1. Introduction

Recently, high-repetition-rate (>1 GHz) mode-locked fiber lasers operating at the 2 µm eye-safe wavelength have attracted wide research interest for the advantages of larger longitudinal mode spacing and dramatically enhanced acquisition rate, which are promising for various applications, such as high speed optical sampling, biomedical imaging, laser-driven particle acceleration, and so on [1–5]. Although a harmonic mode-locked fiber laser can easily achieve a higher repetition rate, the presence of large supermode noise hinders its practical applications [1,6]. In contrast to the harmonic mode-locking scheme, a fundamental mode-locking can provide a better spectral purity, which is essential for high-precision ultrafast photonic applications [7]. For obtaining a fundamental repetition rate of >1 GHz, the use of a high-gain fiber for shortening the laser cavity to the order of cm is a key factor [8].

Among the rare earth (RE) ions, Tm³⁺ and Ho³⁺ are particularly suitable for the generation of 2 µm laser due to the energy level transitions Tm³⁺: ³F₄ → ³H₆ and Ho³⁺: ⁵I₇ → ⁵I₈ [9,10]. Both transitions operate in the three-level quantum scheme, and the luminescence spectrum emitted from these transitions is relatively broad [10]. During the past decades, Tm³⁺ has gained significant attention for its inherent advantages: the strong absorption band around 790 nm that could be directly excited by commercial available laser diode (LD) through ³H₆ → ³F₄ transition and high quantum efficiency (up to 200%) benefited from the cross relaxation process (³H₄ + ³H₆ → ³F₄ + ³F₄) [11–13]. However, the laser efficiency of 790 nm LD pumped Tm³⁺ doped fiber lasers (TDFLs) operating at ∼1.95 µm is somewhat less than when operating beyond 2 µm [14,15]. Compared with Tm³⁺, Ho³⁺ has a larger stimulated emission cross section, longer fluorescence lifetime, and its emission is red-shifted by 200 nm, making it more suitable for wideband tunable 2 µm lasers with high-gain characteristic [10,11,16]. However, Ho³⁺ cannot be pumped directly with commercial 808 or 980 nm LDs due to the lack of an appropriate ground absorption band. To tackle this issue, an attractive way has been proposed by introducing Tm³⁺ as an ideal sensitizer to obtain efficient broadband tunable 2 µm laser output through the energy transfer from Tm³⁺: ³F₄ level to Ho³⁺: ⁵I₇ level [17–19].

It is well-known that high RE ions doping levels of optical fibers make it possible to obtain a high-gain. However, pure-silica glass has poor solubility of RE ions (10¹⁸-10¹⁹ ions/cm³) due to its well-defined glass network, which means that high-gain pure-silica fibers are not achievable [20,21]. In contrast, multicomponent oxide glasses such as silicate and germanate, which have much higher RE ions solubility (∼10²¹ ions/cm³), have been successfully used to fabricate high-gain 2 µm fibers [6,22]. It is noted that the probability of non-radiative transition is proportional to the phonon energy of glass. For this reason, silicate glass is not a suitable host glass for 2 µm lasers due to its high phonon energy (∼1100 cm^-1), which can lead to fast multi-phonon relaxation, reducing the luminescence performance and decreasing quantum efficiency [21,23]. In contrast, germanate glass with outstanding infrared transparency and comparatively low phonon energy (∼850 cm^-1), especially the BaO-Ga₂O₃-GeO₂ (BGG) glass known as a window for high energy laser (HEL) systems, has been intensively studied as promising candidates for drawing the high-gain optical fibers [10,21]. In addition to the need for high concentration of RE ions doping, low OH^- content is another key factor that must be considered in the fabrication of high-gain 2 µm fibers. The OH^- impurities play a key role of quenching centers in the energy transfer processes of Tm³⁺ ions due to the overtone absorption with Tm³⁺: ³F₄ → ³H₆ emission [24]. Therefore, it is essential to remove OH^- content in laser glasses to achieve excellent emission property. In 2019, Kochanowicz et al. fabricated a Tm³⁺/Ho³⁺ co-doped BGG glass double-clad fiber, which has a broadband 2 µm ASE spectrum with a FWHM of 377 nm [10]. However, the RE ions (Tm³⁺ and Ho³⁺ in this case) doping concentrations are very low, resulting in a low gain per unit length. Up to now, broadband high-gain Tm³⁺/Ho³⁺ co-doped germanate glass fiber has never been reported.

In this work, a Tm³⁺/Ho³⁺ co-doped germanate glass with low OH^- content of 4.56 parts per million (ppm) was prepared by using the conventional melt-quenching method combined with the RAP dehydration technique. Then a silicate-clad Tm³⁺/Ho³⁺ co-doped germanate core multimaterial fiber was successfully drawn by using the rod-in-tube method, which has a broadband 2 µm ASE with a FWHM of 247.8 nm. The doping concentrations of the co-doped fiber are 7.1 wt.% Tm₂O₃ and 0.7 wt.% Ho₂O₃, respectively. Because of the high doping concentration of Tm³⁺ ions and low energy transfer efficiency (19.8%) between Tm³⁺ and Ho³⁺ ions, broadband and high-gain 2 µm fiber was achieved. Furthermore, an all-fiber-integrated passively mode-locked fiber laser based on this new developed gain fiber was realized, which has a fundamental repetition rate of 4.329 GHz at 2068.05 nm.

2. Experimental

BGG glasses with molar composition of 92.5(BaO-Ga₂O₃-GeO₂)-5Nb₂O₅-2.5Tm₂O₃-xHo₂O₃ (x = 0, 0.1, 0.125, 0.25, 0.375, 0.5) were prepared by the conventional melting-quenching method using high purity reagents (≥99.99%). Well-mixed raw materials (20 g) were melted at 1350 °C for 0.5 h in a covered crucible. Subsequently the melt was poured onto a preheated steel mold and annealed at 620 °C for 2 h. The annealed glass samples were then cut and optically polished for measurements.

For fiber fabrication, a bulk core glass with molar composition of 92.5(BaO-Ga₂O₃-GeO₂)-5Nb₂O₅-2.5Tm₂O₃-0.25Ho₂O₃, in quantity of 600 g powder, was fabricated. The mixed batches were melted in a covered crucible at 1350 °C with the optimized RAP dehydration process. The reactive dehydration agent (CCl₄ in this case) was transported by bubbling with pure O₂ into the glass melts to remove OH^- in the optimized RAP dehydration process. After this dehydration process, stirring and clarifying the glass melt must be ongoing under the atmosphere of high-purity O₂ in order to cut off the dehydrated glass melt from the H₂O in the ambient atmosphere. More details can be found in previous studies [21,24]. Thereafter, the melt was cast into the preheated steel mold and annealed. The conventional rod-in-tube technique was used for fiber fabrication, as described in our previous work [6]. Continuous silicate-clad Tm³⁺/Ho³⁺ co-doped germanate core multimaterial fibers were successfully drawn inside a commercial drawing tower (SG Controls 288600A) under N₂ controlled atmosphere.

The refractive indexes of the glass samples were tested on a prism coupling apparatus (Metricon Model 2010). The density of the glass was measured by Archimedes’ liquid-immersion method in distilled water. The characteristic temperatures of glass transition (T_g) and onset crystallization peak (T_x) were measured by using a Netzsch SAT 449C Jupiter differential scanning calorimeter (DSC) under Ar atmosphere at a heating rate of 10 °C/min. The infrared transmittance spectrum was measured using a Vector-33 FTIR spectrometer (Bruker, Switzerland). The absorption spectra of the glass samples were recorded by a Perkin-Elmer Lambda 900 UV-Vis-NIR spectrophotometer (Waltham, MA). Fluorescence spectra were measured by a computer controlled Triax 320 type spectro-fluorometer (Jobin-Yvon Corp.) with a lock-in amplifier. The luminescence lifetime was obtained from the first e-folding time of emission intensity in the decay curve recorded with a digitizing oscilloscope. The electron micrograph and the distribution of elements of the as-drawn fiber in a cross section were measured by an electron probe X-ray micro-analyzer (EPMA-1600, Shimadzu, Japan). The all-fiber mode-locked laser performance was evaluated under excitation of a home-made 1570 nm fiber laser. The laser spectrum was recorded by using an optical spectrum analyzer (AQ6375, YOKOGAWA, Japan). A high-speed photodetector (PD, 818-BB-51F, Newport, USA) and a real-time oscilloscope (DSOV204A, Keysight, USA) with a bandwidth of 20 GHz were used to investigate the temporal features of the laser output. The radio frequency (RF) spectra of the electric signal delivered from the PD were measured by a RF signal analyzer (FSWP26, Rohde & Schwarz, Germany) with a bandwidth of 26.5 GHz. All measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows absorption spectra of 0.25 mol.% Ho₂O₃ singly doped, 2.5 mol.% Tm₂O₃ singly doped, and 2.5 mol.% Tm₂O₃-0.25 mol.% Ho₂O₃ co-doped germanate glasses in the wavelength range from 400 to 2200 nm. The characteristic absorption bands corresponding to the transitions from the ground states to higher states of Tm³⁺ and Ho³⁺ ions are labeled in the spectral curves. Compared with Tm³⁺, the absorption peaks of Ho³⁺ are not obvious due to the concentration of Ho³⁺ is relatively low. For Ho³⁺ singly doped glass, five absorption bands centered at 451 nm, 537 nm, 642 nm, 1150 nm, and 1948 nm can be ascribed to the typical transitions from the ground state ⁵I₈ to the exited states ⁵F₁, ⁵S₂, ⁵F₅, ⁵I₆, and ⁵I₇ of Ho³⁺, respectively. Similarly, the Tm³⁺ singly doped glass has five absorption peaks observed at 468 nm, 685 nm, 792 nm, 1211 nm, and 1651 nm, corresponding to transitions from the ground state ³H₆ to high levels ¹G₄, ³F_2,3, ³H₄, ³H₅, and ³F₄ of Tm³⁺, respectively. The obvious absorption bands around 792 nm and 1651 nm suggest that the Tm³⁺/Ho³⁺ co-doped germanate glasses can be efficiently pumped by 808 nm LDs or 1.5 µm lasers.

 

Fig. 1. Absorption spectra of Ho³⁺ singly doped, Tm³⁺ singly doped, and Tm³⁺/ Ho³⁺ co-doped germanate glasses.

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The emission spectra of the prepared germanate glasses in the wavelength region of 1500-2250 nm pumped by an 808 nm LD are shown in Fig. 2(a). The glass doped exclusively with Tm³⁺ ions (marked as Tm) are characterized by strong luminescence with the peak at the wavelength of 1810nm due to the strong transition of ³F₄ → ³H₆. Co-doping Ho³⁺ ions into the glass (marked as Tm/xHo) results in a decrease of the level of luminescence coming from Tm³⁺ ions, at the same time, a strong emission line appears in the region of 2 µm (peaked at 1960 nm and 2015 nm) corresponding to the transition ⁵I₇ → ⁵I₈ of Ho³⁺. Moreover, with the increment of Ho₂O₃ concentration, the emission intensity from the Ho³⁺: ⁵I₇ → ⁵I₈ increases gradually, which can be accounted for shortening the distance between Tm³⁺ and Ho³⁺ and thus increasing the probability of energy transfer from Tm³⁺ to Ho³⁺ before it reaches the maximum value, when 0.25 mol.% Ho₂O₃ is added. And then emission intensity reduces gradually with further increasing Ho₂O₃ concentration because of the diffusion transitions of the donor-donor type or concentration quenching [25,26]. Figure 2(b) presents the normalized emission spectra of the prepared germanate glasses. A significant broadening of the luminescence spectrum is observed in the Tm³⁺/Ho³⁺ co-doped germanate glasses caused by overlapping the Tm³⁺: ³F₄ → ³H₆ and Ho³⁺: ⁵I₇ → ⁵I₈ emission transitions. The FWHM of the Tm/0.25Ho glass is 384 nm, while only 252 nm for the Tm³⁺ singly doped glass. What is more, the doping concentration of Tm₂O₃ is as high as 2.5 mol.% (9.8 × 10²⁰ ions/cm³) in the co-doped glass, enabling the glass has high-gain at 2 µm [6]. Therefore, the Tm³⁺/Ho³⁺ co-doped germanate glass (Tm/0.25Ho) is expected to be a broadband and high-gain 2 µm laser material.

 

Fig. 2. (a) Emission spectra of the Tm³⁺ singly doped and Tm³⁺/Ho³⁺ co-doped germanate glasses. (b) Normalized emission spectra in (a).

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Figure 3(a) shows the transmittance spectrum of Tm³⁺/Ho³⁺ co-doped germanate glass (Tm/0.25Ho) from 2.5 µm to 6.5 µm. It can be observed that the maximum transmittance can reach as high as 99%. The absorption cut-off edge is up to 6.5 µm, which is much longer than that of silicate glass (5.1 µm) and fluorophosphates glass (4.5 µm) [27,28]. It is known that the broad absorption near 3 µm is corresponding to the stretching vibration of free OH^- groups. An optimized RAP dehydration technique was used to eliminate OH^- during the bulk glass melting process, in which gas bubble containing CCl₄ and high purity O₂ chemically reacts with the OH^- groups [24]. To evaluate the OH^- content of glasses, the OH^- absorption coefficient (α_OH) can be calculated by using the Eq. (1) [27]:

 

Fig. 3. (a) Transmittance spectrum of Tm³⁺/Ho³⁺ co-doped germanate glass. (b) Fluorescence decay curves of 1810nm emission in Tm³⁺ singly doped and Tm³⁺/Ho³⁺ co-doped germanate glasses. The inset of Fig. 3(a) shows fluorescence decay curve of 2015nm emission in Tm³⁺/Ho³⁺ co-doped germanate glass.

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The T_g and the T_x of the core glass (Tm/0.25Ho) were measured to be 711.6 °C and 867.4 °C, respectively. The criterion, ΔT = T_x- T_g, is often used as an important parameter to evaluate glass thermal stability [24]. Generally, a larger ΔT above 100 °C will facilitate the fiber drawing. It can be seen that the ΔT of the core glass is about 155.8 °C, which is much larger than 100 °C, indicating a wide operating temperature range and excellent glass stability against crystal nucleation and growth during the fiber drawing. Then continuous silicate-clad Tm³⁺/Ho³⁺ co-doped germanate core multimaterial fibers were successfully drawn in-house by using the conventional rod-in-tube technique. Figure 4(a) presents the cross section of the polished as-drawn multimaterial fiber. It can be observed that the core-clad structure of fiber is preserved completely. The fiber core shows bright white color, and the boundary between the core and clad is clear. The diameters of fiber clad and core are about 127.3 µm and 8.8 µm, respectively. More importantly, there exist no vacuum bubbles and microcracks in the core, and no obvious discontinuities at the core-clad interface due to the mismatch of thermal expansion coefficient between the core and clad glasses. In order to determine element distribution of the fiber core and clad after high temperature drawing, EPMA measurements were performed on the cross section of the as-drawn multimaterial fiber, as shown in Figs. 4(b)-(e). The distribution of each element (Ge, Si, Tm, Ho) forms a circle and exhibits similar size as that of the fiber core shown in Fig. 4(a). The Si is mainly distributed in the clad region, while the Ge, Tm, and Ho are confined in the core region. In addition, Si and Ge have a low concentration diffusion layer at the core-clad interface, demonstrating a minor amount of element diffusion between the core and clad after high temperature drawing. The refractive indexes of the core and clad glasses at 1533 nm were measured to be 1.7280 and 1.5010, respectively. Therefore, the numerical aperture (N.A.) of the as-drawn multimaterial fiber is calculated to be 0.856. Then, the normalized frequency of the as-drawn multimaterial fiber is calculated to be 12.45, suggesting the multimode guiding property at above 1.9 µm. The transmission loss of the newly developed fiber at 1310 nm was measured to be 0.01 dB/cm, which is lower than that of the pure germanate glass fiber (0.05 dB/cm) [21].

 

Fig. 4. (a) Electron micrograph image of the as-drawn multimaterial fiber. (b)-(e) EPMA images of the marked area in (a).

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Figure 5(a) shows the ASE spectrum of 0.9 cm long multimaterial fiber excited with a 1570 nm fiber laser. The pump power is about 600 mW. Intensive and flat broadband ASE spectrum in the region of 2 µm originating from the transitions of Tm³⁺: ³F₄ → ³H₆ and Ho³⁺: ⁵I₇ → ⁵I₈ is clearly observed. It is worth noting that the FWHM of the ASE spectrum is about 247.8 nm, which is much larger than those of Tm³⁺/Ho³⁺ co-doped double clad silicate fiber (70 nm), RE ions doped silicate fiber with multi-section core (160 nm), and Ho/Cr/Tm: YAG crystal derived all-glass fiber (234 nm) [32–34]. To evaluate the gain per unit length of the as-drawn multimaterial fiber, a small-signal gain was measured. A 1570 nm fiber laser was also used as the pump source. A homemade 1950 nm single-frequency fiber laser was used as the signal light with an input signal power level of 0.2 mW. The signal enhancement was obtained with pumping. Figure 5(b) exhibits the measured gain per unit length as a function of the pump power in the as-drawn multimaterial fibers with various fiber lengths. The inset of Fig. 5(b) shows the optical spectrum of the amplified signal at a pump power of 650 mW. It can be observed that the intensity contrast between the signal and pump power is larger than 21 dB, ensuring the validity of the small-signal gain measurements. It can be found that the highest gain per unit length of 4.52 dB/cm at 1.95 µm was achieved in the 2 cm long as-drawn gain fiber, which is lower than that of Tm³⁺ singly doped germanate glass fiber in our previous work (6.11 dB/cm) [6]. It can be explained that after co-doping Ho³⁺ ions, part of the energy on the Tm³⁺: ³F₄ level is transferred to the Ho³⁺: ⁵I₇ level, thereby reducing the gain at 1.95 µm generated by the Tm³⁺: ³F₄ → ³H₆ transition, while increasing the gain at the long wavelength of 2 µm generated by the Ho³⁺: ⁵I₇ → ⁵I₈ transition. Above all, broadband high-gain 2 µm Tm³⁺/Ho³⁺ co-doped germanate glass fiber was successfully fabricated.

 

Fig. 5. (a) ASE spectrum the as-drawn multimaterial fiber. (b)-(e) Gain per unit length versus pump power for different fiber lengths. The inset of Fig. 5(b) shows the optical spectrum of the amplified signal.

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By taking advantage of this broadband high-gain Tm³⁺/Ho³⁺ co-doped germanate glass multimaterial fiber, a typical all-fiber passively mode-locked fiber laser was constructed. The experimental setup is shown in Fig. 6, wherein the laser cavity incorporates a semiconductor saturable absorber mirror (SESAM), 2 cm long gain fiber (GF), and a fiber-type dichroic film (DF). The DF has a high transmittance of up to 95% at the pump wavelength of 1570 nm and a high reflectivity of at the signal wavelength. The SESAM (SAM-2000-20-10ps, Batop GmbH, Germany) was used as the other reflector, which has a modulation depth of 12%, saturation fluence of 65 µJ/cm², and recovery time of 10 ps. A 1570 nm fiber laser was used as the pump and coupled into the fiber resonator via a 1550/1950 nm wavelength division multiplexer (WDM). A fiber isolator (ISO) was utilized to prevent backward-propagation light and assure stable operating status.

 

Fig. 6. Experimental setup of the mode-locked fiber laser.

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The output power as a function of the pump power is shown in Fig. 7(a). The continuous wave laser oscillation starts at the pump power of 85 mW. When the pump power is in the range of 155 mW to 245 mW, the laser operates in Q-switched mode-locking (QSML) and output power increases near linearly with the pump power. Further increasing the pump power can enhance intracavity pulse energy, and thus give rise to the transition from the QSML to continuous-wave mode-locking (CWML) regime [35,36]. Therefore, above injected pump power of 245 mW, a CWML can self-start and the function dependence also keeps linear but with slightly higher slope. The output power of the fiber laser is ∼0.748 mW at a pump power of 245 mW. Then the characteristics of the passively mode-locked fiber laser at a pump power of 245 mW were measured. Figure 7(b) shows the measured oscilloscopic trace, suggesting that the pulse train has a period of about 234 ps, corresponding to a fundamental repetition rate of about 4.3 GHz. The CWML optical spectrum is presented in Fig. 7(c). It can be observed that the center wavelength of the mode-locked fiber laser is near 2068.05 nm with a 3 dB bandwidth of ∼0.7 nm. The RF spectrum of the mode-locked fiber laser in Fig. 7(d) was measured at a resolution bandwidth (RBW) of 100 Hz. It can be found that the fundamental repetition rate is located at 4.329 GHz with a signal-to-noise ratio (SNR) of 64 dB, demonstrating a good short-term stability of the fundamental mode-locking. In addition, the scan in full span (26.5 GHz) at a RBW of 1 kHz presented in the inset of Fig. 7(d) shows the fundamental repetition rate and harmonics, and confirms the absence of unwanted frequency components. In addition, the integrated relative intensity noise (RIN) is calculated to be less than 0.2%. To the best of our knowledge, this is the highest fundamental repetition rate so far reported for the passively mode-locked fiber laser above 2 µm.

 

Fig. 7. (a) Variation of the laser output power with the pump power. (b) Oscilloscope trace. Inset: pulse trace in a wider range. (c) Optical spectrum. (d) Measured RF spectrum of the fundamental mode-locking operation. Inset: the RF spectrum in a span of 26.5 GHz.

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4. Conclusions

In conclusion, a broadband high-gain Tm³⁺/Ho³⁺ co-doped germanate glass multimaterial fiber was successfully fabricated, which has a broadband ASE with a FWHM of 247.8 nm and a high gain per unit length of 4.52 dB/cm at 1.95 µm. The doping concentrations of the co-doped germanate glass are 7.1 wt.% Tm₂O₃ and 0.7 wt.% Ho₂O₃, respectively. Furthermore, a low OH^- content of 4.56 ppm is achieved in the co-doped germanate glass. What is more, an all-fiber-integrated passively mode-locked fiber laser based on a 2 cm long multimaterial fiber was built. The mode-locked pulses operate at 2068.05 nm with a period of ∼234 ps. In particular, the pulse repetition rate of the fundamental mode-locking is as high as 4.329 GHz, which is, to the best of our knowledge, the highest fundamental repetition rate for the passively mode-locked fiber laser above 2 µm. It is anticipated that this newly developed Tm³⁺/Ho³⁺ co-doped fiber can be a promising gain medium for high-repetition-rate mode-locked fiber lasers above 2 µm.