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Fiber laser emits light radially

06 Jun 2006

Applications ranging from in vivo molecular imaging to photodynamic therapy could benefit from the development of a new radially-emitting fibre laser.

Unlike previous fiber lasers, which emit light along their axes, the new device developed by scientists in the US emits light radially from its circumferential surface. Thanks to its larger surface area, the new laser could be ideal for medical applications such as fluorescence imaging and diffusive optical imaging (Optics Express 14, 3929).

"Our surface emitting fiber laser provides continuous illumination over a large area to give a much higher resolution," Ofer Shapira of the Photonic Bandgap Fibers and Devices Group at MIT's Research Laboratory of Electronics told optics.org. "It is also simpler to construct mechanically to match the geometry of the imaged object."

As the team explains, axially emitting fiber lasers offer a much smaller light emitting area that can limit the performance of techniques such as fluorescence imaging. For example, researchers sometimes have to place several fiber tips around the imaged object to simultaneously excite multiple fluorescent samples.

Other applications cited by the MIT group include flexible displays, as well as light emitting textile clothing that operates both in the visible and in the near infrared (NIR), the latter with a number of potential military uses.

MIT's radially-emitting laser features a cylindrical hollow Photonic Band Gap (PBG) fiber transmitting in the visible and NIR, an organic dye gain medium introduced into the fiber's core and a linearly polarized axial pump.

"The large photonic bandgap of this fiber allows axial transmission of the pump as well as the transverse confinement of the dye emission, which provides the laser's resonant cavity," said Shapira. "The emission is azimuthally polarized in the transverse direction and has a dipole-like wave front due to the interplay between the cylindrical cavity, the anisotropic emission of the dye, and the linearly polarized pump."

The cylindrical PBG fiber is composed of 58 alternating layers of As2S3 and poly(etherimide) (PEI) with individual layer thicknesses of 59 nm and 89 nm, respectively. Researchers can control both the direction and polarization of the lasing by simply rotating the pump polarization.

"With the dye molecules oriented randomly, the excitation of a molecule oriented in the direction of the pump polarization is stronger than molecules oriented at other angles, giving stronger fluorescence in the direction of the pump polarization," added Shapira.

Darius Nikbin is Science/Technology Reporter on Optics.org and Opto & Laser Europe magazine.

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