Note: Descriptions are shown in the official language in which they were submitted.
1 32323~
.- 1
PHASE-CONTROLLED THIN FILM_MULTILAYERS
FOR MICHELSON INTERFEROMETERS
This invention relates in general to thin film
multilayers, and more particularly to an achromatically
phase-controlled mirror comprising a specific
arrangement of optical thin film multilayers.
Space based studies of upper atmospheric conditions
are currently being undertaken with the use of optical
Doppler imaging refer to Michelson interferometers123.
Speci~ically, two wide angle Michelson Doppler imaging
interferometers will be flown in the near future on the
Space Shuttle and on the Upper At~ospheric Research
Satellite to study wind3, temperatures, and volume
emission rates of the upper atmosphere. By way of
background, in a generic prior art Michelson
interferometer, the ends of the two arms are usually
coated with a reflective metal such as aluminum or
silver. Since each mirror has the same coating, the
phase change experienced by the i.ncident light on
~0 reflection i5 also the same from each arm. The
difference in the lengths o~ the arms delay the phase of
the electromagnetic ~ave in one arm with respect to the
other. On recombination, this phase differ2nca is
manifested in the ~orm of an inte~ference pattern. As
the path difference varie~, the int2rference pattern
varies.
For one measurement, four consecutive images are
taken with the Michelson mirror moved by an eighth of a
wavelength to create a quarter wavelength change in the
optical path di~ference between each image. From the
four signal value~, the fringe phase is determined on a
plxel by pixel ba is.
However, the derived phase i5 susceptible to
measurement errors arising from atmospheric source
changes in po~ition, intensity, or shape, for example,
if the aurora is being observed. ~150, errors can be
.
.
~ ~.
` ' "`
, :, ' :- ' "~ `
:
1 323234
introduced if the space-craft carrying the
interferometer moves during the measurement,
Researchers have concluded that atmospheric
measurement~ would be greatly improved i~ it were
possible to produce quarter wavelengths of optical path
dif~erences simultan~ously, achromatically~ and without
mirror motion. Many types of solutions to this problem
have been investigated. The most obviou~ solution
consists of a single fixed mirror at the end of one arm
of the Michelson interferometer and four separate
mirrors at the end of the other arm, each of which is
displaced an eighth of wavelength from the adjacent one.
However, this worXs for only one wavelength. In order
to make the steps achromatic, four scanning mechanisms
would be required. For space-borne instruments, the
addQd cost and complexity of hardware and software for
~ontrolling four moving mirrors instead o~ one makes
this solution practical.
The phase change on reflection ~R at a surface can
be any value between O and 2~ radians (modulus 2~). It
is dependant on the wavelength, polarization and angle
of incidence, and on the nature of the surface, the
incident and emergent media.
Within a M~chslson interferometer, re~lection from,
and transmission through surfaces alter the phase of the
incident light . The phase changes on reflection 6 ~ are
disper~ivs. Th~s can crea~e problems in
interferometers. For example, beam splitters can create
phase error~ between ths two arms and cause fringe
shifts if the total phase shi~t on transmission and
reflection of-each beam from the two arm~ is not the
same. For this reason, efforts are frequently made to
minimize the dispersisn of the phasa change.
It i~ an object of an aspect o the present
invention to use 6~ as a tool, to provide a solution to
the pro~lem of taking all four 90 phase stepped images
: .. . ~ , -:
.-, -:. . . ~ : - ::
.~:: , :
1 323234
simultaneously, at any wavelength and without mirror
motions4. According to the present invention, the
dispersion of phase change is exploited to replace the
path differences traditionally produced by mirror
motions in a Michelson interferometer, which contrasts
with previous thinking in this regard.
In general, according to the present invention, an
achromatically phase-controlled mirror is provided
comprising a plurality of optical thin film multilayers.
The present invention has specific application to the
field of Michelson interferometers. In particular,
four high-reflactance mirrors ca~ be constructed in
accordance with the principles of the present invention
whose values of phas~ change on reflection differ by
90, that is, the phase equivalent of a quarter wave
optical path differsnce, for the wavelength between 0.5
and 0.9 micrometers.
In addition to its application to the field of
Michelson interferomQters, numerous other applications
of ths achromatically phase controlled mirror of the
present invention are contemplated.
one advantaga of tha achromatically phase-
controllQd mirror of the present invention is that
predetermined eighth phase change~ on reflection are
produced without moving part~, in contrast with the
prior art interferomet~r~ which require mechanical
motion to accomplish the same result. The elimination
of moving mechanisms is advantageous ~or two reasons.
Firstly, with any complex electromechanical system there
30 i5 always thQ possibility of damags or failure, and if
severe enough it could render an instrument incapable of
taking measurements. This is particularly important for
~pace-borne instruments such as Michel~on
interferometers which are located on satellites launched
from the space shuttle, which means that there i5 no
astronaut available to fix a problem with the mechanism
'
.
. : :
1 323234
if it ~ails. Secondly, there is a limiting accuracy
with which the moving mechanism can reproduce
predetPrmined wavelength st2ps. This reduces the
reproducability of measurements. Thirdly, the cost o~
these electric transducers for effecting mechanical
movement of the mirrors is very high.
The s~cond main advantage of the present in~ention
is that a plurality of phase-shifted inter~erograms may
be produced simultaneously, whereas prior art
interfexometers produce the images consecutively. The
latter is a disadvantage since the derived images o~
emission line intensity, temperature and wind ar~
created from multiple (i.e. ~our) phase images. There
is necessarily a time lag between the beginning of the
first and the end of the last phase images. The
problems with the time lag are that the space platform
(e.g. space shuttle, satellite. etc.) on which the
interferometer is ~lown moves during that time lag, and
since the interferometers are not tracking instruments,
the atmospheric source image moves across the field of
view. This effect cannot be igna,red and software must
be written to account for it before combining the four
phase image~. Additional change~ in the observed
radiance can be caused in the evelnt that the spacecraPt
rolls. Secnndly, the atmospheric: ~ource might change
it~ position, intensity and ~hape throughout the time
period of the four image observations. If all four
images are taken simultaneouRly, as in the present
invention, this error is substantially reduced.
The third main advantase of the pres~nt invention
is that the ~imultaneity o~ phasa images is accomplished
over a range of wavelength~ in contrast with prior art
polarizing inter~erometers which operate at only one
wavelength or otherwise require a mechanical movement of
mirrors.
:.
. ~
1 323234
A preferred embodiment of the present invention
will be described in greater detail below with reference
to the following drawings in which:
Figure l illustrates the construction parameters of
an optical thin film multilayer systemt
Figure 2 is a schematic repxesentation of
achromatically phase-controlled thin ~ilm multilayer
mirror in accordance with the present invention;
Figure 3 i5 graph illustrating ideal curves of
phasie chanqe on reflection ~or multilayers A, B, C and D
in Figure 2 over a predetermined range of wavelengths;
Figure 4 is a schematic illustration of a basic
optical arrangement for a ~ichelson interferomater
incorporating the novel mirrors of the present
invention; and
Figure 5 is a graph illustrating superimposed,
calculated curves o~ phase changla on xeflection, for
multilayers A, 8, C and D, as deaigned6. The full lines
are desired values, and the points are calculated phase
changes on reflection o~ the actual multilayers.
A ~ew generalizations can be made about the phase
change on reflection e~. Fir~t, its value depends on
the properties o~ the incident ~lactromiagnetic wavQ (the
wavelength, angle of incidence on the multilayer, and
~tate of polarization) and on the construction
parameters of the thin film multilayer.
By way of background, an optical thin film is a
layer of an element or inorganic compound deposited on a
substrate, whose thicXness i5 comparable to the
wavelength of intere~t. It is characterized,by its
thicknessi t, index of re~raction n and absorption
coe~ficient k as will bs discussed in greater detail
below with reference to Figure 1. Thus, a thin film
multilayer is a stack of thin films deposited on a
substrate.
`
1 323234
. 6
A second generalization is that since the general
expressiGn of e R is based on electromagnetic theory and
uses the inverse tangent ~unction such that ~ ha~
values between 0 and 360~, modulus 360, the
uncertainty of the modulus is acceptable for the
multilayers of the application descxibed herein. For
atmospheric studies it is not e~sential that the exact
modulus bs known, only that the absolute value of the ~R
differences between adjacent mirrors be reasonably
close.
The reflectance R and phase change on reflection 6R
will now be defined. Consider the multilayer system
depicted in Figure 1, showing an incident material 4, a
plurality of thin film multilayers 3 and a substrate 5.
The construction parameters available for design are the
refractivle indices nm, ns and extinction coe~ficient~
km~ kR of the incident medi~m 4 and substrate 5, and
refractive indices nj, extinction coefficient k~, and
metric thicknesses t;, tl ~ ~ ~ Q) of the Q homogeneous
layers 3 of the system.
Assuming normal incidence and non-absorbing medium
layers of the system in combination with a metallic
absorbing substrate, the charact,eristic matrix of the
jth layer at a wavelQngth ~ i3 ~iven by
r ~, (i~n~) ~il . '
l(in~) ~WI co~l ¦
~hc~ )njt;~di~. Thopr~uctm~eri~
Mof~m~yor~gNonby
- mll im~;
~ .
Iho~np~tud~r~n~tion~fficio~trof~am~t~yer
~von~ ~ofth~a~vo~-~olome~b~
(n",mll ~ n,"h,m1~ - n~ i(n",n,m~ ,m~
lt ~ n~) ~ i(n,~,ml~ + m
R and ~ are given by R = ¦r¦2 and 6R = arg (r),
and hence can be r~adily calculated. Note that even
. . .
:" , . . , ' ~
: , ' " '' '
1 323234
. 7
though QR is desi.gnated as at the first boundary, it
depends on the construction parameters of all o~ the
layers in the sy~tem.
A5 discussed above, according to the present
invention, use is made o~ the fact that electroma~netic
wave~ exhibit a phase change when reflected from
surfaces and the nature of the phase changes depend on
the nature of the surface. For example, from an
air/glass interface a reflected wavs is shifted by ~
radians with respect to the incident wave, and from an
air/silver interface it is 0.8~ radians at particular
wavelengths. This phase change on reflection is
determined by the following factors: properties o~ the
incident wave, the propertie of the incident medium and
the properties of the reflecting surface, In theory,
any phase change on reflection i~ possibl~ to achieve.
Thus, based on an understanding of the characteri~tics
which influence phase change on reflection, an
achromatically phase controlled mirror was constructed
as shown in Figure 2. The number and thickness of the
multilayers A, B, C and D were chosen so as to exhibit
respeative pha~e changes on reflection over a
predetermined range of wavelengtll~ to match the ideal as
shown in Figure 3, a~ close as was practically possible.
Thus, the phase difference ~Eor the mirror of Figure
2 i governe~ by the formula; .
~ ~R = arg(r)
Wher~ ~ ls the phase change on reflection and r is
thQ Yresnel re~lection coefficient for a multilayer
(each multilayer having different values o~ these
antities for different wavelength~ and angles of
incidence). It can be seen, by compariny the phases o~
the different multilayers o~ Figure 3 with each other,
at different wavelengths, that the phase relationship is
maintainPd, and that ~his is done with no moving mirrors
as in prior art devices.
.: . , - . : ~
1 32323~
. 8
Turning to Figure 4, the basic optical arrangement
is shown ~or a Michelson interferometer employing the
achromatically phase-controlled mirror of the present
invention. More particularly, the mirror 7 is shown
schematically without illustrating the three dimensional
variations in the multilayers A, B, C and D or the
associated substrate and incident mediums. A further
mirror 9 is shown comprising a single multilayer Z in
all four sector~ where % can be either A, B, C or D.
The interferometer further include3 a beam splitting
block 6 for splitting an incident beam into two separate
beams along respective arms 1 and 2 towards tha mirrors
7 and 9, respectively. Upon reflection of the split
incident beam from mirrors 7 and 9, the b@am splitting
block 6 f~mction-~ to recombine the reflected beams and
generate an output beam for detection by a ~CD sensor or
other suitable detector device, in a well known manner.
The ends of the arms 1 and 2 are coated with
reflecting multilayers (i.e. mult:ilayers A, B, C and D
on mirror 7, and multilay2r Z on mirror 9). As shown
with re~erence to Figure 2, the multilayers A, B, C and
D are different such that the phase change on
reflection 6 1 and 6 2 are different, and also ths
reflectance r1 and r2 are different~
In general, the vectorial representation of an
electromagnetic wave is given aR ~ei~ where ~ is the
amplitude and ~ is the phase of the wave. The reflected
electromagnetic wave in arm~ 1 and 2 would therefore be:
,
~.
1 32323~
. g
,ci~
9 1 ~eU13+i3~ (2
~ here:
s~t, r~ ~o th~ ~e~el reflcetio~ coef~d~a~ os~ ~ultil~
1 asld 2
~d c~ e t~ ph~e ch~e on re~leetio~ firQDl ~lgil~ye58 iYI ~mJ
1 &n~ 2
0 ~ ~ ~d B~ ~re t~e elcetromsgnet;c fidd~ ill ~1 and ~
t~e opti~l p8th~ diffe~c~ a~ated lt~ dif~ a~ l~gt~u.
In mo~t Michelson inter~erometer applications
r1 = r2 and 6 1 - 6 2~ but this is not the case with
interferometer of the present invention. The net
electric field after recombination by the beamsplitter
is:
E - El + E~ (3)
The net intensity, I, of the recombined beam i5 yiYen
by:
I - E E* (4)
After a few line~ of algebra thi reduces to the
equation which descri~es the net intensity of the
interferogram for a Michelson interferometer with
mirrors that do not yield egual inten~itie~ or equal
phase ch~nge~
~ -r~r~+2rl~c~~ t~2~ (5)
where ~ i8 the optical path dif~erence between the arms.
Since the argument of the cosine function represents the
net phase of the wave after recombination, this eguation
: . . ~,, -
. . . . : :
. i: - . ,
, ~ ; .
. . .:
1 323234
shows that the phase is changed by two mechanisms at
different places within the interferometer:
1. Part of the net phase di~ference results when
each half of the split beam encounters a
different phase change upon reflection at the
end o~ each arm, delaying one beam with
respect to the other by ~2 ~ ~1 radians.
2. The other contribution to the net phase
difference results from the difference in the
optical paths of each ar~, delaying one beam
with respect to another by 2~aa radians.
Note that although the causes of phase change are
designated above as two separate ~echanisms, on the
microscopic level they are all due to one phenomenon
which is the phase change produced when an
electromagrQtic wave encounters an optical path o~
refractive index n and metric thic:kness t. This is
because tha phase change on reflec:tion from a multilayer
is rsally due to an accumulation of phase changes that
occur when the wave passes through every layer in the
multilayer on both its incident and returning journey.
Al~o to be noted is that when tho reflectances and
phase changes are equal (rl = r2 =5 r, and ~2 = 6 1) the
equation reduces to the familiar form used ~or prior art5 ~ichelson interferometers, as follows:
I = 2r2(l~cos~o~) ~6
Thus, in accordancQ with the preferred embodiment
of the present invsntion a sat o~ four optical thin film
~ultilayer ~irrors were constructed with high
reflectivity in ths spectral r~gion o~ interest and
whose value~ of pha~e change on reflection are 90 away
from that of adjacent ones of the multilayers (i.e., x,
90 -~ x, 180C ~ x, and 270 ~ x, where x is the aR for
the first mirror at a pecific wavelength). The most
ideal ~our coatings should have flat phase dispersion
curves with a single value of phasQ across the entire
': -
1 323234
11
region, with each value 90 apart as shown in Figure 3.
According to the present invention, it was discovered
that a set of four broadband reflectors with phase
dispersion curves of the sa~e slope and equal spaced
intercepts on the wavelength axis is sufficient as shown
in Figure 5. The slope and interval can be chosen so
that ~or any wavelength ~R is 90 away from the ~R arms
of the adjacent mirrors, as shown in Figure 5.
The actual design of multilayers for the mirror in
accordance with the pre~ent invention was accomplished
with the aid of a thin film design program at the
National Research of Canada (NRCC) known as FILTER.7~8
FILTE~ is a general purpose program written in
Fortran for calculations o~ thin film coatings
consisting o~ absorbing and non-absorbing layers.
Several input parameters for FILTER were shared by all
the designs generated in accordance with the present
invention. The design wavelenqths were chosen to
correspond to four atmospheric line3 of interest to
space scientists: 0.5577, 0.6300, 0.7320 and 0.7620
micrometers. The desired reflecta!nce was specified to
be 1.0 for all four wavelength~. The phase chanye on
th~ reflection criterian mentionecl above, i.e., x, 90 +
x, 180 ~ x, and 270 + x) had to b~ satis~ied for t~e
set of four re~lectors A, B, C ancl D at each design
wav~length. For these designs, the initial high, low
and intermediate layex indices were chosen to be 1.35
(cryolite), 2.35 (2inc sulphide), and 1.85 (the mean).
The final design~ involved refractive indices with
intermediate values~
Optimum design performance was eskablished by
designing the first reflector o~ a layer to have the
same refractive index as the medium an~ on a sil~er
substrate. That is, with referencQ to Figure 1, the
incident medium 4 and the first layer of multilayer 3
were made o~ glass, and the substrate 5 was an opaque
:
,
,,
1 32323~
12
layer of silver. The resulting mirror exhibited a phase
change which is linear on a wavenumber scale and a slope
which is a function of the optical ~hickness of the
film. Next, four mirrors were designed, each for a
silver substrate, glass medium, and minimum number of
layers (A=l, B=3, C=5 and D=5). The reflectance
exceeded 0.97 at all design wavelengths in the
succ~ssful prototype. The maximum ~R deviation from gO
for all design wavelengths was only 6 for the
prototype.
A discussion of tha preferred embodiment and
success~ul prototype specification will now be
discussed. As mentioned above, four new multilayers A,
B, C and D were de~ign~d u~ing the program FILTER. The
starting design ~or A was made with mica as the medium,
and the sub~;trates for all of the multilayers A, B, C
and D wer~ ~llver. The construction parameter~ and the
calculated result~ ~ ~R aro shown in table l ~or A, ~,
C and D.
~ ~w~n~
5~6~ A Soatn~ 11 Soa~r C S~t~r o
_~ _ . . . == __
~Jo. _~ n nt n nt _n nt r~
Su~ . _ ~ _ Aa _ 1~ . .~
1 0.~12~ 1.)12 0.21~ ~ 2 o.o~ l.~S2 o.l~ ~ 2
2 o.~ ~ 0.~1 2.~ 0.~2~ 2.~ ~.02~0 2.~
~ o.lo~ ~.~12 0.27~ ~.~ 0.2~9~ ~.~12 0.22~ ~.3~2
8:11~ 2 3~ 0.û23? 2.11~ 0.0170 2.35
i o.~ 2.~ .
t~ __ ~ 9D _ 1.~0 _ 1.590 _ 1.590
,__ . . . __ __ _ __ _ __ . --
. ~. IR C~ . o~ ! c~. O~ C-lc. O~lr~l ~lc.
.ss~ 2-7.~ 2~7.~ lSY.~ 157.~ 61.; 70.1 3~2.6 3~3.5
.5~W ~21.~ ~22.~ ~S~.7 2~ ~ l~Y.~ Si.7 ~
.1~23 ~ ~9.~ ~oo.l ~ 2~ 219.0 1~ l~.a
.7~ ~7.~ ~ J~7.~ ~10-2 7~7.~ 2~ I-l ~ 15~.
O~-lr~l O~lo. Oaclte~t Cslo. D~-lr~ C-lo. D3-lrrd t
.5577 1.000 ¦ 0.901 l.OCtO 0.919 l.COO 0.9611 1.000 0.9
.6~00 I.GOO I 0.9H 1.0~ 0.991 1.000 0~96S l.GOO 0.980
.7~2~ 1.000 1 0.98~ l.Ooo 0.9~ ~.COO 0.972 1.~00 0.97
.7~20 ~.COO I 0.9U 1.000 0.99~ ~.aDo 0.~73 I.oaû 0.?7a
_I_ __ ___ ____ _
,.
1 323234
13
Optical constants were chosen to be dispersive, and
the indices were initially allowed to vary anywhere
between 1.35 and 2.35. Next, a program using the Herpin
equivalent index concept wa~ used9 to convert final
designs into multilayers which had only the two indices
1.35 and 2.35. These designs were refined, and the
thicknesses allowed to vary. Th~ final result was again
a set of fsur multilayer~, each consisting of a small
nu~ber of films. Each of the multilayers exhibited high
reflectance, and the phase change at the design
wavelengths were equal to 90 to within +/- 51. the
superimposed curves of phase change on reflection were
discussed above and shown with reference to Figure 3.
As discu~sed above with reference to Figure 4, the
Michelson :interferometer construct~d in accordance with
the present invention utilizes a pair of mirrors 7 and 9
each divided into four sectors, each sector of which can
be described by equation 6 noted above.
The choice of Z for all four sectors of mirror 3
was arbitrarily made for the purposes of this model.
the superpo~ition of radiation in corresponding sectors
re~ults in an interference patter:n. Where 6 Si
represents the phase change o~ reflection of the i'th
superimpos2d s~ctor of output (where i = 1, 2, 3, 4),
then the resulting phases (which are known to within a
modulus 2~) ara:
~ol = c.l~ + ~s~t,~ = t~a~ SC + ~ = e~
There ar~ no physical stops designating the borders of
eac~ coa~ing in the actual mirror~. Th~ coating edges
define the stops the thus the shaps of the output of the
superimposed electromagnetic waves.
Using equation 6 and nomenclature just defined for
each sector o~ the mirrors 7 and 9, the model can now be
constructed. The intensities of all four interference
patterns the interferometer will create are:
1 32323l~
14
I" _ (r"t~)~ + (r~)a ~ (2Pq~ )~((e~a~ 2~3 (8)
I.2=(r~3J)3+(P~ +~2r~"~ )c~t~t~J ~ + 2~o~
I" = (r~ + (r~,~)a + (2r ~ c~o[(t~G ~ 2~o~1(10)
Id~--~r.~)~ + (~a + (2~"~d)c~ e~ + 2~o ~(11)
The interferometer would ideally have steps of
phase change on re~lection that di~fer by multiples o~
~/2. When the following substitutions are made to the
above equations:
c.~ =S~a=~
~s~ -2~ = ~/2 (13~ -
5/~L~ 4 = 3~/~ (15)
then a~tar a few lines of algebra each of th~ resulting
five equations would reduce to the following. It i~ the
simplest model of the interferometer, and it desaribes
the ~our intensity patterns produc:ed all at the same
time (~or a fixed optical path dil'~erence), and at any
wavelength.
I,t--f ~(1 t 2co~('27ro~
I0~=g3+fa-2fg(Jin(2~o~3~ (17)
I~ = h~+f~- 2hf(co~2~o~18~
r,~ + ~ - 2jf~di~8(2~
w~
0 ~ =r~
~ ~P~
P ~t =
~j=r~
35The ideal interferomeker has all reflectances e~ual
to 1 for every multilayer. In this case the equations
.! , . ' , , :
. ' ' . :
. '
1 32323~
r~duce even further to the ~ollowing simplest possible
~orm:
~1 3 1+ ~(2~ 0)
~3 = 2(1-J~(a~o~3(21)
2(1--co~(2~223
~ 1+~(2~o~) (23)
The traditional prior art interferometer can be compared
to the interferometer o~ the pre~ent invention, for a
better understanding of the distinguishing features.
Consider the prior art Michelson interferometer which
has two mirrors to the same re~lectivity r that is
stepped at three ~/4 steps of optical path difference
~1~2~3~4 $he resulting four intensity measurements
(produced one after the other) would be:
Il=2r~ co~(2~o~) (24)
r, = ~r~+ C~-(2~ )) (25)
I~ _ 2rJ(1~ C01~(2~!0f~5)
I~ = 2r~ COJ(?.R0~ (27)
Note that in th~ prior art it: is ~ that i~
changing, and the phase change on reflection 6 cancel
out because they are the sa~e Por each mirror~ How~er,
in the interf~rom~ter of the present invention, as
descrl~ed by equation3 2~23, the ~ i~ the quantity that
changes to producs each inten~ity measurement and the a
remains fixed.
A per~on understanding the present invention may
conceive of other embodiments o~ variations therein.
For example, although the preferred embodiment of the
present invention iB configured a~ four optical thin
multilayer mirror~ who~e value~ of phase changQ on
re~lection are 90 away from that o~ an adjacent mirror,
..: . ,,, .: ~ .
' ' . ' -..... :: -
1 32323~
16
the preferred embodiment is only one o~ plurality of
confi~urations in accordance with the invention, since
phase disper~ion curves with other slopes and
corresponding shifts in wavelength are equally possible.
However, the optimum set of ~our mirrors has been found
to be the one with the small slope, that is, with the
most horizontal curve~ o~ phase change on reflection.
Such coatings have been found to be less sensitive to
deposition errors.
Moreover, it is contemplated that the principles of
the present invention may be used in numerous
applications outside of the field of Michelson
interferometers. Such applications would be for all
multi-wavelength optical devices in which control over
phase is important. Some applications include~
phase controlled mirrors for Fabry-Perot interferometers
(either conventional, or fibre-optic), which can be
useful as industrial sensors, (2) phase controlled
mirrors for lasers, and for laser experiments in which
phase effects can be significant, such as in nonlinear
optics experiments, like double-pass harmonic-generation
experiments; and (3) rever~ible fringe counting
interfometers (a type of ~lch21som interferometer)~
use~ul for measurinq laser heat frequancies.
2~ All such ~odifications, variations are believed to
be within the sph~re and scope of the present invention
as defined by the claims appsnded hereto.
1 323234
17
REFER~NCES
1. G. G. Shepherd et al., "WAMDII- Wide-Angle
Michelson Doppler Imaging Interferometer for Spacelab,"
Appl. Oct. 24, 1571-1584 (1985).
2. G. Thuillier and G. G. Shepherd, I'Fully
Compensated Michelson Interferometer of Fixed-Path
Difference," Appl. Opt. 24, 1599-1603 (1985).
3. G~ G. Shepherd, "Optical Doppler Imaging with
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