2
Overview
2.1 Operating Principle
The Doric
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LISER
TM
light source is inspired by the “white” LED concept. LEDs (Light Emitting Diodes) are solid state
light sources emitting light by the direct conversion of an electric current passing through a semiconductor junction.
LEDs are now available at many central wavelengths and bandwidths. Properly speaking, no LEDs can emit over the
entire visible spectrum to produce white light. Many so-called “white” LEDs are in fact a combination of a blue LED
and a fluorescent crystal. A part of the blue LED light is absorbed by a Cerium doped Yttrium Aluminum Garnet crystal
(shortly: Ce:YAG crystal) which, after absorption of blue light, spontaneously re-emits over a significant part of the
visible spectrum, mostly green, yellow and red light. The combination of the green-yellow-red fluorescence and the
unabsorbed blue light from the pumping LED results in white light.
“White” LEDs are designed for energy efficient white lighting applications. For scientific applications involving optical
fiber coupling, high spectral brightness (W/mm
2
/sr/nm) sources are required. In order to increase the spectral bright-
ness, the “white” LED operating principle must be scaled in term of pump power. The power scaling first requires a
higher brightness pump source emitting blue light. In the Doric
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light source, high power blue laser diodes
(LDs) emitting at 450 nm are used to provide a much smaller pumped volume of the Ce:YAG crystal. Such a direct
power scaling of the pumping source leads to thermal failures. Indeed, a high brightness pumping leads to a local
heating of the Ce:YAG crystal. When the crystal temperature locally reach about 400°C, a phenomenon called “tem-
perature quenching” leads to a significant drop of the fluorescent emission due to non radiative relaxation of blue-light
excited Ce
3+
ions. The temperature increase ultimately leads to a catastrophic failure of the crystal. Heat generated
by high brightness pumping thus have to be managed properly using passive/active cooling and crystal geometry.
The
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LISER
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light source emits over a broad and continuous visible light spectrum. A typical fluorescence spectrum of
a Ce:YAG crystal is shown in Fig.
. The source thus provides a speckle-free light beam at the output of multimode
optical fibers. Also, direct modulation of the emitted light intensity is easily achieved through the modulation of the
injection current of all pump laser diodes, and this, without any detrimental spiking like with diode-pumped solid-state
(DPSS) laser systems.
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