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Abstract
We have demonstrated the continuous-wave operation of a highly efficient 2.8 μm Er-doped Lu2O3 ceramic laser at room temperature. An Er:Lu2O3 ceramic with a doping concentration of 11 at.% afforded a slope efficiency of 29% and an output power of 2.3 W with pumping at 10 W. To our knowledge, these are the highest slope efficiency and output power obtained to date for an Er:Lu2O3 ceramic laser at 2.8 μm. In addition, we prepared ceramics with various doping concentrations and determined their emission cross sections by fluorescence lifetime measurements and emission spectroscopy.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-IR lasers with wavelengths of around 3 μm have many potential industrial and medical applications, such as surgery [1] and spectroscopy [2], because of the strong absorption of these wavelengths by water. The laser processing of glasses and plastics using these wavelength sources is also attractive due to the specific absorption properties of these materials [3,4]. Er3+-doped lasers are currently the most efficient way to obtain lasing in the 3 μm wavelength region [5]. We have developed 24 W continuous-wave (CW) output [6] and 12 W Q-switched Er-doped ZBLAN fiber lasers [7]. However, fluoride glass has disadvantages, including unfavorable thermal properties, low mechanical strength, and poor moisture resistance. Recently, Er-doped cubic rare-earth sesquioxide crystals (e.g., Lu2O3, Y2O3, and Sc2O3) have attracted attention as high-power mid-IR laser sources, owing to their lower phonon energies and higher thermal conductivities compared with those of fluoride glass and yttrium aluminum garnet (YAG) [8]. In particular, crystalline Er:Lu2O3 is a suitable host material because of its high thermal conductivity, even at high doping levels, due to the similar atomic masses and ionic radii of the Lu3+ and Er3+ ions [9]. In the case of Er3+-doped lasers, a high doping level that causes energy transfer upconversion (ETU) from the 4I13/2 state is more favorable for efficient lasing at 2.8 μm. A maximum CW power of 5.9 W and a slope efficiency of 27% at 2.85 μm have been reported for a 7% Er-doped Lu2O3 laser [9]. However, the fabrication of high-quality Lu2O3 single crystals is generally difficult, owing to the high melting point of approximately 2490 °C and slow growth rate. High-quality polycrystalline transparent ceramics of the sesquioxide have recently become available and possess numerous advantages compared with the single crystals, such as excellent mechanical strength and thermal properties, opening up the possibility of efficient high-power mid-IR lasers. Polycrystalline ceramics can also be mass-produced cheaply as large-volume crystals. Ceramic lasers using YAG, Lu2O3, Y2O3, Sc2O3, and LuAG with Yb3+, Nd3+, and Tm3+ dopants have been demonstrated and their output power is increasing [10–13]. In addition, the CW operation at 2.8 μm of an Er:Y2O3 ceramic laser with an output power of 14 W has been reported with cooling at 77 K [14]. Er:Lu2O3 ceramic lasers in CW operation with an output power of up to 1.3 W [15,16] and Q-switched operation [17] have also been reported by Shen and associates. In a previous work, we demonstrated the room-temperature CW operation of an Er:Lu2O3 ceramic laser with a slope efficiency of 22% and an output power of 0.5 W [18]. This slope efficiency is the highest value reported so far for an Er:Lu2O3 ceramic laser. In addition, we performed time-resolved spectroscopy, which revealed that the emission wavelengths exhibited a redshift.
In this letter, we report on the preparation of Er:Lu2O3 ceramics with doping concentrations of 5, 10, 11, and 15 at.% and the evaluation of their lasing characteristics and optical properties. As a result, a slope efficiency of 29% and 2.3 W output by 10 W pumping were achieved for the ceramic laser with a doping concentration of 11 at.%. To the best of our knowledge, these are the highest values of slope efficiency and output power obtained so far for an Er:Lu2O3 ceramic laser. The lifetimes of the upper (4I11/2) and lower (4I13/2) laser levels of Er3+ in Lu2O3 were found to be strongly affected by the Er3+ concentration, as reported previously [13]. The ETU process led to a more favorable ratio of the lifetimes of the upper and lower laser levels for high doping concentrations. The fluorescence lifetimes were also measured and the emission cross sections were determined. The effects of the Er3+ concentration on the lasing performance were elucidated and a variety of techniques such as optical loss measurement and emission spectrum analysis were used to successfully distinguish the properties of these high-quality ceramics for laser emission.
2. Optical properties of Er:Lu2O3 transparent ceramics
Er-doped Lu2O3 polycrystalline transparent ceramics (Konoshima Chemical Co., Ltd.) with various Er3+ concentrations of 5, 10, 11, and 15 at.% were used. An efficient high-power laser was fabricated [11] with Yb-doped Lu2O3 ceramic by the same method [19], owing to its excellent optical properties. The optical properties of the Er:Lu2O3 ceramics were measured using a halogen lamp and an optical spectrum analyzer (Q8381A, Advantest) with a spectral resolution of 0.2 nm.
Figure 1 shows the absorption spectra of the ceramics at the 0.97 μm absorption band due to the 4I15/2 → 4I11/2 transition of Er3+ ions used for laser pumping. The absorption coefficient was calculated using the Fresnel reflection loss of an undoped Lu2O3 crystal estimated from the refractive index [20]. A typical transmittance spectrum of an Er:Lu2O3 ceramic with a broad wavelength range was shown in our previous report [18]. Here, regardless of the doping concentration, the crystals were completely transparent (<0.3% loss) at the laser emission wavelength of 2.8 μm. absorption bands due to the Er3+ ions were observed, the baselines revealed a high transmittance level with less than 1% loss at wavelengths greater than 900 nm. The spectra contained many narrow peaks compared with Er-doped fluoride glasses [21], and these separated peaks arising from the Stark effect show good agreement with the reported spectra for Er:Lu2O3 single crystals [9]. In the spectra shown in Fig. 1, two typical absorption peaks exhibiting relatively-low absorbance where measurement error is small are indicated by purple and red circles. The absorption coefficients of these peaks are plotted against the doping concentration of Er3+ in the inset of Fig. 1. The high reliability of these Er3+ concentration values was demonstrated because the absorption coefficient was proportional to the concentration in . The absorption cross sections at a wavelength of 970 nm were in the range 2.2–2.4 × 10−21 cm2 for all of the doping concentrations tested.
Fig. 1 Absorption spectra of the Er:Lu2O3 ceramics at room temperature. Inset: Absorption coefficients at the wavelengths corresponding to the two peaks indicated in the spectra plotted against the Er3+ doping concentration.
Even though the transmission loss increased with decreasing wavelength, the loss at 500 nm was still only approximately 5% for the 11 at.% sample, which is much smaller than the values reported for other Lu2O3 transparent ceramics [22–24]. Figure 2 shows plots of the extinction curves of the absorption baselines in the visible to near-infrared region against the wavelength. The plotted wavelengths were selected based on they have no Er3+ absorption and are longer than the UV absorption edge. The power-law fitting for the extinction curves was performed using the following equation:
where λ is the wavelength (nm) and the parameters a, b, and c obtained by the fitting are summarized in Table 1. For all of the doping concentrations, the extinction was found to be in good agreement with a fitting curve inversely proportional to the fourth power of wavelength. This indicates that the optical loss was derived from Rayleigh scattering due to nanometer-sized defects [25]. The parameter a, which indicates the scattering strength, had a range of values for the four doping concentrations and the 11 at.% sample exhibited a particularly low loss, as also shown in Fig. 2. The small residual porosity probably caused the Rayleigh scattering and decreased the optical quality [26], which may affect the laser performance [27].Fig. 2 Extinction curves of the absorption baselines of the Er:Lu2O3 ceramics with various doping concentrations.
Table 1. Power-law fitting parameters for the extinction curves of the Er:Lu2O3 ceramics.
3. Room-temperature CW operation of Er:Lu2O3 ceramic lasers
To investigate the lasing properties, samples of the Er:Lu2O3 ceramics with a length of 8 mm were pumped using a fiber-coupled laser diode (LD; model:e06.0550976105, nLight) with a center wavelength of 971 nm in a plane–plane resonator, as shown in Fig. 3. The pump laser was passed through a pump mirror (high transmission at 970 nm, high reflection at 2.8 μm) and focused on the ceramic as a spot about 300 μm in diameter. An output coupler (OC) with a transmittance of 2%, 5%, or 8% at 2.8 μm and a 2.5‒3.1 μm band-pass filter were used. The ceramics were actively cooled in the crystal holder using a water flow at 20 °C. The output power and beam profile were measured using a thermopile power meter (3A, Ophir) and an IR camera (Pyrocam III, Spiricon), respectively. The lasing spectra were also measured using an optical spectrum analyzer (OSA205C, Thorlabs) with a wavelength resolution of about 0.1 nm. The cavity length was 20 mm, which was optimized to achieve the highest laser performance.
Figure 4 shows the output power as a function of the absorbed pump power at the three OC transmittances for CW operation of the four Er:Lu2O3 ceramic lasers with different doping concentrations. The absorbed pump power was defined by measured transmitted LD power. The output power increased linearly as the pump power was increased, and the laser threshold increased with increasing OC transmittance. The use of the OC with a transmittance of 5% afforded the highest slope efficiencies of 22%, 23%, 29%, and 12% for the doping concentrations of 5 at.%, 10 at.%, 11 at.%, and 15 at.%, respectively. For the 15 at.% sample, laser emission was not observed using the OC with a transmittance of 8% owing to the high optical loss. The highest slope efficiency of 29% and output power of 1.0 W at a pump power of 4.7 W were obtained for the 11 at.% Er:Lu2O3 ceramic. To the best of our knowledge, this is the highest slope efficiency obtained to date for an Er:Lu2O3 ceramic laser. Furthermore, it is higher than the maximum efficiency of an LD-pumped Er:Lu2O3 single-crystal laser [9], which confirms that this polycrystalline transparent ceramic has excellent optical and thermal properties. However, a much higher slope efficiency of 36% has been reported for a Er:Lu2O3 single crystal using a narrow-band pump source (<0.3 nm) with a wavelength of 971 nm because the bandwidth of the absorption peak is narrow (<1 nm) and the excited-state absorption is relatively weak at this wavelength [9]. In our plane-plane system, the output power was almost stable less than 30 mm length because of thermal focusing, and it decreased with increasing cavity length due to diffraction loss. Figure 5 shows the lasing spectra of the 11 at.% Er:Lu2O3 ceramic at various pump powers. At a pump power of 1.2 W, which is close to the laser threshold, the output wavelengths were 2715 and 2725 nm. At a pump power of 1.5 W, the output wavelengths moved to 2740 and 2845 nm. Finally, the laser emission at 2845 nm became dominant at pump powers greater than 1.5 W. This jump from a shorter to a longer wavelength is derived from a regime change from four-level to quasi-three-level lasing. We performed a detailed time-resolved spectroscopy study of this phenomenon in Er:Lu2O3 ceramics in our previous report [18]. The output beam was similar to a Gaussian beam and a typical output beam profile is presented in the inset of Fig. 5. Under CW conditions, the effective focal length of thermal focusing [28] in the Er:Lu2O3 ceramics was estimated to be about 50 mm, assuming that the absorbed pump power was 4 W. For a resonator with a length of 20 mm, the calculated Gaussian mode diameter was about 340 μm, which was close to the pump beam diameter of 300 μm; thus, good spatial mode matching in the gain medium was expected. The slopes of the output curves using the OC with a transmittance of 8% shown in Fig. 4(a)–(c) and the two curves in Fig. 4(d) clearly changed in the middle. This slope change was caused by the wavelength jumping from 2725 nm to 2845 nm. For the OCs with transmittances of 2% or 5%, this wavelength jump occurred at a much lower pump power.
Fig. 4 Output power as a function of absorbed pump power at various OC transmittances for CW operation of the Er:Lu2O3 ceramic lasers. The Er3+ doping concentrations were (a) 5 at.%, (b) 10 at.%, (c) 11 at.%, and (d) 15 at.%.
Fig. 5 Laser output spectra for the 11 at.% Er:Lu2O3 ceramic at various pump powers. Inset: Typical intensity profile of the output beam measured at a distance of 300 mm from the OC.
To achieve an output power higher than 1 W, a sample of the 11 at.% Er:Lu2O3 ceramic with a length of 5 mm was pumped up to 10 W absorbed power. The effective focal length of thermal focusing decreases with increasing pump power, resulting in a reduction of the stable resonator length. As such, the cavity length was reduced to 12 mm, the shortest value possible, to suppress the diffraction loss. Figure 6 shows the output power as a function of absorbed pump power. An output power of 2.3 W was achieved under 10 W pumping, which represents the highest CW output power obtained for an Er:Lu2O3 ceramic laser. The total slope efficiency was 26%, though the curve had an undulating shape, and the laser threshold was 1.0 W. The undulating curve shape was ascribed to the fact that the laser diode used in this experiment was not wavelength stabilized, and the center wavelength was found to undergo a redshift from 970 to 973 nm with a linewidth of about 4 nm in the output range of 10 W. Such a redshift of the excitation wavelength results in absorption modulation, causing the undulating shape of the output curve. Considering the absorption peaks observed in Fig. 1, the excitation wavelength would be expected to overlap with the absorption peaks at 970 or 974 nm for the high-slope regions (pump powers of <4 W and >8 W). Conversely, the excitation wavelength was located within the depression between the two absorption peaks when the slope was relatively low at a pump power of around 6 W. The slope efficiency for pump powers in the range of 8–10 W was 29%, which is close to the efficiency at pump powers below 5 W. The possibility exists to achieve much higher output power by increasing the pump power, since no reduction of the curve slope due to thermal loading was observed even at a pump power of 10 W. The efficiency of our ceramic laser could also be further improved by using a suitable wavelength-stabilized pump source with a narrow bandwidth. Furthermore, the ceramics used in these experiments were uncoated, resulting in approximately 10% Fresnel reflection at both surfaces, which would be expected to decrease the efficiency. The application of antireflective coatings to the ceramic surfaces may therefore further improve the laser performance.
Fig. 6 Output power as a function of absorbed pump power for the 11 at.% Er:Lu2O3 ceramic laser using the OC with a transmittance of 5%.
4. Emission cross sections of the Er:Lu2O3 ceramics
We have demonstrated the highest output power and slope efficiency reported to date using the 11 at.% Er-doped Lu2O3. The lifetimes of the upper (4I11/2) and lower (4I13/2) laser levels of the Er3+ in Lu2O3 will be strongly affected by the Er3+ concentration, owing to the ETU process that occurs at high doping concentrations [9]. To investigate the effects of the Er3+ concentration on the lasing performance, the fluorescence lifetimes and emission spectra were measured at various Er3+ concentrations. The Er:Lu2O3 ceramics used in the lasing experiments were pumped using the same LD with a pulse duration of 100 µs and a fall time of <10 μs, which is sufficiently shorter than the fluorescence lifetime. The temporal decay of the fluorescence was measured at the 2.8 µm and 1.6 µm wavelength bands using an InAs photodetector (C12492-210, Hamamatsu Photonics) and an oscilloscope fitted with band-pass filters with wavelengths of 2500-3000 nm and 1500-2000 nm. The emission spectra were measured with CW pumping and detected using an optical spectrum analyzer (OSA205C, Thorlabs).
Figure 7 shows the lifetimes of the upper and lower levels plotted against the Er3+ concentration. The fluorescence at 2.8 µm and 1.6 µm corresponds to the 4I11/2 → 4I13/2 and 4I13/2 → 4I15/2 transitions, respectively. The lifetime of the upper laser level was determined by a single-exponential fitting of the fluorescence decay curve at 2750 nm. For the fluorescence kinetics at 1540 nm, double-exponential fitting of the rise and decay was performed, and the decay-time constant was considered to represent the lifetime of the lower level. The rise-time constant agrees well with the upper level lifetime, because the 4I13/2 level is populated by depopulation of the 4I11/2 level after pumping at 970 nm. The measured lifetimes are plotted as star symbols in Fig. 7 and were generally found to decrease with increasing Er3+ concentration in the ranges of 1.5–1.2 ms (upper level) and 5.3–3.6 ms (lower level) for concentrations from 5 to 15 at.%. The reduction observed for the lower level is due to the ETU process or concentration quenching. In contrast, the ETU process should increase the lifetime of the upper level, indicating that the main reason for the observed decrease is concentration quenching. The lifetimes of Er:Lu2O3 ceramics [29] and single crystals [9] have been reported previously and for comparison these results are also plotted in Fig. 7 as circles and squares, respectively. For the previously reported single crystals, the lifetime of the lower level was found to decrease dramatically between 2 and 9 at.% owing to the ETU effect. Simultaneously, concentration quenching also became dominant at higher concentrations since the lifetime of the upper level exhibited a drastic decline between 2 and 9 at.%. For the previously reported ceramics, the lifetimes of both levels decreased relatively gradually compared with that of the single crystals, although the change was still drastic compared with our results. The comparatively minor changes in the lifetimes observed here indicate that the Er3+ ions were distributed relatively homogeneously in the Lu2O3 ceramic even at high doping concentrations [30]. In our ceramics, the lifetime ratios between the upper and lower laser levels were in the range of 0.29–0.35, which is considered unfavorable for laser transitions even at the high end of this range [31,32]. The lasing performance may not be affected by such a small difference in lifetime ratio. The lifetimes of the upper and lower levels increased only for the 11 at.% sample owing to reduced concentration quenching.
Fig. 7 Lifetimes of the upper (4I11/2, red symbols) and lower (4I13/2, blue symbols) laser levels plotted against the Er3+ concentration for the Er:Lu2O3 ceramics. The results reported in the literature for Er:Lu2O3 ceramics [33] and single crystals [13] are also plotted for comparison.
The emission cross sections were determined from the measured fluorescence lifetimes and emission spectra using the Füchtbauer-Ladenburg equation [35]:
where Iα(λ) is the emission intensity for α (π or σ) polarization, τr is the radiative lifetime, β is the branching ratio, n is the refractive index, and c is the speed of light in vacuum. The branching ratio of 2800 nm emission were calculated by Judd–Ofelt theory [33,34] using the Judd–Ofelt parameter estimated from absorption coefficient spectrum, which were 10.43, 8.97, 9.91 and 8.22% at 5, 10, 11 and 15 at.%, respectively. These ratios were close to the reported value of 10.4% in literature [36] and decreased with increasing doping concentration (except 11 at.%) due to concentration quenching. Figure 8 shows the emission cross-section spectra of the Er:Lu2O3 ceramics at various doping concentrations for the 2.8 μm and 1.6 μm wavelength bands. Similar to the absorption spectra shown in Fig. 1, the emission spectra at 2.8 μm shown in Fig. 8(a) also contained many narrow separated peaks arising from the Stark effect and agree well with the reported emission spectra for Er:Lu2O3 single crystals [9]. The spectral shapes and cross sections at 2.8 μm were consistent for all of the Er3+ concentrations studied, and the cross section at 2845 nm was approximately 1.3‒1.5 × 10−20 cm2. Emission cross-sections of various Er3+-doped crystals were summarized in the reference [37] and their values are in the range of 0.43‒2.9 × 10−20 cm2 at 2.8 μm wavelength band. In contrast, differences were observed between the emission spectra at 1.6 μm recorded for the ceramics with different Er3+ concentrations, as shown in Fig. 8(b). For the 10 and 15 at.% samples, the shorter wavelength emission in the range of 1520–1560 nm was clearly suppressed, owing to overlap with the ground-state absorption with a large cross section of about 5.0 × 10−21 cm2. This suppression was caused by re-absorption, which is one of the reasons for concentration quenching. Indeed, a different spectrum with a large emission cross section in the shorter wavelength region was observed for the 11 at.% sample despite the high doping concentration. This result indicates that the Er3+ ion distribution was homogeneous and the degree of ion clustering was low in the 11 at.% sample, resulting in the suppression of quenching in accordance with the results of the fluorescence lifetime measurements.Fig. 8 Emission spectra of the Er:Lu2O3 ceramics with various doping concentrations for the (a) 2.8 μm and (b) 1.6 μm wavelength bands.
Considering the optical properties discussed in Section 2, the 11 at.% ceramic stands out from all of the other samples in terms of its transparency and reduced concentration quenching, owing to the homogeneous distribution of Er3+ ions. These characteristics of the high-quality transparent ceramic enabled the superior lasing performance discussed in Section 3. In contrast, the sample with the highest doping concentration of 15 at.% exhibited quite low performance, which was ascribed to concentration quenching derived from ion clustering [32]. The reason why the slope efficiencies of 2 and 8% transmittance quite low in 10 at.% sample is also were considered to be concentration quenching as with 15 at.%. In this study, we were able to successfully distinguish the high-quality ceramics for laser emission using a variety of techniques including optical loss measurement, fluorescence lifetime determination, and emission spectrum analysis.
5. Conclusion
We have successfully demonstrated the room-temperature CW operation of Er:Lu2O3 ceramic lasers with a wavelength of 2.8 μm. For the ceramic with a doping concentration of 11 at.%, a slope efficiency of 29% and an output of 2.3 W with pumping at 10 W were achieved. To the best of our knowledge, these are the highest slope efficiency and output power obtained for an Er:Lu2O3 ceramic laser. The maximum slope efficiencies for the ceramics with doping concentrations of 5 at.%, 10 at.%, 11 at.%, and 15 at.% were 22%, 23%, 29%, and 12%, respectively. From the optical loss measurements in the visible to near-infrared region, the 11 at.% sample was found to exhibit particularly low scattering loss in the visible region caused by residual porosity in the ceramics. The emission cross sections at wavelengths of 2.8 μm and 1.6 μm were determined by fluorescence lifetime measurements and emission spectroscopy. The 11 at.% sample, which demonstrated the best lasing performance, indeed exhibited a notably different emission spectrum that indicated low concentration quenching as a result of the homogeneous distribution of Er3+ ions. Based on these results, we suggest the use of a variety of techniques to distinguish high-quality ceramics for laser emission. There still exists the possibility to achieve much higher output power and efficiency by increasing the pump power and using a suitable wavelength-stabilized pump source with a narrow bandwidth. The development of a high-quality Er:Lu2O3 ceramic that can be mass-produced cheaply as large-volume crystals will be of great value for the fabrication of efficient high-power mid-IR lasers.
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI (26709072, 15K13386, 15K04696); Japan Science and Technology Agency (JST) PRESTO (15666084).
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