Generation of Terahertz-Rate Trains of Femtosecond Pulses by Phase-Only Filtering

We describe a simple linear filtering technique for transforming individual femtosecond light pulses into terahertzrepetition-rate bursts of femtosecond pulses. By using phase-only filtering, high efficiency is achieved. Pulse repetition rates approaching 6 THz are obtained. Future high-speed optical communication and computation systems will transmit, route, and manipulate extremely high-rate streams of picosecond and femtosecond pulses. Recently, researchers have been exploring techniques for generating optical pulse trains at repetition rates beyond those obtainable by mode locking or direct modulation. One main approach uses the modulational instability based on nonlinear pulse propagation in optical fibers in the anomalous dispersion regime. Our experiments are based on an apparatus for high-resolution femtosecond pulse shaping. 6 ' 7 A dispersion-compensated, colliding-pulse mode-locked ring dye laser 8 with a typical pulse duration of 75 fsec at a 0.62-gm wavelength is the source of fsec pulses

High-Power Broadly Tunable Electrooptic Frequency Comb Generator

Broadband traveling-wave electrooptic modulators made of lithium niobate have reached a high level of technological maturity. They can provide simultaneously low V pi, sustain high power (both optical and RF) and yet provide low propagation loss. By combining together these features, we present a high-power handling, broadly tunable, electrooptic frequency comb generator. The device produces between 60 and 75 lines within -10 dB bandwidth over its full tuning range-from 6 to 18 GHz- and can handle up to 1 W of optical input power. This optical frequency comb platform is very well suited for applications in RF photonics and optical communications that require independent RF and optical tuning as well as high-repetition rates but moderate bandwidth

High-power femtosecond optical pulse compression by using spatial solitons

We demonstrate a novel pulse-compression technique that uses the self-confinement of two-dimensional spatial solitons propagating in bulk nonlinear media to increase the spectral bandwidth followed by a grating pair for recompression. Output pulses of 19-fs duration with 0.6-,J energies are routinely obtained at a repetition rate of 8.6 kHz. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. Femtosecond pulse compression techniques that employ self-phase modulation in an optical fiber to generate spectral bandwidth have developed to the point where it is now possible to generate optical pulses as short as 6 fs.1 However, fiber damage thresholds and parasitic higher-order nonlinear processes typically limit the amount of energy that can effectively be compressed to less than 10 nJ. Applications such as mode-selective excitation of coherent phonons by means of impulsive stimulated Raman scattering 2 and strong-field physics'-' require new methods of compression that produce shortduration optical pulses while maintaining high energies. In recent years progress has been made in extending the energy range of compressed pulses. Efforts by Rolland and Corkum, who used self-phase modulation in bulk materials, have succeeded in generating 100-,J, 24-fs pulses. In this Letter we report on a new method of pulse compression, which produces 19-fs, 0.6-,uJ optical pulses at a repetition rate of 8.6 kHz. Our method relies on the self-trapping and stable propagation of two-dimensional bright spatial optical solitons in bulk nonlinear media. In close analogy with temporal solitons, in which the balancing of group-velocity dispersion and self-phase modulation lead to dispersion-free propagation, 9 the balancing of diffraction by the spatial nonlinear index profile results in diffraction-free propagation.' 0 Although selftrapping of beams in three dimensions is unstable and leads to catastrophic self-focusing, recent experiments have demonstrated the stable propagation of two-dimensional spatial solitons in CS 2 liquid"' and in guided-wave geometries.12l' 4 The self-trapped propagation of the spatial soliton itself maintains the high intensity necessary for large phase modulation, which generates the necessary bandwidth for pulse compression. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. The basic experimental apparatus for generating and compressing spatial solitons is as follows. Pulses of 75-fs duration and 0.1-nJ energies from a balanced colliding-pulse mode-locked ring dye laser operating at 620 nm were amplified to 30 /.tJ at a repetition rate of 8.6 kHz in a two-stage optical amplifier pumped by a 20-W copper-vapor laser. To achieve these pulse energies, we used a dye cell in the second stage.' 5 Following recompression to 75 fs with a two-prism sequence in a double-pass geometry, the pulses were spatially filtered to improve beam quality and ensure the formation of clean spatial solitons. The energy throughput of the prism sequence-spatial filter was 11 /%J. We chose an 8-mm-thick piece of bulk fused silica as the nonlinear medium, which has a positive nonlinear index (n 2 = 2.7 x 10-16 cm 2 /W), as required for bright spatial solitons as well as minimal linear and twophoton absorption. Pulses were focused on the front face of the glass in an elliptical profile by a cylindrical-spherical lens combination. We used beam diameters of w = 900 gum (l/e peak intensity) in the long dimension (which we denote x; see the graph o

Characterization of Coherent Quantum Frequency Combs Using Electro-Optic Phase Modulation

We demonstrate a two-photon interference experiment for phase coherent biphoton frequency combs (BFCs), created through spectral amplitude filtering of biphotons with a continuous broadband spectrum. By using an electro-optic phase modulator, we project the BFC lines into sidebands that overlap in frequency. The resulting high-visibility interference patterns provide an approach to verify frequency-bin entanglement even with slow single-photon detectors; we show interference patterns with visibilities that surpass the classical threshold for qubit and qutrit states. Additionally, we show that with entangled qutrits, two-photon interference occurs even with projections onto different final frequency states. Finally,we showthe versatility of this scheme for weak-light measurements by performing a series of two-dimensional experiments at different signal-idler frequency offsets to measure the dispersion of a single-mode fiber

Persistent energy–time entanglement covering multiple resonances of an on-chip biphoton frequency comb

We investigate the time-frequency signatures of an on-chip biphoton frequency comb (BFC) generated from a silicon nitride microring resonator. Using a Franson interferometer, we examine the multifrequency nature of the photon pair source in a time entanglement measurement scheme; having multiple frequency modes from the BFC results in a modulation of the interference pattern. This measurement together with a Schmidt mode decomposition shows that the generated continuous variable energy–time entangled state spans multiple pair-wise modes. Additionally, we demonstrate nonlocal dispersion cancellation, a foundational concept in time–energy entanglement, suggesting the potential of the chip-scale BFC for large-alphabet quantum key distribution

Optical dual-comb Vernier division of an octave-spanning Kerr microcomb

We measure the repetition rate of a 900 GHz octave-spanning soliton microcomb based on Vernier dual-comb frequency division implemented with two silicon nitride microresonator combs fabricated on the same wafer

Optical Division of an Octave-Spanning Comb on an All-Silicon Nitride Platform

We demonstrate optical frequency division of an octave-spanning large repetition rate microcomb to an electronically-detectable frequency in an all-silicon nitride dual microcomb platform

Spectral Line-by-Line Pulse Shaping of an On-Chip Microresonator Frequency Comb

We report, for the first time to the best of our knowledge, spectral phase characterization and line-by-line pulse shaping of an optical frequency comb generated by nonlinear wave mixing in a microring resonator. Through programmable pulse shaping the comb is compressed into a train of near-transform-limited pulses of \approx 300 fs duration (intensity full width half maximum) at 595 GHz repetition rate. An additional, simple example of optical arbitrary waveform generation is presented. The ability to characterize and then stably compress the frequency comb provides new data on the stability of the spectral phase and suggests that random relative frequency shifts due to uncorrelated variations of frequency dependent phase are at or below the 100 microHertz level.Comment: 18 pages, 4 figure