PLC Products for FTTH Systems
by Takahiro Ono * , Qiang Chen * , Toshiaki Tsuda * , Tomohiro Watanabe * , Taketsugu Sawamura * ,Kanji Tanaka * , Hiroshi Kawashima * 2 , Haruki Urabe * 2 , Tamotsu Shinada * 2 and Kazutaka Nara * 2

Two types of planar lightwave circuit (PLC) products--chips and modules--havebeen developed to serve the rapidly expanding area of fiber-to-the-home (FTTH)
systems. PLC technology, which has for some time past been incorporated in products for WDM
systems, has here been extended to FTTH applications, making possible the development of
PLC products that combine low cost and high performance. Reliability tests have also been
carried out, confirming that they also offer high long-term reliability. This paper reports on
PLC products that are representative of those for FTTH applications--a wavelength-insensitive
coupler (WINC), and 1 x 4, 1 x 8 and 1 x 16 splitters.

ABSTRACT1.

INTRODUCTION

Recently Japan has seen rapid diffusion of broadband
services, of which the asymmetric digital subscriber line
(ADSL) is currently the most widely used. But the numberof subscribers to FTTH systems, growing more than twice
as fast as ADSL 1), has already reached the 1 million markand is expected to increase at rates even faster than
before.
Among broadband services FTTH systems are thefastest and most stable, but growth has been impededby higher initial cost and usage tariffs. At the present
time initial costs are dropping and usage tariffs havealso come close to those for ADSLs, so that rapid
diffusion has begun. To reduce tariffs, telecom carriers
are proceeding with the introduction of passive optical
network (PON) systems, shown in Figure 1, in which a
number of subscribers can use a single fiber, and thesesystems involve the use of large numbers of componentsthat split the optical signal (optical splitters).
In NTT’s B-Flets system, for example, there are
8-arrayed 1 x 4 splitters and 4-arrayed of 1 x 8 splittersused inside the exchange office, and 1 x 4 and 1 x 8
splitters outside the office. This system is also a world
leader in having an optical monitoring system that uses a
WINC to insert a monitoring signal into the system 2), 3).
And in any future transition from the 100-Mbps PON
systems that are today’s standard to a giga-PON orgiga Ethernet-PON (G-PON or GE-PON), picture image
delivery will also be possible over optical fibers, and thiswill require components with WDM functions, to split theIP signal and picture image signal.

* Optical Components Dept., Information and Communications

Broadband services are also undergoing similar
expansion outside Japan, and field tests have started
for the introduction of FTTH systems. Since it is to
be expected that within a few years diffusion of FTTH
systems will also get under way overseas, we cananticipate significant growth in optical components thatare FTTH-related.
FTTH components will differ, however, from thecomponents that have in the past been the mainstay oftrunk line systems in that since so many are used theymust have very low unit costs.
That is why we have developed chips using PLC
technology, which are suited to the fabrication of
FTTH system components. Since PLC technology,
like semiconductor processes, permits the batchedfabrication of circuits on silicon or silica substrates, it
is possible for a large number of chips at one time tobe supplied cheaply. The fabrication process involves
forming the glass film by flame hydrolysis deposition(FHD), core channelization using a resist and opticalmask, patterning, and dry etching by RIE. In addition we
have developed a highly reliable low-cost module suited
to this chip.
Subscribers
1 x 4 splitter1 x 8 splitter

Exchange officeWINC
4-arrayed 1 x 4 splitter8- arrayed 1 x 4 splitterCo.

* 2 FITEL-Photonics Lab., R&D Div.

Figure 1

Passive optical network (PON) system.

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PLC Products for FTTH Systems

This paper reports on a wavelength-insensitive coupler(WINC) 3), 8-arrayed 1 x 4 splitters--components for use inNTT’s B-Flets, and 1 x 4, 1 x 8 and 1 x 16 splitters. We arealso scheduled to proceed with the development of otherproducts for FTTH systems using PLC technology, suchas a 1 x 32 splitter and WDM coupler.

2.

PLC CHIP FOR FTTH SYSTEMS

Here we describe a wavelength-insensitive coupler(WINC) and splitters developed as PLC chips for FTTH
systems.
Phase shifterPort CPort A

DC1

DC2
Port D

Gap

Length
DC: Directional Coupler

Figure 2

WINC circuit configuration.

2.1 WINC
The WINC is a coupler that has virtually the samebranching ratio in both the 1.3- and 1.55-µm bands,
and is a component of line monitoring systems usedin the NTT exchange offices. Figure 2 shows the circuit
configuration, which consists of a Mach-Zehnderinterferometer incorporating two directional couplers(DCs) and a phase shifter. By checking the difference
between the gaps and lengths of the two DCs and thelength of the waveguide of the phase shifter, it is possible
to cancel out the wavelength dependence of one of theDCs and obtain virtually the same coupling efficiency overa broad bandwidth.
Referring to Figure 2, coupling efficiency R may be
defined by
R

PD
PC PC

(1)

where:
PC is the power of the signal input from Port A andoutput from Port C, andPD is the power of the signal output from Port D.
2.1.1 WINC Design
FTTH system components must be low in cost, andyields must be high. Thus a design was adopted inwhich coupling efficiency remains stable despite process
variations.

Here coupling efficiency may be represented by 3)
R cos2(φ )sin2(θ 1 θ 2) sin2(φ )sin2(θ 1 θ 2)(2)

where:
2φ is the phase difference generated by the phaseshifter,θ 1 is the phase representation of the coupling
efficiency of DC1; and similarly
θ 2 is the phase representation of the coupling
efficiency of DC2.
The first term of this Equation can, in the design, bemade extremely small relative to the second term bysuitable selection of the phase shifter phase. In thiscase it is sufficient in considering variation in coupling
efficiency R, to look at the second term only. φ is a valuethat is determined by the length of the phase shifter andsince this is determined by the precision of the mask
used in patterning, may be considered as not varying. θ isa function of the gap and length of the DC, but since thelength is determined by the precision of the mask it does
not vary. It can thus be seen that the thing to be taken
into account at the design stage is deviations in DC gap.
The gap will change due to deviations in etching andthe conditions of burying of the upper clad, but throughour experience with the process we have found thefollowing. Where G1 and G2 represent the gaps of the two
DCs and ∆ G1 and ∆ G2 the variations in them, we observethat when G 1 and G 2 are not equal ∆ G 1 and ∆ G 2 are
generally not equal, and when G1 and G2 are equal ∆ G1
and ∆ G2 are generally equal.
Thus when the gaps of the two DCs are equal theamount of variation is substantially the same, so we may
consider that the change in (θ 1-θ 2), the second term ofEquation (2) becomes less.
Calculations were carried out to confirm this. Assumingthat ∆ G1=∆ G2 for both cases-- G1 equal to G2 and G1 notequal to G2. Figure 3 plots ∆ G1 (=∆ G2) on the horizontal
axis against the amount of variation in coupling ratio withrespect to the target coupling efficiency of 21 % on thevertical axis. It can be seen that the variation was smallerwhen G1 and G2 were equal than when they were not.
In Figure 4, considering the variation that could occur
when G1 is not equal to G2, we plot the case in which
G1 is 0 and only G2 varies, in an analogous manner toFigure 3, with G2 on the horizontal axis and the variationin coupling efficiency on the vertical axis. Both show the
opposite movement, and this too points up the fact thatstable characteristics are difficult to achieve in a design inwhich G1 and G2 are not equal.
2.1.2 WINC Fabrication
Based on these design-stage considerations, we
designed and fabricated a WINC in which coupling
efficiency would be constant over a broad bandwidth
under the condition that G 1 and G 2 are equal. For
purposes of comparison we also fabricated a WINC
designed with G1 and G2 not equal. Figures 5 and 6 show

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PLC Products for FTTH Systems

0.08

Experiment

0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
1.2

0.02

1.3

1.4

1.5

1.6

1.7

Wavelength (µm)

0Figure 7

-0.02
-0.04
-0.2

0

0.2

0.4

G1=G2, λ =1.31 µm

G1=G2, λ =1.55 µm

G1 ≠ G2, λ =1.31 µm

G1 ≠ G2, λ =1.55 µm

Variation in coupling ratio between WINCs with sameand different DC gap.
0.08

Difference of coupling ratio

Calculation

0.26

0.04

∆ G1, ∆G2 (µm)

0.06

Wavelength-dependence of coupling ratio.

Figure 7 shows a comparison between the wavelength
dependence calculated for the design and that obtained
experimentally for the devices we fabricated. There wasextremely good agreement between results for the design
and for the fabricated devices.
As can be seen from the above, the design allows highyield fabrication of WINCs even when processing wasinconsistent. Modules were also made using these chips,
and it was confirmed that there was no problem in termsof reliability.

0.04
0.02
0
-0.02
-0.04
-0.06
-0.4

-0.2

0

0.2

0.4

∆ G2 (µm)∆G1=∆ G2, λ=1.31 µm∆G1=0, ∆G2≠0, λ =1.31 µm

Figure 4

0.28

0.06

-0.06
-0.4

Figure 3

0.30

Couping ratio

Difference of coupling ratio

the results, and it can be seen that in both the 1.31- and1.55-µm bands the WINCs fabricated with G 1 and G 2
equal had a far lower incidence of deviation in coupling
efficiency.

∆G1=∆ G2, λ=1.55 µm∆G1=0 ∆G2≠0, λ=1.55 µm

Variation in coupling ratio in WINCs with different DC
gap.

2.2 Splitters
Splitters are an indispensable component of PON
systems, and depending on subscriber conditions orcable length, various types of splitter--1 x 4, 1 x 8, 1 x 16,
1 x 32, etc.--may be used. In terms of their characteristics,
they must offer extremely low levels of insertion loss andpolarization-dependent loss (PDL). And since they are
frequently installed outdoors they must also be relatively
insensitive to variations in temperature and humidity.
Further a number of PLC devices can be fabricatedtogether, so that these multiple splitters are also
integrated, as in 4-arrayed 1 x 8, 8-arrayed 1 x 4, etc.

Frequency (%)

70
60

G1=G2

50

G 1 ≠ G2

40
30
20
10
0

-8

-6

-4

-2

0

2

4

6

8

Variation in coupling ratio (%)

Figure 5

1.31-µm coupling ratio distribution with same anddifferent DC gap.

Frequency (%)

60
50
40
30

2.2.1 Developing a Method of Splitter Fabrication
In developing the FTTH splitter described here, we
decided to use a silica substrate, with its promise of lower
PDL, in preference to the silicon substrates, with which
Furukawa Electric has a lengthy track record 4), 5).
As a development procedure it was possible to use asilica substrate as the under-cladding layer, and, in orderto fabricate a low-loss core layer, the conditions for FHD--such as gas flow rate and consolidation temperature-were optimized. Conditions of core channel etching
and for over-cladding layer fabrication were reviewed
to find conditions conducive to the fabrication of a lowloss waveguide. Finally prototypes having various typesof circuit patterns were fabricated and conditions for
optimum characteristics were derived.

20
10
0
--8

--6

--4

--2

0

2

4

6

8

Variation in coupling ratio (%)

Figure 6

1.55-µm coupling ratio distribution with same anddifferent DC gap.

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PLC Products for FTTH Systems

0.23 dB for the 1.31-µm band, and 9.9 dB and 0.19 dB
for the 1.55-µm band; and the average PDL and standard
deviation were 0.13 dB and 0.05 dB for the 1.31-µm band,and 0.07 dB and 0.07 dB for the 1.55-µm band.

50
40
Quartz sub.
30
20

PDL (dB)

1.31-µm PDL distribution for 4-arrayed 1 x 8 splitters.

Figure 8 compares the PDL distribution for 4-arrayed1 x 8 splitters using conventional silicon substrates andsilica substrates. It can be seen that using the silica
substrate gave substantially lower values, both for
absolute values and for dispersion.
2.2.2 Results of Splitter Fabrication
Using these process conditions we fabricated 8-arrayed1 x 4, 1 x 8 and 1 x 16 splitters.
Extremely good results were obtained. For the total
number (2080) of 8-arrayed 1 x 4 splitters, the averageinsertion loss and standard deviation were 6.5 dB and
0.16 dB for the 1.31-µm band, and 6.5 dB and 0.13 dBfor the 1.55-µm band; and the average PDL and standard
deviation were 0.06 dB and 0.03 dB for the 1.31-µm band,and 0.05 dB and 0.03 dB for the 1.55-µm band.

1.35

1.45

1.55

1.65

Wavelength (µm)

Figure 11 Wavelength dependence of insertion loss for 1 x 8
splitter.

Figure 11 shows the wavelength dependence ofinsertion loss for a 1 x 8 splitter. Over a wide range, fromthe 1.31- to 1.55-µm band, low-loss characteristics wereobtained.
4540

1.31 µm351.55 µm

30
25
20
15
10

13.8

13.6

13.4

13.2

13.0

12.8

12.6

12.4

12.2

12.0

0

1.55 µm

35

Insertion loss (dB)30

Figure 12 Insertion loss distribution for 1 x 16 splitter.

25
20
15

11.0

10.8

10.6

10.4

10.2

10.0

1.55 µm

9.81.31 µm

40

9.6

45

Insertion loss (dB)

Insertion loss distribution for 1 x 8 splitter.

35
30
25
20
15

0.40

0.35

0.30

0.25

0

40

0.20

5

1.55 µm

0.15

501.31 µm

0.10

10

60

0.05

0

9.4

50

5
9.2

10

9.0

Frequency (%)

9.5

9.0
1.25

1.31 µm

40

Frequency (%)

10.0

5

45

Figure 9

10.5

0.00

Figure 8Frequency (%)

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0

Insertion loss (dB)

11.0

10

Frequency (%)

Frequency (%)

Silicon sub.

PDL (dB)

30

Figure 13 PDL distribution for 1 x 16 splitter.

20

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

0.00

10

PDL (dB)

Figure 10 PDL distribution for 1 x 8 splitter.

Figures 9 and 10 show the insertion loss distribution
and PDL distribution for a total of 2008 1 x 8 splitters,
and satisfactorily low values were obtained. The averageinsertion loss and standard deviation were 9.7 dB and

Figures 12 and 13 show the insertion loss distribution
and PDL distribution for a total of 368 1 x 16 splitters,
and the results obtained were sufficiently low for practical
purposes. The average insertion loss and standard
deviation were 12.8 dB and 0.21 dB for the 1.31-µm band,and 12.8 dB and 0.18 dB for the 1.55-µm band; and theaverage PDL and standard deviation were 0.13 dB and
0.06 dB for the 1.31-µm band, and 0.09 dB and 0.04 dB
for the 1.55-µm band. Improvements are on-going toachieve even lower insertion loss and PDL in the future.

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PLC Products for FTTH Systems

PLC MODULES

Modules were fabricated using the chips describedabove and their characteristics were confirmed. There
are two sizes, which are Furukawa Electric standards.
The compact module accommodates the 1 x 4, 1 x 8 and1 x 16 splitters, and measures 3.9 x 3.9 mm, with lengthsof 38.0, 41.5 and 50.0 mm respectively. The regularmodule is used for 1 x 32 and 8-arrayed 1 x 4 splitters,
and measures 8.0 x 6.4 x 76.0 mm. The former (compact)
module is suited to accommodating closures etc. having
a low space requirement. Figure 14 is a photographshowing a compact module for a 1 x 8 PLC splitter.

Figure 14 Compact 1 x 8 PLC splitter module.

The method used in fabricating the modules is to joina fiber array, in which fibers are precisely positioned bymeans of glass V-grooves, to the chip using a UV curable
adhesive designed for fiber-optic components. The wholeis then enclosed in a case to protect the adhesive joint
from ambient conditions. The end faces of the fiber arrayand chip are treated to prevent optical loss or reflection.
The chips developed in this project are also of asize that can be accommodated in one or other of themodules described above, and using the above method
the 1 x 8 splitter can be packaged in the compact
module and the 8-arrayed 1 x 4 splitter fits in theregular module. As predicted from chip characteristics,
module characteristics for insertion loss and PDL weresatisfactory, and return loss averaged 55.6 dB for the
1 x 8 splitter modules and 56.4 dB for the 8-arrayed 1 x 4
splitter modules. Good reliability was also demonstrated,with a loss variation of no more than ±0.1 dB over atemperature range of −40 to +85˚C.

4.

confirmed by the fact that variations in insertion loss and
PDL before and after the test were less than 0.1 dB.
The PLC modules were also subjected to reliability tests
in accordance with Telcordia GR-1221 and GR-1209, withthe results shown in Table 1. Passes were achieved for all
tests confirming that the modules have high reliability.
Table 1Time

Max loss
deviation
(dB)

Pass/
Fail

Temperaturehumidity cycling−40 to +85°C, 10%
to 80%RH

42
cycle

0.07

Pass

Water immersion+43°C, pH 5.5

168 h

0.11

Pass

Fiber and cableretention

0.45 kg, 5 s,
3 times

0.19

Pass

Mechanical shock

5 times,
6 directions,
500 G, 1 ms

0.10

Pass

Vibration

20 G, 20~2000 Hz
4 min/cycle
4 cycle/axis

0.09

Pass

Damp heat (nonhermetic)

85°C/85%RH

2000 h

0.23

Pass

Low temperaturestorage−40°C

2000 h0.19

Pass−40 to +85°C

500
cycle

0.22

Pass

Temperature
cycling

As a representative test, Figure 15 shows the variation
in insertion loss for a total of 13 8-arrayed 1 x 4 splittermodules in a damp heat test (85°C 85 % RH for 2000hrs) at the elapse of 336, 1000 and 2000 hrs. It shows
the port exhibiting the greatest increase in loss among
the loss variations on 32 ports, 1 module. It can be seenthat characteristics remained stable at all times, andthat reliability was high at high temperature and relative
humidity.

RELIABILITY TESTS

Because of the overriding importance of the long-term
reliability of components for use in telecommunications
systems, reliability tests were carried out both on PLCs
using silicon substrates and on those using silica
substrates. Here we report on the reliability of 8-arrayed1 x 4 splitters that use silica substrates.
In reliability tests for testing individual chips, a pressure
cooker test was carried out under conditions of 120°C, 100 % RH for 100 hrs (equivalent to 85°C, 85 % RH
for 8000 hrs), and it was confirmed that no externalcracking or breakage occurred. Chip reliability was further

Reliability test results.
Conditions

Test

Insertion loss variation (dB)

3.

0.5
0.4
0.3
0.2
0.1
0.0
--0.1
--0.2
--0.3
--0.4
--0.5

0

500

1000

1500

2000

2500

Time (hour)

Figure 15 Variation in insertion loss for 8-arrayed 1 x 4 splittermodule in damp heat test.

5.

CONCLUSION

WINCs and 1 x 4, 1 x 8 and 1 x 16 splitters for FTTH
services have been developed using PLC technology.

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PLC Products for FTTH Systems

The possibility of fabricating with good characteristics
and high yield has been confirmed. The results ofreliability tests were also good, completing the device
development. It is proposed in future to apply thistechnology to the development of other products, suchas 1 x 32 splitters or WDM couplers for FTTH service
applications.

REFERENCES
1) Press information by the Ministry of Public Management, Home
Affairs, Posts and Telecommunications: “Changes in the number ofusers of Internet-access service,” March 31, 2004. (in Japanese)
2) Enomoto et al.: SB-8-7, General Conference of IEICE '97, (inJapanese)3) K. Jinguji et al.: J. Lightwave Technology, 14, 2301 (1996).
4) Nara et al.: C-3-95, Society Conference of IEICE ‘97. (in Japanese)5) Nekado et al.: C-3-41, Society Conference of IEICE ‘97. (inJapanese)

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