Chi-Fung Jeff Pun

Highly reflective Bragg gratings in slightly etched step-index polymer optical fiber.

During the past few years, a strong progress has been made in the photo-writing of fiber Bragg gratings (FBGs) in polymer optical fibers (POFs), animated by the constant wish to enhance the grating reflectivity and improve the sensing performances. In this paper, we report the photo-inscription of highly reflective gratings in step-index POFs, obtained thanks to a slight etching of the cladding. We demonstrate that a cladding diameter decrease of ~12% is an ideal trade-off to produce highly reflective gratings with enhanced axial strain sensitivity, while keeping almost intact their mechanical resistance. For this, we make use of Trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. FBGs are inscribed at ~1550 nm by the scanning phase mask technique in POFs of different external diameters. Reflectivity reaching 97% is achieved for 6 mm long FBGs, compared to 25% for non-etched POFs. We also report that a cladding decrease enhances the FBG axial tension while keeping unchanged temperature and surrounding refractive index sensitivities. Finally and for the first time, a measurement is conducted in transmission with polarized light, showing that a photo-induced birefringence of 7 × 10(-6) is generated (one order of magnitude higher than the intrinsic fiber birefringence), which is similar to the one generated in silica fiber using ultra-violet laser.

Tilted Bragg gratings in step-index polymer optical fiber

We report the first slightly tilted Bragg gratings photo-inscription in polymer optical fiber (POF). For this, we make use of trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. Tilted fiber Bragg gratings (TFBGs) are inscribed in the near-infrared wavelength range using the scanning phase mask technique with a tilted phase mask in the plane perpendicular to the laser beam direction. The transmitted amplitude spectrum evolution of a 3° TFBG is analyzed as a function of the surrounding refractive index. A maximum sensitivity close to 13  nm/RIU (refractive index unit) is obtained in the range 1.42-1.49.

Single-Mode Perfluorinated Polymer Optical Fibers With Refractive Index of 1.34 for Biomedical Applications

We demonstrate a technique for the fabrication of single-mode perfluorinated polymer optical fiber (PPOF). The PPOF preform is composed of poly-methyl-methacrylate (PMMA)-based outer cladding and a graded-index multimode PPOF as the core. A photosensitive graded-index single-mode PPOF with a core diameter of about 6.6 ¿m and cladding diameter of 400 ¿m was fabricated. The fiber has a cutoff wavelength of 854 nm and exhibits single-mode characteristics at wavelengths of 1310 and 1550 nm. The transmission loss is less than 0.2 dB/m in the wavelength range of 1410-1540 nm and less than 0.5 dB/m for wavelengths up to 1610 nm, significantly less than the typical transmission loss of ~100 dB/m for PMMA fiber. Another important feature of the PPOF is its low refractive index of 1.34, close to aqueous solution of biomaterials, permitting strong optical coupling for biomedical applications.

Side-Illumination Fluorescence Dye-Doped-Clad PMMA-Core Polymer Optical Fiber: Potential Intrinsic Light Source for Biosensing

A specially designed laser-dye-doped polymer optical fiber (POF) was drawn and investigated experimentally. The cladding of the POF was doped with Rhodamine 640 and pumped with a compact and low-cost 532-nm diode-pumped solid-state laser. By using the side-illumination fluorescence technique, any section along the POF can act as an intrinsic light source. The fluorescence power of the dye-doped POF under side-illumination was improved by more than 10 times by a bending method. With this enhancement, the issue of coupling light into the POF becomes simpler, cheaper, and more consistent, constructing a practical platform for a sensing system.

Ultra-fast polymer optical fibre Bragg grating inscription for medical devices

We report the extraordinary result of rapid fibre Bragg grating inscription in doped polymer optical fibres based on polymethyl methacrylate in only 7 ms, which is two orders of magnitude faster than the inscription times previously reported. This was achieved using a new dopant material, diphenyl disulphide, which was found to enable a fast, positive refractive index change using a low ultraviolet dose. These changes were investigated and found to arise from photodissociation of the diphenyl disulphide molecule and subsequent molecular reorganization. We demonstrate that gratings inscribed in these fibres can exhibit at least a 15 times higher sensitivity than silica glass fibre, despite their quick inscription times. As a demonstration of the sensitivity, we selected a highly stringent situation, namely, the monitoring of a human heartbeat and respiratory functions. These findings could permit the inscription of fibre Bragg gratings during the fibre drawing process for mass production, allowing cost-effective, single-use, in vivo sensors among other potential uses.

Optimization of 3-hole-assisted PMMA optical fiber with double cladding for UV-induced FBG fabrication

We propose a new hole-assisted polymer optical fiber design to eliminate the influence of dopant diffusion and to increase the UV writing efficiency in fiber Bragg grating inscription. The optical waveguide is formed inside a solid core surrounded by a ring of 3 large air holes in enhanced UV photosensitive PMMA with double-cladding. We determined a map of the single-mode and multi-mode phase transitions using a finite-element- based vectorial optical mode solver. We obtained a wide range of geometrical configuration for the single-transverse-mode (HE11) propagation in the visible. The design is optimized to operate at the minimum optical loss wavelengths of 580 nm and 770 nm.

Amplified Spontaneous Emission of Side-pumped Step-index Rh640 perchlorate-dye-doped Polymer Optical Fiber

Step-index Rhodamine 640 perchlorate-dye-doped polymer optical fiber with a lightly dye-doped core and relatively high dye-doped cladding is fabricated, and its amplified spontaneous emission performance by side pumping is demonstrated.

Hybrid Silica and Laser-dye-doped Polymer Fiber

We report the fabrication of a hybrid optical fiber with silica core and laser-dye-doped UV curable polymer cladding. Its potential to act as a combined intrinsic light source and FBG sensor is demonstrated.