Tunable filter
A tunable band-pass filter passes a Gaussian band about its centre wavelength. What it does not pass is lost. Put it after an amplifier to cut its spontaneous emission, or sweep it across a spectrum.
Ports
Ports 1 and 2, one each side.
The model
The filter’s power transmission at wavelength \(\lambda\), for a centre \(\lambda_c\) and a full width at half maximum \(w\), is
\[T(\lambda) = \exp\!\left(-4\ln 2\,\left(\frac{\lambda - \lambda_c}{w}\right)^{2}\right),\]and its matrix, both ways, is
\[\mathbf S_{12} = \mathbf S_{21} = a\sqrt{T(\lambda)}\;\mathbf I,\]with \(a = 10^{-\ell/20}\) for an insertion loss \(\ell\) (dB). Broadband light is followed in slices fine enough to resolve the filter (see Fibre optics: broadband light), so the filter shapes its spectrum, and a photodiode after it sees only the band it passes.
Settings
| Field | Default | Meaning |
|---|---|---|
centre | 1550 | Centre (nm) |
fwhm | 1 | Width (nm) |
loss | 1 | Insertion loss (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- The pass band is Gaussian, adds no phase across it (no dispersion) and is the same for both polarisations.
- What is not passed is lost, not reflected. For a filter that sends the rest on, use a WDM; for one that reflects its band, a Bragg grating.
Related
- WDM and fibre Bragg grating.
- Optical spectrum analyser.
- Filter, the free-space filter.