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Multiple Raman peak properties in a 130 m tellurite fiber are demonstrated in this paper. When the tellurite fiber is pumped by a nanosecond laser at ∼1545 nm, a second-order Raman shift with multiple Raman peaks is observed. Especially for the first-order Raman shift, six obvious Raman peaks are observed. When the tellurite fiber is pumped by a picosecond laser at ∼1064 nm, a second-order Raman shift with only dual Raman peaks is obtained. Furthermore, the evolution of Raman shift with the variation of fiber length is investigated.
© 2016 Optical Society of America
1. Introduction
Stimulated Raman scattering (SRS) is an attractive nonlinear optical process that produces the power transfer from a given frequency to one or several down-shifted Stokes beams [1–4]. Being first observed in silica optical fibers by Stolen et al. [3], SRS has been widely applied in sensors, slow light generation, broadband Raman amplifiers and tunable Raman lasers, etc [5–14]. If the Raman gain and the pump power are sufficiently high, multiple-order cascaded Raman shift can extend the frequency shift. Since Cohen and Lin observed sixth-order cascaded Raman shift in a silica fiber [15], more and more researchers devoted to the multiple-order cascaded Raman shift study [16–19]. Recently, soft-glass fibers attracted much attention due to the wide transmission window and the high nonlinear optical properties [20–28]. Particularly, they are highly advantageous for generating multiple-order cascaded Raman shift because they have higher Raman gain coefficients than silica fibers [29–31]. Kulkarni et al. reported the third-order cascaded Raman shift in an As2S3 fiber pumped by a 2 ns, 1.55 μm laser [32]. Troles et al. presented the third-order cascaded Raman shift generation in an As38Se62 microstructured optical fiber (MOF) at the pump power of 4 W [33]. White et al. investigated the fouth-order cascaded Raman shift in As2S3 and As2Se3 optical fibers pumped by a near-IR nanosecond laser [34]. Duhant et al. demonstrated the fourth-order cascaded Raman shift in an AsSe chalcogenide suspended-core fiber [35]. Among the soft-glass fibers, tellurite fibers exhibit better chemical and thermal stability. Furthermore, their Raman gain spectra are much wider and with at least two peaks [36–38], which are more desirable for obtaining wide and multiple-peak cascaded Raman shift. However, relatively few researches concerning this have been carried out on tellurite fibers. Liao et al. observed wide SC generation based on the fifth-order cascaded Raman shift in a tapered tellurite fiber [39], but in their work only single Raman peak was observed for each order Raman shift.
In this work, multiple Raman peak properties in a 130 m tellurite fiber were demonstrated. With a nanosecond laser operated at ∼1545 nm as the pump source, second-order Raman shift with multiple Raman peaks was observed. Especially for the first-order Raman shift, six obvious Raman peaks appeared. With a picosecond laser operated at ∼1064 nm as the pump source, second-order Raman shift with only dual Raman peaks was obtained. The influence of fiber length on the evolution of Raman shift was further investigated using a 2.5 m tellurite fiber.
2. Properties
TeO2–Bi2O3–ZnO–Na2O (TBZN) glass was adopted to fabricate the tellurite fiber with step-index structure. The Raman gain spectrum of TBZN glass was measured by a Raman spectrometer (JASCO, model NRS 2100), as shown in Fig. 1. The inset is the TBZN glass sample with the thickness of ∼1 mm, which was excited using a CW-diode pumped-solid state laser (Coherent, Verdi) at the wavelength of 532 nm with the power of ∼500 mW. We can see that there are three main peaks: the one at 437 cm−1 comes from the stretching vibration of Te-O-Te bond, the one at 665 cm−1 from TeO4 trigonal bipyramids and the one at 740 cm−1 from TeO3 trigonal pyramid [31, 40–42].
The tellurite fiber was fabricated by the rod-in-tube drawing technique. Figure 2(a) shows the chromatic dispersion of the fundamental mode from 800 to 2100 nm, which was calculated by a commercial software (Lumerical MODE Solution) using the full-vectorial mode solver technology. We can see that the whole wavelength region locates in the normal dispersion regime of the tellurite fiber. The core diameter of the fiber was ∼2.55 μm and the refractive index of the core was ∼2.01. At ∼1550 nm, the refractive index difference between the core and the cladding was ∼2.2%, and the attenuation of the tellurite fiber was ∼0.02 dB/m. The nonlinear coefficients are shown in Fig. 2(b), which were calculated based on the effective mode areas and the nonlinear refractive index of the tellurite glass ∼5.9 × 10−19 m2/W.
Fig. 2 (a) Calculated chromatic dispersion of the fundamental mode. (b) Calculated effective mode areas and the nonlinear coefficients from 800 to 2100 nm.
3. Experiments and results
The experimental setup for investigating multiple Raman peak properties in the tellurite fiber is shown in Fig. 3. The pump source was a nanosecond laser with the center wavelength of ∼1545 nm, the pulse width of ∼4.1 ns and the repetition rate of ∼25 kHz, and a picosecond laser with the center wavelength of ∼1064 nm, the pulse width of ∼15 ps and the repetition rate of ∼80 MHz. Because both of the pump wavelengths were in the normal dispersion regime, no soliton effect occurred, which created favorable condition for SRS generation. The pump pulse was coupled into the fiber core by an aspheric objective lens with a focal length of ∼6.24 mm and an NA of ∼0.40 (NEWPORT, F-LA11, 510−1550 nm). The output signal was then butt-coupled into a 0.3 m-long large-mode-area (LMA) fluoride (ZBLAN) fiber with a core diameter of ∼105 μm and a transmission window from ∼0.4 to 6 μm. The nonlinear effect in ZBLAN fiber could be ignored due to the large core size. Finally, the LMA ZBLAN fiber was connected to an optical spectrum analyzer (OSA, Yokogawa) to record the spectra. In this work, although the transmission efficiency of the lens was higher than 90%, the coupling efficiency from the lens to the fiber was only ∼10%. There were several possible reasons: the spot after the lens was larger than the core; the numerical aperture (NA) of the fiber and the lens did not match well, and the surface of the tellurite MOF was not smooth.
First, the nanosecond laser operated at ∼1545 nm was used as the pump source. Figure 4(a) shows the Raman shift generation in the 130 m tellurite fiber at the average pump powers of ∼2, 6, 12, 18, 28 and 52 mW. Considering the coupling efficiency, the peak powers were ∼2.0, 5.9, 11.7, 17. 6, 27.3 and 50.7 W. At the low average pump power, only the first-order Raman shift was observed. When the average pump power exceeded ∼12 mW, the power of the first-order Raman shift reached the threshold of Raman scattering, which induced the generation of the second-order Raman shift. With the average pump power increasing to ∼18 mW, six Raman peaks were obtained in the first-order Raman shift, the center wavelengths of which were ∼1588 (Peak 1), 1653(Peak 2), 1720(Peak 3), 1748(Peak 4), 1769 (Peak 5) and 1796 nm(Peak 6). Raman shift of the six peaks were ∼175, 423, 658, 751, 817 and 904 cm−1, respectively. To the best of our knowledge, this was the first demonstration of multiple Raman peaks in tellurite fibers. Peak 1 came from the intermolecular asymmetric motion of the Te-O band [43]. Peak 2, Peak 3 and Peak 4 came from the stretching vibration of Te-O-Te bond, TeO4 trigonal bipyramid and TeO3 trigonal pyramid, respectively, which corresponded well with the Raman shift of TBZN glass in Fig. 1. Peak 5 and Peak 6 can be attributed to the stretching vibration of Te-O- and Te = O and containing non-bridging oxygen in TeO3tps and TeO3+1 polyhedra [44]. Compared with Fig. 1, the number of Raman peak is difference due to the large Raman gain in the hundred-meter fiber length. Due to the high loss in the long wavelength, the six Raman peaks in the second-order Raman shift were not obvious. With the average pump power further increasing to ∼28 and 52 mW, the six Raman peaks became more obvious and the spectrum broadened due to the self-phase modulation (SPM) effect. In this experiment, second-order Raman shift with multiple Raman peaks were observed, which was different from Ref. 39. One of the reasons was due to the large Raman gain in the 130 m tellurite fiber, and the other was that the low peak power led to weak SPM effect, which cannot cover Raman shift. Figure 4(b) is the dependence of the average output power on the average pump power.
Fig. 4 (a) Multiple Raman peaks in the 130 m tellurite fiber at the average pump powers of 2, 6, 12, 18, 28 and 52 mW. (b) Dependence of the average output power on the average pump power in the 130 m tellurite fiber.
Second, the picosecond laser operated at ∼1064 nm was used as the pump source. The average pump powers were ∼0.025, 0.62, 1.05, 2.2, 2.9, 4.2, 5.05, 5.9, 6.5 and 7.7 W, and the peak powers were ∼2.1, 51.6, 87.5, 183.3, 241.7, 350, 420.8, 491.7, 541.7 and 641.7 W. Figure 5(a) shows the evolution of Raman shift in the 130 m tellurite fiber, and the peak at ∼1079 nm came from the spontaneous emission (SE) of the laser. At low average pump power, only the first-order Raman shift was observed, which had two peaks: Peak 1 and Peak 2. When the average pump power increased to ∼2.9 W, the second-order Raman shift was generated, which also had two peaks: Peak 3 and Peak 4. The center wavelengths of Peak 1, Peak 2, Peak 3 and Peak 4 were ∼1114, 1156, 1216 and 1263 nm, respectively. The Raman shift of Peak 1 and Peak 2 in the first-order Raman shift were ∼422 and 748 cm−1, and Peak 3 and Peak 4 in the second-order Raman shift were ∼1175 and 1480 cm−1. Peak 1 and Peak 3 came from the stretching vibration of Te-O-Te bond, and Peak 2 and Peak 4 came from TeO3 trigonal pyramid. Compared with the former experiment, only dual Raman peaks appeared in each order Raman shift. This is because SPM effect was so marked that it covered some Raman peaks. With the further increase of the average pump power, all peaks broadened due to SPM and SRS. Moreover, at the average pump power of ∼5.05 W, four-wave mixing (FWM) was observed with the Stokes wave at ∼1135 nm and the anti-Stokes wave at ∼998 nm. Peak 2 was selected for further investigation because it was the most prominent among the four peaks. From ∼0.62 to 4.2 W, Peak 2 evolved simultaneously to the blueshift and redshift regions mainly under the effect of SPM. From ∼4.2 to 7.7 W, Peak 2 only evolved to the redshift region mainly due to SRS. Figure 5(b) is the dependence of the average output power on the average pump power, and Fig. 5(c) is the dependence of 3 dB spectral bandwidth of Peak 2 on the average pump power. From the figure we can see with the average pump power increasing from ∼2.2 to 7.7 W, the 3 dB spectral bandwidth increased from ∼6 to 49 nm. Peak 2 with such wide 3 dB spectral bandwidth can be highly applicable for tunable lasers and amplifiers [10, 32–34].
Fig. 5 (a) Dual Raman peaks in the 130 m tellurite fiber at the average pump powers of 0.025, 0.62, 1.05, 2.2, 2.9, 4.2, 5.05, 5.9, 6.5 and 7.7 W. (b) Dependence of the average output power on the average pump power in the 130 m tellurite fiber. (c) Dependence of the 3 dB spectral bandwidth of Peak 2 on the average pump power.
To investigate the influence of fiber length on the evolution of Raman shift, the 130 m tellurite fiber was replaced by a 2.5 m tellurite fiber. Figure 6(a) shows the Raman shift generation with the nanosecond laser as the pump source. The average pump powers were ∼7, 20, 30, 38, 60, 77, 110, 160 and 200 mW, and the peak powers were ∼6.8, 19.5, 29.3, 37.1, 58.5, 75.1, 107.3, 156.1 and 195.1 W. The first-order Raman shift with Peak 1 and Peak 2, and the second-order Raman shift with Peak 3 and Peak 4 were observed. The center wavelengths of Peak 1, Peak 2, Peak 3 and Peak 4 were ∼1652, 1745, 1894 and 2007 nm, and the Raman shift were ∼420, 742, 1193 and 1489 cm−1, respectively. The dependence of the average output power on the average pump power is shown in Fig. 6(b). After that, the 2.5 m tellurite fiber was pumped by the picosecond laser and Raman shift spectrum is shown in Fig. 6(c). The average pump powers were ∼0.8, 2.1, 3.4, 5.4, 6.0, 7.0, 8.4 and 9.6 W, and the peak powers were ∼66.7, 175, 283.3, 450, 500, 583.3, 700 and 800 W. In this case only the first-order Raman shift with two peaks was obtained. The center wavelengths of two peaks were ∼1114 and 1156 nm, and Raman shift were ∼422 and 748 cm−1. The dependence of the average output power on the average pump power is shown in Fig. 6(d). Comparing Fig. 6(b) and (d) with Fig. 4(b) and Fig. 5 (b), we can see that the average output power of the 2.5 m tellurite fiber was higher than that of the 130 m tellurite fiber due to the lower fiber loss. Nevertheless, only dual peaks in each order Raman shift was observed because the fiber length was greatly reduced, which induced a lower Raman gain. Also due to the low Raman gain, only the firs-order Raman shift was formed at the pump of the picosecond laser.
Fig. 6 (a) Raman shift spectra in the 2.5 m tellurite fiber pumped by the nanosecond laser at the average pump powers of ∼7, 20, 30, 38, 60, 77, 110, 160 and 200 mW. (b) Dependence of the average output power on the average pump power in the 2.5 m tellurite fiber. (c) Raman shift spectra in the 2.5 m tellurite fiber pumped by the picosecond laser at the average pump powers of 0.8, 2.1, 3.4, 5.4, 6.0, 7.0, 8.4 and 9.6 W. (d) Dependence of the average output power on the average pump power in the 2.5 m tellurite fiber.
4. Summary
In summary, multiple Raman peak properties in a 130 m tellurite fiber was investigated using different pump sources. When a nanosecond laser operated at ∼1545 nm was used as the pump source, second-order Raman shift with multiple Raman peaks was observed. Especially for the first-order Raman shift, six obvious Raman peaks were obtained. To the best of our knowledge, this was the first demonstration of multiple Raman peaks in tellurite fibers. When a picosecond laser operated at ∼1064 nm was used as the pump source, second-order Raman shift with only dual Raman peaks was observed, which can be applied to tunable lasers and amplifiers. Furthermore, the influence of fiber length on the evolution of Raman shift was investigated using a 2.5 m tellurite fiber. The reduction in fiber length led to a lower Raman gain, which induced only dual Raman peaks in each order Raman shift.
Funding
Japan Society for Promotion of Science (JSPS KAKENHI Grant 15H02250); National Natural Science Foundation of China (NSFC) (Grants 11374084 and 61307056).
Acknowledgments
Tonglei Cheng acknowledges the support of the JSPS Postdoctoral Fellowship.
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