1. 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: