Coupled Lines Coupled Transmission Lines Problems 593. , τ = L. τ = Z 0C


 Virgil Booth
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1 .6. Probles 593 where T d /c is the oneway travel tie to the fault. Show that the corresponding tie constant τ /a is in the four cases: τ Z C, τ Z C, τ L Z, τ L Z For a resistive fault, show that Γ Z /R + Z ), or, Γ R/R + Z ), for a shunt or series R. Moreover, show that Γ Z Z )/Z + Z ), where Z is the parallel in the shuntr case) or series cobination of R with Z and give an intuitive explanation of this fact. For a series C, show that the voltage wave along the two segents is given as follows, and also derive siilar expressions for all the other cases: V ut z/c)+v e at+z/c T ) ut + z/c T ), for z<d Vt, z) V e at z/c) ut z/c), for d <z d + d Make a plot of V d t) for t 5T, assuing a for the C and L faults, and Γ corresponding to a shorted shunt or an opened series fault. The MATLAB file TDRovie. generates a ovie of the step input as it propagates and gets reflected fro the fault. The lengths were d 6, d 4 in units such that c ), and the input was V.. Coupled Transission Lines Coupled Lines Coupling between two transission lines is introduced by their proxiity to each other. Coupling effects ay be undesirable, such as crosstalk in printed circuits, or they ay be desirable, as in directional couplers where the objective is to transfer power fro one line to the other. In Sections..3, we discuss the equations, and their solutions, describing coupled lines and crosstalk In Sec..4, we discuss directional couplers, as well as fiber Bragg gratings, based on coupledode theory Fig... shows an exaple of two coupled icrostrip lines over a coon ground plane, and also shows a generic circuit odel for coupled lines. Fig... Coupled Transission Lines. For siplicity, we assue that the lines are lossless. Let L i,c i, i, bethe distributed inductances and capacitances per unit length when the lines are isolated fro each other. The corresponding propagation velocities and characteristic ipedances are: v i / L i C i, Z i L i /C i, i,. The coupling between the lines is odeled by introducing a utual inductance and capacitance per unit length, L,C. Then, the coupled versions of telegrapher s equations.5.) becoe: C is related to the capacitance to ground C g via C C g + C, so that the total charge per unit length on line is Q C V C V C gv V g)+c V V ), where V g.
2 .. Coupled Transission Lines Coupled Lines V z L I t L I t, I z C V t + C V t..) V z L I t L I t, I z C V t + C V t When L C, they reduce to the uncoupled equations describing the isolated individual lines. Eqs...) ay be written in the atrix fors: V z L L I L L t..) I z C C V C C t where V, I are the colun vectors: V V V, I I For sinusoidal tie dependence e jωt, the syste..) becoes: dv jω L L I L L di jω C C V C C I..3)..4) It proves convenient to recast these equations in ters of the forward and backward waves that are noralized with respect to the uncoupled ipedances Z,Z : a V + Z I Z, b V Z I Z a V + Z I Z, b V Z I Z a a a, b b b..5) The a, b waves are siilar to the power waves defined in Sec The total average power on the line can be expressed conveniently in ters of these: P ReV I ReV I + ReV I P + P a b ) + a b ) a + a ) b + b ) a a b b..6) where the dagger operator denotes the conjugatetranspose, for exaple, a a,a. Thus, the awaves carry power forward, and the bwaves, backward. After soe algebra, it can be shown that Eqs...4) are equivalent to the syste: da jf a + jg b db jg a + jf b d a j b F G G F a b..7) with the atrices F, G given by: β κ χ F, G κ β χ..8) where β,β are the uncoupled wavenubers β i ω/v i ω L i C i, i, and the coupling paraeters κ, χ are: ) κ ω L C Z Z L β β Z Z L L ) χ ω L + C Z Z L β β + Z Z L L C ) C C C )..9) C C A consequence of the structure of the atrices F, G is that the total power P defined in..6) is conserved along z. This follows by writing the power in the following for, where I is the identity atrix: I a P a a b b a, b I b Using..7), we find: dp ja, b F G G F I I I I F G G F ) a b the latter following fro the conditions F F and G G. Eqs...6) and..7) for the basis of coupledode theory. Next, we specialize to the case of two identical lines that have L L L and C C C, so that β β ω L C β and Z Z L /C Z, and speed v / L C. Then, the a, b waves and the atrices F, G take the sipler fors: a V + Z I Z, b V Z I a V + Z I Z, b V Z I..) β κ χ F, G..) κ β χ where, for siplicity, we reoved the coon scale factor Z fro the denoinator of a, b. The paraeters κ, χ are obtained by setting Z Z Z in..9): κ β L L C ), χ C β L L + C ),..) C The atrices F, G coute with each other. In fact, they are both exaples of atrices of the for: a a A a a a I + a J, I, J..3)
3 .. Coupled Transission Lines Coupled Lines where a,a are real such that a a. Such atrices for a coutative subgroup of the group of nonsingular atrices. Their eigenvalues are λ ± a ± a and they can all be diagonalized by a coon unitary atrix: Q e +, e, e +, e..4) so that we have QQ Q Q I and Ae ± λ ± e ±. The eigenvectors e ± are referred to as the even and odd odes. To siplify subsequent expressions, we will denote the eigenvalues of A by A ± a ± a and the diagonalized atrix by Ā. Thus, A+ a + a A QĀQ, Ā..5) A a a Such atrices, as well as any atrixvalued function thereof, ay be diagonalized siultaneously. Three exaples of such functions appear in the solution of Eqs...7): B F + G)F G) Q F + Ḡ) F Ḡ)Q Z Z F + G)F G) Z Q F + Ḡ) F Ḡ) Q..6) Γ Z Z I)Z + Z I) Q Z Z I) Z + Z I) Q Using the property FG GF, and differentiating..7) one ore tie, we obtain the decoupled secondorder equations, with B as defined in..6): d a d b B a, B b However, it is better to work with..7) directly. This syste can be decoupled by foring the following linear cobinations of the a, b waves: A a Γ b B b Γa A B I Γ Γ I a b The A, B can be written in ters of V, I and the ipedance atrix Z as follows: A D) V + ZI) B D) V ZI) V DA + B) ZI DA B)..7) D Z + Z I Z..8) Using..7), we find that A, B satisfy the decoupled firstorder syste: d A B j B B A B da jba, with solutions expressed in ters of the atrix exponentials e ±jbz : db jbb..9) Az) e jbz A), Bz) e jbz B)..) Using..8), we obtain the solutions for V, I : Vz) D e jbz A)+e jbz B) ZIz) D e jbz A) e jbz B)..) To coplete the solution, we assue that both lines are terinated at coon generator and load ipedances, that is, Z G Z G Z G and Z L Z L Z L. The generator voltages V G,V G are assued to be different. We define the generator voltage vector and source and load atrix reflection coefficients: V G VG V G, The terinal conditions for the line are at z and z l : Γ G Z G I Z)Z G I + Z) Γ L Z L I Z)Z L I + Z)..) V G V)+Z G I), Vl) Z L Il)..3) They ay be reexpressed in ters of A, B with the help of..8): A) Γ G B) D ZZ + Z G I) V G, Bl) Γ L Al)..4) But fro..9), we have: e jbl B) Bl) Γ L Al) Γ L e jbl A) B) Γ L e jbl A)..5) Inserting this into..4), we ay solve for A) in ters of the generator voltage: A) D I Γ G Γ L e jbl ZZ + ZG I) V G..6) Using..6) into..), we finally obtain the voltage and current at an arbitrary position z along the lines: Vz) e jbz + Γ L e jbl e jbz I Γ G Γ L e jbl ZZ + ZG I) V G Iz) e jbz Γ L e jbl e jbz I Γ G Γ L e jbl Z + ZG I) V G..7) These are the coupledline generalizations of Eqs..9.7). Resolving V G and Vz) into their even and odd odes, that is, expressing the as linear cobinations of the eigenvectors e ±, we have: V G V G+ e + + V G e, where V G± V G ± V G Vz) V + z)e + + V z)e, V ± z) V z)±v z)..8) In this basis, the atrices in..7) are diagonal resulting in the equivalent solution: Vz) V + z)e + + V z)e e jβ+z + Γ L+ e jβ+l e jβ+z Z + Γ G+ Γ L+ e jβ+l V G+ e + Z + + Z G The atrices D, Z, Γ G,Γ L,Γ,B all coute with each other. + e jβ z + Γ L e jβ l e jβ z Γ G Γ L e jβ l Z Z + Z G V G e..9)
4 .. Coupled Transission Lines 599 where β ± are the eigenvalues of B, Z ± the eigenvalues of Z, and Γ G±,Γ L± are: Γ G± Z G Z ±, Γ L± Z L Z ±..3) Z G + Z ± Z L + Z ± The voltages V z), V z) are obtained by extracting the top and botto coponents of..9), that is, V, z) V + z)±v z) / : V z) e jβ+z + Γ L+ e jβ+l e jβ+z Γ G+ Γ L+ e jβ+l V + + e jβ z + Γ L e jβ l e jβ z Γ G Γ L e jβ l V V z) e jβ+z + Γ L+ e jβ+l e jβ+z Γ G+ Γ L+ e jβ+l V + e jβ z + Γ L e jβ l e jβ z Γ G Γ L e jβ l V..3) where we defined: ) Z ± VG± V ± Z ± + Z G 4 Γ G±)V G ± V G )..3) The paraeters β ±,Z ± are obtained using the rules of Eq...5). Fro Eq...), we find the eigenvalues of the atrices F ± G: F + G) ± β ± κ + χ) β ± L ) ω L ± L ) Z F G) ± β ± κ χ) β C ) ωz C C ) C Then, it follows that: β + F + G) + F G) + ω L + L )C C ) β F + G) F G) ω L L )C + C ) F + G) + L + L Z + Z F G) + Z Z F + G) F G) L C C L L C + C..33)..34) Thus, the coupled syste acts as two uncoupled lines with wavenubers and characteristic ipedances β ±,Z ±, propagation speeds v ± / L ± L )C C ), and propagation delays T ± l/v ±. The even ode is energized when V G V G, or, V G+,V G, and the odd ode, when V G V G, or, V G+,V G. When the coupled lines are iersed in a hoogeneous ediu, such as two parallel wires in air over a ground plane, then the propagation speeds ust be equal to the speed of light within this ediu 65, that is, v + v / μɛ. This requires: L + L )C C ) μɛ L L )C + C ) μɛ L μɛc C C L μɛc C C..35) Therefore, L /L C /C, or, equivalently, κ. On the other hand, in an inhoogeneous ediu, such as for the case of the icrostrip lines shown in Fig..., the propagation speeds ay be different, v + v, and hence T + T. 6. Coupled Lines. Crosstalk Between Lines When only line is energized, that is, V G,V G, the coupling between the lines induces a propagating wave in line, referred to as crosstalk, which also has soe inor influence back on line. The nearend and farend crosstalk are the values of V z) at z and z l, respectively. Setting V G in..3), we have fro..3): V ) Γ G+ ) + Γ L+ ζ+ ) Γ G+ Γ L+ ζ+ V V l) ζ + Γ G+) + Γ L+ ) Γ G+ Γ L+ ζ + V Γ G ) + Γ L ζ ) Γ G Γ L ζ V ζ Γ G ) + Γ L ) Γ G Γ L ζ V..) where we defined V V G / and introduced the ztransfor delay variables ζ ± e jωt± e jβ±l. Assuing purely resistive terination ipedances Z G,Z L, we ay use Eq..5.5) to obtain the corresponding tiedoain responses: V,t) Γ G+) Vt)+ + ) Γ G+ Γ L+ ) Vt T + ) Γ G+ Γ G Γ G ) Vt)+ + ) Γ G Γ L ) Vt T ) V l, t) Γ G+) + Γ L+ ) Γ G+ Γ L+ ) Vt T + T + ) Γ G ) + Γ L ) Γ G Γ L ) Vt T T )..) where Vt) V G t)/. Because Z ± Z, there will be ultiple reflections even when the lines are atched to Z at both ends. Setting Z G Z L Z, gives for the reflection coefficients..3): Γ G± Γ L± Z Z ± Γ ±..3) Z + Z ± In this case, we find for the crosstalk signals: V,t) + Γ +) Vt) Γ+ ) Γ + Vt T + ) + Γ ) Vt) Γ ) Γ Vt T )..4) V l, t) Γ +) Γ ) Γ + Vt T + T + ) Γ Vt T T ) Vt) is the signal that would exist on a atched line in the absence of line, V Z V G /Z +Z G ) V G /, provided Z G Z.
5 .. Crosstalk Between Lines 6 6. Coupled Lines Siilarly, the nearend and farend signals on the driven line are found by adding, instead of subtracting, the even and oddode ters: V,t) + Γ +) Vt) Γ+ ) Γ + Vt T + ) + + Γ ) Vt) Γ ) Γ Vt T )..5) V l, t) Γ + ) Γ+ Vt T + T + ) + Γ ) Γ Vt T T ) These expressions siplify drastically if we assue weak coupling. It is straightforward to verify that to firstorder in the paraeters L /L,C /C, or equivalently, to firstorder in κ, χ, we have the approxiations: β ± β ± Δβ β ± κ, Γ ± ± ΔΓ ± χ β, Z ± Z ± ΔZ Z ± Z χ β, T ± T ± ΔT T ± T κ β v ± v v κ β..6) where T l/v. Because the Γ ± s are already firstorder, the ultiple reflection ters in the above suations are a secondorder effect, and only the lowest ters will contribute, that is, the ter for the nearend, and for the far end. Then, V,t) Γ + Γ )Vt) V l, t) Vt T+ ) Vt T ) Γ+ Vt T + ) Γ Vt T ) Using a Taylor series expansion and..6), we have to firstorder: Vt T ± ) Vt T ΔT) Vt T) ΔT) Vt T), Vt T ± ) Vt T ΔT) Vt T) ΔT) Vt T) V dv dt Therefore, Γ ± Vt T ± ) Γ ± Vt T) ΔT) V Γ ± Vt T), where we ignored the secondorder ters Γ ± ΔT) V. It follows that: V,t) Γ + Γ ) Vt) Vt T) ΔΓ) Vt) Vt T) V l, t) Vt T) ΔT) V Vt T) ΔT) V dvt T) ΔT) dt These can be written in the coonly used for: V,t) K b Vt) Vt T) V l, t) K f dvt T) dt near and farend crosstalk)..7) where K b,k f are known as the backward and forward crosstalk coefficients: K b χ β v ) L + C Z, K f T κ ) 4 Z β v T L C Z Z..8) where we ay replace l v T. The sae approxiations give for line, V,t) Vt) and V l, t) Vt T). Thus, to firstorder, line does not act back to disturb line. Exaple..: Fig... shows the signals V, t), V l, t), V, t), V l, t) for a pair of coupled lines atched at both ends. The uncoupled line ipedance was Z 5 Ω L /L.4, C /C.3 line near end line far end line near end line far end t/t L /L.8, C /C.7 line near end line far end line near end line far end t/t Fig... Near and farend crosstalk signals on lines and. For the left graph, we chose L /L.4, C /C.3, which results in the even and odd ode paraeters using the exact forulas): Z Ω, Z Ω, v +.v, v.3v Γ +.7, Γ.9, T +.99T, T.88T, K b.75, K f.5 The right graph corresponds to L /L.8, C /C.7, with paraeters: Z +.47 Ω, Z 7.5 Ω, v +.36v, v.7v Γ +.4, Γ.49, T +.73T, T.58T, K b.375, K f.5 The generator input to line was a rising step with risetie t r T/4, that is, Vt) V Gt) t t r ut) ut tr ) + ut t r ) The weakcoupling approxiations are ore closely satisfied for the left case. Eqs...7) predict for V,t) a trapezoidal pulse of duration T and height K b, and for V l, t), a rectangular pulse of width t r and height K f /t r. starting at t T: V l, t) K f dvt T) dt K f t r ut T) ut T tr ) These predictions are approxiately correct as can be seen in the figure. The approxiation predicts also that V,t) Vt) and V l, t) Vt T), which are not quite true the effect of line on line cannot be ignored copletely.
6 .3. Weakly Coupled Lines with Arbitrary Terinations Coupled Lines The interaction between the two lines is seen better in the MATLAB ovie xtalkovie., which plots the waves V z, t) and V z, t) as they propagate to and get reflected fro their respective loads, and copares the to the uncoupled case V z, t) Vt z/v ). The waves V, z, t) are coputed by the sae ethod as for the ovie pulseovie. of Exaple.5., applied separately to the even and odd odes..3 Weakly Coupled Lines with Arbitrary Terinations The evenodd ode decoposition can be carried out only in the case of identical lines both of which have the sae load and generator ipedances. The case of arbitrary terinations has been solved in closed for only for hoogeneous edia 6,65. It has also been solved for arbitrary edia under the weak coupling assuption 7. Following 7, we solve the general equations..7)..9) for weakly coupled lines assuing arbitrary terinating ipedances Z Li,Z Gi, with reflection coefficients: Γ Li Z Li Z i, Γ Gi Z Gi Z i, i,.3.) Z Li + Z i Z Gi + Z i Working with the forward and backward waves, we write Eq...7) as the 4 4 atrix equation: a β κ χ dc jmc, c a b, M κ β χ χ β κ b χ κ β The weak coupling assuption consists of ignoring the coupling of a,b on a,b. This aounts to approxiating the above linear syste by: β dc j κ β χ ˆMc, ˆM.3.) β χ κ β Its solution is given by cz) e j ˆMz c), where the transition atrix e j ˆMz can be expressed in closed for as follows: e jβz κ e j ˆMz ˆκe jβz e jβz ) e jβz ˆχe jβz e jβz ) ˆκ β β, e jβz χ ˆχ ˆχe jβz e jβz ) ˆκe jβz e jβz ) e jβz β + β The transition atrix e j ˆMl ay be written in ters of the zdoain delay variables ζ i e jβil e iωti, i,, where T i are the oneway travel ties along the lines, that is, T i l/v i. Then, we find: a l) a l) b l) b l) ζ ˆκζ ζ ) ζ ˆχζ ζ ) ζ ˆχζ ζ ) ˆκζ ζ ) ζ a ) a ) b ) b ).3.3) These ust be appended by the appropriate terinating conditions. Assuing that only line is driven, we have: V )+Z G I ) V G, V )+Z G I ), which can be written in ters of the a, b waves: V l) Z L I l) V l) Z L I l) a ) Γ G b ) U, b l) Γ L a l) a ) Γ G b ), b l) Γ L a l), U Γ G ) V G Z.3.4) Eqs..3.3) and.3.4) provide a set of eight equations in eight unknowns. Once these are solved, the near and farend voltages ay be deterined. For line, we find: Z V ) a )+b ) + Γ Lζ Γ G Γ L ζ V Z V l) a l)+b l) ζ + Γ L ) Γ G Γ L ζ V where V Γ G )V G / Z V G /Z + Z G ). For line, we have: V ) κζ ζ )Γ L ζ + Γ L ζ )+ χ ζ ζ ) + Γ L Γ L ζ ζ Γ G Γ L ζ ) Γ G Γ L ζ ) V l) κζ ζ ) + Γ L Γ G ζ ζ )+ χ ζ ζ )Γ L ζ + Γ G ζ Γ G Γ L ζ ) Γ G Γ L ζ ).3.5) ) V ) V l.3.6) where V + Γ G )V + Γ G ) Γ G )V G / and V l + Γ L )V, and we defined κ, χ by: κ χ Z ˆκ Z Z ˆχ Z Z Z Z Z κ β β χ β + β ) ω L C Z β β Z ) ω L + C Z β + β Z.3.7) In the case of identical lines with Z Z Z and β β β ω/v, we ust take the liit: e jβl e jβl li d e jβl jle jβl β β β β dβ Then, we obtain: κζ ζ ) jωk f e jβl jω l ) L C Z e jβl Z χ K b v ).3.8) L + C Z 4 Z where K f,k b were defined in..8). Setting ζ ζ ζ e jβl e jωt, we obtain the crosstalk signals:
7 .4. CoupledMode Theory Coupled Lines V ) jωk f Γ L + Γ L )ζ + K b ζ ) + Γ L Γ L ζ ) V Γ G Γ L ζ ) Γ G Γ L ζ ) V l) jωk f + Γ L Γ G ζ )ζ + K b ζ )Γ L + Γ G )ζ Γ G Γ L ζ ) Γ G Γ L ζ ) V l.3.9) The corresponding tiedoain signals will involve the double ultiple reflections arising fro the denoinators. However, if we assue the each line is atched in at least one of its ends, so that Γ G Γ L Γ G Γ L, then the denoinators can be eliinated. Replacing jω by the tiederivative d/dt and each factor ζ by a delay by T, we obtain: V,t) K f Γ L + Γ L + Γ L Γ G ) Vt T) + K b + Γ G ) Vt) Vt T) + K b Γ L Γ L Vt T) Vt 4T) V l, t) K f + ΓL ) Vt T)+Γ L Γ G Vt 3T) + K b Γ L + Γ G + Γ L Γ L ) Vt T) Vt 3T).3.) where Vt) Γ G )V G t)/, and we used the property Γ G Γ L to siplify the expressions. Eqs..3.) reduce to..7) when the lines are atched at both ends..4 CoupledMode Theory In its siplest for, coupledode or coupledwave theory provides a paradig for the interaction between two waves and the exchange of energy fro one to the other as they propagate. Reviews and earlier literature ay be found in Refs , see also for the relationship to fiber Bragg gratings and distributed feedback lasers. There are several echanical and electrical analogs of coupledode theory, such as a pair of coupled pendula, or two asses at the ends of two springs with a third spring connecting the two, or two LC circuits with a coupling capacitor between the. In these exaples, the exchange of energy is taking place over tie instead of over space. Coupledwave theory is inherently directional. If two forwardoving waves are strongly coupled, then their interactions with the corresponding backward waves ay be ignored. Siilarly, if a forward and a backwardoving wave are strongly coupled, then their interactions with the corresponding oppositely oving waves ay be ignored. Fig..4. depicts these two cases of codirectional and contradirectional coupling. Fig..4. Directional Couplers. Eqs...7) for the basis of coupledode theory. In the codirectional case, if we assue that there are only forward waves at z, that is, a) and b), then it ay shown that the effect of the backward waves on the forward ones becoes a secondorder effect in the coupling constants, and therefore, it ay be ignored. To see this, we solve the second of Eqs...7) for b in ters of a, assuing zero initial conditions, and substitute it in the first: z bz) j e jfz z ) G az ) da z jf a + Ge jfz z ) G az ) The second ter is secondorder in G, or in the coupling constant χ. Ignoring this ter, we obtain the standard equations describing a codirectional coupler: da jf a d a β κ a j.4.) a κ β a For the contradirectional case, a siilar arguent that assues the initial conditions a ) b ) gives the following approxiation that couples the a and b waves: d a β χ a j.4.) b χ β b The conserved powers are in the two cases: P a + a, P a b.4.3) The solution of Eq..4.) is obtained with the help of the transition atrix e jfz : e jfz e jβz cos σz j δ sin σz σ j κ sin σz σ j κ sin σz σ cos σz + j δ.4.4) sin σz σ where β β + β Thus, the solution of.4.) is: a z) a z), δ β β e jβz cos σz j δ sin σz σ j j κ sin σz σ cos, σ δ + κ.4.5) κ sin σz σ σz j δ a ) a sin σz ) σ.4.6) Starting with initial conditions a ) and a ), the total initial power will be P a ) + a ). As the waves propagate along the zdirection, power is exchanged between lines and according to: P z) a z) cos σz + δ σ sin σz P z) a z) κ σ sin σz P z).4.7) Fig..4. shows the two cases for which δ/κ and δ/κ.5. In both cases, axiu exchange of power occurs periodically at distances that are odd ultiples of z π/σ. Coplete power exchange occurs only in the case δ, or equivalently, when β β. In this case, we have σ κ and P z) cos κz, P z) sin κz.
8 .5. Fiber Bragg Gratings Coupled Lines Co directional coupler, δ /κ P z) P z).5.5 σ z /π Fig Fiber Bragg Gratings Co directional coupler, δ /κ.5 P z) P z).5.5 σ z /π Power exchange in codirectional couplers. As an exaple of contradirectional coupling, we consider the case of a fiber Bragg grating FBG), that is, a fiber with a segent that has a periodically varying refractive index, as shown in Fig..5.. Fig..5. Fiber Bragg grating. The backward wave is generated by the reflection of a forwardoving wave incident on the interface fro the left. The grating behaves very siilarly to a periodic ultilayer structure, such as a dielectric irror at noral incidence, exhibiting highreflectance bands. A siple odel for an FBG is as follows : d az) j bz) β κ e jkz κe jkz β az) bz).5.) where K π/λ is the Bloch wavenuber, Λ is the period, and az), bz) represent the forward and backward waves. The following transforation reoves the phase factor e jkz fro the coupling constant: Az) e jkz/ Bz) e jkz/ d Az) j Bz) az) e jkz/ az) bz) e jkz/ bz) δ κ κ δ Az) Bz).5.).5.3) where δ β K/ is referred to as a detuning paraeter. The conserved power is given by Pz) az) bz). The fields at z are related to those at z l by: A) Al) δ κ e jfl, with F B) Bl) κ.5.4) δ The transfer atrix e jfl is given by: e jfl cos σl + j δ sin σl σ j j κ sin σl σ cos κ sin σl σ σl j δ U U U sin σl U σ.5.5) where σ δ κ.if δ < κ, then σ becoes iaginary. In this case, it is ore convenient to express the transfer atrix in ters of the quantity γ κ δ : e jfl cosh γl + j δ sinh γl γ j j κ γ sinh γl cosh κ sinh γl γ γl j δ.5.6) sinh γl γ The transfer atrix has unit deterinant, which iplies that U U. Using this property, we ay rearrange.5.4) into its scattering atrix for that relates the outgoing fields to the incoing ones: B) Γ T Al) T Γ A), Γ U, Γ U, T.5.7) Bl) U U U where Γ, Γ are the reflection coefficients fro the left and right, respectively, and T is the transission coefficient. We have explicitly, j κ sin σl Γ σ cos σl + j δ, T σ sin σl cos σl + j δ.5.8) σ sin σl If there is only an incident wave fro the left, that is, A) and Bl), then.5.7) iplies that B) ΓA) and Al) TA). A consequence of power conservation, A) B) Al) Bl),is the unitarity of the scattering atrix, which iplies the property Γ + T. The reflectance Γ ay be expressed in the following two fors, the first being appropriate when δ κ, and the second when δ κ : Γ T κ sin σl σ cos σl + δ sin σl κ sinh γl γ cosh γl + δ sinh γl.5.9) Fig..5. shows Γ as a function of δ. The highreflectance band corresponds to the range δ κ. The left graph has κl 3 and the right one κl 6. As κl increases, the reflection band becoes sharper. The asyptotic width of the band is κ δ κ. For any finite value of κl, the axiu reflectance achieved
9 .5. Fiber Bragg Gratings Coupled Lines Γ Fiber Bragg Grating, κ l 3 Γ Fiber Bragg Grating, κ l 6 where we replaced U Γ/T and U /T. Assuing a quarterwavelength spacing d λ B /4 Λ/, we have βd δ + π/λ)d δd + π/. Replacing e jβd e jδd+jπ/ je jδd, we obtain: Γ cop Γ T e jδd Te jδd) T e jδd Γ Te jδd.5.3) At δ, we have T T / cosh κ l, and therefore, Γ cop. Fig..5.4 depicts the reflectance, Γ cop, and transittance, Γ cop, for the case κl... Copound Grating, κ l Copound Grating, κ l δ /κ Fig δ /κ Reflectance of fiber Bragg gratings. at the center of the band, δ, is given by Γ ax tanh κl. The reflectance at the asyptotic band edges is given by: Reflectance Transittance Γ κl, + κl at δ ± κ The zeros of the reflectance correspond to sin σl, or, σ π/l, which gives δ ± κ + π/l), where is a nonzero integer. The Bragg wavelength λ B is the wavelength at the center of the reflecting band, that is, corresponding to δ, or, β K/, or λ B π/β 4π/K Λ. By concatenating two identical FBGs separated by a spacer of length d λ B /4 Λ/, we obtain a quarterwave phaseshifted FBG, which has a narrow transission window centered at δ. Fig..5.3 depicts such a copound grating. Within the spacer, the A, B waves propagate with wavenuber β as though they are uncoupled. Fig..5.3 Quarterwave phaseshifted fiber Bragg grating. The copound transfer atrix is obtained by ultiplying the transfer atrices of the two FBGs and the spacer: V U FBG U spacer U FBG, or, explicitly: V V U U e jβd U U V V U U e jβd U U.5.) where the U ij are given in Eq..5.5). It follows that the atrix eleents of V are: V Ue jβd + U e jβd, V U U e jβd + Ue jβd).5.) The reflection coefficient of the copound grating will be: Γ cop V U U e jβd + Ue jβd) V Ue Γ T e jβd + Te jβd) jβd + U e jβd T e jβd + Γ Te jβd.5.) δ /κ Fig δ /κ Quarterwave phaseshifted fiber Bragg grating. Quarterwave phaseshifted FBGs are siilar to the FabryPerot resonators discussed in Sec Iproved designs having narrow and flat transission bands can be obtained by cascading several quarterwave FBGs with different lengths Soe applications of FBGs in DWDM systes were pointed out in Sec Diffuse Reflection and Transission Another exaple of contradirectional coupling is the twoflux odel of Schuster and KubelkaMunk describing the absorption and ultiple scattering of light propagating in a turbid ediu 95. The odel has a large nuber of applications, such as radiative transfer in stellar atospheres, reflectance spectroscopy, reflection and transission properties of powders, papers, paints, skin tissue, dental aterials, and the sea. The odel assues a siplified parallelplane geoetry, as shown in Fig..6.. Let I ± z) be the forward and backward radiation intensities per unit frequency interval at location z within the aterial. The odel is described by the two coefficients k, s of absorption and scattering per unit length. For siplicity, we assue that k, s are independent of z. Within a layer, the forward intensity I + will be diinished by an aount of I + k due to absorption and an aount of I + sdue to scattering, and it will be increased by an aount of I sarising fro the backwardoving intensity that is getting scattered
10 .6. Diffuse Reflection and Transission 6 6. Coupled Lines Fig..6. Forward and backward intensities in stratified ediu. The reflectance and transittance corresponding to a black, nonreflecting, background are obtained by setting R g in Eq..6.4): R U s sinh βl U β cosh βl + α sinh βl T.6.5) β U β cosh βl + α sinh βl The reflectance of an infinitelythick ediu is obtained in the liit l : R s α + β s k + s + kk + s) k s R ) R.6.6) forward. Siilarly, the backward intensity, going fro z + to z, will be decreased by I k + s) ) and increased by I + s ). Thus, the increental changes are: or, written in atrix for: d di + k + s)i + + si di k + s)i + si + I+ z) k + s s I z) s k s I+ z) I z).6.) This is siilar in structure to Eq..5.3), except the atrix coefficients are real. The solution at distance z l is obtained in ters of the initial values I ± ) by: I+ l) e Fl I+ ) k + s s, with F I l) I ) s k s The transfer atrix e Fl is: U e Fl cosh βl α s sinh βl β s sinh βl β cosh sinh βl β βl + α sinh βl U U U U β.6.).6.3) where α k + s and β α s kk + s). The transfer atrix is uniodular, that is, det U U U U U. Of interest are the input reflectance the albedo) R I )/I + ) of the lengthl structure and its transittance T I + l)/i + ), both expressed in ters of the output, or background, reflectance R g I l)/i + l). Using Eq..6.), we find: For the special case of an absorbing but nonscattering ediu k,s ), we have α β k and the transfer atrix.6.3) and Eq..6.4) siplify into: e U e Fl kl e kl, R e kl R g, T e kl.6.7) These are in accordance with our expectations for exponential attenuation with distance. The intensities are related by I + l) e kl I + ) and I l) e kl I ). Thus, the reflectance corresponds to traversing a forward and a reverse path of length l, and the transittance only a forward path. Perhaps, the ost surprising prediction of this odel first pointed out by Schuster) is that, in the case of a nonabsorbing but scattering ediu k,s ), the transittance is not attenuating exponentially, but rather, inversely with distance. Indeed, setting α s and taking the liit β sinh βl l as β, we find: sl sl U e Fl, R sl + sl)r g, T.6.8) sl + sl + sl slr g + sl slr g In particular, for the case of a nonreflecting background, we have:.7 Probles R sl + sl, T + sl.6.9). Show that the coupled telegrapher s equations..4) can be written in the for..7).. Consider the practical case in which two lines are coupled only over a iddle portion of length l, with their beginning and ending segents being uncoupled, as shown below: R U + U R g U U R g T s sinh βl + β cosh βl α sinh βl)r g β cosh βl + α sr g )sinh βl β U U R g β cosh βl + α sr g )sinh βl These are related to the noralized Kubelka variables a α/s, b β/s..6.4) Assuing weakly coupled lines, how should Eqs..3.6) and.3.9) be odified in this case? Hint: Replace the segents to the left of the reference plane A and to the right of plane B by their Thévenin equivalents.
11 .7. Probles 63.3 Derive the transition atrix e j ˆMz of weakly coupled lines described by Eq..3.)..4 Verify explicitly that Eq..4.6) is the solution of the coupledode equations.4.)..5 Coputer Experient Fiber Bragg Gratings. Reproduce the results and graphs of Figures.5. and Ipedance Matching 3. Conjugate and Reflectionless Matching The Thévenin equivalent circuits depicted in Figs... and..3 also allow us to answer the question of axiu power transfer. Given a generator and a lengthd transission line, axiu transfer of power fro the generator to the load takes place when the load is conjugate atched to the generator, that is, Z L Z th conjugate atch) 3..) The proof of this result is postponed until Sec Writing Z th R th + jx th and Z L R L +jx L, the condition is equivalent to R L R th and X L X th. In this case, half of the generated power is delivered to the load and half is dissipated in the generator s Thévenin resistance. Fro the Thévenin circuit shown in Fig..., we find for the current through the load: I L V th V th Z th + Z L R th + R L )+jx th + X L ) V th R th Thus, the total reactance of the circuit is canceled. It follows then that the power delivered by the Thévenin generator and the powers dissipated in the generator s Thévenin resistance and the load will be: P tot ReV th I L) V th 4R th P th R th I L V th 8R th P tot, P L R L I L V th P tot 8R th 3..) Assuing a lossless line realvalued Z and β), the conjugate atch condition can also be written in ters of the reflection coefficients corresponding to Z L and Z th : Γ L Γ th Γ G ejβd conjugate atch) 3..3) Moving the phase exponential to the left, we note that the conjugate atch condition can be written in ters of the sae quantities at the input side of the transission line:
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