Instrumentation & Measurement Magazine 24-5 - 77

Table 1 - Main specifications of the optical
interrogator I-MON 256 USB by Ibsen Photonics
Parameter
Unit
Maximum number of FBG sensors
Minimum FBG spacing
Wavelength range
Wavelength fit resolution
Repeatability (over any pol. state)
Wavelength accuracy
Wavelength drift
Dynamic range
Input optical power range
Measurement frequency
Interface
Current consumption
Size (LxWxD)
Weight
Connector
Operating temperature
deg C
mA
g
pm
nm
pm
pm
pm
Value
> 37
1200
1525 - 1570
< 0.5
3 (5 max)
5 (typ)
pm/deg C 1 (3 max)
dB
dBm
Hz
30 (typ)
-80 to -20
6000
USB 2.0
250
mm 110x94x49
660
FC/APC
0 to 50
a narrow (45 nm) range of wavelengths (1525-1570 nm). The
main specifications of the optical interrogator are reported in
Table 1.
The optical interrogator needs an external light source to
illuminate the fiber. For this purpose, a superluminescent
LED source DL-BP1-1501 produced by DenseLight Semiconductors
[27] with a wide band DL-CS5169A LED centered at
1550 nm was used. The optical splitter used in the test prototype
was an Anfiber-FSW-1x4 having a wavelength range that
extends from 1260 nm to 1650 nm and features low losses. The
information from the sensors was transmitted from the optical
interrogator to a PC locally through a USB connection and then
remotely by a wireless transmitter.
The narrow bandwidth of the optical interrogator repreTable
2 - Main characteristics of the BendBright fiber by HBM FiberSensing
Sensor
Strain sensitivity (K factor)
Measurement range
FBG length
CTE
Ref Temperature
Termo-optic coeficient (TOC
Optical
Central wavelength
Spectral width (FWHM)
Reflectivity
Side lobe suppression
Fiber
Fiber type
Coatings
Cable type
Connectors
Environmental
Operation temperature
Mechanical
Recoating
Fatigue resistance
Fiber failure tension
August 2021
)
0.79 (1.2 pm/με @ 1550 nm)
± 5000 με
4-10 mm
11 ppm
20 °C
6.2
1512.9±0.5 nm to 1586.0±0.5 nm
< 0.2 nm
> 65%
> 10 dB (<20% of peak (dB))
SMF28 Single Mode Fiber
Acrylate - Polyimide
Ø 0.9 mm laboratory (hytrel)
Ø 3 mm indoor (kevlar)
Ø 3 mm outdoor (armor)
FC/APC - SC/APC - NC (No Connectors)
Acrylate -20 to +80 °C
Polyimide -20 to +350 °C
Standard: polyimide - Upon request: acrylate/no recoating
200 M cumulative fatigue stress cycles with ±450 μεl
>20 N (250 kPSI)
IEEE Instrumentation & Measurement Magazine
sented a serious constraint on the design of the measurement
system and caused the need to adopt Bragg gratings with custom
wavelengths. Moreover, as it will be shown in the next
section, constructive constraints on the sensors imposed small
bending radii of the fiber. It was then necessary to opt for a
flexible fiber that would ensure low attenuation of the optical
signal. The BendBright fiber produced by HBM FiberSensing
[28] was adopted for its suitable features. The main characteristics
of the optical fiber are shown in Table 2.
To set the nominal (central) wavelength of each custom
FBG sensor and allocate the
suitable bandwidth to avoid
overlapping, a careful design
of the sensing elements
followed by a preliminary
experimental investigation
of the mechanical strain to be
measured was performed.
To this aim, dedicated prototypes
employing the same
transduction principle, geometries,
and materials, and
piezoresistive strain gauges
were developed.
To take into account
the effect of temperature,
according to instructions
provided by the experts, a
range of temperature variation
between 0 °C and 50 °C
was considered. The nominal
wavelength of the single
FBG sensor λ0
, centered to
its variation band, was then
defined, considering the
variation range of the strain
of the individual sensor,
the temperature variations,
and the optical band of the
interrogator.
In fact, note the mechanical
strains S generated by
77
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