Patent Publication Number: US-9835436-B2

Title: Wavelength encoded multi-beam optical coherence tomography

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of, and claims priority to U.S. patent application Ser. No. 14/925,993, titled “MULTI-CHANNEL OPTICAL COHERENCE TOMOGRAPHY,” filed on Oct. 29, 2015, all of which is incorporated herein by reference in its entirety for all purposes, which is a continuation of, and claims priority to U.S. patent application Ser. No. 14/069,626, titled “MULTI-CHANNEL OPTICAL COHERENCE TOMOGRAPHY,” filed on Nov. 1, 2013, all of which is incorporated herein by reference in its entirety for all purposes, and also claims the benefit of Japanese Patent Application No. 2014-221411 titled “MULTI-CHANNEL OPTICAL COHERENCE TOMOGRAPHY,” filed on Oct. 30, 2014, all of which is incorporated herein by reference in its entirety for all purposes. 
     If any definitions (e.g., figure reference signs, specialized terms, examples, data, information, definitions, conventions, glossary, etc.) from any related material (e.g., parent application, other related application, material incorporated by reference, material cited, extrinsic reference, etc.) conflict with this application (e.g., abstract, description, summary, claims, etc.) for any purpose (e.g., prosecution, claim support, claim interpretation, claim construction, etc.), then the definitions in this application shall apply. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of optical coherence tomography and, more particularly, to systems and methods for multi-channel wavelength encoded optical coherence tomography. 
     BACKGROUND 
     Various different methods have been published regarding multi-beam systems for optical coherence tomography (“OCT”) that aim at absolute velocity measurement or contrast imaging enhancement of motion occurring inside a sample. In particular, these methods provide a non-invasive way of quantifying blood velocity and improving blood vessel visualizations for biological samples. 
     First, OCT techniques that provide absolute velocity measurements are typically based on laser Doppler velocimetry. Such techniques typically involve illuminating the sample at a given location, with beams having different incident angles. For example, in the case of spectral domain OCT, one approach is to use polarization multiplexing and a beam displacer to generate two beams, as shown by R. Werkmeister, et al., in “Bidirectional Doppler Fourier-domain optical coherence tomography for measurement of absolute flow velocities in human retinal vessels,” Opt. Lett., Vol. 33, Issue 24 (2008), pp. 2967-2969. Similarly, using a knife edge mirror, two beams can be generated as shown by N. Iftimia et al., in “Dual-beam Fourier domain optical coherence tomography of zebrafish,” Opt. Express, Vol. 16, No. 18 (2008), pp. 13624-13636. These techniques typically require two detectors for acquiring both signals. 
     Another approach is to encode the two beams with different path lengths, as demonstrated by Pedersen et al. in “Measurement of absolute flow velocity vector using dual-angle, delay-encoded Doppler optical coherence tomography,” Opt. Lett., Vol. 32, No. 5 (2007), pp. 506-508. In this technique a glass plate is positioned midway into the OCT beam path. This technique has the advantage of using a single detector but has the disadvantage of dividing by three the depth range of acquired OCT signal. Additionally, in systems that use only two incident beams, the angle between the measured velocity vector and the plane formed by the two incident beams must be close to zero. If the angle is not close to zero, this method is prone to large velocity measurement errors. 
     Therefore, other techniques using three beams have been developed, such as, for example, W. Trasischker et al., in “In vitro and in vivo three-dimensional velocity vector measurement by three beam spectral-domain Doppler optical coherence tomography”, J. Biomed. Opt., Vol. 18, No. 11, (2013), pp. 116010-1-116010-11. W. Trasischker et al. utilized three sources and three detectors in the case of spectral domain OCT. 
     Second, OCT techniques that provide imaging contrast enhancement of motion inside a sample are typically performed with two scanning beams having the same incident angle on the sample. This technique typically involves scanning the sample such that the two beams scan the same location at two different instants. Depending on the delay between the two instants, the motion contrast can be modified. Typically, slow motion is better visualized with a larger delay. Such techniques provide, in living tissues for example, image of blood vessels, namely angiographies. 
     Various different OCT methods have been published for generating and acquiring the two beams. For example, methods with polarization multiplexing have been demonstrated by Makita et al., in “Dual-beam-scan Doppler optical coherence angiography for birefringence-artifact-free vasculature imaging,” Opt. Express, Vol. 20, No. 3 (2012), pp. 2681-2692 (US Patent US20120120408 A1). In that method, one light source and two detectors were used. However, issues with the sample birefringence may cause contrast deterioration. Another variant uses non-polarizer elements, such as shown by S. Zotter, et al., in “Visualization of microvasculature by dual-beam phase-resolved Doppler optical coherence tomography,” Opt. Express, Vol 19, No. 2 (2011), pp. 1217-1227. But in this case, significant losses of the signal exist. Moreover, the previous method was performed with two sources and two detectors, increasing cost and complexity. 
     Another approach for generating two beams from a single light source involves encoding each beam with a free-space acousto-optic frequency shifter, as demonstrated by S. Kim et al., “Multi-functional angiographic OFDI using frequency-multiplexed dual-beam illumination”, Opt. Express, Vol. 23, No. 07 (2015), pp. 8939-8947. One disadvantage of this method is the limitation of the distance between beams on the sample due to limited optical bandwidth of the frequency shifter for larger beams. 
     Therefore, there is a need for a system and/or method for optical coherence tomography that addresses at least some of the problems and disadvantages associated with conventional systems and methods. 
     SUMMARY 
     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking into consideration the entire specification, claims, drawings, and abstract as a whole. 
     In one aspect, an optical coherence tomography apparatus includes a first electro-magnetic radiation (EMR) source provides EMR to a first optical path and a second optical path, wherein the first optical path is associated with a sample and the second optical path is associated with a reference. A multi-beam generator unit (MBGU) couples to the first optical path and a scanning system. The MBGU generates a first EMR beam and a second EMR beam, the first EMR beam having different wavelength contents than the second EMR beam. The MBGU provides the first EMR beam and the second EMR beam to the scanning system. The scanning system illuminates the sample with the first EMR beam and the second EMR beam, at a first time and a second time, at a first location and a second location of the sample, the second location being near to the first location. In one embodiment, the second location is near to the first location when the EMR beams illuminating the first and second location are at least partially overlapping. An interference module couples to the first optical path and the second optical path. The interference module generates interference signals based on received EMR returning from the reference and the first EMR beam and the second EMR beam returning from the sample. A detector coupled to the interference module generates detection signals based on received interference signals. A processor coupled to the detector processes detection signals received from the detector and generates optical coherence tomography data based on the processed detection signals. 
     In one embodiment, the MBGU generates the first EMR beam and the second EMR beam to comprise variable relative directions. In one embodiment, the scanning system comprises a pivot point and the MBGU provides the first EMR beam and the second EMR beam so as to intersect with the pivot point. In one embodiment, generating the first EMR beam and the second EMR beam to comprise variable relative directions comprises manipulating optical components including at least one of the following: selecting or moving one of the plurality of optical paths wherein each optical paths comprises a dedicated optical fiber; a fiber Bragg grating, a blazed grating; a dispersive medium; an optical switch; a dichroic mirror; a wavelength division multiplexer; and moving a mirror to change relative directions or distances between the first EMR beam and the second EMR beam. 
     In one embodiment, the MBGU generates the first EMR beam and the second EMR beam to comprise overlapping wavelength contents. In one embodiment, the MBGU generates the first EMR beam and the second EMR beam without regard to a polarization relationship between the first EMR beam and the second EMR beam. In one embodiment, the optical coherence tomography data comprises at least one of the following: a phase difference; an absolute phase difference; a square of the phase difference; a phase variance; an amplitude decorrelation and a speckle decorrelation between at least two complex OCT signals originating from the second location. 
     In one embodiment, the processor reduces motion artifacts. In one embodiment, the first optical path and the second optical path share common optical components. In one embodiment, the processor compares at least two complex OCT signals originating from the second location. 
     In another aspect, an optical coherence tomography method includes providing electro-magnetic radiation (EMR) to a first optical path and a second optical path, wherein the first optical path is associated with a sample and the second optical path is associated with a reference. A first EMR beam and a second EMR beam are generated, the first EMR beam having different wavelength contents than the second EMR beam. The first EMR beam and the second EMR beam are provided to a scanning system. The sample is illuminated with the first EMR beam and the second EMR beam, at a first time and a second time, at a first location and a second location of the sample, the second location being near to the first location. In one embodiment, the second location is near to the first location when the EMR beams illuminating the first and second location are at least partially overlapping. Interference signals are generated based on received EMR returning from the reference and the first EMR beam and the second EMR beam returning from the sample. Detection signals are generated based on the interference signals. Detection signals are processed to generate optical coherence tomography data based on the processed detection signals. 
     In another aspect an optical coherence tomography apparatus includes a first electro-magnetic radiation (EMR) source provides EMR to a first optical path and a second optical path, wherein the first optical path is associated with a sample and the second optical path is associated with a reference. A multi-beam generator unit (MBGU) couples to the first optical path and a scanning system. The MBGU generates a first EMR beam, a second EMR beam, and a third EMR beam, the first EMR beam, the second EMR beam, and the third EMR beam having different wavelength contents. The MBGU being further provides the first EMR beam, the second EMR beam, and the third EMR beam to the scanning system. The scanning system illuminates the sample with the first EMR beam, the second EMR beam, and the third EMR beam such that, at the sample surface, the first EMR beam, the second EMR beam, and the third EMR beam comprise linearly independent vectors. An interference module coupled to the first optical path and the second optical path. The interference module generates interference signals based on received EMR returning from the reference and the first EMR beam, the second EMR beam, and the third EMR beam returning from the sample. A detector coupled to the interference module generates detection signals based on received interference signals. A processor coupled to the detector processes detection signals received from the detector and generates optical coherence tomography data based on the processed detection signals. 
     In one embodiment, the MBGU generates the first EMR beam, the second EMR beam, and the third EMR beam to comprise variable relative distances. In one embodiment, the MBGU generates the first EMR beam, the second EMR beam, and the third EMR beam by manipulating optical components including at least one of the following: one of a plurality of optical paths wherein each optical path comprises a dedicated optical fiber; a fiber Bragg grating, a blazed grating; a dispersive medium; an optical switch; a dichroic mirror; a wavelength division multiplexer; and a mirror. In one embodiment, the MBGU generates the first EMR beam, the second EMR beam, and the third EMR beam to comprise overlapping wavelength contents. 
     In one embodiment, the MBGU generates the first EMR beam, the second EMR beam, and the third EMR beam without regard to a polarization relationship between the first EMR beam, the second EMR beam, and the third EMR beam. In one embodiment, the processor calculates at least three phase differences of at least six complex OCT signals originating from at least one nearby position on the sample. In one embodiment, a position is nearby on the sample when the EMR beams illuminating the sample at a given position are at least partially overlapping. In one embodiment, the processor reduces motion artifacts. In one embodiment, the first optical path and the second optical path share common optical components. 
     In another aspect, an optical coherence tomography method includes providing electro-magnetic radiation (EMR) to a first optical path and a second optical path, wherein the first optical path is associated with a sample and the second optical path is associated with a reference. A first EMR beam, a second EMR beam, and a third EMR beam are generated, the first EMR beam, the second EMR beam, and the third EMR beam having different wavelength contents. The first EMR beam, the second EMR beam, and the third EMR beam are provided to a scanning system. The sample is illuminated with the first EMR beam, the second EMR beam, and the third EMR beam such that the first EMR beam, the second EMR beam, and the third EMR beam comprise linearly independent vectors. Interference signals are generated based on received EMR returning from the reference and the first EMR beam, the second EMR beam, and the third EMR beam returning from the sample. Detection signals are generated based on received interference signals. Detection signals received from the detector are processed and optical coherence tomography data are generated based on the processed detection signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
         FIGS. 1 a  and 1 b    are block diagrams showing a multi-channel optical coherence tomography apparatus in accordance with one embodiment; 
         FIG. 2  is a block diagram showing a multi-channel optical coherence tomography apparatus in accordance with another embodiment; 
         FIGS. 3 a  and 3 b    are block diagrams showing two scanning types with a plurality of beams, in accordance with one embodiment; 
         FIG. 4  is a block diagram showing a multi-beam generator unit in accordance with one embodiment; 
         FIGS. 5 a  and 5 b    are block diagrams showing a multi-beam generator unit in accordance with another embodiment; 
         FIGS. 6 a  and 6 b    are block diagrams showing a multi-beam generator unit in accordance with yet another embodiment; 
         FIGS. 7 a  and 7 b    are block diagrams showing a multi-beam generator unit in accordance with still another embodiment; 
         FIG. 8  is a block diagram showing a multi-channel optical coherence tomography method in accordance with one embodiment; 
         FIG. 9  is a block diagram showing a multi-beam generator unit in accordance with another embodiment; 
         FIG. 10  is a block diagram showing a multi-beam generator unit in accordance with another embodiment; 
         FIG. 11  is a block diagram showing a multi-beam generator unit in accordance with another embodiment; 
         FIG. 12  is a block diagram showing a multi-beam generator unit in accordance with another embodiment; and 
         FIG. 13  is a block diagram showing a multi-channel optical coherence tomography method in accordance with one embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope of the invention. 
     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. Those or ordinary skill in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning routine operations or devices such as light sources, fiber coupling techniques, optical scanning techniques, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art. 
     Generally, the various embodiments described herein provide technical advantages over legacy systems and method. For example, In accordance with exemplary embodiments of the present invention, a multi-beam optical coherence tomography (OCT) apparatus can provide incident beams with different wavelength contents in order to enhance motion imaging contrast and/or to measure absolute velocity of motions inside a sample under study. The disclosed embodiments can be applied, for example, to spectral domain OCT or swept-source OCT. In some embodiments, because each beam is encoded according to its wavelength bandwidth, the resulting interference signals can be detected by a single detection module. In the case of some spectral domain OCT embodiments, each beam is simultaneously detected. In the case of some swept-source OCT embodiments, detection is time dependent and therefore light source sweep speed should be configured to be smaller than motion speed inside the sample under the study. With such exemplary particular arrangements, a smaller wavelength bandwidth implies larger axial resolution. 
     However, in some embodiments, smaller wavelength bandwidths are only used for motion contrast and velocity measurements. In such embodiments, larger axial resolution is usually not an issue since these measurements typically involve depth averaging before display. Regarding the OCT structure imaging, for a given A-line, the axial resolution is not typically affected as long as the location is scanned by all the beams whose combination reconstitutes the full wavelength bandwidth. 
     According to one exemplary embodiment of the present invention, the optical coherence tomography system can be configured to implement a method to measure absolute velocity measurements inside a sample under study. In one embodiment a multi-beam generator is configured to generate three beams with different wavelength bandwidths. In one embodiment, the beams are generated by positioning two dichroic mirrors in series. In one embodiment, the two dichroic mirrors have different reflection and transmission properties such that the original wavelength bandwidth is split into three beams with different sub-bandwidths. In one embodiment, these three beams are non-coplanar and are directed along a same direction onto a scanning system. In one embodiment, an optical system following the multi-beam generator and the scanning system allows focusing these beams onto the sample. Generally, back-reflected lights from the sample interfere with a reference light and the interference signal is detected by a detection module. In one embodiment, comparing the complex OCT signals between the three detected interference signals and knowing the directions of these three beams in a reference frame allow the measurement of absolute velocity of motions inside the sample. 
     According to another exemplary embodiment of the present invention, the optical coherence tomography system can be configured to implement a method to measure absolute velocity measurements inside a sample under study. In one embodiment a multi-beam generator is configured to generate three beams with different wavelength bandwidths. In one embodiment, three dichroic mirrors are positioned perpendicularly to an incoming beam and positioned at different positions such that output transmitted beams do not overlap between each other. In one embodiment, the three dichroic mirrors have different reflection and transmission properties such that the original wavelength bandwidth is split into three beams with different sub-bandwidths. In one embodiment, these three beams are directed along a same direction onto a scanning system and are non-coplanar. In one embodiment, an optical system following the multi-beam generator and the scanning system allows focusing these beams onto the sample. Generally, back-reflected lights from the sample interfere with a reference light and the interference signal is detected by a detection module. In one embodiment, comparing the complex OCT signals between the three detected interference signals and knowing the directions of these three beams in a reference frame allow the measurement of absolute velocity of motions inside the sample. 
     According to one exemplary embodiment of the present invention, the optical coherence tomography system can be configured to implement a method to enhance motion contrast imaging inside a sample under study. In one embodiment is a multi-beam generator is configured to generate two beams with different wavelength bandwidths. In one embodiment, the beams are generated by a dichroic mirror. In one embodiment, by means of optical components, these two beams intersect at the pivot point of a scanning system following the multi-beam generator. In one embodiment, the multi-beam generator is configured to enable altering the angle between these beams on the scanning system by translating the two beams. In one embodiment, an optical system following the multi-beam generator and the scanning system allows focusing these beams onto the sample. In one embodiment, as the multi-beam generator alters the incident angle onto the scanning system, distance between beams on the sample is modified. In one embodiment, for a given scanned position on the sample, these beams impinge with a similar incidence angle but at different instants. In one embodiment, back-reflected lights from the sample interfere with a reference light and the interference signal is detected by a detection module. In one embodiment, comparing the complex OCT signals between the two detected interference signals allows motion contrast enhancement. 
     According to one exemplary embodiment of the present invention, the optical coherence tomography system can be configured to implement a method to enhance motion contrast imaging inside a sample under study. In one embodiment, a multi-beam generator is configured to generate two beams with different wavelength bandwidths. In one embodiment, the beams are generated by a dichroic mirror. In one embodiment, by means of optical components, these two beams intersect at the pivot point of a scanning system following the multi-beam generator. In one embodiment, the multi-beam generator is configured to enable altering the angle between these beams on the scanning system by rotating the two beams. In one embodiment, an optical system following the multi-beam generator and the scanning system allows focusing these beams onto the sample. In one embodiment, as the multi-beam generator alters the incident angle onto the scanning system, distance between beams on the sample is modified. In one embodiment, for a given scanned position on the sample, these beams impinge with a similar incidence angle but at different instants. In one embodiment, back-reflected lights from the sample interfere with a reference light and the interference signal is detected by a detection module. In one embodiment, comparing the complex OCT signals between the two detected interference signals allows motion contrast enhancement. 
     According to one exemplary embodiment of the present invention, the optical coherence tomography system can be configured to implement a method to enhance motion contrast imaging inside a sample under study. In one embodiment a multi-beam generator is configured to generate two beams with different wavelength bandwidths. In one embodiment, the beams are generated by a dichroic mirror. In one embodiment, by means of optical components, these two beams intersect at the pivot point of a scanning system following the multi-beam generator. In one embodiment, the multi-beam generator is configured to enable altering the angle between these beams on the scanning system by rotating one beam and keeping fixed the other beam. In one embodiment, an optical system following the multi-beam generator and the scanning system allows focusing these beams onto the sample. In one embodiment, as the multi-beam generator alters the incident angle onto the scanning system, distance between beams on the sample is modified. In one embodiment, for a given scanned position on the sample, these beams impinge with a similar incidence angle but at different instants. In one embodiment, back-reflected lights from the sample interfere with a reference light and the interference signal is detected by a detection module. In one embodiment, comparing the complex OCT signals between the two detected interference signals allows motion contrast enhancement. 
     Turning now to various specific embodiments,  FIG. 1  is a block diagram showing a multi-channel optical coherence tomography apparatus in accordance with one embodiment. More specifically,  FIGS. 1 a  and 1 b    are a high-level block diagrams illustrating certain components of a system  100  for multi-channel optical coherence tomography, in accordance with a preferred embodiment of the present invention. Generally, in the illustrated embodiment, system  100  is configured to perform multi-beam OCT with one detection unit, providing a plurality of electro-magnetic radiation (EMR) beams onto a sample, where each EMR beam has different wavelength content. In the following descriptions, one having ordinary skill in the art will understand that systems that allow the synchronization between the light source, the scanning system, the optical switch, and the detection have been omitted for ease of discussion. 
     One of ordinary skill in the art will understand that light is a form of EMR and that OCT techniques typically use beams of light. For ease of description, EMR is sometimes described herein as “light,” or a “beam,” and sometimes as a “light beam.” One of ordinary skill in the art will understand that the properties of EMR are complex and the use of the terms “light,” “beam,” or “light beam” is not intended to limit the disclosed electro-magnetic radiation in any way, such as in terms of quantity, intensity, wavelength, etc. 
     In one embodiment, a first EMR beam has “different wavelength content” from a second EMR beam when the range of wavelengths of the first EMR beam does not overlap with the range of wavelengths in the second EMR beam. In one embodiment, a first EMR beam has “different wavelength content” from a second EMR beam when the first EMR beam comprises wavelengths that include some, but not all, wavelengths of the second EMR beam. 
       FIG. 1 a    is a block diagram showing a multi-channel OCT system  100  in accordance with one embodiment. In the illustrated embodiment, system  100  includes an EMR source  101 . In the illustrated embodiment, EMR source  101  is a light source configured to generate light having a wavelength bandwidth Δλ. In an alternate embodiment, EMR source  101  is configured to sweep in time wavelengths over bandwidth Δλ. 
     In the illustrated embodiment, an otherwise conventional optical fiber  102  couples to EMR source  101  and a fiber coupler  103 . In the illustrated embodiment, fiber coupler  103  is an otherwise conventional fiber coupler and splits light from EMR source  101  into two paths: a sample arm (beginning with fiber  104 ) and a reference arm (beginning with fiber  105 ). One of ordinary skill in the art will understand that sample and reference arms are typical components of an interferometer and that different interferometer types can be used to perform OCT techniques, such as, for example, a Michelson type interferometer, a Mach-Zehnder interferometer, or a Fizeau type interferometer. 
     In the illustrated embodiment, light in the reference arm passes through a delay line  106 . One of ordinary skill in the art will understand that there are various techniques available to produce a delay line, such as a simple length of coiled fiber, for example. Generally, a single reference arm with a single delay line typically generates a single reference signal. In one embodiment, system  100  is configured to generate two reference signals. 
     For example, as shown in  FIG. 1 b   , in one embodiment, system  100  is configured to generate two reference signals with two different path-lengths. One having ordinary skill in the art will appreciate that generating two reference signals having different path lengths can help reduce or avoid cross-talk between the plurality of beams when the reference signal path-length difference is larger than the coherence length of light. In the illustrated embodiment, an otherwise conventional fiber  116  guides light exiting delay line  106  to an otherwise conventional fiber coupler  117 . Fiber coupler  117  splits the received light into two paths: fiber  118  and fiber  119 . As shown in the illustrated embodiment, fiber  118  and fiber  119  are configured with different lengths. Both fiber  118  and fiber  119  connect to a fiber coupler  120 . As in  FIG. 1 a   , fiber coupler  120  connects to fiber  113 . Light from fiber  113  connects to the otherwise conventional fiber coupler  112 , which also receives light returning from the sample, as described in more detail below. 
     In the sample arm, as shown in  FIG. 1 a   , fiber  104  connects fiber coupler  103  to a multi-beam generator unit (MBGU)  107 . Generally, in the illustrated embodiment, MBGU  107  generates, from light received from fiber  104 , a plurality of beams distinguishable from each other by their wavelength content. That is, in one embodiment, each generated beam will comprise light have a plurality of wavelengths, thus comprising that beam&#39;s “wavelength contents.” In one embodiment, the wavelength contents of the generated beams do not overlap. In one embodiment, wherein MBGU  107  comprises band pass filters, the wavelength contents of the generated beams include some overlapping bandwidths. In one embodiment, the wavelength contents are sufficiently different that the number of overlapping bandwidths does not significantly impair OCT accuracy. 
     In the illustrated embodiment, the generated beams from MBGU  107  are directed into a scanning unit  108 . Generally, scanning unit  108  illuminates the sample and collects light returning from the sample. As shown in the illustrated embodiment, scanning unit  109  focuses the plurality of beams  109  and scans sample  110 . As described in more detail below, depending on the MBGU  107  configuration, the directions and locations of the beams  109  illuminating sample  110  can be adjusted. 
     One having ordinary skill in the art will understand that light from beams  109  is back-reflected by the sample  110 . Scanning unit  108  receives the back-reflected light returning from sample  110  and directs the returning light to the MBGU  107 . As shown in the illustrated embodiment, MBGU  107  redirects returning light to coupler  103  via fiber  104 . Fiber coupler  103  directs returning light to fiber coupler  112  via fiber  111 . 
     As described above, fiber coupler  112  also receives light from the reference arm. As such, in the illustrated embodiment, light from the reference arm and light returning from the sample interfere in coupler  112 , thereby generating interference signals. Generally, in the illustrated embodiment, the interference signal is acquired as a function of the wavelength, wherein different wavelength bandwidths correspond to different beams. A detection module  114  coupled to coupler  112  detects the interference signals, and generates detection signals. A processing unit  115  coupled to detection module  114  processes received detection signals, thereby generating OCT data (or passing processed detection signals on for further processing into OCT data) that can subsequently be organized, transformed, etc. for presentation to a user, in many cases as an image. One having ordinary skill in the art will appreciate that interference signals, detection signals, and processing can be achieved in a variety of ways, customizable depending on the desired application, general sample characteristics, use environment, etc. 
     Generally, as used herein, a “processing unit” or “processing system” is a collection of one or more components configured to OCT analysis based on received input. In one embodiment, a processing system is configured to perform OCT motion analysis, which one of ordinary skill in the art will understand to be analysis of the velocity (relative or absolute) of particles inside the sample. In one embodiment, a processing system is further configured to perform volumetric or structural analysis, which one of ordinary skill in the art will understand to be analysis of the physical structure and/or volume of the sample. 
     The embodiments illustrated in  FIGS. 1 a  and 1 b    include a single detector. In an alternate embodiment, multiple detectors can also be employed. For example,  FIG. 2  is a block diagram showing a multi-channel OCT apparatus in accordance with a two-detector embodiment. Specifically,  FIG. 2  shows a multi-channel OCT system  200  that includes multiple detectors. Specifically, system  200  can be configured for multi-beam OCT with two detection units, providing onto a sample multiple beams having different wavelength content. One having ordinary skill in the art will understand that using two interferometers and different fiber lengths improves decreasing the effect of cross-talk noise between the measured OCT signals. 
     As shown in the illustrated embodiment, system  200  includes EMR source  201 . As with EMR source  101 , EMR source  201  can also be configured in a variety of embodiments, including generating light that has, or that sweeps in time, a wavelength bandwidth Δλ. In the illustrated embodiment, a fiber  202  delivers light to an otherwise conventional fiber coupler  203 . Fiber coupler  203  directs received light into interferometer  204  and interferometer  205 . As with previous embodiments, one of ordinary skill in the art will understand that different interferometer types can be used to perform OCT techniques, such as, for example, a Michelson type interferometer, a Mach-Zehnder interferometer, or a Fizeau type interferometer. 
     In the illustrated embodiment, interferometers  204  and  205  are substantially similar. In alternate embodiments, interferometers  204  and  205  can be configured with, for example, different optical components, interference/detection/processing techniques, operating parameters, etc. For ease of discussion, interferometers  204  and  205  will be described as having similar characteristics. 
     In the illustrated embodiment, fiber  206  ( 207 ), of interferometer  204  ( 205 ), conveys light to a fiber coupler  208  ( 209 ). Fiber coupler  208  ( 209 ) splits light into a sample arm (fiber  210  ( 211 )) and a reference arm (fiber  212  ( 213 ). In the illustrated embodiment, light in the reference arm  212  ( 213 ) passes through a delay line  214  ( 215 ). Reference light exiting delay line  214  ( 215 ) is directed via fiber  216  ( 217 ) to fiber coupler  220  ( 221 ). 
     In the illustrated embodiment, the sample arm fiber  210  ( 211 ) directs received light from fiber coupler  208  ( 209 ) to a joint multi-beam generator unit (MBGU)  226 . MBGU  226  generates, from the light received from fibers  210  and  211 , a plurality of beams distinguishable by their wavelength content. In the illustrated embodiment, the generated beams from MBGU  226  are directed into a scanning unit  227 . Generally, scanning unit  227  illuminates the sample and collects light returning from the sample. As shown in the illustrated embodiment, scanning unit  227  focuses the plurality of beams  228  and scans sample  229 . As described in more detail below, depending on the MBGU  226  configuration, the directions and locations of the beams  228  illuminating sample  229  can be adjusted. 
     One having ordinary skill in the art will understand that light from beams  228  is back-reflected by the sample  229 . Scanning unit  227  receives the back-reflected light returning from sample  229  and directs the returning light to the MBGU  226  and then to interferometers  204  ( 205 ). As shown in the illustrated embodiment, MBGU  226  redirects returning light to coupler  208  ( 209 ) via fiber  210  ( 211 ). Fiber coupler  208  ( 209 ) directs returning light to fiber coupler  220  ( 221 ) via fiber  218  ( 219 ). 
     As described above, fiber coupler  220  ( 221 ) also receives light from the corresponding reference arm. As such, in the illustrated embodiment, light from the corresponding reference arm and light returning from the sample interfere in coupler  220  ( 221 ), thereby generating interference signals. Generally, in the illustrated embodiment, the interference signal is acquired as a function of the wavelength, wherein different wavelength bandwidths correspond to different beams. A detection module  222  ( 223 ) coupled to coupler  220  ( 221 ) detects the interference signals, and generates detection signals. A processing unit  224  ( 225 ) coupled to detection module  222  ( 223 ) processes received detection signals, thereby generating OCT data. 
     As described above, systems  100  and  200  can be configured to illuminate the sample in a variety of configurations.  FIGS. 3 a  and 3 b    illustrate two such configurations, each using a plurality of beams. 
       FIG. 3 a    is a block diagram showing a MBCU/scanning unit in accordance with one embodiment. More particularly,  FIG. 3 a    illustrates a system  300  configured to deliver light beams having the same incident angle when scanning the same position of the sample. In  FIG. 3 a   , light beams from an associated MBGU enter a scanning unit  302  at interface  301 . In the illustrated embodiment, two beams ( 303 ,  304 ) enter the scanning unit  302 . 
     In the illustrated embodiment, scanning unit  302  includes a scanning mirror  305  and a lens  306 . One having ordinary skill in the art will understand that other configurations, including more complex systems, can also be employed. In the illustrated embodiment, beams  303  and  304  intersect on scanning mirror  305  and are reflected towards the lens  306 . Lens  306  focuses the beams onto a sample  307 . 
     In the illustrated embodiment, by changing the intersection angle  308  between beams  303  and  304 , the distance  309  between beams (at their focusing positions on sample  307 ) can be adjusted. As the beams  303  and  304  are transversally scanned onto sample  307 , the delay between beams to scan the same position on the sample depends on the scanning speed and the distance  309 . One having ordinary skill in the art will understand that this adjustable delay can be used in Doppler OCT to contrast motions inside the sample. For example, increasing the delay generally improves observation of slower motions in the sample. 
       FIG. 3 b    is a block diagram showing a MBGU/scanning unit in accordance with one embodiment. More particularly,  FIG. 3 b    illustrates a system  350  configured to deliver light beams that impinge onto the sample at the same location with different incident angles. In  FIG. 3 b   , light beams from an associated MBGU enter a scanning unit  311  at interface  310 . In the illustrated embodiment, three beams ( 312 ,  313 , and  314 ) enter the scanning unit  311 . 
     In the illustrated embodiment, scanning unit  311  includes a scanning mirror  315  and a lens  316 . One having ordinary skill in the art will understand that other configurations, including more complex systems, can also be employed. In the illustrated embodiment, beams  312 ,  313  and  314  impinge on the scanning mirror  315  in parallel directions and are reflected towards the lens  316 . Lens  316  focuses the beams at a given location  317  onto a sample  318 . 
     In the illustrated embodiment, by changing the distances between beams  312 ,  313  and  314 , the incident angles  320  between the beams at focusing position  317  on sample  318  can be adjusted. One having ordinary skill in the art will understand that combining the OCT signals originating from the beams  312 ,  313  and  314 , and the knowledge of the incident beam geometry, provides the absolute velocity of motions inside the sample. 
       FIG. 4  is a block diagram showing a multi-beam configuration unit in accordance with another embodiment. More particularly,  FIG. 4  illustrates an MBGU  400  suitable for use as, for example, MBGU  107  of  FIG. 1   a.    
     In the illustrated embodiment, fiber  401  corresponds to, for example, fiber  404  of  FIG. 1 a   . As shown in the illustrated embodiment, fiber  401  directs light to a collimator  402 , which generates a beam  403 . In the illustrated embodiment, the beam  403  wavelength contents comprise a wavelength bandwidth A. In the following, it is assumed that wavelength bandwidth Δλ can be decomposed into three sub band-widths Δλ 1 , Δλ 2  and Δλ 3 , such as, for example, Δλ 1 ≦Δλ 2 ≦Δλ 3 . 
     In the illustrated embodiment, beam  403  hits a dichroic mirror  404 . In the illustrated embodiment, dichroic mirror  404  reflects beam  405  (having wavelength bandwidth Δλ 1 ) and transmits beam  406  (having wavelength bandwidths Δλ 2  and Δλ 3 ). A collimator  407  collects beam  405 , directing the light from beam  405  along fiber  413  to collimator  416 . 
     In the illustrated embodiment, beam  406  hits a dichroic mirror  408 . In the illustrated embodiment, dichroic mirror  408  reflects beam  409  (having wavelength bandwidth Δλ 2 ) and transmits beam  410  (having wavelength bandwidth Δλ 3 ). A collimator  411  collects beam  409 , directing the light from beam  409  along fiber  414  to collimator  418 . A collimator  412  collects beam  410 , directing the light from beam  410  along fiber  415  to collimator  420 . One having ordinary skill in the art will appreciate that other configurations can also be used to generate beams  405 ,  409 , and  410 , such as, for example, using directly output beams  405 ,  409  and  410  with additional optical elements. 
     In the illustrated embodiment, the outputs beams  417 ,  419 , and  421 , each having different wavelength contents, are directed onto a scanning unit (not shown), such as scanning unit  108  of  FIG. 1 a   , for example. One having ordinary skill in the art will appreciate that these three beams will ordinarily have parallel directions before hitting at three different locations of the scanning unit, for example, as shown in  FIG. 3 b   . Moreover, in the illustrated embodiment, the three are linearly independent before impinging upon the sample. In the illustrated embodiment, adjusting the length of fibers  413 ,  414 , and  415  allows a user to change the optical path of each output beam. One having ordinary skill in the art will appreciate that this configuration can be employed to help reduce or remove cross-talks between beams  417 ,  419  and  421 . 
     For example, assuming that using dichroic mirrors ( 404 ,  408 ) there are wavelength overlaps between Δλ 1  and Δλ 2  and between Δλ 2  and Δλ 3 , but that there is no overlap between Δλ 1  and Δλ 3 . One having ordinary skill in the art will understand that these relationships depend on the transition slope between transmission and reflection of the dichroic mirrors. Consequently, cross-talks between beams  417  and  419  or between beams  419  and  421  might exist, but cross-talks between  417  and  421  can be expected to be negligible. 
     As such, the path-lengths of beams  417  and  421  can be adjusted to be similar (or at least having a path-length difference less than the coherence length) to each other (changing fiber lengths of fibers  413  and  415  for example) and to be different from the path-length of beam  419  (fiber  414 ). One having ordinary skill in the art will appreciate that using two different path-lengths for the sample beams requires two path-lengths for the reference light. One such embodiment has been described above with respect to  FIG. 1 b   , for example. Moreover, one having ordinary skill in the art will also appreciate that these embodiments can be extended to provide more than two different path-lengths. 
       FIGS. 5 a  and 5 b    are block diagrams showing an MBGU in yet another embodiment. The embodiments shown in  FIGS. 5 a  and 5 b    can be configured to serve as, for example, MGBU  107  in  FIG. 1 a   . In the embodiment shown in  FIG. 5 a   , MBGU  500  includes fiber  501 . Fiber  501  corresponds to, for example, fiber  104  of  FIG. 1 a   . As shown in the illustrated embodiment, fiber  501  connects to a collimator  502 , which generates a beam  503 . In the illustrated embodiment, beam  503  is configured with a wavelength bandwidth Δλ. 
     In the illustrated embodiment, beam  503  hits unit  504 . In the illustrated embodiment, unit  504  is configured to transmit output beams  505 ,  506 , and  507  such that output beams  505 ,  506 , and  507  have different wavelength contents and different spatial locations. For example, in one embodiment, the wavelength bandwidths of output beams  505 ,  506 , and  507  are Δλ 1 , Δλ 2  and Δλ 3 , respectively. 
     Generally, outputs beams  505 ,  506 , and  507  are configured to be directed to a scanning system, such as, for example, scanning unit  108  of  FIG. 1 a   . One having ordinary skill in the art will appreciate that these three output beams are typically linearly independent and typically have parallel directions before hitting at three different locations of the scanning system (e.g., as shown in  FIG. 3 b   ). Additional geometry of one embodiment is illustrated in  FIG. 5   b.    
       FIG. 5 b    displays a view  550  of unit  504  in the plane perpendicular to the propagation direction, in one embodiment. In the illustrated embodiment, unit  504  comprises three band-pass filters  508 ,  509 , and  510 . As described above, in one embodiment, the forward input path to unit  504  is beam  503 , which comprises light having wavelength bandwidth Δλ. In the illustrated embodiment, band-pass filters  508 ,  509 , and  510  transmit wavelength bandwidths Δλ 1 , Δλ 2  and Δλ 3 , respectively. As such, the output beams  505 ,  506 , and  507  comprise light having wavelength bandwidths Δλ 1 , Δλ 2  and Δλ 3 , respectively, arranged in space according to the geometry of the band-pass filters in the illustrated embodiment. 
     One having ordinary skill in the art will also understand that this embodiment assumes that the incident power of the EMR/light source (e.g., EMR source  101 ) is high enough to tolerate losses in the forward path to the sample. For example, in the illustrated embodiment, if beam  503  is assumed circular with surface area given by surface area  511 , losses in the forward path will be equal to the ratio between the surface area of the band-pass filters and the surface area  511 . Accordingly, the EMR source can be configured to provide sufficient incident power to tolerate such losses. Moreover, one having ordinary skill in the art will appreciate that the backward path (light returning from the sample) is not affected by such losses. 
       FIGS. 6 a  and 6 b    are block diagrams showing an MBGU in yet another embodiment. The embodiments shown in  FIGS. 6 a  and 6 b    can be configured to serve as, for example, MGBU  107  in  FIG. 1 a   . In the embodiment shown in  FIG. 6 a   , MBGU  600  includes fiber  601 . Fiber  601  corresponds to, for example, fiber  104  of  FIG. 1 a   . As shown in the illustrated embodiment, fiber  601  connects to a collimator  602 , which generates a beam  603 . In the illustrated embodiment, beam  603  is configured with a wavelength bandwidth Δλ. In the illustrated embodiment, bandwidth Δλ is configured to be split into three sub-bandwidths labeled: Δλ 1 , Δλ 2 , and Δλ 3 . In one embodiment, the three sub-bandwidth wavelength contents do not overlap. 
     In the illustrated embodiment, beam  603  hits unit  604 . In the illustrated embodiment, unit  604  is shown as a cube comprising faces having different coatings configured to realize beam splitting according to beam wavelength. For example, in the illustrated embodiment, face  605  of unit  604  reflects Δλ 1  and transmits Δλ 2  and Δλ 3 . In the illustrated embodiment, face  610  reflects Δλ 2  and transmits Δλ 1  and Δλ 3 . And in the illustrated embodiment, face  613  reflects Δλ 3  and transmits Δλ 1  and Δλ 2 . One having ordinary skill in the art will understand that different methods can also be utilized to achieve a similar splitting. 
     In the illustrated embodiment, face  605  reflects Δλ 1  as beam  606  and transmits Δλ 2  and Δλ 3  as beam  609 . As illustrated, beam  606  hits mirrors  607  and  608 . In the illustrated embodiment, mirrors  607  and  608  are mirrors and are fixed on mount  618 , which can move along direction  619 . So configured, mount  618  can be employed to adjust the path-length of beam  606 . Additionally, in the illustrated embodiment, mirror  608  is fixed on mount  620 , which can move with respect to mount  618  along direction  621 . As illustrated, after reflection on mirror  608 , beam  606  transmits through face  610  and face  613  to location  614 . 
     As described above, in the illustrated embodiment, the transmitted beam  609  comprises two sub-bandwidths Δλ 2  and Δλ 3 . In the illustrated embodiment, sub-bandwidth Δλ 2  of beam  609  is reflected by face  610  to generate beam  611 . Similarly, beam  609  is transmitted through face  610  to generate beam  612 . As shown, beam  611  (having sub-bandwidth Δλ 2 ) is transmitted through face  613  to location  614 . 
     In the illustrated embodiment, beam  612  hits mirrors  615  and  616 . In the illustrated embodiment, mirrors  615  and  616  are fixed on a mount  622  that can move along direction  623 . Similarly, mirror  616  is fixed on mount  624  that can move along direction  625  with respect to mount  622 . One having ordinary skill in the art will appreciate that these movements can be configured to adjust the path length of beam  612 . 
     After reflecting from mirror  616 , beam  612  hits a plate  617  and is reflected by face  613  to location  614 . In the illustrated embodiment, plate  617  is configured with a 45 degree angle with respect to the direction of propagation of beam  612 . As such, one skilled in the art will understand that, in the illustrated embodiment, depending on its thickness, plate  617  therefore displaces beam  612  in the direction  626  as shown in view  650  of  FIG. 6   b.    
     In the illustrated embodiment, the three beams  606 ,  611 , and  612 , having respective sub-bandwidths Δλ 1 , Δλ 2  and Δλ 3 , exit MBGU  600  and are directed onto the scanning system, such as scanning unit  108  of  FIG. 1 a   , for example. One having ordinary skill in the art will appreciate that these three beams are typically linearly independent and typically have parallel directions before hitting at three different locations of the scanning system (as shown in  FIG. 3 b   , for example). 
       FIG. 6 b    shows a view  350  illustrating the three output beam locations in a plane perpendicular to their propagation direction at location  614  of  FIG. 6 a   . In one embodiment, these locations can be changed by, for example, displacing mount  620  and mirror  608  to shift beam  606  with respect to beam  611  in the direction  621 . Similarly, in one embodiment, by displacing mount  624  and mirror  616 , beam  612  is shifted with respect to beam  611  in the direction  625 . As described above, changing the thickness of plate  617  modifies the displacement along direction  625  of beam  612  with respect to beam  611 . Accordingly, the illustrated embodiment, can be configured to help reduce the cross-talks between sub-bandwidths Δλ 1  and Δλ 3  interfering with the same reference light. Similarly, in one embodiment, sub-bandwidths Δλ 2  can be detected with a different (e.g., different reference path-length) reference light, cross-talks with other sub-bandwidths can be expected to be minimized. 
       FIGS. 7 a  and 7 b    are block diagrams showing an MBGU in yet another embodiment. The embodiments shown in  FIGS. 7 a  and 7 b    can be configured to serve as, for example, MGBU  226  in  FIG. 2 . In the embodiment shown in  FIG. 7 a   , MBGU  700  includes fiber  701 . Fiber  701  corresponds to, for example, fiber  210  of  FIG. 2  and fiber  705  corresponds to, for example, fiber  211  of  FIG. 2 . As shown in the illustrated embodiment, fiber  701  connects to a collimator  702 , which generates a beam  703 . Similarly, fiber  705  connects to a collimator  706 , which generates a beam  707 . In the illustrated embodiment, beams  703  and  707  are configured with a wavelength bandwidth Δλ. In the illustrated embodiment, bandwidth Δλ is configured to be split into three sub-bandwidths labeled: Δλ 1 , Δλ 2 , and Δλ 3 . In one embodiment, the three sub-bandwidth wavelength contents do not overlap. 
     In the illustrated embodiment, beam  703  hits unit  704 . In the illustrated embodiment, unit  704  is configured to reflect wavelengths Δλ 2  and transmit wavelengths Δλ 1  and Δλ 3 . As such, in the illustrated embodiment, the transmitted beam  708 , towards unit  709 , is composed of two sub-bandwidths Δλ 1  and Δλ 3 . Similarly, only sub-bandwidth Δλ 2  of beam  707  is reflected by unit  704  towards unit  709 . In the illustrated embodiment, beams  707  and  703  do not hit unit  704  at the same location and therefore, the transmitted and reflected beams generally do not spatially overlap and are not co-planar. 
     In the illustrated embodiment, unit  709  reflects sub-bandwidths Δλ 1  and Δλ 2  and transmits sub-bandwidth Δλ 3 . Therefore, in the illustrated embodiment, beam  707  is reflected by unit  709  towards mirror  713 . As shown, mirror  713  reflects beam  707  back to another location of unit  709 . In the illustrated embodiment, unit  709  is shown as a single unit. In an alternate embodiment, unit  709  can be configured as two separate units with similar or different reflective properties. In the illustrated embodiment, unit  709  again reflects beam  707 , to position  714 . 
     In the illustrated embodiment, beam  708  is composed of sub-bandwidths Δλ 1  and Δλ 3 . As such, when beam  708  hits unit  709 , sub-bandwidth Δλ 1  is reflected as beam  710  towards mirror  713  and sub-bandwidth Δλ 3  is transmitted as beam  711  towards mirror  712 . In the illustrated embodiment, after reflection on mirror  713 , beam  710  is directed to unit  709 , which reflects beam  710  to position  714 . Similarly, in the illustrated embodiment, beam  711  is reflected by mirror  712  towards unit  709 , which transmits beam  711  to position  714 . 
     In the illustrated embodiment, the three beams  710 ,  707 , and  711 , having respective sub-bandwidths Δλ 1 , Δλ 2  and Δλ 3 , exit MBGU  700  (at position  714 ) and are directed onto the scanning system, such as scanning unit  227  of  FIG. 2 , for example. One having ordinary skill in the art will appreciate that these three beams are typically not co-planar and typically have parallel directions before hitting at three different locations of the scanning system (as shown in  FIG. 3 b   , for example). 
       FIG. 7 b    shows a view  750  illustrating the three output beam locations in a plane perpendicular to their propagation direction at location  714  of  FIG. 7 a   . In one embodiment, these locations can be changed by, for example, displacing collimator  706  to move beam  707  and mirror  712  to move beam  711 . Accordingly, the illustrated embodiment can be configured to help reduce the cross-talks between overlapping sub-bandwidths. In one embodiment, adjusting the fiber lengths  701  and  705  allows reducing cross-talks between sub-bandwidths Δλ 2  and Δλ 1  and between sub-bandwidths Δλ 2  and Δλ 3 . 
     So configured, the embodiments described herein can be configured for OCT operations that overcome at least some of the disadvantages associated with legacy systems and methods. For example,  FIG. 8  is a block diagram showing a process for obtaining absolute velocity of motions inside the sample from the signal acquired by, for example, the embodiments of FIGS.  1   a / 1   b  and  2 . As described above, these embodiments allow scanning the sample transversally with three incident beams on the same location on the sample, having three different directions for each acquired position. 
     As shown in  FIG. 8 , two positions are considered: P ( 801 ) and Q ( 808 ). Incident scanning beams on the sample are denoted in  FIG. 8  by A, B, and C. The directions of beams A, B, C are {right arrow over (a)}, {right arrow over (b)}, and {right arrow over (c)} respectively. In the illustrated embodiment, these directions are linearly independent. Additionally, in the illustrated embodiment, it is assumed that measurements at positions P and Q onto the sample are acquired at time t and time t+T, respectively. Moreover, location Q is assumed near or “close to” P. One having ordinary skill in the art will understand that “Q close to P” means that the two locations are close enough to enable overlap between scanning beams, thereby allowing significant correlation between OCT signals. 
     In the illustrated embodiment, the acquired interference signal for positions P and Q is encoded in wavelengths ( 802  and  809 ). Wavelength bandwidths Δλ 1 , Δλ 2 , and Δλ 3  correspond to beams A, B, and C respectively. In the illustrated embodiment, a first step is to apply band-pass filters on the interference signal such as to isolate the three bands corresponding to each beam ( 803  and  810 ). One having ordinary skill in the art will appreciate that different shape of band-pass filtered can be applied to perform this filtering. Then, for each band-pass filtered signal, the optical coherence tomography (OCT) process is applied to obtain a complex OCT signal composed of an amplitude and a phase information as a function of the depth z ( 804  and  811 ). Therefore, phases φ A (P,z,t), φ B (P,z,t), and φ C (P,z,t) are obtained for beams A, B and C, respectively. Therefore, at location Q, phases φ A (Q,z,t+T), φ B (Q,z,t+T), and φ C (Q,z,t+T) are obtained for beams A, B, and C, respectively. For each beam the following phase differences are calculated ( 805 ):
 
Δφ A =φ A ( P,z,t )−φ A ( Q,z,t+T ), Δφ B =φ B ( P,z,t )−φ B ( Q,z,t+T ), and Δφ C =φ C ( Q,z,t+T )−φ C ( Q,z,t+T ).
 
     After correction of sample motion artifacts ( 806 ), previous phase differences Δφ A , Δφ B , and Δφ C  are changed to Δ′φA, Δ′φB, and Δ′φC. Then, velocity V A , V B , and V C  along beam direction {right arrow over (a)}, {right arrow over (b)}, and {right arrow over (c)}, respectively, can be computed as: 
     
       
         
           
             
               
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     with λ being the central wavelength of the light source, n being the sample refractive index ( 807 ). Assuming that directions {right arrow over (a)}, {right arrow over (b)}, and {right arrow over (c)} are known in a given reference frame, the velocity of motions inside the sample can be retrieved. 
     As described above, the disclosed embodiments can be configured for a variety of OCT techniques. For example,  FIG. 9  is a block diagram showing an MBGU  900  in another embodiment, which can be configured such as MBGU  107  of  FIG. 1 a   , for example. In the illustrated embodiment, MBGU  900  can be configured to provide two beams whose incident angle is the same for a given location on the sample and that scan the sample with a variable delay between each other, allowing imaging, via OCT, of motions inside the sample with different velocities. 
     In the illustrated embodiment, fiber  901  can be configured to correspond to fiber  104  of  FIG. 1 . In the illustrated embodiment, fiber  901  directs light having wavelength bandwidth Δλ to collimator  902 . As shown in the illustrated embodiment, collimator  902  generates a beam  903  that is directed to a dichroic mirror  904 . In the illustrated embodiment, dichroic mirror  904  splits beam  903  into two bandwidths: Δλ 1  is transmitted and forms beam  905 , and Δλ 2  is reflected and forms beam  906 . 
     Beam  905  is collected by collimator  907 , which passes beam  905  via fiber  909  to collimator  911 , exiting as beam  912 . In the illustrated embodiment, beam  912  exits collimator  911 , hits the lens  913 , is transmitted through dichroic mirror  904 , and reaches lens  919 . In the illustrated embodiment, collimator  911  and lens  913  are attached to a moving plate  914  that allows moving the beam  912  back and forth as desired along direction  921 . 
     In the illustrated embodiment, beam  906  is collected by collimator  908 , which passes beam  906  through fiber  910  to collimator  915 , exiting as beam  916 . In the illustrated embodiment, beam  916  exits collimator  915 , hits the lens  917 , is reflected by the dichroic mirror  904 , and reaches lens  919 . In the illustrated embodiment, collimator  915  and lens  917  are attached to a moving plate  918  that allows moving the beam  916  back and forth along direction  922 . 
     In the illustrated embodiment, after passing through lens  919 , beams  912  and  916  intersect at the focal point  920  of lens  919 . In one embodiment, focal point  920  corresponds to the pivot point of a scanning system, such as scanning system  108  of  FIG. 1 a   , for example. However, one having ordinary skill in the art will appreciate that off-pivot positioning can be used, for example, for full-range imaging. Similarly, displacing plates  914  and  918  along directions  921  and  922  respectively allows changing the intersecting angle of beams  912  and  916  on the scanning system. 
       FIG. 10  is a block diagram showing an MBGU  1000  in another embodiment, which can be configured such as MBGU  107  of  FIG. 1 a   , for example. In the illustrated embodiment, MBGU  1000  can be configured to provide two beams that scan the sample with a variable delay between the beams, allowing imagining, via OCT methods, of motions inside the sample with different velocities. 
     In the illustrated embodiment, fiber  1001  can be configured to correspond to fiber  104  of  FIG. 1 a   . In the illustrated embodiment, fiber  1001  directs light having wavelength bandwidth Δλ to collimator  1002 . As shown in the illustrated embodiment, collimator  1002  generates a beam  1003  that is directed to a dichroic mirror  1004 . In the illustrated embodiment, dichroic mirror  1004  splits beam  1003  into two bandwidths: Δλ 1  is transmitted and forms beam  1005 , and Δλ 2  is reflected and forms beam  1006 . 
     In the illustrated embodiment, beam  1005  is reflected by mirror  1007  to mirror  1009 . In the illustrated embodiment, mirror  1009  is configured to be rotated so as to change the direction of reflected light. In the illustrated embodiment, mirror  1009  reflects beam  1005  to mirror  1008 . As shown, mirror  1008  redirects beam  1005  to dichroic mirror  1004 . In the illustrated embodiment, dichroic mirror  1004  transmits beam  1005  to lens  1011 . 
     In the illustrated embodiment, beam  1006  is reflected from dichroic mirror  1004  to mirror  1008 . Similarly, mirror  1008  reflects beam  1006  to mirror  1009 . As shown, mirror  1009  redirects beam  1006  to mirror  1007 . And mirror  1007  reflects beam  1006  to dichroic mirror  1004 , which reflects beam  1006  to lens  1011 . 
     Before entering lens  1011  in the forward direction, an angle  1010  describes the angle between the directions of beams  1005  and  1006 . In the illustrated embodiment, the mirror  1009  pivot point is positioned at the focal point of lens  1011  such that when mirror  1009  is rotated, the directions of beams  1005  and  1006  after lens  1012  intersect at the lens  1012  focal position  1013 . In one embodiment, focal position  1013  corresponds to the pivot point of a scanning system, such as scanning system  108  of  FIG. 1 a   , for example. However, one having ordinary skill in the art will appreciate that off-pivot positioning can be used, for example, for full-range imaging. Additionally, in the illustrated embodiment, rotating the mirror  1009  allows changing the intersecting angle of beams  1005  and  1006  on the scanning system. Consequently, in one embodiment, changing this intersecting angle alters the distance between beams on the sample. 
       FIG. 11  is a block diagram showing an MBGU  1100  in another embodiment, which can be configured such as MBGU  226  of  FIG. 2 , for example. In the illustrated embodiment, MBGU  1100  can be configured to provide two beams that scan the sample with a variable delay between the beams, allowing imagining, via OCT methods, of motions inside the sample with different velocities. 
     In the illustrated embodiment, the EMR source is configured to provide light having a wavelength bandwidth Δλ that can be split into two sub-bandwidths labeled: Δλ 1  and Δλ 2 . In the illustrated embodiment, fiber  1101  can be configured to correspond to fiber  210  of  FIG. 2 . In the illustrated embodiment, fiber  1105  can be configured to correspond to fiber  211  of  FIG. 2 . 
     In the illustrated embodiment, collimator  1102  ( 1106 ) emits a beam  1103  ( 1107 ) that is directed to a dichroic mirror  1104 . In the illustrated embodiment, dichroic mirror  1104  transmits wavelength bandwidth Δλ 1  and reflects wavelength bandwidth Δλ 2 . Accordingly, as shown, dichroic mirror  1104  transmits bandwidth Δλ 1  of beam  1103 , labeled as beam  1108 . Dichroic mirror  1104  reflects bandwidth Δλ 2  of beam  1107 . Both beams  1108  and  1107  hit dichroic mirror  1109 . 
     In the illustrated embodiment, dichroic mirror  1109  is configured, for example, with similar reflective properties as dichroic mirror  1104 , therefore beam  1108  is transmitted through dichroic mirror  1109  and is directed to mirror  1110 . As shown, beam  1107  is reflected by dichroic mirror  1109  towards mirror  1112 . 
     In the illustrated embodiment, beam  1108  is consecutively reflected by mirrors  1110 ,  1111 , and  1112  to reach dichroic mirror  1109 , where it is transmitted to lenses  1114  and  1115 . Similarly, beam  1107  is consecutively reflected by mirrors  1112 ,  1111 , and  1110  to reach dichroic mirror  1109 , where it is reflected to lenses  1114  and  1115 . 
     Before entering lens  1114  in the forward direction, an angle  1113  describes the angle between directions of beams  1107  and  1108 . In the illustrated embodiment, the mirror  1111  pivot point is positioned at the focal point of lens  1114  such as when mirror  1111  is rotated, the direction of beams  1107  and  1108  intersect at the lens  1115  focal position  1116 . In one embodiment, focal position  1116  corresponds to the pivot point of a scanning system, such as scanning system  227  of  FIG. 2 , for example. However, one having ordinary skill in the art will appreciate that off-pivot positioning can be used, for example, for full-range imaging. 
     In the illustrated embodiment, rotating the mirror  1111  allows changing the intersecting angle of beams  1107  and  1108  on the scanning system. As such, in the illustrated embodiment, changing this intersecting angle alters the distance between beams on the sample. In comparison with the embodiment described with respect to  FIG. 10 , the embodiment described with respect to  FIG. 11  allows reduction of the cross-talks by adjusting lengths of fiber  1101  and  1105 , such that their half-difference is longer than the coherence length of the light source. 
       FIG. 12  is a block diagram showing an MBGU  1200  in yet another embodiment, which can be configured such as MBGU  107  of  FIG. 1 a   , for example. In the illustrated embodiment, MBGU  1200  can be configured to provide two beams that scan the sample with a variable delay between each other, allowing imaging, via OCT, of motions inside the sample with different velocities. 
     In the illustrated embodiment, fiber  1201  can be configured to correspond to fiber  104  of  FIG. 1 . In the illustrated embodiment, fiber  1201  directs light having wavelength bandwidth Δλ to collimator  1202 . As shown in the illustrated embodiment, collimator  1202  generates a beam  1203  that is directed to a dichroic mirror  1204 . In the illustrated embodiment, dichroic mirror  1204  splits beam  1203  into two bandwidths: Δλ 1  is transmitted and forms beam  1205 , and Δλ 2  is reflected and forms beam  1206 . 
     In the illustrated embodiment, beam  1205  is collected by collimator  1207 , which passes beam  1205  via fiber  1209  to collimator  1211 , exiting as beam  1213 . In the illustrated embodiment, beam  1213  exits collimator  1211  and hits mirror  1215 . In the illustrated embodiment, mirror  1215  reflects beam  1213  to dichroic mirror  1216 . In the illustrated embodiment, beam  1213  is transmitted through dichroic mirror  1216  and reaches lens  1217 . As shown, lens  1217  deflects beam  1213  to lens  1218 . Similarly, lens  1218  redirects beam  1213  to its image focal point  1219 . 
     In the illustrated embodiment, beam  1206  is collected by collimator  1208 , which passes beam  1206  via  1210  to collimator  1212 . One having ordinary skill in the art will understand that, in one embodiment, collimators  1211  and  1212  and fibers  1209  and  1210  can be replaced by bulk optics. One having ordinary skill in the art will understand that, in an alternate embodiment, components from collimator  1202  to collimators  1212  and  1211  could be replaced by a simple fiber coupler or a wavelength division multiplexer. Additionally, in an alternate embodiment, a single dichroic mirror can be used instead of two. 
     In the illustrated embodiment, beam  1214  exits collimator  1212  and hits dichroic mirror  1216 . As shown, beam  1214  is reflected by dichroic mirror  1216  and is directed to lens  1217 . Similarly, beam  1214  is transmitted through lens  1218  and reaches image focal point  1219  of lens  1218 . In the illustrated embodiment, the mirror  1215  pivot point corresponds to the object focal point of lens  1217 . In one embodiment, the mirror  1215  pivot point is the conjugate of point  1219 . As such, in one embodiment, when mirror  1215  is rotated, the angle between beams at point  1219  is changed with conservation of the intersecting point. In one embodiment, the focal point  1219  corresponds to the pivot point of a scanning system, such as scanning system  1408  of  FIG. 1 a   , for example. Accordingly, one having ordinary skill in the art will understand that, in one embodiment, changing this intersecting angle alters the distance between beams on the sample. 
     So configured, the embodiments described herein can be configured for OCT operations that overcome at least some of the disadvantages associated with legacy systems and methods. For example,  FIG. 13  is a block diagram showing a process for obtaining absolute velocity of motions inside the sample from the signal acquired by, for example, the embodiments of  FIGS. 1 a   / 1   b  and  2 . As described above, these embodiments allow scanning the sample transversally with two incident beams at two different locations on the sample. As such, by scanning the sample, an overlap exists between locations of these two beams allowing improved motion contrast imaging. 
     As shown in  FIG. 13  ( 1301 ), the two beams are denoted by A and B. At instant t ( 1301 ) a measurement is carried where beams A and B are at positions P and Q respectively. At instant t+T ( 1308 ), a measurement is carried out where beams A and B are positions Q′ and R. In the illustrated embodiment, it is assumed that positions Q and Q′ onto the sample are “near” or “close to” each other. One having ordinary skill in the art will understand that “close” means that the two locations are close enough to enable overlap between scanning beams allowing significant correlation between OCT signals. 
     In the illustrated embodiment, the acquired interference signal at time t for positions P (beam A) and Q (beam B) is encoded in wavelength ( 1302 ) where beam A and beam B correspond to wavelength bandwidths Δλ 1  and Δλ 2 , respectively. Similarly, the acquired interference signal at time t+T for positions Q′ (beam A) and R (beam B) is encoded in wavelength ( 1309 ), where beam A and beam B correspond to wavelength bandwidths Δλ 1  and Δλ 2 , respectively. 
     In the illustrated embodiment, a first step is to apply band-pass filters on the interference signal such as to isolate the two bands corresponding to each beam ( 1303  and  1310 ). One having ordinary skill in the art will understand that different shapes of band-pass filters can be applied to perform this filtering. Then, for each band-pass filtered signal, the optical coherence tomography (OCT) process is applied to obtain a complex OCT signal composed of an amplitude and a phase information as a function of the depth z ( 1304  and  1311 ). 
     In the illustrated embodiment, because positions Q and Q′ are close, their OCT signals are correlated and comparison is possible. In the following OCT phases are compared but one having ordinary skill in the art will understand that different methods using the intensity information are feasible such as, for example, amplitude decorrelation of speckle decorrelation methods. In the illustrated embodiment, for position Q, the phase of beam B acquired at time t is φ B (Q,z,t) (where z corresponds to the depth) and for position Q′, the phase of beam A acquired at time t+T is: φ A (Q′,z,t+T). In one embodiment, a phase difference is computed ( 1305 ) as:
 
Δφ=φ B ( Q,z,t )−φ A ( Q′,z,t+T ).
 
     After correcting the phase difference for motions artifacts Δφ is written Δ′φ ( 1306 ). Finally, computing, for example, the absolute value |Δ′φ| or the square Δ′φ2, motion contrast imaging is obtained. Additionally, one having ordinary skill in the art will understand from ( 1307 ) that 
                      Δ   ′     ⁢   ϕ          =         4   ⁢           ⁢   π   ⁢           ⁢   n   ⁢           ⁢   T     λ     ⁢   V   ⁢           ⁢   z           
where n is the refractive index of sample, λ is the central wavelength of the light source and V z  is the axial velocity of motions inside the sample. In the illustrated embodiment, if the velocity V z  is small, a large delay T is needed to increase |Δ′φ| above the measurement noise.
 
     One skilled in the art will appreciate that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Additionally, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those having ordinary skill in the art, which are also intended to be encompassed by the following claims.