Generation of Pulsed Ultra-Violet and Mid-Infrared Super-Continua in Standard Single-Mode Fiber Renata Bartula, Chris Hagen, Joachim Walewski, and Scott Sanders Motivation: white light  broad spectra Problem: traditional sources have too low spectral radiance [Wm-2sr-1nm-1] for high performance instrumentation 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: Mid-IR, Most Relevant Work: Mid-IR Supercontinuum Generation: 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 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 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: wavelengths where most absorption occurs in the gas phase % Absorption 100 50 10 mm 1 mm 100 nm Wavelength 5 mm 500 nm most common super-continua wavelength Lin et al., Appl.Phys.Lett., 28 (1976): Differences from Literature: 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) 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 UV Supercontinuum Generation: Summary of Approaches to Supercontinuum Generation: pulse duration medium ultra-fast (< 1 ps) pulsed (> 1 ns) micro-structured fiber bulk material e.g., gas only discrete frequencies standard fiber 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: