Question of the Week

Stronger lasers with mirrors?

Sun, 2nd Sep 2012

Part of the show Is there life under Antarctica?


Jay Shah asked:

Is it possible to make an infinitely powerful laser with the use of mirrors?


Hannah -   This week, we probe with the powers of reflection.

Jay -   My name is Jay Shoal and I'm from Greenhithe in Kent.  My question for the Naked Scientists is whether you can make an infinitely powerful laser, just using mirrors.  Thank you.

Hannah -   So, first up, how does a laser actually work?  If you give an atom energy, it can give that energy back out as a photon of light in two different ways.  Normally, it does it spontaneously in a random direction, but if itís hit by another photon of light of exactly the same energy, it will release its energy as a second photon which is identical to the first photon and in the same direction.  So, amplifying the light signal.  That means that in a medium full of excited atoms one photon can get amplified into millions of photons all going in the same direction.  Dr. Martin Austwick from University College London explains how the use of mirrors further amplifies this signal in lasers.

Argon-ion and He-Ne laser beamsMartin -   Now a way that you can make the amplification even more powerful is by having parallel mirrors.  So, the light passes through the medium and gets amplified, bounces off the mirror on the right hand side and then passes all the way down to the left hand side and get amplified and then repeats and bounces backwards and forwards, and as itís passing through the medium, it gets more and more powerful.


Hannah -   And so, given that, can we simply line up more and more parallel mirrors to make an all-powerful laser?  Apparently, not.

Martin -   You need one of the mirrors to be partially reflective.  It needs to have a little bit of light leakage out, otherwise light will be permanently trapped inside that cavity and youíll never get a shining out of the laser itself.  So, that means some energy is lost on each cycle.  So itís not perfectly reflective.  Each bounce of that light losses a little bit of energy.

Hannah -   In fact, both mirrors are not perfect.  They will absorb some energy rather than reflecting all of it.  And this limits the power of the laser beam output. And most laser beam powers are limited, since if you donít put energy into the laser fast enough to excite the atoms in the cavity medium, they can start absorbing light rather than amplifying it.  Back to MartinÖ

Martin -   The way that lasers work is that they have to have the material which is in some sort of excited state.  When a laser amplifies itself, it takes energy out of the medium.  So it takes excited molecules or atoms in it.  Those atoms and molecules lose energy and they go to a lower energy state.  So, at some point, when youíve got a very, very high degree of amplification, itís constantly de-exciting molecules and eventually, youíll end up in a situation where there arenít any more molecules to de-excite.  So, there's a limit to how powerful a laser beam can be.

Hannah -   But John Tish, Professor of Laser Physics at Imperial College London adds that there is a different way that you can use mirrors to increase laser power.

John -   While an individual laser will have its definite power limit, mirrors can actually be used to combine the beams from multiple lasers to boost the output.  This combining of multiple laser beams is the approach taken by the National Ignition Facility or NIF for short which is in Livermore, California where 192 pulse laser beams are combined using loads of mirrors to deliver huge amounts of laser energy to a hydrogen fuel target, with the hope of achieving fusion.  Just a few weeks ago, NIF fired a record 1.85 mega Joules onto a target, corresponding to a staggering peak laser power of 500 trillion Watts which is about 1,000 times more than the total US power consumption.

Hannah -   So Jay, yes.  Very high power laser beams are possible and it can all be done with mirrors, but only by using mirrors to merge different individual laser beams.  We next reflect on the beginnings of life on earth.

Barry -   Hi, this is Barry Wary calling from Richmond, Virginia.  I was wondering: if earth is such a great place to live, why to our knowledge has life only begun once on this planet?  Why donít we see new life beginning here every day?  And why do we believe that there is only one tree of life?

Hannah -   What do you think?  You can tweet @nakedscientists, write on our Facebook page, email, or join in the debate on our forum which is


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The output power of a laser is always less than the power going into the laser (usually a lot less).
You can't have an infinitely powerful continuous laser beam without an infinite power source to drive it.
Pulsed lasers can reach very high power levels, but only for a small fraction of the time.

Lasers use mirrors facing each other at the ends to ensure that the photons are all "in-step" with each other, as they bounce repeatedly off the mirrors. When the laser material is "pumped" with energy, a photon (ie Light) passing by will Stimulate the Emission of another photon (ie Radiation) with the same direction, frequency and phase. Hence the acronym LASER = Light Amplification by Stimulated Emission of Radiation.

One end of the laser is a semi-transparent mirror, allowing some of the laser energy to be emitted, while bouncing most of it back through the laser.

Lasers always lose some of their input energy - the pump source is not turned into laser output with 100% efficiency. The laser material is never perfectly transparent, so more energy is lost the more times it bounces backwards & forwards. This lost energy is turned into heat - often causing the laser material to shatter or burn out if the power levels get too high. No mirror is perfectly reflective, so the mirror is likely to crack at high power levels.

By making the mirrors more reflective, the power levels in the laser will reach higher levels, but is likely to make the laser fail sooner.

The US National Ignition Facility is a pulsed laser with a peak optical power of 500 TeraWatts, for a few picoseconds - but it takes about 6 hours before it can cool down enough for another shot. The average output power is similar to a hand-held LED flashlight. See evan_au, Thu, 6th Sep 2012

no, imagine a flashlight  if you point at a wall 5 feet away you will be able to see it then 15 it would look fainter then 30 you could barley see it the same idea works with a  laser  O8) jacksllvn0, Fri, 25th Jan 2013

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