Study into the spread of heat from thermo-optic silicon photonic elements.

Phase modulators based upon the thermo-optic effect are used widely in silicon photonics for low speed applications such as switching and tuning. The dissipation of the heat produced to drive the device to the surrounding silicon is a concern as it can dictate how compact and tightly packed components can be without concerns over thermal crosstalk. In this paper we study through modelling and experiment, on various silicon on insulator photonic platforms, how close waveguides can be placed together without significant thermal crosstalk from adjacent devices.

Nonconservative Coupling in a Passive Silicon Microring Resonator.

The authors report on nonconservative coupling in a passive silicon microring between its clockwise and counterclockwise resonance modes. The coupling coefficient is adjustable using a thermo-optic phase shifter. The resulting resonance of the supermodes due to nonconservative coupling is predicted in theory and demonstrated in experiments. This Letter paves the way for fundamental studies of on-chip lasers and quantum photonics, and their potential applications.

Suspended mid-infrared waveguides for Stimulated Brillouin Scattering.

We theoretically investigate a new class of silicon waveguides for achieving Stimulated Brillouin Scattering (SBS) in the mid-infrared (MIR). The waveguide consists of a rectangular core supporting a low-loss optical mode, suspended in air by a series of transverse ribs. The ribs are patterned to form a finite quasi-one-dimensional phononic crystal, with the complete stopband suppressing the transverse leakage of acoustic waves, confining them to the core of the waveguide. We derive a theoretical formalism that can be used to compute the opto-acoustic interaction in such periodic structures, and find forward intramodal-SBS gains up to 1750 m-1W-1, which compares favorably with the proposed MIR SBS designs based on buried germanium waveguides. This large gain is achieved thanks to the nearly complete suppression of acoustic radiative losses.

Ultra-sharp asymmetric Fano-like resonance spectrum on Si photonic platform.

In this paper, we report the generation of an ultra-sharp asymmetric resonance spectrum through Fano-like interference. This generation is accomplished by weakly coupling a high-quality factor (Q factor) Fabry-Pérot (FP) cavity and a low-Q factor FP cavity through evanescent waves. The high-Q FP cavity is formed by Sagnac loop mirrors, whilst the low-Q one is built by partially transmitting Sagnac loop reflectors. The working principle has been analytically established and numerically modelled by using temporal coupled-mode-theory (CMT), and verified using a prototype device fabricated on the 340 nm silicon-on-insulator (SOI) platform, patterned by deep ultraviolet (DUV) lithography. Pronounced asymmetric resonances with slopes up to 0.77 dB/pm have been successfully measured, which, to the best of our knowledge, is higher than the results reported in state-of-the-art devices in on-chip integrated Si photonic studies. The established theoretical analysis method can provide excellent design guidelines for devices with Fano-like resonances. The design principle can be applied to ultra-sensitive sensing, ultra-high extinction ratio switching, and more applications.

Silicon-on-insulator free-carrier injection modulators for the mid-infrared.

Experimental demonstrations of silicon-on-insulator waveguide-based free-carrier effect modulators operating at 3.8 µm are presented. PIN diodes are used to inject carriers into the waveguides, and are configured to (a) use free-carrier electroabsorption to create a variable optical attenuator with 34 dB modulation depth and (b) use free-carrier electrorefraction with the PIN diodes acting as phase shifters in a Mach-Zehnder interferometer, achieving a VπLπ of 0.052 V·mm and a DC modulation depth of 22 dB. Modulation is demonstrated at data rates up to 125 Mbit/s.

49.6 Gb/s direct detection DMT transmission over 40 km single mode fibre using an electrically packaged silicon photonic modulator.

We present the characterization of a silicon Mach-Zehnder modulator with electrical packaging and show that it exhibits a large third-order intermodulation spurious-free dynamic range (> 100 dB Hz2/3). This characteristic renders the modulator particularly suitable for the generation of high spectral efficiency discrete multi-tone signals and we experimentally demonstrate a single-channel, direct detection transmission system operating at 49.6 Gb/s, exhibiting a baseband spectral efficiency of 5 b/s/Hz. Successful transmission is demonstrated over various lengths of single mode fibre up to 40 km, without the need of any amplification or dispersion compensation.

Angled multimode interferometer for bidirectional wavelength division (de)multiplexing.

We have demonstrated a bidirectional wavelength division (de)multiplexer (WDM) on the silicon-on-insulator platform using two 4-channel angled multimode interferometers (AMMIs) sharing the same multimode interference waveguide. An excellent match of the peak transmission wavelength of each channel between the two AMMIs was achieved. The input and output access waveguides were arranged in a configuration such that the propagation of light of one AMMI in the multimode interference waveguide suffered minimal perturbation by the input and output waveguides of the other AMMI. This type of device is ideal for the WDM system for datacom or telecom applications, e.g. an integrated optical transceiver, where the transmission wavelengths are required to match with the receiving wavelengths. The device also benefits from simple fabrication (as only a single lithography and etching step is required), improved convenience for the transceiver layout design, a reduction in tuning power and circuitry and efficient use of layout space. A low insertion loss of 3-4 dB, and low crosstalk of -15 to -20 dB, was achieved.

Mid-infrared wavelength division (de)multiplexer using an interleaved angled multimode interferometer on the silicon-on-insulator platform.

A low-cost and high-performance wavelength division (de)multiplexing structure in the mid-IR wavelength range is demonstrated on the silicon-on-insulator platform using an interleaved angled multimode interferometer (AMMI). As compared to a single AMMI, the channel count was doubled and the channel spacing halved with negligible extra insertion loss and crosstalk and with only a slight increase in device footprint. The device requires only single lithography and etching steps for fabrication. Potential is also shown for achieving improved performance with further optimized design.

Planar surface implanted diffractive grating couplers in SOI.

Grating couplers are used to efficiently couple light from an optical fibre to a silicon waveguide as they allow light to be coupled into or out from any location on the device without the need for cleaving. However, using the typical surface relief grating fabrication method reduces surface planarity and hence makes further processing more difficult. The ability to manufacture high quality material layers on top of a grating coupler allows multiple active optical layers to be realized for multi-layer integrated optical circuits, and may enable monolithic integration of optical and electronic circuits on separate layers. Furthermore, the nature of the refractive index change may enable removal via rapid thermal annealing for wafer scale testing applications. We demonstrate for the first time a coupling device utilising a refractive index change introduced by lattice disorder. Simulations show 44% of the power can be extracted from the waveguide by using uniform implanted gratings, which is not dissimilar to the performance of typical uniform surface relief gratings currently used. Losses determined empirically, of 5.5 dB per coupler have been demonstrated.

Sonographic determination of normal Conus Medullaris level and ascent in early infancy.

INTRODUCTION: Controversial reports exist in the literature regarding both the spinal level of the conus medullaris (CM) in normal infants and the age at which the CM achieves its adult level. Autopsy studies have demonstrated ascent continuing into early infancy while more recent imaging study series' suggest the adult conus level is attained by the 40th postmenstrual week. METHODS: The authors conducted a retrospective review of 1,273 screening lumbar ultrasound studies performed over 5 years at a pediatric tertiary referral center. All patients were infants referred for initial imaging to rule out the presence of a tethered spinal cord. Referral sources included urban academic, urban private practice, and rural private practice pediatricians. After excluding studies lacking sufficient documentation (n = 90) and those reported as abnormal (n = 106), 1,077 remained for review. The CM level and patient age in days were recorded from each study. Statistical analysis was performed using unpaired t testing and ANOVA for continuous variables; chi-square for categorical data. RESULTS: The mean CM level for infants in group I (ages 0-30 days) was compared to those in groups II (31-60 days) and group III (61-100 days). Group I had a mean CM level of 0.125 and 0.2 vertebral segments lower than groups II and III (p = 0.0005 and <0.0001, respectively). ANOVA comparison of all three groups confirmed a rostral migratory trend (p < 0.001). The prevalence of CM level caudal to L2 in group I was 13 %, group II 11.4 %, and group III 4.7 %; also indicating a significant rostral trend (p = 0.004). CONCLUSIONS: Rostral migration of CM level continues through the first few months of post-natal life, albeit of limited extent. Documentation of continued ascent in a neonate may obviate the need for magnetic resonance imaging.

High-contrast 40 Gb/s operation of a 500 µm long silicon carrier-depletion slow wave modulator.

In this Letter, we demonstrate a highly efficient, compact, high-contrast and low-loss silicon slow wave modulator based on a traveling-wave Mach-Zehnder interferometer with two 500 µm long slow wave phase shifters. 40 Gb/s operation with 6.6 dB extinction ratio at quadrature and with an on-chip insertion loss of only 6 dB is shown. These results confirm the benefits of slow light as a means to enhance the performance of silicon modulators based on the plasma dispersion effect.

Silicon slow-light-based photonic mixer for microwave-frequency conversion applications.

We describe and demonstrate experimentally a method for photonic mixing of microwave signals by using a silicon electro-optical Mach-Zehnder modulator enhanced via slow-light propagation. Slow light with a group index of ~11, achieved in a one-dimensional periodic structure, is exploited to improve the upconversion performance of an input frequency signal from 1 to 10.25 GHz. A minimum transmission point is used to successfully demonstrate the upconversion with very low conversion losses of ~7 dB and excellent quality of the received I/Q modulated QPSK signal with an optimum EVM of ~8%.

Wavelength division (de)multiplexing based on dispersive self-imaging.

We proposed and experimentally demonstrated wavelength division (de)multiplexers (WDMs) utilizing the wavelength dispersive nature of self-imaging multimode interferometers. Proof-of-principle devices fabricated on the silicon-on-insulator platform operated as 4-channel WDMs with a free spectral range of >90 nm, an averaging cross talk of <-20 dB for a 1 nm band, and an insertion loss of <2.0 dB. The potential for higher channel counts and smaller channel wavelength spacing was also predicted. This type of WDM is easy to design and fabricate. The underlying concept is applicable to all planar waveguide platforms.

Modulation of the absorption coefficient at 1.3 µm in Ge/SiGe multiple quantum well heterostructures on silicon.

We report modulation of the absorption coefficient at 1.3 µm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290-1315 nm.

High speed silicon electro-optical modulators enhanced via slow light propagation.

While current optical communication networks efficiently carry and process huge amounts of digital information over large and medium distances, silicon photonics technology has the capacity to meet the ceaselessly increasing demand for bandwidth via energy efficient, inexpensive and mass producible short range optical interconnects. In this context, handling electrical-to-optical data conversion through compact and high speed electro-optical modulators is of paramount importance. To tackle these challenges, we combine the attractive properties of slow light propagation in a nanostructured periodic waveguide together with a high speed semiconductor pn diode, and demonstrate a highly efficient and mass manufacturable 500 µm-long silicon electro-optical device, exhibiting error free modulation up to 20 Gbit/s. These results, supported by modulation rate capabilities reaching 40 Gbit/s, pave a foreseeable way towards dense, low power and ultra fast integrated networks-on-chip for future chip-scale high performance computing systems.

High contrast 40Gbit/s optical modulation in silicon.

Data interconnects are on the verge of a revolution. Electrical links are increasingly being pushed to their limits with the ever increasing demand for bandwidth. Data transmission in the optical domain is a leading candidate to satisfy this need. The optical modulator is key to most applications and increasing the data rate at which it operates is important for reducing power consumption, increasing channel bandwidth limitations and improving the efficiency of infrastructure usage. In this work silicon based devices of lengths 3.5mm and 1mm operating at 40Gbit/s are demonstrated with extinction ratios of up to 10dB and 3.5dB respectively. The efficiency and optical loss of the phase shifter is 2.7V.cm and 4dB/mm (or 4.5dB/mm including waveguide loss) respectively.

40 Gb/s silicon photonics modulator for TE and TM polarisations.

A key device in future high speed short reach interconnect technology will be the optical modulator. These devices, in silicon, have experienced dramatic improvements over the last 6 years and the modulation bandwidth has increased from a few tens of MHz to over 30 GHz. However, the demands of optical interconnects are significant. Here we describe an approach based on a self-aligned wrap around p-n junction structure embedded in a silicon waveguide that can produce high-speed optical phase modulation, whilst at the same time, capable of a high extinction ratio. An all-silicon optical modulator using a CMOS compatible fabrication process with a data rate of 40 Gb/s and extinction ratio up to approximately 6.5 dB for TE and TM polarisations is demonstrated. This technology is not only compatible with conventional complementary MOS (CMOS) processing, but is also intended to simplify and improve the reliability of, the fabrication process.

High speed silicon optical modulator with self aligned fabrication process.

With the imminent commercialisation of silicon photonic devices comes the requirement for a fabrication process capable of high yield and device performance repeatability. The precise alignment of the different elements of a device can be a major fabrication challenge for minimising performance variation or even device failure. In this paper a new design of high speed carrier depletion silicon optical modulator is introduced which features the use of a self-aligned fabrication process to form the pn junction. Experimental results are presented from an initial fabrication run, which has demonstrated a 6 dB modulation depth at 10 Gbit/s from a 3.5 m long device.

High-speed modulation of a compact silicon ring resonator based on a reverse-biased pn diode.

High speed modulation based on a compact silicon ring resonator operating in depletion mode is demonstrated. The device exhibits an electrical small signal bandwidth of 19 GHz. The device is therefore a candidate for highly compact, wide bandwidth modulators for a variety of applications.

Fabrication of low-loss silicon-on-oxidized-porous-silicon strip waveguide using focused proton-beam irradiation.

We have successfully fabricated low-loss silicon-on-oxidized-porous-silicon (SOPS) strip waveguides with high-index contrast using focused proton-beam irradiation and electrochemical etching. Smooth surface quality with rms roughness of 3.1 nm is achieved for a fluence of 1x10(15)/cm(2) after postoxidation treatment. Optical characterization at a wavelength of 1550 nm shows a loss of 1.1+/-0.4 dB/cm and 1.2+/-0.4 dB/cm in TE and TM polarization respectively, which we believe is the lowest reported loss for SOPS waveguides. This opens up new opportunities for all-silicon-based optoelectronics applications.