Abstract
This dissertation presents three all-fiber designs of passively mode-locked lasers in order to achieve high pulse energy, environmentally-stable dissipative soliton (DS) operation in all-normal-dispersion cavities. A numerical model for DS mode-locked fiber lasers based on the nonlinear Schrodinger equation has been used to guide the experimental designs. Firstly, an environmentally-stable and ultra-compact SESAM mode-locked fiber laser is demonstrated. The all-fiber design is realized using a mode-field-adaptor (MFA) to couple light onto the SESAM. A polarization-maintaining fiber loop mirror serves multiple functions as a highly reflective mirror, an output coupler and polarization selector. Self-starting and stable DS mode-locking operation is achieved with 1.7 nJ pulse energy and a 22 ps pulse width. Secondly, an ultra-stable DS mode-locking was demonstrated in a long cavity ring laser with a nonlinear amplified loop mirror (NALM) as a mode-locking device. The output pulses of 32 nJ, 615 fs de-chirped pulse width were obtained with the Raman signal suppressed below -20 dB in a 81 m long cavity. The mode-locking is self-starting and the mode-locked pulse train shows excellent stability. Thirdly, the mode-laser cavity was extended with a piece of large-mode-area (LMA) fiber with a low dispersion to further scaling up the pulse energy to 56.8 nJ. The laser pulses were compressed to 750 fs by a pair of volume gratings. In the processing of scaling-up the pulse energy of the NALM mode-locked fiber laser, some interesting physical phenomena were observed, such as the operation regime transition from noise-like to DS with a sudden reduction of Raman signal and a unique waves-splitting with a stable temporal spacing. The phenomena were studied and explained in this dissertation. In addition to the mode-locked fiber laser, a CW and a Q-switched fiber lasers were also designed with a single-mode- multimode- single-mode (SMS) filter as an effective mean of overcoming nonlinear effects. The transmission spectral property of the SMS was studied which fits well with theoretical calculation. One high efficiency SMS CW fiber lasers and one SMS Q-switched fiber laser were designed which showed the effectiveness of the SMS filter for inhibiting the SRS and significantly reducing SPM.
Highlights
Ultrafast optics have for decades been a very remarkable research field, and for last ten years ultrafast mode-locked fiber lasers have found themselves numerous applications in fundamental researches as well as in industrial and medical applications, such as micromachining [1], optical imaging [2], fiber frequency combs [3], supercontinuum generation [4] and terahertz generation [5]
Self-starting and stable dissipative soliton mode-locking operation is achieved with 1.7 nJ pulse energy and 22 ps pulse width
Our results further show that, in addition to the interactions between gain, nonlinearity and dispersion in the main loop, the transfer function of nonlinear amplified loop mirror (NALM) at different power levels is critical to the state of mode-locking
Summary
Ultrafast optics have for decades been a very remarkable research field, and for last ten years ultrafast mode-locked fiber lasers have found themselves numerous applications in fundamental researches as well as in industrial and medical applications, such as micromachining [1], optical imaging [2], fiber frequency combs [3], supercontinuum generation [4] and terahertz generation [5]. Broderick et al reported an all-fiber mode-locked laser with an NALM which achieved environmental-stable output with pulse energy of 16 nJ [31, 35, 36], much higher than those of SESAM mode-locked lasers Their studies suggested that increasing the cavity length could be an effective method for scaling-up pulse energy and avoiding the wave-breaking as the NALM would amplify the stretched pulses. Unlike traditional solitons which can be formed by the balance of nonlinear and anomalous dispersion inside the laser cavity [18], DS is based on a composite balance between several factors, such as chromatic dispersion, Kerr nonlinearity, bandwidth-limited gain from doped gain fibers, and intensity-dependent loss from saturated absorbers and spectral filtering [79] The interplay among these physical effects would lead to stable single pulse dissipative solitons, and produces some multi-pulsing phenomenon, such as soliton molecules, soliton rains and harmonic mode-locking [79]. The stability of such multi-pulsing has not been reported
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