pulse pickers (original) (raw)

Definition: electrically controlled optical switches used for extracting single pulses from a pulse train

Categories: photonic devices, light pulses

• optical switches
  • fiber-optic switches
  • Q-switches
  • cavity dumpers
  • pulse pickers
  • photoconductive switches
  • electro-optic switches
  • acousto-optic switches
  • mechanical switches
  • thermo-optic switches
  • (more topics)

Related: optical switcheselectro-optic modulatorsacousto-optic modulatorspulse generationpulse repetition rateregenerative amplifiersultrashort pulsesPockels cellsPockels cell drivers

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Contents

What are Pulse Pickers?

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 needed 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, 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 used 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

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 21 suppliers for pulse pickers. Among them:

⚙ hardware

Based on proprietary technology, ALPHALAS offers most advanced pulse pickers for selecting single pulses from a train of picosecond or femtosecond optical trains. Large amplitudes of > 10 kV, pulse widths FWHM of 7 … 10 ns and repetition rates up to 10 kHz in combination with a specially designed short-crystal Pockels cell are best suited for selecting individual pulses from various mode-locked lasers. Low jitter of < 40 ps and short delay of typically 30 ns guarantee a precise synchronization for perfect pulse selection.

⚙ hardware

GWU’s UHG-PSK Series has been designed for ultrafast laser oscillators. It offers pulse picking with highest output powers and optimum contrast ratio. The system is suitable for femtosecond and picosecond pulses with high input powers, without requiring any attenuation. Integral pulse selecting from single shot to 40 MHz with adjustable contrast ratio is possible by a convenient software control. The UHG Pulse Selector is either available as standalone version or it can be integrated in our Harmonic Generator, where it is positioned before all harmonic stages to offer all features in a single compact housing.

UHG Pulse Selector features and benefits:

• modular design: standalone version or integrated in our Harmonic Generator
• high pulse picking efficiency
• adjustable contrast ratio
• suitable for high fs/ps power without any attenuation
• convenient and user-friendly software control

⚙ hardware

With the EKSMA Optics opto-electronic laser pulse picking system, femtosecond pulses can be picked from a pulse train at up to 2 MHz rate. The pulse picker UP2 and digital synchronization and delay pulse generator pMaster 4.2 can be synchronized with lasers operating at up to 100 MHz rate. The pulse picker MP1 can select pulses at up to 600 kHz repetition rate and be synchronized with lasers generating pulse trains with up to 60 MHz repetition rate.

⚙ hardware

pulseSelect is an acousto-optic pulse picker, developed for the special demands of femtosecond laser technology. Pulse distortion is minimized by a low dispersive design and the use of reflective optics. High repetition rates and high contrast rates are achieved by using acousto-optical elements. The pulse repetition rate can be reduced by an adjustable internal frequency divider or by external triggering.

APE offers pulse pickers for either a wavelength range from 500 nm to 1600 nm or dual wavelength bands from 340 nm to 540 nm and from 680 nm to 1080 nm.

⚙ hardware

SHIPS TODAY: Pulse Picker is a synchronization tool for driving external modulation devices such as EOMs, AOMs or SOAs in a pulse-picking mode of operation. It has been designed to work with low level electrical or optical input pulse power like typically the signal coming from a photodiode. It includes 3 pulse picking modes (frequency divider, external triggering & high frequency clock, burst shaping). The Pulse Picker board includes a software-adjustable input voltage level threshold and an (optional) photodiode. This board also includes a special AWG function synchronized with the input signal to perform burst shaping of mode-locked lasers.

NEW: A turn-key pulse-picker solution is now offered covering the full wavelength range between 700 and 1750 nm. It includes the synchronization board described above with our high performances AOMs and drivers.

See our detailed application note for Pulse Picker.

⚙ hardware

The ModBox-OPP optical pulse picker allows to pick and optionally shape any sub-nanosecond pulse pattern or pulse sequence in an incoming pulse train. It acts as a fast gate with low insertion loss and high extinction from 30 dB to above 55 dB with high stability over time, and with user-adjustable optical pulse duration and optical pulse train repetition rate (when pre-scaler option is embedded).

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