December 2004
Space-Based Laser Research
Researchers are developing a reliable and versatile high-powered, space-based laser for missile defense systems
AFRL's Directed Energy Directorate, Laser Division,
High Power Gas and Chemical Lasers Branch,
Kirtland AFB NM

Unfortunately, both devices have properties that make them less than ideal for space-based applications. Researchers demonstrated the fundamental HF laser (baseline technology of the SBL program) at very high powers. The fundamental HF laser operates at 2.6 to 3.0 µm and is well suited for targets in space. The utility of the HF laser for applications that require long path lengths through the atmosphere is slightly diminished by atmospheric water absorption at approximately 2.7 µm. This problem is partially mitigated by the overlap between a strong line and a transmission window at approximately 2.9 µm that allows a fraction of the HF laser radiation to pass through the atmosphere without complete absorption. In addition, researchers expect the large optics required to direct the beam across the very long path lengths to be extremely heavy and difficult to manufacture.COIL, on the other hand, operates at very high energies and at an atmospherically friendly wavelength of 1.3 µm. Unfortunately, contemporary COIL devices use a two-phase chemical mechanism that is ill suited for the zero gravity environment in space. Furthermore, the key COIL ingredient of basic hydrogen peroxide is difficult to store for extended periods of time.

Although the concept of generating electronically excited oxygen via electric discharge originated in the 1950s,4 advances in plasma generation techniques fueled an increased interest in recent years in the development of electrical discharge sources of electronically excited oxygen for COIL applications.5,6,7,8 The goal of this work is to generate sufficient quantities of the energy carrier at high density and at conditions suitable for a laser device. Researchers have examined a variety of discharge techniques over the last 10 to 15 years with limited success. In general, the most difficult aspect of this work is to engineer and build a discharge that operates at both the proper electro- and thermo-dynamic conditions. If successful, this work could revolutionize the utility of oxygen iodine lasers by significantly reducing the logistics trail and simultaneously improving the maintainability and supportability of the laser device. In fact, if researchers can eliminate the chemical plant used to fuel a traditional COIL device, a space-based COIL may be viable. The directorate is leading this effort through collaborative efforts with international partners, as well as domestic experts, by leveraging funding provided by the MDA's Small Business Innovation Research program.

In addition to this advanced COIL work, the HF overtone and all gas-phase iodine lasers (AGIL) are two concepts the directorate is developing that address key space-based laser issues. The HF overtone laser uses the same chemistry as the fundamental HF laser, but it operates on 1.3 µm vibrational overtone transitions rather than 2.7 µm fundamental vibrational transitions. Even though quantum mechanical constraints make overtone transitions 50 to 100 times weaker than fundamental transitions, extensive work by the US Army,9,10,11 TRW,12 and the University of Illinois13 shows that 50 to 90% of the power available in an HF fundamental laser can be extracted on the overtone. Unfortunately, HF overtone devices typically have small signal gains. As a result, all known HF overtone research devices use stable resonators to extract the energy. Large aperture, stable resonator-based laser systems have large divergences and are not well suited for long-range propagation applications. Improving the small signal gain is the focus of the directorate's work. Using state-of-theart diagnostics, researchers are characterizing HF laser devices and optimizing the overtone gain. Most recently, the directorate's team used a small-scale, discharge-driven device and measured peak gains14,15,16 in the range of 1.0 to 1.7 × 10-3 cm-1. The highest previously reported value was 8.5 × 10-4 cm-1 in 1993.13 Future work will utilize a combustion-driven supersonic HF device that should produce even higher gain values. In addition to the development of an overtone small signal gain probe, researchers are working on other important diagnostic devices for HF lasers including a fundamental small signal gain probe and advanced flow visualization tools.

The directorate is also pioneering the development of new chemical lasers that have the potential for use in space. In 2000, researchers in the directorate's High Power Gas and Chemical Lasers Branch, demonstrated the first new chemical laser in nearly 25 years.17 The AGIL is a laser technology exclusive to AFRL. Because AGIL operates on the same electronically excited iodine transitions as COIL, there are no atmospheric absorption issues. Furthermore, instead of COIL's twophase chemistry, AGIL produces electronically excited nitrogen chloride molecules by all gas-phase reactions. While the initial demonstrations achieved only 180 mw of output power, researchers produced an output power in excess of 30 W in more recent experiments with a larger, improved AGIL device.18 In addition to scaling the power, the AGIL Team increased the small signal gain from 2.5 × 10-4 to 4.2 × 10-4 cm-1. The next-generation AGIL will incorporate HF laser technology to demonstrate an AGIL that uses fully scalable technology. Instead of using non-scalable and density-limited discharge tubes to produce fluoride atoms, researchers will integrate an HFlaser- like chemical combustor into the design. The directorate hopes the combination of new diagnostics and laser device options will pave the way for more reliable, versatile, and technologically mature laser devices for future SBL missile defense systems.

Dr. Gerald C. Manke II, of the Air Force Research Laboratory's Directed Energy Directorate, wrote this article. For more information contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document DE-03-02.