Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering (Holcomb Dept. of)
The emerging applications of LiDAR, microresonator based frequency comb, and photon pair generation in photonic integrated circuits (PICs) have attracted lots of research interests recently. The single frequency, high power, narrow-linewidth, tunable semiconductor lasers are highly desired for the implementation of these emerging applications in future PICs. In this dissertation, we use the hybrid integration via edge coupling to obtain the integrated diode lasers for future PICs, since the active chip and the passive chip can be fabricated and optimized independently.
We demonstrate hybridly integrated narrow-linewidth, tunable diode lasers in the Indium Phosphide/Gallium Arsenide-silicon nitride (InP/GaAs-Si3N4) platform. Silicon nitride photonic integrated circuits, instead of silicon waveguides that suffer from high optical loss near 1 µm, are chosen to build a tunable external cavity for both InP and GaAs gain chips at the same time. Single frequency lasing at 1.55 µm and 1 µm is simultaneously obtained on a single chip with spectral linewidths of 18-kHz and 70-kHz, a side mode suppression ratio of 52 dB and 46 dB, and tuning range of 46 nm and 38 nm, respectively. The resulting dual-band narrow-linewidth diode lasers have potential for use in a variety of novel applications such as integrated difference-frequency generation, quantum photonics, and nonlinear optics. We also demonstrate one potential application of the dual-band diode laser in beam steering. The dual-band diode laser combined with a waveguide surface grating can provide the beam steering by tuning the wavelength of the light signal.
However, the output power of the hybridly integrated diode lasers is still limited. Integrated coherent beam combining (CBC) is a promising solution to overcome this limitation. In this dissertation, coherently combined, integrated diode laser systems are experimentally demonstrated through hybrid integration. A chip-scale coherently combined laser system is experimentally demonstrated in the InP-Si3N4 platform through the manipulation of optical feedbacks at different output ports of the coupled laser cavities. Coherent combining of two InP-based reflective semiconductor amplifiers is obtained by use of the cross-coupling provided by an adiabatic 3 dB coupler in silicon nitride, with a combining efficiency of ~92%. The novel system not only realizes the miniaturization of coherent laser beam combining but also provides a chip-scale platform to study the coherent coupling between coupled laser cavities.
Besides, the emerging platforms (i.e., gain chips based on semiconductor quantum dots, silicon-carbide-on-insulator and lithium-niobate-on-insulator) have attracted intense interests in recent years. The hybridly integrated diode lasers through edge coupling are demonstrated in these emerging platforms.
In addition, we study the Parity-Time (PT) symmetry in the chip-scale hybrid platform. PT symmetric coupled microresonators with judiciously modulated loss and gain have been widely studied to reveal many non-Hermitian features in optical systems. The phase transition at the exceptional points (EPs) is a unique feature of the PT symmetric non-Hermitian systems. In this dissertation, we propose and demonstrate an electrically pumped, hybridly integrated chip-scale non-Hermitian system, where the optical gain, loss and coupling are separately controlled to allow for the PT symmetry breaking and direct access of the EPs. We use the coupled Fabry-Perot resonators through the hybrid integration of two InP active chips with one Si3N4 passive chip to realize the versatile control of the gain and loss. We first demonstrate the PT symmetry breaking and access of the EPs by investigating the spectral and spatial transition processes of the hybrid system induced by the asymmetric gains in the InP active chips. We then control the loss distribution in the Si3N4 passive chip so that the system loss contrast exceeds the coupling coefficient, which leads to the PT symmetry breaking and coherent addition of the two coupled lasers. Our integrated non-Hermitian optical system in the chip-scale hybrid integration platform successfully bridges the non-Hermitian physics and photonic integrated circuits and is able to expand the practical applications of non-Hermitian optical systems to a whole new stage.
Zhu, Yeyu, "Hybridly Integrated Diode Lasers for Emerging Applications: Design, Fabrication, and Characterization" (2019). All Dissertations. 2510.