What Is an SLD (Super-Luminescent Diode) Light Source?
Overview
An SLD (Super-Luminescent Diode/SLED) light source provides output power equivalent to a laser diode and a wide
oscillation spectrum width equivalent to an LED (Light Emitting Diode), as well as low coherence. Since it emits
light with a narrow active layer equivalent to a laser diode, it is excellent for joining with optical fiber, and
has properties between an LD and LED. A performance comparison of an SLD and LD/LED and spectral example of SLD are
indicated below.
|
Laser Diode |
SLD |
Light Emitting Diode |
| Emitting State |
End facet reflectivity
R1 < R2 |
Both end facets
Non-reflective coating |
 |
| Emitted Light |
Stimulated emission light |
Amplified spontaneous emission ligh |
Spontaneous emission light |
| Spectral Half Width |
Several nm or less
|
10 to 50 nm |
Up to 100 nm |
| Coherence length |
Several dozen cm to several m |
40 to 50 µm |
Up to 20 µm |
| Optical Output |
Several hundred mW |
Up to 10 mW |
Several mW |
| Fiber Affinity |
Yes |
Yes |
No |
Spectral example of SLD
(Spectral width: 14 nm, Gaussian type)
Spectral example of SLD
(Spectral width: 50 nm, flat top type)
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Difference Between an SLD and LD or LED >
Applications
OCT (Optical Coherence Tomography)
OCT (Optical Coherence Tomography) is technology that uses the interference phenomenon of light to precisely measure
the surface roughness of an object and perform biological tomographic imaging without destroying or contacting the
object. When compared to X-rays, OCT achieves a resolution of several micrometers compared to the 0.1 to 1 mm
resolution of X-rays, and since there is no concern about radiation, it is widely used for medical applications. The
basic structure of an interferometer is indicated below.
Basic Structure of OCT
An SLD light source is optimal for OCT. Since an SLD light source emits spontaneous emission light like an ASE light
source, it achieves a wide spectrum width and low coherence. The wider spectrum width enables a higher measurement
resolution and particularly accurate imaging.
OCT for Industry
- Raw material product inspection: Measuring the thickness and surface roughness of steel plate or film Checking
workpieces for burrs and scratches
- Semiconductor defect inspection: Checking the uniformity of resist thin film, the waveguide height for etching,
and the height of cream solder and adhesive
OCT for Medicine
- Ophthalmology OCT: Ocular fundus cross sectional structure, retina inspection, and eye axial length measurement
- Intravascular OCT: Higher resolution tomography than IVUS (Intravascular Ultrasound)
AFM (Atomic Force Microscope)
An atomic force microscope is a type of SPM (Scanning Probe Microscope) where a probe follows the surface of a
material to detect the atomic force between the probe and the material to generate an image of the material surface.
A scanning probe microscope has an extremely high spatial resolution compared to an optical microscope, and enables
the contours of a surface to be checked at the atomic level. Furthermore, an atomic force microscope, which is a
type of scanning probe microscope, is able to measure an insulating material.
By emitting light from an SLD into the rear side of a cantilever with probe attached and checking the reflected laser
light with a positional sensor, the movement of the probe can be accurately checked to enable observation of a
substance surface at nanometer precision.
Atomic Force Microscope Sensor
An SLD light source is optimal for an atomic force microscope. Since an SLD light source emits spontaneous emission
light like an ASE light source, it achieves low coherence. The low coherence reduces noise due to interference to
provide accurate imaging.
Encoders
Encoders are devices detecting motion and producing an output signal. Encoders normally refer to rotary encoders,
which check the positional changes of a rotating object with a sensor and encode it as positional information, and
linear encoders, which encode positional changes on a straight line.
With an optical type of encoder, electrically subdividing the interference signal by reflecting light on the
microfabricated hologram grating realizes ultra-high resolution and high-speed response.
An SLD light source has a wide wavelength band and outputs low coherence light to suppress interference noise during
measurement, thus the SLD light source is most suitable for high-resolution encoders. Using SLD of single-mode fiber
output separates the light source from the measuring unit which will be hot.
Optical linear encoder (reflection type)
Other
- Displacement measurement (precision measurement/displacement measurement): Measuring displacement and height
using changes in light of optical interference and PSD (Position Sensitive Detector), etc.
- Laser scales
- Current sensors
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Difference Between an SLD and LD or LED >
FAQ about SLD >
