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Recent years have seen renewed interest in the study of nonlinear fibre laser and propagation dynamics through the use of real-time measurement techniques for non-repetitive ultrafast optical signals. In this paper we review our recent work in this field using dispersive Fourier Transform and Time Lens techniques.

Ghost imaging is a non-direct measurement technique that uses the correlation between multiple probing patterns and the integrated signal measured after transmission (or reflection) through an object. [1] The resolution of the technique is determined by the characteristic length scale of the probing patterns fluctuations and/or the resolution with which they can be measured. Originally demonstrated in the spatial domain to reconstruct images of real physical objects, ghost imaging has been recently extended to the time-domain to measure ultrafast signals at telecom wavelengths [2]. In this work, we experimentally demonstrate ultrafast ghost imaging in the time-domain where the correlated intensities have completely different wavelengths. This wavelength-conversion temporal ghost-imaging scheme exploits fully the benefit of ghost imaging opening up new possibilities for ultrafast imaging in spectral regions where sensitive and/or fast detectors are not available and in particular the mid-infrared or THz range.

Interferometrie autocorrelation (IAC) and intensity autocorrelation (AC) are simple methods commonly used for short pulse characterization [1]. Unlike AC, IAC is sensitive to the temporal phase of optical pulses and provides information about the coherence properties. Recently, it has been shown that a broadband, compact, and cost-effective IAC measurement can be realized by two-photon absorption (TPA) in a semiconductor photodetector, and which is not limited by phase-matching considerations as in nonlinear crystals [2].

Supercontinuum light (SC) is a broadband source with unique properties generated via cascaded nonlinear processes when intense light propagates in a nonlinear material [1]. Depending on the nonlinear material and pump source used, the spectrum of a SC source can span from the visible to the mid-infrared with up to several Watt-average power. Recently, the generation of broadband SC sources operating in the mid-infrared (MIR) has attracted significant interest due to a wide range of potential applications in spectroscopy [2], microscopy [3], molecular fingerprinting [4], environmental monitoring and LIDAR [5], just to a name few.

Light Detection and Ranging (LIDAR) is a remote-sensing technique based on collection of back scattered (optical frequency) electromagnetic radiation from a target point. The interaction of this radiation with surrounding gases and aerosol particles pave way for the measurement of various surrounding variables, including temperature, pressure, humidity and trace gases concentration. Although the LIDAR technique has been consistently used to measure various observables, the technique is typically limited to one specie at a time. However, this limitation can be mitigated using supercontinuum (SC) sources, which provides a broad spectrum [1] covering the absorption bands of all the investigated gases. Thus, opening the door for simultaneous detection of multiple species [2].

In contrast with direct measurement methods, correlation imaging measures the correlation function between the intensity emitted by a light source and the total (integrated) intensity transmitted (or reflected) by an object, such that the image can be reconstructed without the object to be actually seen. The essential nature of such "ghost imaging" [1] lies in the mutual correlation of the two non-interacting beams, and the image is obtained by summing over the different probing patterns weighted by the integrated signal from the detector [2]. Here, we demonstrate for the first time broadband ghost imaging in the spectral domain using a supercontinuum source [3]. We demonstrate the approach by using a gas cell filled with pure methane as the object in the test arm and reconstruct its absorption spectrum. The advantage of the correlation approach is that it detects the total integrated signal after the object and is thus very sensitive even in low light conditions or in spectral regions where no fast detector exists. It is also inherently insensitive to any perturbation occurring between the object and the detector.

Ghost imaging is a novel imaging technique based on correlation measurements between a structured illumination pattern (the reference) and the total intensity transmitted or reflected by an object [1]. The reference illumination patterns may be either randomly generated by a spatially incoherent light source, or pre-programmed e.g. with a spatial light modulator. Light transmitted (or reflected) by the object is measured by a single-pixel “bucket” detector with null spatial resolution. A unique feature associated with ghost imaging is that (i) neither the bucket detector nor the reference measurement caries enough information to retrieve the object shape and (ii) it is insensitive to distortions placed after the object.

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.