Rugate optical filter systems

A method for fabricating graded refractive index (rugate, optical filters as well as complex rugate filters having prespecified refractive index verses thickness profiles is disclosed. A plurality of at least two different compatable thin film deposition source components of different refractive index, which are stoichiometrically combinable in variable proportion, such as silicon nitride and silicon oxide, or zinc sulfide and zinc selenide, are used to form a thin film material with a refractive index which varies as a function of the proportions of the components. Each source component is separately laser flash evaporated and codeposited on a substrate to form a coating while monitoring the physical and optical thicknesses of the coating, to allow adjustment of the respective laser flash evaporation rates of the source components.

BACKGROUND OF THE INVENTION 
The present invention is directed to methods and apparatus for the design 
and fabrication of graded refractive index (rugate) optical filters as 
well as to complex rugate filters having prespecified refractive index 
versus thickness profiles. 
The term "filter" is used herein in a comprehensive sense to include active 
and passive absorption devices, transmission devices and/or reflection 
devices. Typically, such optical filter devices function in a wavelength 
range which is at least in part within the visible, near UV and/or near IR 
electromagnetic spectrum range. 
Optical filters are conventionally provided by depositing alternating 
layers of dielectric materials of different refractive index, having the 
respective layers being of a thickness determined in the desired 
constructive or destructive interference desired at the wavelength(s) of 
interest. In order to provide various optical filter functions, a 
plurality of dielectric stacks each of different optical filtering 
properties are conventionally provided in adjacent array. However, such 
composite optical filter devices have disadvantages of undesirably low 
efficiency and discrimination capability. Improved optical filters of 
increased efficiency, filter function complexity and/or discrimination 
capacity would be desirable. 
Optical filters are conventionally manufactured by depositing dielectric 
layers on a suitable substrate, such as a reflective or transparent 
substrate. The monitoring and controlling of the deposition process is a 
limiting factor in the manufacture and performance of optical filters. 
Monitoring of deposit thickness and refractive index during deposition have 
utilized reflectance monitoring combined with a crystal thickness 
monitoring, elipsometric monitoring, interferometric monitoring or a 
combination of these techniques. Conventionally, reflectance monitoring 
measures only the optical thickness (nt) rather than the physical 
thickness (t). In order to obtain individual values for both the index of 
refraction (n) and physical thickness, a crystal thickness monitor which 
varies in oscillation frequency with increasing deposit thickness may be 
used to measure the physical thickness (t) of the deposited films, 
permitting calculation of the index of refraction from the reflectance 
monitor. However, crystal monitors tend to have limited accuracy, and can 
accommodate only a limited deposit thickness. Removing the deposit or 
replacing the crystal monitor may require breaking of the deposition 
chamber vacuum, thereby increasing the potential for impurities in the 
deposited film. 
Elipsometric measurement techniques utilize iterative solution of 
transcendental functions to obtain values for n and t, which prolongs 
measurement response time and expense, and presents difficulties in 
respect to deposition-time error correction. In addition, elipsometric 
measurement accuracy decreases with increasing film thickness, which 
conventionally may require the use of witness coupons that must be 
replaced after a short number of fabrication cycles. 
Interferometric monitors are also conventionally utilized to control or 
monitor layer deposition in the fabrication of optical filters. However, 
while interferometry potentially has several advantages over reflectance 
and elipsometric monitoring techniques, conventional interferometric 
monitoring systems have disadvantages with respect to the efficient, rapid 
and accurate determinationof both refractive index and thickness of the 
deposited layer(s) of an optical filter during fabrication. 
Accordingly, it is an object of the present invention to provide new 
optical filters, particularly such filters which provide complex optical 
filtering functions, and methods for designing and fabricating such 
filters. It is a further object to provide methods and apparatus for 
rapidly and accurately measuring the thickness and the refractive index of 
optical filters. It is another object to provide methods and apparatus for 
fabricating optical filters in which the layer thickness and refractive 
index are continuously monitored and in which the deposition is controlled 
in response to the measured thickness and refractive index values. These 
and other objects wil be apparent from the following description and the 
accompanying drawings.

DESCRIPTION OF THE INVENTION 
Various aspects of the present invention are directed to graded refractive 
index (rugate) optical filters and methods and apparatus for designing and 
fabricating such filters. Graded refractive index optical filter devices 
generally comprise film material with a refractive index continuously 
varying as a function of thickness to provide a desired optical filter 
functional response. The refractive index variation with film thickness 
can be designed to cause a reflection or transmission band at a desired 
wavelength, as will be described in more detail hereinafter. 
The attainment of rugate filter bands occurs due to the addition of inphase 
reflections from multiple layers of varying refractive index. At 
particular wavelengths, the reflected radiation adds in-phase to provide 
high transmission or reflection. The design condition for high reflectance 
is that the period of the rugate structure equals half the wavelength of 
the radiation to be reflected. For low reflectance, the period should be 
equal to the wavelength of the radiation to be transmitted. The width of 
the spectral range over which high transmission or reflection is to occur 
increases with the amplitude of the rugate index profile. The magnitude of 
transmission or reflectance increases with the index amplitude and the 
number of rugate cycles in the coating. This is similar in some respects 
to conventional stack coatings in which similar relations hold as the 
difference in index of alternating layers and the number of layers 
increases. However, there are major differences in the way conventional 
stack and rugate coatings are physically realized. In rugate coatings, the 
discontinuous material interfaces are replaced by a controlled homogeneous 
refractive index profile in the coating. This is achieved by changing the 
stoichiometry of the material as the film is grown. 
Rugate filters provide several significant advantages over quarterwave 
dielectric stacks. Below is a comparison of properties between the two 
filters: 
______________________________________ 
Quarter-Wave Stack 
Rugate 
______________________________________ 
Structure 
Many layers of stressed 
Nearly homogeneous 
dissimilar materials 
Optical Limited to existing film 
Graded through 
Index materials alloy mixing 
Selection 
Bandwidth 
Limited by practical 
Very narrow 
materials indices to 
&gt;2.5% .lambda..sub.rej 
Optical Limited by number of 
With increased 
Density (OD) 
layers to .ltoreq. 3 
thickness, higher 
ODs are achievable 
Field of Comparable to rugates 
Comparable to 
view stacks 
Sideband Reduced through complex 
Substantially 
effect stack designs Eliminated through 
gaussian damping 
of index profile 
Multiple not practical Multiple bands 
rejection 
(except by sandwiching) 
permitted 
bands 
Physical Limited by adhesion, 
Approaches that of 
integrity 
residual stress, and 
monolithic optical 
thermal mismatch crystals 
Laser Limited by dissimilar 
Potentially very 
Damage material interfaces 
high 
Threshold 
______________________________________ 
Suitable optical filter devices include narrow band reflection filters 
(high OD), narrow band transmission filters, multi-line (pass/stop) 
filters, anti-reflection (AR) coatings, high-reflection (HR) coatings, 
tuned filters, beam splitters, aperture sharing elements, broad stop band 
filters, and side band suppression filters. 
The present invention is also directed to methods and apparatus for 
fabricating rugate filters having a predetermined refractive index and 
thickness profile. Important components of the synthesis process include 
the synthesis system, a real-time monitoring system for measuring both 
thickness and refractive index during deposition, and a control system for 
controlling the deposition process based on the measured values. The 
filter design system is also provided for design of the thickness and 
composition parameters of rugate filters having preselected optical 
properties. As synthesis proceeds, the real-time optical monitoring system 
independently determines the refractive index (n) and the thickness (t). 
The measured values of n and t are then compared with the desired values 
(previously stored) and an error signal is generated to drive the process 
parameters of the synthesis system. The process is continued until the 
complete filter is fabricated. 
The manufacture of optical filters and the like requires the ability to 
accurately monitor both the thickness and refractive index during 
deposition of optical film materials. The accuracy and speed with which 
this monitoring must be accomplished depends on the center wavelength of 
the filter, the complexity of the index profile, the rate of deposition, 
and the refractive index of the materials being deposited. In the simple 
example of a single line filter, the total optical path of one cycle is 
smaller for shorter wavelengths implying a more rapid variation of the 
refractive index. Similarly, if high index materials are being deposited, 
the physical thickness of one cycle is smaller than for low index 
materials. For more complex index profiles, e.g., a multiline filter, the 
refractive index is also likely to vary rapidly. 
The interferometer reads physical and optical thickness directly and n is 
determined from a closed form equation involving only one division 
operation. This means that deposition-time error correction is quite 
simple. Interferometers are provided in accordance with the present 
invention which directly monitor the optical filter and its substrate, 
eliminating possible errors in calibration between the substrate and a 
witness. The provision of such interferometers permits monitoring of the 
optical filter throughout the entire deposition process without breaking 
vacuum. If a heterodyning interferometer is used, as will be more fully 
described hereinafter, the accuracy is of the order of Angstroms. The 
speed of operation of such heterodyning interferometers is dependent on 
the heterodyne frequency, and accordingly, millisecond sampling times may 
be provided, which are beneficial to rapid and efficient process feedback 
control. 
The determination of the thickness and refractive index profile for a 
desired rugate device may be carried out, for example, by Fourier 
synthesis or sinusoidal design techniques. In this regard, computer 
algorithms may be utilized which take a desired input spectrum, and 
calculate the refractive index profile that will provide the closest 
approximation to the desired optical filter function for the intended 
spectrum by transforming the index requirement from the frequency to the 
spatial domain. The design of the rugate coating may be defined in terms 
of four independent parameters: the number of cycles (M), the average 
index (N.alpha.), the index difference (.DELTA.N) and the period (P). 
In the manufacture of a rugate device for a particular spectral filtering 
purpose, a refractive index versus film thickness profile is calculated 
which will meet the desired spectral requirements. By such suitable 
Fourier synthesis or sinusoidal design techniques, the performance of an 
optical filter for particular parameters of the change in peak-to-peak 
refractive index, number of modulation cycles, peak reflectivity and 
fractional bandwidth may be maximized for a selected material system. 
The range of the achieveable refractive index for a selected material 
system is an important factor in the rugate design and performance. In 
accordance with various aspects of the present invention, multi-maxima 
graded refractive index optical filter devices are provided in which the 
refractive index of the device varies along the optical path in a manner 
which provides a desired optical processing function. Such optical devices 
will generally have at least about 5, and typically from about 10 to about 
100 or more local refractive index maxima in the refractive index profile 
along the optical path of the optical filter device. By designing suitable 
refractive index profiles, efficient, as well as complex optical 
processing functions may be provided. Such refractive index gradations may 
be produced by depositing materials of varying composition in which the 
refreactive index is a function of the composition deposited. For example, 
the system ZnS.sub.x Se.sub.1-x (mixed zinc, sulfide-selenide) provides 
compositions to be deposited which vary smoothly in composition and 
refractive index from that of pure zinc sulfide (ZnS), to that of pure 
zinc selenide (ZnSe), with appropriately varying refractive index. 
Similarly, the silicon dioxide-silicon nitride system permits deposition 
of homogeneous compositions which range from that of substantially pure 
silicon dioxide (SiO.sub.2) to that of substantially pure silicon nitride 
(Si.sub.3 N.sub.4). Another useful material system is the aluminum 
oxide-aluminum nitride system with compositions and refractive indices 
which range from substantially pure alumina to substantially pure aluminum 
nitride. A wide variety of other material systems similarly exhibit an 
index of refraction variation by stoichiometric variation and may be 
deposited on a suitable substrate in accordance with the present invention 
to provide rugate optical devices. As will be discussed hereinafter, a 
wide range of substrate materials may also be utilized for the rugate 
coating. 
Having generally described the various aspects of the present invention, 
the invention will now be more particularly described with respect to the 
various specific embodiments of the drawings. 
FIG. 1 is a schematic illustration of an embodiment of rugate deposition 
apparatus 10 which continuously monitors the thickness and refractive 
index of a deposited optical filter, and which controls the deposition 
parameters in a process controlled feedback loop based on the measured 
thickness and refractive index to provide complex graded refractive index 
filters having a predetermined refractive index/thickness profile. Rugate 
coatings may be provided which have very high reflectance and increased 
resistance to environmental and laser damage. 
The apparatus 10 may be utilized to produce high reflectance graded 
refractive index (rugate) coatings on large optical surfaces. As shown in 
FIG. 1, the apparatus comprises a vacuum deposition chamber system 100 
which includes multiple target boats or supports 102, 104 adapted to 
receive components which are to be evaporated for deposition in the 
manufacture of rugate optical filters by means of apparatus 100. The 
substrate 106 on which layers of continuously varying composition, blended 
from the target materials positioned within the holders 102, 104, is held 
by substrate holder 108. The target materials in the target holders 102, 
104 (such as pure silicon dioxide, and pure silicon nitride, respectively) 
are evaporated by laser beam introduced into the vacuum chamber 100 
through respective optical ports 110, 112. The laser evaporation beam is 
directed through the ports 110, 112 by a motorized mirror system. The 
laser beams are provided by laser evaporation apparatus comprising a 
carbon dioxide laser 114, attenuators 115, 116 and detectors 117, 119, 
121, which system will be described in more detail hereinafter with 
respect to FIG. 2. The apparatus 10 further includes internal shields 118, 
120 and a quartz crystal monitor 122. 
An important feature of the apparatus 10 is heterodyning interferometer 124 
which provides real time measurements for simultaneously determining the 
thickness and refractive index of the deposited film upon the substrate 
106, as will be described in more detail hereinafter. The apparatus 10 
further includes control systems for the various components including a 
shield control system 140 for the shields 118, 120, and a motor control 
142 for directing the laser beam mirrors. In addition, a vacuum control 
144 is provided to control the vacuum pump system of the vacuumized 
deposition chamber 100, an interferometer control system 146 is provided 
for the interferometer 124, a detector control system 148 is provided for 
the laser evaporation system detectors 117, 119, 121 and an attenuator 
control system 150 is similarly provided for the laser evaporation system 
attenuators 115, 116. In a similar manner, as shown in FIG. 1, the carbon 
dioxide laser 114 is provided with a laser power supply 152 for 
controlling the output of the laser 114. A stepping motor control 160 is 
utilized to position the target holders 102, 104 during evaporation of the 
target materials retained therein, and the output from the quartz crystal 
monitor is analyzed by a monitor control apparatus 162. 
The shield control system 140, the motor control system 142, the vacuum 
control system 144, the interferometer control system 146, the detector 
control system 148 and the attenuator control system 150 all indirectly 
provide data to and receive instructions from a microprocessor control 
apparatus 170 through a microprocessor interface 172. The stepping motor 
control, the quartz crystal monitor data input and the laser power supply 
are all directly under the control of the microprocessor 170 as shown in 
FIG. 1. 
While the illustrated embodiment 10 utilizes a laser flash evaporation 
system, rugate filters may also be fabricated using other deposition 
techniques such as chemical vapor deposition (CVD), and physical vapor 
deposition (PVD) techniques. However, using laser flash evaporation, 
nearly any metal, semiconductor or dielectric material system can be 
deposited. Also, virtually any material (at temperatures from ambient to 
500.degree. C.) can be used as a substrate, including plastics which must 
be kept at cool temperatures. 
Substrates of large, flat and curved dimensions may be coated using systems 
such as the apparatus of FIG. 1. Substrates in excess of 20 inches in 
diameter with curvatures as low as f15 may be coated with uniform rugate 
films if the substrates are spun at moderate rates. Only the size of the 
deposition chamber limits the maximum substrate size. Rugate coatings are 
useful as hardened high reflectance films. 
The laser flash evaporation system of the apparatus 10 of FIG. 1 is shown 
in more detail in FIG. 2. As shown in FIG. 2, the high intensity carbon 
dioxide laser beam from the carbon dioxide laser 114 is directed to a 
first mirror M1 through a lens L1 to a second mirror M2 and lenses L2, L3 
to provide a highly collimated beam which is split by beam splitter BS 
into two beams for respective evaporation of multiple components. The 
first beam is passed through attenuator 115, while the second beam is 
passed through attenuator 116 which are independently controlled by 
attenutator control 150 so that the evaporation rate produced by each 
respective beam is independent of the other. Beam splitters BS1, BS2 
provide monitoring beams directed through a chopper apparatus through 
respective pyroelectric detectors 117, 119 for monitoring the intensity of 
the respective beams. The evaporation beams are respectively transmitted 
through ports 110 and 112, and reflected by mirrors M4, M5 to a 4-mirror 
beam stirring system 240, through respective lenses L4, L5 to mirrors M6, 
M7 and through the ports 110, 112 of the vacuum chamber system 100. 
The computer-controlled Apollo 575W tunable CO.sub.2 laser 114 
(CW/pulsed/chopped/Q-switched) mounted to the bottom of a 4 foot by 6 foot 
NRC optical table (with vertical and horizontal isolation legs) is used as 
the laser source for deposition. All transmitting optics are ZnSe. Mirrors 
are 99% reflective, coated Pyrex. The CW/pulsed CO.sub.2 laser 114 
provides high deposition rates and fine control of the deposition process. 
The laser beam is steered up and onto the top of the table where it is 
deflected into the optical path. The resulting 8 mm diameter laser beam is 
then directed by mirrors M1 and M2 through the beam attenuation optical 
path. Before attenuation, 5% of the beam is diverted to a calorimeter 
detector that measures the power of the laser output. This power reading 
is fed into the control computer 170, and feedback to the laser maintains 
a constant laser output. The resulting 95% of the light is focused and 
recollimated. 
As shown in FIG. 2, the laser beam is split into two equal components and 
guided through shutters. Both beams pass through CdTe modulator 
attenuators 115, 116 to control the power levels of each leg 
independently. The CdTe modulators rotate the polarization of incoming 
light as voltage is applied across the crystal. Computer-controlled 
0-10,000 V power supplies are used to drive the modulators. The emerging 
light then passes through a polarizer which causes attenuation of the beam 
as its polarization changes. 
To control the laser power to the deposition materials (power will be 
different for the two beams), the pyroelectric detectors 117 and 119 (2 J 
max. energy, S/N=10.sup.6) sample 5% of each component beam and input 
their readings to a computer for control of the attenuation levels of the 
two beams using the input readings and index and thickness information. 
The beam steering system (two mirrors for each beam with x, y, z mounts) 
guides the laser beams to positions above the chamber 100. The beams are 
then reflected through the top laser ports by mirrors M6 and M7. In the 
illustrated embodiment, lenses of 30 in. focal length focus the laser 
beams onto the deposition materials. These materials are evaporated onto 
the substrate 106 placed 14 in. above the targets. 
The two rotating shields 118, 120 protect the laser ports 110, 112 from 
deposition. The shield window rotates past a small window area. When that 
area is coated, a clean window section is rotated to the opening, allowing 
the laser intensity to be reduced by only a miniscule amount. 
Two mechanisms work together to expose fresh source material to the laser 
beam: the sources are rotated and the laser beam is rastered across the 
source. The source is stepped slowly around at an empirically determined 
speed to give a maximum deposition rate. After each revolution, a mirror 
outside the vacuum system diverts the laser beam to a new position 
resulting in illumination of a new concentric circle on the source. This 
sequence is repeated until the deposition is complete. 
Different refractive indices in the evaporated material deposited on the 
substrate 106 are obtained by varying the stoichiometry of a deposited 
material system. The composition of the deposited mixture is controlled by 
varying the evaporation rates from the two material sources. The 
evaporation rate can be controlled by controlling laser output power, 
pulsed beam pulse width and/or pulse repetition frequency. 
The choice of a particular control parameter or combination of parameters 
can affect the overall deposition rate and the composition of the species 
nucleating on the substrate. Controlling the deposition rate by adjusting 
the power incident on the source is analogous to direct thermal 
evaporation where the source is conventionally heated by resistive or 
e-beam heating, except that only a small portion of the source is heated 
and a fresh area of unperturbed composition can be continuously chosen for 
irradiation by scanning the beam across the source surface. The use of 
this technique to control evaporation rate can in some cases lead to 
decomposition of the source material. Whether the desired stoichiometry in 
the film can be obtained will depend on the mixing of atomic constituents 
in the vapor phase and the mobility of atomic species on the substrate 
prior to nucleation. 
In accordance with conventional practice, compounds having the same 
composition as the source material have been deposited by control of the 
pulse rate. To obtain films congruent with the starting material the 
pulses should be fast enough to prevent decomposition of target material 
during heating. Therefore, to evaporate molecular species, the peak power 
pulses should be high enough to flash the molecular species from the 
target surface in one or a few pulses. 
While the refractive index can be controlled by varying the mixing ratio of 
molecular species on the substrate, considerations other than the 
refractive index may influence the desired form of the deposited material. 
Laser output power, pulse width, and pulse repetition frequency all have 
strong influences on the physical state of a deposited material. For 
example, long-term compositional stability, mechanical properties and 
physical integrity of the film can be enhanced by selecting the form of 
deposition rate control. 
In operation of the illustrated embodiment, the Apollo 575 CO.sub.2 laser 
114 output beam is diffraction-limited and vertically polarized. The 
minimum spot size of the 8 mm beam when passed through a 30 inch focal 
length lens, and from the focal length and diameter (from r.sub.s 
=2F.lambda./.pi.w) is 0.64 mm. For operation at 20 W (average) power 
reaching each target, the peak laser power density at the target is 6.2 
kW/cm.sup.2, 31 kW/cm.sup.2 or 180 kW/cm.sup.2, depending on whether the 
laser is operating CW, pulsed or Q-switched, respectively. The laser power 
at the target is controlled by computer system 170, 150, 152 by applying 
appropriate voltages to the power supply driving the CdTe modulators. 
Minimum pressure of the illustrated system is 5 .times.10.sup.-9 torr. 
Typical base pressure for the chamber is 2.times.10.sup.-7 torr, and 
deposition pressure is typically varied from 7.times.10.sup.-7 torr to 
2.times.10.sup.-6 torr. 
For a source-to-substrate separation of 14 inches and a laser power of 400 
W, the maximum deposition rate is usually between 1000 Angstroms/min and 
2000 Angstroms/min. Typical rates during deposition have averaged between 
400-600 Angstroms/min. 
Both deposition rate and film quality can be affected dramatically by the 
particle size of the source material. The use of pressed discs as sources 
resulted in no deposition due to the high thermal conduction away from the 
irradiated spot, while fine powder proved problematic because of powder 
eruption. The most satisfactory source configuration that deposited good 
quality films was 1-2 mm size lumps of powder. 
In deposition operation, the substrate can be maintained at temperatures 
between 23.degree. C. and 500.degree. C. (depending on the temperature 
limits of the substrate material and/or desired deposition temperature) 
allowing in situ annealing of the film. 
As previously discussed, the desired refractive index profile for a given 
material system may be calculated from the desired spectral profile of the 
optical filter by means of an appropriate computer algorithm. The method 
of calculation is schematically illustrated in FIG. 3. In this regard, the 
desired spectral profile date 302 (such as the desired filter performance 
data for a particular input spectrum) and the material constraint data 304 
(such as the transmission and refractive index properties of the component 
materials and intermediate compositions prepared therefrom) may be 
processed by an appropriate computer algorithm to provide a calculated 
refractive index profile for fabrication of an optical filter device 
having the desired spectral profile within a predetermined degree of 
accuracy. As shown schematically in FIG. 4, the calculated refractive 
index profile data 308 may be utilized to control the operation of rugate 
fabrication apparatus to produce the desired optical filter. In this 
regard, the index profile data 308 may be introduced into a control 
computer 170 which controls the laser flash evaporation synthesis 
equipment by means of a process control feedback loop system utilizing 
data from the real time interferometric measurement of deposit thickness 
and refractive index to control the deposition process to provide an 
optical filter having the desired refractive index profile and optical 
performance. In this regard, illustrated in cross-section in FIG. 5, in 
greatly enlarged form, is a rugate filter 500 of arbitrary refractive 
index variation with thickness. The rugate coating 502 of varying 
composition, such as may be deposited under control of apparatus 10 to 
prescribed parameters as previously described, is deposited on a suitable 
substrate 504 of constant dielectric constant. As shown in FIG. 5, the 
index of refraction (r) of the deposited rugate coating 502 may be varied 
smoothly and arbitrarily within the limits of the material system 
components being deposited. 
Rugate filters having desirable optical properties may be readily provided 
in accordance with the present invention. In this regard, illustrated in 
FIG. 6 are the visible-near infrared spectra of a fabricated rugate filter 
which has been deposited on a polycarbonate plastic substrate. 
As previously discussed, the heterodyning, real-time interferometer for 
simultaneously determining physical thickness and optical thickness (and 
therefore, index of refraction) is an important part of the apparatus of 
FIG. 1. Such interferometers may also find significant use in other 
deposition and film measurement systems and applications. 
As shown in FIGS. 8A, and 8B respectively (G.E. Sommargren, JOSA, 65, 960, 
1975), incoming linearly polarized light of optical frequency .nu. 
selected to be within the functional spectral range of the optical filter 
device which is to be fabricated, impinges on a half-wave plate 704 
spinning at frequency .omega.. This causes the polarization vector to spin 
at 2.omega.. The incoming linearly polarized light responds as the linear 
combination of right and left circularly polarized light whose 
polarization vectors are rotating at .nu., such that the spinning 
half-wave plate 704 upshifts the rotation rate of the left circularly 
polarizing vector by 2.omega. and downshifts the rotation rate of the 
right circularly polarized light by the same amount. When this light is 
passed through a quarter-wave plate 706 with axis oriented at 45.degree., 
The result is orthogonal linear polarization, one upshifted and the other 
downshifted by 2. 
In the interferometer 124, as shown in FIGS. 8A, and 8B respectively the 
polarized laser light is passed through a frequency shifter as described 
above, and directed into a Twyman-Greene type interferometer with a 
polarization neutral beam splitter. In the illustrated embodiment, 
vertical polarization is selected in the reference leg, and horizontal 
polarization is selected in the sample leg. After recombining at the beam 
splitter, a parallel component of each beam is transmitted to the detector 
through a linear polarizer at 45.degree.. Since these two beams are 
separated in frequency by virtue of the frequency shifter, a beat or 
heterodyne signal is seen by the detector. The phase of this signal with 
respect to that of a signal picked off the incoming beam depends on the 
relative optical paths of the sample and reference legs. Thus, a change in 
optical thickness of the sample of 1/2 wavelength will cause a 360.degree. 
phase shift in the heterodyne signal since the interferometer is double 
pass. 
The thickness monitor 124 utilizing these principles is shown in FIG. 7. 
The thickness and refractive index data provided by the optical monitor is 
used in the laser flash evaporation system to control the deposition in 
real time. The laser beam passes through the frequency shifter and a small 
portion is diverted by a beam splitter to establish the reference signal. 
The beam is then directed into the deposition chamber through a high 
quality fused silica window and steered to the beam splitter. The sample 
and reference beams are steered to the sample and reference substrates by 
four mirrors and the appropriate polarizers. The retro-reflected beams 
recombine at the beam splitter and are detected at the physical thickness 
detector. The transmitted beams also recombine at the beam splitter and 
are detected at the optical thickness detector. 
As the film grows on the sample substrate, the path length in the 
retro-reflected sample beam decreases by 2t and the path length of the 
transmitted sample beam increased by (n-1)t. The resultant phase shift is 
read by phase meters with an accuracy of .+-.0.1 degrees. The values of n 
and t are calculated from these phase shifts throughout the entire 
deposition. 
The illustrated heterodyne interferometer 124 (FIGS. 1, 7) measures both 
physical thickness and optical thickness in situ, and in real time. The 
measurements are carried out for each incremental layer, thereby 
minimizing cumulative errors. The measurement accuracy is estimated to be 
in the 20-30 Angstrom range. 
The optical monitor can be used with almost a wide variety of materials 
which have at least limited transmission. The physical thickness leg of 
the interferometer can be used with metallic materials also. 
In accordance with the present invention, the composition, deposited 
thickness and refractory index may be rapidly and accurately monitored and 
controlled to deposit films which have a predetermined compositional 
variation along the thickness of the film. This compositional variation in 
turn provides a graded refractive index profile resulting in desired 
spectral properties. 
To deposit rugate structures, the individual components of a material 
system are evaporated and subsequently deposited at varying rates, under 
control of the feedback control system and the measured physical and 
optical thickness values, which are provided to the real time monitor 
system. The system has demonstrated a precision of .-+.0.2 degrees of 
phase or .+-.3 Angstroms and an accuracy of better than .+-.1% on 
deposited films. 
The use of laser flash evaporation processes for the deposition of thin 
films may be carried out in a pulsed, or continuous (CW) manner, with the 
evaporation and corresponding deposition controlled by the laser pulse 
rate, pulse width and/or intensity. The flash evaporation is easily 
precisely controlled, with the deposition process stopping substantially 
immediately with cessation of the laser energy input. Very high rates of 
deposition may be provided, depending on the material to be evaporated. 
Contamination free films may be deposited, and the substrate may be 
maintained at room temperature. Most refractory materials, including 
substantially all materials that can be deposited by conventional PVD 
methods may be deposited by Laser Flash Evaporation techniques. Laser 
flash evaporation processes also produce a minimization of pressure rise 
in the vacuum chamber, which permit the deposition of thin films in ultra 
high vacuum. Depending on the particular deposition conditions, reactive 
sputtering may be conducted in laser flash evaporatin systems. On the 
other hand, laser flash evaporation may be carried out with elimination of 
deleterious static charges at high active gas pressures. Laser flash 
evaporation may also be carried out under conditions which are favorable 
for the growth of epitaxial layers, and provide for reduction of impurity 
deposition to a minimum due to inherent cleanliness of the process. 
In operation, the interferometer, quartz crystal monitor, and the laser 
detectors provide information to the computer. These measured values of 
index, thickness and laser power are compared to theoretical values. Error 
and change algorithms then drive the CO.sub.2 laser and attenuators to 
give the proper deposition rates of the two source materials which produce 
the predetermined, desired index of refraction and thickness. The sources, 
laser port shields, and beam raster mirrors are moved and adjusted to give 
maximum deposition rate. Vacuum pumpdown and control is also automatic. 
The system utilizes the starting parameters and a starting prompt to 
fabricate the desired rugate filter.