Crystals for Femtosecond Diode-Pumped Lasers
Disordered Oxide Crystals for Femtosecond Diode-Pumped Solid-State Lasers
Tech Area / Field
- PHY-SSP/Solid State Physics/Physics
- PHY-OPL/Optics and Lasers/Physics
3 Approved without Funding
TRINITI, Russia, Moscow reg., Troitsk
- Russian Academy of Sciences / Institute of Radioengineering and Electronics / Fryazino Branch, Russia, Moscow reg., Fryazino\nGNPO Polyus, Russia, Moscow
- Max-Born-Institut für Nichtlineare Optik und Kurzzeitspectroskopie, Germany, Berlin\nNorfolk State University / Center for Materials Research, USA, VA, Norfolk\nUniversité de Paris VI / Laboratoire pour l'Utilisation des Lasers Intenses (LULI)
Diode-pumped solid-state lasers based on crystals doped with rare-earth ions have recently become wide-spread and replace appreciably gas-discharge lamp- pumped lasers. The use of laser diodes with high energy efficiency and high life time for pumping, equipment compatibility of solid-state lasers of such type with power and control electronics provide their further wide use in scientific, medical and industrial instrument-making. The use of synchronization of longitudinal modes due to Kerr nonlinearity of active material as well as new types of “inertialess” passive mode synchronizers allowed production of lasers generating pulses of units of femtoseconds and achievement of extremely high laser intensities. The recent investigations have demonstrated the unique possibilities of such lasers in technology, medicine and scientific research.
As a rule, as an active medium for solid-state lasers, oxide or fluoride crystals such as YAG, YAP, YLF doped with rare-earth ions of neodymium, thulium, erbium, ytterbium are used. The active materials containing rare-earth ions have very strong absorption bands in near-IR spectral region which allows the use of conventional laser diodes, for example, GaAlAs diodes, to excite generation.
In the previous years the efforts of the developers of laser materials were mainly aimed on development of ordered “single-center” crystals to obtain extremely narrow luminescence lines and maximum stimulated-emission cross-sections due to the limited brightness of the pump sources in use, such as flashlamps or arc gas-discharge lamps. To use “single-center” crystals in femtosecond lasers is impossible since a spectrally limited pulse width in such active medium is tens of picoseconds. For the same reason the intrinsic Kerr nonlinearity of a laser material fails to be used to realize self-longitudinal-mode-locking. At present diode-pumped lasers based on vanadate and scandium tetraborate crystals are widely investigated. These crystals have more wide spectral bands compared to YAG and allow introduction of high concentrations of Nd ions and other rare-earth ions. However, insufficient gain bandwidth for laser transition hinders the use of these materials in femtosecond lasers.
When using laser diodes as a pump source, the spectral brightness of these diodes exceeding that of gas-discharge lamps hundreds of times, the requirements to the value of stimulated-emission cross-sections are considerably reduced, and a possibility to use “multicenter” disordered laser materials having a wide gain spectrum is opened up. Limiting cases of disordered laser matrix are rare-earth ion-doped laser glasses. As is known, high-energy femtosecond pulses are produced when using laser glasses, so the interest of scientific groups in the study of a possibility to use laser glasses in diode-pumped lasers is natural . The results of the study show that inadequate thermooptical and thermophysical characteristics of laser glasses prevent their practical use.
Therefore, for development of high-efficiency diode-pumped solid-state femtosecond lasers it is necessary to develop new laser crystals doped with rare-earth ions, particularly Nd3+, Yb3+, Tm2+, Er3+, having a wide luminescence spectrum, strong absorption bands in near-IR spectral range, and thermophysical characteristics close to those of conventional laser crystals.
The attainment of the goal to be sought is possible by development of crystal media with disordered cation sublattice or with ordered cation sublattice, in which in case of introduction of activating ions (especially in high concentrations) the effect of disruption of translation invariance of the cation sublattice takes place. In both media a considerable inhomogeneous broadening of luminescence lines of activating ions due to disruption of translation invariance is observed.
In the previous years POLYUS RDI SE and TRINITI have been engaged with a special purpose in development of laser materials including crystals and glasses for high-power solid-state lasers. The investigations carried out before made it possible to relate the main spectral and luminescent characteristics of rare-earth ions to a matrix composition. In the course of the previous work the properties of solid solutions of a number of binary and ternary oxide R2O3-Al2O3-SiO2 systems have been studied. These properties show the possibility to solve the above problem based on disordered crystals of complex aluminates and silicates. In particular, Nd3+ or Yb3+-doped monocrystals have been produced that had a FWHM of the luminescence line more than 200 cm-1 in the ground-state transition. The possibility to grow these crystals by Czochralski method stimulates further interest in development of the technology for growth of such materials and in detailed investigation of their laser properties. These results have not been needed before due to the absence of practical interest. However, the development of diode-pumped lasers as well as of equipment and technologies based on ultrashort pulses strongly supports the advisability of carrying out such investigations.
At TRINITI and FIRN STC the work on the ISTC Project #251-96 was carried out to develop active media for diode-pumped solid-state lasers based on binary borate crystals [2, 3]. The most universal medium, lanthanium-scandium borate (LSB), admits introduction of neodymium activating ions into the lanthanium sublattice in concentrations up to 1...2·1021 cm-3 without considerable luminescence concentration quenching. Due to disruption of translation invariance of the lanthanium sublattice even at neodymium concentrations of 4.5·1020 cm-3 the gain spectrum bandwidth for the ground-state laser transition is 40 cm-1. This value grows with increase in the neodymium concentration and according to preliminary estimation can achieve 60...70 cm-1. More considerable increase in the gain spectrum bandwidth would be expected with binary borate crystals that admit introduction of a neodymium activating ion into each cation lattice.
With diode-pumped Nd:LSB lasers with a content of neodymium up to 1021 cm-3 the near-maximum energy efficiency has been realized. The Nd3+ pump band structure at ~800 nm in scandium borates allows development of diode-pumped lasers, the performance of which is little sensitive to temperature change.
With Yb and Er-doped LSB crystals, generation is produced at laser transitions of Er ions (l=1.56 mm) both with lamp and diode pumping. The decrease in efficiency of energy transfer from Yb to Er lesser than for Yb-Er glasses in case of attainment of inverse population of the 4I11/2 level of Er3+ characteristic of Yb, Er:LSB crystals allows development of pulsed periodic Q-switched microlasers. A wide absorption band for Yb3+ significantly reduces the requirements to the thermal stability of laser diodes.
For a Yb activating ion, of the most interest is the crystal matrix of yttrium-calcium borate. In this crystal the cation positions are filled with yttrium and calcium ions (statistical average). In the preliminary investigations of Yb:Y2Ca3(BO3)4 the crystals with Yb concentration of 4·1021 cm-3 and absorption coefficient of 120 cm-1 at about 0.98 mm have been grown. The spectral luminescence FWHM linewidth is no less than 300 cm-1. Since the long-wave edge of the absorption band falls within the short-wave edge of the luminescence band, the effective gain spectrum bandwidth is about 200 cm-1.
As a result of realization of the Project, the technology for growth of disordered monocrystals based both on the R2O3-Al2O3-SiO2 crystal matrix and on binary scandium and calcium borates doped with such rare-earth ions as Nd3+, Yb3+, Tm3+, Er3+ suitable for use in diode-pumped lasers including generation and amplification of femtosecond pulses will be developed. The development of an experimental sample of laser oscillator with pulses less than 300 fs and the optimization of optical configurations and designs of pulsed periodic microlasers to provide pulse width from hundreds of microseconds to picoseconds is intended.
The laser materials under development are of obvious interest that is caused by the fact that at present there are no available compact femtosecond lasers in the world market. The assumed commercial availability of such lasers is due to the simplicity of their design and the use of available diode pump sources in production. The wide pump bands of the laser materials under development automatically reduce the requirements to the monochromaticity of diode emission, which decreases considerably the cost of diode pumping systems.
The execution of this Project will allow expansion of the range of laser materials and devices and solution, at a new technical level, of a number of existing problems in such areas as optical communication, atmospheric monitoring, range measurement, medicine, snap analysis of quality of foods, etc.
The activity on the Project is oriented to achieved following purposes:
– development technology of growing crystals for femtosecond lasers with diode pumping;
– manufacture active elements for diode pump lasers;
– optimization of optical RSB-microlasers schemes including frequency intracavity conversion;
– design single mode pulse-frequency lasers with stable radiation parameters and mean power radiation ~1 Watt;
– creation model of femtosecond laser with diode pumping.
1. J.Aus der Au, F.H.Loesel, F.Morier-Genoud, M.Mose, and U.Keller “Femtosecond diod-pumped Nd:glass lase with more then 1 W of average output power”, Opt.Lett.23, 271 (1998);
2. S.T.Durmanov, J.P.Rudnitskiy, G.V.Smirnov, A.J. Abazadze, M.N. Gruden, J.P.Koval, J.M.Kolbatskov, O.V.Kuzmin, S.A.Kutovoi, V.L.Panyutin “Diode Side Pumped Nd:CSB-Laser», Proceedings of the International Conference on “Lasers’98”, 1999;
3. O.V.Kuzmin, S.A.Kutovoi, V.L.Panyutin, S.T.Durmanov, J.P.Rudnitskiy, G.V.Smirnov “Spectroscopic and laser properties of LaSc3(BO3)4:Cr, Yb, Er-crystals”, Proceedings of the International Conference on “Laser Optics’98 1999.
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