In optical communication, the measurement and analysis of high-speed optical communication signals are essential in developing high-speed optical communication devices, equipment, and systems. At present, the common equipment for time-domain measurement of high-speed optical signals is an optoelectronic hybrid broadband oscilloscope, which has a signal processing circuit limit bandwidth of about 90 GHz and requires complex clock synchronization circuitry. Additionally, this type of oscilloscope also has disadvantages such as opaque signal rate and modulation format, complex system composition, and expensive price. To overcome this electronic bottleneck, we develop an optical sampling oscilloscope prototype based on optical domain sampling technology. The oscilloscope adopts a software synchronization algorithm, and the measurable signal bandwidth is up to THz without the requirement for high-speed photodetectors, which lowers the bandwidth requirements of the clock synchronization circuit and subsequent processing circuits. The limitation of the electronic bottleneck is also overcome. However, since we previously adopt a software synchronization algorithm based on chirped z-transform (CZT), its complexity affects the signal processing timeliness. To improve the signal processing efficiency and enhance the equipment practicability, it is necessary to study a less complex software synchronization algorithm suitable for optical sampling oscilloscopes.

Generally, after the optical signal is asynchronously down-frequency optically sampled with a fixed frequency difference, the eye diagram reconstruction algorithm based on software synchronization can be employed to realize parameter measurement of high-speed optical data signals related to the eye diagram recovery, constellation diagram, and signal statistical characteristics (Fig. 2). The key to the entire software synchronous eye diagram reconstruction algorithm is to accurately obtain the down-frequency equivalent sampling time step parameter Δt of asynchronous down-frequency optical sampling from the sampled digital signal. To reduce the complexity of software synchronization, based on the CZT software synchronization method proposed by our research group, we put forward a software synchronization method based on the zoom fast Fourier transform (ZoomFFT). The proposed software synchronization algorithm is divided into two steps of coarse synchronization based on FFT and fine synchronization based on ZoomFFT (Figs. 4-6). After the FFT coarse synchronization, ZoomFFT is adopted to refine the spectrum near the peak of the amplitude spectrum to obtain a more accurate peak frequency point of the amplitude spectrum. Then a more accurate down-frequency equivalent sampling time step parameter Δt is obtained to realize fine synchronization. Among them, after replacing the low-pass filter in the ZoomFFT transform with time-domain averaging, the computational complexity of ZoomFFT is lower than that of CZT.

First, we measure the four-level pulse amplitude modulation (PAM4) signal and quadrature phase-shift keying (QPSK) signal at different rates through an optical sampling oscilloscope prototype. In the measurement of the PAM4 signal, two rates of 6.259 GBaud and 9.696 GBaud are sent respectively. To compare with the downsampling signal, a high-speed broadband digital sampling oscilloscope with a sampling rate of 50 GSa/s and a bandwidth of 20 GHz is utilized to oversample the two-rate PAM4 signal. The results show that the software synchronous optical sampling oscilloscope can measure the eye diagram which is in good agreement with the oversampling broadband oscilloscope (Figs. 8-9). In the measurement of the QPSK signal, two rates of 10 GBaud and 20 GBaud are sent respectively. With the results measured by Agilent's real-time oscilloscope as a comparison, the software synchronous optical sampling oscilloscope can adaptively measure the eye diagram and constellation diagram of the QPSK signal with different symbol rates (Figs. 11-12). Meanwhile, we investigate the effect of the background noise in an optical sampling oscilloscope prototype, and the change curve of the Q value is measured by changing the input optical power. The results show that when the Q value decreases by 3 dB, the corresponding input optical power reduces by about 10.3 dB, and the influence of background noise is small (Fig. 14). It is worth noting that benefiting from the proposed ZoomFFT-based software synchronization algorithm, the complexity can be greatly reduced. Compared with the CZT algorithm, the complexity is reduced by 68.8%.

Based on the previous research results of the software synchronization algorithm of the CZT transform, our paper proposes a software synchronization algorithm of the ZoomFFT transform. The experimental results show that the software synchronization algorithm based on ZoomFFT reduces the complexity by 68.8% compared with the CZT algorithm. With the developed optical sampling oscilloscope prototype, the optical PAM4 signals of 6.259 GBaud and 9.696 GBaud rates, and the optical QPSK signals of 10 GBaud and 20 GBaud are measured. The measurement results are compared with those of a broadband electrical sampling oscilloscope with a sampling rate of 50 GSa/s and a bandwidth of 20 GHz. The measurement results verify that the optical sampling oscilloscope can adaptively measure intensity-modulated signals and phase-modulated signals at different rates. Additionally, the effect of the background noise in the optical sampling oscilloscope is investigated. The results demonstrate that when the measured input optical signal power drops by 10.3 dB, the measured Q factor decreases by 3 dB. Thus, the influence of the background noise is small.