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Applications
The Agilent 11980A can enhance the capability of a lightwave signal analyzer,
such as the Agilent 71400C, 71401C, or 83810A, to allow measurement of laser
linewidth and chirp. Normally the lightwave signal analyzer by itself is capable
of measuring only intensity modulation (IM). However, with the
Agilent 11980A accessory, it is possible to measure the total spectrum of a
modulated single-mode laser. Two examples are described in this section:
• The static linewidth of a single-mode laser is measured using the delayed self-
homodyne technique.
• Chirp is measured by using the gated delayed self-homodyne technique.
Static Linewidth Measurement
Figure 4-1
shows the experimental setup for measurement of the unmodu-
lated linewidth of a single-mode laser. In using the delayed self-homodyne
technique, the underlying assumption is that the delay in the interferometer is
long enough to display less than 10% error in the linewidth due to the coher-
ence length of the laser. Errors greater than 10% occur when the expression
is greater than 5, where ∆ν is the linewidth of the laser under test, and
2π∆ντ
o
τ
is the delay of the interferometer.
o
Thus, the standard Agilent 11980A is capable of measuring static laser line-
widths down to approximately 250 kHz with its delay of 3.5 µs, while
Agilent 11980A Option 005 (with more than 5 km of fiber) can measure line-
widths to 32 kHz. Linewidths narrower than these may be measured with
larger errors in the displayed linewidth (2π∆ντ
and 20% error).
Note
Interferometric errors are in the direction such that the linewidth appears wider than it
actually is.
Applications
= 4 will produce between 10%
o
Operation
4-3
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