# Fiber Bragg Grating-Based Optical Signal Processing: Review and Survey
## 4 Discussion  
In the past two decades, much research has been carried out in the development of all optical signal processors with high bandwidth (even up to the THz regime), aimed at replacing the currently employed electronic processors. Methods for performing analysis and synthesis of FBG enabled the development of a myriad of different filters and optical pulse processors based on FBG technology. This research line was highly fruitful along the first 15 years of this century. From 2016, the research interest in FBG as signal processors started to decrease. The use of optical processors in a fiber platform is currently limited to dispersion compensation devices based on linearly-chirped FBGs, and band pass filtering components (i.e., those described in Section 3.1). Nowadays, the interest on these processing components is mainly focused on the development of Bragg gratings in integrated platforms. In particular, the need for particular filtering operations to implement complete functionalities on a chip, e.g., in microwave photonics processors, beamforming networks, arbitrary waveform generation, etc. is widely recognized [18; 19; 20; 96]. The development of optical processors on integrated Bragg gratings is following a similar path to the one that occurred in fiber platforms. At present, most of the implemented functionalities have been developed via approximations. Recently, analysis tools have been formulated with the aim to predict the effect of particular apodization or period variations on the grating spectral response prior the fabrication of the devices [97]. The next natural step is the development of synthesis tools that enable the design of energy efficient, high precise spectral filtering responses in integrated waveguide Bragg gratings. The design tools described in this review paper can serve as a baseline for the development of the required tools in integrated platforms.  
Through the years, one of the most criticized features of FBG has been the difficulty in producing reconfigurable processors. When talking about reconfigurability, we not only mean the processing operation, but also the features related to their performance, e.g., the fact that the processor can be employed to produce output waveforms with different temporal features. The only parameter that is relatively simple to alter is the central frequency of operation, which is dependent on the temperature or the strain on the structure. Based on the possibility of altering the operation frequency of the grating, there have been attempts to attain programmable and reconfigurable components [98; 99; 100]. An example is the development of a second order optical differentiator based on linearly chirped FBG and a digital thermal print head [100]. Advances towards reconfigurable structures are being researched nowadays through the use of innovative methods for modifying the grating physical parameters in a programmable means or by using static FBGs in more complex interferometric schemes, even in integrated platforms [101].  
Figure 12: Ideal temporal input pulse (blue line) and pulse employed in the characterization of a flat-top pulse shaper implemented on a FBG in transmission (green line); Ideal temporal output (black line) and experimentally measured output (red line). The output signal from the input employed in the experimental test (green line) and the ideal device response is also plotted (magenta line), to show deviations from the ideal output owing to the employed pulse.