Research On Long Distance Stable Soliton Transmission And Triangular-Shaped Optical Pulses

Along with the requirement for continually increased large capacity and high speed communication, optical soliton communication has been researched intensively and developed forward a lot. The optical soliton is a kind of optical pulse train. It can propagate in a fiber for a long distance without intensity profile variation, which is the foundation of optical soliton communication. With the rapid development of all optical signal processing, optical pulses with more exotic intensity profiles are becoming more important for a number of photonics applications. The topic of this thesis is "Research on long distance stable soliton transmission and triangular-shaped optical pulses". The long-distance soliton transmission through ultralong Raman fiber laser and Generation & application of triangular-shaped optical pulses has been researched in this thesis.The main points are as follows:1. Propagation of picosecond optical pulse in single-mode fibers is described using the nonlinear Schrodinger equation (NLSE). Influences of group velocity dispersion and self-phase modulation induced by fiber nonlinearity on propagation of optical pulses with different shapes are discussed in detail. Conditions for soliton formation are analyzed, and the effect of fiber loss on soliton transmission is discussed.2. Properties of a long-distance transmission link based on ultralong Raman fiber laser are investigated detailedly. Using second-order Raman amplification, quasi-lossless transmission can be realized in this long-distance transmission system. Over 83km transmission distance, the variation of the signal power can be reduced to 1.5dB, whereas the signal would experience 16.6dB variation under a conventional lumped amplification scheme. The performance of this long-distance transmission system is affected by basic Raman laser variables such as cavity length, Fiber Bragg Grating (FBG) reflectivity and input signal power. For the long span case, the signal power excursion can be improved through reducing FBG reflectivity, without significant pump efficiency penalties. Even more importantly, the required pump powers for realizing quasi-lossless transmission are well within the capabilities of moderate pump lasers.3. The experimental research of long-distance unperturbed fundamental optical soliton transmission is carried out using the ultralong-Raman-fiber-laser-based transmission system. For the first time, we experimentally present the 4.0ps unperturbed fundamental optical soliton transmission over 22km conventional single-mode optical fiber, which is approximately equivalent to 16 soliton periods. Second-Harmonic-Generation-based Frequency-resolved optical gating (SHG-FROG) technique is used to characterize the soliton pulse at different positions along the transmission span. The soliton spectrogram remains exactly undistorted across the whole length of the 22km fiber, which is the full proof of long-distance transmission of fundamental optical solitons over multiple soliton periods without amplitude, temporal width, or phase variation. The soliton transmission distance is then extended to more than 50 soliton periods for a 72km span.4. Using the long-distance transmission system, we experimentally investigate the effects of total pump power, pump power ratio between forward and backward pump, FBG reflectivities and fiber span length on properties of transmission system and soliton transmission. The operational limit of stable soliton transmission in this quasi-lossless system is discussed. In our experiment, by optimizing FBG reflectivities,2.8ps stable soliton transmission is realized along 100km distance, which is the longest distance for the stable transmission of 2.8ps optical soliton. It has 'breathing-soliton-like' propagation along the 100km distance. The soliton transmission has a relatively large tolerance to FBG reflectivities.5. We propose a new method for passive nonlinear optical pulse shaping in the time domain that relies upon pulse prechirping and nonlinear propagation in a section of normal dispersion (ND) fiber. Group velocity dispersion and self-phase modulation in ND fiber will lead to various reshaping processes of an initial, conventional pulse according to the prechirping value and power level at the input of the ND fiber.6. We experimentally research the generation of triangular-shaped optical pulse using the method of passive nonlinear pulse shaping. High-quality triangular-shaped optical pulses can be obtained for a wide range of different pulse prechirping values. There is a good tolerance of triangular pulse formation to the ND fiber length and this tolerance is affected by the pulse prechirping value. Some primary experimental researches are implemented as well, on the applications of triangular-shaped optical pulses in wavelength converters based on self-phase modulation (SPM) and cross-phase modulation (XPM) in fiber.