Tunable Dispersion Compensator Using PLC Type Optical
Transversal Filter
by Hiroshi Kawashima *, Noritaka Matsubara * and Kazutaka Nara *In high bit rate optical transmission systems such as 40 Gbps, dispersion tolerances will be smaller than the dispersion fluctuations due to such factors as temperature change or power variation. To compensate for these fluctuations, tunable dispersion compensators (TDCs) are needed. Various types of tunable dispersion compensators have been demonstrated. Of these, integrated optic finite impulse response filter type
TDCs in silica-based planar lightwave circuits (PLCs) are promising because of their flexibility
with respect to dispersion curve shape, tunability of center wavelength and ability to compensate
for multiple channels. Among these, the transversal filter, consisting of a splitter, N delay lines(called taps) connected in parallel and a combiner, is capable of having a large number of taps
for high resolution and high controllability using monitor waveguides.
Owing to these advantages, we have fabricated an 8-tap transversal filter type TDC in a silicabased PLC with a high index contrast of 1.5 %, and have confirmed its operating principle using
two types of dispersion curve shapes. We have been able to obtain good performance: chromatic dispersion of -71 ~ +109 ps/nm, insertion loss of <10 dB and operating wavelength range of
0.5 nm. We have also fabricated a 16-tap transversal filter type TDC and have confirmed a higher
resolution than the 8-tap TDC.

ABSTRACT1.

INTRODUCTION

High bit rate optical transmission systems such as 40Gbps systems are about to be introduced in backbone
networks to meet rapid traffic increases in broadband
access networks with FTTH or xDSL.
In such high bit rate systems, dispersion tolerances will
be smaller than the dispersion fluctuations due to suchfactors as temperature change or power variation, andthis will cause deterioration in transmission quality. To
compensate for these fluctuations, adaptive tunable dispersion compensators (TDCs) are needed. Various typesof TDCs based on FBGs, bulk optics and integrated
optics have been demonstrated 1)~3). Of these, integrated
optic finite impulse response (FIR) filter type TDCs suchas lattice filters and transversal filters in silica-based planar lightwave circuits (PLCs) are promising because of
their flexibility with respect to dispersion curve shape, tunability of center wavelength and ability to compensate formultiple channels 3). The FIR filters have N delay lines(called taps) configured with multiple Mach-Zehnder interferometers (MZIs), and the desired filter characteristics
can be obtained according to arbitrary tap coefficientsthat determine the amplitude and phase of optical signals
in each tap. Among these, the transversal filter, which
consists of a splitter, N taps connected in parallel and a* FITEL-Photonics Lab., R&D Div.

combiner, has a number of advantages such as the possibility of having a large number of taps for high resolution
and high controllability of tap coefficients by use of preexisting idle ports of MZIs in the splitters and combiners
as monitor waveguides without additional losses 4).
Owing to these advantages, we have designed a transversal filter type TDC for 40-Gbps systems using discrete
Fourier transformation, have fabricated an 8-tap transversal filter type TDC in a silica based PLC with a high index
contrast of 1.5 %, and have confirmed the operating principle using two types of dispersion curve shapes. We
have thus been able to obtain good performance: chromatic dispersion of -71 ~ +109 ps/nm, insertion loss of<10 dB and operating wavelength range of 0.5 nm 6). In
addition, we have fabricated a 16-tap transversal filter
type TDC and have confirmed a higher resolution than the
8-tap TDC.

2.

CIRCUIT CONFIGURATION

Figure 1 shows the configuration of the 8-tap transversal
filter type TDC that we have fabricated. The filter has three
sections: a variable splitter, delay lines with variable phaseshifters (VPSs) and a variable combiner. The variable splitter and the variable combiner consist of the same threestage cascaded variable couplers (VCs) using MZIs, andare designed symmetrically having mirror image characteristics so that the filter has a low theoretical loss. In thisFurukawa Review, No. 29

2006

13

Tunable Dispersion Compensator Using PLC Type Optical Transversal Filter

OUT

IN
Variable splitter

Delay lines

Variable combiner

Figure 1 Schematic configuration of transversal filter type TDC.

circuit configuration, we can easily increase the resolution
by adding VC stages and increasing the number of taps.
The circuit has monitor waveguides for adjusting the circuit characteristic, which connects pre-existing idle portsof the cascaded VCs to the end of the PLC chip.

3.

FILTER SYNTHESIS

where λ is wavelength, λc is centre wavelength, and ε isa coefficient determining the inclination of the dispersion
characteristics.
Based on the above discussion, we calculated filter
characteristics using Gl1 and Gl2.
Figure 2 and Figure 3 show examples of target spectra
and calculated tap coefficients. The circuit parameters areshown in Table 1.
In the case of Gl1, the calculated tap coefficients are
multiplied by a window function called the “Hamming window” 5) in order to reduce group delay ripples in the passband, caused by the use of an insufficient number of taps
to exactly express the target phase spectrum with sharpedges as Fourier series.
Figure 4 shows calculated group delay responses andpower transmittances of the filter using Gl1 (ε= -2.0 ~+2.0 ) and Gl2 (ε= -0.7~ +0.7). As shown in Figure 4,group delay shapes were varied as ε values in both target
types. As a result, a chromatic dispersion (CD) of ±93
ps/nm, a 3-dB bandwidth of about 0.3 nm and an operat-

The transfer function G of the filter is expressed by a
Fourier series as,(1)

where neff is the effective refractive index of the waveguides, f is the optical frequency, n∆L is a fixed delay toeach tap (n= 0,1...N-1), c is the velocity of light, and j is
the imaginary unit; gn is the tap coefficient of the filter
given bygn = γ n exp (jθn)

where Gl is a target frequency response, and N’ is thenumber of sampling data.
In this study, we used two types of target frequency
response in order to ensure the flexibility of the dispersion
curve shape of the filter. The first target Gl1, determined byequation (4), has a quadratic phase spectrum that results
in a constant chromatic dispersion value in each free
spectral range (FSR) of the filter. The second target Gl2,
determined by equation (5), has a sinusoidal phase spectrum that results in a pseudo-constant chromatic dispersion value around the center of each FSR and a continuous connection with adjacent order spectra.

 2 2 (λ� −λ c�)2 G l1 = exp  jεπ��  FSR 
λ −λ c Gl2 = exp  j επ sin  2 π�
FSR  



0.5

5

0

0

-0.5

-5

-1.0

-0.4 -0.2

(5)

-15

0

0.2

0.4

-20
0.8

0.6

Relative wavelength (nm)(a) Using Gl1
2.0

20

Phase

Amplitude

1.5

Group delay

15

1.0

10

0.5

5

0

0

-0.5

-5

-1.0

-10

FSR

-1.5
-2.0
-0.8 -0.6-0.4 -0.2

-15
0

0.2

0.4

-20
0.8

0.6

Relative wavelength (nm)(b) Using Gl2
Figure 2
Table 1
Target phase shape
Tap number

(4)

-10

FSR

(2)

(3)

15
10

-2.0
-0.8 -0.6

Amplitude/Phase (rad)N'−1 2π 
1
Gl exp  jnl
∑N' l =0
 N' 

Group delay

Phase

1.0

-1.5

where γn is the coupling ratio of the variable splitter andvariable combiner, and θn is additional phase to each tap.
Discrete Fourier transformation of equation (1) gives gn as
gn =Amplitude

1.5
Amplitude/Phase (rad)

N−1
 2π

G =∑ gn exp  −jneff fn∆ L
c
n=0



20

2.0

Group delay (ps)Monitor waveguides

Group delay (ps)

VPS
VC

∆L

Target spectra.

Circuit parameters.
Quadratic (Gl1)Sinusoidal(Gl2)882055 µm

2055 µm

FSR

100GHz100GHzλC

1545nm1545nmε-2 ~ +2-0.7 ~ +0.7

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Tunable Dispersion Compensator Using PLC Type Optical Transversal Filter
1.2

4

1.0

3
2

0.6

γn

0.4

θn

1
0
-1

0.2
0

θn

γn

0.8

-2
0

2

4

8

6

-3

Tap no.

(a) Using Gl1 (FSR=100 GHz, 8 taps, Gl1, ε=-1)
1.2

4

1.0

3

γn

0.6

γn

0.4

θn

1
0
-1

0.2
0

θn

2

0.8

-2
0

2

4

6

8

-3

Tap no.

ing wavelength range of 0.5 nm were obtained at ε= ±2.0
using Gl1 and CD of ±200 ps/nm, 3-dB bandwidth ofabout 0.4 nm and operating wavelength range of 0.35 nm
were obtained at ε= ±0.7 using Gl2 around the centre
wavelength.
The narrower 3-dB bandwidths than the operating
wavelength ranges at large ε values using Gl2 are due tothe window function and that limits the usable wavelength
range.
We then calculated the spectra using tap numbers of 8,
16 and 32, and FSRs of 25, 50, 100 and 200 GHz. Figure
5 shows calculated maximum CD and 3-dB bandwidth vs.
FSR. In the case of 32 taps in Gl1 and the case of Gl2, 3-dB
bandwidths are fixed to be equal to FSR and FSR/2,
respectively, because these spectra are over -3 dB in all
operating wavelength ranges. As shown in Figure 5, there
is a trade-off between CD and bandwidth: that is, CD isinversely proportional to FSR2 whereas bandwidth is proportional to FSR. In addition, the 3-dB bandwidth
becomes wider with an increase in the number of taps in(b) Using Gl2 (FSR=100 GHz, 8 taps, Gl2, ε=-4)60-5Relative group delay (ps)

40

-15

30
20

-20

10

-25

0

-30

-10
-20

-35
Group Delay-40-50-0.4 -0.3 -0.2 -0.1

0.1

0.2 0.3

1.4

-2
-1.5
-1
-0.7
-0.4
-0.2
0.2
0.4
0.7

1.0
0.8

100

0.6
0.4
0.2
1

2

-50
0.4

1.2

1000

10

1
1.5

-45
0

1.6

10000

-40

-30

CD- 32 tap
3 dB-BW- 32 tap

<ε>

-10

TransmittanceCD-16 tap
3 dB-BW -16 tap

0

25

50

75

100

125

150

175

200

3dB bandwidth @max. CD (nm)0Max. CD (ps/nm)

7050

CD- 8 tap
3 dB- BW- 8 tap

Calculated tap coefficients.

Transmittance (dB)Figure 30

FSR (GHz)

Relative wavelength (nm)(a) Using Gl1 (ε=-2)(a) Using Gl1 target (FSR=100 GHz, 8 taps)

60

-5

40

Transmittance

30

-15

20

-20

10

-25

0

-30

-10

-35

-20
-30

Group Delay-40-50-0.4 -0.3 -0.2 -0.1

-0.7

-10

-40
-45

0

0.1 0.2

CD-32 tap
3 dB-BW-32 tap
1.6

10000

1.4

-0.2
0.2
0.4
0.7

1.2

1000

1.0
0.8

100

0.6
0.4

10

-50
0.3 0.4

Relative wavelength (nm)

0.2
1

0

(b) Using Gl2 target (FSR=100 GHz, 8 taps)
Figure 4

CD-16 tap
3 dB-BW -16 tap-0.4
Transmittance (dB)

Relative group delay (ps)50

CD- 8 tap
3 dB-BW-8 tap

<ε>

25

50

75

100

125

150

175

200

3dB bandwidth @max. CD (nm)0Max. CD (ps/nm)

70

0FSR (GHz)

Calculated spectra.

(b) Using Gl2 (ε=-7)
Figure 5

Calculated maximum CD and 3-dB bandwidth
vs. FSR.

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Tunable Dispersion Compensator Using PLC Type Optical Transversal Filter

4.

FABRICATION

5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0

SETTING PROCEDURE OF TAP
COEFFICIENTSIn order to realize the calculated filter response exactly,
we needed to set the tap coefficients precisely. However,each VC and VPS has fabrication errors. We therefore
controlled the TDC by the following procedure. First wemeasured each variable characteristic of the VCs using
monitor waveguides, and then measured the characteristics of VPSs by measuring interference spectra between
adjacent VPSs. After that, we calculated phase errors
caused by fabrication errors from the measured wavelength shift, and compensated for them by driving VPSs
with adequate power. Finally, we set VCs and VPSs to thecalculated tap coefficients using characteristics actually
measured 5).
Figure 8 shows filter spectra of the 8-tap transversal filter fabricated here before and after phase error correctionof the VPSs. Gray plots in each graph indicate calculated
spectra. In the corrected spectra, we found that the transmittance and peak position corresponded well to the calculated spectra, showing that error correction was successfully performed.
0
-5

(a)

Before correct
Calc.

(b)

After correct
Calc.

-10
-15
-20
-25
-30
0
-5
-10
-15
-20
-25
-30
-0.5 -0.4 -0.3 -0.2 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

Relative wavelength (nm)TE 1.55
TM 1.55
0

5

10

15

Figure 86.

Insertion loss versus waveguide length of 1.5% PLC.

Delay lines
with variable
phase shifters

Figure 6

Variable splitter
24 mm

Variable combiner
OUT
68 mm

Figure 7

16-tap transversal filter type TDC chip.

Error correction result of phase shifters.

20

Waveguide length (cm)IN

5.

Transmittance (dB)

Insertion loss (dB)

Based on the above design, we fabricated a transversal
filter type TDC. In order to reduce chip size, we adopted a
GeO2-doped silica based PLC with a high index contrastof ∆=1.5 %, a minimum bending radius of 2 mm and acore size of 5 x 5 µm2 using flame hydrolysis deposition
and reactive ion etching. On the MZI arms of the variable
couplers and delay lines, thin film heaters were depositedas thermo-optic phase shifters. Around the phase shifters,
heat-insulating grooves were formed so that we could
reduce power consumption.
Figure 6 shows the insertion loss of the high index contrast waveguide. We found that the propagation loss of
the waveguide was about 0.06 dB/cm and this value isfully applicable to a large-scale circuit like this TDC.
Figure 7 shows the 16-tap TDC chip that we fabricated.
Because of the small bending radius of the high index
contrast waveguide, we were able to use the folding
arrangement illustrated in Figure 7 and realized a small

chip size of 68.0 x 24.0 mm2.

Transmittance (dB)the case of Gl1, although CD shows little change with thenumber of taps.
From these results, we found that the suitable values for
40-Gbps systems, in which a bandwidth of 0.6 nm and a
CD range of ±100 ps/nm are required, were an FSR of
100 GHz and a tap number of 16 or larger in Gl1, and an
FSR of 150 GHz in Gl2.
However, a large number of taps means needing tocontrol a large number of variable couplers and variable
phase shifters, and this makes control more complicated.
We therefore used an 8-tap transversal filter for the first
prototype to confirm the operating principle, and after
that, we tried fabricating a 16-tap transversal filter to realize higher filter resolution.

RESULT OF MEASUREMENTS FOR
8-TAP TDC

Next, we set the tap coefficient to the values calculated for
the TDC, and measured group delay and transmittanceusing the phase shift method. All the measurements weredone using a TE polarization light around 1545 nm.
During measurement the chip temperature was kept at 60°C using a Peltier cooler in order to fix the center wavelength.
Figure 9 shows the measured spectra using Gl1 and Gl2.
Gray plots in each graph indicate calculated spectra. As
shown in Figure 9, the group delay shapes were success-Furukawa Review, No. 29

2006

16

<ε>

-0.4

TransmittanceGroup delay

<ε>

-1

TransmittanceGroup delay
<ε>
-0.3

-2

2

-0.2 -0.1
00.1
0.2
Relative wavelength (nm)fully varied as ε value and mostly corresponding to thecalculated ones.
Figure 10 shows CD vs. ε, with open plots indicating
calculated values. As can be seen, CD values of -71~
-109 ps/nm with Gl1 and -203~+121 ps/nm with Gl2 wereobserved, and showed good agreement with the calculated values. The measured values for insertion loss, 3-dB
bandwidth and operating wavelength range were about
10 dB, 0.2 nm and 0.5 nm for Gl1 and 13 dB, 0.2 nm and0.35 nm for Gl2.
120
CD (Meas.)

40

-80
-120
-3

<ε>

-0.4

TransmittanceGroup delay

-0.3

0.2

<ε>
-0.7
0
-0.2 -0.1
0.1
0.2
Relative wavelength (nm)

-10
-15
-20
-25
-30
-35
-40
-45
0.4 -50
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
0.7
-55
0.3
0.4

(b) Using Gl2 target (ε=±0.2, 0.4, 0.7)
Figure 9

Measured spectra.

1

2

3

CD (Calc.)

200
100
CD (ps/nm)

Transmittance (dB)

-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-5

0
-100
-200
-300
-1.0

Transmittance (dB)Group delay0ε(a) Using Gl1 target

Transmittance (dB)

Relative group delay (ps)Relative group delay (ps)Relative group delay (ps)Transmittance-1

CD (Meas.)

Group delay

-0.2

-2

300

Transmittance

<ε>

0
-40

(a) Using Gl1 target (ε=±0.4, 1.0, 2.0)
80
70
60
50
40
30
20
10
0
-10
-20
-30
80
70
60
50
40
30
20
10
0
-10
-20
-30
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-0.4

CD (Calc.)80

CD (ps/nm)

Group delay

Transmittance (dB)

Transmittance

-5
-10
-15
-20
-25
-30
-35
-40
-45
0.4 -50
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
1
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
0.3
0.4

Transmittance (dB)

80
70
60
50
40
30
20
10
0
-10
-20
-30
80
70
60
50
40
30
20
10
0
-10
-20
-30
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-0.4

Transmittance (dB)

Relative group delay (ps)Relative group delay (ps)Relative group delay (ps)Tunable Dispersion Compensator Using PLC Type Optical Transversal Filter

-0.5

0
ε

0.5

1.0

(b) Using Gl2 target
Figure 10 Measured CD versus ε.

From these results, we confirmed that the 8-tap transversal filter that we fabricated here works as a TDC. As
has been described, this TDC can form various dispersion
curve shapes using a single filter by changing tap coefficients, showing that it is suitable to compensate for thevarious dispersion properties in actual transmission lines.

7.

HIGH-RESOLUTION 16-TAP TDC

As described above, even if the tap coefficient is set up
precisely, an 8-tap TDC shows a narrower bandwidth andlarger ripples than required in 40-Gbps systems, because
the number of taps is insufficient to express sharp spectraat large ε values. In order to solve this problem, we tried

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Tunable Dispersion Compensator Using PLC Type Optical Transversal Filter

fabricating a 16-tap transversal filter type TDC with highresolution to realize a large bandwidth and smaller ripples
simultaneously.
Figure 11 shows the measured spectra of a 16-tap TDC
at ε=±2.0 using Gl1 target. Relative to the spectra of the
8-tap TDC in Figure 9(a), the 16-tap TDC shows a larger
bandwidth, smaller ripples and the same level of CD. As aresult, a CD of ±97 ps/nm, 3-dB bandwidth of about 0.5
nm, operating wavelength range of 0.6 nm and insertion
loss of 17 dB were obtained.
From these results, we confirmed that increasing thenumber of taps is a practical method for increasing resolution, widening bandwidth and reducing ripples.
80

-5

<ε>

70

2

-2

-10
-15

50

-20

Transmittance

40
30

-25

20

-30

10

-35

0

-40

-10

Group delay

1) Y. Painchaud, M. Lapointe, and M. Guy, “Slope-Matched Tunable
Dispersion Compensation over the Full C-Band Based on Fiber
Bragg Gratings”, ECOC2004, We3.3.4, 2004.
2) H. Ooi, K. Nakamura, Y. Akiyama, T. Takahara, T. Terahara, Y.
Kawahara, H. Isono, and G. Ishikawa, “40-Gb/s WDM Transmission
With Virtually Imaged Phased Array (VIPA) Variable Dispersion
Compensator”, J. Lightwave Technol., vol.20, no.12, pp.2196, 2002.
3) K. Takiguchi, K. Okamoto, and K. Moriwaki, “Planar Lightwave
Circuit Dispersion Equalizer”, J. Lightwave Technol., vol.14, no.9,
pp.2003, 1996.
4) K. Nara, H. Kawashima, and K. Kashihara, “Variable Dispersion
Compensator using Thermo-optic Even Functional Distributed
Phase Shifters formed on Arrayed-waveguides”, ECOC2003, We4.
P.45, 2003.
5) N. Matsubara, H. Kawashima, and K. Nara, “Low-loss optical transversal programmable filter with symmetrical arrangement of cascade variable couplers using silica-based PLC”, proc., APOC2004,
SPIE Vol. 5623-52
6) W. H. Press, et al., “Numerical Recipes in C : The Art of Scientific
Computing”.
7) H. Kawashima, N. Matsubara, and K. Nara “Tunable Dispersion
Compensator using Optical Transversal Filter in 1.5%-delta silicabased PLC”, proc., OECC2005, 7E3-4, 2005

-45

-20
-30
-40
-0.4 -0.3

Transmittance (dB)

Relative group delay (ps)60

REFERENCES-50-0.2 -0.1

0

0.1

0.2

0.3

-55
0.4

Relative wavelength (nm)Figure 11

8.

Measured spectra of 16-tap TDC using Gl1 target.

CONCLUSION

We have designed and fabricated a transversal filter type
TDC in a silica-based PLC with a high index contrast of
1.5 % for dispersion compensation in high bit rate opticaltransmission system such as 40 Gbps.
As a result, we have been able to obtain good performance: CD of -71~+109 ps/nm, insertion loss of 10 dB,
3-dB bandwidth of 0.2 nm and operating wavelength
range of 0.5 nm using a target spectrum with quadraticphase shape, and CD of -203~+121 ps/nm, insertion
loss of 13 dB, 3-dB bandwidth of 0.2 nm and operating
wavelength range of 0.35 nm using a target spectrum withsinusoidal phase shape.
We have also fabricated a 16-tap transversal filter type
TDC with higher resolution, and have been able to obtainbetter performance: CD of ±97 ps/nm, insertion loss of 17
dB, 3-dB bandwidth of 0.5 nm and operating wavelength
range of 0.5 nm.
For future practical use, we need to suppress ripples byimproving the accuracy of tap coefficient setting, toincrease resolution and pass band by increasing thenumber of taps, to compensate for polarization dependence, and to reduce insertion loss.

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