Appl. Phys. B 77, 621–623 (2003)
Applied Physics B
DOI: 10.1007/s00340-003-1290-8
Lasers and Optics
m.o. ram´ırez1,u d. jaque1 j.a. sanz garc´ıa1 l.e. bausa´ 1 ˜ santiuste2 j.e. munoz
74% Slope efficiency from a diode-pumped Yb3+ : LiNbO3 : MgO laser crystal 1 Departamento de F´ısica de Materiales, Universidad Aut´ onoma de Madrid, Cantoblanco 28049 Madrid, Spain 2 Departamento
de F´ısica, Escuela Polit´ecnia Superior, Universidad Carlos III de Madrid, Av. de la Universidad, 30, 28911 Leganes, Madrid, Spain
Received: 7 April 2003/Revised version: 19 June 2003 Published online: 22 October 2003 • © Springer-Verlag 2003
An optimization of the laser action performance from a diode-pumped Yb3+ -doped LiNbO3 : MgO crystal has been carried out. In this sense, efficient laser action at 1.06 µm when pumping with a fiber-coupled laser diode at 980 nm has been demonstrated, achieving laser slope efficiencies as high as 74%. The influence of output mirror transmittance on both pumping threshold and laser slope efficiency has been investigated, and the parameters of relevance in laser dynamics (emission cross section and optical losses) have been determined. Under the experimental conditions leading to maximum slope efficiency, the pump power at threshold was 300 mW, and the pump-to-laser conversion efficiency was 40%. ABSTRACT
PACS 42.55.Xi;
1
42.55.Rz; 42.60.Lh
Introduction
Since the development of strained InGaAs diode lasers emitting around 980 nm, Yb3+ -doped materials have received a renewed interest as diode-pumpable solid-state laser materials. Yb3+ shows, as an active ion, some attractive properties, such as a relatively long radiative lifetime, absence of concentration quenching and a high quantum efficiency of the metastable state. In addition, Yb3+ shows broad emission bands, which give the possibility of a certain tunability range in the infrared, as well as of mode-locking operation [1, 2]. All these properties have strongly motivated the research on new Yb3+ -doped laser materials, leading to the demonstration of laser action in a large number of Yb3+ -doped crystals [3]. As is well known, the LiNbO3 system is an attractive and widely used dielectric medium in integrated optics due to its excellent electro-optic, acousto-optic and non-linear properties, also allowing the fabrication of optical wave guides [4]. The potential of this system as a multifunctional optical material has expanded since the demonstration of laser action from rare-earth-ion optically activated crystals [5]. For this application, the presence of MgO appears to be needed to stabilize the photorefractive damage during laser action [6]. In particular, tunable infrared laser action and efficient self-frequency doubling (SFD) of the fundamental nearu Fax: 34-913978579, E-mail:
[email protected]
infrared laser radiation have been previously demonstrated in Yb3+ : LiNbO3 : MgO using both type-I birefringent phase matching and a quasi-phase matching configuration [7–9]. In spite of high slope efficiencies and the possibility of low threshold values, which were demonstrated for laser oscillation in the near-infrared region by end-pumping with a Ti-sapphire laser, the previous results, relative to laserdiode pumping, should be considered as a preliminary ones, since low slope efficiencies (26%) and high threshold values (750 mW of absorbed pump power) were reported. In this sense, no data concerning the diode-pumped Yb3+ : LiNbO3 : MgO crystal as an efficient infrared laser source have been reported. This letter presents results on stable and particularly efficient 1.06-µm continuous-wave laser action in a Yb3+ : LiNbO3 : MgO single crystal under diode pumping. The laser slope efficiency (ηL ), as well as the absorbed pump power at threshold ( Pth ), have been measured as a function of output mirror transmittance. From the analysis of the data, some parameters of relevance in laser dynamics (such as emission cross section and optical losses) have been estimated. Finally, we have demonstrated that laser slope efficiencies as high as 74% can be obtained. 2
Experimental
The Yb3+ : LiNbO3 : MgO crystal used in this work was grown in the same laboratory and by the same method (Czochralski technique) as those of refs. [7–9]. Yb3+ and MgO concentrations in the crystal were estimated to be 1.3 and 5 mol %, respectively. To perform the laser experiments, a 1.3-mm-thick crystal slab was cut and polished to laser quality. The c-optical axis of the laser element was parallel to the crystal surface and perpendicular to the cavity axis. Crystal faces were polished up to laser quality. It is important to note that no antireflection coatings were used. In Yb3+ end-pumped lasers, optimum mode matching is achieved when the confocal length of the pump is longer than the crystal absorption length [10]. In addition, and in order to improve the beam overlap, the beam waist of the pump (wp ) should be similar or smaller than the waist size of the laser cavity mode (wL ) [11]. That is, a = wp /wL ≤ 1 [11].
622
FIGURE 1
Applied Physics B – Lasers and Optics
Experimental set-up used in this work
The optimum pump radius, wpo is given by [10]: M 2 λpl , wpo = 2nπ
(1)
where l is the crystal length, λp = 980 nm is the pump wavelength, M 2 is the beam quality factor of the pump beam and n is the crystal refractive index (n ≈ 2.2 in our case). We have used this equation as a guide to design our pump and cavity geometries. The experimental set-up used for the lasergain experiments is shown in Fig. 1. We used a double-pass quasi-hemispherical cavity with a flat dichroic input mirror (IM, R > 99.9% at 1.06 µm and T = 90% at 980 nm). The output coupler (OC) was of 10-cm radius of curvature. Six different OC transmittances (0.3, 1, 2, 4, 6 and 10%) were used. All were also of high reflectance ( R > 90%) at the pump wavelength. End pumping was performed by a 50-µm fibercoupled 2-W laser diode (Unique-Mode VDM 38) emitting at 980 nm. The fiber output was first collimated (CL) and then focused by a single 3-cm focal lens (FL) into the crystal. The pumping mode radius of about 45 µm was close to the optimum pump-beam size (46 µm) calculated from (1). It was measured that almost 40% of the pump power was absorbed by the crystal. The cavity length was 9.9 cm, leading to a laser beam waist of about 56 µm, so that a = wp /wL ≈ 0.8 ≤ 1, as is required for optimum pump and laser mode overlap [11]. Stable laser generation was achieved with all the output mirrors OC used. The wavelength of the output laser (1063 ± 1 nm) was found to be almost independent of the output mirror transmittance. In all the cases, the output beam was 100% π -polarized (electric field parallel to the c-optical axis). The output π -polarisation can be explained by taking into account the previous results showing that the stimulated emission cross section around 1.06-µm is around twice as large in π polarisation than in the σ configuration ( E electric field perpendicular to the c-optical axis) [7–9]. 3
Results and discussion
Figure 2a shows the absorbed pump power at threshold as a function of output mirror transmittance. In a quasi-three-level laser system, the absorbed power at pumping threshold is given by [11]: hvp π w2L + w2p (T + L + 2L Yb) Pth = , (2) 4σem τF where υp is the pump frequency, wp and wL are the pump and laser beam radii along the length of the crystal, respectively, σem is the net emission cross section, τF is the fluo-
rescence lifetime (540 µs [7]), T is the output coupler transmittance, L is the round-trip loss and L Yb is the one-pass reabsorption loss in the crystal ( L Yb ≈ 0.7%, as calculated from the room-temperature absorption spectrum [7]). Equation (2) predicts a linear relationship between the absorbed pump power at threshold and the output mirror transmittance. This linear relationship is, indeed, observed in Fig. 2a. From the fit of experimental data to a linear regression behavior, the net emission cross section at the laser wavelength, σem (1.06 µm), can be estimated by simply applying (2), and by taking into account the absence of antireflection coatings on the crystal surfaces. We have obtained a value σem (1.06 µm) = 0.7 × 10−20 cm2 , in reasonably good agreement with that obtained from spectroscopic measurements (σem (1.06 µm) = 0.55 × 10−20 cm2 ) [7]. From this analysis it is also possible to estimate the round-trip losses. We have obtained L ≈ 3%, a value similar to those previously obtained from laser gain experiments under Ti : sapphire pumping [7–9], but a slightly higher value than those values reported for other laser materials, (usually around 1% [12]). The presence of some defects inside the laser crystal could be the origin of the losses obtained. Table 1 shows, for the sake of comparison, the laser threshold values as well as the laser slope efficiencies obtained previously from several Yb3+ -doped crystals. All the values shown in Table 1 refer to absorbed pump power. As can be observed, the laser threshold values obtained in this work are of the same order as those previously obtained from a significant number of Yb3+ -doped materials. Figure 2b shows the laser slope efficiency as a function of the output mirror transmittance. Laser slope efficiency grows monotonically with the output mirror transmittance, following the well-known ηL ∝ T/(T + L) trend [11]. Maximum slope efficiency is obtained when the T = 10% output coupler is used. Due to the saturation in the laser slope efficiency for high transmittances (in our case this saturation is clearly observable for T > 6%), further enhancement in the laser slope efficiency by using higher output coupler transmittances is not expected [11]. Figure 3 shows the fundamental output power versus the absorbed pump power of the Yb3+ : MgO : LiNbO3 crystal obtained when the T = 10% output coupler was used. The pumping absorbed power at threshold was 300 mW. The high-
Material
η L (%)
Pth (mW)
Yb3+ : GGG Yb3+ : YAl3 (BO3 )4 Yb3+ : YAG Yb3+ : KYW Yb3+ : KYW Yb3+ : KGW Yb3+ : KGW Yb3+ : Ca4 YO(BO3 )3 Yb3+ : Sr5 Ba(PO4 )3 F Yb3+ : BaCaBO3 F KYb(WO4 )2 Yb3+ : MgO : LiNbO3
64 71 36 10 78 53 72 73 72 38 44 74
850 200 300 450 300 330 35 64 35 100 30 300
Pump source Reference Diode Diode Diode Diode Ti : sapphire Diode Ti : sapphire Diode Diode Ti : sapphire Ti : sapphire Diode
13 14 10 15 16 16 15 17 18 19 20 This work
TABLE 1 Laser slope efficiencies and laser thresholds obtained from several Yb3+ -doped materials (all these values deal with absorbed pump power)
RAM et al.
74% Slope efficiency from a diode-pumped Yb3+ : LiNbO3 : MgO laser crystal
623
curves, the value reported here being in good agreement with previous work. Laser radiation at 1064 nm was obtained with slope efficiencies as high as 74%. Better efficiencies are expected by improving the optical quality of the laser crystals as well as by using antireflection coatings on the crystal. The results obtained in this work makes Yb3+ : LiNbO3 : MgO a promising material for optoelectronic devices requiring efficient diode-pumped laser light generation in the near-infrared region. ACKNOWLEDGEMENTS This work has been supported by the Comunidad Autonoma de Madrid (CAM) under Project No. 07 N-0020-2002 and by the CICyT under Project No. MAT2001-0167. D. Jaque thanks the Ministerio de Ciencia y Tecnolog´ıa of Spain for a Ramon y Cajal contract.
REFERENCES FIGURE 2 Pumping threshold (a) and laser slope efficiency (b) as a function of output mirror transmittance
1.06-µm laser power as a function of absorbed pump power. Output mirror transmittance was T = 10%
FIGURE 3
est output power of 260 mW was achieved at an absorbed pump power of 660 mW. The light–light conversion efficiency is close to 40%. From the linear fit also shown in Fig. 4, we have obtained a laser slope efficiency as high as 74%. This is the highest laser slope efficiency ever reported, to the best of our knowledge, for a Yb3+ : LiNbO3 : MgO crystal. In addition, this is among the highest values reported in the literature for several Yb3+ -doped materials (see Table 1). 4
Summary
In summary, we have demonstrated the efficient operation of a diode-pumped Yb3+ : LiNbO3 : MgO laser. The emission cross section has been estimated from the laser gain
1 C. Hönninger, F. Morier-Genoud, M. Moser, U. Keller, L.R. Brovelli, C. Harder: Opt. Lett. 23, 126 (1998) 2 C. Hönninger, G. Zhang, U. Keller, A. Giensen: Opt. Lett. 20, 2402 (1995) 3 A. Brenier, G. Boulon: J. Alloys Compd. 323, 210 (2001) 4 E. Cantelar, R.E. De Paolo, J.A. Sanz Garc´ıa, P.L. Pernas, R. Nevado, G. Lifante, F. Cuss´o: Appl. Phys. B 73, 515 (2001) 5 D. Jaque, J. Capmany, J. Garcia Sole, A. Brenier, G. Boulon: Appl. Phys. B 70, 11 (2000) 6 G.G. Zhong, J. Jian, Z.K. Wu: IEEE Cat. No. 80 CH 1561, 631 (1980) 7 E. Montoya, J. Capmany, L.E. Bausa, T. Kellner, A. Diening, G. Huber: Appl. Phys. Lett. 74, 3113 (1999) 8 E. Montoya, J.A. Sanz-Garc´ıa, J. Capmany, L.E. Bausa, A. Diening, T. Kellner, G. Huber: J. Appl. Phys. 87, 4056 (2000) 9 J. Capmany, E. Montoya, V. Bermudez, D. Callejo, E. Dieguez, L.E. Bausa: Appl. Phys. Lett. 76, 1374 (2000) 10 T. Taira, J. Saikawa, T. Kobayashi, R.L. Byer: IEEE J. Selec. Top. Quantum Electron. 3, 100 (1997) 11 W.P. Risk: J. Opt. Soc. Am. B 5, 1412 (1988) 12 W. Koechner: Solid State Laser Engineering, 5th edn. (Springer Verlag, Berlin 1999) 13 S. Chenais, F. Drun, F. Balembois, P. Georges, A. Brenier, G. Boulon: Opt. Mat. 22, 99 (2003) 14 P. Wang, J.M. Dawes, P. Dekker, J.A. Piper: Opt. Comm. 174, 467 (2000) 15 N.V. Kuleshov, A.A. Lagatsky, V.G. Shcherbitsky, V.P. Mikhailov, E. Heumann, T. Jensen, A. Diening, G. Huber: Appl. Phys. B 64, 409 (1997) 16 A.A. Lagatsky, N.V. Kuleshov, V.P. Mikhailov: Opt. Comm. 165, 71 (1999) 17 H. Zhang, X. Meng, P. Wang, L. Zhu, X. Liu, R. Cheng, J. Dawes, P. Dekker, S. Zhang, L. Sun: Appl. Phys. B 68, 1147 (1999) 18 A.J. Bayramian, C.D. Marshall, K.I. Schaffers, S.A. Payne: IEEE J. Quantum Electron. QE-35, 665 (1999) 19 K.I. Schaffers, L.D. deLoach, S.A. Payne: IEEE J. Quantum Electron. QE-32, 741 (1996) 20 P. Klopp, U. Griebner, V. Petrov, X. Mateos, M.A. Bursukova, M.C. Pujol, R. Sole, J. Gavalda, M. Aguilo, F. Güell, J. Massons, T. Kirilov, F. Diaz: Appl. Phys. B 74, 185 (2002)
