Long-path supercontinuum absorption spectroscopy for measurement of atmospheric constituents

Abstract

A supercontinuum source has been proposed as a new tool for measurement of minor species concentrations on long paths through the atmosphere. The present work describes results from recent experiments that demonstrate the potential for Differential Absorption Spectroscopy (DAS) and Spectral Pattern Recognition Differential Absorption Lidar (SPR-DIAL) measurements utilizing a supercontinuum source. As an initial example of this measurement approach, the results include the quantification of water vapor concentration through indoor and outdoor path absorption measurements using a collimated supercontinuum source. Experimental spectra are compared with equivalent simulations from MODTRAN??? versions 4 and 5 to examine the water vapor band between 1300 and 1500 nm to demonstrate the feasibility of the approach.

Measurement of atmospheric oxygen using long-path supercontinuum absorption spectroscopy

“…Prior to the current work, the 1-cm −1 resolution available in the earlier MODTRAN™ 4 version was used for comparison with atmospheric water vapor absorption measurements at lower resolution. 13 The lower-resolution MODTRAN calculation had previously masked the difference caused by using the vacuum refractive index. 21 Converting the MODTRAN™ 5 outputs to the air value using Eq.…”

“…13,21 In the case of the present work, the MLE algorithm provides a way to experimentally determine the concentration of oxygen along the measurement path. This algorithmic approach was originally developed by following the work of Warren 24 and was later expanded by Yin and Wang.…”

“…13 The second-generation SAS system, which is described in the following paper, has increased signal-to-noise and therefore supports measurements along longer atmospheric paths. Furthermore, a high-resolution spectrograph on the receiver side permits measurements of species with fine absorption features at atmospheric pressure and temperatures.…”

“…With this configuration, we are able to reduce the environmental effects on the transmitted signal intensity, which were routinely observed in signals measured by the first-generation SAS system. 13 Prior to this relocation, we had typically experienced up to 50% optical power variations during tens of minutes to an hour of operation due to thermal effects when the supercontinuum source itself was located in the rooftop laboratory. This measurement was performed with a broadband power meter placed at the output of the 18-m Blaze Photonics's photonic crystal fiber used to generate the supercontinuum light source.…”

“…This technique of supercontinuum generation was originally reported by Wadsworth et al, 12 and we have used it for a variety of studies in the laboratory and outdoor environment tests. 13,14 In fact, our prior work 13 describes the first time this technology was applied to outdoor measurements of atmospheric constituents. In that effort, SAS measurements of the water vapor absorption spectrum in the near-infrared (NIR) were measured and analyzed using a maximum likelihood estimation algorithm to quantify the concentration of water vapor over a several hundred meter outdoor path.…”