modern acousto-optics: physics and applications · 2012-06-02 · modern acousto-optics: physics...
TRANSCRIPT
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Modern Acousto-Optics: Physics and Applications
Dr. Alexandre S. Shcherbakov
Image Science Group, Optics Department,
National Institute for Astrophysics, Optics & Electronics ( INAOE )
Part 1: Acousto-optical technique for spectrum analysis
of radio-wave and optical signals in astrophysics.
Part 2: Multi-wave coupled states in acousto-optics.
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Part 1:
Acousto-optical technique
for multi-channel spectrum analysis
of ultra-high frequency radio-signals and
optical signals over all the visible range;
possible applications to the astrophysical needs.
In collaboration with: 1) Astrophysics Department of the INAOE,
2) Molecular Technology GmbH ( Berlin, Germany ),
3) Saint-Petersburg State Polytechnic University (St. Petersburg, Russia),
4) Institute for Applied Physics (Madrid, Spain).
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Outline
Introduction: Regimes of operation for the acousto-optic cells.
The Bragg acousto-optical interaction in anisotropic medium.
A one-phonon non-collinear acousto-optical interaction:
The number of resolvable spots. Estimations for a tellurium dioxide crystal.
A two-phonon non-collinear light scattering:
The number of resolvable spots. Estimations for a tellurium dioxide crystal.
Experimental data related to a multi-phonon light scattering in tellurium dioxide cell.
General schematic arrangement for prototype of an advanced acousto-optical spectrometer.
The first experimental data related to the spectrum analysis
of analogous high-frequency radio-wave signals within a one-phonon light scattering.
Collinear acousto-optical interaction and the opportunity of filtering analogous optical signals.
Potential spectral resolution of a calcium molybdate crystal based collinear filter.
Conclusive remarks.
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Regimes of operating for the AO- cells
npsinsin 0p
1LQ2
1. Raman-Nath regime ( short L ),
in which the diffraction maxima
are given by
n2sin B
1LQ2
Scattering light by a thin dynamic
acoustic grating.
There are two limiting cases for light diffraction (scattering) regimes :
2. Bragg regime ( long L ),
in which the Bragg condition
is given by
where and
=V / f.
, ,
where .
Scattering light by a thick dynamic
acoustic grating.
The parameter Q is the Klein – Cook factor characterizing the regime.
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1SPM2)cos(q
The Raman-Nath limit; Q << 1.
Conservation laws inherent in such interactions are
or
or 321 kkk
kp
321 E
Since in conventional acousto-optics, 1 , 3 10 14 Hz and 2 = 10 9 Hz, so
that / 10 –5 and Ephoton 10 -5 Ephonon.
Because c = 3 10 10 cm/s and V 3 10 5 cm/s, we yield
V / c 10 5, 1 / c / V, and kphoton Kphonon.
Plots for the light intensities in various orders of scattering
The Bragg limit; Q >> 1.
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Realizing the parallel spectrum analysis by
a TeO2 acousto-optical cell in the Bragg regime
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A one-phonon light scattering
1. Normal scattering 2. Optimized anomalous scattering
,Vn2
fsin 1
The comparison for TeO2 -crystal shows that generally: D f1 < D f0 .
,f
V1
Vn2
fsin
2
1
2
22
1,0
11,0
D
2
1
2
0
2nn D
D Vf0
2L0
)Lf(V 1
)Lf(V 0
1
2
1fL
Vn2f
D
L
n2V2f 0
0
D
Lx
Lx
2L0
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A two-phonon acousto-optical interaction
General arrangement of optical beams and a two-phonon acousto-optical
interaction in a TeO2 single crystal
Kkk m1m
,nnnsin21
20
100
.nnV2f2
21
20
122
Conservation laws: m+1 = m + and ( m = 0,1,2 ).
A two-phonon Bragg scattering of light occurs when
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General schematic arrangement of the advanced
prototype of acousto-optical spectrometer for the
Large Millimeter Telescope (GTM)
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The light beam passing through glass prisms.
2 2 21
10
(n sin φ)[1 n sin (α δ)]dB ,
d n cos φ cos (α δ)
sin φδ arcsin .
n
A 4-prism beam expander
A multi-prism beam expander
.)B(Bm
1m
;mm1d0 ;mm35D
;5.1n .30o
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The flatness can be estimated by about 25% ;
The experimentally estimated average transmittance is about 70%.
Experimental plot for a one-dimensionally expanded beam profile.
Transmittance of a 4-prism BK-7 glass beam expander
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Optical aperture of a cell: D = 3.5 cm ; Length of interaction: L = 1.0 cm ;
Slow shear elastic wave velocity: V = 0.660 .10 5 cm/s ;
Operating light wavelength: = 633 nm ; Refractive indices: no = 2.30 , ne = 2.46 ;
Central acoustic frequency: f0 = 75 MHz ; Frequency bandwidth: D f = 45 MHz ;
Ideal theoretical frequency resolution: f = V/D = 18.8 KHz ;
The estimated maximum number of resolvable spots, i.e.
the number of parallel frequency channels for spectrum analysis: N = 2360.
The Bragg tellurium dioxide
acousto-optic cell
Efficiency and frequency bandwidth of a TeO2-cell
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Intensity of the scattered light versus
the applied electric power.
This diagram represents
the obtained degree of
linearity in a dependence
between the electric power
and the irradiance of light.
Experimental light scattering efficiency of a TeO2 - cell
The observed variations are
potentially conditioned by
the interplay between an
electronic impedance of the
matching circuits and an
acoustic impedance of the
crystalline piezo-electric
transducer.
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Experimental frequency bandwidth of a TeO2 - cell
MHz65f D
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Thus, at the current step of our researches an advanced prototype of
the acousto-optic spectrometer can be characterized by:
1. Total frequency bandwidth: D f = 65 MHz ;
2. Frequency resolution: f = 42 KHz ;
3. The number of resolvable spots or
the number of parallel frequency channels for spectrum analysis: N = 1550.
An advanced prototype of the acousto-optic spectrometer.
The resulting resolution characterization
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Scheme of the collinear acousto-optical interaction
in a uniaxial crystal
The transmitted
light The incident
light
z
The acoustic (elastic) wave beam
x
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Collinear acousto-optical interaction
in terms of the wave vectors
,kKk 10
11 ,k
10 ,kp
,E
00 ,k
,K
Acoustic wave is slow v << c , so that , but10 ,10 kKk
In the special case of mismatched wave numbers, when detunings
and are connected with the slow wave and , ,
we yield and .
Thus, the total detuning can be described via only a mismatch related
to just the wave vectors
K
D11 ,k
00 ,k
KD
D KK D D
1,0kKK D 1,0D
,K
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Schematic arrangement for the experiments
with filtering optical signals by just collinear AO-cell
Collinearacousto-optical cell
U H Fgenerator
Oscilloscope Band-pass filter
Pulsegenerator
Polarizer 1 Polarizer 2 Continuous-wave laser
Photodetector
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Physical parameters of the collinear acousto-optical cell used
currently in experiments for filtering analogue optical signals:
.A7.5nm569.0Lf
V o
0
00
This cell is successfully exploited during our experiments in the INAOE.
Tetragonal single crystal CaMoO4 (along the x - axis):
L = 4.5 cm, T = 15 ms, V = 2.95 105 cm/s, D = 0.4 – 4.0 mm,
M2 = 6.16 10 -16 s3/ kg, and f0 = 61.3 MHz for 0 = 532 nm.
In this particular case of the CaMoO4 - crystal, one can expect
the following tuning dependence and spectral resolution:
,f
VnD
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Conclusive remarksThe current situation looks like:
1a. At the moment, an advanced prototype of an acousto-optic spectrometer has been created
in INAOE using a TeO2-crystal cell, which operates in the frequency range of 60 MHz
with the frequency bandwidth of 65 MHz and frequency resolution of 42 KHz.
1b. Nevertheless, this prototype of an acousto-optic spectrometer is under construction and
testing in INAOE . This prototype is mainly oriented on applying multi-phonon regimes
of light scattering to improve the frequency resolution ( CONACyT project # 61237 ).
2. The second step can be done with involving a LiNbO3-crystal cell operating in the frequency
range of 1.0 - 1.5 GHz with the frequency resolution of ~ 200 KHz ( under consideration ).
3. The third step can be developed in the frequency range of 3 - 4 GHz with the frequency
resolution of ~ 1.0 MHz ( for the needs of cosmology; currently exists as a proposal ).
In parallel, one can say:
a. At the moment, the other prototype of an acousto-optic spectrometer is under construction and
testing in INAOE . This prototype is based on the specifically designed a TeO2-crystal cell
and is mainly oriented on applying multi-phonon regimes of light scattering to improve the frequency
resolution
b. Finally, the researches directed on creating a prototype of an optical filter based on
a CaMoO4-crystal collinear acousto-optic cell, which is potentially capable of overlapping
both visible and near infrared optical ranges, have been already initiated in the INAOE.
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Part 2:
Solitary multi-wave coupled states
in acousto-optics
Alexander S. Shcherbakov
Image Science Group, Department of Optics,
National Institute for Astrophysics, Optics & Electronics,
Puebla, Mexico.
In collaboration with: Molecular Technology GmbH, Berlin, Germany and
Saint-Petersburg State Polytechnic University, St.Petersburg, Russia.
Financial support: CONACyT, project # 61237, Mexico;
and Molecular Technology GmbH, Berlin, Germany.
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Abstract and general survey.
Physical background and formulation of theory.
Analytical model and its simplifications.
Effect of the phase mismatches.
A one-phonon light scattering.
A two-phonon light scattering.
A three-phonon light scattering.
Revealing and observing five-wave multi-component
non-collinear acousto-optical coupled states.
Revealing and observing three-wave dissipative
collinear acousto-optical coupled states.
Conclusion.
Outlines
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The physical principles of realizing the Bragg regime of one-, two-
and three-fold scattering of light by acoustic phonons in optically
anisotropic crystals in specially elaborated case are considered.
The exact and closed analytical models with slightly mismatched
wave numbers for describing such regimes are developed. Both the
analysis and the computer simulations are oriented to revealing the
specific acousto-optical solitary waves in the form of multi-component
coupled states, which can appear within a multi-phonon Bragg light
scattering in optically birefringent media.
These solitary waves have been observed and identified in the
core of various experiments with a multi-phonon Bragg light scattering
in uniaxial crystals.
Abstract
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Vector diagrams for the wave vectors of light
and acoustic wave in a TeO2 – single crystal
When these cross-points are lying equidistantly, the two- or three-fold
scattering of light can be provided by only one harmonic acoustic wave,
as shown here for a tellurium dioxide crystal. By this is meant that under
certain conditions, i.e. at a set of the angles of light incidence on
selected crystal cut and at the fixed frequency of the acoustic wave, one
can observe the Bragg scattering of light caused by sequential
participation of one, two or even three identical phonons in a crystal.
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A three-phonon light scattering:
The conservation laws and angles of scattering
,mp1p ,Kkk mp1p
.3and,2,1,0p
A two-phonon light scattering:
;nnV2f221
20
122
;nn2Vf221
20
133
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Dielectric permittivity and boundary conditions
Under action of the elastic wave, the dielectric permittivity takes the form
.tyKsint,y mm10
Generally, in crystalline materials, both 0 and 1 are tensors.
We assume that area of propagation for the elastic wave is bounded by
two planes x = 0 and x = L, and that a triplet or a quartet of the plane
electromagnetic waves with various states of polarization
pppppp
3or2
0p
pin tsinykcosxkiexpAE
strike the plane x = 0 at the angles P ( p = 0, 1, 2, 3 ) to the x-axis.
,p0p .ckk121
0ppp
Here:
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A one-fold or one-phonon regime of light
scattering in an acousto-optical modulator
The other regime of light diffraction occurs with a large L. In this case,
the dynamic acoustic grating is rather thick, so during the analysis of
diffraction one has to take account of the phase relations between waves
in different orders. Such a regime can be realized only when the angle
of light incidence on a thick acoustic grating meets the Bragg angular
condition and Q = L/L2 >> 1. Usually, the Bragg regime
includes the incident and scattered light modes and the acoustic mode.
B
B
L n2sin B
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A one-phonon light scattering
With p = ( 0, 1 ) , the set of Eqs.(8) gives
,xi2expxCq
dx
xdC011
0
.xi2expxCqdx
xdC000
1
Using the boundary conditions C0 ( x = 0 ) = 1, C1 ( x = 0 ) = 0 and the
notation q2 = q0 q1, one can find
20
22
20
2
22
1 qxsinq
qС
1
2
1fL
Vn2f
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A two-fold or two-phonon light scattering
With p = ( 0, 1, 2 ) , equations can be rewritten as
,xi2expxCq
dx
xdC01
0
.xi2expxCqxi2expxCq
dx
xdC1200
1
,xi2expxCq
dx
xdC11
2
There are two different frequency mismatches 0 and 1. We assume
precise angular alignment and extend 0 and 1 into a series in powers
of only the frequency detuning f – f2 for the current frequency f relative
to the central frequency f2 for a two-phonon light scattering.
In the first order approximation, we obtain from the vector diagram:
.00
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A two-phonon light scattering;
the frequency bandwidth
The frequency detuning is
The first unity-level maximum for |C2|2 can be reached at
One can estimate the bandwidth of a two-phonon light
scattering as:
.2xq
.f2Vnff1
22
02
.f4
1f 12
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A three-phonon light scattering
With p = ( 0, 1, 2, 3 ) , equations can be rewritten as
,xi2expCqdx
dC01a
0
,xi2expCqxi2expCqdx
dC12n00a
1
There are three different frequency mismatches 0 , 1, and 2 . We
assume a precise angular alignment and extend these mismatches into
a series in powers of only the frequency detuning f – f3 for the current
frequency f relative to the central frequency f3 for a three-phonon light
scattering. In the first approximation, we find from vector diagram that
3321
00 fffVn2
,xi2expCqxi2expCqxd
Cd23a11n
2
.xi2expCqxd
Cd22a
3
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A three-phonon light scattering; simulations
and estimations of the frequency bandwidth
The frequency detuning is
The first unity-level maximum for |C3|2 can be reached at
Taking 2 0L = / 6 from the figure at x = L , one can estimate the
bandwidth of a two-phonon light scattering as:
.xqn
.fVn2ff1
32
003
.f6
1f 13
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Originating five-wave Bragg non-collinear
weakly-coupled acousto-optical states.
The solutions to the equations governing the three-phonon interaction:
,2
xqPcosSq2
2
xqScosPq2
q412
IxqC n22n22
2n0
,2
xqPsinPq2S
2
xqSsinSq2P
)q41(2q2
I
xqC
n22n22
2
n1
,2
xqPcos
2
xqScos
q41
IqxqC
nn
2n2
.2
xqPsinS
2
xqSsinP
)q41(2
IxqC nn
2n3
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Having two sets of the localization conditions, which are given by
where k and m are the whole numbers; m > k, because P > S.
Putting, k ≠ 0 and using the parameters P and S, one may obtain
Pm22Sk22Lqn 1) ,
correlating k and m and the parameter q.
2222q41q21kq41q21m ,
Graphical interpretation of the 1-st set of localization conditions.
The localization conditions
(19)
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The last two figures show various interplays between the whole number m
and the parameter q for a set of the whole numbers k.
Graphical interpretation of the 2-nd set of localization conditions.
,P/)m2(2S/)k2(2Lqn 2)
In this case, the inequality m > k as valid, but now k = 0 is acceptable,
and the interralation between k and m and the parameter q is given by:
2222q41q21k21q41q21m21 .
The localization conditions
(20)
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Spatial-frequency distributions for a quartet of optical
components in a multi-pulse five-wave weakly coupled
state with q = 4.5 and
The distributions are completely locked with 0 = 0.
an eight-pulse component in
the zero order of scattering
a nine-pulse component in
the first order of scattering
a nine-pulse component in
the second order of scattering
an eight-pulse component in
the third order of scattering
.2Lqn
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Experimental results:
the tellurium dioxide acousto-optical cell
1. A Te02 -crystal has rather dispersive refractive index: n0 = 2.26 at
633 nm, n0 = 2.33 at 488 nm, and n0 = 2.35 at 442 nm. The
ultrasound velocity equals v = 0.6 105 cm/s for the slow shear acoustic
mode running exactly along the [110]-axis with the displacement vector
directed along [110]-axis. The figure of acousto-optical merit for this
shear mode wave in a TeO2-crystal is M2 = 1.2 10-15 s3/g , which is the
highest one for solid-state acousto-optical materials in the visible range
known up to now.
2. At first, one has to check the realization of just Bragg regime for light
scattering in the chosen cell. In such a regime, the Klein-Cook
parameter Q should exceed unity. Operating at the light-blue optical
wavelength 488 nm and at the lowest expected acoustic wave
frequency 40 MHz with L = 1 cm one can calculate Q = 10, which
confirms the Bragg character of light scattering in the regime selected
within the visible range of light spectrum.
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Scheme of the collinear acousto-optical interaction
in a uniaxial crystal
The transmitted
lightThe incident
light
z
The acoustic (elastic) wave
x
en
on
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A triplet of nonlinear differential equations describing the regime of
a weak coupling with linear acoustic losses is given by
,Ut
U
V
1
x
U
,xi2expUCq
t
C
c
1
x
C11
00
,xi2expUCqt
C
c
1
x
C00
11
where describes losses of the non-optical wave.
Co-directional collinear acousto-optical interaction
in the presence of linear acoustic losses.
The exact solutions to this equations in the case of a weak coupling are given by
;0GxGcosxС2
22
2
22
22
0
;0GxGsinq
qxС
2
22
2
1
02
1
(21)
(22)
(23).Uqq2
102
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the localization process may has 3 stages:
1) The localizing non-optical pulse is incoming through the facet x = 0 ;
2) The pulse is passing along the crystal ;
3) The pulse is issuing through the output facet x = L .
Such a process can be illustrated for the scattered light waves under conditions:
.)Vc()Vc(xTor0,0
;)Vc()Vc(xT)Vc()Vc(x,xxGLG
;)Vc()Vc(xT,xxGxG
;T0,0GxG
0
0
If the spatial length x 0 of the non-optical pulse is much shorter than L , i.e. whenVLTVxT 00 .
The localization condition can be written as
N)0(G)x(G , where: N = 1, 2, …
The localization conditions
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a) b)
c) d)Intensity of the scattered light components vs. and L with = 2 and = 0.05.
a) = 1.5, the beginning of one-pulse dissipative coupled state; b) = 2.4, a one-pulse dissipative
coupled state; c) = 3.5, an intermediate stage; and d) = 6.0, a two-pulse disspative coupled state.
An example: Numerical simulations of the light wave 21C
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Schematic arrangement for the experiments
with the collinear AO-cell
Collinearacousto-optical cell
U H Fgenerator
Oscilloscope Band-pass filter
Pulsegenerator
Polarizer 1 Polarizer 2 Continuous-wave laser
Photodetector
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We have developed a special approach to Bragg scattering of light in
optically uniaxial crystals marked by the inclusion of multi-fold
processes. In particular, the configurations related to one-, two-, and
three-phonon scattering processes have been analyzed in details to
highlight both the angular-frequency conditions peculiar for their
realization and the characteristics that can be optimized from the
viewpoint of potential applications.
The analysis performed has given us the opportunities for revealing
and observing various multi-wave collinear and non-collinear multi-
component acousto-optical coupled states.
Thank you
Conclusion