AOM
An acousto-optic modulator is a Bragg cell: sound at the drive frequency makes a moving grating in a crystal. A share of the light goes into one diffracted order, turned by a small angle and shifted in frequency by the drive. The rest goes straight through, unshifted.
The model
The grating
Sound of speed \(v\) at the drive frequency \(f\) makes a grating of period \(\Lambda = v/f\). The chosen order, +1 or −1, takes a share \(\eta\) of the light (the efficiency), turned from the straight-through beam by
\[\sin\theta = \pm\frac{\lambda f}{v},\]anticlockwise for order +1 and clockwise for −1. The rest, \(1 - \eta\), carries on in order 0. Both are branches of their own. Tellurium dioxide carries sound at about 4200 m/s, fused silica at 5960 m/s.
The frequency shift
Because the grating moves, the diffracted light is Doppler shifted by the drive:
\[\nu_{\pm 1} = \nu \pm f.\]Light at a different frequency offset from its laser makes no steady fringes with the rest of the laser’s light: it is a different launch as far as adding fields goes. On a photodiode, though, it beats with the unshifted light at the drive frequency, \(f\). That is how a heterodyne interferometer or an optical frequency shifter works.
A drive
The AOM’s RF level is also an electrical input. Wired to a signal from the electronics, it sets the efficiency, and so the power in the order, through time, as an amplitude modulator.
Settings
| Field | Default | Meaning |
|---|---|---|
length | 4 | mm aperture. |
rf | 80 | MHz drive: the order is shifted by this. |
v | 4200 | m/s sound speed (4200 for TeO₂ shear is typical of fibre AOMs; 650 slow shear). |
eff | 80 | % into the order. |
order | 1 | +1 or −1. |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- Only one diffracted order: a Bragg cell, not a Raman–Nath one, so there are no higher orders.
- The efficiency is set, not worked out from the RF power and the Bragg angle. The cell diffracts its share whatever the angle the light meets it at.
- The diffracted beam keeps the incident beam’s shape.
Related
- EOM, for phase and amplitude modulation without deflection.
- Photodetector, which reads the beat notes.
- Grating.
- Fibre AOM, the fibre-coupled one.