Spectral Characteristics Of Phase-only Sampled Fiber Bragg Gratings

Multichannel fiber Bragg gratings (FBGs) have attracted great interests in dense-wavelength-division-multiplexed (DWDM) systems. A single sampled FBG can show high performance in the operation for filtering and/or chromatic dispersion compensation simultaneously in multiple channels. Such sampled multichannel FBGs can be implemented by modulating the seeding gratings with either a periodic amplitude or phase sampling function or both. In such structures, the free spectral range (FSR) of the comb filter is solely determined by the sampling period P. Recently, it has been shown both theoretically and experimentally that, in an amplitude sampled and chirped fiber grating, a spectral Talbot phenomenon appears if certain conditions between the grating period chirp and the sampling period are satisfied. With the Talbot effect, the FSR can be arbitrarily tuned by changing the grating chirp for a fixed sampling period. The spectral Talbot effect provides great flexibility in fiber grating designs and advantages in energy efficiency enhancement compared to the conventional amplitude sampled uniform gratings.Compared with the amplitude sampled FBGs, phase-only sampled FBGs show its attractive features due to its much higher energy efficiency than that of the amplitude sampled counterpart due to the limited duty cycle of the seeding grating over the sampling period in the amplitude sampled FBGs. However, the fabrication of the phase modulated refractive index is a challenging work. J. E. R. et al first demonstrated a 45- and 81-channel phase-only sampled FBGs. However, to obtain channel numbers as many as possible with phase transitions as less as possible in a sampling grating period is always the aim of researchers. In this paper, we proposed a novel method that utilizes the spectral Talbot effect in the phase-only sampled fiber Bragg gratings to obtain the broadband high-count channels with possibly much less phase transitions. Specifically, we first demonstrate the Talbot effect in the linearly chirped phase-only sampled FBGs, in which integer and fractional Talbot effect are observed in both Dammann and multilevel type of phase-only sampled FBGs. The optimized locations and the phase shifts are implemented by using simulated annealing algorithm such that an array of spectral channels with equal and maximal reflectivity appears. It is shown that the Talbot effect based on phase-only sampled FBGs is capable of generating superior high-count DWDM channels in terms of channel bandwith, channel number, channel uniformity and energy efficiency. Furthermore, the Talbot effect can be implemented in phase-only sampled fiber Bragg gratings (FBGs) with an arbitrarily chirped grating period. We derived the general theoretical conditions and made simulations using the transform matrix method under which spectral Talbot phenomenon occurs. Cascaded chirped and phase-only sampled fiber Bragg gratings have also been proposed to enhance the channel energy which is limited by energy saturation under Talbot condition with the grating length. All the studies provide new techniques for improving the design flexibility and the energy efficiency of the Talbot effect based multichannel comb filters.