Section 16: Metrology
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Laser pulse Signal decay
Intensity
Time
Resonant cavity
Figure 16.1: Cavity ring down spectrometers measure the intensity decay
rate in the resonant cavity, allowing for higher accuracy measurements
than techniques that just measure absolute intensity values
Metrology is crucial for ensuring optical components consistently
meet their desired specifi cations and function safely. This reliability is
especially important for systems utilizing high-power lasers or where
changes in throughput may cause inadequate system performance. A
wide range of metrology is used to measure laser optics including cavity
ring down spectroscopy, atomic force microscopy, diff erential interference
contrast microscopy, interferometry, Shack Hartmann wavefront
sensors, and spectrophotometers.
Section 16.1:
Cavity Ring Down Spectroscopy
Cavity ring down spectroscopy (CRDS) is a technique used to determine
the composition of gas samples, but for laser optics it is used to make
high sensitivity loss measurements of optical coatings. In a CRDS system,
a laser pulse is sent into a resonant cavity bounded by two highlyrefl
ective mirrors. With each refl ection, a small amount of light is lost to
absorption, scattering, and transmission while the refl ected light continues
to oscillate in the resonant cavity. A detector behind the second
mirror measures the decrease in intensity of the refl ected light (or “ring
down”), which is used to calculate the loss of the mirrors (Figure 16.1).
Characterizing the loss of a laser mirror is essential for ensuring a laser
system will achieve its desired throughput.
The intensity of the laser pulse inside the cavity (I) is described by:
I0 is the initial intensity of the laser pulse, τ is the total cavity mirror loss
from transmission, absorption, and scattering, t is time, c is the speed of
light, and L is the length of the cavity.
The value determined in CRDS is the loss of the entire cavity. Therefore,
multiple tests are required in order to determine the loss of one mirror.
Two reference mirrors are used to make an initial measurement (A), and
then two more measurements are taken: one with the fi rst reference mirror
replaced by the mirror being tested (B) and one with the other reference
mirror replaced by the test mirror (C). These three measurements
are used to determine the loss of the test mirror.
16.1
16.2
16.3
16.4
Measuring Refl ectance Measuring Loss
Mirror #1 #2 #1 #2
Nominal 99,98% 99,99% 200 ppm 100 ppm
Uncertainty ±0,1% ±0,1% ±10% ±10%
Loss Range – – 180 - 220 ppm 90 - 110 ppm
Min. Refl ectance 99,88% 99,89% 99,978% 99,989%
Max. Refl ectance 100,08% 100,09% 99,982% 99,991%
Sensitivity (dR/R) 1 x 10-3 1 x 10-3 2 x 10-5 1 x 10-5
Table 16.1: The sensitivity of measuring the refl ectance of a mirror
directly with an uncertainty of ±0,1% is two orders of magnitude greater
than measuring the mirrors loss with an uncertainty of ±10%. This demonstrates
that loss measurements for highly refl ective mirrors are much
more accurate than refl ectance measurements