In this paper, we demonstrate a compact Silicon photonics-based on-chip integrated interference vibrometer. Unlike conventional readout methods, the demonstrated system is alignment-free and offers multiplex sensing. The intensity that is modulated by the cantilever motion by a photodetector. We present the static and dynamic response of the cantilever by electrostatic excitation validated using ac commercial Laser-Doppler-Vibrometer. We also present a detailed simulation, optimisation and sensitivity analysis of the proposed on-chip vibrometer. Furthermore, the tunability of the sensor to achieve maximum sensitivity is demonstrated.
Graphene has emerged as an attractive nonlinear-optical material due to the high coefficient of two-photon absorption and four-wave mixing. Four-wave mixing in graphene has been previously studied in silicon-photonic platform. Enhancement of the four-wave mixing using optical cavities such as silicon micro-ring resonator (MRR) has been demonstrated. Recently, similar experiments have been extended to silicon-nitride (SiN) waveguides and micro-ring resonators. Electrostatic tuning of the four-wave mixing, and generation of frequency combs have been demonstrated using SiN MRRs having a Q-factor of 106 at input pump powers ≥ 1 W. On-chip pump powers of the order of 10 mW to 100 mW are desirable to obtain high conversion efficiency of the four- wave mixing. However, such high on-chip powers are challenging to handle in integrated-optic platforms. We report preliminary experimental result of four-wave mixing in graphene-on-SiN MRRs with CW pump power of 120 μW, which is coupled to the MRR. The MRR used has a modest Q-factor of the order of 103 after transferring graphene. We observe four-wave mixing even with a 50 % coverage of monolayer graphene on the MRR. Such low power level allows low-power on-chip nonlinear process. Furthermore, low photon count could be used for quantum photonic process and fundamental research where high conversion efficiency may not be necessary.
Wavelength-selective integrated photonic devices in silicon-photonic platform require tuning to match the operating wavelength of multiple devices. The operating wavelengths are generally in the near-IR band. The conventional method of choice is to thermally tune the refractive index of silicon using metal micro-heaters. However, metals absorb near-IR wavelengths and must be placed away from the waveguides to avoid optical losses. This significantly reduces the power-efficiency of the heaters. Graphene-based local heaters on top of waveguides have been recently explored. Although the absorption in graphene is less than that of metals, it is still large enough to necessitate the placement of a thin spacer between the waveguide and the heater. We observe that metallic carbon-nanotubes (CNTs) are comparatively more transparent in the C-band. We implement heaters made of solution-processed metallic CNTs directly on top of a silicon-on-insulator micro-ring resonator. We demonstrate thermo-optic tuning of 60 pm/mW on a micro-ring resonator having a free-spectral range (FSR) of 1.75 nm. The estimated power efficiency is 29 mW/FSR, which is at par with previously implemented graphene-based heaters, that has higher absorption and better than conventional metal heaters. The proposed configuration offers compact and efficient thermal-tuner integration.
In this paper, we demonstrate a compact silicon photonics based vibrometer using an on-chip photonic grating (OPG) based sensor. OPG works on the principle of interference where the motion of the cantilever is captured at the output as an intensity variation. The advantage of OPG based sensor over conventional Laser Doppler vibrometer is increased tolerance to alignment errors as both the grating and the cantilever can be integrated on a single chip. The grating parameters were optimized using 2D-FDTD to achieved maximum sensitivity to the displacement of a cantilever. OPG with on-chip germanium photodetector is studied, which indicates a sensitivity of 54 μW/nm. We experimentally demonstrate the feasibility of the proposed sensor that can achieve a displacement sensitivity of 5.3 μW/nm.