Tunable diode-pumpedand frequency-doubled Cr:LiSAF lasers

Pierre Laperle, Kevin J. Snell, Alain Chandonnet, Pierre Galarneau, and Real Vallee

The performance of a diode-pumped Cr:LiSAF laser and its intracavity frequency-doubled operation isreported. Close to 1 W of quasi-cw power at 870 nm was obtained in multimode operation. Power inexcess of 20-mW cw of TEM00 second-harmonic power at 435 nm has also been obtained by using a lithiumtriborate crystal with an infrared-to-visible conversion efficiency of 33%. This result is believed to be thehighest yet reported for a true cw blue output from a diode-pumped Cr:LiSAF system. However, themaximum attainable output power was limited by thermal fluorescence quenching. The effect of thepump size on the temperature rise and on the small-signal gain is investigated. © 1997 Optical Societyof America

Key words: Infrared laser, second-harmonic generation, thermal fluorescence quenching.

1. Introduction

Many applications in the fields of imaging, medicaldiagnosis, and information storage require compactblue-laser sources. One method that is commonlyused to realize such sources is based on frequency dou-bling of an infrared laser by means of either an exter-nal resonant cavity for doubling laser diodes or byintracavity doubling. The latter approach offers theadvantage of giving access to the high power circulat-ing in the laser cavity. The popularity of the Cr31:LiSrAlF6 ~Cr:LiSAF! crystal has grown in the past fewyears, mainly because of its wide tuning range and thepotential it offers for laser-diode pumping. Its emis-sion from 780 to 1000 nm makes it an ideal candidatefor frequency doubling in the visible region.

The Cr:LiSAF crystal is a uniaxial crystal withthree absorption bands, with two in the visible lo-cated in the regions of 400–500 nm and 600–700 nm,allowing efficient pumping by visible laser diodes.In a Cr:LiSAF laser the excited ions in the 4T2 state,by pumping in the red or blue absorption bands, havea laser transition toward vibrational states of the

P. Laperle, K. J. Snell, A. Chandonnet, and P. Galarneau arewith the National Optics Institute, 369 rue Franquet, Sainte-Foy,Quebec G1P 4N8, Canada. R. Vallee is with the Department dePhysique, Centre d’Optique, Photonique et Laser, Universite La-val, Sainte Foy, Quebec G1K 7P4, Canada.

Received 22 November 1996; revised manuscript received 2April 1997.

0003-6935y97y215053-05$10.00y0© 1997 Optical Society of America

ground level. This constitutes a four-level laser sys-tem.1 The Cr:LiSAF crystal offers the advantage oflow thermal lensing, but its low mechanical strength,low thermal conductivity, and temperature quench-ing of the upper laser level limit the maximum powerthat can be extracted in the cw regime. However,this can be circumvented to some degree by using alow-dopant concentration and efficient cooling of thecrystal. A recent report on the highest maximumpower yet achieved from a diode-pumped Cr:LiSAFlaser producing 1.1 W of cw infrared output powermakes it possible to achieve hundreds of milliwatts ofblue light in a compact tunable laser source.2

Intracavity frequency doubling of Cr:LiSAF in aQ-switched mode using an iodate ~LiIO3! crystal3 andin a diode-pumped cw regime using a potassium nio-bate ~KNbO3! crystal has been previously reported.4Although KNbO3 has the largest effective nonlinearoptical coefficient ~d32 5 18 pmyV at 860 nm!, it ismechanically fragile, and its phase-matching andtemperature bandwidths for type I noncritical phasematching are very narrow ~respectively, 0.045 nm cmat 860 nm and 0.3 °C cm at 1.064 mm!. We relaxedthese constraints by the use of a LiB3O5 ~LBO! crystalto frequency double a diode-pumped Cr:LiSAF laser.

The LBO crystal used was cut for type I phasematching. Its general characteristics are an effec-tive nonlinear optical coefficient d32 ' 1 pmyV at 860nm, a wide external acceptance angle ~78 mrad cm at860 nm!, a small walk-off angle ~'0.32°! ~necessaryfor good conversion efficiency in intracavity dou-bling!, a relaxed phase-matching bandwidth ~0.64 nmat 860 nm!, a relaxed temperature bandwidth ~'4 °C

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cm at 1.064 mm!, a high damage threshold ~1-GWycm2 cw mode locked at 1.064 mm!, and a high opticalquality ~transparency range from 160 nm to 2.6 mm!.The phase-matching condition was realized by tem-perature tuning the crystal for wavelengths shorterthan 880 nm.

In this paper, we report on the laser performance ofa diode-pumped Cr:LiSAF crystal and of an intracav-ity frequency-doubled diode-pumped Cr:LiSAF laser.The temperature dependence of the fluorescence life-time of chromium ions in LiSAF is measured to de-termine the effect of temperature rise on the outputlasing power.

2. Experiments and Results

A schematic diagram of the Cr:LiSAF laser setup isshown in Fig. 1. The crystal was 2 mm long anddoped 3-wt. % Cr. It was placed in a near-hemispherical cavity, where a high reflectivity planemirror from 780 to 950 nm was deposited directlyonto its surface. The output coupler had a radius ofcurvature of 10 cm and a minimum transmission of0.6% at 870 nm. The pumping source ~AppliedOptronics LM300-665! consisted of 19 fiber-coupledlaser diodes emitting at a nominal wavelength of 665nm for a total power of 4 W.5 Each laser diode wascoupled onto an optical fiber with a numerical aper-ture of 0.14 to form a 700-mm-diameter fiber bundlein a hexagonal closely packed configuration. Theend of the fiber bundle was imaged to a spot sizediameter of approximately 300 mm into the laser crys-tal with two laser-diode collimating lenses with nu-merical apertures of 0.6. Mode matching of thepump-beam waist and the laser-beam waist wasachieved by reducing the cavity length, which has theeffect of increasing the laser-beam waist. The crys-tal was maintained at a temperature of 20 °C with athermoelectric cooler.

A maximum cw power of 102 mW at 870 nm for 760mW of absorbed pump power with a threshold of 310mW and a slope efficiency of 24% ~Fig. 2! was ob-tained in a multimode beam ~cavity length of 9.9 cm!.Slightly better performance was obtained in Ref. 4with the same concentration of 3-wt. % Cr with amaximum of 190-mW cw for 680 mW of absorbedpower and a slope efficiency of 33%. The differencein performance for both cavities is probably caused bynonoptimum mode matching and the lower bright-ness ~compared with Ref. 4! of our pumping source,

Fig. 1. Cr:LiSAF laser experimental setup.

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leading to a lower small-signal gain.6 Althoughfiber-coupled pumping configurations are generallyless efficient than direct-diode pumping owing to cou-pling losses and larger mode volume, the fiber-coupled scheme offers the advantages of higher totalpump power and greater system reliability.7 In theTEM00 beam configuration, a maximum cw power of65 mW with a threshold of 350 mW and a slopeefficiency of 18% was obtained by reducing the cavitylength to 9.5 cm. The maximum power obtainablewas limited by the thermal gradient in the Cr:LiSAFcrystal because of its low thermal conductivity andconsequently to the lifetime reduction of the upperlaser level. Decreasing the temperature of the crys-tal to approximately 10 °C only slightly increased theoutput power. As can be seen in Fig. 2, roll-off of theoutput power occurs at approximately 800 mW ofabsorbed cw pump power and is attributed to theonset of these thermal effects. To investigate theseeffects more thoroughly the fluorescence lifetime wasmeasured both as a function of the pump power ab-sorbed and as a function of the crystal temperature.The lifetime in the upper laser level was measured bymonitoring the decaying fluorescence from the Cr:LiSAF crystal pumped by a chopped laser beam, andthe lifetime determined from the 1ye intensity point.For the temperature measurements, the absorbedpump power was maintained at 100 mW to keep thetemperature rise at the center of the crystal as low aspossible while producing enough fluorescence to bedetectable. As can be observed in Fig. 3, the fluo-rescence lifetime of the Cr ions at 20 °C is 68 ms anddecreases almost linearly above 30 °C to half of itsvalue at approximately 66 °C, which is in agreementwith a temperature of 69 °C reported in Ref. 8. Withthe absorbed power value of 800 mW, at which roll-offoccurs in the cw regime ~see Fig. 2!, we obtained anaverage lifetime of 52 ms from Fig. 3, which corre-sponds to an average temperature rise along the crys-tal of 23 °C. The critical temperature at which roll-off occurs is expected to be dependent on the small-signal gain of the laser.

Fig. 2. Cr:LiSAF laser multimode output power at 870 nm versusabsorbed power.

In a first-order approximation, the effect of a vari-ation of the pump spot size and fluorescence lifetimeon the small-signal gain can be evaluated as follows.The small-signal gain of a four-level laser in the caseof a TEM00 signal mode is described by the relation6,9

~excluding any limitations that are due to upconver-sion10!

g0 5stPabs

hnP~AP 1 AL!, (1)

where s is the emission cross section, t is the fluo-rescence lifetime, hvP is the pump photon energy,Pabs is the absorbed pump power, and AP and AL arethe pump- and laser-mode areas, respectively. Asfor the fluorescence lifetime dependence on the tem-perature, it is expressed as8

t~T! 5 @1ytR 1 ~1ytNR!exp~2DEykT!#21, (2)

where tR is the radiative decay time, tNR is the non-radiative decay time, DE is the activation energy, k isthe Boltzmann constant, and T is the temperature inkelvins. The temperature rise at the input of thecrystal can be estimated with

DT 5Pabs

4pkaF1 1 2 lnSwC

wPDG , (3)

which can be readily derived from a simple model ofhomogeneous heat flow.11 k is the thermal conduc-tivity of the crystal, a is the absorption coefficient, wCis the crystal radius maintained at a fixed tempera-ture, and wP is a core radius uniformly heated by theabsorbed pump power. According to Eq. ~3!, reduc-ing wP ~and therefore AP 5 pwP

2! increases the tem-perature of the crystal therefore causing a reductionof the fluorescence lifetime. However, from Eqs. ~2!

Fig. 3. Fluorescence lifetime of Cr31 at 870 nm as a function oftemperature of the LiSAF crystal and as a function of the absorbedpower at 665 nm.

and ~3! it can be shown that, for a given pump area

reduction, the term 1yAP appearing in Eq. ~1! in-creases faster than t decreases, which leads to a netincrease of the small-signal gain. Thus tighter fo-cusing or the use of a higher-brightness pump sourcewill result in a higher small-signal gain and higherconversion efficiency for less absorbed pump power~see, for example, in Ref. 4!.

The temperature rise at the center of the pumpscales linearly with the absorbed power @Eq. ~3!#.Thus, reducing the average absorbed pump power bya certain amount will increase by the same amountthe roll-off cw pump power. After this, the pumpwas chopped to reduce the temperature gradient.The maximum quasi-cw powers obtained at 870 nmfor duty cycles of 50% and 25% are, respectively, 285and 980 mW with thresholds of 420 mW ~210 mWaverage! and 430 mW ~245 mW average! of absorbedpump power for a multimode beam ~Fig. 2!. Thesmaller thermal gradient also affects the slope effi-ciency, which increases from 24% for a cw regime to28% and 37% for duty cycles of 50% and 25%, respec-tively. As can be seen in Fig. 2, the cw absorbedpower at which roll-off occurs for a duty cycle of 50%is two times greater than the observed value in the cwregime because the average absorbed power is cutdown by half. No output power decrease is observedat a duty cycle of 25% because the roll-off peak ab-sorbed power is 3.2 W ~that is, 4.6 W of incident light!,which is above the maximum power delivered by ourpumping source.

The laser was tunable over 86 nm, from 830 to 916nm ~Fig. 4!, using an 0.8-mm-thick birefringent filterinserted at the Brewster angle in the laser cavity.Figure 5 shows the output spectrum centered at870.3 nm. The FWHM is 0.7 nm. The structuralpeaks are 0.27 nm apart and are the result of inter-ference effects in the coatings of the LiSAF crystal.Similar results were obtained in Ref. 2 where a truecw output has been demonstrated by the use of a low

Fig. 4. Tuning curves of ~a! the diode-pumped Cr:LiSAF and ~b!the frequency-doubled diode-pumped Cr:LiSAF lasers.

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concentration of 0.8-wt. % Cr, efficient cooling, andgood mode matching between laser beam and pumpbeam. The power drop around 880 nm in the tun-ing curve of Fig. 4 is most probably an artifactcaused by an increase of reflectivity in the antire-flection coating on the Cr:LiSAF crystal that areused. As a matter of fact, we used a Ti:sapphirelaser covering the range from 860 to 900 nm to scanthe transparency of the crystal, and no absorption-related impurity was detected at 880 nm. Also,this particular power drop cannot be related to thebandwidth of the birefringent filter or other opticalcomponents, because they were used with other Cr:LiSAF crystals without any sharp decrease in theoutput over a similar tuning range.5

A schematic diagram of the frequency-doubled Cr:LiSAF laser is shown in Fig. 6. The same pumpsource as that used in the Cr:LiSAF laser was em-ployed. A three-mirror cavity was formed using aplane mirror on the Cr:LiSAF crystal and two 10-cm-radius mirrors coated for high reflectvity from 800 to900 nm with high transmission at 400–500 nm. Thebirefringent filter was inserted near the internalwaist in the Cr:LiSAF crystal. The LBO crystal was10 mm in length and was inserted at the secondcavity waist. The end surfaces of the LBO crystalwere antireflection coated from 780 to 1000 nm.

Fig. 6. Frequency-doubled Cr:LiSAF laser experimental setup.

Fig. 5. Output spectrum ~0.1-nm resolution! of ~a! the diode-pumped Cr:LiSAF laser and ~b! the frequency-doubled diode-pumped Cr:LiSAF laser.

5056 APPLIED OPTICS y Vol. 36, No. 21 y 20 July 1997

Both Cr:LiSAF and LBO crystals were temperaturestabilized with thermoelectric coolers.

The maximum power obtained at 435 nm was 20mW in the cw regime and 250 mW at a 25% dutycycle ~Fig. 7!, with an infrared-to-visible conversionefficiency of 33% and a total optical conversion ef-ficiency .5%. The laser beam was TEM00. Theseresults are believed to be the highest achieved yetfor a true cw blue output from a diode-pumped Cr:LiSAF system. The latest reported performance ofsuch a system was 13 mW of output power at 435nm for 680 mW of absorbed power, with a totaloptical conversion efficiency of 2%.4 It should benoted that, for 680 mW of absorbed power, we havealso demonstrated ;13 mW of output. The tuningof the laser was affected by temperature control ofthe LBO crystal to preserve the phase-matchingconditions and by adjusting the birefringent filter tovary the wavelength. As can be seen in Fig. 4, thetuning range extends from 432 to 442 nm, which issmaller than what is reported for KNbO3 ~427 to443 nm!,4 and was limited by the temperature tun-ing range of the LBO crystal ~0.075 nmy°C! and thestabilization range of the thermoelectric cooler ~0–100 °C!. A temperature variation of 70 °C appliedto the KNbO3 crystal corresponded to a tuningrange of 16 nm,4 although here a maximum tem-perature variation of 100 °C corresponds to an LBOtuning range of 7.5 nm. This range was extendedto 10 nm by simultaneously increasing or decreas-ing the temperature of the LiSAF crystal to tune thefundamental wavelength. The profile of thesecond-harmonic tuning curve closely matches thefundamental-wavelength tuning curve. As can beseen in Fig. 5, the FWHM spectral bandwidth of thesecond harmonic is approximately 0.4 nm and wasobserved to be the same over the entire tuningrange. Moreover, the structural peaks in the spec-trum can be explained in terms of the structuralpeaks in the spectrum of the fundamental.

Fig. 7. Frequency-doubled Cr:LiSAF laser output power at 435nm versus absorbed power.

3. Conclusion

The use of a LBO crystal for frequency doubling aCr:LiSAF laser has given the highest cw blue outputpower yet observed ~to our knowledge! with more than20 mW and with an infrared-to-visible conversion ef-ficiency of 33%. Thermal quenching of lasing was ob-served at about 800 mW of absorbed cw pump power.To maintain the fluorescence lifetime constant over awider region of absorbed power, the pump beam waschopped. This affected both the maximum power thatcould be extracted from the cavity and the efficiency ofthe laser. Reducing the temperature gradient in theCr:LiSAF crystal by better cooling or using a higher-brightness pump source in conjunction with a low Cr-doped LiSAF crystal should increase the maximumoutput power achievable in the cw regime for bothinfrared and frequency-doubled Cr:LiSAF lasers.

P. Laperle acknowledges the financial support pro-vided by the Natural Sciences and Engineering Re-search Council of Canada, the National OpticsInstitute, and Universite Laval during his graduatestudies.

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