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ENE 623 Optical Networks

Learn about the different types of semiconductor photodetectors, their performance characteristics, and methods to improve their signal-to-noise ratio. Explore the impact of quantum noise, thermal noise, dark current, and avalanche multiplication noise on non-ideal optical receivers.

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ENE 623 Optical Networks

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  1. ENE 623 Optical Networks Lecture 9

  2. Semiconductor Photodetectors • There are three material systems used: • Si for λ 1 μm • InGaAs for λ 1 μm • Ge for λ 1 μm

  3. Semiconductor Photodetectors • There are two types of photodiodes: • PIN (most used & without internal gain) • Avalanche Photodiode (with internal gain)

  4. Non-ideal Optical Receivers • Consider effects of quantum noise, thermal noise, dark current, and avalanche multiplication noise (if applied).

  5. Non-ideal Optical Receivers

  6. Non-ideal Optical Receivers • We then could find a signal-to-noise ratio as:

  7. Non-ideal Optical Receivers • Consider effect of avalanche gain M • M = 1 for ‘PIN’ and M > 1 for APD.

  8. Non-ideal Optical Receivers • Consider thermal noise power = 4kTB • k = Boltmann’s constant = 1.38 x 10-23 J/K • T = absolute temperature

  9. Non-ideal Optical Receivers • “RC limited bandwidth” • ‘thermal noise dominates at high bandwidth/data rate’.

  10. Non-ideal Optical Receivers • “RC limited bandwidth” • ‘thermal noise dominates at high bandwidth/data rate’.

  11. Non-ideal Optical Receivers • Relate signal current to an optical incident power on photodetector.

  12. Non-ideal Optical Receivers

  13. Example • If λ= 1.55 μm and quantum efficiency η = 0.5. What is Is in term of Pout?

  14. Example • If the receiver has following parameters: PIN photodiode, B = 500 MHz, Is = 1 μA, Id = 10 nA, T = 293K, R = 50Ω. Find the SNR.

  15. Non-ideal Optical Receivers • How to improve the SNR. • Increase load resistor

  16. Non-ideal Optical Receivers • How to improve the SNR. • Use photodetector with gain

  17. Example • A photodetector with unity internal gain and no thermal noise has a dark current of 1 x 10-13 A. The photodetector is used for digital transmission at a rate of 500Mbps so that a bit interval has a duration of 2ns. • On the average, how many dark current electrons pass through the load resistor during a bit interval?

  18. Example (b) What is the smallest threshold value of electrons per bit interval for which the error rate will be less than 10-5 when a ‘zero’ is transmitted?

  19. Example (c) Using the threshold value for number of electrons per bit interval determined in (b), what is the average number of photoelectrons per bit interval for which that same error rate is achieved when a ‘one’ is transmitted? A tolerance of  0.2 electrons per bit interval is allowed for a ‘correct’ answer.

  20. Example (d) What is the average optical power in W incident upon the photodetector required to achieve a bit error rate of 10-5, assuming an optical wavelength of 0.85 μm, equal numbers of ‘ones’ and ‘zeroes’ are transmitted, and photoelectrons are produced by 23% of the incident photons.

  21. Example • Consider a non-ideal optical receiver, with unity internal gain in which thermal noise is the dominant noise source. The operating temperature is 300K. For a binary data transmission rate of 1 Gbps, and a photodetector capacitance of 1 pF, how many photoelectrons per bit are needed on the average for a bit error rate of 10-10? Assume that B = 1/(2T), where B is the receiver bandwidth and T is the bit time interval.

  22. Example

  23. Data Transmission Standards • Original standard for digital communication is based on coaxial cable/twisted wire pairs (introduced in early 1960’s by AT&T/Bell telephone system). • T1 or DS-1 is then introduced in pulse code communication format.

  24. Data Transmission Standards • T1 or DS-1 is then introduced in pulse code communication format. • DS-0: 64 kbps (one 8 ksamples/s x 8 bits/sample voice channel) • DS-1: 24 DS-0 + 8 kbps = 1.544 Mbps. • DS-2: 6.312 Mbps  96 voice channels. • DS-3: 44.736 Mbps  672 voice channels.

  25. Data Transmission Standards

  26. Data Transmission Standards • SONET – Synchronous Optical Network: international standard developed during 1980’s for high data rate fiber optic transmission.

  27. Data Transmission Standards • SONET – Synchronous Optical Network: international standard developed during 1980’s for high data rate fiber optic transmission. • SONET is robust in that redundant path are provided to maintain transmission after node or cable failure called “self-healing”. • SONET combines, consolidates, and segregates traffic from different locations through same ‘hardware’ machine.

  28. Data Transmission Standards • ATM – Asynchronous Transport Mode • Time axis divided into 125 μs frames. • Each frame contains many 53-octet cells.

  29. Data Transmission Standards • ATM supports voice, video, and data over one medium and platform. • ATM supports LAN, MAN, and WAN with one platform. • ATM cells can be combined or multiplexed for transmission over SONET/SDH. • Note: SDH (Synchronous Digital Hierarchy) – compatible with SONET primarily used in Europe.

  30. Long-haul (intercity, transoceanic) • SMF has been cost-effective medium for: • 1983: new long-distance installations. • 1986: transoceanic systems. • 1988: transoceanic systems. • Initially, installations were at 1.3 μm. Presently, new installations are designed for 1.53 μm to take advantage of lower fiber loss (0.2 – 0.25 dB/km) and Er-doped fiber amplifiers. • These systems use dispersion shifted fiber.

  31. Long-haul (intercity, transoceanic) • Light source: DFB InGaAsP laser. • Modulation: direct modulation, LiNbO3 external modulator, electroabsorption external modulator. • Receiver: InGaAs PIN or APD (M = 20-50) • Fiber optics used for point-to-point transmission of time-division-multiplexed binary information.

  32. Long-haul (intercity, transoceanic) • Time division multiplexing, demultiplexing, switching, routing, buffering are all done electronically. • Trends: Within the past years, WDM has been being employed with channel spacing of 100 GHz (0.78 nm) or 200 GHz (11.56 nm).

  33. Cable TV • Use SMF for analog signal distribution (coaxial cable from distribution modes to homes). • Trends: Fiber-to-the-home (FTTH) and WDM will play a key role in future cable. • HDTV favors digital distributions and optical fibers.

  34. LAN • Both single-mode and multimode fiber are used for computer networks, industrial, and university campuses. • Optical fibers are mainly used for point-to-point transmission. • Trend: More SMF, star buses, and WDM will play important role.

  35. FTTH • Services to homes: telephone, picture, data, entertainment on demand, and etc. • Passive optical networks (PONs) with cascade stars are used. • More Competition between traditional telephone companies and cable TV companies. • Installed fibers to the curb will be extended to homes as demand for broadband services ncrease.

  36. Analyzing Optical Networks • System requirements: Data rate, SNR (analog), BER (digital), number of nodes, maximum length between nodes. • Component specifications: • Light source: output power, wavelength, bandwidth. • Fiber: attenuation and dispersion. • Receiver: quantum efficiency, capacitance, dark current, and gain. • Others such as couplers, connectors, filters, and etc. • Optical power budget.

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