Xiujie Tian

Photoinduced reversible gel–sol transitions of dicholesterol-linked azobenzene derivatives through breaking and reforming of van der Waals interactions

A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as DCAZO0, DCAZO2 and DCAZO6, respectively. A gelation test reveals that a subtle change in the length of the spacer can produce a dramatic change in the gelation behavior of the compounds. DCAZO2 obtains the minimum gelation concentration among the three gelators. For cyclopentanone gel of DCAZO2, the reversible gel–sol transitions by irradiation with UV and visible light are investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD analyses. Upon UV irradiation of the gel, trans–cisphotoisomerization of the azobenzene groups occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in the gel–sol transition. The gel can be recovered by the reverse cis–transphotoisomerization after the exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble into one-dimensional fibers with diameters 50–100 nm in an anticlockwise direction, which further crossed-linked to form three-dimensional networks.

Super-High Sensitivity of Fiber Temperature Sensor Based on Leaky-Mode Bent SMS Structure

A simple temperature sensor based on a bent single mode-multimode-single mode (SMS) fiber structure fixed on a polymer plate frame is proposed and experimentally investigated. The cladding of the multimode fiber in SMS is substituted with an E7 whose refractive index is a little higher than that of the core of the fiber. This construction brings two characteristics for the sensor: one is the leaky mode interference of the multimode fiber and the other is the preset curvature of the bent SMS fiber. When the temperature rises, the curvature of the SMS fiber is altered through the expanding of the polymer plate frame. The shift of central wavelength is observed on the output spectra of the fiber sensor along with temperature changing in the range from 51°C to 65 °C. The proposed fiber sensor offers sensitivity of 6.5 nm/°C, which is significantly higher than that of a normal straight SMS structure or a grating-based fiber structure.

High-sensitivity temperature sensor based on Bragg grating in BDK-doped photosensitive polymer optical fiber

A single-mode polymer optical fiber (POF) with highly photosensitive core doped with benzil dimethyl ketal (BDK) is fabricated and used for writing Bragg grating through the two-beam interference method. The Bragg wavelength of the grating is about 1570 nm, while the full-width at half-maximum (FWHM) of the reflection peak is 0.3 nm. The temperature response of POF Bragg grating is theoretically analyzed and experimentally measured in contrast to silica optical fiber Bragg grating (FBG). The result shows that the temperature character of POF Bragg grating is negative, which is opposite to the silica optical FBG. The absolute value of the temperature response of POF Bragg grating is one order of magnitude higher than that of the silica optical FBG, making POF Bragg grating appear to be very attractive for constructing temperature sensors with high resolution.

Reversible All-Optical Modulation Based on Evanescent Wave Absorption of a Single-Mode Fiber With Azo-Polymer Overlay

An etched single-mode fiber with azo-polymer overlay is made by the chemical etching method. The fibers cylindrical symmetry is maintained and the refractive index of the azo-polymer is controlled to be lower than that of the fiber to keep the waveguide structure. By altering the refractive index of the azo-polymer in course of its photoisomerization, the evanescent wave absorption is controlled, based on the penetration depth changing of the evanescent wave, and eventually modulates the optical power transmitted though the fiber. Under cyclical irradiation of UV light and visible light, reversible modulation of the transmitted optical power at 1550 nm is achieved.

Optically Tunable Polarization State of Propagating Light at 1550 nm in an Etched Single-Mode Fiber With Azo-Polymer Overlay

An optically tunable and rotatable polarization state of light at 1550 nm is achieved in an etched single-mode fiber (SMF) with azo-polymer overlay. The SMF is etched by hydrofluoric acid to access the evanescent field, and then immobilized by the azo-polymer. With irradiation of 435-nm polarized light, photoinduced orientation of the azo-polymer occurs, causing birefringence in the fiber, and finally makes the polarization state of the 1550-nm propagating light tunable. Changing the polarization directions of 435-nm polarized lights, the polarization state of 1550-nm propagating light is tuned from right-handed elliptically polarized light to left-handed elliptically polarized light.

Optical amplification of Eu(TTA) 3 Phensolution-filled hollow optical fiber

A liquid core optical fiber (LCOF) composed of hollow fiber and a solution of Eu(TTA)(3)Phen (TTA=2-thenoyltrifluoroacetone, Phen=1, 10-phenanthroline) in dimethyl sulfoxide (DMSO) has been fabricated, in which the concentration of Eu(TTA)(3)Phen in DMSO is 0.8 wt.%, the core diameter of the LCOF is 10 μm, and the fiber length is 8.1 cm. By the end pumping with a diode-pumped solid-state laser at 355 nm, a small optical signal at 613 nm was amplified with a gain of 8.2 dB at a pump power of 203 mW. Based on this experimental result, a liquid core optical fiber amplifier can be realized by the LCOF, which has wide potential applications in many optical devices.

Ultraviolet Imaging Based on Surface Plasmon Resonance With Azo-Polymer Sensing Layer

An innovative surface plasmon resonance (SPR) sensor is constructed to achieve ultraviolet (UV) imaging. The SPR sensor is fabricated using a gold-coated prism, which is then overlaid with an azo-polymer film as a UV sensing layer. The refractive index of the azo-polymer decreases during UV light irradiation. The changes of the refractive index influence the intensity of the light reflected from the SPR sensor. With a CCD camera, both the images of the light beams reflected from the SPR sensor before and after UV irradiation are recorded. The shape of UV light beam intensity distribution can then be calculated by comparing the difference between these two images.

Optical modulation of polarization state based on an etched single mode fiber with azo-polymer overlay

A large modulation range of polarization state at 1550nm is achieved by using an etched single mode fiber (SMF) with azo-polymer overlay. With irradiation of polarized light, the photo-reorientation of azo-polymer occurs, causing the birefringence in the fiber. This optically induced birefringence results in the change of polarization state which is continuously recorded on the Poincare sphere, finally makes the state of polarization (SOP) tunable by the irradiation of 365nm polarized light. In our experiment the SOP changes from-handed elliptical polarization to left-handed elliptical polarization and the ellipticity varies from 16.2° to 55.6° during the irradiation. The trajectory of the SOP can also be recovered to the initial polarization state when irradiated by unpolarized light. The experimental results show that the modulation is not only in large range, but also dynamic, reversible.

Colorimetric Sensor for Fine Differentiation of Organic Solvents Based on Only One Kind of Polydiacetylene Coated on Polymer Optical Fiber

The hydrogen-bonding complexes of poly (4-vinylpyridine) and diacetylene derivatives 10, 12-tricosadiynoic acid are deposited outside of the side-polished optical fiber as part of the cladding. In the evanescent field of the optical fiber, the blue-to-red colorimetric transition can be monitored by the change of the propagating light in the fiber when the polydiacetylene (PDA) overlay is exposed to organic solvents. As the organic solvents have different diffusivities which cause different colorimetric transition in the evanescent field, when the coated fiber is put in different solvent atmospheres the change trends of optical intensity in the fiber are different accordingly. Using this kind of structure, the colorimetric differentiation of some organic solvents in the vapor state based on just one DA monomer is realized. Together with the remote measurement character of the fiber sensor, it has potential applied value in the future.