When Lower Quality Is Better: The Splendidly Sneaky Art Of Q-Switching

Good afternoon, Darien. 

As you know, I work with a Ti-Sapphire ultrafast laser system.  I suppose that to be precise I should say that I work with two such systems: one for each of my projects.  While we are being precise, though, one might call into question my usage of the term “work” here.  I shall move on quickly without addressing this, and hope that nobody notices.  Anyway, my point is that my lasers generate femtosecond pulses using a cool process called mode locking.  You are familiar with this, I am quite sure.  We are both also aware that there are other lasing mechanisms that are capable of producing pulses.  One of these is the Q-switch, which produces very high power pulses that are not nearly as short as the pulses we use in our research.  Shortly after joining the Dlott group, I was given an introductory book on lasers, which provided a brief overview of the mechanism of laser action various pulsed lasers.  This book contained a brief explanation of various types of pulsed lasers, but I found myself quite confused by it.  I asked Will Shaw (alias “Old Will”) to explain this material to me, and he kindly obliged.  What follows is my attempt to reproduce his excellent explanation of Q-switching.  He actually uses a Q-switched laser, by the way.  The high power apparently comes in handy for blasting aluminum foil pellets at panes of glass.

As you know, lasers have three important parts.  The crystal which produces the light, the pump which energizes the crystal so that it can produce light, and the cavity, which stores the light so that it passes through the crystal repeatedly.  the “Q” in “Q-switching” stands for “Quality”, and the quality being referred to is the quality of the cavity.  In this context, a high quality cavity is one that stores light very efficiently, while a low quality cavity is one that either absorbs the emitted light or lets it escape without being reflected back through the crystal.  For most types of lasers, it is uber important to have a high quality factor, since you need the beam to pass through the crystal many times before population inversion is achieved.  In Q-switching, though, you want your laser quality to be very low most of the time, and only become good occasionally.  This could be easily accomplished by sticking an aluminum block in front of one of the cavity end-mirrors, in order to absorb any light that came from the crystal.  When the block is in place, the quality factor would be low, but it would “SWITCH” to high quality the moment you removed the block.  Thus the term “Q-switching”.  In practice, there are more sophisticated ways of doing this, but the idea is the same.

If I was reading this, I would have become confused and given up during the past paragraph, because it kept going on about Q-switching, but never explained how a low quality factor can allow for lasing to happen.  I will try to remedy that here.  It turns out that Q-switching works by achieving a VERY inverted population in the crystal.  Not some wimpy, ordinary population inversion where you barely have more excited state species than ground state species.  I’m talking about the vast majority of the species being excited.  Most lasing mechanisms don’t allow for such a state to be achieved, because as soon as you get a little bit of population inversion, you start building up an increasingly powerful pulse train that “sweeps out” the excited state species from the crystal via stimulated emission.  In Q-switching, though, you don’t have any pulse train, because you have made the quality super low.  You are pumping your crystal, and some of the excited species in the crystal undergo spontaneous emission, but the fluorescence generated this way gets absorbed once it leaves the crystal, rather than reflecting back to be amplified by stimulated emission.  Sure, a photon spontaneously emitted in the center of the crystal might cause a little stimulated emission in the process of exiting the crystal, but spontaneous emission is rare enough that the population inversion isn’t substantially depleted by this process.  With no way to leave (other than rare spontaneous emission events), the energy pumped into the crystal just builds up.  The population inversion gets more and more extreme.  When it finally approaches a maximum, you are ready to fire your pulse!  You remove the aluminum block (or activate your sophisticated mechanism for switching the quality) and PRESTO! Nothing happens.  At first.  It takes a bit for the stimulated emission to start building up.  Once it does, though, you get a rapid, exponential growth of the pulse power in the cavity.  All that energy stored in the form of an inverted population gets swept out as a tremendously powerful pulse, which you release from the cavity as soon as the population inversion is depleted.  Then you start all over again.  I find it terribly cool that you can store up energy in a crystal this way!  The idea of a nearly complete inversion of population is enough to plaster a sloppy grin on my face.  I hope you find this as cool as I do.  If you don’t, I’ve clearly communicated it poorly.

If you wanted to generate high power pulses using another method, you would run into trouble.  If I remember correctly, maintaining a really powerful pulse train in the cavity ends up damaging it… or something like that.  I think that the advantage of Q-switching here is that the pulse is only very powerful for a brief instant (a few trips through the cavity) before it is released.  I’m not certain of this, though.  I should probably read up on it some more.  But then, there are many things that I should read up on, and I should be reading some of them right now.  I’m afraid I have dashed this off rather quickly, so please enquire concerning anything which is unclear. 

Have a fabulous afternoon!




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