6.3 Experimental setup for grating coupling
6.4.2 Performance of gratings
In order to test the gratings and their functionality, a prototype set of gratings were written on TeO2 onα-Al2O3 (nef f = 1.76). The refractive index of α-Al2O3 is close to Pr3+:Y2SiO5, and so the simulation results were similar. To obtain the most efficient grating possible, 8 different grating depths starting from 110 nm with steps of 10 nm up to 180 nm, were written onto the thin film surface.
Figure 6.16 shows one of the TeO2 on α-Al2O3 thin films. The light coupled to the thin film and the scattering beam propagating along the thin film can be observed in this picture. The efficiency could not be calculated using the light collected from the thin film edge, because the edges of these films were not optically polished, and so the output light was scattered. The transmitted light through
Figure 6.16: Prototype grating coupling, TeO2 onα-Al2O3. Input light was coupled to the binary gratings on the thin film and propagated along the thin film. The coupling point and the scattering beam propagating along the thin film can be observed.
Figure 6.17: Grating coupling, TeO2 on Pr3+:Y2SiO5. Input light was coupled to the binary gratings on the thin film and propagated along the thin film. The coupling point to the 170 nm deep grating and the scattering beam propagating along the thin film are shown.
the thin film was measured and from this number, as mentioned in Section 5.3.2.1, the coupling efficiency could be calculated. The trend of the grating efficiency was similar to efficiency calculations performed in Chapter 5, with the grating with the depth of about 180 nm being the most efficient grating. But the expected efficiency was about 70%, where as the calculated efficiency was much less at about 1%. The possible reasons for such low efficiency are explained in Section 7.1.
After obtaining reasonable results on the prototype thin films on Sapphire sub- strates, gratings were written on the TeO2 on Pr3+:Y2SiO5 thin film. Figure 6.17 shows light coupled to this thin film and the scattering beam propagating along the thin film. The efficiency of coupled light to these thin films followed the increasing trend of the curve in Figure 5.8, but were reduced by an order of magnitude for each grating as shown in Figure 6.18. Also, as can be observed from Figure 6.18, the model predicted a sharper increase in the efficiency as the grating depth was in- creased, which was not observed in the efficiency measurements. The 170 nm grating was calculated to be the most efficient in the TeO2 on Pr3+:Y2SiO5 thin films, with an efficiency of 63%. But the measured efficiency of this depth was only 2±0.1%.
§6.4 Grating coupling 113
Figure 6.18: Measured efficiency of the 8 gratings with depths of 100 nm to 170 nm, with 10 nm steps, written on the TeO2 on 0.05%Pr3+:Y2SiO5 thin film. The lin- early increasing trend of Figure 5.8 can be seen as shown by the red line, but the measured efficiencies were an order of magnitude smaller than expected. The calcu- lated efficiency was scaled down 10×for visibility. Input laser power on the gratings was 2.4±0.1 mW. This measurement was done at room temperature, outside the cryostat.
The efficiency did not change significantly between room temperature measurements and cold measurements. Some possible reasons for these low coupling efficiencies are presented in Section 7.1 of Chapter 7.
6.4.3
Experimental setup
After the gratings were written and the carbon layer was etched off, the exper- imental setup illustrated in Figure 6.19 was constructed. The dye laser used in these experiments was a Coherent 699-29, that was frequency stabilized by lock- ing to an external Ultra-Low Expansion (ULE) cavity via the Pound-Drever-Hall technique [180]. Using this laser locking mechanism, sub-kilohertz linewidths on a millisecond timescale are produced. In addition, the spectral line drifts over longer timescales at a rate of 100 kHz per hour.
The laser was tuned to the 3H
4 ↔1 D2 transition of Site 1 of Pr3+ ions at 605.977 nm (494.73 THz). The substrate in this part of the experiment was a 0.05%Pr3+:Y
2SiO5 crystal, with the dimensions of 3×10×0.5 mm. As before, the excitation beam was gated with a double pass acousto-optic modulator (AOM) to create the pulse sequences for two pulse echo experiments.
... 1 2 3 4 2 5 6 7 8 9 10
Figure 6.19: Grating coupling experimental setup, Consisting of 1- Laser, 2- HWP, 3- Polarizing Beam Splitter (PBS), 4- AOM, 5- Fiber coupler, 6- Goniometer setup, 7- 5 cm lens, 8- Cryostat, 9- Detector, 10- Digital oscilloscope.
The modulated light was then coupled to the planar slab waveguide using the periodic binary gratings written on the film. In order to couple to the TE00 mode, vertically polarized input light was required. The angle of incidence of the input beam on the gratings was calculated to be 11±0.5◦, that was tuned with a goniometer setup outside the cryostat. The emitted light out of the end of the crystal was guided outside the cryostat using a right angle prism shaped mirror. This output light was collected from an edge of the crystal which had not been optically polished, therefore there was a lot of scattering and the coupling efficiency could not be easily calculated. The emitted light out of the end face of the film was directed to the cryostat window and out of the cryostat by a mirror. This light was then detected using a Thorn EMI PhotoMultiplier Tube (PMT) and the signals were recorded using a digital oscilloscope. Also, these experiments were performed with a small magnetic field in the z direction, which was used to null the earth’s magnetic field in this direction. The magnetic field was made by wrapping a piece of wire around the bottom of the cryostat where the sample was sitting.