DAZZLER
TM
system manual Part I : installation & operation
2.1
where
α
depends on the crystal properties (crystal birefringence) and the phase-matching ge-
ometry and is weakly dependent on
ω
. Fastlite has three standard geometries for TeO2 crystals,
named WB, WR and HR. In the presentation most examples will be given using the HR ge-
ometry. For example, if
λ
= 800nm, the optical frequency is
ν
opt
= 375THz and the matched
acoustical frequency is:
ν
ac
= 85
.
93MHz,
α
= 2
.
3 10
−
7
. The small value of
α
enables to control
optical signals in the hundred of Terahertz range with electrical signals in the tens of megahertz
range.
2.1.3
Pulse-shaping mechanism
Two different approaches can be used to understand the pulse-shaping mechanism involved in
AOPDFs: group delay control and time convolution. The first interpretation isn’t rigorous but
provides a direct and clear insight of this mechanism. The only rigorous approach is the time
convolution one, which is practical for computations but less self explanatory.
Explanation in terms of group delay
A schematic of the AOPDF is shown in
An acoustic wave is launched by a trans-
ducer excited by a time-dependent radio-frequency (RF) electronic signal. The acoustic wave
propagates with a velocity
V
along the
z
axis and hence reproduces spatially the temporal
shape of the RF signal.
Figure 2.2: Explanation of the Dazzler principle in terms of group delay control
Let’s take the case of a linearly chirped acoustic wave (
). If there is only one spatial
frequency in the acoustic grating at position
z
, then only one optical frequency can be diffracted
at position
z
. The incident optical short pulse is initially ordinary polarized. Every optical
frequency
ω
travels a certain distance before it encounters a phase-matched spatial frequency
in the acoustic grating. At this position
z
(
ω
), part of the energy is diffracted and gives rise to
an extraordinary polarized component. At the crystal output, the diffracted pulse will be made
of all the spectral components that have been diffracted at various positions. Since ordinary
V3.00 - 8
th
April 2019
