Supercontinuum Source Research Articles

Numerical investigation of a wideband supercontinuum source based on a large-mode-area photonic crystal fiber pumped at 1.3 μm

A numerical investigation was conducted to obtain a supercontinuum spanning about two octaves using a large mode area photonic crystal fiber (PCF) pumped at 1.3 μm. In our study, a 1.3 μm femtosecond laser and a silica-based large mode area PCF were selected as the pump source and nonlinear medium, respectively. The nonlinear Schrodinger equation was solved with split-step Fourier method to simulate the evolution of pulse and the broadening of spectrum. The effect of several parameters including the length of PCF, the pulse width, and the average pump power on characteristics of the output spectrum was studied. The simulation results revealed that the supercontinuum extended from near 600 nm to over 2450 nm at 20 dB with length of 30 cm, pulse width of 100 fs and average power of 12 W, respectively. This work proved this large mode area PCF a potentially excellent medium for supercontinuum source and provided some theoretical guidance for future experiments.

40 W of supercontinuum generated by a self-pulsed pump-sharing oscillator-amplifier.

We demonstrate an all-fiber supercontinuum (SC) source delivering up to 40W of average power ranging from 750to 2200nm. The laser source is based on a self-Q-switched pump-sharing oscillator-amplifier. The self-Q-switched master oscillator generates giant pulses, amplified in the high-power stage. Finally, a passive fiber acts as a nonlinear stage, improving the spectrum flatness as well as the spectral broadening. To the best of our knowledge, this is the first time that a pump-sharing oscillator-amplifier is used for SC generation and is based on the use of a submeter Ytterbium-doped fiber length inside the oscillator.

Mid-IR supercontinuum generation using a slab with D-shaped/semi-ellipse core waveguide at 4μm

In the present article, we designed a D-shaped optical waveguide for supercontinuum generation. The designed waveguide consists of a slab with a D-shape core (or semi-ellipse) made of As2Se3 which is sandwiched between top and bottom claddings made of GeAsS chalcogenide glass. Pulse confinement, transmission loss, and nonlinear parameters are taken to be in consideration while optimizing the waveguide parameters. In this work, we have numerically calculated the effect of waveguide parameters on dispersion. In addition, we have studied the effect of the D-shaped parameter ‘b’ on the supercontinuum spectrum. For b = 1.9 μm, the proposed waveguide achieves a broad-band supercontinuum spanning from 1.9 μm to 24.5 μm by pumping 50 fs pulse duration sech pulses at the central wavelength of 4 μm, with a peak power of 5 kW. Apart from this, we have also evaluated the supercontinuum spectra with 1 kW, and 2 kW peak powers. The designed waveguide in this paper is easy to fabricate because of its semi-ellipse core shape. From the numerical results, it is pointed out that the proposed waveguide should be useful in the generation of supercontinuum sources, telecommunications, and spectroscopy in the mid-infrared region.

Exact Solution of the Raman Response Function of Chalcogenide Fiber and Its Influence on the Mid-Infrared Supercontinuum

We fabricated a core-cladding Ge–Sb–Se glass fiber with a Ge12.5Sb15Se72.5 core and Ge15Sb10Se75 cladding, achieved a supercontinuum spectrum spanning from 2 μm to 9 μm by pumping the Ge–Sb–Se fiber with a core diameter of 11 μm using a femtosecond laser pump at 3.8 μm, and numerically simulated the supercontinuum generation using the generalized nonlinear Schrödinger equation. In particular, we investigate the effect of the different Raman response functions that were calculated using the traditional single Lorentzian model and a multiple vibrational mode model on the evolution of the supercontinuum by comparing the supercontinua obtained from simulation and experimental results. We demonstrate that the Raman response function generated by the multiple vibrational mode model captures the actual response behavior of the material, and the supercontinuum generated using this model has more accuracy. To the best of our knowledge, this is the first reported study on supercontinuum generation in Ge–Sb–Se fiber utilizing a Raman response function calculated using the multiple vibrational mode model. This significant advancement enables more accurate simulation of supercontinuum generation in fibers with a multi-peaked structured Raman gain spectrum and holds great potential for optimizing the performance of various mid-infrared supercontinuum sources.

Nonlinear dynamics of relative intensity noise transfer at fiber amplification and supercontinuum generation

Supercontinuum sources via femtosecond pumping in highly nonlinear photonic crystal fibers considerably impact scientific research and industrial applications. Herein, we investigate the complex transfer function of relative intensity noise in a broadband fiber supercontinuum source generated using an ytterbium-doped mode-locked laser. A designed intensity noise was artificially added to the system through the introduction of weak sinusoidal intensity modulation to the seed pulses. Owing to the influence of nonlinear gain dynamics and soliton dynamics during power amplification and spectral broadening, the transfer functions exhibit unique frequency and wavelength dependencies. Systematic measurements indicate that the seed transfer functions exhibit the characteristics of damped high-pass filters and saturate at low Fourier frequencies within the fiber amplifier. Moreover, supercontinuum broadening induces a nonlinear transformation of fluctuations, resulting in considerable amplification and negative correlation within the dispersion wave and soliton self-frequency shift regions. Clarifying the mechanism and bandwidth of this system is imperative to achieving low-noise supercontinuum generation because they provide insights into the nonlinear laser dynamics associated with broadband spectral broadening and noise transformation.

Supercontinuum generation in all-normal dispersion regime of C7H8-core photonic crystal fibers with different lattices

We present a new design of three photonic crystal fibers (PCFs) made of fused silica glass, infiltrated with toluene (C7H8) with different lattices in cladding including circular lattice (CL), hexagonal lattice (HL) and square lattice (SL). The optimal structures that operate in the all-normal dispersion regime, were selected by investigating the dependence of the lattice parameters in the first ring surrounding the core on the PCF characteristics. In addition, we punctuate the possibility of coherent octave-wide supercontinuum (SC) generation in the wavelength range of 0.75–1.68[Formula: see text][Formula: see text]m, 0.76–1.91[Formula: see text][Formula: see text]m (1.3[Formula: see text][Formula: see text]m pump wavelength), and 0.65–1.45[Formula: see text][Formula: see text]m (1.064[Formula: see text][Formula: see text]m pump wavelength) with 90[Formula: see text]fs pulses and 30[Formula: see text]pJ of low energy in-coupled into the core of the selected PCFs. The highest SC generation efficiency is obtained with HL-PCF. The proposed fibers are good candidates for all-fiber SC sources, as a new source of SC light.

Optimization and Dispersion Tailoring of Chalcogenide M-Type Fibers Using a Modified Genetic Algorithm

M-type optical fibers in which a core is surrounded by a thin ring layer with a higher refractive index have attracted increasing attention in recent years. One of their advantageous features is the ability to operate a non-fundamental LP02 mode possessing unusual dispersion properties, namely, a zero-dispersion wavelength (ZDW) shifted to the short wavelength region relative to the material ZDW. The LP02 mode can be selectively excited since it is predominantly localized near the core, while the fundamental LP01 and other higher modes are localized near the ring (for proper fiber parameters). In this paper, we present a comprehensive theoretical analysis of effective dispersion tailoring for the HE12 mode of highly nonlinear chalcogenide glass fibers (for which the LP mode approximation fails due to large refractive index contrasts). We demonstrate fiber designs for which ZDWs can be shifted to the spectral region < 2 μm, which is of great interest for the development of mid-IR supercontinuum sources and frequency-tunable pulse sources with standard near-IR pumping. We obtained the characteristic equation and solved it numerically to find mode fields and dispersion characteristics. We show the possibility of achieving dispersion characteristics of the HE12 mode with one, two, three, and four ZDWs in the wavelength range of 1.5–5.5 μm. We used a modified genetic algorithm (MGA) to design fibers with desired dispersion parameters. In particular, by applying an MGA, we optimized four fiber parameters and constructed a fiber for which HE12 mode dispersion is anomalous in the 1.735–5.155 μm range.

Parallel multispectral ghost imaging data acquisition with supercontinuum

A wavelength multiplexing with spectral supercontinuum source in computational ghost imaging is implemented and investigated. In our proof-of-concept experiment, we used two diffraction gratings and one spatial light modulator to form up to 10 independent spectral channels. Registration of data by a spectrometer in such a scheme makes it possible to reduce the time of data acquisition by a factor of ten. It was established that an increase the number of spectral channels from 1 to 10 leads to the increase of the contrast-to-noise ratio by the factor of 2.8. This makes the proposed technique attractive for high-speed demanding applications such as communications and remote sensing.

Supercontinuum Generation from Airy-Gaussian Pulses in Photonic Crystal Fiber with Three Zero-Dispersion Points

The supercontinuum generation and manipulation of Airy-Gaussian pulses in a photonic crystal fiber with three zero-dispersion points are studied using the split-step Fourier method. Firstly, the spectral evolution of Airy-Gaussian pulses in four photonic crystal fibers with different barrier widths was discussed, and the optimal fiber was determined after considering the factors of width and flatness. By analyzing the mechanism of supercontinuum generation in photonic crystal fibers with single, double and three zero-dispersion points, it is found that the photonic crystal fiber with three zero-dispersion points have a larger spectral width due to the component of tunneling solitons. Then, the effects of four characteristic parameters (truncation factor a, distribution factor χ0, initial chirp C and central wavelength λ) on forming the supercontinuum spectrum of Airy-Gaussian pulses are analyzed in detail. The results show that the spectral width and energy intensity of the dispersive wave and tunneling soliton generation can be well controlled by adjusting the barrier width and initial parameters of the pulse. These research results provide a theoretical basis for generating and manipulating high-power mid-infrared supercontinuum sources.

Imaging human macular pigments with visible light optical coherence tomography and superluminescent diodes.

We demonstrate superluminescent diodes (SLDs) for visible light optical coherence tomography (OCT) of the human retina. SLDs are less costly than supercontinuum sources and have lower intrinsic excess noise, enabling imaging closer to the shot noise limit. While single SLDs are not broadband, they provide power concentrated at specific wavelengths relevant to retinal function. As a new, to the best of our knowledge, application, we image human macular pigments (MPs), which are thought to both aid vision and protect against advanced age-related macular degeneration. Using the unique depth-resolved capabilities of OCT, we localize MPs in depth to Henle's fibers beneath the foveal pit in the living human retina. Our approach reduces the cost of visible light OCT to nearly that of near-infrared (NIR) OCT while also providing information about clinically relevant MPs which cannot be measured in the NIR.

Corneal damage threshold induced by supercontinuum source: A further study.

Previous report shows that corneal spot size is an important influence factor on damage threshold induced by supercontinumm (SC) source. However, damage thresholds were determined for the spot size of only 0.37 mm due to the low output of the employed SC source at that time. The objectives of this study are to determine the lowest possible corneal damage threshold at a large corneal spot size using a more powerful SC source and provide data for the future possible refinements of laser safety standards. A series of experiments was conducted in the New Zealand white rabbit model to determine the corneal damage threshold induced by a 900-2000 nm SC source, with corneal 1/e beam diameter of about 1.2 mm. Slit-lamp biomicroscope was employed to reveal the corneal damage characteristics. By employing the action spectra determined through the analysis of current laser safety guidelines and standards, the effective damage threshold could be calculated. The determined damage threshold given in terms of the peak radiant exposure for the exposure duration of 0.14 s was 44.3 J/cm2 . At threshold level, corneal damages involved the epithelium and the shallower stroma. The safety factor between the effective damage threshold and the corresponding maximum permissible exposure (MPE) was about 45. Present corneal MPEs in the wavelength range of 700-1200 nm may be conservative and corneal damage thresholds for the infrared A wavelengths should be determined in future studies.

All-fiber ultra-flat supercontinuum source covering 1.85–3.57 μm based on germania-core fiber

All-fiber ultra-flat supercontinuum source covering 1.85–3.57 μm based on germania-core fiber

Visible to Mid-IR Supercontinuum Generation in Cascaded PCF-Germanate Fiber Using Femtosecond Yb-Fiber Pump

Broadband supercontinuum (SC) fiber sources covering the mid-IR range have many significant applications, largely due to their compactness, reliability, and ease of use. However, most of the existing SC fiber sources cannot boast of either high reliability or a wide bandwidth. Thus, supercontinuum sources based on silica fibers are robust, but are not capable of generating SC in the mid-IR range. Sources based on soft glasses (tellurite, chalcogenide, etc.) generate broadband SC in the mid-IR range but are not used commercially, due to the poor mechanical and chemical characteristics of such fibers. In this work, we propose a new approach consisting of cascade generation of a supercontinuum sequentially in a silica photonic crystal fiber (PCF) and a germanate fiber. Using a standard ytterbium chirped-pulse amplification (CPA) laser system for pumping, we have demonstrated a supercontinuum in the range of 450–2950 nm in PCF and germanate fiber firmly connected by a standard fusion splicing technique. Further optimization of the cascade pump will make it possible to create a compact and reliable all-fiber SC source from the visible to mid-IR range.

Broadband supercontinuum generation in hollow-core photonic crystal fibers infiltrated with chloroform

The chloroform-infiltrated square lattice photonic crystal fibers (PCFs) are designed with the difference of the air hole diameters in the cladding to improve chromatic dispersion and nonlinear properties. Based on numerical simulation results, two optimal structures with flat dispersion, high nonlinear coefficient, and small confinement loss are used for supercontinuum (SC) generation. The first fiber has an all-normal dispersion regime, a nonlinear coefficient of 486.355[Formula: see text]W[Formula: see text].km[Formula: see text] generating an SC spectrum of 640.3[Formula: see text]nm within 30 dB levels at a pump wavelength of 0.945[Formula: see text][Formula: see text]m with peak power as low as 1.44[Formula: see text]kW. With anomalous dispersion properties, the second fiber gives a very broad SC spectrum of 1471.8[Formula: see text]nm within 30 dB levels at a 1.4[Formula: see text][Formula: see text]m pumping wavelength with a peak power of 20[Formula: see text]kW. All-fiber SC sources operating with low-power pump lasers can use these fibers to effectively replace glass-core fibers.

Supercontinuum generation in As2S3 chalcogenide waveguide pumped by all-fiber structured dual-femtosecond solitons.

Supercontinuum sources with high compactness are essential for applications such as optical sensing, airborne detection and communication systems. In the past decades, the adoption of bulky optical parametric amplifier to pump various chalcogenide glass waveguides are widely reported for on-chip mid-infrared supercontinuum generation, but this usually leads to a large volume of the whole system, and is not practical. Therefore, integrating advanced femtosecond fiber lasers with optical waveguides using nano-fabrication technology are highly desired. However, the scarcity of compact pump sources and the dispersion-matched high-nonlinearity waveguide in short wavelength regions have hindered the advancement of integrated supercontinuum source performances in the near and mid-infrared region. In this study, we demonstrate a broadband supercontinuum source from As2S3 waveguide pumped by a compact dual-femtosecond solitons pulse source. The laser is completely fiber structured, and its wavelength can be readily tuned from 2 to 2.3 µm using Raman soliton self-frequency shift technology in a Tm3+-doped fiber amplifier. Furthermore, the As2S3 waveguide is designed with controllable dispersion and high nonlinearity for a broadband supercontinuum generation. These results will advance the development of on-chip supercontinuum sources based on chalcogenide waveguides.

Unveiling the properties of liquids via photothermal-induced diffraction patterns

The interaction of a laser with a liquid can cause temperature changes in the liquid from which some properties of the latter can be indirectly obtained. However, from just temperature changes, a sample cannot be identified. Here, we report on the interaction of tightly focused femtosecond infrared light into secondary hydrogen-bonded liquids like water, organic compounds, and binary mixtures. Such interaction induces a local change in the sample’s index of refraction. The latter alters the wavefront of a white-light probe beam, giving rise to unique diffraction patterns that can be observed in the far field. The specific diffraction patterns may be considered as the optical signatures or fingerprints of the liquids studied. The technique proposed here is noninvasive and simple to implement with commercially available supercontinuum sources and digital cameras. Thus, it may be useful for the development of compact thermal lens spectroscopic instruments for a number of practical applications.

The potsdam arrayed waveguide spectrograph: A demonstrator of a cross‐dispersed integrated photonic spectrograph developed for the astronomical H‐band

The Potsdam Arrayed Waveguide Spectrograph (PAWS) is based on an integrated photonic spectrograph designed and developed by innoFSPEC to work in the astronomical H‐band. PAWS demonstrates how a traditional bulk optics spectrograph can be miniaturized and additionally serves as a tool for testing in‐house developed astrophotonical devices. The main element is a second‐generation Arrayed Waveguide Grating (AWG) with unprecedented performance in terms of spectral resolution and throughput that is fiber‐coupled and works as a first dispersive component. The pre‐dispersed light is fed into a free‐space optical system located in a cryostat, where the overlapping spectral orders are separated by cross‐dispersion. The resulting echellogram is recorded by a Teledyne 2k×2k H2RG near‐infrared array. Once the cryogenic cross‐dispersion optics inside the vacuum chamber were appropriately aligned to the AWG coupling optics outside of the chamber, different light sources were used to test the functionality of the complete system. A tunable laser source was used to create an echellogram through frame stacking. Additionally, the cross‐dispersed output of a supercontinuum source was recorded using different input waveguides of the AWG, and first light was achieved with laboratory measurements. PAWS represents a pioneering demonstration of an astronomy‐optimized AWG chip, exhibiting the advantageous capabilities of integrated photonic spectrographs, in particular in the context of space applications.

Enhancement of blue and ultraviolet components in PCF-based supercontinuum generation through inter-modal dispersive-wave radiation.

Broadband supercontinuum (SC) light sources generated through nonlinear effects in solid-core photonic crystal fibers (PCFs) have been widely used in spectroscopy, metrology, and microscopy, leading to great application successes. The short-wavelength extension of such SC sources, a longstanding challenge, has been the subject of intensive study over the past two decades. However, the exact mechanism of blue and ultraviolet light generation, especially for some resonance spectral peaks in the short-wavelength regime, is not yet fully understood. Here, we demonstrate that the effect of inter-modal dispersive-wave radiation, which results from phase matching between pump pulses at the fundamental optical mode and packets of linear waves at some higher-order modes (HOMs) propagating in the PCF core, might be one of the critical mechanisms that can result in some resonance spectral components with wavelengths much shorter than that of the pump light. We observed in an experiment that several spectral peaks resided in the blue and ultraviolet regimes of the SC spectrum, whose central wavelengths can be tuned by varying the PCF-core diameter. These experimental results can be interpreted well using the inter-modal phase-matching theory, providing some useful insights into the SC generation process.

Supercontinuum generation in a square-lattice photonic crystal fiber using carbon disulfide infiltration

This study proposed new liquid-filled photonic crystal fiber structures as a new supercontinuum source with low pumping energy. The designed fiber has square lattices on its cross-section and its core being infiltrated by carbon disulfide. We studied numerically optical properties of the square lattice fiber with the modification of its structural parameters including lattice constant (Λ) and hole diameter (d). From the simulated results, two PCF structures supporting the best broadband supercontinuum generations were selected. The first optimized fiber #F1 having lattice pitch Λ = 1.0 µm and filling factor d1/Λ = 0.7 provides an all-normal dispersion. This fiber offers the supercontinuum spectrum broadening from 860 nm to 1738 nm (resulting in a bandwidth of 878 nm) by pumping at a wavelength of 1300 nm, 80 fs pulse duration and input energy of 1.5 nJ, corresponding to the input power of 18.75 kW. The second fiber #F2 (lattice pitch Λ = 1.5 µm, filling factor d1/Λ = 0.3) exhibits mostly anomalous dispersion region. In this fiber, the octave-spanning of the supercontinuum spectrum was formed with a bandwidth of 1935 nm from 594 nm to 2529 nm for an input pulse are 1670 nm pump wavelength, 80 fs duration and energy of 0.5 nJ, corresponding to the input power of 6.25 kW. The proposed fibers are good candidates for low-cost all-fiber sources for potential application in the fields of telecommunications and mid-infrared detection.

Simple experimental realization of optical Hilbert Hotel using scalar and vector fractional vortex beams

Historically, infinity was long considered a vague concept—boundless, endless, larger than the largest—without any quantifiable mathematical foundation. This view changed in the 1800s through the pioneering work of Georg Cantor, who showed that infinite sets follow their own seemingly paradoxical mathematical rules. In 1924, David Hilbert highlighted the strangeness of infinity through a thought experiment now referred to as the Hilbert Hotel paradox, or simply Hilbert’s Hotel. The paradox describes a “fully” occupied imaginary hotel having an infinite number of single-occupancy rooms. The manager can always find a room for new guests by simply shifting current guests to the next highest room, leaving the first room vacant. The investigation of wavefield singularities has uncovered the existence of a direct optical analogy to Hilbert’s thought experiment. Since then, efforts have been made to investigate the properties of Hilbert’s Hotel by controlling the dynamics of phase singularities in “fractional” order optical vortex beams. Here, we have taken such proposals to the next level and experimentally demonstrated Hilbert’s Hotel using both phase and polarization singularities of optical fields. Using a multi-ramped spiral-phase-plate and a supercontinuum source, we generated and controlled fractional order vortex beams for the practical implementation of Hilbert’s Hotel in scalar and vector vortex beams. Using a multi-ramped spiral-phase-plate, we show the possibility for complicated transitions of the generalized Hilbert’s Hotel. The generic experimental scheme illustrates the usefulness of structured beams in visualizing unusual mathematical concepts and also for fractional vector beams driven by fundamental and applied research.