Reverse Exchange

The final step in the process is the reverse exchange. Reverse proton exchange of PE or APE waveguides (see Fig. 4.3, right-most picture) leads to the realisation of buried waveguides. In addition to the creation of a buried waveguide, an ordinary index waveguide is created at the surface by using the PE region as an index barrier. Buried waveguides in PE:LiNbO3 by reverse-exchange were firstly demonstrated by Ganshinet

al.20 In order to reintroduce lithium at the surface, the PE or APE sample is immersed in a lithium rich melt. For this purpose, lithium nitrate is chosen for the melt due to its stability for long-duration usage, but reverse-exchange in LiNO3 alone damages the

surface of the crystal. It was discovered that mixtures of nitrates containing LiNO3 do

not damage the surface of LiNbO3. A mixture of KNO3 and NaNO3 has been used as

an inert carrier for LiNO3 since both substances do not result in measurable changes in

the waveguide.

Reverse-exchange is carried out by immersing the PE:LiNbO3 sample in an eutectic

melt of nitrate mixtures LiNO3:KNO3:NaNO3with a mole percent ratio of 37.5:44.5:18.0

(the melting point of this eutectic mixture is 1200C) at temperatures varying from 2500 to 3300_C.21 _{After the reverse exchange, a buried waveguide with a graded index profile}

near the surface is created but with a reduced refractive index compared to the PE waveguide. Note that at the same time, a layer of pure LiNbO3 is restored on the

surface, and thus restoring the nonlinearity. Reverse-exchange can be applied to both PE and annealed waveguides. The latter is preferred since the additional annealing step allows better tailoring of the refractive index profile.

4.3

Characterisation of Proton Exchange and Annealing

Characterisation of the device used throughout this research work, presented in Sec- tion 4.4 and 4.5, revealed that its quality is still lower than that of the best state-of-the- art devices.2 Therefore, improving the quality of the device would yield more sensitive results than the ones described in Chapter 6 and 7. Complete knowledge of the fab- rication processes would allow one to tailor the waveguide properties at will, in order to produce highly efficient devices.22 In my research work, much work was spent on characterising the diffusion process in the proton exchange and the subsequent anneal- ing processes. Unfortunately, the characterisation of reverse proton exchange process were not completed, mainly due to the lack of time. The characterisation aimed to obtain the refractive index profile evolution of the waveguide during each process, as it is associated with the evolution of proton concentration in the substrate by either a linear or a nonlinear relationship. It was found17,23,24 that there is essentially a linear relationship between these quantities, i.e.

∆n(x)_∝C(x), (4.1)

where ∆n(x) is the refractive index profile of the waveguide, and C(x) is the position dependent proton concentration. Since the refractive index profile evolution is based on the diffusion of the protons into the substrate, the characterisations was simpler if done on slab waveguides. Generalisation into channel waveguides can be readily done by assuming an isotropic diffusion.

β

substrate

θ

ς

input

Figure 4.4: Illustration of prism coupling principle.

4.3.1 Prism Coupling

The main characterisation technique of the fabrication processes was the prism coupling technique,25_{done using a commercial instrument ”Metricon”. Figure 4.4 gives an illus-}

tration of the basic principle of prism coupling technique. A slab waveguide is put in contact with a prism whose refractive index is higher than the waveguide. When light is shone into the prism, it undergoes total internal reflection at the prism base, unless the waves in the prism and in the waveguide are coupled through their evanescent fields. This occurs when the propagation wavevector of the reflected light and the waveguide are equal. Changing the incident angle of the light allows a specific propagating mode to be preferentially excited in the waveguide. Therefore, placing a detector at the other side of the prism allows one to measure the angleθ at which this coupling occurs. The effective refractive index of propagating modeq of the waveguide is given by:

N(q) =npsin ς+ arcsin sinθ np , (4.2)

whereϕis the base-angle of the prism,θis the incident angle of the beam with respect to the normal of the prism, andnp is the refractive index of the prism. If the refractive

index analytical profile is known, the indices can be used to calculate the parameters of the profile, as shown in Fig. 4.5. Otherwise, an inverse-Wentzel-Kramers-Brillouin (IWKB) method can be used to reconstruct the profile, as discussed later.

A rutile (TiO2) prism with a base angle of 600 was used in the experiment. A

Helium-Neon laser with a wavelength of 632.8 nm was used as the source. The extraor- dinary and ordinary refractive indices for rutile at this wavelength werene= 2.865 and

2.22 2.24 2.26 2.28 2.3 2.32 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Layer thickness (µm) Eff ecti v e R e f racti v e I n d e x 0 0.88 3.53 7.95 14.14 22.09 31.81 43.43 56.00 Exchange Time (hours) q=0 q=1 q=2 q=3

Figure 4.5: Measured effective refractive indices of proton-exchanged waveguides (blue

dots) and theoretically fitted dispersion curves (solid lines) of a slab LiNbO3 waveguide

with a simple step index profile with a refractive index increase of ∆ne = 0.128. The

exchange time is shown by the bottom abscissa, whilst the estimated proton exchanged layer thickness by the top abscissa.

no = 2.584, respectively. The prism and the waveguide were configured in such a way

that the optic axis of the prism is perpendicular to the optic axis of the waveguide, so that the ordinary (extraordinary) refractive index of the prism was used for measuring the transverse magnetic (transverse electric) effective refractive indices.