In an application at a French research base in Antarctica, a one-box ultrafast laser enabled direct spectroscopic monitoring of reactive atmospheric species with better than one part per trillion sensitivity by using mode-locked CEAS.

In cooperation with scientists from LGGE (Laboratoire de Glaciologie et Geophysique de l'Environnement), a team from Laboratoire Interdisciplinaire de Physique (LIPhy) in Grenoble set out to directly measure the concentration of three free radicals (IO, NO₂ and BrO) in real time at an Antarctic location. Because of their high reactivity, these three free radicals are normally only present at the parts-per-trillion by volume level or less.  Yet, this same high reactivity means that even at this ultra-low concentration, they play a key role in atmospheric chemistry, specifically on the oxidation of dimethyl sulfide (DMS) produced by marine phytoplankton.

In theory, the concentration of a target molecule or radical can be measured in a gas-phase sample, in this case ambient air, simply by measuring the strength of one or more known absorption lines as a laser or other light source passes through a cell containing the sample.  However, if the concentration of the target species is very low, then these absorption lines may be impossible to measure by conventional absorption spectroscopy, because the change in the measured laser intensity is below the noise floor of the measurement.   A solution is to use cavity enhanced absorption spectroscopy (CEAS), where the measurement cell is an optical cavity usually based on two superpolished mirrors with extremely high (>99.9%) reflectivity.  The light is essentially trapped in this cavity making many passes through the sample and thus typically creating kilometric effective absorption pathlength.

There are several types of CEAS based on both continuous-wave and pulsed lasers, including cavity ringdown spectroscopy (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS).  However, no commercial CEAS spectrometer offered the required wavelength range, sensitivity and flexibility for this particular application.

Building and Testing a Rugged ML-CEAS Instrument

 Instead, the team designed and built a detection system based on a technique called Mode-Locked, Cavity Enhanced, Absorption Spectroscopy (ML-CEAS) first demonstrated at LIPhy in 2002. The spectrum emitted by a femtosecond laser with pulses of 100 fs is several nanometers broad and constitutes a comb of modes (about 100 000 "teeth") separated by the repetition rate of the laser (e.g. 80 MHz).  In ML-CEAS, when the cavity frequency is exactly equal to an integer (1, 2, 3..) of the repetition rate of the laser, the entire laser spectrum is then transmitted through the cavity at the same time with no distortions.  This condition is called a "magic point" configuration.  In order to stay at this magic point, the cavity length is actively dithered around the magic point by using a transient injection scheme.

A high resolution spectrum of the sample is then obtained by dispersing the broadband output from the ML-CEAS cell with a high resolution grating spectrometer equipped with a cooled CCD array detector. This gives ML-CEAS a multiplex detection advantage, simultaneously sampling an extended spectral region.  In the specific application reported here, this arrangement enables simultaneous measurement of IO and NO₂ through their absorption lines around 436 nm and measurement of BrO by its absorption lines around 338 nm. It was possible to go from one wavelength to the other very rapidly thanks to the fast tuning characteristics of the laser.

Key Laser Requirements

A technically complex method like ML-CEAS places very demanding requirements on the output stability of a femtosecond laser.  And the successful deployment of a ML-CEAS system in this harsh remote location puts even more stringent requirements on the laser in terms of rugged immunity to shipping, and the ability to perform at extreme ambient temperatures and temperatures changes.

The first requirement is the output wavelength.  BrO strongly absorbs around 338 nm.  So the application needed a laser that can reliably reach the relatively short 676 nm wavelength enabling efficient frequency-doubling to 338 nm.  (Many commercial titanium:sapphire lasers cannot reach wavelengths shorter than 690 nm).  Second, the laser comb has to be stable enough so that it is possible to lock the cavity to the laser.  The Chameleon Ultra II from Coherent was chosen as the femtosecond laser that best met all these requirements.

In terms of practical reliability and rugged performance, this application also demanded a laser that could withstand the rigors of long distance shipping, and which could operate out of the box over an extended range of ambient temperatures.  All Coherent Chameleon lasers are subject to extensive HALT/HASS testing including vibrational tests, impact tests, and temperature cycling.  And they are shipped in proven packaging that has also been similarly rigorously tested.

A Successful Application

After extensive testing in the LIPhy lab at Grenoble and two test field campaigns in France, the laser and other components were shipped to the Dumont D'Urville location, including a two month journey on the icebreaker research ship, Astrolabe.  This was followed by a helicopter lift (see photo) and finally a ground sled.  Nonetheless, the laser worked perfectly to specification immediately after unpacking at its final destination.

Unexpectedly, in the small cramped lab (see photo), the temperature quickly reached 32 °C, because of all the equipment.  Even though the system was able to operate at this elevated temperature, system performance was adversely affected.  So the decision was made to open a small window letting some air exchange with the outside.  During a typical 24 hour period, this meant that the lab temperature varied by nearly 10 °C, as the outside temperature varied by 18 °C (from -12 °C to +6 °C).

In spite of these challenging ambient conditions, the ML-CEAS system worked spectacularly well, analyzing ambient air that was continuously and gently pumped through the sample cell.  As the team recently reported in a paper in Physical Review, the laser and system stability supported measurement intervals spanning as long as 10 minutes.  During this interval, the absorption spectral baseline reached the theoretical minimum; it was limited only by photon shot-noise.  As a result, the instrument achieved a minimum absorption coefficient of 7x10^-13 cm^-1 per spectral element, which is very impressive.  In the case of IO this translated into a minimum sensitivity of 0.02 parts per trillion!

Clearly, ultrafast lasers have come a long way from the cumbersome, delicate specialty instruments of just a few years ago - even further than the journey from Grenoble to Dumont D'Urville.