Parallel beam frequency doubling of 170 fs, NIR pulses is demonstrated by placing a thin beta barium borate (BBO) nonlinear crystal after a spatial light modulator. Computer-generated holograms applied to the spatial light modulator create 18 parallel diffracted beams at the fundamental wavelength λ = 775 nm, then frequency doubled to λ = 387 nm and focussed inside the poly(methyl methacrylate) (PMMA) substrate for refractive index structuring. This procedure, demonstrated for the first time in PMMA, requires careful attention to phase matching of multiple beams and opens up dynamic parallel processing at UV wavelengths where nematic liquid crystal devices are more sensitive to optical damage. By overlapping filamentary modifications, an efficient, stable volume phase grating with dimensions 5 × 5 × 2.0 mm 3 and pitch Λ = 15 µm was fabricated in 18 minutes and reached a first-order diffraction efficiency of 70 % at the Bragg angle.

Techniques to directly write localised refractive index structures in polymer optical fibres (POF) are presented, using UV (400nm) ultrafast laser with pulse lengths of 100 fs to create in-fibre gratings for sensing. No doping is necessary for photosensitisation so commercially available POF is used. An in-fibre grating consisting of a 1.8 µm wide refractive index structure with a periodicity of 189 nm was demonstrated in single mode polymer fibre with optimised laser processing parameters.Keywords: femtosecond laser, poly(methyl methacrylate), PMMA, polymer optical fibre, refractive index modification INTRODUCTIONPeriodic refractive index (RI) structures within optical fibres can sensitise them to strain or temperature (eg In Fibre Bragg grating), create filters and reflectors within the fibre, or access core modes for interaction with cladding measurands (Long Period Grating). Poly(methyl methacrylate) (PMMA) or Perspex based polymer optical fibres (POF)is of interest for strain sensing due to its increased sensitivity by 14% 1 compared with glass based fibre, biocompatibility, and good optical transmission in the visible region coincident with high brightness, low cost visible LEDs and laser diodes now commercially available. In 1999, the first inscription of tunable Bragg gratings in doped, single mode POF by excimer laser radiation, exhibiting Δn~10 -4 was reported 2 . The study of Fibre Bragg grating (FBG) based devices in POF was recorded, such as wavelength tunable filter 3 and fibre interferometry 4 . Permanent RI structures within bulk undoped PMMA directly written by femtosecond (fs) laser (Δn~5×10 -4 ) at 800nm, 40fs, 1 kHz repetition rate were demonstrated by Scully 5 , with longevity of several years. The photochemical mechanism was elucidated and effect of wavelength and pulse length on the multi-photon process related to bandgap 6 . A minimum diffraction limited feature size of 0.42 μm was achieved within bulk clinical grade PMMA by holographic writing at 387 nm 7 .This paper describes attempts to direct write photonic structures within the core of single mode POF under control at the required spatial resolution, within the constraints posed by the fibre curved surface, and the fibre itself acting as a cylindrical focusing optic, plus self focusing, and writing structures at depth below the surface. The overall aim is to evaluate the feasibility of inscribing highly localised Δn at an appropriate spatial resolution within the POF fibre core. EXPERIMENTAL DETAILSThe experimental arrangement for fs laser micromachining is shown in Fig.1. The fundamental output from a Spectra-Physiscs Spitfire Ti:Sapphire fs laser located in the Photon Science Institute at the University of Manchester with repetition rate of 1 kHz, maximum pulse energy of 3 mJ and pulse duration of 100 fs, was measured using a scanning autocorrelator. The 6 mm diameter laser beam was attenuated by a diffractive optic attenuator and wavelength halved to

A novel fiber optic localized plasmon resonance (FO-LPR) sensor composed of a D-shape optical fiber was proposed and demonstrated in the present study. The D-shape optical fiber was fabricated by a femtosecond laser micromachining system. The dimensions of the D-shape zone were 100 μm in depth measured from the surface of the polymer jacket layer, and the total length was 6 mm. After annealing treatment, the mean roughness of the surface was 231.7 nm as determined by AFM. The exposed surface of the D-shape fiber was modified with self-assembled gold nanoparticles to produce the FO-LPR sensor. The response of the sensor shows that the signal increases linearly with increasing refractive index. The sensor resolution of the sensor was determined to be 4.1×10 -4 RIU.

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A fast laser texturing technique has been utilized to produce micro/nanosurface textures in Silicon by means of UV femtosecond laser. We have prepared good absorber surface for photovoltaic cells. The textured Silicon surface absorbs the incident light greater than the non-textured surface. The results show a photovoltaic current increase about 21.3% for photovoltaic cell with two-dimensional pattern as compared to the same cell without texturing.