Abstract:
The invention generally relates to a polarization sensitive optical coherence tomography (OCT) system that includes both polarization maintaining and single mode optical fiber and methods of use thereof. Generally, OCT systems of the invention have a light source and a differential path interferometer that includes a sample arm and a reference arm. A splitter is used to split light from the light source to the sample arm and the reference arm. Reflected light from the sample arm and the reference arm is recombined at a detector. The polarization maintaining fiber is used in the reference arm to ensure that optical field intensity is equalized between two orthogonal detection channels of the OCT system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of, and priority to, U.S. Provisional Application Ser. No. 61/745,190, filed Dec. 21, 2012, the contents of which are incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to a polarization sensitive optical coherence tomography (OCT) system that includes both polarization maintaining and single mode optical fiber and methods of use thereof. 
     BACKGROUND 
     Development of depth-resolved light reflection or Optical Coherence Tomography (OCT) provides a high resolution imaging technique for analyzing tissue. OCT is an imaging technique that captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). OCT uses a narrow line width tunable laser source or a superluminescent diode source to emit light over a broad bandwidth. The light is sent to an interferometer that includes a sample arm and a reference arm. A majority of the light is sent to the sample arm to be directed onto a sample, while the remainder of the light is sent to a reference arm. The light reflects from both the sample and the reference arms and is recombined to make in situ tomographic images. Those images typically have an axial resolution of less than 10 μm and tissue penetration of 2-3 mm. OCT provides tissue morphology imagery at much higher resolution than other imaging modalities such as MRI or ultrasound. Further, with such high resolution, OCT can provide detailed images of a pathologic specimen without cutting or disturbing the tissue. 
     Conventional, intensity based OCT cannot directly differentiate between tissues. However, the light&#39;s polarization state can be changed by various light-tissue interactions, allowing it to be used to generate tissue specific contrast. Those effects are used by polarization sensitive (PS) OCT. With PS-OCT, a sample is typically illuminated either with circularly polarized light or with different polarization states successively, and the backscattered light is detected in two orthogonal polarization channels. 
     With polarization diverse detection, optical field intensity of the reference arm of the interferometer needs to be approximately equalized between two orthogonal detection channels. Using single mode fiber in the reference arm causes its light to propagate in an uncontrolled polarization state. To address that concern, a polarization controller is typically used to adjust the polarization state to have equalized intensities in the detection channels. The polarization control component is typically somewhat bulky, it must be adjusted for every system produced, and its control function must be automated (with sensor feedback) if thermal changes in the fiber polarization state are expected. For those reasons, a polarization controller is disadvantageous. 
     Polarization maintaining optical fiber is an option that maintains the polarization state of light as it propagates in the fiber and therefore an external polarization control component is not needed. However, use of polarization maintaining fiber has other disadvantages including higher cost, tight angular tolerances on splices/connectors, non-zero crosstalk between polarization modes, and the inherent polarization specific differential group delay incurred in the light propagating in the two polarization modes within the fiber. 
     SUMMARY 
     The invention provides a polarization sensitive optical coherence tomography (OCT) system that includes both polarization maintaining and single mode optical fiber. Generally, OCT systems of the invention have a light source and a differential path interferometer that includes a sample arm and a reference arm. A splitter is used to split light from the light source to the sample arm and the reference arm. Reflected light from the sample arm and the reference arm is recombined at a detector. The polarization maintaining fiber is used in the reference arm to ensure that optical field intensity is equalized between two orthogonal detection channels of the OCT system. Accordingly, the OCT systems of the invention do not require a polarization controller. Other connections in the system where the polarization state of the light is not important use single mode fiber, thereby saving costs, increasing angular tolerances on certain splices/connectors, and addressing crosstalk between polarization modes. 
     Systems of the invention may be used to analyze a target tissue. Those methods generally involve directing light from the OCT system onto a target tissue, and determining polarization properties of the light reflected from the target tissue. The target tissue may be any tissue and exemplary target tissue includes a vessel wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a PS-OCT interferometer in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The invention provides a polarization sensitive (PS) optical coherence tomography (OCT) system that includes both polarization maintaining and single mode optical fiber. In one embodiment, the PS-OCT configuration includes an interferometer and a light source that produces light over a multiplicity of optical frequencies. The light source may be any light source generally used with OCT. Exemplary light sources include a narrow line width tunable laser source or a superluminescent diode source. Examples of narrow line width tunable laser sources include, but are not limited to, lasers having a Bragg diffraction grating or a deformable membrane, lasers having a spectral dispersion component (e.g., a prism), or Fabry-Pérot based tuning laser. 
     The interferometer includes at least one optical fiber through which the light is transmitted to the sample. The interferometer includes a receiver that receives the light reflected from the sample. The interferometer includes a computer coupled to the receiver that determines depth-resolved polarimetric properties of the sample. Depth-resolved includes either measuring in the depth dimension or the local variation in a parameter versus depth. PS-OCT systems and their methods of use are further described, for example in Milner et al., (U.S. patent application number 2008/0291463) and Kemp (U.S. Pat. No. 7,929,148), the content of each of which is incorporated by reference in its entirety. 
       FIG. 1  provides an exemplary embodiment of the invention.  FIG. 1  shows a Mach-Zehnder interferometer in a PS-OCT configuration  200 , which measures the complex mutual coherence function (magnitude and phase) between two non-reciprocal optical paths, one path encompassing an object under test (i.e. “the sample”) and the other a reference path. The PS-OCT system and calculations for the OCT interferometer are generally described and explained in U.S. patent application Ser. No. 11/446,683, and Provisional Application Ser. No. 60/932,546, herein incorporated by reference. 
     As shown in  FIG. 1 , The PS-OCT system has a light source  210  with cascaded fiber optic couplers to subdivide the source light into three primary modules (1) the primary OCT interferometer, (2) an auxiliary wavemeter interferometer  260 , and (3) an optical trigger generator  262 . In one embodiment, the light source  210  is a High Speed Scanning Laser HSL-2000 (Santec) with an instantaneous coherence length of over 10 mm. The swept laser source  210  includes emitted light with a mean frequency of the output spectrum that varies over time. The mean frequency of light emitted from the swept source may change continuously over time at a tuning speed that is greater than 100 terahertz per millisecond and repeatedly with a repetition period. The swept laser source may be any tunable laser source that rapidly tunes a narrowband source through a broad optical bandwidth. The tuning range of the swept source may have a tuning range with a center wavelength between approximately 500 nanometers and 2000 nm, a tuning width of approximately greater than 1% of the center wavelength, and an instantaneous line width of less than approximately 10% of the tuning range. Polarization maintaining fiber  2001  is used as the connection between the swept source laser  210  and the coupler  212 . 
     As shown in  FIG. 1 , the auxiliary wavemeter  260  and the optical trigger generator  262  are for clocking the swept light source in order for providing an external clock signal to a high speed digitizer  270 , as disclosed in U.S. Pat. No. 8,049,900, herein incorporated by reference. The Uniform Frequency Sample Clock signal is repeatedly outputted for each subsequent optical trigger that occurs as the laser is sweeping and the optical trigger is generated. The optical trigger is generated from the optical trigger generator  262 . The high-speed digitizer card  270  is coupled to the output of the OCT interferometer, output of the auxiliary interferometer  260 , the trigger signal from the trigger generator  262 , and the arbitrary waveform generator. The high-speed PCI digitizer card  270  can be a dual-channel high resolution 16 bit, 125 MS/s waveform for a PCI bus. The external sample clock signal is derived from an auxiliary optical wavemeter photoreceiver during a start-up calibration step, and then repeatedly outputted by the arbitrary waveform generator for each subsequent optical trigger signal that occurs as the laser is sweeping. The external clocking system allows for the wavemeter-generated clock signal to be filtered and processed in software before being outputted on the arbitrary waveform generator. Thus, the external clock derived from the auxiliary wavemeter is regenerated by the arbitrary waveform generator (Gage CompuGen) to allow acquisition of interferometer output data directly in wavenumber (k) space. 
     Coupler  212  splits 90% of the light source power into the primary OCT interferometer and 10% into the coupler  218  for the auxiliary wavemeter  260  and trigger generator  262 . Fiber  2002  between coupler  212  and coupler  218  may be either polarization maintaining fiber or single mode fiber. 
     Polarization maintaining fiber  2003  is used between coupler  212  and coupler  214 . Coupler  214  then splits the light 90% directed to port  1  of a 3-port polarization sensitive optical circulator  220  for the sample path and 10% of the light is directed to port  1  of a 3-port polarization sensitive optical circulator  222  for the reference path. Single mode fiber  2004  is used for the sample path to the circulator  220 , and polarization maintaining fiber  2005  is used for the reference path to the circulator  222 . 
     Port  2  of circulator  220  for the sample path uses single mode fiber  2006  to a sample  240 . The sample path can be coupled to a probe or catheter  242  via a fiber optic rotary junction. An exemplary catheter for OCT systems is disclosed in common assigned provisional application Ser. No. 60/949,511, filed Jul. 12, 2007, herein incorporated by reference. Examples of a rotating catheter tip for the sample path include, a turbine-type catheter as described in Patent Cooperation Treaty application PCT/US04/12773 filed Apr. 23, 2004; or a rotating optical catheter tip as described in U.S. patent application Ser. No. 11/551,684; or a rotating catheter probe as described in U.S. patent application Ser. No. 11/551,684; or an OCT catheter as described in Provisional Application Ser. No. 61/051,340, filed May 7, 2008, each herein incorporated by reference for the methods, apparatuses and systems taught therein. Single mode fiber  2007  is used for that connection. 
     The catheter  242  can be located within a subject to allow light reflection off of subject tissues to obtain optical measurements, medical diagnosis, treatment, and the like. The light reflected back from catheter  242  travels back through single mode optical fiber  2006  to coupler  216 . Single mode fiber  2009  is used for the connection between circulator  220  and coupler  216 . 
     Port  2  of circulator  222  for the reference path uses polarization maintaining fiber  2010  to a variable delay line  246 . The VDL  246  comprises of an input fiber, a retro-reflecting mirror on a translation stage, and an output fiber. A dial controls the variable length, or delay, inserted into the optical path. The typical length variance is about 6 cm, while the typical time delay is about 300 picoseconds. Alternatively, an adjustable phase delay system can be included to modulate phase, which includes a piezo-operated stage, to provide much finer phase control, e.g., in the sub-wavelength range. In contradistinction, the VDL provides for larger path-length adjustments with micron-size adjustment being the smallest increments. Optionally, the VDL may be coupled to an OCT implementation  252  that allows for a single detection path or receiver, which is generally described in U.S. patent application Ser. No. 12/018,706, incorporated by reference herein. 
     The light reflected back from the reference path travels back through polarization maintaining fiber  2010  to circulator  222 . Light from circulator  222  is sent through polarization maintaining fiber  2011  to the coupler  216 . Light from the coupler  216  is sent through polarization maintaining fiber  2012  to the photoreceiver  250 . The photoreceiver  250  includes a detection element, such as an InGaAs photodiode and a transimpedance amplifier that converts the electrical current signal generated by photons absorbed by the photodetector element into a voltage signal that can be read by the digitizer. In one embodiment, a polarizing beam splitter divides horizontal and vertical polarization components returning from the sample and reference paths. Dual photoreceivers measure horizontal and vertical interference fringe intensities versus depth, γ h (z) and γ v (z), respectively. 
     INCORPORATION BY REFERENCE 
     References and citations to other documents, such as patents, patent applications, patent publications, journals, books, papers, web contents, have been made throughout this disclosure. All such documents are hereby incorporated herein by reference in their entirety for all purposes. 
     EQUIVALENTS 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.