Passive wavelength selective polarization rotator in a hybrid waveguide platform.

Polarization rotation and wavelength filtering are key functionalities used to build complex photonic integrated circuits. Both these functionalities have been demonstrated in various material and device platforms. We propose, for the first time, a fully passive wavelength selective polarization rotation in silicon nitride/amorphous silicon hybrid waveguide. We demonstrate TE0 → TM0 and TM0 → TE0 wavelength selective polarization rotator-cum-filter with a measured 3dB bandwidth of 14.8 nm. Further, we experimentally demonstrate a proof of concept for simultaneous coarse wavelength division multiplexing and polarization rotation for the first time in a passive configuration. We also show the feasibility of bandwidth engineering from 0.59 nm to 81 nm, enabled by the unique flexibility of the proposed hybrid waveguide.

Embedded silicon gratings for high-efficiency light-chip coupling to thin film silicon nitride waveguides.

Thin film silicon nitride (<150 nm) waveguide has emerged as a dominant ultra-low-loss platform for many loss-critical applications. While thin-film silicon nitride propagation loss is a crucial characteristic, coupling light between an optical fiber and the waveguide is still challenging. While the larger mode size of the decoupled thin waveguide offers better coupling than a highly-confined waveguide, the coupling efficiency is still sub-optimal. The poor diffraction efficiency of such thin films limits the scope of implementing standalone surface gratings. We demonstrate an efficient way to couple into thin film silicon nitride waveguides using amorphous silicon strip gratings. The high contrast gratings provide an efficient means to boost the directionality from thin films leading to an enhanced coupling performance. In addition, we incorporate a bottom reflector to further improve the coupling. We present an optimal design for uniform strip gratings with a maximum coupling efficiency of -1.7 dB/coupler. We achieved a maximum coupling efficiency of -0.28 dB/coupler by engineering the scattering strength along the grating through apodization. We have experimentally shown the highest coupling efficiency reported yet of -2.22 dB/coupler and -1.84 dB/coupler for uniform and apodized grating couplers in the C-L band. We present a detailed design strategy, simulation, fabrication and characterization data on the effect of various parameters on the coupling efficiency.

Linear and nonlinear characterization of a broadband integrated Si-rich silicon nitride racetrack ring resonator for on-chip applications.

We demonstrate the linear and nonlinear characterization of a plasma-enhanced chemical vapor deposited silicon-rich silicon nitride (SRSN) racetrack ring resonator for on-chip applications within the telecommunication wavelength range. The SRSN waveguide parameters are optimized by employing the refractive index profile measured by ellipsometry to achieve flat dispersion in the telecom band. Furthermore, we measure the thermo-optic coefficient of the micro-resonator by analyzing the temperature-dependent transmission spectra and assess it to be 3.2825×10-5 ∘ C -1. Additionally, we study power-dependent transmission spectra to investigate the effect of local heating and nonlinear absorption. The power-dependent transmission spectra exhibit a blueshifting of the resonance peak in the visible and near-IR regions, which indicates the presence of nonlinear losses in that range. The power-dependent transmission spectra almost remain unchanged in the telecom band, revealing the absence of nonlinear losses and excellent thermal stability in that wavelength range. Our experimental results reveal that the SRSN-based structure can be employed potentially to realize linear and nonlinear applications in the telecom band.

Polarization independent grating in a GaN-on-sapphire photonic integrated circuit.

In this work, we report the realization of a polarization-insensitive grating coupler, single-mode waveguide, and ring resonator in the GaN-on-sapphire platform. We provide a detailed demonstration of the material characterization, device simulation, and experimental results. We achieve a grating coupler efficiency of -5.2 dB/coupler with a 1 dB and 3 dB bandwidth of 40 nm and 80 nm, respectively. We measure a single-mode waveguide loss of -6 dB/cm. The losses measured here are the lowest in a GaN-on-sapphire photonic circuit. This demonstration provides opportunities for the development of on-chip linear and non-linear optical processes using the GaN-on-sapphire platform. To the best of our knowledge, this is the first demonstration of an integrated photonic device using a GaN HEMT stack with 2D electron gas.

Thin-film wafer-scale mid-IR Fabry Perot cavity gas sensor.

We demonstrate a miniaturized wafer-scale optical gas sensor that combines the gas cell, an optical filter, and integrated flow channels. We present the design, fabrication and characterization of an integrated cavity-enhanced sensor. Using the module, we demonstrate absorption sensing of ethylene down to 100 ppm level.

Nonlinear spectral broadening of a dual-carrier electro-optic frequency comb in a graphene oxide cladded silicon-rich nitride hybrid waveguide.

We demonstrate a detailed theoretical analysis describing the generation of an electro-optic comb (EOC) in the near-IR range through discrete phase and amplitude modulation driven by radio frequency (RF) signal generators. Furthermore, the generated EOC spectra suffer nonlinear spectral broadening while propagating through a hybrid Si-rich nitride (SRN) waveguide structure integrated with two-dimensional (2D) layered graphene oxide (GO) films. We perform a detailed analysis to investigate the influence of GO layers, pump wavelength detuning, and other waveguide parameters on the evolution of comb spectra propagating through the hybrid waveguide structure. Owing to the strong modal overlapping between the SRN waveguides and the highly nonlinear GO films, the nonlinearity of the system is enhanced effectively, and broadband comb spectra have been achieved in the near-IR range. Furthermore, we investigate the spectral coherence of the generated comb spectra under different input conditions. The results exhibit strong potential to generate a tunable frequency comb with high spectral coherence in the near-IR range by employing the hybrid waveguide structure.

Guided mode resonance aided polarization insensitive in-plane spectral filters for an on-chip spectrometer.

We demonstrate an on-chip in-plane polarization independent multi-spectral color filter in the visible to near-infrared wavelength band. We experimentally show a four-channel transmission and in-plane spectral filter characteristics spanning a 400-nm spectral range. Engineered 2D guided mode resonance structures in a silicon nitride-on-sapphire substrate are used to realize the filters. The in-plane color filters could provide the necessary impetus for developing robust integrated photonic platforms for on-chip devices and applications.

Polar Semiconducting Scandium Nitride as an Infrared Plasmon and Phonon-Polaritonic Material.

The interaction of light with collective charge oscillations, called plasmon-polariton, and with polar lattice vibrations, called phonon-polariton, are essential for confining light at deep subwavelength dimensions and achieving strong resonances. Traditionally, doped-semiconductors and conducting metal oxides (CMO) are used to achieve plasmon-polaritons in the near-to-mid infrared (IR), while polar dielectrics are utilized for realizing phonon-polaritons in the long-wavelength IR (LWIR) spectral regions. However, demonstrating low-loss plasmon- and phonon-polaritons in one host material will make it attractive for practical applications. Here, we demonstrate high-quality tunable short-wavelength IR (SWIR) plasmon-polariton and LWIR phonon-polariton in complementary metal-oxide-semiconductor compatible group III-V polar semiconducting scandium nitride (ScN) thin films. We achieve both resonances by utilizing n-type (oxygen) and p-type (magnesium) doping in ScN that allows modulation of carrier concentration from 5 × 1018 to 1.6 × 1021 cm-3. Our work enables infrared nanophotonics with an epitaxial group III semiconducting nitride, opening the possibility for practical applications.

On-chip silicon nano-slab photodetector integrated wavelength division de-multiplexer in the 850 nm band.

We present an on-chip photodetector integrated wavelength filter on a SiN-on-silicon-on-insulator (SOI) platform in the 850 nm wavelength window. The wavelength filter is designed using an echelle grating with a distributed Bragg reflector as the grating reflectors. We present the design and experimental realization of a six-channel wavelength filter with a channel spacing of 10 nm. Experimentally, we achieve an insertion loss of 4.3 dB and an adjacent channel cross talk of 22 dB. We demonstrate a silicon nano-slab waveguide integrated metal-semiconductor-metal photodetector with a maximum responsivity of 0.56 A/W and dark current of 217 nA. Furthermore, we demonstrate the integration of the echelle grating with the detector and show the feasibility of a CMOS compatible SiN-on-SOI platform for various applications, including short-reach communication and sensing applications.

Comprehensive grating enabled silicon nitride fiber-chip couplers in the SNIR wavelength band.

We present silicon nitride grating enabled fiber-chip coupling in the sub-near-infrared band. We present a comprehensive design and simulation and experimental demonstration of uniform and apodized grating couplers, with and without bottom reflectors. The mode engineering yields a best efficiency of -1.6 dB for apodized grating design, which is further improved to -0.66 dB with a bottom reflector. Experimentally, we demonstrate a coupling efficiency of -2.2 dB for the optimized design. Furthermore, we present a detailed simulation and measurement comparison of various grating parameters and the effect of fabrication tolerances on the grating performance.

High-efficiency vertical fibre-to-polymer waveguide coupling scheme for scalable polymer photonic circuits.

Polymer photonic circuits offer a versatile platform for various applications, including communication, sensing and optical signal processing. Though polymers offer broadband, linear and nonlinear optical properties, the coupling between an optical fibre and a polymer waveguide has been a challenge. In this work, we propose and demonstrate a wafer-scale vertical coupling scheme for polymer waveguides. The scheme uses a silicon nitride grating coupler with an inverse taper to couple between an optical fibre and a SU8 polymer waveguide. We demonstrate a maximum coupling efficiency of -3.55 dB in the C-band and -2.92 dB in the L-band with a 3-dB bandwidth of 74 and 80 nm, respectively. A detailed design and simulation, fabrication, and characterisation results are presented. The scheme demonstrates a scalable and efficient surface grating approach for polymer photonic integrated circuits.

Compact ring resonator enhanced silicon metal-semiconductor-metal photodetector in SiN-on-SOI platform.

We present a compact on-chip resonator enhanced silicon metal-semiconductor-metal (MSM) photodetector in 850 nm wavelength band for communication and lab-on-chip bio-sensing applications. We report the highest responsivity of 0.81 A/W for a 5 µm long device. High responsivity is achieved by integrating the detector in a silicon nitride ring resonator. The resonance offers 100X responsivity improvement over a single-pass photodetector due to cavity enhancement. We also present a detailed study of the high-speed response of the cavity and single-pass detector. We report an electro-optic bandwidth of 7.5 GHz measured using a femtosecond optical excitation. To the best of our knowledge, we report for the first time silicon nitride resonator integrated Si-MSM detector in SiN-SOI platform.

On-chip silicon photonics based grating assisted vibration sensor.

We present a compact, highly sensitive and scalable on-chip photonic vibration measurement scheme for vibration sensing. The scheme uses a silicon photonic diffraction-grating based sensor integrated underneath a silicon cantilever. We demonstrate a static and dynamic measurement sensitivity (ΔT/Δgap) of 0.6 % change in intensity per nm displacement. The electrostatically driven dynamic response measurement of the grating based sensor shows an excellent agreement with commercial Laser Doppler Vibrometer (LDV) measurement. We demonstrate the thermo-mechanical noise measurement on the cantilever in ambience, which is verified using LDV. A minimum displacement of 1.9 pm is measured with a displacement sensitivity of 10 µW/nm for a measurement bandwidth of 16 Hz. The demonstrated sensitivity is 2 orders of magnitude better than that obtained from measurements of static displacement. We also present a detailed 2D-FDTD simulation and optimization of the grating-based sensor to achieve maximum displacement sensitivity.

Enhanced nonlinear spectral broadening and sub-picosecond pulse generation by adaptive spectral phase optimization of electro-optic frequency combs.

We utilize adaptive optimization to enhance the spectral broadening of an amplified electro-optic frequency comb with a 25 GHz repetition rate in a highly nonlinear fiber and subsequently generate sub-picosecond pulses. The spectral phase of the comb is adaptively optimized by a Fourier pulse shaper in a closed control loop with the HNLF output spectrum as the process variable to be optimized. Enhanced spectral broadening also increases the stimulated Brillouin scattering threshold allowing increased power scaling and thereby boosting the bandwidth by a factor of more than 13 times over the initial comb. System versatility to varying conditions is demonstrated by achieving consistent bandwidth enhancement (nearly or more than 100 lines) in varying operating conditions that distort the temporal profile of the comb. In all cases, the optimization yields a near transform limited pulse that enters the nonlinear fiber. Sub-picosecond pulse generation is achieved with a short length of single mode fiber post the nonlinear fiber.

Microwave power induced resonance shifting of silicon ring modulators for continuously tunable, bandwidth scaled frequency combs.

We demonstrate a technique to continuously tune center frequency and repetition rate of optical frequency combs generated in silicon microring modulators and bandwidth scale them. We utilize a drive frequency dependent, microwave power induced shifting of the microring modulator resonance. In this work, we demonstrate center frequency tunability of frequency combs generated in silicon microring modulators over a wide range (∼8nm) with fixed number of lines. We also demonstrate continuously tunable repetition rates from 7.5GHz to 15GHz. Further, we use this effect to demonstrate a proof-of-principle experiment to bandwidth scale an 8-line (20dB band) comb generated from a single ring modulator driven at 10GHz to a comb with 12 and 15 lines by cascading two and three ring modulators, respectively. This is accomplished by merging widely spaced ring modulator resonances to a common location, thus coupling light simultaneously into multiple cascaded ring modulators.

Broadly tunable and low power penalty radio frequency phase shifter using a coupled silicon microcavity.

We present a continuously tunable silicon photonics assisted radio frequency (RF) phase shifter using a coupled microring resonator. Using the coupled cavity, we demonstrate a sub-1 dB power penalty for a RF bandwidth of 34.5 GHz (9-43.5 GHz) and a phase shift of π over the reported frequency range. Rigorous optimization of the cavity design using the coupled-mode theory is carried out to realize ultranarrow resonance peaks with a low-extinction ratio and large phase shift. Thermal tuning of the cavity is used to tune the phase while all-optical tuning is exploited to achieve broadband operation. We present a detailed simulation and experimental study of the proposed configuration. The proposed device configuration exhibits a configurable resonance linewidth and extinction ratio that allows for a broad bandwidth and an extremely low power penalty microwave phase shifter. We believe the demonstration would allow better integration of the on-chip functional elements of integrated microwave photonics.

Optical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers.

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase modulated electro-optic 25 GHz frequency combs sourced from individual seed carriers offset by 100 GHz, to within 6.7 MHz of each other. We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb. We obtain $\sim 86 $∼86 lines in a 20 dB band spanning over 2 THz.

High efficiency DBR assisted grating chirp generators for silicon nitride fiber-chip coupling.

Silicon Nitride (SiN) is emerging as a promising material for a variety of integrated photonic applications. Given its low index contrast however, a key challenge remains to design efficient couplers for the numerous platforms in SiN photonics portfolio. Using a combination of bottom reflector and a chirp generating algorithm, we propose and demonstrate high efficiency, grating couplers on two distinct SiN platforms. For a partially etched grating on 500 nm thick SiN, a calculated peak efficiency of -0.5 dB/coupler is predicted, while for a fully etched grating on 400 nm thick SiN, an efficiency of -0.4 dB/coupler is predicted. Experimentally measured coupling efficiencies are observed to be -1.17 and -1.24 dB/coupler for the partial and fully etched grating couplers respectively in the C-L band region. Furthermore, through numerical simulations, it is shown that the chirping algorithm can be implemented in eight additional combinations comprising SiN film thickness between 300-700 nm as well as alternate claddings, to achieve a per coupler loss between -0.33 to -0.65 dB.

Alignment-tolerant broadband compact taper for low-loss coupling to a silicon-on-insulator photonic wire waveguide.

We experimentally demonstrate a broadband, fabrication-tolerant compact silicon waveguide taper (34.2 µm) in a silicon-on-insulator wire waveguide. The taper works on multimode interference along the length of the taper. A single taper design has broadband operation with coupling efficiency >70% over 700 nm that can be used in O-, C-, and L-bands. The compact taper is highly tolerant to fabrication variations; ±100 nm change in the taper and end waveguide width varies the taper transmission by <5%. The footprint of the device, i.e., the taper along with linear gratings, is ≈442 µm2, 11.5× smaller than the adiabatic taper. The taper with linear gratings provides coupling efficiency comparable to standard focusing gratings. We have also experimentally compared the translational and rotational alignment tolerance of the focusing grating with linear grating couplers.

High-speed waveguide integrated silicon photodetector on a SiN-SOI platform for short reach datacom.

We present a waveguide integrated high-speed Si photodetector integrated with a silicon nitride (SiN) waveguide on an silicon-on-insulator (SOI) platform for short reach data communication in a 850 nm wavelength band. We demonstrate a waveguide couple Si pin photodetector responsivity of 0.44 A/W at 25 V bias. The frequency response of the photodetector is evaluated by the coupling of a femtosecond laser source through an SiN grating coupler of the integrated photodetector. We estimate a 3 dB bandwidth of 14 GHz at 20 V bias which, to the best of our knowledge, is the highest reported bandwidth for a waveguide integrated Si photodetector. We also present detailed optoelectronic DC and AC characterization of the fabricated devices. The demonstrated integrated photodetector could enable an integrated solution for scaling of short reach data communication and connectivity.