Aerospace & Defense Technology - April 2023 - 42

Tech Briefs
small sensing size (<1 cm), and
a huge absolute dynamic range,
all in standard off-the-shelf telecom
fiber. These sensors function
by measuring the resonance
frequency of the
non-linear Brillouin interaction
in fiber which shifts linearly
with strain and temperature.
However, existing Brillouin
sensors are hampered by fundamentally
poor response resulting
in small frequency shifts
compared to the linewidth of
the interaction. To overcome
this limitation we introduced
a technique known as distributed
Brillouin fiber laser sensing
(DBFLS) which establishes
a series of narrowband lasing
modes that experience Brillouin
gain at discrete locations.
Brillouin fiber sensors are
Pump period is set to
cavity round trip time
Pump
1
Measure lasing
frequency vs time
Lasing modes experience SBS at
same position each round-trip
1 2 3
N
Lasing modes circulate CCW
Pump
SBS gain spectra
Lasing modes
f SBS-3
f SBS-2
f SBS-1
(a)
Residual
Pump
(b)
Here we use a pulsed pump to
excite a series of lasing modes
in a fiber cavity. The pump
pulse period is carefully tuned
to match the round trip time
in lasing cavity such that the
lasing modes only experience
Brillouin gain at distinct locations
in the fiber under test
(FUT). The frequency of the
lasing modes then matches the
Brillouin resonance frequency
and is responsive to changes in
temperature or strain at only
one position each.
Frequency
The distributed Brillouin laser sensor is formed by periodically pumping a
fiber ring cavity. The pump pulse repetition period is matched to the roundtrip
time in the cavity, ensuring that Brillouin scattered light will experience
Brillouin amplification at the same position in the fiber after each round trip.
(Image: Naval Research Laboratory)
able to make fully distributed absolute
strain and temperature measurements
using standard telecom fiber. Brillouin sensors
have been demonstrated with large
dynamic range and operating at long distances
with high spatial resolution. These
aspects have made Brillouin sensors well
suited for a number of applications, particularly
in structural health monitoring.
However, these sensors exhibit poor sensitivity
when compared to other fiber sensing
modalities such as fiber Bragg gratings
or Rayleigh scattering. Ultimately this
weak sensitivity comes from the inherently
low strain and temperature response of
the Brillouin frequency shift compared to
the linewidth of the interaction (despite
the relatively narrow linewidth of the Brillouin
resonance, ~30 MHz), resulting in
poor signal to noise ratio (SNR).
To overcome this limitation we proposed
to use the linewidth narrowing
effects associated with the lasing transition
to greatly improve the SNR. Brillouin
fiber lasers have previously been demonstrated
with very narrow linewidths
by leveraging the already narrowband
nature of the interaction. However these
fiber cavities, while sensitive to strain and
temperature, are sensitive to these changes
anywhere in the cavity and could not
be used to make distributed measures.
My Karle fellowship was
proposed to create, demonstrate,
and investigate the
performance of a distributed
Brillouin fiber laser sensor.
During the fellowship I produced
an initial prototype
and presented the basic noise
performance, bandwidth and dynamic
range. This was followed up with investigations
into noise scaling with sensor
spatial resolution. This work was then
extended to practical lengths and numbers
of sensors while addressing a weakness
in the initial design where the fiber
was not contiguously sampled.
This work was performed by Joseph
Murray, for the Naval Research Laboratory.
For more information, download the
Technical Support Package (free white
paper) at mobilityengineeringtech.
com/tsp under the Sensors category.
NRL-5670
Electro-Optic Materials Research
Army Research Laboratory, Adelphi, MD
Developing single photon UV detection for compact chemical and biological sensors.
T
42
his report summarizes the main
lines of effort for the Electro-Optics
Materials Research (EOMR) program
including its goals and major accomplishments,
focusing on the past 5 years.
This EOMR program was an effort within
601102A.31B.1 titled " Optoelectronic
and Integrated Photonic Materials and
Device Research " for FY16-FY19 and
611102A.AA8.1 titled " Photonic Materials
and Device Research " for FY20FY21.
The focus of this EOMR for most
of the program was to develop novel
semiconductor optoelectronic devices
to reduce the size, weight, power, and
cost (SWaP-C) of chemical and biological
detection and identification systems.
Specifically, the program addressed
the need for high sensitivity photodetectors
in the near-UV (NUV) spectrum
mobilityengineeringtech.com
between 300 and 350 nm for biological
agent detection using light-induced
fluorescence techniques employed by
the Tactical Biological (TAC-BIO) detector,
developed by the US Army Combat
Capabilities Development Command
Chemical Biological Center, as well as
in the deep UV spectrum (220-240 nm)
important for standoff chemical detection
based upon fluorescence-free Raman
Aerospace & Defense Technology, April 2023
http://mobilityengineeringtech.com/tsp http://www.mobilityengineeringtech.com
Aerospace & Defense Technology - April 2023

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