Principle Of Fiber Femtosecond Laser And Its Application In The Length Measurement

Nowadays, ultrashort pulse laser source is an active research front not only in nonlinear optics and ultrafast optics but also in, as a key technology, realizing ultra high speed optical communications. Research on fiber-based ultrashort pulse laser has great significance. By using the nonlinear polarization rotation technique, the fiber laser can operate with tunable wavelength, multiwavelength, or mode-locked. Thus the fiber laser attracts most researchers because of their many advantages such as structure compact, a good beam quality, a high coupling efficiency and a low price.Starting from the time domain mode-locking master equation and NLSE (Nonlinear Schrodinger Equation), we analyze the mechanism of fast saturable absorber for obtaining ultrashort pulses and fully discussed the influences of cavity parameters on the realization of mode-locking. The parameters of mode-locked pulse are simulated numerically by the Master equation. We conclude that stable mode-locked pulse can form in the cavity with both positive and negative dispersion. According to coupled nonlinear Schrodinger equation, we simulate the evolvement process of pulse in the cavity and the affect on pulse width by the change of dispersion in the cavity.We developed mode-locked laser based on optical fiber polarization controller, the nonoptical fiber polarization controller, and the saturated absorber, and obtained the pulse which the pulse width for femtosecond magnitude, the average energy of the pulse output can get to dozens of mw. Also we have observed a variety of pulse output state.Since the transit time in cavity corresponding to the fundamental frequency is exactly linearly proportional to the cavity length of the mode-locked fiber laser, a novel simple method to measure the fiber length has been experimentally demonstrated using a mode-locked fiber laser configuration. Compared to presented methods, there is no limitation in measuring fiber length over hundreds of kilometers due to low insertion loss and direct splicing into an all-fiber system. It has a large measurement range and a high resolution on the order of centimeters. We call this as the mode-locking method.