We report the fabrication of a chalcogenide all-solid hybrid microstructured optical fiber and its application in supercontinuum generation for the first time, to the best of our knowledge. The fiber possesses all-normal and flattened chromatic dispersion, making it highly potential for broad and coherent supercontinuum generation. By pumping the fiber with a femtosecond laser at 3, 4, and 5 μm, broad supercontinua with good spectral flatness are generated. The broadest SC spectrum extending from 2.2 to 10 μm at -20 dB level was obtained when the fiber was pumped at 5 μm with an input power of 3.9 mW.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Mid-infrared (MIR) supercontinua (SC) have attracted a lot of research interest in recent years because of their potential applications in spectroscopy, biomedical imaging, optical coherence tomography and remote sensing [1–5]. Among various media for MIR supercontinuum generation (SCG), chalcogenide optical fibers are usually the media of choice due to their wide transmission in the MIR as well as high nonlinearity and the generated SC are the broadest SC that have been reported so far [6–8]. The zero-dispersion wavelengths (ZDW) of chalcogenide glasses are larger than 5 μm, thus chalcogenide fibers were usually pumped at long wavelengths (6 ∼ 10 μm) to be benefited from the soliton dynamics for efficient expansion of the SC spectrum. By pumping a step-index (SI) As2Se3/Ge10As23.4Se66.6 fiber at 6.3 μm, Petersen et al. reported an SC from 1.4 to 13.3 μm . Cheng et al. reported an SC from 2 to 15.1 μm by pumping a step-index As2Se3/AsSe2 fiber at 9.8 μm . Zhao et al. reported an SC from 2 to 16 μm by pumping a double-clad Te-based chalcogenide fiber at 7 μm . For such a long-wavelength pumping scheme, the pump sources are usually difference frequency generation (DFG) laser systems which consist of many amplification stages and have very large footprints. This limits the applications of the generated SC within only the laboratory environment. Besides, there is little possibility for scaling the SC power because the DFG laser power is normally just a few to a few tens milliwatts. To scale up the power, the pump source can be high-power fiber lasers whose central wavelengths are around 1.5 or 2 μm. To utilize these short-wavelength pumping for SCG, chalcogenide fibers could be concatenated with another fiber such as silica or fluoride fiber and SCG would be cascaded from near to mid-infrared [9,10]. Another approach is to control the chromatic dispersion of the fibers, i.e. shifting the ZDW to shorter wavelengths so that they can be directly pumped using short-wavelength pulses [11,12].
In addition to the SC bandwidth, coherence is also an important factor for practical applications. In the above-mentioned research, SCG has taken advantage of the soliton dynamics. However, SCG based on soliton dynamics suffered from large pulse fluctuation in both phase and amplitude or in other words, the generated SC has a low temporal coherence . On the other hand, a pumping scheme in the all-normal dispersion regime of the fiber has been proven to produce highly coherent SC by suppression of noisy processes such as modulation instability or spontaneous Raman scattering . Considering the advantage of robust commercial fiber lasers at 1.5 and 2 μm, coherent supercontinuum generation with short-wavelength pumps around 1.5 and 2 μm has been demonstrated using silica, tellurite and chalcogenide fibers [15–22]. By pumping in the all-normal dispersion regime, coherent SC have been obtained. However, the long-wavelength edge could not reach 4 μm. Recent advancement in fiber lasers has extended the central wavelength beyond 3 μm with average powers of a few tens Watts [23–25]. Besides, Praseodymium-doped, and Dysprosium-doped chalcogenide fibers have shown emission spectra up to 5, and 6 μm making them attractive as potential media for MIR fiber lasers [26–28]. Thus, it is worth investigating SCG in the all-normal dispersion regime in this range of pump wavelengths, i.e. 3 to 5 μm.
In Ref. , we have proposed a novel all-solid hybrid microstructured fiber consisting of three tellurite glasses. Such a fiber possesses excellent chromatic dispersion controllability. It is attractive to apply that structure using chalcogenide glasses and find out whether the excellent dispersion controllability is still applicable. Then, it will be very useful for extending the SC long-wavelength edge further into the MIR, especially for covering the entire transparent atmospheric windows, i.e. 3-5 μm and 8-13 μm. In this report, a chalcogenide all-solid hybrid microstructured optical fiber is fabricated and SCG with such a fiber is performed. The obtained SC are among the broadest SC using a short-wavelength pump in the all-normal dispersion regime.
2. Fiber design and chromatic dispersion
The designed fiber structure is shown in Fig. 1(a). It has a central core surrounded by six additional rods. We call it the all-solid hybrid microstructured optical fiber (ASHMOF). The core, cladding and rod glasses are As2Se3, AsSe2 and As2S5, respectively. These three glasses were chosen because of their good transmission in the MIR as shown in Fig. 1(b) and their compatible thermo-mechanical properties for fiber drawing as well as suitable refractive index difference. Figure 1(c) shows the refractive indices of the glasses . At 3 μm, the refractive index difference of As2Se3 and AsSe2 is 0.072, and the refractive index difference of As2Se3 and As2S5 is 0.543. Such a large refractive index difference of the core material and the rod material ensures efficient modification of chromatic dispersion. As shown in Fig. 1(d), the ZDWs of these glasses are around 6∼7 μm. With a suitable fiber design, it is possible to shift the ZDW to shorter wavelengths or to obtain an all-normal chromatic dispersion profile which is of our interest in this report. Calculation of chromatic dispersion was performed by the full-vectorial finite element method with a commercial software (Lumerical Mode Solutions).
Figure 2 shows the chromatic dispersion of a chalcogenide step-index (As2Se3/AsSe2) fiber. When the core diameter is smaller than 10 μm, the fiber possesses all-normal chromatic dispersion. However we can see that the chromatic dispersion at long wavelengths decreases and has a large absolute value. This disadvantage can be removed with the ASHMOF.
By adding solid rods around the core, the optical field in the core is disturbed and the waveguide dispersion is modified. As a result, the total chromatic dispersion D can be flexibly controlled over a wide wavelength range. Figures 3(a) and 3(b) show the chromatic dispersion of the fundamental mode of the chalcogenide ASHMOF, the core diameter in the designed fiber was 8 μm. As a reference, chromatic dispersion of a step-index fiber with the same core diameter (8 μm) is plotted in Fig. 3(a). In Fig. 3(a), the rod distance of ASHMOF was kept constant at 11 μm, and the rod diameter was changed from 1.8, to 2.5, 3.0, 4.6, 6.0, and 7.8 μm. With a larger rod diameter, the chromatic dispersion is shifted up on the long-wavelength side, while the short-wavelength side remains unchanged. This makes chromatic dispersion of the ASHMOF smaller and achieves a more flattened chromatic dispersion profile as compared to that of a step-index fiber. When the rod diameter is larger than 4.6 μm, the dispersion becomes anomalous and the ZDW is shifted to the shorter wavelengths with larger rod diameter. In Fig. 3(b), the ratio Λ/drod was kept constant at 3.4, and the rod diameter was changed from 2.0 μm to 2.6, 3.2, 4.0 μm, and 5.0 μm. Changing the rod diameter from 2 to 5 μm makes the chromatic dispersion curve change from a quadratic curve to a quartic curve and then a cubic curve. In the middle, when drod = 3.2 μm, a flattened chromatic dispersion profile is obtained. This profile is valuable for coherent SCG due to its all-normal dispersion and small dispersion value. The flattened chromatic dispersion value is about − 5 ps/km/nm in the wavelength range from 5.2 to 11.2 μm with variation smaller than ± 1 ps/km/nm.
With such a large core and a large refractive index difference between the core and the cladding, the ASHMOF with drod of 3.2 μm shown in Fig. 3(b) is a multi-mode fiber. Figure 4 shows the modal intensity profiles (top) and effective indices (bottom) of the first two modes of the ASHMOF. The effective index difference between the two modes increases with wavelengths. At 3 μm, the effective index difference is 0.016, and at 5 μm, the effective index difference is 0.032. Such large effective index differences together with the high loss of the mode 2 allow the ASHMOF operate as a quasi-single-mode fiber by suppressing the mode coupling . Moreover, the nearest high-order mode has a cut-off wavelength of ∼8 μm which means the optical field can not propagate in this mode for wavelengths longer than 8 μm.
It is possible to optimize the structural parameters, i.e the core diameter, the rod distance and the ratio of rod distance to rod diameter of this chalcogenide ASHMOF as shown in Table 1 to obtain different flattened chromatic dispersion profiles with flatness of ±1 ps/km/nm as shown in Fig. 5. These results imply the excellent chromatic dispersion controllability of chalcogenide ASHMOF. For example, a flattened all-normal chromatic dispersion with value of 1 ± 1 ps/km/nm is obtained when dcore = 9.2 μm, Λ = 12.32 μm and Λ/drod = 3.225. Especially, when dcore = 9.74 μm, Λ = 13.2 μm and Λ/drod = 3.3, an ultra-flattened chromatic dispersion profile with flatness as small as ± 0.4 ps/km/nm from 6 to 13.2 μm can be obtained. To the best of our knowledge, this is the most-flattened chromatic dispersion ever suggested with chalcogenide fiber over such a wide wavelength range.
Such a fiber with an ultra-flattened and near-zero chromatic dispersion profile is highly suitable for applications such as MIR optical fiber parametric amplification or frequency comb expansion. These will be our future study. In this paper, to demonstrate SCG in the all-normal dispersion regime, we study SCG in a fiber with a flattened dispersion profile shown in Fig. 6(a).
3. Simulation of SCG in the chalcogenide ASHOMF with flattened all-normal chromatic dispersion profile
We modeled the pulse evolution by solving the generalized nonlinear Schrodinger equation in the frequency domain, with noise-included input pulse :
The linear operator including dispersion and loss is given by:
The Raman response function is modeled as:
The employed noise model is one-photon-per-mode with random phase noise . Coherence was assessed by calculating the modulus of the complex degree of first-order coherence g12 whose values are from 0 to 1; when |g12 | is unity, it means perfectly coherent:
3.1. SCG with different pump wavelengths
To show the potential of the chalcogenide ASHMOF in SCG, the pump wavelength was changed from 3 to 10 μm. In the simulation, the simulated peak power was 20 kW, the pulse width was 200 fs and the fiber length was 10 cm.
As shown in Fig. 6(a), the chromatic dispersion profile of ASHMOF is flattened with a value of ∼ – 5 ps/km/nm in the wavelength range from 5.2 to 11.2 μm. The fiber structural parameters are dcore = 8 μm, Λ = 11 μm, and Λ/drod = 3.412. Figures 6(b) and 6(c) show the simulated output SC spectra of ASHMOF with two pumping schemes: short-wavelength pumping [Fig. 6(b)], and long-wavelength pumping [Fig. 6(c)]. When the pump wavelength is 3 μm, the SC spectrum is from 2 to 5.9 μm at – 40 dB level which is more than one octave. When the pump wavelength is changed to 4 and 5 μm, the SC long-wavelength edge continues to be shifted to longer wavelengths and the SC bandwidth became broader. When the pump wavelength is 5 μm, the SC spectrum of ASHMOF spans a wavelength range from 2.5 to 9.2 μm at – 40 dB level which was nearly two octaves. These results show the potential of chalcogenide ASHMOF in SCG using short-wavelength pumping. With longer pump wavelengths, e.g. 6 to 10 μm, the long-wavelength edge reaches further to the MIR region, e.g. 10.2 to 15.9 μm. Thus, the ASHMOF can also be applied for SCG using long-wavelength pumping. It should be noted that the simulation is valid for single-mode operation for the reason discussed in Section 2. If mode-coupling occurs, the SC bandwidth will be narrower as compared to the case of fundamental mode propagation. The reason is that modal dispersion will reduce the pulse intensity quickly. As a result, spectral broadening based on self-phase modulation will become less efficient.
Figure 7 shows the spectral evolution along the fiber when the pump wavelength is 5 μm. The SC spectrum reaches its maximum bandwidth when the travel distance is around 3.5 cm. Further propagation of the pulse along the fiber does not change significantly the bandwidth. In the next simulation, we studied SCG with different pump powers but limited the fiber length to 5 cm. This does not affect the discussion about the dependence of SC bandwidth on pump power.
3.2. SCG with different pump powers
Figure 8 shows the output SC spectra when the pump peak power at 5 μm is 20, 30, 50, 80 and 100 kW. When the pump peak power is increased from 20 to 30 and 50 kW, both short- and long-wavelength edges are expanded to both sides. At 50 kW of peak power, the SC spectrum is from 1.9 to 10.8 μm at – 40 dB level which is more than 2 octaves. Further increase of the pump peak power to 80, and 100 kW does not shift the short-wavelength edge significantly. This can be explained by the low material transmission of chalcogenide and the large absolute value of chromatic dispersion for wavelengths shorter than 2 μm. However, the long-wavelength edge continues to be shifted to longer wavelengths with higher pump powers. At 100 kW of pump peak power, the long-wavelength edge can reach 12.4 μm. The SC also has a good spectral flatness with the spectrum expanding from 2 to 11.5 μm at – 10 dB level.
To have an insight in the spectral broadening dynamics, the spectrograms at different travel distances z along the fiber when the pump peak power is 30 kW are shown in Figs. 9(a)–9(d). At z = 0.185 cm, under self-phase modulation (SPM), the spectrum was broadened to both sides as shown in Fig. 9(a). At z = 0.555 cm, optical-wave breaking (OWB) happened first in the short-wavelength region as shown in Fig. 9(b). The spectral side lobe on the left side of the spectrum and oscillation on the trailing edge of the pulse in the time domain are characteristics of OWB . At z = 2.22 cm, OWB happened at the right side of the spectrum. Here, we can see the effect of flattened chromatic dispersion in the long-wavelength region. With a flattened and small chromatic dispersion on the long-wavelength side, OWB happened at a later time as compared to the short-wavelength side. Thus, the spectrum was expanded more to the long-wavelength region. After OWB happened, the energy transfer among different wavelength components helped smooth the spectrum as shown in Fig. 9(d). During the spectral broadening, SPM and OWB were the main processes, thus the coherence of the pulse was kept to unity as shown in Fig. 9(e). To clarify the potential of this newly proposed fiber, fiber fabrication and SCG experiments are shown in the next sections.
4. Fiber fabrication
Figure 10(a) shows the fabrication process of a chalcogenide ASHMOF. It has three main steps. In the first step, the As2Se3 and As2S5 rods were elongated to obtained rods with diameters of 2.5 mm and 1.5 mm, respectively (1A and 1B). Then these rods were inserted into a drilled AsSe2 rod (1C) and further elongated to make a preform (2A). The drilling was performed with an ultrasonic drill. Finally, the preform was inserted into an AsSe2 tube (2B) for fiber drawing. A negative pressure of -3∼-5 kPa was applied in the space between the preform and the tube in order to remove defects between the interfaces. Drawing tension was set constant by controlling the heating temperature. The final fiber has an outer diameter of 170 μm. The fiber cross-section image taken with a scanning electron microscope (SEM) is shown in Fig. 10(b). Although using high accelerating voltage created some charging effect on the rod periphery, it helped us distinguish the core from the cladding. The core and rod diameters and rod distance are 7.95, 4.46 and 12.15 μm, respectively. Based on the SEM image, chromatic dispersion was calculated and shown in Fig. 10(c). Compared to the chromatic dispersion of the designed fiber, that of the fabricated fiber has some discrepancy in the range of wavelengths larger than 10 μm. However, the difference is small, and basically, we obtained the targeted chromatic dispersion profile. Compared to a double-cladding fiber (DCF) which has been fabricated in our previous research , the ASHMOF has better chromatic dispersion controllability with a more flattened dispersion profile. It is expected to generate broader SCG using short-wavelength pumping.
The fiber loss is shown in Fig. 10(d). It was calculated by including both the material loss and the confinement loss of the fiber. There was very high loss of ∼ 50 dB/m for wavelengths longer than 13 μm. Thus a short fiber of 3.7 cm in length was employed for SCG experiments. The fiber far-field light spot is shown in Fig. 10(e). It was captured with a mid-infrared beam imager (WinCamD FIR2-16-HR) when a beam with wavelength of 3 μm was injected into the fiber. The light spot shows that most of light intensity was distributed in the fundamental mode of the fiber.
5. Supercontinuum generation
Figure 11(a) shows the experimental set-up for SC spectrum measurement. The pump source was a DFG laser system pumped by a Ti:Sapphire mode-locked laser (Verdi– Coherent Mira 900 – Legend – Topas). It can generate femtosecond pulse with tunable wavelength from 2.5 to 12 μm at a repetition rate of 1 kHz. The pulse width is ∼ 170 fs (full-width at half-maximum). The DFG average powers at different wavelengths are shown in Fig. 11(b) with a maximum of 4.5 mW at 4 μm. Kept in mind that the future fiber lasers can operate in the wavelength range of 3 ∼ 5 μm which is the emission wavelength range of Pr or Dy doped chalcogenide fibers, we studied SCG using pump wavelength in this range.
In the experiments, the pulses with different central wavelengths from 3 to 5 μm were free-space coupled into the fiber with an aspheric lens (Thorlabs, C028 TME-E, AR 3-5 μm). With a pump wavelength of 3 μm, the input pulse power was adjusted by using a neutral density filter (Newport 50FS02DV.2). The estimated coupling efficiency was around 20%. The fiber length was 3.7 cm. It was put in a metal cylinder holder of 3.5 cm in length to reduce the bending loss. The fiber and optical components were put in a nitrogen-gas-filled chamber to prevent water vapor and CO2 absorption from affecting the measured SC spectra. The output SC from the fiber was directed into a monochromator by a ZnSe lens (f = 25 cm). Before the monochromator slit, an appropriate long-pass filter was used with respect to the measurement range (range < 2.8 μm: filter 1.6 μm, 2.8–4.8 μm: filter 2.5 μm, 4.8–9.0 μm: filter 4.65 μm, range > 9.0 μm: filter 8.6 μm). The detector was a mercury-cadmium-telluride detector cooled with liquid nitrogen and had a measurement range of 1 ∼ 22 μm. The electric signal was analyzed with a computer-based spectrometer after being amplified by a pre-amplifier and a lock-in amplifier (NF LI-5640).
Figure 12 shows the SC spectra with various pump powers when the pump wavelength was 3 μm. With a pump power of 0.5 mW, the bandwidth was about 0.7 μm from 2.6 to 3.3 μm, and the left side of the spectrum was more flattened (2.7–3.0 μm at – 5 dB level). Initially, SPM would broaden the spectrum to both sides. Then optical-wave breaking would happen on the left side first because of the steeper slope of the chromatic dispersion profile on this side. When the pump power was increased to 1 and 1.5 mW, the left side showed insignificant expansion while the long-wavelength edge continuously moved to the right and reached 3.7 μm. At 1.5 mW of pump power, the bandwidth was 1.3 μm. With a maximum pump power of 3.7 mW, the output SC power was 0.3 mW, and the bandwidth was 3.2 μm expanding from 2.2 μm to 5.4 μm, and the flat-top profile was from 2.5 to 4.5 μm at – 10 dB level. To the best of our knowledge, this is the broadest SC pumped at 3 μm in an all-normal chromatic dispersion chalcogenide fiber. Although there were significant flows of N2 in the laser cavity (4 L/min), in the monochromator (20 mL/min) and in the measuring chamber (10 L/min), we could not remove the absorption of water and CO2 completely. Absorption at ∼2.9 μm (water) and ∼4.2 μm (CO2) can be seen in the figures. To further reduce the absorption, higher flows of N2 in a small measuring chamber can be employed and the gas lines as well as the equipment set-up must be redesigned. For that reason, the other measurements were carried out in normal laboratory air ambience without affecting the result discussion.
At the pump wavelength of 4 μm and the pump power of 4.5 mW, the SC power was 0.37 mW and the SC spectrum was from 2.2 to 8.0 μm as shown in Fig. 13(a). The spectrum was smooth and had a flat-top profile which are characteristics of SCG dynamics by SPM and OWB. At the pump wavelength of 5 μm, although the pump power was 3.9 mW, smaller than the pump power at 4 μm, the SC power in this case was 0.32 mW and the SC bandwidth was broader as shown in Fig. 13(b). The SC long-wavelength edge reached 10.2 μm with a flattened and smooth spectrum profile and a bandwidth of 5.7 μm (from 2.8 to 8.5 μm) at – 10 dB level. The pump wavelength in this case was in the flattened region of the chromatic dispersion profile and the chromatic dispersion value at this wavelength was small (-6.5 ps/km/nm) resulting in efficient spectral broadening by SPM.
From the above results, it could be expected that pumping at longer wavelengths would produce broader SC spectra. Thus, we performed pumping the fiber at 8, 9, and 10 μm. For these experiments, a different lens (Thorlabs C028TME-F) with anti-reflection coating from 8 to 10 μm was used. The respective spectra are shown in Fig. 14. It is interesting to find out that while the long-wavelength edge could be expanded to ∼ 13 μm (pumped at 8 μm), and 14 μm (pumped at 10 μm), the short-wavelength edge also moved to the right making the SC spectrum not much broader as compared to that with the pump wavelength of 5 μm. Such a similar trend was also observed in Ref . but has not been explained. In our case, there were two reasons for this trend. First, while the chromatic dispersion is almost constant from 5 to 10 μm, the effective mode area is larger at longer wavelengths as shown in Fig. 10(c). This would reduce the nonlinear coefficient. Second, the pump power was smaller at longer wavelengths. These two factors resulted in a decrease in the SC bandwidth.
It is worth comparing SC bandwidth in this research to other related research. As can be seen from Table 2, even with smaller input power, e.g. about 4 to 5 times smaller to those reported in Refs [39,40], the generated SC in this research are among the broadest SC reported when the fiber was pumped in the normal dispersion regime. These results show the great potential of the chalcogenide ASHMOF in coherent and broad SCG. Moreover, the generated SC have good spectral flatness making them highly applicable for spectroscopy and optical coherence tomography.
For the first time, to the best of our knowledge, we reported the fabrication of a chalcogenide all-solid hybrid microstructured optical fibers using three glasses – As2Se3, AsSe2 and As2S5. The fiber possesses a flattened all-normal chromatic dispersion profile with value of – 5 ps/km/nm and variation of ± 1 ps/km/nm over the wavelength range from 5.2 to 11.2 μm. Broad mid-infrared supercontinuum generation has been demonstrated with various pump wavelengths. The generated SC were among the broadest SC with good spectral flatness when the fiber was pumped at 3, 4, and 5 μm in the normal dispersion regime. Although the output SC powers were relatively low because of the low input powers, it is possible to scale up the power by using fiber lasers with higher repetition rate as the input pump.
Japan Society for the Promotion of Science (17K14671, 18H01504).
The authors declare no conflicts of interest
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