Silicon Nanowaveguide Research Articles

Stimulated Brillouin Scattering Gain of Waveguides Calculated with Acoustic Perturbation Method

Abstract Stimulated Brillouin Scattering (SBS) is a phenomenon of energy transfer from an optical pump beam to longer wavelengths of light through its interaction with the medium via acoustic phonons. Previous studies have reported a calculation of the SBS gain based on the optical force approach, where the gain is not only affected by the classical paradigm of intrinsic material’s nonlinear effects in the form of electrostriction but also due to radiation pressure. In this work, the acoustic perturbation approach is applied in analyzing the optical and mechanical response of the waveguide, wherein calculating the scattering process, the changes of the waveguide structure due to photoelasticity (PE), termed Moving Boundary (MB), are considered. This acoustic perturbation method avoids uncertainties related to optical force contributions. To validate its applicability, the present method is used to calculate the SBS gain of various waveguide structures, namely an unsuspended Silicon nano-waveguide, ridge Lithium Niobate (LiNBO3) nano-waveguide on Sapphire (Al2O3) substrate and buried Arsenic trisulfide (As2S3) in Silicon dioxide (SiO2). The forward and backward SBS gain obtained using the present method of acoustic perturbation are similar to the reported values obtained from other methods of calculation as well as experiments.

Graded-index thin-film stack for cladding and coupling.

A graded-index multilayer thin-film stack is optimized to act as a cladding layer on top of a silicon (Si) nanowaveguide and also a collimator for chip coupling where the waveguide ends. The numerical example shows an optimized graded-index profile from 2.35 to 1.45 provides an optical coupling to the standard single-mode fiber with efficiency close to 90% while retaining tight light confinement for the Si nanowaveguide. The corresponding material realization of a graded-index profile with a Si-rich nitride SiNx/SiON/SiO2 system is explored using inductively coupled plasma chemical vapor deposition, and a SiNx cladded Si waveguide is demonstrated.

40 Gb/s reconfigurable optical logic gates based on FWM in silicon waveguide.

Here we experimentally demonstrate reconfigurable logic gates via four-wave-mixing (FWM) in silicon waveguide with an operating speed of up to 40 Gb/s. After demodulated by a 40-GHz delay interferometer (DI), four non-return-to-zero differential phase shift keying (NRZ-DPSK) signals with carefully selected wavelengths are launched into the waveguide at the same time. Thanks to the effective FWM in silicon nano-waveguide, a full set of the two-input logic minterms can be generated simultaneously, and arbitrary combinational logic functions are able to be realized by properly combining these minterms.

Planar-type polarization beam splitter based on a bridged silicon waveguide coupler.

We demonstrate a compactly integrated polarization beam splitter (PBS) with high polarization extinction ratios greater than 20 dB over the full C-band wavelength range based on a simple bridged silicon nanowaveguide directional coupler. The PBS device is designed via three dimensional finite-difference time-domain (3D-FDTD) simulation, and fabricated experimentally. The optimum dimension of the bridge waveguide is determined to be 7.5-μm-long and 500 nm-wide for 250-nm thick silicon core. At the 1,550-nm wavelength, the measured polarization extinction ratios (PERs) of the PBS device are 22.5 dB and 22.9 dB for TE and TM polarization modes, respectively, and its corresponding insertion losses (ILs) are about 2.1 dB and 1.8 dB, both PERs and ILs within the maximum error range of ± 2.0 dB.

Bandwidth scaling of a phase-modulated continuous-wave comb through four-wave mixing in a silicon nano-waveguide.

We demonstrate an on-chip four-wave mixing (FWM) scheme in a silicon nanowaveguide to scale the bandwidth of a frequency comb generated by phase modulation of continuous-wave (CW) lasers. The FWM process doubles the bandwidth of the initial comb generated by the modulation of a CW laser. For example, a wavelength-tunable frequency comb with >100 comb lines spaced by 10 GHz within a bandwidth of 5 dB is generated.

Direct Temperature Mapping of Nanoscale Plasmonic Devices

Side by side with the great advantages of plasmonics in nanoscale light confinement, the inevitable ohmic loss results in significant joule heating in plasmonic devices. Therefore, understanding optical-induced heat generation and heat transport in integrated on-chip plasmonic devices is of major importance. Specifically, there is a need for in situ visualization of electromagnetic induced thermal energy distribution with high spatial resolution. This paper studies the heat distribution in silicon plasmonic nanotips. Light is coupled to the plasmonic nanotips from a silicon nanowaveguide that is integrated with the tip on chip. Heat is generated by light absorption in the metal surrounding the silicon nanotip. The steady-state thermal distribution is studied numerically and measured experimentally using the approach of scanning thermal microscopy. It is shown that following the nanoscale heat generation by a 10 mW light source within a silicon photonic waveguide the temperature in the region of the nanotip is increased by ∼ 15 °C compared with the ambient temperature. Furthermore, we also perform a numerical study of the dynamics of the heat transport. Given the nanoscale dimensions of the structure, significant heating is expected to occur within the time frame of picoseconds. The capability of measuring temperature distribution of plasmonic structures at the nanoscale is shown to be a powerful tool and may be used in future applications related to thermal plasmonic applications such as control heating of liquids, thermal photovoltaic, nanochemistry, medicine, heat-assisted magnetic memories, and nanolithography.

Effects of SiO2 hard masks on si nanophotonic waveguide loss for photonic device integration

As the basic building block for photonic device integration, silicon nanophotonic waveguide requires low-loss propagation for high-performance ultra-compact photonic device. We experimentally study silicon dioxide hard masks grown by two different methods, i.e., thermal oxidation and plasma-enhanced chemical vapor deposition (PECVD) for silicon nano-waveguides fabrication and their effects on the propagation loss. It is found that the denser and smoother quality of thermally grown silicon dioxide increases the etch selectivity against silicon and reduces the line edge roughness transferred to the silicon nano-waveguide sidewalls, hence resulting in a lower loss as compared with the PECVD silicon dioxide hard mask. With thermally grown silicon dioxide as a hard mask, the silicon nano-waveguides loss can be halved for a 650-nm-wide nano-waveguide, and the loss is comparable with a waveguide fabricated with a resist etch mask.

Ultra-compact optical 90° hybrid based on a wedge-shaped 2 × 4 MMI coupler and a 2 × 2 MMI coupler in silicon-on-insulator

We propose an ultra-compact optical 90° hybrid with the smallest length of 107μm, consisting of a wedge-shaped 2 × 4 MMI coupler connected with a 2 × 2 MMI coupler using silicon nanowaveguide technology. Neither cascaded phase shifters nor waveguide crossings are attached to the proposed 90° hybrid in coherent receiving system. The proposed device is demonstrated on silicon-on-insulator (SOI) with 220nm thick top-silicon layer and 2μm thick buried oxide layer. A high performance of the proposed 90° hybrid is exhibited experimentally with a high extinction ratio larger than 20dB, an excess loss mostly less than 0.5dB, a common mode rejection ratio better than -20dB and phase deviation within the range of 5° over C-band spectral range.

All-optical signal-conversion efficiency with a parameter-dependent four-wave-mixing process in a silicon nanowaveguide

We report on experimental measurements of the signal-wavelength conversion efficiency through the four-wave-mixing (FWM) process in a silicon strip nanowaveguide (SiNW) compared with theoretically-calculated results. The conversion efficiency has been investigated as a function of various parameters, such as the pump power and the pump and signal wavelengths. The measured and the calculated results indicate that a significant variation of the chromatic dispersion (CD) of our test SiNW device among the pump, signal and idler beam wavelengths and a high insertion loss in the device cause a very low FWM conversion efficiency. Our simulation tool can provide a direction for further improvement in the waveguide design by providing optimized CD values for the SiNW in desired ranges of the operation wavelengths.

Ultralow-power all-optical processing of high-speed data signals in deposited silicon waveguides

Utilizing a 6-mm-long hydrogenated amorphous silicon nanowaveguide, we demonstrate error-free (BER < 10(-9)) 160-to-10 Gb/s OTDM demultiplexing using ultralow switching peak powers of 50 mW. This material is deposited at low temperatures enabling a path toward multilayer integration and therefore massive scaling of the number of devices in a single photonic chip.

Wavelength conversion and unicast of 10-Gb/s data spanning up to 700 nm using a silicon nanowaveguide

We report extremely large probe-idler separation wavelength conversion (545 nm) and unicast (700 nm) of 10-Gb/s data signals using a dispersion-engineered silicon nanowaveguide. Dispersion-engineered phase matching in the device provides a continuous four-wave-mixing efficiency 3-dB bandwidth exceeding 800 nm. We report the first data validation of wavelength conversion (data modulated probe) and unicast (data modulated pump) of 10-Gb/s data with probe-idler separations spanning 60 nm up to 700 nm accompanied with sensitivity gain in a single device. These demonstrations further validate the silicon platform as a highly broadband flexible platform for nonlinear all-optical data manipulation.

Characterization of Nonlinear Optical Crosstalk in Silicon Nanowaveguides

We investigate optical crosstalk on a signal in a silicon nanowaveguide due to the presence of another signal by direct radio frequency crosstalk level measurements in a pump-probe configuration and by bit-error-rate-based characterization. We quantify this degradation as a function of the modulation frequency and power of the auxiliary signal. Our results indicate that two-photon and free-carrier absorption are primary facilitators of crosstalk in silicon nanowaveguides.

A New Method of Measuring Localized Chromatic Dispersion of Structured Nanowaveguide Devices Using White-Light Interferometry

We report on a simple and direct method of measuring localized chromatic dispersion (CD) profiles of structured nanowaveguide devices using white-light interferometry. Phase change in the interference fringe of a white-light interferometer (WLI) caused by introducing a sample-under-test into one of the interferometer arms was used to determine a spectral profile of its accurate CD coefficients. This method was tested by measuring the spectral CD profiles of a localized silicon nanowaveguide and of a nonmembrane-type photonic crystal waveguide (PhCW) excluding their optical coupling sections. The measured local CD values of a 500-nm-wide single-line-defect (W1) PhCW with a lattice period of 460 nm and the hole radius of 165 nm formed on 220-nm-thick silicon layer of a silicon-on-insulator (SOI) wafer varied from 0.38 to 0.22 ps/(nm ·cm) for a wavelength range from 1530 to 1570 nm. This method will be very useful in determining the accurate CD profiles of nanophotonic waveguide devices.

Compact all-optical switch for WDM networks based on Raman effect in silicon nanowavegide

We propose an all-optical switching scheme based on Raman gain in a silicon nanowaveguide suitable for multichannel optical communication. Raman gain is used for amplification of a control pulse with a higher wavelength, which depletes the tuned channel signal. Separation between control and signal pulses should be equal to the Raman shift in silicon. By employing a 3 mm channel nanowaveguide, we demonstrate a channel attenuation of about 12 dB, while the suppression ratios for the first and second neighboring channels are about 1.6 dB and 1 dB, respectively. This scheme can be used as an all-optical switch in dense wavelength division multiplexing networks. Moreover, we demonstrate that the depleted channel can be retrieved by a control pulse with lower wavelength in which the pulse amplifies the channel in contrast to the prior situation.

Observation of spectral and temporal polarization oscillations of optical pulses in a silicon nanowaveguide

We observe both spectral and temporal oscillations in the polarization state of optical pulses propagating through a silicon nanowaveguide. The spectral oscillations are linear in nature and result from polarization-mode dispersion (PMD). The temporal oscillations are nonlinear in nature, and theoretical simulations clarify that they result from the combined effects of two-photon absorption generated free carriers and PMD.

Spectrum reshaping of amplified pulse in silicon nanowaveguide

A device consisting of a cascaded semiconductor optical amplifier (SOA) and silicon on insulator (SOI) optical waveguide is presented to amplify and reshape the frequency spectrum of optical pulses in the picoseconds time duration. Numerical simulations show that the output spectrum of the amplified pulse by SOA can be effectively reshaped by utilizing the SOI waveguide. The length of the SOI waveguide may be judiciously adjusted to significantly reduce the frequency chirp of the output pulse from the SOA resulting in reshaping of the output spectrum. We find that the property of pulse spectrum is sensitive to the input pulse power and its temporal width.

All-optically tunable waveform synthesis by a silicon nanowaveguide ring resonator coupled with a photonic-crystal fiber frequency shifter

A silicon nanowaveguide ring resonator is combined with a photonic-crystal fiber (PCF) frequency shifter to demonstrate an all-optically tunable synthesis of ultrashort pulse trains, modulated by ultrafast photoinduced free-carrier generation in the silicon resonator. Pump–probe measurements performed with a 50-fs, 625-nm second-harmonic output of a Cr:forsterite laser, used as a carrier-injecting pump, and a 1.50–1.56-μm frequency-tunable 100-fs soliton output of a photonic-crystal fiber, serving as a probe, resolve tunable ultrafast oscillatory features in the silicon nanowaveguide resonator response.

Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides

We demonstrate reduction of the free-carrier lifetime in a silicon nanowaveguide from 3 ns to 12.2 ps by applying a reverse bias across an integrated p-i-n diode. This observation represents the shortest free-carrier lifetime demonstrated to date in silicon waveguides. Importantly, the presence of the p-i-n structure does not measurably increase the propagation loss of the waveguide. We derive a figure of merit demonstrating equal dependency of the nonlinear phase shift on free-carrier lifetime and linear propagation loss.

Frequency conversion over two-thirds of an octave in silicon nanowaveguides

We demonstrate ultrabroad-bandwidth low-power frequency conversion of continuous-wave light in a dispersion engineered silicon nanowaveguide via four-wave mixing. Our process produces continuously tunable four-wave mixing wavelength conversion over two-thirds of an octave from 1241-nm to 2078-nm wavelength light with a pump wavelength in the telecommunications C-band.

Optical Switching in Silicon Nanowaveguide Ring Resonators Based on Kerr Effect and TPA Effect

We analyze theoretically the 1×2 low-power all-optical switching in silicon nanowaveguide ring resonators (RR) based on the Kerr effect and two-photon absorption (TPA), and give a comparison between both the all-optical switches. The calculation shows that the switching power of the TPA-RR switch is 3 orders smaller than that of the Kerr-RR switch. The switching time for both the switches is about 100 ps.