Single photon detector (SPD) has a maximum count rate due to its dead time, which results in that the dynamic range of photon counting optical time-domain reflectometry (PC-OTDR) de-creases with the length of monitored fiber. To further improve the dynamic range of PC-OTDR, we propose and demonstrate an externally time-gated scheme. The externally time-gated scheme is realized by using a high-speed optical switch, i.e. a Mach-Zehnder interferometer, to modulate the back-propagation optical signal, and to allow that only a certain segment of the fiber under test is monitored by the SPD. The feasibility of proposed scheme is first examined with theoretical analysis and simulation; then we experimentally demonstrate it with our experimental PC-OTDR testbed operating at 850 nm wavelength. In our studies, a dynamic range of 30.0 dB is achieved in a 70 meters long PC-OTDR system with 50 ns ex-ternal gates, corresponding to an improvement of 11.0 dB in dynamic range comparing with no gating operation. Further-more, with the improved dynamic range, a successful identifica-tion of a 0.37 dB loss event is detected with 30-seconds accu-mulation, which could not be identified without gating operation. Our scheme paves an avenue for developing PC-OTDR systems with high dynamic range.

Optical time-domain reflectometry (OTDR) is a widely employed instrument for monitoring the property of fiber links. Traditional OTDR always suffers from the trade-off between its spatial resolution and the sensitivity of light detection. Therefore, it cannot be applied in critical applications, such as in aviation, where OTDR with a high spatial resolution of several centimeters is required. In this paper, we develop a cost-effective photon-counting OTDR based on our homemade gain-switching pulsed laser at 850 nm. Thanks to the short pulse width of the laser and the relatively small time jitter of the photon detector, our photon-counting OTDR achieves a spatial resolution of less than 9 cm and satisfies the requirements for monitoring short fiber links in various types of airplanes. Finally, we realize a fully running system for monitoring an optical cable with 32 fiber channels on a plane.

Photon-counting optical time-domain reflectometry (PC-OTDR) based on single photon detection is an effective scheme to attain the high spatial resolution for optical fiber fault monitoring. Currently, due to the spatial resolution of PC-OTDR being proportional to the pulse width of a laser beam, short laser pulses are essential for a high spatial resolution. However, short laser pulses have a large bandwidth, which would be widened by the dispersion of fiber, causing inevitable deterioration in the spatial resolution, especially for long-haul fiber links. In this Letter, we propose a scheme of dispersion independent PC-OTDR based on an infinite backscatter technique. Our experimental results—with more than 45 km long fiber—show that the spatial resolution of the PC-OTDR system is independent with the total dispersion of the fiber under test. Our method provides an avenue for developing long-haul PC-OTDR with high performance.

To improve the peak power and extinction ratio and produce ultra-short pulses, a novel approach is presented in this paper offers a highly effective modulated method for a gain-switched semiconductor laser by using step-pulse signal modulation. For the purpose of single pulse output, then the effects on the output from the gain-switched semiconductor laser are studied by simulating single mode rate equation when changing the amplitude and width of the modulated signal. The results show that the proposed method can effectively accelerate the accumulation speed of the population inversion and we can acquire the output pulse with higher peak power and shorter width. Compared with the traditional rectangular wave modulation, this method is advantageous to obtain a high gain switching effect by increasing the second modulation current and reduce the pulse width to saturation at the best working point. It can be incorporated as a practical and cost-effective approach for many fields which need high extinction ratio short pulse, such as the optical time domain reflectometry.

Photon counting optical time-domain reflectometry (PC-OTDR), leveraging single photon detection, offers superior spatial resolution and high dynamic range in the testing of optical fiber links. However, the dead time and afterpulsing of the single photon detector could cause distortions on the PC-OTDR trace, especially for the fiber links with strong local reflections, such as the Fresnel reflection from fiber connection, the influence of which has yet to be investigated. In this work, we study the influences of local reflections with different strengths, then propose and demonstrate a scheme of multiple time-gating with narrow temporal filtering to mitigate these influences. Distortions are observed in a simulation system with a relative intensity of local reflection greater than 25.0 dB. To mitigate these distortions, we employ an optical intensity modulator driven by periodic electronic signals with different relative delays to manipulate the optical signal, and then process the measured traces with narrow temporal filtering. A trace without distortion is obtained in the simulation system, where a local reflection with a relative intensity of 37.5 dB is tolerated using our scheme. Furthermore, a PC-OTDR trace of a 70-m-long fiber link is obtained without distortion, where the relative intensity of the Fresnel reflection reaches 35.2 dB. Our results reveal the influence of local reflection on the PC-OTDR trace, and pave an available avenue for the testing of fiber links with strong local reflections.

We present a high SNR signal processing system for coherent solid-state LIDAR. A receiving frequency processing system is initially developed. In this LIDAR system, a frequency-modulated continuous wave (FMCW) laser is used as the transmitter, and balanced detectors array based on silicon photonic technology is used as the laser echo receiver. The receiving processing system includes multi-channel low-noise transimpedance amplifier, band-pass filter, high resolution ADC and output buffers. Based on the signal processing system, frequency signal processing with a high signal-to-noise ratio is realized, and the distance detection is realized to confirm on-chip balanced-photodetector-based coherent ranging. The system can be integrated by CMOS technology in the future and realizing three-dimensional integration through through-silicon-via (TSV) with the silicon photonic chip to get low integration complexity, low power consumption, low optical loss, and large array integration.