Patent Description:
Current ophthalmic refractive surgical methods, such as cataract surgery, intra-corneal inlays, laser-assisted in situ keratomileusis (LASIK), and photorefractive keratectomy (PRK), rely on ocular biometry data to prescribe the best refractive correction. Historically, ophthalmic surgical procedures used ultrasonic biometry instruments to image portions of the eye. In some cases, these biometric instruments generated a so-called A-scan of the eye: an acoustic echo signal from all interfaces along an imaging axis that was typically aligned with an optical axis of the eye: either parallel with it, or making only a small angle. Other instruments generated a so-called B-scan, essentially assembling a collection of A-scans, taken successively as a head or tip of the biometry instrument was scanned along a scanning line. This scanning line was typically lateral to the optical axis of the eye. These ultrasonic A-scans or B-scans were then used to measure and determine biometry data, such as an ocular axial length, an anterior depth of the eye, or the radii of corneal curvature.

In some surgical procedures, a second, separate keratometer was used to measure refractive properties and data of the cornea. The ultrasonic measurements and the refractive data were then combined in a semi-empirical formula to calculate the characteristics of the optimal intra-ocular lens (IOL) to be prescribed and inserted during the subsequent cataract surgery.

More recently, ultrasonic biometry devices have been rapidly giving way to optical imaging and biometry instruments that are built on the principle of Optical Coherence Tomography (OCT). OCT is a technique that enables micron-scale, high-resolution, cross-sectional imaging of the human retina, cornea, lens or other eye structure. Optical waves are reflected from an object or sample and a computer produces images of cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection.

OCT may be performed based on time-domain processing or Fourier-domain processing. The latter approach includes a technique known as swept-source OCT, where the spectral components of the optical signal used to illuminate the sample are encoded in time. In other words, the optical source is swept (or stepped) across an optical bandwidth, with the interference signal produced by the combination of the source signal and the reflected signal being sampled at several points across this optical bandwidth. A receiver receives the source signal (also called the reference signal or the signal that traverses the reference arm) and the sample signal (the signal reflected from the sample) and produces the interference signal. The interference signal (that is the interference pattern when the reference and sample signals are combined or interfere with each other) is then directed to a detector.

OCT technology is now commonly used in clinical practice, with such OCT instruments are now used in <NUM>-<NUM>% of all IOL prescription cases. Among other reasons, their success is due to the non-contact nature of the imaging and to the higher precision than that of the ultrasound biometers.

Even with these recent advances, however, substantial further growth and development is needed for the functionalities and performance of biometric and imaging instruments. The document <CIT>) discloses an imaging apparatus configured to image an object to be examined. The document <CIT>) discloses a reproduction device of a holographic memory capable of reproducing multi-value phase information without being influenced by noise.

According to a first alternative of the invention, an Optical Coherence Tomography (OCT) receiver receives a sample beam and a reference beam and combines the sample beam and the reference beam into an interference beam. The OCT receiver comprises first and second prisms aligned with each other, with two faces in contact and having a beam splitting non-polarizing interface in-between. The first prism is arranged to receive one of the sample beam or the reference beam, and the second prism is arranged to receive the other of the sample beam or the reference beam. The interference beam is created and split into a first interference beam and a second interference beam at the non-polarizing contact interface between the first and second prism. First and second polarizing beam splitters are aligned with each other. The first and second polarizing beam splitters are arranged adjacent to the first prism. The first and second polarizing beam splitters are arranged to split the first interference beam into first and second polarization states. A delay path is arranged adjacent to the second prism. The delay path is configured to receive the second interference beam. Third and fourth polarizing beam splitters are arranged adjacent to each other. The third and fourth polarizing beam splitters are arranged adjacent to the delay path. The third and fourth polarizing beam splitters are arranged to split the second interference beam into first and second polarization states. A photodetector array is configured to receive the first polarization state of the first interference beam, the second polarization state of the first interference beam, the first polarization state of the second interference beam, and the second polarization state of the second interference beam. The first and second prisms, the delay path, the first, second, third, and fourth polarizing beam splitters, the four lenses in front of the detector array, and the detector array, comprise a single assembly that forms a compact package with a volume of less than <NUM> millimeters by <NUM> millimeters by <NUM> millimeters.

The OCT receiver may also comprise a first collimating lens arranged to receive one of the sample beam or reference beam and collimate and direct the one of the sample beam or reference beam to the first prism; and a second collimating lens arranged to receive the other of the sample beam or reference beam and direct the other of the sample beam or reference beam to the second prism.

The OCT receiver may also comprise four lenses: a first lens arranged to receive the first polarization state of the first interference beam; a second lens arranged to receive the second polarization state of the first interference beam; a third lens arranged to receive the first polarization state of the second interference beam; and a fourth lens arranged to receive the second polarization state of the second interference beam. In some cases, these lenses may be ball lenses.

In some cases, the delay path may be a prism, the prism having first and second faces, the first face of the prism adjacent to the second prism, and the second face of the prism adjacent to the third and fourth polarizing beam splitters. In other cases, the delay path may be a pair of reflectors located between the second prism and the third and fourth polarizing beam splitters.

A first optical path length from the adjacent faces (or non-polarizing contact interface) of the first and second non-polarizing splitters (which may be a prism) to a first photodetector of the photodetector array that receives the first polarization state of the first interference beam is equal or nearly equal to a second optical path length from the adjacent faces (or non-polarizing contact interface) of the first and second non-polarizing splitters (which may be a prism) to a second photodetector of the photodetector array that receives the first polarization state of the second interference beam.

A third optical path length from the adjacent faces (or non-polarizing contact interface) of the first and second non-polarizing splitters (which may be a prism) to a third photodetector of the photodetector array that receives the second polarization state of the first interference beam is equal or nearly equal to a fourth optical path length from the adjacent faces (or non-polarizing interface) of the first and second non-polarizing splitters (which may be a prism) to a fourth photodetector of the photodetector array that receives the second polarization state of the second interference beam.

In some cases, the single assembly is tilted by an angle of between one and five degrees from a horizontal plane containing the sample beam and the reference beam. An angle of incidence of the both the sample beam and the reference beam on the first face of the first prism and the first face of the second prism is between one and five degrees.

According to a second alternative of the invention, an Optical Coherence Tomography (OCT) receiver receives a sample beam and a reference beam and combines the sample beam and reference beam into an interference beam. The OCT receiver comprises an assembly comprising a non-polarizing beam splitter prism, a first reflector, and a second reflector. The first and second reflectors are arranged on opposite sides of the prism. The prism is arranged to receive one of the sample beam or the reference beam, and the second reflector is arranged to receive the other of the sample beam or the reference beam. The interference beam is created and split into a first interference beam and a second interference beam in the prism. First and second polarizing beam splitters are aligned with each other. The first and second polarizing beam splitters are arranged adjacent to the first reflector. The first and second polarizing beam splitters are arranged to split the first interference beam into first and second polarization states. A delay path is arranged adjacent to the non-polarizing beam splitter prism and second reflector. The delay path is configured to receive the second interference beam. Third and fourth polarizing beam splitters are arranged adjacent to each other. The third and fourth polarizing beam splitters are arranged adjacent to the delay path. The third and fourth polarizing beam splitters are arranged to split the second interference beam into first and second polarization states. A photodetector array is configured to receive the first polarization state of the first interference beam, the second polarization state of the first interference beam, the first polarization state of the second interference beam, and the second polarization state of the second interference beam. The first and second reflectors, the non-polarizing beam splitter prism, the delay path, and the first, second, third, and fourth polarizing beam splitters comprise a single assembly that forms a compact package with a volume of less than <NUM> millimeters by <NUM> millimeters by <NUM> millimeters.

The OCT receiver may also comprise a first collimating lens arranged to receive one of the sample beam or reference beam and direct the one of the sample beam or reference beam to the prism; and a second collimating lens arranged to receive the other of the sample beam or reference beam and direct the other of the sample beam or reference beam to the second reflector.

The OCT receiver may also comprise four lenses: a first lens arranged to receive the first polarization state of the first interference beam; a second lens arranged to receive the second polarization state of the first interference beam; a third lens arranged to receive the first polarization state of the second interference beam; and a fourth lens arranged to receive the second polarization state of the second interference beam. These lenses may all be ball lenses.

In some cases, the delay path comprises a prism having first and second faces. The first face of the prism is adjacent to the non-polarizing beam splitter prism, and the second face of the prism is adjacent to the third and fourth polarizing beam splitters. In other cases, the delay path may be a pair of reflectors located between the prism and the third and fourth polarizing beam splitters.

A first optical path length from the non-polarizing splitter interface to a first photodetector of the photodetector array that receives the first polarization state of the first interference beam is equal or nearly equal to a second optical path length from the non-polarizing splitter interface to a second photodetector of the photodetector array that receives the first polarization state of the second interference beam.

A third optical path length from the non-polarizing splitter to a third photodetector of the photodetector array that receives the second polarization state of the first interference beam is equal or nearly equal to a fourth optical path length from the non-polarizing splitter to a fourth photodetector of the photodetector array that receives the second polarization state of the second interference beam.

The single assembly is located in a plane. The plane is tilted by an angle of between one and five degrees from a horizontal plane containing the sample beam and the reference beam.

The embodiments described herein may be used to provide and/or operate an all-in-one device to achieve optimized OCT performance for each of several different application modes. Other advantages and variations of the above-summarized embodiments are described below.

In the following description, specific details are set forth describing certain embodiments. It will be apparent, however, to one skilled in the art that the disclosed embodiments may be practiced without some or all of these specific details. The specific embodiments presented are meant to be illustrative, but not limiting.

<FIG> illustrates an example SSOCT system <NUM>, which comprises a swept optical source <NUM>, an interferometer subsystem <NUM>, and a detector receiver <NUM>. It will be appreciated that the details shown here are an example only; other systems may vary in well-known ways.

Swept optical source <NUM> is typically designed for wavelength tuning, to generate swept optical signals that repeatedly scan over a predetermined optical tuning range, e.g., over an optical wavelength range of <NUM> or greater, at a scanning repetition rate of <NUM> kilohertz (kHz) or greater. The bandwidth of the optical emission, i.e., the full-width half-maximum (FWHM) bandwidth is typically less than <NUM>. Interferometer subsystem <NUM>, in this particular example implemented as a Mach-Zehnder-type interferometer designed for operation at, for example, central wavelengths around <NUM>, and receiver <NUM>, are used to analyze the optical signals reflected from the imaged object <NUM>, which may be a human eye. It will be appreciated that interferometer subsystem <NUM> may be based on a different design when designed for different wavelengths. Other central wavelengths may include those around <NUM> or <NUM>.

As seen in <FIG>, the swept optical output from the swept optical source <NUM> is coupled to an optical fiber coupler <NUM> in interferometer subsystem, via optical fiber <NUM>. Optical fiber coupler <NUM> may be a <NUM>/<NUM> optical fiber coupler, for example. The swept optical signal is divided by the coupler <NUM> between a reference arm <NUM> and a sample arm <NUM>.

The optical fiber of the reference arm <NUM> terminates at a fiber end-face <NUM>. The light 102R exiting from the reference arm fiber endface <NUM> is collimated by a lens <NUM> and reflected by a mirror <NUM>, in the illustrated implementation. Mirror <NUM> has an adjustable fiber-to-mirror distance, in one example. This distance determines a reference point in the depth range being imaged, i.e., the position in the sample <NUM> of the zero-path length difference between the reference arm <NUM> and the sample arm <NUM>. This distance may be adjusted, in some embodiments, for different sampling probes and/or imaged samples. Light returning from the reference mirror <NUM> is returned to a reference arm circulator <NUM> and directed to receiver <NUM>.

The fiber on sample arm <NUM> terminates at the sample arm probe <NUM>. The exiting swept optical signal <NUM> is focused by the probe <NUM> onto the sample <NUM>. Light returning from the sample <NUM> is returned to a sample arm circulator <NUM> and directed to the receiver <NUM>. The reference arm signal and the sample arm signal are combined in the receiver <NUM> to generate an optical interference signal as more clearly described below.

In this context, the sample beam is the light beam reflected from the sample, and the reference beam is the light beam reflected from the mirror in the reference arm. The sample beam is associated with the sample arm, and the reference beam is associated with the reference arm. In an example interferometer, the light source (in some cases a swept optical source or swept source laser) produces a beam of light at a central wavelength (in some cases a central wavelength of <NUM>). The beam of light is then split into two beams - one of which is directed at the sample (the sample arm), the other of which is directed at the reference path (the reference arm). The optical paths of the reference arm and the sample arm are generally of similar lengths. The sample beam and the reference beam are combined to produce an interference beam.

<FIG> shows an example of an OCT detector (or OCT receiver in a top view). In <FIG>, a sample beam <NUM> is reflected from the sample <NUM>. A reference eam <NUM> returns from the reference arm. Sample beam <NUM> passes through collimating lens <NUM>. Reference beam <NUM> passes through collimating lens <NUM> and polarizer <NUM>. Collimated sample beam <NUM> then enters prism <NUM>, and the collimated reference beam <NUM> enters prism <NUM>. Collimated sample beam <NUM> is reflected from a surface (or non-polarizing beam splitting contact interface) of prisms <NUM> and <NUM>, and the collimated reference beam <NUM> is reflected from a surface of prism <NUM> as shown in <FIG>. Interference between the sample beam <NUM> and the reference beam <NUM> occurs at point A where prism <NUM> and prism <NUM> meet. The interference beam is split into two beams, B and C, by prism <NUM> and prism <NUM>. Beam B enters polarizing beam splitter (PBS) <NUM> where it is split into two beams BP and BS. Beam BP exits PBS <NUM> and enters lens <NUM> where it is focused onto photodetector <NUM>. Beam BS exits PBS <NUM>, is reflected by reflector <NUM>, and enters lens <NUM> where it is focused onto photodetector <NUM>. In a similar manner, beam C is also split into beam CP and CS. Beam C exits prism <NUM> and enters a delay path <NUM>. In this example, delay path <NUM> comprises a prism with a pair of opposite facets acting as reflectors aligned to direct beam C into PBS <NUM>. Beam C enters polarizing beamsplitter (PBS) <NUM> where it is split into two beams CP and CS. Beam CP exits PBS <NUM> and enters lens <NUM> where it is focused onto photodetector <NUM>. Beam CS exits PBS <NUM>, is reflected by reflector <NUM>, and enters lens <NUM> where it is focused onto photodetector <NUM>.

Structurally, the example OCT receiver of <FIG> comprises a pair of collimating lenses <NUM> and <NUM> aligned with a pair of prisms <NUM> and <NUM> so as to produce an interference of the sample beam <NUM> and the reference beam <NUM> at point A. A polarizer <NUM> is in the optical path of the reference arm between collimating lens <NUM> and NPBS / prism <NUM>. The interference beam is split into two beams B and C. Prism <NUM> is aligned with PBS <NUM> so as to split the interference beam B into two polarization states (beams BP and BS). A reflector <NUM> is aligned with PBS <NUM> to direct the beam BS to lens <NUM>. Lens <NUM> is aligned with photodetector <NUM>. PBS <NUM> is aligned with lens <NUM> to direct the beam BP onto photodetector <NUM>. In a similar manner, prism <NUM> is aligned with delay path <NUM>. Delay path <NUM> is aligned with PBS <NUM> so as to split the interference beam C into two polarization states (beams CP and CS). A reflector <NUM> is aligned with PBS <NUM> to direct the beam CS to lens <NUM>. Lens <NUM> is aligned with photodetector <NUM>. PBS <NUM> is aligned with lens <NUM> to direct the beam CP onto photodetector <NUM>.

Functionally, the example OCT receiver of <FIG> receives sample beam <NUM> and reference beam <NUM>, causes the two beams to interfere with each other so as to produce an interference pattern, splits the interference beam into two beams B and C. The two beams B and C take two parallel optical paths and are further split into two polarization states. Each of the two polarization states (BP, BS and CP, CS) for each of the two beams (B and C) are then directed to an array of photodetectors (<NUM>, , <NUM>, <NUM>, and <NUM>). The optical path length of the first polarization state BP from the non-polarizing interface A up to its detector <NUM> is close-to-equal to the optical path of the first polarization state CP from the non-polarizing interface A up to its detector <NUM>. The optical path length of the second polarization state BS from the non-polarizing interface A up to its detector <NUM> is close-to-equal to the optical path of the second polarization state CS from the non-polarizing interface A up to its detector <NUM>.

Prisms <NUM> and <NUM> are coupled together along a surface to form a non-polarizing beam splitter (NPBS). In this example, prisms <NUM> and <NUM> are attached rhomboid prisms with a non-polarizing beam-splitting coating at their interface. The adjacent surfaces of prisms <NUM> and <NUM> form an NPBS. Sample beam <NUM> and reference beam <NUM> are aligned to enter the NPBS such that they are each <NUM>/<NUM> power-split and simultaneously combined by the NPBS non-polarizing interface. Two spatially separated, orthogonal interference beams are created, each one carrying <NUM>% of the sample beam and <NUM>% of the reference beam. A first face of prism <NUM> is adjacent to a first face of prism <NUM>. The interference beam is created and split into a first interference beam and a second interference beam where the first face of the first prism and the first face of the second prism meet.

In the example of <FIG>, collimating lenses <NUM> and <NUM> may be any type of lens or optical element that collimates a light beam. Other types of lenses may also be used in place of collimating lenses <NUM> and <NUM>. In another example of OCT receiver, collimating lenses <NUM> and <NUM> may be absent. Sample beam <NUM> and reference beam <NUM> may be directed to prism <NUM> and prism <NUM>, respectively without passing through collimating lenses or other optical elements inside the OCT receiver. In the example of <FIG>, collimating lenses <NUM> and <NUM> are a matched pair of lenses or lenses of the same type. In one example, collimating lenses <NUM> and <NUM> have an effective focal length (EFL) of <NUM> and collimate a beam with a diameter between <NUM> micrometers and <NUM> micrometers.

In the example OCT receiver of <FIG>, prisms <NUM> and <NUM> may be implemented by rhomboid prisms, cubic prisms, rectangular prisms, or prisms of other shapes. Instead of prisms, other types of optical elements suitable for the example OCT receiver of <FIG> include plate beam splitters, cube beam splitters, or the like. Functionally, prism <NUM> and prism <NUM> together facilitate the interference of a sample beam and a reference beam as well as splitting the interference beam into two interference beams. In the example of <FIG>, the interface between adjacent surfaces of prisms <NUM> and <NUM> (at point A) act as a non-polarizing beam splitter.

In the example OCT receiver of <FIG>, PBS <NUM> and <NUM> are beam splitters, and may be implemented by prisms, including cubic prisms, rectangular prisms, or prisms of other shapes. Other types of PBS suitable for the example OCT receiver of <FIG> include plate beam splitters, cube beam splitters, or the like. Functionally, PBS <NUM> and PBS <NUM> both split the interference beam into two interference beams with different polarization states (generally denoted as polarization states P and S). In this case, the polarization states are orthogonal to each other. In the example of <FIG>, PBS <NUM> and PBS <NUM> are a matched pair of beam splitters or beam splitters of the same type.

In the example OCT receiver of <FIG>, reflectors <NUM> and <NUM> may be implemented by mirrors, including mirrors designed to reflect light of certain wavelengths. In other examples, reflectors <NUM> and <NUM> may be implemented with prisms or beam splitters of various types. For example, reflectors <NUM> and <NUM> may be implemented with a surface of a prism or beam splitter. In <FIG>, reflectors <NUM> and <NUM> have the same form factor as PBS <NUM> and PBS <NUM>. This form factor allows for a compact design of receiver <NUM>.

In the example OCT receiver of <FIG>, lenses <NUM>, <NUM>, <NUM>, and <NUM> may be implemented with lens element to collimate, focus or otherwise alter the optical path of a light beam. Numerous types of lenses may be used to direct the light beams to photodetectors <NUM>, <NUM>, <NUM>, and <NUM>. In one example, a <NUM> BK7 ball lens is used for each of the lenses <NUM>, <NUM>, <NUM>, and <NUM> in order to focus the light beams into a spot size of approximately <NUM> micrometers.

In the example OCT receiver of <FIG>, delay path <NUM> introduces a delay in the path of interference beam C to compensate for the longer optical path of interference beam B. In other words, delay path <NUM> introduces an optical path length so that the optical path from collimating lens <NUM> or point (interface) A to photodetector <NUM> is equal or nearly equal to the optical path from collimating lens <NUM> or point (interface) A to photodetector <NUM>. Likewise, delay path <NUM> introduces an optical path length so that the optical path from collimating lens <NUM> or point (interface) A to photodetector <NUM> is equal or nearly equal to the optical path from collimating lens <NUM> or point (interface) A to photodetector <NUM>. Delay path <NUM> may be implemented using a pair of mirrors, a beam splitter, a prism, or other optical element designed to introduce a delay or extra length in an optical path.

In the example OCT receiver of <FIG>, photodetectors <NUM>, <NUM>, <NUM>, and <NUM> receive polarized light beams BP, CS, BS, and CP. Typically, photodetectors <NUM>, <NUM>, <NUM>, and <NUM> are semiconductor devices that convert photons into electric current. In this case, photodetectors <NUM>, <NUM>, <NUM>, and <NUM> may be polarization sensitive elements.

<FIG> depicts optical path lengths of the example OCT receiver of <FIG>. In the example of <FIG>, The optical path length from A to B equals or nearly equals the optical path length from A to C, and the optical path length from A to D equals or nearly equals the optical path length from A to E. In this example, the optical glass used in the optical paths shown in <FIG> is BK7 glass with a refractive index of approximately <NUM> at <NUM>. For purposes of showing the optical path lengths, in <FIG>, c = c* = c' and the other path lengths are as follows:.

In this case, in order to ensure equal path lengths, the optical components such as NPBS and PBS are selected from the same manufacturing lots or are presorted and matched to minimize path length differences introduced by variations in optical components.

According to the invention, the OCT receiver shown in <FIG> and <FIG> provide a compact, stable package that may be used in an interferometer. With the dimensions given above, a compact package containing all of the elements occupies a volume of less than <NUM> x <NUM> x <NUM>. In addition, the use of lens elements, NPBS elements, and PBS elements with the geometric shapes described and shown in the Figures allow for compact assembly. This compact assembly provides optical stability as well as a physically small package that may be incorporated into a portable OCT instrument. In the example of <FIG>, prism <NUM> and prism <NUM> may each be implemented with a rhomboid prism, PBS <NUM>, PBS <NUM>, PBS <NUM>, and PBS <NUM> can each be implemented with cubic beam splitters, and delay path <NUM> may be implemented with a prism. These geometric components may be assembled and aligned into a small and stable package. As shown in <FIG> and <FIG>, the prisms <NUM> and <NUM>, PBS <NUM>, <NUM>, <NUM>, <NUM>, and delay path <NUM> may be arranged adjacent to each other. Since the prisms, PBS, and delay path are each geometric elements (e.g., prisms) in this example, the faces of these elements may be placed adjacent to one another as shown to form a stable assembly. Moreover, the location of the lenses <NUM>, <NUM>, <NUM>, <NUM> and photodetectors <NUM>, <NUM>, <NUM>, <NUM> may be fixed in relation to the other elements in a single compact package.

<FIG> are side views of the OCT receiver of the example of <FIG>. In <FIG>, sample beam <NUM>, collimating lens <NUM>, prism <NUM>, prism <NUM>, PBS <NUM>, lens <NUM> and photodetector <NUM> are depicted. Behind collimating lens <NUM> is collimating lens <NUM>. Behind prism <NUM> is prism <NUM>. Behind PBS <NUM> is delay path <NUM>. Behind lens <NUM> are lenses <NUM>, <NUM>, and <NUM>. Behind photodetector <NUM> are photodetectors <NUM>, <NUM>, and <NUM>. A polarizer <NUM> may also be located in the reference path between collimating lens <NUM> and prism <NUM>.

Polarizer <NUM> may be located at a <NUM> degree angle in the reference path (or optical path of the reference arm) as shown in the example of <FIG>. In such a case, polarizer <NUM> will ensure proper signal matching of the P and S channels and that the P and S channels will have equal or approximately equal power. Polarizer <NUM> may be oriented such that proper signal matching of the P and S channels occurs - or that the P and S channels have equal or approximately equal power. In this case, light in the reference arm passes through polarizer before being split into P and S polarization. In this manner the light in the reference arm is polarized or aligned before being further split into the two polarization states (P and S).

<FIG> show a tilt introduced to the optical components of the OCT receiver <NUM>. If <FIG> depicts the horizontal plane of OCT receiver <NUM> top view, then <FIG> depict the vertical plane of OCT receiver <NUM> side view. A tilt of approximately two degrees from the horizontal plane <NUM> is introduced to the assembly comprising the prisms <NUM> and <NUM>, the delay path <NUM>, and the polarizing beam splitters <NUM>, <NUM>, <NUM>, <NUM>. The tilt angle effectively becomes the angle of incidence for the horizontally directed sample beam <NUM> and reference beam <NUM> at the input surface/facet of the NPBS (i.e. the adjacent sides of prisms <NUM> and <NUM>). The tilt angle is shown in <FIG> as the angle between planes <NUM> and <NUM>. In this example, plane <NUM> is the plane containing the sample beam and the reference beam. The assembly comprising the prisms <NUM> and <NUM>, the delay path <NUM>, and the polarizing beam splitters <NUM>, <NUM>, <NUM>, <NUM> is tilted by an angle with respect to plane <NUM>. The tilt introduced in the vertical plane of OCT receiver <NUM> may be in the range of one degree to five degrees, with a tilt of approximately two degrees shown in the example of <FIG> (the angle between planes <NUM> and <NUM>). <FIG> depicts the forward beams, and <FIG> depicts the forward and the retro-reflected beams. In order to eliminate ghosting in the system, the retro-reflected beams return at such an angle as not to pass back hrough collimating lenses. It has been found that a tilt of two degrees is sufficient to eliminate double retro-path interference due to the geometric separation from the forward beam.

<FIG> shows another example of an OCT receiver. In <FIG>, a birefringent crystal <NUM> is used instead of PBS <NUM>, mirror <NUM>, delay path <NUM>, PBS <NUM>, and mirror <NUM>. Lenses <NUM>, <NUM>, <NUM>, and <NUM> are omitted for simplicity. In <FIG>, the location of photodetectors <NUM>, <NUM>, <NUM>, and <NUM> may be adjusted so that the optical paths from A to B and A to D are equal or nearly equal and the optical paths from A to C and A to E are equal or nearly equal.

<FIG> shows an example of an OCT receiver. In <FIG>, NPBS <NUM>, reflector <NUM>, and reflector <NUM> replaces PRISM <NUM> and PRISM <NUM> in <FIG>. In <FIG>, a sample beam <NUM> is reflected from the sample <NUM>. A reference beam <NUM> returns from the reference arm. Sample beam <NUM> passes through collimating lens <NUM>. Reference beam <NUM> passes through collimating lens <NUM> and polarizer <NUM>. Collimated sample beam <NUM> is reflected from reflector <NUM> or directed toward NPBS <NUM>, and the collimated reference beam <NUM> is reflected from reflector <NUM> toward NPBS <NUM>. Collimated sample beam <NUM> then enters NPBS <NUM>, and the collimated reference beam <NUM> enters NPBS <NUM>. Interference between the sample beam <NUM> and the reference beam <NUM> occurs at point A in NPBS <NUM>. The interference beam is split into two beams, B and C, by NPBS <NUM>. The remainder of the OCT receiver is the same as that depicted in <FIG> and described above.

Functionally, the example OCT receiver of <FIG> provides optical paths of the same or nearly the same lengths (i.e., matched lengths) from point A (i.e. the adjacent faces of prisms <NUM> and <NUM> or non-polarizing interface inside NPBS <NUM>) to the photodetectors <NUM>, and <NUM>. It also provides optical paths of the same or nearly the same lengths (i.e., matched lengths) from point A to the photodetectors <NUM> and <NUM>. The example OCT receiver of <FIG> receives sample beam <NUM> and reference beam <NUM>, causes the two beams to interfere with each other so as to produce an interference pattern, splits the interference beam into two beams B and C. The two beams B and C take two parallel optical paths and are further split into two polarization states. Each of the two polarization states (BP, BS and CP, CS) for each of the two beams (B and C) are then directed to an array of photodetectors (<NUM>, <NUM>, <NUM>, <NUM>, and <NUM>).

It will be appreciated that the examples described provide an OCT receiver in a compact and optically stable package. The optical elements are assembled to produce matched optical paths of the same or nearly the same length.

Claim 1:
An Optical Coherence Tomography, OCT, receiver (<NUM>) that receives a sample beam (<NUM>) and a reference beam (<NUM>) and combines the sample beam and the reference beam into an interference beam, the OCT receiver comprising:
first and second prisms (<NUM>, <NUM>) aligned with each other, a first face of the first prism (<NUM>) adjacent to a first face of the second prism (<NUM>), the first prism arranged to receive one of the sample beam or the reference beam, and the second prism arranged to receive the other of the sample beam or the reference beam, the interference beam created and split into a first interference beam (B) and a second interference beam (C) where the first face of the first prism (<NUM>) and the first face of the second prism (<NUM>) meet;
first and second polarizing beam splitters (<NUM>, <NUM>) aligned with each other, the first and second polarizing beam splitters arranged adjacent to the first prism (<NUM>), the first and second polarizing beam splitters arranged to split the first interference beam (B) into first and second polarization states (BP, BS);
a delay path (<NUM>) arranged adjacent to the second polarizing beam splitter, the delay path configured to receive the second interference beam (C);
third and fourth polarizing beam splitters (<NUM>, <NUM>) arranged adjacent to each other, the third and fourth polarizing beam splitters arranged adjacent to the delay path (<NUM>), the third and fourth polarizing beam splitters arranged to split the second interference beam ( C) into first and second polarization states (CP, CS) ; and
a photodetector array (<NUM>, <NUM>, <NUM>, <NUM>) configured to receive the first polarization state (BP) of the first interference beam (B), the second polarization state (BS) of the first interference beam (B), the first polarization state (CP) of the second interference beam (C), and the second polarization state (CS) of the second interference beam (C);
wherein, the first and second prisms (<NUM>, <NUM>), the delay path (<NUM>), and the first, second, third, and fourth polarizing beam splitters (<NUM>, <NUM>, <NUM>, <NUM>), comprise a single assembly that forms a compact package with a volume of less than <NUM> millimeters by <NUM> millimeters by <NUM> millimeters.