Silicon-rich Nitride Waveguides Research Articles

All-Optical Modulation Format Conversions from PAM4 to QPSK and 16QAM Using Silicon-Rich Nitride Waveguides

Quadrature phase-shift keying (QPSK) and 16-quadrature amplitude modulation (16QAM) formats are deployed in inter-data center networks where high transmission capacity and spectral efficiency are required. However, in intra-data center networks, a four-level pulse amplitude modulation (PAM4) format is deployed to satisfy the requirements for a simple and low-cost transceiver configuration. For the seamless and effective connection of such heterogeneous networks without an optical-electrical-optical conversion, an all-optical modulation format conversion technique is required. In this paper, we propose all-optical PAM4 to QPSK and 16QAM modulation format conversions using silicon-rich nitride waveguides. The successful conversions from 50-Gbps-class PAM4 signals to 50-Gbps-class QPSK and 100-Gbps-class 16QAM signals are demonstrated via numerical simulations.

Wideband Simultaneous Wavelength Conversion of Multiple WDM Channels Using Silicon-Rich Nitride Waveguide

The triple-band (S+C+L) transmission system using only C-band transceivers and all-optical wavelength conversions (AOWCs) without S- and L-band optical transceivers have been proposed. Although an AOWC employs highly nonlinear fibers, it is not applicable to photonic integrated circuits (PICs). For a device with successful wavelength conversion, a silicon-rich nitride (SRN) waveguide provides superior performance, as it is applicable to PIC and facilitates a higher Kerr nonlinearity than conventional stoichiometric silicon nitride waveguides. Thus, in this study, we propose an all-optical simultaneous wavelength conversion method that can be cost-effectively fabricated on a PIC based on four-wave mixing in SRN waveguides. Following an explanation regarding the design of an SRN waveguide for wideband AOWC, the successful operation of a double-stage AOWC of 64-channel 64-Gb/s QPSK signals between the C+L bands and the S-band is demonstrated, which is useful for wideband transmission systems.

Widely Tunable Single-Photon Frequency Conversion in a Nanomechanically Reconfigurable Si-Rich Nitride Waveguide

An on-demand single-photon source based on self-assembled semiconductor quantum dots (QDs) is a key building block for quantum communication and quantum information processing. To scale up, a challenge lies in frequency converting the emitted photons to the telecom wavelengths and eliminating the wavelength difference between multiple QDs. In this paper, we propose a novel nanomechanical silicon-rich nitride waveguide device, which can be reconfigured to realize efficient frequency conversion from the emission wavelength band of InAs/GaAs quantum dots at 900 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 950 nm to the telecom C-band around 1550 nm, by exploiting the Bragg scattering process in nonlinear four-wave mixing. Numerical results show that an optimal conversion efficiency over 99% and a signal-to-noise ratio up to 27 dB can be achieved when the waveguide loss is negligible, and the efficiency only reduces to 78% when a realistic loss of 1 dB/cm is considered. We conduct a detailed study on the unwanted noise processes, and provide design guidelines to improve the conversion efficiency and suppress the noises, e.g., by optimizing the power and wavelength of the pump laser.

Edge coupling for hybrid mono-crystalline silicon and lithium niobate thin films

The hybridization of mono-crystalline silicon and lithium niobate thin films (Si-LNOI) combines the excellent electrical properties and mature micro-nano processing technology of Si, as well as the remarkable optical properties of LN. The Si-LNOI platform will drive new and promising integrated photonics devices. High-efficiency fiber-waveguide optical coupling is necessary to realize the full potential of devices and practical applications. In this study, a spot-size converter (SSC) was designed and demonstrated for efficient edge coupling between a Si-LNOI waveguide and lens fiber. The SSC was fabricated by standard semiconductor process, which consisted of an inverted-tapered Si and a silicon-rich nitride (SRN) waveguide overlying the inverted-tapered Si. At a wavelength of 1550 nm, the TE and TM light achieved coupling losses of 1.9 and 2.1 dB/facet, respectively. The coupling efficiency was stable in the wavelength range of 1500–1600 nm. The tolerance of alignment was also evaluated.

Optical characterization of deuterated silicon-rich nitride waveguides

Chemical vapor deposition-based growth techniques allow flexible design of complementary metal-oxide semiconductor (CMOS) compatible materials. Here, we report the deuterated silicon-rich nitride films grown using plasma-enhanced chemical vapor deposition. The linear and nonlinear properties of the films are characterized, and we experimentally confirm that the silicon-rich nitride films grown with SiD4 eliminates Si–H and N–H related absorption. The performance of identical waveguides for films grown with SiH4 and SiD4 are compared demonstrating a 2 dB/cm improvement in line with that observed in literature. Waveguides fabricated on the SRN:D film are further shown to possess a nonlinear parameter of 95 W−1 m−1, with the film exhibiting a linear and nonlinear refractive index of 2.46 and 9.8 times 10–18 m2W−1 respectively.

Investigation on optical integration between LED Mid-IR light sources and Si-based waveguides for sensing applications

Research on mid-IR silicon-based waveguides has recently received strong interest. Particularly, this paper focuses on one of the critical issues in micron-scale photonic integrated circuits, which is to efficiently couple a mid-IR LED (light emitting diode) light source to an external micron-scale waveguide. The optical coupling scheme is crucial for the exploitation of LED light sources in waveguide-based spectroscopic sensing applications. This paper reports optical coupling scheme between an LED mid-IR light source and a silicon rich silicon nitride (SiN) waveguide that could enable the use of LED-based light sources. Finally, the detection limit of the investigated device for carbon dioxide gas detection is calculated.

Silicon/silicon-rich nitride hybrid-core waveguide for nonlinear optics.

A silicon/silicon-rich nitride hybrid-core waveguide has been proposed and experimentally demonstrated for nonlinear applications to fill the gap between the pure silicon waveguide and the pure silicon nitride waveguide with respect to the nonlinear properties. The hybrid-core waveguide presented here leverages the advantages of the silicon and the silicon-rich nitride waveguide platforms, showing a large nonlinearity γ of 72 ± 5 W-1 m-1 for energy-efficient four-wave mixing wavelength conversion. At the same time, the drawbacks of the material platforms are dramatically mitigated, exhibiting a reduced two-photon absorption coefficient βTPA of 0.023 cm/GW resulting in an increased nonlinear figure-of-merit as large as 21.6. A four-wave-mixing conversion efficiency as large as -5.3 dB has been achieved with the promise to be larger than 0 dB. These findings are strong arguments supporting the silicon/silicon-rich nitride hybrid-core waveguide to be used for energy-efficient nonlinear photonic applications.

Low-loss high-Q silicon-rich silicon nitride microresonators for Kerr nonlinear optics.

Silicon nitride is a dielectric material widely used for applications in linear and nonlinear optics. It has an ultra-broad transparency window, low intrinsic loss, and a refractive index that allows for moderate optical field confinement in waveguides. The chemical composition of this material can be precisely set during the fabrication process, leading to an extra degree of freedom for tailoring the optical and mechanical properties of photonic chips. Silicon-rich silicon nitride waveguides are appealing for nonlinear optics, because they have a higher nonlinear Kerr coefficient and refractive index than what is possible with stoichiometric silicon nitride. This is a direct consequence of the increased silicon content. However, silicon-rich silicon nitride waveguides typically display higher absorption losses. In this Letter, we report low-loss (∼0.4 dB/cm) silicon-rich silicon nitride waveguides. The structures feature high optical confinement and can be engineered with low anomalous dispersion. We find an optimum silicon composition that, through an annealing process, overcomes optical losses associated to N-H bonds in the telecom band. Based on this technology, we successfully fabricate microresonators with mean quality factors (Q) ∼0.8×106 in the C and L bands. Broadband coherent microresonator frequency combs are generated in this platform, indicating its potential for efficient Kerr nonlinear optics.

Si-rich Si nitride waveguides for optical transmissions and toward wavelength conversion around 2 μm.

We show that subwavelength Si-rich nitride waveguides efficiently sustain high-speed transmissions at 2 μm. We report the transmission of a 10 Gbit/s signal over 3.5 cm with negligible power penalty. Parametric conversion in the pulsed pump regime is also demonstrated using the same waveguide structure with an efficiency as high as -18 dB.

An Octave-Spanning Optical Parametric Amplifier Based on a Low-Dispersion Silicon-Rich Nitride Waveguide

Optical amplification is one of the main applications of nonlinear optics, and broadband optical parametric amplifiers are useful not only for multiple data channels in a communication system but for ultrashort optical pulses in ultrafast optics as well. We propose an octave-spanning parametric amplifier in a dual-pump scheme, enabled by advanced dispersion flattening and strong Kerr nonlinearity in a silicon-rich nitride waveguide. A comprehensive nonlinear model is used to study cascaded (non)degenerate parametric processes, by taking wavelength-dependent nonlinearity and all-order dispersion into account. The obtained gain spectrum has a 3-dB bandwidth of one octave from 126 to 244 THz (i.e., 1151 nm), and the gain reaches 11 dB. We show that the proposed amplifier is in principle tolerant to variations of physical parameters, such as pump power, propagation loss, and dispersion. We also examine the amplification of an ultrafast 13-fs soliton pulse with a sech2 waveform. The gain can reach 8 dB, with the pulse shape well maintained.

Silicon-rich nitride waveguides for ultra-broadband nonlinear signal processing.

Silicon nitride (SixNy) waveguides constitute a technology platform to realize optical signal processing based on the nonlinear Kerr effect. Varying the stoichiometry of the core (i.e., x and y in silicon nitride) provides an additional degree of freedom for engineering the waveguide properties, such as nonlinear Kerr parameter and dispersion. We demonstrate low-stress high-confinement silicon-rich nitride waveguides with flat and anomalous dispersion over the entire C and L optical wavelength transmission bands for optical signal processing based on cross-phase modulation. The waveguides do not show any nonlinear loss for a measured optical input intensity of up to 1.5 × 109 W/cm2. In particular, we achieve wavelength conversion of 10 Gb/s signals across the C band; XPM broadening is also observed in the O band. In addition, we highlight the use of SixNy waveguides for nonlinear microwave photonics. Specifically, we demonstrate radio-frequency spectral monitoring of optical signals with a bandwidth of hundreds of gigahertz.

Linear and nonlinear characterization of low-stress high-confinement silicon-rich nitride waveguides: erratum.

We correct the value for the nonlinear Kerr effect of the silicon-rich nitride waveguide presented in [Opt. Express23, 25828 (20152015)].

Octave-spanning supercontinuum generation in a silicon-rich nitride waveguide.

We experimentally show octave-spanning supercontinuum generation in a nonstoichiometric silicon-rich nitride waveguide when pumped by femtosecond pulses from an erbium fiber laser. The pulse energy and bandwidth are comparable to results achieved in stoichiometric silicon nitride waveguides, but our material platform is simpler to manufacture. We also observe wave-breaking supercontinuum generation by using orthogonal pumping in the same waveguide. Additional analysis reveals that the waveguide height is a powerful tuning parameter for generating mid-infrared dispersive waves while keeping the pump in the telecom band.

Linear and nonlinear characterization of low-stress high-confinement silicon-rich nitride waveguides.

In this paper we introduce a low-stress silicon enriched nitride platform that has potential for nonlinear and highly integrated optics. The manufacturing process of this platform is CMOS compatible and the increased silicon content allows tensile stress reduction and crack free layer growth of 700 nm. Additional benefits of the silicon enriched nitride is a measured nonlinear Kerr coefficient n(2) of 1.4·10(-18) m(2)/W (5 times higher than stoichiometric silicon nitride) and a refractive index of 2.1 at 1550 nm that enables high optical field confinement allowing high intensity nonlinear optics and light guidance even with small bending radii. We analyze the waveguide loss (∼1 dB/cm) in a spectrally resolved fashion and include scattering loss simulations based on waveguide surface roughness measurements. Detailed simulations show the possibility for fine dispersion and nonlinear engineering. In nonlinear experiments we present continuous-wave wavelength conversion and demonstrate that the material does not show nonlinear absorption effects. Finally, we demonstrate microfabrication of resonators with high Q-factors (∼10(5)).

Front Cover: Supercontinuum generation in bandgap engineered, back‐end CMOS compatible silicon rich nitride waveguides (Laser Photonics Rev. 9(5)/2015)

Front Cover: Supercontinuum generation in bandgap engineered, back‐end CMOS compatible silicon rich nitride waveguides (Laser Photonics Rev. 9(5)/2015)

Guided photoluminescence study of Nd-doped silicon rich silicon oxide and silicon rich silicon nitride waveguides

Planar waveguides made of Nd3+-doped silicon rich silicon oxide (SRSO) and silicon rich silicon nitride (SRSN) have been fabricated by reactive magnetron sputtering and characterized with special emphasis on the comparison of the guided photoluminescence (PL) properties of these two matrices. Guided fluorescence excited by top surface pumping at 488 nm on planar waveguides was measured as a function of the distance between the excitation area and the output of the waveguide, as well as a function of the pump power density. The PL intensity increased linearly with pump power without any saturation even at high power. The linear intensity increase of the Nd3+ guided PL under a non-resonant excitation (488 nm) confirms the efficient coupling between either Si-np and rare-earth ions for SRSO or radiative defects and rare earth ions for SRSN. The guided fluorescences at 945 and 1100 nm were observed until 4 mm and 8 mm of the output of the waveguide for Nd3+ doped SRSO and SRSN waveguides, respectively. The guided fluorescence decays of Nd3+-doped-SRSO and -SRSN planar waveguides have been measured and found equal to 97 μs ±7 and 5 μs ± 2, respectively. These results show notably that the Nd3+-doped silicon rich silicon oxide is a very promising candidate on the way to achieve a laser cavity at 1.06 μm.

Population inversion and low cooperative upconversion in Er-doped silicon-rich silicon nitride waveguide

Single-mode, strip-loaded silicon-rich silicon nitride (SRSN) waveguide with 11 at.% excess Si and 1.7×10(20) cm(-3) Er was fabricated and characterized. By using a 350 nm thick SRSN:Er core layer and a 850 nm wide SiO2 strip, a high core-mode overlap of 0.85 and low transmission loss of 2.9 dB/cm is achieved. Population inversion of 0.73-0.75, close to the theoretical maximum, is estimated to have been achieved via 1480 nm resonant pumping, indicating that nearly all doped Er in SRSN are optically active. Analysis of the pump power dependence of Er3+ luminescence intensity and lifetime indicate that the Er cooperative upconversion coefficient in SRSN:Er is as low as 2.1×10(-18) cm3/sec.

Absorption bleaching by stimulated emission in erbium-doped silicon-rich silicon nitride waveguides

Stimulated emission from sensitized erbium ions in silicon-rich silicon nitride is demonstrated by pump-probe measurements carried out in waveguides. A decrease in the photoinduced absorption of the probe at the wavelength of erbium emission is observed and is attributed to stimulated emission from erbium excited indirectly via localized states in the silicon nitride matrix.

Measurement of optical nonlinearity in silicon rich nitride waveguide ring resonators

The optical nonlinearities of amorphous silicon rich nitride corresponding to χ(3) in a static electric field have been measured by tracking wavelength shifts in the resonances of optical waveguide ring resonators as a function of an applied electrical field. The magnitude of the nonlinearity measured agrees well with expected values.