NAT L INST. OF STAND & TECH R.I.C. REFERENCE

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NIST

PUBLICATIONS

NIST Monograph 176

Modified Airy Function

and WKB Solutions

to the Wave Equation

I A. K. Ghataky R. L. GallawUy and 1. C. Goyal

- QC100

U556 M United States Department of Commerce

#176 National Institute of standards and Technology

1991

NIST Monograph 176

Modified Airy Function

and WKB Solutions

to the Wave Equation

QClO

mi

A. K. Ghatak

R. L. Gallawa

I. C. Goyal

Electromagnetic Technology Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80303

This monograph was prepared, in part, under the auspices of the Indo-US Collaborative Program in Materials Sciences. Permanent Affiliation of Professors Ghatak and Goyal is Physics Department, Indian Institute of Technology

New Delhi, India. November 1991

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards and Technology Monograph 176

Natl. Inst. Stand. Technol. Mono. 176, 172 pages (Nov. 1991)

CODEN: NIMOEZ

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1991

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
TABLE OF CONTENTS

PREFACE vii

1. INTRODUCTION 1

2. WKB SOLUTIONS TO INITIAL VALUE

PROBLEMS 6

2.1 Introduction 6

2.2 The WKB Solutions 6

2.3 An Alternative Derivation 11

2.4 The General WKB Solution 14

Case I: Barrier to the Right 15

Case II: Barrier to the Left 19

2.5 Examples 21

Example 2.1 21

Example 2.2 27

3. AIRY FUNCTIONS 32

3.1 Introduction 32

3.2 The Airy Functions 32

3.3 Asymptotic Forms and the Connection

Formulas 37

4. MAF SOLUTION TO INITIAL VALUE

PROBLEMS 42

4.1 Introduction 42
iv Introduction

4.2 The MAF Solution 43

4.3 Examples 47

Example 4.1 47

4.4 Summary 49

5. WKB SOLUTIONS TO EIGENVALUE

PROBLEMS 53

5.1 Introduction 53

5.2 The WKB Quantization Condition 59

5.3 Examples 63

Example 5.1 . 63

Example 5.2 69

Example 5.3 77

Example 5.4 80

Example 5.5 88

5.4 The WKB Method for Three-Dimensional

Problems 92

Example 5.6 94

6. MAF SOLUTIONS TO EIGENVALUE

PROBLEMS 97

6.1 Introduction 97

6.2 Formulation of the Problem 98

6.3 Examples 100

Example 6.1 100

Example 6.2 106

Example 6.3 122

Example 6.4 128

Example 6.5 135
Introduction v

6.4 Conclusions 140

7. EXTENSION OF MAF METHOD FOR

EIGENVALUES 143

7.1 Introduction 143

7.2 Formulation and Example 7. 1 143

Example 7.2 149

Example 7.3 152

7.3 Conclusions 158

PREFACE

The WKB method has been known and used by students of

mathematical physics since the mid- 1920s. It has served amazingly well in a variety of problems, especially in problems of

quantum mechanics and nonuniform optical waveguides, the

discipline treated in this monograph. The success and popularity

of the method occurs in spite of a serious deficiency, namely that

it does not hold at the turning points unless suitable modifications

are invoked. It is, at once, intuitive and powerful, and it sometimes yields very accurate eigenvalues in spite of its inherent

approximate nature. Its use by the optics community is well

documented. The real power of the method lies in its utility; it is useful in an amazing array of physics problems.

The WKB method springs from a knowledge of the solution to the

governing differential equation when the nonuniform refractive

index (for waveguide problems) or potential function (for quantum

well problems) is uniform. The resulting method yields the exact

solution if the nonuniform region becomes uniform. In an

analogous manner, this monograph concentrates on a method that

becomes exact if the nonuniform region takes on a linear variation.

The method is thus based on the solution to the governing equation

when the nonuniform region varies linearly with x; this suggests

that the result is a higher order approximation than the WKB

approximation. Numerical tests indicate that this seems to be the

case. In addition, and in contrast to the WKB method, these

equations have the significant advantage of being valid even in the
viii Preface

vicinity of and at the turning point. For problems of interest in

optical telecommunications devices, there is, in fact, a turning

point, at which there is a first order zero. It is this class of

problems that is discussed in this monograph.

What we present in this monograph uses a modification of the Airy

functions to describe the wave functions; we have, therefore,

chosen to refer to the method as the Modified Airy Function or MAF method. The method used here is not new. It was

developed 60 years ago in a classic paper by Langer (1931), in

which he developed an asymptotic solution to the Helmholtz

equation. The MAF equations can be recast in the form used by

Langer. Langer used Bessel functions; we choose to use Airy

functions. The two are equivalent. In working with Langer's

equations, we have found, to our dismay, that the method has been

dramatically underutilized by the scientific community. It is,

therefore, appropriate to show, through illustrative examples, the

power, the simplicity, and the profound utility of the MAF method.

Herein lies the purpose of this monograph.

This monograph discusses the WKB and the MAF methods in

considerable detail, to the end that the reader gains an appreciation

of the strengths and weaknesses of each. We treat eigenvalue

problems as well as initial value problems. The two methods

could be illustrated via any of several disciplines, but we chose to use problems that are of current interest to the optical waveguide

community. We have also used Schrodinger's equation for typical

potential well problems to further illustrate the method. Both the

wave functions and the eigenvalues are found. We have also
Preface ix

included several exercises, to be solved by the reader. We trust and hope that, by giving several examples to illustrate the method,

the reader will appreciate the utility of the method.

Advancements in the understanding of physical science often

spring from the harmonic convergence of fortuitous but

unpredictable (sometimes serendipitous) events. Techniques in

mathematical physics, being intimately related to the physical

sciences, follow a similar trend. The fortuitous events that led,

ultimately, to the writing of this monograph are traceable to the

vision, encouragement, and dynamic leadership of colleagues and

friends to whom we are profoundly indebted. They include (in

alphabetical order) Dr. B. Stephen Carpenter (NIST Director of

International Relations), Dr. P.L.M. Heydemann (Science

Counselor, U.S. Embassy, New Delhi), Dr. R.A. Kamper (NIST,

Boulder), Professor N.C. Nigam (Director, IIT New Delhi), and

Dr. Alfons Weber (NIST, Gaithersburg). Their encouragement

and support (both financial and moral) is deeply appreciated.

Finally, a special thanks to Mr. Aaron A. Sanders (NIST,

Boulder), whose enthusiastic support was pivotal to the writing of

this monograph.

1. INTRODUCTION

The purpose of this monograph is to revisit a basic equation of

mathematical physics,

^ + THx)^(x) = 0 , (1)

and to give approximate solutions based on the WKB method and on a modification of the Airy function. All of the examples that

we will use to illustrate the methods are based on optical waveguides and quantum mechanical problems. We hope that this

monograph will prove to be tutorial, giving insight and

understanding to the use of Airy functions in addressing the scalarwave equation.

Equation (1) is encountered in many areas of physics and

engineering. In quantum mechanics, for example, the one- dimensional Schrodinger equation is of the same form as Eq. (1)

with

THx) = ^[E-V(x)], (2)

where m is the mass of the particle, E the total energy, V(x) the

potential energy function and = h/lir, h being Planck's constant. In Eq. (2) E appears as an eigenvalue in a bound state problem and
2 Introduction

as a given (variable) parameter in a scattering problem [see, e.g.,

Schiff (1968)]. Similarly, in optical waveguide theory, for a medium characterized by the refractive index variation n ^(x), the

y-component of the electric tield can be written in the form

Ey{x,z,t) = i|f(A:)exp[i(a)f- Pz)], (3)

where T(x) satisfies Eq. (1) with

r^Cjc) = kln^{x) - p2 (4)

and ^0 ~ is the free space wave number. Equation (3)

describes the TE modes of the slab waveguide. In Eqs. (3) and

(4), 13 is the propagation constant which forms a discrete set for

guided modes and a continuum for radiation modes [see, e.g.,

Ghatak and Thyagarajan (1989)].

For some specific profiles, that is, for some specific forms of r

^

(x), we can obtain analytical solutions of Eq. (1). However, for

profiles which do not lead to exact solutions, we usually resort to one of the three approximate methods: the perturbation method,

the variational method, and the WKB method. The perturbation

method is based on a closely related problem which yields an exact

solution and we usually resort to first-order perturbation; even then

it is extremely difficult to calculate the perturbed eigenfunction as

it would involve the summation over an infinite series. On the

other hand, the variational method can give a good estimate for the

lowest-order mode by choosing an appropriate trial function and

carrying out an optimization; the method becomes quite

cumbersome when one has to apply it to higher-order modes.
Introduction 3

The WKB method was first proposed in the mid- 1920s by Wentzel

(1926), Kramers (1926), Brillouin (1926), and Jeffreys (1923);

indeed because of the contributions made by Jeffreys, the method

is often referred to as the WKBJ or even JWKB method.

The traditional WKB method is well known and has been used

extensively [Froman and Froman (1965), Heading (1962), Pauli

(1980), or almost any text on quantum mechanics]. While several

improvements have been suggested [see Hecht (1957), Bahar

(1967)], the method remains primarily the tool that was originally

proposed.

Our intent is to be primarily tutorial, discussing both the traditional

WKB method and a relatively unknown competing and much more

powerful technique that uses Airy functions. This method is WKBlike and was first suggested by Langer (1931). The advantages

and disadvantages of each method will be demonstrated by way of

examples.

Langer's 1931 paper is pivotal to what is presented here. In our

introduction to that method, we will give a derivation that is less

esoteric than that of Langer. We will illustrate the method with

examples, in the hope that the method will be more widely known

and more widely used. We note in this regard that the Modified

Airy Function method (or MAF, which is how we will refer to the

method formulated by Langer) is not well known and is seldom

used in modem mathematical physics. The analysis of nonuniform

optical waveguides (eigenvalue problems), for example, benefits

greatly from this approach, a fact that will be illustrated. Initial-
4 Introduction

value problems are also amenable to the MAF method.

There is a profound and glaring paucity of literature on the MAF

method, in spite of its importance, its range of applications, and

its accuracy. It is, happily, quite easy to use, even with a desktop

computer; the ease is attributable to the ease with which Airy

functions (the basis of the solution) can be evaluated.

The framework of the MAF solution appears in a few books but

the method is not exploited. An important exception is the book

by Bender and Orszag (1978), where the method is described in a very understandable form. What we present in this monograph is a supplement to the presentation of Bender and Orszag. We use

current examples to illustrate the method.

Because the MAF solutions use Airy functions, it may be thought

that they are the traditional WKB result, wherein the Airy functions are used to derive the connection formulas. However, the MAF

solutions give, in a single closed form, an approximation that holds

throughout the region of interest, including (but not restricted to)

the vicinity of and at a turning point; a turning point is where

T^{x) becomes zero. The WKB solutions are valid in regions far

removed from the turning point, but not in its vicinity.

While solving the eigenvalue problem we will show that use of the

first-order perturbation theory, using the present Modified Airy

Function solutions, yields extremely accurate eigenvalues. Thus

the MAF solution gives an accurate description of the wave

function as well as of the eigenvalues.
Introduction 5

The monograph contains many examples to illustrate the methods

and to allow a comparison between the WKB and the MAF

solutions. In addition, we include a few problems for the benefit

of those who want additional insight.
2. WKB SOLUTIONS TO INITIAL VALUE PROBLEMS

2.1 Introduction

This chapter is devoted to the WKB approximation to the onedimensional wave equation,

^1:^ + r^U) = 0 , (1)

where T represents the electromagnetic field or the wave function

and r^(jc) is assumed to be "slowly varying" (we will quantify this

assumption later). These WKB solutions are not valid near the

turning points, where r^(jc) = 0. We then discuss the formulas

that facilitate the connection between the two WKB solutions, one

on each side of the turning point. Finally, we will illustrate the use of the connection formulas in solving initial value problems.

In Chapters 4 and 6 we will show how the use of modified Airy

functions leads to an improved solution (in most cases) that

precludes the need for connection formulas.

2.2 The WKB Solutions

If a plane electromagnetic wave propagating in the x direction is incident normally onto an inhomogeneous medium having

refractive index n(x), the electric field satisfies the equation

^lli^ ,^nHx)^(.x)-Q, (2)
WKB Solutions to Initial Value Problems 7

where T{3cj is either or E^, the E^ component is taken to be 0.

Thus, t\x) of Eq. (1) is THx) = ^nHx) -- ^^^lltli^ , (3)

where

\ = ^

(4)

represents the free-space wavelength. If t\x) is a constant (that

is, if the medium is homogeneous), the solution (aside from a

multiplying constant) is given by

i|r(:c) = exp(±irx )

. (5)

This suggests that, for the nonuniform medium [r^ = r^(;c)], we

try a solution of the form

^^{x) = exp[±iM(;:)] , (6)

where u(x) is to be determined. Substituting Eq. (6) into Eq. (1)

yields

i u'^ - {u'f + r^(;c) = 0 , where primes denote differentiation with respect to x. Equation

(7) is exact. If is a constant, then u" = 0; thus, when t\x) is slowly varying, we may assume u" to be negligible in comparison

to {u')^ to obtain

[u'Y = V\x) . (8)

The solution of this equation is
8 WKB Solutions to Initial Value Problems

(9)

(10)

u(x) = ±1'' r(jc) dx . Thus, in this approximation, we obtain

^(x) « %(x) = exp [±if" r(x)dx] , which is known as the zeroth-order WKB approximation. The

physical interpretation of Eq. (10) is obvious: the inhomogeneous

region is divided into a series of homogeneous slabs, each of

which has thickness Ax in which r = r(jc). The total phase

change is then the summation of the incremental phase changes

which, in the limit Ax ^ 0, becomes the integral on the right side

of Eq. (10).

To proceed, we use the differential equation satisfied by To(x);

differentiation gives

+ dx^

r^(x) T I —dx

(11)

Equation (11) is the same as Eq. (1), provided that we assume

dr

dx

(12)

or 1

^

r dx

« r(x) . (13)

This is the condition for the validity of the WKB solution. It quantifies the "slowly varying" condition referred to in the opening

paragraph of section 2.1. If we use the relation
WKB Solutions to Initial Value Problems 9

^0

Eq. (12) becomes

or

1 dn

n dx

dn^

dx

« n ,

(14)

(15)

(16)

where A. = k^ln is the wavelength in the medium. Equation (16)

implies that the change in the refractive index in a distance of a wavelength should be very small compared to the refractive index.

This is another condition for the validity of the WKB

approximation or the "slowly varying" condition.

To obtain the more accurate higher-order solution, we modify the

original assumption [Eq. (10)] and write

^{x) = Fix) i|f,(jc) = Fix) exp[±//' Vix)dx] , d^)

where, in the spirit of the WKB approximation, F(x) must be

slowly varying. To determine F(x), we substitute Eq. (17) into

Eq. (1) and neglect the term involving d^F/dj^ to obtain

1^ = dnx) (18)

F dx ITix) dx

'

or ^[ln(F/rw)] = 0. (19)

Thus,
10 WKB Solutions to Initial Value Problems

Fix) = . (20)

The first-order WKB solutions thus become

^(x) = —i— exp ifvixydx]. (21)

Equation (21) is the equation normally meant when reference is made to the WKB solutions. Differentiating twice, we obtain

dx^

1 d^r dr

2r dx^ 4rM dx ]

^(x) = 0 .

(22)

Equation (22) is the same as Eq. (1), provided that

1 d^r

2r dx^

3 (dry < < V-ix) (23)

Clearly the first and second derivatives of r(x) must both be much

smaller than r^(jc) in this approximation.
WKB Solutions to Initial Value Problems 11

2.3 An Alternative Derivation

We now give an alternative (and a more rigorous) derivation of the

WKB solution. For this purpose we assume a scenario according

to Eq. (2). Thus, we consider the equation:

2,1, ^2

where

dx

^ = —

+ —— i|f(jc) = 0 ,

and

We try a solution of the form

= exp

r(x)

i S(x)

n(x)

(24)

(25)

(26)

where S(x) is to be determined. Substituting Eq. (26) into Eq

(24), we obtain

c// «

2

+ I — + —^ = 0 (27)

^0 ^

We next expand S(x) in a power series in Xq:

S(x) = S,(x) + S,(x) + S^(x) + . (28)

The logic of this expansion is as follows: If 0» then virtually

any refractive index variation is slowly varying and the WKB

approximation is accurate. Indeed, in the limit of -^Tq 0, we have

the geometrical optics approximation. Substituting Eq. (28) into

Eq. (27), we obtain
12 WKB Solutions to Initial Value Problems

- i [5o" + *o < + ^0 S2 + •••] - nHx) = 0 .

(29)

Since Eq. (29) must be satisfied for all values of Xq, the

coefficient of each power of Aq, must be 0. This leads to

< dx }

dS. dS

= n\x) , d^S.

dx dx dx

0 ,

[dx )

dS. dS, d^S,

+ 2 —5 - i i = 0

dx dx dx

and so forth. The solution of Eq. (30) is:

dx

= ±n(x) ,

(30)

(31)

(32)

(33)

so So(x) = ± nix) dx (34)

From Eq. (31) we obtain

dS, _ _ i 1 d (dS,'

dx 2 idSJdx) dx\dx , (35)

i dn

In dx

so
WKB Solutions to Initial Value Problems 13

S^(x) = — ]n [n(x)] + constant (36)

Therefore,

exp [i =

const

(37)

Retaining terms up to S^, we obtain for 7

ijr(x) = exp —i So

2 + f 5

const

const

exp ± Y"n {x) dx

exp ± i T (x) dx

(38)

where we have used Eq. (25). These again represent the WKB

solutions [cf. Eq. (21)]. Using Eq. (35) and its differential in Eq.

(32), we readily obtain

S^ix) = ± f gn

(«') 1 2 // -n^ n^^ 4

dx (39)

Similarly,
14 WKB Solutions to Initial Value Problems

5,(x) = i -—3 (n')^

+

1 n

II (40)

16

These equations give further insight into the concept of "slowly

varying" since they are the terms that are neglected in the WKB

solution.

2.4 The General WKB Solutions

The general WKB solution is the linear combination

We could also have used the sine and cosine solutions:

We have, up to now, assumed that V^{x) is positive. For

v\x) < 0, we write Eq. (1) as

(41)

(42)

(43)

where
WKB Solutions to Initial Value Problems 15

Proceeding in a manner similar to that used to obtain Eq. (21), we

obtain the following WKB solution of Eq. (43):

^{x) = exp[ f\(x)dx] + —^^ exp\-f\(x)dx

/Too ^ yf<x) ^ ^ (45)

This solution is valid when an equation similar to Eq. (12) is satisfied for k(jc). Obviously, they will not be valid near a turning

point, where or t\x) = 0. To be specific, we consider a

variation of T^{x) of the form shown in Fig. 2.1.

The point x = a represents the turning point. Near x = a, the

WKB solutions are not valid. For jc < < a, the solutions are given

by Eqs. (41) and (42) and by Eq. (45) for x > > a. These

solutions on one side of the turning point must now be matched to the solutions on the other side. The matching is done through the

connection formulas given below; these formulas are justified in

the next chapter.

Case I: Barrier to the Right

For reasons that will become clear later, the variation of (jc) given in Fig. 2.1 is often called "barrier to the right." The

connection formulas are:
16 WKB Solutions to Initial Value Problems

sin

V <x) (46)

/T(x)

COS

exp

/;rwd.*fl<-»

' \W) (47)

exp 1 +

Thus, the WKB solution given by

/TOO

sm

Ja 4

WKB Solutions Not Valid

Exponentially

Growing/ Decaying

WKB Solutions

Figure 2. 1 r^(x) variation corresponding to barrier to the right.
WKB Solutions to Initial Value Problems 17

becomes an exponentially decaying solution and the cosine solution

becomes the exponentially growing solution. The connection

formulas are safe to use only in the direction of the double arrow.

If they are used in the direction of the single arrow, an error may

be introduced. This can be understood by introducing a small

phase term in the argument of the sine function.

/TOO

sin r (x)dx + - + e 4

sin ''r(x)dx + ^

X 4

cos €

("nxydx^^

frTx) [ •'^ 4 —^ cos € r / \ J > exp - / k{x) dx

sin €

(48)

2 sin € r fx , . , + ——^

exp + / K (x) ax

Eventually the exponentially growing term overwhelms the

exponentially decaying term so we may neglect the latter; we thus

obtain
18 WKB Solutions to Initial Value Problems

sin f'rix) djc + - + €

(49)

2 sin 6

exp ^j\(x)dx

Hence, a negligible phase change in the wave function that is valid

for X < < a [left side of Eq. (46)] leads to an exponentailly

increasing wave function for jc > > a (instead of an exponentially

decreasing function) if the connection formula is used in the

direction of the single arrow. Similarly, we add a term

1 exp -j K(x)dx

on the right side of Eq. (47). With this term, which is negligible

for;c > > a, and using Eq. (46), we obtain — cos / V dx ^ —

2 . + — sm — exp [ + K dx — exp [ - f^K dx

(50)

or
WKB Solutions to Initial Value Problems 19

where a = sin"^ — . (52)

v/5

Hence, in this case, the addition of a negligible decaying term in

the solution for the region x > > a changes the corresponding

solution in the region x < < a significantly if we use the

connection formula in the direction of the single arrow.

It is for this reason that the connection formulas should generally

be used in the direction of the double arrow [Froman and Froman

(1965) and Pauli (1980)]. Nevertheless, the connection formulas

[given by Eqs. (46) and (47)] have been extensively used in the reverse direction also and accurate results have been obtained (see

Chapter 5).

Case n: Barrier to the Left

Consider next the variation of r^(x) as given by Fig. 2.2; this is often called "barrier to the left. " The solutions are exponential on

the left of X = a and sine or cosine functions on the right of

X = a. The connection formulas are
Figure 2.2 r^(jc) variation corresponding to barrier to the left.
WKB Solutions to Initial Value Problems 21

1 <<• •>

(54)

1 f'rix)dx + 4 . J a 4

COS

These formulas are again safe to use only in the direction of the

double arrow; however, we will use them in both directions.

We illustrate the use of the connection formulas through examples.

2.5 Examples

Example 2.1

In this example (see Fig. 2.3)

and the boundary conditions are T(0) = 1 and T'(0) = 0. Thus,

X = 2 represents the turning point. In the region x < 2 the

solutions are oscillatory and for x > 2, the solutions are exponentially growing and exponentially decaying. To find the

WKB solutions in the region jc < 2, consider the following. Let

(X) = 2 - JC, (55)
22 WKB Solutions to Initial Value Problems

Ti(x) = p Fix) dx

(56) = f (2 - x)"^ dx = ^(2- x)'!^

J X 3

Thus, for jc < 2
WKB Solutions to Initial Value Problems 23

WKB sin

[ \'

/TOO

T{x) dx a

r(0) sin [ti(x) + g]

M r(x) sin [Ti(0) + a]

2 -x)

1/4 Sin - (2 - ;c)^/2 + a

3

sin Y

(57)

where

4^2

= -4- + a . (58)

and we have used the condition T(0) = 1. The other initial condition (T'(0) = 0) gives

In the region x > 2, we define

K^(jc) = - T^(x) = X - 2 ,

(59)

(60)

C(;c) = j"" k(jc) = l\x - dx

3

(61)

We now use the connection formulas to obtain the exponentially

decaying and growing solutions forjc > 2. To that end, we write

the solution given by Eq. (57) in terms of the left side of Eqs. (46)

and (47). Thus
24 WKB Solutions to Initial Value Problems — sin [ f

^ T{x) Jjc + a

1 . \ ( F2 ——^

Sin

/TOO '

= sin

I 4 j cos /TOO

+ cos I a - —

^ sin

r(x) dx + ^ fr(x) + 4

> sin f a - — I ^ exp [ k(jc)

+ COS I a - — I ——— exp [ - f* k(x) dx

(62)

The WKB solution for x > 2 is thus given by

21/4

sin Y

sin a

4

(x - 2) 1/4

COS a - —

il

2(x - ly*

exp [-CW]

exp[C(x)]

for a: > 2

(63)
WKB Solutions to Initial Value Problems 25

The exact solution of Eq. (55) is given by

i|f = 71 [Ai(x - 2) Bi\-2) - Bi(x - 2) Ai\-2)] , (^4)

with Ai'(-2) = 0.618 259 02 and 5z'(-2) = 0.278 795 17. The

exact solution given by Eq. (64) is plotted in Fig. 2.4 together

with the WKB solution given by Eqs. (57) and (63). The WKB

solution agrees with the exact solution away from the turning point

X = 2.

Figure 2.4 Exact and WKB solutions for Example 2.1.
26 WKB Solutions to Initial Value Problems

Care is required in using the WKB (or other approximate)

solutions. Consider, for example,

r^(jc) = 1 - jc

The plots of the WKB and exact solutions (subject to the same

initial conditions) are shown in Fig. 2.5. The significant

difference in the large x behavior in Fig. 2.5 is because x = -1 is very close to the zero of Ai'{x), Indeed,

AV{-\.Q) = -0.010 160 57, Ai'(-IA) = +0.046 029 15, and

Ai\-1.0) is the coefficient of the growing term Bi(x - 1) in the

exact solution.

t(x)

Figure 2.5 Exact and WKB solutions for r^(x) = I - x. Compare with Fig. 2.4.
WKB Solutions to Initial Value Problems 27

Example 2.2

We next consider the equation

^ + 4

[exp(- X + a) - 1] i|r(jc) =0, 0 ^ jc < ~ ,

(66)

subject to the initial conditions

i|f(0) = 1 and i|f^(0) = 0 . (^7)

The exact solutions of Eq. (66) are /^(O and Y^d), where

$=exp(^j. (68)

This can easily be seen by writing the equation for T in terms of

$2

^ + $^ + (5^ - 1) i|r (5) = 0 . (69)

Applying the initial conditions, we obtain

^'EXAcr(^) = P[(W) - i'2(P)}-'i(P exp(-;c/2))

- {MP) - •/2(P)} I'l (P exp(-x/2))] , ^"^^^

where p = exp (a/2). Notice that the arguments of /j and are simply $ . To obtain the WKB solutions, we note that x = ais the
28 WKB Solutions to Initial Value Problems

turning point. For x < a, the solutions are oscillatory and for

X > a they are exponential. Now

T^(x) = -[exp(-jc + a) - 1]. (71)

4

Therefore,

Ti(x) = f'r(x) Jjc = ^

f'[exp(-;c+a) - 1]'^^ dx

(72)

y y^dy

Jo 1 + y - tan'^y ,

where

= exp( -x+a) - 1 = 4 r^(jc) . f^^)

Therefore,

Tl(jc) = r(x) dx = 2 r(x) - tan"^ [2 T(x)] . (74)

J X

In the region 0 < x < we write the solution as A

sin r(jc) die + a

r(0) sin [x] (x) + g]

N| r(jc) sin [Ti(0) + a]

(75)

where we have used the initial condition Y(0) = 1 and

r(0) = 1 (e^ - 1)^^ . 2

(76)

Using the condition T'(0) = 0, we readily obtain
WKB Solutions to InitiaJ Value Problems 29

a - tan- liMilMfj _ ^ (O) (77)

[1+4 r^(0)

Thus, the solution in domain 0 < x < a is completely

determined. We now use the connection formulas [Eqs. (46) and

(47)] to obtain the solution in the region x > a. However, to use

the connection formulas we must cast the solution given by Eq.

(75) in terms of the left side of Eqs. (46) and (47); thus, we write

1 Sin + a

sin

sin ( ^]

a - — COS i

[ 4j 4)

+ COS

^ n\ 1 a - —

4y /r(^ Too y" ^1

^ , It 1 1 > sin I a - — exp

+ COS a - —

4

exp - jy^dx (78)
30 WKB Solutions to Initial Value Problems

The WKB solution for x > a is therefore given by

WKB

r(0) 1/2

cos

K (;c) J sin [t| (0) + a ]

a - exp[ C W] sin - -^j

^ (79)

where

C(jc) = r\(jc) dx = tan"^ {2 k(jc)} - 2 k(jc) (80)

and

K^(:<c) = \[l - exp(-A:+a)]

4

(81)

The WKB solution given by Eqs. (75) and (79) is plotted in Fig.

2.6 together with the exact solution, given by Eq. (70). The WKB

solution again deviates from the exact solution near the turning

point.
WKB Solutions to Initial Value Problems

Figure 2.6 Exact and WKB solutions for Example 2.2.
3. AIRY FUNCTIONS

3.1 Introduction

In this chapter we will discuss Airy functions and their properties.

Airy functions are used in the next chapter to develop the MAF

(Modified Airy Function) solutions of the one-dimensional wave

equation. We will also use the asymptotic forms of the Airy

functions to justify the connection formulas used in the previous

chapter.

3.2 The Airy Functions

The Airy functions Ai(x) and Bi(x) are the two independent

solutions of the differential equation

The Airy functions are defined through the following equations:

- X i|f(x) = 0 . (1)

dx^

^i(x) = aj{x) - a^gix) , (2)

Bi(x) = ^3 [ aj(x) + a^gix) ]

, (3)
Airy Functions

1 3 . 1-4 . ^ 1-4-7 o f(x) = I + — x' + x" + 3! 6! 9!

X' +

0 13V i Ik (3ik)!

.X 2 4 2-5 7 4! 7! 10!

V l3j^(3* + l)!

(4)

(5)

where

a .4 =1.

3* a + -

3 j = (3a + l)(3a + 4)... (3a + 3*; -2)

(a arbitrary; k = 3,... ) ,

n = Ai(0) = = JJ!L = 0.35502 80538 87817 , ^ r(|)
34 Airy Functions

a, = Ai'(0) = Bi'jO) _

3-'^'^ /3

0.25881 94037 92807 . (7)

These equations and most of the equations that follow have been

adapted from the handbook by Abramowitz and Stegun (1970).

The Airy functions and their derivatives are related to the Bessel

functions through the following:

3

= 71 M 3

Aii-X) = ^

/I [7i,3 (C) +

/.,/3 (C)]

(8)

(9)

Ai'(x) = - ^ a:

[ 7.2/3(0 -VO]

V3

(10)

(11)

(12)
Airy Functions 35

Bi'(-ac) =

-j: [J,^1,(0 + ^2/3(0]

(13)

(14)

(15)

The asymptotic forms of the Airy functions and their derivatives are follows:

Aiix) - lj,-y^x-'l*e-^ E (-1)* c, C-* . 2 n

sm -2A:

(17)

cos

2 A

Ai\-x) - - Tz-'f-x 1/2 V 1/4 COS C +

+ sin

(19)
36 Airy Functions

0

+ Sin

0

Bi'( -x)~n-"^x"* 2k

cos| C + ^JS.~(-1)M,,,,C

2k-l

where
Airy Functions 2n

r(3Jt + -)

2 _ (2A: + l)(2k + 3y..(6k - 1)

54'k\r(k^^) 216*^!

^^^^ ^0 = 1'

6ik

For numerical evaluation in the following chapters we used the

following procedure: (i) for I < 1, we used Eqs. (2) and (3)

with the first 10 terms of Eqs. (4) and (5); (ii) for 1 < Ix I < 4

we used the first 50 terms of Eqs. (4) and (5); (iii) for I a: I > 4,

we used the first 10 terms of Eqs. (16) through (23). This gave

an accuracy of 1 in 10^.

3.3 Asymptotic Forms and the Connection Formulas

Figures 3.1 and 3.2 show that both Ai{x) and Bi(x) have damped

oscillatory behavior for jc < 0; for jc «> the function Ai(x)

approaches 0 exponentially and the function Bi(x) increases

exponentially. Consider more closely the first term of the

asymptotic series given in Eqs. (16) and (17):
38 Airy Functions

X

Figure 3.1 Airy function Ai(x).
Airy Functions 39

Ai(x)

1

1

Sin

exp(-C) . (25)

where

From Eq. (25), we note that the asymptotic forms of Ai(x) obey

the connection formula

1

1^

1/4 Sin <<-

1 2\x\ 1/4 exp(-C) . (26)

Similarly, examination of the asymptotic forms of Bi(x), given in

Eqs. (20) and (21) leads to

1 ^1 1/4 COS ->>

1 ^1 1/4 exp ( + C ) . (27)

Consider next the WKB solutions of Eq. (1). Here,

r^(x) = - X . (28)
40 Airy Functions

Thus, for jc < 0,

2

(29)

and, therefore, the WKB solutions are [see Eq. (42) of Chapter 2]

^ sin C and — — cos C (31)

or any linear combination of them (x = 0 represents the turning

point). Similarly, forx > 0,

k2(jc) = X , (32)

implying

K(x)d:r = ^x^f' (33)

and

|k(jc)|-i/^ = \x\-'f' . (34)

Therefore, the WKB solutions of Eq, (1) are

^ exp(C) and —-— exp(-C). (35)

\x\'f' \x\'l'

The asymptotic forms of the Airy functions are indeed the WKB

solutions and we may write Eqs. (26) and (27) as
Airy Functions

2

sin p r(x)dx + — 1 << >

Jx 4

[-7;k(x>/x;,

(36)

• exp

and

1 cos >>

(37)

1

These equations justify the connection formulas given by Eqs. (46)

and (47) of Chapter 2. (The significance of the double arrow was

discussed in the previous chapter.) Similarly, by assuming T^(x) = X, we can justify the connection formulas given by Eqs. (53)

and (54) of Chapter 2.
4. MAF SOLUTION TO INITIAL VALUE PROBLEMS

4.1 Introduction

The use of the WKB method in quantum mechanical and optical

waveguide problems is well established. The method is useful, in

spite of its limitations, because it is inherently simple.

Improvements have been offered to the basic method in the past

few years, the most notable being the work of Langer (1931). He

showed that improvements come about because the Airy function

is the exact solution to the equation of interest when is linear

in X, This is significant inasmuch as it prompts us to seek a

solution that becomes exact (that is, reduces to the Airy function)

when is a linear function of x. This would be in direct

correspondence with the WKB methodology, for which the solution

becomes exact (that is, reduces to the exponential or trigonometric

functions) when r^(x) is constant.

In this chapter we will lay the groundwork for a method of solving

the one-dimensional scalar wave equation based on the work of

Langer. The solution we will discuss is the product of a function

of X and the Airy function, the argument of which is also a

function of x.

The MAF solution is quite powerful for problems of practical

interest. It does not suffer the disadvantages that the WKB solution

suffers at the turning point. We will see from the examples in this

and the following chapters that this solution gives a very good
MAF Solution to Initial Value Problems 43

prediction of the eigenfunction as well as the eigenvalue. With

regard to the latter, we will find that the method does not work as well as the WKB method in finding the eigenvalues for the lowerorder modes when the wave equation involves a parabolic index

profile (or parabolic potential function in the case of quantum well

problems.) This will be seen in the later chapters. For most other

profiles, the MAF method works much better than the WKB

method. The WKB method, although approximate, is special in

this regard since it yields the exact eigenvalues for the untruncated

parabolic profile. In Chapter 7, we will show that use of the first- order perturbation theory, using the present Modified Airy

Function solutions, yields extremely accurate eigenvalues. Thus

the MAF solution gives an accurate description of the wave

function as well as of the eigenvalues.

4.2 The MAF Solution

The equation of interest is ^ + rHx)^(x) = 0 , (1)

where r ^ depends on the details of the problem at hand. In this

chapter, we will consider Eq. (1) as an initial value problem, and,

in this section, we will develop the formulation to be used

repeatedly in this and the following chapter.

We assume a solution to Eq. (1) of the form

tl
44 MAF Solution to Initial Value Problems

»|r(x) =

' Fix) At [$(x)] or Gix) Bi [Ux)],

(2)

where, the Airy functions Ai(x) and Bi(x), discussed in the

preceding chapter, are solutions of the equation

dx^

xf(x) = 0 . (3)

Substitution of Eq. (2) into Eq. (1) gives

F"(x)Aia) + 2F'ix)Ai'a)V(x) + F(x)Ai'a)^"

+ +r2(x)] = 0,

(4)

where primes denote differentiation with respect to the argument

and we have used the relation Ai" ($) = $ Ai(^). Equation (4) is rigorously correct. We choose i{x) so that

^{x)[e(x)Y *rHx) = 0 ,

the solution of which gives

(6)

In fact, the general solution of Eq. (5) leaves arbitrary the lower

limit of integration in the expression for ^(x). We take it to be the

turning point Xq, for tactical reasons; that choice renders the

solution [Eq. (2)] exact if r ^(r) is a linear function of x.
MAF Solution to Initial Value Problems 45

If we neglect the term that is proportional to F" in Eq. (4) (which

is the only approximation we will make), Eq. (4) becomes

(7) or -f [In Fix) ] = -lA [in ^' (x)] , dx Idx

the solution of which is Fix) = . (8)

An identical expression is obtained for G(x), Equations (2), (6),

and (8) represent the solution that we will discuss.

Equation (6) is valid when r^(jc) is positive or negative. Indeed,

if we assume is positive for x < Xq and negative for x > jcq,

then

5 (^) = -

I (x)dxj" for x<x^, (9)

5 W = ||

y:(x)dxj" for x > x, , dO)

where

K (x) = V

- r^(jc) for X > Xq . (11)

Interchanging the limits of the integrals in Eqs. (9) and (10) does

not change the value of Simple manipulations yield
46 MAF Solution to Initial Value Problems

3 f'^T(x)dx

2

1/3 Tix) for x< Xq , (12)

3 f K(x)dx

1/3 K (x) for X > Xq , (13)

and thus

1/4 ^x) ir^W 1/4 (14)

The error introduced by neglecting the term proportional to F" in

Eq. (4) is seen by substituting

i|r(jc) = ($0"'^'<l>(O

into Eq. (1). This gives

/\-3

<l>(0 = 0 .

(16)

The method is therefore valid if a')

/\-3 3(r)^ 1 ,///^

I 45' 2

(17)

$ is given by an integral [see Eq. (6)], so we expect it to be a slowly varying function of x; its higher derivatives are therefore

likely to be small. Although the inequality does not hold at the

turning point, the resulting error is small.
MAF Solution to Initial Value Problems 47

The approximate solution that we will discuss, then, is ^ix) - c, ^iiiiMJ . c, ^M^wi . (18)

where

5w = {/;(|)/^^p. ^^^^

and jcq is the turning point, where T^{x) changes sign. and C2

are constants, to be determined from the initial conditions.

4.3 Examples

Example 4.1

To demonstrate the applicability of Eq. (18), we consider an initial value problem. We will give the exact and the MAF results and

compare them graphically with the WKB method. Consider the

equation

+ \ [ exp(-jc + a) - 1

] ^{x) = 0 , 0 ^ x < ~ , dx^ 4

(20)

subject to the conditions

i|r(0) = 1 and i|r^(0) = 0 . (^1)

The exact solution is given by
48 MAF Solution to Initial Value Problems

1'exactW = 7 P

{[ ^o(P) - I'2(P)

I-', [Pexp(

•[/o(P) -•^2(P)]l'i[P«p(-f)]} for 0 ^ X < <» . (22)

where

P

= exp

(f)

(23)

The corresponding WKB solutions were given by Eqs. (75) and

(79) in Chapter 2. They will not be repeated here.

The MAF solution is given by

where

Mil) + Bia), (24)

Cj = It [ $'(0) Bi'am

(25)

2 [5'(0)]'/2
MAF Solution to Initial Value Problems 49

q = - n

5"(0)

2 [5'(0)]^'^

f

[exp(-x + a) - Iji/^Jx

2/3

(26)

(27)

or

and

^n(x)

2/3

for 0 ^ jc ^ a

few

2/3

fOT a ^ X ^ oo

(28)

(29)

where ri{x) and C(^) are defined by Eqs. (72) and (80) of

Chapter 2. Figure 4.1a shows the three solutions for a = 6; the

MAF approximation agrees very well with the exact results,

whereas the WKB solution deviates very much near and at the

turning point. In fact, the very small difference between the exact

and the MAF solutions cannot be easily seen in Fig. 4. la. For that

reason, we also show the difference between the two solutions in

Fig. 4.1b.

4.4 Summary

The MAF method is capable of yielding an accurate solution to

initial value problems. The veracity of the method was

demonstrated by using an example that has an exact solution.
50 MAF Solution to Initial Value Problems

15.00 1

10.00

5.00

0.00 -

-5.00

WKB

Exact and

MAF

~ 10-00 I , V y-T-n I I I M 1 I I I I I I I I

I I I I I 1 I I I I I I I f I I I I I

I 0.00 2.00 4.00 6.00 8.00

Figure 4.1(a) Exact, MAF, and WKB solutions for Example 4.1

The derivation in this chapter, being based on intuitive concepts,

was intended to appeal to the non-mathematically inclined. The

original work by Langer (1931), on the other hand, is somewhat

pedantic. The salient features of the method, which will be

demonstrated further in the following chapters, are these:

1. Equation (18) gives, in a single closed form, an approximation

that holds throughout the region of interest, including (but not

restricted to) the vicinity of and at the turning points. The first-
MAF Solution to Initial Value Problems 51

0.05 q

-0.01

—0.02 I

I I » 1 I I I I I

I I I I I I

I I I I I I I I I I

' ' I '''' »' I 0.00 2.00 4.00 6.00 8.00

X

Figure 4. 1(b) Difference between the exact and the MAF solutions

for Example 4.1.

order WKB solutions fail at and in the vicinity of the turning point.

The conventional method of rendering the solution valid

everywhere is to approximate r ^(jc) by a linear function, r^(jc) = + ft jc, in the vicinity of the turning points. With this, the

approximate solution to the differential equation, aside from

constants, is given in terms of Airy functions having argument

-(a H- b x)/b^'^. The total solution is expressed in terms of (i)
52 MAF Solution to Initial Value Problems

oscillatory functions in the regions where r^(x) > 0, (ii) exponential functions in the regions where r^(jc) < 0, and (Hi)

Airy functions of argument -(a + b x)/b '^'^ in the vicinity of the

turning points, where = 0. The solutions are then connected

at the boundaries of the regions. There is invariably a degree of

arbitrariness in the selection of the boundary. In contrast, the

MAF solution requires no such connection.

2. The solution given here is a single expression in terms of Airy

functions, but the argument is not obtained by assuming r^(;c) to be a linear function of x. In Eq. (18), the argument of the Airy

function is $(jc) and not {a b x)/h^^^. i(x) depends on actual

r\x) and not on a linear approximation to it.

3. The integration limits of Eq. (6) are chosen to ensure that Eq.

(18) reduces to the exact solution if r^(jc) is linear in x. Thus,

while the usual WKB method becomes exact if is constant, Eq.

(18) becomes exact if r\x) is linear in x.

All of the numerical results of this and the following chapters were

obtained on a desktop computer.
5. WKB SOLUTIONS TO EIGENVALUE PROBLEMS

5.1 Introduction

For continuity, we again give the equation of interest:

^lli^ 4- r^(jc)i|f(;c) = 0 . (1)

In Chapter 2, we used the WKB method to solve the initial value

problem. In this chapter, we consider Eq. (1) to be an eigenvalue

problem. The eigenvalue equation appears in many branches of

physics and engineering. In quantum mechanics, for example, the

one-dimensional Schrodinger equation (see any text on quantum

mechanics) is,

^,lll[E-V(x)]^(x)=0, (2)

where T represents the wave function describing the particle (of mass /Tz), E the total energy (which represents the eigenvalue) and

V(x) the potential energy distribution; h = hlln, h (= 6.626

X

10"^"^ J-s) is Planck's constant.

Often Eq. (2) is written as

where

(3)
54 WKB Solutions to Eigenvalue Problems

H = + V(x) = ~ ^ V(x) (4)

2m dx"-

represents the Hamiltonian of the problem and p is the x

component of the momentum that, in quantum mechanics, becomes

the linear operator -ih didx. The application of the boundary

conditions leads to the eigenvalue equation, as will be discussed

below.

The linear harmonic oscillator is one of the most important

problems in quantum mechanics. Its potential energy distribution

is characterized by

where o) is the "classical frequency" of the oscillator. Substituting

Eq. (5) into Eq. (2), we obtain

(5)

<f^t|r ^ 2m - mw^x^ = 0 (6)

2r

and

V) ^a) =0, (7)

where

(8)

and
WKB Solutions to Eigenvalue Problems 55

A = 2E

h 0)

If we apply the boundary conditions that should approach 0

as i -loo, we find that [see, for example, Schift (1968)]:

These are the eigenvalues for the problem. Only if A = 1, 3, 5,

7, . . . , will the corresponding approach 0 as $ ±oo;

indeed, for A ^ = 1, 3, 5, 7, . . . , tends to ±oo as

$ becomes large without limit. (We shall show this in

examples.!; see Figs. 5.5 and 5.6.) The corresponding

orthonormal eigenfunctions are

A = (2n + 1); n = 0, 1, 2, ... . (10)

/I = 0, 1,2, ... ,

(11)

where

/ \l/2 a (12)

represents the normalization constant and H^(^) are the Hermite

polynomials,
56 WKB Solutions to Eigenvalue Problems

H^a) = 1; H,a) =2$; H^a) =4^-2; (jj)

H,a) = - 121 ; H^a) = 165* - 48^2 + 12; ... . Higher-order Hermite polynomials can be obtained from the recurrence relations

=2xH„{x) -2nH„_,{x) .

The functions form a complete set, and

!^'<M^nMdx = b„„. (14)

Before considering the WKB solutions, we mention here that in

describing the propagation of electromagnetic waves in a slab

waveguide having a nonuniform refractive index distribution n(x)

the solution of Maxwell's equation for TE modes is given by

E/x,z,t) = i|fWexp[i(G>r - Pz)] , (15)

where represents the electric field, o) the angular frequency,

and p the propagation constant. The field satisfies the

equation [Ghatak and Thyagarajan (1989)]

^ll^ + n\x) - p2] ^(^x) = 0 , (16)
WKB Solutions to Eigenvalue Problems 57

where = co/c is the free-space wave number. Equation (16) is in the form of Eq. (1) with

The propagation constant {p) represents the eigenvalue, the

determination of which is of fundamental importance in waveguide

theory. For an untruncated parabolic index,

where Ui is the refractive index on the axis (atx = 0), A and a are

constants.

The infinitely extended (or untruncated) profile of Eq. (18) is a

physical and a practical impossibility. In practice, Eq. (18) is valid only for |jc| < a, that is, inside the core. The parabolic

profile is, in this sense, truncated. Outside the core region, the

refractive index is normally constant.

The untruncated parabolic profile of Eq. (18) is of practical

interest in spite of its physical impossibility. Its practical interest

comes about because it yields an exact solution and can therefore

provide guidance when the truncated counterpart "looks"

untruncated. This happens, for example, when the mode in

question is tightly bound to the core, that is, when most of the

modal energy is in the core. This, in turn, happens when the

mode is well beyond cutoff. Under these conditions, guidance is

(17)

(18)
58 WKB Solutions to Eigenvalue Problems

influenced almost exclusively by the refractive index in the core. The approximate solutions (and eigenvalues) of the truncated

profile should begin to replicate the exact solutions (and

eigenvalues) of the untruncated profile as this tightly bound

condition is approached.

For n\x) given by Eq. (18), Eq. (16) can be transformed to the

form of Eq. (7) with

2 2

1/4

2 2 A =

n^ \l 2 A/a

(19)

Therefore, the eigenvalue equation of the type given by Eq. (2) or Eq. (16) should be of profound importance in physics and

engineering. For the parabolic profile of Eqs. (5) and (18),

analytical solutions are possible; however, in general one must

resort to numerical or approximate methods. One of the most used

approximations is the WKB method, which will be discussed in

this chapter. In the Chapter 6, we will discuss the method that uses Airy functions. As discussed in Chapter 2, the WKB

solutions are not valid at or near the turning points; on the other

hand, the MAF solutions remain accurate for all values of x.
WKB Solutions to Hgenvalue Problems 59

5.2 The WKB Quantization Condition

We consider first a general r^(x) variation of the type shown in

Fig. 5.1. For a bound state in quantum mechanics or for a guided

mode in optical planar waveguide theory, the wave function

must decay exponentially as x ±00. Therefore, r^(x) must be

negative as x tends to ±00. A typical behavior is shown in Fig.

5.1; the points x = x^, and x = X2 represent the turning points

where r^(jc) = 0. For the harmonic oscillator problem [see Eq.

(6)], r\x) is an inverted symmetric parabola with

Figure 5.1 A typical variation of r^(x).
60 WKB Solutions to Eigenvalue Problems

-^1 = IE (20)

Similarly, for the waveguide problem, for x < (Region I) and

for X > X2 (Region III) the solutions must be exponentially

decaying. We start from Region I and use the connection formulas

to determine the solution in Region III and obtain the condition for

which the coefficient of the exponentially increasing solution in

Region III is 0.

The exponentially decaying solution in Region I is given by

(21)

where, as in the previous chapter.

kHx) = -r^(jc)

,

which is positive in Regions I and III. If we now use the

connection formula given by Eq. (53) of Chapter 2, the solution

becomes

2A

sin P r(x)dx + 7i/4 (22)

To obtain Tuj, we must "look" toward the turning point x = X2'

To do so, we write
WKB Solutions to Eigenvalue Problems

(' Tdx + 7t/4 = p r(x)dx - r(,x)dx + n/4

= I

+ e - (f^r(x)dx + 7i/4j

,

61

(23)

where

0=1"^ r(x)dx . (24)

Substituting Eq. (23) into Eq. (22) and simplifying, we obtain

>|r„(x) = (2Acose)

+ (2^ sine)

1 cos

mx)

sin

T(x)dx ^ ^

f"" r(x)dx + -

Using the connection formulas given by Eqs. (46) and (47) of

Chapter 2, we find

t|r^(jc) = 2A cos e — exp

+ ^ sin 6 — exp

K(x)dx

(25)

The first term on the right side of this equation represents an

exponentially growing solution that should not be present; we

therefore require cos 6 = 0 e = (n + -)7r

2

or
62 WKB Solutions to Eigenvalue Problems

e = r(x)dx = (n + -)7c; n = 0, 1, 2, ... , (26)

Jx^ 2

which represents the quantization condition. After finding the

eigenvalues [from Eq. (26)], T(x) is completely known and

therefore the complete solution is given by

i|r(x) =

2A

s/r(x)

exp

sm

2A

A(-ir

(-1)" sin

exp

[-/;'KWd.

f'r(x)dx + ^

[^T(x)dx * ^ f K(x)dx

X < X^,

, Xj < X <

, Xj < X <

X > ^2 . (27)

These equations are simple to use. They also readily show the

importance of using the correct eigenvalue. If the eigenvalue does

not satisfy Eq. (26), the solution will diverge in the region x > x^-

This is the principle upon which most numerical methods are based. We choose an eigenvalue and study the behavior of the

solution at large values of jc; if it starts diverging, another value is chosen and the process continues thus until the solution goes to zero as X 00. We will show this explicitly through the following

example. ^
WKB Solutions to Eigenvalue Problems

5.3 Examples

63

Example 5.1

We consider the linear harmonic oscillator, for which

r^(jc) =A-x^ = X^-x\ (28)

[see Eq. (7)] where = A. The turning points are given by

and Eq. (26) becomes

- dx = (n ^ . (29)

If we use the substitution x = A. sin 6, we readily get - + - sin 26

2 4

+11/2

J-7I/2 (n + -)n

2

or

= 2n + 1 (30)

which incidentally represents the exact eigenvalues of the problem

[see Eq. (10)]. This exact eigenvalue is, at once, interesting and
WKB Solutions to Eigenvalue Problems

puzzling. We do not know why the WKB method should yield

exact results.

As a by-product of this happenstance, we expect the WKB

formalism to yield highly accurate eigenvalues for a truncated

parabolic profile, provided that the mode in question is bound

tightly to the core region of the waveguide, where the refractive

index varies in a parabolic fashion. The modes are tightly bound

if they are far from cutoff (that is, high refractive index contrast,

large core, or short wavelength).

To determine the eigenfunctions, we note that since

r^(-x) = T^(x), the eigenfunctions must be either symmetric or

antisymmetric, that is, either

We consider only the antisymmetric case for which the WKB

solution in the region 0 < x < k is given by

where we have chosen the multiplicative constant so that

T'(0) = 1. To use the connection formula, we write Eq. (32) as

^(-x) = i|f(jc), symmetric

i|f(-x) = -i|r(jc), antisymmetric

(32)
WKB Solutions to Eigenvalue Problems

1 sin a

1

' P rix)dx +

sin a cos

cos a sin

I

y\ M +

where

a = /' r(x)dx + ^ = Ti(0) + ^,

11 W = f'r(x)dx

J X

Thus, in Region III (see Fig. 5.1) we have

sin a exp[C(Jc)]

where

- cos a exp[-CU)]
66 WKB Solutions to Eigenvalue Problems

For r^(jc) given by Eq. (28), the integrals can be evaluated to obtain

and

T](X) = /

- dx =

2 . X

^ - — sin M —

1 .2

In

(36)

(37)

The WKB wave functions are now known. The exact

wavefunctions are

EXACT =2^"^^(n-l)/2

«!

n = 1,3,5,... ,

H(x) exp

( i\

-_

r 2 J ,(38)

where we have chosen the multiplicative constant so that

T'(0) = 1. In Figs. 5.2, 5.3, and 5.4 we have plotted the exact

(viz. the Hermite-Gauss functions) and the WKB wavefunctions for

A = 3, 7, and 11.

The WKB solutions obviously fail near the turning points

{x = ± 73 for A = 3, jc = ± Jl for A = 7, and jc = ± 711 for

A = 11). The two solutions are in good agreement for jc not close
Figure 5.2 Exact and WKB wavefunctions for A = 3 (n = 1),

which corresponds to a = tt. T^(x) = 3 - x^.

to the turning points. In Figs. 5.5 and 5.6 we show how the large

|jc| behavior changes significantly if we use a value of A that is slightly different from the correct eigenvalue. These figures

illustrate the sensitivity of the solution to the eigenvalue. Even a

small error in the eigenvalue causes a dramatic change in the

behvaior of the wavefunction. The large deviation is due to the

exponential term in the WKB solution. A slight error has profound

consequences. The figures also show the qualitative improvement

in the WKB solutions for increasing mode number (that is,

increasing values of A).

Problem 5.1. In continuation of Example 5.1, obtain the WKB

eigenfunctions for the symmetric case and compare with the exact
68 WKB Solutions to Eigenvalue Problems

2.T ^(x)

1.5

X

r'(x)=7-x' WKB

-1.5-

Figure 5.3 Exact and WKB wavefunctions for A = 7 (n = 3),

which corresponds to a = 27r. v\x) = 7 - x^.

eigenfunction given by Eq. (11) with n = 0, 2, and 4.

Problem 5.2. Use the connection formulas to show that the WKB

wavefunction in the region x < - A is given by [cf. Eq. (35)]

i|f(x) = - cos a

• exp [ - C C^)] - sin a exp [ + C (^)] , where
WKB Solutions to Eigenvalue Problems 69

2.T ^(x)

r^(x)=ii-x^ 1.5+

Figure 5.4 Exact and WKB wavefunctions for A = 11 (n = 5),

which corresponds to a = Stt. t\x) = II - x^.

Example 5.2

We next consider the asymmetric profile

where

(39)
70 WKB Solutions to Eigenvalue Problems

Figure 5.5 WKB solutions for A = 3 and A = 3.05. For A =

3.05, the coefficient of the exponential term is not

zero, causing the wavefunction to flare up for \x\

> /3,

fix) = f^(x) = a + b + — X,

2a

fJx) = a + b cxp

X

d)

X < 0

X > 0 . (40)

a, b, and d are constants and k is the eigenvalue. The exact

solution for this profile is
WKB Solutions to Eigenvalue Problems 71

Figure 5.6 WKB solutions for A = 7 and A = 7.07. For A =

7.07 the coefficient of the exponential term is not zero resulting in the flaring up of the wavefunction

for |jc| > /7.

EXACT (X) =

1/3 Ai (z, - br X ) Ai(z,)

Z2 exp

X

Yd

X < 0

jc > 0 ,

(41)

where = a + b - X, b^ = b/2d, v = 2d (k - a)^^^, Zi = - Uilib^p-'^, Z2 = 2d (b)^'^, and the multiplying constant has been

chosen to ensure that Texact(^) ^ ^- Continuity of Y and at X = 0 yields
72 WKB Solutions to Eigenvalue Problems

-^^''fei) .1/3 _ y[b -

(42)

Ai(z,) 2 J, (z,)

From Eq. (42), we can find the (exact) eigenvalues. The

corresponding (exact) eigenfunctions are found from Eq. (41).

The WKB eigenvalues are found from

f^"" r(x)dx = r,(x)dx +

f^"" T^(x)dx = (m + |)7c ^f^^

where

r^(jc) = T^(x) = f^(x) - k for jc < 0

(44) = rhx) = /,(jc) - k for jc> 0 ,

and Xi and X2 are again the two turning points,

ri(jci) = 0 = r2(JC2). For tHx) given by Eqs. (39) and (40), Eq.

(43) simplifies to

-2- . 2d

3b,

- v/A. -a tan"^

/ \ «1

,k-a^

71 .

(45)

We have used typical waveguide parameters in the profile of Eq.

(40) and found the eigenvalues. We used a = 4, b = 0.2, and

d = 10. Figure 5.7(a) shows a plot off{x), and Fig. 5.7(b) shows

r^(x) for k = 4.14; this is the approximate value of the first eigenvalue. Indeed, k for the first two modes is
WKB Solutions to Eigenvalue Problems 73

Figure 5.7(a) Variation of f(jc) for a = 4, b = 0.2 3nd d = 10.

EXACT WKB

A.(n = 0) 4.144 976 4.139 262

k(n = 1) 4.079 228 4.079 617

The corresponding WKB solutions are readily obtained. For

X <

=

. exp

3b

' (46a)

where
where
WKB Solutions to Eigenvalue Problems

1/2

For 0 < ;c < :

a + b - X + —

2d

t|r(x) = —-— sin

- a tan

a

75

(47b)

(48a)

where

a + b exp

[ dj

1/2 (48b)

a = yjX - a

(48c)

Vox X >

exp

where

2d

i|r(x) =

In a - K2(jc)

1 - - 6 exp

(-j)r-

(49a)

(49b)

In writing Eq. (48a), we have made use of the eigenvalue Eq.

(43). The value of /4
76 WKB Solutions to Eigenvalue Problems

(a + b

-

sin

^

(a + 6 - X)'f^ + ^

3b 4

(50)

is determined from the condition T(0) = 1. Figures 5.8 and 5.9

show the exact and the WKB solutions for the first two modes for

the profile shown in Fig. 5.7b.

X

Figure 5.8 The exact and WKB eigenfunctions corresponding to the first mode for the profile shown in Fig. 5.7.

r
WKB Solutions to Eigenvalue Problems 11

Figure 5.9 The exact and WKB eigenfunctions corresponding to the second mode for the profile shown in Fig. 5.7.

Example 5.3

We next illustrate the use of the WKB method for solving the

Schrodinger equation [see Eq. (2)] corresponding to the potential

energy distribution shown in Fig. 5.10(a). For simplicity, we

assume V(x) to be symmetric inx [Vf-xj = ^(x)]\ this immediately

implies that the eigenfunctions are either symmetric or antisymmetric in x\

ilr(-x) = ^]f{x) (symmetric) - ij;^(0) = 0 , (^^^
78 WKB Solutions to Eigenvalue Problems

V(x)

X

Figure 5. 10(a) Potential energy variation having two wells.

i|f(-x) = -i|r(jc) (antisymmetric) i|f(0) = 0 . (^2)

The energy eigenvalue E is to be determined. The variation of

T^{x) is shown in Fig. 5.10(b); there are now four turning points.

A bound state must correspond to an exponentially decreasing in

Region V. Thus,

Using the connection formula [see Eq. (46) of Chapter 2], we get
To get to Region III, we must "look" toward the turning point
80 WKB Solutions to Eigenvalue Problems

2A

2X cos e

+ 24 sin 0

-

(/.:

^w-^ * 7),

cos if' r(x)dx + -]

-^sinffrcx).^],

where

0 = P rwdx . We now use Eqs. (53) and (54) of Chapter 2 to obtain

^m(x) = 24 cos 0 — exp [ K{x)dx

(55)

+ ^ sin 6 — exp

(56)

Applying the conditions given by Eqs. (51) and (52), we obtain

cot V(x)dx = ± -^^ exp [-2 p K{x)dx (57)

where the upper and lower signs correspond to symmetric and

antisymmetric solutions, respectively.

Example 5.4

In the previous example we considered the general behavior of

V(x) as shown in Fig. 5.10(a). In this example, we use a specific

form of V(x) given by [see Fig. 5.11(a)]
81

X

Figure 5.11(a) The potential energy variation for a double

oscillator. The corresponding variation of

for E < m G) V/2 and E > m co V/2

are shown in Figs. 5.11(b) and 5.11(c).

V(x) = 1 mo^ [|jc| - . (58)

This potential distribution is usually called the double oscillator.

The corresponding variations of r^(jc) are shown in figures 5. 1 1(b)

and 5.11(c) for E < m(i)V/2 and for E > /720)V/2,

respectively. Notice that for E < An(*)^a^/2, there are four turning

points and for E > m(^V/2 there are only two turning points.

For E < /72G) V/2,
0 =

J"^

rix)dx

2m

2

J -a. 2

(59)

where

a IE

M Ag)

(X - a) (60)
WKB Solutions to EigenvaJue Problems 83

2 2 E > ? moj a

Figure 5.11(c) r\x) for E > m o)V/2

Other integrations can easily be carried out. For E > mo) a /2,

we use Eq. (26). The final results are, for 0 < a < Qq,

cot^a^ 1 ±— exp

2

(61)

and for a > a^.

|- + tto (a^ - ao)

2\l/2 2 . -1 ^0

+ Sin ^ —

a m + — \n

2 (62)

where
84 WKB Solutions to Eigenvalue Problems

a = 2E

and an = ^

m(A>a (63)

Recall that a is the eigenvalue to be determined from Eqs. (61)

and (62) and aQ is a constant. Before plotting the wavefunctions,

it is convenient to introduce the dimensionless variable $ [see Eq.

(60)]; thus,

K[x)ax =

I ^ J X2 J a

a^ln a

Hence,

2x1/4 exp

(64)

2

5 + ($2 - a2)i/2

a

Similarly,

V(0 •2C

(a2 _ ^2)1/4

sin —

It It 2 + — a'' 4 4

2x1/2 V)

(65)

1 2 . -1 $ - —a^sin * —

2 a

and
WKB Solutions to Eigenvalue Problems 85

^m(0 = exp(C)

(66)

where

= p K(x)dx = (e - OL-'fl^dl

(67)

2 2

5 -/F^

a

Figure 5.12 shows the normalized energy eigenvalues for the

double oscillator as a function of ckq^; a large value of (and

hence of a) implies a large distance between the centers of the two

oscillators. The solid and the dashed curves correspond to the

exact and the WKB calculations, respectively. As « -»> oo, (= 2E / h(x>) approaches (2n + 1) and the levels become twofold

degenerate; thus, the system becomes equivalent to two

independent harmonic oscillators. Further, for a 0, we have the

eigenvalue structure of a single harmonic oscillator. The exact

solution for the double oscillator problem is given in terms of the

confluent hypergeometric functions. For V(x) given by Eq. (58),

the Schrodinger equation is ill ^ ^

[e - o>n|jc| -a]^ I ilr = 0 . (68)

In terms of the variables
86 WKB Solutions to Eigenvalue Problems

Figure 5.12 Normalized energy eigenvalues for the double

oscillator as a function of Oq^.

a

and

(x - a) for X > 0,

—J- ix + a)forx<0

(69)

Eq. (68) becomes

1 1 2 V + — - — O

2 4

i|t(a) = 0 for jc> 0 ,(70)
WKB Solutions to Eigenvalue Problems 87

where

1 + —

2

V = —

2

4

for X < 0 , (71)

1 / IE

Aw

(a^ - 1) . (72)

The boundary conditions for solving Eqs. (70) and (71) are Y 0

as (7 00 and r - oo. For (2 = 0, a = r and Eqs. (70) and (71)

become equivalent to the linear harmonic oscillator problem and

V will then be 0, 1, 2, ... . The solution of Eq. (70) that vanishes

for a 00 is the parabolic cylinder functions Dj^(a), [Magnus,

Oberhettinger, and Soni (1966)] which are related to the confluent

hypergeometric function through the relation

l-v\ ' H 2' 2' 2 J

ri -1

a 2

/2 n -1

1^1

'l-v 3 c^'

2

' 2 j

(73)

Since Dy(a) 0 as cj « we must choose D^(a) for a: > 0 and

Dj^(-t) for T < 0. In any case, since V{-x) = V(x), the

wavefunctions will be either symmetric or antisymmetric in x;

thus, we need only the solution fo > 0. The eigenvalue

equation for the symmetric and antisymmetric wavefunctions are
88 WKB Solutions to Eigenvalue Problems

(^) L=o ^ ^ symmetric (74a)

and

^v(^)L=o ^ ^ antisymmetric . (74b)

The solutions given by Eqs. (74) are plotted in Fig. 5.12. As

mentioned earlier, for a 0, the solutions correspond to a single

oscillator and for a «> we have twofold degeneracy corresponding

to two noninteracting oscillators.

Example 5.5

In this and the following examples we illustrate the use of a

convenient procedure that allows the solution to the WKB

quantization condition [see Eq. (26)] in special cases. We consider

the following potential energy distribution (see Fig. 5.13)

(75)

Thus,

r2 ix) =

0 E

+ sech^ (76)

and the quantization condition [Eq. (26)] becomes

(77)

\

where
WKB Solutions to Eigenvalue Problems 89

V(x)

X

Figure 5.13 Potential energy distribution for Fig. 5.5.

1=1^ r = -A and a = sech-^/F . (78)

a

For bound states £ < 0 and, since E can never be less than the

minimum value of V, we have

Equation (77) requires the solution of the integral
90 WKB Solutions to JSgenvalue Problems

(80)

If

then

dJ

dx

J(x) = (^^"^ F(x,y)dy , (81)

*; |f<i, * F[x.fM]^ - Flx,sM]^ . (82)

8(x) dx dx dx

Therefore

AL = -1 [sech^5 - dl . (83)

d^ 2 •'-a

In our case, the last two terms vanish because the limits are the

turning points where the integrand vanishes. In fact, since this

always happens, this technique is very often used in the evaluation

of the integral appearing in the WKB quantization condition. If we

now make the transformation

T| = sinh $ , then
WKB Solutions to Eigenvalue Problems 91

dy\ = cosh ^ ^

and Eq. (83) becomes

dl ^ _l

f

- ^

1/2 (l+ii^)

1 sin

In 1-^ sinh $

Simple integration gives

= 7C(1 (84)

where we have used the condition /(I) = 0 [see Eq. (80]). Thus,

the energy eigenvalues are given by

2

1 ^ (85)

2ma

\l/2

h

- n +

In this case we can also obtain an exact solution of the

Schrodinger equation; the final result is
92

where

WKB Solutions to Eigenvalue Problems

V = 5_ + 1

1/2 (87)

The WKB eigenvalues are recovered if the first term in the

brackets is large with respect to 1. The eigenfunctions are

v-l

2 iKo) = [1 - a] ' W{o) , (88)

where o = -sinh^ {x/a) and W{p) is the polynomial solution of the

hypergeometric equation.

Problem 5.2. Obtain the WKB eigenfunctions of Example 5.5 and

compare them with the exact solutions.

5.4 The WKB Method for Three-Dimensional Problems

For a spherically symmetric potential energy distribution V(r), the

radial part of the Schrodinger equation is given by [Schiff (1968)]

d^R ^ 1 dR + 1)^^ ^ Im

dr^ r dr

E - V(r) - R(r) = 0

(89)

where £(£ + l)ti^ (with £ = 0, 1, 2, ... ) represents the

eigenvalues of the square of the angular momentum operator

and m is the mass of the particle. If we make the transformation

u(r)

R(r) (90)
WKB Solutions to Eigenvalue Problems

then Eq. (89) becomes

V(r)

2mr

u(r) = 0 .

93

(91)

Although Eq. (91) has the same form as Eq. (1), the independent

variable r goes from 0 to «>. To use the asymptotic forms, we

introduce a new variable x such that r = 0 corresponds to x = -

oo; this is achieved through the transformation

r = exp (x) . (92)

In terms of the variable jc, Eq. (91) becomes

d^u

dx"" dx

_ Ug + 1)A^

2m

—du ,rs X 2m + exp(2jc) E - V

exp( -2x) tt(jc) = 0

(93)

To remove the first derivative [so it is in the form of Eq. (1)], we

introduce the transformation

X(x) = «(x) expf-|l . (94)

The function x(x) satisfies the equation

^

+r2(jc)x(^) = 0,

dx^

(95)

which is of the desired form, where
94 WKB Solutions to Eigenvalue Problems

r^(jc) = exp(2x)

2m

2m

E - F(e^)

exp ( -2jc)

(96)

In terms of the original variable r, l"T(x)dx = ['r,(r)dr,

where

rf(r) = 1^ E - V(r) - 2

2ffir

(97)

(98)

To find the bound state energies, we must solve the equation

/^^

r,(r)dr = (n + |)7: . (99)

where and r2 are the turning points, where rj(r) vanishes.

Equation (99) shows that the WKB solution to the radial part of the

Schrodinger equation is equivalent to Eq. (1) when £(£ + 1) is replaced by (£ + 1/2) ^.

Example 5.6

We consider the case of the hydrogen atom, for which

V(r) = -Z^ . (100)

r
WKB Solutions to Eigenvalue Problems 95

The WKB quantization condition is given by

2m

\

•''1 (.1

r 2mr^

(101)

1 dr = (n + —)n , 2

or

where

(102)

= dl , (103)

with

E

Z a

+ —

m e

$ =

r me

(104)

2 2
96 WKB Solutions to Eigenvalue Problems

Following the method used in Example 5.5, we obtain

(105)

Further, the minimum value of V^ff (0 is - / 4a, which occurs

at $ = 2a/Z. Thus,

j\ r

4aJ

= 0 . (106)

because $j becomes equal to $2- Equation (105) can now be

integrated. Using Eq. (106), we obtain

r = — , (107)

2(n + « + 1)2

which is also the exact result with total quantum number

(« + £ + 1).
6. MAF SOLUTIONS TO EIGENVALUE PROBLEMS

6.1 Introduction

In this chapter we reconsider the eigenvalue problem, but we use

the Modified Airy Function (MAF) solutions that were introduced

in Chapter 4. As in previous chapters, we will consider examples

that yield to exact solutions, to demonstrate the utility and strength

of the method. The formulation is based on papers by Goyal,

Gallawa, and Ghatak (1991a,b) and Ghatak, Gallawa, and Goyal

(1991).

The equation of interest is again

^ + r^x) Hr(Jc) = 0 , (1)

where r^{x) includes the (unknown) eigenvalue. If the equation

represents an integrated optical waveguide problem, will

include the propagation constant for the mode in question as the

unknown eigenvalue. For quantum mechanical problems, the

unknown eigenvalue will be the characteristic (allowed) energy

levels. The equations are identical but the parameters represent

different physical quantities.
98 MAF Solutions to Eigenvalue Problems

6.2 Formulation of the Problem

The eigenvalue problems follow the format already established.

To be definite, we will formulate our approach around a planar

waveguide problem, wherein we consider a planar optical

waveguide having a nonuniform refractive index profile in the core of the waveguide. As before, we assume and suppress exp[i((i)r - pz)] variation with time and z. Propagation is thus assumed to be

in the z direction and, because of this exponential variation with z, we explicitly assume that the waveguide is uniform in z. The core of the waveguide is the region \x\ < a. The problems of interest

have refractive index, n = n(x) in the core and n - constant

outside the core (in the cladding) of the guide. n(x) may be quite

general but we will restrict attention to those variations that lend

themselves to exact solutions of the wave equation. The

relationship between the modal propagation constant, /?, and r is

where = IttIXq^ and is the free-space wavelength. For

future reference we define here the effective refractive index n^ for

the mode in question:

The sign of r^(jc) depends on whether n(x) is greater than or less

than n^. The solutions of eq(l) are growing or decaying

exponential functions when is negative and the solutions are

oscillatory when the is positive. The turning point is the point
MAF Solutions to Eigenvalue Problems

(on the X axis) at which changes sign.

99

The form of the oscillatory functions depend on n(x). If n(x) is constant, trigonometric functions are encountered. For n(x) that

is nonconstant and not amenable to a closed form solution, we

again propose the Modified Airy Function (MAF) solutions.

Following the development of Chapter 4, we take the MAF

solution to Eq. (1) as

(4)

As before,

Jx^ 2

2/3

(5)

where x^ is the turning point. Choosing the lower limit of

integration to be the turning point causes the solution to become

exact if happens to be linear in x.

As seen previously.

j. 1 _ 151^

(6)

Equation (4) is identical to the WKB solution if we use the
100 AfAF Solutions to Eigenvalue Problems

asymptotic expansion of Airy functions together with Eqs. (6) and

(5). That is, this solution and the WKB solution are identical in

the region far from the turning point. The eigenvalue equations

are obtained when the boundary conditions are applied.

6,3 Examples

Example 6.

1

Consider first the profile shown in Fig. 6.1, for which an exact

solution is known. The square of the refractive index decreases

exponentially for x > 0 and is constant for jc < 0.

Mathematically,

n^(jc) = nl + (nf - 712) exp (- —) for x > 0 , ^ (7)

n^(x) = for JC < 0 ,

where n^, and are the refractive index values shown

symbolically in Fig. 6.1 and d is the diffusion depth of the

waveguide. It will be convenient later to use the normalized

frequency V defined asV=^d sjnl - nl . (8)

Xq is the operating free-space wavelength. This profile was chosen

because it has an exact solution, making it useful to demonstrate

the accuracy of the MAF solutions.
MAF Solutions to Eigenvalue Problems 101

X

Figure 6. 1 Refractive index profile for the waveguide of

Example 6.1.

We rewrite the governing equation in terms of normalized

(dimensionless) quantities using a convenient change of variables.

For the profile given by Eq. (7), we get

+ (exp (-X) - = 0 for X > 0 , W

d

^ - V^(b + J5)i|; = 0 for X < 0 , (10)

where we have introduced additional dimensionless variables,
102 AIAF Solutions to Eigenvalue Problems

X = x/d ,

B = «2 -

»1 - «2

(11)

2 2 b = -^^—^, 2 2 «1 -«2 (12)

The exact solution of Eqs. (9) and (10) are [Conwell (1973)]

f X

i|r(X) = 2Kexp

for X > 0,

i|r(X) = exp{xV^Jb + B) for X < 0 ,

(13)

where

V = 27v/& (14)

Equation (13) satisfies the boundary condition, T(A) 0 as X-+ +

00 and the multiplying constants have been chosen so that

T(0) = 1. Requiring the continuity of T'(X) at X = 0 yields the

eigenvalue equation for the normalized propagation constant b:

JA2V)

I 2

2 "2

J

(15)

It
MAF Solutions to Eigenvalue Problems 103

The MAF solutions are now written directly, using the results of

Chapter 4. We take

i|r(X) = F(X)/4i[ 5(^)1 , for X > 0 (16)

where

F(X) = constant

12/3

(17)

|/[-r^(X)]"^Jx

and X = X^i% the turning point. For X < 0, the solution given by

Eq. (13) holds. We have neglected the solution that is proportional to Bi{Q as it diverges for large X. We can now write

the solution that gives T(0) = 1

i|r(X)

1/2 for X > 0 , (18)

and $(X) can be written in closed form, as
104 MAF Solutions to Eigenvalue Problems

3K

^ exp (-X) _ J b I J

2/3

(19a)

for X < X, ,

37 v/fc - exp(-X)

^ /I In {b ^ 4b- exp(-X)

2 /ft - /ft - exp(-X) J

2/3

(19b)

for X > X, .

The turning point is located at X, = - In (ft). The subscript 0

indicates the value of the function at X = 0. The eigenvalue

equation can now be written by invoking the continuity of Y'(X) at X = 0:

K(B + ft) "2

_ ^i'($o) 1 C

^i(U '\2

(20)

(Q
MAF Solutions to Eigenvalue Problems 105

To illustrate the method, we used n2 = 2 All, = I, and

n\ - n2 = 0. 187 in our calculations. The value of d was varied to obtain results as a function of V. The calculated values of b are given in Table 6. 1 together with the exact values [obtained by

solving Eq. (15)] and those calculated by the WKB method. The

agreement is very good compared to WKB for all values of V. Even at y = 1.5 (very near to cutoff), the error is only 3 percent.

The corresponding error of the WKB method is 8 percent.

Table 6.1.

Normalized propagation constants.

V Exact MAF WKB

1.5 0.035007 0.036088 0.037833

2.0 0.104954 0.105896 0.108613

2.5 0.171442 0.172159 0.175311

3.0 0.229188 0.229736 0.233076

3.5 0.278650 0.279081 0.282486

4.0 0.321179 0.321520 0.324927

5.0 0.390292 0.390522 0.393845

6.0 0.444075 0.444241 0.447436

7.0 0.487244 0.487369 0.490429

8.0 0.522776 0.522874 0.525803
106 MAF Solutions to Eigenvalue Problems

We call attention to the fact that the error Ab in b is related to error A)S in p by the relation [see Eq. (12)],

At « M ^1 1 . (21)

b ^ - b

Thus, an error of 3 percent in b in this case will correspond to an

error of about 0.(X)2 percent in p.

Figure 6.2 shows T versus X for V = 4 as calculated by Eq. (18)

{MAF method) and Eq. (13) (exact). The figure shows no

discemable difference between the two curves, even at the turning

point.

In Fig. 6.3 we give a plot of (Yj^^ - Yexact)- ^^^^ figure

illustrates the strength of the MAF method since the results are

quite good even at the turning points. In Fig. 6.4 we give the

fractional error in as a function of V,

The profile of the foregoing example will be encountered again in

Chapter 7 to illustrate how the perturbation method used in

conjunction with the MAF solution can improve (quite

dramatically) the accuracy of the calculated eigenvalues.

Example 6.2

For this example we consider a symmetric profile for which
MAF Solutions to Eigenvalue Problems 107

* * * * MAF

X

Figure 6.2 Wavefunctions for the waveguide of Example 6.1,

y = 4.

r2(-x) = r^w . (22)

For a bound waveguide mode, the wave function must approach 0

as X approaches ±oo, so we again reject the solution that is proportional to the Bi function. Hence, we take
108 MAF Solutions to Eigenvalue Problems

4

o

x/d

Figure 6.3 Difference between the exact and the MAF solutions

for Example 6.1, V = 4.

*U) = -^Ai\l(x)} , jc>0.

Equation (22) suggests that the solutions are either symmetric or antisymmetric in x, so

i|r^(0) = 0 (symmetric solution) , or
MAF Solutions to Eigenvalue Problems 109

O

X

-Q

<

10.00 -I

8.00

6.00

4.00

2.00

0.00

n X)

^ I 'I 1 1 1

1

1 1

1

1 1

1 M 1 11

i 1

1

1

1

1) 1

1

1

1 1 1

1 1 1 1

1

1 M n

0.00 2.00 4.00 6.00 8.00 10.00

V

Figure 6.4. Fractional error in normalized propagation constant

for Example 6.1, as a function of V.

i|f (0) =0 (antisymmetric solution) .

Invoking these conditions leads to the eigenvalue equations

(symmetric solution) , (23)

and

[ 5 (0) ] = 0 (antisymmetric solution) . (^'^)
110 MAF Solutions to Eigenvalue Problems

The eigenvalue equation for the antisymmetric solution [Eq. (24)] can be put in a simple form. Let -Z^ (n = 1, 2, 3, . . .) denote

the nth zero of the Ai function; i.e.,

MZ) = 0 , (25)

where the subscript a refers to the antisymmetric solution. Using

Eqs. (5), (24), and (25) and assuming T^(x) to be positive from 0

to Xj, we obtain

12/3 1/ T{x)clx = -Z (26)

an

where

3/2

1/2 (28)

Table 6.2 gives the values of [as obtained from Abramowitz

and Stegun (1970)] and the corresponding values of Can» which are

universal constants. Equation (27) is similar to the WKB

quantization condition except that (for the WKB case) Can

replaced by odd integers.
AfAF Solutions to Eigenvalue Problems 1 1

1

Table 6.2.

Zeros appropriate to MAP quantization condition.

Antisymmetric modes Symmetric modes

n CUl c all r

^ sn

1 2.33811 1.01734 1.20348 0.06033

2 4.08795 3.00790 3.27162 2.01150

3 5.52056 5.00508 4.83082 4.00630

4 6.78671 7.00374 6.16988 6.00435

5 7.94413 9.00295 7.37677 8.00331

6 9.02265 11.00244 8.49195 10.0027

7 10.04017 13.00208 9.53820 12.0023

8 11.00852 15.00181 10.52992 14.0020

9 11.93602 17.00161 11.47696 16.0017

10 12.82878 19.00144 12.38642 18.0015

Similarly, we may consider the symmetric solution [Eq. (23)].

From Eq. (5) of Chapter 4,

Differentiation with respect to x yields
1 12 MAF Solutions to Eigenvalue Problems

a'? ^lii'l" = --fiTHx)). (30)

dx

The right side of this equation will vanish at ;t = 0 for a symmetric profile [see Eq. (22)]. Hence, at jc = 0, Eq. (30)

becomes

1 e'(0) _ 1 4^(0)

(31)

Equation (23) therefore becomes

^(0)Ai'[^(0)] + ^Ai[^(0)] = 0. (32)

4

Consider the function % Ai' (%) + Ai(x)/^ (which is plotted in

Fig. 6.5). Let -Z^j^ be the nth zero of this function (n = 1, 2, 3,

. . . ; that is

sn ^ sn ^

^ sn ^ '

where the subscript s refers to the symmetric solutions. Using

Eqs. (5), (23), and (33), and assuming t\x) to be positive from

0 to jCf, we obtain [see Eq. (26)]

2/3

$(0) =

(34)

sn
MAF Solutions to Eigenvalue Problems 113

15+

Figure 6.5 Plot of xAi(x) - Va Ai {%) as a function of %.

The values of Z^^ and hence Csn universal constants, given in

Table 6.2. Equation (35) is again similar to the WKB quantization

condition except that (for the WKB case) Csn replaced by even

integers. Thus, we see that there is a clear parallel between the
114 MAF Solutions to Eigenvalue Problems

MAF and the WKB methodologies for a symmetric profile and the

eigenvalues can be found by the MAF or the WKB method with

equal ease.

As an example, consider the untruncated parabolic profile

where, following the convention of quantum mechanics, is the

eigenvalue. This profile is instructive because the exact and the

WKB eigenvalues are identical: = (2n + 1); n = 0, 1, 2, ... (exact and WKB) . (^8)

We consider first the antisymmetric wave functions. The

symmetric functions will subsequently be illustrated as well. For

the antisymmetric case, the exact and the WKB wave functions are given by

(37)

(39)

(n = 1,3,5, ... ) ,

where H^(x) are the Hermite polynomials.
MAF Solutions to Eigenvalue Problems 115

Sin

4

for 0 < ;c < A, , (40)

(-1) (ii-l)/2

2/1 (x^-X^)!/'*

exp [ - C (x) ] for a; > A, ,

r\ (x) = f

v/A^ - x^dx n X /A^ - x'

(42)

and the constants have been chosen to ensure that T'(0) = 1. The

MAF solution is given by
116 MAF Solutions to Eigenvalue Problems

2 1 1/4 At

/ 3 \2/3

l2

MAF (x) = — ^— Ai

for 0 < X < X

1/6 3

(43)

for X > X ,

where ?7(x) and CW are the same as given by Eqs. (42) and (41),

respectively. The constant C is given by

1 (44)

C = 8X 8

which ensures that Y'(0) = 1. The wave functions for the second

antisymmetric mode are plotted in Fig. 6.6a. We see excellent

agreement between the exact and the MAF solution but, as expected, the WKB method fails near the turning points. The

difference between the exact and the MAF solution is so small that

it is not apparent in Fig. 6.6a. We have therefore plotted the

difference in Fig. 6.6b.

This untruncated parabolic profile provides an interesting case

study. We have noted, for example, that the WKB method yields

the exact eigenvalues. The profile is interesting for other reasons

as well. The truncated parabolic profile, considered in the next
MAF Solutions to Eigenvalue Problems 111

0.40 -1

0.00

X

-0.40

0.80

-1.20

Exact and

MAF

AWKB

M I 1 M > I I

I 1 I 1 I t I I I T

I M I M T t I I I

1 M I M I I I

I M M I I I I 1

I 0.00 1.00 2.00 3.00 4.00 5.00

X (dimensionless)

Figure 6.6(a) Exact, WKB and MAF wavefunctions for the optical

waveguide having a symmetric profile, Example

6.2, second antisymmetric mode.

example, is more realistic since it has the parabolic profile only in

the waveguide core and is thus more representative of a geometry

that is encountered in practice. The index is constant outside the

core. We expect the untruncated profile to provide trend

information for the truncated profile. For the truncated profile,

for example, eigenfunctions and eigenvalues will begin to replicate

those of the untruncated profile as the modes in question become
118 AlAF Solutions to Eigenvalue Problems

10.00 -I

5.00 -

<

u

o

X

LU

0.00

-5.00

-10.00

— 15.00 I r t I I T I t M I t 1 I I I M I I

I

1 I I I > I t i T

I

I I I I I I I M I

I I I I M r I I

I 0.00 1.00 2.00 3.00 4.00 5.00

X (dimentionless)

Figure 6.6(b) Difference between the exact and the MAF

wavefunctions for the profile of Example 6.2,

second antisymmetric mode.

more tightly bound. This will be seen in the next example.

For the symmetric case, the exact wave function is

EXACT

n! (45)

n = 0,2, 4,... , and the MAF wave function is given by Eqs. (41) through (43)
MAF Solutions to Eigenvalue Problems 119

with C given by

2/3'

Ai

( 3nxA

1

(46)

[ I 8 J

The value of C and the constants in Eq. (45) have been chosen to ensure that T(0) = 1.

Table 6.3

Eigenvalues for the untruncated

parabolic profile.

11n ^ WKB ~ ^ Exact MAF

0 (Symmetric) 1 1.12067

1 (Antisymmetric) 3 3.03470

2 (Symmetric) 5 5.02300

3 (Antisymmetric) 7 7.01580

4 (Symmetric) 9 9.01260

5 (Antisymmetric) 11 11.01016

6 (Symmetric) 13 13.00870

7 (Antisymmetric) 15 15.00747

8 (Symmetric) 17 17.00628

9 (Antisymmetric) 19 19.00591
120 AfAF Solutions to Eigenvalue Problems

The exact and MAF eigenvalues are given in Table 6.3. The WKB

and the exact eigenvalues are the same for this profile. As

expected, the accuracy of the MAF method improves for the

higher-order modes.

The corresponding exact and MAF wave functions for the

symmetric functions are plotted in figures 6.7a (for n = 0) and

6.8a (for = 4). We see excellent agreement between the exact

and the MAF solutions. The WKB method is not plotted but is known to fail near and at the turning points. The difference

between the exact and the MAF solutions is so small that it is not

apparent in figures 6.7a and 6.8a. We have therefore plotted the

corresponding difference between the wave functions for each case

in Figs. 6.7b and 6.8b.

-i H 1 i i i-

• 1 2 3 4 5 x6

Figure 6.7(a) Exact and MAF wavefunctions corresponding to n = 0 in Example 6.2.
MAF Solutions to Eigenvalue Problems 121

Figure 6.8(a) Exact and MAF wavefunctions for n = 4 in

Example 6.2.
122 MAF Solutions to Eigenvalue Problems

Figure 6.8(b) Difference between exact and MAF wavefunctions

corresponding to n = 4 for Example 6.2.

Problem 6.

1

For the profile of Example 6.2, determine the MAF

eigenvalues for the symmetric modes by using the values of -Zgj^ as zeros of Ai '(x) instead of x Ai' (%) + Ai(x)/4 [see Eq. (33)].

Compare the vaues so obtained with those given in Table 6.3.

Why does the agreement become better with increasing mode

numbers?

Example 6.3

Consider next the truncated parabolic profile shown in Fig. 6.9.

For 0 < X < a,
MAF Solutions to Eigenvalue Problems 123

Figure 6.9 Truncated parabolic profile, Example 6.3.

where and /Z2 are as shown in the figure, is the refractive

index of the cover region. For x < 0 and x > a, the refractive

index is constant. We introduce here, for use in what follows, the

normalized frequency, defined as V=^a sjn\ - nl . (48)
124 MAF Solutions to Eigenvalue Problems

This normalized frequency determines how tightly the modes are bound to the waveguide core. As V increases, the modes become

more tightly bound; that is, a greater fraction of the energy is in

the core region (x < a) of the guide. Thus, as V increases, the

waveguide begins to look like a waveguide that has an untrucated

parabolic refractive index profile. This leads to a useful test of the

approximations being suggested. If we allow V to increase, the

solutions that we propose (the approximate MAF solutions being

tested) must approach the known exact solution for the untruncated

parabolic profile. If it does not, it is not very accurate.

The governing wave equation can be transformed to a convenient

dimensionless form, as follows:

^CS - a^)t(0 =0, C<0, (49)

'^^

(50)

^^-(C^K)t(C)=0, /F.C, (51)

where

a

(52)
A£AF Solutions to Eigenvalue Problems 125

2 2 »1 - «2

Co = ''(I - • ^^"^^

X = a corresponds to C = V ''2. The solutions to Eqs. (49) and

(51) are

i|r(C) = exp(/a^ "

Co c) . C^O, (^5)

and

i|;(C) =C3exp(-/K- Co c), C ^ .

The exponentially diverging solutions have been neglected to ensure that Y 0 as ;c (and hence 0 ^ ± ^. The multiplying

constant in Eq. (55) is taken to be unity so that T(0) = 1.

The exact solution in the nonuniform region, where the refractive

index is a parabolic profile can be written in terms of the HermiteGauss functions. Exact evaluation and analysis then requires that

the boundary conditions be applied at the two boundaries with the

uniform regions, wherein the solutions are decaying exponential

functions. We will not evaluate the exact solution, but rely instead on a simple matrix method, for which the numerical precision is well established [Ghatak, Thyagarajan, and Shenoy, (1987)]. In

the numerical results to be given later, then, comparison of the

MAF method will be with the matrix method.
126 MAF Solutions to Eigenvalue Problems

Turning to the MAF solution, we note first that the solutions given

by Eqs. (55) and (56) are again valid outside the core region where

the refractive index is constant. In the core of the waveguide

(0 < C < yv), we take

^(C) = \c,Aia) + qBiCOl , (57)

where, for 0 < C < Cq,

UO =

-

(58)

sin 1

\

/

and for Co ^ C < V

UO = Co, c —In —

2

The boundary conditions require that T(C) and ^ (0 be

continuous at C = 0 and at C = V' (that is at ;c = 0 and x =

a). Applying these conditions, we get the required eigenvalue

equation:
MAF Solutions to Eigenvalue Problems 111

where

(60)

(61)

(62)

and subscripts 0 and 1 refer to values at C = 0 and C = V

Solution of Eq. (60) yields the normalized propagation constant b

as desired.

As in the previous example, the AfAF-predicted electromagnetic

field for this profile is again extremely accurate and for brevity we

have not plotted it here. We will concentrate instead on the

accuracy of the normalized propagation constant b. This is an

interesting example in this respect because the WKB method gives

the exact eigenvalue when the parabolic profile is untruncated.

The MAF method will therefore give very accurate results for large

y.

Figure 6.10 shows Lb/b for this truncated profile where AZ? is the

difference in the values of b using the exact (as determined by the

matrix method) and MAF method or the difference between the

WKB and the exact values. We see that the MAF method is more

accurate than the WKB method at low values of V, but for high V

values, where the errors are small for either method, the WKB
128 MAP Solutions to Eigenvalue Problems

100,00

O 80.00

-~ O

X

_Q

jD

60.00

40.00

20.00

0.00 I

» m HI M j1

1

III I > 11 1 m m 1 1 n 1 1 M l 1 1 n I M > M 111 n t m M I M 1

1 M I m I n

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

V

Figure 6.10 Fractional error in MAF normalized propagation

constant for the truncated parabolic profile.

results are more accurate. This is undoubtedly related to the fact

that WKB yields exact eigenvalues for the infinite (untruncated)

parabolic profile.

Example 6.4

In this example, the MAF technique is extended to profiles which
MAF Solutions to Eigenvalue Problems 129

are not symmetric in x. Without loss of generality, we assume

that in every case, r^{x) will have a maximum which we take to be at = 0. Let

Then

(x) = rf (x) for X > 0 ,

(x) = (x) for X < 0 .

(63)

WW

ri(0) Aig.ix))

for x > 0

for X < 0 ,

(64)

where

dx

2/3

2/3

0 ^ X < oo

^

< X ^ 0 ,

(65)

(66)

and now x = x^^ and x = x^2 turning points, one in the region

X > 0 and the other in the region ;c < 0. Continuity of T and

at X = 0 will give the following transcendental equation, from

which the eigenvalues can be determined:
130 MAF Solutions to Eigenvalue Problems

Ai' (^x(O)) ,/ 1 (0)

^'(5,(0)) ' 2

5^(0)

_ ^f^($^(0)) , 1 ^2 (0)

" A/(52(0)) ' 2^/(0)

We now consider the asymmetric profile:

rf = c + exp f-- 1 - Y , ;t > 0 , (68)

r^'W = a + * - Y + ^ , ^<0,

2 a

(69)

where a and Z» are constants and y is the eigenvalue. The exact

solution for this profile is

r .4i (z, -

(^)

Z2 exp

2d

jc < 0

jc > 0 ,

(70)

where flj = « + - y> *1 = */2d, v = Id {y - a)^'^, ^1 = - Qi/bi^^^, Z2 = 2d {b)^'^, and the multiplying constant has been

chosen to ensure that Y^xact (0) = 1- Continuity of T and T' at X = 0 yields
MAF Solutions to Eigenvalue Problems 131

(71)

From this we can find the eigenvalues. The corresponding

eigenfunctions can be obtained from Eq. (70). The WKB

eigenvalues are found from

r T,(x)dx + r,(x)dx

/w = 0, 1,2, ...

2 j (72)

where and x^2 ^^e turning points. For this profile, Eq. (72)

simplifies to 2 3/2 ^ , a/ + 2£?

3i> -

\ly -a tan

1/2

1

I Y

- a j

1 \ + It . 2j

(73)

From Eqs. (65), (66), (68), and (69), we obtain

lAx) = -{3d)2/3 feexp(-- ) + a - Y

1/2

\ y

- a )

1/2 2/3

(74)
132 AiAF Solutions to Eigenvalue Problems

fcexpl

1/2

+ — In

1 + ^ be x/d

y - a

\

be xld

y - a

2/3

(75)

a, + b.x

^1

Equations (67) and (64) yield the eigenvalues and eigenfunctions

using the MAF method. We used typical values of the parameters

in the profile of Eqs. (68) and (69). We used a = 4, b = 0.2,

and d = 10. The values of y for the first two modes are given in

Table 6.4.

The MAF method yields values correct to one part in 10^ and are more accurate than those found by WKB.

Now, define a normalized eigenvalue as
MAF Solutions to Eigenvalue Problems 133

A = ^ . (77)

b

Since the bound state eigenvalues necessarily lie between {a+b)

and a, the values of A are more meaningful to compare than the

values of y. The values of A for the first two states are given in

Table 6.5. Again, the values calculated by the MAF method are much more accurate than those from the WKB method. Figure

6.11a gives a plot of T(jc) for the exact and the MAF solutions

corresponding to the first mode. We have plotted the field only

for positive jc.

Table 6.4

Y values for first two modes.

EXACT MAF WKB

Yl 4.144976 4.145002 4.139262

Y2 4.079228 4.079254 4.079617

Table 6.5

A values for the first two modes.

EXACT MAF WKB

Ai 0.72488 0.72501 0.69631

A2 0.39614 0.39627 0.39808
134 MAF Solutions to Eigenvalue Problems

The MAF and the exact solutions are identical for negative x

because the profile (r^(x) is linear there. Since the difference

between the two solutions is not apparent in Fig. 6.11a, we have

plotted the difference in Fig. 6.11b. T^^KB plotted

because it is known to fail near the turning points.

1.10 n

0.40 -

0.30 -J , , , ^ , , 0.00 1.00 2.00 3.00 4.00 5.00 6.00

X

Figure 6.11(a). Exact and MAF wavefunction for the first mode, Example 6.4.
AfAF Solutions to Eigenvalue Problems 135

O

o

D

X

LJ

1.00

•0.00

-1.00

2.00

-3.00 -

4.00

-5.00 'I M 'I I I I I I I

I I I I I I I I I I

I

I I I I I I I I I

I I I I I I M n I M I M I I M I 0.00 1.00 2.00 3.00 4.00 5.00 6.00

X

Figure 6.11(b) Difference between the exact and MAF

wavefunctions for the first mode, Example

6.4.

Example 6.5

For a spherically symmetric potential V(r) the radial part of the

wave function R(r) satisfies Schrodinger's wave equation
136 MAF Solutions to Eigenvalue Problems

1 d 2 dR

r^ — E

dr dr

Vir)

5(C + 1)V

2\ir

R(r)=0, (78)

where E is the energy eigenvalue and the other symbols have their

usual meaning [Ghatak and Lokanathan (1984)]. If we make the

transformation _ IM 9 r Rir) (79)

and consider the I =0 case, we readily obtain

dr

where

^2 L V.J

(80)

(81)

Following the results of Chapter 4, we obtain the following MAF

solution to Eq. (80):

nr) = C, ^iilM^ , (82)

where

Ur) dr^^ , (83)

and r = represents the turning point (r(r^) = 0). We have

neglected the Bi solution, which diverges at r = oo. The quantity

I [as defined through Eq. (83)] is negative for r < [where r'^(r)

is positive] and positive for r > (where r^(r) is negative). Thus
MAF Solutions to Eigenvalue Problems 137

and

C(r) = ||/^^K(r)Jr|^ r > r, . (85)

where

K^(r) = -r^(r)

is positive in the region r>r^.

(84)

Because of Eq. (79),

ijr(O) = 0 . (87)

The eigenvalues are therefore obtained from the solution of the

following transcendental equation

Ai[^(0)] = 0 . (88)

Following the discussion of example 6.2 [see Eq. (24)], the MAF

eigenvalues are found from the following equations:

where Can» given in the third column of Table 6.2, are nearly odd

integers. The reason is seen from the following: Recall that for

antisymmetric modes corresponding to a symmetric one- dimensional potential.

T(0) = 0,
138 MAF Solutions to Eigenvalue Problems

and the corresponding WKB quantization condition [see Eq. (26)

of Chapter 5]

(90) /

r(x)dx = + IJti /I = 1,3,5,... ,

simplifies to

f T(x)dx = fnf.^lliL + -1- . (91)

Thus, Eq. (89) is expected to have approximately the same

accuracy as the WKB analysis. In fact, for the potential function

considered next, our Eq. (89) gives an error in the eigenvalue of

0.84 percent, whereas the WKB quantization condition gives an

error of 3.2 percent.

Consider now the profile

V(r) = -V^txp(-L) . (92) a The exact eigenvalues are obtained by solving the transcendental

equation [Ghatak and Lokanathan (1984)]:

JAg) = 0 , (93)

where
AfAF Solutions to Eigenvalue Problems 139

g

= (94)

Consider the deuteron problem, for which « 40 MeV « 6.409

X 10"^^ J, « 1.85x10"^^ m, M « 8.398 x kg (/x represents

the reduced mass), so ^ « 3.64.

Equation (93) gives u = 0.858 628. The corresponding value of

D obtained from Eq. (89) is 0.865 815. In Chapter 7 we will

revisit this problem and show how the well-known perturbation

theory, used in conjunction with the MAF solutions, gives results

that agree extremely well with the exact result. The corresponding

WKB value is given by u^^^ = 0.886 341. Using u =

0.858 628, we get E = 2.223 MeV, which represents the correct

binding energy of the deuteron.

Figure 6.12 shows the exact wave function along with the WKB

and MAF solution. Not surprisingly, the WKB solution fails at the

turning point but the MAF solution agrees well with the exact

solution throughout the region of interest. Because of the good

agreement, we also show, in Fig. 6.13, the difference between the

exact and the MAF solutions.

For g = 3.64, there is only one bound state. If there are more

bound states, the MAF solutions are more accurate for the higherorder states. We have also analyzed the three-dimensional harmonic oscillator
140 AfAF Solutions to Eigenvalue Problems

1.4

1.2

1.+

0.0.8

^0.6

0.4

0.21

WKB

Exact and

MAF Solutions

4 6

P( = r/a)

10

Figure 6.12 Exact, WKB, and MAF solutions to Schrodinger's

equation (( = 0) for the potential function of Eq.

(92).

and found excellent agreement between the MAF solution and the

exact wave functions. Unfortunately, for the Coulomb potential

the eigenvalues found from the MAF method for the first two or

three modes are in considerable error. This may be due to the

singularity of the potential at r = 0.

6.4 Conclusions

In this chapter, we have shown that the MAF solution gives

extremely accurate wave functions for problems of practical

interest and should be of considerable use in many cases. Unlike

the WKB solutions, the MAF solutions hold throughout the region
MAF Solutions to Eigenvalue Problems 141

of interest, including the turning point.

In Examples 6.2 and 6.5, we gave a simple analogy to the wellknown WKB quantization condition. Our analogy holds provided

that the profile is symmetric. In particular, we showed that the

eigenvalue equation takes the familiar form

/r«...(c.4)f. (95)
142 MAF Solutions to Eigenvalue Problems

where is the turning point. The values of Cn universal

constants and are given in Table 6.2, where they are labeled Can

and Csn to denote antisymmetric and symmetric modes,

respectively.

The form of Eq. (95) suggests that the result gives accuracy of

about the same order as the WKB method. For Example 6.5, our

accuracy is considerably better than that of the WKB method; the

error is about 3.2 percent (WKB) as opposed to 0.84 percent

(MAF),
7. EXTENSION OF M4F METHOD FOR EIGENVALUES

7.1 Introduction

In this chapter we derive and use an improvement to the MAF

method; it is based on well-known perturbation techniques. The

method dramatically improves the accuracy of the eigenvalues and

hence represents a powerful tool for use in those special cases when improved accuracy is required. The formulation follows

closely the paper by Goyal, Gallawa, and Ghatak (1991c).

We will first demonstrate the method by examining a nonuniform

planar waveguide having a profile that is amenable to exact

solution. The profile was used in Example 6. 1 of Chapter 6. We

will then further demonstrate the concept by applying

Schrodinger's equation to a three-dimensional symmetrical

potential well. This example was also considered in Chapter 6. In this chapter, we will discuss only the determination of

eigenvalues.

7.2 Formulation and Example 7.1

Consider the scalar wave equation appropriate to a twodimensional optical waveguide,

^ + r^Wi^Jc) =0, (1)
144 Extension ofMAF Method for Eigenvalues

where r ^(x) = kQ ^r?' (x) - p^, is the free-space wavenumber,

n(x) is the x-dependent refractive index variation, and p is the

propagation constant. We consider the following index profile:

n\x) =

nl + (/if - nl) exp {--)

a

for X > 0

for jc < 0 ,

(2)

where n^, n2, n^ are the refractive indices shown in Fig. 7. 1. We

can use Eq. (2) to rewrite Eq. (1) in normalized form as follows:

0

Figure 7. 1 Refractive Index Profile for Example 7.

1
Extension ofMAP Methodfor Eigenvalues 145

+ K2[exp(-X) - &]t|; = 0 for X > 0 , dX (3a)

where

- V^(b + =0 for X <0 , dX^

kid^in\-n\), X = xld,

B

2 2

2 2 Mj -/I2

(3b)

(3c)

2 2 = ——- , and = p/*:^, . 2 2 «l-«2

(3d)

The exact solution of Eqs. (3) was given in Example 6.1. It is repeated here for completeness.

t(X)

J^(2Fexp (-X/2)

JA2V) forX>0

(4a) exv {V ^fVTBX) for X < 0 ,

where

V = 2V/b . <4b)

Equation (4a) satisfies the boundary condition for a guided mode;
146 Extension of MAP Method for Eigenvalues

that is, 0 as X ± «>. The multiplying constants have

been chosen so that T(0) = 1. The continuity of at X = 0

will yield the following eigenvalue equation for the normalized

propagation constant b:

JU2V)

JA2V) n

1/2 (5)

The MAF solution to Eq. (3a) is

1/2 Ata) i|r(X) = _ for X > 0

(6)

For X < 0, the solution is the same as that given in Eq. (4a).

C(X) is given by

3V

4b tan"^

v/exp i-X) - b

exp(-X) _ J b

2/3 (7a)

for X < X,,

3F <Jb - exp(-X)

(7b) ^ /b^^b - exp(-X)

2 /fe - v'i> - exp(-X) j

2/3

for X > X, .
Extension of MAP Method for Eigenvalues 147

is the turning point, that is, ft = exp(-X^), and the subscript 0

indicates the value at X = 0. Equation (6) satisfies the boundary

condition T(± oo) 0 and the continuity condition Y(0) = 1 at X = 0.

The continuity of at X = 0 yields the MAF eigenvalue

equation,

Equation (6) is an exact solution of the differential equation

(8)

J 2 + [exp - 6]i|r

1r = 0 , 1 _ 3

2 2 4

(9)

as can be seen by substitution.

The MAF solution will thus be a good approximation provided that

the last term in Eq. (9) is small. This is so in practical cases, for

which i'" and i" are small. This will be corroborated by the

present example (cf. Eq. (2)). Comparing Eqs. (9) and (3a) and

considering the last term in Eq. (9) as a perturbation, we get a

first-order correction Aft to the normalized propagation constant

[Ghatak and Lokanathan (1984)]
148 Extension ofMAF Method for Eigenvalues

/Jo

Ab

1 3

2 2 4 .7. (10)

To illustrate the accuracy of the method we have used the

following values of the parameters in Eq. (2): n2 = 2A11,

n^ = 1.0, and nj - nl = 0.187 [Ghatak, et. al. (1987), Gedeon

(1974)]. The value of d has been varied to obtain results as a

function of V.

Table 7. 1 gives the resulting MAF values of b as well as the exact

values obtained from Eq. (5) and values obtained using the WKB

method. The MAF values are very good compared to WKB for all values of V, but the correction given by Eq. (10) gives extremely

accurate eigenvalues. The error is only about 0.03 percent even

at y = 1.5, that is, very near cutoff. The error Aft in is related

to error A/3 in /5 by the relation [see Eq. (21) of Chapter 6],

Ab ^ Al n, I ^^^^ b ^ n^-n^ b

Thus, an error of 0.03 percent in b in this case will correspond to an error of only about 0.000 02 percent in p.
Extension of MAF Method for Eigenvalues 149

Table 7.1

Normalized propagation constants for different values of V. V Exact

Eq. (5)

MAF

Eq. (8)

MAF with

perturbation

Eq. (10)

WKB

1.5 0.035 01 0.036 09 0.035 02 0.037 83

2.0 0.104 95 0.105 90 0.105 03 0.108 61

2.5 0.171 44 0.172 16 0.171 52 0.175 31

3.0 0.229 19 0.229 74 0.229 26 0.233 08

3.5 0.278 65 0.279 08 0.278 72 0.282 49

4.0 0.321 18 0.321 52 0.321 24 0.324 93

5.0 0.390 29 0.390 52 0.390 37 0.393 85

6.0 0.444 08 0.444 24 0.444 11 0.447 44

7.0 0.487 24 0.487 37 0.487 27 0.490 43

8.0 0.522 78 0.522 87 0.522 80 0.525 80

Example 7.2

We next revisit Example 6.5 of Chapter 6. We consider

Schrodinger's equation for a spherically symmetric potential

corresponding to £=0. We saw in Example 6.5 that the

eigenvalues can be determined from the tabulated zeros of the Airy
150 Extension of MAP Method for Eigenvalues

function and, in the process, we noted that we should expect that

accuracy of the MAF eigenvalues should be of the same order as the WKB eigenvalues; however, the first-order perturbation theory

(discussed in the preceding example) allows us to obtain very good

accuracy for the eigenvalues.

For a spherically symmetric potential V(r) the radial part of the

wave function R(r) satisfies the equation

_L A.

dr

dR

dr

[E - V{r)

(12)

R{r) = 0 ,

where E is the energy eigenvalue and the other symbols have their

usual meaning. Following the analysis given in Example 6.5, we

fmd that for £ = 0, T(r) given by Eq. (82) of Chapter 6 is an

exact solution of the differential equation

^ + [T^{r) + F(r)] i|r(r) = 0 , (13)

dr^

where

T(r) = rR(r) and r ^ (r) is given by Eq. (81) of Chapter 6.

Because of the last term in Eq. (13) is a perturbation term, we get
Extension ofMAF Method for Eigenvalues 151

JO

(15)

for the first-order correction AE to the energy eigenvalue.

As in Chapter 6, we use the profile

The exact eigenvalues are obtained by solving the transcendental

equation

[See Eqs. (93) and (94) of Chapter 6.] For the deuteron problem

(cf Example 6.5, Chapter 6),

Equation (17) yields u = 0.858 628. The corresponding MAF

value of I) obtained from Eq. (89) of Chapter 6 is 0.865 815. The

perturbation correction [see Eq. (15)] yields u = 0.858 73, which

agrees extremely well with the exact result. The corresponding

WKB value is 0.886 341.

For this example, then, accuracy of the eigenvalue using the MAF

method is considerably higher than that of the WKB method. In

this regard, see Table 7.2, where the error is 3.2 percent (WKB)

versus 0.84 percent. The value of the eigenvalue obtained when

(16)

J, (g) = 0 . (17)

g - 3.64.
152 Extension of MAP Method for Eigenvalues

using the correction given by Eq. (15) is very impressive. The

error is only 0.012 percent for this example.

Table 7.2

Comparison of eigenvalues for deuteron problem.

^ EXACT ^PERTURBATION ^WKB

^EXACT ^ EXACT ^EXACT

0.858628 1.0084 1.00012 1.032

For g = 3.64, there is only one bound state. In general, if there

is more than one bound state the MAF solutions will be more

accurate for higher-order states.

Example 7.3

In this example we use the MAF method together with the first- order perturbations theory to evaluate the energy eigenvalues for an anharmonic oscillator, characterized by the following potential

V(x) = - kx^ ^ ax^ ,k> Omda>0 , (18)

2

This potential is important in many physical problems. Very

accurate eigenvalues for this potential have been obtained by Hioe

and MontroU (1975). Later, Kesarwani and Varshni (1981) used
Extension ofMAF Method for Eigenvalues 153

the five-term (eighth order) WKB approximation to calculate the

eigenvalues and compared them with those obtained by Hioe and

Montrol (HM).

We start with the Schrodinger equation

^ + 1^ - v{x)] i|r = 0 , (19)

where the symbols have their usual meanings. For the potential

given by Eq. (18), the Schrodinger equation can be transformed

to the following form,

^ + T\X) i|f(X) = 0 , (20)

dX^

where

r^(X) = 4 ^ Pi)(P2 -^').
154 Extension ofMAP Method for Eigenvalues

k = ah

sj mk

1 +

1 - 2o)-

P. = 4 1 +

1 - 2w

(0 J. J. 2 21 + l6aE

-1/2 1/2

In the same units as those used by Hioe and Montroll, the

eigenvalue for the state with quantum number n is given by

1

2 -.2 1 (1 - 2oO

(21)

where is determined by solving Eq. (20).

The MAP solution of Eq. (20) for a bound mode is given by
Extension ofMAF Method for Eigenvalues 155

(22)

where

or

/ s/-rHx)dx

2/3

; X > 0

2/3

o<x</p;,

(23)

2/3

(24)

In writing Eq. (22), we used the fact that for a bound mode

Y(x) 0 as x ±00, hence the omission of the term proportional

to Bi(x), The solution given by Eq. (22) is arbitrary to the extent

of a multiplicative constant.
156 Extension ofMAF Method for Eigenvalues

Since the potential given by Eq. (18) is symmetric, we will have

symmetric and antisymmetric modes. For brevity, we consider

only the antisymmetric modes, for which T(0) = 0 and, therefore,

Az[$(0)] = 0 . (25)

Following Example 6.2, the above equation readily leads to

/

r(x) dx - [l^ . Ij ^

(26)

where Can given in Table 6.2. The corresponding first-order

WKB quantization condition is given by

m = 1,3,5,... .

It is obvious from Eqs. (26) and (27) that the numerical effort

required to calculate the eigenvalues by the MAP method is the

same as that required by using the first-order WKB method.

Following Examples 7.1 and 7.2, the first-order perturbation

correction to E^ is given by
Extension ofMAF Method for Eigenvalues 157

where primes denote differentiation with respect to X.

Examination of the five-term (eighth order) WKB formulas for

determining the eigenvalues [see Kesarwani and Varshni (1981)]

illustrates the strength of the MAF method. Clearly, the MAF

results (with perturbation correction) are much easier to use than

the much more complicated five-term WKB expansion.

Using the MAF method, we have calculated the eigenvalues E^ for

/z = 1, X = 0.(X)2, 1, 2, and 50. The results are given in

Table 7.3, column 4. We have also given the eigenvalues

calculated using the first-order (column 3 of Table 7.3) and the

five-term (eighth-order) WKB method, taken from Kesarvani and

Varshni (1981) and labeled KV in the table. The values given by

Hioe and Montroll (1975) are also reproduced in the table and are labeled HM. It is these results (HM) against which comparison

will be made.

Except for X = 0.(X)2, the MAF results are more accurate than the

first-order WKB results. The accuracy of WKB results at k = 0.002 is due to the fact that A. = 0 corresponds to the case of

a simple harmonic oscillator for which the WKB method yields

exact results. Though in comparison to the MAF results, the five-
158 Extension ofMAF Method for Eigenvalues

term WKB results are in better agreement with the HM results, the

numerical effort involved in the latter is much more. We have

also found the first-order perturbation correction to the MAF

eigenvalues. With this correction, the MAF eigenvalues agree

better with HM results than the five-term WKB results.

Table 7.3 Energy eigenvalues for the anharmonic oscillator,

AI = 1.

A £WKB eMaf £MAF+PERT

0.002 1.506 739 7 1.524 198 59 1.507 419 39 1.507 419 39 1.507 432 12

1 2.703 483 41 2.742 703 74 2.739 744 61 2.737 892 27 2.737 985 64

2 3.250 959 01 3.299 115 86 3.296 372 21 3.292 867 82 3.293 025 36 50 8.802 547 14 8.937 821 22 8.933 571 14 8.915 096 36 8.915 747 79

7.3 Conclusions

In summary, the MAF method not only allows an accurate

description of the wave function, but of the eigenvalues as well.

The results given in the tables are extremely good. The MAF

method in itself gives good agreement with the exact eigenvalues.

The correction introduced in this chapter, while mildly

complicated, gives results that are very nearly equal to the exact

values. When good accuracy is required, the method given here
Extension ofMAF Method for Eigenvalues 159

is viable.

We saw in Chapter 6 that, in some cases, there is a clear MAP

analog to the WKB quantization condition, suggesting that the

accuracy of the MAF method will be about the same as the WKB

method. In fact, the MAF eigenvalues, without the perturbation

improvement, are considerably more accurate than the WKB

method.

REFERENCES

Abramowitz, M., and Stegun, I. A. (1970), Handbook of

Mathematical Functions with Formulas, Graphs and Mathematical

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Chapter 10.

Bahar, E. (1967), Generalized WKB method with applications to problems of propagation in nonhomogeneous media, J. Math.

Phys., Vol. 8, 1735-1746.

Bender, Carl M., and Orszag, S.A. (1978), Advanced

Mathematical Methods for Scientists and Engineers, McGraw-Hill

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and Marcuvitz, N. (1973), Radiation and Scattering of Waves,

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(1964), The Theory of Electromagnetism, The Macmillan

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Brillouin, L. (1926), The undulatory mechanics of Schrodinger,

Compt. Rend. Acad. Sci. Paris, Vol. 183, 24-26.

Conwell, E.M. (1973), Modes in optical waveguides formed by

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162 References

Froman, N., and Froman, P.O. (1965), JWKB Approximation:

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Gedeon, A. (1974), Comparison between rigorous theory and

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Ghatak, A.K., and Lokanathan, S. (1984), Quantum Mechanics:

Theory and Applications, Macmillan Press, New Delhi.

Ghatak, A. K., and Thyagarajan, K. (1989), Optical Electronics,

Cambridge University Press, Cambridge, UK.

Ghatak, A.K., Thyagarajan, K., and Shenoy, M.R. (1987),

Numerical analysis of planar optical waveguides using matrix

approach, IEEE J. Lightwave Tech., Vol. LT-5, 660-667.

Ghatak, A.K., Gallawa, R.L., and Goyal, I.C. (1991), Accurate

solutions to Schrodinger's equation using modified Airy functions,

submitted for publication.

Ginzburg, V.L. (1964), The Propagation of Electromagnetic

Waves in Plasmas, Pergamon Press, New York.

Goyal, I.e., Gallawa, R.L., and Ghatak, A.K. (1991a), An

approximate solution to the wave equation-revisited, J. Electromagnetic Waves and Appl., Vol. 5, 623-636.
References 163

Goyal, I.e., Gallawa, R.L., and Ghatak, A.K. (1991b),

Approximate solution to the scalar wave equation for planar optical

waveguides, Appl. Opt., Vol. 30, 2985-2990.

Goyal, I.e., Gallawa, R.L., and Ghatak, A.K. (1991c), Methods

of analyzing planar optical waveguides. Opt. Lett., Vol. 16, 30-

32.

Heading, J. (1962), An Introduction to Phase Integral Methods,

Methuen and Co., London.

Hecht, C.E., and Mayer, I.E. (1957), Extension of the WKB

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Hioe, F.T., and MontroU, E.W. (1975), Quantum theory of

anharmonic oscillators, I. Energy levels of oscillators with positive

quartic anharmonicity, J. Math. Physics, Vol. 16, 1945-1955.

Irving, J., and Mullineux, N. (1959), Mathematics in Physics and

Engineeringy Academic Press, New York.

Jeffreys, H. (1923), On certain approximate solutions of linear

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Kesarwani, R.N., and Varshni, Y.P. (1981), Eigenvalues of an anharmonic oscillator, J. Math. Physics, Vol. 22, 1983-1989.
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and Theorems for the Special Functions ofMathematical Physics,

Springer-Verlag, Berlin.

Pauli, W. (1980) General Principles of Quantum Mechanics, in

English, translated from German by P. Achuthan and K.

Venkatesan, Allied Publishers Private Ltd., Bombay.

Schiff, L.I. (1968), Quantum Mechanics, McGraw-Hill Book Co.,

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quantenbedingung fiir die zweeke der wellenmechanik, Zeits. f. Physik, Vol. 38, 518-529.
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2. PERFORMINQ ORGANIZATION REPORT NUMBER BIBLIOGRAPHIC DATA SHEET B91-0206

3. PUBUCATION DATE November 1991

4. TITLE AND SUBTITLE Modified Airy Function and I-TKB Solutions to the Wave Equation

5. AUTHOR (S) A.K. Ghatak, R.L. Gallawa, and I.C. Goyal

6. PERFORMINQ ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

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9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET. CITY, STATE, ZIP) This monograph was prepared, in part, under the auspices of the Indo^US Collaborative Program in Materials Sciences.

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DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-1 85, FIPS SOFTWARE SUMMARY, IS ATTACHED. OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMEI

NTION IT HERE.) The purpose of this monograph is to revisit a basic equation of mathematical physics,

r^(x)ilr(x) = 0

and to give approximate solutions based on the WKB method and on a modification of the Airy function. All of the examples that we will use to illustrate the meUiods are based on optical waveguides and quantum mechanical problems. We hope that this monograph will prove to be tutorial, giving insight and

understanding to the use of Airy functions in addressing the scalar- wave equation.

^2. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS) Airy functions; approximations; optical waveguides; scalar-wave equation; SchrBdinger ' s equation; wave equation; WKB.

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