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Supercontinuum generation in optical fibers is one of the most dramatic nonlinear effects discovered, allowing short pulses to be converted into multi-octave spanning coherent spectra. However, generating supercontinua that are both coherent and broadband requires pulses that are simultaneously ultrashort with high peak power. This results in a reducing efficiency with increasing pulse repetition rate, that has hindered supercontinua at microwave line spacing, i.e. 10s of GHz. Soliton microcombs by contrast, can generate octave-spanning spectra, but with good conversion efficiency only at vastly higher repetition rates in the 100s of GHz. Here, we bridge this efficiency gap with resonant supercontinuum, allowing supercontinuum generation using input pulses with an ultra-low 6 picojoule energy, and duration of 1 picosecond, 10-fold longer than what is typical. By applying synchronous pulse-driving to a dispersion-engineered, low-loss Si$_3$N$_4$ photonic chip microresonator, we generate dissipative Kerr solitons with a strong dispersive wave, both bound to the input pulse. This creates a smooth, flattened 2,200 line frequency comb, with an electronically detectable repetition rate of 28 GHz, constituting the largest bandwidth-line-count product for any microcomb generated to date. Strikingly, we observe that solitons exist in a weakly bound state with the input pulse, stabilizing their repetition rate, but simultaneously allowing noise transfer from one to the other to be suppressed even for offset frequencies 100 times lower than the linear cavity decay rate. We demonstrate that this nonlinear filtering can be enhanced by pulse-driving asynchronously, in order to preserve the coherence of the comb. Taken together, our work establishes resonant supercontinuum as a promising route to broadband and coherent spectra.
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The ability to amplify optical signals is of pivotal importance across science and technology. The development of optical amplifiers has revolutionized optical communications, which are today pervasively used in virtually all sensing and communication applications of coherent laser sources. In the telecommunication bands, optical amplifiers typically utilize gain media based on III-V semiconductors or rare-earth-doped fibers. Another way to amplify optical signals is to utilize the Kerr nonlinearity of optical fibers or waveguides via parametric processes. Such parametric amplifiers of travelling continuous wave have been originally developed in the microwave domain, and enable quantum-limited signal amplification with high peak gain, broadband gain spectrum tailored via dispersion control, and ability to enable phase sensitive amplification. Despite these advantages, optical amplifiers based on parametric gain have proven impractical in silica fibers due to the low Kerr nonlinearity. Recent advances in photonic integrated circuits have revived interest in parametric amplifiers due to the significantly increased nonlinearity in various integrated platforms. Yet, despite major progress, continuous-wave-pumped parametric amplifiers built on photonic chips have to date remained out of reach. Here we demonstrate a chip-based travelling-wave optical parametric amplifier with net signal gain in the continuous-wave regime. Using ultralow-loss, dispersion-engineered, meter-long, silicon nitride photonic integrated circuits that are tightly coiled on a photonic chip, we achieve a continuous parametric gain of 12 dB that exceeds both the on-chip optical propagation loss and fiber-chip-fiber coupling losses in the optical C-band.
We present the first demonstration of a narrow linewidth, waveguide-based Brillouin laser which is enabled by large Brillouin gain of a chalcogenide chip. The waveguides are equipped with vertical tapers for low loss coupling. Due to optical feedback for the Stokes wave, the lasing threshold is reduced to 360 mW, which is 5 times lower than the calculated single-pass Brillouin threshold for the same waveguide. The slope efficiency of the laser is found to be 30% and the linewidth of 100 kHz is measured using a self-heterodyne method.
We investigate supercontinuum generation in several suspended-core soft-glass photonic crystal fibers pumped by an optical parametric oscillator tunable around 1550 nm. The fibers were drawn from lead-bismuth-gallium-cadmium-oxide glass (PBG-81) with a wide transmission window from 0.5-2.7 micron and a high nonlinear refractive index up to 4.3.10^(-19) m^2/W. They have been specifically designed with a microscale suspended hexagonal core for efficient supercontinuum generation around 1550 nm. We experimentally demonstrate two supercontinuum spectra spanning from 1.07-2.31 micron and 0.89-2.46 micron by pumping two PCFs in both normal and anomalous dispersion regimes, respectively. We also numerically model the group velocity dispersion curves for these fibers from their scanning electron microscope images. Results are in good agreement with numerical simulations based on the generalized nonlinear Schrodinger equation including the pump frequency chirp.
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We propose a concept of chiral photonic limiters utilising topologically protected localised midgap defect states in a photonic waveguide. The chiral symmetry alleviates the effects of structural imperfections and guaranties a high level of resonant transmission for low intensity radiation. At high intensity, the light-induced absorption can suppress the localised modes, along with the resonant transmission. In this case the entire photonic structure becomes highly reflective within a broad frequency range, thus increasing dramatically the damage threshold of the limiter. Here we demonstrate experimentally the principle of operation of such photonic structures using a waveguide consisting of coupled dielectric microwave resonators.
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