Continuous-wave Supercontinuum Research Articles

130 W continuous-wave supercontinuum generation within a random Raman fiber laser

We have demonstrated a 134 W, octave-spanning, continuous-wave (CW) supercontinuum (SC), which is generated within a random Raman fiber laser (RRFL) pumped by Ytterbium (Yb) fiber sources. A piece of 600-m long standard telecom fiber with a zero-dispersion wavelength (ZDW) around 1.3 μm was used to provide Raman gain and random distributed feedback for the RRFL. High power SC is excited when the wavelength of the cascade RRFL reaches the ZDW. 20-dB spectrum ranging from 852 nm to 2055 nm is achieved with an average power spectral density over 111 mW/nm. To the best of our knowledge, this is the highest power of CW SC source ever reported. Moreover, two different 1064 nm Yb-doped fiber sources were used as pumps, which are seeded with a normal oscillator and amplified spontaneous emission, respectively. The difference of the SC generated by the two pump sources are compared in detail, including spectrum, efficiency, and relative intensity noise.

130 W Continuous-Wave Supercontinuum Generation within a Random Raman Fiber Laser

130 W Continuous-Wave Supercontinuum Generation within a Random Raman Fiber Laser

Emergence of breathers in non-linear pulse compression

We report on the universality of the emergence of Akhmediev breathers in soliton effect pulse compression dynamics by using explicit analytic solutions of breathers and solitons in the non-linear Schrödinger equation. We show that at maximum compression point, a higher-order soliton and a breather solution are equivalent with a nearly identical full-width at half maximum duration. From this, we demonstrate numerically how breathers that develop in the initial stage of continuous wave supercontinuum generation transform into an ensemble of solitons in the presence of system perturbation.

Octave-spanning, continuous-wave supercontinuum generation with record power using standard telecom fibers pumped with power-combined fiber lasers: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 1172 (2020).OPLEDP0146-959210.1364/OL.384690.

Octave-spanning, continuous-wave supercontinuum generation with record power using standard telecom fibers pumped with power-combined fiber lasers.

We have demonstrated a record output power of ∼72W, octave-spanning, nearly single-mode, continuous-wave supercontinuum with a bandwidth of ∼1050nm using standard telecom fiber as the nonlinear medium in an all-fiber architecture. We have utilized the recently proposed nonlinear power combining architecture by which power scaling is achieved using multiple independent Ytterbium lasers operating at different wavelengths. In this Letter, Raman conversions in the fiber assist in combining multiple input laser lines into a single wavelength which then undergoes supercontinuum generation. The architecture is based on the recently proposed grating-free, cascaded Raman lasers based on distributed feedback. Here all Raman conversions are well seeded, thereby enhancing the efficiency of supercontinuum generation to ∼44%. In this Letter, we have obtained power spectral densities (PSDs) of >3mW/nm from 850 to 1350 nm and a high PSD of >100mW/nm from 1350 to 1900 nm. Here we have also investigated the power-combined supercontinuum generation for different pump wavelength combinations demonstrating the flexibility of this technique.

Visible light generation in the cladding of optical fibers carrying near-infrared continuous-wave lasers due to Cherenkov-phase matched harmonic conversion.

In this work, we report and analyze the cause of the surprising observation of visible light generation in the cladding of silica-based continuous-wave (CW), near-infrared fiber lasers. We observe a visible rainbow of hues in a cascaded Raman fiber laser, which we attribute to second and third harmonic conversion of the different wavelength components propagating in the core of the fiber. The light in the cladding of the fiber occurs through Cherenkov-type phase matching, and a mathematical analysis is presented to estimate the power of the harmonic light generated. We then extend this theory to visible light generation in other types of fiber lasers. Specifically, we analyze the case of a CW supercontinuum generated in standard telecom fibers, and verify our theoretical predictions with experimental results through visible spectra collected.

A stimulated Stokes Raman scattering-based approach for continuous wave supercontinuum generation in optical fibers

We report on a new and simple approach for continuous wave supercontinuum generation in optical fibers. Our new approach uses the effect of stimulated Stokes Raman scattering in a low loss fiber ring laser. By continuously pumping this ring laser with up to 19 W optical power we excited up to six Stokes orders and covered a wavelength range of 500 nm. Due to the feedback mechanism of the ring layout additional nonlinear effects occurred next to the plain generation of individual Stokes peaks. Eventually, these effects broaden and merge the separated Stokes peaks and create a single, connected continuous supercontinuum. By using the effect of stimulated Stokes Raman scattering, we do not rely on anomalous dispersion and modulation instability as typically required for continuous wave supercontinuum generation.

Stability analysis of high power, octave spanning, continuous-wave supercontinuum sources based on cascaded Raman scattering in standard telecom fibers

Long term spectral and temporal stability of a recently proposed high power, continuous-wave, supercontinuum source has been characterized. The supercontinuum laser, based on telecom fibers as the nonlinear medium and delivering >35W of CW power over an octave spanning bandwidth (880 to >1900nm), was operated continuously for extended periods of time to investigate its spectral stability. The dependence of stability on various parameters such as the wavelength of pumping and output power was studied by pumping the supercontinuum at 3 different wavelengths and at 3 different output power levels. The RMS value of the difference spectrum (spectral change) was used as the metric for comparison. The spectrum was stable with <1 dB variation over a duration of 60 minutes of continuous operation. This small variation is attributed to heating of the fiber and can be further reduced by properly heat sinking the fiber. When the fiber was cooled down to ambient temperature during power cycling tests, the change in spectrum was ~0.4dB. The supercontinuum output power fluctuations were characterized using a fast photo detector and was measured to be within ± 3% in nanosecond time-scales. The stability measured by these experiments demonstrates the efficacy of the source for a variety of applications.

High-power continuous-wave supercontinuum generation in highly nonlinear fibers pumped with high-order cascaded Raman fiber amplifiers.

A novel method for efficient generation of a high-power, equalized continuous-wave supercontinuum source in an all-conventional silica fiber architecture is demonstrated. Highly nonlinear fiber is pumped in its anomalous dispersion region using a novel, high-power, L-band laser. The L-band laser encompasses a sixth-order cascaded Raman amplifier which is pumped with a high-power Ytterbium-doped fiber laser and amplifies a low-power, tunable L-band seed source. The supercontinuum generated 35W of power with ∼40% efficiency. The supercontinuum spectrum was measured to have a high degree of flatness of better than 5dB over 400nm of bandwidth (1.3-1.7μm, limited by spectrum analyzer range) and a power spectral density in this region of >50 mW/nm. The extent of the SC spectrum is estimated to be up to 2μm.

High power, high efficiency, continuous-wave supercontinuum generation using standard telecom fibers

We demonstrate a simple module for octave spanning continuous-wave supercontinuum generation using standard telecom fiber. This module can accept any high power ytterbium-doped fiber laser as input. The input light is transferred into the anomalous dispersion region of the telecom fiber through a cascade of Raman shifts. A recently proposed Raman laser architecture with distributed feedback efficiently performs these Raman conversions. A spectrum spanning over 1000nm (>1 octave) from 880 to 1900nm is demonstrated. The average power from the supercontinuum is ~34W with a high conversion efficiency of 44%. Input wavelength agility is demonstrated with similar supercontinua over a wide input wavelength range.

Continuous wave supercontinuum generation pumped in the normal group velocity dispersion regime on a highly nonlinear fiber

We have obtained spectral broadening by pumping a nonmicrostructured highly nonlinear fiber with a continuous wave signal from a Raman fiber laser. The experiment was simulated using a generalized Schr&#xF6;dinger equation containing the actual Raman response of the fiber as calculated from the experimental Raman gain. A different input-noise model, that reproduces well the power spectral density of the laser, was used and compared with others previously proposed.

All Fiber Continuous-Wave Supercontinuum Source for Sensing

A broadband supercontinuum source is demonstrated with phosphosilicate-doped Raman fiber laser. By lunching 1495nm continuous-wave pump into a short length of microstructure fiber with high nonlinearity and flat dispersion, high efficiency continuum spanning from 1450nm to 1650nm is generated, covering the whole communication windows. The average output power is as high as 2W when the pump is set as 4.14W, corresponding to a spectral density of 10mW/nm. This all -fiber light source can be applied in fiber sensing system.

Fission of solitons in continuous-wave supercontinuum

We use numerical simulations to revisit the generation of fiber supercontinua pumped by partially coherent continuous-wave (CW) sources. Specifically, we show that intensity fluctuations characteristic of temporal partial coherence can be described as a stochastic train of high-order solitons, whose individual dynamics drive continuum formation. For sources with sufficiently low coherence, these solitons actually undergo fission rather than modulation instability, changing the nature of the CW supercontinuum evolution.

Continuous Wave Supercontinuum Generation Through Pumping in the Normal Dispersion Region for Spectral Flatness

We demonstrate the generation of a spectrally flat continuous wave supercontinuum by pumping deep in the normal dispersion region. A Raman cascade forms up to the anomalous dispersion region, where modulation instability supercontinuum dynamics are initiated. The Raman cascade seeds the frequency up-shifted part of the continuum, leading to a spectral flatness of 5 dB from 1.15 to 2.15.

Optimization of continuous-wave supercontinuum generation

We provide an overview of the progress in the field of high power continuous-wave supercontinuum generation over the past 5years. In particular, we show how the development of dispersion-engineered photonic crystal fibers in our group has allowed the demonstration of a continuous-wave white-light supercontinuum source covering more than two octaves.

Spectrally Sampled OCT Imaging Based on 1.7-μm Continuous-Wave Supercontinuum Source

We demonstrate a novel multiwavelength light source around the 1.7-μm region based on continuous-wave (CW) supercontinuum (SC) generation for the deeper imaging of low-water-absorption sample with a spectrally sampled optical coherence tomography (OCT) system. This self-regulated Gaussian spectrum of CW SC source has an extremely smooth flat width with more than 210-nm full-width at half-maximum bandwidth centered at the 1.7-μm region, a high output power of 500 mW, and a low temporal instability with less than 0.1 dB for an hour. In order to improve the OCT signal sensitivity without increasing the optical illumination power, a multiwavelength light source based on this CW SC was implemented by using a tunable fiber Sagnac comb filter. When it was applied to the spectrally sampled OCT imaging, the multiwavelength light source enabled a dynamic range improvement, compared to conventional continuous spectral light source with the same average optical power.

Role of pump coherence in the evolution of continuous-wave supercontinuum generation initiated by modulation instability

We report in detail, both experimentally and using numerical simulation, the efficiency of generation of supercontinua in optical fiber driven by modulation instability of a continuous-wave (CW) pump source. It is shown that the degree of pump coherence has a dramatic effect on the resulting spectral expansion and it is discussed how this can be explained by having the proper conditions for efficient modulation instability to break the CW pump light into a train of fundamental solitons that subsequently undergo self-Raman shift to longer wavelengths. It is proposed that an optimal pump bandwidth exists corresponding to the optimal degree of pump incoherence, defined as a function of the modulation instability period.

Phosphorus-Doped Photonic Crystal Fibers for High-Power (36 W) Visible CW Supercontinuum

We report the fabrication and characterization of photonic crystal fibers having a core doped with 10 wt% of phosphorus and a zero dispersion wavelength (ZDW) around 1 μm. These fibers were successfully used for high-power visible continuous wave (CW) supercontinuum generation with an output average power up to 36 W and spectral power densities of 20 mW/nm at 1550 nm and 2 mW/nm at 650 nm. We also show that the spectral power density in the whole region below the pump wavelength significantly increases as the ZDW moves closer to the pump wavelength, at the expense of the short-wavelength edge that slightly redshifts.

Optical Coherence Tomography Based on a Continuous-wave Supercontinuum Seeded by Erbium-doped Fiber's Amplified Spontaneous Emission

In this study, the use of a continuous-wave (CW) supercontinuum (SC) seeded by an erbium-doped fiber's amplified spontaneous emission (ASE) for optical-coherence tomography imaging is experimentally demonstrated. It was shown, by taking an in-depth image of a human tooth sample, that due to the smooth, flat spectrum and long-term stability of the proposed CW SC, it can be readily applied to the spectral-domain optical-coherence tomography system. The relative-intensity noise level and spectral bandwidth of the CW SC are also experimentally analyzed as a function of the ASE beam power.

Microwave frequency measurement using a finite impulse response system that incorporates Lorentzian profile-weighted optical taps

We propose an advanced photonic microwave processing scheme useful for the accurate measurement of incoming microwave signal frequency over a wide frequency bandwidth. Our scheme is based on a finite impulse response system using Lorentzian profile-weighted multiple optical wavelength taps. Unlike the conventional schemes based on two optical taps, our scheme allows for well-defined, linear mapping of the incoming microwave frequency onto the output electrical power utilizing a log scale. In order to verify the principle, an experimental demonstration was carried out by using a multiwavelength channel beam, which was produced by the combination of a broadband, continuous wave supercontinuum and a digital micromirror device-based spatial light modulator. An excellent frequency measuring performance over a range of 1–9 GHz was achieved.