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Abstract
We present the development of a pair of silica-based thulium-doped fiber amplifiers working together in a broad spectral range from 1.65 µm to 2.02 µm. For the one optimized for shorter wavelengths, we designed and prepared optical fiber with a depressed cladding. We show the performance of the amplifiers achieving small-signal gain of at least 10 dB over 350 nm range from 1670 nm to 2020 nm, maximum gain of 40.7 dB with a noise figure as low as 6.45 dB and an optical signal-to-noise ratio of up to 50 dB. To the best of our knowledge, it is the first time that thulium fiber amplifiers of straightforward design without using redundant spectral filters operating efficiently in such a wide spectral region are demonstrated.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Amplifiers based on rare-earth-doped optical fibers play an indispensable role in various areas of human activity. For instance, erbium-doped fiber amplifiers (EDFA) are the key elements of current telecommunications infrastructure operating namely in the C- (1530 nm–1565 nm) and L-band (1565 nm–1625 nm) [1]. Due to the increasing demands on the Internet network, both in terms of the volume of transferred data and the transmission speed, the current infrastructure is slowly reaching its technical limits [2]. Therefore, original approaches for improving the robustness of current telecommunications infrastructure are being investigated. Aside from continuing efforts to improve transmission systems, such as using low-loss hollow core fibers [3], another interest lies in the attempts to extend the telecommunications window to longer wavelengths beyond the L-band [4,5].
Thanks to a large gain bandwidth (1.6–2.2 µm) and fluorescence lifetimes reaching up to 0.8–1 ms [6–8], thulium-doped fiber amplifiers (TDFA) based on thulium-doped fibers (TDF) are promising devices for applications in a broad spectral range 1650–2050 nm [9–11]. TDFA can also benefit from already existing well-established silica-based optical fiber technology [12,13].
Apart from the telecommunication applications, the short-wavelength part (1650–1800 nm) is also interesting in terms of bio-photonics or material processing because of existing prominent absorption peaks of water [14], cholesterol [15], or hydrocarbon-containing materials [16]. The long-wavelength tail (1800–2050 nm) is significant for applications in free-space optical communication [17], LIDAR-based gas sensing [18], pumping of solid-state lasers, and generation of supercontinuum [19], label-free biological imaging using nonlinear microscopy [20], or for defense applications [21].
For effective operation of TDFA in the whole spectral range, certain obstacles need to be overcome. Drawbacks of TDFA working around 2 µm lie in higher background losses due to mainly infrared absorption related to SiO2 molecular vibrations, as well as on account of significant bend-induced losses in standard single-mode fibers [22]. Owing to strong quasi-three-level nature of thulium-doped silica matrix [23], as well as due to larger absorption cross-section of thulium ions [24], stable operation at shorter wavelengths requires high population inversion between corresponding energy levels and suppressing amplified spontaneous emission (ASE) at longer wavelengths at the same time. Three-level systems also suffer from re-absorption losses [25] whereby a signal photon is absorbed and subsequently re-emitted at different wavelength. These shortcomings can be outdone by using fibers that suppress emission at longer wavelengths and to promote emission at short wavelengths. This could be achieved by the advanced design of TDF with a depressed cladding (dc-TDF) [26].
For the above-mentioned reasons, stable operation of TDFAs, especially at both edges of the thulium emission band, is still a scientific challenge. To date, TDFAs covering the whole spectral window (1650–2020 nm) have been published only to a limited extent. Moreover, the progress of short-wavelengths TDFAs (<1750 nm) has been rather modest [25].
TDFAs operating in a limited spectral range of 1800–2050 nm were presented e.g. in [17,27]. In some other papers, the authors published the TDFA operating over a broad spectral range (1650–2050 nm), however, they used the supplementary dispersion compensating fibers [28,29], or holmium-doped fibers [30] as the spectral filters to enhance the short wavelength gain. For a change, more complex, multi-stage TDFA systems were published in [31,32]. To enhance the performance and stability of TDFAs in the spectral region below 1750 nm, some authors used TDF co-doped with Tb [33] and Ge [9], or used Bi-doped fibers [34,35].
In this paper, we present the development of the TDFAs operating together in the spectral range from 1.65 µm to 2.02 µm. For a short-wavelength operation, we use a specially designed optical fiber with depressed cladding. To the best of our knowledge, experimental verification of the theoretical model [26] as well as fabrication of such thulium-doped silica-fiber structure have not been published. Moreover, no such simple, all-fiber arrangements without any additional spectral filters, using silica-based fibers with a conventional and a depressed cladding, with the output parameters suitable for optical amplification in the whole spectral range have been demonstrated yet.
2. Experimental
2.1 Thulium-doped fibers
The fibers were drawn from preforms made in-house using modified chemical vapor deposition technology [36] extended with the ceramic nanoparticles doping method [13]. While preforms for optical fibers optimized for amplification at longer wavelengths (TDF) were co-doped only with Al2O3, preform for fiber designed for amplification at shorter wavelengths (dc-TDF) was prepared by additional deposition of fluorine-doped layers.
Before fiber drawing, the prepared preforms were analyzed by both an electron probe microanalyzer (EPMA) to verify the element and concentration composition and a refractive index profiler (Photon Kinetics A2600) to measure refractive index differences between doped central part and undoped cladding. Drawn fibers were then measured regarding their absorption, transmission, and attenuation properties. Background attenuation, absorption band around 1640 nm as well as propagation losses especially at the long-wavelength edge of the thulium emission band were measured using a tungsten halogen lamp used as a broadband light source and a Fourier transform infrared (Thorlabs, OSA203C) or monochromator-based (Ando AQ6317B) spectrometers. One-dimensional as well as two-dimensional refractive index profiles (RIP) of drawn fibers were measured using a commercially available optical fiber analyzer (IFA-100, Interfiber Analysis Inc.). The procedure for measuring fluorescent decay is described in detail, e.g., in [37,38].
2.2 Long-wavelength TDFA-$\mathscr{l}$ for spectral range 1820 nm-2020 nm
The design and construction of TDFA optimized for the spectral range from 1820 nm to 2020 nm (TDFA-$\mathscr{l}$) was based on our previous experience gained during the development of the broadband thulium ASE source working around 2 µm [39]. The experimental setup used for the TDFA-$\mathscr{l}$ is shown schematically in Fig. 1. The input signal (Seed) and pump power were coupled to the TDF via a spectrally flat fused wavelength division multiplexer (WDM) [40] that had as low as possible insertion losses in the whole considered spectral region. The selected TDF was core-pumped by an in-house built erbium-doped fiber laser (EDFL) emitting at 1565 nm with a maximum output power of 2.3 W whose time stability varied by less than 2 percent per hour of operation at maximum output power. The input and output ports of the amplifier were protected against unwanted reflections by non-PM optical isolators (ISO) that ensured unidirectional operation of the amplifier.
2.3 Short wavelength TDFA-$\mathscr{s}$ for spectral range 1650 nm-1820 nm
In contrast to TDFA-$\mathscr{l}$, a TDFA optimized for shorter wavelengths of the thulium emission band (denoted as TDFA-$\mathscr{s}$) was based on a dc-TDF that was bent and twisted in the shape of figure eight, see Fig. 1. The pumping source was again the EDFL, in this case with a maximum output power of 3.5 W. Input and output isolators as well as the WDM were selected with respect to the best possible spectral properties in the tested region.
2.4 Characterization setup
For spectral and performance characterization purposes, the testing setup schematically shown in Fig. 2 was used. The source of the input signals was an in-house built tunable thulium-doped fiber laser (TTDFL) working in Fabry-Perot configuration with a plane ruled diffraction grating (DG, Edmund, 600 grooves/mm) used as a wavelength selective element in a Littrow arrangement (for long-wavelength range denoted as TTDFL-$\mathscr{l}$, for short-wavelength range as TTDFL-$\mathscr{s}$). In the case of TTDFL-$\mathscr{l}$, a broadband output coupler was formed by perpendicularly cleaved fiber end. To ensure output stability of the TTDFL-$\mathscr{s}$ emitting at wavelengths below 1700 nm, it was necessary to both increase the reflectivity of the broadband output coupler by using a fiber loop and a polarization controller (PC) and to use the dc-TDF in a “clover” shape. Characterization of the TTDFL-$\mathscr{l}$/$\mathscr{s}$ included testing possibilities of its tunability, measurement of maximum output power over the whole tuning range, and investigation of its time stability. The quality of the signals from both tunable lasers was identical when measured using the settings described above. The laser beam from the TTDFL-$\mathscr{l}$/$\mathscr{s}$ was launched through a digital variable optical attenuator (VOA, OZ Optics) and a spectrally flat optical coupler (OC 50/50) which splits the signal into two identical beams. A power meter (PM, Thorlabs S148C) was then used to monitor the intensity of the input signal. The amplified (output) signal was split by another coupler (OC 99/1), whose low-percentage branch was connected to an optical spectral analyzer (OSA, Yokogawa AQ6375B). To improve the quality of the laser radiation, the pumping direction of TTDFL-$\mathscr{s}$ was changed, i.e., the output signal was less affected by the unabsorbed pump radiation.
Fig. 2. Experimental arrangement of the TTDFL-$\mathscr{l}$/$\mathscr{s}$, L: plano-convex lens, experimental setup for testing TDFA-$\mathscr{l}$/$\mathscr{s}$.
The performance of TDFA-$\mathscr{l}$/$\mathscr{s}$ was investigated mainly in terms of achieved gain (G) and related noise figure (NF), and optical signal-to-noise ratio (OSNR) parameters over the whole spectral range from 1650 nm to 2020 nm. The G and NF parameters were evaluated using build-in functions of the Yokogawa analyzer and subsequently verified using the method comparing input and amplified spectral intensities [41]. The calculation of the NF was influenced by the selection of the interpolation function to determine the ASE noise level. The LINEAR function was set as default, the 3RD POLY setting was used in special cases where estimation using the LINEAR setting wasn’t calculated correctly. The input (output) offset was defined as the losses occurring between the signal source (the amplifier) and the OSA during the measurement of input (amplified) signal. The estimated loss was formed by the combined losses of attenuators at the input of the OSA.
3. Results and discussion
3.1 Characterization of manufactured fibers
Selected information relevant to the comparison of used fibers is summarized in Table 1 and shown in Fig. 3(a)-(d). The TDF (7/125 µm, 7.7 mol.% Al2O3) was designed generally for core-pumped laser applications as well as for low-loss splicing with other fiber-optic components implemented in the testing setup. In contrast, the dc-TDF (6.4/125 µm, 5 mol.% Al2O3) was designed specifically for the purpose to effectively suppress ASE at longer wavelengths (>1800 nm) and shifting gain to shorter wavelengths (<1800 nm) of the thulium emission band. The splice losses between the doped fibers and the passive components were similarly low in both cases (on the order of tenths of dB) and did not significantly affect the performance of the individual amplifiers. A depressed cladding doped with fluorine was created around the core, see Fig. 3(a) and Fig. 3(b), for tailoring the waveguide properties. Achievement of high population inversion in the dc-TDF and optimization for core-pumped applications were other necessary requirements for its design. In general, the working range of such dc-TDF can be adjusted by altering the bend radius of the fiber [26]. Based on iterative simulation processes and on the estimation of transmission losses at longer wavelengths as a function of fiber bending radius, the individual parameters of the dc-TDF optimized for a working regime below 1800 nm were designed [42]. A comparison of the refractive index differences of the two tested fibers is shown in Fig. 3(a), together with the values of depressed claddings parameters as well as with 2-dimensional profile of the dc-TDF, see Fig. 3(b).
Fig. 3. Comparison of used fibers (TDF: red color, dc-TDF: blue color):1D RIP (a), 2D RIP of dc-TDF (b), absorption spectra (c), and ASE spectra for both tested fibers, blue curve corresponds to a shape of figure eight similar as shown in Fig. 1(d).
Table 1. List of selected properties of used fibers
3.2 Characterization and output parameters of TDFA-$\mathscr{l}$
The spectral characterization of the achieved external G, NF and OSNR of the developed TDFA-$\mathscr{l}$ for two selected signal levels, is displayed in Fig. 4(a). The G values reported and shown in the following sections are always referred to the values achieved at the maximum applied pump power and measured for three signal levels (-20 dBm; -10 dBm, 0 dBm). The term “external” expresses the net gain of the amplifiers including insertion losses of internal components. The small-signal gain corresponds to the input signal power of -20 dBm.
Fig. 4. Performance of TDFA-$\mathscr{l}$. Spectral dependance of G, NF, and OSNR for two signal levels (-20 dBm, 0 dBm) (a). Typical output spectra for -20 dBm of input signal (b).
The peak values of G for all tested signal levels were measured at 1880 nm. For the small-signal gain, we reached values of up to 40.7 dB with at least a 30-dB window extending from 1840 nm to 1980 nm. For the gain at the highest tested signal level (0 dBm), we achieved values of at least 25 dB in the same spectral window, with a peak value of 27.2 dB. Regardless of wavelength shift direction, the gradual reduction of G is observed. The reduction at the long-wavelength tail (>1980 nm) results from the length of used fiber (see Table 1) and relatively small emission cross-section of Tm ions [24]. Fiber length generally plays a role in the gain spectrum determination [17] and is simultaneously affected by the fiber dopant concentration. These facts lead to the need to use a sufficiently long fiber. On the other hand, the longer the fiber, the more re-absorption signal losses occur, and hence the G decreases at shorter wavelengths. Next, the spectral reduction of the G is affected by using commercially available silica-based optical fiber components, generally optimized for a specific wavelength. When selecting these components, the spectral flatness over the widest possible spectral range was the main consideration. Bend-induced and infra-red absorption losses of these components [1] also contribute to worsening of the G parameter. In addition, the decrease of G at shorter wavelengths (<1860 nm) was also caused by significantly higher re-absorption losses [43].
In terms of spectral dependence of OSNR or NF, maximal (minimal in case of NF) values were achieved at 1950 nm, and vice versa minimum (maximal in case of NF) values at the short-wavelength edge of the investigated spectral range. Note that the OSNR has decreasing trend with the decreasing signal wavelength for all the signal input power levels. The OSNR value denoted in Fig. 4(a) by an asterisk, i.e., the OSNR value for the lowest input signal power and the shortest wavelength, was accidentally measured for a 3 dB higher input signal power, see black line in Fig. 4(b). These observations correspond to the spectral dependence of insertion losses of the used components. The final length of used TDF (5 m) was chosen as a trade-off between amplifier's performance parameters at short and long wavelengths and re-absorption losses. The forward pumping scheme was chosen because of the lower noise level, namely when amplifying small signals. The achieved parameters of TDFA-$\mathscr{l}$ are summarized in Table 2.
Table 2. Summarized properties of the TDFA-$\mathscr{l}$
Typical examples of spectral intensities (for signal level of -20 dBm) obtained from the OSA measurements with a 0.2 nm resolution are displayed in Fig. 4(b). This figure also serves as a confirmation that TTDFL-$\mathscr{l}$ can be tune over the whole investigated spectral range. It should be noticed that TTDFL-$\mathscr{l}$ was characterized by sufficient OSNR for G and NF measurement purposes, in all cases exceeding the optical rejection ratio of the used OSA. The occasional fluctuations in output power were sufficiently fast and could not have affected the evaluated parameters.
3.3 Characterization and output parameters of TDFA-$\mathscr{s}$
The spectral characterization of the TDFA-$\mathscr{s}$, for the same parameters and signal levels as in the case of the TDFA-$\mathscr{l}$, is displayed in Fig. 5(a) for the dc-TDF length of 1.5 m. As the spectral dependence of the G significantly depends on the length of dc-TDF, this fiber length could be considered as a trade-off for efficient amplification over the entire wavelength range. While the use of longer lengths (>1.5 m) of dc-TDF was more effective for amplifying the signal at longer wavelengths, shorter fiber sections (<1.5 m) amplified more effectively at shorter wavelengths. The peak small-signal gain of 37.7 dB was measured at 1760 nm with at least a 30-dB window extending from 1720 nm to 1800 nm. In the same spectral window, the signal gain for the highest tested signal level ranged from 25 dB to 23.4 dB.
Fig. 5. Performance of TDFA-$\mathscr{s}$. Spectral dependance of G, NF, and OSNR for two signal levels (-20 dBm, 0 dBm) (a). Typical output spectra for -20 dBm of input signal (b).
In order to evaluate the performance of the TDFA-$\mathscr{s}$ in more detail at wavelengths below 1700 nm and getting closer to 1650 nm, we used the dc-TDF in the TTDFL-$\mathscr{s}$ in a clover shape and in the TDFA-$\mathscr{s}$ in the shape of figure eight (see Fig. 1 and Fig. 2). It is in this spectral region, at the shortest wavelengths of the investigated spectral range, where the positive effect of the modified waveguide structure was the most evident. The small-signal gain achieved at 1650 nm was approximately 3 dB (9 dB for the wavelength of 1660 nm). Increasing ASE level naturally depletes the upper laser level and thus reduces the population inversion, which is necessary for efficient amplification at shorter wavelengths [26]. The measured parameters of the TDFA-$\mathscr{s}$ are summarized in Table 3.
Table 3. Summarized properties of the TDFA-$\mathscr{s}$
Typical examples of spectral intensities (for signal level of -20 dBm) obtained from the OSA measurements with a 0.2 nm resolution are displayed in Fig. 5(b).
3.4 Summary results
Figure 6(a) demonstrates the spectral characteristics (G and NF parameters) for both amplifiers together in the spectral window from 1650 nm to 2020 nm. By combining both types of amplifiers, using conventional thulium-doped fiber in one case and the fiber with depressed cladding in the other case, we achieved the spectral window width of 260 nm (1720 nm–1980 nm) with a small-signal gain of at least 30 dB and a width of 350 nm (1670 nm–2020 nm) with a small-signal gain of at least 10 dB. The NF values ranged 5–8 dB in the wavelength ranges from 1680 nm to 1800 nm and from 1860 nm to 2020 nm, respectively. Outside these intervals far from the designed working areas of used components, the NF values increased by about 4 dB, as is evident from Fig. 6(b). In contrast to [9,33,44], we achieved slightly worse amplification parameters at the edge of the studied spectral range (< 1700 nm), however our approach offers the advantages of simplicity of fiber manufacturing and its availability. For the gain at the highest tested signal level (0 dBm), we achieved a relatively flat gain spectrum of ∼25 dB spanning a 260 nm window in the range of 1720–1980 nm. It is worth to note, in contrast to [27,28], we observed this flat gain curve without insertion of any additional spectral shaping component or filter. Moreover, we reached the reported top values of G using a one-directional pumping scheme with only limited input pump power. Only the gain at 1820 nm, does not exactly fit this dependence as it is strongly influenced by re-absorption losses and the fiber length. A slightly increased NF (by 1 dB) value at the same wavelength also confirms this observation.
Fig. 6. Performance of TDFA-$\mathscr{l}$ and TDFA-$\mathscr{s}$ in the whole spectral range (a), spectral dependence of cumulative IL and NF (b).
The increasing performance at the short-wavelength edge of the given region and its subsequent shift even closer towards the L-band should be expected by advanced modification of the waveguide structure of the used dc-TDF and using optical components specially designed for this gain window.
4. Conclusion
We have developed all-fiber TDFAs operating over a broad spectral window from 1650 nm to 2020 nm. In a configuration optimized for the long-wavelength part of the Tm emission band, we obtained peak small-signal gain of 40.7 dB at 1880 nm with noise figure of 6.45 dB. In the case of the amplifier built for effective amplification in the short-wavelength part of the Tm emission band below 1800 nm, we used a novel designed thulium-doped fiber with a depressed cladding. This amplifier operated in the spectral range from 1650 nm to 1800 nm with maximum gain of 37.7 dB and noise figure of 7.18 dB. By combination of these two configurations, we were able to demonstrate amplification with the spectral window width of 260 nm with small-signal gain of at least 30 dB. For the gain at the highest tested signal level, the spectral window with the value of at least 25 dB was spread from 1720 nm to 1980 nm. To the best of our knowledge, we have presented for the first-time all-fiber amplifiers operating with sufficient gain over the spectral range from 1650 nm to 2020 nm based solely on silica fibers with standard and depressed claddings, without the need for more complicated designs, redundant spectral filters, or other unconventional fiber components. In addition, we have proposed the possible use of these amplifiers for telecommunication purposes beyond the L-band and shown sufficient performance for other practical applications in this spectral range.
Funding
CESNET Development Fund (672/2021); Operation Programme - Jan Amos Komensky (OP JAK) (LasApp CZ.02.01.01/00/22_008/0004573).
Acknowledgments
The authors thank the Faculty of Electrical Engineering, Czech Technical University in Prague (Matej Komanec) for the instrumental assistance during the measuring of the spectral properties. The work was co-funded by European Union and the state budget of the Czech Republic under the project LasApp CZ.02.01.01/00/22_008/0004573.
Disclosures
The authors declare no known conflicts of interest.
Data availability
The data supporting the results in this study are available in [45].
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