[time-nuts] laser as clock source

[Michael Wouters]

(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
<michaeljwouters at gmail.com> 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 <info at iliaplatone.com> 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:
>>>
>>> jimlux at earthlink.net 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 -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.