Search

We proposed a compact self-injection locked fiber laser, employing a weak fiber Bragg grating (wFBG) to achieve external feedback linewidth compression. The simulated results indicated that the output laser linewidth decreased with the increase of the length of the wFBG. According to the experimental results, the output laser linewidth was compressed to 300 Hz with an 8-mm long wFBG. However, a further increase in the length of the wFBG led to linewidth broadening, which can be attributed to the enhanced interference between the main cavity and wFBG. Additionally, the corresponding phase noise was suppressed by approximately 20 dB within the frequency range of 0 Hz - 100 Hz, and the relative intensity noise decreased to -123 dB/Hz. Furthermore, the amplitude of beat frequency fluctuated within 20 MHz over 1000-second period, and the output power exhibited fluctuations of less than 1% over a 7-hour period under laboratory conditions. Ideally, the compact fiber laser design will facilitate the miniaturization of sub-kHz-linewidth fiber lasers.

Real-time high spatial resolution fiber sensing is crucial for achieving high-precision strain measurement for various applications. Bulky optical frequency-domain reflectometry is one of the mainstream devices capable of achieving spatial resolution at the micrometer level, but their relatively expensive tunable laser and poor real-time performance are becoming the bottlenecks to hinder the development of ultracompact and fast response strain field measurement systems that are highly demanded in practice. Herein, we proposed a real-time and low-cost accurate strain field measurement methodology based on dense fiber Bragg gratings (FBGs) array with wavelength division multiplexing, in which the system showed a flexibly adjustable spatial resolution down to 10 mm and ultrahigh strain resolution of $0.83~\mu \varepsilon $ . Such strain field measurement methodology has been successfully applied to 2-D irregular shape sensing with a maximum relative reconstruction error of 0.16% and dynamic strain field monitoring with 50-Hz sampling rate. The high-density FBGs array based on strain field measurement systems could have the capability of cost-effective and robust practicality.

In recent years, the unmanned aerial vehicle (UAV) has been widely applied in daily life, but the “black flight” incidents happen frequently and even endanger public safety. Although the electrical microphone can cover the low-altitude and non-line-of-sight (NLOS) limited vision areas of radar or camera system, it is quite difficult to achieve large-scale networking and surveillance. In this article, a low-altitude UAV detection and positioning scheme based on fiber-optic distributed acoustic sensing (DAS) is proposed and demonstrated, which has the superiorities of large-scale networking and antielectromagnetic interference in applications. The DAS system assisted with the highly sensitive sensing cable fabricated by uniformly winding backscattering enhanced optical fiber (BEOF) around sensitization cable can collect the acoustic signals induced by low-altitude UAV, and it can be easily deployed with a large scale. The acoustic signals collected by DAS are further divided into continuous frames and denoised for low-altitude UAV positioning and tracking, in which the time difference of arrival (TDOA) algorithm based on the generalized cross-correlation (GCC) method and least-squares estimation (LSE) method is employed. In the field test, the low-altitude UAV positioning and tracking are both demonstrated and achieved with an ultralow position estimation error of less than 0.5 m in low-altitude airspace. It has exhibited that the proposed surveillance scheme has a great potential for low-altitude airspace early warning in large-scale applications such as border and airport. Then, reporting and monitoring intrusive low-altitude UAV instantaneously allow the controller to take quick action to prevent potential danger and damage from happening in key areas.