Supercontinuum Generation Process Research Articles

All-fiber coherent supercontinuum generation in a cascade of silica, fluoride, and chalcogenide fibers

Abstract We present an all-fiber coherent supercontinuum spanning the spectral range of 1.7–5.0 µm from a cascade of silica, ZBLAN, and chalcogenide (ChG) nonlinear fibers (NLFs). Coherence is maintained by the combined use of femtosecond pump pulses as well as by allowing deterministic spectral broadening mechanism at every stage of the cascade. The use of femtosecond pump pulses enables avoiding modulation instability (MI) at the onset of the supercontinuum generation process and thus prevent subsequent MI-seeded random noise. Once in the NLF cascade, the pump pulse is instead converted into a soliton of order maintained at N < 6 in the silica and ZBLAN NLFs, ensuring soliton fission followed by self-frequency shift of a few solitons. Finally, in the ChG NLF, spectral broadening is facilitated through self-phase modulation and dispersive wave generation. The deterministic nature of these nonlinear phenomena results in the generation of a coherent supercontinuum. The supercontinuum delivers an average power of 54 mW from an average pump power of 300 mW, yielding a power conversion efficiency of 18%. The experimental results closely align with numerical simulations, from which coherence is estimated. Such a coherent supercontinuum with a megahertz repetition rate is essential for spectroscopic systems based on optical frequency combs and applications in high-precision optical coherence tomography.

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.

Flat visible spectrum by a genetic algorithm optimized photonic crystal fiber in the GHz comb spacing.

Coherent and flat supercontinuum (SC) sources are demanded for applications of metrology, spectroscopy, and bio-imaging. However, the process of SC generation is usually very complicated. We demonstrated a convenient and efficient method based on a genetic algorithm (GA). According to an objective spectrum, this algorithm could reverse-design the geometry of a fiber or waveguide without knowing the specific non-linear processes involved. Using this method, we designed a dispersion-managed photonic crystal fiber (PCF) for SC generation at 1 GHz comb spacing. With an input pulse of ∼150 fs, 450 pJ at 1050 nm, a 3 dB fluctuation spectrum ranging from 510 nm to 850 nm is obtained, which is absolutely fit to the calibration of an astronomical spectrograph.

Benefits of cascaded nonlinear dynamics in hybrid fibers for low-noise supercontinuum generation.

The recent development of fiber supercontinuum (SC) sources with ultra-low noise levels has been instrumental in advancing the state-of-the-art in a wide range of research topics. However, simultaneously satisfying the application demands of maximizing spectral bandwidth and minimizing noise is a major challenge that so far has been addressed with compromise, found by fine-tuning the characteristics of a single nonlinear fiber transforming the injected laser pulses into a broadband SC. In this work, we investigate a hybrid approach that splits the nonlinear dynamics into two discrete fibers optimized for nonlinear temporal compression and spectral broadening, respectively. This introduces new design degrees of freedom, making it possible to select the best fiber for each stage of the SC generation process. With experiments and simulations we study the benefits of this hybrid approach for three common and commercially available highly nonlinear fiber (HNLF) designs, focusing on flatness, bandwidth and relative intensity noise of the generated SC. In our results, hybrid all-normal dispersion (ANDi) HNLF stand out as they combine the broad spectral bandwidths associated with soliton dynamics with extremely low noise and smooth spectra known from normal dispersion nonlinearities. Hybrid ANDi HNLF are a simple and low-cost route for implementing ultra-low noise SC sources and scaling their repetition rate for various applications such as biophotonic imaging, coherent optical communications, or ultrafast photonics.

Programmable and tunable flat-top supercontinuum laser sources via electro-optic intensity and phase modulation scheme

In this study, we presented flat-topped coherent supercontinuum lasers with tunable repetition rates and programmable spectral bandwidths. Supercontinuum sources with ultra-broadband and high-repetition-rate coverage can be achieved by merging nonlinearly broadened electro-optic optical frequency combs with optical line-by-line spectrum shaping. Spectral bandwidth programming is implemented by iterative spectrum shaping and input power control of highly nonlinear stages, whereas repetition rate tuning is performed by modulation speed control in optical frequency combs. Herein, we implemented a programmable and tunable flat-topped supercontinuum with a maximum bandwidth and repetition rate of 55 nm at 10 dB and 50 GHz, respectively. To clarify the coherence of the supercontinuum during tuning and programming, we performed a phase-noise analysis. We proposed a remarkably modified self-heterodyne method to measure the phase noise of each mode precisely by filtering specific supercontinuum taps in a Mach–Zehnder interferometer. With this method, it has been proved that the single-sideband spectra in each mode are almost similar to that of the RF clock, indicating that our programmable and tunable supercontinuum generation process added minimal degradation to the phase noise properties. This study shows possibilities for generating hundreds of programmable and tunable flat-topped optical carriers with robustness and coherence.

Quantum model for supercontinuum generation process

A quantum theory is established for the propagation of electromagnetic waves in highly nonlinear dispersive optical fibers. By applying the method recently presented dispersion terms and retarded response of the medium are included for the propagation of light in a fiber in this work. A coupled stochastic generalized nonlinear Schrödinger equation (GNLSE) is obtained via the coherent positive-P representation to describe the supercontinuum generation process. This coupled quantum-stochastic equation is applied to obtain the linearized fluctuation equation for studying quantum noise and the fluctuation in the vicinity of the formed solitons in the supercontinuum generation process in the region of anomalous dispersion. Also, these equations can be used to study the soliton self-frequency shift quantum mechanically. Finally, we simulate the obtained coupled stochastic generalized nonlinear Schrödinger in the mean case and compare our simulation results with experimental results.

Characteristics of spectral peaking in optical fibers

We investigated multiple spectral peak generation during a coherent supercontinuum generation process with normal-dispersion highly nonlinear fibers both numerically and experimentally. Wideband multiple spectral peak generation was achieved in the 1.6–1.7 μm range using an ultrashort-pulse fiber laser, a CH4 gas cell, and highly nonlinear fiber. The maximum signal-to-background ratio was ∼8.4. Thanks to the normal-dispersion fibers, the induced phase shifts in the absorption spectra were clearly observed on the spectra during the spectral peak generation. The intensity and phase noise properties of the generated spectral peaks were examined, and low noise properties were confirmed. The spectral peaking phenomenon was investigated experimentally in a fiber amplifier. Periodical spectral peaking was successfully observed for a soliton pulse with Kelly sidebands, and the optical pulse experienced absorption in HCN gas. It is expected that spectral peaking occurs for pulses that experience absorption in many different kinds of gas species or those with spectral peaks. This light source and phenomenon will be useful for developing novel optical frequency comb techniques and optical wavelength standards.

Supercontinuum Generation in Photonic Crystal Fibers Infiltrated with Liquids

In this paper we present the development of a new direction in so-called optofluidics , namely the research of photonic crystal fibers (PCF) infiltrated with liquids. In particular we concentrate on the flagship application of PCF, the process of Supercontinuum Generation (SG), in which injected monochromatic pulse may be dramatically broadened (spectrally), which creates a coherent beam generation of high brightness comparable to that of monochromatic lasers. The supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam. Explanation of this process is based on numerical simulations for Generalized Nonlinear Schrödinger equation (GNLSE) which describes the rich nonlinear dynamics of pulse propagation in nonlinear dispersive media. All nonlinear phenomena involved in SG will be analyzed. We present specially activity of the Polish-Vietnamese Group from the beginning in 2016 to recent time in this domain.

Wideband tuning of four-wave mixing in liquid-filled photonic crystal fiber

A wideband tuning of parametric four-wave mixing generation in a liquid-filled photonic crystal fiber is theoretically demonstrated, and influence of its temperature is investigated. The broadband spectrum is generated via a supercontinuum generation process induced by degenerate four-wave mixing (DFWM), for which the thermo-optic coefficient of the oil (the refractive index matching fluid) is considered. The signal band tuning from 734 nm to 923 nm, and the idler band from 1931 nm to 1255 nm, are observed when the temperature increases from 30°C to 90°C. The findings can be applied in many fields, such as CARS.

Nonlinear Optical Method of Determination the Chirp of Broadband Femtosecond Laser Pulse in IR-Range

A new method for determining the chirp of a femtosecond broadband IR laser pulse at a central wavelength of 2.5 μm, based on the generation of a spectral supercontinuum in the field of a spectrally limited femtosecond laser pulse, propagating in a single mode fiber based on chalcogenide glass, and subsequent noncollinear generation of radiation at summary frequency by spectrally limited and spectral broadened supercontinuum femtosecond pulses is proposed. The time dependences of the instantaneous frequency of a femtosecond broadband pulse, obtained both from the results of numerical integrations of the equation describing the process of spectral supercontinuum generation and from the results of two-dimensional distributions of dynamic spectrograms are presented. It is shown that the proposed method allows one to determine the chirp of a broadband femtosecond IR laser pulse, formed during the soliton propagation mode in a fiber, with a relative error of no more than 5%, but the chirp of a broadband femtosecond IR laser pulse, formed in the process of non-soliton propagation in a fiber, with a relative error of no more than 10%.The presented results can be used in the development of a nonlinear optical phase correlator for determination the phase and time profile of a femtosecond laser pulse in the mid-IR range.

Theoretical study of coherent supercontinuum generation in chalcohalide glass photonic crystal fiber

Mid-infrared supercontinuum (SC) generation in normally dispersive photonic crystal fiber (PCF) made of GeS2-Ga2S3-CsI chalcohalide glass, is theoretically investigated. The finite difference method is employed to compute and optimize the different linear and nonlinear parameters of the fundamental guided mode. Simulation results indicate that the all-normal regime of dispersion over the entire wavelength range is achieved by properly reducing the diameter of the core neighboring air holes. The pulse propagation within the core of the proposed PCF and the SC generation process are accurately modelled and optimized, by numerically solving the generalized nonlinear Schrodinger equation. Both of Self Phase Modulation (SPM) and Optical Wave Breaking (OWB) effects are identified as the dominant nonlinear mechanisms involved in the pulse spectral broadening process. By launching at 3 μm, a 3 nJ energy and 50 fs duration input pulse into 15 mm long PCF, a broadband and highly coherent mid-infrared SC spectrum spanning the spectral region from 1.45 μm to 5.95 μm at 20 dB, is successfully generated.

Evolution of non-frequency shift components of pulse tail in normal dispersion region of highly nonlinear fiber

Supercontinuum generated in normal dispersion region of highly nonlinear fiber (HNLF) is widely used in signal processing and communication benefiting from its good flatness and high coherence. Because of the normal dispersion, optical wave breaking (OWB) occurs when non-frequency shift components and frequency shift components caused by self-phase modulation (SPM) overlap in time domain, and ends when non-frequency shift components disappear. The evolution of non-frequency shift components at the front and rear edge of optical pulse play an essential role in the supercontinuum generation process. In this paper, the evolution of non-frequency shift components in normal dispersion region is numerically calculated and analyzed based on generalized nonlinear Schrödinger equation. The results demonstrate that non-frequency shift components shrink gradually as the pulse propagates in the normal dispersion region. Cross-phase modulation (XPM) and stimulated Raman scattering (SRS) play a major role in this process, while the third-order dispersion imposes little effect on it. Because of XPM, non-frequency shift components at the front and rear edge shrink gradually, and keep red shifting and blue-shifting respectively. The influence of XPM on the non-frequency shift components at both edges is symmetrical. However, the influence of SRS on the evolution of non-frequency-shift components at both edges is asymmetric. At the front edge, SRS transfers the energy from non-frequency shift component to frequency shift component, which is opposite to that at the rear edge. At the front edge, SRS accelerates the shrinking process of the non-frequency shift component, while it slows down the shrinking process at the rear edge. And this asymmetric effect is more obvious when the peak power of the pulse is higher and SRS is more efficient. The evolution of the non-frequency shift components of chirped pulses propagating in the normal dispersion region is studied. Comparing with the unchirped pulse, the non-frequency shift components at the front and rear edge of the chirped pulse have different wavelengths. For the negative chirped pulse, the wavelength spacing between the overlapped frequency-shift components and non-frequency shift components is larger, which is easier to satisfy the SRS gain range. Therefore, the evolution of non-frequency-shift components at the front and rear edge of the negative chirped pulse are more asymmetric due to the higher SRS efficiency. For positive chirped pulses, the wavelength spacing between the overlapped components is difficult to satisfy the SRS gain range. The evolution of non-frequency-shift components in the positive chirped pulses is more symmetrical due to the lower SRS efficiency.

High-power short-wavelength infrared supercontinuum generation in multimode fluoride fiber

We demonstrate the generation of octave-spanning supercontinuum generation from 1200 nm to over 2500 nm with 600 mW average power in a short length of multimode fluoride fiber with 100 μm core diameter. We perform a detailed study of the supercontinuum generation process as a function of the pump wavelength and for two different fiber lengths. Beam profile characterization at the fiber output in different wavelength bands is also carried out. Our results open up new possibilities for the generation of high-power supercontinuum sources from the near- to mid-infrared.

Dispersion properties of a single-mode windmill single crystal Sapphire optical fiber and its broadband infrared supercontinuum generation

The effective refractive indices of each mode of a windmill single crystal sapphire fiber as function of wavelength can be obtained by finite element method. Recently, it is shown that this fiber operates in single-mode condition for a wide wavelength range. The dispersion and the slope of dispersion for the fundamental mode of a windmill single crystal sapphire fiber are properly obtained making use of the measured values of the refractive index as a function of the wavelength. Subsequently, the pulse evolution along this windmill fiber is investigated for low peak power pulses. Due to high applicability of broadband light sources, here, supercontinuum generation process is also studied when high peak power pulses are launched into the single-mode windmill single crystal sapphire fiber. Due to its unique optical properties, the single-mode windmill single crystal sapphire fiber can be used to generate broadband IR supercontinuum light compared with the conventional single crystal sapphire fiber. Our simulation results indicate that generated output spectrum contains dense frequency combs in a wide range. This broadband IR light source facilitates IR spectroscopy while its frequency combs are important in the development and improvement of the optical metrology.

Numerical Study on the Supercontinuum Generation in an Active Highly Nonlinear Photonic Crystal Fiber With Flattened All-Normal Dispersion

We numerically study the dynamics of supercontinuum generation (SCG) in an ytterbium-doped highly nonlinear photonic crystal fiber (HNL-PCF) with flattened all-normal dispersion (FAND) in the sub-picosecond pulse regime. We discuss the enhancement of the energy spectral density and the recovery of the peak power depletion in the SCG process through the fiber in comparison with the SCG based on a passive-type counterpart. As a unique application of the novel characteristics of the active HNL-PCF with FAND, we also analyze the direct amplification of an SC pulse through it, showing that the incident SC pulse can be amplified by 10 dB without undergoing significant degradations in terms of spectral bandwidth and flatness. Our numerical investigations on the active HNL-PCF with FAND will be helpful for opening up new opportunities for fiber-based SCG technology in the sub-picosecond regime.

Controlled supercontinua via spatial beam shaping

Recently, optimization techniques have had a significant impact in a variety of fields, leading to a higher signal-to-noise and more streamlined techniques. We consider the possibility for using programmable phase-only spatial optimization of the pump beam to influence the supercontinuum generation process. Preliminary results show that significant broadening and rough control of the supercontinuum spectrum in the visible region are possible without loss of input energy. This serves as a proof-of-concept demonstration that spatial effects can controllably influence the supercontinuum spectrum, leading to possibilities for utilizing supercontinuum power more efficiently and achieving excellent spectral control.

Observation of laser pulse propagation in optical fibers with a SPAD camera

Recording processes and events that occur on sub-nanosecond timescales poses a difficult challenge. Conventional ultrafast imaging techniques often rely on long data collection times, which can be due to limited device sensitivity and/or the requirement of scanning the detection system to form an image. In this work, we use a single-photon avalanche detector array camera with pico-second timing accuracy to detect photons scattered by the cladding in optical fibers. We use this method to film supercontinuum generation and track a GHz pulse train in optical fibers. We also show how the limited spatial resolution of the array can be improved with computational imaging. The single-photon sensitivity of the camera and the absence of scanning the detection system results in short total acquisition times, as low as a few seconds depending on light levels. Our results allow us to calculate the group index of different wavelength bands within the supercontinuum generation process. This technology can be applied to a range of applications, e.g., the characterization of ultrafast processes, time-resolved fluorescence imaging, three-dimensional depth imaging, and tracking hidden objects around a corner.

Coherent supercontinuum generation up to 2.2 µm in an all-normal dispersion microstructured silica fiber.

For the first time to our knowledge, we demonstrate a coherent supercontinuum in silica fibers reaching 2.2 µm in a long wavelength range. The process of supercontinuum generation was studied experimentally and numerically in two microstructured fibers with a germanium doped core, having flat all-normal chromatic dispersion optimized for pumping at 1.55 µm. The fibers were pumped with two pulse lasers operating at 1.56 µm with different pulse duration times equal respectively to 23 fs and 460 fs. The experimental results are in a good agreement with the simulations conducted by solving the generalized nonlinear Schrödinger equation with the split-step Fourier method. The simulations also confirmed high coherence of the generated spectra and revealed that their long wavelength edge (2.2 µm) is related to OH contamination. Therefore, improving the fibers purity will result in further up-shift of the long wavelength spectra limit.

Simulation of picosecond pulse propagation in fibre-based radiation shaping units

We have performed a numerical simulation of picosecond pulse propagation in a combined stretcher consisting of a segment of a telecommunication fibre and diffraction holographic gratings. The process of supercontinuum generation in a nonlinear photonic-crystal fibre pumped by picosecond pulses is simulated by solving numerically the generalised nonlinear Schrödinger equation; spectral and temporal pulse parameters are determined. Experimental data are in good agreement with simulation results. The obtained results are used to design a high-power femtosecond laser system with a pulse repetition rate of .

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.