Broadband mid-infrared supercontinuum generation in 1-meter-long As 2 S 3-based fiber with ultra-large core diameter

In this study, the supercontinuum (SC) generation in a 1-m-long As2S3 fiber with a 200 μm core diameter was demonstrated experimentally. The high-purity As2S3 fiber we used exhibited very low optical loss with a background loss of approximately 0.1 dB/m at a wavelength of 2–5 μm. SC generation was studied by pumping the fiber at different wavelengths and different peak powers. A strong spectral broadening with a 30 dB spectral flatness spanning from 1.4 to 7.0 μm was obtained when the fiber was pumped with 150 fs short pulses at 5.0 μm. The SC generation in bent fiber was also studied. The result showed that the bending radius of the fiber will significantly affect the SC spectra bandwidth and the output power. The SC spectra in the used fiber could still be maintained when it was bent to a radius of 5 cm. © 2016 Optical Society of America OCIS codes: (190.4370) Nonlinear optics, fibers; (060.2390) Fiber optics, infrared; (140.3070) Infrared and far-


Introduction
Recent years have seen a growing interest in producing broadband supercontinuum (SC) sources in mid-infrared (MIR) regions for several applications [1,2], such as biomedical sensing [3], metrology [4], spectroscopy [5], optical tomography [6], and microscopy [7].The typical route to achieve a broadband SC source involves sending ultrafast laser pulses into dispersion-engineered MIR transparent fibers.The broadening process also involves the Kerr effect, Raman effect, and dispersion [8].Chalcogenide glass fibers are excellent candidates for broadband SC generation because of their excellent MIR transparency and high thirdorder nonlinearity [1,9].
Efficient and broadband SC generation can be obtained by pumping in the anomalous dispersion regime close to the material zero-dispersion wavelength (ZDW) of the fiber [10].Since bulk chalcogenide glass usually has a ZDW in the MIR region; for instance, the ZDW of As 2 Se 3 is at ∼7.4 μm [10] and that of As 2 S 3 at ∼5.0 μm [11], considerable efforts have been made to tune ZDW to short wave region in order to produce all-fiber mid-infrared supercontinuum sources from commercially available fiber lasers [12][13][14].Dispersion engineering can be realized by scaling down the fiber core to a few micrometers, either by tapering the fiber [15][16][17][18][19][20][21][22] or through suspended-core microstructured fibers [23][24][25][26][27]. Tapered and suspended-core chalcogenide fibers, which allow enhanced nonlinearity and dispersion engineering, have been successfully demonstrated for low-threshold, octave-spanning SC sources in near-infrared (NIR) and MIR regions.However, tapered fibers are more likely to be damaged, when pumped with high peak power lasers.Suspended-core structures are also associated with difficulties of fabrication and power handling at present.Except for the factors of dispersion and transmission losses, most SC generation based on tapered and suspended-core chalcogenide fibers are limited to 1-5 μm spectral region by pumping with NIR sources, which are far from the transmission limit of these materials [19][20][21][22][23]. Therefore, a long pump wavelength is essential to extend the SC spectrum to a wavelength of ∼10 μm [28][29][30][31].
Broadband SC generation in the chalcogenide fibers has been recently achieved by employing a tunable optical parametric amplifier (OPA).Zhang et al. achieved SC generation spanning 1.8-9.8μm by pumping a 13.5 cm step-index small-core fiber with ∼320 fs pulses at 4.1 μm [32].Petersen et al. extended SC generation to 13.3 μm using an 85 mm large-core As 2 Se 3 optical fiber pumped with MW pulses at 6.3 μm [33].Dai et al. generated a 1.8-14 μm SC source in a 15 cm long Ge-Sb-Se fiber by pumping at 6.0 μm [34].Chen et al. achieved SC broadening of 2.0-15.1 μm by pumping a 3 cm step-index As 2 Se 3 with ∼170 fs pulses at 9.8 μm [35].Among these works, short fibers with a centimeter scale are widely used, but SC generation in bending fibers is barely studied.However, in many applications where long propagation lengths or random changes in the direction of the light are required, such as remote sensing or the connection of different devices, long fibers many be better alternative that SC generation and propagation can be achieved in the same fiber.However, the use of long chalcogenide fibers for broad SC generation is now still limited because of the facts that, an SC spectrum from a few-centimeter-long chalcogenide fiber is broad enough due to large nonlinearity of the material [29][30][31][32][33][34][35], and the fabrication of high-quality chalcogenide fibers with extremely low optical losses in the long wavelength region is challenging.
Recently, Gattass et al. studied the SC generation in 2-m-long step-index core-clad As 2 S 3 fiber with a 10 µm core diameter, and SC spanning from 1.9 to 4.8 µm was obtained [14].However, the intrinsic fiber loss due to the multiphoton absorption in the long fiber showed great limitations for the further broadening of the SC spectra.Théberge et al. performed a SC generation in low-loss and large-core As 2 S 3 fibers with different fiber lengths and transmission losses [28].The results showed that the losses and the length of the fiber have great effects on SC generation.However, the effect of bending of long fiber to the SC generation in chalcogenide fibers is rarely studied, but this is practically important for the application of the SC sources.
We aimed to develop practical SC sources based on chalcogenide fibers for various applications.Therefore, we explored SC generation in a 1-m-long low-loss As 2 S 3 step-index fiber with a core diameter of 200 μm in this study, and performed the investigation on the effect of the bending of the fiber to the SC generation.The used high-purity As 2 S 3 fibers exhibited a background loss of approximately 0.1 dB/m and the lowest optical losses of 0.06 dB/m at 2.8 µm and 0.09 dB/m at 4.8 µm.The large core diameter of the fiber could significantly reduce the coupling difficulty; it could also endure much higher power.SC was generated by pumping the fiber at different wavelengths from 3.5 μm to 6.0 μm with ∼150 fs pulses from an OPA.A strong spectral broadening with a 30 dB spectral flatness spanning from 1.4 to 7.0 µm was obtained when the fiber was pumped at 5 µm.Especially, the SC generation in bent fiber was also studied.We demonstrated the SC generation in the fiber with different bending radius.It was found that the SC spectra in the used fiber could still be maintained when the fiber was bent to a radius of less than 5 cm.

Experimental setup and optical parameters of fiber
A low-loss step-index multimode As 2 S 3 fiber (IRflex, IRF-S-200) was used in our experiment.It has a core diameter of 200 µm and a cladding diameter of 250 µm.The fiber was protected by an acrylic material coating to improve its mechanical flexibility, so the fiber can be bent to extremely small diameter.The numerical aperture (NA) of the fiber is about 0.28-0.30between 1.5 to 6.5 μm.In the experiment, the arcylic coating was first softened in dichloromethane (CH 2 Cl 2 ) and then stripped carefully.The bare fiber was then cleared with isopropyl alcohol and cleaved for measurement.The transmission losses of the fiber were measured by the cut-back method with a Fourier-transform infrared spectrophotometer (FTIR, Nicolet 5700, USA) with the help of an external HgCdTe (MCT) detector cooled with liquid nitrogen.Figure 1 shows the transmission loss of the fiber.It was found that the fiber exhibited a high optical transmission in the MIR region, which had a background loss of approximately 0.1 dB/m, except at the S-H absorption peak at around 4.1 μm.The effective refractive indices of the fiber core and the GVD curves of the fiber were calculated using the infinitesimal method [36], as shown in Fig. 2. The ZDW of the fiber used in our work was approximately 4.9 μm.The pump was coupled into the fiber with calcium fluoride lens.The output SC from the fiber was injected into the input slit of a monochromator, and the boxcar integration system was used to improve the signal-to-noise ratio.
The fiber was pumped with a tunable OPA system (Mirra 900 + Legend Elite + OperA Solo).The pump pulses had a duration of ∼150 fs (full width at half maximum [FWHM]) and a repetition rate of 1 kHz.The experimental setup is shown schematically in Fig. 3.The beam from the OPA was first sent through a polarizer pair to control the polarization and power.The light was then coupled via calcium fluoride lens with a focal length of a 75 mm into the 1-m-long As 2 S 3 fiber.The output SC spectrum from the end of the fiber was directly injected into the input slit of grating monochromator with a liquid nitrogen-cooled MCT detector (FPAS, Infrared systems development, USA).Given that the output NA of the fiber was not excessively large (0.28~0.30), it could match the focal length of the monochromator without collimation.Long-pass filters were applied as order-sorting filters to eliminate high-order signals.The monochromator was equipped with a 75 lines/mm diffraction grating that provided a spectral resolution around 10 nm.The MCT detector can provide the spectral measurement range of 1.0 to 16 μm.The signal of the detector was processed with a boxcar integration system before it was recorded automatically with a custom Labview program to obtain a high dynamic range.

Experimental results and discussion
The effects of pumping in the normal and the anomalous GVD regions were first observed.Figure 4 shows the measured SC spectra when pumped at different wavelengths from 3.5 μm to 6 μm.The coupled peak power of about P = 3 MW into the fiber was kept constant for all these tests.This coupled peak power into the fiber was evaluated with the measured output power and fiber loss.From Fig. 1, it is shown that the average loss of the fiber in the whole transmission range is estimated to be about 0.3 dB/m.So the coupled power is evaluated by 1.1 times of the output power from the end of the fiber.When the fiber was pumped at 3.5 μm in the normal dispersion region, the SC spanned from 3.0 μm to 4.5 μm at the 30 dB level.The spectrum spanned from 2.4 μm to 5.6 μm, when the pump wavelength was shifted from 3.5 μm to 4.0 μm.SC generation in this large core and multi-moded fiber was more complex than the case in single mode fiber.SPM, four-wave mixing (FWM), and Raman effects induce the initial spectral broadening before dispersion causes temporal laser pulse fission.As the peak power of the input pulse is about 10 times higher than the critical power for self-focusing (P cr ≅0.3 MW) at 5 μm in As 2 S 3 , the self-focusing distances for a 200 μm diameter laser beam is just several millimeters [28].The balance of self-focusing and plasma defocusing contributes to the dynamic equilibrium of laser filamentation in large core As 2 S 3 at MIR wavelengths.Then the laser pulse fission into multiple subpulses or solitons, the individual solitons are redshifted by the RSS effect and the emission of the phase-matched dispersive waves at wavelengths shorter than the ZDW [28,37].The solitonic long wavelength edge would move to longer wavelengths when the pump wavelength is further shifted to the region close to the ZDW of the fiber (∼4.9 μm), and the short wavelength edge consisting of the phase-matched dispersive waves shiftes accordingly to short wavelengths.Thus, the broadest SC spectrum spanning from 1.4 μm to 7.0 μm was obtained when pumping at 5 μm, which was close to the short-wave transmission limit of the fiber.The SC broadening in the fiber began to be compressed when the pump wavelength was further shifted away from the ZDW to a long wavelength at 6 μm.This phenomenon may be caused by the larger dispersion at the longer pump wavelength as shown in Fig. 2, which may impede initial broadening due to SPM and FWM, and further affect the soliton dynamics at the long-wavelength edge and the corresponding formation and blueshift of the dispersive waves at short wavelength.Therefore, in order to maximize the spectral broadening, the initial laser central wavelength should be close to the fiber ZDW, but slightly shifted toward the anomalous dispersion.
Figure 5 presents the measured SC spectra with different incident powers by pumping at 5 μm.In general, spectral broadening increased continuously when the pump power increased.For the lowest peak power of P = 50 kW, the broadening of the SC spectrum was very weak.As the pump peak power increased from 50 kW to 3 MW, we observed a significant increase of the blueshift.This increase attributes to the dependence of generation and development of dispersive waves at the short wavelength on the peak power of the input pulse.The SC spectrum extended to 1.4 μm at the 30 dB level when the pump peak power increased to P = 3 MW.This level was close to the wavelength where the loss started to rise (see Fig. 1).The largest SC spanning from 1.4 to 7.0 μm was obtained at the peak power of 3 MW.Minimal broadening was yielded by further increasing the pump peak power beyond 3 MW.The SC generation in our fiber was also studied by numerically solving the generalized nonlinear Schrödinger equation with an adaptive split-step Fourier routine [38].Figure 6(a) shows the simulated SC evolution along the fiber when the pump has a peak power of 3 MW at 5 µm in the anomalous dispersion region.Figure 6(b) presents the obtained SC spectra profile in the fiber with different fiber lengths of 2 cm, 4 cm, 8 cm, 50 cm and 1 m, as marked in Fig. 6(a).It was seen that the SC broadens quickly in the first 5 cm of the fiber, and reaches maximum at about 8 cm.After then, the broadening will no longer continue, but the SC spectrum maintains the bandwidth with an increase in the length in the fiber.The black line in Fig. 6(b) presents the experimentally obtained SC spectra at the end of the 1 m fiber, which was agree with the simulation results.Thus, a few centimeters of this chalcogenide fiber is long enough to generate broadband SC.However, in many practical applications where long propagation length and the flexibility to bend the fiber are required, a low-loss fiber with a length of tens of centimeters to a few meters becomes considerably important.The advantage of generating SC in the large core fiber included the possibility of injecting high-energy laser pulses into fibers to achieve high-power output.The long fibers possessed the flexibility of bending to adjust the direction of light propagation.In this work, we carried out the output power measurement from the fiber as the function of the bending radius.The input peak power of 3 MW was kept constant in all the tests, the output power with different bending radius was measured and normalized to the output from the straight fiber.For the straight fiber, the output average power was recorded to be only 0.41 mW due to the low 1 kHz pump repetition rate.However, the average output power can be scaled up by increasing the repetition rate using a megahertz OPA system [30,31].The results are shown in Fig. 7(a).It was found that the output power from the fiber almost unchanged when the bending radius was larger than 10 cm, then reduced slowly with further decreasing bending radius.The measured output power could still exceeded 94% when the fiber was bent to a radius of 5 cm.When the bending radius was further reduced, the output power decreases rapidly, only half of the output power was left when the fiber was bent to a radius of 1.5 cm. Figure 7 (b) presents the spectral distributions of the generated SC spectra in the straight and bent fibers.It was found that, nearly identical SC spectral broadening could be achieved when the fiber was bent to a radius of 5 cm, which indicates the excellent mechanical flexibility of the fiber used in our work.After then, the SC broadening in the fiber began to be compressed when the fiber was further bent, especially in the long wavelength region.It is known that the mode area in the fiber will increase dramatically when moving to long wavelength region, and the higherorder modes will also begin to leak when the fiber was bent to small diameter; this will lead to an increasing bending loss and decreasing nonlinear coefficient, which is harmful to the SC broadening and propagation.Since soliton division and frequency-shift effect plays a major role in the mid-infrared SC broadening, the decreasing of nonlinear coefficient in the midinfrared region will slow down the speed of soliton self-frequency shift, thereby limiting the spectral broadening.In the case of bent fiber, when the soliton center frequency is close to the bending loss border, soliton self-frequency shift will be suppressed, and the spectral broadening is stopped.Therefore, this effect shows a possibility to control the shape of the SC spectra with bent fibers.

Conclusions
In conclusion, we have presented experimental results on SC generation in a 1-m-long lowloss As 2 S 3 fiber with a core diameter of 200 μm.The fiber was pumped at different dispersion regions from 3.5 μm to 6.0 μm with 150 fs pulses from an OPA source.The 30 dB spectral flatness of the generated SC spanning from 1.4 to 7.0 μm was obtained by pumping at 5 μm.This long fiber possesses good mechanical flexibility, which can be bent to an extremely small diameter.The SC generation in bent fiber was also studied, which showed that the bending radius of the fiber will significantly affect the SC broadening width.SC broadening in the used fiber could still be maintained when the fiber was bent to a radius of 5 cm.These properties showed great potential for some applications, such as remote sensing and spectroscopy.

Fig. 2 .
Fig. 2. The calculated effective refractive indices of the As 2 S 3 core (black line) and the group velocity dispersion curves of the fiber (red curve).The inset was the intensity distribution of the LP 01 mode in the fiber.

Fig. 3 .
Fig.3.Experimental setup of the MIR SC generation measurement.The MIR pump source was generated with a noncollinear difference frequency generation (NDFG) unit pumped through an OPA.The pump was coupled into the fiber with calcium fluoride lens.The output SC from the fiber was injected into the input slit of a monochromator, and the boxcar integration system was used to improve the signal-to-noise ratio.

Fig. 4 .
Fig. 4. The SC spectra generated in 1 m As 2 S 3 fiber pumped at different wavelengths.The red and black arrow represents the position of the pump pulse and that of the ZDW, respectively.

Fig. 5 .
Fig. 5. Spectra of the SC generated in 1 m As 2 S 3 fiber at different pump powers.

Fig. 6 .
Fig. 6.(a) Simulation of the SC evolution when pumping with a peak power of 3 MW at 5 µm and (b) simulated SC spectrum with different fiber length, the black line is the experimentally obtained SC spectra at the end of the 1 m fiber.

Fig. 7 .
Fig. 7. (a) The normalized output power with the decrease of bending radius and (b) the measured spectra of the SC generated in straight and bent As 2 S 3 fiber.