## 3 Experimental Setup  
Figure 2 shows the experimental setup of an 8-bit optical comparator. The experimental setup mainly consisted of two components: (A) a QPSK signal generator with optical gate and (B) an optical comparator. The QPSK generator utilizes a tunable laser diode (TLD) (Santec, Tokyo, Japan), pulse pattern generator (PPG) (Anritsu, Kanagawa, Japan), lithium-niobate Mach-Zehnder modulator (LN-MZM) (Sunicem, Tokyo, Japan), and dual-parallel LN-MZM (Sunicem, Tokyo, Japan), respectively. In the generator, a probe light at 1550 nm was generated from the TLD, and the probe was QPSK modulated by using the dual-parallel LN-MZM and the PPG with a \(2^{9}-1\) pseudo random binary sequence (PRBS) at 10.72 Gbaud. The QPSK signal was then return-to-zero (RZ) modulated by the second LN-MZM driven by the PPG with a 10.72 GHz clock. The optical gate consisted of an LN-MZM and a PPG. The generated QPSK RZ-signal was injected into the third LN-MZM, and four successive QPSK symbols were selected from \(2^{9}-1\) PRBS by using the second PPG with a 400 ps gate pulse. Then, the selected symbols were injected into the following optical comparator.  
Figure 2: Experimental setup of an optical comparator for 8-bit QPSK signal. In this figure, the comparator comprises a serially cascaded two delay line interferometers (DLIs), whose FSRs are 5 and 10 GHz, respectively. The comparator generates a coupled symbol from four QPSK symbols with predetermined phase rotations. BPD: balanced photo detector; DP-LN-MZM: dual-parallel lithium-niobate Mach-Zehnder modulator; EDFA: erbium-doped fiber amplifier; ODL: optical delay line; OF: optical filter; OSC: oscilloscope; PC: polarization controller; PPG: pulse pattern generator; PS: phase shifter; TLD: tunable laser diode; VOA: variable optical attenuator.  
The optical comparator comprised two serially cascaded DLIs. The first and second DLIs had 5 and 10 GHz of free spectrum ranges (FSRs), respectively. In the four QPSK symbols at 10 Gbaud, the first and third symbols were optically overlapped in the 5 GHz DLI as well as the second and fourth symbols. The overlapped first + third and second + fourth symbols were then overlapped in the following 10 GHz DLI. As a result, the successive QPSK symbols were converted in parallel and optically coupled, and then the coupled symbol was detected at a following balanced photo detector (BPD).  
In this experiment, the author prepared four types of optical comparators to set the phase shifters in each DLI. Each DLI had only one phase shifter on one side of delay line. The optical comparators for \"00 00 00 00\", \"11 00 11 00\", \"11 10 10 00\", and \"00 10 01 00\" code were arranged by utilizing sets of the (0, 0, 0, 0), (\(\pi\), 0, \(\pi\), 0), (\(\pi\), \(\pi\)/2, \(\pi\)/2, 0), and (0, \(\pi\)/2, 3\(\pi\)/2, 0) phase-shifts. In each DLI, the phase shifter offered 0, \(\pi\)/2, \(\pi\), and 3\(\pi\)/2 phase-shift amounts in the condition that the applied voltage was set at 2.3, 2.8, 3.2, and 3.7 V for the 5 GHz DLI, and 0.6, 1.2, 1.8, and 2.4 V for the 10 GHz DLI, respectively. Table 1 summarizes the applied voltage of the phase shifters in each DLI against the four comparators.