Optical coherence tomography system

In a polarization-sensitive optical coherence tomography system, an interferometer includes single mode fibers and a plurality of polarization controllers. At least one of the plurality of polarization controllers is disposed on each of the fibers of the interferometer. The fibers include a fiber for sample light beam, a fiber for reference light beam, and a fiber for detection light beam. An image forming unit determines a pixel value from a data set consists of two orthogonal polarization components simultaneously obtained at substantially identical spatial positions of a test sample, wherein a measurement light beam has static single polarization state.

TECHNICAL FIELD

The present invention relates to optical coherence tomography systems, particularly, to an optical coherence tomography system that can capture information on polarization characteristics of a target eye.

BACKGROUND ART

In these years, optical coherence tomography (OCT) systems (referred to as OCT systems, below) that utilize interference due to low coherence light have been practically used. The OCT systems can noninvasively capture a tomograph image of a test sample with high resolution. The OCT systems have thus been growing into indispensable systems, particularly in ophthalmology, to obtain a tomograph image of the ocular fundus of the target eye. Besides ophthalmology, the OCT systems are used for purposes such as for observing a tomograph of the skin or capturing a tomograph image of the wall surfaces of the digestive organ or the circulatory organ in the form of an endoscope or a catheter.

An ophthalmologic OCT system that can capture functional OCT images besides normal OCT images (also referred to as intensity images) has been developed where the normal OCT images image the shape of ocular fundus tissues while the functional OCT images image optical characteristics, actions, or other information of ocular fundus tissues. Particularly, polarization-sensitive OCT systems that can draw a nerve fiber layer or a retinal layer have been developed as one of functional OCT systems and the systems for diseases such as glaucoma or age-related macular degeneration have been increasingly studied.

A polarization-sensitive OCT system can form a polarization OCT image using a polarization parameter (retardation and orientation), which is one of optical characteristics of ocular fundus tissues, to discriminate or segment the ocular fundus tissues. Generally, a polarization-sensitive OCT system includes an optical system in which a wave plate (for example, λ/4 wave plate or λ/2 wave plate) is used to appropriately change the polarization states of light measured by the OCT system and reference light. A polarization-sensitive OCT system forms a polarization OCT image by controlling polarization of light emitted from a light source, using light that has been modulated into a desired polarization state as a measurement light beam for observing a specimen, splitting an interference light beam into two orthogonal straight polarized light beams, and detecting the polarized light beams (NPL 1: Biomedical Optics Express 3 (11), Stefan Zotter et al. “Large-field high-speed polarization sensitive spectral domain OCT and its applications in ophthalmology”).

As a method for controlling polarization, a method for modulating a polarized state using an electro-optic modulator (EOM) has been provided (NPL 2: Optics Express 5 (15), Barry Cense et al. “Polarization-sensitive spectral-domain optical coherence tomography using a single line scan camera”). This method enables formation of polarization OCT images on the basis of polarization information of multiple polarized states by applying light beams of multiple polarized states to the same position, whereby more accurate polarization OCT images can be captured.

Meanwhile, size reduction of OCT systems is required at medical institutions with the needs of installing various inspection devices. Thus, a polarization-sensitive OCT system that is smaller than and has a more flexible optical system than existing systems by including an optical fiber as an optical system has been developed (NPL 3: Journal of Biomedical Optics 19 (2), Hermann Lin et al. “All fiber optics circular-state swept source polarization-sensitive optical coherence tomography”).

Existing polarization-sensitive OCT systems include components such as a polarization maintaining (PM) fiber (referred to as a PM fiber, below), a wave plate, and an EOM for controlling polarization. Components such as PM fibers, wave plates, and EOMs, however, are extremely expensive and consequently make the polarization-sensitive OCT expensive. Moreover, conventional OCT systems are unable to easily accept additional components and an additional polarization-OCT-image capturing function. Thus, medical institutions that have already had a conventional OCT system have to purchase a new system, causing a heavy burden. Furthermore, a large space is required to install two OCT systems.

NPL 1 discloses the configuration of a polarization-sensitive OCT system that includes an interferometer for which a PM fiber is used and a wave plate for controlling polarization of a measurement light beam and a reference light beam. This OCT system can facilitate polarization adjustment but cannot be formed at a low cost due to the use of expensive optical elements.

NPL 2 discloses a polarization-sensitive OCT system including an EOM for controlling polarization. However, as in the system of NPL 1, the polarization-sensitive OCT system cannot be formed at a low cost because of a very expensive EOM.

NPL 3 discloses a polarization-sensitive OCT system including an interferometer for which a PM fiber is used. The use of the PMF reduces the size of the system but prevents cost reduction of the system because the PMF is a very expensive optical component.

In addition, the polarization-sensitive OCT systems of the above-described NPLs have configurations basically different from the configuration of conventional OCT systems and thus conventional OCT systems are highly unlikely to be extensible, specifically, to be changed into polarization-sensitive OCT systems. Changing conventional OCT systems into the above-described polarization-sensitive OCT systems involves replacement or addition of most of the components, making it impossible to easily add functions. Consequently, a polarization-sensitive OCT system is installed along with a conventional OCT system, preventing space saving.

SUMMARY OF INVENTION

In view of these problems, one embodiment of the present invention provides a polarization-sensitive OCT system that can be formed in a small size at a low cost. Furthermore, one embodiment of the present invention provides a polarization-sensitive OCT system that can easily add a polarization-OCT-capturing function to a conventional OCT system.

According to one aspect of the present invention, there is provided an optical coherence tomography system that includes the following configuration: an interferometer includes single mode fibers and a plurality of polarization controllers, at least one of the plurality of polarization controllers is disposed on each of the fibers of the interferometer, which include a fiber for a measurement light beam, a fiber for a reference light beam, and a fiber near a detector, and an image forming unit determines a pixel value from a data set including two orthogonal polarization components concurrently obtained at substantially identical spatial positions of a test sample, wherein a measurement light beam has static single polarization state.

According to another aspect of the present invention, there is provided a polarization-sensitive optical coherence tomography apparatus comprising: a plurality of single mode fibers; a first polarization control means for controlling polarization state of measurement light; a second polarization control means for controlling polarization state of a mixed light generated by mixing a light returned from a test sample and a reference light corresponding to the measurement light, wherein the light returned from the test sample is obtained by applying the measurement light to the test sample through an optical system comprising the plurality of single mode fibers, the measurement light having a polarization state controlled by the first polarization control means; and detecting means for detecting the mixed light having a polarization state controlled by the second polarization control means, for each polarization component.

One aspect of the present invention allows a polarization-sensitive OCT system to be formed in a small size at a low cost. In addition, one aspect of the present invention facilitates addition of a polarization OCT function to a conventional OCT system.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is described in detail referring to the drawings.

First Embodiment

A configuration of a polarization-sensitive OCT system according to the embodiment will be described referring toFIG. 1.

[Entire Configuration of System]

FIG. 1is a schematic diagram of the entire configuration of the polarization-sensitive OCT system according to the embodiment. In this embodiment, the configuration of a spectral domain (SD)-OCT system is described.

The Polarization-Sensitive OCT System100according to the first embodiment includes a plurality of single mode fibers and a plurality of polarization controllers, wherein at least one of the plurality of polarization controllers is disposed on each of the fibers of the interferometer, which include a fiber (107) for a measurement light beam, a fiber (115) for a reference light beam, and a fiber (122) near a detector. An image forming unit (132) determines a pixel value from a data set including two orthogonal polarization components concurrently obtained at substantially identical spatial positions of the test sample (114). A measurement light beam has static single polarization state. The configuration of a polarization-sensitive OCT system100will be described in detail now.

A light source101is a super luminescent diode (SLD) light source, which is a low coherent light source, and emits a light beam having, for example, a center wavelength of 850 nm and a band width of 50 nm. Although a SLD is used as the light source101, any light sources that can emit a low coherent light beam, such as an amplified spontaneous emission (ASE) light source, may be used.

A light beam emitted from the light source101passes through a single mode fiber (referred to as an SM fiber, below)102, a polarization controller103, a connector104, and an SM fiber105and is guided to a beam splitter106, at which the light beam is split into a measurement light beam (also referred to as an OCT measurement light) and a reference light beam (also referred to as a reference light beam corresponding to the OCT measurement light). The beam splitter106is an example of dividing means. The ratio of the reference light beam to the measurement light beam at the beam splitter106is 90:10. Although the ratio of the reference light beam to the measurement light beam at the beam splitter106is 90:10 in this embodiment, it is not limited to this. The splitting ratio of the beam splitter106may be set to other value so that signal to noise ratio of the OCT system is optimized. The polarization controller103can change the polarization of light emitted from the light source101to a desired polarization state. In this embodiment, the polarization controller103adjusts the light in such a manner that the light is linearly polarized. Although not described in this embodiment, when the light source101does not polarize light to a large degree, a polarizer may be disposed between the polarization controller103and the connector104to increase the degree of polarization to which the light emitted from the light source101is polarized. In this case, the amount of light that passes through the polarizer can be adjusted by adjusting the polarization controller103. Alternatively, a configuration may only include a polarizer on the SM fiber102instead of the polarization controller103. In this case, only the degree of polarization to which the light emitted from the light source101is polarized can be increased without the need for adjusting the polarization state of the emitted light. However, whether the amount of light is sufficient has to be checked since the amount of light guided to the interferometer may become short depending on the polarization state of the light.

The split measurement light beam is emitted through an SM fiber107and changed into a parallel light beam by a collimator109. A polarization controller108, which is an example of first polarization control means, disposed at a portion on the SM fiber107, can appropriately change the polarization state of the emitted measurement light beam. In this embodiment, the polarization controller108alters the polarization state to a circularly polarized light beam incident on the sample. The measurement light beam changed into a parallel light beam enters a target eye114as a test sample via a galvano scanner110, which scans an ocular fundus Er of the target eye114with the measurement light beam, a scan lens111, and a focus lens112. Here, the galvano scanner110is described as being a single mirror, but two galvano scanners may be provided so as to raster scan the ocular fundus Er of the target eye114. Although we use the galvano scanners in this embodiment, it is not limited to this. For example, polygon scanners, resonant scanners, MEMS mirrors are usable. The focus lens112, fixed onto a stage113, is capable of focus adjustment by moving in the optical axis direction. The galvano scanner110and the stage113are controlled by a drive controller133, so that a desired range of the ocular fundus Er of the target eye114(also referred to as a tomography-image capturable range, a tomography-image capturable portion, or a measurement-light applied portion) can be scanned with the measurement light beam.

The measurement light beam enters the target eye114via the focus lens112on the stage113and is focused on the ocular fundus Er. The measurement light beam that has been applied to the ocular fundus Er is reflected and scattered by each retinal layer and is returned to the beam splitter106through the above-described optical path.

On the other hand, the reference light beam split by the beam splitter106is emitted through an SM fiber115and changed into a parallel light beam by a collimator117. A polarization controller116, disposed at a portion on the SM fiber115, can appropriately change the polarization state of the reference light beam. In this embodiment, the reference light beam is in a linearly polarized state at the entrance to the polarizing beam splitter126which is an example of polarization dividing means, the orientation of the linear polarization orientation being inclined at 45° to each of the orthogonal polarization axes of the polarizing beam splitter126. Although the polarization state of the reference beam is linear in this embodiment, other polarization states are possible, e.g. elliptic, with the axis of the ellipse being inclined at 45°. The reference light beam passes through a dispersion compensating glass118and a ND filter119, is reflected off a mirror120on a coherence gate stage121, and is returned to the beam splitter106. The coherence gate stage121is controlled by the drive controller133so as to allow for variations in axial length of the eye of a subject or other variations.

The measurement light beam and the reference light beam returned to the beam splitter106are mixed and become an interference light beam, which enters a polarization beam splitter126through an SM fiber122, a polarization controller123which is an example of second polarization control means, a connector124, and an SM fiber125. At the polarization beam splitter126, the interference light beam is split into two light beams, that is, a vertical polarization component (referred to as a V polarization component, below) and a horizontal polarization component (referred to as an H polarization component) in accordance with the two orthogonal polarization axes. The V component of the split interference light beam enters the detector128through the SM fiber127, while the H component of the split interference light beam enters the detector130through the SM fiber129. The light beams received by each of the detectors128and130are output as electric signals and received by a signal processor132.

In this embodiment, the light beam is equivalently split into the V polarization component and the H polarization component because the reference light beam is linearly polarized at 45°. Since the measurement light beam is changed into the circularly polarized light beam in this embodiment, data on the ocular fundus Er of the target eye114can be simultaneously obtained regardless of the cells or the direction of fibers of the ocular fundus Er. Consequently, data on the ocular fundus Er for all the polarization directions can be collectively obtained at once, whereby data can be obtained by only a single image capturing operation for a given sample position without performing multiple image capturing operations for different polarization directions.

Referring now toFIGS. 1 to 4, how the polarization controllers108,116, and123control the polarization state will be described.

Firstly, the polarization controller108controls the polarization state of light entering the target eye114. The incident light entering the target eye114has static single polarization state and not changed temporally. In this embodiment, the measurement light beam linearly polarized by the polarization controller103is altered by the polarization controller108into a circularly polarized light beam. A polarimeter201is placed behind the focus lens112and the polarization controller108is adjusted in a manner such that the polarimeter201detects circular polarization (FIG. 2). Here, a detector such as a polarimeter is used for confirming the polarization state, but the polarization state may be confirmed by using other components, such as an optical power meter and a polarizer or a wave plate. The light beam entering to the target eye114is circularly polarized in this embodiment; however the present invention is not limited to. The polarization state can be elliptically polarized.

Subsequently, the polarization controller123is adjusted. The polarization controller123is adjusted using only the measurement light beam. A mirror301is placed behind the focus lens112and the angle of the mirror301is adjusted in a manner such that light emitted through the SM fiber107is guided to the mirror301via the collimator109, the galvano scanner110, and lenses111and112and then reflected by the mirror301to be returned to the beam splitter106(FIG. 3A). The measurement light beam that has entered the beam splitter106passes through the SM fiber122, the polarization controller123, the connector124, and the SM fiber125and is guided to the polarization beam splitter126. The light beam is split into two polarization components, which are the V polarization component and the H polarization component, at the polarization beam splitter126. The spectral distribution of light intensity detected by the detectors128and130is displayed on a display134and the polarization controller123is adjusted so that the light beam is linearly polarized at the polarization beam splitter126and only one of the detectors128and130detects the light (FIG. 3B).

Finally, the polarization controller116is adjusted. The polarization controller116is adjusted using only the reference light beam. The reference light beam passes through the SM fiber115, the polarization controller116, the collimator117, the dispersion compensating glass118, and the ND filter119and is reflected by the mirror120to be guided back to the beam splitter106. Here, the polarization controller116is adjusted so that the amounts of light detected by the detectors128and130become substantially the same (FIG. 4). Consequently, the reference light beam guided to the polarization beam splitter can be linearly polarized in a manner such that the ratio of the V polarization component to the H polarization component is 1:1, that is, linearly polarized so as to be inclined at 45° with respect to the two orthogonal polarization axes. The reference light beam has single polarization state and not changed temporally.

The controller131for controlling the entire system will be described.

The controller131includes the drive controller133, the signal processor132, and the display134. The drive controller133controls each component in the above-described manner.

The signal processor132forms an image, analyses the formed image, and forms visualized information of the analytical results on the basis of signals output from the detectors128and130. For example, the signal processor132forms an image of the test sample (the target eye114) on the basis of a degree of depolarization, or forms an image of the test sample (the target eye114) on the basis of a change in phase. The method for forming an image and the method for analyzing the formed image are the same as those described in NPL 1 and thus are not described here.

An image formed by or results analyzed by the signal processor132are displayed on a display screen of the display134(for example, a display made of liquid crystal or the like). Image data formed by the signal processor132may be transmitted to the display134in a wired or wireless manner.

Although the display134and other members are included in the controller131, the present invention is not limited to this configuration and the display134and other members may be provided separately from the controller131. In that case, it is preferable that a display have a touch-panel function, with which the position at which an image is displayed on the touch panel is movable, the image can be enlarged or contracted, an image displayed on the touch panel is switchable, or other operations can be made.

In the configuration described above, the polarization state is appropriately set by the polarization controllers disposed on the optical paths of the interferometer. Thus, a polarization-sensitive OCT system can be formed at a low cost by using a reasonable SM fiber, not an expensive PM fiber. Although the case of an SD-OCT is described in this embodiment, the present invention is not limited to this case. A polarization OCT image can be similarly formed even by an SS-OCT, structured using a swept source (SS) light source (SS light source, below), if the SS-OCT is formed into a similar configuration. That is, a swept light source can be used as the light source101. In this embodiment, the Michelson interferometer is used but the use of the Mach-Zehnder interferometer would bring about similar effects. Although this embodiment does not include any polarizer, a polarizer may be provided between the SM fiber102and the SM fiber105depending on the degree of polarization to which the light source polarizes light. In such a case, the connector104is removed from the SM fibers102and105and an input terminal of the polarizer is connected to the SM fiber102, while an output terminal of the polarizer is connected to the SM fiber105. Thus, an alternative configuration can be made. Here, the method for directly connecting the SM fibers102and105and a polarizer is described, but the present invention is not limited to this method. When a component in which an optical fiber and a polarizer are integrated is to be added to the system, a polarizer can be added by removing the SM fiber102from the connector104, connecting a polarizer input-side optical fiber to the SM fiber102using a new connector, and connecting a polarizer output-side optical fiber to the connector104.

Second Embodiment

In this embodiment, referring toFIGS. 5 and 6, a method for forming a polarization-sensitive OCT system by adding components to a conventional OCT system will be described. In this embodiment, an SS-OCT is used for a light source501, as an example. Since the basic configuration of an SS-OCT is a publicly-known configuration, the SS-OCT is not described in detail here.

<Addition of Components to conventional OCT System>

FIG. 5illustrates a conventional SS-OCT system. The configuration shown by reference numerals503to518,520and527to530inFIG. 5are similar to the configuration shown by reference numerals104to119,121and131to134inFIG. 1respectively. InFIG. 5, 519are mirrors,521,524and525are SM fibers,523is a polarization beam splitter as an example of polarization dividing means,526is a detector and531is a collimator. A detector526and SM fibers524and525are detachable from each other. Additional components are attached only to a portion on an SM fiber502, which guides light emitted from a light source501to an interferometer, and a detector that detects an interference light beam.

Firstly, a polarization controller601is disposed at a portion on the optical fiber502. Thus, the light emitted from the light source can be linearly polarized (FIG. 6).

Because polarization controller changes polarization state by clipping both sides of the fiber in general, it can be installed without detaching the SM fibers.

Then, polarization controllers602and603which are an example of the second polarization control means, polarization beam splitters604and605, SM fibers606,607,608,609,613, and614, a detector610, and connectors611and612are added to the detector that detects an interference light beam.

Firstly, the SM fibers524and525are removed from the detector526and respectively connected to the connectors611and612. At this time, the polarization controllers602and603are respectively disposed on the SM fibers524and525.

Subsequently, the SM fibers613and614are respectively connected to the connectors611and612. Other ends of the SM fibers613and614are respectively connected to the polarization beam splitters604and605.

Thereafter, the SM fiber606is connected in such a manner that an interference light beam emitted from the polarization beam splitter604enters the detector526. In addition, the SM fiber607is connected in such a manner that another interference light beam emitted from the polarization beam splitter604enters an additional detector610. Similarly, the SM fiber608is connected in such a manner that an interference light beam emitted from the polarization beam splitter605enters the detector526. In addition, the SM fiber609is connected in such a manner that another interference light beam emitted from the polarization beam splitter605enters the detector610.

At this time, the SM fibers606,607,608, and609are connected in such a manner that the detectors526and610receive light having different polarization components. For example, the SM fibers606and608are connected in such a manner that, when a V polarization component split by the polarization beam splitter604enters the detector526through the SM fiber606, the V polarization component split by the polarization beam splitter605enters the detector526through the SM fiber608. Similarly, the SM fibers607and609are connected in such a manner that, an H polarization component split by the polarization beam splitter604enters the detector610through the SM fiber607and the H polarization component split by the polarization beam splitter605enters the detector610through the SM fiber609.

By adding and connecting components in the above-described manner, a conventional OCT system can be changed into a polarization-sensitive OCT system that can form a polarization OCT image.

In this embodiment, the method for sequentially connecting components to the detector of the interferometer is described. However, a configuration in which components from the connectors611and612to the detectors526and610are integrated into a unit so as to be collectively detachable is more preferable.

In that case, conventional SS-OCT system can be changed to polarization-sensitive OCT system easily, only by connecting the fibers524and525to the connectors611and612which are components of the unit. The interference light beam is formed and split into two parts at the beam splitter523, which are respectively guided to the polarization beam splitters604and605through the SM fibers524and525. The split parts of the interference light beam are interference light components having opposite phases (referred to as a positive component and a negative component, below). The positive part of the split interference light beam is further guided to the polarization beam splitter604, at which it is split into a positive H polarization component and a positive V polarization component. Similarly, the negative part of the split interference light beam is split into a negative H polarization component and a negative V polarization component at the polarization beam splitter605. The positive H polarization component generated at the polarization beam splitter604and the negative H polarization component generated at the polarization beam splitter605respectively pass through the SM fibers606and608and are guided to the detector526, at which the differential is detected. On the other hand, the positive V polarization component generated at the polarization beam splitter604and the negative V polarization component generated at the polarization beam splitter605pass through the SM fibers607and609and are guided to the detector610.

Subsequently, polarization state of the light beam is adjusted using the polarization controllers507(an example of the first polarization control means),515,602and603. In the SS-OCT500, the reference light beam and the sample light beam need to be adjusted to same polarization state so that strength of interference light beam at the beam splitter523becomes as high as possible.

However, the polarization state of the light beam entering to the target eye513needs to be circularly polarized in the polarization-sensitive OCT. In addition, H and V polarization components of the reference light beam need to have equal strength to each other. Therefore, polarization state adjustment described in the first embodiment is applied.

Interference signals detected by the detectors526and610are converted into electric signals and transmitted to a signal processor528. The signal processor528forms a polarization OCT image on the basis of information from the detectors. The method for forming polarization OCT images is not described here since the method has been described in NPL 1.

In the above-described manner, a conventional OCT system can be easily changed into a polarization-sensitive OCT system that can form polarization OCT images. In the embodiment, an SS-OCT is described as an example. The present invention, however, is not limited to this configuration. An SD-OCT can be similarly changed into a polarization-sensitive OCT system that can form polarization OCT images by detaching and adding components in the similar manner. Specifically, by making a spectroscope, which is a detector, detachable, the alternative system can have the effects similar to those in the embodiment. In this embodiment, the Mach-Zehnder interferometer is used as an example, but the use of the Michelson interferometer would bring about similar effects. Although all the fibers are SM fibers in this embodiment, PM fibers may be partially used as needed. Although the embodiment does not include a polarizer, a polarizer may be disposed between the polarization controller601and a connector503depending on the degree of polarization to which the light source polarizes light.

This application claims the benefit of Japanese Patent Application No. 2014-003700, filed Jan. 10, 2014, which is hereby incorporated by reference herein in its entirety.