The primary components of two new candidate events (GW190403 051519 and GW190426 190642) fall in the mass gap predicted by pair-instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (q < 0.61 and q < 0.62 at 90% credibility for GW190403 051519 and GW190917 114630 respectively), and find that 2 of the 8 new events have effective inspiral spins χ eff > 0 (at 90% credibility), while no binary is consistent with χ eff < 0 at the same significance.

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We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.

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We study the largely unexplored transition between coherent and noise-seeded incoherent continuum generation in all-normal dispersion (ANDi) fibers and show that highly coherent supercontinua with spectral bandwidths of one octave can be generated with long pump pulses of up to 1.5 ps duration, corresponding to soliton orders of up to N = 600. In terms of N, this corresponds to an approximately 50 times increase of the coherent regime compared to anomalous dispersion pumping. In the transition region between coherent and incoherent spectral broadening we observe the manifestation of nonlinear phenomena that we term incoherent cloud formation and incoherent optical wave breaking, which lead to a gradual or instantaneous coherence collapse of SC spectral components, respectively. The role played by stimulated Raman scattering and parametric four-wave mixing during SC generation in ANDi fibers is shown to be more extensive than previously recognized: their nonlinear coupling contributes to the suppression of incoherent dynamics at short pump pulse durations, while it is responsible for non-phasematched parametric amplification of noise observed in the long pulse regime. We further discuss the dependence of SC coherence on fiber design, and present basic experimental verifications for our findings using single-shot detection of SC spectra generated by picosecond pulses. This work outlines both the further potential as well as the limitations of broadband coherent light source development for applications such as metrology, nonlinear imaging, and ultrafast photonics, amongst others.

Abstract:We report the first experimental realization and detailed characterization of thulium doped fiber amplifiers (TDFAs) specifically designed for optical communications providing high gain (>35 dB), noise figure as low as 5 dB, and over 100 nm wide bandwidth around 2 µm. A maximum saturated output power of 1.2 W was achieved with a slope efficiency of 50%. The gain dynamics of the amplifier were also examined. Our results show that TDFAs are well qualified as high performance amplifiers for possible future telecommunication networks operating around 2 µm.

Supercontinuum spanning over an octave from 900 - 2300 nm is reported in an all-normal dispersion, soft glass photonic crystal fiber. The all-solid microstructured fiber was engineered to achieve a normal dispersion profile flattened to within -50 to -30 ps/nm/km in the wavelength range of 1100 - 2700 nm. Under pumping with 75 fs pulses centered at 1550 nm, the recorded spectral flatness is 7 dB in the 930 - 2170 nm range, and significantly less if cladding modes present in the uncoated photonic crystal fiber are removed. To the best of our knowledge, this is the first report of an octave-spanning, all-normal dispersion supercontinuum generation in a non-silica microstructured fiber, where the spectrum long-wavelength edge is red-shifted to as far as 2300 nm. This is also an important step in moving the concept of ultrafast coherent supercontinuum generation in all-normal dispersion fibers further towards the mid-infrared spectral region.

We demonstrate nonlinear pulse compression based on recently introduced highly coherent broadband supercontinuum (SC) generation in all-normal dispersion photonic crystal fiber (ANDi PCF). The special temporal properties of the octave-spanning SC spectra generated with 15 fs, 1.7 nJ pulses from a Ti:Sapphire oscillator in a 1.7 mm fiber piece allow the compression to 5.0 fs high quality pulses by linear chirp compensation with a compact chirped mirror compressor. This is the shortest pulse duration achieved to date from the external recompression of SC pulses generated in PCF. Numerical simulations in excellent agreement with the experimental results are used to discuss the scalability of the concept to the single-cycle regime employing active phase shaping. We show that previously reported limits to few-cycle pulse generation from compression of SC spectra generated in conventional PCF possessing one or more zero dispersion wavelengths do not apply for ANDi PCF.