# Hybrid Silicon Photonics for Low-Cost High-Bandwidth Link Applications

Authors: B. Jonathan Luff; Dazeng Feng; Daniel C. Lee; Wei Qian; Hong Liang; Mehdi Asghari
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

Current discrete optical solutions for high data-rate link applications, even with potentially high manufacturing volumes, are too costly. Highly integrated, multifunction modules are a key part of the solution, reducing size and cost while providing improved reliability. Silicon, with its proven manufacturability and reliability, offers a solid foundation for building a cost-efficient path to successful products. In this paper, recent work on the development of silicon photonic enabling components for multichannel high data-rate links is presented.


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## Body

## 1. Introduction

High-speed
consumer connections are becoming more prevalent due to the introduction of newer
services  such as online video, HDTV, IPTV, and the increase of personalized
data content. This is leading to significant connectivity bottlenecks in the
network infrastructure of, for example, datacenters,
campus networks, and storage area networks. Internet exchanges and
network centers have expanded to as many as100⁢000 optical fibers routing
traffic at speeds of 10 Gbps. The availability of cost-effective interface
solutions operating at data rates of multiples of 10 Gbps or above over a
single fiber will enable the proliferation of these new services by greatly
reducing the wiring complexity, instrumentation, and operating cost in
high-capacity fiber-optic networks. In this paper, we discuss the development
of components based on a silicon photonics hybrid platform that will create a
compact and low-cost solution for multiple channel transmission over single mode
fiber. Initial work focuses on a 10 channel/10 Gbps per channel solution, but
extension of channel count and/or modulation speed is envisioned, enabling aggregate
data rates of up to 1 Tbps for a 25 channel, 40 Gbps solution.The
approach is illustrated in Figure1. On the transmit (Tx) side of the link,
light at multiple wavelengths from a hybridized laser array is multiplexed onto
a single fiber. After transmission over the fiber, the light is coupled onto
the receiver (Rx) chip where the wavelength channels are demultiplexed and
detected using a hybridized photodiode array. The silicon platform is able to
support the formation of low-loss RF traces onchip to enable space-efficient
positioning of driver and receiver electronics. Transimpedance amplifiers
(TIAs) may also be hybridized onchip if that is appropriate to a particular
packaging solution. Laser diode output power monitoring may be accomplished by
hybridized back-facet monitor photodiodes. Alternatively, as illustrated in Figure
1, the silicon photonics platform offers the option of front-facet monitoring through
the fabrication of tap coupler and the hybridization of PIN photodiodes [1]. It
is also possible to monolithically fabricate monitor diodes in silicon using defect
engineering [2]. A useful function that may be included in future devices is
per channel variable attenuation, a well-established monolithic functional element
in silicon technology [3].Figure 1
Multichannel optical transmission link.The refractive index of silicon is similar
to that of active materials such as GaAs and InP, facilitating efficient
coupling between silicon waveguides, and hybridized active devices such as
lasers and photodetectors. For powered devices such as lasers, the high thermal
conductivity of silicon enables efficient heat removal and good equalization of
temperature across a chip.The basic silicon-on-insulator (SOI) waveguide
ridge structure used in this work is shown in Figure2. Vertical confinement is
provided by the buried layer of oxide inside the wafer (SOI) and a top layer of
oxide deposited on the waveguide during fabrication. Lateral confinement is
provided by the ridge structure. Such waveguides can be designed to be single
mode based on correct choice of ridge height and width.Figure 2
Silicon-on-insulator waveguide structure.The
manufacturing process for the basic waveguide is based on pattern definition by
an appropriate masking layer and subsequent photolithography. RIE etch is used
to achieve the required topology. Following the removal of the masking layer,
an oxide cladding layer is deposited followed by a protective nitride thin
film.A key challenge for a Si photonics platform
is coupling light efficiently on and off the chip. The large index of Si drives
small waveguide dimensions, which enables highly compact structures, but in
turn can cause high beam divergence off chip and high mode mismatch to other
optical components such as fibers. Although not implemented for the devices
described in this paper, the Si photonics platform enables the fabrication of three dimensional tapers
(Figure3) that can significantly
reduce fiber coupling loss to standard single-mode fiber. If necessary, taper
structures can also be used to improve coupling efficiency to hybridized optical
components.Figure 3
3D mode transforming taper.


## 2. Silicon Mux and Demux

Silicon-based multiplexers  and
demultiplexers are the critical passive elements in a multichannel link system
using a single fiber. The key challenges are the design and fabrication of low
loss, low crosstalk, and small footprint devices. The echelle grating-based
design (Figure4) is favored for use in the link architecture because of its
much smaller footprint compared to, for example, an arrayed waveguide grating (AWG)
design [4]. A wavelength channel spacing of 20 nm was chosen for fabrication of
a demonstration device. This channel spacing is typically used in coarse
wavelength division multiplexing (CWDM) systems, and will ensure sufficient
passband width to accommodate the spectral deviation of laser wavelength and
filter passband position due to temperature and polarization variations.Figure 4
10-channel echelle grating demultiplexer in
silicon.The loss of echelle grating arises mainly
from nonvertical grating facets and facet roughness. It is difficult to make
both a smooth and vertical grating facet because of fabrication limitations. Grating
performance variation due to wall roughness and verticality are shown in Figures5(a) and 5(b), respectively. For this work, the roughness of the grating facets
is estimated from SEM measurements as ~11 nm, and the verticality as ~1.5
degrees. Projected process developments are likely to improve these values to
~7 nm and 0.5 degrees, respectively.Effect of grating wall roughness (a) and
verticality (b) on grating insertion loss.
(a)(b)Another important factor influencing the
loss of the mux and demux element is the rounding of the grating teeth element
within the echelle grating design. This can be reduced with better
photolithographic tools and etching processes as indicated in
Figure6.Figure 6
Effect of grating teeth roundness on
insertion andanticipated potential improvement.Nonguided stray light and scattered light
from the rough grating facet can generate high optical crosstalk in an echelle
grating. To reduce crosstalk and enhance performance, proprietary absorber structures
were utilized.To make the silicon demux polarization
independent, a relatively large silicon waveguide core size is used (waveguide
height and waveguide width are both ~3μm). The buried oxide thickness is approximately
0.4 μm. The drawback of using large area waveguides
is that the minimum waveguide bending radius for single mode operation is large
(~5 mm) and although the echelle grating itself can be very small, the waveguide
fan-in and fan-out can take up much of the available space because of the bend
radius limitation. For this work, sharp waveguide bends using a deep-etched
multimode curved waveguide with tapered transitions into the bend to maintain
power in the fundamental mode were used. Using this method, the bending radius
was reduced to approximately 0.25 mm so that the overall silicon mux and demux
footprints were reduced to <1 cm2 for 10-channel devices with 20 nm channel spacing. Because the outputs of the receive waveguides in the
architecture of Figure 1 can be coupled directly to hybridized photodiodes, for
the demux design a flat-top output spectrum was produced by using multimode output
waveguides. Measured transmission spectra from fabricated multiplexer and demultiplexer
devices are shown in Figure 7.(a) Multiplexer
TE transmission spectra; (b) demultiplexer transmission spectra for TE and TM
polarization states.
(a)(b)The
mux demonstrated 6.5 dB insertion loss over the passband of 6.4 nm; PDL was
<0.5 dB. Because the refractive index of silicon has a similar temperature
variation to that of InP, this passband is sufficient to ensure low
transmission loss is maintained over a wide temperature change because the mux
passband will track the drift in the emission wavelength of the DFB laser.
Figure8 shows expected temperature variations for typical InP/InGaAsP DFB lasers from two different manufacturers compared with
the calculated variation for a silicon multiplexer device. For comparison, the
much smaller temperature variation expected for a silica-based device is
included. A large channel spacing ensures that only very coarse and thus
low-cost heater-based temperature control solution is required for stable
operation over normal ambient conditions (typically 0–70°C). The demux device shown in
Figure 7(b) demonstrated 4 dB
insertion loss over a passband of 12 nm, <0.5 dB PDL, and better than 22 dB
crosstalk.Figure 8
Wavelength shift as a function of
temperature for typical DFB lasers compared to that of the passband of silicon-
and silica-based multiplexers.Although a 10 channel device has been demonstrated, the
design can be readily extended to higher channel counts. The optimum channel
count for a given application will depend on multiple factors including per channel
modulation speed, laser wavelength range, required bandwidth over a given
temperature range, and minimum chip footprint.


## 3. Hybrid Integration

### 3.1. Laser

To produce an interface for the highly
efficient optical transition between a semiconductor laser and the silicon
waveguide, the mode profile mismatch and surface reflection between the laser
and the waveguide should be reduced as much as possible. This modal mismatch
can be improved by appropriate tailoring of the laser structure.The silicon
ridge waveguide structure can be optimized to obtain low coupling loss to the
laser.  Even at 5μm separation between the two facets, the
coupling loss can potentially be lower than 3 dB for silicon waveguides with
different ridge width ranging from 3 to 7 μm.  A
specific
ridge width within this range can thus be chosen
to well match the diverging laser mode and provide some tolerance to alignment
errors for a targeted gap size. Coupling loss can theoretically be less than 1 dB if the gap size (z-direction
gap) is smaller than 2 μm. However, the coupling of the laser to the
waveguide in the direction perpendicular to the plane of the chip (y) and along
the plane of the chip (x) requires alignment to a few tenths of a micron to
obtain low coupling loss. Figure 9 shows the calculated coupling loss taking
into account possible alignment errors as a function of silicon waveguide width
(W) for a typical DFB laser.Calculated DFB laser coupling loss as a function of silicon waveguide widthW
for various alignment errors in x and y.
(a)(b)To reduce the amount of reflected light coupled
back into the laser cavity, the waveguide facet can be etched in such a way so
that it is tilted at an angle (>10°) away from the optical axis of the
laser beam. A nitride film of thickness
~200 nm on the waveguide surface further reduces the reflectivity of the facet.
When the laser facet is 4 μm away, the total reflected light coupled back
into the laser can be kept below −30 dB over a
wavelength range of 200 nm (Figure 10).Calculated back-reflection of light into the laser as a function of waveguide
tilt angle andz-separation (D).
(a)(b)


### 3.2. Photodetector

The effective aperture of a high-speed (≥10 Gbps) detector is typically smaller than 30μm in diameter to obtain low capacitance and
large bandwidth. To prevent large beam divergence and achieve high detection
efficiency, the gap between the output facet of a waveguide and the detector
surface must be as small as possible.The electronic connection employing
conventional wire bonds between the detector die to the transimpedance
amplifier (TIA) circuits may introduce too much unwanted parasitic capacitance
and inductance that limits the speed of the receiver, especially for higher
than 10 Gbps detection speed. Two possible methods of hybridization can be considered:
edge coupling and flip-chip coupling.When the detector is vertically attached to
the edge of the waveguide chip, the waveguide facet can be almost in direct
contact with the detector surface. The detector to waveguide alignment is
relatively simple because the effective detection area can be a few times
larger than the waveguide mode size. The relatively large aperture of the
photodiode covers the entire mode profile at the waveguide facet and ensures
high optical detection efficiency. A ceramic submount can be used to fasten the
detector (e.g., at the p-side) with necessary metal contacts and traces that can
readily be connected to a separated amplifier IC chip through wire bonding. The
main disadvantage of this approach is that the waveguide edge needs to be
polished, and hence it is not suitable for low-cost, high-volume manufacture.A more manufacturable  approach is to
flip-chip bond the detector onto the silicon chip surface (p-side up), where an
etched reflective mirror redirects the output beam to the detector surface, as
shown in Figure11. A smooth 〈111〉  direction crystal plane with a 54.7°
angle results from the anisotropic etching on the silicon surface; it is coated
with aluminum to enhance reflectivity. The etched waveguide facet is coated
with a thin nitride antireflection film to minimize the reflection loss at the silicon-to-air
interface.Figure 11
SEM photos of an etched waveguide facet
and turning mirror.All electronic contacts can be made through
the solder pads and metal traces directly plated on the silicon surface. A
separate amplifier IC chip with conducting via holes can be attached onto the
silicon chip surface to facilitate low parasitic electronic contact for
high-speed operation. Alternatively, the detector can be integrated with the
amplifier circuits before being mounted on the silicon chip.


## 3.1. Laser

To produce an interface for the highly
efficient optical transition between a semiconductor laser and the silicon
waveguide, the mode profile mismatch and surface reflection between the laser
and the waveguide should be reduced as much as possible. This modal mismatch
can be improved by appropriate tailoring of the laser structure.The silicon
ridge waveguide structure can be optimized to obtain low coupling loss to the
laser.  Even at 5μm separation between the two facets, the
coupling loss can potentially be lower than 3 dB for silicon waveguides with
different ridge width ranging from 3 to 7 μm.  A
specific
ridge width within this range can thus be chosen
to well match the diverging laser mode and provide some tolerance to alignment
errors for a targeted gap size. Coupling loss can theoretically be less than 1 dB if the gap size (z-direction
gap) is smaller than 2 μm. However, the coupling of the laser to the
waveguide in the direction perpendicular to the plane of the chip (y) and along
the plane of the chip (x) requires alignment to a few tenths of a micron to
obtain low coupling loss. Figure 9 shows the calculated coupling loss taking
into account possible alignment errors as a function of silicon waveguide width
(W) for a typical DFB laser.Calculated DFB laser coupling loss as a function of silicon waveguide widthW
for various alignment errors in x and y.
(a)(b)To reduce the amount of reflected light coupled
back into the laser cavity, the waveguide facet can be etched in such a way so
that it is tilted at an angle (>10°) away from the optical axis of the
laser beam. A nitride film of thickness
~200 nm on the waveguide surface further reduces the reflectivity of the facet.
When the laser facet is 4 μm away, the total reflected light coupled back
into the laser can be kept below −30 dB over a
wavelength range of 200 nm (Figure 10).Calculated back-reflection of light into the laser as a function of waveguide
tilt angle andz-separation (D).
(a)(b)


## 3.2. Photodetector

The effective aperture of a high-speed (≥10 Gbps) detector is typically smaller than 30μm in diameter to obtain low capacitance and
large bandwidth. To prevent large beam divergence and achieve high detection
efficiency, the gap between the output facet of a waveguide and the detector
surface must be as small as possible.The electronic connection employing
conventional wire bonds between the detector die to the transimpedance
amplifier (TIA) circuits may introduce too much unwanted parasitic capacitance
and inductance that limits the speed of the receiver, especially for higher
than 10 Gbps detection speed. Two possible methods of hybridization can be considered:
edge coupling and flip-chip coupling.When the detector is vertically attached to
the edge of the waveguide chip, the waveguide facet can be almost in direct
contact with the detector surface. The detector to waveguide alignment is
relatively simple because the effective detection area can be a few times
larger than the waveguide mode size. The relatively large aperture of the
photodiode covers the entire mode profile at the waveguide facet and ensures
high optical detection efficiency. A ceramic submount can be used to fasten the
detector (e.g., at the p-side) with necessary metal contacts and traces that can
readily be connected to a separated amplifier IC chip through wire bonding. The
main disadvantage of this approach is that the waveguide edge needs to be
polished, and hence it is not suitable for low-cost, high-volume manufacture.A more manufacturable  approach is to
flip-chip bond the detector onto the silicon chip surface (p-side up), where an
etched reflective mirror redirects the output beam to the detector surface, as
shown in Figure11. A smooth 〈111〉  direction crystal plane with a 54.7°
angle results from the anisotropic etching on the silicon surface; it is coated
with aluminum to enhance reflectivity. The etched waveguide facet is coated
with a thin nitride antireflection film to minimize the reflection loss at the silicon-to-air
interface.Figure 11
SEM photos of an etched waveguide facet
and turning mirror.All electronic contacts can be made through
the solder pads and metal traces directly plated on the silicon surface. A
separate amplifier IC chip with conducting via holes can be attached onto the
silicon chip surface to facilitate low parasitic electronic contact for
high-speed operation. Alternatively, the detector can be integrated with the
amplifier circuits before being mounted on the silicon chip.


## 4. Power Budget

The
available power budget for a point-to-point transmission system determines the
optical loss performance targets for the components making up the system. The
maximum optical loss permissible for the system is determined by the output
power of the laser source and the sensitivity of the detector. However, the
sensitivity of the detector is not only dependent upon the speed, but also on
the extinction of the source signal. For example, if the output power of a DFB
laser is +6 dBm and the maximum sensitivity of a PIN detector is −17 dBm at 10 Gbps
at an extinction of ~5 dB, then the maximum optical loss budget is +23 dB for
10 × 10 Gbps
transmission. Table1 summarizes the projected loss budget assuming use of the
mux/demux components demonstrated in this paper.Table 1
Loss
budget estimation for a 2 km transmission link.
Source of loss10 × 10 Gbps DFBLaser chip power6 dBmTo Mux−3In Mux−6.5Mux to fiber−1.5Fiber to DeMux−1.5DeMux−4DeMux to PD−0.5Maximum fiber loss (2 km)−2Signal quality (CDP/PMD)−2Power to detector−15 dbmDetector sensitivity−17 dbmMargin2 db


## 5. Conclusions

We
have discussed the feasibility of using the silicon photonics platform to
provide a viable route to low-cost and compact single-fiber transmission
solutions through hybridization of actives with low coupling loss and high-performance
mux-demux capability. Although this work concentrated on a 10-channel solution
and 10 Gbps data rate per channel, implementation with a greater number of
channels and higher data rates is readily achievable.



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