Flat Supercontinuum Generation Research Articles

High-Power Flat Supercontinuum Generation by Using UHNA7 Fiber With Spectrum of 1.77-3.31 μm

High-Power Flat Supercontinuum Generation by Using UHNA7 Fiber With Spectrum of 1.77-3.31 <i>μ</i>m

Phosphorus-doped fiber for flat octave spanning supercontinuum generation.

In a fiber supercontinuum (SC) source, the Raman scattering effect plays a significant role in extending the spectrum into a longer wavelength. Here, by using a phosphorus-doped fiber with a broad Raman gain spectrum as the nonlinear medium, we demonstrate flat SC generation spanning from 850 to 2150 nm. Within the wavelength range of 1.1-2.0 µm, the spectral power density fluctuation is less than 7 dB. Compared to a similar SC source based on a germanium-doped fiber with narrower Raman gain spectrum, the wavelength span is 300 nm broader, and the spectral power density fluctuation is 5 dB lower. This work demonstrates the phosphorus-doped fiber's great advantage in spectrally flat SC generation, which is of great significance in many applications such as optical coherence tomography, absorption spectroscopy, and telecommunication.

20 GHz fiber-integrated femtosecond pulse and supercontinuum generation with a resonant electro-optic frequency comb

Frequency combs with mode spacing of 10–20 GHz are critical for increasingly important applications such as astronomical spectrograph calibration, high-speed dual-comb spectroscopy, and low-noise microwave generation. While electro-optic modulators and microresonators can provide narrowband comb sources at this repetition rate, a significant remaining challenge is a means to produce pulses with sufficient peak power to initiate nonlinear supercontinuum generation spanning hundreds of terahertz (THz) as required for self-referencing. Here, we provide a simple, robust, and universal solution to this problem using off-the-shelf polarization-maintaining amplification and nonlinear fiber components. This fiber-integrated approach for nonlinear temporal compression and supercontinuum generation is demonstrated with a resonant electro-optic frequency comb at 1550 nm. We show how to readily achieve pulses shorter than 60 fs at a repetition rate of 20 GHz. The same technique can be applied to picosecond pulses at 10 GHz to demonstrate temporal compression by 9× and achieve 50 fs pulses with a peak power of 5.5 kW. These compressed pulses enable flat supercontinuum generation spanning more than 600 nm after propagation through multi-segment dispersion-tailored anomalous-dispersion highly nonlinear fibers or tantala waveguides. The same 10 GHz source can readily achieve an octave-spanning spectrum for self-referencing in dispersion-engineered silicon nitride waveguides. This simple all-fiber approach to nonlinear spectral broadening fills a critical gap for transforming any narrowband 10–20 GHz frequency comb into a broadband spectrum for a wide range of applications that benefit from the high pulse rate and require access to the individual comb modes.

Effect of rotational Raman response on ultra-flat supercontinuum generation in gas-filled hollow-core photonic crystal fibers.

We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By comparing results obtained with either argon or nitrogen we determine the role of the rotational Raman response in the supercontinuum formation. When using argon, a supercontinuum extending from 350 nm to 2 µm is generated through modulational instability. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, we find that the energy density of the continuum in the normal dispersion region is significantly lower when using nitrogen. Using numerical simulations, we find that due to the closely spaced rotational lines in nitrogen, gain suppression in the fundamental mode causes part of the pump pulse to be coupled into higher-order modes which reduces the energy transfer to wavelengths shorter than the pump.

Spectrally flat mid-infrared supercontinuum pumped by a high power 2 µm noise-like pulse.

We demonstrate spectrally flat high-power mid-infrared supercontinuum (MIR SC) generation with record-breaking power of 33.1 W and power conversion efficiency of 75.06%. It is pumped by a 2 µm master oscillator power amplifier system consisting of a figure-8 mode-locked noise-like pulse seed laser and dual-stage Tm-doped fiber amplifiers with repetition rate of 4.08 MHz. Through cascading a piece of ZBLAN fiber with 13.5 µm large core diameter by direct-low-loss fusion splicing, SCs with spectral ranges of 1.9-3.68 µm, 1.9-3.84 µm, 1.9-4.02 µm and average powers of 33.1 W, 29.8 W, 25.9 W are generated. To the best of our knowledge, all of them have achieved the highest output power under the same condition of MIR spectrum range. This high-power all-fiber MIR SC laser system has relatively simple architecture, high efficiency and flat spectrum, demonstrating the advantages of 2 µm noise-like pulse pump in high-power MIR SC generation.

Flat supercontinuum generation via wave-free breaking pulse in the optical fiber

Flat supercontinuum generation via wave-free breaking pulse in the optical fiber

Broadband Supercontinuum Generation in Dispersion Decreasing Fibers in the Spectral Range 900–2400 nm

The spectrally flat supercontinuum generation in the wavelength range of 900–2400 nm is demonstrated in silica-based fibers of variable core diameter and dispersion. It is shown that, in comparison with standard optical fibers of the same length, supercontinuum spectra 200 nm wider can be realized in the samples under study. The significant difference between the spectral and temporal transformations of radiation depending on the direction of propagation is demonstrated in the researched fiber samples.

INFLUENCE OF STRUCTURAL PARAMETERS ON OPTICAL CHARACTERISTICS OF PHOTONIC CRYSTAL FIBERS WITH CIRCULAR LATTICE

We demonstrate in this study that near-zero, ultra-flattened chromatic dispersion can be achieved over a wide range of wavelengths in photonic crystal fibers (PCFs) by means of slight variations in the geometrical parameters of the cladding. To do that, a new solid-core circular PCF design with various air hole diameters and lattice constants is presented, and the design features are numerically analyzed in detail. After 40 simulations, we determined three structures that possess optimal dispersion with the following lattice constants (Ʌ) and filling factors for the first ring (d1/Ʌ): Ʌ = 0.8 µm, d1/Ʌ = 0.45 for #F1, Ʌ = 0.9 µm, d1/Ʌ = 0.45 for #F2, and Ʌ = 1.0 µm, d1/Ʌ = 0.45 for #F3. High nonlinearity and low attenuation are outstanding features of our model. With these advantages, the proposed fibers are targeted for smooth flat broadband supercontinuum generation for near-infrared applications.

Ultra-Compact 50 W Flat Supercontinuum Generation in Single-Stage Self-Q-Switched Fiber Laser With Photonic Crystal Fiber

An all-fiber supercontinuum (SC) laser source with 50.7 W average output power and spectrum ranging from 500 nm to 2400 nm is demonstrated. With the generation of high peak power nanosecond pulses and photonic crystal fiber (PCF) in the self-Q-switched fiber laser, the output SC shows good spectral width and flatness which has a 20 dB bandwidth of 1900 nm except the 1064 nm peak. The effect of the PCF length on the spectral characteristics and output power of SC were carefully studied. To the best of our knowledge, this is the highest power of SC generated in a single-stage fiber laser with such a broadband spectrum. This ultra-compact SC generation method has the merits of simple structure, low cost and good robustness, which provide an easily available SC generation method for various practical applications.

Flattened 2.5-Octave Supercontinuum in a Silicon Nitride Waveguide Pumped With Picosecond Pulses

Supercontinuum generation has been extensively studied in optical waveguides pumped with femtosecond pulses, while there are few reports using picosecond pump pulses with a small spectral power variation. In this paper, we numerically demonstrate the supercontinuum generation in a dispersion-engineered silicon nitride waveguide pumped with 10-ps pulses at 1550 nm wavelength. A supercontinuum from 656 to 3867 nm (2.5 octaves) is enabled by uniquely tailored dispersion. The spectrum has a power variation of < 15 dB and shows a very flat pedestal varying by < 5 dB. The spectra generated from different dispersion profiles are compared and analyzed, which verifies the unique role of dispersion tailoring in flattened supercontinuum generation. To the best of our knowledge, it is the first study on such broad and flat supercontinuum generation on a chip using picosecond pump pulses, which would be highly desirable for many applications.

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.

Longitudinally thickness-controlled nanofilms on exposed core fibres enabling spectrally flattened supercontinuum generation

Nonlinear frequency conversion is a pathway to unlock undiscovered physics and implement tailored light sources for spectroscopy or medicine. A key challenge is the establishment of spectrally flat outputs, which is particularly demanding in the context of soliton-based light conversion at low pump energy. Here, we introduce the concept of controlling nonlinear frequency conversion by longitudinally varying resonances, allowing the shaping of soliton dynamics and achieving broadband spectra with substantial spectral flatness. Longitudinally varying resonances are realised by nanofilms with gradually changing thicknesses located on the core of an advanced microstructured fibre. Nanofilms with engineered thickness profiles are fabricated by tilted deposition, representing a waveguide-compatible approach to nano-fabrication, and inducing well-controlled resonances into the system, allowing unique dispersion control along the fibre length. Key features and dependencies are examined experimentally, showing improved bandwidth and spectral flatness via multiple dispersive wave generation and dispersion-assisted soliton Raman shifts while maintaining excellent pulse-to-pulse stability and coherence in simulations, suggesting the relevance of our findings for basic science as well as tailored light sources.

Arsenic Sulfide Suspended-core Fiber Simulation with Three Parabolic Air Holes for Supercontinuum Generation

Highly nonlinear suspended-core fibers (SCFs) with tunable dispersion have attracted much attention in the fields of Raman amplification, optical frequency combs, broadband and flat supercontinuum generation (SCG). To address the limitation of applications due to its fragile suspension arms, this study proposes the design of a fiber structure with three parabolic air holes. Numerical simulations are performed to optimize an arsenic sulfide SCF in terms of dispersion management and SCG in the wavelength range from 0.6 µm to 11.6 µm. Results show that the proposed SCF has dual zero-dispersion wavelengths (ZDWs) that can be shifted by adjusting the parabolic coefficient of the air-hole and the equivalent diameter of the suspended core. By means of structural optimization, an SCF with 1 μm equivalent diameter and a parabolic coefficient of 0.18 μm−1 is proposed. The first ZDW of the SCF is blue-shifted to 1.541 μm, which makes it possible to use a commercial light source with a cheaper price, more mature technology and smaller volume as the pump source. SCG is studied by solving the generalized nonlinear Schrödinger equation using the split-step Fourier method, and a 0.6–5.0 μm supercontinuum spectrum is obtained at a pump source peak power of 40 kW.

Flat Supercontinuum Generation in Tellurite Fiber Pumped in Deep Normal Dispersion Region

We demonstrate flat supercontinuum (SC) generation in a piece of step-index tellurite fiber pumped by a noise-like mode-locked fiber laser in the deep normal dispersion region. The noise-like pulse emitted from a simple all-fiber mode-locked laser is used as the pump directly and the cascaded stimulated Raman scattering leads to the spectral broadening to the long wavelength region. Flat SC spanning from $\sim 1570$ nm to $\sim 1900$ nm is obtained and no obvious spectral defects are observed. These results show the great advantage of noise-like pulse pumping in the deep normal dispersion region and the potential of noise-like pulse pumped mid-infrared SC generation.

Accurately Shaping Supercontinuum Spectrum via Cascaded PCF.

Shaping is very necessary in order to obtain a wide and flat supercontinuum (SC). Via numerical simulations, we accurately demonstrated shaping the SC using the fiber cascading method to significantly increase the width as well as the flatness of the spectrum in silica photonic crystal fiber (PCF). The cascaded PCF contains two segments, each of which has dual zero-dispersion frequencies (ZDFs). The spectral range of the SC can be expanded tremendously by tuning the spacing between the two ZDFs of the first segmented cascaded PCF. Increasing the pump power generates more solitons at the red edge, which accelerates solitons trapping and improves the spectral flatness of the blue edge. Furthermore, cascading the second segmented PCF by choosing appropriate fiber parameters ensures the flatness of the red end of SC. Therefore, a cost-effective alternative method for broad and flat supercontinuum generation in the near-infrared range is proposed here, which can be implemented easily in any photonics laboratory, where dual ZDFs PCFs are commonly found.

1.6-Octave Coherent OAM Supercontinuum Generation in As2S3 Photonic Crystal Fiber

We design and simulate an all-normal dispersion arsenic trisulfide (As 2 S 3 ) photonic crystal fiber (PCF) with high nonlinearity to enable a flat and coherent orbital angular momentum (OAM) supercontinuum (SC) generation. The photonic crystal fiber features a near-zero and flat negative dispersion with variation between -96.5 and -36.5 ps/(nm·km) over a 940-nm wavelength range from 1740 to 2680 nm. A 1946-nm supercontinuum forms from 959 to 2905 nm at -20 dB level which covers a 1.6-octave bandwidth, by launching a 100-fs 5-kW chirp-free hyperbolic secant pulse with wavelength at 2000 nm into a 1.0-cm designed fiber. The generated supercontinuum of the other two vortex modes (TE 01 and TM 01 ) can cover more than two octaves by optimizing the proposed fiber structures. The coherence of the generated supercontinuum of the three modes all shows nearly perfect property over the whole bandwidth. In general, we found that the designed ring-core As 2 S 3 PCF with all-normal dispersion could be used for broadband coherent supercontinuum generation of various vortex modes.

Investigation of wagon wheel fiber characteristics and flattened supercontinuum generation

In this paper, the dispersion coefficients and the nonlinear parameter of a wagon wheel fiber is obtained by finite element method. Efficient broadband near infrared supercontinuum generation is predicted in a 15 cm of the single-mode small core silicon wagon wheel fiber by solving the generalized nonlinear Schrodinger equation when the fiber is pumped by femtosecond pulses in the abnormal dispersion regime. Here, it is shown that the flattened supercontinuum spectrum over 850 nm wide is achieved.

Supercontinuum Generation From a Thulium Ultrafast Fiber Laser in a High NA Silica Fiber

A broadband supercontinuum is generated from a thulium ultrafast soliton fiber laser in a high numerical aperture silica fiber with low nJ seed pulse energies. Amplification of a thulium based ultrafast soliton ring fiber laser in two customized thulium doped fiber amplifiers provides pulses varying in energy between 1.8 nJ and 13.5 nJ. Coupling these seed pulses into a high numerical aperture silica fiber UHNA 7 of varying length, a systematic study of the impact of pulse energy, pulse duration and nonlinear fiber length is conducted. Based on the normal dispersion, self-phase modulation dominates for lower pump power values and results in a symmetric spectral broadening process. For higher pulse energies, four-wave mixing and Raman scattering contribute to a strong red-shift of the spectrum beyond 2.3 μm. For a fiber length of 20 m of UHNA 7, a broadband supercontinuum spanning from 1.7 μm to 2.33 μm is generated with a 20 dB spectral bandwidth of 502 nm with an output pulse energy of 4.4 nJ, corresponding to an output power of 92 mW. This represents a low seed energy threshold for efficient flat supercontinuum generation at infrared wavelengths.

All-Fiber Supercontinuum Source Pumped by Noise-Like Pulse Mode Locked Laser

We demonstrate flat supercontinuum generation in a piece of highly nonlinear fiber pumped with a stable noise-like pulse mode-locked fiber laser. The compact all-fiber laser realizes mode locking based on a nonlinear amplifying loop mirror and emits noise-like mode-locked pulse with high peak power and high average power. The output pulse has a 3-dB bandwidth of ~35.8 nm with the central wavelength of ~1581 nm. Pumping a 102-m highly nonlinear fiber with the noise-like pulse mode-locked fiber laser directly, a flat and weak residual-pump supercontinuum spanning from ~1370 to ~2200 nm is generated. The spectrum within 1865 to 2200 nm contains more than 45% of the whole energy of the supercontinuum.

Spectrally Flat Supercontinuum Generation in a ZBLAN Fiber Pumped by Erbium-Doped Mode-Locked Fiber Laser

We have experimentally demonstrated the flat supercontinuum (SC) generation using a 10-m-long ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fiber pumped by an erbium-doped mode-locked fiber laser incorporating carbon-nanotube-based saturable absorbers. In order to improve the spectral flatness of SC, the standardized single-mode fiber with different lengths is connected to the output of the mode-locked fiber laser before the pulse amplification. The generated SC with ZBLAN fiber exhibits the best spectral flatness with fluctuation less than 1.29dB over the wavelength of 1571.8nm–1803.1nm, showing potential applications in optical sensing.