High Water-Resistant and Thermal Stable Fluoride Fibers for Mid-Infrared Laser

High-power mid-infrared (MIR) fiber laser sources exhibit excellent application potential in space communications, remote sensing, material processing, spectroscopy, and medical applications. Despite recent developments in high power MIR laser sources using fluorozirconate glass fibers, the lack of fluoride fibers with good chemical and thermal stability remains the main restriction for laser power scaling at MIR. Within the scope of this study, a fluoroaluminate glass fiber was formulated using the AlF3-YF3-BaF2-CaF2-SrF2-MgF2 (AYF) composition. This demonstration, characterized by enhanced corrosion resistance and an elevated thermal-mechanical properties, is detailed and presented herein. By using Ho3+-doping single-cladding AYF fibers as the gain medium and 1150 nm laser as the pump source, a cascaded laser operating at <inline-formula> <tex-math notation="LaTeX">$\lambda \sim 3~\mu $ </tex-math></inline-formula>m and <inline-formula> <tex-math notation="LaTeX">$\lambda \sim 2~\mu $ </tex-math></inline-formula>m with a total unsaturated maximum output power of 11.6 W and slope efficiencies of 29%, both at <inline-formula> <tex-math notation="LaTeX">$2~\mu $ </tex-math></inline-formula>m and at <inline-formula> <tex-math notation="LaTeX">$3~\mu $ </tex-math></inline-formula>m were obtained. The results show that AYF fibers can be used as viable, high performance gain media for MIR fiber lasers.

tip from ambient air.In 2018, Aydin et al. reported an Er: ZBLAN fiber laser at 2.82 µm with an output of 41.6 W [1] using a section of fluoroaluminate multimode fiber as an endcap to slow down OH-diffusion at the fiber tip.In 2021, Newburgh et al. demonstrated an Er: ZBLAN fiber laser at 2.8 µm with a Quasi-CW output power of ∼70 W [2], in which all laser experiments were conducted using dry nitrogen to enclose the fiber tip.As for MIR laser applications is laser surgery, a combination of a λ∼2µm and a λ∼3µm laser, with an optimized power ratio between them, has proved to be an excellent soft tissue surgical knife.λ∼3µm laser could provide precision cutting, while a λ∼2µm laser could penetrate tissue more deeply than the 3µm laser beam, as a means to cauterize blood vessels [3].
In this letter, we report on the characterization of AYF glasses and its applications at high-power cascaded lasers.The results substantiate that AYF fibers exhibit superior properties, establishing them as exceptional gain media for MIR lasing, especially for the λ∼3 µm.

II. GLASS CHARACTERIZATION
Firstly, to further explore stable glass system with high water-resistant and laser-damaged ability, AYF glasses were selected prepared.The fiber cladding glass composition was AYF while the core glass composition of AYF-PbF 2 -xHoF 3 .Then glasses characterization in terms of thermal and chemical stability by Differential scanning calorimetry (DSC) tests, thermal-mechanical properties tests, water-immersion experiments, were performed in details.DSC curve was tested with a Netzsch STA449F5 analyzer with a heating rate of 10 K/min.The glass sample used for water immersion tests has dimensions of ∼ 15 × 15×3mm.Furthermore, 1 mol% and 0.5 mol% Ho 3+ doped AYF glass fibers were fabricated by using rod-in-tube method.The fibers' core diameter is ∼13µm.The background loss of the doped AYF glass fibers was ∼0.5 dB/m at 1.570 µm and ∼1.0 dB/m at 2.87 µm using the cut-back method.The elevated fiber loss can be attributed to impurities present in the initial glass materials The refractive indices for the fiber cladding and core are 1.417 and 1.429 respectively at 1150 nm, which provides an NA of 0.18.The DSC curves for the undoped core and cladding glasses are shown in Fig. 1.The T g of the AYF glasses was ∼160 • C higher than that of ZBLAN glass and is almost the highest available in the fluoride glass family.
The figure-of-merit parameter (R s ) [14] of a gain material, which is used to characterize the thermal-mechanical properties, is an evaluation for the material's thermal performance and is defined as: where k is the thermal conductivity, v is the Poisson's ratio, α is the coefficient of thermal expansion, E is the elastic modulus, and σ F is the fracture toughness.The individual parameters and calculated R s for AYF glasses along with some alternative glasses for comparison are listed in TABLE I. AYF glasses have higher thermal conductivity and transition temperature, which implies that AYF fiber lasers can endure much higher thermal loads arising from quantum loss.Based on the individual parameters, the R s of AYF glass (0.367) is much higher than that of ABCYSMLZ (0.296), AlF 3 -BaF 2 -CaF 2 -YF 3 -SrF 2 -MgF 2 -TeO 2 (ABCYSMT) glass (0.263), ZrF 4 -BaF 2 -YF 3 -AlF 3 (ZBYA) glass (0.197) and ZBLAN glass (0.138).The results indicate that AYF glass fibers could support high laser power and is an excellent fiber medium for high-power lasing.
To investigate the chemical durability of typical AlF 3 -based glasses, ABYPM, ABCYSMLZ, AlF 3 -BaF 2 -CaF 2 -YF 3 -PbF 2 -MgF 2 (ABCYPM), and AYF glass were subjected to waterimmersion tests.The glass's initial transmission spectrum and weight were recorded, then the glass sample was immersed in deionized water for 24h at room temperature.After that, the sample was dried at 100 • for 12 h and weight and transmission spectrum were measured again.The recorded transmission spectra and the corresponding weight loss percentage are shown in Fig. 2. The AYF glass presents better water-resistant ability.
Fig. 2 indicates that bound water on the glass surface reduces transmittance across the entire wavelength range.AYF glasses were compared with other AlF 3 -based glasses, and their water-resistant properties were comprehensively evaluated against conventional ZrF 4 -based glasses.The most significant transmission decrease occurs in the visible and 3-5 µm bands, especially for ZBLAN and ZBYA glasses [16].AYF glass, demonstrating superior water resistance than those ZrF 4 -based glasses, holds significant promise for practical applications.
Phonon energy significantly influences the fluorescence efficiency of rare earth doped glass matrix materials.Lower phonon energy results in reduced non-radiative relaxation rates and increased fluorescence efficiency.Raman spectra for AYF and ZBLAN glasses were experimentally examined and presented in Figure 3. Notably, the Raman spectra of AYF glass reveal a prominent Raman shift peak at approximately 590 cm −1 , indicative of characteristic vibrations associated with [AlF 6 ] 3− [4], [18].Consequently, the calculated phonon energy for AYF glass is approximately 0.076 electron volts (eV).Conversely, the ZBLAN glass exhibits a maximum Raman shift peak situated at 578 cm −1 , thereby corresponding to a phonon energy of approximately 0.072 eV.
The AYF glasses underwent comprehensive characterization concerning their thermal properties, mechanical properties, chemical durability, and phonon energy.Analysis reveals that AYF glasses exhibit comparable or even superior performance when juxtaposed with ZBLAN glasses.This underscores their Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.suitability as outstanding contenders for integration into MIR fiber lasers, where stringent requirements for efficiency, power, and stability are imperative.

III. APPLICATION IN FIBER LASERS
The experimental setup for the laser system is shown in Fig. 4. All experiments were performed at room temperature and 40% humidity under normal atmospheric conditions.A collimated Raman fiber laser at λ∼1150 nm with a maximum output of 22.1 W was used as the pump source.The pump beam was launched into the fiber core through a molded glass aspherical lens (F∼11 mm, NA=0.2) and a dichroic mirror (DM) (HT at λ∼1.15 µm, HR at λ∼2.9 µm and a part reflectance 43% at λ∼2 µm).The DM and the Fresnel reflection of the fiber end form the laser cavity.The fiber was aligned with the high-reflective (HR) surface of the DM, achieving a coupling efficiency of ∼90% between the collimated pump laser and the gain fiber.
Output powers were measured using a Thorlabs (S310C) power meter with dedicated 2.4 and 1.5 µm long-pass (LP) filters.The 2.4 µm LP filter has transmittance values of 90% at 2870 nm, 0% at 2057 nm, and 0% at 1150 nm, while the 1.5 µm LP filter shows transmittance values of 63% at 2870 nm, 89% at 2057 nm, and 0% at 1150 nm.Outputs at 3 and 2 µm were then calculated based on the experimental data.The laser spectrum was monitored using an optical spectrum analyzer (YOKOGAWA AQ6377).Initially, lasing performance was investigated using 1 mol% Ho 3+ doped fibers.Fig. 5(a) illustrates the relationship between the output power and pump power for different fiber lengths for emission at 2 µm and 3 µm.With an increasing pump power and prior to laser emission at λ∼2µm, the slope efficiencies for the λ∼3µm laser are in the order of 10%.As pump power exceeds the threshold for lasing at 2 µm, the slope efficiency of the λ∼3µm laser increases substantially, from about 11% below threshold to 26% above threshold for a 50 cm fiber length.The reason is that the onset of lasing at a wavelength of ∼2 µm leads to depopulation of the lower energy state ( 5 I 7 ) for lasing at λ∼3µm, thereby significantly enhancing the laser efficiency and thus increasing the slope efficiency at ∼3 µm.The laser threshold at a wavelength of increases with the length of the gain fiber, which is mainly a result of increased reabsorption at λ∼2µm from the ground state Ho 3+ : 5 I 8 to Ho 3+ : 5 I 7 for longer fiber lengths.The slope efficiency for lasing at λ∼2µm also decreases from 21% for a fiber length of 40 cm to about 12% for a fiber length of 71 cm.
Regarding the laser output spectra, from Fig. 5(c), one can see that as the fiber length increases, the λ∼3µm and ∼2µm lasers center wavelengths undergo a red-shift.For instance, for a fiber length of 40 cm, the center wavelengths are at 2870 nm and 2058 nm, whereas at a length of 71 cm, the center wavelengths have shifted to 2931 nm and 2062 nm.For fiber lengths longer than 50 cm, the shift in the peak wavelength of the λ∼3 µm brings the lasing wavelength closer to the OH − absorption peak around 2940 nm (evident in Fig. 2.), increasing the propagation loss in the fiber.The slope efficiency at 3 µm decreases from 26% for a fiber length of 50 cm to approximately 19% for a fiber length of 71 cm.Given the impact of longer fiber lengths on the slope efficiencies for lasing at ∼2 µm and ∼3µm, it can be concluded that longer fibers are not suitable for use in efficient Ho 3+ : cascaded lasers.Increasing the fiber length results in greater reabsorption loss for the λ∼2µm laser near the output end of the fiber, consequently raising the lasing threshold for the λ∼2µm laser.In addition, the reabsorption at λ∼2µm leads to a higher population for the 5 I 7 state.Therefore, reabsorption from the 5 I 7 to the 5 I 6 state is increased also.
The laser performance was subsequently studied using 0.5 mol% Ho 3+ doped fibers.The relationship between the output power and the pump power is illustrated in Fig. 5(b).Compared to 1 mol% Ho 3+ doped fibers, the λ∼2µm laser threshold is closer in value to that of the λ∼3µm laser threshold.The λ∼2µm laser threshold is significantly lower due to the lower concentration of rare-earth ions.The maximum slope efficiency observed was 27.3% (λ∼3µm) and 21.4% (λ∼2µm) at fiber lengths of 106 cm and 88 cm, respectively.The laser spectra obtained for an absorbed pump power of are depicted in Fig. 5(d), as the fiber length increases, both lasers' emission wavelengths shift towards longer wavelengths, in similar way to the 1 mol% Ho 3+ doped AYF fibers, discussed previously.
To examine the power scaling ability of this fiber laser, further tests were carried out in a 95 cm section of the 0.5 mol% Ho 3+ doped AYF fiber.Laser output powers at λ∼3µm and λ∼2µm as a function of the pump power for this fiber is shown in Fig. 6(a).This single-cladding fiber was able to withstand a pump power of approximately 20W, close to the maximum available from the 1150 nm Raman laser used as the pump source.The maximum slope efficiencies achieved for lasing at both the ∼3µm and ∼2µm wavelengths are approximately 29%, resulting in unsaturated maximum output powers of 5.82W and 5.76W, respectively.To our knowledge, this is the highest output for Ho 3+ -doped cascaded lasers to date.Fig. 6(b) presents the laser spectrum at the highest pump power.The laser demonstrated good stability over this period with a power fluctuation of less than 1.7%.The laser output performances could be improved further.While the homemade AYF fibers display slightly higher attenuation, manufacturing fibers with reduced attenuation could improve laser slope efficiencies and outputs.Utilizing a dichroic mirror (DM) with higher reflectivity at ∼2µm would notably improve the efficiency of the 2µm laser.Furthermore, the butt positioned DM might not meet the criteria for super-high output stability, as minor vibrations could result in substantial power fluctuations.Therefore, integrating an all-fiber laser structure with fiber Bragg gratings (FBGs) would significantly enhance output stability.

IV. CONCLUSION
In conclusion, our study highlights the superior characteristics of AYF fiber compared to the commonly used ZBLAN glass, including higher water resistance, improved thermal stability, and enhanced thermal-mechanical properties.Low-loss single-cladding fibers utilizing AYF glass were successfully fabricated.A cascaded laser operating at wavelengths of λ∼2 µm and 3 µm was successfully realized, yielding a total output exceeding 10W.This is the first study to report on the laser output using the AYF glass fiber and represents the highest output power reported to date among Ho 3+ -doped cascaded lasers, suggesting excellent potential for developing higher power MIR lasers utilizing this type of fiber.

Fig. 1 .
Fig.1.DSC curves for the AYF fiber undoped core and cladding composition.T g : glass transition temperature; T x : crystallization temperature; T=T x -T g .

Fig. 3 .
Fig. 3. Raman spectra of the AYF and ZBLAN glasses.The dashes represent the multiple peaks' fit results.

Fig. 5 .
Fig. 5. Laser power as a function of the absorbed pump power with different fiber lengths in (a)1 mol% Ho 3+ doped AYF fibers (b)0.5 mol% Ho 3+ doped AYF fibers.Dashed lines are for emission at 2 µm and solid lines are for emission at 3 µm.The typical laser spectra with different fiber lengths in (c) 1 mol% Ho 3+ doped AYF fibers(d) 0.5 mol% Ho 3+ doped AYF fibers.

Fig. (
Fig. (a) Laser output power as a function of the pump power.(b) The laser spectrum at the highest pump power of ∼20W.(c) Temporal dependence of the maximum output power at λ∼3µm.