Abstract
We report on the observation of efficient and ultra-broadband white light supercontinuum generated by focusing femtosecond pulses from an optical parametric amplifier at 1.5 microm in silica glass. The characteristic white light spectrum is extending from 400 nm up to at least 1750 nm. At sufficiently high input powers stable white light patterns associated with the interference of spatially coherent filamentary sources were observed and analyzed. Unlike focusing with 800 nm pulses from a Ti-sapphire laser, the stable fringes formed for each spectral component were pronounced owing to significantly reduced destructive impact of optical breakdown on filamentation of femtosecond pulses at 1.5 microm. By taking advantage of this property, the formation of optical waveguides in silica glass with considerably broader range of writing parameters as compared to those fabricated with 800 nm pulses, was demonstrated.
Highlights
Propagation of intense ultra-short laser pulses through transparent media is usually accompanied by a spectrally broad radiation so called white light super-continuum (SC) which extends from the ultraviolet to the infrared range
The filamentation and white light generation resulting from the propagation of intense ultra-short laser pulses in various gases and condensed media were experimentally demonstrated and the underlying physical mechanisms were analyzed through various theoretical models [4,5,6,7,8,9]
We demonstrate remarkably broadband white light SC generation with a high efficiency upon focusing femtosecond pulses from an optical parametric amplifier at 1.5 μm inside silica glass
Summary
Propagation of intense ultra-short laser pulses through transparent media is usually accompanied by a spectrally broad radiation so called white light super-continuum (SC) which extends from the ultraviolet to the infrared range. For peak powers sufficiently higher than the critical power, the beam cross section appears as a collection of randomly distributed spots resulting from small-scale multiple self-focusing of the pulse Such spots are identified as small-scale filaments acting as sources of the white light. We show that for the case of focusing very near the input glass surface in air, the interference fringes were observed by excitation at both 800 nm and 1.5 μm, whereas no such fringes were observed with 800 nm irradiation when focusing inside the glass We attribute this feature to significantly reduced destructive influence of femtosecond breakdown and structural damage on filamentation of pulses at 1.5 μm. Such remarkable resistance of the glass against breakdown and physical damage at 1.5 μm allowed us to realize efficient smallsize core optical waveguides in silica based on pure filamentation process within a broader range of writing parameters as compared to the 800 nm case
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