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3-22 Measuring Modulated Lasers
Measuring Modulated Lasers
A laser that is amplitude modulated at low frequencies
(for example, modulated in the audio frequency range)
can cause spurious wavelengths to be displayed below
and above the correct wavelength. The power of these
spurious wavelengths is below that of the correct wave-
length. These spurious signals can be eliminated by
decreasing the peak threshold.
laser-line peaks" on page
amplitude modulated, the correct wavelength and
power is displayed.
The spurious wavelengths caused by low frequency
amplitude modulation will be located above and below
the correct wavelength by the following wavelength
spacing:
spacing
where F is the modulation frequency in Hz, and λ is the
correct wavelength in nm. For example, an amplitude
modulation of 10 kHz on a 1550 nm laser will produce
spurious wavelengths spaced by 15 nm from the correct
wavelength, and the spurious wavelengths will be at
1535 and 1565 nm.
Low frequency (10 kHz) AM modulation graph showing
rounded sideband spurs.
The graphical display is useful for locating these spuri-
ous wavelengths. Their amplitude will be below that of
the correct wavelength and they will be broad, rounded
peaks compared to the sharp peak of the correct wave-
length. Use the Peak Threshold function to place the
dotted line above the spurious peaks so they will not be
displayed in the table.
Refer to "To define
3-11. Even when the laser is
–
10
×10
2
=
Fλ
6
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