How does that work? I've never heard of optical energy storage using blocks of glass...
It's not
exactly optical storage of energy, but its a close enough description.
I'm trying to figure out how to convey the concept without going on a waffle about lasers... but I seem to be failing.
Ok, when an atom or molecule is raised out of its ground state to an excited state, there are three possible outcomes -
1) Relaxation - the energy of the atom is shed slowly from the atom, typically via bond vibration. This is the sort of behaviour that NMR spectroscopy takes advantage of. Basically, the molecule releases non-light energy.
2) Spontaneous emission - the atom/molecule just drops back to the ground state, ejecting a photon in the process.
A molecule can absorb a photon and then go on to release a photon after some time has passed. Given that it absorbed a photon in the first place to do this, the energy remains the same. However, the wavelength of the expelled photon is dependant on the energy levels and band gaps of the molecule, and this is basically what gives rise to the colour of opaque surfaces.
3) Stimulated emission – a molecule in an already exited state absorbs more radiation, and then spits out a pair of photons as a result.
Essentially the incoming radiation in this case is a photon itself, so an excited molecule absorbs one photon, and then releases two as it drops back to a ground state.
This third behaviour is what is desired to generate a lasing effect.
As it happens, the rate constants of these behaviours were characterised by Einstein, and he went on to show that the rate of absorption is proportional to that of spontaneous emission.
This is why only a select few materials generate a lasing effect – if you just shine light on most materials, they do absorb and then emit photons, but they can only ever reach an equilibrium where the rate at which photons are being emitted is the same as the rate that they are being absorbed (in practice they rarely become exactly equal.)
So, using some molecular trickery that I’m sure you all don’t need to know about, you need to generate a population inversion in which there are more molecules in an exited state than the ground state. Generally this is achieved with molecules with a number of energy levels (things like neodymium) so that it has to fall through several higher states before returning to the ground state, allowing the creation of population inversions between the excited states, rather than one excited state and the ground state.
So, this is where the actual energy storage part is – once the molecules are in an excited state, you can continue to pump them with more photons (often, from another laser) A crucial part of the system is a pair of mirrors, one at either end of the laser cavity – photons emitted along the axis of the cavity with be reflected back in and produce further stimulated emission, while those off axis will be lost.
This is how a laser generates its highly collimated beam of photons.
Essentially, you get a bunch of photons bouncing back and forth inside the laser cavity generating more and more photons as you continue to pump it with your feeder laser, or gas collisions, or whatever.
In a continuous wave laser, one mirror is slightly transmitting, so that you get some of the photons escaping (thus giving you a steady stream of photons, ie, the laser beam)
One way to pulse this beam is just to turn the input energy on and off – however, this is limited by the time it takes the population inversion to decay.
A better way to pulse the light is to change the optical properties inside the laser cavity so the photons can no longer bounce back and forth.
Basically you can use a crystal who’s refractive index changes as you change the voltage, so you can make it opaque or transparent as needed.
This disrupts the
Quality of the laser resonance by effectively changing the length of the laser cavity – and is called Q-spoiling (or Q-switching)
By doing this, when you ‘un-spoil’ the system, the photons resonate back and forth again, and all the population inversions emit at one time, rather than having a dispersion of molecules at either the ground or excited state, with some undergoing stimulated emission, and some undergoing absorption.
This is what generates the much higher instantaneous power.
The short answer after all that is that lasers can store their energy in the higher energy levels of the atoms making up the lasing material, maintaining it in a solid state, more or less as Admiral Valdemar said.
Although typically the emit photons at a rate proportional to the energy going in, they can be made to release their energy in a series of brief, concentrated bursts.
Given that power equals work over time, and the brief time over which these lasers output their energy (i.e., as short as 10^-9 seconds) they end up with that whole ‘mind boggling instantaneous power’ thing…
Incidentally – I had a look around and you can use strobe lamps to pump a laser – and these
would require capacitors. No idea what they use to pump the NIF lasers however.
