Cavity spatial filter

Figure 2.5a Wideband frequency sweep of a Fabry-Perot cavity without a spatial filter.

Figure 2.5b Wideband frequency sweep of a Fabry-Perot cavity with a spatial filter in place. Note how the non-TEMooq modes are suppressed. In this scan the mode number q ranges from 111 to 114...

The sample was confined within a hollow PTFE cylindrical cup with one mirror serving as its cap. The gaseous sample was pumped into the enclosure made by the cavity and mirror via a needle valve restrictor, through an inlet port 6 mm in diameter near the periphery of the mirror. A diametrically opposite port was connected to the vacuum pumps. The cup base was a flat disc, 5 mm thick, halfway between the mirrors in the region of the beam waist. We soon discovered that this disc caused considerable dielectric loss, and therefore reduced its thickness to 1 mm over an area 30 mm diameter that gave a big improvement, and simultaneously formed a spatial filter which helped suppress non-axial modes see Figure 2.5. 

Figure 6.3 The Mark II cavity spectrometer viewed through its Perspex cover. The spatial filter is visible in the centre of the case. The translation stage and piezoelectric actuator are seen in the foreground and operate via the bellows to move the near mirror. The coupling iris is in the centre of the far mirror. The waveguide couplings holding the mica vacuum windows can be seen either side of the case in the background. Sample inlet is on the left wall of the case not visible and outlet is on the right via the pressure sensor...

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