Selected Papers

A High-speed Tunable Optical Filter
Using a Semiconductor Ring Resonator
Shinji Matsuo†, Yoshitaka Ohiso, and Toru Segawa
Abstract
A high-speed tunable optical filter incorporating a semiconductor ring resonator is a key device in photonic packet switching. To achieve a wide tuning range we designed a double-ring structure, in which
two ring resonators are connected in series, and fabricated it using the InGaAsP-InP material system.
This device exhibits a total free spectral range (FSR) of 1.7 THz and contrast ratio of 9.5 dB. The ringradii are 25.2 and 17.8 µm, which correspond to FSRs of 340 and 425 GHz, respectively. The switchingtime of the device is 2.5 ns.

1. Introduction
The wavelength division multiplexing (WDM)
transport network has emerged as the best way tomake the large-capacity networks needed for futureInternet traffic. In such a network, photonic packet
switches are expected to offer high-throughput
switching because they are based on optical technologies for long-haul transmission. Several photonic
packet switches using WDM technologies have been
proposed. In many of the systems using them, a highspeed tunable optical filter is a key device because
optical packets with different wavelengths must be
selected packet by packet [1], [2]. In those reports, thefilters were fabricated by integrating the arrayedwaveguide grating (AWG) filters and semiconductor
optical amplifier (SOA) gate switches. Although they
exhibited low insertion loss, low crosstalk, and highspeed-switching, they seem likely to be expensive
because they require many components.
Optical ring resonators are very useful for opticalfilters, and it should be possible to fabricate them at alow cost because of their simple structure. Recently,
semiconductor ring or microdisk resonators with afree spectral range (FSR) of more than 20 nm havebeen demonstrated for GaAs-AlGaAs and InGaAsPInP material systems [3], [4]. However, when current† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: mash@aecl.ntt.co.jp

50

injection is used for high-speed tuning of the resonant
wavelength, the tuning range is limited to a few
nanometers to prevent thermal effects. To overcome
this problem, we used a double-ring structure inwhich two single-ring resonators with different FSRs
are connected in series: we call this a double-ring resonator. In this paper, we describe a large-capacity
photonic packet switch using a tunable optical filter
and the fabrication of a high-speed tunable optical filter using the ring resonator.
2. Large-capacity photonic packet switch using
WDM technology
2.1 Configuration of the large-capacity switch
using the photonic switch
The most important problems to be solved are thelimited scalability of the optical buffer and the competition control of the packets because while light canbe switched quickly, it is hard to buffer. Furthermore,when some functions are implemented optically, they
will need complicated optical circuits because thelogic functions are poor. To overcome these problems, one group designed a high-throughput switch
using the features of both photonic and electricalswitches efficiently [5]. The required number of optical buffers could be reduced by complementing optical buffers with electrical buffers. However, a fiber
delay line used as an optical buffer was required forevery output port because the optical buffer was connected to the latter part of the optical switching block.
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Thus, many fiber delay lines were neededin the switching fabric.
In this paper, we describe a photonic
packet switch using the high-speed tunable optical filter with fewer fiber delaylines. Figure 1 shows a schematic diagram of the switch. It consists of the photonic packet switch, consisting of the
optical buffer and the optical switch, andmany electrical routers. The photonic
packet switch switches the input packets
to corresponding electrical routers.

Photonic packet switch

Optical buffer

2.2 Configuration of the photonic
packet switch
Figure 2 shows the configuration of thephotonic packet switch. The opticalbuffer consists of optical couplers, fiber
delay lines, fiber amplifiers, and highspeed wavelength selectors. The fiber delay lines
have different lengths to achieve different buffering
times. The optical switch consists of an optical coupler and several high-speed tunable optical filters.
An input packet is converted to the wavelength forthat input port at a wavelength converter. The optical
coupler divides the optical power of input packets
entering the fiber delay lines. Since each wavelength
Optical buffer

Optical switch

Electrical routerFig. 1. Schematic diagram of the switch.

converter changes the input packet to a different
wavelength, the input packets are multiplexed without wavelength collision at the optical coupler. Therefore, all input ports can share one fiber delay line.
This means that the optical buffer has just one set offiber delay lines for the whole photonic packet
switch. The wavelength selector is connected to the
output port of each fiber delay line. It must selectOptical switch

Controller

0T

L
3T
4T
λ1

Fiber
amplifiersλnWavelength
converter

Opticalcoupler

Photodetector

Fiber delay line

Wavelength selector(AWG + SOA)

High-speed
tunable filter

Fig. 2. Configuration of the photonic packet switch.

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many wavelengths simultaneously from the multiplexed input packets because the required delay timemust be set independently for each input packet. For
this reason, we considered using the combination of
AWG filters and SOA gates as a wavelength selector[6]. In this optical buffer, the packets can be selectedwith any desired delay time (i.e., random accessmemory). This avoids head of line blocking, whichcauses a problem with input-buffer-type switches. In
this optical switch, packets that have the same wavelength and different delay time are not output to theoptical switch part in the same time slot, so one input
port transmits one packet in one time slot to the optical switch part. Thus, the packets selected by the
wavelength selectors are distributed to all output
ports by the optical coupler. Therefore, this opticalswitch part operates as a non-blocking broadcastand-select optical switch, in which the input packets
are filtered by the high-speed wavelength tunable filter placed in front of each output port.
2.3 Effect of speeding up photonic packet
switching
We estimated the packet-loss probability characteristics of the optical buffer part for various numbers Lof tunable filters and photodetectors for one output
port. Figure 3 shows the calculation results. Note that
although the dividing number of the optical couplerin the optical switch part is multiplexed by L, the opti100
p=0.8
N=∞
Traveling-type

10–2

Packet-loss probability

3.1 Operating principle
Optical ring resonators are very useful for opticalfilters and it should be possible to fabricate them at
low cost because of their simple structure. In a ring
resonator, the resonant wavelength can be controlled
by controlling the refractive index of the ring waveguide. When the ring waveguide’s length is l and its
effective refractive index is neff, the resonant wavelength λ0 is given byneff l
,
m

FSR =

10–6

L=3

34 buffers
for PLP <10–12

L=4

∆λ =

5

10
15
Number of optical buffersc0,
neff l(2)where c0 is the speed of the light in vacuum. From (1),the change in resonant wavelength ∆λ is given by

10–8

0

(1)

where m is an integer. The FSR is given by

10–4

10–10

20

Fig. 3. Packet-loss probability characteristics of the
optical buffer part for various numbers of tunablefilters and photodetectors for one output port.

52

3. Device structure and fabrication of tunablefilter using the ring resonator

λ0 =

L=2

10–12

cal buffer part does not change. We assumed that theload p was 0.8, the switch size N was infinite, andpacket arrivals were random. We also assumed thatthe optical buffer was a first-in first-out (FIFO) buffer
to simplify the calculation. To obtain a packet loss
probability of less than 10–12, we need 10 opticalbuffers when L is 3. As shown in this figure, speedingup the optical switch by increasing L has the great
advantage of reducing the number of optical buffers.
As explained before, since our optical buffer is a random access memory, the required number of buffers
is small even if we assume traffic is bursty.
The problem with this solution is, of course, theincrease in the number of tunable filters and photodetectors. Thus, it is very important to make a low-costhigh-speed tunable optical filter. Our high-speed
wavelength tunable filter using a semiconductor ring
resonator will reduce the cost.

∆neff l,m(3)where ∆neff is the change in effective refractive index
in the ring waveguide. From (1) and (3), we obtain∆λ ∆neff
=
.
neff
λ0

(4)

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Equation (4) shows that the wavelength change of thering resonator is independent of the FSR, l, and m anddepends on the effective refractive index change.
In the photonic packet switch, the switching time of
the tunable filter must be less than a few nanoseconds. We can achieve this by controlling the effective
refractive index change using the applied voltage or
injection current on a semiconductor. The effective
refractive index change produced by the applied voltage is as small as 0.1%, although it does provide a fastswitching time of less than 100 ps. Therefore, wechose the current injection effect in semiconductors,
although its refractive-index change is still only
0.3%. However, when the current injection isincreased, heat is generated. This causes the refractive index to change on the order of milliseconds, so
the selected wavelength (i.e., the resonant wavelength
set by the current injection) changes gradually. Thus,
the wavelength tunable range of one ring resonatormust be restricted to a few nanometers to avoid thisthermal effect.

These selected WDM signals are selected again by
R2. Thus, the resonant output light, which passes
through the double-ring resonator, is only obtained
when the resonant frequency of the two single-ringresonators is the same. Thus, the double-ring resonator has a wide tuning range without increasing thetuning range of single resonator. The FSR of double
ring resonator is given by
FSR = N·FSR1= M·FSR2,

3.3 Calculation of frequency responses
Figure 5 shows the calculated frequency responses.
We assumed that the coupling coefficient of the optical coupler, κ, was 0.29, the propagation loss of thering waveguide was 20 dB/cm, and the FSRs of thering resonators were 340 and 425 GHz. Thus, N and
M in Eq. (5) are 5 and 4. The dashed lines are the calculated optical frequency responses of the single ring
resonators. The output from each resonator is a maximum at the resonant frequency. The solid line is thecalculated optical frequency response of the doublering resonator. As shown in this figure, resonantpeaks were observed at 0 and 1700 GHz. Spurious
resonant peaks, corresponding to the resonant wavelength of either R1 or R2, were also observed. We

3.2 Structure of double ring resonator
To overcome these problems, we use the double
ring structure [7], [8]. Figure 4 is a schematic of ourdouble ring resonator. The filter consists of two ring
resonators R1 and R2, each with different FSRs, toachieve a wide tuning range. Each ring resonator is
coupled with the straight waveguide via an optical
coupler. In this device, input WDM signals, whichhave resonant frequencies of ring resonator R1, passthrough R1 and are input to the ring resonator R2.

0
R1

OutputR2–10Transmittance (dB)

λ7

(5)

where N and M are natural and co-prime numbers and
FSR1 and FSR2 are the FSRs of R1 and R2 , respectively.

R2λ2, λ7, λ12 • • •

–20

–30

–40

–50
R1
–60

0

500

1000

1500

2000

Relative optical frequency (GHz)

Input

λ1-λn

Optical coupler

Fig. 4. Schematic of the double ring resonator.

Vol. 1 No. 7 Oct. 2003

Fig. 5. Calculated frequency responses. Dashed lines
are the calculated optical frequency responses ofthe single ring resonators. The solid line is thecalculated optical frequency response of the
double-ring resonator.

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Selected Papers

define the contrast ratio as the power difference
between the resonant and spurious resonant peaks. In
this calculation, the loss of the device was 7.3 dB andthe contrast ratio was 18.6 dB.
3.4 Wavelength tuning
When current is injected into R1 in Fig. 4, the effective refractive index of the waveguide is decreased.
Thus, the resonant wavelength of R1 decreases andthe resonant frequency correspondingly increases. As
a result, the transmission spectrum of R1 in Fig. 5
shifts to the right. The transmissivity of the double
ring resonator is the product of the transmissivity ofeach ring resonator. Thus, the next large transmissivity appeared at 425 GHz in relative optical frequency.
In the same way, when the current injection to R1 isincreased, a large resonant peak appears at 850 and
1275 GHz. On the other hand, increasing the injection current to R2 produces a large resonant peak at
1360, 1020, 680, and 340 GHz. By changing therefractive indices of both ring resonators simultaneously, we can obtain any desired resonant frequency.
3.5 Device design
The loss and crosstalk of the device are very important in determining the device performance. Figure 6
shows the transmission spectrum near the resonancefrequency as a function of the coupling coefficient of
the optical coupler. As the coupling coefficientincreased, the transmissivity and 3-dB bandwidth
increased while the contrast ratio decreased. Thus
controlling the coupling coefficient of the optical

coupler is very important because the contrast ratio
and loss of the ring resonator strongly depend on it.
A suitable way to control the coupling coefficient is
to use a directional coupler. However, to obtain awide tuning range, the FSR of a ring resonator mustbe large. For example, when we assumed that the
maximum effective index change was 0.3%, thewavelength shift was 4.65 nm, which corresponds toan optical frequency range of 580 GHz. In this case,the total waveguide length was about 144 µm and thediameter of the ring waveguide was less than 45.7 µm
even if we assumed that the length of optical coupler
was zero. Thus, we used a high-mesa waveguidestructure to reduce the radiation loss. In this case, thedirectional coupler needs a small gap of less than 0.1µm between two high-mesa waveguides to obtain ashort optical coupler. The processing to make a 0.1µm gap, by electron beam lithography and dry etching, has low productivity and reproducibility, so we
decided to use a multimode interference (MMI) coupler.
However, controlling the coupling coefficient of the
MMI coupler is difficult for a simple small structure.
To overcome this problem, we etched a groove in thecenter of the MMI coupler [9]. Figure 7 shows thecoupling coefficient calculated by the beam propagation method (BPM) as a function of the groove depth.
For this calculation, we assumed a waveguide consisting of a 0.4-µm-thick InGaAsP (1.3Q) layer sandwiched between the 1.5-µm-thick InP layers. The
waveguide and groove widths were 1.8 and 0.8 µm,
respectively. The etching depth is defined as 0 µm foretching through the top of the InGaAsP core layer.

0
0.8
–10

w= 1.8 µm
Core thickness=0.4 µm

0.6
Coupling coefficient, κTransmittance (dB)

0.7
–20
–30
–40
–50

κ = 0.4

0.3

–60

0.5

∆x=0

0.4
0.3

Cross-sectional view

0.2

0.2
–70
1200

0.1
1400
1600
1800
Relative optical frequency (GHz)2000

Fig. 6. Transmission spectrum near the resonancefrequency as a function of coupling coefficient of
the optical coupler.

54

0
–0.1

0

0.1
0.2
Groove depth, ∆x (µm)

0.3

0.4

Fig. 7. Calculated coupling coefficient as a function of thegroove depth.

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The coupling coefficient decreases with increasing
groove depth, indicating that the coupling coefficient
can be controlled by adjusting the groove depth.
4. Device fabrication and characteristics
Figure 8(a) shows a photograph of the fabricateddouble-ring resonator, and Fig. 8(b) is a scanningelectron microscope image of the ring resonator taken
after the dry-etching process. The epitaxial wafer was
grown by metalorganic chemical vapor deposition on
an n-doped InP substrate. It consisted of a p+-InGaAs
contact layer, a 1.2-µm-thick p-InP cladding layer(doping concentration: 2×1017 cm–3), a 0.4-µm-thick
InGaAsP layer (1.3Q), and a 0.5-µm-thick n-InP
buffer layer. The high-mesa waveguide structure wasformed by Br2-N2 reactive beam etching [10]. For
current injection, Ni/Zn/Au electrodes were deposited on the p+-InGaAs contact layer. The waveguide
width and height were 1.8 and 3.5 µm, respectively.
The radii of the ring resonators were 25.2 and 17.8
µm, which correspond to FSRs of 340 and 425 GHz,
respectively. The device size was 500 × 500 µm2
including the bonding pads.
Figure 9 shows the optical frequency response of
the double-ring resonator for transverse electric (TE)polarization. The light source was a tunable laser
diode and the current in R1 was set to 1.3 mA. Resonant peaks were observed at 1541.8 and 1555.3 nm.
Thus, the total FSR of the double-ring resonator was

13.5 nm, which is approximately 1700 GHz in thefrequency domain. Spurious resonant peaks, corresponding to the resonant wavelength of either resonator R1 or R2, were also observed. The crosstalk
between the resonant and spurious resonant peaks
was 9.5 dB, as determined from the crosstalk of eachsingle-ring resonator.
Figure 10 shows the tuning characteristics of the
double-ring resonator. We changed the injection current of R1 from 0 to 6.3 mA while keeping that of R2
constant at 0 mA. The refractive index of the ring
waveguide in R1 decreased with increasing injection
current, so the resonant wavelength of R1 gradually
decreased. As described before, resonant output fromthe double-ring resonator occurs where both rings
have a common resonant wavelength. Thus, resonant
outputs were obtained for currents of 0, 1.3, 3.3, and6.3 mA and they appear every 425 GHz. This is consistent with the FSR of R2.
The dynamic response of the ring resonator isshown in Fig. 11. In this experiment, the device wasa single-ring resonator. The current was driven
between 0 and 1.8 mA at intervals of 400 ns. In Fig.
11(a), the wavelength of the tunable laser diode was1550.1 nm, which is the resonant wavelength for
injection current of 0 mA. In Fig. 11(b), the wavelength of the tunable laser diode was 1549.1 nm,
which is the resonant wavelength for injection currentof 1.8 mA. As these figures show, fast and stableswitching was achieved by current injection. The

OutputR2

R=17.8 µm

R1

Input

R=25.2 µm

MMI

(a)

(b)

Fig. 8. (a) Photograph of the fabricated double-ring resonator and (b) SEM image of the ring resonator taken after the
dry-etching process.

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switching time was 2.5 ns.
Normalized output power (dB)

IL=1.3 mA
0

5. Discussion–5

The characteristics of the fabricated device were
not as good as the calculated values. As Fig. 12shows, the calculated transmissivity at the resonant
wavelength and the contrast ratio both decreased withincreasing MMI coupler loss. When the loss was 1.0
dB, they were –27.0 and 10.4 dB, respectively
because the light in the ring resonator passed throughthe MMI coupler until it reached the output port.
Thus, it is important to reduce the MMI coupler loss.
In the BPM calculation, the MMI coupler loss wasestimated to be as low as 0.2 dB. However, in the fabricated device, the etching width of the center of MMI
coupler was increased and this changed the optimal
length of the MMI coupler, resulting in higher MMI
coupler loss. It should be possible to reduce the MMI
coupler loss by optimizing.
Another problem is obtaining a low-cost tunablefilter. In general, the semiconductor WDM devices
require precise temperature control, which is expensive. An arrayed device, consisting of many tunableoptical filters and photodetectors, could solve thisproblem.

–10

–15

–20

–25
1540

1544

1548
1552
Wavelength (nm)1556

Fig. 9. Optical frequency response of the double-ringresonator for TE polarization.

IL(mA)=6.3

Normalized output power (dB)

0

3.3

1.3

0

–2
–4
–6
–8

6. Conclusion

–10
–12
1545

1550
1555
Wavelength (nm)1560

We have demonstrated a high-speed tunable opticalfilter using a semiconductor double-ring resonator.
The device shows total FSR of 13.5 nm and contrast

Fig. 10. Tuning characteristics of the double-ringresonator.

20

0
–5

0
∆I=1.8 mA

1549.1 nm

–25
5

–35
–40

0.5

0

500

1000
Time (ns)

1500

2000

Fig. 11. Dynamic response of the ring resonator.

56

10

–20

–30

1.0

0

–15

Contrast ratio (dB)

Output power (a.u.)

0.5

(b)

15

–10

∆I=1.8 mA

1550.1 nm

Transmittance (dB)

(a)
1.0

0

0.5
1.0
1.5
Loss of MMI coupler (dB)0
2.0

Fig. 12. Calculated transmissivity at the resonant
wavelength and contrast ratio as a function of
the loss of MMI coupler.

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ratio of 9.5 dB between desired and spurious resonantpeaks. Its switching time is 2.5 ns. This is the firsttime a high-speed tunable optical filter has been fabricated using semiconductor ring resonators.
Acknowledgments
We thank Junji Yumoto and Chikara Amano forstimulating discussions, and Kazuaki Ishibashi and
Toru Kodaira for device fabrication and measurements.

Shinji Matsuo
Senior Research Engineer, Advanced Optoelectronics Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Hiroshima University,
Hiroshima in 1986 and 1988, respectively. In
1988, he joined NTT Opto-electronics Laboratories, Atsugi, where he was engaged in researchon photonic functional devices using MQW-pin
modulators and VCSELs. In 1997, he researched
optical networks using WDM technologies at
NTT Network Innovation Laboratories, Yokosuka. Since 2000, he has been researching a highspeed tunable optical filter and laser for photonic
packet switching at NTT Photonics Laboratories,
Atsugi. He is a member of Japan Society ofApplied Physics and the IEEE Lasers and Electro-Optics Society.

References[1]

A. Misawa, K. Sasayama, and Y. Yamada, “WDM knockout switch
with multi-output-port wavelength-channel selectors,” J. Lightwave
Technol., Vol. 16, No. 12 , pp. 2212-2219, 1998.
[2] C. Guillemot, M. Renaud, P. Gambini, C. Janz, I. Andonovic, R.
Bauknecht, B. Bostica, M. Burzio, F. Callegati, M. Casoni, D. Chiaroni, F. Clerot, S. L. Danielsen, F. Dorgeuille, A. Dupas, A. Franzen,
P. B. Hansen, D. K. Hunter, and A. Kloch, “Transparent optical packet switching: the European ACTS KEOPS project approach,” J.
Lightwave Technol., Vol. 16, No. 12, pp. 2117-2134, 1998.
[3] M. K. Chin, C. Youtsey, W. Zhao, T. Pierson, Z. Ren, S. L. Wu, L.
Wang, Y. G. Zhao, and S. T. Ho, “GaAs Microcavity Channel-Dropping Filter Based on a Race-Track Resonator,” IEEE Photon. Technol. Lett., Vol. 11, No. 2, pp. 1620-1622, 1999.
[4] Y. Ma, S. H. Chang, S.S. Chang, and S. T. Ho, “Improved optical filter responses in cascaded InGaAsP/InP microdisk resonators,” Electron. Lett., Vol. 37, No. 9, pp. 564-565, 2001.
[5] T. Okugawa, T. Matsunaga, and K. Habara, “Composite optical/electrical buffer configuration for photonic ATM switching systems,”
Electron. Lett., Vol. 33, No. 16, pp.1398-1400, 1997.
[6] H. Ishii, M. Kohtoku, Y. Shibata, S. Oku, Y. Kadota, Y. Yoshikuni,
H. Sanjoh, Y. Kondo, and K. Kishi, “Zero insertion loss operations inmonolithically integrated WDM channel selectors,” IEEE Photon.
Technol. Lett., Vol. 11, No. 2, pp. 242-244, 1999.
[7] S. Suzuki, K. Oda, and Y. Hibino, “Integrated-Optic Double-Ring
Resonators with Wide Free Spectral Range of 100 GHz,” J. Lightwave Technol., Vol. 13, No. 8, pp. 1766-1771, 1995.
[8] S. Matsuo, Y. Ohiso, K. Tateno, T. Segawa, M. Kohtoku, and S. Oku,
“A high-speed tunable optical filter using a semiconductor doublering resonator,” IPRM 2002, pp. 467-470, 2002.
[9] G. Griffel, J. H. Abeles, R. J. Menna, A. M. Braun, J. C. Connolly, andM. King, “Low-Threshold InGaAsP Ring Lasers Fabricated Using
Bi-Level Dry Etching,” IEEE Photon. Technol. Lett., Vol. 12, No. 2,
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[10] S. Oku, and Y. Shibata, “Semiconductor photonic waveguide structures made by a reactive beam etching technique,” IPRM 2001., pp.
75-78, 2001.

Vol. 1 No. 7 Oct. 2003

Yoshitaka Ohiso
Senior Research Engineer, Advanced Optoelectronics Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Keio University, Yokohama in 1989 and 1991, respectively. Since joining the NTT Laboratories, Kanagawa Japan in
1991, he has been engaged in the research anddevelopment of 0.85-µm bottom-emitting
VCSELs on an AlGaAs substrate. His current
research interests include MOCVD epitaxial
growth and device fabrication process techniques
for long wavelength VCSELs. He is a member of
JSAP and the Institute of Electronics, Information and Communication Engineers of Japan.

Toru Segawa
Researcher, Advanced Opto-electronics Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Keio University, Yokohama in 1999 and 2001, respectively. In 2001, hejoined NTT Photonics Laboratories and has beenworking on a high-speed tunable optical filter forphotonic packet switching.

57