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Abstract
In this letter, we demonstrate efficient cascaded Raman amplification in fluorotellurite fibers pumped by a 1550 nm nanosecond laser. All-solid fluorotellurite fibers with a core diameter of ∼ 5 µm are fabricated by using a rod-in-tube method. By using a continuous wave (CW) 1765 nm fiber laser as the signal source for first-order Raman amplification, the amplified 1765 nm laser with an average output power of ∼ 190.9 mW and a pulse width of ∼ 3.42 ns is obtained from a 3 m long fluorotellurite fiber for an average pump power of ∼ 519.35 mW and an input signal power of ∼ 1.05 mW, corresponding to a net gain of ∼ 22.6 dB and an optical-to-optical conversion efficiency of 36.76%. Furthermore, a CW 2049.2 nm fiber laser combined with the above 1765 nm laser are used as the signal sources for second-order cascaded Raman amplification. As the power of the 1765 nm and 2049.2 nm lasers are set as 1.05 and 1.00 mW, respectively, the amplified 2049.2 nm laser with an average output power of ∼ 135.35 mW and a pulse width of ∼ 3.36 ns is obtained from the fluorotellurite fiber for an average pump power of ∼ 558.37 mW, corresponding to a net gain of ∼ 21.3 dB and an optical-to-optical conversion efficiency of 24.24%. As the length of the fluorotellurite fiber is increased to 10 m, third-order cascaded Raman amplification is observed by using a 2442.6 nm signal generated by four-wave mixing and the above 1765/2049.2 nm lasers as the signal sources, causing the generation of the amplified 2442.6 nm laser with an average output power of ∼ 18.84 mW. Our results show that fluorotellurite fibers are promising gain media for constructing cascaded Raman fiber amplifiers with a wide range of wavelengths.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Raman fiber amplifiers (RFAs) can provide broad gain bands in any wavelength region required by properly setting the pump wavelengths, which is inaccessible for active ions (rare earth ions or transition metal ions) doped fiber amplifiers. Owing to the intrinsic advantages of wavelength flexibility, RFAs have been widely applied in optical communications, nonlinear frequency conversion, and in-band pumping [1–3]. Recently, RFAs have been used to extend the operation wavelength of fiber lasers. For silica or phosphosilicate fiber-based RFAs, V. R. Supradeepa et al. reported a high-efficiency 1480 nm cascaded Raman fiber laser with an output power of 301 W, by using a single-pass cascaded Raman amplifier seeded at all intermediate Stokes wavelengths [4]. Later, near infrared fiber lasers with over kilowatt output power have been achieved through RFAs employing different types of silica fibers, including multi-clad fibers [5], multimode graded index fibers [6], and ytterbium-doped fibers (YDFs) (hybrid ytterbium–Raman gains) [7–9]. However, silica or phosphosilicate fibers are not suitable for generating mid-infrared (MIR, 2.2-5 µm) RFAs because of very high material loss in the MIR spectral region.
To construct MIR RFAs or Raman lasers, several types of specialty optical fibers including tellurite, fluoride, and chalcogenide fibers with low transmission loss in mid-infrared region have been employed for this purpose [10–15]. For tellurite fibers, Masuda et al. demonstrated hybrid tellurite + silica RFAs with a gain bandwidth of more than 130 nm over S + C+L bands [10]. A widely tunable ring-cavity tellurite fiber Raman laser covering the S + C+L + U band was demonstrated experimentally by one of the authors [11]. In the case of fluoride fibers, V. Fortin et al. reported the first Raman laser at 2185 nm based on a fluoride glass optical fiber [12]. Later, V. Fortin et al. demonstrated 3.7 W fluoride glass Raman fiber laser operating at 2231 nm [13]. For chalcogenide fibers, M. Bernier reported a chalcogenide fiber Raman laser with an operation wavelength of ∼ 3.34 µm and a maximum average output power of ∼ 47 mW [14]. Subsequently, a 3.77 µm chalcogenide fiber Raman laser with a maximum average output power of ∼ 9 mW based on cascaded Raman gain was demonstrated by the same group [15]. Despite recent progress, it is still necessary to search new fiber materials with good chemical, thermal and mechanical stability for realizing MIR RFAs and Raman lasers with improved performances.
Recently, to further improve the performances of tellurite fiber-based amplifier and laser sources, fluorotellurite fibers based on TeO2 ‒ BaF2 ‒ Y2O3 (TBY) glasses with a broadband transmission window of 0.4∼5.5 µm, low hydroxyl content and highly water resistance have been developed by us. The transition temperature of the TBY glass was about 424 °C, and the figure-of-merit parameter for characterizing the thermal mechanical properties of a laser material was also measured for TBY glasses, which indicated that the fluorotellurite fibers might bear stronger thermal shock than fluoride glass fibers and had a potential for constructing high power MIR laser sources [16,17]. By using the fluorotellurite fiber as a nonlinear medium, an all-fiber mid-infrared supercontinuum laser source with an output power of about 25.8 W was achieved [18]. Very recently, the properties of Raman scattering in fluorotellurite fibers were investigated by us [19]. However, until now, fluorotellurite fiber-based RFAs have not yet been demonstrated.
In this work, we demonstrated efficient cascaded Raman amplification in fluorotellurite fibers pumped by a 1550 nm nanosecond laser. For first-order Raman amplification, a continuous wave (CW) 1765 nm fiber laser was used as the signal source, the amplified 1765 nm laser with average output power of ∼ 190.9 mW and a pulse width of ∼ 3.42 ns was obtained from a 3 m long fluorotellurite fiber, corresponding to a net gain of ∼ 22.6 dB and an optical-to-optical conversion efficiency of 36.76%. For second-order cascaded Raman amplification, a CW 2049.2 nm fiber laser combined with the above 1765 nm laser were used as the signal sources, the amplified 2049.2 nm laser with average output power of ∼ 135.35 mW and a pulse width of ∼ 3.36 ns was obtained, corresponding to a net gain of ∼ 21.3 dB and an optical-to-optical conversion efficiency of 24.24%. For third-order cascaded Raman amplification, the amplified 2442.6 nm laser with average output power of ∼ 18.84 mW was observed in a 10 m long fluorotellurite fiber by using a 2442.6 nm signal generated by four-wave mixing (FWM) and the above 1765/2049.2 nm lasers as the signal sources. The mode characteristics of the amplified 1765 nm and 2049.2 nm laser were also investigated.
2. Experiments and results
To obtain cascaded Raman amplification, the fluorotellurite fibers were fabricated by using a rod-in-tube method based on the 70 TeO2 - 20 BaF2 - 10 Y2O3 (70 TBY, core material) and 65 TeO2 - 25 BaF2 - 10 Y2O3 (65 TBY, cladding material) glasses. Figure 1 showed the Raman gain coefficient of TBY glass for 1550 nm pumping [19]. The 70 TBY glass has a usable Raman shift of ∼ 785 cm-1 (∼ 23.5 THz) and a Raman gain coefficient of ∼ 1.65×10−12 m/W at 1550 nm, which is about 25.4 times larger than that of silica glass [19]. Such a large Raman gain coefficient and a large usable Raman shift are beneficial for constructing broadband RFAs with a wide range of operating wavelengths. The detailed fiber drawing process was similar to that mentioned in [16]. The fiber had a step-index structure and its core diameter was about 5 µm, as seen in Fig. 2(a). Figure 2(b) shows the refractive indices of 70 TBY and 65 TBY glasses. The normalized frequency was calculated to be ∼ 2.184 at 2049.2 nm, indicating that the fabricated fluorotellurite fibers have single mode propagation characteristics in mid-infrared band. The transmission losses of the fiber were measured to be ∼ 0.65, 0.62, and 0.59 dB/m at 1550, 1765, and 2049 nm, respectively, by using a cut-back method.
Fig. 2. (a) Experimental setup of the cascaded Raman amplification. (b) Refractive indices of 70 TBY and 65 TBY glasses. (c) Output spectrum of the signal sources.
Figure 2(a) shows the experimental setup for the generation of cascaded Raman amplification, which consists mainly of a 1550 nm nanosecond laser (pump source), two CW fiber lasers (signal sources) and a piece of fluorotellurite fiber (gain medium). Two signal sources comprising conventional Tm-doped fiber lasers are power combined with the 1550 nm nanosecond laser and sent through the fluorotellurite fiber. The 1765 nm signal source consists of a pair of 1765-nm fiber Bragg gratings (FBG1 & FBG2), a fiber-pigtailed diode laser at 1570 nm (pump source), a piece of 7 cm Tm-doped fiber (TDF, Nufern, SM-TSF-5/125) (gain medium), and a wavelength division multiplexer (WDM). The 2049.2 nm signal source has the same configuration except for the operating wavelength (at 2049.2 nm) of the FBG3 & FBG4 and the gain fiber length (50 cm). Figure 2(c) shows the output spectrum with only the signal sources turned on. There is no residual 1570 nm excitation light remaining due to the absorption of thulium ions and the loss of the WDM. As the pump light is turned on, the signal lights are amplified through stimulated Raman scattering in the fluorotellurite fiber. The output end of the fluorotellurite fiber was mechanically connected to a 1 m long ZBLAN (ZrF4 - BaF2 - LaF3 - AlF3 - NaF) fiber cable with a core diameter of 400 µm and an operating wavelength of 0.285-5 µm. The output end of the fluoride fiber cable was connected directly to an optical spectrum analyzer with a measurement range of 1200-2400 or 1900-5500 nm (Yokogawa, 75 or 77). The total output power was directly measured by using a power meter at the output end of the fluorotellurite fiber. The output powers of the Stokes light were obtained by multiplying the total output power by the percentages of the integral areas of the corresponding Stokes light.
We first investigated the performance of first-order Raman amplification in fluorotellurite fiber by using a CW 1765 nm laser with an output power of ∼ 1.05 mW as signal source and a 1550 nm laser with a pulse width of 12 ns and a repetition of 80 kHz as pump source. Figure 3(a) shows the dependence of the net gain at 1765 nm on the average power of the 1550 nm nanosecond laser. Figure 3(b) shows the dependence of the average output power of the amplified 1765 nm laser and the conversion efficiency on the average power of the 1550 nm nanosecond laser. The total average output power of the amplified 1765 nm laser and the un-absorbed 1550 nm nanosecond laser grew almost linearly with the increase of the average power of the 1550 nm nanosecond laser. Among them, the average output power of the amplified 1765 nm laser increased rapidly after reaching the Raman threshold. The average output power of the amplified 1765 nm laser reached about 190.9 mW for an average pump power of about 519.35 mW, corresponding to a net gain of ∼ 22.6 dB (with a fluctuation of ±0.05 dB in twenty minutes) and an optical-to-optical conversion efficiency of 36.76%, which is much higher than that (15.70%) obtained for the spontaneous Raman scattering process [19] and can be further improved by reducing the transmission loss of the fluorotellurite fiber. Figure 3(c) shows the comparison of output spectra for an average pump power of 0 and 519.35 mW. Owing to the narrow linewidth of the seed laser (0.08 nm), the obtained 3 dB spectral linewidth of the 1765nm laser (∼ 0.2 nm) was much narrower than that (∼ 9.5 nm) obtained for the spontaneous Raman scattering process [19]. After amplification, the spectrum of the 1765nm laser was broadened. To give a quantitative description of the spectral broadening of the amplified 1765 nm laser, we measured the corresponding 3 dB spectral linewidth of the 1765 nm laser at different average output powers, as shown in Fig. 3(d). The spectral linewidth increased from 0.08 nm to 0.2 nm, which might be caused by self-phase modulation and spontaneous Raman scattering. In addition, the second-order Stokes light at 2049 nm appeared in the output spectrum for an average pump power of 519.35 mW, accompanying with the generation of the anti-Stokes light at 1383 nm. It is evidently that, the above Stokes (∼ 2049 nm) and anti-Stokes (∼ 1383 nm) light were generated through nondegenerate FWM between the 1550 nm laser and the amplified 1765 nm in the fluorotellurite fiber. Meanwhile, the Stokes light at 2049 nm generated by FWM could be amplified via second-order cascaded Raman scattering in the fluorotellurite fiber as the amplified 1765 nm laser is strong enough, causing the generation of second-order cascaded Raman amplification at 2049 nm. It clearly showed that efficient second-order cascaded Raman amplification might be obtained in the fluorotellurite fiber.
Fig. 3. (a) Dependence of the net gain on the launched pump power of the 1550 nm nanosecond laser. (b) Dependence of the output power and corresponding conversion efficient on the launched pump power of the 1550 nm nanosecond laser. (c) Output spectrum with pump power of 0, 519.35 mW. (d) Output spectral linewidths of 1765nm laser at different output powers.
To investigate the performance of second-order cascaded Raman amplification in fluorotellurite fiber, a CW 2049.2 nm fiber laser with an output power of ∼1.00 mW combined with the above 1765 nm laser were used as signal sources, and a 1550 nm laser with a pulse width of 10 ns and a repetition of 80 kHz was used as pump source. When the first-order Stokes light (∼ 1765 nm) became strong enough, it would serve as the pump source and cause the amplification of second-order Stokes light (∼ 2049.2 nm). Figure 4(a) shows the dependence of the net gain at 2049.2 nm on the average power of the 1550 nm nanosecond laser. Figure 4(b) shows the dependence of the average output power of the amplified 2049.2 laser and the conversion efficiency on the average power of the 1550 nm nanosecond laser. After the average power of the 1550 nm nanosecond laser reached the first-order Raman threshold, the power of the amplified 1765 nm laser increased rapidly. As the average power of the 1550 nm nanosecond laser was increased to over 354.0 mW, the power of the amplified 1765 nm laser reached the second-order Raman threshold, and the power of the 1765 nm laser was transferred to the 2049.2 nm laser, causing the generation of second-order cascaded Raman amplification at 2049.2 nm. The output power of the amplified 2049.2 nm laser reached about 135.35 mW as the average power of the 1550 nm nanosecond laser was about 558.37 mW, corresponding to a net gain of ∼ 21.3 dB (with a fluctuation of ±0.05 dB in twenty minutes) and an optical-to-optical conversion efficiency of 24.24%, which is much higher than that (13.70%) obtained for the spontaneous Raman scattering process [19] and can be further improved by reducing the transmission loss of the fluorotellurite fiber. Figure 4(c) shows the comparison of output spectra for an average pump power of 0 and 558.37 mW. Owing to the narrow linewidth of the seed laser (0.08 nm), the obtained 3 dB spectral linewidth of the 2049 nm laser (< 0.2 nm) was much narrower than that (∼ 14.3 nm) obtained for the spontaneous Raman scattering process [19]. Figure 4(d) shows the measured 3 dB spectral linewidth of the 2049.2 nm laser at different average output powers. The 3 dB spectral linewidth increased from 0.08 nm to 0.19 nm, which might be caused by self-phase modulation and spontaneous Raman scattering, as with the amplification of the 1765 nm laser.
Fig. 4. (a) Dependence of the net gain on the launched pump power of the 1550 nm nanosecond laser. (b) Dependence of the output power and corresponding conversion efficient on the launched pump power of the 1550 nm nanosecond laser. (c) Output spectrum with pump power of 0, 558.37 mW. (d) Output spectral linewidths of 2049.2 nm laser at different output powers.
Since the pump source is a nanosecond pulsed laser at 1550 nm, the amplified 1765 nm and 2049.2 nm laser should be pulsed lasers owing to the transient response characteristics of RFAs [20]. Figure 5(a) shows the measured pulse profiles of the amplified 1765 nm laser via first-order Raman amplification. The inset of Fig. 5(a) shows the oscilloscope trace of the pulse trains. The pulse width of the amplified 1765nm laser was about 3.42 ns and the repetition rate was ∼ 80 kHz for an output power of ∼ 190.9 mW, corresponding to the maximum peak power of about 698 W. Figure 5(b) shows the measured pulse profiles of the amplified 2049.2 nm laser (red line) and 1765nm laser (black line) via second-order cascaded Raman amplification. The inset of Fig. 5(b) shows the oscilloscope trace of the 2049.2 nm laser pulse trains. The pulse width of the amplified 2049.2 nm laser was about 3.36 ns and the repetition rate was ∼ 80 kHz for an output power of ∼ 135.35 mW, corresponding the maximum peak power of about 504 W. The dip in the pulse profile of the amplified 1765 nm laser via second-order cascaded Raman amplification indicated the relative portion of the amplified 1765nm laser was transferred to the 2049.2 nm laser. The position of the dip, where the trailing edge of the pulse, was not matched with the peak of the 2049.2 nm laser pulse, which might be a result of the asymmetry in the input pump at 1765 nm and pump depletion.
Fig. 5. (a) Measured pulse profiles of amplified 1765nm laser at maximum output power of 190.9 mW. Inset: the corresponding pulse trains. (b) Measured pulse profiles of cascaded Raman amplified 2049.2 nm laser (red line) at 135.35 mW and 1765nm laser (black line) at 37.8 mW. Inset: the corresponding pulse trains.
As another key parameter for fiber lasers and amplifiers, beam quality was also measured in this experiment. Figures 6(a) and 6(b) show the dependence of the spot size on the position of the amplified 1765nm and 2049.2 nm laser, respectively. And the inset gave out their beam shape. For the amplified 1765nm laser, for an output power of ∼ 190.9 mW, the M2factor in the X-axis and Y-axis direction were measured to be 1.75 and 1.72, respectively. For the amplified 2049.2 nm laser, for an output power of ∼ 135.35 mW, the M2 factor in the X-axis and Y-axis direction were measured to be 1.32 and 1.50, respectively, which were smaller than that of the amplified 1765nm laser. The above results indicated that high efficient Raman amplifier with high beam quality could be realized by using our fluorotellurite fibers.
Fig. 6. (a) Measured dependence of the spot size of the amplified 1765nm laser on the position. Inset: Measured beam shape of the amplified 1765nm laser. (b) Measured dependence of the spot size of the amplified 2049.2 nm laser on the position. Inset: Measured beam shape of the amplified 2049.2 nm laser.
Furthermore, as the length of fluorotellurite fiber was increased to 10 m, third-order cascaded Raman amplification at 2442.6 nm was also observed. Figure 7(a) shows the output spectra for an average pump power of 0, 276.9 and 488.6 mW. As the average power of the 1550 nm nanosecond laser reached 276.9 mW, Stokes light at 2442.6 nm was generated through nondegenerate FWM between the amplified 1765nm and 2049.2 nm lasers in the 10 m long fluorotellurite fiber. By using the generated Stokes light at 2442.6 nm and the above 1765/2049.2 nm lasers as the signal sources, third-order cascaded Raman amplification at 2442.6 nm was observed in the 10 m long fluorotellurite fiber. The output power of the amplified 2442.6 nm laser was about 18.84 mW as the average power of the 1550 nm nanosecond laser was about 488.6 mW, corresponding to an optical-to-optical conversion efficiency of ∼ 3.86%, as seen in Fig. 7(b). Since the Stokes light at 2442.6 nm generated by FWM was not so strong, the conversion efficiency for the 2442.6 nm laser was quite low (∼ 3.86%). Compared to our previous work [19], efficient cascaded Raman amplification with higher conversion efficiencies, narrower linewidths and better beam qualities were achieved for cascaded Raman amplifiers based on the fluorotellurite fibers. In the future, we will further improve the performances of the cascaded Raman amplifiers by optimizing the parameters of the fluorotellurite fibers and the pump lasers.
Fig. 7. (a) Output spectrum of third-order cascaded Raman amplification from 10-m long fluorotellurite fiber. (b) Dependence of the output power of 1765nm, 2049.2 nm and 2442.6 nm light on the launched pump power of the 1550 nm nanosecond laser.
3. Conclusions
In summary, we demonstrated efficient cascaded Raman amplification in all-solid fluorotellurite fibers pumped by a 1550 nm nanosecond laser. By using a continuous wave (CW) 1765nm fiber laser as the signal source for first-order Raman amplification, the amplified 1765nm laser with an average output power of ∼ 190.9 mW and a pulse width of ∼ 3.42 ns was obtained from a 3 m long fluorotellurite fiber, and the corresponding net gain was ∼ 22.6 dB and the optical-to-optical conversion efficiency was about 36.76%. For second-order cascaded Raman amplification, a CW 2049.2 nm fiber laser combined with the above 1765nm laser are used as the signal sources, the amplified 2049.2 nm laser with an average output power of ∼ 135.35 mW and a pulse width of ∼ 3.36 ns was obtained from the above fluorotellurite fiber, and the corresponding net gain was ∼ 21.3 dB and the conversion efficiency was ∼ 24.24%. Third-order cascaded Raman amplification at 2442.6 nm was also observed in a 10 m long fluorotellurite fiber, and the amplified 2442.6 nm laser with an average output power of ∼ 18.84 mW was obtained. Our results show that fluorotellurite fibers are promising gain media for constructing near-infrared or mid-infrared cascaded Raman fiber amplifiers.
Funding
National Key Research and Development Program of China (2020YFB1805800); National Natural Science Foundation of China (62090063, 62075082, U20A20210, 61827821, 11774132, 61960206004); the Opened Fund of the State Key Laboratory of Integrated Optoelectronics.
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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