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_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.@en

We experimentally demonstrate an interrogation procedure of a ring-resonator ultrasound sensor using a fiber Mach-Zehnder interferometer (MZI). The sensor comprises a silicon ring resonator (RR) located on a silicon-oxide membrane, designed to have its lowest vibrational mode in the MHz range, which is the range of intravascular ultrasound (IVUS) imaging. Ultrasound incident on the membrane excites its vibrational mode and as a result induces a modulation of the resonance wavelength of the RR, which is a measure of the amplitude of the ultrasound waves. The interrogation procedure developed is based on the mathematical description of the interrogator operation presented in Appendix A, where we identify the amplitude of the angular deflection Φ0 on the circle arc periodically traced in the plane of the two orthogonal interrogator voltages, as the principal sensor signal. Interrogation is demonstrated for two sensors with membrane vibrational modes at 1.3 and 0.77 MHz, by applying continuous wave ultrasound in a wide pressure range. Ultrasound is detected at a pressure as low as 1.2 Pa. Two optical path differences (OPDs) of the MZI are used. Thus, different interference conditions of the optical signals are defined, leading to a higher apparent sensitivity for the larger OPD, which is accompanied by a weaker signal, however. Independent measurements using the modulation method yield a resonance modulation per unit of pressure of 21.4 fm/Pa (sensor #1) and 103.8 fm/Pa (sensor #2).@en

Photonic sensors have recently attracted much attention in both industry and academia. High accuracy, low weight and the possibility of building a large sensor network are key benefits of photonic sensors. Another benefit is installing optical sensors in harsh environments where electronic sensors' usage is not plausible: aerospace applications where ionizing radiation is present or gas and oil pipes are some examples. Integrated photonics brings new challenges to the interrogation of multiplexed sensors in WDM. Unlike FBG sensors, whose resonance wavelength can be chosen to an accuracy better than 1.0 nm, the resonance wavelength of integrated micro-ring resonators cannot be chosen during the design stage. The main reason is the imperfections of the manufacturing process. The fact that the resonance wavelength is unpredictable is a problem for interrogators based on interferometry. Such interrogators perform the demultiplexing and demodulation in different stages: first, a spectrometer separates the optical channels; subsequently, outputs of the spectrometer are conveyed to interferometers. From the photo-receiver voltages connected to MZI outputs, the signal from the sensors can be demodulated. As the resonance value of sensors cannot be determined during design, two sensors may have their resonances in the same spectrometer's channel. As a result, the demultiplexing step fails, compromising the interrogator's operation. In Chapter 4 of this thesis, a new interrogation method is proposed. Much of the effort of interferometric interrogators is to separate the spectrum of the sensors correctly. In the Fourier Transform Interrogator, the spectrum of all sensors is sent to an array of Mach-Zehnder interferometers (MZI) with different OPDs. Using the output voltages from the photo-receivers attached to the MZIs, we derive a system of non-linear equations, whose solution provides the signal from each sensor. The demodulation and demultiplexing steps are performed simultaneously for the Fourier interrogator, which guarantees the interrogator's unique flexibility. On the other hand, the computational cost is high since the system of non-linear equations is solved using Newton's method. For each set of voltages sampled over time, a different system of equations is obtained. Chapter 4 leaves some unanswered questions: Does the system of non-linear equations have a unique solution? How many solutions are there? What is the physical meaning of each of the solutions? Is it possible to solve non-linear systems of equations for fast sensors in real-time? All these questions are answered in Chapter 5. As a consequence of the new algebraic formulation, it is possible to solve about 1 000 000 algebraic systems of equations in about 10 ns, i.e., allowing the real-time interrogation of high-speed sensors. The interrogator is a candidate for interrogating arrays of ultrasound ring resonator sensors in the tens of MHz range. @en

A new method for fast, high resolution interrogation of an array of photonic sensors is proposed. The technique is based on the integrated Fourier transform (FT) interrogator previously introduced by the authors. Compared to other interferometric interrogators, the FT-interrogator is very compact and has an unprecedented tolerance to variations in the nominal values of the sensors’ resonance wavelength. In this paper, the output voltages of the interrogator are written as a polynomial function of complex variables whose modulus is unitary and whose argument encodes the resonance wavelength modulation of the photonic sensors. Two different methods are proposed to solve the system of polynomial equations. In both cases, the Gröbner basis of the polynomial ideal is computed using lexicographical monomial ordering, resulting in a system of polynomials whose complex variable contributions can be decoupled. Using an NVidia graphics processing card, the processing time for 1 026 000 systems of algebraic equations takes around 9 ms, which is more than two orders of magnitude faster than the interrogation method previously introduced by the authors. Such a performance allows for real time interrogation of high-speed sensors. Multiple solutions satisfy the algebraic system of equations, but, in general, only one of the solutions gives the actual resonance wavelength modulation of the sensors. Other solutions have been used for optimization, leading to a reduction in the cross-talk among the sensors. The dynamic strain resolution is 1.66 nε/√Hz.@en