Agilent Technologies 86120C, User Manual

Page: 63 / 294

2-35
Making Measurements
Measuring Modulated Lasers
The graphical display is useful for locating these spurious wavelengths.
Their amp litude will be below that of the correct wavelength and they
will be broad, rounded p eaks comp ared to the sharp p eak of the cor-
rect wavelength. Use the Peak Threshold function to p lace the dotted
line above the sp urious p eaks so they will not be disp layed in the List
by WL or List by Power table.
A laser modulated at high frequency (in the RF or microwave range)
can also cause sp urious wavelengths to be disp layed, esp ecially when
the modulation is of a rep etitive nature such as that of PRBS or
SONET digital formats. In general, no sp urious wavelengths will be dis-
played using preset instrument conditions. The preset condition
includes peak excursion, peak threshold, and wavelength range limit-
ing. However, increasing peak threshold can cause spurious wave-
lengths to be displayed.
Even when the laser being tested is modulated with repetitive formats,
the carrier’s correct wavelength and p ower is disp layed; the wave-
length and p ower of the sp urious sidebands are incorrect.
The grap hical disp lay is useful to see the effects of high frequency
modulation. Without modulation, the noise floor is typically 45 dB
below the laser power. In general, high frequency modulation will raise
the noise floor to about 25 dB below the laser p ower. The noise floor
is typ ically flat, or white. The actual level of the noise floor dep ends
on the typ e of data format and the data rate.
PRBS modulation graph showing raised noise floor.