Transparent Oxyfluoride Glass Ceramics
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G 4 level of Pr 3 + that emits at 1310 nm. The rate of nonradiative decay due to phonons WMp(T) can be expressed as
exp(hw (2) where WMp(0) is the low-temperature multiphonon emission rate, k is Boltzman's constant, T is absolute temperature, hu> is the maximum phonon energy of the host, and p is the integer number
25
20
1 I
o o o o
350 ran Pump
15
1310 nm Emission
5.
I 10 LU
u
-1014 ^ _ Pump
Visible Emission
Decay via Phonons 1014 nm Pump
> >
:3
-
H6
1
G4
3
F4
3
H6
3
H5
3
H4
;
(1)
where the reduced mass μ = mAm-e,/ (mA + mB). Thus heavy elements with weak bonding will provide the lowest phonon-energy glasses. Unfortunately the weak bonding also implies poor glass stability, strength, and durability. Table I lists the phonon energies for various glass systems and the quantum efficiency (number of phonons out per electron excited to the : G 4 level) of the
MRS BULLETIN/NOVEMBER 1998
J
of phonons required to bridge the energy gap AE between the level of interest and the next lower lying level (p = AE/(ft&>). Table I shows that heavy-metal fluoride glasses such as ZBLAN (53ZrF4 • 20BaF2 • 4LaF 3 • 3A1F3 • 20NaF) and PIGLZ (43PbF2 • 17InF3 • 17GaF, • 4LaF3 • 19ZnF2) have half the maximum phonon energy of silicates and thus take twice as many : 3+ phonons to quench the G 4 level of Pr . Fluoride glasses have been demonstrated as 1.3-/am amplifiers but require a high pump power because of their low quantum efficiency. Fluoride glasses are also very expensive, toxic, corrosive and unstable, and must be processed in a dry oxygen-free atmosphere. In addition they have poor durability and are not fusion-spliceable to conventional telecommunications fiber (all of which are SiO2-based), which gives rise to device reliability issues that are inhibiting their use in the telecommunications network. Oxyfluoride glass ceramics can offer the best of both worlds: the low phonon energy of a fluoride and the durability and mechanical properties of an oxide glass. In a properly engineered oxyfluoride glass ceramic, the active ion will partition into the low-phonon-energy fluoride crystals that form upon heat treatment. The crystals must be small
3
H6 >1
•3HS 3
• H4
Figure 1. Energy-level diagram of Pr3+ showing pumping and emission scheme of (a) 1310-nm fluorescence and amplification, (b) nonradiative decay via phonons, and (c) two-frequency up-conversion.
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Transparent Oxyf luoride Glass Ceramics
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