Stabilization mechanism for optical interferometer

A stabilizer for the relative mirror separation and inclination in a Michelson (two beam) or a Fabry-Perot (multiple beam) interferometer that generates at least three reference beams and in one embodiment an optional monitor beam. The reference beams are disposed substantially around the interferometer signal beam and are directed through the interferometer. The resultant intensity from the interference within each reference beam (transmission) is detected and compared to a fixed reference, in one embodiment the monitor beam. Based upon the ratio of reference intensity to fixed reference, the mirror position is controlled to maintain a constant spacing with respect to the wavelength of the reference source. A stepped mirror surface in the interferometer is used in one embodiment and provides a constant difference between effective mirror spacing encountered by a signal beam and the spacing encountered by reference beams. A servo system is provided to adjust mirror spacing to maintain a constant transmission ratio for each of the reference beams in response to distortion in the interferometer or changes in reference wavelength. A method of stabilizing a Michelson and a Fabry-Perot interferometer utilizing reference beams derived from an independent reference source of nearly monochromatic radiation is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical interferometers of the type 
used to measure the spectrum of a narrow-band light source and, more 
particularly, to a method and apparatus for maintaining the separation of 
the reflecting plates at a desired relationship to a reference optical 
wavelength. 
Spectral characteristics of light sources can be conveniently measured with 
optical interferometers. Two of such devices are the Fabry-Perot 
interferometer (multiple beam interference) and the Michelson 
interferometer (two beam in interference). In these interferometers, when 
the mirror separation or optical path length is changed in some way, the 
radiant power detected on the optical axis changes especially in the case 
of sources with interesting spectra. A substantial body of scientific and 
technological work relates to inferring the spectrum of the incident 
radiation (radiant power in a spectral interval vs. wave-length or 
frequency) from the interferogram (radiant power vs. optical path length). 
In order to accurately measure an interferogram, the optical path length 
difference between optical elements of the interferometer (usually 
mirrors) must be known; and the angular alignment of the optical elements 
precisely maintained. One method of accurate positioning of the 
interferometer mirrors is mechanical or passive stabilization. Scan 
distance is determined by a lead screw or fixed spacer of some sort and 
very careful mechanical design is used to reduce the effects of creep, 
vibration, thermal changes, and other sources of dimensional error. Mirror 
alignment is also relatively stabilized by careful design and, in the case 
of the Michelson interferometers, retroreflectors of the "cat's eye" or 
cube corner type are sometimes used. 
A second method for accurate positioning of optical elements is that of 
active stabilization. Active stabilization of interferometers is based on 
various techniques for sensing mirror separation coupled with 
electro-mechanical adjustment of mirror position by means of piezoelectric 
translators or motorized screws or wedges. One method of sensing spacing 
and parallelism of interferometer elements is by means of capacitance 
micrometers. Changes in distance, caused by physical factors, between 
capacitor plates, located on the mirror and some fixed point, 
respectively, are sensed. This technique has been applied, for example, to 
a Fabry-Perot interferometer manufactured by Queensgate Instruments Ltd., 
Franklin Road, London, England SE20 8MW. There is no fundamental 
relationship between the capacitance of the sensors and the wavelength of 
the incident spectrum so the relationship must be established empirically. 
Another active stabilization method is to use the interferogram itself as a 
measure of separation and alignment. This method is feasible only if the 
spectrum contains a single sharp, distinct peak. The interferometer plate 
separation is changed by a small amount, and a logic circuit determines if 
the contrast of the interferogram has increased or decreased and so 
adjusts the direction of the subsequent change. A Fabry-Perot 
interferometer using this method is available from Burleigh Instruments, 
Inc., Fishers, N.Y. 14453. An additional disadvantage is that it is 
difficult to make quantitative spectral intensity measurements when the 
interferogram contrast is being changed for stabilization sensing. 
Scanning Michelson interferometers are available where the interference 
fringes from a reference laser (usually HeNe) are counted to determine the 
optical path difference at any point of the scan. A reference 
interferometer is attached to the main interferometer such that the main 
and reference mirrors move together. Devices of this type are available 
from Nicolet Instrument Corp., 5225 Verona Road, Madison, Wis. 53711, 
Eocom Corp., 19722 Jamboree Boulevard, Irvine, Calif. 92664, and Bomem 
Inc., 910 Place Dufor, Vanier, Quebec, Canada. In these instruments the 
reference interferometers are used to measure the scan rather to stabilize 
the units at a fixed spacing. 
Many very high resolution spectroscopy applications involve scattering from 
a laser source. Examples include Brillouin and Rayleigh spectroscopy in 
solids, liquids, and gases. Often these spectra are relatively simple, and 
many important features of the spectra can be inferred from one or a few 
interferometers operating at fixed optical path differences as specialized 
filters. In order to be useful, the intereferometers must have optical 
path differences set in a precise relationship to the wavelength of the 
laser source. One way to achieve this relationship, which is usually to 
operate at an extremum (a maximum or minimum) of the interferogram, is to 
tune the laser wavelength to match some multiple of the interferometer 
path difference as that path length difference changes from outside 
influences. A. Olsson et al. in Applied Optics, Vol. 19, No. 12, 1897 
(1980) describe a device to tune a laser to an extremum of a passive 
interferometer by "dithering" the laser frequency around a central valve. 
This instrument is restricted to operate with a single interferometer and 
requires a tunable laser to work. Additional limitations of the instrument 
are that there is no provision for active alignment stabilization of the 
interferometer and that it must rely upon passive techniques for mirror 
tilt correction. A further disadvantage is that it is difficult to make 
quantitative transmission measurements when the interferometer 
transmission and laser wavelengths are being changed rapidly (dithered). 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide an apparatus 
and method for the stabilization of an interferometer at an extremum of 
the interferogram, i.e., with the optical path difference in a fixed 
relationship to the wavelength of a single laser or other source or nearly 
monochromatic electromagnetic radiation used as a reference. 
Another object of the present invention is to provide an apparatus and 
method for maintaining the fixed relationship between the optical path 
difference in the interferometer and the reference wavelength irrespective 
of changes in the reference wavelength. 
A further object of the invention is to provide a method and apparatus for 
maintaining a relationship between interferometer path differences and 
reference wavelength without introducing an artificial fluctuation in the 
radiant power transmitted by the interferometer, i.e., without 
"dithering". 
Still another object of the invention is to provide an apparatus and method 
to permit an interferometer to be stabilized to a laser source or 
reference when the stabilization device has no control of the laser source 
wavelength or radiant power output. 
A still further object of the invention is to provide a quick response by a 
very accurate measuring of the departure of the interferometer optical 
path difference from the path difference giving an extremum of the 
interferogram so that corrections to the interferometer mirror position 
can be made before the transmission of the interferometer changes 
significantly. 
The above and other objects are achieved in the present invention by 
providing a stabilized interferometer that maintains an optical path 
difference in a fixed (transmission extremum) relationship to the 
wavelength of a reference laser or other source of nearly monochromatic 
electromagnetic radiation, which may also serve as a source for a 
spectrometric experiment such as scattering. In a preferred embodiment, 
the reference beam (usually from a laser) is collimated and split into at 
least four separate beams. One beam is used to monitor the reference flux 
density, (providing a fixed reference) and the other three or more beams 
are introduced in a suitable way into the interferometer so that the 
reference beams are generally surrounding and at the periphery of the 
central signal beam, which has been directed into the interferometer for 
spectral analysis. 
The mirrors of a Michelson interferometer or the reflective plates of a 
Fabry-Perot interferometer, for example, interact with the signal beam in 
the usual way. However, in a preferred embodiment, one of the mirrors is 
stepped in such a way that a known optical path difference is introduced 
between the signal and the reference paths. Being stepped means that the 
surface of the interferometer mirror is not smoothly flat or spherical, 
but that selected portions (those that interact with the reference beam) 
are raised or lowered a small amount with respect to the surface that 
interacts with the signal beam. The stepped mirror creats an optical phase 
difference between the signal and reference paths. 
When the separate portions of each reference beam are recombined 
interferometrically, the resulting radiant power in proportion to a fixed 
reference (the flux density monitor beam in one embodiment) is a sensitive 
measure of the optical delay encountered by the reference beam in its 
portion of the interferometer. A number of electronic servo systems (one 
for each interferometer reference beam, usually three) then act on the 
mirror position to adjust the mirror position such that the ratio of the 
radiant power in each reference beam to the monitor power is held to a 
constant value. 
If the reference laser drifts in wavelength, all reference-to-monitor 
ratios would tend to drift, and the servo system would change the mirror 
position to account for the wavelength drift. Similarly, if the 
interferometer framework distorts from thermal or mechanical causes, the 
servo system adjusts the electromechanical (usually piezoelectric) mirror 
mounts to compensate for the distortion and maintain a constant optical 
path difference (in terms of wavelength) and precise mirror alignment. 
The size of the step in one interferometer mirror is chosen so that when 
the signal path part of the interferometer is at an extremum, the 
reference part of the interferometer is set to an optical path difference 
so that the transmitted power is approximately midway between the 
transmitted power at a minimum and maximum of the interferogram. If the 
interferometer is set at a minimum of the interferogram, for example, both 
an increase and a decrease in the optical path difference will increase 
the detected signal power. However, if the reference optical delay is set 
at a transmission midpoint, an increase in optical path will increase the 
reference-to-monitor ratio and a descrease in path will decrease the ratio 
(or conversely, depending on the phase of the reference-to-signal 
difference). Thus, the servo correction can be made in the proper 
direction without the need for dithering. In addition, with the reference 
interferometer at a transmission midpoint, the change in 
reference-to-monitor ratio for a given change in optical path difference 
will be much larger than the fractional change in transmitted signal at an 
extremum for the same change in optical path. 
The stabilization method and apparatus is applicable to a number of 
interferometers operating from a common reference source, because no 
control of the wavelength or radiant power output of the reference source 
is required.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now more particularly to the drawings, wherein like numerals 
represent like elements throughout the several views, FIG. 1 depicts a 
modified Michelson interferometer. Use of the term "optical" in this 
specification is not intended to limit the invention to visible 
wavelengths of electromagnetic radiations but is intended to be inclusive, 
because the invention is applicable to all wavelengths of electromagnetic 
radiation for which interferometers can be used. 
1. MODIFIED MICHELSON INTERFEROMETER 
A reference source 10, which can be a laser, vapor discharge lamp, or other 
source of narrow-band (nearly monochromatic) radiation, is divided into at 
least three and preferably four beams. One method for producing the beams 
is to use a collimating lens 11 and aperture mask 12 with a plurality of 
holes. A fixed reference is provided by monitoring the flux density of the 
reference source through monitor beam 8. The three required reference 
beams (only beams 13 and 15 are shown in FIG. 1 for clarity) are arranged 
around the periphery of the interferometer. The reference beams 13 and 15 
and monitor beam 8 are shown stippled to identify them separately from the 
signal beam 24, which is shown in outline. Mirror 16 with a central hole 
for the signal beam introduces the reference beam around the signal beam. 
After the reference beams pass through the interferometer, they are 
separated from the signal beam by mirror 17 also having a central hole 
therein. Components of each of the reference beams represented by beams 13 
and 15 interfere on and are detected by separate detectors for each beam, 
represented by 18, 19 and 20. Detector 21 measures the flux density of the 
monitor beam 8 to provide a fixed (relative to the reference source) 
reference. 
The Michelson interferometer in FIG. 1 with the exception of mirror 26 and 
the reference beam system is of the type often used in the art. 
Electromgnetic radiation from a signal source 1, which is to be studied, 
is formed into a signal beam 24 and collimated by the use of aperture 22 
and lens 23. The signal beam 24 is divided into two approximately equal 
beams by beamsplitter 25. Each of the two beams is reflected at its 
corresponding mirror 26 and 27, and the beams are recombined by 
beamsplitter 25. Both components of beam 24, one delayed with respect to 
the other by the different lengths of the two arms of the interferometer, 
are focused by lens 28 onto detector 29 where the interference of the two 
components of beam 24 is detected. There are electromechanical transducers 
30, 31 and 32, corresponding to each reference beam, in order to position 
one of the interferometer mirrors (26 or 27) with respect to the other. 
The transducers of which there must be at least three are represented by 
components 30, 31, and 32 which serve to translate and tip one mirror with 
respect to the other. A cross-section of the beams in the interferometer 
is shown in FIG. 2. Reference beams represented by 13, 14 and 15 are shown 
stippled. At least three reference beams surround the signal beam 24. An 
optical tubular shield 44 may be used to separate signal and reference 
paths. All beams are reflected by the interferometer mirrors, of which 
mirror 27 is shown for example. 
Mirror 26 is an important and unique feature of the instant invention. The 
optical path difference between the two arms of the interferometer is 
different for signal and reference beams by a fixed amount that depends on 
the configuration of the mirror. Mirror 26 is stepped; i.e., the portion 
of the mirror surface that reflects the reference beams is raised or 
lowered a small amount with respect to the portion that reflects the 
signal beam. The functional effect of the stepped mirror can be seen with 
the help of FIG. 4. If the position of one mirror with respect to the 
other is some distance X, the optical path length will be 2X and the 
interferogram is as given in FIG. 4. It is an object of the present 
invention to stabilize the mirror separation at a distance corresponding 
to an extremum of the interferogram, for example, the minimum of the 
interferogram indicated at X.sub.m. If the step in mirror 26 is 
approximately .lambda./8, where .lambda. is the wavelength of the 
reference source, the mirror position for the reference beams is X.sub.o 
as shown. At position X.sub. o, the transmitted radiant power I is 
approximately one-half the maximum radiant power in the reference beam. 
Transmission of one half is a ratio of reference power in detectors 18, 19 
and 20 to monitor power in detector 21 of one half, if detector gains and 
areas are properly matched. 
A step in mirror 26 of .lambda./8 permits a mirror position so that the 
signal path is maintained at X.sub.m while the reference-to-monitor beam 
power ratio is maintained at 1/2. A servo system adjusts the position of 
mirror 27 by means of three or more electromechanical transducers, 
represented by 30, 31 and 32. If some outside influence tends to change 
X.sub.o (and thereby X.sub.m), the reference/monitor power ratio will 
change, causing the servo system to drive the electromechanical 
positioners 30, 31 and 32, moving the mirror 27 in such a way that the 
reference monitor ratio retains its original value of 1/2. As shown, an 
increase in X.sub.m and X.sub.o will reduce the reference/monitor ratio 
and cause the position transducers to reduce the X position such that the 
power ratio increases to 1/2. 
This embodiment provides for the stabilization of any number of 
interferometers to a common source because the stabilization apparatus 
acts only on each individual interferometer. No dithering of the mirror 
position is required because the invention provides an error signal to the 
correction system with a sign that depends on the sign of the departure 
from the desired position. If the wavelength of the reference source 
changes, the interferogram will shift. For example, an increase in 
reference wavelength will stretch the interferogram, having the same 
effect as a decrease in X.sub.m and X.sub.o because X.sub.m is a certain 
multiple of .lambda./4 at an extremum of the interferogram in FIG. 4. An 
increase in wavelength will increase the reference/monitor ratio and cause 
the position transducers 30, 31 and 32 to increase the mirror separation 
until the power ratio is reduced to 1/2 and the mirror position again 
provides a minimum of the signal interferogram. This operation of the 
apparatus also insures that the stabilized interferometer follows changes 
in reference source wavelengths. Note that the reference/monitor power 
ratio is measured at the steepest part of the interferogram. That is, a 
change in X.sub.o and (X.sub.m) will cause the largest possible change in 
the reference/monitor ratio if the ratio is maintained near 1/2. 
Conversely, the fractional change in transmitted signal intensity I is 
relatively much less than the change in reference/monitor ratio if the 
signal path is maintained at an extremum of the interferogram. This 
accounts for the extreme accuracy of the device and method. 
Many alternative forms of mirror 26 are possible to provide the difference 
in optical path length between the signal and reference beams in 
accordance with the invention. The reference portion of mirror 26 may be 
raised as shown in FIG. 1. The raised portion may be an annulus about the 
signal portion or may be separate areas corresponding to the reference 
beams. The reference portion of 26 may be lower than the signal portion 
shown in FIG. 7a. Another way to introduce the optical path difference 
between reference and signal beams is to use a flat mirror 26A and 
retarding element 26B as shown in FIG. 7b. The retarding element can be 
isotropic or a birefringent waveplate, depending on the polarization state 
of the reference beams. Either the signal beam may be retarded with 
respect to the reference, or the converse arrangement may be used. The 
effective difference in mirror separation for signal and reference beams 
is not constrained to be .lambda./8 but may be any odd multiple of 
.lambda./8 depending on manufacturing convenience. In fact, a separation 
distance of exactly .lambda./8 is not necessary although this is a 
preferred embodiment. The goal is to operate with the reference/monitor 
power ratio near 1/2. Departure of the dimension of the mirror step from 
.lambda./8 can be trimmed out by adjustment of the servo system such that 
the mirror position is correctly maintained. Reference beams 13, 14 and 15 
may propagate counter to the signal beam 24 as shown in FIG. 1 or may 
propagate in the same direction depending on details of the scattered 
light in the interferometer chosen. Mirrors 16 and 17 may be annular or 
may have separate segments for each reference beam. A variety of methods 
exist, and are well known to those of ordinary skill in the art, to obtain 
separate reference beams from the reference source. Rather than the lens 
11 and mask 12 shown in FIG. 1, for example, various arrangements of 
beamsplitters or other elements could be used. Further, electromechanical 
positioners may be attached to either mirror 27 or 26. 
2. MODIFIED FABRY-PEROT INTERFEROMETER 
The invention may also be embodied in a form suitable for stabilization of 
a Fabry-Perot (multiple beam) interferometer to a reference source. A 
schematic diagram of part of the optical arrangement is shown in FIG. 3, 
which shows only the interferometer part of the apparatus. Elements 10, 11 
and 12 for producing reference beams 13 and 15 (two instead of three are 
shown for clarity) and monitor beam 8 can be the same for the Fabry-Periot 
in FIG. 3 as for the Michelson in FIG. 1. Similarly, reference beam 
handling by mirrors 16 and 17 and detection by 18, 19 and 20 can be the 
same. Elements 1, 22 and 23 of FIG. 1 for handling source beam 24 are also 
applicable to the arrangement in FIG. 3, as are signal detection elements 
28 and 29. 
In FIG. 3, Fabry-Perot partially reflecting plates 33 and 34 comprise the 
interferometer. Electromechanical positioners, of which there must be at 
least three, are represented by 30 and 31 (beam 14 and positioner 32 are 
omitted for clarity). A cross-section of the interferometer showing signal 
beam 24 surrounded by reference beams represented by 13, 14 and 15 would 
be indentical to FIG. 2 except mirror 27 would be considered to be 
reflecting plate 34. An optical tubular light shield 44 may also be used 
to separate signal and reference sections to the interferometer. 
Again, one key element of the instant invention for a stabilized 
Fabry-Perot interferometer in FIG. 3 is stepped reflecting plate 34. This 
stepped plate 34 is analogous to the mirror 26 of the previously described 
embodiment for a Michelson interferometer except that the step size will 
not be .lambda./8. The interferogram showing the intensity I transmitted 
by a Fabry-Perot interferometer as a function of the position X of one 
mirror with respect to plate 34 is shown in FIG. 5. The sharpness of the 
transmission peaks depends on the finesse of the interferometer, which in 
turn depends on mirror reflectivity and flatness among other factors. A 
maximum of the interferogram occurs at mirror separation X.sub.M, and it 
is desirable to hold the mirror separation for the signal path at X.sub.M 
with respect to the wavelength of the reference source in spite of changes 
in the interferometer support dimensions or reference wavelength. The 
separation between the reference beam portions of plates 33 and 34 is set 
to X.sub.o in FIG. 5 so that the transmission of reference beams 13, 14 
and 15 is approximately 1/2, i.e., so that the reference/monitor detected 
radiant power ratio is 1/2. The mirror step size X.sub.M --X.sub.o is 
determined by a particular Fabry-Perot finesse so as to place X.sub.o at a 
midpoint on the intensity curve. Of course the X.sub.M --X.sub.o step size 
need not be chosen to be much smaller than .lambda./2 as shown because 
X.sub.M and X.sub.o could be on different peaks of the interferogram. The 
sharpness or steep slope of the Fabry-Perot peaks means that a closer 
tolerance on X.sub.M -X.sub.o is required for a Fabry-Perot than for a 
Michelson interferometer in order to achieve a reference/monitor ratio 
near 1/2. 
The functioning of the invention as applied to a Fabry-Perot interferometer 
is the same as in the case of a Michelson interferometer. Changes in the 
mirror support structure or reference source wavelength are corrected for 
by a servo system acting on positioners 30, 31 and 32 for example to 
maintain the power ratios of representative detectors 13, 14 and 15 to 
monitor detector 8 at a preset level near 1/2. 
3. MIRROR CONTROL SYSTEM 
A preferred embodiment of a control system of the present invention is 
shown in FIG. 6. Reference source 10 provides reference beams 13, 14, 15 
and monitor beam 8 into the interferometer, which has stepped mirrors and 
may be of either Michelson or Fabry-Perot type. Signal source 1, beam 24, 
detector 29, and data processing system 42 for the signal path do not 
enter directly into the stabilization apparatus except that it is desired 
that the interferogram for the signal be maintained at an extremum for a 
reference source wavelength. Detectors 18, 19 and 20 for reference beams 
13, 14 and 15 are compared to the output of detector 21 for monitor beam 
8. The radiant power in each reference beam 13, 14 and 15 is compared to 
the power in monitor beam 8 in comparators 35, 37 and 39 for each 
reference channel in order to determine the reference/monitor ratio for 
each channel. If the ratio departs from a preset level near 1/2, an error 
signal is fed into servo 36, 38 or 40 for each channel. The servo then 
changes the drive voltage to electromechanical positioners 30, 31 and/or 
32 to change the mirror position and thus correct the reference/monitor 
ratio. 
Some typical components that may comprise the elements of the disclosed 
invention include: for mirrors 26 and 34, flat, dielectric 
(vacuum-deposited) coated mirrors with the step applied to the substrate 
by vacuum deposition with a partial mask covering the signal portion of 
the mirror before coating the entire mirror with a high-reflection 
coating; for mirrors 27 and 33, flat, dielectric coated mirrors; for 
mirrors 16 and 17, standard front-surface aluminized flat mirrors; for 
detectors 18, 19, 20 and 21, silicon photodiode with integrated amplifier 
model OS15-K available from Centronic, 1101 Bristol Road, Mountainside, 
N.J. 07092; for reference source 10, an argon laser model 95-4, available 
from Lexel Corporation, 928 E. Meadow Drive, Palo Alto, Calif. 94303; for 
signal source 1, the argon laser light scattered from a fluid target of 
interest; for electromechanical positioners 30, 31 and 32, PZAT 
piezoelectric drive model PZ-81, available from Burleigh Instruments, 
Inc., Fishers, N.Y. 14453; for comparators 35, 37 and 39, integrated 
circuit operational amplifiers type LN0042 with differential input; for 
servos 36, 38 and 40, LN0042 op amps with adjustable bias as offset 
drivers for high voltage operational amplifiers model 604 available from 
Trek, Inc., 1674 Quaker Road, Barker, N.Y. 14012. 
It will be obvious to one of ordinary skill in the art that the above 
method and apparatus can be utilized other than as specifically discussed 
above. For example, the method of using a difference in signal beam path 
and reference beam path to provide a reference signal which varies 
substantially when the relative mirror spacing or reference wavelength 
changes very slightly can be used not only to control servo actuators to 
position mirrors to maintain an interferometer at an extremum of the 
interferogram but also to provide a signal output indicative of the 
distance and/or wavelength change. This may well be useful in the 
automatic processing of optical equipment and in other fields. It is the 
applicant's recognition that by providing a difference in signal and 
reference beam path length, a large error output can be generated when the 
signal interferogram is at an extremum, and is a major aspect of the 
present invention. The application of this method to interferometers other 
than those above will be obvious to one of ordinary skill in the art in 
view of the above teachings. Therefore, the present invention is not 
limited to the embodiments and applications expressed herein and is only 
limited in accordance with the appended claims.