High-speed organic gas sensor using a microfiber Mach-Zehnder interferometer interrogated based on microwave photonic pulse compression

A high-speed fiber-optic real-time organic gas sensor based on a microfiber Mach-Zehnder interferometer (MZI) interrogated based on microwave photonic pulse compression is proposed and experimentally demonstrated. A taper-based microfiber MZI (mMZI) with a nonuniform free spectral range (FSR) is used as a crucial sensor element. The mMZI is designed to be sensitive to the surrounding refractive index (RI) in the air. When the mMZI is exposed to an organic gas ambiance with varying concentrations, the optical spectrum of the mMZI will shift with the gas concentration change. Instead of measuring the wavelength shift of the mMZI in the optical domain, which has a low interrogation resolution and a slow interrogation speed, we convert the mMZI spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping by using a frequency swept laser (FSL) source. The time-domain waveform is then processed by a digital signal processor (DSP), where the time-domain waveform is compared with a reference waveform through cross-correlation. A narrow correlation peak indicating the time shift of the waveform is obtained. Since the time shift is linearly proportional to the wavelength shift of the mMZI, the wavelength shift of the mMZI is measured. In the experiment, the surrounding RI of ethanol gas is measured at a scanning speed as high as 1.6 MHz. The results show that the sensor has a sensitivity and resolution of -0.3663 ps/ppm and 54.6 ppm, respectively, without any surface modification. The proposed sensing system used to monitor the diffusion of ethanol vapor in real time and to mark the gas concentration after the diffusion has also been studied.