Toward Hybrid Integrated Rare-Earth Doped Passively Mode-Locked Lasers on a Chip

Abstract

Mode-locked lasers (MLLs) have transformed a wide range of scientific fields and real-world applications, from precision material processing to advanced medical surgery, due to their ability to generate ultrashort pulses with high peak power. The significance of MLL technology is highlighted by the awarding of four Nobel Prizes for MLL-related research in the last 25 years. However, commercial benchtop MLLs face challenges in scalability and mass production, making integrated solutions more appealing for today's advanced applications like portable devices. Rare-earth doped MLLs stand out as a candidate due to the longer upper-state lifetime compared to semiconductor gain medium, but fully integrated versions are yet to be demonstrated as no single material system can conveniently offer all the required functionalities, and hybrid integration of multiple technologies is inevitable.

This PhD research focused on realising the essential components for constructing an integrated rare-earth doped passively MLL other than the pump laser. To achieve this, firstly, the fabrication tolerant passive functionalities, such as a wavelength division multiplexing (WDM) coupler and a Mach-Zehnder interferometer (MZI) based spectral filter, were demonstrated on the 3% index contrast low loss germanosilicate (GeSiO2) platform, which serves as a base for stacking the Er-doped tellurium dioxide (TeO2) amplifier.

A breakthrough was achieved in fabricating Er-doped TeO2 films at room temperature. Through a comprehensive investigation of the contamination effect on the film, a lifetime of ~1 ms was preserved at high Er3+ dopant concentrations (1021 ions/cm3), showing performance comparable to bulk glasses for the first time. This led to the development of etch-less in-line Er-doped amplifiers using vertical taper technology via shadow mask deposition. Before fabricating the amplifier, the fully etched pure TeO2 waveguide was studied for the first time, demonstrating an experimental propagation loss of 0.12 dB/cm with normal dispersion, making it suitable in realising the MLLs with large pulse energy output. By overlaying the Er-doped TeO2 film on the fully etched TeO2 waveguides, the initial integrated Er-doped amplifier experimentally achieved over 15 dB net gain in a 6 cm Er:TeO2 waveguide using 1480 nm pumping, corresponding to a net gain per unit length of 2.5 dB/cm.

The waveguide saturable absorber (SA) using graphene oxide (GO) was also fabricated, showing a modulation depth of about 6 %, which is comparable to previously demonstrated GO-based SA used in fibre MLLs.

Numerical studies using the performance from the previously demonstrated essential components confirmed the feasibility of the proposed integrated MLL, even under challenging fabrication scenarios. The optimal laser cavity design can produce an output peak power exceeding 1 kW with a dechirped pulse duration of ~700 fs. Unfortunately, due to the severe interruption caused by COVID, the experimental realisation of the on-chip MLL is not complete. Show more