Side-line tunable laser transmitter for differential absorption lidar measurements of CO_2: design and application to atmospheric measurements

Abstract

A 2 microm wavelength, 90 mJ, 5 Hz pulsed Ho laser is described with wavelength control to precisely tune and lock the wavelength at a desired offset up to 2.9 GHz from the center of a CO(2) absorption line. Once detuned from the line center the laser wavelength is actively locked to keep the wavelength within 1.9 MHz standard deviation about the setpoint. This wavelength control allows optimization of the optical depth for a differential absorption lidar (DIAL) measuring atmospheric CO(2) concentrations. The laser transmitter has been coupled with a coherent heterodyne receiver for measurements of CO(2) concentration using aerosol backscatter; wind and aerosols are also measured with the same lidar and provide useful additional information on atmospheric structure. Range-resolved CO(2) measurements were made with <2.4% standard deviation using 500 m range bins and 6.7 min? (1000 pulse pairs) integration time. Measurement of a horizontal column showed a precision of the CO(2) concentration to <0.7% standard deviation using a 30 min? (4500 pulse pairs) integration time, and comparison with a collocated in situ sensor showed the DIAL to measure the same trend of a diurnal variation and to detect shorter time scale CO(2) perturbations. For vertical column measurements the lidar was setup at the WLEF tall tower site in Wisconsin to provide meteorological profiles and to compare the DIAL measurements with the in situ sensors distributed on the tower up to 396 m height. Assuming the DIAL column measurement extending from 153 m altitude to 1353 m altitude should agree with the tower in situ sensor at 396 m altitude, there was a 7.9 ppm rms difference between the DIAL and the in situ sensor using a 30 min? rolling average on the DIAL measurement.

All-fiber versatile laser frequency reference at 2 μm for CO2 space-borne lidar applications

“…For comparison with other frequency-stabilized lasers in the 2.05-μm wavelength range reported in the literature, we have also calculated a root-mean-square (rms) standard deviation of ~190 kHz for our frequency-stabilized laser over a period of more than 3 h. Koch et al reported an rms deviation of 870 kHz for a cw solid-state laser frequency-stabilized around 1.2 GHz away from the center of a CO 2 absorption line at 2053 nm [13], whereas Refaat et al achieved an rms deviation of 650 kHz for a semiconductor laser stabilized to the center of the CO 2 R(30) transition at 2051 nm, as estimated from a statistical analysis and a fitting procedure [14]. One should notice in these comparisons that different measurement instrumentation and data processing were used in each case to retrieve the frequency instability.…”

“…The VCO frequency (here set around 1 GHz) was measured with a frequency counter, after frequency division by a factor 20 and down-conversion with a mixer to be in the operation range of the counter Fig. 13 Frequency stability (Allan deviation) of the laser locked with a detuning of ~1 GHz from the center of the CO 2 R(30) transition using the developed modulation sideband locking scheme locked conditions, by analyzing the heterodyne beat signal with the frequency comb using a frequency discriminator [30]. Results are displayed in Fig.…”

“…The drawback is that CO 2 transitions are much weaker and spectrally broader than the sub-Doppler atomic Rb absorption line used in the aforementioned 1572-nm laser stabilization scheme (where the considered transition linewidth was in the range of 15 MHz). Therefore, rootmean-square (rms) frequency deviations of several hundred kilohertz have typically been reported so far for cw lasers frequency-stabilized to Doppler-broadened CO 2 transitions at 2053 nm [R (22) line] [13] or 2051 nm [R (30) line] [14].…”

“…These methods have proven their efficiency to transfer the frequency stability of the master to the slave laser. Using such a scheme, frequency stabilization of a (slave) cw laser to the side of a CO 2 transition at 2053 nm was for instance realized for further injection seeding of a pulsed oscillator [13]. However, two distinct laser sources are needed, each with its own driving electronics, which can be a drawback in some conditions, e.g., for space applications.…”

Airborne 2-Micron Double Pulsed Direct Detection IPDA Lidar for Atmospheric CO2Measurement

“…To obtain the wavelength accuracy and stability, a master wavelength reference against a sample of CO2 in a gas cell is established. One of the CW lasers, called the center-line reference, is locked on the center of the CO2 absorption line R(30) by a frequency modulation spectroscopic technique [4]. A second laser, called the tunable side-line, is referenced to the center-line laser by a heterodyne technique.…”