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T AKAI, A ., K AT O, T., YAM ASHIT A. S., H A N A T A N I , S.,
M OT E GI. Y., ITO, K .,
Amplitude mask patterned on an excimerlaser mirror for h igh
intensity writ ing of longperiod fibre gratingsH.J. Patrick, C.G.
Askins, R.W . McElhanon andE.J. Friebele
Indexing terms: Gratings in fibres, Mu.slzsMasks have been
produced for long period fibre gratingfabrication from commercial
dielectric laser mirrors. The masks,which were produced by direct
patterning of a photoresist usingan argon laser, can withstand in
excess of 200mJ/cm2 per 15nspulse of 248nm laser light. The use of
these masks decreasedexposure times by 90% and nearly doubled the
attenuation (dB)of a long period grating produced by a given U V
fluencecompared to chrome-on-silica masks.
Introduction: Long-period fibre gratings (LPGs) provide
promi-nent attenuation bands at specific wavelengths in optical
fibre, andhave been applied for band-rejection [11, gain-flattening
of erbium-doped fibre amplifiers [2], sensing of strain,
temperature, andrefractive index [3, 41, and fibre Bragg grating
sensor demodula-tion [5] .An LPG is a periodic modulation of the
index of refrac-tion in the fibre core, typically with a period
>1 O O p a nd a lengthof a few cm, which is induced by patterned
irradiation of the fibrewith an intense UV source, such as a K rF
excimer laser. The pat-tern is usually defined by a mask, but can
also be defined by usingpoint-by-point exposure [6]. An amplitude
mask permits therepeated use of a precision manufactured optic to
produce multi-ple LP Gs with little requirement for precision
during the writingexposure.Producing LPG s of a few centimetres in
length with attenuationbands of greater than a few decibels
requires large refractive indexmodulations (An>1W). This is
generally achieved by enhancing
the fibre photosensitivity by hydrogen- or deute rium-lo ading,
fol-lowed by exposure to total fluences of several kJ/cm2
[7].Chrome-on-silica photolithography masks have been used,
although theirdamage threshold is -100mJ/cm2 [l]. The erratic
intensity profileof an excimer laser can limit average intensities
to 99% reflecting layers could be easily and selec-tively removed,
with minimal effect on the silica substrate. Wehave patterned the
etching by directly exposing a photoresist resinwith a focused
argon laser beam. To our knowledge, this is thefirst published
report of direct laser patterning of a dielectric mir-ror am
plitude mask for long period grating fabrication.Fabrication: A
commercial photoresist (Hoechst AZ 5214) wasspu n to a thickness of
1 . 2 ~n a clean, 5cm diameter high powerKrF excimer laser mirror
(CVI par t #KRF-2037-O), then bakedat 90C fo r 35min in a
convection oven. An argon laser (CoherentInnova 90-6) configured
for multi-line, single transverse-modeoperation in the 351-363nm
range was expanded with a 3x tele-scope to improve intensity
uniformity at the image position. Onedimension of the collimated,
Gaussian beam was restricted by a-1 slit placed before a 50 focal
length fused silica cylindri-cal lens oriented with the cylinder
axis perpendicular to the slit.These optics formed a line image of
-1 x 10pn at the sub-strates surface. A motorised stage translated
the mask in theplane of its surface and perpendicular to the axis
of the line at aconstant rate so that the focused line swept out a
1 mm wide pathon the photoresist. To produce the desired periodic
pattern, anelectro-mechanical shutter placed before the telescope
imposed asquare-wave modulation on the beam in synchrony with
themotion of the translation stage.Exposures ranging fro m 40 to
130mJ/cm2 were sufficient fortotal removal of the exposed
photoresist during developing, with-out broadening the exposed
areas through blooming. After devel-oping the pho toresist, the
exposed areas of the dielectric mirrorwere etched away using a
solution of 5% HF acid in deionisedwater; the remaining photoresist
was then removed with solvents.
Fig. 1 Photogruph ofxection qf,fini.shed dielectric nm k
Results und discussion:Fig. 1is a photograph of a completed m
askwith a period of 3OOpm taken under low m agnification w ith
visiblelight. Most of the surfkc is still covered by the dielectric
stack(dark grey), while the areas where the mirror has been
etchedaway appear light grey. Transmission of the patterned region
at248nm was measured to be 4196, while 46% would be expected fora
50 % duty cycle mask with two 4% Fresnel reflections. The m askwas
found to withstand several dozen 15ns pulses at
200mJ/cm2transmitted intensity per pulse without damage. At
250mJ/cmZ perpulse, ablation of the reflecting layers at the
boundary betweenetched and un-etched areas was seen after a few
tens of pulses.
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7/30/2019 Amplitude mask patterned on an eximer laser.pdf
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Considering the presence of hot spots in the excimer beam,
theactual dam age threshold was proba bly higher than 250m
J/cm2.J=-TI-10
U
-10 1 W5 I 1 51180121Ll3 , p m
Fig. 2 Transmission spectra of 3 0 0 p ~eriod LPGs written using
eithera chrome or dielectric muska C hr om e , IOHz, 23m J/cm2 per
pulse, IOminb Dielectr ic, lOHz, 120mJ/cm2 per pulse, lm inc
Dielectric, lOHz, 120m J/cm2 per pulse, IOmin
1 .2
To measure the advantage of writing LPGs with a
dielectricagainst chrome-on-silica mask, each was used for writing
2.1cmlong LPGs with a 300pn period in Lucent Technologies
disper-sion-shifted fibre. The fibre had been hydrogen loaded
under12OOpsi at 70C for several weeks, for an estimated [HJ
ofO.Smol%. As shown in Fig. 2, using the chrome mask at
IOHz,23mJ/cm 2 per pulse, the transmission of the deepest a
ttenuationband was -1 dB after lOmin (Fig. 2u). In contrast, the
attenuationof the grating written using the dielectric mask at
lOHz, 120mJ/cm2 per pulse, reached -1dB after only lm in (Fig. 2b)
and -9dBafter lOmin (Fig. 2c). In other exposures we have seen as
much as-25dB attenuation after 10 min at lSHz, with 18SmJ/cm 2
perpulse with the dielectric mask . We also mo nitored the d epth
of thelargest attenuation band against UV fluence for high and
lowintensity exposures, as shown in Fig. 3. After 1.15kJ/cm2
perpulse, attenuation of the g rating written at low intensity
(chromemask, 28m in at 30H z, 23mJ/cm2 per pulse) had only reached
-6dB, while that of the grating written at a higher intensity
(die-lectric mask, 8min at 5Hz followed by 4min at 30Hz, 120mJicm2
per pulse) was -1 1dB. Intensities were measured after
trans-mission through the masks to ensure that this effect was not
dueto a difference in mask attenuation, and similar results
wereobtained when the dielectric mask was used for both the high
andlow intensity exposures. Ths confirms tha t the use of higher
inten-sity pulses not only decreases exposure times, but also
generatesLPGs with a greater attenuation for a given UV
fluence.
fluence, J /cmLFig. 3 Transmission of largest uttenuation band
aguinst U V puenceusing chrome and dielectric musks-A- 23mJ/cm2 per
pulse, chrom e--O-- 20mJ/c m2 per pulse, dielectric
Conclusions: We have demonstrated a technique which
allowsresearchers with access to a UV-adapted argon laser and a
pho-toresist spinner to produce robust masks for LPG fabrication
in-house. The same mirror can be coated with photoresist,
exposed,and etched many times. A single pattern is exposed in less
than a1168 ELEC
minute, and >20 masks per m irror can be developed and etched
ina day. The masks enable rapid writing of LPGs with up to
25dBattenuation in fibre with moderate H2-loading.Acknowledgments:
We gratefully acknowledge helpful conversa-t ions with T. Erdogan
and A.D. Kersey, and thank D .L. Griscomfor his assistance with the
etching studies. We thank Bell Ldbord-tories, Lucent Technologies
for providing the fibre used in thiswork. H .J. Patrick
acknowledges the support of an Am ericanSociety for Engineering
Education postdoctoral fellowship. Thiswork was sup ported in part
by the Office of Nav al Researc h.0 EE 1997Electrorzics Letters
Online N o : 19970780H.J. Patrick, C.G. Askins , R . W . McElhanon
and E.J . Fr iebele (NavalResearch Laboratory, Opticul Sciences
Division, Code 5600, 4555Overlook Avenue SW, Washington, D C 20375,
USA)
2 M a y 1997
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COST 241 intercomparison of nonlinearrefractive index
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measurements of the nonlinear refractive index, n2,exhibit a large
scatter depending on the specific measurementtechnique. This is
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nonlinearity, as is revealed by the resonant behaviourof n, (with
peak values up to 3.9 1W0ni2W-) observed withsignal modula tion
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Introduction: The intensity-dependent contribution to the
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parison of differentmeasurement methods for evaluating n2 was
activated in the frame
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