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Abstract
Lasers at ${\sim}{900}\;{\rm nm}$ have been of vital importance in various fields, including material processing, underwater communications, and strong-field physics. Although ${{\rm Nd}^{3 +}}$-doped materials have been employed for the ${\sim}{900}\;{\rm nm}$ laser, the ${\sim}{900}\;{\rm nm}$ emission is in strong competition with the often more dominating ${\sim}{1060}\;{\rm nm}$ emission, which strongly limits the output power and applications. This paper proposes a direct coordination engineering approach, which introduces halogen to the nearest coordination of ${{\rm Nd}^{3 +}}$ in glass for increasing the bond covalency, leading to stronger emissions at ${\sim}{900}\;{\rm nm}$ than at ${\sim}{1060}\;{\rm nm}$. Iodide-incorporated ${{\rm Nd}^{3 +}}$-doped silica fibers show prevailing ${\sim}{900}\;{\rm nm}$ emission rarely observed in ${{\rm Nd}^{3 +}}$-doped materials. Using the created fibers, a power (113.5 W) 50 times higher than the current record is accomplished based on an all-fiber structure. Our approach holds the potential for regulating the spectroscopic properties of other rare-earth-doped laser materials.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
High-power near-infrared (NIR) lasers play an important role in the field of photonics owning to their broad applications in laser communication, material processing, and strong-field physics [1–3]. Recently, there has been a strong interest in high-power laser systems emitting in the NIR region around 900 nm, which can be used to pump ${{\rm Yb}^{3 +}}$ laser materials directly, for atmospheric detection, and in image-guided surgery [4–7]. Moreover, the frequency doubling of ${\sim}\;{900}\;{\rm nm}$ laser leads to a deep blue laser of ${\sim}{450}\;{\rm nm}$, which has attracted significant attention from researchers for its potential applications in the fields of deep-sea communication, laser storage, laser display, and atomic physics [8–10]. At present, several methods have been adopted to obtain ${\sim}{900}\;{\rm nm}$ high-power lasers, including semiconductor, solid-state, and ${{\rm Nd}^{3 +}}$-doped fiber (NDF) lasers [11,12]. Owing to their unparalleled tunability, compactness, and beam quality, the NDF lasers have been receiving increasing research attention [13–16].
However, it remains a tremendous challenge to develop a high-efficiency ${\sim}{900}\;{\rm nm}$ laser based on either NDF or other ${{\rm Nd}^{3 +}}$-doped materials, because of the strong competition between the ${^4{{\rm F}}_{3/2}}\; \to {^4{{\rm I}}_{11/2}}$ (${\sim}{1060}\;{\rm nm}$) transition and the ${^4{{\rm F}}_{3/2}}\; \to {^4{{\rm I}}_{9/2}}$ (${\sim}{900}\;{\rm nm}$) transition, leading to the emission at ${\sim}\;{1060}\;{\rm nm}$ that commonly overwhelms the transitions at ${\sim}{900}\;{\rm nm}$ [17,18]. Therefore, the key to achieving a high-efficiency ${{\rm Nd}^{3 +}}\;\sim{900}\;{\rm nm}$ laser is to manipulate the competition. Directly, the problem of transition competition can be solved by inhibiting the ${\sim}{1060}\;{\rm nm}$ emission intensity passively using bandpass filters near ${\sim}{900}\;{\rm nm}$, yet this scheme sacrifices the compactness and stability of the laser device [19]. An alternative way is to design a fiber waveguide structure with a loss up to ${\sim}{1060}\;{\rm nm}$ but not to ${\sim}{900}\;{\rm nm}$, such as with special W-type fibers and photonic crystal fibers (PCFs) [20–23]; however, the W-type index profile is only applicable to small-diameter cores (${\lt}{6}\;{\unicode{x00B5}{\rm m}}$) that suffer from a low threshold for nonlinear optical effects, and PCFs, which are complicated and expensive to prepare, are difficult to fuse with conventional commercial passive optical fibers for device integration [24,25]. In addition, operating at low temperatures or reducing the clad-to-core ratio of NDF is also an effective approach to reducing the gain difference between ${\sim}{900}$ and ${\sim}{1060}\;{\rm nm}$ [26,27]. However, after a long struggle, the maximum output power of all-fiber lasers operating at ${\sim}900 \text{-} {\rm nm}$-based NDFs is still only 2.7 W [28].
Obviously, the aforementioned methods for overcoming the transition competition by passively suppressing the 1060 nm ASE have encountered a bottleneck in boosting the ${\sim}{900}\;{\rm nm}$ laser power of NDF. Therefore, a novel method to actively enhance the ${\sim}{900}\;{\rm nm}$ fluorescence branching ratio of core glass should be developed. Here, we propose an innovative method to change the direct coordination of ${{\rm Nd}^{3 +}}$ through non-oxygen anion co-doping, which results in a huge change in the fluorescence characteristics of ${{\rm Nd}^{3 +}}$. In particular, the ${\sim}{900}\;{\rm nm}$ fluorescence intensity of the ${{\rm NdI}_3}$-doped silica glass exceeded that at ${\sim}{1060}\;{\rm nm}$, leading to a giant enhancement of the ${\sim}{900}\;{\rm nm}$ laser power. Using this glass as a fiber core, a 30/125 µm NDF was fabricated, and it is used to build a ${\gt}{100}\;{\rm W}$ all-fiber CW laser operating at 927 nm. To the best of our knowledge, the 113.5 W output power reported here is the highest output power achieved with an all-fiber structure operating in the ${\sim}{900}\;{\rm nm}$ spectral region, and also a record value among all NDF lasers operating at ${\sim}{900}\;{\rm nm}$. Our findings not only aid in the development and application of ${\sim}{900}\;{\rm nm}$ high-power fiber lasers, but also provide a new avenue for the regulation of the spectral properties of ${{\rm RE}^{3 +}}$-doped materials.
2. THEORETICAL ANALYSIS AND GLASS STRUCTURE DESIGN
We began with the consideration of how to rationally create a suitable local coordination environment of ${{\rm Nd}^{3 +}}$ using the Judd–Ofelt theory [29,30]. According to this theory, various local environments of ${{\rm Nd}^{3 +}}$ lead to different intensity parameters ${\Omega _t}\;({t} = {2},\;{4},\;{6})$, resulting in different spontaneous emission probabilities $A({J \to J^\prime})$ and fluorescence branching ratios (${\beta _x}$, $x = {900}$, 1060, or 1330 nm). The $A({J \to J^\prime})$ of ${{\rm Nd}^{3 +}}$ can be determined using the following equation:
${\beta _x}$ is defined as
Table 1. Doubly Reduced Matrix Elements of ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{9/2}}$, ${^4{{\rm I}}_{11/2}}$, ${^4{{\rm I}}_{13/2}}$ Transitions [31]
We note that ${| {{\varphi ^a}{|}{U^{(2)}}{|}{\varphi ^b}} |^2}$ corresponding to all three transitions equals zero (Table 1). Consequently, $A ({J \to J^\prime})$ are mainly dominated by ${\Omega _{4,6}}$, yet without a direct connection with ${\Omega _2}$, as suggested by Eq. (1). In particular, the coefficients ${\beta _x}$ will be governed only by the ratio of ${\Omega _4}/{\Omega _6}$ according to Eq. (2); as the ${\Omega _4}/{\Omega _6}$ increases, only ${\beta _{900\;{\rm nm}}}$ increases, whereas both ${\beta _{1060\;{\rm nm}}}$ and ${\beta _{1330\;{\rm nm}}}$ decrease. More importantly, if ${\Omega _4}/{\Omega _6}$ is greater than unity, the intensity of the ${\sim}{900}\;{\rm nm}$ emission is stronger than that of the ${\sim}{1060}\;{\rm nm}$ emission [32]. Consequently, to obtain a larger ${\beta _{900\;{\rm nm}}}$, a larger ${\Omega _4}$, or a smaller ${\Omega _6}$ is desired. The ${\Omega _6}$ is related to the overlap integrals of the 4f and 5d orbitals and decreases as the outermost electron cloud density of ${{\rm Nd}^{3 +}}$ increases, which can be accomplished by increasing the covalency between ${{\rm Nd}^{3 +}}$ and the bound anion [33]. Therefore, we took the view that the key to enhancing ${\beta _{900\;{\rm nm}}}$ is to judiciously create a coordination environment of ${{\rm Nd}^{3 +}}$ featuring stronger covalency with the bound anion.
However, the rigid network structure of silica glass makes the doping concentration of ${{\rm RE}^{3 +}}$ very low, and the coordination environment is difficult to adjust. The doping concentration of ${{\rm RE}^{3 +}}$ in silica glasses can be increased by co-doping Al and P as dissolving agents, which can eliminate the formation of ${{\rm RE}^{3 +}}$ clusters and improve the luminescence intensity of ${{\rm RE}^{3 +}}$ to a certain extent [34,35]. Unfortunately, in NDF, the addition of these co-dopants tends to decrease the Stark splitting of the ground state manifold by reducing the distribution asymmetry of ${{\rm Nd}^{3 +}}$, shifting the ${^4{{\rm F}}_{3/2}}\to {^4{{\rm I}}_{9/2}}\;(\sim{900}\;{\rm nm})$ emission spectrum to shorter wavelengths, which is detrimental to the ${\sim}{900}\;{\rm nm}$ lasers [35]. Another method to regulate the luminescence characteristics of ${{\rm RE}^{3 +}}$ is to grow microcrystals in a matrix glass by means of heat treatment and femtosecond laser direct writing, which change the crystal field around the luminescent ion [36,37]. However, this approach is difficult to apply to the rigid network of silica glass, and the larger scattering loss due to the nanocrystals could be detrimental to high-power lasers [38].
Fig. 1. Snapshots of the MD simulated structure of the ${{\rm Nd}^{3 +}}$-doped silica glass without and with halogen anions. The upper left snapshot shows the halogen-unincorporated ${{\rm Nd}^{3 +}}$-doped silica glass with a characteristic local coordination environment as shown in the upper middle snapshot (denoted NO). The upper right snapshot and the lower ones from the left to the right correspond to the local coordination environments of ${{\rm Nd}^{3 +}}$ in the silica glasses doped with ${{\rm I}^ -},\;{{\rm Br}^ -},\;{{\rm Cl}^ -}$, and ${{\rm F}^ -}$, respectively. The polyhedron is the local structure within the 0.5 nm range around ${{\rm Nd}^{3 +}}$.
Fig. 2. (a) Image of the ${{\rm Nd}^{3 +}}$-doped silica glass rod with a dimension of $\Phi{6} \times {100}\;{\rm mm}$. (b) Refractive index distribution of the ${{\rm Nd}^{3 +}}$-doped silica glass rod with pure silica glass tube cladding. The fluctuation in the center (${\pm 0.2}\;{\rm mm}$) is an instrument signal error. (c) Transmission electron microscopy (TEM) image and energy-dispersive X-ray spectrometry (EDS) analysis of the NI sample.
According to the electronegativity theory, the smaller the electronegativity difference between cations and the direct adjacent anions, the more covalent the bond is. Consequently, modifying the direct ligand anion of ${{\rm Nd}^{3 +}}$ ions that can change the covalency of the bonding could be a viable route to modulate its spectral behaviors. Besides ${{\rm O}^{2 -}}$, the most common anions are the halogen ions (${{\rm X}^ -},\;{\rm X} = {\rm F},\;{\rm Cl},\;{\rm Br},\;{\rm I}$). The electronegativity values of Nd, O, F, Cl, Br, and I are 1.2, 3.5, 4.0, 3.0, 2.8, and 2.5, respectively, and the electronegativity differences of Nd-O, Nd-F, Nd-Cl, Nd-Br, and Nd-I bonds are 2.3, 2.8, 1.8, 1.6, and 1.3, respectively. The ${{\rm NdX}_3}$-doped silica glasses (labeled as NF, NCl, NBr, and NI) were used as samples for classical molecular dynamics (MD) simulations to verify that the direct ligand anions of ${{\rm Nd}^{3 +}}$ are changed by the introduction of halogens.
As shown in the snapshots of MD simulations in Fig. 1, ${{\rm Nd}^{3 +}}$ disperse randomly in the silica glass matrix both before and after introducing halogens. In the silica glass, ${{\rm Nd}^{3 +}}$ preferentially occupied the gap position of the Si-O network, where non-bridge oxygen is present, and part of the Si atoms are replaced by Al atoms. It is noted that introducing halogens causes partial replacement of the oxygen ions (${{\rm O}^{2 -}}$) located nearest to ${{\rm Nd}^{3 +}}$ by ${{\rm X}^ -}$, suggesting that ${{\rm X}^ -}$ tend to directly coordinate with ${{\rm Nd}^{3 +}}$. We underscore that our MD simulation results strongly suggest that halogen ions lead to a significant change in the coordination environment of ${{\rm Nd}^{3 +}}$, and we expect that the spectroscopic properties could be notably altered.
3. RESULTS AND DISCUSSION
We chose the M-Sol-Gel strategy to prepare the ${{\rm Nd}^{3 +}}$-doped silica glass with a high doping homogeneity [39–41]. ${{\rm X}^ -}$ was introduced through the use of different halide compounds of Al and Nd. The content of ${{\rm Nd}^{3 +}}$ in all samples is 2500 ppm. NX is the general name for all the samples containing ${{\rm X}^ -}$. As the control group, an NO sample without ${{\rm X}^ -}$ was also prepared (Fig. S2). The detailed experimental section is provided in the Supplement 1 (Fig. S1).
A. Homogeneity Characterization
Figure 2(a) shows a large-size ($\Phi{6} \times {100}\;{\rm mm}$) ${{\rm Nd}^{3 +}}$-doped silica glass preform with high transmittance and no bubbles. The preform shows good radial and axial optical uniformity with refractive index fluctuations of $\le {2} \times {{10}^{- 4}}$ [Fig. 2(b)], which means that silica fiber performs with good optical quality, and doping homogeneity can be prepared by M-Sol-Gel. The refractive index difference $\Delta n$ with respect to the pure undoped silica glass was $({1.0}\;{\pm}\;{0.2}) \times {{10}^{- 3}}$. The NI sample is the most difficult one to preserve ${{\rm X}^ -}$ in silica glass because ${{\rm I}^ -}$ is the most easily oxidized anion in ${{\rm X}^ -}$. Hence, among NX samples, we chose NI as a representative in the study of homogeneity. The uniform distributions of Si, Al, Nd, and I in the glass rod are shown in Fig. 2(c), as measured by transmission electron microscopy combined with energy-dispersive X-ray spectrometry. The iodine content in the silica glass was small, but its distribution was homogeneous. The micron-scale homogeneity of silica glass and the presence of iodine elements were further examined using an electron probe micro-analyzer and a laser ablation inductively coupled plasma mass spectrometer (see Fig. S3), respectively. We note that, with more stable ${{\rm X}^ -}$, other NX samples have the same or better homogeneity than NI, which indicates that they have good optical quality and are suitable for NIR lasers.
B. Role of ${{\rm X}^ -}$ in the Direct Coordination of ${{\rm Nd}^{3 +}}$-Doped Silica Glass
We then investigate the changes in the binding energy and local asymmetry of ${{\rm Nd}^{3 +}}$ after ${{\rm X}^ -}$ partially occupy its direct coordination sites because the luminescence properties of ${{\rm RE}^{3 +}}$ are sensitive to the local structure asymmetry and chemical state [42]. As the most difficult sample to preserve ${{\rm X}^ -}$ in silica glass, NI was selected again as a representative. X-ray photoelectron spectroscopy (XPS) of Nd 3d levels was performed, and the results are shown in Fig. 3(a). This figure reveals in detail how the chemical state of ${{\rm Nd}^{3 +}}$ changes when its near-neighbor O atoms are replaced by I. Two XPS peaks are located at 980 and 1000 eV, which correspond to Nd ${{3d}_{5/2}}$ and ${{3d}_{3/2}}$ levels, respectively. According to Gaussian fitting, the band at 980 eV exhibited a two-line structure, whereas the band at 1000 eV consisted of three parts. In particular, as pointed out by the arrow in Fig. 3(a), the high-energy parts of both bands were enhanced when the nearest-neighbor atoms of ${{\rm Nd}^{3 +}}$ changed from O to I. The shift of these two bands to high energies indicates a higher covalent nature of ${{\rm Nd}^{3 +}}$ [43]. This phenomenon is attributable to the fact that I atom can only provide a negative charge to ${{\rm Nd}^{3 +}}$ by sharing extra-nuclear electrons and cannot directly provide unpaired electrons like non-bridge oxygen. With the same valence, other halogen ions play the same role as ${{\rm I}^ -}$ in ${{\rm Nd}^{3 +}}$-doped silica glass.
Fig. 3. (a) High-resolution X-ray photoelectron spectroscopy (XPS) curves in the Nd-3d region and Gaussian fitting of the ${{\rm Nd}^{3 +}}$-doped silica glass with different anions. (b) Absorption spectra of NX, NO, NdSG, NdPG, and NdYAG in the range of 400–1000 nm. (c) Normalized absorption of the ${^4{{\rm I}}_{9/2}} \to {^2{{\rm G}}_{7/2}} + {^4{{\rm G}}_{5/2}}$ hypersensitivity transition in the range of 550–620 nm for NO and NX. (d) Normalized absorption of the ${^4{{\rm I}}_{9/2}} \to {^4{{\rm F}}_{3/2}}$ transition in the range of 840–950 nm for NO and NX.
Absorption spectra also provide worthwhile information on the local environment of ${{\rm RE}^{3 +}}$. To intuitively investigate the spectral characteristics of NX, the spectra of commercial ${{\rm Nd}^{3 +}}$-doped silicate glass (NdSG), phosphate glass (NdPG), and yttrium aluminum garnet (NdYAG) were tested for comparison. Figure 3(b) shows the absorption spectra of NX, NO, NdSG, NdPG, and NdYAG in the 400–1000 nm range. Eight characteristic absorption bands can be observed, and the corresponding energy levels are marked in the spectra. The absorption characteristics of NX differ significantly from those of NdSG, NdPG, and NdYAG owing to differences in local site symmetry and bonding strength. As the anion ions change from ${{\rm O}^{2 -}}$ to ${{\rm F}^ -}$, ${{\rm Cl}^ -}$, ${{\rm Br}^ -}$, and ${{\rm I}^ -}$, the ${^4{{\rm I}}_{9/2}} \to {^2{{\rm G}}_{7/2}} + {^4{{\rm G}}_{5/2}}$ hypersensitive transition is red-shifted and broadened [Fig. 3(c)], which suggests that the crystal field strength around ${{\rm Nd}^{3 +}}$ increased [44]. In addition, the laser upper energy level (${^4{{\rm F}}_{3/2}}$) shifts to a lower energy state [Fig. 3(d)], which can be attributed to the nephelauxetic effect as ${{\rm Nd}^{3 +}}$ is incorporated into a more covalent glass network [45]. The conclusions drawn from the absorption are consistent with those obtained from the structural analysis.
Fig. 4. Spectroscopic properties of ${{\rm Nd}^{3 +}}$-doped silica glass. (a) Photoluminescence (PL) spectra of NO and NX. (b) Integrated fluorescence intensities of the transitions from the ${^4{{\rm F}}_{3/2}}$ level to each lower level of NO and NX. All integrated fluorescence intensities are normalized to the same ${\sim}{1060}\;{\rm nm}$ emission intensity of each sample. (c) PL spectra of NI, NdSG, NdPG, and NdYAG. (d) PL decay curves and fluorescence lifetimes of NX, NdSG, NdPG, and NdYAG.
We calculate ${\Omega _t}$ of ${{\rm Nd}^{3 +}}$ through the least-squares fitting method, and the results are summarized in Table 2. The detailed calculation process can be found in Reference [46]. The low root-mean-square deviation ${\delta _{\rm rms}}$ (of the order of ${{10}^{- 7}}$) indicates high reliability of the calculated results. As presented in Table 2, ${\Omega _t}$ changes in the order ${\Omega _{2}} \gt {\Omega _{4}} \gt {\Omega _6}$ for NX, and this is different from NO, NdSG, NdPG, and NdYAG. This rare phenomenon has only been observed in chalcogenide and halide glasses [47]. Specifically, NX have a larger ${\Omega _2}$, implying a higher asymmetry around the sites of ${{\rm Nd}^{3 +}}$ and a higher covalence degree between ${{\rm Nd}^{3 +}}$ and the bonding anions, which has also been confirmed by structural analysis and absorption spectra. Furthermore, NX have a lower ${\Omega _6}$, which contributes significantly to the emission intensity at ${\sim}{1060}\;{\rm nm}$ originating from the ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{11/2}}$ transition (as suggested by Eq. (1) and Table 1). Meanwhile, for NX samples, ${\Omega _2}$ increases but ${\Omega _{4,6}}$ decreases slightly as the anion ions change from ${\rm F}^-$ to ${{\rm Cl}^ -},\;{{\rm Br}^ -}$, and ${{\rm I}^ -}$. Combined with the conclusion obtained from the microstructure and absorption spectra, we infer that ${{\rm X}^ -}$ breaks the Si-O rigid network and enters the direct neighbor coordination of ${{\rm Nd}^{3 +}}$, creating more sites for ${{\rm Nd}^{3 +}}$, while the sharing of extra-nuclear electrons increases its covalency degree. In addition, the 6 s electron cloud density of ${{\rm Nd}^{3 +}}$ increases with the increasing radius of ${{\rm X}^ -}$, which shields 5d orbits or repels 5d electrons close to 4f orbits, resulting in a decrease in ${\Omega _6}$ [33].
Table 2. Judd–Ofelt Intensity Parameters ($\Omega _t$), Root-Mean-Square Deviation ${\delta _{\rm rms}}$ (${{10}^{-6}}$), and Fluorescence Branch Ratio of the ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{9/2}}$ (${{\beta}_{900\;{\rm nm}}}$) Transition
The ${\Omega _4}/{\Omega _6}$ ratio and ${\beta _x}$ were also calculated and listed in Table 2. With a lower ${\Omega _6}$, NX have a ${\Omega _4}/{\Omega _6}$ ratio greater than 1, and it is much larger than that of NO, NdSG, NdPG, and NdYAG, leading to a larger ${\beta _{900\;nm}}$. Interestingly, ${\Omega _4}/{\Omega _6}$ and ${\beta _{900\;nm}}$ increase as the radius of ${{\rm X}^ -}$ increase. Therefore, the introduction of ${{\rm X}^ -}$ in silica glass is beneficial for increasing ${\beta _{900\;nm}}$, particularly for anions with large radius. The above results indicate that NI is an ideal material for obtaining a ${\sim}{900}\;{\rm nm}$ laser.
C. Spectroscopic Properties
Afterward, we study the emission properties of the core glass to confirm our hypothesis directly. As shown in Fig. 4(a), the photoluminescence (PL) spectra of NO and NX ranging from 830 to 1500 nm were measured under the same conditions. The three fingerprint emission bands at 902, 1058, and 1331 nm are associated with the ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{9/2}}$, ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{11/2}}$, and ${^4{{\rm F}}_{3/2}} \to {^4{{\rm I}}_{13/2}}$ transitions of ${{\rm Nd}^{3 +}}$, respectively. As the anion ions changed from ${{\rm O}^{2 -}}$ to ${{\rm F}^ -}$, ${{\rm Cl}^ -}$, ${{\rm Br}^ -}$, and ${{\rm I}^ -}$, the emission bands were red-shifted and broadened, which implies strong crystal field splitting that correlates with the high covalent level of ${{\rm Nd}^{3 +}}$. Importantly, the intensity around 900 nm was dramatically enhanced, and it became stronger than that around 1060 nm without significant change in spectral shape. Under optimal conditions, the emission intensity of NI at ${\sim}{900}\;{\rm nm}$ was higher than that at ${\sim}{1060}\;{\rm nm}$. This anomaly has only been reported in ${{\rm Ge}^{4 +}}$-co-doped silica glass with a low ${{\rm Nd}^{3 +}}$ concentration and unstable ${{\rm Nd}^{3 +}}$-doped chalcogenide glass [48].
Fig. 5. (a) Experimental setup of the monolithic ${{\rm Nd}^{3 +}}$-doped fiber MOPA system. LD: laser diode; CPS: cladding power stripper; PM: power meter; OSA: optical spectrum analyzer. (b) Laser output power versus launched pump power. The inset shows the cross section of the NDF with octagonal cladding. (c) The laser spectrum of the MOPA with a 113.5 W output power. The inset shows a photograph taken at the maximum power. (d)–(e) The beam quality on each axis, respectively. The inset in (d) shows the beam profile.
Relative integrated emission intensity ($R{I_x}$) is a comprehensive parameter that describes the emission trend of luminescent ions in an asymmetrical environment. The $R{I_x}$ of each emission band relative to the ${\sim}{1060}\;{\rm nm}$ band can be defined as
To compare the emission performance at ${\sim}{900}\;{\rm nm}$ of NI with that of commercial ${{\rm Nd}^{3 +}}$-doped materials, Fig. 4(c) illustrates the PL spectra (normalized to ${\sim}{1060}\;{\rm nm}$, respectively) of NI, NdSG, NdPG, and NdYAG. Obviously, $R{I_{900\;{\rm nm}}}$ of NI is much greater than that of the others, and the emission bandwidth of NI at 900 nm is close to that of NdYAG and much wider than that of NdSG and NdPG, which makes the ${\sim}{900}\;{\rm nm}$ transition closer to a four-level scheme and is less affected by reabsorption [49]. Compared to other materials, NI glass shows significant emission enhancement and tunable wavelength range broadening at ${\sim}{900}\;{\rm nm}$. In addition, as Fig. 4(d) shows, ${{\rm Nd}^{3 +}}$ has a lifetime of ${\sim}{460}\;\mu {\rm s}$ in NX, which is much longer than that of typical NdSG, NdPG, and NdYAG. The calculated gain coefficients of NI with different population inversions ($p$) are shown in Fig. S4. When $p\; \ge \;{0.4}$, a broad gain range of 880–950 nm was obtained. Ultimately, we conclude that, with a large $R{I_{900\;{\rm nm}}}$, long lifetime and broad gain range at 900 nm, the NX samples, especially the NI sample, could be a potential material that achieves a highly efficient, tunable ${{\rm Nd}^{3 +}}\;\sim{900}\;{\rm nm}$ laser output.
D. Optical Parameters and Laser Performance of NDF
We finally chose the NI sample as the core glass to fabricate 30/125 NDF using the rod-in-tube method for studying the laser performance. The NDF has a core numerical aperture (NA) of 0.056, and its transmission loss at 1200 nm is 0.36 dB/m (see Fig. S5b). Figure 5(a) shows the experimental setup of the high-power monolithic all-fiber master oscillator power amplifier (MOPA). To overcome the limitations of the ${\sim}{900}\;{\rm nm}$ amplified spontaneous emission and parasitic oscillation at ${\sim}{1060}\;{\rm nm}$ on the output power, a 927 nm seed laser with a maximum output power of 11.5 W was used. The output fiber was a commercial 20/125 passive silica fiber. Four 808 nm multimode laser diodes (LDs) with 105/125 pigtail fibers were used in the experiment, which provided a maximum pump power of 320 W. The 30/125 NDF length was optimized to be 4.3 m to further suppress the ${\sim}{1060}\;{\rm nm}$ parasitic oscillation and maximize the output power of the MOPA. The optimization details are shown in Fig. S6. A 20/125 $({6} + {1}) \times {1}$ combiner was adopted to connect the seed laser, pumping LDs, and NDF by splicing to construct an all-fiber structure. The NDF was bent into 35-cm-diameter rings and fixed on a metal plate for cooling. A 30/125 fiber-pigtailed cladding power stripper was used to eliminate the residual pump power and the leakage of the signal power from the fiber core. To prevent the ${\sim}{1060}\;{\rm nm}$ parasitic oscillation due to the feedback from the fiber end-face, the output fiber was cleaved at an angle of ${\sim}{8}^\circ$.
The output power of the MOPA versus the launched pump power is plotted in Fig. 5(b). The maximum output of 113.5 W was achieved at a pump power of 300.4 W, and the slope efficiency was 36.2%. By subtracting the seed power, a pure output power of ${\gt}{100}\;{\rm W}$ was achieved in our 30/125 NDF amplifier. Further increasing the output power leads to parasitic oscillations at 1060 nm. To the best of our knowledge, this is the highest output power for lasers operating at ${\sim}{900}\;{\rm nm}$ based on ${{\rm Nd}^{3 +}}$-doped laser materials [11,50]. The output power and laser efficiency can be further improved by decreasing the transmission loss of the NDF. Figure 5(c) shows the laser spectrum of the MOPA at a maximum output power of 113.5 W. The peak-to-peak contrast between 927 nm and ASE around 1060 nm was ${\gt}{32}\;{\rm dB}$, and the energy ratio of the 927 nm laser was as high as 99.2%. Owing to the relatively large core NA and core-to-clad ratio, a beam quality of ${\rm M}_x^2 = {3.1}$ and ${\rm M}_y^2 = {2.9}$ was obtained from the MOPA [Figs. 5(d)–5(e)]. We think the beam quality can be improved by decreasing the core NA via co-doping an appropriate amount of ${{\rm F}^ -}$ or ${{\rm P}^{5 +}}$ with the ${\rm M}$-Sol-Gel method [51]. The variation in the laser spectra with increasing output power is shown in Fig. S7 in the Supporting Information. In another MOPA experiment, we achieved a 51.4 W output at 915 nm with a slope efficiency of 33% using a 4 W seed laser (Fig. S8). Undoubtedly, the NI-based 30/125 NDF is an excellent gain material for achieving tunable high-power output at 900 nm.
4. CONCLUSION
In summary, we have shown that direct coordination engineering can be used as a novel and effective approach for modulating the spectral behavior of ${{\rm Nd}^{3 +}}$-doped glass matrix. We theoretically and experimentally demonstrated that the PL behavior of the ${{\rm Nd}^{3 +}}$-doped silica glass was significantly modified by engineering the direct coordination environment of ${{\rm Nd}^{3 +}}$ ions. Importantly, we find that the emission intensity at ${\sim}{900}\;{\rm nm}$ from ${{\rm Nd}^{3 +}}$, with respect to that at ${\sim}{1060}\;{\rm nm}$, goes up with an increase in the radius of doped halogen ions and becomes dominant for the ${{\rm I}^ -}$ doped case. We emphasized that the stronger ${\sim}{900}\;{\rm nm}$ emission from the ${{\rm Nd}^{3 +}}$ ion compared with its ${\sim}{1060}\;{\rm nm}$ emission is rarely observed in most traditional ${{\rm Nd}^{3 +}}$-doped laser media. Using the developed ${{\rm I}^ -}$ incorporated ${{\rm Nd}^{3 +}}$-doped silica fibers, we achieved a record output power of 113.5 W operated at 927 nm based on an all-fiber structure, over 50 times higher than previously reported power. We highlight that judiciously regulating the direct coordination and bonding features of ${{\rm Nd}^{3 +}}$ in the glass matrix via the introduction of foreign anions not only helps us attain unconventionally stronger ${\sim}{900}\;{\rm nm}$ emission that leads to a record laser power at this peculiar and important spectral range but also holds potential to be extended to regulate the spectroscopic properties of other rare-earth-doped laser materials.
Funding
National Natural Science Foundation of China (Grant Nos. 61975216, 62205356, 62122027 and 62075063).
Acknowledgment
The authors acknowledge Yan Feng of the Hangzhou Institute for Advanced Study for lending equipment for laser testing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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