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Abstract
Mid-infrared cascaded stimulated Raman scattering (SRS) is experimentally investigated in an As-S optical fiber which is fabricated based on As38S62 and As36S64 glasses and whose fiber loss is ∼0.08 dB/m at1545 nm. Using a nanosecond laser operated at ∼1545 nm as the pump source, mid-infrared cascaded SRS up to eight orders is obtained in a 16 m As-S fiber. To the best of our knowledge, this is the first demonstration of SRS of such high order in non-silica optical fibers, and it may contribute to developing tunable mid-infrared Raman fiber lasers using C-band pump sources.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Stimulated Raman scattering (SRS) is a nonlinear effect in the media which can transfer a fraction of power from one optical field to another and whose frequency is downshifted by an amount determined by the vibrational modes of the medium [1, 2]. Optical fibers have many outstanding advantages, for example, they have low attenuation and are easy to produce high Raman gain, which prepare a good platform for SRS generation. Since R. H. Stolen et al. first reported SRS in optical fibers in 1972 [3], researchers have observed it in fibers of varied materials and structures, including silica [4, 5], fluoride [6, 7], tellurite [8–11], chalcogenide [12, 13] and photonic crystal fibers [14, 15], and have put it into practice [16–20].
When Raman gain of the optical fiber is high and pump power into it is sufficient, cascaded SRS effect can be achieved. Cohen et al. observed a sixth-order cascaded SRS in optical fibers in 1978 [21], and since then more and more researchers have devoted to the related study [22–24]. Recently, Yin et al. presented a third-order cascaded Raman shift pumped by an all-fiber master oscillator power amplifier system at 2050 nm [25]. Zhang et al. investigated a cascaded Raman random fiber laser continuously tunable from 1070 to 1370 nm using a ytterbium-doped fiber laser as its pump source [26]. Babin et al. demonstrated for the first time the high-order random Raman lasing in a polarization-maintaining optical fiber [27]. Although cascaded SRS in optical fibers has been well studied in the visible and near-infrared fields, very few are reported in the mid-infrared field due to the limitation of the transmission window of the traditional fiber material: silica.
Contrary to silica, chalcogenide glasses have wider transmission window (from the visible up to the infrared region of 12 or 15 μm depending on the composition), and their nonlinear material indices can be up to tens or hundreds of times those of silica, fluoride or tellurite [28–32]. They also have higher Raman gain coefficients (4.3∼5.7 × 10−12 m/W for As2S3 at 1.5 μm, and 2∼5 × 10−11 m/W for As2Se3 at 1.5 μm) [33–35]. As a result, optical fibers fabricated based on chalcogenide glassses are more promising for generating cascaded SRS in mid-infrared region. Troles et al. presented a third-order cascaded SRS in an As38Se62 suspended core fiber using a 1995 nm pump at 4 W [36]. Gao et al. demonstrated the SRS effects in an all-solid chalcogenide microstructured optical fiber pumped by the picosecond pulses at 1958 nm [37]. Duhant et al. reported a fourth-order cascaded SRS in an AsSe suspended-core fiber using a nanosecond pump at 1995 nm [38]. Yao et al. reported mid-infrared supercontinuum generation based on a fifth-order cascaded SRS generated in a chalcogenide optical fiber using a thulium-doped fiber laser at 2050 nm [39]. Despite this research boom, the reported SRS was confined to 5 orders and the cascaded Raman frequency shift was limited, because it was not practical to use long fiber length to enhance Raman gain in case the fiber loss ran too high.
In this paper, a low-loss chalcogenide optical fiber was designed and fabricated based on AS-S glass, and a 16 m long fiber was experimentally used to enhance Raman gain for boosting cascaded SRS generation. A nanosecond laser at ∼1545 nm was used as the pump source and mid-infrared cascaded SRS up to eight orders was observed.
2. Fabrication and properties of chalcogenide optical fiber
An As38S62 and an As36S64 glass rod were respectively prepared by a direct synthesis from elements with a purity of 99.999% in an evacuated silica ampoule at 650°C. The rods were in turn annealed near the glass transition temperature for 2 hours to stabilize their structure and to relieve internal stresses. Because the composition of As38S62 and As36S64 glasses was similar, only the former was measured for its various properties. Its linear material refractive indices were measured by the ellipsometry (J. A. Woolam IR-Vise and M-2000DI) and the material dispersion is presented in Fig. 1, in which the zero-dispersion wavelength (ZDW) is shown to be ∼5.246 μm.
The thermal expansion coefficient of As38S62 glass was obtained using the thermo mechanical analyzer (Rigaku, Thermo Plus TMA 8310) where the deformation of the glass under a constant load was measured as a function of temperature. The result is shown in Fig. 2(a), in which the softening temperature (Ts) is Ts = 184 °C, defined by the temperature of the maximal expansion. Using the cut-back technique and a Fourier-transform infrared (FT-IR) spectrophotometer (PerkinElmer Spectrum 100), the As38S62 rod loss was measured and the result is shown in Fig. 2(b). We can see it is less than 0.2 dB/m from 2 to 7 μm, except that two absorption peaks appear around 2.9 and 6.3 μm due to the O-H and H2O pollution.
Fig. 2 (a) Measured thermal expansion coefficient of As38S62 glass. (b) Measured As38S62 rod loss by cut-back technique within 2 ∼7 μm.
The spontaneous Raman spectrum of the As38S62 bulk glass sample was measured by a Raman spectrometer (JASCO NRS 2100), as shown in Fig. 3. The sample with the thickness of ~1.5 mm was excited by a CW laser with a wavelength of 532 nm at the average power of ~500 mW. The Stokes Raman spectrum was recorded in the back scattering alignment mode with co-polarization of incident and scattered light mode. In Fig. 3 it is clear a third-order SRS is formed, the Raman shift being 347, 688 and 1040 cm−1, respectively. The intensity of each peak dwindles while its spectral width (FWHM) increases. The Raman gain coefficient of As-S fiber is taken to be ~4.3∼5.7 × 10−12 m/W around 1.5 μm [33], much higher than that of silica fiber (~10−13 m/W at 1.5 μm) [2]. All these features made the proposed chalcogenide fiber a more favorable host for generating cascaded SRS.
The As-S optical fiber was fabricated by rod-in-tube drawing technique. An 8 cm long As36S64 rod was ultrasonically drilled by a Ultrasonic Drilling Machine (UDM) to form a tube with a diameter of ~1.5 mm. An As38S62 rod with a diameter of ~12 mm was elongated into capillary (~1.48 mm in diameter) to match the As36S64 tube. Both the As36S64 tube and As38S62 rod are shown in the insets of Fig. 4. The As38S62 capillary was then inserted into the As36S64 tube to form a preform which was further drawn into optical fiber at ~198 °C. During the fiber-drawing process, the preform was filled with a pressure 1~2 kPa lower than the standard atmospheric pressure to avoid the interstitial hole formation.
The core diameter of the As-S optical fiber was 4.2 μm and numerical aperture (NA) was 0.35, as shown in Fig. 4. The fiber loss was ∼0.08 dB/m at 1545 nm, which was measured by the cutback technique. The chromatic dispersion of the fundamental mode from 2 to 8 μm was calculated by a commercial software (Lumerical MODE Solution) using the full-vectorial mode solver method, as shown in Fig. 5(a). The zero-dispersion wavelength (ZDW) was 5.42 μm. Figure 5(b) shows the nonlinear coefficients, which were calculated based on the effective mode areas and the nonlinear-index coefficient of As2Se3 glass (n2 = 1.1 × 10−17 m2W−1 in [40] and [41]).
Fig. 5 (a) Calculated chromatic dispersion of the fundamental mode. (b) Calculated nonlinear coefficients from 1500 to 3500 nm.
3. Experiments and result discussion
The experimental setup for investigating SRS generation in a 16 m AS-S optical fiber is shown in Fig. 6. The pump source was a nanosecond laser operated at ∼1545 nm with a pulse width of ∼4.1 ns and a repetition rate of ∼25 kHz. Because the pump wavelength was in the normal dispersion regime, no soliton effect occurred, which also created favorable condition for the cascaded SRS generation. A polarization controller (PC) was used to align the polarization state of the pump. After the fiber collimator (FC), the pump pulse was coupled into the fiber core by a aspheric lens (AL) with a focal length of ∼6.24 mm and an NA of ∼0.40 (NEWPORT, F-LA11, 510-1550 nm). The output end of AS-S fiber was connected directly to a 0.5 m large-mode-area (LMA) ZBLAN fiber which had a core diameter of 105 μm and a transmission window from ~0.4 to ~5.0 μm. The nonlinear effect in the LMA ZBLAN fiber could be ignored due to its large core diameter. The generation SRS spectrum was recorded by an optical spectrum analyzer (OSA) (Yokogawa) and a Fourier-transform infrared (FT-IR) spectrometer.
Fig. 6 Experimental setup for investigating SRS generation in a 16 m As-S optical fiber. PC, polarization controller; FC, fiber collimator; AL, aspheric lens; OSA, optical spectrum analyzer; FT-IR, Fourier-transform infrared.
Figure 7 shows the cascaded SRS spectra generated in the fiber at the average pump power of ∼1.6, 7.9, 10, 11.2, 13.4, 17.8, 25.8, 37.3 and 100 mW. Considering the coupling efficiency (∼10%), the peak powers were calculated to be ∼1.56, 7.7, 9.8, 10.9, 13.1, 17.4, 25.2, 36.4 and 97.6 W. At ∼1.6 mW, a first-order SRS (R1) was obtained at ∼1631 nm and the Raman shift was ∼341 cm−1, which corresponded well with the spectral line (R1) in Fig. 3. Meanwhile, the modulation instability (MI) with sidebands (Stokes & anti-Stokes) was clearly observed at ∼1550.6 and 1539.3 nm. At ∼7.9 mW, the power of the first-order SRS reached the threshold of Raman scattering, which induced the generation of a second-order SRS (R2) at ∼1729 nm (∼689 cm−1). R2 power grew steadily. At ∼10 mW, a third-order SRS (R3) was obtained at ∼1838 nm (∼1032 cm−1), and R1 peak power depleted to −29.7 dBm while most of its power converted to R2, contributing to its growth to as high as −23.9 dBm. With the average pump power increasing to ∼11.2 mW, R3 took the turn to show a steady growth. And at ∼13.4 mW, a fourth-order SRS (R4) was observed at ∼1963 nm (∼1378 cm−1) and R3 (peak power −24.1 dBm) became predominant. At ∼17.8 and 25.8 mW, a sixth-order SRS (R6) appeared at ∼2280 nm (∼2086 cm−1), where R4 and a fifth-order SRS (R5) predominated in turn. When the average pump power further increased to ∼37.3 mW, a seventh-order SRS (R7) was obtained at ∼2473 nm (∼2428 cm−1), and the high-order Raman peaks flattened, becoming barely distinguishable. At ∼100 mW, an eighth-order SRS (R8) was formed at ∼2698 nm (∼2766 cm−1) but R5, R6 and R7 cannot be recognized at all. Here R8 became predominant and the spectrum induced by high-order SRS was broadened to ∼3800 nm. To the best of our knowledge, this was the first demonstration of eighth order cascaded SRS in non-silica optical fibers, which was mainly due to the low fiber loss and the enhanced Raman effect resulted from the long fiber length. It may contribute to the development of tunable Raman fiber lasers and random Raman lasers in the mid-infrared region based on the C-band pump sources.
Fig. 7 Cascaded SRS in the 16 m As-S optical fiber at the average pump power of ∼1.6, 7.9, 10, 11.2, 13.4, 17.8, 25.8, 37.3 and 100 mW with a nanosecond laser operated at ∼1545 nm.
From the above experimental investigation we can see only R1, R2, R3 and R4 stay recognizable throughout the whole process of power increase, so only their output Raman spectra were selected for the following analysis. Figure 8 shows the individual Raman spectrum as a function of the average pump power. We can see the individual Raman spectral width expanded with the increase of the pump power. It is also to be noted that the high-order Raman spectrum surpassed its low-order ones in width, especially for R3 and R4, which almost flattened at ∼25.8, 37.3 and 100 mW. This spectral characteristics of the cascaded SRS generation was the combined result of its own Raman gain spectrum and the effect of self-phase modulation (SPM) [27, 42].
Figure 9 shows the individual peak power (a) and FWHM (b) of R1, R2, R3 and R4 as a function of the input average pump power. From Fig. 9(a) we can see that R1 peak power keeps on growing till the input pump power reaches its higher order Stokes threshold, namely R2 threshold, and after that, it starts to deplete. This also occurs to R2, R3 and R4. Figure 9(b) shows FWHM of R1, R2, and R3, and the spectral lines behave similarly, exhibiting the Schawlow-Townes narrowing near the threshold and slight broadening with the power increase. The FWHM of R1 and R2 vary within the range of 3.8-5.8 nm and 4-11.8 nm with the average power increasing from ∼1.6 mW to 100 mW. However, for R3, because the Raman spectrum almost flattened from ∼37.3 mW to 100 mW, its FWHM was difficult to distinguish. As a result, only FWMH within the power range of ∼1.6 mW to 25.8 mW was depicted in Fig. 9(b), which shows a variation from 9 nm to 11.4 nm. For R4, the Raman spectrum became flattened since ∼17.8 mW, leaving only FWHM at ∼13.4 mW identifiable (8.1 nm), as shown in Fig. 9(b). The FWHM evolution comprised rather large constant value at the threshold and small power-variable part, especially for the higher orders [42]. The kinetic equations describing the effect of SPM can be used to explain, in which SPM linewidth is a slowly-growing cubic root function of the power for all orders [27].
Fig. 9 Individual peak power (a) and spectral width (FWHM) (b) of R1, R2, R3 and R4 as functions of the input average pump power.
4. Summary
In summary, an As-S optical fiber was designed and fabricated based on the As38S62 and As36S64 glass and its fiber loss was measured to be ∼0.08 dB/m at 1545 nm. Using a nanosecond laser operated at ∼1545 nm as the pump source, mid-infrared cascaded SRS up to eight orders was experimentally observed, and individual Raman spectral property including peak power and spectral width was investigated with the average pump power increasing from ∼1.6 to 100 mW. It was the first demonstration of SRS of such high order in non-silica optical fibers, and could be useful in developing tunable mid-infrared Raman fiber lasers using C-band pump sources.
Funding
This work is supported by National Natural Science Foundation of China (61775032 and 11604042), Fundamental Research Funds for the Central Universities (N170405007, N160404009), JSPS KAKENHI Grant (15H02250), and 111 Project (B16009).
Acknowledgment
The authors thank the national “Young 1000 Talent Plan” program of China.
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