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This is he accep ed e sion o he ollowing a icle: I o Ba on e al., "Fab ica ion o nega i e
cu a u e hollow co e op ical ibe s capable o acous ic sensing," in * P oc. IEEE ICTON 2025*
The inal e sion is a ailable a : h ps://doi.o g/10.1109/ICTON67126.2025.11125404
Fab ica ion o nega i e cu a u e hollow co e
op ical ibe s capable o acous ic sensing
I o Ba on
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
Ond ej Pod azky
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
Ali. A. Jasim
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
And ei Bo odkin
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
Ma in G abne
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
Yauhen Ba a e s
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences
P ague, Czech Republic
[email p o ec ed]
Pa el Honza ko
Ins i u e o Pho onics and Elec onics,
Czech Academy o Sciences)
P ague, Czech Republic
[email p o ec ed]
Abs ac —Nega i e cu a u e hollow-co e op ical ibe s
(NCHCFs) a e one o he g ea in en ions o mode n imes, as
hey a e capable o ansmi ing ligh o e a b oad ange o
wa eleng hs, om UV o mid-in a ed, wi h losses ha can be
lowe compa ed o hose o s anda d solid silica ibe s. By
exploi ing an an i- esonance mechanism, low a enua ion is
achie ed wi hin he ansmission windows loca ed in he icini y
o wa eleng hs de e mined by a capilla y hickness. La ely,
no el applica ions ha e eme ged, as hese ibe s show signi ican
po en ial o unc ion as dis ibu ed acous ic senso s, since hei
sensi i i y o acous ic wa es may su pass ha o con en ional
ibe s. Howe e , he ab ica ion o NCHCFs o en p o es o be
complica ed due o he di icul y o con olling a ibe ’s capilla y
diame e , ibe co e diame e , and capilla y wall hickness. In
his con ibu ion, we epo on he ab ica ion p ocess o in-
house d awn NCHCFs wi h di e en s uc u es, he splicing
p ocess o NCHCF wi h con en ional solid ibe s, and he
capabili y o ou NCHCFs o se e as acous ic senso s.
Keywo ds—op ical ibe ab ica ion, hollow co e ibe s,
ibe splicing, acous ic sensi i i y
I. INTRODUCTION
Nega i e cu a u e hollow- co e op ical ibe s (NCHCFs)
ha e su passed s anda d solid ibe s as a guide o ligh in ai
a he han in glass, which gi es hem excep ional p ope ies
such as low la ency, a low nonlinea coe icien , and a high
damage h eshold [1]. I also means ha hey can guide ligh
a wa eleng hs whe e solid ma e ials ha e high a enua ion
due o Rayleigh sca e ing, IR abso p ion, o pho oda kening
[2], [3]. S anda d solid ibe s wi h an op ical loss o below 0.2
dB/km emain applicable in cu en long- ange ansmission
sys ems; howe e , bulk ma e ial nonlinea i y p esen s a mo e
undamen al obs acle o capaci y scaling. NCHCF exhibi s
low-loss guidance and a high lase damage h eshold, making
i sui able o deli e ing high-powe lase s. They can handle
lase powe s exceeding wo kilowa s wi hou damaging he
cladding mic os uc u e [4]. HCFs can achie e excep ional
pola iza ion pu i y, ansmi ing o hogonal pola iza ion
modes wi h minimal c oss-coupling. This p ope y is c ucial
o high-pe o mance pho onics applica ions like
in e e ome e s and quan um in o ma ion expe imen s.
Ano he niche applica ion o NCHCF is lase gas
spec oscopy, which enables he con ol o HCF guidance
pe o mance and he ai -co e s uc u e, allowing o illing any
gas sample, including single gases o gas mix u es. The e o e,
low- olume ibe -based gas abso p ion cells wi h op ical pa h
leng hs p ecisely ailo ed o he speci ic applica ion can be
cons uc ed [5]. NCHCFs can se e as e ec i e low- olume
abso p ion cells in lase -based gas senso s, enabling he
de ec ion o gases such as me hane and ca bon dioxide wi h
high sensi i i y [6]. In he majo i y, hese ibe s a e d awn
down om high-pu i y used silica glass; howe e , o he
glasses, such as bo osilica e [7] and ellu ide [8] ha e also
been used.
Low o e lap o he guided mode wi h he ibe s uc u e
leads o e y small Rayleigh sca e ing [9]. Backsca e ing is
p edominan ly due o Rayleigh sca e ing in he ai , wi h a
backsca e ing coe icien o -100 dB/m. O he con ibu ions
a e smalle . The backsca e ing coe icien om he su ace o
capilla ies is -115 dB/m, and he backsca e ing coe icien
om he ma e ial is -150 dB/m [10]. This low Rayleigh
sca e ing enables low-loss p opaga ion and makes he hollow-
co e ibe (HCF) less suscep ible o dis ibu ed acous ic
sensing, which elies on backsca e ed signals. This ea u e is
pa icula ly impo an in oday's massi ely deployed passi e
op ical ne wo ks. Howe e , acous ic wa es can s ill modula e
he op ical pa h in HCF. We in es iga ed his e ec o se e al
HCFs and ound ha he ibe design signi ican ly impac s he
acous ic sensi i i y [11]. The acous ic sensi i i y o HCF is
c ucial o deploying p ecise equency signals and con olling
he ulne abili y o passi e op ical ne wo ks o ea esd opping.
NCHCF pos p ocessing was used o adjus he ansmission
bands' posi ions o sho -leng h ibe s [12]. This echnique
allows c ea ing ibe pig ailed pass-band il e s and weaking
he ansmission band o a pa icula wa eleng h.
In his wo k, we epo on he ab ica ion p ocedu e used
o p epa e NCHCFs and NANFs. We p esen a me hod o
splicing bo h ypes o ibe s wi h s anda d ibe s ha exhibi
low losses. We p esen measu emen s o he no malized
esponse o NCHCFs and NANFs o acous ic wa es
p opaga ing h ough ai , e ealing he ela ionships be ween
he hollow-co e ibe s and hei acous ic sensi i i y.
II. FABRICATION OF NCHCFS
NCHCFs a e ab ica ed using he commonly employed
s ack-and-d aw echnique [2]. We ha e ab ica ed in-house
ibe s om syn he ic silica glass (He aeus F300). Following
he scheme shown in Fig. 2, we depic ed he ab ica ion
p ocess in o s ages A-F. We s a ed by elonga ing an 18/15.2
mm silica ube in o a capilla y ube wi h dimensions 2.97/2.5
mm. In he nex s ep, we assembled a p ima y p e o m by
s acking 8 capilla ies inside ano he ube wi h OD/ID=18/15.2
mm. Acco ding o he c oss-sec ional design shown in Fig.
3A, we used cen al space s wi h a diame e o 9.26 mm o
secu e he capilla ies o he wall o he ou e ube and eigh
smalle space s wi h a diame e o 2 mm o main ain equal
dis ances be ween he capilla ies.
Fig.2. Scheme o ab ica ion NCHCF8 -A) Tube 18/15.2mm,
B)Capilla y ube 2.97/2.5 C) P ima y P e o m, D) Cane, E) Final p e o m, F)
NCHCF8 ibe
We elonga ed he assembled p ima y p e o m a 1870°C
o a 4.8 mm ou e diame e cane. We assembled he inal
p e o m by placing he cane inside a 10/5 mm jacke ing silica
ube. We used a hin silica ube wi h a diame e o 2/1 mm o
p essu ize he co e o he cane and a b ass T- i ing o
p essu ize he capilla ies. We e acua ed he space be ween he
coa ing ube and he cane o ensu e a secu e connec ion
be ween he cane and he jacke ing ube. Finally, we ha e
d awn he p e o m in o a ibe wi h an ou e diame e o 197
μm, a co e diame e o 57 μm, and a capilla y wall hickness
o 1.3 μm a 1855°C wi h a d awing ension abo e 300 cN.
The ibe co e was p essu ized o 1.96 kPa, and he capilla ies
we e p essu ized o 8.33 kPa. SEM pho o o he d awn ibe is
shown in Fig. 3B.
Fig.3. A) Scheme o ab ica ion NCHCF8, B)SEM pho o o NCFCF 8
Fo he second ype o s uc u e, we p epa ed in-house
NANF wi h i e nes ed ings. The ab ica ion p ocess,
ou lined in he scheme, was di ided in o s ages A–H. Fi s , we
elonga ed an F300 ube wi h an 18/15.2 mm diame e o a
diame e o 12.5/10.6 mm a 1900 °C. Then, we ook ano he
F300 ube, 18/15.2 mm, and inse ed an elonga ed ube inside
i . In he nex s ep, we elonga ed he nes ed ube a 1900 °C
in o he nes ed capilla y, which had ou e capilla y dimensions
o 4.8/4.4 mm and inne capilla y dimensions o 3.5/3.1 mm.
The p ima y p e o m ( he design o he s uc u e is depic ed
in Fig.5A) was hen assembled by a anging i e nes ed
capilla ies wi h wo se s o i e small space s wi h a diame e
o 2.6 mm and one cen al space wi h a diame e o 5.6 mm,
bo h being 4 cm long. A e assembling, he subp e o m was
elonga ed a a low empe a u e o 1850 °C, enabling he cane
o be d awn wi hou collapsing he capilla ies inside. No
p essu iza ion was used in his s ep. The inal p e o m (G) was
assembled by inse ing he cane in o a hick 10/5 mm silica
ube.
Fig.4. Scheme o ab ica ion NANF: A) Tube 18/15.2 mm, B)Tube
12.5/10.6 mm, C) Nes ed ube, D) Nes ed capilla y, E) P ima y p e o m, F)
Cane, G)Final p e o m, H)Final ibe
Simila ly, as in he case o NCHCF8, he co e o he p e o m
was p essu ized h ough a hin 2/1 mm ube, and capilla ies
we e p essu ized h ough a special T- i ing ha enabled he
sepa a e p essu iza ion o bo h ou e and inne capilla ies. The
space be ween he cane and he silica ube was e acua ed o
ensu e he using o he cane and ube. The inal ibe was
d awn a 1850°C wi h a d awing ension abo e 300 cN. The
ibe co e was p esu ized o 0.98 kPa, and he p essu e
di e ence in he ou e and inne capilla ies was 4.81 kPa. A
SEM pho o o he NANF5 b ibe is shown in Fig. 5 B.
A
B
Fig.5. A) Scheme o ab ica ion NANF5a B) C oss-sec ion o
subp e o m design, C)SEM pho o o NANF5a
III. SPLICING NCHCFS
A widesp ead applica ion o HCFs equi es an e icien
in e connec ion be ween HCF and a s anda d single-mode
ibe (SSMF) used in mos exis ing op ical sys ems. Fibe
splicing is he mos popula , eliable, and epea able me hod
o in e connec ing ibe s. Howe e , he e a e se e al
challenges based on he s uc u e o hollow co e ibe s. HCFs
ypically ha e a la ge mode ield diame e , which makes i
necessa y o use a mode ield adap e (MFA) o dec ease
inse ion loss. We employ a wo-s ep app oach o achie e
simul aneous mode ield and ou e cladding diame e
adap a ion. We p epa e he MFAs using s anda d single-mode
ibe s such as SMF28 and 1060XP. In he i s s ep, we use he
splice LZM-100 LAZERMas e o expand he ibe co e
g adually by hea ing. The ibe is slowly mo ed in he hea ing
egion, wi h he hea ing powe inc easing om 20 W up o
22.5 W o achie e an adiaba ic inc ease in he mode ield
diame e . The o al leng h o he hea ing egion is abou 30
mm, and he ibe speed is 10 μm/s. We epea his p ocess
se e al imes o achie e a esul ing mode ield diame e
compa able o he MFD o he HCF. In he second s ep, we
dec ease o inc ease he diame e o he SSMF in he he mally
ea ed egion o achie e he same cladding diame e as he
HCF. I is an obliga o y p ocess in case o HCF wi h a
diame e mo e han 125 μm and a hin cladding. Indeed, he
only place whe e HCFs ha e enough ma e ial o splice wi h is
cladding, so he cladding diame e o he HCF should be equal
o o smalle han he cladding diame e o he ibe you splice
wi h.
Fig.6. Re ac i e index p o ile o he o iginal SMF28 ibe and a e one,
wo and h ee hea ing p ocesses and h ee hea ing p ocesses and one e e se
ape ing p ocess ill 160 μm.
The e ac i e index p o ile was measu ed by he
In e e ome ic Fibe Analyse IFA-100 in o de o calcula e
he esul ing mode ield diame e (Fig.6). Mo eo e ,
ansmission p ope ies o nega i e cu a u e HCFs highly
depend on he geome y o hin inne capilla ies. Capilla ies
a e qui e sensi i e o he splicing pa ame e s because hey can
be mel ed and collapsed a high splicing powe . To p e en
such a p ocess, we calib a e splicing powe a he minimum
le el equi ed o splice ibe s only by cladding wi hou any
isible changes in he capilla ies geome y o e e y HCF. We
achie e he le el inse ion loss o 0.4 dB o he splicing o
HCF wi h SMF28 ibe om bo h sides.
A ansmission spec um o nega i e cu a u e HCFs
con ains a se o bandpass lines wi h posi ions de e mined by
he diame e and hickness o inne capilla ies. Such ibe s
ha e no ansmission be ween hese bands. In mos op ical
applica ions, i is necessa y o ha e a ansmission a a speci ic
wa eleng h o some spec al ange, which could be ou o he
ansmission bands o an HCF. Howe e , we can shi hese
ansmission bands o ano he posi ion by hea ing o ape ing
a long segmen o HCF in he LZM-100 splice . In he case o
hea ing, we achie e a egime o sligh ly dec easing inne
capilla y diame e wi h a co esponding inc ease in hei
hickness. As a esul , he posi ions o he ansmission lines
shi o he longe wa eleng hs. The opposi e shi can be
achie ed by ape ing he HCF a he low empe a u e. The
ape ing p opo ionally dec eases he ull s uc u e o he ibe ,
so capilla ies become smalle and hinne , and ansmission
lines shi o sho e wa eleng hs. The maximum leng h o he
shi ed HCF ha we achie ed is 15 cm and is limi ed only by
he mo emen sys em used. Mo eo e , i is possible o do a
sho all- ibe bandpass il e based on he HCF by shi ing he
ansmission bands in he opposi e di ec ion on he di e en
segmen s o he HCF (Fig.7). The supe posi ion o he
esul ing ansmission bands gi es a new band wi h a
p ede ined wid h and posi ion. We achie e he minimum ull
wid h a hal maximum o 33 nm in ou labo a o y [16].
Fig.7. Fil e ing NC-HCF’s ansmission band a 1.05 μm by
hea ing a ape ing ibe segmen . Black line – o iginal ansmission
spec um; ed line – spec um a e hea ing a 2 cm segmen ; g een
line – spec um a e ape ing ano he 2 cm segmen om 125 o
112 μm.
IV. MEASUREMENT OF ACOUSTIC SENSITIVITY
The acous ic sensi i i y o op ical ibe s is a key ac o o
conside when deploying p ecise equency signals o e
op ical ibe s and planning new op ical ibe ins alla ions. I is
well-known ha polyme ibe coa ings signi ican ly a ec he
acous ic sensi i i y o s anda d op ical ibe s. The ela ionship
be ween he hickness o ibe coa ings and ibe sensi i i y o
acous ic wa es has been exploi ed in cons uc ing
hyd ophones [17]. We in es iga ed how he inne s uc u e o
hollow co e ibe s in luences hei acous ic sensi i i y. Fo his
pu pose, we inse ed he ibe unde es (FUT) in o he
sensing b anch o a he e odyne in e e ome ic analyze (Fig.
1 in [11]). The FUT was placed in an acous ic chambe , well
acous ically isola ed om he es o he in e e ome e . To
p eclude he in luence o he suppo ing ay, he ibe was
a anged in o coils hanging inside he chambe wi hou any
con ac wi h he chambe walls. A loudspeake exci ed a sound
ield, and a calib a ed mic ophone in he plane o he FUT
A
B
measu ed he sound p essu e le el. We ound a s ong
dependence on he hickness o he ibe glass jacke ube. The
NCHCF/NANF wi h a hin glass jacke we e obse ably mo e
sensi i e o acous ic wa es due o hei lowe s i ness. Fig. 8
shows he mos and leas sensi i e ibe s' no malized acous ic
sensi i i y. The di e ence in sensi i i ies is 12 dB.
Addi ionally, he esponse o he ibe wi h a hin jacke is
esonan ly enhanced a 5.9 kHz and is s onge by 34 dB
compa ed o he less sensi i e ibe . The no malized acous ic
sensi i i y o he SMF28 ibe is shown o compa ison. SEM
images o c oss-sec ions o he es ed ibe s a e also included
in Fig. 8.
Fig.8. (A) No malized acous ic sensi i i ies o he (B) hin
jacke and (C) hick jacke NANF.
V. CONCLUSION
In his wo k, we epo on de eloping and demons a ing
ab ica ion p ocedu es o NCHCFs and NANFs. We
demons a ed a wo-s ep splicing echnique ha enables low-
loss in eg a ion o hollow-co e ibe s wi h s anda d single-
mode ibe s, simul aneously adap ing bo h mode ield and
ou e cladding diame e s. Expe imen al in es iga ions in o he
acous ic sensi i i y o NCHCFs and NANFs con i med ha
acous ic wa es can modula e he op ical pa h despi e he
p edominan ai guidance in hollow-co e ibe s. We obse ed
ha he NCFCFs and NANFs design in luences acous ic
sensi i i y, wi h a signi ican dependence on he hickness o
he silica jacke ube. Speci ically, HCFs wi h hinne silica
jacke s exhibi ed highe sensi i i y o acous ic wa es due o
hei educed s i ness compa ed o SMF28. In he case o
HCFs wi h hicke o e coa ed silica cladding, sensi i i y o
acous ic wa es was shown o be lowe han ha o SMF28.
The esul s p esen ed highligh he c i ical ole o ibe
geome y and s uc u al pa ame e s in he ab ica ion o HCF
o he acous ic esponse.
ACKNOWLEDGEMENTS
This wo k was suppo ed by he Minis y o he In e io o he
Czech Republic unde G an No. VK01030114 wi hin he
p og am OPSEC. De elopmen o he esea ch in as uc u e
was co- unded by he Eu opean Union and he s a e budge o
he Czech Republic unde he p ojec LasApp
CZ.02.01.01/00/22 008/0004573
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