Recognition of Bragg Wavelength Disturbed by Time Delay of Fiber Length in Prepositive Tunable Filter

doi:10.4236/opj.2013.32B054

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Optics and Photonics Journal, 2013, 3, 232-235

doi:10.4236/opj.2013.32B054 Published Online June 2013 (http://www.scirp.org/journal/opj)

Recognition of Bragg Wavelength Disturbed by Time

Delay of Fiber Length in Prepositive Tunable Filter

Chuan Li, Xiaoyong Chao, Yingna Li, Tao Xie, Zhengang Zhao, Xin Xiong

Faculty of Information Engineering and Automation, Kunming University of Science and Technology, Kunming City, China

Email: [email protected]

Received 2013

ABSTRACT

The wavelength shift in fiber Bragg grating does not depend directly on the total light levels, losses in the connecting

fibers and couplers, or source power. However, if the tunable Fabry-Perot filter is place on the end of incident fiber, the

detected time delay of modulation light is occurred due to the unmatch between the scanning time and light transmis-

sion time in the transmission fiber. Consequently, the detected peak wavelength shifts with the length of transmission

fiber. Thus, the peak wavelength shift effect of Bragg reflective light transmitted in fiber with different fiber length can

be obvious in the demodulator with a prepositive tunable Fabr y-Perot filter. The experiment indicates the shift rates of

0.109 - 0.126 nm/km increase approximately linearly with the original peak wavelength of 1532.917 - 1560.300 nm at

the fiber length of 0 - 6 km. To certify the consistency of measurement data, the criterion correction is introduced. By

using the differential method of two fiber Bragg gratings with an optical path, the differential worth is compensated

from the disturbance modulated by the time delay of fiber length.

Keywords: Optic Fiber; Fiber Length; Bragg Wavelength; Fabry-Perot Filter; Time Delay

1. Introduction

Fiber photosensitivity was first observed in germanium-

doped silica fiber in experiments performed by K. Hill

and coworkers at the communication research center in

Canada in 1978 [1,2]. Since then, the fiber Bragg grat-

ings represent a key element in the established and

emerging fields of optical communications and optical

fiber sensing. The principal advantage is that the meas-

urand information is wavelength-encoded (an absolute

quantity, or a state value), thereby making the sensor

self-referencing, rendering it independent of fluctuating

light levels and the system immune to source power and

connector losses [3-5]. In the sensing system of fiber

Bragg grating, the precision of shift of peak wavelength





B should be superior to 0.001 nm for proving the

measurement precision of 0.1℃ or 1  Thus, the meas-

urement precision 



B defined the measurement preci-

sion of whole system. However, the intensity variations

in the light source, the bending on lead/in-out fibers, and

a tunable filter can cause signal deviations when a tun-

able filter scans the Bragg grating sensors [6-8]. H. G.

Limberger etc. found a reflectivity of 94% and a band-

width of 1.7 nm in communication fiber with three side-

lobes [6]. V. Gaillarda etc. research the optical spectrum

feature in the fiber of low coherence interferometer and

fiber Bragg grating [7]. Y. L. Lo etc. found that the in-

tensity variations from the macrobending of the lead-in/

out fibres during high frequency disturbances can cause

deviations of the signal when a tunable filter scans the

Bragg wavelength [ 8] .

2. Peak Wavelength Shift with the Fiber

Length in a Prepositive Tunable Filter

The fiber Bragg grating is a permanent perlodic modula-

tion of the refractive index along a given length of opti-

cal fiber. Due to the coupling between the forward and

backward propagating modes, the specific wavelength

light depending on the modulation period of the refrac-

tive index is reflected at the location of the fiber Bragg

grating. The other wavelength lights are transmitted

through the fiber Bragg grating. The incident light is re-

flected when its peak wavelength is equal to the Bragg

wavelength [1-5]:

max 2

Bragg eff





 (1)

where, neff is the effective refractive index of reverse

couple mode,



is the period of grating. The most im-

portant property of FBG is that it will reflect the incident

light with particularly predetermined wavelengths, while

passing all the other wavelengths of light at the same

time. As the wavelength of the reflected light varies with

the strain, temperature and the other environmental fac-

C. LI ET AL. 233

tors, detection of the wavelength will yield information

about these quantities. Figure 1 is the schematic diagram

of fiber Bragg grating sensing modulation system, where-

into, (a) is the reflection-type measurement, and (b) is the

transmission-type measurement. In the modulation sys-

tem, the light sources can be broadband light, tuning light,

pulse light, and laser etc., the connectors are coupler and

circulator, the sensing gratings are fiber Bragg grating

and long period fiber grating, and the photoelectric de-

tector is used [5].

In the modulation process, the incident light directly

accessed the sensing grating through the transmission

optical path or the optical coupler. Under the static, quasi-

static, and time varying action of external filed, such as:

strain filed, temperature filed ect., the incident light is

modulated; subsequently, the reflected (or transmitted)

modulated light is detected by the photoelectric detector.

A very important character of fiber Bragg grating is that

the peak wavelength



B of reflected light will change

when there is a change in strain, temperature and other

environment factor. The reason is that change of the

strain of the grating or its environmental temperature can

result in change in the efficient refractive index of the

core neff as well as the period of index modulation



Therefore, the the shift of Bragg wavelength 



B can be

expressed as:

Bragg effeff



 (2)

where,  is the elastic deformation of fiber; neff is the

elasto-optical effect of fiber. Eq. (2) indicate that the ef-

fective measurement of 



B determine the measurement

precision of the whole system.

The peak wavelength shift effect of Bragg reflective

light transmitted in fiber is observed, as shown in Figure

2. In this experiment, the fiber is the G.652 standard

communication monomode fiber, and the fiber Bragg

grating is inscribed in the Hydrogen-loaded G.652 fiber.

Figure 1. The schematic diagram of fiber Bragg grating

sensing demodulation system.

In Figure 2, the incident broadband light source is

modulated to the narrowband light by the tunable Fabry-

Perot (TFP) filter. The filter is characterized by bandpass

resonances of Lorentzian lineshape and bandwidth of

typically 0.3 nm, with a wide operating range of 40 nm,

depending on the spacing between the mirrors of Fabry-

Perot etalon. Electrical control of this mirror spacing via

piezoelectric stacks allows for tuning the passband

wavelength. In operation, the passband light is injected to

each fiber Bragg grating through two couplers, a circula-

tor. The return passband light modulated by the grating is

detected by the detector through the circulator. As the

filter is tuned, the passband scans over the return signal

from the grating, and the wavelength can be determined

and recorded from the matching peak wavelength of re-

flective light.

In this scanning optical filter, the wavelength range is

1525 - 1565 nm, the wavelength resolution is 0.001 nm,

and the wavelength repetition is 0.01 nm. In this experi-

ment, the tunable Fabry-Perot filter discovers that the

peak wavelength is shifted with the variation of fiber

length, as shown in Figure 3.

In Figure 3, the original peak wavelengths of grating

are



max(l = 0 ) = 1532.917 nm, 153 7.0 91 nm, 139. 899 nm,

1540.568 nm, 1545.063 nm, 1550.444 nm, 1555.573 nm,

and 1560.300nm, and the widths of gratings are 



= 0.2

nm. As the fiber length is l = 6 km, the peak wavelength

shifts are 0.654 - 0.759 nm. By using fitting, the peak

wavelength shift of Bragg reflective light



max(l) -



max(0)

is related to the fiber length l:

Figure 2. The schematic diagram of the fiber Bragg grating

measurement system demodulated via a prepositive tunable

Fabry-Perot passba nd filte r.

Figure 3. The peak wavelength shift of Bragg reflective light

related to the fiber length.

C. LI ET AL.

234

 





max max

max

0.10901532.9176.823 10







 





(3)

where, the standard error between the fitting results and

the experimental results is 0.009 nm, the shift rates of the

Bragg wavelength to the fiber length are 0.109 - 0.126

nm/km.

3. Transmission Component of Bragg

Reflective L igh t

This wavelength shift effect of Bragg reflective light

caused by the fiber length is important to the fib er Bragg

grating sensor network. Figure 4 shows the strain ex-

periment based on the cantilever beam of constant bend-

ing rigidity [9,10]. The bottom of the beam is fixed on

the bracket, and the top is hung through the blade, the

pothook, and the weight. The cantilever beam is made

from the stainless steel material at the size of l = 300.0

mm, h = 3.0 mm, and B = 45.9 mm.

In Figure 4, the fiber Bragg gratings mounted on the

up and down surfaces of cantilever are separately suf-

fered the strains of



and -



, but is located in the same

temperature field T. Thus, the strain



can b e expressed

as the function of the shifts of Bragg wavelength in the

gratings [ 7- 1 0]:

 



 





 (4)

where, S = 1.22 × 10-3 nm/ is the strain sensitivity co-

efficient of grating, 





, T) and 



B(-



, T) are the

wavelength shifts of gratings mounted on the up and

down surfaces of the cantilever. The strain of the beam

caused by the loading can be measured directly by the

tunable Fabry-Perot detector (TFP detector), as shown in

Figure 5.

In Figure 5, the Bragg wavelengths of gratings mounted

on the up and down surface of cantilever are separately

subject to wavelength levels of 1550.2 - 1552.2 nm and

1538.8 - 1540.8 nm. In the loading, the solid and virtual

line groups denote the wavelength sensitivities of the

gratings mounted on the up and down surface with the

Figure 4. The schematic diagram of loading test.

Figure 5. The loading-wavelength variaiton diagram of

fiber Bragg gratings mounted on the cantilever with the

fiber length of l = 0 km, 2 km, 4 km, and 6 km.

fiber length of l = 0 km, 2 km, 4 km, and 6 km. Accord-

ing to Eq. (4), through the least-square algorithm fitting,

the strain can be formatted by the weight g:

163.1





(5)

where, the strain precision of the gratings is 0.5% in the

strain range of 0 - 815.6 . Eq. (5) indicates the Bragg

wavelength shift of sensing grating can be obtained by

processing these Bragg wavelengths of gratings in the

same fiber length, i.e., the Bragg wavelength shift of

sensing grating is independent to the Bragg wavelength

shift of transmission fiber with different lengths. By us-

ing the differential method of two fiber Bragg gratings

with an optical path, the differential worth is compen-

sated from the disturbance modulated by the time delay

of fiber length.

4. Conclusions

In this paper, the shift effect of peak wavelength of

Bragg reflective light is produced by changing the length

of transmission fiber. The experiment indicates that the

peak wavelength shifts of Br agg ref lec tive lig ht ar e 0.109

- 0.126 nm/km, which increased with the original peak

wavelength of 1532.917 - 1560.300 nm at the fiber

length of l = 0 km. The further sensing experiment indi-

cates that the shift of Bragg wavelength in the prepositive

tunable filter includes the sense component of grating

and the transmission component of Bragg reflective lig ht.

The measured peak wavelength should be corrected

when the transmission shift distributes the measurement

accuracy. It is noteworthy that the differential worth is

compensated from the disturbance modulated by the time

delay of fiber length by using the differential method of

two fiber Bragg gratings with an optical path.

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