Alexandre D. Simard

Apodized Silicon-on-Insulator Bragg Gratings

An accurate control of the apodization profile is still an issue for integrated Bragg grating filters fabricated in silicon-on-insulator because of the high modal confinement of these waveguides. In this letter, we present two fabrication-friendly apodization techniques that are compatible with deep UV lithography and can be used in mass-production of photonic-integrated circuits. These techniques are reliable even for weak effective index modulation amplitude, thus opening the door to the fabrication of long and elaborate grating structures.

Impact of Sidewall Roughness on Integrated Bragg Gratings

A major issue in the fabrication of integrated Bragg grating (IBG) filters in highly confined waveguides is the average effective index fluctuations caused by sidewall roughness. In this work, we model the impact of this effect on IBG spectral responses and we identify key parameters that need to be controlled in order to minimize distortions. We show that only low spatial frequency components of the noise are relevant to the calculation of the IBG spectral response, which decreases considerably the computation time. Furthermore, we present an IBG emulator that allows estimation of expected fabrication yield of specific gratings given that the fabrication process is well characterized. The analysis of apodized gratings is used as an example to illustrate how this modeling can help to reduce development cost by first studying robustness of IBG designs to fabrication limitations. Finally, we study analytically the impact of sidewall roughness having short correlation lengths and small roughness variances on the spectral response of weak gratings.

Characterization and reduction of spectral distortions in Silicon-on-Insulator integrated Bragg gratings

A major issue in the fabrication of integrated Bragg grating filters in highly confined waveguides is the average effective index fluctuations caused by waveguide dimension variations. Lateral variations are caused by the sidewall roughness created during the etching process while vertical variations are coming from the wafer silicon layer thickness non-uniformity. Grating spectral distortions are known to result solely from the low spatial frequency components of these variations. As a result, in this work, we present an experimental method to quantify such relevant spatial components by stitching a hundred high-resolution scanning electron microscope images. Additionally, we propose two techniques to reduce, in the design, the phase noise impact on integrated Bragg gratings without relying on fabrication process improvements. More specifically, we show that the use of hybrid multimode/singlemode waveguides reduce by more than one order of magnitude the effect of sidewall roughness on integrated Bragg gratings while we show that the fabrication of ultra-compact gratings in spiral waveguides mitigate the impact of the silicon layer thickness variations.

Integrated Bragg gratings in spiral waveguides

Over the last two decades, many filters requiring custom spectral responses were obtained from photo-inscribed fiber Bragg gratings because of the flexibility inherent to this technology. However, Bragg gratings in silicon waveguides have the potential to provide faster and more efficient tuning capabilities when compared to optical fiber devices. One drawback is that Bragg gratings filters with elaborate spectral amplitude and phase responses often require a long interaction length, which is not compatible with current integration trends in CMOS compatible photonic circuits. In this paper, we propose to make Bragg gratings in spiral-shaped waveguides in order to increase their lengths while making them more compact. The approach preserves the flexibility of regular straight grating structures. More specifically, we demonstrate 2-mm long gratings wrapped in an area of 200 µm x 190 µm without any spectral degradation due to waveguide curvature. Furthermore, we interleave three spiral waveguides with integrated gratings thereby tripling the density and demonstrate good phase compensation for each of them. Finally, we show that this approach is compatible with phase-apodization of the grating coupling coefficient.

Bandpass integrated Bragg gratings in silicon-on-insulator with well-controlled amplitude and phase responses

Bandpass filters with square shape amplitude responses and well-controlled dispersion characteristics are achieved by accurate apodization of Bragg grating structures in silicon-on-insulator waveguides. For these devices, precise tailoring of their frequency response typically requires low coupling coefficients and relatively long on-chip propagation lengths. These challenges are addressed by implementing apodization by phase-modulation and using wider strip waveguides to reduce phase noise. This design approach is demonstrated with a dispersion-less narrowband filter and a chirped bandpass filter.

Dual phase-shift Bragg grating silicon photonic modulator operating up to 60 Gb/s

We demonstrate PAM-4 and OOK operation of a novel silicon photonic modulator. The modulator design is based on two phase-shifts in a Bragg Grating structure driven in a push pull configuration. Back-to-back PAM-4 modulation is demonstrated below the FEC threshold at up to 60 Gb/s. OOK modulation is also shown up to 55 Gb/s with MMSE equalization and up to 50 Gb/s without equalization. Eye diagrams and BER curves at different bit rates are provided for both PAM-4 and OOK modulations. To our knowledge, this structure is the fastest silicon photonic modulator based on Bragg gratings, reaching modulation speed comparable to the fastest Mach-Zehnder modulators and micro-ring modulators.

Segmented silicon MZM for PAM-8 transmissions at 114 Gb/s with binary signaling.

We experimentally demonstrate PAM-8 generation from binary electrical signals driving a silicon multi-electrode Mach-Zehnder modulator acting as an optical digital-to-analog converter. Measured BER in back-to-back configuration is used to evaluate signal quality. We demonstrate 38 GBd PAM-8 transmission below the forward error correction (FEC) threshold using minimum mean square error (MMSE) equalization. The results show that modulators with segmented phase shifters can be advantageously used to eliminate the need for high bandwidth electronic digital-to-analog converters in the generation of multilevel signals. These modulators, that can be designed and fabricated with standard CMOS compatible tools and processes, are of high interest for short range high-speed data links.

Long Integrated Bragg Gratings for SoI Wafer Metrology

Phase-sensitive photonic devices fabricated in silicon-on-insulator platform can have their response significantly modified by wafer height fluctuations. In this letter, we demonstrate a novel metrology technique for measuring on-chip thickness variations using long integrated chirped Bragg gratings. Thickness variations are recovered from the measurement of the complex transmission and reflection spectra using an inverse scattering algorithm. With 1-cm long sidewall gratings on 1.2-μm wide waveguides, we measure waveguide height variations with nanometer-scale precision and tens of micrometer of longitudinal resolution. The results are confirmed by atomic force microscopy measurements.

Complex apodized Bragg grating filters without circulators in silicon-on-insulator.

Bragg gratings operating in reflection are versatile filters that are an important building block of photonic circuits but, so far, their use has been limited due to the absence of CMOS compatible integrated circulators. In this paper, we propose to introduce two identical Bragg gratings in the arms of a Mach-Zehnder interferometer built with multimode interference 2 x 2 couplers to provide a reflective filter without circulator. We show that this structure has unique properties that significantly reduce phase noise distortions, avoid the need for thermal phase tuning, and make it compatible with complex apodization functions implemented through superposition apodization. We experimentally demonstrate several Bragg grating filters with high quality reflection spectra. For example, we successfully fabricated a 4 nm dispersion-less square-shaped filter having a sidelobe suppression ratio better than 15 dB and an in-band phase response with a group delay standard deviation of 2.0 ps. This result will enable the fabrication of grating based narrowband reflective filters having sharp spectral responses, which represents a major improvement in the filtering capability of the silicon platform.

Characterization of Integrated Bragg Grating Profiles

Spectral responses of gratings in SOI are extracted using time windowing to eliminate parasitic reflections. Filtering high spatial frequencies of the phase profile, obtained by layer peeling, allows examination of the wafer thickness uniformity.