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By pumping a 1.7-m-long As2S3 fiber at 2050 nm directly, a fiber-based mid-infrared supercontinuum (SC) source with an output power of 366 mW is demonstrated. This is the first experimental demonstration to obtain such a mid-infrared SC in a piece of chalcogenide fiber pumped at 2 μm directly. The cut-off wavelength of the As2S3 fiber is 3.5 μm, indicating that it could support several modes at around 2 μm. It is found that nonlinear spectral broadening mechanisms in the few-mode chalcogenide fiber could be affected through adjusting the butt-coupling position. That is because different positions will excite different modes that correspondingly possess different nonlinearity and dispersion characteristics. When stimulated Raman scattering (SRS) corresponding to the excitation of the fundamental mode becomes dominant in this few-mode fiber, an efficient cascaded SRS-based SC is obtained with five Stokes peaks ranging from 2 μm to 3.4 μm. Results from numerical simulation are in accord with the experimental results, showing that it is feasible to obtain an SRS based mid-infrared SC in a step-index As2S3 fiber by using a 2 μm high peak power picosecond laser to pump directly.
© 2016 Optical Society of America
Introduction
After a series of successes in the field of mid-infrared supercontinuum (SC) generation [1–4], it has become desirable to develop SC techniques for generation of longer wavelength. Particularly, chalcogenide fibers benefit from their broad mid-infrared transmission region and high nonlinearities so that could be much more suitable for mid-infrared SC generation. Among various types of chalcogenide fibers, As2S3 fibers and As2Se3 fibers are the most well-known ones. It is known that a broadband SC can be obtained by pumping a nonlinear fiber in the anomalous dispersion regime close to zero-dispersion wavelength (ZDW). Since the ZDWs of the material in the As2S3 fibers and As2Se3 fibers locate at 4.8 μm and 8 μm respectively [5], some effective post-processing technologies or appropriate pump sources operating at long wave infrared region become essential. Through dispersion engineering like designing tapered fibers and suspended-core fibers, the ZDWs of chalcogenide fibers can be changed to near infrared band or shortwave infrared region [6, 7]. More specifically, by tapering the outer diameter of the multimaterial chalcogenide fiber to approximately two microns, the ZDW of the fiber was converted to below 3.3 μm [6]. When pumping the tapered fiber with 100 fs short pulses at 3.4 µm, a SC spanning from 1.5 μm to over 4.8 μm has been demonstrated. However, such tapered fibers and suspended-core fibers are relatively fragile with high insertion losses, which would limit their usage in commercial applications.
Adopting an optical parametric amplifier (OPA) operating at long wave infrared region is another approach to dealing with this problem. When pumping the chalcogenide fiber in the anomalous dispersion region with femtosecond pulses, the long wavelength generation is dominated by soliton-related propagation effects, which is of benefit to long-wavelength generation. In the first example, an SC spanning from 1.6 µm to 5.9 µm at −20 dB level with the average power of 8 mW was generated from an As2S3 fiber pumped by an optical parametric chirped pulse amplifier operating at 3.1 μm [8]. Last year, by pumping an 11-cm-long chalcogenide fiber with low pump peak power (~3 kW at 4 μm) from an OPA, Yu et al reported a SC source spanning from 1.8 μm to 10 μm [9]. To date, the longest wavelength of SC has been achieved in an ultra-high numerical aperture (NA) step-index chalcogenide fiber. The system was based on a noncollinear difference frequency generation unit pumped by an OPA to produce intense ultra-short pulses with a central wavelength of 6.3 μm. A broadband SC source from 1.4 μm to 13.3 μm was obtained when the pump peak power reached about 2.29 MW [10].
However, the above-mentioned pump sources also have their limitations such as complex structure and low average power available. As a result, the average output power of these SC has so far been limited below 10 mW. Therefore, to achieve an SC with higher output power and wider bandwidth, alternative approaches need to be proposed. When a picosecond shortwave infrared fiber laser was used as the pump source in the normal dispersion regime, the generated spectrum is mainly attributed to stimulated Raman scattering (SRS) in a chalcogenide fiber. In the work of Gattass et al, a 2-m-long step-index As2S3 fiber with the diameter of 10 μm was pumped at 2.45 μm [11] which produced a high-power mid-infrared SC with an output power of up to 565 mW and the spectrum spanning from 1.9 μm to 4.8 μm. However, the pump wavelength was shifted to 2.45 μm with another nonlinear fiber firstly, indicating that the efficiency of the power transferring was limited by the high loss from silica absorption. Therefore, it brings a question that whether it is possible to obtain such a broadband SC source directly pumped by a near-infrared short pulse source? In fact, Richard T. White et al reported a cascaded Raman shifting in large-core chalcogenide fibers with near-infrared nanosecond pump pulses at 1.9 μm in 2011 [12]. With the pump peak power of 95 kW, the long wavelength edge of the output spectrum was shifted beyond 2.6 μm which corresponds to the generation of 4 orders cascaded Raman Stokes waves with a total output power of 1.1 mW. However, the whole system was not optimized further which might limit its performance for long-wavelength generation. Recently, with the advancement of Tm-doped fiber lasers (TDFL) technology, pulses of higher peak power at 2 μm could be produced, which gives hope to the obtaining of SRS of more orders by directly pumping at 2 μm, thus achieving a broadband mid-infrared SC.
In this study, we investigate mid-infrared SC generation in a step-index As2S3 glass fiber directly pumped by a picosecond TDFL at 2 μm from both experiments and numerical simulations. The core diameter of the step-index As2S3 fiber is 9.2 μm (170 μm cladding) while the NA of it is 0.3. Given the relative large core diameter and high NA, the As2S3 fiber is few-moded at 2 μm. By adjusting the butt-coupling position to avoid excitations of higher order modes and cladding mode meanwhile increasing coupled power of fundamental mode, an efficient cascaded SRS related SC with at least five Stokes peaks and 366 mW output power was achieved. Numerical simulating results are in accord with the experimental results.
2. Experimental setup
The experimental setup is shown in Fig. 1. The pump source was a 2050 nm TDFL similar to the laser source [13] with a pulse duration and a repetition rate of 32 ps and 1 MHz. The fundamental mode field diameters of the output fiber of the TDFL and As2S3 fiber were 11.5 μm and 7.52 μm. Considering the mode field mismatch between these two fibers, a short piece of ultra-high numerical aperture (UHNA) fiber was spliced to the output fiber of the TDFL as the matching fiber. Then the end of the UHNA fiber was butt coupled to the As2S3 fiber. Both of the fibers were perpendicularly cleaved and placed in a pair of parallel V-grooves. The air gap between the two fibers was small enough to minimize the coupling loss and the divergence of the light beam. The output beam of the As2S3 fiber was collimated by a black diamond (BD) infrared lens (f = 3.05 mm) with an antireflection coating for 1.8-3 μm band. A liquid nitrogen-cooled HgCdTe (MCT) detector embedded in the monochromator was applied to detect the spectrum with the aid of a chopper and a lock-in amplifier to obtain high dynamic range in the SC spectrum measurement.
Fig. 1 Experimental set-up for generating and measuring MIR SC. UHNA: ultra-high numerical aperture; BD lens: black diamond lens; MCT detector: liquid nitrogen-cooled HgCdTe detector.
3. Result and discussion
Due to the relatively large core diameter and high NA of the step-index As2S3 fiber, the coupling efficiency and the power handling were greatly improved. However, the cutoff wavelength of the fiber was calculated to be 3.5 μm, which means that it would support few-mode operation at 2 μm. LP11, LP21 and LP02 were the corresponding higher-order modes with the group velocity dispersions of these modes presented in Fig. 2. Note that all modes possessed normal dispersion over the mid-infrared region, with the ZDW of the fundamental mode calculated to be 6.7 μm. The Sellmeier equation of bulk As2S3 was used with the same in [14]. For the case of SC generation in normal dispersion regime pumped by picosecond pulses, the dominant broadening mechanism would be attributed to self-phase modulation (SPM), four-wave mixing (FWM) and SRS [15].
Fig. 2 Calculated group velocity dispersions of the four dominant guided modes of the As2S3 fiber, LP01 (purple) LP11 (blue) LP21 (red) and LP02 (black)
Figure 3 depicts the output spectral evolution while the V-groove was aligned precisely in the X direction with the pump peak power of 2 kW. In position 1, the output spectrum contained only the pump light when the X-direction position was deviated from the optimal position where the incident pump light is absolutely vertical to the core of the As2S3 fiber. After careful adjustment of the UHNA fiber with the aid of a power monitor at the end of the As2S3 fiber, several sidebands appeared at 2037.5 nm and 2062.5 nm along with the generation of first order Stokes wave as shown in position 2 of Fig. 3. When continuing adjusting the V-groove in X direction only, the sidebands became weak while the SRS was strengthened as shown in position 3 of Fig. 3. When SRS became the dominant nonlinear effects, the strength of first order Stokes waves peaked as shown in position 4 of Fig. 3. Once the position was deviated from position 4, SRS would become weak and the sidebands were strengthened as shown in position 5 and 6 of Fig. 3. Finally the output spectrum contained only the pump light as shown in position 7 of Fig. 3. The same process can be repeated in the Y direction. Noted that while there were great changes happened on spectrum during the adjustment process, little fluctuation occurred in the output power of the As2S3 fiber. When the pump peak power was higher than 2 kW in Position 4, the second-order Stokes peak at around 2380 nm could be observed, but it can hardly be acquired by increasing the pump power in other positions. Thus position 4 was identified as an optimized position for long-wavelength generation.
Fig. 3 Spectral evolutions in the few-mode fiber with pump peak power of 2 kW pumped at 2 μm while adjusting the coupling position in the X direction.
The output spectrum showed regular changes at a constant coupling efficiency. The peak located at 2205 nm was identified as the Stokes peak of the fundamental mode corresponding to 10.3 THz Stokes frequency shift in As2S3 fibers [16]. There was also a pair of sidebands located at 2037 and 2062 nm on both sides of the pump wavelength. With the adjustment of coupling position, the output power of the As2S3 fiber remained stable while great changes existed on the spectrum, suggesting that the generation of the pair of sidebands might relate to the excitations of higher order modes, such as LP11 and LP21.
The fibers were carefully prepared for the purpose of guaranteeing the quality of the end surfaces. However, the ideal perpendicular end faces could not be obtained. Besides, although the V-grooves were used to keep the two fibers in parallel as close as possible, ideal coaxial cannot be kept. Therefore, it is practicable to alter the power rates of different modes in the effectively few-mode fiber by adjusting the position of V-groove. Since the mode fields of higher-order modes were larger than that of the fundamental mode, the nonlinear coefficients of them would be lower. As a result, the excitations of higher order modes were not beneficial to spectral broadening to the long wavelength region and would reduce the coupled power of the fundamental mode. The mentioned above would be the reason for the variation of the output spectrum during the adjusting process. Considering that the pair of sidebands on both sides of the pump wavelength always appeared at the same time, FWM of higher-order modes and inter-mode FWM are identified as the reason of the sidebands generation. Additionally, the existence of cladding mode is inevitable, which is another factor that influences the bandwidth of the SC. Consequently, in order to generate efficient cascaded SRS-induced SC, the coupling position should be well adjusted to avoid the excitations of higher order modes and cladding mode meanwhile increase the coupled power of the fundamental mode.
Figure 4 shows cascaded SRS generation in the As2S3 fiber with different pump peak power at the optimized condition. The incident peak powers were 1.6, 2.11, 3.23, 4.96, 6.29 and 11.8 kW corresponding to the average pump power of 62.63, 82.15, 126, 193.5, 245 and 460 mW respectively. The first order Stokes light centered at 2205 nm was detected at the peak power of 1.6 kW. With an incident peak power of 2.11 kW, the second order stokes light was formed and it became stronger as the pump power increased to 3.23 kW. When the peak power of the input pulses reaches 4.96 kW, the long wavelength edge of the output spectrum was shifted beyond 3 μm which can be attributed to 5th Stokes shifts (at ~2.8 μm). With the pump power been continually increased to 6.29 kW, the spectral widths of the generated higher-order Stokes waves got wider and the process of spectral broadening began to slow down. With 30 ps pump pulses at 2 μm, the walk-off lengths for the pump and Stokes wavelengths were calculated through the formula [17], in which TFWHM is the pump pulse width, Vg is the group velocity, λ1 and λ2 are the wavelengths of the pump and Stokes waves. All of the walk-off lengths among the pump and the Raman peaks were longer than 1.5 m, ensuring efficient energy shifting among these lights.
Fig. 4 Spectral evolution of the SC as a function of input pump peak power (measured output power) with a pump pulse width of 32 ps pumped at 2 μm.
Figure 5 depicts the SC output spectrum at the optimized position with the maximum pump peak power. It is seen that when the pump peak power reached 11.8 kW, the long wavelength edge of the output spectrum was shifted to 3.4 μm with 366 mW total output power at 460 mW average pump power (the loss from Fresnel reflection (1.62 dB) removed). The result demonstrated that when the UHNA fiber and the As2S3 fiber was aligned properly to control the excitations of higher-order modes and cladding mode, the cascaded SRS effect could be greatly strengthened in the As2S3 fiber. To certify our analysis, an infrared-camera was adopted to observe the beam shape so as to detect the mode operations in the effectively few-mode fiber. In the inset of Fig. 5, the output beam profile corresponding to the spectrum at all wavelengths was shown in the inset (a). It can be seen that the output beam mainly appeared to be fundamental mode at all wavelength. With the help of long-pass filter and attenuator, the output beam profile of the spectrum above 2.4 μm was measured. It is depicted that the light of long wavelengths was still confined in the core (as shown in the inset (b) of Fig. 5). However, after slightly adjusting the coupling point, significant excitations of higher order modes were observed as shown in the inset (c) in Fig. 5. Meanwhile, the output spectral bandwidth was greatly reduced.
Fig. 5 Measured spectrum of the cascaded SRS in the 1.7-m-long As2S3 fiber at the optimized position with the pump peak power of 11.8 kW. Inset: (a) Output beam profile for all wavelengths, (b) Output beam profile for wavelengths above 2.4 µm only, (c) Output beam profile for wavelengths above 2.4 µm only after slightly adjusting the coupling position.
In order to confirm that the generated spectrum is mainly attributed to cascade SRS in the step-index As2S3 fiber numerically, we solved the generalized nonlinear Schrödinger equation (GNLSE) for the propagation of the fundamental mode. The dispersion and the nonlinear coefficient of the As2S3 fiber for the fundamental mode are shown in Fig. 6. The Raman response function was used with the same in [18]. A value of n2 = 0.9 × 10−18 m2/W and a Raman fraction of fR = 0.18 were used. The material loss for the As2S3 fiber was neglected. The spectral broadening was simulated by adjusting the peak power of 30 ps pulses at 2050 nm. Figure 7 shows the evolution of the spectrum at a peak power of 1.2 kW. The system provides an SC spanning from 2.0 μm to 3.4 µm at the −30 dB level that agrees with the experimental result. As the evolution of the spectrum shows, with light propagating along the fiber, discrete Raman peaks were generated gradually and finally combined to form an SC. As the pump power was further increased to 2 kW, the spectral evolution is shown in Fig. 8, revealing that the spectrum expanded beyond 4.2 μm at the −30 dB points through the generation of higher-order Stokes peak.
Fig. 6 (a) The chromatic dispersion profiles and (b) nonlinear coefficient of the As2S3 fiber for the fundamental mode.
Fig. 7 Simulated results of SC generation in the As2S3 fiber when pump peak power is 1.2 kW with a pump pulse width of 30 ps at 2 μm. (a) Evolution of the spectrum along the fiber. (b) Spectral slices at the end of the fiber.
Fig. 8 Simulated results of SC generation in the As2S3 fiber when pump peak power is 2 kW with a pump pulse width of 30 ps at 2 μm. (a) Evolution of the spectrum along the fiber. (b) Spectral slices at the end of the fiber.
Note that the peak power required for the same spectral broadening in the numerical simulation is quite different from that in the experiment. At the same spectral broadening from 2 μm to 3.4 μm, the simulation needs a peak power of 1.2 kW while the experiment require peak power of 11.8 kW. The discrepancy can be explained as follow: Firstly, these SC modelling results are based on the assumption that all pulse energy is coupled into the fundamental mode of the fiber. When higher-order modes and cladding mode are excited in the few-mode fiber, the power of the fundamental mode will be reduced. As cascaded SRS of the fundamental mode is the dominant spectral boarding mechanism, the existence of higher-order modes and cladding mode are detrimental for spectral boarding, which can also explain why the pump residues of the SC we got in the experiment is much higher than those of the simulation results. Recent simulations done by Irnis Kubat et al. also point out that the excitations of high modes have a significant impact on SC generation [19]. Once the excitations of high modes were taken into consideration, much less spectral broadening could be obtained. Secondly, the pump wavelength located in the shortwave infrared region, which is under the influence of several characteristic absorption peaks in mid-infrared region of the fiber [20]. As we know, a piece of interaction length between different Stokes waves is necessary for the generation of SRS, thus the spectral redshifts will be affected by the loss peaks. Thirdly, the nonlinear optical properties of the As2S3 fiber are various in the current literature available [18, 21–23]. Thus a measured transmission loss spectrum and detailed nonlinear optical properties of the fiber used in the experiment are vital for the accuracy of numerical simulation. On the whole, both numerical simulations and experiment have proved that a broadband SC could be obtained in a step-index As2S3 fiber pumped at 2 μm directly. Numerical simulations are conducive to acquiring a clear perspective of experimental research, revealing that a low-loss, robust fusion splicing of silica fiber to chalcogenide fiber is expected to achieve a better mode control. Moreover, a pump source with high peak power and low pulse energy is needed, which could avoid laser-induced damage of the As-S fiber.
4. Conclusion
In conclusion, both numerical simulation and experiment have demonstrated that it is possible to generate a mid-infrared SC in a step-index As2S3 fiber pumped by a picosecond TDFL at 2 μm. By carefully adjusting the butt-coupling position to ensure a dominant state for SRS effect in this few-mode chalcogenide fiber, an efficient cascaded SRS related SC with at least five Stokes peaks is realized with the output power of 366 mW.
Acknowledgment
This research is funded by “National Natural Science Foundation of China” (61435009, 61235008 and 61405254) and “National High Technology Research and Development Program of China” (2015AA021101).
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