Laser & Photo-Medicine Definitions

Super-Luminous/Superluminescent Diode (SLD)

Super-luminous - or superluminescent - light emitting diodes are an optical source whose properties are intermediate between LEDs/IREDs and Laser Diodes. In the field of laser physics, the terms super-luminous and superluminescent mean 'amplified spontaneous emission': the emission of flourescence which experiences significant optical gain within the emitting device.

SLDs are similar to laser diodes, in that they contain an electrically-driven p-n junction and an optical waveleguide, but lack optical feedback so no laser action can occur. Optical feedback in such devices is suppressed by tilting the output facet of the p-n junction relative to the waveguide, and by using anti-reflection coatings. SLDs use a double heterostructure to confine the active region under conditions of high current density, creating a population inversion and enabling amplification of light. However, as SLDs do not have positive optical feedback, only spontaneous emission of radiation is achieved.

The optical output of an SLD is more powerful and more sharply confined than a standard LED, but not as monochromatic, directional or coherent as a laser diode. Most superluminescent diodes emit radiation in the wavelength regions around 800 nm, 1300 nm, and 1550 nm, however, other wavelengths are available. The optical bandwidth is usually some tens of nanometers, sometimes even above 100 nm. The coherence length are often a few tens of microns, sometimes even only a few microns. The typical output power range for SLDs is a few microwatts to a few tens of milliwatts, similar to that of a single-mode laser diode.

SLDs are supplied in device packages similar to those used for laser diodes, such as butterfly mounts (Figure 1) and metal cans (Figure 2). The most common packages for laser diodes used in light therapy devices are 5.6mm and 9mm metal cans (Figure 2). Conversely, LEDs and IREDs are almost always mounted in molded plastic packages (Figure 3) which are not capable of the higher heat loads generaterd by SLDs and laser diodes.

Figure 1	Figure 2	Figure 3

Radiation emitted by SLDs has a much shorter coherence length than that produced by lasers, thus they can produce speckle-free beams. This a feature that is of use in telecommunications, sensor technologies, and some imaging applications such as optical coherence tomography, but of no practical importance in light therapy.

Despite the marketing claims of many manufacturers, SLDs are not used in Light Therapy devices due to their lower power (compared to similarly-priced laser diodes), and larger package sizes (compared to LEDs) which, combined with their higher cost, make them a low-value, no-benefit proposition compared to standard Laser Diodes or LEDs/IREDs.

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