Pockels effect

Last updated
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. Pockels cell modulaliing light polarization.png
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.

Contents

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

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

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

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

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 to adjust the transmitted laser intensity at a time scale of microseconds. [11]

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. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Nonlinear optics</span> Branch of physics

Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typically observed only at very high light intensities (when the electric field of the light is >108 V/m and thus comparable to the atomic electric field of ~1011 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.

An electro–optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light. The term encompasses a number of distinct phenomena, which can be subdivided into

<span class="mw-page-title-main">Electro-optic modulator</span> Type of optical device

An electro-optic modulator (EOM) is an optical device in which a signal-controlled element exhibiting an electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the beam. Modulation bandwidths extending into the gigahertz range are possible with the use of laser-controlled modulators.

Electro–optics is a branch of electrical engineering, electronic engineering, materials science, and material physics involving components, electronic devices such as lasers, laser diodes, LEDs, waveguides, etc. which operate by the propagation and interaction of light with various tailored materials. It is closely related to photonics, the branch of optics that involves the application of the generation of photons. It is not only concerned with the "electro–optic effect", since it deals with the interaction between the electromagnetic and the electrical states of materials.

<span class="mw-page-title-main">Polarization (waves)</span> Property of waves that can oscillate with more than one orientation

Polarization is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string (see image), for example, in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, and transverse sound waves in solids.

Q-switching, sometimes known as giant pulse formation or Q-spoiling, 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 (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave mode. Compared to modelocking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. The two techniques are sometimes applied together.

<span class="mw-page-title-main">Birefringence</span> Refractive property of materials

Birefringence means double refraction. It is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

The Kerr effect, also called the quadratic electro-optic (QEO) 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 for the Kerr effect is directly proportional to the square of the electric field instead of varying linearly with it. All materials show a Kerr effect, but certain liquids display it more strongly than others. The Kerr effect was discovered in 1875 by Scottish physicist John Kerr.

<span class="mw-page-title-main">Acousto-optic modulator</span> Device which diffracts light via sound waves

An acousto-optic modulator (AOM), also called a Bragg cell or an acousto-optic deflector (AOD), uses the acousto-optic effect to diffract and shift the frequency of light using sound waves. They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric transducer is attached to a material such as glass. An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the material. These can be thought of as moving periodic planes of expansion and compression that change the index of refraction. Incoming light scatters off the resulting periodic index modulation and interference occurs similar to Bragg diffraction. The interaction can be thought of as a three-wave mixing process resulting in sum-frequency generation or difference-frequency generation between phonons and photons.

This is a list of acronyms and other initialisms used in laser physics and laser applications.

An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide. Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase modulators, polarization modulators, etc. The easiest way to obtain modulation of intensity of a light beam is to modulate the current driving the light source, e.g. a laser diode. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason light modulators are, e.g. in fiber-optic communications, called external light modulators.

<span class="mw-page-title-main">Acousto-optics</span> The study of sound and light interaction

Acousto-optics is a branch of physics that studies the interactions between sound waves and light waves, especially the diffraction of laser light by ultrasound through an ultrasonic grating.

Electro-optical sensors are electronic detectors that convert light, or a change in light, into an electronic signal. These sensors are able to detect electromagnetic radiation from the infrared down to the ultraviolet wavelengths. They are used in many industrial and consumer applications, for example:

<span class="mw-page-title-main">Optical rectification</span>

Electro-optic rectification (EOR), also referred to as optical rectification, is a non-linear optical process that consists of the generation of a quasi-DC polarization in a non-linear medium at the passage of an intense optical beam. For typical intensities, optical rectification is a second-order phenomenon which is based on the inverse process of the electro-optic effect. It was reported for the first time in 1962, when radiation from a ruby laser was transmitted through potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (KDdP) crystals.

EKSPLA is a laser and laser electronics manufacturing company based in Vilnius, Lithuania. EKSPLA is known for their lasers and laser systems, as well as other photonics components. The company is supplying their products for scientific, OEM and industrial applications.

There are various classifications of the electro-optical modes of liquid crystal displays (LCDs).

An electro-absorption modulator (EAM) is a semiconductor device which can be used for modulating the intensity of a laser beam via an electric voltage. Its principle of operation is based on the Franz–Keldysh effect, i.e., a change in the absorption spectrum caused by an applied electric field, which changes the bandgap energy but usually does not involve the excitation of carriers by the electric field.

An optical modulator is an optical device which is used to modulate a beam of light with a perturbation device. It is a kind of transmitter to convert information to optical binary signal through optical fiber or transmission medium of optical frequency in fiber optic communication. There are several methods to manipulate this device depending on the parameter of a light beam like amplitude modulator (majority), phase modulator, polarization modulator etc. The easiest way to obtain modulation is modulation of intensity of a light by the current driving the light source. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason, light modulators are called external light modulators. According to manipulation of the properties of material modulators are divided into two groups, absorptive modulators and refractive modulators. Absorption coefficient can be manipulated by Franz-Keldysh effect, Quantum-Confined Stark Effect, excitonic absorption, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called electro-absorptive modulator. Refractive modulators most often make use of electro-optic effect, other modulators are made with acousto-optic effect, magneto-optic effect such as Faraday and Cotton-Mouton effects. The other case of modulators is spatial light modulator (SLM) which is modified two dimensional distribution of amplitude & phase of an optical wave.

<span class="mw-page-title-main">Acousto-optic programmable dispersive filter</span>

An acousto-optic programmable dispersive filter (AOPDF) is a special type of collinear-beam acousto-optic modulator capable of shaping spectral phase and amplitude of ultrashort laser pulses. AOPDF was invented by Pierre Tournois. Typically, quartz crystals are used for the fabrication of the AOPDFs operating in the UV spectral domain, paratellurite crystals are used in the visible and the NIR and calomel in the MIR (3–20 μm). Recently introduced lithium niobate crystals allow for high-repetition rate operation (> 100 kHz) owing to their high acoustic velocity. The AOPDF is also used for the active control of the carrier-envelope phase of few-cycle optical pulses, as a part of pulse-measurement schemes and multi-dimensional spectroscopy techniques. Although sharing a lot in principle of operation with an acousto-optic tunable filter, the AOPDF should not be confused with it, since in the former the tunable parameter is the transfer function and in the latter it is the impulse response.

References

  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. Bibcode:1922PhRv...20..639V. 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. ↑ "Conoptics Model 350-80LA Electro-Optic Modulator". Conoptics Homepage. Retrieved 30 August 2024.
  12. ↑ "How NIF Works". lasers.llnl.gov. Retrieved 25 April 2023.


This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.