Highly sensitive and integrated optical multi-band CO₂ sensors are significant at the shortwave infrared (SWIR) region and still lack research. A compact CO₂ sensor composed of a Au-disk/TiO₂-cylinder/Au-film metasurface coated by polyhexamethylene biguanide (PHMB) film, functioning at multi-band resonances as well as having high sensitivity to gas concentrations, is presented. It can be employed as a dual-band narrowband absorber, producing two strongly resonant modes at the SWIR region under a reflection-type framework of linearly polarized incidence. Moreover, the metasurface sensor possesses high refractive index sensitivity of 109.25 pm/ppm at around 1040 nm and 42.57 pm/ppm at around 1330 nm in the range of 200–600 ppm, which is suitable for detecting atmospheric CO₂. Furthermore, the numerical results show that the sensitivity increases with a thicker PHMB film and optimizes at a thickness above 600 nm. The physical mechanism reveals that the higher order mode exhibits more extended near-field energy than the lower order mode, resulting in more sensitivity towards the surroundings. The design and results of our investigation show high-quality CO₂ sensing performance which functions at dual spectrum bands in the SWIR region and is promising for integrated photonic applications.

An open-cavity optical fiber gas pressure sensor based on the Vernier effect is proposed. The sensor consists of two Fabry-Perot interferometers (FPIs) connected in parallel. As a sensing FPI, FPI-1 is an open-cavity FPI and composed of single-mode fiber (SMF), hollow core capillary (HCC) and twin-hole and dual-core fiber (THDCF). The open-cavity FPI structure is constructed using the air holes of THDCF. FPI-2 consists of SMF and HCC with a large inner diameter and works as a reference FPI. The manufacturing cost of the sensor is low, and easy to manufacture. This study explored the relationship between the magnification factor and the cavity lengths of the sensing FPI and the referencing FPI. After optimizing, the sensor has a sensitivity of -168.82 nm/MPa in the gas pressure range of 0.1-1.6 MPa, which is 48.37 times the sensing sensitivity of single FPI. In addition, the temperature crosstalk of the sensor is 2.285×10 ^-4 MPa/°C. Repeated experiments have demonstrated that the sensor has good repeatability and stability. Therefore, the proposed sensor structure provides a new idea for the design and application of optical fiber gas pressure sensors with ultra-high sensitivity and lower temperature cross-sensitivity.

To take full advantage of lab-on-fiber technologies for gas sensing, we propose a reflective Fabry-Perot interferometer (FPI) CO ₂ gas sensor composed of the functional material (polyhexamethylene biguanide, PHMB) and a single-mode fiber (SMF)-hollow core capillary (HCC) structure. The theoretical analysis of the FPI cavity mode in the PHMB film is discussed. The experiment results show a maximum sensitivity of 286 pm/ppm in the low concentration region of 0-1000 ppm detection range and a high sensitivity of 6.88 pm/ppm in the atmosphere CO ₂ concentration range (200-600 ppm). The response time of the CO ₂ sensor is about 10 min. Repeated experiments show that the sensor has good sensing stability and a high selectivity for CO ₂ gas. In addition, the sensitivity of our CO ₂ sensor is stable to temperature changes. Moreover, the results of monitoring the real in-door atmosphere verify the repeatability and desirable sensing performance of our sensor. This reflective lab-on-fiber CO ₂ sensor has a concise structure, low cost, and high sensitivity possessing more possibilities in various and complex applications.

Ultrafast humidity interrogation is valuable in industrial production, healthcare, and environmental monitoring due to its fast response and high sensitivity. We present an ultrafast humidity sensor based on an agarose-coated fiber Bragg grating (AG-FBG) structure combined with time stretching method. Leveraging AG’s ability to swell or contract in response to humidity variations results in changes to the effective grating period, which in turn causes a shift in the Bragg wavelength. Using a home-made high-power femtosecond all-polarization-maintaining fiber laser, the time-stretching technique can be achieved through a long dispersion-compensating fiber. According to wavelength-to-time mapping, the wavelength shift associated with humidity response can be converted into the time domain, resulting in a humidity sensitivity of −0.017 ns/%RH at an interrogation speed of 76.75 MHz and a humidity resolution of 0.007 %RH. The proposed humidity monitoring system has demonstrated high stability and repeatability. Therefore, with the advantages of high sensitivity, high accuracy and ultrafast response time, our proposed humidity sensor shows significant potential in the field of real-time monitoring.

Respiratory rate is an important indicator in respiratory monitoring. To achieve accurate measurement of respiratory rate, we have proposed a novel all-fiber sensor based on a lab-on-fiber microfiber knot resonator. The sensor is made of precisely knotted micro-optical fibers, which enhance its performance in detecting small changes in humidity caused by breathing. When the humidity changes, the refractive index of the humidity-sensitive polyvinyl alcohol film alters accordingly. Experimental results demonstrate a humidity sensitivity of 0.069 dB/%RH. With a response time of 412 ms and recovery time of 318 ms enable dynamic respiratory rate monitoring. The investigation of various heart rates and breathing patterns demonstrate that the sensor can accurately detect and distinguish changes in breathing patterns. The sensor’s continuous monitoring capability and the impact of airflow crosstalk have been thoroughly evaluated, demonstrating its remarkable stability and reliability. Owing to its high sensitivity, low power consumption and a high-quality factor, the sensor can accurately monitors breathing in harsh environments.