MIT Lincoln Laboratory creates bright diode lasers

Researchers at MIT Lincoln Laboratory have created the most powerful coherently combined diode lasers yet demonstrated. These lasers address a long-standing challenge for laser technology—merging multiple laser beams coherently to produce a single-beam, high-brightness output.

SCOWL arrayA SCOWL array module used in COCHISE stage I, the demonstration of 7 W of high brightness, is shown. The view is of the front output end of the diode laser.

Brightness, which is a measure of the power that can be focused on a distant target, is a key parameter for diode lasers. Although diode lasers are efficient, compact, powerful, and reliable, they generally have poor beam quality at high power levels, leading to low brightness, when compared to other types of high-power lasers, including diode-pumped solid-state lasers and fiber lasers. It is possible to combine the output power of many diode laser elements through coherent beam combination (phased-array) approaches. However, coherent combination has generally been regarded as difficult, since phasing (coordinating the beams) lasers usually requires extraordinary phase control. 

If high-brightness diode lasers could be developed so that they are practicable and cost-effective, they may enable advancements for a variety of applications. Improvements to industrial lasers for materials processing, such as cutting, welding, and brazing, would provide higher precision and increased capability. Free-space optical communications, in which lasers transmit information to distant receivers, would become more reliable, and work over greater distances. Other applications of interest to Lincoln Laboratory's Department of Defense sponsors include target-tracking lasers, laser countermeasures, and laser radar. 

The Defense Advanced Research Projects Agency (DARPA) funded the Coherently Combined High-Power Single-Mode Emitters (COCHISE) program to spur the development of coherently combined diode lasers. Lincoln Laboratory researchers developed slab-coupled optical waveguide lasers (SCOWLs) as diode laser elements with nearly ideal single mode output beams. The SCOWL concept is based upon slab coupling, in which a large, multimode waveguide is converted to a large, single-mode waveguide by means of slab coupling of the higher-order waveguide modes.

Graph of the far-field profile from the array of SCOWLsThe nearly ideal coherently combined far-field profile from an array of SCOWLs is shown. The horizontal axis is the position along the detector used to record the far-field intensity. The five sharp peaks in the far-field profile are the unique signature of coherent beam combination, indicating a phase-coherent spatial far-field intensity distribution.

In stage I of the COCHISE program, the Laboratory demonstrated 7 W of high brightness, coherently combined output power from an array of ten SCOWL devices. This output represents the highest coherently combined beam power yet achieved using a diode laser array. In stage II, which continues through 2009, the objective is to produce a two-dimensional array of SCOWL devices with a high brightness, coherently combined output of 100 W.

The results of the DARPA-sponsored COCHISE program were reported in an invited paper at the Conference of Lasers and Electro-Optics (CLEO 2009) held in Baltimore in June 2009. 

Posted October 2009

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