Variable delay line
A variable delay line adds a set delay to the light: free space inside a fibre-coupled housing, so its group and phase delays are one. It sets the delay between two arms, to find white-light fringes or overlap pulses.
Ports
Ports 1 and 2, one each side.
The model
A delay \(\tau\) (ps) is a path \(L = c\tau\) in air. The light through it is delayed by that path in the network and takes the phase it gives:
\[\mathbf S_{12} = \mathbf S_{21} = a\, e^{-i 2\pi L/\lambda}\;\mathbf I,\]with \(a = 10^{-\ell/20}\) for an insertion loss \(\ell\) (dB). The delay is kept with the light, so it changes how the network weighs two paths by the laser’s coherence (see Fibre optics: solving the network). Sweep it in an interferometer lit by broadband light and the fringes appear where the two arms are equal, within the source’s coherence length.
Settings
| Field | Default | Meaning |
|---|---|---|
delay | 0 | Delay (ps) |
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 path inside is air with no dispersion, so group and phase delay are the same.
- The loss does not change with the delay, as a real one’s coupling would a little.
Related
- Fibre stretcher, for a phase without a delay.
- Fibre laser, for the coherence that sets how far the fringes reach.