Ultrafast, Tunable, High-power and Intelligent Laser Pulse Generation by Fiber Laser

Abstract

This year marks the 64th year since the laser was invented. A laser is a coherent radiation source in the light spectrum region with many distinctive features which have enabled their use in numerous practical applications that have been of tremendous benefit to humanity. Continuous efforts are still being paid on advancing laser performance in several foremost aspects. Ultrafast laser pulse with high pulse energy is one main goal for laser development. Novel wavelength enables laser to solve more diverse and complex problems. Tailoring laser operation regimes makes laser suitable for certain specific applications to improve their performance. Intelligent fast laser design and operation increases effectiveness and efficiency of operating a laser. This thesis work presents the investigations into the above several aspects of laser development.

Wave-breaking free laser pulses hold the key to accommodate high pulse energy. Similariton fiber laser is found to have the resistance to wave-breaking and tolerate high nonlinear phase shift accumulation. A 1 μm Yb-doped similariton fiber laser is built using a hybrid mode-locking technique incorporating both a nonlinear polarization rotation mechanism and a frequency-shifting feedback mechanism. It produces a 7.8 ps pulse with 20.5 nm spectral bandwidth after the laser cavity and the pulse can be further compressed to 140 fs. The starting dynamics of the mode-locked laser are investigated using the dispersive Fourier transformation method. The numerical simulation reveals the pulse formation dynamics within the laser cavity.

To get access to the traditionally forgotten short wavelength emission band (<1800 nm) of Tm-doped fiber lasers, which is highly desirable for multiple applications, wavelength-tunable dissipative soliton operation from 1.7 μm to 1.9 μm is achieved in a dispersion-managed fiber laser cavity. The picosecond laser pulse after the laser cavity can be compressed to hundreds of fs using a grating pair. The chirped pulse amplification technique is subsequently applied to an optimized seed laser, and a wavelength-tunable chirped pulse amplification system is realized from 1720 nm to 1800 nm. This is the first wavelength-tunable chirped pulse amplification system beyond 1.1 μm, providing a new technically feasible route for constructing a wavelength-tunable chirped pulse amplification system. A numerical simulation was performed to investigate the pulse evolution dynamics in the chirped pulse amplification system.

Noise-like pulse operation is distinct due to the relatively low coherence property of the emitted pulses. The property can help in imaging applications to mitigate speckle formation and in optical coherence tomography to increase the axial resolution. Using an all-anomalous-dispersion Tm-doped fiber laser cavity, wavelength-tunable noise-like pulse operation covering the full-gain-bandwidth of the Tm-doped fiber from 1650 nm to 2100 nm is realized. The spectral widths of the noise-like pulse after the laser cavity range from 13.8 nm to 18.8 nm with an average output power from 63.3 mW to 213 mW. The dispersive Fourier transformation method is used to reveal how the noise-like pulse builds up. The wavelength-tuning range is the broadest among tunable noise-like pulse lasers.

Neural network algorithms as a general model to solve regression problems can tackle traditional physics questions in a much more efficient way. The ability of a feed-forward neural network to directly predict laser cavity output from the laser cavity parameters is demonstrated, paving the way for intelligent laser cavity design. Laser output spectra and temporal pulse profiles can be accurately predicted with a normalized root mean square error of less than 0.04 within only a 5 ms time frame. In contrast, a conventional iterative laser cavity simulation based on a generalized nonlinear Schrödinger equation may take thousands of times longer. The influence of the number of neurons and layers on prediction performance is also studied.

The studies improve fiber laser performance in different aspects, deepening the laser physics for laser design and fulfilling different requirements for practical applications. The results also open broad perspectives for further advancement in the field of the ultrashort pulse fiber lasers with innovative and intelligent operation.