Biomedical Engineering Reference
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photolithographic techniques, depth is controlled via diffusion time, and
change in index of refraction of the waveguide versus the surrounding crys-
tal is a linear function of the deposited Ti thickness.
There is an additional process step involved in the actual mechanism
of diffusion. It has been discovered that Ti does not directly diffuse into
the LiNbO 3 crystal but rather goes through an intermediate oxide phase
before entering [64]. Important intermediate products are TiO 2 , LiNb 3 O 8 and
(Ti 0.65 Nb 0.35 )O 2 [65]. It has been noted experimentally that if the deposited
Ti layer is first oxidized at temperatures of 300°C-500°C for approximately
4 h the resultant waveguides display better performance characteristics with
regard to energy loss due to in-plane scattering [66]. It has been noted that
this oxidation step should be done in an O 2 or dry air atmosphere [67] to pre-
vent the excess stripping of O 2 from the LiNbO 3 crystal, and to prevent the
formation of faults within the crystal [68]. It has been found that following
complete TiO 2 formation a LiNb 3 O 8 phase forms [69]. These two may then
combine to form (Ti 0.65 N 0.35 )O 2 . The latter ternary compound begins to form
at temperatures above 700°C and its formation rate increases up to 950°C.
This compound has been identified as the true source of Ti for diffusion
into the LiNbO 3 crystal, the ultimate compound being (LiNb 3 O 8 ) 0.75 (TiO) 0.25
wherein counter current diffusion of Nb +5 and Li +1 and indiffusion of Ti +4
occur.
The LiNb 3 O 8 appears to be the resultant of Li or LiO 2 outdiffusion, and will
form at temperatures of 650°C < T < 950°C regardless of whether or not Ti is
present. As stated previously outdiffusion of Li or LiO 2 causes changes in the
index of refraction. The uncontrolled outdiffusion of these compounds in Ti
indiffused waveguides is a major concern. This is because if left unchecked
surface waveguides (the equivalent of electrical shorts) could form and limit
the usefulness of the device. A number of studies have been undertaken
[70] on the nature of structural faults which occur during the formation of
Ti:LiNbO 3 waveguides. What is typically noted is that Li does outdiffuse
and Nb molecules take their place in the waveguide portion of the crystal.
Vacancies exist where the Nb had been. The faults are present in undoped
(no Ti) Li deficient waveguides as point defects. In Ti indiffused systems
they tend to form structural faults on the order of tens of microns [71]. Faults
of this size are possible scattering sights for photons and thus their forma-
tion must be controlled. The growth kinetics of LiNb 3 O 8 may be controlled
via annealing of the waveguide in an atmosphere rich in Li in the presence
of O 2 or in an atmosphere of steam [72]. Typical device parameters for the
formation of a Ti:LiNbO 3 waveguide device would require channel widths
of 8-10 μm, a coated metal (Ti) thickness of 0.3-1.0 μm, diffusion tempera-
ture of 980°C-1050°C, and a diffusion time of 4-12 h. This results in diffusion
depths on the order of 5-10 μm. This process is used to form waveguides for
operation in the 1.3-1.55 μm wavelength range where common silica-based
fiber exhibits its best properties. Propagation losses as low as 0.2 dB cm −1
have been noted in waveguides produced via these techniques [73]. After
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