1. Introduction

The development of an efficient and robust green laser source for an increasingly broad range of applications including laser displays, microscopy, and spectroscopy remains a challenge [1,2]. Green semiconductor lasers have been mainly investigated at green wavelengths ranging of 500–550 nm [3–5], called the “green gap”. However, laser performance beyond 535 nm remains the lowest among the green spectral range due to the inherent issues in the component materials [4–6]. Therefore, green lasers based on second-harmonic generation are currently being used [7,8]. In addition to frequency-conversion, upconversion from the near-infrared (NIR), specifically direct generation from blue, is a simple method for generating green laser radiation [9–12].

As a three-level laser system generally requires more than 50% population inversion, high pump intensity is always required to realize laser oscillation. Diode-pumped fiber lasers are more favorable for three-level laser systems because the inherent waveguide confinement of the pump, and other advantages including good heat dissipation capability, high efficiency, and high-power scalability. Many efficient rare-earth-doped three-level fiber lasers under direct laser-diode (LD) pumping have been reported in the NIR spectral range. For example, efficient 938-nm operation of an Nd³⁺-doped fiber laser with a slope efficiency of 36% was demonstrated in 1987 [13]. Recently, an improved slope efficiency of 55% was achieved at 925 nm utilizing a scalable waveguide design in an Nd³⁺-doped photonic crystal fiber [14]. In addition, an 880-nm Nd³⁺-doped fiber laser with a slope efficiency of 42.8% was reported [15]. In 2001, 48% slope efficiency was achieved at approximately 978 nm for a Yb³⁺-doped double-clad fiber laser [16]. Recently, the efficiency of Yb³⁺-doped all-solid photonic bandgap fiber laser operating at approximately 978 nm has been improved to more than 60% [17,18]. Furthermore, efficiencies of 53% and 64% were demonstrated in diode-pumped Er³⁺-doped and Er³⁺/Yb³⁺ co-doped double-clad fiber lasers operating at 1558 nm and 1562 nm, respectively [19,20]. In comparison, with the rapid development of low-cost blue LDs, optical pumping of rare-earth-doped fluoride glass fibers using these LDs is a straightforward and efficient solution to realize green laser oscillation, providing strong impetus for further commercial development [21,22].

Over the past few years, there have been several reports on Pr³⁺-doped fluoride fiber lasers operating in the green spectral region. For example, high slope efficiency of 53% was demonstrated at 521 nm from a Pr³⁺-doped zirconium fluoride glass (Pr:ZFG) fiber directly pumped by a GaN LD [21], and a watt-level green fiber laser at 522 nm was developed by using Pr³⁺-doped aluminium fluoride glass (Pr:AFG) fiber [23]. Also, a broadband tunable green laser operating at 515–548 nm was reported in the Pr:ZFG fiber [10]. In particular, Ho³⁺-doped zirconium fluoride glass (Ho:ZFG) fiber has been recently used to convert the radiation from the blue pump light to the green spectral range [24]. Ho³⁺ ions are of great interest as active ions because they exhibit intense absorption at the visible wavelengths of the blue/red bands and convert these radiations into laser emission at green wavelengths. The highest efficiency reported for green Ho³⁺-doped fiber laser was 39.4% under direct blue LD pumping [24]. This slope efficiency is less than half of the theoretical Stokes-limited efficiency of approximately 82%. Its wavelength tunability is limited to Δλ = 7 nm between 543 nm and 550 nm. With upconversion pumping, the laser efficiency is inherently limited in sequential two-photon absorption upconversion, and the highest reported efficiency is 38.9% [25–29], which is considerably lesser than the Stokes-limited efficiency of approximately 58%. Although it represents the broadest tuning range of Δλ = 13 nm beyond 539 nm, a relatively expensive and complicated pump system was typically required [27,28]. Table 1 summarizes the research developments on continuous-wave (CW) green fiber lasers with two active ions Ho³⁺ and Pr³⁺.

Table 1. Research developments on the output performance of CW green Ho³⁺- and Pr³⁺-doped fiber lasers

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In this study, we present the direct generation of a highly efficient Ho³⁺-doped fiber laser in the green spectral range and demonstrate broadband green wavelength tuning. Utilizing a commercial blue LD as a direct pump, the free-running laser provides an output power of more than 360 mW at 543.9 nm with a slope efficiency of 59.3% with respect to the launched pump power. The slope efficiency is approximately 73% of the Stokes efficiency limit. For the wavelength-tuning laser, a broadband tuning range of Δλ = 13 nm is achieved at 536–549 nm employing customized active fiber lengths, through the fine adjustment of the reabsorption effect; the obtained output power is as high as 118 mW at 543.4 nm.

2. Active-fiber characterization

In the experiments, commercial Ho:ZFG fiber (Le Verre Fluoré Inc.) was used as the active fiber with a Ho³⁺-ion concentration of 5000 ppm, core/cladding diameter of 7.5/125 µm, and cutoff wavelength of 2.40 µm. The core numerical aperture (NA) of the Ho:ZFG fiber was 0.24. As shown in Fig. 1(a), we characterized a core absorption of approximately 0.2 cm⁻¹ at a pump wavelength of approximately 450 nm, corresponding to an absorption coefficient of approximately 0.86 dB/cm. It should be noted that the broadband absorption spectrum of the blue pump band relaxes the required wavelength tolerance on the LD considerably, when pumping a three-level laser, in particular. In this study, we selected a commercial single beam-shaped GaN LD operating at 447–451 nm as the pump source (LSR450CPD, ∼3 W, Lasever Inc.). The corresponding power-dependent wavelength properties are depicted in Fig. 1(a). A commercial ZFG passive fiber with a core/cladding diameter of 7.5/125 µm and NA of 0.26 was employed to estimate the pump launching efficiency. The fiber background attenuation at green wavelengths was measured to be <0.04 dB/m, which is remarkable, and plotted in Fig. 1(b). This renders the ZFG fiber particularly suited for laser emission in the green spectral range. With the pump arrangement in our experiments, the launching efficiency into the ZFG passive fiber was estimated to be 35.1%, as shown in Fig. 1(c). The maximum available launched pump power was measured to be 996 mW.

 

Fig. 1. (a) Absorption spectrum of the Ho:ZFG fiber at blue wavelengths and wavelength evolution of the blue LD with the launched pump power. (b) Measured attenuation of the ZFG passive fiber at green wavelengths. (c) Launched power as a function of the operating power of the blue LD for the ZFG passive fiber.

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To explore the potential of the Ho:ZFG fiber for green laser development, we consider a direct pumping scheme. The scheme was selected based on the energy-level diagram depicted in Fig. 2(a) [24]. Pumping at 450 nm populates the ⁵F₄,⁵S₂ levels rapidly from various upper levels, with broad green emission from the transition back to the ground-state ⁵I₈ level, effectively constituting a three-level system. The fluorescence decay curves for green emission are depicted in Fig. 2(b) for the 5000-ppm Ho:ZFG fiber. They are well fitted using the multiexponential model and the effective lifetime of the thermalized ⁵F₄,⁵S₂ levels is 261 µs at 545 nm and 550 nm.

 

Fig. 2. (a) Simplified energy-level diagram of the Ho:ZFG fiber for green laser transition through direct blue pumping. (b) Fluorescence decay curve of the Ho:ZFG fiber for the ⁵F₄,⁵S₂ levels; λ_ex = 450 nm and λ_em = 545 nm and 550 nm.

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Laser evaluation was first performed by measuring the amplified spontaneous emission (ASE) of the Ho:ZFG fiber. The analysis of the ASE spectrum detailed below gives a qualitative description of the effect of reabsorption, which is the origin of the difference between the forward and backward cases. Three Ho:ZFG fiber samples with lengths of 13 cm, 17 cm, and 28 cm were prepared. The green ASE output spectra shown in Fig. 3 were obtained when the active fiber was directly excited by a blue LD using the setup depicted in Fig. 4. In the forward case in Fig. 3(a), because of the increased reabsorption, the green ASE spectra exhibit progressive vanishing of the spectrum peak around 535 nm with the increase in the active fiber length. This indicates that a short length of Ho:ZFG fiber can be used, potentially enabling the realization of a tunable green fiber laser with a shorter operating wavelength. The green ASE spectra measured at the same launched pump power for L_AC = 13 cm and L_AC = 28 cm are shown in Figs. 3(b) and 3(c), respectively. For the shorter length, L_AC = 13 cm, comparison between the forward and backward spectra shows negligible qualitative difference, except for the operation intensity. The forward case for L_AC = 28 cm shows a more drastic effect, in which the 535 nm peak disappears (identical to Fig. 3(a)). The disappearance of the 535 nm peak due to reabsorption causes the appearance of a dip at 535 nm in the ASE spectrum. This effect is less pronounced in the backward case because of the smaller absorption effect. It is worth noting that the backward ASE intensity is always higher than that of the forward case. The changes in the output ASE spectrum explain the variation in the lasing wavelength with the active fiber length [30,31].

 

Fig. 3. Green ASE outputs from the blue-excited Ho:ZFG fiber (with cavity mirror #1 removed in Fig. 4) for different lengths of 13, 17, and 28 cm.

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Fig. 4. Schematic of the blue diode-pumped green Ho³⁺-doped fiber laser in a free-running configuration.

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3. Results and discussion

3.1 Green free-running laser operation

Figure 4 depicts the experimental setup for the blue LD counter-pumped green Ho³⁺-doped fiber laser in a free-running configuration. The pump beam was directed through a dichroic mirror (DM, T>95% at 450 nm and T<0.1% at 530–550 nm; see Fig. 4(a)) and launched into the Ho:ZFG fiber through a 22.5 mm focal length aspheric lens. Feedback for laser oscillation was provided by Fresnel reflection of approximately 4% from a high-quality polished fiber end-facet at one launch end of the Ho:ZFG fiber, and at the opposite end, by a homemade single-mode fiber (SMF) end-facet mirror (i.e., cavity mirror #1; see Fig. 4(b)) with broadband high reflectivity covering both the signal (T<5% at 545 nm) and pump bands (T<7% at 450 nm). This results in near-total pump absorption and high inversion per unit length, facilitating a minimal value for the oscillation threshold. A broadband SMF end-facet mirror was fabricated by directly depositing a multilayer coating of SiO₂ and Ta₂O₅. Here, a ceramic sleeve with 2.5-mm inner diameter was employed to realize efficient butt coupling between the Ho:ZFG fiber and SMF. The laser output power was measured in the backward direction through the DM using a thermal power sensor head (S425C-L) along with a power meter console (PM100D, Thorlabs Inc.). The corresponding spectrum of the green fiber laser was measured using an optical spectrum analyzer (OSA, Ando AQ-6315B).

We characterized the green fiber laser for several fiber lengths under free-running operation to determine the maximum slope efficiency and output power achievable with the Ho:ZFG. As displayed in Fig. 5(a), the maximum slope efficiency is approximately 59.3% for L_AC = 20 cm, and the maximum output power is 376 mW for L_AC = 17 cm. The high efficiency is because of direct blue pumping scheme and pump retroreflection from HR cavity mirror #1 (similar to bidirectional pumping). It is comparable or superior to previous NIR laser systems under direct pumping [13–20]. As the Stokes efficiency limit was approximately 82%, further improvement of the laser efficiency was feasible by optimizing the Ho:ZFG fiber design (e.g., fiber multimode behavior at blue pump wavelengths) together with the laser resonator scheme [32,33].

 

Fig. 5. (a) Dependences of the slope efficiency and output power on the active fiber length for the green free-running Ho:ZFG fiber laser. (b), (c) Output power of the 543.5-nm and 543.9-nm fiber lasers versus the launched pump power for different active fiber lengths of L_AC = 17 cm and L_AC = 20 cm, respectively. (d), (e) Corresponding output spectra at a maximum output power of 376 mW and 363 mW, respectively.

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Correspondingly, the laser thresholds at approximately 350 mW and 400 mW of the launched pump powers are depicted in Figs. 5(b) and 5(c), respectively. The linearity of the output power as a function of the launched pump power for both lengths of the Ho:ZFG fiber in the free-running cavity configuration suggests that there is no severe thermal induced deformation of the active fiber, even at the highest launched pump power. This result indicates that further power scaling can be expected employing cladding pumping scheme in passively and actively cooled setups [34,35].

At the maximum output power of 376 mW and 363 mW, respectively, the output spectra of the green fiber laser were measured at a spectrum analyzer resolution of 0.1 nm, as shown in Figs. 5(d) and 5(e) for L_AC = 17 cm and L_AC = 20 cm, respectively. Both these operation wavelengths were stable at 543.5 nm and 543.9 nm over the output power ranges, and performance degradation over a period of several weeks of intermittent use was not observed. As expected, the operating wavelength red-shifted with the increase in the active fiber length due to the balance between gain ground-state absorption in the three-level system [36,37]. For longer fibers, the signal absorption increases most significantly near the shorter-wavelength edge of the spontaneous emission, and the center shifts to a longer wavelength. This result is in accordance with the above-described green ASE from Ho:ZFG fibers under blue pumping (see Fig. 3).

3.2 Green wavelength-tuning laser operation

We investigated the tuning characteristics of the green Ho³⁺-doped fiber laser using the same active fiber used previously. The co-pumped configuration of the tunable fiber laser cavity is depicted in Fig. 6. The cavity is defined by an HR flat mirror (T<0.1% at 530–550 nm; see Fig. 4(a)) aligned to reflect light back into the cavity, and a broadband fiber end-facet mirror (T∼6% at 530–550 nm) acting as the cavity output mirror (i.e., cavity mirror #2; see Fig. 6(b)). The green output from the input end facet of the polished Ho:ZFG fiber was collimated by an aspheric lens, directed through the DM, and passed to a bandpass filter (#86737, Edmund Optics). Wavelength tuning was accomplished by simply rotating the angle of the intracavity bandpass filter to change the central transmission wavelength (see Fig. 6(a)). The laser output power was measured in the forward direction through the pump filter using a thermal power detector (S425C-L, Thorlabs). The corresponding spectrum of the green tunable fiber laser was measured using an OSA (AQ-6315B, Ando).

 

Fig. 6. Schematic of the blue diode-pumped green Ho³⁺-doped fiber laser in a wavelength-tuning configuration.

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The output spectra of tunable fiber lasers with L_AC = 13 cm and L_AC = 28 cm were measured at various pump power, as shown in Fig. 7. The tuning range of the green fiber laser with L_AC = 13 cm increases with the increase in the launched pump power, similar to the performance of a general wavelength-tunable fiber laser [33,38]. When the launched pump power is 514 mW, the tuning range Δλ is only 3.2 nm from 540.3–543.5 nm. As the launched pump power increases to 603 mW, the tuning range increases to 8.1 nm from 538.3–546.4 nm. The tuning range increases to 10.1 nm from 536.3–546.4 nm at a launched pump power of 759 mW. At the peaks of the tuning curves, the green fiber laser exhibits a maximum output power of 3.8 mW at 543.5 nm, and the output power intensity decreases as the wavelength reduces or increases.

 

Fig. 7. Output spectra of the green tunable Ho³⁺-doped fiber laser with L_AC = 13 cm and L_AC = 28 cm from 536.3 nm to 549.3 nm at various launched pump power.

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For comparison, the tunable range of the green fiber laser with L_AC = 28 cm at a launched pump power of 759 mW was measured from 541.9 nm–549.3 nm with Δλ = 7.4 nm. As previously discussed, the absorption increases with the increase in the active fiber length at short wavelengths, and the short operation wavelength is limited by reabsorption below 540 nm, which is consistent with the reduced green ASE intensity at short wavelengths (see Fig. 3).

To investigate the possibility of higher output power from the green tunable fiber laser, the cavity mirror #2 was removed from the arrangement shown in Fig. 6; i.e., higher output coupling was implemented with only approximately 4% Fresnel reflection from the end facet of the polished active fiber. For L_AC = 17 cm, the output power of the tunable fiber laser pumped at 656 mW and 887 mW were measured for different operation wavelengths, as shown in Fig. 8. At the peaks of the tuning curves, the green fiber laser exhibits maximum output power of 46 mW at 543.6 nm, and 118 mW at 543.4 nm. The output power is highly dependent on the operation wavelength, and reduces as the wavelength decreases or increases. This is because the green Ho³⁺-doped fiber laser under blue pumping is a directly pumped three-level laser system. At short wavelengths, the active fiber can provide optical gain only when highly pumped, and a large population inversion is achieved. At long wavelengths, as the emission cross-section is less, the unit gain is small. As a result, the output power of the laser beyond 543 nm decreases with the increase in wavelength.

 

Fig. 8. Output power of the green tunable Ho³⁺-doped fiber laser with L_AC = 17 cm at various wavelengths for various launched pump power.

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Compared to the previously reported green tunable Ho³⁺-doped fiber laser pumped by a blue LD [24], the operating wavelength in this case is slightly shorter at 539.9–545.2 nm, originating from the ⁵F₄→⁵I₈ transition alone [27]. The main reason for the shorter operating wavelength is the comparatively lower reflectivity of the output coupler (∼4%) used in this investigation. For this cavity, higher output coupling forces a higher value of the launched pump power at the threshold compared to the lower output coupling case. Consequently, the current operating wavelength is shifted to a shorter wavelength because the fractional population inversion increases under a higher launched pump power.

4. Conclusion

In conclusion, we demonstrated highly efficient CW green laser oscillation in a Ho³⁺-doped fiber in this study. Using a direct diode-pumped linear cavity, more than 360 mW was generated at 543.9 nm with 59.3% slope efficiency (relative to the launched pump power), which is the highest efficiency realized for any green fiber laser to date, to the best of our knowledge. Moreover, broadband wavelength tuning of 536–549 nm targeting power operation of more than 100 mW was achieved by employing various active fiber lengths. An extended tuning range at the short-wavelength edge is possible by reducing the length of the active fiber, thereby changing the degree of signal reabsorption in the cavity. We believe that this demonstration will inspire the development of next generation of rare-earth-doped fiber laser systems operating in the green spectral range, which can address the unfulfilled needs of green laser applications beyond 535 nm.