Optical Nyquist Pulses Generation Based On Optical Frequency Comb

Combining the advantages of the microwave and optical technology, microwave photonics is developing rapidly in the last few decades and being widely applied in all kinds of aspects of the microwave and optical fields, and Optical Nyquist pulse is also one of the key technologies in microwave photonics. In recent years, with the coming of “Internet Plus” Age, the fiber transmission network, which is the foundation of the Internet and communication networks, is under an increasing pressure to take actions to improve the system capacities. Traditionally, widening the channel bandwidth is used to solve these problems, but the frequency sources are always limited. Therefore, how to improve the spectrum utilization efficiency becomes one of the important questions attracting public attentions. Theoretically, Nyquist signal has the smallest signal bandwidth, but limited by the sampling rate and bandwidth of electronic devices, it is very difficult to generate high speed Nyquist pulses in the electrical fields. All optical Nyquist signals, which can not only realize high speed signal generation but also ensure high spectrum efficiency, have a great application prospect comparing with other techniques. In addition, these pulses can be considered as ideal rectangle filters in the frequency domain, which can be used in the fields of all-optical signal processing. Furthermore, it is also very important to the spectroscopy and optical storage, attracting more and more attentions at home and abroad.There are many ways to generate optical Sinc pulses, some point on designing simple and flexible architecture, and others concentrate in the quality and parameters of the generated pulses. This article mainly focuses on the directed generation of optical Nyquist Sinc pulses(optical Sinc pulses) based on the time-frequency duality of the Fourier transformation. Firstly, optical-electrical modulator has been used to generate uniformly spaced optical frequency comb(OFC) with narrow linewidth, equal amplitude and linear-locked phase. After time-to-frequency conversion, the sequence of optical Nyquist Sinc pulses is directly generated in the time domain, which can be named as optical Sinc pulses.Firstly we study the generations of 3? 5?9 and 25 lines optical combs with different frequency separations using the scheme combines with a single intensity modulator(IM),a single dual parallel Mach-Zehnder modulator(DPMZM), two cascaded IMs and two cascaded DPMZMs. The Sinc pulse can be obtained by the time-frequency transformation.Then we analyses the parameters of the modulator under cascade conditions. We propose a new scheme using cascaded DPMZM and IM to generate Sinc pulse with 15 lines optical combs under the above conditions, and the parameters of the Sinc pulses can be changed by tuning the input RF frequency imparted on the modulators. We analyses these schemes by theoretical derivation and simulation with VPI software, and we also study the corresponding time-domain waveforms of different optical combs using Matlab under normalized frequency.Secondly we study and propose a new scheme to generate Sinc pulses with 5 lines OFC based on the Sagnac effect, which puts an intensity modulator(IM) in a Sagnac loop,defined as the S-IM. Then we design the structures of cascade IM and S-IM based on the cascade modulation theory. The S-IM and S-IM structure could produce optical Sinc pulses with 15 lines and 25 lines OFC. We also use the Matlab and VPI software to do the simulations. The proposed scheme breaks the limits that IM can only generate Sinc pulse with 3 lines optical comb, it is more simple, flexible and controllable than the traditional schemes. In addition, the environmental changes have limited impact on the system stability because of the use of a Sagnac loop, in which the two counter propagation light waves are traveling within the same fiber loop. The proposed scheme has some values both in the researches and applications.