Integrated Optics - Applied Optics Fundamentals and Device Applications - page 256

diffusion the crystal end faces are polished to allow for coupling to a fiber.
Waveguide devices for operation at a wavelength of 2.6 μm for use with the
ultra low-loss fluoride based glass fiber have also been developed [74]. For
these devices the optimal channel width is 13-15 μm, the required thickness
is 0.8 μm, and the diffusion conditions are 1050°C for 12 h.
5.9.3 Implantation and Proton Exchange Techniques
A great deal of research continues with the goal of supplementing or replac-
ing Ti diffusion. One technique is to ion implant Ti into the LiNbO 3 crystal.
The waveguides regions are defined lithographically and implanted. This
causes an amorphous layer to form in the waveguide regions. Solid phase
epitaxy is then used starting from the undamaged underlying LiNbO 3 sub-
strate in order to restore crystallinity and produce a functional waveguide.
This process produces good waveguides with higher Ti concentrations and,
therefore, greater changes in index of refraction than are possible via ther-
mal diffusion [75]. It also promises sharper geometries and tighter Ti concen-
tration gradients than are otherwise possible.
Another means for altering the index of refraction of LiNbO 3 is proton
exchange. In this process the crystal (with defined waveguide regions) is
submersed in a solution of benzoic acid at a temperature of 210°C-245°C for
1-4 h. The benzoic acid is the proton (H + ) source and the diffusion of protons
into the substrate causes the index of refraction to rise in those areas [76]. The
process has the advantage of being a simple low temperature process, and
is capable of achieving greater changes in the index of refraction than with
Ti indiffusion. The drawback is that devices fabricated in this manner typi-
cally cannot achieve the same performance levels (losses) as LiNbO 3 devices.
Annealing after the proton exchange improves the optical quality but also
causes a drop in the achieved index of refraction.
Another exciting fabrication technique is known as Titanium Indiffused
Proton Exchange (TIPE). This technique has the capability to produce devices
not possible by either TI of PE processes alone. In TIPE a standard Ti indif-
fusion process is first applied to the LiNbO 3 substrate to define some regions
with higher indices of refraction. The piece is then masked with a suitable
image and immersed in a benzoic acid solution. This results in the areas of
the crystal exposed to both Ti indiffusion and PE having relative indices of
refraction considerably greater than achieved by either process alone. The
most important and immediate application for this technology is in the fabri-
cation of planar waveguide microlenses [77] (Figure 5.23a and b) and micro-
lens arrays. Desirable properties of TIPE lenses include: very short focal
lengths, large numerical apertures, small focal spot size, and low insertion
losses [78]. The utility of these TIPE lenses has already been demonstrated
via the production of an integrated acousto-optic Bragg modulator module
on a LiNbO 3 substrate. Some preliminary testing utilizing the modulator
for optical systolic array processing has also been undertaken [79]. These