University of Wisconsin Engine Research Center

Generation of Pulsed Ultra-Violet and Mid-Infrared

Super-Continua in Standard Single-Mode FiberRenata 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

Summary of Approaches to Supercontinuum Generation:

Method used for Supercontinuum Generation in both the UV and 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

UV, Most Relevant Work:

UV Supercontinuum Generation:

Mid-IR, Most Relevant Work:

Mid-IR Supercontinuum Generation:

Applications:

• 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

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

pulse duration

medium

ultra-fast

(< 1 ps)

pulsed

(> 1 ns)

micro-structured fiber

bulk material

e.g., gasonly discrete

frequencies

standard fiber

~ ~

Lin et al., Appl.Phys.Lett., 28 (1976):

• 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

Differences from Literature:

Our Approach:

Mid-IR Supercontinuum Generation:

Sanghera et al., Laser Focus World, 41 (2005):

• 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)

• convenient telecom pump wavelength

• all-fiber system

Our Approach:

• 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%

Outlook:

wavelengths where most absorption occurs in the gas

phase

% A

bso

rpti

on 100

50

010 m 1 m 100 nm

Wavelength5 m 500 nm

most common super-continua

wavelength

Strong Absorption in the Mid-IR and UV is Important:

Challenge:

• all fibers have large dispersion

Solution:

• use a long pulse to minimize pulse walk-off

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

ULTR

AV

IOLET

MID

-IN

FR

AR

ED

Ultraviolet: Ultraviolet:

Mid-Infrared:

• 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

Mid-Infrared: