Frequency Modulated Spectroscopy
FMS is an ultrasensitive variation of absorption spectroscopy that has been developed to measure stable isotope ratios of CO2 (Fessenden et al., 2010). FMS instruments capable of stable isotope field measurements for carbon sequestration (Fessenden et al., 2010) as well as measuring the carbon isotopes in CO2 and CH4 on Mars (Webster & Mahaffy, 2011). Most FMS applications are accomplished in an in situ configuration with a multipass cell (Webster & Mahaffy, 2011). However, FMS is also capable of operating in a remote configuration as discussed below. The technique involves directing a laser through a modulator that produce sidebands that are shifted from the laser wavelength by kilohertz to gigahertz frequencies as depicted in Figure 3.2. The entire system can be tuned, the laser wavelength and the frequency shift, so that one sideband interacts with an absorption feature while the carrier laser and the other sideband are unaltered. The detector senses the changes in the sideband and the derivative shaped function. As the CO2 concentration increases, the peak‐to‐peak amplitude of the recorded response increases. This allows for very small changes in concentration to be measured, effectively increasing the signal to noise ratio where a large contrast in harmonic amplitude change can be seen on a dark background. This is in contrast to absorption spectroscopy where one is recording a relatively small change relative to a bright background. Consequently, one can theoretically achieve quantum noise levels, and it is conservatively a factor of 100 to 1,000 times more sensitive than absorption spectroscopy under identical conditions. Finally, this increased sensitivity also enables the detection of stable isotopes such as 13CO2.
Figure 3.2 A FMS instrument produces sidebands (ωc ± ωm) shifted from the carrier (laser) wavelength (ωc) (based on Bjorklund & Levenson, 1983).
3.2.2. Standoff or Remote Methods
The number of remote sensing possibilities is limited compared with the in situ methods, and we are not aware of any commercially available instruments. Various LIght Detection And Ranging (LIDAR) methods have been developed and demonstrated including Differential Absorption LIDAR (DIAL). The FMS technique has been developed and demonstrated in several configurations including LIDAR.
LIDAR fundamentally involves directing a pulsed nanosecond laser through the gaseous sample in the atmosphere (Johnson et al., 2013). Some of this laser light is scattered off dust and aerosols and some of that scattered laser light is directed back toward a collection telescope and recorded. The amount of scattered light is very weak and requires very sensitive detectors. The scattered laser intensity is recorded as a function of time and one can convert the signal as a function of time to distance based on the speed of light. The intensity of scattered light is also proportional to the concentration of species that absorbed the light over the path length of the light. With most laser powers, one can achieve LIDAR detection at hundreds to thousands of meters. However, the range resolution required to identify the location of a leak requires short and accurate temporal resolution and is generally limited by the detector sensitivity.
DIAL is typically used to calculate the CO2 concentration from a LIDAR experiment. DIAL involves collecting a LIDAR spectrum where the laser is tuned to a CO2 absorption feature to determine the “I” term in the Beer‐Lambert law (equation 3.1). The same laser (or a second laser) is tuned to a wavelength off of the absorption feature to determine the baseline intensity, Io. The CO2 concentration is proportional to the difference between these two signals.
At Los Alamos National Laboratory (LANL), FMS has been developed into both a “remote” and LIDAR instrument that was designed to measure both the major and minor carbon isotopes. Here, we define remote as an experiment that involves directing the laser to a hard target such as a wall, a geologic surface, or a mirror/corner cube. The intensity of light scattered or reflected off a hard surface is significantly brighter than the light scattered off dust and aerosols used to detect a LIDAR instrument. The same modulated laser discussed in the in situ instrument above is directed through the sample in the field and the signal returned from the hard target is recorded rather than recording the range resolved signal. While this method produces the most sensitive record of CO2 concentration over the long path length, this technique is limited to locations where a hard target is available.
An FMS LIDAR‐capable instrument fundamentally can accomplish all of the requirements discussed in the introduction. A LIDAR instrument capable of stable isotope sensitivity would distinguish natural CO2 sources from CO2 seepage to the surface and would have some capability to locate the source with the ranging capability.
3.3. FREQUENCY MODULATED SPECTROSCOPY
The development of field deployable FMS instruments started with the construction of an in situ instrument similar to the one depicted in Figure 3.3. The instrument uses a New Focus tunable diode laser (TDL) that scans over the 1,604–1,609 nm CO2 absorption band. A Stanford Research Systems DS345 function generator was used to directly modulate the TDL at 2 GHz. The laser is fiber optically coupled to an Infrared Analysis, Inc., 8 m long multipass White cell. Air samples containing CO2 are pumped into the White cell for analysis. The laser beam exits the White cell, is launched into a second optical fiber that directs the laser into a New Focus detector. The output of the director is filtered and amplified with a Stanford Research Systems SR560 before being recorded on a PC.
Figure 3.3 The in situ FMS instrument built at Los Alamos National Laboratory enclosed in a weatherproof case for field deployment.
An open‐path remote instrument was developed using the same optical system discussed for the in situ system above. The same model New Focus TDL was fiber optically coupled to a 5x beam expander used to collimate an enlarged beam. The modulated beam was directed to a retroreflector placed up to hundreds of meters away from the beam expander. The returned laser beam was collected with a second beam expander, coupled to a second optical fiber and onto the New Focus detector.
3.4. FMS PHYSICS AND MODELING
Superimposing a periodic (sinusoidal) frequency upon a light wave having a base, or carrier, frequency of ω o results in a frequency modulated wave described by
(3.2)
where E o is the electric field amplitude, t is time, M is the modulation index (or strength) of the imposed periodic variation, and ω m is the frequency of the periodic “dithering” frequency. When passed through a sample, both absorption and dispersion of the wave occur. To account for these, convention is to use a complex valued frequency‐dependent transmission
