speed of light

I'm sending a light ray with a laser. The ray will follow some complicated path. Sometimes, the ray will be splitted into 2 rays and each of them will follow some path.

All rays have the same destination, but they will reach there at different moments in time (probably), depending on the length of the path that they have followed.

I know the exact moment when a particular ray should get to the destination, but I need to know if it actually got there (or it has got stuck somewhere on the path).

This is why I need some device, which can tell me if there is a fluctuation in the light, at the destination point, at a given moment in time (that moment must be specified with high precision ~ 10^-9). If I have a fluctuation in the signat at that moment I know for sure that the particular ray which I was expecting has arrived there.

Thanks, Laura

Optical sensors with 1ns response time are readily available.

I'm not researching metallurgy.

I'm a computer scientist.

best regards, L

If you are working with bright lasers, and can expect to see lots of photons within your critical sub-nanosecond time interval, life is pretty straightforward.

If you are in the position to have to detect whether there was one photon there from time to time you need a single-photon detector, and it could be worth your while to read "Performance otimisation of active quenching circuits for picosecond timing with single photon avalanche diodes" by A. Lacaita, S. Cova, C, Smaori and M. Ghioni in Rev. Sci. Instrum. volume 66 pages 4289-95(1995). Sergio Cova really does know what he is talking about, and over the years he has published an interesting series of papers on the subject in Rev. Sci. Instrum. and Applied Optics. I've got his e-mail address - it is ten years since I've used it, but he still seems to be publishing steadily.

--------- Bill Sloman, Nijmegen

Seems pretty obvious to me too. Anybody who uses language like the OP does not stand a chance in hell of setting up and using a laser, optical beamsplitters, and photodetectors.

Mark

Dear Bill,

Thanks for you advice.

I don't want to implement the device now. Right now I'm looking for some possible solutions which could help me.

You say that if I use some bright lasers things are easier? Could you give me some more details about that?

I don't plan to use only one photon.

Thanks, L

You could though, if you wished, at least with respect to timing its arrival to under a nansecond.

Search " laser autocorrelator", and modify that technique, ie run your two laser pulses (assuming your using a picosecond or other pulsed laser) into a second harmonic generating crystal, when you have a maxima from the doubled light, your pulses are in phase. This is easy to do with common IR lasers, you havent told us about your light source, which makes a big difference of how you go aboput this. probably time to find a skilled physics person, too!

your really tryint to look at the leading edges of your light pulse, aren't you?

Quantum Computing?

Steve Roberts

I don't mind to use only one photon.

I mean, the problem is that if I use only one photon, maybe it's hard to be detected...

And what happens if the photon is divided into 2 photons? Is this possible? Sometimes, in my device I need to do this. And the photons will go on different paths. And some of them will get earlier to the destination and others will get later....

I don't know what light source should I use. Right now I'm looking for some solutions to my problem. What I know is that I send a signal (probably a light ray) into some device... the light will be divided into some nodes of the device and because of that I will have multiple rays at the destination. But I know the moment when a particular ray will arrive there. I need only to know whether that particular ray has arrived there (or it was stuck somewhere).

Do you think that I could use another kind of signal (instead of light). At a theoretical level I work with signals, but if I want to make an implementation probably (???) the light is the best.

Any suggestions are welcomed !

Thanks, L

Within limits yes. First you need a sufficiently fast and sensitive sensor. If this sensor is delivering a still measureable current diffeence then the electronics can do it.

Rene

So you don't think she is the L. Brandusan that researches metallurgy?

Why don't you use two detectors at the receive end?

"laura"

There are heaps of Martian Engineers just waiting to supply you with a missing photon detector.

Those cute, green alien dudes know all about pesky, random, micro size black holes eating up innocent, unwary photons for several millennia.

This outrage has got to stop !!!!

When a cute photon says no - it means no.

........ Phil

"Anthony Fremont"

The post was a troll.

.......... Phil

Not that I know of - at least not in any useful way. Non-linear crystals allow frequency-doubling, where two photon fuse to make one photon of twice the frequency (half the wavelength), but I don't think that works the other way around. In theory, you could hit a molecule with the right energy to promote it into a energy state that was two quantum steps above the ground state, and hope that it would emit two photons in the process of relaxing back to the ground state, but this isn't an instantaneous process.

Are you thinking about quantum cryptography? Page 18 of the review below discusses single-photon detectors ....

---------- Bill Sloman, Nijmegen

"redbelly"

The name and email addy have not been used on usenet before.

Walks like a duck .......

......... Phil