## 1 Introduction  
Atmospheric CO\({}_{2}\) is presently regarded as the largest anthropogenic forcing factor for climate change, but the global CO\({}_{2}\) budget remains considerably uncertain [1; 2]. To be useful in reducing uncertainties about carbon sources and sinks, atmospheric CO\({}_{2}\) measurements need to have a high resolution, with 1% precision [3]. To meet this need, differential absorption lidar (DIAL) or laser absorption spectrometer (LAS) systems, which use conventional coherent or incoherent lidar methods, have recently been developed. Many publications have reported the successful measurement of the atmospheric CO\({}_{2}\) column or profile concentrations by ground-based or airborne instruments using laser transmitters at wavelengths of near 2.0 \(\upmu\)m or 1.57 \(\upmu\)m. [PERSON] et al. and [PERSON] et al. demonstrated a range-resolved backscatter CO\({}_{2}\) profiling lidar using a line near 2.0 \(\upmu\)m, a pulsed Ho:Tm:YLF laser, and a coherent receiver [4; 5; 6; 7]. However, the detected power fluctuated obviously, due to the speckle effects in these systems. This is quite a serious problem, as it implies that a large number of pulses are necessary to obtain signals with a stable intensity [8]. [PERSON] et al. reported an integrated path differential absorption (IPDA) lidar operating at 1.57 \(\upmu\)m using direct-detection, where an injection seeded KTP -OPO system pumped by an Nd:YAG laser serves as the transmitter [9]. Comparative measurements on diurnal CO\({}_{2}\) variations performed by this lidar and an in situ device showed good agreement, but the maximum deviation was 9% (rms). [PERSON] et al. reported on airborne CO\({}_{2}\) column absorption measurements with a pulsed direct-detection multi-wavelength IPDA lidar operating at 1572.33 nm [10]. In all the pulsed lidar systems mentioned above, the on and off lasers are separated in time or space. Consequently, the sensing columns and footprints of the laser beam are different, which will introduce measurement errors, especially in space applications [8]. Recently, a continuous laser DIAL system with fiber transmission has been developed, which presents advantages compared with the scheme of a pulsed laser propagating in free space, such as continuous laser tenability and compactness. Another attractive feature of this technology is that the fiber interconnects allow for the straightforward development of system configurations [11]. [PERSON] et al. developed an intensity modulated, continuous-wave (IM-CW) LAS system by using the backscatter from a geometric target, and which operated at a wavelength of 1.6 \(\upmu\)m [12]. Diode lasers operating at discrete on and off wavelengths are intensity modulated at different frequencies, as well as combined and amplified simultaneously in a single erbium-doped fiber amplifier (EDFA). This instrument has been successfully flight tested in many airborne campaigns conducted in different geographic regions over the past few years [13; 14; 15; 16]. They improved their system by using a swept modulation frequency technique to provide range discrimination, due to the ubiquitous nature of cirrus clouds for space applications, and achieved a very high accuracy: the contrast error between the lidar and in situ CO\({}_{2}\) measurements was less than 0.6%. They found that the CO\({}_{2}\) measurement precision improved considerably after transitioning to the more thermally controlled cabin environment associated with the aircraft [14]. However, details about the influence of the cabin's ambient temperature on system errors was not reported in their reports. [PERSON] et al. developed a dual-wavelength IM-CW IPDA lidar for CO\({}_{2}\) measurements at the CO\({}_{2}\) absorption line near 1.6 \(\upmu\)m, where the transmitting and monitoring beams were all in fibers [17]. They added a horizontal rotator and vibrator on the optical antenna unit to average the speckle patterns which were independent between two wavelengths.