Japanese Journal of Applied Physics
Vol. 45, No. 12, 2006, pp. 9100–9102#2006 The Japan Society of Applied Physics

Apodization Method Owing to the Finite Length of UV Laser Coherencein Fabricating Fiber Bragg Gratings
Yoshifumi S UZAKI, Hiromu I WATA1 , Keisuke N AKAYAMAy, Yasuo M IZUTAMI2 ,Makoto Y AMAUCHI2 , Eiji I ZUMIKAWA2 , Takashi Y OKOUCHI and Seiki E JIMA
Kagawa University, 2217-20 Hayashi-cho, Takamatsu 761-0396, Japan
1Takamatsu National College of Technology, 355 Chokushi-cho, Takamatsu 761-8058, Japan
2Shinko Electric Wire Co., Ltd., 1298-12 Shido, Sanuki, Kagawa 769-2101, Japan(Received May 24, 2006; accepted August 21, 2006; published online December 7, 2006)

A very eﬀective apodization, in which side lobes are suppressed to below a particular noise level, is found to operate
automatically in a two-beam interference device for fabricating ﬁber Bragg gratings when a pair of mirrors deﬂecting twobeams are oﬀ parallel to each other. This is considered to be a consequence of the short temporal coherence length of the KrF
excimer laser used. [DOI: 10.1143/JJAP.45.9100]
KEYWORDS: fiber Bragg grating, optical fiber, two-beam interference, excimer laser

1.

Introduction

Because ﬁber Bragg gratings (FBGs) have very sharp
narrowband reﬂection spectra, they are well suited toapplications involving narrowband optical signals, such as
optical ﬁlters in optical telecommunication systems, ﬁber
laser cavities, and various types of ﬁber sensor.
An FBG has a periodic refractive index structure inside an
optical ﬁber in its longitudinal direction. It is commonly
fabricated by exposing the optical ﬁber to UV laser lightto create a photoinduced refractive index variation. Since
the grating period is very short, on the order of 0.5 mm,
irradiation is normally performed by exposing the ﬁber tothe interference patterns of UV laser light so that the patterns
are transprinted into the ﬁber. There are two standard
methods for generating these interference patterns: the phasemask and the two-beam interference methods.1) In the phasemask method, the interference pitch is deﬁned by the grating
period of the mask; hence, the resulting FBG wavelength
is ﬁxed. On the other hand, in the two-beam interference
method, the crossing angle of the beams is controlled byadjusting mirror reﬂection angle. This has the substantial
advantage of permitting FBG production with diﬀerent FBG
wavelengths. To let the two beams interfere each other in thetwo-beam interference method, the path lengths of the twobeams must be the same within the coherence length of thelaser. For this reason, lasers with a high coherence quality,
such as an argon ion laser, are generally preferred for FBG
fabrication.
In addition to selecting a suitable laser source in terms ofcoherence, spectral shaping is important in fabricating highquality FBGs for generating a clean spectral peak in an FBG
reﬂection signal. Apodization is a well-known technique forsuppressing side lobes in a reﬂection spectrum by forminga spatial proﬁle of refractive index modulation that varies
smoothly in the longitudinal direction, rather than abruptly
as in the case of a rectangular proﬁle. Apodization iscommonly performed by shaping the proﬁle of the incident
UV laser beam intensity on the ﬁber so that the proﬁle of thedeposited energy varies smoothly along the FBGs during
E-mail address: suzaki@eng.kagawa-u.ac.jp
Present address: MicroCraft Inc., 630-2 Tanaka, Okayama 700-0951,Japan.

yfabrication. Some examples explored are a double-exposuretechnique,2) the dithering control of a phase mask3) and theuse of a self-apodized phase mask.4) Our method yielded anear-perfect apodization without the geometric and opticalconstraints of the previous techniques.
We performed a two-beam interference experiment using
a high-power excimer laser with a rather short coherence
length, as opposed to the general preference for highcoherence laser sources. We found that we could obtain
FBGs with a high reﬂectivity and without side lobes in theirreﬂection spectrum or below a particular noise level. Thatis, a near-perfect apodization was easily attained. The originof this new type of apodization appears to be related to thecoherence length of the excimer laser employed in theexperiments.
2.

Experimental Methods and Results

The two-beam interference method for FBG fabrication
was originally developed by Meltz et al.5) who used a half
mirror and two mirrors. Since one of the two beams
underwent mirror reﬂection once whereas the other underwent two mirror reﬂections in their experiment, the beams
did not cross each other with the same handedness; thus, theinterference was incomplete. Askins et al.6) corrected this byadding another mirror reﬂection to the ﬁrst beam to ensure
an equal number of mirror reﬂections for both beams.
However, adding another mirror made the optical setup
rather complex to control. Instead of a half mirror, Kashyap7)
used a phase mask as a beam splitter. This ensured asymmetrical arrangement of optical components, such asmirrors and their controls, which vastly eased the ﬁneadjustment and control of parameters, such as mirror angle.
The geometry of our two-beam interference experiment isshown in Fig. 1. The excimer laser light incident on thephase mask is diﬀracted into 1st-order beams. The twobeams are reﬂected by the mirrors and cross at the positionof the optical ﬁber. The grating period of the phase mask is1072.34 nm in the experiments reported here. This gives adiﬀraction angle of 13.37 for KrF excimer laser light ( ¼248 nm). The distance 2s between the two mirrors at theiraxes of rotation is 70.3 mm. We made FBGs in three
diﬀerent wavelength bands, 1.55, 1.3, and 1.0 mm, byadjusting mirror angle. The ﬁber was positioned at the point
where the two beams crossed when the mirrors were rotated,9100

Jpn. J. Appl. Phys., Vol. 45, No. 12 (2006)

Exicimer laser

Phase mask

10Reflection (dB)θ -αθMirrorY. SUZAKI et al.

Mirrorangle αα

θ -2αα2sIncidence angle
β = θ - 2αFiber-10-20-30-40-50Mirror
Transmission (dB)

L0(a) No apod.
#1050-5-10-15-20-25-30-3510(b) Apod.
#250(c) Apod.
#3-5-10-15-20-25
500-10-5-10-20-10-20-30-401548

1553-301558 1305 1308 1311

λ (nm)

Fig. 1. Two-beam interference geometry.

-15-201082.5

λ (nm)

10851087.5

λ (nm)

Fig. 2. Spectra of three diﬀerent FBGs.

Table I. Parameters used in experiments.
FBG

#1

#2

#3Mirror angle  (deg)Fiber to mask distance L (mm)

0.00
295.8

1:27271.2

2:97
248.2

Incidence angle  ¼   2 (deg)

13.36

15.91

19.31

Optical path diﬀerence  (mm)

0.00

0.332

FBG (nm)1552.71309.4(a)Two excimer
beams

0.781

P

1085.0Fiberand its distance from the phase mask is listed in Table I,
together with the other parameters used in the experiments.
We used a KrF excimer laser (Lambda Physik, COMPex102MJ with an unstable resonator). The beam was shapedusing an optical stop to a size of 7:5  7:5 mm2 , and theenergy delivered to the phase mask is 30 mJ per pulse at arepetition rate of 20 Hz. The energy density at the ﬁber wasincreased by a factor of 12 using a cylindrical lens. The
length of the FBGs is the same as the beam width of 7.5 mm.
The zeroth order and higher-order beams generated using
the phase mask were blocked using optical stops. SMF-28
optical ﬁbers were used for FBGs #1 and #2, and an HI-1060
ﬁber was used for FBG #3. The ﬁbers were loaded withhydrogen at a pressure of 100 atm for 10 days prior to theexperiments to increase their UV sensitivity. The duration ofexcimer laser irradiation was set to be 3 min. The transmission and reﬂection spectra of the three FBGs are shownin Fig. 2. Many side lobes were observed in for FBG #1[Fig. 2(a)], which was fabricated with the two mirrors inparallel. As the mirrors were rotated from  ¼ 0 to theangles corresponding to the FBG reﬂection wavelengths of
1.3 and 1 mm, the side lobes disappeared completely in thereﬂection spectra, falling below the noise level.
This result indicates that a very eﬀective apodizationeﬀect is created only by rotating the mirrors. Our interpretation of the apodization eﬀect is shown schematically inFig. 3. When the two mirrors are rotated symmetrically at
the same angle  in Fig. 1, the image of the phase mask at
the ﬁber position after reﬂection by a mirror is tilted at
the angle 2. Since the two images are tilted in oppositeIRIL

2dcoherence(b) Refractiveindex

∆

2αxQ

Optical path
differencealong fiberFig. 3. Schematic of apodization physics model.

directions, they intersect as shown in Fig. 3, where theimages are depicted as IR and IL with depths equal to thetemporal coherence length dcoherence in the direction of beam
propagation. The optical path diﬀerence  is generated
away from the FBG center. Interference occurs in the region
where the two images IR and IL overlap, but not outside theregion, where the diﬀerence  becomes larger than thecoherence length dcoherence . Since the path diﬀerence  iszero at the center (x ¼ 0), the interference is maximal andfalls oﬀ toward the edge of the FBG, resulting in a refractive
index pattern that creates the apodization eﬀect. It should
be noted that, in this case, the index averaged over theinterference period is uniform along the ﬁber, because theﬁber is exposed to the beam even with a partial or nointerference; that is, the apodized ac component of refractive
index modulation is superimposed on its dc component. This
yields a perfect apodization, which is symmetric in both
positive and negative directions of the index modulation
pattern.

9101Jpn. J. Appl. Phys., Vol. 45, No. 12 (2006)

Y. SUZAKI et al.

The coherence length of the excimer laser is estimatedfrom our data using the apodization condition, dcoherence < for FBGs #2 and #3. The  values listed in Table I imply
that dcoherence is less than 0.33 mm, which appears to be ingood agreement with the length derived from the spectral
width of the KrF excimer laser emission reported in ref. 8,that is,  2 = = (248 nm)2 /(0.5 nm) = 0.12 mm. Our ﬁrst
estimate for the coherence length may be longer than
0.12 mm because our excimer laser is equipped with an
unstable resonator.
The transmission minimum becomes shallower as themirror angle increases from #1 to #2 and from #2 to #3,
because the eﬀective FBG length becomes shorter with an
apodized refractive index proﬁle.
Here, we have demonstrated apodization for 1.3 and 1 mm
FBGs using a phase mask that produces a 1.55 mm FBG
in the phase mask method. We also have conﬁrmed thatapodization is realized in the opposite case, i.e., apodization
in a 1.55 mm FBG is realized using a phase mask that
produces a 1.3 mm FBG in the phase mask method.
3.

Discussion

It is commonly assumed that UV lasers with better
coherence should be used to facilitate the formation of aninterference pattern for FBG fabrication. Thus, high-quality
UV lasers such as argon ion lasers and injection-locked
excimer lasers, have been used. However, our analysis
shows that the present geometric apodization technique may
not work as well for these lasers with a long coherence
length. That is, if the coherence length is longer than thepath diﬀerence at the edge of an FBG, the interference
pattern will be uniform. Thus, lasers with a relatively shortcoherence length, such as excimer lasers, are better suitedfor FBG fabrication by this method.
Frohlich and Kashyap9) suggested the possibility of FBG
apodization based on the ﬁnite coherence length of a UVlaser. That is, if the laser beam is split into two and brought
together with a predetermined path length diﬀerence, theinterference fringe contrast will be aﬀected by the degree ofincoherence. To achieve this, they proposed, as an example,

a scanning technique applied to a mirror interferometer like
ours, but no experimental proof was provided. Here, we have
utilized the ﬁnite coherence length of a UV laser for FBG
apodization, but our approach completely avoids the complex mechanical control implied in ref. 9. Our method ismuch simpler, in that it involves tilting the two mirrors oﬀ
parallel, as demonstrated above. The method is novel andprovides a very powerful means of near-perfect apodizationof FBGs.
4.

Conclusions

A two-beam interference device was constructed, andvery eﬃcient narrowband FBGs were fabricated with a highreproducibility. When two mirrors are symmetrically rotatedfrom their parallel position, the two images of a phase mask
at the position of an optical ﬁber cross each other obliquely.
This generates an optical path diﬀerence along the length of
an FBG such that, when the coherence length of the laser
used is shorter than this path diﬀerence, apodization isautomatically induced. This apodization technique is considered very eﬀective in suppressing side lobes to below aparticular noise level.
Acknowledgement
We acknowledge Dr. D. H. McNeill for carefully proof
reading our manuscript.
1) R. Kashyap: Fiber Bragg Grating (Academic Press, London, 1999)
Chap. 3.
2) B. Malo, S. Theriault, D. C. Johnson, F. Bilodeau, J. Albert and K. O.
Hill: Electron. Lett. 31 (1995) 223.
3) M. J. Cole, W. H. Loh, R. I. Laming, M. N. Zervas and S. Barcelos:Electron. Lett. 31 (1995) 1488.
4) J. Albert, K. O. Hill, B. Malo, S. Theriault, F. Bilodeau, D. C. Johnson
and L. E. Erickson: Electron. Lett. 31 (1995) 222.
5) G. Meltz, W. W. Morey and W. H. Glenn: Opt. Lett. 14 (1989) 823.
6) C. G. Askins, T.-E. Tsai, G. M. Williams, M. A. Putnam, M. Bashinskyand E. J. Friebele: Opt. Lett. 17 (1992) 833.
7) R. Kashyap: Electron. Lett. 34 (1998) 2025.
8) R. Patzel: The Photonics Design and Applications Handbook (LaurinPublishing, Pittsﬁeld, MA, 1998) 44th International ed., p. H-247.
9) H. G. Frohlich and R. Kashyap: Opt. Commun. 157 (1998) 273.

9102