Optical Add-Drop Multiplexers for Metro/Access Networks
by Masayoshi Kagawa *, Hiroyuki Tsukada * and Masafumi Yoneda *As multi-media fields develop, metro/access networks are required to allow for
flexible configuration changes as well as higher capacity and speed. An optical
network configuration that integrates optical add/drop multiplexers (OADMs) with wavelength division multiplexing (WDM) system began to be introduced in these networks for its suitability. The
authors have developed OADMs to be used in metro/access networks, which are capable ofdealing with one to several channels arbitrarily selected.

ABSTRACT1.

signals are separated from a large amount of signals tocarry out sending and receiving of information with theusers. See Figure 1. Consequently, when conventional
systems based on optical-electrical signal conversion areused, an enormous scale of equipment would be required,leading to an increase in system maintenance costs.
Recently, the coarse WDM (CWDM) communications
technology is drawing attention since the technology
allows a network configuration using devices with lower
costs. However, because mutual entry between CWDM
and the dense WDM (DWDM) for trunk networks is impossible, it is required to provide optical-electrical-optical conversion at switching nodes between the CWDM and
DWDM systems.
To reduce the cost of large capacity transmission,
whereas conventionally most signal processing has been
done after optical-to-electrical conversion, it is required toprocess signals in optical form. Optical add/drop multiplex-

INTRODUCTION

Presently, high-speed and wide-bandwidth broadband
access systems such as ADSL, CATV and FTTH havebecome widespread. With respect to the data sent andreceived in such systems, those not only of text but also
from multi-media fields --such as moving images andaudio-- have grown in volume, and tend to increase hereafter. Thus, metro/access networks that connect subscribers with a trunk network are required to have scalability enabling rapid changes of communications capacity inresponse to the requirement of subscribers, in addition totransmission capacity enhancement and cost reduction,
so that WDM systems have begun to be introduced.
Many metro/access networks adopt a ring configuration
where the nodal bases are connected to form a ring, taking into consideration the reliability, transmission efficiency and ease of operation. At each nodal base, required
λ1

λ1Band Mux/Demux

Band OADMλnλnλ 1 .. λ 8

EDFA

λ 1 .. λ 4Metro Ring

AccessOADM

λ 5 .. λ 8
Splitter

λ4

Figure 1*

Typical system configuration of metro/access network.

Optical Components Dept., FITEL Products Div.

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Optical Add-Drop Multiplexers for Metro/Access Networkser (OADM) is one of the key devices to implement such
optical signal processing where signals with arbitrary
wavelengths are multiplexed to, or demultiplexed from,wavelength multiplexed signals. Use of this device makes
it possible to freely add or drop signals with arbitrary
wavelengths over multiplexed optical signals by assigninga wavelength to each destination. Moreover, it is possible
to simplify the component configuration of optical amplifiers through reduction of the optical attenuation for the
express channels --optical channels neither add nor dropat nodes-- in OADMs, thereby decreasing the total cost of
networks.

2

CONFIGURATION OF OADM

2.1 Configurations Using TFF and FBG
The basic configuration of an OADM using dielectric thinfilm filter (TFF) is shown in Figure 2.
An arbitrary signal wavelength is branched (i.e.,
dropped) from wavelength-multiplexed signals via a narrow band-pass filter (BPF), whereby only the desired signal wavelength being transmitted while others reflected.
Meanwhile, an arbitrary signal wavelength can be inserted(i.e., added) into wavelength-multiplexed signals via a narrow BPF, whereby the desired signal wavelength beingtransmitted is combined with the reflected signal wavelengths.
Figure 3 shows the basic configuration of an OADM
using a fiber Bragg grating (FBG).
The wavelength-multiplexed signals enter an FBG
through a circulator, where only one arbitrary signal wavelength is reflected while others are transmitted. The
reflected signal wavelength is branched (i.e., dropped)
into a port other than that where the wavelength-multiplexed signals enter. In the case of wavelength multiplex-

ing an arbitrary signal wavelength, the signal wavelength
incident on the circulator is reflected by the FBG, and isinserted (i.e., added) into the wavelength-multiplexed signals that are transmitted via the circulator.
Features of these configurations are as follows. The
configuration using the TFF is simple in construction andlow in cost since it comprises only a TFF module. On theother hand, the configuration using the FBG can reduce
the level of the signal wavelength contained in the wavelength-multiplexed signals down to 0.1 % or less --whereas it is about 0.3 % for the one using the TFF, so that it
advantageously reduces the interference with the signal
wavelength to be added after dropping, thereby decreasing signal noises.
In actual applications of OADMs for metro/access networks, it is typically required to add/drop four channels. In
this report, accordingly, configurations for four consecutive
channels using TFF-based OADMs will be described indetail.
2.2 TFF Module
Figure 4 shows the basic configuration of a TFF module
for OADM. It consists of two single-mode optical fibers,
dual-core ferrule, first lens, TFF, second lens, single-coreferrule and single-mode optical fiber. Whereas each ofthese components is fixed using laser welding or adhesive, the latter is not used in the optical paths to make thisdevice resistant against an optical input of the watt-classpower. The optical path is such that, in the case of a λ1reflecting and λ2-transmitting filter, the light with wavelength λ1 incident on the dual-core ferrule side is collimated by the first lens to be totally reflected by the filter; thereflected light with wavelength λ2 is again incident on andcollimated by the first lens to enter another fiber on the
dual-core ferrule; the transmitted light with wavelength λ2
is coupled into the fiber on the single-core ferrule via thesecond lens. Figure 5 shows the appearance of a TFFbased optical combiner module.

First lensNarrow Band Pass Filter

Drop

Add

Second lensDual-core ferrule

Single-core ferrule
Dielectric TFF

Figure 2

Circulator

Basic configuration of TFF-based optical combiner
module for OADM.

Figure 5

Appearance of TFF-based optical combiner module.

FBG
Drop

Figure 3

Figure 4
Configuration of OADM using TFF.

Add

Configuration of OADM using FBG.

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Optical Add-Drop Multiplexers for Metro/Access Networks

Incident

Band-pass filter
Transmitted

Table 1

Target specifications for four-channel OADM with
100-GHz spacing.
Min.

Max.

Operating temp.

Item

0°C

70°C

Pass band

±0.11

NA

Pass band loss

NA

3.5 dB

Express channels loss

NA

0.6 dB

Ripple (Drop channel)

NA

0.5 dB

Adjacent channel isolation

25 dB

NA

Polarization dependent loss

NA

0.3 dB

Chromatic dispersion@pass bandNA

±70 ps/nm

Reflected

Express channels

Express channels

Band-separation filter

0

0.0

-10

-0.5

-20

-1.0

-30

-1.5

-40
1549

1550

1551

1552

1553

1554

(dB)

Basic configuration of TFF-based four-channel
OADM, where four BPFs are connected in series.

Insertion loss (dB)Figure 6

-2.0
1555

Wavelength (nm)Figure 7

Configuration of TFF-based four-channel OADM
using band-separation filter.

2.3 Four-Channel OADM
Figure 6 shows the basic configuration of a TFF-basedfour-channel OADM for branching (i.e., dropping). A signal
wavelength for one channel is transmitted through a BPF
and the rest signal wavelengths are reflected to enter the
next BPF. By repeating these steps four times, four channels are branched from the multiplexed signal wavelengths, whereby four TFF modules are connected inseries to constitute an OADM. But this configuration is at a
disadvantage in terms of the high insertion loss imposedon the express channels due to the repeated reflections
by the four filters. Higher insertion loss for the express
channels has an adverse influence on the cost such thatthe system requires optical amplifiers of higher gain in thelater stage of signal transmission.
2.4 Use of Band-Separation Filter
In order to improve the insertion loss on the express channels, an alternative configuration shown in Figure 7 is suggested, where a band-separation filter (i.e., 4-skip-0 filter)
is used to separate four channels in block from multiplexed signal wavelengths. With this configuration, theexpress channels undergo only one reflection, so that theinsertion loss is greatly improved leading to a reduction in
total system cost.
When BPFs are used to configure a WDM system, theinsertion loss difference of each channel depends on thenumber of reflection at the BPFs. Accordingly, the use offour-channel band-separation filter reduces the passing
times through the reflection filters, thus improving the

Figure 8

Transmission spectrum of 4-skip-0 filter module.

insertion loss characteristics. The insertion loss differences between the signal wavelengths added or dropped
are also improved by a factor of four.

3.

TARGET SPECIFICATIONS

Target specifications for a four-channel OADM with 100GHz spacing using 4-skip-0 band-separation filters havebeen specified as shown in Table 1.

4.

CHARACTERISTICS OF PRODUCTS

4.1

Characteristics of 4-skip-0 Band-Separation
Filter Module
Figure 8 shows the transmission spectrum of a 4-skip-0 filter module. Over the four channels with a 100-GHz spacing (327.5 GHz) in the transmission band from 193.0 THz
to 193.3 THz, the insertion loss is 0.49 dB --a low value.
The flatness of insertion loss over the transmission bandis 0.13 dB --excellent performance, and the isolation
between adjacent channels is 25 dB or more.
Figure 9 shows the reflection spectrum. The insertion
loss is 0.3 dB or less --a low value. The isolation over thetransmission band is 18 dB or more, ensuring a dropchannel isolation of 36 dB or more --large crosstalk-when used as an OADM for add/drop.
Figure 10 shows the transmission chromatic dispersion,
giving a low value of dispersion of ±25 ps/nm or less overFurukawa Review, No. 23

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61

0.0

-10

-0.5

-20

-1.0

-30

-1.5

40

30

∆λ c (pm)0(dB)

Insertion loss (dB)

Optical Add-Drop Multiplexers for Metro/Access Networks

20

10

0
-40
1548

1550

1552

1554

Wavelength (nm)

20

100

80

100

0

-50

1550

1551

1552

1553

1554

-10

-20

-30
1550

1555

1551

1552

1553

1554

Wavelength (nm)Wavelength (nm)Figure 10 Transmission chromatic dispersion of 4-skip-0 filter
module.

Figure 13-1 Transmission spectrum of four-channel OADM
module.

0100

Insertion loss (dB)

Chromatic dispersion (ps/nm)

60

0

50

50

0

-50

-100
1549

40

Figure 12 Temperature dependence of transmission spectrumof 4-skip-0 filter module.

Reflection spectrum of 4-skip-0 filter module.

-100
1549

0

Temperature (°C)

Insertion loss (dB)

Chromatic dispersion (ps/nm)

Figure 9

-20

-2.0
1556

1550

1551

1552

1553

1554

-1
-2
-3
-4
-5
1550

1555

1551

1552

Wavelength (nm)Figure 11 Reflection chromatic dispersion of 4-skip-0 filter
module.

the transmission band.
Figure 11 shows the reflection chromatic dispersion,
giving a low value of dispersion of ±10 ps/nm or less over
the reflection band.
Figure 12 shows the temperature dependence of transmission spectrum. The conventional dependence of 1
pm/°C has been improved to 0.5 pm/°C.
4.2 Characteristics of 4-skip-0 OADM
Figures 13-1 and 13-2 show the transmission spectrum ofa four-channel OADM module.
The insertion loss for each channel is as low as
1.24~2.21 dB and its ripple is 0.20 dB or less for the
wavelength band of ITU grid ±0.11 nm. Other optical characteristics are shown in Table 2, and the appearance inFigure 14.

1553

1554

Wavelength (nm)Figure 13-2 Transmission spectrum of four-channel OADM
module. (Magnification)

Table 2

Optical characteristics of four-channel OADM with
100-GHz spacing.

Drop channel (THz)

193.3

193.2

193.1

193.0

Pass band loss (dB)

1.24

1.33

1.66

2.21

Pass band ripple (dB)

0.17

0.16

0.14

0.19

Band width@-0.5 dB (nm)

0.50

0.52

0.49

0.45

Band width@-25 dB (nm)

1.03

1.05

1.03

0.88

Chromatic dispersion (Min)(ps/nm)-27.36-31.44-33.12-46.90Chromatic dispersion (Max)(ps/nm)

39.60

15.56

12.46

11.07

Pass band: λITU ±0.11 nm
Express channels loss: 0.28 dB

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Optical Add-Drop Multiplexers for Metro/Access Networks

Used Ch.

Figure 14 Appearance of four-channel OADM module.

5.

APPLICATION EXAMPLES INMETRO/ACCESS NETWORKS

In conventional long-haul transmission systems, emphasis
has been placed on how much capacity and how far thesystem can transmit. In metro/access networks, however,
low cost and system flexibility are strongly required.
In the metro-ring system, certain signal wavelengths areadded to or dropped from wavelength-multiplexed signals
at several nodes. Figure 15 shows the configuration of a
DWDM metro system using the C-band that comprises
four channels by eight nodes --32 channels in total-- witha signal spacing of 100 GHz. This configuration assigns
four consecutive signal wavelengths to one node, wavelength multiplexes by four channels, amplifies the signal
light using low-cost optical amplifiers, and combines thesignals using eight 4-skip-1 band-separation filter modules
based on TFF to carry out transmission. The 4-skip-1
module transmits four consecutive channels to do
add/drop, while reflecting all the other channels except thefour consecutive channels and one adjacent channel.
When required, optical amplifiers may be used according
to the transmission distance.
Advantages of this system configuration are: 1) Lowcost optical amplifiers without gain flattening filters (GFFs)
to equalize the amplifier gain within the signal bandwidth
can be used; 2) Optical components used for wavelength
multiplexing and those used for add/drop multiplexing at
each node --such as 4-skip-1 filter module, optical amplifier, 100-GHz combiner module, etc.-- can be standardized
for use.
However, this configuration using 4-skip-1 filters has the
disadvantage that the number of available signal wavelengths in the C-band decreases by about seven channels. When the 4-skip-0 band-pass filter described in
Section 2.4 is used as shown in Figure 16, the number ofsignal wavelengths increases from 32 to 40 --the numberof node increases from eight to ten-- thereby implementing an advantageous and efficient system configuration.

Tx

59
:
56
55
54
:
51
50
49
:
46
45
44
:
41
40
39
:
36
35
34
:
31
30
29
:
26
25
24
:
21

Rx

Node

Drop Add
Up to 8 nodes

DWDM

VOA

4ch ATT

4-skip-1

Figure 15 Schematic diagram of DWDM metro network systemusing TFF-based OADM with 4-skip-1 filter.

Used Ch.

Tx

Rx

Node

60
:
57
56
:
53
52
:
49
48
:
45
44
:
41
40
:
37

Drop Add
Up to 10 nodes

DWDM

VOA

4ch ATT

4-skip-0

36
:
33
32
:
29
28
:
25
24
:
21

Figure 16 Schematic diagram of DWDM metro network systemusing TFF-based OADM with 4-skip-0 filter.

6.

CONCLUSION

We have developed the OADM that substantially improves
the insertion loss for express channels by using, unlike
conventional OADM systems comprising series connection of 100-GHz optical combiner modules, 4-skip-0 bandseparation filter modules. We have also established flexi-

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Optical Add-Drop Multiplexers for Metro/Access Networks

0.0

Insertion loss (dB)

0

-10

-20

-0.5

-30

-40
1546

ble system configurations taking advantage of the characteristics of this module.
Hereafter, we plan to develop an 8-skip-0 filter modulethat enables the add/drop of consecutive eight signal
wavelengths with 100-GHz spacing in block, as shown in
Figure 17.

-1.0
1548

1550

1552

1554

Wavelength (nm)Figure 17 Transmittance spectrum of 8-skip-0 filter module
designed.

REFERENCES
1) R.Riehier, A.Fornier and E.Pelletier: “Optical Monitoring of ThinFilm Thickness” in “Thin Films for Optical Systems” edited by
F.R.Flory (1995)
2) Ishiguro et al.: Optical Thin Film, Kyoritsu Shuppan Publisher (inJapanese)
3) H. Takahashi: Temperature stability of thin-film narrow-bandpass
filters produced by ion-assisted deposition, Applied Optics,
34(1995)
4) N. Shimojoh et al.: New gain equalization scheme in WDM opticalamplifier repeated transmission systems, OECC’96, Technicaldigest, 17B3-3 (1996), 120-121.

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