Femtosecond pulses generated within optical fiber--no resonator cavity needed

IMAGE: Scientists at the Institute of Experimental Physics, Faculty of Physics, University of Warsaw have created a fiber-optic-based femtosecond laser. Above: Ph.D. student Jan Szczepanek at the lab. (Image credit: UW Physics, Grzegorz Krzyżewski)

Researchers in the faculty of physics at the University of Warsaw (Warsaw, Poland) have created a laser capable of generating ultrashort pulses of light even under extremely difficult external conditions.¹ The teams says that this unique combination of precision and resilience is due to the fact that the whole process of generating femtosecond laser pulses takes place within a special optical fiber.

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The compact tens-of-centimeters-square device is the first pulsed laser of its type, capable of generating femtosecond light pulses under truly extreme environmental conditions as the entire laser-generating activity occurs directly within the optical fiber itself, unlike typical ultrafast fiber lasers that require an optic resonator--a precision set of mirrors that is sensitive to external conditions.

"In our laser, the ultrashort pulses are generated directly in the fiber optic cable. The design is so simple that there is nothing that might break down," says Yuriy Stepanenko (UW Faculty of Physics and IPC PAS). And he admits that his team treated the new laser in ways highly unrecommended by the manufacturers of normal precision optical instruments: "We turned on the laser and then heated up a segment of the optical fiber to more than 120 degrees Celsius. The temperature gradient was therefore really large, and the laser still worked well. We also put it into a shaker, with acceleration in excess of 7 g. It still worked afterwards, and most interestingly it also worked during the testing."

The femtosecond laser from the UW Faculty of Physics generates pulses in an ytterbium-doped optical fiber. The wavelength of the light emitted is close to a micron (1030 nm), which can then be multiplied by generating higher-order harmonics.

The optical fiber itself is flexible, and so laser pulses can be easily led into places inaccessible to traditional laser techniques. For industrial applications, it is no less important that the laser beam still preserves excellent spatial quality irrespective of how the fiber optic cable is positioned. Here, the laser cross-section still shows the optimal bell curve (Gaussian distribution).

The "spaghetti noodle" laser, as its designers jokingly describe it, also has one more advantage: the simplicity of its design will make it a relatively inexpensive instrument. Built using commerciallyavailable components (a pump semiconductor diode and its driver), it would cost just a few thousand euro. Companies interested in commercializing the device could also seek additional ways to cut the cost, for instance by using a custom-designed driver.

In addition to materials processing applications, the laser could also be an important element of devices generating terahertz radiation, such as airport scanners, as well as refined measurement devices (such as in two-photon microscopy) and medical equipment (such as in optical coherence tomography, used to study soft tissues like the retina).

REFERENCE: 1. J. Szczepanek et al., Optics Letters 40, 15, 3500-3503 (2015).

SOURCE: University of Warsaw; http://www.fuw.edu.pl/tl_files/aktualnosci/2015/2015-08-19_a_ang.pdf