Novel Approaches to Power Scaling In Mode-Locked Laser Diode Based Ultrashort Pulse Sources
In this thesis, two novel approaches to upscaling the average output power of mode-locked laser diode ultrashort pulse sources were investigated. This was done with the motivation of developing mode-locked laser diode based light sources for nonlinear bioimaging applications to which high repetition rate pulse sources such as diode lasers could be of great benefit. The first approach to power scaling that was examined built upon the common approach of amplifying pulses from a mode-locked laser diode using a separate semiconductor optical amplifier. In this work, a tapered amplifier was used in a double-pass regime to amplify picosecond pulses for the first time. The double-pass amplifier regime was compared to a standard single-pass amplifier set-up using the same semiconductor optical amplifier and was found to have an increased signal gain of 4dB on average, produce higher peak and average power pulses as well as reduce the amplified spontaneous emission noise in the amplified signal. The tapered amplifier that was used in this study had a two section contact layout which allowed inhomogeneous pumping of two separate sections of the amplifier. This allowed the pulse characteristics such as the duration, centre wavelength and spectral bandwidth of amplified pulses to be tuned within a given range while maintaining a constant average power. The tapered amplifier had a chirped quantum dot active region which sparked a further investigation into the superluminescence output of the solitary two-section tapered amplifier. The second approach to boosting the power of diode based ultrashort pulse sources involved increasing the power of mode-locked laser diodes directly. This was done by implementing a multi-active region structure in which two separate quantum well active regions were cascaded together in a single monolithic device via a thin tunnel junction. A simple two-section, narrow ridge laser which had 2 active regions that were cascaded together via a thin tunnel junction was used to demonstrate mode-locking in a device with a multi-active region structure for the first time. The mode-locking dynamics of the multi-active region laser was investigated and several common behaviours were observed along with less common behaviours such as bistability within the mode-locking region. This demonstration of mode-locking in a double active region laser could be the first step towards nonlinear imaging by a stand-alone laser diode.
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