Pockels effect
Encyclopedia
The Pockels effect 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 in 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.
s.
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.
A transverse cell consists of 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.
Pockels cells may be used to rotate the polarization of a passing beam. See Applications below for uses.
≈ 36 of the crystal electric field changes propagate only with 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 (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 not important 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.
. 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 a 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 Ohm, which matches standard coaxial cable
s, and the Mosfet can thus placed remotely. Some of the 50 Ohm is 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.
And then there the little problem, that the IRF delivers a negative pulse, but the Y690 needs a positive pulse,
which is solved by a transformer.
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
in an optical medium induced by a constant or varying electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. It is distinguished from the Kerr effect
Kerr effect
The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...
by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is quadratic in the field. The Pockels effect occurs only in crystals that lack inversion symmetry, such as lithium niobate
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
or gallium arsenide and in other noncentrosymmetric media such as electric-field poled polymers or glasses.
Pockels cells
The Pockels effect is used to make Pockels cells, which are voltage-controlled wave plateWave plate
A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it.- Operation :A wave plate works by shifting the phase between two perpendicular polarization components of the light wave. A typical wave plate is simply a birefringent crystal...
s.
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
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
and to a large phase shift across the long crystal. This leads to polarization rotation
Optical rotation
Optical rotation is the turning of the plane of linearly polarized light about the direction of motion as the light travels through certain materials. It occurs in solutions of chiral molecules such as sucrose , solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms...
if the alignment is not exactly parallel or perpendicular to the polarization.
A transverse cell consists of 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.
Pockels cells may be used to rotate the polarization of a passing beam. See Applications below for uses.
Dynamics within the cell
Due to the high relative dielectric constant of ε ≈ 36 of the crystal electric field changes propagate only with 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
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
may employ a traveling wave design to reduce current requirements and increase speed.
Usable crystals also exhibit the piezoelectric effect to some degree (RTP has the lowest, BBO
Beta Barium Borate
Beta barium borate is a crystal frequently used for frequency mixing and other nonlinear optics applications...
and Lithium niobate
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
are high), after a voltage change sound waves start propagating from the sides of the crystal to the middle. This is not important 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 capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
. 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
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, or the gates are driven by a large transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
.
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 a 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 Ohm, which matches standard coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
s, and the Mosfet can thus placed remotely. Some of the 50 Ohm is 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
RLC circuit
An RLC circuit is an electrical circuit consisting of a resistor, an inductor, and a capacitor, connected in series or in parallel. The RLC part of the name is due to those letters being the usual electrical symbols for resistance, inductance and capacitance respectively...
, 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
Common gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a current buffer or voltage amplifier...
configuration and not in common source
Common source
In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal...
configuration.
Transistors, which switch 40 V are typically faster, so in the previous stage a current gain is possible.
And then there the little problem, that the IRF delivers a negative pulse, but the Y690 needs a positive pulse,
which is solved by a transformer.
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 nanosecondNanosecondA nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s. The same technique can be used to impress information on the beam by modulating the rotation between 0° and 90°; the exiting beam's intensityIntensity (physics)In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
, when viewed through the polarizer, contains an amplitude-modulatedAmplitude modulationAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
signal. - Preventing the feedbackFeedbackFeedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
of a laserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
cavityOptical cavityAn optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
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-switchingQ-switchingQ-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in a...
, chirped pulse amplificationChirped pulse amplificationChirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification...
, and cavity dumping use this technique. - Pockels cells can be used for quantum key distribution by polarizingPhoton polarizationPhoton polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photons are completely polarized...
photonPhotonIn physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s. - Pockels cells in conjunction with other EO elements can be combined to form electro-optic probes.
- A Pockels Cell was used by MCA Disco-Vision (DiscoVisionDiscoVisionDiscoVision is the name of several things related to the video laserdisc format. It was the original name of the "Reflective Optical Videodisc System" format later known as LaserVision or LaserDisc....
) engineers in the optical videodisc mastering system. Light from an Argon-Ion laser was passed through the Pockels Cell to create pulse modulations corresponding to the original FM video and audio signals to be recorded on the master videodisc. MCA used the Pockels Cell in videodisc mastering until the sale to Pioneer Electronics. To increase the quality of the recordings, MCA patented a Pockels Cell stabilizer that reduced the 2nd Harmonic Distortion that could be created by the Pockels Cell during mastering. MCA used either a DRAW (Direct Read After Write) mastering system or a Photoresist system. The DRAW system was originally preferred, since it didn't require clean room conditions during disc recording, and allowed instant quality checking during mastering. The original single-sided test pressings from 1976/77 were mastered with the DRAW system as were the "educational", non-feature titles at the formats release in December 1978.
External links
- Paper on ultrafast switching Pockels Cell Drivers
- Pockels Cell Primer- Article on Pockels cell basics.
- Electro-Optic Devices in Review- Article about Pockels cell configurations and drive voltage calculations.