Green Laser Pointers
Samuel M. Goldwasser
http://misty.com/people/don/laserdio.htm#diodlm2a
January 6, 2003
Red laser pointers are by far the most common and now quite inexpensive. Pretty soon,
they will be given away free in specially marked boxes of corn flakes. :) Seriously, prices
under $5 aren't uncommon and dropping rapidly. Search on eBay and you'll probably find
them for less than $1 each in bulk. However, except for various shades of red (depending
on wavelength), all other colors are very expensive. In fact, there is really only one other
color of any practical consequence - green. And this is a much different type of laser than
the simple diode lasers used in red laser pointers.
Currently, nearly all green laser pointers are based on Diode Pumped Solid State
Frequency Doubled (DPSSFD) laser technology. They are not just red laser pointers with
a different laser diode or green lens! (See the section: Diode Pumped Solid State Lasers.)
The exceptions are older models using green helium-neon (HeNe) lasers. I bet you didn't
know HeNe lasers came in green, huh? :) These had power outputs of less than 1 mW and
were much bulkier than modern laser pointers. And they are now about as common as
raw dinosaur eggs. (See the section: HeNe Tubes of a Different Color if you are curious.)
The wavelength of the DPSSFD lasers is 532 nm based on the intracavity frequency
doubling of a Nd:YVO4 (vanadate) chip using a Potassium Titanyl Phosphate, KTiOPO4
(KTP) crystal inside the laser cavity. Their output may either be CW, quasi-CW, or
pulsed. CW means "continuous wave" which results in a constant intensity spot. QuasiCW and pulsed both result in a spot that varies in intensity (so they are really both pulsed
output) but the pulses for the quasi-CW variety may be at a much higher frequency (e.g.,
5 kHz versus 300 Hz). You can tell which you have by moving the spot rapidly across a
screen - the trace from the quasi-CW and pulsed types will break into discrete spots.
However, the spot spacing for the quasi-CW pointers may be so small for normal use that
for all intents and purposes, they will appear continuous. However, a quasi-CW pointer
would not be a good choice to use in a laser show application. (Note that there is no
standard for calling a particular pointer quasi-CW or pulsed so your advertising blurb
mileage may vary!)
Visibility of these green pointers is 4 to 5 times that of 635 nm diode lasers or 632.8 nm
red HeNe lasers, which in turn appear 6 or 7 times brighter than the older 670 nm laser
diode based laser pointers for the same power output. The maximum legal green laser
pointer power is still only 5 mW but this would be equivalent in brightness to something
like a 150 mW, 670 nm device! And, the sellers of these things don't let you forget it! :)
Battery life of any green pointer is likely to be much worse than that of the simpler red
variety though for actual uses as a *pointer* (what a concept!), it probably doesn't matter
all that much. The quasi-CW and pulsed variety should be somewhat better in this regard.
(The "spec" sheet that comes with the Edmund Scientific L54-101 green laser pointer
claims a 3 to 4 hour battery life from a CR2 lithium cell though I'm not sure I believe it.)
There is no functional advantage to the pulsed system (it's actually less desirable since
the spot breaks up into dots when swept over a screen) but it can be made much more
efficient reducing the need for thermal management and extending battery life at the
same perceived brightness for these current hogs. Quasi-CW (frequency in the kHz
range) pointers may use either a pulsed pump diode, a passive Q-switch (sometimes
called FRQS - Free Running Q-Switch), or both, to improve the efficiency. Pulsed
pointers (frequency in the hundreds of Hz range or less) use a pulsed diode.
Note that since there is no real control of temperature, power output may change
significantly (up or down or both) if the pointer is kept on for an extended period of time.
Usually, since pointers are really intended to be used for brief periods of time for pointing
at something, if any optimization was done, the manufacturer would attempt to select the
laser diode wavelength to match the vanadate's absorption band when the components are
cool. As the laser diode heats up, its wavelength increases (about 0.3 nm/°C) and drifts
away from the optimal value. (Even though the absorption band is quite broad, there may
still be some noticeable effect.) However, if the wavelength was low to begin with, the
power would increase as the wavelength moved toward the peak absorption for the
crystal and then decrease if it went far enough. From my experience with these as well as
other basic green DPSS lasers, unlike red laser pointers whose output is either constant or
gradually dropping in intensity until the batteries poop out, expect a modest amount of
slow cyclical and even possibly some sudden power fluctuations as the temperature of
key components increase and lasing characteristics change. So, a typical green pointer
may actually dip to less than 2/3rds of its rated power at times, hitting the rated power
only occasionally. None are rock steady at their rated power. For that, you pay really big
bucks. :) Maybe the next wave of green pointer technology is a light sensor to maintain
constant output power! And don't forget that just because the CDRH safety sticker may
say 5 mW max, your actual model may not come anywhere near that - ever. The actual
power rating would be listed elsewhere. But providing it at all is rare, partially due to the
fluctuation problem, but mostly because the manufacturers figure you're better off not
knowing how mediocre the pointer realy is!
With the much higher prices for green pointers, make sure you get a decent written
warranty. Prices are currently averaging about $300 (in 2001) though I've seen some 3
mW models advertized on the Web for as little as $180, lower on eBay). And supposedly,
though I haven't tried to buy one, there is at least one company (Leadlight Technology,
Inc., Taiwan) who will sell 1 to 3 mW green pointers for as low as $88, quantity 1.
Although some may consider it unethical, ordering several pointers and only keeping the
best may be the only way to assure satisfactory performance as they are quite variable in
output and stability. The additional complexity and more delicate nature of the individual
components means that reliability and robustness may not be as good as for their red
cousins (to the extent that these are reliable and robust!). This means that while those
fancy polished wood cases look impressive, transporting the pointer in a well padded case
is probably a better idea. Comparing the detailed diagrams of a Typical Red Laser Pointer
and the Edmund Scientific L54-101 Green DPSS Laser Pointer, or the single diagram
Comparison of Red and Green Laser Pointer Complexity. (The L54-101 is a $395 model,
but even so, it's amazing prices aren't a lot higher as it has all the sophistication of a much
more expensive DPSS laser.) Even a failed switch just out of warranty (assuming there is
a warranty that will be honored in the first place!), can render a $300 pointer useless
since there is often no non-destructive way of getting inside to repair it. (And, I've heard
that the switches they use on these things are often not adequately rated for the much
higher current green laser pointers use compared to red ones.)
For more information on DPSS lasers and green laser pointers including details of the
L54-101, see the sections starting with: Diode Pumped Solid State Lasers.
And, what about those other colors? As a practical matter, there isn't much need for
anything beyond green since its wavelength (532 nm) is near the peak (555 nm) of the
human eye's response curve. However, to impress those high flying corporate executives,
blue might be cool - but expect to spend a $2,000 for one using DPSSFD technology that
isn't as bright as a $5 red pointer. I think yellow would look nice on dark color slides, but
the only way to do this now would be to use a yellow HeNe laser (yep, they come in
yellow also!) as there are no yellow laser diodes or practical DPSSFD alternatives. Same
for orange. At the other end of the spectrum, violet (which would be really hard to see)
laser pointers using the Nichia violet (400 to 415 nm) laser diodes could be built
inexpensively like red ones since the circuitry is about as simple - except for one minor
detail: the cost of these violet laser diodes is presently (February, 2001) still around
$1,000 each! A violet pointer might impress the corporate big-wigs also but due to the
lack of visibility, would be quite useless for presentations unless the projection screen
had a coating that glowed when hit by violet light. Hmmm, now that's an idea. :)
There are inexpensive LED-based key chain pointers in bright blue and other colors but
these are not true lasers and the divergence is typically 5 to 10 degrees instead of 1 or 2
milliradians (1 degree = 17 mR). But, if all you want to do is impress management types,
that may be good enough. :)
And, no, there is currently no technology capable of producing a variable color laser
pointer.
Additional Precautions with Respect to Green DPSS Laser Pointers
Unfortunately, these usually don't come with any sort of useful user manual.
Much of the following applies to any laser pointer but especially to the expensive green
variety:

Avoid physical shock: Due to the additional complexity and precise component
alignment, green DPSS laser pointers are much more susceptible to damage from
being dropped or whacked on a hard surface.

Use ONLY the recommended power source: Substituting a different battery
type or external power supply may result in immediate damage or reduced life.
Without analyzing the driver circuit, there is no way to know for sure what is safe.

Do NOT attempt to operate if very cold: The pump output will be greater and
the components and adhesives will be more brittle. I don't know how to determine
whether your pointer will be affected and at what precise temperature. However,
I've heard of the vanadate being shattered from operation at low temperatures.

Do NOT attempt to disassemble the pointer: At least, not if you value it as a
working pointer. While getting at the driver and DPSS module may be relatively
low risk, taking the DPSS module apart is definitely to be avoided. Damage is
almost certain.

Do NOT remove the IR filter: A substantial amount of the 808 nm pump light
can leak through to the output (10s of mW which is potentially dangerous) and
this is nearly totally invisible and swamped by the green output. Some totally
invisible IR at 1,064 nm from the vanadate lasing process also leaks through the
output mirror and its power may be similar to that of the green output! The IR
filter is typically a greenish piece of glass buried inside the collimating optics but
on some models, unscrewing the end-cap also removes the IR filter. The risk of
the additional IR isn't worth the very slightly increased green output that may
result. It's best to avoid these models entirely or somehow make sure that the endcap can never be left off accidentally.

Do NOT attempt any modifications to the driver: While it is tempting to tweak
up the drive current just a bit, or replace the pulsed driver with a CW one to get
more power, my recommendation is to avoid any of this unless you won't mind
turning your sleek green pointer into a sleek piece of junk.