Erbium-Doper Fiber Amplifiers

Transcription

1 Seminar presentation Erbium-Doper Fiber Amplifiers Ville Pale

2 Presentation Outline History of EDFA EDFA operating principle Stimulated Emission Stark Splitting Gain Gain flatness Gain Saturation Er 3+ -Er 3+ interaction effect Pumping Pumping configurations Multistage designs Summary 2

3 History of EDFA Before the invention of EDFAs regenerators we're used to amplify signal expensive and cumbersome to use Idea for EDFA invented in 1960s First commercial viable EDFA invented in 1987 by researchers from Southampton University and AT&T Bell Laboratories. E. Desurvide et Al., High-gain erbium-doped traveling-wave fiber amplifier, OPTICS LETTERS, Vol. 12, No. 11, November 1987 Pros of EDFA: EDFAs are able to amplify many wavelengths simultaneously without crosstalk EDFAs can be integrated straigth to the fiber 3

4 EDFA operating principle Part of the fiber doped with erbium ions Amplification achieved by stimulated emission in Er 3+ - doped SM-fiber 4

5 EDFA operating principle - Stimulated Emission Pump laser excites the Er 3+ ions Incoming signal beam is amplified by stimulated emission by Er 3+ ions 5

6 EDFA operating principle - Stark Splitting In reality, doped Er 3+ ions in the glass induce a local electric fields called Stark splitting This causes the 4 I 13/2 to split into 7 levels and 4 I 15/2 into 8 sub-levels 6

7 Gain Stark splitting increases the range of the signals that can be amplified Within the energy bands, Er 3+ ions are distributed in nonuniform manner Due to this process, EDFA are able to amplify many wavelengths simultaneously Lifetime of energystate E 3 is 1μs -> EDFA can be approximated as 2-level system Prequisite for amplifying: Population inversion (N 2 > N 1 ) Gain can be presented in terms of population levels, cross-sectional area and overlap integral: 7

8 Gain Population levels are different inside the band -> Gain is a function of wavelength Amplification is not even for all wavelengths Unwanted noise is produced by amplified spontaneous emission (ASE) Signal noise ratio varies with wavelength ASE also reduces amount of gain 8

9 Gain 9

10 Gain Flatness Ways to improve gain flatness: Erbium-doped fluoride fiber amplifiers (EDFFA) Naturally flat gain Cannot be pumped with 980nm -> poor SNR Very brittle, difficult to splice with another fibers and susceptible to moisture Filters Use L-band with EDFA(1565nm 1625nm) EDFA gain flatter in L-band 3 times lower gain -> higher pumping, fiber length or erbium concentration needed Need for separate systems for C- and L-band 10

11 Gain saturation When signal power is increased gain decreases When gain is saturated and gain is increased, only noise is increased in relation to signal -> SNR increases 11

12 Er 3+ -Er 3+ interaction effects If the quantity of Er 3+ ions is too high, this causes problems (ions located too close to each other) This causes four different phenomena Resonant energy transfer mechanism Upconversion process Cooperative luminescence Cooperative energy transfer and simultaneous photon absortion 12

13 Pumping Pumping with 980nm Higher population inversion -> lower noise Most of pump power is absorbed in the beginning of the fiber Pumping with 1480nm Lower population inversion -> higher noise Low loss inside silica -> can propagate long distances Higher power lasers available -> high output powers 13

14 Pumping configurations Three different types Copropagating pump Counterpropagating pump Bidirectional pump Application dependant 14

15 Multistage designs In practice, most of the system are more complicated Cascaded amplifiers First for low gain (980nm pumping) Second for high power (1480 pumping) Loss element can filter, add/drop or dispersion compensator 15

16 Summary High gain (30 50 db) Simultaneous amplification of many wavelengths All optical amplification High output power (10 20 dbm) High efficiency (40% - 80%) Polarization insensitive Multistage design can enable better systems 16