Spectral slicing of femtosecond pulses using semiconductor modulator arrays

Abstract

ABSTRACT Unique properties of ultrashort laser pulses open new poss ibilities for broadband optical communications in both space and terrestrial systems. Spectral slicing offers a promising approach to wavelength multiplexing using a coherent broadband source such as a modelocked femtosecond laser. We have realized a free-space spectral slicing and transmission system, with a spectrally sliced modelocked laser delivering ~100 fs pulses at 806 nm as the “frequency comb” source. Spectral slicing was performed using monolithic arrays of electro-absorption modulators (EAM) fabricated from quantum-well GaAs/AlGaAs semiconductor material with a bandgap energy falling within the fs pulse spectrum. The array bars contained between 2 and 10 individually addressable EAM channels and were packaged into modules with cylindrical micro-optics for efficient coupling of light into and from the semiconductor waveguide. By performing absorption measurements as a function of wavelength and voltage bias on the EAM, we identified the spectral region where modulation depth was the largest. Wavelength slicing was achieved by fanning out the fs pulse beam with a diffraction grating and coupling it across the full width of the EAM array. A modulation depth >12 dB was achieved by probing adjacent spectral channels using ON/OFF keying. In summary, we have demonstrated spectral slicing of femtosecond pulses with EAM arrays for free-space communications. The technology can find use in other areas, e.g., instant chemical analysis and remote sensing, as EAMs can modulate both the intensity and phase of randomly selectable spectral channels, allowing complex spectra and waveforms to be generated in real time. Keywords: free-space communications, femtosecond pulse, spectral slicing, frequency comb, electro-absorption modulator, EAM, modulator array, quantum-well intermixing.