FOSS Required Configuration reflector (distance 0) Bragg gratings L 1 L 2 L 3 L ref reference signal data signal Electronics: reference signal data signal A/D c data signal sampled according to reference length fringes
FFT To Separate Gratings power (cm) -1 FFT sampled set with w=-(2nL ref ) -1 spacing between samples number of samples = N sam power cm L 1 L 2 L 3 “Spatial” set with ((N sam -1) w) -1 between points
FFT -1 To Find Grating Spectra power cm L 1 L 2 L 3 “Spatial” set with spat=((N sam -1) w) -1 point spacing FFT -1 power (cm) -1 b ) -1 “Spectral” set with spec=((N spec -1) spat) -1 point spacing
Bragg Grating Reflection Spectrum reflection(%) 0 0.1 0.1 nm wavelength(nm) b = center reflection wavelength of Bragg grating b ) -1 = center reflection wavenumber
Fundamentals of Photonic Crystals - What are photonic crystals? - Theory of Photonic Crystals Photonic Crystal Fibers (PCF) Fabrication of PCF Advantages and Challenges Photonic Crystals & Fibers
Photonic Crystals & fibers What are photonic crystals? Features of a photonic crystal: Made of low-loss periodic dielectric medium Optical analog to the electrical semiconductors Able to localize light in specified areas by preventing light from propagating in certain directions – optical bandgap.
In 2D photonic crystal structures it is possible to confine light within a cavity. Photonic band gaps appear in the plane of periodicity and in 2D we can achieve linear localization. By introducing a defect, i.e. removing one column, we may obtain a peak in the density of states localized in the photonic band gap – similar to semiconductors. The defect mode cannot penetrate the crystal in the xy- plane because of the band gap but extends in the z- direction
Photonic crystal fibers By making a suitable geometry of the periodic dielectric medium, a phonic crystal can be used as an optical fiber.
PCF Fabrication PCF Preform Construction PCF Drawing -similar to drawing capillary tubes
Preform Construction Tubes are packed in a hexagonal shape with hollow, solid, birefringent, doped or tubular core elements.
Advantages: PCF with high-index core is more flexible than conventional fiber: - Possible to make very large core area to send high power - Possible to make core very small compared to conventional fibers. Designer wavelengths possible. Air-guiding PCF (hollow core of fiber): - Possible to send high power - No entrance or exit reflectance Challenges: PCF is difficult to fabricate PCF is limited to specific frequencies
Draw Tower Capabilities Graphite Furnace Multiple heat zone furnace elements Fiber diameter feedback control IR and UV thermal fiber coating capability Excimer Laser Ability to write gratings while drawing fiber for increased fiber strength