Compression Of Pulse Spectrum By Cosinoidal Phase Modulations In Highly Nonlinear Fiber

It is very important to obtain high-brightness spectrum and high-resolution light source in photonics.It has a wide range of applications in nonlinear microscopes,ultra-fast spectrum and high-speed communications.The method of obtaining a high-brightness spectrum and a high-resolution light source is to spectrally compress the pulse and redistribute the energy to a narrower spectral range.Firstly,for Super-Gaussian pulses with nonlinear negative chirp,we propose a two phase modulation method to optimize the spectral compression process in highly nonlinear optical fibers.The results show that the temporal unlevel phase of the super-Gaussian pulse resulted by the mismatch between initial nonlinear negative chirp of the pulse and self-phase modulation induced nonlinear positive chirp,spectral splitting and side lobes occur,and the spectrum cannot be sustained and efficiently compressed.So the spectrum quality is limited.In order to optimize the quality of spectrum compression,two cosine phases are introduced to compensate the phase of the initial super-Gaussian pulse so that to prolong the spectrum compression distance and reduce the spectrum side lobe.In this case,the amplitude and frequency of the cosine phase used for phase modulation come from the nonlinear phase shift analysis of super-Gaussian pulses.At the same time,we also studied the compression of super-Gaussian pulses with different stretching factors and different orders.The results show that the spectral compression quality can also be optimized in a largrer range of stretching factors.In addition,the compression quality of the spectrum can be improved by increasing the number of cosine phase modulations.Next,this cosine-phase-modulated spectral compression method can be applied to other forms of pulses.We take the example of a Gaussian pulse with a linear chirp and a hyperbolic secant pulse to investigate the spectrum compression.The results indicate that the split spectrum can be effectivelycompressed with external cosine phase modulation.It also shows that this method of cosine phase modulation is suitable for various pulse forms.For more complex pulse forms,this method is also applicable.