All-normal Dispersion Fiber Research Articles

Dynamic counterpropagating all-normal dispersion (DCANDi) fiber laser

The fiber single-cavity dual-comb laser (SCDCL) is an emerging light-source architecture that opens up the possibility for low-complexity dual-comb pump-probe measurements. However, the fundamental trade-off between measurement speed and time resolution remains a hurdle for the widespread use of fiber SCDCLs in dual-comb pump-probe measurements. In this paper, we break this fundamental trade-off by devising an all-optical dynamic repetition rate difference (Δf rep ) modulation technique. We demonstrate the dynamic Δf rep modulation in a modified version of the recently developed counterpropagating all-normal dispersion (CANDi) fiber laser. We verify that our all-optical dynamic Δf rep modulation technique does not introduce excessive relative timing jitter. In addition, the dynamic modulation mechanism is studied and validated both theoretically and experimentally. As a proof-of-principle experiment, we apply this so-called dynamic CANDi (DCANDi) fiber laser to measure the relaxation time of a semiconductor saturable absorber mirror, achieving a measurement speed and duty cycle enhancement factor of 143. DCANDi fiber laser is a promising light source for low-complexity, high-speed, high-sensitivity ultrafast dual-comb pump-probe measurements.

Coherence evolution of multi-pulse pumped supercontinuum generation in all-normal dispersion fibers.

We numerically calculate the coherence of supercontinuum (SC) spectrum generated in all-normal dispersion (ANDi) fibers by multiple picosecond pulses. We show that multi-pulse pumping significantly mitigates the phenomenon of coherence degradation and maintains high spectral coherence over a broader wavelength range compared to the single pumping. It is found that inter-pulse four-wave-mixing (FWM) and stimulated Raman scattering (SRS) significantly contribute to the superior coherence characteristics of the SC. We further discuss the impact of key parameters on the multi-pulse pumped SC. Results indicate that a large temporal interval between pulses reduces spectral broadening efficiency, while closely spaced pulses generate a significant number of incoherent photons, severely degrading spectral coherence. Introducing initial chirp effectively mitigates this coherence degradation. A slight increase in the dispersion parameter notably suppresses the spectral broadening efficiency of the multi-pulse generation. Pulse shape primarily affects the SC shape without significantly impacting the spectral broadening efficiency.

Low-noise supercontinuum generation in chiral all-normal dispersion photonic crystal fibers.

We present the advantages of supercontinuum generation in chiral, therefore circularly birefringent, all-normal dispersion fibers. Due to the absence of nonlinear power transfer between the polarization eigenstates of the fiber, chiral all-normal dispersion fibers do not exhibit any polarization instabilities and thus are an ideal platform for a low-noise supercontinuum generation. By pumping a chiral all-normal dispersion fiber at 802 nm, we obtained an octave-spanning, robustly circularly polarized supercontinuum with a low noise.

Pump power induced instability and hysteresis in an all-normal dispersion linear mode-locked fiber laser

In this manuscript, experimental studies on the instability and hysteresis in an all-normal dispersion (ANDi) fiber laser have been presented. The laser was mode-locked by using a semiconductor saturable absorber mirror and a chirped fiber Bragg grating in linear cavity configuration and under the stable conditions it delivered stationary dissipative soliton pulses with characteristic rectangular-shaped steep-edged spectrum. With increasing the pump power, the laser transits to a non-stationary state with a near trapezoidal-shaped spectrum with significant temporal instabilities. The hysteresis associated with the state transition and variations in spectral characteristics has been studied including dispersive Fourier transform based analysis. Pump power induced state transition in an ANDi linear cavity with a physical saturable absorber without the influence of any physical polarization controller or apparent limitation due to spectral filtering is the key observation presented in this paper.

High power ytterbium rod-type fiber laser delivering tunable picosecond pulses.

We designed an all-normal dispersion ytterbium rod-type fiber laser oscillator delivering picosecond pulses which are continuously tunable both in central wavelength and pulse duration. This system delivers self-mode-locked pulses with an average power up to 25 W. At a repetition rate of 78 MHz, it yields picosecond laser pulses, of which the central wavelength and pulse duration can be tuned between 1010 nm - 1060 nm and 4.5 ps - 1.8 ps, respectively. The tunability is obtained by adjusting the position and the width of a slit which acts as a spectral bandwidth filter, placed close to the center of a 4f-folded zero dispersion line inserted in the laser cavity. This oscillator delivers nearly Fourier limited pulses with at most a 1.2 time-bandwidth product. A numerical model accounts well for the behavior of this all-normal dispersion fiber oscillator.

High-energy Q switched Yb-doped fiber laser based on a ternary layered structured Ti2AlC saturable absorber.

Ti2AlC is a kind of ternary layered structured ceramic metal compound, combining the advantages of both ceramic and metal. Herein, the saturable absorption performance of Ti2AlC at the 1-µm wave band is investigated. The Ti2AlC behaves with excellent saturable absorption, which has a modulation depth of 14.53% and a saturable intensity of 13.27 MW/cm2. An all-normal dispersion fiber laser based on the Ti2AlC saturable absorber (SA) is constructed. The repetition frequency of the Q switched pulses increased from 44 to 49 kHz as the pump power rose from 276 to 365 mW, and the corresponding pulse width decreased from 3.64 to 2.42 µs. The maximum output single Q switched pulse energy is as high as 169.8 nJ. Our experiments prove that the MAX phase Ti2AlC has potential as a low-cost, simple preparation, and broadband SA material. To the best of our knowledge, this is the first demonstration of Ti2AlC serving as a SA material achieving Q switched operation at the 1-µm wave band.

Improving the formation probability and stability of noise-like pulse by weakening the spectrum filtering effect.

Noise-like pulses (NLP) are extremely sought after in low-coherence tomography and supercontinuum, etc. Here, we propose an effective method to form the NLP in the all-normal-dispersion (ANDi) fiber laser by weakening the spectrum filtering for the first time. Numerical explorations are performed in detail and demonstrate that the NLP can be originated from the clustering behavior of amplified random sub-pulses led by the saturable absorber. By simulating the pulse-pattern distribution in the two-dimensional parameter space, it is further found that this kind of NLP pattern is widely distributed where are with a weak spectrum filtering. Since, the weaker the filtering, the harder the dissipative system to achieve balance, which helps to avoid the evolution from the pulse cluster to other coherent states and supports the NLP stability. To prove the feasibility experimentally, we built an ANDi fiber laser based on the nonlinear polarization rotating (NPR). The built laser can operate at the stable dissipative soliton (DS) state with a spectrum filter, which also with a 45 nm wavelength tuning performance. Replacing the filter, only NLP with a 40.2 ps pedestal and 237 fs spike can be attained. The experiments agree well with the numerical predictions. This exploration significantly broadens the design possibilities for ultrafast lasers, making them much more accessible to produce desired pulse patterns.

Direct observation of the build-up and extinction of similaritons in an all-normal dispersion fiber laser

Similaritons have attracted considerable attention thanks to their outstanding output performance in recent years. In this paper, we explored the entire dynamic evolution process of similaritons in an all-normal dispersion fiber laser through the time-stretch dispersive Fourier transform technique. Particularly, the narrow-band spectral filtering effect on the pulse evolution dynamics has been discovered. The “memory” ability in the relaxation oscillation state has disappeared in similariton fiber lasers compared with the traditional soliton mode-locked fiber lasers, and the build-up time is much shorter than other fiber lasers mode-locked by the nonlinear polarization evolution. Furthermore, the extinction process of the similariton fiber laser is observed and evaluated for the first time as well. An unstable time period accompanied by a high spectral peak over the spectrum is captured during the extinction process, and then the damped relaxation oscillation state is observed. These results can provide new perspectives into the ultrafast transient process of similaritons in all-normal dispersion fiber lasers and the dynamics of sophisticated nonlinear systems.

Relative timing jitter in a counterpropagating all-normal dispersion dual-comb fiber laser

The counterpropagating all-normal dispersion (CANDi) fiber laser is an emerging high-energy single-cavity dual-comb laser source. Its relative timing jitter (RTJ), a critical parameter for dual-comb timing precision and spectral resolution, has not been comprehensively investigated. In this paper, we enhance the state-of-the-art CANDi fiber laser pulse energy from 1 nJ to 8 nJ. We then introduce a reference-free RTJ characterization technique that provides shot-to-shot measurement capability at femtosecond precision. The measurement noise floor reaches 1.6 × 10 − 7 f s 2 / H z , and the corresponding integrated measurement precision is only 1.8 fs (1 kHz, 20 MHz). With this characterization tool, we are able to study the physical origin of the CANDi laser’s RTJ in detail. We first verify that the cavity length fluctuation does not contribute to the RTJ. Then we measure the integrated RTJ to be 39 fs (1 kHz, 20 MHz) and identify the pump relative intensity noise (RIN) to be the dominant factor responsible for it. In particular, pump RIN is coupled to the RTJ through the Gordon–Haus effect. Finally, solutions to reduce the free-running CANDi laser’s RTJ are discussed. This work provides a general guideline to improve the performance of compact single-cavity dual-comb systems such as the CANDi laser, benefitting various dual-comb applications.

Mechanisms of dual-wavelength dissipative soliton mode-locking in net-normal dispersion fiber lasers

Dual-wavelength mode-locked fiber lasers have attracted great attention due to their enormous potential in facilitating dual-comb spectroscopy. However, mechanism for dual-wavelength dissipative soliton mode-locking and the intracavity dynamics are still elusive. In this paper, we construct a simple and universal model to numerically investigate the intracavity buildup dynamics of dual-wavelength mode-locking in an all-normal dispersion fiber laser. By resolving the pulses evolution and their interactions, we find that the chromatic dispersion induced pulse collision plays an important role in dual-wavelength pulses formation and propagation. At the chaotic stage, the collision-induced energy upheaval results in drastic nonlinear coupling effects, which contribute to the dual-wavelength mode-locking and formation of dual-wavelength soliton molecules. While at the stable dual-wavelength mode-locking stage, the two asynchronous pulses periodically collide, accompanying extremely intense wave generation and spectral explosion. Moreover, cavity energy and gain competition are modeled and optimized to realize the self-started pulse-splitting-free dual-wavelength mode-locking.

Direct Observation of the Build-Up and Extinction of Similaritons in an All-Normal Dispersion Fiber Laserdirect Observation of the Build-Up and Extinction of Similaritons in an All-Normal Dispersion Fiber Laser

Direct Observation of the Build-Up and Extinction of Similaritons in an All-Normal Dispersion Fiber Laserdirect Observation of the Build-Up and Extinction of Similaritons in an All-Normal Dispersion Fiber Laser

Generation of pulse bursts in an Yb-doped all-normal dispersion passively mode-locked fiber laser with Raman scatting

Pulse bursts are generated in an Yb-doped all-normal dispersion (ANDi) passively mode-locked fiber laser with Raman scatting based on nonlinear polarization rotation (NPR). The number of pulses can be continuously adjusted from 1 to 8 by adjusting the pump power. The output power varies stepwise with linearly increasing of input power and pulse numbers in the bursts. We explain that the pulse bursts in ANDi fiber laser are formed by the clamping of pulse width and peak power. The energy evolution of the pulses burst with Raman component is experimentally realized in Yb-doped fiber laser, which is excepted to benefit energy controlling in applications such as laser material ablation and so on.

Impact of reverse saturable absorption on pulse dynamics in the ultrafast fiber laser

In this work, we present a numerical investigation of the pulse dynamics in an all-normal dispersion fiber laser mode-locked by a physical saturable absorber. We erected a theoretical model including the reverse saturable absorption (RSA) and spectral filtering effects. Utilizing the double-parameter scanning method, the pulse state distribution is mapped in the two-dimensional parameter space. The simulation results emphasize that the RSA greatly affects the transitions of pulse states and pulse properties. Moreover, we explore the multipulse region in detail and then demonstrate two completely different multipulse evolution dynamics. Based on this, we offer an analysis of dissipative soliton resonance multipulse generating in the physical saturable absorber-based ultrafast fiber laser.

Highly coherent visible supercontinuum generation in a micrometer-core borosilicate glass photonic crystal fiber

Highly coherent visible supercontinuum (SC) sources are demanded for many applications such as bio-sensing and imaging. Either dispersion management on the fiber or optimizing ultrafast pulsed pump parameters can work for achieving broadband coherent fiber SC. Normally, highly coherent SC with medium bandwidth can be obtained in an all-normal-dispersion (ANDi) nonlinear photonic crystal fiber (PCF), which has normal dispersion for all the wavelengths by tailoring the structure parameters of the microstructured cladding to sub-wavelength scale. Hence, such an ANDi fiber requires a submicron core diameter, and it makes precise fabrication challenging. Instead, using a standard anomalous dispersion pumping scheme and an ultrafast pulsed pump source, broadband SC with a high degree of coherence can also be obtained. In this study, we report broadband visible SC with a high degree of coherence approaching unity in a low-index borosilicate glass air-suspended PCF with a micrometer-core diameter, by the anomalous dispersion pumping scheme. The pulse duration of the 800 nm Ti:sapphire femtosecond laser is 29 fs. The experimental results are in good agreement with the numerical simulations, indicating that ultrashort pump pulses are the primary cause for the high degree of the generated visible SC, while the weak Raman effect of the borosilicate glass host plays a non-negligible but secondary role in the procedure of coherent SC generation.

Recent advances in supercontinuum generation in specialty optical fibers [Invited

The physics and applications of fiber-based supercontinuum (SC) sources have been a subject of intense interest over the last decade, with significant impact on both basic science and industry. New uses for SC sources are also constantly emerging due to their unique properties that combine high brightness, multi-octave frequency bandwidth, fiber delivery, and single-mode output. The last few years have seen significant research efforts focused on extending the wavelength coverage of SC sources towards the 2 to 20 µ m molecular fingerprint mid-infrared (MIR) region and in the ultraviolet (UV) down to 100 nm, while also improving stability, noise and coherence, output power, and polarization properties. Here we review a selection of recent advances in SC generation in a range of specialty optical fibers, including fluoride, chalcogenide, telluride, and silicon-core fibers for the MIR; UV-grade silica fibers and gas-filled hollow-core fibers for the UV range; and all-normal dispersion fibers for ultralow-noise coherent SC generation.

Shot-noise limited, supercontinuum-based optical coherence tomography

We present the first demonstration of shot-noise limited supercontinuum-based spectral domain optical coherence tomography (SD-OCT) with an axial resolution of 5.9 μm at a center wavelength of 1370 nm. Current supercontinuum-based SD-OCT systems cannot be operated in the shot-noise limited detection regime because of severe pulse-to-pulse relative intensity noise of the supercontinuum source. To overcome this disadvantage, we have developed a low-noise supercontinuum source based on an all-normal dispersion (ANDi) fiber, pumped by a femtosecond laser. The noise performance of our 90 MHz ANDi fiber-based supercontinuum source is compared to that of two commercial sources operating at 80 and 320 MHz repetition rate. We show that the low-noise of the ANDi fiber-based supercontinuum source improves the OCT images significantly in terms of both higher contrast, better sensitivity, and improved penetration. From SD-OCT imaging of skin, retina, and multilayer stacks we conclude that supercontinuum-based SD-OCT can enter the domain of shot-noise limited detection.

Near-infrared nanospectroscopy using a low-noise supercontinuum source

Unlocking the true potential of optical spectroscopy on the nanoscale requires development of stable and low-noise laser sources. Here, we have developed a low-noise supercontinuum (SC) source based on an all-normal dispersion fiber pumped by a femtosecond fiber laser and demonstrate high resolution, spectrally resolved near-field measurements in the near-infrared (NIR) region. Specifically, we explore the reduced-noise requirements for aperture-less scattering-type scanning near-field optical microscopy (s-SNOM), including inherent pulse-to-pulse fluctuation of the SC. We use our SC light source to demonstrate the first NIR, spectrally resolved s-SNOM measurement, a situation where state-of-the-art commercial SC sources are too noisy to be useful. We map the propagation of surface plasmon polariton (SPP) waves on monocrystalline gold platelets in the wavelength region of 1.34–1.75 μm in a single measurement, thereby characterizing experimentally the dispersion curve of the SPP in the NIR. Our results represent a technological breakthrough that has the potential to enable a wide range of new applications of low-noise SC sources in near-field studies.

Multipulsing states management in all-normal dispersion fiber laser with a fiber-based spectral filter

We report an experimental study of multipulsing states in an all-normal dispersion fiber laser using an all-fiber, tunable birefringent spectral filter. Various multipulsing states such as harmonic mode-locking and soliton bunches are observed by tuning the spectral filter. The multipulse evolution in this experiment relies only on the spectral filter characteristics, and it is independent of input pump power and saturable absorber effects. This laser provides an attractive option of controlling the pulse dynamics of a fiber laser with the use of a simple spectral filter.

All normal dispersion nonlinear fibre supercontinuum source characterization and application in hyperspectral stimulated Raman scattering microscopy

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful label-free, chemical-specific technique for biomedical and mineralogical imaging. Usually, broad and rapid spectral scanning across Raman bands is required for species identification. In many implementations, however, the Raman spectral scan speed is limited by the need to tune source laser wavelengths. Alternatively, a broadband supercontinuum source can be considered. In SRS microscopy, however, source noise is critically important, precluding many spectral broadening schemes. Here we show that a supercontinuum light source based on all normal dispersion (ANDi) fibres provides a stable broadband output with very low incremental source noise. We characterized the noise power spectral density of the ANDi fibre output and demonstrated its use in hyperspectral SRS microscopy applications. This confirms the viability and ease of implementation of ANDi fibre sources for broadband SRS imaging.

Noise amplification in all-normal dispersion fiber supercontinuum generation and its impact on ultrafast photonics applications

Highly coherent and low-noise supercontinuum (SC) sources based on nonlinear spectral broadening of femtosecond pulses in all-normal dispersion (ANDi) fibers are attractive for many applications in ultrafast photonics. By simulating a real nonlinear pulse compression experiment, we numerically investigate the impact of shot noise and technical pump laser fluctuations on the quality and stability of single-cycle pulse generation and other multi-shot experiments based on the manipulation of the SC spectral phase. We find that for pump pulse durations of less than 600 fs, input relative intensity noise < 1 %, and correctly chosen fiber lengths, the initial fluctuations of the pump laser are at most amplified by a factor of three. We also show that the usual strong correlation between SC coherence and quality of the compressed pulses collapses in the presence of technical noise, and that in this situation the coherence is not a useful figure of merit to quantify pulse quality, noise amplification, or decoherence due to incoherent nonlinear dynamics. Our results highlight the very limited impact of technical pump laser noise on ANDi SC generation and are of practical relevance for many ultrafast photonics applications that require high-quality, low-noise SC sources.