What Is an SLD Light Source?
An SLD light source is a broadband light source that has the characteristics of both a light-emitting diode (LED) and a semiconductor laser (LD).
SLD stands for “super luminescent diode” and combines a broad spectrum like a light-emitting diode, and high brightness like a semiconductor laser.
However, unlike semiconductor lasers, SLDs are also characterized by low coherence. Because of these characteristics, SLDs are widely used in measuring machines and sensor devices.
Applications of SLD Light Sources
SLD light sources are used in the following equipment, taking advantage of the two characteristics of LEDs and LDs.
OCT
OCT, also called optical coherence tomography, is an instrument that uses light interference to measure a tomographic image of an object. It can measure the interior of an object without touching it. Compared to X-rays, which can also observe the inside of an object, OCT has higher resolution and no risk of radiation exposure.
Atomic Force Microscope
This is a microscope that observes surface conditions by moving a pointer on the surface of a material and measuring the interatomic force between the pointer and the material. Compared to an optical microscope, it has a very high resolution and can even check the atomic-level irregularities on the surface.
Principle of SLD Light Sources
This section describes the principle behind the light emitted by an SLD light source.
Like LEDs and LDs, SLD light sources emit light when a forward voltage is applied to the pn junction. When they are excited, there are many electrons in the conduction band and holes in the valence band. The energy generated in the process of recombination of these electrons and holes is emitted as light.
Thus, SLDs, like LEDs and LDs, generate light, but they differ from the two in that they amplify the generated light. The spectral width of SLDs is wider than that of LDs, but narrower than that of LEDs. Thus, SLDs can have characteristics that fall somewhere in between those of LEDs and LDs.
This principle enables SLDs to emit light with a broad spectrum like LEDs and coherence like LDs, making them an ideal light source for medical and research applications.
Application of SLD Light Source to OCT
OCT stands for optical coherence tomography, a technology that uses light interference to measure the surface roughness of an object or a tomographic image of a living body in a nondestructive and non-contact manner.
OCT makes it possible to obtain cross-sectional images of the human body without taking X-rays.
The SLD Light Source is an indispensable component of OCT.
OCT light sources must have low temporal coherence and high spatial coherence.
Temporal Coherence
First, let us introduce temporal coherence. Laser light is a monochromatic light emitting at a fixed wavelength and propagating as a sinusoidal wave in the direction of light propagation. Since the intensity of this sinusoidal wave is maintained over a long distance, multiple beams of light are generated along the direction of light propagation that are separated by an integer multiple of the wavelength. This is observed as noise, and LED light is more suitable for OCT light sources than laser light because of its lower temporal coherence.
Spatial Coherence
Next, we introduce spatial coherence. Laser light has excellent spatial linearity, so it is easy to illuminate a desired light intensity. However, in the case of LED light, it is difficult to illuminate a desired object with a desired amount of light because of the strong diffusion of light.
Therefore, SLD light sources, which have both properties, are attracting attention as a light source of low temporal coherence and high spatial coherence.
Emission Wavelength of SLD Light Sources
As with LDs and LEDs, the emission wavelength of an SLD Light Source is determined by the band gap of the semiconductor material used. Semiconductors with a large band gap enable the development of light sources with short wavelengths, while semiconductors with a small band gap enable the development of light sources with long wavelengths.
As mentioned above, the SLD light source is expected to be applied to OCT. This is because there is a region where water absorption is minimal at wavelengths between 1 and 1.1 μm, commonly known as the biological window, and high SNR may be obtained by applying OCT near the biological window while reducing the effect of water in the human body.