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 - gainG=Pout/Pin (dB), gain efficiency (G is a function ofPin ) (dB/ mW), gain bandwidth, gain saturation (the maximum output powerPout,sat 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),Pout,sat510 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 1μs after excitation. Excited atoms will relax after 10ms (spontaneous lifetime). For fast signal, no chance for atoms to leak.
• EDFA characteristics: From energy conservation,PoutPin+λpPp,in/λs wherePout is output signal power,Pin is input signal power,Pp,in is pump power,λp is pump wavelength,λs is signal wavelength.
Power conversion eff. (PCE)=(PoutPin)/Pp,inPout/Pp,inλp/λs
• Gain:G=Pout/Pin1+λpPp,in/(λsPin) IfPinλpPp,in/λs , we have gain saturationG1
Consider gain as a function of fiber length L,Gmax=exp(ρσeL) whereσe is the gain cross section area,ρ ~ is the volume density of rare-earth element.
Combine effect,Gmin[exp(ρσeL),1+λpPp,in/(λsPin)]
• Amplifier noise (sec 4.4.5): Major source - amplified spontaneous emission (ASE). Power spectral density of ASE,SASE=PASE/Δνopt=hνnsp(G(f)1) wherePASE is noise power in an optical bandwidthΔνopt ,nsp=n2/(n2n1) is the spontaneous factor,n1 is the population in low energy state,n2 is the population in high energy state.
• Noise currents: Beating of ASE gives rise to 3 more noise components.

itot2¯=iT2¯+iS2¯+isigASE2¯+ishotASE2¯+iASEASE2¯

where

iS2¯=2qRGPinΔf,

ishotASE2¯=2qR(SASEΔνopt)Δf,

isigASE2¯=4(RGPin)RSASEΔf,

iASEASE2¯=R2SASE2(2ΔνoptΔf)ΔfisigASE2¯

dominates when G >> 1 andΔνopt is limited by optical filter.SNR=GRPin/[2Δf(2hνnsp(G1)R+q)]=[RPin/(2qΔf)][G/(2ηnsp(G1)+1)]
Notice thatSNRin=RPin/(2qΔf) is the SNR of an ideal photodetector.
Noise figureFampl=SNRin/SNRout=(2ηnsp(G1)+1)/G
• PDFA (Praseodymium-Doped Fiber Amplifier):G30dB ,Psat=20dBm ,λ : 1280-1340 nm,Pp,in300mW at 1017 nm (uncommon wavelength).
• SRS and SBS: Two pump config - opposite to signal propagation direction (counterdirection); bidirectional.

λ : 1300-1600 nm; it has low gain coef. 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 relationλi+j=λo(i+j)Δλ where i is the input port index and j is the output port index.


Last Modified: April 11, 2018
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