Jpn. J. Appl. Phys. Vol. 41 (2002) pp. L 599–L 601
Part 2, No. 5B, 15 May 2002c2002
The Japan Society of Applied Physics

Fabrication of Fiber Gratings with Different Bragg Wavelengths
Using a Single Phase Mask
Kiyoshi NAKAGAWA∗ , Yutaka TAKEMURA1 , Ryou K UNIMOTO1 , Yasuo M IZUTANI1 , Susumu K IMURA1 ,Yoshiyuki F UKUYAMA1 , Yoshifumi S UZAKI2 and Seiki E JIMA2
Department of Reliability-Based Information Systems Engineering, Faculty of Engineering, Kagawa University,
2217-20 Hayashimachi, Takamatsu 761-0396, Japan
1 Shinko Electric Wire Co., Ltd., 1298-12 Shido, Sanuki 769-2101, Japan
2 Department of Advanced Material Science, Faculty of Engineering, Kagawa University,
2217-20 Hayashimachi, Takamatsu 761-0396, Japan(Received February 25, 2002; accepted for publication March 26, 2002)

We present a method of fabricating multichannel fiber Bragg gratings using a single mask under control of the applied tension
to the optical fiber. The Bragg wavelength of fiber Bragg gratings is investigated as a function of the amount of tension applied
to an optical fiber during the photoimprinting process. The Bragg wavelength changes linearly with the amount of tension
applied to the optical fiber. Using the linear relation, an optical wavelength filter consisting of five fiber Bragg gratings withfive different wavelengths is fabricated successfully using one phase mask. [DOI: 10.1143/JJAP.41.L599]
KEYWORDS: fiber Bragg grating, Bragg wavelength, optical fiber filter, optical fiber sensor

Fiber Bragg gratings (FBGs) are known as important components of fiber optics owing to the large number of devices
functions in which they can be used.1) Wavelength-divisionmultiplexed systems (WDMs), fiber lasers, and fiber sensors
all use fiber gratings.2–4) In order to use the functions of FBGs
in these optical communications or sensing systems, functions of FBGs should be precisely controlled during fabrication. Particularly, it is necessary that the Bragg wavelength of
FBG is accurately adjusted to the desired wavelength for optical filtering or the wavelength of the light source be used.
Precise tuning of Bragg wavelength is carried out during thefiber grating fabrication process.
Currently, fiber grating fabrication techniques have two different categories: the holographic and the phase mask approaches.5, 6) The phase-mask technique is popular and attractive because it is highly reproducible, it offers easy alignmentof the fabrication system, it requires low coherence light forphotoimprinting the phase mask, and it makes per unit grating fabrication cost low. One drawback of the phase-mask
technique is that the period of the phase mask imposes toostrong a restriction on the grating wavelength. In short, a separate phase mask is required for each Bragg wavelength. Tocompensate this drawback, an approach has been recently reported.7) Another compensation technique for this drawback
is to apply tension to the optical fiber. It is experimentally
known that applying tension to the optical fiber during thephotoimprinting process enables us to tune the Bragg wavelength of FBG.1)
In this letter, the Bragg wavelength of FBGs has been investigated as a function of the amount of the tension applied
to the optical fiber during the fabrication process. It is shownthat the Bragg wavelength of FBG can be tuned by changing
the tension applied to the fiber. Using the tension dependenceof the Bragg wavelength, an optical wavelength filter based
on FBG, which exhibits multichannel filtering, is fabricatedusing one phase mask.
The FBGs fabrication setup is shown in Fig. 1. FBGs werefabricated using a zero-order nulled diffraction phase mask.
The optical fiber was placed almost in contact with the peri∗ E-mail address: nakagawa@eng.kagawa-u.ac.jp

Laser
beam
Phase Mask
Clamp

Clamp
H2 loaded
fiber

Fixed Stage

Movable StageWeightFig. 1. Optical setup for drawing the grating structure with a phase mask.
The amount of applied tension is controlled by the weight.

odic grating corrugation of the phase mask. Both ends of theoptical fiber were fastened to two translation stages. One ofthe stages was fixed and the other was movable. The movable stage was pulled by a constant force owing to a weight,
to apply constant tension to the optical fiber during the photoimprinting process. Coated resin of the optical fiber was removed between the two stages prior to ultraviolet light exposure.
Ultraviolet light normal to the phase mask passes throughand is diffracted by the periodic corrugations of the phasemask. The two diffracted ±1 order beams interfere with eachother to produce a periodic pattern that photoimprints a corresponding grating in the optical fiber. When the period of thephase mask grating is mask , the period of the photoimprinted
index grating is mask /2. The Bragg wavelength λB is givenby
λB = 2n eff (mask /2),(1)where n eff is the effective diffraction index of the opticalfiber. The Bragg wavelength is fundamentally determined by
the pitch of the phase mask used. However, any change infiber properties, such as strain, temperature, or polarization,
which changes the diffraction index or grating pitch, will also

L 599

L 600Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 5Bchange the Bragg wavelength of the FBG.
We used a pulsed KrF excimer laser as the ultraviolet lightsource. The fluence of the laser beam at the optical fiber surface was around 60 mJ/cm2 with a repetition rate of 20 Hz
and the pulse duration was 30 ns. Exposure time was fixed to1 min in this experiment. We used only a single phase mask inthis experiment. The pitch of the phase mask was 1073.1 nm,and the diffraction efficiency in the zero order was minimized (< 3%) at a wavelength of 248 nm. FBGs with center
wavelength of around 1553 nm were fabricated in a standard
single-mode fiber (Corning SMF-28 fiber), which was loadedwith hydrogen (12 MPa, 373 K, 8 days) to enhance its photosensitivity.
The transmission spectrum of each FBG was monitoredin situ during and after the laser exposure with an unpolarized
broadband light and an optical spectrum analyzer set at 0.1 nm
resolution. In this letter, Bragg wavelength was measured by
the center wavelength at which the transmission spectrum of
the FBG exhibited maximum attenuation.
The Bragg wavelengths of FBGs, which were fabricatedunder different amounts of applied tension during photoimprinting process, were measured individually. Measurement
results of the relationship between the Bragg wavelength of
FBGs and the amount of the applied tension during the photoimprinting process are shown in Fig. 2. The Bragg wavelength of each FBG was measured under a tension-free condition. The amount of tension during the ultraviolet light exposure is also shown in the figure. The amount of the tension was calculated for the optical fiber clad diameter of
250 µm. The Bragg wavelength linearly decreases with theapplied weight, that is, the tension. The rate of the change
in the Bragg wavelength per unit weight was 15.7 pm/g. It is
known that this property is due to the residual stress in the optical fiber. From this relationship, when the amount of tension
from 0 g to 250 g is applied to the optical fiber, we can fabricate FBGs, whose Bragg wavelengths are from 1553.2 nm
to 1549 nm using a single phase mask of mask = 1073.1 nm
only. Strictly speaking, these Bragg wavelengths also depend

K. NAKAGAWA et al.

on the amount of the ultraviolet light exposure.
Using the tension dependence of the Bragg grating, a multichannel optical wavelength filter was successfully fabricatedusing a single phase mask. First, an FBG with a Bragg wavelength of 1549.6 nm was fabricated in an H2 -loaded optical fiber under the condition that tension corresponding to aweight of 250 g was applied to the optical fiber. After thisfabrication, the weight was changed to 200 g. Under this condition, the next FBG with a Bragg wavelength of 1550.3 nm
was fabricated at a different position in the same optical fiber.
Subsequently, the weight was changed to 150, 100 and 50 g,and FBGs were respectively fabricated with different Bragg
wavelengths at different positions under each condition of
the applied tension. As a result, each FBG was fabricated asa cascade in the optical fiber. The length of each FBG was15 mm and the separation between any two FBGs was 5 mm.
Therefore, the total length of five FBGs was 100 mm. The
transmission and reflection spectra of the multichannel opticalfiber filter with FBGs are shown in Fig. 3. Excellent reflection
spectrum properties at each desired wavelength are visible in
the figure. This result shows that this multichannel optical
wavelength filter can be utilized in dense wavelength-divisionmultiplexed optical communication systems (DWDMs) in thenear future, and these fabrication techniques are useful to precisely control the Bragg wavelength of FBGs.
Finally, we fabricated many single channel FBGs using asingle phase mask under different amount of applied tension.
Then, we measured the Bragg wavelength of each FBG as afunction of the amount of applied tension, for the purpose ofusing these FBGs for a strain sensor. It is well known thatFBG is useful sensor for strain measurement.8) Recently, several techniques have been investigated for multiplexing FBG
sensors together in a serial array, in order to measure straindistribution.9) In these techniques, it is necessary to fabricate
many FBGs with a number of Bragg wavelengths, which areelements of a sensor array. In this case, the Bragg wavelengthof each FBG is also exactly controlled.
The measurement results are shown in Fig. 4. In this figure,the change in Bragg wavelength versus the amount of applied
tension is shown, because the FBGs used here exhibit dif-2Tension (gf/cm )5 105 1 106 1.5 106 2 106 2.5 106

1553
Tension free
1552
1551
1550
1549
1548

050100 150 200 250 300
Weight (g)Fig. 2. Relationship between the center wavelength of FBGs and theamount of applied tension during the photoimprinting process. The center wavelength under tension and no tension are shown.

Reflection, Transmission (dB)

Bragg wavelength (nm)155405Transmission
0-5-10-15-20
Reflection-251548 1549 1550 1551 1552 1553
Wavelength (nm)Fig. 3. Transmission (a dotted line) and reflection (a solid line) spectra ofa multichannel optical fiber filter with FBGs used a single phase mask.

Change in Bragg wavelength (nm)Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 5B

K. NAKAGAWA et al.

4.0
3.0

2.0

1.0
0.00

5 105 1 106 1.5 106 2 106 2.5 106
2Applied Tension (gf/cm )Fig. 4. Change in center wavelength of five FBGs with different centerwavelengths, which were fabricated using a single phase mask under different amounts of applied tension.

ferent Bragg wavelengths owing to the different tensions applied during the photoimprinting process. In these measurements, 11 kinds of FBGs were used and 45 plots are drawn
on this graph. Although the Bragg wavelength was different
for each FBG due to the effect of the different tensions applied during the photoimprinting process, the change of theBragg wavelength varies linearly with the amount of the applied tension and the behavior is all same for all FBGs. Therate of the change in Bragg wavelength per unit applied tension was 1.57×10−6 nm/(gf/cm2 ). The variation of the change
in Bragg wavelength is due to atmospheric temperature variation. FBGs, which exhibit different Bragg wavelengths, show
the same tension dependence of the change in Bragg wavelength. Thus, these results show that the applied tension tech-L 601

nique enables us to tune the Bragg wavelength of FBGs foran optical fiber strain sensor. We believe that this technique is
useful for fabricating FBGs for sensor arrays in strain distribution sensing, because the Bragg wavelength of each FBG
element should be controlled to obtain the spatial resolutionof the distributed strain.
In conclusion, it is shown that the Bragg wavelengthchanges linearly with the amount of tension applied to theoptical fiber during the photoimprinting process. Using therelationship between Bragg wavelength and applied tension,
Bragg wavelength, which is fundamentally determined fromthe pitch of the phase mask, can be tuned. Multichannel optical wavelength filters of FBGs can be fabricated with one
phase mask. FBGs with different Bragg wavelengths show thesame tension dependence of the change in Bragg wavelength,
although these FBGs were fabricated under different amountsof applied tension. We believe that these results are useful for
fabricating FBGs for optical communication systems or optical fiber sensing systems.
This research was supported by The New Energy andIndustrial Technology Development Organization (NEDO) ofJapan.

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