Hi
I have a copy of the Laser Cookbook. I got it because I was intrigued by
the idea of a laser-based
seismograph.
The web page that Ron Thompson referred us to was for a class project
(presumably for a class
that dealt with lasers). Some of the other projects look interesting by the
way. It looks like the two
students who wrote the web page pretty much borrowed the design directly
from the Laser
Cookbook.
Apparently, in a stepped-index fiber, laser light travels through the fiber
in several groups of light
beams. At the transmitting laser, all the light beams have exactly the same
phase (a basic property
of laser light known as coherency). As the light beams travel along the
stepped-index fiber, the
phase of each group of light beams changes relative to the other groups.
Apparently, if the fiber
is long enough, the light coming out the far end is completely incoherent
(i.e. the optical equivalent
of mush, or ordinary light).
By using only 10 - 20 feet of fiber in the seismograph, coherency is only
partly degraded. When
the laser light is detected by a phototransistor and the resulting
electrical signal is amplified, the
slight differences in phase relationship between the groups of light beams
creates low frequency
audio noise. Based on the description given in the Laser Cookbook, I am
guessing that the audio
noise is fairly constant in the absence of changes in geometry of the fiber.
The changes in phase relationship are sensitive to length of the fiber as
mentioned and curvature.
I wouldn't be surprised if temperature causes changes as well. Vibration
dynamically changes
the curvature of the fiber causing dynamic changes in phase relationship.
The audio noise
generated by the phototransistor changes. The Laser Cookbook mentions that
the noise "changes
pitch and makes odd thuds, pings, and thrums". The Cookbook does not
provide a precise
analysis of the changes in phase relationship, so the mathematical
relationship between the
changing noise and the original vibration is unclear.
Figure 4 (16-3 in the Cookbook) gives the mechanical structure of the
seismograph. As Bob
Avakian has pointed out, the device is non-directional because the fiber is
wound around
four bolts in a rectangle, so the fiber will have both N-S and E-W
components of motion. Because
the fiber can move in a vertical direction, there will also be a vertical
component.
Some degree of directionality could be achieved by using only two bolts
oriented in a N-S or E-W
direction. You would unfortunately still have a vertical component mixed
in. The other problem I
can see is that the fiber motion is not damped.
Figure 5 (15-9 in the Cookbook) provides the schematic for the
phototransistor and amplifier. In
Figure 15-9 in the Cookbook there is an LM386 audio amplifier chip that is
fed by the output of
the LM741. In Figure 5 from the web page, the potentiometer wiper output
lead is connected to the
positive input of the 386 and the negative input of the 386 is grounded as
indicated by the other
output lead in Figure 5. The output of the 386 is capactively coupled (220
uF) to a headphone
jack.
The author of the cookbook built the seismograph and detected a quake in the
Los Angeles area
on October 1, 1987. The M6.1 quake was 50 miles away from the seismograph.
Hopefully that was interesting. If you have any other questions that I can
answer by digging
through the Cookbook, let me know.
Ron Westfall
-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@.................
Behalf Of ChrisAtUpw@.......
Sent: Tuesday, November 06, 2001 4:42 PM
To: psn-l@..............
Subject: Re: laser seismograph
A LASER SEISMOGRAPH
EE488
Department of Electrical Engineering
University of Washington
Autumn Quarter, 1994 Proposal prepared by: Leslie Woitte and Richard
Anderson
Hi All,
With the date 1994, there should be some record if this was ever
tried and what, if any, results were obtained. The article seems to lack any
logical relationship between a mechanical vibration and the 'noise' on the
output. The use of a lot of long words and a 741 amplifier does not
immediately inspire confidence in the scholarship of the proposal. Perhaps
someone who has a copy of the 'Laser Cookbook' would look it up the text
associated with Fig 16-9 and comment, please?
Regards,
Chris Chapman
Hi
I have=20
a copy of the Laser Cookbook. I got it because I was intrigued by =
the idea=20
of a laser-based
seismograph.
The=20
web page that Ron Thompson referred us to was for a class project =
(presumably=20
for a class
that=20
dealt with lasers). Some of the other projects look interesting by =
the=20
way. It looks like the two
students who wrote the web page pretty much borrowed the design =
directly=20
from the Laser
Cookbook.
Apparently, in a stepped-index fiber, laser light travels =
through the=20
fiber in several groups of light
beams. At the transmitting laser, all the light beams =
have exactly=20
the same phase (a basic property
of=20
laser light known as coherency). As the=20
light beams travel along the stepped-index fiber, =
the
phase=20
of each group of light beams changes relative to the other groups. =
Apparently, if the fiber
is=20
long enough, the light coming out the far end is completely incoherent =
(i.e. the=20
optical equivalent
of=20
mush, or ordinary light).
By=20
using only 10 - 20 feet of fiber in the seismograph, =
coherency is only=20
partly degraded. When
the=20
laser light is detected by a phototransistor and the resulting =
electrical=20
signal is amplified, the
slight=20
differences in phase relationship between the groups of light beams=20
creates low frequency
audio=20
noise. Based on the description given in the Laser Cookbook, I am =
guessing=20
that the audio
noise=20
is fairly constant in the absence of changes in geometry =
of the=20
fiber.
The=20
changes in phase relationship are sensitive to length of the fiber as =
mentioned=20
and curvature.
I=20
wouldn't be surprised if temperature causes changes as well. =20
Vibration dynamically changes
the=20
curvature of the fiber causing dynamic changes in phase =
relationship. The audio noise
generated by the phototransistor changes. The Laser =
Cookbook=20
mentions that the noise "changes
pitch=20
and makes odd thuds, pings, and thrums". The Cookbook does not =
provide a=20
precise
analysis of the changes in phase relationship, so the =
mathematical=20
relationship between the
changing noise and the original vibration is =
unclear.
Figure=20
4 (16-3 in the Cookbook) gives the mechanical structure of the=20
seismograph. As Bob
Avakian has pointed out, the device is non-directional because =
the fiber=20
is wound around
four=20
bolts in a rectangle, so the fiber will have both N-S and E-W components =
of=20
motion. Because
the=20
fiber can move in a vertical direction, there will also be a vertical=20
component.
Some=20
degree of directionality could be achieved by using only two=20
bolts oriented in a N-S or E-W
direction. You would unfortunately still have a vertical =
component=20
mixed in. The other problem I
can=20
see is that the fiber motion is not damped.
Figure=20
5 (15-9 in the Cookbook) provides the schematic for the phototransistor =
and=20
amplifier. In
Figure=20
15-9 in the Cookbook there is an LM386 audio amplifier chip that is fed =
by the=20
output of
the=20
LM741. In Figure 5 from the web page, the potentiometer wiper =
output lead=20
is connected to the
positive input of the 386 and the negative input of the 386 is =
grounded=20
as indicated by the other
output=20
lead in Figure 5. The output of the 386 is capactively coupled =
(220 uF) to=20
a headphone
jack.
The=20
author of the cookbook built the seismograph and detected a quake in the =
Los=20
Angeles area
on=20
October 1, 1987. The M6.1 quake was 50 miles away from the=20
seismograph.
Hopefully that was interesting. If you have any other =
questions=20
that I can answer by digging
through the Cookbook, let me know.
Ron=20
Westfall
-----Original Message-----
From: =
psn-l-request@.................
[mailto:psn-l-request@...............On Behalf Of=20
ChrisAtUpw@.......
Sent: Tuesday, November 06, 2001 4:42 =
PM
To: psn-l@..............
Subject: Re: laser=20
seismograph
A LASER SEISMOGRAPH
EE488
Department of Electrical Engineering=20
University of Washington
Autumn Quarter, 1994 Proposal =
prepared by:=20
Leslie Woitte and Richard Anderson
Hi All,=20
With the date 1994, there =
should be=20
some record if this was ever tried and what, if any, results were =
obtained.=20
The article seems to lack any logical relationship between a =
mechanical=20
vibration and the 'noise' on the output. The use of a lot of long =
words and a=20
741 amplifier does not immediately inspire confidence in the =
scholarship of=20
the proposal. Perhaps someone who has a copy of the 'Laser Cookbook' =
would=20
look it up the text associated with Fig 16-9 and comment, please?=20
Regards,=20
Chris Chapman
Larry Cochrane <cochrane@..............>