| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 102 === With the flexible fiber patch cord as the output coupler, it is considerably convenient to focus the soliton pulses delivered from the passively mode-locked erbium-doped fiber lasers (EDFLs) on where the users want. Therefore, it is great potential to utilize the passively mode-locked EDFL as the ultrafast light source for many applications. To fulfill the demands of compact and inexpensive ultrafast lasers, passively mode-locked EDFLs based on nonlinear polarization rotation mode-locking (NPRML) mechanism have been developed.
In this thesis, we demonstrated that with the assistance of weak or strong polarization-dependent loss (PDL) in the cavity, NPRML-EDFL shows the transformation on the pulse from single to multiple bunched state with nearly one order of magnitude reduction on pulsewidth. With the bent intracavity fiber providing the weak PDL, the NPRML only shortens the pulsewidth from 5.3 ps to 4.9 ps and correspondingly broadens the spectral linewidth from 0.43 nm to 0.56 nm when enlarging the pump power from 100 mW to 325 mW. With the use of an inserted polarizer providing strong PDL in the EDFL, the fundamental soliton pulsewidth is significantly compressed to 390 fs with the spectral linewidth as wide as 7.14 nm. In particular, the parameters of the soliton pulses is nearly unchanged at different pump powers; however, the soliton pulses splits itself to form the tightly bunched pulses circulating in the EDFL cavity. There are as many as 18 solitons tightly bunched together at the maximum pump power of up to 325 mW. Such a tightly bunched package can be elucidated by the soliton energy quantisation and the long-range soliton interaction according to the perturbation theory in a passively mode-locked EDFL.
Moreover, based on the combined effects of the fiber birefringence and the cavity filtering in the EDFL, the central wavelength tunable NPRML-EDFL is developed. By bending the intracavity fiber to induce the refractive index difference between the fast and slow axes, the wavelength tunable range is 2.9 nm at 1570 nm regime and 10.2 nm at 1600 nm regime. The difference of the tunable range at these two bands is attributed to the fact that the gain narrowing effect is influenced by the shape of the gain profile. On the other hand, the central wavelength shift under the different pumping geometries with the same pump power is also investigated. At 1570 nm regime, the central wavelength offset between the forward pumping and backward pumping condition is 5.9 nm, whereas the forward and backward pumping induced central wavelength offset is only 1.1 nm when mode-locking at 1600 nm regime. Such a difference originates from that the gain spectrum is dependent on the spatial distribution of the excited erbium ions under different pumping schemes. Besides the gain-saturation induced refractive index change, the birefringence variation and the gain spectral variation induced dual-band central wavelength tenability of the NPRML-EDFL is observed and discussed.
Finally, the timing jitter and the intensity stability of the output pulse-train at different intracavity nodes are investigated by operating the NPRML-EDFL under the tightly bunched multi-soliton state. When measuring the output delivered from the node after the polarized mode-locker, the average output power is 3.59 mW and the pulsewidth of the solitons is 400 fs at the pump power of 325 mW. It shows that the multiple soliton pulses at the tightly bunched state are relatively stable, which exhibits the energy fluctuation of 2.7×10-3 and the timing jitter of 31 ps. In contrast, when the output node of the pulse-train is just selected after the EDF amplification, it leads to a larger average output of 7.98 mW at the same pump power. The external quantum ef&;#64257;ciency can be enhanced by 2.1 times. At this node, the pulsewidth of the tightly bunched solitons can be compressed from 400 fs to 380 fs due to the gain-induced GVD of the EDF and the negative chirp parameter of the soliton pulse. Moreover, it exhibits even lower energy fluctuation of 1.6×10-3 and smaller timing jitter of 12 ps. These observations signify that the soliton pulses in the tightly bunched state delivered from the output port after the EDF exhibit the better external quantum ef&;#64257;ciency and less intensity noise and phase noise.
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