Yuan, Bocheng (2025) Millimetre-wave and terahertz signal generation based on monolithic integrated dual-wavelength semiconductor DFB lasers. PhD thesis, University of Glasgow.
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Abstract
The high-frequency RF signals generated using the optical beat frequency method exhibit significant design flexibility in terms of frequency. With the advancement of photonic integration technology, these devices achieve a high level of integration, enabling the generation and modulation of signals to be completed chip level devices. Typically, the beat frequency generation process requires two independent single-wavelength lasers, necessitating additional design considerations to ensure the polarization and phase relationship between the two devices. Additionally, at least two power sources are needed to pump the two devices separately, which means that external noise is more likely to affect the phase correlation between the two independent optical signals.
This thesis presents the design, fabrication, and characterisation of a series of monolithically integrated dual-wavelength lasers. The main feature is that two lasing wavelengths are simultaneously emitted within a single resonant cavity, resulting in the two lasing modes naturally having the same polarisation. Lasers and their arrays which achieved beat frequencies range from 50 GHz to 1 THz were fabricated based on three different grating structures: two π phase shifts with equivalent chirped gratings, sidewall grating with lateral modulation of grating coupling coefficient, and superimposed sampled Bragg gratings. For devices operating at terahertz frequencies, the frequency interval can be precisely controlled. With a grating pattern precision of 0.5 nm, the theoretical minimum beat frequency interval can reach 1.1 GHz.
Furthermore, by injecting an external optical frequency comb’s sub-harmonic to the integrated electro-absorption modulator, the phases of the two lasing modes are locked. The experiment recorded two frequencies, 67.75 GHz and 136 GHz, both of which exhibited a linewidth of 1 Hz in the locked state. The phase noise test results indicate that the phase noise of the generated signal adheres to the inherent attenuation law of the frequency comb. This confirms that the injection locking of a dual-wavelength laser can successfully multiply the frequency of an external RF signal without introducing additional noise.
Experimental results confirm that the monolithically integrated dual-wavelength laser possesses a wide design range for beat frequencies. Moreover, after locking, the generated millimetre-wave and THz signals exhibit extremely narrow linewidths and very low phase noise, making the system highly stable and precise for high-frequency applications. This device has significant potential to become a crucial component in microwave/THz photonic integration.
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