Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach-Zehnder interferometer system

This paper describes the design, fabrication and testing of a pigtailed integrated optical (IO) phase-modulated Mach–Zehnder interferometer (MZI) including both the optical chip and the electronics. The optical chip is realised in SiON technology. The IO components (the sensing function, the straight waveguiding channels, the phase modulator, the polariser, the splitter, the combiner and the fibre-to-chip connection unit) are individually optimised and interconnected by using transversal adiabatic tapers. To obtain a high waveguide evanescent field sensitivity, the sensor is designed for — but not limited to — a wavelength of 632.8 nm. The integrated MZI is actively phase-modulated by virtue of the electro-optic effect of the incorporated material zinc oxide (ZnO). The electro-optical voltage–length product Vπ is 16 V cm at frequencies above 10 Hz. The polariser is a distributed function, that effectively filters TM-polarised light (TE/TM polarising ratio >30 dB). The fibre pigtail, affording remote optical sensing, is based on a cheap, easy-to-use fibre-to-chip connection with a typical coupling efficiency of 50%, while the device throughput (“insertion loss”) is −20 dB. The drive- and demodulation electronics enable a phase resolution 5×10−5×2π, corresponding to a refractive index resolution of 2×10−8. The sensing system as has been realised up to now shows a phase resolution of 1×10−4×2π, its long-term stability (hours) being ≤3×10−4×2π. This corresponds to a refractive index resolution of 5×10−8, and a long-term stability of 10−7.