The substrates used were 25 mm diameter, 1 mm thick discs of GLS. The test gratings were 3 mm by 3 mm in size, small enough to be relatively quick to manufacture and to allow several gratings to be inscribed on a single sample, while still big enough to be easily tested with a collimated beam with ~2 mm beam diameter. The gratings were arranged in a 4 by 4 array with a 1 mm gap between gratings (Figure 4.3 a). The gratings were inscribed by translating the sample back and forth through the focal spot at a speed of 10 mms-1, with each adjacent line separated by 3 μm for a total of 1000 lines.
To allow for easy testing the first gratings were designed to work well at 633 nm, and as such the grating thickness was designed to be 41 μm thick, which with the predicted refractive index change induced by the ULI in GLS would produce efficient gratings at this wavelength. The grating thickness was built up by inscribing 17 layers of grating, one above the other by moving the stages down by 1 μm after each layer, equating to 2.4 μm spacing between layers within the material (due to the refractive index of GLS, 2.4 at the laser wavelength). When viewed under the shadowgraph the gratings are a uniform colour with the darker gratings indicating those that are most efficient in the first order (Figure 4.3 b). Viewing the gratings under a microscope the ULI process is seen to produce straight uniform lines, with the modified material appearing darker (Figure 4.3 c).
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Figure 4.3 – (a) Photograph of GLS sample showing 16 inscribed gratings. (b) Shadowgraph image of sample showing uniform colour of gratings, marks in some gratings caused by dirt on sample surface, samples need to be very clean before performing ULI. (c) Close up microscope image of grating showing straight linear lines of modification, dark lines. (d) Photograph of HeNe laser incident on grating showing clear diffraction orders with very little scatter and a few ghost images. Image taken by Dr David Lee in a darkened room using a long exposure and a piece of black card moved through the laser beam by hand, showing the laser beam. At the end of the exposure the camera flash is used to illuminate the apparatus.
To determine the optimum pulse energy for the modification a sample was written with 16 gratings written from 11 to 177 nJ in approximately 11 nJ steps. The samples were initially tested with a 633 nm laser and very clear diffraction orders were visible with little visible scatter and only a few faint ghost images (Figure 4.3 d). The contribution of the ghost images and scattered light was determined by measuring the total integrated transmittance, and subtracting from this the measured efficiency of the expected diffraction orders. The total integrated transmittance of the grating was measured by placing the aperture of an integrating sphere within a few millimetres of the grating to capture the entire output beam. The efficiency of the diffraction orders was measured with the integrating sphere placed at a distance of a few centimetres from the grating to capture only the light contained within the individual diffraction order. The gratings were measured to produce less than 5 % scattered light at 633 nm, with longer wavelengths expected to produce less scatter and having < 1 % contribution at 2500 nm. Of interest the clearest ghost images were present equidistant between the expected, bright
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diffraction orders of a 3 μm grating period and correspond to the diffraction pattern expected from a 6 μm grating period. The origin of this is expected to be an interference effect within the grating, the exact nature of which is currently unknown and could merit further investigation, however for this work parameters were chosen that reduced this effect as much as possible.
The grating efficiency was tested, with the angle of the grating altered to maximise the first order diffraction efficiency for each pulse energy (Figure 4.4 a). The measured diffraction efficiency of any given grating was deemed to be quite accurate with errors of ± 2 %, determined taking into account; repeat measurements; the stability of the light source; measurement error and dark counts of the detector; and positional dependency of the beam on the grating. There may however be larger differences between gratings written with nominally identical design and inscription parameters, but manufactured in different batches at different times due to the performance of the ULI process being susceptible to laboratory issues, such as room temperature, beam alignment and dust.
Figure 4.4 – (a) Plot of diffraction efficiency measured at 633 nm for the -2 to +2 diffraction order vs. laser pulse energy for gratings written with a broad PE scan by the Fianium laser. (b) Plot of diffraction efficiency measured at 633 nm vs. pulse energy for the 1st diffraction order for a broad and fine PE scan.
The optimum pulse energy was found to be 77 nJ producing a 42.5 % absolute diffraction efficiency in the first order. A second sample was inscribed with a much finer pulse energy scan of 2 nJ steps from 62 to 92 nJ. The optimum pulse energy in this sample was 72 nJ, producing a first order diffraction efficiency of 44.4 % (Figure 4.4 b). The slight difference between the optimum pulse energy found from the broad and fine pulse energy (PE) tests is due to the alignment of the system being adjusted between the two runs. After the fine PE scan the system alignment was not altered and subsequent tests showed that 72 nJ consistently produced gratings with diffraction efficiencies of 44 %.
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