From - Fri Dec 06 08:51:34 1996 Xref: nsc sci.optics:23825 sci.physics:211184 Path: nsc!voder!nntp-hub3.barrnet.net!su-news-hub1.bbnplanet.com!news.bbnplanet.com!cpk-news-hub1.bbnplanet.com!feed1.news.erols.com!howland.erols.net!surfnet.nl!news.tue.nl!usenet From: h.m.m.dejong@phys.tue.nl (Herman de Jong) Newsgroups: sci.phsics.research,sci.physics,sci.optics Subject: Re: photon statistics for LEDs and diode lasers? Date: Fri, 06 Dec 1996 10:45:25 GMT Organization: Eindhoven University of Technology, Dept. of Physics Lines: 73 Message-ID: <588tck$38s@tuegate.tue.nl> References: NNTP-Posting-Host: pc36.etp.phys.tue.nl X-Newsreader: Forte Free Agent 1.0.82 Bill Simpson wrote: >If I deliver a weak flash from >a) LED >b) diode laser >what distribution do the photons follow? (If it matters, the flash >duration will be on the order of microsec to say 100 millisec) >(BTW, is it possible to deliver short pulses from an "ordinary" laser? >I looked at Melles Griot and it appeared the answer was "no") There are many ways to modulate lasers. There are many types of lasers that might be called ordinary today but each has its ows curiousities. A HeNe (Helium-Neon) continueos gas laser (standing for all cw gas/plasmalasers) would be modulated poorly by switching on and of the supply. The plasma is hard to ignite. some types of these glass plasma tubes are however thin (2 mm internal diam. and 6-7 mm external diam) and it is possible to disturb the plasma with a magnetic field from a coil, so the allignment for the discharge with the mirrors is less effective for lasing. an external shutter could be used like an LCD shutter (best 200 us) or a pockels cell (better then 100 ns) CW lasers are diodelasers. They are not hard to switch and have virtually no limitation to switching by supply-interruption. It is bad for these lasers to be overpowered even for a very short time. The optical facets/mirrors at both sides of the laser are vulnerable to optical damage and you turn a laser into a poor LED by that. It might be an idea to have a nice constant supply current and bypass the laserdiode by a switch (transistor, unipolar or FET) to divert the current. Don't reflect (a fraction of) the beam back into the laser. The laser-cavity is redefined and tends to get instable unless you know what you're doing. >If the light is from a tungsten filament, I know that the resulting >stream of photons is a spatiotemporal Poisson process (approx). >Therefore the distribution of photons in a flash is Poisson. >Are LED and diode laser giving rise to spatiotemporal Poisson processes? >If not, what? >Finally with incandescent light and image of fixed size, one can vary the >total number of delivered photons by >- use fixed "intensity", vary pulse length >- use fixed pulse length, vary "intensity" >In both cases the number of delivered photons is distributed as Poisson >(due to spatiotemporal Poisson process). Will this also be true for >LED and diode laser? >I have a poor physics background. Can anyone suggest references? I have >Louden, The quantum theory of light. Usually it is over my head, and >it does not discuss LEDs or diode lasers. >Thanks very much for any help. >Bill Simpson The following disclaimer was written by a company lawyer. I take no responsibility for it. Standard Disclaimer______________________ Opinions expressed herein are my own and may not represent those of my employer. E-mail H.M.M.deJong@Phys.TUE.NL Homepage http://www.etp.phys.tue.nl/herman/herman.htm Phone (031) 40 2473472 Fax (031) 40 2456442 Snail Eindhoven University of Technology, Dept. of Phys. P.O.Box 513 5600 MB Eindhoven The Netherlands