Optical interconnect technology is
perceived as a key solution to solving major performance limitations in high
performance computing caused by bus bandwidth bottleneck, latency and power
consumption issues.
While these issues have caused the clock frequencies to saturate at a few GHz,
the use of multi-thread and multi-core processor technologies have enabled
processor performance to scale forward. However, the parallelism resulting from
such architectures and the resulting aggregate bandwidth, place stringent
requirements on the power and footprint of the interconnecting technology. It
is widely accepted that Si photonics, due to its electronics integration
capability, proven manufacturing record and price volume curve, will form the
platform of choice for this interconnect technology [5].
A key component to enabling this is a high-speed, efficient modulator requiring
a low drive voltage and a small footprint. Although significant progress has
been made in this area ,
previously demonstrated silicon modulators do not fully satisfy these
requirements for on-chip applications. Here, we present a high-speed silicon
optical modulator that is the closest to date of any reported device to
demonstrating the combined system level performance requirements placed on such
a device.
On chip applications of silicon
modulators require excellent electrical and optical performance in terms of
speed, power/energy consumption, dynamic voltage swing, modulation depth,
insertion loss, and device size. Particularly, a compact silicon modulator with
a low Vpp and low energy for which optical performance is not
sacrificed is highly desired.
In recent years, high-speed waveguide based silicon modulators have been
developed using the plasma dispersion effect ,
where the refraction index of silicon is changed by injecting/removing free
carriers. The demonstrated modulators were based on either a MOS capacitor,
a forward-biased p-i-n diode,
or a reverse-biased pn junction.
Due to the weak index change induced by this effect, previously demonstrated
silicon modulators suffer from either high power consumption and high Vpp,
or high insertion loss.
summarizes the device performance for various silicon-integrated
modulators including a reversed-biased Mach-Zehnder interferometer (MZI)
modulator,
a forward-biased MZI modulator,
a forward-biased p-i-n based microring modulator,
a GeSi electro-absorption modulator,
a reversed-biased disk modulator,
and the device presented in this work. MZI based modulators usually exhibit a
large footprint, have a energy consumption greater than 5 pJ per bit, and
require a large Vpp of 6-8 V. Microring based modulators can
dramatically reduce the power consumption by taking advantage of strong light
confinement in the resonator, however, the demonstrated forward-biased p-i-n
modulator requires a Vpp of ~3.5 V and a complex pre-emphasis
driving signal to achieve high speed operation.
Recently demonstrated SiGe electro-absorption modulators integrated with Si
waveguides
exhibit low power consumption, but suffer from large insertion loss due to
residual material absorption and fabrication complexity due to GeSi growth.
Another modulator example which has ultralow energy is a silicon disk modulator
using reverse-biased vertical pn junction reported in ref.
This disk device exhibits a Vpp of 3.5 V and power consumption of 85
fJ/bit, and may suffer from high-order resonance associated with the disk
cavity structure, which limits possible wavelength channels if wavelength
division multiplexing is used. In comparison, the reverse-biased microring
modulator in this work demonstrates a 3dB bandwidth of 11 GHz with a low Vpp
of 2 V and ultralow energy consumption of 50 fJ per bit without suffering from
optical performance.
The modulator reported here consists
of a ring resonator coupled to a neighboring bus waveguide. The bus and ring waveguides have a width of 0.5 µm, a height of
0.25 µm and a slab height of 50 nm. Such waveguide geometry enables very tight
waveguide bend radii, down to a few microns. The optical resonance condition is
satisfied when the circumference of the ring corresponds to an integer multiple
of the guided light wavelength. The transmission of an optical signal at the
resonant wavelength can be dramatically reduced, and the extinction ratio
between on/off resonance can be greatly enhanced under the critical condition
where the power coupling between the ring and the waveguide equals the round
trip loss of the ring. Tuning the effective index of the ring waveguide
modifies the resonant wavelength which induces a strong modulation of the
transmitted signal. To achieve this, we design a lateral pn diode along the
ring waveguide. For a reverse-biased pn junction, a high electrical field is
built up across the junction and sweeps out the free electrons and holes in the
depletion region.
The depletion width depends on the bias voltage and the doping concentrations .
Altering the bias voltage modifies the depletion width and hence the effective
index of the ring waveguide through the plasma dispersion effect.
In general, increasing the doping concentration increases the index change with
a fixed bias voltage,
however, it would also increase the optical loss of the ring waveguides. By
optimizing the overlap between the optical mode and the depletion region, we
designed an asymmetric pn junction with a p doping concentration of 5x1017
cm−3 and an n doping concentration of 1x1018 cm−3.
The pn junction position is set at 50 nm offset from the mode centre because hole
concentration changes induce a larger index change than electron concentration
changes.
In order to ensure a good ohmic contact between silicon and metal, heavy doped
p + + and n + + regions with doping concentrations of ~1x1020 cm−3
are utilized at the metal to silicon interface. In addition, the metal contacts
are positioned over 250 nm silicon regions rather than 50 nm slab areas, as
shown in. This reduces possible metal-induced optical loss due to the proximity
of the metal to the waveguide.
Transmission spectra under different bias voltages demonstrate a large resonance shift which enables high modulation depth with a low Vpp. Figure 2(a) presents the transmission spectra with bias voltages of 0V, −1V and −2V, respectively. The on/off extinction ratio of the ring resonance exceeds 15 dB, demonstrating the ring is operated almost under the critical coupling condition. The quality factor of the ring resonator is estimated at ~14500. The resonance shift per unit voltage is 18 pm/V. This value is significantly larger than that of a similar racetrack resonator silicon modulator (2.3 pm/V) using lateral reverse biased pn junction . Another figure of merit for modulation efficiency usually used in MZI modulators, V·Lπ (Lπ is the device length needed to achieve a π phase shift) is ~1.5 V·cm for this device. If we use Vpp = 2 V to drive the modulator, the on/off modulation ratio and insertion loss can be obtained from the spectral measurement at 0 V and −2 V. One can see that, from Fig. 2(b), there are two modulation ratio peaks, which arise due to two different resonant wavelengths at 0 V and −2 V respectively. This modulator can operate at either of these peaks. At a wavelength of 1551.84 nm, we achieve a modulation depth of 6.5 dB and an insertion loss of 2 dB, which are values comparable to typical modulator performance requirements seen in the optical industry. If we can tolerate more insertion loss (~3 dB), a modulation depth of ~12 dB is achievable.
Ultra-low energy consumption for high-speed modulation
relies on low device capacitance. For a reverse biased pn diode, the energy
consumption for dynamic modulation is mainly determined by the transient energy
consumption during the on/off transitions since the DC current is extremely low
[< 1 nA in the device shown in Fig.
1(a). For non-return-to-zero (NRZ) modulation, the energy
consumption per bit can be expressed as
where C is the device capacitance (the factor 4 is due to the fact that
the 0-1 transition only happens with a probability of 0.25 for all bit
sequences [22]). Considering an experimentally measured junction capacitance of ~50 fF for our device and a Vpp
of 2 V, we can obtain the energy consumption per bit as 50 fJ/bit. This
can be further reduced by scaling down the ring radius to a few
microns.
The device 3 dB modulation bandwidth is determined by the RC time and the photon lifetime, as expressed by
Fig. 3 Dynamic EO response of the device. The main panel shows the EO response in the frequency range of 50 MHz to 20 GHz with a small signal drive. The 3 dB bandwidth is determined as 11GHz and no frequency roll-off is observed up to 6 GHz. The transmission spectrum (inset) is taken during the EO response measurement. We chose a wavelength for this measurement such that that the normalized transmitted power is ~0.5 (the most linear regime in the ring resonance).
Fig. 4 High-speed modulation of the device. The eye diagrams are measured with NRZ signal (PRBS 223 −1, Vpp = 2 V, Vdc = −1 V) at 5 Gbps (a) and 10 Gbps (b).
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