Development of Ultra-Compact Highly Nonlinear Fiber Modules
by Masanori Takahashi *, Yu Mimura *, Yuki Taniguchi *, Jiro Hiroishi *,Masateru Tadakuma *, Ryuichi Sugizaki *, Misao Sakano *2 and Takeshi Yagi *Highly nonlinear fiber (HNLF) is a key component holding great promise in applications in the all-optical signal processing devices, broadband light sources,
pulse compressors etc., which are essential to high-speed transmission systems.
Improvements in HNLF functionality have been realized in recent years, but there is a need formore compact modules when these fibers are to be accommodated.
In this paper we provide a theoretical discussion of the limits of reducing cladding diameterusing the finite element method (FEM), and have fabricated fiber with a cladding diameter of
51 µm. This fiber shows satisfactory characteristics in tests for mechanical strength and environment performance, and delivers ample reliability while fitting into coin-sized modules. Using prototype ultra-compact modules that we fabricated, we carried out supercontinuum (SC) light generation tests and have confirmed satisfactory characteristics.

ABSTRACT1.

INTRODUCTION

While nonlinear phenomenon occurring in an optical fiber
is a source of signal waveform degradation in the transmission path, the optical signal processing 1) that is considered essential in the anticipated ultra-highspeed transmission systems above 160 Gbps is a technique thatmakes use of this phenomenon of nonlinearity. Amongwell-known applications are optical wavelength conversion 2) or optical pulse shaping 3), and there have also
been reports, outside of signal processing, concerning
broadband light sources 4), pulse compressors 5), and soon. In such applications as these, fiber that is highly nonlinear constitutes a key component.
At present there are numerous reports of HNLFs of differing materials and structures 6)~9). In order to produce
nonlinearity in a fiber with high efficiency, it is necessary
to have a large nonlinear coefficient (γ=2πn2/A eff ) and lowtransmission loss. Compared to silica-glass HNLFs, nonsilica-glass HNLFs have an extremely large value of γ, but
are also characterized by extremely large values of transmission loss. For this reason, it has been reported 10) thatwhen comparing the maximum nonlinear phase shift calculated from transmission loss and γ, silica-glass HNLF is
the most efficient.
HNLF must also have, in addition to low loss and large
γ, dispersion characteristics that are specific to the application. In the communications wavelength band, dispersion for silica-glass HNLF can be controlled to zero. And
since a low dispersion slope can also be achieved, theregion having the specified dispersion value can be main-tained over a broad bandwidth. Dispersion characteristics
that are stable in the lengthwise direction of the fiber also
have an effect on an effective nonlinear process 11). In thisrespect also, silica-glass HNLF, which shows good controllability during fiber fabrication, is also advantageous.
Most of the numerous reports concerning applications
using HNLF published in recent years are at the research
institute level. To produce commercially viable HNLF
requires more compact modules. From the standpoint ofcompact packaging of HNLF, non-silica-glass HNLF having a large value of γ can be used in lengths as short asseveral meters 7), 9), making it advantageous in terms ofachieving more compact modules. However when using
HNLF in modules, which ultimately will be connected toexisting optical systems, it must be spliced to ordinary
single-mode fiber (SMF) or dispersion-shifted fiber (DSF),
so that the large splice loss for these fibers presents problems. Since γ is smaller for silica-glass HNLF than for nonsilica-glass HNLF, the length of fiber required is greater.
On the other hand, not only is the nonlinear process more
efficient, dispersion more easily controlled and fabricationeasier for silica-glass HNLF, but also its reliability is established and low-loss splicing (<0.2 dB) can be obtainedwith ordinary SMF, rendering it suitable for actual use.
In this paper we report on the down-sizing of silicaglass HNLF with a view to commercial viability. In Section
2 we discuss theoretical and mechanical design, in
Section 3 the characteristics of the prototype fiber thatwas fabricated, and in Section 4 the characteristics ofmodules using the prototype fiber. In Section 5 we go on
to report on SC light generation using a prototype ultracompact module.

* FITEL Photonics Laboratory, R&D Division.
*2 Yokohama Laboratories, R&D Division.

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Development of Ultra-Compact Highly Nonlinear Fiber Modules

2.1

Theoretical Approach to Down-Sized Design(FEM Simulation)
Transmission in optical fibers occurs generally by utilizing
the difference in refractive index between the core and the
cladding to confine the light in the area of the core. The
intensity of this light decreases exponentially as the distance from the core becomes greater, but when the cladding diameter (cladding thickness) is insufficient, thetransmission mode reaches the interface between thecladding and the coating and large leakage loss occurs.
Thus in reducing cladding diameter it is necessary to clarify the cladding diameter required with respect to thedesign of the core.
By means of simulation using the finite element method(FEM) 13), we analyzed the relationship between cladding
diameter and leakage loss. In this simulation we used therefractive index profile of the core actually adopted in sili-

Leakage loss（dB/km）

The length of silica-glass HNLF when used in a nonlinear
device is determined by the characteristics of the fiberand by the application, but is generally in the order ofseveral hundred meters. If such lengths of fiber are to beaccommodated in a module the fiber is generally wound
on a bobbin, so that reducing bobbin size is important toachieving a more compact module. For a given length offiber, bobbin size is dependent on the outer diameter ofthe fiber (coating diameter) and the inner diameter of thebobbin (the fiber spooling diameter).
One approach to reducing the outer diameter of thefiber is to reduce the thickness of the coating.
Conventionally the diameter of the coating is 250 µm but
this is based on the assumption that the fibers are to bemade into cables and used in a variety of fields. Since
HNLF is formed into coils and used inside equipment, thethickness of the coating can be reduced to the extent thatstress applied to the fiber by cabling, cable laying etc.
need not be considered. Another effective way to reduce
coating diameter without changing coating thickness is todecrease fiber diameter (cladding diameter).
In reliability design for optical fibers, it is important thatthe strain on the fiber be kept low. One way to keep coilsize small is to reduce the fiber spooling diameter, but thesmaller the spooling diameter the greater will be thebending strain on the fiber. Since the bending strain on
the fiber is inversely proportional to fiber diameter, reducing fiber diameter can limit the bending strain and at thesame time allow a smaller spooling diameter.
For these reasons reducing cladding diameter is aneffective way to achieve a more compact module.
Previously it was possible, using 90-µm HNLF, to develop
a module size of a 3.5-inch floppy disk 12), but any further
reduction in module size would require even finer fiber.
Since HNLF has an extremely small mode field diameter(MFD) and light confinement to the periphery of the coreis strong, it is advantageous in terms of down-sizing but
any major reduction in cladding diameter would require
investigation of the theoretical and mechanical design.

ca-glass HNLF and that used in ordinary SMF. The HNLF
profile was W-shaped, having a center core with highdensity Ge doping and a side core with F doping, while
the SMF had a single-peak profile. For each core design,
simulations were carried out for leakage loss when cladding diameter was changed with no change in core diameter. Since in these simulations loss was taken as theamount of light leaking from the cladding into the coating,it did not include Rayleigh scattering or absorption, microbending loss etc.
Figure 1 shows the relationship between cladding diameter and leakage loss obtained by simulation, demonstrating that the thinner the cladding diameter the greater the
leakage loss. Comparing HNLF and SMF we can see thateven an HNLF with a cladding diameter of 40 µm has
lower loss than 125-µm SMF.
Figure 3 shows the leakage loss under macrobending.
It should be noticed that as the cladding becomes thinnerand light confinement weaker, the ability to withstand
1E＋6
SMF

1E＋3

HNLF

1E＋0
1E−3
1E−6
1E−9

20

40

60

80

100

120

Cladding diameter（mm）

Figure 1

Relationship between cladding diameter and leakage
loss.

Cladding
Core
Field distribution

Coating

Figure 2

Field distribution under macrobending (SMF with 80-µm cladding, r = 20 mm).

Leakage loss（dB/km）

2. DESIGNING DOWN-SIZED HNLF

SMF 125 µm
SMF 40 µm
HNLF 30 µm

1.E＋09

SMF 80 µm
HNLF 40 µm
HNLF 20 µm

1.E＋06
1.E＋03
1.E＋001.E−03
1.E−06
1.E−09

0

10

20

30

40

50

Bending radius（mm）

Figure 3

Leakage loss under macrobending.

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Development of Ultra-Compact Highly Nonlinear Fiber Modules

bending changes. However it can be seen that an HNLF
having a core designed for extremely strong light confinement will be only slightly susceptible to the effects ofbending, and even when the cladding is thinner the ability
to withstand bending will not change to any degree.
Thus from the results of a theoretical study based onthese simulations, we may conclude that for HNLF, aslong as the cladding diameter is 40 µm or more, leakage
loss characteristics will be as good as or better than SMF
with a cladding diameter of 125 µm.
2.2 Investigation of Mechanical Strength
In fabricating fibers of greatly reduced cladding diameter,
it is necessary to consider mechanical strength in addition
to optical characteristics. Generally speaking, when handling optical fiber, a tensile strength of the order of 10 N isrequired. The tensile strength of optical fiber with a cladding diameter of 125 µm is approximately 50~60 N, andthis tensile strength is substantially proportional to thecross-sectional area of the cladding. Figure 4 shows thedependence of tensile strength on cladding diameter, andfrom it we see that to obtain adequate tensile strength ofat least 10 N we need a cladding diameter of about
50-55 µm.

3.

CHARACTERISTICS OF DOWN-SIZEDHNLF

Tensile strength（N）

60

40

20

0
40

60

80

100

4.

DESIGN OF ULTRA-COMPACT
MODULES

4.1 Mechanical Strength Reliability Test
Making a smaller bobbin for spooling the fiber is an effective means of enclosing it more compactly. It would thus
be desirable if the spooling diameter (inner diameter ofthe bobbin) were as small as possible but if the spoolingdiameter is too small, we also must consider the effect of
the bending strain applied to the fiber on its failure life.
From the theory of optical fiber failure life 14), the probabili1.E＋02

Accumulated failure ratio（%）

Table 1 shows the characteristics of prototype down-sized
HNLFs designed and fabricated based on our study ofdown-sizing. Fibers A and B are of differing core design

but both have a cladding diameter greater than 55 µm,
and a tensile strength in excess of 10 N. Both A and B
were given a 2-layer coating with a view to achieving
ample reliability. Compared to fiber A, fiber B has a lower
dispersion slope, and loss is also less. The basic opticalcharacteristics shown in Table 1 are similar to those for an
HNLF with a cladding diameter of 125 µm having thesame core design.
We also evaluated the mechanical characteristics of the
prototype fibers. Figure 5 is a Weibull distribution plot.
The Weibull distribution coefficients m1 (high strength)
and m2 (low strength) were 44.7 and 1.2, respectively.
Figure 6 shows the result of dynamic fatigue tests. The
dynamic fatigue coefficient n for the prototype down-sizedfibers was 27.6 (by the IEC and TIA/EIA methods).
From these results it was confirmed that the prototype
down-sized HNLFs had satisfactory mechanical strengthcharacteristics comparable to ordinary-sized fibers.

120

1.E＋01

1.E−01High strength：m1=44.7
Low strength：m2=1.2

1.E−02
1.E−03
1.0E−01

Dependence of tensile strength on cladding diameter.

Figure 5

1.0E＋00
Applied stress（GPa）

1.0E＋01

Weibull distribution in prototype HNLFs.

Characteristics of prototype HNLF (@ 1550 nm).
Item

Unit

A

0.25

B

Cladding diameter

(µm)

56

58

Coating diameter

(µm)

130

124

Dispersion slope(ps/nm2/km)

0.026

0.013

Dispersion(ps/nm/km)

-0.29

0.63

λ0

(nm)

1539

1617

γ (XPM)(W−1km−1)

13.3

12.1

Attenuation loss

(dB/km)

0.64

0.50

Bending loss (15 mm dia.)(dB/m)

<0.1

<0.1

PMD

(ps/nm1/2)

0.06

0.13

0.20
LOG（σf）

Table 1

Low strength

1.E＋00

Cladding diameter（μm）

Figure 4

High strength

0.15
0.10

n=27.6（IEC, TIA/EIA method）

0.05
0.00
−1.50

−1.00

−0.50

0.00

0.50

1.00

LOG（dσ/dt）

Figure 6

Result of dynamic fatigue tests.

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Development of Ultra-Compact Highly Nonlinear Fiber Modules

ty of fiber failure may be stated asλ �� ������������ε ��ε�� � �β��� � ��� β�������

⑴�

SymbolSignificanceam/np-2Frequency of fiber failure

NP
BP/E（B/E2）β

Environmental parameterεEquivalent applied strain

εP

Applied strainnP

Fatigue parameter

N

Fatigue parametertPTime of εP appliedtSTime of ε applied

2

0.5

90 µm
125 µm
60 µm （1%）
60 µm （2%）

0.4
0.3
0.2

4.2 Splicing Characteristics
An important parameter with respect to HNLF is spliceloss. It is assumed that this type of fiber will mainly be
used as a component inside devices that make up equipment, but it will ultimately have to be connected to the
SMF of existing optical systems. With HNLF, increasing
the nonlinearity of the fiber improves the efficiency of thenonlinear process, promising a reduction in the power
required, but if splice loss is too great, the input power tothe HNLF will decrease so that this advantage will not be
fully realized.
Using commercially available splicing tools and varying
splicing conditions, the prototype down-sized HNLF canbe fusion spliced directly to ordinary SMF. Splice loss canbe kept to 0.2 dB or less.
4.3 Environment Resistance
Using the prototype ultra-compact module, environment
tests in conformity with Telcordia GR-63-CORE were carried out. The temperature pattern used is shown in Figure
10, the results of the environment tests in Table 2, and theresult of measurements of operating temperature in
Figure 11.
As can be seen from Table 2, in all environment tests
the change in loss was less than 0.1 dB and the change
in polarization mode dispersion (PMD) was less than 0.1
ps. Further, in the operating temperature tests the variation in power for all values of temperature and humidity
was less than 0.02 dB. It was thus confirmed on the basisof these results that the prototype ultra-compact module
fabricated possesses satisfactory environment character-

Splice loss＜0.2 dB

125 µm SMF

0.1
0.0

Figure 7

5

15

25
35
45
55
Spooling diameter（mm）

65

Figure 9

Relationship between spooling diameter and failure
ratio.
for 200m

25mm

56 µm HNLF

Fusion splice between prototype HNLF and SMF.

70℃, 72 hours

Temperature（℃）

Failure ratio（%/20years）

Based on Equation ⑴ we calculated the failure ratio forthe down-sized HNLFs from parameters obtained fromthis prototype and its evaluation. Figure 7 shows the relationship between spooling diameter and failure ratio.
When a 125-µm fiber is spooled to a 60-mm diameter, the20-year failure ratio is about 0.25 %. The prototype downsized fiber fabricated here can be spooled to a diameterof 20 mm with the same failure ratio. By setting the proof
test level at the time of fabrication at 2 %, the down-sizedfiber can be spooled to a diameter of 12 mm.
Thus by reducing cladding diameter to reduce spoolingdiameter and decreasing fiber diameter (coating diameter) (i.e., reducing the space factor of the fiber), it is possible to accommodate the down-sized fiber in a smallerbobbin. Figure 8 shows small bobbins for down-sized
HNLFs. Using these bobbins 200 m of fiber can beaccommodated in the size of a coin (photo shows a 500yen coin).

40℃, 90-95% RH,
96 hours
Transition＜5 min−40℃, 72 hours

for 500m

Figure 8

for 300m

Small bobbins for down-sized HNLFs.

Time（hour）

Figure 10 Temperature and humidity patterns under GR-63CORE.

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Development of Ultra-Compact Highly Nonlinear Fiber Modules

istics.

5.

SC LIGHT GENERATION BY DOWNSIZED HNLF

We conducted supercontinuum (SC) light generation
tests using the ultra-compact module achieved by meansof down-sized HNLF. SC light generated in the normal dispersion domain of HNLF is low in noise, and since it has agood signal-to-noise ratio (SNR), it is promising for applications in optical signal processing or as a broadband
light source 4). For these applications to be actually usedin the field, it is anticipated that the fiber will be accommodated within the device as one of the components making
up the device, so that achieving a more compact module
is considered essential. The ultra-compact module developed in this work will, it is suggested, prove extremely
useful in such applications.
5.1 A Fiber Design Suited to SC Light Generation
To improve the efficiency of SC light generation requiresthat the nonlinear phenomenon be generated at high efficiency, and this necessitates a large value of γ and lowtransmission loss. A lower value of fiber dispersion at the
wavelength of pumping light incidence is also preferable.
However to generate emission of broadband low-noise
SC light, it is necessary to obtain a normal dispersion
domain over a broad band. For this reason a fiber having
a low dispersion slope is desirable. And by increasing
fiber length it can be anticipated that efficiency will behigher 15). Lengthening of the fiber is, however, counterproductive in terms of module down-sizing.
Taking these conditions into account, we carried out
optimization by means of numerical calculation. The main
Table 2

Results of environment tests.
Loss

PMD

(dB)

(ps)

<0.1

<0.1

Item
Low-temperature and thermal shock
High-temperature and thermal shock

<0.1

<0.1

High relative humidity exposure

<0.1

<0.1

Operating temperature and humidity

<0.02

−

Loss
Temperature

32% RH

Loss（dB）

0.25

40
90% RH

50% RH
0

20

-0.25

-0.5

0

60

<15% RH

50

100

150

0

Temperature（℃）

0.5

−20
200

Time（hour）

Figure 11 Result of measurements of operating temperature.

parameters used in this calculation were: pumping wavelength 1560 nm, repetition rate 10 GHz, pulse width 2 ps,
average pump power 100 mW, and γ 14 km-1W-1. As aresult we designed and fabricated a prototype HNLF witha fiber length of 1 km having a dispersion value of -0.8
ps/km/nm at 1560 nm.
5.2 Characteristics of Prototype Fiber
Table 3 shows the characteristics of the prototype fiberthat we fabricated. It was possible to obtain an HNLF having substantially the same value of dispersion and low
dispersion slope as the design. The γ of this fiber was
13.8 km-1W-1 by the CW-SPM method. Furthermore cladding diameter was reduced to 51 µm, and by optimizing
coating structure it was possible to reduce the coating
diameter (outer diameter of the fiber) to 85 µm. In this way
the space factor was reduced to about half that of the
HNLF shown in Table 1, and it was possible to accommodate about twice the fiber length on the bobbins shown in
Figure 8.
Table 4 shows the characteristics of the module madeusing this fiber. A 1-km length of the prototype HNLF wasspooled on a bobbin 50 mm in diameter and 5 mm thick(labeled for 500 m in Figure 8). It was possible to connect
this fiber to SMF using a commercially available fusion
splicer as described in Section 4.2, and splice loss was
kept down to 0.2 dB or less.
5.3 SC Light Generation Experiments
SC light generation experiments were carried out using
the prototype HNLF module. Figure 12 shows the experimental set-up, which was extremely simple, consisting ofa mode-lock laser (MLL), EDFA, compact HNLF module,
and optical spectrum analyzer (OSA). In all the experiments the repetition rate of the light source was 10 GHz,
the pulse width (FWHM) was 2.1 ps, the pulse configuration Sech2 was ∆ t x ∆ν 0.35, and the wavelength was1560 nm.
Figure 13 shows the SC spectrum at an input power ofTable 3Characteristics of prototype HNLF for SC module.
Item

Cladding diameter(µm)

Coating diameter

(µm)

51
85

Dispersion slope(ps/nm2/km)

0.017

Dispersion(ps/nm/km)

0.82

Attenuation loss

(dB/km)1.14

γ (SPM)(W−1km−1)

13.8

PMD

(ps/nm1/2)

0.05

Table 4Characteristics of SC module.
Item

Fiber length

(m)

1000

Insertion loss

(dB)

<1.5

PMD

(ps/nm1/2)

0.05

Bobbin size

(mm)

φ 50

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Development of Ultra-Compact Highly Nonlinear Fiber Modules

100 mW. OSA resolution was set at 0.01 nm. Ripple
around the pumping light input wavelength was sufficiently small at about 3 dB, and by using the normal dispersion domain of the HNLF the spectrum was observed tohave excellent flatness. The spectrum spread (bandwidth)
at the points where power decreased 10 dB and 20 dB
from peak power was 28.1 nm and 32.1 nm respectively.
It is thought that by increasing input power it will be possible to further increase the bandwidth.

6.

CONCLUSION

By means of simulations using the finite element method
we have clarified the theoretical limits of down-sizing forsilica-glass HNLF. As a result of this theoretical investigation and of mechanical strength design, we designed andfabricated a prototype HNLF with a cladding diameter of
51 µm and a coating diameter of 85 µm, and by using thisfiber were able to realize coin-size HNLF modules. In various reliability tests the prototype down-sized HNLF andultra-compact modules exhibited characteristics that weresatisfactory compared to those of conventional fibers. SC
light generation experiments were also carried out using
the prototype ultra-compact modules, and satisfactorycharacteristics were confirmed.

λ=1560 nm
MLL
EDFA

10GHz
2.1 ps

50 µm
HNLF

OSA

Power（10 dB/div.）

Figure 12 Experimental set-up.

1530

1540

1550

1560

1570

1580

1590

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Wavelength（nm）

Figure 13 SC spectrum at input power of 100 mW.

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