Acousto-optic programmable dispersive filter

Last updated

An acousto-optic programmable dispersive filter (AOPDF) is a special type of collinear-beam acousto-optic modulator [1] capable of shaping spectral phase and amplitude of ultrashort laser pulses. AOPDF was invented by Pierre Tournois. [2] 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 (up to 4 μm) 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 [3] , as a part of pulse-measurement schemes [4] and multi-dimensional spectroscopy techniques [5] [6] . 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.

Contents

Image illustrating the principle of spectral phase and amplitude shaping by an acousto-optic programmable dispersive filter. AOPDF principle.png
Image illustrating the principle of spectral phase and amplitude shaping by an acousto-optic programmable dispersive filter.

Theory of operation

Traveling acoustic wave induces variations in optical properties, thus forming a dynamic volume grating.

Pulse shaping

AOPDF is a programmable spectral filter. From signal processing point of view, the AOPDF corresponds to a time-variant passive linear transversal filter with a programmable finite impulse response. Phase and amplitude filtering in the AOPDF is achieved by virtue of birefringent acousto-optic effect and can be represented by a convolution between the amplitude of the input optical signal Ein(t) and a programmable acoustical signal S(t/α) proportional to the electrical signal S(t) applied to the Piezoelectric transducer (made typically from lithium niobate). Here, α is a scaling factor equal to the ratio of the speed of sound v to the speed of light c times the index difference Δn between the ordinary and the extraordinary waves taken along the propagation axis in the crystal. In the limit of low diffraction efficiency the AOPDF behaves as a linear filter and the small value of the α (typically 10−7) allows for the quantitative control of optical signals with frequencies of tens to hundreds of terahertz with electrical signals of tens of megahertz, which are readily produced by commercial waveform generators.

Polarization

Owing to its birefringent nature, the AOPDF is intrinsically polarization-sensitive. Furthermore, polarization of the diffracted wave, created as the result of the interaction between the incident optical wave and the acoustic wave in the crystal, is rotated by 90° with respect to the incident wave polarization. For the single-beam optical input there could be up to 4 beams at the output of the AOPDF: two transmitted (non-diffracted) beams arising from double refraction and (in the presence of a suitable acoustic wave in the crystal) two diffracted beams corresponding to each linear polarization component (ordinary and extraordinary) of the input beam. Typically, an ordinary-polarized beam is used at the input and so, only two beams are observed at the output: an ordinary-polarized transmitted beam and an extraordinary-polarized diffracted beam.

Diffraction efficiency

Spectral intensity of the diffracted wave depends on the spectral intensity of the acoustic wave (which depends, in turn, on the RF power applied to the transducer). Ratio between the diffracted intensity and the input one represents the diffraction efficiency. Maximum diffraction efficiency is limited by nonlinear effects. Linear regime persists up to diffraction efficiencies of about 50% [ citation needed ]. Total efficiency is altered by Fresnel losses at the input and output faces of the crystal unless anti-reflection coating is used.

Spectral bandwidth

Spectral bandwidth of the AOPDF is defined as a range over which the device can operate. One can distinguish intrinsic bandwidth, which is limited by absorption of the acousto-optic crystal, total device bandwidth, limited by impedance matching between the piezoelectric transducer and the radio-frequency generator, and instantaneous bandwidth defined by maximal simultaneous spectral width diffracted with reasonable efficiency.

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.

<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.

<span class="mw-page-title-main">Waveplate</span> Optical polarization device

A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are the half-wave plate, which rotates the polarization direction of linearly polarized light, and the quarter-wave plate, which converts between different elliptical polarizations

<span class="mw-page-title-main">Birefringence</span> Property of materials whose refractive index depends on light polarization and direction

Birefringence 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.

In physics, coherence expresses the potential for two waves to interfere. Two monochromatic beams from a single source always interfere. Physical sources are not strictly monochromatic: they may be partly coherent. Beams from different sources are mutually incoherent.

<span class="mw-page-title-main">Prism (optics)</span> Transparent optical element with flat, polished surfaces that refract light

An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite.

<span class="mw-page-title-main">Pockels effect</span> Linear change in the refractive index of optical media due to an electric field

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. 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.

<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.

<span class="mw-page-title-main">Polarimetry</span> Measurement and interpretation of the polarization of transverse waves

Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object.

A Lyot filter, named for its inventor and French astronomer Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. Lyot filters are used in astronomy, particularly for solar astronomy, lasers, biomedical photonics and Raman chemical imaging.

A photoelastic modulator (PEM) is an optical device used to modulate the polarization of a light source. The photoelastic effect is used to change the birefringence of the optical element in the photoelastic modulator.

Digital holography is the acquisition and processing of holograms with a digital sensor array, typically a CCD camera or a similar device. Image rendering, or reconstruction of object data is performed numerically from digitized interferograms. Digital holography offers a means of measuring optical phase data and typically delivers three-dimensional surface or optical thickness images. Several recording and processing schemes have been developed to assess optical wave characteristics such as amplitude, phase, and polarization state, which make digital holography a very powerful method for metrology applications .

<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.

An acousto-optical spectrometer (AOS) is based on the diffraction of light by ultrasonic waves. A piezoelectric transducer, driven by the RF signal, generates an acoustic wave in a crystal. This acoustic wave modulates the refractive index and induces a phase grating. The Bragg-cell is illuminated by a collimated laser beam. The angular dispersion of the diffracted light represents a true image of the IF-spectrum according to the amplitude and wavelengths of the acoustic waves in the crystal. The spectrum is detected by using a single linear diode array (CCD), which is placed in the focal plane of an imaging optics. Depending on the crystal and the focal length of the imaging optics, the resolution of this type of spectrometer can be varied.

<span class="mw-page-title-main">Radial polarization</span>

A beam of light has radial polarization if at every position in the beam the polarization vector points towards the center of the beam. In practice, an array of waveplates may be used to provide an approximation to a radially polarized beam. In this case the beam is divided into segments, and the average polarization vector of each segment is directed towards the beam centre.

<span class="mw-page-title-main">Cross-polarized wave generation</span>

Cross-polarized wave (XPW) generation is a nonlinear optical process that can be classified in the group of frequency degenerate processes. It can take place only in media with anisotropy of third-order nonlinearity. As a result of such nonlinear optical interaction at the output of the nonlinear crystal, it is generated a new linearly polarized wave at the same frequency, but with polarization oriented perpendicularly to the polarization of input wave

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">Polarization rotator</span> Optical device

A polarization rotator is an optical device that rotates the polarization axis of a linearly polarized light beam by an angle of choice. Such devices can be based on the Faraday effect, on birefringence, or on total internal reflection. Rotators of linearly polarized light have found widespread applications in modern optics since laser beams tend to be linearly polarized and it is often necessary to rotate the original polarization to its orthogonal alternative.

<span class="mw-page-title-main">Vitaly Voloshinov</span> Soviet-Russian physicist (1947–2019)

Vitaly Borisovich Voloshinov was a Soviet and Russian physicist, one of the world's leading experts in the field of acoustoptics, honored teacher of Moscow State University

Spectral interferometry (SI) or frequency-domain interferometry is a linear technique used to measure optical pulses, with the condition that a reference pulse that was previously characterized is available. This technique provides information about the intensity and phase of the pulses. SI was first proposed by Claude Froehly and coworkers in the 1970s.

References

  1. I.C. Chang (1992). "Collinear beam acousto-optic tunable filters". Electronics Letters. 28 (13): 1255–1256. Bibcode:1992ElL....28.1255C. doi:10.1049/el:19920793.
  2. Pierre Tournois (1997). "Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion in laser systems". Optics Communications. 140 (4–6): 245–249. Bibcode:1997OptCo.140..245T. doi:10.1016/S0030-4018(97)00153-3.
  3. L. Canova; et al. (2009). "Carrier-envelope phase stabilization and control using a transmission grating compressor and an AOPDF". Optics Letters. 34 (9): 1333–5. Bibcode:2009OptL...34.1333C. doi:10.1364/OL.34.001333. PMID   19412263.
  4. N.Forget (2010). "Pulse-measurement techniques using a single amplitude and phase spectral shaper". JOSA B. 27 (4): 742–756. doi:10.1364/JOSAB.27.000742.
  5. Z.Zhang (2012). "Phase-cycling schemes for pump–probe beam geometry two-dimensional electronic spectroscopy". Chemical Physics Letters. doi:10.1016/j.cplett.2012.08.037.
  6. O.Schubert (2013). "Rapid-scan acousto-optical delay line with 34 kHz scan rate and 15 as precision". Optics Letters. doi:10.1364/OL.38.002907.