15 August 2001
Optics Communications 195 (2001) 431±436
www.elsevier.com/locate/optcom
1.1 W CW self-frequency-doubled diode-pumped Yb:YAl3(BO3)4 laser Peter Dekker a,*, Judith M. Dawes a, James A. Piper a, Yaogang Liu b, Jiyang Wang b a
Centre for Lasers and Applications, Macquarie University, North Ryde NSW 2109, Australia b National Laboratory of Crystal Materials, ShanDong University, Jinan 250100, China Received 10 April 2001; accepted 7 June 2001
Abstract We report 1.1 W CW green output from a diode-end-pumped self-frequency-doubled (SFD) Yb:YAB laser, with a diode-to-green optical conversion eciency of 10%. This is ®ve times the highest green power reported for any diodepumped SFD laser to date. The beam quality factor
M 2 of the infrared and green were both better than 1.6. We also report plano±plano `microchip' operation of Yb:YAB at these pump levels, resulting in infrared output power near 4 W and green power near 600 mW. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.55.Xi; 42.70.Mp; 42.72.Bj Keywords: Yb:YAB; Self-frequency-doubled laser; Visible laser
1. Introduction Self-frequency-doubled (SFD) crystalline laser materials oer attractive simplicity for the development of compact all-solid-state visible sources. SFD lasers have been demonstrated in a variety of Nd3 doped non-linear crystal hosts, most notably YAB [1], LiNbO3 [2], and the oxoborates (YCOB and GdCOB) [3,4]. The highest visible powers obtained to date for Nd:YAB are 450 mW with a Ti:sapphire pump source, and 225 mW with a diode laser pump [5]. In the oxoborate family the
* Corresponding author. Tel.: +61-2-98508911; fax: +61-298508983. E-mail address:
[email protected] (P. Dekker).
highest diode-pumped visible power reported is 115 mW for Nd:GdCOB [4] and 245 mW for Nd:YCOB [3]. These Nd3 SFD materials, however, suer from low quantum eciencies, high thermal loading factors including thermal dephasing of the non-linear process [6] and reabsorption losses in the visible (and in the case of Nd:YAB, relatively poor crystal quality). The use of Yb3 as an alternate dopant in these same nonlinear host materials [7±10] oers several advantages including a high quantum eciency, a small quantum defect and hence reduced thermal eects. In addition, no concentration quenching or other parasitic eects such as excited state upconversion occur. Moreover Yb3 oers the prospect of broad tunability in the infrared and visible (in SFD operation). In Yb:YAB the broad pump band at
0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 4 7 - 5
432
P. Dekker et al. / Optics Communications 195 (2001) 431±436
977 nm is well matched to high power InGaAs diodes and there is no visible reabsorption, the latter favouring intracavity SFD operation. Finally substitution of Yb3 for Nd3 substantially ameliorates the crystal growth problems for YAB [7], which is otherwise an excellent material, with a high non-linear coecient, good mechanical hardness and thermal conductivity. In order for a material to be considered as a good candidate for power scaling it should have a high thermal conductivity, high quantum eciency and a small Stokes factor
kp =kl . To be ecient the material should have a strong and broad bandwidth pump absorption, as well as a relatively large gain cross-section. The broad absorption allows the wavelength tolerance on the pump sources to be relaxed, while still obtaining near complete pump absorption. The material and optical properties of Yb:YAB meet these criteria and are in fact superior to other Yb doped SFD crystals (see for example, recent comparisons of SFD lasers by Brenier [11,12]). We have recently reported ecient infrared and SFD laser operation [13,14] of the new material Yb:YAB at moderate input powers (1.4 W) giving visible output powers of 160 mW and a tuning range of more than 30 nm. We now report power scaling of a miniature SFD Yb:YAB laser giving record powers of 1.1 W CW green and over 4.3 W in the infrared. 2. Experimental details The laser design was an end-pumped linear hemispherical resonator, as illustrated in Fig. 1.
The pump source was a 15 W ®bre-coupled InGaAs diode laser, ®bre diameter 400 lm, numerical aperture of 0.16, operating at 977 nm and with a 5 nm bandwidth. The pump light was collimated and focused using two aspheric lenses with an eective magni®cation of 0.73, giving a minimum pump beam diameter at the laser crystal of 290 lm. An uncoated 10 at.% Yb:YAB crystal
3 3 3 mm3 cut for type I phase-matched operation at 1064 nm at normal incidence
h 31° was used for these experiments. Like pure YAB and YbAB, Yb:YAB crystallises in the R32 space group with little lattice distortion. Detailed characterisation of Yb:YAB can be found in Ref. [7] and an overview of the borate family of crystals in Ref. [17]. The Yb:YAB crystal used for these experiments was grown, cut and polished at the Institute of Crystal Materials, Shandong University, China. The crystal was held in a copper block that was maintained at room temperature. The resonator comprised a 1 mm thick ¯at input mirror (coating HR at 1040 nm, and 90% transmission at 977 nm), and a 10 cm radius of curvature (RoC) output coupler coated HR at 1040 nm and 80% T at 520 nm (for SFD green output). The maximum incident pump power at the Yb:YAB crystal was 12.4 W. The cavity length was typically 2±3 cm. The peak absorption coecient of Yb:YAB at the pump wavelength (976 nm) was 13 cm 1 (averaged for both the `o' and `e' polarisations), and the absorption bandwidth was 22 nm. In order to determine the pumping eciency of Yb:YAB at these pump levels we compared measurements of the absorbed power in the 3 mm uncoated Yb:YAB crystal as a function of diode
Fig. 1. Experimental arrangement of end-pumped Yb:YAB laser. The dichroic is used to protect laser diode from backward propagating green and accounts for 20% of total green emission.
P. Dekker et al. / Optics Communications 195 (2001) 431±436
433
wavelength, power and crystal temperature. The absorbed power varied more due to pump saturation than due to the other parameters, 7% (i.e. at maximum pump power only 88% of the incident power was absorbed compared with 95% at the 1 W pump level), although in practice this number will be less as these measurements were made in the non-lasing case. There was no dierence in absorbed power for a diode wavelength shift of 8 nm and approximately a 1.7% decrease in absorbed power for a 50°C increase in crystal temperature. Clearly the strong and broad pump absorption in Yb:YAB allows a wide tolerance in the pump conditions. 3. Infrared laser operation in Yb:YAB Infrared laser operation was obtained using a 10 cm RoC, 4% T output coupler. At maximum pump power the laser operated near 1040 nm with a bandwidth of 4 nm. Less than 1 mW of green light was obtained using this arrangement. The slope eciency of the infrared output with respect to the incident diode power was 48% with a maximum infrared output power of over 4.3 W and a pump power threshold of 2 W. To verify the wide pump-wavelength tolerance, the infrared power was measured as a function of pump power for dierent diode temperatures and hence wavelengths, as shown in Fig. 2. Even with a variation in pump wavelength of up to 6 nm, there was little variation in infrared output power (taking into account the decrease in power with diode temperature). Fig. 2 also shows that because the output power increased linearly over the full range of pump powers, at these relatively high incident powers there is no evidence of having reached a thermal limit in Yb:YAB. The magnitude of the induced thermal lens was greater than 25 cm at maximum pump power. This was deduced by increasing the cavity length up to the stability limit determined by a 25 cm RoC output coupler (in a hemispherical resonator) and observing no deterioration in the laser output power over the full range of pump powers. Further evidence of a lack of signi®cant thermal distortions can be found by measurements of the infrared beam quality that
Fig. 2. Infrared output power as a function of incident pump power over a range of diode temperatures.
resulted in M 2 values of 1.6. The amplitude stability, as evaluated by 2 standard deviations/average
2r=av, was very good with less than a 2% variation over a 10-min period. Another important criterion for a material to be a good candidate for power scaling is a low sensitivity of laser output power to crystal temperature. This is particularly true in quasi three-level systems where the ground state is thermally populated. In Yb:YAB, a quasi three-level laser, the infrared power increased by 10% for a 21°C decrease in the crystal mount temperature (28°C reduced to 7°C), when the laser was operating 4.7 times above threshold (inversion ratio). This relatively small eect indicates that the ground state is not substantially thermally populated under these conditions.
4. Self-frequency-doubled operation in Yb:YAB For SFD laser operation, an output coupler highly re¯ecting over the infrared wavelengths and with high transmission in the green
80% was used. At maximum pump power
11 W, green output powers of over 1.1 W were obtained. The
434
P. Dekker et al. / Optics Communications 195 (2001) 431±436
quoted output power for the green includes the power extracted from the pump-input mirror: for the unoptimised coating used, this was typically 20% of the total green power. The green power was found to be very sensitive to the alignment between the input mirror, uncoated crystal and output coupler, which suggests the formation of a low-loss coupled cavity. At these high powers, relaxation oscillation spiking damaged the input mirror, but no damage was sustained to the Yb: YAB crystal or output coupler. The green and residual infrared power are plotted in Fig. 3 as a function of diode pump power. The green power increased quadratically with an approximate slope eciency of better than 15%, showing no sign of saturation. The overall diode-to-green conversion eciency was 10%. The Yb:YAB crystal mount was held at 22°C over the range of pump powers shown in Fig. 3. The temperature of the crystal is estimated to rise by approximately 15°C over this range [6] indicating the lack of sensitivity of the laser, and in particular SFD operation, to the crystal internal temperature. In comparison the on-axis temperature rise in Nd:YAB, under similar pump conditions, is estimated to be 90°C. The dierence
Fig. 3. Second harmonic and residual infrared power as a function of incident pump power.
Fig. 4. Envelope of second harmonic emission spectra over a 10-min period (approximately 40 spectra).
between these materials is primarily due to the thermal loading factors (n 0:06 for Yb, compared with n 0:35 for Nd [6]). Given the large temperature acceptance bandwidths of the borates (25°C cm) [15], the increase in internal crystal temperature is not expected to limit the SFD performance at the pump powers used here. The green emission bandwidth was approximately 4±5 nm, with emission concentrated in up to four bands as shown in Fig. 4 (the IR emission bandwidth was 8±10 nm). The peak spacing is determined primarily by the air etalon spacing between the input mirror and uncoated crystal. The green emission continually jumped between bands within the overall emission bandwidth. The green power stability
2r=av varied between 2.5% over 2.5 min to 7.4% over a 2.5 h period. The stability of the second harmonic emission in any intracavity frequency-doubled laser is typically dependent on the number of longitudinal modes operating. Stable operation is obtained when either one or hundreds of modes lase. In the Yb:YAB laser presented here the number of longitudinal modes oscillating has been calculated to be approximately 20, with the exact number depending on the number of emission bands (typically 3 or 4) and the bandwidth of each band
P. Dekker et al. / Optics Communications 195 (2001) 431±436
(typically 25 times) than those recently reported for a microchip styled Nd:GdCOB laser [4] albeit with higher pump powers. The key dierence with Yb:YAB being the ability to scale to higher pump powers with reduced thermal loading factors over Nd based materials.
6. Conclusions Yb:YAB is a good candidate for power scaling and also microchip operation. We have demonstrated a high beam quality
M 2 1:6, highpower SFD diode-pumped laser with 1.1 W of output in the green at a diode-to-green conversion eciency of 10%. This is ®ve times the highest green power reported for any diode-pumped SFD laser to date. When optimised for infrared output more than 4.3 W at 1042 nm was obtained, with a diode-to-infrared conversion eciency of 36%. For a microchip style cavity, a record SFD green output power of 600 mW was obtained in a 10 mm plano±plano cavity. The results demonstrate Yb:YAB is a good candidate for power scaling to multi-watt levels in the green and also for microchip operation at the 1 W level.
436
P. Dekker et al. / Optics Communications 195 (2001) 431±436
References [1] S. Amano, T. Mochizuki, Diode-pumped NYAB green laser, Nonlinear Opt. 1 (1991) 297±306. [2] J. Capmany, D. Jaque, J.A. Sanz Garcia, J. Garcia Sole, Continuous wave laser radiation and self-frequency-doubling in ZnO doped Nd:LiNbO3 , Opt. Commun. 161 (1999) 253±256. [3] D.A. Hammons, M. Richardson, B.H.T. Chai, A.K. Chin, R. Jollay, Scaling of longitudinally diode-pumped selffrequency-doubling Nd:YCOB lasers, IEEE J. Quant. Electron. 36 (2000) 991±999. [4] G. Lucas-Leclin, F. Auge, S.C. Auzanneau, F. Balembois, P. Georges, A. Brun, F. Mougel, G. Aka, D. Vivien, Diodepumped self-frequency-doubling Nd:GdCa4 O(BO3 )3 lasers: toward green microchip lasers, J. Opt. Soc. Am. B 17 (2000) 1526±1530. [5] J. Bartschke, R. Knappe, K.-J. Boller, R. Wallenstein, Investigation of ecient self-frequency-doubling Nd: YAB lasers, IEEE J. Quant. Electron. 33 (1997) 2295± 2300. [6] T. Omatsu, Y. Kato, M. Shimoseqawa, A. Hasegawa, I. Ogura, Thermal eects in laser diode pumped self-frequency-doubled Nd:YAB microchip laser, Opt. Commun. 118 (1995) 302±308. [7] P. Wang, J. Dawes, P. Dekker, D. Knowles, J. Piper, B. Lu, Growth and evaluation of ytterbium-doped yttrium aluminium borate as a potential self-doubling laser crystal, J. Opt. Soc. Am. B 16 (1999) 63±67. [8] E. Montoya, J.A. Sanz-Garcia, J. Capmany, L.E. Bausa, A. Diening, T. Kellner, G. Huber, Continuous wave infrared laser action, self-frequency-doubling and tunabil-
[9] [10]
[11] [12] [13] [14]
[15]
[16] [17]
ity of Yb3 :MgO,LiNbO3 , J. Appl. Phys. 87 (2000) 4056± 4062. L. Shah, Q. Ye, J. Eichenholz, D. Hammons, M. Richardson, B. Chai, R. Peale, Laser tunability in Yb:YCOB, Opt. Commun. 167 (1999) 149±153. F. Druon, F. Auge, F. Balembois, P. Georges, A. Brun, A. Aron, F. Mougel, G. Aka, D. Vivien, Ecient, tunable, zero-line diode-pumped, continuous-wave Yb3 :Ca4 LnO(BO3 )3 (Ln Gd, Y) lasers at room temperature and application to miniature lasers, J. Opt. Soc. Am. B 17 (2000) 18±22. A. Brenier, The self-doubling and summing lasers: overview and modelling, J. Lumin. 91 (2000) 121±132. A. Brenier, A new evaluation of Yb3 -doped crystals for laser applications, J. Lumin. 92 (2001) 199±204. P. Wang, J. Dawes, P. Dekker, J. Piper, Highly ecient diode-pumped ytterbium-doped yttrium aluminium borate laser, Opt. Commun. 174 (2000) 467±470. P. Wang, P. Dekker, J. Dawes, J. Piper, Y. Liu, J. Wang, Ecient continuous-wave self-frequency-doubling green diode-pumped Yb:YAl3 (BO3 )4 lasers, Opt. Lett. 25 (2000) 731±733. D. Jaque, J. Capmany, J. Rams, J. Garcia Sole, Eects of pump heating on laser and spectroscopic properties of the Nd:YAB self-frequency-doubling laser, J. Appl. Phys. 87 (2000) 1042±1048. M.W. Sassnet, in: D.R. Hall, P.E. Jackson (Eds.), Physics and Technology of Laser Resonators, Adam Hilger, Bristol, 1989, pp. 133±142. N.I. Leonyuk, L.I. Leonyuk, Growth and characterisation of RM3 (BO3 )4 crystals, Progr. Growth Charact. Mater. 31 (1995) 179±278.
