Keysight Technologies 86120B, User Manual

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79
Measurements Applications
Measuring Coherence Length
Coherence length
(L
c
)
The interferogram of the laser being tested is sampled and the envelope of the inter-
ferogram is found. This envelope has peaks (regions of high fringe visibility) at zero
optical path delay and at delays equal to multiples of the laser cavity round-trip opti-
cal length. This is shown in the following figure of the interferogram envelope:
The amplitudes of the peaks decreases exponentially from the largest peak at zero
path delay. The exponential decay constant is defined as the coherence length, Lc.
The curve that connects the tops of the envelope peaks is given by the following
equation:
OPD is the optical path delay and Lc is the coherence length. Thus, at an optical
path delay equal to the coherence length, the envelope peaks are down to
of
their value at zero path delay peak. All envelope peaks found are used to determine
the exponential decay constant (coherence length) using a least squares fit.
Round trip optical
length of diode laser
cavity (2nLd) The average optical path delay spacing of the envelope peaks is measured. This is
equal to the diode laser cavity round trip optical length, 2nLd.
Alpha factor The alpha factor is defined as the height of the first envelope peak away from zero
path delay relative to the height of the envelope peak at zero path delay. The alpha
factor is always between 0 and 1.
decay curve e
OPD
L
c
------------
–
=
1 e
⁄