the subject application is directed to systems and methods for visualizing subsurface regions of samples , and more specifically , to a frequency domain optical coherence reflectometer and frequency domain optical coherence tomography device that provide internal depth profiles and depth images of samples . modifications of the common path frequency domain optical coherence reflectometer are illustrated by means of examples of optical fiber devices being part of an apparatus for optical coherence tomography , although it is evident that they may be implemented with the use of bulk optic elements , and may be used as independent devices . the optical fiber implementation is preferable for use in medical applications , especially in endoscopy , where flexibility of the optical fiber provides convenient access to different tissues and organs , including internal organs via an endoscope . turning now to fig1 , there is shown a block diagram of an embodiment of the common path frequency domain optical coherence reflectometer 100 . as shown in fig1 , the reflectometer 100 includes a source 102 of optical radiation , and a directional element 104 . in a preferred embodiment , the source 102 operates in the visible or near ir range . a skilled artisan will appreciate that the source 102 is , for example , and without limitation , a semiconductor superluminescent diode , doped - fiber amplified spontaneous emission superlum , solid state and fiberoptic femtosecond laser . a skilled artisan will also appreciate that directional element 104 is capable of being implemented as any suitable directional element known in the art . the reflectometer 100 is specified by a longitudinal range of interest 106 at least partially overlapping with an associated sample 108 . the longitudinal range of interest 106 has a proximal boundary 110 and a distal boundary 112 . the common path frequency domain optical coherence reflectometer 100 also includes a portion of optical fiber with predetermined optical properties adapted for producing two eigen modes of the optical radiation propagating therethrough with a predetermined optical path length difference . this portion of optical fiber is illustrated in fig1 as a portion of polarization maintaining optical fiber 114 . the polarization maintaining optical fiber 114 is optically coupled with the source 102 of optical radiation . the embodiment of the common path frequency domain optical coherence reflectometer 100 of fig1 includes a polarization controller 116 placed between the source 102 of optical radiation and the polarization maintaining optical fiber 114 . the polarization controller 116 is adapted for controlling a power ratio between the two eigen polarization modes of the optical radiation propagating through the polarization maintaining optical fiber 114 . as will be understood by a skilled artisan , the polarization controller 116 is capable of being implemented as any suitable polarization controller known in the art . the common path frequency domain optical coherence reflectometer 100 also includes a delivering device adapted for forming and delivering an optical radiation beam to an associated sample 108 . in the embodiment of fig1 , the delivering device is implemented as an optical fiber probe 118 that includes a proximal part 120 , a distal part 122 , and an optical fiber 124 extending therethrough . the optical fiber 124 includes a tip 126 placed in the distal part 122 of the optical fiber probe 118 . the tip 126 of the optical fiber 124 is adapted for performing a function of a reference reflector . the directional element 104 is optically coupled with the optical fiber probe 118 and with the polarization maintaining optical fiber 114 . in the embodiment illustrated in fig1 , the directional element 104 is optically coupled with the optical fiber probe 118 through an optical fiber 128 , and is coupled with the polarization maintaining optical fiber 114 through an optical fiber 130 . in the embodiment of fig1 , the directional element 104 is adapted for directing two replicas of the optical radiation to the proximal part of the optical fiber probe 118 . the common path frequency domain optical coherence reflectometer 100 of fig1 , is specified by an optical path length difference of a first value for the optical radiation beam propagating to the tip 126 of the optical fiber 124 and to the proximal boundary 110 of a longitudinal range of interest 106 , and by an optical path length difference of a second value for the optical radiation beam propagating to the tip 126 of the optical fiber 124 and to the distal boundary 112 of a longitudinal range of interest 106 . those skilled in the art will recognize that the above mentioned predetermined optical properties of the polarization maintaining optical fiber 114 are chosen such , that the value of the optical path length difference for the two eigen modes of the optical radiation propagating through the polarization maintaining optical fiber 114 is , preferably , one of the following : less than the first value , and exceeds the second value . the common path frequency domain optical coherence reflectometer 100 further includes a frequency domain optoelectronic registering unit 132 optically coupled with the directional element 104 . the frequency domain optoelectronic registering unit 132 includes a data processing and displaying unit ( not shown in the drawing ). a skilled artisan will appreciate that the frequency domain optoelectronic registering unit 132 is capable of being implemented as any suitable registering unit known in the art . turning now to fig2 , there is shown a block diagram of another embodiment of the common path frequency domain optical coherence reflectometer 200 . as shown in fig2 , the reflectometer 200 includes a source 202 of optical radiation , and a directional element 204 . a skilled artisan will appreciate that the source 202 and the directional element 204 are capable of being implemented analogous to respective elements referred to in the description of the embodiment shown in fig1 . the reflectometer 200 is specified by a longitudinal range of interest 206 at least partially overlapping with an associated sample 208 . the longitudinal range of interest 206 has a proximal boundary 210 and a distal boundary 212 . the common path frequency domain optical coherence reflectometer 200 also includes a portion of optical fiber with predetermined optical properties adapted for producing two eigen modes of the optical radiation propagating therethrough with a predetermined optical path length difference . this portion of optical fiber is illustrated in fig2 as a portion of polarization maintaining optical fiber 214 . the polarization maintaining optical fiber 214 is optically coupled with the source 202 of optical radiation . the embodiment of the common path frequency domain optical coherence reflectometer 200 of fig2 includes means adapted for modifying the two replicas outgoing from the portion of polarization maintaining optical fiber 214 such , that the two replicas entering the directional element 204 are parallel - polarized replicas of the optical radiation . in the embodiment illustrated in fig2 , the means for modifying the two replicas is implemented as a polarizer 216 placed between the polarization maintaining optical fiber 214 and the directional element 204 . the polarizer 216 is capable of being further adapted for controlling a power ratio between the two replicas of optical radiation . alternatively , a polarization controller is capable of being additionally placed between the source of optical radiation 202 and the polarization maintaining fiber 214 ( not shown in the drawing ) for controlling a power ratio between the two replicas of optical radiation . as will be understood by a skilled artisan , the polarizer 216 is capable of being implemented as any suitable polarization controller known in the art . preferably , the polarizer 216 is implemented as a 45 degree polarizer . the common path frequency domain optical coherence reflectometer 200 also includes a delivering device adapted for forming and delivering an optical radiation beam to an associated sample 208 . in the embodiment of fig2 , the delivering device is implemented as an optical fiber probe 218 that includes a proximal part 220 , a distal part 222 , and an optical fiber 224 extending therethrough . the optical fiber 224 includes a tip 226 placed in the distal part 222 of the optical fiber probe 218 . the tip 226 of the optical fiber 224 is adapted for performing a function of a reference reflector . the directional element 204 is optically coupled with the optical fiber probe 218 and with the polarization maintaining optical fiber 214 . in the embodiment illustrated in fig2 , the directional element 204 is optically coupled with the optical fiber probe 218 through an optical fiber 228 , and is coupled with the polarization maintaining optical fiber 214 through an optical fiber 230 and the polarizer 216 . in the embodiment of fig2 , the directional element 204 is adapted for directing two replicas of the optical radiation to the proximal part of the optical fiber probe 218 . the common path frequency domain optical coherence reflectometer 200 of fig1 , is specified by an optical path length difference of a first value for the optical radiation beam propagating to the tip 226 of the optical fiber 224 and to the proximal boundary 210 of a longitudinal range of interest 206 , and by an optical path length difference of a second value for the optical radiation beam propagating to the tip 226 of the optical fiber 224 and to the distal boundary 212 of a longitudinal range of interest 206 . those skilled in the art will recognize that the above mentioned predetermined optical properties of the polarization maintaining optical fiber 214 are chosen such , that the value of the optical path length difference for the two eigen modes of the optical radiation propagating through the polarization maintaining optical fiber 214 is , preferably , one of the following : less than the first value , and exceeds the second value . the common path frequency domain optical coherence reflectometer 200 further includes a frequency domain optoelectronic registering unit 232 optically coupled with the directional element 204 . the frequency domain optoelectronic registering unit 232 includes a data processing and displaying unit ( not shown in the drawing ). a skilled artisan will appreciate that the frequency domain optoelectronic registering unit 232 is capable of being implemented analogous to the frequency domain optoelectronic registering unit 132 of the embodiment illustrated in fig1 . turning now to fig3 , there is shown a block diagram of another embodiment of the common path frequency domain optical coherence reflectometer 300 . as shown in fig3 , the reflectometer 300 includes a source 302 of optical radiation , and a directional element 304 . a skilled artisan will appreciate that the source 302 and the directional element 304 are capable of being implemented analogous to the source 102 and directional element 104 , respectively , of the embodiment shown in fig1 . the reflectometer 300 is specified by a longitudinal range of interest 306 at least partially overlapping with an associated sample 308 . the longitudinal range of interest 306 has a proximal boundary 310 and a distal boundary 312 . the common path frequency domain optical coherence reflectometer 300 also includes a portion of optical fiber with predetermined optical properties adapted for producing two eigen modes of the optical radiation propagating therethrough with a predetermined optical path length difference . this portion of optical fiber is illustrated in fig3 as a portion of two - mode optical fiber 314 . thus , the two eigen modes produced by the two - mode optical fiber 314 are parallel - polarization modes . the two - mode optical fiber 314 is optically coupled with the source 302 of optical radiation . the common path frequency domain optical coherence reflectometer 300 also includes a delivering device adapted for forming and delivering an optical radiation beam to an associated sample 308 . in the embodiment of fig3 , the delivering device is implemented as an optical fiber probe 316 that includes a proximal part 318 , a distal part 320 , and an optical fiber 322 extending therethrough . the optical fiber 322 includes a tip 324 placed in the distal part 320 of the optical fiber probe 316 . the tip 324 of the optical fiber 322 is adapted for performing a function of a reference reflector . the directional element 304 is optically coupled with the optical fiber probe 316 and with the two - mode optical fiber 314 . in the embodiment illustrated in fig3 , the directional element 304 is optically coupled with the optical fiber probe 316 through an optical fiber 326 , and is coupled with the two - mode optical fiber 314 through an optical fiber 328 . in the embodiment of fig3 , the directional element 204 is adapted for directing two replicas of the optical radiation to the proximal part of the optical fiber probe 316 . the common path frequency domain optical coherence reflectometer 300 of fig3 , is specified by an optical path length difference of a first value for the optical radiation beam propagating to the tip 324 of the optical fiber 322 and to the proximal boundary 310 of a longitudinal range of interest 306 , and by an optical path length difference of a second value for the optical radiation beam propagating to the tip 324 of the optical fiber 322 and to the distal boundary 312 of a longitudinal range of interest 306 . those skilled in the art will recognize , that the above mentioned predetermined optical properties of the two - mode optical fiber 314 are chosen such , that the value of the optical path length difference for the two eigen modes of the optical radiation propagating through the two - mode optical fiber 314 is , preferably , one of the following : less than the first value , and exceeds the second value . the common path frequency domain optical coherence reflectometer 300 further includes a frequency domain optoelectronic registering unit 330 optically coupled with the directional element 304 . the frequency domain optoelectronic registering unit 330 includes a data processing and displaying unit ( not shown in the drawing ). a skilled artisan will appreciate that the frequency domain optoelectronic registering unit 330 is capable of being implemented analogous to the frequency domain optoelectronic registering unit 132 of the embodiment illustrated in fig1 . in accordance with another aspect of the invention , the embodiments of fig1 , fig2 , and fig3 are capable of further including means for changing relative positions of the optical radiation beam being delivered to an associated sample , and the associated sample ( not shown in the drawing ). in this embodiment , the common path frequency domain optical coherence reflectometers illustrated in fig1 through 3 each are part of a common path frequency domain device for optical coherence tomography . those skilled in the art will recognize , that in these devices the means for changing relative positions of the optical radiation beam being delivered to the associated sample , and the associated sample is suitably capable of being implemented in any way known in the art , for example and without limitation , as a lateral scanner incorporated into the delivering device , or as an element for changing the position of the associated sample . referring now to operation of the common path frequency domain optical coherence reflectometer 100 in accordance with the present invention shown in fig1 , the operation of the reflectometer 100 commences by placing the delivering device , preferably implemented as the optical fiber probe 118 , at a predetermined position with respect to the sample 108 . depending basically on the tasks performed , the optical fiber probe 118 is placed in the vicinity of the sample 108 , in contact with the sample 108 , or at a predetermined distance from the sample 108 . in all cases , there is a distance between the tip 126 of the optical fiber 124 , the tip 126 serving as a reference reflector , and the proximal boundary 110 of the longitudinal range of interest 106 , which is specified as an optical path length of a first value ( reference offset ). the distance between the tip 126 of the optical fiber 124 and the distal boundary 112 of the longitudinal range of interest 106 , will be referred to here as an optical path length of a second value . hence , in the preferred embodiment the tip 126 of the optical fiber 124 is positioned at a distance having a first optical length value from the proximal boundary 110 of the longitudinal range of interest 106 ( reference offset ), or , in other words , having a second optical length value from the distal boundary 112 of the longitudinal range of interest 106 . next , an optical radiation from the source 102 is directed to the polarization maintaining optical fiber 114 . those skilled in the art will recognize that the polarization maintaining optical fiber 114 produces two eigen modes of the optical radiation propagating therethrough , which are cross - polarization modes of the optical radiation . as will be appreciated by a skilled artisan , the cross - polarization modes of the optical radiation experience an optical path length difference , which is defined by the optical properties of the polarization maintaining optical fiber 114 . in one preferred embodiment , the value of this optical path length difference is less than the first optical path length value between the tip 126 of the optical fiber 124 and the proximal boundary 110 of the longitudinal range of interest 106 ( reference offset ). in another preferred embodiment , the value of this optical path length difference exceeds the second optical path length value between the tip 126 of the optical fiber 124 and the distal boundary 112 of the longitudinal range of interest 106 . those skilled in the art will appreciate that the optical path length difference for the cross - polarization modes of the optical radiation is equivalent to an interferometer offset in a common path frequency domain reflectometer with a secondary interferometer . this optical path length difference is suitably adjusted in the process of manufacturing and assembling . a typical length range for the polarization maintaining optical fiber 114 is capable of being from several meters to several tens of meters . as will be recognized by those skilled in the art , the value of the optical path length difference for the cross - polarization modes of the optical radiation propagating in the polarization maintaining optical fiber 114 , being less than the reference offset , or exceeding the distance from the tip 126 of the optical fiber 124 to the distal boundary 112 of the longitudinal range of interest 106 , nonetheless stays in the vicinity of the value of the reference offset . thus , outgoing from the polarization maintaining optical fiber 114 are two replicas of the optical radiation propagating with an optical path length difference generally equal to the predetermined optical path length difference for the two eigen cross - polarization modes of the optical radiation . in the embodiment illustrated in fig1 , the two replicas enter the optical fiber probe 118 through the common optical fiber 130 , the directional element 104 , and the common optical fiber 128 . the optical fiber probe 118 is adapted for forming and delivering an optical radiation beam to the associated sample 108 . thus , one part of a portion of the optical radiation beam corresponding to each replica is delivered to the associated sample 108 and is reflected or backscattered from it ( the sample portion ). another part of each portion of the optical radiation that enters the optical fiber probe 118 does not reach the associated sample 108 , but is instead reflected at the tip 126 of optical fiber 124 of the optical fiber probe 118 , at some distance from the associated sample 108 ( the reference portion ). those skilled in the art will appreciate that due to the mentioned above relationship between the reference offset and the optical path length difference between the two replicas , the tip 126 of optical fiber 124 produces a combination optical radiation in a manner similar to that of the directional coupler in a previously known common path optical coherence reflectometer with a secondary interferometer . the tip 126 of optical fiber 124 combines an optical radiation returning from the associated sample 108 of one replica of optical radiation with a reference optical radiation being reflected from the tip 126 of the other replica . the combination optical radiation returning from the optical fiber probe 118 is directed to a frequency domain optoelectronic registering unit 132 including a data processing and displaying unit ( not shown in the drawing ) by the directional element 104 . the combination optical radiation is registered by the frequency domain optoelectronic registering unit 132 . the optical spectrum of the combination optical radiation registered by the frequency domain optoelectronic registering unit 132 , has all necessary information about the in - depth coherent reflection profile by including a component that is fourier conjugate of the in - depth profile of the sample . thus , the profile is extracted from fourier transformation of the optical spectrum of the combined optical radiation by the data processing and displaying unit of the frequency domain optoelectronic registering unit 132 . no depth ambiguity problem arises since the optical path difference for the interfering reference and any part of sample portion belonging to the longitudinal range of interest of the two replicas of the optical radiation is not reduced to zero . as will be appreciated by those skilled in the art , since the two replicas of the optical radiation outgoing from the polarization maintaining optical fiber 114 are cross - polarized replicas , the frequency domain optoelectronic registering unit 132 registers a combination optical radiation responsive only to a portion of the reflected or backscattered optical radiation that is depolarized by the associated sample 108 . the non - depolarized portion of the optical radiation reflected from the associated sample 108 does not produce interference fringes and is not registered . in a preferred embodiment illustrated in fig1 , the polarization controller 116 controls a power ratio between the two eigen polarization modes of the optical radiation propagating through the portion of polarization maintaining optical fiber 114 , and , hence between the two replicas of the optical radiation . typically , a ratio of 1 : 1 is considered desirable . in one embodiment , the source 102 of optical radiation is narrowband and tunable , whereas the frequency domain optoelectronic registering unit 132 includes at least one photodetector connected with the data processing and displaying unit ( not shown in fig1 ). in another embodiment the source 102 is broadband and implemented as a low - coherence source of optical radiation . in this embodiment a spectrometer instead of a single photodiode is used in the frequency domain optoelectronic registering unit 132 , therefore parallel registration is performed instead of sequential . referring now to operation of the common path frequency domain optical coherence reflectometer 200 in accordance with the present invention shown in fig2 , those skilled in the art will recognize , that the operation of the reflectometer 200 proceeds , essentially , in the same manner as the operation of the reflectometer 100 depicted in fig1 , as described in detail above . however , the two replicas outgoing from the portion of polarization maintaining optical fiber 214 are modified by the polarizer 216 such , that the two replicas entering the directional element 204 are parallel - polarized replicas of the optical radiation . the two parallel - polarized replicas propagate further with an optical path length difference generally equal to the predetermined optical path length difference for the two eigen cross - polarization modes of the optical radiation . in this embodiment , the frequency domain optoelectronic registering unit 232 registers a combination optical radiation responsive to a portion of the reflected or backscattered optical radiation that is not depolarized by the associated sample 208 . the depolarized portion of the optical radiation reflected or backscattered from the associated sample 208 does not produce interference fringes and is not registered . as will be appreciated by those skilled in the art , the own axis of the polarizer 216 is capable of being oriented such , as to provide a desired power ratio between the two replicas of optical radiation . alternatively , a desired power ratio between the two replicas of optical radiation is capable of being provided by suitably controlling a power ratio between the two eigen cross - polarization modes of the optical radiation propagating through the polarization maintaining optical fiber 214 . the latter is achieved using a suitable polarization controller placed between the source of optical radiation 202 and the polarization maintaining fiber 214 ( not shown in the drawing ). referring now to operation of the common path frequency domain optical coherence reflectometer 300 in accordance with the present invention shown in fig3 , those skilled in the art will recognize , that the operation of the reflectometer 300 proceeds , essentially , in the same manner as the operation of the reflectometer 100 depicted in fig1 , as described in detail above . as will be appreciated by a skilled artsan , the two - mode optical fiber 314 produces two eigen modes of the optical radiation propagating therethrough , which are parrallel - polarization modes of the optical radiation . analogous to that described with reference to the common path frequency domain optical coherence reflectometer depicted in fig1 , and in fig2 , the optical spectrum of the combination optical radiation registered by the frequency domain optoelectronic registering unit 330 , has all necessary information about the in - depth coherent reflection profile by including a component that is fourier conjugate of the in - depth profile of the sample . thus , the profile is extracted from fourier transformation of the optical spectrum of the combined optical radiation by the data processing and displaying unit of the frequency domain optoelectronic registering unit 330 . no depth ambiguity problem arises since the optical path difference for the interfering reference and any part of sample portion belonging to the longitudinal range of interest of the two replicas of the optical radiation is not reduced to zero . in this embodiment , the frequency domain optoelectronic registering unit 330 registers a combination optical radiation responsive to a portion of the reflected or backscattered optical radiation that is not depolarized by the associated sample 308 . the depolarized portion of the optical radiation reflected or backscattered from the associated sample 208 does not produce interference fringes and is not registered . turning now to fig4 , there is shown an illustration 400 of producing a combination optical radiation in an embodiment of the invention depicted in fig1 . for illustration purposes the optical radiation is represented by an imaginary short pulse propagating therethrough and placed along a time axis t in fig4 . thus , fig4 a illustrates the optical radiation entering the optical fiber probe 118 through the directional element 104 of fig1 , after the optical radiation is divided into two replicas shifted along the time axis by the polarization maintaining fiber 114 . the two replicas are illustrated in fig4 a as respective short pulses 402 and 404 . as will be recognized by a skilled artisan , the time shift between the two replicas of the optical radiation is defined by the optical properties of the polarization maintaining fiber 114 . fig4 b illustrates the two replicas after each of them was split into two portions ( a reference portion and a sample portion ) by the tip 126 of the optical fiber 124 of the optical fiber probe 118 . as shown in fig4 b , the reference portion 408 of the first replica has a shift ( reference offset 416 ) with respect to the sample portion 410 of the same replica . also , the reference portion 412 of the first replica has a shift ( reference offset 416 ) with respect to the sample portion 414 of the same replica . those skilled in the art will appreciate that reference portion of one replica interferes with the sample portion of the other replica . a skilled artisan will understand , that the illustration provided in fig4 a and fig4 b for the embodiment of fig1 , is equally applicable to the embodiments of fig2 and fig3 . the foregoing description of the preferred embodiments of the subject application has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the subject application to the precise form disclosed . obvious modifications or variations are possible in light of the above teachings . the embodiments were chosen and described to provide the best illustration of the principles of the subject application and its practical application to thereby enable one of ordinary skill in the art to use the subject application in various embodiments and with various modifications as are suited to the particular use contemplated . all such modifications and variations are within the scope of the subject application as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly , legally and equitably entitled .