Compact waveguide splitter networks
Yusheng Qian, Jiguo Song, Seunghyun Kim, Weisheng Hu, and Gregory P. Nordin
Electrical and Computer Engineering, Brigham Young University,Provo, UT 84602 USA
nordin@ee.byu.edu
Abstract: We demonstrate compact waveguide splitter networks in siliconon-
insulator (SOI) rib waveguides using trench-based splitters (TBSs) and
bends (TBBs). Rather than a 90‹ geometry, we use 105‹ TBSs to facilitate
reliable fabrication of high aspect ratio trenches suitable for 50/50 splitting
when filled with SU8. Three dimensional (3D) finite difference time
domain (FDTD) simulation is used for splitter and bend design. Measured
TBB and TBS optical efficiencies are 84% and 68%, respectively. Compact
105‹ 1 ~ 4, 1 ~ 8, and 1 ~ 32 trench-based splitter networks (TBSNs) are
demonstrated. The measured total optical loss of the 1 ~ 32 TBSN is 9.15
dB. Its size is only 700 ƒÊm ~ 1600 ƒÊm for an output waveguide spacing of
50 ƒÊm.
c2008 Optical Society of America
OCIS codes: (130.0130) Integrated optics; (230.1360) Beam splitters; (260.6970) Total
internal reflection; (230.7370) Waveguides; (130.1750) Components; (250.5300) Photonic
integrated circuits.
References and Links
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1. Introduction
Waveguide splitter networks that divide an optical signal into N outputs (1 ~ N) are important
elements in a variety of applications including power splitters for planar lightwave circuits
(PLCs) [1,2] and periodic optical sources for integrated microfluidic devices [3,4]. Such
splitter networks are primarily based on either cascaded Y-branch splitters [1-3,5,6] or
multimode interference (MMI) splitters [4,7-9]. In this paper we report an alternate approach
using trench-based splitters (TBSs) [10,11] and trench-based bends (TBBs) [12]. We focus on
silicon-on-insulator (SOI) rib waveguides that have low in-plane core/clad refractive index
contrast and hence require relatively large bend radius (1.2 mm for the waveguides considered
in this paper) which limits achievable size reduction for traditional splitter networks. The use
of TBSs and TBBs to create trench-based splitter networks (TBSNs) results in a large
decrease in required chip area. This is particularly important in our ultimate application of
sourcing light into many SOI microcantilevers for a new in-plane photonic transduction
mechanism [13] to enable single-chip microcantilever sensor arrays [14-16].
In this paper we first discuss modification of our previously-reported SOI TBSs [10] to
achieve 50/50 reflection/transmission splitting ratios in fabricated splitters with SU8 as the
trench fill material by changing the splitter angle from 90‹ to 105‹. We then report fabrication
and measurement of 105‹ 1 ~ 4 and 1 ~ 32 TBSNs, followed by an examination of total
splitter network loss. For an output waveguide spacing of 50 ƒÊm, the 1 ~ 32 network occupies
an area of only 700 ƒÊm ~ 1600 ƒÊm.
2. Design
As shown in Fig. 1(a), we consider an SOI rib waveguide with rib width of 1.6 ƒÊm, rib
thickness of 0.75 ƒÊm, and slab thickness of 0.65 ƒÊm. The underclad is SiO2 and the overclad
is SU8, which is the same material used to fill the TBS and TBB trenches. The waveguide
supports only the fundamental TE polarization mode (electric field in the plane of the silicon)
at a wavelength of 1550 nm.
Figures 1(b) and 1(c) show the geometry of a TBB and a TBS. The TBB bend angle, ƒ¿1,
is defined as the angle between the original waveguide direction and the direction of the
output waveguide. Similarly, the TBS bend angle, ƒ¿2 , is defined as the angle between the
transmission output direction and the reflection output direction. In both cases, D is defined as
the distance from the intersection of the waveguide center lines to the first interface of the
trench. In Ref. 12 we reported fabrication and measurement of TBBs with a 90‹ bend angle in
which the trench is filled with SU8 and the measured optical efficiency (i.e. fraction of the
incident waveguide mode power reflected into the mode of the output waveguide) is 93%. In
Ref. 10 we reported the development of SOI TBSs with a 90‹ bend angle. TBSs filled with air
(n = 1.0), SU8 (n = 1.57), or refractive index matching fluid (n = 1.733) are characterized at
1550 nm. A 49/51 (reflection/transmission) splitting ratio is reported for a trench width of 82
nm with index matching fluid as the trench fill material. However, TBSs with SU8 as the
trench fill material need a trench width of 67 nm to achieve a 50/50 splitting ratio, which is
too small for us to reliably fabricate since the trench etch depth must be 750 nm. To realize
50/50 TBSs with SU8 as the trench fill material, in this paper we explore a new design by
(C) 2008 OSA 31 March 2008 / Vol. 16, No. 7 / OPTICS EXPRESS 4982
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increasing the TBS bend angle to 105‹ so that a 50/50 splitting ratio can be achieved with a
wider trench.
(a)
(b) (c)
(d)
Fig. 1. (a). Rib waveguide cross section. (b) 105‹ TBB and (c) TBS geometry (i.e., ƒ¿1 = ƒ¿2 =
105‹ and ƒÆ1 = ƒÆ2 = 37.5‹). (d) Required trench width for 50/50 splitting using SU8 filled TBSs
(right axis) and total splitter efficiency (left axis) as a function of splitter bend angle.
As discussed in Ref. [10], TBSs operate based on frustrated total internal reflection (FTIR)
in which the trench width is small enough that part of the optical field is transmitted through
the trench while the rest undergoes total internal reflection. For a given incidence angle,
ƒÆ 2 = 90‹ .ƒ¿ 2/2, (1)
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the ratio between the reflected and transmitted power is a function of trench width.
Alternatively, for a given trench width, the splitting ratio can be altered by changing the
incidence angle (i.e., splitter bend angle). We use the three dimensional (3D) finite difference
time domain (FDTD) method [17,18] with Berenger PML boundary conditions [19] to
explore the relationship between trench width and splitter angle to achieve 50/50 splitting for
the case of SU8 trench fill, which is also the overclad material of the SOI rib waveguide. The
refractive indices used for numerical simulation are 3.476 for silicon, 1.444 for SiO2, and
1.570 for SU8 at a wavelength of 1550 nm. The result is shown in Fig. 1(d) in which the
trench width (right axis) is shown as a function of splitter bend angle for 50/50 splitting. Also
shown is the total optical efficiency (i.e., sum of transmitted and reflected mode power
divided by incident mode power) on the left axis. Note that as the splitter bend angle increases
the required trench width also increases, but the total optical efficiency is reduced. Based on
fabrication considerations, we choose a splitter bend angle of 105‹ such that the desired
trench width is 116 nm while the total optical efficiency is 84% (reflection 42% and
transmission 42%). To account for the Goos-Hanchen shift, D is chosen to be -97 nm. The
TBS trench has an aspect ratio (depth:width) of 6.5:1, which is relatively straightforward for
us to fabricate. A plot of the magnitude of the time-averaged magnetic field is shown at a
plane 0.325 ƒÊm above the SiO2 underclad (i.e., nearly in the middle of the rib waveguide) in
Fig. 2(a).
Changing the splitter bend angle, ƒ¿2, to 105‹ necessitates changing ƒ¿1 for the TBBs to
105‹ to maintain the desired geometry of the TBSNs (shown in later sections). We similarly
use 3D FDTD to design the 105‹ bends. Figure 2(b) shows the magnitude of the timeaveraged
magnetic field in a plane 0.325 ƒÊm above the SiO2 underclad for a 105‹ SU8 filled
TBB (D = -85 nm), which has an optical efficiency of 82%.
(a) (b)
Fig. 2. Magnitude of the time-averaged magnetic field for (a) 105‹ TBS and (b) 105‹ TBB.
3. Measured 105‹ TBB and TBS optical properties
The 105‹ TBBs and TBSs are fabricated with the same process as the 90‹ geometry devices
reported in Refs. [10] and [12]. Electron beam lithography (EBL) with a Nanometer Pattern
Generation System (JC Nabity NPGS) and field emission environmental scanning electron
microscope (FEI/Philips XL30 ESEM-FEG) is used for trench patterning. A water soluble
conductive polymer (aquaSAVE53za) is spin coated on top of the electron-beam resist (ZEP
520A) to prevent charging during EBL. After developing, trenches are etched in an
inductively coupled plasma reactive ion etcher (ICP RIE) with a fluorine-based etch
chemistry. Finally, SU8 is spin coated to fill the trenches and also act as the upper cladding.
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The optical source for characterization of TBBs and TBSs is an amplified spontaneous
emission (ASE) source with a center wavelength of 1550 nm connected to an erbium-doped
fiber amplifier (EDFA). A non-laser source source is chosen because the ASE bandwidth
(~30 nm) results in a short coherence length which eliminates Fabry-Perot effects in the
waveguides that would otherwise be present because of light reflected from the chip output
and input endfaces. Note that the bandwidth of the source does not significantly affect our
measurement results because the bend and splitter performance are only weakly dependent on
wavelength. Light from the EDFA passes through a linear polarizer and is coupled into a
polarization maintaining (PM) fiber, which in turn is butt coupled to an input waveguide on
the chip under test. A single mode fiber is butt coupled to an output waveguide to direct light
to a detector. A Newport auto-align system is used to maximize the coupling through the
input and output fibers [10,12].
Fig. 3. Measured loss of 105‹ TBB as a function of number of bends in a set of equal-length
waveguides. The average error for each data point is +/-0.09 dB. The insertion loss is ~37 dB,
with almost all of this (~36 dB) due to the fiber/waveguide mode mismatch in getting light on
and off chip.
The optical properties of the 105‹ TBBs and TBSs are characterized as discussed in Refs.
10 and 12 for 90‹ devices. The optical loss for 105‹ TBBs is measured with a set of equal
length waveguides that have different numbers of bends. Figure 3 shows the measured optical
loss as a function of the number of bends. The measured loss of 105‹ TBBs is -0.77 dB } 0.02
dB (84%) per bend. Curiously, the measured efficiency is slightly higher than the 3D FDTD
prediction of 82%. However, this is consistent with our experience for 90‹ TBBs in which the
measured efficiency is 93% while the 3D FDTD prediction is 89%. We have not yet
discovered the source of this discrepancy.
For TBSs, the splitter ratio and efficiency is measured using sets of 105‹ 1 ~ 2 network
structures that contain one TBS and one TBB. Figure 4(a) shows a fabricated 105‹ 1 ~ 2
network before coating SU8. The two etched circular regions at each end of the splitter trench
are intended to facilitate filling SU8 into the trench. The other etched circles are present to
scatter stray light in the silicon slab which originates from butt coupling the input fiber to the
input waveguide. Measurement results for the reflection and transmission splitting ratio (i.e.,
reflected or transmitted optical power divided by the sum of the two) for individual splitters
with different trench widths are shown in Fig. 4(b). Also shown are 3D FDTD simulation
results. The short dashed lines are linear fits to the measured data. While the slope of these
lines is comparable to the 3D FDTD results near the 50/50 splitting ratio region, the actual
trench width at which 50/50 splitting occurs is 95 nm for the measured data compared to 116
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nm for the simulations. The reason for this discrepancy is the fabricated trench widths are
measured nondestructively by scanning electron microscope (SEM) imaging of the top of the
trenches (i.e., looking down on the trenches from above the plane of the silicon). However,
when an etched trench is cleaved and imaged in cross section as shown in Fig. 4(c), the trench
sidewalls are seen to exhibit bowing. The center of the trench is 25% wider than the top
trench width and therefore the effective trench width as experienced by the waveguide mode
is larger than predicted by top-view SEM imaging.
(a)
(b) (c)
Fig. 4. (a). SEM image of a fabricated 1 ~ 2 network before SU8 spin coating. The separation
between transmission and reflection waveguides is 50 ƒÊm. (b) Measurement and 3D FDTD
simulation results for 105‹ TBS splitting ratio as a function of trench width. (c) Cross sectional
SEM image of a cleaved trench.
The optical efficiency,ƒÅTBS , of 105‹ TBSs can be experimentally determined based on
[10]
ƒÅTBS =
PTBS _ reflection ƒÅTBB + PTBS _ transmission
PStraight_ waveguide
(2)
where ƒÅTBBis the optical efficiency of a 105‹ TBB, PTBS _ reflection and PTBS _ transmissionare
the measured TBS reflected and transmitted power, respectively, and PStraight_ waveguideis the
measured power through a separate straight waveguide. The measured splitter efficiency
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based on Eq. (2) is 67.8% } 9.9% (-1.79 dB } 0.66 dB). Increasing the verticality of etched
trench sidewalls to remove the observed bowing should significantly improve TBS efficiency.
(a) (b)
Fig. 5. (a). Microscope image and (b) 1D output fiber scan of SU8 coated 1 ~ 4 105‹ TBSN.
4. 1 ~ N 105‹ TBSN Measurements
With 105‹ TBBs and TBSs successfully demonstrated, we combine them to make 1 ~ N
networks. We use 1 cm ~ 1 cm die designed such that we can fabricate 1 ~ 4, 1 ~ 8, or 1 ~ 32
105‹ TBSNs. The TBSs of the network are fabricated to have a top-view trench width of ~95
nm to account for sidewall bowing. Figure 5(a) shows a microscope picture of a fabricated 1
~ 4 network with 50 ƒÊm output waveguide spacing with SU8 on top. Figure 5(b) shows the
measured optical power as a fiber is scanned along the output waveguides. The measured
optical power through a straight waveguide is 23.7 ƒÊW so the optical efficiencies for outputs
1-4 are 12%, 9%, 12%, and 9%, respectively.
Figure 6(a) shows a 1 ~ 32 TBSN. The output waveguide spacing is 50 ƒÊm except for
outputs #16 and #17 which have a spacing of 100 ƒÊm. The total 1 ~ 32 network region
occupies an area only 700 ƒÊm ~ 1600 ƒÊm. Figure 6(b) is an infrared camera image of the 32
corresponding outputs. The optical power of each output is measured and plotted in Fig. 6(c).
The 1~32 network has an average output power of 0.12 ƒÊw and a standard deviation (STD) of
0.03 ƒÊw. The normalized STD (STD divided by the mean) of the measured 32 outputs is 0.26.
The optical power through a nearby straight waveguide is 32.8 ƒÊW so the average fraction of
the input light that exits a given output waveguide is 0.37%.
Due to the asymmetry of our TBSN structure, light in different output waveguides passes
through different numbers of TBBs. Consequently, there will be variation in the output optical
powers due to losses from the TBBs. To estimate the expected variation for an ideal TBSN,
we calculate the normalized output power of a 1 ~ 32 TBSN using the measured 105‹ TBB
and TBS efficiency reported in Section 3, and assume that all of the TBSs in the network have
a 50/50 splitting ratio (i.e., TBB efficiency 84% and TBS transmission and reflection
efficiencies both 34%). The result is a normalized STD of 0.20. Comparing with the
normalized STD of the measured 32 output powers (0.26), the variation of output power in the
fabricated 1 ~ 32 network is ~30% higher than the theoretical value, which is most likely due
to variations between individual splitters because of fabrication process nonuniformities and
variations introduced by the quality of the endface polish. We note that TBSN output
uniformity can be improved by using a symmetric 105‹ network geometry in which the
number of TBBs in each output path is the same. However, the achieved level of uniformity
reported in this paper is entirely adequate for our application since differential signals are
used to transduce microcantilever deflection [13].
As a final comment on output uniformity, light exiting each output waveguide goes
through a different waveguide propagation length. For our 1 ~ 32 network, the longest path
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(output waveguide #32) is 2 mm longer than the shortest one (#1). Since the measured
propagation loss is 1.1 dB (measured with the cut-back method using a straight waveguide
sample at 1550 nm), this length difference causes an extra loss of only 0.22 dB. Hence the
network output power variation due to waveguide length difference is negligible compared to
the variation caused by the different number of TBBs in each output path.
(a) (b)
(c)
Fig. 6. (a). Microscope image of SU8 coated 1 ~ 32 TBSN and corresponding (b) IR camera
image of output waveguides and (c) fiber-based output waveguide power measurement as a
function of output waveguide number.
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5. 1 ~ N 105‹ TBBSN loss
An important parameter to evaluate 1 ~ N network performance is the total optical loss of the
network. We analyze this loss by assuming an ideal case in which the TBBs and TBSs of the
network have same optical efficiency,ƒÅ, and the TBS splitting ratio is 50/50. The total optical
efficiency of a 1 ~ 2 network is the sum of the efficiency for the reflection path, ƒÅ2 2 , and
the efficiency for the transmission path, ƒÅ 2 . The total network loss can therefore be
calculated as
Lcalc = 10 * log((ƒÅ2
2
+ƒÅ
2
)M ) (3)
where M is the number of layers in the network, which is defined as the number of splitters
that each waveguide passes from input to output. The output number N and the layer number
M are related by N = 2M .
The experimentally measured total network loss is
Lmeas = 10 * log(
P1 ~ N network
Pstraight.waveguide
) (4)
where the network total output power, P1 ~ N network , is the sum of all N output powers.
Lcalc is plotted as a function of N and M (top and bottom axes, respectively) in Fig. 7 for
ƒÅ= 60%, 70%, 80%, 90%, and 95%. The measured total network loss, Lmeas, is also shown (-
3.82 dB, -5.9 dB, and -9.15 dB for 1 ~ 4, 1 ~ 8, and 1 ~ 32 TBSNs, respectively). In the case
of the 1~8 network, the total output power is an estimated value based on only seven outputs
(multiplying the average power of the seven outputs by eight) because one output waveguide
of the network has a waveguide defect such that no output power can be measured. Note that
the measured data indicates an average TBB/TBS efficiency between 70% and 80%, and that
the data points are consistent with each other (i.e., nearly linear).
Fig. 7. Measured and calculated 1 ~ N network total loss as a function of number of network
layers (bottom axis) and network outputs (top axis) (see text for details).
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6. Conclusions
In summary, we have demonstrated 105‹ TBBs and TBSs with SU8 as the trench fill material.
The measured optical efficiencies are 84% and 68% respectively. With a 105‹ splitter bend
angle we are able to achieve 50/50 splitting for reasonable trench widths at the cost of
somewhat lower total efficiency. Based on these 105‹ components, we have fabricated 1 ~ N
networks up to 1 ~ 32, which occupies an area of only 700 ƒÊm ~ 1600 ƒÊm for output
waveguide spacing of 50 ƒÊm. The total network loss for the 1 ~ 32 network is 9.15 dB, which
is consistent with the measured TBB and TBS efficiencies. The normalized standard deviation
of the output power in the networkfs 32 outputs is 0.26, which is only ~30% higher than what
is expected based only on the asymmetry of the network.
Acknowledgment
This work was supported in part by NSF grants IIS-0641973 and ECS-0602261, and DARPA
grant N66001-04-8933.
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