Re: Building a Light Clock
From: Gauge (gauge5_at_hotmail.com)
Date: 12/01/04
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Date: 1 Dec 2004 14:01:57 -0800
"Randy M. Dumse" <rmd@newmicros.com> wrote in message news:<4Bkrd.429$LL3.5687@eagle.america.net>...
> "Pmb" <someone@somewhere.com> wrote in message
> news:ZcmdnU1-_v9FVjDcRVn-1Q@comcast.com...
> > Anyone know of some specs on light emitters and light detectors
> > that can operate at 1 MHz - 150 MHz? What complications arise
> > when constructing circuitry that operates at 1MHz? Is that
> > something "easily" done at home?
>
>
> I think today it probably is reasonable. In 1995 I made a barrel racing
> timer ...
What is a "barrel racing timer"?
> ... with a Quarton laser. I modulated the beam by digital switching
> the applied power, and checked the output. I found it rolled off rather
> quickly above 500 kHz, and it was still operational at 600 kHz, but with
> half the power output. I also found, with some difficulty, a detector
> that worked well in those frequency ranges.
Are their any optical devices that can modulate a laser beam at a
higher rate? E.g. are their any devices that exist that allows laser
light to pass through and come out modulated? Its been close to 20
years since I studied optical electronics (but when I did I had a very
well known expert as a prof in the field of optics - I recall that he
worked on one of the cameras that astronauts took to the moon!).
Hmmmm! Now that I mention it - I should contact him since he's an
expert in the field of optics.
> Things have progressed quite a bit in optoelectronics with the push to
> fiber communications.
Do you know of a web site that has this kind of data and a list of
fancy shmancy devices?
> For instance if you go to www.digikey.com and search on Optoisolators ...
Thanks! I looked at the site but didn't understand all the
terminology. Please e-mail me or call me at home and you can walk me
through the specs.
> ...you will find there are a few with less than 100nS on/off specs, which
> would mean there's an opto pair (emitter - detector) there which
> approach 10MHz capacity.
>
> A quick check of fiber optic transceivers shows some around 125
> Megabits/sec, and lots in the 8 and 13 MBPS range.
>
> So, I think it is possible to construct a 1MHz transmitter/receiver in
> one's (electronics lab equipped) basement today.
I think to determine that I need to see exact device specs.
The real underlying purpose of this thread was to get a very rough
idea of the kinds of devices and engineering difficulties that would
arise in that would be required (tom roberts failed to understand that
so he posted his usual condescending remarks and masked insults - i.e.
roberts reverted to his wiseass mode. Shame - his post started off
mature but it went downhill from there).
As you know, the absolute theoretical minimum frequency response is
related to the round trip time of the pulses. If a single light pulse
is sent through an instrument for which the optical path length length
is about 150 meters then the time between emission and detection is
500 nanoseconds. Therefore to have a "light clock" we must send light
pulses every 500 nanoseconds. The pulse width must, of course, be less
than 500 nanoseconds. That is the theoretical minimum. In practice the
pulse width must be less than 500 nanoseconds. E.g. if the pulse width
is 250 nanoseconds then the detector must be able to detect a train of
pulses of that width.
Pete
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