Production Engineering of Polarization Beam Combiner
by Hiroshi Matsuura *, Takuma Aoki *, Yasuhiro Watanabe *, Daeyoul Yoon *,Mamichi Tsuyuki *2, Hidehito Kagiwada *2, Masaru Abe * and Toshihiko Ohta *2

A Polarization beam combiner has been developed, which is capable of doubling
the output power of semiconductor pumping lasers by way of polarization multiplexing, thereby making it applicable to Raman amplifiers and high-power optical amplifiers for
DWDM (Dense Wavelength Division Multiplexing) systems. The combiner has high reliability
since its mechanical parts are high-strength welded using a YAG laser and also no adhesives areused in its optical paths. By optimizing optical parts characteristics and optical coupling, a broad
bandwidth ranging from 1410 nm to 1580 nm has been realized together with a low insertion loss
of 0.2 dB on average.

ABSTRACT1.

INTRODUCTION

At the beginning of the 1990s, the Internet that had beendeveloping in North America showed an explosive growth
worldwide thus requiring even greater capacity of opticalfiber transmission lines. To deal with this requirement,
fiber-optic telecommunications technologies including, inparticular, erbium-doped fiber amplifier (EDFA) that was
put into practical use at about the same period wereapplied to increase the transmission capacity. Fortunately,
the gain bandwidth of EDFAs is comparatively wide, sothat wavelength division multiplexing (WDM) using all the
optical signals included in this wavelength band was identified as a method superior to time division multiplexing(TDM), and was rapidly implemented.
Optical amplification that comprises the core of WDM
technology started from 1550 nm, where gain flattening iseasily accomplished, but soon spread to the 1530-nm
band and has recently been employed in the 1580-nm
band, where the small gain coefficient had made it impracticable. Since wider gain bandwidth with larger number ofwavelengths necessitates higher pumping power, EDFAsof these days increasingly require pumping sources ofhigher output power because of the above-mentioned
movement toward wider bandwidth and more channels.
In addition, Raman amplifier that operates outside the
EDFA bandwidth is recently drawing attention. In the
Raman amplifiers the gain bandwidth is determined by the
wavelength of the pumping light, making it possible toamplify any desired wavelength simply by selecting theappropriate pumping wavelength. Although muchresearch was done in the 1970s to take this advantage,practical application was abandoned because several* WP Team, FITEL-Photonics Research Lab.
*2 Optical Parts Dept., FITEL Products Div.

hundred milliwatts of pumping power was required at thattime. As high-power pumping sources have improved
along with the improvement of EDFAs, however, a new
outlook for practical application is emerging. Recent
research has identified such advantages unattainable with
EDFAs as low noise and wide bandwidth in amplification,
so that development activities aimed at practical application are accelerated together with a keen competition fordeveloping pumping light sources of higher output power.
Furukawa Electric has been developing, since the earlydays of optical communication, pumping laser modules(LDMs) as pumping light source, and is now capable ofsupplying the world's top ranked products with an output
power of as high as 300 mW. Moreover, we have developed wide bandwidth, high-power pumping units (HPUs)
with an output power exceeding 1W using silica glass planar light circuitry (PLC), thus contributing a great deal tothe practical application and development of optical communication technology.
This paper reports on the development and engineering
technology of polarization beam combiner (hereafter
called PBC) of low insertion loss and wide bandwidth,
which are capable of obtaining, in an easy and cost-effective manner, an optical output power of 500-mW class bypolarization multiplexing the outputs of LDMs having
polarization maintaining optical fibers.

2.

STRUCTURE AND PERFORMANCE OFPBC

A PBC is basically structured, with reference to Figure 1,
such that two polarized beams that are input through twopolarization maintaining fibers --PMF1 and PMF2-- arepolarization multiplexed using birefringent crystal into an
output single mode fiber --SMF. The end facet on themodule side of each fiber is antireflection (AR) coated and11

is provided with an aspherical lens, making the diverging
beams from the fiber ends parallel in shape. A beam-separating element is used to secure the distance between
PMF1 and PMF2. All mechanical parts are high-strength
welded using a YAG laser eliminating any adhesive on the
optical paths. Figure 1 shows the appearance of a PBC
module.
2.1 Application
Figure 2 illustrates an application of PBC for the DWDM
amplifier. The 1480 nm pumping lights from LDMs arepolarization multiplexed by a PBC and the multiplexed
light beam with built-up power is fed to a backward pumping unit (BPU). The BPU is incorporated with a high passfilter (HPF) which reflects pumping lights ranging from1450 nm to 1500 nm and transmits signal lights ranging
from 1520 nm to 1570 nm, so that the pumping light isreflected by the HPF into an EDF (erbium-doped fiber)
while the signal light amplified by the EDF is transmitted
through the BPM.
2.2 Comparison with Different PBCs
Some manufacturers use polarization beam splitter (PBS)
in their PBC products as a polarization-multiplexing element and these products are commercially available. The
PBS is known for its easy and low-cost manufacturingmethod such that the long side of a right-angled triangle
prism is deposited, and the prism is bonded with the longside of another prism using organic adhesive. Generally

speaking, however, the PBC using a PBS has a narrowtransmission bandwidth. In addition, such a PBC has low
reliability in terms of optical power rating since it is likely to
be damaged, when an optical power of as high as several
hundred mW is input, due to optical absorption of thebonding adhesive or burning of foreign inclusions contained during manufacturing. Moreover, there is fear thatthe insertion loss might increase due to deflection of collimated beam, which sometimes occurs unless the characteristics and thickness of the bonding adhesive are controlled with extreme preciseness to avoid uncontrollable
displacement of the prism caused by temperaturechanges.
We use, in contrast to this, birefringent crystal with small
wavelength dependence and low absorption for the PBC
products, taking into consideration future high-power
applications including Raman amplifiers. Figure 3
schematically compares the structures of PBCs that use
birefringent crystal and PBS.

3.

SPECIFICATIONS

We have designed a PBC considering the conditions ofwide bandwidth for Raman amplifiers and low insertion
loss for efficient pumping together with requirements forhigher power handling capability. Subsequently, performance deviation and yield rate were calculated based ontrial production so as to determine the product specifications as shown in Table 1.

SMF
Adhesive
Optical film

Optical axis
PMF2

Port B
Port CPMF1

Figure 1

Port A
Birefringent crystal

Figure 3

Appearance of polarization beam combiner

PBS

Schematic structures of PBCs using birefringent
crystal and PBS

Table 1BPM
ItemSpecifications of PBC

Input / Output

Criteria

Measured value

port(Average)

Port A to C

EDF

Insertion loss

0.18 dB<= 0.5 dB

Port B to C
LDMPort A
Extinction ratio

PBC

12

Schematic diagram of erbium-doped amplifier, anapplication of PBC

29 dB>= 17 dB

Port BLDM

Figure 20.22 dB

Reflection loss

Port A,B,C

Allowable input power

Port A,B,C

Dimensions

28 dB>= 45 dB

56 dB

< 600 mW

1W

L33×W11×T6 (mm)λ = 1410-1580 nm

Furukawa Review, No. 20

2001

4.

PRODUCT PERFORMANCE

Every item of product performance is described below
based on the evaluation of 800 PBC products that havebeen consecutively manufactured.
4.1 Insertion LossPort A in Figure 3 corresponds to the port designated as
PMF1 in Figure 1. A polarized beam entering this port isadjusted to be an ordinary ray in the beam-combining
crystal, while another polarized beam enters port B or
PMF2 to be an extraordinary ray so that the combined
beam exits from port C equipped with an SMF. Figure 4
shows the histogram of insertion loss, where the beam
entering port B that passes the beam-separating element
is seen to have an averaged insertion loss 0.04 dB greater
due to wave front distortion and the AR coat.
4.2 Temperature Characteristics of Insertion Loss
Judging from the purpose of its application, it is very likelythat PBC devices are placed near to the LDM in device
arrangement. Accordingly, appropriate care should betaken to make the device performance least influenced by
the heat emitted by the LDM, while not only PBCs but also
all components to be used in optical amplifiers are
required to be checked, before shipping, for the losschanges due to temperature. Figure 5 shows the histogram of the insertion loss changes, i.e., the differences
between the maximum and the minimum losses measured

in the storage temperature range of between -40°C and
85°C. All products showed an insertion loss change of
0.15 dB or less at the operating temperature range of
between 0°C and 65°C. This loss change due to temperature presumably results from refractive index changes of
optical materials and fixed position changes of optical elements including the beam collimating mechanism.
4.3 Reflection Loss
Figure 6 shows the histogram of reflection loss changes
for all ports consisting of ports A, B and C. The measured
values fully meet the specifications and it turned out thatlittle influence was seen when the measuring wavelengths
were changed.
4.4 Extinction Ratio
Extinction ratio is one of the most important items that
ought to be degradation free since it is directly related withinsertion loss increase. Accordingly, the PBC module ismanufactured using a production line equipped with a
special light source and a phase-controlling algorithm foradjustment so that a high extinction ratio is attained in ashort time. Figures 7 and 8 show the histogram of extinction ratios for ordinary ray (designated as no) and extraordinary ray (designated as ne), respectively. The performance of ordinary ray is slightly better in terms both of
averaged value and variance since the ray does not pass
the beam-separating element.

350

300
Port B to C ; Ave = 0.22 dB
Port A to C ; Ave = 0.18 dB

300

λ = 1480 nm

250

200
λ = 1480 nm

150

Frequency

Frequency

250

100

200
150
100

50

50

0

0
0

0.08

0.16

0.24

0.32

0.4

45

0.48

47.5

Histogram of insertion loss

Figure 6

55

57.5

60

62.5

65

Histogram of reflection loss

150

400
350

λ = 1480 nm

300
250

λ = 1480 nm

Frequency

Frequency

52.5

Reflection loss (dB)

Insertion Loss (dB)Figure 4

50

200
150

100

50

100
50
0

0
0

0.05

0.1

0.15

0.2

0.25

Insertion loss change (dB)Figure 5

Histogram of insertion loss change due to temperature

17

21

25

29

33

37

Extinction ratio (dB)Figure 7

Histogram of extinction ratio for ordinary ray (no)13

Table 2150
λ = 1480 nm

Frequency

100

Results of the Bellcore 1221 and 1209 tests

Heading

Test

Criteria

PassedVibration

PassedThermal shock

PassedMechanicalintegrity

50

High temp storage
Endurance

0

17

21

25

29

33

Loss change (dB)Figure 8

Histogram of extinction ratio for extraordinary ray(ne)1

No. 544 ne

0.8

No. 544 no0.6

No. 546 ne

0.4

No. 546 no0.2

No. 030 ne0

No. 030 no

- 0.2
1430

No. 541 ne
1480

1530

Wavelength (nm)Figure 9

1580

No. 541 no

Wavelength characteristics of insertion loss forselected PBCs

4.5 Wavelength Characteristics of Insertion Loss
The wavelength characteristics of transmission are rather
flat as shown in Figure 9, while a small gradient can beseen around a wavelength of 1480 nm probably resulting
from the dispersion of glass material parameters. Thus,
the insertion loss can be minimized when module assembly is done at a wavelength for use.

RELIABILITY

Table 2 shows the results of Bellcore 1221 tests including,at the bottom, those of fiber pulling tests under Bellcore1209 which are essential for evaluating the reliability offiber termination. Needless to say, all products passed
Bellcore 1209 tests.

6.

CONCLUSION

We have developed and put into the marketplace a polarization combiner which is provided with high reliability,
compactness, wide bandwidth and ultra-low insertion loss.
The product is expected to contribute a great deal to multiplexing of pumping powers for Raman amplifiers alongwith upgrading the output power of optical amplifiers for
DWDM systems.

14

PassedLow temp storage

PassedTemperature cycling

PassedFiber termi-

Side pull

Passednation (1209)

Cable retention

Passed37

Extinction ratio (dB)

Passed± 0.2 dB

Damp heat5.

StatusMechanical shock

ACKNOWLEDGMENTS
The substance of this paper belongs to the achievementsof the members of Functional Optical Components Groupand Optical Components Department of FITEL Products
Division, and has been compiled by the authors who represent these members. The authors would like to thankthe members of Optical Equipment Department andOptical Communication Material Research Group in addition to Dr. Shii, Senior Staff Researcher at the Yokohama
Laboratories, Mr. Fukazawa, General Manager of the
FITEL-Photonics Laboratory and Mr. Miyazawa, Manager
of WP Team for their helpful advice and kind cooperation
during the development.

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Manuscript received on December 14, 2000.

Furukawa Review, No. 20

2001