TECHNOLOGY AREAS: Sensors, Electronics, Weapons

ACQUISITION PROGRAM: PMS 405 Ultrashort Pulse Laser Development. ACAT level N/A

OBJECTIVE: The goals of this STTR: (1) to develop a concept for a compact, environmentally stable approach to the temporal control of the pulse quality of USP lasers; (2) to evaluate this approach in an eye-safer 1.5 micron USP laser with greater than 1 mJ per pulse energy. Studies should include the evaluation of hardware for control and sensing, investigation of robust real time control algorithms, and testing with a compact fiber laser source with autonomous software control. Optical solutions that could be exploited for future spatial beam correction methods are preferred.

DESCRIPTION: The purpose of this topic is to innovate a compact and robust active control method to scale the energy of ultrashort pulse (USP) fiber laser sources. The results will enable solutions for applications of interest to the US Navy, including, but not limited to, directed energy weapons, ECM, and LADAR.

Fiber lasers offer many advantages for Navy applications: high efficiency, high average power and good beam quality with a small form factor. Single fiber lasers with average power up to several kilowatts have been demonstrated, and even higher power levels have been obtained from arrays of such lasers. Key advantages for fiber lasers are direct diode pump configurations as well as beneficial geometry for thermal management.

While fiber lasers easily scale to high average power, pulsed laser emission is inherently more susceptible to temporal distortions. Two main methods are commonly employed to mitigate the distortions. First, chirped pulse amplification (CPA) reduces peak power by up to three orders of magnitude. Second, much work has been done to reduce distortions by changes in the fiber geometry, primarily by deploying larger core sizes and shorter fibers with higher dopant concentration. However, both methods are ultimately limited. Longer stretched pulses require increasingly large and complex optics, and fiber geometry improvements are limited by fiber fabrication and beam quality issues.

The energy levels of USP fiber lasers could potentially be further extended by exploiting control methods in the time domain similar to those employed in correcting for spatial distortions. An example of the enabling power of such systems is the spatial control method exploited in very large terrestrial telescopes. These telescopes would not otherwise be feasible as they would be limited by atmospheric distortions. Studies and bench top experiments have shown the potential of temporal control in USP lasers. Nonetheless, due to complexity of the solutions and available lasers, these methods have not been commercialized into environmentally stable and compact lasers. Innovative approaches to this temporal control are required in order to pave the way for compact USP lasers with energies in the mj and higher range.

PHASE I: Conduct research, analysis, and studies for a compact high energy short-pulse fiber laser architecture at 1.5 micron eye-safer wavelength with active temporal control. Develop measures of performance and document results in a final report. The Phase I effort should include modeling and simulation results supporting performance claims. The effort should also produce a concept for evaluating the proposed laser architecture in the phase II effort.

PHASE II: Evaluate the concept developed in Phase I for a temporal control and sensing system with a compact 1.5 micron eye-safer USP laser with autonomous software control, to show scaling to pulse energies of 1 mJ and greater output.

PHASE III: Develop a rugged, deployable temporal correction and sensing system suitable for deployment in both civilian and military applications. Specific requirements will be based on the specific application.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: USP fiber lasers are a broadly enabling technology with a multitude of applications in the private sector, including manufacturing, medical technology, and life sciences. Temporal correction and sensing enables USP fiber lasers to reach higher peak power and pulse energy. Increased peak power and pulse energy give additional capabilities in these fields. Accordingly, it is expected that this technology will be of high value for USP laser vendors and USP laser users.

REFERENCES:
1. G. P. Agrawal, Nonlinear Fiber Optics, Third Edition, San Diego, CA: Academic Press, 2001.

2. TMT Science Advisory Commission, Thirty Meter Telescope Detailed Science Case: 2007.

3. A. Braun, S. Kane and T. Norris, "Compensation of self phase modulation in chirped-pulse amplification laser systems. Optics Letters, Vol. 22, No. 9 pp. 615-617, 1997.

4. J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, C. Jakobsen, "High-power rod-type photonic crystal fiber laser," Opt. Express, vol. 13, no. 4, pp. 1055-1058, 2005.

KEYWORDS: Ultra-short pulses; Millijoule pulses; High-energy amplifiers; High peak power pulses; Compact fiber amplifiers; Eye-safer fiber amplifiers; Pulse control; Environmentally robust