Lidar instrument for O2 remote sensing
Toward a Lidar Instrument for O2 Remote Sensing
July 2012 – Andrea Martin, NASA ESTO
Understanding the various components that drive climate change and how they interact within the environment has been, and continues to be, a major focus for NASA Earth science. To research one of the drivers of the Earth’s climate, the upcoming ASCENDS mission (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) – which could launch as early as 2021 – will study the variability of natural and man-made carbon sources/sinks and the transportation of carbon dioxide (CO2) within the Earth’s atmosphere.
To more fully understand Earth’s atmospheric carbon cycle there is a need to measure the conserved quantity of CO2 mixing ratio (or how many CO2 molecules there are relative to all other molecules of air). In order to determine the mixing ratio, one needs to first determine the total number of air molecules. One way to do this is to measure the total number of CO2 molecules and the total number of oxygen (O2) molecules in the atmosphere. With the appropriate algorithms, the O2 measurement can be used to determine the surface pressure that, when combined with other information about temperature and relative humidity, can be used to convert the total CO2 measurement into CO2 mixing ratio.
An ESTO Advanced Component Technology (ACT) project, “Laser Remote Sensing of O2 for Determination of CO2 Mixing Ratio and Sensing of Climate Species,” is tasked with developing the instrumentation and algorithms that could make O2 measurements possible from space in order to gain a more complete picture of Earth’s changing climate.
Jeremy Dobler of ITT Exelis’ Geospatial Systems is leading this effort in conjunction with teams at Exelis (Ft. Wayne, IN), TIPD, LLC (Tucson, AZ), and AER, Inc. (Lexington, MA) with the support from ESTO, and leveraging prior investments by Exelis. The task team is developing an integrated path differential-absorption lidar (IPDA) instrument to meet the requirements for measuring O2 as part of the suite of integrated instruments envisioned for the ASCENDS mission.
As part of the IPDA, the team has developed the first high-power, narrow spectral linewidth fiber laser source at a wavelength of 1262 nm, where O2 has an absorption feature that can be used for spectroscopic lidar measurements. The overall laser source consists of low-power single-frequency semiconductor lasers boosted by a fiber Raman amplifier (FRA), the optical fiber amplifier technology most capable of meeting the power and linewidth requirements for remote sensing IPDA measurements at this wavelength.
To be viable for use in space, the team needed to engineer a custom optical fiber that could achieve five watts of optical power efficiently. Fiber amplifiers increase the optical power of a small input signal by transferring energy into this signal beam from a secondary light source at a different wavelength known as a pump. Therefore efficiency for fiber amplifiers is often specified according to the percentage of light converted from the pump into the amplified signal. For the ASCENDS instrument – assuming currently available pump laser efficiencies – this optical to optical efficiency must be greater than fifty percent based on limitations for available power to the proposed instrument suite from the spacecraft.
Above: Fisheye lense photo of CO2 and O2 components integrated on the NASA DC-8 aircraft, from 2011. (image credit: J. Dobler)
Below: The O2 transmitter aboard the DC-8 in 2011.
(image credit: J. Dobler)
Meeting this power and efficiency is an extremely difficult task because of mechanisms inherent in optical fibers that set an upper limit on the amplified power that can be achieved. The most significant limiting process in fiber amplifiers is stimulated Brillouin scattering (SBS) in which energy is transferred from an intense light signal into the generation of acoustic (sound) waves along the fiber. For light traveling in one direction in the optical fiber, these acoustic waves ultimately reflect the light back into the opposite direction, which limits the amplification of light by fiber above certain power levels. SBS is particularly problematic to fiber Raman amplifiers, where long fibers are desired for improved efficiency, and needs to be addressed in order to achieve the requirements necessary for this laser transmitter to be considered as part of the ASCENDS mission.
The power limitations imposed by SBS can be overcome by using techniques that inhibit the formation of acoustic waves or by separating or reducing the spatial overlap between optical and acoustic waves within the optical fiber. Both of these options necessitate the engineering, development, and testing of entirely new fibers to determine which methods could best overcome the limiting power issues in FRAs.
Under the initial 2008 ACT funding, Dobler’s team first developed phosphorus doped silica glass (phosphosilicate) fibers whose size varied longitudinally along the length of the entire fiber. Varying the core sizes was meant to increase the light intensity at which significant acoustic waves begin to form. Using this fiber, a Raman amplifier was constructed that transmitted two watts of continuous wave optical power at 1262 nm, the first such high power narrow linewidth laser source at this wavelength. As part of an ASCENDS test flight campaign in 2011 aboard a NASA DC-8, Dobler and the Exelis team demonstrated for the first time airborne measurements of atmospheric O2 in the 1260 nm spectral band with this FRA.
While the power of this initial amplifier is lower than the five watts necessary to make the same measurements from space, the development teams from Exelis and TIPD developed additional fiber designs. The phosphosilicate fibers were doped with additional elements such as fluorine in order to separate optical and acoustic waves traveling within the fiber in an attempt to reduce the limitations imposed by SBS.
After modeling, manufacturing, and testing several different fibers, Dobler and the team members at TIPD have found a fiber that they believe will be able to further suppress SBS in order to increase the current power limitations. This would allow for the construction of an amplifier with output power near five watts and high optical to optical efficiency, moving the project team closer towards the possibility of taking O2 measurements in the 1260 nm spectral band from space.
Now funded for a second ACT task to increase the output power and efficiency of the fiber, Dobler and his team are further refining the specially doped fibers and testing Raman amplifiers containing these fibers developed during the first ACT project.
Considerable progress has been made recently in reaching the task’s goals. In addition to the laser amplifier and transmitter development, the team members at Exelis and AER have made significant advances in the algorithms that will be used to process the data generated from the O2-lidar instrument. The processed data will be applied to the retrieval of CO2 mixing ratio to gain a better understanding our Earth’s climate.
For more information on emerging technologies for Earth science, visit the NASA Earth Science Technology Office website.