transmission response of the four optical output channels (**a**) _ch-1_, (**b**) _ch-2_, (**c**) _ch-3_ and (**d**) _ch-4_. Figure 12: **(a)** Overlapped transmission response of the four optical output channels. (**b**) Zoomed view of the optical response shows 14.5 dB crosstalk for _ch-2_. Polarization controllers (PC-1 to PC-4) were inserted at each of the CW channel paths to maximize the extinction ratio of the modulated signal from the polarization sensitive electro-optic LiNbO\({}_{3}\) modulator. Prior to the external optical modulator, the four CW signals were multiplexed onto a single fiber using a 4:1 optical power coupler. The 4:1 optical power coupler has an insertion loss of around 6.4 dB. We measured around 9.5 dBm of total optical power from the output of the 4:1 optical coupler. The optically multiplexed four CW channels were given as input to the commercial LiNbO\({}_{3}\) modulator. The modulator has a measured insertion loss of 7 dB. The optical modulator was driven by a 40 Gb/s non-return to zero (NRZ) on-off keying (OOK) electrical data generated from four 10 Gb/s pulse pattern generators (PPG). The four 10 Gb/s data generated from the PPGs were time multiplexed by the electrical multiplexer (MUX) to create the 40 Gb/s data. The electrical signal is a pseudorandom binary sequence (PRBS) data with a bit pattern of length \(2^{31}-1\). To compensate for the insertion loss of the optical modulator and the device under test, two EDFAs (EDFA-1 and EDFA-2) with 5 dB noise figure were used before and after the chip. A polarization controller (PC-5) was used before the chip to control the polarization state of the input signal to the polarization sensitive Si\({}_{3}\)N\({}_{4}\) waveguide. A variable optical attenuator (VOA) was used to control the received optical power by the commercial photodetector such that BER performance of the demultiplexer can be evaluated in terms of the average received power. A 90/10 optical coupler was used after the VOA to measure the amount of received power by the photodetector and to record the optical spectrum of the output signals. Figure 14 shows the recorded eye diagrams from the DCA for each of the four channels of the demultiplexer. Figure 13: Experimental setup to capture eye diagrams and to evaluate BER performance of the optical demultiplexer. EDFA: Erbium doped fiber amplifier, PM: power meter, OSA: Optical spectrum analyzer, DCA: digital communication analyzer. Inset shows the optical spectrum of the four modulated channels. To measure the BER of all four channels, we used an electrical demultiplexer after the commercial photodetector to electronically demultiplex each of the 40 Gb/s channel to a 10 Gb/s channel. The 10 Gb/s data rate signal was used to measure the BER performance of each channel using a BER tester (Anritsu MU181040A-002). Figure 15 shows that an average received power of \(-2.5\) dBm is required at the photodetector input to achieve \(1\times 10^{-12}\) BER for all four channels. It should be mentioned that the BER performance result does not include errors due to the crosstalk between channels, as all the channels were modulated with same PRBS data. In the case of the back-to-back link, only one CW signal source is modulated without using the 4:1 optical coupler, where the device under test is replaced by an optical attenuator set to the insertion loss of the Si3N4 demultiplexer (approximately 11.1 dB).