Single-Photon Level Dispersive Fourier Transform: Ultrasensitive Characterization of Noise-Driven Nonlinear Dynamics.

Dispersive Fourier transform is a characterization technique that allows directly extracting an optical spectrum from a time domain signal, thus providing access to real-time characterization of the signal spectrum. However, these techniques suffer from sensitivity and dynamic range limitations, hampering their use for special applications in, e.g., high-contrast characterizations and sensing. Here, we report on a novel approach to dispersive Fourier transform-based characterization using single-photon detectors. In particular, we experimentally develop this approach by leveraging mutual information analysis for signal processing and hold a performance comparison with standard dispersive Fourier transform detection and statistical tools. We apply the comparison to the analysis of noise-driven nonlinear dynamics arising from well-known modulation instability processes. We demonstrate that with this dispersive Fourier transform approach, mutual information metrics allow for successfully gaining insight into the fluctuations associated with modulation instability-induced spectral broadening, providing qualitatively similar signatures compared to ultrafast photodetector-based dispersive Fourier transform but with improved signal quality and spectral resolution (down to 53 pm). The technique presents an intrinsically unlimited dynamic range and is extremely sensitive, with a sensitivity reaching below the femtowatt (typically 4 orders of magnitude better than ultrafast dispersive Fourier transform detection). We show that this method can not only be implemented to gain insight into noise-driven (spontaneous) frequency conversion processes but also be leveraged to characterize incoherent dynamics seeded by weak coherent optical fields.

High-temperature wave thermalization spoils beam self-cleaning in nonlinear multimode GRIN fibers.

In our experiments, we reveal a so-far unnoticed power limitation of beam self-cleaning in graded-index nonlinear multimode optical fibers. As the optical pulse power is progressively increased, we observed that the initial Kerr-induced improvement of the spatial beam quality is eventually lost. Based on a holographic mode decomposition of the output field, we show that beam spoiling is associated with high-temperature wave thermalization, which depletes the fundamental mode in favor of a highly multimode power distribution.

Efficient second-harmonic generation through cascaded optically poled fibers.

We report the experimental demonstration of efficient second-harmonic generation by splicing optically poled fiber segments. A device made from five segments each 20 cm-long exhibits, at a fundamental average power of 4.2 mW, a maximum increase of 5.5 in the conversion efficiency with respect to a single 20 cm poled fiber, corresponding to a conversion efficiency of 5.4%. For any considered fundamental power, the conversion efficiency at the output of a five-segment device is larger than the efficiency of a single poled fiber having the same length.

Spatiotemporal beam self-cleaning for high-resolution nonlinear fluorescence imaging with multimode fiber.

Beam self-cleaning (BSC) in graded-index (GRIN) multimode fibers (MMFs) has been recently reported by different research groups. Driven by the interplay between Kerr effect and beam self-imaging, BSC counteracts random mode coupling, and forces laser beams to recover a quasi-single mode profile at the output of GRIN fibers. Here we show that the associated self-induced spatiotemporal reshaping allows for improving the performances of nonlinear fluorescence (NF) microscopy and endoscopy using multimode optical fibers. We experimentally demonstrate that the beam brightness increase, induced by self-cleaning, enables two and three-photon imaging of biological samples with high spatial resolution. Temporal pulse shortening accompanying spatial beam clean-up enhances the output peak power, hence the efficiency of nonlinear imaging. We also show that spatiotemporal supercontinuum (SC) generation is well-suited for large-band NF imaging in visible and infrared domains. We substantiated our findings by multiphoton fluorescence imaging in both microscopy and endoscopy configurations.

Coherent combining of self-cleaned multimode beams.

A low intensity light beam emerges from a graded-index, highly multimode optical fibre with a speckled shape, while at higher intensity the Kerr nonlinearity may induce a spontaneous spatial self-cleaning of the beam. Here, we reveal that we can generate two self-cleaned beams with a mutual coherence large enough to produce a clear stable fringe pattern at the output of a nonlinear interferometer. The two beams are pumped by the same input laser, yet are self-cleaned into independent multimode fibres. We thus prove that the self-cleaning mechanism preserves the beams' mutual coherence via a noise-free parametric process. While directly related to the initial pump coherence, the emergence of nonlinear spatial coherence is achieved without additional noise, even for self-cleaning obtained on different modes, and in spite of the fibre structural disorder originating from intrinsic imperfections or external perturbations. Our discovery may impact theoretical approaches on wave condensation, and open new opportunities for coherent beam combining.

Nonlinear beam self-imaging and self-focusing dynamics in a GRIN multimode optical fiber: theory and experiments.

Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain a new exact solution for the nonlinear evolution of first and second order moments of a laser beam of arbitrary transverse shape carried by a graded-index multimode fiber. We have experimentally directly visualized the longitudinal evolution of beam self-imaging by means of femtosecond laser pulse propagation in both the anomalous and the normal dispersion regime of a standard telecom graded-index multimode optical fiber. Light scattering out of the fiber core via visible photo-luminescence emission permits us to directly measure the self-imaging period and the beam dynamics. Spatial shift and splitting of the self-imaging process under the action of self-focusing are also revealed.

Nonlinear polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber.

We experimentally study polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber. We show that spatial beam cleaning is accompanied by nonlinear polarization rotation and a significant increase of the degree of linear polarization.

Spectro-temporal shaping of supercontinuum for subnanosecond time-coded M-CARS spectroscopy.

A supercontinuum laser source was designed for multiplex-coherent anti-Stokes Raman scattering spectroscopy. This source was based on the use of a germanium-doped standard optical fiber with a zero dispersion wavelength at 1600 nm and pumped at 1064 nm. We analyzed the nonlinear spectro-temporal interrelations of a subnanosecond pulse propagating in a normal dispersion regime in the presence of a multiple Raman cascading process and strong conversion. The multiple Raman orders permitted the generation of a high-power flat spectrum with a specific nonlinear dynamics that can open the way to subnanosecond time-coded multiplex CARS systems.

Publisher's Note: Observation of Geometric Parametric Instability Induced by the Periodic Spatial Self-Imaging of Multimode Waves [Phys. Rev. Lett. 116, 183901 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.183901.

Observation of Geometric Parametric Instability Induced by the Periodic Spatial Self-Imaging of Multimode Waves.

Spatiotemporal mode coupling in highly multimode physical systems permits new routes for exploring complex instabilities and forming coherent wave structures. We present here the first experimental demonstration of multiple geometric parametric instability sidebands, generated in the frequency domain through resonant space-time coupling, owing to the natural periodic spatial self-imaging of a multimode quasi-continuous-wave beam in a standard graded-index multimode fiber. The input beam was launched in the fiber by means of an amplified microchip laser emitting sub-ns pulses at 1064 nm. The experimentally observed frequency spacing among sidebands agrees well with analytical predictions and numerical simulations. The first-order peaks are located at the considerably large detuning of 123.5 THz from the pump. These results open the remarkable possibility to convert a near-infrared laser directly into a broad spectral range spanning visible and infrared wavelengths, by means of a single resonant parametric nonlinear effect occurring in the normal dispersion regime. As further evidence of our strong space-time coupling regime, we observed the striking effect that all of the different sideband peaks were carried by a well-defined and stable bell-shaped spatial profile.

Simultaneous broadband Raman cascading and parametric conversion in potassium titanyl phosphate.

We generated a broad spectrum of light between 1064 and 1300 nm in the infrared by cascading stimulated Raman scattering in a potassium titanyl phosphate crystal while broadband conversion of the infrared Raman cascade was simultaneously achieved in the visible through second-harmonic generation (SHG) and sum-frequency mixing. We observed that odd- and even-order cascaded Stokes components were spatially addressed at different angles of propagation in the crystal. The efficiency of SHG and sum-frequency mixing is discussed as a function of the pump polarization. We also report on significant spatial distortions of the output Stokes beams.

Bragg-scattering conversion at telecom wavelengths towards the photon counting regime.

We experimentally study Bragg-scattering four-wave mixing in a highly nonlinear fiber at telecom wavelengths using photon counters. We explore the polarization dependence of this process with a continuous wave signal in the macroscopic and attenuated regime, with a wavelength shift of 23 nm. Our measurements of mean photon numbers per second under various pump polarization configurations agree well with the theoretical and numerical predictions based on classical models. We discuss the impact of noise under these different polarization configurations.

Time-frequency resolved analysis of a nanosecond supercontinuum source dedicated to multiplex CARS application.

In this paper, we describe and investigate the properties of a broadband source designed from a nanosecond microchip laser operating at high repetition rate and dedicated to multiplex-CARS application. We demonstrate that a strong reshaping of the initial pulse profile drastically affects the Stokes wave and therefore represents an important limitation in CARS experiment. In particular, we emphasize the saturation effect of the peak power of the Stokes wave resulting from supercontinuum generation. However, we show that this type of compact system can be particularly suitable for achieving CARS measurement.

Phase closure retrieval in an infrared-to-visible upconversion interferometer for high resolution astronomical imaging.

This paper demonstrates the use of a nonlinear upconversion process to observe an infrared source through a telescope array detecting the interferometric signal in the visible domain. We experimentally demonstrate the possibility to retrieve information on the phase of the object spectrum of an infrared source by using a three-arm upconversion interferometer. We focus our study on the acquisition of phase information of the complex visibility by means of the phase closure technique. In our experimental demonstration, a laboratory binary star with an adjustable photometric ratio is used as a test source. A real time comparison between a standard three-arm interferometer and our new concept using upconversion by sum-frequency generation demonstrates the preservation of phase information which is essential for image reconstruction.

Unprecedented Raman cascading and four-wave mixing from second-harmonic generation in optical fiber.

We experimentally demonstrate strong second-harmonic-generation from a self-induced all-optical poling in germanium-doped fiber with a subnanosecond laser pump at 1064 nm. The large second-harmonic conversion efficiency allows nonlinear spectral broadening at visible wavelengths so that up to nine distinct Raman sidebands have been obtained. In this work we emphasize how the Raman scattering, induced from the pump in the IR region, can drastically affect the optical poling effect, limiting in turn second-harmonic generation.

Shortening pulses from subnanosecond to picosecond by means of ultrafast temporal filtering in an optical fiber.

Taking advantage of the combination of stimulated Raman scattering and nonlinear polarization rotation in a single-mode optical fiber, a temporal reshaping of subnanosecond pulses is achieved by means of both spectral and polarization filtering. Shortening of 650 ps down to 43 ps temporally and spectrally stable pulses is experimentally obtained. Numerical simulations supporting this new temporal filtering are presented.

Controlling intermodal four-wave mixing from the design of microstructured optical fibers.

Intermodal four-wave mixing (FWM) in microstructured optical fibers (MOF) is studied theoretically and experimentally. The dependance of FWM frequency detuning on the geometrical parameters of the fiber, namely the pitch, the core width and the air-filling fraction is derived. We propose to use the results of this investigation to control the position of the Stokes and anti-Stokes waves directly from the fiber transverse structure drawing without the need for time-consuming simulations as in usual design procedures. Stokes sideband can then be freely tuned within the S-, L-, and C- bands with great potential for infrared applications.

Picosecond polarized supercontinuum generation controlled by intermodal four-wave mixing for fluorescence lifetime imaging microscopy.

We present the generation of a picosecond polarized supercontinuum in highly birefringent multimodal microstructured fiber. The initial steps of the spectral broadening are dominated by intermodal four-wave mixing controlled by the specific fiber design. Using a low repetition rate ultra-stable solid state laser, a pulse train well-suited for versatile time-domain fluorescence lifetime imaging applications is obtained.

Robustness of 40 Gb/s ASK modulation formats in the practical system infrastructure.

In this work, we theoretically and experimentally analyzed the resilience of 40 Gb/s amplitude shift keying modulation formats to transmission impairments in standard single-mode fiber lines as well as to optical filtering introduced by the optical add/drop multiplexer cascade. Our study is a pre-requisite to assess the implementation of cost-effective 40 Gb/s modulation technology in next generation high bit-rate robust optical transport networks.

Noise evolution and soliton internal modes in dispersion-managed fiber systems.

We perform a linear stability analysis of the Ablowitz-Biondini equations to explain the dynamic evolution of the noise squeezing, caused by the interplay among optical solitons and noise components, of initially uncorrelated statistics in fiber systems with dispersion management.