Fibre mirror
A partly reflecting mirror inside the fibre, such as a coated fibre end or a broadband reflector. It reflects a share of the light back from either side and passes the rest. Two of them make a fibre Fabry–Perot.
Ports
Ports 1 and 2, one each side.
The model
With a reflectance \(R\) and an excess loss \(\ell\) (dB), \(a = 10^{-\ell/20}\), the mirror reflects and transmits, for each polarisation alike,
\[r = a\sqrt R, \qquad t = a\sqrt{1 - R},\] \[\mathbf S_{11} = r\,\mathbf I, \qquad \mathbf S_{22} = -r\,\mathbf I, \qquad \mathbf S_{12} = \mathbf S_{21} = t\,\mathbf I.\]The reflection from the second side has the opposite sign, as a lossless mirror’s must for its matrix to be unitary. That sign is what puts a Fabry–Perot’s transmission peaks where they should be.
In a cavity
Two fibre mirrors a fibre apart are a Fabry–Perot. The network follows the light round between them until what is left is negligible (see Fibre optics: solving the network), so the transmission shows its free spectral range \(c/(2 n_g L)\) and its finesse.
Settings
| Field | Default | Meaning |
|---|---|---|
rmax | 50 | Reflectance (%) |
loss | 0 | Excess loss (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- The reflectance is flat in wavelength and the same for both polarisations. For a narrow band, use a Bragg grating.
- The mirror is thin: it adds no delay of its own.
Related
- Fibre Bragg grating, a mirror for a narrow band.
- Faraday mirror.
- Mirror, the free-space mirror.