A 30 mW Laser Oscillator at 2.72 μm and 2.8 μm Wavelengths Based on Er³⁺-Doped Tungsten–Tellurite Fibers

Abstract

The purpose of this paper was to develop fiber lasers in the 2.7–2.8 μm range based on the tungsten–tellurite glass fiber that is technically robust compared to the other fibers currently used in laser engineering. Using an advanced technology for producing ultra-dry tellurite glasses, we manufactured Er³⁺-doped tungsten–tellurite glass preforms with extremely low absorption and obtained active single-mode tungsten–tellurite fibers. Based on a 70 cm long fiber, we developed a laser oscillator pumped by a low-cost, high-efficiency diode laser at 976 nm. At the highest used pump power, the laser output reached 33 mW, which may be interesting for practical applications. We also measured the single-pass on/off gain of the fibers and showed that with increasing pump power amplification, as high as 5 can be reached, showing that such active fibers may also be used for increasing laser output.

1. Introduction

Fiber lasers in the 3 µm range are in high demand, mainly due to their applications in spectroscopic studies of gas molecular components as well as the remote sensing of the atmosphere, since the range of ~2.7–3 µm is characterized by a very high absorption of hydroxyl groups and contains many molecular vibrational absorption lines [1,2]. It is widely recognized that the most suitable candidates for this wavelength range are erbium ions with the ⁴I_11/2-⁴I_13/2 transition of Er³⁺. Moreover, this transition can be easily pumped by commercial laser diodes. However, it is very important to find a host material which, on the one hand, meets both the optical and physical requirements for efficient lasing and, on the other hand, is simple enough to be implemented in practice. Currently, the most significant progress has been made for laser systems based on erbium-doped fluoride fibers, particularly with the use of ZBLAN fibers [3,4,5,6,7]. However, despite the great success in using fluoride fibers in lasers, their commercialization is a formidable task, stemming mainly from the drawbacks of such fibers, e.g., mechanical brittleness, a tendency to crystallization, absorption of atmospheric moisture, and a low softening temperature, which result in a lack of commercial proposals. At the same time, there is another class of more-stable glasses and, accordingly, fibers based on such glasses, namely tellurite glasses, which can also be used in the 3 µm range. Tellurite glasses show a good thermal property and high doping concentration of rare-earth ions without clustering, which make them a good luminescent material [8,9,10,11]. It should be noted that tellurite-based fiber lasers have recently been realized, demonstrating their new potential and flexibility in the near-infrared range [12,13,14]. Of course, the ⁴I_11/2-⁴I_13/2 transition of Er³⁺ in these host materials has poor generation properties due to the short lifetime of the upper laser level, but their more acceptable operational properties can be useful for creating commercially available fiber lasers. There are two main categories of tellurite glass, each of which has advantages of its own for use in fiber laser development. These are TeO₂-ZnO-Na₂O (TZN)- and TeO₂-WO-La₃O₂ (TWL)-based systems. The TZN-based system has a rather low phonon energy, mainly due to the vibration of Te-O banding at 760 cm⁻¹ [8,15]. Recently, a laser output of about 70 mW at 2.8 µm was realized based on Er³⁺-doped zinc tellurite glass fibers and broadband amplification in the wavelength range of 2.6–2.9 µm was studied [16,17]. On the contrary, the TWL system with W-O banding located at 930 cm⁻¹ in Raman spectra provides a high phonon energy [8,15]. Correspondingly, the disadvantage of tungsten–tellurite glass as a host for erbium ions is the short lifetime of the upper laser level related to the ⁴I_11/2-⁴I_13/2 transition in singly Er³⁺-doped glass. It is about 100–110 μs, half of that compared with zinc tellurite glass, in which the lifetime of the upper laser level is about 210–220 μs [18]. However, the TWL system has a very good thermal stability due to the W-O banding in the network. There is no obvious crystallization temperature in some TWL glasses; therefore, they are quite beneficial for fiber drawing. Lasing in Er³⁺-doped tungsten–tellurite glass fibers was demonstrated only in 2022 and a few mW output was obtained [19]. In the presented work, we continued the efforts to develop fiber lasers based on erbium-doped tungsten–tellurite fibers by both improving the properties of the fibers and increasing the input power of the diode pump, which can increase the emission characteristics of the active medium, since the ⁴I_11/2-⁴I_13/2 transition of Er³⁺ is known to be self-terminating. We manufactured Er³⁺-doped tungsten–tellurite glass preforms with extremely low absorption using an advanced technology for producing ultra-dry tellurite glasses [20] and obtained active single-mode tungsten–tellurite fibers. We measured the luminescence spectra of the fibers and showed that with increasing pump power their efficiency drastically changed towards longer wavelengths, from 2.72 μm towards longer wavelengths, up to 2830 nm. We developed a fiber oscillator pumped by a low-cost, high-efficiency diode laser at 976 nm. At the highest used pump power, the laser output reached 33 mW, which may be of interest for practical applications. We also measured the single-pass on/off gain of the fibers and showed that with increasing pump power a fivefold gain can be obtained with a 70 cm long fiber, showing that such active fibers may also be used for increasing laser output.

2. Materials and Methods

2.1. Tungsten–Tellurite Erbium-Doped Fiber

2.1.1. Glasses and Preform Preparation

Preforms for the manufacture of an erbium-activated fiber-optic core and two shells were obtained from specially synthesized tungsten–tellurite glasses with a low content of hydroxyl groups according to the technique described in [20]. For more efficient generation of laser radiation, it is advisable to further reduce the concentration of OH groups by increasing the duration of melting in dry oxygen using bubbling or special reagents.

A photo of the preform is shown in Figure 1a. The preform consists of a core glass with the composition 70.35TeO₂-23.45WO₃-2.4La₂O₃-2.2Bi₂O₃-1.6Er₂O₃ activated by erbium ions in the concentration of ~6 × 10²⁰ Er³⁺ cm⁻³ and an inactive cladding with the composition 71.1TeO₂-23.7WO₃-4La₂O₃-1.2Bi₂O₃. The jacketing tube (Figure 1b), molded from the 72TeO₂-24WO₃-4La₂O₃ glass, was used to obtain a second cladding of the fiber.

The fiber manufacturing procedure involved stretching the preform into a rod with a diameter of several millimeters, inserting a piece of the rod into a jacketing tube, and drawing the fiber out of the resulting structure at reduced pressure in the hollow.

The refractive indices of the glasses constituting the fiber were measured at a wavelength of 1539 nm; they were equal to 2.0926 for the core, 2.0893 for the first cladding, and 2.0832 for the second cladding.

2.1.2. TWL Fiber Production and Its Properties

From the preform described earlier, we drew an optical fiber with a core diameter of 12 µm, and the first and second cladding diameters of 60 and 190 µm, respectively. A photograph of the cross-section of the resulting fiber is shown in Figure 2.

The core diameter was chosen to provide single-mode propagation of radiation at a wavelength of 2.7 µm. Accordingly, the corresponding calculated numerical aperture for the fiber core was 0.12. The first cladding was a light guide for the pump radiation with a numerical aperture of 0.16, which allowed us to use commonly used multimode laser diodes for pumping.

Table 1. Er-Doped Tungsten–Tellurite Fiber Parameters.

Fiber Parameter	Parameter Value
Er2O3 concentration	1.6 mol.%
Core diameter	12 µm
First cladding diameter	60 µm
Second cladding diameter	190 µm
Core numerical aperture	0.12
First cladding numerical aperture	0.16
Core OH absorption coefficient near 3 µm	0.01 cm−1
Cladding OH absorption coefficient near 3 µm	0.006 cm−1

shows the detailed parameters of the fiber under study.

It is worth noting that the dark red spots in the center of the fiber in Figure 2 are due to dust sticking during the cleaving process. In all subsequent experiments, the fiber ends were additionally cleaned of dust.

2.2. Experimental Setup

The optical scheme of the experimental setup for studying the laser and luminescent characteristics of the erbium-doped TWL fiber is shown in Figure 3. The radiation of a multimode laser diode with a fiber-optic output of optical power was used to pump erbium ions. The output power of the diode radiation varied in the range from 0 to 130 W. The laser diode radiation was stabilized at a wavelength of 976 nm. A multimode optical fiber with a core-to-cladding diameter ratio of 105/125 μm was used to output the radiation. The fiber had a numerical aperture of 0.18. A plano-convex lens with an antireflective coating was used to collimate the laser pumping radiation. The focal length of the lens was 35 mm. The pump radiation was coupled into the first cladding of the erbium-doped TWL fiber by focusing the radiation using an uncoated aspherical calcium fluoride (CaF₂) lens with a focal length of 20 mm. The two-lens system compressed the pump radiation diameter by a factor of 1.75, which increased the efficiency of pump radiation coupling.

To analyze laser radiation at the output, we used a dielectric dichroic mirror (DM) with a reflectivity of ~90% in the wavelength range of 2.7–2.8 μm and a transmittance of diode pump radiation of ~85%. The mirror was positioned at a slight angle to the pump and signal radiation. To block unwanted pump reflections and parasitic luminescence at a wavelength of 1.5 μm, we used 5 mm thick crystalline germanium plates placed in front of the photodetector (PD) and power meter (PM), which completely blocked any radiation with a wavelength shorter than 1.7 μm. In all of the studies below, pumping was performed in the pulsed mode with a fixed pulse duration of 130 μs and different repetition rates. The duration of the pump pulses was chosen to be somewhat longer than the lifetime of erbium ions at the upper laser level of ⁴I_11/2, since the maximum output power of laser radiation was experimentally observed at this duration of the pump pulses. The length of the active erbium fiber used in the studies was 70 cm. This length was chosen to ensure optimal absorption of pumping in the fiber. With a shorter length, a large amount of unabsorbed pumping is present at the fiber output, which reduces the gain; whereas with a longer length, a section of weakly pumped fiber remains, which introduces additional radiation losses inside the cavity. In the luminescence studies, the pump radiation was input and the radiation under study was output through a perpendicularly cleaved end of the fiber, while the opposite end was broken to avoid unwanted lasing. In the lasing studies, the fiber was cleaved perpendicularly at both ends, one of which was butt-coupled to a dielectric mirror with a reflectivity of 97% (2.7–2.8 μm range), while the other provided a Fresnel reflection of 12%. The laser output power was measured using a pyroelectric power meter (S401C, Thorlabs, Newton, NJ, USA) and calculated considering the reflectivity of the dichroic mirror (DM) and the transmittance of the germanium filter (LPF). The laser and luminescence spectra were measured using a scanning monochromator (MS2004i, SOL Instruments, Augsburg, Germany) with an InSb IR photodetector (IS-0.50, Infrared Associates, Stuart, FL, USA) cooled with liquid nitrogen.

3. Results and Discussion

3.1. Luminescent Properties

Luminescence in the 2.7–2.8 μm region at the ⁴I_11/2-⁴I_13/2 transition in tellurite glasses was actively studied, for example, see [15,18,21]. For various compositions of tungsten–tellurite glasses, the lifetime of the ⁴I_11/2 level (~100 μs) is much shorter than the lifetime of the ⁴I_13/2 level (several ms) due to the moderate phonon energy of 930 cm [8,16]. With continuous wave (CW) pumping at the ⁴I_15/2-⁴I_11/2 transition, this leads to a high population at the ⁴I_13/2 level and a small population at the ⁴I_11/2 level. Under diode pumping at 976 nm in Er³⁺-doped tungsten–tellurite glasses, the luminescence intensity in the 2.7 μm region increases with an increasing concentration of Er₂O₃ [15]. The main channel of nonradiative relaxation in tellurite glasses activated with Er³⁺ is quenching on vibrations of OH groups. Due to the presence of an OH absorption band of about 3 μm, the internal energy of Er³⁺ at the ⁴I_13/2 level is converted to the vibration energy of two hydroxyl groups. As a result, OH ions cause both nonradiative relaxation of the excited energy level and absorption of Er³⁺ luminescence radiation at a wavelength of about 1.5 μm. These effects are even more pronounced at the ⁴I_11/2-⁴I_13/2 transition, since only one hydroxyl group is involved [22]. Thus, to obtain an active medium for lasing, it is especially important to synthesize only glass with a minimum hydroxyl concentration.

As a starting point in our studies, we also measured the luminescence spectra in a 70 cm long TWL fiber under different pumping conditions. Since the pump pulse duration was fixed in all our experiments, hereinafter we will operate with the pump energy, which was proportional to the peak power of the pump pulse in our case. Figure 4 demonstrates the luminescence spectra in the wavelength range of 2.6–2.9 μm depending on the pump energy. The pump pulse repetition rate was 100 Hz and was chosen to completely unload the lower laser level ⁴I_13/2, the lifetime of which is 5 ms.

Figure 4 shows the luminescence spectra of an erbium-doped TWL fiber exposed to pulsed pump radiation with a wavelength of 976 nm. One peak at pump pulse energies of less than 1 mJ is observed in the luminescence spectrum in the region of 2.72 μm. The long-wavelength wing of the spectrum decreases to a wavelength of 2.8 μm. With an increase in the pump pulse energy, a second peak appears in the luminescence spectrum at a wavelength of 2.78 μm. The intensity of the two peaks in the spectrum becomes comparable at pump radiation energies above 3 mJ. We associate these spectral changes in luminescence with the processes of energy transfer up-conversion (ETU) of ⁴I_13/2 + ⁴I_13/2 → ⁴I_15/2 + ⁴I_9/2 with an increase in the pump pulse energy. The ETU process results in broadband filling of the upper laser level, which, as will be shown below, significantly affects lasing behavior.

3.2. Lasing in the TWL Fiber

3.2.1. Lasing Efficiency

To demonstrate the operation of an erbium-doped TWL fiber, we created an oscillator based on a piece of such a fiber and studied its spectral and temporal characteristics. The laser cavity was formed by a multilayer dielectric mirror deposited on a zinc selenide substrate with a reflectivity of ~97% in the wavelength range of 2.7–2.8 μm, on one side, and a perpendicular cleavage of the optical fiber, providing a Fresnel reflectivity of ~12%, on the other side of the active fiber (see Figure 3). The laser diode pump radiation was introduced into the active light guide opposite to the output of laser radiation in order to avoid optical damage to the highly reflective dielectric mirror.

Laser generation was studied at a diode pump pulse repetition rate of 50, 100, 200, and 300 Hz and a pulse duration of 130 μs. The dependence of the laser output energy on the pump pulse energy at different repetition rates is shown in Figure 5.

The minimum lasing threshold for the pump energy was ~0.6 mJ at a pump pulse frequency of 50 Hz, while the slope efficiency calculated in the most linear part of the graph (pump energy range of 1–3 mJ) was ~2.2%. The maximum output laser pulse energy was ~0.16 mJ, which corresponds to an average radiation power of ~8 mW at a pulse repetition rate of 50 Hz. The maximum average output power of the laser was obtained at a pump pulse repetition rate of 300 Hz. The maximum energy of the generated laser pulses in this mode was ~0.11 mJ, and the average output power of the laser was up to 33 mW. The laser output power (see Figure 5) increased at a pulse repetition rate of 300 Hz compared to 50 Hz due to the fact that while the laser pulse energy decreased from 0.16 mJ to 0.1 mJ, the pulse repetition rate increased by a factor of 6. We believe that the drop in pulse energy with an increase in the repetition rate was caused by incomplete unloading of the lower laser level during the time between pump pulses. The possibility of further increasing the pump pulse repetition rate to increase the output laser power was limited by the lifetime of erbium ions at the lower laser level ⁴I_13/2, which in our case was ~5 ms.

3.2.2. Lasing Spectra

The laser output spectra at different pump energies were recorded in the wavelength range of 2.65–2.85 μm at a pump pulse repetition rate of 100 Hz, and they are shown in Figure 6.

As can be seen in Figure 6, at low pump energies of ~1 mJ, near the threshold, lasing predominates in the spectral range of 2.72 μm. With an increase in the pump pulse energy to 1.6–12 mJ, intense lasing was observed in two spectral wavelength ranges of 2.72 μm and 2.8 μm. The width of the optical spectrum in the region of 2.8 μm measured at half the maximum value was ~12–15 nm, which became possible due to the expansion of the luminescence spectrum and an increase in laser gain in the fiber due to the up-conversion of erbium ions Er³⁺ from the excited state ⁴I_13/2, as well as a wide reflection band of the dielectric mirror in the laser cavity. It should be noted that at all pump pulse energy levels, the short-wavelength peak in the lasing spectrum remained at a wavelength of 2718 nm and did not shift with increasing pump energy, while the position of the lasing peak in the 2.8 μm region depended on the pump pulse energy. In our case, the initial position of the lasing peak in the long-wavelength region of the spectrum was at a wavelength of 2792 nm, and then shifted toward longer wavelengths by 14 nm, as the pump energy increased from 0.6 mJ to 12 mJ. We believe that this long-wavelength shift was due to the broadening of the luminescence spectrum and the appearance of the second peak, as shown in Figure 4.

The optical spectra of the laser in the Er:TWL fiber with a high erbium oxide concentration (~1.6 mol.% Er₂O₃) when pumped by 130 ms pulses depended significantly on the pump pulse energy and differed from the spectra for fibers with a low erbium oxide concentration studied earlier [10,11]. For the Er:TWL active fiber with an erbium oxide Er₂O₃ concentration of ~(0.4 mol.%), the optical spectrum of the laser radiation was located only in the region of 2.72 μm. The maximum lasing power was observed in the wavelength range of 2.8 μm with a spectral band of ~15 nm at a pump pulse energy of ~12 mJ. We believe that efficient lasing in the 2.8 μm region with increasing pump pulse energy became possible due to the activation of the pumping mechanism of the upper laser level ⁴I_11/2 by means of the up-conversion process (ETU) in erbium ions from the long-lived laser level ⁴I_13/2 to the excited state ⁴I_11/2. We also measured the transverse profile of the laser beam using an Ophir Pyrocam IV beam profiler (inset in Figure 5). The laser beam had a nearly symmetric intensity distribution, indicating that the propagation of light in the fiber was close to the single-mode regime.

3.2.3. Temporal Profile of Laser Pulses

Since the obtained results show a qualitative difference at pump energies higher than 4 mJ, we decided to investigate the temporal structure of the lasing process during the pump pulse. The shape of the laser pulses was studied at two wavelengths of 2718 and 2795 nm, which were selected using a scanning monochromator. The pulse shape was recorded at the output of an InSb photodetector (IS-0.50, Infrared Associates, Stuart, FL, USA) using an oscilloscope (TDS 3052B, Tektronix, Beaverton, OR, USA). Figure 7 shows the pulse shapes at the laser output at different wavelengths and pump energies.

Near the lasing threshold at a pump pulse energy of ~1.1 mJ, lasing started with a single pulse of about a 2 μs duration. Figure 7a shows a sequence of laser pulses like that observed earlier for zinc tellurite fibers. The laser pulse was generated with a delay of ~55 μs from the start of the pump pulse at a wavelength of 2718 nm. With increasing pump pulse energy, the amplitude of the generation pulse increased, and then the transition to the multi-pulse sequence (train) of pulses occurred. At a pump pulse energy of ~4.6 mJ, the dynamics of pulse generation changed significantly.

As is shown in Figure 7b, with an increase in the pump pulse energy, the lasing pulses appear at wavelengths in the region of 2.8 μm. We believe that the lasing process in the long-wavelength region of the spectrum is caused by the up-conversion mechanism of the erbium ion transfer from the excited state from the ⁴I_13/2 level. With an increase in the pump pulse energy, the time shift of the lasing pulses in the region of 2.8 μm is observed from the trailing edge of the pump pulse to the leading edge due to an increase in the population inversion. With an increase in the pump pulse energy to 5.2 mJ, the lasing at a wavelength in the region of 2.72 nm occurs in the time interval from the beginning of the leading edge of the pump pulse with a delay of ~11 μs. The lasing pulses in the region of 2795 nm are in the time range from 80 to 115 microseconds from the leading edge of the pump pulses. Thus, near the trailing edge of the pump pulse, lasing at long waves in the 2.8 µm region is observed, which predominates at high pump powers.

3.3. On/Off Gain

To gain an insight into the amplification properties of the Er³⁺-doped TWL fibers, our next step was to measure the on/off gain. We used a coherent supercontinuum source as a seed in our experiments, and the gain measurement was performed in a pulsed mode, considering the lifetime of the lower laser level. The on/off gain measurement technique is described in more detail in our earlier work [23]. Since the luminescence spectra had two clearly defined peaks at wavelengths of 2.7 and 2.8 µm, the on/off gain coefficients were also measured at these two wavelengths. Figure 8 shows the experimental dependences of the gain on the pump pulse energy.

The pump pulse energy in this experiment was limited to 1 mJ, since at higher energies, we observed parasitic lasing, which did not allow correct measurement of the gain. As follows from Figure 8, at low pump pulse energies of ~0.4 mJ, the on/off gain in our fiber at both wavelengths was almost the same. With increasing pump energy in the range of 0.4–1 mJ, the gain at a wavelength of 2795 nm grew faster than at a wavelength of 2718 nm and reached a value of ~5. We believe that the sharp increase in gain in the long-wavelength region of the spectrum was associated with the processes of up-conversion of erbium ions at the lower laser level with an increase in the energy of diode pump pulses. Summarizing the above, we can conclude that at a high pump energy, the laser dynamics should be investigated not only in the wavelength range of 2.7 μm, but also in the range of 2.8 μm.

4. Conclusions

The main goal of this work was to develop fiber lasers in the 2.7–2.8 μm range based on the technically robust tungsten–tellurite glass fiber. The results obtained can be summarized as follows: First, single-mode Er³⁺-doped tungsten–tellurite fibers with extremely low absorption have been manufactured using an advanced technology for producing ultra-dry tellurite glass preforms. It has been shown that the luminescence spectra of erbium-doped tungsten–tellurite glass fibers with a doping concentration of 1.6 mol.% Er₂O₃ (and higher) can be effectively controlled by pump energy. With increasing pump energy, the long-wavelength wing can be effectively extended, covering more than 2.8 μm, which points to an important role of the ETU process. Second, we have developed a laser oscillator based on a 70 cm long fiber, pumped by a low-cost, high-efficiency diode laser at 976 nm. To gain an insight into the nature of laser generation, we have investigated the temporal dynamics of the process of generating spectrally resolved pulses in detail. We have shown that when using broadband mirrors, lasing starts at a wavelength of 2718 nm, corresponding to the maximum luminescence during the first ~65 μs from the start of the pump pulse. The generation of laser pulses with a greater delay relative to the leading edge of the pump pulse occurs in the region of 2.8 μm at high pump pulse energies. Thus, we have experimentally demonstrated how the transition from the generation process at a wavelength of 2.7 μm to a longer-wavelength generation at 2.8 μm occurs with an increase in the pump pulse energy. It has also been shown that the highest generation efficiency is achieved in the wavelength range of 2.8 μm, which agrees well with the second luminescence peak caused by the ETU process. The conducted studies have allowed us to create an erbium-doped tungstate–tellurite-fiber oscillator with a pulse energy of ~0.1 mJ and a pulse repetition rate of 300 Hz, which is ~30 mW of the output power and may be of practical interest. Third, we have demonstrated that in a 70 cm long Er³⁺-doped TWL fiber, an on/off gain as high as 5 can be reached, which allows for implementation of the MOPA configuration to obtain a more powerful laser output.

As compared to the previous results, we can say that the most efficient lasing (60 mW average power) was obtained in a zinc tellurite fiber with an erbium oxide concentration of 2.5 mol.% [16,17]. As for tungstate–tellurite fibers, the best achieved lasing was only 12 μW in a fiber with an erbium oxide concentration of 0.4% [19]. Thus, in this work, we have shown that increasing the concentration of erbium oxide (up to 1.6 mol.%) together with a correct cavity design and high peak pump power allow for creating a laser with an average power comparable to that of a zinc tellurite fiber. We believe that thanks to better mechanical and thermal properties, erbium-doped tungstate–tellurite fibers can become good candidates for constructing both lasers and high-power amplifiers in the mid-IR range.