1. Introduction

Solid state laser sources operating in the eye-safe ~2 µm spectral region have attracted a great deal of interest owing to their numerous applications in areas such as laser remote sensing, medicine, atmospheric monitoring, and eye-safe lidar [1,2]. Among various fiber host glasses, tellurite (TeO₂) glasses have attracted a considerable interest because they are promising hosts for fiber lasers operating in the mid-IR [3–5]. They combine the attributes of high refractive indices, high rare earth solubility, low phonon energies, wide transmission region of 0.35-5 μm (up to 5.2 μm in this study) and high gain per unit length [6]. Compared to fluoride glasses, which also feature low phonon energies, they exhibit a higher thermal stability and improved corrosion resistance [6]. First fabricated and reported by Wang et al. in 1994 [6], tellurite glass fibers with the composition of TeO₂-ZnO-Na₂O (TZN) have attracted great research interest for laser and infrared nonlinear applications [7–9]. CW lasing has been demonstrated at 1.06 μm [10] in 1994, and at 1.6 μm in 1997 [3] for Nd³⁺ and Er³⁺ doped tellurite fibers, respectively. In 2008, a breakthrough of 280 mW ~2 μm laser output was achieved in a Tm³⁺-doped TZN glass fiber, there was no damage to the fiber and the output power was only limited by the available pump power [4]. Nevertheless, the low glass transition temperature (T_g, ~300°C) and large coefficient of thermal expansion (CTE, ~190 × 10⁻⁷/°C) of TZN glass make it less durable to the large amount of heat generated in a fiber laser [11,12]. In 2010, a tungsten tellurite fiber was developed based on the ternary TeO₂-WO₃-La₂O₃ (TWL30) glasses, and the thermal properties were significantly improved, watt level ~2 μm laser output in a piece of 40 cm long Tm³⁺ doped tellurite multimode fiber was demonstrated [13], the results offered the prospects of making possible new fiber devices based on the tungsten tellurite glass. However, it is necessary to develop a single mode fiber for achieving single frequency seed laser and ultra-fast fiber laser.

Holmium is another well-known rare-earth material with ⁵I₇→⁵I₈ transition can generate ~2 μm emission [14]. The emission peak of Ho³⁺ is 200 nm longer than the emission peak of Tm³⁺. Hence, holmium ions have a much higher gain at 2050 nm. But the limitation of Ho-doped laser is the lack of commercial laser diode, Therefore, co-doping with Tm³⁺ ions is an ideal way to achieve ~2.1 μm laser by using a commercial 800 nm laser diode.

In this work, we investigated the ~2 μm spectroscopic and lasing performance of Tm³⁺ and Tm³⁺-Ho³⁺ co-doped tungsten tellurite glass single mode fibers with a commercial 800 nm laser diode. The detailed process of fabricating the single mode tungsten tellurite fiber was also described.

2. Experimental techniques

The tungsten tellurite glass double cladding fibers were fabricated based on the glass composition 60TeO₂-30WO₃-10La₂O₃ (TWL30). The starting chemicals for the core and cladding glasses were weighed, ground, mixed and then placed into platinum crucibles and melted in a separated electric furnace in a dry O₂ atmosphere. The melts were casted at 820°C into a preheated 280°C stainless steel mold and annealed at 430°C for 5 h, after which they were allowed to cool slowly in the furnace to room temperature. The preforms were well polished and then rod-in-tube method was used three times to achieve a smaller core to cladding diameter ratio. The fiber had a ~9 μm core diameter with a numerical aperture (NA) of 0.14, and a ~100 μm inner cladding diameter with a NA of 0.29. The diameter of the fiber was ~210 μm. The propagation loss of the fiber was ~2.5 dB/m at 1310 nm.

The coefficient of thermal expansion was measured using a NETZSCH 402EP with a heating rate of 5°C/min. The glass transition temperature (T_g) and onset crystallization temperature (T_x) were analyzed by differential thermal analysis (DTA), using NETZSCH STA 409 PC with a heating rate of 10°C/min. The refraction index of glass was measured by using Metricon Model 2010/M Prism Coupler. The near infrared luminescence signals of the glasses were measured using FLS920 combined fluorescence lifetime and steady state spectrometer (Edinburgh Instruments), excited by a 800 nm laser diode (LD). The fluorescence and lasing spectra of the fibers were measured using StellarNet RED-Wave NIRx and Blue-Wave spectrometers. The beam quality factor was measured by using Thorlabs BP109-IR2 beam profiler. All the measurements were carried out at room temperature.

3. Results and discussion

The thermal properties, refractive indices and numerical apertures of core and cladding glasses are summarized in Table 1 . The T_g of this novel tungsten tellurite glass is ~50% higher and the CTE is ~36% lower than that of the conventional TZN glass [11]. It is well recognized that higher T_g and lower CTE are favorable for alleviating the problem of thermal damage in a fiber laser [12].

Table 1. Thermal properties, refractive indices and numerical apertures of the core and cladding glasses

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In order to efficiently eliminate the OH⁻¹ content in the fabricated tungsten tellurite glasses, the melts were bubbled with dry O₂ for 1 hour and then stirred at 850°C for 2 hours in a dry O₂ atmosphere protected furnace. The OH⁻¹ absorption coefficient in the core glass is calculated to be less than 0.5 cm⁻¹, which can be seen in Fig. 1 . The optical transmission range of the core glass is 0.42-5.2 μm.

 

Fig. 1 Transmittance spectrum of core glass in the range of UV-vis and mid-IR. (Thickness of the test sample is 1.0 mm).

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Figure 2 shows the emission spectra of a 12.5 cm length of 1 mol% Tm³⁺ doped fiber with varying absorbed pump power, using a 800 nm laser diode (LD). The emission spectrum of the bulk sample with the same doping concentration is for comparison. The spectra have been normalized at 1.82-μm which is attributed to ³F₄→³H₆ transition of Tm³⁺. The peak shifts to longer wavelengths in the fiber due to radiation trapping where light is absorbed from the ground state to the ³F₄ level and then re-emitted [15]. It is also observed that the peak narrows in fiber with the increase of the absorbed pump power. It is important to note the emission intensity at ~1900 nm shows a significant enhancement when the absorbed pump power reaches 500 mW, which means the population inversion is easily occurred at around 1900 nm in this Tm³⁺ doped tungsten tellurite double cladding fiber.

 

Fig. 2 The emission spectra of bulk glass and 12.5 cm length of tellurite fiber with varying absorbed pump power, using a 800 nm laser diode (LD). The spectra have been normalized with respect to the peak at 1.82 μm.

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Figure 3 (left) shows the emission spectra of 1 mol% Tm³⁺ and 1 mol% Tm³⁺-0.5 mol% Ho³⁺ co-doped bulk glasses, the intensity of 1.8 μm emission of Tm³⁺:³F₄→³H₆ decreases drastically because of the co-doping of Ho³⁺ ions, the measured lifetime of ³F₄ level in Tm³⁺ doped bulk glass is 3.6 ms, which decreases to 1.9 ms in Tm³⁺-Ho³⁺ co-doped sample, indicating the energy transfer from Tm³⁺ to Ho³⁺. The lifetime of Ho³⁺:⁵I₇ level in the bulk glass is 3.3 ms. Figure 3 (right) shows the emission spectra of varying lengths of Tm³⁺-Ho³⁺ co-doped tellurite fiber. In fiber the peak narrows and shift to longer wavelength and also appears to a single peak instead of a double peak in the bulk glass, the spectra have been normalized with respect to the peak at 1.93 μm, it can be found that both the intensities of 2.0 μm and 1.47 μm increase in longer lengths of fiber, which means that besides the forward energy transfer from Tm³⁺ to Ho³⁺, there is a strong back energy transfer from Ho³⁺ to Tm³⁺. This also suggests there must be ESA processes occurring in Ho to populate the ⁵I₄ and ⁵S₂/⁵F₄ and higher levels. Meanwhile, energy transfer from Tm:³H₄ to Ho:⁵I₅, and Ho:⁵I₆ to Tm:³H₅ may be possible in this co-doped system.

 

Fig. 3 The emission spectra of 1 mol% Tm³⁺ and 1 mol%Tm³⁺-0.5 mol%Ho³⁺ co-doped bulk glasses (left); The emission spectra of Tm³⁺-Ho³⁺ co-doped tellurite fibers (right).

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Figure 4 shows strong visible up-conversion emission of Tm³⁺-Ho³⁺ co-doped tellurite fiber, the two peaks at 480 nm and 540 nm are due to Tm³⁺:¹G₄→³H₆ and Ho³⁺:(⁵S₂,⁵F₄)→⁵I₈, respectively. The 480 nm blue emission is hard to be detected in Tm³⁺ doped tellurite glasses and fibers, but the backward energy transfer from Ho³⁺ to Tm³⁺ makes the blue and green emissions be easily detected. The up-conversion emission has a negative impact on the Ho³⁺ fiber laser.

 

Fig. 4 The visible up-conversion emission spectrum of Tm³⁺-Ho³⁺ co-doped tellurite fiber (left); the fiber with strong visible emission (right).

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Figure 5 is the sketch of laser experiment setup, for the fiber laser performance reported here, the Tm³⁺ doped or Tm³⁺-Ho³⁺ co-doped fibers were straightly cleaved on both ends. The multimode output from the laser diode was collimated and then focused on the tungsten tellurite single mode fiber, using a 10 × microscope objective. The wavelength of the pump laser was 793 nm. The optical cavity comprised a reflective mirror coated with a dichroic thin film with high reflectivity near 2050 nm or 1900 nm and anti-reflect near 800 nm. Fresnel reflection of approximately 13% from the other cleaved end of the Tm³⁺ or Tm³⁺-Ho³⁺ co-doped tellurite glass fiber functioned as the partially reflective mirror of the laser cavity. Figure 6 (right) shows the lasing spectrum from a 20 cm length of the Tm³⁺ doped tungsten tellurite single mode fiber, the lasing wavelength is centered at 1900 nm, which is ~80 nm longer than the emission peak in the bulk glass. The maximum laser output power is 494 mW, the slope efficiency is 26%. The beam quality and beam profile are shown in Fig. 6(left), the beam quality factor M² is 1.09. A 48 mW laser output has also been achieved in a 4 cm length of this fiber, making the fiber a promising active medium for 2 μm single frequency seed laser. Figure 7 shows the lasing spectrum of a 7 cm length of the Tm³⁺-Ho³⁺ co-doped tellurite single mode fiber, the lasing wavelength is centered at 2046 nm, the laser output power is 35 mW, the inset is the cross section of Tm³⁺-Ho³⁺ co-doped tellurite SM fiber, the core diameter is ~9 μm.

 

Fig. 5 Sketch of Tm³⁺ and Tm³⁺-Ho³⁺ co-doped tungsten tellurite fiber laser experiment setup.

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Fig. 6 The Tm³⁺ doped tungsten tellurite SM fiber beam quality measurement, the inset in the beam profile (left); the spectrum of Tm³⁺ doped tellurite fiber laser, the inset is the laser output power versus pump power (right).

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Fig. 7 The spectrum of the Tm³⁺-Ho³⁺co-doped tungsten tellurite fiber laser, the inset is cross section of the fiber.

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4. Conclusion

We have successfully developed Tm³⁺ doped and Tm³⁺-Ho³⁺ co-doped tungsten tellurite single mode fiber. The fiber has a ~9 μm core diameter with a NA of 0.14, and a ~100 μm inner cladding diameter with a NA of 0.29. The propagation loss of the fiber was ~2.5 dB/m at 1310 nm. ~2 μm spectroscopic and lasing performance of the single mode fibers were investigated with a commercial 800 nm laser diode. A 494 mW laser operating at ~1.9 μm was achieved in a Tm³⁺ doped 20 cm long fiber, the slope efficiency was 26%, and the laser beam quality factor M² was 1.09, a 48 mW laser output has also been achieved in a 4 cm length of this fiber. A 35 mW ~2.1 μm laser output was demonstrated in a 7 cm long of Tm³⁺-Ho³⁺ co-doped tungsten tellurite SM fiber. The results indicated that the fiber could be a promising active medium for 2 micron single frequency seed laser.