Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Michael Jackson; Lyndon R. Zink

doi:10.3791/53399

Method Article

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

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DOI:

10.3791/53399

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December 18th, 2015

Michael Jackson¹^,³ , Lyndon R. Zink²

¹Department of Physics, Central Washington University, ²Department of Physics, University of Wisconsin-La Crosse, ³College of Science and Technology, Millersville University

Summary

We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH₂F₂) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

Abstract

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is ± 5 parts in 10⁷. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH₂F₂, was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO₂ pump laser, and strength.

Introduction

The measurement of far-infrared laser frequencies was first performed by Hocker and co-workers in 1967. They measured the frequencies for the 311 and 337 μm emissions from the direct-discharge hydrogen cyanide laser by mixing them with high order harmonics of a microwave signal in a silicon diode¹. To measure higher frequencies, a chain of lasers and harmonic mixing devices were used to generate the laser harmonics². Eventually two stabilized carbon dioxide (CO₂) lasers were chosen to synthesize the necessary difference frequencies^3,4. Today, far-infrared laser frequencies up to 4 THz can be measured with this technique....