Graphene based new physics phenomena are leading to a variety of stimulating graphene-based photonic devices. In this study, the enhancement of surface evanescent field by graphene cylindrical cladding is observed, for the first time, by using a graphene-coated microfiber multi-mode interferometer (GMMI). It is found theoretically and experimentally that the light transmitting in the fiber core is efficiently dragged by the graphene, hence significantly enhancing the evanescent fields, and subsequently improving the sensitivity of the hybrid waveguide. The experimental results for gas sensing verified the theoretical prediction, and ultra-high sensitivities of ~0.1 ppm for NH₃ gas detection and ~0.2 ppm for H₂O vapor detection are achieved, which could be used for trace analysis. The enhancement of surface evanescent field induced by graphene may pave a new way for developing novel graphene-based all-fiber devices with compactness, low cost, and temperature immunity.

Graphene and its derivative graphene oxide (GO) have been the focus of attention in the field of chemical and biological sensing. In this paper, we report a fiber-optic sensor for chemical gas sensing by using graphene oxide coated microfiber knot resonator (GMKR). The refractive index of GO was changed when the gas molecules were adsorbed to the surface of GO, and the gas concentration varying induced refractive index change can be detected by measuring the interference fringes shift of the GMKR. The experimental results show the sensitivities of ~0.35pm/ppm for NH₃ and ~0.17pm/ppm for CO detection, due to the different adsorption energy and charge transfer ability between the gas molecules and GO. Experimental results show GO is a promising candidate for gas sensing and can be combined with various fiber-optic devices due to the easy transfer process.

Graphene has become a bridge across optoelectronics, mechanics, and bio-chemical sensing due to its unique photoelectric characteristics. Moreover, benefiting from its two-dimensional nature, this atomically thick film with full flexibility has been widely incorporated with optical waveguides such as fibers, realizing novel photonic devices including polarizers, lasers, and sensors. Among the graphene-based optical devices, sensor is one of the most important branch, especially for gas sensing, as rapid progress has been made in both sensing structures and devices in recent years. This article presents a comprehensive and systematic overview of graphene-based microfiber gas sensors regarding many aspects including sensing principles, properties, fabrication, interrogating and implementations.

A graphene-coated microfiber (GCM)-based hybrid waveguide structure formed by wrapping monolayer graphene around a microfiber with length of several millimeters is pumped by a nanosecond laser at $\sim 1550\mathrm{nm}$, and multi-order cascaded four-wave-mixing (FWM) is effectively generated. By optimizing both the detuning and the pump power, such a GCM device with high nonlinearity and compact size would have potential for a wide range of FWM applications, such as phase-sensitive amplification, multi-wavelength filter, all-optical regeneration and frequency conversion, and so on.

A miniature fiber-optic microphone (FOM) based on a periodic corrugated microelectro-mechanical-system (MEMS) diaphragm has been developed in this paper. A micro Fabry-Perot cavity is an acoustic sensor structure made up of a single-mode fiber(SMF) end face and a MEMS diaphragm. The achieved signal-to-noise ratio (SNR) of the FOM is 79.08 dB under the sound pressure of 37.76 mPa at the 1 kHz acoustic signal, and the corresponding minimum detectable pressure is as low as 2.97 μPa/Hz ^1/2 . In addition, the FOM has a flat frequency response and a linear sound pressure response within the audible sound frequency range. These results indicate that this FOM shows the extreme sensitivity and miniature size, making it a promising technology for acoustic detection.

In this Letter, a graphene-coated D-shaped fiber (GDF) chemical gas sensor is proposed and demonstrated. Taking advantage of both the graphene-induced evanescent field enhancement and the in-fiber multimode interferometer, the GDF shows very high sensitivity for polar gas molecule adsorptions. An extinction ratio of up to 28 dB within the free spectrum range of $\sim 30\mathrm{nm}$ in the transmission spectrum is achieved. The maximum sensitivities for ${\mathrm{NH}}_3$ and ${\mathrm{H}}_2\mathrm{O}$ gas detections are $\sim 0.04$ and $\sim 0.1\mathrm{ppm}$, respectively. A hybrid sensing scheme with such compactness, high sensitivity, and online monitoring capabilities may pave the way for others to explore a series of graphene-based lab-on-fiber devices for biochemical sensing.