Chirped Fiber Improves Pulse

By introducing a radial chirp into a photonic crystal, researchers in Germany and Russia have developed a novel optical fiber that can transmit ultrashort light pulses with very little distortion over extended distances. The fiber could prove useful in medical applications that require femtosecond pulses to be delivered with minimal distortions through sharp bends, such as through an endoscope.

Scientists at the Max Born Institute for Nonlinear Optics and Short-Pulse Spectroscopy (MBI) in Berlin, working in collaboration with those at the Institut fur Festkorpertheorie (Institute for Solid Theory and Optics) at Friedrich Schiller University in Jena, the Institute for Applied Photonics in Berlin and Saratov State University in Saratov, Russia, transmitted light pulses of 13 femtoseconds in duration (1 femtosecond is one million-billionth of a second) over a distance of 1 meter, with the pulses only stretching to about double that of the initial duration.

Cross-section of the Max Born Institute for Nonlinear Optics and Short-Pulse Spectroscopy's chirped fiber in a scanning electron microscope. (Image copyright Institute for Applied Photonics Assoc.)

Chirping is caused by wavelength changes over the duration of the pulse. The concept of cell-size chirping has been successfully applied in one-dimensional photonic structures such as chirped mirrors and chirped fiber Bragg gratings, but this research represents the first time that a chirped optical fiber has guided sub-100 femtosecond pulses over extended distances, allowing a new degree of freedom in photonic crystal fiber design and eliminating much of the pulse-duration restrictions of previous approaches, the researchers said.

“Currently, no other fiber-based technique is capable of such little distortion,” said Dr. Günter Steinmeyer of the Max Born Institute. Pulse stretching to more than 50 times the original duration was observed in similar fibers of a more conventional design.

The MBI fiber, manufactured at Saratov State, consists of five circular layers of glass tubes of different diameters, with each layer consisting of 30 identical cells. In contrast to conventional hollow fibers, which consist of capillaries of equal diameter, the diameter changes in MBI’s fiber, resulting in what looks like rings of straws with increasing diameters glued together. Launching ultrashort laser pulses into such a fiber, the chirped structure acts to distribute detrimental resonances over a wide wavelength range, which would otherwise add up at one wavelength if the capillaries had all the same diameter.

The researchers said one promising medical application for the fiber could be in photodynamic therapy (PDT), in which a drug works as a photosensitizer or photosensitizing agent in conjunction with light. When photosensitizers are exposed to a specific wavelength of light, they produce a form of oxygen that kills nearby cells. Because their method using ultrashort laser pulses rather than continuous light, the researchers said, it could significantly improve the selectivity of the therapy, with photoexcitation limited to the immediate vicinity of the target area, leaving tissue layers immediately above or below unharmed.

The chirped fiber structure could also be beneficial for diagnostic applications in biology and medicine, such as in two-photon microscopy, a method that allows for 3-D resolution of the smallest biological structures, the researchers said.

The work was published online Oct. 12 by Nature Photonics.

For more information, visit: www.mbi-berlin.de/index_en.html