Paper Review Laser & Photonics Reviews, 2009 I. Photodiode review II. Motivations III. Principle of UTC PD IV. Design of UTC PD V. Applications Special Topics in Optical Engineering II (15/1) Minkyu Kim
Photodetection β’ Photodetection -Produces current when βπ > πΉ π -Different absorption coefficient with different material( πΉ π ) π½ -R(Responsivity) = π Special Topics in Optical Engineering II (15/1) Minkyu Kim
Phototectors Photoconductor PN junction photodiode PIN junction photodiode βπ β’ β’ β’ Easy to make Reverse bias Reverse bias β’ β’ β’ Slow Small dark current Solve efficiency problem β’ β’ Dark current Drift in depletion region of PN junction PD β’ Thin depletion region ο¨ bad efficiency Special Topics in Optical Engineering II (15/1) Minkyu Kim
Demands for High Power RF PD β’ Power amp can be followed by PD ο¨ Only when amplifier is available at frequencies of interest β’ High frequency optical signals ο¨ High speed PD required Special Topics in Optical Engineering II (15/1) Minkyu Kim
Principle of UTC PD (a) UTC PD (b) PIN PD * UTC : Uni Traveling Carrier β’ Light absorption layer(p-type) + Carrier collection layer(transparent) β’ Generated hole ο¨ respond very quickly within in the dielectric relaxation time β’ Generated electron ο¨ exhibit velocity overshoot in carrier collection layer (Condition : quasi-field in the absorption layer by doping grading, bias for overshoot velocity) β’ Electron diffusion time mainly determines the operation speed Special Topics in Optical Engineering II (15/1) Minkyu Kim
Pulse Response (a) UTC PD (b) PIN PD β’ Current tails observed due to the slow response of holes in PIN PD β’ No current tails in UTC PD due to the fast response of holes Special Topics in Optical Engineering II (15/1) Minkyu Kim
Bandwidth β’ Conventional PIN PD -Trade-off between transit time vs RC time ππ πππ‘ππ’ π’πππ πππππ’ππ πΆπ β 1 π ππ· π’πππ πππππ’ππ πΆπ β 1 ππ· β π π : Thickness of absorption layer β’ UTC PD -No trade-off in thickness of layer due to diffusive electron transport in absorption layer ο¨ Thinner absorption layer increases BW Special Topics in Optical Engineering II (15/1) Minkyu Kim
Output Saturation Current (a) UTC PD (b) PIN PD β’ Photogenerated carriers(electron & hole) are stored in the absorption layer in conventional PIN PD ο¨ Decrease E field ο¨ Current saturation β’ Only electrons whose velocity at overshoot stored in UTC PD ο¨ Current saturation occurs at much higher intensity Special Topics in Optical Engineering II (15/1) Minkyu Kim
Structure Design Distributed Waveguide(WG) PD Refracting Facet(RF) PD Surface(vertical) illuminated PD Waveguide-fed Waveguide(WG) PD evanescently coupled PD Distributed Waveguide-fed evanescently coupled PD Improve responsivity while maintaining BW Improve responsivity & BW Special Topics in Optical Engineering II (15/1) Minkyu Kim
Circuit Design β’ Integration with a short-stub matching circuit ο¨ Increase output power, bandwidth Special Topics in Optical Engineering II (15/1) Minkyu Kim
Applications β’ High-speed wireless communications - Promising technique for βuncompressedβ HDTC broadcasting -UTC PD can make transmitter core very compact and light-weight Schematic diagram of wireless link β’ Spectroscopic measurement systems -Using photonic LO is best combination due to low noise Experiment setup for spectroscopic system Special Topics in Optical Engineering II (15/1) Minkyu Kim
Conclusion β’ High power RF photodetector required in microwave photonics β’ UTC PD -Larger bandwidth -Higher saturation current -Better pulse response β’ Structure & circuit design ο¨ Improvement of responsivity & bandwidth β’ Microwave photonics applications Special Topics in Optical Engineering II (15/1) Minkyu Kim
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