A low frequency-noise ring random fiber laser (RFL) based on random Bragg grating array (RBGA) feedback is proposed. A dual-cavity fiber Bragg grating Fabry-Perot (FBG-FP) filter, which matches the wavelength of RBGA, is specially designed. Due to the narrower bandwidth, flatter response and steeper rolloff, the dual-cavity FBG-FP filter can significantly suppress the competition-induced frequency jitter and undesired residual random lasing modes, and contribute to establish stable lasing and further reduce frequency noise of RFL. The frequency noise of the proposed RFL is ∼3 Hz/√Hz above 1 kHz, and the frequency jitter is ∼3.11 × 10 ⁻¹¹ over 10 s. The 3-dB linewidth is ∼490 Hz with a ∼40 dB of optical signal-to-noise ratio.

A stabilized narrow-linewidth random fiber laser for strain detection, based on a three-grating ring (TGR) resonator and half-open-cavity structure, is proposed and investigated experimentally. The half-open-cavity structure proved to provide double optical gain of erbium-doped fiber, which was beneficial to increase the photon lifetime as well as further narrow the linewidth. Meanwhile, the stability and frequency noise of narrow lasing output was improved by suppressing the competition-induced undesired residual random lasing modes with the TGR resonator. The TGR resonator is composed of a double-cavity fiber Bragg grating Fabry–Perot (FBG-FP) interferometer, a section of single-mode fiber, and a circulator. The specially designed double-cavity FBG-FP interferometer embedded in the TGR resonator acted as the strain-sensing element and improved the resolution of the dynamic strain. A stable ultra-narrow linewidth of about 205 Hz was obtained. The frequency noise was reduced to about 2 Hz/√Hz. A high dynamic strain measuring resolution of 35 femto-strain (fε)/√Hz was achieved.

A high-resolution optic fiber dynamic strain sensor based on a two-cavity fiber Bragg grating Fabry-Perot (FBG-FP) interferometer and random fiber laser (RFL) is demonstrated. The specially designed two-cavity FBG-FP interferometer acts as a sensing element and can effectively suppress the competition-induced multi-spike lasing modes and frequency jitter, which contributes to establish single-wavelength laser and suppress frequency noise of RFL. The frequency noise is reduced from about 12 to 3 Hz/ $\surd $ Hz above 4 kHz. An ultrahigh dynamic-strain detecting resolution of 30 femto-strain ( $\text{f}\varepsilon)/\surd $ Hz is achieved. To the best of our knowledge, it is the first time that two-cavity FBG-FP interferometer has been used in sensing field.

The embedded high-resolution random fiber laser acoustic emission (RFL-AE) sensor for in-situ monitoring of buoyancy material is demonstrated. Our proposed embedded RFL-AE sensor can directly couple inside the solid buoyant materials (sBm) and be expected to detect different AE signals mechanism of SBM without any medium. The randomly chirped grating array (RCGA) provides effective random distributed feedback and is embedded inside the SBM to act as an AE sensing element. Many steep peaks can be provided by the multiple-interfering refection spectrum of RCGA over a broad spectral range, which significantly suppresses chirping phenomenon caused by the thermal stress of SBM and realizes in-situ performance monitoring of buoyant materials under high pressure. The test results show that our proposed RFL-AE sensor is highly responsive to continuous and burst AE signals of SBM. The measured resolution of AE signal is 215 fε/√Hz, which provides a potential embedded laser AE monitoring scheme for damage detection of SBM-associated composite material.

In this paper, we proposed the temperature compensation theory using a pair of unbalanced interferometers composed of different temperature coefficients' optical fibers. The difference between the temperature sensitivities of the two interferometers was analyzed in detail. The larger the difference of temperature coefficients of the sensing arms is, the longer the sensing arm is, and the higher the phase demodulation resolution is, the higher the temperature resolution is. The bend-insensitive single-mode fiber and Boron/Germanium co-doped fiber were used to develop two kinds of unbalanced Michelson interferometers for the experiment. The result showed a temperature sensitivity difference of 3.93 rad/°C and a temperature resolution of better than the order of 10 ^–3 °C, which is suitable for achieving strain measurement of sub-nε for crustal deformation measurement.