Laser pointer experiments

Pointers are now a cheap commodity item and, although the quality is pretty variable, most can be persuaded to produce a
spot at least as good as the centre/edge finder versions.
The pointers incorporate three main items plus the battery.
The laser diode itself which is a near "point" source and emits a diverging beam.
A lens which changes this diverging beam to a collimated beam (i.e. a parallel beam focussed at infinity)
A small bunch of electronics which monitors the beam intensity and keeps it safely below laser diode burnout point independent of battery voltage and laser temperature.
This is all organised to produce the brightest possible beam. For center/edge finder work this is far too bright and neutral density filters or crossed polaroids are commonly used to reduce this to a manageable level. Without these, because the eye saturates, the spot appears much larger
Another way of reducing brightness is to add a series resistor to the battery to reduce the operating current (typically 150mA). The amount required is dependent on the laser characteristics - the useful range is about 50 to 500 ohms. In adding wires to the pointer be sure to remember that the metal case is POSITIVE.
A second change is to refocus the lens to produce its focussed image at a more useful distance. The lens is normally in a threaded metal mount which can be rotated to alter focus. Because the initial beam diameter is small, the depth of focus is large and not critically dependent on the distance to the image point. The lens position needs to be typically 30% to 50% further away from the laser diode.
Changing the lens position is the best option but it can be a fiddly business. Much the same result can be obtained by adding a second lens. Because the initial beam is parallel the position of this lens is not critical - close to the existing lens is convenient.
Ideally it should be plano convex with the convex side facing the existing laser lens but the the biconvex lens fitted many eye loupes performs pretty well. The focal distance depends on the power of the eye loupe - a x5 loupe has a focal length of 2" and this is a convenient working distance. Higher power loupes produce proportionately smaller working distance and spot size.
This reduction in spot size produces a substantial increase in brightness so reduction of beam power is essential to prevent eye saturation effects.
This spot is now a LOT smaller than the original short range pointer collimated beam but, because it is now an image of the actual emitting surface of the laser, the shape can be a bit variable dependent on the particular laser diode. Two out of three laser pointers produced acceptably shaped spots the third produced an odd shaped elongated spot. Some improvement should be possible with a strategically placed pinhole but I've not tried this. The best pinhole position is likely to be very close the the laser diode window. It's not easy to mount here and it would require accurate alignment of the pinhole position.
It's pretty easy to check out the odd pointer so it would be interesting to learn how well different pointers performed and whether pinholes are a practicable improvement.
Jim
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Great..!! I have been experimenting with a pointer for aligning purposes on my lathe. Thanks, this is what I was waiting for. Best regards, Dirk
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I'm not sure how you plan to use these but may be able to help as I've played around with laser pointers for a good few years now and hang out at alt.lasers and sci.optics quite a bit.
Given the low power requirements I'm guessing you'll want to use red laser diode based pointers at around 650nm or 635nm. These do produce a far from a circular spot. Given the low power requirements for this application the easiest solution is to use a suitable round aperture in the light path as mentioned. I can't see that any benefit will be gained from placing a tiny aperture near to the laser diode itself. I'd have thought either side of the collimating lens is best. In fact normally the lens is actually smaller than that required to capture the entire diode output so it already acts to partially clean up the beam shape by excluding the extreme ends of the (very roughly) rectangular beam.
Cheap laser pointer tend to have a bare laser diode without a windowed containing package.
Laser diodes are very susceptible to static electricity or overpowering even for short durations. Bear this in mind if you plan to rewire one.
Hope this is of use,
Scrim

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Not wishing to detract anything said, might I mention that a series ran in Model Engineers Workshop.
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On Mon, 2 Jun 2008 00:12:15 -0700 (PDT), ravensworth2674

Which issue, Norm?
Regards, Tony
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Tony, I'm a bit hazy about issues but Peter Rawlinson did it. Going on his Spark erosion machine article his phone is 01233-712158 That was No60 so this will be an early series of articles-where you also contributed!
( you now owe me a beer)
Cheers
N
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On Mon, 2 Jun 2008 05:20:20 -0700 (PDT), ravensworth2674
I'll rent one for you <G>
Regards, Tony
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On Mon, 2 Jun 2008 05:20:20 -0700 (PDT), ravensworth2674
Yep - issue 79 in Jan 2002.
Thanks!
Regards, Tony
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With unmodified pointers, producing a collimated approximately parallel beam, I agree that limiting the beamwidth by a pinhole close to the collimating lens is the best bet. This position is easily accessible and has the advantage that it produces a roughly round spot.
In initial work I played around with pinholes in this position. However none of these produced a spot which was as small, bright and well defined as the convergent beam from the laser refocussed for an image distance of about 2".
The "spot" is now an image of the light emitting surface of the laser. With a perfect laser diode one would expect an elongated rectangular emission surface. However, on the few samples I have played with, only parts of the length appear to lase properly and it is easy to pick out the dominant part of the image.
A pinhole close to, or at the laser diode would eliminate this problem This requires precise positioning in a pretty inaccesible location and I hadn't got the patience to try it. For braver souls it would be worth using a slit rather than a pinhole as this would be much easier to align.
Your comments on the fragility of laser diodes are well taken because lasers are much more sensitive to mishandling than the general run of semiconductor devices. The modifications proposed should be reasonably safe because there is no direct connection to the laser diode and the applied voltage is always less than the designed battery voltage.
Jim
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On Mon, 02 Jun 2008 23:39:53 +0100, snipped-for-privacy@yahoo.com wrote:

Greetings Jim, I messed around with lenses and laser pointers a while back. I used a negative and a positive lens. This gave me the best results. I'm no optics guy but from reading the book from Edmunds I thought that my best hope would be to expand the beam and then focus it. By changing lens positions I was able to get a very small spot about 15 feet away which was my goal. Cheers, Eric
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On Tue, 03 Jun 2008 02:38:28 GMT, snipped-for-privacy@whidbey.com wrote:

Hi My experiments were aimed at short distance work for edge finding or scribed point location. For long range work your negative expander followed by a positive focusser is a much better arrangement.
The lens system is all outside the laser pointer so it's easy to organise. The downside is that a suitable lens pair may not be easy to obtain.
I'm sure that some of the group would be interested in long range work so further information on the setup and your supply sources would be welcome.
On a different matter I would like to correct a error I made in an earlier post.
I commented that, if crossed polaroids were used reduce beam brightness, it really didn't matter whether they were placed between the laser and the spot or between the eye and the spot.
While this is true as far as spot brightness is concerned it neglects the illumination required for the target. If you're using this for edge finding or scribed line location you need all the external illumination you can get on the target. The polaroids are best located between laser and target, not eye and target.
Jim
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On Wed, 04 Jun 2008 17:07:44 +0100, snipped-for-privacy@yahoo.com wrote:

Greetings Jim, The lenses were purchased surplus from Surplus Shed. I think. Anyway, I have lots of lenses kicking around that I play with learning optics and all have been bought surplus. However, I will try to get the time in the next week to measure the lenses for diameter and focal length and post the results here. Cheers, Eric
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wrote:

Hi Jim,
So are you using the pointers built in collimating lens refocused to a spot at 2 inches? If you can use a much closer focus, although it doesn't need to be near the actual laser, I think a pair of identical collimating lenses correctly oriented would do a very good job of focussing the laser spot back down to its original micron scale. The spot might be small enough for you not to worry about it's shape! It's beyond my expertise to be certain but I suspect this arrangement cancels out a lot of lens aberrations like the similar lens pair arrangement in a projector 'condenser' so it should work really well. It's also easy to try: take two identical laser pointers. Access the lens in one and note it's orientation. Remove it and place it in the collimated beam (at any distance) coming from the other laser orientating the lens so the side that faced the diode now faces the focussed spot (so the lenses are symmetrical about a plane between themselves).
Diode damage by static discharge and power supply spikes: A well designed laser driver circuit should incorporate protection for the diode from power supply spikes and static, but laser pointer manufacturers know that batteries are free from voltage spikes and batteries are rarely changed. They save money every way they can, reaching an extreme with laser pointers that simply use a resistor for current limiting. (Aside:These seem to be special laser diodes designed to be used this way as most laser diodes wouldn't reliably operate at all like this due to the sensitivity of the threshold voltage to temperature and the variation of threshold current from unit to unit combined with the fact that the diodes operating current is normal (relatively) only just above the threshold current and the minute optical facets of the diode chip are running very close to the limit where they will be damaged if the output gets any brighter than the design limit) Anyway, a simple resistor as a current limiter isn't going to provide any protection from power supply spikes or static. I learnt the hard way that my bench power supply produced large momentary spikes as its switched on and off. No problem normally, but laser diodes are run at the limits of possibility and are very tiny, so they can't cope with it. If you have no problems, fine, but if your laser dies or loses power mysteriously, consider adding a simple RC low pass filter. The R should not drop too much of your supply voltage and the C should probably be a combination of a small high frequency ceramic decoupling capacitor and a small electrolytic capacitor to cover both fast and slow spikes (say 10 ohms with 100nF + 10uF). Better still, just add the capacitors directly across the diode so the R is provided by the diodes current limiting resistor, but only if you're confident you can add them without damaging the diode. Personally I modified a cheap laser pointer by drilling a hole in the screw fitting batter access cap for a coaxial cable and fitting an RC filter in the battery compartment using a much larger electrolytic capacitor than I just suggested because it happened to be the same diameter as the original batteries,. I was able to make it so the capacitor just sat in the battery compartment just the way the batteries were meant to, using the original battery contact spring :-)
One useful feature of a laser diodes output is that it's polarised so you can use a single polarising filter as a rotatable brightness control. After reflection from a metal surface the polarisation may be lost or weakened so that two crossed filters would than be required, another reason for locating the polarising filter at the laser rather than the observer.
Scrim
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