Laser Quest


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Eckhouse, Friesem, Davidson and Shimshi. Well-focused beam
You can't have your cake and eat it too. Or maybe sometimes you can, at least according to scientists at the Weizmann Institute of Science who set out on a quest to devise lasers that are both high-powered and razor-sharp - properties that are generally mutually exclusive.
Usually, powerful lasers have low beam quality, while high beam quality is attainable only in weak lasers. As opposed to light waves from a light bulb, which are emitted in all different wavelengths and directions, laser beam light waves are emitted in a single wavelength and direction. The more closely matched the waves, the higher the beam quality. A high-quality beam can be focused to a point like that of a sharp dagger, while a low-quality beam spreads out more like a blunt butter knife.

But what would happen if several weak lasers were combined? Would they turn into one strong, sharp laser? And the big question: Would the high quality of the separate weak beams be maintained once combined, or would some quality be lost in the process? A team headed by Profs. Asher Friesem and Nir Davidson of the Physics of Complex Systems Department, including Ph.D. students Amiel Ishaaya, Vardit Eckhouse and Liran Shimshi, sought answers to just these questions when they created a laser joining 16 individual weak beams into one powerful beam. In a paper published in Optics Letters, they showed their combination laser sustained a beam quality as high as that of the original weak beams.

To combine beams, the properties of all the light waves have to be coherent (identical in every way). Then the light waves must be superimposed in such a way that the peaks and troughs line up exactly with one another. But this is not so simple in practice: These conditions are very difficult to achieve, and the minutest disturbance can knock the waves out of their superimposed state.

The team - with a bit of "magic" - has managed to design a laser that largely overcomes such problems. In their device, 16 beams of light are produced and, like darts, they all travel straight toward a special optical element located within the laser cavity. This element is positioned such that beams that aren't coherent or properly aligned won't pass through; but if they are exactly the same and superimposed to boot, the element efficiently combines their individual powers. "And here comes the magic," says Davidson. "As if the laser beams have personalities and can make choices, they automatically choose to conform to one another. They seem to know their alternative is to be cast out, and they are able to rapidlyandcontinuouslyself-conform even under unstable conditions." The end result: an uncompromisingly stable, powerful and sharply focused combined beam.

Although this is not the first time laser beams have been joined, the team has taken laser combining to new heights. Other techniques face restrictions in beam numbers, but with this new innovation, the possibilities are unlimited, at least in theory. The design of this laser is also more stable and robust - crucial traits when translated to practical purposes. Their research, therefore, has significant implications in a wide range of commercial fields, from laser radars, optical communications, space exploration and material processing to laser treatments and surgery. Their next steps, already in progress, are to refine their design to eliminate practical constraints to combining even more beams, and to apply it to additional types of laser systems.

Prof. Nir Davidson's research is supported by the Fritz Haber Center for Physical Chemistry; the Levine Institute of Applied Science; the Rosa and Emilio Segre Fund; and the Cymerman-Jakubskind Prize. Prof. Davidson is the incumbent of the Peter and Carola Kleeman Professorial Chair of Optical Sciences.