Depolariser
A Lyot depolariser makes broadband light unpolarised. Any polarisation in leaves as four parts, each a delay apart. For light whose coherence length is shorter than the delay, the parts add as powers.
Ports
Ports 1 and 2, one each side.
The model
The depolariser is two birefringent sections. The first has its axes at ±45° to s and a delay \(\tau\) between them (its differential group delay, DGD). The second has its axes along s and p and is twice as long, \(2\tau\). So the light leaves as four parts, one for each pair of axes it travelled, delayed by
\[0,\quad \tau,\quad 2\tau,\quad 3\tau.\]Each part is the light projected onto one axis of each section:
\[\mathbf S_{12} = a\sum_{i=0}^{1}\sum_{j=0}^{1} \mathbf P_j^{(2)}\,\mathbf P_i^{(1)}\; e^{-i k\,(i + 2j)\,c\tau},\]with \(\mathbf P\) the projectors onto each section’s axes, \(k = 2\pi/\lambda\), and each part carrying its own delay \((i + 2j)\,\tau\) into the network. Backwards the order of the sections is reversed. \(a\) is the insertion loss.
When it depolarises
The network weighs the parts against each other by the light’s coherence at their delays (see Fibre optics: solving the network). For light with a coherence length well under \(c\tau\) (an LED, an amplifier’s ASE, a superluminescent diode) the four parts add as powers, and the light is unpolarised. Light narrower in its line, which is most lasers, stays polarised, as through a real one. The default DGD of 5 ps is a path of 1.5 mm.
Settings
| Field | Default | Meaning |
|---|---|---|
dgd | 5 | Delay between axes (ps) |
loss | 0.3 | Insertion loss (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- The sections’ axes and lengths are exact, and their DGD does not change with wavelength.
- The phase across each section’s axes is \(k\) times its delay in path, as for a birefringence of no dispersion.
Related
- Fibre laser, for broadband and LED sources.
- Polarisation controller.