Keysight Technologies 86120B, User Manual

Page: 62 / 273

60
Using the Multi-Wavelength Meter
Measuring Modulated Lasers
Measuring Modulated Lasers
Lasers modulated at
low frequencies 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 wavelength. These spurious signals can be eliminated by
decreasing the peak threshold.
Refer to “Defining Laser-Line Peaks” on page 52 .
Even when the laser is 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 spac-
ing:
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 spuri-
ous wavelengths will be at 1535 and 1565 nm.
Low frequency (10 kHz) AM modulation graph showing rounded sideband spurs.
spacing 6
10 –
×
10 F
λ 2
=