An international team with the participation of the Higher Council for Scientific Research (CSIC) has achieved the first fiber laser that achieves high energies and peak powers in pulses in the femtosecond range (billionths of a second) without the help of additional amplification stages. The advance, published in the journal Optics & Laser Technology, achieves a more compact, simple, cheap and robust design of the laser, and will allow multiple applications in areas that require high power, such as materials processing, telecommunications, biomedicine, metrology or spectroscopy, among others.

Lasers are now 60 years old and yet they are in full force and constantly evolving due to the high demand for ultrafast radiation in various fields of science and industry. On the one hand, research tries to obtain lasers with increasingly shorter pulses: ultrafast, and on the other, they try to make them have greater intensity.

Among the characteristics of this type of source, fiber lasers with passive mode blocking ─a type of laser that emits light in the form of pulses or flashes in extraordinarily short times, on the order of femtosecond (10-15s) ─ stand out especially for its many advantages, such as its compact design, low cost, stability and ease of use. "They are also characterized by not depending on external signal modulators, hence the passive", explains Juan Diego Ania, CSIC researcher at the Instituto de Óptica Daza.
In this study, in which the CSIC Optics Institute, the University of Alcalá and the Commissariat of Atomic Energy of Grenoble in France (CEA) have collaborated, ultra-fast fiber lasers with passive blocking have been developed. Thanks to them, for the first time peak powers (laser output intensity) above one megawatt are achieved, with pulses in the femtosecond range, without the help of additional amplification stages, as occurs, for example, with the technique chirped pulse amplification (CPA).

This system, winner of the Nobel Prize in 2018, consists of creating high intensity laser pulses, widening them to reduce their peak power and so that the medium does not suffer damage through a subsequent optical amplification that serves to increase its energy. The pulse is then compressed again, allowing it to reach very high powers. "The CPA technique and other derivatives have allowed a real revolution in photonics in recent years and are absolutely essential in applications that require very high energies", explains Ania. "What we show is that, for certain power ranges and applications, you can find other simpler, more compact, robust and affordable solutions," she adds.
The researchers have designed a femtosecond pulse laser in a ring configuration and independent of polarization, that is, of the direction of the oscillation of the emitted radiation, which facilitates its application, assembly and integration. "It is based on standard optical components using a cavity much longer than usual (2.4 km), making it possible to obtain pulses below 250 femtoseconds and peak powers greater than one megawatt (while this type of lasers are usually below one hundred kilowatts), as well as an energy per pulse greater than 250 nanojoules ”, adds Ania.

The high powers of the new laser have been obtained by increasing the length of the laser cavity to several kilometers and keeping the duration of the pulses in the range of hundreds of femtoseconds at all times, thanks to the use of adequate dispersion control and of the powers inside the cavity to avoid widening of the pulses.
These properties open the possibility of applying this type of sources to new fields that need high power (such as materials processing and other industrial applications), without the need, for the first time, of external amplification stages, while allowing their use direct in areas such as the measurement of atmospheric pollutants, the development of laser measurement and detection systems (LIDAR) or high precision metrology.

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The work is a collaboration between the Institute of Optics, the photonics engineering group of the electronics department of the University of Alcalá and the Grenoble-Alpes University, CEA-RIG-PHELICS of Grenoble - France