Analysis of Operation of Photonic Integrated Interrogators for Fiber Optic Sensor Networks

Summary form only given. In this work we present and discuss our solutions of photonic integrated interrogators for application in a novel fiber-optic sensing system, dedicated to monitoring condition of a patient exposed to magnetic resonance imaging (MRI) scanning procedure. The monitoring system makes use of multiple fiber Bragg grating (FBG) based sensors, which offer the advantages of high sensitivity and immunity to strong magnetic fields and simultaneously do not affect the results of an MRI scan. Furthermore, the fact that many gratings can be inscribed within a single optical fiber simplifies the architecture of the sensing network. Basically, two main approaches to interrogate FBG-based sensors can be distinguished. The first uses a broadband optical signal, which is coupled into the fiber-optic sensing network, while the reflected spectrum is monitored using a spectrometer, typically equipped with a number of detectors. In the second approach a monochromatic, wavelength-swept light is used to scan the sensors and the reflected signal is detected using a single photodetector. The solutions presented and discussed in this work are based on the first approach and make use of generic technology of photonic integration It should be noted that the majority of commercially available photonic interrogators are realized in a form of bulky devices, consisting of a number of discrete optoelectronics components. That is why they have significant disadvantages: relatively large dimensions, bulky design and considerable power consumption. On the contrary, photonic integrated circuits (PICs) offer compact design, energy efficiency, high reliability as well as reduced fabrication and packaging costs. In our research we are trying to benefit from all these features. In this work we present and discuss the properties of a test series of integrated interrogators manufactured using generic InP technology [1] and differing in details of design and the number of wavelength channels available. R&D works enabled comparison of the performance parameters of all devices and, finally, determination of the most promising solution, based on 36-channel arrayed waveguide gratings (AWG) with 75 GHz and 50 GHz channel spacing, shown in Fig. 1. The photonic integrated chip shown comprises two interrogators, both equipped with two optical inputs - one with built-in semiconductor optical amplifier (SOA) and the second without SOA. A series of experiments that was carried out confirmed design assumptions. Fabricated structures with 50 GHz and 75 GHz AWG have 0.4 nm (Fig lb) and 0.6 nm channel spacings and free spectral ranges (FSR) 14.4 mu and 21.6 nm respectively, as designed. Furthermore, application of the built-in SOA appeared to increase signal by 10 dB, allowing interrogators working better with low-level input optical signals. The signal reflected from an FBG was reconstructed successfully by the interrogator. Finally, the Bragg wavelength shift due to strain applied to an FBG was reconstructed, demonstrating a possibility of monitoring the changes of strain induced on the sensors on the MRI bed, which is a result of the patient's breath movements.