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SLD (Super-Luminescent Diode) Light Sources

Optical Sensing for Industry

SLD (Super-Luminescent Diode) Light Sources
Overview

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 Emitting state;Laser Diode
End facet reflectivity
R1 < R2
Emitting state;SLD
Both end facets
Non-reflective coating
Emitting state;LED
Emitted Light Stimulated emission light Amplified spontaneous emission ligh Spontaneous emission light
Spectral Half Width Spectral width;Laser Diode
Several nm or less
Spectral width;SLD
10 to 50 nm
Spectral width;LED
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 example of SLD
(Spectral width: 14 nm, Gaussian type)
Spectral example of SLD
Spectral example of SLD
(Spectral width: 50 nm, flat top type)

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Difference Between an SLD and LD or LED >

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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
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
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 for encoding information. 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 encoder, the light that passes through or is reflected a slitted grating is detected to check displacement. This provides higher precision than a magnetic type encoder and enables high-speed responses because an incremental output method can be adopted to omit arithmetic processing.

An SLD light source is optimal for encoders. 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 with the light that is sent and received to provide more accurate detection of positional changes.

An optical type encoder
An optical type encoder

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 >

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Lineup
Category Series/Model Number Package Optical Output [mW] Center Wavelength [nm] Spectral Half Width [nm]
0.8 µm SLD AS8K215GY30Mpdf CAN 5 830 15
AS8E210GP30Mpdf Cylindrical 1 830 15
AS8B112G230Mpdf Butterfly 2 830 14
AS8B115G230Mpdf Butterfly 5 830 14
AS8B115L240Mpdf Butterfly 5 840 50
1.31 µm SLD AS3E113HJ10Mpdf Cylindrical 3 1,310 53
AS3B119GM10Mpdf Butterfly 15 1,310 55
1.55 µm SLD AS5B125EM50Mpdf Butterfly 25 1,550 60
AS5B310KM50M  NEW
pdf
Butterfly 3 typ.
IF=200 mA
1,550 60 typ.
IF=200 mA
AS5B320EM50M  NEW
pdf
Butterfly 25 typ.
IF=500 mA
1,550 55 typ.
IF=500 mA
1.65 µm SLD AS6B118GM50Mpdf Butterfly 10 1,650 70

SLD unit with drive circuits
Category Series/Model Number Package Optical Output [mW] Center Wavelength [nm] Spectral Half Width [nm]
0.88 µm SLD AM8P8006SL45Gpdf Built-in drive circuit 7 max. 880 50
Library
White Paper: Super Luminescent Diodes (SLD)

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