Low-loss Silicon Waveguides Research Articles

Fabrication, Characterization and Loss Analysis of Silicon Nanowaveguides

Low loss silicon waveguides are the key to the realization of high performance photonic integrated circuits. In this paper, fabrication, characterization and loss analysis of silicon nanowaveguides are presented. Silicon nanowaveguides are fabricated on silicon-on-insulator (SOI) wafers with 0.13 μm complementary metal-oxide-semiconductor (CMOS) technology. To reduce the propagation loss, both photolithography and etching processes are optimized to make the waveguide sidewalls smooth. Propagation losses of 2.4 ± 0.2 and 0.59 ± 0.32 dB/cm are obtained at 1550 nm wavelength for TE and TM modes, respectively. A theoretical method is used to estimate the propagation losses for TE and TM modes. Scattering losses from both sidewalls and top/bottom surface are considered. The calculated results show that loss comes from sidewall roughness is the main source of propagation loss for TE mode while for TM mode, losses from both sidewall and top/bottom surface contribute comparably to the total propagation loss. The theoretically estimated propagation loss agrees well with the measured results.

Optimization of electron beam patterned hydrogen silsesquioxane mask edge roughness for low-loss silicon waveguides

We carried out a multiparameter fabrication study designed to reduce the line edge roughness (LER) of electron beam (e-beam) patterned hydrogen silsesquioxane resist for the purpose of producing low-loss silicon strip waveguides. Reduced mask roughness was achieved for 50°C pre-exposure baking, 5000 μC/cm 2 dose with a beam spot size more than twice as large as the electron beam step size, development in 25% tetramethylammonium hydroxide and postdevelopment baking with rapid thermal annealing in an O 2 ambient at 1000°C. The LER caused by pattern fracturing and stage stitches was reduced with multipass writing and per-pass linear and rotational offsets. Si strip waveguides patterned with the optimized mask have root-mean-square sidewall roughness of 2.1 nm with a correlation length of 94 nm, as measured by three-dimensional atomic force microscopy. Measured optical propagation losses of these waveguides across the telecommunications C-band were 2.5 and 2.8 dB/cm for the transverse magnetic and transverse electric modes, respectively. These reduced loss waveguides enable the fabrication of advanced planar lightwave circuit topologies.

A CMOS-Compatible, Low-Loss, and Low-Crosstalk Silicon Waveguide Crossing

We demonstrated a waveguide crossing for submicron silicon waveguides with average insertion loss of 0.18±0.03 dB and crosstalk of -41±2 dB, uniform across an 8-inch wafer. The device was fabricated in a CMOS-compatible process using 248 nm lithography, with only one patterning step.

Electronically controlled optical beam-steering by an active phased array of metallic nanoantennas

An optical phased array of nanoantenna fabricated in a CMOS compatible silicon photonics process is presented. The optical phased array is fed by low loss silicon waveguides with integrated ohmic thermo-optic phase shifters capable of 2π phase shift with ∼ 15 mW of applied electrical power. By controlling the electrical power to the individual integrated phase shifters fixed wavelength steering of the beam emitted normal to the surface of the wafer of 8° is demonstrated for 1 × 8 phased arrays with periods of both 6 and 9 μm.

Silicon nanophotonic devices for chip-scale optical communication applications [Invited

This paper reviews recent work in the area of silicon photonic devices and circuits for monolithic and heterogeneous integration of circuits and systems on a chip. In this context, it presents fabrication results for producing low-loss silicon waveguides without etching. Resonators and add-drop distributed filters utilizing sidewall modulation fabricated in a single lithography and etching step are demonstrated. It also presents an optical pulse compressor that monolithically integrates self-phase modulation and anomalous dispersion compensation devices on a silicon chip. As an example of heterogeneous integration, we demonstrate vertical emitting metallo-dielectric nanolasers integrated onto a silicon platform. Future research directions toward large-scale photonic circuits and systems on a chip also are discussed.

Monolithic Silicon Waveguides in Standard Silicon

A methodology allows fabrication of silicon-photonic devices in standard silicon wafers, eliminating the need for silicon-on-insulator (SOI) wafers. Using this technology, the authors demonstrate low-loss silicon waveguides (2.34 dB/cm), comparable to conventional SOI channel waveguides. The ease and flexibility of this fabrication method simplify integration of electronics and photonics and make it a possible alternative to SOI-based technology for implementation of silicon-photonic devices and systems.

Nonlinear distortion of optical pulses by self-produced free carriers in short or highly lossy silicon-based waveguides

An explicit analytical solution for the asymmetric attenuation of optical pulses by self-produced free carriers in silicon waveguides is derived. It allows us to quantify the pulse distortion and to calculate explicitly the free-carrier density and the nonlinear phase shifts caused by the Kerr effect and by free-carrier refraction. We show that omitting two-photon absorption (TPA) as a cause of attenuation and accounting only for free-carrier absorption (FCA) as done in the derivation appropriately models the pulse propagation in short or highly lossy silicon-based waveguides such as plasmonic waveguides with particular use for high-energy input pulses. Moreover, this formulation is also aimed at serving as a tool in discussing the role of FCA in its competition with TPA when used for continuum generation or pulse compression in low-loss silicon waveguides. We show that sech-shaped intensity pulses maintain their shape independently of the intensity or pulse width and self-induced FCA may act as an ideal limiter on them. Pulse propagation under self-induced free-carrier absorption exhibits some features of superluminal propagation such as fast or even backward travelling. We find that input pulses need to have a sufficiently steep front slope to be compressible at all and illustrate this with the FCA-induced pulse broadening for Lorentzian-shaped input pulses.

Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks

An etch-free fabrication technique for creating low loss silicon waveguides in the silicon-on-insulator material system is proposed and demonstrated. The approach consists of local oxidation of a silicon-on-insulator chip covered with a e-beam patterned hydrogen silsesquioxane mask. A single oxidation step converts hydrogen silsesquioxane to a glass-like compound and simultaneously defines the waveguides, bypassing the need for any wet or dry etching steps. The spectral response of ring resonators fabricated using this technique was used to characterize the waveguide losses. Intrinsic Q-factors as high as 1.57 × 10(6), corresponding to a waveguide loss of 0.35 dB/cm, were measured.

Low loss silicon waveguides for the mid-infrared

Silicon-on-insulator (SOI) has been used as a platform for near-infrared photonic devices for more than twenty years. Longer wavelengths, however, may be problematic for SOI due to higher absorption loss in silicon dioxide. In this paper we report propagation loss measurements for the longest wavelength used so far on SOI platform. We show that propagation losses of 0.6-0.7 dB/cm can be achieved at a wavelength of 3.39 µm. We also report propagation loss measurements for silicon on porous silicon (SiPSi) waveguides at the same wavelength.

Integrated Microwave Photonic Filter on a Hybrid Silicon Platform

A hybrid silicon photonic integrated filter is proposed and demonstrated with a novel structure. This filter incorporates a ring resonator in one arm of a Mach-Zehnder interferometer making it possible to obtain a programmable filter response. The optical filter consists of a 5-mm-long delay loop made of low-loss silicon waveguides with integrated thermal modulators resulting in a 0.164-nm free spectral range with absolute phase tunability and gain elements that allow for the tuning of the filter factor. The microwave response of this integrated filter is measured and display tunability of 20 GHz.

Low-loss amorphous silicon waveguides grown by PECVD on indium tin oxide

Low-loss hydrogenated amorphous silicon (α-Si:H) waveguides were realized by plasma enhanced chemical vapour deposition (PECVD) on a transparent conductive oxide (TCO) layer which is intended to provide the buried contact for the application of an external bias in active devices, e.g. switches and modulators. In particular we propose a technological solution to overcome both the strong reduction in optical transmittance due to the very high extinction coefficient of metal contacts and, at the same time, the optical scattering induced by the texturization effect induced in α-Si:H films grown on TCO. The very high optical propagation losses were minimized by depositing a spin-on-glass (SOG) layer between the α-Si:H core-layer and the TCO bottom contact. In this case, propagation losses of 2.5 dB/cm at 1550 nm were measured. All the fabricated samples were optically characterized and the surface roughness was accurately measured using a mechanical profilometer. We observed that, for an α-Si:H core-layer directly deposited on the TCO contact, the surface roughness is of the order of 100 nm leading to totally opaque waveguides. The experimental performances have been compared to those obtained through calculations using an optical simulation package. The results are found to be in agreement with the experimental data.

Advances in lasers and optical micro-nano-systems

Lasers represent a well consolidated technology: nevertheless, research in this field remains very active and productive, in both basic and applied directions. At the moment significant attention is given to those sources that bring together high power and compactness. Such high power lasers find important applications for material treatments and such applications are presented by Ehsani et al and Saiedeh Saghafi et al, in the treatment of dielectric thin films (Alteration of optical and morphological properties of polycarbonate illuminated by visible/IR laser beams) or of biological tissues like pistachio seeds (Investigating the effects of laser beams (532 and 660 nm) in annihilation of pistachio mould fungus using spectrophotometry analysis). In particular the latter paper show how laser sources can find very important applications in new domains, preserving goods and food without the need for preservatives or pesticides by simply sterilizing them using light. Optical Micro and Nano Systems presents a new domain for exploration. In this framework this special issue is very attractive, because it assembles papers reporting new results in three directions: new techniques for monitoring integrated micro- and nano-systems, new integrated systems and novel high performance metamaterial configurations. Integrated micro-components can be monitored and controlled using reflectance measurements as presented by Piombini et al (Toward the reflectance measurement of micro components). Speckle formation during laser beam reflection can also be a very sophisticated tool for detecting ultra-precise displacements, as presented by Filter et al (High resolution displacement detection with speckles : accuracy limits in linear displacement speckle metrology). Three dimensional integrated optical structures is indeed a big challenge and a peculiarity of photonics, they can be formed through traditional holography or using more sophisticated and novel technologies. Thus, special manipulation of the writing-reading optical beam can push holography toward storages at higher data densities, as presented by Norihiko Ishii et al (Wavefront compensation method using novel index in holographic data storage). Along a similar direction Furlan et al describe a very innovative technique for producing optical traps using novel Devil micro-lenses (Volumetric multiple optical traps produced by Devil’s lenses). Vynnyk et al presented an interesting application of electron microscopy for monitoring sub-micrometric structures in 3D configurations (3D-measurement with the stereo scanning electron microscope on sub-micrometer structure). Finally, S. Rao et al present two interesting papers on integrated structures compatible with silicon technology: one describes the realisation of low-loss waveguides using amorphous silicon, a relatively novel material with many applications in very different domains (Low-loss amorphous silicon waveguides grown by PECVD on indium tin oxide), and one on the realisation of a electrically drivable device with affective compatibility with CMOS technology (Electro-optical modulating multistack device based on the CMOS-compatible technology of amorphous silicon). We hope that this special issue of the Journal of the European Optical Society will reflect the interest of the European Scientific Community toward these fundamental and applied topics and will demonstrate to readers some of the actual directions of research. We express our full appreciation to the authors that participated to this initiative which acts only as a primer for the vast amount of work now being undertaken in laser physics and applications in micro- and nano-systems. We would like to give a special thank to the paper reviewers for their important role in the paper selection process and all the journal staff for their very professional support, dedication and energy, which made this special issue feasible.

Low loss etchless silicon photonic waveguides

We demonstrate low loss silicon waveguides fabricated without any silicon etching. We define the waveguides by selective oxidation which produces ultra-smooth sidewalls with width variations of 0.3 nm. The waveguides have a propagation loss of 0.3 dB/cm at 1.55 microm. The waveguide geometry enables low bending loss of approximately 0.007 dB/bend for a 90 degrees bend with a 50 microm bending radius.

An integrated recirculating optical buffer

This paper reports an integrated optical buffer consisting of a low loss silicon waveguide delay line and a silicon evanescent gate matrix switch. The integrated device demonstrates an error free operation at 40 Gb/s data rate with a packet delay of 1.1 ns. This demonstration also highlights the silicon evanescent device platform to realize new types of photonic integrated devices by combining the low loss silicon passive components with the silicon evanescent photonic active devices.

Raman amplification of 40 Gb/s data in low-loss silicon waveguides

We demonstrate on-chip Raman amplification of an optical data signal at 40 Gb/s in a silicon-on-insulator p-i-n rib waveguide. Using 230 mW of coupled pump power, on/off gain of up to 2.3 dB is observed, while signal integrity is maintained. In addition, the gain is measured as a function of signal wavelength detuning from the Stokes wavelength. The Lorentzian linewidth of the Raman gain profile is determined to be approximately 80 GHz. This provides applicability for the selective amplification of individual DWDM optical channels.

Optical amplification and lasing by stimulated Raman scattering in silicon waveguides

Achieving light amplification and lasing in silicon is one of most challenging goals in silicon-based optoelectronics. As a nonlinear optical effect, stimulated Raman scattering (SRS) provides a means to generate optical gain in silicon. Recent results of a nonlinear optics approach to optical amplification and lasing in silicon at the Photonics Technology Laboratory of Intel Corporation are reviewed. This paper starts with the description of the underlying physics related to the Raman scattering in silicon and experimental results of SRS in silicon waveguides. Then, it is shown that nonlinear optical absorption associated with the two-photon absorption (TPA)-induced free carrier absorption (FCA) is a dominant loss mechanism limiting optical gain in a silicon waveguide in addition to the linear optical scattering loss due to the waveguide sidewall roughness. The design and fabrication of a low-loss silicon waveguide containing a p-i-n diode to reduce the nonlinear optical loss are described. It is demonstrated that the free carrier density inside the waveguide can be reduced significantly with a reverse bias of the p-i-n diode. As a result, net optical gain in a silicon waveguide is achieved. The design, fabrication, and characterization of a Raman silicon laser are also described. Both pulsed and continuous-wave (CW) lasing in silicon are achieved using SRS.

Raman gain and nonlinear optical absorption measurements in a low-loss silicon waveguide

We fabricated a low-loss (∼0.22dB∕cm) rib waveguide (WG) in silicon-on-insulator with a small effective core area of ∼1.57μm2 and measured the stimulated Raman scattering gain in the WG. We obtained 2.3dB Raman gain in a 4.8-cm-long S-shaped WG using a 1455nm pump laser with a cw power of 0.9W measured before the WG. In addition, we observed nonlinear dependence of Raman gain and optical propagation loss as a function of the pump power. Our study shows that this mainly is due to two-photon absorption (TPA) induced free carrier absorption in the silicon WG. We experimentally determined the TPA induced free carrier lifetime of 25ns, which agrees well with our modeling.

Net optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering

We observe for the first time net optical gain in a low loss silicon waveguide in silicon-on-insulator (SOI) based on stimulated Raman scattering with a pulsed pump laser at 1.545 microm. We show that pulsed pumping with a pulse width narrower than the carrier recombination lifetime in SOI significantly reduces the free carrier generation rate due to two-photon absorption (TPA) in silicon. For a 4.8 cm long waveguide with an effective core area of ~1.57 microm2, we obtained a net gain of 2 dB with a pump pulse width of ~17 ns and a peak pump power of ~470 mW inside the waveguide.

Low-loss polycrystalline silicon waveguides for silicon photonics

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly-Si), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as-deposited bulk poly-Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly-Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to about 4–6 nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of transmission electron microscopy. Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1–0.4 μm range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature electron-cyclotron resonance hydrogenation step. After hydrogenation, we obtain the lowest reported poly-Si loss values at λ=1.54 μm of about 15 dB/cm.