In, 21st century , photonics is main field. LASER has revolutionized almost every field. Winners of Nobel Prize in physics,2018 has developed a revolutionary techniques to create LASER pulse of extremely extremely high peak and which lasts for a millionth of a billionth second. that method is called” Chirped Pulse Amplification”. diffraction grating is so important tool. This method has tons of application in research as well as industrial field.
Why do we need LASERs?
It is no doubt to say that 20th century was the era of electronics (making powerful computers and machines). But now, in 21st century, photonics is the future. It is (much fascinating) branch of experimental physics of using LASER as a tool in medical surgeries, manufacturing, computer technologies, astrophysics (in short, everywhere.)
Ever since 1960s after the introduction of LASER pulse ( which is far more better than Continuous LASER), our run is always towards getting better, more powerful LASERs. Because scientists and engineers know: more powerful the LASER, more awesome stuff we can do!
In one line , by awesome stuff I mean creating miniature star (like the one you see in night sky) in the laboratories as a source pf enormous clean energy, here on earth. Yes, that’s how awesome LASERs can be. But we need so powerful LASERs.
A Run Towards Powerful LASERs
Its no wrong to say that we have developed good techniques like Q-switching and Mode locking to produce LASER pulses.
An ultrashort pulse laser emits light in short bursts i.e it emits laser light in pulses (and not continuously) .It persists for extremely small time duration – from picoseconds to 10 femtoseconds (1 trillionth of a second).And secondly, they are extremely Powerful ranging from gigawatt (1 million watts) to terawatt ( 1 million million watts).
These methods are used in eye surgeries, tattoo removal etc.
Here’s the problem.
Whenever we attempt to increase the peak power, energy density delivered by the laser pulse gets so high that it damages the optical components of the LASER system itself. but we need high peak power.
So, what next? How can we increase the peak power, without damaging the LASER system .
Here’s the solution :
This is the Nobel Prize winning idea:
In 2018, Genard Morou and Donna Strickland were awarded Nobel prize in physics, for developing a method called “ chirped pulse amplification” which has the potential to produce LASER pulse with peak power of petta watt (1000000000000000 watts) than ever before (without damaging the system). Yes!
Chirped pulse amplification
It works in three stages: stretching the pulse, amplifying it and then compressing it before it hits the target.
First a seed pulse is generated by q-switching or mode locking techniques.
Stretching the pulse: it means, increasing the pulse million times it’s original length. Now it lasts for nanoseconds.
Amplification: the stretched pulse is then amplified upto the energy density just less than the threshold to damage the optical components.
Compressing it: Before the LASER hits the target, pulse is compressed in time and thus increasing the energy density even more.
Boom! You get a pettawatt LASER pulse.
Ofcourse the question coming in your mind is. How this stretching and compressing works? (It’s a great engineering challenge)
When there is a task to spread out light , diffraction grating is always a tool of choice. It breaks up light. It deflects different wavelength of light by different amounts.
question: But if LASER contains light of just one wavelength, they all will be deflected by same amount. How will it spread out?
Answer: the truth is, 100% accuracy is never achieved. LASER actually contains light of a very narrow band of wavelength. So when you put it through diffraction grating, it will spread out.
This Diffraction Grating is so important to Chirped Pulse Amplification.
And spreading it out in space, will make pulse lasts longer in time.
You now have all tools ready. Produce seed LASER, stretch it, amplify it, compress it, hit the target.
There is no short of application of this ultra-short LASER pulse :
- National Ignition Facility : as mentioned earlier, LASER pulse can help us ignite FUSION reaction (that happens inside stars) and if we achieve that, we will have enormous clean energy. I have dedicated a whole blog to that:
- Medical Applications are more than you would expect: medical diagnostics, radiotherapy, pharmaceutical research,DNA research, electron beam material processing etc.
- Astrophysics also adds up to the list.