Agilent Technologies Agilent 86120C manual PRBS modulation graph showing raised noise floor

Making Measurements

Measuring Modulated Lasers

The graphical display is useful for locating these spurious wavelengths. Their amplitude will be below that of the correct wavelength and they will be broad, rounded peaks compared to the sharp peak of the cor- rect wavelength. Use the Peak Threshold function to place the dotted line above the spurious peaks so they will not be displayed 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 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 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 power is displayed; the wave- length 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 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.

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