Lecture_18

Lecture_18 - 8. Modulators Modulators are used to encode...

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1 Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Modulators are used to encode information on lasers and for switching in optical networks Optical modulators can change transmission or refection of a light beam, or, re-route the light beam along a different path (in some cases), by applying a voltage to the modulator The major advantage of independently modulating a cw laser is to avoid "chirping" (imposing a frequency sweep on the beam as power changes). This also allows a single optical power source to be used for a large number of information-carrying beams. Two categories Absorptive Change amount of incident light absorbed in the structure Refractive Change optical path length of a beam which relies on wave interference or propagation to thereby change transmission, reFection, or direction of beam 8. Modulators Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Capable of high speeds, even into sub-picosecond range Relatively small e.g. waveguides a few hundred microns long Advantages over some other waveguide modulators (e.g., lithium niobate electro-optic modulators) • Smaller and lower power • Can be integrated directly with other optoelectronic devices (e.g., III-V based lasers, optical ampli±ers, and photodetectors) Disadvantages losses can be higher, both from coupling into semiconductor waveguides, and from propagation within the waveguides. Notable non-waveguide modulator is the quantum well absorption modulator, which allows useful modulation in ~ 1μm of material. This enables modulators that work for light propagating perpendicular to the surface. Semiconductor modulators
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2 Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Two main mechanisms • Franz-Keldysh effect • Quantum-Con±ned Stark Effect (QCSE) Both of these so-called "electroabsorption" effects are seen near the bandgap of semiconductors when electric ±elds are applied Franz-Keldysh effect is seen in conventional, bulk semiconductors QCSE is seen only in quantum well structures (QCSE is actually the quantized version of the Franz-Keldysh effect) Absorptive modulators Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Concept of Franz-Keldysh effect (at least as originally conceived) solutions for electron and hole wavefunctions with constant ±eld are Airy functions. Wavefunctions now "tunnel" into the bandgap region allowing overlap of electron and hole wavefunctions even for photon energies less than the bandgap energy, hence allowing optical absorption below the bandgap energy. Franz-Keldysh effect
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3 Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Absorption spectrum now has a "tail" at lower energies and some oscillations above the bandgap Franz-Keldysh effect (2) Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Effects of excitons are, however, important and possibly dominant in spectral region just below the bandgap The change magnitude of absorption and introduce peaks in
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This note was uploaded on 06/05/2010 for the course EE 243 taught by Professor Harris,j during the Winter '10 term at Stanford.

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Lecture_18 - 8. Modulators Modulators are used to encode...

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