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Abstract
High efficiency continuous-wave laser operation of an in-house made Nd:Y2O3 ceramic was demonstrated. The Nd:Y2O3 laser ceramic was fabricated by vacuum sintering plus the hot isostatic pressing process, whose in-line transmittance reaches 81.6% at the wavelength of 1000 nm. The 1.08 and 1.36 µm CW laser oscillation of an uncoated ceramic sample was experimentally investigated. Both achieved the highest slope efficiency and output power so far reported for the Nd:Y2O3 ceramic lasers. For the operation of 4F3/2-4I11/2 transitions, simultaneous oscillations at 1074.6 nm and 1078.8 nm were observed. At an absorbed pump power of 7.0 W, a maximum output power of 3.01 W was obtained with a slope efficiency of 49.6%. For the laser operation of 4F3/2-4I13/2 transitions, a 1.68 W single wavelength at 1357.7 nm with the slope efficiency of 29.3% was achieved at the absorbed pump power of 6.7 W.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Sesquioxide materials have attracted increasing attention as solid-state laser hosts for their favorable properties such as stability, ruggedness, refractoriness and optical clarity over a broad spectral region [1–4]. Among the sesquioxides, researches on using Y2O3 single crystals as laser hosts could even be traced back to 1964 [1]. The thermal conductivity of Y2O3 is about twice of that of the YAG, and their thermal expansion coefficients are similar [5]. The high thermal conductivity and small thermal expansion coefficient of Y2O3 are especially important for high power solid-state laser development. The Nd3+ doped sesquioxides have attractive emission lines around 0.9 μm, 1.08 μm and 1.4 μm. In particular, in Nd:Y2O3 because the transitions at 1075 nm and 1079 nm have comparable stimulated emission cross section, simultaneous laser emission of the two wavelengths could be easily achieved. Such a dual-wavelength laser could have potential applications in coherent terahertz generation based on the difference frequency generation technique [6,7]. It was also proposed to use such a dual-wavelength laser in the differential absorption lidar systems for remote sensing of ozone [8].
However, it is extremely difficult to grow large-size and high optical quality Y2O3 single crystal due to its high melting point of 2430°C, and a structure phase transition at 2280°C [1,2,9]. It is possible to fabricate high optical quality Y2O3 transparent ceramics, because ceramic sintering is carried out at a temperature much lower than its melting point [10]. In addition, compared with single crystal growth, laser ceramic fabrication has the advantages of low production cost, short production period, large sample size and possible of flexible doping profiles and composite structures [11–14]. Different transparent ceramic fabrication methods such as vacuum sintering [15], hot isostatic pressing [16] and Spark plasma sintering [17,18] have been developed. In 2001, Lu et al reported the first Nd:Y2O3 ceramic laser with a 1.5 at% Nd3+-doping concentration ceramic sample fabricated using the vacuum sintering method [9]. Under the pump power of 742 mW, the laser achieved a maximum output power of 160 mW with a slope efficiency of 32%. Although since then a number of works on the fabrication and characterization of Nd:Y2O3 ceramics were conducted [19–23], no further progress on the Nd:Y2O3 ceramic laser was reported. Most recently, laser oscillations of the Nd:Lu2O3 ceramics fabricated by the Spark plasma sintering method were reported [6]. Xu et al demonstrated 1.25 W dual-wavelength operation from the 4F3/2-4I11/2 transitions and 200 mW single wavelength emission at 1360 nm from the 4F3/2-4I13/2 transitions.
We have been intensively working on the fabrication of high quality sesquioxide based laser ceramics [24]. Recently we have successfully developed a fabrication procedure to produce high lasing efficiency Nd:Y2O3 ceramic based on the vacuum sintering plus hot isostatic pressing method. In this paper we report on the CW laser performance of the in house fabricated 0.6 at% Nd3+-doping concentration Nd:Y2O3 laser ceramics. Highly efficient 1.08 and 1.36µm CW laser operation of the laser ceramics based on the 4F3/2-4I11/2 and 4F3/2-4I13/2 transitions were demonstrated, respectively. In our experiments an uncoated Nd:Y2O3 ceramic sample was used. For the 4F3/2-4I11/2 transition dual-wavelength lasing simultaneously at 1075 nm and 1079 nm was observed, and a maximum output power of 3.01W was achieved under an absorbed pump power of 7.0 W. The slope efficiency of the laser is as high as 49.6%. For the 4F3/2-4I13/2 transitions a maximum output power of 1.68 W with a slope efficiency of 29.3% in single wavelength at 1358 nm was obtained under an absorbed pump power of 6.7 W. In both cases the output power could be further increased if higher pump power was used. Our experimental results clearly show the high quality of the Nd:Y2O3 ceramics fabricated. It also shows the great potential of the laser gain medium for the practical high efficiency high power laser applications and other laser applications such as remote sensing.
2. Nd:Y2O3 ceramic and experimental setup
Nd:Y2O3 ceramics with 0.6 at.% Nd3+ concentration were fabricated in-house by using vacuum sintering plus hot isostatic pressing process, which is similar as that used to fabricate the Ho:Y2O3 ceramics [24]. Briefly, Nd(NO3)3 and YCl3 solutions (derived from the 99.995% purity Nd and 99.999% purity Y2O3 powders, Jiahua Advanced Material Resources Co., Ltd, China) were mixed according to the formula (Nd0.006Y0.994)2O3 as mother solution. Then, precipitants, the mixed solution of NH4HCO3 + NH4OH was added dropwise into the mother solution, until the pH value reached 8. After aging for 24 h, the precipitate was washed with deionized water and ethanol, respectively. After that, it was dried and calcined at 1200 °C for 5 h in a muffle furnace. The calcined powders were then formed into green compacts. The compacts were vacuum sintered at 1525 °C and HIPed at 1570 °C at 198 MPa in argon. Finally, the sintered and HIPed ceramics were annealed at 1050 °C for 10 h to remove the color centers. A typical annealed Nd:Y2O3 ceramic sample is shown in the inset in Fig. 1.
The in-line transmittance of a 3.0 mm thickness Nd:Y2O3 ceramic sample was measured by a UV-VIS-NIR spectrometer (Carry 5000, Agilent, Santa Clara, CA) at room temperature. Figure 1 shows the transmission spectrum of the Nd:Y2O3 ceramic over the wavelength range of 200–2000 nm. The sample exhibited excellent in-line optical transmittance. The measured transmittance at 1000 nm wavelength is 81.6%, which is the same as the calculated value of the Y2O3 single crystals. Based on the dispersion formula of the Y2O3 single crystal [25], its refractive index at 1000 nm is 1.9028. Based on the formula T = 100[1 − ((n − 1)/(n + 1))2]2 (n is the refractive index), the inline transmittance at the wavelength is estimated 81.6%. As reported in ref [6], the maximum absorption peak of the Nd:Y2O3 ceramic in the 800 nm band is at about 820 nm, the secondary absorption peak is at 806.6 nm.
The measured fluorescence spectrum of the Y2O3 ceramic is shown in Fig. 2. The group with strong fluorescence emission lines corresponds to the 4F3/2-4I11/2 transitions. The two strong emission peaks, located at 1075 nm and 1079 nm, respectively, have comparable spectral intensity and are closely spaced with each other. The two lines correspond to the Stark transition R2-Y1 and R2-Y2, respectively. The two lines could easily oscillate simultaneously and leads to dual wavelength emission of the laser. There is also a group of fluorescence emission lines corresponding to the 4F3/2-4I13/2 transitions with the strongest emission peak located at 1358 nm. In the following work, both the laser oscillations from the 4F3/2-4I11/2 and 4F3/2-4I13/2 transitions are studied.
Figure 3 shows the schematic diagram of the diode end-pumped Nd:Y2O3 ceramic laser. The Nd:Y2O3 ceramic was uncoated and had a Nd3+-doping concentration of 0.6-at. % and 3 × 3 × 3 mm3 in size. The ceramic was wrapped with indium foil and fixed on a copper holder whose temperature was controlled at 20°C. A fiber-coupled laser diode array with a numerical aperture of 0.22 and a fiber core diameter of 200 µm was used as the pump source. The pump diode has a line width of about 3 nm and its central wavelength can be tuned from 803 to 808 nm depending on the current level. The pump beam was re-imaged by two achromatic lenses with focal lengths of 50 and 80 mm, respectively, and focused into the Nd:Y2O3 ceramic. The laser cavity composed of two mirrors IM and OC and the cavity length was kept at about 15 mm. The input mirror IM was high-transmission (HT)-coated at 808 nm (T>96%) and high-reflection (HR)-coated at the oscillating laser wavelength. The output mirror OC was partial reflection coated at the oscillating laser wavelength. In the experiment we used mirrors with different specification parameters to optimize the laser operation with the 4F3/2-4I11/2 and 4F3/2 - 4I13/2 transitions.
3. Experimental results and discussion
Firstly, the continuous-wave laser operation of the 4F3/2-4I11/2 transitions was investigated. A plane-concave mirror with a radius of curvature of 320 mm was used as the pump input mirror IM, and six plane output couplers with different transmittance (T = 10, 15, 20, 30, 45 and 50%) around 1.08 µm were used for comparison. Figure 4 shows the output power versus the incident pump power with the output power of the laser at 13.2 W incident pump power being optimized. The lowest threshold and the highest output power of 2.6 W was obtained with the output coupler whose transmittance is 10%, suggesting that the output power of the laser might be further improved if an output coupler with smaller transmittance could be used. The absorbed pump power was measured when the laser system was in operation. The thresholds with respect to the absorbed pump power of the lasers were 0.77, 1.04, 1.33, 1.76, 3.07 and 3.9 W for laser setup using above six output couplers, reflectivity. We have estimated the total round-trip cavity loss by using the Findlay and Clay method [26]. As the fitting line shown in Fig. 5, the estimated total cavity round-trip losses were about 0.025. In order to examine the optical homogeneity of the ceramic sample, at the maximum output power operation of the laser of around 2.6 W, we moved the position of the Nd:Y2O3 ceramic in the cavity and forced the laser to use different portion of the ceramic. The output power only exhibited fluctuation of less than 0.1W, indicating that the fabricated ceramic also has good homogeneity.
Fig. 4 Output power versus incident pump power of the laser with output coupler of different reflectivity at 1.08 µm.
We then focused the experiment with the output coupler of 10% transmittance. Figure 6 shows the output power of the laser versus the absorbed pump power for the 4F3/2-4I11/2 transitions. A maximum output power of 3.01 W was achieved at an absorbed pump power of 7.0 W under the optimized laser operation condition. In our experiment the absorbed pump power couldn’t be further increased as the pump wavelength deviated from the absorption peak of 806.6 nm. No pump saturation was observed in our experiment, suggesting that even higher output power could be obtained if the absorbed pump power could be further increased. In Fig. 6 the slope efficiency with respected to the absorbed pump power was about 49.6%.
Fig. 6 Output power and laser spectrum of the laser operation with 4F3/2-4I11/2 transition by increasing the pump power.
The laser oscillation spectrum was monitored by a monochromator with a resolution of 0.05 nm. Dual-wavelength laser oscillation with emission wavelength at 1074.6 and 1078.8 nm, respectively, was detected. The ratio of their spectral intensities varied with the pump power. The insets of Fig. 6 show the laser spectra measured under the absorbed pump power of about 1.2 and 7.0 W. As the absorbed pump power increased, the laser began to oscillate at 1075 nm from the threshold, the 1079 nm line started to oscillate at an absorbed pump power of about 1.2 W, then the 1079 nm line intensity increased with the pump power. Following the trend it is expected that balanced intensity emission at 1075 and 1079 nm could be achieved if higher absorbed pump power is available.
The continuous-wave laser operation on the 4F3/2-4I13/2 transitions was also experimentally investigated by replacing the cavity mirrors with those coated for the 1.3 µm laser operation. In this experiment two concave output couplers with 100 mm radius of curvature and different transmittance (T = 4% and 8%) around 1.36 µm was used for comparison. Figure 7 shows the output power of the laser versus the absorbed pump power. The thresholds were about 0.7 and 1.3 W, and the output power increased with the absorbed pump power with slope efficiencies of 29.3% and 25.2% for the output couplers with transmittance of 4% and 8%, respectively. A maximum output power of 1.68 W was achieved at an absorbed pump power of 6.7 W. The measured laser spectrum was shown in the inset of Fig. 7. The stronger fluorescence emission lines in the 4F3/2-4I11/2 transitions was suppressed, only single wavelength emission at 1357.7 nm was detected.
4. Conclusion
In conclusion, we have demonstrated highly efficient continuous-wave laser operation of a diode pumped Nd:Y2O3 ceramic laser. A 0.6 at.% Nd3+ concentration Nd:Y2O3 ceramic was in house fabricated using the vacuum sintering plus hot isostatic pressing method. The Nd:Y2O3 ceramic has not only excellent in-line transmittance which is as good as its single crystal counterpart, but also excellent optical homogeneity. Both the CW laser performance on the 4F3/2-4I11/2 and the 4F3/2-4I13/2 transitions was experimentally investigated. For the 4F3/2-4I11/2 transitions, simultaneous emission at two spectral lines at 1074.6 nm and 1078.8 nm was obtained. A maximum output power of 3.01 W with slope efficiency of 49.6% was obtained at an absorbed pump power of 7.0 W. For the 4F3/2-4I13/2 transitions, single wavelength oscillation at 1357.7 nm was obtained. A maximum output power of 1.68 W with slope efficiency of about 29.3% was achieved at an absorbed pump power of 6.7 W. To the best of our knowledge, these are the highest output powers and slope efficiencies of the Nd:Y2O3 ceramic lasers ever achieved at the lasing wavelengths. Our experimental results demonstrated that excellent quality Nd:Y2O3 laser ceramics could be fabricated by the vacuum sintering plus hot isostatic pressing process, and the Nd:Y2O3 laser ceramics can be promise for the high power and high efficient laser applications.
Funding
A*Star AME IRG, Singapore (A1883c0003); Minister of Education (MOE), Singapore (2016-T1-001-026); National Natural Science Foundation of China (NSCF) (61505147); Zhejiang Province Public Welfare Technology Application Research Project (2017C34008); Research Funds of College Student Innovation of Zhejiang Province (2017R426020).
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