Diode-pumped all-solid-state lasers have been extensively investigated during the past two decades [1–4]. Different kinds of laser crystals were used as the gain media [5–10]. Specific attention has been paid to the single-crystal fibers (SCFs) due to their good pump guidance, light weight, and small size [11–13]. At present, there are several methods to fabricate SCFs, including the edge-defined film-fed growth (EFG) method [14], micro-pulling-down (μ-PD) method [15], and the laser heated pedestal growth (LHPG) method [16].

Bismuth germanate (${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$, BGO) crystal has wide applications in particle scintillation detectors for high-energy physics and high-resolution positron emission tomography, and its scintillation properties have been the focus of many studies [17–21]. Also, as a potential laser host crystal, Feng et al. demonstrated a continuous-wave (CW) laser operation in ${\mathrm{Nd}}^{3+}$-doped BGO (Nd:BGO) crystal with an output power of 40 mW and a slope efficiency of 13% [22]. After that, there were few reports on Nd:BGO crystal. Recently, we have successfully grown the Nd:BGO SCF by the μ-PD method with the resistance heating system and presented a CW laser operation in the Nd:BGO SCF laser.

The Nd:BGO crystals have been mostly grown by the conventional vertical Bridgman (VB) method, and often problems were encountered in obtaining single crystals with high quality. Alternatively, we have used a μ-PD method and achieved high quality and transparency of the fiber crystals grown.

In this Letter, we report on diode-pumped, true CW laser operation in Nd:BGO SCF gain, grown by the μ-PD method. The output power of 3.37 W and slope efficiency of 31.2% was demonstrated. To the best of our knowledge, this is the first time to achieve laser operation in the rare-earth ion-doped BGO SCF.

The Nd:BGO SCF was grown using a (211) BGO seed. A platinum crucible was resistively heated in air. The raw materials were melted in the platinum crucible and allowed to pass through the micro-nozzle. The SCF was formed by attaching the seed crystal to the tip of the micro-nozzle and slowly pulling downward with a constant velocity. The crystal diameter was maintained constant by controlling the temperature of the main and after-heaters during the growth process. ${\mathrm{Bi}}_2{\mathrm{O}}_3$, ${\mathrm{GeO}}_2$ powders of 99.999% and ${\mathrm{Nd}}_2{\mathrm{O}}_3$ powders of 99.99% purity were used as starting materials without preliminary starting. The ${\mathrm{Nd}}^{3+}$ doping concentration in the starting melt was 0.3 mol. %.

The crystal structure of BGO belongs to the cubic space group $I$ 43d, where the ${\mathrm{Bi}}^{3+}$ ion occupies the center of a distorted octahedron of oxygen ions. The site symmetry of ${\mathrm{Bi}}^{3+}$ is $C_3$ one [23,24]. The ionic radii of ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ are 0.96 Å and 1.04 Å, which are close to each other. Considering that the ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Bi}}^{3+}$ ions have similar ionic radii and the same valency, it is expected that the ${\mathrm{Nd}}^{3+}$ ion will substitute for the ${\mathrm{Bi}}^{3+}$ ion. In fact, this expectation has been confirmed by the subsequent electron paramagnetic resonance (EPR) experiment showing the ${\mathrm{Nd}}^{3+}$ impurity located exactly at the octahedral ${\mathrm{Bi}}^{3+}$ site with trigonal symmetry [25]. It can be suggested that the ${\mathrm{Nd}}^{3+}$ ions substitute for the ${\mathrm{Bi}}^{3+}$ ions because of their similar crystallochemical behavior.

The transmission spectra of 0.3at.% Nd:BGO SCF, which are real concentrations in the crystal measured by inductively coupled plasma optical emission spectrometry, are shown in Fig. 1. The transmission spectrum shows an obvious absorption peak at 808 nm, which means that it is suitable to be pumped by a commercial 808-nm laser diode (LD).

 

Fig. 1. Transmission spectra of Nd:BGO SCF.

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Fluorescence spectra of the media are presented in Fig. 2 with different pump powers. The character of the band at 1064 nm is similar to that of Nd:BGO crystal [22]. However, the peak position at 915 nm disappears, which may be due to the fact that the intensity of the peak is too weak compared with other peaks, or a too strong re-absorption effect.

 

Fig. 2. Emission spectra of the samples excited at 808 nm with different pump powers.

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The CW laser operation was conducted with the setup presented in Fig. 3. We adopted a commercial 808-nm LD as the pump source, delivering a maximum power of 30 W from a pigtailed fiber with a core diameter of 200 μm and a numerical aperture (NA) of 0.22. The pump beam was collimated and focused by a pair of lenses with a same focal length of 75 mm, resulting in a waist diameter of 200 μm on the incident surface of the SCF. The Rayleigh length of the pump beam was calculated to be 1.1 mm. Although it was much shorter than the length of SCF (21.5 mm), the pump beam could be constrained in SCF via total reflection in the fiber, resulting in effective absorption for the pump beam. The laser cavity consisted of a plane-concave mirror with a radius of curvature of $-100\mathrm{mm}$ and an output coupler (OC) with a transmission of 1.5%, 5%, or 25%, respectively. Here, the uncoated SCF had a nearly circular cross section of 2-mm diameter, which was wrapped with indium foil and tightly mounted in a copper holder. The holder was cooled to 12°C by a thermo-electric cooler.

 

Fig. 3. Schematic diagram of the laser diode array pumped Nd:BGO SCF laser.

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Figure 4 shows the dependence of output power on the absorbed pump power for different OCs ($T=1.5\%$, 5%, and 25%, respectively). The slope efficiency increased with the transmission of OC and the maximum slope efficiency reached 33.3% with respect to the absorbed pump power. Under absorbed pump power of 15.25 W, the maximum output power of 3.37 W was delivered with an OC of 5%. Compared with Nd:BGO crystal [22], a higher slope efficiency was obtained in our experiment. The CW spectrum was measured by an optical spectrum analyzer (Ocean Optics, USB4000+), as shown in Fig. 5. The SCF laser had a central wavelength of 1064 nm.

 

Fig. 4. Output power as a function of the absorbed pump power with different output couplers of 1.5%, 5%, and 25%, respectively.

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Fig. 5. CW spectrum of the Nd:BGO SCF laser recorded at the maximum output power of 3.37 W.

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In conclusion, the Nd:BGO SCF was successfully grown with the μ-PD method. The CW output power of 3.37 W was demonstrated with a slope efficiency of 31.2%. To the best of our knowledge, this is the first time to achieve laser operation in the rare-earth ion-doped BGO SCF.