Microphones have been widely applied in fields such as unmanned aerial vehicle (UAV) detection and tracking, noise monitoring, medical devices, and disaster warning. Optical microphones based on grating Fabry-Perot (FP) interferometry have the advantages of high sensitivity, easy integration, and low power consumption. Therefore, they provide a development direction for integrated grating interferometric microphones. There are two challenges in implementing integrated grating interferometric microphones: 1) how to optimize the design of the microphone structure to guarantee a small size and a high performance; 2) how to stabilize the microphone at the quadrature operating point for high-fidelity and high-sensitivity detection of acoustic signals. Owing to the thermal expansion effect on the FP cavity length of the grating interferometric microphone, the operating point of the microphone inevitably drifts with the ambient temperature. A conventional method to overcome this drawback is to modulate the FP cavity length through electrostatic force to compensate for the temperature-induced cavity length variation. However, this method is effective merely when the drive voltage is greater than 10 V. Moreover, the electrostatic force between the diaphragm and the back electrode will affect the elasticity of the diaphragm and thus make the frequency response characteristic of the microphone change. In this work, we develop an integrated grating interferometric microphone, which consists of a micro-electro-mechanical system (MEMS) diaphragm on the silicon on insulator (SOI) substrate, a metal grating on the glass substrate, a vertical cavity surface emitting laser (VCSEL), and a miniature photodetector (PD). In addition, we propose a simple method to compensate for the temperature-induced cavity length variation, which is to modulate the VCSEL wavelength to match the quadrature operating point of the microphone. We also analyze the effect of the divergence angle of the VCSEL on the design parameters of the microphone, conclude a rule of designing the integrated grating interferometric microphone, and successfully match the operating point to the quadrature operating point by modulating the wavelength. Our work can overcome the current challenges in preparing integrated grating interferometric microphones.

In order to optimize the design of the microphone structure for a small size and a high performance, a VCSEL and a miniature PD are adopted to build a microphone. Since the VCSEL still has a certain divergence angle, an optical design model is established by analyzing the reflection diffraction and transmission diffraction processes of the grating interferometer using geometric optics. Under the given PD active area, grating period, and FP cavity length, the optical design model can derive the best placement position of PD to ensure the high integration of a grating interferometric microphone. In addition, a laser wavelength tuning method by applying the thermoelectric effect of VCSEL is proposed to match the operating point to the quadrature point. The method discriminates the operating point by monitoring the normalized light intensity and the frequency-domain harmonic components. The amplitudes of the fundamental frequency (FF) component and second harmonic (SH) component at different operating points are analyzed. The FF component with a larger amplitude is selected as a reference, which can make the method have higher accuracy. The method can avoid the problems of high voltage and low reliability introduced by conventional methods.

By using the geometric optical design model proposed in this work, an integrated grating interferometric microphone is fabricated. The overall size of the microphone is 10 mm×10 mm×2.5 mm, and the sensing core size is 0.93 mm×0.34 mm×2.5 mm (volume of 0.79 mm³) (Fig. 8). An experimental setup for the acoustic test is built to verify the operating point adjustment method and characterize the microphone performance (Fig. 9). The operating point determination test results show that the laser wavelength tuning method is realized under the conditions of an operating voltage of 5 V and a drive current of no more than 15 mA (Fig. 10). The method features low voltage and low current. Thus the proposed microphone can be applied to cases with low voltage. The performance characterization results show that the proposed integrated microphone has a high-quality response waveform in the low-frequency range of 50-100 Hz (Fig. 11). Moreover, the sensitivity of the proposed integrated microphone is 22.10 mV/Pa at 251.2 Hz, and the microphone has a flat response with a fluctuation range of no more than ±3 dB in the low-frequency range from 50 Hz to 300 Hz (Fig. 12). Therefore, the proposed microphone can be applied to the field of low-frequency acoustic detection.

In this work, an integrated grating interferometric microphone with a sensing core size of 0.93 mm×0.34 mm×2.5 mm (volume of less than 0.8 mm³) has been proposed and experimentally demonstrated. In order to optimize the design of the microphone structure, the effect of laser divergence angle is analyzed, and thus an optical design model for grating interferometers is proposed. According to the optical design model, an integrated microphone with an overall size of 10 mm×10 mm×2.5 mm is fabricated. The experimental results of the operating point determination test show that the proposed operating point adjustment method is realized in low voltage conditions. The experimental results of performance characterization show that the microphone has a flat frequency response curve in the low-frequency range from 50 Hz to 300 Hz with a sensitivity of -33.11 dBV/Pa at 251.2 Hz. Compared with the existing studies, the proposed integrated grating interferometric microphone has a smaller sensing core and a low-voltage modulated operating point and thus can be widely applied. Moreover, the proposed microphone has an excellent response in the low-frequency range and thus shows a great application prospect in the field of low-frequency acoustic detection.