Cluster Control for Construction of Bismuth‐Doped Glass Fiber with Broadband Optical Response

Bismuth(Bi)‐doped optical fibers are widely used in various aspects such as fiber lasers and amplifiers. Despite extensive research efforts to explore high‐gain Bi‐doped fibers, the pursuit of Bi‐doped fibers with emission over an extremely wide wavelength range remains unfulfilled. Here, the strategy of cluster control via regulating glass structure is proposed. It is applied to construct Bi‐doped photonic glass with ultra‐broadband near‐infrared (NIR) emission and a full width at half maximum (FWHM) of 343 nm. The strategy is further combined with the modified chemical vapor deposition (MCVD) technique with solution doping. The fiber exhibits ultra‐broadband amplified spontaneous emission (ASE) covering the entire fiber communication band (1260–1625 nm). Finally, the fiber exhibits higher optical gain and bandwidth compared with the fiber without cluster control. This device can be utilized to potentially enhance the capacity of telecommunication systems.


Introduction
The increasing density of informational interaction has accelerated the capacity crisis in fiber communication systems.To resolve this crisis, it is critically significant to widen the communication bandwidth and develop broadband, high-efficiency DOI: 10.1002/apxr.2023001413][4][5] The vital strategy for this avenue is to develop novel gain fiber materials that can support broadband optical amplification.In the past several decades, researchers have extensively studied rare-earth doped gain fibers.8][9][10][11] Raman fiber amplifiers may potentially help to achieveamplification in a wide waveband, but the derived fiber amplifier usually has a complex structure and requires high pumping power which strictly limits their commercial applications. [2,4,12][15][16] Since the E band fiber amplifiers based on Bi-doped fibers have been demonstrated, the Bi-activated center for E band emission and the evolution behavior of the Bi-center in glass and glass fiber still remains unknown and the gain coefficients of these gain fibers are relatively low, ranging from ≈0.05 to 0.15 dB m −1 . [1,17,18]In recent years, several efforts have been made to enhance the optical performance of the Bi-doped fiber for amplification.[33][34][35] However, due to the unknown of the Bi active center for E band emission, it is a great challenge to select the matched active centers and design the proper host chemical environment.[38][39] But it also leads to high-level loss, further decreasing the optical performance of fibers.
In this study, we propose a cluster control strategy to construct novel Bi-doped glasses and fibers with broadband optical response.We demonstrated that the chemical state of the Bi center such as dispersed clusters, aggregated clusters, and isolated ions can be controlled by modification with alkali ions.It is also found that the amorphous Bi cluster might be the most effective active center for E band emission.Guided by these results, we successfully fabricated cluster control fibers with high efficiency and ultra-broadband emission.These fibers demonstrate intense gain and wide bandwidth compared to the fiber without cluster control.This strategy holds promise for developing novel highgain Bi-doped broadband fiber amplifiers or tunable lasers.

Results and Discussion
The gain of Bi-activated centers is mainly associated with the state of Bi (valence state, clusters, etc.), and the state of Bi strictly re-lies on the glass network structure.Alkali metal oxides are considered to be excellent modifiers for Bi-doped glass, which might lead to the modification of the local network structure for Bi.As a typical example, the structure of a series of Cs 2 O-modified germanosilicate glass (GS) would be first discussed to investigate the influence of alkali on the evolution of glass network structure.
Figure 1a illustrates the X-ray diffractometer (XRD) patterns of the samples, which exhibit the typical amorphous wide peaks.GSC0.5, GSC1, GSC2, and GSC3 represent Cs 2 O doping concentrations were 0.5, 1, 2, and 3 (in mol%), respectively.The shape of the peak does not change with the introduction of Cs 2 O.This phenomenon indicates the amorphous glass was successfully prepared and the addition of Cs 2 O would not result in crystallization of Bi.
The oxygen vacancies were characterized by electron paramagnetic resonance (EPR) spectroscopy.As shown in Figure 1b, the g-factor of the clearly visible peak is 2.003, which represents oxygen vacancy.In Bi-activated glass, the intensity of the oxygen vacancy signal was inversely proportional to the Cs doping level, indicating that the introduction of Cs 2 O brought a large amount of free oxygen and filled the oxygen vacancy defects, further indicating the glass network structure was broken with the increasing amount of Cs 2 O.
The transformation of the glass micro-structure was further characterized by fourier transform infrared (FTIR).As shown in Figure 1c, with the increase of the Cs doping level, the shape of the absorption band changed from "W" to "V" at 450 cm −1 , which is associated with the absorption of O-Si-O bond vibrations.The band at 800 cm −1 represents the Si-O absorption of non-bridging oxygen, and the enhancement of the absorption means increased effectiveness of alkali metal ions in breaking the network structure of glass.This is consistent with the results of the EPR analysis.The absorption band at 1100 cm −1 is related to the asymmetric stretching vibration of Si-O-Si bonds.Due to the destruction of the glass network structure, this absorption band has converted from wide to narrow.The band at 1600 cm −1 is attributed to the absorption of hydroxyl groups.With the changes in the microstructure of GSC glass, the hydroxyl content increases, which also confirms that high levels of Cs doping are detrimental to fiber performance in the following content.
Figure 1d-f displays high-resolution transmission electron microscopy (HRTEM) images of GS, GSC1, and GSC3 glasses, respectively.The image of the GS sample shows that particles uniformly appeared in the glass.Further statistical analysis (Figure S1, Supporting Information) shows that the particle size is mainly at ≈10 nm.These particles are referred to as dispersed Bi clusters, and it is believed that Bi clusters are the source of NIR emission.Different from the GS glass sample, in GSC1 glass, Bi clusters aggregated within the white border area, which might be ascribed to the breakage of the glass network structure.The loose network structure promotes Bi clusters to migrate, and then form aggregated Bi clusters.With a further increase in the Cs 2 O doping level, it is difficult to find Bi clusters in the glass, as shown in Figure 1f.
In order to have a clearer understanding of this evolution process, this strategy is illustrated in the schematic diagram (Figure 1g Furthermore, the optical properties of the various alkali metalmodified GS glass was studied.Li + ions usually play the role of glass network accumulation, [40] which is contrary to our cluster control strategy.Therefore, it was not discussed.Figure 2a displays the emission sepctra of sample GS doped with 1 mol% different alkali metal ions.It is worth noting that as the radius of alkali metal ions increases, the NIR emission located at E band of the glass sample is gradually enhanced.Simultaneously, the full width at half maximum (FWHM) of the emission also exhibits a notable increase from 197 to 343 nm.All spectra show the same trend of variation, proving that the structure of glasses doped with other alkali metals is similar to Cs-doped glasses.In order to further investigate the evolution of NIR emission intensity influenced by the types and contents of alkali metals, a series of Bi-doped glasses were prepared.As shown in Figure 2b, as the doping concentration of alkali metals increases, the nearinfrared (NIR) emission intensity of glass initially increases and then gradually decreases.Finally, the glass would be completely deactivated when the alkali metal content exceeds 4 mol%.This phenomenon can be attributed to the change in the number of Bi active centers in the glass.When a small amount of alkali metal is doped, the network modification effect causes Bi clusters to transition from a dispersed state to an aggregated state.When an excessive amount of alkali metal is doped, isolated Bi ions increase in order to maintain a stable glass network structure.Therefore, the aggregated cluster is the most effective Bi active center for E band emission.
In addition, at the same doping concentration, the NIR emission intensity monotonically enhances with the increase of alkali metal radius.Cs-doped GS glass exhibits the most excellent optical response and strongest ultra-broadband NIR emission, which has the potential to be used as a gain medium for broadband fiber amplifiers.Therefore, the NIR emission spectra of Cs-doped glass were discussed.Figure 2c,d correspond to the NIR emission spectra and normalized spectra of GSC glass under 808 nm excitation, respectively.It can be seen that the NIR emission intensity and width exhibit the same variation trend, both enhance first and then decrease with the increase of Cs content.In order to compare the evolution trend of excitation and emission spectra in Bi-activated glass more clearly, 2D excitation-emission spectra were analyzed.Figure 2e,f correspond to the 2D excitation-emission spectra of GS and GSC1 samples, respectively.The emission peak at 950 nm is suppressed by Cs doping, while the emission intensity and bandwidth in the NIR region of 1200-1600 nm increases.
The results of glass optical performance and structure testing demonstrate the feasibility of the cluster control strategy.In the next step, we further tried to construct cluster control fiber preforms.The synthesis strategy of the fiber preform is a combination of solution doping and modified chemical vapor deposition (MCVD), as shown in Figure 3.For details, oxygen was used as a reactant gas to chemically react with SiCl 4 (or GeCl 4 ) to generate SiO 2 (or GeO 2 ) micro-particles at high temperatures.Meanwhile, it served as a carrier gas to deposit the micro-particles on the inner wall of the quartz tube.The specific reaction process is as follows: (1) During the deposition process, it was necessary to precisely control the flow rate of the gas.The Bi and Cs elements were carried into the core position of the preform by the solution doping method.The deposited preform was immersed in a mixed salt solution of Bi(NO 3 ) 3 and CsCl.The concentration of the immersion solution and immersion time were changed to control the contents of Bi and Cs elements.Finally, the loose preform was collapsed to obtain a solid cluster control fiber preform.Based on this strategy, we ultimately obtained GS fiber preform and GSC fiber preform.
By using the constructed fiber preform, a continuous cluster control fiber was successfully fabricated.Figure 4a shows an optical microscope image of the cross-section of GSC fiber, which exhibits an excellent core-cladding structure.Figure 4b displays a typical electron probe microanalysis (EPMA) image of the crosssection of the obtained GSC fiber.The diameter of the GSC fiber is 125 μm, and the diameter of the fiber core is 4 μm.The refractive index distribution of GSC fiber was characterized, and the maximum difference in refractive index was 0.0306, as shown in Figure S2 (Supporting Information).According to Equation (1), the numerical aperture (NA) value of GSC fiber was estimated to be ≈0.3.When the operating wavelength () is 1.4 μm, the normalized frequency value (V) can be obtained from Equation (2) as 2.693.According to Equation (3), the number of transmission modes (M) of GSC fiber near 1.4 μm is ≈4, which indicates that GSC fiber is a few-mode fiber. (3) where n 1 is the core refractive index, n 2 is the cladding refractive index and a is the radius of the fiber.This configuration is compatible with those of commercial single-mode transmission fibers such as G652.The GSC fiber cross-section was analyzed by EPMA. Figure 4c-f displays the elemental mappings of Ge, Si, Cs, and Bi, respectively.Various elements were homogeneously distributed, with Ge and Si enriched in the core region.The distribution of Cs and Bi in the fiber core was not observed clearly, primarily because of the relatively small doping amount of Cs and Bi in the preform that was lower than the detection limit of the test equipment.Figure 4g-j displays the linear distribution analysis results of various elements in the cross-section of the optical fiber.The distribution is consistent with the elemental mapping results.The deposition of Cs elements in the fiber core can be observed.The Bi element is still invisible, indicating that its doping level is lower compared to the Cs element.
A principal device with a backward pumping scheme was designed, as displayed in Figure 5a.This device comprised a tunable laser source (TLS), active optical fibers, and an optical spectrum analyzer (OSA).The optical signal from the TLS was injected into the Bi active fiber through an isolator (ISO) and a wavelength division multiplexer (WDM), and then received by the OSA after amplification.laser diode (LD) with different wavelengths were employed as the pumping source.The pumping power of LD is 250 mW.The Bi-activated fiber was backward pumped using the LD through an ISO and WDM. Figure 5b displays the loss spectra of GS and GSC fiber.The background loss of GS fiber remains at 50 dBkm −1 , while GSC fiber has reached 170 dB km −1 .The formation of Bi clusters directly leads to an increase in the level of background loss.Meanwhile, the loss of hydroxyl groups also significantly increases, which is consistent with the FTIR analysis results of GSC glass.The introduction of Cs leads to the increase of the hydroxyl group content in the glass.Therefore, the step of removing hydroxyl groups and adjusting Bi doping concentration should be further designed in the preparation process of fiber preform.An 808 nm LD was used to pump a 50 m length Bi-doped fiber, and the amplified spontaneous emission (ASE) spectrum of the fiber was comprehensively evaluated.Obviously, Cs play a role in suppressing ASE at 1100 nm.Although the hydroxyl loss of GSC fiber has significantly increased, its broadband ASE at 1400 nm still maintains a high level, which is closely related to the number of activation centers in the gain medium.The gain performance of the fiber was analyzed.The GSC fiber exhibited ultra-broadband on-off gain performance and reached 4 dB at 1420 nm, and its gain bandwidth covers the NIR region of 1250-1500 nm.However, the GS fiber not only showed insignificant on-off gain but also showed excited state absorption ≈1300 nm, which is detrimental to the gain performance.Nearband pumping can mitigate the effect of excited state absorption, which is beneficial to the gain performance of Bi-doped fiber.A backward pumping device with a longer wavelength 1240 nm LDwas constructed.Figure 5e shows the ASE spectrum at a pump power of 250 mW, and the GSC fiber exhibits more potential spectral properties throughout the communication band.Further evaluation of its gain performance is shown in Figure 5f, where the 1240 nm LD pumping suppresses the effect of excited state absorption and enables the GS fiber to achieve weaker onoff gain.The GSC fiber achieves a gain of 9.4 dB at 1405 nm, and the gain coefficient is 0.19 dB m −1 .The evolution of ASE and gain performance with pump power change is presented in Figure 5g.Both ASE and gain performance of GSC show more potential.However, the gain of GSC fiber is prone to saturation.The gain performance of the fiber does not change significantly with the increase in pump power after reaching 200 mW.

Conclusion
In summary, we comprehensively analyzed the optical response of cluster control Bi-doped glass and elucidated the underlying mechanisms for its interesting optical properties.Cluster control glass is considered as a promising gain material for broadband optical gain fiber.Importantly, cluster control Bi-doped fiber has been successfully fabricated using a combined strategy of solution doping and MCVD.Furthermore, it has been demonstrated that cluster control fiber exhibits better performance than fiber without cluster control in terms of optical gain performance.It is necessary to note that the proposed strategy of cluster control could be potentially and generally fit to other types of unusual active centers (e.g., Te, Sb, and Se) that are sensitive to the local chemical environment.This creates an effective avenue for the development of novel broadband fiber amplifiers.

Experimental Section
30GeO 2 -70SiO 2 -0.01Bi 2 O 3 (in mol%) glass was selected based on a systematic analysis of glass composition-dependent responses.The matrix glass was named GS.The samples hybridized with Na 2 O, K 2 O, Rb 2 O, and Cs 2 O were named GSN, GSK, GSR, and GSC, respectively.GSC0.5, GSC1, GSC2, and GSC3 represent Cs 2 O doping concentrations were 0.5, 1, 2, and 3 (in mol%), respectively.High-purity chemical reagents, including GeO 2 (Aladdin 99.99%), SiO 2 (Aladdin 99.99%), Bi 2 O 3 (Aladdin 99.99%), and M 2 O (M = Na, K, Rb and Cs, Aladdin 99.99%), were used as raw materials.The raw powder was uniformly mixed and then compressed into a circle with a diameter of 2 cm.The precursor was obtained by sintering the circular powder at 800 °C for 5 h.A small amount of the precursor was placed in the reaction chamber of a laser melting furnace, and N 2 gas was sprayed through the nozzle to suspend the precursor in the reaction chamber.The precursor was melted with a high-power CO 2 laser at a temperature of 1800 °C for ≈10 s.After the laser was turned off, a glass ball sample with a diameter of 3 mm was obtained.The spectral and structural testing of the glass was carried out in the powder state.The fiber preform was prepared by using the MCVD method and the solution doping method, and the specific strategies were described in the results and discussion section.The fiber preform was suspended in the drawing tower furnace.The drawing process can be summarized as follows: the temperature was increased from room temperature to 2030 °C at a heating rate of 200°C min −1 ; after the preform was stayed at 2030 °C for 15 min, the fiber was drawn at a rate of 20 m min −1 .
NIR emission were obtained on a Zolix Omni 5015i spectrometer equipped with an InSb photodetector and a Stanford Research SR830 lockin amplifier under excitation with an 808 nm LD.The excitation-emission spectra were captured using an Edinburgh FS920 fluorescence spectrometer monitored by an InGaAs detector (800-1600 nm) equipped with a 450 W Xe lamp.Crystalline phase identification was performed using a Bruker D8 Advance XRD.The EPR measurement was carried out on the Bruker E500-10/12 spectrometer, and for each test, the same quartz tube and the same mass of sample were used.FTIR spectra at a wavenumber of 400-2000 cm −1 were measured on the FTIR spectrometer (Nicolet iS50, Thermo Scientific).HRTEM (JEM-2100F) was used to analyze the glass structure and morphology.The elemental distribution in the fiber was determined using an electro-probe micro-analyzer (EPMA-1600, Shimadzu) system.The ASE spectra of the fibers were captured on a fiber optic spectrometer (AQ6370C) excited with the 808 and 1240 nm LD.All the measurements were performed at room temperature.

Figure 1 .
Figure 1.Microstructural characterization of Bi-doped glass.a-c) XRD spectra (a), EPR spectra (b), and FTIR spectra (c) of glass with different doping levels of Cs 2 O. d-f) HRTEM images of GS d), GSC1 e), and GSC3 f).g) Schematic diagram of the cluster control strategy.
) below.Germanosilicate glass has a stable and complete network structure, and Bi clusters are uniformly dispersed in structure.Alkali metal oxides promote the fracture of connections among [SiO 4 ] or [GeO 4 ] in glass.As shown in Figure 1g.It is speculated that at low doping levels, dispersed Bi clusters are more likely to form; at moderate doping levels, aggregated clusters are formed; and at high doping levels, isolated ions are presented.The appearance of non-bridging oxygen ultimately leads to a porosity network structure.Dispersed Bi clusters may migrate due to the destruction of the glass network structure.The luminescence properties of glass change with the evolution of Bi aggregation states.The increasing level of destruction of the glass network structure leads to a decrease in the degree of poly-merization of the glass.When the degree of destruction, Bi 2 O 3 , as a network intermediate, participates in the formation of glass networks and reduces the number of Bi clusters.

Figure 2 .
Figure 2. Luminescence properties of Bi-doped glasses.a) NIR emission spectra of different alkali metals doped GS glasses, excited by 808 nm; b) Comparison of 1.4 μm emission intensity of GS glass doped with different alkali metals and concentrations, excited by 808 nm;c) The NIR emission spectra and d) normalized spectra of GSC glass, excited by 808 nm; e,f) 2D excitation-emission spectrum of glass, GS e) and GSC1 f).

Figure 3 .
Figure 3. Schematic diagram of the synthesis strategy for fiber preforms.

Figure 4 .
Figure 4. Structural characterization of GSC fiber.a) Cross-sectional optical microscope image of GSC fiber.b) Cross-sectional EPMA image of GSC fiber.c-f) Distribution of Ge, Si, Cs, and Bi in the fiber cross-section.g-j) The linear distribution of various elements in the cross-section.

Figure 5 .
Figure 5. Fiber amplifier device based on GS and GSC fiber.a) Schematic diagram of backward pumping fiber amplifier device.b) The loss spectra of Bi-activated fiber.c) ASE spectra under backward pumping 808 nm LD pumping, 250 mW).d) On-off gain under backward pumping (808 nm LD pumping, 250 mW).e) ASE spectra under backward pumping (1240 nm LD pumping, 250 mW).f) On-off gain under backward pumping (1240 nm LD pumping, 250 mW).g) The dependence of ASE signal and gain on pump power Bi-activated fiber (1240 nm LD pumping).