W-type and Graded-index bismuth-doped fibers for efficient lasers and amplifiers operating in E-band

A. S. Vakhrushev; A. A. Umnikov; A. S. Lobanov; E. G. Firstova; E. B. Evlampieva; K. E. Riumkin; A. V. Alyshev; A. M. Khegai; A. N. Guryanov; L. D. Iskhakova; M. A. Melkumov; S. V. Firstov

doi:10.1364/OE.444360

1. Introduction

Fiber-based laser and amplifier systems have been studied as promising tools for a wide range of applications, including telecom, sensing, imaging, industry, etc. Such systems are based on an active medium, which optical properties depend on the used laser-active dopant. The fibers, where bismuth ions are used as an active element incorporated in glass core, provide amplification in broad bands of near IR region [1–3]. To a large extent, the variation of their optical properties has been made possible by unique features of active centers forming in these fibers. Specifically, the variation of fiber core glass composition has enabled the development of media for use in amplifiers and lasers operating in the range from 1.15 µm to 1.75 µm. The progress in this direction was a driver for considerable attention to these kinds of fibers.

Recently, there has been increased interest in creating efficient amplifiers operating in E- and S- bands. This could enable one to expand the possibilities of the existing infrastructure of fiber-optic telecom systems. In this regard, Bi-doped fibers with germanosilicate glass core are attractive materials providing good performance in this spectral region. In particular, the promising results were obtained in paper [4], where a 20-dB Bi-Doped Fiber Amplifier (BDFA) near 1430 nm was demonstrated using a forward pump configuration with a pump power of 65 mW. Then, to demonstrate applicability of such an amplifier for telecommunication networks, it was tested for data transmission in experimental optical links [5]. In [6], detailed measurements of BDFA parameters in different configurations were carried out. In addition, using these fibers it was possible to develop the most efficient Bi-doped fiber lasers with a maximum efficiency of 60$\%$ was achieved [7,8].

Despite the significant progress made in this field, there is still room for further advancement, in particular, improving the parameters of the active material as well as the design of active fiber. All the mentioned results were achieved using Bi-doped germanosilicate fibers as an active medium, which is characterized by the formation of bismuth-related active centers associated with silicon (BACs-Si) with radiative transitions at wavelengths of 420, 820, 1430 nm [7]. However, this type of BACs can also be formed in different glass matrices. Besides Bi-doped pure-silica [9] and germanosilicate fibers, amplification in the wavelength range near 1430 nm can be provided by Bi-doped aluminosilicate [10], phospho-, and phosphogermanosilicate [11], and high-germania silica-based fibers [12]. By now, it was revealed that the BACs-Si concentration strongly depends on the glass matrix composition. Taking into account the results presented before, one can conclude that variation of glass composition of a Bi-doped fiber core may significantly influence the BACs formation process. The design of active fiber can also affect the parameters of the produced devices such as gain efficiency, compatibility, etc. Recently, it was shown that modification of glass cladding of Bi-doped fibers is one of the efficient ways to increase $\Delta$n without noticeable change of their optical properties. It is clearly shown in the paper [13], where an O-band BDFA based on a Bi-doped fiber with depressed cladding structure was proposed.

In this paper, we report the study of new designs of Bi-doped germanosilicate fibers, namely W-type and Graded-index fibers. The former design is considered as a way of increasing the pump efficiency of the amplifiers whereas the latter is concerned with the effect of variation of the chemical composition over the core cross-section on the BACs formation and output parameters of the lasers.

2. Experimental

The preforms of the Bi-doped fibers were fabricated by the conventional modified chemical vapor deposition technique. As substrate and jacket tubes, we used high-purity SiO$_2$ glass tubes of F-300 Heraeus. The fabrication process of the preforms consists of a series of stages. For W-type fiber, the initial stage includes the formation of a depressed cladding using layer-by-layer deposition at temperature 1650 $^\circ$C and further sintering at T $\approx$2000 $^\circ$C of 10 fluorine-doped porous layers. After that, follows the deposition of several layers of germanosilicate glass soot, which are sintered in a flow of BiBr$_3$ gas forming the Bi-doped glass core. For Graded-index fiber, we initially deposed (at 1300 $^\circ$C) and sintered (at 1800 $^\circ$C) 8 layers with low GeO$_2$ content, then 4 layers with high GeO$_2$-content all doped with Bi. The flow of Bi precursor was the same during vitrification of all the deposited layers. Before Bi-doped glass layers, we also made two layers doped with fluorine. At the final stage, the tubes with doped glass layers were consolidated into glass rods (preforms) at temperature 2100 $^\circ$C. The refractive index profiles (RIPs) of the preforms measured by a Photon Kinetics PK2600 preform profiler are presented in Fig. 1(a,b). It is seen that the first cladding to core radii ratio of W-type fiber is close to 6. The RIP of the Graded-index fiber preform has a double-step-like design. The thickness of an inner part of RIP with a higher refractive index difference of 0.0137 is similar to that of an outer part of RIP with a lower refractive index difference of 0.0096. The central dip is due to the evaporation of Ge during the collapsing process. It is well known that RIP modification takes place during the drawing process. That is why we demonstrate the RIP of the single-mode fibers under study (Fig. 1(c,d)). The observed differences between RIP shapes of W-type fiber and preform are negligible. Only a decrease of the refractive index in the Ge-doped region is observed caused by uncontrolled drawing-induced diffusion of fluorine atoms from the surrounding ring region. In the case of Graded-index fiber, one can observe a noticeable change of RIP form, in particular, a double-step-like form in the preform transformed into parabolic form in the fiber. In our opinion, this is due to the Ge atoms diffusion that took place between low and high refractive index regions. It should be noted that RIP shapes in the studied samples measured by an S14 Photon Kinetics profiler are in agreement with the measurement of dopant distribution across the fiber core obtained using the Energy Dispersive X-ray Spectroscopy (EDX) technique as shown in Fig. 1(e,f). The Bi concentration distribution along the preform was not homogeneous. This allows us to use different parts of the preform for drawing the fibers with various BACs concentrations. These fibers were used for measuring the BACs concentration using the luminescence saturation approach. The fiber core diameters were 5 (W-type) and 9 (Graded-index) µm, while the cladding of these fibers had a standard diameter of 125 µm. The cut-off wavelengths were 1.2 and 1.15 µm for Graded-index and W-type fibers, correspondingly. The absorption spectra and unsaturable loss of the studied Bi-doped fibers were measured by the cut-back technique using small and large signals, correspondingly. We also measured the radial distribution of BACs in the preforms by means of the luminescence spectroscopy described in [14]. The luminescence intensity of BACs as a function of input power excitation at 1330 nm was measured using the detection scheme presented in [15]. A Bi-doped laser was assembled using a linear Fabry-Perot cavity design with a high-reflection Bragg grating at 1460 nm and right-cleaved end of active fiber. The core-pumping of the tested Bi-doped fiber laser was provided by a Raman fiber laser operating at 1.31 µm with an output power of several watts. A Bi-doped fiber amplifier was tested in a counter-pumping configuration. Signal and pump powers were combined and split with an optical coarse WDM filter spliced to the active fiber. Optical isolators were spliced to the input and output of the amplifier. A Fianium supercontinuum equipped with an acousto-optic tunable filter was used as a signal source for measuring the gain and noise figure spectra of the amplifier. The gain saturation experiments were performed by using a fiber-coupled laser diode operating at 1430 nm in combination with a digital variable attenuator of OZ Optics for changing the input signal power. The detection, averaging, and recording of initial and amplified signals as well as the amplified spontaneous emission was carried out by an HP 70950B spectrum analyzer. The output signal and pump powers were measured by power meters of Ophir Nova II and FHP-2A04.

Fig. 1. RIP of the preforms (a, b) and fibers (c, d); (e, f) the radial distributions of germanium, fluorine along a cross-section of the Bi-doped fiber measured by means of the Energy Dispersive X-ray Spectroscopy (EDX).

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3. Results and discussion

Figure 2(a,b) shows the radial distribution of luminescence intensity at 1430 nm and 1680 nm belonging to BACs-Si and BACs-Ge, correspondingly, and RIPs of the Bi-doped fiber preforms. In the W-type preform, the luminescence intensity of both BACs types achieved a maximum at a half radius from the center. The central part of the preform was depleted in BACs content, which is caused by the decrease of the total Bi content induced by its evaporation. It is worth noting that the obtained distribution of BACs is close to that of Ge concentration in the core. This is even more prominent on the graph of Graded-index fiber preform (Fig. 2(b)). In this case, one can observe that the distribution of two kinds of BACs are similar to the RIP induced by GeO$_2$ addition. Taking into account all the presented data it is concluded that the formation process of BACs, associated with silicon, is in some way assisted by Ge atoms. It may be related to the intensive formation of oxygen-deficient centers, which are considered to be an essential part of BACs [16]. Nevertheless, it is not excluded that the obtained distribution of BACs reflects only the distribution of the total Bi content along the preform cross-section, although the flow of BiBr$_3$, as well as the other parameters, remained unchanged during the MCVD process. It should also be noted that we did not detect any effect of fluorine on the BACs formation. The obtained BACs distribution of the preform may differs that of fiber due to significant RIP transformation, especially in Graded-index fiber.

Fig. 2. Refractive index difference (line) and BACs luminescence intensity (symbols) as a function of radial coordinate along the preform cross-section of the studied fibers (a – W-type fiber preform; b – Graded-index fiber preform).

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In the following experiments, we used an approach based on the determination of the saturation power $P_{sat}$($\lambda _s$) of the luminescence intensity of BACs-Si to estimate their concentration. The dependence of luminescence intensity of BACs-Si (W-type fiber) on the excitation input power is shown in Fig. 3. A typical luminescence spectrum of W-type fiber is given in Fig. 3(a,inset). To derive the information on BACs content using the data in Fig. 3(a) one can employ the following considerations. Pump power, $P(z)$, propagating through a fiber can be described by a rate equation of the form [17]:

(1)$$\frac{\text{d}P(z)}{\text{d}z}={-}\sigma_{a}\cdot\bar{N}_{BAC}\cdot\Gamma(0)\cdot\frac{\Gamma(P)}{\Gamma(0)} \cdot P(z) - \alpha_{UL}\cdot P(z).$$

with the initial condition of $P(z=0)=P_{in}$, $P_{in}$ is the launched pump power. Here, $z$ is the longitudinal coordinate along the fiber axis, $\sigma _{a}$ is absorption cross-section at the pump wavelength, $\bar {N}_{BAC}$ is maximum concentration of BACs along the transverse coordinate $r$, such that the doping profile of the fiber can be presented as $N_{BAC} (r)= \bar {N}_{BAC}\cdot n(r)$, where $n(r)$ is a dimensionless function of $r$, and $\alpha _{UL}$ is the unsaturable loss in the fiber (see Fig. 4(b)). A power dependent overlap factor (overlap integral), $\Gamma (P)$, is given by the equation:

(2)$$\Gamma(P)=\frac{2}{\omega_p^2}\int_{0}^{\infty}\frac{n(r)\cdot\psi(r)}{{1+\frac{P}{P_{sat}}}\cdot\psi(r)} rdr.$$

where $\psi (r)$ is the so-called, mode-envelope, introduced in such a way, so the intensity of the pump light can be represented in the form $I(z,r) = P(z) / \pi \omega _p^2 \cdot \psi (r)$, $\omega _p$ is the so-called, power radius, determined according to the formula $\omega _p^2 = 2\int _{0}^{\infty }\psi (r)rdr$, and $P_{sat}$ is the saturation power defined as:

(3)$$P_{sat}(\lambda)=\frac{hc/\lambda \tau}{\sigma_a(\lambda)\cdot (1+\eta(\lambda))}\cdot\pi\omega_p^2.$$

where $\tau$ is the lifetime of the metastable level and $\eta (\lambda ) = \sigma _e (\lambda ) / \sigma _a (\lambda )$ is the emission-to-absorption cross-section ratio. Other letters have their usual meaning. The factors $\sigma _a$, $\bar N_{BAC}$, and $\Gamma (0)$ are grouped together, so they constitute the small-signal absorption due to active centers:

(4)$$\alpha_{BAC} = \sigma_{a}\cdot \bar N_{BAC} \cdot \Gamma (0) = \sigma_{a}\cdot \bar N_{BAC} \cdot \frac{2}{\omega_p^2} \int_{0}^{\infty} n(r) \psi (r) rdr.$$

which is a quantity, easily determined in a direct experiment (see Fig. 4(a)). The intensity of the luminescence is known to be proportional to the population of BACs in the excited state. In our experiments, the setup allowed us to acquire luminescent signal from the entire length of the active fiber backward propagating in the cladding modes, thus avoiding possible distortions through reabsorption (for more detail, see [15]). That is, the luminescence signal is proportional to the population of BACs in the excited (metastable) state, integrated over both transverse and longitudinal coordinates, $r$ and $z$, which can be shown to be expressed in the form:

(5)$$I_{Lum} \propto \int_{0}^{L} \Gamma (P(z)) \cdot P(z) dz.$$

So, through Eq. (1), (2), and (5), luminescence intensity, $I_{Lum}$, can be considered as a function of an independent variable $P_{in}$, while other characteristics, such as $n(r)$, $\psi (r)$, $\alpha _{BAC}$, $\alpha _{UL}$, and $P_{sat}$ can be viewed as parameters. $\alpha _{UL}$, $\alpha _{BAC}$, and $n(r)$ were measured independently (see, Fig. 2 and Fig. 4). $\psi (r)$ was calculated using the refractive index profile shown in Fig. 1. So, we were left with $P_{sat}$, which was determined via fitting the implicit function $I_{Lum}$ ($P_{in}$) to the experimental data in Fig. 3(a). The best-fit curve is presented in Fig. 3(a) by the solid line. In this case, $P_{sat}$ turned out to be equal to 1.7 mW. Using, now, the parameters $\eta$ ($\lambda _{p}$ = 1330 nm) = 0.25, $\sigma _{a}$ = 1.9 $\cdot 10^{-24}$ m$^2$, derived by the approach described in [14], and $\tau$ = 640 µs , we can employ Eq. (3) to calculate $\sigma _{a}$, and then Eq. (4) to determine $\bar N_{BAC}$. The result of the calculation is presented in Fig. 3(b) as a plot of $\bar N_{BAC}$ versus $\alpha _{BAC}$. It is determined that the average BACs concentration in our fibers ranges in $(0.5 - 1.5)\cdot 10^{23}$ m$^{-3}$. Taking into account the data for step-index Bi-doped fibers with higher absorption value, we obtained that the concentration dependence of active absorption is close to linear with a coefficient of 1 dB m$^{-1}$ per $\approx$1.2 $\cdot$ 10$^{23}$ m$^{-3}$. The data obtained for W-type and Graded-index fibers show that the new types of fibers do not have significant advantages in the formation of BACs compared to the step-index fibers. The main benefits of the studied fibers are rather related to the improvement of unsaturable loss which is considered below.

Fig. 3. (a) Dependence of luminescence intensity at 1420 nm on the excitation input power (log-log scale). Inset: typical luminescence spectrum of the investigated fiber excited at 1.33 µm; (b) The calculated BACs concentration in Bi-doped GeO$_2$-SiO$_2$ fibers with various active absorption.

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Fig. 4. (a) Absorption spectra of the Bi-doped fibers; (b) Power dependence of optical loss of the studied Bi-doped fibers.

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Figure 4(a) illustrates the small-signal absorption spectra of the investigated fibers in the spectral region of 1150 – 1700 nm. It is seen that the absorption value in the band at 1410 nm assigned to the BACs-Si is equal to 1.1 dB/m for Graded-index fiber and 0.8 dB/m for W-type fiber. It should be noted that although the Ge concentration in Graded-index fiber was greater compared to W-type fiber, the absorption at 1650 nm attributed to BACs-Ge is higher for W-type fiber. The curves showing the variation of optical loss of Bi-doped fiber with respect to input power at 1.33 µm are given in Fig. 4(b). The total absorption to the residual loss ratio for both fibers was more than 47 times. The saturation powers of the fibers are significantly different: 1.5 mW for W-type fiber and 4.8 mW for Graded-index fiber. This is explained by the difference in core diameters of the fibers.

Taking into account the obtained characteristics of W-type fiber we expected an increase of the gain efficiency (i.e. gain value per mW) of the BDFA. In this case, we used the 120-m-long active fiber in the BDFA configuration, which was described before. The small-signal gain and noise figure spectra of this BDFA for different pump powers of 45, 65, and 85 mW are presented in Fig. 5(a). As it can be observed, the peak gain in 20 dB at a noise figure of 5.3 dB and a 40-nm bandwidth can be achieved even with 45 mW of pump. With increasing pump power from 45 to 85 mW, the peak gain increases from 22.5 to 29.5 dB, and the noise figure reduces up to 4.6 dB. Figure 5(b) shows the variation of gain on the pump power for different input signals. All the obtained curves can be characterized by monotonic growth of gain with increasing pump power. At a low signal, the gain efficiency of BDFA achieves 0.52 dB/mW, whereas it decreased to 0.1 dB/mW at a high-power signal when BDFA operated in the saturation regime. Figure 5(a,b) demonstrates the gain as a function of input and output signal powers. The presented data allow one to determine the characteristics, which are important for practical applications (pre-amplifier or booster) of BDFA. It is seen that for the small-signal powers, signal gain is almost constant, however, for the larger powers of the input signal gain saturation occurs. It should be noted that the saturation power of the input signal varied in the range of 0.02-0.07 mW depending on the used pump power. In this regard, the more pronounced changes of saturation power of output power took place. In particular, output power from 1 mW to 30 mW can be achieved after BDFA. Analyzing the obtained results, it can be concluded that W-type fiber allows us to build an optical amplifier with improved parameters.

Fig. 5. (a) Gain (solid) and noise figure (dashed) of BDFA vs wavelength at different pump powers at 1.33 µm (input signal power of <10 µW); (b) Gain of BDFA as a function of pump power at various input signals; dependencies of gain on the input (c) and output (d) signal powers at various pump powers.

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In our experiments, we also tested the BDFA based on the Graded-index fiber. However, the results for its gain efficiency were noticeably worse than expected, especially in relation to the pump power required for 20-dB gain that was caused by a large core diameter. Nevertheless, this fiber has shown excellent results in the laser experiments. Because the absorption of this fiber was higher than that of others studied before, its length for a laser was chosen to be about 75 m. It allowed us to provide both nearly full absorption of the pump radiation and achieve the necessary population inversion along all the active fiber for providing lasing. The laser at 1460 nm constructed in a simple configuration described above provided a high efficiency of 72 $\%$ with respect to the input pump power at 1310 nm. The experimentally obtained dependence of output power at 1460 nm on the input pump power is shown in Fig. 6. The pump power threshold was $\approx$250 mW. The achieved output power was almost 3 W and was limited by the available pump power. Figure 6(inset) presents the output emission spectrum of the laser where one can observe two narrow lines corresponding to the unabsorbed pump at 1.31 µm and the laser radiation at 1.46 µm.

Fig. 6. The output power of laser based on Graded-index fiber doped with bismuth versus launched pump power. Inset: output emission spectrum of the laser.

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4. Conclusion

In conclusion, we presented the novel designs of Bi-doped germanosilicate fibers promising for E-telecommunication band, namely, W-type and Graded-index fibers. Using the luminescence spectroscopy of these fibers we determined the radial distribution BACs-Si and their maximum concentration, which turned out to be $\approx$(0.5 – 1.5)$\cdot$10$^{23}$ m$^{-3}$. From absorption spectroscopy, it was revealed that the investigated fibers have a low level of unsaturable loss even if the total absorption is higher compared to step-index Bi-doped fibers. Also, it was demonstrated that BDFA based on W-type fiber is capable of providing >20 dB gain and a noise figure of 5.3 dB (a minimum noise figure of 4.7 dB) even at the lowest used pump power of 45 mW. This BDFA exhibits a gain efficiency of 0.52 dB/mW that is the highest known value for Bi-doped fiber amplifiers. A record slope efficiency of 72$\%$ with respect to the launched pump power was achieved for the Bi-doped fiber laser utilizing the Graded-index fiber. We have concluded that the approach concerning the variation of the design of Bi-doped fibers is promising for further improvements in Bi-doped fiber devices.

Funding

Russian Science Foundation (19-72-10003).

Acknowledgments

The authors are grateful to A.N. Abramov and N.N. Vechkanov for assistance in the fabrication of the single-mode Bi-doped fiber.

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.

References

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W-type and Graded-index bismuth-doped fibers for efficient lasers and amplifiers operating in E-band

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusion

Funding

Acknowledgments

Disclosures

Data availability

References

Data availability

Cited By

Figures (6)

Equations (5)

Optics Express