Multimode supercontinuum generation in chalcogenide glass fibres

Spectral-temporal-spatial customization via modulating multimodal nonlinear pulse propagation

Multimode fibers (MMFs) are gaining renewed interest for nonlinear effects due to their high-dimensional spatiotemporal nonlinear dynamics and scalability for high power. High-brightness MMF sources with effective control of the nonlinear processes would offer possibilities in many areas from high-power fiber lasers, to bioimaging and chemical sensing, and to intriguing physics phenomena. Here we present a simple yet effective way of controlling nonlinear effects at high peak power levels. This is achieved by leveraging not only the spatial but also the temporal degrees of freedom during multimodal nonlinear pulse propagation in step-index MMFs, using a programmable fiber shaper that introduces time-dependent disorders. We achieve high tunability in MMF output fields, resulting in a broadband high-peak-power source. Its potential as a nonlinear imaging source is further demonstrated through widely tunable two-photon and three-photon microscopy. These demonstrations provide possibilities for technology advances in nonlinear optics, bioimaging, spectroscopy, optical computing, and material processing.

Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M² factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

Higher order mode supercontinuum generation in tantalum pentoxide (Ta2O5) channel waveguide

We fabricated tantalum pentoxide (Ta₂O₅) channel waveguides and used them to experimentally demonstrate higher-order mode supercontinuum (SC) generation. The Ta₂O₅ waveguide has a high nonlinear refractive index which was in an order magnitude of 10^-14 cm²/W and was designed to be anomalously dispersive at the pumping wavelength. To the best of our knowledge, this is the first time a higher-order mode femtosecond pump based broadband SC has been measured from a nonlinear waveguide using the phase-matching method. This enabled us to demonstrate a SC spectrum spanning from 842 to 1462 nm (at - 30 dB), which corresponds to 0.83 octaves, when using the TM₁₀ waveguide mode. When using the TE₁₀ mode, the SC bandwidth is slightly reduced for the same excitation peak power. In addition, we theoretically estimated and discussed the possibility of using the broadband higher-order modes emitted from the Ta₂O₅ waveguide for trapping nanoparticles. Hence, we believe that demonstrated Ta₂O₅ waveguide are a promising broadband light source for optical applications such as frequency metrology, Raman spectroscopy, molecular spectroscopy and optical coherence tomography.

Higher-order mode supercontinuum generation in dispersion-engineered liquid-core fibers

Supercontinuum generation enabled a series of key technologies such as frequency comb sources, ultrashort pulse sources in the ultraviolet or the mid-infrared, as well as broadband light sources for spectroscopic methods in biophotonics. Recent advances utilizing higher-order modes have shown the potential to boost both bandwidth and modal output distribution of supercontinuum sources. However, the strive towards a breakthrough technology is hampered by the limited control over the intra- and intermodal nonlinear processes in the highly multi-modal silica fibers commonly used. Here, we investigate the ultrafast nonlinear dynamics of soliton-based supercontinuum generation and the associated mode coupling within the first three lowest-order modes of accurately dispersion-engineered liquid-core fibers. By measuring the energy-spectral evolutions and the spatial distributions of the various generated spectral features polarization-resolved, soliton fission and dispersive wave formation are identified as the origins of the nonlinear broadening. Measured results are confirmed by nonlinear simulations taking advantage of the accurate modeling capabilities of the ideal step-index geometry of our liquid-core platform. While operating in the telecommunications domain, our study allows further advances in nonlinear switching in emerging higher-order mode fiber networks as well as novel insights into the sophisticated nonlinear dynamics and broadband light generation in pre-selected polarization states.

22.7 W mid-infrared supercontinuum generation in fluorotellurite fibers

In this Letter, we demonstrate 22.7 W mid-infrared (MIR) supercontinuum (SC) generation in all-solid fluorotellurite fibers. All-solid fluorotellurite fibers based on ${{\rm TeO}_2} {\text -} {{\rm BaF}_2}{\text -}{{\rm Y}_2}{{\rm O}_3}$TeO₂-BaF₂-Y₂O₃ and ${{\rm TeO}_2}$TeO₂ modified fluoroaluminate glasses are fabricated by using a rod-in-tube method. By using a 0.6 m long fluorotellurite fiber with a core diameter of 11 µm as the nonlinear medium and a high-power 1.93-2.5 µm SC fiber laser as the pump source, we obtain 22.7 W SC generation from 0.93 to 3.95 µm in the fiber for a pump power of 39.7 W. The 10 dB bandwidth is about 1633 nm, and the corresponding spectral range is from 1890 to 3523 nm. The optical-to-optical conversion efficiency is about 57.2%. Our results show that all-solid fluorotellurite fibers are promising nonlinear media for constructing high-power MIR SC light sources.

Coherent Mid-IR Supercontinuum Generation using Tapered Chalcogenide Step-Index Optical Fiber: Experiment and modelling

Mid-infrared region of electromagnetic spectrum has increased a lot of scientific and technical interest because of its utility to figure out the molecular fingerprints. Current mid-infrared light sources including quantum cascade lasers, thermal-emitters, and synchrotron radiation are not suitable for various potential applications where we require coherent, portable and broadband light sources. During the current decade, several efforts have been put forwarded to extend the spectral range of the supercontinuum. However, the coherent mid-infrared supercontinuum spectrum in the mid-infrared region has been demonstrated rarely. Here, we demonstrate a coherent mid-infrared supercontinuum using a tapered chalcogenide fiber pumped at various wavelength ranging from 2 µm to 2.6 µm. Experimental observations show that the supercontinuum spectrum extending from ~1.6 µm to 3.7 µm can be achieved using a 3 cm long tapered chalcogenide step-index optical fiber pumped with femtosecond laser pulses at 2.6 µm. To the best of our knowledge, using short pump wavelengths at 2 µm to 2.6 µm in an all-normal dispersion engineered chalcogenide glass fiber, the coherent supercontinuum spectrum has been reported first time. Such coherent broadband light source has its key prominence for the various prospective applications in the fields of bio-medical, sensing, and multiplex coherent anti-Stokes Raman scattering microspectroscopy.

High-power supercontinuum generation in the mid-infrared pumped by a soliton self-frequency shifted source

We report the demonstration of a fiber-based supercontinuum source delivering up to 825 mW of average output power between 2.5 and 5.0 µm generated in all-normal dispersion regime. The pumping source consists of an amplified ultrafast Er^3+:ZrF₄ fiber laser providing high peak power femtosecond pulses at 3.6 µm with an average output power exceeding the watt-level. These pulses are spectrally broadened through self-phase modulation using commercial chalcogenide-based step-index fibers. Al₂O₃ anti-reflection coatings were sputtered on chalcogenide fiber tips to increase the launching efficiency from 54% to 82%, making this record output power possible, and thus confirming that such coatings can support watt-level pumping with intense femtosecond pulses. To the best of our knowledge, this result represents the highest average output power ever achieved from a As₂Se₃-based mid-IR supercontinuum source with the potential of a high degree of coherence.

1.2-15.2 μm supercontinuum generation in a low-loss chalcohalide fiber pumped at a deep anomalous-dispersion region

A novel low-loss selenium-based chalcohalide fiber, with a low zero-dispersion wavelength, was prepared by an innovative preparation process. The composition optimized fiber has a wide transmission range of up to 11.5 μm, a lowest fundamental mode zero-dispersion wavelength of 4.03 μm, and a minimum optical loss of 1.12 dB/m at 6.4 μm, which provides a possibility to replace As₂S₃ and As₂Se₃ in a cascade of ZrF₄-BaF₂-LaF₃-AlF₃-NaF(ZBLAN)-As₂S₃-As₂Se₃ fiber in the practical all-fiberized supercontinuum (SC) source. Meanwhile, the broadest SC spectrum, ∼1.2 to 15.2 μm, was achieved by pumping a 12-cm-long fiber with a femtosecond laser at a deep anomalous-dispersion region. Furthermore, simulations are adopted to interpret the results as well as to demonstrate spectral evolution along the fiber. To the best of our knowledge, this is the broadest SC spectrum reported in any selenium-based chalcogenide fiber.

Ultrabroadband and coherent mid-infrared supercontinuum generation in Te-based chalcogenide tapered fiber with all-normal dispersion

We demonstrated an ultrabroadband supercontinuum (SC) generation with high coherence property in all-normal-dispersion (ANDi) Te-based chalcogenide tapered fiber. The fibers made of Ge₂₀As₂₀Se₁₅Te₄₅ core and Ge₂₀As₂₀Se₂₀Te₄₀ cladding glasses were fabricated via isolated stacked extrusion. The waist diameter and length can be accurately controlled by a homemade tapering platform. When the core diameter of the waist was ≤14 μm, the fiber showed an ANDi characteristic in the wavelength range of 1.7-14 μm. A coherent SC generation covered 1.7-12.7 μm was generated in a 7-cm-long tapered fiber, pumped at 5.5 μm. To the best of our knowledge, this is the first SC experimental demonstration in Te-based step-index tapered fiber and the broadest SC generation in chalcogenide tapered fiber when pumped in the normal dispersion regime so far.

Mid-infrared flattened supercontinuum generation in all-normal dispersion tellurium chalcogenide fiber

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.

Generation of near-diffraction-limited, high-power supercontinuum from 1.57 μm to 12 μm with cascaded fluoride and chalcogenide fibers

We generate a supercontinuum (SC) spectrum ranging from 1.57 μm to 12 μm (20 dB bandwidth) with a soft glass fiber cascade consisting of ZrF₄-BaF₂-LaF₃-AlF₃-NaF fiber, As₂S₃ fiber, and As₂Se₃ fiber pumped by a nanosecond thulium master oscillator power amplifier system. The highest on-time average power generated is 417 mW at 33% duty cycle. We observe a near-diffraction-limit beam quality across the wavelength range from 3 μm to 12 μm, even though the As₂Se₃ fiber is multimode below 12 μm. Our study also shows that parameters of the As₂Se₃ fiber, such as numerical aperture, core size, and core/cladding composition, have significant effects on the long wavelength edge of the generated SC spectrum. Our results suggest that the high numerical aperture of 0.76 and low-loss As₂Se₃/GeAs₂Se₅ core/cladding material all contribute to broad SC generation in the long-wave infrared spectral region. Also, among our results, 10 μm core diameter selenide fiber yields the best spectral expansion, while the 12 μm core diameter selenide fiber yields the highest output power.

Mid-infrared supercontinuum generation in multimode As2 Se3 chalcogenide photonic crystal fiber

We rigorously investigated the mid-infrared supercontinuum generation in a multimode As₂Se₃ chalcogenide photonic crystal fiber (PCF). We studied the impact of the intermodal nonlinear effects on the nonlinear propagation of the fundamental and high-order modes. By solving the multimode generalized nonlinear Schrödinger equation, we have predicted the generation of a very broadband supercontinuum in both polarizations of the fundamental mode spanning from 2 to 11 μm at -20  dB in only 5 cm PCF length. The proposed study confirms that the energy transfer occurs only between the optical degenerate modes when propagating in the multimode chalcogenide PCF in the mid-infrared region.

1.5-14 μm midinfrared supercontinuum generation in a low-loss Te-based chalcogenide step-index fiber

We have experimentally demonstrated midinfrared (MIR) supercontinuum (SC) generation in a low-loss Te-based chalcogenide (ChG) step-index fiber. The fiber, fabricated by an isolated extrusion method, has an optical loss of 2-3 dB/m at 6.2-10.3 μm and 3.2 dB/m at 10.6 μm, the lowest value reported for any Te-based ChG step-index fiber. A MIR SC spectrum (∼1.5 to 14 μm) is generated from the 23-cm fiber pumped by a 4.5 μm laser (∼150  fs, 1 kHz). To the best of our knowledge, this is the first SC experimental demonstration in Te-based ChG fiber and the broadest MIR SC generation pumped in the normal dispersion regime in the optical fibers.

Mid-infrared supercontinuum generation based on cascaded Raman scattering in a few-mode As2S3 fiber pumped by a thulium-doped fiber laser

By pumping a 1.7-m-long As₂S₃ fiber at 2050 nm directly, a fiber-based mid-infrared supercontinuum (SC) source with an output power of 366 mW is demonstrated. This is the first experimental demonstration to obtain such a mid-infrared SC in a piece of chalcogenide fiber pumped at 2 μm directly. The cut-off wavelength of the As₂S₃ fiber is 3.5 μm, indicating that it could support several modes at around 2 μm. It is found that nonlinear spectral broadening mechanisms in the few-mode chalcogenide fiber could be affected through adjusting the butt-coupling position. That is because different positions will excite different modes that correspondingly possess different nonlinearity and dispersion characteristics. When stimulated Raman scattering (SRS) corresponding to the excitation of the fundamental mode becomes dominant in this few-mode fiber, an efficient cascaded SRS-based SC is obtained with five Stokes peaks ranging from 2 μm to 3.4 μm. Results from numerical simulation are in accord with the experimental results, showing that it is feasible to obtain an SRS based mid-infrared SC in a step-index As₂S₃ fiber by using a 2 μm high peak power picosecond laser to pump directly.

Visible supercontinuum generation in a graded index multimode fiber pumped at 1064 nm

We observe efficient supercontinuum generation that extends into the visible spectral range by pumping a low differential mode group delay graded index multimode fiber in the normal dispersion regime. For a 28.5 m long fiber, the generated spectrum spans more than two octaves, starting from below 450 nm and extending beyond 2400 nm. The main nonlinear mechanisms contributing to the visible spectrum generation are attributed to multipath four-wave mixing processes and periodic spatio-temporal breathing dynamics. Moreover, by exploiting the highly multimodal nature of this system, we demonstrate versatile generation of visible spectral peaks in shorter fiber spans by altering the launching conditions. A nonlinearly induced mode cleanup was also observed at the pump wavelength. Our results could pave the way for high brightness, high power, and compact, multi-octave continuum sources.

Mid-IR supercontinuum from 2.4 to 5.4 μm in a low-loss fluoroindate fiber

A mid-infrared supercontinuum extending up to 5.4 μm is generated in a low-loss fluoroindate fiber. It is pumped with an erbium-doped fluoride fiber amplifier seeded with 400 ps pulses at 2.75 μm. Both fibers are fusion spliced to increase the robustness and long-term stability of the system. With more than 82% of the total power beyond 3 μm, this approach is promising for efficient mid-IR light generation.