# Real-Time Eye Diagram Monitoring for Optical Signals Based on Optical Sampling
## 1 Introduction  
An eye diagram, which consists of samples of the temporal intensity of a digital signal, is regarded as one of the most effective optical signal quality characterization methods [1, 2]. Thanks to the ability to reflect the effects of various sources of impairments on digital optical signal qualities, eye diagram monitoring has been widely used in areas of modern optical techniques [3, 4], such as optical communications, ultrafast optics, and cloud computations. In particular, real-time eye diagram monitoring techniques have grown in importance with full-time optical information transmission services. Traditionally, an eye diagram is achieved through photodetection and electrical signal processing. One example is converting the optical signal into an electrical signal via a wide-band photodetector (PD) and processing the electrical signal with a high-speed sample-and-hold circuit [5]. However, the rapidly increasing optical digital signal data rate presents a significant challenge to the bandwidth of the conventional photodetection and electrical signal processing technique. Additionally, achieving real-time eye diagram monitoring at a low operating cost is essential, ensuring optimum resource utilization and guaranteeing dynamic management of optical networks. The problem of the bandwidth limitation of the conventional photodetection and electrical signal processing technique can be solved by using the photonic signal processingtechnique, such as optical sampling, which has the advantages of broad bandwidth, low loss, and immunity to electromagnetic interference [6; 7]. In general, conventional optical sampling techniques can be divided into two categories: linear optical sampling [8; 9; 10] and nonlinear optical sampling [11; 12; 13; 14; 15; 16; 17; 18]. For the linear-optical-sampling-based eye diagram monitoring systems, the gating function is achieved by a short optical pulse. The optical sampling is implemented using a linear optical process in a low-speed PD, where the optical pulse and the optical signal under test are coherently homodyne mixed [8]. The linear-optical-sampling-based eye diagram measurement system can analyze the optical signal under test with a remarkable minimum peak power of about 60 \(\upmu\)W. However, the coherent homodyne mixing process puts forward a high requirement for the wavelength controlling the optical pulse and the optical signal under test. For the nonlinear-sampling-based eye diagram monitoring systems, the gating function is achieved by the same short optical pulse used in a linear sampling system. Still, the optical sampling is performed through a second- or third-order nonlinear effect. In general, second-order effect-based sampling can be implemented in nonlinear crystals [14; 15]. The third-order effect-based sampling can be achieved in a semiconductor optical amplifier (SOA) [16], a dispersion-shifted optical fiber [17], or a highly nonlinear fiber (HNLF) [18]. Though the nonlinear optical sampling-based eye diagram monitoring systems can measure the eye diagram of a high-speed optical signal, the sensitivity of the system is limited to an mW level due to the low efficiency of the nonlinear effects. In addition, the low repeat frequency limits the eye diagram update time. An eye diagram monitoring method with the advantages of high sensitivity, large dynamic range, and fast update speed is urgently needed. In this paper, a real-time eye diagram monitoring method for optical signals is proposed and experimentally demonstrated based on a gated on-off optical sampling in a Lithium niobate modulator. The gating function is achieved by an optical pulse train, which is generated by gating on-off modulation of a Lithium niobate Mach-Zehnder modulator (MZM) with a narrow pulse width and a high repetition frequency. The sampling for the optical signal under test is then implemented on the MZM, where the optical signal under test is injected.