Electroless nickel plating on optical fiber probe

As a component of near-field scanning optical microscope （NSOM）, optical fiber probe is an important factor influncing the equipment resolution. Electroless nickel plating is introduced to metallize the optical fiber probe. The optical fibers are etched by 40% HF with Turner etching method. Through pretreatment, the optical fiber probe is coated with Ni-P film by electroless plating in a constant temperature water tank. Atomic absorption spectrometry （AAS）, scanning electron microscopy （SEM）, and energy dispersive X-ray spectrometry （EDXS） are carried out to characterize the deposition on fiber probe. We have reproducibly fabricated two kinds of fiber probes with a Ni-P film： aperture probe and apertureless probe. In addition, reductive particle transportation on the surface of fiber probe is proposed to explain the cause of these probes.

Finite element analysis on stress-induced birefringence of polarization-maintaining optical fiber

Influence of thermal-mechanical properties on the features of the panda polarization-maintaining optical fiber (PMF-) in fabrication process is studied in detail by finite element method (FEM). The stress birefringence is 2.13443×10-4 obtained by the static analysis and 2.1269×10-4 by dynamic analysis. The difference in simulation by two methods is around 0.4%. The non-uniformity of stress birefringence in the fiber core is about 1.6%. Predicted results demonstrate that effect of the thermal conductive parameter on fiber thermal stress dominates. The high and uniform stress birefringence in the fiber core is obtained by appropriately selecting suitable stress region area and position.

Multi-hole plastic optical fiber force sensor based on femtosecond laser micromachining

We study the theoretical and experimental effects of hole quantity and inter inter-hole spacing on insertion loss for using femtosecond laser to make bend-sensitive multi-hole plastic optical fiber （POF）, and also analyze the mechanism of bending loss in multi-hole POF. A force sensor based on bending loss of the multi-hole POF is fabricated. The measurement ranges from 0 to 65 N, and the maximum output change exceeds 15.51 dB with good linearity and repeatability, and the sensitivity is 0.24 dB/N.

Highly efficient fiber-to-chip evanescent coupling based on subwavelength-diameter optical fibers

A novel, compact, and highly efficient fiber-to-chip evanescent coupling structure is proposed based on a subwavelength-diameter fiber. The coupling structure is characterized by a large misalignment tolerance and easy fabrication. The dependence of coupling efficiency on various parameters is calculated and analyzed. The simulation results show that a coupling efficiency as high as 95% can be obtained within a coupling length of 4 μm.

Temperature-insensitive refractive index sensor based on an optical fiber extrinsic Fabry–Perot interferometer processed by a femtosecond laser

An optical fiber extrinsic Fabry-Perot interferometer （EFPI） is designed and fabricated for refractive index （RI） sensing. To test the RI of liquid, the following two different methods are adopted： the wavelength tracking method and the Fourier-transform white-light interferometry （FTWLI）. The sensitivities of sensors with cavity lengths of 288.1 and 358.5 μm are 702.312 nm/RIU and 396.362 μm/RIU, respectively, by the two methods. Our work provides a new kind of RI sensor with the advantages of high sensitivity, mechanical robustness, and low cross sensitivity to temperature. Also, we provide a new method to deal with gold film with a femtosecond laser.

Fabrication of fiber-embedded multi-core photonic crystal fibers

A novel fabrication method of multi-core photonic crystal fibers is proposed on the basis of a fiber-embedded technique. A taper tower is used to modify the structures of the fiber preform, and four steps of fiber fabrication and different structures of fiber samples are given. The mode structures and beating characteristics of a photonic crystal fiber sample with two successive cores are investigated in detail with the help of a supercontinuum light source, a charge-coupled device （CCD） camera, and an optical spectrum analyzer. The test results show a clear beating phenomenon between two orthotropic polarization modes with a 2.8-nm peak interval in wavelength.

Calibrating an optical fiber humidity sensor and applying it in real-time monitoring of relative humidity in fresh concrete

A doubly cladding single-mode fiber humidity sensor is fabricated by agarose. The sensor has an insertion loss of -0.08 dB and a power change of -17.83 dB. The responses of the sensor to a relative humidity （RH） range form 30% to 100% at a temperature range form 25 to 34 ~C are validated. The experiments demonstrate that the absorbability of agarose gel to moisture decreases with increasing RH in measured gas. We propose a calibration method that uses lookup tables and construct a corresponding calibration matrix. Using the sensor, we conduct real-time monitoring of RH in fresh concrete during its hardening nrocess.

Temperature-insensitive Mach-Zehnder interferometric strain sensor based on concatenating two waist-enlarged fiber tapers

A novel temperature-insensitive strain sensor, based on an in-line Mach-Zehnder interferometer, is fabri- cated by concatenating two waist-enlarged fiber tapers separated by a short piece of photonic crystal fiber. The interference spectrum of the proposed sensor is analyzed in detail. Experimental results demonstrate that this sensor has a strain sensitivity of 3.02 pm/μe and maintains the temperature insensitivity feature. The proposed sensor has great potential in diverse sensing applications due to its advantages, such as its compact size, low cost, and simple fabrication process.

High speed sub-micrometric microscopy using optical polymer microlens

We report the high speed scanning submicronic microscopy （SSM） using a low cost polymer microlens integrated at the extremity of an optical fiber. These microlenses are fabricated by a free-radical photopolymerization method. Using a polymer microlens with a radius of curvature of 250 nm, a sub-micrometric gold pattern is imaged experimentally by SSM. Different distances between the tip and the sample are used with a high scanning speed of 200 cm/s. In particular, metallic absorption contrasts are described with an optical spatial resolution of 250 nm at the wavelength of 532 nm. Moreover, finite-difference time-domain （FDTD） simulations concerning the focal lengths of microlenses with different geometries and heights support the experimental data.

Study of heat transfer characteristics in PCFG fabrication technology using heat method

The fiber gratings fabrication technology with the heating method in a photonic crystal fiber （PCF） based on structural change is examined. The principle of photonic crystal fiber gratings （PCFGs） is analyzed in theory. The heat transfer theory and finite element method are used to examine the thermal field distribution in the fiber and the influence of the air hole structure in the cladding, and the parameters of the laser beam in the process of grating fabrication are discussed. The results show that gratings can be formed by the periodic collapse of air holes in the cladding of PCFs. Under double-point heating condition, the energy is uniformly distributed in the radial direction and is approximate to Gaussian distribution in the axial direction. With the same size of the luminous spot, as the layers and radius of the air holes increase, the laser power needed to make the air holes collapse decreases. With the same laser power, as the luminous spot radius increases, the needed heating time increases. Moreover, the relationship between the laser power needed and the air filling rate is obtained as the number of layers of the air holes changes from 1 to 7. This kind of PCFG can overcome the long-term thermal instability of conventional gratings in substance and thus has great potential applications in the related field of optical fiber sensors.

Free-space 1×2 wavelength-selective switches for wavelength-division multiplexing optical networks

A free-space 1 ×2 wavelength-selective switch （WSS） based on thin-film filter technology is proposed. The 1 × 2 WSS is fabricated with an electromagnetic actuator, a reflecton prism, a narrow-band thin-film filter, and three fiber collimators. The working principle and the configuration of WSS are illuminated. The experimental results indicate a fiber-to-fiber insertion loss ranging from 1.109 to 1.249 dB with 2-V voltage input, which satisfies the application of optical fiber communication.

Study on transmission characteristics of photonic crystal fiber gratings with periodic air hole deformation

The paper studies a kind of improved photonic crystal fiber gratings fabricated by CO2 laser heating method. The effective refractive index of cladding induced by periodic air hole deformation is computed using multipole method, and the rela-tionship between the effective refractive index and the collapse of air-holes is discussed, thereby the modulation expression of effective refractive index is obtained. The grating transmission characteristics are simulated. The results indicate that with the diameter of air-holes increasing from 3.3 ~m to 3.7 μm, the resonance wavelength shows blue-shift, the resonance peak intensifies, and the bandwidth becomes narrow. As the collapse degree of cladding enhances, the resonance wave- length shows red-shift, the transmission increases, and the bandwidth tends to narrow.

Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 μm

A robust design for a photonic crystal fiber （PCF） based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is, presented. This design is suitable for flat broadband supercontinuum （SC） generation in the 1.55-μm region. The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W^-1·km^-1 at 1550 nm. Ultraflat dispersion with a value between -1.65 and -0.335 ps/（nm·km） is obtained ranging from 1375 to 1625 nm. The 3-dB bandwidth of the SC is 125 nm （1496-1621 nm）, with a fiber length of 80 m and a corresponding input peak power of 43.8 W. The amplitude noise is considered to be related to SC generation. For practical fabrication, the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.

Design of a fiber polarizer based on an asymmetric dual-core photonic crystal fiber

We propose a novel optical polarizer based on an asymmetric dual-core photonic crystal fiber(PCF) with triangular lattice air-holes.The fiber is designed as that the effective indices of modes in the two cores are matched at one polarized state but mismatched at another polarized state.As a result,one of the polarization states is coupled to the other core and transferred into a high-order mode.The transmission properties of the polarizer are investigated by the semi-vectorial beam propagation method(SV-BPM).Numerical results demonstrate that a device length of 11.3 mm shows extinction ratio as low as-20 dB with bandwidth as great as 80 nm ranging from 1.51 mm to 1.59 mm.

Design of large-core single-mode Yb^(3+)-doped photonic crystal fiber

The effective index of the cladding fundamental space-filling mode in photonic crystal fiber (PCF) is simulated by the effective index method. The variation of the effective index with the structure parameters of the fiber is achieved. For the first time, the relations of the V parameter of Yb3+-doped PCF with the refractive index of core and the structure parameters of the fiber are provided. the single-mode characteristics of large-core Yb3+-doped photonic crystal fibers with 7 and 19 missing air holes in the core are analyzed. The large-core single-mode Yb3+-doped photonic crystal fibers with core diameters of 50 μm, 100 μm and 150 μm are designed. The results provide theory instruction for the design and fabrication of fiber.

Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities

We propose a temperature-insensitive refractive index（RI） fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber（PCF） between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.

Temperature dependence of the PER in PM-PCF coil

A piece of domestic polarization-maintaining photonic crystal fiber （PM-PCF, 964 m in length） is made into a fiber coil, and its polarization extinction ratio （PER） is measured in a temperature range of –45– 80 ℃ before and after PM-PCF is wound and solidified. A fiber coil made of commercial panda PM fiber （PMF） is also fabricated and measured for comparison. Our experiments show that the PER variation of the PM-PCF coil （2.25 dB） is far smaller than that of the panda PMF coil （10 dB） in the whole temperature range because PM-PCF is intrinsically insensitive to the temperature variation and stress in the fiber coil induced by the winding and solidification process. This characteristic is important for the real application of PM-PCFs in temperature-insensitive fiber interferometers, fiber sensors, and optical fiber gyroscopes.

Cascaded PCF tapers for flat broadband supercontinuum generation

We report the fabrication of cascaded photonic crystal fiber （PCF） tapers in monolithic design. Flat broadband supercontinuum （SC） generation in cascaded PCF tapers pumped by sub-nanosecond pulses from a 1 064-nm microchip laser is demonstrated. The spectral width （20 dB） extends from 0.47 to 1.67 μm. In the optimal configuration, an ultraflat （3 dB） spectrum from 500 to 1 000 nm is achieved.