The idea of using optical fibers to generate pulse lasers has grown in demand due to the ability of these lasers to be compact in size but have a high output and high optical quality as well as the durability to work in harsh environments.
Researchers at the Warsaw Laser Centre of the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Physics of the University of Warsaw have developed a method to create ultrashort laser pulses using optical fibers that are simple to design.
The scientists have forced one of the types of optical fiber lasers to generate ultrashort high energy pulses without the use of mechanically sensitive external parts. The invention could greatly shorten the time of processing materials in industrial laser machines, researchers say.
“Fiber lasers can be built so that all the processes important for the generation and shaping of the ultrashort pulses take place in the fiber itself,” says Dr. Yuriy Stepanenko of the IPC PAS. “Such devices, without any external mechanically sensitive components, operate in a very stable manner and are ideal for working in difficult conditions.”
Researchers say the laser action in the fiber leads to the generation of a continuous light beam, however, the release of energy in short pulses is more favorable because it produces a great increase of power. These pulses are generated by a saturable absorber that when light intensity is low, the absorber blocks light, when it is high it lets light through. The absorber can also be adjusted so that it only lets through pulses.
“Up to now, graphene sheets, among others, have been used as the saturable absorbers, in a form of a thin layer deposited on the tip of the fiber. But the diameters of optical fibers are in the order of single microns. Even a little energy cramped in such a small cross-section has a significant density per unit area, affecting the lifetime of the materials,” says Jan Szczepanek, a PhD student from the Faculty of Physics of the University of Warsaw. “Therefore, if an attempt was made to increase the power of the femtosecond pulses, the graphene on the tip of the connector was destroyed. Other absorbers, such as carbon nanotubes, may also undergo degradation.”
How It Works
The Warsaw researchers were able to generate a higher energy pulse in the optical fiber by improving saturable absorbers using nonlinear effects that caused a change in the refractive index of glass.
The nonlinear artificial saturable absorber works by dividing linearly polarized light into a beam with low intensity and a beam with high intensity. The medium of the absorber can be chosen for both light beams to travel at slightly different phase velocities. The result is the plane of polarization starts to rotate and at the output of the absorber a polarization filter lets in only waves oscillating perpendicularly to the plane of polarization of the incoming light.
When in continuous mode, the light in the beam is of low intensity, the polarization does not change and the output filter blocks the light. At high enough intensity for pulsing lasers, the rotation of polarization causes the pulse to pass through the polarizer.
In order to get the saturable absorber with polarization rotation to work, researchers cut the optical fiber into segments of an appropriate length and then reconnected them, rotating each successive segment 90 degrees in relation to its predecessor. The more rotated segments there are, the better the quality of the pulses generated in the fiber.
Warsaw researchers created a laser with a saturable absorber that consists of a fiber with a length of about three meters and divided it into three segments and a filtering polarizer. Researchers believe the rotated segments can be increased up to a dozen.
The laser generated by Warsaw University produces energy that is 1,000 times larger than typical for lasers with material absorbers.