Infrared Super-continuum Light Sources and Their Applications

Summary form only given. In the long pulse regime, supercontinuum (SC) generation is very sensitive to the input noise of the pump pulses resulting in poor coherence properties and stability [1]. Recently, the susceptibility of the dynamics to the input noise was shown to induce large shot-to-shot fluctuations when the initial stage of the SC mechanism is driven by modulation instability, leading to the generation of rare pulses with abnormally high intensity [2]. Recently, the noise amplification characteristics of modulation instability and subsequent stages of SC generation were observed in real time using a time-stretching technique which allows to capture single-shot SC spectra [2,3]. In this way, significant insight can be obtained into the SC dynamics that would be otherwise impossible to observe from average quantities, and indeed wavelength correlations in the SC have been shown to exhibit a rich structure that can be connected to the energy exchange dynamics along propagation [3].Here, we report on the experimental investigation of the spectral correlations in the SC regime of cascaded stimulated Raman scattering (SRS) where a large number of Stokes orders are generated leading to the formation of a broadband SC spectrum. We launch 355 nm nanosecond pulses from a 10 kHz repetition rate Q-switched laser deep into the normal dispersion region of highly nonlinear fiber single-mode at the pump wavelength and above. These pulses lead to the generation of a SC whose dynamics are mainly dominated by cascaded SRS [1]. Using and a direct single-shot detection technique where the output end of the fiber is connected to the entrance aperture of a spectrograph with an ICCD camera mounted on the spectrograph exit aperture, we can capture the spectra produced by individual pulses. Significant variations are observed and the captured single-shot spectra allow us to study in detail the SC spectral correlation map at the fiber output as shown in Fig 1.The correlation map analysis reveal how the pump and all the Stokes orders are generally highly correlated except at the long wavelength edge of the mean spectrum, beyond 400 nm, where the Stokes lines appears to be somewhat anti-correlated with the lower Stokes orders. Such correlations are indicative of the energy exchange dynamics between the different cascaded Raman orders and allow to interpret the noise dependence between the different Stokes components. Specifically, the region of high correlations are indicative of the saturated regime of cascaded SRS with little wavelength jitter from noise effects whilst the unsaturated regime of SRS at the edge of the SC is associated with a region of anti-correlations and correspond to large shot-to-shot fluctuations. Similar correlation/anti-correlation features were recently observed in modulation instability dynamics using [4]. We anticipate the new type of correlation analysis reported here to provide new insights into the study of broadband nonlinear propagation dynamics and noise amplification processes.

With the continuous improvement in output power and beam quality, the application scope of supercontinuum (SC) laser sources has been significantly expanded. However, the propagation characteristics of SC lasers in atmospheric environments remain insufficiently studied. In this work, we conducted an 8 km field experiment to quantitatively investigate the propagation behavior of SC lasers through atmospheric turbulence. The measurements focused on intensity fluctuations, scintillation index, and beam wander after long-distance propagation. Numerical simulations based on the Gaussian-Schell model (GSM) were also performed for comparison with the experimental data. The analysis reveals that, under identical propagation conditions, the scintillation index at each SC wavelength is substantially lower than that of a fully coherent laser. Moreover, the far-field scintillation index increases monotonically with off-axis distance. The scintillation behavior of each SC wavelength is found to be similar to that of a monochromatic partially coherent beam with a spatial coherence length on the order of several millimeters. In contrast, the beam wander statistics of SC lasers closely resemble those of monochromatic coherent beams, indicating that beam wander is essentially independent of wavelength and spatial coherence. These findings provide valuable references for deploying SC lasers in free-space optical communication and active optical countermeasure systems.

Modulation instability gain and wave patterns in birefringent fibers induced by coupled nonlinear Schrödinger equation

The noise properties of supercontinuum (SC) generation have attracted a lot of attention due to a large application demand for low noise SC sources and in the more fundamental context of clarifying links with instabilities in other systems. In typical commercial SC sources the pulse break-up is initiated by noise-driven modulational instability (MI). The resulting large spectral shot-to-shot fluctuations can be significantly reduced by modulating the pump with a phase coherent seed, which leads to a coherent pulse break-up through the amplification of a cascade of four-wave mixing (FWM) side-bands. The paper demonstrates that the noise properties of the generated SC are highly sensitive to the degree of phase noise of the seed and that a nearly coherent seed pulse is needed to achieve a coherent pulse break-up and low noise SC. This limits the mechanisms that can be used to generate the seed, which is important in designing next-generation low-noise SC sources.

The supercontinuum laser light source has many advantages over other light sources, including broad spectral range. Transmission images of paired normal and malignant breast tissue samples from two patients were obtained using a Leukos supercontinuum (SC) laser light source with wavelengths in the second and third NIR optical windows and an IR- CCD InGaAs camera detector (Goodrich Sensors Inc. high response camera SU320KTSW-1.7RT with spectral response between 900 nm and 1,700 nm). Optical attenuation measurements at the four NIR optical windows were obtained from the samples.

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.

We numerically demonstrate that noise properties of picosecond mid-infrared (mid-IR) supercontinuum (SC) in the anomalous region of a chalcogenide photonic crystal fiber (PCF) can be effectively controlled by a weak continuous wave (CW) trigger. Detailed simulation results show that in the mid-IR region, the characteristics of SC, including spectral bandwidth, temporal coherence, and signal-to-noise ratio (SNR), can be greatly enhanced by the CW trigger at selected wavelengths. The present study provides valuable insight into the CW-triggered mid-IR SC under the influence of modulation instability (MI) and stimulated Raman scattering (SRS).

The possibility to perform broad wavelength generation and conversion is of great interest in classical and quantum optics. With the introduction of integrated nonlinear silicon photonics, these tasks have been made even more viable thanks to the combination of the high nonlinear refractive index of silicon with the high confinement provided by the photonic waveguides. Exploiting this platform, several solutions for wavelength conversion have been investigated, for both optical processing [1] and applications in the Mid Infrared (MIR) [2]. The possibility to perform broad and controllable wavelength generation can be exploited also for creating integrated sources of quantum states of light based on single photons [3] or correlated photon pairs [4]. Both the generation and conversion can be accomplished by means of Four Wave Mixing (FWM). FWM is a nonlinear optical process in which two pump photons at frequency are converted into signal and idler photons at frequencies and ω/ respectively (with ω I /ω S ). In the particular case of wavelength conversion, FWM is stimulated by a seed wave at the same wavelength of the signal (idler) that will be converted to the wavelength of the idler (signal). FWM has to satisfy the energy conservation, according to which the relation ω P + ω P = ω S + ω I holds, and its efficiency is ruled by the phase matching condition. This latter condition is related to the phase mismatch among the waves involved in the FWM process, that is evaluated as Δk = k p + k p −k S −k I , where k P , k S , k I are the wavevectors for the pump, the signal and the idler, respectively. When Δk = 0, the phase matching condition is fulfilled and the efficiency of FWM is maximized. What is important for wavelength generation and conversion is the spectral position of the phase matching condition, which determines the wavelength at which the maximum efficiency of the conversion or generation process is achieved. Because of this, techniques for the control of the phase matching condition have been developed, focusing on the possibility to tailor the Group Velocity Dispersion (GVD) through a proper engineering of the waveguide geometry [5]. While these techniques consider only the first order waveguide mode, here we propose to exploit the higher order modes propagating inside a multimode waveguide. The dispersion of the higher order modes can be used to tune the spectral position of the phase matching condition for the FWM process, achieving controllable and large spectral translation in both Stimulated Four Wave Mixing (sFWM) and Spontaneous Four Wave Mixing (SFWM).

We investigated the effect of Raman scattering in multimode whispering gallery mode (WGM) microcavities. First, we discuss the competition between the effects of four-wave mixing (FWM) and stimulated Raman scattering (SRS). Thanks to the different gain bandwidths of FWM and SRS, we can switch between the FWM and SRS dominant states by changing the pump power or by changing the coupling quality factor (Q). Next, we investigated the transverse mode interaction that occurs during SRS comb formation. We found that transverse mode coupling occurred when we pumped in a low-Q mode but a comb with a single-mode family was generated when we pumped in a high-Q mode. This finding will allow us to obtain or suppress a dual comb in a single WGM microcavity. Finally, we demonstrated broad bandwidth visible light generation by third harmonic generation (THG) following the generation of a broadband SRS comb. The generation and good understanding of the SRS comb will offer us various possibilities such as dual comb generation and broad bandwidth visible light generation.

Ultra-broadband high-power supercontinuum (SC) light sources are currently being applied to a wide range of biooptical applications, in which noise is a severely limiting factor. With picosecond pumping, as we consider here, noise is inherent in the SC generation process, because the generation is initiated by modulation instability (MI), which amplifies random fluctuations in the pump pulse. Above a certain threshold power the original pulse breaks up into a train of solitons, which evolves into a supercontinuum [1]. Recently it has been shown that by seeding the four-wave mixing (FWM) gain wavelengths, SC generation close to the threshold can be accelerated and the noise can be greatly reduced [2,3]. The noise is reduced because the seed initiates the MI in a controlled manner, thereby removing a great deal of the randomness in the SC generation process [3]. The theory behind it builds on the recent discovery of Rogue waves in SC [4,5].

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F −2 color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F −2 crystals and the Stokes radiation amplification in the color center crystal.

We investigate modulation instability (MI) in the distributed fiber amplifier based on a modified Ginzburg-Landau equation. The role of gain dispersion and stimulated Raman scattering (SRS) in MI is identified. It is found that, due to SRS, the MI gain spectrum consists of two parts: the conventional MI gain spectrum and the Raman gain spectrum. Gain dispersion exerts little influence on the conventional MI spectrum, yet it deforms the Raman gain spectrum seriously, mainly by reducing its growth rate. Moreover, as the signal power increases, the bandwidths of the conventional MI and the Raman gain spectrum are simultaneously extended, with the latter spectrum being extended more quickly.

A new approach is introduced to enhance supercontinuum (SC) spectrum ina dispersion-flattened/decreasing fibre with a convex dispersionprofile. A flat SC spectrum nearly extending from 1200 nm to 2100 nm canbe generated based on this scheme. It is found that group-velocitydispersions (GVD) and self-phase modulation (SPM) effects are theprimary factors for pump pulse compression and SC spectrum generation,stimulated Raman scattering (SRS) effect plays an essential role on thefinal SC-spectrum bandwidth and flatness, but self-steepening effectcan be ignored.

Supercontinuum (SC) light sources are nowadays a very common way to access a large span of wavelengths, usually ranging from the near ultraviolet (around 400 nm) to the infrared (around 2.4 μm). It corresponds to a range of interest for many applications in optics for measuring transmission, dispersion, or in biophotonics for achieving fluorescencemicroscopy, optical coherence tomography... Indeed these sources are really promising because they should allow to replace the N laser sources used in these experimental setups to access to all theses wavelengths by a single broad one and a spectral filtering apparatus. Most of these results have been obtained by using powerful pump lasers of several kilo-Watts peak power, operating from the femtosecond (Titane:Saphire) to the nanosecond regimes (Nd:YAG), launched in the low dispersion region of a microstructured optical fiber. Although these fibers are short enough (typically from 1 to 10 m) to neglect the linear absorption during the propagation of the pump, the spectral power density is relatively low (few hundreds of μw/nm)which could limit the implementation of SC sources inmany application devices. This is related to a technological limitation of the pump source because it is not easy to combine strong peak power and high average power. One of the simplest solutions to increase the the spectral power density of SC sources is to replace pulsed sources with continuous-wave (CW) light sources whose available average powers are much more important. Wewill see that the dynamics of SC formation is considerably different in this case, requiring to perform intensive numerical studies to optimize the fiber parameters. Indeed, longer fibers are required (from tens to hundreds of meters) which heightens sensitivity to fiber attenuation, namely of the OHpic absorption, that strongly impacts the soliton evolution. However extremely powerful SCs have been reported with more than 10 mW/nm of spectral power density. Furthermore, these pump sources are usually all-fiber that leads to a second advantage against most of pulsed SC because CW pump can be directly spliced on the PCF. It is also important to point out that these SC sources have different temporal properties than the ones of pulsed SCs. The first experimental demonstration of CW SC have been realized at the end of the nineties with a Raman laser launched in a standard telecommunication fiber. The spectral broadening was relatively restricted (around 200 nm) because it was mainly due to Raman effect Gonzalez-Herraez et al. (2003); Persephonis et al. (1996); Prabhu et al. (2000). A breakthrough was reached a few years later when stronger pump lasers (from more than one order of magnitude) based on Ytterbium doped fibers were combined with photonic crystal fibers 4

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].