Japanese Journal of Applied Physics
Vol. 43, No. 1, 2004, pp. 147–151#2004 The Japan Society of Applied Physics

Evolution of Optical Fiber Temperature during Fiber Bragg Grating Fabrication
Using KrF Excimer Laser
Mitsuo N AKAMURA, Chisa K OMATSUy, Yuji M ASUDA, Keio F UJITA, Makoto Y AMAUCHI1 , Yasuo M IZUTANI1 ,Susumu K IMURA1 , Yoshifumi SUZAKI, Takashi Y OKOUCHI, Kiyoshi N AKAGAWA and Seiki EJIMAz
Department of Advanced Material Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan1
Shinko Electric Wire Co., Ltd., 1298-12 Shido, Sanuki, Kagawa 769-2101, Japan(Received May 19, 2003; accepted October 9, 2003; published January 13, 2004)

The temperature distribution in an optical ﬁber during the fabrication of ﬁber Bragg gratings (FBGs) using a KrF excimer laser
with a phase mask has been analyzed experimentally for typical fabrication conditions. The ﬂuence of UV laser light at theﬁber surface has been varied up to 420 mJ/cm2 . These experiments show that (1) scanning electron microscope (SEM) imagesof the ﬁber surface facing the excimer laser beam reveal partial physical damage apparently owing to partial melting, so thatthe surface temperature has increased beyond the softening point of silica glass, which is approximately 1200 C, (2) the
optical spectrum transmitted through the FBG during laser irradiation contains spikes coincident in time with the laser pulsesthat correspond to a near instantaneous shift of the FBG spectrum to a higher temperature spectrum representing ﬁber coreheating of approximately 8 C, and (3) analysis of the energy absorbed by the ﬁber indicates a bulk temperature rise of
approximately 3 C in the ﬁber. The resulting large variation in the temperature rise over the ﬁber cross section from a few  C
to 1200 C, along with partial physical damage on the surface, will certainly induce large internal stresses in the ﬁber material
and reduce the mechanical strength of the FBG. [DOI: 10.1143/JJAP.43.147]
KEYWORDS: optical fiber, fiber Bragg grating, KrF excimer laser, heat conduction, phase mask

1.

that the eﬀect of UV irradiation should be very small there.
However, when the laser has a high power on the order of
MW/cm2 , the eﬀect may become appreciable although thefractional absorption is very low.
If the laser beam is well focused onto the optical ﬁber, theeﬀect may become serious. Severe physical damage isreported in ref. 1 where an FBG was made by exposing aﬁber positioned at the focal point to a single excimer laser
pulse using the two-beam interference method. Reference 1
revealed that the surface of the optical ﬁber was melted by
the laser with corrugations corresponding to the FBG grating
period. Also, many cracks were observed across the ﬁber.
The condition for FBG fabrication reported in ref. 1, whichis intended to generate a type II FBG by a single excimer
laser pulse, may be an extreme case. Since an FBG is usually
made by exposing a ﬁber to an excimer laser for about one or
few minutes at a 5 to 20 Hz repetition rate, such severe
damage as shown in ref. 1 may not occur under such mild
condition. We cannot, however, rule out the possibility ofpartial but similar damage under the standard conditions for
FBG fabrication.
If an FBG is damaged in this way during the fabrication
process, its mechanical strength will certainly be degraded.
The mechanical strengths of optical ﬁbers exposed to two
diﬀerent lasers under the condition of FBG fabrication withthe same amount of laser irradiation doses are compared inref. 2. One was exposed to an Arþ 240 nm CW laser and theother, to an excimer laser. Weibull plot analysis of themechanical strength of the ﬁbers showed that the ﬁber
exposed to the CW laser had almost exactly the same
strength as the pristine ﬁber (5 GPa), while the strength of
the ﬁber exposed to the excimer laser was reduced by morethan half (to 1–2 GPa). This result appears to be verysigniﬁcant because the lifetime of FBGs fabricated using
excimer lasers will be shortened signiﬁcantly in many
applications.
To clarify the damage process in optical ﬁbers, the veryﬁrst consideration is how much the temperature rises andIntroduction

Because of their simple structure and very sharp spectral
response, ﬁber Bragg gratings (FBGs) are widely used formany optical telecommunication applications. Nowadays
they are used in optical strain gauges, resonators of ﬁber
lasers, and numerous types of sensors.
The wide ranging applications of FBGs in many ﬁelds
may be explained partially because they can be easily
fabricated by exposing an optical ﬁber to a UV excimer laser
beam through a phase mask grating. They can also be
fabricated using CW UV lasers such as the Arþ 240 nm laser
with a typical power of approximately 100 mW by a twobeam interference without using a phase mask. However, in
this method, accurate tuning is required to intersect the angle
between the two beams to obtain the desired central
wavelength of the FBG and, hence, it is very sensitive tomechanical vibrations of the optical setup and even to air
current. The optical arrangement requires precision alignment. The phase mask method with excimer lasers such as
KrF (248 nm) and ArF (193 nm) is more commonly used inindustry, primarily because of the ease of the optical setup,
but also because of the high power of these lasers.
The excimer laser normally provides 100 mJ to a few
100 mJ energy per pulse at a repetition rate up to
approximately 20 Hz. Its pulse width is 20–30 ns. Its output
power is several MW to above 10 MW, or 107 times higher
than that of the CW Arþ laser. An FBG is created byinducing defects in the chemical bonding of the germanium
dopant in the optical ﬁber core by exposing it periodically to
UV laser light along the axis of the ﬁber.
Since the cladding is made of pure silica, which is highly
transparent and has very low UV absorption, it might seem
Present address: Material Science, Graduate School of Engineering, Osaka
University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
yPresent address: Fujitsu Kochi Systems Engineering Ltd., 1-4 Hotarugaoka, Nankoku, Kohchi 783-0060, Japan.
zE-mail address: ejima@eng.kagawa-u.ac.jp
147

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Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)

KrF
Excimer
Laser

3X Beam
ExpanderMirrorM. NAKAMURA et al.

(a)

Stop
Cylindrical Lens
Stop

Phase mask
FiberFig. 1. Schematic of FBG fabrication.

how it evolves dynamically across the ﬁber during exposure
to an excimer laser pulse. This paper provides experimental
information on the temperature behavior of FBGs during
fabrication.
2.

(b)Experiments

The schematic of our experimental apparatus3) is shown in
Fig. 1. The excimer laser (Lambda Physik COMPex-102)
delivers up to 200 mJ of energy at 248 nm with a pulse widthof 25 ns at a repetition rate of 1–20 Hz. The 8  24 mm2
rectangular laser output beam is shaped by a 5  5 mm2 stop
to select a higher quality uniform portion of the beam. This
limits the power to one-quarter of the laser output. After
passing through a 3X beam expander, the beam is deﬂected
downward by a mirror and focused by a cylindrical lens witha focal length of 100 mm. A bare optical ﬁber is positioned
80 mm away from the cylindrical lens, i.e., 20 mm in front of
the focal point. A phase mask grating is placed above theﬁber at a distance of less than 1 mm from the ﬁber. A secondstop is placed above the mask. The beam size in the plane of
the ﬁber is 12 mm along the ﬁber and 0.65 mm in thetransverse direction. This yields an FBG length of 12 mm.
The ﬂuence of the excimer laser light at the ﬁber has beenvaried up to 420 mJ/cm2 .
The optical ﬁber used is a standard single mode ﬁber(Corning SMF-28), whose core and cladding diameters are
8.3 and 125 mm, respectively. The eﬀective refractive index
neff is about 1.4456. Prior to the experiment, the ﬁber was
kept under 10 MPa hydrogen gas for ten days for hydrogenloading to increase the sensitivity of the ﬁber to UV light.
The resin coating of the ﬁber is removed and the ﬁber isultrasonically cleaned just before it is placed underneath thephase mask. Grating period p of the phase mask is1070.3 nm giving a grating pitch ¼:p=2 in the ﬁber core;hence, the FBG central wavelength is expected to be 0 ¼
2neff  ¼ 2neff p=2  1547 nm.
We normally monitor FBG formation by injecting abroadband light signal from an ampliﬁed spontaneousemission (ASE) source (Thorlabs ASE-7701-AP) and analyzing the signal transmitted through the FBG with an
optical spectrum analyzer (Advantest Q8384).3) In theexperiment described here, we have monitored the FBG
formation using a wavelength-scannable LD light source(Ando AQ4321A, wavelength resolution 1 pm) and a highspeed photodetector (Thorlabs SIR5-FC, 5 GHz bandwidthand 70 ps minimum rise time) to achieve high resolution in
the dynamic evolution of the spectrum.

Fig. 2. SEM images of FBG surface exposed to the excimer laser.
Magniﬁcation is 1000X in (a) and 3300X in (b).

When the ﬁber is exposed to the excimer laser beam at
180 mJ/pulse at a 20 Hz repetition rate, a transmission
spectrum with 30 dB at the FBG wavelength and 0.2 nm
width can be easily obtained within about two minutes. A
scanning electron microscope (SEM) image of the surface ofan FBG fabricated in this manner is shown in Fig. 2. The
magniﬁcation is 1000X in (a) and 3300X in (b). Scratchlike
damage is clearly visible with many stripes in the circumferential direction over approximately 30 mm, or 1/8 of the
ﬁber circumference, i.e., over a 30 angle. The damaged
portion is magniﬁed in Fig. 2(b). Although weak and faint,
many stripes corresponding to the pitch of the FBG, which isapproximately 0.5 mm, i.e., one-half of the grating pitch p of
the phase mask, are visible. Since the softening temperatureof pure silica glass is 1100–1200 C, the surface of thedamaged portion has exceeded that temperature. Since theﬂuence is typical of the level commonly used in FBG
fabrication, we consider that FBGs fabricated by excimer
laser irradiation will have physical damage relatively similar
to that shown in Fig. 2.
We usually monitor the transmission spectrum as the FBG
grows during exposure of the ﬁber to the excimer laser, and
we always observe spikes in the spectrum corresponding to
the excimer laser pulse. The spikes are positive at wavelengths below the FBG central wavelength and negative at
wavelengths above the FBG central wavelength. A typical
spectrum obtained with the wavelength scannable LD lightJpn. J. Appl. Phys., Vol. 43, No. 1 (2004)

M. NAKAMURA et al.

149

1

B

(arb. units)

Signal transmitted

Signal Intensity
Transmitted through FBG (v)

10

A

A

-20

B

0.08nm

0

20

40

60

80

100

120

140

Time (µs)

01546.81546.91547.01547.11547.2

Fig. 5. Temporal evolution of the light signal transmitted through the
FBG. Wavelength of the signals (A) and (B) are those marked in Fig. 3.

Wavelength (nm)Fig. 3. Spectrum of optical signal transmitted through the FBG. Spikes
correspond to excimer laser pulse. Dashed line is generated by smoothly
connecting peak points of the spikes. Temporal evolutions of the signal
with wavelengths ﬁxed at A and B are shown in Fig. 5.

FBG central wavelength (nm)source and the fast photodetector is shown in Fig. 3. The
excimer laser pulse energy is 180 mJ/pulse, and the FBG has
been fabricated by running the excimer laser at a 20 Hz
repetition rate for 2 min. Subsequently, the spectrum shownin Fig. 3 has been obtained with the excimer laser operating
at a 1 Hz repetition rate. The sweep time is about 10 s in thisﬁgure and spikes appear every second when the excimer
laser is turned on. These spikes can be interpreted as a
momentary shift of the spectrum to longer wavelengths by
the laser pulse, as indicated by a dashed line in the ﬁgure
which is generated by smoothly connecting the peak pointsof the spikes. The wavelength shift is approximately
0.08 nm. After a series of experiments, the temperaturedependence of the FBG wavelength was determined separately. The result is shown in Fig. 4 and 10 pm/ C isobtained. Thus, an instantaneous shift of 0.08 nm corresponds to a temperature increase of 0.08 nm/(10 pm/ C)
8 C. Since this information is derived from the spectrumof light transmitted through the FBG formed in the ﬁbercore, the temperature increase of approximately 8 C occurs
at the ﬁber core.
To examine the dynamic evolution of the temperature, the1547.6
1547.41547.21547.01546.81546.6
0

20

40

60

Teperature (°C)
Fig. 4. Temperature dependence of FBG central wavelength.

80

temporal response of the transmission signal is measured byﬁxing the wavelength of the LD light source. This is shownin Fig. 5 for two wavelengths, 0.02 nm shorter (labeled A)
and 0.16 nm longer (B) than the FBG central wavelength
shown in Fig. 3. The temperature of the FBG ﬁber core rises
within 15–20 ms to its maximum, which is approximately
8 C as shown above, and cools down within about 100 msec.
The temperature increase of the FBG as a whole can be
deduced from the thermal energy balance and the physicalproperties of silica glass, which are described byE ¼   V  c  T;

ð1Þ

by measuring the energy E absorbed by the FBG, where is the density, V is the FBG volume, and c is speciﬁc heat.
The excimer laser pulse energy is measured at the laser
output, above the cylindrical lens, and underneath the ﬁberusing a UV power meter. The result of these measurements
is shown in Fig. 6(a). The size of the laser beam is 0:65 
12 mm2 at the plane of the ﬁber, for an energy density of
32.5 mJ/(0:65  12 mm2 )  420 mJ/cm2 for the highest
energy data point. The diﬀerence between energies with andwithout the ﬁber inserted, which gives the energy absorbedby the ﬁber, is small but clearly detectable. The absorbed
energy is plotted in Fig. 6(b), which is about 3% for all theapplied energies, as can be seen in Fig. 6(c). Using physicalproperties of pure silica glass, i.e., its density  ¼ 2:2 
103 kg/m3 and speciﬁc heat c ¼ 0:74  103 J/(kgK), together with the ﬁber volume V ¼ ð125 mm=2Þ2  15 mm, we
ﬁnd T ¼ 3:38 C for E ¼ 1 mJ. This value of 3.38 C is,of course, the temperature increase averaged over the FBG
volume.
By summarizing the above results, our study of thetemperature rise in an FBG under typical fabrication
conditions shows that
(1) SEM observation of the FBG surface yields T ¼
1100{1200 C or more at the surface where the excimer
laser beam is incident;(2) The spikes in the transmission spectrum indicateT  8 C at the ﬁber core of the FBG; and,(3) Measurements of the absorbed energy yield T equal
to a few degrees averaged over the FBG volume.
The temperature increment has very large range over theﬁber cross section.

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Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)

M. NAKAMURA et al.

KrF excimer laser(a)Measured Energy (mJ)

150

Excimer Laser Output

Most heatedregion100
Undeneath phase mask
without fiber ( )with fiber ( )

50

Fiber
core
Heat flux

Above cylindrical lens

Energy Absorbed by Fiber (mJ)0
1.2

Temperature
contour(b)
1
0.8

Optical ray

Air

0.6
0.4

Fig. 7. Schematic of temperature and heat ﬂow evolutions on the ﬁber
cross section of the FBG.

0.2
0Energy Absorbed
by Fiber (%)

6
(c)

5
4
3
2
1
0
0

50

100

150

200

Setting Enegy per Pulse (mJ)
Fig. 6. Energy of excimer UV laser measured at various locations. (a) is at
the laser exit, at above the cylindrical lens and at the position beneath theﬁber with and without its presence. (b) is the energy absorbed by the ﬁberand (c) shows its fraction in percentage.

3.

Discussion

The temperature increase during excimer laser irradiationof FBGs during fabrication show that it ranges from a few
C to above 1000 C over the ﬁber cross section. The
temperature distribution and heat ﬂow on and around theﬁber cross section are illustrated schematically in Fig. 7. A
surface area corresponding to approximately 30 of thecircumference is heated rapidly to a temperature close to or
exceeding the softening point for silica glass. The ﬁber coreis heated directly by the excimer laser beam and/or by theheat conducted from the surface, which has reached about
8 C. The temperature increase for the FBG as a whole isonly a few  C.
A surface temperature above 1000 C is unusually high.
The thickness of the surface layer can be estimated from eq.
(1) assuming that most of the absorbed energy is depositedin this layer. Taking the volume of the surface layer as equal
to the FBG length of 15 mm, multiplied by the 30 mm
circumferential length of the scratch in the SEM images,
multiplied by the thickness d, and taking T ¼ 1200 C as
the softening point of silica glass, yields a thickness d of
0.57 mm if half of the laser energy is absorbed there, i.e.,E ¼ 1=2 mJ. If 10% of the energy is absorbed, thethickness is d  0:11 mm. This estimate, although crude,

indicates that the layer is extremely thin.
If the temperature increase at the ﬁber core is solely due tothe energy absorbed there, the transmission signal should nothave the time delay shown in Fig. 5. The delay time of 15–
20 ms appears to be the time for conduction of heat depositedat the surface to the ﬁber core. Heat transport and removalby the surrounding air will, of course, require a moredetailed analysis.
One might suspect that the large surface temperature rise
originates from foreign material on the surface which may
have a high absorption eﬃciency for excimer laser light.
Since the ﬁber surface is cleaned by cleaning tissue withalcohol after removing the coated resin and is further
cleaned ultrasonically in alcohol before being mounted in
the optical system, we consider that this is unlikely. On theother hand, the phase mask placed above the ﬁber is used formany months in our experiments without any cleaning, andthere is no particular deterioration in the FBG characteristics
or any evident damage on its surface. The mask is also madeof silica glass but is ﬂuorine-doped to ensure higher UVtransmission and lower absorption. If the ﬁber is made ofmaterial with ﬂuorine doping, we expect that strong surface
absorption of the laser light may not occur.
Our result that a very thin surface layer, perhaps less than
1 mm thick, has exceeded 1000 C and the FBG as a bulk has
increased by a few  C indicates that large internal stresses
are induced in the ﬁber. The stress can be removed byannealing and is discussed in many articles.4) in which thediscussions are focused on the defects induced by UV laser
light on Ge chemical bonding in the ﬁber core. However,there is little or no discussion, to our knowledge, on ﬁber
cladding which occupies the bulk of the ﬁber volumewise
and is made of pure silica. Our results suggest that an
investigation of stress removal by annealing for the bulk
FBG is required. The scratchlike stripes may also certainly
reduce the mechanical strength of the ﬁber. Reference 2
shows by Weibull plot analysis a large deterioration in thestrength of the ﬁber exposed to the excimer laser, whereasthat exposed to a CW laser of Arþ 240 nm has very little orno deterioration at all. Further study is required to determineJpn. J. Appl. Phys., Vol. 43, No. 1 (2004)

whether the damage to the ﬁber in the form of scratchlike
stripes is the cause of this kind of degradation in mechanicalstrength.
4.

Conclusions

We have conducted an experimental study of the temperature rise in an FBG during exposure to KrF excimer laser
light using phase mask under typical FBG fabrication
conditions. The irradiation energy is 420 mJ/cm2 at theposition of the ﬁber. The following results have beenobtained:(1) SEM observation of the FBG surface has revealed
weak but clear physical damage with scratchlike marks
stretching over 30 mm in the direction of the ﬁber
circumference on the side where the laser light isincident. This indicates that a very thin surface layer ofthe ﬁber has reached or exceeded the glass softening
temperature, i.e., 1200 C.
(2) The spectrum of light transmitted through the FBG
contains spikes corresponding to the time of the laser
pulses, implying a nearly instantaneous shift of thespectrum to one at a higher temperature. The temperature increase at the ﬁber core implied in thisM. NAKAMURA et al.

151interpretation is approximately 8 C.
(3) Measurement of the UV laser light absorbed by theﬁber together with an energy balance analysis has
yielded a temperature increase of a few  C averaged
over the entire FBG.
The temperature distribution over the ﬁber cross section,with a very large range from a few  C to over 1000 C, may
induce large internal stresses in the ﬁber. Whether the stresscan all be removed by annealing requires further study. Also,
whether the physical damage appearing as scratchlike marks
is responsible for the degradation of mechanical strength
reported in ref. 2 awaits further study.
Acknowledgement
We thank Dr. D. H. McNeill for his careful correction of
our manuscript.
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