Huihui Lu

Reduced graphene oxide for fiber-optic humidity sensing

Graphene-based electrical chemical vapor sensors can achieve extremely high sensitivity, whereas the comparatively slow sensing response and recovery, the research focused on only low concentration detection, have been known as drawbacks for many applications requiring rapid and high concentration detection. Here we report a novel graphene-based fiber-optic relative humidity (RH) sensor relying on fundamentally different sensing mechanism. The sensor can achieve power variation of up to 6.9 dB in high relative humidity range (70-95%), and display linear response with correlation coefficient of 98.2%, sensitivity of 0.31 dB/%RH, response speed of faster than 0.13%RH/s, and good repeatability in 75-95%RH. Theoretical analysis of sensing mechanism can explain the experimental result, and reveal the broad applying prospect of the sensor for other kinds of chemical vapor detection. This novel graphene-based optical sensor provides a beneficial complement to the existing electrical ones, and will promote the employment of graphene in chemical sensing techniques.

Lithium niobate photonic crystal wire cavity: Realization of a compact electro-optically tunable filter

We report an electro-optically tunable filter using a lithium niobate photonic crystalcavity configuration with an efficient optical guiding geometry. The compact device (5.5 μm × 2.8 μm) was made on a hybrid waveguide combining an annealed proton exchange waveguide and a ridge waveguide realized by focused ion beam with vertically deposited electrodes. Due to the slow light and nonlinear effect in lithium niobate photonic crystal, experimental results show an enhanced tunability of ∼0.56 nm/V. This compact tunable photonic crystalcavity demonstration opens the path for the development of micro and nano-scale low-power driving active photonic devices.

Optical fiber with nanostructured cladding of TiO 2 nanoparticles self-assembled onto a side polished fiber and its temperature sensing

We demonstrated temperature sensing of a fiber with nanostructured cladding, which was constructed by titanium dioxide TiO2 nanoparticles self-assembled onto a side polished optical fiber (SPF). Significantly enhanced interaction between the propagating light and the TiO2 nanoparticles (TN) can be obtained via strong evanescent field of the SPF. The strong light-TN interaction results in temperature sensing with a maximum optical power variation of ~4dB in SPF experimentally for an external environment temperature varying from -7.8°C to 77.6°C. The novel temperature sensing device shows a linear correlation coefficient of better than 99.4%, and a sensitivity of ~0.044 dB/°C. The TN-based all-fiber-optic temperature sensing characteristics was successfully demonstrated, and it is compatible with fiber-optic interconnections and high potential in photonics applications.

All-optically reconfigurable and tunable fiber surface grating for in-fiber devices: a wideband tunable filter

A fiber surface grating (FSG) formed from a photosensitive liquid crystal hybrid (PLCH) film overlaid on a side-polished fiber (SPF) is studied and has been experimentally shown to be able to function as an all-optically reconfigurable and tunable fiber device. The device is all-optically configured to be a short period fiber surface grating (SPFSG) when a phase mask is used, and then reconfigured to be a long period FSG (LPFSG) when an amplitude mask is used. Experimental results show that both the short and long period FSGs can function as an optically tunable band-rejection filter and have different performances with different pump power and different configured period of the FSG. When configured as a SPFSG, the device can achieve a high extinction ratio (ER) of 21.5dB and a wideband tunability of 31nm are achieved. When configured as a LPFSG, the device can achieve an even higher ER of 23.4dB and a wider tunable bandwidth of 60nm. Besides these tunable performances of the device, its full width at half maximum (FWHM) can also be optically tuned. The reconfigurability and tunability of the fiber device open up possibilities for other all-optically programmable and tunable fiber devices.

Hybrid optical fiber add-drop filter based on wavelength dependent light coupling between micro/nano fiber ring and side-polished fiber

In this paper, we report our experimental study on directly coupling a micro/nano fiber (MNOF) ring with a side-polished fiber(SPF). As a result of the study, the behavior of an add-drop filter was observed. The demonstrated add-drop filter explored the wavelength dependence of light coupling between a MNOF ring and a SPF. The characteristics of the filter and its performance dependence on the MNOF ring diameter were investigated experimentally. The investigation resulted in an empirically obtained ring diameter that showed relatively good filter performance. Since light coupling between a (MNOF) and a conventional single mode fiber has remained a challenge in the photonic integration community, the present study may provide an alternative way to couple light between a MNOF device and a conventional single mode fiber based device or system. The hybridization approach that uses a SPF as a platform to integrate a MNOF device may enable the realization of other all-fiber optical hybrid devices.

Optical and RF Characterization of a Lithium Niobate Photonic Crystal Modulator

We optically characterize a micrometric radio frequency electro-optic modulator based on a lithium niobate photonic crystal. With the vertically deposited sidewalls electrodes, the experimental results show a half-wave voltage length product Vπ*L figure of merit of 0.0063 V-cm, a bandwidth of ~1 GHz for an active interaction area of ~37 μm2. This is the smallest figure of merit measured on an electro-optic modulator to date by several orders of magnitude with respect to bulk lithium niobate.

Fabrication of Side-Polished Single Mode-Multimode-Single Mode Fiber and Its Characteristics of Refractive Index Sensing

This paper presents a low-cost, flexible, and highly efficient wheel polishing techniques for the fabrication of sidepolished single mode-multimode-single mode fiber (SP-SMSF). The evolution of transmission spectrum of SP-SMSF is measured, simulated, and discussed. The good linear relationship between polished depth (PD) and polish-induced loss has relatively high linear correlation at 95%, allowing us to monitor and control the critical parameter PD in line and in real time. Several desirable SPSMSF with PD = 9.6 , 15, 20.6 μm were fabricated successfully. Their characteristics of refractive index (RI) sensing are investigated experimentally. The results show that SP-SMS sensitivity increases as RI increases, approaching its maximum when the latter gets close to its core. The maximum sensitivity of the SP-SMSF with PD = 20.6 μm is 1190 nm/RIU, comparable to that of chemically etched SMSF. The dependence of the sensitivity on the PD of SP-SMSF is also measured and discussed, showing that an increase in PD can improve the sensitivity of SP-SMSF. In addition, such novel structure of SP-SMSF will provide a flat platform to implement various fiber devices.

Reduced graphene oxide for fiber-optic toluene gas sensing

A fiber-optic toluene gas sensor based on reduced graphene oxide (rGO) is demonstrated and its sensing property is investigated experimentally and theoretically. The rGO film is deposited on a side polished fiber (SPF), allowing the strong interaction between rGO film and propagating field and making the SPF sensitive to toluene gas. It is found that the sensor has good linearity and reversibility and can work at room temperature with the response and the recovery time of 256 s and the detection limit of 79 ppm. Moreover, a theoretical model for the sensor is established to analyze the sensing mechanism. Theoretical analysis indicates this type of sensor could work in a wide range of toluene gas concentration and shows that a significant rise in its sensitivity can be expected by adjusting the doping level or chemical potential of graphene.

Microfiber With Methyl Blue-Functionalized Reduced Graphene Oxide and Violet Light Sensing

This letter introduces a methyl blue-functionalized reduced graphene oxide (MB-rGO) deposited on a microfiber (MF) and demonstrates its absorption characteristics can be tuned by violet light illumination. MB-rGO is fabricated by rGO stacked to noncovalent MB, exhibiting intriguing optical, electrical properties, and soluble in water. The MB-rGO was utilized as sensitive materials and MF as a substrate. The enhanced interaction between the light and the MB-rGO can be achieved via strong evanescent light of the MF. The experiments show that when the violet light is illuminated onto the MB-rGO film with power ranging from 0.03 to 12.77 mW, the transmitted optical power of the MF changes with a relative variation of 2.5~3 dB in broadband (1520-1620 nm) operation. The device has a sensitivity of ~0.235 dB/mW (1520 nm) and illustrates that the MB-rGO possesses high potentiality in photonic application, such as fiber-optic sensor and other photonic devices.

Large spatial and angular spin splitting in a thin anisotropic ε-near-zero metamaterial

We show theoretically that after transmitted through a thin anisotropic ε-near-zero metamaterial, a linearly polarized Gaussian beam can undergo both transverse spatial and angular spin splitting. The upper limits of spatial and angular spin splitting are found to be the beam waist and divergence angle of incident Gaussian beam, respectively. The spin splitting of transmitted beam after propagating a distance z depends on both the spatial and angular spin splitting. By combining the spatial and angular spin splitting properly, we can maximize the spin splitting of propagated beam, which is nearly equal to the spot size of Gaussian beam w(z).