Pockelsceller

A Pockels cell is an electro-optic modulator that uses the Pockels effect, a linear electro-optic effect, to change the refractive index of a non-centrosymmetric crystal in response to an applied electric field. When an electric field is applied along a crystal axis, the ordinary and extraordinary indices shift differently, producing a controllable phase difference between polarization components. In a typical setup, the crystal is placed between polarizers so that the induced phase shift converts into intensity modulation; the voltage needed to produce a half-wave phase shift is called the half-wave voltage, Vπ.

The underlying relation is Δn ≈ −(1/2) n^3 r E, where n is the refractive index, r is the relevant electro-optic coefficient, and E = V/d for a crystal of thickness d. By varying V, the device can operate as a fast optical modulator, with potential bandwidth extending into the gigahertz range in suitably designed crystals and electrodes.

Materials commonly used for Pockels cells include lithium niobate (LiNbO3), potassium dihydrogen phosphate (KDP), potassium titanyl phosphate (KTP), and other non-centrosymmetric crystals. Each material presents trade-offs in terms of electro-optic coefficients, optical losses, photorefractive effects, and environmental sensitivity (temperature and humidity).

Applications span laser pulse shaping and Q-switching, high-speed polarization control, and optical communication modulators. Practical design considerations include crystal cut orientation, electrode geometry, dielectric coatings, temperature stabilization, and precise alignment of polarization optics. Limitations include the need for relatively high drive voltages, sensitivity to temperature and wavelength, and potential photorefractive or hygroscopic issues depending on material.