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Multimode lasers get brightness boost

18 Apr 2005

Israeli scientists report a method for producing multimode lasers with much better beam quality.

A way to dramatically improve the beam quality of a multimode laser has been demonstrated by researchers in Israel. In tests, the technique increased the brightness of a multimode Nd:YAG laser by a factor of ten (Optics Express 13 2722).

The leap in performance is achieved by splitting the multimode beam within the laser cavity into an array of smaller, higher-quality beams which are then coherently combined before leaving the laser.

“Essentially our technique should be useful whenever there is need for high output powers combined with good beam quality and compact dimensions,” Amiel Ishaaya from the Weizmann Institute of Science in Rehovot told Optics.org. “Many potential applications exist in industry, medicine, military and scientific fields.”

In Initial tests the Weizmann team split the multimode beam of a flash-lamp pumped Nd:YAG laser into four Gaussian beams by inserting a mask containing four small circular apertures (1.4 mm diameter and spaced by 2.4 mm apart) into the laser cavity.

Just before the output coupler, these beams were then brought together by the use of two interferometric beam combiners which effectively fold the separate beams on top of each other.

The first combiner performs horizontal folding while the second performs vertical folding, in each case the phase is carefully preserved so that the beams add coherently. At low pump powers the combining efficiency was measured to be as high as 90% but dropped to 80% at higher pump powers.

Operating the laser with a single large square aperture and no beam combiners resulted in a large square multimode beam with M2 values in the x and y axes of 6.3 and 6.0 respectively.

In contrast the 4-beam combined design produced a high quality almost circular beam with M2 values of 1.23 and 1.31, resulting in a brightness enhancement of 10.5

The team is now experimenting with splitting the multimode beam into a larger array of sub-beams and also applying the technique to different kinds of lasers, “We certainly believe that our technique can be scaled up from 4 sub-beams,” said Ishaaya. “Indeed, we are currently experimenting with 9 and even 16 beams with very promising results.”

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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