Design and simulation of integrated photonic devices based on tilted Bragg gratings

Lead Research Organisation: University of Southampton
Department Name: Optoelectronics Research Centre

Abstract

In traditional optics, instruments are made out of bulk components (mirrors, lenses, etc.) that are used to manipulate light. Unfortunately, it is very difficult and time consuming to align these components precisely and such devices tend to be large and heavy.
The solution is to integrate the entire apparatus in a single optical chip. Light is routed around the chip in what are known as waveguides that confine light in a finite number of optical "states", which we call modes of the waveguide. The components that are used to process the light signals are themselves integrated into the chip. This is known as integrated photonics.
One such component is the Bragg grating which consists of a periodic modulation of the refractive index of the material along a waveguide. The grating is able to couple light between different modes of the waveguide, for example between forward and backward propagating light in the same waveguide like a mirror operating at a certain frequency. A Bragg grating can also couple light between the waveguide and the surrounding material.
A newly developed fabrication technology allows us to introduce a tilt angle to the planes of this periodic refractive index modulation. This gives rise to a new class of integrated photonic devices with higher efficiency, a wider range of design parameters, and completely new functionality.
The aim of this project is to develop and simulate new devices based on tilted Bragg gratings for use in integrated photonic chips. We have already derived an equation quantifying the reflection of light by a grating to the surrounding material and have designed a new device that can transfer light between two waveguides with 100% efficiency. Other potential applications are in polarizing beam splitters, beam focusing, spectrometry, and quantum information processing.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509747/1 01/10/2016 30/09/2021
1921409 Studentship EP/N509747/1 29/09/2016 31/03/2020 Mathias Weisen
 
Description I have derived a general analytical expression for the reflection of light by a Bragg grating of arbritrary tilt angle from the bound mode of a single mode waveguide to an unbound radiation mode using a ray tracing method. I have compared my results to numerical simulations made using the finite element method and found good agreement provided that the tilt angle and waveguide width are sufficiently large. I have assumed that the waveguide has a Gaussian index profile and that the profile of the fundamental mode of a Gaussian waveguide is also Gaussian. I have found that the latter approximation holds better as the waveguide width gets larger.

I have also investigated a new type of waveguide-to-waveguide coupler using two parallel single-mode waveguides with identical tilted Bragg gratings that are inscribed into a single ridge structure. The gratings are used to couple light between the waveguide modes using one of the ridge modes as a bus. Full details including the data are in the accompanying JOSA B publication. I simulated the system in Python code I developed myself. I also calculate the effect of fabrication parameters on this device, the effect of using an effective refractive index oil, the group delay properties of the device, the effect of apodizing the gratings and the effect of phase shifting the gratings to simulate noise. I also simulated the device using parameters decided together with members of the Optical Engineering and Quantum Photonics Group to later compare with experimental results.

I have also investigated a similar device to the above only with two modes in each of the waveguides leading to four modes in total. By superimposing two gratings in each waveguide I'm able to couple all four core modes together via a single cladding mode and achieve any arbitrary power splitting. By chaining multiple couplers, I am able to achieve any unitary matrix, such as a Walsh-Hadamard and quantum Fourier transforms. More details including examples of power splitting and transformations that can be achieved are available on the poster that was presented at CLEO/Europe-EQEC 2019 in Munich. In order to arrive at useful device parameters to simulate this device, I have made mode calculations sweeping the device width and the waveguide width, seperation and index contrast.

Finally, I investigated another similar device, only exploiting two single-mode waveguides and both of their propagation directions. By connecting circulators at all the ends of the waveguides we have a device with four input and four output ports. Using a combination of both long-period and short-period gratings, to couple via the cladding mode in both the co-propagating and counter-propagating direction, I am able to achieve any 4 by 4 matrix that is both unitary and symmetric. Full details on this device are in an as yet unpublished paper. I also found a way to implement the Walsh-Hadamard transform using only three gratings. I also simulated a set of devices on this platform that have intricate mathematical behaviour but are extremely sensitive and therefore not practical.
Exploitation Route The Optical Engineering and Quantum Photonics Group at the Optoelectronics Research Centre are planning to use my results in order to fabricate devices based on tilited gratings such as the waveguide-to-waveguide coupler in the paper "Low-loss wavelength-selective integrated waveguide coupler based on tilted Bragg gratings".
Sectors Digital/Communication/Information Technologies (including Software)

URL https://www.researchgate.net/profile/Mathias_Weisen
 
Title Dataset for "Low loss wavelength-selective integrated waveguide coupler based on tilted Bragg gratings" 
Description Dataset supports the paper "Efficient wavelength-selective integrated waveguide coupler based on tilted Bragg gratings" published in Journal of the Optical Society of America B. https://doi.org/10.1364/JOSAB.36.001783 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact This is the database for the paper: "Low-loss wavelength-selective integrated waveguide coupler based on tilted Bragg gratings" 
URL https://eprints.soton.ac.uk/431648/