fig1 illustrates my invention in its most general form . it consists of collecting optics 10 , a polarization interferometer 11 , focusing optics 12 , detector 13 , processor 14 , and modulation signal source 15 . together , the collecting optics 10 and focusing optics 12 serve to image the field of observation onto the detector 13 in such a way that the light of interest 16 passes through the polarization interferometer 11 and onto detector 13 . division of collecting and focusing optics into assemblies before and after the interferometer is done to best accomplish this goal for the specific application . it should be understood that for some applications , the entire operation of collecting the light , directing it through the interferometer , and imaging the light onto the detector may be accomplished by either the collection optics or the focusing optics . hence , each of the collecting optics and the focusing optics may consist of any combination of mirrors , lenses , and optical fibers or either may be omitted . the polarization interferometer 11 is driven by a modulation signal s ( t ), supplied by modulation signal source 15 via a line 19 . the signal s ( t ) causes modulation of the interferometer &# 39 ; s optical properties , as described hereinbelow . the electrical output signal generated by the detector 13 is provided via line 17 to processor 14 , which recognizes the presence or absence of the desired optical input . the general polarization interferometer is illustrated in fig2 . the purpose of the interferometer is to apply amplitude modulation to temporally coherent light , while effecting no amplitude modulation on incoherent light . it consists of polarizer 20 , birefringent modulator 21 , and analyzer 22 . the orientation and purpose of these devices is described in detail below . like much of the apparatus in the prior art , the interferometer imposes a path length difference which exceeds the coherence length of the incoherent light but not that of the coherent light . unlike most prior art approaches , however , my invention applies modulation whose form is precisely known . the interferometer 11 will be described as it operates on a single light ray 24 passing through it , though in a practical application it will operate on more complex and realistic optical disturbances in a similar manner . it should also be understood that while my invention is described herein in terms of light , it is capable of operating on radiation throughout the electromagnetic spectrum and this disclosure should be viewed in this more general context . a light ray 24 entering the interferometer is incident on polarizer 20 which passes only the component having linear polarization in the direction 45 degrees to both the y and z axes . see fig2 for the definition of these components . let the incident light have temporal stationary coherence function γ ( τ ), where τ is time difference . in this description , the spatial variation is suppressed , as this analysis is carried out on a ray of light . light 25 leaving polarizer 20 may be resolved into its two components having time - varying complex magnitudes a y ( t ) and a z ( t ), the linear polarizations parallel to the y and z axes , respectively . these two magnitudes are equal and their mutual coherence function is the two polarizations then pass through birefringent modulator 21 . birefringent modulator 21 is aligned with its principal axes parallel with the y and z axes so that each polarization passes through the modulator unrotated and emerges in the same polarization . light 26 leaving the modulator has polarization components b y ( t ) and b z ( t ), respectively . a modulation signal s ( t ) is presented to the modulator by way of line 19 . the modulator imposes a different effective path length for each polarization . in particular , ## equ1 ## where s ( t ) is the modulation signal described above , c is the speed of light , and the constants a y , a z , b y , and b z depend upon the size and optical properties of the modulator . ( practical examples of these constants are given below .) thus , the mutual coherence function of the two polarizations is ## equ2 ## light leaving the birefringent modulator encounters analyzer 22 oriented 45 degrees to both the y and z axes . this analyzer produces linearly polarized light whose amplitude is the sum of equal contributions from the two impinging polarizations , reduced by a factor of √ 2 . thus , the light 27 leaving the analyzer has intensity ## equ3 ## now let the modulator be described by the more convenient parameters ## equ4 ## the detector 13 responds in proportion to this intensity . from the above discussion , one may see that an interferometer is formed by polararizer 20 acting as a beam splitter , analyzer 22 acting as a beam combiner , and birefringent modulator 21 providing two optical paths , one for each polarization . let v denote the maximum absolute value of signal s ( t ). in order to reject incoherent light of coherence length l , the values of a and b are chosen so that conversely , when light of much longer coherence length l is present , say of wavelength λ , then ## equ5 ## provided that so that the condition for obtaining the stated responses to incoherent and coherent light is with this condition satisfied , coherent light may be recognized by the time varying behavior of the intensity . accordingly , processor 14 operating on the detected signal received over line 17 is designed to look for the time - varying behavior described above . incoherent light , which does not produce such time variation , is largely ignored . consider the case of coherent illumination with wavelength λ and sinusoidal modulation signal although it is understood that other forms of s ( t ) may be used in my invention , a sinusoid is the prefered choice . in this case , the intensity consists of terms oscillating at frequencies which are multiples of the excitation frequency f . these terms are weighted by bessel functions whose arguments depend upon b and λ : ## equ6 ## where the functions { j k } are the bessel functions of the first kind . in particular , the first four harmonics of the drive frequency have magnitudes ## equ7 ## respectively . the processor 14 is designed to measure h 1 and h 2 to detect the presence of coherent radiation . in addition , h 3 and h 4 may be measured to obtain an approximation of the wavelength of radiation . a block diagram of one embodiment of processor 14 to be used with sinusoidal modulation is illustrated in fig3 . the modulation signal s ( t ), provided to polarization interferometer 11 over line 19 , is also provided to processor 14 via line 18 . processor 14 uses this signal as a reference for detecting the modulation in the detected signal 17 . harmonics of the reference s ( t ) are generated by a harmonic generator 31 for each term to be measured and these are used to downconvert the detected signal to baseband . the nth harmonic of s ( t ) is a tone at frequency nf . the detected signal arriving via line 17 is routed through a plurality of bandpass filters 35 - 35 to provide a group of signals filtered about the desired harmonics of s ( t ): the signal to be downconverted by the nth harmonic of s ( t ) is a bandpass filtered about the frequency nf . this rejects much of the noise which would otherwise overload the mixers 34 - 34 . each harmonic of s ( t ) is routed over one of the lines 30 - 30 to one of the mixers 34 - 34 , where it is mixed with its corresponding filtered detected signal on one of the lines 33 - 33 . the phases of the tones on lines 30 - 30 are adjusted by generator 31 to insure they are appropriate to compensate for phase changes throughout the rest of the electrical and optical system . the downconverted signals on lines 36 - 36 are filtered through low pass filters 37 - 37 of cutoff frequency b . this cutoff frequency is small to obtain good rejection of noise . the signal power exiting each low pass filter , produced by squarers 38 - 38 , is measured , producing on lines 39 - 39 an estimate , ( h i ) 2 of ( h i ) 2 , where i is 1 , 2 , 3 , or 4 , depending upon whether the first , second , third , or fourth harmonic of the reference was used for downconversion , respectively . the estimates ( h 1 ) 2 and ( h 2 ) 2 are weighted by multipliers 40 - 40 and summed by summer 41 to produce on line 42 a statistic , ## equ8 ## and where ( λ ) is the detector responsivity , r is the detector load resistance , and w 1 and w 2 are constants set to make f ( λ ) be as near to unity as possible over the wavelengths of interest . the statistic s has a mean which increases with the coherent light power and may be used to determine whether or not coherent light is present and to provide a crude estimate of the light &# 39 ; s power . denote the set of wavelengths of interest by λ . to overcome noise , the product vb is chosen so that j 1 ( vb / λ ) and j 2 ( vb / λ ) have apreciable values for all λελ . in practice , a is large number compared with the optical wavelengths , and the sine and cosine in the function f cause the major contribution to oscillate between h 1 and h 2 as the wavelength is varied . to see how to pick w 1 and w 2 , define the extrema ## equ9 ## for i = 1 , 2 . to minimize the maximum possible multiplicative error , one chooses ## equ10 ## then the maximum possible multiplicative error ( neglecting noise ) is a factor of ## equ11 ## as an example , let λ be the interval of 0 . 55 to 0 . 82 micron . then a choice of vb = 2 microns results in vb / λε [ 2 , 3 ], with 0 . 33 & lt ; j 1 ( vb / λ ) & lt ; 0 . 58 and 0 . 35 & lt ; j 2 ( vb / λ )& lt ; 0 . 49 . with a flat responsivity , the maximum error of measuring the light power is about ± 2 . 4 db . the values of ( h 3 ) 2 and ( h 4 ) 2 may be used to determine the wavelength by comparing them to ( h 1 ) 2 and ( h 2 ) 2 , for in the absence of noise , ## equ12 ## either of these ratios may be used to determine λ , provided that vb and λ are such that unique values are obtained . to avoid numerical instability , one chooses the first or second ratio , depending upon whether ( h 1 ) 2 or ( h 2 ) 2 is larger , respectively . the operation of computing r 1 and r 2 and using these values to determine an estimate 43 of λ is carried out by wavelength estimator 44 . for the numerical example above , both r 1 and r 2 are monotonic in λ , ensuring unique determination of λ in λ . in fact , over this range , r 1 and r 2 are nearly exponential in vb / λ , covering a range of about 12 db for r 1 and 9 db for r 2 . note that the value of b may vary somewhat with wavelength , causing small modifications to the described behavior , and perhaps requiring some modification to the interpretation of results . the processing apparatus pictured in fig3 need not be implemented in analog hardware . indeed , the squaring operation , performed on very low bandwidth signals , would be realized in digital hardware much more easily . moreover , a system which attempts to obtain extreme sensitivity would require large dynamic range in the mixers -- a requirement met more practically in the digital domain . in general , the preferred approach is to implement at least a portion of the processing in digital hardware , perhaps using software , and perhaps to the point of digitizing immediately and implementing the entire processing operation digitally . now consider the details of the birefringent modulator 21 . several embodiments of this portion of the apparatus shall be discussed here , illustrated in fig4 , 6 and 8 , in which the entire birefringent modulator is shown . according to the embodiment illustrated in fig4 birefringent modulator 21 includes a fixed retarder 50 and an electro - optic modulator 51 arranged along an axis such that a ray of light 25 will pass through them serially . electro - optic modulator 51 possesses no birefringence in the absence of an applied voltage . electro - optic modulator 51 and retarder 50 both have their optic ( z ) axes perpendicular to the direction of optical propagation . retarder 50 serves to introduce the constant path delay identified as αin the above analysis . this delay arises from the difference between the ordinary index n o experienced by light of the y polarization and the extraordinary index n e experienced by light of z polarization . after passing through the retarder of length s , the difference in path lengths between the two polarizations is electro - optic modulator 51 consists of a block of electro - optic material 53 fitted with parallel plates 54 - 54 perpendicular to the z axis . wires 55 - 55 attached to the plates allow the modulation voltage to be applied . the material 53 possess no quiescent birefringence , but when an electric field is applied via the plates , a birefringence is induced which is proportional to the field strength e . in particular , if the material is cubic having symmetry 43 m , then the change in index in the z polarization is ## equ13 ## where n is the quiescent index and r 41 is the electro - optic coefficient . similarly , the change in the y - polarization index is ## equ14 ## now , let h be the height ( z extent ) of the electro - optic material block and l be the length ( x extent ). with the voltage s ( t ) applied to the modulator , the path length difference imposed by the modulator is ## equ15 ## so that the quantity b used in the analysis above is given by the product of a material constant and a geometric factor : ## equ16 ## as an example of this type of birefringent modulator , consider the use of gap as an electro - optic material 53 and quartz as the retarder 50 . at visible red wavelengths , gap has the figure of merit r 41 n 3 = 35 pm / v . thus , to achieve the vb of 2 microns suggested earlier , one would require ## equ17 ## in quartz , n o = 1 . 54 and n e = 1 . 55 in the red . thus , a =- 0 . 01s , and a 100 micron decorrelating distance could be achieved with a retarder of 1 cm thickness . ( the indices do change with wavelength , an effect which would not be significant here .) the polarization interferometer described here is similar to some standard electro - optic modulators , with the exception that a small path delay ( but much longer than optical wavelengths ) has been deliberately introduced by inclusion of retarder 50 . according to the embodiment illustrated in fig5 birefringent modulator 21 includes a first electro - optic modulator 60 and a second electro - optic modulator 61 , arranged along an axis such that a ray of light 25 will pass through them serially . modulator 60 is constructed of a block of electro - optic material 65 fitted with parallel plates 62 - 62 perpendicular to the crystalline z axis of material 65 . modulator 61 is constructed of a block of electro - optic material 66 fitted with parallel plates 64 - 64 perpendicular to the crystalline z axis of material 66 . the electro - optic material in block 65 of this embodiment is the same as the material in block 66 . this material differs , however , from the material in block 53 of modulator 51 of the previous embodiment . the difference is that block 65 and 66 of this embodiment each exhibit birefringence even in the absence of applied field resulting from a voltage on lines 63 - 63 . light passes serially through the two blocks perpendicular to their z axes . the blocks 65 and 66 are oriented such that the two crystalline z axes are perpendicular , so that the portion of light 25 polarized parallel to the optic axis in the first block is polarized perpendicular to the optic axis in the second block . the first block 65 is of length l 1 , while the second block 66 is l 2 long . in the absence of applied voltage , then , the path difference experienced by light of the two polarizations passing through the combination is ## equ18 ## the difference in length is chosen to effect the desired decorrelation length . the plates 62 - 62 of modulator 60 and plates 64 - 64 of modulator 61 are connected in parallel by wires 63 - 63 , but with opposing polarity . wires 63 - 63 are used to apply the modulation signal . combined with the orthogonal orientation , this connection causes the induced index changes to reinforce . the structure illustrated in fig5 is similar to electro - optic modulators which have been constructed for temperature compensated modulation . the difference here is that l 1 is deliberately made different from l 2 . consider the use of a material with 3m symmetry with field e applied along the optic ( z ) axis as described . then the y and z polarizations experience the induced index changes where r 33 and r 13 are electro - optic coefficients . with a crystal height of h , the difference in path length induced by the applied voltage s ( t ) is ## equ19 ## as an example , consider linbo 3 as the electro - optic material for modulators 60 and 61 . in the red , r 33 n e 2 - r 13 n o 2 = 215 pm / v and n o - n e = 0 . 086 . thus , to achieve vb = 2 microns , one must have ## equ20 ## in addition , to achieve a decorrelating distance of a = 100 microns , one would want this value , of course , is not critical . it is only necessary that the incoherent light be decorrelated , the coherent light of interest remain correlated , and the imaging system be able to tolerate the differences in path length . it should be understood that while the use of two modulators 60 and 61 is preferred , my invention will function with only one such modulator . indeed , though using one birefringent electro - optic modulator will result in inferior performance , such an embodiment may be used to reduce complexity and cost . the modified embodiment of the birefringent modulator 21 is pictured in fig6 comprising only birefringent electro - optic modulator 66 . modulator 66 , like modulators 60 and 61 , is comprised of a block 67 of electro - optic material which is birefringent in the absence of an electric field . block 67 is fitted with plates 68 - 68 to apply a field in the direction of the optic axis , via lines 69 - 69 . the birefringent modulator 21 so constructed behaves like the apparatus of fig4 with the quiescent birefringence equivalent to that introduced by retarder 50 , and the field - induced birefringence being equivalent to that introduced by electro - optic modulator 51 . as a alternative to constructing my invention with electro - optic modulators which use rectangular prisms with electrodes on the sides , my invention may be constructed using electro - optic plate modulators , wherein the optical path through the electro - optic material is small compared to the lateral extent of the modulator . such a device is pictured in fig7 . the electro - optic plate modulator 70 is comprised of a thin plate 71 of electro - optic material whose faces are fitted with electrodes 72 - 72 . ( in the illustration , only one face is fitted with electrodes , through both sides may , in general , have electrodes .) wires 73 - 73 are provided to apply a voltage to the electrodes . the electrodes are interdigitated so that an electric field which is substantially parallel to the optic axis of the material may be introduced by applying a voltage to wires 73 - 73 . modulators similar to 70 of fig7 may be used in place of modulator 51 of fig4 modulators 60 and 61 of fig5 or modulator 66 of fig6 . in particular , fig8 illustrates an embodiment of the birefringent modulator using electro - optic plate modulator 70 in place of electro - optic modulator 51 of fig4 . the plate modulator more easily accommodates a large angular aperture . it should also be understood that my invention may exploit the quadratic electro - optic effect , rather than the above - described linear electro - optic effect , in the electro - optic modulators . the quadratic electro - optic effect produces optical pathlength variations which are proportional to the square of the applied field . if quadratic electro - optic materials are used in place of the linear electro - optic materials , then my invention will behave in a similar fashion , but with the desired optical intensity components being produced on higher harmonics . thus , when using quadratic electro - optic materials , detection of a coherent light source is accomplished by examining the second and fourth harmonic of the modulation signal . in particular , plzt may be used as a quadratic electro - optic material to achieve high efficiency with a small pathlength . plzt is thus well suited for use in a plate modulator , and the preferred embodiment of my invention using plzt as an electro - optic material uses the birefringent modulator illustrated in fig8 . as one of the primary objects of my invention is to detect coherent light while ignoring the perturbations due to incoherent light , it is necessary to consider the effects of noise arising from detector dark current , johnson noise , and shot noise . the following model will be chosen for analysis : the detector is characterized by a sensitive area a , a responsivity ( λ ), which may vary with wavelength , a load resistance r , and a dark electron generation rate of λ d . it is operated at absolute temperature t . the detector is illuminated by a thermal ( and incoherent ) source supplying an average intensity i , and a coherent source whose intensity is sinusoidally modulated ( 100 %) at a frequency of f . the detector &# 39 ; s output is then beat down to baseband by multiplication with a sine wave of unit amplitude at frequency f . the result is then passed through an ideal low pass filter of cutoff frequency b . the filter &# 39 ; s output is squared and sampled to produce the statistic x . it is also assumed that the coherence time of the incoherent light is much less than the inverse of the low pass filter &# 39 ; s bandwidth -- a nearly universal situation . using standard analytical techniques known to those skilled in the art , one finds that the output mean is the sum of a &# 34 ; bias &# 34 ; μ b ( which does not depend on the coherent illumination ) and a &# 34 ; signal &# 34 ; μ s ( which depends only on the coherent illumination ). in particular ## equ21 ## where e is the electron charge , k is boltzmann &# 39 ; s constant , λ i is the mean photoelectron generation rate due to the incoherent light , and λ c is the mean photoelectron generation rate due to the coherent light . ( the latter contribution to μ s is not likely to be significant .) hence , ## equ22 ## the output variance , in the absence of the coherent light , is found to be ## equ23 ## in terms of the illumination , ## equ24 ## is the mean responsivity , i ( λ )/ i is the normalized intensity as a function of wavelength ( whose integral is 1 ), and λ is the coherent light wavelength . to ensure that μ s is some s standard deviations above the bias , one requires that μ s & gt ;( 1 +√ 2s ) μ b , that is , that ## equ25 ## the minimum sensitivity ( achieving the required number of standard deviations above bias ), in the absence of incoherent background radiation , is ## equ26 ## when the incoherent background is strong enough to dominate the internally generated noise , then the minimum coherent light power is ## equ27 ## as an example , consider a detector operated at 290 ° k . with a load resistance of 10kω , a dark current swamped by johnson noise , a responsivity of 0 . 5 a / w , and an aperture of 1 cm 2 . suppose that a low pass filter with a cutoff frequency of 1 hz is chosen , and that one desires the signal to be 10 standard deviations above the bias . then with no incoherent background , the coherent light must have power p c & gt ; 28 pw . now suppose that the detector receives incoherent illumination of 100 mw / cm 2 . then the coherent light must have power p c & gt ; 2 . 8 nw , a factor of 100 higher than with no background . the results above apply to any one of the four estimates calculated in the detector processing . to combine such estimates one observes that each of the four estimates { h i } is independent of the others . ( this is a slight approximation : there is a dependence introduced by the &# 34 ; excess noise &# 34 ; of the thermal source . this contribution is too small to be significant .) in particular , then , the means and the variances add when the statistics are summed . consider now the detection of coherent light using the statistic s . the system is illuminated by incoherent light of intensity i o and coherent light of power p o . the coherent light is of wavelength λ , has a polarization which is at an angle of θ with respect to the polarizer at the entrance to the interferometer , and is assumed to be collected completely by the detector . the incoherent light is captured by an aperture of size a . the coherent light powers oscillating at the first and second harmonic of f are ## equ28 ## respectively . with incident incoherent intensity of i o , the detector sees 1 / 4 of this light ( due to loss at the polarizers ), so that i = i o / 4 . the bias of s ( no signal ) is ## equ29 ## and the signal component of the mean is ## equ30 ## the variance is ## equ31 ## to ensure that μ s is some s standard deviations above the bias , one requires that ## equ32 ## specifies the unbalance between the weights . in particular , the minimum power detectable ( at s standard deviations above bias ) with no incoherent background is given by ## equ33 ## with a strong incoherent background whose shot noise dominates the internal noise , the minimum coherent power is ## equ34 ## in these equations , it is helpful to note that f ( λ )≈ 1 , r ≈ 1 , and w 1 and w 2 are on the order of 4 ( r m ) - 2 . as a numerical example , again let λ be the interval of 0 . 55 to 0 . 82 microns with vb = 2 microns so that 0 . 33 & lt ; j 1 ( vb / λ )& lt ; 0 . 58 and 0 . 35 & lt ; j 2 ( vb / λ )& lt ; 0 . 49 . with a flat responsivity , 0 . 57 & lt ; f ( λ )& lt ; 1 . 74 . also , r = 1 . 00 . now suppose that the detector is operated at 290 ° k . with a load resistance of 10 kω , a dark current swamped by johnson noise , a responsivity of 0 . 5 a / w , and an aperture of 1 cm 2 . suppose that a low pass filter with a cutoff frequency of 1 hz is chosen , and that one desires the signal to be 10 standard deviations above the bias . then with no incoherent background , the coherent light must have a power of 30 to 50 pw , depending on its wavelength . now suppose that the detector receives incoherent illumination of 100 mw / cm 2 . then the coherent light must have a power of 1 . 5 to 2 . 6 nw . the numerical examples above assumed a flat responsivity . the inclusion of more realistic responses will not change the order of magnitude of the results . except for pathological cases , a coherent source which is modulated will produce nearly the same result as an unmodulated source with equal average power . for general ( time varying ) coherent power p c ( t ), the mean output becomes ## equ35 ## where h f ( t ) is the low pass filter &# 39 ; s impulse response . the first integral above is just the average coherent power ( averaged through the low pass filter ). the second integral only contributes if p c ( t ) has significant components near the frequencies f or 2f . in particular , if the coherent source pulses at a rate which is not harmonically related to the system &# 39 ; s drive frequency , the system will respond as if the source were of constant power over the effective observation time imposed by the low pass filter . many variations of the hereinabove embodiments may be made . one may , for example , remote the detector from the interferometer , joining them by an optical fiber bundle which may be incoherent . this would avoid any radio frequency coupling between the modulating field and the detector which might arise . the detection of tones in the output may be accomplished by superheterodyne receivers , that is , by translating each tone to an intermediate frequency for amplification before detecting its amplitude . a portion or all of the detection process may be implemented in digital hardware . the detector &# 39 ; s output , after amplification , may be digitized directly , along with the modulation source . alternatively , the modulation may be derived from the digital clock , allowing very simple digital downconversion . the interferometer may be placed in an imaging system to view the coherent contribution to an entire scene . since imaging detectors usually sample the scene slowly , a slow modulation must be employed . downconversion and integration would then be performed on the detected scene by multiplying the detected pixel values by the modulation function before accumulating them in a video memory . wavelength information could be provided as described above . the materials and numerical values suggested hereinabove serve only to illustrate my invention &# 39 ; s embodiment and power and are not to be considered exclusively . though the specification hereinabove described my invention in terms of light , it should be understood that my invention is applicable to other areas of the electromagnetic spectrum . those skilled in the art will recognize that embodiments may vary substantially from those illustrated herein without deviating from the spirit of my invention . i intend that my invention be limited only by the following claims .