Development of PBS-Integrated Coherent Mixer Using
Silica-Based Planar Lightwave Circuit
Takashi Inoue*, Hiroshi Kawashima*, Noritaka Matsubara*, and Kazutaka Nara*

A silica-based planar lightwave circuit (PLC) that is required for a receiver frontend used in digital coherent telecommunication system is developed byintegrating a polarization-beam splitter (PBS) and a 90-degree hybrid mixer. At first, byconfiguring a PBS for signal with a double-pass Mach-Zehnder interferometer, a PBS-integrated
coherent mixer chip is developed and a polarization extinction ratio (PER) higher than 30 dB isachieved. In the next stage, downsizing of a PLC chip is studied. For this purpose, the relativerefractive index difference Δ is improved to 1.8%, and a circuit layout of folded waveguide isemployed. As a result, an ultra-small PBS integrated mixer chip with dimension of
12 mm×12 mm is realized.
Using the developed chips in digital coherent receivers, experiments to demodulate dualpolarization quadrature phase-shift keying (DP-QPSK) signals at 40 Gbit/s and 100 Gbit/s are
conducted, and excellent performances were confirmed in both cases.

ABSTRACT1.

INTRODUCTION

The polarization-multiplexed quadrature phase-shift keying transmission system combined with the digital signal
processing shows great promise as the next-generationtransmission system with the bit-rate higher than
100 Gbit/s in one channel. Especially, the DP-QPSK isrecognized as a standard format in 100 Gbit/s Ethernet
systems, and related specifications for transmitters andreceivers are standardized in Optical Internetworking
Forum (OIF) 3). Among them, the specifications of an optical receiver front-end have been developed, which comprise a PBS, 90-degree hybrid mixers, balanced photodetectors (BPDs) and transimpedance amplifiers in a single module, and plays the roles of splitting the multiplexed polarizations , demodulating a phase-modulated
optical signal, and converting the optical signal to theelectrical one. In a front-end module, the PBS and the
90-degree hybrid mixers are required to be small so thatthey can be packaged in the specified size of the module.
In addition, optical features such as low loss, high accuracy, and stability are desired for the devices. Finally, theyshould be capable of mass-production at low cost. To
realize a product which satisfies these requirements, weinvestigated the development of the optical circuit which
integrated the PBS and the 90-degree hybrid mixer in aplanar lightwave circuit 4) ~ 6). This report shows the specificdevelopment results.
In Section 2, we propose a PLC integrating a PBS consisting of cascaded Mach-Zehnder interferometers (MZIs)

and 90-degree hybrid mixers, where the polarizationextinction ratio for the dual-polarization optical signals isdrastically improved.
Section 3 focuses on downsizing of a PBS-integrated
coherent mixer chip, and shows a fabrication result of
ultra-small PLC chip with the dimension of 12 mm
×12 mm. To downsize the chip dimension, we increase
the Δ of a waveguide to 1.8% in the PLC fabricating process, and develop the circuit layout of folded waveguideconfiguration.

2.

DEVELOPMENT OF THE DOUBLE-PASS
PBS-INTEGRATED COHERENT MIXER

Both the stable operation and the downsizing are possible by applying a PLC to passive optical components
such as PBS and 90-degree hybrid mixer. In fact, various
experimental results are reported 7) ~ 9). Combining thesetechnologies, a PLC which has a PBS and a 90-degreehybrid mixer integrated in one chip is proposed 10).
Reference 10) reports that the PBS comprised of a single
MZI has the PER of approximately 25 dB. In this section,
for design of PBS-integrated coherent mixer, we propose
to compose a PBS of a double-pass cascaded-type MZI
to improve the PER of polarization-multiplexed optical signal 9).

* Fitel Photonics Laboratory R&D Div.

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Development of PBS-Integrated Coherent Mixer Using Silica-Based Planar Lightwave Circuit

Upper mixer for TE-signal
Δθ = 90deg.

HWP

Signal
(TE/TM)TEPBS (2nd stage) for TE-signal
PBS (1st stage)

LO
(TM)

TM

PBS (2nd stage) for TM-signal

Δθ = 90deg.

TM

Lower mixer for TM-signal

Outputport #
1
2
3
4
TE

TM

5
6
7
8

（b）

Figure 1(a) Schematic diagram of the proposed double-passPBS-integrated coherent mixer based on PLC, and(b) picture of a fabricated PLC-chip.

Figure 1(a) shows a schematic diagram of the circuit
layout of the proposed PLC chip. The input signal propagates through the first stage PBS and the signal is split
into the TE and the TM polarizations. The second stage
PBS improves the PER of the TE/TM signals. We note in
Figure 1(a) that the waveguides after the first- and second-stage double-pass Mach-Zehnder PBSs are folded,
which contributes to a significant downsizing of the longitudinal dimension of the chip.
Between the second PBS and the mixer for theTE-signal, a half-wave plate (HWP) with 45-degree
inclined principal axis is inserted. And then, the TE-signalof the optical signal is converted to the TM-mode andinjected into the upper mixer of the chip. Meanwhile, theTM polarization component of the optical signal after
propagating through the PBS is directly injected into thelower mixer of the chip without any polarization conversion.
On the other hand, the local oscillator (LO) with the TM
polarization is injected into the PLC and guided to theupper and the lower 90-degree hybrid mixers after splitting. The split LOs interfere with the optical signals at
upper and lower mixers and are emitted from the eight
output ports of the PLC. The lights emitted from the PLC
chip are injected into four BPDs through free-space optics
or optical fibers.
Figure 1(b) shows a picture of the fabricated chip withthe dimension of 25 mm×21 mm. The maximum insertion
loss between the input port and the output port is10.1 dB: this includes the 6 dB of the intrinsic loss of the
90-degree hybrid mixer and the bonding loss associatedwith the optical fibers used for the loss evaluation. In asimilar way, the maximum insertion loss between LO andeach output port is 11.6 dB: this includes the 3 dB of the
intrinsic loss of the power splitter, 9 dB of the intrinsic lossof the 90-degree hybrid mixer and the bonding loss associated with the input and the output optical fibers. The
minimum PER of the optical signal passes is 33.2 dB at
1.55 μm. An extremely large PER is obtained thanks to
the double–pass cascaded-MZI structure for the PBS.
In order to evaluate the phase characteristics of a90-degree hybrid mixer, we configure MZIs combining themixer with an extra PLC chip consisting of a single input
waveguide, a beam splitter, and two optical paths led tothe output with different lengths. Note here that we adjust
the path-length difference so that the MZIs have the sinusoidal interference pattern in the frequency domain withthe free-spectral range (FSR) of 100 GHz. Figure 2 showsthe “MZI intensity transfer function vs. the wavelength” ofthe upper and the lower mixers. From this measurement
result, we confirm that the upper and lower mixers welloperate as 90-degree optical hybrid mixers.

Port#
1
2
3
4

Spectrum (a.u.)

TM

Port#
5
6
7
8

Spectrum (a.u.)

（a）

1551

1551.51552

1552.51553

Wavelength (nm)Figure 2

The interference patterns of MZIs comprising
the integrated coherent mixers.

We conduct a transmission experiment of a 40-Gbit/s
DP-QPSK signal using the fabricated PLC chip shown
above in a digital coherent receiver. Figure 3 shows theexperimental setup. The wavelength of the optical signal
and the local oscillator was 1552 nm. A 200-km single
mode fiber(SMF) was used for the transmission line. The
developed double-pass PBS-integrated coherent mixer
was allocated in front of the BPDs in the receiver, andoptical fibers were used to connect the PLC and the
BPDs. In this experiment, instead of using the HWP
shown in Figure 1 (a), the input LO was adjusted to contain the same optical power for both of the TE/TM modes.
In this case, the optical demodulation operation works
well, though the optical loss for LO has been increasedby 3 dB.

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Development of PBS-Integrated Coherent Mixer Using Silica-Based Planar Lightwave Circuit

Optical line
Electrical linePPG
DFB-LD

40 Gbit/s
EDFA
QPSK

SMF
200 km

IQM
PBS

DSP
circuit(Offline)

ADC

BPD

ADC

BPD

ADC

BPD

ADC

BPD

PBC

DFB-LD: Distributed-feedback laser diode
IQM: IQ modulator
PPG: Pulse pattern generator
PBS: Polarization-beam splitter
PBC: Polarization-beam combiner
EDFA: Erbium-doped fiber amplifier

Figure 3

EDFA
PLC-based
coherent mixer
w/double-pass
PBS

VOA

DFB-LD (LO)SMF: Single-mode fiber
VOA: Variable optical attenuator
LO: Local oscillator
BPD: Balanced photo detector
ADC: Analog-to-digital converter
DSP: Digital signal processing

The setup for the 40-Gbit/s DP-QPSK signaltransmission experiment.

(a) TE signal-27 dBm, B-to-B-37 dBm, B-to-B-37 dBm, after 200 km-37 dBm, B-to-B-37 dBm, after 200 km(b) TM signal-27 dBm, B-to-B
Figure 4

BER

10

-1

The constellation maps of back-to-back (B-to-B)
and transmitted DP-QPSK signals.

Back-to-back (TM)
Back-to-back (TE)
After 200 km (TM)
After 200 km (TE)
Shot-noise limit

10 -2
10 -3
10 -4
10 -5
10 -6
-50

Figure 5-45-40Received power (dBm)-35

Figure 4 shows the constellation maps of the received
40-Gbit/s DP-QPSK signals at back-to-back and after a
200-km transmission. In Figure 4, clear constellation
maps are observed for both the TE/TM modes of thepolarization-demultiplexed signal. Figure 5 shows the biterror rate (BER) characteristics. The BER is observed
even for the signal transmitted over the long distance,
and the satisfactory performance of the developed PLC
as a component in a coherent receiver is confirmed.
DEVELOPMENT OF AN ULTRA SMALL PBSINTEGRATED COHERENT MIXER USING 1.8% Δ
PROCESSThe chip dimension of the double-pass PBS-integrated
coherent mixer which is reported in the previous section
is 25 mm×21 mm. In consideration of the practicalaspect, a more compact dimension is required. In particular, for use with enough room in the receiver front-end
module standardized by OIF 11), a dimension of less than
15 mm square is desired.
In this section, development of a significant downsizingof the chip is investigated. For this purpose, we improve a
PLC fabrication process with a higher Δ so that the bending radius of the waveguide is reduced and thus a compact circuit layout can be designed. In the previous section, the PBS with the double-pass MZI structure is integrated only for the signal, and not for the LO. On the other
hand, in this section, we develop a PLC chip that integrates PBSs for both the signal and the LO, and90-degree hybrid mixers.
First, Δ of the waveguide is optimised. Generally, ahigher Δ makes the optical confinement of a guided mode
stronger and the bending radius of the waveguide canthen be reduced. Figure 6(a) shows the calculated minimum bending radius of the silica-based PLC for Δ. The
minimum bending radius is monotonically decreased for
an increased Δ. The relation between Δ and the thermalexpansion coefficient α of the SiO2-GeO2 glass is shown in
Figure 6 (b), where α increases with Δ. In the range whereΔ > 2.5%, α of the glass exceeds that of the silicon substrate. In this range, a tensile stress is induced in theglass and the reliability of the core glass is severely deteriorated. Furthermore, a highly doped GeO2 for higher Δ,
decreases the melting point of the core glass. As a result,the core shape is subject to deformation, and hence thefabrication repeatability becomes more difficult. For thesereasons, a PLC with Δ higher than 2.5% is not realistic
and Δ has to be carefully selected for practical use.
Here, we adopt 1.8% as an optimal value. With thisvalue, the minimum bending radius is 1200 μm which isapproximately a half of the conventional case, then thechip dimension can be more compact and enough reliability and repeatability in the fabrication process is guaranteed. In Figure 6, red points indicate Δ of 1.8%.
3.

BER characteristics of 40-Gbit/s DP-QPSK signals.

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Development of PBS-Integrated Coherent Mixer Using Silica-Based Planar Lightwave Circuit

(a)

(a)

6
Min. bending radius (mm)TE-signal

53

Si gnal
input

2

0

1

2

3

PBS forLO
TM-signal

Refractive-index difference, ∆ (%)
TM-LO

(b)

4

Δθ = 90deg.

P5
P6
P7
P8

Lower mixer for
TM signal and LO

Thermal-expansion coefficient ofSi-substrate3

α ×10 -6 (deg -1)

Output
port
P1
P2
P3
P4

PBS forsignal

LO
input

1
0

Δθ = 90deg.

TE-LO

4

Upper mixer for
TE signal and LO

(b)

2
1
0
0

1

2

3

Refractive-index difference, ∆ (%)

Figure 6(a) A minimum bending radius and (b) a thermalexpansion coefficient of the silica-based PLC vs. Δ.

Second, a circuit layout to further reduce the dimensionof a chip of PBS-integrated coherent mixer is studied.
When two PBSs for the signal and the LO, and two
90-degree hybrid mixers for TE/TM polarization components are allocated straightforward along the direction
parallel to the input and the output waveguides of thechip, the chip inevitably has a slender-rectangle shape in
the longitudinal direction 10). In this case, the circuit layout
with a longitudinal length less than 15 mm is difficult even
if the bending radius is reduced by increasing Δ. We thenpropose a folded configuration for a circuit layout shownin Figure 7 (a). By folding the waveguides at the input andoutput portions of the PBSs, the longitudinal dimension of
the chip can be drastically reduced. We note here that
there is no skew among any of the TE/TM components ofthe signal and the LO thanks to the symmetric structure of
the four folded waveguides coming from the output of thetwo PBSs.
Based on the above considerations, we fabricated a
PBS-integrated coherent mixer chip with the Δ of 1.8% andthe circuit layout shown in Figure 7 (a). Figure 7 (b)depicts a picture of the fabricated chip, and the dimension
is 12 mm×12 mm. The length on a side of the chip is lessthan 15 mm, which means the chip is enough small to be
used in the OIF-compliant receiver front-end module.

Figure 7(a) The schematic diagram of the proposed
circuit layout with a folded configuration, and(b) picture of the fabricated PLC-chip.

The optical characteristics of the fabricated chip areshown as follows. Note that those characteristics are evaluated by connecting SMFs at both ends of the chip.
Figure 8 shows the insertion loss. The insertion loss foreach port is about 10 dB, which is estimated to comprise
6 dB for the intrinsic loss of a 90-degree hybrid, 2.2 dB for
the connection loss between the PLC chip and the SMFs
at both ends, and 1.8 dB for the excess circuit loss. Figure
9 shows the measurement results of the PER of the PBSs
for the signal and the LO. For the both PBSs, the PER
over 20 dB are obtained in the entire C-band (15301570 nm). Finally, Figure 10 (a) shows the intensity transfer functions in the frequency domain for MZIs comprising
the integrated 90-degree hybrid mixers and an extra PLC
chip similar to that used in the previous section. From theresults, one can confirm that the integrated mixers work
well as 90-degree hybrid. Figure 10 (b) shows the phaseerror of the 90-degree hybrid mixers calculated from theresult of Figure 10 (a). The error is less than 5 degrees inabsolute value, and the specification for OIF-compliant
receiver front-end is then satisfied. In summary, the developed PLC chip has an ultra-small dimension and reasonable optical properties enough to be applied to an OIFcompliant receiver front-end module.

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Development of PBS-Integrated Coherent Mixer Using Silica-Based Planar Lightwave Circuit(a) Signal input port

(a)

-5

P2
P4
P6
P8

-10
-15
-20
1530

1540

1550
1560
Wavelength (nm)

1570

P1
P2
P3
P4

Power (a.u.)

P1
P3
P5
P7

P5
P6
P7
P8

Power (a.u.)

Insertion Loss (dB)

0

1547

(b) LO input port

1548

1549

P1
P3
P5
P7

-5

P2
P4
P6
P8

(b)

1551

1552

1553

-10
-15

1540

1550
1560
Wavelength (nm)3Phase error (deg.)

Insertion Loss (dB)

0

-20
1530

1550

Wavelength (nm)1570

Upper mixer
Lower mixer

2
1
0
-1
-2
-3
1530

1540

1550

1560

1570

Wavelength (nm)Figure 8

The insertion loss from input ports for (a) the signal
and (b) the LO to output ports P1-P8.

(a) PBS for signal

Figure 10 The phase characteristics for the upper and lower
mixers in the chip(a) The interference patterns in the vicinity of1550 nm(b) The phase error for the wavelength

PER (dB)

40
30
20
P1-P4
P5-P8

10
0
1520

1540

1560

1580

Wavelength (nm)A digital coherent receiver is built with the developed
PLC chip, and evaluated by demodulating a 100-Gbit/s
transmission signal with DP-QPSK format. Figure 11
shows the experimental setup. The wavelength of the
optical signal and the LO is 1552 nm, and 100-km SMF isused for the transmission line. The developed PLC chip isused in the front end of the receiver, and the PLC and the
BPDs are connected with the optical fibers.

(b) PBS for LO

PER (dB)

40
30

PPG
EMUX

20
P1-P4
P5-P8

10
0
1520

1540

1560

1580

Wavelength (nm)Figure 9

The polarization extinction ratio of PBSs for(a) the signal and (b) the LO.

DFB-LD
1552 nmADC
DSP ADCcircuit(offline) ADC
ADC

EMUX

25-Gsymbol/s(50-Gbit/s)
QPSK signal

100-Gbit/s
DP-QPSKsignal100-kmSMF~
10-ns
fiber delay

IQM
PBS PBC

BPD
BPD
BPD
BPD

EDFA
EDFA

12 mm×12 mm
PLCof 90- degreehybrid w/PBS

VOA OBPF

DFB-LD(LO)
1552 nm

Figure 11 The setup for a 100-Gbit/s DP-QPSK signaltransmission experiment.

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Development of PBS-Integrated Coherent Mixer Using Silica-Based Planar Lightwave Circuit

Back-to-backTE-pol, -30 dBm TM-pol, -30 dBm

TE-pol, -35 dBm TM-pol, -35 dBm

After 100-km transmission

transmission experiment to demodulate a 40-Gbit/s
DP-QPSK signal. On the other hand, for downsizing of thechip size, Δ was increased to 1.8% and the minimum
bending radius has been decreased. Furthermore, a folded waveguide configuration was adopted. Hence, anultra-small PBS integrated coherent mixer chip with thedimension of 12 mm×12 mm was achieved. Satisfactory
performance of this chip in a 100 Gbit/s DP-QPSK signal
demodulation experiment was also confirmed.

ACKNOWLEDGEMENT
TE-pol, -30 dBm TM-pol, -30 dBm

TE-pol, -35 dBm TM-pol, -35 dBm

Figure 12 The constellation maps of a back-to-back (B-to-B)
and a transmitted DP-QPSK signals.

Back-to-back (TM)
Back-to-back (TE)
After 100 km (TM)
After 100 km (TE)
Shot-noise limit

BER

10 -1

REFFERENCE

10 -2

10 -3
10 -4
10 -5
-45

-44

-43

-42 -41 -40 -39 -38
Received power (dBm)-37 -36-35

Figure 13 The BER characteristics of 100-Gbit/s DP-QPSK
signals.

Figure 12 shows the constellation maps of DP-QPSK
signals at back-to-back and after 100 km transmission,
obtained after the digital coherent detection. Figure 12
shows the observation result of the clear constellation
maps as a QPSK signal for the both TE/TM polarization
components. Figure 13 shows the BER characteristics ofthe received signal. The BER is properly observed on a
100-Gbit/s DP-QPSK signal after a long distance transmission, which confirms that the developed PLC performs
well as a component of the digital coherent receiver.

4.

We would like to thank and to express our sincere gratitude to the members of the laboratory of Prof. Kikuchi
and Prof. Igarashi of the department of electronic engineering of the school of engineering at the University ofTokyo, for their cooperation in conducting the DP-QPSK
transmission experiment.

1) D.-S. Ly-Gagnon et al, “Coherent detection of optical quadraturephase-shift keying signals with carrier phase estimation,” J.
Lightwave Technol. 24, 12-21 (2006).
2) S. J. Savory et al., “Electronic compensation of chromatic dispersion using a digital coherent receiver,” Opt. Express 15, 2120-2126(2007).
3) http://www.oiforum.com/
4) T. Inoue et al., “Double-Pass PBS-Integrated Coherent Mixer
Using Silica-based PLC,” Proc. OFC2010, paper OThB2 (2010).
5) T. Inoue et al., “Ultra-small PBS-Integrated Coherent Mixer Using
1.8%-Delta Silica-Based Planar Lightwave Circuit,” Proc.
ECOC2010, paper Mo.2.F.4 (2010).
6) http://www.furukawa.co.jp/what/2010/comm_10318_1.htm
7) S. Norimatsu et al., “An optical 90°-hybrid balanced receiver moduleusing a planar lightwave circuit,” Photon. Technol. Lett. 6, 737-740(1994).
8) Y. Hashizume et al., “Integrated polarisation beam splitter using
waveguide birefringence dependence on waveguide core width,”
Electron. Lett. 37, 1517-1518 (2001).
9) N. Matsubara et al., “Silica-based PLC-type polarization beam splitter with >30 dB high extinction ratio over 75 nm band width,”
MOC2005, paper C2 (2005).
10) Y. Sakamaki et al., “One-chip integrated dual polarization opticalhybrid using silica-based planar lightwave circuit technology,” Proc.
ECOC 2009, paper 2.2.4 (2009).
11) http://www.oiforum.com/public/documents/OIF_DPC_RX-01.0.pdf

CONCLUSION

This paper illustrates the technical development resultsof a PBS-integrated coherent mixer using a silica-based
planar lightwave circuit, which can be used in the digital
coherent receiver front-end. The double-pass PBSintegrated coherent mixer realizes a polarization demultiplexing with the PER over 33 dB for the optical signal,and the satisfactory performance was confirmed in the

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