2um Laser Propagation Facility

One of the most attractive properties of thulium as a fiber laser dopant, relative to more established ytterbium and erbium hosts, is the extremely wide wavelength range of laser output from ~1.8 ? 2.1 ?m. This ?eye safe? wavelength range is interesting for a variety of reasons and potential applications.

For example:

• There are several regions of very low atmospheric absorption within the wavelength range of Tm:fiber, well-suited for long-range propagation for applications such as LIDAR or directed energy.
• Vibrationally resonant absorptions from trace water vapor and CO2 occur within the emission band of Tm:fiber, making this a useful band for trace gas analysis.
• The wide laser tunability range is sufficient to enable the amplification of 100 fs pulses to high peak powers using all-fiber architecture.

For all of these cases, it is necessary to characterize the propagation of Tm:fiber emission. Towards this goal, we have conducted 1 km laser range experiments using a tunable Tm:fiber MOPA system producing >200 W output power with narrow linewidth (<200 pm) from ~1.94 ? 2.1 ?m at the Innovative Science & Technology Facility (ISTEF) in Merritt Island Florida. By tuning the laser wavelength, we measured the relative transmission over the 1-km range and compared it with MODTRAN simulation. These measurements confirm that there atmospheric transmission windows from 2.03 ? 2.05 ?m and for wavelengths >2.07 ?m.

We have also conducted short range transmission measurements using broadband Tm:fiber sources for illumination and our Yokogawa AQ6375 Optical Spectrum Analyizer for data collection. This provides us with a diagnostic with <50 pm spectral resolution and high dynamic range. This diagnostic is capable of detecting CO2 absorption features after ~2 m propagation in air, despite the low ~0.04% percentage of atmospheric CO2.