Diode Laser Transmitter For Water Vapor DIAL Measurements Kevin S. Repasky, Joseph A. Shaw, John L. Carlsten Michael D. Obland, Lei S. Meng, and David S. Hoffman Physics and Electrical Engineering Departments Montana State University Bozeman, MT 59717 Repasky@physics.montana.edu Abstract -- The design and performance of two diode laser based transmitters for differential absorption LIDAR (DIAL) are presented. The first laser transmitter uses a tunable external cavity diode laser (ECDL) in the Littman-Metcalf cavity configuration to injection seed a flared amplifier. Water vapor absorption measurements are demonstrated with this laser transmitter operating in a continuous wave mode. The second diode based laser transmitter uses an ECDL in the Littrow cavity configuration and angle-angle semiconductor optical amplifiers to produce amplified laser pulses. Both designs are easily adaptable to wavelengths in the visible to the near infrared spectral region. Keywords: Diode Lasers, Optical Amplifiers, Laser Transmitter, LIDAR, DIAL I. INTRODUCTION Differential absorption LIDAR (DIAL) [1-3] is a technique used for measuring concentration profiles of atmospheric trace gases. Measurements are made in the following way. First, backscattered light is collected as a function of time with the laser source tuned onto an absorption feature of interest. Next backscattered light is collected as a function of time with the laser source tuned off of the absorption feature of interest. The ratio of these two measurements can be used to determine a profile of the trace gas concentration. DIAL requires a tunable laser transmitter capable of tuning onto and off of the absorption line of interest. Solid state lasers have limited spectral coverage and are difficult to tune making these lasers impractical for DIAL measurements. Solid state pumped dye lasers and optical parametric oscillators have good spectral coverage and can be tuned but these laser systems are difficult to tune, complex, expensive, and require high levels of maintenance. Current work on compact DIAL instruments are focused on using semiconductor based laser transmitters [2-7]. Diode lasers are compact and inexpensive and have good spectral coverage in the visible and near infrared spectral region. Recent work by Janet Machol et.al. [6] has demonstrated a micro-pulsed DIAL instrument for water vapor studies based on a distributed feedback (DFB) semiconductor laser at 823 nm with 0.8 nm of tuning. This laser was amplified using a tapered flared semiconductor amplifier up to a maximum of 500 mW and a pulsed with a pulse repetition frequency of up to 8 kHz. Successful DIAL measurements were made up to three kilometers. However, as noted in this paper, new laser transmitter designs are needed for better spectral coverage and larger tuning ranges. In this paper, two laser transmitter designs based on amplified external cavity diode lasers (ECDL’s ) are presented. The first laser transmitter is based on a Littman- Metcalf ECDL and a flared optical amplifier. This laser transmitter has a tuning range from 824 nm to 841 nm. The 17 nm tuning range allows the laser transmitter to access many water vapor absorption lines with varying absorption strengths. The output from the ECDL is used to seed a commercially available flared amplifier to produce an amplified output of up to 500 mW. Pulsed operation of the laser transmitter is achieved by pulsing the drive current to the flared amplifier. The laser transmitter was used in initial experiments to make horizontal path water vapor measurements for 0.5 km to 2.4 km at both 829.02 nm and 831.62 nm. This data compares favorably with theoretical calculations based on the HitranPC software [8]. The second laser transmitter design presented in this paper is based on a Littrow ECDL with a tuning range from 1041 nm to 1062 nm. The ECDL is used to seed an angle- angle semiconductor optical amplifier (SOA) that can provide an amplified pulsed output. The ECDL can also be used to seed multiple SOA’s while maintaining the spectral characteristics of the seed laser. This allows the output power of the laser transmitter to be scaled by adding additional SOA’s. Both tunable diode laser transmitters have been incorporated into DIAL instruments. These DIAL instruments utilize high pulse repetition rates, up to 10 KHz, to overcome their relatively low average power output. The detectors used in these DIAL instruments utilize commercially available Schmidt-Cassagrain telescopes and avalanche photodiodes for photon counting. This paper is organized as follows. Section II contains a description of the 830 nm laser transmitter. In section III, initial water vapor measurements are presented. The 1050 nm laser transmitter is discussed in section IV. In section V, the performance of the 1050 nm laser transmitter is discussed. 0-7803-8742-2/04/$20.00 (c) 2004 IEEE 1947