TECHNOLOGY AREA(S): Sensors, Electronics, Battlespace
OBJECTIVE: Demonstrate Vertical Cavity Surface Emitting Lasers as a possible source to excite mesospheric sodium at 589 nm and 1141 nm to provide cheaper, more useful, and more powerful cooperative sources for adaptive optics.
DESCRIPTION: The purpose of this development is to investigate and implement novel techniques to develop single color (589 nm) and two color (589 nm and 1141 nm) Vertical External Cavity Surface Emitting Laser (VECSEL) for use as a sodium guidestar. A guidestar is a cooperative source in the mesosphere created by the excitation of neutrally charged mesospheric sodium metal. The sodium atoms are excited to their first excited state by 589 nm laser light. These atoms then fluoresce and create an artificial star at the edge of the atmosphere. This fluorescence is used as a cooperative beacon for large aperture telescopes' adaptive optics systems. A 589 nm guidestar is the primary excitation source for all current sodium guidestars; however, these guidestars are generally very complex with > 30 optical elements and very expensive > $1M in acquisition cost. Because of the wavelength selectability, small footprint, and simplistic design a VECSEL could provide a cheaper, less complex source for a sodium guidestar at 589 nm. Current developments of 589 nm VECSELs have focused on frequency doubling of an 1178 nm VECSEL to provide 589 nm light. Such a system would require narrow linewidth (20 MHz) and > 10W of output power. During the same development effort, an 1141 nm VECSEL source could also be grown to provide excitation of the next excited state in sodium when used with a 589 nm source. Such an 1141 nm VECSEL source could be paired with a traditional sodium laser guidestar at 589 nm or with a VECSEL guidestar at 589 nm. An 1141 nm guidestar would be utilized to create a sodium polychromatic laser guidestar (PLGS). A PLGS system would allow for the correction of atmospheric induced Tip and Tilt aberrations without the use of a natural guidestar. A PLGS VECSEL must be narrowband at 1141 nm (500 MHz) and must have an output power >10W. PLGS guidestars do not currently exist and would be a monumental increase in corrective ability for AO systems, especially for dim objects. Phase 1 of this development would involve demonstrating the technologies and growth of VECSEL chips for a VECSEL guidestar at 589 nm and 1141 nm. Phase 2 of this development would constitute laboratory prototype development and system robustness improvement. Phase 3 of this development would involve production of a facility grade laser guidestar system capable of being used with a large aperture telescope.
PHASE I: 1. Develop VECSELs emitting at 1141 nm and 1178 nm 2. Develop High Power (>10W) VECSEL emitting at 1141 nm and/or 1178nm 3. Develop high power (>10W) VECSEL emitting at 1141 nm and/or 589 nm (via SHG) 4. Develop Narrow linewidth ( < 1 GHz), tunable (5 GHz tuning), high power (> 10W) VECSELs emitting at 1141 nm and/or 589 nm (via SHG)
PHASE II: 1. Develop laboratory demonstration system of Phase 1 as a proof of concept capable of pumping sodium through use of an evacuated sodium cell 2. Develop wavelength stability, system robustness, and system concept of use on a telescope at 1141 nm and 589 nm 3. Produce a proof of concept system capable of attaching to a telescope for an on-sky test of this system at 1141 nm or 589 nm
PHASE III: 1. Deliver a working prototype VECSEL guidestar PLGS system capable of attaching to a telescope for an on-sky test of this system at 1141 nm 1. Deliver a working prototype VECSEL guidestar capable of attaching to a telescope for an on-sky test of this system at 589 nm
1. Fallahi, M., Fan, L., Kaneda, Y., Hessenius, C., Hader, J., Li, H., Moloney, J. V., Kunert, B., Stolz, W., Koch, S., Murray, J., and Bedford, R., 5W Yellow Laser by intracavity frequency doubling of high-power vertical-external-cavity surface-emitting laser, IEEE Photonics Technology Letters, Vol. 20, No. 20, Oct (2008).
2. Ranta, S., Tavast, M., Leinonen, T., Van Lieu, N., Fetzer, G., and Guina, M., 1180 nm VECSEL with output power beyond 20W Electronics Letters vo. 49, Jan (2013).
3. Kantola, E., Leinonen, T., Ranta, S., Tavast, M., and Guina, M., High-efficiency 20 W yellow VECSEL, Optics Express, Vol 22. Issue 6, March (2014).
KEYWORDS: VECSEL Guidestar, 1141 Nm Guidestar, Sodium Guidestar, 589 Nm VECSEL