The Pockels effect (after Friedrich Carl Alwin
Pockels who studied the effect in 1893), or Pockels electro-optic
effect, produces birefringence
in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is
proportional to the electric field, whereas in the Kerr effect it is quadratic
to the field. The Pockels effect occurs only in crystals that lack inversion symmetry,
such as lithium niobate
or gallium arsenide
and in other noncentrosymmetric media such as electric-field poled polymers or
glasses.
Pockels Cells
Pockels Cells are voltage-controlled wave plates. The Pockels effect is the basis of Pockels Cells operation. Pockels Cells
may be used to rotate the polarization of a passing beam. See Applications below for uses.
A transverse Pockels Cell comprises two crystals
in opposite orientation, which give a zero order wave plate when voltage is
turned off. This is often not perfect and drifts with temperature. But the
mechanical alignment of the crystal axis is not so critical and is often done
by hand without screws; while misalignment leads to some energy in the wrong
ray (either e or o – for example, horizontal or vertical), in contrast to
the longitudinal case, the loss is not amplified through the length of the
crystal.
The electric field can be applied to the crystal
medium either longitudinally or transversely to the light beam. Longitudinal
Pockels Cells need transparent or ring electrodes. Transverse voltage
requirements can be reduced by lengthening the crystal.
Alignment of the crystal axis with the ray axis
is critical. Misalignment leads to birefringence and to a large phase shift across
the long crystal. This leads to polarization rotation if the alignment is not exactly parallel
or perpendicular to the polarization.
Dynamics within the cell
Because of the high relative dielectric constant
of εr ≈ 36 inside the crystal, changes in the electric field
propagate at a speed of only c/6. Fast non-fiber optic cells are thus
embedded into a matched transmission line. Putting it at the end of a
transmission line leads to reflections and doubled switching time. The signal
from the driver is split into parallel lines which lead to both ends of the
crystal. When they meet in the crystal their voltages add up. Pockels Cells for
fibre optics may employ a traveling wave design to
reduce current requirements and increase speed.
Usable crystals also exhibit the piezoelectric effect
to some degree[1] (RTP has the lowest, BBO and lithium niobate are high). After a voltage change
sound waves start propagating from the sides of the crystal to the middle. This
is important not for pulse
pickers, but for boxcar windows.
Guard space between the light and the faces of the crystals needs to be larger
for longer holding times. Behind the sound wave the crystal stays deformed in
the equilibrium position for the high electric field. This increases the
polarization. Due to the growing of the polarized volume the electric field in
the crystal in front of the wave increases linearly, or the driver has to
provide a constant current leakage.
The driver electronics
The driver must withstand the doubled voltage
returned to it. Pockels Cells behave like a capacitor. When switching these to high voltage a
high charge is needed; consequently, 3 ns switching requires about 40 A for a
5 mm aperture. Shorter cables reduce the amount of charge wasted in
transporting current to the cell.
The driver may employ many transistors connected
parallel and serial. The transistors are floating, and need DC isolation for
their gates. To do this, the gate signal is connected via optical fiber, or the gates are driven by a large transformer. In this case, careful compensation
for feedback is needed to prevent oscillation.
The driver may employ a cascade of transistors
and a triode. In a classic, commercial circuit the last transistor is an IRF830
MOSFET and the triode is an Eimac Y690 triode.
The setup with a single triode has the lowest capacity; this even justifies
turning off the cell by applying the double voltage. A resistor ensures the
leakage current needed by the crystal and later to recharge the storage
capacitor. The Y690 switches up to 10 kV and the cathode delivers 40 A if the
grid is on +400 V. In this case the grid current is 8 A and the input impedance
is thus 50 ohms, which matches standard coaxial cables, and the MOSFET can thus be placed
remotely. Some of the 50 ohms are spent on an additional resistor which pulls
the bias on -100 V. The IRF can switch 500 volts. It can deliver 18 A pulsed.
Its leads function as an inductance, a storage capacitor is employed, the 50
ohm coax cable is connected, the MOSFET has an internal resistance, and in the
end this is a critically damped RLC circuit, which is fired by a pulse to the gate
of the MOSFET.
The gate needs 5 V pulses (range: ±20 V) while
provided with 22 nC. Thus the current gain of this transistor is one for 3 ns
switching, but it still has voltage gain. Thus it could theoretically also be
used in common gate configuration and not in common source configuration. Transistors, which
switch 40 V are typically faster, so in the previous stage a current gain is
possible.
Applications of Pockels Cells
Pockels Cells are used in a variety of scientific
and technical applications:
- A Pockels Cell, combined with a polarizer, can be used for a variety of applications. Switching between no optical rotation and 90° rotation creates a fast shutter capable of "opening" and "closing" in nanoseconds. The same technique can be used to impress information on the beam by modulating the rotation between 0° and 90°; the exiting beam's intensity, when viewed through the polarizer, contains an amplitude-modulated signal.
- Preventing the feedback of a laser cavity by using a polarizing prism. This prevents optical amplification by directing light of a certain polarization out of the cavity. Because of this, the gain medium is pumped to a highly excited state. When the medium has become saturated by energy, the Pockels Cell is switched, and the intracavity light is allowed to exit. This creates a very fast, high intensity pulse. Q-switching, chirped pulse amplification, and cavity dumping use this technique.
- Pockels Cells can be used for quantum key distribution by polarizing photons.
- Pockels Cells in conjunction with other EO elements can be combined to form electro-optic probes.
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