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On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23â nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87â ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57â dB/ns and the total insertion loss of the device is 6â dB at the central wavelength near 1550â nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.
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Integrated photonic devices play a key role in modern optical communications, of which optical resonators are important fundamental structures. This work proposes and experimentally demonstrates compact integrated photonic devices based on a traveling wave-like Fabry-Perot (TW-like FP) resonator(s) coupled with waveguides. Add-drop filters based on a single TW-like FP resonator have been realized with a high drop efficiency and the same output direction for the through and drop ports. Particularly, their transmission response can be either symmetric Lorentzian or asymmetric Fano line shape, through adjusting the shift between the two bus waveguides and the waveguide widths. Fano resonance has been demonstrated in a TW-like FP resonator with a very high extinction ratio and large slope rate. The second-order optical filter exhibits low-loss flat-top passbands with small ripples. Owing to the compact size, easy scalability, and large flexibility, TW-like FP cavity-based devices using Fano and Lorentzian resonances will provide new potential applications in integrated photonics.
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Diffraction gratings with high upward diffraction efficiency and large effective length are required for chip-scale light detection and ranging. We propose a diffraction grating based on a multilayer silicon nitride waveguide, which theoretically achieves an upward diffraction efficiency of 92%, a near-field effective length of 376 µm, and a far-field divergence angle of 0.105° at a wavelength of 850 nm. The diffraction grating has a high tolerance to process variations based on Monte Carlo analysis. When the conditions are ±5% layer thickness variation, ±50nm lithographic variation, and ±20nm wavelength drift, more than 71% of the grating samples have a diffraction efficiency higher than 80%, and 100% of the samples have an effective length larger than 200 µm (corresponding to a far-field divergence <0.2∘). Furthermore, the near-field effective length of the grating with an upward diffraction efficiency above 90% can be adjusted from hundreds of microns to centimeters by changing the etching layer thickness and the grating duty cycle. This diffraction grating has a potential application in optical sensing and imaging from visible to near-IR wavelengths.
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An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.
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We have theoretically and experimentally demonstrated electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) phenomena in a compact silicon ring-bus-ring-bus (RBRB) system. The two ring resonators in our RBRB system are both directly coupled and indirectly coupled through an asymmetric tricoupler. The coherent interference between a radiant mode and a subradiant mode in the two rings results in EIT and EIA effects at the through and drop ports, respectively. A theoretical model is established to analyze the proposed system based on temporal coupled mode theory. Finite-difference time-domain method is also employed to simulate the characteristics of this system. Consequently, RBRB structures were fabricated on a silicon-on-insulator platform and EIT and EIA transmissions have been observed simultaneously in the two outputs. The experimental results agree with our theoretical modeling and numerical simulations.
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We propose and demonstrate a novel two-mode grating assisted contra-directional coupler (TGACC), which is capable of filtering two modes channels simultaneously by superposed grating with two superposed grating components. Finite-difference time-domain simulation is employed to study the structure. The influences of main structural parameters are analyzed, and apodization is employed to reduce the band sidelobes, crosstalk and back-reflections. We experimentally present a mode-channel switchable TGACC for 2.54nm-wide wavelength band centered at 1548.0nm by 50K thermal-optic tuning. With two channels combined into one device, the TGACC can help to enrich the functionality and reduce the footprint of mode-division multiplexing (MDM) systems.
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A femtogram scale nanobeam optomechanical crystal (OMC) resonator operating in water is designed and demonstrated. After immersing the device in water, the mechanical Q-factor reduces to 6.6 from 2285 in air. The thermomechanical motion of the highly damped mechanical resonance in water can be resolved using the sensitive cavity optomechanical readout. The mechanical frequency is shifted to 5.251 GHz from 5.3 GHz in air due to the added motional inertia. From the thermomechanical noise spectrum of the mechanical resonance, a noise floor of 9.33am/Hz is achieved in water. Through 2D finite element method (FEM) simulations, the acoustic dissipation dominates the low mechanical Q-factor of the device during the interaction between the mechanical resonance and surrounding water. The mass sensitivity of the present device is estimated to be 1.33ag/Hz in water.
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In this paper, we perform an investigation of terabit-scale data transmission in silicon subwavelength grating (SWG) waveguides for wavelength-division multiplexing (WDM) optical signals. Silicon SWG waveguide is capable of decreasing the light confinement in silicon core by engineering the geometry, leading to relatively lower optical nonlinearity compared to silicon wire waveguide. We demonstrate ultrahigh-bandwidth 2.86 Tb/s data transmission through the fabricated 2-mm-long silicon SWG waveguide over a wide range of launch powers. In the experiment, 75 WDM channels are utilized with each carrying 38.12 Gb/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. With the benefit of efficient reduction on optical nonlinearity, the optimum launch power is increased by 8 dB in SWG waveguide, indicating higher tolerance to the nonlinear impairments, compared to a silicon wire waveguide with identical length. With the optimum launch power, all 75 channels exhibit bit-error rate (BER) values less than 4e-5 after SWG waveguide transmission. We also evaluate the terabit-scale data transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). The required optical signal-to-noise ratios (OSNRs) to achieve BER level of 1e-3 are around 15.27, 15.47, 16.66 and 20.38 dB, respectively.
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We have proposed and studied a novel channel add-drop filter (ADF) based on a single Fabry-Perot resonator. The resonator consists of two mode-conversion Bragg grating reflectors separated by a wide waveguide that laterally coupled to two narrow waveguides. It behaves like a traveling-wave resonator where fields are coupled to the buses in one direction. Compact and narrowband ADFs are achieved with dropping efficiencies higher than 95%, as shown by the three-dimensional finite-difference time-domain simulations. In addition, the proposed device is applied to realize an eight-channel add-drop multiplexer in the C-band by cascading the ADFs with adjusted channel wavelengths.
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In this paper, ultra-compact, broadband tunable optical bandstop filters (OBSFs) based on a multimode one-dimensional photonic crystal waveguide (PhCW) are proposed and systematically investigated. For the wavelengths in the mini-stopband, the input mode is coupled to a contra-propagating higher order mode by the PhCW and then radiates in a taper, resulting in a stopband at the output with low backreflection at the input. Three-dimensional finite-difference time-domain method is employed to study the OBSFs. The influence of main structural parameters is analyzed, and the design is optimized to reduce the back-reflection and band sidelobes. Using localized heating, we can shift the stopband and tune the bandwidth continuously by cascading the proposed structures. Due to the strong grating strength, our device provides a more compact footprint (40 µm × 1 µm) and much broader stopband (bandwidth of up to 84 nm), compared to the counterparts based on microrings, long-period waveguide gratings, and multimode two-dimensional PhCWs.
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We experimentally demonstrate enhanced third harmonic generation (THG) using a silicon metasurface, which is consist of symmetric spindle-shape nanoparticle array. Relying on the trapped mode supported by the high quality factor all-dielectric metasurface, the conversion efficiency of THG is about 300 times larger than that of bulk silicon slab. The maximum extinction ratio of the intensity of THG reaches about 25 dB by tuning the polarization of incident light. The simulation results agree with the experimental performances.
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We demonstrate that enhanced linear absorption coefficient (LAC) of in-plane monolayer graphene is determined by the optical transmission spectra of a graphene layer coated symmetrically coupled add-drop silicon microring resonator (SC-ADSMR), of which the value is around 0.23 dB/µm. In contrast to the traditional cut-back method, the measured results aren't dependent on the coupling efficiency between the fiber tip and the waveguide. Moreover, precisely evaluation of graphene layer coated silicon microring resonator (SMR) is crucial for future optoelectronic devices with compact footprint and low power consumption.
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We present the design of a double-slot photonic crystal cavity as an optomechanical device which contains a nanomechanical resonator with an effective mass as small as 6.91 fg. The optical Q-factor is optimized to 2 × 10(5). Using phononic crystals, the mechanical vibration is confined in a small volume to form a mechanical mode of 4 GHz with a high mechanical Q-factor and a femtogram effective mass. The localized mechanical mode overlaps with the optical field and strengthens the optomechanical coupling with a vacuum optomechanical coupling rate g0/2π exceeding 600 kHz. Considering fabrication imperfections, structures with deviation from ideal design are studied. The symmetry breakage of the structures and the displacement fields makes the mechanical effective masses reduced and close to 4 fg. The devices can be used in ultrasensitive sensing of mass, force and displacement.
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In this paper, a single microring resonator structure formed by incorporating a reflectivity-tunable loop mirror is demonstrated for the tuning of resonance spacing. Autler-Townes splitting in the resonator is utilized to tune the spacing between two adjacent resonances by controlling the strength of coupling between the two counter-propagating degenerate modes in the microring resonator. A theoretical model based on the transfer matrix method is built to analyze the device. The theoretical analysis indicates that the resonance spacing can be tuned from zero to one free spectral range (FSR). In experiment, by integrating metallic microheater, the tuning of resonance spacing in the range of the whole FSR (1.17 nm) is achieved within 9.82 mW heating power dissipation. The device has potential for applications in reconfigurable optical filtering and microwave photonics.
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We demonstrate four-wave mixing (FWM) in a 10-µm-radius silicon microring resonator with the assistance of giant nonlinearity of the monolayer graphene. A maximum enhancement of 6.8 dB of conversion efficiency in the silicon-graphene microring (SGM) resonator is observed. A nonlinear propagation model is established and the optical Kerr coefficient of the silicon-graphene hybrid waveguide is three times larger than that of the silicon waveguide.
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A silicon light emitter in telecom-band based on a single germanium quantum dot precisely embedded in a silicon photonic crystal nanocavity is fabricated by a scalable method. A sharp resonant luminescence peak is observed at 1498.8 nm, which is enhanced by more than three orders of magnitude. The Purcell factor for the fundamental resonant mode is estimated from enhancement factor and increased collection efficiency. The cavity modes coupled to the ground state and excited state emission of germanium quantum dot are identified in the luminescence spectrum. Our devices provide a CMOS-compatible way of developing silicon-based low-power consuming light emitters, and are promising for realizing on-chip single photon sources.
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We theoretically investigate the coherent interaction between two orthogonal travelling-wave modes in a microdonut resonator symmetrically coupled to two bus waveguides. An analytical model has been developed to describe this structure using transfer matrix method. The simulation reveals that the two-mode microdonut can exhibit either a flat-top response or all-pass transmission, governed by the resonance spacing. Then, we implement analytical simulations to characterize the device and analyze the influence of coupling efficiencies and propagation losses of two resonant modes on behavior. Consequently, finite difference time domain simulations have been performed. The numerical results validate our theoretical analysis, and optical buffering effect is demonstrated in a pulse propagation simulation, when the two resonances are aligned. In addition, we show that the device function can be switched between flat-top filtering and all-pass filtering by tuning the local refractive index in a microdonut. Hence, this structure is promising for on-chip optical filtering and buffering applications.
Asunto(s)
Dispositivos Ópticos , Fenómenos Ópticos , Simulación por Computador , Procesamiento de Señales Asistido por Computador , Análisis Espectral , Factores de TiempoRESUMEN
We observe theoretically and experimentally electromagnetically induced transparency (EIT)-like effect in a single microdisk resonator (MDR) evanescently coupled with two bus waveguides. This structure is modeled using transfer matrix method, and it is revealed that the EIT-like spectrum originates from the coherent interference between two nearby low-order whispering-gallery modes (WGMs) with comparable quality factors. The EIT-like properties have been investigated analytically with respect to coupling efficiency, round-trip power attenuation, as well as phase spacing between two resonances. The resonance spacing and mode coupling are adjustable by varying the effective indices of WGMs and waveguide mode. Consequently, fully integrated MDRs were fabricated in silicon. Resonant modes and coupling efficiency are studied in one-bus waveguide coupled MDRs. Finally, EIT-like resonance is observed in a two-bus waveguides coupled MDR of 3 µm in radius with a quality factor of 4,200 and central transmission larger than 0.65. The experimental results agree with our modeling well and show good internal consistency, confirming that two WGMs coupled in a point-to-point manner are required for EIT-like effect.
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We have experimentally demonstrated a microdisk-resonator (MDR)-based filter for filtering and buffering applications utilizing two-mode interference. Optical filtering or buffering behavior can be generated by the coherent interference between two orthogonal whispering-gallery modes (WGMs) in a MDR symmetrically coupled to two bus waveguides. MDRs were fabricated using nanofabrication processes on a silicon-on-insulator platform. Multiple resonances of WGMs are excited in a MDR and unique filter characteristics have been observed for two spectrally nearby resonances. The measured results agree well with the theoretical simulation. Bandpass filtering is achieved with a 3 dB bandwidth of 0.66 nm and an in-band ripple of less than 1 dB when the two resonances are detuned, while the filter can function as an optical buffer with an insertion loss of 5.85 dB and a predicted time delay of 15 ps when the two resonances are approximately aligned. Furthermore, the influence of resonance spacing is analyzed on the performance of this filter.
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An all-silicon passive optical diode based on optical nonlinearity in cascaded photonic crystal (PhC) L3 cavities is proposed and demonstrated. A nonreciprocal transmission ratio (NTR) of 30.8 dB and insertion loss of 8.3 dB are realized in the device. The device has a relatively broad 17 dB operation bandwidth of 0.08 nm, and at least 16 dB of NTR is achieved when input power varies between -6.25 and -2.95 dBm. A nonlinear couple mode model for cascaded PhC cavities is established to analyze the behavior of the device, and numerical simulation results are in good agreement with the experiment results.