1. Introduction

Diode pumped laser devices based on neodymium-doped laser materials operating on energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions is a promising approach to meet the growing demands for biology, medical detection, optical sensor and communication [1]. Moderate energy gap between the ⁴F_3/2 level and the fundamental ⁴I_9/2 level in Nd³⁺ ions is crucial in selecting appropriate laser hosts. The terminal laser energy level should be high enough to avoid strong re-absorption owing to the thermal population of the Stark levels. Various factors influencing the positions of the electronic levels are taken into accounts. Firstly, hosts with ionic matter presenting a weak nephelauxetic effect are preferred. Secondly, oxides or fluorides matrices containing low covalence and high electron-electron interactions are beneficial with increased nephelauxetic effect and anionic co-ordination number due to long distance of Nd – O [2]. Thirdly, Nd doped matrices with moderate crystal field splitting could limit the competitive splitting between fundamental and excited state levels. Finally but not the least, suitable thermal conductivity and moderate splitting on energy level ⁴F_3/2 are advantageous.

Laser operations based on transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions have been investigated in aluminum hosts [3–5]. In this letter, we report a diode pumped laser operation at 914 nm in monoclinic silicate family of neodymium doped scandium silicate (Nd:Sc₂SiO₅, Nd:SSO) crystal.

2. Gain cross section on energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions in Nd:SSO crystal

1 at.% Nd:SSO single crystal was grown by Czochralski technique. The orientation of the crystal used for the absorption and emission spectra measurement is b-cut sample parallel to Y axis where b represents for crystallographic axis and Y represents for optical indicatrix axis. Branching ratio for energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions with central wavelength at 914 nm was calculated to be 30% by Judd-Ofelt theory [6]. The room temperature absorption spectra were carefully measured with light changeover at 861 nm in order to get rid of covering the band signals of the related energy transfer. The absorption cross section σ_abs and emission cross section σ_em is shown in Fig. 1.The absorption cross section σ_abs at 803 nm was calculated to be 1.69 × 10⁻²⁰ cm² and that at 808 nm was calculated to be 0.95 × 10⁻²⁰ cm². The stimulated emission cross section σ_em at 914 nm was estimated to be 1.13 × 10⁻²⁰ cm² by using the Füchtbauer-Ladenburg equation [6].

 

Fig. 1 Room-temperature absorption and emission cross section in Nd:SSO crystal.

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Gain cross section, denoted as σ_g and measured in units of area, is an important parameter in a laser design and operation. σ_g determines the transition populations from the upper levels to lower levels caused by a particular flux of photons. σ_g can be estimated with spectroscopic measurements and fluorescence lifetime. The gain cross section could be given by Eq. (1) [7].

It can be noted that β is the inversion coefficient defined as the population ratio on ⁴F_3/2 level over the total Nd³⁺ ion population densities. Figure 2 shows the gain cross section σ_g around 914 nm obtained for different inversion ratio β. For β = 1, gain cross-section σ_g is equal to the emission cross section σ_em. It can be indicated from Fig. 2 that laser inversion at 914 nm may happen at β = 0.25 with obtained σ_g of 1.42 × 10⁻²¹ cm².

 

Fig. 2 Gain cross section of energy transition ⁴F_3/2→⁴I_9/2 in Nd:SSO crystal.

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3. Spectroscopic parameters of energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ in Nd:SSO crystal

Laser performance at 900 nm has been reported in Nd doped Sr_1-xLa_xMg_xAl_12-xO₁₉ (Nd:ASL) crystal due to the favorable spectroscopic parameters [3]. Laser operation at 946 nm in Nd:YAG core ceramics composites has recently been realized [8]. Frequency doubling blue laser based on the transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions has been demonstrated in Nd:ASL crystal and Nd:YAG ceramics [1,8]. Infrared laser emission based on energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions in SSO crystal was performed. Accordingly, the spectroscopic parameters in Nd:SSO crystal are compared with those in Nd:ASL crystal and Nd:YAG ceramics as shown in Table 1.

Table 1. Spectroscopic parameters of energy transition ⁴F_3/2→⁴I_9/2 of Nd ions in Nd:SSO crystal and Nd:ASL crystal, Nd:YAG ceramics.

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As shown in Table 1, the energy Stark splitting in Nd:SSO crystal is 526 cm⁻¹ and the value of τ_exp × σ_em is 2.49 × 10⁻¹⁸ μs·cm². The value of energy Stark splitting in Nd:ASL crystal is 553 cm⁻¹ but the value of τ_exp × σ_em is 8.7 × 10⁻¹⁹ μs.cm² which is lower than that in Nd:SSO crystal. The energy Stark splitting in Nd:YAG ceramic is the largest (851 cm⁻¹) while the value of τ_exp × σ_em (2.33 × 10⁻¹⁷ μs.cm²) is the largest [9,10]. Nd:SSO crystal possess moderate value of τ_exp × σ_em when compared with those in Nd:ASL crystal and Nd:YAG ceramic, while the energy splitting is close to that in Nd:ASL crystal. Taking thermal property into account, SSO crystal possess the advantage of minus thermal-optics coefficient (dn/dT = −6.3 × 10⁻⁶ K⁻¹) which is beneficial for releasing thermal lens effect [12]. With the favorable thermal conductivity (4.1 W·m⁻¹·K⁻¹) and moderate splitting (526 cm⁻¹) in Nd:SSO crystal, efficient laser performance on energy transition ⁴F_3/2→⁴I_9/2 of Nd³⁺ ions in SSO crystal can be expected.

4. Laser operation at 914 nm in Nd:SSO crystal

Laser setup for energy transition ⁴F_3/2→⁴I_9/2 in Nd:SSO crystal is shown in Fig. 3.Fiber coupled laser diode (LIMO) with a numerical aperture of 0.22 and core diameter of 100 μm was used as pump source with maximum output power of 35 W, where central emitting wavelength could be tuned by a temperature controller. The coupling optics consists of two identical plano - convex lenses with focal lengths of 100 mm used to reimage the pump beam into the laser crystal at a ratio of 1:1. The resonator was consisted of an input coupler M1 (plane), Nd:SSO crystal and output coupler M2 (BK7). M1 is coated with HT @ 800 nm - 810 nm & 1083 nm and HR @ 914 nm. M2 is coated with HR @ 800 nm - 810 nm & Toc = 2.5% @ 914 nm with radius curvature of 100 mm. 1.0 at.% Nd:SSO sample with aperture of 5 mm × 5 mm and length of 10 mm was polished. The orientation of the polished crystal is b-cut. The propagation of the light is along b direction. Filter RG850 (SHOTT company) was used to cut the pump power left after the laser crystal. To further remove the generated heat during laser oscillation, the Nd:SSO crystal was wrapped with indium foil and mounted in a water-cooled copper heat sink. Temperature was controlled at 5 °C using a thermo-coupler device.

 

Fig. 3 Laser setup for transition ⁴F_3/2→⁴I_9/2 in Nd:SSO crystal.

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Before moving to laser operation at 914 nm in Nd:SSO crystal, laser behavior at 1083 nm on the ⁴F_3/2 → ⁴I_11/2 transition were recorded with different transparency of output coupler T_OC = 2%, T_OC = 6% and T_OC = 10% as shown in Fig. 4. The cavity mirrors were replaced with coatings covering 1083nm accordingly. The highest output power of 2.54 W and slope efficiency of 33.3% was obtained with T_OC = 10% and absorbed pump power of 11.69W. In the case of T_OC = 6% and T_OC = 2%, output power of 482 mW and 266 mW was obtained.

 

Fig. 4 Laser performance at 1083 nm in Nd:SSO crystal with different T_OC.

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According to the absorption spectra shown in Fig. 1, the absorption band was centered at 811 nm and 803 nm. The commercially available laser diode pump with central wavelength of 808 nm was adopted. Unfortunately, the maximum pumping wavelength could not go further to 811nm even with temperature adjustment. A second laser diode pump source with central wavelength of 804 nm was employed. With the temperature adjustment on the laser diode pump source, the best laser performance at 914 nm was obtained when the laser diode emitting wavelength was centered at 803 nm. In the following discussion, Nd:SSO laser pumped by different laser diode pump was compared with central wavelength of 803 nm and 808 nm.

The stable oscillation was maintained at cavity length of 11 cm. The relationships between output power at 914 nm in Nd:SSO crystal and the absorbed pump power at 803 nm and 808 nm are shown in Fig. 5.The laser beam profile at 914 nm is also shown in Fig. 5. When laser diode pump is centered at 803 nm, 271 mW of output power with 11.9 W of absorbed pump power corresponding to a slope efficiency of 5.4% was obtained. On the other hand, the use of laser diode pump centered at 808 nm led to 581 mW of output power with 13.3 W of absorbed pump power, while the slope efficiency was found to be 8.6%. Laser diode pump source with central wavelength of 808 nm is beneficial for higher laser output when compared to that of 803 nm, although the absorption cross section at 808 nm is lower than that at 803 nm according to Fig. 1. The slope efficiency of 8.6% obtained could be affected by crystal quality, crystal length, as well as the cavity conditions. Higher output power and slope efficiency based on transitions of ⁴F_3/2→⁴I_9/2 of Nd ions could be expected by shortening sample length, improving crystal quality and laser diode pump source with central wavelength of 811 nm. These experiments are now under progress. Tracking back to the data in Table 1, hosts with branch ratio higher than 30% for energy transition ⁴F_3/2→⁴I_9/2 of Nd ions seems to be positive for laser performance.

 

Fig. 5 Laser operation at 914 nm in Nd:SSO crystal by laser diode pump centered at 803 nm and 808 nm.

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5. Conclusion

In conclusion, Nd:SSO with the advantage of minus refractive index versus temperature, favorable thermal conductivity (4.1 W·m⁻¹·K⁻¹), moderate energy Stark splitting (526 cm⁻¹) were firstly reported with laser performance at 914 nm. The realization of diode pumped laser operation at 914 nm in monoclinic Nd:SSO crystal opens the way to prolific hosts for second harmonic generation of blue lasers. Improvement of the optical quality of the Nd:SSO crystal should help enhance the laser performances at 914 nm. This work is now in progress.