(replying to myself here) ... one other way to use a laser as a clock is to use a pulsed laser like a frequency comb, where the pulse period is locked to an external reference like a Cs. A system like this has been used for synchronization along a particle accelerator beamline, to give one example.
Cheers Michael On Sat, May 7, 2016 at 6:05 PM, Michael Wouters <[email protected]> wrote: > Dear Ilia > > Emission of light is a quantum mechanical process. It is fundamentally > statistical in nature and as someone commented earlier, makes a good > random number generator. Here's one, for example: > > http://www.nature.com/articles/srep10214 > > If you attenuate any light source, lasers included, to the point that > you can count individual photons, it will just be noise. > > Your professor is right about a laser having a narrow linewidth > compared to other sources. Its essential property though is that its > light is coherent. When viewed as a particle (photon), this means that > the wave functions of the photons are phase coherent. When viewed as > an electromagnetic wave, this means that there is phase coherence in > the electromagnetic wave. > > Here's a simple picture that may help: > > A laser consists of some active medium, usually placed in an optical > resonator to increase the overall gain of the lasing process. You > 'pump' the medium, putting energy into it via an electric current, a > flashlamp, or another laser. Excited atoms start to emit this absorbed > energy via spontaneous emission - this is random and is not laser > light. However, there is another process that takes place (this is one > of Einstein's insights), stimulated emission. A photon passing by an > excited atom will cause that atom to de-excite (with a certain > probability), emitting a photon whose wave function is coherent with > the incident photon. These photons circulate around the cavity, > causing other phase coherent photons to be emitted, in a kind of > avalanche - this is the laser light. You make one part of the cavity > slight transparent so that some light leaks out for you to use. > > Imagine now that your active medium only has a few atoms in it, > randomly scattered along the length of the cavity. As a photon travels > along the cavity, the photons that are caused by stimulated emission > will be emitted at random times determined by their random positions > (and you don't have to make any assumptions about probabilistic > emission to see this). The light that we see coming out of the cavity > is therefore emitted at random times. > > You may be thinking, OK that's a gas laser where the atoms are moving > around. What about a solid state laser like a diode laser ? The > crystal is of course not perfect, but really it comes down to my > initial statements that emission and absorption of light is a > probabilistic process. So the circulating photon effectively causes a > sequence of emissions that are random in time. > > The only way to use a laser as a clock is to lock it to some reference > like an atomic transition or stable cavity and then use that as source > to heterodyne other lasers suitably close in frequency with, or to > lock an optical frequency comb to it, which transfers the optical > frequency back into the RF domain. > > > Cheers > Michael > > > On Sat, May 7, 2016 at 3:14 PM, Ilia Platone <[email protected]> wrote: >> Wait... no telescopes, very close distances... >> >> only a laser, with a photon limiter like a dark window, "close" like 10mm or >> so... just the space required for the laser optics plus the "limiter", and a >> photon counting detector that can be an APD or a PMT, it depends on the size >> required and scale of integration. >> >> The "idea" came because my professor told me that laser is a light source >> composed by a limited number of harmonics, so close the ones as some nm >> wavelengths, to get these lights can be directional and manouverable: if >> these should be the carachteristics of lasers (a laser expert can correct >> me), photons emitted by this type of light source should hit the detector at >> a constant rate. The (very dark) limiter serves to regulate the photon flux >> so a very limited number of photons reach the sensor part. >> >> The question was if the photodetector could use the individual photon >> detection as clock tick, and if these ticks can be regular in frequency. >> Many have replied that it would be noisy: phase noise? I don't think a >> single photon can cause AM noise, because I was talking about single photon >> pulses into the photon counting region, not into the analog region. Please >> correct me if I'm wrong. >> >> Ilia. >> >> >> Il 05/05/2016 21:22, Hal Murray ha scritto: >>> >>> [email protected] said: >>>> >>>> Well, in deep space optical comm, we send many photons with a laser, and >>>> we >>>> use pulse position modulation at the receiver detecting single photons >>>> (or >>>> "few photons"), by which we can send "many bits/photon" (e.g. if you >>>> have >>>> 256 possible time slots in which the photon can arrive, you have 8 bits/ >>>> photon) >>> >>> Neat. Could you please say a bit more. >>> >>> What sort of distance? Bandwidth? Error rate? >>> >>> How big is the laser and telescope? What sort of optics on the receiver? >>> How hard is it to point the receiver in the right direction? How hard is >>> it >>> to point the transmitter telescope? ... >>> >>> How does the receiver get timing? >>> >>> >> >> -- >> Ilia Platone >> via Ferrara 54 >> 47841 >> Cattolica (RN), Italy >> Cell +39 349 1075999 >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
