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Simple scheme combines multiple beams

19 Jan 2006

Sixteen laser beams are combined into one high-power source thanks to four specially-designed components.

Scientists in Israel have come up with a simple way to coherently recombine 16 laser beams using a clever intracavity optical scheme. The researchers say the combined beam has both a high output power and a good beam quality making it ideal for applications such as laser range finders, free-space optical communications and materials processing. (Optics Letters to appear)

The team's idea involves inserting four specially-designed interferometric combiners into a laser resonator. Made from fused-silica parallel plates, half of the front surfaces of the combiners are coated with a 50% beam-splitter layer and half of the back surfaces are coated with a highly reflective layer. The other two halves are anti-reflection coated.

Nir Davidson, Asher Friesem and their colleagues from the Weizmann Institute of Science believe this approach has several distinct advantages over other beam combining schemes.

"It is stable against mechanical and thermal vibrations since the entire interferometer is formed on a single substrate via multiple coatings," Davidson told Optics.org. "The coherent combining is also performed outside the gain medium so the higher power of the combined beam cannot damage the more sensitive parts of the laser cavity. The number of elements required to combine a large number of beams is also small."

Davidson adds that common longitudinal modes have to exist between all the beams so that they can be coherently combined. This problem was solved by placing common end mirrors at either end of the resonator.

To test its idea, Davidson's team used a resonator containing a 9 mm diameter Nd:YAG rod and a pair of common end mirrors to create 16 Gaussian beams. Without the combiners, the resonator generated 2.6 mJ pulses with a beam quality M2 of 1.3.

After inserting the first pair of combiners into the resonator, which merged four beams, the researchers measured an output of 10 mJ per pulse, a combining efficiency of 96% and a beam quality of 1.3. When the remaining two combiners were used and 16 beams were combined, the output rose to 37 mJ per pulse, the combining efficiency fell to 88% and the beam quality stayed constant at 1.3.

"We have demonstrated efficient and robust multi-channel coherent combining both for free running pulses with a pulse width of around 200 microseconds as well as for Q-switched operation where the pulse width as a few tens of nanoseconds."

The team also says its intracavity combining approach will work for continuous wave (CW) sources. "We have already demonstrated it for two and four channels," said Davidson. "However, since CW lasers usually have smaller gain they are more sensitive to intracavity losses and more care must be taken to align the combiners."

Having successfully demonstrated the potential of the technique, the team is keen to explore a number of new avenues of research. "We want to add high order modes and even multimode channels," concluded Davidson. "We also want to apply the technique to other laser systems, in particular diode and fiber lasers, and to different wavelength ranges. Finally, we want to increase the number of channels by adding more intracavity combiners."

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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