Altered Light Property: Polarization
The two orthogonal linear polarisation states that are most important for reflection and transmission are referred to as p- and s-polarisation. P (derived from the German word parallel) and S (from the German word senkrecht) are linear polarisations defined by their relative orientation of the electric field (either parallel or perpendicular) to the plane of incidence.
Coupling: DC / Broadband
The electro-optic crystal is directly coupled to the RF modulation input connector, allowing the full bandwidth of the crystal to be utilised. The bandwidth of the Pockels Cell is limited by the output impedance of the driver that forms with the capacitance of the crystal a low-pass filter. For typical retarders with 50Ohms coupling (via e.g. SMA) the limit is about 150MHz. Applications with nano-second response times require much higher bandwidths. In such cases the high voltage is coupled directly to the crystal via short, flying leads in order to keep the capacitance low.
Effect on the Laser Light: Polarization Shift
Pockels cells are voltage-controlled wave plates. The basis of the operation is the Pockels effect which describes the change or generation of birefringence in an optical medium induced by a linear electric field. Pockels cells are commonly used in science and industry to rotate the polarisation of a beam that passes through.
The electric field can be applied to the crystal medium either in longitudinal or transverse configuration with respect to the light beam. Typical longitudinal Pockels cells possess no natural birefringence and thus achieve high extinction ratios. Their halfwave voltage is high but independent (in first order) of the crystal geometry which is useful for high power applications. Transverse arrangements allow geometry dependent halfwave voltage adjustments but suffer from imperfect natural birefringence compensations which limits the extinction ratio and temperature stability.
Alignment of the crystal axis with respect to the ray axis is critical. Misalignment leads to unwanted birefringence which reduces the extinction ratio.
QUBIG provides Pockels Cells with large clear apertures, very low optical insertion loss, high optical quality and damage thresholds that enable a wide range of applications such as:
Pulse Picking & Q-Switching
In industry and science Pockels Cells are used as fast and precise electrically controlled optical switches. Main applications are a) Pulse Picking for extracting single pulses from a pulse train and b) o ptical laser pulse generation either via optical chopping of cw light or by rapid Q-Switching.
LiDAR
In contrast to the "phase shift" method, in which continuous beams of different wavelengths are emitted and the distances are calculated from the phase offset between the transmitted and received signals, the “Time Of Flight” method is based on sending short, staggered light pulses. The transit time of the portion of the emitted radiation reflected by an object serves as an indication of the distance. LiDAR is the analogy to Radar, using laser instead of radio wave. Pockels Cells from QUBIG are useful tools to generate fast laser intensity modulations.
Magnetometry
The atomic magnetometer detects an external magnetic field by exciting atomic magnetic dipole transitions via modulated light (Bell–Bloom magnetometer) and measuring the coherent precession frequency of atomic spins about the external magnetic field. Besides SQUIDs, atomic magnetometer are the most sensitive magnetometers which are used in many significant fields, such as medicine, tests of fundamental symmetries, space exploration, and detection of nuclear magnetic resonance signals.
Research / Science
Electro-optic polarisation shifters find versatile use in many areas of science and research. Besides the use as optical switch for active (de-)multiplexing of single photons, which is a promising candidate for large-scale quantum networks, very often they are implemented for active intensity control of optical dipole traps and lattices. The latter can be used to confine neutral atoms which serve as basis of many exciting topics in fundamental research. Potential applications can be found in ultrafast spectroscopy, high-speed communications and even optical quantum science and technology.
