An integrated optical microcalorimeter

Abstract

Microcalorimetry is the detection and quantification of small changes of heat in a system. Interferometry as a measuring technique is known for its high degree of sensitivity. Bulk optical interferometry is impractical for microcalorimetric measurements. Interferometric fiber calorimeters suffer from a wide range of environmentally induced perturbations that cannot be distinguished from measurand signals. The current work presents a hybrid integrated optical interferometer intended to measure chemical heats of reaction. The device incorporates a novel design to discriminate between measurand-induced signals and environmentally induced signals. The device is composed of two interferometers that share a single sensing arm.The overall layout of the device together with detailed design calculations for its individual components are presented. The fabrication process contracted to Photonics Integration Research Inc. (Columbus, Ohio), using flame hydrolysis and reactive ion etching is discussed. Phase resolution is enhanced using an up-sampling interpolation routine. Quantification of phase modulation is carried out using a cross correlation algorithm developed for this work. Details for both signal-processing routines are described. Program source code is included in the appendices. Results from tests evaluating basic operation and characterization of phase response are presented. Initial studies using an electronic heater show the relationship between phase modulation and surface-temperature to be 1.98 deg/°C-mm and 1.61 deg/°C-mm for the two interferometers. Further characterization studies of the device using 10 mul water droplets at fixed temperatures are described. Results from water droplet studies show the phase modulation as a function of temperature to be 2.74 deg/°C-mm and 2.88 deg/°C-mm for the two interferometers. The relationship between phase modulation and heat from the interferometers is calculated to be 274.33 deg/calorie and 288.04 deg/calorie. The experimental results reported provide a measure of device sensitivity to temperature. Further characterization is necessary to determine the heat capacity of the device and ultimately, the device sensitivity to heat. Finally, suggestions for future work are given along with specific modifications of the system for improved performance.