CA1323234C - Phase-controlled thin film multilayers for michelson interferometers - Google Patents

Abstract

ABSTRACT
An achromatically phase-controlled mirror comprising a mosaic of optical thin film multilayers deposited on a substrate, each of the thin film multilayers being characterized by a predetermined number of layers each of predetermined optical thickness for establishing predetermined reflectance and phase changes on reflectance for respective ones of the thin film multilayers. The difference between the phase changes for respective ones of the multilayers remains substantially constant over a predetermined range of optical wavelengths.

Description

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 .
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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 : .. . ~ , -:
.-, -:. . . ~ : - ::

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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 ' .
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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.

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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.

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. 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 . . .

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: , ' " '' ' . 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.

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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:

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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: - . , , ~ ; .

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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 art 5 ~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 ': -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 :

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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_ __ ___ ____ _ ,.

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~

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 .! , . ' , , :
. ' ' . :

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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~

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.

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 Field-Widened Michel~on Interferometers," Surv. Geophys.
9, 185~195 (1987).

4. S.H.C. Piotrowski, Thin Film Multilayers for Optical Doppler Imaging Interferometers, ~c. Thesis, Centre for Research in Experimental Space Science, York U., Toronto, Canada, (Sept. 198~) 5. J.A. Dowbrowolski, "Coatings and Filters," in Hand~ook of optics, W.G. Driscoll, Ed. (McGraw-Hill, New York, 1978), Cap. 8.

6. S.~.C. Piotrowski McCall, J.A. Dobrowolski, and G.
G. Shepherd, "Pha~e Shifting Thin film Multilayer~ for Michelson Interferometer ", Appl. Opt., 28, 2854-2859, No. 14, 15 July, 1989.

7. J.A. Dobrowol~ki, "Completely Automatic Synthe~is o~ Optical Thin Film Systems," ApE~l. Opt. 4. 937-946 (1965~.
80 J.A. Dobrowolski, "Versatile Computer Program for Absorbing Optical Thin Film Systems," Appl. Opt. 20, 74-81 tl981~.
9. J.A. Dobrowol~ki, and S.H.C. Piotrowski, "Refractive Index as a Variabla in the Numerical Design of Optical Thin Film System~," Appl. Opt. 21, 1502-1511 (1982)o J.A. Dobrowolski, "~ica Interference Filters with Transmis~ion Bands o~ Very Low N~rrow Half Widths," J.
Op~. Soc. Am. 4g, 794-806 (1~59).

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Claims (10)

1. An achromatically phase-controlled mirror, comprising a plurality of optical thin film multilayers deposited on a substrate, each of said thin film multilayers being characterized by a predetermined number of layers each of predetermined optical thickness for establishing predetermined reflectance and phase changes on reflectance for respective ones of said thin film multilayers, the difference between said phase changes for respective ones of said multilayers remaining substantially constant over a predetermined range of optical wavelengths.

2. The achromatically phase-controlled mirror of claim 1, wherein said difference between said phase changes is approximately 90- and said predetermined range of optical wavelengths varies from 0.5 to 0.9 µm.

3. The achromatically phase-controlled mirror of claim 1, wherein said substrate is silver and respective layers of said plurality of multilayers are characterized by indices of refraction varying between 1.35 and 3.25.

4. The achromatically phase controlled mirror of claim 1, wherein said phase change on reflectance (.epsilon.R) is characterized by the relation hip .epsilon.R = arg(r); where r is the Fresnel reflection coefficient.

5. A Michelson interferometer comprising a pair of orthogonally displaced mirrors located at the end of respective arms and a beamsplitter for splitting an incident beam and recombining corresponding reflected beams along the axes of said respective arms, wherein at least one of said pair of mirrors comprises a plurality of optical thin film multilayers deposited on a substrate, each of said thin film multilayers being characterized by a predetermined number of layers each of predetermined optical thickness and reflection coefficient for establishing predetermined phase changes on reflectance for respective ones of said thin film multilayers, the difference between said phase changes for respective ones of said multilayers remaining substantially constant over a predetermined range of optical wavelengths.

6. The Michelson interferometer of claim 5, wherein said difference between said phase changes is approximately 90- and said predetermined range of optical wavelengths varies from 0.5 to 0.9 µm.

7. The Michelson interferometer of claim 5, wherein said substrate is silver and respective layers of said plurality of multilayers are characterized by indices of refraction varying between 1.35 and 3.25.

8. The Michelson interferometer of claim 5, wherein said phase change on reflectance (.epsilon.R) is characterized by the relationship .epsilon.R = arg(r); where r is the Fresnel reflection coefficient.

9. The Michelson interferometer of claim 5, wherein said at least one of said pair of mirrors is substantially circular and divided into four sectors of said optical thin film multilayers and the other of said pair of mirrors is substantially circular and characterized by a single optical thin film multilayer having equivalent optical characteristics to a predetermined one of said four sectors of said optical thin film multilayers on said one of said pair of mirrors.

10. The Michelson interferometer of claim 9, wherein each of said four sectors includes a different predetermined phase change on reflection such that upon recombining said reflected beams four adjacent output beams are produced each differing in phase by 90°, whereby said interferometer simultaneously produces four phase shifted beams over said predetermined range of optical wavelengths.