PhD Defence Roel Botter | Stimulated Brillouin Scattering in Silicon Nitride Based Waveguides

Stimulated Brillouin Scattering in Silicon Nitride Based Waveguides

Roel Botter is a PhD student in the department Laser Physics & Nonlinear Optics. (Co)Promotors are prof.dr.ir. D.A.I. Marpaung and prof.dr. K.J. Boller from the faculty of Science & Technology.

Stimulated Brillouin scattering (SBS) is a nonlinear optical process that is mediated by an acoustic wave. SBS has shown many interesting applications, including signal processing, sensing, and the creation of narrow-linewidth laser sources. One big challenge in the field is unlocking the effect in a mature, densely integrated photonic platform. In this thesis we describe ways to unlock enhanced SBS in silicon nitride, a well-established platform. So far, the Brillouin gain coefficients measured in this platform have been 0.1 m⁻¹W⁻¹. We show that SBS can be enhanced by multiple orders of magnitude, and demonstrate the first applications of SBS in this platform.

In Chapter 1 we give an insight to the current state of the field, and introduce the different types of SBS, as well as some related processes. Chapter 2 provides a theoretical background of SBS. Here we not only discuss the Brillouin process, but also the underlying physical phenomena.

Chapter 3 delves into the first method of SBS enhancement, geometrical optimization. Here we show that we can increase the strength of the interaction by designing the waveguide such that acoustic guidance is made possible. This way we have measured a Brillouin gain coefficient of 0.4 m⁻¹W⁻¹. We also show, through simulations, that we can further increase this to 1.5 m⁻¹W⁻¹. This represents an increase of the gain coefficient of more than an order of magnitude.

We use this enhanced Brillouin response to show possible applications in signal processing. We use two different approaches to create a microwave photonic notch filter. The first method combines the Brillouin gain with a special modulation spectrum, creating a notch filter through the cancellation between the two sidebands. The second method uses a microring resonator to shift the phase, combined with the Brillouin gain to compensate losses in the ring. Here again we get a notch through cancellation. Furthermore, we investigate the requirements for a Brillouin laser in this platform.

In Chapter 4 we use the Brillouin gain from the previous chapter to create a Brillouin dynamic grating (BDG). In the BDG we probe the acoustic wave created in the SBS process with a lightwave in the orthogonal polarization. The acoustic wave acts as a grating, only reflecting part of the probe. The unique property of a BDG is that the grating can be tuned, in location and length, purely through optical means. This makes it an interesting phenomenon for use in for example signal processing. We managed to observe it for the first time in silicon nitride, making this only the second integrated photonic platform that has shown a BDG.

In Chapter 5 we show an alternative way of enhancing SBS. Here we add a layer of tellurite (TeO₂) on top of the waveguide. Tellurite is a soft material, which helps in the guiding of the acoustic waves, and it has a high photoelastic coefficient, increasing the opto-acoustic interaction. We were able to leverage the strengths of tellurite and measure a gain coefficient of up to 4.5 m⁻¹W⁻¹. We realized that the cladding of the waveguide, a soft polymer, absorbed the acoustics, dampening them and reducing the Brillouin interaction. We were able to reduce this by adding a thin layer of silicon oxide, increasing the gain coefficient to 8.5 m⁻¹W⁻¹. We further optimized the geometry and cladding using simulations, showing that gain coefficients of up to 155 m⁻¹W⁻¹ are possible. This is two orders of magnitude more than we saw in the waveguides without tellurite.

Finally, in Chapter 6 we discuss the conclusions we can draw from the work presented here. We also take a look into the future, and where the field might head in the coming years.