# On-chip interrogator based on Fourier Transform spectroscopy

## TL;DR

This paper introduces a novel integrated Fourier transform spectrometer for photonic sensor interrogation, demonstrating high sensitivity, drift compensation, and potential for compact sensing systems.

## Contribution

It presents the first integrated FT spectrometer using a planar spatial heterodyne design with MZIs for sensor interrogation, including a numerical solution for sensor modulation.

## Key findings

- Achieved 92% compensation of thermal phase drift.
- Minimum modulation amplitude of 400 fm, surpassing spectrometer resolution.
- First demonstration of integrated FT spectrometer for photonic sensors.

## Abstract

In this paper, the design and the characterization of a novel interrogator based on integrated Fourier transform (FT) spectroscopy is presented. To the best of our knowledge, this is the first integrated FT spectrometer used for the interrogation of photonic sensors. It consists of a planar spatial heterodyne spectrometer, which is implemented using an array of Mach-Zehnder interferometers (MZIs) with different optical path differences. Each MZI employs a 3$\times$3 multi-mode interferometer, allowing the retrieval of the complex Fourier coefficients. We derive a system of non-linear equations whose solution, which is obtained numerically from Newton's method, gives the modulation of the sensor's resonances as a function of time. By taking one of the sensors as a reference, to which no external excitation is applied and its temperature is kept constant, about 92$\%$ of the thermal induced phase drift of the integrated MZIs has been compensated. The minimum modulation amplitude that is obtained experimentally is 400 fm, which is more than two orders of magnitude smaller than the FT spectrometer resolution.