Abstract:
Common path frequency domain optical coherence reflectometry/tomography devices include 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. The two replicas of the optical radiation outgoing from the portion of the optical fiber are then delivered to an associated sample by an optical fiber probe. The tip of the optical fiber serves as a reference reflector and also serves as a combining element that produces a combination optical radiation by combining an optical radiation returning from the associated sample with a reference optical radiation reflected from the reference reflector. The topology of the devices allows for registering a cross-polarized or a parallel-polarized component of the optical radiation reflected or backscattered from the associated sample. Having the optical path length difference for the two eigen modes of the optical radiation (which is an equivalent of an interferometer offset in previously known devices) differ from the reference offset in the devices of the present invention allows for relieving the requirements to the spectral resolution of the FD OCT engine and/or data acquisition and processing system, and substantially eliminates depth ambiguity problems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 11/456,414 which was filed on Jul. 10, 2006, which is based on and claims priority to provisional U.S. patent application Ser. No. 60/697,714, which was filed on Jul. 8, 2005. 

   BACKGROUND OF THE INVENTION 
   The present invention relates 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 (OCT) device that provides internal depth profiles and depth resolved images of samples. 
   Optical coherence reflectometry/tomography involves splitting an optical radiation into at least two portions, and directing one portion of the optical radiation toward a subject of investigation. The subject of investigation will be further referred to as a “sample”, whereas the portion of optical radiation directed toward the sample will be further referred to as a “sample portion” of optical radiation. The sample portion of optical radiation is directed toward the sample by means of a delivering device, such as an optical probe. Another portion of the optical radiation, which will be further referred to as “reference portion”, is used to provide heterodyne detection of the low intensity radiation, reflected or backscattered from the sample. 
   Typically, any optical coherence reflectometer or OCT device is specified by a longitudinal (in-depth) range of interest, whereas the longitudinal range of interest and the sample overlap, at least partially. The longitudinal range of interest includes a proximal boundary and a distal boundary, and in time domain systems is equivalent to the longitudinal scanning range. In traditional time domain optical coherence reflectometry, at every moment only a small part of the sample portion of the optical radiation, reflected or backscattered from some point located inside the boundaries of the longitudinal range of interest is utilized. In-depth profiling of the sample is provided by introducing a variable optical path length difference for the sample and reference portions of the optical radiation. 
   A well known version of time domain optical coherence reflectometry and tomography is the “common path” version, also known as autocorrelator or Fizeau interferometer based OCR/OCT. In this version, the reference and sample portions of the optical radiation do not travel along separate optical paths. Instead, a reference reflection is created in the sample optical path by introducing an optical inhomogenuity in the distal part of the delivering device, the inhomogenuity serving as a reference reflector. Resulting from that, the reference and sample portions of the optical radiation experience an axial shift only. The distance between the reference reflector and the front boundary of the longitudinal range of interest will be considered here as “reference offset”. The entire combination of the sample portion of the optical radiation and axially shifted reference portion is combined with the replica of the same combination, shifted axially, so the reference portion of one replica has a time of flight (or optical path length) matching that of the sample portion of another replica. These portions interfere in a very similar way to the traditional “separate path” time domain optical coherence reflectometry/tomography embodiments. The interference signal is formed by a secondary interferometer, the two arms of which have an optical length difference (“interferometer offset”) equal to the reference offset. By scanning an optical delay between the two replicas, a time profile of the interference signal is obtained, which represents the in-depth profile of the coherent part of the reflected sample optical radiation. The later is substantially equivalent to the profile obtained in traditional separate path embodiments. 
   Common path frequency domain reflectometry/tomography has a lot of intrinsic advantages over separate path frequency domain reflectometry/tomography. These advantages are based on the fact that reference and sample portions of the optical radiation propagate in the same optical path and therefore experience substantially identical delay, polarization distortions, optical dispersion broadening, and the like. Therefore, the interference fringes are insensitive to the majority of the probe properties, including the optical fiber probe length, dispersion properties and polarization mismatch. In separate path frequency domain reflectometry/tomography, the length and dispersion of the sampling arm should be closely matched with the reference arm and the polarization mismatch should be prevented (using PM fiber or other means) or compensated (using polarization diversity receiver or other means). 
   The optical spectrum of the combined reference and sample portions of the optical radiation, both in the separate path and the common path reflectometry and OCT designs 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 capable of being extracted from Fourier transformation of the optical spectrum of the combined optical radiation. 
   Fourier transformation of the optical spectrum of the reference and sample optical radiation combination is actually well known and has been utilized in frequency domain optical coherence reflectometry and tomography (also known as spectral domain and Fourier domain) since 1995. In frequency domain optical coherence reflectometry, the reference and sample portions of the optical radiation have a substantially similar optical path. The optical spectrum of the combined optical radiation can be registered using parallel means (such as a spectrograph) or sequential scanning means using a swept frequency optical source. 
   However, it took several years for the scientific community to realize that frequency domain optical coherence reflectometry/tomography has a fundamental, major advantage in signal-to-noise ratio (SNR) over traditional time-domain reflectometry/tomography. The frequency domain reflectometry/tomography SNR advantage can be explained by a simultaneous use of light coming back from all in-depth pixels, whereas in time domain reflectometry/tomography, only light from one in-depth pixel is used at a time and all the rest is wasted. Therefore, the SNR for frequency domain reflectometry/tomography is capable of being improved by a factor equal to the number of in-depth pixels (which for a system with moderate in-depth resolution of 15 μm and scanning depth of 2 mm will be a factor of 133). It should be also noted that this advantage increases with improving in-depth resolution for the same depth, reaching a factor of 1000 for 2 μm resolution. 
   Common path frequency domain optical coherence reflectometry and tomography are well known in the art. However, previously known devices typically employ an optical layout where reference reflection occurs in the vicinity of the sample. In these devices the combination of reference and sample reflection is directly spectrally analyzed without any additional optical processing, such as using an additional interferometer. This approach works very well if stable reference reflection can be obtained from a point axially close to the sample. Unfortunately, in many situations, and in particular, in a probe design for medical application it is very difficult or even impossible to obtain reference reflection from the vicinity of the sample and instead, reference reflection can only be obtain from a point located far from the sample. 
   A limitation to such common path frequency domain OCR/OCT systems without a secondary interferometer is the great value of required spectral resolution of the frequency domain OCR/OCT processing engine. This limitation becomes especially important in medical applications. The problem is that even for miniature optical fiber endoscopic probes known in the art that use the optical fiber tip of the optical fiber probe as a reference element, the reference offset could be as big as 10 mm, since the optical fiber probe inevitably includes a lens system in its distal part. This distance may be greater if a bigger probe with a larger field of view is required, such as for laparoscopy. It is known that the larger the in-depth distance is between the most remote points involved in the optical interference (which is the reference offset plus intended depth range), the finer the spectral resolution of the system should be, in order to resolve the highest frequency spectral fringes. 
   The later can be illustrated referring to the spectrum of two pairs of pulses with different time separation. Each pair of pulses (for OCR/OCT corresponding to a pair of reflecting surfaces separated in depth) produces interference fringes in the spectrum. The frequency of spectral fringes increases accordingly with increasing of the delay between pulses. To restore the in-depth profile, the spectral resolution of the frequency domain OCR/OCT engine should be sufficient to resolve the most frequent fringes in the optical spectrum. In spatial-temporal terminology, the effective coherence length should be sufficient to provide interference between the most distant points. Therefore, a large reference offset creates unnecessary high spectral resolution requirements for the spectrometer or unnecessary strict instantaneous line width requirements for the tunable source. It also puts an additional burden on the data acquisition and real time signal processing system, where a several times increase of data flow is required for the same image acquisition rate. Additionally, the system design would require substantial changes if another probe with different reference offset is needed. All of the described is capable of making questionable the advantage of using common path topology in a frequency domain OCR/OCT system. 
   One solution would be to add an additional interferometer in the manner known for time domain OCT/OCR systems. Unfortunately, applying frequency domain registration to earlier separate path OCR/OCT systems creates a serious problem—the “depth ambiguity problem” (also referred to as mirror artifact or depth degeneracy). The problem is well known and is associated with Fourier transformation&#39;s inability to differentiate between positive and negative depth coordinates in a case of the optical path difference for the interfering reference and sample portions of the optical radiation being reduced to zero. The same problem would arise for a common path frequency domain OCR/OCT system utilizing a secondary interferometer since in a system of this type, as discussed above, the interference signal is formed by reducing to zero the optical path difference for the interfering reference and sample portions of the two replicas of the optical radiation. There are several ways known to deal with the depth degeneracy problem, all of them being cost consuming and rather complicated for being used in a medical device. 
   Another limitation to previously known common path frequency domain reflectometry/tomography devices is that the registered interference signal is responsive only to the non-depolarized portion, or in other words, responsive only to the parallel-polarized component of the optical radiation reflected or backscattered from the associated sample. The portion of the optical radiation depolarized by the associated sample and reflected or backscattered from it (the cross-polarized component), does not produce interference fringes and is not registered. However, in many cases OCR/OCT images created from the depolarized portion of the optical radiation demonstrate enhanced contrast and could be successfully used for biomedical diagnostics. 
   As will be appreciated by those skilled in the art, the concept of “parallel-polarized” and “cross-polarized” is applied here for elliptical polarization. “Parallel-polarized” is used for components with elliptical polarizations having the same eccentricity, same orientation of the long axis (ellipse tilt angle), and same rotation direction for the electric field. “Cross-polarized” is used for components with elliptical polarizations having the same eccentricity, orthogonal orientation of the long axis, and opposite rotation direction for the electric field. As in the case of linear or circular polarization these parallel-polarized components produce strongest interference, while cross-polarized components do not interfere at all. 
   Thus, there exists a need for common path frequency domain OCR/OCT devices that use the advantages of a common path optical interferometer design together with the advantages of frequency domain registration of the optical spectrum of the combined reference and sample portions of the optical radiation, overcoming at the same time limitations of both approaches. 
   There also exists a need for common path frequency domain OCR/OCT devices that provide registration of the portion of the optical radiation depolarized by an associated sample, i.e. of the cross-polarized component of the optical radiation reflected or backscattered from an associated sample. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there are provided improved common path frequency domain OCR/OCT devices that use the advantages of a common path optical interferometer design together with the advantages of frequency domain registration of the optical spectrum of the combined reference and sample portions of the optical radiation, overcoming at the same time limitations of both approaches. 
   Further, in accordance with the present invention, there are provided common path frequency domain OCR/OCT devices that provide registration of a portion of the optical radiation depolarized by an associated sample, i.e. of a cross-polarized component of the optical radiation reflected or backscattered from an associated sample. 
   Still further, in accordance with the present invention, there are provided common path frequency domain OCR/OCT devices that provide registration of a portion of the optical radiation not depolarized by an associated sample, i.e. of a parallel-polarized component of the optical radiation reflected or backscattered from an associated sample. 
   According to one aspect of the present invention, a common path frequency domain optical coherence reflectometer is provided that includes a source of an optical radiation, a directional element, and a delivering device. The common path frequency domain optical coherence reflectometer is specified by a longitudinal range of interest having a proximal boundary and a distal boundary, and at least partially overlapping with an associated sample. The common path frequency domain optical coherence reflectometer also includes a portion of optical fiber with predetermined optical properties optically coupled with the source of optical radiation. The portion of optical fiber with predetermined optical properties is adapted for producing two eigen modes of the optical radiation propagating therethrough with a predetermined optical path length difference. 
   The directional element is optically coupled with the delivering device and with the portion of optical fiber that has predetermined optical properties. The directional element is adapted for directing two replicas of the optical radiation to the proximal part of the delivering device. The two replicas propagate with an optical path length difference generally equal to the predetermined optical path length difference for the two eigen modes of the optical radiation. The delivering device is adapted for forming and delivering an optical radiation beam to an associated sample. 
   The delivering device includes a proximal part and a distal part, wherein the distal part of the delivering device includes a reference reflector. The reference reflector is adapted for producing a combination optical radiation by combining an optical radiation returning from an associated sample with a reference optical radiation reflected from the reference reflector. The delivering device is further adapted for delivering the combination optical radiation to the directional element. The directional element is further adapted for directing the combination optical radiation to the frequency domain optoelectronic registering unit. 
   In addition, the common path frequency domain optical coherence reflectometer of the present invention includes a frequency domain optoelectronic registering unit that includes a data processing and displaying unit, and is optically coupled with the directional element. 
   The common path frequency domain optical coherence reflectometer of the present invention is specified by an optical path length difference of a first value for the optical radiation beam propagating to the reference reflector and to the proximal boundary of a longitudinal range of interest and by an optical path length difference of a second value for the optical radiation beam propagating to the reference reflector and to the distal boundary of a longitudinal range of interest. The value of the optical path length difference for the two eigen modes of the optical radiation propagating through the portion of optical fiber with predetermined optical properties is, preferably, selected from the group consisting of: less than the first value, and exceeds the second value. 
   In accordance with another aspect of the present invention, the portion of optical fiber with predetermined optical properties is a portion of polarization maintaining optical fiber. Thus, the two eigen modes produced in the portion of polarization maintaining optical fiber are cross-polarization modes. In this embodiment, two replicas of optical radiation outgoing from the portion of polarization maintaining optical fiber are cross-polarized replicas of the optical radiation. The cross-polarized replicas propagate with an optical path length difference generally equal to the predetermined optical path length difference for the two eigen polarization modes of the optical radiation. 
   In one embodiment, the common path frequency domain optical coherence reflectometer includes a polarization controller placed between the source of optical radiation and the portion of polarization maintaining optical fiber. The polarization controller is adapted for controlling a power ratio between the two eigen polarization modes of the optical radiation propagating through the portion of polarization maintaining optical fiber. 
   In another embodiment, the common path frequency domain optical coherence reflectometer includes means adapted for modifying the two replicas outgoing from the portion of polarization maintaining optical fiber such, that the two replicas entering the directional element are parallel-polarized replicas of the optical radiation. The replicas propagate with an optical path length difference generally equal to the predetermined optical path length difference for the two eigen polarization modes of the optical radiation. The means adapted for modifying the two replicas is, preferably, implemented as a suitable polarizer. In this embodiment, a polarization controller is capable of being additionally included between the source of optical radiation and the portion of polarization maintaining optical fiber. The polarization controller is adapted for controlling a power ratio between the two eigen polarization modes of the optical radiation propagating through the portion of polarization maintaining optical fiber. 
   In accordance with another aspect of the present invention, the portion of optical fiber with predetermined optical properties is a portion of two mode optical fiber. In this embodiment, the two eigen modes produced in the portion of the two mode optical fiber are parallel-polarization modes. Thus, the two replicas of optical radiation outgoing from the portion of the two mode optical fiber are parallel-polarized replicas of the optical radiation. The parallel-polarized replicas propagate with an optical path length difference generally equal to the predetermined optical path length difference for the two parallel-polarization modes of the optical radiation. 
   The delivering device is, preferably, an optical fiber probe including an optical fiber extending therethrough. The optical fiber includes a tip placed in the distal part of the optical fiber probe. The tip of the optical fiber is suitably adapted for performing a function of a reference reflector. 
   In one preferred embodiment, the source of optical radiation is tunable. In this embodiment, the frequency domain optoelectronic registering unit includes at least one photodetector connected with the data processing and displaying unit. 
   In another preferred embodiment, the source of optical radiation is a low-coherence source of optical radiation. In this embodiment, the frequency domain optoelectronic registering unit includes a spectrometer connected with the data processing and displaying unit. 
   In yet another preferred embodiment, the common path frequency domain optical coherence reflectometer further includes means adapted for changing relative positions of the optical radiation beam being delivered to an associated sample, and an associated sample. In this embodiment, the common path frequency domain optical coherence reflectometer is part of a common path frequency domain device for optical coherence tomography. 
   In accordance with a further aspect of the present invention, there is provided a common path frequency domain optical coherence tomography device, specified by a longitudinal range of interest having a proximal boundary and a distal boundary, and at least partially overlapping with an associated sample. The common path frequency domain optical coherence tomography device includes a source of optical radiation, an optical fiber probe, and a directional element. The optical fiber probe is adapted for forming and delivering an optical radiation beam to an associated sample. The optical fiber probe includes a proximal part, a distal part, and an optical fiber extending therethrough. The optical fiber includes a tip placed in the distal part of the optical fiber probe. The tip of the optical fiber is adapted for performing a function of a reference reflector. 
   The common path frequency domain optical coherence tomography device also includes a portion of optical fiber with predetermined optical properties, which is optically coupled with the source of optical radiation. The portion of optical fiber with predetermined optical properties is adapted for producing two eigen modes of the optical radiation propagating therethrough with a predetermined optical path length difference. In one embodiment, the portion of optical fiber with predetermined optical properties is a portion of polarization maintaining optical fiber, wherein the two eigen modes produced in the portion of polarization maintaining optical fiber are cross-polarization modes. In another embodiment, the portion of optical fiber with predetermined optical properties is a portion of two mode optical fiber. 
   The directional element, included in the common path frequency domain optical coherence tomography device, is adapted for directing 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 modes of the optical radiation, to the proximal part of the optical fiber probe. The directional element is optically coupled with the optical fiber probe and with the portion of optical fiber with predetermined optical properties. In addition, the common path frequency domain optical coherence tomography device includes means adapted for changing relative positions of the optical radiation beam being delivered to an associated sample, and an associated sample. Also included in the common path frequency domain optical coherence tomography device is a frequency domain optoelectronic registering unit including a data processing and displaying unit, and optically coupled with the directional element. 
   Thus, in accordance with the subject invention, unlike previously known common path frequency domain OCT/OCR devices, optical radiation from a source is first split into two replicas by 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. The two replicas of the optical radiation outgoing from the portion of the optical fiber are then delivered to an associated sample by a delivering device, the delivering device being, preferably, an optical fiber probe. The tip of the optical fiber of the optical fiber probe serves as a reference reflector and also serves as a combining element that produces a combination optical radiation by combining an optical radiation returning from the associated sample with a reference optical radiation reflected from the reference reflector. The topology of the devices of the subject invention allows for registering a cross-polarized component of the optical radiation reflected or backscattered from the associated sample, as well as a parallel-polarized component. Having the optical path length difference for the two eigen modes of the optical radiation (which is an equivalent of an interferometer offset in previously known devices) differ from the reference offset in the common path frequency domain optical coherence reflectometry and optical coherence tomography devices of the present invention allows for relieving the requirements to the spectral resolution of the FD OCT engine and/or data acquisition and processing system, and substantially eliminates depth ambiguity problems. 
   Still other objects and aspects of the present invention will become readily apparent to those skilled in this art from the following description wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of the best modes suited for to carry out the invention. As it will be realized by those skilled in the art, the invention is capable of other different embodiments and its several details are capable of modifications in various obvious aspects all without departing from the scope of the invention. Accordingly, the drawings and description will be regarded as illustrative in nature and not as restrictive. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of one preferred embodiment of the common path frequency domain optical coherence reflectometer in accordance with the subject application. 
       FIG. 2  is a block diagram of another preferred embodiment of the common path frequency domain optical coherence reflectometer in accordance with the subject application. 
       FIG. 3  is a block diagram of another preferred embodiment of the common path frequency domain optical coherence reflectometer in accordance with the subject application. 
       FIGS. 4   a  and  4   b  illustrate producing of a combination optical radiation in accordance with the subject application. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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  FIG. 1 , there is shown a block diagram of an embodiment of the common path frequency domain optical coherence reflectometer  100 . As shown in  FIG. 1 , 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  FIG. 1  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  FIG. 1  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  FIG. 1 , 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  FIG. 1 , 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  FIG. 1 , 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  FIG. 1 , 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  FIG. 2 , there is shown a block diagram of another embodiment of the common path frequency domain optical coherence reflectometer  200 . As shown in  FIG. 2 , 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  FIG. 1 . 
   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  FIG. 2  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  FIG. 2  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  FIG. 2 , 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  FIG. 2 , 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  FIG. 2 , 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  FIG. 2 , 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  FIG. 1 , 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  FIG. 1 . 
   Turning now to  FIG. 3 , there is shown a block diagram of another embodiment of the common path frequency domain optical coherence reflectometer  300 . As shown in  FIG. 3 , 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  FIG. 1 . 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  FIG. 3  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  FIG. 3 , 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  FIG. 3 , 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  FIG. 3 , 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  FIG. 3 , 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  FIG. 1 . 
   In accordance with another aspect of the invention, the embodiments of  FIG. 1 ,  FIG. 2 , and  FIG. 3  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  FIGS. 1 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  FIG. 1 , 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  FIG. 1 , 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  FIG. 1 , 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  FIG. 1 ). 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  FIG. 2 , 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  FIG. 1 , 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  FIG. 3 , 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  FIG. 1 , 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  FIG. 1 , and in  FIG. 2 , 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  FIG. 4 , there is shown an illustration  400  of producing a combination optical radiation in an embodiment of the invention depicted in  FIG. 1 . For illustration purposes the optical radiation is represented by an imaginary short pulse propagating therethrough and placed along a time axis t in  FIG. 4 . Thus,  FIG. 4   a  illustrates the optical radiation entering the optical fiber probe  118  through the directional element  104  of  FIG. 1 , 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  FIG. 4   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 . 
     FIG. 4   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  FIG. 4   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  FIG. 4   a  and  FIG. 4   b  for the embodiment of  FIG. 1 , is equally applicable to the embodiments of  FIG. 2  and  FIG. 3 . 
   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.