Spectrally-resolved statistical characterization of seeded supercontinuum suppression using optical time-stretch

Spectral characterization of a supercontinuum source based on nonlinear broadening in an aqueous K2ZnCl4 salt solution

We report on investigations concerning the shot-to-shot spectral stability properties of a supercontinuum source based on nonlinear processes such as self-phase modulation and optical wave-breaking in a highly concentrated K₂ZnCl₄ double salt solution. The use of a liquid medium offers both damage resistance and high third-order optical nonlinearity. Approximately 40 μJ pulses spanning a spectral range between 390 and 960 nm were produced with 3.8% RMS energy stability, using infrared input pulses of 500±50  fs FWHM durations and 2.42±0.04  mJ energies with an RMS stability of 2%. The spectral stability was quantified via acquiring single-shot spectra and studying shot-to-shot variation across a spectral range of 200-1100 nm, as well as by considering spectral correlations. The regional spectral correlation variations were indicative of nonlinear processes leading to sideband generation. Spectral stability and efficiency of energy transfer into the supercontinuum were found to weakly improve with increasing driver pulse energy, suggesting that the nonlinear broadening processes are more stable when driven more strongly, or that self-guiding effects in a filament help to stabilize the supercontinuum generation.

Ultrafast measurements of optical spectral coherence by single-shot time-stretch interferometry

The palette of laser technology has significantly been enriched by the innovations in ultrafast optical pulse generation. Our knowledge of the complex pulse dynamics, which is often highly nonlinear and stochastic in nature, is however limited by the scarcity of technologies that can measure fast variation/fluctuation of the spectral phase (or coherence) and amplitude in real-time, continuously. To achieve this goal, we demonstrate ultrafast interferometry enabled by optical time-stretch for real- time spectral coherence characterization with microsecond-resolution. Accessing the single-shot interferograms continuously, it further reveals the degree of second-order coherence, defined by the cross-spectral density function, at high speed-a capability absent in any existing spectroscopic measurement tools. As the technique can simultaneously measure both the high-speed variations of spectrally resolved coherence and intensity, time-stretch interferometry could create a new arena for ultrafast pulse characterization, especially favorable for probing and understanding the non-repetitive or stochastic dynamics in real-time.

Direct comparison of shot-to-shot noise performance of all normal dispersion and anomalous dispersion supercontinuum pumped with sub-picosecond pulse fiber-based laser

Coherence of supercontinuum sources is critical for applications involving characterization of ultrafast or rarely occurring phenomena. With the demonstrated spectral coverage of supercontinuum extending from near-infrared to over 10 μm in a single nonlinear fiber, there has been a clear push for the bandwidth rather than for attempting to optimize the dynamic properties of the generated spectrum. In this work we provide an experimental assessment of the shot-to-shot noise performance of supercontinuum generation in two types of soft glass photonic crystal fibers. Phase coherence and intensity fluctuations are compared for the cases of an anomalous dispersion-pumped fiber and an all-normal dispersion fiber. With the use of the dispersive Fourier transformation method, we demonstrate that a factor of 100 improvement in signal-to-noise ratio is achieved in the normal-dispersion over anomalous dispersion-pumped fiber for 390 fs long pump pulses. A double-clad design of the photonic lattice of the fiber is further postulated to enable a pump-related seeding mechanism of normal-dispersion supercontinuum broadening under sub-picosecond pumping, which is otherwise known for similar noise characteristics as modulation instability driven, soliton-based spectra.

High-performance multi-megahertz optical coherence tomography based on amplified optical time-stretch

As the key prerequisite of high-speed volumetric structural and functional tissue imaging in real-time, scaling the A-scan rate beyond MHz has been one of the major pursuits in the development of optical coherence tomography (OCT). Along with a handful of techniques enabling multi-MHz, amplified optical time-stretch OCT (AOT-OCT) has recently been demonstrated as a viable alternative for ultrafast swept-source OCT well above MHz without the need for the mechanical wavelength-tuning mechanism. In this paper, we report a new generation of AOT-OCT demonstrating superior performance to its older generation and all other time-stretch-based OCT modalities in terms of shot-to-shot stability, sensitivity (~90dB), roll-off performance (>4 mm/dB) and A-scan rate (11.5 MHz). Such performance is mainly attributed to the combined contribution from the stable operation of the broadband and compact mode-locked fiber laser as well as the optical amplification in-line with the time-stretch process. The system allows us, for the first time, to deliver volumetric time-stretch-based OCT of biological tissues with the single-shot A-scan rate beyond 10 MHz. Comparing with the existing high-speed OCT systems, the inertia-free AOT-OCT shows promises to realize high-performance 3D OCT imaging at video rate.

Spectral and spatial full-bandwidth correlation analysis of bulk-generated supercontinuum in the mid-infrared

We report the measurement of spectral and spatial correlations in supercontinua generated by focusing microjoule pulses from a femtosecond ytterbium-doped fiber amplifier laser in bulk YAG. The measurement is full-bandwidth at a repetition rate of 1 MHz owing to the use of time-stretch dispersive Fourier transform technique. In contrast with fiber-based supercontinuum generation, our results show an excellent stability of the spectral and spatial properties of the output supercontinuum, with an essentially correlated behavior in the 1.4-1.7 μm wavelength range. These results provide strong ground for the development of supercontinuum-seeded ultrafast optical parametric amplifier systems in the mid-infrared using ytterbium lasers as pump sources.

Spectral coherence in all-normal dispersion supercontinuum in presence of Raman scattering and direct seeding from sub-picosecond pump

Intensity stability and wavelength correlations of near-infrared supercontinuum generation are studied in all-normal flattened dispersion, all-solid soft glass photonic crystal fiber. We use dispersive Fourier transformation method to measure shot-to-shot resolved spectra under pumping from a sub-picosecond, fiber-based chirped pulse amplification (CPA) system. For the first time to our knowledge, we demonstrate how unconverted radiation from pump, propagating in the photonic cladding of the fiber, improves the measured degree of coherence in the spectrum and influences its wavelength correlation by seeding of multiple four-wave-mixing / Raman scattering components. The presented results suggest a convenient and simple way of stabilizing of shot-to-shot coherence in sub-picosecond fiber laser pumped, normal-dispersion supercontinuum sources by direct, pump-related seeding.