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

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.

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).

Polarization-insensitive antisymmetric multimode waveguide Bragg grating filter based on an SiN-Si dual-layer stack

A polarization-insensitive multimode antisymmetric waveguide Bragg grating (MASWBG) filter based on an SiN-Si dual-layer stack is demonstrated. Carefully optimized grating corrugations patterned on the sidewall of a silicon waveguide and SiN overlay are used to perturbate TE and TM modes, respectively. Furthermore, the lateral-shift apodization technique is utilized to improve the sidelobe suppression ratio (SLSR). A good overlap between the passbands measured in TE and TM polarization states is obtained. Insertion losses, SLSRs, and 3-dB bandwidths of measured passbands in TE/TM polarizations are 1/1.72 dB, 18.5/19.1 dB, and 5.1/3.5 nm, respectively.

Tunable narrow band add-drop filter design based on apodized long period waveguide grating assisted co-directional coupler

Tunable add/drop filter based optical interconnects are an integral part of data centers as well as optical communications. Although add/drop filters based on ring resonators and waveguide Bragg gratings are well developed, long period waveguide grating (LPWG) based add/drop filters have little been investigated so far. In this article, we propose an apodized LPWG assisted co-directional coupler for narrow band add/drop filtering by combining silicon (Si) waveguide with titanium dioxide (TiO₂) waveguide geometry. The proposed structure has been analyzed by combining the finite element method (FEM) and transfer matrix method (TMM), showing a good side lobe suppression ratio (SLSR) equal to 25.7 dB and an insertion loss of 0.6 dB. Owing to the high group index difference of Si and TiO₂ waveguides, a narrow band response of 1.4 nm has been achieved with 800µm long LPWG. The opposite thermo-optic coefficients of Si and TiO₂ ensures a good thermal tunability of the central wavelength. Considering a thin metallic heater of titanium nitride (TiN) the thermal tuning efficiency is found to be 0.07 nm/mW. Further, two LPWGs have been cascaded to realize a tunable dual channel filter with a minimum channel spacing of 185 GHz and a channel crosstalk better than 20 dB, showing its potential application towards dense wavelength division multiplexing.

C-band four-channel CWDM (de-)multiplexers on a thin film lithium niobate-silicon rich nitride hybrid platform

A four-channel coarse wavelength division multiplexing (CWDM) (de)multiplexer on a thin film lithium niobate-silicon rich nitride hybrid platform has been designed, fabricated, and experimentally measured. Enabled by cascaded multimode waveguide Bragg gratings, the (de)multiplexer has a box-like spectral response, wide 1-dB bandwidth (10 nm), low excess loss (<1.08dB), and low channel cross talk (<-18dB). The central wavelengths of the (de-)multiplexer are 1531/1551/1571/1591 nm, which align to the wavelength grids stipulated by the standard ITU-T G.694.2.

Polarization Splitter-Rotator Based on Multimode Waveguide Grating

We demonstrate a polarization splitter rotator (PSR) based on multimode waveguide grating (MWG) on a silicon-on-insulator (SOI) platform. Bloch mode hybridization in mini-stopband is exploited to achieve high polarization conversion efficiency. The fabricated device yields a high extinction ratio of > 53 dB and > 31 dB, low crosstalk of < −26.4 dB and < −40 dB for the injected TE0 and TM0 mode, with average insertion loss of 1.2 dB and 1.5 dB in the wavelength regime 1552 nm–1562 nm. Such a device shows great design flexibility and an easy fabrication process, serving as a good candidate in integrated polarization diversity circuits, especially for applications requiring spectra manipulation. Additionally, the polarization conversion approach provides opportunities to develop novel polarization management devices.

Hitless and gridless reconfigurable optical add drop (de)multiplexer based on looped waveguide sidewall Bragg gratings on silicon

Reconfigurable optical add-drop filters in future intelligent and software controllable wavelength division multiplexing networks should support hitless wavelength switching and gridless bandwidth tuning. The hitless switching implies that the central wavelength of one channel can be shifted without disturbing data transmissions of other channels, while the gridless tuning means that the filter bandwidth can be adjusted continuously. Despite a lot of efforts, very few integrated optical filters simultaneously support the hitless switching of central wavelength and the gridless tuning of bandwidth. In this work, we demonstrate a hitless add-drop filter with gridless bandwidth tunability on the silicon-on-insulator (SOI) platform. The filter comprises the two identical multimode anti-symmetric waveguide Bragg gratings (MASWBG) which are connected to a loop. The phase apodization technique is utilized to weaken the intrinsic sidelobe interference of grating-based devices. By sequentially manipulating central wavelengths of the two MASWBGs with the thermo-optical effect, we can reconfigure the spectral response of the filter gridlessly and hitlessly. Specifically, the central wavelength of the device is shifted by 14.5 nm, while its 3 dB bandwidth is tuned from 0.2 nm to 2.4 nm. The dropping loss and the sidelobe suppression ratio (SLSR) are dependent on the bandwidth selected. Measured variation ranges of dropping loss and SLSR are from -1.2 dB to -2.5 dB and from 12.8 dB to 21.4 dB, respectively. The hitless wavelength switching is verified by a data transmission measurement at a bit rate of 25 Gbps.

Silicon-based wavelength division multiplexer using asymmetric grating-assisted couplers

A wavelength division multiplexer (WDM) based on asymmetric grating-assisted couplers is proposed, which can flexibly adjust the bandwidth by changing the corrugation width of the grating. The simulation results show that, compared with asymmetric uniform grating-assisted couplers, asymmetric unilateral amplitude apodization grating-assisted couplers and asymmetric bilateral amplitude apodization grating-assisted couplers can effectively suppress the sidelobes. The experimental results show that the insertion loss of each wavelength channel is between 0.23dB and 0.58dB, and the sidelobe suppress ratio of both unilateral amplitude apodization grating-assisted couplers and bilateral amplitude apodization grating-assisted couplers is larger than 10dB, which reduces channel crosstalk and proves the feasibility of the wavelength division multiplexers.

Silicon-based polarization-insensitive optical filter with dual-gratings

A silicon-based polarization-insensitive optical filter is proposed and demonstrated. For the present on-chip polarization-insensitive optical filter, there is a dual-polarization mode (de)multiplexer, a TE-type multimode waveguide grating (MWG) with triangular corrugations and a TM-type MWG with rectangular corrugations. Here the triangular corrugations are introduced to reduce the undesired reflection and suppress the Fabry-Parot resonance. Furthermore, lateral-shift apodization is introduced for both two types of MWGs to suppress the sidelobes. For the fabricated device, the measured 3 dB-bandwidth is as large as ∼11 nm and the excess loss is ∼1.5 dB for both polarizations, while the sidelobe suppression ratios are 23 dB and 17 dB for TE and TM polarizations, respectively.

Silicon photonic add-drop filter based on a grating-assisted co-directionally coupled vertical hybrid structure

Add-drop filters (ADFs) are an essential component in optical interconnection using dense wavelength-division multiplexing. Silicon photonic ADFs based on contra-directional coupling have been well developed, but those based on grating-assisted co-directional coupling (GACC) have never been studied. This paper reports an ADF based on GACC in a vertical hybrid structure (VHS). which consists of two width-modulated silicon strip waveguides with a large lateral gap and a wide silicon nitride strip waveguide above them. The VHS makes it possible for the ADF to have a narrow 3-dB bandwidth as well as a short grating length. An efficient analysis method for design is explained, and the ADF is designed. Theoretical investigation of the ADF demonstrates that the ADF has a 3-dB bandwidth of 1.16 nm and a grating length of 1.13 mm, which are similar to those of ADFs based on contra-directional coupling. As an application, the ADF is used for a nonvolatile switchable ADF by adding an optical phase change material strip above the silicon nitride waveguide. The nonvolatile switchable ADF is shown to have an extinction ratio larger than 30 dB. The investigated ADF requires neither waveguides in close proximity nor grating teeth with dimensions close to the resolution of deep UV lithography. In this regard, it has the advantage of ease of fabrication as compared to ADFs based on contra-directional coupling. Therefore, the ADF is expected to play a key role in optical interconnection using dense wavelength-division multiplexing, prevailing over ADFs based on contra-directional coupling.

Characterization and compensation of apodization phase noise in silicon integrated Bragg gratings

Precise and reliable apodization of silicon integrated Bragg gratings (IBGs) is the key to realizing their spectral tailoring for many optical applications such as optical signal processing and wavelength-division multiplexing systems. However, apodization in a silicon IBG that is typically realized by modifying the physical waveguide grating structure can also introduce unwanted grating phase variations that can affect the grating response. In this paper, we present a model to characterize apodized silicon IBGs which can take such apodization phase noise (APN) into account, based on direct synthesis of the physical grating structure. The model is used to characterize a set of different silicon IBGs apodized by lateral misalignment (ΔL) and duty-cycle (DC) modulations and designed with different responses, and the results show that the APN can greatly distort the complex responses of the gratings. Then, we develop a methodology to compensate the APN and thus to correct the distorted grating responses. The designed silicon IBGs were fabricated and tested experimentally. The accuracy of the model is examined by comparing the measured grating spectra with those predicted by the model. Spectral corrections are then demonstrated in Gaussian-apodized gratings based on ΔL- and DC-modulated silicon IBGs and a square-shaped filter developed on a ΔL-modulated IBG. Finally, a complex spectral correction of a photonic Hilbert transformer developed on a ΔL-modulated silicon IBG is achieved.

Low-loss and low-crosstalk silicon triplexer based on cascaded multimode waveguide gratings

A low-loss and low-crosstalk silicon triplexer is proposed and realized for wavelength-division-multiplexed optical systems with the wavelength channels of 1310, 1490, and 1550 nm. The proposed on-chip triplexer is composed of three cascaded multimode-waveguide-grating (MWG)-based filters, which consist of an MWG and a two-mode (de)multiplexer. For all three wavelength channels, flat-top spectral responses are achieved experimentally, and their 3 bandwidths are 89, 19, and 7 nm, respectively. The excess losses are less than 1 dB. The crosstalks are -36--27 and -32--28  dB for channels 1490 and 1550 nm, respectively.

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).