C.D. Walton

Inscription of fibre Bragg gratings in non-sensitised fibres using VUV F2 laser radiation

We report the inscription of fibre Bragg gratings in non-sensitised SMF 28 and HI 980 fibre by exposure to VUV F2 laser radiation at 157 nm. The modulated effective refractive index change Deltan(eff) deduced from the shift in the grating reflection peaks was Deltan(eff) = 2.8x10(-4) and 1.7x10(-4) in SMF 28 and HI 980 fibre respectively. The possible influence of non-uniformity of core exposure and VUV cladding absorption loss on these results is discussed.

Glass micromachining using the VUV F2 laser

Theoretical and experimental studies of the surface quality in 157 nm F2 laser-ablated glasses are reported. Limitations set by statistical fluctuations in the multi- mode beam and by stationary beam non-uniformity are explored together with materials issues such as laser-induced surface cracking. Experimental work on ablating polymethyl methacrylate, used as a low threshold medium for recording of the VUV beam, and soda lime glass are described. Use is made of the probe beam deflection technique to determine ablation thresholds, and a variety of methods adopted for characterizing and assessing the quality of ablated surfaces e.g. scanning-electron microscopy, mechanical and optical interference profiling and atomic force microscopy. Preliminary roughness measurements are compared with theoretical expectations and the implication for glass micromachining with the F2 laser discussed.

Ablation of pristine and radiation exposed CR-39 polymer using a 157nm laser

The polymer CR-39 (allyl diglycol carbonate) finds application in optical components because of its excellent transparency in the visible region and its good scratch resistance. It is also widely used as a detector for heavy ionizing particles, based on revealing their damage tracks by selective chemical etching (‘track-etch’ detector). The influence of coherent and incoherent radiation on the polymers chemical etching properties have been reported previously [1], principally in relation to track-etch detection. Here we describe an investigation of the ablation of CR-39 using 157nm F2 laser radiation, motivated by an interest in producing optical microstructures and the possibility of revealing radiation damage tracks by photoablation.

High current photoelectric effects with VUV F2 laser irradiated metals

We report investigations of photoemission from a bulk Ni target using an ∼10 ns pulse duration, 157 nm F 2 laser source. Photoelectric response may provide information relevant for understanding the basic laser-solid interaction mechanisms and also has potential practical applications. One example of this is the development of a simple, solar-blind F 2 laser detector based on photoemission, which allows fast time response measurement of the laser pulse shape. It has also been demonstrated that with a suitably designed diode, current pulses of several amperes can be generated from Ni using the F 2 laser at very low cathode fluence (<1 mJ cm -2 on the cathode) where surface heating is negligible.

Interaction of VUV F2 laser radiation with glasses

The 157nm F2 laser wavelength is strongly absorbed by glasses, even those with high silica content, making it potentially well suited for machining these materials by ablation. This is of interest for fabricating micro-optics and micro-devices in glass, provided crack-free surfaces with minimal laser-induced stress and surface roughness can be produced. Experimental studies are reported on the ablation threshold, ablation rate and surface quality of N-BK7 and soda lime glass for exposure with the VUV F2 laser. Optical probe techniques and etching are employed to determine the ablation threshold and removal rate and scanning electron microscopy to assess the surface quality of the glass following laser exposure. The interaction is discussed within the framework of a thermal vaporization model and the surface thermal loading is used to make a preliminary assessment of resolution attainable in micro-feature definition.