Lasers & Optics

We use light as part of the detection system in almost every instrument we build. Light of appropriate frequencies can be used to excite molecules, causing them to fluoresce, or the light may be absorbed, with the depth of the absorption indicating the molecule's concentration. Having a stable, well-understood light source is critical to making these measurements. Plasma lamps and lasers are the sources we use in most of our experiments.

Plasma lamps are constructed to produce light at frequencies characteristic of specific molecules. Their frequencies are quite stable, but their intensity can vary with time. Since the lamp output is steady, experiments must be designed to induce other changes in order to make a measurement. In the Lyman Alpha water vapor instrument, the lamp light is blocked intermittently by a chopper. In the Chlorine Monoxide (ClO) instrument, NO is periodically added to the flow to react with and remove the ClO, and the signal with and without ClO is compared to determine the ClO concentration.

Lasers offer direct control of frequency, so measurements can be made by changing the laser frequency. This can simplify the experimental design. Historically, laser assemblies have been large and difficult to maintain and operate, but in recent years there have been significant advances in laser technology. The results are smaller, lighter, and more stable lasers, all important considerations for us as we try to fit more instruments onto smaller aircraft.

Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers based on so-called intersubband transitions, i.e. radiative transitions which take place between confined electronic states of the conduction band of multiple quantum-well (QW) heterostructures, as shown in Figure 3.

A key feature of QCLs is that the emission wavelength is primarily a function of the QW thickness. As a consequence, QCLs can cover a wide spectral range using the same material system. The Anderson Group integrates QCLs emitting at various wavelengths from 4.3 – 10 microns for in situ measurement of H2O, CO2, and CH4 and their isotopes.  These lasers are based on InGaAs/InAlAs alloys lattice-matched to InP, which is a material system widely used in optoelectronics. State-of-the-art single frequency QCLs are now available over much of the Mid-IR and can produce over 100 mW of continuous wave power at room temperature.  Our group has developed QCL-specific electro-optical and mechanical packaging systems for integrating these breakthrough devices into airborne Earth Science instrumentation.  Figure 4 shows a photograph of our air-cooled QCL housing for fielding CW QCLs in environments ranging from the tropical stratosphere to the arctic planetary boundary layer.

QCLs were invented by fellow SEAS Professor Federico Capasso, and our groups continue to collaborate on the design and integration of devices that are fabricated specifically for airborne Earth Science applications.  Currently we are working with Professor Capasso’s group to increase the output power of single frequency QCLs into the hundreds of mW by integrating tapered power amplifiers on the end-facet of the devices.