We demonstrate a nanosecond-pulse erbium-doped fiber laser that is passively mode locked by a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution. Owing to the good solution processing capability of few-layered graphene oxide, which can be filled into the core of a hollow-core photonic crystal fiber through a selective hole filling process, a graphene saturable absorber can be successfully fabricated. The output pulses obtained have a center wavelength, pulse width, and repetition rate of 1561:2 nm, 4:85 ns, and 7:68 MHz, respectively. This method provides a simple and efficient approach to integrate the graphene into the optical fiber system. [14] have been used for ultrafast lasers. The common method of integrating graphene SAs in laser cavities is to sandwich a graphene SA film between two fiber connectors with a fiber adaptor [10][11][12][13][15][16][17]. However, it is known that solubility and/or processability are the first issues for many prospective applications of graphene-based materials [19]. Although GO has a good solubility in water and many organic solvents, unfortunately carbon atoms bonded with oxygen groups are sp 3 hybridized and disrupt the sp 2 conjugation of the hexagonal graphene lattice in GO and thus destroy the linear dispersion of the Dirac electrons and influence the unique optical properties of graphene [20]. This makes GO unsuitable as a broadband SA in laser cavities for ultrafast pulse generation. Thus, a solution-phase graphene with large SA becomes crucially important because it can easily be integrated into a range of photonic systems. Furthermore, a solution-phase graphene can provide a robust method of nonlinear interaction of the guided mode, which has the potential benefit of increasing the damage threshold of the SA for high power pulse formation in laser cavity.Here, using a selective hole filling technique, we report an erbium-doped fiber laser mode locked by a hollowcore photonic crystal fiber (HC-PCF) filled with fewlayered graphene oxide (FGO) solution. The good solution processing capability of FGO makes it possible to be filled into the core of HC-PCF. The cavity comprises sections with normal and anomalous dispersion and the output pulses of 4:85 ns are obtained at the center wavelength of 1561:2 nm.Our all-fiber mode-locked ring laser setup is shown in Fig. 1. A 1:5 m heavily erbium-doped fiber (OFS EDF-80) is used as the gain medium, pumped by a 1480 nm high power laser diode through a wavelength division multiplexer coupler. A polarization controller (PC) is used to optimize the mode-locking operation while a polarization independent isolator maintains the unidirectional

We demonstrate a wavelength-tunable, passively mode-locked erbium-doped fiber laser based on graphene and chirped fiber Bragg grating. The saturable absorber used to enable passive mode-locking in the fiber laser is a section of microfiber covered by graphene film, which allows light-graphene interaction via the evanescent field of the microfiber. The wavelength of the laser can be continuously tuned by adjusting the chirped fiber Bragg grating, while maintaining mode-locking stability. Such a system has high potential in tuning the mode-locked laser pulses across a wide wavelength range.

Ultraviolet (UV) photodetectors with high responsivity and speed are highly desirable for imaging and remote sensing applications. Limited by the crystalline quality of a GaN-based material, which is ideal for UV photodetection, the further improvement of the performance is minimal. A hybrid graphene/unintentionally doped (UID) GaN UV photodetector with both high responsivity and high speed is reported. Holes in graphene, which are induced by the photogenerated electrons trapped at the graphene/UID GaN interface according to the capacitive effect, have a long lifetime owing to the electron-hole pair separation in space. Graphene acts as a carrier transport channel and greatly increases the charge collection efficiency under an external bias voltage. The responsivity of a hybrid graphene/UID GaN photodetector with a photosensitive area of 2 mm2 reaches 5.83 A/W at −10 V with a specific detectivity of ∼1011 Jones. The response time is ∼5 ms, which is faster than that of traditional GaN photodetectors. These results will provide a feasible route to UV detection with high performance.

Photodetectors based on two‐dimensional (2D)/ three‐dimensional (3D) semiconductor heterojunction structures are emerging as appealing candidates for high‐sensitivity applications. The performances of these hybrid photodetectors are closely correlated with their current gain mechanism. Carrier recirculation is the most commonly reported mechanism. Recently, a Fermi level alignment mechanism was proposed for 2D graphene/0‐dimensional (0D) quantum dot heterostructures because of the easy Fermi level tunability of the quantum dot. In this article, an interface‐induced gain mechanism using this Fermi level alignment process is proposed and identified based on a 2D graphene/3D GaAs hybrid structure with comparative measurement configurations. Because of the high surface state density of GaAs, the photo‐excited holes tend to become trapped at the graphene/GaAs interface, which can easily lower the interface Fermi level and the Fermi level in graphene via an alignment process. When combined with the high carrier mobility characteristics of graphene, a maximum current gain of 2520 and responsivity of 1321 A W−1 are achieved in the devices. This study clarifies the role of the interface states in the gain characteristics of some 2D/3D hybrid devices, with results that are instructive for optimal device design.

The theoretical basis and experimental evidence for a Stem-Volmer type relationship using the fluorescence ratio (Ibi/i) in the absence (ij,i) and presence (I) of CO2 as response function for fluorescence-based fiber-optic CO2 sensors is explored. By choosing the right indicator, appropriate bicarbonate concentration, and a thin enough sensing layer, a fiber-optic CO2 sensor with a usable linear range can be easily achieved using Ibi/I as the response function. This allows the possibility of one-point calibration, which cannot be accomplished if the measured fluorescence intensity (I) is used directly as the response function.The carbon dioxide electrode and carbon dioxide optrode, each possessing its own merits and shortcomings, are the two major C02 sensors developed to date.1•2 Among all the CO2 optrodes explored so far, the fluorescence-based fiber-optic C02 optrodes, which were first described by Zhang and Seitz,3 have the most advantageous features and therefore are preferred in practical usage. Compared to the commercially available CO2 electrodes, CO2 optrodes offer advantages for remote sensing and have minimum electrical or magnetic interferences. However, they suffer two major limitations, e.g., the lack of a linear response function, which is a major drawback for biosensor use, and the variation in measured fluorescence intensities due to light source intensity changes or indicator photobleaching. Addressing this latter problem, Zhang and Seitz4 5developed a two-wavelength excitation method which can make the fluorescence-based pH sensor insensitive to the effect of photobleaching, wash out, and light source intensity variations. This method also applies to the fluorescence-based fiber-optic C02 optrode, which is based on pH sensing. Since it requires more complicated instrumentation, however, applications using this method are limited. As far as the response function is concerned, either the fluorescence intensities at one wavelength or the ratio of fluorescence intensity at two wavelengths as in the two-wavelength method are commonly adopted as indicators for C02 concentration.3,5•6 **In both cases, the calibration curves are not theoretically linear.In this work, we propose the use of the ratio of fluorescence intensities in the absence (/bi) and the presence (I) of C02 as the (1) Turner, A P.