frequencies
f
and a small acoustooptic interaction
sin.9
MHz and 1< 1cm ). This
length I (typically, f <lO
type of diffraction takes place at an arbitrary incidence
.9
angle of light 0 (Fig. 1,a). The diffraction pattern can
contain many diffraction orders with symmetrical
distribution of light intensity. By contrast, the Bragg
regime is observed at high acoustic frequencies
exceeding usually 100MHz. The diffraction pattern,
.
(a)
(b)
However, even these maxima appear only at
definite incidence angles near the so-called Bragg
.9
angle B (Fig. 1,b). In this case, the side maximum
(Bragg maximum) is formed as a selective reflection of
light from wave fronts of ultrasound. The Bragg angle
is determined by the expression
.
sm.9
B
=
- [
--
in the region where d.9B / df
]
900
(4)
where A is the optical wavelength in vacuum,
diffracted light, respectively. There is no well-defined
boundary between these two diffraction regimes. With
increasing the acoustic frequency the angular
selectivity of acousto-optic interaction increases and
the number of observed maxima is gradually reduced.
Traditionally, the Raman-Nath and Bragg regimes are
stated by the conditions Q«l and Q» 1 accordingly,
Q 27rAlf 2 nv2
where
is the Klein-Cook parameter.
/
Since only one diffraction maximum is used in
acousto-optic devices (the first order, as a rule), the
Bragg regime is more preferable because of smaller
light losses. However, the acousto-optic selectivity
peculiar to the Bragg diffraction restricts the frequency
range of acousto-optic interaction and, as a
consequence, speed of operation of acousto-optic
devices.
If the acoustooptic medium is optically
nd
==
=
0
(point d in Fig.2).
:
_900 �-c____________________�
Figure
2.
Bragg angles as a function of acoustic
frequency
Analogously, optimal areas for modulators and
fIlters are situated close to points m and f, respectively,
.9
d.9
0
' I soIutlOn
.
B / df 00 An ana IytIca
where B
and
of the problem of acousto-optic interaction can be
obtained only for the limiting cases of Raman-Nath
and Bragg diffraction. In the latter case, assuming
additionally that the optical beam falls on the acousto­
optic cell at the Bragg angle, the following expression
for the diffraction efficiency can be derived:
=
=
j
o ���__�____________�
k
=a j� f
nj and nd are the refraction indices for the incident and
isotropic, then nj
nd (0 =? 0 type of scattering) or
( 0 =?e or e=?o scattering). Therefore, nj =1= nd and
.9 f
B ( ) becomes much more
the dependence
complicated. Curves 2 and 3 in Fig. 2 demonstrate
these dependencies for a relatively simple variant when
the acousto-optic interaction plane is perpendicular to
the optical axis of an uniaxial crystal. From the
viewpoint of practical usage, all advantages of
anisotropic diffraction result from these complicated
dependences of the Bragg angle on the acoustic
frequency. It has been shown that the best
characteristics of acousto-optic deflectors are achieved
Figure 1. diffraction ofRaman-Nath and Bragg
2
Af 1+-v2
2
nj -nd
A 2f 2
2njv
=
interaction is known as isotropic diffraction. In the
other case known as anisotropic diffraction, the optical
mode is transformed during the diffraction process
z
diffraction
light
(5 )
nj "'" nd (e=?e scattering) and the Bragg angle is
determined by Eq. (5). This type of acousto-optic
x
incident
light
_ Af
2 nv
interaction, then nj
.
acoustic
=
.9
Curve 1 in Fig. 2 shows the dependence B (1) for
this case. In an anisotropic medium, two types of
acousto-optic interaction are possible. If the optical
mode does not change during the acousto-optic
even at a Iarge acoustIc power p conSIsts, as a ruIe,
of two diffraction maxima of the zeroth and first orders.
a,
B
=
-
1] _
nand Eq. (4) is simplified to
337
=
•
- [� )
. 2 (. 2 7r MP)
7rlA
sm
tin ) _ sm
2b
A
A
(6)
where I x b is the cross-section of the acoustic beam.
The parameter M defined by the expression
M=
p2n6
--
pv
3
p
by the hopping and drift of the light source strength. It
can also achieve a high accuracy of the measurement
of the diffraction angle. Besides the measurement of
oscilloscope, it can also be used to collect and process
the experiment data.
Acousto-optic
4.
modulation
(7)
where
is the medium density, is known as an
acousto-optic figure of merit; it is the main parameter
of the acousto-optic medium. The greater the figure of
communication model
By varying the parameters of the acoustic wave,
including the amplitude , phase , frequency and
polarization , properties of the optical wave may be
modulated. The acousto-optic interaction also makes it
possible to modulate the optical beam by both
temporal and spatial modulation.
The acousto-optic medium must be designed
carefully to provide maximum light intensity in a
single diffracted beam. The time taken for the acoustic
wave to travel across the diameter of the light beam
gives a limitation on the switching speed, and hence
limits the modulation bandwidth. So to increase the
bandwidth the light must be focused to a small
diameter at the location of the acousto-optic interaction.
This minimum focused size of the beam represents the
limit for the bandwidth.
Acousto-optic crystal
merit, the less the required acoustic power P" .[ 1]
r;
-+
o ��� ----
Figure
3.
t
The features of Bragg Acousto-optic
modulation
The features of Bragg Acousto-optic modulation are
shown in Figure 3. From the figure we can see that
diffraction efficiency'7 and acoustic power P" are just
curve form of nonlinear modulation. To prevent the
modulation from distorting, the bias acoustic power is
needed to make it work in the area of linear.[2]
3.
CCD
light
strength
distribution
measurement system
In the experiment of acousto-optic communication,
the strongest first order diffraction beams is needed. In
the past we just judge the strength of diffraction beams
by eyes. Because of the different ability of sense the
light, the error is easily caused.
-
C
cliffraction
beams
C
D
-
Figure
CPU
5 Acousto-optic communication
System
System model shown in Figure 5. The Acousto­
optic modulation is achieved through the audio signals
loaded on the Optical Carrier based on acousto-optic
effect. The strength distribution of every diffraction
beams is measured by the CCD light strength
distribution measurement machine. By adjusting the
bias voltage and deflection angle, the distribution of
diffraction beams can be changed. After the strongest
No.1 diffraction beams goes through the pinhole
aperture , it coupling with one of the light fibre
through focusing lens. Then, the other end will
coupling with photodiode. The signal received by the
photodiode will be sent to the speakers by the
operational Amplifier. Thus the acousto-optic
communication is completed.
Scanning
-j .A.mplifier
I Signal
CCD light strength distribution
measurement system
model
S ynchronous
-
Audio Signal
oscilloscope
4. Internal circuit diagram of the system
Therefore,
the
light
strength
distribution
measurement system is designed with the CCD as the
detecter. CCD light strength distribution measurement
system can measure and show the relative light
strength distribution Real-time without being affected
Figure
338
5. Experiment and Analysis
After the system through optical fiber coupled input
;and out ut waveforms.
·
·
·
·
·
.
.
.
.
.
.... . . ... .. . .. . ...
A
..
·
I
.
..
.
..... / ....
.
..........•......... ; .
.
..
I
I
I
I
I
I
I
I
I
I
I
I
I
II
II
I
.
\
- .............. , .... , ..J l .................. , .
M1200)Jsj At ChI f 1.00 VI
tiiIIl 500mVQ J
Figure9. Waveforms ofzeroth order diffraction
beams intensity is strongest
.
6. Conclusion
The
Bragg
acousto-optic
modulation
communication model is based on the theory of
Acousto-optic effect of crystal and the light strength
auxiliary measurement system of LO and CCO is
completed. After the test, it can Work steadily and
continuously.
l.The tradition He-Ne LO is replaced by LO, which
can avoid the system high-pressure, improve the safety
of experiment and lengthen the using life of the device.
What's more, LO can fulfill the continuous modulation
of laser power and save the power.
2 . The light fibre is adopted as the communication
media, which makes the long-distance communication
of modulation signal possible.
3.The CCO light strength distribution measurement
system replaces the observation by eyes, which
increases the accuracy of experiemtn result. The
system can be used in the measurement of the strength
distribution of every diffraction beams of Bragg
acousto-optic diffraction and the laser beam of solid­
state laser and gas laser. It is very practical.
r
· ··· · ·
.�.
.
-H-+++-+--t--!-..+-+·+++-+-i·++..-!-+·+++++-i--+++--i-+-+-+-+··+-+--!-+++-i···+-+++-++-++-+···
.
.
.
.
�
.
t
" o
ChI
50.0mV
[!l!l 1.00 V
M
200)Js
AI
ChI f
110mVj
Figure 7. Without distortion when the audio
input and output waveforms
Measured results of CCO light strength distribution
measurement system
7. References
[I]
WANG
Chun-yu
, KONG
H i hg -rpe noite -reta
Y o
ng
, GUO
LD-pumpde
niM -g xiu
solid
steta
, et
laser
al.
[J]
.
Chinese Journal ofLasers , 2005 ,32 (5) :601-603.
2
[ ] NGOI
B K A, VENKA
A n gular
used
dispersion
for
T AKRISHNAN
compensation
for
ultrashort-pulsed
laser
spatial
las-re scna ning
YER
V,
and
LOSA
VIO
temporal
microscopy[J].
Optics,2003,8(3): 460-471.
rim
Figure
8.
M 200)Js
AI ChI
f
328mvi
Waveforms of first order diffraction
beams intensity is strongest
339
for
al.
OPTICS
B E,SAGGAU
dispersion
B, te
devices
microahining[J].
EXPRESS,2001,9(4).
[3]L
K, TAN
acousto-optic
acousto-optic
P. Compensation
Journal
of
of
multiphoton
Biomedical