Abstract:
Optical coherence tomography (OCT) probe and system designs are disclosed that minimize the effects of mechanical movement and strain to the probe to the OCT analysis. It also concerns optical designs that are robust against noise from the OCT laser source. Also integrated OCT system-probes are included that yield compact and robust electro-opto-mechanical systems along with polarization sensitive OCT systems.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. application Ser. No. 13/568,717, filed on Aug. 7, 2012, which is a Divisional of U.S. application Ser. No. 12/466,993, filed on May 15, 2009, which claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/053,241, filed on May 15, 2008, all of which are incorporated herein by reference in their entirety. 
         [0002]    This application is related to U.S. Continuation application Ser. No. 13/568,507, filed on Aug. 7, 2012, by Bartley C. Johnson el al., entitled “OCT Combining Probes and Integrated Systems,” now U.S. Patent Publication No. US 2012/0300215 A1, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of an object of interest. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging that can provide images of objects such as biological tissue structures, for example, on the microscopic scales in real time. Optical waves are sent through an object and a computer produces images of cross sections of the object by using information on how the waves are changed. 
         [0004]    The original OCT imaging technique is the time-domain OCT (TD-OCT), which uses a movable reference mirror in a Michelson interferometer arrangement. Another type of optical coherence analysis is termed Fourier domain OCT (FD-OCT). Other terms are time encoded Frequency Domain OCT and swept source OCT. These techniques use either a wavelength swept source and a single detector, sometimes referred to as time-encoded ED-OCT or TEFD-OCT, or a broadband source and spectrally resolving detector system, sometimes referred to spectrum-encoded FD-OCT or SEFD-OCT. FD-OCT has advantages over time domain OCT (TD-OCT) in speed and signal-to-noise ratio (SNR). 
         [0005]    TEFD-OCT has advantages over SEFD-OCT in some respects. The spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. 
         [0006]    Probe design is an important aspect of OCT system design, especially on systems that are intended to analyze the human body, such as medical diagnostic systems. On one hand, the probes must be mechanically robust to withstand use and possibly repeated use by medical care delivery personnel such as doctors, nurses and technicians in clinical settings. The probes should also be robust against noise generated from the use in their intended application. For example, OCT probe systems for intravascular analysis applications are typically long, extending from at least the point of access, such as the femoral artery to the coronary or carotid artery that is to be scanned. Moreover, the probes are often spun at high speed within a sheath while being pulled-back through the artery section of interest to generate a cylindrical scan. Any concomitant mechanical stress on the fiber can induce length changes and birefringence due to twisting. Probes for dental applications typically include a long umbilical that connects the handpiece/optical interface to the OCT analysis system or console; noise introduced in the OCT analysis due to mechanical shock to both the umbilical and handpiece/optical interface should be minimized. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention concerns probe and OCT system designs that minimize noise and interference due to the effects of mechanical movement and strain on the OCT system. It also concerns optical designs that are robust against amplitude noise from the OCT laser source. In embodiments, this is achieved by combining the OCT signals from the reference arms and signals arms of the OCT interferometer in the handpiece itself. This combining is performed by fiber couplers that are easily integrated into compact handpieces and connected to scanning units and fiber reference arms. Thus, noise due to movement and stress to the system, such as to the umbilical that connects the analysis system to the probe, does not corrupt the OCT analysis and/or image since the noise is common and does not appear on only the reference or signal arms of the interferometer. In examples, amplitude referencing is performed and delay matched to the interference signals to compensate for the optical delay associated with the umbilical and other components. Also integrated OCT system-probes are included that yield compact and robust electro-opto-mechanical systems along with polarization sensitive OCT systems. 
         [0008]    In general, according to one aspect, the invention features, an optical coherence tomography probe, comprising: a handpiece housing; an optical window in the handpiece housing; a reference arm reflector in the handpiece housing; an interference signal fiber coupler in the handpiece housing that receives an optical coherence tomography (OCT) signal from an OCT analysis system and divides the OCT signal between a reference optical fiber arm and a signal optical fiber arm; and an optical window in the handpiece housing through which the OCT signal from the signal optical fiber arm is transmitted to an object of interest and through which an object OCT signal is received from the object of interest and coupled into the signal optical fiber arm. The object OCT signal is mixed or combined with the OCT signal from the reference optical fiber arm that is reflected by the reference arm reflector to generate an interference signal that is transmitted from the handpiece housing to the OCT analysis system. 
         [0009]    In general according to another aspect, the invention features an optical coherence tomography method. This method comprises receiving an OCT signal from an OCT analysis system in an interference signal fiber coupler located within a handpiece housing and dividing the OCT signal between a reference optical fiber arm and a signal optical fiber arm, transmitting the OCT signal on the signal optical fiber arm from the handpiece housing to an object of interest and receiving an object OCT signal from the object of interest into the handpiece housing and coupling the object OCT signal onto the signal optical fiber arm, combining the object OCT signal with the OCT signal from the reference optical fiber arm to generate an interference signal, and transmitting the interference signal from the handpiece housing to the OCT analysis system. 
         [0010]    In general, according to still another aspect, the invention features an optical coherence tomography system. This system comprises a swept source laser for generating the OCT signal that is transmitted to a handpiece, a detector system that detects the interference signal received from the handpiece and a controller that uses the response of the detector system to generate an image of an object of interest. 
         [0011]    In general, according to another aspect, the invention features, an integrated optical system for detecting an interference signal generated by an OCT probe. The integrated optical system comprises an hermetic package, an optical bench in the hermetic package, a detector system attached to the bench for detecting the interference signal, and a beam splitter system attached to the bench that couples an OCT signal from a swept laser source to the OCT probe and couples the interference signal from the OCT probe to the detector system. 
         [0012]    In general, according to another aspect, the invention features, an integrated OCT system. The system comprises a hermetic package having an optical window, an optical bench in the hermetic package, a swept source laser system attached to the optical bench for generating an OCT signal, a detector system attached to the bench for detecting an interference signal. A beam splitter system is attached to the bench that couples the OCT signal from the swept laser source through the optical window to an object of interest, couples a portion of the OCT signal to a reference arm, couples light returning from the reference arm to the detector system, and directs light returning from the object of interest to the detector system. 
         [0013]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0015]      FIG. 1  is a schematic view of an optical coherence tomography (OCT) probe according to a first probe embodiment of the present invention; 
           [0016]      FIG. 2  is a schematic view of an OCT probe according to a second probe embodiment of the present invention; 
           [0017]      FIG. 3  is a schematic view of an OCT probe according to a third probe embodiment of the present invention; 
           [0018]      FIG. 4  is a schematic view of an OCT system according to a first system embodiment of the present invention; 
           [0019]      FIG. 5  is a schematic view of an OCT system according to a second system embodiment of the present invention; 
           [0020]      FIG. 6  is a schematic view of an OCT system according to a third system embodiment of the present invention; 
           [0021]      FIG. 7  is a schematic view of an OCT system according to a fourth system embodiment of the present invention; 
           [0022]      FIG. 8A  is a schematic view of an OCT probe that provides for polarization sensitivity according to a first polarization probe embodiment of the present invention; 
           [0023]      FIG. 8B  illustrates the polarization of input signal, reference signal and return signals; 
           [0024]      FIG. 8C  is a schematic view of an OCT probe that provides for polarization sensitivity according to a second polarization probe embodiment of the present invention; 
           [0025]      FIG. 9  is a plan view of the optical components of an OCT probe including an integrated reference path; 
           [0026]      FIG. 10A  is a schematic view of a polarization sensitive OCT system according to a first polarization system embodiment of the present invention; 
           [0027]      FIG. 10B  is a schematic view of a polarization sensitive OCT system according to a second polarization system embodiment of the present invention; 
           [0028]      FIG. 11A  is a schematic plan view of an integrated OCT engine according to the present invention; 
           [0029]      FIG. 11B  is a perspective view of the integrated OCT engine according to the present invention; and 
           [0030]      FIG. 12  is a schematic plan view of an integrated OCT engine/probe according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  shows an optical coherence tomography (OCT) probe  100 A that has been constructed according to the principles of the present invention (first probe embodiment). 
         [0032]    Generally, the probe  100  comprises a handpiece housing  160 . This handpiece housing is typically grasped by an operator of the OCT system. It is characterized by a rigid portion that connects to an OCT analysis unit by an intervening flexible or articulated umbilical. 
         [0033]    The housing  160  comprises an optical window element  164 , which is typically tilted and anti-reflection coated to prevent spurious reflection back into the OCT system. This optical window element  164  is transmissive to the optical frequencies at which the OCT system operates. In one example, the OCT system operates in the near infrared. In some embodiments, the optical window  164  is also transmissive to visible optical frequencies to enable a visible targeting beam to pass through the window to indicate where the non-visible infrared OCT signal is impinging on the object of interest  10 . 
         [0034]    The handpiece  160  in one implementation includes an electro-optical connector  110 . This electro-optical probe connector  110  enables operator connection to and disconnection from an OCT analysis system. In one example, the electro-optical probe connector  110  provides the OCT and interference signals between the probe system  100  and the analysis system along with electrical control signals. 
         [0035]    In other embodiments, the umbilical is integral with the probe such that the connector  110  is not used. 
         [0036]    In more detail, the OCT signal, such as light from a swept source laser, is received from the OCT analysis system via the connector  110  and coupled onto an OCT/interference signal optical fiber  106 . The OCT/interference signal fiber  106  couples the OCT signal received from the OCT analysis system to an interference/OCT signal coupler  112 . In one example, the interference/OCT signal coupler  112  is a 90/10 percent fiber coupler, and thus does not divide the light evenly between the two output ports. Specifically, the interference/OCT signal coupler  112  provides the OCT signal received on the OCT/interference signal fiber  106  to a reference arm optical fiber  130  and a signal arm optical fiber  132 , with most of the light, i.e., 90% or greater, in this current example, on the signal arm optical fiber  132 . 
         [0037]    The reference arm optical fiber  130  forms the reference arm of an interferometer that is implemented, preferably entirely, within the handpiece housing  160 . Specifically, the reference arm optical fiber  130  terminates in a reflector  116 . In one example, the reflector  116  is simply a highly reflective coating at the end of the reference arm optical fiber  130 . Exemplary highly reflective coatings include dielectric stack coatings and metalized endfacet coatings that are deposited on the endfacet of the reference arm optical fiber  130 . In other examples, the reflector  116  is implemented as a discrete mirror element, and possibly including a discrete lens to collimate and couple light between the endfacet of the reference arm optical fiber  130  and the mirror reflector. 
         [0038]    The signal arm optical fiber  132 . transmits the received OCT signal to a scanning unit  150 . The scanning unit  150  couples the OCT signal between the object of interest  10  and the signal arm optical fiber  132 . 
         [0039]    In the illustrated embodiment, the scanning unit  150  comprises an optional glass or transmissive spacer  152  that is secured to the endfacet of the signal arm optical fiber  132 . This spaces the endfacet of the signal arm optical fiber  132 . from a GRIN (graded refractive index) lens element  154 , which has an angled output facet to prevent parasitic reflections. The GRIN lens  154  focuses the OCT signal from the signal arm optical fiber  132  onto the sample  10 . The free-space light beam  156  is directed to a fold mirror  158  that directs the OCT signal beam  156  through the optical window  164  to the object of interest  10 . Then light returning from the object of interest  10  is coupled back through the optical window  164  to reflect off of the fold mirror  158  and be coupled back into the signal arm optical fiber  132  via the GRIN lens  154  and the spacer element  152 . 
         [0040]    In a preferred embodiment, the fold mirror  158  is a scanning mirror. Specifically, it is driven to both tip and tilt in the x and y axes as indicated by arrow  134 . In one implementation, this is a micro electro mechanical system (MEMS) mirror that scans the OCT signal beam  156 , such as raster scans, over the object of interest  10  in order to generate a three-dimensional image of the object of interest  10 . 
         [0041]    In the typical embodiment, the handpiece housing  160  also supports one or more electrical control switches  162 . These control switches  162  are coupled to the OCT analysis system via the opto-electrical connector  110  via control line  170 . The switches are used by the operator to begin and end OCT scanning and activate a visible targeting laser during the OCT analysis of the object of interest  10 . Preferably, the switches  162  are also used to electronically drive and control the scanning mirror  158 . 
         [0042]    The light returning from the object of interest  10  on the signal arm optical fiber  132  is combined with the light returning from the reflector  116  on the reference arm optical fiber  130  in the interference/OCT signal coupler  112 . This combination generates the interference signal that is transmitted to the OCT analysis system on the OCT/interference signal optical fiber  106  via the electro-optical connector  110 . 
         [0043]    Since the typical fiber coupler is a four port system, some interference signal light is also coupled onto the fourth arm that terminates in the termination  114 . This light is lost in this exemplary embodiment. Otherwise, a three-port coupler is used in other implementations. The length of the reference arm optical fiber  130  is important to control the scanning depth in the object. Specifically, the length of the reference arm optical fiber  130  is sized so that plane  175  is the zero distance virtual reference plan of the OCT system. Thus, the optical path length of the reference arm optical fiber  130  is made equal to the sum of the optical path lengths of the signal arm optical fiber  132 , transmissive spacer  152 , GRIN lens element  154 , and the free space path to the reference plane  175 , including window  164 . 
         [0044]    The probe  100  in some sense a “common path” probe, with one fiber connection back to the OCT system. It would typically be used with some sort of relative intensity noise (RIN) reduction system. One option is to use a balanced receiver to accept input from the probe in one detector and a laser amplitude signal in the other (US2009/0046295 A1, Kemp, et al., Feb. 19, 2009, FIG. 13). Another option is to ratio the probe signal with that of a laser power monitor (Normalization detection scheme for high-speed optical frequency-domain imaging and reflectometry, Sucbei Moon and Dug Young Kim, 12 Nov. 2007/Vol. 15, No. 23/OPTICS EXPRESS 15129). 
         [0045]      FIG. 2  shows a second embodiment of the OCT probe  100 B. This embodiment is generally similar to the first probe embodiment but uses two OCT/interference signal fibers  106 ,  108  to optically connect the OCT probe  100  to the OCT analysis system. This probe is compatible with standard balanced receiver/relative intensity noise (RIN) reduction scheme, and would also suppress autocorrelation artifacts from the sample signal interfering with itself. 
         [0046]    In more detail, the OCT signal from the OCT analysis system is received via the electro-optical connector  110  typically through a flexible umbilical on a first OCT/interference signal fiber  106  and a second OCT/interference signal fiber  108 , or only one of these fibers. 
         [0047]    The light is then coupled to via a 50/50 interference/OCT signal coupler  112  between the reference arm optical fiber  130  and the signal arm optical fiber  132 . 
         [0048]    The OCT signal on the reference arm optical fiber is transmitted to a partial reflector  118 . In one example, this partial reflector reflects back less than 10%, such as 4% or less, of the OCT signal light that carried on the reference arm optical fiber  130 . In one example, this partial reflector  118  is implemented as a dielectric stack or metal coating on the endfacet of the optical fiber  130 . 
         [0049]    Light on the signal arm optical fiber  132  is transmitted to the scanning unit  150 . This directs the light as described previously through the optical window  164  to the object of interest  10 . Returning light in turn passes through the optical window  164  and is coupled by the scanning unit  150  to the signal arm optical fiber  132 . 
         [0050]    The OCT signal returning on the reference arm optical fiber  130  and the light from the object of interest returning on the signal arm optical fiber  132 . is combined in the 50/50 interference/OCT signal coupler  112 . This combination generates the interference signal that is transmitted back to the OCT analysis system on the first and second OCT/interference signal fibers  106 ,  108  via the electro-optical connector  110 . 
         [0051]      FIG. 3  shows a third embodiment of the OCT probe system  100 C. This embodiment is similar to the second embodiment OCT probe system of  FIG. 2 . The difference lies in the configuration of the reference arm. In this example, the reference arm optical fiber  130  includes an attenuator  120  that attenuates the OCT signal carried on the reference arm optical fiber  130 . The light passing through the attenuator  120  is then reflected by a highly reflecting endfacet  116 . This highly reflecting end facet is typically implemented as described in connection with the first probe embodiment of  FIG. 1 . The OCT light returning from the reflector  116  passes through the attenuator  120  and then on the reference arm optical fiber  130  to the interference/OCT signal coupler  112 . 
         [0052]    The potential problem associated with the embodiment of  FIG. 2  is dissipating the light that is transmitted through the partial reflector  118 . This transmitted light is then potentially within the handpiece housing  116  and can potentially serve as an interference source: either being coupled back into the reference arm optical fiber  130  creating multipath interference or possibly interfering with the OCT signal that is transmitted to and from the object of interest  10 . This potential problem is addressed in the embodiment of  FIG. 3  by using the attenuator  120  to absorb the excess OCT signal light in the reference arm to ensure that it does not create interference. In examples, the attenuator  120  is a lossy element that is implemented by fiber microbending, through a lossy fiber splice, or other means. 
         [0053]      FIG. 4  illustrates an OCT analysis system  200 A that is compatible with the OCT probe of  FIG. 1 . Specifically, the OCT analysis system  200 A provides electrical and optical connection to the probe  100  via a typically flexible or articulated umbilical  205 . Specifically, this umbilical extends between an OCT analysis system electro-optical connector  218  and the probe connector  110 . This flexible umbilical  205  allows the reference probe  100  to be moved around the object of interest, such as the patient, to enable analysis of regions of interest of the patient, such as the patient&#39;s teeth or skin in some examples. 
         [0054]    The OCT signal receive by the probe  100  is generated in the preferred embodiment by a swept laser source  212 . An exemplary source is that described in U.S. patent application Ser. No. 12/396,099, filed on 2 Mar. 2009, entitled Optical Coherence Tomography Laser with Integrated Clock, by Flanders, et al., which is incorporated herein by this reference. 
         [0055]    The OCT signal generated by the swept source laser is transmitted to a 50/50 OCT/amplitude reference fiber coupler  214  on a swept source optical fiber  235 . The 50/50 coupler  214  divides the OCT signal from the swept source  212  between an amplitude reference fiber  216  and the OCT probe optical fiber  240 . This OCT probe optical fiber  240  transmits the OCT signal from the 50/50 coupler  214  to the unit connector  218 . Similarly, the returning interference signal from the reference probe  100  is received via the unit connector  218  on the probe optical fiber  240  and is then divided by the 50/50 OCT/amplitude reference fiber coupler  214 . 
         [0056]    The path match optical fiber  216  has a length that corresponds to twice the optical delay between the OCT/amplitude reference fiber coupler  214  and the reference probe  110  plus the delay from coupler  214  to interference signal photodiode detector  230 . In this way, the delay induced by the path match optical fiber is consistent with the combined delay associated with OCT signal to the probe  100  and the interference signal returning on optical fiber  240  from the probe. The OCT signal light transmitted through the path match optical fiber  216  is then detected by an amplitude reference photodiode detector  220  which is then sampled by the controller  210  and used to remove amplitude noise in the system from the swept source  212 . 
         [0057]    The interference signal returning from the OCT probe  100  and received on OCT probe optical fiber  240  is transmitted through the 50/50 OCT/amplitude reference fiber coupler  214  to the interference signal detector  230 . This detector detects that light which is then sampled by the controller  210 . 
         [0058]    In one example, the amplitude reference detector  220  and the interference detector  230  are combined into a balanced detector system for rejection of the amplitude noise from the swept source  212  in the interference signal. In this case, the optical power levels at the two detectors need to be balanced (For example, see a similar RIN reduction scheme in US2009/0046295 A1, Kemp, et al., Feb. 19, 2009, FIG. 13). Alternately, the signal from the amplitude reference detector  220  can be digitally divided in the controller  212 , for example, by the interference signal from detector  230  for RIN reduction (Normalization detection scheme for high-speed optical frequency-domain imaging and reflectometry, Sucbei Moon and Dug Young Kim, 12 Nov. 2007/Vol. 15, No. 23/OPTICS EXPRESS 15129). 
         [0059]      FIG. 5  illustrates a second system embodiment  200 B of the OCT analysis system that is also compatible with the probe of  FIG. 1 . This system makes more efficient use of the available optical power, but has more expensive components. 
         [0060]    The second embodiment  200 B uses an unbalanced. OCT/amplitude reference fiber coupler  214  to divide the OCT signal from the swept source  212  between the amplitude reference path match fiber  216  and OCT probe optical fiber  240 . The OCT signal light on the OCT probe optical fiber  240  passes through interference signal circulator  242  to be transmitted to the reference probe  100  via the unit electro-optical connector  218 . In turn, the interference signal returning from the reference probe  100  is directed by the circulator  242  to interference signal detector  230 . 
         [0061]    The use of the circulator  242  leads to a more optically efficient system relative to  FIG. 4  since the 95/5% OCT/amplitude reference fiber coupler 214 of this embodiment allows most of the OCT signal, greater than 90% and preferably 95% or more, generated by the swept source  212  to be directed to the object of interest with only a small amount being used to generate the amplitude reference. 
         [0062]      FIG. 6  illustrates a third embodiment  200 C of the OCT analysis system that is compatible with the probes of  FIGS. 2 and 3 . In this example, the OCT signal generated by the swept source  212  is transmitted on swept source optical fiber  235  to interference signal circulator  242  and then on OCT probe optical fiber  240  to the optical probe  100  via the unit connector  218 . The interference signal from the OCT probe is then received on interference signal optical fiber  244  and the OCT probe optical fiber  240 . The returning interference signal light on OCT probe optical fiber  240  is directed by the interference signal circulator  242  to the balanced detector  248 . The interference signal received on the interference signal optical fiber  244  is directly coupled to the balanced detector  248 . 
         [0063]    The balanced receiver reduces the effect of RIN on the system&#39;s signal-to-noise ratio. The common-path probe systems in  FIGS. 4 and 5  also have methods to reduce the effects of RIN. A major advantage of the two-fiber probe ( FIGS. 2 and 3 ) and the corresponding system ( FIG. 6 ) is that the autocorrelation image (sample light interfering with itself) is strongly attenuated. 
         [0064]    In implementations, the balanced receiver  248  is an auto-balanced receiver (one example is manufactured by New Focus. Part number 2017), which automatically balances the electrical signals from the two detectors even in the presence of mismatched lightwave signals impinging on the two detectors. 
         [0065]      FIG. 7  illustrates a variant, fourth embodiment  200 B of the OCT system that uses two circulators  252 ,  254  for the two fiber probe embodiments. This configuration is similar to that in  FIG. 6 , except that it incorporates a “dummy” circulator  254  (one port not used) to help balance the lightwave signals present at the two detectors of the balanced receiver  248 . If closely matched, the interference signal circulators  252 ,  254  will have similar optical losses vs. wavelength and balance the lightwave signals to the two detectors. A better match provides improves signal-to-noise performance and attenuation of the autocorrelation image. Better matching by the additional circulation may be preferred to the use of an autobalanced detector for cost and performance reasons. 
         [0066]      FIG. 8A  illustrates a first polarization sensitive embodiment  100 D of the OCT probe  100 . Generally, this OCT probe is similar to the first probe embodiment of  FIG. 1 , thus, the descriptions associated with  FIG. 1  are relevant here. This probe  100 D, however, allows for polarization dependent or sensitive OCT analysis. Specifically, it enables the analysis of the OCT signal and the polarization characteristics of the object of interest  10 . 
         [0067]    In more detail, the OCT signal received on the OCT/Interference signal fiber  106  is a highly polarized signal such as a signal from a semiconductor external cavity laser system. To preserve polarization, the OCT/Interference signal fiber  106  is polarization maintaining optical fiber. 
         [0068]    Specifically, as illustrated in Fig,  813 , the polarization of the swept source OCT signal is polarized according to one, slow, axis of the polarization maintaining that is used for the OCT/Interference signal fiber  106 . See polarization  190 . 
         [0069]    The polarized OCT signal is divided by the interference/OCT signal coupler  112 , which is a 50/50 polarization-maintaining coupler. The polarized OCT light is transmitted over the reference arm optical fiber  130 , which is PM fiber, to the reflector  116 . In this embodiment, there is an intervening quarter wave plate  810 . This rotates the polarization of the light by 22½ degrees. As a result, the returning OCT signal light has both a portion that is polarized parallel to the input OCT signal but also perpendicular to the input OCT signal, see polarization  192  in  FIG. 8B . 
         [0070]    The OCT light that is transmitted through the PM interference/OCT signal coupler  112  onto the signal arm optical fiber  132 , which is PM optical fiber, is directed to the object of interest  10  as described previously via the scanning system  150 . 
         [0071]    Light returning from the object of interest  110 , however, now is potentially polarized according to the birefringence properties of the object of interest  10  and thus will have polarizations aligned along axis  190  and also fast axis  194 , see  FIG. 8B . Thus, the signal light returning from the object of interest  10  is then combined with the two polarizations returning from the reference arm optical fiber  130  by the PM coupler  112 . Thus, this light then returns on the OCT signal/interference signal optical fiber  106  to the OCT analysis system. Interference signal now has two polarizations allowing for the polarization dependent OCT analysis of the object of interest. 
         [0072]      FIG. 8C  shows a second embodiment polarization sensitive probe  100 E that is analogous to the two fiber probes of  FIGS. 2 and 3  and is compatible with standard balanced receiver/relative intensity noise (RIN) reduction scheme, and would also suppress autocorrelation artifacts from the sample signal interfering with itself. 
         [0073]    In more detail, the OCT signal from the OCT analysis system on a first OCT/interference signal fiber  106  and a second OCT/interference signal fiber  108 , or only one of these fibers. These fibers are PM fiber. 
         [0074]    The light is then coupled to via a 50/50 interference/OCT signal PM fiber coupler  112  between the reference arm optical fiber  130  and the signal arm optical fiber  132 , which are both constructed of PM fiber. 
         [0075]    The OCT signal on the reference arm optical fiber  130  is transmitted to a partial reflector  118 . In one example, this partial reflector reflects back less than 10%, such as 4% or less, of the OCT signal light that carried on the reference arm optical fiber  130 . Alternatively, attenuator  120  is used in combination with a highly reflecting reflector. In either case, the intervening quarterwave plate  810  shifts the polarization so that the returning OCT signal now has component polarizations along each axis of the PM fiber. 
         [0076]    Light on the signal arm PM optical fiber  132  is transmitted to the scanning unit  150 . This directs the light as described previously through the optical window  164  to the object of interest  10 . Returning light in turn passes through the optical window  164  and is coupled by the scanning unit  150  to the signal arm PM optical fiber  132 . 
         [0077]    The OCT signal returning on the reference arm optical fiber  130  and the light from the object of interest returning on the signal arm optical fiber  132  is combined in the 50/50 interference/OCT PM fiber coupler  112 . This combination generates the interference signal for each polarization that is transmitted back to the OCT analysis system on the first and second OCT/interference PM fibers  106 ,  108  via the electro-optical connector  110 . 
         [0078]      FIG. 9  illustrates an OCT probe  100 F that includes an integrated reference arm. In this example, the OCT signal from the swept source laser is transmitted on an OCT/Interference signal optical fiber  410 . The OCT signal is coupled to the probe body  422 . In a preferred implementation, an intervening graded index fiber  420  connects the OCT/Interference signal optical fiber  410  to the probe body  422 . The graded index fiber  420  collimates the OCT signal so that the beam  440  that is transmitted through the optical probe body  422  is collimated. The light passes through interface  424  to be directed to a scanning fold mirror  158 , which scans see arrow  134 . This allows the OCT signal beam  156  to be scanned over the object of interest  10 . 
         [0079]    Light returning from the object of interest is directed by the scanning fold mirror  158  through interface  424  to be directed back into the OCT/interference signal fiber  410  via the graded index fiber  420 . 
         [0080]    The probe body  422  includes an integrated reference arm. Specifically, the interface  424  is a partial reflector so that a portion, typically less than 10%, of the OCT signal beam  440  is directed to a reference arm that is within the transmissive probe body  422  to be directed to an interface  428  that has a high reflecting coating on it. This reflects light back to the interface  422  to mix or combine with the light returning from the object of interest to generate the interference signal that is then coupled via the graded index fiber  420  to the OCT/interference signal fiber  410 . 
         [0081]    In one embodiment, this integrated OCT probe performs polarization dependent OCT analysis. In this example, a quarterwave plate  430  is attached to the probe body  422  to the interface  428  to rotate the light so that the light is now polarized along both axes. The OCT/interference signal fiber  410  is then polarization maintaining fiber. 
         [0082]      FIG. 10A  shows a first embodiment of a swept source polarization sensitive OCT system  200 E that is compatible with the polarization sensitive, common path probes of  FIGS. 8A and 9 . In this embodiment, all of the optical fibers in the system are polarization maintaining. 
         [0083]    In more detail, the swept source laser  212 , provides a linearly polarized output aligned to the slow axis of the PM fiber of the system and specifically the PM fiber used for the swept source optical fiber  235 . The OCT/amplitude reference fiber coupler  214  is similarly a PM fiber coupler that divides light between the amplitude path match fiber  216  and the OCT probe PM fiber  240 . Preferably the OCT/amplitude reference fiber coupler  214  is an unbalanced coupler so that most of the OCT signal is transmitted to the sample, i.e., greater than 90% and preferably 95% or more. The OCT signal light on the OCT probe optical fiber  240  passes through interference signal circulator  242  to be transmitted to the reference probe  100  via the OCT probe optical fiber  240  and potentially a unit optical connector  218 , umbilical  205 , and probe connector  110 . 
         [0084]    In turn, the interference signal returning from the reference probe  100  is directed by the circulator  242  through a length of detector PM optical fiber  910 . This fiber has a long length so that mixing of the parallel polarized light and the perpendicular light occurs at a frequency that is cut by an anti-aliasing filter  912  between the optical detectors  918 ,  920  and the analog-to-digital converters of the controller  210  that are used to sample the detector signals. For example, if the anti-alias filter removes any OCT image information at displacements greater than 5 mm, the fiber must be long enough that returns for the slow and fast axis light are separated &gt;5 mm over the propagation distance. A typical fiber length is tens of meters for a few m of displacement. 
         [0085]    An interference signal polarization splitter  914 , which can be implemented with fiber-optic components or bulk optic components, separates the two signals of different polarizations and routes them to separate detectors, a parallel polarization detector  918  and a perpendicular polarization detector  920 . 
         [0086]    The system controller  210  generates and displays two images by separately processing the interference signals of the two polarizations: One where the light scattered from the sample  10  has the same polarization as the illumination light generated by the swept source laser  212 , the parallel light; and a second image where the scattered light is polarized perpendicular to the illumination light. 
         [0087]      FIG. 10B  shows a polarization sensitive OCT analysis system  200 F that is compatible with the polarization dependent, two-fiber probe of  FIG. 8C . 
         [0088]    In this example, the OCT signal generated by the swept source  212  is transmitted on swept source optical fiber  235  to interference signal circulator  252  and then on OCT probe optical fiber  240  to the optical probe  100 , via potentially a unit optical connector  218 , umbilical  205 , and probe connector  110 . The interference signal from the OCT probe is then received on interference signal optical fiber  244  and the OCT probe optical fiber  240 . Returning interference signal light on OCT probe optical fiber  240  is directed by the circulator  252  to the detectors. The interference signal received on the interference signal optical fiber  244  directed to the detectors by circulator  254 . 
         [0089]    Similar to the embodiment of  FIG. 10A , long lengths of PM fiber  910   a,    910   b  are used on the optical paths to the detectors to prevent cross mixing of the parallel and perpendicular waves. On the other hand, the PM detector fibers  910   a,    910   b  should have matched lengths. 
         [0090]    A first interference signal polarization splitter  914   a  separates the polarizations of the interference signal received from interference signal circulator  252 . A second interference signal polarization splitter  914   b  separates the polarizations of the interference signal received from interference signal circulator  254 . 
         [0091]    The perpendicular polarization interference signals from each splitter  914   a,    914   b  are detected by a perpendicular balanced detector  248   b  and the parallel polarization interference signals are detected by parallel polarization balanced detector  248   a.    
         [0092]    This system has the RIN reduction and autocorrelation image suppression properties of the polarization insensitive systems of  FIGS. 2 and 3 , because of the use of balanced detection. The PM fibers  910   a  and  910   b  would have to be long to prevent polarization mixing as described above. They need to be roughly matched in length, so that the propagation delay difference is much less than the reciprocal of the highest electrical frequency generated in the detector systems. 
         [0093]      FIG. 11A  shows an integrated polarization dependent OCT system  500  that has been constructed according to the principals of the present invention and is compatible with the OCT probes of  FIGS. 8A and 9 . 
         [0094]    Generally the integrated polarization dependent OCT system  500  comprises a tunable swept source laser subsystem  510 , which generates a wavelength or frequency tunable optical signal, a clock subsystem  520 , which generates k-clock signals at spaced frequency increments as the OCT signals or emissions of the laser  510  are spectrally tuned over a. spectral scan band, and a detector subsystem  530 , which includes an amplitude references and interference signal detectors. The k-clock signals are used to trigger sampling, typically in an OCT sampling analog to digital converter (A/D) system  505 . 
         [0095]    The detector subsystem  530  and clock subsystem  520  of the integrated polarization dependent OCT system  500  are integrated together on a common optical bench  550 . This bench is termed a micro-optical bench and is typically less than 20 millimeters (mm) by 30 mm in size, and preferably less than 10 millimeters (mm) by 20 mm in size so that it fits within a standard butterfly or DIP (dual inline pin) hermetic package  560 . In one implementation, the bench is fabricated from aluminum nitride. A thermoelectric cooler  561  is preferably disposed between the bench  550  and the package  560  (attached/solder bonded both to the backside of the bench  550  and inner bottom panel of the package  560 ) to control the temperature of the bench  550 . 
         [0096]    To collect and collimate the OCT signal light exiting from polarization maintaining fiber  512  from the tunable laser  510 , an input lens structure  514  is used. Preferably, the input lens structure  514  comprises a LIGA mounting structure, which is deformable to enable post installation alignment, and a transmissive substrate in which the lens is formed. The transmissive substrate is typically solder or thermocompression bonded to the mounting structure, which in turn is solder bonded to the optical bench  550 . 
         [0097]    The input lens structure  514  couples the light from the laser  510  to a partially reflecting 10/90 substrate that functions as input beam splitter  516 . A majority of the beam enters the detector subsystem  530  and the remaining beam is directed to the clock subsystem  520 . In one example, greater than 90% of the input beam from the laser  510  is directed to the detector subsystem  530 . 
         [0098]    The OCT signal light is divided in the clock subsystem by a clock beam splitter  522 , which is preferably a 50/50 splitter. The clock beam splitter  522  divides the light between to a clock etalon  524  and a k-clock detector  526 . Any light not reflected by the splitter  522  is directed to a beam dump component that absorbs the light and prevents parasitic reflections in the hermetic package  560 . 
         [0099]    The clock etalon  524  functions as a spectral filter. Its spectral features are periodic in frequency and spaced spectrally by a frequency increment, termed free spectral range (FSR), that is related to the refractive index of the constituent material of the clock etalon  524 , which is fused silica in one example, and the physical length of the clock etalon  524 . The etalon can alternatively be made of other high-index and transmissive materials such as silicon for compactness, but the optical dispersion of the material may need to be compensated for with additional processing inside the controller/DSP  505 . Also, air-gap etalons, which are nearly dispersionless, are another alternative. 
         [0100]    The contrast of the spectral features of the etalon is determined by the reflectivity of its opposed endfaces. In one example, reflectivity at the etalon endfaces is provided by the index of refraction discontinuity between the constituent material of the etalon and the surrounding gas or vacuum. In other examples, the opposed endfaces are coated with metal or preferably dielectric stack mirrors to provide higher reflectivity and thus contrast to the periodic spectral features. 
         [0101]    in the illustrated example, the clock etalon  524  is operated in reflection. The FSR of the clock etalon is chosen based on the required scanning depth in an OCT system. The Nyquist criterion dictates that the periodic frequency spacing of the clock etalon that defines the sample rate be twice the largest frequency period component of the sample, thus setting the optical thickness of the clock etalon to twice the required imaging depth. However, as is typically done with clock oscillators, the periodic waveform can be electrically frequency doubled, tripled, etc, see doubler  528 , or can be halved to obtain the desired sample rate while choosing an etalon of a length that is convenient for handling and that easily fits within the package  560  and on the bench  550 . A thicker etalon compensates better for nonlinear frequency scanning than a thinner one due to its finer sample rate, but it is larger and more difficult to fabricate, so a tradeoff is made depending upon the laser tuning linearity, system depth requirements, and manufacturing tolerances. Moreover, a thicker etalon requires a laser of comparable coherence length to generate stable clock pulses, so the laser coherence length can also help dictate the design of the etalon thickness. 
         [0102]    The light returning from the clock etalon  524  and not reflected by beamsplitter  522  is detected by detector  526 . The light detected by detector  526  is characterized by drops and rises in power as the frequency of the tunable signal scans through the reflective troughs/reflective peaks provided by the clock etalon  524 . 
         [0103]    The detector photocurrent is amplified and conditioned. The clock signal is multiplied or divided in frequency by multiplier/divider  528 , depending on the needs of the OCT system&#39;s application and the requirement for a convenient etalon (or other clock interferometer) size within the butterfly package  560 . A digital delay line is also added to the doubler circuitry  528  is some embodiments to compensate for any round-trip optical delay to the probe  400 . 
         [0104]    The OCT signal that is transmitted through the input beam splitter  516  enters the detector subsystem  530 . The detector subsystem  530  comprises an amplitude reference splitter  562  that directs a portion of the OCT signal, typically less than 10%, to an amplitude reference detector  564 . This detector  564  is used to detect amplitude noise in the OCT signal. 
         [0105]    Light transmitted through the amplitude reference splitter  562  passes through a parallel detector splitter  566 , a polarization beam splitter  568  and is coupled onto OCT/Interference signal optical fiber  410  to the polarization dependent OCT probe  400  by output lens structure  518 . 
         [0106]    The returning interference signal from the OCT probe  400  is separated into its two polarizations by the polarization beam splitter  568 . The portion of the interference signal that is perpendicular to the polarization of the OCT signal from the laser  510  is directed to and detected by a perpendicular interference signal detector  570 . The portion of the interference signal that has a polarization that is parallel to the polarization of the polarization of the OCT signal from the laser  510  and that passed through the polarization beam splitter  568  is directed by the parallel detector splitter  566  and detected by the parallel interference signal detector  572 . 
         [0107]    The k-clock signal is used by the digital signal processing and analog-detector sampling system  505  as a sampling clock to trigger the sampling of the amplitude reference signal, the parallel detector signal, and the perpendicular detector signal. This information is used to perform the Fourier transform to reconstruct the image of the object including a polarization dependent OCT image at the two polarizations. 
         [0108]      FIG. 11B  shows one physical implementation of the integrated polarization dependent OCT system  500  in a butterfly package  560 . In this example, the lid of the package  560  is removed to expose the components of the bench  560 . This view also shows the LIGA structures S that attach the lens substrates L to the bench  560 . 
         [0109]      FIG. 12  shows another integrated OCT system  600  that has been constructed according to the principals of the present invention. This system integrates the swept source  610 , k-clock system  520 , detector system  530 , and reference arm  660  on a bench  550 , and within a hermetic package  560 . 
         [0110]    Generally the integrated laser clock system  600  comprises a tunable laser swept source subsystem  610 , which generates a wavelength or frequency tunable OCT signal, a clock subsystem  520 , which generates k-clock signals at spaced frequency increments as the tunable signals or emissions of the laser  610  are spectrally tuned over a spectral scan band, and a detector subsystem  530 . The clock signals are generally used to trigger sampling of detector system. 
         [0111]    The tunable laser subsystem  610 , clock subsystem  520 , and the detector subsystem  530  are integrated together on a common optical bench  550 . This bench is termed a micro-optical bench and is usually less than 20 mm by 30 mm and preferably less than 10 mm by 20 mm in size so that it fits within a standard butterfly or DTP (dual inline pin) hermetic package  560 . In one implementation, the bench is fabricated from aluminum nitride. A thermoelectric cooler  562  is disposed between the bench  550  and the package  560  (attached/solder bonded both to the backside of the bench and inner bottom panel of the package  560 ) to control the temperature of the bench  550 . 
         [0112]    In more detail, the tunable laser  610  in the preferred embodiment in based on the tunable laser designs disclosed in U.S. Pat. No. 7,415,049 B2, which is incorporated herein in its entirety by this reference. 
         [0113]    In the current implementation, the tunable laser  610  comprises a semiconductor gain chip  652  that is paired with a micro-electro-mechanical (MEMS) angled reflective Fabry-Perot tunable filter  654  to create external cavity laser (ECL) with the tunable filter  654  being an intracavity tuning element and forming one end, or back reflector, of a laser cavity of the tunable ECL. 
         [0114]    The semiconductor optical amplifier (SOA) chip  652  is located within the laser cavity. In the current embodiment, both facets of the SOA chip  652  are angled relative to a ridge waveguide  58  running longitudinally along the chip  652  with the back facet  651  and the front facet  655  being anti-reflection (AR) coated. A partially reflecting substrate  662  provides reflectivity to define the front reflector of the laser cavity. 
         [0115]    To collect and collimate the light exiting from each end facet of the SOA  652 , two lens structures  660 ,  662  are used. Each lens structure  660 ,  662  comprises a LIGA mounting structure, which is deformable to enable post installation alignment, and a transmissive substrate in which the lens is formed. The transmissive substrate is typically solder or thermocompression bonded to the mounting structure, which in turn is solder bonded to the optical bench  550 . 
         [0116]    The first lens component  660  couples the light between the back facet of the SOA  652  and the tunable filter  654 . Light exiting out the front facet of the SOA  652  is coupled by a second lens component  662  to the detector subsystem  530 . 
         [0117]    The angled reflective Fabry-Perot filter  654  is a multi-spatial-mode tunable filter having a curved-flat optical resonant cavity that provides angular-dependent, reflective spectral response back into the laser cavity. This effect is discussed in more detail in incorporated U.S. Pat. No. 7,415,049 B2. In the referred embodiment, the curved mirror is on the MEMS membrane and is on the side of the filter  654  that adjoins the laser cavity. The flat mirror is on the opposite side and faces the laser cavity. The flat mirror preferably has a higher reflectivity than the curved mirror. Currently the reflectivities for the flat and curved mirrors are typically 99.98% and 99.91%, respectively, in order to achieve the desired reflectivity and requisite linewidth of the filter  654  in reflection. 
         [0118]    The light transmitted by the tunable filter  654  is coupled out of the laser cavity and into the clock subsystem  520  by fold mirror  614 , which are reflective coated substrates that are solder bonded to the bench  550 , fold the beam of the light from the tunable laser subsystem  610 , allowing for a dimensionally compact system. 
         [0119]    The light then passes to a beam splitter  522 , which is preferably a 50/50 splitter to a clock etalon  524 . Any light transmitted by the splitter  522  is preferably directed to a beam dump component that absorbs the light and prevents parasitic reflections in the hermetic package  560  and into the laser cavity and detectors. 
         [0120]    The light returning from the clock etalon  524  is detected by detector  526  to form the k-clock signal. 
         [0121]    The detector subsystem  530  receives the OCT signal from the tunable laser subsystem  610 . The OCT signal passes through an amplitude reference splitter  562 , and interference/reference splitter  620 . The OCT signal is focused by an output lens  622  on the object of interest  10 . The OCT signal exits the hermetic package  560  via a transmissive window  630  that is provided in the side of the package  560 . 
         [0122]    The OCT signal that is reflected by the interference/reference splitter  620  is directed to a reference arm  660  including reference arm fold mirror  624  to a reference arm mirror  626 . 
         [0123]    Light returning from the reference arm mirror  624  is mixed or combined with light from the sample  10 , which is received by received by window  630  and focused by lens  622 , at interference/reference splitter  620  to form the interference signal that is detected by interference signal detector  628 . 
         [0124]    Signal to noise ratio (SNR) improvement by reducing the effects of RIN is performed by digitally dividing the interference signal from detector  628  by the amplitude reference signal from detector  564  before ITT processing. This is a compact system for performing A-scans, but movement of the package  560  or the sample  10  would allow B-scans to be made. Additionally, a MEMS mirror scanner could be incorporated before the package&#39;s output lens to perform this function without movement of the sample or the package in some implementation. 
         [0125]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.