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Abstract
This paper presents an experimental study of broadband mid-IR amplification that is carried out, for the first time, to the best of our knowledge, in an erbium-doped tungsten tellurite fiber. A simple, robust supercontinuum source based on a tapered germanate fiber is developed as a seed input in the region of 1.5–3 µm. We show that gain by a factor of 5 on one pass can be achieved in the 2.7 µm range by pumping a low-cost, high-efficiency diode laser at 976 nm using high-purity tellurite glass fibers.
© 2022 Optica Publishing Group
1. INTRODUCTION
Fiber-based mid-infrared laser sources are of great interest due to tremendous applications in spectroscopy, remote sensing, and eye-safe radars, in medical, chemical, and biochemical fields [1,2]. Significant progress was demonstrated in recent years for laser systems based on nonlinear optical conversion in chalcogenide, tellurite, fluoride, and germanate fibers, mainly due to supercontinuum generation [3–10]. However, the use of gain fibers doped with rare-earth (RE) ions promises still more progress in creating powerful mid-IR fiber laser sources and simplicity of design. There are several rare-earth ions employed to generate radiation in the 3 µm waveband based on fluoride glass fiber lasers with active ions, such as ${{\rm Er}^{3 +}}$ [11], ${{\rm Ho}^{3 +}}$ [12], and ${{\rm Dy}^{3 +}}$ [13]. The ${{\rm Er}^{3 +}}{^4}{{\rm I}_{11/2}}\to {^4}{{\rm I}_{13/2}}$ transition at 2.7 µm is currently most convenient because of the available high-power commercial laser diodes for the 976 nm absorption band [14]. At present, the technologies to create CW and pulsed fiber lasers operating at about 3 µm are well developed for fluoride ZBLAN fibers [2,15]. The record output power was achieved from an ${{\rm Er}^{3 +}}$-doped fluoride fiber laser [16]. However, despite the great success of using fluoride fibers in lasers, their commercialization is a formidable task stemming mainly from the disadvantages of such fibers, such as mechanical brittleness, tendency to crystallization, absorption of atmospheric moisture, and low softening temperature. Therefore, the development of acceptable glasses and technologies for their production and activation with rare-earth ions to create active media is still the most important task in the development of mid-IR fiber lasers [17].
To find viable alternatives, researchers have turned their attention from fluoride glass to multicomponent oxide glass with lower phonon energy and moderate strength. Until now, 2.7 µm fluorescence of ${{\rm Er}^{3 +}}$ has been frequently reported from many oxide glass hosts (e.g., silicate, fluorophosphate, germinate, and tellurite glasses), but, to the best of our knowledge, there has been no report on their fiberization and laser operation [18–23]. Therefore, it is essential to develop a multicomponent oxide glass with superior physical, mechanical, and photoluminescence performance for practical application.
As a promising alternative, tellurite glasses play an important part in fiber lasers, amplifiers, waveguides, and photonic crystal fibers due to their unique optical and physical properties that include lower phonon energy and better thermal ability [24,25]. Tellurite-based lasers operating from 1.0 to 2.1 µm with various material shapes, such as bulk glasses and fibers were implemented in past decades, demonstrating their great potential and flexibility in the field of optoelectronic functional materials [26–30]. However, the most interesting region near 3 µm is still far from fiber laser realization, mostly due to glass impurities, primarily hydroxyl groups, which strongly affect optical losses in this wavelength region [31,32].
Recently, by paying special attention to reducing impurity concentration using pure starting materials and original synthesis technology, we produced ultradry preforms with cores of ${{\rm TeO}_2}{-} {{\rm WO}_3} {-} {{\rm La}_2}{{\rm O}_3} {-} {{\rm Bi}_2}{{\rm O}_3}$ (TWL) glasses doped with ${{\rm Er}_2}{{\rm O}_3}$ and undoped ${{\rm TeO}_2} {-} {{\rm WO}_3} {-} {{\rm La}_2}{{\rm O}_3}$ claddings. This allowed us to fabricate high-quality multimode fibers and achieve a broadband 2.7 µm spontaneous emission by diode pumping at 976 nm. The study of the transmission spectra of multimode fibers also shows that the region of low losses and, consequently, the effective application of fiber-optic devices, is limited to ${\sim}{4}\;\unicode{x00B5}{\rm m}$. Thanks to the use of an original melt dewatering technique in the spectrum of a fiber sample, the absorption band of hydroxyl groups with a maximum of ${\sim}{3}\;\unicode{x00B5}{\rm m}$ and the overtone with a maximum of ${\sim}{2.3}\;\unicode{x00B5}{\rm m}$, characteristic of conventional tellurite glass samples, are almost imperceptible [33,34]. To understand the potential to use the developed tellurite fibers as an active medium, we will concentrate in this work on the study of their emission and amplifying properties.
This paper has a total of five sections. In Section 2, we present luminescence spectra in a single-mode, erbium-doped tungsten tellurite fiber in the ultrabroad range of 1.5–3 µm range. In Section 3 we demonstrate a supercontinuum source based on a tapered germanate fiber covering the same region. In Section 4 we experimentally study the broadband mid-IR amplification and show that gain as high as ${\sim}{5}$ times over one pass step can be achieved in the 2.7–2.8 µm range by pumping a low-cost high efficiency diode laser at 976 nm. Finally, the results are summarized in Section 5.
2. HIGH-PURITY TWL FIBER
To study the fiber laser and amplifier capabilities of the developed high-purity, erbium-doped tellurite glasses, we fabricated single-mode fibers with different cores, and first and second cladding diameters. The fiber core glass composition was ${{\rm TeO_2 {-} WO}_3} {-} {{\rm La}_2}{{\rm O}_3}{ -} {{\rm Bi}_2}{{\rm O}_3}$ with ${23.7}\;{\rm mol}.\;\% \;{{\rm WO}_3}$, ${3.6}\;{\rm mol}.\;\% \;{{\rm La}_2}{{\rm O}_3}$, and ${1.1}\;{\rm mol}.\;\% \;{{\rm Bi}_2}{{\rm O}_3}$, doped with ${0.4}\;{\rm mol}.\;\% \;{{\rm Er}_2}{{\rm O}_3}$, while the cladding’s composition was ${{\rm TeO}_2}{\rm n} {-} {{\rm WO}_3} {-} {{\rm La}_2}{{\rm O}_3}$ with 4 mol. % and ${6}\;{\rm mol}.\;\% \;{{\rm La}_2}{{\rm O}_3}$ in the first and second claddings, respectively. This composition provided a numerical aperture of 0.15 at 2.7 µm for the core and 0.2 at 0.98 µm for the first cladding. The luminescence power spectra of the fibers were measured in the ultrabroad range of 1.5–3 µm using a scanning monochromator (SOL Instruments MS2004i) and an InSb IR photodetector (InfraRed Associates IS-0.50) cooled by liquid nitrogen. To do this, a 976 nm laser diode pump source was used. Since the lifetimes of the upper ${{\rm I}_{11/2}}$ and lower ${{\rm I}_{13/2}}$ laser levels of the ${{\rm Er}^{3 +}}$ ion are about 100 µs and 7 ms, respectively, we choose a pulse duration of 110 µs with a pulse repetition rate of 80 Hz, keeping in mind the subsequent study of the lasing properties of these fibers. Figure 1 shows the luminescence spectra of a 50 cm long tungsten tellurite fiber with an erbium-doped core 7.5 µm in diameter and two claddings with diameters of 23 µm and 66 µm (${{\rm Er}^{3 +}}{:}\,{\rm TWL}$ fiber) in the range up to 3 µm at various peak pumping powers: 7, 8.5, and 10 W, which correspond to 62, 75, and 88 mW of average powers, respectively. As expected, we see a very broadband luminescence spectrum in the 1.5–1.6 µm range and in the 2.7–2.8 µm range. It is interesting to note that, at a low pump power, most of the luminescence power is concentrated in the long-wavelength range of 2.7–2.8 µm, where the maximum spectral luminescence intensity is 2.72 µm. As the pump power increases, the luminescence fraction in the 1.5–1.6-µm range also increases and then exceeds the long-wavelength part in magnitude. A two-peak spectral distribution also is clearly pronounced. We believe that such spectral characteristics, together with excellent thermal stability and mechanical properties, can make the ${{\rm Er}^{3 +}}$-doped tellurite fibers a good active medium for fiber lasers and amplifiers, particularly for broadband amplification of supercontinuum. Since the gain characteristics of tellurite-based, erbium-doped fiber amplifiers for the 1.5 µm range were well studied in [26], the main attention in this work will be paid to amplification in the 2.7 µm range.
3. SUPERCONTINUUM SOURCE
To study gain in tellurite fibers doped with erbium ions in the spectral range of 2.7 µm, we first developed an all-fiber supercontinuum source based on nonlinear conversion of femtosecond pulses, as shown in Fig. 2. Compared to other works [35–37], here we will demonstrate the most compact and energy-efficient scheme, consisting of a master oscillator, one amplification stage, and a tapered germanate fiber as a nonlinear converter. We started with an erbium femtosecond fiber oscillator, built according to a well-known scheme with passive mode locking via nonlinear polarization rotation [38]. This laser generated 200 fs pulses at a wavelength of 1560 nm with a fundamental repetition rate of 50 MHz.
Fig. 2. Schematic of supercontinuum source based on nonlinear wavelength conversion in germanate tapered fiber.
Next, the femtosecond pulses were amplified in a core-pumped erbium fiber amplifier with an active fiber length of 2.1 m to an energy of 2 nJ. Due to self-phase modulation and a specially selected length of a passive fiber (2 m) with anomalous dispersion in the 1.5 micron range (SMF 28), the amplification of femtosecond pulses led to a broadening of their spectrum and their compression in the time domain to a duration of 70 fs. To generate supercontinuum, we used a germanate fiber with a tapered geometry. Compared to silicate fibers, such fibers, have a transparency region up to a wavelength of 3 µm, a larger coefficient of Kerr nonlinearity, and can be easily spliced to silicate fibers by arc fusion splicers [39]. The tapered geometry, in turn, makes it possible to provide supercontinuum generation almost in the entire transparency range of the germanate fiber, even when using a femtosecond source with a relatively low peak power.
The fibers used in this work were manufactured using modified chemical vapor deposition technology and have a core containing 97 mol% ${{\rm GeO}_2}$, while their claddings were made of silica glass. The core/cladding diameters of all the fibers had a ratio of 1/30. The total length of the first fiber was 1.8 m. At the thick end, the fiber had a core diameter of 6 µm, which gradually decreased along the length of the fiber to a value of 3 µm. The total length of the second fiber was 1.3 m, and its core diameter changed from 8 µm at the thick end to 3 µm at the thin end. Figure 3 shows the dependence of the diameter of the core of the studied germanate fibers on their length. The germanate fibers were spliced from the thick end to the output of the femtosecond erbium fiber system using a standard arc splicer (Ericsson FSU 995). To reduce welding losses, we repeatedly heated the welding spot with an arc in a fusion splicer. Figure 4 shows the supercontinuum spectrum at the output of germanate tapered fibers recorded with a scanning monochromator (SOL Instruments MS2004i) and a PbSe photodetector (Thorlabs PDA20H).
At the output of the tapered germanate fibers, we obtained a more than an octave spanning supercontinuum, as shown in Fig. 4. The average output power of the supercontinuum was 45 mW for fiber 1 and 48 mW for fiber 2. The low power at the output of the fiber compared to the input (100 mW) is explained by the large losses in the germanate fiber in the long-wavelength region, as well as by the splicing losses due to the large difference in the numerical apertures of the germanate and silicate fibers. Even though fiber 2 had a larger diameter drop and a shorter length, the broadest band supercontinuum (1.5–3 µm) was obtained in the longer fiber 1. It should be noted that, in both cases, more than 60 percent of the supercontinuum power was concentrated in the wavelength region of 2–3 µm. In addition, an increase in the peak power of 1.5 micron pump pulses in our case would most likely not lead to a significant broadening of the supercontinuum spectrum due to losses in the germanate fiber, but would lead to a significant complication of the pump circuit. Thus, we have created a compact broadband single-mode supercontinuum source that can find many applications, including the study of the amplification properties of various fibers doped with rare-earth elements, which we will demonstrate below.
4. GAIN IN THE 3 µm RANGE
A. Experimental Setup
To study the amplification properties of the produced erbium-doped tellurite fibers, we developed the experimental setup that is shown in Fig. 5. Since the lifetime of the upper laser level ${{\rm I}_{11/2}}$ of ${{\rm Er}^{3 +}}$-ion is much shorter than the lifetime of the lower laser level $^4{{\rm I}_{13/2}}$ (110 µs versus 7 ms, as shown in Fig. 6), lasing in such fibers is possible only in the pulsed mode.
Fig. 5. Experimental setup to measure the on–off gain in erbium-doped tellurite fibers. LD, laser diode; L1, plano-convex silica lens; L2–L4, plano-convex ZnSe lenses; BS, beam splitter; and LPF, crystalline germanium low-pass filter.
A large difference in the lifetimes of the upper and lower laser levels leads to the need to work with short pump/signal pulses (${\sim}{100}\;\unicode{x00B5} {\rm s}$) with a low repetition rate (${\lt}{140}\;{\rm Hz}$), which makes it difficult to directly measure the gain, especially when its values are low. However, the on–off gain in this case can be easily measured. Thus, we have created a setup to measure the on–off gain in the pulsed mode. In this setup, which is shown in Fig. 5, the supercontinuum source described above was used as a seed. Its radiation was coupled with a pair of uncoated plano-convex zinc selenide lenses with a focal length of 25 mm (L3, L4) into the tested fiber.
The input end of the tellurite fiber, as well as the output fiber of the supercontinuum source, were placed on precision three-coordinate optical stages (Thorlabs MBT612) and further adjusted based on the maximum gain. To prevent the short-wavelength part of the supercontinuum from entering the fiber under test, a low-pass filter made of crystalline germanium with a cutoff wavelength of 1.8 µm was placed between the lenses. This filter is necessary because radiation in the 1.5-micron wavelength range falls into the amplification range of erbium ions and can affect the gain measurement result in the long wavelength (2.7 µm) region. An erbium-doped tellurite fiber was cladding-pumped at a wavelength of 976 nm using a multimode laser diode with a core/cladding diameter ratio of 105/125 and an NA of 0.18, which delivered a maximum optical power of 60 W. The pump radiation was collimated using an AR-coated at 976 nm plano-convex lens with a focal length of 30 mm (L1) and then coupled into the tellurite fiber using an uncoated ZnSe lens with a focal length of 25 mm (L2). The output end of the tellurite fiber was also placed on a three-coordinate optical stage (Thorlabs MBT612) and adjusted based on the maximum luminescence level at a wavelength of 2.7 µm, which corresponds to the best pumping conditions for the fiber. We used a dichroic beam splitter located between lenses L1 and L2 to extract the amplified radiation of the seed source from the tellurite fiber.
B. Time-Dependent Amplification
To measure an on–off gain in pulsed regime, we used a mechanical chopper placed just after the beam splitter. It modulated the supercontinuum radiation emerging from the fiber, which allowed us to measure the signal level with the pump turned off. The clock signal from the chopper was fed to a pulse generator with an adjustable delay, which formed the pump pulses in such a way that the pump pulse would arrive in time in the middle of the open half-cycle of the chopper. Thus, we could observe a pulse of amplified radiation against the background of a passing nonamplified radiation. The chopper was tuned to a frequency of 80 Hz, which corresponds to the formation of pulses with a duration of 6.25 ms with a period of 12.5 ms. Such times were chosen based on the depletion of the lower laser level during the time between pump pulses. The duration of the pump pulses was chosen based on the lifetime of the upper laser level and amounted to 110 µs. The rise and fall times of the optical pump pulse measured with a Thorlabs DET08C photodiode amounted to 3 µs, so the total pulse energy difference from a perfectly rectangular pulse was less than 3 percent. The radiation at the output of the tellurite fiber was recorded using a cooled InSb detector (InfraRed Associates IS-0.50) and a digital oscilloscope. To ensure the spectral selectivity of measurements, a scanning monochromator (SOL Instruments MS2004i) was installed in front of the photodetector.
The resulting oscillogram from the output of the photodetector for the case of an on–off gain of 1.5 at the 2.72 µm wavelength is shown in Fig. 7. Note that there is a sharp dip in the background radiation immediately after the end of the pump pulse. We believe that its cause is the absorption of radiation at a wavelength of 2.72 µm from the excited state from the laser level $^4{{\rm I}_{13/2}}$. The on–off gain was calculated as the ratio of the amplitude of the amplified pulse (${{\rm V}_{{\rm amp}}}$) to the background one (${{\rm V}_{{\rm bkg}}}$). Since the repetition rate of the pulses forming the supercontinuum (50 MHz) is more than two orders of magnitude greater than the bandwidth of the photodetector and its amplifier (150 kHz), in our experiments we can approximate the supercontinuum radiation by a CW.
Fig. 7. Oscillogram at the output of InSb detector in the gain measurement circuit (red line). Pump triggering pulses are shown by the blue line.
Since the duration of the pump pulse remained fixed in all experiments (110 µs), below we will operate with the values of the peak pump power. While measuring the on–off gain, we varied the peak pump power in the range from 0 to 25 W (0 to 220 mW average power). Here and below, the pump power values were measured at the output of the fiber of the pump diode after the collimating lens. At high pump powers (${\gt}{25}\;{\rm W}$), the instability of the results was observed, which, we believe, was associated with the heating of the end face of the tellurite fiber. We also estimated the pump absorption in an active TWL fiber by measuring the pump power at the input and output of the fiber and considering the Fresnel reflections at the fiber edges. The absorption value in our case was practically independent of the pump power and amounted to 44 dB/m.
To better understand the processes that occur during amplification in a tellurite fiber, we studied the changes in the time profile of the amplified pulse from the peak pump power. The dynamics of the amplified pulse shape as a function of the peak pump power measured at 2.72 µm wavelength is shown Fig. 8. For better clarity, part of the data before and after the amplified pulse was discarded. For comparison, the blue dashed line in Fig. 8 shows the pulses triggering the pump, which coincide in duration with the pump pulses. At a low peak pump power, less than ${\le} {10}\;{\rm W}$ (respectively, ${\le} {88}\;{\rm mW}$ of average power), small gains (${\le} {2}$) are observed, while the maximum gain is reached at the trailing edge of the pump pulse, when the population inversion between the levels $^4{{\rm I}_{11/2}}$ and $^4{{\rm I}_{13/2}}$ reaches its maximum. At medium peak pump powers of 15–20 W (132–176 mW of average power), the amplified pulse maximum shifts from the trailing edge of the pump pulse to the leading edge. This occurs because, starting from a certain pump power, most of the excited ions turn out to be at the laser level $^4{{\rm I}_{13/2}}$, and pumping is no longer able to create a population inversion between the levels $^4{{\rm I}_{11/2}}$ and $^4{{\rm I}_{13/2}}$. The gain decreases and, after the end of the pump pulse, takes on negative values due to the absorption of radiation from the level $^4{{\rm I}_{13/2}}$ to $^4{{\rm I}_{11/2}}$.
Fig. 8. Shape of the amplified pulse (red line) versus pump power. The blue dashed line shows the trigger pump pulses.
In this case, the on–off gain saturates and changes slightly with increasing power pumping. At a high peak pump power of about 25 W (220 mW of average power), a further increase in the gain is observed. We believe that it is related to the unloading of the lower laser level $^4{{\rm I}_{13/2}}$ due to the upconversion effect caused by the excited-state absorption that occurs at a high pump intensity [14,40].
C. On–Off Gain
As part of the study of the amplifying properties of Er:TWL fibers, we first studied the gain spectrum to determine the wavelength of maximum gain. To do so, in the scheme shown in Fig. 5, we measured the on–off gain at five different pump powers by changing the monochromator wavelength in the range of 2.6–2.9 µm.
The gain spectra of a 50 cm long erbium-doped tellurite fiber are presented in Fig. 9. The spectra were recorded at a peak pump power varying from 1 to 17 W. The overall gain increases monotonically with increasing pump power. One can see the broadening of the gain spectrum with increasing pump power. The maximum gain wavelength is in the range of 2.7–2.72 µm. At low pumping, it shifts to the short-wavelength region with increasing pump power. However, at high pumping, the maximum gain wavelength shifts to the long wavelength region. Also note that some oscillations are present in the gain spectrum at high pump power. We assume that the reason for this may be the redistribution of the populations of the energy sublevels caused by the high pump power.
We also studied the dependence of the on–off gain on the pumping for two lengths of the Er:TWL fiber: 25 and 50 cm.
Measurements were made in the scheme described in Fig. 5 at a fixed wavelength of 2720 nm, while the peak pump power was varied in the range of 1–25 W. As can be seen from Fig. 10, the effect of gain saturation, which we described above, is observed both with an active fiber length of 25 and 50 cm. However, with a shorter fiber length, gain saturation occurs earlier (10–15 W versus 15–20 W), moreover due to lower losses at low pump powers, a higher gain is observed in a short fiber. At high pump powers (${\gt}{15}\;{\rm W}$), a higher gain is observed in a longer piece of fiber. Thus, at a maximum pump power of 25 W, we obtained an on–off gain of 4.4. As can be seen in Fig. 10, the amplification saturation region is replaced by a further increase of this region due, in our opinion, to the upconversion effect. We believe that the obtained amplification results demonstrate a high potential of the developed high-purity, erbium-doped tellurite glass fibers for their use in fiber lasers and amplifiers.
5. CONCLUSION
The goal of this work was threefold. First, we proposed an experimental method to study the gain characteristics of active ion-doped fibers in the ultrabroadband range based on the supercontinuum seed. Second, we developed a simple, robust all-fiber supercontinuum source based on a tapered germanate fiber that covered the particular region of 1.5–3 µm. Third, using the recently developed high-purity, erbium-doped tungsten tellurite glass fibers, we experimentally studied the broadband mid-IR amplification and showed that gain as high as ${\sim}{5}$ times over one pass step can be achieved in the 2.7 µm range by pumping a low-cost, high-efficiency diode laser at 976 nm. Note that the level of doping is undoubtedly a very important parameter that significantly affects the effective length and magnitude of gain in the fiber. We plan to study this issue in our future work. However, the results already obtained show that tellurite-based, erbium-doped fibers are attractive for the development of broadband tunable fiber lasers.
Funding
Ministry of Science and Higher Education of the Russian Federation (075-15-2022-316).
Acknowledgment
The work is supported by the Center of Excellence “Center of Photonics” funded by the Ministry of Science and Higher Education of the Russian Federation. The authors thank Dr. V. V. Koltashev for fruitful discussions about the measurement of fiber luminescence.
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.
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