Broadband Supercontinuum Research Articles

High contrast glasses for all-solid fibers fabrication

We present composition development of borosilicate glasses for fabrication of high refractive index contrast, all-solid photonic crystal fibers. An oxide system composed of SiO2B2O3Al2O3Li2ONa2OK2O was adjusted to match thermal properties of selected highly nonlinear, lead bismuth gallium silicate glass. A high difference of refractive index of 0.376 is achieved at a wavelength of 1550 nm. We proved experimentally that the developed pair of glasses enables to draw optical fibers, and we propose a design of a photonic crystal fiber structure for broadband supercontinuum generation.

Broadband supercontinuum generation with femtosecond pulse width in erbium-doped fiber laser (EDFL)

We demonstrate two flat plateaus and the low-noise spectrum of supercontinuum generation (SCG) in a highly nonlinear fiber (HNLF), injected by an amplified picosecond pulse seed of a carbon nanotube-based passively mode locked erbium-doped fiber laser. A broad spectrum of width approximately 1090 nm spanning the range 1130–2220 nm is obtained and the pulse width is compressed to the shorter duration of 70 fs. Variations of the injected peak power up to 33.78 kW into the HNLF are compared and the broad spectrum SCG profiles slightly expand for each of the injected peak powers. This straightforward configuration of SCG offers low output power and ultra-narrow femtosecond pulse width. The results facilitate the development of all fiber time-domain spectroscopy systems based on the photoconductive antenna technique.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores.

Photonic crystal fibers (PCFs) made from ZBLAN glass are of great interest for generating broadband supercontinua extending into the ultraviolet and mid-infrared regions. Precise sub-micrometer structuring makes it possible to adjust the modal dispersion over a wide range, making the generation of new frequencies more efficient. Here we report a novel ZBLAN PCF with six cores, each containing a central nanobore of a diameter ∼330 nm. Each nanobore core supports several guided modes, and the presence of the nanobore significantly modifies the dispersion, strongly influencing the dynamics and the extent of supercontinuum generation. Spectral broadening is observed when a single core is pumped in the fundamental and first higher order core modes with 200 fs long pulses at a wavelength of 1042 nm. Frequency-resolved optical gating is used to characterize the output pulses when pumping in the lowest order mode. The results are verified by numerical simulations.

Flat-Top Supercontinuum and Tunable Femtosecond Fiber Laser Sources at 1.9–2.5 μm

We report the high-energy flat-top supercontinuum covering the midinfrared wavelength range of 1.9–2.5 μm as well as electronically tunable femtosecond pulses between 1.98 and 2.22 μm directly from the thulium-doped fiber laser amplifier. Comparison of experimental results with numerical simulations confirms that both sources employ the same nonlinear optical mechanism—Raman soliton frequency shift occurring inside the Tm-fiber amplifier. To illustrate that we investigate two versions of the compact diode-pumped SESAM mode-locked femtosecond thulium-doped all-silica-fiber-based laser system providing either broadband supercontinuum or tunable Raman soliton output, depending on the parameters of the system. The first system operates in the Raman soliton regime providing femtosecond pulses tunable between 1.98 and 2.22 μm. Wide and continuous spectral tunability over 240 nm was realized by changing only the amplifier pump diode current. The second system generates high-energy supercontinuum with the superior spectral flatness of better than 1 dB covering the wavelength range of 1.9–2.5 μm, with the total output energy as high as 0.284 μJ, and the average power of 2.1 W at 7.5-MHz repetition rate. We simulate the amplifier operation in the Raman soliton self-frequency shift regime and discuss the role of induced Raman scattering in supercontinuum formation inside the fiber amplifier. We compare this system with a more traditional 1.85–2.53-μm supercontinuum source in the external highly-nonlinear commercial chalcogenide fiber using the Raman soliton MOPA as an excitation source. The reported systems 1 1 Patent pending, ATLA Lasers AS, Trondheim, Norway.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications.

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

Acousto-optic tunable filter for dispersion characterization of time-domain optical coherence tomography systems.

A broadband supercontinuum light source with an acousto-optic tunable filter (AOTF) are used to characterize dispersion in two time-domain OCT systems, at 850 and 1300 nm. The filter is designed to sweep across two spectral ranges, which are restricted here from 800 to 900 nm and from 1200 to 1500 nm, respectively. Dispersion compensation for 850 nm was achieved with a spectral delay line. Dispersion compensation for 1300 nm was achieved using BK 7 rod glasses in the reference arm. The AOTF allows evaluation of dispersion in under as well as overcompensated systems. The AOTF method is based on wavelength dependence of the optical path difference corresponding to the maximum strength of the interference signal recorded using a mirror as object. Comparison is made between the AOTF method and the more usual method based on measurement of the full width at half-maximum of the autocorrelation peak. This comparison shows that the AOTF method is more accurate in terms of evaluation of the dispersion left uncompensated after each adjustment. The AOTF method additionally provides information on the direction of dispersion compensation.

High Brightness 2.2–12 μm Mid‐Infrared Supercontinuum Generation in a Nontoxic Chalcogenide Step‐Index Fiber

An environment friendly nonlinear chalcogenide glass fiber with a Ge‐Sb‐Se core and a Ge‐Se cladding is fabricated for bright broadband mid‐infrared (MIR) supercontinuum (SC) generation. The fabricated Ge‐Sb‐Se/Ge‐Se fiber with a core diameter of 6 μm shows zero group velocity dispersion at ~4.2 μm and ~7.3 μm. By pumping the fiber with a length of 11 cm at 4.485 μm with 330 fs pulses, we achieve a SC covering the 2.2–12 μm spectral range and with an output average power of ~17 mW. This bright broadband SC source is promising for high‐resolution MIR spectroscopy.

Invited Article: Multiple-octave spanning high-energy mid-IR supercontinuum generation in bulk quadratic nonlinear crystals

Bright and broadband coherent mid-IR radiation is important for exciting and probing molecular vibrations. Using cascaded nonlinearities in conventional quadratic nonlinear crystals like lithium niobate, self-defocusing near-IR solitons have been demonstrated that led to very broadband supercontinuum generation in the visible, near-IR, and short-wavelength mid-IR. Here we conduct an experiment where a mid-IR crystal is pumped in the mid-IR. The crystal is cut for noncritical interaction, so the three-wave mixing of a single mid-IR femtosecond pump source leads to highly phase-mismatched second-harmonic generation. This self-acting cascaded process leads to the formation of a self-defocusing soliton at the mid-IR pump wavelength and after the self-compression point multiple octave-spanning supercontinua are observed. The results were recorded in a commercially available crystal LiInS2 pumped in the 3-4 μm range with 85 fs 50 μJ pulse energy, with the broadest supercontinuum covering 1.6-7.0 μm. We measured up 30 μJ energy in the supercontinuum, and the energy promises to scale favorably with an increased pump energy. Other mid-IR crystals can readily be used as well to cover other pump wavelengths and target other supercontinuum wavelength ranges.

Broadband supercontinuum generation in lead silicate photonic crystal fibers employing optical pulses of 50 W peak power

A unique solid core lead silicate photonic crystal fiber has been designed which promises to simultaneously exhibit large optical nonlinearity and almost uniform low dispersion. At 1064 and 1550nm wavelengths, the fiber is expected to exhibit nonlinear coefficient ~ 1420 W-1 km-1 and ~ 923 W-1 km-1, respectively, and dispersion ~ 7ps/km/nm and ~ 40ps/km/nm, respectively. Employing numerical simulation, we have investigated supercontinuum (SC) generation in this fiber by pumping 1ps and 50fs pulses. An octave-spanning SC from 680 to 1500nm is achievable at the end of 15cm long fiber by pumping 50fs pulses of 50W peak power at 1064nm. SC spectra from 1200 to 2600nm are also achievable by pumping 50fs pulses of 100W peak power at 1550nm. The simulated SC spectra due to 50fs pulses are attributed to self-phase modulation. A flat spectrum from 650 to 980nm is predicted due to pumping in the normal dispersion regime at 800nm using 1ps pulses of 300W peak power in a 75cm long fiber.

Ultraflat-top midinfrared coherent broadband supercontinuum using all normal As 2 S 5 -borosilicate hybrid photonic crystal fiber

We report more than two octave spanning mid-IR flat-top supercontinuum (SC) generation using all normal As2S5-borosilicate hybrid photonic crystal fiber. Our design is based on a chalcogenide As2S5 photonic crystal fiber (PCF), where the first ring composed of six air holes is made by borosilicate glass. By injecting 50-fs pulses with 1.6 nJ energy at 2.5 μm in the all normal dispersion (ANDi) regime, a flat-top broadband SC extending from 1 to 5 μm with high-spectral flatness of 8 dB is obtained in only 4-mm fiber length. To the best of our knowledge, we present the broadest flat mid-IR spectrum generated in the ANDi regime of an optical fiber. The self-phase modulation and the optical wave breaking are identified as the main broadening mechanisms. The obtained broadband light source can be potentially used in the field of spectroscopy and in high-resolution optical coherent tomography owing to the high-spectral SC flatness generated by our designed fiber.

Supercontinuum based absorption spectrometer for cycle-resolved multiparameter measurements in a rapid compression machine.

A broadband supercontinuum (SC) based absorption spectrometer capable of cycle-resolved multiparameter measurements at internal combustion (IC) engine conditions is presented. Three parameters, temperature, pressure and water mole fraction, were extracted from broadband near-infrared H2O absorption spectra, spanning the wavelength-range from 1340 to 1405.5 nm, which exhibits a large number of specific H2O transitions. The spectrometer is based on spatial domain detection and features a near-infrared line scan camera as a detector. Measurements were performed during a compression cycle of a rapid compression machine comprising a pressure and temperature range from 2.5 to 65 bar and 300 to 900 K, respectively. With the new spectrometer, we are for the first time, based on the authors' knowledge, able to perform measurements based on SC radiation over a complete compression and expansion stroke at measurement rates up to 50 kHz. A detailed overview is provided about the best match algorithm between theory and experiments, including parameters from two different spectral databases, namely the Barber-Tennyson database (BT2) and HITRAN2012. The results indicate that spectral broadening effects are not properly described by theory, especially at pressure levels exceeding 20 bar, which culminates in a clear underestimation of the derived pressure data by SC absorption spectroscopy. Nevertheless, temperature can be determined accurately by performing a three-parameter fit based on water mole fraction, temperature, and pressure. In contrast, making use of pressure transducer data as look-up values and varying only temperature and H2O mole fraction to find the best match leads to a clear overestimation of temperature at elevated pressures.

Supercontinuum Generation in a Microstructured Fiber with an Irregular Cladding

A broad-band supercontinuum generation was obtained at excitation of a microstructured optical fiber with an irregular cladding by femtosecond laser pulses. To explain the experimental data, calculations of the mode characteristics of microstructured fibers were performed. It was shown that the creation of air channels with different radii in the fiber cladding makes it possible to involve both the fundamental and high fiber modes in the supercontinuum generation that helps to increase the width of the generation spectrum.

Artificially anisotropic core fiber with ultra-flat high birefringence profile

We present a highly birefringent fiber with a core made of artificial anisotropic glass material. The fiber core is composed of interleaved subwavelength layers of two types of soft glasses ordered in a rectangular structure. A pair of thermally matched glasses, a low refractive index borosilicate glass and a high refractive index lead oxide glass, are used. The fiber has a unique flat profile of birefringence over one octave, weakly dependent on wavelength. The group birefringence and effective mode area were measured in a broadband range across the visible and the near infrared for the fundamental mode and were found to be equal 1.8 × 10−3 and 20 μm2, respectively. The group birefringence is uniquely flat over the wavelength range of 0.8-1.7 μm and the relative difference of birefringence is below 0.2 × 10−3. The measured dispersion shows also relatively flat characteristics varying from −60 ps/(nm × km) at 1150 nm to 20 ps/(nm × km) at 1690 nm with Zero Dispersion Wavelength at 1520 nm. We demonstrated an application of the fiber for polarization maintaining broadband supercontinuum generation in the range of 1210-1830 nm when pumped with a subpicosecond fiber-based laser at 1560 nm.

Optical solitons and supercontinuum generation in a tellurite microstructured optical fiber

We demonstrate soliton self-frequency shift (SSFS), multiple optical solitons and supercontinuum (SC) generation in a tellurite microstructured optical fiber (TMOF). By using an optical parametric oscillator (OPO) with the pulse of ∼80MHz and ∼200fs as the pump source, the evolution of SSFS is investigated at the pump wavelengths of ∼1730, 1750, 1810 and 1900nm with the fiber length of ∼20cm. At the pump wavelength of ∼1730nm, SSFS with the soliton center wavelength from ∼1850 to 1995nm is observed. Increasing the pump wavelength to ∼1920nm and the fiber length to ∼100cm, stable multiple optical solitons and dispersive waves (DWs) are obtained. Changing the pump source to a SC source which is generated in a single mode fiber (SMF) pumped by a nanosecond laser with the wavelength of ∼1550nm, a broadband SC spectrum from ∼580 to 2796nm is obtained at the pump power of ∼202mW. To the best of our knowledge, this is the first demonstration of SC generation in the TMOF by using SC light generated by a nanosecond laser.

Studies of the modal properties of circularly photonic crystal fiber (C-PCF) for high power applications

The guiding properties of a new type of photonic crystal fibers where air-holes are arranged in a circular pattern (C-PCF) with a silica matrix have been investigated. The dispersion properties of the fiber with different spacing of circle and air-hole diameter have been studied in detail. It is shown that C-PCFs with smaller values of radius and higher air-filling fraction can be used as dispersion compensating fiber. A comparison between fibers with circular and triangular lattice has also been performed, taking into account the dispersion properties and the effective area in the wavelength range between 1200nm and 1600nm. C-PCF can better compensate the inline dispersion for both single wavelength and broadband wavelength applications which is a unique property not observed by regular triangular-lattice or square-lattice PCFs. The fiber provides higher effective area, making it a better candidate for high power accumulations in the core of the fiber. The fiber also shows red-shifting of the first zero dispersion wavelength (ZDW), flatter dispersion slope and lower Group Velocity Dispersion (GVD) in the normal dispersion region thereby making it a better candidate for high power nonlinear applications like super-continuum generation, soliton pulse propagation etc. With the above advantages, we have considered a series study of these circular-lattice structures for various geometrical parameters and temporal pulses in order to explore the characteristics of broadband supercontinuum generation. This design study for high power supercontinuum generation will be very helpful for potential application of new sources in various fields like astronomy, climatology, spectroscopy optical tomography and sensing etc. to name a few.

Tapered fluorotellurite microstructured fibers for broadband supercontinuum generation.

Fluorotellurite microstructured fibers (MFs) based on TeO2-BaF2-Y2O3 glasses are fabricated by using a rod-in-tube method. Tapered fluorotellurite MFs with varied transition region lengths are prepared by employing an elongation machine. By using a tapered fluorotellurite MF with a transition region length of ∼3.3 cm as the nonlinear medium and a 1560 nm femtosecond fiber laser as the pump source, broadband supercontinuum generation covering from 470 to 2770 nm is obtained. The effects of the transition region length of the tapered fluorotellurite MF on supercontinuum generation are also investigated. Our results show that tapered fluorotellurite MFs are promising nonlinear media for generating broadband supercontinuum light expanding from visible to mid-infrared spectral region.

Multimode supercontinuum generation in chalcogenide glass fibres.

Mid-infrared supercontinuum generation is considered in chalcogenide fibres when taking into account both polarisations and the necessary higher order modes. In particular we focus on high pulse energy supercontinuum generation with long pump pulses. The modeling indicates that when only a single polarisation in the fundamental mode is considered the obtainable supercontinuum bandwidth is substantially exaggerated compared to when both polarisations are taken into account. Our modeling shows that if the pump pulse is short enough (≤ 10 ps) then higher order modes are not important because of temporal walk-off. In contrast long pump pulses (≥ 40 ps) will efficiently excite higher order modes through Raman scattering, which will deplete the fundamental mode of energy and limit the possibility of obtaining a broadband supercontinuum.

Optimal operational conditions for supercontinuum-based ultrahigh-resolution endoscopic OCT imaging.

We investigated the optimal operational conditions for utilizing a broadband supercontinuum (SC) source in a portable 800 nm spectral-domain (SD) endoscopic OCT system to enable high resolution, high-sensitivity, and high-speed imaging in vivo. A SC source with a 3-dB bandwidth of ∼246 nm was employed to obtain an axial resolution of ∼2.7 μm (in air) and an optimal detection sensitivity of ∼-107 dB with an imaging speed up to 35 frames/s (at 70 k A-scans/s). The performance of the SC-based SD-OCT endoscopy system was demonstrated by imaging guinea pig esophagus in vivo, achieving image quality comparable to that acquired with a broadband home-built Ti:sapphire laser.

Ultra-compact Watt-level flat supercontinuum source pumped by noise-like pulse from an all-fiber oscillator.

We demonstrate Watt-level flat visible supercontinuum (SC) generation in photonic crystal fibers, which is directly pumped by broadband noise-like pulses from an Yb-doped all-fiber oscillator. The novel SC generator is featured with elegant all-fiber-integrated architecture, high spectral flatness and high efficiency. Wide optical spectrum spanning from 500 nm to 2300 nm with 1.02 W optical power is obtained under the pump of 1.4 W noise-like pulse. The flatness of the spectrum in the range of 700 nm~1600 nm is less than 5 dB (including the pump residue). The exceptional simplicity, economical efficiency and the comparable performances make the noise-like pulse oscillator a competitive candidate to the widely used cascade amplified coherent pulse as the pump source of broadband SC. To the best of our knowledge, this is the first demonstration of SC generation which is directly pumped by an all-fiber noise-like pulse oscillator.

Broadband supercontinuum generation in photonic crystal fibers using cosh-Gaussian pulses at 835 nm wavelength

This paper presents broadband supercontinuum generation in photonic crystal fibers employing cosh-Gaussian pulses. The supercontinuum spectrum of cosh-Gaussian pulses consists of a large number of internal oscillations which increases with the increase in the value of cosh factor. Unlike the supercontinuum spectra created by Gaussian pulses, which consists of two distinct minima, the spectra created by cosh-Gaussian pulses of equal peak power and pulse duration characterized by virtually flat and broadband supercontinuum. Improvement in the flatness of the generated supercontinuum increases with the increase in the value of cosh factor.