1、信息傳輸材料與技術(shù) 光放大器,Schematic diagram of the eight-channel, dense wavelength-division multiplexing 80 Gbps system,Experimental setup of the 80 Gbps, DWDM fibre-optics circulating loop,Signal gain and (b) noise figure of the 5 EDFAs employed in the circulating loop test-bed,(a),(b),全波光纖最適合于在城域網(wǎng)中應(yīng)用。 新開(kāi)辟的第5

2、窗口（1.351.53m）把波長(zhǎng)譜擴(kuò)大了大約180nm。 這段波長(zhǎng)處于第2窗口與第3窗口之間，光纖的衰減小于1310nm處的衰減，而色散則小于1550nm區(qū)域內(nèi)的數(shù)值。能更好地利用光纖的低色散區(qū)。 在這樣寬闊的波長(zhǎng)區(qū)內(nèi)，采用粗波分復(fù)用（CWDM）代替DWDM往往能夠取得相當(dāng)大的經(jīng)濟(jì)效益。,波長(zhǎng)m,Yb-doped ASE source,Optical amplifier,Rare earth doped fiber amplifier Pr, Er Semiconductor amplifier Nonlinear fiber amplifier SRS, SBS, OPO,Er energy

3、 level,摻鐠光纖放大器（PDFA）,摻鐠光纖放大器（PDFA）是工作于1300nm波長(zhǎng) 的，以摻鐠光纖作為增益介質(zhì)，以1017nm附近波長(zhǎng) 的激光器作為泵浦光源的一種光纖放大器。,圖1 Pr3+的簡(jiǎn)化能級(jí)結(jié)構(gòu)與能級(jí)間的躍遷,Raman amplifier,BROAD BAND High output power High stability Low polarization dependence Low noise figure, There is amplification at any wavelength, provided the appropriate pump sources

4、 are available. A fiber itself can be used as an active medium. A pump spectrum determines a gain spectrum., distributed Raman amplifiers; discrete Raman amplifiers; Raman fiber lasers.,Na is the number of molecules per unit volume in the lower energy state. EL is the amplitude of the laser light wa

5、ve, Nb is the number of molecules in the upper energy state, nLand nsare the indexes of refraction at the laser and Stokes frequencies. ab is the width of a Stokes line. The normalized gain cm / W is often used (where is intensity of laser light).,Among other important characteristics of Raman fiber

6、s one should point out the following: Raman frequency shift and Raman gain bandwidth; effective mode area; optical losses; photosensitivity; catastrophic damage threshold; compatibility with other fiber elements of communication systems and with todays technology.,consider doped silica fibers as Ram

7、an fibers. The relative Raman cross sections of vitreous SiO2 , GeO2 , B2 O3 , and P2 O5 widely used in the fiber technologywere accuratelymeasured by Galeener et al.,F. L. Galeener, J. C. Mikkelsen, R. H. Geils, andW. J. Mosby, “The relative Raman cross sections of vitreous SiO , GeO , B O and P O

8、,” Appl. Phys. Lett., vol. 32, pp. 3436, 1978.,Germania glass had the highest cross section, approximately nine times higher compared with silica. The Raman frequency shift of germania glass was measured to be approximately 420 cm-1 . Vitreous P2 O5 is also of great interest because this glass has t

9、wo Raman scattering bands shifted by 650 and 1300 cm , the cross section for these bands being 5.7 and 3.5 times higher compared with silica. The presence of the Raman scattering band at 1300 cm is the most important feature of the P2 O5 glass.,The Raman gain bandwidth in the glasses is approximatel

10、y 100 cm-1 . The Raman gain of doped silica glasses depends linearly on the dopant concentration.,3.2 Tbit/s (40 x 80Gb/s) transmission over 1000km with 100km span (25dB loss) and 0.8bit/s/Hz of spectral efficiency,Counterpumped, distributed Raman amplifiers can achieve better signal-level excursion

11、 because the signals tend to be amplified only when their power level is lowered because of fiber span loss.,the average path signal level is low, allowing nonlinear effects to be avoided,Multi-wavelength pump,Raman assisted four-wave mixing,Multi-wavelength laser,Parametric amplifier,Four wave mixi

12、ng is one of the parametric processes Parametric processes lies in the nonlinear response of bound electrons of a material to an applied optical field. Be classified as second- or third- order processes depending on whether or plays role.,Typical experimental setup,Fiber parameters: = 2.4 km1W1 0 =

13、1560 nm = 0.26 dB/km,Wideband amplification (A) 200-nm gain bandwidth (M.C. Ho et al, JLT, p.977, 2001),Pulsed pump 9 W peak power HNL-DSF,Recent advances (1),(B) 400-nm gain bandwidth, or tunable spectrum (K. K. Y. Wong et al., OECC04, 15D1-2),Pulsed pump 12 W peak power HNL-DSF,Recent advances (1)

14、,Pulsed pump 10 W peak power DSF,Recent advances (2),Tunable narrowband spectrum (K. K. Y. Wong et al., OAA2004, OTuA5),Large CW gain 49 dB(J. Hansryd et al, PTL, p. 194, 2001) 60 dB(K. K. Y. Wong et al, PTL, p. 1707, 2003),1.2 W CW pump Isolator in the middle (to reduce SBS),Recent advances (3),Gai

15、n slope = 125 dB/W/km,Low noise figure,(a) S. French co-propagating waves,Recent advances (5),Signal gain spectrum,Large pump depletion 92% in 1-pump OPA (M. E. Marhic et al., Opt. Lett., p.620, 2001),11 km DSF 100 mW pump,Recent advances (6),15 km DSF 280 mW pump,Recent advances (6),88% in 2-pump O

16、PA (J. M. Chavez Boggio et al., PTL, p.620, 2003),Full-bandwidth utilization (M. E. Marhic et al., OFC03, p.620, 2001) for wide WDM spectra, signals gain coefficient: g 2. Orthogonal linearly-polarized pumps = Polarization-independent; gain coefficient: g/3 3. Orthogonal circularly-polarized pumps =

17、 Polarization-independent; gain coefficient: 2g/3 need spun fibers to implement! (M. Marhic et al, JOSA B, p. 2425, 2003 Q. Lin et al., JOSA B, p. 1216, 2004.),Recent advances (13),Random fiber birefringence Need to average OPA equations over SOP distributions Uniform over Poincare sphere: g = (8/9)

18、 g Uniform over great circle: g = (2/3) g (M. Marhic et al., OFC04, TuC2) PMD can lead to: Large gain reductions Distortion of gain spectrum (F. Yaman et al., PTL, p.1292, 2004; Q. Lin et al., Opt. Lett., p. 1114, 2004),Recent advances (14),(Q. Lin et al., Opt. Lett., p. 1114, 2004),Recent advances

19、(14),(F. Yaman et al., PTL, p.1292, 2004),Recent advances (15),Fiber-to-fiber variations in the FOPA gain spectra caused by random variations in l0 along the fiber for the pump wavelength separation of 98 (left) and 50 nm (right).,Random l0 variations can lead to spectrum distortions:,(Application =

20、 Amplifier),CW Polarization independent Flat gain spectrum, broadband ( 50 nm) Under-served spectral window Medium small-signal gain ( 20 dB) Low NF ( 20 dBm) Reasonable pump power ( 1 W) Low nonlinear effects in WDM systems (FWM, XPM) Reasonable cost,Features desirable in a single OPA,Effective, lo

21、w-cost SBS suppression Improved fibers Cost-effective pumps Reduction of unwanted nonlinear effects in WDM systems,Desirable developments,Currently: pump frequency dithering ( 3 GHz) problem for -conversion Possible alternatives: isolators air gaps stress heating special fibers?,SBS suppression,Lase

22、r diodes Single frequency High power ( 100 mW) Anywhere in 1200-1700 nm range Some tunability MOPA configurations ( 1 W?) Raman-shifted fiber lasers Double-clad Yb laser near 1 m Nested FBG Raman cavities Seed with LD = narrow linewidth?,Pumps,Need: Better uniformity Higher nonlinearity coefficient

23、n /Aeff Reduction in Increasing Increasing n : non-silica glasses Reducing Aeff holey fibers (microstructured, etc.),Fibers,Many major features demonstrated Large bandwidth High gain Low noise figure Pulsed operation etc. Recent emphasis on second-order effects Undesired nonlinear interactions Fiber

24、 imperfections,Conclusion,Progress made possible due to advances in Pump sources (high power EDFAs) Highly-nonlinear fibers Additional progress in Pump sources Highly-nonlinear fibers SBS suppression could lead to practical fiber OPAs,Conclusion,Semiconductor Optical Amplifier (SOA),A Semiconductor Optical Amplifier (SOA) is an optical device that can be used to compensate for Optical losses introduced by loss from a transmission