Cavity dumping
A technique in which a laser runs with no output coupling, so the circulating energy builds up, and a fast switch then deflects the intracavity pulse out in one round trip. It raises the pulse energy several-fold at a reduced repetition rate: a Ti:sapphire oscillator giving 6.25 nJ at 80 MHz can deliver about 44 nJ per pulse at 1 MHz when dumped.
Cavity dumping extracts laser output through a switch instead of a partially transmitting mirror. Both end mirrors are highly reflecting, so the power circulating inside the cavity is much larger than would be available through a conventional output coupler; at a chosen moment an acousto-optic modulator or a Pockels cell with a polarizer deflects the light out of the cavity in about one round-trip time. It is used in two regimes: with mode locking, to pick single femtosecond or picosecond pulses of higher energy at kHz to MHz rates, and with Q-switching, to produce nanosecond pulses whose duration equals the cavity round-trip time.
Mode-locked cavity dumping
In a mode-locked oscillator the intracavity pulse energy is the output pulse energy divided by the output coupler transmission. A Ti:sapphire laser with a 10% output coupler, giving 500 mW at 80 MHz, emits 6.25 nJ per pulse, so about 62.5 nJ circulates inside. Replacing the output coupler with a high reflector and dumping with an efficiency of 70% gives
seven times the normal pulse energy. At a 1 MHz dumping rate the average output is about 44 mW, less than a tenth of the original 500 mW, so the gain in pulse energy comes at the cost of average power. The intracavity energy is in practice somewhat higher than this estimate because the cavity loss is lower, but it must rebuild after each dump, over a number of round trips set by the net gain, which limits the dumping rate to a few MHz in most systems.
The switch must open and close within one round trip, 12.5 ns for an 80 MHz cavity (1.87 m long). In an acousto-optic dumper the rise time is roughly the acoustic transit time across the beam; for a 50 µm focused spot in fused silica, with a sound speed of 5960 m/s,
Acousto-optic dumpers therefore place the crystal at an intracavity focus, and they are often driven with an RF burst synchronized to the pulse train so that the acoustic wave is present only when the pulse arrives. A Brewster-cut fused-silica cell with a double pass through the focus is a common arrangement, and dumping efficiencies of 50–80% are typical.
Q-switched cavity dumping
In a Q-switched laser with cavity dumping, sometimes called pulse transmission mode, the cavity is first held at high loss while the gain medium is pumped, then switched to zero output coupling so that the intracavity field builds up with no output, and finally switched to full output coupling at the peak of the circulating energy. The entire contents leave in one round trip, so the pulse duration is
10 ns for a 1.5 m cavity and 1 ns for a 15 cm cavity, independent of pump level. An ordinary Q-switched pulse, in contrast, lasts several round trips and depends on the gain; cavity dumping therefore gives shorter and more reproducible pulses from short, high-gain cavities, and allows high repetition rates because only part of the inversion need be used. Electro-optic switches are used here because they can switch the whole beam in 1–2 ns.
Where it is used
Cavity-dumped mode-locked lasers were a standard source for time-resolved spectroscopy and time-correlated single-photon counting before amplified systems became common, since a lower repetition rate leaves time for slow fluorescence to decay between pulses. Cavity-dumped dye and Ti:sapphire oscillators, and later cavity-dumped Yb thin-disk oscillators, provide tens of nanojoules to microjoules without a separate amplifier. Q-switched cavity dumping appears in compact nanosecond lasers for ranging and in microchip lasers where sub-nanosecond pulses are wanted.
Pitfalls
Incomplete switching leaves satellite pulses one round trip before or after the main pulse. The dumping disturbs the mode-locked steady state, so energy and timing can fluctuate for some round trips after each dump; the dumping rate is kept low enough for the laser to recover. Intracavity peak intensities are high, and the dumper crystal adds dispersion that must be compensated in a femtosecond cavity.
Common questions
How does cavity dumping compare with a regenerative amplifier?
A regenerative amplifier uses the same kind of switched cavity but amplifies an injected seed over many round trips, reaching millijoules. Cavity dumping extracts only what the oscillator itself has built up, typically nanojoules to a few microjoules.
Does cavity dumping shorten the pulse?
In the mode-locked case the pulse duration is essentially that of the oscillator. In the Q-switched case the output length equals the round-trip time, which is usually shorter than the ordinary Q-switched pulse from the same cavity.
Why not just use a pulse picker outside the laser?
An external pulse picker lowers the repetition rate but keeps the pulse energy of the oscillator output, 6.25 nJ in the example above. Cavity dumping takes the pulse from the high-energy intracavity field, so each selected pulse carries several times more energy.
References: Siegman, Lasers (University Science Books, 1986); Saleh & Teich, Fundamentals of Photonics 3rd ed. (Wiley, 2019); W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006).