Laser-line
bandpass flter
Object
SpectrometerLaser
Optical filter performance keeps
improving—measurement
techniques must keep pace
AMBER CZAJKOWSKI and STEPHAN BRIGGS
The telecom boom of the late 1990s
drove advances in many different
technologies, not the least being optical filter technology. Telecom needed
dense wavelength division multiplying
(DWDM) filters with superior perfor-
mance, which spurred optical coating
innovation. Since then, life sciences
and space exploration have continued
to drive performance requirements and
revolutionize expectations of how we
can manipulate light. These fields have
further challenged filter manufacturers to deliver on ever-more demanding specifications. System designers
now often demand tight requirements
such as deep opti-
cal density (OD)
blocking coupled
with nearly vertical transitions
from low signal to
high transmission.
Filter manu-
facturers are be-
coming adept at
meeting these tough re-
quirements—in fact, fil-
ter fabricators often
manufacture their com-
ponents to specifications
so tight they can’t be rou-
tinely measured. The
question with today’s
high-performance filters
becomes: How do you
verify they meet their specifications?
Is your filter made to spec?
Verifying that your filter is made to
spec is not simple. Most commercial
spectrophotometers do not simply pro-
vide an accurate spectral curve; the op-
erator must have some insight into the
measurement method. In other words,
without proper understanding of how
the spectrophotometer works, and op-
tical limitations of its components, the
measurement may not be complete-
ly accurate using just “standard” set-
tings. That puts a heavy burden on
the user to acquire information about
the inspection activities. The same is
true for performing root cause anal-
ysis of problems with multi-element
systems, where accurate measurement
at the component level is critical for
troubleshooting. Any measurement is
a function of instrument accuracy, but
the issue is particularly challenging for
measuring precision filters, where the
scale of the spectral features can be fin-
er than the resolution of the instrument.
Because modern spectrophotometers
are easy to use, operators often over-
look the fact that spectral measure-
ments are not a one-size-fits-all prop-
osition. The measurement approach
can vary depending on the type of filter
and which feature of the curve requires
further analysis. For most high-per-
formance filters, some optics knowl-
edge goes a long way to assess spec-
tral conformance or, at the very least,
understanding of the limitations of
most commercial spectrophotome-
ters. The following example discuss-
es a few common limitations of most
Measuring the new generation of steep
bandpass filters with bandwidths
less than 1 nm tests the resolution
capabilities of instrumentation; more
credible measurements in these high-
performance regimes are now required
to advance the boundaries of optical
filter technology.
