Electro-optic Q-switch & drivers0 pages

Sintec Optronics Pte Ltd

Sintec Optronics Technology Pte Ltd
10 Bukit Batok Crescent #07-02 The Spire Singapore 658079
Tel: +65 63167112 Fax: +65 63167113

Pockels Cells (EO Q-switches)
Technical Overview

The Electro-Optic Effect
The linear electro-optic effect, also known as the Pockels effect, describes the variation of the refractive
index of an optical medium under the influence of an external electrical field. In this case certain
crystals become birefringent in the direction of the optical axis which is isotropic without an applied
voltage.
When linearly polarized light propagates along the direction of the optical axis of the crystal, its state of
polarization remains unchanged as long as no voltage is applied. When a voltage is applied, the light
exits the crystal in a state of polarization which is in general elliptical.
In this way phase plates can be realized in analogy to conventional polarization optics. Phase plates
introduce a phase shift between the ordinary and the extraordinary beam. Unlike conventional optics,
the magnitude of the phase shift can be adjusted with an externally applied voltage and a λ/4 or λ/2
retardation can be achieved at a given wavelength. This presupposes that the plane of polarization of
the incident light bisects the right angle between the axes which have been electrically induced. In the
longitudinal Pockels effect the direction of the light beam is parallel to the direction of the electric field.
In the transverse Pockels cell they are perpendicular to each other. The most common application of
the Pockels cell is the switching of the quality factor of a laser cavity.
Q-Switching
Laser activity begins when the threshold condition is met: the optical amplification for one round trip in
the laser resonator is greater than the losses (output coupling, diffraction, absorption, scattering). The
laser continues emitting until either the stored energy is exhausted, or the input from the pump source
stops. Only a fraction of the storage capacity is effectively used in the operating mode. If it were
possible to block the laser action long enough to store a maximum energy, then this energy could be
released in a very short time period.
A method to accomplish this is called Q-switching. The resonator quality, which represents a measure
of the losses in the resonator, is kept low until the maximum energy is stored. A rapid increase of the
resonator quality then takes the laser high above threshold, and the stored energy can be released in a
very short time. The resonator quality can be controlled as a function of time in a number of ways. In
particular, deep modulation of the resonator quality is possible with components that influence the state
of polarization of the light. Rotating the polarization plane of linearly polarized light by 90°, the light can
be guided out of the laser at a polarizer. The modulation depth, apart from the homogeneity of the 90°
rotation, is only determined by the degree of extinction of the polarizer.
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