Actually, what is a diode laser?

Diode lasers and their applications – Part 1: The basics

The question doesn’t seem to be up-to-date. But isn’t the answer supposed to be clear by now? At the very least, this technology has not only been since yesterday in the market and is also firmly established in the industry. What did the composer Robert Schumann once say? “There is no end to learning.” In the ‘century of light’ in which diode lasers also play a key role, a deeper examination of the basics and possible application fields of this technology is most appropriate. Starting today, we want to do this in a mini series.

So, what is a diode laser? Maybe we should start by taking a step back and ask: What is a laser in general? The answer begins with Albert Einstein who first defined the principle of stimulated emission in 1917. It states that an excited electron or molecule can deliver energy in the form of light. This stimulated emission is triggered by supplying energy to an ideally light amplifying material (the so-called laser-active material or medium) and thus bring it to a higher energy level (an energetically excited state). This energy supply is called ‘pumping’ in laser technology. When the excited electrons or molecules then fall back into their initial state, the previously absorbed energy will be delivered as a light beam. Today, the self-evidently used acronym LASER stands for exactly this phenomenon: Light Amplification by Stimulated Emission of Radiation.

Which material is suitable as a laser-active medium? This can be a gas or gas mixture, a crystal or liquid. The expression ‘gas or solid-state laser’ has to do exactly with these material options and the resulting specifications of the laser. And as to the diode laser? Here, the laser-active material is a semiconductor, namely the laser diode. This was already developed in 1962, and it creates the laser light via small crystal plates that are supplied with stimulating energy from a power source; hence, it is pumped by electrical energy.

Thus, the diode laser is a semiconductor laser which therefore defines a particular type of laser. However, at the beginning there was doubt whether you can do something productive with this laser or with any other laser for that matter. Even the US American physicist Theodore Maiman, who had constructed the first functioning laser in 1960 (a ruby laser, i.e. a solid-state laser), did not at first expect much from his invention, and merely considered it “a solution that is searching for its problem”. The laser diode was not valued any better. It was not until the laser technology was initially used industrially in 1969, namely for welding watch springs, did a rethink take place. Indeed, in the following decades laser diodes would go on to conquer important application fields in stage technology and consumer electronics.

Their use in industrial material processing was, however, only possible due to a change in the laser’s construction, namely, the development of the diode bar. This is a heat sink, on which many laser diodes are mounted side by side, so that one diode laser can work with hundreds of single laser emitters. This approach was further expanded during the technological development. High power diode lasers, like those that Laserline produces and sells, are made of stacked diode bars, namely the so-called stacks. In any laser, and depending on the target output power, there are several of those stacks. In all laser diodes, the emitted light in the bars and stacks is optically combined while laser power is thus added to a high-performance system. At the dawn of the industrial diode laser technology, people were already proud of 1 kW. Today, Laserline's diode lasers are offered as standard in power ranges between 500 W and 25 kW, and with special configurations, even up to 60 kW are attained in tests, while 100 kW are possible with the current construction method.

However, diode lasers not only prove themselves with their high output powers, but are also popular because of their high energy efficiency. With a socket output efficiency of about 50 percent, they achieve the best efficiency of all current laser types. And although a beam of hundreds of single emitters must be combined at the diode laser, the technology reaches even with respect to focusing very good values. Laserline's diode lasers prove themselves with their high brilliance; in other words, they combine their high output powers with a high beam quality, i.e. an excellent focus-ability. With the beam parameter product – a physical parameter that provides information about the focus-ability of laser beams – they can attain excellent values of up to 4 mm·mrad.* Special concepts in the laser that were developed by Laserline have made this possible. Together with high-quality processing optics, with which the laser beam can be specifically formed, you receive a variable tool, because the laser beam not only meets the workpiece in its classical round form, but optionally also in its linear, rectangular or square form.

With their high output powers, brilliance, and excellent energy efficiency, diode lasers are suitable for numerous applications. The emphasis here is on the joining, heat treatment or cladding of metals. However, plastics or print products are also processed with the help of diode lasers. Need more details? Please take a look at the next part of our series, where we will talk about applications and branches. But if you can hardly wait, feel free to browse our website already. On our application site, you will find numerous practical examples.

*With the beam parameter product, the tightest cross section (beam waist; in millimeters/mm) and half opening angle (beam expansion; in milliradians/mrad) of the laser beam are multiplied by each other. The lower the product, the higher the focus-ability, and hence better beam quality.