The thesis consists of four chapters. In Chapter 1, the motivation of this research and the overview of mode-locked fiber lasers will be introduced.
Chapter 2 is devoted to the principles of mode-locking techniques. Chapter 3 shows the structure of the experiment and the discussion of our results. Finally, Chapter 4 gives the conclusions and future expectations.
Reference
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Chapter 2 Principles
2.1 Polarization Additive Pulse Mode-Locking
The additive pulse mode-locking (APM) [2.1] is a common technique for passive mode-locking of lasers. It employs nonlinear Kerr effects in the cavity to attain pulse shortening. The Kerr effects induce different nonlinear phase shifts on the peak and the wings of the pulses. Constructive interference then occurs near the peak of the pulses and destructive interference occurs near the wings. In this way, the pulse-width is shortening. APM is easily achieved in fiber lasers because of the strong nonlinear self-phase modulation (SPM) effects that arise from the small mode field diameter of the fiber.
The particular scheme used in fiber lasers for implementing APM is called the polarization additive pulse mode-locking (P-APM) [2.2]. If the optical pulse in an isotropic optical fiber is with an elliptical polarization state, the nonlinear polarization rotation (NPR) effects will occur for this polarization state due to the nonlinear Kerr effects in the fiber. Fig.2-1 shows the illustration of NPR.
Fig. 2.1 Nonlinear polarization rotation in Kerr medium
Fig. 2.2 P-APM technique in a fiber ring laser
Nonlinear polarization rotation induces different nonlinear phase shifts on different circular polarization components. The orientation of the polarization is rotated by the nonlinear medium and the quantity of rotation depends on the intensity of the light. Fig. 2.2 shows the schematic of one realization of a fiber ring laser. A polarization dependent isolator (PD-ISO) is for single direction propagation and also serves as a polarizer-analyzer. PC is the inline polarization controller. The following is the principle of the P-APM technique. The light wave is first linearly polarized by the PD-ISO in position ○1 . Then, the PC1 elliptically polarizes the light wave in position ○2 .The ellipse can be
When the light wave propagates from ○2 to ○3 , if the light intensities between two circular components are different, the lightwave experiences different amounts of nonlinear phase shifts for each component. The light polarization orientation starts to rotate, which is known as the nonlinear polarization rotation.
Finally, at the position ○4 of the ring cavity, the PC2 orients the pulse and selects the lightwave polarized in a particular direction so that the peak of the pulse passes through the analyzer while the wings of the pulse are extinguished.
Therefore the ultra-short pulse can be created.