Yiwen Ou

1. A fibre Bragg grating (FBG) is a passive optical device with a periodic refractive index modulation in the fibre core. In recent years, FBGs have attracted considerable interest for temperature ...

A high-resolution temperature sensor based on space-domain active fiber cavity ring-down sensing technology was proposed for the first time, to our knowledge. In this scheme, a bidirectional erbium-doped fiber amplifier is employed to compensate for the intrinsic cavity loss, significantly improving the temperature resolution. Meanwhile, a temperature sensing head based on a single-few-single mode fiber structure is utilized to enhance the mode interference effect, thereby effectively increasing the temperature sensitivity. The differential interference signal is captured by a balanced detector and processed via fast Fourier transform to obtain the space-domain ring-down signal, thereby establishing a quantitative relationship between the reciprocal difference of ring-down distance and temperature. Experimental results demonstrate a good linear relationship between the reciprocal difference of ring-down distance and temperature within the range of 90°C to 130°C, with a sensitivity of 0.182km ⁻¹ ⋅ ^∘ C ⁻¹ . Benefiting from the advantages of cavity loss compensation technology and enhanced sensitivity, the proposed sensor achieves a temperature resolution of 0.033°C, which is one order of magnitude higher than traditional time-domain passive cavity ring-down temperature sensors and twice that of space-domain passive fiber cavity ring-down temperature sensors.

We present a simple way to enhance bidirectional broadband modulation in a traveling wave Mach–Zehnder electro-optic modulator (EOM) by removing matched impedance and effecting a reflection of the microwave signal. A model was devised for determining the modulation efficiency (ME) in the presence of microwave reflection, without access to the physical parameters of the EOM. By using the model, the simulated MEs matched well with the measured ones, which verified the correctness of the model and demonstrated that the internal parameters of the modulator could be inferred from the model. Compared with the traditional unidirectional EOM, the measured ME of the bidirectional modulator for the optical signal that counter-propagates with the incident microwave signal was improved by 10 times at least. When the velocities of microwave and optical signals were matched, the MEs at the frequency higher than 3.3 GHz were further improved. Our model was also used to derive the optical interference signal utilizing bidirectional modulation. It was found that the signal intensity was enhanced significantly, and further improved after introducing the velocity match. This work can be expected to open up many microwave photonics applications where bidirectional optical modulation is utilized in novel sensing and metrology.

Trace gas sensors have received extensive attention and have become particularly attractive for applications in toxic gas detection, medical diagnostics, pollution monitoring, industrial emission measurement, and so on. However, it is still a critical challenge for gas sensors to simultaneously achieve high sensitivity and low detection limits while performing well in stability. Here, a space-domain active fiber cavity ring-down (FCRD) gas sensing technique was proposed to develop a high-performance gas sensor. Taking advantage of the active FCRD sensing technique and frequency-shifted interferometry, the proposed gas sensor exhibited a high sensitivity of 1.122km⁻¹/%. Importantly, introducing a bidirectional EDFA into the fiber cavity further enhances the light–gas interaction, resulting in a low detection limit of 55 ppm, which is much lower than that of most reported gas sensors. Moreover, a good stability of 0.58% can be achieved due to the combined effect of the proposed technology using differential detection to eliminate DC noise, as well as common-path interference to eliminate common-mode noise and the use of continuous light to stabilize the optical power in the fiber cavity for suppressing the gain fluctuations of EDFA.

A fiber Bragg grating (FBG) sensor utilizing space-domain fiber cavity ringdown (FCRD) scheme is presented and experimentally demonstrated for high-resolution temperature measurement. Different from the existing FBG temperature sensors based on time-domain or frequency-domain FCRD techniques, this scheme senses temperature in the space domain by measuring the ringdown distance rather than the ringdown time or the frequency response of the system over a frequency span, without pulse modulation/microwave modulation nor fast photoelectric detection. It integrates frequency-shifted interferometry (FSI) and FCRD so that it can achieve high sensitivity, good stability and thus high measurement resolution. By incorporating a standard single-mode bare FBG in the fiber ringdown cavity as the sensing element, the linear response of the sensor to temperature in the range of 25-35 °C was experimentally investigated, and a sensitivity of 0.1641/ (km·°C) was attained. Through repetitive testing, the ringdown baseline stability of the sensor was determined to be 0.42%, and the temperature resolution was 0.077 °C which is the best among the ones of the current bare-FBG-based temperature sensors. In addition, the FBG sensor’s behaviors in other temperature regions were observed by adjusting the operating wavelength of the laser, and the approximately same sensing performances were achieved. The experimental results show that the space-domain-FCRD-based FBG temperature sensor offers a promising candidate for the temperature measurements in civil engineering, environmental protection, biomedicine and other fields.