High-visibility energy-time entanglement system enabled by a low-loss silicon-integrated platform.

Energy-time (E-T) entanglement is widely employed in long-distance quantum entanglement distribution due to its strong robustness against transmission fluctuations. In this Letter, we report what we believe to be the first silicon monolithically integrated E-T entanglement system, which integrates the photon sources, wavelength demultiplexers, and Franson interferometers on a single chip. Also, by utilizing low-loss multimode waveguides in Franson interferometers, we measured an on-chip quantum interference visibility of 99.66% (±0.47%), to our knowledge one of the highest values for integrated E-T entanglement systems reported to date. The quantum interference after 1- and 5-km fiber propagation shows visibilities of 96.72% (±0.78%) and 97.46% (±1.23%), respectively. These results demonstrate the potential of using silicon monolithic integration for advance E-T entanglement-based quantum communication networks.

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.

Low-crosstalk and fabrication-tolerant four-channel CWDM filter based on dispersion-engineered Mach-Zehnder interferometers.

The O-band coarse wavelength-division (de)multiplexing (CWDM) has been extensively used in data-center optical communications, whereas it's still challenging to reduce crosstalk and enhance fabrication tolerances for a CWDM filter. In this paper, we propose and experimentally demonstrate a low-crosstalk and fabrication-tolerant four-channel CWDM filter by utilizing dispersion-engineered Mach-Zehnder interferometers. The multi-sectional phase shifters are exploited to eliminate the phase errors induced by width deviations, leading to ultra-precise phase shifts and ultra-large width-error tolerances. The random-phase errors are also inhibited by using multi-mode waveguides as phase-shifting sections. The two-stage-coupler scheme is utilized to flatten the strong coupling-ratio dispersions for directional couplers, so that low crosstalk can be achieved over the whole O-band. The experimental results show both low insertion losses (< 1.2 dB) and low crosstalk (< -22.2 dB) over the whole working wavelength range. The measured width-error tolerance is also as large as ≈ 70 nm.

Proposal for an ultra-broadband polarization beam splitter using an anisotropy-engineered Mach-Zehnder interferometer on the x-cut lithium-niobate-on-insulator.

We propose and theoretically demonstrate an integrated polarization beam splitter on the x-cut lithium-niobate-on-insulator (LNOI) platform. The device is based on a Mach-Zehnder interferometer with an anisotropy-engineered multi-section phase shifter. The phase shift can be simultaneously controlled for the TE and TM polarizations by engineering the length and direction of the anisotropic LNOI waveguide. For TE polarization, the phase shift is -π/2, while for TM polarization, the phase shift is π/2. Thus, the incident TE and TM modes can be coupled into different output ports. The simulation results show an ultra-high polarization extinction ratio of ∼47.7 dB, a low excess loss of ∼0.9 dB and an ultra-broad working bandwidth of ∼200 nm. To the best of our knowledge, the proposed structure is the first integrated polarization beam splitter on the x-cut LNOI platform.

Subwavelength-grating-assisted silicon polarization rotator covering all optical communication bands.

We propose an ultra-broadband and ultra-compact polarization rotator (PR) structure on the silicon-on-insulator platform. The subwavelength gratings (SWGs) are introduced at the waveguide corner in order to excite the hybridized modes and realize the polarization rotation. The dispersion-engineered SWG can dramatically reduce the polarization conversion length deviation. High polarization extinction ratio > 20 dB and low excess loss < 1 dB can be achieved over 1.26-1.675 µm wavelength range, which covers O-, E-, S-, C-, L-, and U-bands. The total device size is as small as 4.8 × 0.34 µm2. To the best of our knowledge, the proposed structure is the first silicon PR that could cover all of the optical communication bands.

Polarization-insensitive four-channel coarse wavelength-division (de)multiplexer based on Mach-Zehnder interferometers with bent directional couplers and polarization rotators.

A polarization-insensitive four-channel coarse wavelength-division multiplexing (CWDM) (de)multiplexer based on Mach-Zehnder interferometers is proposed and demonstrated. By utilizing the bent directional couplers and the polarization rotators, the CWDM (de)multiplexer could work for both transverse electric and transverse magnetic polarizations. The measurement results of the fabricated devices show that the 1-dB bandwidth is ∼15 nm for dual polarizations, and the polarization dependent losses are <∼0.5 dB for all four channels.

Ultra-broadband silicon polarization splitter-rotator based on the multi-mode waveguide.

We propose and demonstrate an ultra-broadband polarization splitter-rotator (PSR) on the Silicon-On-Insulator (SOI) platform. The proposed PSR consists of a straight multi-mode waveguide, an asymmetrical directional coupler and a bent directional coupler. The multi-mode waveguide enables highly-efficient TM0-TE1 polarization rotation. The excited TE1 mode is then converted to be TE0 mode by the asymmetrical directional coupler. The remained TM0 mode is filtered out by the bent directional coupler. On the other hand, the incident light of TE0 mode goes through the PSR with negligible conversion and coupling. Only one-step etching is required for the proposed PSR. The fabricated PSR shows a high extinction ratio > 30.82 dB and a low loss < 0.57 dB at the central wavelength. The extinction ratio is > 20 dB over an ultra-broad wavelength band > 85 nm.

Ultra-compact and highly efficient polarization rotator utilizing multi-mode waveguides.

An ultra-compact and highly efficient polarization rotator is proposed and experimentally realized on the silicon-on-insulator (SOI) platform. The polarization rotation (TM0-TE1) process is obtained by utilizing a straight multi-mode waveguide, while the mode conversion (TE1-TE0) process is realized by a bent multi-mode waveguide. For the proposed structure, only one etching step is required for the fabrication. The measured extinction ratio and insertion loss at the central wavelength are 19.8 and 0.86 dB, which agree well with the simulations.

Ultra-compact polarization-independent directional couplers utilizing a subwavelength structure.

Ultra-compact polarization-independent directional couplers are proposed and demonstrated on the silicon-on-insulator (SOI) platform. By using the subwavelength structure in the coupling region, the coupling strength is greatly enhanced only for TE polarization, so that the coupling strength could be equivalent between TE and TM polarizations. Both complete coupling and 3 dB splitting have been demonstrated. The coupling length could be as short as ∼3.75 µm. The measured excess losses and polarization dependence losses are <∼1 and <∼0.5 dB.

Ultra-broadband 16-channel mode division (de)multiplexer utilizing densely packed bent waveguide arrays.

We demonstrate an ultra-broadband 16-channel mode division (de)multiplexer utilizing densely packed bent waveguide arrays (DPBWAs). The phase mismatch is realized by using bent waveguides with a different radius instead of changing the waveguide width. The proposed structure shows theoretical insertion losses (ILs) that are <0.1 dB and crosstalk (CT) that is <-30 dB over a 100 nm wavelength range for all the channels. The fabricated device, which consists of a multiplexer, DPBWAs, and a demultiplexer, exhibits ILs that are <1 dB and crosstalk that is <-20 dB over an ∼80 nm wavelength band.

Ultra-broadband dual-mode 3 dB power splitter based on a Y-junction assisted with mode converters.

A dual-mode 3 dB power splitter for application in an on-chip mode (de)multiplexing system is proposed and demonstrated. This device is composed of mode converters cascaded with a Y-junction. The simulation results show that ∼50∶50 splitting ratio can be achieved for both TM0 mode and TM1 mode over a wide wavelength range from 1.52 to 1.62 µm. The fabricated device, which consists of a multiplexer, a dual-mode 3 dB power splitter, and a demultiplexer, has insertion losses of 0.54 dB for TM0 mode and 0.86 dB for TM1 mode at 1.55 µm wavelength. The measured cross talk is 15.7 and 17.2 dB at 1.55 µm wavelength for the TM0 mode and TM1 mode, respectively.

Dual-mode waveguide crossing utilizing taper-assisted multimode-interference couplers.

A dual-mode waveguide crossing is proposed and realized for the application of on-chip multimode interconnection. The present structure is realized by two 90° crossed multimode-interference (MMI) couplers. By properly choosing the width and length of the MMI section, the self-images for both the incident TM0 mode and the TM1 mode could be at the center of the crossing. The characterization results for the fabricated device show that low insertion loss below ∼1.5 dB and low crosstalk below ∼-18 dB can be achieved over a large bandwidth more than 80 nm for both the TM0 and TM1 modes.

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.

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.