Electro-optic modulators operate based on the principle of voltage-induced birefringence.
Solaris Optics manufactures electrooptic modulators – Pockels cells in its premises in Poland.
Pockels cell is a device which can produce controllable birefringence by applying voltage to the cell. The cell contains an uniaxial crystal, which becomes biaxial when electric field is applied. If new axes induced by the applied electric field are at 45° to the plane of polarization of the incident beam, the incident beam is split into two equal orthogonally polarized components. They propagate in a crystal with different velocities because of induced different refraction indices. Induced birefringence Δn is proportional to the applied electric field and on the crystal length l it gives controlled phase retardation Γ between these components. This phenomenon is used to modulate the incident beam.
Fig. 1. Electrooptics modulators – phase retardation formula
where λ is the wavelength of the incident beam.
The voltage sensitivity of Pockels cells is given by the half-wave voltage U λ/2. This is a voltage required to get a phase shift of 180°. The quarter wave voltage U λ/4 is required for a phase shift of 90 °.
There are two basic designs of Pockels cells:
- Pockels cell with longitudinal electrooptic effect, employing a crystal in which electric field is applied along the direction of the optical beam.
- Pockels cell with transverse electrooptic effect, utilizing a crystal in which electric field is perpendicular to the direction of the optical beam.