Directional coupler
Photonic chips are an experimental feature. Switch them on under Help › Experimental features.
A directional coupler is two waveguides run close enough together that light crosses from one to the other. It is the chip's beam splitter, and the coupler of a ring.
Ports
Ports 1 and 2 on one side, 3 and 4 on the other. Port 1 goes through to 3 and across to 4; port 2 goes through to 4 and across to 3.
The model
A share \(\kappa\) of the power crosses over. It can change linearly with wavelength, about \(\lambda_0 = 1550\) nm:
\[\kappa(\lambda) = \kappa_0 + \kappa'\,(\lambda - \lambda_0),\]clamped to 0 to 100 %. With \(t = \sqrt{1 - \kappa}\), \(x = \sqrt{\kappa}\) and an excess loss \(a\) in amplitude,
\[S = a\begin{pmatrix} 0 & 0 & t & i x \\ 0 & 0 & i x & t \\ t & i x & 0 & 0 \\ i x & t & 0 & 0 \end{pmatrix},\]the same both ways. The light crossing over leads by a quarter wave, as in the fibre coupler, so two equal arms of a Mach–Zehnder recombine wholly into one port.
Settings
| Field | Default | Meaning |
|---|---|---|
kappa | 50 | Coupling (%) |
dkappa | 0 | Change (%/nm) |
loss | 0 | Excess loss (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Readouts
Its ports and what they join, and the powers where the fibre network reaches them. The chip’s spectrum between any two of its ports is drawn below, with the lines in it measured (see Photonic chips: spectra and lines).
Assumptions
- The coupling is linear in wavelength, with no reflection and no phase of its own beyond the quarter wave.
- The coupler’s own length adds no delay: put a waveguide either side for that.
Related
- 2×2 MMI, a 50:50 coupler made another way.
- Ring, all-pass.