Time delay signature (TDS) of a semiconductor laser subject to dispersive optical feedback from a chirped fibre Bragg grating (CFBG) is investigated experimentally and numerically. Different from mirror, CFBG provides additional frequency-dependent delay caused by dispersion, and thus induces external-cavity modes with irregular mode separation rather than a fixed separation induced by mirror feedback. Compared with mirror feedback, the CFBG feedback can greatly depress and even eliminate the TDS, although it leads to a similar quasi-period route to chaos with increases of feedback. In experiments, by using a CFBG with dispersion of 2000ps/nm, the TDS is decreased by 90% to about 0.04 compared with mirror feedback. Furthermore, both numerical and experimental results show that the TDS evolution is quite different: the TDS decreases more quickly down to a lower plateau (even background noise level of autocorrelation function) and never rises again. This evolution tendency is also different from that of FBG feedback, of which the TDS first decreases to a minimal value and then increases again as feedback strength increases. In addition, the CFBG feedback has no filtering effects and does not require amplification for feedback light.

We propose a combined attenuation and high-resolution fault measurement method using chaos optical time-domain reflectometry (OTDR). Utilizing the pulse-modulated chaotic light and correlation operation, this technique realizes fiber attenuation measurement and a distance-independent spatial resolution simultaneously. We demonstrate a proof-of-concept experiment for measurements of a single-mode fiber (G.652) and a fiber link consisting of a single-mode fiber (G.652) and a conventional (62.5-μm) multimode fiber. The measurement results show that different attenuation coefficients can be measured, and the coefficients agree well with the values obtained by using conventional OTDR. A 0.6-m resolution is achieved with a data acquisition bandwidth of 200 MHz.

Laser chaos is promising for constructing high range-resolution and anti-jamming radars, whereas the range resolution and anti-jamming capability are unfortunately limited by laser relaxation oscillation and external-cavity resonance, respectively. Here, we theoretically propose using a broadband white chaos generated by optical heterodyne of two chaotic external-cavity semiconductor lasers as source signal to construct a radar system with enhanced range resolution and anti-jamming capability. The white chaos not only has a white-noise-like wide spectrum without relaxation oscillation, but also has no signature of external-cavity resonance. Benefitting from these merits, the proposed white-chaos radar shows a marvelous unambiguous detection performance with subcentimeter range resolution. Moreover, the elimination of external-cavity resonance enables the randomness improvement of source signal, and resultantly, the anti-jamming capability is enhanced as well.

Chaos optical time domain reflectometry (chaos OTDR) has an advantage of high spatial resolution without tradeoff of measure distance by utilizing correlation detection based on broadband chaotic laser. However, achieving a high spatial resolution usually requires a wideband photodetector and a high-speed data acquisition card. In this letter, we propose a significant-bit correlation method to improve spatial resolution of chaos OTDR instead of increasing hardware bandwidth. In this method, the reference and echo chaotic signals are quantized with multi-bit ADC, and some least significant bits (LSBs) are extracted to construct new time series for calculating cross correlation. Our experimental results demonstrate that the spatial resolution is significantly enhanced as the number of LSBs decreases. Under a detection bandwidth of 200 MHz, the spatial resolution is approximately improved 5 times from 21 cm to 4 cm with 6-LSBs correlation. The improvement of spatial resolution is attributed to bandwidth enhancement caused by LSBs extraction.

We experimentally demonstrate a real-time high-speed physical random bit generator using the time-interleaved sampling of broadband white chaos. The white chaos is generated by optical heterodyning of two external-cavity feedback laser diodes. It has a white-noise-like spectrum with a 3-dB bandwidth of 16.7 GHz and a symmetric amplitude distribution and has no time-delay signatures of the external feedback cavities. An analog-to-digital converter (ADC) chip integrated by two time-interleaved sub-ADCs with a maximum sampling rate of 7 GHz is utilized to discretize the white chaos into parallel binary streams. After real-time interleaving combination and exclusive-OR operation in the field-programmable gate array (FPGA), a 14-Gbps binary stream with verified randomness is achieved.

With the large-scale application and high-quality development demands of optical fiber cables, higher requirements have been placed on the corresponding measurement technologies. In recent years, optical fiber testing has played a crucial role in evaluating cable performance, as well as in the deployment, operation, maintenance, fault repair, and upgrade of optical networks. The Optical Time-Domain Reflectometer (OTDR) is a fiber fault diagnostic tool recommended by standards such as the International Telecommunication Union and the International Electrotechnical Commission. It is used to certify the performance of new fiber links and monitor the status of existing ones, detecting and locating fault events with advantages including simple operation, rapid response, and cost-effectiveness. First, this paper introduces the working principle and system architecture of OTDR, along with a brief discussion of its performance evaluation metrics. Next, a comprehensive review of improved OTDR technologies and systems is provided, categorizing different performance enhancement methods, including the enhanced measurement distance with simple structure and low cost in 2024, and the high spatial resolution measurement of optical fiber reflection events and non-reflection events in 2025. Finally, the development trends and future research directions of OTDR are outlined, aiming to achieve the development of low-cost, high-performance OTDR systems.