Widely-tunable and ultra-stable hybrid-integrated diode lasers
Albert van Rees is a PhD student in the department Laser Physics & Nonlinear Optics. (Co)Promotors are prof.dr. K.J. Boller and dr. P.J.M. van der Slot from the faculty of Science & Technology.
This thesis describes our investigation into diode lasers based on hybrid integration with frequency-selective waveguide feedback circuits. Deploying intra-cavity ring resonators as feedback filters, implemented using low-loss Si3N4 waveguides, enables single-mode laser operation with wavelength tunability over very wide ranges. Using high-Q ring resonators for cavity length extension is also very effective for improving the laser’s short-term frequency stability, which shows as an ultra-narrow intrinsic (Schawlow-Townes) linewidth. We demonstrate in this thesis that these lasers are also continuously tunable, that the spectral coverage can be extended from the infrared to the visible range, and that these lasers can be frequency locked to optical references to greatly enhance their long-term frequency stability.
In chapters 1 and 2 we describe an introduction to the field of chip-integrated lasers and the relevant theory regarding optical feedback circuits and laser frequency stability. In chapter 3, we present a novel method to increase the range of continuous tuning for hybrid-integrated diode lasers, based on feedback from ring resonators. Although laser cavity length extension using high-Q ring resonators is highly effective for obtaining ultra-narrow linewidths, it also decreases the wavelength range of continuous laser tuning that can be achieved with a given phase shift of an intracavity phase tuning element. To increase the range of continuous tuning to that of a short equivalent laser cavity, while maintaining the ultra-narrow linewidth of a long cavity, we describe an analytical model for synchronous tuning of the ring resonators with an intracavity phase tuning element. This method is confirmed with an experimental demonstration by recording Doppler broadened absorption lines of acetylene (C2H2) molecules using a hybrid-integrated InP-Si3N4 diode laser. The laser has a 120-nm coverage around 1540 nm, a maximum output power of 24 mW, and a lowest intrinsic linewidth of 2.2 kHz. We demonstrate a six-fold increased continuous and mode-hop-free tuning range of 0.22 nm (28 GHz) as compared to the free-spectral range of the laser cavity.
In chapter 4 we present the first realization of a hybrid-integrated diode laser in the visible spectral range. Before this, all previous realizations of such lasers had been restricted to the infrared. Providing chip-sized diode lasers to generate visible light with wide tunability and high frequency stability is of great interest for applications in biophotonics, precision metrology and quantum technology. For this demonstration in the 685-nm (red) wavelength range, we coupled an AlGaInP optical amplifier with a Si3N4 circuit for feedback from a ring resonator-based spectral filter. The laser delivers up to 4.8 mW output power, has a spectral coverage of 10.8 nm around 684.4 nm wavelength and exhibits a low intrinsic linewidth of 2.3±0.2 kHz.
In chapter 5 we describe our results on long-term frequency stabilization of a hybrid-integrated laser with a ring-resonator based extended cavity, by electronic locking to two optical references. Although feedback from high-Q ring resonators improves the laser’s short-term frequency stability, long-term stability is often also required. By locking a widely tunable hybrid-integrated laser with a central wavelength of 1550 nm to suitable references, namely a fiber-based optical frequency discriminator (OFD), and an acetylene absorption line, we greatly enhance the laser’s long-term frequency stability. Locking the laser to the OFD improves the laser's fractional frequency stability down to 1.5∙10-12 over an averaging time of 0.5 ms. Locking the laser frequency to an acetylene absorption line reduces the residual frequency deviations of the laser to a range of less than 12 MHz within 5 days.
The thesis is concluded with chapter 6, where we provide an outlook to the exciting developments that can be expected regarding these widely-tunable and ultra-stable hybrid-integrated diode lasers.
