MDL’s Tunable Laser Spectrometer Essential to Mars Science Laboratory Mission
The MDL-developed interband cascade semiconductor laser at 3.27-μm wavelength is at the heart of the tunable laser spectrometer (TLS) instrument on board Mars Science Laboratory (MSL). MSL will assess whether Mars ever was, or is still today, an environment able to support microbial life. The methane and carbon dioxide measurements provided by the Tunable Laser Spectrometer instrument will add unique and essential information needed to answer this fundamental question. It is capable of measuring the abundance of methane and its isotope ratio with very high precision (see graph below).
Studies of the source and evolution of the Martian atmosphere depend on high-precision measurements of abundances and isotope ratios of atmospheric gases. TLS scans over selected rotational lines within a given vibrational band. Target spectral regions are chosen for strong lines but with minimal spectroscopic interference and comparable temperature dependence important for isotope ratio determinations. Combined with a relatively long pathlength Herriott cell, the high spectral resolution results in high-sensitivity detection with limits at the parts-per-billion level.
About MDL’s Tunable Semiconductor Lasers
Tunable semiconductor lasers produce a very specific wavelength of light tuned to a fundamental or overtone harmonic frequency of the target gas molecule in the near infrared band. The light causes the molecule to vibrate and therefore, absorb energy. Once adjusted to the specific frequency of the molecule, the laser is minutely tuned to different wavelengths on either side of the original wavelength. The light energy being absorbed at the target gas absorption frequency is compared to the light energy at the surrounding frequencies, resulting in an extremely precise measurement. New data are integrated every second, making the system quick to respond to variations in the target gas.
In this technique, trace molecules in the Earth’s atmosphere or the atmosphere surrounding another planet may be precisely identified and studied by measuring their infrared absorption spectrum. Such measurements can reveal a wealth of information about the atmosphere: its composition, chemistry, evolution, and winds. However, the availability of tunable infrared lasers with the characteristics needed for a particular measurement or mission are often very limited. Due to the strategic importance of the tunable lasers for spectroscopy a development activity was formed at MDL which began in the early 1990s.
Spectroscopy in the mid-infrared (3-10 µm) is particularly powerful since the fundamental vibrational transitions (the strongest absorption) of many molecules fall in this range. The earliest devices (mid-1960’s) used exotic low bandgap “lead-salt” semiconductors and required low cryogenic temperatures - a severe limitation for a space project. While the lead-salt devices were used by JPL in atmospheric balloon-borne instruments, a much better solution appeared in 1994, when a new laser operating at higher temperatures was demonstrated by a group at Bell Laboratories - the “quantum cascade laser”.
In 2002 a strong effort was made at MDL which led to the successful development of mid-infrared lasers in the 3.0-4.0 micron wavelength range. These lasers were integrated into laser spectrometers and used in JPL aircraft and balloon experiments for measuring atmospheric hydrochloric acid and methane profiles in the atmosphere. The development of this type of laser for detection of the isotopes of methane proved to be an important factor for the selection of the Tunable Laser Spectrometer currently on board the Mars Science Laboratory.