Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Summary

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Abstract

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

Introduction

Whispering gallery mode resonators are disks or spheres of micro- or millimetric radius^1,2,3,4. Provided that the resonator is almost perfectly shaped (nanometer-size surface roughness), laser light can be trapped by total internal reflection within its eigenmodes, which are usually referred to as whispering-gallery modes (WGMs). Their free-spectral range (or intermodal frequency) can vary from GHz to THz depending on the resonator's radius, while their quality factor Q can be exceptionally high⁵, ranging from 10⁷ - 10¹¹. Due to their unique property of stockpiling and slowing down light, WGM optical resonators have....

Protocol

The protocol consists in 5 main stages: In the first one, the whispering-gallery-mode resonator is made. In order to control the progress of the polishing of the resonator, surface state measurements are carried out. In the third stage, we fabricate the tool that will launch light in the resonator. Once those two main tools are manufactured, we use them to visualize optical high-Q resonances. Finally, using a high-power input laser beam, the resonator behaves in a nonlinear fashion and Kerr combs are produced.

1. Polishing the Resonator

In this stage, an optical window crystalline resonator (MgF₂

Results

This five-step protocol enables to obtain WGM resonators with very high quality factors for microwave photonic applications.

The first step aims to give to the resonator the desired shape, as represented on Scheme 2. The main difficulty here is to manufacture a disk whose rim is sharp enough so that can it strongly confine the trapped photons, without leading to structural fragility from a mechanical standpoint. This polishing tower also possesses remarkable versatility as it .......

Discussion

This protocol allows producing high-Q optical resonators, to couple light into them and trigger nonlinear phenomena for various microwave photonics applications.

The first step of rough grinding should give its shape to resonator. After an hour of grinding with the 10 μm abrasive powder, one side of the rim of the resonator should be conveniently shaped (see Scheme 2). The following step will smooth the surface of the resonator and when reaching the stage of the 1 .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Step motors 50 mm course	Thorlabs
3 axis nanostage	Physik Instrumente
TUNICS tunable laser source	Yenista
Optical spectrum analyzer APEX	APEX Technologies