Coherent light generators – Particular active media – Insulating crystal
Reexamination Certificate
2001-02-14
2003-01-21
Paladini, Albert W. (Department: 2827)
Coherent light generators
Particular active media
Insulating crystal
C372S069000, C359S341500, C359S345000
Reexamination Certificate
active
06510169
ABSTRACT:
The present invention relates to a method for generating laser light, a laser crystal, and a laser Which uses said laser crystal.
A laser is an amplifier and generator of electromagnetic waves. The amplification of light thus relates to the procedure of induced emission. In general, a suitable material that has appropriately arranged energy levels is arranged within a resonator and optically pumped such that a population inversion of the energy levels occurs.
A whole range of different materials is employed as the active material, or respectively laser material. Semi-conductor lasers are known, for example. Different types of conductivity are generated by doping the semi-conductor crystal with impurity atoms. If a p-n junction is charged in the direction of flow, electrons and holes are driven towards one another by means of the voltage applied, and can recombine to radiate, emitting a light quantum. In order to obtain laser operation, that is to say population inversion, there has to be sufficiently strong doping of one side of the p-n junction for the associated Fermi level to lie in one band. Gallium arsenide is used in this instance, for example.
A further class of laser materials is represented by the so-called dye lasers. In this case, dye molecules with a very high molecular weight, for example, rhodamine, sodium fluorescein and others are used as the active medium, which are dissolved in water, alcohol or other solvents. Because of the numerous vibrational states of the molecules, the emission spectrum displays a wide band, so the dye laser can be tuned across a large wavelength range. This is done, for example, by replacing a cavity mirror with a diffraction grating.
A third class of lasers is represented by so-called gas lasers. In this case, the excitation is done by impact in a gas discharge. A mixture of different gases is often used, as excitation energy is stored in the non laser active gas, and can be transferred by impacts of a second type to the active gas. Examples of gas lasers are helium-neon lasers, noble gas lasers, and CO
2
lasers.
A further class of lasers is represented by optically pumped solid state lasers. In order to obtain a population inversion between two lasers here, the solid state is optically pumped, that is to say more atoms are intentionally excited by supplying energy than would correspond to the thermal balance. For example, the “active medium” in which inversion is possible can be illuminated using an intensive flash light.
With most laser materials, difficulties arise in that during the pumping procedure not all atoms end up in the same excited state, and the pumping energy used is distributed over several states. Thus, despite a high degree of energy expenditure, only a few atoms are available for amplification of a specific frequency.
In recent years, interest has centred on laser materials doped with ytterbium, as it is hoped that with the aid of these materials, efficient diode-pumped lasers can be manufactured. Because of its electronic structure, the ytterbium ion has, on the one hand, the advantage of a broad absorption and fluorescence band, and on the other hand shows only low thermal stress. This is because, inter alia, ytterbium does not have a high energy level. Ytterbium can be easily doped with relatively high concentrations. Moreover, the absorption band in ytterbium doped crystals can be covered by standard, generally available diode lasers which emit in the wavelength range between 930 and 990 nanometers.
Compared to other rare-earth doping such as, for example, neodymium, ytterbium doped materials show broader emission bands such as are necessary for so-called ultra-fast lasers which emit short pulses. When generating short and ultra-short laser pulses, there are two opposing classes of ytterbium doped materials in the prior art. On the one hand, ytterbium doped glasses are used, with which pulse widths of less than 100 femtoseconds are generated. On the other hand, ytterbium ions are doped in a crystalline matrix. For example, for Yb:YAG, a pulse length of 340 femtoseconds is obtained. The shortest pulse length in a crystalline matrix was obtained in Yb:GdCOB material, with 90 femtoseconds. The reason for this considerable difference in the smallest obtainable pulse duration is in the greater smoothness and greater breadth of the amplification spectrum of the glass compared to the crystals. Compared to the crystals, however, Ytterbium doped glasses display a lower thermal conductivity and a smaller emission cross-section. For example, in the case of ytterbium doped phosphate glasses, the emission cross-section is approximately 0.05×10
−20
cm
2
with a wavelength of 1060 nm, with a band width of 62 nm, compared to an emission cross-section of 2×10
−20
cm
2
with a band width of 12 nm for Yb:YAG, and of 0.5×10
−20
cm
2
with a band width of 44 nm for Yb:GdCOB. With the glasses, the large band width results in not insignificant thermal stress problems during operations with a high average power. In addition, the small-signal amplification and laser efficiency are clearly reduced. For example, with the Yb:YAG and YB:GdCOB crystals, a cut-off efficiency of more than 75% is obtainable, whereas the Yb phosphate glasses display a cut-off efficiency of less than 50%.
There is therefore a need for a laser material which, on the one hand, has an amplification spectrum and a fluorescence spectrum which is as flat as possible, and on the other hand, has high thermal conductivity and a large emission cross-section. Such a material is particularly desirable precisely with regard to a high average initial power and a short pulse duration.
The object of the present invention is therefore to provide such a material and respectively a laser with such material.
This object is solved in accordance with the invention with a crystal with the chemical composition M
3
RE
1-x
Yb
x
(BO
3
)
3
, wherein M is an element from the group Mg, Ca, Sr, Ba, Ra, wherein M is optionally partially replaced with at least one further element of this group, and RE is either Y or Lu or Y partially substituted by Lu or Sc.
These crystals belong to a large structural family which is formed by replacement of a large number of atoms at the A and M or M′ positions in the general formula A
6
MM′(BO
3
)
6
. According to the invention, such a structure is used as the base crystal for doping with Yb
3+
ions.
A particularly preferred embodiment provides that the crystal is a single crystal.
Advantageously, M is either Sr or Ba, or Sr partially substituted by Ba. With the partial substitution of strontium with barium, care must be taken that during crystallisation in the molten bath, no parasitic phases develop. The barium content in the strontium is therefore limited to a value in which M
3
RE
1-x
Yb
x
(BO
3
)
3
phase shows a congruent molten mass.
The value x can be between 0 and 1, regardless of the effect to be achieved. In a preferred embodiment, x is between 0 and 0.6, preferably between 0.1 and 0.4, particularly preferably approximately 0.2.
The laser crystals according to the invention belong to two different crystal structures according to the alkaline earth metals used. For example, the crystal with the chemical formula Sr
3
Y
1-x
x
Yb
x
(BO
3
)
3
with 0≦x≦1 belongs to the space group R-3, while Ba
3
Lu
1-x
Yb
x
(BO
3
)
3
with 0≦x≦1Y belongs to the space group P6
3
crn. All the bonds form single axis crystals and melt congruently. They are preferably grown from molten mixtures of stoichiometric compositions according to the Czochralski method. Naturally, a non-stoichiometric solution can also be used when, for example, a seed crystal is used. Y
3+
or Lu
3+
can be replaced with Yb
3+
with high doping concentrations without having to take into account any substantial deterioration in quality in the grown crystal.
The molten mixtures are produced by mixing the starter materials. These are alkaline earth metal carbonate or alternatively alkaline earth metal oxide (MCO
3
Aka Gerard
Gaume Romain
Haumesser Paul Henri
Viana Bruno
Vivien Daniel
Forschungsinstitut fur Mineralische und Metallische Werkstoffe E
Paladini Albert W.
Paul & Paul
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