An Optical Combiner Module for DWDM Systems
by Hiroshi Matsuura *, Yasuhiro Watanabe *2, Masayoshi Kagawa *,Hajime Kazami *, Kazumi Ida *2 and Nobuaki Sato *2

An optical combiner module with two input ports has been developed which is
aimed at applications in Raman amplifiers, high-power optical amplifiers for
DWDM (dense-wavelength-division multiplexing) systems, and ADM (add-drop module). An averaged insertion loss of 0.2 dB has been achieved by optimizing the coupling system for the filtering element and fiber as well as by preparing a common optical platform for the module, thereby
eliminating the excess loss of the filter. Moreover, the module permits easy integration of an optical isolator, so that responding to ever-diversifying system requirements will be made possible.

ABSTRACT1.

INTRODUCTION

At the beginning of the 1990s, erbium-doped fiber amplifiers (EDFA) came into practical application, bringing
about a dramatic leap in optical communications technology. However the explosive growth in the Internet, whichbegan in North America at about the same period,
increased the volume of public telecommunications traffic
to the point where even greater capacity was required.
Fortunately the gain band of EDFAs is comparatively
wide, so that wavelength-division multiplexing (WDM)
using all the optical signals included in this wavelengthband was identified as a method that could provide a better solution to the problem than increasing the transmission speed of optical signals, and was rapidly implemented.
The WDM technology using EDFAs started from the1550-nm band, where transmission loss of optical fiber is
low and gain flattening is easily accomplished, but soon
spread to the 1530-nm band and has recently been
employed even in the 1580-nm and 1500-nm bands,
where gain coefficient is quite small. In addition, Raman
amplification that can amplify optical signals outside thewavelength band of EDF (erbium-doped fiber) amplification is drawing attention, and is beginning to enter into
practical application. At the same time, development of
optical modules is also proceeding actively to cope withsuch requirements of next-generation optical communications systems as high density, wide bandwidth, wavelength multiplexing, high power, and ease of extension.
Among these, the optical combiner module with two input
ports is a multi-use module that can constitute a variety of
optical devices, by changing the characteristics of thedielectric multi-layered filter (hereafter called "filter") usedin the device, such as wavelength combiners for Raman

amplifiers, wavelength multiplexers and demultiplexers for
multiplexed optical signals as well as wavelength combiners for pumping and signal lights. Consequently, it isessential that the combiner module provide design freedom to change its structure flexibly depending on its specific application.
This contribution reports on the development of an optical combiner module with two input ports, in which thebasic performance of the input portion has been improvedto achieve high design flexibility, low insertion loss, andhigh power capacity 1)~7).

2.

OPTICAL COMBINER MODULE WITHTWO INPUT PORTS

Figure 1 shows the basic configuration of an optical combiner module with two input ports. The module consists of
two single-mode fibers (SMF1 and SMF2), a two-core ferrule, the first lens, a filter, the second lens, a single-coreferrule, and a single-mode fiber (SMF3); and an isolator isinserted between the filter and lens in case of use forpumping modules. Each component is securely fixedeither by laser welding or using adhesive, but the latter is
not used in the optical paths to make the device capableof withstanding a high-power optical input of Watt-class.
An example of ray tracing will be described below, inwhich a λ1-reflecting and λ2-transmitting filter is used. A
light with a wavelength λ1 launched into the SMF1 on the

Two-core ferrule

FilterSingle-core ferrule

SMF1
SMF3
First lensSecond lens

SMF2
*
*2

WP Team, FITEL-Photonics Lab.
Optical Components Dept., FITEL Products Div.

Figure 1

Basic configuration of optical combiner module with
two input ports.

17

two-core ferrule is collimated by the first lens and then is
totally reflected by the filter. The reflected light is again
collimated by the first lens before it is launched into the
SMF2. Meanwhile, a light with a wavelength λ2 that islaunched into the SMF3 on the single-core ferrule is collimated by the second lens and is transmitted through thefilter before being launched into the SMF2.
Figure 2 shows an appearance of this optical combiner
module 8)~10).
2.1 Application Examples
Figure 3 shows an application of the optical combiner
module with two input ports as wavelength combiner for a
Raman amplifier. The wavelength combiner No. 1 in the
Figure incorporates a band-pass filter (BPF) that reflects apumping light of 1420 nm and transmits that of 1440 nm
radiated from the semiconductor laser modules (LDM), sothat the combiner combines wavelength-wise the two
pumping lights. Similarly, the wavelength combiner No. 2
combines wavelengthwise the pumping lights of 1460 nm
and 1480 nm. Furthermore, the wavelength combiner No.
3 combines the pumping lights from the wavelength combiners No. 1 and No. 2, thereby outputs four pumping
lights of 1420 nm, 1440 nm, 1460 nm, and 1480 nm. The
combined pumping light ranging from 1420 nm to 1480
nm is then launched into the backward pumping module,

which incorporates a long-wave pass filter (LWPF) thatreflects pumping lights (from 1400 nm to 1500 nm) andtransmits signal lights (from 1525 nm to 1570 nm). Thus,
the pumping lights are reflected by the LWPF to belaunched into an SMF to effect optical amplification, andthe amplified signals emitted from the SMF are transmitted through the backward pumping module.
Figure 4 shows another application of the optical combiner module in a Mux/Demux (multiplexer/demultiplexer)
unit for ADM. The 100-GHz BPF incorporated in every
optical combiner module comprises dielectric multi-layer
filters of five-cavity structure, featuring a broad transmission bandwidth of 550 pm between the points 0.5 dB
down from the peak wavelength, and the chromatic dispersion within the bandwidth is quite small. These filters
are selected from those that are very precisely measured
for their spectral characteristics using an incident beamwith an accuracy of 0.001 degree or less with respect tothe design incident angle, so that the peak wavelengthachieves a good match with the signal wavelength. Figure
5 shows the measured results of transmission characteristics.
When a light containing eight signals of different wavelengths is incident upon the common port of the combiner
module No. 1, the BPF No. 1 retrieves the signal light of
194.7 THz and reflects other seven signal lights to send
them into the common port of the combiner module No. 2.
Connecting such a stage in cascade, a Demux unit can be
configured enabling to retrieve every signal light.
Meanwhile, a Mux unit can be structured by injecting signal lights with different wavelengths from the reversedway.
2.2 Comparison with Other Module Configurations
When a high optical power of Watt-class is injected into an
optical combiner module, the temperature of the modulerises due a small fraction of optical power that isFigure 2

Appearance of optical combiner module.
Combiner module 1
194.7 THz

SMF
Backward pumping module

Combiner module 2

194.6 THz

Combiner module 3

194.5 THz

Combiner module 4

194.4 THz

Combiner module 5

194.3 THz

Combiner module 6

194.2 THz

Combiner module 7

194.1 THz

SignalWavelength Combiner 3

Wavelength Combiner 1

Wavelength Combiner 2

LDM (1480 nm)
LDM (1460 nm)

LDM (1440 nm)
LDM (1420 nm)

194.0 THz

Figure 318

Configuration of Raman amplifier, an application of
the optical combiner module.

Figure 4

Configuration of Mux/Demux unit for ADM, an application of the optical combiner module.

Furukawa Review, No. 21

2002

0

60

-5

40

-10

20

-15

0

-20

-20

-25

-40

-30

-60

-35

-80

-40
1532.651532.851533.05

1533.25

1533.451533.65

Table 1

Specifications of wavelength combiner for WDM systems.

Operating Temp.

Transmission (dB)

Chromatic Dispersion (ps/nm)

80

λ op

0 - 70°C

Notes

1400 - 1620 nm

R-, S-, C-, L-band< 0.25 dB@R.T., ASOP, λ c< 0.45 dB@Top, ASOP, λ op

PDL

< 0.05 dB@Top, ASOP, λ op

Directivity> 60 dB@Top, ASOP, λ op

Optical power<3W@Top, ASOP, λ op

Return loss

> 50 dB@Top, ASOP, λ op

Insertion loss

1533.85

Wavelength (nm)0150Frequency

Insertion Loss (dB)

-5
-10
-15

ITU Grid

ITU Grid≤0.6 nm≥0.6 nm-20
ITU Grid±0.11nm

-25

100

50

0
~0.1

~0.2

~0.3

-30
-35
-1

-0.8

-0.6

-0.4

-0.2

0

ITU Grid (nm)

0.2

0.4

0.6

A

B

0.8

1

Figure 6

FITEL

Typical transmission and dispersion characteristics
of 100-GHz BPF based on optical combiner module.

~0.5

~0.6

~0.7

absorbed. In an experiment where a pumping light of 6 W
was injected into a module with a coupling loss of 0.2 dB,
the temperature rise of the module package recorded
more than 10°C suggesting a higher temperature riseinside.
Because of their simple structures and ease of design,
optical collimators are often used, in which a lens having a
distributed refractive index profile is adhesively bondedwith a fiber end. However, these types of collimators are
obviously less reliable in terms of optical-power resistancethan the air-gap type based on welding or solder bonding,because minute foreign particles contaminating the organic adhesive may absorb optical power thus resulting inpossible burning of the adhesive. Meanwhile, solder bonding is likely to lead to loss increase caused by the accelerated creep phenomenon of solder due to temperature rise.
In contrast, the developed optical combiner module is stabilized in performance because it entirely employs laser
welding reducing sources of insertion loss change against
heat. Furthermore, the module is very reliable against
high-power operation because it uses no adhesive in its
optical paths thus eliminating possibilities of burning.
It is known that an ultra-narrow band BPF can notachieve desired transmission characteristics unless the
thickness of every filter layer is controlled with a precisionof 0.01 %. We have established a measuring method thatenables precise measurement of filter chips used in the
combiner module, making it possible to manufacture ultranarrow band BPF modules rather easily.

Histogram of insertion loss of pumping modules for
Raman amplifiers. (Ports SMF1 to SMF2)

150

Frequency

Figure 5~0.4

IL (dB)

100

50

0

~0.1

~0.2

~0.3

~0.4

~0.5

~0.6

~0.7

IL (dB)

Figure 7

3.

Histogram of insertion loss of pumping modules for
Raman amplifiers. (Ports SMF2 to SMF3)

SPECIFICATIONS

Wide bandwidth, low insertion loss, and sufficient power
resistance that are expected to be strongly required hereafter have been taken into consideration in determining
the specifications of the optical combiner module forwavelength combining as shown in Table 1.

4.

PRODUCT CHARACTERISTICS

Optical characteristics of optical combiner products suchas pumping module, wavelength combiner, pumping module with an isolator, and polarization-maintaining pumping
module will be described below based on the results ofmeasurements.
4.1 Insertion Loss Distribution
Figures 6 and 7 show histograms of insertion loss of
selected pumping modules that combine signal light andpumping light under conditions of full temperature range,
full wavelength band, and unpolarized state of polariza-

19

500

Insertion Loss (dB)

0.6

Frequency

400
300
200

70.0~

~70.0

~67.5

~65.0

~62.5

~60.0

~57.5

~55.0

~52.5

~50.0

~45.0

0

~47.5

100

0.5
0.4
0.3
0.2

L-2iso-01

0.1

L-2iso-02

01550

RL (dB)

Figure 8

1570

1590

1610

Wavelength (nm)Histogram of return loss of pumping modules for
Raman amplifiers. (Ports SMF1, SMF2, and SMF3)

Figure 10 Wavelength dependent insertion loss of L-band
pumping modules.

Frequency

8

Accordingly, the insertion loss must be as flat as possible
in the signal wavelength band, and the developed pumping module is designed to provide an insertion loss flatness of 0.05 dB or less within the band 9), 10).

6
4
2
0
18

20

22

24

26

28

30

32

34

36

38

40

Extinction Ratio (dB)Figure 9

Histogram of extinction ratio of polarization-maintaining pumping modules.

tion. Figure 6 relates to the loss between ports SMF1 and
SMF2, and Figure 7 between ports SMF3 and SMF2,
respectively. Because the module uses aspherical lenses,
both the reflection port and transmission port have an
averaged insertion loss of 0.2 dB with a dispersion of 0.03
dB --excellent performance.
4.2 Return Loss Distribution
Figure 8 shows a histogram of return loss of selected
pumping modules that combine signal light and pumping
light, including the measured results of port SMF1 throughSMF3. The return loss satisfactorily meets the specifications and its wavelength dependence is small.
4.3 Extinction Ratio
Figure 9 shows a histogram of extinction ratios of signal
lights in polarization-maintaining pumping modules thatare used in polarization-maintaining optical amplifiers having polarization-maintaining fibers (PMF) on input- andoutput-optical ports. The extinction ratio should be as highas possible, because it substantially affects the amplification efficiency and the insertion loss of optical amplifiers.
The two-core ferrule used in the developed combiner
module is designed to be less susceptible to degradationin extinction ratio even when PMF is used on all ports 8).
4.4 Wavelength Characteristics
Figure 10 shows the wavelength dependent insertion lossof selected pumping modules that incorporate an isolator.
If the insertion loss changes by 0.1 dB per module within
the signal wavelength band, an optical amplifier for nextgeneration communications systems that are provided
with four or more stages of EDFAs may have a wavelength dependent insertion loss of 0.4 dB or more, thereby
becoming unable to meet the performance requirements.

20

4.5 High-power Performance
A pumping light of 1 W or more is sometimes injected intoa wavelength combiner for Raman amplifiers. In view ofthis, the developed combiner module was tested for its
high-power performance, in which 12 modules were tested with a pumping light of 3 W at 1480 nm for 1000 hours
in a high-temperature, high-humidity tank. The insertion
loss degradation of each combiner module before andafter the test was 0.1 dB or less.
Next, the temperature rise in a wavelength combiner
module under pumping light injection was checked using a
Thermo-viewer. Figures 11 and 12 show the results. From
Figure 11, because the module is provided with low insertion loss and countermeasures against high-power optical
input, only a small temperature rise of 2 to 3°C evenly distributed is seen even at an optical input of 3 W. Figure 12
shows the result of an optical power injection test of 6 W
into a wavelength combiner module having an insertion
loss of 0.29 dB. Although the temperature at the two-coreferrule rose by 10°C, no change in insertion loss was
seen.

5.

RELIABILITY

Reliability tests were carried out in accordance with
Telcordia standards GR-1209 and GR-1221 to confirm thereliability of the optical combiner module with two input
ports. In each test, loss and its changes were seen to satisfy the specified values, thereby confirming sufficient reliability.

6.

SUMMARY

An optical combiner module with two input ports has beendeveloped, which features high reliability, compactness,and ultra-low insertion loss. The product is expected to be
used in wide areas of application such as pumping lightcombiners for Raman amplifiers, optical amplifiers for
DWDM systems, and Mux/Demux units.

Furukawa Review, No. 21

2002guides", Appl. Phys. Lett. vol.22, P276-278 (1973)
5) Edakawa et al.: "Amplification characteristics in optical fiber
Raman amplification", Trans. IEICE, OQE88-33 (1988) (inJapanese)
6) M.Nissov: "100Gb/s (10×10Gb/s) WDM transmission over
7200km using distributed Raman amplification", Proc. European
Conference on Optical Communication, vol.5 (1998)
7) Y. Emori, Y. Akasaka, and S. Namiki: "Less than 4.7dB noise figure broadband in-line EDFA with a Raman amplified 1300ps/nm
DCF pumped by multi-channel WDM laser diodes", OAA'98, PD3(1998)
8) Yariv: "Optical wave in crystals"
9) K. Kono: "Fundamentals and Applications of Optical Systems for
Optical Devices", Gendai-Kogaku-sha (in Japanese)10) Yariv: "Optical Electronics in Modern Communications", Oxford
Uni. Press. (1997)

Manuscript received on December 14, 2001.
Figure 11 Temperature rise of a combiner module when an
optical power of 3 W is injected.

Figure 12 Temperature rise of a combiner module when an
optical power of 6 W is injected.

ACKNOWLEDGEMENTS
The content of this report is the achievement of the staff in
the Functional Optical Components Group of the FITELPhotonics Laboratory and the Optical Components
Department of the FITEL Products Division, and theauthors have compiled this paper as representatives. The
authors would like to thank the staff of the Optical
Equipment Department and the Research Group for
Optical Communication Materials together with Mr.
Ninomiya of the WP Team for their advice and cooperation in conducting this research.

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3) Y. R. Shen: The Principles of Nonlinear Optics, John Wiley &Sons, Inc., (1984)
4) R. H. Stolen and E. P. Ippen: "Ramam gain in glass optical wave-

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