56 OPTICS LETTERS / Vol. 29, No. 1 / January 1, 2004

High-brightness 2.4-W continuous-waveNd:GdVO4 laser at 670 nm

Antonio Agnesi, Annalisa Guandalini, and Giancarlo Reali

Dipartimento di Elettronica dell’Universitá di Pavia, Via Ferrata 1, Pavia 27100, Italy

Stefano Dell’Acqua and Giuliano Piccinno

Bright Solutions S.r.l, Via Paiola 3, Cura Carpignano (PV) 27010, Italy

Received July 15, 2003

We report on a diode-pumped 1.3-mm Nd:GdVO4 cw laser, intracavity doubled for highly efficient generationof red light. We obtained as much as 2.4 W of power at 670 nm (corresponding to 26% optical-to-opticalefficiency) in a nearly TEM00 mode and with small amplitude noise. To the best of our knowledge, theseresults represent the highest performance at this wavelength for cw solid-state lasers. © 2004 Optical Societyof America

OCIS codes: 140.3480, 140.3580, 140.7300, 190.2620.

Several applications requiring multiwatt power levelsin the red region of the visible spectrum, such as laserdisplays and biomedical technology, have driven thestrong development of broad-area InGaAlP diodes anddiode arrays1 as a natural replacement for relativelyinefficient krypton-ion lasers. Applications such aspumping high-power tunable Cr31 lasers would alsobenefit from a nearly TEM00 all-solid-state red source.However, the present laser diode technology doesnot offer cost-effective commercial multiwatt deviceswith nearly diffraction-limited red-light output likekrypton-ion lasers. Furthermore, it is known thathigh-power InGaAlP laser diodes show a reducedlifetime with respect to broadly available laser diodesfor pumping solid-state lasers near 808 nm.

Recently we proposed an alternative approachfor eff icient generation of high-quality red light,exploiting intracavity-doubled cw neodymium laserspumped with the highly reliable and relatively low-cost808-nm high-power diode arrays.2 Therein, a nearlydiffraction-limited Nd:YVO4 laser emitting 780 mWat 671 nm was reported and successfully used todemonstrate efficient operation of a widely tunableCr:LiSAF oscillator.

In this Letter we report on a significant improve-ment in the development of a compact solid-state sourceat 670 nm, yielding a 2.4-W, nearly diffraction-limitedbeam (M2 1.1) at 670 nm. With 9.2-W absorbedpump power, the optical-to-optical eff iciency was ashigh as 26% (3 times that achieved in Ref. 2).

Key steps for signif icant improvement over our pre-vious results were three different actions: (1) designof a compact three-mirror resonator for minimizingthe intracavity passive losses; (2) choice of Nd:GdVO4as the active laser material, by virtue of its reducedpower-dependent diffractive loss; and (3) choice ofLiBO3 crystal for type I angular phase matching(nearly noncritical) in the x z plane. The layout ofthe compact intracavity-doubled resonator is shownin Fig. 1. As a pump source we used a 10-W fiber-coupled diode array emitting at 806 nm from a 300-mm

0146-9592/04/010056-03$15.00/0

fiber core with a numerical aperture of 0.2 (suppliedby Bright Solutions S.r.l). The laser crystal wasan 8-mm-long, a-cut Nd:GdVO4, doped at 0.5%, withantiref lection coatings (at 808 and 1340 nm) on bothfaces. The crystal was mounted in a copper holderand cooled through the resonator baseplate, which waskept at a constant temperature of 20 ±C by a thermo-electric chiller. The fiber-coupled output f ield wasreimaged in the laser crystal with a pair of asphericlenses. The optimum pump waist diameter in thevanadate crystal was 450 mm, and the maximumabsorbed power at 806 nm was 9.2 W.

The LiBO3 crystal was antiref lection coated at 1340and 670 nm. It was finely temperature tuned witha separate low-power thermoelectric device that alsoserved to stabilize the critical type I phase matchingof the second-harmonic generation process.

The cavity was assembled with 100-mm radius-of-curvature concave mirror M1, 45± folding mirror M2(both mirrors had high ref lectivity at 1340 nm and

Fig. 1. Resonator schematic: M1, input mirror, r 100 mm (concave), with high transmission at 808 and1064 nm and high ref lectivity at 1340 nm; M2, 45± foldingmirror, high transmission at 670 and 1064 nm and highref lectivity at 1340 nm; M3, plane total ref lector at 1340and 670 nm.

© 2004 Optical Society of America

January 1, 2004 / Vol. 29, No. 1 / OPTICS LETTERS 57

high transmission at 808 and 1064 nm), and plane mir-ror M3 with high ref lectivity at both 1340 and 670 nm.The laser crystal and the nonlinear crystal were lessthan 2 mm from mirrors M1 and M3, respectively.Minimizing the number of mirrors in an intracavity-doubled cw laser is particularly desirable to reduce thepassive intracavity loss that is due to scattering andother coating imperfections. The L-shaped resonatorallows convenient extraction of a single p-polarizedsecond-harmonic beam through the 45± folding mirror.Instead, the fundamental infrared field oscillating inthe resonator was s polarized, thus minimizing itstransmission loss at the folding mirror.

The 70-mm-long folded resonator was designedto allow nearly optimum overlap in the active crystalbetween the longitudinally integrated gain distribu-tion (radius Wp, eff) and the resonator mode (radiuswg). Our design was optimized for wgWp, eff 0.8 atfull pump power. Owing to the good thermo-opticalproperties of Nd:GdVO4,3 improved performance isallowed in the multiwatt-pumped cw laser with respectto Nd:YVO4. In particular, since the resonant fun-damental mode in the Nd:GdVO4 crystal experiencessmaller spherical aberrations near the boundary of thepump channel,3,4 one gets highly eff icient operationwith excellent beam quality, which is very importantfor intracavity second-harmonic generation with cwlasers. A further benefit of Nd:GdVO4 is its widerabsorption linewidth. When we replaced mirror M3with a 5% output coupler, the optimized infrared laseroperated at 1340 nm, generating 3.7 W at 9.2 W ofabsorbed pump power, with a slope eff iciency of 43%.This represents one of the best results reported sofar for a diffraction-limited 1.3-mm diode-pumpedvanadate laser.3,5

The particular orientation chosen for the 15-mm-long LiBO3 nonlinear crystal allows a suff icientlylarge angular acceptance, 11 mrad cm (FWHM),with respect to the widely used noncritical type IIphase matching.2 Furthermore, the small walk-offangle of 3.1 mrad is not a limiting factor in ourdesign. It is worth noting that LiBO3 cut for criticaltype I phase matching in the x z plane has never, toour knowledge, been considered before for high-powerintracavity second-harmonic generation at 670 nm invanadate lasers. Although type II noncritical phasematching allows larger angular and temperatureacceptances, with no walk-off, type I phase matchinghas the distinct advantage of a significant reductionof the amplitude noise level in a short resonatorcompared with that affecting type II interactions (seeRef. 6 for a detailed discussion).

Figure 2 shows the results obtained with a 10-Wfiber-coupled pump laser: 2.4 W at 670 nm weregenerated at 9.2 W of absorbed power, with a beamquality factor M2 1.1 measured by the knife-edgetechnique (ISO standard). The conversion efficiencyto second harmonic from the maximum power ex-tracted by the optimized infrared laser at 1340 nmwas 65%, an excellent result even compared withthe best performances reported for the more commonwavelength of 532 nm. No roll-over up to full pumppower was observed, suggesting that the performance

at 670 nm might be further upscaled with more power-ful pump diodes. Thermal lensing was characterizedby extracavity measurements of both the output beamdivergence and the waist size when the resonator wasoptimized for 1.3-mm operation by use of standardABCD modeling.7 For the intracavity-doubled laser,the waist radius on M3 at full power was measuredto be w0 100 mm, in fair agreement with modelpredictions. Such a value allows nearly optimummatching of the crystal length with the confocalparameter, with negligible walk-off and negligiblefiltering effects of the angular spectrum. In fact, weobserved a perfectly circular output beam at 670 nm,with no signs of walk-off asymmetry. Our numericalresults further confirm that the second-harmonicefficiency is only slightly affected by the 7.3-mradangular acceptance of the 15-mm-long LiBO3 crystal.

To estimate the pump-induced thermal diffractivelosses in the intracavity-doubled laser, we employedthe saturated-gain model first proposed by Smith,8 in-cluding pump-induced thermal diffractive losses (theirmodeling is discussed in detail, for example, in Refs. 9and 10):

P2v Is2Ag

4kn

Ω2

µkn 1

LIs

∂

1

∑µkn 1

LIs

∂2

14kng0 2 L

Is

∏12æ2, (1)

where Ag pWp, eff22 is the effective pump area, Is

is the laser saturation intensity, g0 is the small-signalgain (proportional to the absorbed pump power, Pabs),and L is the total intracavity loss (including thepump-induced contribution). The nonlinear coeffi-cient kn includes the limited angular acceptance, thewalk-off angle, and the beam focusing into the LiBO3crystal.11

First, we calibrated the gain coefficient g0Pabs byfitting the 1.3-mm output power curve as a functionof Pabs, according to the standard four-level saturated-gain model.11 The passive losses affect the visiblelaser more significantly, since the effective outputcoupling provided by the nonlinear crystal is smaller

Fig. 2. Second harmonic generated as a function of theabsorbed pump power.

58 OPTICS LETTERS / Vol. 29, No. 1 / January 1, 2004

Fig. 3. Amplitude f luctuations of the 670-nm out-put beam on different time scales: (a) 5 msdivision,(b) 20 msdivision.

than the optimum output coupling employed for theoptimized infrared resonator. Therefore, Eq. (1)was eventually used to refine the value of kn, whichalso depends on the double-pass phase effects inthe nonlinear crystal,8 and to determine the totalloss L. The reduced thermal lensing [80% thatof Nd:YVO4 (Ref. 3)] and the smaller accompanyingaberrations in this novel design produce negligiblethermally induced diffractive losses in the intracavity-doubled laser, as suggested by a comparison of theexperimental results and our numerical analysisbased on standard best f it to the experimental resultswith a fixed (constant) loss of 0.5% and a diffractive

contribution smaller than 0.1%. Figure 3 shows theamplitude noise recorded on different time scales,typical for the random or periodic red problem.12 Theamplitude rms f luctuation on such scales was #1.6%.Over much longer time scales of hours of operation,the powermeter recorded a maximum drift variationof 62% at full power operation of the laser. Since nocare was taken for the selection of a single axial mode,the intracavity-doubled laser is expected to operatewith a linewidth comparable to that of the 1.3-mm cwlaser.

In conclusion, we have shown that both high powerand high eff iciency can be achieved by a novel 670-nmcompact source with excellent beam quality. Thissource employs, for what is believed to be the f irsttime, a combination of Nd:GdVO4 and LiBO3 cutfor type I phase matching in the x z plane. Thiscould represent the f irst successful replacement oftraditional krypton-ion laser technology.

A. Guandalini’s e-mail address is guanda@ele.unipv.it.

References

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