The acoustooptic
effect allows controlling any parameters of an optical wave: amplitude,
phase, frequency and polarization. Therefore, different modifications
of the acoustooptic modulators can be realized. Besides, it should be
taken into account that acoustooptic interaction makes it possible to
introduce information into an optical beam by means of both temporal and
spatial modulation.
At the present time, the temporal modulators of light intensity are the
most widespread. These modulators employ a progressive acoustic wave modulated
in amplitude by an informative signal. The principle of operation of the
modulators is based on the dependence of the diffracted light intensity
_{} on the acoustic power
_{}. A scheme of the modulator
operating in the Bragg regime of diffraction is shown in Fig. 1m. Electrical
oscillations of a frequency _{} from a HF generator
(1) are modulated in amplitude by an informative signal _{} and then enter a piezotransducer
(2) of an acoustooptic cell (3). The regime of travelling acoustic waves
is provided by an acoustic absorber (4). The carrier frequency _{} is chosen equal to
the central frequency of the transducer. Modern technology permits attaining
the transducer bandwidth _{} as great as _{}; so that _{}
A basic characteristic of the modulator is the modulation bandwidth _{}. It is obvious that
_{} cannot be greater
than _{}. However, there exist
additional restrictions of the modulation band defined by peculiarities
of acoustooptic interaction. For the modulators, the parameter _{} is of considerable
importance, where _{} and _{} are the optical and
acoustic beam divergences, _{} is the optical wavelength
in vacuum, n is the refractive index, v is the acoustic velocity, f is the acoustic frequency, d and l
are the optical and acoustic beam widths in the acoustooptic interaction
plane. If the condition _{} is satisfied, the
modulation bandwidth is determined by the relationship _{}. This expression
shows that in the limit _{} the modulation band
is defined exclusively by the time _{} required for ultrasound to cross the optical beam.
With increasing the parameter G the modulation
band becomes narrower because of volume effects in acoustooptic interaction.
In this case the ratio _{} plays the decisive
role.
The optimization of modulator parameters is conventionally produced with
taking into consideration both the modulation band and the consumed power.
Such optimization leads to the following value of the parameter G:
_{} which defines the
condition of optical and acoustic beam divergences matching. In this case
. 
(1m) 
If the diffraction efficiency _{} does not exceed 30%,
the consumed acoustic power _{} can be calculated
from the equation
, 
(2m) 
where
M is the acoustooptic figure of merit,
_{} is the crosssection
of the acoustic beam. Eqs (1m) and (2m) enable defining optimal parameters
of the modulating acoustooptic cell. Supposing the acoustic bandwidth
_{} to given, one can
yield:

(3m) 
Here the reasonable assumptions _{} and _{} are taken into account.
Graphic results of the calculations for _{} and for different
acoustooptic materials are presented in this section.
