Aligna
®
4D User Manual
33 / 84
6.1.2 CW Lasers with Intensity modulation (5 kHz to 200 kHz)
((Demodulation, PSD LID … Description in preparation))
6.1.3 Pulsed Lasers with Slow Repetition Rates (1 Hz … 3 kHz)
Those lasers are the most difficult to handle: The information of the momentary pointing is only
available with the laser repetition rate. This is smaller than or around the servo bandwidth.
Each amount of information should be used to achieve a servo bandwidth as high as possible,
e.g. for compensating air fluctuations. However, the effective servo bandwidth will be smaller
compared to CW lasers.
Precision Measurement of Pulsed Signals
It is much more complicated to measure position signals of short laser pulses with very high
precision, compared to CW signals. Without compensation networks, the steep flanks of the
laser light pulses drive the detector amplifiers into their non-linear “Slew Rate Limitation”, even
at small intensities. For creation of the position signals many signals have to be calculated by
means of additions, subtractions and divisions. If this is done with signals that contain non-
linearities, the residual precision decreases significantly.
To avoid (or reduce) these effects we introduce analog networks around the detector circuitry
and we carefully match the time constants. However, this can only be done to a certain degree
because of some position dependent asymmetries of time constants of the detector signals of
the PSDs.
Sample & Hold Stages
A simple low pass filtering would cause unwanted phase shifts that lead to a servo loop band-
width far beyond the repetition rate. In addition, a high residual amount of the repetition rate
signal appears in the piezo signals; that leads to acoustical excitation of the mirror actuators.
For this reason, we use position sensitive detectors (PSDs) with a special differential three
stage Sample & Hold circuitry (PSD S&H) for this category of lasers. (See detector description
for more details). With these detectors, we get highest possible information speed with lowest
possible phase shift and low residual repetition frequency amounts.
In addition the differential S&H offers the nearly perfect compensation of environmental light
(daylight and 50/60, 100/120 Hz of artificial light sources,…), equivalent to a “Lock-In”-
technique.
The repetition rates of course influences the following parameters, which have to be adapted
to a changing repetition rate:
1. The filtering networks at the detectors and the transimpedance amplifiers (see description
of PSD S&H)
2. The timing within the S&H logic circuitry (see description of PSD S&H)
3. The servo loop parameters of the four PIDT
2
Regulators in the Aligna
®
4D electronics
Therefore, when ordering an Aligna
®
system, designated for pulsed lasers, you should tell us
the target repetition rate and the possible variation range of the repetition rate. The detector
and servo parameters will be pre-aligned to these frequencies before delivery.
6.1.4 Pulsed Lasers with medium repetition rates (3 kHz … 15 kHz)
The lasers of this category can be controlled similarly to CW lasers. However, the basic detec-
tor signals are pulsed and residual amounts of the repetition frequency in the piezo signals will
remain. They may lead to acoustical beeping of the actuators.
To Next Page
Loading...