1. Introduction

Two micrometer fiber lasers have received increasing attention recently due to its potential applications in remote sensing, ranging, medical surgery, and as pumping sources for mid-infrared lasers [1–4]. For ~2 μm fiber lasers, thulium or holmium have been widely used as doping rare earth ions. Till now, Tm³⁺-doped fiber laser have drawn wide attention due to available commercial diode laser pump source and high quantum efficiency [5, 6]. However, one favorable LIDAR wavelength of 2053 nm is far away from the gain peak of thulium doped glass fiber [7]. Compared to thulium, the emission peak of Ho³⁺ is 200 nm longer. Hence, holmium ions have a much higher gain at 2050 nm. Though there is no commercial diode pump laser available, great progress have been made in pumping Ho³⁺ with Tm³⁺-doped fiber laser or Yb³⁺-doped fiber laser [8, 9].

Silica fiber laser have gained great success in power scaling owing to its high strength and low loss. Until now, kilowatt-level laser output has been achieved in thulium-doped silica fiber lasers [10], and 407 W laser output has also been obtained in Ho³⁺-doped silica fibers pumped by Tm³⁺-doped fiber lasers [11]. However, doping concentration of rare earth ions is relatively low in silica, and thus hinders its gain ability [12]. Till now, to the best of our knowledge, single frequency fiber lasers have not been reported in Ho³⁺-doped silica fiber. Multi-component glass fiber gained wide attention for single frequency glass fiber laser [13, 14]. Ho³⁺-doped fiber laser have been achieved in various kinds of multi-component glass materials. In 2009, Wu et al demonstrated a single frequency fiber laser operating near 2 μm with over 50 mW output power using a short piece of holmium-doped germanate glass fiber [7]. In 2012, single frequency fiber laser has also been obtained with gain-switch method in a Ho³⁺-doped silicate fiber by Geng et al [15]. Recently, laser output has been realized in a Ho³⁺-doped flurotellurite fiber by Qin et al [16].

Rare earth ion doped lead silicate glass fibers have been fewly investigated, mainly ascribed to hardness of melting corresponding glass. Earlier, Tm³⁺-doped lead silicate glass fiber laser has been realized [17]. Here we investigated a Ho³⁺-doped lead silicate glass for ~2 μm fiber lasers. A single-clad fiber was prepared successfully. An all-fiber configuration was constructed. With in-band core-pumped by Tm³⁺-doped fiber laser, a maximum 60 mW single-mode signal laser was realized from a 6 cm-long single-clad fiber. This is the first report of ~2 μm laser output from a Ho³⁺: SPANK fiber.

2. Experiments

2.1 Material synthesis and fiber drawing

Ho³⁺-doped glass system SiO₂-PbO-Al₂O₃-Na₂O-K₂O was prepared, which was named SPANK afterwards. Na₂O and K₂O were introduced by corresponding carbonates. The glass was melted in a 500 mL crucible made of high purity silica. The glass melt was cleared at 1400°C for 1 h and then stirred at 1200°C for 3 h. After that, the glass melt were poured into a preheated stainless iron mould and placed in a muffle furnace for annealing. The annealing temperature was 10°C above T_g of corresponding glass sample. The whole melting process was protected by high purity oxygen. Finally, the glass sample was machined to a size of 10 × 10 × 2 mm for analysis.

2.2 Measurements

The refractive index was measured by the prism coupler method, while the density was measured by the Archimedes method using distilled water as the immersion liquid. The transmittance spectra in the 2000-4000 cm^–1 range were measured on a Nicolet 6700 series Fourier transform infrared (FTIR) spectrometer. In addition, the absorption spectra were recorded using a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer in the 300-2200 nm range. Doping concentration of Ho³⁺ in glass sample was measured using an ICP-AES 6300 apparatus. The emission spectrum of the core glass was measured upon excitation at 640 nm using a Triax 320 type spectrometer (Jobin-Yvon Co, France). Lifetime measurement of the Ho³⁺: ⁵I₇ state was conducted in a FLSP 920 (Edinburgh instruments Ltd, UK) instrument. Laser experimental setup will be given in section 3.2. All the experiments were carried out at room temperature.

3. Results and discussions

3.1 Luminescent properties of Ho³⁺-doped SPANK

Doping concentration of Ho³⁺ was confirmed by ICP-AES to be 5.91 × 10²⁰ cm⁻³, much higher than that of Ho³⁺-doped silica fiber [18]. Figure 1 shows absorption spectra of Ho³⁺ in SPANK in the range of 500-2200 nm. Absorption peaks around 536, 640, 880, 1150, and 1938 nm are ascribed to transitions from ground state ⁵I₈ to excited states ⁵I₇, ⁵I₆, ⁵I₅, ⁵I₄, and ⁵F₅, respectively. Integrated areas of ⁵I₆, ⁵I₅, ⁵I₄, and ⁵F₅ are calculated for J-O parameters simulation. J-O parameters Ω₂, Ω₄, and Ω₆ of Ho³⁺ for SPANK is calculated to be 5.52, 2.19, and 0.59, respectively. The room-mean-square error deviation of intensity parameters is 0.6 × 10⁻⁶, which indicates the validity of the J-O theory for predicting the spectral intensities of Ho³⁺ and the reliable calculations. As listed in Table 1, compared to other glass matrices, Ω₂ is much higher owing to high polarizability of PbO in Ho³⁺: SPANK. As a result, spontaneous transition rate A₁₀ is higher than silicate and ZBYA glass though smaller than that of germanate glass. However, A₁₀ of Ho³⁺ is still much lower than that of Tm³⁺ in the same matrix [17].

 

Fig. 1 Absorption spectrum of Ho³⁺-doped SPANK glass.

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Table 1. J-O parametersΩ₂, Ω₄, Ω₆, spontaneous transition rate (A₁₀), radiative lifetime τ_rad in various glass matrices

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OH^– is one of the dominant quenching centers in Ho³⁺-doped glasses [22], especially for populations in ⁵I₇ level, since only a few hydroxyl groups (2700 cm^–1-3700 cm^–1) are required for non-radiative de-excitation through multi-phonon relaxation. In this work, this glass was melted in a low hydroxyl atmosphere with 8% humidity. Bubbling with liquid oxygen was used to eliminate OH^– from the glass melt. The OH^– content in the glass could be expressed by the absorption coefficient of OH^– vibration band at 2880 cm^–1, which is given by [23]:

Figure 2(a) shows transmittance spectra of 2 mm-thick Ho³⁺: SPANK glass sample in the range of 2000-4000 cm⁻¹. According to Eq. (1), absorption of hydroxyls was calculated to be 0.4 cm⁻¹. The relatively low content of hydroxyls contribute to a long lifetime of Ho³⁺: ⁵I₇ energy level, which is fitted to be 1.41ms.The lifetime of Ho³⁺: ⁵I₇ in SPANK is much longer than that in Ho³⁺: silica though doping concentration of Ho³⁺ is much higher, indicating fine distribution of Ho³⁺ in the glass [18]. The relatively long lifetime contributes to population inversion during laser operation.

 

Fig. 2 (a) Transmittance spectra of Ho³⁺: SPANK glass in the range of 2000-4000 cm⁻¹. Inset is decay curve of Ho³⁺: ⁵I₇ . (b) Normalized absorption spectrum and emission spectrum of Ho³⁺-doped SPANK in the range of 1700-2200 nm.

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The absorption cross section is calculated according to σ_abs = 2.303*OD/N₀*l and emission cross section is calculated according to Füchtbauer–Ladenburg (F-L) equation [24]. Figure 2(b) shows absorption cross section and emission cross section of Ho³⁺: SPANK glass in the range of 1700-2200 nm. The peak absorption cross section reaches 3.94 × 10⁻²¹ cm² around 1938 nm while peak emission cross section arrives at 3.07 × 10⁻²¹ cm² around 2055 nm. Thus, pump laser source at 1940 nm was picked up to pump single-clad Ho³⁺: SPANK fiber in the laser experiment. A FBG with central wavelength at 2040 nm is chosen owing to relatively high emission cross section here.

3.2 Laser properties of Ho³⁺: SPANK single-clad fiber

The Ho³⁺: SPANK single-clad fiber was prepared with stack-and-draw method [17]. The core glass was Ho³⁺: SPANK while the cladding glass was ZF-2, provided by CDGM GLASS CO.,LTD. Inset of Fig. 3 shows cross section of Ho³⁺: SPANK single-clad fiber with a diameter of 125 μm. This fiber has a core of 13 μm and a NA of 0.30 at 2040 nm. Loss of the fiber was confirmed to be 0.05dB/cm at 1310 nm using cut-back method, which is much lower than our previous fiber [17]. Absorption around 1940 nm of this fiber was theoretically estimated to be 9.94 dB/cm. However, the practical absorption at 1940 nm was measured to be 2.34 dB/cm. The large discrepancy between the calculated and measured is ascribed to off-center distribution of the fiber core. That is to say, part of light was injected into the cladding. A length of 6 cm could make the best use of the pump light due to high absorption of the fiber.

 

Fig. 3 Setup of test platform for Ho³⁺-doped single-clad SPANK fiber. Cross section of Ho³⁺: SPANK single-clad fiber is shown inset.

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Ho³⁺: SPANK single-clad fiber was in-band pumped by a Tm³⁺-doped silica fiber laser. The Tm³⁺: silica fiber was provided by FUTONG GROUP. A schematic of built-in-house in-band pump test platform is depicted in Fig. 3. In the Tm³⁺-doped silica fiber laser, a 793 nm LD was chosen as pump source. The pump light was collimated and focused into the fiber with two aspheric mirrors. A FBG with high reflectivity at 1940 nm on the front end and a FBG with 55% reflectivity at 1940nm on the back end were used to form a resonant cavity. A 4 m-long Tm³⁺-doped silica fiber could make the best of pump laser. An isolator was placed after the output end in case of back-reflection of signal fiber laser. For Ho³⁺: SPANK fiber laser, a FBG with high reflectivity at 2040 nm on the front end and a FBG with 40% reflectivity at 2040nm on the back end were used to form a resonant cavity. All FBGs used here are written on commercial single-mode silica fibers. A 6 cm-long Ho³⁺: SPANK fiber was butted to the fiber gratings to form an all-fiber configuration. Here, a reflector with high reflection (99%) at 1940nm was chosen to filter the 1940 nm pump laser for power and luminescence measurement.

As is shown in Fig. 4(a), the black curve is spectra detected from the output fiber end at a LD pump current of 2.8 A. ASE of Ho³⁺-doped fiber covers from 1981 nm to 2086 nm. The wide ASE indicates feasibility of a short pulse for mode-locking [25]. It should be noticed that signal of Tm³⁺: silica fiber laser could not be entirely blocked by the reflector. With LD pump power increasing to 3.5 A, a stable signal laser peaking at 2040 nm arises. The signal laser has a FWHM of 6 nm, which was ascribed to narrow reflection band of FBG (Δλ - 0.766 nm). Furthermore, at this point, signal of Tm³⁺: silica fiber laser was very weak and could hardly be detected by the fiber spectrograph.

 

Fig. 4 (a) Normalized spectra of pump laser, ASE, and signal laser; (b) Efficiency of signal power with respect to LD pump current.

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Figure 4(b) shows efficiency of signal power with respect to LD pump power. It is hard to determine how much pump light was coupled into core of the fiber owing to off-center distribution of the fiber core in the Ho³⁺: SPANK fiber. Thus, pump current of 793 nm LD laser was chosen as x axis. The signal laser has a threshold of 3.4 A. With increasing pump current, a maximum signal power of 60 mW was generated from Ho³⁺: SPANK single-clad fiber without saturation. It is believed that signal out-power could be further scaled. Though large NA of this single-clad fiber does not supports single-mode operation, the signal laser was single-mode outputted owing to the single-mode FBG was pigtailed. The fiber shows high potential for single-frequency fiber lasers if a more complete fiber structure was picked up.

4. Conclusion

A heavily Ho³⁺-doped lead silicate glass with low content of hydroxlys was prepared. Doping concentration of Ho³⁺ was confirmed to be 5.91 × 10²⁰ cm⁻³. Luminescent properties of this glass were characterized. Peak emission cross section reaches 3.07 × 10⁻²¹ cm² around 2055 nm and lifetime of Ho³⁺: ⁵I₇ was fitted to be 1.31 ms. A single-cald fiber was prepared using this glass as the core. Under in-band core-pump of a silica fiber laser, a single-mode maximum 60 mW was realized in a 6 cm-long fiber. This is the first report of ~2 μm laser output from a Ho³⁺: SPANK fiber. All these results show that Ho³⁺: SPANK fiber has high potential for single-frequency fiber lasers applications.