Generation of Pulsed Ultra-Violet and Mid-Infrared Super-Continua in Standard Single-Mode Fiber

1. Generation of Pulsed Ultra-Violet and Mid-Infrared Super-Continua in Standard Single-Mode Fiber 100 % Absorption 50 0 100 nm 1 mm 500 nm 10 mm 5 mm Wavelength most common super-continua wavelength wavelengths where most absorption occurs in the gas phase Renata Bartula, Chris Hagen, Joachim Walewski, and Scott Sanders • Solution: • super-continua are like fiber coupled light bulbs/white lasers • broadband light, useful for optical sensing • spectra can span more than three octaves (e.g., 200 – 1800 nm) • superior spectral radiance (~1,000 × larger) compared to traditional broadband sources such as incandescent lamps Method used for Supercontinuum Generation in both the UV and Mid-IR: • Motivation: • white light broad spectra • Problem: • traditional sources have too low spectral radiance [Wm-2sr-1nm-1] for high performance instrumentation Mid-IR, Most Relevant Work: Mid-IR Supercontinuum Generation: • choose pump wavelength to be at the blue end of the desired spectrum • colors are red-shifted primarily by stimulated Raman scattering as pump power increases • Challenge: • dispersion can easily cause pulse walk-off for ultra-fast pulses • Solution: • many fibers with different dispersions are available in the near to mid-IR Sanghera et al., Laser Focus World, 41 (2005): Our Approach: • Challenge: • all fibers have large dispersion • Solution: • use a long pulse to minimize pulse walk-off • convenient telecom pump wavelength • all-fiber system • Ti:Sapphire laser, red-shifted to ~2.5 μm in nonlinear crystal • 100 pJ pulses, 100 fs pulse duration (before fiber) • coupled into chalcogenide fibers • 2.1 – 3.2 μm supercontinuum generation (spans ~1650 cm-1) MID-INFRARED ULTRAVIOLET Mid-IR Supercontinuum Generation: • Soliton self-shift in fiber (positive dispersion) • fused silica absorbs in mid-IR  difficult SC generation • fluoride is ideal, but there is no high-power laser above 1.6 μm (zero dispersion wavelength) • red shift Er laser (1.55 μm) in fused silica beyond 1.6 μm • couple into fluoride fiber for continued shifting • 1.5 μJ pulses, 37.5 ps pulse duration in positive-dispersion fiber • spans ~ 4200 cm-1 • Ge filter (1.8 μm cut off) • blue shift to ~ 1.4 μm (estimated) • max input power ~ 300 mW, coupling efficiency ~ 65% UV, Most Relevant Work: Our Approach: Strong Absorption in the Mid-IR and UV is Important: Lin et al., Appl.Phys.Lett., 28 (1976): Differences from Literature: • pump with Nitrogen laser only (337 nm) • coupled into a UV-grade single mode fiber • coupled into a ~50 m-long, 2 μm core fiber • 46 nJ pulses, 4 ns pulse duration (after fiber) • 4 % coupling efficiency • Nitrogen laser (337 nm) pumping dye laser (373 – 399 nm) • 10 μJ pulses, 10 ns pulse duration (before fiber) • coupled into ~20-m, 7-μm core multi-mode silica fiber • M2 > 1 • 392 – 537 nm supercontinuum generation (spans ~ 6900 cm-1) UV Supercontinuum Generation: Summary of Approaches to Supercontinuum Generation: Applications: Outlook: ~ ~ Ultraviolet: Ultraviolet: • use pump laser with higher repetition rate • use deeper UV wavelength (below 230 nm, solarization will be a problem) • increase energy per pulse using coiled multimode fiber • generation of CW supercontinuum • mircrochip lasers available at < 1/10 the cost • no support for soliton self shift, but Raman shifting prevalent • ergo, high spectral radiance at low cost • atmospheric sensing • absorption spectra in combustion (formaldehyde, OH, NO) • absorption spectra in combustion (H2O, CO, CO2) • free-space communications • spans ~ 5600 cm-1 • peaks spaced 13.2 THz apart (up to 12th Stokes shift) • spectral width of pump only 0.1 nm, but Raman broadening produces a continuum Mid-Infrared: Mid-Infrared: