To maintain an advantage in today’s battlespace, our forces require the ability to engage targets at longer ranges and see them with higher resolution. Active electro-optical (EO) systems address this need, providing important mission capabilities such as ranging, tracking, marking, designating, 3-D imaging (ladar), chemical and biological agent detection, and laser defense using high-energy lasers (HELs). Compared to passive EO systems (FLIR or camera) or active radio frequency (RF) systems (radar), active EO systems allow us to increase both range and resolution and also to perform new types of missions. A critical component of any active EO system is the laser used to illuminate the target. To offer our customers the highest performance systems, we must utilize advanced lasers meeting ever more challenging requirements in the areas of laser power, beam quality, efficiency, size and weight. Lasers are inherently inefficient, converting only a portion of the electrical input power into useful laser output power. The size, weight and input power of a laser system are largely driven by two factors: 1) the average output power of the laser and 2) its efficiency. In the design of our active EO systems, we typically optimize the system design to minimize the required average power from the laser and then optimize the laser design for high efficiency. All lasers require a “gain medium,” a material that can emit a laser beam, and a “pump source,” a means of exciting atoms in the gain medium so that they emit light. Historically, most military laser systems have used arclamps to excite the laser gain medium with resulting efficiencies of just a few percent. During the past decade, efficiencies of 20 percent or more have been achieved by diode-pumped solid-state lasers due to development of high-power laser diodes as a more efficient means of exiting the gain medium, as well as advances in the gain medium configurations utilized. While laser diode pumping is a key factor in enabling high efficiency, scaling laser-output power while maintaining high beam quality necessitates improvements in the geometry of the gain medium itself. Due to the fact that lasers are not 100 percent efficient, significant amounts of waste heat are generated in the gain medium during laser operation. This waste heat can create distortions in the gain medium, adversely affecting important properties of the laser beam. Raytheon has long been at the forefront of laser technology development for military applications, and is currently focused on advanced laser architectures that leverage the benefits of laser diode pumping and address the shortcomings of conventional laser gain medium Feature Next-GenerationLasersforAdvancedActiveEOSystems architectures such as the venerable cylindrical rod and, more recently, bulk slab geometries. The optimal gain medium geometry for a given application varies, depending on the average power and laser waveform, but all of the advanced gain medium geometries employed by Raytheon seek to minimize the amount of waste heat and remove the waste heat from the gain medium in a manner that minimizes adverse effects on the quality of the laser beam. The goal of minimizing adverse thermal gradients within the gain medium has led Raytheon to focus on three primary gain medium geometries for advanced laser systems: microchip lasers, fiber lasers and planar waveguide lasers. Microchip lasers are very simple, robust devices for applications requiring up to ~1W of average laser power. They can be operated in a pulsed mode with pulse energies up to ~1mJ and pulse widths as short as ~1 nanosecond, enabling peak powers up to 1MW. Fiber lasers and planar waveguide lasers both enable scaling of laser average power up to the kW level by using gain medium geometries with large surface- Continued on page 10 RAYTHEON TECHNOLOGY TODAY 2008 ISSUE 1 9
Feature Next-GenerationLasers Continued from page 9 area-to-volume ratios that provide efficient cooling of the gain medium and minimize adverse thermal effects. Both can also be efficiently pumped by laser diodes. In a fiber laser, the gain medium is configured as a long filament, while in a planar waveguide (PWG), the gain medium is configured as a thin sheet. Fiber lasers are well suited to applications with average powers up to 1kW when the pulse energy does not exceed a few mJ. Planar waveguide lasers are currently being developed by Raytheonfor applications with average powers ranging from 10kW. The PWG has the potential to scale in average power to the MW level and produce pulse energies up to >1 J. Fiber lasers evolved out of the telecom community beginning in the late 1980s, when they were invented to enable massive increases in data throughput by directly amplifying the packets of laser light that carry information in fibers around the planet and under the oceans. During the past 15 years or so, the power capability of fiber lasers has increased five orders of magnitude, from 10s of mW to several kW. Raytheon is now actively exploring how these efficient, versatile laser sources can be inserted into advanced defense systems. Figure 1 shows a fiber-based master oscillator, power amplifier configuration. The fiber gain medium is formed into a 10 cm coil, Micro-laser Remote fiber pigtailed pump diodes Passively cooled fiber ~ 10 cm coil 10 2008 ISSUE 1 RAYTHEON TECHNOLOGY TODAY and it is excited by several pump diodes. Note that the pump power is directly coupled into the gain fiber through conventional passive fibers, thereby avoiding any free-space optics in the pump coupling function. A micro-laser generates a weak signal containing the properties appropriate for the intended application: wavelength, spectral bandwidth, temporal profile, beam quality, etc.; the fiber amplifier adds the power. We see that, unlike most other types of lasers, there are essentially no freespace optics in the signal channel — just robust, flexible fibers — and there is no need for a rigid, thermally stable optical bench. The inset shows the cross-section of a state-of-the-art cladding-pumped fiber amplifier. The active core occupies just a fraction of the fiber, with most of the cross-sectional area being made available to receive the diode pump power. The fiber is made sufficiently long that nearly all of the pump power is ultimately absorbed by the core, despite the small relative size of the core. Typical dimensions are core diameter ~ 20 μm, and pump cladding diameter ~ 400 μm. In addition to the packaging features of fiber lasers, they are also among the most efficient lasers ever built. One major factor leading to the high efficiency has to do with the tiny core along the fiber axis that contains the laser ion (typically Yb or Er), the pump light and the signal light. Since the pump and signal are closely confined Cladding-pumped fiber amplifier Cross-section Robust single-mode output beam quality Figure 1. Schematic diagram showing a fiber-based master oscillator, power amplifier laser system. Inset shows the cross-section of a cladding-pumped fiber amplifier. Core Pump cladding Outer cladding (typ. polymer) within the fiber, and the interaction length can be made very long (many meters, if necessary), very efficient conversion can occur from the pump power to signal power. Commercial fiber lasers typically demonstrate more than 70 percent power conversion efficiency from the pump to signal. Another feature is that the tiny core size does not allow anything but the lowest-order transverse spatial profile, which rigorously forces the beam divergence of the output signal to the minimum value allowed by fundamental physical laws. These and other features are summarized in Table 1. Table 1. Features of Fiber Lasers High efficiency, due to the excellent spatial overlap of the pump and signal Rigorously single-mode outputs Favorable thermal geometry with a large surface-to-volume ratio Compact size and considerable packaging flexibility Recent technical breakthroughs allowing power scaling to > 1kW with a single fiber Evolving all-fiber architectures free of any free-space propagation of signal beams Common pump-diode technology developed for bulk crystalline solid-state lasers A foundation in the telecom culture, with mature materials and processes that offer robust components with long operational lifetimes Not listed in Table 1 is “high power.” This is because the same tiny core that makes fiber lasers efficient and ensures excellent beam quality also makes it difficult to produce high-peak or average powers without either degrading some other performance parameter of interest, or causing severe damage to the fiber medium. However, the laser community is actively working on innovative fiber laser designs that, hopefully, will retain all of the key performance features listed in Table 1, while allowing power scaling by several orders of magnitude. Raytheon is pursuing a proprietary approach to accomplishing these objectives, and we anticipate significant new power capabilities in the next few years.