Surveillance still plays a critical role in the detection and assessment of hostile threats to the United States. High-resolution imaging satellites have been deployed for more than 50 years to provide critical data for U.S. defense experts over denied airspace. The progress in optical sensors over the past decade has created an exponential growth in intelligence, surveillance, and reconnaissance (ISR) data from both passive and active sensors. This progress includes not just an increase in area coverage rate, but also an increase in sensor capabilities and performance. Material advances have made collection at new wavelengths feasible, and improved components provide new data signatures including vibrometry, polarimetry, hyper-spectral signatures, and three-dimensional data that mitigate camouflage for targets of interest.
A key advance since the NRC’s 1998 study is the dramatic increase in the application of active optical sensors for surveillance. The primary impetus for this increase has been the advances made in laser technology, including advances in robustness, efficiency, and optical power (see Figure C.1) for many wavelengths. In order for optical sensors to be widely fielded, they must also meet eye-safety requirements, which have driven advances in sources and amplifiers for 1.5 and 2 µm wavelength lasers.
In order to maximize atmospheric transmission, there has been a push for longer wavelength amplifiers in regimes with efficient detectors. Recent advances in thulium (Tm)-doped fiber amplifiers enable laser sensor operation in the 1.9 to 2.1 µm wavelength range. Average power levels are approaching the kilowatt level, and pulsed amplifiers with peak powers approaching 100 kW have been demonstrated.2,3,4,5 Several vendors are offering lasers with output powers up to 150 W. There are also several vendors offering narrow line-width, rapidly tunable 2.1 µm laser sources. The atmospheric transmission at this wavelength combined with the availability of commercial amplifiers, sources, and detectors makes 2.1 µm an attractive wavelength for long-range laser sensing. The efficiency of the current
2 Cristensen, S., G. Frith, and B. Samson. 2008. “Developments in Thulium-Doped Fiber Lasers Offer Higher Powers.” SPIE Newsroom. DOI: 10.1117/2.1200807.1152. Available at http://spie.org/x26003.xml. Accessed July 31, 2012.
3 Moulton, P.F., G.A. Rines, E.V. Slobodtchikov, K.F. Wall, G. Firth, B. Samson, and A.L.G. Carter. 2009. Tm-doped fiber lasers: Fundamentals and power scaling. IEEE Journal of Selected Topics in Quantum Electronics 15(1):85-92.
4 Sudesh, V., T.S. McComb, R.A. Sims, L. Shah, M. Richardson, and J. Stryjewsky. 2009. Latest developments in high power, tunable, CW, narrow line thulium fiber laser for deployment to the ISTEF. Proceedings of SPIE 7325:73250B.
5 McComb, T.S., R.A. Sims, C.C.C. Willis, P. Kadwani, V. Sudesh, L. Shah, and M. Richardson. 2010. High-power widely tunable thulium fiber lasers. Applied Optics 49(32):6236.