the invention uses a multiplexed optical coherence domain reflectometer in a catheter or endoscope or other tubular inspection device for guidance and for optical sensing of in vivo cavity structures during minimally invasive medical procedures or for similar exploration of nonmedical systems . the catheter / device guidance and optical sensing s system 40 is illustrated in fig2 a . the device is based on an optical coherence domain reflectometer ( ocdr ) which has been multiplexed . except for the multiplexed feature , the system is similar to the prior art system 10 of fig1 . output from a low coherence light source 12 is split at the 2 × 2 fiber optic coupler 14 and directed through a multiplexed sample arm 42 toward the sample 24 and through a reference arm 18 to reference mirror 26 . reflections from the mirror 26 and backscattered light from the sample 24 are recombined at the coupler 14 and propagated to the detector 30 ( and light source 12 ). constructive interference creates a signal at the detector 30 when the sample and reference reflections have traveled approximately the same optical group delay . the shorter the coherence length of the source , the more closely the sample and reference arm group delays must be matched for constructive interference to occur . by imposing a changing optical delay in the reference arm 18 with a known velocity , either by scanning mirror 26 in the z - direction or with a piezomodulator 28 ( with fixed mirror 26 ), the amplitudes and longitudinal positions of reflections from the sample 24 can be measured with high precision . the sample arm 42 contains a multiplexer 44 for switching between several ( e . g ., 8 ) optical fibers 20 - 1 . . . 20 - 8 , allowing sequential spatially distinct regions to be diagnosed consecutively using the same basic ocdr system . the fibers can be placed anywhere in the device 22 . an alternate embodiment , catheter optical sensing system 50 , is shown in fig2 b . catheter sensing system 50 is similar to catheter sensing system 40 of fig2 a , except that an optical circulator 52 is added to the system and detector 30 is replaced by balanced detector unit 54 . balanced detector unit 54 includes a pair of detectors 56 , 58 with associated processing electronics and produces a backscatter profile on display 32 . ocdr / oct systems are based on white light michelson interferometers in which light from a source is split via a beamsplitter into two arms , a reference arm and a sample arm . light is then reflected back to the beamsplitter in both arms . the light returning to the beamsplitter is then split , half returning to the source and the rest going to a detector . the light returning to the source is wasted and can cause the source to lase , reducing the bandwidth of the source . the optical circulator 52 has three ports , as shown in fig2 b . the first port is connected to the output of source 12 and the second port is connected to coupler 14 . thus light from source 12 passes through optical circulator 52 to coupler 14 and into reference arm 18 and multiplexed sample arm 42 , as before . in system 40 of fig2 a , the light returning to coupler 14 from reference and sample arms 18 , 42 would be split , with some going to detector 30 , where useful information is obtained , and some going back to source 12 . in system 50 of fig2 b , some of the light passing back through coupler 14 goes to detector unit 54 and some goes back to the second port of optical circulator 52 . but light returning to the second port of optical circulator 52 cannot pass back through the first port to source 12 . instead , the light passes through the third port to detector unit 54 . thus putting an optical circulator 52 in the source arm between source 12 and coupler 14 allows the light that would have returned to the source 12 to be sent to another detector . detector unit 54 contains a pair of balanced detectors 56 , 58 . detector 58 receives the light which passes directly from coupler 14 while detector 56 receives the light which passes back through optical circulator 52 . thus detector unit 54 can utilize all the reflected light . in the balanced detection scheme , the signal on the second detector is subtracted from the first . the signal caused by heterodyning between light in the reference and sample arms is 180 degrees out of phase on the two detectors . the use of optical circulator 52 provides three benefits : ( 1 ) it protects source 12 from optical back reflections which can cause it to lase ; ( 2 ) it allows detector unit 54 to collect twice as much light , enhancing system sensitivity ; ( 3 ) balanced detection is achieved by subtracting the signal on one detector from the other which eliminates source or ring noise as fluctuations in source intensity appear equally on both detectors and thus cancel when the two signals are subtracted . another embodiment , catheter optical sensing system 49 , is shown in fig2 c . catheter sensing system 49 is similar to catheter sensing system 40 of fig2 a , except that the polarization of the light through the system is controlled by polarization maintaining ( pm ) fibers and optics . mismatches between the polarization states of the light returning from the reference and sample arms 42 , 18 in system 40 causes reduction in the coherent interference between light from the two arms and thus losses of signal . control of the polarization state of the light in the system can both eliminate losses in signal due to depolarization of the light and provide the additional capability of measurement of the birefringence of the sample 24 . in this embodiment , linearly polarized light is introduced into the system either through use of a linearly polarized broadband light source 12 or by placing linear polarizer 51 directly after an unpolarized source 12 . the linear polarization of the light is then maintained through the use of pm fibers and a pm fiber optic coupler 14 where the linear polarization is one of the two modes of the pm fiber and pm coupler 14 . the polarization state of the light returning from the reference arm 18 is modified by either a waveplate or faraday rotator 53 so as to be equally split between the two modes ( orthogonal polarizations ) of the pm fiber . a polarization beam splitter 55 in the detector arm splits the two polarizations and directs them to two separate detectors 57 , 59 of detector unit 54 . in one embodiment , the optical fibers 20 - 1 . . . 20 - 8 in the sample arm 42 are not polarization maintaining . in this case , the polarization beam splitter 55 ensures that the polarization state of the light from the reference and sample arms 42 , 18 is matched on each detector 57 , 59 , thus eliminating the losses due to depolarization of the light . the light returning from the sample arm 42 is then measured by summing the signals from the two detectors 57 , 59 . in another embodiment , the optical fibers 20 - 1 . . . 20 - 8 in the sample arm 42 are polarization maintaining . the fibers 20 - 1 . . . 20 - 8 can be oriented such that the light leaving the fibers is linearly polarized at an angle approximately 45 ° relative to the fast axis of birefringence of the sample 24 . alternatively a quarter waveplate 85 ( shown in fig4 b ) can be placed at the distal end of each fiber 20 - 1 . . . 20 - 8 to cause the light entering the sample to be circularly polarized . in either case , the total light in all polarization states returning from the sample 24 is once again determined by summing the signal from the two detectors 57 , 59 . in addition , detector unit 54 includes means for ratioing the output signals from detectors 57 , 59 ; the birefringence of the sample 24 is determined based on the arc tangent of the ratio of the signals from the two detectors 57 , 59 . as previously described , a variable optical delay can be produced in reference arm 18 by scanning reference mirror 26 back and forth in the z - direction ( see fig1 a - b ). however , there are two key issues in varying the axial length of the reference arm : linearity of the axial scan and duty cycle . a rotating helix reference mirror 60 , shown in fig3 a , b , can be used to smoothly vary the path length in the reference arm of the ocdr system . mirror 60 is formed of a disk 62 with a radius r which varies from r 1 to r 2 over its entire circumference . lateral edge surface 64 of disk 62 is a highly reflective mirror so that a collimated light beam 66 incident thereon at normal incidence will be reflected back . collimated light beam 66 is formed by collimating the diverging light from optical fiber 70 , which forms the reference arm of the ocdr system . lens 68 is used to collimate the output of fiber 70 . when the beam is reflected back by surface 64 , lens 68 focuses the light back into fiber 70 . when mirror 60 is positioned so that beam 66 is incident on point 72 , at which the radius r = r 2 , the longest radius , the path length δz between lens 68 and surface 64 is the shortest . as mirror 60 is rotated about shaft 74 , which fits into central opening 76 and is turned by motor 78 , the path length δz increases as r decreases . as mirror 60 completes an entire 360 degree revolution , r = r1 , the shortest radius , is reached and δz has increased by δr = r2 − r1 . beam 66 then returns to point 72 and starts a new cycle . in each cycle , the path length δz changes by δr , or the optical path length change in the reference arm δl changes by 2δz = 2δr = 2 ( r2 − r1 ). disk 62 can typically be about 2 inches in diameter and 0 . 2 inches thick , with a δr of about 0 . 2 inches . thus the optical path length will be varied by about half an inch on each cycle . as shown in fig4 a , b , the fibers 82 are embedded in plastic cover or catheter wall 84 around the circumference of the catheter or device 80 to maximize available space for other devices . the number of fibers 82 surrounding the core 86 is dependent on the limit of the device size , the fiber optic diameter , the desired speed of acquisition , and the necessary radial resolution . either single or multiple mode optical fibers can be used . single mode fibers are preferable for maximizing the longitudinal resolution . however , multimode fibers can be made smaller , thus maximizing radial resolution and catheter flexibility . average sizes for single mode fibers are on the order of 100 μm diameter , while an average catheter is 1 to 3 mm in diameter . thus , although eight fibers are shown in fig4 a , a maximum of about 30 to 100 single mode fibers could be used . miniature optics 88 , e . g . grin lenses 90 and mirrored corner cubes 92 , as shown at the top of fig4 b , can be used for collimating and directing the light emerging from the fiber tips onto the arterial or cavity wall . the optical elements 88 extend through cover 84 , or cover 84 is optically transparent to allow light to be transmitted to and received from the surrounding area . miniature optics 88 can be eliminated and just the bare fiber tip can be used , as shown at the bottom of fig4 b ; also different combinations of optical elements , e . g . grin lens 90 without corner cube 92 or corner cube 92 without grin lens 90 , can also be used . thus with different optical arrangements , foreward and / or side viewing can be obtained . the scan data can be displayed , as shown in fig5 as a radial pie slice 96 for each fiber containing either a single line of data , or multiple adjoining lines portraying a history of the data collected by the fiber . each segment 96 is the scan obtained by one of the side viewing fibers , which have been multiplexed to produce a 360 degree view . the boundaries 98 represent the artery walls . since there are only a discrete number of fibers and - sectors 96 , there are some discontinuities in the boundaries 98 . however , boundary 100 is clearly much farther away and represents a junction with a secondary artery . an inflatable balloon catheter device 110 comprising a catheter tube 112 having an inflatable balloon 114 attached thereto is shown in fig6 . optical fibers 116 are mounted on ( as shown at top of fig6 ) or embedded in ( as shown at bottom of fig6 ) the balloon 114 . additional fibers 118 may be mounted on ( as shown at top of fig6 ) or embedded in ( as shown at bottom of fig6 ) the catheter tube 112 inside balloon 114 . by including miniature optics 120 , e . g . grin lens 122 and corner cube 124 , at the ends of fibers 116 , 118 , the fibers can be side viewing . thus fibers 116 can be used to detect the arterial wall 126 while the internal fibers 118 can be used to detect the balloon 114 . a catheter device 130 as shown in fig7 may have a plurality of fibers 132 - 1 . . . 132 - 6 mounted on ( or embedded in ) catheter tube 134 with individual fibers extending to different lengths along the tube 134 . each fiber may terminate in optical elements 136 , e . g . grin lens 138 and corner cube 140 , for side viewing , or some of the fibers can be forward viewing . thus features found a different locations along the length of the catheter can be viewed without moving the catheter . applications for the invention include any method or procedure where accurate catheter or device positioning is beneficial , including angioplasty , stroke treatment , aneurysm , arteriovenous malformations , ophthalmic surgery , laparoscopic surgery , arthroscopic surgery , treatment of colorectal disorders , sinus disorders , ear surgery , pneumothoracic surgery , spinal surgery , bladder surgery , esophageal surgery , uteral disorders , essentially any treatment that requires accurate information about tissue structures while using a catheter or other tool inside a body cavity . in addition to medical applications , the invention can be used for non - medical instruments which can be used to inspect and probe in situ locations . changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention , which is intended to be limited only by the scope of the appended claims .