Lecture 8 Intro to Optical Amplifiers ECE228B Prof D J Blumenthal Lecture 8 Slide 1 1R Optical Regeneration Analog amplification Faithfully reproduces input signal with minimal distortion Can be used as a linear repeater by periodically boosting optical power Can be used in nonlinear region as a level clamping amplifier Single amplifier can be used as a multichannel amplifier ideally with minimal crosstalk and distortion ECE228B Prof D J Blumenthal Lecture 8 Slide 2 Waveband Operation EDFA GS EDFA EDWA ETDFA TDFA GC SOA GC SOA PDFFA RFA Short Reach Band 1260 1270nm 1360 EDFA EDWA ETDFA TDFA PDFFA GS EDFA GC SOA RFA ECE228B Prof D J Blumenthal S Band 1450 S Band 1490 C Band 1530 L Band L Band 1570 Erbium doped fiber amplifier 1530 1570 nm Erbium doped waveguide amplifier Telluride based erbium doped fiber amplifier 1532 1608nm Thulium doped fluoride based fiber amplifier Praseodymium doped fluoride fiber amplifiers Gain shifted EDFA Gain clamped semiconductor optical amplifier Raman fiber amplifier 1610 1650 Optically Pumped Electrically Pumped Lecture 8 Slide 3 OA Figures of Merit and Design Parameters Figure of Merit Unsaturated Gain G0 Gain Flatness Noise Figure Fn Maximum amplifier output power Pout max Design Parameter Pump Power Erbium Doped Fiber Length Operation in Saturation Erbium Doped Fiber Length Co Propagating pump Counter Propagating Pump Erbium Doped Fiber Length Pump Power Erbium Doped Fiber Length Pump Power ECE228B Prof D J Blumenthal Impact Sets the number of photons available for gain increase in G0 with increased pump power but reaches an asymptope Increased G0 with increased length for moderate pump power Higher Fn at shorter wavelengths Gain sensitivity to channel add drop Optimal length for pump and signal powers Lower Fn than counter Propagating Higher Fn than co Propagating Fn increases with increase in fiber length Fn decreases with increase in pump power Pout max increases with increased pump power Lecture 8 Slide 4 1R Optical Regeneration 1R Optical Analog amplification without reshaping or retiming Eout t G Ein t n t Amplifier optical Gain Amplifier emitted optical noise Faithfully reproduces input signal with minimal distortion Can be used as a linear repeater by periodically boosting optical power Can be used in nonlinear region as a level clamping amplifier Available solutions Erbium Doped Fiber Amplifiers EDFA Semiconductor Optical Amplifiers SOA ECE228B Prof D J Blumenthal Lecture 8 Slide 5 Optical Amplifiers Three classes Booster power amplifiers Boost power into transmission fiber low NF high Psat In line amplifiers Periodically amplify signal due to fiber attenuation high G high Psat Receiver preamplifiers Boost power into receiver low NF high G Tx Rx Booster OA Tx Rx In Line Amplifier Tx Rx Receiver Preamplifier ECE228B Prof D J Blumenthal Lecture 8 Slide 6 Optical Amplifiers Gain Characteristics Define Unsaturated amplifier gain G0 as the gain achieved at low signal levels and in the linear amplifier regime Define Output saturation power as the output power needed to decrease the amplifier gain by a factor of 2 Pout G linear nonlinear G0 3dB Slope G0 Pin Region I Linear G0 ECE228B Prof D J Blumenthal Pout Pin Pout sat Pout Region II Nonlinear Saturated G 1 Pout G G0 exp G PS Lecture 8 Slide 7 Optical Amplifier Physics An atomic system with two energy levels can absorb light amplify light spontaneously emit light Absorption Stimulated emission Spontaneous emission Stimulated and spontaneous emission are achieved by pumping the amplifier electrically or optically ECE228B Prof D J Blumenthal Lecture 8 Slide 8 Erbium Doped Fiber Amplifier EDFAs Energy levels for Er ions in silica glass E3 32 1 s 980nm E2 1480nm 1520nm 1570nm 21 10 ms E1 Two pumping options 980 nm pump Complete population inversion Low noise figure 1480 nm pump Low population inversion high quantum efficiency in converting pump photons to signal photons ECE228B Prof D J Blumenthal Lecture 8 Slide 9 EDFA Gain Spectrum The gain coefficient for a single atomic transition in the unsaturated regime is given by the peak gain g0 and the dipole relaxation time T2 as g g0 2 1 0 T22 Averaging the gain over the distribution of atomic transition frequencies yields the effective gain geff g f d 0 0 0 Gain dB An illustration of the effective gain is given below Note the presence of a gain peak around 1530nm and a semi flat gain region with optical bandwidth 20 30nm 1530 nm 20 30 nm ECE228B Prof D J Blumenthal Lecture 8 Slide 10 EDFA Theory Basics Using a simple two level model for the EDFA assumes that ASE and excited state absorption are negligible Also this model assumes the top excited energy level empties instantly negligible excited state lifetime The population densities of states E1 and E2 are given by N1 and N2 with the cross a e a e section emission and absorption p p s s for the pump and signal photon flux p and s T1 is about 10ms for EDFAs N 2 N y ap N1 ep N 2 p sa N1 se N 2 s 2 t T1 x N1 N ep N 2 ap N1 p se N 2 sa N1 s 2 t T1 If we ignore ASE the evolution of the pump and signal powers along the fiber in direction z can be approximated by taking into account the fiber loss at signal and pump wavelengths PS S se N 2 sa N1 PS PS z Pp p ep N 2 ap N1 Pp Pp z ECE228B Prof D J Blumenthal Lecture 8 Slide 11 EDFA Theory Basics For short amplifiers 10 20m optical loss can be ignored 0 Let N1 N2 Ntotal and ad be the cross sectional area of the doped portion of the fiber core The steady state solution for the rate equations reduces to N 2 z T1 Ps T1 Pp ad h s z ad h p z ad s as p a p Substituting this equation into the power evolution equations and integrating over the length of fiber the gain can be computed by taking the ratio of output to input power L G S exp se N 2 sa N1dz 0 ECE228B Prof D J Blumenthal Lecture 8 Slide 12 EDFA Basics From the figure below we observe that For a given amplifier length the gain initially increases with pump power then saturates For a given pump power the amplifier gain becomes maximum at optimum L then rolls off sharply as the pump photons have all been absorbed Both L and Pp must be optimized for a particular amplifier design ECE228B Prof D J Blumenthal Lecture 8 Slide 13 EDFA pumps 1480 nm pumping it was the choice for most of the first commercial solutions Mainly due to the fact that 1480 nm laser were more resilient and commercially available at high output power which is usually in the order of 200 400 mW Generally less expensive From a pure