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In the optical mode two adjacent different atoms move against each other, while in the acoustic mode they move together. The speed of propagation of an acoustic phonon, which is also the speed of sound in the lattice, is given by the slope of the acoustic dispersion relation, ∂ωk∂k (see group velocity.)
An “acoustic” phonon is a vibration in the crystal lattice where all moving components are the same mass. So if you take the crystal’s unit cell as a single object, the three different ways the unit cells can move w/r/to each other (two transverse one longitudinal) are the acoustic phonon modes.
Transverse modes occur in radio waves and microwaves confined to a waveguide, and also in light waves in an optical fiber and in a laser’s optical resonator. Transverse modes occur because of boundary conditions imposed on the wave by the waveguide.
The difference between TE and TM mode is that TE stands for transverse electric mode while TM stands for transverse magnetic mode. TE mode is also known as H mode as there is only a magnetic field along the direction of propagation.
Waveguides have certain optical field distributions that stay constant during propagation, except for a change in the overall phase and possibly the optical power. Such field distributions, which depend on the refractive index profile, correspond to so-called waveguide modes.
5. In a waveguide, always which condition holds good? Explanation: In air medium, the phase velocity is assumed to be the speed of light. For waveguides, the phase velocity is always greater than the speed of the light.
The waveguide acts as a high pass filter in that most of the energy above a certain frequency (the cutoff frequency) will pass through the waveguide, whereas most of the energy that is below the cutoff frequency will be attenuated by the waveguide.
In view of the mechanical constraints this means that waveguides are only used for microwave frequencies. Although it is theoretically possible to build waveguides for lower frequencies the size would not make them viable to contain within normal dimensions and their cost would be prohibitive.
The cutoff frequency of an electromagnetic waveguide is the lowest frequency for which a mode will propagate in it. In fiber optics, it is more common to consider the cutoff wavelength, the maximum wavelength that will propagate in an optical fiber or waveguide.
A rectangular waveguide supports TM and TE modes but not TEM waves because we cannot define a unique voltage since there is only one conductor in a rectangular waveguide. A rectangular waveguide cannot propagate below some certain frequency. This frequency is called the cut-off frequency.