Contra-directional Coupling Research Articles

Spectral tailoring of silicon grating-assisted contra-directional couplers

Spectral tailoring of silicon grating-assisted contra-directional couplers

Tunable silicon integrated quantum light source with on-chip FSR-free filters.

In this work, we design and fabricate a telecom band quantum light source (QLS) on a silicon photonic chip, which integrates a piece of a long silicon waveguide as the nonlinear medium for spontaneous four-wave mixing (SFWM) and five narrow FSR-free bandpass filters based on a grating-assisted contra-directional coupler (GACDC). Two optical filtering functions of the silicon integrated QLS have been demonstrated. First, the QLS supports two tunable outputs of photon pair generations by four GACDC filters. A wavelength tunable range of 6 nm is demonstrated. Second, one GACDC bandpass filter is designed as an on-chip pump filter before the silicon waveguide. The performances of the QLSs with and without the on-chip pump filter are measured and compared. It shows that the on-chip pump filter has the effect to enhance the performance of the QLS by suppressing the Raman noise photons generated when a pump light propagated in optical fibers before it is injected into the chip. These results show that FSR-free filters would play important roles in developing silicon integrated QLSs.

Hardware assurance with silicon photonic physical unclonable functions

In the modern landscape of optical communication networks, ensuring robust security is increasingly critical, particularly for applications requiring seamless integration and minimal attack surfaces. Photonic Physical Unclonable Functions (PUFs) leverage the response from the photonic devices that are prone to inherent physical variations to generate unique and unpredictable signature identifiers which are then utilized by an authentication system for identification or encryption purposes. These photonic PUFs can be cohesively integrated into systems that use optical communication, whereas using electronic PUFs would introduce additional vulnerabilities due to the need for signal-domain conversions between optical and electronic signals. In this paper, we present the design, fabrication, and experimental evaluation of advanced silicon-photonic-based PUFs utilizing Contra-Directional Coupler (CDC) structures. These structures offer a complex design space and are intrinsically sensitive to fabrication-process variations, making them ideal for creating unique and secure responses. We introduce several innovative design enhancements, including randomized corrugation functions, perforated designs, and ring-assisted CDCs, to increase the complexity and unpredictability of the CDC response. Measurement results from the fabricated CDCs demonstrate their capability to achieve an average Hamming distance threshold of over 0.2, effectively distinguishing between legitimate devices and their copies. We rigorously tested these fabricated designs against three different machine-learning-based attack scenarios. The results showed a Hamming distance of over 0.4 with a standard deviation of less than 0.01 at a quantization level of three, using 10,000 samples of challenge-response pairs. These findings underscore the potential of silicon photonic PUFs in enhancing security for optical communication systems of different scales. The integration of such photonic PUFs offers robust and reliable security solutions for applications where traditional electronic methods introduce additional attack surfaces and fail to provide adequate protection.

Prediction through Simulation-Based Corrective Models of Contra-Directional Couplers’ Experimental Results

Lithography variation presents one of the biggest challenges for photonic component optimization, especially for fabless designers. Lithography prediction models are a crucial tool for minimizing the necessary number of fabrication iterations for a device’s optimization. This paper presents one of these models specifically adapted for the contra-directional coupler structure. Through the experimental characterization of devices with a specific range of design parameters, it was possible to observe how the lithography process impacts their performance. A correction model based on effective refractive index variation and its impact on the Bragg condition of the structure was developed to predict the performance variation of a device based on the expected design variation induced by fabrication. The contra-directional couplers fabricated at CORNERSTONE foundry show a tendency to be redshifted as the gap decreases, due to an increase in waveguide width as a result of a diffraction-limited lithography process. Based on these and other findings, it was possible to correlate the design parameters to the posterior fabricated structure and ultimately predict the expected experimental response.

Large-Scale and Multiband Silicon Photonics Wavelength Cross-Connect Switch With FSR-Free Grating-Assisted Contra-Directional Couplers

Large-Scale and Multiband Silicon Photonics Wavelength Cross-Connect Switch With FSR-Free Grating-Assisted Contra-Directional Couplers

Continuously tunable silicon waveguide optical switched delay line based on grating-assisted contradirectional coupler

The integrated optical delay line plays a crucial role in microwave photonic chips. Continuous tunability is a growing trend in filtering and beamforming techniques of microwave photonics. Based on the silicon platform, we present and experimentally demonstrate an integrated continuously optical tunable delay line (OTDL) chip, which contains a 4-bit optical switched delay line (OSDL) and a thermally tunable delay line based on grating-assisted Contradirectional coupler (CDC). The OSDL can achieve stepwise optical delays, while the CDC is introduced to improve delay tuning resolution within one step delay of the OSDL. The combination of the two modules can realize tuning delays from 0 to 160 ps. Additionally, it is easy to increase the maximum delay by cascading more optical switches. The experimental results demonstrate that the proposed OTDL shows outstanding performance and good expansibility.

Topologically Protected Quantum Logic Gates with Valley-Hall Photonic Crystals.

Topological photonics provide a promising way to realize more robust optical devices against some defects and environmental perturbations. Quantum logic gates are fundamental units of quantum computers, which are widely used in future quantum information processing. Thus, constructing robust universal quantum logic gates is an important way forward to practical quantum computing. However, the most important problem to be solved is how to construct the quantum-logic-gate-required 2×2 beam splitter with topological protection. Here, the experimental realization of the topologically protected contradirectional coupler is reported, which can be employed to realize the quantum logic gates, including control-NOT and Hadamard gates, on the silicon photonic platform. These quantum gates not only have high experimental fidelities but also exhibit a certain degree of tolerances against certain types of defects. This work paves the way for the development of practical optical quantum computations and signal processing.

Broadband, Flat-Top, Silicon Photonic Contra-Directional Couplers Assisted by Asymmetric Multimode Photonic Crystals

Broadband, Flat-Top, Silicon Photonic Contra-Directional Couplers Assisted by Asymmetric Multimode Photonic Crystals

Anti-reflection grating-assisted contra-directional coupler with large corrugation width

The grating-assisted contra-directional coupler (contra-DC) is one of the commonly used add-drop filters serving for coarse wavelength-division-multiplexing (CWDM) systems due to its flat-top and wide-band response. However, the free-spectral-range (FSR) of contra-DC is limited by the back reflection to the input port caused by the intra-waveguide coupling. In this paper we propose an anti-reflection grating-assisted contra-DC by simply increasing the sidewall corrugation width. The intra-waveguide coupling coefficient can be decreased to 0 when the corrugation width is increased to a certain value. The simulation result shows that the transmission at drop port is −0.42 dB, while the back reflection to the input port is only −22.0 dB. The 4-channel cascaded contra-DCs with wavelength range of ∼60 nm are also simulated, all the channels are not affected by the intra-waveguide coupling. We believe the proposed contra-DC with large corrugation width can be applied to the CWDM system requiring a broad spectrum range.

Broadband, compact and reflection-less silicon polarizer and polarization beam splitter using chirped anti-symmetric multimode nanobeams.

We present chirped anti-symmetric multimode nanobeams (CAMNs) based on silicon-on-insulator platforms, and describe their applications as broadband, compact, reflection-less, and fabrication-tolerant TM-pass polarizers and polarization beam splitters (PBSs). The anti-symmetric structural perturbations of a CAMN ensure that only contradirectional coupling between symmetric and anti-symmetric modes is possible, which can be exploited to block the unwanted back reflection of the device. The new possibility of introducing a large chirp on an ultra-short nanobeam-based device to overcome the operation bandwidth limitation due to the coupling coefficient saturation effect is also shown. The simulation results show that an ultra-compact CAMN with a length of ∼4.68 um can be used to develop a TM-pass polarizer or a PBS with an ultra-broad 20 dB extinction ratio (ER) bandwidth of >300 nm and an average insertion loss of <1.3 dB. The CAMN-based polarizer and PBS were fabricated and experimentally characterized in a wavelength range from 1507 to 1575 nm. The measured ERs were >20 dB over the entire tested wavelength range and the average insertion losses were <0.5 dB for both devices. The mean reflection suppression ratio of the polarizer was ∼26.4 dB. Large fabrication tolerances of ±60 nm in the waveguide widths of the devices were also demonstrated.

Nonlinear phase shifts induced by pumping spin waves in magnonic crystals

A nonlinear phase shift of low-power spin waves (SWs) induced by a high-power pumping SW excited both inside and outside the magnonic band-gaps of a magnonic crystal has been studied. The magnonic crystal with spatially periodic thickness is fabricated from an yttrium iron garnet film by chemical etching. The results show that the phase shift of the low-power SWs can be effectively controlled by variation of power level of the pumping SW. This induced nonlinear phase shift is weakened if the pump frequency lies in the magnonic bandgap. The data obtained are well explained by contradirectional coupling of the high-power forward and reflected spin waves. A theoretical model for this effect is presented. Our findings are important for the further progress in SW computing.

Simultaneous Generation of Two Negatively Correlated Linearly Frequency-Modulated Pulses Based on an Integrated Anti-Reflection Spectral Shaper

Grating-based spectral shaping (SS) combined with wavelength-to-time (WTT) mapping has proved to be an essential approach to the photonic generation of linearly frequency-modulated (LFM) pulses due to its potential to yield the large time-bandwidth product (TBWP) and to be fully integrated. One of the most challenging issues of chip-level integration is the grating-induced reflection which may significantly impact the laser source. Since the on-chip nonreciprocal devices such as optical isolators and circulators remain difficult to realize, a silicon-integrated anti-reflection spectral shaper based on a Mach–Zehnder interferometer (MZI) comprising two linearly chirped grating-assisted contra-directional couplers (LCGA-CDCs) has been proposed and demonstrated. Thanks to the isolation between the input and output ports of the contra-directional couplers, the reported shaper presents an effective suppression of the reflecting power returned to its input port with a measured rejection ratio of around −10 dB covering the entire C-band. To predict the instantaneous frequency responses of the generated microwave pulses, an equivalent asymmetric MZI configuration with wavelength-dependent length difference has also been introduced to explain the mechanism of SS. Two negatively correlated LFM waveforms with up-chirped profiles and large TBWPs of 45.60 and 53.52 are simultaneously generated using a fabricated sample. Our work paves the way for an all-optical integration of the photonic-assisted isolator-free LFM pulse generation, presenting great potential for multiple application scenarios in pulsed radar systems.

Straight and curved distributed Bragg reflector design for compact WDM filters.

Grating-assisted contra-directional couplers (CDCs) wavelength selective filters for wavelength division multiplexing (WDM) are designed and experimentally demonstrated. Two configuration setups are designed; a straight-distributed Bragg reflector (SDBR) and curved distributed Bragg reflector (CDBR). The devices are fabricated on a monolithic silicon photonics platform in a GlobalFoundries CMOS foundry. The sidelobe strength of the transmission spectrum is suppressed by controlling the energy exchange between the asymmetric waveguides of the CDC using grating and spacing apodization. The experimental characterization demonstrates a flat-top and low insertion loss (0.43 dB) spectrally stable performance (<0.7 nm spectral shift) across several different wafers. The devices have a compact footprint of only 130µm2/Ch (SDBR) and 3700µm2/Ch (CDBR).

Contra-Directional Coupling Phase-Shifted Grating with High Q-Factor and Ultra-Narrow Bandwidth

Contra-Directional Coupling Phase-Shifted Grating with High Q-Factor and Ultra-Narrow Bandwidth

Compact High-Directivity Contra-Directional Coupler

This paper presents a novel design of a compact contra-directional coupler with high directivity for high-power monitoring in high frequency. Microstrip parallel coupled lines are widely used for directional couplers; however, they show poor directivity inherently. Their directivity has been improved by many works. However, the suggested approaches often result in other limitations, such as a weak structure for high-power monitoring, or a larger size to be integrated with other circuits. The design approach proposed in this study starts from a ring-type four-port network to avoid weak components that are vulnerable to high power, and uses a 60° electrical length of coupled line for a compact size. The design equations for the initial dimensions are derived from the ring-type four-port network model. The weak coupling of the 20 dB coupler was designed and measured. The measurement shows 20 dB directivity from 12.8 GHz to 14.8 GHz, covering the Ku-band satellite uplink communication and peak directivity of about 45 dB. The coupler’s active area is 4 mm by 5.5 mm; this is a compact size compared with other works.

Angled Flip-Chip Integration of VCSELs on Silicon Photonic Integrated Circuits

An investigation of angled flip-chip integration of a singlemode 850 nm vertical-cavity surface-emitting laser (VCSEL) on a silicon nitride photonic integrated circuit (PIC) is presented. Using numerical FDTD simulations, we consider the conditions under which the VCSEL can be integrated at an angle over a grating coupler with high coupling efficiency and low optical feedback. With both coupling efficiency and feedback decreasing with increasing angle, there is a trade-off. With co-directional coupling, first-order diffraction loss sets in at a critical angle, which further reduces the coupling efficiency. No such critical angle exists for contra-directional coupling. We also experimentally demonstrate angled flip-chip integration of GaAs-based 850 nm single transverse and polarization mode VCSELs over grating couplers on a silicon-nitride PIC. At the output grating coupler, light is either collected by an optical fiber or converted to a photocurrent using a flip-chip integrated GaAs-based photodetector. The latter forms an on-PIC optical link. We measured an insertion loss of 21.9, 17.6 and 20.1 dB with a singlemode fiber, multimode fiber and photodetector over the output grating coupler, respectively.

Design of a grating-assisted silicon hybrid mode-, polarization-, and wavelength-division (De)multiplexer for on-chip optical interconnects.

This paper presents a 12-channel hybrid mode-, polarization-, and wavelength-division (de)multiplexer. The proposed device is designed on a silicon-on-insulator platform. The device structure constitutes three coupling sections with a bus waveguide at the middle and two single-mode waveguides on each side. The periodic grating structures are present in between the waveguides for contra-directional coupling of the three TE modes and three TM modes of the bus waveguide to respective fundamental modes of the single-mode waveguides at 1550 and 1560 nm. The device is simulated using the 3D finite-difference time-domain technique, and the resulting insertion loss, cross talk, and return loss are <1.56dB, <-21.74dB, and >12.22dB, respectively. Moreover, the period of the grating structures is varied to perform the fabrication tolerance study.

Side-lobe reduction by cascading Bragg grating filters on a Si-photonic chip.

Bragg-grating based cavities and coupler designs present opportunities for flexible allocation of bandwidth and spectrum in silicon photonic devices. Integrated silicon photonic devices are moving toward mainstream, mass adoption, leading to the need for compact Bragg grating based designs. In this work we present a design and experimental validation of a cascaded contra-directional Bragg-grating coupler with a measured main lobe to side-lobe contrast of 12.93 dB. This level of performance is achieved in a more compact size as compared to conventional apodized gratings, and a similar design philosophy can be used to improve side-lobe reduction in grating-based mirror design for on-chip lasers and other cavity-based designs as well.

Anti-Reflection Grating-Assisted Contra-Directional Coupler with Large Corrugation Width

Anti-Reflection Grating-Assisted Contra-Directional Coupler with Large Corrugation Width

Ultra-Broadband Tunable Filter Based on Cascaded Chirped Subwavelength Grating Assisted Contra-Directional Coupler

Ultra-Broadband Tunable Filter Based on Cascaded Chirped Subwavelength Grating Assisted Contra-Directional Coupler