## 1 Introduction  
The knowledge of the distribution of atmospheric water vapor and its rapid variations is key information for both weather forecast and climate science [1]. A promising technique for measuring the water vapor distribution with good accuracy and high spatial resolution is differential absorption LIDAR (DIAL), where alternating laser pulses at different wavelengths are utilized. One emission wavelength is set within an absorption line (online wavelength--\(\lambda_{\text{on}}\)), and the other one nearby to a non-absorbing spectral range (offline wavelength--\(\lambda_{\text{off}}\)). From the time delay between the outgoing pulses and the backscattered signals, the distance to scatterers can be determined. Besides real-time detection, post-processing of the returned signals allows also to characterize the spatial distribution of the water [2]. The DIAL technique has the advantage that the knowledge of the absorption cross sections suffices for the determination of the water vapor density from the ratio of the returned intensities at the online and offline wavelengths. However, the DIAL technique requires a pulsed laser with high wavelength stability and small spectral linewidth, capable of operating at two different wavelengths. The spectral linewidth of the laser must be at least smaller than the width of the absorption line at atmospheric pressure. Assuming a pressure broadening at normal pressure of 0.1 cm\({}^{-1}\) measured as full-width at half-maximum (FWHM), this leads to a maximal width of about 10 pm. To maintain low systematic errors, the emission linewidth should be about one order of magnitude smaller, e.g., 1.2 pm (\(<\) 400 MHz), reported by [PERSON] and [PERSON] [3]. Potential absorption bands for the detections are in the spectral regions around 720, 820, 930 and 973 nm [4; 5; 6]. The peak power of the pulses should exceed 10 W, providing sufficient backscattered intensity in the case of a ground-based measurement. The development of DIAL has been strongly connected with the progress in laser technology. Initial modeling indicated that a low pulse energy with a high pulse repetition rate is suitable to measure the water vapor profile in the lower troposphere [7]. This opened the possibility to use all-diode laser-based transmitters in DIAL instruments for inexpensive and reliable water vapor profiling. Advanced internally wavelength-stabilized diode lasers, such as distributed feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers, provide a stabilized wavelength, a high spectral purity, and are stable against mechanical and thermal disturbances. Both DFB and DBR lasers can operate mode-hop free with a small spectral line width and excellent side mode suppression ratio (SMSR). Their emission wavelengths can be tuned by changing the injection current or temperature. They fulfill not only the requirements for ground-based water vapor DIAL, but also those for space-borne online/offline injection seeders for laser transmitters. Semiconductor optical amplifiers can be used to generate and subsequently amplify optical pulses. In 2004, the first diode-laser-based water vapor DIAL was demonstrated by [PERSON] et al. [8], using a DFB laser and a flared amplifier operating around 823 nm. By changing the current injected into the DFB laser, online and offline wavelengths were generated. A peak power of 0.5 W with a pulse width of 600 ns at a repetition frequency of 8 kHz was obtained. For faster switching between the two wavelengths, several groups developed DIAL systems, utilizing two diode lasers for the online and offline wavelengths, either DFB lasers [9] emitting near 820 nm or distributed Bragg reflector (DBR) lasers [10; 11; 12; 13; 14; 15] emitting near 830 nm. The usage of semiconductor optical amplifiers was demonstrated, too. For example, [PERSON] et al.