Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 6933–6934
Part 1, No. 11, November 2003
#2003 The Japan Society of Applied Physics

Short Note

Spectral Response of Fiber Bragg Gratings Connected in Series with Diﬀerent Wavelengths
Yuji M ASUDA, Mitsuo N AKAMURA, Chisa KOMATSUy, Keio FUJITA1 , Makoto Y AMAUCHI, Yasuo M IZUTANI,Susumu K IMURA2 , Yoshifumi SUZAKI, Takashi Y OKOUCHI, Kiyoshi N AKAGAWA and Seiki EJIMA1 z
1Department of Advanced Material Science, Faculty of Engineering, Kagawa University,
2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan2
Shinko Electric Wire Co., Ltd., 1298-12 Shido, Sanuki, Kagawa 769-2101, Japan(Received July 2, 2003; accepted August 5, 2003; published November 10, 2003)

When ﬁber Bragg gratings (FBG) with diﬀerent reﬂection wavelengths are connected in series, the reﬂection spectrum may be
aﬀected by cladding modes depending on the order in which the FBGs are connected and on which side the signal light isincident. It is analyzed and experimentally demonstrated that, in order to obtain proper reﬂection signals from multiple FBGs,
the FBGs must be connected in order from those with shorter wavelengths to those with longer wavelengths and signal lightmust be incident on the shortest wavelength FBG. [DOI: 10.1143/JJAP.42.6933]
KEYWORDS: fiber Bragg grating, optical circuit, optical fiber, cladding mode, fiber sensor, reflection spectrum

Because the ﬁber Bragg grating (FBG) has a very narrowband sharp-reﬂection spectrum, it is well suited for applications that deal with narrow-band optical signals in itsspectrum, such as optical ﬁlters in dense wavelength divisionmultiplexing (WDM) systems, external reﬂectors of a ﬁber
laser cavity and various types of ﬁber sensors. In theseapplications, the FBGs with diﬀerent reﬂection wavelengths
are often used by connecting them in series. For example, inorder to detect temperatures or stresses at several diﬀerent
locations, FBGs with diﬀerent reﬂection wavelengths areassigned to each location and are all connected in series so as
to simultaneously measure parameters sensed at diﬀerent
locations from multiple spectra reﬂected through a singleﬁber. An FBG fabricated from a ﬁber that is commonly usedin optical telecom systems normally has a so-called claddingmode.1) Depending on the order in which the FBGs areconnected and also on which side the signal light is incident
from, the overall reﬂection spectrum may become distortedby the cladding modes. Othonos2) has pointed out this eﬀect
in his review article by stating brieﬂy that the cladding-mode
problems become very serious at wavelengths shorter thanthe peak reﬂection wavelength. However, experimentaldemonstration has not been given. In this work, we analyze
the conditions under which distortion occurs and we
demonstrate them experimentally.
The cause of the cladding modes is as follows. The FBG isusually fabricated by inducing defects in the ﬁber core,
which increases the refractive index. The defects are
periodically induced along the ﬁber by exposing the ﬁber
to periodically modulated UV laser light which is generated
by interference of the UV laser light diﬀracted by a phasemask grating. The defect is induced in Ge chemical bonding
in a silica glass network by its UV photosensitivity. Thisoccurs only in the ﬁber core where Ge is doped. The
cladding of the ﬁber is not aﬀected by the UV laser lightbecause Ge is not doped there. Light transmitted in a singlemode ﬁber has its intensity concentrated mostly near thecenter of the ﬁber core with a distribution slightly smearedout into the ﬁber cladding around the core. The grating of thePresent address: Material Science, Graduate School of Eng. OsakaUniversity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
yPresent address: Fujitsu Kochi Systems Eng. Ltd., 2-2-34 Honmachi,Kochi, Kochi-ken 780-0870, Japan.
zE-mail address: ejima@eng.kagawa-u.ac.jp

FBG reﬂects light of a particular wavelength deﬁned by thegrating pitch in the ﬁber core, and the light smeared out inthe ﬁber cladding is not reﬂected by the FBG. This results inpartial mode conversion of the fundamental mode intohigher order modes, which are called cladding modes. Sincea higher order mode cannot propagate in a single-mode ﬁber,
the cladding modes are lost and cause attenuation of thefundamental mode. The cladding modes usually show up atwavelengths in the range from approximately 1 nm to 10 nm
shorter than the FBG central wavelength. This cladding
mode can be suppressed by using a special ﬁber with Ge
doping in the cladding around the core, which needs
additional doping such as ﬂuorine to compensate for therefractive index there. Such a ﬁber is for a special purpose
and is expensive with limited availability. Standard telecom
ﬁbers, for example Corning SMF-28, are commonly used tofabricate FBGs.
When two FBGs are connected in series having centralwavelengths 1 (FBG#1) and 2 (FBG#2) with 1 < 2 , andwhen light is incident on FBG#1, 1 is reﬂected by FBG#1
without being aﬀected by FBG#2. Conversely, if light isincident on FBG#2, 1 goes through FBG#2, is reﬂected byFBG#1 and passes back through FBG#2 again. During thetwo passes through FBG#2, it may suﬀer cladding mode
attenuation from FBG#2 because 1 may overlap with thecladding mode wavelengths of FBG#2.
In order to conﬁrm the distortions discussed above wehave fabricated two FBGs, whose central wavelengths are1 ¼ 1552:9 nm (FBG#1) and 2 ¼ 1555:9 nm (FBG#2),
using standard telecom single-mode ﬁbers (Corning SMF28) by exposing each ﬁber to a UV excimer laser through aphase mask grating. The length of the FBG section is 15 mmfor both. The reﬂection and transmission spectrum of theseFBGs, measured individually, are shown in Fig. 1(a) forFBG#1 and (b) for FBG#2. Narrow-band sharp-reﬂection
spectra are seen with nearly 100% reﬂectivity, and cladding
modes are clearly visible in the transmission spectra atwavelengths shorter than the central wavelength by approximately 1 nm to 10 nm. The cladding mode signals are
simply lost and appear as an attenuation of the transmission
spectra.
When these two FBGs are connected in series by fusion
splicing, the transmission and reﬂection spectra become likethose shown in Fig. 2(a) when broad-band light is incident

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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 11(a)12080

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(b)

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Cladding
modes

Claddingmodes

#140

#240

Transmission
01548

1550

1552

1554

1556

01548

1550

1552

1554

1556

Wavelength (nm)Wavelength (nm)

Reflection & Transmission (%)

Fig. 1. Reﬂection and transmission spectra of two FBGs #1 and #2. The
spectra are taken individually.

(a)

(b)

12012080804001548#1

FBG#1 #2

1550

#240

1552

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Wavelength (nm)

1556

01548

#1

FBG#1 #21550

1552

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Wavelength (nm)Fig. 2. Reﬂection and transmission spectra of two FBGs #1 and #2
connected in series. Signal light is incident on FBG#1 ﬁrst in (a) and onFBG#2 ﬁrst in (b).

on FBG#1 ﬁrst. Both wavelengths 1 and 2 are reﬂectedwith little attenuation and with nearly identical spectra as
those in Figs. 1(a) and 1(b), respectively. Conversely, whenlight is incident on FBG#2 ﬁrst, the reﬂection spectrum of 1
is attenuated by approximately 30% as seen in Fig. 2(b). The
reason for this attenuation is because 1 goes throughFBG#2 twice in its round trip and is aﬀected by the cladding
mode of FBG#2 as explained above.
The eﬀect on the reﬂected light signal of how the FBGs
are connected and in which direction the signal light isincident also applies to multiply connected FBGs with
diﬀerent reﬂection wavelengths. This is demonstrated inFig. 3, where ﬁve FBGs are connected in series. The FBGs
are fabricated in the same way as those of Fig. 1. But forthese, one single-phase mask grating is used and diﬀerent
tension has been applied to the ﬁber. Releasing the tension

Reflection & Transmission (%)

Reflection & Transmission (%)

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FBG#3 #4 #5 #6 #7

FBG#3 #4 #5 #6 #780

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404001547

1549

1551

Wavelength (nm)155301547

1549

1551

1553

Wavelength (nm)Fig. 3. Reﬂection spectra of ﬁve FBGs #3–# 7 connected in series. Signal
light is incident on FBG#3 ﬁrst in (a) and on FBG#7 ﬁrst in (b).

after imprinting the FBG shifts the central wavelength.3)
Five FBGs (FBG#3#7) with 3 ¼ 1549:1 nm, 4 ¼1549:8 nm, 5 ¼ 1550:7 nm, 6 ¼ 1551:2 nm and 7 ¼1552:1 nm are consecutively fabricated in one ﬁber with a
20 mm separation between each. Reﬂection spectra areshown in Fig. 3 with a broad band incident ﬁrst on (a)FBG#3 and on (b) FBG#7. When the light is incident ﬁrst on
the short wavelength FBG, it is reﬂected with little
attenuation and without being aﬀected by the cladding
modes of other FBGs as seen in Fig. 3(a). However, when itis incident ﬁrst on the longer wavelength FBG, all but theﬁrst wavelength, that is 7 , suﬀer attenuation from the otherFBG cladding modes. That is, 7 is reﬂected immediately by
FBG#7 but 3 is attenuated by the cladding modes ofFBG#4–#7 and 4 by the cladding modes of FBG#5–#7 asseen in Fig. 3(b).
From these results, in order to obtain proper reﬂection
signals from multiple FBGs connected in series without
attenuation by cladding modes, it can be concluded that theymust be connected in order from the shortest wavelengthFBG to the longest wavelength FBG and the signal lightmust be incident ﬁrst on the shortest wavelength FBG.
We thank Dr. James C. DeBoo of General Atomics Co.,San Diego, U. S. A. for his careful proofreading.
1) V. Mizrahi and J. E. Sipe: J. Lightwave Technol. 11 (1993) 1513.
2) A. Othonos: Rev. Sci. Instrum. 68 (1997) 4309.
3) K. Nakagawa, Y. Takemura, R. Kunimoto, Y. Mizutani, S. Kimura, Y.
Fukuyama, Y. Suzaki and S. Ejima: Jpn. J. Appl. Phys. 41 (2002) L599.