Jpn. J. Appl. Phys. Vol. 41 (2002) pp. L 1461–L 1463
Part 2, No. 12B, 15 December 2002c2002
The Japan Society of Applied Physics

H-Tree-Type Optical Clock Signal Distribution Circuit Using a Si Photonic Wire Waveguide
Tatsuhiko F UKAZAWA, Atsushi S AKAI and Toshihiko BABA
Department of Electrical and Computer Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan(Received May 17, 2002; revised manuscript received October 18, 2002; accepted for publication October 30, 2002)

We fabricated an ultra small H-tree-type optical signal distribution circuit on a silicon-on-insulator substrate. We used a micronsize bent-waveguide-type optical branch in a Si photonic wire waveguide and observed clear light output from eight output ports
for laser light of 1.5 µm wavelength. The fluctuation of distributed light intensity was 2–5 dB. It was caused by a nonuniform
branching ratio, and can be reduced by improving asymmetric corner shapes of each branch. [DOI: 10.1143/JJAP.41.L1461]
KEYWORDS: integrated optics, photonic circuit, optical waveguide, branch, photonic wire, SOI, FDTD

The clock skew will be a serious problem in the nextgeneration high-speed VLSI circuit. The clock signal distribution by an intra chip optical interconnection is a key solution
to this problem. The free-space interconnection by diffractive
optical elements has a simple configuration, but requires asmall diffraction angle of the distributed light beam to obtain a high diffraction efficiency. Therefore, it is difficult toinstall into a VLSI chip of square centimeter area and millimeter thickness. On the other hand, the interconnection byan optical waveguide can be completely integrated, if an ultra small waveguide is realized, which produces high-density
optical wirings and low-loss small branches for signal distribution. Here, let us consider an H-tree circuit, as shown inFig. 1. It distributes a signal to all output ports via waveguides of the same length. Assuming that there are 128 ports
inside a 10 mm2 area, a branch much smaller than the area
occupied by each distribution section (< 1 mm2 ) is essential.
From the input port to one output port, light successively
passes through seven branches and an approximately 15-mmlong waveguide. Let us denote the excess loss at a branch tobe A [dB] and the propagation loss of the waveguide to beB [dB/mm]. The incident light suffers 3 × 7 + 7A + 15B [dB]loss before reaching the output port. Providing that the incident power is 10 mW and the output power required is of
10 µW order, A must be of 0.1 dB order and B of 0.1 dB/mm
order. Of course, all the wirings should be in the singlemodefor stable branching characteristics.
Such an H-tree circuit has not been realized yet, except
for a large prototype composed of a polymer waveguide withmillimeter-size branches and bends.1) For the drastic miniaturization of this circuit, the photonic wire waveguide on aFig. 1. Schematic of H-tree-type optical clock signal distribution circuitcomposed of Si photonic wire waveguide. Magnified figure shows theschematic of bent-waveguide-type branch.

silicon-on-insulator (SOI) substrate2–5) is effective. An ultrahigh relative refractive index difference of more than 40% between the Si core (index n = 3.5) and the SiO2 (n = 1.45)
or air (n = 1.0) cladding achieves strong optical confinement
and flexible wirings. Previously, we fabricated a singlemodewaveguide with a 0.5 µm × 0.32 µm rectangular cross section. We measured a propagation loss of ∼ 10 dB/mm and abend loss of ∼ 0.2 dB at a bend radius of 2.5 µm at a wavelength λ = 1.55 µm.4) We also proposed a bent-waveguidetype branch,5) as shown in Fig. 1, whose dimensions are lessthan 6 µm × 6 µm. We theoretically calculated an excess lossof less than 0.2 dB and 0.5 : 0.5 branching ratio, which isinsensitive to the position of the input waveguide relative
to a central axis of the branch. In addition, we experimentally demonstrated an excess loss of 0.3 dB. This is much
lower than those reported for reflection-type and resonanttype small branches.6, 7) It is attributed to its simple structure,a small wavelength dependence and a large fabrication tolerance. Based on these results, we aimed at demonstrating
an ultra small H-tree circuit by using this branch. In this letter, we describe the fabrication and optical distribution characteristics of this circuit, and discuss two intensity fluctuations caused by the Fabry-Perot resonance and a nonuniform
branching ratio.
We designed a 35 µm × 25 µm circuit composed of a
0.5-µm-wide waveguide and eight output ports. It included
a total of seven branches, so that light successively passed
through three branches between the input port and one output
port. The bend radius of the branches was 2.75 µm, which
completely suppressed the bend loss. The length of the slabregion5) at the center of the branch was designed to be 0.8 µm.
The theoretical calculation showed that this length causes anexcess loss of 0.5 dB, which is slightly larger than its minimum of 0.2 dB. However, this length ensures a stable 0.5 : 0.5
branching ratio even against the position shift of the input
waveguide in the branch.5) Each of the two branches was connected by a straight waveguide of 5 µm length. Each output port was terminated by a straight waveguide of 3 µm
length. The fabrication process is the same as that reportedpreviously.4) We used a unibond-type SOI wafer (SOI TECInc.), which had an SiO2 layer of 1.0 µm thickness and atop Si layer of 0.32 µm thickness. For the pattern drawing,
we used field-emission-type electron beam (EB) writer ELS7300 (Elionix Inc.). For the formation of Si channels, we used
CF4 /Xe inductively coupled plasma etching. An important issue in this process is the fine lithography of complex patterns
including straight and bent waveguides. A standard vector

L 1461

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 12B

0

Relative Intensity [dB]scanning EB exposure and a continuous dot exposure wereused for straight and round patterns, respectively. However,these patterns were sometimes disconnected. To avoid this,
we prepared a 1.0 to 1.5-µm-long overlap region betweenthem.
We formed the cleaved end facet of the input waveguide,
and focused transverse electric (TE)- or transverse magnetic(TM)-polarized 1 mW laser light to a spot diameter of 1 µmon this facet. We observed light output from the top of thesample using a vidicon camera and measured the output intensity using an optical power meter through a lens system.
Figure 2 shows a scanning electron micrograph of the fabricated circuit and the near field pattern of light output for
TM-polarized light at λ = 1.55 µm. Clear outputs are observed from the eight ports without any irregular scattering at
the branches. Figure 3 shows transmission spectra. Here, theintensity level includes a calculated coupling loss of 6 dB at
the input end, experimental waveguide losses of 1.2 dB and
1.6 dB for TE and TM polarizations, respectively, an experimental total excess loss of 2 dB at the branches and a calculated reflection loss of 3 dB at the output end. On the otherhand, it excludes the loss in the optical setup and the intensity attenuation by power distribution of 9 dB. The intensity
fluctuation of 7–8 dB is primarily caused by the Fabry-Perot
resonance either between input and output ends of waveg-T. F UKAZAWA et al.

5Coupling loss at input end

6 dB

Waveguide loss

1.2 dB

Excess loss at branches

2 dB

Reflection loss at output end
3 dB
1015
5 dB
20

1.50

1.55
Wavelength [ m]
(a)

1.60

0Coupling loss at input end
Waveguide loss
Relative Intensity [dB]

L 146256 dB
1.6 dB

Excess loss at branches

2 dB

Reflection loss at output end

3 dB

1015
2 dB
20

1.50

1.55
Wavelength [ m]
(b)

1.60

Fig. 3. Measured transmission spectra for eight ports. (a) TE polarization.
(b) TM polarization. Fast oscillation seen in one output port in (b) is notdue to the waveguide but to the unstable condition of the optical power
meter.

Fig. 2. Top view of H-tree circuit and corresponding near field pattern oflight output for TM-polarized light at λ = 1.55 µm. Dashed curves areadded for better understanding of waveguide position.

uides, between a waveguide end and a branch or between twobranches. The typical wavelength space of the resonance is
15 nm. This corresponds to a resonator length of 16 µm, whena group velocity index is assumed to be 4.5.4) This length isthe same as that between the second branch and an output end.
This indicates that the branches in vertical directions havesome imperfections, while the branches in horizontal directions are almost perfect. This Fabry-Perot resonance is not aserious problem, since it is easily removed by some structuralmodifications. We experimentally observed that it was completely suppressed by tapered ends of waveguides. Except forthis resonance, there still remain intensity fluctuations among
eight ports of ∼ 5 dB for TE polarization and ∼ 2 dB for TM
polarization, which must be due to an unstable branching ratio. The inset of Fig. 4 shows the magnified view of a fabricated branch. Let us denote the shift of the input waveguidefrom the center axis of the branch as w. The normalizedshift w/w was measured to be 0–9% in the branches. The
length of the slab region g was 0.71–0.75 µm. Moreover, theJpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 12B

Fig. 4. Magnified top view of fabricated branch, and branching ratio calculated with normalized shift w/w. The top view shows a shift w ofthe input waveguide against the center axis of the branch and asymmetriccorner shape of the slab region, which is indicated by white circles. The
calculation method is the same as that described in ref. 5. Solid and dashedcurves indicate asymmetric and symmetric corners, respectively.

corner shape of the slab region was asymmetric. The left corner of the slab region was round and its position was shiftedby ∼ 80 nm from the right corner. Figure 4 also shows thebranching ratio calculated with w/w. Imperfections in w
and g only give a total intensity fluctuation of less than 0.5 dB
without the asymmetric corner. This value dose not agree withthe experimental value. On the other hand, the branching ratio
is 0.58 : 0.42 with the asymmetric corner. This ratio yields a
total fluctuation of ∼ 3 dB, which almost agrees with the experimental value. This asymmetric corner shape seems to becaused by anisotropic scanning in the EB exposure.
In addition to this intensity fluctuation, other remaining problems of this study are a large waveguide loss of
10 dB/mm order and a large coupling loss of ∼ 6 dB at theinput end. The origin of the waveguide loss is light scattering
caused by the process roughness at waveguide sidewalls.3, 4)
Therefore, it can be reduced by simple technical improve-

T. F UKAZAWA et al.

L 1463ments as already demonstrated in other reports. For example,
0.08 dB/mm was achieved in a thin-film channel by an additional oxidation process8) and 0.6 dB/mm was achieved ina square channel by optimized EB exposure.9) The couplingloss can also be reduced using a spot size converter9) or a
grating coupler.10) Thus, the H-tree circuit discussed here isexpected to become a practical and useful tool by the integration of photodetectors sensitive to 1.5 µm wavelength.11)
In conclusion, we fabricated an H-tree optical distribution circuit within an area of 35 µm × 25 µm by combining low-loss branches in Si photonic wire waveguides, andsuccessfully demonstrated clear light output from eight ports.
The transmission spectra showed an intensity fluctuation of
2–5 dB among output ports, except for the oscillation by theFabry-Perot resonance. This is primarily caused by the asymmetric corner shape of the branch, and will be reduced bycontrolling the scanning in the EB exposure process.

1) L. Wu, B. Bihari, J. Gan and R. T. Chen: Proc. Top. Meet. Integrated
Photonics Research (Optical Society of America, Washington, DC,1998) p. 282.
2) J. Foresi, P. R. Villeneuve, J. Ferrera, E. R. Thoen, G. Steinmeyer, S.
Fan, J. D. Joannopoulas, L. C. Kimerling, H. I. Smith and E. P. Ippen:
Nature 390 (1997) 143.
3) K. K. Lee, D. R. Lim, H.-C. Luan, A. Agarwal, J. Foresi and L. C.
Kimerling: Appl. Phys. Lett. 77 (2000) 1617.
4) A. Sakai, G. Hara and T. Baba: Jpn. J. Appl. Phys. 40 (2001) L383.
5) A. Sakai, T. Fukazawa and T. Baba: IEICE Trans. Electron. E85-C(2002) 1033.
6) C. Monolatou, S. G. Johnson, S. Fan, P. R. Villeneuve, H. A. Haus andJ. D. Joannopoulos: J. Lightwave Technol. 17 (1999) 1682.
7) R. U. Ahmad, F. Pizzuto, G. S. Camarda, R. L. Espinala, H. Rao and R.
M. Osgood: IEEE Photon. Technol. Lett. 14 (2002) 65.
8) L. C. Kimerling: Proc. Top. Meet. Integrated Photonics Research (Optical Society of America, Washington, DC, 2002) IFB1.
9) K. Yamada, M. Notomi, A. Shinya, I. Yokohama, T. Shoji, T.
Tsuchizawa, T. Watanabe, J. Takahashi, E. Tamedhika and H. Morita:Proc. SPIE (The Society of Photo-Optical Instrumentation Engineers,
Bellingham, 2002) Vol. 4870, p. 324.
10) D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I.
Moerman, S. Verstuyft, K. De Mesel and R. Baets: IEEE J. Quantum
Electron. 38 (2002) 949.
11) L. Colace, G. Masini, G. Assanto, H.-C. Luan, K. Wada and L. C.
Kimerling: Appl. Phys. Lett. 76 (2000) 1231.