YASKAWA AQ6370B - Page 54

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IM 735302-01E
Preparing for Measurement
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App
Index
Input Optical Fiber Numerical Aperture (NA) and Level Measurement Values
The level measurement error of the instrument changes as shown in the figure below,
according to 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(SSMAtypeinJISC6835,withPCpolishing,9.5μmmodefielddiameter,
and 0.104 to 0.107 NA). Even if a single-mode optical fiber is used, the level accuracy
will be outside the specifications if the NA is not in the range shown above.
Input optical fiber numerical aperture and level error (typical characteristics)
-0.50
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
0.40
0.101 0.102 0.103 0.104 0.105 0.106 0.107 0.108 0.109 0.110
0.50
Level error (dB)
Numerical Aperture
Absolute Level Accuracy
Theinstrument’sabsoluteleveliscalibratedwitha9.5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.
3.8 Important Points During Measurement