Supercontinuum Generation in Nanophotonic Silicon Nitride Waveguides

The generation of supercontinuum (SC) is a long standing research field with applications in metrology, imaging, spectroscopy and telecommunication. While SC in fibers and bulk media have dominated the field, in recent years generating SC in on-chip waveguides have been a heavily researched topic. Especially the on-chip generation of octave spanning SC for stabilized optical frequency combs (OFC) has been a major driving force.

On-chip SC is vital component for broadband integrated photonics. The miniaturization of optical components would enable devices with smaller footprint, lower power consumption and reduced cost. On-chip SC has been generated in waveguides of various materials. The material used throughout this thesis is silicon nitride (SiN).

It is a CMOS compatible material with low loss and high nonlinearity. It does not suffer from two photon absorption (TPA) and fabrication techniques are mature enough that a large range of waveguide sizes can be fabricated. Throughout the thesis, both Si3N4 and silicon-rich nitride (SiRN) waveguides are used.

Increasing the silicon content increases the nonlinearity, but also increases the loss. Because of this it is necessary to use the low loss Si3N4 for long waveguides, while in short waveguides the increased nonlinearity of (SiRN) makes it preferable. Research has tended to focus on using the waveguide geometry to dispersion engineering waveguides with anomalous dispersion.

This method of supercontinuum generation (SCG) suffers from a number of limitations, as it is governed by the dynamics of the soliton. Here, we have studied several different techniques for generating SC in silicon nitride waveguides. By engineering the dispersion profile of the waveguides dramatically different SC can be generated.

Initially, the use of SiRN waveguides for SCG is investigated. The increased silicon content increases the nonlinear effect and modifies the dispersion. In these anomalous dispersion waveguides the well known combination of soliton compression and dispersive wave (DW) emission generates an octave spanning SC.

It is numerically shown that by increasing the silicon content, the phase matching wavelength of the mid infrared (MID-IR) DW is pushed further into the infrared. The concept of increasing silicon content, as a way to dispersion engineer the waveguide, is shown to be a simple way of enabling a larger range of DW wavelengths with phase matching all the way to 5 µm.

Previous results in on-chip SC are mainly based on anomalous dispersion waveguides. We have performed a thorough investigation of SC generated in an all normal dispersion (ANDi) Si3N4 waveguide. A 20 cm Si3N4 spiral waveguide is designed with all normal dispersion. The SC is found to retain very low relative intensity noise (RIN) for all pump powers while generating a SC from 1150nm to 1950nm.

This is compared to the spectrum and noise of an anomalous dispersion waveguide. It is shown that the RIN of the anomalous dispersion waveguide rapidly increases after soliton fission. Even at soliton fission simulations show that the anomalous waveguide cannot retain low noise over the full bandwidth.

One key result is that an octave spanning SC is generated using 22 pJ of pulse energy in an anomalous dispersion spiral waveguide. The ANDi SC has a much flatter spectrum, and numerical simulations show that the flatness could be improved by shaping the input laser pulse to a gaussian. Additionally, it is numerically shown that ANDi waveguides retain coherence for longer pump pulses, and the main decoherence is caused by the noise seeded Raman gain, which is expected to be significantly weaker in SiN waveguides when compared to silica fibers.

It is clear from these simulations that for long pulse SCG it is necessary to use long waveguides. These waveguides could be implemented using a spiral design. The observations confirm that ANDi waveguides enable a flatter SC to be generated with a lower RIN using longer input pulses. The key limiting factors are the need for higher pulse energies and a flat dispersion profile.

Finally, we numerically study SCG by pumping a SiRN waveguide with two zerodispersion wavelengths (ZDW). The waveguide dispersion is designed so that the pump pulse is in the normal dispersion region (NDR). We show, that because of self phase modulation (SPM), the initial pulse broadens into the anomalous dispersion region (ADR) and forms a soliton.

The interaction of the soliton and the broadened pulse in the NDR causes additional spectral broadening through formation of DWs by non-degenerate four-wave mixing (FWM). It is shown that pumping in either NDR results in broadening towards the other NDR. This effect is reproduced in a micro structured fiber (MSF) with a similar dispersion profile.

Finally, it is shown that by increasing the input pulse power, the soliton generated can be launched through the ZDW into the opposite NDR. We are confident that the presented research sheds new insight on generating supercontinuum in on-chip waveguides using any of the mentioned concepts. This paves the way for small footprint supercontinuum sources with improved performance in terms of bandwidth and noise.