Soliton-effect compression of picosecond pulses on a photonic chip.

We report soliton-effect pulse compression of low energy (∼25pJ), picosecond pulses on a photonic chip. An ultra-low-loss, dispersion-engineered 40-cm-long waveguide is used to compress 1.2-ps pulses by a factor of 18, which represents, to our knowledge, the largest compression factor yet experimentally demonstrated on-chip. Our scheme allows for interfacing with an on-chip picosecond source and offers a path towards a fully integrated stabilized frequency comb source.

Performance scaling of a 10-GHz solid-state laser enabling self-referenced CEO frequency detection without amplification.

A simple and compact straight-cavity laser oscillator incorporating a cascaded quadratic nonlinear crystal and a semiconductor saturable absorber mirror (SESAM) can deliver stable femtosecond modelocking at high pulse repetition rates >10 GHz. In this paper, we experimentally investigate the influence of intracavity dispersion, pump brightness, and cavity design on modelocking with high repetition rates, and use the resulting insights to demonstrate a 10.4-GHz straight-cavity SESAM-modelocked Yb:CALGO laser delivering 108-fs pulses with 812 mW of average output power. This result represents a record-level performance for diode-pumped femtosecond oscillators with repetition rates above 10 GHz. Using the oscillator output without any optical amplification, we demonstrate coherent octave-spanning supercontinuum generation (SCG) in a silicon nitride waveguide. Subsequent f-to-2f interferometry with a periodically poled lithium niobate waveguide enables the detection of a strong carrier-envelope offset (CEO) beat note with a 33-dB signal-to-noise ratio.

Gigahertz frequency comb offset stabilization based on supercontinuum generation in silicon nitride waveguides.

Silicon nitride (Si3N4) waveguides represent a novel photonic platform that is ideally suited for energy efficient and ultrabroadband nonlinear interactions from the visible to the mid-infrared. Chip-based supercontinuum generation in Si3N4 offers a path towards a fully-integrated and highly compact comb source for sensing and time-and-frequency metrology applications. We demonstrate the first successful frequency comb offset stabilization that utilizes a Si3N4 waveguide for octave-spanning supercontinuum generation and achieve the lowest integrated residual phase noise of any diode-pumped gigahertz laser comb to date. In addition, we perform a direct comparison to a standard silica photonic crystal fiber (PCF) using the same ultrafast solid-state laser oscillator operating at 1 µm. We identify the minimal role of Raman scattering in Si3N4 as a key benefit that allows to overcome the fundamental limitations of silica fibers set by Raman-induced self-frequency shift.

Octave-spanning coherent supercontinuum generation in a silicon nitride waveguide.

We demonstrate the generation of a supercontinuum spanning more than 1.4 octaves in a silicon nitride waveguide using sub-100-fs pulses at 1 µm generated by either a 53-MHz, diode-pumped ytterbium (Yb) fiber laser or a 1-GHz, Yb:CaAlGdO(4) (Yb:CALGO) laser. Our numerical simulations show that the broadband supercontinuum is fully coherent, and a spectral interference measurement is used to verify that the supercontinuum generated with the Yb:CALGO laser possesses a high degree of coherence over the majority of its spectral bandwidth. This coherent spectrum may be utilized for optical coherence tomography, spectroscopy, and frequency metrology.

Strong polarization mode coupling in microresonators.

We observe strong modal coupling between the TE00 and TM00 modes in Si3N4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si3N4 microresonators due to polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode crossings can be used as a tool for dispersion engineering in microresonators.

Microresonator-based comb generation without an external laser source.

We demonstrate a fiber-microresonator dual-cavity architecture with which we generate 880 nm of comb bandwidth without the need for a continuous-wave pump laser. Comb generation with this pumping scheme is greatly simplified as compared to pumping with a single frequency laser, and the generated combs are inherently robust due to the intrinsic feedback mechanism. Temporal and radio frequency (RF) characterization show a regime of steady comb formation that operates with reduced RF amplitude noise. The dual-cavity design is capable of being integrated on-chip and offers the potential of a turn-key broadband multiple wavelength source.

Modelocking and femtosecond pulse generation in chip-based frequency combs.

We investigate simultaneously the temporal and optical and radio-frequency spectral properties of parametric frequency combs generated in silicon-nitride microresonators and observe that the system undergoes a transition to a mode-locked state. We demonstrate the generation of sub-200-fs pulses at a repetition rate of 99 GHz. Our calculations show that pulse generation in this system is consistent with soliton modelocking. Ultimately, such parametric devices offer the potential of producing ultra-short laser pulses from the visible to mid-infrared regime at repetition rates from GHz to THz.

Asynchronous single-shot characterization of high-repetition-rate ultrafast waveforms using a time-lens-based temporal magnifier.

We demonstrate asynchronous, single-shot characterization of an ultrafast, high-repetition-rate pulse source using a time-lens-based temporal magnifier. We measure a 225 GHz repetition-rate pulse train from a microresonator-based frequency comb. In addition, we show that such a system can be used as a frequency compressor for real-time, high-speed RF spectral characterization.

Chip-based frequency combs with sub-100 GHz repetition rates.

By fabricating high-Q silicon-nitride spiral resonators, we demonstrate frequency combs spanning over 200 nm with free spectral ranges (FSRs) of 80, 40, and 20 GHz using cascaded four-wave mixing. We characterize the RF beat note for the 20 GHz FSR comb, and the measured linewidth of 3.6 MHz is consistent with thermal fluctuations in the resonator due to amplitude noise of the pump source. These combs represent an important advance towards developing a complementary metal-oxide-semiconductor (CMOS)-based system capable of linking the optical and electronic regimes.