Comparison of cw laser generation in Er 3+, Yb 3+ : glass microchip lasers with different types of glasses

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1 OPTO ELECTRONICS REVIEW 19(4), DOI: /s Comparison of cw laser generation in Er 3+, Yb 3+ : glass microchip lasers with different types of glasses J. MŁYŃCZAK *1, K. KOPCZYŃSKI 1, Z. MIERCZYK 1, M. MALINOWSKA 2, and P. OSIWIAŃSKI 3 1 Institute of Optoelectronics, Military University of Technology, 2 Kaliskiego Str., Warsaw, Poland 2 Institute of Micromechanics and Photonics, Warsaw University of Technology, 1 Politechniki Sq., Warsaw, Poland 3 Institute of Telecommunications, Teleinformatics and Acoustics, Wrocław University of Technology, 27 Wybrzeże Wyspiańskiego Str., Wrocław, Poland The paper describes comparison of laser generation of concentrated and SELG glasses developed at the General Physics In stitute of Russian Academy of Sciences in Moscow and most recent EAT14 glass worked out by the CLaser Photonics Com pany in Shanghai. The laser generation was carried out using 976 nm pump wavelength in cw and quasi cw regimes. Com parison of efficiencies and thresholds as well as of dissipative losses was made with a view to choose the best glass for eye safe microchip laser range finder. The generated wavelengths by each sample were measured. Keywords: microchip laser, erbium glass, eye safe laser radiation. 1. Introduction The first works concerning eye safe generation in erbium glasses were presented in 1965 [1]. For these experiments, sili cate glasses were used but soon phosphate glasses turned out to be much more efficient [2]. Eye safe 1.5 μm lasers with these active media were used in many different applications [3,4]. Development of new pumping laser diodes, with very high output power has increased the requirements for mecha nical, optical, and thermal resistance of new active media. Higher concentrations of dopants are required, especially for diode pumped lasers. Intensive works have been made in order to find a new type of laser glasses with very high thermal resis tance and optimal concentrations of dopants. During the last several years QX/Er, QE 7, and QE 7S glasses, manufactured by KIGRE, have been most often applied to lamp and diode pumped laser heads generating at 1.5 μm. At the General Physics Institute, Laser Materials and Technology Research Centre Russian Academy of Scien ces in Moscow, under Professor Denker leadership, two new types of erbium glasses were developed. One of them is concentrated Yb Er laser glass (GLASS) with concentration of ytterbium ions as high as cm 3 [5]. The other one is strong erbium laser glass (SELG), especially deve loped for microchip laser applications and characterized by enhanced thermal damage threshold [6]. These glasses have thermo mechanical properties of silicate glasses as well as high generation efficiency of phosphate glasses. * e mail: jmlynczak@wat.edu.pl Recently, the CLaser Photonics seated in Shanghai, China developed EAT14 Er Yb phosphate glass for laser diode pumping. Concentration of ytterbium ions in this glass is cm 3. In the literature, one can find the results of laser genera tion in microchip lasers with SELG and concentrated glasses but there is still lack of paper concerning application of EAT14 glasses to microchip lasers. In this paper we present generation investigations of concentrated and SELG glasses as well as EAT 14 glasses pumped by 976 nm laser diode. We made attempt to compare the glasses with a view to choose the best glass for eye safe microchip laser range finder. 2. Experiment and discussion Three samples of concentrated glasses and three samples of SELG glasses with optimal thickness of 1 and 1.5 mm, respectively, as well as 24 samples of EAT14 glasses with two different thicknesses were examined. Thickness of con centrated and SELG glasses were assumed to be optimal as it was shown in Ref. 7. All samples had 6 mm diameter. Concentrations of ytterbium and erbium ions were as follows: SELG 1: Yb cm 3, Er cm 3, thick ness 1.5 mm SELG 2: Yb cm 3, Er cm 3, thickness 1.5 mm SELG 3: Yb cm 3, Er cm 3, thickness 1.5 mm Opto Electron. Rev., 19, no. 4, 2011 J. Młyńczak

2 Comparison of cw laser generation in Er 3+, Yb 3+ : glass microchip lasers with different types of glasses GLASS 1: Yb cm 3, Er cm 3, thickness GLASS 2: Yb cm 3, Er cm 3, thickness GLASS 3: Yb cm 3, Er cm 3, thickness EAT14 1: Yb cm 3, Er cm 3, thick ness, (3 samples) EAT14 2: Yb cm 3, Er cm 3, thick ness, (3 samples) EAT14 3: Yb cm 3, Er cm 3, thickness, (3 samples) EAT14 4: Yb cm 3, Er cm 3, thickness, (3 samples) EAT14 5: Yb cm 3, Er cm 3, thick ness 1.5 mm, (3 samples) EAT14 6: Yb cm 3, Er cm 3, thick ness 1.5 mm, (3 samples) EAT14 7: Yb cm 3, Er cm 3, thickness 1.5 mm, (3 samples) EAT14 8: Yb cm 3, Er cm 3, thickness 1.5 mm, (3 samples) Absorption coefficient spectra of the investigated media were calculated on the basis of the measured transmission as a function of wavelength taking into account multiple reflec tions inside the probes. Peaks of absorption at 976 nm relate to the 2 F 7/2 2 F 5/2 transition in Yb 3+ ions. The absorption coefficient is higher for concentrated glasses (32 cm 1 for 975 nm) [5] than for SELG glasses (22 cm 1 for 975 nm) [6], while for EAT14 glass it is 16 cm 1. The curves of the absorption coefficient for the investigated samples are presented in Fig. 1. Absorption coefficient can be expressed as a product of concentration and absorption cross section. Thus, assuming that absorption cross section for Er Yb glasses at 976 nm wavelength amounts to cm 2, one can calculate that concentration for the investigated glasses equals cm 3, cm 3, cm 3 for concentrated, SELG, and EAT14 glasses, respectively. It means that the concen trations of Yb ions, calculated on the basis of the absorption Fig. 1. Absorption coefficient of investigated glasses as a function of wavelength. coefficient, differ only a little from the ones stated by the manufacturers. Lifetimes of the 4 I 13/2 level of the Er 3+ ions for all the samples is around 9 ms [7,8]. The most important parame ters of the glasses are presented in Table 1 [8 10]. For laser generation, dichroic input mirrors (AR@975 nm, HR@1535 nm) were deposited on one side of the sam ple and AR@1535 nm coatings on the other one (Fig. 2). As an output coupler we used plane parallel mirrors with four Fig. 2. Glass sample prepared for laser generation. Table 1. Main parameters of samples. Parameter SELG Concentrated EAT14 Fluorescence lifetime (ms) Absorption coefficient at 975 nm (cm 1 ) Concentration of ytterbium ( cm 3 ) stated by the manufacturer Calculated concentration of ytterbium ( cm 3 ) T thermal expansion coefficient (20 40 C) ( 10 7 K 1 ) W o thermo optical parameter (30 C) W o = dn/dt + (n 1) ( 10 7 K 1 ) dn/dt (20 40 C) ( 10 7 K 1 ) density (g/cm 3 ) thermal conductivity (W/mK) Hardness (kgf/mm 2 ) Opto Electron. Rev., 19, no. 4, SEP, Warsaw

3 Fig. 3. Experimental setup for investigations of laser generation. different transmissions (98.70%, 98.15%, 97.64%, and 96.49%). Laser investigations were carried out in experi mental setup presented in Fig. 3. For pumping the laser, diode LIMO25 F100 LD976 generating at 976 nm was used. The length of the laser resonator was determined by the length of the active media. The dependence of the output power on the incident pump power was examined in cw and quasi cw (duty cycle 50%) pumping mode without cooling of the active media. Generation was achieved in all samples with the exception of EAT14 1, EAT14 2, and EAT14 5 which means that due to low concentration of erbium ions, the achieved gain was too low. In Figs. 4 and 5, output power vs. pump power for three samples of EAT14 8 glas ses (Yb cm 3, Er cm 3, thickness 1.5 mm) with two extreme transmissions of the output coupler in quasi cw and cw pumping mode are presented, respectively. In Figs. 6 and 7, the best results for SELG, GLASS, and EAT14 8 in quasi cw and cw pumping mode are shown. The symbols on the picture represent the name of the sam ple (SELG 1, GLASS 3, EAT14 8,...), the number of the sample only for EAT14 (1,2,3). Transmission of the out put coupler is represented as T. During the experiments, the coatings on two samples have been broken down as a result of too high pump power. The pump powers and the evaluated power densities which brought about the damage of the coatings are shown in Table 2. Table 2. Damage thresholds of coatings on the samples. Sample Pump power (mw) Power density (MW/cm 2 ) GLASS GLASS For each of these microchip lasers, the generation spec trum was examined using the optical spectrum analyzer AQ6319. The exemplary spectra generated by sample Fig. 4. Output power vs. pump power for three samples of EAT14 8 glasses (Yb cm 3, Er cm 3, thickness 1.5 mm) with two extreme transmissions of the output coupler in quasi cw pump ing mode. Fig. 6. The best results for SELG, GLASS, and EAT14 8 in quasi cw pumping mode. Fig. 5. Output power vs. pump power for three samples of EAT14 8 glasses (Yb cm 3, Er cm 3, thickness 1.5 mm) with two extreme transmissions of the output coupler in cw pumping mode. Fig. 7. The best results for SELG, GLASS, and EAT14 8 in cw pumping mode. Opto Electron. Rev., 19, no. 4, 2011 J. Młyńczak 493

4 Comparison of cw laser generation in Er 3+, Yb 3+ : glass microchip lasers with different types of glasses SELG 1 and EAT14 3 are shown in Figs. 8 and 9, respec tively. The lasers generated at several longitudinal modes around 1535 nm for EAT14 glasses and around 1535 nm and nm for SELG and concentrated glasses. For con centrated and SELG glasses, if the transmission of the out put mirror increases, the wavelength shifts from nm to 1535 nm (wavelength switching). This phenomenon was explained on the basis of an analysis of absorption and emission spectra described in Ref. 11. The generation at only 1535 nm wavelength by EAT14 glasses may be caused by their higher losses compared with the SELG or GLASS glasses. The generated wavelengths for different transmis sion of output couplers are presented in Table 3. Table 3. Results of investigations of wavelength shifting. Reflectivity of output coupler Sample 98.70% 98.15% 97.64% 96.49% SELG SELG SELG 3 GLASS GLASS GLASS EAT EAT EAT EAT EAT Fig. 8. Generated spectra by SELG 1. Figures 4 and 5 show that the main lasing parameters, like slope efficiency and threshold for EAT14 8 glasses, range from several percent to over twenty percent for diffe rent samples with the same concentrations of dopants. It can be seen from Figs. 6 and 7 that for the best sample of EAT14, the main lasing parameters do not differ very much from the parameters of SELG and GLASS glasses which show that they are as good as SELG and GLASS glasses. Table 4 shows the values of slope efficiencies and thresholds for the highest transmission of the output coupler (T = 98.70%) for SELG, GLASS, and EAT14 glasses work Fig. 9. Generated spectra by EAT14 3. ing in quasi cw pumping mode. For EAT14 glasses, the mentioned parameters were arithmetically averaged for three samples. Also, concentrations of erbium and ytterbium ions, the ratios of concentration of ytterbium to erbium ions, and the length of the media are presented. Table 4. Comparison of SELG, concentrated, and EAT14 glasses. Parameter SELG 1 SELG 2 SELG 3 GLASS 1GLASS 2GLASS 3 EAT14 3 EAT14 4 EAT14 6 EAT14 7 EAT14 8 Yb concentration ( cm 3 ) Er concentration ( cm 3 ) Yb/Er Length (mm) Slope efficiency (%) Threshold (mw) Generated wavelength (nm) Opto Electron. Rev., 19, no. 4, SEP, Warsaw

5 3. Conclusions Looking at Table 4, one can see that EAT14 glasses have comparable parameters to SELG and concentrated glasses. Of course, there are small differences in efficiency which is higher for SELG and concentrated glasses but thresholds are lower for EAT14 glasses. From the point of view of range finder application, the possible change of the wavelength is disadvantageous due to filters applied to the detection optics so, EAT14 glasses seem to be more appropriate. The best sample out of EAT glasses is EAT14 8 (Yb cm 3,Er cm 3, thickness 1.5 mm) and it was the sample with the highest measured slope efficiency (22.85%) from all glasses. References 1. E. Snitzer and R. Woodcock, Yb 3+ Er 3+ glass laser, Appl. Phys. Lett. 6, (1965). 2. E. Snitzer, R. Woodcock, and J. Segre, Phospate glass Er 3+ laser, J. Quant. Electron. 4, (1968). 3. Laser Rangefinders, JANE s Armour and Artillery Up grades, Eleventh Edition , Jane s Information Group Ltd., Couldson, Surrey UK, Z. Mierczyk, Investigations of saturable absorbers for eye safe giant pulse laser systems, Optics and Opto electron ics, Theory, Devices and Applications 2, (1998). 5. J. Młyńczak, K. Kopczyński, and Z. Mierczyk, Generation of 1.54 μm laser radiation in Er,Yb:glass, Proc. SPIE 6599, 65990E (2007). 6. J. Młyńczak, K. Kopczyński, and Z. Mierczyk, Investiga tions of optical and generation properties of Yb Er laser glasses (SELG) designed for 1.5 μm microlasers, Proc. SPIE 6599, 65990D (2007). 7. J. Młyńczak, K. Kopczyński, and Z. Mierczyk, Generation investigation of eye safe microchip lasers pumped by 974 nm and 939 nm wavelength, Opt. Appl. 38, (2008) sh.com/?file=product_detail&lang=2& ID=39 9. B. Denker, B. Galagan, V. Osiko, and S. Sverchkov, Mate rials and components for miniature diode pumped 1.5 μm er bium glass lasers, Laser Phys. 12, (2002). 10. G. Karlsson, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, and S. Sverchkov, Development and character ization of Yb Er laser glass for high average power laser di ode pumping, Appl. Phys. B Lasers O. 75, (2002). 11. J. Młyńczak, K. Kopczyński, and Z. Mierczyk, Wavelength tuning in Er 3+,Yb 3+ :glass microchip lasers, Opto Electron. Rev. 17, (2009). Opto Electron. Rev., 19, no. 4, 2011 J. Młyńczak 495