Highly Sensitive Pressure Measurement based on Multimode Fiber

Tip Fabry-Perot Cavity

Wei Ping Chen* and Dongning Wang

College of Optical and Electronic Technology, China Jiliang University, Hangzhou, China

Keywords: Fabry-Perot Interferometer, Multimode Fiber, Pressure Sensors.

Abstract: In this work, a highly sensitive pressure measurement device based on multimode fiber filled with

ultraviolet adhesive is presented. The experimental results show that the device has a high gas pressure

sensitivity of -40.94nm/MPa, a large temperature sensitivity of 213 pm/°C within the range from 55 to

85°C, and a relatively temperature cross-sensitivity is 5.2kPa/°C. The fiber device is miniature, robust and

low cost, in a reflection mode of operation, and has high potential in monitoring environment of high

pressure.

1 INTRODUCTION

A variety of Fabry–Perot interferometric sensors

based on optical fiber have recently been studied and

applied in various industrial areas to measure

temperature, refractive index, strain, pressure, and

fluid dynamics (Roriz P., et al., 2013. Optical fiber

FP interferometer (FPI) pressure sensors can be

divided into two types: operated on cavity length

change (Pevec S. and Donlagic D., 2012; Guo F et

al., 2012; Xu F., et al., 2012; Ma J., et al., 2012; Jin

L., et al., 2013; Eom J., et al., 2015; Ran Z., et al.,

2015; Hou M., et al., 2014.) or mainly rely on cavity

refractive index (RI) variation (Yuan W., et al.,

2011; Coelho L., et al., 2012; Hu G., Chen D., 2012;

Liu Z., et al., 2012; Wu C., et al., 2010). Many FPI

sensors worked on cavity length variation possess

relatively low pressure sensitivity compared with

those based on thin diaphragm. However, the

diaphragm based FPI sensors have a small

measurement range, typically a few tens of kPa (Xu

F., et al., 2012; Ma J., et al., 2012) and relatively

poor mechanical strength. Although the FPI sensors

worked on cavity RI change usually have a large

measurement range and good robustness, their

pressure sensitivity is relatively low, typically on the

order of tens of pm/MPa (Coelho L., et al., 2012; Hu

G., Chen D., 2012; Liu Z., et al., 2012, Wu C., et al.,

2010), except sensor head with a carefully designed

(Xu B., et al., 2015), which increases system

complexity and manufacturing difficulty.

In this work, a novel fiber tip FPI pressure sensor

based on etched end of multimode fiber (MMF)

filled with ultra-violet (UV) adhesive is

experimentally demonstrated. The fiber tip FP cavity

is compact in size, robust in structure, simple in

manufacturing, and convenient in operation.

Because of the special design of the sensing head,

the proposed sensor is effectively exhibits high gas

pressure sensitivity of -40.94nm/MPa within the

measurement range between 0 and 1 MPa (limited

by the pressure meter we used). On the other hand,

the device can measure temperature and the

refractive index, and their sensitivities are 213

pm/°C within the range from 55 to 85°C, ~-73.54

nm/RIU (RI unit) within the range from 1.332 to

1.372, respectively, which shows its versatile

measurement capability. Moreover the fiber tip FP

cavity possesses a relatively low temperature

cross-sensitivity of 5.2 kPa/°C.

2 DEVICE FABRICATION AND

OPERATION PRINCIPLE

The optical fiber FP cavity sensor head is fabricated

by an etched MMF filled with UV adhesive. During

the fabrication process, the end face of MMF with a

core diameter of 62.5 μm and a nominal effective RI

of 1.4682 (at 1550 nm) is etched by use of

hydrofluoric (HF) acid to form a tapered hole cavity.

The UV adhesive (Norland, NOA68) employed can

Chen W. and Wang D.

Highly Sensitive Pressure Measurement based on Multimode Fiber Tip Fabry-Perot Cavity.

DOI: 10.5220/0006103501470151

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 147-151

ISBN: 978-989-758-223-3

147

sustain the temperature change from -80°C to 90°C,

and has the RI of 1.54. The steps of fabrication are

illustrated in Fig. 1.

a) Firstly, a section of MMF is etched for ~10

minutes by use of HF acid with the

concentration of 40% before a taper-shaped

hole of several tens micrometers in depth is

formed at the end face of the MMF.

b) Secondly, the taper-shaped hole created is

filled with UV adhesive. As shown in the

figure, an inner air-cavity is formed inside the

UV adhesive due to the remaining air in the

taper-shaped hole region during the process

of UV adhesive filling. Then, employing the

UV light to solidify the UV adhesive.

The two reflection surfaces of R

and R

create an

FP cavity (C

). The light lunched into the inner

air-cavity is split into three beams: one traveling

along the MMF arrives at R1, part of the light is

reflected back which is denoted by I1, and the rest

continues to propagate until reaching R

and

experiencing partial reflection, denoted by I

. The

light beam transmitted through R

finally arrives at

and is partially reflected again which is denoted

by I

. The two reflection surfaces of R

and R

thus

forming another FP cavity (C

). Meanwhile, the two

reflection surfaces R

and R

can also form an FP

cavity (C

) with a cavity length that is equal to the

sum of cavity lengths of C

and C

. The cavity

lengths for C

and C

are ~37.7 and ~18.7 μm,

respectively.

Fig. 3(a) displays the reflection spectrum of the

device sample fabricated. And the fiber tip FPI

sensor corresponding spatial frequency spectrum

obtained by use of fast Fourier transform as shown

in Fig. 3(b), there are two dominant side peaks in the

spatial frequency spectrum, located at ~0.0239 and

~0.0563 nm

-1

, respectively, which indicates an

interference pattern of two FP cavities.

The well-known expression of free spectral range

(FSR) is written as

FSR=λ

/2nL (1)

where λ is the wavelength, n is the RI of the

cavity, and L is the cavity length. As the RI of UV

adhesive is 1.54 and the cavity lengths of C

and C

are ~37.7 μm, ~18.7 μm, respectively, by taking the

wavelength of 1550 nm, the spatial frequency peak

positions for three FP cavities C

, C

and C

can be

determined as ~0.0314, ~0.0239 and ~0.0554 nm-1,

respectively. This reveals that C

and C

are the

dominant FP cavities as the experimentally obtained

results agree well with those obtained from the

calculations. Thus, the output spectrum of the device

consists of two superimposed FP spectra of C

and

The reflection intensity (I) of the MZI can be

expressed by three beam interference theory as

I=I

+2I

√

cos Φ

√

cos Φ

(2)

where I

, I

, and I

are the light intensity reflected

from adhesive-air surface R

, air-adhesive surface

, and adhesive-air surface R

, respectively, and

and Φ

are the introduced phase shifts of the

cavity C

and C

, respectively, given by



=

4πn

 (3)

4π(n

)

 (4)

where n

and n

are the RI of air, UV adhesive, L

and L

are cavity length of C

and C

, respectively.

From Eqns. (2) - (4), by taking n

=1, n

=1.54, the

cavity lengths L

=37.7 μm, L

=18.7 μm, and the

reflection coefficient of the surface of R

, R

and R

as (n

-n

)

/(n

)

=0.0452, the reflection spectrum

of the FP cavity device can be simulated and the

result obtained is displayed in Fig. 4, together with

that obtained from the experiment as shown

previously in Fig, 3(a), to facilitate the comparison.

It can be seen from Fig. 4 that the two reflection

spectra have nearly the same waveform, and their

intensity difference comes from the insertion loss of

the device while the wavelength shift existed in the

experimental fringe pattern is likely due to the initial

phase of the fringe pattern and the dispersion effect

of the optical fiber.

3 EXPERIMENTAL RESULTS

AND DISCUSSION

Fig. 5 shows the schematic of the experimental setup

used to test the response of FP sensor head to gas

pressure. A broadband source (BBS) centered at

1550 nm is used to illuminate the FPI through a

circulator. The reflection spectrum of the FPI is

observed by an optical spectrum analyzer (OSA)

with the resolution of 0.05 nm to monitor the

spectrum. The sensor head is placed in a gas

chamber, where the gas pressure can be adjusted by

use of an air pump (Wisdom Billiton, Y039),

measured by a pressure meter (ZHITUO, YB-150).

The sensor output is directed to an optical spectrum

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

148

analyzer (OSA) with the resolution of 0.05 nm to

monitor the spectrum.

We investigated the responses of the fiber tip FPI

pressure sensor samples with different pressure. In

the measurement, the fiber in the chamber was kept

in a straight line to avoid any bending-induced

effects, gradually increasing the pressure from 0.02

to 0.04 and then to 0.92 MPa with a step of 0.04

MPa, and the reflection spectrum was monitored in

real-time by use of an OSA. Fig. 6 and its inset

demonstrate the shift of dip wavelength in the

reflection spectrum with the pressure variation. It

can be seen from the figure that the dip wavelength

is decreased with the increase of pressure, and a

linear response with a sensitivity of -40.94nm/MPa

is obtained in the range of 0.02 to 0.92 MPa.

The high temperature sensing capability of the

device are tested by placing the sensor head in an

electrical oven and gradually increasing the

temperature from room temperature to 85°C with a

step of 5°C. During the experiment, to make sure the

temperature was stabilized in the chamber, the

temperature was stayed for 10 minute at each step.

Fig. 7 presents the dip wavelength variation with the

temperature change and its inset demonstrates the

reflection spectra at different temperatures. A fringe

dip near ~1540 nm at the temperature of 30°C is

found to experience a red shift with the increase of

temperature. The highest sensitivity obtained is ~213

pm/°C within the temperature range from 55°C to

85°C. However, considering of the pressure

sensitivity of -40.94nm/MPa obtained in the

experiment, the temperature cross-sensitivity is

calculated to be only 5.2 kPa/°C, which is much

smaller than that of the sensors based on side-hole

dual-core PCF (Hu G. and Chen D., 2012) (1

MPa/°C) and on FBG in the SMF (2.3 MPa/°C) (Wu

C et al., 2010).

To test the system response to the RI change, the

fiber device was fixed on a translation stage, and

immersed by concentrations of salt water and the

reflection spectra recorded had a resolution of 0.05

nm. Each time after the liquid sample was measured,

the fiber sensor head was rinsed with methanol

carefully until the original spectrum (i.e., the

reference spectrum) could be restored and no residue

liquid was left on the sensor head surface. Fig. 8

shows the interference fringe dip wavelength shift

with the RI change and the sensitivity of ~73.54

nm/RIU was achieved. In the inset of Fig. 8, the

wavelength variation as a function of RI is plotted.

Currently, the pressure measurement range

achieved in the experiment is limited by the air

pump used, which only provides a pressure value up

to 1 MPa. However, our device has the potential of

achieving much higher pressure measurement range

due to its robust structure. As a number of

wavelength dips exist in the reflection spectrum as

shown in Fig. 3(a), and the device is sensitive to a

range of physical parameters, a simultaneous

multiple parameter measurement can be expected.

4 CONCLUSIONS

In summary, we demonstrated and fabricated an

optical fiber FP interferometer which is composed of

etched MMF filled with UV adhesive. The gas

pressure change induces the air-cavity length

change, which causes the change in optical path

difference of the MZI, and in turn leads to the

reflection spectrum shift. The sensor device exhibits

a high pressure sensitivity of -40.94nm/MPa and a

good temperature sensitivity of 213 pm/°C within

the range from 55°C to 85°C, and a RI sensitivity

of~-73.54 nm/RIU within the range from 1.332 to

1.372. The temperature cross-sensitivity of the

device is 5.2kPa/°C. Such a device is based on low

cost MMF, compact in size, robust in structure,

simple in fabrication, convenient in operation, which

makes it highly attractive for pressure sensing.

ACKNOWLEDGMENTS

This work is supported by the National Natural

Science Foundation of China (Grant No. 61377094).

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APPENDIX

Figure 1: Schematic diagram of the fabrication process of

the optical fiber sensor head. (a) A taper-shaped hole at

the end of MMF is formed by HF etching. (b) The

taper-shaped hole at the end of MMF is filled by UV

adhesive.

Figure 2: (a) Schematic diagram and (b) the microscope

image of the sensor head.

Figure 3: (a) Reflection spectra of the device sample. (b)

Spatial frequency spectra of the device sample.

Figure 4: Reflection spectra of the FP cavity device

obtained from theoretical simulations and experiment.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

150

Figure 5: Schematic diagram of experimental setup.

Figure 6: Dip wavelength shift with the increase of

pressure, and the inset shows the reflection spectra of the

device at different pressures.

Figure 7: Fringe dip wavelength shift with the temperature

variation. Inset shows the reflection spectra of the device

at different temperatures.

Figure 8: Dip wavelength shift with RI, and the inset

shows the reflection spectra of the device at different RI

values.

Highly Sensitive Pressure Measurement based on Multimode Fiber Tip Fabry-Perot Cavity

151