In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations.It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time.Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate,based on wavelength or spatial frequency encoding.However,further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams.For pump–probe experiments realized within solids or liquids,because of the difference in group velocities between pump and probe,the determination of the absolute pump–probe delay depends on the sample position.In addition,pulse-front tilt is a widespread issue,unacceptable for ultrafast imaging,but which is conventionally very difficult to evaluate for the low-intensity probe pulses.Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse.It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection.Our approach is valid whatever the transparent medium with nonnegligible Kerr index,whatever the probe pulse polarization and wavelength.Because it is nondestructive and fast to perform,this in-situ probe diagnostic can be repeated to calibrate experimental conditions,particularly in the case where complex wavelength,spatial frequency or polarization encoding is used.We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro-and nanoscale.

High-gain integrated in-line few-mode amplifier enabling 3840-km long-haul transmission

An integrated few-mode erbium-doped fiber amplifier(FM-EDFA)with high modal gain is suitable for the in-line amplification in mode-division multiplexing transmission(MDM)systems.We first experimentally demonstrate a dual-stage integrated FM-EDFA supporting three linear polarization modes.Consisting of integrated passive components with low insertion losses,the FM-EDFA has a similar structure and performance to widely used commercial single-mode EDFAs.The averaged modal gain of 25 dB,the differential modal gain(DMG)of<1.1 dB,and noise figures of 5–7 dB are simultaneously achieved.In addition,the DMG of the 3M-EDF itself is~0.3 dB.Moreover,an MDM transmission experiment with the in-line few-mode amplification by our proposed FM-EDFA over a 3840-km few-mode fiber link for a 28-Gbaud quadrature phase-shift keying(QPSK)signal is demonstrated.

Robust cavity soliton formation with hybrid dispersion

Microresonator-based Kerr frequency combs have attracted a great deal of attention in recent years, in which mode locking of the generated combs is associated with bright or dark cavity soliton formation. In this paper,we show that, different from soliton propagation along a waveguide, cavity solitons can be robustly formed under a unique dispersion profile with four zero-dispersion wavelengths. More importantly, such a dispersion profile exhibits much smaller overall dispersion, thus making it possible to greatly reduce the pump power by five to six times.

Robust generation of frequency combs in a microresonator with strong and narrowband loss

Octave-spanning frequency comb generation in microresonators is promising, but strong spectral losses caused by material absorption and mode coupling between two polarizations or mode families can be detrimental. We examine the impact of the spectral loss and propose robust comb generation with a loss of even 300 dB/cm.Cavity nonlinear dynamics show that a phase change associated with spectral losses can facilitate phase matching and Kerr comb generation. Given this unique capability, we propose a novel architecture of on-chip spectroscopy systems.