Resolving absolute depth in cyclic ranging optical coherence tomography

Abstract

An apparatus includes: an electromagnetic radiation source that generates radiation to illuminate a sample located at an optical path depth, the electromagnetic radiation source providing radiation to the sample to facilitate determination of the optical path depth within the sample; an interferometer including: a reference arm to which a first portion of the radiation is delivered; a sample arm to which a second portion of the radiation is delivered; a first optical subsystem coupled to the sample arm to interrogate the sample using the radiation delivered to the sample arm and to collect backscattered radiation from the sample; a second optical subsystem coupled to the reference arm and the first optical subsystem to generate interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm; and a data collection and processing system communicating with the interferometer and configured to calculate the optical path depth of the sample based on the received interference fringes.

Description

[0001] Cross-referencing related applications

[0002] This application is based on and claims priority to U.S. Patent Application Serial No. 62 / 929,390, filed November 1, 2019, the entire disclosure of which is incorporated herein by reference.

[0003] Explanation of federally funded research

[0004] This invention was made with government support under grant number P41EB015903 granted by the National Institutes of Health in the United States. The government holds certain rights in this invention. Background Technology

[0005] Subsampling-based OCT is an imaging technique that offers high speed and can operate over long distances. It achieves this by using optical subsampling to perform optical domain compression of the generated interference signal. Traditionally, this is done using a single frequency comb source (i.e., a source comprising spectral lines equally spaced at optical frequencies). However, these techniques cannot determine absolute depth, which can be a limitation for some applications. Summary of the Invention

[0006] Therefore, new systems, methods, and media are desired for resolving absolute depth in cyclic ranging optical coherence tomography.

[0007] In one embodiment, the present invention provides an apparatus comprising: an electromagnetic radiation source that generates radiation for illuminating a sample located at an optical path depth, the electromagnetic radiation source providing radiation to the sample to facilitate determination of the optical path depth within the sample; an interferometer comprising: a reference arm to which a first portion of the radiation is delivered; a sample arm to which a second portion of the radiation is delivered; a first optical subsystem coupled to the sample arm to interrogate the sample using the radiation delivered to the sample arm and to collect backscattered radiation from the sample; a second optical subsystem coupled to the reference arm and the first optical subsystem to generate interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm; and a data collection and processing system communicating with the interferometer and configured to calculate the optical path depth of the sample based on the received interference fringes.

[0008] In some embodiments of the device, the electromagnetic radiation source may include a frequency comb source. In various embodiments of the device, the frequency comb source may generate a first frequency comb having a first free spectral range (FSR) and a second frequency comb having a second FSR different from the first FSR. In some embodiments of the device, the data collection and processing system may be configured to: obtain a first set of interferometric data using the first frequency comb, obtain a second set of interferometric data using the second frequency comb, determine the phase shift between the first and second sets of interferometric data, and determine the optical path depth of the sample based on the phase shift.

[0009] In certain embodiments of the device, the frequency comb source may include a degenerate frequency comb source. In some embodiments of the device, the degenerate frequency comb source may include a chirped frequency comb source. In various embodiments of the device, the data collection and processing system may be configured to: analyze interference fringes to generate a first point spread function (PSF) and a second PSF, calculate the offset between the first PSF and the second PSF, and determine the optical path depth within the sample based on the offset. In some embodiments of the device, the frequency comb source may include a stepped frequency comb with a free spectral range, the frequency comb may include multiple frequency comb lines, and the frequency comb lines may be modulated by a specific amount to generate frequency variations. In some embodiments of the device, the data collection and processing system may be configured to: acquire a first set of interferometric data without modulating the frequency comb lines, acquire a second set of interferometric data with modulated frequency comb lines, determine the phase shift between the first and second sets of interferometric data caused by the modulated frequency comb lines, and determine the optical path depth of the sample based on the phase shift.

[0010] In some embodiments of the device, the electromagnetic radiation source may further include a continuously swept source, and the data collection and processing system may be configured to: obtain a first set of interferometric data using a frequency comb source, obtain a second set of interferometric data using a continuously swept source, determine the optical path depth of the swept source within the sample based on the second set of interferometric data, and determine the optical path depth of the sample based on referencing the optical path depth of the swept source to the first set of interferometric data.

[0011] In some embodiments of the device, the electromagnetic radiation source may include a broadened pulsed active mode-locked laser. In various embodiments of the device, the electromagnetic radiation source may include a dispersive Fabry-Perot etalon filter. In some embodiments of the device, the Fabry-Perot etalon filter may include a silicon wafer. In some embodiments of the device, the first optical subsystem may include an optical circulator circuit to route radiation to the sample and backscattered radiation from the sample to an optical waveguide. In various embodiments of the device, the reference arm may include an active phase modulator to perform complex demodulation of interference fringes. In some embodiments of the device, the data acquisition and processing system, when calculating the optical path depth of the sample, may be further configured to: calculate multiple optical path delays of the sample corresponding to multiple optical path depths within the sample.

[0012] In some embodiments of the method, the electromagnetic radiation source may include a frequency comb source. Various embodiments of the method may further include: generating a first frequency comb having a first free spectral range (FSR) and a second frequency comb having a second FSR different from the first FSR from the frequency comb source. Some embodiments of the method may further include: obtaining a first set of interferometric data using the first frequency comb by a data collection and processing system; obtaining a second set of interferometric data using the second frequency comb by the data collection and processing system; determining a phase shift between the first and second sets of interferometric data by the data collection and processing system; and determining the optical path depth of the sample based on the phase shift by the data collection and processing system.

[0013] In some embodiments of the method, the frequency comb source may include a degenerate frequency comb source. In various embodiments of the method, the degenerate frequency comb source may include a chirped frequency comb source. Some embodiments of the method may include: analyzing interference fringes by a data collection and processing system to generate a first point spread function (PSF) and a second PSF; calculating an offset between the first PSF and the second PSF by the data collection and processing system; and determining the optical path depth within the sample by the data collection and processing system based on the offset.

[0014] In some embodiments of the method, the frequency comb source may include a stepped frequency comb with a free spectral range, and the frequency comb may include multiple frequency comb lines. The method may further include modulating the frequency comb lines by a specific amount to generate a frequency variation. Specific embodiments of the method may further include: obtaining a first set of interferometric data by a data collection and processing system without modulating the frequency comb lines; obtaining a second set of interferometric data by the data collection and processing system with modulated frequency comb lines; determining a phase shift between the first and second sets of interferometric data caused by the modulated frequency comb lines; and determining the optical path depth of the sample by the data collection and processing system based on the phase shift.

[0015] In some embodiments of the method, the electromagnetic radiation source may further include a continuously swept frequency source, and the method may further include: obtaining a first set of interferometric data using a frequency comb source by a data collection and processing system; obtaining a second set of interferometric data using a continuously swept frequency source by a data collection and processing system; determining the optical path depth of the swept frequency source within the sample based on the second set of interferometric data by the data collection and processing system; and determining the optical path depth of the sample by referencing the optical path depth of the swept frequency source to the first set of interferometric data by the data collection and processing system.

[0016] In various embodiments of the method, the electromagnetic radiation source may include a broadened pulsed active mode-locked laser. In some embodiments of the method, the electromagnetic radiation source may include a dispersive Fabry-Perot etalon filter. In some embodiments of the method, the Fabry-Perot etalon filter may include a silicon wafer. In a particular embodiment of the method, the first optical subsystem may include an optical circulator circuit to route radiation to the sample and to route backscattered radiation from the sample to an optical waveguide. In some embodiments of the method, the reference arm may include an active phase modulator to perform complex demodulation of the interference fringes. In various embodiments of the method, calculating the optical path depth of the sample may further include calculating multiple optical path delays of the sample corresponding to multiple optical path depths within the sample.

[0017] In another embodiment, the present invention provides a method comprising: generating radiation from an electromagnetic radiation source for illuminating a sample located at an optical path depth, the electromagnetic radiation source providing radiation to the sample to facilitate determination of the optical path depth within the sample; delivering a first portion of the radiation from the electromagnetic radiation source to a reference arm of an interferometer; delivering a second portion of the radiation from the electromagnetic radiation source to a sample arm of the interferometer; interrogating the sample by a first optical subsystem coupled to the sample arm using the radiation delivered to the sample arm to generate backscattered radiation; collecting the backscattered radiation from the sample by the first optical subsystem; generating interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm by a second optical subsystem coupled to the reference arm and the first optical subsystem; and calculating the optical path depth of the sample by a data collection and processing system in communication with the interferometer based on the received interference fringes. Attached Figure Description

[0018] The various objectives, features and advantages of the disclosed subject matter can be more fully understood when considered in conjunction with the following figures and with reference to the following detailed description of the disclosed subject matter, in which the same reference numerals identify the same elements.

[0019] Figure 1A and Figure 1B The following is shown: ( Figure 1A Non-degenerate frequency comb sources and ( Figure 1B A comparison of the mapping between the physical and measured CR-OCT of degenerate (chirped) frequency comb sources. ZPL (zero pathlength) = zero path length.

[0020] Figure 2A An experimental CR-OCT setup is shown, illustrating an SPML laser with degenerate frequency comb output. Fresnel reflections from the Si etalon produce a finesse of 2.1 per etalon, and a combined finesse of 5.1 after four etalons. LD, laser diode driver; DDG, digital delay generator; PG, mode generator; A, amplifier; EOM, electro-optic modulator; PC, polarization controller; CFBG, continuous fiber Bragg grating; SOA, semiconductor optical amplifier; FP, Fabry-Perot etalon spectral filter; OSA, spectrometer; ISO, optical isolator; PG, signal generator. Photographs of the silicon wafer are included with permission from WaferPro.

[0021] Figure 2B The transmittance of the standard etalon at 232.5 THz is shown sequentially for silicon wafers 1 (green line), 2 (red line), and 4 (blue line).

[0022] Figure 2C The standard etalon FSR is shown across the entire spectrum. Red dots indicate experimental data, and blue lines indicate theoretical curves.

[0023] Figure 2D The degenerate frequency comb spectrum at the SPML output is shown.

[0024] Figure 2E An enlarged view of the three comb lines at the SPML output is shown.

[0025] Figures 3A-3D Numerical simulations of CR depth signals using a degenerate frequency comb and a dispersive silicon-etalon are shown. Figure 3A The analog mirror signal is shown for each setorder of the degenerate frequency comb spectrum centered at 1290 nm with a bandwidth of 100 nm. Figure 3B The blue (S) is shown as a function of setting order. b (Centered at 1265nm) and red (Centered at 1315nm, S) r The superimposed mirror signal. Figure 3C The CR A line shows the blue and red dotted spread function (PSF), and the walk-off of the blue and red dotted spread function (PSF) at the 9th setting order. Figure 3DThe diagram shows the use of the order parameter of recovery and D = 0.54 x 10⁻⁶. -3 ps 2 The PSF width extracted before and after correcting the degenerate frequency comb chirp.

[0026] Figures 4A-4D An experimental implementation of absolute delay ranging using a degenerate frequency comb is shown. Figure 4A The PSF calculated based on the full spectrum is shown. Figure 4B The superimposed PSF from the blue and red subbands is shown. Figure 4C As shown Figure 4A The order parameter used in the calculation is D = 0.54 x 10. -3 ps 2 The PSF after dechirping. Figure 4D The PSF width, measured before (red, square) and after (black, circle), is shown as a function of the setting order.

[0027] Figures 5A-5C The absolute height topography of a pile of cents is shown using the proposed CR-OCT system. Figure 5A The photo was shown. Figure 5A (upper part) and intensity depth projection ( Figure 5A (the lower part). Figure 5B It shows the relationship with Figure 5A The lower part corresponds to the circular depth map. Figure 5C It shows the relationship with Figure 5A The absolute depth of the reconstruction corresponding to the lower part.

[0028] Figure 6 Images depicting the absolute position of surgical instruments resolved using the proposed CR-OCT system are shown. Surgical forceps were positioned above a tissue (chicken skin) sample. The images depict an en-face intensity projection (top row), a cyclic depth map (middle row), and a recovered absolute depth map (bottom row), with color / shading ratios representing the depth (see the ratios on the right side of the middle and bottom rows).

[0029] Figure 7 The stripes and frequency comb tracks of the source are shown when the frequency comb is modulated, tuned, and / or shifted.

[0030] Figure 8 The method of using sources including stepped lasers and swept lasers to determine depth is shown.

[0031] Figure 9 Schematic diagrams of interferometric measurement systems that can be used in conjunction with the various embodiments disclosed herein are provided.

[0032] Figure 10Examples of systems for resolving absolute depth in CR-OCT are shown, based on some embodiments of the disclosed subject matter.

[0033] Figure 11 Examples of hardware that can be used to implement computing devices and servers, based on some embodiments of the disclosed subject matter, are shown.

[0034] Figure 12 Examples of a process for resolving absolute depth in CR-OCT are shown, based on some embodiments of the disclosed subject matter. Detailed Implementation

[0035] According to some embodiments of the disclosed subject matter, a mechanism (which may include a system, method, and medium) is provided for resolving absolute depth in cyclic ranging optical coherence tomography.

[0036] Subsampling OCT is an imaging technique capable of operating at high speeds and over long distances. It achieves this by using optical subsampling to perform optical domain compression of the generated interference signal. Traditionally, this is done using frequency comb sources (i.e., sources comprising spectral lines equally spaced at optical frequencies).

[0037] A potential drawback of subsampling OCT is the lack of measurement of the absolute delay of the sample (and therefore the position of the sample). Instead, the relative position of sample features is measured. In this invention, methods and apparatus are presented that provide both compression and absolute delay / position sensing. In some embodiments, this can be based on the use of an imperfect (or degenerate) frequency comb, i.e., a frequency comb with spectral lines intentionally not arranged on an equidistant grid in optical frequencies. This method can be used in many different applications, including any application of subsampling OCT, for which knowing the absolute position of the sample has additional value. For example, this can be used to measure the distance between surgical instruments and tissue samples, or to measure the morphology of a sample. The latter, for example, can be used in a variety of fields, such as performing dynamic autofocus.

[0038] In Fourier domain optical coherence tomography, an interference signal is generated that spans an RF bandwidth proportional to the product of three parameters: imaging range, imaging speed, and the reciprocal of axial resolution. A circular ranging (CR) OCT architecture is introduced to simplify long-range imaging by decoupling the imaging range from the signal RF bandwidth. Therefore, current CR-OCT systems resolve the relative depth position of the scattered data, rather than the absolute depth position. A modified implementation of CR-OCT is presented here, in some embodiments of which uses a degenerate frequency comb source that allows for the recovery of absolute depth information while minimally impacting the previously described RF bandwidth compression benefits of CR. Results show that such a degenerate frequency comb can be created with relatively simple modifications to the excitation frequency comb source design, and absolute ranging capabilities are demonstrated through imaging studies and simulations.

[0039] In Fourier domain optical coherence tomography (OCT), acquisition systems with limited electronic bandwidth are used to capture the generated optical signals. In applications requiring a long depth range compared to the 1–2 mm imaging depth of most samples by OCT, a large portion of the acquired signal bandwidth is dedicated to measuring signal gaps located before or beyond the deepest imageable depth of the sample surface. CR-OCT captures these signals with greater data efficiency by overlapping equally spaced depth points into a single measurement depth. This is done in the optical domain, allowing electronic acquisition to operate with reduced bandwidth and less noise. Therefore, CR-OCT decouples the imaging range and acquisition bandwidth, making simultaneous high-speed and long-distance imaging more feasible.

[0040] CR-OCT technology such as Figure 1A The diagram illustrates the cyclic mapping from the physical delay coordinates (z-axis) to the measured cyclic delay coordinates (xy-plane). Note that each measured delay is a superposition of a set of equally spaced physical delays. This spacing is referred to as the cyclic delay range. Importantly, it can be understood from this illustration that the distribution of reflected signals spanning only delay ranges smaller than the cyclic delay range can be resolved in the measured / cyclic delay space without overlapping artifacts. Composite (in-phase and orthogonal) detection of the frequency comb source combined with interference fringes results in cyclic ranging, and the cyclic delay range is proportional to the reciprocal of the free spectral range (FSR) of the frequency comb source (Δτ = 1 / (2fsr)). For convenience, the corresponding cyclic depth range can be defined as the product of the cyclic delay range and the velocity of light in the imaging material. It should be noted that the term delay in this work is always defined in relation to the reference arm delay.

[0041] Figure 1AOne consequence of the cyclic mapping shown is that the absolute delay of the sample is not measured. This work presents a method for resolving absolute delay in CR-OCT. Absolute ranging is achieved by employing a degenerate frequency comb (i.e., a frequency comb with unequally spaced optical frequency lines). Specifically, it is shown that a chirped frequency degenerate comb source allows for the measurement of absolute position while maintaining a high compression factor (the core motivation behind CR-OCT). Furthermore, it is demonstrated that modifying an existing CR-OCT source to generate a degenerate comb output is straightforward.

[0042] As a starting point, it is beneficial to consider a simple approach to resolving absolute delay in a CR-OCT system using a “dual FSR” method. Here, consider a first measurement using a first frequency comb source with a given FSR (fsr1). A given signal can then be located in a delay influenced by an unknown integer period (called the order) of the corresponding cyclic delay range [Δτ = 1 / (2fsr1)]. If a second measurement of the same sample is obtained using a different FSR (fsr2), the signal may appear at different cyclic delays depending on its absolute position. Therefore, it is possible to detect the absolute delay by measuring the relationship between the cyclic delays of each measurement using fsr1 and fsr2, thus detecting the integer order parameter in each measurement. This can be analogized to measuring the elapsed time of an event using a stopwatch with a second hand but no minute hand. A single stopwatch cannot distinguish between, for example, 17 seconds and 1 minute and 17 seconds. By adding a second stopwatch that runs a known number faster than the original stopwatch, the minute (i.e., the order) can now be calculated using the difference between the second hand measurements of the two watches.

[0043] While simple to understand, the dual-FSR method can have drawbacks, at least for certain applications. First, generating two frequency combs with different FSRs is possible, although it can complicate the entire source module; this could be done, for example, by using two frequency comb sources to illuminate the sample simultaneously or continuously, or by using a single frequency comb source (where the free spectral range can be adjusted (changed) between images or A-lines). Second, two A-lines need to be collected to add absolute depth measurements, doubling the number of measurements and diminishing the efficiency / compression advantage of the CR method. However, for some applications, this trade-off of a more complex source and a slightly slower acquisition rate for absolute depth information can be considered acceptable.

[0044] This work demonstrates an implementation based on a similar principle, but with a simpler source implementation and minimized compression loss. Instead of using two different FSRs, a frequency comb with a continuously chirped FSR, i.e., a degenerate frequency comb (DFC), is used. The principle of this approach is similar to that of the dual FSR described above, where variations in the optical frequency of the FSR provide clocks with different speeds, which can be used to decipher the absolute delay. However, the information is now contained within a single depth measurement (A-line). Furthermore, the chirped FSR can be created simply using a dispersive Fabry-Perot etalon within the laser source, fsr(ω) = c / (2n g (ω)l), where ω is the angular optical frequency, n g is the group refractive index of the etalon, l is the thickness of the etalon, and c is the speed of light. Figure 1A and Figure 1B The proposed methods were compared. Figure 1B Compared to the traditional single FSR CR-OCT method ( Figure 1A The absolute to measured delay mapping of the physical delay. By using a degenerate frequency comb, the spiral mapping from physical delay to measured delay becomes dependent on the optical frequency, thus producing, as Figure 1B The chirp is illustrated in the diagram with three discrete optical frequencies and three delays. Dispersion in the cyclic delay (now dependent on the optical frequency) from spectral analysis is used to calculate the absolute delay. Subsequently, knowing the absolute delay of the scattered signal, dechirping the measured fringes makes it possible to generate an image that is resolved on the absolute delay and has transform-constrained axial resolution.

[0045] In various embodiments, the CR-OCT system can be implemented by placing a dispersive Fabry-Perot etalon within a 1.29 μm broadened pulse active mode-locked (SPML) laser architecture and using a degenerate frequency comb. Figure 2A The DFC-SPML boasts a sweep rate of 194 THz / μs and a repetition rate of 7.6 MHz at 76% duty cycle. It delivers approximately 1750 ps at 1.3 μm. 2 A silicon (Si) wafer with a group velocity dispersion of / km was used as the etalon. The 280μm thick wafer provides an FSR with a center optical frequency of 146.7GHz, thus offering a circulation depth range of approximately 1mm in air. Single-pass transmittance and frequency-dependent FSR are shown in the figure. Figure 2B and Figure 2C As shown in the image. A magnified view of the spectrum and three comb lines at the DFC-SPML output is shown below. Figure 2D and Figure 2EAs shown, the coherence length of the source was measured to be approximately 2 cm (two-sided FWHM near zero path length). This source, combined with a simple interferometer and acquisition system, provided the measured fringe signal. Complex demodulation of the fringe signal was achieved using an active (LiNbO3) phase modulator in the reference arm.

[0046] A numerical forward model for generating CR-OCT fringe signals was generated from the aforementioned system (i.e., one with the same light source properties and interferometer design). Using this model, a processing pipeline was constructed and validated to extract absolute depth information from the mirrored fringe signal. As a starting point, Figure 3A The analog point spread function (PSF) for mirrored signals generated by processing degenerate frequency comb signals using a conventional CR-OCT method is presented. The mirrors are located at the same cycle depth (0 mm), but at different orders within the cycle depth range (where the cycle depth range is defined by the average FSR of the DFC). Here, the expected chirp (blurring) of the PSF due to the use of the degenerate frequency comb can be seen. Next, a dual-band processing method is implemented, which refers to the "blue" and "red" spectral bands (centered at 1265 nm (237.2 THz) and 1315 nm (228.1 THz), respectively) to analyze the detected fringes individually. The difference in average FSR within each band is approximately Δ = 1.3 GHz, where fsr r >fsr b (Δτ r <Δτ b ). Simulated PSF for the same mirror position, such as Figure 3B As drawn in [the text]. Here, Figure 3A The overall chirp can be resolved into two PSFs that are moving away from each other. The measured depth offset ΔL between the blue and red band PSFs is used to... Figure 3C The order parameter k is calculated using the following formula:

[0047]

[0048] fsr b,r These are the average FSRs across the blue and red bands, respectively.

[0049] Next, a technique was developed to eliminate the chirp (axial blurring) caused by the use of degenerate frequency combs (see...). Figure 3A By measuring the complex fringes s(ω) i ) and exp[iDk(ω-ω0) 2 Multiplying the given correction vectors, the corrected fringe signal s'(ω) is calculated. i ), where k is an integer depth-order parameter ( Figure 3DThe parameter D is given by the properties of the degenerate frequency comb as follows:

[0050]

[0051] The processing pipeline was then applied to the experimental data. Figure 4 shows PSF measurements at a cycle depth of approximately 250 μm using a degenerate frequency comb, obtained similarly to the simulated data in Figure 3, for order parameters ranging from ±10. Figure 4A and Figure 4B The PSF is shown using the full band and the red / blue spectral bands, respectively. PSF walk-off is extracted via cross-correlation of the A-line. It should be noted that in practice, PSF wrap-around at the edges of the cycle depth range must be considered. At these boundaries, the true PSF offset is given by the measured offset ΔL' and the blue band cycle depth range (because fsr...). r >fsr b ), ΔL=ΔL'±cΔτ b It has been confirmed that, according to Figure 4B The order of the measured PSF offset is matched to the set order for each measurement (set order from -10 to 10). Using this resolved order parameter, the chirp (broadening) of the full-spectrum data is corrected as described in Equation 1, and a dechirped PSF is generated. Figure 4C The restored PSF width before and after correction is as follows: Figure 4D As presented in the diagram. Due to the asymmetry of the PSF, the root mean square width δz rms =(Σ((d-d0)) 2 S) / ΣS) -1 / 2 Here it is adjusted (adapted), where d is the depth, d0 is the PSF center depth, and S is the signal amplitude.

[0052] To test the reconstruction of absolute depth information from the samples, a pile of 9 cents was imaged. Figure 5A The cyclic wrapping topography image and the absolute topography image are shown respectively in... Figure 5B and Figure 5C It should be noted that the absolute image spans a much larger depth range, but retains the same height resolution (given by axial resolution) as the regular loop-wrap image depth range. Errors in the measured height are thought to stem from low measured SNR; research into SNR requirements and noise mitigation strategies in CR-OCT with absolute delay resolution is ongoing and beyond the scope of this work.

[0053] In the second experiment, the technique was used to provide feedback on the absolute positioning of surgical instruments relative to the tissue surface. Figure 6 ). Figure 6Images depicting the absolute position of surgical instruments resolved using the proposed CR-OCT system are shown. Surgical forceps are positioned above a tissue (chicken skin) sample. The images depict anterior-facing intensity projection (top row), a circumferential depth map (middle row), and a recovered absolute depth map (bottom row), where color / shading ratios depict depth (see the ratios on the right of the middle and bottom rows). The absolute topographic image (bottom row), instead of the conventional CR topographic image with circumferential wrapping (middle row), allows for the measurement of instrument proximity to the tissue. This absolute depth mapping technique enables applications such as surgical guidance using high-depth sensing, where visualization of complex organs within the surgical area during surgery benefits from real-time stereoscopic vision.

[0054] Finally, a brief commentary is provided on the magnitude of the standard etalon dispersion required to perform absolute ranging. Returning again to the dual FSR technique as a simplified example, the minimum FSR difference Δ required to generate a detectable offset can be estimated by setting ΔL = δz for k = 1 in Equation 1. min Where δz is the axial resolution constrained by the transformation for each measurement, thus producing

[0055]

[0056] in This is the average FSR between all measurements. Applying this to the degenerate frequency comb technique presented in this work, it is possible to set... And bz is set to be equal to the axial resolution of the transform limit for each subband. This is to estimate the maximum difference FSRΔ. max It should be noted that the maximum order k of the system is designed for accurate measurement. max The offset ΔL caused by the sample should not exceed half of the cyclic delay range (cΔτ). r / 2),

[0057]

[0058] Where k max =l c / (2ΔL k＝l ), where l c It is the coherence length, and ΔL k＝l This refers to ΔL for k=1, i.e., the difference within the cycle depth range. It should be noted that in the degenerate frequency comb method, this limitation can be overcome by shifting the spectral window closer to the center frequency, thereby reducing the differential FSR between measurements.

[0059] This work numerically and experimentally describes and verifies that a degenerate frequency comb constructed from dispersive Fabry-Perot etalons can be used to recover the absolute delay position without significantly affecting measurement compression. The current work focuses on principles and first-order implementation, and therefore has some limitations. First, uncoated silicon etalons (silicon wafers) are used, and multiple etalons are cascaded due to the resulting low fineness. Custom-designed silicon-coated (or other dispersive material) etalons providing higher fineness values ​​would improve system performance, including coherence length and measurement SNR. The processing method uses simple red / blue subband analysis and cross-correlation-based PSF offset measurements. It is likely that this can be enhanced to better utilize the continuous chirps occurring through the spectral bands. Relatedly, the construction of an absolute delay resolving algorithm with minimal processing cost relative to conventional CR-OCT is crucial and is the subject of ongoing research.

[0060] In various embodiments, although several different types of electromagnetic radiation sources can be used to perform the above procedure, in some cases the procedure can be modified according to the type of source used:

[0061] Dual FSR

[0062] In some embodiments, the source can be modified to generate a frequency comb with two different FSRs, as disclosed in the above-disclosed "dual FSR" method. Using this method, two A-lines are collected for each data point, one for each of the two different FSRs, and the A-line data corresponding to the two FSRs are combined to determine the absolute depth.

[0063] Degenerate frequency comb

[0064] In embodiments such as those disclosed herein, the source that generates the degenerate frequency comb can be used to determine absolute depth. In addition to the embodiments disclosed above where a dispersive etalon is used to create the degenerate (chirped) source, in other embodiments, a prism or programmable laser can alternatively be used to create the degenerate frequency comb. In various embodiments, the techniques disclosed above for generating degenerate frequency combs using SPML lasers can also be implemented using other types of laser sources such as PCML, FDML, or conventional external cavity lasers. Furthermore, the degenerate frequency comb created using any of the techniques in the specific art need not be continuous.

[0065] Frequency comb modulation / tuning / offset

[0066] In some embodiments, a source that generates a stepped frequency comb with a known FSR can be used, wherein the frequency comb lines are offset, tuned, and / or modulated by a known amount (Δf) in the spectral domain using a frequency shifter, phase modulator, etalon tilt, or other suitable means. For example, in some embodiments, the frequency shifter or phase modulator may be located at the laser output before the interferometer. In other embodiments, etalon tilt can be achieved within the laser cavity by tilting the spectral filter that generates the frequency comb itself (or changing the refractive index or otherwise). Similar to the dual FSR method discussed above, the two A-lines (depth signals) are detected and acquired with and without comb line offset / tuning / modulation.

[0067] Comb offset in the spectral domain produces a phase shift associated with the optical delay (τ) between the measured fringe signals (depth signals). Figure 7 An example of a continuous sweep fringe with two different delays τ1 and τ2 is shown in both the time and optical frequency domains, where the frequency domain trajectory represents the offset of the frequency comb between the two delays. The measured phase shift... It is possible Figure 7 The stepped fringes were observed and used to solve for the absolute depth. The phase difference at the baseband edge is given by the following equation:

[0068]

[0069] Absolute depth is determined by the following formula:

[0070]

[0071] Typically, the frequency shift Δf must be small enough to avoid π-phase ambiguity in the measured phase shift at the maximum target depth.

[0072]

[0073] Where N k It is the order within the coherence length.

[0074] Stepped laser plus sweeping laser

[0075] In some embodiments, a combination of a stepped laser source and a swept-frequency laser source can be used to obtain absolute depth information from a sample. A stepped laser with a known FSR (laser 1, ...) Figure 8 This can be used to detect A-lines (depth signals) from samples. As mentioned above, this generates depth ambiguity about the cyclic range (Δτ = 1 / FSR, LB = c / FSR / 2) in the A-lines already collected with laser 1. To supplement this data, a narrow-linewidth (long coherence length) continuously swept laser (laser 2) can be used. Figure 8 To obtain the second A-line (depth signal) of the same sample (e.g.) Figure 8 The cornea and / or lens are shown in the upper right. The relative depth (step) of the depth signal obtained with a step-wise laser (laser 1) can be referenced to the absolute depth obtained with a swept-frequency laser (laser 2) to determine the absolute depth of the data obtained using the frequency comb of laser 1.

[0076] Generally, the axial resolution of laser 2 should be half or better than the loop range (LB) of laser 1. Furthermore, the optical bandwidth of laser 2 should be kept to a minimum to maximize its sweep rate; ideally, the A-line rates of lasers 1 and 2 should be the same or very similar. Finally, the coherence length of laser 2 should be equal to or better than that of laser 1.

[0077] Figure 9 A schematic diagram of an interferometric measurement system that can be used in conjunction with various embodiments of the present invention is provided. Figure 9 This illustrates a Mach-Zehnder type interferometer, which can utilize free-space optics. Figure 9 Panel A) or fiber optic arrangement ( Figure 9 This can be achieved using panel B. Other types of interferometers (e.g., Michelson) can also be used. Figure 9 The light source LS in panel A or panel B can be: an SPML laser with a degenerate frequency comb as shown in Figure 2 or other types of lasers (e.g., PCML, FDML or conventional external cavity lasers); a source in which the lines of the stepped frequency comb are offset, tuned or modulated; a dual FSR source; or a source that combines a stepped frequency comb with a swept frequency laser.

[0078] The beam B9 emitted from LS is guided to the interferometer input, where a beam splitter (BS3) splits B9 into two approximately equal-length paths. B10 is then guided toward the sample S. The backscattered light from the object of interest is subsequently guided toward the interferometer output (B11). In the reference arm, beam B12 is guided toward the phase modulator (PM). The beam after PM (i.e., beam B13) is guided to the interferometer output to interfere with beam B11 after being combined by BS4. The output beam B14 is then detected by a detector D (e.g., a photodiode). Alternatively, Figure 2B The fiber-optic interferometer shown readily allows for balanced detection due to the π-phase shift between the output beams B14 and B15. A data acquisition and processing system (which may include a data acquisition board or a real-time oscilloscope (DAQ)) is used at a sampling rate f. S The detected signal is digitized. Several wavelength sweeps (A1, A2, ..., An) can be acquired to form a two-dimensional or three-dimensional image.

[0079] Turning Figure 10 Example 1000 of a system (e.g., a data collection and processing system) for resolving absolute depth in CR-OCT is shown, according to some embodiments of the disclosed subject matter. Figure 10 As shown, computing device 1010 can receive interferometric data from optical interferometry system 1000. In some embodiments, computing device 1010 can execute at least a portion of system 1004 for resolving absolute depth to determine absolute depth based on the interferometric data received from optical interferometry system 1000. Additionally or alternatively, in some embodiments, computing device 1010 can transmit information related to the interferometric data received from optical interferometry system 1000 to server 1020 via communication network 1006, server 1020 can execute at least a portion of system 1004 for resolving absolute depth to determine absolute depth based on the interferometric data. In some such embodiments, server 1020 can return information indicating the output of system 1004 for resolving absolute depth, such as absolute depth information, to computing device 1010 (and / or any other suitable computing device). This information can be transmitted and / or presented to users (e.g., researchers, operators, clinicians, etc.) and / or can be stored (e.g., as part of a research database or medical records associated with a subject).

[0080] In some embodiments, computing device 1010 and / or server 1020 can be any suitable computing device or combination of devices, such as desktop computers, laptop computers, smartphones, tablet computers, wearable computers, server computers, virtual machines executed by physical computing devices, etc. As described herein, system 1004 for resolving absolute depth can present information and / or absolute depth information related to interferometric data to users (e.g., researchers and / or physicians).

[0081] In some embodiments, the optical interferometry system 1000 may include an electromagnetic radiation source 1002, which may be any source suitable for optical interferometry, such as CR-OCT. In other embodiments, the electromagnetic radiation source 1002 may be local to the computing device 1010. For example, the electromagnetic radiation source 1002 may be integrated with the computing device 1010 (e.g., the computing device 1010 may be configured as part of a device for capturing and / or storing optical interferometry information). As another example, the electromagnetic radiation source 1002 may be connected to the computing device 1010 via a cable, a direct wireless link, or the like. Additionally or alternatively, in some embodiments, the electromagnetic radiation source 1002 may be located locally and / or remotely from the computing device 1010 and may transmit information to the computing device 1010 (and / or server 1020) via a communication network (e.g., communication network 1006).

[0082] In some embodiments, the communication network 1006 may be any suitable communication network or combination of communication networks. For example, the communication network 1006 may include a Wi-Fi network (which may include one or more wireless routers, one or more switches, etc.), a peer-to-peer network (e.g., a Bluetooth network), a cellular network (e.g., a 3G network, 4G network, etc., conforming to any suitable standard (such as CDMA, GSM, LTE, LTE Advanced, WiMAX, etc.), a wired network, etc. In some embodiments, the communication network 1006 may be a local area network (LAN), a wide area network (WAN), a public network (e.g., the Internet), a private or semi-private network (e.g., a corporate or university intranet), any other suitable type of network, or any suitable combination of networks. Figure 10 The communication links shown can be any suitable communication link or combination of communication links, such as wired links, fiber optic links, Wi-Fi links, Bluetooth links, cellular links, etc.

[0083] Figure 11 Example 1100 illustrates hardware that can be used to implement computing device 1010 and server 1020 according to some embodiments of the disclosed subject matter. Figure 11 As shown, in some embodiments, computing device 1010 may include processor 1102, display 1104, one or more inputs 1106, one or more communication systems 1108, and / or memory 1110. In some embodiments, processor 1102 may be any suitable hardware processor or combination of processors, such as a central processing unit, graphics processing unit, etc. In some embodiments, display 1104 may include any suitable display device, such as a computer monitor, touchscreen, television, etc. In some embodiments, input 1106 may include any suitable input device and / or sensor that can be used to receive user input, such as a keyboard, mouse, touchscreen, microphone, etc.

[0084] In some embodiments, the communication system 1108 may include any suitable hardware, firmware, and / or software for transmitting information over the communication network 1006 and / or any other suitable communication network. For example, the communication system 1108 may include one or more transceivers, one or more communication chips and / or chipsets, etc. In a more specific example, the communication system 1108 may include hardware, firmware, and / or software that can be used to establish Wi-Fi connections, Bluetooth connections, cellular connections, Ethernet connections, etc.

[0085] In some embodiments, memory 1110 may include any suitable storage device or device that can be used to store instructions, values, etc., which may be used, for example, by processor 1102 to present content using display 1104, to communicate with server 1020 via communication systems(multiple) 1108, etc. Memory 1110 may include any suitable volatile memory, non-volatile memory, storage device, or any suitable combination thereof. For example, memory 1110 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid-state drives, one or more optical drives, etc. In some embodiments, memory 1110 may have a computer program encoded thereon for controlling the operation of computing device 1010. In such embodiments, processor 1102 may execute at least a portion of the computer program to present content (e.g., images, user interfaces, graphics, tables, etc.), receive content from server 1020, transmit information to server 1020, etc.

[0086] In some embodiments, server 1020 may include processor 1112, display 1114, one or more inputs 1116, one or more communication systems 1118, and / or memory 1120. In some embodiments, processor 1112 may be any suitable hardware processor or combination of processors, such as a central processing unit, graphics processing unit, etc. In some embodiments, display 1114 may include any suitable display device, such as a computer monitor, touchscreen, television, etc. In some embodiments, input 1116 may include any suitable input device and / or sensor that can be used to receive user input, such as a keyboard, mouse, touchscreen, microphone, etc.

[0087] In some embodiments, the communication system 1118 may include any suitable hardware, firmware, and / or software for transmitting information over the communication network 1006 and / or any other suitable communication network. For example, the communication system 1118 may include one or more transceivers, one or more communication chips and / or chipsets, etc. In a more specific example, the communication system 1118 may include hardware, firmware, and / or software that can be used to establish Wi-Fi connections, Bluetooth connections, cellular connections, Ethernet connections, etc.

[0088] In some embodiments, memory 1120 may include any suitable storage device or device that can be used to store instructions, values, etc., which may be used, for example, by processor 1112 to present content using display 1114, to communicate with one or more computing devices 1010, etc. Memory 1120 may include any suitable volatile memory, non-volatile memory, storage device, or any suitable combination thereof. For example, memory 1120 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid-state drives, one or more optical drives, etc. In some embodiments, memory 1120 may have server programs encoded thereon for controlling the operation of server 1020. In this embodiment, processor 1112 may execute at least a portion of the server program to transmit information and / or content (e.g., organizational identification and / or classification results, user interface, etc.). In this embodiment, processor 1112 may execute at least a portion of the server program to transmit information and / or content (e.g., organizational identification and / or classification results, user interface, etc.) to one or more computing devices 1010, receive information and / or content from one or more computing devices 1010, receive instructions from one or more devices (e.g., personal computer, laptop computer, tablet computer, smartphone, etc.), etc.

[0089] In some embodiments, any suitable computer-readable medium may be used to store instructions for performing the functions and / or processes described herein. For example, in some embodiments, the computer-readable medium may be transient or non-transient. For example, a non-transient computer-readable medium may include media such as magnetic media (e.g., hard disks, floppy disks, etc.), optical media (e.g., compressed discs, digital video discs, Blu-ray discs, etc.), semiconductor media (e.g., RAM, flash memory, electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), any suitable medium that is not transient during transmission or does not have any persistent appearance, and / or any suitable tangible medium. As another example, a transient computer-readable medium may include signals on a network, in wires, conductors, optical fibers, circuits, or any suitable medium that is transient during transmission and does not have any persistent appearance, and / or any suitable intangible medium.

[0090] In some embodiments, the optical signal is detected by a photodiode. It should be recognized that any photoelectric conversion device, including but not limited to photodetectors, photodiodes, line scanners and 2D cameras, and photodiode arrays, can be used to perform this detection function.

[0091] It should be noted that the terminology used herein may cover hardware, software, firmware, or any appropriate combination thereof.

[0092] Figure 12 Example 1200 illustrates a process for resolving absolute depth in CR-OCT according to some embodiments of the disclosed subject matter. (e.g.) Figure 12 As shown, at 1202, process 1200 can generate radiation to illuminate the sample located at the optical path depth, wherein an electromagnetic radiation source can provide radiation to the sample to facilitate the determination of the optical path depth within the sample. At 1204, process 1200 can deliver a first portion of the radiation to the reference arm of the interferometer. At 1206, process 1200 can deliver a second portion of the radiation to the sample arm of the interferometer. At 1208, process 1200 can use the radiation delivered to the sample arm to interrogate the sample to generate backscattered radiation. At 1210, process 1200 can collect the backscattered radiation from the sample. At 1212, process 1200 can generate interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm. Finally, at 1214, process 1200 can calculate the optical path depth of the sample based on the received interference fringes.

[0093] It should be understood that Figure 12 The above steps of the process can be implemented or performed in any order or sequence, and are not limited to the order shown and described in the figures. Furthermore, Figure 12Some of the steps in the above process can be performed substantially simultaneously or in parallel where appropriate to reduce latency and processing time.

[0094] Therefore, although the invention has been described above in conjunction with specific embodiments and examples, the invention is not necessarily limited thereto, and many other embodiments, examples, uses, modifications, and deviations from the said embodiments, examples, and uses are intended to be included in the appended claims.

Claims

1. An optical apparatus comprising: a source of electromagnetic radiation that generates radiation for illuminating a sample located at an optical path depth, the source of electromagnetic radiation providing the radiation to the sample to facilitate determining the optical path depth within the sample; an interferometer comprising: a reference arm to which a first portion of the radiation is delivered, a sample arm to which a second portion of the radiation is delivered, a first optical subsystem coupled to the sample arm to interrogate the sample with the radiation delivered to the sample arm and collect backscattered radiation from the sample, and a second optical subsystem coupled to the reference arm and the first optical subsystem to generate interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm; and a data collection and processing system in communication with the interferometer and configured to calculate the optical path depth of the sample from the generated interference fringes, wherein the source of electromagnetic radiation comprises a chirped frequency comb source, and wherein the data collection and processing system is configured to: analyze the interference fringes to generate a first point spread function (PSF) associated with a first waveband and a second PSF associated with a second waveband different from the first waveband, calculate a shift between the first PSF and the second PSF, and determine the optical path depth within the sample based on the shift.

2. The optical device of claim 1, wherein, the frequency comb source generates a first frequency comb having a first free spectral range (FSR) and a second frequency comb having a second FSR different from the first FSR.

3. The optical device of claim 2, wherein, the data collection and processing system is configured to: obtain a first set of interferometric measurement data using the first frequency comb, obtain a second set of interferometric measurement data using the second frequency comb, determine a phase shift between the first set of interferometric measurement data and the second set of interferometric measurement data, and determine the optical path depth of the sample based on the phase shift.

4. The optical device of claim 1, wherein, the frequency comb source comprises a stepped frequency comb having a free spectral range, wherein the frequency comb comprises a plurality of frequency comb lines, and wherein the frequency comb lines are modulated by a known amount to generate a frequency variation.

5. The optical device of claim 4, wherein, the data collection and processing system is configured to: obtain a first set of interferometric measurement data without modulating the frequency comb lines, obtain a second set of interferometric measurement data with modulating the frequency comb lines, determine a phase shift between the first set of interferometric measurement data and the second set of interferometric measurement data resulting from modulating the frequency comb lines, and determine the optical path depth of the sample based on the phase shift.

6. The optical device of claim 1, wherein, the source of electromagnetic radiation further comprises a continuous swept source, and wherein the data collection and processing system is configured to: obtain a first set of interferometric measurement data using the frequency comb source, obtain a second set of interferometric measurement data using the continuous swept source, determine a swept source optical path depth within the sample based on the second set of interferometric measurement data, and determining the optical path depth of the sample based on referencing the swept source optical path depth to the first set of interferometric measurement data.

7. The optical device of any one of claims 1-6, wherein, The electromagnetic radiation source comprises a stretched pulse actively mode-locked laser.

8. The optical device of any one of claims 1-6, wherein, The electromagnetic radiation source comprises a dispersive Fabry-Perot etalon filter.

9. The optical device of claim 8, wherein, The Fabry-Perot etalon filter comprises a silicon wafer.

10. The optical device of any one of claims 1-6, wherein, The first optical subsystem comprises an optical circulator circuit to route the radiation to the sample and to route the backscattered radiation from the sample to an optical waveguide.

11. The optical device of any one of claims 1-6, wherein, The reference arm comprises an active phase modulator to perform complex demodulation of the interference fringes.

12. The optical device of any one of claims 1-6, wherein, The data collection and processing system is further configured to, in computing the optical path depth of the sample, compute a plurality of optical path delays of the sample corresponding to a plurality of optical path depths within the sample.

13. An optical method comprising: generating, by an electromagnetic radiation source, radiation for illuminating a sample located at an optical path depth, the electromagnetic radiation source providing the radiation to the sample to facilitate determining the optical path depth within the sample; delivering, by the electromagnetic radiation source, a first portion of the radiation to a reference arm of an interferometer; delivering, by the electromagnetic radiation source, a second portion of the radiation to a sample arm of the interferometer; inquiring, by a first optical subsystem coupled to the sample arm, the sample with the radiation delivered to the sample arm to generate backscattered radiation; collecting, by the first optical subsystem, the backscattered radiation from the sample, generating, by a second optical subsystem coupled to the reference arm and the first optical subsystem, interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm; computing, by a data collection and processing system in communication with the interferometer, the optical path depth of the sample from the generated interference fringes, wherein the electromagnetic radiation source comprises a chirped frequency comb source, and analyzing, by the data collection and processing system, the interference fringes to generate a first point spread function (PSF) associated with a first waveband and a second PSF associated with a second waveband different from the first waveband, computing, by the data collection and processing system, a shift between the first PSF and the second PSF, and determining, by the data collection and processing system, the optical path depth within the sample based on the shift.

14. The optical method of claim 13, wherein, further comprising: generating, by the frequency comb source, a first frequency comb having a first free spectral range (FSR) and a second frequency comb having a second FSR different from the first FSR.

15. The optical method of claim 14, wherein, further comprising: obtaining, by the data collection and processing system, a first set of interferometric measurement data using the first frequency comb, obtaining, by the data collection and processing system, a second set of interferometric measurement data using the second frequency comb, determining, by the data collection and processing system, a phase shift between the first set of interferometric measurement data and the second set of interferometric measurement data, and determining, by the data collection and processing system, the optical path depth of the sample based on the phase shift.

16. The optical method of claim 13, wherein, the frequency comb source comprises a stepped frequency comb having a free spectral range, and wherein the frequency comb comprises a plurality of frequency comb lines, and wherein the frequency comb comprises a plurality of frequency comb lines, and wherein the method further comprises: modulating the frequency comb lines by a known amount to generate a frequency change.

17. The optical method of claim 16, wherein, further comprising: obtaining, by the data collection and processing system, a first set of interferometric measurement data without modulating the frequency comb lines, obtaining, by the data collection and processing system, a second set of interferometric measurement data with modulating the frequency comb lines, determining, by the data collection and processing system, a phase shift between the first set of interferometric measurement data and the second set of interferometric measurement data resulting from modulating the frequency comb lines, and determining, by the data collection and processing system, the optical path depth of the sample based on the phase shift.

18. The optical method of claim 13, wherein, the electromagnetic radiation source further comprises a continuous swept source, and wherein the method further comprises: obtaining, by the data collection and processing system, a first set of interferometric measurement data using the frequency comb source, obtaining, by the data collection and processing system, a second set of interferometric measurement data using the continuous swept source, determining, by the data collection and processing system, a swept source optical path depth within the sample based on the second set of interferometric measurement data, and determining, by the data collection and processing system, the optical path depth of the sample based on referencing the swept source optical path depth to the first set of interferometric measurement data.

19. The optical method of any one of claims 13-18, wherein, the electromagnetic radiation source comprises an extended pulse actively mode-locked laser.

20. The optical method of any one of claims 13-18, wherein, the electromagnetic radiation source comprises a dispersive Fabry-Perot etalon filter.

21. The optical method of claim 20, wherein, the Fabry-Perot etalon filter comprises a silicon wafer.

22. The optical method of any one of claims 13-18, wherein, the first optical subsystem comprises an optical circulator circuit to route the radiation to the sample and to route the backscattered radiation from the sample to an optical waveguide.

23. The optical method of any one of claims 13-18, wherein, the reference arm comprises an active phase modulator to perform complex demodulation of the interference fringes.

24. The optical method of any one of claims 13-18, wherein, calculating the optical path depth of the sample further comprises calculating a plurality of optical path delays of the sample corresponding to a plurality of optical path depths within the sample.

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