University of Twente Student Theses


Continuous wavelength tuning of an integrated extended-cavity laser beyond its mode spacing limit

Rees, Albert van (2019) Continuous wavelength tuning of an integrated extended-cavity laser beyond its mode spacing limit.

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Abstract:Chip-sized diode lasers having both an ultra-narrow linewidth and a large continuous wavelength tuning range are of high interest for many applications. Ultra-narrow intrinsic laser linewidths can be obtained by hybrid integration of a semiconductor amplifier with a low-loss dielectric waveguide-based feedback circuit. This integrated extended-cavity provides spectrally-selective feedback and a significant extension of the cavity length, which narrows the laser linewidth. However, as the length of the laser cavity increases, the cavity mode spacing decreases, which causes undesired instabilities in the form of mode hops when tuning the laser wavelength. We overcome these instabilities and experimentally demonstrate a new approach towards mode-hop-free tuning of such lasers, well beyond the standard limit that is formed by the cavity mode spacing. This approach addresses the conflicting demands on the laser cavity length resulting from the requirement of both a narrow intrinsic linewidth and a large continuous tuning range. Furthermore, we present for the first time an analytical model that describes the mode-hop-free tuning as realized with this approach. After calibration, this model can be used to accurately predict the laser wavelength anywhere within its mode-hop-free tuning range. We demonstrate this by measuring several absorption lines of acetylene with a high accuracy and resolution. The hybrid laser used in the experiments comprises an InP amplifier and a Si3N4 waveguide feedback circuit with a tunable phase section, and a Vernier filter comprising two tunable microring resonators of slightly different radii. When tuning the laser by tuning the phase section only, this results in mode-hop-free tuning only over a single free spectral range of the laser cavity, which is 0.034 nm. To overcome this tuning limit, we synchronously control the phase section and both rings with a proper ratio, resulting in a mode-hop-free tuning range increase by a factor of 6 to 0.22 nm. This increase is not only due to the synchronization of the three involved tuning elements. In addition, the synchronous tuning results in a 2.8 times larger tuning slope of the laser wavelength versus phase section heater power, which corresponds to much shorter laser cavity where the rings are not present. These results indicate that both an ultra-narrow linewidth and a broad mode-hop-free tuning range can be achieved with a fully integrated extended-cavity laser based on microring resonators.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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