# IoT: A Long-Reach Fully Passive Low-Rate Upstream PHY for IoT over Fiber
## 2 Materials and Methods
### FBG-Based IoToF: A Fully Passive Optical Architecture  
A FBG-based IoToF concept is presented in Figure 2a, which is composed of five main blocks. Starting from IoT devices, their information undergoes codification-modulation device (CMD) before reaching the acousto-optical modulators (AOMs) elements, which are composed of FBGs mechanically driven by an acoustic signal containing the IoT information. In other words, the CMD receives the information (analog or digital format) from the IoT devices and codifies this information into analog signals which fed the AOM. The data of IoT devices is translated from the electrical domain to the optical domain into wavelength codification. Provided that FBG are completely passive elements, the optical excitation stage sends a broadband signal, covering all the FBG individual backscattering peaks. An optical circulator enables the use of a single fiber to send the broadband signal, and simultaneously, collect the individual FBGs backscattered responses (i.e., Bragg wavelength which contends the IoT devices information) along the fiber to, in turn, feed the receiver module. To better understand how our FBG-based AOMs work, Figure 2b illustrates a fiber grating changing its backscattered spectrum according to mechanical waves. A longitudinal movement imposes either a contraction (\(\varepsilon<\varepsilon_{0}\)) or an expansion (\(\varepsilon>\varepsilon_{0}\)) in the FBG when a mechanically attached transducer (e.g., buzzer or loudspeaker) is fed with an external electrical signal. Hence, launching an excitation optical broadband source in the fiber, a blueshift or redshift is observed in the Bragg wavelength of the FBG backscattered spectrum as function of the applied electric signal. Whether the transducer is not fed (\(\varepsilon=\varepsilon_{0}\)), the Bragg wavelength shift depends mainly on temperature effects. Since the temperature variations occurs at very low frequency when compared with strain variations (which occurs from 100 Hz to 800 Hz in the current work), the temperature cross-sensitivity can be neglected or filtered by a high-pass filter [12]. In Figure 2a, the receiver stage performs the optical to electrical conversion task. Note that there are several FBG-based AOMs along the same optical fiber, being each one associated with a different Bragg wavelength (\(\lambda_{1},\lambda_{2},...,\lambda_{n}\)). A low-cost solution for optical spectrum monitoring is to employ edge-filtering [11]. A wavelength demultiplexing device is proposed here to act as such wavelength discriminator. It is necessary to guarantee that the Bragg wavelength is located in the edge of the corresponding channel from the demultiplexing element. In other words, it is possible to obtain a wavelength-to-amplitude conversion, allowing that the FBGs' spectral shift are translated into an optical power variation, which in turn are converted into the electrical domain by the photodetectors (PD\({}_{1}\), PD\({}_{2}\),...,PD\({}_{n}\)). Edge filtering processes can be performed with matched FBG-based filters [30], in-fiber Fabry-Perot micro-cavities [11], long period gratings [31], chirped-FBGs [32], tilted-FBGs [33], and a large length of fiber used to create the edge-filters such as Erbium doped fiber [34] or multimode fiber [35] based edge-filters. As presented by the authors in [11], the most equilibrated relation between visibility and dynamic range is achieved with the micro-cavities approach, which is also used in the current work. Since the proposed approach is based on edge-filtering, the number of AOMs is limited and pre-determined by the demultiplexer ports at the receiver stage. To optimize the bandwidth of each AOM, each CMD must integrate more than one IoT device. On the other hand, the technique of edge-filtering is vulnerable to optical power fluctuations. Therefore, parameters such as bandwidth and visibility of the filter, excursion of Bragg wavelength shift, and sensitivity of the receiver need to be into account to improve the signal to noise ratio [12].