RINGDOWN CAVITY FOR ISOTOPIC RATIO MEASUREMENTS OF CARBON AND OXYGEN

Todd Sauke and Joe Becker

Molecular and isotopic spectroscopy in the mid-infrared (3 - 7 micrometer wavelength) has been extremely useful for many quantitative gas detection applications in fields as diverse as astrobiology, geology, atmospheric science, pollution control, environmental monitoring, and industrial process control. Variations in isotopic ratios of ¹²C/¹³C and ¹⁶O/¹⁸O in Martian soil samples could be important clues to the planet's geologic and biologic history. Such variations would be expected to be generated in a sample by any process of elemental transfer whose rate limiting step is diffusion controlled. This could include past or present volcanism, freeze thaw cycles, incorporation of carbon dioxide into the soil from the Martian atmosphere, enzymatic reactions, or respiration. Isotopic variability could also be caused in a sample by its having been mixed with other reservoirs of carbon or oxygen.

The typically strong absorption lines in the mid-infrared spectral region allow for sensitive detection without the need for complex, alignment sensitive, multipass sample absorption cells. The diode laser light sources used for spectroscopy in this spectral region typically require cryogenic cooling making them difficult, cumbersome and expensive, limiting their usefulness. On the other hand, in the near-infrared at 1.3 and 1.55 micrometer wavelengths, where inexpensive room temperature laser sources are readily available, the molecular absorption lines are orders of magnitude weaker than those in the mid infrared and can only be used with long path multi-pass absorption cells. Typical long path multi-pass cells, such as White cells, Harriot cells, etc. are large, cumbersome, and alignment sensitive. The relatively new technique of cavity ringdown spectroscopy affords another solution to the problem of achieving very long effective absorption pathlengths.

Laser spectroscopy offers important advantages over conventional mass spectrometry for measurements on a planet's surface. Importantly, because of the high spectral resolution of the laser spectrometer, the detailed and complex sample preparation and purification required for reliable mass spectrometry is unnecessary, because contaminant gasses do not interfere with the measurement. The goal is to develop a prototype instrument for laser spectroscopic isotope analysis of planetary soils and ices on possible missions to Mars and/or Europa.

This project makes use of a ringdown cavity to provide the ultra long effective pathlength needed for spectroscopy with weak infrared absorption lines, but will achieve high light throughput and high spectral resolution by locking the ringdown cavity to the narrow spectral linewidth of a diode laser source.

We have designed and constructed a near-infrared spectrometer consisting of a 1.6 micrometer near-infrared room temperature diode laser, an optical isolator, a spatial filter, and a tuned ringdown cavity which ultimately will be frequency locked to the continuous wave laser source, affording both high spectral resolution of isotopic absorption lines and high optical throughput for high sensitivity measurements.