Ultrashort pulses are in most cases generated by a mode-locked laser in the form of a pulse train with a pulse repetition rate of the order of 10 MHz – 10 GHz. For various reasons (see below), it is often necessary to pick certain pulses from such a pulse train, i.e., to transmit only certain pulses and block all the others. This can be done with a pulse picker, which is essentially an electrically controlled optical switch.
Operation Principles of Pulse Pickers
A pulse picker is in most cases either an electro-optic modulator or an acousto-optic modulator, combined with a suitable electronic driver. In the case of an electro-optic device, a pulse picker consists of a Pockels cell and some polarizing optics, e.g. a thin-film polarizer; the Pockels cell manipulates the polarization state, and the polarizer then transmits or blocks the pulse depending on its polarization.
The principle of an acousto-optic pulse picker is to apply a short RF pulse to the acousto-optic modulator so as to deflect the wanted pulse into a slightly modified direction. The deflected pulses can then pass an aperture whereas the others are blocked.
In any case, the required speed of the modulator is determined by the temporal distance of pulses in the pulse train (i.e. by the pulse repetition rate of the pulse source), rather than by the pulse duration, which may be far shorter.
The electronic driver of a pulse picker may fulfill additional functions. For example, it may use the signal from a fast photodiode, sensing the original pulse train, in order to synchronize the switching with the input pulses. A trigger signal may then come at any time, and the electronics will act on the switch at the proper time to transmit the next arriving input pulse.
Applications of Pulse Pickers
Some typical applications of a pulse picker are described in the following:
- For obtaining high pulse energies in ultrashort pulses, it is frequently necessary to reduce the pulse repetition rate before amplification. This can be achieved by placing a pulse picker between the seed laser and the amplifier. The amplifier will then act only on the wanted pulses. The blocked pulses do not necessarily constitute a strong energy loss, since the average power of the seed laser may be small compared with the average output power of the amplifier, and the remaining average power can be sufficient for saturating the amplifier.
- In a cavity-dumped mode-locked laser, a pulse picker (then often called cavity dumper) extracts the circulating pulse from the cavity in only every <$N$>th round trip. During all the other round trips, the pulse experiences low optical losses and can be amplified to a high energy.
- A kind of pulse picker is part of any regenerative amplifier, where it is used for injection and extraction of pulses. One may also use an additional pulse because for better suppressing parasitic pulses.
Important Properties of Pulse Pickers
Depending on the application, different properties of a pulse picker can be critical:
- the switching time (particularly for high input pulse repetition rates)
- the maximum repetition rate for the switching
- the insertion loss, i.e., the energy loss of transmitted pulses
- the degree of suppression of unwanted pulses
- the optical bandwidth (particularly for broadband pulses)
- the chromatic dispersion (particularly for broadband pulses, e.g. with durations well below 100 fs)
- the optical nonlinearity (particularly for pulses with high peak powers)
- the size of the open aperture
- the outer dimensions
- the alignment sensitivity (acceptance angle)
- the capabilities of the corresponding electronic driver, e.g. concerning synchronization
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