Note: Descriptions are shown in the official language in which they were submitted.
~ ~ ~3~
"~nterferometer"
The invention relates to an interferometer
- comprising a radiation source, which produces a radiation
beam, a beam splitter for deriving a first and a second
subbeam from the radiation beam, which two subbeams,
af-ter the first subbeam has been incident on the surface
- of an object to be examined are made to coincide in the
plane of a radiation-sensitive detection s~s-tem.
Such an interferometer, used as a displacement
measuring device is known, for example -from "Philips
l0 Technical Review", 30, No. 6/7, pages 160-l65. In the
path of each of -the subbeams there is arranged a reflector.
One of these reflectors is arranged stationarily9 whilst
` the second one ls fi~edly connected to the object whose
displacement is to be measured. Upon reflection of the
two subbeams they are recombined by the beam splitter,
said beams, which have covered substantially the same
pathlengths, interfering with each other. The resulting
intensity depends on the relative phase of the subbeams
and will therefore vary periodically if the optical path-
length of the second subbeam increases or decreases
continuously owing to a displacement of the object. One
period of the interference pattern corresponds to a path-
length variation of the magnitude of a half wavelength
of the light being used. By means of a radiation-sensitive
detector a periodic electric signal can be obtained,
whose number of periods, which is a measure of the dis-
; placement of the object, can be counted.
The ar-ticle in "Philips Technical Review" 30,
No. c>/7, pages 160-165 describes a special interferonleter
with excellent properties, such as an accuracy down to
a fraction Or 1/um, simple digital clisplay and discrimi-
nation between forward and reverse displacements. 1-Iot~ever,
.~
~ ~ ~3V~
said properties can only be ob-tained if use is macle of
a special frequency-stabilised laser source, which
produces a laser beam with two oppositely circularly
polarised components of equal intensity bu-t different
frequencles.
It is -the object of the present invention to
provide an interferometer which is of substantially
simpler construction, but which has substantially the
same good properties. The interferometer according to
the invention is characterized in that the radiation-
sensitive detection system is constituted by a multiple
photo-cell, comprising a linear array of pho-todiodes,
which are sequentially connected to an electronic circuit
for processing the signal produced in -the photodiodes
by means of an electronic switch, so that the detection
system functions as a moving grating-like de-tector~ the
grating period of the multiple photo-cell corresponding
to the period of the interference line pattern of the two
superimposed subbeams.
The invention uti:Lizes the fact that in an
interferometer a sinusoidal :interference pat-tern can be
produced and that the intens:ity variation in the inter-
ference pattern, which variation is caused by a variation
in the pathlength of a subbeam, may be regarded as a dis-
placement of said intarference pattern. The displacement
of such a pattern of light and dark lines, which in its
turn may be regarded as a grating, can be determined with
the aid o~ a reference grating in the rorm of a multiple
photo-cell.
By means of the electronic switch it is
achieved that a reference grating apparently "travels"
over the surface of the multiple photo-cell. The radiat-
ion-sensitive elements of the multiple photo-cell, the
electronic switc:h and the electronic processing circuit5 may be integra-ted on one chip of a semiconductor [naterial.
The said multiple photo-cell is described in
United States Patent Specification No. 3,~73,11~. In
3 0 9 l~
accordance with said Paten-t Specification the displace-
rnent of an object can be measured by projecting a first
grating, which is connected to the object, on a reference
grating constituted by the multiple photo-cell. ~he
accuracy of this displacement measurement is determined
by the grating period of the first grating. This grating
period is for example 625/um. By suitable signal
processing and interpolation within the signal period a
displacement measurement with an accuracy down to in
principle O.5/um is possible.
In the interferometer in accordance with the
invention the periodicity in the electric signal depends
on a periodicity of half the wavelength of the radia-tion
used instead of on a periodic structure having a period
of 635/um. Thus, using the same multiple photo-cell,
a substantially higher accuracy can be obtained than
attainable with the displacement meter in accordance with
United States Patent Specification No. 31973~119.
The invention will now be described in more
detail with reference to the drawing. In the drawing:
Figure 1 shows a first embodiment of an inter-
feronieter in accordance with -the invention,
Figure 2 shows the interference pa-ttern ~`ormed
; in said interferometer and a grating,
Figures 3 and ~ sho-~ t-~o other embodiments o~
the interferometer,
`~ ~igure 5 is a block diagram of the circuit
arrangement employed in said in-terferometers,
~igure 6 shows a device for determining -the
linearity of the movement of an object providecl with an
interferometer in accordance with the invent:ion, and
Figure 7 shows an optical writing apparatus
provided with an interferometer in accordance ~ith -the
invention.
In the interferometer shown in Figure 1 the
radia-tion source 1 emits a beam Z. Depen~ling on the use
of the interferometer, said beam has a srnaller or greater
.
~ ~ 6 ~
coherence length. For measuring displacernents over longer
distances the beam should have a greater coherence length.
The source 1 is then a laser, ~or example a helium-neon
laser. ~y means o~ the beam splitter 3 a part of beam 2
is reflected to a stationary re~erence mirror 4 as a
subbeam a. The subbeam b which is transmitted by beam
splitter is incident on a second mirror 5, which is
arranged on or secured to an objec-t 6, or is a part o~
said object whose displacement is to be determined.
Of the beam a which is reflected by the mirror 4
a part (a~) is transmitted by the beam splitter 3, whilst
a part (b') of the beam b, which is re~lected by the
mirror 5, is re~lected by the beam splitter. The beams a'
and b~ then ~orm an inter~erence pattern I. The intensi-ty
of the intarference pattern depends on the relative phase
of the subbeams a~ and b'. Consequently, -this intensity
~; will vary periodically if the op-tical pathlength of the
subbeam b increases or decreases owing to a displacement
of the object 6.
In known interferometers the intensity of the
~-~ inter~erence pattern is measured locally by means of a
radiation-sensitive detector, which is for example
arranged on the optical axis of the system. I~ the
object 6 moves, a periodic or pulse-shaped signal is
produced on the output o~ the detector. By counting a
number of pulses the magnitude of the displacement can
be determined.
In the interferometer in accordance with the
invention steps are taken to ensure that the subbeams
a~ and b' make a small an~le with each other. This is
possible, as is shown in Figure 1, by arranging -the mirror
` 5 at an angle relative to the beam b which differs
slightly -from 9O. This results in an interference pattern
I with a spatial intensity distribution, which pattern
is schematically represented in Figure 1. The interfererlce
lines are perpendicular to the plane of drawing in Figure 1.
Figure ~ again shows the interference pattern, but now
in plan view.
A
If the beams a ancl b traverse equal optical
pathlengths the intensity in a point c1, for example a
point on the optical axis o~ the system, is maximum,
and so is the intensity in points 2 and C3~ whilst
S the intensity in points d1 and d2 is minimum. The inter-
ference pattern is then as represented by the curve 11.
If the object 6 is displaced, the intensity in points C1,
C2 and C3 will clecrease and that in points d1 and d2 will
increase. If, for example, the object has moved over a
1n distance equal to a quarter wavelength of the beam ~, the
intensity in points C1, c2 and C3 will be minimum and
that in points d1 and d2 maximum. The interference
pattern will then be as represented by the curve 12.
In the interferometer in acco-rdance with the
lS invention use is made of the fact that the change of the
intensity distribution may be regarded as "travelling"
of the interference pattern relative to point C1, c2,
C3, d1~ d2- Furthermore, the interference pattern itself
may be regarded as a grating with gradual transitions
- 20 from the light to the clark grating stripes. In the inter-
ferometer now proposed the displacement of the interfe-
rence pattern, and -thus the displacement of the objact,
is determined with the aid of a reference grating ~,
which is arranged in the plane of the inter~erence pattern.
Use can -then be made of techniques employed in known
measurement systems, in which two physical gratings move
relative to each other. Since the period o~ a physical
grating, which period is for example a few hundreds of /um,
is now replaced by a period equal to hal~ a wavelengtll o~
30 the radia-tion used, ~or example 0.3164/um for a helium-
neon laser, substantially smaller displacements can be
measured than with a gra-ting measuremen-t system.
As is shown in l~igure 1, a moving reference
grating can be formed with the aid of a mul-tiple photo-cell
comprising a linear array of substantially identical photo-
diodes, which are sequentially connec-tecl to an electronic
processing circui-t by means o~ an electronic switch. This
:
1 ~ 63~4
interferometer uses neither a reference grating nor any
mo~ing parts ~or imparting a uni~orm motion to said
grating, so that this interferometer i5 0~ a simple con-
struction and is highly vibration-proof.
Figure 3 shows a second embodiment o~ an inter-
ferometer in accordance with the invention. ~here the
embodiment shown- in ~igure 1 employs a semitransparent
mirror 3 as beam splitter, the embodiment shown in Fig. 3
employs a prism o~ special shape as beam splitter. Said
lO prism may be thought o~ as comprising a normal semi-
transparent prism, represented by dashed lines in Fig. 3,
with a semi-reflecting sur~ace 15, but on which now a
second ~ully re~lecting sur~ace 16 and a third semi-
reflection sur~ace 17 are ground. By a suitable choice
l5 of the angle between the sur~aces l6 and 17 it can be
achieved that the re~lected subbeams a' and bt make a
small angle with each other.
In this embodiment the reflecting elements are
. .
so-called retro-reflectors 20 and 21. Such elemen-ts may
20 be cons-tituted by a prism having three re~lecting sur-
faces which are perpendicular to each other, a so-called
"corner-cube" prism. ~ beam which is consecuti~ely
re~lected by said three surfaces has the same direction
as the beam which enters the prism, regardless o~ the
; 25 angular position of the prisrn. Consequently, an ali~nment
m of said angular position is not necessary. ~ similar
e~ect can be obtained with a so-called "cats' eye"
mirror system, comprising a lens and a mirror arranged
in the ~ocal plane o~ said lens.
Figure 4 shows an embodiment of -the inter~ero-
meter in which a l~ollaston prisT~ 26 is empLoved in order
to introduce a small angle between -the subbeams a' and
b' which inter~ere with each other. In this embodiMen-t
the beam splitter is a polarisation-sensitiYe splitting
prism 22 with a polarisation-sensitive surface 23. The
beam 2 now comprises two components which are polarised
perpendicularly to each other, o~ which one component a
- ~ 3 G30~
is reflected by the surface 23, whilst the o-ther com-
ponent is transmitted by the surface 23 There may be pro-
vided a polariser ~8 in order to adapt the direction
of polarisation of the beam produced by the radia-tion
source 1. The subbeam a passes through a ~/4 pla-te 24,
is reflected by -the element 20 and -then traverses the
~/4 plate 24 again. The direction of polarisa-tion is
then rotation through 90 in to-tal, so that the subbeam
-- a~ is transmitted by the surface 23. The component of
` lO the beam 2 which is transmitted by the surface 23 tra-
verses the ~/4 plate 25 two times and is then reflected
by the surface 23. The coincident subbeams a~ and bl, which
have directions of polarisation which are perpendicular to
each other~ pass through a l~ollaston prism, which deflects
lS the beam depending on their directions of polarisation.
The subbeams emerging from the prism then make a small
angle with each other. After the subbeams have passed
through an analyser, they can form an interference pattern
with a spatial intensity-distribution at the location of
~` 20 the detector, the multiple photo-cell -~9.
In comparison with a semitransparent mirror,
~ a polarisation-sensitive beam splitter has the advantage
; that in principle no racliation is lost in the process
of beam splitting and recomb:ining the subbeams. In the
25 interferometer shown in Figure 3 the surfaces 15 and 17
may alterna-tively be polarisation-sensitive surfaces. In
that case ~/4 plates 18 and 19 should be arranged in the
paths of the subbeams a and b and an analyser 27 bet~een
the prism 14 and the detector 29.
l~hen a polarisation-sensitive beam splitter is
employed steps must be taken to preclude additional
polarisation changes during reflections on the surfaces
of the reflecting prisms. For this purpose layers of
silver may be provided on the reflecting surfaces of
said prisms.
As stated previously, the interferometer in
accordance with the invention employs a multiple photo-
" ~ 1 630~1
cell as detection system.
Figure 5 shows a front view of the mul-tiple
photo-cell 29 and a block diagram of the circuit. The
photo-cell comprises a comparatively great number of
photo-sensitive elements, such as photodiodes 30, ~hich
are divided into a comparatively small number of groups.
~ach group consequently comprises a comparatively great
number of photodiodes. Each photodiode of a group
corresponds to a period, one light and one dark line,
of the interference pattern I. As a result of this, a
number of periods of the interference pa-ttern is scanned
; equal to the number of photodiodes in a group. The numbar
of photodiodes per period of the interference pattern
should be as great as possible for an optimum electrical
reproduction of the optical signal. On the other hand,
an as large as possible part of the interference pattern
should be scanned.
In one version of the multiple photo-cell -the
number of photodiodes was 200 and the length of each
photodiode 1.8 mm. The width of each photodiode was
10/um, and -the spacing of the photodiodes also 10/um. The
number of photodiodes per period of the interfe~ence
pattern was 10, so that -the field of view covered 20
periods of the interference pattern. Correspo-nding photo-
diodes o~ each set of 10 consecutive photodlodes ~ere
interconnected, so that there were 10 groups of 20 photo-
diodes each.
A stationary grating with a black-white ratio
of 1 : 1 in the area of the multiple photo-cell 29 is
simulated by activating five consecutive groups of photo-
diodes (five groups of 20 photodiodes in the present em-
bodiment). A travelling grating is obtained l~hen the
set of five groups each time advances by one group.
In -the processing circuit, ~hich is block-
schematically represented in ~igure 5, the clock pu:Lses
32, generated in the clock-pulse generator 31, are
supplied -to a divider 33 and a clivider 34. The cli~icler 33
~ IT ~ 3 ~ ~ ~
supplies pulses 35, which drive a ring coun-ter 36. The
multiple photo-cell 29 ls activated by the ring counter 36
and supplies the measuring signal 37. The divider 3L~
supplies pulses 38 (generally of another repetition fre-
quency than -the control pulses 35 from the divider 34),
. which constitute the reference signal. In the buffer
counter 39 the measuring signal 37 and -the reference
pulses 38 are compared with each o-ther. The output pulses
; of -the buffer 39 are .for example applied to an indicator.
The ring counter 36 activates the consecutive
groups of photodiodes of the multiple photo-cell 29,
so that in effect a grating is obtained which travels
~1 over the sur~ace of the pholto-cell ~ with a constant
:. velocity. The period of the grating is equal -to the
period of the interference pa-ttern I. If the interference
pattern is stationary relative to the photo-cell 29, the
measuring signal has a constant ~requency. If the inter-
ference pattern moves in the same direc-tion as the
apparent grating activated by the ring counter 36, the
frequency of the measuring signal 37 will decrease, ~ihilst
if it travels in -the opposite direction the frequency of`
the measuring signal 37 will increase. Thus, -the direction
and magnitude of the displacement o~ the interference
pattern, and thus the displacement of the object 6, can be
determined.
Within a range of one period of -the interference
pattern I the position of the multiple photo-cell 29 re-
lative to the interference pattern can be determined in an
absolute manner by measuring the phase c1ifference bet~een
the measuring signal 37 and the reset signa:l of the ring
counter 36. The ring counter 36 is to be rese-t upon every
start of a measurement in order to guarantee -that the
counter 39 starts in a specific initial pOSitiOIl.
~Iowever, the circuit arrangement becomes simpler
and more reliable if the ring counter 36 is reset upon
each period. The reset signal is obtained by dividing
the pulses 35 in -the divider 4O. The frequency of the
`~ 3 ~309~
reset pulses is selected to be equal to the nominal
frequency of the measuring signal 36.
lrhen the multiple photo-cell and processing
circuit shown in Figure 5 are employed in a gra-ting
measuring system in which the measuring grating has a
period of 635/um, it is in principle possible to detect
displacements of the measuring grating down to O.5/um.
By employing the multiple photo-cell and the circuit in
accordance with the invention in an interferometer using
,lO a helium-neon laser beam with a wavelength of 0.6328/um
; it is in principle possible to detect the displacements
down to ~ x O.5/um = 0.25 nm.
The displacement measuring device described in
the foregoing can be used in all cases where small
displacements have to be measured accurately, such as
in machine tools, for example lathes, for measuring slide
and shaft notions. An example of this is the numerically
controlled lathe mentioned in "Optics Le-tters", ~ol. 14,
no. 2, pages 7O-72, by means of which bi-aspherical
objective lenses, that is lenses having two aspherical
surfaces, can be manufacturecl.
The interferometer in accordance with the in-
v~ntion may also be used for measuring the linearity of
the movement of an object. For this use, as is shown in
Figure 6, the radiation source l, the beam splitter 3,
the reference mir~ror 4 and the radiation~sensitivity
detection system ~ are all accommodated in one housing
41. This housing~ whose dimensions can be small, is moved
wi-th the same velocity as the object 6 in the direction
of the arrow 46. The movement of the object in this
direction will not cause any change in the interference
pattern. However, if the object moves obliquely relative
to the arrow 46, the reflected subbeam b' will move rela-
tive to the subbeam a~, so that the distribution within
the interference pattern changes. The in-terference pattern
-then begins to "travel" relative to the radiation-sensi-
-tive detection system. This movement can be measured
3~94
1 1
by counting the number of periods in the output signal
of the detection system 29.
For a simultaneous movement of the housing 41
and the object 6 this housing may be mounted on the slide
43 by means of which the object is moved. I-t is alter-
natively possible, as is shown in Figure 6, to provide
the housing ~1 with separate drive mens 42 which via
the connection 45 are energized by the motor 44 which
drives the object slide.
In recent years there have been significant
developments in the field of optically readable record
carriers. On these record carriers a large amount of
information, such as video- and/or audio information or
digital information is stored, the information details
having dimensions of the order of 1/um or smaller. In
an apparatus for inscribing said record carriers an
interferometer in accordance with the invention may be
used for controlling the movement of the ~ri-te head
transverse of the tracks, which movement may be very slo~
especially when audio information is recorded.
Figure 7 schematically represents such an
apparatus. The record carrier 50 to be inscribed is
placed on a table 51, which can be rotated by means of
a motor 52. The write head 53 contains a laser L~4, ~hoje
Z5 beam 61 is directed at the record carrier via the mirrors
55, 56 and 57, said beam being focused to a small write
spot by means of an objective 58. The information to be
recorded is applied to the terminals 60 of a modulator 59,
which modulates the intensity of the beam in accordance
with the information to be recorded. On the wall of the
write head 53 a reflecting prism 6~ is arranged, which
prism is accommodated in the measuring arm of an intcrfero-
meter. This interferometer further comprises a splitting
prism 62 and a second reflecting prlsrn 63. The beains a' and
b' reflected by -the reflecting prisms have the same
direc-tions as and are slightly shifted relative to the
beams a and b incident on the prisrns. ~ ~edge 65 sliglltly
~ ~ 6309~
12
deflects the beam b~, so that the beams which emerge from
the prism 6~ make a small angle with each o-ther.
Besides in displacemen-t measuring devices,
and obviously also in velocity meters, in which the
number of periods per unit of time of the measured signal
is determined, the invention may be used in all cases
where interferome-ters can be used. ~xamples of this
are surface roughness meters, de-vices for measuring
extremely small magnetostricti~e or electros-trictive
effects, etc.
