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*"*€AU Of '' NBS TECHNICAL NOTE 1001
NATIONAL BUREAU OF STANDARDS

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MATIONaL bure

'• f>» STANDARD!

Ubrarv

AC/G 2 3 197

Laser Far-Field Beam-Profile Measurements ^^

By The Focal Plane Technique ^^T

-tr

G.W. Day

C.F. Stubenrauch

Electromagnetic Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80303

•

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

i NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1978
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1001

Nat. Bur. Stand. (U.S.), Tech. Note 1001, 52 pages (Mar 1978)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. G03-003-01902-0 Price $2.20 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

1

.

INTRODUCTION 1

2. BASIC RELATIONSHIPS 2 2.1 Propagation Laws 2 2.2 Transform Properties of a Focusing Element 5

2 .

3

Approximations 7

2 .4 Equivalence 8

3. THE QUESTION OF DEFINITION 8 3.1 Single Transverse Modes g 3.2 Multimode Beams 10

4. SELECTION OF FOCUSING ELEMENT n

4.1 Refracting Elements H

4.1.1 Single Element Lenses 11

4.1.2 Multi-element and Aspheric Lenses 16

4.2 Reflecting Elements 16

4.2.1 Parabolic Mirrors 16

4.2.2 Spherical Mirrors 16

[5. ESTIMATION OF CERTAIN ERROR SOURCES 20

5.1 Computational Technique 20

5.2 The Fresnel Approximation 22 5.3 Lens Aberration 22

5.4 The Position of the Observation Plane 25

5.5 Summary of Computations 28

[6. EXPERIMENTAL IMPLEMENTATION 29

6.1 Characterizing the Focusing Element 29 6.1.1 Lenses 29

6.1.2 Mirrors 33

6.2 Other Experimental Considerations 37

7. SPATIALLY RESOLVED MEASUREMENTS 39

8

.

SUMMARY 4

1

9

.

REFERENCES 4

1

10 . ACKNOWLEDGMENT 42 APPENDIX ,- 43

iii

LASER FAR-FIELD BEAM-PROFILE MEASUREMENTS

BY THE FOCAL PLANE TECHNIQUE

An analysis of laser far-field beam-profile measurements by the focal plane

technique is given. Particular attention is paid to systems at '^^lpm wavelength

and having apertures up to 10 cm. The basic mathematics is reviewed and approximations are evaluated. Using geometrical optics techniques, it is shown that an

f/20 plano-convex lens is an appropriate choice for the focusing element. For

two arbitrarily chosen laser beam profiles the errors associated with the choice

of this lens are discussed through the use of computed far-field and focalplane irradiance distributions. Experimental procedures including methods of

testing the optical elements are also given.

Key words: Beam divergence; beam profile; lasers; optical propagation.

1. INTRODUCTION

The accelerating use of pulsed lasers for ranging, target designation, and similar

applications where maximizing the radiation on a distant target is desired leads to the need

for methods of accurately characterizing beam profiles. The Q-switched Nd:YAG laser emitting

intense 1.06 ym wavelength pulses of 20-30 ns duration and low repetition rate ('^' 10 Hz) is

typical of the sources used in these applications. Beam profiles at the source are generally

complex and vary from pulse to pulse. Moderately large optics ('^ 10 cm) are used to obtain

-4 -2 beam divergence in the range of 10 to 10 radians.

For such a source the far-field pattern is only observed beyond several thousand meters

from the source. Thus direct measurement is not generally possible, and some form of nearfield to far-field transformation technique must be applied. By transformation technique we mean a method whereby the far-field patterns can be determined by measurements taken

relatively close to the source.

Given the complex amplitude distribution of the field across a plane near the source,

one can, in principle, compute the far-field distribution from diffraction theory. The

relationship, as shown in a later section, is essentially that of a Fourier transform."

In antenna measurements, where similar problems arise in obtaining far-field patterns,

it is common to measure the complex field amplitude and compute the far-field pattern using

fast Fourier transform algorithms. At optical wavelengths, however, two problems make this

approach difficult. First, experimental determination of the complex amplitude distribution

is difficult in this region of the spectrum, especially for pulsed sources. Second, the

amount of data required (that is, the grid spacing over which data are taken) is inversely

related to the wavelength. At optical wavelengths the volume of data processing becomes

prohibitive.

A second approach is to use the fact that the field distribution in the focal plane of

a lens or mirror is related to the far-field distribution. It is useful to think of the

focusing element as simply a means of information processing analogous to that described in

the preceding paragraph. An analysis of this technique and its experimental implementation

are the primary subjects of this document. The mathematical background is reviewed in

section 2 with attention to necessary approximations. Section 3 discusses some questions of

definition, relative to beam characterization. Sections 4 and 5 analyze the limitations of

the technique from both a geometrical and physical optics perspective. Section 6 deals with

experimental implementation and section 7 with possible extensions of the technique.

It should be pointed out here that in most cases determining the intrinsic propagation

characteristics of a laser is only the first step in determining the actual irradiance

distribution on a distant target. If propagation is through the atmosphere, the beam may be

disturbed by turbulence, scattering, thermal defocusing (blooming) and other effects.
Determining the intrinsic propagation characteristics is, however, a necessary first step

and should aid an investigation of the complete propagation problem.

2. BASIC RELATIONSHIPS

2. 1 Propagation Laws

The purpose of this section is to draw together some of the basic expressions governing

the propagation of electromagnetic radiation and observe their application to the specific

problem of determining the propagation characteristics of laser radiation. The basic problem

is that given a field distribution over some surface in space (usually a plane) , one wishes

to learn the field distribution over a second surface a distance away. Each polarization is

assumed to be independent and is characterized by a complex scalar amplitude function.

It has become popular to proceed [1,2,3] by recognizing that the source distribution

can be represented as a linear superposition of plane wave solutions to the scalar wave

equation. Through the use of the Fourier transform, the source distribution is decomposed

into plane wave components, each of which is assumed to propagate independently to the

observation plane where they are recombined. Such a procedure is fully analogous to frequency

domain solutions of electronic circuit problems.

The mechanics of the procedure are as follows:

1. The two-dimensional Fourier transform of a complex source distribution, U-,, is

given by

oo U^CCn) = F{U^(x^,y-L)} = // U^(x^,y^)e"^2^^^1^+yi^) dx^ dy^ (2-1)

—00

where x, and y, are the spatial variables in the source plane and E, and n are the

transform variables.

2. The Fourier transform is recognized as a decomposition of U-, (x^,y,) into plane wave components of the form

^-1277 (f t-£;x-ny-?z)

The time dependence is normally dropped and propagation nominally along the z direction is assumed. The z dependence is then understood from the relation

?2 + n2 + ?2 = 1^ .

This leaves

i27r(gx+riy) e

with values of E, and t\ (called spatial frequencies) giving the direction of

propagation of the component through

^ sin 6 sin A

where 6 and (^ are the angles between the direction of propagation of the plane wave component and the y-z and x-z planes, respectively. Note that E, and n take values from -1/A to +1/X. The Fourier transform of the function U(x,y) is

thus commonly called its plane wave spectrum.
In propagating a distance, d, along the z axis through free space each plane wave undergoes a phase shift of the form

^ikd(l-A252_^2^2)^ (2-2)

as shown in figure 2-1.

Thus if one multiplies the plane wave spectrum (2-1) in the x, ,y-, plane by the

propagati

at z = d.

ikd(l-X252_x2^2)^2.

propagation factor (2-2) one obtains the plane wave spectrum in the x„,y- plane

U2(?,n) = U^(?,n) e

5. The resulting amplitude function is then given by the inverse transform

U2(x2,y2) = F"''"{U2(5,n)}

= // U2(?.n)e+"^("25+y2n) ^^ ^^^

Analysis of this form leads to a derivation of the Fresnel-Kirchhof f diffraction

integral

, - exp{ik[d2+(x -X )2+(y -y )2]^}

U2(x2,y2) = IX / / U,(x^,y,) — — dx^ dy^. (2-3)

x-|^,yj^ [d^+(x2-Xj^)^+(y2-y3^)'^]

The steps leading to this expression are somewhat mathematically complex and are summarized

in the appendix.

It is common to consider cases where x and y are small enough relative to d that the

exponential term can be written as exp(ikd) exp{ik[ (x„-x, )2 + (y~-y, )2] /2d} which leads to

ik

2j. 2 ^2+^2

,, , , 1 ikd 2d U2(x2,y2) =J^e e

x^+y^ x^X2+y.^y2

xk^^ ^^ d

X // U^(x^,y^)e e dx^ dy^. (2-4)

If X-. and y, are so small relative to d such that the first exponential term within

the integral may be neglected, the expression (2-4) becomes:

ik

2^ 2 X2+y2

IT c ^ 1 ikd 2d

x^x2+y^y2

-ik

X // U^(x^,y^)e dx^ dy^. (2-5)
^z

Figure 2-1. Propagation of a plane wave component a distance d In the

\, z direction occurs for propagation a distance d(l-sln^0)

along the wave normal. The phase shift undergone by each

J- 3- component Is exp Ikd(l-X^C^) ^ or exp Ikd(l-X^C^-A^ri^) ^ In

three dimensions.
That region where expression (2-4) is appropriate is commonly called the Fresnel

region while the region where (2-5) applies is called the far-field or Fraunhofer region.

The transition between these regions is generally taken to be about d = t— where D is the

diameter over which U-, is non-zero. At this distance, d, the maximum on axis phase error

-4

from neglecting this term is A/8. For a 10 cm diameter source at 10 cm wavelength,

4 expression (2-5) applies by this definition for distances greater than 10 meters.

Examination of (2-5) shows that except for the phase and scaling factors preceding the

integral it simply represents a Fourier transform of the source function. The transform

X2 y2

variables are identified as -rr and y-r- The phase factors imply that the transform lies on

a spherical surface of radius d, rather than a plane at z = d. This distinction is not

generally significant and disappears completely when intensity measurements (that is UU*) are made.

2. 2 Transform Properties of a Focusing Element

As stated in the Introduction it is possible to observe the far-field distribution

much closer to the source through the use of a focusing element. In order to indicate the

conditions under which this is true, we provide the following analysis [4]

.

Consider figure 2-2, which shows a lens inserted between the source plane and the

observation plane, at distances d, and d„, respectively, from the two planes. For present

purposes we assume that the lens introduces a phase factor of

P(x,y) = e

and that it is sufficiently large that its aperture is not important.

Using the Fresnel approximation (2-4) the complex amplitude function in the plane of

the lens is

"2(^2^2^ = iXd^^'"''^' h(x2,y2;d^)

X // U^(x-|^,y^) h(x^,yj^;dj^)exp[ik(x^X2+y-,^y2)/2d^] dx^ dy^ (2-6)

ik x-^+y

where the notation h(x,y;d) = e is used for simplicity.

The distribution U„(x„,y„) is given in terms of U„(x„,y„) as

^ikd2

X // U2(x2,y2)P(x2,y2)h(x2'^2''^l^

"3^^3'y3^ = lAdT h(x3,y3;d2)

-ik(x2X2+y2y3)

^^ dx2 dy2. (2-7)
A

V

Figure 2-2. Geometry for demonstrating the Fourier transforming properties

of lenses. The lens is assumed to have zero thickness and a

2 2 phase factor exp -ik(x2 + j^)/2l, where the y^^, y^ plane is

that of the lens.
Substituting (2-6) into (2-7), changing the order of integration, and evaluating the

integral over X2 and 72 yields

U,(x,,y,) = 7TJ-^i- exp^l^ (x^ + y^) (l - 3' 3'-^3' j-] iXd^d2

fik

// U^(x^,y^)exp[^|^ (x^ + yj (l - f-]

exp -ik(x^X3 + y^y3] ^J dx^ dy^ (2-8)

where

w

=

d3_ d2 "f

•

By specializing to the case where w = d^ , which gives d„ = f, we find a Fourier transform

! and the x„,y_ plan

||(x^ + y2) (1 _ J:.]

X // U^(x^,y3^) exp|-ik

relationship between the x, ,y, plane and the x„,y_ plane

"3^^3y3) = iXf ^^p

x^X3 y^y3

I dx^ dy^ (2-9)

The transform variables are seen to be — and —. The phase factor preceding the integral

disappears for d, = f, that is, the Fourier transform thus derived contains no phase error

if the source and observation planes are each located one focal length from the focusing

element. If this condition is not satisfied, the transform contains a spherical phase

error. In experiments where only intensity, UU*, is detected the phase error, as in the case of the far-field expression, is of no consequence.

2. 3 Approximations

It is important to summarize, at this point, the approximations made in reaching

equation (2-9). In addition to the use of scalar theory we have assumed the following:

1. That the approximations leading to the Fresnel form of the diffraction integral

are valid. Essentially this means that terms like [d^ + (x.-x )2 + (y--y,)^]^

can be approximated by d + [(x2-x,)^ + (y2-y-| ) ^] /2d, even in exponentials where

the approximation represents a phase error.

2. That the effect of the focusing element can be adequately represented by the

phase factor exp -ik[ (x2+y^)/2f ] . This basically implies that the focusing

element converts plane wavefronts into parabolic wavefronts instead of spherical

wavafronts which would be obtained if the phase factor of a perfect lens,

exp -ik[(f2+x2472) 2 - f]^ were used.
For the wavelength and dimensions we expect to use here, these approximations can be shown

to be marginally valid. However, due to the fact that these terms appear with opposite

sign In exponents the combined effect of the two approximations is much less than either

alone. This is verified by numerical computation in section 5.

2.4 Equivalence

Acceptance of equation (2-9) and a comparison of it with expression (2-5) form the

basis for the use of a focusing element to obtain far-field distributions. The intensity

functions calculated from the two equations are identical if the appropriate scaling

X X factor is applied. Since the transform variables are of the form -r-^ and -r-r, respectively,

Ar Ad we see that the focal plane distribution is compressed relative to the far-field distribution by the factor -j. For example, the distribution in the focal plane of a 1 m focal

4 length lens is identical in shape but 10 times smaller (and more Intense) than the dlstrl4 bution 10 m from the source.

It is generally more convenient to express the far-field distribution in terms of

angle. If we Interpret x/d In the far-field as an angle (-r = tan Q - d) , the equivalent

point in the focal plane distribution is x- = f9. Using the 1 m focal length lens as an example again, the field amplitude (U) or Irradlance (UU*) occurring at a far-field angle

of 1 mrad will occur in the focal plane a distance of 1 mm from the axis. (Consistent

normalization is required if absolute rather than relative values are to be obtained.)

3. THE QUESTION OF DEFINITION

Manufacturers' specifications of laser beam characteristics are often vague and

Inadequate. The problem arises both from inappropriate definitions and from lack of

precision in describing the chosen definition.

3.1 Single Transverse Modes

For a laser emitting radiation in certain well characterized modes a single quantity,

precisely but somewhat arbitrarily defined, can convey complete Information about the beam

propagation. Specifically, for a TEM (Gaussian) mode given by [5,6]

U(x,y) = f ^ exp -Ik x^+y^

2R exp _ x^+y^ ^

where w^ = vj^[l + (Az/ttw^)^]

o o R = z[l + (Trw2/Az)2]

and w is the minimum 1/e^ (intensity) radius,

it is conventional to define "beam divergence" as that far-field half-cone angle (envelope)

Inside of which the irradlance is greater than 1/e^ of its peak value. This angle is

determined from the above equations to be A/ttw , and Is shown graphically in figure 3-1.

The selection of the l/e^ intensity points is totally arbitrary, but for the Gaussian is

convenient. Integration shows that 86% of the energy in the beam is encompassed by that

cone angle. Thus simple measurement of fractional energy through a known aperture along

with the known wavelength is sufficient to fully characterize the propagation of the beam.
Figure 3-1. Propagation of a Gaussian beam. A single measurement of farfield divergence completely characterizes beam propagation.
Similar techniques [6,7] can in principle be used to characterize higher order modes,

for if the mode order TEM is known, the propagation laws can be described analytically

and related to measurements of energy through an aperture.

3.2 Multimode Beams

Many lasers do not emit in a single transverse mode but rather in a combination of

modes often distorted by non-uniform thermal effects. It is for this class of lasers that

the specification of beam characteristics by a single parameter becomes difficult if not

inappropriate and also for which transformation procedures are most important.

It is common practice with multimode lasers to follow a procedure similar to that for

single mode lasers and to specify that half-cone envelope containing some arbitrarily

chosen (often 90%) fraction of the energy. This is obtained by placing a series of apertures in the equivalent far-field, the focal plane, and applying the scaling factor given

in the previous section. Placement of the aperture is usually for maximum transmission.

For sources emitting nominally circular beams with nominally central maxima and for

applications where the primary concern is the shape of the beam near the centroid such

a procedure may be adequate. A certain precision of definition is required, and we therefore use the notation

to denote that half-cone angle containing fraction f of the energy of a single pulse.

Later, we will also use the notation

to denote divergence measured in the x-z plane. In this case f is defined as though there were no variation of the beam in the y-z plane.

Two principal problems arise with this form of measurement.

1. The output of a Q switched solid-state laser is often grossly as3mimetric. It is

frequently elliptical and may have multiple maxima, well displaced from the centroid.

2. Both the propagation direction (of, say, the centroid) and/or the profile may

vary from pulse to pulse.

If problem 1 is severe, any form of envelope measurements may yield little information

of use to the designer, the manufacturer, or the user. In this case it is probable that one should proceed to use a means of obtaining spatially resolved measurements.

If problem 2 is severe, a relatively crude solution is to define an average

divergence

V2,ave(^)

where the conditions of the average (e.g., first 10 pulses, 50th to 100th pulses, etc.)

should be clearly specified.

A greatly superior solution to both problems 1 and 2 is to replace the focal plane

aperture with an imaging device such as a videcon, CCD, or detector array, possibly with

some additional optics for magnification. This has the advantage of providing information

on both complex beam profiles and beam wander. A later section is devoted to a more thorough

discussion of these possibilities.

10
4. SELECTION OF FOCUSING ELEMENT

In choosing a focusing element to provide accurate near-field to far-field transforms

and to perhaps satisfy other requirements (e.g., power handling capability) one faces a variety of element types and an infinite selection of parameters. Some guidance can be

obtained from optics texts on minimizing aberration. Additional information may be obtained

by employing computer-generated spot diagrams. In this section we use the spot diagram

method to illustrate the merits of some possible choices.

To use the spot diagram technique we assume that a circular bundle of parallel rays

strikes the focusing element at a specified angle. By employing the rules of geometrical

optics, each ray is traced through the focusing element to the focal plane and the intersection of each ray with the focal plane is plotted. If this spot diagram is smaller than

the spot size calculated to result from diffraction (Airy disc) the performance of the

focusing element is expected to approach that of an ideal element. Further calculations

(section 5) can then be undertaken to estimate the errors in the transform.

4.1 Refracting Elements

4.1.1 Single Element Lenses

Lens selection involves not only a choice of size and focal length, but also of shape

and to some extent index of refraction. We consider here single element thin lenses with

spherical or plane surfaces. For such lenses (properly aligned) the principal defect is

spherical aberration.

Analysis [8] shows that to minimize spherical aberration in a lens designed to focus

nearly collimated light (i.e., an infinite conjugate ratio) a shape factor, q, of '^ +0.71

should be chosen (n=1.5). The shape factor is defined as

q

= ^2 + ^1

v^ - r^

where r^ and r2 are the radii of the first and second lens surfaces, respectively. Planoconvex lenses with the plane surface nearer the focus (q = +1.0) are sufficiently near the

optimum that they are frequently used to meet the condition. This shape factor is also

near optimum for minimizing coma, so that such a lens should not be particularly sensitive

to alignment.

[It is of great importance that the lens be used with the plane side nearer the focus.

A good rule of thumb is that the rays should undergo equal bending at the two surfaces for

minimum aberration.]

Figure 4-1 shows spot diagrams for an f/20 plano-convex lens (n = 1.5). In each case

the spot diagram is compared with the diffraction limited spot (Airy disc), which for a 10 cm dia aperture at X = 1.0 ym corresponds to a far-field half-angle of 1.2 x 10 rad.

For on-axis illumination the spot diagram suggests that the lens is essentially diffraction

limited. Coma resulting from misalignment is also small for misalignments up to about

one degree.

An f/10 lens shows significantly more spherical aberration than the f/20 lens (figure

4-2 (a) compared to figure 4-1 (a)) and more coma. A further problem arises from the presence

of spherical aberration. The outer zones of the lens focus inside the paraxial focal plane

with the result that the location of the minimum spot size depends on how much of the lens

is illuminated. For a 1.0 m focal length f/10 lens the ambiguity (longitudinal spherical

aberration) is about 2 mm, which can be significant (see section 5.4).

From analysis of this type we conclude that at a wavelength of 1.0 ym a simple planoconvex lens with an f-number of 20 or greater is a good candidate for transform measurements if reasonable care is taken in alignment. Quantitative assessments of such a choice

11
(a)

(b)

Figure 4-1. Focal plane and spot diagrams for an f/20 plano-convex lens,

Circles represent the diffraction limit (Airy disc) which

corresponds to a far-field half-angle of 1.2 x 10"-* rad for

a 10 cm dia aperture and wavelength of 1.0 ym,

(a) on axis; (b) 0.5 degree misaligned.

12
(c)

(d)

Figure 4-1. Continued

(c) 1.0 degree misaligned; (d) 2.0 degrees misaligned,

13
(a)

(b)

Figure 4-2. Spot diagrams for an f/10 plano-convex lens. Circles

represent the diffraction limit (Airy disc) which cor- responds to a far-field half-angle of 1.2 x 10 rad

for a 10 cm dia aperture and wavelength of 1.0 ym.

(a) at the paraxial focus; (b) 0.9985 f.

14

\ I
* V I V *

(c)

(d)

Figure 4-2 . Continued

(c) 0.998 f; (d) 0.9955 f

15
will be presented in section 5. At wavelengths shorter than 1.0 ym, the spot diagram

analysis indicates that the f/20 lens will cease to be diffraction limited and some other

choice should perhaps be made.

4,1.2 Multi-element and Aspheric Lenses

If one wishes to use a low f-number system for transform measurements, more sophisticated lens designs may be utilized. This may also be necessary at short wavelengths.

Spherical aberration can be completely eliminated from a single element by correcting the

shape (aspherizing) . Alternately, both spherical aberration and coma can be greatly re- duced by using a two element lens composed entirely of spherical surfaces. No analysis of

such systems is presented here because the single element high f-number lens provides the

presently required performance and is, as well, more generally available. The techniques

employed in this section are, however, directly applicable to more sophisticated systems

and could be used to select from various designs.

4.2 Reflecting Elements

In some measurement systems it may be desirable to consider the use of a mirror as the

focusing element. A mirror may withstand higher power densities; its focal length is

independent of wavelength and may therefore be more easily measurable; and it may result in

a more compact measurement system. Mirrors may be analyzed by the same type of geometrical

optics approach used for lenses.

4.2.1 Parabolic Mirrors

Among mirrors, the most likely candidate is an off-axis paraboloid, which lacks

primary spherical aberration. The geometry of such a system is shown in figure 4-3. By

operating off-axis, it is possible to examine the focal plane distribution without disturbing

the input beam.

Figures 4-4 and 4-5 show spot diagrams for f/20 and,f/10 paraboloids. For an input

parallel to the parabolic axis the spot diagrams consist of a single point (no spherical

aberration), but for slight misalignment substantial coma appears. The notation of the

figures is that of figure 4-3 with a denoting misalignment in the tangential plane (the

plane of the figure) and 3 denoting misalignment in the saggital plane (perpendicular to the figure). Dq. is given in units of the focal length. As before, for a 10 cm dia

aperture at A = 1.0 ym the diffraction limit is 1.2 x 10 rad.

From figure 4-4 we see that for an f/20 reflector a misalignment of 0.5° produces a comatic focal plane spot which is significantly larger than the diffraction limit. For an

f/10 reflector (figure 4-5) the coma is about a factor of four greater than for an f/20

reflector.

Stated in a more useful form, analysis of this type shows that to be diffraction

limited an f/20 off-axis paraboloid must be aligned to better than about 0.3° (tangential

plane) and an f/10 paraboloid must be aligned to better than about 0.07°. Alignment in the

saggital plane is less critical. Given the greater difficulty of aligning an off-axis

reflector compared to a lens (procedures for both cases are discussed in section 6), the

above results seem to favor the use of a lens rather than a reflector. Accordingly, the

quantitative analysis presented in section 5 assumes the use of a long focal length lens.

4.2.2 Spherical Mirrors

For beam divergence measurements, spherical mirrors must be used off-axis (input not

parallel to radius) so that the focal spot is located outside the input beam. This results

in substantial aberrations except where very high f-numbers are used. To assess this problem

more quantitatively, we have generated spot diagrams for spherical mirrors with various

f-numbers and angles of incidence. The results are summarized in the following table.

16

.11
/ MIRROR /

Figure 4-3. The use of an off-axis parabolic mirror for focusing laser

radiation.
(a)

(b)

Figure 4-4. Focal plane spot diagram for an f/10 off-axis paraboloid.

D = 0.1; (a) a = 0.5°, g = 0.0°; (b) a = 0.5°, 3 = 2.0°

Circles represent equivalent far-field half-angles. The

diffraction limit for a 10 cm dia lens at A = 1.0 ym

corresponds to 1.2 x 10 rad.

18
(a)

(b)

Figure 4-5 Focal plane spot diagrams for an f/20 off -axis paraboloid,

Dq^ = 0.05; (a) a = 0.5°, 3 = 0.0°; (b) a = 0.5°, 3 = 2.0'

Circles represent equivalent far-field half angles. The

diffraction limit for a 10 cm dia lens at A = 1.0 ym

corresponds to 1.2 x 10~ rad.

19
5 3 X

10"-4

6 7 X

10'-5

3 4 X lO"

-5

1 9 X

10'-5

8 4 X lO'

-6

4 3 X lO'

-6

f# Angle of Incidence Max Spot Dimension/Focal Length

20 6.0 degrees

40 3.0 degrees

50 2,4 degrees

60 2.0 degrees

80 1.5 degrees

100 1.2 degrees

The angle of incidence is defined as the angle between the direction of beam propagation

and the normal to the center of the mirror (radius) . Angles listed above are reasonable

minimums for each f-number. For a diffraction limited 10 cm diameter beam (X = 1.0 ym)

the Airy disc diameter divided by the focal length is 2.4 x 10 . Thus we expect that

those combinations in the above table which yield smaller spot dimensions should be

essentially diffraction limited cases for which aberrations are not significant.

5. ESTIMATION OF CERTAIN ERROR SOURCES

5.1 Computational Technique

It would be impossible to provide an error analysis which would incorporate all possible measurement configurations and beam profiles. We therefore resort to the device of

attempting to define a typical measurement system and a typical beam profile within the

limits of common laser behavior.

Our measurement system is a plano-convex lens, illuminated from the convex side. The

diameter is 10 cm and the focal length 200 cm (i.e., an f/20 lens). This choice is based

on the analysis of section 4, and the fact that certain systems of interest have apertures

this large.

Our "typical" beam profile is chosen more arbitrarily. It consists of a linear combination of several TEM modes having the irradiance distribution shown in figure 5-1 (a). A

wavefront curvature is applied such that the far-field divergence is about

^1/2 x^"^^-^ ~ ^'^ TM rad, within the range of 0.1 to 10 m rad typical for systems of interest.

Application of wavefront curvature is equivalent in an analytical sense to experimentally

defocusing the laser collimator. In addition we also define an "extreme" case. Most of the error sources become more severe when the input beam is larger and when the divergence is

smaller. The "extreme" beam is therefore taken to have the same shape as the "typical" beam,

but has a larger cross section (figure 5-1 (b)) and has a far-field divergence of about

0.13 m rad.

The computations are performed numerically using the approach described in section 1.

1. For the beam under consideration, the far-field distribution is computed.

2. The same distribution is assumed to occur at the input of the lens described

above. An appropriate phase factor for the lens is applied along with the

appropriate phase factor for propagation to the focal plane. The resulting

distribution is then scaled to correspond to the far-field.

3. The irradiance distributions computed from 1 and 2 above are compared to estimate

the quantitative error from various sources.

20
Lens

Diameter

10 cm

Figure 5-1. Irradiance distributions for two cases examined in this

section. In both cases the distribution is formed by a linear combination of TEM laser cavity modes . Phase nm

curvature is applied to each case to set the far-field

divergence at desired levels. (a) "typical" case,

(.9) - 1.3 m rad; (b) "extreme" case, (a) distribu1/2,

X

tion expanded.

1/2,

X

(.9) ^ 0.13 m rad,

21
A fast Fourier transform (FFT) algorithm is used in each of these computations. The

primary source of error for this application of the FFT is aliasing due to insufficiently

close spatial sampling. This is avoided by application of the sampling theorem, but for

the relatively large (in terms of wavelengths) apertures considered here the result is

'^ 10 pieces of data in each dimension ('^ 10 in two dimensions) . For low divergence

beams fewer samples can be taken, but the volume of data still prohibits routine calculations of this form in two dimensions; we therefore resort to single dimension analysis.

If all functions involved were separable in rectangular coordinates, the above restriction would be of no great consequence. In fact, most beam profiles are not separable at least without approximation. The plane wave propagation factor is separable in the Fresnel

approximation as is the phase factor of a lens in the parabolic approximation. When, in

the analysis below, extensions from one-dimensional divergence St/o (^ ) to two-dimensional

^1/2^^ ) are made, it should be understood that certain assumptions of symmetry as lust

outlined are being made. However, even when this symmetry is not present, error estimates

thus obtained should be approximately correct.

5. 2 The Fresnel Approximation

In section 2.3 we pointed out that in the derivation of the focal-plane/far-field

equivalence the Fresnel approximation, though only marginally valid by itself, is largely

compensated by the assumption of a non-ideal phase factor for the focusing element. To verify this argument, we can compute numerically, as described above, the focal plane

distribution without the Fresnel approximation and with the ideal lens phase factor.

We expect this result, properly scaled, to closely coincide with the computed far-field

patterns for our test cases. The result for our "extreme" case is shown in figure 5-2.

Any difference between the two curves is comparable to or less than the width of the line.

We therefore set the following limit to the residual error:

"typical" extreme

'h/lj''^ < 0.1% ^9l/2,x^-^)

< 0.1%

5.3 Lens Aberration

Having shown that the focal-plane/far-field equivalence using an ideal lens is quite

good, we now seek to determine the effect of the non-ideal character of our real lens.

For our chosen lens we expect spherical aberration to be the dominant defect. Using

equation (4.50) of Klein [9] and the additional data that the lens is 1 cm thick and has

an index of refraction of 1.5, we compute a distortion to the ideal phase factor of

-8 4 -3.65 X 10 X . The phase factor for our real lens is thus taken to be:

exp -ik[(d^+x^)^ - d + 3.65 X 10"^x^].

Tl'e resulting error in S-i /^ (f ) depends significantly (in both sign and magnitude) on

f , the fraction of energy encompassed,

extreme case. This is illustrated in figure 5-3 for our

For the two examples used here, we find

"typical" "extreme"

AQi /o (.9) < 0.1%

1/2,

X

^V2,x(-7) =+7.0%

^«l/2,x(-9) =-0-2^

Ae,/2,,(.95) -3.0%

22
-0.1 0.1 0.2 m rad

Figure 5-2. Error due to using the Fresnel approximation. The directly

computed far-field irradiance pattern for the extreme case

is super-imposed upon the scaled focal plane distribution

computed without the Fresnel approximation and with the

ideal phase factor for a lens.

23
-0.2 -0.1 0.0 0.1 0.2 m rad

Figure 5-3. Effect of spherical aberration, "extreme" case, f/20 lens,

10 cm dia, (a) directly computed far-field distribution;

(b) scaled focal plane distribution with spherical

aberration present.

24
The fact that the error is minimum when the fraction of energy encompassed is 0.9 is purely

coincidental and a consequence of the beam shape chosen as an example.

We will not treat coma in this section for two reasons. First, we believe that for

any reasonable alignment of the lens, spherical aberration should be the dominant aberration. Secondly, the wavefront aberration of coma is not separable in rectangular coordinates, and thus not readily tractable in our one-dimensional analysis. Coma in the tangential

plane could be examined, but the results would be of limited usefulness.

5.4 The Position of the Observation Plane

Perhaps the largest quantitative errors result from an improper determination of the

focal length and errors in the location of the observation plane. For small errors these

effects can be separated.

Since the scaling of the focal plane distribution relative to the far-field is proportional to the focal length, an error in its numerical value propagates directly in one dimension. Thus, for a 1% error in determination

^\,2j.9) = 1%.

Appropriate means for establishing the numeric value of the focal length will be discussed

in section 6.

The error resulting from improper placement of the observation depends critically on

the magnitude of the divergence. For the case of a slightly defocused laser beam such as we have been using for test purposes this fact is illustrated in figure 5-4. Specific

calculations yield the following results:

1% misplacement (2 cm)

"typical" "extreme"

"^l/2,x(-9) = * 1-7^ "V2,x(-^) = - f^^t°^ °^ 5

0.05% misplacement (0.1 cm)

"typical" "extreme"

Ae, /„ (.9) < 0.1% Ae,,„ (.9) = ± 6.6% l/i,x 1/2,

x

Figure 5-5 shows irradiance computations for the extreme case.

These results point out the great importance of determining the focal plane properly

and properly placing the observation device. In the infrared, these tasks are more difficult

than in the visible. Further discussion of proper procedures is contained in section 6.

It should also be noted that for beams with relatively low divergence it is possible

to under estimate very substantially the divergence by placing the observation device too

far from the lens.

25
LENS

PLANE OBSERVATION

PLANE

LARGE DIVERGENCE

CASE

FOCAL

PLANE

LOW DIVERGENCE

CASE

Figure 5-4. Effect of incorrectly placed observation plane in the

geometrical approximation. For the same error, A, in

the placement of the observation plane, the error in

spot size becomes much greater for low divergence beams

C A

That is ^ »

g

.

26
l.OOOSf

-0.2 0.2 m rad

Figure 5-5. Effect of misplacement of observation plane,

"Extreme case."

27
5.5 Summary of Computations

The results of computations in this section are tabulated as follows;

Errors in AO, ,„ (f ) 1/2, X x^

(f =0.9 unless specified)

Source "typical" extreme

Fresnel < 0.1% < 1%

Approximation

Spherical (0.7) + 7.0%

Aberration < 0.1% (0.9) - 0.2%

(0.95) - 3.0%

Observation Plane A = ± 1% A = ± 0.05% A = ± 1% A = ± 0. Location:

Scale factor ±1% ± 0.05% ± 1% ± 0.05%

Location ± 1.7% < 0.01% ± X5 ± 6.6%

It is important to remember the difference between the definitions of 6, ,„(f) and

^1/2 x^^x^* ^^^ former is defined as a cone angle (a plane angle of revolution) while

6-| /2 (f ) is defined from the profile cross section in the x-z plane only. For beams

which are generally circular, 9-L/2(f) ^^d 9, , (f ) are comparable [Ex: Gaussian

beam - 6^ ,„(.86) = -^

1/2 TTO) 3, ,„ (.86) = 1.045 ] and therefore Ae-,,„(f) 1/2,

X

TTco 1/2

o 'hnj'^ If,

instead, the beam is substantially elongated in one dimension, 6, /„(f) approaches the one dimensional value for the larger dimension.

Any generalization of the error sources in a measurement process into a single estimate of uncertainty is hazardous. This is particularly true when, as in this case, the

contributions of each error source depend, often critically, on the details of the source

to be measured. The philosophy of this analysis has, therefore, been to examine specific

cases and to observe the error contributions and their sensitivity to the parameters of the

measurement system and to some of the parameters of the source. The extent to which these

chosen cases are typical or worst cases cannot be established. If we nevertheless assume

that what we have called a "typical" case is representative of 1 ym wavelength beams of

several centimeters diameter and nominal divergence of 1 m rad, the above analysis suggests

that measurements of 6-, /«(.9) to uncertainties of 2-3% should be possible with reasonable

care. Further, if our "extreme" case is, in fact, typical of sources nearly filling a 10 cm diameter aperture and having nominal divergence of 0.1 m rad, the above analysis

suggests that very careful attention to detail may be necessary to achieve uncertainties of

less than 10%.

Some improvement in accuracy may be obtainable by employing a lens which is corrected

for spherical aberration, though in most cases this will not be worth the additional cost. The principal source of error will likely remain the positioning of the observation plane.

28
6. EXPERIMENTAL IMPLEMENTATION

6.1 Characterizing the Focusing Element

We turn now to measurements and tests on the focusing element which must be made prior

to use. Of critical Importance Is the numeric value of the focal length and the position

of the focal plane at the wavelength of Interest . In addition, It Is desirable to check

for unexpected aberrations and to appropriately align the element with the optical bench or

test facility to be used. Many techniques for these purposes can be found In the literature.

In this section we describe some of the more simple tests and their adaptation to present

I needs.

6.1.1 Lenses

When accurately specifying the focal length of a lens It Is usually necessary to use

i thick lens terminology. For a more complete discussion of this terminology see chapter 5 of reference [8]

.

The focal length of a lens is properly measured from the secondary principal plane of

the lens, which is defined in a geometrical optics sense as the plane of Intersections

between parallel input rays (extended) and the bundle of rays (extended) which converge to the focal point (figure 6.1). This "focal length" is to be distinguished from the "back

focal length" which is measured from the back surface of the lens.

When the index of refraction on each side of the lens is the same, the secondary

[principal plane contains the secondary nodal point (figure 6.1). The secondary nodal point

[has the useful property that when the lens is rotated through small angles about it as center the focus does not move laterally. This forms the basis for the nodal slide technique

of focal length determination.

The nodal slide is simply a device which allows the lens to be rotated about any point

on its axis. It can be used in conjunction with a high quality collimator (figure 6.2(a))

or more simply in auto-collimatlon (figure 6.2(b)). In the latter case a source at the

appropriate wavelength (perhaps a cw laser) is focused through a pinhole. Using the test

lens and a high quality flat mirror as shown, the image of the pinhole is located along

side the pinhole (as close as reasonably possible) . By successive translation and rotation

an axis of rotation is found which results in no lateral movement of the image. This axis

contains the secondary nodal point and the focal length is measured from this axis to the

pinhole.

As one check on this procedure, the location of the secondary principal plane can be

computed knowing the shape and index of refraction of the lens. The procedure is given in

reference [8], chapter 5. For a 2 m focal length lens (R, = 1 m, R2 = °°, n = 1.5,

I thickness = 1 cm) the secondary principal plane is found to lie Inside the lens 6.7 mm from

the plane surface.

The nodal slide also provides a means of aligning the lens with the optical bench.

With the lens center and the pinhole both centered on the bench, and the same distance

above it, the lens can be rotated a few degrees in each direction until a small amount of coma appears in the image. By symmetry the best orientation of the lens can be determined.

Using this method it is convenient to align the lens described in the previous paragraph to better than one degree.

A second, complementary, approach to evaluating a lens is the use of wave-front

shearing interf erometry. This technique provides a convenient cross-check of focal length

measurements made with a nodal slide. It also provides information about aberrations

present in the system. The setup for this measurement is shown in figure 6.3. As with the

I nodal slide, a pinhole source is placed near the focal plane of the test lens. A shearing

iplate, typically a piece of high quality optical glass or other transparent material having

29
FOCAL

LENGTH

^—SECONDARY

I PRINCIPAL

PLANE

'-^ FOCAL

I PLANE

Figure 6-1. Parameters of a thick lens,

30
ROTATION

COLLIMATOR

TRANSLATION

(a) SCREEN

ROTATION

(b)

FLAT

MIRROR TRANSLATION

SCREEN WITH

PINHOLE

Figure 6-2. The use of a nodal slide to determine the focal length

of a lens. (a) Using a collimated source; (b) in

autocollimation.

31

AtaM.
SHEARING

PLATE

PINHOLE

SCREEN

Figure 6-3. The use of a wavefront shearing, plate to determine

the focal length of a lens.

32
flat and nearly parallel faces, is placed in the collimated beam at an angle to the direction of propagation. Interference fringes are produced on a screen by the reflections from

the front and back faces.

If the pinhole is not in the focal plane of the lens the interference pattern is that

of two offset spherical waves, and consists of linear fringes. As the radius of the spherical

wavefronts increases (the pinhole is moved closer to the focal plane) the spacing of the

fringes increases. For plane wavefronts (pinhole in focal plane) and exactly parallel

faces on the shearing plate the fringe spacing becomes large compared to the beam diameter.

Increased precision can be obtained by polishing the faces of the shearing plate with

a slight wedge. If the apex of the wedge is parallel to the shearing plane (the plane of

the paper in figure 6.3) the fringe spacing does not approach infinity as the pinhole is

passed through the focal plane, but reaches a limit determined by the wedge angle, while

the orientation rotates through 180°. For plane wavefronts, the fringes are parallel to the shearing plane.

This is illustrated in figure 6.4 which shows computer simulated fringes for a 2 m

focal length plane convex lens at 632.8 ym. When the pinhole is at the focal plane, the

fringes at the center of the interference pattern are seen to be parallel to the shearing

plane, while spherical aberration causes slight curvature to the fringes at the edge.

Since the fringe separation corresponds to one wavelength of phase deviation, we see that

the maximum phase error is about X/5.

Figure 6.5 shows the experimental equivalent of figure 6.4. Comparison of the two figures suggests that the lens used is slightly corrected for spherical aberration. Both

figures indicate that a precision of ± 1 mm (± .05%) in determining the focal length of

such a lens is possible by this technique. It should be noted, however, that it is the

focal plane which is found in this manner, and computation (as indicated previously) is

required to get the actual focal length.

The computed and experimental patterns of figures 6.4 and 6.5 were produced at a wavelength of 0.63 ym for ease of illustration. At a wavelength near 1 ym the same results

will hold, except that the fringe spacing will be proportionately larger and the phase

error proportionately smaller.

To conclude this discussion on lens evaluation we present below a summary of data on a single lens of the type described in this and previous sections.

Method

Nodal slide

Wavefront shearing

X = . 63ym

198.84 cm

198.93 cm

Focal length

A = 1.06ym

202.19 cm

202.19 cm

^1.06^^.63

1.0168

1.0164

Based on dispersion data for the glass (BK7) used in the lens we expect the ratio of focal

lengths to be 1.0167. These results further suggest that, with a small loss in confidence

one may perform the focal length determination at some known, available source wavelength

and extrapolate from dispersion data to the desired wavelength.

6.1.2 Mirrors

One advantage of using reflective elements, especially for infrared systems, is that

they can be characterized at any convenient wavelength. In the case of the off-axis

paraboloid, this is offset significantly by the problem of locating the parabolic axis.

For the paraboloid, a simple approach is to use a wavefront shearing plate as shown in

figure 6.6(a). The mirror is moved relative to the pinhole until the interference pattern

consists of straight lines parallel to the shearing plane. At this point, the pinhole is

located at the focal point on the parabolic axis and the collimated beam is parallel to the

33
I

IT)I 1 1 , : „ \ \ \ \ : \\\\ \ \ \\ \ \ \ \

',, \ \ s

', f s • \ \ in \ \ + 1 1 1 m 1

1, 11 i 111 1 1 1 B

', 1 i1 S

', I-H + • • • 1 . <+-i I / /I I I I

.1 I ^\ \ \ I

I,

mI

•HoPc

0)6

0)M

CO

5-1

CO

0)

•u

>-l o

CO

^1

Q) 4J4J

COft

0)60d

•H

d Ti •H 0)'

a!

d HJ

cd M-il 0)1

CO Vj d

0)

CO CN p

CO II •HT3 M^

(U

CO

CDd

(U +J

4-1 CO

T3

QJ •u

CO

CO iH

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CUU

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(1)

34 i
I

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r-iCO>

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dj HI

bO

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35
SCREEN

PINHOLE

(b)

FLAT

MIRROR

IFE EDGE

PINHOLE

(c)

IFE EDGE

Figure 6-6. Techniques for determining the focal length of a mirror.

(a) A shearing plate used with a parabolic mirror; (b) the

knife edge technique for a parabolic mirror; (c) the

knife edge technique for a spherical mirror.

36
axis. As with a lens, residual curvature in the fringes is a measure of the aberrations

introduced by the mirror.

It should be possible to use this same technique to examine a spherical mirror. In this case the least aberration is obtained as near the collimated beam as possible.

As an alternate means of determining the focal length of a mirror, one can employ the

Foucault (knife edge) test. This is illustrated for a parabolic mirror in figure 6.6(b).

The apparatus consists of a pinhole which is co-planar with a moveable knife edge. If the

pinhole is located in the focal plane, the observer will see the mirror darken suddenly and

uniformly as the knife edge is passed through the image of the pinhole. If the pinhole is

located in front or in back of the focal plane, the image will not lie in the plane of the

knife edge. Consequently, when scanning, the observer will see the mirror darken gradually

and non-uniformly.

Imperfections in the lens can be thought of as regions of different focal length.

Thus the uniformity with which the mirror darkens is a measure of mirror quality. Simple

geometric optics suggests that for a 200 cm focal length mirror, deviations in the mirror

of the order of 0.1 pm can be detected.

The Foucault test can also be applied to spherical mirrors, either in a manner like

that of figure 6.6(b) or by placing the pinhole and knife edge at the center of curvature

(figure 6.6(c)). This is possible because the image of an object at the center also lies at

the center of curvature.

6. 2 Other Experimental Considerations

In this section we wish to point out some experimental problems which may arise in

performing envelope divergence measurements. Typical setups are indicated in figure 6.7.

The laser under test must first be aligned collinear with the optic axis of the system

as defined by the lens/mirror axis, the aperture, and the optical bench. Procedures for

performing this will depend greatly on the type and characteristics of the laser under test

and are therefore not discussed further here.

It is desirable that the focusing element be placed as far as possible from the laser

to minimize the collection of scattered light and (if the light is unfiltered) of light from

the pumping source. In the case of the lens, this will also help minimize feedback into the

laser. (A high efficiency anti-reflection coating on the lens should also be employed for

this purpose if expected power densities will allow it.)

In selecting apertures, size and damage resistance are important considerations. The

required determination of aperture dimensions is generally within the capability of a machine

shop although for small apertures some uncertainty may be introduced. An inaccuracy of

0.001 inch in the diameter of a 1 mm aperture results in an inaccuracy in area of 5%.

Optical damage at the edge of the aperture may result in the emission of radiation (which

will be detected along with laser radiation) as well as dimensional changes. If levels high

enough to produce damage are expected, it is well to use apertures of a reflective or damage

resistant material.

Since the data of interest is the ratio of power/energy transmitted through a known

aperture to the total power/energy, calibration accuracy of the monitor is of little concern

as is linearity since the quantities typically differ by only 10-20%. Other factors affecting

precision, for example, spatial uniformity of response, are of much greater importance in

selecting an instrument.

In many cases the reproducibility of the laser, in amplitude, in profile, and in direction of propagation, will set the limit to measurement quality. Variations in amplitude can

be compensated by the use of a beamsplitter and monitor (figure 6.7). Several difficulties

arise with this approach:

(1) The beamsplitter must be wedged to avoid interference effects; this deviates the beam

and makes alignment difficult.

37
ono

o

LlJ LU

r

u ^

LlJ 00

-e^CD q:Q_

Qi

I"

-J OO r

u

CQ

V^ o

\ 6

cc:

\

\

\

cc:

Q_

\<P

^ — Id.

\

QD

cuM

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en

ct) 0)e

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+Ju

o

u

u

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n3

J-4 o

en CU d O

OJ d

iH OJM

CO U

a) bO >

c •H •H 'd

cn3 e

CO

5-1 (U O rQ

M-l t3

cn tH

Cb CU d •H

•u IM

OJ 1 cn u

CO iH M-l COo M

•H CU D- cn >% CO H H

I

CU

5-1 M

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38 i
(2) The reflection coefficient is polarization sensitive; if the laser is randomly polarized

and the angle of incidence is more than a few degrees, the ratio of transmission to reflection will show significant scatter.

(3) The beamsplitter may modify the transmitted beam through phase distortion or multiple

reflections.

Variations in beam deviation can be examined to some degree by mounting the aperture on

a translation stage allowing motion perpendicular to the system axis.

Variations in profile from pulse to pulse are not usefully examined with envelope

measurements. Some possible approaches to this problem are discussed in the next section.

7. SPATIALLY RESOLVED MEASUREMENTS

In the preceding sections we have indicated the deficiencies of envelope divergence

measurements for complex beam profiles and where variations with time occur. An obvious

question is whether detailed beam profiles can be obtained with good resolution and accuracy

from the focal plane transform measurement system. Beyond the approximations of the transform

discussed in this document, the answer rests with the technology of imaging devices.

For beams exhibiting no time dependence, scanning a small detector across the focal

plane distribution may yield sufficient spatial information. In general, however, an array

or imaging device might be placed in the focal plane or in an equivalent image plane to provide magnification (figure 7-1). Introduction of additional optics should not necessarily add additional inaccuracy because, as in a telescope, the quality of the transform

lens (objective) will normally have the dominant effect on image quality.

A suitable image device should be linear over 2 to 3 decades of irradiance and be

simultaneously uniform over its surface. Resolution requirements will depend on the particular application but even as few as 10 elements across a typical beam diameter would provide

substantial improvement over envelope measurements. A suitable storage mechanism must be

employed to observe waveforms of pulsed sources. Exploratory work must be performed to determine whether the above conditions can be met and the extent to which the quality of the

imaging device will effect the complete measurement process.

In the visible and near infrared portions of the spectrum, the most likely candidates

will be among tubes designed for television-type applications. Conventional vidicons do

not, in general, provide either sufficient linearity or uniformity; however, silicon diode

array tubes do show apparently sufficient linearity and may be sufficiently uniform at wavelengths significantly shorter than the band gap [10]. CCD vidicons may provide still better

linearity and uniformity but at some loss in resolution with present commercial tubes. A wide

variety of image processing hardware (readout, storage, analog and digital processing) is

commercially available to be used with such tubes thus making automated measurement techniques

immediately feasible.

At longer wavelengths the choices are less clear. Some image tubes are presently

available but less is known about their performance capabilities. Arrays of individual

detectors should also be considered.

39
IMAGE RECORDING

DEVICE

TRANSFORM

LENS

FOCAL

PLANE

Figure 7-1. Image recording with the focal plane transform technique.

With a second lens, the focal plane irradiance distribution

is imaged with magnification on a suitable recording device.

40
8. SUMMARY

The preceding sections represent an attempt to illuminate the problem of far-field laser

beam profile measurements from the point of view of measurement quality. They are not

intended to dictate measurement technique or define parameters. Rather, we hope to have

made clear the fact that measurement requirements evaluated with a knowledge of various

trade-offs must determine the details of the measurement system and, accordingly, the

quality of measurement.

To improve our ability to make such judgments, we have examined in some detail the

consequences of a particular choice of measurement parameters for a particular quality to be measured. The results show that for this particular set of conditions measurements of

generally acceptable accuracy (less than "^ 10%) are possible with reasonable care. We

urge caution, however, in applying this result to measurement systems with different

parameters or to beams with substantially different characteristics. This specific analysis

should be used, instead, to illustrate the nature of parameter choices and their effect.

The greatest advantage of using focal plane measurements to obtain far-field characteristics is simplicity. We can suggest no comparably simple technique of any sort. The

greatest disadvantage of this technique is the possibility that, without sufficient care, substantially erroneous measurements can be obtained.

9 . REFERENCES

[I] Goodman, J. W., Introduction to Fourier Optics (McGraw-Hill Book Co., Inc., New York,

N.Y., 1968).

[2] Cathey, W. T., Optical Information Processing and Holography (John Wiley, New York,

N.Y., 1974).

[3] Collier, R. J., Burckhardt, C. B., and Lin, L. H., Optical Holography (Academic Press,

New York, N.Y. , 1971).

[4] See Chapter 5 of Reference [2] for greater detail.

[5] Siegman, A. E. , An Introduction to Laser and Masers, Chapter 8 (McGraw-Hill Book Co.,

Inc., New York, N.Y. , 1971).

[6] Yariv, A., Quantum Electronics, Chapter 14 (John Wiley, New York, N.Y., 1967).

[7] Kruger, J. S. Beam Divergence for Various Transverse Laser Modes, Harry Diamond Laboratories Report HDL-TM-71-11, AD 729 299 (1971).

[8] Jenkins, F. A. and White H. E., Fundamentals of Optics, 4th Edition (McGraw Hill Book

Co., Inc., New York, N.Y. , 1976).

[9] Klein, M. V., Optics (John Wiley, New York, N.Y., 1970).

[10] Bieberman, L. M. and Nudelman, S., Editors, Photoelectronic Imaging Devices, Vol. 2 (Plenum Press, 1971).

[II] Jahnke, E. and Emde, F. , Tables of Functions with Formulae and Curves, p. 149, 4th Ed.

(Dover, New York, N.Y., 1945).

[12] Erdelyi, A., Higher Transcendental Functions, Eq. (52), p. 95, Vol. 2 (McGraw-Hill Book

Co., Inc., New York, N.Y., 1953).

41
10. ACKNOWLEDGMENT

This work was supported by the Air Force Guidance and Metrology Center, Newark

Air Force Station, Ohio, and by the Department of Defense Calibration Coordination

Group through its Laser and Infrared Working Group.

42
APPENDIX

In the field of optics, when Fourier transform techniques are applied to obtain diffraction relations, approximations are usually made which lead to expressions valid in the

Fresnel region and beyond. As a background to section 2, we present here a derivation of

the Fresnel-Kirchhoff diffraction integral (2-3) using the Fourier transform approach

without any small angle approximations. The approach is essentially that of

Collier, et al. [3]

.

Using the notation of section 2.1 we write

U2(x2,y2) = F"^{U^(5,Ti)exp[ikz(l-x252_x2^2)^]}

= U^(x^,y^) x F"-'-{exp[ikz(l-A252_x2Ti )^]}

where x denotes convolution.

The inverse transform can be written

00 F"-'-{} =

/ / exp[ikz(l-A252_x2f^2)'2]exp[l2Tr(Cx+ny)]d? dn.

—00 Making the transforms

5 = p cos a x = r cos <{)

ri = p sin a y = r sin ij)

we obtain

-1 °° h

'^^ F { } = / exp[ikz(l-A2p2) pdp j exp[i27rpr cos(a-(j)) ]doi. o o

The integral over a is evaluated using the formula [11]

.-n 2Tr

L271

T / \ i "

7 iz cos d) ind) ,, / n\ J (z) = J— J e ^ e ^ d()) (n=0)

o

yielding

00

F"-'-{ } = 277 / exp[ikz(l-x2p2)'5]j (27rpr)pdp.

o

The integral over p is evaluated using the formula [12]

00/ J^Cbt) exp[-a(t2-y2)'2](t2-y2)-'5 tdt

o

= exp[-iy(a2+b2)'^](a2+b2)"'^

43
which is differentiated with respect to a to obtain

00/ J (bt)exp[-a(t2-y2)'5]tdt

= exp[-ly(a'^+b'^) ]

The inverse transfoirm can then be written

^a2 + b2

,24.^2^'5^ p-lr , _ -ikz exp[-ik(z^+r^) ] (z^+r^)

^(a2 + b2)'

1 +

+ iy

ik(d2+r2)''J

For d greater than a few wavelengths, the second term in brackets is clearly much less than

one so the transform becomes

r~l/ rjT /I -i2r2 a2 2\ 11 -ikz exp[-ik(z2+r2) ] F {exp[ikz(l-A^C'^-X^Ti ) ]> = -^— —'^ ^^ ^—^ • ^^ c 2j_ 2\

Now performing the convolution indicated above and writing r explicitly in terms of x and

y, we have

U2(x2,y2) =J^!! U^(x^,y^)

exp[-ik(z2 + (X„-Xj2 + (y -y )2)'5]

2 "1' '2 ^1^

[z2 + (x2-x^)2 + (y^-Yj)^]

dx^ dy^^

which is the diffraction integral we set out to obtain.

44
S.114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUI fATION OR REPORT NO.

NBS-TN-1001

2. Gov't Acce;

No.

3. Recipient's Accession No.

TITLE AND SUBTITLE

Laser Far-Field Beam-Profile Measurements by the

Focal Plane Technique

5. Publication Date March 1978

6. Performing Organization Code

276.01

AUTHOR(S)

G. W. Day and C. F. Stubenrauch

8. Performing Organ. Report No.

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2761171

11. Contract/Grant No.

Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Air Force Guidance and Metrology Center,

Newark Air Force Station, Ohio and

Department of Defense

CCG, Laser and Infrared Working Group

13. Type of Report & Period Covered

14. Sponsoring Agency Code

SUPPLEMENTARY NOTES

ABSTRACT (A 200-word or less (actual summary of most si^iticant information. If document includes a significant

bibliography or literature survey, mention it here.)

An analysis of laser far-field beam-profile measurements by the focal plane

technique is given. Particular attention is paid to systems at A/lym wavelength

aua having apertures up to 10 cm. The basic mathematics is reviewed and approximations are evaluated. Using geometrical optics techniques, it is shown that

an f/20 plano-convex lens is an appropriate choice for the focusing element.

For two arbitrarily chosen laser beam profiles the errors associated with the

choice of this lens are discussed through the use of computed far-field and

focal-plane irradiance distributions. Experimental procedures including methods

of testing the optical elements are also given.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Beam divergence; beam profile; lasers; optical propagation.

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