Low-threshold supercontinuum generation in semiconductor nanoribbons by continuous-wave pumping

The control of temporal noise of the pump could add an additional degree of freedom to manipulate the spectrum of continuous-wave (CW) pumped SC generation. In this paper, we experimentally tailor the CW-pumped supercontinuum (SC) generation in a cascaded Raman random fiber laser (CRRFL) based on a 1 µm pump with tunable temporal dynamics. The pump is based on a spectrally filtered ytterbium-doped random fiber laser (YRFL) seed laser, which can be amplified to a 10 W level with the tunable temporal noise. By increasing the detuning between the wavelength of a filter and the central wavelength of a YRFL, the temporal noise of the pump can be gradually increased, and we experimentally demonstrate the larger temporal noise of the pump can significantly enhance the spectral performance of SC generation in terms of spectral bandwidth and flatness. As a result, we realize the enhanced SC generation in a CRRFL with a more than doubled -10 dB bandwidth in a 50-km-long single-mode fiber by optimizing the temporal dynamics of the pump. We also demonstrate the enhanced output power of the spectral-optimized SC generation to 1.27 W with a shorter cavity length. We anticipate the combination of a tunable temporal noise ytterbium-doped pump and CRRFL could provide a new route to realize high-power, ultra-broadband, and smooth SC generation in a simple fiber cavity.

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

Supercontinuum (SC) generation in nonlinear photonic crystal fibers (NLF) using continuous-wave (CW) fiber lasers for pumping has been studied before[1,2]. The advantages of CW-SC are high spectral smoothness, high spectral power density, simplicity of the system, and lower noise than the more common picosecond-pumped SC. The cost of these features is increased nonlinear fiber lengths due to less efficient nonlinear processes at the lower power levels. Especially, the generation of light in the visible wavelength range has been a challenge. This has previously been tackled by increasing the nonlinearity of the fiber, zero dispersion shifting along the fiber length, and using 200 m of NLF[1], or alternatively using extremely high powers of 100's of Watts[2].

The noise properties of supercontinuum (SC) sources are of importance in many applications. This intensity noise of the SC arises from nonlinear amplification of the input-pulse shot noise and the spontaneous Raman scattering down the fiber [1]. Low noise femtosecond SC generation (SCG) has previously been demonstrated [2]. However, in commercial SC sources, the SC is generated by pumping with picosecond to nanosecond pulses and the SCG is thus initiated by modulation instability (MI). Therefore, the SC is characterized by low coherence and high shot-to-shot fluctuations, in particular at the spectral edges [3-5]. However, the influence of the noise properties when pumping in the normal dispersion regime has so far not been investigated for long pulse pumping. In this work we have measured and compared the relative intensity noise (RIN) properties of long-pulse MI initiated SCG in three different regimes: (i) all-normal SCG, (ii) normal-pumped SCG where higher-order Raman lines occur in the anomalous dispersion regime, and (iii) anomalous-pumped SCG. We show that the noise properties for the three regimes are similar: when increasing the input pump power, the intensity noise will continuously decrease for a given wavelength, and for a given input power the intensity noise will be lowest at the pump wavelength and increase when moving towards the spectral edge [6].

We demonstrate the supercontinuum (SC) generation in a suspended-core As(2)S(3) chalcogenide microstructured optical fiber (MOF). The variation of SC is investigated by changing the fiber length, pump peak power and pump wavelength. In the case of long fibers (20 and 40 cm), the SC ranges are discontinuous and stop at the wavelengths shorter than 3500 nm, due to the absorption of fiber. In the case of short fibers (1.3 and 2.4 cm), the SC ranges are continuous and can extend to the wavelengths longer than 4 μm. The SC broadening is observed when the pump peak power increases from 0.24 to 1.32 kW at 2500 nm. The SC range increases with the pump wavelength changing from 2200 to 2600 nm, corresponding to the dispersion of As(2)S(3) MOF from the normal to anomalous region. The SC generation is simulated by the generalized nonlinear Schrödinger equation. The simulation includes the SC difference between 1.3 and 2.4 cm long fiber by 2500 nm pumping, the variation of SC with pump peak power in 2.4 cm long fiber, and the variation of SC with pump wavelength in 1.3 cm long fiber. The simulation agrees well with the experiment.

Introduction It is perhaps not surprising that using extremely high power and short pulse duration pump sources leads to dramatic nonlinear processes in optical fibres; in contrast, the generation of a supercontinuum from a continuous pump wave of relatively meagre power is at first sight, astounding. Yet supercontinua spanning over 1000 nm have been generated with pump powers of a few tens of watts – orders of magnitude lower than pulse pumped systems. The key to continuous wave (CW) supercontinuum generation is the utilisation of modulation instability (MI). This is inherent to any anomalously dispersive, nonlinear medium, and has been observed in a wide range of systems. This instability can enable the creation of the extremes of peak power and pulse duration necessary for dramatic nonlinear processes to occur, even from very low power CW pump lasers. But although MI from CW pump lasers was observed in the 1980s by Itoh et al. (1989), other factors required for efficient continuum generation were missing, causing another decade to pass before such results were obtained. A full review of experimental results will be presented later, but for reference, some examples of continuous wave supercontinua are shown in Fig. 8.1, which illustrate the high spectral power and the spectral smoothness and flatness which are characteristic of CW continuum generation. Although significantly different from the physical mechanisms involved in ultra-short – femtosecond based – supercontinuum generation, the basic physical processes underpinning CW continuum generation are the same as for longer pump pulses (greater than a few picoseconds) and the observations and conclusions developed in the 1980s surrounding these type of sources apply to the CW regime.

A theoretical study of flat supercontinuum (SC) generation in a highly nonlinear photonic crystal fiber (PCF) is presented. We propose a chromatic dispersion profile of a highly nonlinear PCF for flatly broadened SC generation around 1550nm. The proposed chromatic dispersion profile of the fiber which is a convex function of wavelengths has small normal group-velocity dispersion at the pump wavelength and the dispersion slope around which is flat. It is found that the fiber with such dispersion characteristics is suitable for generating a relatively flat SC. The initial chirp of the input pulse also has a significant effect on SC generation in photonic crystal fiber. A flatter and wider spectrum can be obtained by the pulse with appropriate positive initial chirp.

Broadband normal dispersion pumping supercontinuum (SC) generation in silica photonic crystal fiber (PCF) is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.

Supercontinuum (SC) generation in bulk materials offers a versatile, broadband coherent light source for ultrafast spectroscopy and nonlinear optics. The filamentation process underlying SC generation is primarily governed by three critical parameters: input pulse energy, focusing numerical aperture, and crystal thickness. Using femtosecond pulses and sapphire crystals, we systematically investigate how these parameters affect SC energy stability. Depending on the number of pulse-splitting events, filamentation can manifest in either single- or multi-filament regimes, with the latter leading to undesirable spectral modulation and thus requiring avoidance. Our study reveals that within the single-filament regime, two distinct dynamical states can arise: a critical state, in which pulse-splitting dynamics and SC spectra evolve rapidly with increasing input pulse energy, and a steady state, which ensures optimal stability. Experimental results show that steady-state SC generation yields the highest stability, exceeding that of the driving laser. These findings deepen the physical understanding of filamentation and provide clear guidelines for parameter selection to achieve stable SC in bulk media, offering practical insights for optimizing broadband coherent sources in ultrafast applications.

Supercontinuum (SC) generation from a highly nonlinear tellurite microstructured fiber with multi-ring holes was demonstrated by pumping with a 2 μm high power mode-locked fiber laser. The chromatic dispersion of the fiber was measured with a homemade white-light spectral interferometer on a wide wavelength range and matched very well with the theoretical calculation. The zero dispersion wavelength (ZDW) of the fiber was measured to be 1358 nm and calculated to be 1390 nm. Although the pumped wavelength was far away from ZDW, with flat dispersion profile of the fiber on the anomalous dispersion, the SC could be expanded from 650 nm to 2850 nm with launched pulse energy of several hundred picojoules. Simulations of SC generation were performed and showed fair agreement with the experimental results.

Supercontinuum (SC) generation has the important applications such as broadband light source, optical coherence tomography, ultra-short pulse compression, and optical frequency metrology, etc. Tellurite glass is transparent in the mid-infrared range, and has a higher n₂ than silica glass by at least one order of magnitude. We have fabricated the hexagonally shaped tellurite air-clad fiber with a core diameter of around 1 μm through controlling the temperature field exactly in the process of fiber-drawing. Since the SC generation strongly depends on the chromatic dispersion, which is determined by the microstructure of fiber, it is interesting to investigate and demonstrate such dependence for such a small core fiber in detail. In this work by pumping a positive pressure of nitrogen gas into the holes of preform, we obtained 1 μm core fibers with diameter ratio of holey region to core (DRHC) varied from 3.5 to 20. The dispersion was tailored effectively by the variation of DRHC. Dependences of SC on the microstructure and dispersion were demonstrated. The pump lasers were picosecond and femtosecond fiber lasers. One octave flattened SC generation was obtained for the fibers pumped by 1064 nm picosecond fiber laser with the pulse energy of several hundred pJ. Intense second and third harmonic generations were obtained under the pump of a 1557 nm femtosecond fiber laser. The correlation of dispersion and SC spectra was analyzed. Such tellurite microstructured optical fibers (MOFs) with high nonlinearity and controlled dispersion are significant in nonlinear applications.

We experimentally investigate continuous-wave (CW) laser-pumped cascaded random Raman fiber-laser (CRRFL) architectures based on standard telecom fiber for low intensity noise supercontinuum (SC) generation. Our objective is to identify conventional pump laser designs that minimize wavelength and radio frequency (RF) resolved relative intensity noise (RIN) - a key determinant of sensitivity of low coherence interferometry application like ultrahigh-resolution optical coherence tomography (UHR-OCT). Three CW Yb-fiber pump configurations were evaluated: a single-stage fiber Bragg grating (FBG)-based bidirectional linear cavity, an FBG and a unidirectional ring-cavity seeded two-stage master-oscillator power-amplifier (MOPA) systems. Each laser delivered ∼75 W at 1064 nm into 2 km of SMF-28 Ultra, generating up to ∼34 W of SC spanning 1000-1700nm (measurement-limited) with power spectral densities (PSDs) exceeding 50 mW/nm. Spectrally resolved RIN was measured from 1200-1700nm across two RF bands: the quantum-noise-dominated regime (1-40 MHz) and the UHR-OCT relevant regime (10-160 kHz). Ring-based CRRFL SC systems consistently exhibited the lowest integrated rms RIN (∼2.5% in the 1-40 MHz band and ∼0.35% in the 10-160 kHz band), outperforming FBG-based implementations (5% and 0.4%, respectively). Importantly, this improvement persists even when the ring-laser free spectral range (FSR) is matched to that of the FBG laser, confirming that the RIN reduction arises from the intrinsic unidirectional lasing dynamics of the ring cavity rather than cavity FSR differences. To assess application-level relevance, we benchmark our best CRRFL-SMF SC sources against a commercial picosecond photonic crystal fiber (PCF)-based SC system. Across 1200-1700nm, the ring-MOPA-pumped SC exhibits at least a four-fold reduction in spectrally resolved RIN while providing comparable spectral flatness and nearly an order-of-magnitude higher PSD (>50 mW/nm vs ∼5 mW/nm). To our knowledge, this is the first systematic wavelength and RF frequency resolved noise characterization, including an explicit assessment of cavity FSR effects of conventional CW pumped CRRFL based SC sources, establishing their strong potential as low-cost, low-noise alternatives to commercial PCF-SCs for UHR-OCT and other low-coherence interferometry applications.

We demonstrate supercontinuum (SC) generation in a 2.8-cm-long chalcogenide double-clad fiber (Ch-DCF). The calculated chromatic dispersion of the fundamental mode shows that the Ch-DCF has flattened chromatic dispersion, which is within −10 ± 10 ps/nm/km from 3.8 to 12.6 in the normal dispersion regime. The variation of SC spectra is investigated by changing the pump wavelength and pump peak power. The broadband SC spectra extending from 2 to 14 μm at the −40 dB level is observed when the pump wavelength of 10 μm and pump peak power of 1.3 MW. The SC generation is simulated by the scalar generalized nonlinear Schrödinger equation. The simulation results are mostly the same as the experimental results and show that the supercontinua generated in the Ch-DCF are highly coherent.

We have theoretically investigated supercontinuum (SC) generation with dual-wavelength pumping. The dual-wavelength-pumped source at 1064/686nm generated through four-wave mixing and SC generation have been realized in an all-fiber device. A good agreement between our simulation and experimental results has proved the conjugate action of cross-phase modulation and soliton self-frequency shift can obviously improve the power at the visible wavelengths with group-velocity matching between the visible wave at 686nm and the infrared wave at 1064nm.

In this work we report the numerical results on the filamentation and supercontinuum (SC) generation of femtosecond flat-top pulses with different steep edges propagating in fused silica. Results show that the filamentation process is determined mainly by the self-steepening effect and ionization, and the SC generation is influenced mainly by the self-steepening effect. The filamentation onset and termination is determined by the leading edge and the tailing edge, respectively. The steeper the leading edge is, the earlier the filamentation onset is; the steeper the tailing edge is, the earlier the filamentation termination is. The cutoff wavelength in the blue-side region of the SC has no obvious difference for the pulses with different leading and tailing edges. But the spectral intensity of the blue extension is much stronger for the pulses with the steeper leading edges. The stronger spectral intensity results from the formation of more steep descending edges during filamentation.