SEMICONDUCTER-GLASS WAVEGUIDE HYBRID LASERS WITH ULTRA-HIGH SPECRAL PURITY
Youwen Fan is a PhD student in the MESA+ research group Laser Physics and Nonlinear Optics. His promoter is Klaus Boller.
In this thesis we describe investigations of a novel class of semiconductor-glass waveguide hybrid lasers. We show that such lasers provide an unprecedented spectral purity as well as tunability among all chip-based diode lasers.
The concept of the investigated hybrid lasers is based on spectral control of light generated with a laser diode using highly frequency selective, tunable optical feedback provided by an integrated photonic circuit. In this work, we have employed InP laser diodes that generate light at around 1.5 μm wavelength, as an important as well as representative example. The integrated waveguide feedback circuits are based on microring resonators (MRRs). The waveguide platform selected for the feedback circuits utilizes Si3N4 as the waveguide core material and SiO2 as cladding material (TriPleXTM), because this provides exceptionally low propagation loss in combination with high index contrast, to enable large optical path lengths (for linewidth narrowing) and sophisticated circuit geometries in a compact, chip-sized format. The main advantages of feedback using low-loss microring resonators, when compared to conventional optical feedback using integrated Bragg gratings, are i) ease in fabrication without requiring complex re-growth nor fine lithography; ii) a significant enhancement of the effective length associated to multiple passes at resonance. Thereby MRRs extend the overall optical length of the laser cavity which narrows the laser emission linewidth; iii) a spectrally sharp filtering characteristic.
A fundamental and well-known property of single-frequency lasers based on resonators is that the spectral purity of the output increases, i.e., its spectral bandwidth decreases, with increasing the photon lifetime in the resonator. In diode lasers this can be achieved to a certain extent by lowering the optical roundtrip losses, while the most effective method is via increasing the resonator length. In Chapter 3, we present a first version of a hybrid laser that possesses reduced roundtrip losses as achieved with an improved optical coupling between the diode and silicon nitride waveguide mode fields. The optical length of the cavity is increased appreciably from about 1.4 mm (of the solitary diode) to about 9.4 mm. In this particular hybrid laser, to achieve single-frequency oscillation over a wide tuning range via exploiting the Vernier effect, the spectral feedback filtering is based on two small, tunable MRRs of slightly different size (49.5 and 54 μm radius). Already with the MRRs having moderate quality factors (Q≈2000) the laser shows a strong linewidth reduction, to a 3-dB linewidth of about 24 kHz. With electric tuning of the optical lengths of the MRRs via the thermo-optic effect, the laser wavelength becomes tunable over a range of 45 nm, thereby covering a wide range of the laser's gain spectrum.
To explore the potential of using narrowband amplification as well as frequency and phase synchronization, such as for application in integrated microwave photonics, in Chapter 4 we report the first injection locking experiments with a hybrid laser. We have used these experiments to estimate the Q-factor of the laser cavity. A possible future use of injection locking is the measurement of losses in laser resonators also of complex design, such as the hybrid lasers described here are based upon.
Most of the investigations thus far reported are carried out with the two chips fixed on separate sub-mounts for alignment with regard to each other. However, the full future application potential of so-called hybrid lasers can only be gained with optical integration, i.e., firmly and permanently mounting the chips to each other with high precision and strong optical coupling. In Chapter 5 we report the first integration of an InP- Si3N4 hybrid laser. Based on mode overlap calculations and laser rate equations we demonstrate that optical integration of such lasers is feasible with current mounting technology. The integrated hybrid laser shows a clear single-frequency oscillation with wide tunability, and with narrow linewidth at the 100 kHz-level throughout the tuning range. This proves that the underlying facet-to-facet (longitudinal) mode coupling between chips is a viable integration concept.
Systematically and efficiently pursuing the concept of semiconductor-glass waveguide hybrid lasers also requires that the output properties of such lasers can be theoretically predicted. Only when the many parameters entering the functional design of the waveguide feedback circuits are properly chosen based on modeling, a proper performance, such as single-frequency operation and a narrow linewidth, can be expected after fabrication and integration. To aid this process, in Chapter 6 we presented the first theoretical analysis of such hybrid lasers that includes a spatially resolved modeling of the gain section. Using numerical solutions of a transmission line model, a hybrid laser based on a dual-MRR Vernier feedback circuit is investigated. We calculate the laser's frequency noise spectra and obtain the laser's intrinsic linewidth vs the output power. We compare the results with previous theory that takes a spatially inhomogeneous gain distribution in the semiconductor into account via a modification of the mean-field approximation. The comparison shows that higher losses in the optical coupling between the chips requires also spatially-resolving models for index related broadening effects, to enable a proper prediction of the laser linewidth.
Finally, in Chapter 7 we describe how the expertise gained with modeling, laser-design and experimental investigation is used to realize a semiconductor-glass hybrid laser with a record-low spectral linewidth. Making use of a feedback circuit that involves three MRRs in series and double pass we demonstrate a laser linewidth as low as 290 Hz, which is clearly the narrowest intrinsic linewidth that has ever been achieved with a chip-based diode laser. The same laser also shows a record-wide wavelength tunability, more than 80 nm, i.e. fully covering the gain bandwidth of the laser diode with appreciable output power (up to 13 mW) coupled out of a single mode fiber attached to the laser.
