Lasers using fibers doped with ytterbium ions
Since 2004, we have launched new developments aimed at studying laser pointeur boutique systems based on double clad Ytterbium doped fibers. These fibers are implemented in different architectures of lasers or amplifiers. We have succeeded in designing a unique special fiber based on the new architecture referred to as “rod-type” fiber having an exceptional core diameter of 80 μm and single mode guiding properties. While implemented in different kind of lasers and amplifiers, this fiber has allowed us to break several world records in terms of laser specifications. One of the fiber laser systems developed here provide now the heart of the Solstice project.
Amplification in the non-linear regime
The main limitation to amplification of short pulses in fibers is the development of non-linearities during the propagation in the small fiber core. Most of the architectures therefore tend to limit these effects by stretching the pulses (see the CPA below). At contrast, we have developed a new strategy based on inducing the maximum amount of non-linearities in a controlled manner in order to produce pulses way shorter than Yb amplification bandwidth allows. Beside the demonstration of the parabolic regime, we have developed several ultrafast femtosecond amplification setups implementing different techniques of compensating the third order dispersion (TOD) term. In collaboration with the Insitut d’Optique, by using an embedded set of gratings and prisms (grisms) we have produced pulses of 63 fs @ 27 MHz (11W). Another experiment managing the TOD by non-linear process and ultrawide asymmetric spectra (60 nm) in rod type fibers allowed us to demonstrate record breaking performances such as pulses of 1 μJ and 50 to 70 fs @ 1 MHz repetition rate.
Chirped pulse amplification scheme
Although, sub 100 fs pulses can be produced in the non-linear regime, generation of high energy femtosecond pulse in fiber systems requires stretching prior to amplification. The systems described below are all based on the chirped pulse amplification (CPA) although we distinguish between real CPA systems in the linear regime and CPA systems in the non-linear regime where the B-integral accumulated can be large. A first setup has been build based on an a 10 MHz oscillator, an Offner stretcher and 2 microstructured fiber amplification stages and transmission gratings compressor. The system delivers 30 W of 270 fs pulses with very high temporal quality and energies varying from 120 μJ (100 kHz) to 30 μJ (1MHz).
Since many years, investigations are carried out to operate Ytterbium doped laser materials on the « zero-phon line » at 976 nm in a genuine 3-level system. The potential of such studies is straightforward since these lasers can be used to optically pump other Yb materials or if frequency doubled at 488 nm replace the expensive bulky Ar lasers. However, single mode laser bleu pas cher powers achieved up to now are limited to few mW in bulk and few W in fibers. We have patented a laser system based on a rod type microstructured fiber that delivers a CW TEM00 beam of 94 W at 977 nm upgrading the highest power so far by almost 2 orders of magnitude. Very recently, we have succeeded to operate the system in a Q-switch regime and efficiently generate blue light with unprecedented powers.