What's achievable with pulsed lasers?

[ehj]

I am in dire need of some information on pulsed lasers. I'm not quite sure if this is the correct forum for this kind of question but here goes. I would like to know if you can buy a commercial pulsed laser system that can deliver a repetition rate of 40Mhz and 20 fs pulse duration while being able to tune it to the signal from a storage ring? This is in relation to an experiment on Compton backscattering. What kind of average wattage would such a laser be able to deliver if it exists?

 

[Cthugha]

This is difficult to say without some more info.

I know there are pulsed lasers delivering sub-20fs pulses at around 75 MHz, so 40 MHz should be possible, too. The output power will depend on how you pump them, but 500-1000 mW are realistic. However, for such short pulses, peak powers or the energy per pulse are quantities giving better information. However, I do not know which wavelength you intend to operate at. The above specs are typical for lasers operating around 800 nm. If you want to go to the IR or the blue, yellow or orange spectral regions, things may be more difficult.

Concerning the tuning to the storing ring signal: Typically the central wavelength of the emission is tunable within certain bounds. However, you should consider that even at an optimal time-bandwidth product, a sub-20fs pulse will give you a very broad spectrum. Also for such short pulses one should carefully consider which optics one uses as chromatic dispersion becomes a problem and even a simple element as a beam splitter may spoil your pulse as the bandwidth of the beam splitter might be limited.

You might want to have a look at the web pages of certain laser suppliers which have ultrafast products. I do not want to advertise a special one, so the following are all companies that immediately come to my mind, which might sell laser systems offering the specs you have in mind: Femtolasers, Coherent, Spectra Physics (though I do not know whether they have lasers with <35 fs pulse duration), Toptica Photonics (if you need IR fiber lasers - output power is way lower, though). There might be more, but these come to my mind immediately.

 

[ehj]

Thank you for the response. I havn't checked some of those websites yet.
The wavelength does not matter that much , anywhere between IR and UV would work. I must however be able to focus it to the diffraction limit (or close). Is this difficult with fs pulses?

You say that the wavelength is tunable within certain limits - but does changing the wavelength change the repetition rate or?

 

[Cthugha]

In principle, you can focus fs pulses spatially just like any other pulse - so short focal lengths and large spot diameters before the lens help. However, you need to be a bit more picky about the optics. As your pulse contains a rather broad spectral range, the refractive index of the lens might be slightly different across the whole range, so you may not be able to focus all wavelengths to exactly the same spot (chromatic aberration). There are achromatic lenses avoiding this problem, but for all optical components for ultrafast pulses, you need to consider the dispersion they introduce. Using 'thick' optical components may broaden your pulse in the time domain and for ultrashort pulses 'thick' already begins at a rather thin level. However, which specs you can really get, depends strongly on the exact experiment you have in mind and the laser used, so you might need to discuss this with someone from a laser company in detail.

The repetition rate is typically given by the length of the laser cavity. As you tune the central wavelength of your beam, you usually do not change the repetition rate by a huge amount. In order to cancel dispersion, you might have a bit more or less glass in your beam path, which might create a slight deviation in the kHz range. If you need a really precise repetition rate, for example for triggering a fs-streak camera, you can still move the cavity end mirror perpendicular to the beam to adjust the exact cavity length and pulse repetition rate for many laser models.

 

[ehj]

But isn't it possible to tune the repetition rate to an external signal? This is the greatest concern at the moment. In the storage ring the particles are bunched up, and the bunches pass a point at a certain rate (40 Mhz in our case). The laser pulse must hit these bunches and even if they match up initially, my concern is that this matching will be lost quickly if there isn't a live feedback system that can slightly adjust the repetition rate of the laser on the go, in order to match the accelerator bunch rate.

 

[Cthugha]

Oh, you have a special interest. Ok, there are systems out there which allow to synchronize two different lasers to each other by actively correcting the repetition rate of one of them. If you can create a low-jitter external signal, I suppose you can use the same system for synchronizing the laser to the accelerator bunch rate. In practice you achieve this by having the laser cavity end mirror mounted on a piezo, so its position and therefore the repetition rate can be actively corrected using a feedback signal.

However, I do not know, whether there is some company which offers this whole range of specs you need as a turnkey solution or you need them to customize an existing system.