3-23
IM AQ6373-01EN
Preparing for Measurement
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App
Index
These waveform steps caused by the changes in effective resolution can be corrected by
using the following functions.
• Power Density Display function
When the Power Density Display function is ON, the level axis shows the power
density per nanometer. That is, it is corrected to be able to obtain a constant
measured value regardless of the setting resolution. In this case, the waveform steps
are eliminated because the change in effective resolution occurring at the order
switching point is also corrected to power density per nanometer.
• Resolution Correction function
When the Resolution Correction function is ON, measured data is processed by
the software so that the set resolution of each wavelength matches the effective
resolution. In this case, the waveform steps are eliminated because the amount of
change in effective resolution occurring at the order switching point is also corrected
so that the effective resolution is constant at each wavelength.
Switching High Order Optical Cut Filters
The instrument uses optical filters to cut orders of light that are higher and lower than
the orders to be measured. Therefore, the higher and lower orders of light not to be
measured are kept to lower levels than those in the correct optical spectrum.
There are three types of optical filters. Because the filter is switched for each wavelength
band, there is a maximum of two filter switching locations during a single sweep.
The filter switching wavelengths are shown in the table.
Filter Switching Wavelengths
Filter switching point Switching wavelength
1
450 nm when the center wavelength is set to 460 nm or higher
470 nm when the center wavelength is set to less than 460 nm
or
480 nm when the center wavelength is set to 490 nm or higher
500 nm when the center wavelength is set to less than 490 nm
2
690 nm when the center wavelength is set to 695 nm or higher
700 nm when the center wavelength is set to less than 695 nm
It is impossible to completely eliminate the unwanted higher and lower orders of light by
using optical filters. The formation of “false” spectra from the unwanted higher and lower
orders is suppressed, but not entirely. Therefore it is important to know where “false”
spectra (hereinafter ghost spectra) can appear.
The figure on the following page contains a graph showing the relationship between
wavelengths as they appear on screen and their actual wavelengths. The horizontal axis
represents the wavelength as displayed on screen, and the vertical axis represents the
actual wavelength. The lines on the graph show the correspondence between displayed
and actual wavelengths. The bold line is the correct optical spectrum correspondence
(naturally, the displayed and actual wavelengths match), and the other lines show the
correspondences with the high and low order light–induced ghost spectra.
The numbers appearing next to the thin lines indicate how much the ghost is suppressed
relative to the correct optical spectrum. Note that these values are approximate, and are
not guaranteed.
For example, when a 400 nm light is input, if a horizontal line is drawn from 400 nm on
the vertical axis, a wavelength is displayed at points where the line crosses one of the
lines on the graph. From this you can see that a ghost can appear at 800 nm in addition
to the correct optical spectrum at 400 nm. Also, you can see that this ghost is suppressed
down to a level of 40 dB or lower relative to the correct optical spectrum.
3.7 Important Points During Measurement