We have prepared a well-structured tellurium chalcogenide (ChG) fiber with a specialized double cladding structure by an improved extrusion method, and experimentally demonstrated an ultra-flat mid-infrared (MIR) supercontinuum (SC) generation in such a fiber. The step-index fiber had an optical loss of <1 dB/m in a range from 7.4 to 9.7 μm with a minimum loss of 0.69 dB/m at 7.87 μm. Simulation showed that an all-normal dispersion profile can be realized in this double cladding tellurium fiber. An ultra-flat MIR SC spectrum (~3.2-12.1μm at −10 dB, ~2-14 μm at −30 dB) was generated from a 22-cm long fiber pumped with a femtosecond laser at 5 μm (~150 fs, 1 kHz). Then the degree of coherence was calculated out based on a simulation, showing that a high coherent MIR SC (from 2.9 to 13.1 μm) can be generated in this double-cladding tellurium fiber.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Supercontinuum (SC) light sources with broad bandwidth, strong brightness, and high coherence are critical for applications such as spectrum analysis, coherent measurement, optical sensing, and optical frequency comb . In fact, coherent SC light sources from the ultraviolet to near infrared have been realized in silica fibers. Further research requires an extension of the SC band into the mid- and far-infrared. Obviously, silica-based fibers are not satisfied for the requirements due to the strong absorption of the silica beyond 3 μm. In contrast, chalcogenide (ChG) glasses are transparent in a wide wavelength range up to 8, 15 and 20 µm in S-, Se- and Te-based ChG glasses, respectively, and thus ideal for SC generation in mid- and far-infrared. For example, Petersen et al reported an SC spectrum covering a range from 1.4 to 13.3 μm by pumping As2Se3/Ge10As23.4Se66.6 fiber with 2.29 MW peak power and 100 fs pulses at 6.3 μm . Cheng et al obtained an SC spectrum from 2.0 to 15.1 μm in a 3 cm AsSe2/As2Se3 fiber with 2.89 MW peak power and 170 fs pulses at 9.8 μm . Tellurium glass fibers also show excellent performance in mid-infrared (MIR). A broadband SC covering 1.5-14 μm was generated in a large-core GeAsSeTe (GAST) step-index fiber .
In terms of coherence, which is a key parameter for the quality of the SC sources , Corwin et al. demonstrated that coherence degradation is mainly caused by amplification of noise . A conventional method for coherent SC generation and noise restrain is pumping fibers with all-normal dispersion profile by femtosecond lasers . All normal dispersion can be achieved via tailoring the waveguide dispersion, and this can compensate the shortage of material dispersion. Photonic crystal fiber has been proposed as an optimal candidate to tail dispersion, but it is challenging for CHG fiber. With optimized double-clad structure, all-normal dispersion profile can be realized in step-index fiber . Kenshiro  et al reported that the dispersion can be tailed by double cladding structure in As-Se step-index fiber, while the flatness of SC can be further improved.
In this work, we have improved the extrusion method and successfully obtained ultra-low loss fibers (core: Ge20As20Se15Te45, inner cladding: Ge19As16Se25Te40, outer cladding: Ge20As20Se17Te43) from double-cladding preform. Here, the contaminated surface of the core glass can be peeled off, and this can decrease the number of the defects and thus reduce the optical loss of the fiber effectively. The fiber exhibits an optical loss of <1 dB/m in a range from 7.4 to 9.7 μm and a minimum loss of 0.69 dB/m at 7.87 μm. For SC generation, broad spectra from 2 to 14 μm at −30 dB are achieved in a 22 cm-long fiber pumped by 5 μm laser with a pump power of 30 mW, and the flattened SC with the intensity flatness of −10 dB bandwidth is ~3.2-12.1 μm. The coherence of the SC is calculated out, as that fit well with the fiber structure.
High purity Ge, As, Se and Te materials were purified to remove oxygen, molecular water and carbon by dynamical distillation. The glasses were prepared by the traditional melt-quenching method. The as-prepared glasses were used to fabricate Ge20As20Se15Te45/Ge19As16Se25Te40/Ge20As20Se17Te43 preform via improved extrusion method. The preform consists of 9 mm core, 26 mm inner cladding and 46 mm outer cladding. The ratio of fiber core/inner cladding/ outer cladding is 1:4:8, just as shown in the flowchart of Figs. 1 (a)-1(c), with a force of around 1 ton, shown as the red arrows in the Fig. 1. Firstly, the inner-cladding glass rod was planted into the outer-cladding one. Secondly, the core glass rod was planted into the inner-clad rod. Then the structured glass rod was co-extruded out with the removal of the surface layer of the core glass rod that could contain any defects or re-oxidized contaminations. Here, the core and claddings were isolated by steel tubes. This improved method can effectively decrease fiber flaws, (i.e. bubbles and interfacial roughness, which can introduce external scattering losses) in the production of step-index ChG glass fiber, accomplished with the transform of glasses to fiber preform.
The fiber was drawn from a homemade drawing-tower at a temperature of 300 °C. The glass transition temperature (Tg) and the softening temperature (Tp) were obtained by thermal dilatometer (DIL402C). The transmission spectrum was recorded by Fourier transform infrared (FTIR) spectrometer (Nicolet 380) in a range from 2 to 25 μm. The refractive index was measured by an IR ellipsometer (IR-VASE MARK II, J.A. Woollam Co.), and the dispersion was obtained by commercial software (RSOFT). The fiber loss was measured with a FTIR spectrometer (Nicolet 5700) using cut-back method. A 1.8-m long fiber was employed to evaluate the attenuation. By removing 80-cm pieces of the fiber using a precision fiber optic cleaver (FK11-LDF, Photon kinetics, Inc.), the cross section of each cleave was observed by the optical microscope (Keyence, VHX-1000) to ensure a flat interface. The attenuation was calculated by Loss = 10*log(P1/P2)/L, where P1 is the input power, P2 is the output power, and L is the removed length of fiber. The pulse was generated by an optical parametric amplifier (OPA) system. For SC measurements, The MIR pulses (~150 fs, repetition rate of 1 kHz) from an OPA system were launched into the fiber using ZnSe lens. Considering the Fresnel reflection and wide-spectrum, the coupling efficiency was estimated to be around 5%. The output beam from the fiber was injected into a monochromator directly. The liquid nitrogen cooled mercury cadmium telluride (HgCdTe) detector (spectral response range: 1–16 μm) was used to detect SC signals that were amplified by a lock-in amplifier.
3. Result and discussion
The transmission spectra of the as-prepared glasses is shown in Fig. 2(a). It is evident that the transmission range is extended to 16 μm . There are no apparent absorptions from oxides or other impurities like Se-H (4.5 μm) and Ge-H (5 μm). The concentrations of Se-H and Ge-H assessed using the known extinction coefficient, are 0.04 ppm (wt) and <0.03 ppm (wt), respectively. Although the glasses are purified using a series of distillation, traces of hydrogen cannot be removed completely, because it starts to enter into the ChG glass melts from the silica tube at high temperature .
The physical parameter of the glasses was listed in Table 1. The glass Tg and Tp was obtained from the intersection point and vertex of tangents, respectively, in the thermal expansion curves. Tg increases with increasing content of Ge in the glasses, as shown in Fig. 2(b). Meanwhile, the higher the Te content is, the larger the refractive index is , and Ge20As20Se15Te45 glass has the largest refractive index.
The step-index fiber consists of a Ge20As20Se15Te45 core (diameter: 20 μm) and two claddings: the first is Ge20As20Se17Te43 and the second is Ge19As16Se25Te40. Double-cladding or multi-cladding layers can be employed to design dispersion-shifted fibers. However, in this paper, given the small difference of refractive indices core and extra cladding, the inner cladding of a Ge19As16Se25Te40 layer was only employed to increase the numerical aperture (NA). Figure 3(a) shows the refractive indices and the calculated NA of the fiber. The fundamental mode (FM) dispersion was calculated by a commercial software (RSOFT) . With the W-type refractive index structure as well as the large material ZDW (10.5 μm), a large waveguide dispersion profile is realized and the total dispersion is red-shifted to long wavelength effectively. Thus, we obtained all-normal dispersion profile successfully in this double-cladding fiber, as shown in Fig. 3(b).
Cross section image of the fiber is shown in Fig. 4(a), from which round shape of the core and cladding is evident. Due to the large difference of the refractive index, the core package interface is clear and no any defects can be observed. The optical loss of the fiber is presented in Fig. 4(b). Compared with the transmission spectrum in Fig. 2(a), absorption peaks become obvious from Se–H and Ge-H impurities at around 4.25 μm and 5 μm, respectively. Increasing loss at shorter wavelength is due to the multi-photon absorption, while that at longer wavelength is due to overtone and combination vibrational absorption bands of the fundamental stretching, anti-stretching and bending absorption bands of ≡Ge-Se, = As-Se, = As-Se-As = and ≡Ge-Se-Ge≡ . The fiber exhibits excellent transmission less than 10 dB/m in a range from 3.8 to 11.5 μm, especially less than 1 dB/m in a range from 7.4 to 9.7 μm. The minimum loss is 0.69 dB/m at 7.87 μm. As far as we know, this is the lowest optical loss in any step-index tellurium fibers.
Figure 5(a) summarizes the SC spectra generated from 22 cm long fiber with a mean input power of 30 mW. When the pump wavelength changes from 4 to 5 μm, strong self-phase modulation (SPM) of pulses leads to optical wave breaking (OWB) via self-steepening and third-order dispersion, and thus significant broadening of the spectrum . However, broadening tendency cannot be observed with further increasing pumping wavelength up to 7 μm. The absence of the trend is due to coupling instability, for example, if launching into the core is slightly diverged, higher-order modes will receive most of the power  thereby restraining the SC spectrum broadening. The spectrum declined drastically beyond 12 μm, which was due to the multi-phonon absorption band of Se-Se. Besides, as shown in Fig. 5(a), the flattest and broadest SC spectrum 2 to 14 μm at −30 dB pumping at 5 μm was obtained, and according to the optical loss spectra (Fig. 4(b)), a strong Ge-H absorption around 5 μm can be observed. The influences of Ge-H absorption should be avoided, which was identical to the SC generation. Thus, the 5 μm was chosen as the pumping wavelength.
Then, the experimental SC spectra pumped by different length (I) 15 cm (II) 22 cm and (III) 35 cm, are shown in Fig. 5(b).
When the length of the fiber is short, nonlinear effects play a major role. The spectral broadening caused by only group-velocity dispersion (GVD) is small. The frequency induced by SPM causes the spectrum broadening, producing a red-shift component near the leading edge of the spectrum and a blue-shift component near the trailing edge of the spectrum, and causing the spectrum to spread toward both ends at the same time. When the length of the fiber is long, the dispersion effect gradually increases, while self-sharpening effect and optical loss, the flatness and width of the SC spectrum are obviously deteriorated. To obtain a flat and wide SC spectrum, the length of the fiber should be smaller than the dispersion distance and larger than the nonlinear length. Therefore, 22 cm long fiber is selected for SC generation.
We use 5 μm laser wavelength to pump 22 cm long fiber length with different pump power to uncover the relationship between pump power and SC spanning width. In Fig. 5(c), the spectral bandwidth at −10 dB bandwidth is ~6.5 μm when the pumping power is ~5 mW, and this increases to ~8.9 μm when the pumping power is ~30 mW. With increasing pumping power, the nonlinear effect becomes stronger. The new spectrum is excited at both the longer and the shorter wavelength region, under the condition that the group velocity matching is satisfied (the GVD of the two new frequency lights are equal), then the two frequency lights undergo cross-phase modulation, thereby the spectrum tends to be flat. Although the pump power shifted from 10 mw to 30 mw, the output difference of the spectrum broadening is small, indicating that the current pump power is already in the margin/saturation state (the spectrum will widen as the pump power increases in the non-surplus/saturated state).
The generalized nonlinear Schrödinger equation for the fundamental mode of the fibers was solved to simulate SC generation. Spectral fluctuations from shot to shot in the input pulse was modeled via the one-photon-per-mode noise model . The pump pulse was supposed as sech2-shaped with a central wavelength of 5 µm and a pulse duration of 150 fs. The measured refractive index of the all normal dispersion tellurium CHG fiber was used in the simulation and the nonlinear coefficient (γ) was calculated by using the commercial software (COMSOL Multiphysics). Raman response function was used with the same in . The material loss of fiber was neglected in the simulation. Coherence was assessed by calculating the modulus of the complex degree of first-order coherence g12  and the equation was mathematically defined in the following form:
where E is the electric field intensity, which includes E1 and E2 along the time changing, t1 and t2 are times, λ is wavelength. Angular brackets indicate an ensemble average over independently generated pairs of SC spectra, and t is the time measured at the scale of the temporal resolution of the spectrometer used to resolve these spectra. To focus on the wavelength dependence of the coherence, t1 – t2 = 0 was adopted.
Figure 5(d) shows the simulation results of the SC generation in 22 cm long fibers along with the corresponding spectral coherence degree for 200 simulations with random noise. It shows that the system provides MIR SC covering 2 to 14 µm with the peak pump power of 180 kW. Since the fiber was pumped by femtosecond pulses in the normal dispersion region, self-phase modulation (SPM) and stimulated Raman scattering (SRS) were the main reason for spectral broadening. The degrees of coherence in Fig. 5(d) suggest that perfect coherence can be obtained in the spectrum spanning from 2.9 to 13.1 μm. This kind of coherent SC becomes more suitable for optical coherence tomography, nonlinear microscopy and those kinds of time-resolved applications.
All the SCs were given for comparing in Table 2, we chose the bandwidth at −10 dB to characterize the flatness of the SC. Petersen et al reported an SC spectrum with the intensity flatness of −10 dB bandwidth is only from ~2.1 to 5.1 μm. Cheng et al obtained an SC spectrum, in which the intensity flatness of −10 dB bandwidth is from ~6.2 to 12.5 μm. In this work, Tellurium CHG glass fiber with a range of ~3.2-12.1 µm at a high power of −10 dB, which near doubled the previous reports .
In summary, we have obtained a fresh low-loss tellurium fiber via an improved extrusion method, and all normal dispersion profile was realized in a specialized w-type double-clad structure. This well-structured fiber exhibits a low loss of <1 dB/m in the 7.4-9.7 μm range, and the minimum loss is 0.69 dB/m at 7.87 μm, which is the lowest value reported for any tellurium fiber. We have studied the SC generation by different pump wavelength, power and fiber length, and obtained a flattest SC spectrum covering ~3.2-12.1 μm at −10 dB by a 22-cm long fiber pumped at 5 μm. To be our best knowledge, this new SC result is the broadest record in MIR. Besides, simulated SC spectrum is in a good agreement with the result of experiment. The output SC spectra exhibit excellent coherence from 2.9 to 13.1 μm. The fiber-based SC has covered most of fundamental molecule vibration absorptions region. Such a broad and flat SC source has great potential to develop a highly coherent and broadband light source running across all the mid-infrared molecular ‘fingerprint region’.
National Natural Science Foundation of China (61705091, 61875097, 61627815, 61775109, 61435009); Zhejiang Provincial Natural Science Foundation of China (LR18F050002); the Opening Project of Key Laboratory of Optoelectronic Detection Materials and Devices of Zhejiang Province, China (2017004); Program for Science and Technology of Jiaxing, China (2017AY13010); the K. C. Wong Magna Fund in Ningbo University, China; and 3315 Innovation Team in Ningbo City, Zhejiang Province, China.
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