DAZZLER
TM
system manual Part I : installation & operation 3.3
• the input face is not the correct face.
• the spectrometer is improperly calibrated
• the setup files are incorrect
If the difference dλ is in the order of 1nm, then the calibration is best corrected by changing
slightly the crystal orientation until the hole position matches the programmed hole wavelength.
Verify that the calibration remains valid for other wavelength positions of the hole feature.
For the next steps of the alignment procedure, refer to subsection 3.3.2 if the Dazzler is operating
at the output of a seed oscillator and to subsection 3.3.4 if not.
3.3.2 Dazzler operating at the output of a seed oscillator
A diode detector and a spectrometer will be placed on the diffracted beam. An oscilloscope is
needed to measure the diode output.
The beam should be fully contained in the diode detector. If the detector area is too small,
focus the beam to achieve this condition. It is not necessary that the detector resolve the
individual pulses from the oscillator. In fact a simple diode loaded with 1kΩ resistor driving a
high impedance scope through a 1 meter coaxial cable will yield a 0.1µs response time adequate
for the measurement. The procedure follows:
1. observe the diffracted beam and align the diode on this diffracted beam.
2. observe the diffracted signal on the scope. Move the x and y position of the crystal to
assess the position of the acoustic beam (approximate dimensions 5 × 8mm
2
). Set the x
and y positions so that the laser beam is approximately in the center of the diffraction
region.
3. change the waveform of the Dazzler to produce a short acoustic signal by reducing the
second order phase coefficient (see Figure 3.7). A typical signal on the oscilloscope is
shown for this example. The duration of this signal is the propagation time through
the crystal t
xtal
, typically 20µs (the exact value is displayed in the ”Trigger and Mode”
panel, refer to section 4.2). Its shape reflects variations of the acoustic field in the crystal
associated with the acoustic diffraction pattern. As you move the crystal, along the
y direction, the scope trace will be shifted in time, due to the change of the acoustic
propagation time from the transducer to the input face. When moving the crystal along
the x direction, the onset of the signal remains constant in time but the shape changes.
Adjust the x position to obtain an approximately flat pattern.
4. place a spectrometer on the diffracted beam and check that the whole input spectral
bandwidth is measured.
5. check that the input beam is still autocollimated.
6. fine adjustment of the synchronization between the input optical wave and the acoustic
wave using the trigger settings are described in chapter 4.
3.3.3 Measurement of the Dazzler diffraction efficiency:
The diffraction efficiency is defined by the ratio between the diffracted optical intensity and
the input intensity (subsection 2.2.1). To determine this ratio we shall use the fact that the
incident energy is equal to the sum of the transmitted and diffracted energies.
V3.00 - 8
th
April 2019 (ContentsTable) (FiguresTable) 21/94
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