OPTICAL COHERENCE TOMOGRAPHY DEVICE AND OPTICAL COHERENCE TOMOGRAPHY METHOD

Abstract

To provide an optical coherence tomography instrument suitable for imaging a retina.SOLUTION: An optical coherence tomography instrument suitable for imaging a retina is disclosed. In the instrument, an adjustable optical frequency shifter, which can be or include an acousto-optic modulator or electro-optic modulator, is arranged (i) between a coupler and a reference optical system, (ii) in the reference optical system, (iii) between the coupler and a front-end optical system, or (iv) in the front-end optical system. The optical frequency of reference light or signal light may adjustably be increased or decreased. In operation, a subject is arranged such that its retina is in the focal depth of the front-end optical system. The increase or decrease in the optical frequency of the reference light or the sample light can be adjusted.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] Exemplary aspects of the present disclosure relate to optical coherence tomography, and more particularly, to optical coherence tomography devices and methods for performing retinal optical coherence tomography measurements. [Background technology]

[0002] Optical coherence tomography (OCT) is an imaging technique that can non-invasively measure and image surface and subsurface structures of human tissue, particularly the retina, at high resolution, for example but not limited to.

[0003] In optical coherence tomography, the measurement light is split into two paths by an optical coupler. The optical coupler directs the light from each path into a different arm of the interferometer. One arm is called the reference arm and the other is called the sample arm. In the sample arm, the sample optics, called the front-end optics, shine light onto the sample to be measured, and the reflected light is collected by the sample optics and returned to the optical coupler. In the reference arm, the light enters the reference optics, which returns it to the optical coupler. The returning light from the sample and reference arms is recombined by the coupler to produce an interference pattern. The interference pattern is recorded by a detector.

[0004] The interference pattern contains information about the path traveled by the reflected sample light and the magnitude of the sample light that propagated a particular path length. Because the wavelength of light is selected to be at least partially transparent to the sample being measured, the interference pattern contains information about the surface and subsurface structures of the sample.

[0005] Various implementations of optical coherence tomography techniques are known in the art. One technique, called swept-source optical coherence tomography (SS-OCT), uses a measurement light whose optical frequency is modulated periodically in a controlled manner over a defined source bandwidth. Typically, a series of rising sweeps over a defined optical frequency band is used as the modulation. The time-varying interference pattern signal is recorded by a detector. The Fourier transform of the recorded signal over one period of the modulation of the optical frequency of the measurement light produces an axial depth profile of the sample with an intensity corresponding to the strength of the reflection.

[0006] By scanning the measurement light in one or two dimensions across the surface of the sample, an axial depth profile can be obtained for each of a number of points on the sample surface, such that a two-dimensional or three-dimensional depth profile of the sample can be obtained.

[0007] The coherence length of the measurement light determines the imaging depth of the system, while the source bandwidth determines the axial resolution of the system, and the optical path length of the reference arm determines the axial position at which the axial depth profile is obtained.

[0008] Conventionally, to position the axial region at or around the surface of the sample for which the axial depth profile is to be measured, the optical path length of the reference arm is either fixed or mechanically adjustable, for example by a movable mirror. However, a fixed reference arm optical path length results in an inflexible system, while mechanically adjusting the reference arm optical path length is not fast, reliable or accurate enough.

[0009] It would be useful to provide an optical coherence tomography apparatus and method that allows for rapid, reliable and precise adjustment of the axial position at which an axial depth profile can be obtained. Summary of the Invention

[0010] According to an exemplary aspect of the present disclosure, an optical coherence tomography apparatus for imaging the retina is provided. The apparatus of the present disclosure includes an optical coupler configured to receive light from a tunable narrowband light source and split the light into at least a signal light and a reference light. The apparatus of the present disclosure also includes a reference optical system configured to return the reference light, and a front-end optical system configured to direct the signal light to the subject's eye and return the signal light reflected from the subject's eye. The apparatus of the present disclosure also includes a detection unit configured to sample a time-varying interference signal between the return reference light and the return signal light. An adjustable optical frequency shifter is disposed either (i) between the optical coupler and the reference optical system, (ii) in the reference optical system, (iii) between the optical coupler and the front-end optical system, or (iv) in the front-end optical system. The adjustable optical frequency shifter is configured to adjustably increase or decrease the optical frequency of the reference light or the signal light.

[0011] In one embodiment of the present disclosure, the reference optics includes a reflector configured to reflect the reference beam back.

[0012] Additionally, in one embodiment of the present disclosure, the reflector is fixed relative to the optical coupler.

[0013] Furthermore, according to one embodiment of the present disclosure, the reference optical system includes an optical loop for returning the reference light. The optical loop can have a fixed optical path length, although this example is not limiting.

[0014] According to one embodiment of the present disclosure, the reference light passes through an optical frequency shifter in a forward and reverse direction.

[0015] According to another embodiment of the present disclosure, signal light passes through an optical frequency shifter in a forward and reverse direction.

[0016] According to one embodiment of the present disclosure, the optical frequency shifter includes an acousto-optical modulator or an electro-optical modulator.

[0017] In yet another embodiment of the present disclosure, the apparatus of the present disclosure further comprises a radio frequency driver configured to drive the acousto-optic or electro-optic modulator to obtain a predetermined optical frequency shift.

[0018] Additionally, according to an embodiment of the present disclosure, the optical coupler includes a beam splitter or a fiber coupler.

[0019] The apparatus of the present disclosure further comprises a tunable narrowband light source configured to emit narrowband light into the coupler.

[0020] In an embodiment of the present disclosure, the narrowband light has a coherence length greater than 0.5 cm, optionally greater than 1 cm, optionally greater than 10 cm.

[0021] In one embodiment of the present disclosure, the tunable narrowband light source includes, but is not limited to, a tunable vertical cavity surface emitting laser.

[0022] In one embodiment of the present disclosure, the tunable narrowband light source is configured to periodically vary the optical frequency of the light emitted thereby.

[0023] Also, according to one embodiment of the present disclosure, the detection unit comprises one of a photodetector or a balanced photodetector.

[0024] According to another exemplary aspect of the present disclosure, a method for optical coherence tomography is provided. The method of the present disclosure includes positioning a subject such that the subject's retina is located at a focal depth of a front-end optical system of an optical coherence tomography apparatus. The method of the present disclosure includes introducing narrowband light having a periodically varying optical frequency into an optical combiner to cause the optical combiner to split the light into at least a signal light and a reference light, the reference light being reflected back by the reference optical system and the signal light being reflected back by the subject's eye. The method of the present disclosure also includes recording a time-varying interference signal between the reflected reference light and the reflected signal light. The recording is based on detection of the time-varying interference signal by a detection unit having a detection bandwidth defined by a sample frequency of the detection unit. The method of the present disclosure further includes adjusting the optical frequency of the reference light or the signal light. According to an embodiment of the present disclosure, the adjustment is performed such that an interferogram representing a depth structure in the retina obtained between the reflected signal light and the reflected reference light falls within the detection bandwidth.

[0025] For a better understanding of the present disclosure, and to show how the same may be carried into effect, reference will be made to the accompanying drawings, which are described below. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram of an optical coherence tomography apparatus (system) according to an exemplary embodiment of the present disclosure. [Diagram 2] FIG. 2 is a graphical representation of a periodic frequency sweep provided by a swept source typical of an optical coherence tomography apparatus. [Diagram 3] FIG. 3 is a diagram illustrating at least a portion of a conventional optical coherence tomography system, including, among other components, a reference arm and a sample arm. [Figure 4] FIG. 4 illustrates the effect of introducing optical delay due to increased optical path length into the periodic frequency sweep of FIG. [Diagram 5]FIG. 5 illustrates an optical coherence tomography system according to an exemplary embodiment of the present disclosure, including, among other components, a reference arm and a sample arm. [Figure 6] FIG. 6 is a diagram showing the effect of introducing an optical frequency downshift by an optical frequency shifter into the periodic frequency sweep of FIG. [Figure 7] FIG. 7 is a diagram illustrating an optical coherence tomography apparatus according to another exemplary embodiment of the present disclosure, including, among other components, a reference arm and a sample arm. [Figure 8] FIG. 8 is a diagram illustrating an optical coherence tomography apparatus according to yet another exemplary embodiment of the present disclosure. [Figure 9] FIG. 9 is a schematic diagram of an optical coherence tomography apparatus using balanced detection according to yet another exemplary embodiment herein. [Figure 10] FIG. 10 is a diagram illustrating another optical coherence tomography apparatus using balanced detection in accordance with another exemplary embodiment of the present disclosure. [Figure 11] FIG. 11 is a schematic diagram of a controller for controlling an optical coherence tomography apparatus according to an exemplary embodiment of the present disclosure. [Figure 12] FIG. 12 is a flow chart illustrating an optical coherence tomography method according to an exemplary embodiment of the present disclosure. [Figure 13] FIG. 13 is a diagram illustrating an optical coherence tomography system according to an exemplary embodiment of the present disclosure, in which an optical frequency shifter is disposed between an optical beam splitter and a reference optics. [Figure 14] FIG. 14 is a diagram illustrating an optical coherence tomography system according to an exemplary embodiment of the present disclosure, where an optical frequency shifter is in the sample optics. [Figure 15] FIG. 15 is a diagram illustrating an optical coherence tomography system according to an exemplary embodiment of the present disclosure, in which an optical frequency shifter is disposed between an optical beam splitter and a sample optics. [Figure 16] FIG. 16 illustrates the effect of introducing inverse optical delay by increasing the optical path length into the periodic frequency sweep of FIG. [Figure 17] FIG. 17 is a diagram showing the effect of introducing an optical frequency upshift by an optical frequency shifter into the periodic frequency sweep of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] FIG. 1 is a schematic diagram of an optical coherence tomography apparatus (also referred to herein as an optical coherence tomography system) constructed in accordance with an exemplary embodiment of the present disclosure.

[0028] In the apparatus of FIG. 1, the swept light source SLS generates a beam of narrowband light with a variable center frequency. The swept light source SLS is configured to repeatedly vary the center frequency of the narrowband light, for example by a repeated periodic frequency sweep over a defined frequency band between a low frequency and a high frequency. Such a periodic frequency sweep is illustrated in FIG. 2, which shows the frequency of the output of such a swept light source SLS versus time. Such a frequency sweep is conventionally called a chirp. The sweep may be an increase in time from a low frequency to a high frequency, or a decrease in time from a high frequency to a low frequency. The sweep may be repeated in succession in time, increasing and decreasing. For example, there may be a forward sweep from a low frequency to a high frequency followed by a reverse sweep from a high frequency to a low frequency. The sweep range may be, for example, on the order of 100 nm. The swept light source SLS may have a high coherence, for example a coherence length of more than 5 mm. In some configurations, the coherence length may be greater than 10 mm, or even greater than 100 mm, although these examples are of course not limiting.

[0029] In an exemplary embodiment of the present disclosure, the swept light source SLS is a tunable laser or a tunable laser diode, including, for example, an external cavity laser, an optical parametric amplifier, a Fourier domain mode-locked laser (FDML), or a tunable vertical cavity surface emitting laser (VCSEL), but these examples are not intended to limit the scope of the present invention. In an exemplary embodiment of the present disclosure, the bandwidth of the swept light source SLS can be selected, for example, to optimally transmit the tissue of the eye lens and the eye retina under examination. Also, in an exemplary embodiment of the present disclosure, the bandwidth is in the infrared region of the optical spectrum, for example, at wavelengths greater than 850 nm. Exemplary wavelengths that may be included in the sweep are, but are not limited to, 850 nm, 1050 nm, 1310 nm, or 1550 nm.

[0030] The beam of narrowband light produced by the swept light source SLS is directed to an optical beam splitter OBS, which acts as an optical combiner to split the beam of narrowband light from the source SLS into two beams. The splitting of the beam may be symmetric, such that the two resulting beams are of equal intensity, or asymmetric, such that the two resulting beams are of unequal intensity.

[0031] The first beam (also referred to herein as the sample beam) is directed to a sample optics SOS. The sample optics SOS includes optical components for shaping and directing the light beam towards a sample S (such as, for example, a retina) and for collecting the reflected light (i.e., light reflected from the sample S) and directing it back to the optical beam splitter OBS. The reflected light follows essentially the same optical path as the sample beam to the optical beam splitter OBS, but in the opposite direction.

[0032] In exemplary embodiments of the present disclosure, the sample optical system SOS includes conventional components known in the field of optical coherence tomography, and in some exemplary embodiments of the present disclosure, such conventional components are adapted according to the imaging operation to be performed. More specifically, in one embodiment of the present disclosure, the sample optical system SOS can include scanning optics that can pivot the sample beam about a pivot point located in front of the eye (e.g., sample S) to scan the sample beam across a wide field of the retina located in the rear of the eye.

[0033] By way of example, the sample optical system SOS may include one or more scanners arranged to scan the beam in one or more directions across the retina, such as a galvanometer scanner, an oscillating plane mirror, a MEMS mirror, a rotating mirror, a prism or polygon scanner, or a resonant scanner.

[0034] The sample optics may also include a scan relay unit, e.g., comprising a lens or a curved mirror, configured to image a first scanner that scans the beam in a first direction onto a second scanner that scans the beam in a second direction to generate a two-dimensional scan pattern originating from an apparent origin located at the second scanner.

[0035] The sample optics may also include a scan transfer unit, such as a lens or a curved mirror, configured to project an apparent origin into space beyond the sample optics SOS, such that the two-dimensional scan pattern resulting from the apparent origin in the second scanner is transferred to an apparent pivot point in space beyond the sample optics such that the sample beam pivots around the scanning pivot point.

[0036] The second beam (also referred to herein as the reference beam) is directed to the reference optics ROS. The reference optics ROS then returns the reference beam to the optical beam splitter OBS. At the optical beam splitter OBS, the return reflected light (i.e., the light reflected from the sample S) and the return reference beam (i.e., the reference beam returned from the reference optics ROS) are interferometrically combined and directed to the detector DET as an interference beam. In one embodiment of the present disclosure, the detector DET is an optical detector, such as a photodiode or an avalanche photodiode, that converts the optical intensity of the interference beam to provide a resultant conversion signal in the form of an electrical signal, such as a voltage or current. In one embodiment of the present disclosure, the resultant conversion signal is a time-varying analog signal. After being output by the detector DET, the resultant conversion signal can be recorded in a time-varying manner, as described below, and the recorded version of the signal constitutes an interferogram between the return reflected light and the return reference beam in terms of the optical frequency of the narrowband light from the swept light source SLS.

[0037] In the exemplary embodiment depicted in Fig. 1, the analog-to-digital converter ADC periodically samples and quantifies the signal output by the detector DET at a predetermined sample frequency and provides digital values ​​of the quantified and sampled signal to the data processing unit DPU, where the digital values ​​are recorded. Thus, the detector DET and the analog-to-digital converter ADC constitute a detection unit DU that samples the time-varying interference signal between the return reference light and the return signal light. With regard to the data processing unit DPU, in one embodiment of the present disclosure, the data processing unit DPU performs a Fourier transform operation, such as a fast Fourier transform, on the time-varying quantified and sampled signal values ​​obtained from the analog-to-digital converter ADC to generate an axial depth profile.

[0038] As a result of the periodic sampling of the output of the detector DET by the analog-digital converter ADC, the highest frequency components of the time-varying analog signal, which are outside the detection bandwidth defined by the sample frequency of the analog-digital converter ADC, are not recorded. Simply put, the beat frequency of these components of the interferogram is too high to be recorded. For example, the highest frequency component of the time-varying analog signal may be greater than the Nyquist frequency of the analog-digital converter and conversely, the sample rate may be less than the Nyquist rate of the highest frequency component of the interferogram.

[0039] The swept source SLS may have a relatively high coherence length such that the interference between the returning reflected light and the returning reference beam contains information about the reflectivity of the sample over a large axial depth range defined by the high coherence length. Nevertheless, from the Fourier transform of the recorded sampled signal in the data processing unit DPU, information is only available about a subrange of this axial depth range, which corresponds to the detection bandwidth centered on the axial position defined by the optical path length of the reference arm.

[0040] In conventional optical coherence tomography apparatus, in view of the generally short coherence length of the light source used, the optical path length of the reference arm is adjusted so that the axial position of the measured axial depth range corresponds to the position of the sample surface to be measured, thereby introducing an optical delay between the sample light and the reference light, which allows the sample light and the reference light to maintain coherence when they are combined and interfere at the detector.

[0041] An exemplary configuration of such an arrangement is shown in Figure 3, where a movable mirror M is provided in the reference optics ROS to change the optical path length traveled by the light in the reference arm. Changing such an optical path length traveled by the light in the reference arm corresponds to a variable optical delay of the frequency chirp generated by the swept light source SLS, as shown in Figure 4. It should be understood that in the disclosed configuration an optical delay is introduced in the reference beam, but this optical delay is relative to the sample beam and can in principle be positive or negative depending on whether the optical path of the sample arm is longer or shorter than that of the reference arm.

[0042] The inventors have discovered that if the SLS swept source has sufficiently high coherence, then a constant frequency shift, such as a frequency upshift or a frequency downshift, applied to the frequency chirp generated by the SLS swept source corresponds to a substantially constant optical delay, and the amount of equivalent optical delay introduced corresponds to the amount of frequency shift introduced.

[0043] According to an exemplary embodiment of the present disclosure, such a frequency shift may be introduced by placing an optical frequency shifter OFS in the reference arm between the optical beam splitter OBS and the mirror M (e.g., in the reference optics ROS), as shown in FIG. 5. The effect on the frequency chirp generated by the swept light source SLS is shown in FIG. 6. A comparison of FIG. 6 with FIG. 4 shows that the introduction of a predetermined frequency shift is equivalent to the introduction of a predetermined optical delay. Although FIG. 4 and FIG. 6 show the effect of introducing a frequency downshift, a frequency upshift is also contemplated, and FIG. 16 and FIG. 17 correspond to FIG. 4 and FIG. 6 for the frequency upshift case.

[0044] Equivalently, an optical frequency shifter may be placed between the optical beam splitter OBS and the reference optical system ROS, as shown in FIG. 13. Similarly, a frequency shift that would correspond to an optical delay in the sample arm may be introduced in the sample arm by placing an optical frequency shifter OFS in the sample optical system SOS. Such a configuration is shown in FIG. 14. Equivalently, an optical frequency shifter may be placed between the optical beam splitter OBS and the sample optical system SOS, as shown in FIG. 15.

[0045] The optical frequency shifter OFS is controllable by a frequency shift controller FSC. The frequency shift controller FSC controls the optical frequency shifter OFS to adjust the amount of frequency shift introduced, which can achieve the same effect as introducing an adjustable optical delay by a movable mirror M as shown in FIG. 3, for example, without using any moving parts. Thus, by introducing a predetermined amount of frequency upshift or downshift in the light in the sample or reference arm, the temporal interference frequency between the return light of the sample beam and the return light of the reference beam can be slowed down so that the interference components originating from the desired axial region of the sample are within the detector bandwidth defined by the sample frequency. In simple terms, the beat frequency of the interferogram components originating from the desired axial region of the sample S is slowed down enough that it can be recorded. As a result, the electrical frequency of the electrical signal output by the detector is also reduced. As an example, the frequency of the interferogram components originating from the desired axial region of the sample can be reduced to be lower than the Nyquist frequency of the analog-to-digital converter.

[0046] According to one embodiment of the present disclosure, an acousto-optic modulator (AOM) may be used as the optical frequency shifter OFS to introduce the optical frequency shift.

[0047] An acousto-optical modulator comprises a driven optical medium to which vibrations are applied at a defined frequency. This vibration induces phonons in the optical medium, which interact with photons passing through the optical medium to generate diffracted light with a frequency shift proportional to the frequency of the vibration, the proportionality constant being determined by the order of diffraction. Thus, by varying the frequency of the vibration, a well-defined optical frequency shift can be introduced. Depending on the diffraction order, and in particular whether the diffraction order is positive or negative, the optical frequency shift will be either an upshift or a downshift. Such an acousto-optical modulator can be operated in a traveling wave configuration or a standing wave configuration. In such a configuration, the frequency shift controller FSC is a high frequency driver arranged to apply a variable high frequency electrical signal to the optical medium by means of a piezoelectric element attached to the optical medium.

[0048] According to an embodiment of the present disclosure, an electro-optical modulator (EOM) may alternatively be used as the optical frequency shifter OFS to introduce the optical frequency shift.

[0049] An electro-optic modulator includes an optical medium that exhibits an electro-optic effect when an electric field is applied. The electric field may be applied to the medium, for example, by placing the medium between the plates of a parallel plate capacitor. The applied electric field induces a corresponding change in the refractive index of the optical medium depending on the strength of the electric field, due to the electro-optic effect. Without being bound by theory, such a change in the refractive index may be caused by forces that distort the position, orientation, or shape of the molecules that typically make up the optical medium. The change in the refractive index induces a change in the phase of the light emitted from the medium. Changing the electric field at a frequency will also change the refractive index at that frequency. The change in the refractive index can induce a corresponding change in the phase of the light emitted from the optical medium.

[0050] For example, if the electric field is sinusoidally varied with a predetermined frequency ω, a time-dependent phase at the predetermined frequency ω may be added to the time dependence of the electromagnetic wave, e.g., light of frequency Ω, emitted from the optical medium. As a result of the addition of this time-dependent phase, a set of sidebands, including at least a first pair of sidebands, is added to the light at frequencies Ω±ω, each sideband of the first pair of sidebands being shifted in frequency by the predetermined frequency ω with respect to the frequency Ω of the light. Thus, by varying the frequency of the electric field, a well-defined optical frequency shift can be introduced into the light. Due to the presence of two sidebands, an optical frequency shift can be obtained that is either an upshift or a downshift.

[0051] In another exemplary embodiment, an electro-optic modulator (EOM) may alternatively be used to introduce the optical frequency shift by amplitude modulation rather than phase modulation.

[0052] In particular, electro-optical modulators of the phase-modulating type can be used to introduce amplitude modulation in incident light by incorporating at least one such electro-optical modulator in at least one corresponding arm of an interferometer, such as a Mach-Zehnder interferometer, and applying the incident light. Such an interferometer has two arms in which the incident light is coherently separated, and produces an output light by coherently combining the light of the two arms. The coherent separation and combination of the light can be achieved by one or more beam splitters. This configuration is sometimes called a Mach-Zehnder Modulator (MZM). In a related configuration, one electro-optical modulator can be provided in each of the two arms of the interferometer.

[0053] Application of an electric field across the optical medium of the electro-optic modulator in the interferometer will, as described above, introduce a phase shift dependent on the strength of the electric field. In response to the introduced phase shift, the amplitude of the light output from the interferometer will also vary depending on the amount of phase shift introduced, in accordance with normal interferometer principles. This allows the light to be imparted with a varying amplitude. The magnitude of the change in the applied electric field may be set such that the difference in the introduced phase shift between the minimum and maximum values ​​of the applied electric field corresponds to half the wavelength of the input light. In such an arrangement, varying the applied electric field at a predetermined frequency ω will also cause the amplitude of the output light to vary at the predetermined frequency ω.

[0054] For example, if the electric field is sinusoidally varied with a predetermined frequency ω, a time-dependent amplitude at the predetermined frequency ω may be added to the time-dependent electromagnetic wave, e.g. of frequency Ω, emitted from the optical medium, e.g. of light. As a result of adding this time-dependent amplitude, a set of sidebands, including only the first pair of sidebands, is added to the light at frequencies Ω ± ω, each sideband of said sidebands being shifted in frequency by the predetermined frequency ω with respect to the frequency Ω of the light. Thus, by varying the frequency of the electric field, a well-defined optical frequency shift can be introduced into the light. Due to the presence of two sidebands, an optical frequency shift can be obtained that is either an upshift or a downshift.

[0055] When sidebands are generated as frequency-shifted light, for example through the use of an electro-optical or Mach-Zehnder modulator, it may be necessary to select only the sideband light for further use. The selection of the high-frequency sideband light can be done, for example, by incorporating a suitable high-pass filter after the interferometer or modulator. On the other hand, the selection of the low-frequency sideband light can be done, for example, by incorporating a suitable low-pass filter after the modulator. However, if the frequency of the sidebands is sufficiently different from the incident light, it may not be necessary to incorporate a filter. For example, by using an electro-optical or Mach-Zehnder modulator, a frequency offset of several gigahertz can be achieved for the shifted light with respect to the input light. In such a case, the filter can be omitted.

[0056] In such a configuration, the frequency shift controller FSC is a radio frequency (which can include microwave frequencies) driver configured to apply a variable radio frequency or microwave frequency electrical signal to the optical medium using a suitable pair of capacitor plates attached to or positioned adjacent to the optical medium, thereby applying a variable electric field across the optical medium.

[0057] The medium for the electro-optic modulator is lithium niobate (LiNbO 3 ) can be used. Other media, including nonlinear and birefringent media, may also be used. Examples of alternative media include potassium dihydrogen phosphate (KD * P or DKDP), potassium titanyl phosphate (KTP), beta barium borate (BBO), lithium tantalate (LiTaO 3 ) and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 In addition to these inorganic media, nonlinear polymeric media such as poled polymers can be used as the medium.

[0058] In other exemplary embodiments of the present disclosure, other nonlinear optical techniques can be used to generate the desired frequency upshift or downshift, for example, instead of an acousto-optic or electro-optic modulator, difference frequency generation or half-harmonic generation using a nonlinear optical crystal may be used to introduce the desired frequency shift.

[0059] According to an example aspect of the present disclosure, the function of the optical frequency shift is to bring the high frequency interference components within the low frequency detection bandwidth. This can be achieved by an optical frequency shift that is either an upshift or a downshift, depending on the laser sweep direction, regardless of whether the frequency shift is applied to the reference or sample arm.

[0060] For example, if the frequency shifter is incorporated in the reference arm, a downshift introduced in the reference arm may be appropriate for a swept light source that sweeps from a low frequency to a high frequency. Also, if the frequency shifter is incorporated in the reference arm, an upshift introduced in the reference arm may be appropriate for a swept light source that sweeps from a high frequency to a low frequency. On the other hand, if the frequency shifter is incorporated in the sample arm, a downshift introduced in the sample arm may be appropriate for a swept light source that sweeps from a low frequency to a high frequency. Furthermore, if the frequency shifter is incorporated in the sample arm, an upshift introduced in the sample arm may be appropriate for a swept light source that sweeps from a high frequency to a low frequency. Thus, a particular frequency shifter can be used to introduce both positive and negative delays. However, the effect of a positive or negative delay can increase or decrease the beat frequency of the interference depending on whether the introduced delay brings the reference and sample arms into phase with respect to each other or out of phase.

[0061] A piezoelectric transducer can be used to apply vibrations to the driven optical medium, itself driven by a radio frequency (RF) voltage applied across the transducer. Thus, in one embodiment of the present disclosure, the optical frequency shifter OFS depicted in FIG. 5 can be or include, for example, an acousto-optic or electro-optic modulator as part of a Mach-Zehnder modulator driven by a variable frequency RF signal generated by a frequency shift controller FSC. Thus, the sample frequency, and thus the detector bandwidth, remains constant, but the amount of frequency shift can be adjusted to reduce the beat frequency of the interferogram and to bring the interferogram region of interest within the detector bandwidth.

[0062] Referring again to Fig. 5, the reference light can pass through the optical frequency shifter OFS twice. In particular, once the reference light propagates in a forward direction from the optical beam splitter OBS via the optical frequency shifter OFS to the mirror M, and once the reference light propagates in a reverse direction from the mirror M via the optical frequency shifter OFS to the optical beam splitter OBS. Each pass through the optical frequency shifter OFS introduces a predetermined frequency shift related to the drive frequency provided by the frequency shift controller FSC. Thus, in the exemplary configuration of Fig. 5, the total shift applied is twice the frequency shift introduced by the optical frequency shifter OFS for each individual optical path.

[0063] In the above description of the exemplary configuration of FIG. 5, the effect of the optical frequency shifter OFS is described in relation to a free-space optical configuration of an optical coherence tomography apparatus. However, this configuration is not intended to limit the scope of the present invention. Indeed, in other exemplary embodiments of the present disclosure, an optical fiber-based configuration can be used, in which an optical fiber is used to propagate the light beam instead of free space.

[0064] For example, FIG. 7 illustrates another exemplary embodiment of the present disclosure of an optical coherence tomography apparatus (also referred to herein as an “optical coherence tomography system”). In FIG. 7, light from a swept light source SLS propagates through an optical fiber to a fiber coupler unit FCU, which provides the function of an optical coupler. The light from the swept light source SLS is thereby split into a reference beam and a sample beam (also referred to as “sample light”). The reference beam travels in the reference arm, circulates through an optical frequency shifter OFS controlled by a frequency shift controller FSC, and is returned to the fiber coupler unit FCU. The sample light travels in the sample arm, and travels through a sample optics SOS to a sample S (not shown in FIG. 7), which reflects the sample light, which is then returned to the fiber coupler unit FCU through the sample optics SOS. In the fiber coupler unit FCU, the return light received from each arm is combined and passed further along the optical fiber to a detector DET. Otherwise the embodiment of Figure 7 operates similarly to the embodiment of Figure 3, except that the light in the reference arm passes through the optical frequency shifter OFS only once, so that only a single frequency shift is applied instead of a twofold frequency shift.

[0065] A further exemplary embodiment of an optical coherence tomography apparatus is shown in FIG. 8. The apparatus of FIG. 8 includes the same components as the apparatus of FIG. 7, except that the reference optical system ROS of the embodiment of FIG. 8 is different from the apparatus of the embodiment of FIG. 7. In FIG. 8, the reference beam propagating in the reference arm is provided to the fiber mirror unit FMU by the optical frequency shifter OFS. The fiber mirror unit FMU reflects the incident light received from the optical frequency shifter OFS and returns the reflected light to the fiber coupler unit FCU through the optical frequency shifter OFS. This introduces a frequency shift of 2 times, similar to the free space optical arrangement of FIG. 5.

[0066] In one embodiment of the present disclosure, the embodiments of Figures 1, 5, 7 and 8 are implemented using so-called unbalanced detection, where the interference frequency components are detected directly from the optical signal of the detector DET. However, in other exemplary embodiments of the present disclosure, a balanced detection configuration can be adopted instead. In balanced detection, the signal arm and the reference arm are independently coupled to different optical inputs of the detector DET, and the signal light and the reference light are coupled to interfere at or within the detector DET via an optical or electrical medium. In such a configuration, the intensities of the signal light and the reference light can also be measured independently, for example, by directing at least a portion of each of the sample light and the reference light to separate detection elements, so that variations in the light intensity can be compensated for by the detection elements.

[0067] FIG. 9 illustrates an exemplary embodiment of the present disclosure of an optical coherence tomography apparatus employing balanced detection. The apparatus of FIG. 9 is similar to the apparatus of FIG. 1, except that in the apparatus of FIG. 9, the detector DET has two optical inputs, and a reference optical system ROS is included in the loop optical system between the output of the optical beam splitter OBS and one of the two optical inputs of the detector DET. The loop optical system includes an optical beam splitter OBS, a reference optical system ROS, and a detector DET. In one exemplary embodiment of the present disclosure, the optical loop has a fixed optical path length, but is not limited to this example. Interference of the returning reference light and the returning sample light in the detector provides an interferogram.

[0068] FIG. 10 illustrates another exemplary embodiment of the present disclosure of an optical coherence tomography apparatus, which has a fiber optic configuration. The apparatus of FIG. 10 includes similar components to the apparatus of FIG. 7, except that the reference optical system ROS is not shown in FIG. 10, and instead of the output of the optical frequency shifter OFS being fed to the fiber coupler unit FCU as shown in FIG. 7, in the apparatus of FIG. 10 the output of the optical frequency shifter OFS is fed to a first one of a number of inputs of the detector DET. Thus, an optical fiber loop is provided that includes the fiber coupler unit FCU, the optical frequency shifter OFS, and the detector DET. In this way, the reference beam output by the fiber coupler unit FCU is fed to the optical frequency shifter OFS, and the output of the optical frequency shifter OFS is fed to a first input of the detector DET. The second input of the detector DET receives the return sample light returning from the sample optical system SOS through the fiber coupler unit FCU. In the detector, an interferogram is provided by the interference of the return reference light and the return sample light in the detector.

[0069] The above configurations have been described in conjunction with separate control and data processing units, such as an internal controller of the swept light source SLS, a frequency shift controller FSC that controls the optical frequency shifter OFS, and a data processing unit DPU that converts the detected optical signals of the detector DET into an axial depth profile by fast Fourier transformation. However, in other exemplary embodiments of the present disclosure, at least some or all of the control and data processing aspects of the above configurations may be provided by an integrated controller that may be instantiated as a programmable logic unit (PLU), an application specific integrated circuit (ASIC), a supervisory control and data acquisition system (SCADA), a general purpose data processor such as a microcomputer, minicomputer, or personal computer (PC), or a mobile device such as a tablet computer or smartphone.

[0070] A schematic diagram of the integrated controller CONT according to an exemplary embodiment of the present disclosure is shown in FIG. 11. In one embodiment of the present disclosure, the integrated controller CONT controls at least some or all of the various components shown in FIG. 1, FIG. 3, FIG. 5, and FIG. 7-10, and may also form all or at least a part of the frequency shift controller FSC. As shown in FIG. 11, all components of the integrated controller CONT are coupled to each other and can therefore communicate with each other. The integrated controller CONT shown in FIG. 11 comprises an analog-to-digital converter ADC to receive, quantify, and sample a signal from the detector DET (not shown in FIG. 11), and in one embodiment of the present disclosure provides the resulting converted signal to one or more other components of the integrated controller CONT, such as, for example, only the master control unit MCU. In one exemplary embodiment of the present disclosure, the integrated controller CONT also comprises a digital-to-analog converter. The digital / analog converter converts digital signals received from one or more other components of the integrated controller CONT, such as, for example only, the master control unit MCU, into analog value(s) and provides them as analog control voltage(s) to the frequency shift controller FSC (not shown in FIG. 11 ), thereby defining the amount of frequency shift introduced by the optical frequency shifter OFS (not shown in FIG. 11 ). The integrated controller CONT also comprises a sweep signal generator SSG that outputs a time-varying signal to control the frequency of the swept light source SLS (not shown in FIG. 11 ). For example, the output of the sweep signal generator SSG may be a time-varying voltage, and the swept light source SLS may accept a voltage input to define the center frequency of the output narrowband light. The integrated controller CONT also comprises a clock generator CLK that defines one or more clocks to provide a common time reference for the operation of the time-varying aspects of the controller CONT. For example, the clock generator CLK may define a sample clock for the analog-to-digital converter ADC and a time reference for the sweep signal generator SSG.The clock generator CLK may also define internal clocks of the controller CONT, such as a memory clock, an instruction clock and a data bus clock. The integrated controller CONT also comprises a fast Fourier transform unit FFT for performing a Fourier transform on the quantification signal and the sample signal obtained via the analog-to-digital converter ADC. The integrated control unit also comprises an arithmetic logic unit ALU for performing arithmetic logic operations on the data processed in the designated controller CONT.

[0071] The integrated controller CONT also comprises a memory MEM, which in one embodiment of the present disclosure is computer readable for storing and retrieving data values ​​(such as recorded data from the analog-to-digital converter ADC, signal waveforms associated with the sweep signal generator SSG, Fourier transform output data generated by the fast Fourier transform unit FFT, and other parameters, instructions and values ​​required to perform the operation of the integrated controller CONT). The memory MEM may include, by way of example and not limitation, a RAM, a ROM, a hard drive, a floppy disk, a memory stick, a buffer, and the like. In one exemplary embodiment of the present disclosure, the memory MEM stores instructions and / or programs for performing the methods and functions described herein and represented in the drawings. The integrated controller CONT also comprises an input / output controller IOC, which transmits and receives values ​​to an external device such as an offline storage instantiated as a hard drive, a flash drive or a disk drive, or an interface such as a network interface, for example, a wired local area network, a wireless area network, or a mobile data network. The integrated controller also comprises a master control unit MCU, which coordinates the operation of the various functional units of the controller CONT and thus controls the other components of the integrated controller CONT. In one exemplary embodiment of the present disclosure, the master control unit MCU (and / or the arithmetic logic unit ALU) can read and write data, instructions and programs from / to the memory MEM and can execute the instructions and programs to perform the methods and functions described herein and represented in the figures. Also, in an exemplary embodiment of the present disclosure, the analog-to-digital converter ADC of FIG. 11 may form and / or be included in the analog-to-digital converter ADC of other figures described herein, and the master control unit MCU and / or the integrated controller CONT may form and / or be included in the frequency shift controller FSC and / or the digital processing unit DPU of other figures described herein.However, the division of various functional tasks into units as shown in FIG. 11 is merely exemplary, and such tasks may be performed by individual functional modules, discrete electronic devices, integrated logic, other hardware, software, and / or other program code, as desired.

[0072] 12 illustrates an optical coherence tomography method according to an exemplary embodiment of the present disclosure, which may be performed by one or more individual ones of the optical coherence tomography apparatuses described above and shown in the drawings. In step S1, a sample (e.g., sample S), such as a subject's retina, is placed at the focal depth of the front-end optics (e.g., sample optics SOS) of the optical coherence tomography apparatus.

[0073] In step S2, a narrowband light having a periodically changing optical frequency is introduced into an optical coupler (e.g., a fiber coupler unit FCU) and split between a sample arm and a reference arm. In the sample arm, the sample light is directed through the front-end optics and illuminated onto the retina. The returning reflected light is captured by the front-end optics and returned to the optical coupler through the sample arm. Meanwhile, the reference light in the reference arm is returned to interfere with the returning light in the sample arm.

[0074] In step S3, a time-varying interference signal between the return reference light and the return reflected light is recorded with or based on a detector (e.g., detector DET). The detector is characterized by a detection bandwidth defined by the detector's sample frequency. The time-varying interference signal recorded by the detector is Fourier transformed to generate an axial depth profile.

[0075] In step S4, an optical frequency shift is introduced / applied to the light in the sample arm or the reference arm by an optical frequency shifter (e.g., an optical frequency shifter OFS) according to a predetermined operating criterion. The optical frequency shift is adjusted by a frequency decrease, for example as shown in FIG. 6, to change the optical frequency of the light passing through the optical frequency shifter. The amount of the optical frequency shift is adjusted until the interferogram between the reflected light from the retina and the reference light falls within the detection bandwidth.

[0076] This allows an axial depth profile to be obtained in the manner described above, the obtained axial depth profile being representative of depth structures in the retina. Of note, the axial depth profile can be obtained substantially without the need to move any part of the sample arm or the reference arm. This can be determined by observing the axial depth profile, or can be detected programmatically, for example, by detecting signals characteristic of retinal surface structures in the axial depth profile.

[0077] According to exemplary embodiments of the present disclosure, the disclosed apparatus and methods may be implemented in a scanning laser ophthalmoscope (SLO). Alternatively, the disclosed apparatus and methods may be implemented to measure tissues other than the retina and other than ocular tissue. For example, the disclosed apparatus and methods may be implemented to measure other biological membranes, such as skin or parts of a plant, or may be applied to measure non-biological structures.

[0078] For convenience, the analog-to-digital converter ADC and the digital processing unit DPU are not shown in Figures 5, 7, 8 and 10, but it should be noted that the devices (systems) of those figures may also include those components, in which case it will be readily understood by those skilled in the art that, for example, the digital processing unit DPU is coupled to the detector DET of the respective device via a converter ADC interposed therebetween.

[0079] In the above description, exemplary aspects are described with reference to several exemplary embodiments. This specification should therefore be considered illustrative rather than restrictive. Similarly, the diagrams shown in the drawings that highlight the functionality and advantages of the exemplary embodiments are presented for illustrative purposes only. The architecture of the exemplary embodiments is sufficiently flexible and configurable that it may be utilized in ways other than those shown in the accompanying figures.

[0080] The example software configurations presented herein may be provided as computer programs or software, such as one or more programs having instructions or sequences of instructions contained or stored in an article of manufacture such as a memory, a machine-accessible medium, a machine-readable medium, an instruction store, or a computer-readable storage device, in one exemplary embodiment, each of which may be non-transitory. The programs or instructions on the non-transitory memory, machine-accessible medium, machine-readable medium, instruction store, or computer-readable storage device may be used to program a computer system or other electronic device. The memory, machine-readable medium, computer-readable medium, instruction store, and storage device may include, but is not limited to, floppy disks, optical disks, and magneto-optical disks, or other types of media / machine-readable media / instruction stores / storage devices suitable for storing or transmitting electronic instructions. The techniques described herein are not limited to any particular software configuration. They may find applicability in any computing or processing environment. As used herein, the terms "memory," "computer-readable," "machine-accessible medium," "machine-readable medium," "instruction store," and "computer-readable storage device" are intended to include any medium capable of storing, encoding, or transmitting instructions or sequences of instructions for execution by a machine, computer, digital processing unit, or computer processor, causing the machine / computer unit / computer processor to perform any one of the methods described herein. Furthermore, it is common in the art to refer to software in any form (e.g., program, procedure, process, application, module, unit, logic, etc.) as performing an action or causing a result. Such expressions are merely a shorthand way of stating that execution of the software by a processing system causes the processor to perform an action or produce a result.

[0081] Some embodiments may be implemented using application specific integrated circuits, field programmable gate arrays, or by interconnecting an appropriate network of conventional component circuits.

[0082] Some embodiments include a computer program product. The computer program product may be a memory, one or more storage media, instruction store(s), or storage device(s) having stored thereon or therein instructions that can be used to control or cause a computer or computer processor to execute any of the procedures of the exemplary embodiments described herein. Such memories / storage media / instruction stores / storage devices may include, by way of example and not limitation, optical disks, ROM, RAM, EPROM, EEPROM, DRAM, VRAM, flash memory, flash cards, magnetic cards, optical cards, nanosystems, molecular memory integrated circuits, RAID, remote data storage / archive / warehousing, and / or any other type of device suitable for storing instructions and / or data.

[0083] Some implementations stored in any one of the memory, one or more computer readable media, instruction store, or storage device(s) include both software for controlling the hardware of the system and software for enabling the system or microprocessor to interact with a human user or other mechanisms that utilize the results of the exemplary embodiments described herein. Such software includes, but is not limited to, device drivers, operating systems, and user applications. Finally, such memory, computer readable media, or storage devices further include software for performing the exemplary aspects of the present disclosure, as described above.

[0084] The programming and / or software of the system includes software modules for implementing the procedures described herein, in some exemplary embodiments herein the modules include software, while in other exemplary embodiments herein the modules include hardware or a combination of hardware and software.

[0085] Although various exemplary embodiments of the present disclosure have been described above, it should be understood that they are presented by way of example and not by way of limitation. It will be apparent to those skilled in the art that various changes in form and details can be made therein. Therefore, the present disclosure should not be limited by any of the exemplary embodiments described above, but should be defined only by the following claims and their equivalents.

[0086] Moreover, the purpose of the Abstract is to enable patent offices and the general public, particularly scientists, engineers, and professionals unfamiliar with patent and legal terminology, to quickly assess the nature and substance of the technical disclosure of the present application at a glance. This Abstract is not intended to be limiting in any way with respect to the scope of the exemplary embodiments presented herein. It should also be understood that any steps recited in the claims need not be performed in the order presented.

[0087] Although many specific embodiment details are described herein, these should not be construed as limiting the scope of the disclosure or what may be claimed, but rather as descriptions of features specific to the particular embodiments described herein. Certain features described herein in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination. Furthermore, although features may be described above as functioning in a particular combination and may initially be claimed as such, one or more features from a claimed combination may, in some cases, be deleted from the combination, and the claimed combination may be directed to a subcombination or a variation of the subcombination.

[0088] In certain circumstances, multitasking or parallel processing may be advantageous. Furthermore, it should be understood that the separation of various components in the above embodiments should not be understood to require such separation in all embodiments, and that the above program components and systems may generally be integrated together in a single software product or packaged into multiple software products.

[0089] Now, having described several exemplary embodiments, it is clear that the above is presented by way of example and not limitation. In particular, while many of the examples presented herein include specific combinations of device or software elements, those elements may be combined in other ways to achieve the same purpose. Operations, elements, and features described only in relation to one embodiment are not intended to be excluded from a similar role in other embodiments.

[0090] The apparatus and methods described herein may be embodied in other specific forms without departing from the characteristics thereof. The above-described embodiments are illustrative rather than limiting of the described systems and methods.

[0091] The scope of the devices and methods described herein is therefore indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. an optical coupler configured to receive light from a tunable narrowband light source and split the light into at least a signal light and a reference light; a reference optical system configured to return the reference light; a front-end optical system configured to direct the signal light toward the subject's eye and return reflected signal light from the subject's eye; a detection unit configured to sample a time-varying interference signal between the returned reference light and the returned signal light; (i) an adjustable optical frequency shifter disposed between the optical coupler and the reference optical system, (ii) within the reference optical system, (iii) between the optical coupler and the front-end optical system, or (iv) within the front-end optical system, configured to adjustably increase or decrease an optical frequency of the reference light or the signal light; Equipped with the wavelength-tunable narrowband light source is a swept light source that changes a center frequency of narrowband light by frequency sweeping, The optical frequency shifter comprises: When the optical frequency shifter is disposed (i) between the optical coupler and the reference optical system or (ii) within the reference optical system, the optical frequency shifter is adjusted so that the reference light is downshifted for the swept light source that sweeps from a low frequency to a high frequency, and is adjusted so that the reference light is upshifted for the swept light source that sweeps from a high frequency to a low frequency; When the optical frequency shifter is disposed (iii) between the optical coupler and the front-end optical system, or (iv) within the front-end optical system, the optical frequency shifter is adjusted so that the signal light is downshifted for the swept light source sweeping from a low frequency to a high frequency, and is adjusted so that the signal light is upshifted for the swept light source sweeping from a high frequency to a low frequency. Optical coherence tomography device.

2. The optical coherence tomography apparatus of claim 1 , wherein the reference optical system comprises a reflector configured to reflect the reference light back to the optical axis.

3. The optical coherence tomography apparatus according to claim 2 , wherein the reflector is fixed relative to the optical coupler.

4. The optical coherence tomography apparatus of claim 1 , wherein the reference optical system includes an optical loop for returning the reference light.

5. The optical coherence tomography apparatus according to claim 1 , wherein the reference light passes through the adjustable optical frequency shifter in a forward direction and a reverse direction.

6. The optical coherence tomography imaging apparatus according to claim 1 , wherein the signal light passes through the optical frequency shifter in a forward direction and a reverse direction.

7. The optical coherence tomography apparatus of claim 1 , wherein the optical frequency shifter comprises an acousto-optic modulator or an electro-optic modulator.

8. The optical coherence tomography apparatus of claim 7 , further comprising a radio frequency driver configured to drive the acousto-optic or electro-optic modulator to obtain a predetermined optical frequency shift.

9. The optical coherence tomography apparatus according to claim 1 , wherein the optical coupler includes a beam splitter or a fiber coupler.

10. 10. The optical coherence tomography apparatus of claim 1, further comprising a tunable narrowband light source, the tunable narrowband light source configured to emit the light into the optical coupler, the light being narrowband light.

11. The optical coherence tomography apparatus of claim 10 , wherein the narrowband light has a coherence length greater than one of 0.5 cm, 1 cm, or 10 cm.

12. The optical coherence tomography apparatus according to claim 10 or 11, wherein the tunable narrowband light source includes a tunable vertical cavity surface emitting laser.

13. 13. An optical coherence tomography apparatus according to claim 10, wherein the tunable narrowband light source is configured to periodically vary the optical frequency of the light emitted thereby.

14. The optical coherence tomography apparatus according to claim 1 , wherein the detection unit comprises one of a photodetector or a balanced photodetector.

15. Positioning a subject such that the subject's retina is located at a focal depth of a front-end optical system of an optical coherence tomography apparatus; introducing narrowband light having a periodically changing optical frequency from a swept light source into an optical coupler, and causing the optical coupler to split the narrowband light into at least a signal light and a reference light, the reference light being reflected back by a reference optical system and the signal light being reflected back by an eye of the subject; recording a time-varying interference signal between the reflected reference beam and the reflected signal beam, the recording being based on detection of the time-varying interference signal by the detection unit having a detection bandwidth defined by a sample frequency of the detection unit; adjusting an optical frequency of the reference light or the sample light, the adjustment being performed such that an interferogram representative of a depth structure in the retina obtained between the reflected signal light and the reflected reference light is within the detection bandwidth; Including, The optical frequency adjustment is performed by an optical frequency shifter; When the optical frequency shifter is disposed (i) between the optical coupler and the reference optical system or (ii) within the reference optical system, the optical frequency shifter is adjusted to downshift the reference light for the swept light source that sweeps from a low frequency to a high frequency, and adjusted to upshift the reference light for the swept light source that sweeps from a high frequency to a low frequency; When the optical frequency shifter is disposed (iii) between the optical coupler and the front-end optical system, or (iv) within the front-end optical system, the optical frequency shifter adjusts the optical frequency shifter so as to downshift the signal light for the swept light source sweeping from a low frequency to a high frequency, and adjusts the optical frequency shifter so as to upshift the signal light for the swept light source sweeping from a high frequency to a low frequency. Optical coherence tomography method.

16. The optical coherence tomography apparatus of claim 4 , wherein the optical loop has a fixed optical path length.

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