referring to fig1 an optical system is shown that uses forward - scatter to determine the presence of a particle in an enclosed volume . a laser 10 generates a polarized optical beam which is focused by a lens 12 onto a beam splitter 14 . the preferred embodiment shown in fig1 employs heterodyne principles , so beam splitter 14 is an acousto - optic device such as a bragg cell . bragg cell 14 upon application of a signal from a local oscillator 15 , creates a divergent beam 16 ( in addition to transmitted beam 18 ) whose frequency is modulated by the signal applied by local oscillator 15 . thus , if the frequency of local oscillator 15 is 80 mhz , then the optical frequency of beam 16 differs from the optical frequency of beam 18 by 80 mhz . beams 16 and 18 are focused by lens 20 through window 26 into enclosed volume 22 wherein a particle ( or particles ) 24 are present . lens 20 is positioned so as to image the point of origin of beam 16 within bragg cell 14 into volume 22 . by utilizing lens ( or lenses ) 20 to image the point of origin of beam 16 , the alignment problem present in the prior art is eliminated due to the fact that beams 16 and 18 will invariably cross at a known point within volume 22 , solely in dependence upon the focal distance of lens 20 . it is preferred that the beam from laser 10 be focused at the center of bragg cell 14 in order for the overlap region of beams 16 and 18 ( within volume 22 ) to encompass the beam foci . since the angular acceptance of bragg cell 14 is likely to be relatively small , the focusing is generally weak and the depth of focus is large . therefore , the positioning of the focus in the bragg cell is easily accomplished . the numerical aperture in the intersection region of beams 16 and 18 can be as large as desired . after passing through the intersection region , beams 16 and 18 pass through a window 28 , with beam 16 being absorbed in a beam dump 30 and beam 18 being directed onto a photocell 32 . the output from photocell 32 is fed to a signal processor 34 that enables the presence of a particle 24 to be indicated . referring to fig2 beams 16 and 18 are shown at their point of intersection within volume 22 . since beam 16 differs in frequency from beam 18 by the modulating frequency applied to bragg cell 14 , a moving fringe pattern 36 is produced at the intersection of the beams . the fringes move in the direction shown by arrow 38 at a rate determined by the frequency of modulating oscillator 15 . thus , when a particle 24 enters intersection region 36 , movement of the particle through the moving fringe pattern induces a doppler shifted burst in the optical frequency of beam 18 . the amount of doppler shift is related to the velocity of particle 24 as it passes through intersection region 36 . photocell 32 is responsive to the forward scattered burst of optical energy to produce an electrical signal having frequency components that encompass the modulating frequency of local oscillator 15 plus and minus the largest expected doppler shift created by particle 24 . in fig3 a schematic is shown of one embodiment of circuitry within signal processor 34 . photocell 32 feeds its output to an amplifier 50 , whose output is , in turn , fed through a band pass filter 52 . in this instance , it is assumed that the local oscillator frequency is 80 mhz , thus the center frequency of filter 52 is also 80 mhz . the output from filter 52 is amplified in amplifier 54 and passed to a mixer 56 , which also has applied the 80 mhz modulating signal from local oscillator 15 . that is the same signal that is applied to bragg cell 14 . as a result , the output from mixer 56 is a base band beat frequency including the doppler tone burst envelope created when particle 24 passes through intersection region 36 . the doppler tone burst is passed through a 200 hertz high pass filter 58 that removes dc , and other slowly varying levels that result from reflections off of the walls of volume 22 and windows 26 and 28 , as well as electronic offsets and drift . the output from filter 58 is then fed to a low pass filter 60 ( 6 khz ) that sets the detection bandwidth of the system and eliminates extraneous noise outside that bandwidth . the filtered signal is then detected in detector 62 and fed , as a base band signal , to threshold detector 64 . if the base band signal &# 39 ; s instantaneous level exceeds a preset threshold within threshold detector 64 , an output appears on line 66 indicating the presence of a particle . turning to fig4 another signal processor embodiment is shown that enables reduction of envelope ripples from the doppler tone burst . in fig4 the circuit of fig3 has been augmented by adding an additional channel wherein the local oscillation signal is displaced by 90 ° from the local oscillator signal fed to mixer 56 . to create the phase shifted local oscillator signal , the output from local oscillator 15 is passed through a 90 ° phase shifter before being applied to a mixer 72 , connected in parallel to mixer 56 . the resulting phase shifted beat frequency ( the envelope shape is not phase shifted ) is applied to filters 74 , 76 and detector 78 ( identical to filters 58 , 60 and detector 62 ). the outputs of detectors 62 and 78 are combined in summer 80 and then applied to threshold detector 64 . the summing , after full wave rectification , of two identical doppler tone bursts ( with the carrier for one shifted by 90 ° from the other ), achieves a substantial reduction in doppler envelope ripple ( an rms summer can further reduce envelope ripple ). as a result , the output from threshold detector 64 exhibits more accurate detection of particles in the beam intersection region . the system of fig1 can also be operated as a homodyne system , with bragg cell 14 being replaced by a beam splitter . in this instance , the signal detected by photocell 32 is the doppler frequency created as a result of the interaction between particle 24 and a set of stationary interference fringes . the circuitry within signal processor 34 is modified by the exclusion of band pass filter 52 and mixer 56 . in other respects , the signal processor circuitry is similar to that shown in fig3 . a detection system which uses a beam splitter , in lieu of an acousto - optic modulator , does exhibit drawbacks ( e . g . sensitivity to stray light , 60 hz pick - up and microphonics ). turning to fig5 a particle detector embodiment is shown which relies upon reflected scattered light from a particle in an enclosed volume . laser 100 produces a highly polarized beam 102 that is directed at a polarizing beam splitter ( pbs ) 104 . the direction of polarization of beam 102 has a component orthogonal to the polarization angle of polarizing surface 106 so that a small portion of beam 102 is reflected towards a dielectric retro - reflector 108 . the remainder of beam 102 is unhindered by surface 106 and passes as beam 109 to quarter wave plate 110 , whose orthogonal axes are oriented at 45 ° to the angle of polarization of beam 109 . as a result , the beam exiting from quarter wave plate 110 is circularly polarized and exhibits a helicity that is either right handed or left handed . the thus circularly polarized beam passes through a lens 112 and is reflected by a pair of scanning mirrors 114 and 116 ( which create a raster scan ) to create beam 118 which enters volume 120 through window 122 . if particles within volume 120 are moving sufficiently rapidly one or both of scanning mirrors 114 and 116 can be eliminated and the beam directly introduced into the volume . if a particle 126 passes through scanned beam 118 , a doppler shift is imposed upon the reflected scattered light that is directed back in the direction shown by dotted beam 128 . due to the reflection , the direction of helicity of beam 128 is reversed from that of beam 118 . when beam 128 thus encounters quarter wave plate 110 , the resulting beam that exits therefrom ( beam 130 ) is linearly polarized , but oriented at an angle that is orthogonal to beam 109 . beam 130 is thus reflected by polarizing surface 106 in a downward direction towards a wollaston prism 132 . those skilled in the art will realize that the other polarizing beam splitters can be substituted for wollaston prism 132 . in the meanwhile , retro reflector 108 has caused the small portion of incident beam 102 that was reflected upwardly by pbs 104 to be reflected back ( anti - parallel to itself ) as beam 134 . if beam 134 had exactly the same polarization as incident beam 102 , it would be entirely reflected by polarizing surface 106 . retro reflector 108 , however , operates by forcing retro reflected beam 134 to undergo three successive total internal reflections from orthogonal surfaces . since the phase shift during internal reflection depends upon angle and polarization , the polarization of beam 134 can be adjusted over a large range by rotating retro reflector 108 about its three fold symmetry axis . this results in an adjustable fraction of retro reflected beam 134 being transmitted through polarizing surface 106 towards wollaston prism 132 . as above indicated , beam 130 exhibits one angle of linear polarization while beam 134 exhibits an orthogonal direction of polarization , both beams combining and interfering at wollaston prism 132 . in fig6 the orthogonal polarization phasors of beams 130 and 134 are illustrated . the axes of wollaston prism 132 are oriented near 45 ° with respect thereto and are designated as w y and w x , respectively . wollaston prism 132 thus directs at photo detector 140 , a beam 136 that has an instantaneous intensity that is represented by the squared modulus of the vector sum of the projections on the w x axes of the phasors representing beams 130 and 134 . similarly , wollaston prism 132 directs a beam 138 towards photo detector 142 , beam 138 having an instantaneous intensity value that varies in accordance with the projections of polarization phasors of beams 130 and 134 on the w y axis . an examination of fig6 will show that the intensities of beams 136 and 138 vary in opposite senses as a result of changes in phase and amplitude of beams 130 and 134 . thus , the projections of beams 130 and 134 onto the w x axis subtract whereas their projections onto the w y axis are additive , thereby creating opposing direction changes in intensity on photo detectors 140 and 142 . in addition to the interference signals produced by photo detector 140 and 142 , an additional signal is employed to remove laser noise from the interference signals . as above indicated , the polarization state of reflected beam 134 ( fig5 ) can be adjusted by the rotation of retro reflector 108 . by causing the angle of polarization of beam 134 to be somewhat divergent from the direction of polarization of inclined surface 106 , a portion of beam 134 is reflected towards a mirror 144 and from there , as beam 134 &# 39 ; to a third photo detector 146 . the signal from photo detector 146 , as will be hereinafter understood , is employed in signal processor 150 in a differential manner to remove laser noise from the outputs of photo detectors 140 and 142 . in fig7 a block diagram illustrates the major components that enable signal processor 150 to indicate the presence of a particle or particles within volume 120 . the back scattering system shown in fig5 can result in large doppler shifts being induced in reflected beam 128 . this is due to the fact that a particle moving radially along the direction of incident beam 118 can create a doppler shift as large as 2 mhz , at expected particle velocities . this is contrasted to doppler shifts for the system of fig1 that are in the 10 khz frequency range . this can better be understood by realizing that the doppler shift frequency is proportional to 2 sin θ s / 2 , where θ s is the angle between the incident beam and the particle - scattered beam . for a forward scatter angle of approx . 0 . 5 °, the value of the sin function is 0 , 01 and for a back scatter of 180 °, the value of the sin function is 2 . thus there is a 200x larger doppler shift for the back scatter system . as a result of the large doppler frequency range in the system of fig5 sensitivity advantages can be gained by splitting the expected doppler signal bandwidth into smaller bands , roughly the same width as the transit time bandwidth , and detecting tone bursts in each band separately . in fig7 photo detectors 140 , 142 and 146 feed their respective signals into noise cancelling circuit 152 . a portion of the current from photo detector 146 is subtracted from the difference of the currents derived from photo detectors 140 and 142 to thereby reduce the laser noise present therein . the resulting signal is then amplified in amplifier 156 and passed through filters 158 and 160 . filter 158 sets the upper bandwidth limit of the detection circuit , and filter 160 removes low frequency components from the doppler signal . this is important as the scanning of beam 118 by mirrors 114 and 116 ( fig5 ) causes considerable speckle to be fed back into the system as the result of reflections from the rear wall of volume 120 . those signals are at a low frequency ( compared with the transit - time bandwidth and doppler shifts of most particles ) and are eliminated by filter 160 . the filtered doppler signal is then broken into a plurality of frequency bands by filters 162 , 164 and 166 and 168 . the output from each filter is detected and fed to a threshold comparator in the manner shown in fig3 and 4 . thus , depending upon where in the dynamic range the doppler signal resides , one of the aforementioned filters isolates and detects the doppler tone burst envelope and feeds the detected envelope to a threshold comparator to enable the determination of the presence of a particle or particles within volume 120 . in lieu of using wollaston prism 132 to make reference beam 134 and scattered beam 130 interfere , a polarizer may be used whose axis is oriented at an angle to the polarization vectors of the two beams . referring to fig7 and 8 , a polarizer 200 is shown oriented nearly along the polarization axis of the scattered light ( see fig8 ). such an orientation allows most of the scattered light to pass through the polarizer 200 to detector 20 , minimizing the degradation in signal - to - noise ratio which results from the loss of scattered photons . this orientation causes polarizer 200 to absorb most of the light in reference beam 134 ; however , since reference beam 134 contains only a small part of the total laser power , increasing its strength to compensate for the loss in the polarizer does not appreciably decrease the sensitivity of the system . however , the increase of laser power does add expense to the system . signal processor 204 is substantially identical to that shown in fig7 except that one photodetector input to noise canceller 152 is eliminated . it will be appreciated by those skilled in the art that the particle detector of fig1 has been described in both heterodyne and homodyne versions . the particle detectors of fig5 and 8 are shown in a homodyne version ; however , they also can be operated in the heterodyne manner through the inclusion of an appropriate acousto - optic modulation means and local oscillator . the signal processor will also require demodulation circuitry to remove the modulation signal and produce intermediate frequency signals containing the desired doppler tone bursts . the systems of fig1 and 8 can also be employed to obtain an indication of particle size . when a particle crosses the interaction region in either of the embodiments , it produces a burst of scattered light at a photocell , with the photocell , in turn , producing a pulse or tone burst whose duration is the transit time of the particle . for a given particle trajectory , the amplitude of the pulse is a measure of the particle size . therefore , the particles can be sorted into size categories using pulse height analysis . the curve relating burst signal amplitude to particle size is monotonic in the case of the forward scattered light system shown in fig1 . however , for the systems of fig5 and 8 , back - scattered light exhibits resonances which render its curve less predictable . nevertheless , size binning by amplitude measurement is useful even in the systems of fig5 and 8 as long as it is remembered that there will be a few large particles which look like small particles because of resonances . fig1 is a block diagram of an alternative preferred embodiment of the invention which is a modification of the embodiment shown in fig5 with the same elements having the same reference numeral in both figures . in fig1 , laser 100 produces a beam of light 102 that is directed at beam splitter 104 . a small portion of the beam 102 is reflected from surface 106 toward a dielectric retro - reflector 108 . the remainder of the beam travels as beam 109 to half wave plate 110 &# 39 ;. the half wave plate 110 &# 39 ; changes the polarization of the beam 109 by π / 2 from vertical to horizontal or vicea versa . the beam 109 passes through lens 112 and is reflected by a pair of scanning mirrors 114 and 116 ( which create a raster scan ) to create beam 118 which enters volume 120 through window 122 . if a particle 126 passes through the scanned beam 118 , a doppler shift is impressed upon the reflected scattered light , a portion of the scattered light is directed back in the direction shown by dotted beam 128 . the scattered light 128 travels back through scanning means 114 and 116 and lens 112 to beam splitter 104 , without passing through half wave plate 110 &# 39 ;. beam 130 is reflected by surface 106 in a downward direction toward a wollaston prism 132 where the beam 130 is combined with the retro - reflected beam reflected from retro - reflector 108 . it will be apparent to those skilled in the art that other polarization analyzers can be substituted for the wollaston prism 132 so long as the two beams overlap and in the overlap region the beams are in substantially the same state of focus , have substantially the same polarization and are substantially parallel in the planes of photodetectors 140 and 142 . the remainder of the signal processing is as described in conjunction with the embodiment per fig5 . one skilled in the art will appreciate that the half wave plate 110 &# 39 ; need only be in either the path of the transmit signal 109 as shown or alternatively the plate 110 need only be in the path of the received beam 130 . the half wave plate 110 &# 39 ; must not be in the path of both beams 109 and 130 . moreover , the cube 104 may be replaced by one or two conventional beam splitters and / or plate mirrors . it is not necessary that the beam from laser 110 be a polarized beam . in an alternative embodiment , only a portion of which is shown fig1 , the internally reflective retro reflector 104 and quarter wave plate 110 are replaced by a non - polarizing beam splitter 220 having a uniform coating along surface 222 . laser 100 produces a beam of light 102 that is directed at non - polarizing beam splitter 220 . a portion of the beam is reflected at surface 222 toward reflectors 224 and 226 where the beam is reflected back to beam splitter 220 from which the beam travels to beam splitter 132 . the remainder of the beam 102 travels through the beam splitter to lens 112 . the quarter wave plate 110 is eliminated in this embodiment . the beam reflected from a particle 126 after passing through lens 112 enters beam splitter 220 . a portion of the beam 130 is reflected by uniformly coated surface 132 toward beam splitter 132 . the remainder of beam 130 passes through the beam splitter 220 to mirror 144 . the beam splitter 132 causes the two beams to overlap and in the overlap region the beams are in the same state of focus , have the same polarization and are substantially parallel in the planes of the photodetectors 140 and 142 . the beam splitter 220 can be a single uniformly coated plate which reflects a portion of an incident light beam and passes the remaining portion of light through the plate . in a modification of the embodiment described in conjunction with fig1 , in fig1 the surface 222 &# 39 ; is coated in a manner such that the reflective / transmissive properties along the surface is non - uniform . in region 230 the surface 222 &# 39 ; is less reflective so that only a small portion of the beam 102 is reflected toward reflector 224 while most of the beam 102 passes through beam splitter 220 as beam 109 . the region 232 is highly reflective so that most of the returning beam 130 is reflected at surface 222 &# 39 ; toward beam splitter 132 while only a small portion of the beam passes through beam splitter 220 toward mirror 144 . alternatively , the surface 222 &# 39 ; including region 230 and 232 of different reflective / transmissive properties can be replaced by two beam splitters with different reflective / transmissive properties or by two separate plates with different reflective / transmissive properties . also , the single beam splitter 220 can be replaced by two beam splitters having different reflective / transmissive properties . in an alternative embodiment , shown in part in fig1 , a wavefront splitting interferometer is used in which a portion of the beam 102 is totally reflected and the remainder of the beam 102 is transmitted through lens 112 , without an intervening quarter wave plate or half wave plate , into the volume 120 . after being reflected by particle 126 , and passing through lens 112 , the reflected beam from particle 126 and the totally reflected beam portion are recombined for analysis as described above . it will be apparent to those skilled in the art that after being reflected in a wavefront splitting interferometer that a portion of the light beam can be transmitted through a first optical fiber 240 and the light resulting from back scatter at a particle after passing through lens 112 can be transmitted through a second optical fiber 242 . the first optical fiber 240 and second optical fiber 242 are coupled at a fiber coupler 244 where the two light beams are combined in optical fiber 246 for further signal processing of the type described hereinabove . it will also be apparent to those skilled in the art that the optical fiber arrangement described above is also applicable for use with amplitude splitting interferometers as well as with wavefront splitting interferometers . it should be understood that the foregoing description is only illustrative of the invention . various alternatives and modifications can be devised by those skilled in the art without departing from the invention . accordingly , the present invention is intended to embrace all such alternatives , modifications and variances which fall within the scope of the appended claims .