Pressure Sensor Based on Mechanically Induced LPFG in Novel MSM Fiber Structure

doi:10.4236/opj.2013.33036

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Optics and Photonics Journal, 2013, 3, 225-228

http://dx.doi.org/10.4236/opj.2013.33036 Published Online July 2013 (http://www.scirp.org/journal/opj)

Pressure Sensor Based on Mechanically Induced LPFG in

Novel MSM Fiber Structure

Sunita Ugale, Vivekanand Mishra

Department of Electronics and Communication Engineering, S. V. National Institute of Technology, Surat, India

Email: [email protected]

Received January 1, 2013; revised February 12, 2013; accepted February 20, 2013

tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly

cited.

ABSTRACT

We have proposed and demonstrated experimentally a novel and simple pressure sensor based on mechanically induced

long period optical fiber gratings. We report here for the first time to our knowledge the characterization of mechani-

cally induced long period fiber gratings in novel multimode-singlemode-multimode fiber structure. The MLPFG in-

duced in single mode fiber and multimode fibers are studied separately and the results are compared with MLPFG in-

duced in MSM fiber structure. MLPFG in MSM structure has much greater sensitivity. We have obtained maximum

transmission loss peak of around 18 dB, and the sensitivity of pressure sensor is 8 dB/Kg.

Keywords: Mechanically Induced Grating; Pressure Sensor; Sensitivity

1. Introduction

Fiber optics sensors based on gratings are still in the de-

velopment stage in laboratories and lot of work is needed

to be done to promote and develop their use in advanced

applications. In-fiber grating sensor technology has be-

come one of the most rapidly progressing sensing topics

of this decade in the field of optical fiber sensors. These

sensors are currently emerging from the laboratory to

find practical applications. Rapid progress has been made

in both sensor system developments and applications in

recent years.

The spectral characteristic of Long period optical fiber

grating (LPFG) is more flexible as compared to fiber

Bragg grating (FBG). LPFG have low insertion loss and

low back reflection. It can offer the advantages of abso-

lute measurement, high sensitivity, all-fiber in-line, small

size, etc. The other advantages of LPFG sensor include

simple fabrication and easiness in adjusting the resonant

wavelength well within the spectrum of optical source by

simply adjusting the grating period. All these attractive

features of LPFGs are the strong points to push towards

detail study of this device. Because of much longer Pitch

Λ (almost 100 times that of FBG) the forward propagat-

ing core bounded modes and cladding-bounded modes

couple to each other. Therefore there are several resonant

peaks depending on the involved number of cladding

modes in the interesting wavelength range and the core

mode is coupled to several copropagating cladding modes.

Many methods have been demonstrated for the fabri-

cation of LPFG. The techniques can be divided into two

groups: the one that enables the fabrication of permanent

gratings, and the other that allows the fabrication of re-

versible or mechanically induced gratings, i.e. by remov-

ing the external perturbation the grating disappears [1].

As the period of LPFG is of the order of micrometers, it

can be induced through microbending using various me-

chanical means. The important advantage of this tech-

nique over other techniques is that it can be applied to

any kind of fiber and is also simple, flexible and low-cost.

These are very sensitive to external pressures enabling a

good control over their transmission characteristics [2,3].

2. LPFG Mathematical Model

If a periodical pressure is applied on the waveguide, a

long period grating is formed owing to the photo elastic

effect and the microbending effect. The energy of the

core mode LP01 is coupled into that of the cladding mod-

es LP1m if the phase matching condition as follows is

satisfied [4].

22 2

co cl

eff eff









 (1)

where : is the effective index of the core mode, :

Is the effective index of cladding mode.

eff

ncl

eff

S. UGALE, V. MISHRA

226

For a given periodicity Λ one can induce mode-coupl-

ing between the fundamental mode and several different

cladding modes, a property that manifests itself as a set

of spiky losses at different wavelengths in the transmis-

sion spectrum. In design of optical filters concatenation

of gratings are required and the relatively close spaced

resonance peaks of cladding modes can cause serious

difficulties to generate a desired spectrum.

The coupled mode equations describe their complex

amplitude, Aco(z) and Acl(z) [5].

  

co iz

co cococo clcl

cl iz

cl clclcl coco

Az s

iKAzi KAz

Az s

iKAzi KAz









(2)

where Aco and Acl are the slowly varying amplitudes of

the core and cladding modes, Kco-co, Kcl-cl and --co clclco

are the coupling coefficients, s is the grating modulation





depth and 1

co cl

eff eff







 









is the detuning from

the resonant wavelength. The coupling is determined by

the transverse fields of the resonant modes EI and the

average index of the grating nI

 

iji j

nrE rErr







 (3)

According to coupled mode theory, grating transmis-

sion is a function of coupling coefficient Kij

Assuming the detuning from resonant wavelength is

balanced by the dc coupling, simplified expression for

grating transmission is given by







cosTZ KZ (4)

Cross coupling coefficient к depends on the grating

index profile and field profiles of the resonant modes.

The analysis given by Erdogan (Erdogan 1997) [6] is

followed for the calculation of core and effective clad-

ding refractive index.

Consider a step index fiber with three layers: central

core with refractive index n1, cladding with refractive

index n2 and the external medium with refractive index

n3 is considered. The core radius is a and the cladding is

assumed to extend to infinity.

Variation of effective index of fundamental LP01

guided mode as a function of wavelength in a fiber

shown in Figure 1 is calculated by using the following

equations.

eff

Figure 1. Multimode-Single mode-Multimode (MSM) Fiber

Structure.

The normalized frequency of the fiber is given by V.



2a



 (5)

Normalized index difference



 (6)

The approximate value of index as a function of wave-

length is given by Sellmeier equation







 

 (7)

The commonly used waveguide parameters u and w

are

uk 1



 (8)

01 2





 (9)

where

12 01

eff











  (10)

The characteristic equation for a LP0m guided propaga-

tion in a weakly guiding fiber (n1 ≈ n2) is given by







uak wa

uJ uawk wa

 (11)

where m is radial order of mode. Jp, kp are Bessel and

modified Bessel functions of order p.

Calculation of effective indices of the circularly sym-

metric, forward propagating cladding modes.

Consider a multimode step index structure ignoring the

presence of core.

The Eigen value equation for the LP0m cladding mode

can then be approximated by that of a uniform dielectric

cylinder surrounded by an infinite medium.



 



 



 



 



111

122

12 22

111 1

mmm

clcl cl

cl cl

mmm mmmm mmm

cl clclclcl clclclcl cl

JubJub Kwb

Kwb KK b

uJub wKwbuJubwKwbuw





 







 

 



 



 



 



 







(12)

S. UGALE, V. MISHRA 227



u and are the waveguide parameters for clad-

ding



 







(13)

 





k (14)

 

mcl







 (15)

and



cl m

eff jm

nn b



 



 



 

(16)

where jm are the roots of the Bessel function of order

zero (J0(jm) = 0).

3. Experiment and Results

The Reversible LPFG with period of 600 μm and length

= 70 mm is induced in single mode fiber in Multimode-

Single mode-Multimode (MSM) structure. Light is laun-

ched from a broadband source to the lead-in MMF,

through the device (MLPFG) to the lead-out MMF and

spectrally resolved using an optical spectrum analyzer

(OSA) (Prolite60).

A schematic diagram of the MSM structure used in

experiment is shown in Figure 1. The sample is prepared

by splicing a 15 cm long section of SMF (SMF-28™)

using a Sumitomo Type39 fusion splicer in between two

MMFs (62.5/125). The loss at both splices was 0.02 dB.

The MLPFG induced in single mode fiber and multi-

mode fibers are also studied separately. It is observed

that single mode grating produced resonant loss peaks of

up to ~7 dB and multimode grating produced resonant

loss peaks of up to ~5 dB. The transmission spectrum of

MLPFG in MSM structure is plotted in Figure 2, the

input power spectrum is also shown for comparison pur-

pose. The peak loss of around 17 - 18 dB is obtained,

which is much greater than MLPFG in Single mode and

multimode fiber.

Thus the MSM structure has a higher sensitivity than

just writing the MLPFG the single mode or multimode

fiber individually.

The transmission spectra of the MLPFG in SMS struc-

ture with periods of 600 μm and length L = 70 mm for

different pressure applied on it is shown in Figure 3. We

can see that a high pressure has a deep notch in the trans-

mission spectrum and the high coupling efficiency at the

resonant wavelength. The results of those measurements

are given in Table 1, and plotted in Figure 4.

The curve fitting polynomial P for pressure sensor is

0.20870.94422.6336 0.0449Px xx.

Resonant loss peaks with strengths of up to 17 dB

have been generated in the MLPFG in MSM structure.

There is no change in resonance wavelength with exter-

nal applied pressure. Therefore MLPFGs, can find appli-

cation as pressure gauges.

Sensitivity of pressure sensor = Change in transmis

Figure 2. Spectral response of MLPFG in MSM fiber struc-

ture.

Figure 3. Complete transmission spectrum of MLPFG in

MSM structure (Λ = 600 µm) with different pressures

Table 1. Transmission loss to applied pressure.

Applied Weight (Kg) Transmission loss (dBm)

0 45.0

2.0 48.5

3.5 51.0

4.0 53.5

4.4 56.7

4.6 57.8

4.8 59.3

5.0 60.8

5.2 62.0

S. UGALE, V. MISHRA

228

Figure 4. Response of grating to external applied pressure.

sion loss/ change in applied weight

56.7 53.53.28dB Kg

4.4 4.00.4

S





4. Conclusion

We report here, for the first time to our knowledge, the

characterization of mechanically induced LPFGs in MSM

fiber structure. MLPFG in MSM structure gives single

transmission dip. Resonant loss peak strength is around

18 dB, which is much greater than maximum loss of 8

dB in Single mode MLPFG and 5 dB in multimode

MLPFG. The MLPFG in MSM structure with  = 600

μm and length = 70 mm can be used as a pressure sensor.

5. Acknowledgements

This work is partially supported by the Department of

Science and technology of India.

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