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IM AQ6373-01EN
Input Optical Fiber Numerical Aperture (NA) and Level Measurement Values
Level measurement error can occur in the instrument depending on the numerical
aperture (NA) of the optical fiber connected to the input connector. The instrument’s
absolute level is calibrated using a 9.5/125 μm single-mode optical fiber (typical 0.14
NA).
Absolute Level Accuracy
The instrument’s absolute level is calibrated with a 5 μm single-mode optical fiber.
In terms of actual capability, even single-mode optical fibers that do not have a core
diameter of 9.5 μm provide nearly the same level of accuracy.
Multimode (GI) fiber provides a relatively accurate spectrum if the light source is low-
coherent light such as white light, natural light, or an LED. If the light source has high
coherency as in the case of a laser beam, interference will occur inside the optical fiber,
and the intensity distribution of light radiating from the fiber tip will vary according to the
fiber form. As a result, the spectrum (measurement level) may fluctuate if the fiber is
moved.
When an optical fiber with a large core diameter or large NA value is used, a low fraction
of the light emitted from the optical fiber is received. Therefore, the measurement level is
lower than the true value, but the optical spectrum is accurate in relative terms.
Level Accuracy below the Cutoff Wavelength (Short Wavelength) of a
Connected Optical Fiber
With wavelengths at or below the cutoff level of the connected fiber, light propagates
through the optical fiber in multiple modes. When high-coherent light from a light source
such as a gas laser or DFB-LD light source propagates in multiple modes, speckle noise
contained in the optical fiber output light may become unstable by the optical fiber’s form,
resulting in an inaccurate measurement level.
In such cases, improving the coupling between the light source and the optical fiber will
lessen the level inaccuracy.
Measurement Sensitivity and Vertical Axis Effective Range
When the measurement sensitivity is set to NORMAL HOLD, the internal amplifier has
a fixed gain. Five different gains are set automatically according to the reference (REF)
level setting. However, the effective range of measurement data is limited to the following
range, using the reference (REF) level (dBm) as a reference.
REF-20 dBm < (effective range) < REF+10 dBm
If the level scale is set to 10 dB/DIV, the display would exceed the effective range, so the
areas at 10 dB from the screen stop and 20 dB from the bottom are inaccurate.
When measurement sensitivity is set to NORMAL HOLD, we recommend setting the
level scale to 5 dB/DIV or less.
Under the measurement sensitivity settings NORMAL AUTO, MID, and HIGH 1–3, an
automatic gain is used, permitting measurements over a wide level range through a
single sweep. Select the appropriate sensitivity level based on the light reception level
required for the particular measurement application.
Brightness of the installation location
The instrument’s shielding specifications indicate use in common office or factory
environments (with illumination of 300 to 1000 Ix). Therefore, depending on the
illumination in the installation location, the instrument may receive some ambient light
which can affect measurements of light with small level measurement values. In such
cases, turn lights around the instrument Off to reduce inaccuracies.
3.7 Important Points During Measurement