Optical biosensors with a high sensitivity and a low detection limit play a highly significant role in extensive scenarios related to our daily life. Combined with a specific numerical simulation based on the transfer matrix and resonance condition, the idea of novel single-waveguide-based microresonators with a double-spiral-racetrack (DSR) shape is proposed and their geometry optimizations and sensing characteristics are also investigated based on the Vernier effect. The devices show good sensing performances, such as a high quality factor of 1.23×105, a wide wavelength range of over 120 nm, a high extinction ratio (ER) over 62.1 dB, a high sensitivity of 698.5 nm/RIU, and a low detection limit of 1.8×10 5. Furthermore, single-waveguide-based resonators can also be built by cascading two DSR structures in series, called twin-DSRs, and the results show that the sensing properties are enhanced in terms of quasi free spectral range (FSR) and ER due to the double Vernier effect. Excellent features indicate that our novel single-waveguide-based resonators have the potential for future compact and highly integrated biosensors.

The uses of optical fibers are numerous, and over the past few decades, they have extended from optical fiber communications to a wide variety of sensing applications. In particular, fiber Bragg grating (FBG) sensors inscribed in single-mode optical fibers offer significant advantages over more conventional electrical sensors and have been successfully deployed in many different industries. In this Review, we review the applications of intrinsic FBG pressure and flow sensors in oil and gas and the deployment of FBG sensing networks in railways. The promising prospect of using polymer FBGs in wearable medical devices is also described.

An ultrasensitive biosensor based on hybrid structure and composed of long-range surface plasmon polariton (LRSPP) and dielectric planar waveguide (PWG) modes is proposed. Both PWG and LRSPP modes have strong resonances to form strong coupling between the two modes, and the two modes can couple to enhance sensitivityof sensors. In the hybrid structure, PWG is composed of cytop–Si–cytop multilayers and the LRSPP configuration is composed of cytop–metal–sensing medium multilayer slabs. The highest imaging sensitivities of 2264 and 3619 RIU?1 were realized in the proposed sensors based on Au and Al-monolayer graphene, respectively, which are nearly 1.2 and 1.9 times larger than the 1910 RIU?1 sensitivity of the conventional LRSPR sensor (LRSPP sensor). Moreover, it is demonstrated that the PWG-coupled LRSPP biosensor is applicable to the sensing medium, with refractive index in the vicinity of 1.34.of Guangdong Province (2016B050501005); Science and Technology Project of Shenzhen (JCYJ20140828163633996, JCYJ20150324141711667); Natural Science Foundation ofSZU (201452, 201517, 827-000051, 827-000052, 827-000059).

We propose a reflection-type infrared biosensor by exploiting localized surface plasmons in graphene ribbon arrays. By enhancing the coupling between the incident light and the resonant system, an asymmetric Fabry–Perot cavity formed by the ribbons and reflective layer is employed to reshape the reflection spectra. Simulation results demonstrate that the reflection spectra can be modified to improve the figure of merit (FOM) significantly by adjusting the electron relaxation time of graphene, the length of the Fabry–Perot cavity, and the Fermi energy level. The FOM of such a biosensor can achieve a high value of up to 36/refractive index unit (36/RIU), which is ～4 times larger than that of the traditional transmission-type one. Our study offers a feasible approach to develop biosensing devices based on graphene plasmonics with high precision.

A CO2 sensor for capnography, based on a hollow waveguide (HWG) and tunable diode laser absorption spectroscopy (TDLAS), is presented; the sensor uses direct absorption spectroscopy and requires neither frequent calibration nor optical filters, giving it a significant advantage over existing techniques. Because of the HWG, the CO2 measurement achieved a concentration resolution of 60 ppm at a measurement rate of 25 Hz, as characterized by Allan variance. The length of the HWG was selected to efficiently suppress the optical fringes. This setup is perfectly suited for the detection of CO2 by capnography, and shows promise for the potential detection of other breath gases.