High brightness broadband sources in mid-infrared (MIR) atmospheric transmission windows have attracted a significant scientific interest in the past decade due to their wide range of potential uses in various fields, such as spectroscopy, metrology, as well as defense applications ^[1–3]. Supercontinuum (SC) sources using fluorozirconate-based fibers have reached watt level powers ^[4–9], some of which extended up to 4.7 μm by using short ZBLAN fibers ^[10], but usually the SC extensions are restricted to wavelengths below ∼4.2μm when using long ZBLAN fibers. A wavelength extension of the SC above 5 μm is possible when using fluoroindate glass fibers ^[11–13], allowing to completely cover the infrared atmospheric windows at 1.5–1.8 μm, 2.0–2.4 μm, and 3–5 μm for the aforementioned applications.

The intrinsic fiber loss due to the multiphonon absorption edge in fluorozirconate fibers is the main limitation on the long-wavelength side below ∼4μm of the generated SC. Emission beyond the multiphonon absorption edge can be achieved using small-core fibers of short lengths along with high pump intensities ^[14–17]. SC is also efficiently produced in chalcogenide (ChG) and tellurite fibers, given their very high nonlinear responses that are a few hundred times larger than in fluoride fibers ^[18,19]. Demonstrations in ChG fibers yielded broader MIR supercontinuum; however, given that their zero-dispersion wavelength is usually above 4.5 μm, direct pumping with standard fiber lasers operating at wavelengths below 3 μm is not optimum. Extending SC beyond 4.5 μm in fluoride fibers is also important so as to be able to pump step-index ChG fibers and to further extend the SC through nonlinear propagation in cascaded fibers ^[20–22].

As shown in Table 1, there have been few demonstrations of infrared SC generation with fluoroindate fibers ^{[6,11–13,23–26]} with some of them standing out due to their output powers or spectral bandwidths. Furthermore, SC flatness, which is important for many applications, has often been neglected. Previous works ^[6,24–26] reported that InF3 fibers can sustain high average powers, while ^[13] showed that broadband and high spectral flatness MIR SC can be achieved by injecting picosecond laser pulses with a central wavelength around 2 μm.

In parallel to these demonstrations, a lot of effort has been made to produce higher purity fibers in order to decrease the detrimental effects of the MIR background loss on the SC broadening and its output power.

In this paper, we present, to the best of our knowledge, the first demonstration of watt-level, spectrally flat, and broadband MIR SC in step-index InF3 fiber pumped by an all-fiber laser source. An average output power of 1.0 W spanning from 1 μm to 5 μm is demonstrated with high-spectral flatness. The recent production of high-purity InF3 fibers allowed these achievements, and these fibers exhibited an MIR background loss of 2 dB/km.

The fiber-laser source used is shown in Fig. 1(a) and is based on a 1.55 μm pigtailed semiconductor laser diode delivering 50 ps pulses at 1 MHz repetition rate, pre-amplified in an erbium (Er)-doped fiber and amplified in an erbium–ytterbium (Yb)-doped (10 μm core) double-clad silica fiber pumped at 980 nm. A broadband spectrum ranging from 1.2 μm to 2.5 μm with 1.9 W of average power was generated in the 1.5 m long undoped silica fiber (8 μm core diameter) from the combiner spliced at the output of the Er–Yb amplifier [see blue line in Fig. 1(b)].

The spectral components generated around 1.9 μm were preliminarily amplified in a thulium (Tm)-doped single-clad silica fiber and then spectrally filtered with a fiber isolator [see green line in Fig. 1(b)]. The signal around 1.9 μm was then further amplified in a 10 μm core diameter Tm-doped double-clad silica fiber having an effective absorption of 3 dB/m at the 791 nm pump wavelength. The unconverted part of the 1.55 μm pulse injected into the Tm amplifier was absorbed in the Tm-doped fiber and did not generate any significant output power. At low pump power, the amplified laser pulses have a narrower spectrum than at maximum pump power [see black and red lines in Fig. 1(b)]. The broadening of the amplified laser pulses is due to the Tm-doped fiber acting as a nonlinear and active medium. The 2.3 W output laser was butt-coupled using flat cleaves into an InF3 fiber, as well as into a ZBLAN fiber for comparison, and both fluoride fibers were wound on a spool having a radius of 10 cm. The alignment of the butt-coupling was optimized by monitoring the SC output power and the spectral extension. Table 2 summarizes the main parameters for the fluoride fibers used in the experiment. The computation of ZBLAN dispersion was calculated with commercial software (Mode Solutions, Lumerical Inc. ^[27]) and was based on the Sellmeier equation with the coefficients from Ref. ^[28]. The index of refraction of the InF3 fiber was measured from a bulk sample with an infrared ellipsometer. The theoretically estimated dispersion of the InF3 fiber was confirmed for wavelengths between 2.3 μm and 2.5 μm by measuring the dispersion of an 81-cm long InF3 fiber sample with a white-light interferometer.

The SC distribution was measured with an f=12.5cm monochromator purged with dry nitrogen and equipped with a 300 lines/mm diffraction grating to provide a spectral resolution of around 5 nm. The spectral fluence was measured with a PbSe detector for wavelengths between 1 μm and 4.8 μm and with a liquid nitrogen cooled HgCdTe detector from 2 μm to 13 μm. A set of long-pass filters was properly used in order to block any higher diffraction orders of the grating from overlapping with the SC profile. The complete spectral response was calibrated with a blackbody radiation source (IR-508, Infrared Systems) for each combination of a detector and a long-pass filter.

At low powers from the Tm amplifier, the butt-coupling efficiencies between the Tm amplifier and the fluoride fibers were around 85%. At the maximum power from the Tm amplifier, the coupling efficiencies decreased to 56% for the ZBLAN fiber and to 60% for the two InF3 fibers tested. The decrease of the coupling efficiency when increasing the pump power is due to the spectral broadening of the Tm-amplifier output and the increase of its mode field diameter (MFD) at longer wavelengths. At the output of the Tm-amplifier combiner [numerical aperture (NA)of0.15, core diameter of 10 μm], the MFD increases from 11 μm to 14.5 μm for laser wavelengths from 2 μm to 2.7 μm, respectively, while the MFDs of ZBLAN and InF3 fibers remain below 10 μm for this range of wavelengths. Therefore, as the pump power is increased, the output spectrum of the Tm amplifier broadens and the effective MFD increases, which reduce the butt-coupling efficiency with the following ZBLAN or InF3 fibers. For the maximum output power of 2.3 W from the Tm amplifier, 1.3 W of pump power was coupled into InF3 fibers and 1.27 W into the ZBLAN fiber. The corresponding SCs produced into the 20-m long fiber for the three fluoride fibers described in Table 2 are presented in Fig. 2(a). As a reference, the output spectrum of the Tm amplifier is also shown in Fig. 2(a). The corresponding attenuation, dispersion, and MFD spectra of InF3 and ZBLAN fibers are shown in Figs. 2(b), 2(c), and 2(d), respectively.

The dynamics of the SC generation in optical fibers is well known. When a fiber is pumped with picosecond or nanosecond pulses in the anomalous dispersion region, modulation instability (MI) and self-phase modulation (SPM) are the main phenomena involved in the first step of SC generation. This MI leads to the temporal breakup of injected pulses into a distributed spectrum of many shorter subpulses, which then propagate through the fiber. Subsequently, each subpulse undergoes further spectral broadening through MI, SPM, as well as Raman soliton self-frequency shift ^[29].

It is interesting to note that for similar injected powers into those fluoride fibers, the MIR SC extension into ZBLAN fiber is limited to 4.3 μm with an output power of 0.75 W, while the SC generated in the low loss InF3 fiber (sample A) extended up to 5 μm with 1 W of output power. When examining the MIR SC extension of ZBLAN and InF3 (sample A) fibers in Fig. 2(a) along with their respective attenuations in Fig. 2(b) (blue and red lines), we clearly see that the multiphonon absorption edge was the main limitation to their long-wavelength extension. For all fluoride fibers used in this work, the bending loss at long wavelength is negligible. The lower NA of the ZBLAN fiber induces a larger MFD at longer wavelength [see Fig. 2(d)], which could also have contributed to reducing the spectral extension by decreasing the nonlinearity. Since the ZBLAN fiber and the first InF3 fiber (sample A) exhibit a similar MIR background loss, the lower output power for the ZBLAN fiber is mainly due to the absorption of its MIR SC extending beyond 4 μm. As shown in Fig. 2(d), the MFD as a function of wavelength for both InF3 fibers is very similar. However, in the case of the SC produced with the second InF3 fiber (sample B), the output power and the MIR extension were lower because its MIR background loss was almost 10 times higher than for the first InF3 fiber (sample A). Indirectly, the higher MIR background loss also reduced the fiber’s MIR SC extension [see green line in Fig. 2(a)] since spectral broadening is driven by numerous nonlinear intensity-dependent processes. As a result of the MIR background loss, the laser pulse intensity decreased as it propagated along the fiber, reducing both the nonlinear effects and the MIR SC extension. Comparing both supercontinua from InF3 fibers [green and red lines in Fig. 2(a)] clearly points to the importance of reducing the MIR background loss for SC generation in long fibers.

Figure 3(a) presents the SC distribution at the output of the InF3 fiber (sample A) for increasing powers. Both the output average powers from the InF3 fiber and the Tm amplifier (in parenthesis) are given in the legend of Fig. 3(a). As expected, when the injected power into the InF3 fiber increases, the MIR SC extends towards longer wavelengths, and the spectral flatness improves. Since the spectral broadening was asymmetric and accentuated towards longer wavelengths, it can be inferred that the Raman soliton self-frequency shift was a dominant process for the SC generation in these fibers with the pump source used.

For the first InF3 fiber (sample A), the experimentally observed optimum length in terms of spectral broadening is 20 m. The use of a longer InF3 fiber, e.g., 30 m long, did not increase the SC span, but reduced its output power by ∼210mW due to the MIR background loss [see Fig. 3(b)]. On the other hand, the use of InF3 fiber length shorter than 20 m increased the SC output power, but both the MIR SC extension and its spectral flatness were reduced. An example is shown in Fig. 3(b) for 10-m long InF3 fiber (sample A) where the SC output power increased to 1.22 W. Similarly for ZBLAN, the optimum length in terms of the spectral broadening and the output power was experimentally observed for 8-m long fiber. This length is shorter than the optimum InF3 fiber length in order to minimize the impact on the SC of the stronger absorption in ZBLAN fiber at longer wavelengths [see Fig. 2(b)].

For comparison purposes, Fig. 4(a) depicts the 10-dB spectral flatness at the output of the InF3 (sample A, 20 m long) and ZBLAN fibers (8 m and 20 m long) as a function of their output powers. In both cases, the SC bandwidth increased at a slower rate when the SC MIR extension reached their respective multiphonon absorption edges. With 20 m long fibers, the SC bandwidth increase slowed down when output powers exceeded ∼700mW in the case of InF3 and ∼500mW for ZBLAN. Using a shorter ZBLAN fiber (8 m long) allowed to reduce the detrimental effect of the long-wavelength absorption edge and to increase its output power up to 1.03 W [see Fig. 4(b)], a similar output power as for the 20-m long InF3 fiber (sample A). However, optimizing the ZBLAN fiber length did not yield a broader SC output. As seen in Figs. 4(a) and 4(c), both lengths of ZBLAN fibers (8 m and 20 m) resulted in a similar MIR extension up to 4.3 μm, while the 20 m InF3 fiber produced an SC up to 5 μm.

In summary, we demonstrated the first watt-level output power, spectrally flat and >2-octave SC generated in a step-index InF3 fiber pumped by an all-fiber laser source. The longer multiphonon absorption edge in InF3 fibers allowed to extend the SC to a longer MIR wavelength (up to 5 μm) as compared to ZBLAN fiber. The high output power and high-spectral flatness achieved here show the potential of InF3-based fibers as reliable all-fiber SC sources covering important infrared atmospheric windows. In addition to their transmission, step-index InF3 fibers exhibit favorable dispersion properties for SC generation when pumped within the Tm gain window, around 2 μm. Finally, for the scheme where fluoride fibers are concatenated with ChG fibers for further infrared extension of the SC ^[20–22], the use of InF3 fiber instead of ZBLAN can lead to further extension of the MIR SC ^[30].

Acknowledgment. The authors acknowledge technical support from Pascal Duchesne. We also thank Pierre Mathieu and Denis Vincent for their fruitful discussions.