Application of supercontinuum radiation for mid-infrared spectroscopy

Optical fiber based supercontinuum light sources combine the brightness of lasers with the broad bandwidth of incandescent lamps and thus are promising candidates for sources in spectroscopic applications requiring high brightness and broad bandwidth. Herein, near-infrared (IR) Fourier transform (FT) spectrometry with a supercontinuum (SC) light source is investigated. The efficient, collimated propagation of broad bandwidth SC light through an 18 m path length multipass cell is demonstrated. A normalized spectral difference is calculated for the SC spectrum on consecutive FT mirror scans and is found to vary by less than 0.5%, indicating excellent spectral stability. The rms noise on zero absorbance lines is obtained as a function of the number of mirror scans at 0.125, 2, and 16 cm(-1) resolution for both the SC and conventional tungsten lamp source. The SC source has approximately a factor of ten times more noise than the lamp under comparable conditions for each resolution and data acquisition time. This clearly indicates that spectral acquisition with the SC source is not detector noise limited. NIR-FT spectra of methane and methyl salicylate, acquired with both the SC and lamp source, are reported. These spectra illustrate the advantage the SC source has over the incandescent source in that it can efficiently traverse long path lengths, thus providing a sensitivity advantage. The spectra also demonstrate the disadvantage of the SC source with respect to the lamp in the increased level of amplitude noise. Prospects for the future use of SC sources in absorption spectroscopy, including possible noise mitigation strategies, are briefly discussed.

In this work, a high-power all-fiber mid-infrared (MIR) supercontinuum (SC) laser source was generated in a short piece of centimeter level germania-doped fiber (GDF). The MIR SC generation in GDF was pumped by a 2 μm chirped pulse amplification (CPA) system. The seed source was a passively mode-locked oscillator by semiconductor saturable absorber mirror (SESAM) with a pulse width of 29.7 ps, a spectrum width of 0.3 nm and a fundamental repetition rate of 44.31 MHz. A high-power pump source with a spectral coverage of 1.9~2.2 μm was obtained in thulium-doped fiber amplifier (TDFA). Adopted by 19 cm-long 64 mol. % GDF, we obtained a broadband SC source spanning from 1850 to 3000 nm. The generated SC had a maximum output power of 31.2W with a pump slope efficiency from 56.2% to 30.9%. By optimizing fusion-spliced parameters, a high optical coupling efficiency was obtained between single-mode fiber (SMF) and GDF. In addition, we measured the pulse characteristics of SC laser source in different peak power of pumping source and different length of GDF. It paved the way for further realization of high power and wide spectrum MIR SC laser source.

With the rapid developments and widespread applications of supercontinuum (SC) sources, ocular damage induced by this new light source becomes possible and receives our concern. To explore the ocular damage effect of an SC source, a series of experiments were conducted in a chinchilla grey rabbit model to determine the in-vivo retinal damage thresholds induced by a 420-750 nm SC source and a 532 nm laser. For the SC source, the beam divergence and the corneal 1/e2 beam diameter were 3.8 mrad and 2.45 mm, respectively. The determined ED50 values given in terms of total intraocular energy (TIE) for exposure durations of 0.1, 1.0, and 10.0 s were 1.57, 12.1, and 86.0 mJ, respectively. For the 532 nm laser, the beam divergence and the corneal 1/e2 beam diameter were 0.9 mrad and 2.25 mm, respectively. The determined ED50 value for an exposure duration of 0.1 s was 1.39 mJ. By employing the retinal thermal action spectrum in the ICNIRP guidelines, the damage thresholds for SC sources could be compared with the exposure limits for incoherent and laser radiation. Between the 420-750 nm SC source and the 532 nm laser, no significant difference could be found for the damage effects including damage threshold, retinal lesion size, and histological damage characteristics.

Mid-infrared (MIR) fiber-based supercontinuum (SC) sources, with their broadband spectral coverage, high coherence, and integrability, have shown great potential in molecular spectroscopy, environmental monitoring, remote sensing, and biomedical imaging. In particular, MIR sources enables strong molecular absorption and reduced scattering, which allows high-contrast, label-free imaging with enhanced sensitivity. Therefore, the development of compact and stable broadband MIR sources is of significant importance. In this work, we propose and experimentally demonstrate an all-fiber SC source based on fluorotellurite fiber. Compared with commonly used fluoride fibers, fluorotellurite fiber exhibits a higher damage threshold, broader transmission window, and lower hygroscopicity, making it well-suited for generating high-power and stable MIR laser output. The calculated nonlinear coefficient of the fiber is 19 W^-1km^-1 at 2 μm, and its low loss in the 2–4 μm region further supports efficient MIR generation. The system is pumped by amplified broadband nanosecond pulses covering 1.9–2.4 μm, where the wide spectral width and flat-top temporal profile of the pump improve both the flatness and spectral extension of the generated SC. By cascading a specially designed fluorotellurite fiber with a 20 μm core and triple-cladding structure, we achieve an SC spanning from 1.9 to 3.9 μm, with a 10 dB bandwidth of ~1400 nm, an average output power of 949 mW, and a power conversion efficiency of ~25%. The demonstrated source combines compactness, stability, and alignment-free operation, providing a reliable platform for MIR photonics. It is expected to play an important role in advancing biophotonics and other interdisciplinary applications.

Fluoroindate glass fibers have shown intriguing application prospects in the field of mid-infrared (MIR) fiber lasers and supercontinuum (SC) sources because of their wide MIR transmission window, low phonon energy, and nonlinear characteristics. This paper reviews the representative research achievements of fluoroindate glass from the 1980s to the present, comprising the composition optimization, structure, glass melting conditions, optical fiber fabrication process, MIR laser emissions, and SC of fluoroindate glass. Notably, the MIR laser output of the fluoroindate bulk glass and optical fiber was beyond 3 µm, and the power reached to 100 mW levels. SC generated in fluoroindate-based fibers has been greater than 4 µm, and has achieved its highest output power at a 10 W level. Fluoroindate-based fibers are expected to produce high-power MIR laser and SC output based on the reduction of fiber loss and optimization of the pump source.

We demonstrate a 143.4 W high power combined white supercontinuum (SC) source using a (7 × 1) SC fiber combiner. Based on the criteria of adiabatic tapering and brightness conservation, a (7 × 1) SC fiber combiner is designed and theoretically investigated, and the simulated results of the transmission efficiencies of laser at different wavelengths of the combiner verify that the combiner has the ability of combining white SC sources efficiently. Then, the (7 × 1) SC fiber combiner is fabricated, and three white SC fiber laser sources with average power of about 50 W are set up for beam combining experiment. Finally, a 143.4 W high power combined white SC source is achieved with high combining efficiency of 97.4%. and the corresponding combined SC spectrum ranges from 450 nm to 1700 nm, which remains consistent with the overall spectral range of the three SC sources. The experiment result indicates that the (7 × 1) SC fiber combiner is greatly capable of combining high power white SC sources efficiently, thereby increasing the average power of the white SC source significantly. To the best of our knowledge, this is the first demonstration of a fiber combiner for high power white SC sources with such a high combining efficiency.

Diabetes mellitus 1 requires tight control of the blood glucose levels to avoid harmful effects of either too high (hyperglycemia) or too low (hypoglycemia) blood sugar. Due to the availability of low-cost components, fiber- coupled near infrared (NIR) absorption spectroscopy could be a feasible measurement method. From the molar absorptivity of glucose, it is shown that to achieve high accuracy using near infrared spectroscopy for glucose sensing, relative noise levels should not exceed 0:003 %. Two supercontinuum (SC) sources and one broadband lamp were investigated with a low-cost portable spectrometer. The SNR of the two SC sources was limited by amplitude fluctuations and could be improved by averaging. The SNR of the broadband source was found to be largely limited by the detector noise due to the weak intensity. 16 aqueous glucose samples ranging from 0 to 500mm were measured with the broadband source and an SC laser. A partial least squares regression (PLSR) model was built for both measurement sets, yielding root mean square errors of 49 and 54mm, illustrating how the limit of detection is restrained by the high relative intensity noise. A reference arm setup was built and could account for much of the variability of the SC source. A glucose measurement series using this setup and five samples (100 to 500mm) yielded a root mean square error of 10:6mm. The results indicate that an SC source can be feasible for absorption spectroscopy in a reference arm setup.

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.

We have determined the cause of performance variation as a function of system parameters in supercontinuum (SC) sources in a normal-dispersion fiber by numerically analyzing the noise evolution of the SC. High-order nonlinearity was considered in such an analysis for the first time to our knowledge. We found that the evolution of noise along the SC fiber has two stages: First, the noise is governed by dispersion, which is different from that of the signal, whereas self-steeping leads to an asymmetric noise distribution across the spectrum. Second, four-wave mixing generates new noise components, whereas Raman scattering augments the asymmetry. We evaluated the qualities of the spectrally sliced pulses at several stages to verify the analysis and found an asymmetric profile of the sliced-pulses' quality versus frequency.

Mid-infrared (Mid-IR) supercontinuum (SC) generation with a large optical bandwidth and an easily fabricated device is poised to become an enabling technology for Mid-IR SC devices.In this work, we numerically demonstrate Mid-IR SC generation in all-anomalous dispersion AsSe 2 -As 2 S 5 chalcogenide (ChG) hybrid microstructured optical fibers (HMOFs).To achieve Mid-IR SC spectra extending beyond 10 m, As 2 S 5 and AsSe 2 ChG materials were considered, owing to their high transparency in this spectral range.The optimized fiber, consisting of an AsSe 2 core (radius r = 1.9 m) and an As 2 S 5 cladding (radius R = 4.4 m), exhibits all-anomalous dispersion with small magnitudes ranging from +4.08 ps/nm/km to +49.78 ps/nm/km over the wavelength range from3.2m to 10 m and supports a well-confined fundamental TE mode (horizontally polarized), using a two-dimensional finite-difference eigenmode solver.Numerical simulations performed using the split-step Fourier method show that, when pumped with 200-fs pulses and a peak power of 3000 W at a wavelength 3.4 m, the 4.0-mm-long AsSe 2 -As 2 S 5 HMOFs can generate SC spectra spanning from 1.7 m to 9.3 m (-40 dB bandwidth of 7600 nm).By increasing the pulse duration to 300 fs and using the same pump source, the 5.8-mm-long fiber achieves an even broader spectrum, covering 1.7 m to 10.2 m.The nonlinear broadening mechanisms are dominated by self-phase modulation and four-wave mixing.The demonstrated high-performance Mid-IR SC generation-combining ultra-broadband operation, compact structure, and low-energy (< 1 pJ) pulse requirements-shows strong potential for enabling novel on-chip Mid-IR SC device architectures.

Supercontinuum (SC) light sources are nowadays a very common way to access a large span of wavelengths, usually ranging from the near ultraviolet (around 400 nm) to the infrared (around 2.4 μm). It corresponds to a range of interest for many applications in optics for measuring transmission, dispersion, or in biophotonics for achieving fluorescencemicroscopy, optical coherence tomography... Indeed these sources are really promising because they should allow to replace the N laser sources used in these experimental setups to access to all theses wavelengths by a single broad one and a spectral filtering apparatus. Most of these results have been obtained by using powerful pump lasers of several kilo-Watts peak power, operating from the femtosecond (Titane:Saphire) to the nanosecond regimes (Nd:YAG), launched in the low dispersion region of a microstructured optical fiber. Although these fibers are short enough (typically from 1 to 10 m) to neglect the linear absorption during the propagation of the pump, the spectral power density is relatively low (few hundreds of μw/nm)which could limit the implementation of SC sources inmany application devices. This is related to a technological limitation of the pump source because it is not easy to combine strong peak power and high average power. One of the simplest solutions to increase the the spectral power density of SC sources is to replace pulsed sources with continuous-wave (CW) light sources whose available average powers are much more important. Wewill see that the dynamics of SC formation is considerably different in this case, requiring to perform intensive numerical studies to optimize the fiber parameters. Indeed, longer fibers are required (from tens to hundreds of meters) which heightens sensitivity to fiber attenuation, namely of the OHpic absorption, that strongly impacts the soliton evolution. However extremely powerful SCs have been reported with more than 10 mW/nm of spectral power density. Furthermore, these pump sources are usually all-fiber that leads to a second advantage against most of pulsed SC because CW pump can be directly spliced on the PCF. It is also important to point out that these SC sources have different temporal properties than the ones of pulsed SCs. The first experimental demonstration of CW SC have been realized at the end of the nineties with a Raman laser launched in a standard telecommunication fiber. The spectral broadening was relatively restricted (around 200 nm) because it was mainly due to Raman effect Gonzalez-Herraez et al. (2003); Persephonis et al. (1996); Prabhu et al. (2000). A breakthrough was reached a few years later when stronger pump lasers (from more than one order of magnitude) based on Ytterbium doped fibers were combined with photonic crystal fibers 4

We numerically investigate the mid-infrared (MIR) supercontinuum (SC) and SC-based optical frequency comb (OFC) generations when the three optical modes (LP01, LP02, and LP12) are considered in a multimode tellurite photonic crystal fiber (MM-TPCF). The geometrical parameters of the MM-TPCF are optimized to support the multimode propagation and obtain the desired dispersion characteristics of the considered three optical modes. When the pump pulse with center wavelength λ = 2.5 μm, width T = 80 fs, and peak power P = 18 kW is coupled into the anomalous dispersion region of the LP01, LP02, and LP12 modes of the MM-TPCF, the –40-dB bandwidth of the generated MIR SCs can be up to 2.56, 1.39, and 1.12 octaves, respectively, along with good coherence. Moreover, the nonlinear dynamics of the generated SCs are analyzed. Finally, the MIR SCs-based OFCs are demonstrated when a train of 50 pulses at 1-GHz repetition rate is used as the pump source and launched into the MM-TPCF.

A strip titanium dioxide (TiO2) waveguide is designed for highly coherent mid-infrared (MIR) supercontinuum (SC) generation. For the designed TiO2 waveguide, three zero-dispersion wavelengths (ZDWs) are obtained through adjusting the waveguide structure parameters. The three ZDWs are located at 1.53, 3.96, and 5.43 μm, respectively. The nonlinearity coefficient γ is calculated as 1.12 W − 1 m − 1 at wavelength 3.1 μm. By optimizing the pump pulse parameters, the highly coherent MIR SCs are generated when the hyperbolic secant pump pulse with a duration of 80 fs, peak power of 1 kW, and wavelength of 3.1 μm is launched into the TiO2 waveguide and propagated 4.2-mm in length. The obtained SC covers a wavelength range from 1.71 to 9.90 μm (more than 2.5 octaves). Our research results can find important applications in MIR photonics and spectroscopy, biophotonics, optical precision measurement, etc.

The noise properties of supercontinuum (SC) generation have attracted a lot of attention due to a large application demand for low noise SC sources and in the more fundamental context of clarifying links with instabilities in other systems. In typical commercial SC sources the pulse break-up is initiated by noise-driven modulational instability (MI). The resulting large spectral shot-to-shot fluctuations can be significantly reduced by modulating the pump with a phase coherent seed, which leads to a coherent pulse break-up through the amplification of a cascade of four-wave mixing (FWM) side-bands. The paper demonstrates that the noise properties of the generated SC are highly sensitive to the degree of phase noise of the seed and that a nearly coherent seed pulse is needed to achieve a coherent pulse break-up and low noise SC. This limits the mechanisms that can be used to generate the seed, which is important in designing next-generation low-noise SC sources.

A high-power supercontinuum (SC) laser source with 10 dB spectral range of 1.95-3 pm is demonstrated based on a piece of germania-core fiber (GCF). The pump source is a master oscillator power amplifier (MOPA) system composed of a 1.96 pm mode-locked fiber laser and a two-stage thulium-doped fiber amplifier (TDFA) chain. The maximum output power of the SC source is up to 30.1 W, which is, to the best of the authors' knowledge, the highest output power level ever reported for a GCF based SC source as well as a 2-3 pm SC laser source.