SPIE Proceedings [SPIE SPIE LASE - San Francisco, California, USA (Saturday 21 January 2012)] Solid State Lasers XXI: Technology and Devices - Influence of undoped YAG cap on diode-pumped

Influence of undoped YAG capon diode pumped composite YAG/Er:Yb:glass laser

Jan Šulca, Helena Jelínkováa, Karel Nejezchlebb, Václav Škodab

aCzech Technical University in Prague, 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

Two samples of Er-Yb doped phosphate glass were tested as a gain medium of longitudinally diode pumpedlaser. One sample was a simple Er:Yb:glass rod (length 2.8mm), second sample was composite rod consisting of2.8mm long Er:Yb:glass and 6mm long YAG crystal. Diameter of both samples was 6mm. Dopant concentrationfor Er:Yb:glass was 0.75× 1020 cm−3 Er and 1.7× 1021 cm−3 Yb. The goal of the experiment was to investigatean effect of the undoped YAG cap on the Er:Yb:glass laser operation. The active medium, fixed in cupreousheatsink, was placed inside the 150mm long resonator consisted of a flat pumping mirror (HR @ 1.52− 1.65μm,HT @ 0.97μm) and curved output coupler (r = 150mm, R = 97% @ 1.52−1.61μm). For Er:Yb:glass pumping afiber coupled laser diode, operating in pulsed regime, was used. The pumping pulse width, energy, and wavelengthwere 1ms, 10mJ, and 975 nm, respectively. The decrease of Er:Yb:glass laser output pulse energy with increasingpumping repetition rate was observed for both samples. In case of simple Er:Yb:glass the energy dropped from1.4mJ to 0.6mJ after pumping duty cycle increase from 0.5% to 6%. In case of composite YAG/Er:Yb:glass activemedium the relative output energy decrease was only 20% for pumping duty cycle increase from 0.5% to 10%.This result showed that the slope of the output energy decrease with increasing duty cycle was approximatelyfour times slower for composite active media in comparison with simple Er:Yb:glass.

Keywords: Er:Yb:Glass laser, diode pumping, composite.

1. INTRODUCTION

An erbium-based diode pumped solid state lasers, operating on ion Er3+ ion transmission 4I13/2 → 4I15/2, belongto well established direct laser sources of eye-safe radiation in range from 1.5μm up to 1.7μm. 1 There existswide range of applications for such radiation, including remote sensing, rangerfinding, or telecommunications, etc.Commonly used matrices for Er3+ ions are phosphate glasses.2–4 Erbium doped glass offers high and broad gainat 1.5μm spectral range, efficient absorption around wavelength 0.97μm (even supported by Yb3+ co-doping)suitable for diode pumping, low excited-state absorption, and long upper laser level lifetime.5 Glass itself hasmany interesting properties, including excellent optical quality, good mechanical properties and possibility toproduce “unlimited” sizes and shapes.6 Combination of all these properties give the rise of fibre lasers, includingEr-doped fibre lasers, referring high mean and peak power output.7–9 In bulk configuration, unfortunately, theoutput mean power is limited due to poor thermo-mechanical properties of glass, mainly by too low thermalconductivity (thermal conductivity of YAG crystal is 13W/K.m, thermal conductivity of glass is∼ 0.2W/K.m).10The constructed diode-pumped miniature Q-switched Er:glass lasers are significantly limited in repletion rate toseveral Hz and low pumping duty cycle.11–14

One possible way how to increase heat removal from low thermal conducting glass medium and decreasethermal effects (such as thermal lensing and thermal stress-induced birefringence) and enhance the performance

Further author information: (Send correspondence to J.Š.)J.Š.: E-mail: [email protected], Tel.: +420 224 358 672, 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

Solid State Lasers XXI: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 8235, 82351Z · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.909125

Proc. of SPIE Vol. 8235 82351Z-1

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of a laser system is to use advanced solid-state laser composite crystals15 or highly thermally-conducting heat-spreader (like diamond or quartz plate).16 Using doped and undoped laser rod components enlarges the activematerial cooling surface and improves the thermal uniformity and heatsink of laser active media. In case of glass,the combination of doped and undoped parts from the same matrix has no significant effect – for heat-spreaderother high thermally conducting material has to be used.

In presented study we have investigated influence of undoped YAG (Y3Al5O12) crystal cap on Er-Yb dopedphosphate glass, used as a active medium of longitudinally diode pumped 1.54μm laser.

2. MATERIALS AND METHODS

2.1. Er:Yb:glass and YAG/Er:Yb:glass composite samples

Two samples of Er-Yb doped phosphate glass were tested. One sample was a simple Er:Yb:glass rod, 2.8mmlong with diameter 6mm, second sample was composite rod consisting of 2.8mm long Er:Yb:glass and 6mmlong YAG crystal. Photos of these samples are presented on Figure 1. The dopant concentration was the samefor both glasses: 0.75× 10−20 cm−3 Er and 1.7× 10−21 cm−3 Yb. The absorption coefficient of Er:Yb:glass was27 cm−1 @ 975 nm. The measured fluorescence lifetime for upper laser level vas 6.4ms. The measured absorptionand fluorescence spectra are shown on figure 2 and 3.

Figure 1. Photograph of the tested single Er:Yb:glass sample (a) and YAG/Er:Yb:glass composite sample (b).

Figure 2. Measured absorption spectra of the tested Er:Yb:glass sample.



Figure 3. Measured fluorescence spectra of the tested Er:Yb:glass sample.

In case of YAG/Er:Yb:glass composite the undoped part made form YAG crystal (Y3Al5O12) was bonded tothe pumping face of the Er-Yb doped phosphate glass part. This composite was prepared by optical contactingof YAG and doped phosphate glass followed by diffusion bonding at elevated temperatures. It was confirmed17, 18

that using of such undoped part of laser rod enlarges the active material cooling surface and improves the laseractive media thermal field uniformity and heatsink. Also, the contact of highly thermally conductive YAG crystal(thermal conductivity k = 13W/K.m) with low thermally conducting phosphate glass (thermal conductivityk ∼ 0.15W/K.m) will reduce the hot-spot inside the glass. No AR coating were placed on glass faces. The YAGface on composite sample was AR coated for generated and pumping radiation.

2.2. Experimental setup

The Er:Yb:glass active medium and YAG/Er:Yb:glass composite were fixed in cupreous water-cooled heatsink(water temperature ∼ 20 ◦C). An indium foil was used to achieve as good as possible thermal contact to thesample. Samples in heatsing were placed inside the 150mm long hemispherical resonator close to the resonatorfocus. Diagram of laser resonator and pumping optics arrangement is shown on Figure 4. The laser resonatorconsisted of a flat pumping mirror (HR @ 1.52 − 1.65μm, HT @ 0.97μm) and a curved (r = 150mm) outputcoupler with the reflectivity of 97% @ 1.52− 1.61μm. For Er:Yb:glass pumping a fiber (HLU20F200, maximumunpolarized output power 20W, core diameter 200μm, NA = 0.22) coupled laser diode was used. The laser diodewas working in the pulsed regime. The pumping pulse width, maximum energy, and wavelength were 1ms, 13mJ,

Figure 4. Experimental setup of the longitudinally diode pumped Er:Yb:glass laser.



and 975 nm, respectively. The pulse repetition rate was set in range from 0.5 up to 100Hz. The diode radiationwas focused into the active Er:Yb:glass by two achromatic doublet lenses (Thorlabs, Inc., AC508-075-B) withthe focal length f = 75mm – see Figure 4. The measured diameter of pumping beam focus was 170μm. Thepumping beam waist was ∼ 3mm long. The active part of the laser media absorbed more than 94% of pumpingradiation.

2.3. Measuring instrumentsThe mean output power was measured by Molectron energy/power meter EMP2000 with two Molectron Power-Max probe PM3. Output energy was calculated dividing the mean power by frequency. For frequencies lowerthan 10Hz the output energy was measured directly using Molectron probe J25LP1 (sensitivity 16.4V/J). Aspectrum of the generated radiation was investigated with the help of an IR fibre spectrometer (OceanOpricsNIR512).

3. RESULTS

3.1. Laser output characteristics measurement

To characterize the laser performance, the generated laser radiation energy was measured in dependence onpumping energy. For this measurement, pumping repetition rate was set to 10Hz. The pumping pulse width was1ms, which corresponds to pumping duty cycle 1%. The obtained results, measured for both samples, are shownon Figure 5.

Figure 5. The laser output characteristics for 1% duty cycle pumping: glass sample (a) and composite sample (b).

From the results follows that better performance at 1% duty cycle was obtained using simple Er:Yb:glasssample. Energy up to 1.7mJ was obtained for absorbed energy 10.5mJ. The laser threshold and slope efficiencyin respect to absorbed energy were 2.9mJ and 21.2%, respectively. Using the composite YAG/Er:Yb:glass samplethe obtained results were significantly worst. The maximum output energy was approximately three times lower(0.66mJ laser output for 11.3mJ absorbed pumping energy). The lasers threshold was higher about 50% higher(4.5mJ of absorbed pumping energy) and the slope efficiency was more than twice lower (9.5%). All theseresults can be explained by some additional losses present in laser with composite sample. These losses canraise form the possible defects, scattering, or Fresnel reflection on YAG-glass boundary. The expected minimalboundary absoption/scatering losses for single pass are 0.85%. The Fresnel losses for YAG/phosphate glass canbe calculated to be ∼ 0.7% (depending on exact phosphate glass refractive index). The measured laser emissionwavelength was 1532 nm for both active medium samples. it can be expected, that for higher outcoupling thedifference between simple glass sample and composite sample will be lower.



3.2. Laser output energy in dependence on duty cycle of pumpingTo investigate the effect of undoped diffusion bounded YAG cap on active glass cooling, both samples weretested under pumping with increasing duty cycle (constant pumping pulse length 1ms, increasing repetition rate,constant pumping energy 10mJ) and the generated laser pulse energy was measured. The obtained dependenceof absolute and relative output pulse energy on increasing pumping pulses duty cycle is shown on Fig. 6 for bothactive medium samples.

Figure 6. The dependence of absolute (a) and relative (b) output pulse energy on increasing pumping pulses duty cycle.

From the results it follows that the highest output energy was obtained for the lowest duty cycle 0.5% forboth samples. Corresponding with the previous results, the output energy with simple Er:Yb:glass sample wasapproximately three time higher. With increasing pumping duty cycle the output laser energy drops down forboth samples. This can be explained as a negative impact of elevated temperature inside the lasing region of activemedium, followed by changes in absorption and emission and also in stronger thermal lensing. Comparing theresults for simple Er:Yb:glass sample and composite YAG/Er:Yb:glass sample it is evident a significantly sloweroutput laser energy decrease in case of composite YAG/Er:Yb:glass sample. After increasing of pumping dutycycle from 0.5% to 6% decreased the output energy in case of simple Er:Yb:glass sample about 60% while in casefor composite YAG/Er:Yb:glass sample the corresponding relative energy decrease was only 10%. The furtherincrease of pumping duty cycle caused in case of simple Er:Yb:glass sample fast decrease of energy. In case ofcomposite YAG/Er:Yb:glass sample the laser was operating up to duty cycle 10%. The following sudden decrease



of output energy was later explained by debonding of YAG/Er:Yb:glass composite. The observed deviation ofenergy dependence on duty cycle form monotone decrease can be explained by laser diode emission tuning withduty cycle changes.

4. CONCLUSION

Two samples of Er-Yb doped phosphate glass – simple Er:Yb:glass rod and composite rod consisting of sameEr:Yb:glass and YAG crystal – were tested as a gain medium of longitudinally diode pumped laser. The goalof the experiment was to investigate an effect of the undoped YAG cap on the Er:Yb:glass laser operation. Itwas observed that the boundary between YAG crystal and Er:Yb:glass can cause significant losses which coulddegrade significantly the laser performance in case of low outpcoupling. The performed experiments also provedthe positive effect of undoped YAG cap on laser generation for elevated pumping duty cycle. It was observed,that the output energy of Er:Yb:glass laser decreased with increasing pumping repetition rate for both samples.The slope of the output energy decrease with increasing duty cycle was approximately four times slower forcomposite active media in comparison with simple Er:Yb:glass. It can be conclude that further improvement ofYAG/Er:Yb:glass composite together with using of higher laser resonator outcoupling can significantly improvethe performance of diode pumped eye-safe Er:Yb:glass laser.

ACKNOWLEDGEMENT

This research has been supported by the Grant of the Czech Ministry of Education No. MSM6840770022 “Lasersystems, radiation, and modern optical applications”, and by the Czech Ministry of Industry and Trade, FR-TI1/356, program TIP .

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SPIE Proceedings [SPIE SPIE LASE - San Francisco, California, USA (Saturday 21 January 2012)] Solid State Lasers XXI: Technology and Devices - Influence of undoped YAG cap on diode-pumped