PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services

PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for
FTTH Video Distribution Services
by Kazutaka Nara *, Junichi Hasegawa * and Toshiaki Tsuda *With the rapid spread of FTTH services in recent years, there have been proposals for a broadband passive optical network (B-PON) system that combines datatransmission services with simultaneous video distribution services. Taking
advantage of the new wavelength assignments —1.31, 1.49 and 1.55 µm— which have become
international standards under ITU-T G983.3, requires a WDM filter that is economical and of highfunctionality to multiplex these three wavelengths. Specifically, central offices require compact
WDM modules that contain WDM filter arrays.
Accordingly we have developed an 8-channel 3-wavelenth wideband WDM filter array using a
new optical circuit configuration based on a Mach-Zehnder interferometer (MZI) that uses silicabased planar lightwave circuit (PLC) technology, and have been able to obtain good performance: insertion loss of <1.96 dB, loss ripple of <0.77 dB and isolation of >32 dB.

ABSTRACT1.

INTRODUCTION

In recent years, as Internet connections have reached into
every home, there has been widespread adoption of highspeed low-cost services including always-on connections,megabit-class transmission speeds and monthly fees of a
few tens of dollars. And greater use is being made of thekind of large-volume content distribution and video chatrooms consistent with broadband connections.
To provide this type of broadband service, a broadband
passive optical network (B-PON) system has been madean international standard under ITU-TG983.3, capable ofdelivering data transmission speeds of 100 Mbps (peak)simultaneously with up to 500-channel video distribution
over a single optical fiber. Figure 1 shows the structure ofa typical system 1).
The principal feature of this system is that due to new
optical wavelength assignments, it permits reception of
two wavelengths used for transmission of high-speed

Central office

Subscriber
ITU-T G.983.3

V-OLT

1.49 µm-band
1.55 µm-band

OpticalsplitterFigure 1

Opticalsplitter

ONU
WDM filter

OLT

WDM filter

1.31 µm-band

V-ONU

Structure of typical B-PON system.

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

data, and, by superimposing one more optical wavelength, simultaneous distribution of multi-channel video.
Ordinarily the wavelengths for high-speed data transmission and reception are the 1.49-µm band (1.48~1.50 µm)
for the downstream and the 1.31-µm band (1.26~1.36µm) for the upstream, and video data is distributed on the
1.55-µm band (1.55~1.56 µm), which is the EDFA amplification band. For this reason it is necessary to have awavelength division multiplexing (WDM) filter to simultaneously demux the three wavelengths—the high-speed
data transmission wavelengths and the video signal wavelength—on a single optical fiber. In the past there havebeen various candidates for this WDM filter. The mostgenerally used type, one that adopts a dielectric multilayer film filter, has been reported to have excellent characteristics including low loss and wideband operation 2),
but to improve economy in central offices there is a needfor array installations and in this respect it is not satisfactory.
In contrast, a WDM filter that adopts silica-based planar
lightwave circuit (PLC) technology is promising from thestandpoint of array design, and up to the present timeseveral methods have been considered. One of these is a
WDM filter with a dielectric multi-layer filter inserted at thecrossing point of intersecting waveguides formed of silicabased PLC 3). However because of the formation of the slit
for filter insertion and the complexity of the processes offilter insertion and mounting, it is not satisfactory from thestandpoint of economy. And in terms of characteristics,
while it achieves low loss and low loss ripple, it presents a
problem in terms of the characteristics of the dielectric
multi-layer filter in that return port isolation, at <20 dB, is

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PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services

unsatisfactory.
Another method that has been considered is a WDM filter based on a Mach-Zehnder interferometer (MZI) 4), 5).
Since the PLC-MZI-type WDM filter achieves its characteristics in the optical circuit only, it does not need any of theadditional processing that has been described above,and is therefore generally considered to be the most suitable method in terms of economy. At the same time as
offering a wider band, however, it does present problems
in terms of degradation of loss ripple in the pass bandand degradation of isolation in the stop band, andimprovements must be made before practical characteristics can be achieved.
In this paper, accordingly, we propose a novel opticalcircuit based on the MZI, present details of its design, andreport on the results of fabricating an 8-channel PLC-MZItype 3-wavelenth wideband WDM filter array.

First of all we carried out a formulation of a PLC-MZItype wideband WDM filter. From Figure 3 as mentioned
above, the PLC-MZI-type wideband WDM filter may be
represented as in Figure 4.

2.

we obtain

OPTICAL CIRCUIT CONFIGURATION
AND DESIGN

Structure and Formulation of PLC-MZI-type
Wideband WDM Filter
Figure 2 shows a schematic of the optical circuit configuration for the proposed PLC-MZI-type wideband WDM
filter. It has a cascaded MZI consisting of four identicaldirectional couplers and two different phase shifters, connected point-symmetrically with respect to its output
waveguide.
We then proceeded to formulation of a concrete opticalcircuit design. First we broke down the WDM filter circuit
into its component elements, as shown in Figure 3. That
is to say optical coupler S1 is connected in a fully pointsymmetrical manner with respect to its output waveguide,
and that optical coupler S1 can be considered as a PLCMZI in which optical coupler S 3 is connected throughphase shifter T φ1 in a point-symmetrical manner withrespect to its output waveguide, and further, optical coupler S3 consists of two identical directional couplers S5
and phase shifter Tφ0.

1

P1
Optical coupler S1Figure 4aS1 = 
b− b* 
,a* 

Schematic optical circuit configuration for proposed
PLC-MZI-type wideband WDM filter.

In

Directional
coupler S3

Figure 3

Through port
Cross port

Breakdown of optical circuit for PLC-MZI-type wideband WDM filter into component elements.

a 2+ b 2=1(1)

where: * designates a complex conjugate.
Accordingly the optical intensity of the cross-port, thatis to say the coupling efficiency P of the PLC-MZI-type
wideband WDM filter, may be determined by
P2 2 = P = 2a b 2 = 4b2 ( 1 − b2 ) = 4η ( 1 − η )(2)where: η is the coupling efficiency of optical coupler S1.
Next, Figure 5 shows the circuit arrangement for optical
coupler S1.
Phase Shifter
Tφ1

1

Q1

C

Figure 5

Optical coupler
S4

η

Q2

Circuit arrangement for optical coupler S1.

Taking the transfer matrix for the case in which optical
coupler S3 is connected through phase shifter Tφ1 in apoint-symmetrical manner asαS3 = 
β

Optical coupler S1
Optical coupler S3
Phase
Phase
shifter Tφ1
shifter Tφ0

− b*
,
a

 P1 
 1 0
 1
 a* a − b* b  = S2 
 S1   = 2ab  P2 
 0 1
 0
Through port

Directional coupler

Figure 2 a*
S2 = 
b

Optical coupler
S3

Cross port

P2

Circuit arrangement for PLC-MZI-type wideband
WDM filter.

Cascaded MZI
Phase
Phase
shifter2
shifter1

P

Taking the transfer matrix for the case in which optical
coupler S1 is connected in a fully point-symmetrical manner as

2.1

In

Optical coupler S2− β *
,
α* 

α *
S4 = 
β− β *,α 

α 2 + β 2 =1

andφ

 − j 21e
Tφ 1 = 

 0


0 
φ
j 1 
e 2 

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PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services

we obtain

φ1
φ 1
 P1
 1   α 2 e− j 2 − β 2 e j 2   = S4Tφ 1 S3   = 

φ
 P2
 0 

2αβ cos 1


2

(3)

where: φ1 is the amount of phase shift of phaseshifter Tφ1.
Accordingly the optical intensity of the cross-port, thatis to say the coupling efficiency η of optical coupler S1,
may be determined by
φ1 2
φ= 4 β 2 ( 1 − β 2) cos2 1
22
φ= 4 C ( 1 − C ) cos2 12η ≈0.98, or 2 % divergent from the design value.
Next let us consider the case where η ≈0. In this case itis sufficient from Equation (4) to satisfy only one of 4C(1C)≈0 or cos2(φ1/ 2)≈0, but when they are satisfied simultaneously more stable characteristics will be obtained. For
example even in a case in which 4C(1-C)≈ 0.01 andcos2(φ1/2)≈0.01, each parameter diverging from the design
value by 1 %, we see that we will get η ≈0.0001, virtuallyno divergence (only 0.01 %) from the design value.
Furthermore with this effect the change in η due to wavelength will be made slower and the band will be widened.
From this we can obtain as a first condition

Q2 2 = η = 2αβ cos

(4)

and

where: C is the coupling efficiency of optical coupler S3.
Next, Figure 6 shows the circuit arrangement for optical
coupler S3.
1

Phase ShifterTφ0

Directional coupler
S5

Figure 6

R1

R2
C

Circuit arrangement for optical coupler S3.

And in the same way as above described, the opticalintensity of the cross-port, that is to say the coupling efficiency C of optical coupler S3, may be determined by
R2 2 = C = 4κ (1 − κ ) cos2

φ0
2

(5)

where: κ is the coupling efficiency of directional
coupler S5 and φ0 is the amount of phaseshift of phase shifter Tφ 0.
From the above discussion we can represent the characteristics of the PLC-MZI-type wideband WDM filter using
Equations (2), (4) and (5).
2.2

Design of PLC-MZI-type 3-Wavelength Wideband
WDM Filter
Next we proceeded to the design of a 3-wavelenth wideband WDM filter consisting of PLC-MZI wideband WDM
filters.
First of all the through-port is set to output light of λ=1.31 µm and 1.49 µm. From Equation (2) we can thenobtainP ≈ 0 ⇔ 4 η ( 1 − η ) ≈ 0 ⇔ η ≈ 0 or η ≈ 1(6)First let us consider the case where η ≈1. In this case itis necessary from Equation (4) to satisfy 4C(1-C)≈1 andcos2(φ1/2)≈1 simultaneously. Now if we consider the casein which 4C(1-C)≈0.99 and cos2(φ1/2)≈0.99, each parameter diverging from the design value by 1 %, we will get

4C (1 − C) ≈ 0
4C (1 − C) ≈ 0
φ 
cos2  1  ≈ 0 ⇔ φ 1 ≈ (2m + 1 )πφ
cos2  21  ≈ 0 ⇔ φ 1 ≈ (2m + 1 )π2
@ λ =1.31µm, 1.49 µm@ λ =1.31 µm, 1.49 µm(7)

Next let us consider 4C(1-C)≈0 in the first half ofEquation (7). As in Equation (6) we can obtain C ≈ 0 orC≈1, but from the above discussion we select C≈0. Here,when designing a 3-wavelenth wideband WDM filter, particularly at λ=1.31 µm, a much wider band of ±50 nm is
required. Accordingly in this instance, limited to λ=1.31µm, we make settings such that C ≈ 0, and pursue bandwidening in that band of wavelengths. That is to say, from
Equation (5), we can obtain as a second condition
4κ (1 − κ ) ≈ 0
4κ (1 − κ ) ≈ 0

and

φ 
cos2  φ 00  ≈ 0 ⇔ φ 0 ≈ ( 2n + 1 )π2
cos  2  ≈ 0 ⇔ φ 0 ≈ ( 2n + 1 )π2@ λ =1.31 µm@ λ =1.31 µm(8)Then from 4κ(1-κ)≈0 in the first half of Equation (8) we
can obtain κ ≈ 0 or κ ≈1. In this case it does not matter
which is used, but since, generally speaking, the longer
the coupling length of the directional coupler becomesthe more subject it is to fabrication errors, here we chooseκ ≈0.
To summarize the design conditions with respect to thethrough-port based on the above, simultaneously satisfying
φ 
cos2  1 ≈ 0 ⇔ φ 1 ≈ ( 2m + 1) π 2@ λ =1.31 µm, 1.49 µm
φ 
cos2  0 ≈ 0 ⇔ φ 0 ≈ ( 2n + 1) π
 2

(9)

@ λ =1.31 µm
κ ≈0@ λ =1.31 µm

is the condition.
where: m and n are integers.

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PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services

P ≈ 1 ⇔ 4η ( 1 − η ) ≈ 1 ⇔ η ≈ 0.5(10)Here, from Equations (4), (5), (9) and (10) we canobtain
κ≈

1
1 ±
2



1−


C

cos ( φ0 2 )2@ λ =1.55 µm(11)which yields
C≈


0.5
1
1± 1−
2
cos2 ( φ 1 2 ) @ λ =1.55 µm(12)Transmittance（dB）

Summarizing the above, from Equation (9) we canobtain φ 1 =23 π , 21 π (when λ = 1.31 µ m and 1.49 µ m
respectively). Then letting the waveguide length for phaseshifter Tφ1 be ∆L1 and the equivalent refractive index of thecore be neff, the amount of phase shift φ1 may be represented as (2π/λ) neff∆L1, and from this we can find that
∆L1= 0.87 µm. Similarly, letting the waveguide length forphase shifter Tφ0 be ∆L0, the amount of phase shift φ0 may
be represented as (2π/λ) neff∆L1, and from Equation (8)we can find that ∆L0 =3.18 µm. From these values for theamount of phase shift and from Equations (11) and (12),we selected directional couplers having κ=0.008, 0.026
and 0.040 (when λ =1.31 µ m 1.49 µ m and 1.55 µ m
respectively).
Moving on, the wavelength response of the PLC-MZItype 3-wavelenth wideband WDM filter calculated on thebasis of the circuit parameters that have been derived isshown in Figure 7. From this it can be seen that light in
the 1.31-µm and 1.49-µm bands is output to the throughport, while that in the 1.55-µm band is output to the crossport, and wideband operation is achieved in the 1.31-µm
band. It can also be seen that a low loss ripple characteristic is obtained in all of the bands.

Through
Cross

In

Through port
Cross port
Additional filter

Figure 8

-5
-15
-20
-25
-30

0
-5
-10
-15
-20
-25
-30

1250

1300

1350

1400

1450

Through
Cross

-35
-40
1200

1250

1300

1350

1400

1450

1500

1550

1600

Wavelength（nm）

Calculated wavelength response using double-passconfiguration.

2.4

Through
Cross-35

Schematic configuration for achieving high isolation.

Figure 10 shows the calculated wavelength responseusing this additional filter. From this we see that by using
an additional filter with wavelength characteristics thatexhibit a valley in the 1.49-µm band we can enhance 1.49µm band isolation compared to that with the double-passconfiguration alone.

-10

1500

1550

1600

Wavelength（nm）

Figure 7

Proposed PLC-MZI-type Proposed PLC-MZI-type
wideband WDM filter
wideband WDM filter

Figure 9

0
-0.2
-0.4
-0.6
-0.8
-1
0

-40
1200

2.3 Design for Achieving High-Isolation
From the results described above, it was possible toachieve wideband, low loss ripple operation through theuse of the novel optical circuit arrangement proposed inthis work. If, however, we examine isolation for each of the
bands, we find that it is about 15~20 dB, which is notentirely adequate. We therefore investigated how to use
the optical circuit configuration shown in Figure 8 as ameans of achieving high isolation 5).
First we adopted a double-pass configuration, in whichlight would pass through two identical PLC-MZI wideband
WDM filters at both the through-port and the cross-port.
Figure 9 shows the calculated wavelength responseusing this double-pass configuration. From this we cansee that isolation of >35 dB was achieved for both the
1.31-µm and 1.55-µm bands, while for the 1.49-µm band,on the other hand, it was only about 22 dB, still inadequate. Accordingly we applied an additional filter consisting of an MZI to the cross-port, thereby improving 1.49µm band isolation.

Transmittance（dB）

Next the cross-port is set to output light of λ=1.55 µm.
From Equation (2) we can then obtain

Calculated wavelength response of PLC-MZI-type
3-wavelenth wideband WDM filter.

Design for Achieving Reduced Fiber CouplingLoss
To further promote loss lowering, we devised a means oflimiting coupling loss to the optical fiber. The PLC-MZI 3wavelenth wideband WDM filter described above has circuit that is of large scale, and we proceeded with ourwork on condition that it would be fabricated on a silica-

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PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services-5

Transmittance
（dB）

-10
-15
-20
-25
Double pass of PLC-MZI WDM filter-30

Additional filter-35

Double pass of PLC-MZI WDM filter andadditional filter

-40
1200

1250

1300

1350

1400

1450

1500

1550

1600

Wavelength（nm）

Figure 10 Calculated wavelength response using additional
filter.

tolithography and reactive ion etching, we fabricated an
8-channel PLC-MZI-type 3-wavelenth wideband WDM filter array on a silicon substrate, which exhibits low loss,
low loss ripple and high isolation. Figure 13 shows the
waveguide layout.
It may be seen that even including all the reference optical circuits, it has been possible to fabricate onto a chipmeasuring only 63 × 5.6 mm2. Figure 14 is a photographshowing the fabricated module.
Next we present the results of an evaluation of wavelength response. Measurements were made using an
optical spectrum analyzer having a wideband light source.
First we compared the calculated values and the measured values to confirm the suitability of the design.
5.6 mm0

63 mm

Optical
fiber
array

Figure 13 Waveguide layout for 8-ch PLC-MZI-type 3-wavelenthwideband WDM filter array.

Lateral tapered waveguide
WT
PLC

Figure 11 Schematic configuration of interface between PLC
and I/O optical fibers.

based PLC with a high refractive index difference ∆ of
0.8 %. Thus by using dispersion-shifted fiber (DSF) for the
input and output fibers and by introducing lateral tapered
waveguides on the input and output ends of the silicabased PLC, we were able to reduce fiber coupling loss.
Figure 12 shows calculated results for the relationshipbetween taper waveguide width WT and coupling loss.
From this we can see that by using DSF and adopting atapered waveguide with a width of 8.75 µm, we may anticipate a reduction in coupling loss calculated to be approximately 0.4 dB/facet compared to a connection with ordinary single-mode fiber.

Coupling loss（dB）/ facet

0.6
0.5

SMF

0.4
1.31 µm（DSF）

0.3

1.55 µm
（DSF）

0.2
0.1
0

Figure 15 compares the calculated result (a) and themeasured result (b) for the 7th optical circuit of the 8channel array. It can be seen that there is an extremely
high level of agreement between the calculated and measured results, confirming the suitability of this design.
Figure 16 shows the results of an evaluation of wavelength response for all 8 channels of the PLC-MZI-type 3wavelenth wideband WDM filter array module fabricatedin this work, and Table 1 presents a summary of its opticalproperties. From this it can be seen that good characteristics were obtained for all wavelength bands concerned,
namely the 1.31-µm band (1.26~1.36 um), the 1.49-µm
band (1.48~1.50 µm) and the 1.55-µm band (1.55~1.56µm) and for all 8 channels. These characteristics were:insertion loss of <1.96 dB, loss ripple of <0.77 dB, In-tothrough-port and In-to-cross-port isolation of >32 dB andthrough-to-cross-port isolation of >50 dB.

4.
2

3

4

5
6
7
8
9
WT of taper waveguide
（µm）

10

11

Figure 12 Relationship between taper waveguide width WT andcoupling loss.

3.

Figure 14 Fabricated module.

RESULT OF FABRICATION

Based on the arguments advanced in Section 2 above,and using the ordinary technology for fabricating silicabased PLC, combining flame hydrolysis deposition, pho-

CONCLUSION

In the run-up to achieving FTTH video distribution services in an economical manner, we have used silica-based
PLC technology to develop an 8-ch PLC-MZI 3-wavelenthwideband WDM filter array that utilizes a novel circuit configuration based on the Mach-Zender interferometer. The
result was that good characteristics were obtained for all
wavelength bands concerned and for all 8 channels, thatis to say insertion loss of <1.96 dB, loss ripple of <0.77
dB, and isolation of >32 dB.

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PLC-MZI-type 3-Wavelength Wideband WDM Filter Array for FTTH Video Distribution Services

Transmittance
（dB）

0
-0.25
-0.50
-0.75
-1.00
0

Table 1Through
Cross

Meas.
(Worst)

-5

Meas.
(Avg)

Design-15

Ins. loss

1.54

1.32

1.42

—

-20

Loss ripple

0.38

0.20

0.26

0.12

Isolation (In-Cr)

33.6

34.6

34.0

41.0

Isolation (Th-Cr)

>50.0

>50.0

>50.0

—

-25
Through
Cross

-35
-40
1200

1250

1300

1350

1400

1450

1.49 µm passband (1.48~1.50 µm)
1500

1550

1600

Wavelength（nm）a) Calculated
-1.00
-1.25
-1.50
-1.75
-2.00
0

Transmittance
（dB）

Meas.
(Best)

1.31 µm passband (1.26~1.36 µm)

-10

-30

Through
Cross

Ins. loss

1.96

1.75

1.85

—

Loss ripple

0.77

0.65

0.71

0.67

Isolation (In-Cr)

32.1

34.4

33.3

34.2

Isolation (Th-Cr)

>50.0

>50.0

>50.0

—

1.55 µm passband (1.55~1.56 µm)

-5
-10
-15

Ins. loss

1.71

1.53

1.63

—

Loss ripple

0.13

0.08

0.10

0.09

Isolation (In-Th)

32.0

33.1

32.2

45.0

Isolation (Th-Cr)

>50.0

>50.0

>50.0

—

-20
-25
-30
-40
1200

REFERENCES

Through
Cross

-35
1250

1300

1350

1400

1450

1500

1550

1600

Wavelength（nm）b) Measured
Figure 15 Comparison of calculated and measured results forwavelength response of the 7th of 8 channels.

Transmittance（dB）

Summary of optical properties of fabricated 8-ch
PLC-MZI-type 3-wavelenth wideband WDM filter array
module.

-1.00
-1.25
-1.50
-1.75
-2.00
0

Through
Cross

-5
-10
-15

1) Y. Maeda, K. Okada and D. Faulkner, “FSAN OAN-WG and futureissues for broadband optical access networks”, IEEE
Communication Magazine, Dec. (2001) pp.126-132.
2) K. Nishiwaki, Y. Noguchi, T. Matsuura, H. Hosoya, “Developmentof a simply structured WDM component for access network”, Proc.
Society Conference 2003, IEICE, C-3-101. (in Japanese)
3) M. Yanagisawa, Y. Inoue, M. Ishii, T. Oguchi, Y. Hida, H. Izumita, N.
Araki and T. Sugie, “Low-loss and compact TFF-embedded silicawaveguide WDM filter for video distribution services in FTTH systems”, OFC2004, Technical Digest TuI4.
4) Y. Hida, N. Takato and K. Jinguji, “Wavelength division multiplexer
with wide passband and stopband for 1.3 µm/1.55 µm using silicabased planar lightwave circuit”, Electron. Lett., vol.35, no.16 (1996)
pp.1377-1378.
5) K. Nara, H. Urabe, J. Hasegawa, N. Matsubara and H. Kawashima,“1.26-1.56 µm/1.65 µm ultra-wide band WDM filter with high isolation using silica-based planar lightwave circuit”, OECC2004,
Technical Digest 15F-1-5, pp.722-723.

-20
-25
-30

Through
Cross

-35
-40
1200

1250

1300

1350 1400 1450
Wavelength
（nm）

1500

1550

1600

Figure 16 Measured wavelength response of all 8 channels of
the PLC-MZI-type 3-wavelenth wideband WDM filter
array module.

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