mid-IR Supercontinuum Research Articles

Covering the vibrational spectrum with microjoule mid-infrared supercontinuum pulses in nonlinear optical applications

Nonlinear spectroscopies focused on molecular vibrations require high-energy [several microjoule (μJ)] laser pulses in the mid-infrared (IR). Current methods of generating μJ pulses in the mid-IR exhibit limited spectral coverage, while pulses that span the mid-IR range display insufficient pulse energies for nonlinear spectroscopy. Here we present the generation of high-energy mid-IR supercontinuum laser pulses by focusing 70 fs, 30 μJ mid-IR pulses in commercially available bulk chalcogenide glasses. The resulting supercontinuum pulses exhibit pulse energies of several μJ, which is sufficient for nonlinear spectroscopic applications, while spanning the important wavelength range from below 2.5 μm to over 10 μm at the 10% level (2.5–11 μm at −20 dB)—more than two octaves within the mid-infrared. This facilitates capturing most of the fundamental transitions (v=0→1) of molecular vibrations in a wide range of applications including nonlinear experiments, as demonstrated with sum-frequency generation spectroscopy.

Mid-infrared Supercontinuum Generation to ∼4.7 μm in a ZBLAN Fiber Pumped by an Optical Parametric Generator

We present mid-infrared (mid-IR) supercontinuum (SC) generation from a step-index fluorozirconate fiber pumped at 2.25 μm. When 70 ps pulses with a peak power of 128 kW were launched into a ZBLAN fiber with a zero dispersion wavelength at 1.603 μm, SC was extended to 4695 nm, with spectral flatness below 10 dB, in the 2.00–4.56 μm range. To the best of our knowledge, this is the first time such a broad and flat mid-IR SC spectrum from a ZBLAN fiber pumped with ps pulses has been achieved. The SC evolution as a function of pump wavelength and pump pulse energy was also experimentally demonstrated.

Ultra-broadband, coherent mid-IR supercontinuum expanding from 1.5 to 12.2 μm in new design of AsSe2 photonic crystal fibre

We propose a new design of AsSe2 photonic crystal fibre (PCF) with all-normal dispersion, nearly zero flat-top used to generate an ultra-broadband supercontinuum spanning from 1.5 to 12.2 μm. Simulated results show that, when we use only 1 mm of AsSe2 PCF, a broadband mid-infrared supercontinuum with the specrum extent from 1.5 to 12.2 μm is obtained with a very low input energy of E = 1.3 nJ at the wavelength of 3.5 μm and pulse duration of 100 fs. We study the temporel and spectral impact of optical wave breaking in the development of the continuum. The influence of the fibre length, the input energy and the full width at half maximum is investigated. Compared to previous research works, we have obtained the broadest, coherent supercontinuum, which could be applicable in biomolecular sensing, cancer diagnostics, infrared spectroscopy and free space communication.

Ultra broadband mid-IR supercontinuum generation in Ge11.5As24Se64.5 based chalcogenide graded-index photonic crystal fiber: design and analysis.

In this paper, we report design and numerical analysis of a Ge11.5As24Se64.5 based chalcogenide glass graded-index photonic crystal fiber structure for mid-IR ultra broadband supercontinuum generation. The proposed dispersion engineered photonic crystal fiber offers a zero dispersion wavelength at a pump wavelength of 2.8 μm. To simulate the supercontinuum generation spectrum, the orders of dispersion coefficient up to the ninth order are considered. Simulated results indicate that an ultra broadband supercontinuum spectrum spanning 1-16 μm has been achieved using a 10 mm long photonic crystal fiber structure pumped with 50 fs secant hyperbolic pulses of 3 kW at a -30 dB spectral intensity level. To the best of our knowledge, this is the first time such broad supercontinuum spectrum has been reported. This ultra broadband mid-IR supercontinuum spectrum is applicable in many diverse fields, including medical, defense, metrology, and spectroscopy.

Multi-watt mid-infrared supercontinuum generated from a dehydrated large-core tellurite glass fiber

We report for the first time multi-watt mid-infrared supercontinuum (SC) output from a dehydrated tellurite step-index fiber. The tellurite glass preform is melted in a dry-atmosphere filled glovebox and the water impurity in the glass has been reduced to 1ppm level. The fiber has a large core diameter of 12 µm and a numerical aperture (NA) of 0.21. Using the amplified Raman-shifted soliton pulses from a 1.55 µm erbium-doped fiber laser as the pump, a broadband 2-3 µm supercontinuum with a 3dB bandwidth of 985 nm has been generated from an 80cm-long dehydrated tellurite glass fiber. The power spectral density of the SC is above 3.5 dBm/nm in the whole range of 2-3 µm. The net output power of the supercontinuum from the tellurite fiber is measured to be 2.1 W. To the best of our knowledge, this is the highest average power of mid-IR SC generated from a tellurite fiber. The spectral density of above 3 dBm/nm is also one of the highest spectral densities of a SC achieved in 2.5-3 μm range from an oxide glass fiber.

Solitonic supercontinuum of femtosecond mid-IR pulses in W-type index tellurite fibers with two zero dispersion wavelengths

We present a detailed experimental parameter study on mid-IR supercontinuum generation in W-type index tellurite fibers, which reveals how the core diameter, pump wavelength, fiber length, and pump power dramatically influence the spectral broadening. As pump source, we use femtosecond mid-IR pulses from a post-amplified optical parametric oscillator tunable between 1.7 μm and 4.1 μm at 43 MHz repetition rate. We are able to generate red-shifted dispersive waves up to a wavelength of 5.1 μm by pumping a tellurite fiber in the anomalous dispersion regime between its two zero dispersion wavelengths. Distinctive soliton dynamics can be identified as the main broadening mechanism resulting in a maximum spectral width of over 2000 nm with output powers of up to 160 mW. We experimentally demonstrated that efficient spectral broadening with considerably improved power proportion in the important first atmospheric transmission window between 3 and 5 μm can be achieved in robust W-type tellurite fibers pumped at long wavelengths by ultra-fast lasers.

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.

Scaling all-fiber mid-infrared supercontinuum up to 10 W-level based on thermal-spliced silica fiber and ZBLAN fiber

We demonstrate an integrated all-fiber mid-infrared (mid-IR) supercontinuum (SC) source generated by a 1.95 μm master oscillator power amplifier system and a single-mode ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF) fiber. The maximum average output power is 10.67 W with spectral bandwidth covering from ∼1.9 to 4.1 μm. The single-mode ZBLAN fiber and silica fiber are thermal-spliced to enhance the robustness and practicability of the system. It is, to the best of our knowledge, the first high-power integrated compacted all-fiber mid-IR SC source based on thermal-spliced silica fiber and ZBLAN fiber.

Mid-IR supercontinuum pumped by femtosecond pulses from thulium doped all-fiber amplifier.

We present a mid-infrared (mid-IR) supercontinuum (SC) light source pumped by femtosecond pulses from a thulium doped fiber amplifier (TDFA) at 2 μm. An octave-spanning spectrum from 1.1 to 3.7 μm with an average power of 253 mW has been obtained from a single mode ZBLAN fiber. Spectral flatness of 10 dB over a 1390 nm range has been obtained in the mid-IR region from 1940 - 3330 nm. It is resulted from the enhanced self phase modulation process in femtosecond regime. The all-fiber configuration makes such broadband coherent source a compact candidate for various applications.

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.

High-peak-power single-oscillator actively Q-switched mode-locked Tm3+-doped fiber laser and its application for high-average output power mid-IR supercontinuum generation in a ZBLAN fiber.

A single-oscillator actively Q-switched mode-locked (QML) thulium-doped silica fiber laser is presented and used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-infrared (mid-IR) supercontinuum (SC) generation. The fiber laser provided high-peak-power levels directly from the oscillator delivering single mode-locked pulse energies up to 48 μJ, being 2-4 orders of magnitude higher than conventional continuous wave mode-locked lasers. By pumping a ZBLAN fiber specially designed for high-output-power SC generation, 7.8 W have been achieved in all spectral bands with a spectrum extending to 4.2 μm.

Mid-infrared surface plasmon polariton chemical sensing on fiber-coupled ITO coated glass.

A novel fiber-coupled indium tin oxide (ITO) coated glass slide sensor for performing surface plasmon polariton chemical monitoring in the ∼3.5 μm mid-infrared (IR) range is reported. Efficient mid-IR fiber coupling is achieved with 3D laser written waveguides, and the coupling of glass waveguide modes to ITO surface plasmon polaritons (SPPs) is driven by the varying phase matching conditions of different aqueous analytes across the anomalous dispersion range determined by their molecular fingerprints. By means of using both a mid-IR fiber supercontinuum source and a diode laser, the excitation of SPPs is demonstrated. The sensor sensitivity is tested by discriminating CH from OH features of ethanol in water solutions, demonstrating an instrumental ethanol limit of detection of 0.02% in a wide concentration range of at least 0%-50%. The efficient optical monitoring of mid-IR SPPs in smart glass could have a broad range of applications in biological and chemical sensing.

Upconversion imaging using an all-fiber supercontinuum source.

In this Letter, the first demonstration, to the best of our knowledge, of pulsed upconversion imaging using supercontinuum light is presented. A mid-infrared (IR) imaging system was built by combining a mid-IR supercontinuum source emitting between 1.8 and 2.6 μm with upconversion detection. The infrared signal is used to probe a sample and mixed with a synchronized 1550 nm laser pulse inside a lithium niobate (LiNbO3) crystal. The signal is thus upconverted to the 860-970 nm range and acquired on a standard silicon CCD array at a rate of 22 frames per second. In our implementation, spatial features in the sample plane as small as 55 μm could be resolved.

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

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

High power broadband mid-infrared supercontinuum fiber laser using a novel chalcogenide AsSe2 photonic crystal fiber

A high power supercontinuum (SC) based on a new type of chalcogenide AsSe2 material for broadband mid-infrared light source is numerically reported. Ultra-broadband coherent mid-IR SC generation with more than 3 octave-spanning from 1.7 to 14μm in a novel design of chalcogenide AsSe2 photonic crystal fiber (PCF) is demonstrated. To the best of our knowledge and aiming to properly model the nonlinear propagation, an accurate fit of the Raman response function and the corresponding Raman gain of the novel AsSe2 chalcogenide glass are proposed numerically for the first time. The obtained SC is generated by pumping at 3.9μm in the anomalous dispersion regime in only 8mm long fiber. Our study shows that the initially generated SC from 150fs pulse duration with 8.8kW peak power exhibits high power proportion of more than 80% for wavelengths beyond 3μm which is very promising for designing high power SC fiber laser sources in the mid-IR atmospheric windows and the molecular fingerprint region.

Mid-infrared supercontinuum generation in telecommunication multimode fiber by nanosecond 2.054 μm laser pulses

We demonstrated a broadband mid-infrared (mid-IR) supercontinuum (SC) directly generated in telecommunication multimode fiber (MMF) pumped by 2.054 μm nanosecond laser pulses. The characteristics of mid-IR SC, including the output spectrum, pulse energy, and optic-to-optic conversion efficiency, were exhaustively investigated at different pump powers and fiber lengths. The nonlinear processes for SC generation in fiber were analyzed, and the frequency shift of the modulation instability (MI) was theoretically calculated, which was close to the experimental data. The output SC pulse shapes were measured in different output powers, and no split was found. The SC laser beam was nearly diffraction limited with an M2 =1.15 in 2.1 μm measured by the traveling knife-edge method, and the laser beam spot was monitored by an infrared vidicon camera.

Theoretical study on generating mid-infrared ultrashort pulse in mode-locked Er3+: ZBLAN fiber laser

Fiber lasers show several advantages over other types of lasers. They are efficient, compact, and rugged since they require few bulk components and are virtually unaffected by the surrounding environment. Mode-locked mid-infrared (mid-IR) lasers are essential for a wide variety of applications. The promising applications of mode-locked fiber lasers at wavelengths near 3 m include combs generation (metrology), spectroscopic sensors, infrared countermeasures, laser surgery, high-efficient pump sources for longer-wavelength oscillators and mid-IR supercontinuum source pumping. Based on the nonlinear Schrdinger equation (NLSE), a theoretical model of passively mode-locked Er3+-doped fluoride fiber laser using a saturable absorber is set up. Some mechanisms for generating mid-IR ultrashort pulse in fiber lasers are investigated. When the dispersion of the cavity is managed properly, the numerical simulation mainly focuses on the evolution process of mid-IR ultrashort pulse in fluoride fiber oscillators. Influences of the intracavity net dispersion and the small-signal gain on the generation of mode-locked pulses are analyzed in detail. And the reasonable parameter windows are given. Just as the simulated results showed, for a case of 4 m Er3+-doped fluoride fiber, small-signal gain g0= 0.6 m-1 and unsaturated loss l0 = 0.7, the stable mode-locked pulses are achieved by tuning the net intracavity dispersion within a certain range from 0.72 ps2 to 0.83 ps2. As the net intracavity dispersion increases, the output pulse duration increases gradually, while the spectrum width (FWHM) and peak power decrease accordingly. In addition, for the case of 4 m Er3+-doped fluoride fiber, unsaturated loss l0 = 0.7 and net intracavity dispersion of 0.8 ps2, the stable mode-locked pulses can also be obtained by tuning the small-signal gain within a certain range from 0.55 to 0.70 m-1. As the small-signal gain increases, the output pulse duration, spectral width, and peak power increase gradually. This work may be beneficial to the design of experiments for achieving more narrow pulse duration, wide spectral width, and high peak power mid-infrared ultrashort pulse.

Highly efficient cascaded amplification using Pr(3+)-doped mid-infrared chalcogenide fiber amplifiers.

We computationally investigate cascaded amplification in a three-level mid-infrared (IR) Pr(3+)-doped chalcogenide fiber amplifier. The overlap of the cross-sections in the transitions (3)H(6)→(3)H(5) and (3)H(5)→(3)H(4) enable both transitions to simultaneously amplify a single wavelength in the range between 4.25 μm and 4.55 μm. High gain and low noise are achieved simultaneously if the signal is at 4.5 μm. We show that 45% of pump power that is injected at 2 μm can be shifted to 4.5 μm. The efficiency of using a mid-IR fiber amplifier is higher than what can be achieved by using mid-IR supercontinuum generation, which has been estimated at 25%. This mid-IR fiber amplifier can be used in conjunction with quantum cascade lasers to obtain a tunable, high-power mid-IR source.

Stable high-spectral-flatness mid-infrared supercontinuum generation in Tm-doped fiber amplifier

We demonstrate a stable high-spectral-flatness mid-infrared (mid-IR) supercontinuum (SC) generation in a thulium-doped fiber amplifier (TDFA) with an average output power of 2.32W in a spectral band of ∼(1875–2700nm). A 1550nm distributed feedback (DFB) pulsed laser diode (LD) with repetition rate of 600kHz and pulse width of 900ps was used as the seed source. The measured long term stability of SC output power is less than 1.1%rms (root mean square) at the average output power of 2.32W. The measured slope efficiency from the 793nm pump power of the TDFA to the total SC output power is ∼14%. The output SC has a 6dB spectral flatness in the wavelength ranging from 1955 to 2505nm (550nm span) at the average output power of 2.32W.

Broadband mid-IR supercontinuum generation in As2Se3 based chalcogenide photonic crystal fiber: A new design and analysis

A new design of a triangular-core photonic crystal fiber in As2Se3-based chalcogenide glass with all-normal, nearly zero flat-top dispersion has been proposed for supercontinuum generation. Simulated results indicate that ultra-broadband supercontinuum spanning 1.9–10µm can be obtained using only 6mm long PCF pumped with 50 femto-second laser pulses operated at 4.5µm. In comparison to previously reported work we have obtained ultra-broadband supercontinuum spectra using relatively very low peak power of incident pulses. Proposed photonic crystal fiber structure is applicable in bio-molecular sensing and infrared spectroscopy.