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. 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. 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. 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).

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).

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