An Adjustable Finesse Fabry-Perot Resonator

In order to optimize the performance of the resonator as samples of various sizes and loss tanagents are studied, it is useful to have a means to vary the loaded Q of the resonator. As we discussed in Section VIII, a poorly coupled resonator reduces the highest achievable signal-to-noise ratio. 

One simple method for varying the coupling is to construct the resonator from two polarizers. We can show (Tudisco, 1988) that the finesse of such a resonator is proportional to cos, where is the relative orientation of the two polarizers. This device is the quasioptical analog of the cavity coupling scheme of Lebedev (1990). There are several limitations to this scheme as pointed out by the author, namely, the radiation must be linearly polarized, which complicates transmit-receive duplexing in a reflection mode spectrometer on resonance, the power minimum occurs in transmission, which precludes using the device in a reflection mode spectrometer if we wish to work with low background levels. 

Treatments of wire meshes can be found in the books of Chantry (1984) and in Goldsmith (1982, Chap. 5) and Holah (1982). These treatments are based mainly on the original work of Ulrich and co-workers (Ulrich et al, 1963 Ulrich, 1968, 1979), who derived an equivalent circuit analysis for wire meshes that works quite well in practice. 

R is a dimensionless correction for ohmic resistance in the mesh material and ft is the generalized frequency  

The quantity R should not be confused with the reflectivity of the surface, which is given by i = 1 — t, where T is defined by eqs. (101)-(105). The wire mesh geometry is shown in Fig. 9a. In Eq. (103), w = g/K and Wg dimensionless correction factor near unity, which has been empirically found to be (Ulrich et al., 1963 Ulrich, 1968, 1979) 

---

X. An Adjustable Finesse Fabry-Perot Resonator XL Optimization of Resonators XII. Summary... 

---