Stronger lasers with mirrors?

09 September 2012

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Question

Jay Shah asked:

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

Answer

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 it’s 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.

Martin -   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 it’s 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 you’ll never get a shining out of the laser itself.  So, that means some energy is lost on each cycle.  So it’s 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 don’t 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 Martin…

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 you’ve got a very, very high degree of amplification, it’s constantly de-exciting molecules and eventually, you’ll end up in a situation where there aren’t 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 don’t we see new life beginning here every day?  And why do we believe that there is only one tree of life?

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