**2. Basic principles of IO for interferometric sensing**

The basic element of any Integrated Optic device is the optical waveguide that can be generated by tailoring the refractive index (n) in the near-surface region of the base material. It must be remembered that the light transmission is confined in the regions where the refractive index is higher than in the surroundings.

Several techniques have been developed to obtain local variation of the refractive index creating the possibility to obtain an integrated optical waveguide. In this chapter, we will only mention some of the most common techniques used for LiNbO3 substrate, which is one of the most used materials in optical device fabrication. One possibility is to use local doping processes obtained by photolithographic definition of the desired waveguide geometry associated with the dopant thermal diffusion (typically, Ti diffusion or Proton exchange processes), to increase the refractive index in the doped region. Alternatively, it is possible to create a waveguide by lowering the refractive index of the base material in the regions outside the waveguide introducing lattice damage through ion bombardment. A third possibility is to pattern the surface with the desired geometry, and etching the surrounding region to obtain a ridge waveguide protruding from the surface. **Figure 1** reports a sketch of the two geometries.

In general, the geometries of the integrated microsystems are designed to reproduce the same physical effects obtained by optical instrumental architectures created in laboratory, assembling several optical elements like mirrors, beam splitters, etc.

*Integrated Optics and Photonics for Optical Interferometric Sensing DOI: http://dx.doi.org/10.5772/intechopen.103770*

**Figure 1.**

*Two basic geometries of integrated optics waveguides: (a) buried waveguide, (b) ridge waveguide.*

Probably, the most frequently used integrated micro-spectrometric devices are based on the Mach-Zehnder Interferometric, (MZI), geometry [1–4] or on Young Interferometer, (YI) or, more recently, on the Staircase Micro Diffractive Gratings, (MDG) first developed by Michelson [5].

All these devices take advantage of the Electro-Optic properties of the LiNbO3 substrate. In fact, the optoelectronic properties of the substrate allow to locally controlling the refractive index of a waveguide by applying a suitable electric field, so creating a Pockels cell that induces a phase modulation in the light transmitted in the specific waveguide (See **Figure 2**).

In particular, it can be useful to recall that, in the case of spectroscopic analysis techniques, the wavelength-dispersive systems, such as prisms or gratings, spatially spread the light wavelengths at different angles allowing the direct measurements of the relative intensities, (wavelength spectrum), by using suitable photodetectors at the corresponding angles.

On the contrary, in the Fourier Transform Spectroscopy the intensity of the total light beam that contains the whole ensemble of wavelength, at the same time, is measured. In this case, the measurement with a traditional Mach-Zehnder instrument is performed by splitting the light beam into two branches that are then recombined giving rise to an interference pattern. The light intensity of the recombined beams is

**Figure 3.**

*Upper: Traditional Mach-Zehnder interferometer geometry. The light beam first crosses the BS1 beam splitter then, through the M1 and M2 mirrors, the two light beams are recombined in BS2 and then arrive in the photodetector PD. lower: Equivalent integrated optic device: The voltage applied to the electrodes controls the phase shift in the light beams propagating in the two arms.*

measured as a function of the phase shift, generated by the respective different optical path-lengths *l = nL*, where n is the refractive index and *L* is the geometrical path length.

**Figure 3(a)**, reports a typical example of Mach-Zehnder equipment, whereas **Figure 3(b)** is shown the equivalent device fabricated with planar technology. Following the optical interference laws, the intensity measured by the photodetector (PD) depends on the phase shift between the two optical paths. Inducing a variation in the optical paths, *l*, of one of the two arms the phase shift changes and, in turn, the correspondent variation of the light intensity is measured by the PD.
