How are Laser Diodes Used?
Laser diodes are used in industry and medicine to generate laser light from 360nm to 25,000nm. This is an extremely broad range of products, but the applications for laser diodes fall into five or six major categories. The list shown below indicates where laser diodes find application.

635-690nm In the visual light range, diodes are used in medicine to generate light for Photo Dynamic Therapy (PDT), bar code readers and for pumping metal vapor gas lasers. The term pumping means to use laser diodes to generate laser light at a specific wavelength that, in turn, starts up a more powerful gas, dye, or tunable solid state laser. In effect, laser diodes at a variety of wavelengths start-up or pump more powerful lasers.
785nm Pumping exotic metal YAG laser systems.
808nm Pumping Nd:YAG laser systems
830nm In general use for laser printers, laser readers.
904nm Short pulse laser diodes are commonly used in ranging (measuring
distances). Continuous wave 904nm diodes are used in the military for target
illumination.
940-980nm Pumping exotic metal, dye or tunable laser systems.
1300-1550nm Fiber optic communications is the main use.
> 1550nm Pumping highly exotic laser systems

A laser diode has never been produced specifically to generate laser light for use in Low Level Laser Therapy (LLLT). Laser therapy manufacturers have been using whatever laser diodes were available from industrial, military or medical applications. For these applications, the laser diode's wavelength, and various other important parameters, were carefully matched to the design needs of printers, fiber optic transmission systems, ranging systems, target illuminators or PDT. Nothing has ever been done to manufacture laser diodes with the wavelength, half power points, slope efficiency and power output required to optimize LLLT treatment.

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A Laser Diode for LLLT

The Luminex® System has been designed with output powers up to 500mW (average power). A 500mW laser diode generates a great number of photons and the treatment time is far shorter than using a diode with less power. Jan Tuner and Lars Hode indicate the advantage of irradiating with higher power lasers is that their photobiological effect is greater and, consequently, the therapeutic effects are felt more quickly and at greater depth.

They make the following observation: "The (research) literature supports the hypothesis that higher power density yields better clinical results."

Second, the Luminex® System has been engineered to deliver high output power per unit of current, resulting in less heat dissipation, a direct cause of laser diode failures. High efficiency laser diodes result in longer mean time to failure rates.

Third, the laser diode in the Luminex® System has a narrow spread of wavelength at the half power points, providing a sharp peak output power (intensity) with little side lobs (wavelength). The result is maximum output power at one wavelength with the half power points, perhaps at ±5nm from the center.

Fourth, the Luminex® System has chosen a wavelength with ideal penetration characteristics, using higher energy photons to accomplish this work faster. As demonstrated by our laboratory tests, the amount of forward light scatter in tissue is high at 904nm. In shorter wavelengths, the energy of the photons and their absorption nearer the surface of the skin is increased while the amount of deep scatter of laser light is reduced.

Ohshiro summarized the comparative transmission of laser wavelengths in tissue, in vivo, and found that the absorption and penetration characteristics of laser light in the 830nm and 904nm window was preferred as a clinical LLLT tool. The summary compared the absorption and penetration characteristics of laser light from argon-pumped dye lasers at 488-514.5nm to carbon dioxide at 10,600nm.

Another notable reference is the Ohshiro & Calderhead report on the relationship between penetration of laser light and wavelength. The report eliminates the absorption component and gives us a clear picture of the penetration, or deep scatter, of laser light. The peak penetration occurs at about 904nm, which supports our laboratory depth of penetration work.

Over the years, researchers have studied laser light absorption at various wavelengths in different media, i.e., water, skin, liver, and blood. From Charschan's report (see table below), typical values of absorption coefficients can be obtained for some basic components of the human body at selective wavelengths:

Absorption Coefficients vs. Λ (cm-1)

Note the large spread in absorption coefficient values in the table above. The absorption factor for water increases rapidly with longer wavelengths from a valley at about 660nm. At 660nm the absorption factor is approximately 0.0073 cm-1. In the region near 860 to 870nm, the absorption value has jumped 40 fold to about 0.30 cm-1. Similar jumps in absorption factors can be observed for all of the components as wavelength increases.

In summary, due to good forward scatter (depth of penetration) and good absorption (a higher proportion of photons are absorbed) wavelengths between 860 and 870nm are highly desirable. Shorter wavelengths (860-870nm) have the advantage of creating photons with more energy per photon, bringing more energy to the cells being photoactivated.

Selecting a Laser Diode for LLLT
The shape of the intensity of the laser light output vs. the wavelength further supports the choice of the 867nm laser diode. In general, a sharp intensity peak at one wavelength with little spread is the most effective. The laser diodes at 867nm only spread ±1.8nm at the half power points, a low value. This means no jagged or odd shaped, multi-wavelength power spikes in the power output, rather a pure beam of laser light at one wavelength. These desirable characteristics can be traced to the purity of the elements and compounds used in the fabrication of the laser diodes, as well as the lack of contamination in the clean room assembly process.

Many LLLT professionals express an interest in the spot size of the laser beam; some prefer tight, narrow beams (focused), whereas others prefer a spread beam. The Luminex® System utilizes a 500mW diode. The laser light output is 500mW (average power) measured at the lens of the diode. The laser light generated within the diode itself is from a pinpoint source measuring 1µm by 50µm; with laser light generated from a narrow slot called the emitter aperture. The light generated from this pinpoint source is dispersed in a wide elliptical pattern from the front lens. This spread of laser light, or beam divergence, is characterized by two divergent angles. One is parallel to the "slot" and the other perpendicular to the "slot", corresponding to the full angle at the point that is ½ of the peak power. The beam divergence of the Luminex® System's 867nm diode is 8° by 35°, a narrow beam in one axis and a wide beam in the other axis. The distribution of light intensity follows a Gaussian curve wherin the laser light is highly concentrated at the center and grows progressively weaker as one moves to the edges of the ellipse. The power density represents an average for the entire spot size area. Because of the Gaussian distribution of light over the area being irradiated, a 500 mW laser is effective for LLLT professionals desiring high laser light intensity, with the greatest amount of laser light concentrated at the center of the spot.

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Laser Penetration Through Fabric: Why we Recommend the Contact Method.

Medical Laser Systems' engineers did a study to observe what happens to laser output when laser light is shined through clothing. Using both an infrared 867nm laser diode and a red beam 670nm laser diode, the engineers used a cotton t-shirt to demonstrate the dramatic power loss when laser light is shined through fabric.

This is why we recommend that practitioners doing laser treatment always use the "contact method", with the laser probe in direct contact with bare skin.

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Laser as a Therapeutic Tool

Lasers have been used safely as a therapeutic tool for over 30 years. Laser Therapy (LT) differs from the use of lasers in surgery due to a lower output power and reduced energy density. Instead of ablating tissue, LT stimulates cellular activity that improves the speed and quality of healing. In over 1,800 publications worldwide, LT has demonstrated its non-invasive, non-toxic quality, and its ability to augment and in some cases, replace, pharmaceuticals and surgical intervention. LT is most often used as a primary medical treatment, but is also effective as a complement to other modalities, such as needle acupuncture and chiropractic adjustment. Acupuncturists, Chiropractors, Physical Therapists, Dentists, Osteopaths and M.D.'s currently use LT for a variety of problems; including the treatment of acute pain and chronic degenerative conditions, improving the speed and quality of wound healing, and for muscle, tendon and ligament injuries.

Laser Therapy as a Healing Stimulus
Lasers commonly in use for LT operate at a wavelength between 600 and 1060 nanometers (nm). Laser devices in this range are known to be safe because they do not include wavelengths in the lower end of the spectrum, which includes X-RAYs and Gamma Rays that cause destructive ionization in the cell. The first lasers used for LT were gas-tube, helium neon lasers at 632nm. Developed in the 1960's, these lasers were very expensive to purchase and too difficult to operate, limiting their availability to just a few well-financed researchers. In the 1980s however, technological advances allowed for the emergence of relatively inexpensive laser diodes with a wide range of wavelengths. Many Therapeutic Lasers were developed and sold during this period, but were all very low power, around 1 miliwatt(mW). The development of devices for LT has proceeded in such small steps because rather than being driven by the demand for LT devices, the production and availability of laser diodes is driven by the massive demand for laser diodes in technology such as compact disc players, laser scanners, and for a wide range of defense applications. As these technologies matured in the 1990's, they were able to handle much higher power outputs, as high as 500mW, resulting in shorter treatment times for LT applications. The availability of more powerful lasers for LT allowed for the treatment of a number of new conditions, and may explain some early clinical studies that showed non-significant results using LT with very low power, sometimes less than 1 miliwatt(mW). Typical power outputs of diode lasers currently available range from 50mW to 500mW.

Wavelengths and Impact of Penetration and Absorption
Portions of these diode laser wavelengths are visible, from 600 up to approximately 780 or 820 nm. Humans have a declining ability to see light above approximately 820nm. Photon energy increases as the wavelength decreases; conversely, penetration through the skin increases as wavelength increases. Thus, certain conditions may benefit from lower wavelengths where most of the energy is absorbed at the surface, and other conditions may benefit more from higher wavelengths that permit deeper penetration. It follows that an ideal wavelength for treating most conditions would be in the midrange.

LT in the Literature

Below we summarize the wide range of effect of LT. A lengthy bibliography can be accessed by downloading or viewing our Laser Bibliography PDF file. More reference can be located by viewing our Links page that connects you to a wide range of sites with even more literature citations.

Cellular: Cellular homeostatis of the mitochondria is modified by laser irradiation, promoting a cascade of events in the respiratory chain of cytochromes, cytochrome oxidase and flavine dehydrogenase that permit absorption of light. The redox status of both mitochondria and cytoplasm are impacted, resulting in improved production of ATP. When cellular membranes are irradiated, the flow of the membrane ion carriers sodium and potassium are altered, affecting the movement of calcium between cytoplasm and mitochondria. (Karu)¹ . Recent study by (Naviaux)² et. al. Demonstrate the affinity of varying mitochondria to varying wavelengths, promoting an enticing model which matches mitochondria of one tissue type with its most effective laser wavelength. (Naviaux) Cell proliferation, motility and secretion are altered when irradiated with laser with specific wavelength, intensity and dose.(Basford)³

Improved micro-circulation after laser irradiation promotes accelerated recovery after injury, resulting from reduced arteriolar and venular vessles and improved blood-flow in nutritional capillaries and activation of angiogenisis. (Zhao⁴ , Skobelkin⁵ , Kozlov⁶ ,and Telfer⁷ )

Collagen synthesis, proliferation of fibroblasts, faster edema reduction and enhanced lymph flow from LT can accelerate recovery after trauma, through improved edema resolution, regenerated blood and lymph vessels and tendon strength (Lievens⁸ ).

The improvements induced by laser on collagen production lead to significant increases in collagen content and tensile strength of wounds at one and two weeks following laser treatment (Lyon⁹ , Abergel¹⁰ ). Similarly, (Braverman¹¹ ) and (Enwemeka¹¹) found improved tensile strength in laser treated wound and tendon groups. Also, Enwemeka found that Laser Therapy not only improved the rate of healing; but led to a better quality of healing. Shoulder tendinitis showed statistical improvement after LT (England¹² ).

Beneficial Effect on Nerve Cells and the Production of B-Endorphins

Laser light has a highly beneficial effect on nerve cells which blocks pain transmitted by these cells to the brain. Studies have shown that laser light increases the activity of the ATP-dependent NaK pump. In this case, laser light increases the potential difference across the cell membrane moving the resting potential further from the firing threshold, thus, decreasing nerve ending sensitivity. A less understood pain blocking mechanism involves the product of high levels of painkilling chemicals such as endorphins and enkephalins from the brain, adrenal gland and other areas, as a result of stimulation by laser light. Lombard concluded that the neuropharmacological analgesic effects of lasers are likely due to the release of serotonin acetylcholine at the site and in higher centers. This pharmacological effect leads Baxter to conclude that laser is the premier pain reliever compared to other electro-therapeutic modalities.

How is Laser Therapy Administered?

LT is usually conducted in an outpatient clinic setting, and requires no unusual equipment or precautions except that safety glasses are normally recommended for the patient and therapist. In the United States, the Food and Drug Administration (FDA) requires that laser devices be measured at a distance of 20cm, through a 7mm aperture stop; this measurement is applied to the laser's label. This standard measurement permits the therapist to assess the potential for eye hazard.

The laser is held against the skin in a contact mode (Oshiro ), applying the maximum amount of laser light to the area of consideration. Many therapists recommend applying light to firm pressure to the area to distress the underlying blood vessels and tissue to improve the penetration of the energy. The laser is applied at a given power output for a specified period of time, to deliver the proper amount of laser energy, measured in joules. Dosages can range from 1 joule up to 30 or more, depending upon the condition being treated and the schedule of treatments. A wavelength is chosen which meets the absorption requirements of the condition, with wounds and aesthetic conditions benefiting from higher absorption (lower wavelengths), and deep tissue benefiting from deeper penetration (higher wavelengths). Normally, multiple treatments are needed to resolve chronic conditions and injuries. Laser can be directed to acupuncture points, trigger points, nerve endings and directly to the specific injury. Recent findings conclude that, with few exceptions, patients do better when treatment begins as quickly as possible.

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¹Karu, T. Molecular mechanism of the therapeutic effect of low-intensity laser irradiation. Lasers in the Life Sciences, 1988:;2:53-74.
²Naviaux, Robert K. Mitochondrial Metabolism and the Injured Cell Response to Near Infrared Light, at the N.A.A.L.T, April 5, 2003, Bethesda, MD..
³Basford, J. Laser Therapy: Scientific Basis and clinical role, Orthopaedics, 1993;16:541-547.
⁴Zhao, Y, Yasudam S, Yamamoto M, et al. He-Ne lasr irradiation against rat adjuvant arthritis. Jap J Assoc.
Phys. Med. Balneol Climatol, 1990;53 No. 2:95-100.
⁵Skobelkin O, Kozlov V, Litwin G, et al. Blood microcirculation under laser physiotherapy and reflexotherapy in patients with lesions in vessels of low extremeties. Laser Therapy, 1190; 2 No.2; 69-77.
⁶Kozlov V, Terman O Builin V,et. al. Structural and functional basis at laser microvessels interaction. Proc of SPIE, 1993:48-55
⁷Telfer J, Filonenki N, Salansky N. Leg ulcers: Plastic surgery descent y laser therapy. Proc of SPIE, 1993;2086:258-261.
⁸Lievens, P. The influence of laser irradiation on the motricity of lymphatical system and on the wound healing process. InLT. Congress on Laser in Medicine and Surgery, Bologna, June 26-28, 1985.
⁹Lyons R, Abergel R, White R, et al. Biostimulation of wound healing in-vivo by a helium neon laser. Annals of Plastic Surgery, 1987;18:47-50.
¹⁰Abergel R, Lyons R, Castel J. Biostimulation of wound healing by lasers: experimental approaches in animal models and fibroblast cultures. J Dermatological Surgery Oncology, 1987;13:127-133.
¹¹Enwemeka, C. Rodriquez O., Gall N., et. al, Correlative Ultrastructural and biomechanical changes induced in regenerating tendons exposed to laser photostimulation. Lasers in Surgery and Medicine, 1990: (Suppl.2)" 12-19.
¹²England S, Farrell J, Coppock G, et al. Low power laser therapy of shoulder tendonitis. Scand J Rheumatologoy, 1989;18:427-431.
¹³Baxter D, Bell A, Allen J. et al. Low Level Laser Therapy. Current clinical practice in Northern Ireland. Physiotherapy, 1991;77:171-178.
¹⁴Oshiro, T. Low Level Laser Therapy, Wiley and Sons, Bath Press, Avon, U.K., 1988, p.16.