DE3222217A1 - Cr<3+>-doped crystals for tunable solid state lasers - Google Patents

Abstract

The invention relates to novel Cr<3+>-doped garnet crystals for tunable solid state lasers in the near-infrared spectral region. Depending on the composition, the broad-band emission varies from 650 nm to 1000 nm. For example, the emission maximum of Gd3.04Cr0.01Ga4.95O12 is at 730 nm, with a half-width of 110 nm. Pulsed operation as well as continuous operation of the lasers were demonstrated at room temperature.

Description

Description.

The invention relates to a crystal for tunable

3 +
Solid-state laser with Cr as an active ion. Alexandria (BeAl-O.:Cr) is known as such a crystal for an emission wavelength between 700 and 820 nm (John C. Walling et al .: Tunable Alexandrite Lasers. IEEE Journal of Quantum Electronics, Volume QE-16 (1980 ), Pages 1302 to 1315 and John C. Walling: Alexandrite Lasers, Physics and Performance, Laser Focus, February 1982, pages 45 to 50).

In the case of alexandrite, the Cr is built into an Al site, which is very narrow for the Cr ion, because the ionic radius of the Al-Ions is smaller. Crystals of optical quality are therefore only obtained with very small amounts of Cr doping maximum 0.4 atomic percent. Therefore, relatively large crystals are required in order to achieve sufficient optical amplification.

Furthermore, the crystal field parameter in alexandrite is Dq / B in the Tanabe-Sugano diagram (Yukito Tanabe, Satoru Sugano: On the Absorption Spectra of Complex Ions II. Journal of the Physical Society of Japan, Volume 9 (1954), pages 766 to 779)

2 4

so large that there is a

-1 4

Energy difference of 800 cm results. The T ₂ band is therefore only weakly occupied thermally at room temperature, and the fluorescence spectrum shows, in addition to the broadband emission

4 4
. »Ion at the transition from T» to A ₂ a double line (R lines) with much greater intensity, which comes from the transition from ^ E to A.-.

Since a considerable part of the excitation power is used for the line emission, the pumping threshold of the is Alt.-xandr it - laser so high that only pulse operation is possible. Ks has already been reported about continuous wave operation (John C. Walling et al .: Tunable CVi Alexandrite Laser. IEEE Journal of Quantum Electronics, Volume QE-16 (1980) pages

Jf

-5-

120 to 121), but it was not a matter of authenticity Continuous wave operation, but just about relaxation oscillations above the surge threshold, the amplitude of which dropped periodically to zero.

The invention was therefore based on the object of creating a crystal in which, on the one hand, for the Cr ion so large a grid space is available that even higher ones Dopings are possible, and in which, on the other hand, the energy

4 2

The difference between the T ₉ band and the Ε line is significantly smaller than 800 cm, so that the line emission probability is smaller and thus the pumping threshold is lower and the conversion of pumping power into laser power is more efficient. This object is achieved according to the invention in that a gallium garnet is provided as the host lattice for the active ion.

The ionic radii for Ga ^{3+ are} 0.620 Å and for Cr 0.615 Å, so they are practically the same, while the ionic radius for

3+ s

Al is only 0.535 A. From this it is immediately clear that

3 + 3 +

Cr can be incorporated in practically any proportion on Ga lattice sites in gallium garnets, while the replacement of Al by Cr in alexandrite or in Al garnets (e.g. Y ₃ Al ₅ O ₁ 2 / YAG) is only possible to a very limited extent .

Measurements on various chromium-doped gallium grenades have shown that the energy difference between the T ₉ -

2
Band and the Ε line, depending on the composition of the Ga garnet, is a maximum of 600 cm and can assume values less than zero at a minimum. Accordingly, the intensity of the R-line emission can be reduced to zero (see FIG. 5). Real continuous wave operation is possible at very low pump thresholds.

In addition, the continuous lowering of the ⁴ T ₂ band with the variation of the crystal composition ensures that the maximum of the emission is variable between 700 nm and 850 nm. This extends the range of application of the Cr laser in Ga garnets considerably compared to the

te.

Alexandrite, which due to its solid crystal composition does not allow any variation in the Cr emission.

Further details and appropriate refinements of the The invention are described and explained in more detail below with the aid of five exemplary embodiments.

Show it:

Fig. 1 common to all the exemplary embodiments Crystal structure,

FIGS. 2 to 5 fluorescence spectra of four exemplary embodiments

6 shows the transmission spectrum of a fifth exemplary embodiment

7 shows an efficiency diagram for the fifth embodiment sbeispieI

and

8 shows the spectrum of the laser emission of the fifth embodiment.

* a »** nJi

The representation of the garnet crystal lattice according to FIG. 1 is from SC Abrahams, S. Geller: Refinement of the Structure of a Grossularite Garnet; Taken from Acta Crystallographica, Volume 11 (1958), pages 437 to 441. For a crystal of the chemical composition Α-, Β-C .0 .. ₂ dodecahedrally coordinated lattice plate, which is occupied by atoms A, furthermore octahedrally coordinated, occupied with atoms B, further tetrahedrally coordinated, lattice sites occupied with atoms C and finally Oxygen-G.itterp.1 etches marked with open circles.

In the case of the gallium grenades according to the invention, the C-grid sites are basically occupied with gallium, while one of the elements Ga, Sc or In is built into the B lattice sites will. One of the elements Y, La, Ce, Gd or Lu can be incorporated.

These elements are suitable because, firstly, they are optically inactive and, secondly, have a large ionic radius. The second property leads to an upgrade of the garnet grille, so that there are lattice sites of the required size for the Cr doping units. These doping ions are preferably installed on B-lattice sites, but can also occupy C-lattice sites if the chromium concentration is higher.

For typical laser application, however, it is because of the fluorescence quenching and the absorption length not expedient to go with the doping above 4 5%, although higher dopings possible are.

To the spectroscopic properties of the crystals, in particular To be able to change the emission wavelength range of the laser as well as possible, is used for crystal growth Mixtures of binary and fcernary compounds.

If you look at the fluorescence curve towards shorter wavelengths If you want to move, you can also build Al ions into the grid positions B and C. You mustn't overdo it Η) ε Ai go, because otherwise, the R-line emission is like that again as big as Alexandrik and the special advantages of the GaIliumgranatos are lost.

The Ga grenades are manufactured using the Czochralski method. For example, the following procedure was used to produce single crystals: The starting materials (for example) for GdScGa garnets: Gd ₃ O ₃ , Sc ₂ O 4, Ga ₃ O ₃ and Cr-O-. v / earth carefully mixed, pressed into tablets and sintered in a platinum crucible for 8 hours at 1200 ° C. The tablets are then melted in an iridium crucible in a closed chamber (atmosphere: 98% M ₃ and 2 vol% O ₂ ). The crystals can be pulled from the Schmeiß (approx. 100 cm volume) with a pulling speed of 7 mm / h. At the same time, the pull rod rotates at 85 rev / min. Ndi: In a heating phase (approx. 24 hours), the crystal is cooled linearly to room temperature within 14 hours.

As a concrete example, crystals of the composition Gd-.Sc are obtained. ₇ pCr _{n nor} Ga-, _iqr 0 _in with the following physical properties:

Symmetry: cubic

Lattice constant: 12.56 A refractive index: 1.95 typical "dimension: 10 cm length, 2.5 cm

Diameter of melting point: approx. 18 6O ⁰ C

The Cr fluorescence spectron of four exemplary embodiments, namely yttrium gallium garnet (YGG or Y-, ₁ Cr _{n nn0} .Ga. - .., O ₁ -), gadolinium gallium garnet (GGG or Gd., _N .Cr _{n n1} Ga.Qi-O ₁ -,), Yttriui ocandium-Gallium-Garnet (YScGG or Y-.Sc. ₄ Cr _{nn 9H} Ga ^ O _n ) and Lant.han-Lutc ti um-Gcill ium-Granal. (IiaLuGG or ^lld 2 l \) 9 ^ilU 2 785 ^ ¹ O Ο16 ^ ^α 3 ° 12 ^ ^{sincl shown in} Figs. 2 to 5. The abscissa is the wool length in ran and

the ordinate is the fluorescence intensity in arbitrary units specified. It can be seen that the K-line emission only has an effect in the first exemplary embodiment according to FIG. 2, but much less than with alexandrite.

At room temperature, broadband emissions predominate in all spectra. The fluorescence spectra according to Fig. 2 to Fiy. 5 simultaneously show the tuning range of the lasers from 6 to 60 nm up to at least 950 nm for the crystals listed as examples.

6 shows the transmission spectrum of the fifth exemplary embodiment, namely a 7 mm thick, Cr-doped crystal of gadolinium-scandium-gallium-garnet (GSGG or Gd ₀ SqGa-, 0- _o ). The two absorptions in the T ₂ level (at 640 nm) and the T ₁ level (at 460 nm) can be clearly seen.

The continuous wave laser properties were examined by placing a crystal Gd ^ Sc ^ Ga ^ O .. _y with a chromium concentration of

19-3 J ^ Ji /
5-10 cm (fifth embodiment) was pumped at 647 nm with a krypton laser. 7 shows the output power rout in mW as a function of the absorbed pump power Pabs. With a coupling out of T = 1% (concentric resonator with 20 cm mirror spacing), the result is an efficiency (slope efficiency) of ti = 7%.

In Fig. 8 the associated Cr laser spectrum is shown with a maximum at 777 nm. The intensity is in measured in arbitrary units. The individual tips come from different resonator modes. For the measurement according to 8 the resonator did not contain a dispersive element.

Because of the broad Cr absorption shown in FIG. 6 , the crystals according to the invention can also be pumped with broadband lamps (flashed and continuous wave).

Finally it should be mentioned that the application of the crystals according to the invention does not apply to the tunable

_B AD ORIGIN * ¹ "COPY

Laser limited. You: the life span of the excited state depends on the tomporalur jb. Jo na ;; in the example of the embodiment Jjegt the service life ii -.- i 4K in the range of a few ms and with KX ^- ) K in the range of some ', j. ΙύΟ, ιΐί- ;. Therefore the crystals can also be used as optical, contactless temperature sensors vcrv / c-'rir ¹ · η. You stimulate it with short pulses and measure the decrease in fluorescence intensity or the intensity ratio of broadband and R-line emission.

Finally, the crystals according to the invention can also in opt j amplifiers are already used.

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Claims (1)

Cr-DOPED CRYSTALS FOR EftiRfeeSTlMÄOkÄE SOLID-STATE LASERS Claims ) Crystal for tunable solid state laser with Cr as active ion, characterized in that a gallium garnet is provided as the host lattice for the active ion. 2.) Crystal according to claim 1, characterized by a garnet host lattice of the chemical composition A ₃ B ₃ C ₃ O ₁₉ , where A is one of the elements Y, La, Ce, Gd or Lu, B is one of the elements Sc, Ga, In or Lu and C is the element Ga. 3.) Crystal according to claim 1, characterized by a host lattice of the chemical composition (AA '"), (BB ¹ -) -Ga _n O ₁₀ , where A, A" is a binary abmix ι -χ ο γ ι -y £. J ι δ ^ Two of the elements Y, La, Ce, Gd, Lu with O = X = 1 and B, B "is a binary blend of two of the elements Sc, Ga, In, Lu with O = y ^ 1. 4.) Crystal according to claim 1, characterized by a host lattice of the chemical composition CA A ¹ A '' .._, _) - (BB 'B''i_ _ ^ 2 ^Ga 3 ° 12' ^w ° k ^{e: i} - ^Λ ' ^{A <} ' ^A '' is a ternary mixture of three of the elements Y, La, Ce, Gd, Lu with 0 = x = 1 and O- ^ z = 1, and B, B ', B' ^{1 is} a ternary Mixture of three of the elements Sc, Ga, In, Lu with 0 = y = 1 and 0 = r = 1. 5.) Crystal according to one of claims 1 to 4, characterized in, that Al occupies a proportion of at most 40% of the grid positions B and / or C, 6.) Crystal according to one of claims 1 to 5, characterized in that Sc has a proportion of at most 5% of the lattice sites A continues. 7.) Crystal according to one of claims 1 to 6, characterized in that Y and / and Gd have a proportion of at most 10 ?. occupy the grid places B. 8.) Crystal according to claim 1, 2, 3, 7 characterized by the chemical composition Y-, (Y Ga,) -Ga ^ O .. “with 0 = y = 0.1. 9.) Crystal according to claim 1, 2, 3, 7 characterized by the chemical composition Gd-, (Gd, ^ Ga,) 2Ga ₃ O _{1 2} with 0 ^ y = 0.05. 10.) Crystal according to claim 1, 2, 3, 6, 7 characterized by the chemical composition ^(Υ 1 ₇ χ ^ 3 VV ^ Iy-rk ^Sa 3 ° 1 2 ^rait 0 = x = 0.05, 0 ^ 0.1, O.1 = r = 1.0. 11.) Crystal according to claim 1, 2, 3 characterized by the chemical composition (La Lu ₁ I ₁ Lu-Ga ₁ O ₁₀ with Xl X-J ^ JEZ .6 ^ x = I.0. 12.) Crystal according to claim 1, 2, 3, 6, 7 characterized by the chemical composition (Gd ₁ Sc), (Gd Sc Ga- I _n s ~ ^ ~ Xj yr ι - y - r i. with 0 ^ -x = 0.0! 3, 0 = y = 0.1, 0.1 = r = 1.0. 13.) Crystal according to one of claims 1 to 12, characterized in that that the active ion Cr occupies a proportion of 0.01% to 4 5% of the lattice sites B and / or C. 14.) Crystal according to einorn dor claims 1 to 13 characterized by its use in an optical amplifier. 113.) Crystal according to claims 1 to 14 characterized by its use as an optical, non-contact temperature sensor. OOP? j -3. 16.) Process for the production of a crystal according to a of claims 1 to 15, characterized in that there / 3 the crystal was grown from a flux with spontaneous nucleation or nucleation. 17.) Method for producing a crystal according to a of claims 1 to 15, characterized;, u..ß the crystal according to the C'.iochralski method from Melt is sucked. BATH