Empowering high-dimensional optical fiber communications with integrated photonic processors

Mode-division multiplexing (MDM) in optical fibers enables multichannel capabilities for various applications, including data transmission, quantum networks, imaging, and sensing. However, high-dimensional optical fiber systems, usually necessity bulk-optics approaches for launching different orthogonal fiber modes into the optical fiber, and multiple-input multiple-output digital electronic signal processing at the receiver to undo the arbitrary mode scrambling introduced by coupling and transmission in a multi-mode fiber. Here we show that a high-dimensional optical fiber communication system can be implemented by a reconfigurable integrated photonic processor, featuring kernels of multichannel mode multiplexing transmitter and all-optical descrambling receiver. Effective mode management can be achieved through the configuration of the integrated optical mesh. Inter-chip MDM optical communications involving six spatial- and polarization modes was realized, despite the presence of unknown mode mixing and polarization rotation in the circular-core optical fiber. The proposed photonic integration approach holds promising prospects for future space-division multiplexing applications.

Giant optical absorption of a PtSe2-on-silicon waveguide in mid-infrared wavelengths

Low-dimensional platinum diselenide (PtSe2) is a promising candidate for high-performance optoelectronics in the short-wavelength mid-infrared band due to its high carrier mobility, excellent stability, and tunable bandgap. However, light usually interacts moderately with low-dimensional PtSe2, limiting the optoelectronic responses of PtSe2-based devices. Here we demonstrated a giant optical absorption of a PtSe2-on-silicon waveguide by integrating a ten-layer PtSe2 film on an ultra-thin silicon waveguide. The weak mode confinement in the ultra-thin waveguide dramatically increases the waveguide mode overlap with the PtSe2 film. Our experimental results show that the absorption coefficient of the PtSe2-on-silicon waveguide is in the range of 0.0648 dB μm-1 to 0.0704 dB μm-1 in a spectral region of 2200 nm to 2300 nm wavelengths. Furthermore, we also studied the optical absorption in an ultra-thin silicon microring resonator. Our study provides a promising approach to developing PtSe2-on-silicon hybrid optoelectronic integrated circuits.

Scalable integrated two-dimensional Fourier-transform spectrometry

Integrated spectrometers offer the advantages of small sizes and high portability, enabling new applications in industrial development and scientific research. Integrated Fourier-transform spectrometers (FTS) have the potential to realize a high signal-to-noise ratio but typically have a trade-off between the resolution and bandwidth. Here, we propose and demonstrate the concept of the two-dimensional FTS (2D-FTS) to circumvent the trade-off and improve scalability. The core idea is to utilize 2D Fourier transform instead of 1D Fourier transform to rebuild spectra. By combining a tunable FTS and a spatial heterodyne spectrometer, the interferogram becomes a 2D pattern with variations of heating power and arm lengths. All wavelengths are mapped to a cluster of spots in the 2D Fourier map beyond the free-spectral-range limit. At the Rayleigh criterion, the demonstrated resolution is 250 pm over a 200-nm bandwidth. The resolution can be enhanced to 125 pm using the computational method.

Multichip multidimensional quantum networks with entanglement retrievability

Quantum networks provide the framework for quantum communication, clock synchronization, distributed quantum computing, and sensing. Implementing large-scale and practical quantum networks relies on the development of scalable architecture and integrated hardware that can coherently interconnect many remote quantum nodes by sharing multidimensional entanglement through complex-medium quantum channels. We demonstrate a multichip multidimensional quantum entanglement network based on mass-manufacturable integrated-nanophotonic quantum node chips fabricated on a silicon wafer by means of complementary metal-oxide-semiconductor processes. Using hybrid multiplexing, we show that multiple multidimensional entangled states can be distributed across multiple chips connected by few-mode fibers. We developed a technique that can efficiently retrieve multidimensional entanglement in complex-medium quantum channels, which is important for practical uses. Our work demonstrates the enabling capabilities of realizing large-scale practical chip-based quantum entanglement networks.

High coupling efficiency waveguide grating couplers on lithium niobate

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency (CE) grating couplers (GCs) in the lithium niobate on insulator photonic integration platform. Enhanced CE is achieved by increasing the grating strength using a high refractive index polysilicon layer on the GC. Due to the high refractive index of the polysilicon layer, the light in the lithium niobate waveguide is pulled up to the grating region. The optical cavity formed in the vertical direction enhances the CE of the waveguide GC. With this novel structure, simulations predicted the CE to be -1.40 dB, while the experimentally measured CE was -2.20 dB with a 3-dB bandwidth of 81 nm from 1592 nm to 1673 nm. The high CE GC is achieved without using bottom metal reflectors or requiring the etching of the lithium niobate material.

Breaking the resolution-bandwidth limit of chip-scale spectrometry by harnessing a dispersion-engineered photonic molecule

The chip-scale integration of optical spectrometers may offer new opportunities for in situ bio-chemical analysis, remote sensing, and intelligent health care. The miniaturization of integrated spectrometers faces the challenge of an inherent trade-off between spectral resolutions and working bandwidths. Typically, a high resolution requires long optical paths, which in turn reduces the free-spectral range (FSR). In this paper, we propose and demonstrate a ground-breaking spectrometer design beyond the resolution-bandwidth limit. We tailor the dispersion of mode splitting in a photonic molecule to identify the spectral information at different FSRs. When tuning over a single FSR, each wavelength channel is encoded with a unique scanning trace, which enables the decorrelation over the whole bandwidth spanning multiple FSRs. Fourier analysis reveals that each left singular vector of the transmission matrix is mapped to a unique frequency component of the recorded output signal with a high sideband suppression ratio. Thus, unknown input spectra can be retrieved by solving a linear inverse problem with iterative optimizations. Experimental results demonstrate that this approach can resolve any arbitrary spectra with discrete, continuous, or hybrid features. An ultrahigh resolution of <40 pm is achieved throughout an ultrabroad bandwidth of >100 nm far exceeding the narrow FSR. An ultralarge wavelength-channel capacity of 2501 is supported by a single spatial channel within an ultrasmall footprint (≈60 × 60 μm²), which represents, to the best of our knowledge, the highest channel-to-footprint ratio (≈0.69 μm^-2) and spectral-to-spatial ratio (>2501) ever demonstrated to date.

Efficient 330-Gb/s PAM-8 modulation using silicon microring modulators

We propose and demonstrate a high-efficiency silicon microring modulator for next-generation optical transmitters operating at line rates above 300 Gb/s. The modulator supports high-order PAM-8 modulation up to 110 Gbaud (330 Gb/s), with a driving voltage of 1.8 V_pp. The small driving voltage and device capacitance yields a dynamic energy consumption of 3.1 fJ/bit. Using the modulator, we compare PAM-8 with ultrahigh baud rate PAM-4 of up to 130 Gbaud (260 Gb/s) and show PAM-8 is better suited for 300-Gb/s lane rate operation in bandwidth-constrained short-reach systems.

Combined polysilicon and silicon gratings for dual-wavelength-band waveguide grating couplers

We proposed a novel, to the best of our knowledge, design for a dual-wavelength-band waveguide grating coupler. The proposed structure works in both the C band and O band. The proposed device is optimized from an initial design of two independent gratings formed on the silicon and polysilicon overlay layers, respectively. We designed the up layer (polysilicon) for the C band and the down layer (silicon) for the O band as the initial optimization seed. After numerical optimization of this structure using a genetic algorithm, the grating coupler has a coupling efficiency of -3.86 dB at the C band and -4.46 dB at the O band. We validate the approach in a commercial foundry using 193-nm photolithography in a multi-project wafer, and the experimental result has coupling efficiencies of -4.37 dB in the C band and -5.8 dB in the O band.

Compact integrated mode-size converter using a broadband ultralow-loss parabolic-mirror collimator

In this Letter, we propose and demonstrate an integrated mode-size converter (MSC) with a compact footprint, low losses, and a broad bandwidth. By exploiting a parabolic mirror, the divergent light from a narrow waveguide (450 nm) is collimated to match the mode size of a wide waveguide (10 µm). The measured insertion loss (IL) is ≈ 0.15 dB over a 100-nm bandwidth. The mode-size conversion is achieved with a footprint as small as ≈ 20 × 32 µm², which is much shorter than the linear taper length required to attain the same level of losses.

Polarization-independent waveguide grating coupler using an optimized polysilicon overlay

We propose and validate a new, to the best of our knowledge, approach to designing a polarization-independent waveguide grating coupler, using an optimized polysilicon overlay on a silicon grating structure. Simulations predicted coupling efficiencies of about -3.6 dB and -3.5 dB for TE and TM polarizations, respectively. The devices were fabricated using photolithography in a multi-project wafer fabrication service by a commercial foundry and have measured coupling losses of -3.96 dB for TE polarization and -3.93 dB for TM polarization.

Optimized shift-pattern overlay for high coupling efficiency waveguide grating couplers

We propose and validate a new, to the best of our knowledge, approach for increasing the coupling efficiency of waveguide grating couplers by introducing an optimized shift-patterned polysilicon overlay above the silicon grating structure. After optimizing the shifts in position and duty cycles of each period in the polysilicon overlay and silicon grating, the silicon grating and polysilicon overlay can form composite subwavelength structures which improve both the mode matching and the directionality of the grating coupler, and enable the design of a high-efficiency perfectly vertical grating coupler (PVGC) with -0.91 dB simulated coupling efficiency. The devices are fabricated using photolithography in a standard commercial multi-project wafer fabrication service by IMEC and have a measured coupling loss of approximately 1.45 dB.

Broadband high-Q multimode silicon concentric racetrack resonators for widely tunable Raman lasers

Multimode silicon resonators with ultralow propagation losses for ultrahigh quality (Q) factors have been attracting attention recently. However, conventional multimode silicon resonators only have high Q factors at certain wavelengths because the Q factors are reduced at wavelengths where fundamental modes and higher-order modes are both near resonances. Here, by implementing a broadband pulley directional coupler and concentric racetracks, we present a broadband high-Q multimode silicon resonator with average loaded Q factors of 1.4 × 10⁶ over a wavelength range of 440 nm (1240–1680 nm). The mutual coupling between the two multimode racetracks can lead to two supermodes that mitigate the reduction in Q factors caused by the mode coupling of the higher-order modes. Based on the broadband high-Q multimode resonator, we experimentally demonstrated a broadly tunable Raman silicon laser with over 516 nm wavelength tuning range (1325–1841 nm), a threshold power of (0.4 ± 0.1) mW and a slope efficiency of (8.5 ± 1.5) % at 25 V reverse bias.

The authors demonstrate a new approach for multimode resonators to maintain high-quality-factor resonances across a broad wavelength range using the detuning between the symmetric and anti-symmetric supermodes in concentric racetrack resonators.

C-band 67 GHz silicon photonic microring modulator for dispersion-uncompensated 100 Gbaud PAM-4

A very-high-bandwidth integrated silicon microring modulator (MRM) designed on a commercial silicon photonics (SiP) platform for C-band operation is presented. The MRM has a 3 dB electro-optic (EO) bandwidth of over 67 GHz and features a small footprint of 24 µm × 70 µm. Using the MRM, we demonstrate intensity modulation-direct detection (IM-DD) transmission with 4-level pulse amplitude modulation (PAM-4)  signaling of over 100 Gbaud. By utilizing the optical peaking effect and negative chirp in the MRM, we extend the transmission distance, which is limited by the fiber-dispersion-induced frequency fading. Using a standard single-mode fiber (SSMF) for transmission across distances of up to 2 km, we measured the data transmission of 100 Gbaud PAM-4 signals with a bit error rate (BER) under the general 7% hard-decision forward-error correction (HD-FEC) threshold. The MRM enables an extended transmission distance for 100 Gbaud signaling in the C-band without dispersion compensation.

Optimal Bezier curve transition for low-loss ultra-compact S-bends

S-bends with a widening of the width at the mid-bend and a Bezier curve transition are proposed and demonstrated for low-loss S-bends. The increased optical confinement and reduced transition loss enable low insertion loss (IL) and compact S-bends with longitudinal offsets as small as 2.5 µm and a wide operating bandwidth (∼100nm) on a 220 nm thick silicon-on-insulator platform. The simulation results show ILs less than (0.22, 0.20, 0.20, 0.20) dB in the wavelength range of (1.5-1.6) µm, while the minimum ILs are (0.13, 0.13, 0.15, 0.16) dB for lateral offsets of (3, 6, 9, 12) µm. The experimental results show that ILs remain less than (0.41, 0.38, 0.36, 0.39) dB for the mid-bend widening Bezier (MWB) S-bends.

Inverse design of multi-band and wideband waveguide crossings

Photonic integrated circuits for wideband and multi-band optical communications will need waveguide crossings that operate at all the wavelengths required by the system. In this Letter, we use the modified gradient decedent method to optimize the dual-wavelength band (DWB) crossings on both single- and double-level platforms. On the single-level platform, the simulation results show insertion losses (ILs) less than 0.07 and 0.11 dB for a crossing working at a DWB of 1.5-1.6 and 1.95-2.05 µm. ILs are less than 0.1 and 0.2 dB for a crossing operating in the DWB of 1.5-1.6 and 2.2-2.3 µm. On the double-layer platform, the simulated results show IL less than 0.08 dB across the wavelength range of 1.25-2.25 µm. We experimentally demonstrate the DWB crossing operating at 1.5-1.6 and 2.2-2.3 µm to have IL less than 0.3 and 0.4 dB and crosstalk of -28 and -26dB in the two bands, respectively.

High-extinction CROW filters for scalable quantum photonics

We report an integrated tunable-bandwidth optical filter with a passband to stop-band ratio of over 96 dB using a single silicon chip with an ultra-compact footprint. The integrated filter is used in filtering out the pump photons in non-degenerate spontaneous four-wave mixing (SFWM), which is used for producing correlated photon pairs at different wavelengths. SFWM occurs in a long silicon waveguide, and two cascaded second-order coupled-resonator optical waveguide (CROW) filters were used to spectrally remove the pump photons. The tunable bandwidth of the filter is useful to adjust the coherence time of the quantum correlated photons and may find applications in large-scale integrated quantum photonic circuits.

Bufferless III-V photodetectors directly grown on (001) silicon-on-insulators

Efficient photodetectors (PDs) and lasers are critical components in silicon photonics technology. Here, we demonstrate bufferless InP/InGaAs PDs, directly grown on (001) silicon-on-insulators. The nano-scale PDs exhibit a high photoresponsivity of 1.06 A/W at 1.55 µm, and a wide operating range from 1450 nm to 1650 nm. The bufferless feature of nano-PDs facilitates effective interfacing with Si waveguides, thus paving the path toward fully integrated silicon photonics circuits.

Compact high-extinction tunable CROW filters for integrated quantum photonic circuits

We describe the use of cascaded second-order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported measured extinction ratios of $ {\gt} {110}\;{\rm dB}$>110dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum-correlated photons that could be measured after the cascaded CROW filters. The CROW filters offer a compact footprint for use in monolithic quantum photonic circuits.

Experimental demonstration of 111.1-Gb/s net information rate using IM/DD probabilistically shaped orthogonal chirp-division multiplexing with a 10-GHz-class modulator

We propose probabilistically shaped quadrature amplitude modulation (PS-QAM) formats to maximize the capacity in fiber transmission systems using orthogonal chirp-division multiplexing (OCDM). OCDM possesses the property of chirp spread spectrum (CSS), leading to improved resilience to system impairments. We further investigate the recently proposed robust channel estimator based on pulse compression and noise rejection and experimentally demonstrate its feasibility in an intensity-modulated/direction-detection (IM/DD) OCDM system. By applying the proposed PS-QAM based OCDM to an IM/DD optical system, a net information rate of 111.1 Gb/s has been successfully achieved using a 10-GHz class Mach-Zehnder modulator (MZM) and has also shown improved performance compared to the conventional PS-QAM based orthogonal frequency-division multiplexing (OFDM) systems. Moreover, due to the superior characteristics of OCDM, there is no need for additional feedback to obtain the prior knowledge of channel state information in the proposed system, leading to reduced complexity and cost.

Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared

We show that dual-wavelength-band (DWB) subwavelength grating couplers (SWGCs) can be designed for simultaneous coupling of the near-infrared and extended shortwave-infrared (SWIR) fundamental transverse electric polarized light at the same diffraction angle. Numerical simulations predict coupling efficiencies (CEs) larger than 34% for the DWB SWGCs operating in the S/C band (1.48/1.55 μm) and extended SWIR band (1.8-2.8 μm) with a widely and continuously tailorable peak wavelength separation between 250 and 1250 nm. The fabricated DWB SWGCs with peak wavelengths of (1.56, 2.255) μm and (1.487, 2.331) μm respectively obtain CEs of (20.2, 25.8)% and (20.6, 26.9)%, 1-dB bandwidths of 38 nm and 54 nm at a diffraction angle of 2°.

Efficient perfectly vertical grating coupler for multi-core fibers fabricated with 193 nm DUV lithography

We propose a novel high-efficiency, low-reflection, and fabrication-tolerant perfectly vertical grating coupler (PVGC) with a minimum feature size >200  nm to allow for fabrication using 193 nm deep-ultraviolet lithography. The structural parameters of PVGC were optimized by a genetic optimization algorithm. Simulations predicted the coupling efficiency to be -2.0  dB (63.0%) and the back reflections to be less than -20  dB in the wavelength range of 1532-1576 nm. The design was fabricated in a multi-project wafer run for silicon photonics, and a coupling efficiency of -2.7  dB (53.7%) with a 1 dB bandwidth of 33 nm is experimentally demonstrated. The measured back reflection is less than -16  dB over the C-band. The PVGC occupies a compact footprint of 30  μm×24  μm and can be interfaced with the multi-core fibers for future space-division-multiplexing networks.

A silicon nitride waveguide-integrated chemical vapor deposited graphene photodetector with 38 GHz bandwidth

We demonstrate a high-speed chemical vapor deposited graphene-on-silicon nitride waveguide photodetector. The device is designed with grating-like metal contact to reduce the channel spacing. Benefiting from the narrow channel spacing, a calculated transit-time-limited bandwidth of 111 GHz is derived. The resistance-capacitance-limited bandwidth is also improved due to the small relative permittivity of silicon nitride. At a wavelength of 1550 nm, we measured an electro-optic bandwidth of 38 GHz under zero bias and an intrinsic responsivity of 13 mA W-1 at 0.1 V reverse bias with a 6 μm detection length.

Integrated germanium-on-silicon Franz-Keldysh vector modulator used with a Kramers-Kronig receiver

We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz-Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800  μm×200  μm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40 Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully re-constructed with a single-ended photodiode in a recently proposed Kramers-Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.

Mid-infrared high-Q germanium microring resonator

Germanium is a promising material for mid-infrared (MIR) integrated photonics due to its CMOS compatibility and wide transparency window covering the fingerprint spectral region (2-15 μm). However, due to the limited quality and structural configurations of conventional germanium-based integration platforms, the realization of high-Q on-chip germanium resonators in the MIR spectral range remains challenging to date. Here we experimentally demonstrate an air-cladding MIR germanium microring resonator with, to the best of our knowledge, the highest loaded Q-factor of ∼57,000 across all germanium-based integration platforms to date. A propagation loss of 5.4 dB/cm and a high extinction ratio of 22 dB approaching the critical coupling condition are experimentally realized. These are enabled by our smart-cut methods for developing high-quality germanium-on-insulator wafers and by implementing our suspended-membrane structure. Our high-Q germanium microring resonator is a promising step towards a number of on-chip applications in the MIR spectral range.

Tailorable dual-wavelength-band coupling in a transverse-electric-mode focusing subwavelength grating coupler

A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with coupling efficiencies of (18.3%, 20.1%) and (14.5%, 17.5%), 3-dB bandwidths of (55.0, 30.5) nm and (44.0, >39.5) nm, respectively. A DWB FSWGC working at (1480, 1830) nm with a wavelength separation of 350 nm and coupling efficiencies of (38.0%, 33.2%) is also numerically predicted.

High-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors

Waveguide photodetectors integrated with graphene have demonstrated potential for ultrafast response and broadband operation. Here, we demonstrate high-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors by enhancing the absorption of light propagating in the transverse-magnetic mode through a metal-graphene junction. A doubling in responsivity is experimentally observed. In our zero-biased metal-graphene junction, a 15  mA W^-1 intrinsic responsivity and a 30 GHz bandwidth are achieved at ∼1550  nm. The results are comparable to those obtained from the best pristine graphene-based photodetectors. Our work enables new architectures for high-performance optoelectronic devices based on the graphene-on-silicon nitride platform.

Characterisation of patients with endoscopy-negative, computer tomography-negative midfacial segment pain using the sino-nasal outcome test

BACKGROUND

The purpose of this study was to qualitatively characterise patients with midfacial segment pain (MSP) using the Sino-Nasal Outcome Test (SNOT). The data will provide a detailed overview of the physical and psychological impact on patients'well-being, and how it compares with the normal, healthy population.

METHODS

Suitable patients were prospectively identified from the Multi-disciplinary Facial Pain Clinic at the Royal Liverpool University Hospital, based on the diagnostic criteria for MSP. The pre-treatment SNOT-22 of these patients were also compared to patients with chronic rhinosinusitis and normal healthy volunteers.

RESULTS

Twenty-nine consecutive patients with a diagnosis of MSP were identified, and compared with 30 CRS patients and 34 healthy volunteers. The average SNOT-22 scores of MSP and CRS patients were higher than normal healthy volunteers. Patients with CRS had the highest rhinological subscale SNOT scores compared to normal healthy volunteers and MSP. Conversely, the reported ear and facial symptoms of MSP patients were most unfavourable. A similar trend was observed in reported sleep function where MSP patients recorded higher subscale scores than the other two cohorts. The subscale mean score for psychological function of MSP patients was not significant when compared to the mean score of patients diagnosed with CRS.

CONCLUSION

MSP has an adverse impact on both physical and psychological well-being. The subtle differences in the SNOT subscores between MSP and CRS have provided greater insight into the character and disease impact of MSP. We propose that the SNOT may be suitably utilised in MSP to document disease severity and measure response to treatment.

Mid-infrared germanium photonic crystal cavity

The mid-infrared (MIR) spectral range holds significant potential for spectroscopic and sensing applications because it encompasses the fingerprint region that unveils the vibrational and rotational signatures of molecules. CMOS-compatible on-chip devices that can achieve strong light-matter interaction in the entire fingerprint region are considered a promising way for such applications, but remain unprecedented. Here we present an on-chip MIR germanium photonic crystal cavity that covers the entire fingerprint region. This is made possible by harnessing a homemade air-cladding germanium platform. Our MIR device creates a new avenue toward integrated nonlinear optics and on-chip biochemical sensing in the fingerprint region.

Focusing subwavelength grating coupler for mid-infrared suspended membrane germanium waveguides

We present a focusing subwavelength grating (SWG) for efficient coupling of mid-infrared (mid-IR) light into suspended membrane Ge photonic integrated circuits (PICs) that enable mid-IR applications in the entire fingerprint region. By virtue of their wide spectral transparency window and air-cladding device configuration, the suspended membrane Ge PICs are expected to be effective for mid-IR applications over the spectral region covering from 2 to 15 μm. Specifically, we demonstrate the maximum coupling efficiency of -11  dB with a 1-dB bandwidth of ∼58  nm at the SWG's center wavelength of 2.37 μm. Our focusing SWG is expected to advance the development of on-chip long-wavelength mid-IR applications such as biochemical sensing, thermal imaging, and nonlinear optics in the fingerprint region.

Cavity-enhanced thermo-optic bistability and hysteresis in a graphene-on-Si3N4 ring resonator

Cavity-enhanced thermo-optic bistability is studied in a graphene-on-Si₃N₄ ring resonator. By engineering the coverage of the monolayer graphene on top of the Si₃N₄ ring resonator, we observed a two-fold enhancement in the thermo-optically induced resonance shift rate and an 18-fold increase in the effective thermal nonlinear refractive index compared with the devices without graphene. The thermo-optic hysteresis loop was also characterized in this hybrid structure, where the experimental results agree well with the theoretical calculations. This Letter paves the way for graphene-on-Si₃N₄-based high-speed photodetectors, modulators, and devices for on-chip nonlinear optical applications.

Fully suspended slot waveguides for high refractive index sensitivity

A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9  dB/cm at the wavelength of 2.25 μm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is limited only by the absorption of silicon. Numerical simulation shows that its sensitivity, defined as the ratio between the change of the effective index and the ambient refractive index, can reach 1.123, which is 9.7% higher than the maximal sensitivity of conventional SOI slot waveguides on BOX.

Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity

Graphene's unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.

Monolithically integrated reconfigurable add-drop multiplexer for mode-division-multiplexing systems

An integrated reconfigurable optical add-drop multiplexer (ROADM) for mode-division-multiplexing systems is proposed and demonstrated for the first time, to the best of our knowledge. The present ROADM with four mode-channels is composed of a four-channel mode demultiplexer, four identical 2×2 thermo-optic Mach-Zehnder switches (MZSs), and a four-channel mode multiplexer, which are integrated monolithically on silicon. All the devices are designed for operation with TM polarization. The ROADM can add/drop any one of the mode channels freely by thermally turning on/off the corresponding MZS. For the added/dropped mode-channels, the excess loss is 1-5 dB, and the extinction ratio is 15-20 dB in the wavelength range of 1535-1565 nm.

High-responsivity graphene-on-silicon slot waveguide photodetectors

We demonstrated strong optical absorption in a graphene integrated silicon slot waveguide. Due to the increase in light intensity and decrease of optical mode confinement in the silicon slot, the graphene experienced an increased interaction to the in-plane light. A waveguide absorption of 0.935 dB μm(-1) was measured at 1.55 μm wavelengths. Based on the graphene-on-silicon slot waveguide, a compact and high-responsivity photodetector was demonstrated. Benefited from the lack of efficient electron cooling in the suspended graphene and the intensity enhancement effect in the nano slot, a maximum responsivity of 0.273 A W(-1) was achieved in the telecommunication band.

Impact of heart disease and calibration interval on accuracy of pulse transit time-based blood pressure estimation

Continuous blood pressure (BP) measurement without a cuff is advantageous for the early detection and prevention of hypertension. The pulse transit time (PTT) method has proven to be promising for continuous cuffless BP measurement. However, the problem of accuracy is one of the most challenging aspects before the large-scale clinical application of this method. Since PTT-based BP estimation relies primarily on the relationship between PTT and BP under certain assumptions, estimation accuracy will be affected by cardiovascular disorders that impair this relationship and by the calibration frequency, which may violate these assumptions. This study sought to examine the impact of heart disease and the calibration interval on the accuracy of PTT-based BP estimation. The accuracy of a PTT-BP algorithm was investigated in 37 healthy subjects and 48 patients with heart disease at different calibration intervals, namely 15 min, 2 weeks, and 1 month after initial calibration. The results showed that the overall accuracy of systolic BP estimation was significantly lower in subjects with heart disease than in healthy subjects, but diastolic BP estimation was more accurate in patients than in healthy subjects. The accuracy of systolic and diastolic BP estimation becomes less reliable with longer calibration intervals. These findings demonstrate that both heart disease and the calibration interval can influence the accuracy of PTT-based BP estimation and should be taken into consideration to improve estimation accuracy.

Experimental demonstration of polarization-insensitive air-cladding grating couplers for silicon-on-insulator waveguides

We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2  dB maximum coupling efficiency with ∼28  nm 1 dB bandwidth is achieved. With the same grating, -4.3  dB maximum coupling efficiency with ∼58  nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32  nm. At the 1525 nm wavelength range, the polarization-insensitive grating is demonstrated with -6.5  dB coupling efficiency. The polarization-insensitive coupling wavelength can be controlled experimentally.

Spectral hole burning in silicon waveguides with a graphene layer on top

We present an experimental study of the nonlinear absorption of graphene-on-silicon waveguides. The wavelength dependence of absorption saturation from a narrow linewidth optical pump is measured. Spectral hole burning (SHB) introduces a decrease in the probe absorption near the pump wavelength, which may be distinguished from the free carrier absorption (FCA) by using the time dependence of the buildup of the free carrier population. Beyond SHB bandwidth, only FCA dominates the waveguide loss. The spectral bandwidth of absorption bleaching from SHB is measured to have a full width at half-maximum of ~12 meV (~20 nm) at 16.8 dBm pump power.

Stable and wavelength-tunable silicon-micro-ring-resonator based erbium-doped fiber laser

In this work, we propose and demonstrate a stable and wavelength-tunable erbium-doped fiber (EDF) ring laser. Here, a silicon-on-insulator (SOI)-based silicon-micro-ring-resonator (SMRR) is used as the wavelength selective element inside the fiber ring cavity. A uniform period grating coupler (GC) is used to couple between the SMRR and single mode fiber (SMF) and serves also as a polarization dependent element in the cavity. The output lasing wavelength of the proposed fiber laser can be tuned at a tuning step of 2 nm (defined by the free spectral range (FSR) of the SMRR) in a bandwidth of 35.2 nm (1532.00 to 1567.20 nm), which is defined by the gain of the EDF. The optical-signal-to-noise-ratio (OSNR) of each lasing wavelength is larger than 42.0 dB. In addition, the output stabilities of power and wavelength are also discussed.

Broadband focusing grating couplers for suspended-membrane waveguides

We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100  nm 3 dB bandwidth with -1.7  dB coupling efficiency for an apodized grating in near-IR. -3.5  dB maximum coupling efficiency and ∼90  nm 3 dB bandwidth are realized experimentally. In mid-IR, -5.5  dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75 μm, and ∼500  nm 3 dB bandwidth is predicted theoretically.

Tunable integrated variable bit-rate DPSK silicon receiver

We integrate a wavelength-tunable microring resonator with a monolithic Ge-on-Si photodetector for use as a variable bit-rate silicon receiver. The integrated receiver has a responsivity of 0.7 A/W. We achieved error-free operation for a wide range of bit rates from 5 Gb/s to 12.5 Gb/s. The wavelength tuning was realized by a TiN-based thermal heater, which enabled tuning of up to ~1 free spectral range.

Demodulation of 20 Gbaud/s differential quadrature phase-shift keying signals using wavelength-tunable silicon microring resonators

We experimentally demonstrate a demodulation scheme for 20 Gbaud/s nonreturn-to-zero differential quadrature phase-shift keying signals using a silicon microring resonator. Either the I or Q channel can be selected by electrical tuning using the carrier depletion effect. Error-free (bit error rate <10(-9)) operations can be achieved for both the I and Q channels with ~1.5 dB power penalty compared with a commercial 20 GHz Mach-Zehnder delay interferometer. The tuning range of the ring resonator can be up to 60 GHz, which enables operation over a wide range of data rates.

Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide

A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.

Long-reach radio-over-fiber signal distribution using single-sideband signal generated by a silicon-modulator

The integration of passive optical network (PON) and radio-over-fiber (ROF) networks could provide broadband services for both fixed and mobile users in a single and low-cost platform. Combining the long-reach (LR)-PON (>100 km) and the LR-ROF can further reduce the cost by simplifying the network architecture, sharing the same optical components and extending the coverage of ROF network. However, the transmission and distribution of ROF signal in LR network is very challenging due to the chromatic dispersion generated periodic power fading and code time-shifting effects in the optical fiber. In this work, we propose and experimentally demonstrate a LR-ROF signal distribution using single-sideband (SSB)-ROF signal generated by a silicon ring-modulator. The silicon modulator is compact and has low power consumption. Besides, one unique feature of the silicon ring-modulator is that it only modulates the signal wavelength at the resonant null. This makes it very suitable for the generation of the SSB-ROF signal. Numerical comparison of the SSB-ROF with the double-sideband (DSB)-ROF and optical carrier suppress (OCS)-ROF signals; as well as the fabrication of the silicon ring-modulator will be discussed.

Dynamic gain-tilt compensation using electronic variable optical attenuators and a thin film filter spectral tilt monitor

An all-optical dynamic gain tilt compensator (DGTC) is proposed and experimentally demonstrated. A single wide-band thin film filter and a pair of photodetector allow the DGTC to distinguish band add/drop position. Power fluctuations from EDFA gain tilt were reduced with fast electronic variable optical attenuators.

Optical label switching of DRZ/DPSK orthogonal signal generated by photonic-crystal fiber

We demonstrate orthogonal label switching by using a dark-return-to-zero (DRZ) payload and differential phase-shift keying (DPSK) label generated by a dispersion-flattened photonic-crystal fiber. The high extinction ratio of both the payload and label improves the receiver margin. The DRZ payload introduces little cross talk to the DPSK label due to the RZ-like output of the demodulated DPSK. Simulations are performed to study the eye-closure penalty of the payload and label at different DRZ pulse widths. We compare the DRZ/DPSK with the RZ and DPSK signals numerically at the same data rate and show that the DRZ/DPSK has a strong tolerance to the polarization-mode dispersion. The DRZ/DPSK has a more compact spectrum suitable for the strong filtering requirements in WDM systems.

Nonlinear polarization rotation in a dispersion-flattened photonic-crystal fiber for ultrawideband (>100 nm) all-optical wavelength conversion of 10 Gbit/s nonreturn-to-zero signals

We study the conversion bandwidth of the cross-polarization-modulation (XPoIM)-based wavelength conversion scheme with a dispersion-flattened highly nonlinear photonic-crystal fiber for signals with a nonreturn-to-zero (NRZ) modulation format. Both theoretical and experimental results show that the conversion bandwidth can be extended to cover a very wide band, including S-, C-, and L-bands for 10 Gbit/s NRZ signals (a total bandwidth of 120 nm is experimentally demonstrated). We also study the theoretical bandwidth limit for 40 Gbit/s NRZ signals. A significant extension of the conversion bandwidth using the XPoIM approach compared with the four-wave mixing approach previously reported is demonstrated.

Nonlinear absorption and Raman gain in helium-ion-implanted silicon waveguides

We study whether helium ion implantation can reduce the carrier lifetime and thus reduce the density of two-photon-absorption-generated free carriers. Our experiments and theoretical model show that helium ion implantation into silicon waveguides can successfully reduce the free-carrier losses and allow net gain to be attained by cw-pumped stimulated Raman scattering without requiring reverse bias to remove the photogenerated free carriers.

Optical label encoding and swapping using half-bit delayed dark RZ payload and DPSK label

Orthogonal labeling is a potential candidate in future opticallabel- switched-networks. Half-bit-delayed-dark-return-to-zero (HBDDRZ) payload with DPSK orthogonal labeling is proposed. The precise payload extinction-ratio adjustment at source nodes and maintenance at each intermediate node is not needed. The high extinction-ratio of both payload and label improves the receiver margin. Owing to the RZ-like nature of the demodulated DPSK label, HBDDRZ payload causes negligible effect on the label. Nonlinear-optical-loop-mirror generates the HBDDRZ payload, which is then encoded by DPSK label. Label swapping is demonstrated by birefringence switching. 10-Gb/s bit-error-rate measurements are performed.