Fizeau fringe light evaluator and method

A beam of light to be evaluated for coherence length is projected through an optical cavity having non-parallel reflecting surfaces to form a Fizeau fringe pattern. A multiple detector array senses the light output of the cavity at various locations along the fringe pattern. The sensed values of the light are compared with one another and the polarity of their difference is used to determine whether the coherence length of the light beam is above a predetermined threshold value.

BACKGROUND OF THE INVENTION 
The invention relates to a method and apparatus for evaluating the 
frequency or wavelength properties of a light beam and, more particularly, 
to a method and apparatus for determining whether a laser beam has 
frequency properties meeting predetermined criteria through the use of a 
fringe pattern of alternating light and dark bands which represent 
particular frequency properties of the beam. 
The utilization of laser light in various fields of technology is becoming 
more and more widespread. In many laser applications, it is necessary to 
know the makeup of the laser light beam in terms of the bandwidth of 
radiation which constitutes the major portion of light beam. This makeup 
is usually referred to as the spectral content of the light beam and is 
often defined in terms of the range of frequencies or wavelengths of 
radiation contained in the beam. 
Ideally, monochromatic light consists of electromagnetic radiation of a 
single frequency .nu. or wavelength .lambda.. In practical applications, 
however, monochromatic radiation is characterized by a center frequency 
.nu..sub.o and a bandwidth .DELTA..nu. such that a frequency interval 
.nu..sub.o -.DELTA..nu./2 to .nu..sub.o +.DELTA..nu./2 contains a large 
part of the energy of the radiation. It is the extent of this range or 
interval of frequencies which is of particular interest in the application 
of laser light. For example, this information is useful in the field of 
holography where light composed of radiation falling within a maximum 
allowable frequency range is necessary for proper imaging. Instruments 
which are responsive to laser light, such as optical countermeasure 
receivers, may require substantially monochromatic light of a narrow 
bandwidth as well. 
A common method of expressing the spectral content of a light beam is in 
terms of its coherence length. The coherence length L.sub.c may be defined 
as L.sub.c =c/.DELTA..nu., where c is the speed of light and .DELTA..nu. 
is known as the temporal bandwidth. Since the coherence length is 
inversely proportional to bandwidth, it can be seen that for applications 
of laser light such as those discussed previously, it is desirable that 
the light beam have a maximum attainable coherence length. Light having at 
least a predetermined minimum coherence length is typically referred to as 
"coherent light", i.e. light comprised of electromagnetic radiation with a 
major portion of the radiation energy falling within a predetermined 
relatively narrow bandwidth of frequencies or wavelengths. 
At present, the only successful method of measuring the coherence length of 
a light beam is through the use of multiple beam interferometers. An 
optical device such as a Fabry-Perot etalon breaks up a beam of light into 
a number of beams which interfere with one another to create a fringe 
pattern indicative of the spectral content as a function of radiation 
wavelength or frequency of the beam, the angle of incidence of the beam on 
the etalon and the distance along the face of the etalon from a 
predetermined edge thereof. The fringe pattern is analyzed to determine 
the coherence length of the light beam. 
When this analysis is to be done electronically with light detectors, 
complex and costly circuitry is required in accordance with prior devices. 
For example, in U.S. Pat. No. 3,824,018 to Crane a frequency 
discriminator, among other components, is called for. Also shown in this 
patent, as is typical of most prior art systems, is the need for an etalon 
having relatively low finesse. 
Finesse is a property of an interferometer which is determined by the 
reflectivity of the reflecting surfaces of the interferometer. It is a 
measure of the width of a band of light in a fringe pattern in relation to 
its distance from the light band of an adjacent order for monochromatic 
light. Use of a low finesse etalon results in a fringe pattern in which 
the light and dark bands are less sharply defined. This limits the degree 
of resolution which can be obtained in the analysis of the fringe pattern. 
Many of the prior art coherence length measurement systems utilize a 
plurality of etalons with each etalon having a light detector associated 
therewith. Such systems present manufacturing problems because only 
extremely low tolerances are acceptable in the design of the etalons in 
order to obtain the small thickness differential between etalons which is 
necessary to achieve a valid coherence length measurement. 
Design problems are also encountered in the prior art systems. A number of 
design variables such as the coherence length threshold (i.e. the minimum 
coherence length that is useful in a particular application), the spectral 
range of measurement for which the system is designed, the number of light 
detectors used and the finesse of the etalon are interactive and cannot be 
independently controlled, resulting in unwanted compromises in measurement 
and extreme complexity. 
OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a novel method and 
apparatus for the detection of the coherence length of a laser beam which 
is capable of simple and economical operation. 
It is another object of the invention to provide a novel method and 
apparatus for coherence length detection which utilizes a single optical 
device having moderate tolerances and a single light detector array. 
It is a further object of the invention to provide a novel method and 
apparatus for coherence length detection utilizing an interferometer 
having a relatively high level of finesse in comparison with prior art 
methods and devices and yet meeting the foregoing objectives. 
It is yet another object of the invention to provide a novel method and 
apparatus for coherence length detection which allow independent control 
of previously interactive design variables for more accurate coherence 
length detection. 
These and other objects of the invention are accomplished with the use of a 
multiple beam interferometer having non-parallel reflecting surfaces to 
generate a Fizeau fringe pattern. The light which makes up the Fizeau 
fringe pattern is sensed at a plurality of points along the fringe pattern 
and the sensed values are compared with one another to determine whether 
the coherence length of the light beam is above a predetermined threshold 
length. The simplicity of the apparatus and its operation results from the 
use of relative intensities rather than the absolute intensity of the 
light in the fringe pattern. 
In accordance with the disclosed embodiment of the invention, the light 
sensing is performed by an array of light detectors located so as to 
intercept the Fizeau fringe pattern. A value proportional to the value of 
the light sensed by one detector has subtracted therefrom the output value 
of another sensor. This is repeated for each detector, and the polarities 
of the differences obtained by these comparisons are evaluated to 
determine the coherence length of the light relative to the predetermined 
value.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 generally illustrates one embodiment of the invention in which an 
optical cavity 10 produces a fringe pattern that is detected by a detector 
array 12. The signals from the detector array are then processed by a 
signal processor described hereinafter in detail to produce an indication 
of the coherence length of a beam of light A directed onto the optical 
cavity relative to a predetermined threshold value. It should be 
understood that the optical cavity 10 and detector array 12 are mounted in 
a suitable housing in actual use of the invention. However, the housing is 
not shown in order to facilitate a description of the cavity and detector 
array. 
As shown in FIG. 1, the optical cavity 10 comprises a wedge-shaped block of 
light transmissive material whose surfaces are partially silvered. In an 
alternative form, the cavity can be made of two flat transparent plates 
with an air space between them, the air space varying in thickness in one 
direction along the plates. The cavity 10 breaks up the beam of light A 
into a plurality of beams which generate an interference fringe pattern, 
commonly called a Fizeau fringe pattern, indicative of the wavelengths of 
light present in the light beam A. Specifically, the distance between 
reflecting surfaces, or thickness, of the cavity at each point where a 
high peak of light intensity occurs in the fringe pattern is an integral 
number of half wavelengths of the light content of the beam. A device such 
as optical cavity 10 which breaks up a beam of light into a plurality of 
light beams to create a fringe pattern is generally known as a multiple 
beam interferometer. 
In accordance with the invention, a light detector assembly 12 is 
positioned to intercept the light exiting the cavity 10. The detector 
assembly 12 senses the light which constitutes the fringe pattern at 
various locations of expeated maximum and minimum intensities for a 
particular coherence length in the direction of varying thickness of the 
wedge along the fringe pattern, as shown by arrow 16. The light sensed at 
these various locations produces signals which are used by a signal 
processor 17 to determine the coherence length of light beam A, as will be 
explained in connection with FIGS. 2 and 7. 
In accordance with the invention, the light detector assembly 12 comprises 
a plurality of spaced light detectors. In the embodiment of the invention 
illustrated in FIG. 1, the light detector assembly 12 is made up of an 
array of three light detectors, E.sub.1, E.sub.2 and E.sub.3. This array 
is positioned to sense the light of three discrete segments of the fringe 
pattern, the segments being divisions of the fringe pattern along the 
length thereof in the direction of arrow 16. The light impinging on each 
detector is compared by the processor 17 with the integrated value 
obtained from the other sensors for a determination of coherence length. 
FIG. 2 illustrates the operation of the invention employing the three 
detector array. Solid line 18 and dotted line 20 represent the intensity 
of light with respect to distance along the length of optical cavity 10 of 
a Fizeau fringe pattern for coherent and non-coherent light, respectively. 
Coherent light, i.e. light having a predetermined minimum coherence 
length, produces peaks of intensity 22 and 24 only at those points along 
the length of the cavity 10 where the thickness of the cavity is an 
integral number of half wavelengths of the content of the coherent light, 
as explained previously. As the bandwidth of the light beam is increased, 
the peaks become broader. Thus when "white" light is projected through the 
cavity, the Fizeau fringe consists of uniform intensity, as shown by 
dotted line 20, since its frequency content is substantially continuous 
across the entire frequency spectrum of light. 
As was previously mentioned, the energy or intensity of the light in each 
particular segment of the fringe pattern is integrated over the area of 
its associated light detector to obtain an output signal indicative of the 
makeup of that segment. For coherent light, it can be seen that the output 
signals of detectors E.sub.1 and E.sub.3 of FIG. 2 are high with respect 
to that of E.sub.2. On the other hand, for noncoherent light as 
represented by dotted line 20, the output signals of each of the three 
detectors are substantially equal. 
In accordance with the illustrated embodiment, the signals processor 17 
compares the detector output signal to determine whether or not the light 
beam is coherent. The signal processor performs the following calculations 
in determining coherence length acceptability: 0.9 times the value of the 
output signal V.sub.1 of detector E.sub.1 has subtracted therefrom the 
value of the output signal V.sub.2 of detector E.sub.2 ; 0.9 times the 
value of the output signal V.sub.2 of detector E.sub.2 has subtracted 
therefrom the output signal V.sub.3 ; and 0.9 times the value of detector 
output signal V.sub.3 has subtracted therefrom output signal V.sub.1. 
Referring to FIG. 2, it can be seen that for coherent light, 0.9V.sub.1 
-V.sub.2 yields a positive result, but 0.9V.sub.2 -V.sub.3 and 0.9V.sub.3 
-V.sub.1 gives negative differences. On the other hand, for non-coherent 
light, 0.9V.sub.1 -V.sub.2, 0.9V.sub.2 -V.sub.3 and 0.9V.sub.3 -V.sub.1 
all yield negative results. It is the presence or absence of a positive 
result in the signal processing technique which is indicative or coherent 
or non-coherent light, respectively. 
The 0.9 multiplication factor utilized in processing the detector output 
signals is reflective of the level of tolerence which is acceptable in the 
coherence length measurement, a higher multiplication factor indicating a 
lower tolerance level. Appropriate selection of the multiplication factor 
allows for differences in gain and the presence of noise in the electrical 
components which perform the sensing and algebraic comparison functions. 
It will be appreciated that the multiplication or tolerance factor also 
has an effect on the threshold value of coherence length for which the 
device is designed. As the multiplication factor is lowered, the 
acceptable bandwidth is broadened. The acceptable bandwidth is also 
broadened by reducing the cavity thickness, as explained in more detail 
hereinafter. The 0.9 factor disclosed here is merely for the purpose of 
illustration. The actual figure to be used would be determined in 
accordance with the specific characteristics and requirements of a 
measurement device constructed in accordance with the invention. 
Another factor to be considered in the construction of the instrument is 
the wavelength of light to be examined. At the outset, a particular 
wavelength, .lambda.max, is selected which will be the maximum wavelength 
for which the instrument is designed. This value is used in determining 
the thickness differential of the optical cavity, which is 
.lambda.max/2.times.n, where n is the index of refraction of the material 
which constitutes the cavity. Use of such a differential limits to one the 
number of bands of light representative of .lambda.max in the generated 
fringe pattern. This assures more accurate evaluation by eliminating 
multiple detections of light of the same wave length. 
It will also be appreciated that the actual thickness of the cavity will 
affect the properties of the instrument. As is known with interferometers 
of this type, the thickness of the cavity is determinative of the order of 
the band of light measured in the fringe pattern. For monochromatic light, 
the zero order band of light is that band of light present in the fringe 
pattern where the two reflecting surfaces intersect, i.e. where the 
optical path length in the cavity is zero. As the path length within the 
cavity increases, successively higher number order bands of light are 
generated in the fringe pattern, the number being indicative of the 
integral number of half wavelengths of the monochromatic light in the 
length of the optical path. 
The order of light bands measured is relevant in that it is determinative 
of the separation of the bands of light in the Fizeau fringe pattern 
representative of different wavelengths of light present in the light beam 
being analyzed. FIG. 3 illustrates this characteristic with respect to two 
thicknesses of optical cavities. 
In FIG. 3, a beam of light composed of radiation of two wavelengths 
.lambda..sub.1 and .lambda..sub.2 is projected onto two optical cavities 
26 and 28 of different thicknesses. Cavity 26 is relatively thin and the 
bands of light present in its generated Fizeau fringe pattern are of low 
order. Curve 30 illustrates the intensity of light in a Fizeau fringe 
pattern generated by optical cavity 26, with the solid line peaks 
representative of light of wavelength .lambda..sub.1 and the dotted line 
peaks representative of light of wavelength .lambda..sub.2. Since the 
length of the optical path within the cavity is relatively close to the 
difference between .lambda..sub.1 and .lambda..sub.2, the peaks of 
intensity occur close to each other in the fringe pattern and appear 
almost as a single peak, as shown by curve 30. On the other hand, a 
thicker cavity 28 which generates higher order bands of light has an 
optical path whose length is greater than the difference between 
.lambda..sub.1 and .lambda..sub.2. This change in path length separates 
the bands of light in the fringe pattern representative of light of 
wavelengths .lambda..sub.1 and .lambda..sub.2, as shown by curve 32. Thus, 
a thicker cavity permits better discrimination between adjacent bands of 
light and therefore provides a longer coherence length threshold value. 
As is known with multiple beam interferometers, the angle at which the beam 
of light is incident upon the interferometer has an effect on the fringe 
pattern generated. For the wedge shaped cavity used in the invention, the 
angle of incidence merely affects the placement of the fringe pattern 
relative to the interferometer. Referring to FIG. 3, varying the angle of 
incidence from normal as shown would merely cause the curves 30 and 32 to 
shift to the left, up the wedge. No significant change in the shape of the 
curves or the separation of peaks of different wavelength would occur. The 
separation of peaks at the same wavelength will increase with the secant 
of the angle of incidence. The shape of the fringe peaks will be changed 
by a minor predictable degree, becoming unsymmetrical at large angles of 
incidence. The degree of non-symmetry increases with finesse and the 
magnitude. 
Referring now to Table I and FIG. 4, the effect of finesse on the coherence 
length discrimination measurement is illustrated for a three detector 
array. Table I lists a range of reflectivities from 0.3 to 0.8 and the 
value of finesse which is associated with each reflectivity value. The 
third and fourth columns list a standardized value of the intensity of 
light in a fringe pattern integrated over the area of detectors D.sub.1 
and D.sub.2 of the three detector array, such as is shown in FIG. 4, for a 
beam of light having a wavelength of 1.0.mu. which is projected onto a 
wedge shaped interferometer having the associated finesse listed. The 
fifth column lists the difference of the values in the third and fourth 
columns. FIG. 4 illustrates the intensity of light in the fringe pattern 
across the detector array for three selected values of finesse. 
As can be seen in FIG. 4, as the value of finesse is increased, the width 
of a band of light decreases and the band becomes more pronounced. It has 
been found that the best results are obtained when the finesse is adjusted 
so that the width of a band of light, as represented by the peak on 
detector D.sub.1, is substantially equal to the width of one photo 
detector in the array, such as is shown in FIG. 4 for the example where 
finesse is approximately equal to 8. This occurs when the refectivity of 
the surfaces of the interferometer is in the range of 0.4 to 0.5. When the 
reflectivity is increased beyond this point, the finesse is increased and 
the band of light is narrowed such that the difference in the values 
obtained from the two adjacent detectors is not as great, as shown in the 
last three examples of Table I, providing a lesser degree of resolution in 
the measurement. 
The values disclosed herein relate to optimum finesse for a three detector 
array. It is apparent that if an array having a larger number of detectors 
is utilized, wherein the width of each detector is decreased, a higher 
finesse value which correlates the width of an intensity peak with 
detector width would provide optimum results. 
TABLE I 
______________________________________ 
Reflectivity 
Finesse D.sub.1 D.sub.2 D.sub.1 -D.sub.2 
______________________________________ 
.3 2.45 .84 .42 .42 
.4 4.44 .75 .30 .45 
.5 8.00 .66 .19 .47 
.6 15.00 .55 .12 .43 
.7 31.11 .43 .06 .37 
.8 50.00 .29 .025 .26 
______________________________________ 
FIG. 5 illustrates how the number of light detectors can be varied to 
change the bandwidth for which the discrimination testing is being done. 
Line 34 represents the intensity of a Fizeau fringe pattern of a light 
beam whose spectral content is broader than that of the beam represented 
by line 18 of FIG. 2 but less than continuous over the entire spectrum. 
For the purpose of illustration, assume that the spectral content of the 
light beam lies within a bandwidth of 80 Angstroms. Further assume that 
the three detector array shown in solid lines is capable of detecting 
whether the light beam is coherent within a range of 50 Angstroms. It can 
be seen that the value of the intensity of the light integrated over the 
area of each of the detectors is approximately the same. Thus, the 
comparison steps of 0.9V.sub.1 -V.sub.2, 0.9V.sub.2 -V.sub.3 and 
0.9V.sub.3 -V.sub.1 will all yield negative results, indicating 
non-coherent light, at least within a 50 A wavelength range. 
For some applications of laser light, however, a less coherent beam of 
light may be acceptable. Testing for coherence length within a broader 
bandwidth can be easily achieved in the context of the present invention 
by merely increasing the number of light samples taken per unit length of 
the Fizeau fringe pattern for the same optical cavity. This is achieved in 
the illustrated embodiment by substituting the five detector array 36 
shown in dotted lines for the three detector array 12. This will increase 
the bandwidth discrimination from 50 A to 100 A, for example. It can now 
be seen that for the same beam of light, the output signal from detector 
E.sub.4 is substantially less than that from any of the other detectors. 
Accordingly, 0.9V.sub.3 -V.sub.4 will yield a positive result, indicating 
a bandwidth of less than 100 A. 
This same effect can alternatively be achieved by keeping the number of 
detectors the same and substituting a different optical cavity having a 
smaller angle between the two reflecting surfaces. This spreads out the 
fringe pattern since the rate of change in optical path length is not as 
great, thereby decreasing the area of sample of each detector. It can be 
seen that this has the same effect as increasing the number of detectors 
per unit length. It is to be noted, however, that this procedure changes 
the thickness differential of the cavity over the length of the pattern 
being sensed, thereby reducing .lambda. max. 
The number of detectors used is also critical in eliminating "holes" in the 
detection of the peaks in the fringe pattern. For monochromatic light of 
wavelength .lambda. max, the spacing of peaks in the fringe pattern is L, 
the width of the optical cavity, which is also the width of the detector 
array, as illustrated in FIG. 6. For light of wavelength 2.lambda. max/3, 
the fringe spacing will be equal to 1/3 of the array width. This can lead 
to a condition where each detector detects an identical intensity for a 
three-detector array. That is, if one peak is present at one edge of the 
array, another peak will be at the junction of the two detectors closest 
to the opposite edge of the array, leading to the condition where each 
detector detects the same amount of light and all three detectors produce 
equal output signals. This results in a "hole" or a point where 
monochromatic light appears to be the same as broadband light which forms 
no fringe pattern. 
It has been found that holes can appear for wavelengths defined by: 
##EQU1## 
where n is an integer greater than zero and N.sub.d is the number of light 
detectors in the array. Accordingly, to insure hole free measurement, the 
number of detector N.sub.d must be: 
##EQU2## 
where .lambda. min is the minimum wavelength in the spectral range over 
which the measurement is desired. 
FIG. 7 illustrates one example of a signal processor which performs the 
light value comparison and detection functions of the invention. The light 
detectors are shown as photodiodes E.sub.1, E.sub.2 and E.sub.3. Each of 
the photodiodes is connected to a common source of D.C. bias potential 38 
and produces a current at its output terminals proportional to the 
intensity of the pulses of the light shining on it. The output current 
from each photodiode passes through two coils whose windings are in the 
ratio 9:10. The coils form part of a three-winding transformer system 
which performs the comparison function. 
Referring to transformer T1 of the transformer system, the primary windings 
consist of two coils P1 and P2 whose turn ratio is 9:10 as mentioned 
previously. The current flowing in P1 is proportional to V.sub.1, the 
output current of detector E.sub.1. The current in P2 flows in the 
opposite direction to that of P1 and is proportional to V.sub.2, the 
output current of detector E.sub.2. Due to the turns ratio and the 
direction of current flow, the output current of the secondary winding of 
transformer T1 is representative of the value 0.9V.sub.1 -V.sub.2. 
Transformer T2 and T3 operate in the same manner to provide the results 
0.9V.sub.2 -V.sub.3 and 0.9V.sub.3 -V.sub.1, respectively. It can be seen 
that it is necessary to maintain the output currents of each of the 
detectors in phase with one another in order to obtain the difference 
between them, and for this reason the common source of potential is used 
for all of the detectors. It will be obvious to one of ordinary skill in 
the art that other circuitry which maintains the phase relationship of the 
output currents or which performs the subtracting function, such as a 
differential amplifiers which are suitably biased to provide the requisite 
signal proportions, can be substituted for the specific components 
disclosed herein. 
The results from each of the detector output value comparisons are fed into 
a series of preamplifiers 40, and from there into a series of main 
amplifiers 42. The amplified difference value is fed as an input signal 
into a bipolar detector and latch circuit 44 for a determination of the 
polarity of the difference signal. The bipolar detector consists of two 
threshold comparators connected to the input line, one having a positive 
threshold value and the other a negative threshold value. Because the 
input signals occur in pulses which can come in at a very fast rate, a 
latch is connected with the threshold comparators to hold the output 
signal of the comparators, indicative of input signal polarity of an 
individual pulse, for a predetermined time necessary to make the coherence 
length determination. The latches are controlled by a reset generator 46 
which, after receiving a signal that the latches have been set, sends an 
output signal to reset the latches when the predetermined time period has 
lapsed to sample a different pulse. 
Also connected with the bipolar detector and latch circuit 44 is a noise 
automatic gain control circuit 48. This circuit measures the level of 
noise in the input signal to the bipolar detector and latch circuit and 
controls the gain of main amplifier 42 accordingly. As the noise level 
increases, the gain is decreased. This enables the threshold comparators 
of the bipolar detector to operate at a predetermined threshold to noise 
voltage ratio. 
The signals stored in each latch of the bipolar detector and latch circuit 
44 are fed as input signals into a parity checking circuit 50. The circuit 
checks for equality of signals and sends an output signal to indicator 52 
indicative of whether such a condition exists. If all of the input signals 
are equal, i.e. all negative, indicator 52 will indicate non-coherent 
light. On the other hand, if at least one input signal is positive, parity 
will not exist and the indicator will be actuated to indicate coherent 
light. It is to be noted that in the disclosed embodiment at least one of 
the inputs to the parity checking circuit will always be negative, and 
therefore one need not be concerned with parity in the sense of all inputs 
being positive. 
FIG. 8 graphically illustrates one example of the results obtained from a 
device constructed in accordance with the present invention. A multiple 
beam interferometer was constructed from a block of quartz 0.007 inch 
thick at the narrow edge with an angle between the two reflecting surfaces 
of 9 arc seconds. 
It has been found that the difficulty in controlling the angle of the wedge 
shaped block during manufacture is most easily overcome by monitoring 
fringe spacing rather than the angle itself. This is done by passing a 
collimated, expanded laser beam of known wavelength through the 
interferometer and observing the interference fringe pattern with the eye 
or recording it photographically. This enables the achievement of high 
accuracy during manufacture since the measurement of a length (i.e. 
spacing between fringes) is inherently easier than measurement of an 
angle. Furthermore, fringe spacing and shape can be monitored in one 
convenient measurement. The observed fringe pattern includes the net 
effect of surface shape and refractive index, and so the uncertainty or 
variations caused by these parameters are eliminated as well. 
The interferometer was cemented between two uncoated cover plates for 
support and protection. While the wedge and surface quality tolerances for 
the cover plates are not as tight as for the interferometer itself, they 
do affect system performance. This is because any imperfections in the 
surfaces of the upper cover plate can distort the wavefronts entering the 
interferometer, causing the fringes themselves to be distorted. The effect 
of the cemented surface of the upper plate is minimal because the 
refractive indices of the cement and upper plate were nearly matched. It 
is the upper surface of the upper plate which is most significant in this 
respect. There is the additional possibility that the cover plates will 
produce their own sets of interference fringes. Their effects can be 
minimized, however, by putting a relatively large wedge in the cover 
plates themselves (10 to 30 arc min). 
The reflecting surfaces of the interferometer were coated with silver 100 A 
thick with the resultant surface reflectivity being approximately 0.55, 
light transmission being 0.37 and absorption within the cavity being 0.08. 
A beam of He-Ne laser light having a center wavelength of 6328 A was 
projected through the interferometer to generate a Fizeau fringe pattern. 
The transmitted intensity of the light in the fringe pattern was measured 
in the area of one order of the bands of light and compared with the 
theoretical value for such a band of light. As can be seen in FIG. 8, the 
curve which was actually obtained from the measurement very closely 
approximates the computed curve of what such measurement should be. From 
such it is apparent that the utilization of a wedge shaped optical cavity 
is highly satisfactory for evaluating the coherence length of a beam of 
light. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. For 
example, a single light detector which is moved relative to the Fizeau 
fringe pattern to sense the light at various locations can be substituted 
for the multidetector array disclosed. The presently disclosed embodiment 
is therefore considered in all respects as illustrative and not 
restrictive. The scope of the invention is indicated by the appended 
claims rather than the foregoing description, and all changes which come 
within the meaning and range of equivalency of the claims are therefore 
intended to be embraced therein.