Aligna
®
4D User Manual
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6 Pulsed Lasers
Aligna
®
can handle both cw (continuous wave) and pulsed lasers.
However, for best performance, pulsed lasers may need other detector electronics than cw
lasers.
The standard actuators of an Aligna
®
system are piezo driven mirrors with first main reso-
nances f
R
of 0.5 … 3 kHz. (We also offer Piezo actuators up to 40 kHz resonance frequency,
working with a servo bandwidth up to 20 kHz) Thus, the closed servo loop bandwidth will be
distinct below these first main resonances (0.3 … 2 kHz). The servo loop cannot compensate
for disturbing frequencies above the servo loop bandwidth.
Pulsed lasers with repetition rates far above the servo loop bandwidth (>100 kHz) can be seen
equivalent to CW lasers, while low repetition rates (< 3 kHz) are a bit more difficult to handle.
6.1 Repetition Rate Categories
We will subdivide the lasers into following categories (let us assume here the main piezo
resonance f
R
to be 1 kHz; scale the named frequencies to other resonance frequencies):
1. CW lasers
2. CW lasers with laser intensity modulation (5 kHz to 200 kHz)
3. Pulsed Lasers with slow repetition rates (1 Hz … 3 kHz)
4. Pulsed Lasers with medium repetition rates (3 kHz … 15 kHz)
5. Pulsed Lasers with high repetition rates (> 15 kHz … > 1 GHz)
The servo signals are created from the momentary laser (periodically pulsing) intensities. Thus,
the piezo signals will contain distinct components with the laser repetition frequency.
If the main resonance is assumed to be 1 kHz, the actuator will cut off frequencies above
1 kHz with approx. 40 dB per decade (system of 2
nd
order). That means the actuator will move
at frequencies of 10 kHz with only 1% compared to low frequencies, but nevertheless these
movements may cause acoustical noise with a periodic signal of the repetition frequency.
This will not disturb the beam pointing regulation: Laser pulses are typically very much shorter
than the repetition time. The laser pulse will hit the moved mirror always at the same phase of
the periodic movement. The laser pointing will not care about the movement of the mirror
outside the very small time window of the laser pulse.
However, acoustical noise is not appreciated because of hearing a continuous beeping.
6.1.1 CW Lasers
Those lasers are the easiest to handle, because the detectors receive continuous information
about the momentary beam pointing. Since the detector bandwidth is normally much higher
than the bandwidth of the actuators, the actuator properties (resonance frequencies, reso-
nance quality, mechanical crosstalk, etc.) will limit the regulation speed.
The smaller and lighter the mirrors are, the higher the servo bandwidth will be.
In many applications, however, there are no strong disturbances with some 100 Hz to some
kHz. The strongest disturbances are slow or quasi-static (air density gradients and thermal
drifts). No high servo bandwidth is necessary.
So only if higher frequencies have to be compensated small light-weighted mirrors are rec-
ommended (10 mm diameter, 2 mm thickness, depending on beam diameter). In most cases,
standard 1 inch mirrors of 6.3 mm thickness are sufficient.
A 4D stabilization system is a rather complicated combination of four coupled servo loops.
With CW lasers, however, the reaction is rather fast. Even if the servo parameters and the
decoupling ( Orthogonalization) are not perfect the system is not critical.
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