Flat Spectral Phase Research Articles

Nonquadratic Spectral Phase Aberration With Quadratic Temporal Phase Modulation in an Actively Modulated Ultrafast Laser System

Ultrashort pulses can be generated through active modulation techniques, such as the time-lens technique. To achieve better pulse compression, in the time-lens source, quadratic temporal phase modulation was naturally considered as the ideal modulation. However, it is unknown that whether a quadratic temporal phase modulation (equivalently, a linear chirp) translates into a quadratic spectral phase modulation, which can be easily eliminated using compressors. Here, we study this phase transfer both analytically and numerically. Our results indicate that depending on the pulse shape, nonquadratic spectral phase aberration arises even if the temporal phase modulation is quadratic. Consequently, there is observable difference between the compressed pulses with a flat spectral phase and with quadratic spectral phase compensation only. However, both analytical and numerical analyzes show that as quadratic temporal phase modulation increases, nonquadratic spectral phase modulation decreases, and pulse compression with quadratic spectral phase tends to that with a flat spectral phase.

Spectral mirror for ultra-short, high peak power, multi-PW Ti:sapphire lasers

A multilayer mirror for spectral filtering adapted to ultra-short and multi-PW Ti:Sa laser has been designed, manufactured and characterized. The method used to determine both the reflectivity shape and the coating design leads to global compensation of gain narrowing, saturation and spectral phase. The result is a spectral control on 200 nm range while keeping a flat spectral phase. This kind of filter will enable obtaining 15 fs pulse duration for multi-PW laser systems based on Ti:Sa.

Pulse-shaper-assisted phase control of a coherent broadband spectrum of Raman sidebands

We report pulse-shaper-assisted phase control of a broad spectrum, aiming to synthesize nonsinusoidal waveforms. We first generate multiple-order coherent Raman sidebands by focusing two laser beams into a piece of diamond. Then we combine five sidebands into a single beam by using a prism. Our setup allows for both coarse, manual phase adjustments and fine tuning of the spectral phases. A flat spectral phase across these five frequency-separated sub-bands is achieved, which implies generation of two to three optical-cycle pulses.

Spectral interference of frequency-shifted solitons in a photonic-crystal fiber

Frequency-shifted solitons in a highly nonlinear photonic-crystal fiber (PCF) are shown to give rise to high-visibility interference fringes in PCF output spectra, indicating flat spectral phase profiles of individual solitons in the PCF output. This experimental finding, supported by numerical simulations, suggests a promising method of fiber-format pulse shaping and an attractive technology for few-cycle pulse synthesis through a coherent addition of frequency-shifted solitons generated in a highly nonlinear fiber.

Towards athermal optically-interconnected computing system using slotted silicon microring resonators and RF-photonic comb generation

We report that completely athermal design of a slotted silicon waveguide is possible by combining the negative thermo-optic (TO) coefficient of, for example, polymethyl methacrylate (PMMA) with the positive TO coefficient of silicon. When used in a microring resonator structure, the filled overcladding slotted waveguide and the unfilled (air-filled) overcladding slotted waveguide can both achieve athermal characteristics. Simulations indicate a wide range of realizations with proper design parameters of the slotted waveguides, namely, the silicon strip and slot widths. Preliminary experimental results on fabricated devices demonstrate that the temperature dependence is reduced from 91 pm/°C for a regular microring resonator to 52 pm/°C for the PMMA-clad microring resonator. Completely athermal realization is expectable in similar devices with improved fabrication techniques. For the external optical source, we demonstrate a stable 3.5 THz wide (175 modes×20 GHz) optical comb source with nearly flat spectral phase. Adjustable mode spacing and wavelength tunability across the C-band are maintained so that comb lines can be matched to the specified wavelength grid of the computing system. With such schemes, temperature controls of individual optical components in the optically interconnected computing chips become unnecessary, greatly reducing the complexity of the computing system.

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance.