Fiber Optic Tutorials

This is a continuation from the previous tutorial - acousto-optic modulators. The acoustic wave of an acousto-optic deflector is frequency modulated. Unlike an acousto-optic modulator, which is an amplitude modulator, an acousto-optic deflector is a frequency modulator, which allows its acoustic frequency to be varied electronically. Acousto-optic deflectors have many applications. A frequency shifter, which has the sole purpose of generating a diffracted optical beam at an optical frequency shifted by the amount of the acoustic frequency from the input optical frequency, can be considered as the simplest form of an acousto-optic deflector. Acousto-optic deflectors are also used in such...

This is a continuation from the previous tutorial - acousto-optic diffraction. The acoustic wave of an acousto-optic modulator is amplitude modulated. The operation of an acousto-optic modulator is based on the dependence of the acousto-optic diffraction efficiency on the intensity of the acoustic wave. The acoustic intensity can be controlled by an electrical signal that generates the acoustic wave in a modulator. An acousto-optic modulator is an electronically addressed amplitude modulator that accepts an electrical modulation signal to vary the intensity of an optical beam accordingly. Acousto-optic modulators have been put to many different applications. The straightforward application is amplitude...

This is a continuation from the previous tutorial - photoelastic effect. We see from the preceding two tutorials that the space- and time-dependent periodic permittivity changes induced by a traveling plane acoustic wave of the form given in (8-1) [refer to the elastic waves tutorial] can be generally expressed as \[\tag{8-34}\Delta\boldsymbol{\epsilon}=\Delta\tilde{\boldsymbol{\epsilon}}\sin(\mathbf{K}\cdot\mathbf{r}-\Omega{t})\] where \(\mathbf{K}\) depends on both the polarization and the propagation direction of the acoustic wave. In general, \(\Delta\tilde{\boldsymbol{\epsilon}}\) is a function of the strain and the rotation generated by the acoustic wave in the medium, the elasto-optic coefficients of the medium, the mode and direction of the acoustic wave,...

This is a continuation from the previous tutorial - elastic waves. Mechanical strain in a medium causes changes in the optical property of the medium due to the photoelastic effect.  The basis of acousto-optic interaction is the dynamic photoelastic effect in which the periodic time-dependent mechanical strain caused by an acoustic wave induces periodic time-dependent variations in the optical properties of the medium. The photoelastic effect is traditionally defined in terms of changes in the elements of the relative impermeability tensor caused by strain: \[\tag{8-7}\eta_{ij}(\mathbf{S})=\eta_{ij}+\Delta\eta_{ij}(\mathbf{S})=\eta_{ij}+\sum_{k,l}p_{ijkl}S_{kl}\] where \(p_{ijkl}\) are dimensionless elasto-optic coefficients, also called strain-optic coefficients or photoelastic coefficients, and they...

This is a continuation from the previous tutorial - guided-wave magneto-optic devices.   Introduction Scattering of light by acoustic waves was first investigated by Brillouin. The acoustic frequencies involved in Brillouin scattering fall in the ultrasonic and hypersonic regions. Hypersonic waves in a medium are caused by thermal excitation, whereas ultrasonic waves can be excited electronically using piezoelectric transducers. The acoustic waves used in acousto-optics are generally ultrasonic waves that have frequencies in the range between about 100 kHz and a few gigahertz. The basic principles of acousto-optic devices are based on the scattering of light by the periodic index...