Multifunctional optical nanofiber polarization devices with 3D geometry.

Here a reliable fabrication process enabling the integration of multiple functions in a single rod with one optical nano/microfiber (ONM) was proposed, which represents a further step in the "lab-on-a-rod" technology roadmap. With a unique 3D geometry, the all-fiber in-line devices based on lab-on-a-rod techniques have more freedom and potential for compactness and functionality than conventional fiber devices. With the hybrid polymer-metal-dielectric nanostructure, the coupling between the plasmonic and waveguide modes leads to hybridization of the fundamental mode and polarization-dependent loss. By functionalizing the rod surface with a nanoscale silver film and tuning the coil geometry, a broadband polarizer and single-polarization resonator, respectively, were demonstrated. The polarizer has an extinction ratio of more than 20 dB over a spectral range of 450 nm. The resonator has a Q factor of more than 78,000 with excellent suppression of polarization noise. This type of miniature single-polarization resonator is impossible to realize by conventional fabrication processes and has wide applications in fiber communication, lasing, and especially sensing.

Miniaturized broadband highly birefringent device with stereo rod-microfiber-air structure.

A wrapping-on-a-rod technique is presented and demonstrated successfully to realize broadband microfiber-based highly birefringent (Hi-Bi) devices with 3D geometry. By wrapping a circular microfiber (MF) on a Teflon-coated rod (2 mm in diameter), a large and broadband birefringence can be obtained utilizing a rod-microfiber-air (RMA) structure. Wavelength scanning method is used to measure the birefringence of the device. Results show that group birefringence as high as 10(-3) can be achieved over 400 nm wavelength range. This compact element presents great potential in sensing and communication applications, as well as lab-on-a-rod devices.

Microfiber-based Bragg gratings for sensing applications: a review.

Microfiber-based Bragg gratings (MFBGs) are an emerging concept in ultra-small optical fiber sensors. They have attracted great attention among researchers in the fiber sensing area because of their large evanescent field and compactness. In this review, the basic techniques for the fabrication of MFBGs are introduced first. Then, the sensing properties and applications of MFBGs are discussed, including measurement of refractive index (RI), temperature, and strain/force. Finally a summary of selected MFBG sensing elements from previous literature are tabulated.

Ultra-flattened and low dispersion in engineered microfibers with highly efficient nonlinearity reduction.

We propose an engineered microfiber with nano-scale slots that produce ultra-flattened and low dispersion of ±10 ps/(nm · km) over a 340 nm wavelength range. It is comparable with the results in photonic crystal fibers and planar slot waveguides, but can be hardly realized in conventional circular microfibers. By confining the light in a low nonlinearity air slot, the nonlinear coefficient can be greatly reduced. With the unique geometry and excellent performance, the slot microfiber offers large potential in miniature fiber devices for high-speed telecom applications.

Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe.

We experimentally demonstrate an all-silica first-order fiber Bragg grating (FBG) for high temperature sensing by focused ion beam (FIB) machining in a fiber probe tapered to a point. This 61-period FBG is compact (~36.6 µm long and ~6.5 µm in diameter) with 200-nm-deep shallow grooves. We have tested the sensor from room temperature to around 500 °C and it shows a temperature sensitivity of nearly 20 pm/°C near the resonant wavelength of 1550 nm. This kind of sensor takes up little space because of its unique geometry and small size and may be integrated in devices that work in harsh environment or for detecting small objects.

Miniature tapered photonic crystal fiber interferometer with enhanced sensitivity by acid microdroplets etching.

We fabricate a miniature tapered photonic crystal fiber (PCF) interferometer with enhanced sensitivity by acid microdroplets etching. This method is very simple and cost effective, avoiding elongating the PCF, moving and refixing the device during etching, and measuring. The refractive index sensing properties with different PCF diameters are investigated both theoretically and experimentally. The tapering velocity can be controlled by the microdroplet size and position. The sensitivity greatly increases (five times, 750 nm/RIU) and the size decreases after slightly tapering the PCF. The device keeps low temperature dependence before and after tapering. More uniformly and thinly tapered PCFs can be realized with higher sensitivity (∼100 times) by optimizing the etching process.

Broadband and highly efficient quadratic interactions in double-slot lithium niobate waveguides through phase matching.

We propose and study in detail a phase-matched quadratic optical interaction in a realizable 2D double-slot lithium niobate (LN) waveguide. As opposed to the single-slot waveguide, the field can be well confined in the nanometric nonlinear material, and it is also more flexible for birefringence and dispersion design. The proposed compact double-slot structure could not only achieve form birefringence phase matching but also effectively enhance the modal overlap integral and expand the working wavelength. The calculated results on second harmonic generation show an extremely large bandwidth of ~40 nm. The modal overlap integral up to 0.035 W(3/2)/V can be realized by optimizing the waveguide geometry, and it is much better than previous results on single-slot waveguides. Its temperature dependence is low--around 25 °C. The geometry is practical considering the current micromachining technique of LN.

Miniaturized fiber taper reflective interferometer for high temperature measurement.

We present an ultra-small all-silica high temperature sensor based on a reflective Fabry-Perot modal interferometer (FPMI). Our FPMI is made of a micro-cavity (approximately 4.4 microm) directly fabricated into a fiber taper probe less than 10 mum in diameter. Its sensing head is a miniaturized single mode-multimode fiber configuration without splicing. The sensing mechanism of FPMI is the interference among reflected fundamental mode and excited high-order modes at the end-faces. Its temperature sensitivity is approximately 20 pm/degrees C near the wavelength of 1550 nm. This kind of sensor can work in harsh environments with ultra-large temperature gradient, but takes up little space because of its unique geometry and small size.

Microfiber-probe-based ultrasmall interferometric sensor.

We report an ultrasmall microfiber-probe-based reflective interferometer for highly sensitive liquid refractive index measurement. It has a 3.5 microm micronotch cavity fabricated by focused ion beam micromachining. A sensitivity of 110 nm/RIU (refractive index unit) in liquid is achieved with over 20 dB extinction ratio. Theoretical analysis shows this kind of device is a hybrid of Fabry-Perot and modal interferometers. In comparison with normal fiber interferometers, this probe sensor is very compact, stable, and cheap, offering great potentials for detecting inside sub-wavelength bubbles, droplets, or biocells.