# How many photons are produced in a laser pulse?

Friday, May 27, 2022

Friday, May 27, 2022

Ever wondered how many photons make up one laser pulse?

Of course, it depends on the laser, but we’ll give you some ballpark numbers, and will also show you how you can compute these values for your own laser setup.

If I showed you a basket full of apples, could you guess the number of apples it contains? What about if I told you the weight of one apple and the weight of the basket, then could you calculate how many apples are in the basket?

Finding how many photons are in one pulse is the same thing! By dividing the total energy of a pulse by the energy of one photon within the pulse, we find the number of photons.

The real question then is how to find the pulse’s energy and the photon’s energy.

Finding the individual photon energy is actually as simple as finding its wavelength, thanks to the formula E = hc/λ, where h is Planck’s constant, c is the speed of light, and λ is the photon wavelength.

Finding the pulse’s energy is even simpler: just plug any laser energy detector into the MIRO Altitude touchscreen display device and you’re set.

Gentec-EO's high-accuracy laser beam measurement instruments help engineers, scientists and technicians in all sorts of laser applications from the factory to the hospital, laboratory and research center. Learn about our solutions for these measurement types:

If we wanted a laser pulse containing as few photons as possible, we would aim for two things. First of all, low pulse energy. Second of all, minimal wavelength, because photons with short wavelengths pack more energy, meaning fewer individual photons would be required to make up a pulse.

Gentec-EO’s PE3B photodiode detectors can accurately measure down to just 24 picojoules of energy, at wavelengths as short as 210 nm. A pulse with these specs would contain just a little over 25 million photons. If that sounds like a lot, read on – You’ll fall off your chair.

In 2021, HiLASE Centre broke the world record for high average power from a high-energy laser. Using a Gentec-EO detector, the team determined that each pulse packed 145 Joules of energy! Knowing that the wavelength of this laser is 1030 nm, let’s find out how many photons are in one pulse.

First, we find the energy of one 1030 nm photon using E = hc/λ:

E = (6.63 * 10^{-34} Js) x (3 * 10^{8} m/s) / (1030 * 10^{-9}m) = 1.93 * 10^{-19} J

Dividing the total energy of one pulse (145 Joules) by that value, we find that one pulse of the HiLASE laser contains as many as 7.5 * 10^{20} photons. That’s 750 000 000 000 000 000 000 photons, or seven hundred and fifty quintillion!

If you can’t quite wrap your head around such a big number, here’s a fun fact. There are about 350 quintillion gallons of water in all of Earth’s oceans. In other words, there are only half as many gallons of water in the ocean as there are photons in *one* of HiLASE’s record-breaking pulses!