Supercontinuum Light Research Articles

Ring shaped fs supercontinuum with a thermally induced self-diffraction effect.

A broadband femtosecond supercontinuum (SC) light with a dark core has been generated in a pump-probe type geometry. It has been achieved by introducing a small thermal perturbation in a highly absorbing medium with a continuous wave pump laser. The SC was generated with the help of a photonic crystal fiber and a nJ femtosecond oscillator. Numerical simulations for the system were done by employing the Fresnel-Kirchhoff formula. The power dependence of the dark core diameter was also studied experimentally, and the results agree well with numerical simulations. The dark core size can be controlled by varying the input power of the pump beam.

Experimental investigation of supercontinuum generation in a birefringence tellurite microstructured optical fiber.

A four-hole birefringence tellurite microstructured optical fiber (BTMOF) was designed and fabricated based on 76.5T e O 2-6Z n O-11.5L i 2 O-6B i 2 O 3 glass, and its core (slow and fast axes were) measured to be approximately 4.74µm and 4.29µm, respectively. The experimentally measured results demonstrated that the maximum supercontinuum (SC) spectra extended from ∼914.1n m to ∼1885.1n m when the polarization state of the pump pulse was parallel to the fast axis at 1400nm with an average power of 460mW. We performed numerical simulations based on the nonlinear Schrödinger equation, which support the experimentally measured results. The SC generation in birefringent silica microstructured fiber with the same geometric parameters was simulated, and the results showed that the enhanced nonlinear refractive index of the BTMOF yielded a spectrum with a significantly larger bandwidth. Furthermore, the two polarization states along the fast axis and slow axis exhibit different dispersion characteristics, which provide a convenient way of tuning the properties of the generated SC. This work highlights BTMOF as a promising platform for the development of a SC light source, which can be widely used in food quality inspection, early cancer diagnostics, gas sensing, and high-spatial-resolution imaging.

Intermodal dispersive waves and soliton collision during multimode supercontinuum generation in chalcogenide glass fiber

Comparing with fused silica fiber, chalcogenide glass fibers are considered as an ideal platform for mid-infrared supercontinuum generation (SCG) because of its higher nonlinear coefficient and wider transparency window. Multimode fiber provides a new way to achieve special spectral characteristics in SCG by introducing intermodal interaction. We investigate the intermodal nonlinear phenomena associated with SCG in multimode chalcogenide fiber by numerically solving multimode generalized nonlinear Schrödinger equation. The results indicate that when the phase matching condition is satisfied, the intermodal interaction leads to the generation of dispersive waves at mid-infrared wavelength. Furthermore, intermodal and intramodal soliton collision are observed, and the bandwidth of the supercontinuum is increased by the intermodal interaction. These results may provide a new way for the generation of mid-infrared supercontinuum light source.

Detection of gaseous nerve agent simulants with broadband photoacoustic spectroscopy

While recent high-profile assassination attempts have once again brought nerve agents (NAs) into the spotlight, the current portfolio of NA sensors lack a sufficient combination of high performance and field-deployability. Here, we report a novel optical sensor for the detection of gaseous NAs with a potential to fill this gap. The technique is based on Fourier transform spectroscopy with a supercontinuum (SC) light source and cantilever-enhanced photoacoustic detection providing fast multi-species gas sensing with high sensitivity and selectivity in a sample volume of 7 ml, which becomes advantageous when analysing limited NA samples in the field. We study the fundamental C–H stretch bands of four known NA simulants and achieve detection limits of 64–530 ppb in one minute and recovery times of a few minutes. In the near future, the technique has significant potential to improve through the development of more powerful SC sources further in the mid-infrared region.

Insights into the charge carrier dynamics in perovskite/Si tandem solar cells using transient photocurrent spectroscopy

Direct bandgap perovskite and indirect bandgap Si, which form the two active layers in a tandem solar cell configuration, have different optoelectronic properties and thicknesses. The charge-carrier dynamics of the two-terminal perovskite-on-Si tandem solar cell in response to a supercontinuum light pulse is studied using transient photocurrent (TPC) measurements. Spectral dependence of TPC lifetime is observed and can be classified into two distinct timescales based on their respective carrier generation regions. The faster timescale (∼500 ns) corresponding to the spectral window (300–750 nm) represents the top-perovskite sub-cell, while the slower timescale regime of ∼25 μs corresponds to the bottom-Si sub-cell (>700 nm). Additionally, under light-bias conditions, the transient carrier dynamics of the perovskite sub-cell is observed to be coupled with that of the Si sub-cell. A sharp crossover from the fast-response to a slow-response of the device as a function of the light-bias intensity is observed. These results along with a model based on transfer matrix formulation highlight the role of charge-carrier dynamics in accessing higher efficiencies in tandem solar cells. The carrier transit times and lifetimes in addition to their optical properties need to be taken into account for optimizing the performance.

25.8 W All-Fiber Mid-Infrared Supercontinuum Light Sources Based on Fluorotellurite Fibers

In this letter, we demonstrate a 25.8 W all-fiber mid-infrared supercontinuum (SC) light source based on fluorotellurite fibers. All-solid fluorotellurite fibers are fabricated by using a rod-in-tube method. A 56 cm long fluorotellurite fiber with a core diameter of <inline-formula> <tex-math notation="LaTeX">$\sim 11~\mu \text{m}$ </tex-math></inline-formula> is used as the nonlinear medium and a high power <inline-formula> <tex-math notation="LaTeX">$1.93\sim 2.5~\mu \text{m}$ </tex-math></inline-formula> SC fiber laser as the pump source. The fluorotellurite fiber is connected to the output silica fiber of the pump source by direct fusion splicing, leading to the formation of the compact all-fiber structure. For a pump power of &#x007E;42.6 W, we obtain a 25.8 W SC light source with a spectral range from 0.93 to 3.99 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>. The corresponding optical-to-optical conversion efficiency is about 60.6&#x0025;. To the best of our knowledge, this is the first time to report all-fiber high power mid-infrared SC light source based on fluorotellurite fibers. Our results pave the way to apply mid-infrared SC light sources based on fluorotellurite fibers for real applications.

Supercontinuum intensity noise coupling in Fourier transform photoacoustic spectroscopy.

We investigate the noise transfer mechanism from the light source intensity fluctuations to the acoustic signal in Fourier transform photoacoustic spectroscopy (FT-PAS). This noise coupling is expected to be reduced in FT-PAS compared with conventional Fourier transform spectroscopy, as only the specific spectral components that are absorbed by the probed sample contribute to the noise level. We employ an incoherent supercontinuum (SC) light source in our experiments and observe a linear relation between the sample gas concentration and the detected noise level, which significantly reduces the influence of the SC noise on the detection limit. Based on our experimental results, we derive a model for the noise level, which establishes the foundation for practical sensitive implementation of FT-PAS.

Ultrahigh resolution spectral-domain optical coherence tomography using the 1000-1600 nm spectral band.

Ultrahigh resolution optical coherence tomography (UHR-OCT) can image microscopic features that are not visible with the standard OCT resolution of 5-15 µm. In previous studies, high-speed UHR-OCT has been accomplished within the visible (VIS) and near-infrared (NIR-I) spectral ranges, specifically within 550-950 nm. Here, we present a spectral domain UHR-OCT system operating in a short-wavelength infrared (SWIR) range from 1000 to 1600 nm using a supercontinuum light source and an InGaAs-based spectrometer. We obtained an axial resolution of 2.6 µm in air, the highest ever recorded in the SWIR window to our knowledge, with deeper penetration into tissues than VIS or NIR-I light. We demonstrate imaging of conduction fibers of the left bundle branch in freshly excised porcine hearts. These results suggest a potential for deep-penetration, ultrahigh resolution OCT in intraoperative applications.

Direct f-3f self-referencing using an integrated silicon-nitride waveguide.

We have achieved the simultaneous generation of a 2.6-octave-wide supercontinuum (SC) spectrum over 400-2500 nm and third-harmonic light solely by a dispersion-controlled silicon-nitride waveguide (SiNW). To increase the visible intensity of the SC light component, we fabricated low-loss 5-mm-long deuterated SiNWs with spot-size converters by low-temperature deposition. We succeeded in measuring the carrier-envelope-offset (CEO) signal with a 34-dB signal-to-noise ratio because this short deuterated SiNW provides a large temporal overlap between the f and 3f components. In addition, we have demonstrated this method of CEO locking at telecommunications wavelengths with f-3f self-referencing generated solely by the SiNW without the use of highly nonlinear fiber and an additional nonlinear crystal. Compared with the method of CEO locking with a highly nonlinear fiber and a standard f-2f self-referencing interferometer, this method is not only simple and compact but also stable.

Selective excitation of different combinations of LP01 and LP11 polarization modes in a birefringent optical fiber using a Wollaston prism.

We present an effective method for free-space selective excitation of different combinations of LP01 and LP11 polarization modes in a birefringent optical fiber using a Wollaston prism, rotatable polarizer, and achromatic half-wave plate. The method is minimally wavelength-dependent and can be used for high-power sources. The relative coupling efficiencies of different modes can be continuously tuned and the suppression rate of the unwanted modes with respect to the targeted mode exceeds 20 dB. We present input system configurations that allow for the excitation of different individual modes and groups of modes and estimate the maximum coupling efficiencies based on numerical simulations. As example applications, we show the generation of Raman sidebands in different modes, gain tunability of intermodal four-wave mixing, and broadband conversion of a supercontinuum light beam from the fundamental to the LP11 mode.

Low-frequency multiplex CARS microscopy with a high-repetition near-infrared supercontinuum laser

We have developed a multiplex coherent anti-Stokes Raman scattering microscope effective for low-wavenumber measurement by combining a high-repetition supercontinuum light source of 1064 nm and a near-infrared high-sensitivity InGaAs diode array. This system could observe the low-wavenumber region down to 55 cm−1 with high sensitivity. In addition, using spectrum shaping and spectrum modulation techniques, we simultaneously realized a wide bandwidth (<1800 cm−1), high wavenumber resolution (9 cm−1), high efficiency, and increasing signal-to-noise ratio by reducing the effect of the background shape in the low-wavenumber region. Spatial variation of a sulfur crystal phase transition with metastable states was visualized.

Active hyperspectral imager using a tunable supercontinuum light source based on a MEMS Fabry-Perot interferometer.

We have developed an active hyperspectral imager based on a tunable near-infrared supercontinuum light source. Non-dispersive wavelength selection of the supercontinuum laser source is achieved with a microelectromechanical Fabry-Perot interferometer. The tunable light source enables the use of any monochromatic imaging sensor with a suitable spectral sensitivity for hyperspectral imaging. The imager is characterized and demonstrated in the laboratory for remote detection of ice.

Si-Based Infrared Detector Using Organic Nano-Dot Array

Silicon-based infrared photodetector is proposed using an organic nano-dot array covered with an Au thin film. The nano-dots were self-assembled structures composed of crystalline CuPc and PTCDA organic semiconductors, and they enhanced the absorption of the incident infrared light. The annealing process at the CuPc and PTCDA deposition formed these semiconductor materials to be a nano-dot array shape. The Au-coated nano-dots presented broadband reflectance decrease in the near- to mid-infrared region. The Arrhenius plot investigation revealed that the proposed photodetector had 0.4-0.43 eV activation energy, a sub-Si-bandgap energy barrier. The photodetection experiments in the near-infrared region presented the proposed photodetector discriminated on and off of 2.07- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -long super continuum light. In addition, band-pass filtered emission from a black body cavity with a wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.05~\mu \text{m}$ </tex-math></inline-formula> could be measured. Since the derivation of the activation energy and the cut-off wavelength showed a good agreement, the organic semiconductor generated a carrier at the photodetection. A possible detection mechanism was thus attributed to carrier excitations at an interface between organic semiconductors, CuPc and PTCDA. The proposed structure contributes to fabricate mid-infrared silicon-based photodetectors with high light absorption performance.

Chromatic-aberration-free multispectral optical-resolution photoacoustic microscopy using reflective optics and a supercontinuum light source.

A supercontinuum (SC) light source enables multispectral photoacoustic imaging at excitation wavelengths in the visible-to-near-infrared range. However, for such a broad optical wavelength range, chromatic aberration is non-negligible. We developed a multispectral optical-resolution photoacoustic microscopy (MS-OR-PAM) setup with a nanosecond pulsed SC light source and a reflective objective lens to avoid chromatic aberration. Chromatic aberrations generated by reflective and conventional objective lenses were compared, and the images acquired using the reflective objective were not affected by chromatic aberration. Hence, MS-OR-PAM with the reflective objective was used to distinguish red blood cells from melanoma cells via spectral subtraction processing.

Time-domain NIRS system based on supercontinuum light source and multi-wavelength detection: validation for tissue oxygenation studies.

We present and validate a multi-wavelength time-domain near-infrared spectroscopy (TD-NIRS) system that avoids switching wavelengths and instead exploits the full capability of a supercontinuum light source by emitting and acquiring signals for the whole chosen range of wavelengths. The system was designed for muscle and brain oxygenation monitoring in a clinical environment. A pulsed supercontinuum laser emits broadband light and each of two detection modules acquires the distributions of times of flight of photons (DTOFs) for 16 spectral channels (used width 12.5 nm / channel), providing a total of 32 DTOFs at up to 3 Hz. Two emitting fibers and two detection fiber bundles allow simultaneous measurements at two positions on the tissue or at two source-detector separations. Three established protocols (BIP, MEDPHOT, and nEUROPt) were used to quantitatively assess the system’s performance, including linearity, coupling, accuracy, and depth sensitivity. Measurements were performed on 32 homogeneous phantoms and two inhomogeneous phantoms (solid and liquid). Furthermore, measurements on two blood-lipid phantoms with a varied amount of blood and Intralipid provide the strongest validation for accurate tissue oximetry. The retrieved hemoglobin concentrations and oxygen saturation match well with the reference values that were obtained using a commercially available NIRS system (OxiplexTS) and a blood gas analyzer (ABL90 FLEX), except a discrepancy occurs for the lowest amount of Intralipid. In-vivo measurements on the forearm of three healthy volunteers during arterial (250 mmHg) and venous (60 mmHg) cuff occlusions provide an example of tissue monitoring during the expected hemodynamic changes that follow previously well-described physiologies. All results, including quantitative parameters, can be compared to other systems that report similar tests. Overall, the presented TD-NIRS system has an exemplary performance evaluated with state-of-the-art performance assessment methods.

Ultra-fast dynamic line-field optical coherence elastography.

In this work, we present an ultra-fast line-field optical coherence elastography system (LF-OCE) with an 11.5 MHz equivalent A-line rate. The system was composed of a line-field spectral domain optical coherence tomography system based on a supercontinuum light source, Michelson-type interferometer, and a high-speed 2D spectrometer. The system performed ultra-fast imaging of elastic waves in tissue-mimicking phantoms of various elasticities. The results corroborated well with mechanical testing. Following validation, LF-OCE measurements were made in in situ and in in vivo rabbit corneas under various conditions. The results show the capability of the system to rapidly image elastic waves in tissues.

Plasmonic heptamer-arranged nanoholes in a gold film on the end-facet of a photonic crystal fiber.

We use the end-facet of a solid-core polarization-maintaining photonic crystal fiber (PM-PCF) as a platform on which to fabricate resonant plasmonic nanostructures. Solid-core PM-PCFs can be excited in a polarization-aligned single mode by supercontinuum light, so they are well-suited to the wavelength-interrogation of resonant plasmonic nanostructures, especially supporting complex spectra over a broad spectral range. The nanostructures implemented consist of an array of heptamer-arranged nanoholes formed in a thin Au film. The nanoholes were milled with a He+ focused ion beam, with the array polarization-aligned in situ to cover the solid core of the PM-PCF. Transmittance spectra, measured using a supercontinuum source coupled to the input of the PM-PCF, reveal a rich set of Fano resonances associated with localized and propagating surface plasmons. The measured spectra are compared to computations in order to identify the resonant modes. The spectra redshift as the medium covering the nanoholes changes from air to oil, anticipating application to sensing.

Multichromatic Polarization-Controlled Pulse Sequences for Coherent Control of Multiphoton Ionization

In this review, we report on recent progress in the generation and application of multichromatic polarization-tailored pulse sequences for the coherent control of multiphoton ionization (MPI) dynamics and present unpublished experimental results that complement our previous findings. Specifically, we utilize single-color, bichromatic, and trichromatic polarization-controlled pulse sequences generated by spectral amplitude, phase and polarization modulation of a carrier-envelope phase (CEP)-stable white light supercontinuum for MPI. The analysis of the number of ionization pathways and the number of distinct final free electron states shows that both increase significantly, but scale differently with the number of absorbed photons and the number of pulses in the sequence. In our experiments, ultrafast polarization shaping is combined with high-resolution photoelectron tomography to generate, control, and reconstruct three-dimensional photoelectron momentum distributions from atomic and molecular MPI. We discuss the use of polarization-controlled single-color and bichromatic pulse sequences in perturbative and non-perturbative coherent control of coupled electron-nuclear dynamics in molecules, atomic spin-orbit wave packet dynamics and the directional photoemission from atoms and chiral molecules. We compare the coherent control of CEP-insensitive intraband multipath interference in the MPI with a fixed number of photons with CEP-sensitive interband multipath interference in the ionization with a different number of photons. The generation and control of free electron vortices with even-numbered rotational symmetry by MPI with single-color pulse sequences is contrasted with the bichromatic control of CEP-sensitive electron vortices with odd-numbered rotational symmetry. To illustrate the potential of multichromatic pulse sequences for coherent control, we present a trichromatic scheme for shaper-based quantum state holography.

Silica-based photonic crystal fiber infiltrated with 1,2-dibromoethane for supercontinuum generation

This study proposes a photonic crystal fiber (PCF) made of fused silica glass with the core infiltrated with 1,2-dibromoethane (C2H4Br2) as a new source of supercontinuum light pulses. Due to the modifications of the PCF's structure geometry, a number of computer simulations investigating their optimized structures has been carried out. This aimed at achieving flat near-zero dispersion and zero dispersion wavelength matching of the pump wavelength for efficient spectral broadening. Based on the obtained results, the structural geometries of two C2H4Br2-core PCFs were optimized using numerical modeling for broadband supercontinuum (SC) generation. The first fiber structure with a lattice constant 1.5 µm and filling factor 0.4 has all-normal dispersion profile. The SC with a broadened spectral bandwidth from 0.64 to 1.70 µm is generated by pump pulses centered at a wavelength of 1.03 µm, 120 fs duration, and energy of 1.5 nJ. The second proposed structure-with lattice constant 1.5 µm and filling factor 0.65-has anomalous dispersion for wavelengths longer than 1.03 µm. We obtained high coherence of the SC pulses in the anomalous dispersion range over wavelengths of 0.7-2.4 µm with the same pump pulse as the first fiber and with input energy of 0.09 nJ. These fibers would be interesting candidates for all-fiber SC sources operating with low-energy pump lasers as cost-effective alternatives to glass core fibers.

Mid-infrared cascaded stimulated Raman scattering and flat supercontinuum generation in an As-S optical fiber pump at 2 µm.

We demonstrate broadband mid-infrared cascaded stimulated Raman scattering (SRS) and flat supercontinuum (SC) generation in a chalcogenide optical fiber made from As2S5 glass. By using a 2 µm nanosecond laser as the pump source, mid-infrared cascaded SRS up to six orders ranging from 2149 to 3425 nm was experimentally observed, and this all-fiber Raman laser operating at 3.43 µm was realized for the first time to our knowledge. By introducing a 2 µm femtosecond laser as the excited source, the broadband flat mid-infrared SC with the spectral range of ∼10dB (from ∼1030 to 3441 nm) was observed. Our results verify that the As2S5 optical fibers possess promising applications for tunable mid-infrared Raman fiber lasers and SC light sources pumped by 2 µm pulsed lasers.