We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation spanning to 15.1 μm in a 3 cm-long chalcogenide step-index fiber. The pump source is generated by the difference frequency generation with a pulse width of , a repetition rate of , and a wavelength range tunable from 2.4 to 11 μm. To the best of our knowledge, this is the broadest MIR SC generation observed so far in optical fibers. It facilitates fiber-based applications in sensing, medical, and biological imaging areas.
© 2016 Optical Society of America
Supercontinuum (SC) generation in optical fibers was first observed in 1976 . Since then, researchers have been engaged in developing optical devices operating at wavelengths in the visible light and near-infrared (NIR) region [2–7]. However, to further realize SC spectral evolution into the mid-infrared (MIR) region in silica optical fibers is a challenging task due to the strong material absorption above 2.4 μm. In order to address this limit, the pursuit of suitable host material is attracting growing interest, and many candidates have been proposed, such as fluoride, tellurite, and chalcogenide glasses [8–13]. To date, the broadest MIR SC generation in fluoride fibers is from ultraviolet to 6.28 μm , and the broadest in tellurite fibers is from 789 to 4870 nm . Chalcogenide glasses prove to be a more promising candidate, for they present a wider transparency window over 20 μm, and possess a higher nonlinear material index up to tens or hundreds of times as those of fluoride and tellurite glasses [11,16–20]. Numerical simulations have demonstrated chalcogenide fibers’ potential for MIR SC generation [21–24], and Petersen et al. in 2014 experimentally observed the broadest MIR SC spectrum, spanning 1.4 to 13.3 μm . Based on previous work, we strive to extend the SC evolution in MIR region from the following aspects: designing chalcogenide fibers with near-zero flattened dispersion, shifting the pump wavelength to the long wavelength region, and decreasing the fiber length to reduce the loss.
In this Letter, we demonstrate MIR SC generation in a 3 cm-long chalcogenide step-index fiber. The step-index fiber with near-zero flattened dispersion was designed based on and , and fabricated by the rod-in-tube drawing technique. The pump source was generated by the difference frequency generation (DFG), which had a pulse width of , a repetition rate of , and a wavelength range tunable from 2.4 to 11 μm. The resulting SC generation was investigated both experimentally and numerically; both methods exhibited agreement with each other.
The fiber design and optimization were carried out to achieve features of high nonlinearity and near-zero flattened dispersion. and glasses were selected for the core and cladding respectively, because they have good compatibility, higher nonlinear index, and wider transparency window compared with other chalcogenide glasses ( and ) [26,27]. Figure 1(a) shows the measured linear material refractive indices of two glasses, as well as the numerical aperture (NA). For the chalcogenide step-index fiber, the variation of the dispersion with the change of the core diameter was analyzed using the full-vectorial mode solver of a commercial software (Lumerical MODE Solution), as shown in Fig. 1(b). We can see that the number of zero-dispersion wavelength (ZDW) reduces from two to one, with the core diameter increasing from 11 to 17 μm. For fibers with two ZDWs, there is a possibility that red-shifted dispersive waves may be emitted by solitons over the second ZDW region. However, the second ZDW would definitely restrain the soliton evolution. Taking this into consideration, fiber with the diameter of 15 μm, one ZDW, and near-zero flattened dispersion was selected. The resulting ZDW was calculated to be and the wavelength range between the dispersion of was from to 20 μm. Figure 1(c) shows the variation of confinement loss with the change of the core diameter, which confirmed that the chalcogenide step-index fiber with the diameter of 15 μm can support MIR transmission.
The chalcogenide step-index fiber was fabricated by the rod-in-tube drawing technique, and the and glass rods were offered by Furukawa Denshi Co., Ltd. The fabrication required four steps. Step 1, an 8 cm-long rod with the diameter of , was ultrasonically drilled to form a tube with a 3.5 mm-diameter hole in the center. Step 2, an 8 cm-long rod with the diameter of , was elongated to the diameter of and inserted into the tube produced in Step 1. Step 3, the tube with rod in the hole, was elongated to get a preform with the diameter of . Finally, the preform was inserted into another tube with the hole diameter of , and drawn into the fiber at the temperature of . During the fiber-drawing process, the nitrogen gas pressure was set as negative to avoid interstitial hole formation. Figure 2(a) shows photos of the tube, the initial rod, and the elongated rod. Figure 2(b) is the measured rod loss and the transmission spectrum of a 2 mm-thick glass sample. The former was obtained through the cut-back technique, and the latter was recorded using a Fourier-transform infrared (FT-IR) spectrophotometer (PerkinElmer Spectrum 100) in the infrared range of 2.5–25 μm. We can see there are several absorption bands from 2.5 to 19 μm, which correspond to the residual O-H, As-O, Se-O, and Se-H pollution in the glass. In particular, the loss resulted from Se-H absorption band centering around 15.2 μm is prominently strong. Consequently, in order to minimize the influence from the loss, the fiber length was reduced to 3 cm in this experiment. Figure 2(c) shows the cross section of the chalcogenide step-index fiber taken by a scanning electron microscope (SEM), in which the core diameter was measured to be . Based on the nonlinear index , the effective mode areas and the nonlinear coefficients of the fundamental mode from 2 to 20 μm were calculated, as shown in Fig. 2(d).
The experimental setup for SC generation in the 3 cm-long chalcogenide step-index fiber is shown in Fig. 3(a). The MIR pump source started from a Ti:Sapphire mode-locked seed laser (Coherent Mira 900), which delivered seed pulses with a spectrum bandwidth of at 800 nm to a Coherent Legend pulse picker regenerative amplifier for boosting the pulse energy to about 1 mJ at a low repetition rate of 1000 Hz. The amplified laser pulse passed through a traveling-wave optical parametric amplifier of superfluorescence (TOPAS) to generate a signal beam tunable from 1160 to 1600 nm and an idler beam tunable from 1600 to 2600 nm. The signal and idler beams were collinearly combined together and passed through a DFG unit to generate a MIR pulse tunable from 2.5 to 11 μm and with a pulse width of (full-width at half-maximum, FWHM). The DFG average powers at different wavelengths are shown in Fig. 3(b). A long-pass filter was used to separate the DFG pulse away from the residual signal and idler. After the filter, the beam was free-space coupled into a 3 cm-long chalcogenide step-index fiber by an aspheric lens (AL) with a focal length of and a NA of (THORLABS, C021TME-F, 8–12 μm). The transmission efficiency of the lens was higher than 80%, and the coupling efficiency was measured to be . The output beam from the fiber was injected into a monochromator by a lens and a gold-coated parabolic mirror (PM, THORLABS, 800 nm–20 mm). And a nitrogen gas filled the monochromator to avoid gas absorption, such as . The liquid nitrogen cooled mercury cadmium telluride (MCT, HgCdTe) detector (HAMAMATSU, P5274-01) had a measurement range of . The SC signal was amplified by a lock-in amplifier and the spectrum was taken by a computer-based spectrometer.
During the experimental process, in order to minimize the fiber loss (especially for Se-H absorption band) and maximize the DFG pump power, the wavelength was chosen, which was in the anomalous dispersion region of the fiber. The average pump power measured directly from DFG was . Considering the Fresnel reflection (), the estimated coupling peak power was . Because the generated MIR SC spectrum was uncommonly wide, three gratings with different blaze wavelengths were used to collect the raw data from the MCT detector. Furthermore, three long-pass filters were used to remove the high-order diffraction peaks of the gratings. From 1.8 to 5.2 μm, the spectrum was recorded by a grating of 120 grooves/mm and 3750 Blaze (JASCO) (), and a 1.8 μm long-pass filter (). From 4.5 to 10 μm, the spectrum was recorded by a grating of 120 grooves/mm and 8300 Blaze (JASCO) (), and a 4.7 μm long-pass filter (). From 9 to 16 μm, the spectrum was recorded by a grating of 40 grooves/mm and 15000 Blaze (BUNKOUKEIKI) (), and a 9.4 μm long-pass filter (). The three raw spectra were stitched, and then calibrated by applying a calibration function.
The spliced MIR SC spectrum and the spectrum of the pump source are shown in Fig. 4. We can see that the SC spectrum covers from to 15.1 μm, which is, to the best of our knowledge, the broadest MIR SC spectrum observed so far in optical fibers. It is of key importance for the development of optical devices operating in MIR and gives a great chance for fiber-based applications in sensing, medical, and biological imaging areas. For the 3 cm-long chalcogenide step-index fiber, the nonlinear length is , where is the nonlinear coefficient and is the peak power. From Fig. 2(d), we get at the pump wavelength of . The dispersion length is , where is the pulse width for hyperbolic-secant shape and at is the dispersion parameter calculated according to Fig. 1(b). For the coupling peak power of , is , and is . Because the fiber length and , the spectrum broadening in the anomalous dispersion region was dominated by the fission of the higher-order solitons. Based on , the order of solitons () in the fiber was . In the normal dispersion region, the spectrum broadening was dominated by the radiation of dispersive waves generated under the phase-matching condition. The recessions in the SC spectrum centering around (1), 5.9 (2) and 10.6 μm (3) perhaps come from the absorption bands of atmospheric water and Se-O. After 11.7 μm, the spectrum declined abruptly (4), which was in accordance with the strong and wide absorption band of Se-H. Moreover, because the wavelength of the SC spectrum was comparable to the fiber core diameter, the output near-field beam profile was measured by a beam profiling camera (WinCamD, FIR2-16-HR) with the measurement range of . The image is shown in the inset of Fig. 4, which confirms that the light was confined in the fiber core.
The SC generation in the chalcogenide step-index fiber was simulated by the generalized nonlinear Schrödinger equation (GNLSE) , as shown in Fig. 5. The total response function including the instantaneous electronic () and the delayed Raman response () is given by
Table 1 lists the parameters used for the simulation: fiber length , peak power , pump wavelength , fiber loss , nonlinear coefficient , pulse width TFWHM, etc. The fiber loss was replaced by the rod loss [Fig. 2(b)], and the nonlinear coefficient can be obtained from Fig. 2(d). However, there are still some differences, probably due to the following: the disparity between the fiber loss and the rod loss; the disparity between the calculated peak power in the simulation and the actual peak power in the experiment. Moreover, the deviation of the simulated dispersion profile in Fig. 1(b) would affect the shape and range of the simulated SC, and there is also the possibility of coupling to other polarizations or spatial modes in the fiber.
In summary, MIR SC spectrum spanning to 15.1 μm is successfully generated in a 3 cm-long chalcogenide step-index fiber. To the best of our knowledge, it is the broadest MIR SC generation observed so far in optical fibers. This study facilitates the development of optical devices operating at wavelengths in the MIR region, and improves the fiber-based applications in sensing, medical, and biological imaging areas.
Ministry of Education, Culture, Sports, Science, and Technology (MEXT) (2011-2015).
Tonglei Cheng acknowledges the support of the JSPS Postdoctoral Fellowship. The authors wish to thank J. A. Woollam Japan Company for measuring the refractive indices of and glasses.
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