Study On Silicon Waveguide Admittance Detecting Circuits

As the physical sizes of photonic devices shrink and the monolithic integration of photonic devices increases,detecting the working state of silicon photonic devices having the compatibility with integrated circuit devices becomes crucial.Due to the non-invasive advantage,an optical power monitor based on surface state absorption effect monitoring the optical intensity in the silicon waveguide by detecting the admittance of the silicon waveguide is attracting much attention.In this paper,electrical parameters are extracted first by analyzing the silicon waveguide admittance sensor.and then several schemes for realizing silicon waveguide admittance detecting circuits are proposed according to the difference between the noise and the target signal associated with the admittance of the silicon waveguide.The study mainly includes:1.Silicon waveguide admittance detecting circuit based on the second-harmonic frequency lock-in architecture.An equivalent circuit of the silicon waveguide admittance sensor using capacitively coupled contactless conductivity detection(C~4D)is presented first.Based on the equivalent circuit,a silicon waveguide admittance detecting circuit based on the second-harmonic frequency lock-in amplification architecture is proposed.In the designed circuit,the target signal is frequency-synthesized,amplified,and extracted.The variation of the extracted voltage signal is used to reflect the variation of the silicon waveguide admittance.As 1/f noise is avoided,the resolution of the admittance is enhanced.2.Silicon waveguide admittance detecting circuit utilizing quadratic cross-correlation method.By analyzing the difference between the target signal and the noise in the detecting circuit,a silicon waveguide admittance detecting circuit utilizing quadratic cross-correlation method is proposed according to the principle of the cross-correlation method.In the proposed circuit,the input target signal is cross-correlated with the external reference signal twice.In other words,the target signal is endowed with frequency characteristics twice to achieve better self-noise suppression.3.Silicon waveguide admittance detecting circuit using the capacitor-integration method.Firstly,an equivalent circuit of the silicon waveguide admittance sensor using an improved scheme of C~4D is presented.Then,according to the periodicity and continuity of the current signal reflecting the variation of the silicon waveguide admittance and the randomness of the noise,a silicon waveguide admittance detecting circuit utilizing quadratic cross-correlation method is proposed.The continuous current signal is integrated and then converted into a voltage signal by an integrator and the noise is not affected,so that the self-noise of the circuit can be suppressed.The proposed three types of silicon waveguide admittance detecting circuits are designed with TSMC 0.18?m process and analyzed by using the Spectre simulator.The results indicate that the designed three types of silicon waveguide admittance detecting circuits have excellent self-noise suppression,good linearity and high resolution.