Pulsed vs continuous-wave lasers: Understand the differences

Pulsed lasers can have powers in the megawatt range and even petawatt range! Yet, if you shop for a Continuous Wave (CW) laser, you’ll typically find powers listed in watts and milliwatts.

Read on to learn why they can deliver such immensely different powers, which require a very broad range of measurement solutions that Gentec-EO has developed over the last 50 years.

As the name implies, a pulsed laser emits pulses of laser light. It emits nothing for a little while, and then it emits a short burst of light, and then that pattern repeats.

The average power is, unsurprisingly, the average of the laser’s power output. It’s a specification you’ll find for both CW and pulsed lasers.

However, when powers as high as a petawatt are listed, you can be sure that this is not the average power, but rather the peak power, in other words the power during a pulse. This specification only makes sense for pulsed lasers.

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How continuous-wave lasers work

Photons and atoms can be emitted/absorbed in 3 ways:

• Absorption: The atom is in the ground state. An incoming photon excites the atom and is absorbed.
• Spontaneous emission: The atom is excited. It emits a photon and goes back to the ground state.
• Stimulated emission: The atom is excited. A photon collides. The atom returns to the ground state, and a second identical photon is produced.

To get a laser working, you need something to pump at least half of the lasing medium’s atoms into an excited state, creating what’s called a population inversion.

The more atoms you excite, the higher the chances that photons will meet excited atoms. And when a photon does meet an excited atom, a 2^nd photon is generated by stimulated emission, which can itself trigger another stimulated emission, starting a chain-reaction, also known as a LASER (Light Amplification by Stimulated Emission of Radiation).

Chain reactions of stimulated emissions will deplete the population of excited atoms, but as long as you keep pumping to replenish the supply of excited atoms, the laser will continue to shine. This is, in short, how CW lasers work.

Pulsed Q-switch Lasers

More excited atoms means higher chances of chain reactions, but chain reactions reduce the number of excited atoms available. Hence, no amount of pumping will allow you to have all atoms excited at the same time.

Unless, of course, you could find a way to temporarily stop photons from creating chain reactions while you pump, but not at other times, which is precisely what Q-switching allows.

By changing the quality factor of the laser’s resonator (hence the name Q-switch) 2 different modes of operation are made available. A “pumping mode”, where chain reactions are inhibited, and a “lasing mode”, where they are not.

Thanks to the “pumping mode”, it is possible to excite just about every atom in the lasing medium. This represents an enormous amount of stored energy, leading to monstrous peak powers when the laser is switched back into “lasing mode”.

Ultrashort mode-locked pulsed lasers

We’ve discussed lasers so far as if photons were particles, but thanks to the wave-particle duality, we can also exploit their wave-like properties. Namely, constructive interference.

When talking about lasers, there tends to be an assumption that everything is perfectly uniform. But in the resonator cavity, there can be more than one mode. It’s possible to synchronize the different modes in such a way that the different modes are all in phase at the laser’s exit aperture for brief moments at a time, resulting in extremely short pulses.

We hope you enjoyed this brief overview of CW and pulsed (Q-switched and mode-locked) lasers. If you need to measure your CW or pulsed laser, Gentec-EO can provide solutions for all power levels.