1 Introduction

Ho3+, Tm3+ ions can be commonly used as the activation ions at the range of 2-3 μm wavelength, which has the advantage of large stimulated-emission cross-section and long fluorescence lifetime. In the military, 2 μm laser has a strong ability to penetrate the smoke of the battlefield with good confidentiality. These two types of activation ions are suitable for military laser ranging, laser radar and electro-optical jamming, etc[1-4]. In the medical field, 2 μm laser can perform precise excision of biological tissues without damaging surrounding tissues, while reducing the anesthetic dosage[5]. In addition, it can be used to detect harmful gases such as formaldehyde with the magnitude of 10-9, as well as carbon monoxide in the environment[6].

Tm3+ ions can be often used as the sensitizing agent of Ho3+ ions since Tm3+ ions have stronger absorption efficient than Ho3+ ions at the range of 785-810 nm. It can achieve the energy transfer of Tm→Ho and then improve the absorption efficiency to pumping light[7-11]. Yb3+ ions have been widely used as a sensitizing agent with the LD pumping popular. The absorption peak of Yb3+ ions located near 980 nm and can be effectively coupled with InGaAs LD. Because the energy level structure of Yb3+ ion is very simple without excitation state re-absorption process, Yb3+ can be served as the sensitizing agent of Ho3+, Tm3+ ions.

KYb(WO4)2 is one of the self-activation laser crystals with a double tungstates structure. In the host of KYb(WO4)2,Yb3+ ions have been served as not only an integral part of the host, but also the active ions. The absorption wavelength of KYb(WO4)2 is 930-980 nm, which can be effectively coupled with the InGaAs LD pumping[12-14]. The laser output wavelength of KYb(WO4)2 is in the range of 980-1 080 nm. Laser emission with multi-wavelength and tenability in this crystal can be achieved by doping Er3+, Tm3+, Ho3+, etc. Re∶KYb(WO4)2 has several merits including high doping concentration, low fluorescence quenching, high conversion efficiency, low threshold value and well stability, etc. It can achieve all-solid-state, miniaturization and integration for lasers and widely used in the fields of human-eye safe, laser communication, medical treatment, remote sensing, etc[15-18]. In this paper, the growth, absorption and fluorescence spectra of KHo0.04Tm0.06-Yb0.9(WO4)2 laser crystal were investigated, and the spectral parameters of this crystal were calculated as well.

2 Experiments

2.1 Crystal Growth

The chemicals included K2CO3(top-grade pure), Yb2O3 (99.99%), Ho2O3 (99.99%), Tm2O3 (99.99%), and WO3 (99.999%). The K2W2O7 was served as solvent. They were mixed and reacted based on the equations below：

（1）制定验收制度。对容易出现问题的施工节点制定验收制度，减少出现钻井施工问题的可能性。如二开固控设备验收制度、水平井下FEWD验收制度、沙河街地层验收制度等，实现问题的提前处理。

0.04Ho2O3+0.06Tm2O3+0.9Yb2O3+K2CO3+

4WO3→2KHo0.04Tm0.06Yb0.9(WO4)2+CO2 ,

(1)

K2CO3+2WO3→K2W2O7+CO2,

(2)

where the mole ratio of KHo0.04Tm0.06Yb0.9(WO4)2 to K2W2O7 was 1∶4. The crystal was grown by the Top Seeded Solution Growth (TSSG) method. The experimental equipment included a MCGE-furnace heated by resistance wire, a platinum crucible with a size of Φ60 mm×50 mm, a Pt-Rh thermocouple, and an AI-808P thermal controller. The mixed materials was put in a platinum crucible and fully melted at a temperature of 80 ℃ above super-saturation for 24 h. The oriented crystallon was dipped at about 910 ℃, and grown gradually with a rotation rate of 15 r/min, a pulling rate of 2 mm/day and a cooling rate of 0.05/h. After 2 weeks, the crystal was removed from the crucible and cooled down to the room temperature at a rate of 20 ℃/h. Finally, the KHo0.04Tm0.06Yb0.9(WO4)2 laser crystal with a dimension of 26 mm×11 mm×9 mm was obtained successfully. Then the crystal sample with a dimension of 3 mm×3 mm×1 mm was cut perpendicularly to b axis for spectral measurement.

2.2 Measurement of Sample

The absorption spectrum of the crystal sample was measured using an ultraviolet (UV) spectrophotometer(Model UV360, Shimadzu Company, Japan) at room temperature. The fluorescence spectrum of the crystal sample was measured with an X-ray fluorescence spectrometer(FLUOROLOG-3, HORIBA Company, Japan) at room temperature.

3 Results and Discussion

3.1 Absorption Spectrum

The fluorescence spectrum of the crystal sample under 980 nm pumping is shown in Fig.2. There are four obvious emission peaks at 950-1 050 nm, 1 100-1 250 nm, 1 400-1 470 nm, 1 750-2 200 nm, which are attributed to 2F5/2→2F7/2(Yb3+)， 3H5→3H6(Tm3+) & 5I6→5I8(Ho3+)， 3H4→3F4(Tm3+)， 3F4→3H6 (Tm3+) & 5I7→5I8 (Ho3+) transitions, respectively. The emission line width at 1 750-2 200 nm amounts to 250 nm, which is due to the overlap between 3F4→3H6 (Tm3+) and 5I7→ 5I8(Ho3+). It is indicated that the crystal can be used as a high quality tunable laser medium at 2 μm.

责任，即分内应做的事；或没有做好分内应做的事,因而应承担的过失[3]。社会责任感是公民在社会生活中，对他人、对集体、对国家乃至人类社会所应承担的，表现在思想、政治、道德和法律上的一种使命、职责、义务[4]。大学生是当代青年的先进代表，是祖国未来繁荣昌盛的建设者，也是中国特色社会主义事业建设与发展的核心人力资源，其社会责任感的有与无、强与弱，直接关系到民族的兴衰、国家的强盛乃至社会主义事业的成败。

Fig.1 Absorption spectrum of KHo0.04Tm0.06Yb0.9(WO4)2 crystal

where D is the optical density, which can be calculated as D=lg(I0/I), based on absorption spectrum measurements; I0 and I represent the intensities of incident light and emergent light, respectively; N represents the concentration of Ho3+, Tm3+ or Yb3+ ions, and L represents the thickness of the sample (L=1 mm). The absorption cross section of the crystal at the strongest peak of 1 000 nm was calculated as 16.92×10-20 cm2. Tab.1 lists the main absorption peaks and absorption parameters of the crystal.

(3)

The absorption cross section σabs of the crystal can be defined as formula (3)[19]:

Yb3+ ions generate a 2F7/2→2F5/2 absorption transition due to the 980 nm excitation. Then the energy is transferred from 2F5/2 (Yb3+) to 5I8 (Ho3+) and 5I8→5I6 transition of Ho3+ ions can be achieved. From 5I6 energy state (Ho3+), the emission at 1 900-2 200 nm(5I7→5I8) can be obtained by the non-radiative transition process from 5I6 to 5I7 energy state.

Yb3+ ions can uptake quantities of pumping light to generate 2F7/2→2F5/2 absorption transition under 980 nm excitation. Then the energy was transferred from 2F5/2 (Yb3+) to 3H6 (Tm3+) and 3H6→3H5 transition of Tm3+ ions can be achieved. From 3H5 energy state(Tm3+), the emission at 1 700-1 900 nm (3F4→3H6) can be obtained by the non-radiative transition process from 3H5 to 3F4 energy state.

Tab.1 Main absorption peaks and absorption parameters of the KHo0.04Tm0.06Yb0.9(WO4)2 crystal

Transition λ/nm N/cm-3σabs/cm2 3H6→1D2 (Tm3+)3782.94×1020 1.17×10-205I8→5G5(Ho3+)4211.96×1020 0.92×10-205I8→5G6(Ho3+)4501.96×1020 3.38×10-203H6→1G4 (Tm3+)4752.94×1020 2.12×10-205I8→5F4 (Ho3+)5381.96×1020 3.89×10-205I8→5F5(Ho3+)6251.96×10201.93×10-206451.96×10201.95×10-206911.96×1020 4.47×10-203H6→3H4 (Tm3+)8052.94×1020 3.28×10-205I8→5I4(Ho3+)8901.96×1020 5.32×10-205I8→5I5(Ho3+)9161.96×1020 7.89×10-202F5/2→2F5/2 (Yb3+)9804.4×102115.27×10-201 0004.4×102116.92×10-205I8→5I6(Ho3+)1 1901.96×1020 5.31×10-201 2231.96×1020 5.46×10-203H6→3F4 (Tm3+)1 690-1 8122.94×1020 10.35×10-205I8→5I7(Ho3+)1 9241.96×1020 3.02×10-20

3.2 Fluorescence Spectrum

The absorption spectrum of the crystal sample is shown in Fig.1. There are several absorption peaks at 378, 421, 450, 475, 538, 625, 645, 691, 805, 890, 916, 980, 1 000, 1 190, 1 223, 1 690, 1 712, 1 749, 1 804, 1 812, 1 924 nm. It can be seen that the absorption peaks at 378, 475, 805 and 1 690-1 812 nm are accounting for 3H6→1D2(Tm3+), 3H6→1G4(Tm3+), 3H6→3H4(Tm3+) and 3H6→3F4(Tm3+) transitions, respectively. Meanwhile, the absorption peaks located at 421, 450, 538, 625-691, 890, 916, 1 190 (1 223), 1 924 nm are due to 5I8→5G5(Ho3+), 5I8→5G6(Ho3+), 5I8→5F4(Ho3+), 5I8→5F5(Ho3+), 5I8→5I4(Ho3+), 5I8→5I5(Ho3+), 5I8→5I6(Ho3+),5I8→5I7(Ho3+) transitions, respectively. The absorption peaks at 981 and 1 000 nm are attributed to 2F7/2→2F5/2(Yb3+) transition. The full width at half maximum (FWHM) at the range of 890-1 000 nm amounts to 90 nm, which is more propitious for InGaAs LD laser pumping. In this absorption spectrum, the most interesting is a broader absorption band (Tm3+) at 1 690-1 812 nm with a FWHM of 118 nm. The effective energy transfer of Yb→Ho, Yb→Tm, Tm→Ho can be easily achieved to produce 2 μm around laser due to these two broader absorption bands at 890-1 000 nm(Yb3+) and 1 690-1 812 nm(Tm3+) in the crystal.

Fig.2 Fluorescent spectrum at 900-2 300 nm of the KHo0.04Tm0.06Yb0.9(WO4)2 crystal

(1)Energy transfer of Yb3+→Tm3+

The energy level diagram sketched in Fig.3 can describe the energy transfer among Tm3+, Yb3+, Ho3+ ions in the crystal. The fluorescence emission at 1 750-2 200 nm is due to the following three energy transmission interaction.

以误差变化量e为例，基本论域[-e，e] 中e表征误差大小的精确量，n是0～e范围内连续变化的误差离散化后分成的档数。它是构成基本论域的元素，一般情况下，档数分的越精细控制效果就会越好，但是档数越高，计算量就越繁琐，通常取n=6或7。

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(2) Energy transfer of Yb3+→Ho3+

Meanwhile, the spectral intensity parameters in the crystal can be obtained based on the Judd-Ofelt theory[20-21] as follows:Ω2=10.24×10-20cm2, Ω4=2.37×10-20 cm2, Ω6=1.43×10-20 cm2.

(3) Energy transfer of Tm3+→Ho3+

This efficient resonant energy transfer happens due to the energy approximate overlap between the 3F4 (Tm3+) and 5I7 (Ho3+) energy levels. Therefore, the emission at 1 900-2 200 nm (5I7→5I8) can be achieved based on the energy transfer from 3F4(Tm3+) to 5I7(Ho3+).

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Fig.3 Energy level diagram of Tm3+, Yb3+, Ho3+ ions and emission channels.

According to the formula (4)[15], the stimulated emission cross section of main emission peak at 2 030 nm can be calculated as 3.47×10-20 cm2.

(4)

where λ represents the fluorescent wavelength (λ=2 030 nm), n refers to the refractive index (n=2.17), c represents the light speed in the vacuum (c= 3×108 m/s), τf is the spontaneous fluorescence time (τf =1.8 ms). can be defined as the normalized line shape function, corresponding to 5I7→5I8 transition of Ho3+ ions.

4 Conclusion

KHo0.04Tm0.06Yb0.9(WO4)2 laser crystal with a dimension of 26 mm×11 mm×9 mm was grown by the TSSG method. The testing result of absorption spectrum shows that Yb3+ ions have a wider absorption band at the range of 890-1 000 nm with a FWHM of 90 nm, and Tm3+ ions have a broader absorption band at 1 690-1 812 nm with a FWHM of 118 nm. The effective energy transfer of Yb→Ho, Yb→Tm, Tm→Ho can be easily achieved to produce 2 μm around laser due to these two broader absorption bands at 890-1 000 nm (Yb3+) and 1 690-1 812 nm(Tm3+) in the crystal. The fluorescent spectrum measurement result indicates that the emission line width at 1 750-2 200 nm amounts to 250 nm, which is due to the overlap between 3F4→3H6(Tm3+) and 5I7→5I8(Ho3+). The spectral strength parameters of the crystal were calculated based on the Judd-Ofelt theory as follows:Ω2=10.24×10-20 cm2, Ω4=2.37×10-20 cm2, Ω6=1.43×10-20 cm2。According to the energy level diagram of Tm3+, Ho3+ and Yb3+ ions, three kinds of energy transfer modes at 1 750-2 200 nm emission were discussed in the crystal. The stimulated emission cross section of main emission peak at 2 030 nm is 3.47×10-20 cm2. It is indicated that KHo0.04Tm0.06- Yb0.9(WO4)2 laser crystal can be widely used as an excellent laser gain medium at 2 μm around wavelength.

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