Scalable, high-power fiber laser produces coherent output

High-power fiber-laser arrays can potentially produce tens or hundreds of kilowatts of output power, providing significant military capabilities. At the May Solid-State and Diode Laser Technology Review workshop, hosted by the Directed Energy Professional Society (DEPS) in Albuquerque, NM, researchers from Northrop Grumman Space Technology demonstrated significant progress toward a seven-element fiber-laser array based on ytterbium (Yb)-doped polarization-maintaining single-mode power amplifiers. The 155-W output from the single fiber laser was a record for this type of coherent-beam-combining design and should prove highly scalable.

The demonstration was sponsored by the Defense Department's High Energy Joint Technology Office, with costs shared by Northrop Grumman (Los Angeles, CA). Previous work was funded by the U.S. Air Force. It is one of several programs by government agencies and the military to develop capabilities based on technologies such as solid-state Nd:YAG slab lasers and single and coupled fiber lasers. Solid-state lasers provide an alternative to chemical lasers for applications on the battlefield, including missile defense, ship protection, target illumination, and attacking ground targets. Fiber lasers have the advantage of high efficiency, simpler design and support logistics, excellent beam quality, and a large surface area for removing heat.

FIGURE 1. The design of the seven-fiber-laser array shows the phase sensor and modulator feedback loop derived from the reference arm. In this experiment, only one signal arm was tested, achieving 155-W output power.Click here to enlarge image

The Northrop Grumman demonstration team, led by Michael Wickham, constructed an experimental setup based on an architecture of massively parallel elements (see Fig. 1).¹ The master oscillator (MO) is a distributed-feedback laser with a modulated drive current that produces a spectrally broad output to suppress stimulated Brillouin scattering and four-wave mixing—parasitic nonlinear processes that limit the output achievable from single-frequency, single-mode fiber lasers. The output is divided among eight paths: one reference arm, and seven amplified signal arms.

The reference arm is frequency-shifted in an acousto-optic Bragg cell and then a sample of each signal arm is interfered with the reference beam to generate a heterodyne beat waveform used to measure the phase of that signal arm relative to that of the reference arm. The phase of each arm is adjusted with a control voltage to individual lithium niobate waveguide phase adjusters.

Two sets of preamplifiers bring the power up to 3 W before the power amplifier; the pump light is injected into the power amplifier through the output end of the fiber. The output light for each signal is then sent to one of the lenses of the output array and phase corrected. For this experiment, only one signal arm is optimized, with an original goal of producing 50 W of output power. The 155 W that the group actually achieved was limited only by the optical pump power available. There should be no degradation of phase-correction capability as more elements are added (see Fig. 2).

FIGURE 2. A three-dimensional model of the experimental layout shows the pump modules and a detail of the pump and output coupling.Click here to enlarge image

"We should be able to phase all seven fiber lasers together by late summer or early fall," according to Jackie Gish, director for directed energy and products at Northrop Grumman. She says that the research group has already phased four fiber lasers together at low power, and she sees no real barrier to completing the project successfully, and no real upper limit on total output power. Gish says the next step would be packaging the fiber-laser array into smaller dimensions.

The lasers are based on Yb-doped polarization-maintaining double-clad optical fibers from Nufern (East Granby, CT). A Nufern team led by David Machewirth also presented a paper at the Directed Energy Professional Society workshop, outlining the basic design of their large-mode-area fibers used for beam combining.² The company has developed "Panda"-type fibers exhibiting numerical apertures as low as 0.06, cladding absorptions exceeding 3 dB/m (near 976 nm), and slope efficiencies of 77%. The large mode areas and polarization-maintaining qualities are critical to overcoming undesirable nonlinear processes and enabling beam combining.