GaAs, HeNe, GaAlAs… What does it mean, and does it really matter?
By Peter A. Jenkins, MBA
Modern therapeutic laser devices are typically constructed using semiconductor lasers. A semiconductor, or diode, is an electronic component consisting of a p-n junction through which electrical current flows in only one direction.
The p-n junction is essentially a sandwich of two materials; one with a surplus of negatively-charged electrons (n type), and one with a shortage of electrons (p type). There are many different types of semiconductors, such as signal diodes, rectifiers, transistors and, of course, laser diodes.
When current flows in a forward direction (from the ‘positive’ anode to the ‘negative’ cathode) through a rectifier diode, for example, a small portion of the electrical current is used up. This is called the forward voltage drop, or just forward voltage. As we know from high-school physics, energy cannot be created or destroyed, so the current that is used up is not lost, but is converted to heat. Designers of rectifier diodes strive to minimize the forward voltage so as to not waste too much energy.
Laser diodes, however, are designed in such a way as to utilize some of the energy supplied by the forward current flow to produce light, not heat. The electrical current ‘charges’ – or raises to a higher energy state – the electrons in the atoms that create the p-n junction. When those electrons ‘fall back’ to their normal energy state they release a packet of energy called a photon. This happens multiple times and produces the laser beam.
In a Gallium Aluminium Arsenide (GaAlAs) laser diode, pure gallium (Ga) is doped with arsenic (As) to create the n type GaAs alloy that forms the substrate material. This alloy is then doped with aluminium (Al) to produce the p type material, and changing the ratio of Al to GaAs changes the wavelength of light that is produced when current flows through the resulting GaAlAs p-n junction.
The first laser diodes were actually created using Gallium Arsenide junction without the Aluminium, giving us the GaAs laser. These devices operate in pulsed mode, with very short-duration, high-current pulses of electrical energy producing equally short-duration, high-intensity pulses of light, and for many years GaAs was the only type of laser diode used in therapeutic laser products.
As semiconductor laser technology developed, with new alloys of GaAs being created that produced light in continuous beams, different output power ranges and a wide variety of wavelengths, so too did their use in laser therapy, and in the therapeutic laser devices of today one can find examples of almost every different type of laser diode based upon that initial GaAs substrate.
Even very high-powered lasers that are typically used for surgery, and which are also currently being repurposed and promoted for laser therapy, contain GaAlAs emitters. To produce such high powers, multiple GaAlAs emitters are built into a single module that is then connected to an optical fibre along which the laser beam is directed to the patient.
Such laser modules may contain only emitters of one particular GaAlAs ‘alloy’ and, so, produce only a single wavelength, but multiple emitters of different alloys can also be combined into a single module to produce a device that emits a beam with two or more wavelengths through the same optical fibre. These additional emitters may be made of different alloys of GaAlAs and used to produce beams intended for therapeutic use (i.e., working beams), or they may employ a different semiconductor material altogether, such as InGaAlP, to produce a visible red aiming beam.
Typically, the working beam wavelengths of therapeutic Class 4 semiconductor lasers range from 800 nm to 980 nm, with 800–810, 940 and 970–980 currently being the most common.
Of course, not all therapeutic lasers are based upon semiconductor technology. HeNe lasers, for instance, are not semiconductor devices, but are instead made from a combination of Helium and Neon gases and, most commonly, emit light at 632.8 nm in the visible red part of the spectrum. However, HeNe lasers are rarely used as therapeutic devices these days because their cost to produce is comparatively high and their power output is relatively low, and they have all but been replaced by InGaAlP and similar laser diodes that can emit at similar wavelengths and higher powers for a fraction of the cost.
Ok, so now we know a whole lot about the construction of lasers!
How much of this does a laser therapist actually need to know?
In a word, none!
As mentioned previously, the specific ‘alloy’ used to create a semiconductor laser emitter will, among numerous other factors such as the forward current and temperature, determine its operating wavelength.
But, while the wavelength of the laser beam is an important factor in the practice of laser phototherapy, the actual construction of the laser which produces that wavelength is of interest value only to nerds and geeks - it has no clinical relevance whatsoever.
Assume, for a moment, that we have two laser systems that emit beams at exactly the same wavelength, with the same coherence length, same operating mode and the same output power.
In one of these devices a GaAlAs module is used to produce the laser beam, while in the other a different lasing medium altogether, such as a gas or liquid, is used.
Despite the fundamental differences in lasing media, however, the two devices are, effectively, still identical from the perspective of their therapeutic potential and, if used the same way, will produce the identical clinical outcomes…