High sensitivity and low loss open-cavity Mach-Zehnder interferometer based on multimode interference coupling for refractive index measurement

An open-cavity Mach-Zehnder interferometer (MZI) liquid refractive index (RI) sensor based on multimode interference (MMI) coupling is proposed and demonstrated. The sensor is fabricated by embedding a small section of no core fiber (NCF) into two sections of multimode fiber (MMF) with large lateral-offset splicing. In term of MMI effect, the lengths of the MMFs are optimized to enhance the exciting and coupling efficiencies of the interference lights. Therefore, this coupling method reduces the insertion loss obviously and allows bigger tolerance of offset distance. The simulated and experimental results all prove it has the most optimized loss compared with other coupling ways that we know. A sensor with 1 mm sensing length is fabricated and its loss is only −11.9 dB which is prominently reduced compared with other open cavity MZIs with the similar cavity length. This sensor has a high RI sensitivity of −1364.343 nm/RIU and shows a good linear response in the measurement range. The temperature sensitivity is about 33 pm/°C, so the cross-sensitivity is only 2.419 × 10−5 RIU/°C. The proposed sensor can promote the development of the open-cavity MZI to satisfy practical demands.