OSA: Spatial Light Measurement

Published Date: 2020

When measuring light emitted into space, the emitted light needs to be captured into an optical fiber before entered in to an optical spectrum analyzer (OSA). However, there are cases where ordinary optical fibers cannot capture a sufficient amount of light in the OSA, such as the output of a light source that emits light from a surface such as an LED or a fluorescent lamp and pulsed light with a low duty ratio (short emission time and long repetition period).

The applicable models have a free space structure inside the optical input port which allows to use an optical fiber with a core diameter of up to 800 µm, so the use of large core fiber helps the OSA collect more light. Please note that the actual minimum wavelength resolution is limited by the core diameter of the optical fiber used (See lower right table) and for wavelengths over 2000 nm, an optical fiber with low infrared absorption (e.g. fluoride fiber) is required. In OSA that uses an optical fiber in the optical input port, even if the core diameter of the external optical fiber is increased, the amount of light that can be taken in is not increased because it is limited by the core diameter of the internal optical fiber.

Core diameter dependence (Reference 50 μm)

Core diameter dependence (Reference 50 μm)

How optical input structure affects optical power measurement

How optical input structure affects optical power measurement

Limitation of minimum wavelength resolution by optical fiber

Optical fiber		Minimum wavelength resolution (nm)
Type	Core (μm)	AQ6360 Telecom	AQ6370 Telecom	AQ6373 VIS	AQ6374 VIS- exNIR	AQ6375 exNIR-SWIR	AQ6376 exNIR-MWIR	AQ6377 SWIR-MWIR
SM	5	0.1	0.02	0.02 *1	0.05	0.05	0.1	SM	5	0.2
SM	9.5	0.1 *1	0.02 *1	0.02	0.05* 1	0.05 *1	0.1 *1	SM	9.5	0.2
GI	50	0.1	0.05	0.05	0.2	0.1	0.1	SM	13	0.2 *1
GI	62.5	0.1	0.05	0.05	0.2	0.1	0.1	MM	50	0.5
SI	80	0.2	0.1	0.1	0.2	0.2	0.2	MM	80	1
SI	100	0.5	0.2	0.2	0.5	0.5	0.5	MM	100	1
SI	200	1	0.5	0.5 *2	0.5 *2	1	1	MM	200	2
SI	400	2	1	1 *2	1 *2	2	2	MM	400	5
SI	800	2	2	2 *2	2 *2	2	2
Approximate values at reference wavelengths 1 Reference optical fiber type 2 Core size setting: Large

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