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Lasers modulated at
high frequencies
Directly modulated
lasers
The graphical display is useful for locating these spurious wavelengths. Their ampli-
tude will be below that of the correct wavelength and they will be broad, rounded
peaks compared to the sharp peak of the correct wavelength. Use the Peak Thresh-
old function to place the dotted line above the spurious peaks so they will not be dis-
played 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
spurious wavelengths to be displayed, especially when the modulation is of a repeti-
tive nature such as that of PRBS or SONET digital formats. In general, no spurious
wavelengths will be displayed using preset instrument conditions. The preset condi-
tion includes peak excursion, peak threshold, and wavelength range limiting. How-
ever, increasing peak threshold can cause spurious wavelengths to be displayed. To
control the wavelength range, refer to
Even when the laser being tested is modulated with repetitive formats, the carrier's
correct wavelength and power is displayed; the wavelength and power of the spuri-
ous sidebands are incorrect.
The graphical display 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 power. The noise floor is typically flat, or white. The actual level of the
noise floor depends on the type of data format and the data rate.
PRBS modulation graph showing raised noise floor.
Directly modulated lasers exhibit a linewidth that is broadband. To measure directly
modulated lasers,
refer to "Measuring broadband devices and chirped lasers" on
page
46.
Using the Multi-Wavelength Meter
Measuring Modulated Lasers
"" on page
44.
61
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