TE and TM modes only exist in certain waveguides with mostly homogeneous dielectrics.  Otherwise they are hybrid modes and contain all six field components.  In dielectric waveguides the modes are mostly linearly polarized and analysis can be approximated this way, although it is not strictly true.  These are usually called LP modes.  Your optical source is probably linearly polarized.  If you excite a waveguide with this linear polarization aligned at 45 degrees with respect to the waveguide, half of your power will go into one LP mode and half into the other.  The important thing to consider is whether the waveguide actually supports both LP modes.  If not, half your source power will go into one guided mode and the other half of your power will escape.  There is also the losses associated with butt coupling.  If the mode from your source does not look similar to the mode in the waveguide or it is not well aligned, you will get some scattering losses as well.

Hope this helps!

Thank you Dr. Raymond.

Yes the source is linearly polarized and structure is oxide/nitride/oxide. This is a symmetric waveguide with 1.46/1.92/1.46 refractive index and 1.5um/0.2um/1.5um thickness respectively.

I didn't find the literature for LP modes in planar waveguide. LP modes are usually associated with optical fiber.

Thanks for the light :-) I will search for literature.

I don't think optical waveguides outside of fibers are discussed in terms of LP modes.  Not sure why, but if you model some waveguides rigorously, you will see that they are very strongly linearly polarized.  See the field plots on Slide 31 here:

http://emlab.utep.edu/ee5390cem/Lecture%2011%20--%20Finite-Difference%20Analysis%20of%20Waveguides.pdf

You calculate the overlap integral between your input mode and modes in the waveguide. The result tells you how much of your input will be coupled to how much of each mode in the waveguide. If the waveguide supports TE and TM, the linearly polarized light at 45° excites both about equally, but we need the calculation done for more accurate number.

First of all,

As Dr. Dr. Raymond mentioned there exists hybrid modes. But under certain conditions, homogeneous medium and weakly guiding approximations (n1~n2) one can say these modes are Transverse modes.

For the planar wave guides, these are TE and TM modes not LP modes.

By virtue of the polarization azimuth is at 45, both, TE and TM modes will exists with equal power in each provided perfect alignment of the source polarization.

The other thing is, check whether your wave guide support both the modes or not. You have to check the V parameter of the wave guide. As u mentioned the refractive indices of your wave guide, the difference in refractive indices is very large ~0.45. The propagation constants of TE and TM modes may differ. You have to be very careful in analyzing the modes of the structure.

More fundamental problems and solutions are discussed in 7th chapter of “Introduction to fiber optics by A. Ghatak and K.Thyagarajan”.I hope you may get more details in this.

Good Luck

It is an essential feature of the sensor that two orthogonally polarised modes are excited (preferably equally) and propagate through the waveguide.  These are referred to as s- and p- polararised field components in the paper.

The evanescent fields of the two modes interact differently with the liquid medium in contact with the waveguide and with molecules adsorbed onto the surface. A differential change in their propagation constants leads to a phase difference at the output.  The resultant change in polarisation state of the output beam is measured using a linear polarizer between the output facet and a fibre-coupled photomultiplier.

Thank you Mr. Bhargava Ram for literature suggestion and Mr. Alan Robinson for make it simple.

Cheers

At normal incidence, the linear polarization at 45 °, equivalent to the sum of both TE and TM polarizations alone. The effect is the same in a waveguide. It is as if we injected polarized wave linearly polarized along the x axis and then along the y axis at the same time.