Hey I'm Dave, welcome to my shop! Today in Dave's
garage it's time to look at lasers and how they work. And not just any old laser, because we're
making the jump from RGB LEDs to directed-beam RGB lasers, often known as show lasers. These are the
ones you see onstage at your favorite concerts or in larger clubs, and today we'll investigate how
they work in detail as we put my new high powered LaserCube Pro through its paces.
The LaserCube
is a fascinating and enigmatic device that promises to revolutionize the way we think about
color laser projectors. With its sleek, metallic exterior, rugged constructions, and advanced
capabilities, it's hard not to be drawn in by its allure. What exactly is it capable of? How does it
all work? That's what we'll find out today.
The finer points of this laser are fascinating, but
even the basics can impress. As you might guess, to make an arbitrary color of the rainbow it
must combine individual elements of red, green, and blue. But how do you combine three lasers into
a single beam with perfect alignment? How do you move that beam around to draw images? We'll look
such challenges and how they solved them when tale a look inside the beast.
As a little kid I kept
coming back to a singular odd thought experiment. What if you had a sphere of glass made from
one-way mirror, such that you could shine light in, but it couldn't bounce back out. Ignoring
inefficiencies and losses, could you leave it out in the sun for a while to make a light bomb? Would
the sphere flash if you broke it in the dark, and so on?
Think about it long enough, that
eternal internal reflection almost leads you to how a laser works, except we want all of our light
going in a single direction. Let's imagine it in 2D for a moment, as a tube where, like my sphere,
everything is a mirror on the inside. Except we leave a hole open on the end, normally in the
form of a partial mirror. Inside this cavity we place a ruby rod aligned lengthwise. Then we wrap
the ruby rod in the brightest lamp we can muster. When we light the lamp, it emits white light in
all directions. A lot of the light goes directly into the ruby, and any that misses will bounce off
the internally mirrored surface for another chance at the ruby.
Two things happen when the light
finally hits the ruby. First, the incoming light triggers a chain reaction within the ruby, causing
many photons of red light to be emitted. Second, the ruby somehow aligns all of that new light in a
single direction.
But why does the light come out of the ruby in a coherent red beam? Well, Ruby is
a crystal. And it turns out that Ruby emits light mostly along its crystal axis when excited because
of the way the ruby crystal is structured. Ruby is made up of aluminum oxide with a small amount
of chromium ions in its lattice structure. Those chromium ions are responsible for the ruby's red
color and its ability to act as a laser medium.
When a ruby crystal is excited with an external
energy source, such as a flash tube, it causes the chromium ions in the crystal lattice to become
excited and move to a higher energy level. When these ions relax back to their lower energy level,
they emit photons of light. The specific energy levels and transitions involved in this process
dictate the wavelength of the emitted light, which in the case of ruby, is mostly in the red
part of the spectrum. So, the ruby crystal is not just tinting the white light to another color, but
instead, it is absorbing energy and re-emitting it as a specific new wavelength of light.
The crystal structure of ruby is such that the chromium ions are oriented along a specific axis,
known as the crystal axis. This means that when the chromium ions emit photons of light, they tend
to do so in a way that is aligned with this axis. This leads to a highly directional laser beam
that is emitted mostly along that crystal axis.
The key takeaway here is that the ruby isn't
taking the white light and somehow aligning it. It's absorbing the energy from the incoming
light and using it to emit completely new photons of a specific wavelength and a particular
direction. It's more like it's eating all the white light it can get and then pooping out little
photons of red light aligned with the cystal.
Any emitted light that does come out in any other
direction is reflected back into the ruby by the mirrors so that it gets endless chances at
aligning on the crystal axis. In the end, it means that any light emitted by the flash
tube ultimately comes out of the laser orifice as a straight beam of red light. The amount that
is lost to imperfect reflections or from hitting opaque parts like the flash tube assembly is
converted to heat. At the end of a day, modern lasers are between 10 percent and 50 percent power
efficient, meaning that if you put one watt in as random white light, you'll get a half watt out
in laser light.
As a side note, by now you've heard of the successful nuclear fusion test at the
National Ignition Facility, or NIF. They focus the beams of numerous powerful lasers onto a hydrogen
fuel pellet and the energy from those lasers ignites, or fuses, the hydrogen. They achieved
more power output than the laser energy that hit the pellet, a major milestone. But there's one
problem: The lasers at the NIF are much older tech dating back to the 1980s, and are only about
1% efficient. So, while the test was successful in that it produced more net output energy than the
laser energy that was pumped into the fuel cell, it actually took up to 100x as much electricity to
generate the laser light. In other words, the test came nowhere near recouping the amount of energy
put into the system as a whole; it only succeeded in generating more power than the actual laser
light hitting the fuel cell. It's a subtle but important difference. The good news is that modern
lasers would be up to 20 times more efficient.
With our basic ruby laser roughly understood,
what about other colors? If a Ruby is red, how do you make a green laser? Well, it turns out they
generally use a "doubly-ionized neodymium-doped yttrium aluminum garnet". Or as I like to call it,
garnet. And for a blue laser, they typically use gallium nitride.
There are also tunable lasers
that can shift their color output, but they're not well suited to doing what we want to do, which
is to make nright color visuals. The LaserCube, then, doesn't contain a single full color laser -
it contains multiple red, green, and blue lasers that are aligned to produce a single beam of the
desired color combination. But how do they combine three separate lasers into a single beam without
all kinds of convergence and alignment issues?
It turns out the answer to this problem is called
a dichromic mirror. A dichromic mirror reflects light of certain wavelengths, or colors, yet it
allows others to pass directly through unchanged. Also known as a dichromatic mirror or
beam-splitter, it is essentially a type of optical filter that reflects certain wavelengths
of light while allowing others to pass through. It is made by depositing thin layers of dielectric
materials onto a glass or plastic substrate.
The unique feature of a dichroic mirror is
that it has different optical properties for different wavelengths of light. This is achieved
by adjusting the thickness and composition of the layers of dielectric materials deposited on the
substrate, which creates a thin-film interference effect. This happens when light bounces off both
the top and bottom layers of the thin film. If the thickness of the film matches the wavelength
of the light, it will reinforce the light, but if it's offset by half of the thickness, it
undergoes destructive interference which cancels out the light.
For our purposes, a typical
dichroic mirror used in a laser may reflect light in the blue and green part of the spectrum, while
allowing light in the red part of the spectrum to pass through. This makes it useful for combining
or separating different colors of light, allowing them to be directed to different detectors or
used for different purposes. Long story short, you can control what gets reflected versus pass on
through by controlling the thickness of the film relative to the wavelength of the light color in
question.
Perhaps a diagram will help. Let's say we have a green laser beam and we want to add, or
combine, our blue and red lasers with it. We place a first dichroic mirror at a 45 degree angle.
The green laser passes through without change, but the blue laser that we feed in at a 90 degree
angle is reflected and directly aligned with the green. We then use a second dichroic mirror,
which this time allows green and blue to pass while reflecting the red. The red is brought in at
a 90 degree angle as well and it reflected along the common path. Now we have all three lasers
combined into a single path. From here on in, it's point and shoot.
And speaking of shooting,
we need to address some basic laser safety. In the United States, the Food and Drug Administration
(FDA) regulates the use of lasers for medical, cosmetic, and research purposes. The FDA
classifies lasers into four different categories based on their potential hazards
to human health:
Class 1: These are low-power lasers that do not pose a risk of eye or skin
injury and are exempt from regulatory controls. This would be your typical laser pointer.
Class 2
lasers are low-power visible lasers that can cause temporary visual impairment but are unlikely to
cause permanent eye damage. They are also exempt from regulatory controls, because they figure your
blink reflex will save you in time.
Class 3R and 3B are medium-power lasers and can cause eye and
skin injury. They require FDA certification and are subject to certain regulatory controls.
Class 4: These are high-power lasers that can cause severe and permanent eye and skin injuries.
They require FDA certification and are subject to strict regulatory controls.
Lasers that fall
under Class 3R, 3B, and 4 categories require FDA exemption or approval, depending on their intended
use. The exemption is typically granted if the laser is used in a controlled environment and with
appropriate safety measures to prevent accidental exposure or injury.
The maximum safe power output
of a Class 1 laser is 4 milliwatts. By comparison, my LaserCube emits 2500 milliwatt, or 625 times as
much as the legal limit for a class 1. That makes it a dangerous class IV laser, capable of both
blinding your vision and burning your skin.
You might think, "Oh, I'll just throw on a pair
of laser protection glasses". But for which color? After all, those glasses are designed to
protect against a specific wavelength like red, green, or blue. We have to be concerned
with the entire visible spectrum, but if you want a pair of sunglasses
that safely blocks the whole spectrum, you might as well make them out of wood because
you won't be able to see anything anyway.
You might then get clever and throw on your welding
helmet, but if it's auto-darkening, it will do nothing to protect you when the laser hits your
eye but misses the light sensor on the helmet.
Put more simply, there are no cheap and easy
shortcuts to protecting yourself from a color laser. Like a firearm, never point it at humans,
animals, or anything you don't intend to shoot. You must be careful about what's behind
the target area and you must also avoid any specular reflections that would hurl your
beam right back at you. So before using a laser, make sure that you read, understand, and follow
all of the manufacturer's safety information.
This is particularly true if you need to take a
laser apart and operate it with the case open. Laser light leakage could damage your vision,
and for that reason, rather than do it myself I'm going to rely on some internal footage used with
the kind permission of the Brainiac75 channel, which I encourage you to check out for a lot more
laser fun. With that, let's have a look inside of a LaserCube.
Here you can see the lid removed
and a piece of paper set on top; as soon as the laser is energized, we can see where dangerous
light leakage would occur, and why we need to be careful. As if protecting your vision isn't enough
incentive, remember it can equally destroy your camera's CCDs as well.
The LaserCube model shown
here has a single powerful blue laser, two greens, and three reds. When it is energized, we can see
the array of mirrors and lenses that are used to focus and direct the beams. There is a lot of
beam spread straight out of the laser diode, and it's much more pronounced in one axis, known
as the fast axis, and a bit less on the slow acis. To correct this, each beam is first sent through
a lens to gather that spreading light back into a focused beam on the fast axis, and then a
second lens to do the same for the slow axis. A third lens collimates the beams into a straight
coherent beam.
The three red lasers are then sent into a knife-edge array that reflects the three
independent beams into what appears as a single united beam. Similarly, the green lasers are
sent through separate set of mirrors that bring the green beam into alignment with the red.
Finally, the blue laser emits directly into a mirror that reflects it on though the output path.
And since the green and red were reflected to be in alignment with the blue, you now have a single
beam of mixed colors.
This complicated beam path and combination array is possible only through the
magic of the dichromic mirrors which I introduced earlier. As you can see in this summary diagram,
the three reds pass unimpeded through the first dichromic mirror, while the green is reflected.
The second dichromic mirror reflects the blue while allowing the red and green to pass through
unmolested, and the resultant beam is a fully converged RGB laser. By adjusting the power of the
various primary laser colors, the laser is able to mix and match any color of visible light.
This
setup also avoids the need for manual convergence, an annoying process that you might remember from
trying to align the electron beams in an old CRT or movie projector. I imagine getting this right
takes some very careful alignment and tuning at the factory, and it's not a product you'd want
to drop very often... that much I'm certain of!
Knowing just what we've learned so far today
would only get us a colored beam - a powerful custom-color laser pointer, in essence. We
want to move that beam around incredibly quickly and to draw pictures with it. Your first
thought might be to do what the yoke on an old CRT does - bend the beam through electromagnets to
place it where you want. Unfortunately, CRT yokes are designed to bend an electron beam in a vacuum,
not photons. Since photons do not have an electric charge, they do not interact with magnetic fields
in the same way that electrons do. Therefore, a yoke from a television would not be effective
at bending a laser beam of photons. Back to the drawing board.
The way the LaserCube actually
aims the light is through the use of what are known as Galvanometer Scanners. These scanners,
also known as "galvos", are a common method used to steer laser beams in laser beam projectors.
They consist of two small mirrors mounted on rotating shafts that are controlled by small
electric motors or other actuators.
To direct a laser beam, the first mirror (referred to as the
X mirror) rotates along the horizontal axis, while the second mirror (referred to as the Y mirror)
rotates along the vertical axis. By precisely controlling the rotation of these mirrors, the
laser beam can be deflected in any direction, allowing it to be directed to different points on
a screen or other surface.
To approximate what's going on, imagine you had two ping pong paddles
with mirrored surfaces. You'd place one this way and one 90 degrees to the first, and with a little
practice, by rotating your wrists you could send the beam in pretty much any direction you wished.
Ping pong paddles are quite big and wrists are slow, however, so the mirrors in a galvo system
are typically very small and lightweight, allowing them to move very quickly and with high precision.
The stepper motors or other actuators used to rotate the mirrors are also highly precise and can
be controlled with great accuracy in software.
The beam emitted by the LaserCube Pro is 4mm in
diameter. It has a divergence of 1 milli-rad, or thousandth of a radian. The galvos in the
LaserCube can draw at a rate of 35,000 unique points per second. The higher the number, the
sharper corners appear in the final drawing. A slower or lazier galvo would need more time to
draw the same result, so you would wind up with either less resolution in the image or a longer
time taken to draw it. The faster that the galvos can draw the image, the higher the framerate and
the less flicker you will perceive as well.
The LaserCube is controlled by a software package
known as LaserOS. The software package includes literally hundreds of different animations and
effects, and you can combine them in creative ways to create completely new shows. You can
import images and songs and lay them out on an editing timeline. You can download and experiment
with LaserOS on any desktop or Android device, and you get a color preview of what your results
would look like even without a laser. If you have an interest in tinkering with it, simply
stop by LaserOS.com and download a copy.
Remember I'm mostly in this for the subs
and likes, so please be sure to leave me one of each before you go today!
If you have any
interest in matters related to autism, asperger's, or ASD, check out the free sample of my book
on Amazon, Secrets of the Autistic Millioniare. It's got nothing to do with money and everything
to do with how to live a successful life on the spectrum. It's everything I know now about autism
and ASD that I wish I'd known back then.
In the meantime and in between time, I hope to see you
next time, right here in Dave's Garage.
