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Hard photon processing (HARP)

Deliverables

The move to 157nm for the inscription of fibre Bragg gratings has been demonstrated. However, at this wavelength, there are some subtle influential effects than can affect the inscription process and which are to some extent, less important at longer wavelengths. The strong optical absorption that takes place in the VUV necessitates clean sample surfaces and highly transparent beam delivery systems. Absorption with very thin layers (nm’s) of organic material (e.g. residual fibre Jacket) or mono-layers of contaminants can cause significant attenuation of the beam. However, in addition to ‘technical’ considerations, bulk and surface absorption becomes non trivial at 157nm in fused silica. With the precautions taken on sample preparation, it is important for the fibre cladding to be transparent in the VUV, i.e. ‘pure’ fused silica cladding has to be carefully tailored during manufacture to minimise absorption. Not only does the dose have to be high enough to modify the silica and thus manifest itself as a change in the refractive index, but, the fringe contrast needs to be preserved in the direction along the fibre. The spatial coherence should be maximised by operating in a configuration that minimises the beam divergence. The successful inscription of fibre Bragg gratings in SMF28, HI980 and GeB co-doped fibre illustrates the possibility of utilising a 157nm VUV laser source. The results presented here, appear to be one of the first results relating to writing fibre Bragg gratings at 157nm. These results are therefore new and encouraging.
157nm laser emission was used to ablate material from a Macor ceramics plate. A linear array of grooves of rectangular cross-section was milled into the substrates. The accuracy of the spacing as well as of the width and the depth of the grooves increased significantly in comparison to fibre holders that were manufactured by standard techniques. Today's tolerance of 10µm could be improved by approximately one order of magnitude. In conjunction with the new generation of high-power high-brightness laser diodes / laser-diode bars, the advanced fiber holders open up new markets, e.g. in medicine, printing technologies, and optical pumping of solid-state and fibre lasers.
Companagnie Deutch has specific manufacturing requirements for the alignment of single mode fibres. For these applications fibres need to be aligned at the sub-micron scale in order to achieve high coupling efficiencies. A mask dragging technique has been used to ablate V-grooves directly onto glass substrates. This type of technique for producing high resolution V-grooves could be advantageous in integrated fibre optics applications. A typical insertion loss for a single mode optical connector is about 0.3dB with a maximum accepted coupling loss of 1.3dB. The difference between 1.3dB and 0.3dB is in fact relatively small in terms of lateral misalignment (less than 0.5µm). To reduce the insertion loss the surface roughness must be kept below about 0.2µm Ra. The depth of the groove must be the order of 150µm with an angle of 90°. The first prototype has been produced with 157nm on several substrates. Initial tests show promising results especially in N-BK7 glass.
A 157nm optical beam path for instalment in micro-machining tools has been developed. The beam path consists of the following components: - Mirrors for beam guiding and transportation, with a high reflectivity (HR) coating for 157 nm, the reflectivity is around 90%. - Lenses for beam preparation with a high transmission (HT) coating, transmission > 90 %. - Attenuator plates with an adjustable transmission. - Beam homogenisation with a 7 x 7 cylindrical micro-lens array. - Projection optic with a demagnification x10, based on an dioptic design. - Projection optic with a demagnification x25, based on Schwarzschild-design. - Projection optic with a demagnification x50, (design and components). For the 157nm wavelength only a few optical materials are available. Here, all optics are based on CaF2, which shows a good transmission in the VUV and is not hydrophil. However, due to its material properties, these materials are difficult to manufacture. The required process technology has been developed, including test tools.
The modification of the refractive index of fused silica by a 157nm excimer laser was used to inscribe an optical diffuser with discrete scattering areas into an optical fibre. Radiation which is launched into the fibre core emits radially from the fibre at the modified areas. Such diffusers are used in minimal-invasive therapies like laser induced thermo-therapy (LITT) and photodynamic therapy (PDT). The patient is punctured and a catheter system is inserted, which allows the placement of the diffusing fibre inside the tumour tissue. In case of LITT, the tumour tissue is destroyed due to thermal damage caused by irradiation with laser light. As for PDT, a medicament, a so-called photosensitizer is administered and cumulates in the target tissue where it is activated by laser radiation of appropriate wavelength (generally in the red spectral region). After excitation a photochemical reaction yields an irreversible damage of the target cells. Diffusers with different emission profiles (decreasing and increasing alongside the fibre axis) were demonstrated. Thus, the developed technique is feasible to manufacture cylindrical diffusers with longitudinally homogeneous emission profile. Additionally, future application will include tailored diffusers whose emission characteristics is adapted to the specific topography of the tumour of each single patient.
This result incorporates the basic process knowledge, necessary for micro machining difficult materials such as fused silica, silicon or fluorinated polymers. These materials can not be precision machined with longer wavelength, the high transmission in the UV wavelength range leads to an energy dissipation into the material and thus temperature gradients, which cause thermal induced cracking. Some fluorinated polymers show high binding energies, which cannot be crack with UV or deep UV wavelength. Here the high photon energy of 7,9 eV is used for direct bond braking and thus for ablation. For these processes, the basic process technology has been developed; micro-machining tools are available for feasibility studies and deeper process understanding. The processing strategies are at hand and the basic results like ablation rate and achievable surface quality are available.
For micro-machining processes with 157nm laser radiation an industrial machine tool has been developed. This micro-machining tool is equipped with special designed optics for VUV-wavelength, including beam homogenisation and high precision projection optics, providing near diffraction-limited resolution. The energy density on the work piece is sufficient for the defined ablation on most of the relevant materials. Currently, energy densities of above 100mJ/cm² are available on the work piece, enough for machining polymers. By improving the purging of the beam line an increase towards the J/cm² regime is expected, to include brittle materials like glasses into the list of feasible materials for the work piece positioning, high precision stages with a resolution of ten nm have been installed. Oxygen sensing, dose monitoring and vision systems are integrated for process monitoring and control. High-level graphical user interface allows operation on factory floor level. The machine tool is especially suited for processes, where minor volumes with a very high precision need to be removed, e.g. for micro optics or in telecom applications.

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