IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 22, NOVEMBER 15, 2013 2153

Directly Diode-Pumped Ho:YAG Ceramic LaserShengli Wang, Yulong Tang, Benxue Jiang, Chongyuan Huang, Nan Yang, Hongqiang Li,

Jianlong Yang, Jiang Li, Yubai Pan, and Jianqiu Xu

Abstract— We present a Ho3+-doped YAG ceramic laser whichis directly diode-pumped at ∼1.13 µm for the first time. TheHo:YAG ceramic slab in this letter has a dimension of 4.2×4.8×22 mm3 and a doping concentration of 1 at%. The fiber coupledlaser diode (LD) used for pumping has central wavelengths of1.123–1.130 µm and a typical FWHM bandwidth of 5 nm. Theefficient absorption of Ho:YAG ceramic is stable for broadbandLD pump sources. The maximum output power was up to 1 Wof continuous wave at the lasing wavelength of 2099 nm. Thelaser has a slope efficiency of 20.6% with respect to absorbedpower and a beam propagation factor of M2 ∼4 at the maximumoutput power.

Index Terms— Diode-pumped, ceramic, Ho:YAG, laser.

I. INTRODUCTION

ONE effective pump wavelength for Ho3+-doped lasers,of which emission wavelengths are ∼2.1 μm, is around

1.9 μm [1]–[3]. Both the Ho3+-doped bulk lasers and fiberlasers have been researched widely with the pump sourcesof Tm3+-doped lasers [4], [5]. The early attempt of diodepumping for Ho:YAG crystal was performed at 1.9 μm beforecommercialization of high power LDs [6], where the CWoutput power was less than 0.7 W even at temperature of−53 °C. With the late development of diode stacks at 1.9 μmwavelength, it is now possible to generate 55 W of outputpower from Ho:YAG crystal lasers [7] or 2.5 W CW outputpower from Ho:Y2O3 ceramic lasers [8]. The electro-opticalefficiency of these diode stacks is restricted because of Augerrecombination, which generates large heat within the diodelaser system [9], and the pump line width is usually largerthan the absorption peak of the Ho3+-doped bulk material,leading to a comparatively low absorption efficiency of thepump emission.

Another available pump wavelength for Ho3+-doped lasersis around 1.1 μm. Ho3+-doped fiber lasers have been pumped

Manuscript received October 18, 2012; revised July 13, 2013; acceptedSeptember 2, 2013. Date of publication September 11, 2013; date of currentversion October 16, 2013. This work was supported in part by the KeyNational Natural Science Foundation of China under Grant 61138006, and inpart by the National Natural Science Foundation under Contract 61275136.

S. Wang, Y. Tang, C. Huang, N. Yang, H. Li, J. Yang, and J. Xu arewith the Key Laboratory for Laser Plasmas, Ministry of Education, andthe Department of Physics, Shanghai Jiaotong University, Shanghai 200240,China (e-mail: wangshengli6@163.com; yulong@sjtu.edu.cn; zzyyhcy@qq.com; ynecho@163.com; igg8@sjtu.edu.cn; nicholasyjl@sjtu.edu.cn;jqxu09@sjtu.edu.cn).

B. Jiang, J. Li, and Y. Pan are with the Key Laboratory of Transparentand Optofunctional Advanced Inorganic Materials, Shanghai Institute ofCeramics, Chinese Academy of Sciences, Shanghai 200050, China (e-mail:jiangsic@foxmail.com; lijiang@mail.sic.ac.cn; ybpan@mail.sic.ac.cn).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2281424

with Yb3+-doped lasers specially designed to the long wave-length range (around 1.148 μm) [10]. Directly 1.1-μm LDpumped Ho3+-doped fiber was also reported, the output powerof 55 mW with a slope efficient of 27% was achieved througha 0.5 wt.% Ho3+-doped silica fiber [11]. For the Ho3+-dopedbulk laser, there is no report on directly diode pumping around1.1 μm to the best of our knowledge.

Currently developed ∼1.1 μm LDs can provide muchhigher pump power, higher electrical efficiency and longerlifetime than ∼1.9 μm LDs. Furthermore, the cross relaxation(CR: 5I5, 5I8 → 5I7, 5I7 and 5I6, 5I8 → 5I7, 5I7, see theinset of Fig. 1) of the Ho3+ ion in pumping at ∼1.1 μmreveals the possibility of the “two-for-one” effect, whichallows one pump photon excite two ions to the upper laserlevel and greatly enhances the pumping efficiency [12]. There-fore, pumping Ho3+-doped bulk laser materials with 1.1-μmLDs has the potential to achieve higher laser performance.Besides, pumping Ho3+-doped laser materials with 1.1-μmLDs can also realize laser emission at the wavelength region of2.9 μm.

In the past decade, much progress has been made inimproving the optical quality of laser ceramics, as well asexploring new fabricating methods [13]–[16]. Ceramic lasershave been developed in a wide wavelength range. The Ho:YAGceramic pumped with Tm3+:YLF lasers or Tm3+ fiber lasers,which was in turn diode pumped around 0.79 μm has beenreported previously [17]. The advantages of ceramic materialalso make it an attractive candidate for directly diode pumpedHo3+-doped lasers.

In this letter we present our results on the Ho:YAG ceramiclaser pumped with a LD at ∼1.13 μm. The maximum CWlaser power of 1 W at 2099 nm was generated with theoutput transmission of 2%. The optical conversion efficiencyof 20.6% and a beam propagation factor of M2∼4 wereobtained.

II. EXPERIMENT, RESULTS AND DISCUSSION

The absorption spectrum of 5I8→5I6 of Ho:YAG ceramicused in our experiments is presented in Fig. 1. As can beseen in Fig. 1, the absorption spectrum of Ho:YAG ceramicshows a group of peaks in the range of 1120∼1150nm, whichcan be utilized for pumping with the commercial LDs withoutrequirement of wavelength control technique. The absorptioncross section of primary peak around 1128 nm is calculatedto be 2.4 × 10−21 cm2.

The pump diode source used in the experiment was a fiber-coupled laser diode module, which consisted of 10 diodeemitters and cooled with a thermoelectric cooler (TEC) at the

1041-1135 © 2013 IEEE

2154 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 22, NOVEMBER 15, 2013

Fig. 1. The 5I8 →5I6 absorption spectrum of Ho:YAG (1 at.%) ceramic andLD emission spectra for various LD output powers.

Fig. 2. Absorption efficiency of the 22-mm Ho:YAG (1 at.%) ceramic slabunder different pump power.

temperature of 25 °C. The central wavelengths and bandwidthsof pump diode varied under different drive currents. Theemission spectra of our pump diode are also shown in Fig. 1.The central wavelength shifted from 1123 nm at the start to1130 nm at maximum drive current, meanwhile its spectralbandwidth (FWHM) increased from 3 to 7 nm. The rate ofchange of central wavelength was about 0.6 nm/W.

In order to measure the absorption characteristics of theHo:YAG ceramic sample, the pump power of the fiber coupledLD was varied by changing the diode laser driver currentand the transmitted pump power was measured with removedoutput coupler for different pump power levels without lasing.No absorption bleaching was observed up to the maximumpump power. For the whole range of available drive currents,the absorption efficiency of 22-mm long slab was found tovary from 44% to 72% as shown in Fig. 2. When the pumpwavelength is longer than 1125 nm, the absorption efficiencyremained stable at between 68% and 72%.

The fluorescence spectrum was measured on a 1-mm thickHo:YAG sample by a spectrometer with 0.5 nm spectralresolution, excited at 785 nm from a Ti-sapphire laser. Thethin sample was used to avoid of re-absorption effects on thespectra [8]. The fluorescence spectrum is shown in Fig. 3.

Fig. 3. The fluorescence spectrum and laser spectra of the Ho:YAG (1 at.%)ceramic laser.

It shows the fluorescence spectrum from 1800 to 2200 nmof Ho:YAG ceramic with doping concentration of 1 at.% atroom temperature (300 K). The FWHM of the emission bandof main peak was about 80 nm with the center wavelengthof near 2090 nm. The emission spectrum was rather smoothto benefit ultra-short pulse generation from mode-locking ofHo:YAG ceramic lasers.

The Ho:YAG ceramic laser was CW pumped by the fibercoupled LD (10W out of a 400 μm core, NA∼0.2) , whichis the inset sketched in Fig. 2. The sample slab was anti-reflection (AR) coated at the both facets (R<0.2% at 2090 nmand R<0.4% at 1128 nm for each coating), and conductivelycooled through a copper heat sink, which was cooled by thecooling water of 15 °C. The laser cavity was formed by a flatmirror M1 (AR at 1128 nm, highly reflective at 2090 nm),which was positioned 1mm to the facet of the sample slab,and a 250-mm radius-of-curvature (ROC) partially reflectiveoutput couplers M2 (98%, 95%, 90% reflectivity at the laserwavelength of 2090 nm, respectively). A dichroic mirrorM3(R>99.5% at 1128 nm and AR at 2100 nm) was placed toremove residual pump power. The fiber-coupled output wascollimated and then focused into a 4.2 × 4.8 mm2 crosssectional area and formed a 600-μm diameter spot at the centerof 22-mm long slab. The size of cavity fundamental mode at∼2 μm was estimated to be 0.6 mm in diameter, providing agood matching between the pump and laser beam.

The output performance of Ho:YAG ceramic lasers is plot-ted in Fig. 4 as a function of absorbed pump power. Themaximum power of 1 W was generated with the 2% outputcoupler, corresponding to a slope efficiency of 20.6% withrespect to absorbed power. The slope efficiency decreasedwith higher output coupling rates, since higher inversionswere required for laser operation leading to higher up-conversion losses and a reduced absorption efficiency. Thethreshold pump power was around 1 W, indicating pumpthreshold intensity of 350 W/cm2 for 1.13-μm pumping.The pump threshold intensity was on the same level asthat of Ho:YAG ceramic laser pumped by Tm:YLF laser at1.9 micron [17].

WANG et al.: DIRECTLY DIODE-PUMPED Ho:YAG CERAMIC LASER 2155

Fig. 4. Output power versus absorbed pump power of Ho:YAG (1 at.%)ceramic lasers.

The quantum defect limit based on pumping and emissionwavelength is 53% for the 1.13-μm pumping of Ho:YAGceramic lasers. Considering the electro-optical efficiency of50% for our LD pump source, the electro-optical efficiencyof whole system can be 26%, which is comparable to thetraditional pumping scheme where Ho:YAG lasers are pumpedby Tm-doped lasers. The relative low slope efficiency of ourexperiments came mainly from large optical losses of under-developed Ho:YAG laser ceramic. The scattering losses of ourHo:YAG ceramic slab measured with the integrating sphereequipment was about α = 0.02 cm−1, leading to 6% singlepass loss for 22-mm long Ho:YAG ceramic sample. Whenthe transmittance of the Ho:YAG is enhanced, higher powerLD pump source and optimized laser designing are used, wecan expect higher output power and optical-optical conversionefficiency.

The laser emission spectrum at the output power of 0.2 Wand 1.0 W were also presented in Fig. 3 in compared withthe fluorescence spectrum. For 0.2-W output power, the laserwavelength was 2096 nm (6 nm FWHM). When the outputpower was 1 W, the spectrum exhibit a smooth spectral profilecentered at 2099 nm with a broad bandwidth δλ(FWHM) =9 nm. The broad emission band reveals the laser operatedin the multi-longitudinal mode regime. It is noted that theemission peak is red-shifted to 2099 nm from the fluorescencepeak of 2090 nm, which was not observed with the 1.9-μmpumping [17]. The possible reason is the temperature increasedin the ceramic slab. As a result of larger quantum defect,the temperature inside the Ho:YAG ceramic slab is higherthan that pumped at 1.9-μm wavelength, which could inducedetrimental thermo-optic effects.

The M2 factor of the 2.099 μm laser was measured at 1-WCW operation with a beam profile analyzer. The beam wascollimated and focused with a 100-mm focal length lens. Thebeam spot size and divergence angel were recorded by thescanning method. The measured beam propagation factor M2

value was ∼4.

III. CONCLUSION

The first demonstration of Ho3+-doped bulk lasers directlydiode pumped at ∼1.13 μm is reported. This Ho3+-doped laserat room temperature has generated 1 W CW output power at2.099 μm with a slope efficiency of 20.6% with respect toabsorbed pump power. The absorption efficiency of Ho:YAGceramic is quite stable for the broadband LD pump source.Broad emission band of 9 nm of Ho:YAG ceramic laser revealsits potential for ultra-short pulse and tunable sources. Theexperimental results show that Ho:YAG ceramic laser directlydiode pumped at ∼1.13 μm can be an attractive source for2.1-μm radiation. Because of high power, high electricalefficiency and long lifetime of 1.1-μm LDs, the lasers directlydiode pumped at ∼1.1 μm can be interest in the lasercommunity.

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