Message-Id: <l03110718b2a83fde9de0@[192.174.2.173]>
Date: Thu, 24 Dec 1998 11:01:05 -0800
To: pinhole@exploratorium.edu
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Cheap laser pointers and their instrumental implications
>X-Sender: jlahr@netmail.home.com
>Date: Wed, 23 Dec 1998 17:10:28 -0700
>To: donrath@aol.com, pauld@exploratorium.edu
>From: The Lahrs <JohnJan@lahr.org>
>Subject: Cheap laser pointers and their instrumental implications
>Mime-Version: 1.0
>
>Hi Don and Paul,
>
>I thought you might get some ideas re laser pointers from this Email
>message that I got from the Public Seismic Network group.
>
>If you get this in time, I hope you have a nice Christmas holiday,
>and in any case I'll wish you a fruitful New Year.
>
>Cheers,
>John
>
>>>To: PSN-L Mailing List <psn-l@seismicnet.com>
>>>From: Roger Baker <rcbaker@mail.eden.com>
>>>Subject: Cheap laser pointers and their instrumental implications
>>
>>Friends,
>> A significant new opportunity for amateur scientists who build
>>scientific instrumentation has recently opened up with the appearance of
>>inexpensive laser pointers.
>>
>>The advent of very small and inexpensive (for example $8 on sale at
>>Walgreens Drug stores in Austin, Texas in December 1998) and of good
>>quality has to be considered an important milestone for those who build
>>many kinds of amateur scientific instrumentation. Until quite recently,
>>they were not so cheap and bright and small.
>>
>>One obvious and appropriate application for such inexpensive laser pointer
>>technology is my Hi-Q Gravimeter/Seismograph, pretty fully described on my
>>web page at www.eden.com/~rcbaker.
>>
>>Note that there is also a newly appended section on my web page that
>>describes a little more fully how to build sensitive magnetometers in the
>>Amateur Scientist column of the latest (Jan., 1999) Scientific American,
>>and laser pointers are an essential part of this magnetometer project.
>>
>>The key property of this new and inexpensive scientific toy are that the
>>powerful and easily electronically detectable red light beam allows one to
>>accurately measure the microscopic motion of many kinds of small objects,
>>down to a nanometer and below, without any appreciable back force.
>>
>>These little key chain pointers, about the size of a lipstick case, have an
>>optical feedback circuit already built into them so that they are quite
>>bright, enough for the spot to be seen at hundreds of yards, yet they do
>>not ordinarily burn out.
>>
>>Actually, these Asian imports are very finely constructed of threaded brass
>>tubes, complete with mini-circuit board with precision appearance and
>>coated window and precision injection molded optics -- really quite amazing
>>for the price. They incorporate a 3 volt circuit that can easily be
>>modified to run off of two alkaline flashlight batteries.
>>
>>An external pulsing circuit can be added if needed for lock-in
>>amplification, etc., by chopping the battery power supply with any circuit
>>having a low on-resistance measured in tens of ohms or less. Suitable ways
>>to do this are by using a power FET or by else by using a quad bilateral
>>switch like a CMOS 4066 chip and paralleling several or all of these
>>switches to give the necessary low on-resistance).
>>
>>One can use them as is, or better with external flashlight batteries, to
>>generate a highly collimated light beam in accord with their normal
>>function. Or one can take them apart to get near to the red laser diode,
>>which shines out through a tiny coated glass window. The beam is in the
>>form of an intensely bright source which is emitted from a microscopic
>>slit-like aperture in one side of the semiconductor chip to give a somewhat
>>divergent beam with most of the light concentrated along one axis like a
>>line.
>>
>>A small biconvex precision injection-molded plastic lens is seen to be
>>mounted a few millimeters in front of the laser diode window inside the
>>threaded brass case to focus the slit-like divergent beam into a slightly
>>convergent beam which is focused to a bright point at a distance of a
>>number of meters, as is most suitable for laser pointer operation.
>>
>>One can also make one's own very short-focus glass bead lens of reasonable
>>optical quality by melting a thin strip of window glass with a propane
>>torch, drawing out a thread of glass and remelting it so it fuses into a
>>round bead a few millimeters in diameter on the end of the glass thread.
>>One may chose the best from a number of these, which can all be made in a
>>few minutes.
>>
>>This laser pointer beam is quite suitable, as is, for many uses, and is
>>nicely detectable with the so-called infrared photo transistors sold for a
>>dollar or so at Radio Shack, used in conjunction with many kinds of op amp
>>circuits (I especially like the 324 quad bipolar op amps except for exotic
>>applications).
>>
>>The laser pointers can be disassembled for further experimentation with the
>>help of a vise and a jewelers saw and rebuilt into various instruments.
>>One first unscrews the cap and battery chamber plug to leave a short
>>cylinder that can rest on top of the slightly opened vise. One should then
>>saw inward through the brass toward the central axis of the
>>cylinder and a few millimeters behind the plastic lens. With a little
>>experimentation, one can free up and remove the laser diode and the little
>>attached (it is finely adjusted too--do not try to readjust the tiny pot!)
>>circuit board and spring battery contact. One can now carefully saw or
>>grind off the rest of the brass collar to expose the flush surface of the
>>little (near point) light source window inside the gold-plated laser diode
>>enclosure. The total dimensions are now only about 1 cm by 2 cm. There is
>>an on-off button that one can tie down with a tight loop of sewing thread
>>tied round the little circuit board, which is probably easier than
>>soldering in a jumper.
>>
>>One could unsolder and reconnect the little circuit board too, to rebuild
>>the laser into small instruments with especially tight spaces, allowing for
>>the total size of the light emitter to be only about one centimeter or so,
>>maximum, in any one dimension; thus allowing for good design flexibility in
>>various kinds of miniaturized instruments.
>>
>>Potential uses:
>>
>>One of my favorite techniques of instrument design has been to mount small
>>infrared or red light emitters very close to silicon photo transistors and
>>to use the intense stream of photons across the gap to detect any minute
>>motion that can be arranged to interrupt this light beam -- such as the
>>motion of the flag on my Hi-Q gravimeter/seismograph.
>>
>>There is no reason not to now apply the same principle to instruments that
>>can use a laser diode for the same purpose, for at least a factor of ten or
>>probably better, improvement in sensitivity and general performance.
>>
>>For such a purpose, it may sometimes be useful to artificially constrict
>>the laser beam to a narrower well-defined slit at its point of origin by
>>using silicone rubber to glue two little rectangles of aluminum a few
>>millimeters on a side, cut from a drink can, over the exposed laser window,
>>and aligned with the inherent slit-like emission of the laser beam. Thus
>>the straight edges of the aluminum snips are thus perhaps 50 microns apart,
>>centered over the laser diode window, and allow a narrow beam of red laser
>>light to leak out of the slit that they define over the glass window.
>>
>>Any minute motion of an object with a straight edge, such as the flag on a
>>seismograph beam mounted very near to the beam, will now choke off or
>>modulate the laser beam and thus generate a powerful signal change in a
>>phototransistor chip mounted a few millimeters away.
>>
>>Many other kinds of instruments like weighing instruments and barometric
>>sensors and vibration sensing instruments can benefit from a small
>>displacement detector with zero back force.
>>
>>As an important example, various kinds of scanning probe microscopes use
>>tiny reflective cantilevers to sense sub-nanometer motion and these can
>>make good use of collimated laser beams. Laser pointers are arranged to
>>focus at a distance some meters in front of the pointer, and since the best
>>configuration for scanning microscopes is to have the beam focus to a point
>>on the tiny cantilever mirror, (these are best made quite optically flat
>>but are typically less than a millimeter across), before it is reflected
>>into a nearby light detector.
>>
>>(I'm pretty sure I have all the mechanical elements of an amateur-friendly
>>scanning vibrating probe microscope hacked now except for the computer
>>interface).
>>
>>It is apparent that an unmodified laser pointer will not be quite optimum
>>for such a purpose. To focus the beam more tightly to a point at a distance
>>of a few centimeters or inches, all that is needed is to mount a small
>>glass supplemental magnifying lens in front of the laser pointer to help
>>the beam converge to a point at the distance needed for instrumental design
>>purposes. This beam, which now converges at a short distance, could be used
>>as is to detect the motion of any object or first passed through a pinhole
>>or narrow slit to define it a little better.
>>
>>In many or most cases, this use of a supplemental lens is probably a better
>>alternative to taking the laser apart and messing with the optics, which I
>>typically could not resist doing.
>>
>> --Yours, Roger Baker
>>
>>
>>
>>_____________________________________________________________________
>>
>>Public Seismic Network Mailing List (PSN-L)
>>
>>To leave this list email listserver@seismicnet.com with the body of the
>>message: leave PSN-L
>>
>>
>>
>
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>* JohnJan@lahr.org *
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>