In a chirped pulse amplifier (CPA) one uses a short femtosecond laser pulse and then amplifies that to the pulse energy scales of terawatts and pentawatts. In doing so a stretcher and a compressor has to be used along with the pulse amplifier. Initial pulse is stretched then amplified and then compressed again. Such ultra short laser pulses have broad spectral bandwidth, a property which is used in it's controlled stretching/re-compression.
This is the process of chirped pulse amplification. But it is not clear what is your question for this discussion.
What is the physical mechanism/principle used during stretching and compressing? Why utrashort pulses have broad bandwidth?
The product of time duration of laser and spectral band width is always constant. This comes from Heisenberg uncertainty principle. This shows that as the time duration of laser decreases, the spectral bandwidth increases.
CPA utilizes dispersion for both stretching and compressing the pulse. Dispersion elements can be either prisms or diffraction gratings.
Some applications of CPA are as follows.
a) In the same way that lasers gave us nonlinear optics, CPA has been integral in the development of high-order harmonic generation which has pushed past the second- or third-order harmonics to happily produce tens or hundreds of harmonics. (The current record goes all the way to harmonic 5,000.)
b) laser-driven electron diffraction, which can be used to image the shapes of molecules as they undergo bending and other vibrations.
c) CPA is also extremely useful in delivering sharply controlled bursts of power to materials. This is extremely useful in laser micromachining .
d) There are several types of eye surgery that exclusively use CPA pulses to provide sharp 'kicks' of power which perform cleaner incisions.
e) Including the manufacturing of glass for cellphone screens.
Hope it suffices.
Thank you very much Rajib, Alex, Virendra. As this is supposed to be a discussion, please feel free to add comments/contents which according to you will be appropriate to this topic.
Dear Biswajoy,
I describe this topic in detail, however it may be confusing.
1) From the Heisenberg uncertainty principle we know that Δt ΔE ≥ h_bar/2.
h_bar = h/(2*pi) and h is Plank constant. So Δt Δω ≥ 1/2 .
From the experimental point of view, this inequality is ΔνΔt = K and K=0.441 for Gaussian-shaped pulses.
Nanosecend sclae Gaussian-shaped pulse (1ns) has a minimum spectral bandwidth of 441 MHz.
Picosecend sclae Gaussian-shaped pulse (1ps) has a minimum spectral bandwidth of 441 GHz.
Femtosecond sclae Gaussian-shaped pulse (1fs) has a minimum spectral bandwidth of 441 THz.
By converting this numbers to meter we can appropriately understand them. I attached the relationship in following.
For example for λ=800 nm we have:
For ns pulses we obtain: 441Mhz= 94*10^-14 m = 0.94 pm.
For ps pulses we obtain: 441Ghz= 94*10^-11 m = 0.94 nm.
For fs pulses we obtain: 441Thz= 94*10^-8 m = 0.94 um.
In practical we can obtain near 10 fs pulses from Ti:sapphire laser which bandwidth become 94 nm.
2)The emission spectrum of Ti:sapphire is from 600 to 1000 nm, which make it possible to produce sub 10 femtosecond pulses.
3) In ns pulses, by using some amplifiers we can amplify pulse amplitude and never reached to damage threshold of amplifying crystals (medium), but in fs regime because of high peak power we cannot amplify by this mentioned method.
4) Energy of fs pulses is about some nano joules which is not applicable in any applications.
5)As you know, dispersive elements such as prisms and gratings are used to stretch and compress pulses. Strength of dispersion in gratings are more powerful than for prisms. For this reason gratings are appropriate in CPA techniaues. Gratings can stretch pulses 1000 times to ps and this reduce pulse peak power and make it possible to amplify fs pulses.
After amplifying several times (depend on the system configuration by using multi-pass/regenerative amplifiers) , pulses need to compress in time using pulse compressors (two diffraction grating) to achieve fs pulses again.
The method of stretching, amplifying and compressing is called Chirped Pulse Amplification (CPA).
At final we obtain femtosecond pulses using CPA with more energy and high peak power. NOW these fs pulses are applicable in some applications.
Where you see fs pulse lasers used in industry, biology, accelerators, spectroscopy, micro-machining, holography, Z-scan, .and so on , the CPA techniques are used in which amplifies pulses to gain high enough energy required in applications.
Bests,
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