Ch11Sec2

Ch11Sec2 - Dr Joseph C Palais 11.2 1 THE FIBER FORUM Fiber...

Info iconThis preview shows pages 1–13. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Dr. Joseph C. Palais 11.2 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY Dr. Joseph C. Palais 11.2 2 Section 11.2 Signal-to-Noise Ratio Dr. Joseph C. Palais 11.2 3 Consider the equivalent circuit of a photodiode receiver. R d v i S C d C d = diode’s junction capacitance (small) R d = diode’s junction resistance (large) R s = diode’s bulk series (n and p) resistance (small) R s Dr. Joseph C. Palais 11.2 4 i S is the photocurrent. As before, it is given by i s = ( η eP/hf) = ρ P For simplicity, assume R s = 0 and R d = infinite. Also neglect C d for purposes of noise calculations, since it does not affect the noise in the circuit. The simplified receiving circuit, including all sources of thermal and shot noise is now: Dr. Joseph C. Palais 11.2 5 R L i S i 2 NT i 2 NS We will use this circuit to compute SNR. Dr. Joseph C. Palais 11.2 6 11.2.1 Constant Power SNR Let the incident optical power P be a constant. This corresponds to a binary 1 in a digital system. Compute the SNR. SNR = average signal power / average noise power These are the electrical powers. From the equivalent circuit, we see that SNR = (R L i 2 S ) / (R L i 2 NS + R L i 2 NT ) SNR = P ES / (P NS + P NT ) SNR = i 2 S / ( i 2 NS + i 2 NT ) Dr. Joseph C. Palais 11.2 7 These equations are general. For the special case where P = a constant: i S = i S = ( η e/hf)P = ρ P P ES = R L i 2 S = ( η e P/hf) 2 R L P NT = R L i 2 NT = (4kT ∆ f /R L ) R L = 4kT ∆ f P NS = R L i 2 NS = 2e[ I D + ( η e P/ hf)] ∆ f R L Then [( η e P/ hf) 2 R L ] {2e[ I D + ( η e P/ hf)] ∆ f R L }+ 4kT ∆ f SNR = (11-8) Dr. Joseph C. Palais 11.2 8 Special cases: Case 1 : signal current >> dark current i s >> I D and shot noise >> thermal noise 2e ∆ f ( η e P/hf) >> (4kT ∆ f /R L ) Then, the SNR equation simplifies to: SNR = [( η e P/hf) 2 R L ] / [2e ∆ f R L ( η e P/hf)] SNR = η P/2hf ∆ f (11-9) This is called the shot-noise limited SNR or the quantum-noise limited SNR . Dr. Joseph C. Palais 11.2 9 This is a very good result (high SNR). It is usually not the actual result, because P is not usually large enough to make the assumptions leading to it valid. If P is large, we have no SNR problems. Since i S = ( η eP/hf) the shot-noise limited SNR (11-9) can be rewritten as SNR = i S /2e ∆ f (11-10) Dr. Joseph C. Palais 11.2 10 Case 2: thermal noise >> shot noise In this case the SNR (11-8) becomes: SNR = [( η eP/hf) 2 R L ] /4kT ∆ f (11-11) This is the thermal-noise limited SNR. It is valid when the received power is low, which is normally the situation. Dr. Joseph C. Palais 11.2 11 Example: Light source is an LED, 10 mW output power, λ = 0.85 μ m. The system losses are: coupling loss = 14 dB fiber loss = 20 dB connector losses = 10 dB Total loss = 44 dB Dr. Joseph C. Palais 11.2 12 Compute the received power....
View Full Document

This note was uploaded on 01/26/2011 for the course EEE 546 taught by Professor Palais during the Spring '10 term at ASU.

Page1 / 56

Ch11Sec2 - Dr Joseph C Palais 11.2 1 THE FIBER FORUM Fiber...

This preview shows document pages 1 - 13. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online