Fiber Supercontinuum Source Research Articles

Nonlinear dynamics of relative intensity noise transfer at fiber amplification and supercontinuum generation

Supercontinuum sources via femtosecond pumping in highly nonlinear photonic crystal fibers considerably impact scientific research and industrial applications. Herein, we investigate the complex transfer function of relative intensity noise in a broadband fiber supercontinuum source generated using an ytterbium-doped mode-locked laser. A designed intensity noise was artificially added to the system through the introduction of weak sinusoidal intensity modulation to the seed pulses. Owing to the influence of nonlinear gain dynamics and soliton dynamics during power amplification and spectral broadening, the transfer functions exhibit unique frequency and wavelength dependencies. Systematic measurements indicate that the seed transfer functions exhibit the characteristics of damped high-pass filters and saturate at low Fourier frequencies within the fiber amplifier. Moreover, supercontinuum broadening induces a nonlinear transformation of fluctuations, resulting in considerable amplification and negative correlation within the dispersion wave and soliton self-frequency shift regions. Clarifying the mechanism and bandwidth of this system is imperative to achieving low-noise supercontinuum generation because they provide insights into the nonlinear laser dynamics associated with broadband spectral broadening and noise transformation.

All-fiber 2–6 [formula omitted] coherent supercontinuum source based on chalcogenide fibers pumped by an amplified mid-IR soliton laser

We numerically and experimentally investigate an all-fiber configuration of coherent mid-infrared supercontinuum generation in the femtosecond pumping regime. In particular, we demonstrate the advantageous combination of various dispersion-engineered chalcogenide (Ge-Se-Te glass) fibers with a turn-key amplified femtosecond fiber laser delivering 175-fs soliton pulses around 3.4 μm and developed from the erbium-doped fluoride fiber technology. Our results show that compact and coherent octave-spanning fiber supercontinuum sources centered on the 2–5 μm infrared atmospheric window can be obtained with a maximum average power of 50 mW at 25 MHz repetition rate.

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.

Ultra-flat, low-noise, and linearly polarized fiber supercontinuum source covering 670-1390 nm: publisher's note.

This publisher's note contains a correction to Opt. Lett.46, 1820 (2021)10.1364/OL.420676.

Next generation mid-infrared fiber: fluoroindate glass fiber

Fluoroindate glass fibers have shown intriguing application prospects in the field of mid-infrared (MIR) fiber lasers and supercontinuum (SC) sources because of their wide MIR transmission window, low phonon energy, and nonlinear characteristics. This paper reviews the representative research achievements of fluoroindate glass from the 1980s to the present, comprising the composition optimization, structure, glass melting conditions, optical fiber fabrication process, MIR laser emissions, and SC of fluoroindate glass. Notably, the MIR laser output of the fluoroindate bulk glass and optical fiber was beyond 3 µm, and the power reached to 100 mW levels. SC generated in fluoroindate-based fibers has been greater than 4 µm, and has achieved its highest output power at a 10 W level. Fluoroindate-based fibers are expected to produce high-power MIR laser and SC output based on the reduction of fiber loss and optimization of the pump source.

Reducing the noise of fiber supercontinuum sources to its limits by exploiting cascaded soliton and wave breaking nonlinear dynamics

The low-noise and phase-coherent nonlinear transformation of a narrowband laser into a broadband supercontinuum (SC) in an optical fiber forms the basis of extremely precise applications ranging from optical frequency comb technology to ultrafast photonics and biomedical imaging. A major challenge of this process is the avoidance of incoherent nonlinear effects that amplify random quantum noise, requiring careful birefringence and dispersion engineering of the fiber. However, fundamental trade-offs exist between working in normal or anomalous dispersion regimes. Here, we combine the benefits of nonlinear dynamics in both regimes by cascading soliton compression and optical wave breaking in a hybrid fiber, formed by joining two widely available, commercial, polarization-maintaining step-index fibers exhibiting anomalous and all-normal dispersion, respectively. We experimentally demonstrate that this hybrid approach results in an ultra-low-noise fiber SC source covering the 930–2130 nm range with phase coherence near unity, spectrally resolved relative intensity noise (RIN) as low as 0.05%, and averaging 0.1% over a bandwidth of 750 nm, approaching the theoretical limits close to the pump laser noise. This corresponds to a doubling of the generated spectral bandwidth and a decrease of RIN by up to 1 order of magnitude compared to direct pumping of the individual fibers, where modulational polarization instabilities play a limiting role. Owing to its simplicity and its scalability to high repetition rates, our hybrid scheme is readily applicable to various laser platforms and could enhance the performance of applications such as hyperspectral nonlinear microscopy, coherent optical communications, and photonic signal processing.

Spatiotemporal thermal analysis of tapered fibers in optical cavity sensing applications at 633 nm and 1550 nm

Fiber optic sensors with tapered silica fibers as sensing heads are attractive for various sensing applications. A mode propagating in a tapered fiber generates heat and induces temperature changes in and along the surface of the tapered fiber. The mode’s effective index also changes due to the thermo-optic coefficients of silica and analyte surrounding the tapered fiber. It is essential to analyze the spatiotemporal thermal response of tapered fibers due to the heat generated by a propagating mode to optimize the sensor’s performance. Here, we investigate the thermal response of air-clad and water-clad tapered fibers in Fabry-Pérot cavity sensors by analytically solving the heat equation in conjunction with finite element method simulations at 633 nm and 1550 nm. We find that the tapered fiber surface temperature and resonant wavelength of the cavity sensor change by ten times more at 633 nm than at 1550 nm. We also find an optimum radius of tapered fiber for sensing in aqueous solutions where thermal error becomes zero. The optimum radius is 0.35 μm at 633 nm and 0.85 μm at 1550 nm for a 25 cm long cavity made of 99.99% reflectivity mirrors with 2 cm long tapered fiber at 10 mW input power. We believe that the present work will give researchers better insight into understanding and controlling thermal-dependent properties of tapered fibers, specifically towards sensing applications and generally towards related technologies such as tapered fiber amplifiers and supercontinuum sources.

Perspective on the next generation of ultra-low noise fiber supercontinuum sources and their emerging applications in spectroscopy, imaging, and ultrafast photonics

A new generation of ultrafast and low-noise supercontinuum (SC) sources is currently emerging, driven by the constantly increasing demands of spectroscopy, advanced microscopy, and ultrafast photonics applications for highly stable broadband coherent light sources. In this Perspective, we review recent progress enabled by advances in nonlinear optical fiber design, detail our view on the largely untapped potential for noise control in nonlinear fiber optics, and present the noise fingerprinting technique for measuring and visualizing the noise of SC sources with unprecedented detail. In our outlook, we highlight how these SC sources push the boundaries for many spectroscopy and imaging modalities and focus on their role in the development of ultrafast fiber lasers and frequency combs with ultra-low amplitude and phase noise operating in the 2 μm spectral region and beyond in the mid-IR.

Seeded optical parametric generation in CdSiP2 pumped by a Raman fiber amplifier at 1.24 µm.

We report a seeded optical parametric generator (OPG) producing tunable radiation from 4.2-4.6 µm. The seeded OPG employs a 13 mm long CdSiP2 (CSP) crystal cut for non-critical phase-matching, pumped by a nanosecond-pulsed, MHz repetition rate Raman fiber amplifier system at 1.24 µm. A filtered, continuous-wave fiber supercontinuum source at 1.72 µm is used as the seed. The source generates up to 0.25 W of mid-infrared (MIR) idler power with a total pump conversion of 42% (combined signal and idler).

Ultra-flat, low-noise, and linearly polarized fiber supercontinuum source covering 670-1390 nm.

We report an octave-spanning coherent supercontinuum (SC) fiber laser with excellent noise and polarization properties. This was achieved by pumping a highly birefringent all-normal dispersion photonic crystal fiber with a compact high-power ytterbium femtosecond laser at 1049 nm. This system generates an ultra-flat SC spectrum from 670 to 1390 nm with a power spectral density higher than 0.4 mW/nm and a polarization extinction ratio of 17 dB across the entire bandwidth. An average pulse-to-pulse relative intensity noise down to 0.54% from 700 to 1100 nm was measured and found to be in good agreement with numerical simulations. This highly stable broadband source could find strong potential applications in biomedical imaging and spectroscopy where an improved signal-to-noise ratio is essential.

Polarization-dependent relative intensity noise of fiber supercontinuum sources

Detailed experimental polarization-dependent relative intensity noise (RIN) measurements of all-normal dispersion (ANDi) and conventional supercontinuum (SC) sources are presented. We show that the polarization-maintaining ANDi fiber suppresses polarization noise, is robust against pump polarization fluctua- tions, and allows ultra-low noise SC generation with RIN identi- cal to the pump laser (< 0.05%).

Supercontinuum source based on all-silica fibers with optimized spectral power from 1100 up to 2300 nm

The authors present here an attainable fiber supercontinuum source with competitive characteristics in the near infrared range. The proposed system is based on a highly nonlinear silica fiber, with nominal zero-dispersion wavelength at 1550 nm. A mode-locked fiber laser was used as the seed. The pump wavelength was shifted to match the zero-dispersion wavelength of the nonlinear fiber, achieving a better performance of the spectral emission. The supercontinuum emission was characterized between 1100 nm and 2300 nm, showing a flat and wide spectrum with a conversion efficiency over 80%. The set-up consists only in silica fiber and can be considered, therefore, as a cost-effective alternative for supercontinuum generation in the near infrared range, which can be easily implemented in any photonics laboratory, where commercial amplifiers are usual devices.

Record power, ultra-broadband supercontinuum source based on highly GeO2 doped silica fiber.

We demonstrate highly germania doped fibers for mid-infrared supercontinuum generation. Experiments ensure a highest output power of 1.44 W for a broadest spectrum from 700 nm to 3200 nm and 6.4 W for 800 nm to 2700 nm from these fibers, while being pumped by a broadband Erbium-Ytterbium doped fiber based master oscillator power amplifier. The effect of repetition frequency of pump source and length of germania-doped fiber has also been investigated. Further, germania doped fiber has been pumped by conventional supercontinuum source based on silica photonic crystal fiber supercontinuum source. At low power, a considerable broadening of 200-300 nm was observed. Further broadening of spectrum was limited due to limited power of pump source. Our investigations reveal the unexploited potential of germania doped fiber for mid-infrared supercontinuum generation. These measurements ensure the potential of germania based photonic crystal fiber or a step-index fiber supercontinuum source for high power ultra-broad band emission being by pumped a 1060 nm or a 1550 nm laser source. To the best of our knowledge, this is the record power, ultra-broadband, and all-fiberized supercontinuum light source based on silica and germania fiber ever demonstrated to the date.

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.

Theoretical models for beam quality evaluation of fiber supercontinuum sources

We propose three theoretical models to evaluate the beam quality of the fiber supercontinuum source as a whole. New definitions of spectral centroid and three factors to evaluate the beam quality of supercontinuum source are introduced. Based on the three factors some supercontinuum sources with different output power and spectra obtained from the experiment are calculated numerically. All the results denote that the three models are feasible and useful for the beam quality evaluation and comparison of the fiber supercontinuum sources as a whole with considering the power spectral distribution and propagation modes.

Broadband nonlinear vibrational spectroscopy by shaping a coherent fiber supercontinuum.

Vibrational spectroscopy has been widely applied in different fields due to its label-free chemical-sensing capability. Coherent anti-Stokes Raman scattering (CARS) provides stronger signal and faster acquisition than spontaneous Raman scattering, making it especially suitable for molecular imaging. Coherently-controlled single-beam CARS simplifies the conventional multi-beam setup, but the vibrational bandwidth and non-trivial spectrum retrieval have been limiting factors. In this work, a coherent supercontinuum generated in an all-normal-dispersion nonlinear fiber is phase-shaped within a narrow bandwidth for broadband vibrational spectroscopy. The Raman spectra can be directly retrieved from the CARS measurements, covering the fingerprint regime up to 1750 cm(-1). The retrieved spectra of several chemical species agree with their spontaneous Raman data. The compact fiber supercontinuum source offers broad vibrational bandwidth with high stability and sufficient power, showing the potential for spectroscopic imaging in a wide range of applications.

A compact seven-core photonic crystal fiber supercontinuum source with 42.3 W output power

We have fabricated a new type of seven-core photonic crystal fiber and demonstrated its all-fiber supercontinuum source pumped with a ytterbium-doped fiber master oscillator power amplifier. The generated supercontinuum covers the wavelength range from 720 nm to beyond 1700 nm, with an output power of 42.3 W. It is the first high-power all-fiber SC source based on seven-core photonic crystal fiber.

Compression of fiber supercontinuum pulses to the Fourier-limit in a high-numerical-aperture focus

A multiphoton intrapulse interference phase scan (MIIPS) adaptively and automatically compensates the combined phase distortion from a fiber supercontinuum source, a spatial light modulator pulse shaper, and a high-NA microscope objective, allowing Fourier-transform-limited compression of the supercontinuum pulses at the focus of the objective. A second-harmonic-generation-based method is employed to independently validate the transform-limited compression. The compressed pulses at the focus of the objective have a tunable duration of 10.8-38.9 fs (FWHM), a central wavelength of ~1020 nm, an average power of 18-70 mW, and a repetition rate of 76 MHz, permitting the application of this source to nonlinear optical microscopy and coherently controlled microspectroscopy.

Fiber supercontinuum sources (Invited)

We review supercontinuum generation in optical fibers for particular cases where the nonlinear spectral broadening is induced by pump radiation from fiber-format sources. Based on numerical simulations, our paper is intended to provide experimental design guidelines tailored ytterbium and erbium-based pumps around 1060 and 1550 nm, respectively. In particular, at 1060 nm, we consider conditions under which the generated spectra are phase and intensity stable, and we address the dependence of the supercontinuum coherence on the input pulse parameters and the fiber length. At 1550 nm, special attention is paid to the case of dispersion-flattened dispersion-decreasing fiber, where we revisit the underlying physics in detail and explicitly examine the use of such fiber for supercontinuum generation with pumps of peak power in the range 200-1200 W and sub-10 m fiber lengths. We show that supercontinuum generation under such conditions can be highly coherent and can be applied to nonlinear pulse compression.