1. Introduction

High-power mid-IR lasers emitting at wavelengths of around 3 μm have many potential applications in industry and medicine, including surgery [1,2], spectroscopy [3,4], and fabrication [5,6], because of the strong absorption of these wavelengths by water. Er lasers are currently the most efficient way to obtain lasing in the 3 μm wavelength region [7]. Fluoride glass fiber is a suitable host material for Er lasers owing to its low phonon energy. We have developed a 24 W output Er-doped ZBLAN fiber laser [8]. However, fluoride glass has disadvantages, including poor thermal properties, mechanical strength, and moisture resistance. Er-doped cubic rare-earth sesquioxide crystals (e.g., Lu₂O₃, Y₂O₃, and Sc₂O₃) are attracting attention as high-power mid-IR laser sources due to their lower phonon energies and higher thermal conductivities compared with fluoride glass and yttrium aluminum garnet (YAG) [9]. In particular, crystalline Er:Lu₂O₃ is a suitable host material because its thermal conductivity remains high, even at high doping levels, owing to the similarity of the atomic masses and ionic radii of Lu³⁺ and Er³⁺ ions [10]. In Er lasers, the high doping level that causes energy transfer upconversion from the ⁴I_13/2 state is more favorable for efficient lasing at 2.8 μm. A maximum continuous wave (CW) power of 5.9 W and slope efficiency of 27% at 2.85 μm have been reported for a 7% Er-doped Lu₂O₃ laser [9]. However, fabrication of high-quality Lu₂O₃ single crystals is generally difficult owing to its high melting point of ~2490 °C and slow growth rate. High-quality polycrystalline transparent ceramics of the sesquioxide, which have become available recently, open up the possibility of efficient high-power mid-IR lasers because of their advantages, such as excellent mechanical strength and thermal properties, compared with single crystals. 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 the output power is increasing [11–14]. In addition, 2.8 μm Er³⁺-doped ceramic lasers have been investigated. CW operation of an Er:Y₂O₃ ceramic laser with an output power of 14 W has been reported with cooling at 77 K [15]. Er:Lu₂O₃ ceramic lasers have also been reported with a slope efficiency of 11.9% and an output power of 1.3 W which are relatively-low compared with single crystals [16,17].

In this letter, we demonstrated a 2.8 μm Er-doped Lu₂O₃ ceramic laser and investigated the lasing dynamics by time-resolved spectroscopy. During room-temperature CW operation, a slope efficiency of 22% was achieved with a high-quality transparent ceramic. To our knowledge, this is the highest value obtained by an Er:Lu₂O₃ ceramic and is comparable to a single crystal. The emission wavelength exhibited a temporal red shift from 2715 to 2845 nm. We suggest that this high-quality optically transparent ceramic of Er-doped sesquioxide has great potential for developing high-power mid-IR lasers.

2. Optical properties of Er:Lu₂O₃ transparent ceramic

An 11 at. % -Er-doped Lu₂O₃ polycrystalline transparent ceramic (Konoshima Chemical Co., Ltd.) was used. An efficient high-power laser was fabricated [12] with Yb-doped Lu₂O₃ ceramic by the same method [18], owing to its excellent optical properties. The optical properties of the Er:Lu₂O₃ ceramic were measured with a halogen lamp and an optical spectrum analyzer (Q8381A, Advantest) with a spectral resolution of 0.2 nm.

Figure 1 shows the transmittance spectrum of a 5-mm-long ceramic sample at room temperature. The predicted maximum transmittance (Fresnel reflection loss) of an undoped Lu₂O₃ crystal calculated from the refractive index [19] is shown (dashed line). The crystal was completely transparent (<0.3% loss) at the wavelength of 2.8 μm used for laser emission. There were absorption bands owing to the Er³⁺ ions, and the base line showed a high transmittance level with less than 1% loss at wavelengths greater than 900 nm. Even though the transmission loss increased with decreasing wavelength, the loss at 500 nm was about 5%, which was much smaller than other reported Lu₂O₃ transparent ceramics [20–22]. The extinction curve of absorption baseline at visible region agrees well with the fitting curve of inversely proportional to the fourth power of wavelength. It means that the optical loss derives from Rayleigh scattering due to nanometer-sized defects. The residual small porous probably decreased the optical quality. The inset in Fig. 1 shows the absorption coefficient of the 1 μm absorption band due to the ⁴I_15/2 → ⁴I_11/2 transition of Er³⁺ ions for laser pumping. The spectrum contained many narrow peaks compared with Er-doped fluoride glasses [23], and these separated peaks arising from the Stark effect agree well with the reported spectra of Er:Lu₂O₃ single crystals [10]. The excitation wavelength range with the laser diode (LD) used in this work was 973 to 979 nm, which overlapped with the one of the strongest absorption peaks at 974 nm with an absorption coefficient of 8 cm⁻¹.

 

Fig. 1 Transmittance spectrum of the 11 at. % Er-doped Lu₂O₃ ceramic 5 mm long at room temperature. Predicted maximum transmittance (Fresnel reflection loss) of an undoped Lu₂O₃ crystal is shown as a dashed line. Inset: Absorption coefficient of the 1 μm absorption band for laser pumping.

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3. Lasing properties and dynamics during quasi-continuous wave operation

To investigate the lasing properties, a Er:Lu₂O₃ ceramic sample 8 mm long was pumped with a fiber-coupled LD (1038929, nLight) with a center wavelength of 976 nm in a plane-plane resonator as shown in Fig. 2. The cavity length was about 12 mm which was reduced as short as possible in order to suppress a diffraction loss. In the first experiment, the LD was operated in quasi-continuous wave (QCW) mode at various pulse durations to avoid thermal effects. The pump laser passed through a dichroic mirror (high transmission at 970 nm, high reflection at 2.8 μm) and was focused on the ceramic as a spot about 300 μm in diameter. An output coupler (OC) with a transmittance of 5% at 2.8 μm and a 2.5‒3.1 μm band-pass filter were used. The output power and temporal waveform were measured with a thermopile power meter (3A, OPHIR) and an InAs photodetector, respectively. The time-resolved spectrum of the laser output was also measured with an InAs photodetector by scanning the detection wavelengths with a grating monochromator (CM110, Spectral Products) with a wavelength resolution of about 1 nm.

 

Fig. 2 Schematic of the setup of the Er:Lu₂O₃ ceramic laser.

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Figure 3(a) shows the output peak power versus the absorbed pump peak power at various pulse durations. The duty cycles were 2% for all pulse durations and the repetition rates were 10, 2, and 1 Hz, resulting in pulse durations of 2, 10, and 20 ms, respectively. The output power increased linearly as the pump power was increased, and the slope efficiency increased and the laser threshold decreased with increasing pulse duration. At a pulse duration of 20 ms, a maximum output peak power of 1.2 W and a slope efficiency of 15% were obtained. The temporal waveforms of the output pulse with an absorbed pump power of 7 W are shown in Fig. 3(b). The zero time when pumping started was defined by the falling edge in the waveform, which corresponded to the end of a QCW square pulse. The Er:Lu₂O₃ ceramic started lasing about 0.5 ms after starting excitation, and large fluctuations in output power were observed until about 2.5 ms under all the pulsed conditions before the waveform became stable. These initial lasing delays meant that it took about 0.5 ms to exceed the lasing threshold with a pump power of 7 W. One of the reasons for the lower efficiency of the shortest pulse duration was this initial delay. The output power fluctuation at 0.5 to 2.5 ms is discussed later by the lasing dynamics results. High-stability CW operation was expected in this ceramic laser system because the output stability is high in the later part (>10 ms) of the waveforms.

 

Fig. 3 (a) Output peak power as a function of absorbed pump peak power for the Er:Lu₂O₃ ceramic laser at various QCW pulse durations. (b) Temporal waveforms of the output pulse with an absorbed pump power of 7 W.

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Figure 4(a) shows a time-resolved spectrum of the laser output with a pump pulse duration of 10 ms and an absorbed pump power of 7 W, which is the same as in Fig. 3(b). Typical spectra chosen from Fig. 4(a) at time region of 1.0, 1.5, 2.5 and 6.0 ms are shown in Fig. 4(b). Laser emission was obtained at four wavelength bands of 2715, 2725, 2740, and 2845 nm, and no other emission was observed in the mid-IR region. There was no emission during the initial 0.5 ms, and then lasing occurred at 2725 nm until 2 ms. A weak emission at 2715 nm was observed instantly around 1.5 ms. Strong emissions at 2740 and 2845 nm were induced at 2 and 3 ms, respectively. From 3 ms, two wavelength emissions of 2740 and 2845 nm were continuously obtained until excitation finished, and the total intensity was almost constant as shown in Fig. 3(b). The spectral peaks of the multi wavelengths indicated different pulses because the measurements were performed by scanning the detection wavelength gradually. The output intensities at 2740 and 2845 nm competed and fluctuation was observed reproducibly. The laser emission for a pump duration of 2 ms included shorter wavelengths of 2715 and 2725 nm with lower emission intensities; therefore, the lasing efficiency was much lower than the longer pulse conditions in Fig. 3(a). This red shift over time has been reported in Er-doped lasers by other groups [12,24,25]. At the beginning of the laser emission, the lower Stark levels of the ⁴I_13/2 state are empty and a shorter wavelength tended to oscillate more because of the large emission cross section. The laser emissions of 2715 nm correspond to the transition from the first Stark level of the ⁴I_11/2 state to the first level of the ⁴I_13/2 state (1 → 1) [17]. The 2725 nm emission is a 2 → 2 or 4 → 3 transition. These shorter wavelength emissions also exhibit output power fluctuations as shown in Fig. 3(b). Then, the lower Stark levels are populated and longer wavelengths are dominant due to reabsorption at the shorter wavelength band. The longer wavelength emissions of 2740 and 2845 nm correspond to the 3 → 3 and 2 → 5 transitions, respectively. Output bursts observed at 2 to 2.5 ms in Fig. 3(b) arise from a typical relaxation oscillation caused by wavelength jumping [25]. Under the CW operation, two bands at 2740 and 2845 nm are expected as the lasing wavelengths and these may change to a single wavelength emission of 2845 nm at a higher pump power.

 

Fig. 4 (a) Time-resolved spectrum of the laser output of the Er:Lu₂O₃ ceramic with a pump pulse duration of 10 ms and an absorbed pump power of 7 W. (b) Time-resolved spectra at typical time regions.

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4. Room-temperature CW operation

CW operation was observed by cooling the crystal holder with water flow of 20 °C. The cavity length was optimized to 20 mm to achieve the highest laser performance. The CW output power and beam profile were measured by using a power meter and an IR camera (Pyrocam3, Spiricon) at various OC transmittances.

The output power as a function of absorbed pump power at various OC transmittances for CW operation of the Er:Lu₂O₃ ceramic laser are shown in Fig. 5. The inset shows a typical intensity profile for the output beam with a diameter of 1.1 mm. The output power increased linearly as the pump power was increased, and the laser threshold increased with increasing OC transmittance. The highest slope efficiency of 22% and maximum output power of 0.48 W were obtained at an absorbed pump power of 3.8 W by using an OC with a transmittance of 5%. The absorbed pump power was around 95% of the incident pump power resulting in an optical-optical efficiency of 20%. In the output curve of 8% OC, the slope clearly changed at around 2.5 W of absorbed pump power. This slope change was probably caused by the wavelength jumping from 2725 nm to 2740 nm which correspond to the jumping point at 2 ms in Fig. 4. In the shorter wavelength regime, the output power was relatively-low compared with the longer wavelength regime as shown in the temporal waveform of Fig. 3(b). In the case of 2% and 5% transmittance, such wavelength jumping was observed at much lower pump power. The highest slope efficiency obtained during QCW operation was 15% by using an OC of 5% in Fig. 3(a). The efficiency improved during CW operation compared with QCW because of the stabilization of the resonator owing to thermal focusing. Under CW conditions, the effective focal length [26] of the Er:Lu₂O₃ ceramic was estimated to be about 50 mm, assuming 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. However, under the QCW pumping with less thermal loading, the resonator will be unstable resonator because of negligible thermal lensing. To our knowledge, our CW slope efficiency of 22% is the highest value obtained by an Er:Lu₂O₃ ceramic laser. Furthermore, it is comparable to the efficiency of an LD pumped Er:Lu₂O₃ single-crystal laser [10], which confirms that this polycrystalline transparent ceramic has high-quality optical and thermal properties. The most likely reason for the high-efficiency is the particularly low optical loss in the 2.8 and 1 μm wavelength regions in Fig. 1. However, a much higher slope efficiency of 36% has been reported in a Er:Lu₂O₃ single crystal by 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 that wavelength [10]. The efficiency of our ceramic laser could also be improved further by using a suitable pump source. The ceramic was uncoated resulting in about 10% Fresnel reflection at the both surfaces which decrease the efficiency. An antireflection coating on the ceramic or Brewster's angle incident may improve the laser performance. In the present system, the output power was limited by the pump power rather than the damage threshold. An absorbed pump power much higher than 4 W was not tried because the cooling capability is currently insufficient (measured temperature of ceramic was higher than 70°C). In the future, we will optimize the cooling system, which is expected to achieve a CW output of >1 W at room temperature because an output peak power of 1.2 W was obtained during QCW operation. The lifetime of the upper (⁴I_11/2) and lower (⁴I_13/2) laser levels of Er³⁺ in Lu₂O₃ are strongly affected by the Er³⁺ concentration as reported in Ref. 10. According to the reference, the lifetime of the lower level decreases drastically at 2 to 8 at. % of Er³⁺ content arising from upconversion energy transfer, leading to a more favorable ratio of the lifetimes of the upper and lower laser level for doping concentrations higher than 7 at. %. However, the lifetime of upper laser level also decreases with increasing concentration.

 

Fig. 5 Output power as a function of absorbed pump power for CW operation of the Er:Lu₂O₃ ceramic laser at various OC transmittances. Inset: Typical intensity profile of the output beam with a diameter of 1.1 mm.

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5. Conclusion

We have demonstrated the operation of an 11 at. % Er-doped Lu₂O₃ ceramic laser at a wavelength of around 2.8 μm. During QCW operation, a slope efficiency of 15% and an output peak power of 1.2 W were obtained, and the lasing dynamics were investigated to elucidate the lasing characteristics and properties. Time-resolved spectroscopy showed that the emission wavelengths exhibited a red shift, which indicates that CW operation could be performed at 2740 and 2845 nm. During room-temperature CW operation, we demonstrated laser emission with a slope efficiency of 22%, which is the highest value achieved by an Er:Lu₂O₃ ceramic laser. The high efficiency probably arises from the particularly low optical loss of the polycrystalline ceramic. A high-quality Er:Lu₂O₃ ceramic that can be mass produced cheaply as large volume crystals will help develop the production of efficient high-power mid-IR lasers.