Pulse picker

A pulse picker reduces the repetition rate of a pulsed laser by transmitting selected pulses and blocking the rest. Mode-locked oscillators run at tens of megahertz because the rate is fixed by the cavity length, while amplifiers, materials processing and many experiments need kilohertz to a few megahertz, or single pulses on demand. The picker is a gate synchronized to the pulse train: it opens for one pulse, closes before the next arrives, and repeats every $N$ pulses. The pulse energy is unchanged and the average power falls by $N$. Picking 1 MHz from a 1 W, 80 MHz oscillator keeps 12.5 nJ per pulse and leaves 12.5 mW of average power; a 1 kHz amplifier fed from the same oscillator selects one pulse in 80 000.

Pockels-cell and acousto-optic pickers

The most common picker is a Pockels cell placed between two polarizers. With no voltage the cell leaves the polarization unchanged and the second polarizer blocks the light; a half-wave voltage pulse rotates the polarization of the selected pulse by 90°, and it passes. BBO and RTP cells are preferred at high repetition rates because they show little piezoelectric ringing. The driver must switch kilovolts in a few nanoseconds and dissipates about $CV^2$ per pulse: for a 5 pF cell at 3.3 kV that is 54 µJ, or 54 mW at 1 kHz and 54 W at 1 MHz, which is why megahertz pickers use specialized drivers.

An acousto-optic modulator gates the diffracted first order by switching its RF drive. Its rise time is the acoustic transit time across the beam, about $0.64\,d/v_a$ for a Gaussian beam of $1/e^2$ diameter $d$: 152 ns for a 1 mm beam in TeO₂, far longer than the 12.5 ns spacing of an 80 MHz train. Picking from such a train requires focusing into the AOM to a diameter of about 65 µm, which brings the rise time near 10 ns. Fiber-coupled AOMs are standard in fiber laser systems, where the beam inside is already small.

Timing

The gate is triggered from the pulse train itself. A photodiode samples the oscillator output, a counter divides the rate by $N$, and an adjustable delay places the switching window around the selected pulse. The window must be shorter than two pulse spacings, so that it opens after the preceding pulse and closes before the following one, and its edges must be placed with margin for the driver's rise time and timing jitter. A photodiode after the picker, viewed on a fast oscilloscope, shows whether the neighbors on both sides are suppressed equally.

Contrast and leakage

The quality of a picker is its contrast, the ratio of transmission for the selected pulse to leakage of the rejected ones. For a Pockels-cell picker this is limited by the extinction ratio of the polarizers and cell, typically several hundred to a few thousand to one. Leakage adds up over all rejected pulses. With a contrast of 1000:1 and $N = 80$, the 79 rejected pulses together carry 7.9% of the selected pulse energy in each period. At $N = 80\,000$ the same contrast lets through 80 times more energy in rejected pulses than in the selected one, spread as a weak background at the oscillator rate. Two pickers in series multiply their contrasts; at 10⁶:1 the background falls to 8% of the selected energy at $N = 80\,000$.

Leakage matters most before an amplifier. In chirped-pulse amplification systems with fiber or multipass amplifiers, the unpicked background extracts gain, and the neighboring pulses immediately before and after the selected one are amplified into pre-pulses and post-pulses. A regenerative amplifier is less sensitive, because its own Pockels cell admits only one seed pulse per cycle, but a second external picker is still used where pre-pulse contrast matters, as in high-intensity laser-matter experiments.

Pitfalls

The crystal adds material dispersion to femtosecond pulses, which the stretcher-compressor design must include. Piezoelectric ringing after each high-voltage step modulates the transmission for microseconds in KD*P and lithium niobate cells. A picker placed after an amplifier switches much higher pulse energies and needs a larger aperture, which raises the cell capacitance and slows the switch.

Common questions

What is the difference between a pulse picker and cavity dumping?

A pulse picker works outside the oscillator and keeps its pulse energy. Cavity dumping switches a pulse out of the intracavity field, so each selected pulse carries several times more energy than a normal output pulse.

Why not simply build an oscillator at the desired repetition rate?

The repetition rate equals the inverse of the cavity round-trip time, so 1 kHz would need a cavity 150 km long. Oscillators at tens of megahertz, as set out under mode locking, followed by a picker, are the practical route to low rates.

References: W. Koechner, Solid-State Laser Engineering, 6th ed. (Springer, 2006); A. E. Siegman, Lasers (University Science Books, 1986); A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications, 6th ed. (Oxford University Press, 2007).