In this paper, we demonstrate a new 2.1-µm Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at room temperature. The absorption and emission spectra of Ho:SYSO crystal were studied at temperature of 300 K. The strongest absorption peak of 5I7 level of Ho ions in SYSO crystal is located at 1943 nm with cross section of 0.79 × 10−20 cm2. The maximum emission cross section of 1.74 × 10−20 cm2 is located at 2032 nm. A 1940.3-nm narrow-linewidth Tm fiber was used to pump the Ho:SYSO crystal. At absorbed pump power of 20.4 W, the continuous wave Ho:SYSO laser yielded 10.3 W maximum output power at 2097.67 nm and 54.7% slope efficiency respect with absorbed pump power. In addition, we have estimated the beam quality factor (M2) of Ho:SYSO laser to be 1.7 near maximum output level.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The solid-state lasers operating around 2-µm spectral region have important application in many technical fields such as materials processing, lidar, spectroscopy, and environment monitoring. Among 2-µm laser materials, the rare-earth holmium (Ho)-doped crystals have properties of large emission cross section and long upper level lifetime. In addition, it can be directly in-band pumped by 1.9-µm lasers, which bring out advantages of low quantum defect and high conversion efficiency . Therefore, the Ho laser becomes an excellent technical way to achieve high efficient 2-µm laser radiation at room temperature. In the past two decades, high-power or high-energy Ho lasers at 2-µm have been demonstrated in different oxide [2–4] and fluoride [5–7] hosts. With excellent physicochemical and mechanical properties, the silicate crystals RE2SiO5 (RE = Sc, Y, Gd, Lu) are promising host materials for rare earth ions. Diode-pumped Yb or Nd-doped silicate lasers have been widely investigated [8–12]. The first 2-µm laser action of rare-earth-doped silicate was reported in a Ti:Al2O3-pumped Tm:YSO laser . Also, diode-pumped Tm-doped LSO  and SSO  lasers have been demonstrated. For Ho-doped silicates, the 2-µm laser performances were presented in LSO  and SSO  crystals.
Recently, yttrium (Y) ions were used to replace fraction of scandium (Sc) ions in the Sc2SiO5 crystal, thus making a new disordered silicate crystal ScYSiO5 (SYSO), which enhanced the inhomogeneous lattice field for Ho ions because of more types of crystallographic sites from Sc and Y. As a result, the stark-splitting of ground state energy level is enlarged and the emission spectrum of Ho ions is widened in SYSO crystal. Simultaneously, the bigger emission cross section and longer level lifetime of Ho ions is achieved in SYSO crystal. Moreover, the monoclinic biaxial SYSO crystal with natural birefringence is conducive to reduce the thermally induced birefringence. In 2012, the first laser action of RE-doped SYSO crystal has been reported in a diode-pumped Nd:SYSO laser with continuous wave (CW) output power of 1.96 W at wavelengths of 1074.8 nm, 1076.6 nm and 1078.2 nm . In 2017, a 580 mW CW diode-pumped Tm:SYSO laser at 2022 nm was demonstrated . In 2018, the optical properties of Tm:SYSO crystal have been demonstrated . However, as the best knowledge of authors, there is no report on the laser performance of Ho-doped SYSO crystal up to now.
In this paper, the spectral properties and laser performance of Ho:SYSO crystal were studied. By using a 1940.3-nm Tm fiber laser as the pump source, we presented the high power Ho:SYSO laser at 2.1 µm. At the absorbed pump power of 20.4 W, the maximum CW output power of 10.3 W at 2097.67 nm was obtained in Ho:SYSO laser with output transmittance of 11%, corresponding to a slope efficiency of 54.7%. The beam quality factor M2 of Ho:SYSO laser was estimated to be 1.7 at output power of 9 W. The Table 1 summarizes the reported output performances of Ho-doped silicate lasers at 2 µm. It can be seen that the highest CW output power and slope efficiency was demonstrated in this work.
2. Spectral properties
The Ho:SYSO crystal with dopant concentration of 0.5 at.% was successfully grown along the a-axis by the Czochralski method. A 1-mm-thickness Ho:SYSO chip was polished and used to measure its absorption spectrum. At room temperature, the unpolarized absorption spectrum in the range from 1800 nm to 2250 nm was recorded by a spectrophotometer (Lambda 750 UV/VIS/IR), and the results were shown in Fig. 1(a). It can be seen that the strongest absorption peak is located at 1943 nm with an effective bandwidth of 33 nm, which was beneficial to be pumped by high power Tm lasers. The unpolarized emission spectrum of Ho:SYSO crystal in the range from 1800 nm to 2200 nm, which was also shown in Fig. 1(a), was measured with a spectrometer (Edinburgh FLS980). It has a broad emission band which centered at 2032 nm with an effective bandwidth of about 190 nm. With the above spectrometer and an additional oscilloscope (Tektronix TDS 3052), the fluorescence decay curve of 5I7 level of Ho ions was measured by using a ns-pulsed 2.05-µm laser as the excitation source, as shown in Fig. 1(b). The decay curve has single-exponential characteristic, and the lifetime was fitted to be 2.32 ms.
The dopant of Ho ions in SYSO crystal was calculated to be 9.59 × 1019 cm−3. The absorption cross section of Ho:SSO crystal can be calculated according to the formula σabs = α(λ)/N. Where σabs is the absorption cross section, α(λ) is the absorption coefficient, and N is the unit volume concentration of Ho ions. Figure 2 shows the absorption cross sections of Ho:SYSO crystal. The absorption cross section at 1943 nm was 0.79 × 10−20 cm2. The stimulated emission cross section was calculated according to the Fuchtbauer-Ladenburg equation (Eq. (1)).Fig. 2. It can be seen that the maximum emission cross section of 1.74 × 10−20 cm2 is located at 2032 nm. Another two emission peaks of 2065 nm and 2086 nm were observed with the emission cross section of 1.71 × 10−20 cm2and 1.66 × 10−20 cm2, respectively. In contrast, the maximum emission cross section and lifetime of 5I7 level of Ho ions in SSO crystal was 1.1 × 10−20 cm2 at 2086 nm and 1.51 ms , respectively. Obviously, the emission cross section and level lifetime of Ho ions were enhanced in SYSO crystal.
3. Experimental setup
The experimental setup of Ho:SYSO laser was shown schematically in Fig. 3. A 30 W unpolarized Tm fiber laser wavelength-locked by two FBGs was used to pump the Ho:SYSO crystal. Its output wavelength and M2 factor was 1940.3 nm and 1.3, respectively. Two lenses (f1 = 8 mm, f2 = 75 mm) were employed to collimate and focus the pump light into the Ho:SYSO crystal. The 1/e2 pump radius was approximately 0.17 mm. The a-cut Ho:SYSO crystal with dopant concentration of 0.5 at% was used as the gain medium, which has dimensions of 4 × 4 mm2 in cross section and 20 mm in length. Both end faces of the crystal were polished and antireflection coated for pump and resonant wavelength. The Ho:SYSO crystal was wrapped with 0.1mm-thickness indium foils and mounted in a cooper heat sink. The heat-sink temperature was controlled at 15 °C using a thermoelectric cooler. A simple linear resonator cavity was used to investigate the output performance of Ho:SYSO laser. The input mirror M1 was a plat mirror with high transmission (~94%) for pump wavelength and high reflective (~99.8%) for resonant wavelength. The output coupler M2 was a plano-concave mirror with radius of curvature of 100 mm. The physical length of whole Ho resonant cavity was about 33 mm. The 45° dichroic mirror M was high transmission for pump light and high reflective for resonant wavelength. We use the ABCD matrix method to calculate the resonant beam waist radius to be about 0.17 mm. Moreover, the calculating result indicates that this cavity is stable when the thermal focal length from Ho:SYSO crystal is more than 16 mm.
4. Experimental results
In this experiment, the actual single-pass pump absorption was measured to be about 80% under lasing conditions. A power meter (Coherent PM30) was employed to measure the output powers of Ho:SYSO laser. The Ho:SYSO laser yielded linearly-polarized beam along c-axis, which was verified by a contrast ratio of about 25 dB. Four output transmittances of 2%, 8%, 11% and 17% were used to investigate the output characteristics of Ho:SYSO laser at room temperature, as shown in Fig. 4(a). For transmittances of 2%, 8%, 11% and 17%, the threshold pump powers were 0.96 W, 1.09 W, 1.35W and 2.29 W, respectively. With output transmittance of 11% and absorbed pump power of 20.4 W, the Ho:SYSO laser yielded the 10.3 W maximum output power and 54.7% slope efficiency with respect to the absorbed pump power. The power stability of the Ho:SYSO laser was estimated over a period of thirty minutes. When the absorbed pump power was 20.4 W, the output power slowly fluctuated between 10.2 W and 10.4 W, which indicated that the power stability was approximately 2.0%. For transmittances of 2%, 8% and 17%, the slope efficiencies were 42.3%, 48.3% and 49.1%, respectively. In this work, some scattered spots were observed inside the Ho:SYSO crystal under He-Ne laser irradiating conditions, so the quality of crystal is not good. With improving of the crystal quality, the better conversion efficiency would be obtained. Furthermore, with optimization of Ho dopant concentrations and cavity structure, significant enhancements in both efficiency and output power should be reached.
The output spectra of Ho:SYSO laser under different output transmittance conditions were measured by an optical spectrum analyzer (Bristol 721A), as shown in Fig. 4(b). With the transmittance of 2%, the longest wavelength of 2111.34 nm with FWHM linewidth of 4.96 nm was obtained in Ho:SYSO laser. In the case of transmittance of 8%, the output wavelength decreases to 2099.44 nm with linewidth of 4.54 nm. When the transmittance increases to 11%, the output wavelength of 2097.67 nm was observed with linewidth of 3.47 nm. With the transmittance of 17%, the central wavelength was located at 2097.64 nm with the linewidth of 3.42 nm. At the fixed any output transmittance, no significant changes were observed in output spectrum of Ho:SYSO laser under both low and high pump level. Compared with the emission peaks of Ho:SYSO crystal, the lasing wavelength is longer than peaks in Fig. 2. This phenomenon was mainly caused by re-absorption loss in quasi-three-level laser system.
With output transmittance of 11%, the M2 factor of Ho:SYSO laser was measured by the 90/10 scanning-knife edge method. The beam radius at the output power of 9 W was measured at different positions and shown in Fig. 5. The M2 factor was calculated to be approximately 1.7, indicating TEM00 mode operation of the Ho:SYSO laser. This was verified by far-field beam profile which was recorded by a camera (Cinogy CR 200HP), as shown in the inset of Fig. 5.
In summary, the spectral properties of Ho:SYSO crystal were investigated in this work. With a Tm fiber laser at 1940.3 nm, we have presented the efficient 2.1-μm laser performance of Ho:SYSO crystal at room temperature. In the case of output transmittance of 11%, the Ho:SYSO laser yielded CW output power of 10.3 W at 2097.67 nm and slope efficiency of 54.7% respect with the absorbed pump power. The experimental results indicate that Ho:SYSO crystal is a new efficient Ho laser gain medium, which makes it a promising candidate for high power 2.1-μm lasers.
National Natural Science Foundation of China (NSFC) (51572053, 61805209, 61775053, U1530152); National Key Research and Development Program of China (2016YFB070100).
We appreciate Dr. He Wen of CREOL at University of Central Florida for improving the quality of the manuscript.
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