SECTION 5: OPTICAL AMPLIFIERS

Transcription

1 SECTION 5: OPTICAL AMPLIFIERS 1

2 OPTICAL AMPLIFIERS In rder t transmit signals ver lng distances (>100 km) it is necessary t cmpensate fr attenuatin lsses within the fiber. Initially this was accmplished with an ptelectrnic mdule cnsisting f an ptical receiver, a regeneratin and equalizatin system, and an ptical transmitter t send the data. Althugh functinal this arrangement is limited by the ptical t electrical and electrical t ptical cnversins. Fiber Optical Signal In OE Rx Electrnic Amp Equalizatin Regeneratin OE Tx Optical Signal Out Fiber Several types f ptical amplifiers have since been demnstrated t replace the OE electrnic regeneratin systems. These systems eliminate the need fr E-O and O-E cnversins. This is ne f the main reasns fr the success f tday s ptical cmmunicatins systems.

3 OPTICAL AMPLIFIERS The general frm f an ptical amplifier: PUMP Pwer Fiber Weak Signal Optical AMP Medium Amplified Signal Fiber Optical Signal In Optical Signal Out Sme types f OAs that have been demnstrated include: Semicnductr ptical amplifiers (SOAs) Fiber Raman and Brilluin amplifiers Rare earth dped fiber amplifiers (erbium EDFA 1500 nm, prasedymium PDFA 1300 nm) The mst practical ptical amplifiers t date include the SOA and EDFA types. New pumping methds and materials are als imprving the perfrmance f Raman amplifiers. 3

4 Characteristics f SOA types: Plarizatin dependent require plarizatin maintaining fiber Relatively high gain ~0 db Output saturatin pwer 5-10 dbm Large BW Can perate at 800, 1300, and 1500 nm wavelength regins. Cmpact and easily integrated with ther devices Can be integrated int arrays High nise figure and crss-talk levels due t nnlinear phenmenn such as 4- wave mixing. This last feature restricts the use f SOAs. Semicnductr Optical Amplifier (SOA) similar t a laser cavity. Used as a discrete amplifiers. They can be integrated int arrays f amplifying switching and gating devices. Finding applicatin in all ptical 3Rregeneratin systems. AR Electrical Drive Current AR Weak Optical Signal In Semicnductr Cavity Amplified Optical Signal Limited in peratin belw 10 Gb/s. (Higher rates are pssible with lwer gain.) 4

5 Rare Earth Dped Fiber Amplifier Characteristics: Rare earth dped fiber amplifiers are finding increasing imprtance in ptical cmmunicatins systems. Perhaps the mst imprtant versin is erbium dped fiber amplifiers (EDFAs) due t their ability t amplify signals at the lw lss 1.55 m wavelength range. Characteristics f EDFAs (advantages): High pwer transfer efficiency frm pump t signal pwer (> 50%). Wide spectral band amplificatin with relative flat gain (>0 db) useful fr WDM applicatins. Saturatin utput > 1 mw (10 t 5 dbm). Gain-time cnstant lng (>100 msec) t vercme patterning effects and intermdulatin distrtins ( lw nise). Large dynamic range. Lw nise figure. Plarizatin independent. Suitable fr lng-haul applicatins. Disadvantages f EDFAs: Relatively large devices (km lengths f fiber) nt easily integrated with ther devices. ASE amplified spntaneus emissin. There is always sme utput even with n signal input due t sme excitatin f ins in the fiber spntaneus nise. Crss-talk effects. Gain saturatin effects. 5

6 An energy level diagram fr Er dped silica is shwn belw. 4I (11/) 0.98 um 4I (13/) 1.53 um Pump Bands 4I (15/) 1.48 um Emissin Wavelengths Pumping is primarily dne ptically with the primary pump wavelengths at 1.48 m and 0.98 m. As indicated atms pumped t the 4I (11/) 0.98 m band decays t the primary emissin transitin band. Pumping with 1.48 m light is directly t the upper transitin levels f the emissin band. Semicnductr lasers have been develped fr bth pump wavelengths mw f absrbed pump pwer at these wavelengths can prduce db f amplifier gain. Pump Efficiencies f 11 db/mw achieved at 980 nm. Pumping can als be perfrmed at 80 and 670 nm with GaAlAs laser dides. Pump efficiencies are lwer but these lasers can be made with high utput pwer. 6

7 Crss Sectin (X 10^(-5)m^) Typical Absrptin/Gain Spectrum fr Erbium Dped Fiber: 10 6 Lss/Gain (db/m) 5 Absrptin 4 Gain Wavelength (um) Since the gain spectrum f erbium resembles a 3-level atm it is pssible t mdel the gain prperties using this apprach. Several different wavelength bands have been designated fr wavelength divisin multiplexing and EDFAs have been designed t perate in these bands. The divisins have been designated as * : S-Band nm C-Band nm L-Band nm (* Nte sme variability in these values is cmmn.) 7

8 General EDFA Amplifier Cnfiguratin: 980 r 1480 pump laser Islatr EDFA Amplified Output Signal Weak Input Signal Cupler Narrw Band Filter Basic Amplifier Characteristics Optical Gain Rare earth dped ptical amplifiers wrk much like a laser. The primary difference is that they d nt have a resnatr. Amplificatin ccurs primarily thrugh the stimulated emissin prcess. The medium is pumped until a ppulatin inversin state is achieved. Pump pwers are typically several 0-50 mw. An islatr is used t reduce reflectins at the input t the amplifier. A narrw band ptical filter is used t reduce transmissin f amplified spntaneus emissin frequency cmpnents. The resultant ptical gain depends bth n the ptical frequency and the lcal beam intensity within the amplifier sectin. Fr basic discussin cnsider a tw-level hmgeneusly bradened medium. 8

9 The gain cefficient can be expressed as: g g ( ) 1 ( ) T P / P s, g is the peak gain, is the ptical frequency f the incident signal, is the transitin frequency, P is the ptical pwer f the incident signal, T is the diple relaxatin time, and P s is the saturatin pwer. Typically T is small < 1 ps, and the saturatin pwer P s depends n gain medium parameters such as the flurescence time and the transitin crss sectin. 9

10 Gain Spectrum and BW: When nt saturated (i.e. P/P s <<1) the gain cefficient g() becmes: g g( ) 1 ( ) T. Gain is maximum when = (i.e. the gain cefficient is at resnance). At nn-resnant frequencies the gain fllws the hmgeneusly bradened characteristics f a tw level atm (i.e. Lrentzian prfile). The gain BW fr this spectrum is typically expressed as the (Full Width at Half Maximum) FWHM g T. g g with T 0.1p s g 3THz Large Spectral BW amplifiers are preferred fr fiber ptic systems t make them less sensitive t dispersed transmitted signals and useful fr WDM systems. EDFA Gain Spectrum: The gain spectrum f erbium ins alne is hmgeneusly bradened and the BW is determined by the diple relaxatin time T. Hwever when placed in a glass hst the spectrum is influenced bth by the silica and any ther dpants. This can result in inhmgeneus bradening cntributins. The cmbined hmgeneus and inhmgeneus BW f EDFAs: ~ 30 nm. 10

11 Amplificatin factr: Define as: G = P ut /P in P ut is the amplifier utput pwer and P in the input pwer f a CW input signal. Pump P in N N1 Gain Medium P ut z=0 z=l Frm the previus discussin f the laser the gain in ptical pwer per length f gain medium (z) with gain g is dp gp dz. Integrating ver a length z f amplifier medium gives the resultant ptical pwer P( z) P(0) exp( gz). 11

12 The amplificatin factr after a length L f OAM (ptical amplifier medium) is G ( ) exp g( ) L Bth g() and G() are a maximum when the frequency is at resnance and decrease when the frequency is detuned frm resnance. Hwever the amplifier factr(g) decreases much faster than the gain cefficient(g). The amplifier BW A is defined as the FWHM f G() A g ln ln( / ) G 0.5 where g is the gain BW, and G = exp(g L). The amplifier BW is smaller than the gain BW. The difference depends n the amplifier gain characteristics. If G = 10, A g 1

13 Gain Saturatin: Since g() depends n the incident ptical pwer when P P S, G will start t decrease with an increase in ptical pwer P. Assume that the incident frequency is tuned fr peak gain ( = ) dp dz g P 1 P / P. s With the cnditins P(0) = P inc and P(L) = P ut = GP inc the large signal amplifier gain becmes G G G 1 Put exp. G PS This expressin shws hw the amplifier gain decreases when P ut P s. Output saturatin pwer the ptical pwer at which G is reduced t G / (3 db) P ut sat G ln Ps. G Typically G =1000 (30 db), P (ln ) P 0. 69P. s ut s s 13

14 Amplifier Nise: Spntaneus emissin in the amplifier will degrade the SNR by adding t the nise during the amplificatin prcess. SNR degradatin is quantified thrugh the amplifier nise figure F n F n SNR SNR in ut where the SNR is based n the electrical pwer after cnverting the ptical signal t an electrical current. Therefre F n is referenced t the detectin prcess and depends n parameters such as detectr bandwidth (B e ) and thermal and sht nise. Cnsider a simple case with an ideal detectr with perfrmance limited by sht nise. The amplifier has an amplificatin factr G (P ut = G P in ). SNR f the input signal: SNR in s RPin Pin RPin Be hbe I, q s in B e q RP. The spntaneus emissin cntributin is amplified alng with the signal. The Spectral density f the spntaneus emissin induced nise is nearly cnstant (white nise) and can be expressed as: S G n h sp 1 Spntaneus emissin ppulatin inversin factr n sp is given by: sp N n sp. N N1 N and N 1 are the ppulatin densities fr the excited and grund states f the amplifying medium. 14

15 Alternatively can express the spntaneus emissin pwer within the receiver bandwidth B e as: P sp S Spntaneus emissin adds fluctuatins t the amplified pwer and is cnverted t current fluctuatins at the detectr utput. Majr cntributin t receiver nise results frm cherent interference (beating) between the spntaneus emissin with the signal. This results in a nise current given by sp B I R GP P 1/ ( in sp ) cs The variance in the phtcurrent after the signal is passed thrugh the amplifier is 4( RGP ) in e RS sp B e where cs is replaced with its average value f ½. (Nte that this relatin assumes several idealizatins n the detectin prcess i.e. ther nise surces are negligible.) The SNR f the amplified signal becmes SNR ut RGP in GP 4S sp in B e and the amplifier nise figure is n sp G 1/ G nsp F n. Fr mst amplifiers F n > 3 db and can be 6-8 db. 15

16 Nise Figure Characteristic plt f gain and nise figure fr an erbium dped fiber amplifier pumped ~30 mw at 980 nm Gain (db) Wavelength (nm) 0 16

17 EDFA Gain Equalizatin 10 Lss/Gain (db/m) 5 Unflattened EDFA Gain Flattened EDFA Gain Wavelength (um) 1.56 Gain equalizatin can be accmplished in several ways: a. Thin film filters b. Lng perid fiber gratings c. Chirped fiber Bragg gratings 17

18 Energy Raman Scattering, Stimulated Raman Scattering, and Raman Amplifiers: Raman scattering is an elastic scattering mechanism. Des nt require a ppulatin inversin. A phtn with energy h 1 traveling thrugh a material can excite a vibratinal transitin f the material frming an ptical phnn with energy h p and a phtn with slightly reduced energy h given by 1 p h 1 h hp Mlecule is raised t a new vibratinal state and the energy f the phtn is reduced. There is a large difference between the phtn and phnn energies. Raman scattering is weak effect. It ccurs thrugh a slight mdulatin f the refractive index thrugh mlecular vibratins f the material. Can derive the effect thrugh a discussin f plarizability f a material. 18

19 The electric field induces a diple mment f the mlecule r p qx p E where is the cmplex plarizability f the mlecule. x +q -q E The bulk plarizability f a material is expressed as P 1 E 1 with the linear susceptibility f the material. Respnse f t an incident harmnic electric field: x x x x is the displacement frm the equilibrium mlecular length x x x t j pt xe 19

20 Energy Energy pt () () tet () j pt j1t xe Ee x x Ee j1t x x xee j 1 p t There are tw frequency cmpnents: a) 1 ; b) 1 p The secnd cmpnent is nnlinear the utput frequency is different frm the input frequency. Stkes Anti-Stkes h 1 h h h 1 hp hp Scattered light with lwer energy ( Scattered light with higher energy ( 1 1 ) Stkes Scattering. ) Anti-Stkes Scattering. Stkes scattering typically dminates due t greater ppulatin f the grund state relative t the vibratinal state when the system is in thermal equilibrium. At lw illuminatin levels the Raman prcess results in lw scattering levels. The mlecules cntributing t the prcess are vibrating independently and the scattered light is nn-directinal. Spntaneus Raman Scattering. 0

21 At higher intensity levels the generated phtns begin t act in phase r cherently i.e. the mlecules scillate as an array f vibrating scillatrs. This gives rise t Stimulated Raman Scattering (SRS). SRS can be can be a prblem but it can als be used as a signal amplificatin prcess. On the negative side it cntributes t dispersin and places an peratinal limit n the amunt f pwer that can be transmitted thrugh a fiber. The Stkes wave is amplified as it prpagates thrugh the medium di dz GII I is the intensity f the Stkes shifted light s 1 vib ; I 1 is the intensity f the pump beam ( 1 ); and G r is the Raman gain term that includes material factrs such as / x and varies as 1/. Fr I <<I 1 and cases where the pump beam is nt significantly depleted: r 1 1 I z I e Gr I z 0 1

22 Prperties f Raman Amplifiers: The peak resnance in silica fibers ccurs abut 13 THz frm the pump wavelength. At 1550 nm this crrespnds t a shift f abut 100 nm. Raman Gain Cefficient p = 1550 nm Frequency Shift (THz) As indicated pwer is transferred frm shrter wavelengths t lnger wavelengths. Cupling with the pump wavelength can be accmplished either in the frward r cunter prpagating directin. Pwer is cupled frm the pump nly if the signal channel is sending a 1 bit.

23 Pump Arrangement t Extend the Range fr Stimulated Raman Amplificatin: An array f laser dides can be used t prvide the Raman pump. The beams are cmbined and then cupled t the transmissin fiber. The pump beams can cunter prpagate t the directin f the signal beams. Transmissin Fiber 14xx/1550 nm WDM Cupler Raman Pump Blck Laser Dide Array 1430 nm 1450 nm 1470 nm 1490 nm 14xx nm Cmbiner 3

24 Difficulties with Raman Amplifiers: The Pump and amplified signals are at different wavelengths. Therefre the signal and the pump pulses will separate due t dispersin (waveguide dispersin) after a certain prpagatin distance. The difference in prpagatin time is given by: L/ c d n/ d / L is the fiber length. A 1 psec pump pulse at 600 nm separates frm a 1 psec Stkes pulse in ~ 30 cm. A secnd prblem is that the pump pwer decreases alng the fiber length due t linear absrptin and scattering Raman gain is greater at the input end. A final prblem results frm amplifying spntaneus Raman phtns. This ccurs when the pump pwer is increased t ffset attenuatin lsses and spntaneus Raman phtns are cupled int the guided mde all alng the length f the fiber. This increases nise. Upper limit n the pwer int a cmmunicatins signal frm SRS amplificatin can be defined as the pint at which the Stkes pwer P r equals the signal pwer P sig. Example: 1.55m p / md e L w 5m A 80m linear G r 16 w P GL eff 0. db / km L 0km m W eff r 1 e eff L 700 mw QUITE LARGE cmpared t nrmal ptical signal pwers (~1 mw). 4