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Nonlinear spectroscopy and passive Q-switching operation of a Co 21 :LaMgAl 11 O 19 crystal K. V. Yumashev, I. A. Denisov, N. N. Posnov, and V. P. Mikhailov International Laser Center, Belarusian State Polytechnical Academy, Building 17, 65 F. Skaryna Avenue, 220027 Minsk, Belarus R. Moncorge ´ Centre Interdisciplinaire de Recherche Ions Laser, Unite ´ Mixte de Recherche 6637 Centre National de la Recherche Scientifique Commissariat a ` l’Energie Atomique Institute des Sciences de la Matiere et du Rayonnement, Universite ´ de Caen, 6 Boulevard Mal. Juin, 14050 Caen, France D. Vivien Laboratoire de Chimie Applique ´ e de l’Etat Solide, Ecole Nationale Superieure de Chimie et Physique, Unite ´ Mixte de Recherche 7574 Centre National de la Recherche Scientifique, Universite ´ Paris VI, 11 Rue P. et M. Curie, 75231 Paris Cedex 05, France B. Ferrand Laboratoire d’Electronique, de Technologie et d’InstrumenstationCommissariat a ` l’Energie Atomique Techniques Avance ´ es, Departement Optronique SMDO, 17 Avenue des Martyrs, Centre d’Etudes Nucleaires de Grenoble 85X, 38041 Grenoble, France Y. Guyot Laboratoire de Physico-Chimie des Mate ´ riaux Luminescents, Unite ´ Mixte de Recherche 5620 Centre National de la Recherche Scientifique, Universite ´ de Lyon 1, 43 Boulevard 11 Novembre 1918, 69622 Villeurbanne, France Received February 23, 1999; revised manuscript received June 29, 1999 A pumpprobe technique with picosecond resolution was used to record and derive the differential and excited- state absorption (ESA) spectra of a Co 21 :LaMgAl 11 O 19 (LMA) single crystal in the visible and near-infrared spectral regions. Time resolution allowed us to observe ESA bands that can be assigned to a 4 T 2 ( 4 F) 4 T 1 ( 4 P) optical transition and to transitions from the thermally populated 2 E( 2 G) excited state to doublet levels that arise from the 2 F free-ion level of the tetrahedrally coordinated Co 21 ion. Intensity-dependent transmission measurements were also carried out at 1.34 and 1.54 mm. Passive Q switching of Nd 31 :YAlO 3 (1.34-mm) and of Er 31 :glass (1.54-mm) lasers by use of the Co 21 :LMA crystal as a saturable absorber was dem- onstrated. The pulse durations (energies) of the Q-switched Nd 31 :YAlO 3 and Er 31 :glass lasers were found to be 75 ns (3.8 mJ) and 50 ns (4.5 mJ), respectively. © 1999 Optical Society of America [S0740-3224(99)01411-3] OCIS codes: 140.3380, 140.3540, 300.6420. 1. INTRODUCTION Crystals doped with Co 21 ions that are located in tetrahe- dral symmetry sites, such as LiGa 5 O 8 , LaMgAl 11 O 19 (LMA), and ZnSe, may exhibit strong and broad lumines- cence bands. 14 The presence of these strong bands has allowed us to consider these crystals, like crystals doped with Cr 41 ions such as Cr:Mg 2 SiO 4 (forsterite) and Cr:Y 3 Al 5 O 12 (YAG), possible candidates for tunable solid- state lasers in the visible and the near infrared. Y 3 Al 5 O 12 , Y 3 Sc 2 Ga 3 O 12 , and LaMgAl 11 O 19 dielectric crystals, 5,6 the semiconductor ZnSe, 7 and the glass ce- ramic SiO 2 ZnOAl 2 O 3 CoO (Ref. 8) doped with tetrahe- drally coordinated Co 21 ions have been proved to be use- ful passive saturable absorbers for flash- or diode-pumped 1.54-mm Er 31 :glass lasers. So, motivated mostly by the need to accommodate eye-safe laser applications, research is now being done to optimize the already available mate- rials and to find new ones. In particular, there is a need for better characterization and a more extensive analysis of the potential of the promising Co:LMA system. Its basic absorption and lu- minescence were reported in Ref. 3; however, its nonlin- ear optical properties were not described. In this paper we study the nonlinear optical properties and saturable-absorber Q-switching operation of two Co 21 :LMA crystals. Differential absorption spectra re- corded by use of 1.08-mm and 540-nm picosecond excita- tion and nanosecond intensity-dependent transmission Yumashev et al. Vol. 16, No. 12 / December 1999 / J. Opt. Soc. Am. B 2189 0740-3224/99/122189-06$15.00 © 1999 Optical Society of America

Nonlinear spectroscopy and passive Q-switching operation of a Co^2+ :LaMgAl_11O_19 crystal

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Page 1: Nonlinear spectroscopy and passive Q-switching operation of a Co^2+ :LaMgAl_11O_19 crystal

Yumashev et al. Vol. 16, No. 12 /December 1999 /J. Opt. Soc. Am. B 2189

Nonlinear spectroscopy and passive Q-switchingoperation of a Co21:LaMgAl11O19 crystal

K. V. Yumashev, I. A. Denisov, N. N. Posnov, and V. P. Mikhailov

International Laser Center, Belarusian State Polytechnical Academy, Building 17, 65 F. Skaryna Avenue,220027 Minsk, Belarus

R. Moncorge

Centre Interdisciplinaire de Recherche Ions Laser, Unite Mixte de Recherche 6637 Centre National de la RechercheScientifique–Commissariat a l’Energie Atomique–Institute des Sciences de la Matiere et du Rayonnement,

Universite de Caen, 6 Boulevard Mal. Juin, 14050 Caen, France

D. Vivien

Laboratoire de Chimie Appliquee de l’Etat Solide, Ecole Nationale Superieure de Chimie et Physique, Unite Mixtede Recherche 7574 Centre National de la Recherche Scientifique, Universite Paris VI, 11 Rue P. et M. Curie,

75231 Paris Cedex 05, France

B. Ferrand

Laboratoire d’Electronique, de Technologie et d’Instrumenstation–Commissariat a l’Energie Atomique TechniquesAvancees, Departement Optronique–SMDO, 17 Avenue des Martyrs, Centre d’Etudes Nucleaires de Grenoble

85X, 38041 Grenoble, France

Y. Guyot

Laboratoire de Physico-Chimie des Materiaux Luminescents, Unite Mixte de Recherche 5620 Centre National de laRecherche Scientifique, Universite de Lyon 1, 43 Boulevard 11 Novembre 1918, 69622 Villeurbanne, France

Received February 23, 1999; revised manuscript received June 29, 1999

A pump–probe technique with picosecond resolution was used to record and derive the differential and excited-state absorption (ESA) spectra of a Co21:LaMgAl11O19 (LMA) single crystal in the visible and near-infraredspectral regions. Time resolution allowed us to observe ESA bands that can be assigned to a 4T2(4F)→ 4T1(4P) optical transition and to transitions from the thermally populated 2E(2G) excited state to doubletlevels that arise from the 2F free-ion level of the tetrahedrally coordinated Co21 ion. Intensity-dependenttransmission measurements were also carried out at 1.34 and 1.54 mm. Passive Q switching of Nd31:YAlO3(1.34-mm) and of Er31:glass (1.54-mm) lasers by use of the Co21:LMA crystal as a saturable absorber was dem-onstrated. The pulse durations (energies) of the Q-switched Nd31:YAlO3 and Er31:glass lasers were found tobe 75 ns (3.8 mJ) and 50 ns (4.5 mJ), respectively. © 1999 Optical Society of America[S0740-3224(99)01411-3]

OCIS codes: 140.3380, 140.3540, 300.6420.

1. INTRODUCTIONCrystals doped with Co21 ions that are located in tetrahe-dral symmetry sites, such as LiGa5O8, LaMgAl11O19(LMA), and ZnSe, may exhibit strong and broad lumines-cence bands.1–4 The presence of these strong bands hasallowed us to consider these crystals, like crystals dopedwith Cr41 ions such as Cr:Mg2SiO4 (forsterite) andCr:Y3Al5O12 (YAG), possible candidates for tunable solid-state lasers in the visible and the near infrared.

Y3Al5O12, Y3Sc2Ga3O12, and LaMgAl11O19 dielectriccrystals,5,6 the semiconductor ZnSe,7 and the glass ce-ramic SiO2–ZnO–Al2O3–CoO (Ref. 8) doped with tetrahe-drally coordinated Co21 ions have been proved to be use-ful passive saturable absorbers for flash- or diode-pumped

0740-3224/99/122189-06$15.00 ©

1.54-mm Er31:glass lasers. So, motivated mostly by theneed to accommodate eye-safe laser applications, researchis now being done to optimize the already available mate-rials and to find new ones.

In particular, there is a need for better characterizationand a more extensive analysis of the potential of thepromising Co:LMA system. Its basic absorption and lu-minescence were reported in Ref. 3; however, its nonlin-ear optical properties were not described.

In this paper we study the nonlinear optical propertiesand saturable-absorber Q-switching operation of twoCo21:LMA crystals. Differential absorption spectra re-corded by use of 1.08-mm and 540-nm picosecond excita-tion and nanosecond intensity-dependent transmission

1999 Optical Society of America

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2190 J. Opt. Soc. Am. B/Vol. 16, No. 12 /December 1999 Yumashev et al.

measurements performed at 1.34 and 1.54 mm are pre-sented. Passive Q switching of Nd31:YAlO3 (1.34-mm)and Er31:glass (1.54-mm) lasers with Co:LMA as the satu-rable absorber are also described and analyzed. We thenuse these data to derive stimulated-emission (SE) andexcited-state absorption (ESA) spectra and to give valuesfor the absorption saturation intensities at 1.34 and1.54 mm.

2. EXPERIMENTSLaMgAl11O19 has a slightly distorted PbFe12O19 (M-type)structure and belongs to the hexagonal space groupP63 /mmc. The lattice constants are a 5 0.5581 nm andc 5 2.1982 nm.9 The transition from a compound withperfect M-type structure, Me21Al12O19, to LaMgAl11O19occurs through the substitution of the La31 ion for theMe21 ion and of the Mg21 ion for one of the Al31 ions.The two Co21:LMA crystals were grown by the Vierneuil(flame fusion) and Czochralski techniques. TheVierneuil-grown sample was in the form of a small plateapproximately 5 mm long, 5 mm wide, and 0.35 mm thick(sample 1), and the Czochralski-grown sample was asmall disk of 5-mm diameter and 1.1-mm thickness(sample 2). The nominal dopant concentration @Co21# insample 1 was 5 at. % (8.4 3 1019 ions/cm3); the concentra-tion in sample 2 was 0.3 at. % (0.5 3 1019 ions/cm3).Sample 1 had its c crystallographic axis perpendicular tothe plate, and sample 2 had it parallel to the disk surface.

The differential absorption spectra were recorded witha pump–probe technique, and the measurements yielded

DOD~l! 5 2log~T/T0!, (1)

where OD is the optical density and T and T0 are theprobe-beam transmission with and without the pumpbeam present, respectively. When DOD(l) is negative,ground-state absorption (GSA) bleaching and SE exceedESA, whereas, for positive DOD(l) values, ESA domi-nates. The pump beam was that of a passively mode-locked Nd31:YAlO3 (YAP) laser delivering 15-ps-durationpulses at 1.08 mm or at 540 nm (second harmonic). Partof the 1.08-mm picosecond pump radiation was amplifiedand used to produce a white-light continuum in a D2Ocell; part was used as a probe beam at variable time de-lays after the pump beam. An optical multichannel ana-lyzer with two photodiode arrays combined with a spec-trometer was used as a recording system. The intensity-dependent transmission of the crystals was measuredwith the aid of an electro-optically Q-switched Nd31:YAPlaser at 1.34 mm and a passively Q-switched Er31:glass la-ser at 1.54 mm that delivered pulses of 75 and 45 ns, re-spectively. The transmission of the samples was mea-sured with two photodetectors. The laser pulse intensityincident onto the crystal was varied with a set of neutral-density filters. The GSA spectra were measured with aconventional dual-beam Cary-17 spectrophotometer, andall the measurements were carried out at room tempera-ture.

3. PUMP–PROBE MEASUREMENTRESULTSPolarized GSA spectra of the Co21:LMA samples studiedhere are shown in Fig. 1. LMA is a uniaxial crystal, and

the GSA spectra were taken for Eic and E'c polariza-tions. The GSA spectrum for Co21:LMA is similar to thatwhich is found in the other materials—spinels, garnets,fluorides, and semiconductors—in which the Co21 ionsalso occupy tetrahedral symmetry sites.1–4,10–17 Figure 2is an energy-level diagram of the Co21 ion (electronic con-figuration d7) in a tetrahedral crystal field. The strongGSA band located near 590 nm is assigned to the spin-and electric-dipole-allowed 4A2 → 4T1(4P) transition; thenear-infrared GSA band centered near 1350 nm, to thespin- and electric-dipole allowed 4A2 → 4T1(4F)transition.1–4 The third GSA band that is due to the

Fig. 1. Polarized GSA spectra of the Co21:LMA crystals forsamples 1 and 2.

Fig. 2. Energy-level diagram of the Co21 ion in LMA crystal inTd symmetry. Solid arrows, observed optical transitions.

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Yumashev et al. Vol. 16, No. 12 /December 1999 /J. Opt. Soc. Am. B 2191

spin-allowed 4A2 → 4T2(4F) transition is expected to liein the 2.5–3-mm region (see, for example, Refs. 10–12).This spectral region is not investigated here, in particularbecause this 4A2 → 4T2 transition is electric-dipole for-bidden in case of pure tetrahedral site symmetry and thusexhibits only a low oscillator strength. By analogy withthe absorption features observed and analyzed in theother Co21-doped crystals LiGa5O8, ZnGa2O4, andY3Al5O12,

1,10–14 the shoulder observed near 545 nm on thehigh-energy side of the 4A2 → 4T1(4P) optical band couldbe due to one of the doublet levels that arise from the 2Gfree-ion level. In the same way, the weak peak observednear 475 nm could be assigned to 4A2 → 2T1(2P),2T2(2H) transitions because similar features were alsoevidenced in the GSA spectra of the tetrahedrally coordi-nated Co21-doped MgAl2O4 and ZnAl2O4 crystals.10–12

However, in the LMA structure9 the oxygen tetrahedraabout the Co21 ion are slightly distorted (C3v local sitesymmetry) and the 4T1(4P) and 4T1(4F) states certainlysplit into various crystal field components, which can per-fectly account for the complicated structure of the GSAbands observed near 590 and 1350 nm.

Figure 3(a) shows the ground-state, the differential-absorption, and the emission spectra obtained with highlydoped Co21:LMA crystal #1. The pump wavelength wasat 540 nm of the frequency-doubled picosecond Nd:YAPlaser. The pump-beam and probe-beam polarizationswere both perpendicular to the c crystallographic axis ofthe crystal ( s polarization). The pump radiation excitedthe 4A2 → 4T1(4P) transition of the Co21 ion. The sharpfeature observed in the DOD spectrum comes from the

Fig. 3. (a) Differential absorption spectrum DOD 5 2log(T/T0)and corresponding GSA and luminescence (dashed curve) spectrafor Co21:LMA crystal sample 1. Experimental conditions: ex-citation with 15-ps laser pulses at 540 nm; delay time betweenpump and probe pulses, 45 ps; pump- and probe-beam polariza-tion perpendicular to the c axis of the crystal. (b) sSE–sESA Spec-trum obtained after subtraction of the sGSA spectrum from theappropriately scaled DOD(l) spectrum.

scattered light of this pump beam in the direction of theprobe light (it can be used, in fact, for wavelength calibra-tion). Bleaching of the 4A2 → 4T1(4P) GSA band is evi-dent, whereas ESA clearly appears only near 450–520nm. GSA bleaching and ESA signals both appearedwithin the instrumentally limited rise time of ;20 ps anddid not decrease significantly for delay times as long as550 ps between pump and probe pulses.

The room-temperature lifetime of the 4T1(4P) level ofthe Co21 ion in LMA is known to be 40 ns in the case of alightly doped sample with 0.5-at. % Co or 0.843 1019 ions/cm3.3 This is considerably longer than thepulse duration (;15 ps) in our measurements. In thiscase the differential absorption signal DOD(l) after thepump pulse can be expressed as18

DOD~l! 5 DNL@sESA(l) 2 sGSA(l) 2 sSE~l!#, (2)

where sESA , sGSA , and sSE are the ESA, GSA, and SEcross sections, respectively, DN is the density of ions inthe excited state, and L is the sample length. sGSA(l)can be determined from absorption measurements withthe known Co21 concentration of 8.4 3 1019 ions/cm3. Toobtain the difference absorption spectrum between SEand ESA, i.e., sEFF(l) 5 sSE(l) 2 sESA(l), one shouldsubtract the GSA bleaching spectrum sGSA(l)DNL fromthe DOD(l) spectrum. If there is a wavelength l0 atwhich ESA and SE are negligibly small @sESA(l0)5 sSE(l0) 5 0#, sEFF(l) can be found from the expres-sion

sEFF(l) 5 sGSA(l0)[DOD(l)/DOD(l0) 2 a(l)/a(l0)], (3)

where a is the linear GSA coefficient. In the case ofCo21:LMA, there is no SE, for example, at 580 nm.3 As-suming that sESA is negligibly small compared with sGSAnear this wavelength, we obtained the sEFF(l) spectrumshown in Fig. 3(b). ESA is now clearly evidenced from;450 to ;620 nm, and a weak SE band is also observednear ;640 nm. According to the emission spectrumshown in Fig. 3(a), two SE bands near 660 and 880 nm,which are related to the 4T1(4P) → 4A2 and 4T1(4P)→ 4T2 transitions, are expected, with maximum SE crosssections of ;5.4 3 10219 and ;0.45 3 10219 cm2,respectively.3 Thus the SE band observed at 640 nm canbe assigned to the 4T1(4P) → 4A2 transition. The4T1(4P) → 4T2 SE band expected near 880 nm is not ob-served, probably because it is too weak.

Because no energetically higher-lying quartet levels[higher than the 4T1(4P) level] exist, there is no in-traionic spin-allowed ESA transition from the 4T1(4P)level. According to the Tanabe–Sugano diagram for theCo21 (d7) ion in a tetrahedral crystal field, the ESA ob-served in the 500–620 nm region can be assigned to aspin-allowed transition from the thermally populatedlevel 2E(2G) to the energetically higher doublet levelsthat arise from the 2F free-ion state. ESA at the shorterwavelengths (l , 500 nm) can be related to charge-transfer transitions or to transitions from level 4T1(4P)to the conduction band of the crystal. These transitionsare possible because the duration of the pump pulse (;15ps) is much shorter than the 4T1(4P) relaxation time (42ns).

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2192 J. Opt. Soc. Am. B/Vol. 16, No. 12 /December 1999 Yumashev et al.

The differential absorption spectrum of Co21:LMA wasalso recorded with a pump wavelength of 1.08 mm, inwhich case the pump radiation excited the 4A2→ 4T1(4F) transition of the Co21 ion. Pump- and probe-beam polarizations were perpendicular to the c axis of thecrystal. As shown in Fig. 4(a), under these conditionsboth bleaching (or gain) and ESA signals appeared.They appeared within the instrumentally limited risetime of ;20 ps, and they did not decay significantly afterour maximum pump–probe delay time of 550 ps. Theshape of the structure at approximately 500–620 nm isidentical to the shape of the 4A2 → 4T1(4P) GSA band, soit can be assigned to a 4A2 → 4T1(4P) bleaching bandthat results from a 4A2 ground-state depletion. Thus theobserved DOD(l) signal can be expressed as

DOD~l! 5 DNL@sESA(l) 2 sGSA~l!#. (4)

Because in the 500–620-nm region the shape of theDOD(l) spectrum is identical to the shape of the 4A2→ 4T1(4P) GSA band, ESA is assumed to be negligible inthis spectral region, and the correction of the DOD(l)spectrum for GSA is made directly by subtraction of theGSA spectrum from the appropriately scaled DOD(l).The resultant sESA spectrum is shown in Fig. 4(b). AnESA band ranging from 630 to 850 nm with a maximumat ;710 nm is then evidenced. This ESA band is likelydue to a 4T2(4F) → 4T1(4P) transition that occurs afterrelaxation of the ions from the directly excited 4T1(4F)level to the 4T2(4F) absorbing level. This assignmentagrees perfectly with the relative energetic positions of

Fig. 4. (a) Differential absorption spectrum DOD5 2log(T/T0) and corresponding GSA spectrum of Co21:LMAcrystal sample 1. Experimental conditions: excitation with15-ps pulses at 1.08 mm; delay time between pump and probepulses, 30 ps; pump- and probe-beam polarization perpendicularto the c axis of the crystal. Inset, DOD at a wavelength of 540nm versus delay time. (b) sESA spectrum obtained after subtrac-tion of the sGSA spectrum from the appropriately scaled DOD(l)spectrum.

the 4T1(4P), 4T1(4F), and 4T2(4F) states, which can bederived from the GSA and luminescence spectra and fromthe Tanabe–Sugano diagram for a Co21(d7) ions in tetra-hedral coordination: According to these data, the4T2(4F) → 4T1(4P) ESA transition should be locatednear 800 nm. However, the 4T1(4F) → 4T1(4P) ESAtransition is expected to lie in the near-infrared regionnear 1.1 mm, which deviates from the 630–850 nm rangein which the ESA is observed. As we mentioned above,both GSA bleaching and ESA signals appear simulta-neously, within ;20 ps, and do not decay for pump–probedelay times of as much as ;550 ps. Thus we are led toassume that the lifetime of the 4T1(4F) state is shorterthan ;20 ps and that the lifetime of the 4T2 state islonger than ;550 ps.

4. TRANSMISSION AND PASSIVEQ-SWITCHING EXPERIMENTSFigure 5 shows the transmission T of Co21:LMA sample 1as a function of the pump fluence of the 1.34-mm-wavelength, 75-ns pulse-duration Nd:YAP and 1.54-mm,45-ns Er31:glass Q-switched lasers mentioned above.The pump wavelengths at 1.34 and 1.54 mm excited the4A2 → 4T1(4F) transition. The polarization of the laserlight was perpendicular to the c axis of the crystal (s po-

Fig. 5. Dependence of transmission on input fluence forCo21:LMA crystal sample 1 with Q-switched (a) Nd31:YAP and(b) Er31:glass lasers operating at l with pulse durations of 75and 45 ns, respectively (laser light polarized perpendicular to thec axis of the crystal). Solid curves, results of fitting with the aidof Eq. (5).

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Yumashev et al. Vol. 16, No. 12 /December 1999 /J. Opt. Soc. Am. B 2193

larization). Bleaching was observed from both pumpwavelengths. Knowing that the lifetime of the meta-stable level 4T1(4P) is <40 ns3 and assuming that the re-laxation time for the ions in the 4T1(4F) excited state ismuch shorter than the pump-pulse duration of 75 ns, weanalyzed the T(I0) dependence within the framework of afast-relaxing absorber model.19 Neglecting ESA near1.34 and 1.54 mm, we can describe the experimentaltransmission data by the equation

ln~T/T0! 5 ~I0 /IS!~1 2 T !, (5)

where T0 is the small-signal transmission and IS is thesaturation intensity. The best fits to the experimental

Fig. 6. (a) Schematic of the passively Q-switched 1.34-mmNd31:YAlO3 laser cavity with a Co21:LMA saturable absorber.HT, highly transmitting, HR, highly reflecting. (b) Input–output characteristics of the 1.34-mm Nd31:YAlO3 laser withCo21:LMA Q switching. (c) Typical Q-switched laser pulse fromthe Nd31:YAlO3 laser at 1.34 mm with Co21:LMA.

Fig. 7. (a) Schematic of the passively Q-switched 1.54-mmEr31:glass laser cavity with a Co21:LMA saturable absorber. (b)Typical Q-switched laser pulse from the Er31:glass laser at 1.54mm with Co21:LMA. P, polarizer; HR, highly reflecting; M1,M2, mirrors.

data were obtained for IS 5 20 MW/cm2 at 1.34 mm andIS 5 56 MW/cm2 at 1.54 mm. Starting from those valuesand using the GSA spectra shown in Fig. 1(b), we can es-timate the saturation intensity for polarization Eic to be;25 and ;15 MW/cm2 at 1.34 and 1.54 mm, respectively.These values can be compared with the saturation inten-sities of 140 and 180 MW/cm2 measured in theCo21:Y3Al5O12 and Co21:Y3Sc2Ga3O12 single crystals at1.54 mm.5

In a second step, Co21:LMA was used as saturable ab-sorber for passive Q switching of a Nd31:YAlO3 laser atwavelength of 1.34 mm and of an Er31:glass laser at wave-length of 1.54 mm. As shown in Fig. 6(a), the three-mirror cavity of the 1.34-mm Nd31:YAlO3 laser consistedof a flat output mirror (M1; reflectivity R, 78% at 1.34 mm)and two identical highly reflecting concave mirrors (M2and M3; radius of curvature, 5 cm). The Co21:LMA crys-tal (sample 1), without any antireflection coating, wasplaced between concave mirrors M2 and M3. As above,the polarization of the laser beam was perpendicular tothe c axis of the crystal. Figure 6(b) shows a typical de-pendence of the output energy of the Q-switched pulses oninput electrical energy. The Q-switched Nd31:YAlO3 la-ser pulses were found to be ;75 ns in duration [Fig. 6(c)],and energies of as much as 3.8 mJ, corresponding to ;5%of the free-running energy at the same pump level, wereobtained. The intracavity focusing parameter Ag /Aa ,where Ag and Aa are the beam cross-section areas in thelaser rod and the absorbing crystal, respectively, was ap-proximately 3.

Passive Q switching of the 1.54-mm Er31:glass laserwith the Co21:LMA crystal as a saturable absorber wasalso demonstrated by means of a 30-cm-long laser cavityconsisting of highly reflecting 100-cm radius-of-curvaturemirror M1 and flat output coupler M2 (R 5 90% at 1.54mm). A glass plate oriented at Brewster’s angle wasplaced in the laser cavity to force linear polarization ofthe laser radiation. The Co21:LMA crystal (sample 2)used for this experiment had no antireflection coating ei-ther and was placed between a laser rod that measuredB3 mm 3 50 mm and output mirror M2. The polariza-tion of the laser beam was parallel to the c axis of theCo21:LMA crystal, for which its small-signal transmissionat 1.54 mm was ;75%. Q-switched pulses of ;45-ns du-ration [Fig. 7(b)] with 3.0-mJ energy (corresponding to'25% of the free-running output at the same pumpinglevel) were obtained. The laser operated in the TEM00transverse mode. By using a flat output coupler of R5 78%, we obtained Q-switched pulses of ;50-ns dura-tion and with 4.5 mJ of energy, corresponding to '20% ofthe free-running output at the same pumping level. In ashorter laser cavity 20 cm long (with R 5 78%), aQ-switched pulse duration (energy) of 40 ns (3.5 mJ) wasobtained.

5. CONCLUSIONSThe differential absorption spectra of theCo21:LaMgAl11O19 crystal were studied under picosecondexcitation at 540 nm and 1.08 mm tuned inside the 4A2→ 4T1(4P) and 4A2 → 4T1(4F) transitions, respectively.GSA bleaching that corresponds to the 4A2 → 4T1(4P)

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2194 J. Opt. Soc. Am. B/Vol. 16, No. 12 /December 1999 Yumashev et al.

transition was observed. For 540-nm pumping, ESA ob-served in the 500–620-nm region was tentatively as-signed to transitions from the thermally populated2E(2G) state to doublet levels that arise from the 2F free-ion level of the tetrahedral Co21 ions. For 1.08-mmpumping, ESA in the 630–850-nm region is most likelydue to the 4T2 → 4T1(4P) transition of the tetrahedrallycoordinated Co21 ions. The absorption saturation inten-sities of the Co21:LMA crystal were estimated to be20 MW/cm2 (E'c) and 15 MW/cm2 (Eic) at 1.34 and 1.54mm, respectively. Passive Q switching of Nd31:YAlO3(1.34-mm) and Er31:glass (1.54-mm) lasers with theCo21:LMA crystal as the saturable absorber was demon-strated. The pulse durations (energy) of the Q-switchedNd31:YAlO3 and Er31:glass lasers were found to be 75 ns(3.8 mJ) and 50 ns (4.5 mJ), respectively.

R. Moncorge’s e-mail address is [email protected].

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