Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications. Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.

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Ultrafast All-Optical Graphene Modulator

Ultra Wideband Signals and Systems in Communication Engineering

Owing to their atomic layer thickness, strong light–material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

2D Materials for Optical Modulation: Challenges and Opportunities.

A novel refractive index (RI) sensor based on a fiber Mach-Zehnder interferometer was realized by concatenating two single-mode fiber tapers separated by a middle section. The proposed device had a minimum insertion loss of 3 dB and maximum interferometric extinction ratio over 20 dB. The resolution (0.171 nm) of the two-taper sensor to its surrounding RI change (0.01) was found to be comparable to that (0.252 nm) of similar structures made from an identical long-period gratings pair, and its ease of fabrication makes it a low-cost alternative to existing sensing applications.

Refractive Index Sensing With Mach–Zehnder Interferometer Based on Concatenating Two Single-Mode Fiber Tapers

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

In this paper, a fiber-optic surface plasmon resonance (SPR) biosensor is presented, in which a sheet of graphene acting as a sensing layer is coated around the gold film. A theoretical study of the proposed fiber-optic biosensor has been carried out by applying four-layer modal, which shows that by incorporating a graphene sensing layer, the sensitivity of the proposed SPR fiber biosensor can be greatly enhanced than the conventional gold film SPR fiber sensors. The relationship between resonance wavelengths and sensitivity of the proposed graphene sensing layer-based SPR fiber biosensor with the number of sensing layer has also been studied.

Graphene Enhances the Sensitivity of Fiber-Optic Surface Plasmon Resonance Biosensor

We demonstrate the fabrication of graphene-deposited tapered fibers (GDTFs), which can be used as saturable absorbers (SAs) for pulsed lasers. The advantages of GDTF SAs include flexibility, all-fiber configuration, and high optical damage threshold. The fabrication process is based on the interaction of the evanescent field of a tapered fiber with graphene. By in situ monitoring the transmitted power, the deposition process can be controlled, and the GDTF with a desirable level of nonsaturable absorption loss can be fabricated. We also study the dynamic deposition process by employing different waist diameters of tapered fibers and the different deposition powers. The results show that the deposition time can be significantly shortened with stronger evanescent field by decreasing the taper diameter or increasing the deposition power. Furthermore, by exploiting the GDTF as an intracavity passive power modulating element, we demonstrate efficient Q-switched and mode-locked erbium-doped fiber lasers, respectively.

Evanescent-Light Deposition of Graphene Onto Tapered Fibers for Passive Q-Switch and Mode-Locker

We propose and experimentally demonstrate simultaneous generation and transmission of downstream multiband signals and upstream data in a bidirectional radio-over-fiber system. The scheme is based on a dual-parallel Mach-Zehnder modulator and a following single-drive Mach-Zehnder modulator. Multiband data including baseband, frequency-doubled, and frequency-quadrupled signals are generated through optical carrier suppression and frequency-shifting techniques. Upstream data transmission is realized by remodulation of the downstream differential phase-shift-keying signal without the need of additional light source and wavelength management at the base station.

Simultaneous Generation and Transmission of Downstream Multiband Signals and Upstream Data in a Bidirectional Radio-Over-Fiber System

An optical liquid-level sensor (LLS) based on a long-period fiber grating (LPG) interferometer is proposed and experimentally demonstrated. Two identical 3-dB LPGs are fabricated to form an in-fiber Mach-Zehnder interferometer, and the fiber portion between two LPGs is exposed to the liquid as the sensing element. The sensitivity and measurement range of the sensors employing different orders of cladding modes are investigated both theoretically and experimentally. The experimental results show good linearity and large measurement range. One of the significant advantages of such a sensing structure is that the measurement level is not limited to the length of the LPG itself. Also, the measurement range and sensitivity of the proposed LLS can be readily tailored for a particular applications.

Implementation and Characterization of Liquid-Level Sensor Based on a Long-Period Fiber Grating Mach–Zehnder Interferometer

A new type of optical refractive-index (RI) sensor is proposed and experimentally demonstrated by using a structure of two single-mode fiber (SMF) Bragg gratings with a multimode fiber (MMF) taper in-between. The loss induced by a mismatch of waveguide structure between SMFs and MMFs is amplified by a tapering process, and is utilized for RI sensing through evanescent field. Experimental results show that the sensor possesses a tailorable sensitivity to the change of external RI and has a good linear response in the simultaneous measurement of external RI and temperature

Hongyan Fu

Papers

Graphene Enhances the Sensitivity of Fiber-Optic Surface Plasmon Resonance Biosensor

Evanescent-Light Deposition of Graphene Onto Tapered Fibers for Passive Q-Switch and Mode-Locker

Simultaneous Generation and Transmission of Downstream Multiband Signals and Upstream Data in a Bidirectional Radio-Over-Fiber System

Implementation and Characterization of Liquid-Level Sensor Based on a Long-Period Fiber Grating Mach–Zehnder Interferometer

Optical Refractive-Index Sensor Based on Dual Fiber-Bragg Gratings Interposed With a Multimode-Fiber Taper