Silicon band-rejection and band-pass filter based on asymmetric Bragg sidewall gratings in a multimode waveguide

Dual-polarization pump rejection filter in silicon nitride technology

On-chip pump rejection filters are key building blocks in a variety of applications exploiting nonlinear phenomena, including Raman spectroscopy and photon-pair generation. Ultrahigh rejection has been achieved in the silicon technology by non-coherent cascading of modal-engineered Bragg filters. However, this concept cannot be directly applied to silicon nitride waveguides as the comparatively lower index contrast hampers the suppression of residual light propagating in the orthogonal polarization, limiting the achievable rejection. Here, we propose and demonstrate a novel, to the best of our knowledge, strategy to overcome this limitation based on non-coherent cascading of the modal- and polarization-engineered Bragg filters. Based on this concept, we experimentally demonstrate a rejection exceeding 60 dB for both polarizations, with a bandwidth of 4.4 nm. This is the largest rejection reported for silicon nitride Bragg gratings supporting both polarizations.

Sidewall Corrugation-Modulated Phase-Apodized Silicon Grating Filter

In this work, phase-apodized silicon grating filters with varying sidewall corrugation width and location were investigated, while the resonance wavelength, extinction ratio, and rejection bandwidth were tuned flexibly. The grating filters with a waveguide width of 500 nm and grating period of 400 nm were fabricated and characterized as a proof of concept. The resonance wavelength of the device can be shifted by 4.54 nm by varying the sidewall corrugation width from 150 to 250 nm. The corresponding rejection bandwidth can be changed from 1.19 to 2.03 nm by applying a sidewall corrugation location offset from 50 to 200 nm. The experimental performances coincide well with the simulation results. The presented sidewall corrugation-modulated apodized grating can be expected to have great application prospects for optical communications and semiconductor lasers.

Local-field engineering in slot waveguide for fabricating on-chip Bragg grating filters with high reflectivity across a flat broadband

On-chip Bragg gratings with high reflectivities have been found to have widespread applications in filters, resonators, and semiconductor lasers. However, achieving strong Bragg reflections with flat response across a broad bandwidth on the popular 220 nm silicon-on-insulator (SOI) platform still remains a challenge. In this paper, such a high performance device is proposed and fabricated, which is based on a slot waveguide with gratings etched on the inner sidewalls of the slot. By manipulating the local field in the slot region using a chirped and tapered grating-based mode transition, the device achieves a flat response with ultra-high reflection and low transmission for the TE mode across a broad operating bandwidth. Leveraging the ultra-high birefringence of the SOI waveguide, the device functions both as a TE slot waveguide reflector and a TM pass polarizer. Simulation results demonstrate that the device exhibits an ultra-high rejection of more than 50 dB and a reflectivity exceeding 0.99 for the TE mode across a 91 nm wavelength range, while maintaining a high transmittance of larger than 0.98 for the TM mode. Experimental results validate that the device performance is consistent with the simulation results. A fabricated device based on such a gratings exhibits a low insertion loss (<0.8 dB) and high polarization extinction ratio (>30 dB) over 100 nm bandwidth (1484 nm-1584 nm), demonstrating that the performance of the present design is competitive with that of the state-of-the-art SOI Bragg gratings.

All-fibre phase filters with 1-GHz resolution for high-speed passive optical logic processing

Photonic-based implementation of advanced computing tasks is a potential alternative to mitigate the bandwidth limitations of electronics. Despite the inherent advantage of a large bandwidth, photonic systems are generally bulky and power-hungry. In this respect, all-pass spectral phase filters enable simultaneous ultrahigh speed operation and minimal power consumption for a wide range of signal processing functionalities. Yet, phase filters offering GHz to sub-GHz frequency resolution in practical, integrated platforms have remained elusive. We report a fibre Bragg grating-based phase filter with a record frequency resolution of 1 GHz, at least 10× improvement compared to a conventional optical waveshaper. The all-fibre phase filter is employed to experimentally realize high-speed fully passive NOT and XNOR logic operations. We demonstrate inversion of a 45-Gbps 127-bit random sequence with an energy consumption of ~34 fJ/bit, and XNOR logic at a bit rate of 10.25 Gbps consuming ~425 fJ/bit. The scalable implementation of phase filters provides a promising path towards widespread deployment of compact, low-energy-consuming signal processors.

Gain Enhancement of the Optical Waveguide Amplifier Based on NaYF4/NaLuF4: Yb, Er NPs-PMMA Integrated with a Si3N4 Slot

A Si₃N₄ slot waveguide has the ability to confine light tightly in the slot, shows weak absorption of 980 nm pump light, and has lower transmission loss compared to a Si slot. Hence, the optical waveguide amplifier based on Er³⁺ and Yb³⁺codoped was proposed to be integrated with a Si₃N₄ slot to increase the gain. The core-shell NaYF₄/NaLuF₄: 20%Yb³⁺, 2%Er³⁺ nanocrystals-polymeric methyl methacrylate covalent linking nanocomposites were synthesized and filled into the slot as gain medium. The concentrations of Er³⁺ and Yb³⁺ were increased compared with traditional physical doping methods. High-efficiency emission at 1.53 μm was achieved under 980 nm laser excitation. The slot waveguide was accurately designed using the semivector finite difference method in combination with the maximum confinement factors and the minimum effective mode area. The optimum width of the slot was 200 nm, and the optimum height and width of the silicon strip waveguide were 400 nm and 400 nm, respectively. The six-level spectroscopic model was presented, and the gain characteristics of the slot waveguide amplifier were numerically simulated. A net gain of 8.2 dB was achieved, which provided new ideas and directions for waveguide amplifiers.

Advancement in Silicon Integrated Photonics Technologies for Sensing Applications in Near-Infrared and Mid-Infrared Region: A Review

Exploration and implementation of silicon (Si) photonics has surged in recent years since both photonic component performance and photonic integration complexity have considerably improved. It supports a wide range of datacom and telecom applications, as well as sensors, including light detection and ranging, gyroscopes, biosensors, and spectrometers. The advantages of low-loss Si WGs with compact size and excellent uniformity, resulting from the high quality and maturity of the Si complementary metal oxide semiconductor (CMOS) environment, are major drivers for using Si in photonics. Moreover, it has a high refractive index and a reasonably large mid-infrared (MIR) transparency window, up to roughly 7 μm wavelength, making it beneficial as a passive mid-IR optical material. Several gases and compounds with high absorption properties in the MIR spectral region are of prodigious curiosity for industrial, medicinal, and environmental applications. In comparison to current bulky systems, the implementation of Si photonics devices in this wavelength range might allow inexpensive and small optical sensing devices with greater sensitivity (S), power usage, and mobility. In this review, recent advances in Si integrated photonic sensors working in both near-infrared (NIR) and MIR wavelength ranges are discussed. We believe that this paper will be valuable for the scientific community working on Si photonic sensing devices.

SOI Waveguide Bragg Grating Photonic Sensor for Human Body Temperature Measurement Based on Photonic Integrated Interrogator

A waveguide Bragg grating (WBG) provides a flexible way for measurement, and it could even be used to measure body temperature like e-skin. We designed and compared three structures of WBG with the grating period, etching depth, and duty cycle. The two-sided WBG was fabricated. An experimental platform based on photonic integrated interrogator was set up and the experiment on the two-sided WBG was performed. Results show that the two-sided WBG can be used to measure temperature changes over the range of 35–42 °C, with a temperature measurement error of 0.1 °C. This approach has the potential to facilitate application of such a silicon-on-insulator (SOI) WBG photonic sensor to wearable technology and realize the measurement of human temperature.

Silicon subwavelength modal Bragg grating filters with narrow bandwidth and high optical rejection

Waveguide Bragg grating filters with narrow bandwidths and high optical rejections are key functions for several advanced silicon photonics circuits. Here, we propose and demonstrate a new, to the best of our knowledge, Bragg grating geometry that provides a narrowband and high rejection response. It combines the advantages of subwavelength and modal engineering. As a proof-of-concept demonstration, we implement the proposed Bragg filters in 220-nm-thick Si technology with a single etch step. We experimentally show flexible control of the filter selectivity, with measured null-to-null bandwidths below 2 nm, and strength of 60 dB rejection with a null-to-null bandwidth of 1.8 nm.

Substrate integrated Bragg waveguide: an octave-bandwidth single-mode hybrid transmission line for millimeter-wave applications

We demonstrate an air-core single-mode hollow hybrid waveguide that uses Bragg reflector structures in place of the vertical metal walls of the standard rectangular waveguide or via holes of the so-called substrate integrated waveguide. The high-order modes in the waveguide are substantially suppressed by a modal-filtering effect, making the waveguide operate in the fundamental mode over more than one octave. Numerical simulations show that the propagation loss of the proposed waveguide can be lower than that of classic hollow metallic rectangular waveguides at terahertz frequencies, benefiting from a significant reduction in Ohmic loss. To facilitate fabrication and characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated, which verifies the properties and advantages of the proposed waveguide. A zero group-velocity dispersion point is observed at near the middle of the operating band, which is ideal for reducing signal distortion. This work offers a step towards a hybrid transmission-line medium that can be used in a variety of functional components for multilayer integration and broadband applications.

Ultra compact Bragg grating devices with broadband selectivity

Current silicon waveguide Bragg gratings typically introduce perturbation to the optical mode in the form of modulation of the waveguide width or cladding. However, since such a perturbation approach is limited to weak perturbations to avoid intolerable scattering loss and higher-order modal coupling, it is difficult to produce ultra-wide stopbands. In this Letter, we report an ultra-compact Bragg grating device with strong perturbations by etching nanoholes in the waveguide core to enable an ultra-large stopband with apodization achieved by proper location of the nanoholes. With this approach, a 15 µm long device can generate a stopband as wide as 110 nm that covers the entire ${\rm C} + {\rm L}$C+L band with a 40 dB extinction ratio and over a 10 dB sidelobe suppression ratio (SSR). Similar structures can be further optimized to achieve higher SSR of $ \gt {17}\;{\rm dB}$>17dB for a stopband of about 80 nm.

On-chip optical narrowband reflector based on anti-symmetric Bragg grating

We propose an on-chip optical narrowband reflector (NBR) based on two cascaded Bragg gratings (BGs). A π phase shifted anti-symmetric Bragg grating (π-PS-ASBG) and a rear uniform Bragg grating (UBG), are in-line connected. The π-PS-ASBG provides a hybrid mode resonance between the even- and odd TE (TE₀ and TE₁) modes, while the UBG is used as a rear reflector to reflect the TE₀ mode that transmitted from the π-PS-ASBG. Different from traditional UBG, the reflection bandwidth decreases when the coupling coefficient increases. The calculated 3-dB bandwidth is 0.16 nm when the whole grating length is 400 µm. The proposed NBR can be applied in the cases requiring narrow reflection such as narrow linewidth semiconductor lasers.

On-chip mode converter based on two cascaded Bragg gratings

We propose an on-chip mode converter via two cascaded Bragg reflection processes. A forward conversion between two guided modes can be achieved with the aid of an additional mode. The proposed structure is theoretically studied and simulated via the rigorous three-dimensional finite-difference time-domain (3D-FDTD) method. The bandwidth and central wavelength of the proposed mode converter can be adjusted according to our theoretical analysis and simulation results. By applying the similar design approaches as fiber Bragg gratings, conversion spectra with different shapes can be obtained. As an example, several mode converters with bandpass and sidelobe-reduced spectra are designed. We also investigate and verify the mode conversion by experiment. Therefore, the proposed method may pave a new path for the mode converters with desired conversion spectra.

High-Q antisymmetric multimode nanobeam photonic crystal cavities in silicon waveguides

Antisymmetric multimode nanobeam photonic crystal cavities (AM-NPCs) are proposed and demonstrated in this paper. Due to transverse symmetry-breaking of the antisymmetric multimode periodic waveguide, anti-crossing of the fundamental mode and 1st-order mode is realized and confirmed by band structure calculation. Two-mode filtering and reflection-free cavity filters based on this characteristic are demonstrated. Experimental results on silicon-on-insulator platform shows that broadband (> 100 nm) reflection suppression (< -10 dB) and high-Q (7 × 10⁴) AM-NPCs can be achieved using existed design methodology and fabrication facility. We also explain resonance splitting of the measured transmission spectra and find resonance-enhanced mode-conversion phenomena in the AM-NPCs.

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters

Bragg filters stand as key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we show that the combination of single-side corrugation asymmetry and subwavelength engineering provides a narrowband response with large corrugations, overcoming minimum feature size constraints of conventional Si Bragg filters. We comprehensively study the impact of the corrugation asymmetry in conventional and subwavelength single-etched SOI Bragg filters, showing their potential for bandwidth reduction. Finally, we experimentally demonstrate novel subwavelength geometry based on shifted corrugation teeth, achieving null-to-null bandwidths and rejections of 0.8 nm and 40 dB for the symmetric configuration and 0.6 nm and 15 dB for the asymmetric case.

Ultra-broadband and compact graphene-on-silicon integrated waveguide mode filters

Increasing bandwidth demands in optical communications necessitates the introduction of mode-division multiplexing (MDM) on top of the existing wavelength-division multiplexing (WDM) systems. Simultaneous management of both multiplexing systems will be a complex task, and there is the possibility of signal degradation through modal crosstalk. Here, we propose graphene-on-silicon (GOS) integrated waveguide mode filters to suppress the propagation of spurious waveguide modes at the telecommunications wavelength. Graphene’s high fabrication tolerance potentially enables surgical tailoring and deployment at targeted segments on the waveguide to absorb the undesired TE₀ or TE₁ modes. The proposed GOS waveguide mode filters can potentially improve the performance and reduce the device footprint of MDM systems.

Silicon optical filter with transmission peaks in wide stopband obtained by anti-symmetric photonic crystal with defect in multimode waveguides

We experimentally demonstrate and characterize a wide optical stopband filter with transmission peaks constituted by two parallel rows of an anti-symmetric one-dimensional (1D) photonic crystal with defect in a multimode waveguide. The working principle is based on mode coupling for the wavelengths that meet the phase matching condition as the fundamental mode is coupled to a higher-order mode and then filtered out with linear tapers as they are reflected back. The defect in the photonic crystal works as a Fabry-Perot like cavity that allows for localized states in the stopband to quantify the effects of the design parameters and we show a Fabry-Perot filter that extends through the 200 nm stopband. A compact 7 μm² device is demonstrated with standard fabrication techniques in Silicon-on-Insulator (SOI), and compared to Fabry-Perot filters in single mode waveguides, we obtain a larger bandwidth and lower back reflections. Applications include refractive index sensors, optical communications and on-chip spectroscopy.

Broadband tunable filter based on the loop of multimode Bragg grating

A broadband tunable silicon filter has been demonstrated on silicon-on-insulator platform. The device is based on the loop of multimode anti-symmetric waveguide Bragg grating. A wide bandwidth tunability about 1.455 THz (0.117-1.572 THz) is achieved. The device, functions like a ring, can realize the bandwidth tunable of the drop port and the through port. And, its feature has simultaneous wavelength tuning and no free space ranges limitation. A high out-of-band contrast of 30 dB is achieved with a bandwidth of 1.572 THz (Δλ = 13 nm). The out-of-band contrast is 18 dB at the minimum bandwidth 0.117 THz (Δλ = 1.0 nm).

Silicon lateral-apodized add-drop filter for on-chip optical interconnection

A lateral-apodized add-drop filter is demonstrated in a multimode asymmetric waveguide Bragg grating. This design utilizes two individual superposed gratings with the same sidewall corrugation depth. The strong side lobes of the grating filter are efficiently suppressed by mapping the target apodization profile into lateral shifts between the periods of the two gratings. Compared with other apodized technology, this device is easier to be realized. Experimental results show that the side-lobes suppression ratio can reach 18.5 dB, and a bandwidth of 9.5 nm is achieved by a large corrugation width of 150 nm. The insertion loss at the drop port is only 0.8 dB, and the extinction ratio is up to 24 dB at the through port.

Planar waveguide moiré grating

We propose a new kind of planar waveguide Bragg grating structure, i.e., planar waveguide moiré grating (MG), which is formed by two transverse adjacent gratings with slightly different Bragg wavelengths. It is found that this kind of structure shows the same light properties as the conventional MG that is realized by superimposing two Bragg gratings. Because the proposed MG structure is a planar pattern, the fabrication becomes much easier if applying a semiconductor microfabrication process, which is very beneficial for its applications in photonic integrated devices. Similar to the well-known Vernier effect, the coupling coefficient distribution can be easily adjusted by the alignment of the two adjacent gratings. Consequently, some special grating profiles can be achieved, such as perfect apodization with two sides of the coupling coefficient approaching zero. One important potential application of these specific features is the distributed feedback (DFB) semiconductor laser for improved properties, such as reduced spatial-hole burning and more power extraction. Some design examples are also given in this paper.

Broad bandwidth and large fabrication tolerance polarization beam splitter based on multimode anti-symmetric Bragg sidewall gratings

A novel polarization beam splitter based on an anti-symmetric sidewall Bragg grating in a multimode silicon-on-insulator strip waveguide is demonstrated. Anti-symmetric spatially periodic refractive-index perturbations are designed for strong coupling between the fundamental (TE₀) and the first-order transverse electric modes (TE₁), while not for transfer magnetic modes. An adiabatic coupler is cascaded at the input-port, so as to drop the TE₁ reflection. The Bragg grating has a compact length of ∼20  μm (55 periods). The polarization isolations of the through- and drop-ports at the wavelength of 1557 nm are 34 and 31 dB, respectively. A broad bandwidth of 64 nm and a large fabrication tolerance of 80 nm for polarization isolation over 20 dB are also achieved.

Silicon wire waveguide TE0/TE1 mode conversion Bragg grating with resonant cavity section

Silicon wire waveguide TE₀/TE₁ mode conversion Bragg grating can be used in wavelength add/drop and polarization rotation Bragg diffraction. The device can implement many filtering functionalities required in wavelength division multiplexing optical communications. In this paper we describe TE₀/TE₁ mode conversion Bragg grating device incorporating resonant cavity section to obtain narrow transmission wavelength peak. Theoretical calculation agreed with measured wavelength response.

Hybrid multimode resonators based on grating-assisted counter-directional couplers

Research thrusts in silicon photonics are developing control operations using higher order waveguide modes for next generation high-bandwidth communication systems. In this context, devices allowing optical processing of multiple waveguide modes can reduce architecture complexity and enable flexible on-chip networks. We propose and demonstrate a hybrid resonator dually resonant at the 1st and 2nd order modes of a silicon waveguide. We observe 8 dB extinction ratio and modal conversion range of 20 nm for the 1st order quasi-TE mode input.

Optical pump-rejection filter based on silicon sub-wavelength engineered photonic structures

The high index contrast of the silicon-on-insulator (SOI) platform allows the realization of ultra-compact photonic circuits. However, this high contrast hinders the implementation of narrow-band Bragg filters. These typically require corrugation widths of a few nanometers or double-etch geometries, hampering device fabrication. Here we report, for the first time, to the best of our knowledge, on the realization of SOI Bragg filters based on sub-wavelength index engineering in a differential corrugation width configuration. The proposed double periodicity structure allows narrow-band rejection with a single etch step and relaxed width constraints. Based on this concept, we experimentally demonstrate a single-etch, 220 nm thick, Si Bragg filter featuring a corrugation width of 150 nm, a rejection bandwidth of 1.1 nm, and an extinction ratio exceeding 40 dB. This represents a 10-fold width increase, compared to conventional single-periodicity, single-etch counterparts with similar bandwidths.

Add-drop filter based on TiO2 coated shifted Bragg grating

We present a titanium dioxide coated shifted Bragg grating in a silicon-on-insulator platform enabling optical add-drop functionality. The device works on the basis of mode conversion due to shifted sidewall structure followed by mode splitting based on an asymmetric Y-coupler. We experimentally demonstrate the working principle of the device. A reflection bandwidth of 2.2 nm with 14 dB extinction ratio is obtained with a 300 μm long shifted Bragg grating. The performance of the device is also compared without the titanium dioxide coating. A scope of spectral tunability with titanium dioxide re-coating (0.8 nm per 1 nm re-coating) by atomic layer deposition is experimentally verified.