Contra-directional Coupling Research Articles

Flat-Top, Sharp-Edge Add-Drop Filters Using Complementary-Misalignment-Modulated Grating-Assisted Contradirectional Couplers

We demonstrate flat-top, sharp-edge, silicon add-drop filters using apodized grating-assisted contradirectional couplers (contra-DCs). Periodic sidewall corrugations are imposed on both sides of two asymmetric strip waveguides for contradirectional coupling between the first two TE supermodes of a two-waveguide system. Gaussian apodization is applied to suppress the sidelobes. By laterally shifting the positions of both waveguide Bragg gratings in a complementary manner, which is called complementary lateral-misalignment modulation, our devices avoid phase errors which impair the spectral responses for conventional lateral-misalignment modulated contra-DCs. We demonstrate a complementary lateral-misalignment modulated (CLMM) contra-DC that has a flat-top, rapid roll-off, drop-port spectral response with a sidelobe suppression ratio (SLSR) of 20 dB. We also demonstrate a series-cascaded CLMM contra-DCs that has a higher SLSR of 39 dB and a sharper filtering edge. ©

Contra-directional pump reject filters integrated with a micro-ring resonator photon-pair source in silicon.

High coincidence-to-accidental ratio (CAR) is crucial for photon-pair sources (PPSs) integrated with pump reject filters (PRFs) in silicon, but CAR values currently reported for integrated PPS/PRF chips still fall short of those achieved using stand-alone sources with external PRFs. Here we report measured and modelled CAR values for a micro-ring resonator PPS integrated with a PRF consisting of a three-stage, cascaded (via their through ports), contra-directional coupler (CDC) that compare favorably even with some stand-alone sources. CDC-based PRFs provide the benefits of compact area and wide reject bands without a need for tuning, in comparison to prior-art implementations.

Broadband, silicon photonic, optical add-drop filters with 3 dB bandwidths up to 11 THz.

We present the designs, theory, and experimental demonstrations of ultra-broadband, optical add-drop filters on the silicon-on-insulator platform, realized using period-chirped contra-directional couplers. Our fabricated devices have ultra-broad 3 dB bandwidths of up to 11 THz (88.1 nm), with flat-top responses at their drop ports. All of our devices were fabricated using a commercial, CMOS-compatible, 193 nm deep-ultraviolet lithography process. By using lithography-prediction models, the measured bandwidths, insertion losses, central wavelengths, and extinction ratios of our devices are all in good agreement with our predicted, simulated results. Such filters are necessary for photonic integrated circuits to operate over multiple optical bands.

Miniaturized integrated spectrometer using a silicon ring-grating design.

We introduce and experimentally demonstrate a miniaturized integrated spectrometer operating over a broad bandwidth in the short-wavelength infrared (SWIR) spectrum that combines an add-drop ring resonator narrow band filter with a distributed Bragg reflector (DBR) based broadband filter realized in a silicon photonic platform. The contra-directional coupling DBR filter in this design consists of a pair of waveguide sidewall gratings that act as a broadband filter (i.e., 3.9 nm). The re-directed beam is then fed into the ring resonator which functions as a narrowband filter (i.e., 0.121 nm). In this scheme the free spectral range (FSR) limitation of the ring resonator is overcome by using the DBR as a filter to isolate a single ring resonance line. The overall design of the spectrometer is further simplified by simultaneously tuning both components through the thermo-optic effect. Moreover, several ring-grating spectrometer cells with different central wavelengths can be stacked in cascade in order to cover a broader spectrum bandwidth. This can be done by centering each unit cell on a different center wavelength such that the maximum range of one-unit cell corresponds to the minimum range of the next unit cell. This configuration enables high spectral resolution over a large spectral bandwidth and high extinction ratio (ER), making it suitable for a wide variety of applications.

Design of a hybrid mode and wavelength division (de)multiplexer based on contra-directional grating assisted couplers on the SOI platform

This paper presents a hybrid mode and wavelength division (de)multiplexer (MDM-WDM), based on the silicon-on-insulator platform using the grating assisted contra-directional coupling principle. The proposed hybrid MDM-WDM is able to (de)multiplex six channels with two wavelengths and three modes. The proposed device is designed by cascading three grating assisted coupler sections. Each coupler section is dedicated to reflecting a specific mode at two wavelengths. The operation of the proposed design is simulated by the 2.5D finite-difference time-domain technique. The analysis shows that the proposed device structure exhibits an insertion loss of < 0.55 d B , return loss of > --> 12.81 d B , extinction ratio of > --> 15.49 d B , and crosstalk of < − 12.74 d B . A fabrication tolerance study for the MDM-WDM device is also performed by changing the inter waveguide distance between the respective multimode bus waveguide and the single-mode waveguide.

Thermally chirped contra-directional couplers for residueless, bandwidth-tunable Bragg filters with fabrication error compensation.

Bandwidth-tunable filters are essential in elastic optical networks for dynamic bandwidth allocation. Existing solutions in silicon photonics face challenges to meet requirements in real-world applications due to design trade-offs and fabrication errors. In this Letter, we propose and experimentally demonstrate a silicon photonic tunable add-drop filter in a single-stage, hyperbolic-tangent-apodized contra-directional coupler with a segmented microheater. It allows to create an arbitrary temperature profile along the device for bandwidth tuning in both through and drop responses. We show that the algorithmic operation of the device can effectively compensate local fabrication nonuniformity and improve the out-of-band suppression ratio by 69%. Applying proper temperature offsets and slopes allows to continuously tune the filter's center wavelength over 8 nm and its drop-port 3 dB bandwidth between 14.0 and 22.4 nm.

Performance analysis of grating-assisted passive mode-division multiplexing device using silicon photonics for high-speed RoF/RoFSO communication

The performance of a bidirectional silicon mode-division (de)multiplexer has been analyzed in the radio over fiber passive optical network system. The device uses two grating-assisted contra-directional coupler sections to demultiplex the transmitted fundamental, first-order, and second-order quasi-transverse electric (TE) modes. A bandwidth of 791 GHz and acceptable crosstalks of −25.73 dB (TE0m), −10.30 dB (TE1m), and −11.84 dB (TE2m) have been achieved from the 2.5D finite-difference time-domain method at a wavelength of 1550 nm. The system uses parabolic index multimode fiber (MMF) and free-space optics channels alternatively. A 120-Gbps data rate has been obtained using differential quadrature phase-shift keying scheme and hybrid mode-division multiplexing techniques. The system maintains an acceptable bit error rate (≤10 − 12) over transmission distances of ≤5.3 km MMF and ≤1.9 km for free-space optics link with receiver sensitivities of ≥ − 26.2 and ≥ − 24.8 dBm, respectively.

Ultracompact bandwidth-tunable filter based on subwavelength grating-assisted contra-directional couplers.

An ultracompact, bandwidth-tunable filter has been demonstrated using a silicon-on-insulator (SOI) wafer. The device is based on cascaded grating-assisted contra-directional couplers (GACDCs). It also involves the use of a subwavelength grating (SWG) structure. By heating one of the heaters on GACDCs, a bandwidth tunability of ∼6 nm is achieved. Owing to the benefit of having a large coupling coefficient between SWG and strip waveguides, the length of the coupling region is only 100 µm. Moreover, the combination of the curved SWG and the tapered strip waveguides effectively suppresses the sidelobes. The filter possesses features of simultaneous wavelength tuning with no free spectral range (FSR) limitation. A maximum bandwidth of 10 nm was experimentally measured with a high out-of-band contrast of 25 dB. Similarly, the minimum bandwidth recorded is 4 nm with an out-of-band contrast of 15 dB.

2 × 2 16-ch silicon photonics wavelength-selective switch based on waveguide gratings.

We demonstrate a 2 × 2 16-ch silicon photonics wavelength-selective switch consisting of contra-directional couplers and thermo-optic Mach-Zehnder switches. The contra-directional couplers are based on sidewall corrugated Bragg gratings with an unlimited free spectral range and thus enable the device to operate over a very wide wavelength range of the C- and L-band. We obtain a fiber-to-fiber insertion loss of 9.2 dB, an on-chip loss of 5.4 dB, a 3-dB bandwidth of 4.2 nm, a bar-port extinction of 15.0 dB, and a cross-port extinction of 23.0 dB, all in 16-ch average. An average trimming power for the bar states and an average switching power for the cross states are 3.8 mW (σ = 2.3 mW) and 15.6 mW (σ = 2.1 mW), respectively. The wavelength dependence of the spectral responses and the resonant effect from the apodization are discussed in detail to show how to further improve the spectral performance.

Silicon photonic chip for 16-channel wavelength division (de-)multiplexing in the O-band

We experimentally demonstrate a silicon photonic chip-scale 16-channel wavelength division multiplexer (WDM) operating in the O-band. The silicon photonic chip consists of a common-input bus waveguide integrated with a sequence of 16 spectral add-drop filters implemented by 4-port contra-directional Bragg couplers and resonant cladding modulated perturbations. The combination of these features reduces the spectral bandwidth of the filters and improves the crosstalk. An apodization of the cladding modulated perturbations between the bus and the add/drop waveguides is used to optimize the strength of the coupling coefficient in the propagation direction to reduce the intra-channel crosstalk on adjacent channels. The fabricated chip was validated experimentally with a measured intra-channel crosstalk of ∼-18.9 dB for a channel spacing of 2.6 nm. The multiplexer/demultiplexer chip was also experimentally tested with a 10 Gbps data waveform. The resulting eye-pattern indicates that this approach is suitable for datacenter WDM-based interconnects in the O-band with large aggregate bandwidths.

Design and experimental demonstration of a silicon multi-dimensional (de)multiplexer for wavelength-, mode- and polarization-division (de)multiplexing

Leveraging the physical dimensions of an optical carrier (e.g., wavelength, mode, or polarization) allows significant scaling of the transmission capacity for optical communications. Here we propose a scheme for implementing on-chip silicon (de)multiplexers with simultaneous wavelength-, mode-, and polarization-division (de)multiplexing capability. The device is constructed by using cascaded subwavelength grating (SWG)-based contra-directional couplers. To verify the feasibility of the proposed structure, we perform a proof-of-concept experiment of an 8-channel (de)multiplexer with two wavelengths, two modes, and two polarizations. The insertion losses are lower than 6.6 dB and the crosstalk values are below -18.7dB at around 1540 nm and 1550 nm for all the eight channels.

Silicon Photonics Wavelength Selective Switch With Unlimited Free Spectral Range

We propose a novel silicon photonics wavelength selective switch (WSS) with sidewall-corrugated contra-directional couplers (C-DCs), which has an unlimited free spectral range and is suitable for broadband operation. Fabricated 2 × 2 1-ch and 2-ch WSSs demonstrate the expected operation with channel wavelengths of 1532 and 1553 nm, a 3-dB bandwidth of ∼3 nm, a flattop spectral shape, and an outband extinction ratio larger than 30 dB over a wavelength range wider than C-band. We discuss a limiting factor for the lower inband extinction of ∼15 dB, which is a leakage to the bar port in the Mach–Zehnder (MZ) switch. Matching the center wavelength of each MZ switch to the Bragg wavelength of the corresponding C-DCs will improve the inband extinction ratio to be larger than 30 dB.

Integrated Optical True Time Delay Network Based on Grating-Assisted Contradirectional Couplers for Phased Array Antennas

Phased array antennas (PAAs) play a key role in many fields such as radar systems and wireless communication systems. Since PAAs based on optical methods outperform the electronic counterparts in beam directivity and instantaneous bandwidth, optical PAAs have aroused wide research interests. Here, we report and experimentally demonstrate an integrated 4-channel optical true time delay network for PAAs. The true time delay unit is based on a grating-assisted contradirectional coupler, assisted by index chirp engineering and asymmetric apodization. A linear group delay with both positive and negative dispersion coefficients can be obtained. For the 4-channel time delay network, the measured dispersion coefficient of each channel is 10.35, 4.00, −4.32 and −14.38 ps/nm with 7 nm available bandwidth, respectively. The calculated beam scanning angle can range from 36.4 degree to −33.9 degree for the beam steering. This time delay network can be extended to large scale with more delay units and cascaded structures, thus wider scanning range and better beam directivity can be achieved.

Computational Lithography for Silicon Photonics Design

We demonstrate a method to predict the photolithography effects of a 193 nm deep ultraviolet (DUV) process, and we build a lithography model using physical measurements on a fabricated test pattern. Our model accurately predicts the proximity and smoothing effects that are characteristic of DUV lithography. We verify the accuracy of the model by visually inspecting the fabricated test patterns and comparing them to the predicted ones. Additionally, using predicted shapes, we compare experimental measurements on a benchmark device (the contra-directional coupler) against those of our predicted, computed responses. Our predictions show good agreement with the fabricated results using both verification methods. Lastly, we illustrate the design-for-manufacturability enabled by our model by demonstrating a design correction method for the contra-directional coupler that improves its compatibility with the DUV process.

Ultra-broadband TE-pass polarizer based on hybrid plasmonic-assisted contra-directional couplers

An ultra-broadband and compact TE-pass polarizer assisted by contra-directional coupling is proposed, which consists of two identical hybrid plasmonic waveguides (HPWs) and a middle strip grating waveguide (SGW). The grating pitch of the SGW is properly designed to make TM polarization satisfy the phase-matching condition. Therefore, the launched TE mode passes through the middle SGW directly with a very low propagation loss, while the launched TM mode is contra-directionally coupled to the two HPWs evenly and finally absorbed by the top metal layers. Simulation results show that the proposed polarizer has a compact coupling length of ∼ 16.48 µ m , and the insertion loss, extinction ratio, and reflection loss are 0.269 dB, 27.7 dB, and − 16.5 d B for the fundamental TE mode at 1550 nm, respectively. In the wavelength range of 1430–1700 nm, the ER is more than 20 dB and the IL is less than 1 dB, extending the operating spectrum to 270 nm.

Contra-directional switching enabled by Si-GST grating.

We present the design, simulation, and experimental demonstration of a Si-GST grating assisted contra-directional coupler for optical switching. The effective refractive index of the GST-loaded silicon waveguide changes significantly when the GST is switched from the amorphous state to the crystalline state, allowing for large tuning of the propagation constant. The two coupled waveguides are designed to satisfy the phase-match condition only at the amorphous state to achieve Bragg reflection at the drop-port. Experimental results show that the device insertion loss is less than 5 dB and the extinction ratio is more than 15 dB with an operation bandwidth of 2.2 nm around the 1576 nm operating wavelength. Due to the nonvolatile property of GST, there is no static power consumption to maintain the two states. It is the first demonstration of a GST-enabled grating coupler that can be switched by phase change material.

Flat-Top CWDM (De)Multiplexers Based on Contra-Directional Couplers With Subwavelength Gratings

A coarse wavelength division multiplexing (CWDM) (de)multiplexer based on the contra-directional couplers (contra-DCs) with subwavelength grating (SWG) waveguide is proposed and demonstrated on the silicon-on-insulator (SOI) platform. By utilizing the SWG waveguides with strong coupling strength, the coupling length can be significantly reduced and the transmission spectra are flat-top. The footprint of the demonstrated device is about $15\times 160\,\,\mu \text{m}^{2}$ . The proposed device is characterized with large bandwidth and low crosstalk at C-band.

Contra-Directional Ridge Waveguide Couplers: Design and Applications

This article presents novel wideband contra-directional couplers based on single conductor ridge waveguide, and their applications. The couplers exhibit wide bandwidths and tight coupling while occupying very small volume due to the compactness of ridge waveguides. The methodology to design the couplers is presented. It is inspired by the even and odd modes used to design TEM coupled lines couplers but implemented with TE modes in this novel approach. The design starts with designing the coupling section in a quadruple-ridge waveguide that supports two independent even and odd TE modes. Proper choice of the cross section allows the designer to achieve the required impedance levels for the required coupling values. An S-band proof of concept prototype of a single coupler was synthesized, fabricated, and tested, demonstrating excellent measured results without any tuning. The couplers can be exploited in versatile applications, such as power sampling, beamforming, or high-power combiners. As an additional proof of concept design, a C-band three-way combiner was also designed and tested, showing the potential of the novel coupler for different applications.

CWDM demultiplexer using anti-reflection, contra-directional couplers based on silicon nitride rib waveguide

We report on the development and fabrication of a 9-channel coarse wavelength-division multiplexing for telecommunication wavelengths (1550 nm) using anti-reflection contra-directional couplers, based on silicon nitride (Si3N4) rib waveguide. The transmitted and reflected spectrum in each channel of the demultiplexer were measured. The average full width at half maximum of the transmitted (reflected) spectra is about 3 nm.

Broadband flat-top SOI add-drop filters using apodized sub-wavelength grating contradirectional couplers.

We report on broadband, flat-top, optical add-drop filters using apodized, sub-wavelength grating contradirectional couplers (SWG CDCs) on a silicon-on-insulator platform. In our device, two asymmetric SWG waveguides, having corrugation-apodized Bragg gratings, are used to couple light contradirectionally between the fundamental and next higher-order transverse electric modes of a two-waveguide system. We demonstrate an apodized, SWG CDC that has a flat-top, drop-port response with a 3dB bandwidth of 32.6nm, a sidelobe suppression ratio of 19dB, and a low excess loss of 0.26dB. We also demonstrate series-cascaded, apodized, SWG CDCs that have square-shaped, drop-port responses with 3dB bandwidths >30 nm, sidelobe suppressions >50 dB, and low excess losses <0.85 dB.