Multi-core Photonic Crystal Fiber Femtosecond Laser System And Its Applications In Frequency Conversion

Fiber lasers have been the subject of intense research and development recently because of their significant advantages, such as compact size, high stability and good beam quality. However, the traditional fibers always have a small mode area. Consequently, the accumulation of excessive nonlinear phase shift appears as an important limitation for power scaling. The work of this thesis focus on fiber laser power scaling based on the multi-core photonic crystal fiber. Firstly, the multi-core photonic crystal fiber structure and mode properties were numerically studied. Then, the mode-locked multi-core photonic crystal fiber oscillator and amplifier were experimentally studied. The high power femtosecond laser applications in frequency conversion were also experimentally investigated. The main points are as follows:1. A hybrid multi-core photonic crystal fiber is proposed. The hybrid fiber has a flat in-phase supermode which keeps Gaussian shaped distribution in the far-field. The maximum gain is achieved for the in-phase supermode and is important for in-phase supermode selection.2. Using the 18-core photonic crystal fiber, the mode-locked multi-core fiber oscillator is experimentally investigated. As far as I know, this is the first time mode-locked multi-core fiber laser is realized. Laser pulses with average power as high as 3.3 W and 70 dB signal-to-noise ratio are obtained. A multi-pass cell is added to reduce the repetition rate and the pulse energy can be as high as 180nJ with 690 fs pulse duration after out of cavity compression.3. Using the Yb-doped large mode area photonic crystal fiber and 7-core photonic crystal fiber, the dual-stage femtosecond fiber amplifier system is experimentally investigated. As far as I know, this is the first time femtosecond laser pulses are propogated and amplified in multi-core fiber. We obtain 24W, 1MHz output pulses which can be compressed to 110fs using a grating pair. The corresponding pulse peak power reaches up to 150 MW and is the highest peak power obtained in the fiber laser amplifier based on the nonlinear amplification mechanism. Based on the 18-core photonic crystal fiber, we have obtained 50W, 50MHz pulse output and the pulse duration is 80 fs after compression. The self-phase-locking mechanism is understood through numerical modeling.4. Based on the large mode area photonic crystal fiber laser amplifier and the low loss fusion technique, we generate power enhanced supercontinuum. The output supercontinuum power is as high as 6.2W which is the highest power at that time.5. A new technique for high power femtosecond pulse frequency conversion is proposed and experimentally investigated with optical crystal and photonic crystal fiber. In the visible regime, with the generated high power supercontinuum and frequency-doubling technique based on BBO crystal, we obtain tunable source with wavelength ranging from 510 to 660 nm; in the infrared, with all-solid photonic bandgap fiber, wavelength-tunable soliton source with wavelength ranging from 1160 to 1260 nm is obtained. The soliton with the central wavelength of 1260 nm has an average power as high as 320mW, pulse energy of 6.4 nJ and pulse duration of 84 fs. This technique does not require complicate adjustment which is great advantage compared to other frequency conversion technique.