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NOEDYMIUM SOLID STATE LASERS SLIGHTLY TUNABLE IN THE INFRA-RED
M. Leduc
To cite this version:
M. Leduc. NOEDYMIUM SOLID STATE LASERS SLIGHTLY TUNABLE IN THE INFRA-RED.
Journal de Physique Colloques, 1987, 48 (C7), pp.C7-315-C7-316. �10.1051/jphyscol:1987777�. �jpa-
00227081�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au n012, Tome
48, decembre 1987NOEDYMIUM SOLID STATE LASERS SLIGHTLY TUNABLE IN THE INFRA-RED
M.
LEDUCLaboratoire de Spectroscopie Hertzienne de I'ENS (UA-18), 24, Rue Lhomond, F-75231 Paris Cedex 05, France
Our interest in developing tunable Nd lasers i n the infra-red arises from the need of appropriate laser sources for optical pumping of helium atoms at A = 1.083 pm ( 2 3 ~ - 2 3 P transition of the helium atom). Manipulating helium atoms with laser light can find applications in several fields of physics, ranging from helium magnetometers, cooling and trapping of helium metastable atoms, studies of quantum Fermi statistrcs with polarized helium 3 fluids and high energy scattering experiments from helium 3 polarized targets. The c. w. tunable power at 1 . 083 wm required for these experiments varies from a few mW (magnetometers) to several 10 W f nuclear targets) C A I .
Neodymium crystals are good candidates as laser sources for helium pumping, due to the many advantages of such YAG-type solid-state lasers i n general : high efficiency (they are 4 level lasers), room temperature operation.
lnflnlte life time, possibility of diode pumping and very high output power i n excitation by lamps Crl. For all these reasons, neodymium lasers are much superior t o colour-centre lasers, which on the other hand exhibit broad band wavelength tunability C 31, whereas neodymium lasers are usually considered as single line emitters Cal. However, we tried several laser crystals in which the Nd3+ ions are embedded in different lattices CY,At.,O,,(YAG), YAt.O,(YAP), LiNbO,: MgO and LaxNd,-xMgA~,,O,,(LNA) I , and showed that some tunability can be achieved around each of their fluorescence lines. Different types of pumping schemes were tried (longitudinal of transverse), various pump sources were used (Ar+ or ~ r + lasers, ~ r + lamps, diode lasers or diode arrays) and several laser cavities were built (linear or ring cavities). Possible laser sources for helium pumping were thus found.
When a longitudinal excitation of the crystal is chosen, the pump source is usually highly focussed onto the crystal, were a large fraction of its power i s absorbed. Only small sizes of crystal are required ( a few m m ) . The beam waist of the pump source at the crystal can be very small ( a few 10 pm when a gas laser is used and about 100 pm when a diode laser or a diode array is used). If pump and cavity beam waists are carefully matched, rather low thresholds can be achieved : for instance a few milliwatts of Kr+ beam at 752 nm are enough to produce laser action of an LNA crystal 5 mm in length. With such laser cavities.
the transverse mode structure is easily controlled and TEMoo output beams are generated, even if the pump beam exhibits a highly multimode pattern, as occurs for Instance with diode arrays. Similarly single mode operation with little frequency jltter can be obtained without difficulty by building ring laser cavities : an LNA ring laser can deliver 25 mW of single frequency power at the helium wavelength with an intrinslc jltter better than 1 MHz without any electronic stabilization. The laser crystal efflciency can be rather high (over 35% for LNA pumped by a Kr+ laser) E51161, but the output power is limited both by the power available for pumping
( a
few watts at most) and by posslble non linear effects, such as reabsorption from the ~d,+ exclted state. Tuning the wavelength around each emission peak can beArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987777
JOURNAL DE PHYSIQUE
achieved by inserting appropriate selective elements in the cavity, for instance both a Lyot birefringent filter and a thcn etalon. For most neodymium crystals, the tuning band ranges between 10 and 100
A
for each line and the number of laser lines in the near infra-red is great : for instance, we have observed 7 laser transitions within the 4~,/,-41ii/z multiplet of a Nd: YAP crystal between 1. 1 and 1. 6 pm C71. A neodymium doped LiNbO, crystal was also studied for its possibility of frequency doubling by the crystal itself Csl : it could be tuned 30A
around the peak at 1.084 pm CsI. Similarly LNA lasers could be tuned 100A
around 1.05 and 1.08 pm C i o I and recently a diode-pumped LNA laser [ + + I gave a continuously tunable output from 1.045 to 1.085 pm (over 400A)
C i r l .A great advantage of neodymium laser crystal is that one can also excite them transversly with intense lamps ( o r diode arrays in the future). The laser output can usually be much larger than with longitudinal pumping schemes. The pumping light from the ~ r + lamps (several kW) is reflected on the side of the crystal rod, the whole volume of the crystal is used and long rods ( a few cm) are required for high power. A 10 cm Nd:YAP crystal excited in an usual Nd: YAG cavity deilvered over 100 W of powev at 1.0795 pm, 10 W at 1.0845 pm and still about 1 W at 1. 088 pm, the helium wavelength, when tuning elements were added in the cavity. LNA crystals cannot yet be grown in very long rods with excellent optical quality. However a crystal from LET1 (Grenoble), 5 cm in length and 5 mm i n diameter, could be excited by lamps and delivered a few watts of power. The laser output could also be tuned as with the longitudinal Ar+ pumping set-up.
One of the main problems related to high power neodymium lasers is the heating of the laser rods caused by the excitation from the lamps, which induces lens effects changing the resonator properties t + 3 l t i + l . Such problems seem to be even more severe with the new generation of solid laser such as CrNd: GCGG Cis3 or Cr doped garnets t i 6 3 . The counterpart of the appealing features of the new laser materials (high gain, broad tunability, etc. ) i s often their poor thermal properties. The low heat conductivity and the large variations of the refractive index with temperature present interesting problems in the design of optimized laser resonators. We have developed calculations of cavities containing self-focussing elements using the Gaussian beam matrix formalism. The laser rods are treated as thlck materials showing a radial quadratic dependence of their refractive index.
Cavities incorporatlng an arbitrary number of lenses, mirrors and crystals (including rods with spherical end faces) are described, with no restriction on the value of the pump power. This generalizes the calculation of references 1+71CiaI. Stability domains and beam size in the resonator and inside the rods are calculated. A few experimental tests with Nd:YAP lasers will be presented.
R ~ f e r e n c e ~
C i l D. Betts and M. Leduc, Annales de Physique ( F r . ) ,
11,
267 (1986)C z I W. Koechner, "Solid State Laser Engineering', Springer-Verlag, Optical Science
( 1976)
Csl C. Pollock, Encyclopedia of science and technology (1987)
C41 A. A. Kaminskii, "Laser crystals', Springer-Veriag, Optical Sccence ( 1981) C 51 K. S. Badgasarow et al. , Sov. J.
m,
1082 ( 1983)C e l V. M. Garmash et ai.
,
Phys. Status Solldi,75,
K l l l ( 1983) C71 L.D. Schearer and M. Leduc, IEEE J. QE22, 756 (19863 Cel T.Y. Fan et al.. J. Opt. Soc. Am. B a , 140 (1986) Cel L. D. Schearer et al., to be published in iEEE J. of QE t i 0 1 L. D. Schearer et al.,
IEEE J. QE22. 713 (1986)C ~ i l J. Hamei et ai., to be published in Optics Commun. (July 1987) E L L ] J. Hamel, private communication
Cis1 W. Koechner, Appl. Opt. 9, 1429 (19701, and
9,
2548 (1970) t ~ r l J. D. Foster and L. M. Osterlnk, J. of Appl. Phys. 41, 3656 (1970) C ~ s l J. E. Reed, IEEE J. Quant. Electronics QE21, 1625 (1985)Lie1 See for instance :
D. Viven, Revue Phys. Appi.
21.
709 (1986), and references quoted in this article.t173 V. Magni. Applied Optics. 25, 107 (1986)
C s e l H.P. Kortz et al., Applied Optics,
