Optical coherence tomography instrument and optical coherence tomography method

Abstract

The present application relates to an optical coherence tomography instrument and an optical coherence tomography method. An optical coherence tomography instrument suitable for imaging the retina is disclosed. In the instrument, an adjustable optical frequency shifter, which may be or include an acousto-optic modulator or an 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 the reference light or the signal light can be adjustably increased or decreased. In operation, the subject is arranged so that its retina is at the focal depth of the front-end optical system. The increase or decrease of the optical frequency of the reference light or the sample light can be adjusted. Therefore, the interference pattern representing the depth structure at the retina obtained between the returned signal light and the returned reference light can be within the detection bandwidth of the instrument.

Description

[0001] field

[0002] Example aspects herein relate to optical coherence tomography (OCT), and more particularly, to OCT instruments and methods for performing OCT measurements of the retina. background

[0003] Optical coherence tomography (OCT) is an imaging technique that can non-invasively obtain high-resolution measurements and images of surface and subsurface structures of, for example but not limited to, human tissue, particularly the retina.

[0004] In optical coherence tomography, the measurement light is split into two paths by an optical coupler. The optical coupler directs the light on each path to different arms of the interferometer. One arm is called the reference arm, and the other is called the sample arm. In the sample arm, light is directed toward the sample being studied by the sample optical system (sometimes called the front optical system), and the reflected light is collected by the sample optical system and returned to the optical coupler. In the reference arm, the light enters the reference optical system, which returns the light to the optical coupler. The return light from the sample arm and the reference arm is recombined by the coupler to produce an interference pattern. The interference pattern is recorded by a detector.

[0005] The interference pattern contains information about the optical path traveled by the reflected sample light and the amplitude of the sample light that has traveled a specific optical path length. Because the wavelength of the light is selected to at least partially penetrate the sample being studied, the interference pattern contains information about the surface and subsurface structure of the sample.

[0006] Different implementations of optical coherence tomography techniques are known in the art. A technique known as swept-source optical coherence tomography (SS-OCT) uses measurement light whose optical frequency is periodically modulated in a controlled manner over a defined source bandwidth. Typically, a series of rising sweeps over a defined optical frequency band is used as modulation. A detector records the interference pattern signal that varies with time. The Fourier transform of the signal recorded over one period of modulation of the optical frequency of the measurement light produces an axial depth profile of the sample, the intensity of which corresponds to the intensity of the reflection.

[0007] Scanning the measurement light one-dimensionally or two-dimensionally over the entire sample surface can obtain an axial depth distribution for each of a plurality of points over the entire sample surface, thereby obtaining a two-dimensional or three-dimensional depth distribution of the sample.

[0008] 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. In addition, the optical path length of the reference arm determines the axial position at which the axial depth distribution is obtained.

[0009] Traditionally, the optical path length of the reference arm has been either fixed or mechanically adjustable, for example by a movable mirror, to position the axial region from which the axial depth profile can be measured at or around the sample surface. However, a fixed reference arm path length results in an inflexible system, while mechanical adjustment of the reference arm path length is not fast, reliable, or precise enough.

[0010] It would be useful to provide an optical coherence tomography instrument and method that can quickly, reliably, and accurately adjust the axial position about which an axial depth profile can be obtained.

[0011] Overview

[0012] According to an exemplary aspect of the present invention, an optical coherence tomography instrument for imaging the retina is provided. The instrument includes an optical coupler arranged to receive light from a tunable narrowband light source and to separate the light into at least signal light and reference light. The instrument also includes a reference optical system arranged to return the reference light, and a front-end optical system arranged to guide the signal light to the subject's eye and return the reflected signal light from the subject's eye. The instrument also includes a detection unit arranged to sample a time-varying interference signal between the returned reference light and the returned signal light. An adjustable optical frequency shifter is arranged (i) between the coupler and the reference optical system, (ii) in the reference optical system, (iii) between the coupler and the front-end optical system, or (iv) in the front-end optical system. The adjustable optical frequency shifter is arranged to adjustably increase or decrease the optical frequency of the reference light or the signal light.

[0013] In an example embodiment herein, the reference optical system includes a reflector arranged to reflect the reference light to return the reference light.

[0014] Also in the example embodiments herein, the reflector is fixed relative to the coupler.

[0015] Furthermore, according to an exemplary embodiment of the present invention, the reference optical system includes an optical loop for returning the reference light. The optical loop may have a fixed optical path length, although this example is not limiting.

[0016] According to example embodiments herein, reference light propagates in forward and reverse directions through an optical frequency shifter.

[0017] According to another example embodiment herein, signal light propagates in forward and reverse directions through an optical frequency shifter.

[0018] In an example embodiment herein, the optical frequency shifter includes an acousto-optic modulator or an electro-optic modulator.

[0019] In yet another example embodiment herein, the apparatus further comprises a radio frequency driver arranged to drive the acousto-optic modulator or the electro-optic modulator to obtain a predetermined optical frequency shift.

[0020] Also according to example embodiments herein, the optical coupler includes a beam splitter or a fiber coupler.

[0021] The apparatus may further comprise a tunable narrowband light source, wherein the tunable narrowband light source is arranged to emit narrowband light towards the coupler.

[0022] In an example embodiment herein, the coherence length of the narrowband light is greater than 0.5 cm, optionally greater than 1 cm, optionally greater than 10 cm.

[0023] In one example embodiment herein, the tunable narrowband light source comprises a tunable vertical cavity surface emitting laser, although this example is not limiting.

[0024] According to example embodiments herein, a tunable narrowband light source is configured to periodically vary the optical frequency of light emitted thereby.

[0025] Furthermore, according to example embodiments herein, the detector includes one of a photodetector or a balanced photodetector.

[0026] According to another exemplary aspect of the present invention, an optical coherence tomography method is provided. The method includes positioning a subject so that the subject's retina is within the focal depth of a front-end optical system of an optical coherence tomography instrument. The method includes introducing narrowband light having a periodically varying optical frequency into a coupler so that the coupler separates the light into at least signal light and reference light, wherein the reference light is reflected back by a reference optical system and the signal light is reflected back by the subject's eye. The method also includes recording a time-varying interference signal between the reflected reference light and the reflected signal light, the recording being based on detection of the time-varying interference signal by a detection unit having a detection bandwidth defined by a sampling frequency of the detection unit. The method also includes adjusting the optical frequency of the reference light or the signal light. According to an exemplary embodiment of the present invention, the adjustment is performed so that an interference pattern representing the depth structure at the retina obtained between the reflected signal light and the reflected reference light is within the detection bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] For a better understanding of the present disclosure, and to show how it may be put into effect, reference will be made to the accompanying drawings, which are now described below.

[0028] Figure 1is a schematic diagram of an optical coherence tomography apparatus (system) according to an example embodiment of the present invention.

[0029] Figure 2 is a graphical representation of the periodic frequency sweep provided by a typical swept source of an optical coherence tomography instrument (prior art).

[0030] Figure 3 At least a portion of a conventional optical coherence tomography instrument (system) is shown, including reference and sample arms and other components (prior art).

[0031] Figure 4 shows that optical delay is introduced into Figure 2 The effect of periodic frequency sweep.

[0032] Figure 5 An optical coherence tomography instrument (system) according to example embodiments herein is shown, the instrument including, among other components, a reference arm and a sample arm.

[0033] Figure 6 The optical frequency downshift is introduced by an optical frequency shifter. Figure 2 The effect of periodic frequency sweep.

[0034] Figure 7 An optical coherence tomography instrument according to another example embodiment herein is shown, the instrument including, among other components, a reference arm and a sample arm.

[0035] Figure 8 An optical coherence tomography apparatus according to yet another example embodiment herein is shown.

[0036] Figure 9 is a schematic diagram of an optical coherence tomography instrument using balanced detection according to yet another example embodiment herein.

[0037] Figure 10 Another optical coherence tomography instrument using balanced detection according to another example embodiment herein is shown.

[0038] Figure 11 is a schematic diagram of a controller for controlling an optical coherence tomography instrument according to an example embodiment of the present invention.

[0039] Figure 12 is a flow chart illustrating an optical coherence tomography method according to an example embodiment of this document.

[0040] Figure 13 An optical coherence tomography apparatus (system) according to example embodiments herein is shown, wherein an optical frequency shifter is arranged between an optical beam splitter and a reference optical system.

[0041] Figure 14 An optical coherence tomography apparatus (system) according to example embodiments herein is shown, wherein an optical frequency shifter is in the sample optical system.

[0042] Figure 15 An optical coherence tomography instrument (system) according to example embodiments herein is shown, wherein an optical frequency shifter is arranged between the optical beam splitter and the sample optical system.

[0043] Figure 16 shows that the reverse optical delay is introduced into Figure 2 The effect of periodic frequency sweep.

[0044] Figure 17 The optical frequency is upshifted by an optical frequency shifter. Figure 2 The effect of periodic frequency sweep. Detailed description

[0045] Figure 1 is a schematic diagram of an optical coherence tomography instrument (also referred to herein as an optical coherence tomography system) constructed according to example embodiments herein.

[0046] exist Figure 1 In an apparatus of the type described herein, a scanning light source SLS generates a beam of narrowband light having a variable center frequency. The scanning light source SLS is configured to vary the center frequency of the narrowband light in a repetitive manner, such as by repeating a periodic frequency sweep over a defined frequency band between a lower frequency and a higher frequency. This periodic frequency sweep is performed in a manner that is consistent with the embodiment of the present invention. Figure 2 For example, Figure 2 The output frequency of such a scanning light source SLS is shown with respect to time. Such a frequency sweep is conventionally referred to as a chirp. The scan may rise from a low frequency to a high frequency in time, or may fall from a high frequency to a low frequency in time. The scan may also rise and fall continuously in time. For example, a forward scan from a low frequency to a high frequency may be followed by a reverse scan from a high frequency to a low frequency. The scanning range may be, for example, of the order of 100 nm. The scanning light source SLS may have high coherence and, for example, may have a coherence length of more than 5 mm. In some configurations, the coherence length may exceed 10 mm, or may even exceed 100 mm. Of course, these examples are non-limiting.

[0047] In an example embodiment herein, the scanning light source SLS is a tunable laser or a tunable laser diode and includes, 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), although these examples are not limiting on the scope of the invention. In an example embodiment herein, the bandwidth of the scanning light source SLS can be selected for optimal penetration, for example, through the tissue of the lens of the eye being examined and the retina of the eye. Also in an example embodiment herein, the bandwidth is in the infrared region of the spectrum, for example, at wavelengths longer than 850 nm. Example wavelengths that can be included in the scan are 850 nm, 1050 nm, 1310 nm, or 1550 nm, although these examples are not limiting.

[0048] The narrowband light beam generated by the scanning light source SLS is directed to an optical beam splitter OBS, which acts as an optical coupler to split the narrowband light beam from the light source SLS into two beams. The beam splitting can be symmetrical, so that equal intensities are directed into the two resulting beams, or asymmetrical, so that unequal intensities are directed into the two resulting beams.

[0049] The first light beam (also referred to herein as the sample beam) is directed to a sample optical system SOS, which includes optical components to shape and direct the light beam to a sample S (such as a retina, for example only), and to collect reflected light (i.e., light reflected from the sample S) and return it to an optical beam splitter OBS. The reflected light arrives at the optical beam splitter OBS along substantially the same path as the sample beam, but in the opposite direction.

[0050] In one example embodiment herein, the sample optical system SOS includes conventional components known in the art of optical coherence tomography, and in some example embodiments herein, such conventional components are adapted depending on the imaging operation to be performed. More specifically, in one example embodiment herein, the sample optical system SOS may include scanning optics that can cause the sample beam to pivot about a pivot point located at the anterior segment of the eye (e.g., sample S) so as to scan the sample beam over a wide field of view of the retina located at the posterior segment of the eye.

[0051] As an example, the sample optical system SOS may comprise one or more scanners arranged to scan a light beam in one or more directions on the retina. Such scanners may comprise oscillating plane mirrors, such as galvanometer scanners, MEMS mirrors, rotating mirrors, prisms, polygon scanners or resonant scanners.

[0052] The sample optical system may further comprise a scanning relay unit comprising, for example, a lens or a curved mirror, the scanning relay unit being arranged to image one scanner that scans a light beam in one direction onto a subsequent second scanner that scans the light beam in a second direction, thereby generating a two-dimensional scanning pattern caused by an apparent origin located at the second scanner.

[0053] The sample optical system may further comprise a scan transfer unit, such as a lens or a curved mirror, arranged to project an apparent origin into a space outside the sample optical system SOS, so that a two-dimensional scan pattern caused by the apparent origin at the second scanner is transferred to an apparent pivot point in a space outside the sample optical system, so that the sample light beam pivots around the pivot point in the scan.

[0054] The second light beam (also referred to herein as the reference beam) is directed to the reference optical system ROS, which then returns the reference beam to the optical beam splitter OBS. At the optical beam splitter OBS, the returned reflected light (i.e., the light reflected from the sample S) and the returned reference beam (i.e., the reference beam returned from the reference optical system ROS) are combined so as to interfere with each other and are directed to the detector DET as an interference beam. In an example embodiment herein, the detector DET is a photodetector, such as a photodiode or an avalanche photodiode, which converts the light intensity of the interference beam to provide a resulting conversion signal in the form of, for example, an electrical signal, such as a voltage or current. In an example embodiment herein, the resulting conversion signal is a time-varying analog signal. After being output by the detector DET, the resulting conversion signal, as will be described below, can be recorded in a time-varying manner, wherein the recorded version of the signal constitutes an interference pattern between the returned reflected light and the returned reference beam with respect to the optical frequency of the narrowband light from the scanning light source SLS.

[0055] exist Figure 1 In the depicted exemplary embodiment, the analog-to-digital converter (ADC) periodically samples and quantizes the signal output by the detector DET at a predetermined sampling frequency, and then provides the digital values ​​of the quantized and sampled signal to a data processing unit (DPU) that records these values. Thus, the detector DET and the analog-to-digital converter (ADC) together constitute a detection unit (DU) that samples the time-varying interference signal between the returned reference light and the returned signal light. In one exemplary embodiment herein, the data processing unit (DPU) performs a Fourier transform operation, such as a fast Fourier transform (FFT), on the time-varying quantized and sampled signal values ​​obtained from the analog-to-digital converter (ADC) to generate an axial depth profile.

[0056] Because the analog-to-digital converter (ADC) periodically samples the output of the detector (DET), the highest-frequency components of the time-varying analog signal, which lie outside the detection bandwidth defined by the sampling frequency of the ADC, are not recorded. Simply put, the beat frequency of these components in 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 ADC. Conversely, the sampling rate may be less than the Nyquist rate of the highest-frequency component of the interferogram.

[0057] The scanning light source SLS can have a relatively high coherence length, so that the interference between the returned reflected light and the returned reference beam contains information about the sample reflectivity over a large axial depth range defined by the high coherence length. Nevertheless, from the Fourier transform of the sampled signal recorded in the data processing unit DPU, only information about a sub-range of this axial depth range is available, which corresponds to the detection bandwidth centered on the axial position defined by the optical path length of the reference arm.

[0058] In conventional optical coherence tomography instruments, the optical path length of the reference arm is adjusted to set the axial position of the measured axial depth range to correspond to the surface of the sample being investigated, taking into account the typically low coherence length of the light source used. Therefore, an optical delay is introduced between the sample and reference beams, which ensures that the sample and reference beams remain coherent when combined to interfere at the detector.

[0059] Figure 3 An exemplary configuration of such an arrangement is shown in FIG, wherein a movable mirror M is provided in the reference optical system ROS to change the optical path length traveled by the light in the reference arm. This change in the optical path length traveled by the light in the reference arm corresponds to the variable optical delay of the frequency chirp generated by the scanning light source SLS, as shown in FIG. Figure 4 In this configuration, an optical delay is introduced to the reference light; it will be understood that the optical delay is relative to the sample light and can in principle be positive or negative, depending on whether the optical path of the sample arm is greater or less in length than the optical path of the reference arm.

[0060] The inventors have recognized that when the scanning light source SLS has a sufficiently high coherence, the constant frequency shift (e.g., a frequency upshift or downshift) applied to the frequency chirp generated by the scanning light source SLS essentially corresponds to a constant optical delay, and the amount of equivalent delay introduced corresponds to the amount of frequency shift introduced.

[0061] According to exemplary aspects of the present invention, such a frequency shift can be introduced by placing an optical frequency shifter OFS into the reference arm, for example in the reference optical system ROS, between the optical beam splitter OBS and the mirror M, as shown in FIG. Figure 5 The effect on the frequency chirp generated by the scanning light source SLS is shown in Figure 6 As shown, through Figure 4 It can be seen from the comparison that the introduction of a predetermined frequency shift is equivalent to the introduction of a predetermined optical delay. Figure 4 and Figure 6 The effect of introducing a frequency down-shift is shown, but a frequency up-shift is also considered. Figure 16 and Figure 17 Corresponding to the case of frequency upshift Figure 4 and Figure 6 .

[0062] Equivalently, the optical frequency shifter can be arranged between the optical beam splitter OBS and the reference optical system ROS, as Figure 13 Similarly, by placing an optical frequency shifter OFS into the sample optical system SOS, a frequency shift can be introduced into the sample arm, which will correspond to an optical delay in the sample arm. Such a configuration is shown in Figure 14 Equivalently, the optical frequency shifter can be arranged between the optical beam splitter OBS and the sample optical system SOS, as shown in Figure 15 shown.

[0063] The optical frequency shifter OFS can be controlled by a frequency shift controller FSC. By controlling the optical frequency shifter OFS with the help of the frequency shift controller FSC to adjust the amount of frequency shift introduced, it is possible to achieve the same as by, for example, Figure 3 The movable mirror M shown introduces the effect of an adjustable optical delay without the use of any moving parts. Thus, by introducing a predetermined amount of frequency up- or down-shift to the light in the sample or reference arm, the temporal interference frequency between the return-reflected light of the sample beam and the return light of the reference beam can be slowed down so that the interference component caused by the desired axial region of the sample falls within the available detector bandwidth defined by the sample frequency. In short, the beat frequency of the interference pattern component caused by the desired axial region of the sample S is sufficiently slowed to enable it to be recorded. Consequently, the electrical frequency of the electrical signal output by the detector is also reduced. For example, the frequency of the interference pattern component caused by the desired axial region of the sample can be reduced to below the Nyquist frequency of the analog-to-digital converter.

[0064] According to an example embodiment herein, as an optical frequency shifter OFS, an acousto-optic modulator (AOM) may be used to introduce an optical frequency shift.

[0065] An acousto-optic modulator (AOM) consists of a driven optical medium to which vibrations are applied at a defined frequency. The vibrations induce phonons in the optical medium, which interact with photons passing through the medium to produce diffracted light with a frequency shift proportional to the vibration frequency, with the proportionality constant being determined by the diffraction order. Thus, by varying the frequency of the vibrations, 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 is either upward or downward. Such an AOM can operate in either a traveling wave configuration or a standing wave configuration. In such a configuration, the frequency shift controller (FSC) is a radio frequency driver arranged to apply a variable radio frequency electrical signal to the optical medium using a piezoelectric element attached to the optical medium.

[0066] According to an example embodiment herein, as the optical frequency shifter OFS, an electro-optical modulator (EOM) may be alternatively used to introduce an optical frequency shift.

[0067] An electro-optic modulator comprises an optical medium exhibiting an electro-optic effect, to which an electric field is applied. The electric field can be applied to the medium, for example, by placing the medium between the plates of a parallel plate capacitor. Due to the electro-optic effect, the applied electric field causes a corresponding change in the refractive index of the optical medium, depending on the strength of the electric field. Without wishing to be bound by theory, this change in refractive index may generally be caused by forces that distort the position, orientation, or shape of the molecules comprising the optical medium. This change in refractive index causes a phase change in light leaving the medium. If the electric field varies at a defined frequency, the refractive index also varies with this defined frequency. This change in refractive index can cause a corresponding phase change in light leaving the optical medium.

[0068] For example, if the electric field varies sinusoidally at a predetermined frequency ω, a time-dependent phase at the predetermined frequency ω can be added to the time-dependent electromagnetic wave of the light, for example, the time-dependent frequency Ω exiting the optical medium. As a result of this addition of the time-dependent phase, a set of sidebands is added to the light, including at least a first pair of sidebands with frequencies Ω±ω, each of which is shifted in frequency by a predetermined frequency ω relative to the light's frequency Ω. 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 upshift or downshift in optical frequency can be achieved.

[0069] In another example embodiment, an electro-optical modulator (EOM) may instead be used to introduce an optical frequency shift through amplitude modulation rather than phase modulation.

[0070] Specifically, phase-modulating electro-optical modulators can be used to introduce amplitude modulation into incident light by incorporating at least one such electro-optical modulator into at least one corresponding arm of an interferometer (such as a Mach-Zehnder interferometer) to which the incident light is applied. Such an interferometer has two arms into which the incident light is coherently separated, and output light is generated by coherently combining the light from the two arms. The coherent separation and combination of light can be performed by one or more beam splitters. This arrangement is sometimes referred to as a Mach-Zehnder modulator (MZM). In a related arrangement, one electro-optical modulator can be provided to each of the two arms of the interferometer.

[0071] As described above, applying an electric field to the optical medium of an electro-optical modulator in an interferometer introduces a phase shift depending on the strength of the electric field. According to the normal principles of interferometry, depending on the introduced phase shift, the amplitude of the light output from the interferometer will also vary depending on the amount of the introduced phase shift. Thus, it is possible to impart a varying amplitude to the light. The amplitude of the applied electric field can be set so that the difference in phase shift introduced between the minimum and maximum applied electric fields corresponds to half a wavelength of the input light. In this configuration, variations in the applied electric field at a predetermined frequency ω cause the amplitude of the output light to also vary at the predetermined frequency ω.

[0072] For example, if the electric field varies sinusoidally at a predetermined frequency ω, the time-dependent amplitude at the predetermined frequency ω can be added to the time-dependent electromagnetic wave of the light, for example, to the time-dependent frequency Ω exiting the optical medium. As a result of this addition of the time-dependent amplitude, a set of sidebands consisting solely of a first pair of sidebands at frequencies Ω ± ω is added to the light, each of which is shifted in frequency by the predetermined frequency ω relative to the light's frequency ω. 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 as either an upshift or a downshift.

[0073] When the sidebands are generated as frequency-shifted light, for example by using an electro-optic modulator or a Mach-Zehnder modulator, it may be necessary to select only the sideband light for further use. The selection of the upper sideband light can be performed by incorporating, for example, a suitable high-pass filter after the interferometer or modulator, while the selection of the lower sideband light can be performed by incorporating, for example, a suitable low-pass filter after the modulator. However, if the frequency of the sidebands differs sufficiently from the frequency of the incident light, it may not be necessary to include a filter. For example, by using an electro-optic modulator or a Mach-Zehnder modulator, a frequency shift of several gigahertz can be achieved for the shifted light compared to the input light. In this case, the filter can be omitted.

[0074] In such a configuration, the frequency shift controller FSC is a radio frequency (which may include microwave frequency) driver that is arranged to apply a variable radio frequency or microwave frequency electrical signal to the optical medium using a pair of suitable capacitor plates attached to or arranged adjacent to the optical medium, thereby applying a variable electric field on the optical medium.

[0075] As a medium for electro-optic modulators, lithium niobate (LiNbO3) can be used. Other media can be used, including nonlinear or birefringent media. Example alternative media include potassium dideuterium phosphate (KD*P or DKDP), potassium titanyl phosphate (KTP), β-barium borate (BBO), lithium tantalate (LiTaO3), and ammonium dihydrogen phosphate (NH4H2PO4 or ADP). In addition to these inorganic media, nonlinear polymer media such as polarized polymers can also be used as the medium.

[0076] In other example embodiments herein, other nonlinear optical techniques may be used to produce the desired frequency up- or down-shift. For example, difference frequency generation or semi-harmonic generation using, for example, nonlinear optical crystals may be used in place of an acousto-optic modulator or an electro-optic modulator to introduce the desired frequency shift.

[0077] According to exemplary aspects of the present disclosure, the function of optical frequency shifting is to bring high-frequency interference components into the low-frequency detection bandwidth. This can be achieved by optical frequency shifting, which is either up-shifting or down-shifting, depending on the scanning direction of the laser and whether the frequency shift is applied to the reference arm or the sample arm.

[0078] For example, if the frequency shifter is incorporated into the reference arm, a downward shift introduced into the reference arm may be appropriate for a sweep source that sweeps from a low frequency to a high frequency. Furthermore, if the frequency shifter is incorporated into the reference arm, an upward shift introduced into the reference arm may be appropriate for a sweep source that sweeps from a high frequency to a low frequency. On the other hand, if the frequency shifter is incorporated into the sample arm, a downward shift introduced into the sample arm may be appropriate for a sweep source that sweeps from a low frequency to a high frequency. Furthermore, if the frequency shifter is incorporated into the sample arm, an upward shift introduced into the sample arm may be appropriate for a sweep source that sweeps from a high frequency to a low frequency. Thus, a specific frequency shifter can be used to introduce a positive or negative delay. However, the effect of the positive or negative delay will increase or decrease the beat frequency of the interference, depending on whether the introduced delay causes the reference arm and the sample arm to move relatively more in-phase or out-of-phase.

[0079] A piezoelectric transducer can be used to impart vibrations to a driven optical medium, wherein the piezoelectric transducer itself is driven by a radio frequency (RF) voltage applied to the transducer. Thus, in one example embodiment herein, Figure 5The optical frequency shifter OFS, shown in FIG, can be or include an acousto-optic modulator or an electro-optic modulator, for example, as part of a Mach-Zehnder modulator, driven by a variable-frequency RF signal generated by a frequency shift controller FSC. Thus, the sampling frequency, and therefore the detector bandwidth, remains constant, while the amount of frequency shift can be adjusted to reduce the beat frequency of the interferogram and bring the region of interest of the interferogram within the detector bandwidth.

[0080] Reference again Figure 5 , the reference light may travel through the optical frequency shifter OFS twice, specifically, in one instance, forwardly propagating from the optical beam splitter OBS to the reflector M through the optical frequency shifter OFS, and in another instance, backwardly propagating from the reflector M to the optical beam splitter OBS through the optical frequency shifter OFS. Each time it passes through the optical frequency shifter OFS, a predetermined frequency shift related to the driving frequency provided by the frequency shift controller FSC is introduced. Therefore, in Figure 5 In the example configuration of , the total offset applied is twice the frequency shift introduced by the optical frequency shifter OFS for each individual pass.

[0081] In the above Figure 5 In the description of the exemplary configuration, the effect of the optical frequency shifter (OFS) has been explained in conjunction with a free-space optical arrangement for an optical coherence tomography instrument. However, this arrangement is not limited to the scope of the present invention. In fact, in other exemplary embodiments herein, a fiber-based arrangement may be employed, in which optical fibers are used instead of free space to propagate the light beam.

[0082] For example, Figure 7 Another example embodiment of the present invention is shown in FIG. 1 , which is an optical coherence tomography apparatus (also referred to herein as an "optical coherence tomography system"). Figure 7 In the embodiment, the light from the scanning light source SLS is transmitted via an optical fiber to the fiber coupler unit FCU that provides the optical coupler function. The light from the scanning light source SLS is thus split into a reference beam and a sample beam (also called "sample light"). The reference beam advances in the reference arm, circulates through the optical frequency shifter OFS controlled by the frequency shift controller FSC, and returns to the fiber coupler unit FCU. The sample light advances in the sample arm and is transmitted to the sample S (not in the sample optical system SOS) via the sample optical system SOS. Figure 7 ), the sample S reflects the sample light, so it returns to the fiber coupler unit FCU through the sample optical system SOS. In the fiber coupler unit FCU, the return light received from each arm is combined and transmitted along another optical fiber to the detector DET. In other aspects, Figure 7 The embodiment of Figure 3 The embodiment of φ operates in the same manner, except that since the light in the reference arm passes through the optical frequency shifter OFS only once, only a single frequency shift is applied instead of a double frequency shift.

[0083] exist Figure 8 Another example embodiment of an optical coherence tomography instrument is shown in FIG. Figure 8 The reference optical system ROS of the embodiment is different from Figure 7 Examples of reference optical systems ROS, Figure 8 The instruments include Figure 7 Same parts. Figure 8 In the reference arm, the reference beam propagating in the reference arm is provided to the fiber mirror unit FMU through 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 via the optical frequency shifter OFS. Figure 5 A double frequency shift is introduced in the same way as the free-space optical arrangement.

[0084] In an example embodiment herein, Figure 1 、 Figure 5 、 Figure 7 and Figure 8 The embodiment is implemented using so-called unbalanced detection, in which the interference frequency component is detected directly from the optical signal of the detector DET. However, in other example embodiments herein, a balanced detection arrangement can be employed instead, in which the signal arm and the reference arm are independently coupled to different optical inputs of the detector DET, so that the signal light and the reference light are combined to interfere at or in the detector DET through an optical or electrical medium. In such an arrangement, the intensities of the signal light and the reference light can also be measured independently, for example, by diverting at least some of each of the sample light and the reference light to a separate detection element, so that variations in the light intensity can be compensated by the detection element.

[0085] Figure 9 An example embodiment of an optical coherence tomography instrument employing balanced detection is shown herein. Figure 9 The instrument is similar to Figure 1 The instrument, except Figure 9 In an instrument, a detector DET includes two optical inputs, and a reference optical system ROS is included in a ring optical system between an output of an optical beam splitter OBS and one of the two optical inputs of the detector DET. The ring optical system includes the optical beam splitter OBS, the reference optical system ROS, and the detector DET. In one exemplary embodiment herein, the optical loop has a fixed optical path length, although this example is not limiting. Interference between the reference light and the sample light returning to the detector provides an interferogram.

[0086] Figure 10 Another example embodiment of the optical coherence tomography instrument herein is shown, wherein the instrument has a fiber optic configuration. Figure 10 The instruments include Figure 7The instrument has similar components, except that the reference optical system ROS is not in Figure 10 In the expression, and instead of Figure 7 As shown in FIG, the output of the optical frequency shifter OFS is provided to the fiber coupler unit FCU. Figure 10 In an instrument, the output of the optical frequency shifter OFS is provided to the first of multiple inputs of a detector DET. Thus, a fiber loop is provided, including a fiber coupler unit FCU, the optical frequency shifter OFS, and the detector DET. Thus, a reference beam output by the fiber coupler unit FCU is provided to the optical frequency shifter OFS, and the output of the optical frequency shifter OFS is provided to a first input of the detector DET. A second input of the detector DET receives returned sample light, which is returned from the sample optical system SOS via the fiber coupler unit FCU. Interference between the returned reference light and the sample light in the detector provides an interferogram.

[0087] The above configuration has been described with respect to discrete control and data processing units, such as an internal controller of the scanning light source SLS, a frequency shift controller FSC that controls the optical frequency shifter OFS, and a data processing unit DPU that transforms the detected optical signal of the detector DET into an axial depth distribution using a fast Fourier transform. However, in other example embodiments herein, at least some or all of the control and data processing aspects of the above configuration may be provided by an integrated controller, which 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, a minicomputer, or a personal computer (PC), or a mobile device such as a tablet or a smartphone.

[0088] exist Figure 11 In one example embodiment of the present invention, the integrated controller CONT controls Figure 1 、 Figure 3 、 Figure 5 and Figure 7-10 At least some or all of the various components shown, and may also form all or at least a portion of a frequency shift controller FSC. Figure 11 As shown, all components of the integrated controller CONT are coupled to one another and can therefore communicate with one another. Figure 11 The integrated controller CONT shown includes an analog-to-digital converter ADC to receive, quantize and sample the data from the detector DET (not shown). Figure 11) and, in an example embodiment herein, provides the resulting converted signal to one or more other components of the integrated controller CONT, such as the main control unit MCU, as an example only. In an example embodiment herein, the integrated controller CONT further comprises a digital-to-analog converter that converts digital signals received from one or more other components of the integrated controller CONT, such as the main control unit MCU, as an example only, into analog values ​​and provides them as analog control voltages to the frequency shift controller FSC (not shown). Figure 11 ), thereby defining the optical frequency shifter OFS (not shown in Figure 11 The integrated controller CONT also includes an output for controlling the scanning light source SLS (not shown). Figure 11 The integrated controller CONT further includes a scanning signal generator SSG that generates a time-varying signal of a frequency (shown in FIG). For example, the output of the scanning signal generator SSG can be a time-varying voltage, and the scanning light source SLS can accept a voltage input to define the center frequency of the output narrowband light. The integrated controller CONT also includes a clock generator CLK, which defines one or more clocks for providing a common time base for the time-varying operations of the controller CONT. For example, the clock generator CLK can define a sampling clock for the analog-to-digital converter ADC, and can also define a time base for the scanning signal generator SSG. The clock generator CLK can also define internal clocks of the controller CONT, such as memory and instruction clocks and a data bus clock. The integrated controller CONT also includes a fast Fourier transform unit FFT for performing a Fourier transform on the quantized and sampled signal obtained via the analog-to-digital converter ADC. The integrated control unit also includes an arithmetic and logic unit ALU for performing arithmetic and logic operations on data processed within the controller CONT.

[0089] The integrated controller CONT also includes a computer-readable memory MEM, which, in one exemplary embodiment herein, is used to store and retrieve data values, such as recorded data from the analog-to-digital converter (ADC), signal waveforms associated with the scan signal generator (SSG), Fourier transform output data generated by the fast Fourier transform (FFT) unit (FFT), and other parameters, instructions, and values ​​required to perform the operations of the integrated controller CONT. By way of example only and not limitation, the memory MEM may include RAM, ROM, a hard drive, a floppy disk, a memory stick, a buffer, and the like. In one exemplary embodiment herein, the memory MEM stores instructions and / or programs for performing the methods and functions described herein and illustrated in the accompanying drawings. The integrated controller CONT also includes an input / output controller (IOC) for sending and receiving values ​​to and from external devices, such as offline storage instantiated as a hard drive, flash drive, or magnetic disk drive, or an interface such as a network interface, e.g., a wired local area network, a wireless local area network, or a mobile data network. The integrated controller also includes a main control unit (MCU), which coordinates the operations of the various functional units of the controller CONT and thereby controls the other components of the integrated controller CONT. In an example embodiment herein, the main control unit MCU (and / or the arithmetic and 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 shown in the accompanying drawings. Also in an example embodiment herein, Figure 11 The analog-to-digital converter ADC may be formed and / or included in the analog-to-digital converter ADC of other figures described herein, and the main control unit MCU and / or the integrated controller CONT may be formed and / or included in the frequency shift controller FSC and / or the digital processing unit DPU of other figures described herein. Figure 11 The shown division of various functional tasks into units is purely exemplary, and these tasks may be performed by separate functional modules, discrete electronic devices, integrated logic, other hardware, software and / or other program code, as desired.

[0090] Figure 12 An optical coherence tomography method according to an example embodiment of the present invention is shown, wherein the method can be implemented by one or more individual optical coherence tomography instruments described above and shown in the accompanying drawings. In step S1, a sample (e.g., sample S) such as a subject's retina is arranged at the focal depth of a front-end optical system (e.g., sample optical system SOS) of the optical coherence tomography instrument.

[0091] In step S2, narrowband light having a periodically varying optical frequency is introduced into an optical coupler (e.g., a fiber coupler unit (FCU)) to be separated between the sample arm and the reference arm. In the sample arm, the sample light is guided by the front-end optical system to illuminate the retina. The returned reflected light is captured by the front-end optical system and returned to the optical coupler via the sample arm. At the same time, the reference light in the reference arm is returned to interfere with the returned light in the sample arm.

[0092] In step S3, a time-varying interference signal between the returned reference light and the returned reflected light is recorded by or based on a detector (e.g., detector DET). The detector is characterized by a detection bandwidth defined by the sampling frequency of the detector. The time-varying interference signal recorded by the detector is Fourier transformed to produce an axial depth profile.

[0093] In step S4, an optical frequency shifter (eg, optical frequency shifter OFS) introduces / applies an optical frequency shift to the light in the sample or reference arm according to a predetermined operating criterion. Figure 6 The optical frequency shift is adjusted by lowering the frequency as shown to change the optical frequency of the light passing through the optical frequency shifter. The amount of optical frequency shift is adjusted until the interference pattern between the light reflected from the retina and the reference light is within the detection bandwidth.

[0094] Thus, an axial depth profile can be obtained in the manner described above, wherein the obtained axial depth profile represents the depth structure at the retina. Notably, obtaining the axial depth profile does not require substantially moving any portion of the sample arm or reference arm. This can be determined by observing the axial depth profile, or programmatically by, for example, detecting signal signatures of retinal surface structures in the axial depth profile.

[0095] According to an exemplary embodiment of the present invention, the disclosed apparatus and method can be implemented in a scanning laser ophthalmoscope (SLO). Alternatively, the disclosed apparatus and method can be implemented to measure tissues other than the retina and tissues other than eye tissue. For example, the disclosed apparatus and method can be implemented to measure other biological membranes, such as skin or plant parts, or can be applied to measure non-biological structures.

[0096] It should be noted that although for convenience, Figure 5 、 Figure 7 、 Figure 8 and Figure 10 The analog-to-digital converter ADC and the digital processing unit DPU are not shown, but based on the description, a person skilled in the art will easily understand that the instruments (systems) of those figures may also include those components, where in such a configuration, for example, the digital processing unit DPU is coupled to the detectors DET of those corresponding instruments via the converter ADC inserted therebetween.

[0097] In the foregoing description, example aspects have been described with reference to several example arrangements. Therefore, the description should be regarded as illustrative rather than restrictive. Similarly, the figures shown in the accompanying drawings, which highlight the features and advantages of the example arrangements, are presented for illustrative purposes only. The architecture of the example arrangements is sufficiently flexible and configurable that it can be utilized in ways other than those shown in the accompanying drawings.

[0098] In one example arrangement, the example software arrangements presented herein may be provided as computer programs or software, such as one or more programs having instructions or sequences of instructions, included or stored on an article of manufacture (such as a memory, a machine-accessible or machine-readable medium, an instruction storage device, or a computer-readable storage device, each of which may be non-transitory). The programs or instructions on the non-transitory memory, machine-accessible medium, machine-readable medium, instruction storage device, or computer-readable storage device may be used to program a computer system or other electronic device. The memory, machine- or computer-readable medium, instruction storage device, and storage device may include, but are not limited to, floppy disks, optical disks, and magneto-optical disks, or other types of media / machine-readable media / instruction storage devices / storage devices suitable for storing or transmitting electronic instructions. The techniques described herein are not limited to any particular software configuration. They may be applied in any computing or processing environment. As used herein, the terms "memory," "computer-readable," "machine-accessible medium," "machine-readable medium," "instruction storage device," and "computer-readable storage device" shall 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 and causing the machine / computer / unit / computer processor to perform any of the methods described herein. Furthermore, it is common in the art to refer to software in one form or another (e.g., program, procedure, process, application, module, unit, logic, etc.) as taking 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 to produce a result.

[0099] Some arrangements may also be implemented by the preparation of application specific integrated circuits, field programmable gate arrays, or by interconnecting an appropriate network of conventional component circuits.

[0100] Some arrangements include computer program products. A computer program product may be one or more memories, storage media, instruction stores, or storage devices having stored thereon or therein instructions that can be used to control or cause a computer or computer processor to perform any of the processes of the example arrangements described herein. The memories / storage media / instruction stores / storage devices may include, by way of example and without limitation, optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memories, flash memory cards, magnetic cards, optical cards, nanosystems, molecular memory integrated circuits, RAIDs, remote data storage / archiving / warehousing devices, and / or any other type of device suitable for storing instructions and / or data.

[0101] Some implementations stored on any of one or more memories, computer-readable media, instruction storage devices, or storage devices include hardware for controlling the system and software for enabling the system or microprocessor to interact with a human user or other mechanism using the results of the example arrangements described herein. Such software may include, without limitation, device drivers, operating systems, and user applications. Finally, as described above, such memories, computer-readable media, or storage devices also include software for executing example aspects of the present disclosure.

[0102] Software modules for implementing the processes described herein are included in the system's programming and / or software. In some example arrangements herein, the modules include software, but in other example arrangements herein, the modules include hardware or a combination of hardware and software.

[0103] Although various example arrangements of the present disclosure have been described above, it should be understood that they are presented by way of example and not limitation. It will be apparent to those skilled in the relevant art that various changes in form and detail may be made. Therefore, the present disclosure should not be limited by any of the above example arrangements, but should be defined only in accordance with the appended claims and their equivalents.

[0104] Furthermore, the purpose of the Abstract is to enable patent offices and the public generally, and especially scientists, engineers, and practitioners in the field who are not familiar with patent or legal terminology or wording, to quickly ascertain the nature and essence of the technical disclosure of the present application based on a cursory inspection. The Abstract is not intended to limit in any way the scope of the example arrangements presented herein. It should also be understood that any process recited in the claims need not be performed in the order presented.

[0105] Although this specification contains many specific arrangement details, these should not be understood as limitations on the scope of any disclosure or content that may be claimed, but rather as descriptions of features specific to the particular arrangements described herein. Certain features described in this specification in the context of separate arrangements may also be implemented in combination in a single arrangement. Conversely, various features described in the context of a single arrangement may also be implemented individually or in any suitable subcombination in multiple arrangements. Furthermore, although features may be described above as acting in a particular combination and even initially claimed as such, one or more features from the claimed combination may be deleted from the combination in some cases, and the claimed combination may be directed to a subcombination or a variant of the subcombination.

[0106] In some cases, multitasking and parallel processing may be advantageous. Furthermore, the separation of the various components in the above-described arrangements should not be understood as requiring such separation in all arrangements, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0107] Now that some illustrative arrangements and configurations have been described, it will be apparent that the foregoing is illustrative rather than restrictive and has been presented by way of example. Specifically, although many of the examples presented herein relate to specific combinations of devices or software elements, these elements can be combined in other ways to achieve the same purpose. Actions, elements, and features discussed in conjunction with only one configuration are not intended to be excluded from similar roles in the configuration or other configurations.

[0108] The apparatus and methods described herein may be embodied in other specific forms without departing from their characteristics.The foregoing arrangements are illustrative, rather than limiting, of the described systems and methods.

[0109] Scope of the devices and methods described herein is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. An optical coherence tomography apparatus comprising: an optical coupler arranged to receive light from the tunable narrowband light source and to separate the light into at least signal light and reference light; a reference optical system arranged to return the reference light; a front-end optical system arranged to direct the signal light toward an eye of a subject and return the signal light reflected from the eye of the subject; a detection unit arranged to sample a time-varying interference signal between the returned reference light and the returned signal light; and An adjustable optical frequency shifter is arranged (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 being arranged to adjustably increase or decrease the optical frequency of the reference light or the signal light.

2. The optical coherence tomography apparatus according to claim 1, wherein: The reference optical system includes a reflector arranged to reflect the reference light to return the reference light.

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 according to 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 4, wherein: The optical loop has a fixed optical path length.

6. An optical coherence tomography apparatus according to any one of the preceding claims, wherein: The reference light passes through the tunable optical frequency shifter in a forward direction and a reverse direction.

7. The optical coherence tomography apparatus according to any one of claims 1 to 5, wherein: The signal light passes through the tunable optical frequency shifter in a forward direction and a reverse direction.

8. An optical coherence tomography apparatus according to any one of the preceding claims, wherein: The tunable optical frequency shifter includes an acousto-optic modulator or an electro-optic modulator.

9. The optical coherence tomography instrument according to claim 8, further comprising a radio frequency driver, the radio frequency driver being arranged to drive the acousto-optic modulator or electro-optic modulator to obtain a predetermined optical frequency shift.

10. An optical coherence tomography apparatus according to any one of the preceding claims, wherein: The optical coupler includes a beam splitter or a fiber coupler.

11. The optical coherence tomography apparatus according to any one of the preceding claims, further comprising the tunable narrowband light source, wherein the tunable narrowband light source is arranged to emit light towards the optical coupler, and the light is narrowband light.

12. The optical coherence tomography apparatus according to claim 11, wherein: The coherence length of the narrowband light is greater than one of 0.5 cm, 1 cm or 10 cm.

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

14. The optical coherence tomography apparatus according to any one of claims 11 to 13, wherein: The tunable narrowband light source is configured to periodically vary an optical frequency of light emitted by the tunable narrowband light source.

15. An optical coherence tomography apparatus according to any one of the preceding claims, wherein: The detection unit includes one of a photodetector or a balanced photodetector.

16. An optical coherence tomography method, comprising: Arranging a subject so that the subject's retina is within the focal depth of a front-end optical system of an optical coherence tomography instrument; introducing narrowband light having a periodically varying optical frequency into a coupler such that the coupler separates the light into at least signal light and reference light, wherein the reference light is reflected back by a reference optical system and the signal light is reflected back by an eye of the subject; recording a time-varying interference signal between the reflected reference light and the reflected signal light, the recording being based on detection of the time-varying interference signal by a detection unit having a detection bandwidth defined by a sampling frequency of the detection unit; An optical frequency of the reference light or the signal light is adjusted, wherein the adjustment is performed so that an interference pattern representing a depth structure at the retina obtained between the reflected signal light and the reflected reference light is within the detection bandwidth.

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