This is a basic tutorial on how to use lenses in order to focus

This is a basic tutorial on how to use lenses in order to focus
An introduction to the use of lenses to
solve optical applications can begin with
the elements of ray tracing. Figure 1
demonstrates an elementary ray trace
showing the formation of an image, using
an ideal thin lens. The object height is y1
at a distance s1 from an ideal thin lens of
focal length f. The lens produces an
image of height y2 at a distance s2 on the
far side of the lens.
We can use basic geometry to look at the
magnification of a lens. In Figure 2, we
have the same ray tracing figure with
some particular line segments
highlighted. The ray through the center of
the lens and the optical axis intersect at
an angle φ. Recall that the opposite
angles of two intersecting lines are equal.
Therefore, we have two similar triangles.
Taking the ratios of the sides, we have
φ= y1/s1 = y2/s2
Rearranging one more time, we finally
arrive at
1/f = 1/s1 + 1/s2.
This is the Gaussian lens equation.
This equation provides the fundamental
relation between the focal length of the
lens and the size of the optical system.
A specification of the required
magnification and the Gaussian lens
equation form a system of two equations
with three unknowns: f, s1, and s2. The
addition of one final condition will fix
these three variables in an application.
Optical Ray Tracing
Focusing and Collimating
This can then be rearranged to give
y2/y1 = s2/s1 = M.
Gaussian Lens Equation
Let’s now go back to our ray tracing
diagram and look at one more set of line
segments. In Figure 3, we look at the
optical axis and the ray through the front
focus. Again looking at similar triangles
sharing a common vertex and, now, angle
η, we have y2/f = y1/(s1-f).
Now we are ready to look at what
happens to an arbitrary ray that passes
through the optical system. Figure 4
shows such a ray. In this figure, we have
chosen the maximal ray, that is, the ray
that makes the maximal angle with the
optical axis as it leaves the object,
passing through the lens at its maximum
clear aperture. This choice makes it
easier, of course, to visualize what is
happening in the system, but this
maximal ray is also the one that is of
most importance in designing an
application. While the figure is drawn in
this fashion, the choice is completely
arbitrary and the development shown
here is true regardless of which ray is
actually chosen.
Rearranging and using our definition of
magnification, we find
y2/y1 = s2/s1 = f/(s1-f).
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In addition to the assumption of an
ideally thin lens, we also work in the
paraxial approximation. That is, angles
are small and we can substitute θ in
place of sin θ.
Optical Invariant
This puts a fundamental limitation on the
geometry of an optics system. If an optical
system of a given size is to produce a
particular magnification, then there is
only one lens position that will satisfy
that requirement. On the other hand, a
big advantage is that one does not need
to make a direct measurement of the
object and image sizes to know the
magnification; it is determined by the
geometry of the imaging system itself.
Three rays are shown in Figure 1. Any two
of these three rays fully determine the
size and position of the image. One ray
emanates from the object parallel to the
optical axis of the lens. The lens refracts
this beam through the optical axis at a
distance f on the far side of the lens. A
second ray passes through the optical
axis at a distance f in front of the lens.
This ray is then refracted into a path
parallel to the optical axis on the far side
of the lens. The third ray passes through
the center of the lens. Since the surfaces
of the lens are normal to the optical axis
and the lens is very thin, the deflection of
this ray is negligible as it passes through
the lens.
Figure 3
Figure 2
By ideal thin lens, we mean a lens whose
thickness is sufficiently small that it does
not contribute to its focal length. In this
case, the change in the path of a beam
going through the lens can be considered
to be instantaneous at the center of the
lens, as shown in the figure. In the
applications described here, we will
assume that we are working with ideally
thin lenses. This should be sufficient for
an introductory discussion. Consideration
of aberrations and thick-lens effects will
not be included here.
The quantity M is the magnification of
the object by the lens. The magnification
is the ratio of the image size to the object
size, and it is also the ratio of the image
distance to the object distance.
Figure 1
This additional condition is often the
focal length of the lens, f, or the size of
the object to image distance, in which
case the sum of s1 + s2 is given by the
size constraint of the system. In either
case, all three variables are then fully
reciprocal relation. For example, to
improve the collimation by a factor of
two, you need to increase the beam
diameter by a factor of two.
Figure 4
Figure 5
This arbitrary ray goes through the lens at
a distance x from the optical axis. If we
again apply some basic geometry, we
have, using our definition of the
θ1 = x/s1 and θ2 = x/s2 = (x/s1)(y1/y2).
Rearranging, we arrive at
y2θ2 = y1θ1.
This is a fundamental law of optics. In
any optical system comprising only
lenses, the product of the image size and
ray angle is a constant, or invariant, of
the system. This is known as the optical
invariant. The result is valid for any
number of lenses, as could be verified by
tracing the ray through a series of lenses.
In some optics textbooks, this is also
called the Lagrange Invariant or the
Smith-Helmholz Invariant.
This is valid in the paraxial
approximation in which we have been
working. Also, this development assumes
perfect, aberration-free lenses. The
addition of aberrations to our
consideration would mean the
replacement of the equal sign by a
greater-than-or-equal sign in the
statement of the invariant. That is,
aberrations could increase the product
but nothing can make it decrease.
Application 1: Focusing a
Collimated Laser Beam
As a first example, we look at a common
application, the focusing of a laser beam
to a small spot. The situation is shown in
Figure 5. Here we have a laser beam, with
radius y1 and divergence θ1 that is
focused by a lens of focal length f. From
the figure, we have θ2 = y1/f. The optical
invariant then tells us that we must have
y2 = θ1f, because the product of radius
and divergence angle must be constant.
As a numerical example, let’s look at the
case of the output from a Newport
R-31005 HeNe laser focused to a spot
using a Newport KPX043 plano-convex
lens. This laser has a beam diameter of
0.63 mm and a divergence of 1.3 mrad.
Note that these are beam diameter and
full divergence, so in the notation of our
figure, y1 = 0.315 mm and θ1 = 0.65 mrad.
The KPX043 lens has a focal length of
25.4 mm. Thus, at the focused spot, we
have a radius θ1f = 16.5 µm. So, the
diameter of the spot will be 33 µm.
This is a fundamental limitation on the
minimum size of the focused spot in this
application. We have already assumed a
perfect, aberration-free lens. No
improvement of the lens can yield any
improvement in the spot size. The only
way to make the spot size smaller is to
use a lens of shorter focal length or
expand the beam. If this is not possible
because of a limitation in the geometry of
the optical system, then this spot size is
the smallest that could be achieved. In
addition, diffraction may limit the spot to
an even larger size (see Gaussian Beam
Optics section beginning on page 484),
but we are ignoring wave optics and only
considering ray optics here.
Application 2: Collimating
Light from a Point Source
Another common application is the
collimation of light from a very small
source, as shown in Figure 6. The problem
is often stated in terms of collimating the
output from a “point source.”
Unfortunately, nothing is ever a true point
source and the size of the source must be
included in any calculation. In figure 6,
the point source has a radius of y1 and
has a maximum ray of angle θ1. If we
collimate the output from this source
using a lens with focal length f, then the
result will be a beam with a radius y2 =
θ1f and divergence angle θ2 = y1/f. Note
that, no matter what lens is used, the
beam radius and beam divergence have a
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Figure 6
Since a common application is the
collimation of the output from an optical
fiber, let’s use that for our numerical
example. The Newport F-MBB fiber has a
core diameter of 200 µm and a numerical
aperture (NA) of 0.37. The radius y1 of our
source is then 100 µm. NA is defined in
terms of the half-angle accepted by the
fiber, so θ1 = 0.37. If we again use the
KPX043, 25.4 mm focal length lens to
collimate the output, we will have a beam
with a radius of 9.4 mm and a half-angle
divergence of 4 mrad. We are locked into
a particular relation between the size and
divergence of the beam. If we want a
smaller beam, we must settle for a larger
divergence. If we want the beam to
remain collimated over a large distance,
then we must accept a larger beam
diameter in order to achieve this.
Application 3: Expanding a
Laser Beam
It is often desirable to expand a laser
beam. At least two lenses are necessary
to accomplish this. In Figure 7, a laser
beam of radius y1 and divergence θ1 is
expanded by a negative lens with focal
length –f1. From Applications 1.1 and 1.2
we know θ2 = y1/|–f1|, and the optical
invariant tells us that the radius of the
virtual image formed by this lens is y2 =
θ1|–f1|. This image is at the focal point of
the lens, s2 = –f1, because a wellcollimated laser yields s1 ~ ∞, so from
the Gaussian lens equation s2 = f. Adding
a second lens with a positive focal length
f2 and separating the two lenses by the
sum of the two focal lengths –f1 +f2,
results in a beam with a radius y3 = θ2f2
and divergence angle θ3 = y2/f2.
Figure 7
y3/y1 = θ2f2/θ2|–f1| = f2/| –f1|,
= 2y1f2/|–f1|
= 2(0.315 mm)(250 mm)/|–25 mm|
or the ratio of the focal lengths of the
lenses. The expanded beam diameter
= 6.3 mm.
located at a distance s1 from a lens of
focal length f. The figure shows a ray
incident upon the lens at a radius of R.
We can take this radius R to be the
maximal allowed ray, or clear aperture, of
the lens.
The expanded beam diameter
The expansion ratio
The divergence angle
2y3 = 2θ2f2 = 2y1f2/|–f1|.
= θ1|–f1|/f2
= (0.65 mrad)|–25 mm|/250 mm
θ3 = y2/f2 = θ1|–f1|/f2
= 0.065 mrad.
Application 4: Focusing an
Extended Source to a
Small Spot
If s1 is large, then s2 will be close to f,
from our Gaussian lens equation, so for
the purposes of approximation we can
take θ2 ~ R/f. Then from the optical
invariant, we have
y2 = y1θ1/θ2 = y1(R/s1)(f/R) or
y2 = 2y1(R/s1)f/#.
where f/2R = f/D is the f-number, f/#, of
the lens. In order to make the image size
smaller, we could make f/# smaller, but we
are limited to f/# = 1 or so. That leaves us
with the choice of decreasing R (smaller
lens or aperture stop in front of the lens)
or increasing s1. However, if we do either
of those, it will restrict the light gathered
by the lens. If we either decrease R by a
factor of two or increase s1 by a factor of
two, it would decrease the total light
focused at s2 by a factor of four due to the
restriction of the solid angle subtended
by the lens.
This application is one that will be
approached as an imaging problem as
opposed to the focusing and collimation
problems of the previous applications. An
example might be a situation where a
fluorescing sample must be imaged with a
CCD camera. The geometry of the
application is shown in Figure 8. An
extended source with a radius of y1 is
Figure 8
As an example, consider a Newport
R-31005 HeNe laser with beam diameter
0.63 mm and a divergence of 1.3 mrad.
Note that these are beam diameter and
full divergence, so in the notation of our
figure, y1 = 0.315 mm and θ1 = 0.65 mrad.
To expand this beam ten times while
reducing the divergence by a factor of
ten, we could select a plano-concave lens
KPC043 with f1 = –25 mm and a planoconvex lens KPX109 with f2 = 250 mm.
Since real lenses differ in some degree
from thin lenses, the spacing between the
pair of lenses is actually the sum of the
back focal lengths BFL1 + BFL2 = –26.64
mm + 247.61 mm = 220.97 mm.
For minimal aberrations, it is best to use
a plano-concave lens for the negative lens
and a plano-convex lens for the positive
lens with the plano surfaces facing each
other. To further reduce aberrations, only
the central portion of the lens should be
illuminated, so choosing oversized lenses
is often a good idea. This style of beam
expander is called Galilean. Two positive
lenses can also be used in a Keplerian
beam expander design, but this
configuration is longer than the
Galilean design.
is reduced from the original divergence by
a factor that is equal to the ratio of the
focal lengths |–f1|/f2. So, to expand a laser
beam by a factor of five we would select
two lenses whose focal lengths differ by a
factor of five, and the divergence angle of
the expanded beam would be 1/5th the
original divergence angle.
The divergence angle of the resulting
expanded beam
Fiber Optic Coupling
Application 5: Coupling Laser
Light into a Multimode Fiber
The objective lens has an effective focal
length of 9 mm. In this case, the focused
beam will have a diameter of 9 µm and a
maximal ray of angle 0.05, so both the
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When we look at coupling light from a
well-collimated laser beam into a
multimode optical fiber, we return to the
situation that was illustrated in Figure 5.
The radius of the fiber core will be our y2.
We will have to make sure that the lens
focuses to a spot size less than this
parameter. An even more important
restriction is that the angle from the lens
to the fiber θ2 must be less than the NA
of the optical fiber.
Let’s consider coupling the light from a
Newport R-30990 HeNe laser into an
F-MSD fiber. The laser has a beam
diameter of 0.81 mm and divergence
.0 mrad. The fiber has a core diameter of
50 µm and an NA of 0.20. Let’s look at the
coupling from the beam into the fiber
when a Newport M-20X objective lens is
used in an F-915 or F-915T fiber coupler.
The problem of coupling light into an
optical fiber is really two separate
problems. In one case, we have the
problem of coupling into multimode
fibers, where the ray optics of the
previous section can be used. In the
other case, coupling into single-mode
fibers, we have a fundamentally different
problem. In this case, one must consider
the problem of matching the mode of the
incident laser light into the mode of the
fiber. This cannot be done using the ray
optics approach, but must be done using
the concepts of Gaussian beam optics
(see page 484).
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