Ultrafine Temperature Measurement Using Optical Whispering- Gallery Modes

This keynote provides a review of the research and development of optical whispering-gallery modes on the measurement and monitoring of temperature at ultrafine and micro scales. INTRODUCTION Optical whispering-gallery mode (WGM) resonators have seen increasing study over the past two decades in both physics and engineering [1-27]. The small mode volume confined at the surface of the resonators allows for strong interactions between resonators and the surrounding environment. Additionally, the potential for high quality factors (Q-factor) of WGM resonators allows for devices making use of WGM resonance to perform with high-precision and high-resolution. WGM devices have been developed in the fields of solid state physics and electrical engineering for the purposes of creating micro-disk lasers [1], studying quantum electrodynamic interactions [5, 6], and for the manipulation of optical signals [7, 8]. One of the most popular applications of WGM devices is for sensing. Sensors designed around WGM resonators are able to take full advantage of both their small mode volume and high quality factor. WGM sensing devices have been demonstrated in a number of different applications including: electric field [9], biological/chemical [10-16], humidity [17], pressure [21], and temperature sensing [18-20, 25-27]. In these applications WGM sensors demonstrate extremely highresolution. Single molecule and individual RNA viruses [11, 12], as well as, pico-molar chemical residues [16] for biological/chemical sensing have been reported in the literature. Pressure sensors with milli-newton force [21] and electrical field sensors [9] with resolutions as high as 0.027pm/V m have been demonstrated. Temperature sensors capable of detecting milli-kelvin temperature changes have also been exhibited [18]. In this article we look to review the work done on developing accurate, on-chip, high-resolution temperature sensors. The work begins with studying resonators fabricated from fused silica. These resonators act as independent sensing devices, and are studied at both the room temperature and cryogenic temperature regimes. After these independent fused silica resonators are studied and understood, the development of sensors for on-chip temperature measurements took place. The goal of these experiments was to find suitable techniques for the fabrication of WGM sensors directly onto electrical components of interest and determine of these sensors could be used for real-time temperature monitoring. In order to perform these experiments innovative techniques capable of creating onchip WGM resonators needed to be developed. After fabrication experiments were performed to understand the physics of these on-chip devices and to test their sensitivities as WGM temperature sensors in both the room temperature and cryogenic temperature regimes. Once the sensors sensitivity was discovered it was then possible to determine if those sensors were capable of on-chip dynamic temperature monitoring. EXPERIMENTAL SETUP Before discussing the results of the various experiments that took place, a brief discussion of the experimental setups used needs to take place. Though a number of different experiments will be reviewed in this paper, there were a number of common elements contained within all the experimental setups used. A 1516nm distributed feedback laser (NEL NLK1556STG) was used as the excitation source. The laser is controlled with a ILX Lightwave LCD-3724B laser controller and a function generator (Agilent 33220A). The standard practice for experiments conducting was to use a ramping function with frequency of 100Hz and amplitude of 3.5Vpp. The laser was injected into a fiber optic cable (Fiber Instruments Sales Inc. SMF28E+). The light was coupled into the WGM resonator via a tapered fiber technique [28], and the final signal was collected using a photo detector (Thorlabs PDA400). The signal from the photo detector was recorded either using a DAQ card and a LabVIEW program, or using a digital oscilloscope (Picoscope 3206B). The standard procedure