Search the CONFOCAL archive at <a onclick="return top.js.OpenExtLink(window,event,this)" href="http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal" target="_blank"> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocalIt's entirely reasonable to be able to tune the laser as often asyou like - I'd often do it several times in an hour. The settlingperiod of half an hour is certainly a wise precaution. Giventhe amount of gear you've got I find it a bit surpriusing that yousay you can't afford an autocorrelator, though!As to the mode of operation, here are the relevant paragraphsfrom my book (details below) which give a brief overview:The generation of pulses in a Ti-S laser is essentially passive (though active devices may be added to get things started). As the light passes through the sapphire crystal the intensity of the electric field modifies the refractive index (the Kerr effect) so that it tends to focus the beam. To encourage this a slit is placed in the optical path, so that focussed light will pass more easily than non-focussed light. The end result is that a single pulse forms in the cavity and travels around it, generating the maximum electric field and therefore the maximum degree of self-focussing. The Ti-S laser (but not its pump laser, of course) will work perfectly well with no electrical power. The repetition rate of the laser is the time taken for light to travel around the laser cavity, which explains why the laser is so large, and has a complex folded light path (Figure 8.3). The actual Ti-S crystal is small.
A Ti-sapphire laser has to be pumped with another laser of quite substantial power. Typically the nominal efficiency will be around 10%, so that a 500 mW laser will require 5 W to pump it. For this solid-state neodymium lasers are used. The output from these is in the infrared, at ~1050 nm, and is then doubled in frequency with an LBO crystal which generates a strong second harmonic at 525nm. These are small enough to be fitted into the casing of the Ti-sapphire laser in modern 'one box' systems. The neodymium laser itself needs to be pumped with red light from two or more powerful diode lasers, which are always mounted externally and coupled by an array of optical fibres.
The pulses of a Ti-sapphire laser are only a few wavelengths long, and therefore are not monochromatic by the standards of continuous wave lasers. In fact it is possible to estimate the pulse length quite accurately by measuring the spread of wavelengths present. This presents a problem when the laser pulses must travel through a dispersive medium, as in optical components. The (slightly) different wavelengths will travel at different speeds, and the pulses will spread out.
Hope this helps.GuyOptical Imaging Techniques in Cell Biology
by Guy Cox CRC Press / Taylor & Francis
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Associate Professor Guy Cox, MA, DPhil(Oxon)
Electron Microscope Unit, Madsen Building F09,
University of Sydney, NSW 2006
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Mobile 0413 281 861
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From: Confocal Microscopy List on behalf of Shalin Mehta
Sent: Tue 4/09/2007 10:55 PM
To: [hidden email]
Subject: Re: what diagnostics to use
Search the CONFOCAL archive at <a onclick="return top.js.OpenExtLink(window,event,this)" href="http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal" target="_blank">http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal Thanks all for reply.
Given that we provide 1/2 hour for laser to settle is it reasonable to expect that one can tune the laser thrice a day for different users and still it works fine? Autocorrelator is too costly for time being but we do have a spectrum analyzer. Upto now, the optical setups have been aligned with aid of only the spectrum analyzer.
Is there a good application note/ reading available that explains the light-path of MIRA 900 (both picosecond and femtosecond configurations) in context of Kerr Lens Modelocking? - I don't know about either. Also some 'protocol' of tuning will be great. We have a manual from Coherent, but I am finding it hard to follow.
Regards
Shalin
On 8/30/07, Guy Cox <[hidden email]> wrote:Search the CONFOCAL archive at <a onclick="return top.js.OpenExtLink(window,event,this)" href="http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal" target="_blank"> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocalShalin wrote;What diagnostics do you use for tuning of two photon laser or OPO? Is laser spectrum analyzer enough?
The laser spectrum analyser should get you back exactly to yourwavelength, and give a pretty good idea of the pulse length.Lastek in Australia make a software system which works with aspectrum analyser to give you pulse length and it seems prettygood provided it is carefully calibrated. (It's probably sold undersome other name in the States).But if you have users who want to know the pulse length preciselyit's really worthwhile buying an autocorrelator.
We have a shared two photon (MIRA 900F Femtosecond configuration) and OPO from Coherent. One of the main user is very afraid of tuning it - so he always has a service engineer tune it to 830nm and longest possible pulse (since he does spectroscopy). There are two more of us who will benefit by tuning it for two-photon and SHG (different wavelengths and shortest possible pulse).830 is quite OK for SHG (brings the harmonic to 415 which is withinthe range of conventional interference filters (they start to absorb a lotat anythong shorter). But it's a bit limiting for 2P.
How is the 'reproducibility' of laser beam profile when working with these lasers? One of the main concern that other user has is that - if you cannot come back to exactly 830 nm after tuning to say 740 nm, the custom optical setup will go out of alignment.
In the early days one make of Ti-S laser was notorious for beamwander as you tuned it but even so it should go back when youreturned it to the original wavelength. Nowadays they seem reallystable, and I don't think your Mira should be a problem there. Theonly thing I find with a Mira is that it's best to let it warm up forhalf an hour before doing any tweaking of the alignments (and oftenthat way you find you didn't really need to do any tweaking anyway).Guy
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Shalin Mehta, Graduate student
Graduate Programme in Bioengineering, NUS, Singapore
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