Optical Sensing for Industry

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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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.

FOG (Fiber Optic Gyroscope)

A gyroscope detects angular velocity, or how much a device has rotated, and enables the position of a moving device to be accurately determined. There are various types of gyroscopes, such as mechanical gyroscopes, oscillation gyroscopes, and optical gyroscopes. Optical gyroscopes utilize the Sagnac effect to enable the detects of rotation speeds as slow as one hundredth of the Earth's rotation in principle, and are used for applications such as aircraft because they do not have any moving parts and are highly durable.

Optical gyroscopes include ring laser gyroscopes and fiber optic gyroscopes, and this page describes the principles of a fiber optic gyroscope.

Fiber Optic Gyroscopes utilize the Sagnac effect to detect the angular velocity from the difference in propagation time between light that branches clockwise and counterclockwise from a looping optical path. When the device is not rotating, the light that passes through the optical path clockwise and counterclockwise return to the original position at the same time. When the device is rotating, a difference occurs in the arrival time of the light because it does not return to the original position at the same time. The angular velocity can be determined from this difference in propagation time to detect the moving direction of the device.

An SLD light source is optimal for a fiber optic gyroscope. 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 in both directions to provide more accurate detection of the rotation speed.

Principle of Fiber Optic Gyroscopes

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

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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What Is an SLD? What Is the Difference between an SLD and LD or LED?>