《集成光波導》ppt課件-全文預覽
【正文】 e n1 and n2 also vary with wavelength, material dispersion exists in addition to waveguide dispersion. The total dispersive pulse spread is the sum of the two. That is: ? ? ? ? ?? ????? gMML/? 因為材料色散 M有可能為負值(例如在石英玻璃中 ， 當工作波長超過 1300nm時 )， 由色散引起的總脈沖展寬實際上有可能會因為波導色散的存在反而減小 。 ? 各個模式到達輸出端的時間不同 ， 從而使得波形被展寬 ， 這種現(xiàn)象稱為 多模失真 或 模式失真 ， 也常稱為 多模色散 。 9 10 Multimode Distortion Consider the worst case by calculating the difference in arrival times between the lowest mode (axial) and the highest mode (? = ?c) for a highly multimode waveguide. Let L = length of the waveguide 11 Multimode Distortion n1 n2 n1 n2 Axial mode L2 n2 ?c Higher order mode L1 12 Multimode Distortion Axial mode travel time: 2 2 112sinL L nLn??? cL1 L2 () cLnvLta x i a l1??For the critical angle ray, 21sin LL? ?c?c 13 Multimode Distortion The total path of the critical angle ray is then LncnnLnvnLn22112121c r i t i c a lt ?????????????????????????() The critical angle ray travel time is: ????????21nnL14 Multimode Distortion The total delay is 2211 )(nnnL????????? ?This is the modal pulse spread per unit length in a multimode waveguide. We can write it as cnnLtt a x i a lc r i t i c a l 1221 ???() 112121 nnnnnL ???????? ????????? ?15 Multimode Distortion 221nL????????? ? 1L ????????? ?to get )( 121nnn ???Use the fractional refractive index change () Modal Pulse Spread If n1 and n2 are nearly equal 16 Multimode Distortion 212NA
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