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{{Short description|Linear change in the refractive index of optical media due to an electric field}}
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.
[[File:Pockels cell modulaliing light polarization.png|thumb|254x254px|A schematic of a Pockels cell modulating the [[Polarization (physics)|polarization]] of light. In this case, the Pockels cell is acting as a quarter wave plate, where [[Linear polarization|linearly-polarized]] light is converted to [[Circular polarization|circularly-polarized]] light. With the addition of a [[Brewster window]] (left), this change in polarization can be converted to a change in the [[Luminous intensity|intensity]] of the beam, by transmitting only the p-polarized vector component.]]

In [[optics]], the '''Pockels effect''', or '''Pockels electro-optic effect''', is a directionally-dependent linear variation in the [[refractive index]] of an [[optical medium]] that occurs in response to the application of an [[electric field]]. It is named after the German physicist [[Friedrich Carl Alwin Pockels]], who studied the effect in 1893.<ref>{{Cite book |author-link=Friedrich Carl Alwin Pockels |last=Pockels |first=F. |title=Ueber den Einfluss des elektrostatischen Feldes auf das optische Verhalten piëzoelektrischer Krystalle |publisher=Dieterich |location=Göttingen |series=Abhandlungen der königlichen Gesellschaft der Wissenschaften zu Göttingen |oclc=55796322 |url={{GBurl|yURAAQAAMAAJ}} |year=1894 |volume=39 |language=de}}</ref><ref>{{Cite book |last=Pockels |first=F. |title=Lehrbuch der Kristalloptik |year=1906 |location=Leipzig |bibcode=1906lekr.book.....P |language=de |publisher=B.G. Teubner |series=B.G. Teubners Sammlung von Lehrbüchern auf dem Gebiete der mathematischen Wissenschaften mit einschluss ihrer Anwendungen |volume=19 |oclc=864091434}}</ref> The non-linear counterpart, the [[Kerr effect]], causes changes in the refractive index at a rate proportional to the square of the applied electric field. In optical media, the Pockels effect causes changes in [[birefringence]] that vary in proportion to the strength of the applied electric field.

The Pockels effect occurs in crystals that lack [[inversion symmetry]], such as [[monopotassium phosphate]] ({{chem2|KH2PO4}}, abbr. KDP), [[potassium dideuterium phosphate]] ({{chem2|KD2PO4}}, abbr. KD*P or DKDP), [[lithium niobate]] ({{chem2|LiNbO3}}), [[beta-barium borate]] (BBO), [[barium titanate]] (BTO) and in other non-centrosymmetric media such as electric-field poled polymers or glasses. The Pockels effect has been elucidated through extensive study of electro-optic properties in materials like KDP.<ref>{{Cite journal |year=1976 |title=Electro-Optics Properties of KH2PO4 and Isomorphs |url=https://cdn.sanity.io/files/8jt7x1sz/production/2b7bac030cd14a9a5554d54001a68f8fa482db8a.pdf |journal=Information Sheet |publisher=Cleveland Crystals, Inc.}}</ref>


==Pockels cells==
==Pockels cells==
Pockels cells are voltage-controlled [[wave plate]]s. 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 of Pockels cells|Applications]] below for uses.
The key component of a Pockels cell is a non-centrosymmetric single crystal with an optic axis whose refractive index is controlled by an external electric field. In other words, the Pockels effect is the basis of the operation of Pockels cells. By controlling the refractive index, the optical retardance of the crystal is altered so the polarization state of incident light beam is changed. Therefore, Pockels cells are used as voltage-controlled [[wave plate]]s as well as other photonics applications. See [[#Applications|applications]] below for uses. Pockels cells are divided into two configurations depending on the crystals' electro-optic properties: longitudinal and transverse.


Longitudinal Pockels cells operate with electric field applied along the crystal optic axis or along incident beam propagation. Such crystals include KDP, KD*P, and ADP. Electrodes are coated as transparent metal oxide films on crystal faces where the beam is propagating through or metal rings (usually made out of gold) coated around the crystal body. Terminals for voltage application are in contact with the electrodes. The optical retardance ''Δφ'' for longitudinal Pockels cells proportional to the ordinary refractive index ''n''<sub>o</sub>, electro-optic constant ''r''<sub>63</sub> (units of m/V), and applied voltage ''V'' and inversely proportional to the incident beam wavelength ''λ''<sub>0</sub>. For an example, the halfwave voltage is approximately 7.6 kV for a KDP crystal with a ''n''<sub>o</sub> = 1.51, ''r''<sub>63</sub> = 10.6X10-12 m/V at ''λ''<sub>0</sub>, and Δφ = π.<ref>{{cite book |last1=Hecht |first1=Eugene |title=Optics |date=2002 |publisher=Addison Wesley |isbn=0-8053-8566-5 |edition=4th}}</ref> The advantage of using longitudinal Pockels cells is that the voltage requirements for quarter wave or half wave retardance is not dependent on crystal length or diameter.
⚫
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{{spaced ndash}}for example, horizontal or vertical), in contrast to the longitudinal case, the loss is not amplified through the length of the crystal.


Transverse Pockels cells operate with electric field being applied perpendicular to beam propagation. Crystals used in transverse Pockels cells include BBO, LiNbO<sub>3</sub>, [[CdTe]], [[ZnSe]], and [[CdSe]].<ref>{{Cite journal |year=1984 |title=Properties of the II-VI Crystals |url=https://cdn.sanity.io/files/8jt7x1sz/production/c4acc2ba55ea779842622b2c93d88ea305853417.pdf |journal=Information Sheet |publisher=Cleveland Crystals, Inc.}}</ref>
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.
The long sides of the crystal are coated with electrodes. Optical retardance ''Δφ'' for transverse Pockels cells is similar to that of longitudinal Pockels cells but it is dependent on crystal dimensions. The quarter wave or half wave voltage requirements increase with crystal aperture size, but the requirements can be reduced by lengthening the crystal.


⚫
Two or more crystal can be incorporated into a transverse Pockels cell. One reason is to reduce the voltage requirement by extending the overall length of the Pockels cell. Another reason is the fact that KDP is biaxial and possesses two electro-optic constants, ''r <sub>63</sub>'' for longitudinal configuration and ''r <sub>41</sub>'' for transverse configuration. A transverse Pockels cell that uses a KDP (or one of its isomorphs) consists of two crystals in opposite orientation, which together give a zero-order waveplate when the 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''{{spaced ndash}} for example, horizontal or vertical), in contrast to the longitudinal case, the loss is not amplified through the length of 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 (waves)|polarization]] [[optical rotation|rotation]] if the alignment is not exactly parallel or perpendicular to the polarization.

⚫
Alignment of the crystal axis with the ray axis is critical, regardless of configuration. Misalignment leads to [[birefringence]] and to a large phase shift across the long crystal. This leads to [[Polarization (waves)|polarization]] [[optical rotation|rotation]] if the alignment is not exactly parallel or perpendicular to the polarization.


=== Dynamics within the cell ===
=== Dynamics within the cell ===
Because of the high relative [[dielectric constant]] of ε<math>_r</math> ≈ 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.
Because of the high relative [[dielectric constant]] of ε<sub>r</sub> ≈ 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 that lead to both ends of the crystal. When they meet in the crystal, their voltages add up.
Pockels cells for [[optical fiber|fibre optics]] may employ a traveling wave design to reduce current requirements and increase speed.
Pockels cells for [[optical fiber|fiber optics]] may employ a traveling wave design to reduce current requirements and increase speed.


Usable crystals also exhibit the [[piezoelectric effect]] to some degree<ref>Joseph Valasek, "Properties of Rochelle Salt Related to the Piezo-Electric Effect", Physics Review, 1922, Vol XIX, No. 478</ref> ([[Rubidium Titanyl Phosphate|RTP]] has the lowest, [[Beta barium borate|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 picker]]s, but for boxcar windows. Guard space between the light and the faces of the crystals needs to be larger for longer holding times.
Usable crystals also exhibit the [[piezoelectric effect]] to some degree<ref>{{cite journal |first=J. |last=Valasek |title=Properties of Rochelle salt related to the piezo-electric effect |journal=Physical Review |volume=20 |issue=6 |page=639 |date=1922 |doi=10.1103/PhysRev.20.639 }}</ref> ([[Rubidium Titanyl Phosphate|RTP]] ({{chem2|RbTiOPO4}}) has the lowest, [[Beta barium borate|BBO]] and [[lithium niobate]] are the highest). After a voltage change, sound waves start propagating from the sides of the crystal to the middle. This is important not for [[pulse picker]]s, but for [[boxcar window]]s. 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.
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
This increases the polarization. Due to the growing of the polarized volume the electric field in the crystal in front of the wave increases
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=== The driver electronics ===
=== The driver electronics ===
A Pockels cell, by design, is a [[capacitor]], and often require high voltages to change the state of the polarization of the laser beam to effectively operate as a switchable waveplate. The voltage required depends on the type of Pockels cell, the wavelength of the light, and the size of the crystal; but typically, the voltage range is in the order of 1-10 kV. Pockels cell drivers provide this high voltage in the form of very fast pulses, which typically have rise times of less than 10 nanoseconds.
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&nbsp;mm aperture.
Shorter cables reduce the amount of charge wasted in transporting current to the cell.


There are basically two types of drivers: a quick or Q drive which has a fast rise time, then slowly decays. A Pockels cell that uses a Q-drive is sometimes referred to as a Q-switch. The other type of driver is referred to as a regenerative or R drive. R drives will have a fast rise time and a fast fall time. The driver's output pulse width can be from nanoseconds to microseconds long, depending on the application. The type of drive and its repetition rate will depend on the laser and the intended application.
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.


⚫
===Applications===
The driver may employ a cascade of transistors and a triode.
Pockels cells are used in a variety of scientific and technical applications. A Pockels cell, combined with a polarizer, can be used for switching between initial polarization state and half-wave phase retardance, creating a fast shutter capable of "opening" and "closing" in [[nanosecond]]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 [[Intensity (physics)|intensity]], when viewed through the polarizer, contains an [[amplitude modulation|amplitude-modulated]] signal. This modulated signal can be used for time-resolved electric field measurements when a crystal is exposed to an unknown electric field.<ref>{{cite journal|last1=Consoli|first1=F.|last2=De Angelis|first2=R.|last3=Duvillaret|first3=L.|last4=Andreoli|first4=P. L.|last5=Cipriani|first5=M.|last6=Cristofari|first6=G.|last7=Di Giorgio|first7=G.|last8=Ingenito|first8=F.|last9=Verona|first9=C.|title=Time-resolved absolute measurements by electro-optic effect of giant electromagnetic pulses due to laser-plasma interaction in nanosecond regime|journal=Scientific Reports|date=15 June 2016|volume=6|issue=1|page=27889|doi=10.1038/srep27889|bibcode=2016NatSR...627889C|pmc=4908660|pmid=27301704}}</ref><ref>{{cite journal|last1=Robinson|first1=T. S.|last2=Consoli|first2=F.|last3=Giltrap|first3=S.|last4=Eardley|first4=S. J.|last5=Hicks|first5=G. S.|last6=Ditter|first6=E. J.|last7=Ettlinger|first7=O.|last8=Stuart|first8=N. H.|last9=Notley|first9=M.|last10=De Angelis|first10=R.|last11=Najmudin|first11=Z.|last12=Smith|first12=R. A.|title=Low-noise time-resolved optical sensing of electromagnetic pulses from petawatt laser-matter interactions|journal=Scientific Reports|date=20 April 2017|volume=7|issue=1|page=983|doi=10.1038/s41598-017-01063-1|bibcode=2017NatSR...7..983R|pmc=5430545|pmid=28428549}}</ref>
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 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.


Pockels cells are used as a [[Q-switching|Q-switch]] to generate short, high-intensity laser pulse. The Pockels cell prevents optical amplification by introducing a polarization dependent loss in the laser cavity. This allows the [[gain medium]] to have a high [[population inversion]]. When the [[Active laser medium|gain medium]] has the desired [[population inversion]], the Pockels cell is switched "open", and a short high energy laser pulse is created. Q-switched lasers are used in a variety of applications, such as medical aesthetics, metrology, manufacturing, and holography.
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.


Pulse picking is another application that uses a Pockels cell. A pulse picker is typically composed of an oscillator, electro-optic modulator, amplifiers, high voltage driver, and a frequency doubling modulator along with a Pockels cell.<ref>{{cite journal |last1=Zhao |first1=Zhi |title=An ultrafast laser pulse picker technique for high-average-current high-brightness photoinjectors |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |year=2020 |volume=959 |page=163586 |publisher=Elsevier |doi=10.1016/j.nima.2020.163586 |bibcode=2020NIMPA.95963586Z |s2cid=213227045 |doi-access=free }}</ref> The Pockels cell can pick up a pulse from a laser induced bunch while blocking the rest by synchronized electro-optic switching.
⚫
==Applications of Pockels cells==
Pockels cells are used in a variety of scientific and technical applications:


Pockels cells are also used in [[Regenerative amplification|regenerative amplifiers]], [[chirped pulse amplification]], and [[Q-switching#Variants|cavity dumping]] to let optical power in and out of lasers and optical amplifiers.<ref>{{cite journal |last1=Pichon |first1=Pierre |last2=Taleb |first2=Hussein |last3=Druon |first3=Frédéric |last4=Blanchot |first4=Jean-Philippe |last5=Georges |first5=Patrick |last6=Balembois |first6=François |title=Tunable UV source based on an LED-pumped cavity-dumped Cr:LiSAF laser |journal=Optics Express |date=5 August 2019 |volume=27 |issue=16 |pages=23446–23453 |doi=10.1364/OE.27.023446 |pmid=31510620 |bibcode=2019OExpr..2723446P |s2cid=201256144 |issn=1094-4087|doi-access=free }}</ref>
*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 [[nanosecond]]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 [[Intensity (physics)|intensity]], when viewed through the polarizer, contains an [[amplitude modulation|amplitude-modulated]] signal.

* Preventing the [[feedback]] of a [[laser]] [[optical cavity|cavity]] by using a [[polarizer#Beam-splitting polarizers|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 [[Q-switching#Variants|cavity dumping]] use this technique.
* Pockels cells can be used for [[quantum key distribution]] by [[Photon polarization|polarizing]] [[photon]]s.
Pockels cells can be used for [[quantum key distribution]] by [[Photon polarization|polarizing]] [[photon]]s.

* Pockels cells in conjunction with other EO elements can be combined to form electro-optic probes.
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 ([[DiscoVision]]) 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.

⚫
A Pockels cell was used by MCA Disco-Vision ([[DiscoVision]]) 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 second-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 format's release in December 1978.

Pockels cells are used in [[two-photon microscopy]].

In recent years, Pockels cells are employed at the [[National Ignition Facility]] located at [[Lawrence Livermore National Laboratory]]. Each Pockels cell for one of the 192 lasers acts as an optical trap before exiting through an amplifier. The beams from all of the 192 lasers eventually converge onto a single target of deuterium-tritium fuel in hopes to trigger a fusion reaction.<ref>{{cite web |title=How NIF Works |url=https://lasers.llnl.gov/about/how-nif-works |website=lasers.llnl.gov |access-date=25 April 2023}}</ref>


==See also==
==See also==
* [[Electro-optic modulator]]
* [[Electro-optic modulator]]
* [[Acousto-optic modulator]]
* [[Acousto-optic modulator]]
* [[Kerr effect]]


== References ==
== References ==
{{reflist}}
{{reflist}}


{{commons category}}
==External links==

* [http://www.eos-optronics.com/documents/Paper-10kV_PCD_PoznanSLR.pdf Paper on ultrafast switching Pockels Cell Drivers]

* [http://www.fastpulse.com/pdf/pcp.pdf Pockels Cell Primer]- Article on Pockels cell basics.
{{Authority control}}
* [http://www.fastpulse.com/pdf/eodir.pdf Electro-Optic Devices in Review]- Article about Pockels cell


{{DEFAULTSORT:Pockels Effect}}
[[Category:Nonlinear optics]]
[[Category:Nonlinear optics]]
[[Category:Polarization]]
[[Category:Polarization (waves)]]
[[Category:Cryptographic hardware]]
[[Category:Quantum information science]]
[[Category:Quantum information science]]

Latest revision as of 04:36, 18 April 2024

A schematic of a Pockels cell modulating the polarization of light. In this case, the Pockels cell is acting as a quarter wave plate, where linearly-polarized light is converted to circularly-polarized light. With the addition of a Brewster window (left), this change in polarization can be converted to a change in the intensity of the beam, by transmitting only the p-polarized vector component.

In optics, the Pockels effect, or Pockels electro-optic effect, is a directionally-dependent linear variation in the refractive index of an optical medium that occurs in response to the application of an electric field. It is named after the German physicist Friedrich Carl Alwin Pockels, who studied the effect in 1893.[1][2] The non-linear counterpart, the Kerr effect, causes changes in the refractive index at a rate proportional to the square of the applied electric field. In optical media, the Pockels effect causes changes in birefringence that vary in proportion to the strength of the applied electric field.

The Pockels effect occurs in crystals that lack inversion symmetry, such as monopotassium phosphate (KH2PO4, abbr. KDP), potassium dideuterium phosphate (KD2PO4, abbr. KD*P or DKDP), lithium niobate (LiNbO3), beta-barium borate (BBO), barium titanate (BTO) and in other non-centrosymmetric media such as electric-field poled polymers or glasses. The Pockels effect has been elucidated through extensive study of electro-optic properties in materials like KDP.[3]

Pockels cells

[edit]

The key component of a Pockels cell is a non-centrosymmetric single crystal with an optic axis whose refractive index is controlled by an external electric field. In other words, the Pockels effect is the basis of the operation of Pockels cells. By controlling the refractive index, the optical retardance of the crystal is altered so the polarization state of incident light beam is changed. Therefore, Pockels cells are used as voltage-controlled wave plates as well as other photonics applications. See applications below for uses. Pockels cells are divided into two configurations depending on the crystals' electro-optic properties: longitudinal and transverse.

Longitudinal Pockels cells operate with electric field applied along the crystal optic axis or along incident beam propagation. Such crystals include KDP, KD*P, and ADP. Electrodes are coated as transparent metal oxide films on crystal faces where the beam is propagating through or metal rings (usually made out of gold) coated around the crystal body. Terminals for voltage application are in contact with the electrodes. The optical retardance Δφ for longitudinal Pockels cells proportional to the ordinary refractive index no, electro-optic constant r63 (units of m/V), and applied voltage V and inversely proportional to the incident beam wavelength λ0. For an example, the halfwave voltage is approximately 7.6 kV for a KDP crystal with a no = 1.51, r63 = 10.6X10-12 m/V at λ0, and Δφ = π.[4] The advantage of using longitudinal Pockels cells is that the voltage requirements for quarter wave or half wave retardance is not dependent on crystal length or diameter.

Transverse Pockels cells operate with electric field being applied perpendicular to beam propagation. Crystals used in transverse Pockels cells include BBO, LiNbO3, CdTe, ZnSe, and CdSe.[5] The long sides of the crystal are coated with electrodes. Optical retardance Δφ for transverse Pockels cells is similar to that of longitudinal Pockels cells but it is dependent on crystal dimensions. The quarter wave or half wave voltage requirements increase with crystal aperture size, but the requirements can be reduced by lengthening the crystal.

Two or more crystal can be incorporated into a transverse Pockels cell. One reason is to reduce the voltage requirement by extending the overall length of the Pockels cell. Another reason is the fact that KDP is biaxial and possesses two electro-optic constants, r 63 for longitudinal configuration and r 41 for transverse configuration. A transverse Pockels cell that uses a KDP (or one of its isomorphs) consists of two crystals in opposite orientation, which together give a zero-order waveplate when the 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.

Alignment of the crystal axis with the ray axis is critical, regardless of configuration. 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

[edit]

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 that lead to both ends of the crystal. When they meet in the crystal, their voltages add up. Pockels cells for fiber optics may employ a traveling wave design to reduce current requirements and increase speed.

Usable crystals also exhibit the piezoelectric effect to some degree[6] (RTP (RbTiOPO4) has the lowest, BBO and lithium niobate are the highest). 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

[edit]

A Pockels cell, by design, is a capacitor, and often require high voltages to change the state of the polarization of the laser beam to effectively operate as a switchable waveplate. The voltage required depends on the type of Pockels cell, the wavelength of the light, and the size of the crystal; but typically, the voltage range is in the order of 1-10 kV. Pockels cell drivers provide this high voltage in the form of very fast pulses, which typically have rise times of less than 10 nanoseconds.

There are basically two types of drivers: a quick or Q drive which has a fast rise time, then slowly decays. A Pockels cell that uses a Q-drive is sometimes referred to as a Q-switch. The other type of driver is referred to as a regenerative or R drive. R drives will have a fast rise time and a fast fall time. The driver's output pulse width can be from nanoseconds to microseconds long, depending on the application. The type of drive and its repetition rate will depend on the laser and the intended application.

Applications

[edit]

Pockels cells are used in a variety of scientific and technical applications. A Pockels cell, combined with a polarizer, can be used for switching between initial polarization state and half-wave phase retardance, creating 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. This modulated signal can be used for time-resolved electric field measurements when a crystal is exposed to an unknown electric field.[7][8]

Pockels cells are used as a Q-switch to generate short, high-intensity laser pulse. The Pockels cell prevents optical amplification by introducing a polarization dependent loss in the laser cavity. This allows the gain medium to have a high population inversion. When the gain medium has the desired population inversion, the Pockels cell is switched "open", and a short high energy laser pulse is created. Q-switched lasers are used in a variety of applications, such as medical aesthetics, metrology, manufacturing, and holography.

Pulse picking is another application that uses a Pockels cell. A pulse picker is typically composed of an oscillator, electro-optic modulator, amplifiers, high voltage driver, and a frequency doubling modulator along with a Pockels cell.[9] The Pockels cell can pick up a pulse from a laser induced bunch while blocking the rest by synchronized electro-optic switching.

Pockels cells are also used in regenerative amplifiers, chirped pulse amplification, and cavity dumping to let optical power in and out of lasers and optical amplifiers.[10]

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.

A Pockels cell was used by MCA Disco-Vision (DiscoVision) 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 second-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 format's release in December 1978.

Pockels cells are used in two-photon microscopy.

In recent years, Pockels cells are employed at the National Ignition Facility located at Lawrence Livermore National Laboratory. Each Pockels cell for one of the 192 lasers acts as an optical trap before exiting through an amplifier. The beams from all of the 192 lasers eventually converge onto a single target of deuterium-tritium fuel in hopes to trigger a fusion reaction.[11]

See also

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References

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  1. ^ Pockels, F. (1894). Ueber den Einfluss des elektrostatischen Feldes auf das optische Verhalten piëzoelektrischer Krystalle. Abhandlungen der königlichen Gesellschaft der Wissenschaften zu Göttingen (in German). Vol. 39. Göttingen: Dieterich. OCLC 55796322.
  2. ^ Pockels, F. (1906). Lehrbuch der Kristalloptik. B.G. Teubners Sammlung von Lehrbüchern auf dem Gebiete der mathematischen Wissenschaften mit einschluss ihrer Anwendungen (in German). Vol. 19. Leipzig: B.G. Teubner. Bibcode:1906lekr.book.....P. OCLC 864091434.
  3. ^ "Electro-Optics Properties of KH2PO4 and Isomorphs" (PDF). Information Sheet. Cleveland Crystals, Inc. 1976.
  4. ^ Hecht, Eugene (2002). Optics (4th ed.). Addison Wesley. ISBN 0-8053-8566-5.
  5. ^ "Properties of the II-VI Crystals" (PDF). Information Sheet. Cleveland Crystals, Inc. 1984.
  6. ^ Valasek, J. (1922). "Properties of Rochelle salt related to the piezo-electric effect". Physical Review. 20 (6): 639. doi:10.1103/PhysRev.20.639.
  7. ^ Consoli, F.; De Angelis, R.; Duvillaret, L.; Andreoli, P. L.; Cipriani, M.; Cristofari, G.; Di Giorgio, G.; Ingenito, F.; Verona, C. (15 June 2016). "Time-resolved absolute measurements by electro-optic effect of giant electromagnetic pulses due to laser-plasma interaction in nanosecond regime". Scientific Reports. 6 (1): 27889. Bibcode:2016NatSR...627889C. doi:10.1038/srep27889. PMC 4908660. PMID 27301704.
  8. ^ Robinson, T. S.; Consoli, F.; Giltrap, S.; Eardley, S. J.; Hicks, G. S.; Ditter, E. J.; Ettlinger, O.; Stuart, N. H.; Notley, M.; De Angelis, R.; Najmudin, Z.; Smith, R. A. (20 April 2017). "Low-noise time-resolved optical sensing of electromagnetic pulses from petawatt laser-matter interactions". Scientific Reports. 7 (1): 983. Bibcode:2017NatSR...7..983R. doi:10.1038/s41598-017-01063-1. PMC 5430545. PMID 28428549.
  9. ^ Zhao, Zhi (2020). "An ultrafast laser pulse picker technique for high-average-current high-brightness photoinjectors". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 959. Elsevier: 163586. Bibcode:2020NIMPA.95963586Z. doi:10.1016/j.nima.2020.163586. S2CID 213227045.
  10. ^ Pichon, Pierre; Taleb, Hussein; Druon, Frédéric; Blanchot, Jean-Philippe; Georges, Patrick; Balembois, François (5 August 2019). "Tunable UV source based on an LED-pumped cavity-dumped Cr:LiSAF laser". Optics Express. 27 (16): 23446–23453. Bibcode:2019OExpr..2723446P. doi:10.1364/OE.27.023446. ISSN 1094-4087. PMID 31510620. S2CID 201256144.
  11. ^ "How NIF Works". lasers.llnl.gov. Retrieved 25 April 2023.
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