Optical Device Guide

Difference Between an SLD and LD or LED

An SLD is an optical device that provides output equivalent to an LD (Laser Diode) and low coherence equivalent to an LED (Light Emitting Diode), as well as a wide spectrum width. Since SLDs reduce coherence noise and enable high-precision measurement, they are used in a wide range of sensing fields, including photo-application measurement and medical imaging.

Comparison of SLD and LD/LED Properties

The table below provides a performance comparison of an SLD and LD/LED.

	Laser Diode	SLD	Light Emitting Diode
Emitting State/th>	End facet reflectivity R1 < R2	Both end facets Non-reflective coating
Emitted Light	Stimulated emission light	Amplified spontaneous emission light	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

SLD Operation Principle

The operation principle of an SLD involves generating spontaneous emission light, then amplifying and outputting that light with optical gain in the active layer. The figure below depicts this schematically.

The spectral width of an SLD is wider than that of stimulated emission light such as LD light. However, since spontaneous emission light such as LED light is amplified by optical gain with a limited bandwidth, the spectral width of an SLD is narrower than that of an LED. As indicated here, the spectral width of an SLD is between that of an LD and LED. The graph below depicts the SLD spectrum in the 800 nm range, as an example.

Benefit of Using an SLD

The benefit of using an SLD is that it has lower coherence than the light of an LD, which enables coherence noise to be reduced. For example, if an SLD is used for light interference measurement, it reduces diffused reflection (speckle noise) on the surface of the object to measure.

Since it has spectral performance with a certain width, it enables high resolution to be obtained for interference measurement. Anritsu provides broadband SLDs developed for OCT.

Since an SLD also has directivity like an LD, it enables light to be focused to optical fiber and sent over a long spatial distance. Therefore, positional light sources can be created in locations away from the light source, which makes the sensor less susceptible to the surrounding temperature when performing sensing. Anritsu provides SLD cylindrical modules that have light already focused in optical fiber, as well as SLD butterfly modules.