Ultra-Compact High Order Ring Resonator Filters Using Submicron Silicon Photonic Wires For On-Chip Optical Interconnects(1).pdf

(465 KB) Pobierz
Ultra-compact high order ring resonator filters
using submicron silicon photonic wires for on-
chip optical
interconnects
Fengnian Xia
*
, Mike Rooks, Lidija Sekaric, and Yurii Vlasov
IBM Thomas J. Watson Research center
Yorktown Heights, NY 10598
*
fxia@us.ibm.com
www.research.ibm.com/photonics
Abstract:
Ultra-compact 5
th
order ring resonator optical filters based on
submicron silicon photonic wires are demonstrated. Out-of-band rejection
ratio of 40dB, 1dB flat-top pass band of 310GHz with ripples smaller than
0.4dB, and insertion loss of only (1.8±0.5)dB at the center of the pass band
are realized simultaneously, all within a footprint of 0.0007mm
2
on a silicon
chip.
©2007
Optical Society of America
OCIS codes:
(130.3120) Integrated optics devices
References and links
1.
B. E. Little, J. S. Foresi, G. Steinmeyer, E. R. Thoen, S. T. Chu, H. A. Haus, E. P. Ippen, L. C. Kimerling,
and W. Greene, “Ultra-compact Si/SiO
2
microring resonator optical channel dropping filters,” IEEE
Photon. Technol. Lett.
10,
549–551 (1998).
T. Barwicz, M. A. Popovi , M. R. Watts, P. T. Rakich, E. P. Ippen, and H. I. Smith, “ Fabrication of add-
drop filters based on frequency-matched microring resonators,” J. Lightwave Technol.
24,
2207–2218
(2006).
F. Xia, M. Rooks, L. Sekaric, and Y. A. Vlasov, “Ultra-compact silicon WDM optical filters with flat-top
response for on-chip optical interconnects,” in Conference on Lasers and Electro-Optics 2007 Technical
Digest (Optical Society of America, Washington, DC, 2007), paper CTuG3.
J. K. S. Poon, L. Zhu, G. A. DeRose, and A. Yariv, “Transmission and group delay of microring coupled-
resonator optical waveguides,” Opt. Lett.
31,
456-458 (2006).
B. E. Little, S. T. Chu, P. P. Absil, J. V. Hryniewicz, F. G. Johnson, F. Seiferth, D. Gill, V. Van, O. King,
and M. Trakalo, “Very high-order microring resonator filters for WDM applications,” IEEE Photon.
Technol. Lett.
16,
2263–2265 (2004).
J. V. Hryniewicz, P. P. Absil, B. E. Little, R. A. Wilson, and P. –T. Ho, “High order filter response in
coupled microring resonators,” IEEE Photon. Technol. Lett.
12,
320-322 (2000).
Y. A. Vlasov and S. J. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,”
Opt. Express 12, 1622-1631 (2004).
F. Xia, L. Sekaric, and Yu. A. Vlasov, “Ultra-compact optical buffers on a silicon chip,” Nature Photon.
1,
65-71 (2007).
A. Yariv, Y. Xu, R. K. Lee, and A. Scherer, “Coupled-resonator optical waveguide: a proposal and
analysis,” Opt. Lett.
24,
711-713 (1999).
Y. Ye, J. Ding, D. Y. Jeong, I. C. Khoo and Q. M. Zhang, “Finite-size effect on one dimensional coupled
resonator optical waveguides,” Phys. Rev. E
69,
056604 (2004).
J. K. S. Poon, J. Scheuer, S. Mookherjea, G. T. Paloczi, Y. Huang, and A. Yariv, “Matrix
analysis of
microring coupled-resonator optical waveguides,”
Opt. Express
12,
90-103 (2004).
F. Xia, L. Sekaric, M. O’Boyle, and Y. A. Vlasov, “Coupled resonator optical waveguides based on silicon-
on-insulator photonic wires,” Appl. Phys. Lett.
89,
041122 (2006).
F. Xia, L. Sekaric, and Y. A. Vlasov, “Mode conversion losses in silicon-on-insulator photonic wire based
racetrack resonators,” Opt. Express
14,
3872-3886 (2006).
E. Dulkeith, F. Xia, L. Schares, W. M. Green, Y. A. Vlasov, “Group index and group velocity dispersion in
silicon-on-insulator photonic wires,” Opt. Express
14,
3853-3863 (2006).
C. Madsen and J. Zhao,
Optical filter design and analysis: A signal processing approach
(New York:
Wiley-interscience, 1999), Chap. 5.
ć
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
#85355 - $15.00 USD
Received 17 Jul 2007; revised 29 Aug 2007; accepted 30 Aug 2007; published 5 Sep 2007
(C) 2007 OSA
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 11934
16.
17.
18.
19.
B. E. Little, S. T. Chu, J. V. Hryniewicz, and P. P. Absil, “Filter synthesis for periodically coupled
microring resonators,” Opt. Lett.
25,
344–346 (2000).
S. L. Chuang,
Physics of Optoelectronic Devices
(New York: Wiley-interscience, 1995), Chap. 8.
M. Popovic, C. Manolatou, and M. Watts, "Coupling-induced resonance frequency shifts in coupled
dielectric multi-cavity filters," Opt. Express
14,
1208-1222 (2006).
T. Barwicz, M. Popovi , P. Rakich, M. Watts, H. Haus, E. Ippen, and H. Smith, "Microring-resonator-
based add-drop filters in SiN: fabrication and analysis," Opt. Express
12,
1437-1442 (2004).
ć
1. Introduction
Optical filters based on high order ring resonators attract a lot of attention due to their
applications in optical signal processing and routing in various optical communication and
interconnect systems [1-4]. One of the important applications of the optical filters is in
wavelength division multiplexing (WDM) systems [5]. In such high order optical filters, light
can resonantly tunnel through a series of coupled resonators if its frequency is in tune within
the pass band of the system, and can be almost completely rejected if its frequency is tuned
out of the pass band, leading to very large out-of-band rejection ratio [4-5]. On the other hand,
the width of the pass band of such filters can be adjusted by tuning the inter-resonator
coupling strengths. Most previous demonstrations in high order ring resonator based optical
filters are realized in silicon oxynitride [5], silicon nitride [2], or polymer systems [4] in which
low-loss and ultra-compact footprints are difficult to realize simultaneously due to the limited
refractive index contrasts. Ultra-compact high order optical filters based on high refractive
index contrast waveguide in GaAs/AlGaAs material system have been demonstrated
previously [6]. However, very deep trench etch (~2µm) is necessary to prevent substrate
leakage and losses reported are much higher than those of silicon waveguides. In this paper,
we report such high order optical filters based on submicron silicon photonic wire
waveguides. Due to extremely high index contrast between silicon and air (oxide) and tight
confinement of the light, low loss micron scale waveguide bends can be realized [7]. Hence,
ultra-compact and low-loss micro-scale ring resonators based on such waveguide bends can
be realized [8]. Using 5 coupled silicon ring resonators with R=4µm radius, 1dB flat-top pass
band of 310GHz with intensity ripples smaller than 0.4dB, out-of-band rejection ratio of
40dB, and transmission loss of only (1.8±0.5)dB at the center of the pass band are realized
simultaneously, all within a footprint of 0.0007mm
2
. Additional CMOS (complementary
metal-oxide-semiconductor) compatibility in material system and fabrication processes makes
such filter an ideal candidate for applications in on-chip optical interconnects.
2. Design
Our design goals of high order optical filters for on-chip interconnect applications are: (i)
wide (>300GHz) and flat (ripples <0.5dB) pass band that allows for accommodating large
optical signal bandwidth and temperature variations in on-chip environment. (ii) the flat-top
pass band should occupy approximately one seventh of the filter free spectral range (FSR)
hence WDM devices with 5 to 7 channels can be constructed using such optical filters with
detuned central wavelengths. (iii) high out-of-band rejection ratio (>30dB) and 1 to 20dB
shape factor [5] approaching unity hence low crosstalk between different WDM channels can
be achieved. (iv) small resonator perimeter hence the filter footprint can be small. On the
other hand, small resonator perimeter leads to large FSR and pass band if the pass band
occupies certain portion of the FSR.
Design of optical filters based on ring resonators consists of two steps: choosing proper
inter-resonator coupling coefficients for given number of resonators and determining physical
dimensions. Analysis method based on tight binding model [9] proposed by Yariv
et al.
predicts a flat pass band for coupled optical systems with infinite number of resonators.
However, in this theory, the existence of Bloch mode in such infinitely long coupled system is
presumed. In realistic coupled system with finite resonators, usually simple strip input/output
#85355 - $15.00 USD
Received 17 Jul 2007; revised 29 Aug 2007; accepted 30 Aug 2007; published 5 Sep 2007
(C) 2007 OSA
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 11935
waveguides are located on both sides of the coupled resonator system, and reflections at strip
waveguide/coupled system interfaces lead to ripples in the pass band even when there are 100
coupled resonators between input and output strip waveguides [8]. In order to achieve flat-top
pass band, the inter-resonator coupling coefficients should be tapered to minimize such
reflections [3, 5, and 10]. Based on this concept of tapering inter-resonator coupling
coefficients, simulations were performed using matrix approach reported previously [11-12].
Fig. 1. Simulated responses of optical filters with 3 and 5 coupled ring resonators. The
responses as functions of both FSR (for ring resonators with arbitrary size, in bottom x-axis)
and absolute wavelength detuning (for ring resonators with properly designed physical
dimensions in section 2, in top x-axis) are shown. Inset: schematic drawings of filters
comprised of 3 and 5 ring resonators.
The objective in the first design step is to achieve flat pass band and desirable pass band
width simultaneously by adjusting inter-ring coupling strengths. In this step, a group index of
4.25 was assumed for such silicon photonic wires as reported in Refs. [13-14]. No physical
dimensions about the resonator were assumed in simulation and hence the frequency response
is normalized to the free-spectral range (FSR) as shown in bottom x-axis of Fig. 1. No loss
mechanisms were introduced in simulation either. Since losses in silicon resonator are
extremely small (0.035dB per round trip) [7, 8], the design of filters is hardly affected by
introduction of such losses. On the other hand, in order to fit 5 to 7 WDM channels using such
optical filters within one free-spectral range (FSR) and at the same time maximizing the pass
band of each filter, we designed the coupling strengths in such a way that flat-top 1dB pass
band of the filter occupies about one-seventh of the FSR. For optical filter with 5 coupled
rings, the inter-ring power coupling coefficient [4, 11],
κ
2
(κ itself defined as the field
coupling coefficient), from left to right (including bus-ring coupling as shown in the inset of
Fig. 1), are designed to be 0.45, 0.09, 0.05, 0.05, 0.09, and 0.45, respectively [15, 16]. For 3-
ring optical filter, the inter-ring power coupling coefficients,
κ
2
, from left to right, are 0.45,
0.09, 0.09, and 0.45, respectively. From the simulation results shown in Fig. 1, the ripples in
the pass band of the 5-ring filter are smaller than ±0.15dB and the out-of-band rejection ratio
is larger than 40dB. 5-resonator filter exhibits sharper rising and falling edges (1- to 20dB
shape factor of around 0.7) in transmission spectra than that of 3-resonator filter (with 1- to
#85355 - $15.00 USD
Received 17 Jul 2007; revised 29 Aug 2007; accepted 30 Aug 2007; published 5 Sep 2007
(C) 2007 OSA
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 11936
20dB shape factor of around 0.35), leading to lower crosstalk level between channels in
WDM system.
Fig. 2. Simulated power beating length, L
B
, between two parallel photonic wires as a function
of air gap distance between them. L
B
represents a length within which optical power transfers
completely from one waveguide to another and is calculated through the index difference
between the even (n
E
) and odd (n
O
) modes. The indices are calculated using a commercial
FimmWave software package, version 4.3.4.
After achieving design goals (ii) and (iii) in first design step using 5 coupled resonators
with proper coupling strength, the second step is to achieve design goals (i) and (iv) by
choosing proper resonator physical dimensions. In fact, given the fixed inter-resonator
coupling coefficients in such optical filters, goals (i) and (iv) are correlated. Since the pass
band of the optical filters with given inter-resonator coupling coefficients occupies certain
fraction of the FSR and the FSR is inversely proportional to the perimeter of the resonator
[13], L
P
, the pass band itself is inversely proportional L
P
. Hence, minimizing the perimeter of
the resonator not only reduces the footprint of the optical filter, but also maximizes the pass
band of the optical filters. The second step in this design process is then determining the
physical dimension of the resonators so that required coupling coefficients, small footprint,
and large pass band can be simultaneously achieved. In this step, there are three parameters to
be determined: bending radius in the ring (r), straight coupling length in the ring (L
C
), and air
gap widths between rings [13]. Bending radius of 4µm is chosen such that a balance between
the footprint and loss is achieved [7]. Desirable straight coupling length L
C
can be inferred
from Fig. 2 in which the calculated power beating length, L
B
, between two parallel photonic
wires as a function of air gap distance between them is plotted. Physically, L
B
is the length
within which optical power transfers completely from one waveguide to another. L
B
can be
calculated using the following simple relation [17]:
λ
(1)
L
=
B
2(
n
E
−
n
O
)
where
λ
is the wavelength of the light (here a wavelength of 1550nm is used), n
E
and n
O
are
the effective indices of the fundamental (even) and first order (odd) modes of the two coupled
parallel photonic wires at a wavelength of 1550nm. Here, n
E
and n
O
are calculated based on 3-
D full vectorial mode matching method using commercial FimmWave software.
If the coupling in the bending region of the resonators is ignored due to the small
waveguide bending radius (4µm), the power coupling coefficient,
κ
2
, is [17]:
π
L
(2)
κ
2
=
sin
2
(
•
C
)
2
L
B
#85355 - $15.00 USD
Received 17 Jul 2007; revised 29 Aug 2007; accepted 30 Aug 2007; published 5 Sep 2007
(C) 2007 OSA
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 11937
As can be inferred from Fig. 2 and Eq. (2), in order to achieve a power coupling
coefficient of 0.45, the straight coupling length has to be as large as 12µm if an air gap
spacing of about 100nm is adopted (L
B
is around 24.5µm in this case). In this case, the
perimeter of each ring resonator, L
P
, is:
(3)
L
P
=
2
π
r
+
2
L
C
The perimeter of each ring will be as large as 49µm, leading to a free spectral range of
around 11nm. Then 1dB pass band is only about 1.5nm (187GHz), which is much smaller
than our design goal (i). In future on-chip optical interconnects, it is desirable to have optical
filters operational without active temperature control, hence a wide pass band is essential and
rings with smaller perimeter are needed. One straightforward method to reduce the coupling
length, L
C
, and hence the perimeter of the resonator is to choose smaller air gap spacing. For
example, the coupling length L
C
can be reduced significantly to around 3µm if a gap width of
20nm is used. However, there are a few issues associated with this simple approach. First, this
introduces certain difficulties in device fabrication. Second, from Eq. (2), the variation of the
coupling coefficient as a function of beating length, L
B
, is:
π
L
1
(4)
sin(
π
•
C
)
2
• Δ
L
B
Δ
(
κ
2
)
=
2
L
C
L
B
L
B
Hence, the coupling is very sensitive to the beating length variation when beating length
itself is small. On the other hand, the beating length, L
B
, almost varies linearly as a function of
the air gap when the gap changes from 0 to 120nm as shown in Fig. 2. This almost linear
dependence together with the high sensitive coupling variation at small beating length lead to
large uncertainty in coupling coefficient even due to very small variation in this designed
20nm air gap. We resolve this issue by replacing the coupling region with a MMI (multimode
interferometer) coupler. The width of the MMI coupler is designed to be around 100nm wider
than the combination of two access waveguides as shown in lower left corner of Fig. 3. Three-
dimensional beam propagation (BPM) method is used to determine the length of the MMI
coupler. By choosing a coupling length of 3.5µm, the input power in one of the waveguide is
split into two output waveguides at a ratio of 45:55. Such a MMI structure does introduce a
few percent of mode conversion loss during this splitting process [13]. Such loss is not
acceptable if optical delay lines will be constructed where a large number of resonators are
involved. Fortunately, in our optical filters with only a few resonators, this mode conversion
loss will not have significant impact on the overall filter performance.
Given a coupling length of 3.5µm, power coupling coefficients
κ
2
of 0.09 and 0.05 can be
achieved using air gap widths of 90nm and 110nm, respectively, as can be inferred from Fig.
2 and Eq. (2). In this design, the perimeter of the individual ring is 32µm, leading to a free
spectral range of 17.8um, which is significantly larger than the previous design with L
C
of
12µm. In this case, the designed 1-dB pass band of filter with 5 rings is 330GHz and the
footprint of such a filter is 0.0007mm
2
. Both design goals (i) and (iv) are then fulfilled in this
second design step.
In the entire design process, CIFS (coupling induced frequency shift) [18-19] is not
considered although in principle it will play a role in filter performance since air gap widths
between different resonators are not identical. As will be shown in Section 4, final results
indicate negligible CIFS effect since the experimental transmission spectra are flat and almost
symmetric. This is probably due to extremely wide pass band in our filters (>310GHz). CIFS
is relatively small when compared with this wide pass band and hence the effect introduced by
CIFS is not significant.
3. Device fabrication
Fabrication processes of the optical filters are similar to those described in Refs. [7, 8, 12-14].
Optical filters consisting of 3 and 5 resonators with parameters mentioned above were
#85355 - $15.00 USD
Received 17 Jul 2007; revised 29 Aug 2007; accepted 30 Aug 2007; published 5 Sep 2007
(C) 2007 OSA
17 September 2007 / Vol. 15, No. 19 / OPTICS EXPRESS 11938

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin