A diode by any other name ...

March 1, 2007

We have all heard the word diode, but few of us, including those who use lasers, actually understand what the term means and how common this device is.

We have all heard the word diode, but few of us, including those who use lasers, actually understand what the term means and how common this device is.

Every day, we encounter at least one example of a diode laser. Diodes can be found in laser pointers, writable CD/DVD drives, DVD players, bar-code scanners, fiber optics for telecommunication, range finders, laser printers and medical lasers. The diode is the most common type of laser, with approximately 733 million in sales in the year 2004 alone.1 So what exactly is this fabulous piece of machinery?

Terminology

A laser diode can be used as a source of energy for another lasing system. If the diode is the actual lasing system then it is referred to as a diode laser.

As we have discussed in previous columns, true lasers are devices that excite photons, with the end result being a monochromatic, collimated and coherent beam of light. Many devices that are called lasers are not truly lasers.

To be a laser, the following three criteria must be satisfied: monochromaticity, collimation and coherence. To produce any kind of laser light, a device needs an excitation source.

There are several ways that these photons of light can be created. The first laser, invented by Theodore Maiman and first operated in 1960, generated light using a flash lamp as its excitation source.2 Lasers can be pumped by optical energy (such as from a flash lamp), chemical energy, thermal energy or electrical energy. The first diode laser was produced by Robert Hall with General Electric in 1962 and was electrically pumped.3

A laser diode can be used as the excitation source or pump for many devices. If the diode is the actual lasing source then it earns the name diode laser.

The active portion of a diode device is about the size of of a pinhead, and some are as small as a grain of sand. A diode consists of a pair, or pairs, of wafer-thin sheets arranged in parallel fashion. Atoms literally bounce between these plates to form energy. Within this tiny device, light energy in the form of photons can be generated more efficiently than with any other stimulator.

The cost of this mechanism is affordable when compared with other light generators, and the technology is very durable. Typically, flash lamps will need to be replaced after 500 to 2,000 hours. A diode, on the other hand, has a lifetime of around 30,000 hours.4

All of these features make the diode a very popular energy generator. Many lasers today use other lasers as energy pumps. Diodes, because of their efficiency at producing energy, have found a niche in this area. These lasers are referred to as diode-pumped lasers. Diode-pumped lasers are efficient energy generators, housed in a relative small device. An example of one of these is the Fraxel (Reliant) laser, which is a diode-pumped fiber laser.

Lasers are named for the active medium in which lasing takes place. The active medium is actually the place where the atoms are collected and stimulated to emit light. Laser types, and hence their active media, include: gas (CO2), solid-state (ruby, Nd:YAG), excimer (UV), dye lasers (pulsed dye) and semi-conductors or diode lasers.

Frequently in dermatology people say they use a diode laser for hair removal. Because we understand lasers and dermatology, we know that what they really mean is that they are using a diode laser that generates light from 800 nm to 810 nm. Other true diode lasers used in dermatology include the 940 nm Dornier laser, used for vascular lesions and endovenous closure.

There are many diode lasers in medical practice today. The most common in dermatology is the 800 nm to 810 nm diode laser which is considered to be the gold standard for hair removal. Diode lasers have been used in dermatology for hair removal, treatment of vascular lesions, spider veins, endovenous closure, photorejuvenation/nonablative laser, acne scarring and cellulite correction. The wavelength emitted will determine the use for the laser, and this is set by the metallic composition of the diode plates themselves.