Amplifiers Apr. 17, 2018

+ Optical amplifier: To extend beyond 100km, we need to remove distortion in phase (timing error) and intensity (attenuation, pulse shape) with optical amplifier.
Parameters - gain [equation] (dB), gain efficiency (G is a function of [equation]) (dB/ mW), gain bandwidth, gain saturation (the maximum output power [equation] beyond which there is no gain, i.e. G=1), noise from spontaneous emission and polarization sensitivity.
Regenerators: It consists of receiver, electrical amplifier and other electronics, transmitter.
1R - amplifier only; 2R - amplifier and pulse reshape; 3R - retiming, amplifier and pulse reshape.

+ Optical amplifier (OA): 1R only. 2 classes,
- semiconductor OA (SOA) require current pump
- fiber based, e.g. erbium-doped (EDFA) and praseodymium-doped (PDFA), require optical pump.
Various configurations - traveling wave, FP laser amplifiers, DFB laser amplifiers, SR amplifiers, SB amplifiers, EDFA, PDFA.

+ SOA: same materials as those for semiconductor LD.
Characteristics - polarization dep. (can be reduced with special config.), high gain (20dB), [equation] dBm, large bandwidth, operate at 3 windows of transmission, easily integrated with other chips and can be produced in arrays.

Handout for EDFA

+ EDFA: A piece of fiber doped with Er ions. Optically pumped at 532, 667, 800, 980 and 1480nm. Output wavelength at 1520 - 1620 nm (C-band, L-band). Amplification built up 1 [equation] after excitation. Excited atoms will relax after 10ms (spontaneous lifetime). For fast signal, no chance for atoms to leak.

+ EDFA characteristics: From energy conservation, [equation] where [equation] is output signal power, [equation] is input signal power, [equation] is pump power, [equation] is pump wavelength, [equation] is signal wavelength.
Power conversion eff. (PCE) [equation].

+ Gain: [equation]. If [equation], we have gain saturation [equation].
Consider gain as a function of fiber length L, [equation] where [equation] is the gain cross section area, [equation] is the volume density of rare-earth element.
Combine effect, [equation].

+ Amplifier noise (sec 4.4.5): Major source - amplified spontaneous emission (ASE). Power spectral density of ASE, [equation] where [equation] is noise power in an optical bandwidth [equation], [equation] is the spontaneous factor, [equation] is the population in low energy state, [equation] is the population in high energy state.

+ Noise currents: Beating of ASE gives rise to 3 more noise components.
[equation] where
[equation], [equation],
[equation] dominates when G >> 1 and [equation] is limited by optical filter. [equation] [equation] [equation]
Notice that [equation] is the SNR of an ideal photodetector.
Noise figure [equation].

+ PDFA (Praseodymium-Doped Fiber Amplifier): [equation], [equation], [equation]: 1280-1340 nm, [equation] at 1017 nm (uncommon wavelength).

+ SRS and SBS: Two pump config - opposite to signal propagation direction (counterdirection); bidirectional.
[equation]: 1300-1600 nm; it has low gain coef. [equation] requires higher pump power (> 1W) and long fiber (1 km). It functions as a distributed amplifier (Fig. 3.38 p. 160).

+ Classification of amplifier: power ampl - right after the transmitter, preamp - right before the receiver, line ampl. - middle of the link.

+ Application: Waveguide grating router (WGR) with optical amplifiers forms a multiple wavelength laser. WGR can be constructed following the wavelength relation [equation] where i is the input port index and j is the output port index.

Last Modified: April 11, 2018
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