Application of extended tuning range for external cavity diode lasers to water vapor differential absorption measurements

Abstract

Recent advances using electronic feedback to control the optical cavity length of external cavity diode lasers (ECDLs) have led to extended continuous tuning ranges. Mode-hop-free tuning over more than 65 GHz has been demonstrated. The ability to tune ECDLs asross a wide range is particularly useful to differential absorption lidar (DIAL) systems that use ECDLs as seed laser sources. Experiments using a multiple-pass gas absorption cell are performed to test a widely tunable, amplified ECDL DIAL transmitter with this extended tuning range system. Experimental results show that the system can be tuned to and maintained at a user-defined wavelength for one hour, then tuned to and maintained at a second user-defined wavelength for one hour without mode hopping. This tuning is successfully accomplished between wavelengths separated by approximately 44 GHz. A computer-controlled feedback loop in the tuning system tunes and holds the laser system to the on- and off-line wavelengths to within ±88 MHz. The laser power transmitted through the gas absorption cell is monitored and used to perform a differential absorption calculation to find the number density of water vapor molecules within the cell. The measured value is in agreement with a HiTRAN prediction of the expected value.