Wideband Tunable Dispersion Compensator
Using a 25-Stage PLC-MZI
Hiroshi Kawashima *, and Kazutaka Nara *In high-bit-rate optical transmission systems that are being introduced into
commercial applications, such as those with 40-Gbps bit-rate, there is a needfor tunable dispersion compensators (TDCs) that enable adaptive compensation for dispersion
fluctuations due to temperature changes and the like in the transmission lines, and accordingly,
a variety of TDCs based on different principles have been reported. In particular, the cascaded
Mach-Zehnder interferometer (MZI) type TDC using a lattice type filter based on the silica-based
planar lightwave circuit (PLC) technology is promising because of its ease of control and large
achievable dispersion. However, the usable bandwidths of this type of TDCs reported to date arethought to be insufficient for their practical application to the 40-Gbps systems. We havedemonstrated here, therefore, a TDC using a 25-stage PLC-MZI that combines a usablebandwidth of 60 GHz or higher and a tunable dispersion range of ±300 ps/nm or more. Its
superior characteristics will be reported in this paper.

ABSTRACT1.

INTRODUCTION

In high-bit-rate optical transmission systems that are
being introduced into commercial applications, such as
those with 40-Gbps bit-rate, there is a need for tunabledispersion compensators (TDCs) in order to additively
compensate for chromatic dispersion (CD) fluctuations
due to temperature changes and the like in the transmission lines. Accordingly, a variety of TDCs based on different principles have been reported so far 1)~7). In particular,
the cascaded Mach-Zehnder interferometer (MZI) type
TDC using a lattice type filter based on the silica-based
planar lightwave circuit (PLC) technology is promisingbecause of its ease of control and large achievable dispersion 5)~7).

Delay line
φ1

θ1

θ2

φ3

θ3

φ2

Figure 1θ4

φn-2
Tunable coupler...........
θn-1

φ4

φn

θn

θn+1

φn-1

Configuration of cascaded PLC-MZI type TDC.

* FITEL-Photonics Lab., R&D Div.

Figure 1 shows the basic configuration of cascaded
PLC-MZI type TDC. Basically this TDC is a finite impulse
response (FIR) filter of lattice filter type, consisting of multiple PLC-based MZIs that are cascaded via tunable couplers, in which, by tuning the phase of each delay line
and the coupling ratio between delay lines, the filter
response shown in Equation (1) can be obtained.

+1

＝Σ
=1

exp

2π
c

- +1

(1)

∆

where, ai is the Fourier coefficient,
j is the imaginary
-i+1
n+1
2π
eff is the effective refractive index, f is the optical freunit, nT(f)＝
a
j
n
f∆
L
(1)
exp
eff
i
Σ
c velocity in vacuum.
i=1 c is the lightquency, andIn this circuit, the degree of freedom of the filter characteristics is determined by the number of stages n in the
MZI, so that arbitrary filter characteristics can be obtained
according to the value of n. And, since periodical waveforms are obtained per every free spectral range (FSR)
depending on ΔL, this circuit can be used in simultaneouscompensation for wavelength multiplexed signals or as acolorless device. However, in order to obtain desired filter
characteristics by using such a general FIR filter, all thephases of delay lines and all the coupling coefficients
must be tuned, and this makes it problematic in terms ofpracticality.
On the other hand, other types of cascaded PLC-MZITDCs have been proposed, in which the delay lines on
the first and last stages have optical path difference of ΔL,
while the rest delay lines have that of 2ΔL, and every tunable coupler is driven in parallel by the same power

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Wideband Tunable Dispersion Compensator Using a 25-Stage PLC-MZI

source 6), 7).
Figure 2 shows the schematic configuration of this typeof TDC. Even though its usable bandwidth is limited toabout half the FSR, this type allows for “single knob control” of tunable couplers, where, by precisely trimming theoptical path difference on each delay line to ΔL or 2ΔL in
advance, every coupler can be driven in parallel, at thetime of practical usage, either to the plus or minus direction, using a single current source. This improved thepracticality in terms of controllability and power consumption. Moreover, by adjusting the ratio of driving power on
the first and last tunable couplers over that on the rest, thelinearity and flatness of group delay spectrum and transmittance spectrum can be controlled. Furthermore, byusing a quarter waveplate combined with a reflection mirror or Faraday rotator mirror, compensation for polarization dependence can be provided simultaneously withdoubling of tunable dispersion range which results fromdouble pass configuration. Thus several demonstrations
have been reported including a TDC with a tunable dispersion range of about ±500 ps/nm based on 8-stage MZI 7).

transmission. Based on the design, we have further fabricated a cascaded PLC-MZI TDC with single knob control,and carried out dispersion compensation experiments on
a 40-Gbps return-to-zero (RZ) on-off keying (OOK) signal.

2.

CONFIGURATION

To design a TDC suitable for 40-Gbps transmission as
mentioned before, we aimed at assuring a usable bandwidth of higher than 60 GHz and a tunable dispersion
range larger than ±300 ps/nm. First, the FSR was set to
200 GHz in order to deal with WDM optical transmission
on the ITU grid while assuring a usable bandwidth higher
than 60 GHz. Next, the relationship between the numberof stages and tunable dispersion range at an FSR of 200
GHz was calculated by simulation. Figure 3 shows therelationship between the maximum dispersion and number of stages, which is obtained from the calculation
results of group delay spectrum with the dispersion maximized for each number of stages. In this calculation, thedriving power ratio of the first and n-th tunable couplers
over the rest was set at 1:1.8.

V+

θ1

θ2

θ3

2

φ3

φn-2

θ4

...........

400
φn

θn-1

θn

φ2

φ4

φn-1

2

2

2

θn+1

VV-

Figure 2

Calculated CD（ps/nm）

φ12Configuration of cascaded PLC-MZI type TDC withsingle knob control.

With these TDCs reported, in order to obtain a tunabledispersion range of about ±300 ps/nm or more that isrequired in the 40-Gbps systems, it is necessary to set the
FSR to 100 GHz and the usable bandwidth to about 40
GHz. However, this bandwidth might be insufficient forpractical application in the 40-Gbps systems. Usablebandwidth of this type of TDC can be expanded byexpansion of the FSR, but since there is a tradeoff relationship between the usable bandwidth being proportional to the FSR and the tunable dispersion range being
inversely proportional to the square of FSR, it is difficult tomake them compatible with each other. On the other
hand, the maximum tunable dispersion range achievable
is approximately proportional to the number of stages.
Thus, by expanding the FSR and simultaneously increasing the number of stages, there is the possibility of making the usable bandwidth compatible with the tunable dispersion range at a high level.
Here, we have accordingly studied designs to satisfy
both of these characteristics, i.e., usable bandwidth ofhigher than 60 GHz and tunable dispersion range of larger than ±300 ps/nm, which are sufficient for 40-Gbps

350
300
250
200
150
100
50
0

5

10

15

20

25

30

Stage number

Figure 3

Calculated maximum dispersion vs. number ofstages.

It can be seen from Figure 3 that the tunable dispersion
range increases approximately in proportion to the number of stages, reaching ±300 ps/nm at 25 stages.
Next, setting the number of stages at 25, the groupdelay and transmittance spectra were calculated at theplus and minus maxima of dispersion and at zero dispersion, and the results are shown in Figure 4.
From Figure 4, the tunable dispersion range is seen to
be 60 GHz (0.48 nm) or more under any conditions. Thus
it was decided to adopt the 25-stage configuration.
Figure 5 shows the schematic configuration of 25-stage
MZI-TDC. This circuit, basically equivalent to the cascaded PLC-MZI TDC with single knob control shown in Figure
2, consists of cascaded 25-stage MZIs, comprising two
MZIs with an FSR of 200 GHz at both ends and 23 intermediate MZIs with an FSR of 100 GHz in between. Each
MZI is connected by a tunable coupler having an initialcoupling ratio of 0%, which consists of an MZI having aphase difference of π and a thin-film heater on its arm.

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Wideband Tunable Dispersion Compensator Using a 25-Stage PLC-MZI

0

Transmittance
60 GHz-312 ps/nm （0.48 nm）

100

-0.5
-1

50

-1.5

0 ps/nm

0

-2

-50

Transmittance（dB）

Group delay（ps）

150

-2.5
312 ps/nm Group delay-100-0.4 -0.3 -0.2 -0.1

0

0.1

0.2

-3
0.3 0.4

than that shown in Figure 5, thus reducing the chip area.
Furthermore, at the time of folding back the 13th delayline, the two delay lines were made to cross with eachother in order to make the optical connection between
adjacent delay lines identical to that in Figure 5. This
makes the control method for the folded structure identical to that for the circuit arrangement shown in Figure 5.
Meanwhile, the delay lines are arranged to cross with anangle of 90° in order to suppress loss increase due tocrossing. By devising such a layout, the circuit area has
been reduced by about 20% compared with the layout
shown in Figure 5.

Wavelength（nm）

Figure 4

Calculated spectra of 25-stage TDC.

The MZIs at both ends are connected to the input andoutput waveguides by means of a Y junction, rather thana 2x2 structured coupler, thereby assuring 1:1 branchcoupling. Moreover, in order to reduce polarization dependence, a polarization diversity scheme is implemented byconnecting a polarization beam splitter (PBS) circulator
via a polarization maintaining fiber.
With this TDC, tunable dispersion characteristics can beobtained by providing in parallel the heaters on each tunable coupler with either plus or minus dispersion driving
power, thereby changing the coupling coefficient within
the range of 0% ~ ±50%.
Variable couplers

FSR=200 GHz
1st
1st

3rd
3rd

5th
5th

7th
7th

9th
9th

FSR=200 GHz

11th
11th 13th
13th 15th
15th 17th
17th 19th
19th 21st
21st

23rd
23rd 25th
25th

Y junction

Y junction
2nd
2nd

4th
4th

6th
6th

8th
8th

10th
10th 12th
12th 14th
14th 16th
16th 18th
18th

FSR=100 GHz

PMF

20th
20th 22nd
22nd 24th
24th

PMF

PBS Circulator

Figure 5

Schematic configuration of 25-stage MZI-TDC withpolarization diversity.

Meanwhile, each MZI has a thin-film heater on its delaylines, so that phase errors due to fabrication variation canbe permanently compensated for by local heating-based
phase trimming 8).
But, in fabricating a 25-stage MZI of such a structure asa practical PLC circuit, it was found that however much
downsizing is strived for by inverse disposing the curvature of MZIs alternately as shown in Figure 5, the resulting
circuit size becomes excessively large, making implementation difficult. Accordingly, a folded arrangement shownin the circuit layout in Figure 6 was newly adopted here to
make the circuit size more compact. In this structure, thecircuit is folded at the delay line on the 13th MZI, the center of the 25-stage MZI, so that the (13+n)th delay line (nis integer, 1≦n<13) is laid along the (13-n)th delay line.
This allows for higher-density arrangement of delay lines

2nd

4th

6th

8th

10th

12th

24th

22nd

20th

18th

16th

14th

1st

3rd

5th

7th

9th

11th

25th

23rd

21st

19th

17th

15th

13th

90deg. Crossing

Figure 6

3.

Waveguide layout of compact 25-stage MZI-TDC withfolded arrangement.

FABRICATION

Based on the design described above, the MZI-TDC chip
was fabricated using usual silica PLC fabricating technologies. Then, a polarization maintaining fiber is connectedto the input and output waveguide ends, and subsequently the PBS circulator is connected thus implementing
polarization diversity. After that, while evaluating the interference characteristics of each delay line using a temperature-controlled stage, phase trimming was carried out
using the thin-film heaters on the delay lines.
Figures 7 and 8 show the fabricated MZI-chip and optical module, respectively. As shown in Figure 7, despite its
multi-stage structure including as many as 25 stages, thechip has a compact size of 25 mm x 70 mm. The opticalmodule measures 130 mm x 100 mm x 16 mm.

Figure 7

Photo of 25-stage MZI-TDC chip.

Figure 8

Photo of 25-stage MZI-TDC module.

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Wideband Tunable Dispersion Compensator Using a 25-Stage PLC-MZI

4.

firmed that, thanks to the polarization diversity using PBS
circulator, PDL and PMD were 0.3 dB or less and 0.3 psor less, respectively, for the whole passband.

EVALUATION RESULTS OF OPTICAL
CHARACTERISTICS

With a single power source connected to drive the heaters on the tunable couplers, the group delay, transmittance and polarization mode dispersion (PMD) were measured. On this occasion, the driving power ratio over the
tunable couplers was set at 1:1.8.
Figure 9 shows the measurement results, as the control
voltage is changed, of the group delay and transmittancespectra, as well as polarization-dependent loss (PDL) and
PMD at CD values of +336 ps/nm and -320 ps/nm. It has
been confirmed from Figure 9 that, as has been targeted
at, CD of ±300 ps/nm or more is achieved with a usablebandwidth of about 60 GHz (0.48 nm). The insertion loss
is about 7 dB at the center wavelength in the passband,and the loss fluctuation was not more than 1.5 dB within
the 60-GHz band under every driving condition. The insertion loss comprises about 4.8 dB from circuit loss, about
1 dB from two fiber connections, and about 1.2 dB from
PBS circulator and connectors. It has also been con-

60 GHz
-6

140
100

-8

Transmittance-320 ps/nm（-7.6 V）

+336 ps/nm（+7.6 V）

-10

60

-12

20

-14

-20

-16
Group delay-60

Transmittance（dB）

Relative group delay（ps）

180

5.

In order to validate the effectiveness of the TDC developed here in high-bit-rate optical communication systems,
dispersion compensation experiments on a 40-Gbps
RZ-OOK signal was carried out using the fabricated TDC
module. Figure 10 shows the experimental setup. In theexperiment, CW light of 1561.0 nm in wavelength from a
distribution feedback laser diode (DFB-LD) array wasmodulated, using a LiNbO3 modulator (LN1), by 40-Gbps
nonreturn-to-zero (NRZ) pseudorandom bit sequence of7
10 -1 in signal length, subsequently was modulated using
LN2 by 40-Gbps RZ pulses, to obtain 40-Gbps RZ-OOK
signals. After amplification using an erbium-doped fiber
amplifier (EDFA), the signal was transmitted over dispersion compensated fibers (DCFs) each having CD of -125,-250, and -330 ps/nm, dispersion compensated at the
TDC, and was received for eye pattern observation andbit error rate (BER) measurement. Figure 11 shows thesignal spectra for the back-to-back and -330 ps/nm transmissions. It can be seen from Figure 11 that good RZ signal spectrum has been generated and that, due to thewide passband of the TDC, practically no spectral narrowing has occurred. Figure 12 shows eye pattern comparisons of the zero, -250, and -330 ps/nm transmission line
dispersions, with and without TDC. From Figure 12 it can

-18
-20

Transmittance（dB）

DGD（ps）

PDL（dB）-100
1-0.4-0.3-0.2-0.100.10.20.30.4
0.8
Relative Wavelength+336 (nm)ps/nm-320 ps/nm
0.6
0.4
0.2
0
1-0.4-0.3-0.2-0.100.10.20.30.4
0.8
Relative Wavelength (nm)-320 ps/nm
0.6+336 ps/nm
0.4
0.2
0
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2

0.3

b to b0-330 ps/nm w.TDC

-10
-20
-30
-40
-1

0.4

Relative wavelength（nm）

Figure 9

DISPERSION COMPENSATIONEXPERIMENTS ON 40-GBPS RZ-OOK
SIGNAL-0.8 -0.6 -0.4 -0.2

Measured transmittance, group delay, PDL, and PMD.

LN1

1nm

1nm

Filter2

Filter3

LN2
DCF

TDC

ATT.

PM

40 GHz

DeMUX

MUX
PPG

1

OEM
Filter1

40 GHz0.2 0.4 0.6 0.8

Figure 11 Measured optical spectra.

2.8nm

DFB-LD
Array

0

Wavelength（nm）

ED

PRBS
7

10 GHzLength=10 -1

Figure 10 Experimental setup for dispersion compensation of 40-Gbps RZ signal.

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Wideband Tunable Dispersion Compensator Using a 25-Stage PLC-MZI

without TDC

with TDC

Voltage（100 mV/div）

0ps/nm0ps/nm(b-to-b)(b-to-b) w/o.
w/o. TDC
TDC-250
ps/nm w/o.
w/o. TDC
TDC-250ps/nm-250
ps/nm, w.
w. TDC,V-250ps/nm,
TDC,6.52
6.52V-330ps/nm-330
ps/nm w/o.
w/o. TDC
TDC-330ps/nm,
7.60V-330 ps/nm, w.
w. TDC,
TDC, 7.60
V

Time（10 ps/div）

Figure 12 Eye opening of 40-Gbps RZ signal with/without TDC.

10

BER

10
10
10
10
10
10

b to b-125 ps/nm-250 ps/nm-330 ps/nm

-4

-5

-6

-7

-8
-9
-10

-35

-30

-25

Pin（dB）

Figure 13 BER measurement results.

be seen that, in contrast to the fact that, without TDC, theeye pattern is completely closed at -250 ps/nm, it is open,with TDC, not suffering significant degradation up to -330ps/nm, thus confirming substantial effectiveness of dispersion compensation. Figure 13 shows the measurement results of BER. From Figure 13, it has been confirmed that, despite power penalty of several dB included,-9transmission with a BER of approximately 10 is possible
by using this TDC. As described above, the TDC developed here has been confirmed to be effective for tunabledispersion compensation in 40-Gbps systems.

the waveguides, thereby succeeding in accommodating
the very large-scale circuit of 25-stage MZI in a compact
chip size of as small as 70 mm x 25 mm. Moreover, polarization diversity was implemented using a polarization
splitter/circulator module in order to reduce polarization
dependence.
As the result, the fabricated TDC showed good characteristics including: insertion loss of about 7 dB, bandwidthof about 60 GHz, tunable dispersion range of ±300 ps/
nm or more, PDL of 0.3 dB or less, and PMD of 0.3 ps or
less.
Furthermore, using this TDC, we conducted dispersion
compensation experiments on 40-Gbps RZ on-off keying
signal, and demonstrated that dispersion compensation
up to -330 ps/nm is possible, thus confirming the applicability of this TDC to 40-Gbps optical transmission systems.

ACKNOWLEDGMENTS
We would like to express our heartfelt thanks to the personnel of Kikuchi-Igarashi Laboratory, Department ofElectrical Engineering and Information Systems, Schoolof Engineering, The University of Tokyo for their making
available the transmission line experimental system, theiradvice and collaboration.

REFERENCES

6.

CONCLUSIONS

We have developed a PLC-MZI tunable dispersion compensator with broad bandwidth targeted at 40-Gbps highbit-rate optical transmission systems.
To satisfy both the wide bandwidth and large tunabledispersion required for 40-Gbps systems, the free spectrum range was designed to be as broad as 200 GHz, andcascaded MZI of as many as 25 stages was adopted. A
folded circuit configuration was used to densely arrange

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