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Review

Laser Sources Based on Rare-Earth Ion Doped Tellurite Glass Fibers and Microspheres

Federal Research Center Institute of Applied Physics of the Russian Academy of Sciences, 46 Ul’yanov Street, Nizhny Novgorod 603950, Russia
Fibers 2020, 8(5), 30; https://doi.org/10.3390/fib8050030
Received: 6 March 2020 / Revised: 27 April 2020 / Accepted: 1 May 2020 / Published: 11 May 2020
(This article belongs to the Special Issue Fiber Laser Sources)

Abstract

In recent years, huge progress has been made in the development of rare-earth ion doped tellurite glass laser sources, ranging from watt- and multiwatt-level fiber lasers to nanowatt level microsphere lasers. Significant success has been achieved in extending the spectral range of tellurite fiber lasers generating at wavelengths beyond 2 μm as well as in theoretical understanding. This review is aimed at discussing the state of the art of neodymium-, erbium-, thulium-, and holmium-doped tellurite glass fiber and microsphere lasers.
Keywords: tellurite fiber laser; tellurite glass fiber; microlaser; microsphere laser; rare-earth ions tellurite fiber laser; tellurite glass fiber; microlaser; microsphere laser; rare-earth ions

1. Introduction

Tellurite glasses (based on tellurium dioxide TeO2) possess chemical stability, have a broad transmission range of ~0.4–6 μm, high linear refractive index n ~ 1.8–2.3 (against n ~ 1.4 for silica and n ~ 1.5 for fluoride glasses), high nonlinear refractive index, and relatively low phonon energies (~700–900 cm−1) [1,2,3,4,5,6,7,8,9]. Many compositions are resistant to crystallization. Additionally, by controlling crystallization, tellurite glass ceramics with promising characteristics can be made [1]. Detailed discussion of the physico-chemical properties of tellurite glasses can be found in the literature [1,2]. Current technologies allow creating high-purity low-loss “ultra-dry” tellurite glasses and fibers [8,9], so with allowance for substantial solubility of rare-earth ions, tellurite glass-based rare-earth-doped elements as active media for laser sources seem very attractive [4,10]. Due to the high refractive index n, the stimulated emission cross sections proportional to (n2 + 2)2/9n for tellurite glasses are ~1.5–2 times higher than for silica and fluoride ones [10]. Low phonon energies allow operation in the spectral range well beyond 2 μm, expanding the possibilities of achieving longer wavelength generation in comparison with silica glasses [11,12]. It should be noted that fluoride glasses make it possible to obtain even longer wavelength lasing than tellurite glasses; however, fluoride glasses are less resistant to atmospheric moisture.
In addition to choosing the material of gain medium, when developing photonic devices, considerable attention should be paid to the geometry of active elements. Fibers and microspheres have significant benefits in this respect. Thanks to the exploitation of waveguide light propagation, fiber lasers have advantages such as high efficiency of converting pump energy into radiation energy, efficient heat removal, and high quality of the spatial profile of the laser beam. Cavities for tellurite fiber lasers can be formed by bulk mirrors, reflective coatings deposited on fiber ends, fiber ends themselves, or by their combinations. Additional optical elements such as lenses or beam splitters can be also placed into a laser cavity. An example of a possible fiber laser scheme is shown in Figure 1.
For microsphere-based lasers, which can be used as miniature photonic devices for basic science and many real applications—for example, for sensing [13], a gain medium and a cavity are the same device that is a microresonator with whispering gallery modes (WGMs) [14]. Such microresonators typically have high Q-factors (quality factors) and small optical WGM volumes which make them suitable for constructing narrow linewidth and low threshold microlasers [13,14]. For efficient excitation of WGMs, evanescent-wave coupling is required. Commonly, coupling can be implemented using a tapered fiber with a waist diameter meeting phase matching conditions [14]. Coupling can be also realized by means of a prism by adjusting the angle of incidence to achieve phase matching [15]. An example of a possible microsphere laser scheme is shown in Figure 2.
In this review, we discuss laser sources utilizing fibers and microspheres based on rare-earth-doped tellurite glasses. Lasing was attained at the radiative transitions of the following ions: Nd3+ [16,17,18,19,20,21], Er3+ [22,23,24,25,26,27,28,29,30,31,32], Tm3+ [11,12,33,34,35,36,37,38,39,40,41,42,43,44,45,46], and Ho3+ [47,48,49,50,51,52,53,54,55,56]. In most works on tellurite fiber and microsphere lasers, generation was achieved in a continuous wave (CW) regime. For tellurite fiber lasers, several works were devoted to pulsed generation including via Q-switch [28,38,48] or pulsed pump [12]. The generation of ultrashort pulses via mode-locking has not been reported yet for tellurite fiber and microsphere lasers. Note that, in [1] there is the chapter “Lasers Utilising Tellurite Glass-Based Gain Media” [10] on the same topic with references to the research works published in 2015 and earlier. However, since 2015, significant progress has been made in the development of such laser sources, including achieving the watt and multiwatt level of laser radiation near 2 μm [52,53] and attaining generation in the range well above 2 μm in tellurite active elements [11,12]. Significant advances have been also achieved in the theory of such sources [8,11,29,53,56,57,58,59]. Therefore, this review is focused on the recent results. Nevertheless, an attempt of summarizing all the data on the reported tellurite glass fiber-based and microsphere-based lasers has also been made.

2. Neodymium

A rare-earth Nd3+ ion has an absorption band near 800 nm corresponding to the wavelengths of commercially available laser diodes and Ti:sapphire lasers [10]. Therefore, these laser sources are widely used for pumping Nd-doped gain media including tellurite glass based ones. Under pumping at a wavelength near 800 nm, Nd3+ ions are excited to the (4F5/2, 2H9/2) states from the ground level 4I9/2 via the ground state absorption. Next, rapid relaxation from short-lived (4F5/2, 2H9/2) states to the upper laser level 4F3/2 occurs due to non-radiative multiphonon decay. Furthermore, the emission band of Nd3+ peaking at about 1060 nm can be observed due to the 4F3/24I11/2 transition. A simplified scheme of Nd3+ energy levels is shown in Figure 3.
Note that Nd3+ ions can also be pumped at shorter wavelengths. For example, the first tellurite bulk glass-based laser operating near 1060 nm using Nd-doped sample pumped by an Argon laser at 514.5 nm was demonstrated in 1978 [60]. However, with the development of Ti:sapphire lasers and laser diodes, other pump sources for Nd-doped gain elements lost relevance.

2.1. Lasing in Nd-Doped Fibers

The first tellurite glass fiber laser was reported in 1994 [16]. A 0.6-m piece of fiber with 76.9TeO2-15.5ZnO-6.0Na2O-1.5Bi2O3-0.1Nd2O3/75TeO2-20ZnO-5Na2O core/cladding glass compositions was pumped by a Ti:sapphire laser at 818 nm. The fiber had an elliptical core and the resonator was organized by Fresnel reflection with a coefficient of 11.9% from each fiber end; external mirrors were not used. The slope efficiency for the power output from one end of the fiber was 23% relative to pump power. Assuming equal emission from both ends, the slope efficiency above threshold was estimated to be 46%. The threshold was approximately 27 mW. Laser generation was single-modal for pumping above the threshold, and became multimodal for pumping well above the laser threshold. The maximal output power from one fiber end exceeded 4 mW [16].

2.2. Lasing in Nd-Doped Microspheres

Lasing in the ~1058–1075 nm range in Nd-doped tellurite microspheres using Ti:sapphire lasers for pumping was reported in several research works [17,18,19,20,21]. The first tellurite microlaser was demonstrated in 2002 [17]. Microspheres with diameters ranging from 50 μm to a few hundred μm were produced by melting the end of a glass wire using a Kanthal wire heater [17]. Microspherical resonators were pumped at 800 nm by a Ti:sapphire laser. The pump light was focused to a point slightly shifted from the center of the microsphere. The output laser radiation at ~1.06 μm was collected using a multimode fiber. The microsphere laser with a diameter of 140 μm had an incident pump threshold of 81 mW, but pump coupling was not optimized [17].
In 2003, it was shown that the oscillation wavelengths of a microlasers could be adjustable using quarter wavelength shifted distributed feedback resonators produced on the surface of microspherical samples [18]. Generated wavelengths agreed with the Bragg wavelengths calculated using the known periods of gratings and the effective refractive indices of WGMs. Tapered optical fibers were used for both coupling the pump light and extracting the output radiation. The threshold of the laser based on a microsphere with a Q-factor of ~105 was lower than 40 mW. Single-mode and multimode generation was observed in the ~1058–1075 μm range for different sphere diameters of ~70–180 μm [18].
Air-bubble-containing Nd-doped tellurite glass microspheres were produced and investigated [21]. The localized laser heating technique was applied for sample fabrication. When the edge part of the microspherical sample with a diameter of 20 μm containing an air bubble with a diameter of 1.6 μm was pumped, periodical spectral peaks were generated. These peaks corresponded to WGMs. However, when the bubble position was pumped, not only WGMs but also a continuous range of other wavelengths were excited. It was shown that the introduction of an air bubble into the microresonator led to “non-WGMs excitation”, reducing the threshold [21]. The lowest reached lasing threshold was 0.034 mW for an air-bubble-containing 4-μm microsphere, which agreed with numerical estimations [21].

3. Erbium

A rare-earth Er3+ ion has an absorption band near 980 nm corresponding to the wavelengths of low cost commercial laser diodes which are standard telecom components. Radiation at ~980 nm via the ground state absorption from the 4I15/2 level excites the 4I11/2 level (see Figure 4). After that, the transition from the 4I11/2 to the 4I13/2 level occurs. For silica glass, due to high phonon energy, the 4I11/2 level is depopulated non-radiatively, but for tellurite glasses, the radiative transition near the 2.7–2.8 μm range is possible [61]. After that, the 4I13/2 level is populated and lasing in the 1.53–1.6 μm range at the 4I13/24I15/2 transition can be attained (see Figure 4). Therefore, tellurite glasses can be used as a gain medium for lasing in the 1.53–1.6 μm range as well as in the 2.7–2.8 μm range. For lasing in the 1.5 μm region, in-band pump by standard laser diodes at 1480 nm or silica fiber lasers can be also used. Er-doped tellurite fibers are promising for L-band lasers and amplifiers [30] due to lower excited-state absorption and higher values of stimulated emission cross-sections in comparison with Er-doped silica fibers for this spectral range [30].

3.1. Lasing in the 1.5 μm Region in Er-Doped Fibers

The first signal amplification and laser oscillation at a wavelength of 1560 nm in Er-doped tellurite fibers was claimed in [22]. A small-signal gain of 16 dB was achieved when a pump power was 130 mW and a pump wavelength was 978 nm. Lasing with a slope efficiency of 0.65% was obtained in a 85-cm fiber. A threshold pump power was 120 mW. A fiber cavity was based on the Fresnel reflection of 12.3% at each end; external mirrors were not used [22]. Since then, Er-doped tellurite fiber lasers have been developed and investigated [23,24,25,26,27,28,29,30].
A CW Er-doped zinc-tellurite fiber laser was recently reported in [29]. A single-mode generation was attained in a 20-cm long piece of fiber with single-mode 975-nm diode pumping. The pump beam was coupled into the core of the fiber using a beam splitter put into a laser cavity which was formed by two aspheric lenses. Laser radiation at 1555 nm was extracted through the beam splitter [29].
L-band wavelength-tunable CW Er-doped tellurite fiber lasers were reported in [30]. Lasing tunable in the 1589–1627 nm range was obtained in the 3-m long fiber made of TeO2–ZnO–La2O3 glass. A fiber laser with a maximum output power of 2.5 W at a wavelength of 1570 nm was used as a pump source [30]. A laser cavity contained a mirror with a high reflectivity, a fiber coupler, a wavelength division multiplexer, an active tellurite fiber, a collimation lens, and diffraction grating. The wavelength tuning was realized with the Littrow configuration [30].
Self-Q-switching behavior of Er-doped tellurite microstructured fiber lasers was reported in [28]. Using a 1480-nm pump with a power of 705 mW, output 282-ns laser pulses with a repetition rate of 1.14 MHz at 1558 nm were obtained in the 14-cm piece of the active fiber using a linear cavity. The maximum average power was 316 mW. The slope efficiency of 72.6% was reached before the saturation. The authors explained the self-Q-switching behavior by the reabsorption originating from the ineffectively pumped part of the Er-doped tellurite microstructured fiber [28].
A high-performance Ce/Er co-doped tellurite fiber laser and a fiber amplifier dual-wavelength pumped at 980 nm and 1480 nm were presented in [23]. A 22-cm long piece of gain fiber in a fiber ring laser configuration was used to generate radiation with a tuning range of 83 nm (from 1527 to 1610 nm). The addition of Ce3+ co-doping with Er3+ was used for a resonant transfer to Ce (the 2F5/22F7/2 energy transition) from Er (the 4I11/24I13/2 energy transition), which reduced excited state absorption from the 4I11/2 level of Er and improved the gain characteristics at ~1.6 μm [23].

3.2. On the Possibilities of Lasing in the 2.7–2.8 μm Region in Er-Doped Fibers

Luminescence in the 2.7–2.8-μm region at the 4I11/24I13/2 energy transition in tellurite glasses was first observed in [62] and was later reported by various research groups both, in glasses and in fibers [61,63,64,65,66,67]. However, master oscillators or amplifiers at 2.7–2.8 µm based on Er-doped tellurite glass fibers or other tellurite glass optical elements have not been published in refereed journals yet, but studies in this area are being conducted. The first reason for the absence of such coherent light sources is associated with high absorption by OH groups in the considered range. The next reason is that the lifetime τ3 of level 4I11/2 is much less than the lifetime τ2 of level 4I13/2 due to moderate phonon energy (~900 cm−1 for tugsten-modified tellurite glasses and ~750 cm−1 for zinc-modified tellurite glasses [1]). For zinc-tellurite glasses, τ3 is about 200–300 μs [29,61,67] but for tugsten-tllurite glasses τ3 is about 100 μs [8], whereas τ2 reaches a few ms for different glass compositions [29,61]. Under continuous wave (CW) pumping at the 4I15/24I11/2 transition, this leads to high population of the 4I13/2 level and small population of the 4I13/2 level. A possible way to solve the first challenge is to synthesize high-quality “ultra-dry” TeO2 glasses [8]. To overcome successfully the second challenge and effectively populate the upper level 4I11/2, different paths shown schematically in Figure 5 were proposed. These include:
  • Using heavy Er3+ concentrations to provide a significant up-conversion rate (4I13/2 + 4I13/24I9/2 + 4I15/2) (see Figure 5a) [61,66].
  • Using pulsed pumping with a duration of the order of τ3 and a low enough repetition rate (<1/τ2) (see Figure 5b) [8].
  • Using dual-wavelength cascade laser schemes at two consecutive radiative transitions 4I11/24I13/2 and 4I13/24I15/2 at 1.56 and 2.8 μm, respectively, see Figure 5c [29].
  • Using two-color pumping with the first pump wavelength of 980 nm (4I15/24I11/2) or 1.56 μm (4I15/24I13/2) and the second pump wavelength of 1.7 μm (4I13/24I9/2) (see Figure 5d) [59].
Regarding the first way, it was proposed to effectively depopulate the level 4I13/2 and populate the level 4I11/2 exploiting energy transfer up-conversion processes [61]. It was found experimentally that energy transfer rate parameters strongly depend on Er3+ ion concentration [61]. The higher the Er3+ content, the larger up-conversion rate is [61]. On the basis of experimental data, it was calculated that for Er3+ concentration >1.2·1021 cm−3 (or Er2O3 ~2.65 mol. % for zinc-telurite glass) and OH concentration equal to zero, a population inversion could be achieved for a pump intensity over 87 kW/cm2 at 976 nm [61]. The first theoretical analysis of ultrashort pulse amplification in the 2.7–3 μm range under diode pumping at 975 nm was performed in [66]. The opportunity of obtaining pulses with the duration shorter than 100 fs and the energy of order 10 nJ (with gain of ~20 dB) was shown numerically for fibers manufactured from high-quality TeO2–ZnO–Na2O–La2O3 glasses with Er3+ concentration of 1021 cm−3 in the core and a low concentration of OH groups [66].
As concerns the second way, a possibility of realizing a diode-pumped gain-switched laser with 100 Hz repetition rate based on specially developed Er-doped single-mode and multi-mode TeO2-WO3-La2O3-Bi2O3 glass fibers was proposed and investigated in [8]. When the pump is switched on, the upper laser level 4I11/2 starts to be populated. The initial population of the 4I13/2 level is equal to 0. When the pump threshold is attained, laser oscillations start to develop. The corresponding condition is that the signal gain for a cavity round trip is equal to the total losses. The total losses depend on the losses associated with radiation output from the cavity and the losses on optical elements including the fiber losses. When the first pump pulse is applied, the population n3 of the level 4I11/2 starts to increase linearly with time. At the moment of spike generation, n3 decreases and the population n2 of the level 4I13/2 increases by the similar value. For the pump duration comparable to τ3, the depopulating the upper 4I11/2 level via spontaneous non-radiative transitions affects significantly. However, the population of metastable level 4I13/2 with a lifetime of a few ms continues to grow. The generation efficiency becomes reducing and vanishes. Thus, the duration of pump pulses should be of the order of τ3. After this, the residual population relaxes from the 4I11/2 level to the 4I13/2 level with a characteristic lifetime τ3. Relaxation from the 4I13/2 level to the ground state 4I15/2 occurs with a characteristic time τ2 >> τ3. Therefore, the repetition rate of the pump pulses should be chosen low enough so that the vast majority of the ions populating the 4I13/2 level transits to the 4I15/2 level by the time the next pump pulse arrives [8].
The third opportunity concerns using dual-wavelength cascade laser schemes at 1.56 μm and 2.8 μm (which was demonstrated for fluoride fiber lasers doped with Er3+ ions with 50% slope efficiency at 2.8 μm exceeding the Stokes limit by 15% [68]). The first theoretical analysis of such a scheme for Er-doped tellurite fibers was performed in the paper [29]. A numerical model calibrated to the experimental data (concerning laser generation and amplification at 1.555 μm in TeO2–ZnO–La2O3–Na2O glass fibers doped with 0.24 mol % Er2O3 [29]) was developed for simulation and optimization of lasing parameters for schematics with different characteristics [29]. This model can describe single-wavelength laser generation and cascade dual-wavelength laser generation at 1.555 μm and 2.8 μm. It was demonstrated in numerical simulation that for the optimal parameters, the maximum slope efficiency at 2.8 μm at the 4I11/24I13/2 transition can reach ~20%. The maximum calculated efficiency at 1.555 μm exceeds 30%.
For the fourth opportunity based on using two-color pumping with the first pump wavelength of 980 nm (4I15/24I11/2) or 1.56 μm (4I15/24I13/2) and the second pump wavelength of 1.7 μm (4I13/24I9/2), the metastable level 4I13/2 plays a role of a virtual ground state [59]. Under pumping at the first wavelength, the intermediate 4I13/2 level is populated, and under pumping at the second wavelength, ions from the metastable 4I13/2 level transit to the short-lived 4I9/2 level, and after that a non-radiative rapid relaxation observed from the 4I9/2 level to the level 4I11/2. The theoretical investigation of lasing at the 4I11/24I13/2 energy transition under two-color pumping based on of the experimentally measured characteristics of an Er-doped core fiber made of TeO2-ZnO-Na2O-La2O3 glass with the concentration of Er3+ ions of NEr = 1020 cm−3 was performed in [59]. It was demonstrated numerically that, for the optimized length of the active tellurite fiber, the small-signal gain could be higher than 18 dB and lasing could be attained with a relatively low pump power of several hundreds of mW, whereas the maximum slope efficiency could reach ~40% [59].
Note that for numerical modeling of different schemes of Er-doped and other rare-earth ion-doped fiber lasers, full systems of equations, corresponding approaches and methods for their solution were presented and described in [69,70,71,72,73].

3.3. Lasing in Er-Doped Microspheres

Lasing in Er-doped tellurite microspheres have been demonstrated only at the 4I13/24I15/2 energy transition using 975-nm pump at the 4I15/24I11/2 transition [31] and using 1480-nm in-band pump at the 4I15/24I13/2 transition [32]. Microspheres were fabricated and investigated by the same research group [31,32]. A spin method was applied for fabrication. TeO2-based glass was melted in a furnace and after that dropped on a spinning platform. This melted glass was spun out under centrifugal forces and quickly cooled, which led to the formation of microspheres with diameters from a few μm to a few hundred μm by surface tension [31,32]. The Q-factors were estimated to be 105–106 [31,32] and 3 × 106 [32]. A silica fiber taper was used for both, pump coupling and extracting the generated radiation from microspheres. The generation was achieved in the 1560–1610 nm range by adjustment of pump power [31]. By tuning a relative position between the microresonator and the fiber taper and selecting the microsphere size, single-mode as well as multimode lasing in the L-band was reached [31]. The temperature dependence of the wavelength and the laser threshold was also studied in [32]. The wavelength tuning in the ~1606–1608 nm range was experimentally attained and numerically verified with allowance for the thermal expansion of the TeO2 -based glass. It was shown that the threshold increased with temperature [32].

4. Thulium

A rare-earth Tm3+ ion in tellurite glass matrixes has several radiative transitions at the 3F43H6 transition in the 1.9–2 μm range, at the 3H43H5 transition near 2.3 μm, and at the 3H43F4 transition near 1.5 μm as shown in Figure 6. To achieve generation at these transitions, pumping by commercial laser diodes at ~800 nm can be used to excite the 3H4 level from the ground level 3H6 via the ground state absorption at the 3H63H4 transition. To achieve generation only in the 1.9–2 μm range, in-band fiber laser pump near 1.6 μm can be used effectively.

4.1. Lasing in the 1.9–2 μm Range in Tm-Doped Fibers

The first Tm-doped tellurite fiber laser was reported in [33]. The generation in the 1.88–1.99 μm range was demonstrated in a 32 cm long fiber with core composition of 80TeO2–10Na2O–10ZnO mol % co-doped with 1.5 wt % Yb2O3 and 1.0 wt % Tm2O3, and the cladding composition of 75TeO2–10Na2O–15ZnO mol %. The cavity of the laser was formed by a high reflectivity mirror at the pumped end as well as Fresnel reflection of 12% at the opposite output fiber end. Tellurite fibers were in-band pumped by an Er/Yb co-doped silica fiber laser generating in the 1570–1610 nm range. The maximum obtained slope efficiency was 76%. This value was similar to the 82% Stokes limit. The maximum output power of 280 mW was a record one for tellurite fiber lasers [33].
In the further studies of Tm-doped tellurite fiber lasers, an increase in output power to ~0.4 W with in-band pump [40] and to ~0.5 W [36] and 1.12 W [35] with laser diode pump at ~0.8 μm was obtained. A watt level CW lasing at 1937 nm was reached from a specially designed highly Tm-doped double-clad fiber made of tungsten-tellurite glass [35]. The pumped end of the 40-cm long fiber was coated with a dielectric thin film, which was highly reflective for a laser wavelength in the range near 1950 nm and antireflective for a pump wavelength in the range near 800 nm. This coating was used as the first mirror of the laser cavity. As the second mirror of the laser cavity at the output, the fiber end with ~13% Fresnel reflection was used. The slope efficiency was 20%, and the optical-to-optical efficiency of 16% was reached relative to the absorbed pump power [35].
The first passively Q-switched Tm3+-doped tellurite fiber laser was reported in [38]. A semiconductor saturable absorber mirror (SESAM) as well as a carbon nanotubes (CNTs) inserting separately into the fiber laser cavity were used as saturable absorbers to obtain Q-switching. Pulsed lasing at a 1.86 μm without any self-mode-locking effects was observed in a piece of 9-cm long tellurite fiber pumped in-band at 1.59 μm. A CNT Q-switched laser generated 860-ns pulses with an average power of 84 mW, while a SESAM Q-switched laser generated 516-ns pulses with an average power of 21 mW. Pulse energies were 736 nJ and 193 nJ for CNT- and SESAM-based configuration, respectively [38].
The transition from supercontinuum generation to 1.887 μm CW lasing in a Tm-doped tellurite microstrucured fiber pumped by a femtosecond fiber laser system at 1.56 μm was observed for the first time in [37]. This transition occurred when varying pulse duration of the pump laser from 290 fs to 3.47 ps. The microstructured 20-cm long fiber made of Tm-doped 78TeO2-5ZnO-12Na2CO3-5Bi2O3 glass had a “wagon wheel” cross-section with a solid core surrounded by six air holes. With a decrease in the pump pulse duration from 3.4 ps to 1.3 ps, the threshold of the fiber laser operating at 1887 nm monotonically decreased from 232 mW to 216 mW. The corresponding slope efficiency varied in the 10.2–14.8% range [37].

4.2. Lasing near 2.3 μm in Tm-Doped Fibers

Achieving laser generation at the 3H43H5 transition near 2.3 μm is a more complicated problem than at the 3F43H6 transition in the 1.9–2 μm range, due to the lifetime of the 3H4 energy level is 10 times shorter than the lifetime of the 3F4 energy level (~0.3 ms against ~3 ms [74]), and the stimulated emission cross-section at ~2.3 μm is smaller than the stimulated emission cross-section in the 1.9–2 μm range [74]. Therefore, when producing new tellurite glass fibers for lasers operating at 2.3 μm, the quality of the samples should meet more strict requirements than for lasers operating only in the 1.9–2 μm spectral range. It is desirable to fabricate a high-quality low loss fiber, with a very low content of OH groups. In addition, the concentration of Tm3+ ions should be selected carefully. On the one hand, heavy concentrations lead to shorter pump absorption lengths, consequently, to less impact of losses. On the other hand, for heavy concentrations, the cross-relaxation process (3H4 + 3H63F4 + 3F4) becomes significant and contributes to depopulation of the energy level 3H4. Note that for laser generation at the 3F43H6 energy transition, cross relaxation is a helpful process (due to increasing the population of the 3F4 level), but for generation at the 3H43H5 transition it is parasitic (due to decreasing the population of the 3H4 level). Therefore, for generating in the ~2.3 μm spectral range, particular attention should be paid to the parameters of glass. As in the case of generation at ~2.7 μm in Er-doped fibers (‘the third opportunity’), the bottleneck problem in Tm-doped fibers with a short lifetime of the upper energy level 3H4 compared with the underlying level 3F4 can be solved by cascade lasing at two transitions 3H43H5 and 3F43H6. The first experimental demonstration of two-color CW cascade laser generation simultaneously near 2.3 μm and 1.9 μm in the tellurite TeO2-ZnO-La2O3-Na2O glass fiber with Tm-doped core having a concentration of 5 × 1019 cm−3 was reported in [11]. A theoretical investigation of laser amplification and generation, which is in a very good agreement with the experimental results, was also performed in [11]. A numerical model calibrated against experimental data was used to simulate power scalability at 2.3 μm in two-color cascade schematics with optimal characteristics under enlarged pump power. It was simulated that the output power at 2.3 μm can exceed 0.6 W for backward 10-W pump. Using the same fiber, experimental ultrabroadband amplification of supercontinuum was also obtained with maximum gain of 7 dB and 30 dB at wavelengths of 2.3 μm and 1.9 μm, respectively [11].
The specific problem of short lifetime of the upper laser level for generation at ~2.3 μm can be also solved by using pulsed pump (analogously to ‘the second way’ of lasing in the 2.7–2.8 μm region in Er-doped fibers). This path to generate laser pulses at 2.3 μm in Tm-doped TeO2-ZnO-La2O3-Na2O glass fiber under diode pulsed pump at 794 nm was experimentally realized in [12] by setting 1 ms pump duration and 10 ms interval between pulses.
Tm-doped tellurite glass elements can be also used as gain media in the S-band near 1.47–1.48 μm at the 3H43F4 transition. The possibility of amplification was demonstrated in several papers including under single wavelength pumping at ~795 nm [75] as well as under dual-wavelength pumping at 795 nm and 1064 nm [75,76], at 1550 nm and at 1047 nm [76], at 1047 nm and at 1605 nm [77], but laser generation at the 3H43F4 transition in a Tm-doped tellurite fiber has not been reported yet.

4.3. Lasing in Tm-Doped Microspheres

Tm-doped tellurite microsphere lasers generating only at the 3F43H6 transition near 1.9 μm as well as at two consecutive transitions 3H43F4 and 3F43H6 simultaneously near 1.5 and 1.9 μm in the cascade scheme were reported in [41,42,43,44,45,46]. Note that such a cascade two-color generation scheme was not implemented for tellurite fiber lasers.
The first Tm-doped tellurite microsphere laser was presented in 2004 [41] by the same research team that constructed the first Nd-doped tellurite microsphere laser [17]. The Tm-doped tellurite microsphere was produced by melting the end of a wire made of tellurite glass. The microlaser was pumped at 800 nm using a tapered fiber. Generation was obtained simultaneously in the 1.9 μm band and in the S-band (near 1.5 μm). The threshold for the 1.9 μm band was lower than for the S-band, and the quantum efficiency for the 1.9 μm band increased with pump power above the threshold of the S-band [41].
A single-mode microlaser at 2 μm pumped by a 973 nm Ti:sapphire laser was demonstrated from a highly Tm-doped tellurite glass microsphere with a diameter of 25 μm [42]. Fiber taper was used for pump coupling and laser signal outputting. Note that in the theoretical study of 2-μm lasing in Tm-doped microspheres presented in [57], the obtained numerical results agreed with the experimental ones [42].
A Tm/Ho co-doped tellurite glass microsphere laser at ∼1.47 μm was implemented at the 3H43F4 energy transition of the Tm3+ ion [46]. Ho3+ was co-doped to Tm3+ to reduce the population of the long-lived 3F4 state of the Tm3+ ion through a resonant energy transfer process. Using a CO2 laser, microspheres with diameters ranging from several μm to several hundred μm were produced. Both ∼802 nm pump light of a laser diode and the lasing emission were efficiently guided through a silica tapered fiber. Multimode lasing near 1.47 μm as well as single-mode lasing at 1494.9 nm were demonstrated [46].
Recently, laser generation at 1.9 μm and visible light of red and green emissions in Tm/Er co-doped tellurite glass microspheres were reached at pumping by 1550 nm broadband amplified spontaneous emission source [45]. Adjusting the relative position between the microsphere and the tapered fiber, a single-mode microlaser was observed. The output power was 210 nW for the laser wavelength of 1891.63 nm, the line width was 0.18 nm, and the ratio of the side-mode suppression was 12.04 dB. The threshold pump power coupled into a microspherical resonator was 1.5 mW. Single-mode and multimode lasing was observed by adjusting the coupling [45].
A theoretical investigation of multi-color CW lasing in Tm-doped tellurite glass microspheres pumped at a wavelength near 0.8 μm was presented in [58]. The numerical model was based on solving a system of equations for intracavity field amplitudes for all fundamental WGMs in the gain bands and rate equations using experimental parameters of Tm-doped tellurite glass. It was shown that, depending on Q-factors and pump powers, different generation regimes can be attained including single-color lasing at a wavelength of ~1.9 μm, two-color lasing at wavelengths of ~1.9 and 1.5 μm and at ~1.9 and 2.3 μm, and three-color lasing at wavelengths of ~1.9 and 1.5 and 2.3 μm [58]. However, unlike fiber lasers generated at 2.3 μm [11,12], tellurite microlasers at 2.3 μm have not yet been reported. The theoretical results obtained in [58] agreed with the experimental data reported in [41,43].

5. Holmium

Generation in the 2–2.1 μm range can be attained with Ho3+ ion at the 5I75I8 transition, which is shown in Figure 7. However, unlike Nd3+, Er3+, and Tm3+ ions considered above, Ho3+ has no absorption bands corresponding to commercial low cost laser diodes at wavelength of ~800 nm or ~980 nm. In-band pumping at the 5I85I7 transition can be implemented by Tm-doped silica fiber lasers. Pumping at the 5I85I6 transition near 1.1 μm by a specially designed fiber laser, including Raman lasers, can be also reasonable. Another way is to use co-doping with a sensitising ion to provide useful absorption bands. A popular ion for co-doping is Tm3+ pumped near 800 nm by a laser diode. In this case, a so-called ‘two-for-one’ cross-relaxation process of Tm ions can be utilized (when an excited ion at the 3H4 level transfers a part of its energy to an ion at the ground state, and after that both ions populate the intermediate 3F4 level), so the energy transfer from the 3F4 level of the Tm3+ ion to the 5I7 level of the Ho3+ ion can be very efficient. Co-doping with Yb3+ ions allows pumping by laser diodes at 980 nm. Nd3+ ions can be also used as sensitizers, which was recently proposed and demonstrated for a 795-nm diode pumped Nd/Ho co-doped tellurite fiber laser [51].

5.1. Lasing in Ho-Doped Fibers

CW lasing at 2.1 μm from a Tm/Ho/Yb triply-doped tellurite fiber was presented in [48]. The generation at the 5I75I8 transition of Ho ions was observed. Using an Yb-doped silica fiber laser at 1.1 μm as a pump source, it was suggested to co-dope tellurite gain fiber for sensitization by both Tm and Yb ions to provide a more efficient energy transfer path in comparison with sensitization by Yb alone. For a 17-cm piece of active fiber and a cavity with a reflectivity of 60–99%, the 15-mW threshold was observed. The slope efficiency was 25%. A maximum output power of 60 mW was obtained for a launched pump power of 270 mW which corresponded to an optical-to-optical efficiency of 22%. The same authors reported CW and Q-switch lasing from a Tm/Ho/Yb co-doped tellurite fiber pumped at ~1.6 μm by an Er/Yb silica fiber laser (the Yb3+ ions were added with other experiments in mind; they have no absorption at wavelengths >1.1 μm and therefore were not thought to play an active role) [48]. For CW operation, the slope efficiency was 62% and the threshold was 100 mW; the maximum output power was 160 mW. Mechanical Q-switching resulted in pulses with an energy of 0.65 μJ, and a duration of 160 ns at a repetition rate of 19.4 kHz was achieved [48].
A watt-level generation at 2051 μm with a slope efficiency of 31.9% was attained with 50-cm long Ho/Tm co-doped 50TeO2-25GeO2-3WO3-5La2O3-3Nb2O5-5Li2O-9BaF2 glass fiber pumped by an Er-doped fiber laser at 1560 nm [52]. The threshold power was 0.336 W. The maximum output power was 0.993 W when the pump power was 2.97 W [52].
A multiwatt laser output from a 30-cm piece of microstructured Ho-doped fluorotellurite fiber was recently reported in [53]. A Tm-doped silica fiber laser at 1980 nm was used for pumping. When the threshold pump power was 61.8 mW, lasing at 2067 nm started. The highest unsaturated output laser power of ~8.08 W was achieved for a 10.56-W pump [53]. The slope efficiency was approximately 77%. Numerical simulation of lasing for experimental parameters was also performed by solving the rate equations and propagation equations. The experimental and numerical results were in a good agreement [53].

5.2. Lasing in Ho-Doped Microspheres

A single-mode Ho-doped tellurite glass microsphere laser directly pumped by a fiber laser at 1150 nm was studied in [55]. The lasing at ~2 μm with a low-threshold of 342 μW was achieved. The dynamic characteristics of the microsphere laser were also studied theoretically by considering rare-earth ion spectroscopy, the rate equation of the rare-earth energy level, and the light-matter interactions in the microsphere [55].
The fabrication and characterization of microsphere lasers operating at wavelengths near 2.1 μm was reported in [54]. A Tm/Ho co-doped tellurite glass was used to produce hundreds of high quality microspheres simultaneously (with Q-factors reaching 106) by a droplet method [54]. Using a laser diode at 808 nm as a pump source, single-mode, and multimode generation was observed as the pump power was increased beyond a threshold of 887 μW. The maximum laser power reached 105.8 nW and the longest wavelength of the laser radiation reached 2099.52 nm for the sphere with a diameter of 59.52 μm. It was also demonstrated that a microlaser could be thermally tuned [54].

6. Opportunities for Other Rare-Earth Ions

In addition to the rare-earth ions discussed above, using other rare-earth ions for the development of tellurite glass based laser elements was investigated and discussed. There are a lot of works reporting the production of tellurite glass samples doped with Yb3+ [78,79], Dy3+ [67,80,81], Pr3+ [81,82,83], Eu3+ [81,84], Tb3+ [81,85], and Sm3+ [85,86] and study of their luminescent and other optical, physical, and chemical properties. However, we could not find information about laser generation in tellurite glass fibers or microspheres operating at the radiative transitions of these rare-earth ions.

7. Summary

The state of the art of tellurite glass fiber and microsphere lasers has been reviewed. It has been demonstrated that high-quality rare-earth doped tellurite fibers are promising novel gain elements for lasers and amplifiers in the L-band and in the range of 2 μm and well beyond. Tellurite microsphere lasers are also an attractive platform for developing ultracompact, low threshold, narrow linewidth light sources. The main experimental results on tellurite glass fiber lasers and microsphere lasers are summarized in Table 1 and Table 2, respectively.

Funding

The review of microsphere lasers was funded by the Russian Science Foundation, grant no. 18-72-00176. The review of fiber lasers was funded by the Russian Foundation for Basic Research, grant no. 20-03-00874.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Variant of experimental fiber laser scheme with forward diode pumping using a mirror at the input end and only the fiber end (with Fresnel reflection) at the output as a cavity.
Figure 1. Variant of experimental fiber laser scheme with forward diode pumping using a mirror at the input end and only the fiber end (with Fresnel reflection) at the output as a cavity.
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Figure 2. Variant of experimental microlaser scheme with a fiber taper used for both pump coupling and extracting the generated radiation from the microsphere.
Figure 2. Variant of experimental microlaser scheme with a fiber taper used for both pump coupling and extracting the generated radiation from the microsphere.
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Figure 3. Simplified energy level diagram for Nd-doped tellurite glass.
Figure 3. Simplified energy level diagram for Nd-doped tellurite glass.
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Figure 4. Simplified energy level diagram for Er-doped tellurite glass.
Figure 4. Simplified energy level diagram for Er-doped tellurite glass.
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Figure 5. Schematic of different ways of effective population of the 4I11/2 level of Er3+ ions in tellurite glasses. (a) 1. Up-conversion for very high Er3+ ion concentrations. (b) 2. Pulsed pump. (c) 3. Dual-wavelength cascade lasing. (d) 4. Two-color pump.
Figure 5. Schematic of different ways of effective population of the 4I11/2 level of Er3+ ions in tellurite glasses. (a) 1. Up-conversion for very high Er3+ ion concentrations. (b) 2. Pulsed pump. (c) 3. Dual-wavelength cascade lasing. (d) 4. Two-color pump.
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Figure 6. Simplified energy level diagram for Tm-doped tellurite glass.
Figure 6. Simplified energy level diagram for Tm-doped tellurite glass.
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Figure 7. Simplified energy level diagram for Ho-doped tellurite glass.
Figure 7. Simplified energy level diagram for Ho-doped tellurite glass.
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Table 1. Summary of performance of rare-earth doped tellurite glass fiber lasers.
Table 1. Summary of performance of rare-earth doped tellurite glass fiber lasers.
DopantGlass CompositionFiber Length, cmPump Laser, Pump Wavelength, nmCW or PulsedLaser Wavelength, nm Maximum Output Power (or Energy)Year, Reference
Nd3+76.9TeO2-6.0Na2O-15.5ZnO-1.5Bi2O3-0.1Nd2O360Ti: sapphire, 818CW10614.2 mW1994 [16]
Er3+Not reported85Ti: sapphire, 978CW15602.5 mW1997 [22]
Er3+, 0.9·1020 cm−3, Ce3+, 2.1·1020 cm−380TeO2-10ZnO-10Na2O22Laser diodes, 980&1480CW1527–16100 dBm (1 mW) @1558 nm2011 [23]
Er3+, 8.12·1019 cm−3, Ce3+, 1.92·1019 cm−379TeO2-13ZnO-8Na2O10Laser diodes, 980&1480CW~15502.6 mW2012 [24]
Er3+, 1019 cm−3TeO2-ZnO-La2O3-Na2O~220Laser diode, 974CW~1550<1 mW2012 [25]
Er3+, (7500 ppm Er2O3)71TeO2-22.5WO3-5Na2O-1.5Nb2O55–16Laser diode, 980CW1530–1565≤−24.39 dBm (≤3.6 μW)2012 [26]
Er3+, (Er2O3 5000 ppm)78TeO2-5ZnO-12Na2CO3-5Bi2O317Laser diode, 1480CW1561140 mW2013 [27]
Er3+, 10000 ppm76.5TeO2-6Bi2O3-6ZnO-11.5Li2O14Fiber Raman laser, 1480Pulsed (self-Q-switch)1558316 mW2014 [28]
Er3+, 1020 cm−3TeO2-ZnO-La2O3-Na2O20Laser diode, 975CW1555>100 μW2019 [29]
Er3+, 1 wt %(50–80) TeO2-(10–40) ZnO-(10 − x) La2O3-xEr2O3~300Fiber laser, 1570CW1589–162752.4 mW @ 1614 nm2019 [30]
Tm3+/Yb3+ (1.5/1.0 wt % Yb2O3/Tm2O3)80TeO2-10ZnO-10Na2O32Fiber laser, 1568–1610CW1880–1990280 mW2008 [33]
Tm3+/Yb3+ (1.5/1.0 wt % Yb2O3/Tm2O3)80TeO2-10ZnO-10Na2O22Fiber laser, 1088CW1910–199467 mW2009 [34]
Tm3+ 3.76 × 1020 cm−360TeO2-30WO3-10La2O340Laser diode, 800CW19371.12 W2010 [35]
Tm3+, 1 mol %60TeO2-30WO3-10La2O320Laser diode, 793CW~1900494 mW2012 [36]
Tm3+, 5000 ppm78TeO2-5ZnO-12Na2CO3-5Bi2O320Femto-second fiber system, 1560CW1887>6.5 mW2014 [37]
Tm3+, 3.76 × 1020 cm−3Not reported9Fiber laser, 1590Pulsed (Q-switch)186084 mW, 736 nJ2015 [38]
Tm3+, 1 mol % Tm2O345GeO2-25TeO2-15PbO-10(La2O3+ Al2O3)-5(CaO+ SrO+ Li2O)26Laser diode, 793CW19680.75 W2015 [39]
Tm3+, 0.5% Tm2O3 70TeO2-10BaF2-9.5Y2O3-0.5Tm2O342.5Fiber laser, 1570CW1887408 mW2017 [40]
Tm3+, 5 × 1019 cm−3TeO2-ZnO-La2O3-Na2O~220Laser diode, 792CW2300&19501.7 mW @2300 nm, ~40 mW @1950 nm2018 [11]
Tm3+, 5 × 1019 cm−3(86 − x) TeO2-xZnO-4La2O3-10Na2O30Laser diode, 794Pulsed2300; 2300&1900A few μW2019 [12]
Tm3+/Ho3+/Yb3+, (1.5/1.0/1.0 wt % Yb2O3/Tm2O3/Ho2O3)80TeO2–10ZnO–10Na2O17Fiber laser, 1088CW~210060 mW2008 [47]
Tm3+/Ho3+/Yb3+, (1.5/1.0/1.0 wt % Yb2O3/Tm2O3/Ho2O3)80TeO2–10ZnO–10Na2O76
79
Fiber laser, 1600
Fiber laser, 1600
CW
Pulsed (Q-switch)
2051–2096
~2100
160 mW
26 mW, 1.3 μJ (train), 0.65 μJ (main pulse)
2008 [48]
Tm3+/Ho3+, 1 mol %/0.5 mol %60TeO2-30WO3-10La2O37Laser diode, 793CW204635 mW2012 [36]
Ho3+, 0.75% Ho2O370TeO2–20BaF2–9.25Y2O3–0.75Ho2O327Fiber laser, 1992CW2077161 mW2015 [49]
Ho3+, 0.5% Ho2O360TeO2–30WO3–9.5La2O3–0.5Ho2O39Fiber laser, 1940CW204034 mW2016 [50]
Nd3+/Ho3+, 0.5% Nd2O3/0.5% Ho2O360TeO2–30WO3–3ZnO–6La2O3–0.5Ho2O3–0.5Nd2O35Laser diode, 975CW205212 mW2016 [51]
Tm3+/Ho3+, Ho2O3: 0.3%mol, Tm2O3: 0.3%mol50TeO2-25GeO2-3WO3-5La2O3-3Nb2O5-5Li2O-9BaF250Fiber laser, 1560CW20510.9932017 [52]
Ho3+, 0.75% Ho2O370TeO2-20BaF2-9.25Y2O3-0.75Ho2O330Fiber laser, 1980CW20678.08 W2019 [53]
Table 2. Summary of performance of rare-earth doped tellurite glass microsphere lasers.
Table 2. Summary of performance of rare-earth doped tellurite glass microsphere lasers.
DopantGlass CompositionSphere Diameter, μmPump Laser, Pump Wavelength, nmSingle-Mode (SM) or Multi-Mode (MM)Laser Wavelength, nmYear, Reference
Nd3+, (1 wt % Nd2O3)70TeO2-20ZnO-10Li2O 201 (50-a few hundred)Ti: sapphire, 800MM~1060 (1061–1067)2002 [17]
Nd3+, (0.2 wt % Nd2O3)75TeO2-20ZnO-5Na2O~70–180Ti: sapphire, 800SM and MM~1058–10752003 [18]
Nd3+, (1 mol % Nd2O3)80TeO2-10K2O-10WO3
air-bubble-containing or solid
~20–50Ti: sapphire, 810MM~1060–10702012 [19]
Nd3+, (1 mol % Nd2O3)80TeO2-10K2O-10WO329 Ti: sapphire, 800–810SM and MM~1060–10702015 [20]
Nd3+, (1 mol % Nd2O3)80TeO2-10K2O-10WO3
air-bubble-containing or solid
~4-200Ti: sapphire, 790–817MM~1060–10702015 [21]
Er3+, (1.7 × 1020 cm−3)Not reported33975SM and MM1560–16102003 [31]
Er3+Not reported311480SM~1606–16082003 [32]
Tm3+, (0.15% Tm2O3)74.85TeO2-20ZnO-5Na2O-0.15Tm2O3 104Ti: sapphire, 800MM~1500&19002004 [41]
Tm3+, (5 wt %)Not reported25Ti: sapphire, 793SM~20002005 [42]
Tm3+, 0.5 wt % Tm2O3Not reportedNot reportedTi: sapphire, 793MM~1500&19002005 [43]
Tm3+, (4.2 × 1020 cm−3)74TeO2-15ZnO-5Na2O-5ZnCl2-1Tm2O3 (mol. %)301504–1629MMCentered at ~19752015 [44]
Er3+/Tm3+ (0.1/0.2% Er2O3/Tm2O3)68.7TeO2-23WO3-8La2O3-0.1Er2O3-0.2Tm2O3 (mol %)110Amplified spontaneous emission source, 1527–1603SM and MM~19002019 [45]
Tm3+/Ho3+ (0.2/0.8% Tm2O3/Ho2O3)72TeO2-20ZnO-5.0Na2CO3-2.0Y2O3-0.8Ho2O3-0.2Tm2O3~100Laser diode, 802SM and MM~14702019 [46]
Tm3+/Ho3+ (1.0/0.7% Tm2O3/Ho2O3)75TeO2-18.3ZnO-5Na2O-1.0Tm2O3-0.7Ho2O3~60Laser diode, 808SM and MM~21002017 [54]
Ho3+, (1 mol % HoF3)72TeO2-20ZnO-5Na2CO3-2Y2O3-1HoF3 (in mol %)~42Fiber laser, 1150SM~20802020 [55]
Ho3+/Yb3+ (0.2/0.8% Ho2O3/Yb2O3)72TeO2-20ZnO-5Na2CO3-2Y2O3-0.8Yb2O3-0.2Ho2O380Laser diode, 980SM and MM~2065–20722020 [56]
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