TEM Aligna 4D - Pulsed Lasers with High Repetition Rates (>15 Khz

Aligna
®
4D User Manual
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To reduce this effect we carefully match the time constants of the networks around the detec-
tor chip and the transimpedance amplifiers. However again, this can only be done to a certain
extent because of some position depending asymmetries of time constants of the detector
signals of the PSDs.
Using Lock-In-Technique with medium repetition rates
In principle, the described “PSD S&H” can be used in this frequency range, too. However, the
switching times of the S&H circuitry are no longer negligible compared to the repetition period;
thus the detection precision would decrease.
Equivalent to modulated CW lasers the pulsed detector signals can be demodulated and low-
passed by a “Lock-In”-technique by help of the detector variant “PSD LID”. This gives the
advantage of the compensation of environmental light effects and electronic offsets (daylight,
50/60/100/120 Hz from artificial light sources, electronic humming,…) The necessary low pass
cut-off frequency has to be selected beyond the repetition rate, but will be high enough not to
decrease the servo bandwidth too much.
6.1.5 Pulsed Lasers with high repetition rates (>15 kHz … > 1 GHz)
This category of lasers (e.g. mode locked lasers) can be handled similarly to CW lasers, be-
cause the detectors or the Aligna
®
-built-in filters will low-pass the AC amount of the light.
However, one has to be more careful not to destroy the detectors: Even if the average intensi-
ty at the detectors is low (let’s say 100 µW) the peak intensity of a 100 MHz 20 fs laser is in
the order of 50 Watts (!)
If 100 µW average power is applied to the detectors at a repetition rate of 1 kHz (measurement
of amplified sliced 50 fs mode-locking lasers) the pulse peak power is unbelievable 5 MW (!!),
the order of an energy power station! This power is focused onto the detector surface!
Up to now, we do not have too much experience with destroying levels of the detectors. (We
have not destroyed detectors in a short pulse system yet ;-) BUT: In any case one should be
very careful during the detector intensity alignment (with optical filters) not to apply much more
laser power to the detectors than needed for operation.
((There is very little information from the detector chip manufacturers regarding short pulse
destroying levels. In future time we will do some detector destroying experiments to get better
values of the safe operating area. If you have knowledge or own experience about this topic,
please let us know!))