Lasers have many uses in our modern world. From measuring fluid flow and reading barcodes, to steering lightning bolts and initiating fusion, lasers are a very useful tool. This article will give a quick overview of what lasers are, how they work, and look into an example of a laser diode and how we can power it.
Contents
Laser Diode History
Lasers have been a staple in science fiction stories since the 40’s, but it only took a few decades before they moved out of the television and into real life. The first type of laser used Ruby as a gain medium for creating light was invented in 1960 by Ted Maiman. For the first design, a flash tube was wrapped around a core rod of ruby and used to excite the electrons. Ruby lasers emit light with a wavelength of 694nm so they look red to us.
Semiconductor lasers were developed in the 1960’s by Robert N. Hall. They are inexpensive and compact compared to the earlier versions of lasers that use ruby or a gas as the gain medium. Alloys of gallium arsenide combined with other metals are used to create semiconductor lasers. When a voltage is applied across the diode junctions it causes stimulated emission of coherent photons. The small sizes of these diodes allowed for mass production and gave power and packaging advantages for many applications.
Laser Diode Theory
Lasers use Spontaneous and Stimulated emissions of photos occur in the gain medium to create coherent light. They are used in barcode scanners, surgery, and also used in unlocking your phone.
In Laser Population Inversion the number of total electrons, the population, that are above the ground state must be enough to create sustained emissions. There must be more electrons at a higher energy level than the ground state as the high energy levels give a greater chance of amplified emissions. The energy for these states can come from externally pumped sources such as another laser diode or a bright lamp . The presence of a population inversion means that the rates of sustained emissions is greater than the absorbtion rates.
The sustained stimulated photon emissions are conditions where there are enough electrons releasing photons to maintain a state of stimulated photon emission. This requires a population inversion in the gain medium.
Laser amplification is when population inversion and stimulated emissions are both present. Laser amplification occurs in a material known as the gain medium which can be a gas such as helium, a solid like ruby, or a doped semiconductor.
Stimulated Photon Emissions occur when an excited electron is already in a high energy state, and it is hit with a photon that has an energy level equal to the difference between the present level and the lower energy level causes the electron to return to its ground state and release a photon. This causes all of the photons that are released to be at the same wavelength, phase, and direction.
Spontaneous Photon Emissions occur when the electrons of an atom are excited to the point that they change their energy state and release a photon.
Energy and frequency of light are related with the following equation
$$E_2-E_1=hf$$
Where $E$ is the energy in $J$, $h$ is Plank’s Constant, $f$ is frequency
Now that we have the basis for the laser theory we will dive deeper into how special diodes can create laser light.
Laser Diodes are a specialized type of semiconductor that produces Laser Light. The junction width is very small and may be as small as a single micrometer. The laser beam that emerges from this gap is then collimated by a lens to form a coherent laser beam. Laser diodes are anywhere between 10%-60% efficient. This makes heat management important for laser diodes, as excessive heat is the cause of most failures for low-powered lasers. (I found this out the hard way as explained below) The degradation of the laser diode increases exponentially with temperature, so for longevity and robustness it is best to not overheat your laser. If you operate the laser diode at a lower power can increase power output and lifetime. Condensation will damage a laser diode and GaN and AlGaN laser diodes will oxidize when exposed to oxygen. Therefore most laser diodes are contained inside a hermetically sealed package. If the wavelength of the laser is longer than expected it may be a sign of an overloaded thermal diode.
Laser diode heat sinks may use either fins or an actively-controlled airflow. Ideally copper is the best heat sink material, but aluminum can also be adequate.
Laser Diode Testing
The laser diode that we are going to look at is a 1mw laser diode with a 650nm wavelength (in the Red part of the visible spectrum). The working voltage is 2.2 to 2.4v, and it comes with a photodiode (PD) in the package. The image below shows the dimensions of the exterior packaging of the laser.
The image below shows the packaged laser diode.
We can look through the window at the front to see the inner components. These are hermetically sealed but have a transparent window to allow the release of light from the diode.
In this next image you can see the photodiode located at the back of the package. We can see a wire connecting the photodiode to pin 3 of the package. From the wiring scheme above we can see that this is the anode.
By tilting the diode a bit, this image also confirms that the wire coming out of the top of the diode is connected to the pin 1 of the package. This is the cathode of the laser diode.
We can also see a wire coming out of the bottom shelf that the diode is sitting on that appears to connect to the wall of the package.
When driven with a very low voltage we can see the intrinsic layer of the diode junction start to produce laser light. The diode is sitting on he shelf near the middle of the transparent window.
With slightly more power we get a stronger light.
Zooming out we can see just how small the diode is as well as how little light is being produced at this low power level. We can also see just how small the laser diode is. For reference, the square pin headers are about 2.5mm apart. In the bottom right corner of the image we can see part of the resistor that is used to deliver power to the diode.
Switching the power on an off we can see how the light-emitting junction changes.
Laser Diode Beam Pattern
Due to the shape of the semiconductor cavity, the laser beam coming from the diode is very divergent with an elliptical shape. We can visualize this by turning up the laser power and pointing it at a nearby wall. This wall is around 1 meter away from the laser diode, and this pattern spans a little more than half of that. I had always thought that lasers were naturally narrow beams of light so this was a fun surprise.
After accidentally burning out the diode to get the previous picture, I switched it out with a fresh one for some lower-power images. In both of these images of the laser beam, we can see the diffraction patterns caused by specks of dust or manufacturing defects in the transparent window of the diode. By measuring the size of these aberrations we could calculate the wavelength of light that was used to make them.
The Collimating Lenses are used to remove the divergence and focus the light from the laser diode into a beam. The lenses that I chose had a 7mm diameter and an 8mm focal length. Aspheric collimating lenses are used to reduce the resulting spherical aberration, for a better collimated beam.
With a collimating lens, the laser diode is able to focus its beam pattern so that the output light rays are parallel. The image below represents the uncollimated light source. You can see the faint shape of the elliptical beam in the center of the red circle. The transparent window on the diode package acts as an aperture stop, preventing light above a specific angle from exiting the package. This is why we see the red circle.
In this image the lens is held much out from the diode than would usually happen to be able to focus the beam on the close index card. This increased distance means that some of the light emitted at larger angles bypasses the lens giving us this cool picture. To focus the light at a further target, the lens is moved closer to the laser diode which has the side effect of capturing more of the emitted light. Most lasers have a focus-able lens that is adjusted by rotating a threaded lens mount to be closer or further away from the diode source.
Laser Diode Circuit Design
It can be easy to damage a laser diode by connecting it directly to a source. The excess current can overheat the diode and destroy it. Most laser diodes need a type of regulator source to prevent this from happening. A constant current source provides a constant current no matter what the load is. It can be used to drive a string of LEDs, bias a transistor, or to power a laser diode. It can be created using a pre-packaged voltage regulator chip like the LM317. The resistor determines the value of the constant current. This circuit still needs to have an input voltage higher than the output voltage to overcome the dropout voltage. This design is not efficient and for high currents, switched-mode supplies should be used instead. Below is an example circuit taken from the LM317 data sheet.
Laser diodes typically use a constant current source. In this case, I’m using an LM317 voltage regulator arranged with a resistor in a constant current source. Typical laser diodes of this size may have an operating current of around 40mA, but just to be safe, I’m limiting the current to around 30mA. This is because I do not have any heat sink ability besides the package itself. This 30mA value was chosen after burning out another diode when running it at high power for one of the earlier photos on the beam pattern. The LM317 is placed between the output of the regulator and the Laser Diode. This node is also connected to the adjustment pin of the regulator. The output current of this arrangement is determined by the equation:
$$I=\frac{1.5}{R1}$$
Therefore to have a desired current of 30mA, the resistor size must be $1.5/0.03$ which is 50 Ohms. The closes resistor that I had was 47 ohms so I substituted that in which would give a current of around 31 mA. This is close enough and still well under the 40mA operating current. I laid this circuit out in EDA software as shown below.
This is a zoomed-out view of the prototype circuit.
PCB Layout Design
The circuit board layout is a straightforward 1-layer design. The second layer is used as a ground plane, and the regulator chip and resistor are mounted to the top layer. We can see the relative size of the diode leads as compared to the header pins, as well as the outline of the diode packaging, as shown on the yellow silkscreen layer. I am only adding the LM317 chip and the resistor to the materials to be added to the board from the assembly plant. I already have the laser diode and the header pins, so I can add them myself when the boards arrive.
A few weeks later, the board arrived pre-assembled at the fabrication plant. All that was left to do was to solder the header pins and the laser diode package.
Sources
admin. “TO-Can Laser Diode Heat Dissipation.” Solid State Lasers and Laser Diodes from RPMC Lasers Inc, August 7, 2023. https://www.rpmclasers.com/blog/to-can-laser-diode-heat-dissipation/.
“Collimating Lenses.” Accessed September 27, 2023. https://www.iadiy.com/collimating-lenses.
Explain that Stuff. “Semiconductor Laser Diodes.” Accessed September 6, 2023. http://www.explainthatstuff.com/semiconductorlaserdiodes.html.
“Fiber Laser Basics and Design Principles (with VIDEOS).” Accessed August 17, 2023. https://www.laserlabsource.com/Solid-State-Lasers/Solid-State-Lasers/fiber-laser-basics-and-design-principles.
Growing Laser Crystals Used in NUCLEAR FUSION!, 2023. https://www.youtube.com/watch?v=UaslPhb9_ls.
Wilk, Stephen R. “A Popular History of the Laser,” n.d.