What’s inside the ‘black box’ of a class 4 therapy laser? Are all therapeutic laser devices the same? This article will describe the components that make up a class 4 therapy laser.
Figure 1. Simplified schematic of a class 4 therapy laser. Inside of the “Black Box” are the laser diodes, diode drivers, diode cooling mechanism, electronics and software, and the optical connection from the diodes to the fiber optic cable.
Anatomy of a Laser
Laser therapy treatments are best delivered as an active modality. While there are some devices marketed as being “hands-free” or “unattended”, laser therapy treatments must always be attended by a clinician or staff member due to laser safety concerns and clinical outcomes.
Would a curious patient – if left alone in the treatment room – look directly into the laser beam’s path? Or – would they claim eye injury? If a patient moves during an unattended laser therapy treatment the treatment beam would not be hitting the intended target tissue, and effectiveness would plummet.
The five principal components of a therapy laser include the following: the laser diodes; the drivers that control the diodes; the cooling mechanism; the electronics that control those first three items; and the optics that channel the laser light from the diodes to the patient.
Diodes are like light bulbs – they produce the laser light. Across all types of lasers, there are different media that can produce laser light. The first laser used a ruby crystal, whereas today there are lasers that use a solid, a liquid, or a gas as the lasing medium.
In therapeutic lasers, it is a semiconductor pumped directly with electric current that produces the laser light. In class 4 therapeutic lasers, the most common lasing medium is called “GalliumAluminumArsenide” (abbreviated GaAlAs), for the three elements that compose the diode.
Laser light is special because it is held to a very tight bandwidth, typically plus or minus fifteen nanometers. But most importantly because it is coherent, meaning that the light waves are matched up in space and in time. It’s primarily the coherency of therapeutic laser light that is responsible for the superior clinical outcomes compared to other light-based treatments.
Although some diodes produce multiple wavelengths, the majority of those used therapy lasers are single-wavelength diodes. Just like light bulbs, diodes come in different power output capabilities. With light bulbs, a 40-watt bulb will cost about the same as a 100-watt bulb.
But with semiconductor laser diodes the cost escalates rapidly as the power output increases. For example, two 5-Watt diodes cost substantially less than one 10-Watt diode. Some laser manufacturers take advantage of this cost vs. power curve and utilize a collection of several lower powered diodes.
In football, five 60-pound PeeWee linemen do not equate to a 300-pound NFL lineman. And in laser diodes, combining several low power diodes is not equivalent to a single, powerful diode. (See Figure 3) This is where the idea of a driver comes into play.
The driver is a device that regulates electrical current to another component of the circuit. Appliances in your house are constant-voltage devices. The electrical outlets in your house are wired in parallel and have the same voltage, but not necessarily the same current.
Diodes are constant-current devices, which means that to control several diodes, they must either a) have a separate driver for each, or b) wire them in series. The first option increases the cost and complexity of the overall laser device, so most laser manufacturers will not do that. The second option means the diodes are like a string of Christmas lights - if one in a series goes out, they all go out.
Figure 2: (Left) A parallel circuit elements 1, 2, and 3 are subject to the same voltage, but possibly different current. (Right) A series circuit elements have the same current. If element 1, 2, or 3 burns out, the circuit is not complete and ell elements will not work
A series also means that all diodes in the chain are controlled together, not independently, so it would be impossible to turn one wavelength down or off without doing the same to all of them. Independently controlled laser diodes have a separate driver for each. This is more expensive, but it opens up options to turn wavelengths on and off, to adjust the power output of wavelengths independently, and to pulse (i.e. turn the current to the diode on/off periodically) different wavelengths differently.
Figure 3: (Left) Three diodes in series, with V being the driver. If the three diodes were 5-watts each, the total power output would be 15 watts. However, if one diode burns out, the circuit is interrupted. Also, the laser light from any one diode is not going to be coherent with the output from the other two diodes. (Right) One diode with one driver, V. A 15-watt diode is significantly more expensive to manufacture than three 5-watt diodes. However, this is optimal as the power output and laser coherency are optimal.
Using our light bulb analogy again, think of the old-style incandescent bulbs, they would get very hot when turned on. Diodes also get very hot and keeping them within normal operating temperatures is very important for two reasons. First, the power output of a properly cooled diode will be more consistent. And second the lifetime of a diode is directly dependent on how long and how often it is operated outside its optimal temperature window.
Passive cooling takes room temperature air and circulates it with fans over the hot surface and out of the unit, thereby dissipating some of the heat. An advanced form of cooling places the semiconductor diode on a bed of copper, which is an excellent conductor of heat. Circulating an alcohol/water solution through the copper bed, and then through cooling fins will increase the efficiency of diode cooling, thus keeping the power output stable and increasing diode longevity.
The components described thus far need a fast computer processor, capable memory cards, solid architecture, and good programming. As with computers or cell phones, all of this can be very small and very powerful, but quality and compact portability comes with a price. Some lasers that have meager power dials and timer knobs have little more electronics than a digital watch.
Many class four therapy laser devices have preset protocols. Details about the body region, body part, condition, chronicity, skin tone, and pain level can be inputted into the laser software, giving the proper power, dosage, wavelength power distribution and pulse frequency settings for that particular patient.
Efficient fiber optic coupling is essential to proper delivery of laser. Light is always lost when combining light sources. And the more sources, the more loss, even if the transition is from thin optical fibers to thicker ones. If the light from four laser diodes is combined into one fiber optic, all four can be coupled at once or in stages. Fusing four cylindrical fibers into one is inefficient (high losses) and yields a heterogeneous beam profile (something like a clover leaf, while fusing two-into-one twice will minimize both these unwanted artifacts.
The flexibility and therefore durability of a fiber optic cable is directly proportional to the diameter of the fiber. The thinner it is, the more flexible it is.
The final piece in the optics chain is the handpiece, the component held by the laser technician in the delivery of therapeutic laser treatments to the patient. The most efficient optics coupling in the handpiece delivers the laser light directly to the skin of the patient, but some manufacturers choose to add another transition from the glass optical fiber to a large round glass ball. This transition, by its nature, adds more light loss. Also, a glass ball can get very heavy after delivering laser treatments for a day and the technician will soon tire of the task.
Photobiomodulation (laser therapy) is a scientifically proven modality and will be a valuable modality in all fields of health care in the years to come. Class 4 therapy lasers could possibly be the single piece of medical technology that has changed the lives of more human and animal patients in the last decade.