Miniaturized photonic sensors based on micro-interferometers

Fiber Bragg gratings (FBGs) sensors present a series of benefits over electric and electronical sensors, such as: immunity to EMI, low cost of production, capability of distributed or quasi-distributed sensing, usability in harsh environments, etc [1]. FBGs reflect a specific wavelength, which shifts depending on the measurand, and it is detected by an interrogator. Commercial interrogators rely on bulk optics, which usually use either a swept laser source or a spectrometer for the detection. Due to the use of bulk optics, these devices have relatively high production cost, high power consumption, and large dimensions. In this work, we present FBGs interrogators based on integrated optics in a silicon chip. The sensing system uses Mach Zehnder interferometers to detect changes in the wavelength of the FBG. However, to avoid the responsivity fading of the interferometers, we introduce a modulating signal in the interferometer, and then we demodulate using a combination of harmonics of the modulating signal, to retrieve the wavelength shift. The technique that we use is called multi-tone-mixing MTM [2]. The photonic integrated chip (PIC) features a 12 channel AWG integrated on-chip, which allows simultaneous detection of multiple FBGs, and integrated Ge photodiodes. The PIC was packaged and mounted on a PCB, with a Peltier cell for temperature control, and a pigtailed fiber array for the optical input, as seen in Fig. 1. A full description of the system can be found in [3]. We demonstrate successful detection of an FBG signal, and we compared it with a commercial interrogator. As seen in Fig. 2, we modulate the wavelength of an FBG at 1540 nm by attaching a piezoelectric to it. The results of the two interrogators are very similar, and they match for the temporal trace, as well as in the FFT for the main frequency and the first harmonics. Therefore, we demonstrate that it is possible to produce integrated devices that can match in performance current commercial bulk devices.