LASER PHYSICS AND ENGINEERING Erbium ... - OSA Publishing

LASER PHYSICS AND ENGINEERING Erbium-glass slab laser with transverse diode pumping G. I. Ryabtsev,a兲 M. V. Bogdanovich, A. I. Enzhievski , and L. L. Teplyashin B.I.Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk, Belarus

A. P. Klishchenko, A. V. Pozhidaev, M. A. Shchemelev, and A. G. Ryabtsev Belorussian State University, Minsk, Belarus

A. S. Kraskovski and S. N. Titovets OAO Peleng, Minsk, Belarus

K. V. Yumashev and A. M. Malyarevich Scientific Research Institute of Optical Materials and Technologies, Belarusan National Technical University, Minsk, Belarus

O. S. Dymshits and A. A. Zhilin Scientific Research and Technological Institute of Optical Material Science, S. I. Vavilov State Optical Institute All-Russia Scientific Center, St. Petersburg

共Submitted March 31, 2008兲 Opticheski Zhurnal 75, 21–25 共November 2008兲

A slab laser based on borosilicate phosphate glass doped with erbium and ytterbium with transverse diode pumping has been developed and investigated. In the passive Q-switching regime, using a shutter based on a glass-crystalline material with nanosize spinel doped with divalent cobalt ions, the energy of the output radiation was 1.3 mJ with a pulse width of 19 ns. With active Q switching, the energy of the developed laser was 2.3 mJ. The internal optical losses are estimated, and the parameters of the passive shutter are determined. © 2008 Optical Society of America. INTRODUCTION

EXPERIMENT

Erbium glass lasers are promising sources of radiation that is provisionally safe for the organs of vision 共1.5– 1.6 ␮m兲.1,2 The problem of increasing the output energy Eout of the radiation of such radiators is currently attracting great attention. One method of achieving high Eout values is to use multipass active elements 共AEs兲 with slab geometry. A zig-zag optical path makes it possible to minimize the influence of the thermal lens and of induced birefringence on the characteristics of the AEs of the lasers. A slab configuration of the AE is widely used when creating high-efficiency solid-state lasers based on Nd, Yb, and Tmcontaining media.3–5 This paper is devoted to the development and investigation of the parameters of a solid-state slab laser 共SSL兲 based on erbium-ytterbium glass with powerful transverse diode pumping, radiating at a wavelength of about 1.54 ␮m. The operation of the laser in the active and passive Q-switching regimes is studied, the internal optical losses are estimated, and the parameters of a passive shutter based on a glasscrystalline material are determined. The characteristics of the SSL sample thus created are compared with the lasing properties of a laser based on erbium-ytterbium glass with a cylindrical AE.

The design of the SSL with transverse diode pumping investigated here is schematically shown in Fig. 1. The AE was fabricated from borosilicate phosphate glass doped with erbium and ytterbium ions 共Er, Yb: BSPG兲. The erbium and ytterbium concentrations were 5 ⫻ 1019 atom/ cm3 4 ⫻ 1021 atom/ cm3, respectively. The AE was made in the shape of a 1 ⫻ 2 ⫻ 9.9-mm3 truncated parallelepiped with a bevel angle of ␸ ⬇ 27°. The geometrical parameters of the AE were chosen so that total aperture/volume filling of the active medium by the laser radiation was achieved when there was a fourfold internal reflection of the generated beam, coming out in a direction parallel to the total-internalreflection faces of the AE. In this case, the bevel angle admitted the radiation into the AE at an angle close to Brewster’s angle, and this eliminated the need to deposit antireflection coatings on the faces. The calculated losses at the input faces did not exceed 2.5% for a complete circuit of the cavity for radiation with polarization perpendicular to the total-internal-reflection faces. For the other polarization component, the calculated losses for a complete circuit were about 60%. For a comparative analysis of the characteristics of the SSLs, AEs in the form of a rod 2 m in diameter and 10 mm long, made from similar Er, Yb: BSPG, were also used in the experiments. Coatings were deposited on the ends of the rod that made them nonreflective at the lasing wavelength.

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J. Opt. Technol. 75 共11兲, November 2008

1070-9762/2008/110704-04$15.00

© 2008 Optical Society of America

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FIG. 1. Layout of a slab laser with transverse diode pumping. 1—Spherical backstop mirror with r = 500 mm, 2—optical shutter 共active or passive兲, 3—active element, 4—linear laser-diode arrays, 5—flat output mirror with R = 88%, 6—transverse profile of the intensity distribution of the output beam.

The AEs were excited with InGaAs/AlGaAs linear laserdiode arrays with a near-field pattern 5 mm wide 共OAO NPP Inzhekt, Saratov, Russia兲. The power of each linear array was 25 W with a pump pulse 5 ms wide and a pulse-repetition rate of up to 20 Hz. The linear diode arrays were placed in pairs on two sides of the AE. In the case of the AE with slab geometry, the pump radiation was introduced through the total-internal-reflection faces of the AE. The characteristics of the erbium radiator were investigated in the free-lasing regime, as well as in the passive/ active Q-switching regime. An electrooptic modulator based on lithium niobate 共LiNbO3兲 was used as an active shutter. The passive shutter was a glass-crystalline material containing nanosize spinel crystals doped with divalent cobalt ions 共Co: MAC兲. The material was fabricated by the variable-rate crystallization of glass containing magnesium and aluminum oxides in equimolar ratio and the necessary concentration of cobalt oxide. The glass was synthesized at a temperature of 1560 ° C and was then annealed at 700 ° C. As a result of the adjustable-rate crystallization of the initial glass, a crystalline phase was formed in it—nanosize spinel, doped with divalent cobalt ions. In order to obtain the required amount of spinel crystals, supplementary crystallization of the glass was carried out at 700– 1000 ° C. The size of the spinel crystals in the fabricated Co: MAC glass-crystalline material was about 10 nm, and this ensured that it was transparent. The transmission spectrum of the passive shutter is shown in Fig. 2. The absorption band in the wavelength region 1.1– 1.7 ␮m is associated with the 4A2 → 4T1共 4F兲 transition of the Co2+ cobalt ions that occupy tetrahedrally coordinated sites in the structure of the MgAl2O4 crystal6–8 共inset to Fig. 2兲. The initial transmission of the shutter was about 79%. The clarification-relaxation time of the passive shutter was measured by the excitation-probe method with excitation by the emission pulses of an erbium laser 共1.54 ␮m兲 70 ns wide. The excitation-energy density at the Co: MAC surface was about 2 J / cm2. The probing was done at a wavelength of 1.35 ␮m with a wide radiation pulse 共about 500 ␮s兲 from a Nd: KGW laser with quasi-continuous diode pumping. A resonator was formed by a hemispherical backstop mirror with radius of curvature r = 500 mm 共or r = 1000 mm兲 and flat output mirrors with various reflectances of R = 78, 88, 95, and 98% at the lasing wavelength 共1540 nm兲. A laser cavity with Q switching was formed by a blind spherical 705


FIG. 2. Transmittance T of glass-crystal material Co: MAC 0.7 mm thick vs wavelength of radiation in the near-IR region. The inset shows the energylevel diagram of the Co2+ 共3d7兲 ion in a tetrahedral crystal field of symmetry Td and shows the transition corresponding to the absorption band in the 1.1– 1.7-␮m region.

mirror 共radius of curvature 500 mm兲 and a flat mirror with R = 88%. The cavity was 60 mm long. The degree of polarization of the SSL was determined from the dependence of the intensity after the radiation passed through a Glan prism on the angle of rotation of the prism. The energy of the radiation transmitted through the polarizer was measured by means of an Ophir Laser Star digital device. The quality parameter M 2 of the radiation beam was determined by a technique based on the hyperbolic approximation of the dependence of the beam diameters on the distance in the beam-waist region formed by a thin lens. The focal length of the lens equaled 130 mm. The diameters of the test beams were found by the method of second-order moments, using the transverse profiles of the radiationintensity distribution, recorded by an ElectroPhysics 7290 Infrared Vidicon camera 共Coherent兲 with a set of attenuators that do not distort the beam profile.9 The width and repetition rate of the radiation pulses were measured by means of an FP-150P high-speed germanium photodiode and a Tektronix TDS 3052B digital oscilloscope. RESULTS AND DISCUSSION

In the free-lasing regime, the energy of an output light pulse developed by the SSL reached values of 7 mJ 共lasing pulse width 4 ms, absorbed pump-radiation power about 30 W, pulse-repetition rate 1 Hz兲. The external differential quantum yield of the lasing was ⬇8 – 15%. As can be seen from Fig. 1 共6兲, the transverse profile of the radiationintensity distribution of the SSL is not circular. Therefore, the quality parameter M 2 of the generated beam was measured in two mutually orthogonal directions, corresponding to the minimum and maximum dimensions of the beam. The M 2 values equalled 2.8 共in the direction of the minimum Ryabtsev et al.

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FIG. 3. Transmittance T of glass-crystal material Co: MAC 3.4 mm thick at a wavelength of 1.54 ␮m vs the energy-flux density W of incident radiation pulses 70 ns wide. The points denote experimental data, and the solid curve shows the calculated values. The transmission data are corrected for radiation losses due to reflection from the sample surfaces.

dimension兲 and 3.4 共in the direction of the maximum dimension兲. The generated radiation of the SSL has virtually 100% plane polarization, with the electric vector E of the light wave being perpendicular to the total-internal-reflection faces. Such a position of the E vector, to all appearances, is given by the polarization conditions of the beam that undergoes total internal reflection inside the AE for which the optical losses for the given orientation of E are minimal. In accordance with the technique explained in Ref. 10, the internal optical-loss factors ␳ were estimated for the developed sample of an SSL and the laser with a cylindrical AE. The cavity configurations were varied by means of backstop mirrors with r = 500 and 1000 mm and output mirrors with R = 78, 88, 95, and 98%. The cavity length was constant and equalled 60 mm. Parameter ␳ equalled 0.03 cm−1 for the AE with slab configuration, and ␳ was 0.07 cm−1 in the case of the cylindrical AE. Such scatter in the internal optical-loss factors was explained both by inhomogeneities in the glass from which the AEs were fabricated and by the difference of the size of the pumping regions in the AE with slab geometry and the cylindrical AE. Investigations of the kinetics of the induced absorption variation for the glass-crystalline material made it possible to establish that the relaxation of the absorption variation in Co: MAC has a single-exponential character with time constant 270⫾ 45 ns, the value of which is determined by the lifetime in the excited state of the 4T2 term of the tetrahedrally coordinated Co2+ ion 共see insert in Fig. 2兲.8 The dependence of the transmittance of Co: MAC on the energy-flux density of the incident pulses is shown in Fig. 3. It can be seen that, as the pumping increases, the transmission of the sample increases from the initial value of 58% to the limiting value of ⬇95% in the totally cleared state, and this corresponds to a comparative quality index of FOM= ln共0.58兲 / ln共0.95兲 = 10.6. The residual absorption in the completely cleared state is mainly caused by the presence of some number of cobalt ions in the glass matrix and possibly in the aluminotitanate crystalline phase. An analysis of the experimental data of Fig. 3 in terms of the model of a slowly relaxing saturable 706


absorber11 is evidence that the residual absorption coefficient of the Co: MAC in the totally cleared state does not exceed 0.15 cm−1, while the transverse absorption cross section from the ground state for the Co2+ ions is ␴ = 4.0 ⫻ 10−19 cm2. The resulting value of ␴ agrees with the values of the effective absorption cross section from the ground state for tetrahedrally coordinated Co2+ ions in single-crystal MgAl2O4 关共2.6– 5.1兲 ⫻ 10−19 cm2兴, which in the case of plane-polarized light depends on the vibration direction of the electric vector and the radiation-propagation direction with respect to the crystallographic axes of the single crystal.12 According to Ref. 12, the value of FOM for Co2+ : MgAl2O4 is about 11. The absorption cross section for Co: MAC thus exceeds the gain cross section for erbium glass 共about 6.2⫻ 10−21 cm2兲 by more than a factor of 60. The relatively high value of ␴ in combination with the long clearing-relaxation time 共about 270 ns兲 and the fairly large FOM= 10.6 is evidence that the glass-crystalline material Co: MAC is an efficient passive shutter for an erbium-glass laser. In the Q-switching regime, the output radiation energy of the SSL with a passive shutter reached 1.3 mJ when the pulse length was 19 ns. In the case of the laser with a cylindrical AE at comparable pump levels and pulse width, the energy of a single laser pulse with passive Q switching was at a level of 1.0 mJ. The use of an electrooptic shutter based on lithium niobate made it possible to increase the output energy of the generated radiation pulse of the SSL to 2.3 mJ. The energy of the output pulse of the laser with a cylindrical AE and an electrooptic shutter did not exceed 1.7 mJ. CONCLUSION

A solid-state slab laser based on erbium-ytterbium glass with powerful transverse diode pumping and emitting at a wavelength of about 1.54 ␮m has been developed and investigated. In the passive Q-switched regime, by using a shutter based on the material Co: MAC, the output radiation of the slab laser reached an energy of 1.3 mJ when the pulse was 19 ns wide. Using the active Q-switching regime, the laserpulse energy of the SSL was able to be increased to 2.3 mJ. The further increase of the output power was mainly restricted by the power of the linear laser-diode arrays that were used. The internal optical-loss factor for the AEs based on Er, Yb: BSPG is 0.03– 0.07 cm−1. The results of a comparative analysis of the characteristics of an SSL and a laser with a cylindrical AE are evidence that, at comparable pumping levels, the use of AEs with slab geometry makes it possible to increase the energy of the output pulses by about a factor of 1.5 with virtually 100% polarization of the generated radiation. a兲

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