20th Day of Christmas: Diode Laser

This Advent during the 24 days leading up to Christmas I thought it would be fun to have a quick introduction of scientific apparatus, in particular tools used in atomic physics research. This is a way to say thanks to their hard work for our research, and a look at how many things are really needed to make a laboratory work!

Diode Laser

After so many optical elements, I really ought to get to lasers in this series, since they make the lab go ’round (and ’round). The most common ones, I believe, are diode lasers, or semiconductor lasers. In essence they are not much different from ubiquous laser pointers, except all the effort that goes into making the source more stable, more reliable, more controllable, higher performance.

The laser diodes come in a small package usually, that contains the piece of semiconductor that will generate the light beam, once there’s current flowing through it. The package is metal for good thermal conductivity – these devices can require quite a bit of cooling, both because how much they heat up during use, and both to have their temperature stabilized. The light leaves through the front window, but actually there’s a “beam” that goes backwards too – but only within the package, onto a photodiode to measure the output of the laser diode for diagnostics.

Laser diode in its package, with 1¢ for scale
Laser diode in its package, with 1¢ for scale (source)

A quick look into the insides of that package, if someone cuts away the outside very carefully, shows this picture below. The light comes from the little black piece in the middle, sitting on the gray block.

Laser diode stripped to the core, the power's in the little black part in the center
Laser diode stripped to the core, the power’s in the little black part in the center (source)

The way these lasers work, the light generated inside has to pass through many times. That is enabled by a setup similar to a reference cavity, and there are a number of different type of devices based on how this cavity is created.

For example, Fabry-Pérot diodes have their front and back surface turned into mirrors, and the diode itself is the reference cavity. Temperature changes change the length of the device and its refractive index as well, effectively adjusting the resonance wavelength – the output wavelength of the laser. This can be useful to tune the device, but not that useful when the output mode-hops from the wavelength being used to the next allowed value because of environmental changes. The FP diodes need pretty good temperature correction, and when trying to find the right wavelength to be used, we have to search in 2 variables: temperature and electric current.

Another design is the anti-reflection coated diode, which has a mirror on the back face, but the front is made “anti-reflective”, transmitting as much light as possible. Then we can use another, external mirror (or mirror-like object like a diffraction grating) to create the reference cavity. These are the “external cavity diode lasers” (ECDL), and one of the designs was mentioned in a writeup here a while ago. The ECDL design allows narrower laser linewidth (more pure “colour”), and controlling the output  in different ways, making these lasers versatile and quite good for a lot of use cases.

There are other designs as well, such as distributed feedback (DFB) diodes, when a diffraction grating (similar to an ECDL) is actually built into the diode (similar to an FP diode). These make devices like fiber lasers possible: while FB and ECDL lasers can be coupled into an optical fiber, DFB lasers can be create inside the fiber, making a much more integrated and different design.

Diode lasers can be quite small pieces and flexible in their use because of that. The most common designs (for FP and ECDL types) are actually relatively straightforward – now, that a lot of smart people spent a lot of time developing them. Almost any lab that has access to a machine shop could make a decent working model (though also depending on the quality of the research that the lab is aiming to do).

A ECDL box (blue) and its control electronics
A ECDL box (blue) and its control electronics

While the laser boxes themselves are pretty small, they need a lot of extra equipment most of the time. The laser diode current controller can be quite complex (since very precise electric current needs to be applied, with as little noise as possible). Then there are feedback loops, wavelength locking circuits, all the optics needed by the wavelength locking and stabilization… The whole setup likely takes up 50x30cm and more on the optical table, and a couple of large boxes on the shelf above the table for the control electronics.

Just like keeping any other machine in working order, they also need a number of tools for diagnostics and adjustments: power meters, beam profilers most often. Also, noting in the lab book the usual parameters during the experiment (supply current, diode temperature) helps to see how the health of the diode is going. They have a limited lifetime, shortened by high currents, high temperatures, and other environmental factors. Fortunately, diode lasers are usually made so that replacing the diodes is a quite routine task – even if it can be a quite nervous event for a student who does that in the lab.

I want to highlight laser safety. These diode lasers usually class 3B types of devices, which means direct light is dangerous, but scattered light (e.g. from a piece of paper or the barriers around the optical table) is not harmful. Still, laser safety goggles are highly recommended!

This is of course is just scratching the surface, and would take a long time to cover everything laser related. Lasers are everywhere in science now (not just in atomic physics, from particle physics to biology, from chemistry to material science…), and also in industry. I think it will just get better from here, and laser techniques will help us to a lot of discoveries. I certainly cannot get tired of them – there’s plenty to learn!

Further info:

If you liked this, come back tomorrow for another apparatus! And send this link to someone who you think would be interested! :)