Ossi  Kimmelma,  Ilkka Tittonen,  and  Scott  C.  Buchter.  2008. Thermal  tuning  of  laserpulse parameters  in passively Q­switched Nd:YAG lasers. Applied Optics, volume 47,number 23, pages 4262­4266.

© 2008 Optical Society of America (OSA)

Reprinted with permission.

Thermal tuning of laser pulse parametersin passively Q -switched Nd:YAG lasers

Ossi Kimmelma,1,* Ilkka Tittonen,1 and Scott C. Buchter2

1Department of Micro and Nanosciences and Center for New Materials, Helsinki University of Technology,Micronova, P.O. Box 3500, FI-02015 TKK, Finland

2Arctic Photonics, Hirsalantie 11, FI-02420 Jorvas, Finland

*Corresponding author: Ossi.Kimmelma@tkk.fi

Received 23 April 2008; revised 27 June 2008; accepted 12 July 2008;posted 15 July 2008 (Doc. ID 95382); published 6 August 2008

Modifying the output pulses of a passively Q-switched Nd:YAG laser, operating at 1064nm, was realizedby heating the laser crystal. With the demonstrated laser setups, a 100K temperature rise led to a morethan 50% increase in the pulse energy and a more than 10% decrease in the pulse length. This methodoffers an effective way to tune the output of the laser without mechanical adjustment or a change ofcomponents. © 2008 Optical Society of America

OCIS codes: 140.3580, 140.3540, 140.3600.

1. Introduction

The key parameters characterizing the output of apulsed laser are pulse width, energy, and peak power.Usually, tuning these pulse properties without me-chanical adjustments or a change of components ina laser is limited. The optimum would be to be ableto independently tune the pulse energy, width, andpeak power. In this work the aim is to show that,when using a passively Q-switched laser, changingthe laser crystal temperature is an effective methodto control the pulse properties. In actively Q-switched lasers the energy of the pulses is stronglycoupled to the pulse repetition rate, whereas in pas-sive Q-switching schemes the repetition rate is con-trolled by the pump power, which has only a minuteeffect on the pulse energy.A common feature that is observed by heating a

crystal is a broadening of the spectral lines and re-duction of the stimulated emission cross sections.In continuous wave (cw) lasers heating the laser crys-tal is usually undesirable because of an increase inthe laser threshold due to the decreasedstimulated emission cross section. However, in

passivelyQ-switched lasers, heating affects the pulseproperties, also. A smaller cross section raises thethreshold for an individual pulse. A higher inversiondensity is built up in the medium before reaching thethreshold, leading to the increased pulse energy.Furthermore, the pulse width is shortened due tothe increased photon density for stimulated emissionin the laser medium. An increase of the crystal tem-perature has a similar effect on the laser output asusing a Q switch with a smaller initial transmit-tance. With the use of four-level lasers the heatingdoes not introduce extra losses inside the cavitybut only lowers the laser gain.

By controlling the laser crystal temperature onecan perform tuning in a continuous way without aneed to replace any of the laser components. Heatingis technically straightforward by using a resistor in-side the crystal mount. The cross section of stimu-lated emission is, however, not the only propertythat is dependent on the actual crystal temperature.In terms of the laser performance, the most impor-tant properties that are affected by the temperatureare absorption and laser-gain spectral lines. In highpower lasers the decrease of thermal conductivity atelevated laser temperatures can also be significant.

Earlier work on temperature dependence on laserperformance includes research on neodymium-doped

0003-6935/08/234262-05$15.00/0© 2008 Optical Society of America

4262 APPLIED OPTICS / Vol. 47, No. 23 / 10 August 2008

lasers in GSGG and YAG host crystals [1,2]. Brauchet al. have also studied cw laser performance on solarand diode-laser side-pumped lasers [3,4]. As far aswe know, the only experimental study on tempera-ture dependence on passively Q-switched lasershas been done by Bass et al. [1] where the pulse en-ergy dependence on temperature in a flashlamp-pumped laser has been reported. In this work westudy the temperature dependence of pulse energy,length, and peak power in three different end-pumped lasers covering a wide range of pulse ener-gies. The temperature inside the laser crystal in theend pumping has been modeled using a finite-element method (FEM) simulation. We also reportour results concerning the beam quality and thresh-old power together with temperature effects on lasercrystal absorption spectrum and laser wavelength.

2. Setup

The general layout of the lasers studied in this workis depicted in Fig. 1. The pump power is provided by afiber-coupled laser diode emitting at 808nm. Thediameter of the pump fiber is 100 μm with a numer-ical aperture of 0.22. The output from the fiber is col-limated and focused onto the laser crystal with a pairof aspheric lenses with focal lengths of 11mm. Thespot from the pump fiber was thus reimaged intothe laser crystal. The rear surface of the crystalhas a dielectric coating that is highly reflective at1064nm and highly transmissive at 808nm. Thelaser cavity is formed between the coated rear sur-face of the crystal and a spherical dielectric mirror.The laser crystal is doped with 1% of neodymium.Passive Q switching of the laser is obtained by pla-cing a Cr4þ:YAG crystal with AR-coated surfaces intothe laser cavity. The physical length of the laser cav-ity, unsaturated transmission, and thickness of theCr:YAG crystal, together with the reflectivity ofthe outcoupling mirror, are listed in Table 1. Lasercrystal length in all the lasers was 2:5mm and theradius of curvature for the outcoupling mirror was10 cm. The lasers were designed to produce relativelyshort pulses with a wide range of pulse energies(from 5 to 32 μJ). With temperature tuning we couldcover continuously the peak power range from 0.7 to

17kW. The pulse width was measured with a Thor-labs DET210 photodetector. Because of the slow risetime of∼1ns for the shortest pulses, a correction wasmade to the measurement results. The correctioncoefficient was obtained by comparing the responseof the photodetector with that of the Newport D-100 detector, which has a 100ps rise time.

The heating is performed with a resistor inside analuminummount. A negative temperature coefficient(NTC) thermistor inside the laser crystal mount wasused for temperature sensing in the pulse parametermeasurements, while a PT100 resistor on the surfaceof the mount was used in the laser wavelength mea-surements. The NTC thermistor resistance was com-pared with a PT100 resistance in order to calibratethe NTC thermistor temperature response for thetemperature rangeused. Silicon pastewasused to im-prove the heat conduction from themount to the NTCthermistor and heating resistor. The remaining partof the pump laser power was filtered from the laserpower of interest with the RG850 long-pass filterwith cutoff wavelength at 850nm. The transmissionspectrum and laser wavelength measurements of theNd:YAG crystal were done by using an optical spec-trum analyzer (Ando AQ-6315E). An incandescentlampwas used as a broadband light source for the ab-sorption measurements.

3. Results

The pulse parameters and the absorbed pump powerin the laser crystal were measured as a function ofthe crystal mount temperature while keeping thepulse period at 348 μs. Beam alignment was opti-mized before measurements. Time traces of the laserpulses of the three lasers without external heatingare presented in Fig. 2. The curves are scaled so thatthe peak power values correspond to the measuredpeak power values.

Information concerning the temperature inside thelaser crystalwhere the lasermode volume is located isof interest. This was simulated with a FEM (ComsolMultiphysics 3.4). The crystal was pressed betweentwo aluminum plates with the other side having anindium sheet to improve the thermal contact. Thiswas simulated by a 0:1mm thick layer of indium onone side; both sides were adjacent to a 10 μm longair gap to model unideal thermal contacts. The verti-cal sides were surrounded by 0:1mm air gaps.

The two-dimensional (2D) model took into accountconvection in finding the temperature distribution in

Fig. 1. (Color online) Schematic of the pulsed Nd:YAG laser setupwith laser crystal heating. Laser crystal mount is heated with aresistor. DMM refers to the digital multimeter that is used to mea-sure the thermistor resistance that gives the temperature of thelaser crystal mount.

Table 1. Laser Specifications for the Measured Lasersa

Mirror Q Switch Cavity Q SwitchR T Length Length

Laser (%) (%) (mm) (mm)

1 88 93 9 0.72 60 77 5.5 1.23 60 62 7 2.8

aThe laser crystal length of all the lasers was 2:5mm; the radiusof curvature for the outcoupling mirror was 10 cm.

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a steady state. The pump beam that created the heatinside the crystal was approximated by a cylinder. Todetermine the size of the heating element, the pumpbeam intensity was measured and the respectivebeam radius was calculated using the second-moment method [5]. With this beam radius andthe M2 value of the fiber, the average beam size inthe crystal was determined [6,7]. The heating ele-ment in the model was a cylinder whose peak inten-sity and heating power were the same as for theaverage beam. The second-moment method gave aFWHM beam width of 73 μm at the focus, whenthe average FWHM beam width was 111 μm. Theheat source cylinder diameter in the model was134 μm. A simulated laser crystal temperature distri-bution with vertical and horizontal temperaturecross sections are presented in Fig. 3 for an alumi-num temperature of 137 °C and a heating power of

1:4W. These conditions are equal to those used inperforming the measurement with laser 3 at thehighest observed pulse energy.

Measured laser pulse parameters as a function ofthe mount temperature and simulated crystal tem-perature are shown in Fig. 4. A fit was performedon the pulse energy data according to

EðTÞ ¼ CσðTÞ ; ð1Þ

where the stimulated emission cross section σ is as-sumed to have a linear temperature dependence [8]σ ¼ Aþ BT and C is a fitting parameter. This form ofthe equation also serves as a solution to the differen-tial equation that describes the pulse as a function oftemperature for a passivelyQ-switched laser, exclud-ing excited state absorption in the Q-switching ele-ment [1]:

dEdT

¼ E

�−

1σdσdT

�: ð2Þ

Fig. 2. (Color online) Time traces of the laser pulse power for thethree lasers with no external heating of the crystal. Each pulseheight was scaled to the measured peak power value. Solid curve:laser 1, dashed–dotted curve: laser 2, dotted curve: laser 3. Thenoise floor, due to limited signal-to-noise ratio, can be seen as rip-ples in the curve for the largest energy pulse.

Fig. 3. (Color online) Laser crystal temperature distribution forlaser 3 with the laser mount temperature of 137 °C or 410K andheating power of 1:4W.

Fig. 4. (Color online) Measured laser pulse parameters as a func-tion of the laser mount and simulated laser crystal peak tempera-ture: (a) pulse energy, (b) pulse length, (c) peak power, and(d) absorbed pump power at the laser threshold. Squares, circles,and diamonds refer to lasers 1, 2, and 3, respectively.

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Equation (2) is derived from the corresponding rateequations [9]. This model gives a good qualitativeprediction but it is not so accurate that the exact va-lue for the emission cross section could be obtainedfrom experimental data, as was also the case in [1].To compare the output of different lasers, the re-

sults are normalized with the measurement valuewithout external heating. Normalized pulse energy,pulse width, and peak power graphs are presentedin Fig. 5 as a function of mount and simulated lasercrystal temperatures. The normalized pulse energybehaves more or less similarly for all three lasers.One can notice a 50% rise in the laser pulse energywhen heating the mount by 90 °C, which is equal to a120 °C temperature rise in the laser crystal. As ex-pected, the result of heating is observed as a short-ening of the pulses. As a consequence of these effects,a temperature rise of 100 °C in the laser mount leadsto at least a 90% increase in the peak power in allthree lasers.

The change of the laser wavelength as a function oftemperature was measured with laser 1. The resultsare presented in Fig. 6, together with the typicalspectrum of laser 1. Extracted slopes obtained fromthe data are 5.0 and 4:6 × 10−3 nm=K for laser mountand crystal temperature results, respectively.

Beam quality has been reported to degrade withincreasing temperature due to stress-induced bire-fringence [3]. The M2 values were measured withoutexternal heating using laser 3 at the temperature of38 °C, giving the results 1.1 and 1.2 for the two per-pendicular axes. At themount temperature of 150 °C,both values were 1.2 for vertical and horizontal axes.The average laser power was kept approximately at100mW in the measurements. With these experi-mental parameters no degradation was observed.

The laser crystal absorption measured at 25 and140 °C is shown in Fig. 7. One can note that the ab-sorption maxima become clearly broadened at highertemperatures.

As a conclusion, heating the Nd:YAG laser crystalof a 1064nm laser was discovered to have significanteffects on the pulse properties within a practicalheating range. The pulse energy can be remarkablyincreased by heating and, together with the reduc-tion of the pulse length, the influence of heating ismost prominent in the peak power. In this work,the lasers were operated near the lasing thresholdto make the direct comparison of different lasers pos-sible. In many practical cases that correspond to hav-ing higher repetition rates, the effect is even moresignificant due to the higher local heating of thepumping laser beam. However, one should note that,even without applied external heating, the contribu-tion of local heating in pulsed-laser output becomesobservable by comparing the pulsed and cw pumpingschemes, leading to different local crystal tempera-tures. We can conclude that heating the laser crystaloffers a practical way to tune the pulse properties inthe case of using passively Q-switched lasers.Furthermore, the temperature of the crystal is oneimportant factor determining the pulsed laser outputand, thus, one should be well aware of spatial and

Fig. 5. (Color online) Normalization to the data point with no ex-ternal heating. (a) Normalized pulse energy, (b) pulse length,(c) peak power, and (d) absorbed pump power at the laser thresholdas functions of change in the mount and laser crystal peak tem-peratures. Squares, circles, and diamonds refer to lasers 1, 2,and 3, respectively.

Fig. 6. (Color online) (a) Laserwavelength change at 1064nmasafunction of the laser mount temperature (squares) and simulatedlaser crystal temperature (circles). (b) Typical spectrum of laser 1normalized so that the peak power corresponds to the value of 0dB.

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temporal heating processes if the details of the pulseparameters are of significance.

This work was supported by the Graduate Schoolof Electrical and Communications Engineering, theFinnish Foundation of Technology, and the FinnishAcademy of Science and Letters. We are gratefulto Esa Räikkönen for helpful discussions.

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Fig. 7. (Color online) Absorption in a laser crystal as a function ofthe wavelength at the mount temperature of 25 (solid curve) and140 °C (dashed curve).

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