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A key challenge for practical laser ignition systems is the need for fiber-optic delivery.
Our interest is to investigate fiber delivery of high-peak-power megawatt pulsed laser beams
in a way that allows spark formation (in the gas phase) after exiting the fiber. This
capability would have general application for the ignition of many other combustion devices
in which laser ignition may have applicability (turbines and aircraft). |
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We have investigated hollow core fibers and photonic crystal fibers for high power pulse
delivery. Above is shown the cross section of a hollow core fiber. The fiber is a cyclic
olefin polymer (COP) coated silver hollow waveguide. The light is guided in the waveguide
by reflection from the highly reflective wall. Below can be seen the cross section of a
large mode area solid core photonic crystal fiber. These fibers can transmit light in a
single mode via modified total internal refection. The refractive index of the solid core
is modified due to the surrounding photonic (hollow cores) structure allowing single mode
light propagation. |
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We have been able to transmit more than 50 mJ of 10 ns 1064 nm light through hollow core
fibers. Although these waveguides are multimode, the output beam quality (M2) as
low as 15 could be achieved through these fibers with a very narrow launch. With these beam
parameters, we have been able to form laser sparks in atmospheric pressure. We have also
been able to transmit 0.55 mJ of 8 ns pulses through LMA fiber with 35 μm core diameter. |
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We are currently working with Corning, Inc. on investigating hollow core photonic band gap
(PBG) fibers, (shown above left), for high power delivery. These PBG fibers work on the
principle of photonic band gap effect which allows single mode propagation of selected
light frequencies through the central hollow core. The photonic band gap effect is achieved
via changing the diameter of the surrounding holes and the inter-hole distance. The picture
below shows the hardware developed for launching high power laser beams through a PBG fiber
with a 8.6 μm core and the above right picture shows the output beam quality (which is
essentially single mode). |
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| References: |
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- S. Joshi, A.P. Yalin, A. Galvanauskas, "Use of Hollow Core Fibers, Fiber Lasers,
and Photonic Crystal Fibers for Spark Delivery and Laser Ignition in Gases", Applied
Optics, 46 19, pp. 4057
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