Performance improvement and analysis of a 16 μm continuous-wave modulation laser absorption spectrometer system for CO_2 sensing

Abstract

In a previous study, we developed a 1.6 μm continuous-wave (cw) modulation laser absorption spectrometer system for CO(2) sensing and demonstrated the measurement of small fluctuations in CO(2) corresponding to a precision of 4 parts per million (ppm) with a measurement interval of 32 s. In this paper, we present the process to achieve this highly specific measurement by introducing important points, which have not been shown in the previous study. Following the results of preliminary experiments, we added a function for speckle averaging on the optical antenna unit. We additionally came up with some ideas to avoid the influences of etalon effects and polarization dependence in optical components. Because of the new functions, we realized a calibration precision of 0.006 dB (rms), which corresponds to a CO(2) concentration precision of less than 1 ppm for a 2 km path. We also analyzed the CO(2) sensing performance after the improvements described above. The measured short time fluctuation of the differential absorption optical depth was reasonably close to that calculated using the carrier-to-noise ratio of the received signal.

Improvement of the 1.57-micron laser absorption sensor with chirp modulation to evaluate spatial averaging carbon dioxide density

“…This system can measure the path-averaged atmospheric CO 2 absorption and range from the system to a target just as the previous system could. The performance of the previous system was already reported in [15,16]. Each module consists entirely of optical fiber circuits except for the transmitting and receiving telescopes.…”

“…The details of the ground-based measurements were described in early studies [15,16]. Following that, the flight measurements in past few years were carried out to evaluate a performance of CO 2 measurements over the various surfaces of lands and oceans [8].…”

An airborne amplitude-modulated 1.57 μm differential laser absorption spectrometer: simultaneous measurement of partial column-averaged dry air mixing ratio of CO<sub>2</sub> and target range

“…Our system employed three narrow linewidth lasers, which are based on continuouswave distributed feedback diode lasers and a fiber amplifier (Kameyama et al, 2011a). The system used two laser wavelengths during measurements, and the output of each laser was amplitude modulated by different sinusoidal waves.…”

“…By taking the ratio of online to offline signals, we could measure τ . Our system obtained round-trip τ and the range (z) from the height of the aircraft above the target (Sakaizawa et al, 2010;Kameyama et al, 2011a):…”

“…We first manufactured an LAS system for ground-based measurements (Kameyama et al, 2011a;Sakaizawa et al, 2009). Figure 2 shows the block diagram of the 1.57 µm prototype LAS system and other instrument settings in the aircraft cabin.…”