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Measuring Modulated Lasers 3
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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
repetitive nature such as that of PRBS or SONET digital
formats. In general, no spurious wavelengths will be dis-
played using preset instrument conditions. The preset
condition includes peak excursion, peak threshold, and
wavelength range limiting. However, increasing peak
threshold can cause spurious wavelengths to be dis-
played.
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
spurious 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 gen-
eral, 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, the data
rate, and the number of lines present.
PRBS modulation graph showing raised noise floor.