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1.44µm giant pulse generation

Jan Šulca, Pavel Arátora, Helena Jelínkováa, Karel Nejezchlebb, Václav Škodab

aCzech Technical University, Faculty of Nuclear Sciences and Physical EngineeringBřehová 7, 115 19 Prague 1, Czech Republic

bCrytur, Ltd. Turnov, Palackého 175, 511 01 Turnov, Czech Republic

ABSTRACT

We have compared two solid-state saturable absorbers for Q-switching of longitudinally diode-pumped Nd:YAGlaser operating at wavelength 1444 nm: vanadium doped garnet (V3+:Y3Al2O5, V:YAG), and cobalt doped spinel(Co2+:MgAl2O4, Co:MALO). V:YAG crystal with initial transmission 91% was 2.2mm thick. Co:MALO crystalwith initial transmission 91% was 2.0mm thick. Q-switched laser consisted of the Nd:YAG composite rod (8mmlong Nd-doped part, 4mm long undoped YAG part) and the saturable absorber placed in 80mm long hemisphericcavity. As an output coupler a concave mirror (r = 150mm) with reflectivity 98% on lasing wavelength was used.Giant pulses were obtained with both passive Q-switches. When V:YAG saturable absorber was used, 55 ns long(FWHM) pulses were generated with peak power 0.47 kW (pulse energy 26µJ). Using Co:MALO, more powerfulpulses were obtained (40 ns long, 1.0 kW peak power, 45µJ energy). Advantage of less efficient V:YAG consistsin possibility of diffusion bonding between Q-switch and laser active medium which allows to prepare miniaturecompact laser device. This concept was demonstrated by using of Nd:YAG/V:YAG monolith crystal (4mm longundoped YAG part, 8mm long Nd:YAG part, 0.5mm long V:YAG part – initial transmission 97% @ 1444 nm).This monolithic crystal, originally designed for 1338 nm lasing, was placed into 23mm long cavity resonatingat wavelength 1444 nm. For output coupler reflectivity 96%, 39 ns long pulses with peak power 0.64 kW weregenerated at wavelength 1444 nm.

Keywords: Nd:YAG, V:YAG, Co:MALO, Q-switching, Nd:YAG/V:YAG monolith

1. INTRODUCTION

Even though the available laser emission of Nd3+:YAG covers tens of lines between wavelengths 0.94µm and1.84µm; the most used lines is 1064 nm.1, 2 In our presented work we were focused on laser line 1444 nm. Althoughat this line the stimulated emission cross-section is more than ten times lower3 in comparison with emission at1064 nm, and quantum defect is ∼ 56%, using low-loss resonator this wavelength can be obtained relatively easilywith reasonable efficiency (multi-watt output was demonstrated previously using diode pumped Nd:YAG laseroperating at 1444 nm4). Unlike 1064 nm and 1318 nm the radiation at 1444 nm is much higher absorbed by water– see Figure 1. The absorption coefficient of liquid water α ≈ 31 cm−1 @ 1.44µm corresponds with soft tissuepenetration depth 320µm and this radiation should be “eye-save”, because it will be absorbed by eye frontalparts sooner than it will caused retina damage. This “shielding effect” will be higher in comparison with 1.54µmradiation but it has to be mentioned that simultaneously lower threshold for cornea lesions can be expected.From the same point of view this radiation is suitable for medical applications because of strong absorption insoft tissues thanks which provides this radiation practically the same effects like a 2.1µm Holmium laser5, 6 andit was used in the surgery.7–9 In comparison with 1.54µm the radiation at 1.44µm has higher absorption in air(Figure 1) which can affect atmosphere propagation in case if this radiation will be used for laser range-findingbut it can be useful for air-mass monitoring.

Further author information: (Send correspondence to J.Š.)J.Š.: E-mail: [email protected], Tel.: +420 224 358 672, Fax: +420 222 512 735P.A.: E-mail: [email protected], Tel.: +420 224 358 538, Fax: +420 222 512 735H.J.: E-mail: [email protected], Tel.: +420 224 358 538, Fax: +420 222 512 735K.N.: E-mail: [email protected], Tel.: +420 481 319 511, Fax: +420 481 322 323V.Š.: E-mail: [email protected], Tel.: +420 481 319 511, Fax: +420 481 322 323

Best Student Award Paper

Solid State Lasers XVI: Technology and Devices, edited by Hanna J. Hoffman, Ramesh K. Shori, Norman Hodgson, Proc. of SPIE Vol. 6451, 64511H, (2007) · 0277-786X/07/$15 · doi: 10.1117/12.717104

Proc. of SPIE Vol. 6451 64511H-1

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Figure 1. Liquid water13 and atmosphere absorption14 vs wavelength in range from 1000 up to 1650 nm. Main Nd:YAGlaser emission lines in this range and emission of Er:glass laser at 1.54µm are marked too.

In this paper we are focused on a passive Q-switching of a diode pumped Nd:YAG laser operating at 1444 nm.We have tested two various saturable absorbers: V:YAG (Vanadium V3+ doped yttrium aluminium garnet), andCo:MALO (Cobalt Co2+ doped spinel MgAl2O4). Previously both these crystals were tested for Q-switching atthis wavelength. V:YAG crystal was used for diode pumped Nd:YAG laser and 50 ns long pulses with energy24µJ were reached.10, 11 Co:MALO crystal was used for Q-switching of flash-lamp pumped Nd:YAG and 100 nslong pulses with energy up to 3mJ were reached.12 We report comparison of both saturable absorbers in thesame laser resonator, and also application of special Nd:YAG/V:YAG monolithic crystal operating at 1.44µm.

2. MATERIALS AND METHODS

2.1. Diode pumped Nd:YAG laser at 1444 nm

The active medium of diode pumped laser operating at 1444 nm was end-pumped composite Nd:YAG crystal.This YAG crystal consists of 8mm long active part doped by Nd3+ ions (1 at.%) and of 4mm long undoped part.The diameter of this crystal was 5mm. The outer frontal part of the laser crystal had broadband antireflectioncoatings (AR) for the wavelengths 0.8µm, 1.06µm, and 1.34µm. The undoped part was bonded to the pumpingface of the crystal doped part. In our previous study15 we have confirmed that using of such undoped part oflaser rod enlarges the active material cooling surface and improves a laser active media thermal field uniformityand heatsink. Laser crystal was mounted in adjustable water-cooled cupreous ring. Temperature of cooling waterwas 15 ◦C.

Figure 2. Longitudinaly diode pumped Nd:YAG laser. RM – rear (pumping) flat mirror, OC – curved output coupler.

The pumping source used was a CW or QCW operating fibre-coupled laser diode HLU20F400 (LIMO LaserSystems) emitting radiation at wavelength 808 nm with the maximum output power 20W at the end of thefiber (fiber core diameter: 400µm; numerical aperture: 0.22). The diode radiation was focused into the activeNd:YAG crystal by two achromatic doublet lenses with the focal length f = 75mm. The measured diameter of



pumping beam focus inside the crystal was 360µm. The active part of laser crystal absorbed approximately 95%of pumping radiation.

The 80mm long resonator of the Nd:YAG laser was formed by a planar dielectric mirror RM with hightransmission for the pumping radiation (T > 98% @ 808 nm) together with the high reflectance for the generatedradiation (RRM = 100% @ 1444 nm), and by a concave (r = 150mm) dielectric mirror OC serving as an

output coupler – see Figure 2. Two couplers with various reflectivities were tested: R(1)OC = 98% @ 1444 nm, and

R(2)OC = 96% @ 1444 nm. All mirrors have as possible reduced reflection at wavelengths 1.06µm and 1.34µm toprevent Nd:YAG lasing at this lines.

2.2. Used saturable absorbers: V:YAG & Co:MALO

For Q-switching of diode pumped Nd:YAG laser operating at 1444 nm two various saturable absorbers havetested: vanadium doped garnet (V3+:Y3Al2O5, V:YAG), and cobalt doped spinel (Co2+:MgAl2O4, Co:MALO).Both these crystals belong to well developed hard and stable solid state saturable absorbers characteristic in abroad vibronic absorption bands typical for transition metal activators. The dependence of V:YAG and Co:MALOabsorption coefficient on the wavelength is plotted in Figure 3. The maximum of V3+ ions absorption can befound at wavelength ∼ 1320 nm, and V:YAG crystal is usually used for Q-switching of lasers based on Nd-dopedmaterials operating around 1.3µm.16–21 Co:MALO with absorption maximum ∼ 1550 nm is the material of choicefor lasers emitting the important eye-safe wavelengths near 1.5 microns, especially Er:glass.22, 23 Nevertheless, ithas useful saturable absorption that covers 1.2 to 1.6 micron laser transitions.

Figure 3. Measured room-temperature absorption coefficient of used V:YAG and Co:MALO crystals.

Table 1. Summary of V:YAG10, 24 and Co:MALO25–27 characteristics at 1.44µm wavelength.

V:YAG (at 1.44µm) Co:MALO (at 1.44µm)

Ground state absorption cross-section σGSA ∼ 1.8× 10−18 cm2 σGSA ∼ 2.9× 10−19 cm2Saturable absorption recovery time τr = 22± 6 ns τr ∼ 130 nsCalculated saturation energy fluence Φsat = 76mJ.cm−2 Φsat = 475mJ.cm−2

Calculated saturation intensity Isat = 3.5MW.cm−2 Isat = 3.7MW.cm−2

Measured absorption coefficient α = 0.423± 0.05 cm−1 α = 0.467± 0.05 cm−1

Major characteristics of V:YAG and Co:MALO saturable absorber at wavelength 1444 nm are summarizedin Table 1. Main difference between this two absorbers is approximately one order longer Co:MALO relaxationtime and about one order smaller Co:MALO ground state absorption cross-section. Absorption coefficients atwavelength 1444 nm of both used absorbers are approximately identical.



For the Q-switching two saturable absorbers with similar initial transmission were used. V:YAG crystal withinitial transmission 91% was 2mm thick. Co:MALO crystal with initial transmission 92% was 2.2mm thick.These crystal were both sides polished and AR/AR coated to obtain a minimum reflectivity on 1.08 and 1.34µm.Crystals were fixed in adjustable air-cooled cupreous ring and placed in the laser resonator between laser crystaland output coupler.

2.3. Diode-pumped Nd:YAG/V:YAG monolith laserFor the construction of compact longitudinally diode-pumped Q-switched Nd:YAG laser operating at wavelength1444 nm a specially developed sandwich crystal was tested. This crystal, designed originally for operation at1.3µm spectral range,28 combines in one piece the cooling undoped part (undoped YAG crystal 4mm long),active laser part (YAG crystal doped with Nd3+ ions 8mm long), and saturable absorber (YAG crystal dopedwith V3+ ions 0.5mm long) – see Figure 4. The diameter of the monolith was 5mm. The outer frontal part ofthe laser crystal had broadband AR coatings for 1064 nm and 1340 nm. The undoped part was bonded to thepumping face of the monolith Nd3+-doped part. The V:YAG saturable absorber was diffusion bonded to the un-pumped face of the Nd:YAG section. The initial transmission of the V:YAG part was approximately T0

.= 97%

at the wavelength 1444 nm.

Figure 4. Nd:YAG/V:YAG monolith crystal used in diode end-pumped laser: undoped part 4mm, Nd3+ doped part8mm and, V3+ doped part 530µm long; initial transition of V:YAG part T0

.= 97% @ 1444 nm.

This monolithic configuration of active crystal and saturable absorber allows us to realize a compact resonator.The shortest tested resonator (limited by the optics and crystal mount) was 23mm long. Except of resonatorlength and active medium the laser configuration was the same like it was described in subsection 2.1. As anoutput coupler, mirror with reflectivity 96% was used. The diagram of the used optical cavity can be seen inFigure 5.

Figure 5. Longitudinally diode pumped Q-switched laser based on Nd:YAG/V:YAG monolith crystal. RM – rear (pum-ping) resonator flat mirror, OC – curved output coupler.

2.4. Measuring instrumentsTo investigate the time structure of generated pulses Tektronix oscilloscope (TDS 3052, 500MHz) with the In-GaAs photodiode FGA04 (Thorlabs, Inc.) was used. The output power was measured by Molectron energy/powermeter EMP2000 with two Molectron PowerMax probes (PM10 and PM3). A spectrum of the generated radiationwas investigated by an IR wavelength meter (StellarNet EPP2000). The Spiricon IR sensitive camera PyrocamIII was used for the laser beam spatial structure investigation.



3. RESULTS AND DISCUSSION

3.1. Nd:YAG CW emission at 1444 nmFirstly, a CW performance of diode-pumped Nd:YAG laser operating at 1444 nm was investigated. Simple two-mirror resonator was constructed consisting of a flat high reflector (HR) and a curved output coupler (OC)with radius of curvature of 150mm (see Figure 2 for a sketch). Two OCs with various reflectivity (R = 96%or 98%) and three resonator lengths were tested. Output laser power in dependence on absorbed pumpingpower was measured. Results are displayed in Figure 6 and 7. The maximal output power of 2W was obtainedwith output coupler reflectivity of 96% with shortest laser resonator (30mm lengths). For longer resonatorsthermally induced-roll-off was observed. The beam profile character was observed throughout the experimentusing a fluorescence card and was assessed to be close to the fundamental mode up to thermally induced-roll-off.More accurate measurements of the beam profile and quality were made using an IR sensitive camera (PyrocamIII, Spiricon) – see Figure 8. In case of OC with 98% reflectivity the laser threshold was about 35% lowerresulting in higher thermally limited output power obtained with 55mm long resonator (Figure 7). The freerunning wavelength was measured to be 1443.9± 1 nm – see Figure 9.

Figure 6. Diode pumped Nd:YAG laser at 1444 nm:output power as a function of the absorbed pump powerfor ROC = 96%. Laser resonator length Lres = 30mm,55mm, and 80mm, OC curvature radius 150mm.

Figure 7. Diode pumped Nd:YAG laser at 1444 nm:output power as a function of the absorbed pump powerfor ROC = 98%. Laser resonator length Lres = 55mm,OC curvature radius 150mm.

Figure 8. CW diode pumped Nd:YAG laser beam pro-file at 1444 nm, absorbed pump power 7W (ROC =96%, Lres = 80mm). Image was recorded in distance750mm from laser OC, magnification 4×.

Figure 9. CW diode pumped Nd:YAG laser outputspectrum, absorbed pump power 7W. Output couplerreflectivity ROC = 98%. Laser resonator length Lres =80mm.



3.2. Q-switching of Nd:YAG laser at 1444 nm

For Q-switching of Nd:YAG laser at 1444 nm two separate saturable absorbers Co:MALO and V:YAG were testedstepwise. Results were summarized in following subsections.

3.2.1. Co:MALO absorber

Co:MALO saturable absorber with initial transmission T0 = 91% at wavelength 1444 nm (thickness 2mm) wasplaced close to the laser output coupler. To accommodate this element mounted in the adjustable holder thelaser resonator had to be 80mm long. Output coupler reflectivity was 98%. This laser was able to operate underCW pumping. In dependence on pumping power the following laser output radiation parameters were measured:a mean output power, repetition rate of generated pulses, and generated giant pulse width (FWHM). Frommeasured data, single pulse energy and peak power were calculated. In Figure 10 the measured laser outputcharacteristics — the mean power, pulse repetition rate, and pulse width versus absorbed power are plotted.Calculated pulse energy and pulse peak power in dependence on absorbed pump power is shown in Figure 11.In this configuration laser threshold is 9.3W. The maximum generated output mean power 107mW and outputpulse frequency 2.4 kHz corresponds to single pulse energy ∼ 45µJ. For 44 ns long pulses it leads to pulse peakpower ∼ 1 kW @ 1444 nm.

Figure 10. Measured output characteristics of CW diode pumped Nd:YAG laser Q-switched by Co:MALO at wavelength1444 nm (mean power – (a), pulse frequency – (b), pulse width – (c), T0 = 91%, Roc = 98%, Lres = 80mm).

Figure 11. Calculated output characteristics of CW diode pumped Nd:YAG laser Q-switched by Co:MALO at wavelength1444 nm (pulse energy – (a), pulse peak power – (b), T0 = 91%, Roc = 98%, Lres = 80mm).



3.2.2. V:YAG absorber

In case of V:YAG saturable absorber similar laser configuration was used like in Co:MALO case. The V:YAGcrystal (2.2mm thick) with initial transmission T0 = 91% at wavelength 1444 nm was placed in to 80mm longNd:YAG laser cavity with OC reflectivity 98%. In spite of very similar condition laser was unable to run underCW pumping and pulsed pumping with duty cycle 50% (pumping pulse length 1ms, period 2ms, pumpingpulse energy 20mJ) had to be applied to reach lasing threshold and to decrease thermal effects in Nd:YAGcrystal. In this condition ∼ 700µs long train of Q-switched pulses was generated inside the pumping pulseenvelope. Estimated single Q-switched pulse energy was 26µJ. Measured pulse width (FWHM) was 55 ns whichcorresponds to pulse peak power 0.47 kW @ 1444 nm.

3.3. Diode-pumped Nd:YAG/V:YAG monolith laser at 1444 nm

For construction of compact Q-switched laser operating at wavelength 1444 nm Nd:YAG/V:YAG monolithiccrystal was tested. Due to relatively high initial transmission of the saturable absorber at operation wavelength(T0 = 97% @ 1444 nm) output coupler with the lower reflectivity was used (ROC = 96%). More compact systemallows us to use shorter laser resonator to 23mm – see Figure 5. Laser was able to operate under CW pumping.Like in the Co:MALO case, following parameters of Nd:YAG laser output radiation in dependence on pumpingpower were measured: the mean output power, repetition rate of generated pulses, and generated giant pulsewidth (FWHM). All these measured characteristics are summarized in Figure 12. From obtained values, singlepulse energy and peak power were calculated. Results are plotted in Figure 13.

From results it follows, that in case of this compact laser, the more stable CWQ-switched output at wavelength1440 nm was obtained. Due to lower active losses (in comparison with Nd:YAG laser Q-switched by Co:MALO)the laser threshold was ∼ 6W. Shortest generated pulse lengths were in range from 30 ns to 40 ns. This result wasobtained mainly owing to resonator shortening. For the lower initial absorption of saturable absorber, generationof significantly shorter pulses can be expected. For absorbed pumping power up to ∼ 10W the laser was operatingin single mode close to TEM00. In this case the maximum obtained mean power, peak power, and pulse energyat wavelength 1444 nm was 183mW, 0.63 kW, and 25µJ, respectively. For more intense pumping, the highermodes were generated simultaneously and laser stability was lower. Nevertheless, mean output power was stillincreasing and the maximum obtained value was 323mW.

Figure 12. Measured Nd:YAG/V:YAG monolith laser output characteristics (mean power – (a), pulse frequency – (b),pulse width – (c)) against absorbed pumping power (T0 = 97%, Roc = 96%, Lres = 23mm).



Figure 13. Calculated output characteristics of CW diode pumped Nd:YAG laser Q-switched by Co:MALO at wavelength1444 nm (pulse energy – (a), pulse peak power – (b), T0 = 97%, Roc = 96%, Lres = 23mm).

4. CONCLUSION

Passive Q-switching provides a successful and compact means of generating short, high-intensity laser pulses.We have compared two solid-state saturable absorbers for Q-switching of longitudinally diode-pumped Nd:YAGlaser operating at wavelength 1444 nm: vanadium doped garnet (V3+:Y3Al2O5, V:YAG), and cobalt doped spinel(Co2+:MgAl2O4, Co:MALO). V:YAG crystal with initial transmission 91% was 2.2mm thick. Co:MALO crystalwith initial transmission 91% was 2.0mm thick.

In the first configuration the composite Nd:YAG crystal was used as a laser active medium. This crystalconsists of 4mm long YAG undoped cooling part and 8mm long Nd3+ ion doped gain part. This crystal itselfallows us to generate up to 2W output power in free-running regime. Used saturable absorbers were placed in80mm long hemispheric laser resonator separately in its own holder. As an output coupler was used concavemirror (r = 150mm) with reflectivity 98% on lasing wavelength. Giant pulses were obtained with both passiveQ-switches. When V:YAG saturable absorber was used, 55 ns long (FWHM) pulses were generated with peakpower 0.47 kW (pulse energy 26µJ). Laser emission in this case was obtained only under pulsed pumping. UsingCo:MALO, more powerful pulses were obtained under CW pumping (40 ns long, 1.0 kW peak power, 44µJenergy).

Advantage of less efficient V:YAG consists in the possibility of diffusion bonding between Q-switch andlaser active medium which allows to prepare miniature compact laser device. This concept was demonstratedin second configuration by using of Nd:YAG/V:YAG monolith crystal (4mm long undoped YAG part, 8mmlong Nd:YAG part, 0.5mm long V:YAG part – initial transmission 97% @ 1444 nm). This monolithic crystal,originally designed for 1338 nm lasing, was placed into 23mm long cavity resonating at wavelength 1444 nm.For output coupler reflectivity 96%, 39 ns long pulses with peak power 0.63 kW were generated at wavelength1444 nm in case of single transversal mode operation. For higher pump power, shorter pulses were obtained(∼ 33 ns), but the mode structure was unstable and pulse energy was lower. Better results can be excepted usingvalue of V:YAG initial transmission optimized for operation at this wavelength.

Comparing the Nd:YAG/V:YAG monolith system with the “non-contact” configuration (when the saturableabsorber is separately from the active medium) some significant advantages of monolith arrangement for allfuture applications are seen:

• The simplest Q-switched laser system construction and adjustment, shorter laser resonator⇒more compactlaser system, smaller sensitiveness to all mechanical disturbance, rugged and low-maintenance system,generation of shorter pulses.



• Contact of V:YAG saturable absorber with the rest of the monolith crystal inherently increases its coolingand improve thermal stability of all system⇒ higher temporal stability of generated radiation (mean powerand pulse frequency stability, pulse-to-pulse stability.)

• The simple arrangement of laser active-passive medium (the connection of two originally independent op-tical elements) results in the reduction of the inner resonator losses⇒ lower obtained threshold, possibilityof using the output mirror with lower reflection, higher power done by shorter length of generated pulse.

• Other improvement of monolith scheme can be expected in the case when the resonator mirrors will beimplemented directly on the crystal faces ⇒ microchip laser based system.

5. ACKNOWLEDGEMENTS

This research has been supported by the Czech Ministry of Industry and Trade, project FI-IM2/129, and by thegrant of the Czech Grant Agency No. 102/05/0414 “Solid State Infrared Lasers for Ophthalmology”.

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