1. Modulators and Semiconductors ERIC MITCHELL

2. Acousto-Optic Modulators Based on the diffraction of light though means of sound waves travelling though a median The quartz crystal has a piezoelectric transducer attached at the end that propagates strong acoustic waves within the crystal. acoustic waves generated by applying an RF signal to the transducer Common Materials Lithium Niobate LiNbO 3 Tellurium Dioxide TeO 2 KTP – Potassium Titanyl Phosphate KTiOPO 4 Lead Molybdate Glass Germanium Quartz

3. Acousto-Optic Modulators

4. Semiconductors Basic Characteristics: Small energy gap between valence and conduction band (~1.1eV) Insulating material have a very large energy gap (~5.0 eV) Semiconductors allow for control of conductivity with impure atoms called dopants Electrons can easily be excited to conduction band leaving behind holes Absorption of light increases strongly with frequency

5. Semiconductors Laser diode: Electrical bias across the laser diode causes electrons and holes to be injected from opposite sides of the p-n junction into depletion region If electron and holes appear in the same region they may recombine and result in spontaneous emission The difference between the photon emitting semiconductor laser and the photon emitting (nonlight emitting) semiconductor junction diodes lies in the use of different types of semiconductors These are the "direct bandgap" semiconductors Examples: Gallium arsenide Indium phosphide Gallium nitride

6. Semiconductors Photodiodes: Capable of detecting radiation energy hv > E g Photodiodes have an intrinsic high resistivity (i) region in which most of the potential drop occurs Photons are absorbed in the intrinsic high resistivity (i) region Carriers can then efficiently contribute to the photo current Thicker depletion region = more efficient collection of carriers =larger quantum efficiency = higher detection bandwidth Most p-i-n junctions are based on silicon

7. Semiconductors Laser amplification: When excited electrons in the active region return to ground state, amplification occurs R 1 and R 2 are responsible for the optical feedback in amplifier Advantages: Integrated optical amplifiers are smaller in length than fiber amplifiers Possibility of very large gains ( >20db) in short semiconductor chip ( <400 micrometers) Disadvantage: The presence of residual reflections and the resulting need for optical isolators reflections can give rise to instabilities and noise

8. Electro-Optic Modulation Combination of properly oriented polarizers and birefringent crystals to control the amount of light transmitted in a optical system The electro-optic effect: When a voltage is applied across a crystal it induces birefringence Birefringence crystal: an incident light rays will separate into two rays and scatter in different directions. Birefringence will be increasing function of applied voltage Transmission will be a an oscillatory function of applied voltage Consists of rotating retardation plate example: “Quarter wave” (Γ = π/2)

9. Electro-Optic Modulation

10. Intensity modulation: can be achieved using crossed polarizers or by means of interference the interference of modulated and unmodulated light can be achieved with a waveguide interferometer. i.e. Mach-Zehnder Modulator high speed telecommunication employ the use of a phase modulator in conjunction with a Mach-Zehnder Interferometer

11. References Yariv, Optical electronics Oshea, Rodes An Introduction to Laser and their Applications Silfvast, T. William Laser Fundamentals Simcick, John ELECTRO-OPTIC AND ACOUSTO-OPTIC DEVICES What When how, Optical Time-Division Multiplexed Communication Networks Part 5