Laser Basics
1. Laser Light
A laser generates a narrow, concentrated beam of light. LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
2. Basic Laser Construction
Ared laser (635 -660nm) or ablue/violet laser(405nm) generates laser generates laser light directly from a laser diode so a red laser module or pointer is relatively simple in construction:
A green laser (532nm) generates the green laser light indirectly and is much more complicated in construction. The laser light originates from an lnfrared laser diode which emits invisible infared laser light, this is then converted by laser crystals to visible green light:
A green laser module(DPSS Module) can be constructed in many different ways, a good quality MCA type module construction is shown below:
3. Laser Visibility
A typical green laser appears much brighter than a red laser of the same power.The relative sensitivity of the eye to different colours of light is shown in the figure below which explains the apparent brightness of green lasers compared to similar powered red lasers - in light conditions green is about 10 times brighter, but in dark conditions 30-40 times brighter:
4. Notes on Green Lasers
Almost all green lasers originate from
China, sometimes they are imported and re-badged. The purpose of this
section is to explain a little about how many cheaper lasers are made
because it explains many of the myths and false claims that surround
cheaper high-power lasers such as:
- Inflated power claims
- Why some lasers have IR or Infrared, and whether it is dangerous
- Whether a modified high power laser is a realistic proposition or a problem waiting to happen.
The
first point is that green lasers are quite complicated compared to red
lasers and yet they are now being made and sold at prices that are a
fraction of the price five years ago and at much higher powers. This has
led to compromises with quality control at the cheapest end of the
market.
The problem is magnified because the selling price is
largely determined by the claimed power of the laser. For instance, the
OFL19 style laser is available with manufacturers rated power outputs
from 5 – 200mW and externally the lasers all look identical but the
prices range from $30 to $150! This has led some unscrupulous dealers
to take advantage of the fact that most people have no easy way of
knowing if the power they buy is correct. Certainly when I started to be
interested in lasers I could not honestly say if I was looking at a 10,
30 or 50mW version, only if I had them side by side could I see which
appeared brighter.
It was then that I got the real surprise: I
had bought five of each power rating of a promising new laser and was
going to take some photographs to show the differences. When I tried to
do this I found that if I was sorting them on how bright they looked
almost half of them looked like they had the wrong label on them and
should have been up or down a group. Finding out how they were produced
at one manufacturer explained a lot:
Green Laser Production
Green
Lasers use an Infrared pump diode to produce the IR laser light that
passes through two laser crystals to produce the green output:
Cut-away
green laser module showing diode pumping the laze crystal that looks
purple and the frequency doubling green crystal. The green laser light
is then **may** pass through a through an IR Filter followed by a
focusing lens, or the filter may be after the lens, or none at all.
The picture below shows an exit filter:
A typical manufacturer I saw produced two module designs, one with a larger diode and crystals but sold six
different power outputs. Basically the modules were sorted after
production and those with the best aligned and matched crystals gave the
highest outputs, the smaller module gave powers of >10, >20,
>30 and >50mW.
All had some IR light emitted that was not converted by the crystals but the amount varied a lot between modules.
With
these modules, the drive current supplied to the pump diode can be
increased by adjusting a potentiometer on the driver board. This
increases the infrared emitted by the diode however the amount of green
532nm light produced is limited by the crystals and their alignment,
too much IR leads to excessive heating and infrared emission from the
laser. A further difficulty is that heating the crystals can change
alignment and lead to lower outputs of the desired 532nm green laser
light.
Practical Tips
Infrared (IR) Emission
Most
small green lasers at the cheaper end of the market have some IR
emission associated with them, in some cases this can be a danger but
for the majority of lasers up to around 30mW the risk is minimal
compared to the green laser emission.
Modified or “pot-modded’ Lasers
If
the laser has been ‘pot modded’ in an attempt to increase the laser
output there is an increased risk that excess infrared is produced that
cannot be converted by the crystals to green 532nm laser light.
Why is IR a potential danger?
The
main problem with IR is that since you do not see it your eye does not
blink to stop it. As the distance from the laser increases the danger
from IR drops rapidly since it diverges a lot, also lots of things do
absorb IR easily, so it's more staring into it that damages your eye,
because you don't blink to stop it.
What about IR filters?
A
separate identifiable IR filter is rare in most cheap lasers. This is
because of several factors including cost, manufacturing issues and the
marketing ploy that some vendors quote the total laser output of IR and
532nm light.
Manufacturing issues
IR filters are of two types, those that adsorb IR and those which reflect it back.
This
gives manufacturers a potential problem: absorbing filters can overheat
and are not really suitable while reflective filters ideally need to be
accommodated in the laser design or the crystals may overheat. This is
difficult for many small manufacturers who buy in components, including
the basic laser module and assemble complete lasers. Often they will
change suppliers depending on component availability consequently adding
a filter and properly tuning the laser for maximum output is simply not
economic for buget low power lasers. This method of manufacturing also
means many lasers look similar but are different in one or two aspects.
Good, Bad and Dangerous Lasers:
Best
Only
a couple of the bigger producers like CNI have world-class quality
control and fit IR filters as standard, the selling price reflects this
extra cost.
Acceptable
Some
vendors add an IR filter before resale, perhaps with their own badge on
the laser, but this is something any buyer can do for themselves do for
a couple of pounds.
Amongst the cheaper 50-200mW lasers there is
considerable variation and these lasers all really need testing to be
sure of what you are getting. It is quite feasible to buy a reasonably
priced 150mW laser that has as little as 10% excess IR if the crystals
are well matched, however, laser to laser variation can be significant
and manufacturers/vendors at the buget end of the market simply cannot
afford to reject too large a proportion of production.
Dangerous
Most
cheap laser pens of 30mW and above are dangerous because nearly all
have much more IR than true green laser output. The danger is that as
the IR exceeds the visible light the user may be squinting trying to use
it to burn or pop something and getting a big dose of IR reflected
back. Because the eye does not respond to IR the full dose burns the
eye.
Green lasers above 50mW need to have decent matched crystals,
alignment that is stable as the laser warms up and adequate heat
dissipation. This is not easily done in very small lasers with the
techniques that are used to make buget lasers - generally if it looks
too cheap for the claimed power it probably is!
Power Scams
IR issues lead to potential problems from some vendors who just buy and sell a laser like other consumer gadgets.
Many
scams simply sell power that is non-existent and rely on the fact that
the majority of customers have no way of checking the power and do not
know what to expect an XmW beam looks like apart from ‘bright’.
Other
scams quote the total power from IR and green laser light and this can
inflate the laser output considerably - if a vendor ‘measures’ the
output with a simple laser power meter that does not distinguish green
and IR wavelengths, these readings can be massive because photosensor
meters are more sensitive to IR and give a falsely high reading. I have
a sample of a laser sold by a manufacturer as 100mW that reads 300mW
using a photosensor meter directly, but only 60mW with an IR filter.
OFL Policy:
OFL
choose lasers up to 50mW where the Laser Power OFL measures does not
vary by more than 20% with and without an external IR filter - on the
basis that reflected or stray IR is not likely to be a safety issue
compared to the green laser light itself.
OFL do not sell high power laser pens because these OFL have tested so far have more IR than green laser light.
For
lasers above 50mW OFL always test a couple of samples before ordering
stock. OFL reject models where the IR output exceeds the visible output
since they seem most dangerous. If the visible laser output exceeds the
IR output, the main danger comes from the laser light which the user is
aware of.
All Lasers above 50mw are PowerChecked before sale using
an IR filter on the power meter so the mW OFL sell is the mW of visible
light.