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Making Measurements
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 wave-
lengths to be displayed below and above the correct wavelength. The
power of these spurious wavelengths is below that of the correct
wavelength. These spurious signals can be eliminated by decreasing the
peak threshold.
Refer to "Defining Laser- Line Peaks" on page 2-
15.
Even when the laser is amplitude modulated, the correct wavelength
and power is displayed.
The spurious wavelengths caused by low frequency amplitude modula-
tion will be located above and below the correct wavelength by the fol-
lowing wavelength spacing:
–
10
×10
2
spacing
=
6
Fλ
where F is the modulation frequency in Hz, and λ is the correct wave-
length 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.
2-34
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