Development of High-Power Stable PLC-Type Pump Combiner
by Koji Seo *, Mitsuhiro Iwaya *, Masato Shiino * and Kanji Tanaka *2

A PLC-type pump combiner has been developed, which is capable of multiplexing for 15 wavelength channels between 1410 and 1510 nm, thereby making it
applicable to Raman amplifiers and high-power optical amplifiers for dense wavelength divisionmultiplexing (DWDM) systems. Our device has a narrow spectral pass-band spacing of 1 THz,and a total insertion loss below 2.0 dB. Both input and output connections between PLC andfibers are physical contact (PC) with a mechanically transferable (MT) connector-like structure
and are adhesive free in the optical path, so they show good stability for a high input power morethan 1.2 W.

ABSTRACT1.

INTRODUCTION

The capabilities of optical communications networks havebeen increased rapidly with the progress and introductionof wavelength division multiplexing (WDM) technology inthe past several years. There has been a slowdown inhigher speed and greater capacity trunk lines for networks
recently, because of the influence of the recession due toover-investment and the collapse of the IT-market in NorthAmerica; however, taking a long and global view, it is certain that the volume of communications traffic will
increase, and construction of optical networks which has
stopped is expected to accelerate if a broader band
access network becomes popular.
By having all optical signals in the gain bandwidth of an
erbium-doped fiber amplifier (EDFA), WDM technology
has been successful so far. On the other hand, EDFAs
with a larger number of wavelengths necessitates higherpumping power 1), 2) . In addition, the Raman amplifier,which operates outside the EDFA bandwidth, has recently
been drawing attention. Although Raman amplifiers wereresearched in the 1970s, practical application was abandoned because several hundred mW of pumping power
were required at that time. However, new proposals forpractical application are emerging as high-power pumping
sources have improved along with the improvement ofEDFAs. The gain wavelength band is determined by theenergy level of erbium ions in EDFAs, whereas it is determined by the wavelength of pumping light in Raman
amplifiers 3). This makes it possible to amplify any desired
wavelength simply by selecting the appropriate pumping
wavelength.
Therefore, the gain bandwidth can be broadened in a

*Optical Components Development Dept., FITEL-Photonics
Lab.
*2 Optical Components Dept., FITEL Products Div.

way that is not achievable with EDFAs by broadening thebandwidth of the pump using WDM 4), 5).
In Raman amplification, around the 1500 nm band, gain
is produced at approximately 110 nm on the long wavelength side, so if the pumping unit is multiplexed around1450 nm to have a broad bandwidth of 100 nm, Raman
amplification also occurs in a broad band of 100 nm
around 1560 nm. Moreover, it is possible to make the gaineven flatter by increasing the number of pumping wavelengths.
Meantime, the high-power durability of optical devices
around the optical amplifiers is an important problem. For
example, when an optical power as high as several hundred mW is input, the end-face of the optical connector issometimes damaged if there is contamination on it, due tooptical absorption of the contamination 6) , and when adevice has a connection using adhesive, it also absorbs
the optical power, heats up the device, and sometimes
lowers its performance. In this paper, we report the development of a high-power stable PLC-type pump combiner,
which is capable of multiplexing for 15 wavelength channels between 1410 and 1510 nm, thereby making it
applicable to Raman amplifiers and high-power opticalamplifiers for DWDM systems.

2.

PLC-TYPE PUMP COMBINER

2.1 Configuration
Figure 1 shows the appearance of the 15-wavelength
PLC-type pump combiner developed.
The small size of 10(W)×8(H)×130(L) mm is realizedusing an integrated wavelength multiplexer applying silicabased planar lightwave circuit (PLC) technology, which isconnected to two ribbonized 8-fibers at the input port andwith a 0.25 mm fiber at the output port.
Figure 2 is a schematic illustration of the PLC used inthis device. Fourteen Mach-Zehnder-interferometers (MZI)

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Development of High-Power Stable PLC-Type Pump Combiner

are fabricated on a silicon substrate, and channel spacing
is designed approximately 7 nm from 1410 to 1510 nm,
which provides a flat gain for Raman amplification.
Using this device can make the composition of a Raman
amplifier simpler, less expensive, and more compact thanusing multilayered dielectric thin film filters or fused fiber
couplers, because we use only one multiplexer to integrate all wavelengths 7)~9).
2.2 Demands for High-Power Durability
Figure 3 shows one of the high-power pumping units(HPU) for a Raman amplifier. Generally, PLC is connected
to fibers arrayed on a V-groove using adhesive after theiralignment to lower the connection losses 10). However, aconnection using adhesive cannot withstand a high power
above 200 mW, because optical absorption of adhesive orconnection loss leads to the connection heating and losses. Nowadays, the power of 1480 nm band laser diodes(LD) for HPU is reaching as high as about 300 mW, soinput multi-channels of this PLC-type pump combiner
should be guaranteed for 600 mW considering that two
LDs may be multiplexed by a polarization beam combiner(PBC). Polarization multiplexing of LDs is designed toincrease pumping unit output, as well as to suppress thepolarization dependence of Raman gain. The output single channel of this combiner should be guaranteed for 3
W under present conditions, because all LDs are notalways used at maximum power, but at the power thatmakes the gain flat for Raman amplification.

3.

STRUCTURE

Both input and output connections of this PLC-type pump
combiner are made of a PC with a MT connector-likestructure, with adhesive-free optical path, and low losses.
So, this pump combiner shows good stability at a highinput power. Figure 4 shows the structure of a 15-wavelength PLC-type pump combiner. Each end of 15×1 PLC
fits with a MT-compatible ferrule, which has two guideholes. They are assembled at a precise position decided
by V-grooves formed in both of them, and this can makethe connection loss lower. A MT ferrule with a single mode
fiber is coupled with the output single channel, and a MT
ferrule with fifteen single mode fibers is coupled with theinput multi-channel. Using guide pins, they are connectedwith passive alignment like the connections of a MT connector. Figure 5 shows the cross-section of the connection
between the waveguide device and a MT connector. The
input multi-channel side and the output single channelside have the same structure. The end-face of the MT
connector is specially polished so that the fibers protrude
slightly, and both end-faces of the waveguide device andMT connector are polished at an angle of 8 degrees. ThePC is realized by mating them with a clip.

λ1

PLC-MZI type Multiplexer

Outputλ1 ~ λ n

λn> 3W> 600mW
Input

PBC

Adhesive
connection> 300mW

LD1Figure 1

Appearance of 15-wavelength PLC-type pump combiner.

LD2

Figure 3

LDn

HPU for Raman amplifier.

Clip
OutputMach-Zehnder interferometer
3dB-DC

Phase shift
L+ ∆L

λ1λ2L

λ 1+ λ 2

Ch1
Ch2
Ch3.
.
.
.
Ch13
Ch14
Ch15

Figure 21MT

PLC

3dB-DC

MT-compatible
FerruleEndface of the waveguide device

Input

Schematic illustration of 15-wavelength PLC.

Figure 4

Waveguide

MT-compatible
FerruleChip

Guide hole

15MT

Structure of 15-wavelength PLC-type pump combiner.

Furukawa Review, No. 23

2003

49

Development of High-Power Stable PLC-Type Pump Combiner

30

PLC chipMT connector

WaveguideN = 80 Ports

25Frequency

Waveguide device

Fiber

AVG = 0.29dB
MAX = 0.54dB

20

STD = 0.11dB
151058°

00.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Connection Loss (dB)

Cross-section of the connection.

Figure 7

N = 80 Ports∆ MAX = 0.02dB

0.30

80.25
0.20
0.15

64

0.10
2
0.05
000.05

0.10

0.15

0.20

0.25

0.30

1400

0.35

Figure 6

Test of physical contact (PC).

OPTICAL PROPERTIES

4.1 Test of Physical Contact
As stated above, except for adhesive, having no contamination is also important for a connection under high power
conditions. However, this can be prevented by thoroughinspections during the manufacturing process. In the caseof this device, it is most significant that multi-reflection for
insufficient PC might heat the device or lead to thedestruction of the fiber end-faces. So, all fiber-to-waveguide PC must be inspected. Suitably designing the configurations of both the waveguide device’s and the MT
connector’s end-face, the clip achieves the quality shownin Figure 6. Figure 6 shows the correlation between theconnection losses without and with low-viscosity indexmatching oil that is dropped on the input end of thedevice. If PC is incomplete and there is a microscopic
space between the mated cores, dropped oil fills thespace and reduces the connection loss. However, the loss
reduction may be regarded as zero, so PC is completelyachieved with these devices.
4.2 Connection Loss
Under high power conditions, connection loss also
becomes one of the causes of lower device quality.
Because the loss of optical power is changed into thermalenergy, it heats the device to a temperature that is above
the heat-resistance of the material and leads to greaterloss changes when the connection loss is large. So, thetarget loss was set below 0.3 dB, considering the heat-1420

1440

1460

1480

1500

1520

Wavelength (nm)Connection loss without index-matching (dB)4.

Connection loss without index-matching.

100.35

Loss (dB)

Connection loss with index-matching (dB)Figure 5

Figure 8

Loss spectrum of a PLC-type pump combiner.

resistance of the epoxy resin that makes up the ferrules.
Figure 7 shows the measured values of connection loss. It
was 0.29 dB on average and 0.54 dB at maximum without
index-matching including two connection losses generated
at the input/output ends of the device.
Then, the value can be reduced by half to be 0.15 dB on
average and 0.27 dB at maximum at a single end, which
is low enough to attain the target loss.
4.3 Loss Spectrum
Figure 8 shows the loss spectrum of a 15-wavelength
PLC-type pump combiner. The insertion losses areapproximately 1.5 dB at all channels, so it bears comparison with the losses multiplied by fifteen wavelengths withseveral multilayered dielectric thin film filters. Besides, allchannels achieve high return losses larger than 50 dB.

5.

RELIABILITIES

5.1 Mechanical and Environmental Characteristics
Table 1 shows the mechanical and environmental characteristics under normal power based on Telcordia GR1209CORE or Telcordia GR1221-CORE. Loss changes were
all below 0.2 dB, and peak wavelength shifts were all within 0.2 nm during the tests.
5.2 High-Power Durability
We also examined high-power durability based on ourguideline 11). The tests were carried out with more than

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200350Development of High-Power Stable PLC-Type Pump Combiner

Category

Results of reliability test.
TestStatus

Criteria

Temperature-Passhumidity cycling
TemperatureTelcordia

Passhumidity aging

GR1209-CORE

Vibration

Loss change< 0.2 dB

Pass

ImpactWavelength change

Pass< 0.2 nmCable retention
Temperature

Telcordia

0.4

Loss change (dB)

Table 10.2

0-0.2

-0.4
0300

600

Elapsed time (hrs)

PassFigure 10 Insertion loss change under high-power input,85°C/85%.

Testingcycling

GR1221-CORE

Testing

Damp heat

3W LS
1480nmTAP

30dB Att

Input

OPM

OutputEnvironment chamber
DUT

Integral sphereOPM≥ 1.2W

Figure 9Table 2
Category

Setup for high-power tests.

High-power durability.
Test

Criteria

High-power Room-temperature
durability

Temperature-humidity aging
Telcordia GR1209-CORE
Temperature cycling
Telcordia GR1221-CORE

0.72 dB

Room temp.

24.5 °C

24.5 °C

Temp. under 3 W

31.0 °C

33.0 °C

Temp. change+6.5 °C+8.5 °C

StatusLoss change

Pass

Figure 11 Thermo-viewer observation at 3 W input power.

< 0.2 dB
Wavelength change

Pass< 0.2 nmTesting

Damp heat
Telcordia GR1221-CORE

f : Output port

0.37 dB

Pass

Temperature-humidity cycling≥ 1.2 W Telcordia GR1209-CORE

a : Input port
Connection loss

Testing

1200 mW, which is twice (= 3 dB up) the required maximum rating of 600 mW of each input multi-channel.
Moreover, a long-term reliability test was carried out under
accelerating conditions with self-heating and dump-heat.
Figure 9 shows the setup for the high power tests. Threesamples were daisy chain connected and confirmed formore than 1200 mW at the output end using a calorie
meter. Table 2 shows the results for high-power durability.
Loss changes during the tests were all below 0.2 dB, andlong-term tests are under evaluation. They are making
good progress for insertion loss change under a highpower input, 85°C/85%RH, as shown in Figure 10.
To guarantee the 3 W durability of the output single
channel connection, we also evaluated them at 3 W forten minutes as a trial, and we observed a small temperature rise. Figure 11 is a thermo-viewer observation at 3 W
input power. Although the observed sample had comparatively high loss connections, the temperature change waswithin 10°C. This result indicates the stability of this sample at 3 W input power, although long-term tests should becarried out.

6.

CONCLUSION

We developed a high-power stable 15-wavelength PLCMZI pump combiner for 1480 nm band pumping for
Raman amplifiers. The low-loss connection and new PC
technique for 15 ports between fibers and PLC haveimproved high-power durability. We evaluated the input
multi-channel connection of more than 1200 mW to guarantee a 600 mW and output single channel connection at3 W as a trial, and will evaluate the output connection at 6
W for a guarantee of 3 W. We expect no failures to occur,considering that the temperature change at 3 W input
power was small.
Finally, we can also develop an 8-wavelength PLC-MZIpump combiner for Raman amplifiers, which is the samestructure as that of 15-wavelength.

Furukawa Review, No. 23200351Development of High-Power Stable PLC-Type Pump Combiner

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