Absolute depth resolution in circular optical coherence tomography

Abstract

To provide a new system for resolving an absolute depth in circular-ranging optical coherence tomography.SOLUTION: An apparatus comprises: an electromagnetic radiation source for generating radiation to irradiate a sample located at an optical path depth, in order to facilitate determination of an optical path depth in the sample; an interferometer; and a data collection and processing system. The interferometer comprises: a reference arm delivering a first portion of radiation; a sample arm delivering a second portion of radiation; a first optical subsystem which is coupled to the sample arm in order to collect backscattered radiation from the sample; and a second optical subsystem coupled to the reference arm and the first optical subsystem, in order to generate interference fringes between the collected backscattered radiation and the radiation delivered in the reference arm. The data collection and processing system calculates an optical path depth of the sample from the interference fringes received in communication with the interferometer.SELECTED DRAWING: Figure 2A

Description

[Technical Field]

[0001] <Cross-reference of related applications> This application is based on U.S. Provisional Patent Application No. 62 / 929,390, filed on November 1, 2019, claiming priority thereunder, and the full disclosure of said Provisional Application is incorporated herein by reference.

[0002] <Description of research and development funded by the federal government> This invention was made with government support under grant number P41EB015903, granted by the National Institutes of Health. The U.S. Government has certain rights in this invention. [Background technology]

[0003] Subsampling optical coherence tomography (OCT) is an imaging technique that can operate at high speeds and over long distances. Subsampling OCT achieves this by performing optical domain compression of interference signals generated through the use of optical subsampling. Typically, this is achieved by using a single-frequency comb light source, i.e., a light source with individual spectral lines equally spaced at optical frequencies. However, these techniques cannot determine absolute depth, which can be a limitation in certain applications. [Overview of the project]

[0004] Therefore, new systems, methods, and media are desired for resolving absolute depth in circular optical coherence tomography.

[0005] In one embodiment, the present invention provides an interferometer comprising: an electromagnetic radiation source that generates radiation for irradiating a sample located at the optical path depth, configured to provide radiation to a sample to facilitate the determination of the optical path depth in the sample; a reference arm to which a first portion of radiation is delivered; a sample arm to which a second portion of radiation is delivered; a first optical subsystem coupled to the sample arm for examining a sample with radiation delivered to the sample arm and collecting backscattered radiation from the sample; a second optical subsystem coupled to the reference arm and the first optical subsystem for generating interference fringes between the collected backscattered radiation and the radiation delivered to the reference arm; and a data acquisition and processing system configured to calculate the optical path depth of the sample from the received interference fringes.

[0006] In some embodiments of the above apparatus, the electromagnetic radiation source may include a frequency comb light source. In various embodiments of the above apparatus, the frequency comb light source can 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 above apparatus, the data acquisition and processing system may be configured to acquire a first interference dataset using the first frequency comb, acquire a second interference dataset using the second frequency comb, determine the phase shift between the first and second interference datasets, and determine the optical path depth of the sample based on the phase shift.

[0007] In certain embodiments of the above apparatus, the frequency comb light source may include a degenerate frequency comb light source. In some embodiments of the above apparatus, the degenerate frequency comb light source may include a chirp frequency comb light source. In various embodiments of the above apparatus, the data acquisition and processing system analyzes interference fringes to generate a first point spread function (PSF) and a second PSF, calculates the shift between the first PSF and the second PSF, and determines the optical path depth in the sample based on the shift. The apparatus may be configured as follows: In some embodiments of the apparatus, the frequency comb light source may include a step frequency comb having a free spectral range, the frequency comb may include a plurality of frequency comb lines, and the frequency comb lines may be modulated by a certain amount to vary the frequency. In some embodiments of the apparatus, the data acquisition and processing system may be configured to acquire a first interference dataset without modulating the frequency comb lines, acquire a second interference dataset by modulating the frequency comb lines, determine the phase shift between the first interference dataset and the second interference dataset resulting from modulating the frequency comb lines, and determine the optical path depth of the sample based on the phase shift.

[0008] In some embodiments of the above apparatus, the electromagnetic radiation source may further include a continuous sweep light source, and the data acquisition and processing system may be configured to acquire a first interference dataset using a frequency comb light source, acquire a second interference dataset using a continuous sweep light source, determine the sweep light source optical path depth in the sample based on the second interference dataset, and determine the optical path depth of the sample based on comparing the sweep light source optical path depth with the first interference dataset.

[0009] In some embodiments of the above apparatus, the electromagnetic radiation source may include a stretched pulsed active mode-locked laser. In various embodiments of the above apparatus, the electromagnetic radiation source may include a dispersed Fabry-Perot etalon filter. In some embodiments of the above apparatus, the Fabry-Perot etalon filter may include a silicon wafer (Si-wafer). In some embodiments of the above apparatus, the first optical subsystem may include an optical circulator circuit for sending radiation to the sample and backscattered radiation from the sample to the optical waveguide. In various embodiments of the above apparatus, the reference arm may include an active phase modulator for complex demodulation of interference fringes. In certain embodiments of the above apparatus, the data acquisition and processing system may be further configured to calculate multiple optical path delays of the sample corresponding to multiple optical path depths in the sample when calculating the optical path depth of the sample.

[0010] In some embodiments of the present method, the electromagnetic radiation source may include a frequency comb light source. Various embodiments of the above method may further include the step of 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 using a frequency comb light source. Specific embodiments of the above method may further include the steps of acquiring a first interference dataset using the first frequency comb with a data acquisition and processing system, acquiring a second interference dataset using the second frequency comb with a data acquisition and processing system, determining a phase shift between the first and second interference datasets with a data acquisition and processing system, and determining the optical path depth of a sample based on the phase shift with a data acquisition and processing system.

[0011] In some embodiments of the above method, the frequency comb light source may include a degenerate frequency comb light source. In various embodiments of the above method, the degenerate frequency comb light source may include a chirp frequency comb light source. A particular embodiment of the above method may include the steps of: analyzing interference fringes by a data acquisition and processing system to generate a first point spread function (PSF) and a second PSF; calculating a shift between the first PSF and the second PSF by the data acquisition and processing system; and determining the optical path depth in the sample based on the shift by the data acquisition and processing system.

[0012] In some embodiments of the above method, the frequency comb light source may include a step frequency comb having a free spectral range, the frequency comb may include a plurality of frequency comb lines, and the method further includes modulating the frequency comb lines by a specific amount to change the frequency. . A particular embodiment of the above method may further include the steps of: acquiring a first interference dataset by a data acquisition and processing system without modulating a frequency comb; acquiring a second interference dataset by modulating a frequency comb by a data acquisition and processing system; determining a phase shift between the first interference dataset and the second interference dataset resulting from modulating the frequency comb by a data acquisition and processing system; and determining the optical path depth of a sample based on the phase shift by a data acquisition and processing system.

[0013] In a particular embodiment of the above method, the electromagnetic radiation source may further include a continuous sweep light source, and the method may further include the steps of: acquiring a first interference dataset using a frequency comb light source with a data acquisition and processing system; acquiring a second interference dataset using a continuous sweep light source with the data acquisition and processing system; determining the sweep light source optical path depth in a sample based on the second interference dataset with the data acquisition and processing system; and determining the optical path depth of a sample based on comparing the sweep light source optical path depth with the first interference dataset with the data acquisition and processing system.

[0014] In various embodiments of the above method, the electromagnetic radiation source may include a stretched pulsed active mode-locked laser. In some embodiments of the above method, the electromagnetic radiation source may include a dispersed Fabry-Perot etalon filter. In some embodiments of the above method, the Fabry-Perot etalon filter may include a silicon wafer. In a particular embodiment of the above method, the first optical subsystem may include an optical circulator circuit for delivering radiation to the sample and backscattered radiation from the sample to the optical waveguide. In some embodiments of the above method, the reference arm may include an active phase modulator for complex demodulation of interference fringes. In various embodiments of the above method, the step of calculating the optical path depth of the sample may further include the step of calculating multiple optical path delays of the sample corresponding to multiple optical path depths in the sample.

[0015] In another embodiment, the method of the present invention comprises the steps of: generating radiation for irradiating a sample located at the optical path depth using an electromagnetic radiation source that provides radiation to the sample to facilitate the determination of the optical path depth within the sample; sending a first portion of the radiation to a reference arm of an interferometer using the electromagnetic radiation source; sending a second portion of the radiation to a sample arm of an interferometer using the electromagnetic radiation source; examining the sample using the radiation sent to the sample arm with a first optical subsystem coupled to the sample arm to generate backscattered radiation; collecting the backscattered radiation from the sample with the first optical subsystem; generating interference fringes between the collected backscattered radiation and the radiation sent to the reference arm with a second optical subsystem coupled to the reference arm and the first optical subsystem; and calculating the optical path depth of the sample from the received interference fringes with a data acquisition and processing system communicating with the interferometer.

[0016] The various purposes, features, and advantages of the disclosed subject matter will become more readily apparent by referring to the following detailed description of the disclosed subject matter in conjunction with the following drawings, which use similar reference numerals for similar elements.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1A and FIG. 1B are diagrams comparing the mapping of physical measurement CR-OCT when using a non-degenerate frequency comb light source (FIG. 1A) and when using a degenerate (chirped) frequency comb light source (FIG. 1B). (ZPL = zero path length). [Figure 2A] An experimental CR-OCT setup showing an SPML laser having a degenerate frequency comb output is shown. The Fresnel reflection of the silicon etalons results in a finesse of 2.1 per etalon and a total finesse of 5.1 after four etalons. LD is a laser module driver, DDG is a digital delay generator, PG is a pattern generator, A is an amplifier, EOM is an electro-optic modulator, PC is a polarization controller, CFBG is a continuous fiber Bragg grating, SOA is a semiconductor optical amplifier, FP is a Fabry-Perot etalon spectral filter, OSA is an optical spectrum analyzer, ISO is an optical isolator, and PG is a signal generator. Includes a photograph of a silicon wafer with permission from WaferPro. [Figure 2B] FIG. showing the measured etalon transmittance at 232.5 THz for 1 (green line), 2 (red line), and 4 (blue line) of a series of silicon wafers. [Figure 2C] Shows the etalon FSR across the spectrum. The red dots indicate experimental data and the blue line indicates the theoretical curve. [Figure 2D] Shows the degenerate frequency comb spectrum at the SPML output. [Figure 2E] Shows an enlarged plot of three comb lines at the SPML output. [Figure 3](A) to (D) show numerical simulations of CR depth signals using a degenerate frequency comb and a dispersive silicon etalon. (A) shows the simulated mirror signals for various set orders for a degenerate frequency comb spectrum centered at 1290 nm with a bandwidth of 100 nm. (B) shows the superimposed mirror signal from blue (Sb, centered at 1265 nm) and red (centered at 1315 nm, Sr) as a function of set order. (C) shows the A line of the CR indicating the band point spread function (PSF) and walk-off for blue and red at a 9th set order. (D) shows the extracted PSF width before and after correcting for the degenerate frequency comb chirp using the recovered order parameter and D = 0.54 × 10⁻³ ps². [Figure 4A] Figures 4A to 4D show an experimental implementation of absolute delay distance measurement using a degenerate frequency comb. Figure 4A shows the PSF calculated from the entire spectrum. [Figure 4B] The superimposed PSF from the blue and red subbands is shown. [Figure 4C] Figure 4A shows the PSF after dechirping using the calculated order parameter and D = 0.54 × 10⁻³ ps². [Figure 4D] The measured PSF width before (red, square) and after (black, circle) dechirpening is shown as a function of set order. [Figure 5A] Figures 5A–5C show absolute height topographic imaging of a stack of US one-cent coins using the proposed CR-OCT system. Figure 5A shows the photograph (top of Figure 5A) and the intensity depth projection (bottom of Figure 5A). [Figure 5B] The circular depth map corresponding to the lower part of Figure 5A is shown. [Figure 5C] The reconstructed absolute depth corresponding to the lower part of Figure 5A is shown. [Figure 6]The image shows the resolution of the absolute position of a surgical instrument using the proposed CR-OCT system. A surgical forceps is placed on a tissue (chicken skin) sample. The image shows the en-face intensity projection (top), a circular depth map (middle), and the recovered absolute depth map (bottom), where the color / shading scales represent depth (see the scales on the right in the middle and bottom panels). [Figure 7] The frequency comb shows interference fringes and frequency comb traces for modulated, tuned, and / or shifted sources. [Figure 8] This demonstrates depth determination using a light source equipped with a step laser and a sweep laser. [Figure 9] This is a diagram of an interferometer system that may be used in conjunction with the various embodiments disclosed herein. [Figure 10] Examples of systems for resolving absolute depth in CR-OCT according to several embodiments of the disclosed subject matter are shown. [Figure 11] Examples of hardware that can be used to implement computing devices and servers according to several embodiments of the disclosed subject matter are shown. [Figure 12] Examples of processes for resolving absolute depth in CR-OCT according to several embodiments of the disclosed subject matter are shown. [Modes for carrying out the invention]

[0018] According to some embodiments of the disclosed subject matter, a mechanism (which may include a system, method, and medium) for resolving absolute depth in circular ranging optical coherence tomography is provided.

[0019] Subsampling optical coherence tomography (OCT) is an imaging technique that offers high speed and can operate over long distances. Subsampling OCT achieves this by performing optical domain compression of the interference signal generated through the use of optical subsampling. Typically, this is achieved by using a frequency comb light source, i.e., a light source with individual spectral lines that are equally spaced at optical frequencies.

[0020] One possible drawback of subsampling OCT is that the absolute delay (and therefore position) of the sample is not measured. Instead, the relative position of the sample features is measured. This invention presents a method and apparatus that provides both compression and absolute delay / position sensing. In certain embodiments, this can be based on using an incomplete (or degenerate) frequency comb, i.e., one with spectral lines that are not intentionally placed on an equally spaced grid in optical frequency. This approach can be used in many different applications, including any application of subsampling OCT where knowing the absolute position of the sample is of additional value. This can be used, for example, to measure the distance between a surgical instrument and a tissue sample, or to measure the topography of a sample. The latter can be used in a variety of fields, such as performing dynamic autofocusing.

[0021] In Fourier-domain optical coherence tomography, an interference signal is generated across an RF bandwidth proportional to the product of three parameters: imaging range, imaging speed, and the reciprocal of the axial resolution. Circular ranging (CR) OCT architectures were introduced to facilitate long-range imaging by decoupling the imaging range from the signal RF bandwidth. As a result, current CR-OCT systems resolve the relative depth location rather than the absolute depth of scattering. An improved implementation of CR-OCT is introduced herein, which, in certain embodiments, uses a degenerate frequency comb light source that enables the retrieval of absolute depth information with minimal impact on the aforementioned RF bandwidth compression advantages of CR. It is shown that this degenerate frequency comb can be fabricated with relatively simple modifications to the excitation frequency comb light source design, and its absolute ranging capability is presented through imaging studies and simulations.

[0022] In Fourier-domain optical coherence tomography, the generated optical signal is acquired using an acquisition system with a finite electron bandwidth. In applications requiring a longer depth range compared to the 1-2 mm imaging depth of OCT for most samples, the majority of the acquired signal bandwidth is dedicated to measuring signal-void regions located in front of the sample surface or beyond the deepest imagingable depth. CR-OCT acquires these signals with higher data efficiency by superimposing equally spaced depth points onto a single measurement depth. This is done in the optical domain, allowing electron acquisition to operate with reduced bandwidth and less noise. In this way, CR-OCT decouples the imaging range and acquisition bandwidth, making simultaneous high-speed and long-range imaging more practical.

[0023] Figure 1A illustrates the CR-OCT technique, showing the circular mapping of the physical delay coordinate (z-axis) to the measured circular delay coordinate (xy-plane). It should be noted that each measured delay is a superposition of equally spaced sets of physical delays. This interval is referred to as the circular delay range. Importantly, this figure shows that only delay ranges smaller than the circular delay range are... This means that the distribution of the reflected signals can be resolved in the measured / circular delay space without overlapping artifacts. Circular distance measurement is obtained by a frequency comb light source combined with complex (in-phase and orthogonal) detection of interference fringes, and the circular delay range is proportional to the reciprocal of the free spectral range (FSR) of the frequency comb light source, i.e., Δτ = 1 / (2fsr). For convenience, a corresponding circular depth range can be defined as the product of the circular delay range and the speed of light in the image-forming material. Note that in this specification, the term delay is always defined in relation to the reference arm delay.

[0024] One consequence of the circular mapping shown in Figure 1A is that the absolute delay of the sample cannot be measured. This specification presents a method for resolving absolute delay in CR-OCT. Absolute distance measurement is achieved by employing a degenerate frequency comb, i.e., a frequency comb with unequally spaced optical frequency lines. It has been specifically shown that absolute position measurement is possible while maintaining a high compression factor (the core motivation underlying CR-OCT) using a chirp frequency degenerate comb light source. Furthermore, it has been shown that it is straightforward to modify existing CR-OCT sources to generate a degenerate comb output.

[0025] As a starting point, it is instructive to consider a simple approach to resolving absolute delay in a CR-OCT system using a "dual FSR" approach. Here, consider a first measurement using a first frequency comb light source with a given FSR, i.e., fsr1. Then, a given signal can be localized to a delay imposed by an unknown integer part (called the order) of the cycle within the corresponding circular delay range [Δτ = 1 / (2fsr1)]. If a second measurement of the same sample is acquired with another FSR, i.e., fsr2, the signal may appear with different circular delays depending on its absolute position. In this way, by measuring the relationship between the circular delays measured using fsr1 and fsr2 respectively, the integer order parameter in each measurement can be extracted, and thereby the absolute delay can be extracted. This can be analogous to measuring the elapsed time of an event using a stopwatch that has 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 operates a known amount faster than the original stopwatch, it becomes possible to calculate the minutes (i.e., the order) using the difference in the second hand measurements of the two stopwatches.

[0026] While easy to understand, the dual FSR method can have drawbacks, at least in certain applications. Firstly, while it is possible to generate two frequency combs with different FSRs, this can complicate the overall light source, for example, by using two frequency comb light sources to illuminate the sample simultaneously or sequentially, or by using a single frequency comb light source that can adjust (change) the free spectral range between images or A-lines. Secondly, acquiring two A-lines is required to add absolute depth measurements, which doubles the number of measurements and reduces the efficiency / compression advantages of the CR approach. Nevertheless, for certain applications, this trade-off may be acceptable in exchange for absolute depth information and a more complex light source and slightly slower acquisition speed.

[0027] This study presents an implementation based on a similar principle, but with a simpler light source implementation and minimized compression penalties. Instead of using two separate 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, namely, the change in FSR across optical frequencies provides a clock with a changing speed that can be used to decode the absolute delay. However, in this case, the information is contained within a single depth measurement (A line). Furthermore, the chirped FSR can be created simply using a dispersed Fabry-Perot etalon in a laser light source, where 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 etalon thickness, and c is the speed of light. Figures 1A and 1B show a conventional single FSR. We compare the absolute-to-measured delay mapping of the proposed approach (Figure 1B) with that of the CR-OCT approach (Figure 1A). By using a degenerate frequency comb, the spiral mapping of the physical delay to the measured delay becomes dependent on the optical frequency, thus resulting in chirps for three discrete optical frequencies and three delays as shown in Figure 1B. The absolute delay is calculated using the variance in the spectrally resolved circular delay (in this case, dependent on the optical frequency). Knowing the absolute delay of the scattered signal allows us to dechirp the measured interference fringes to generate an image with transform-limited axial resolution resolved over the absolute delay.

[0028] In various embodiments, the CR-OCT system may be implemented using a degenerate frequency comb by arranging a dispersed Fabry-Perot etalon at 1.29 μm within a stretched pulsed active mode-locked (SPML) laser architecture (Figure 2A). The DFC-SPML had a sweep rate of 194 THz / μs and a repetition rate of 7.6 MHz at a 76% duty cycle. Approximately 1750 ps at 1.3 μm. 2A silicon (Si) wafer providing a group velocity dispersion of 1 / km was used as the etalon. The 280 μm thick wafer provided an FSR of 146.7 GHz at the central optical frequency and a circular depth range of approximately 1 mm in air. Single-pass transmittance and frequency-dependent FSR are shown in Figures 2B and 2C. The spectrum at the DFC-SPML output and magnified plots of the three comb wires are shown in Figures 2D and 2E. The coherence length of the light source was measured to be approximately 2 cm (both sides FWHM around the zero-pass length). This light source, combined with a simple interferometer and acquisition system, yielded measured interference fringe signals. Complex demodulation of the interference fringe signals was performed using an active (LiNbO3) phase modulator on the reference arm.

[0029] A numerically ordered model of the CR-OCT interference fringe signal was generated from the aforementioned system, i.e., using the same light source characteristics and interferometer design. Using this model, a processing pipeline was constructed and verified to extract absolute depth information from the mirror interference fringe signal. As a starting point, Figure 3(A) presents the simulated point spread function (PSF) for the mirror signal generated by processing the degenerate frequency comb signal using a conventional CR-OCT approach. The mirrors were placed at the same circular depth (0 mm), but the circular depth ranges were of different orders (the circular depth range is defined by the average FSR of the DFC). Here, we can see the expected chirp (blurring) of the PSF due to the use of the degenerate frequency comb. Next, a dual-band processing approach was used to separately analyze the interference fringes detected in separate bands, referred to herein as the "blue" spectral band and the "red" spectral band, centered at 1265 nm (237.2 THz) and 1315 nm (228.1 THz), respectively. The difference in average FSR within each band is approximately Δ = 1.3 GHz, and fsr r >fsr b (Δτ r <Δτ b) It was. The PSF simulated for the same mirror position is plotted in Fig. 3(B). Here, the overall chirp in Fig. 3(A) can be decomposed into two PSFs that walk off from each other. Using the measured depth shift ΔL between the blue-band PSF and the red-band PSF (Fig. 3(C)), the order parameter k is calculated as follows.

[0030] [Number]

[0031] In the formula, fsr b,r are the average FSRs in the blue band and the red band, respectively.

[0032] Next, a technique for removing the chirp (axial blur ) caused by the use of the degenerate frequency comb (see Fig. 3(A)) was developed. The corrected interference fringe signal s’(ω i ) is calculated by multiplying the measured complex interference fringe s(ω i ) by the correction vector given by exp[iDk(ω - ω0) 2 , where k is the integer depth order parameter (Fig. 3(D)). The parameter D is given as follows according to the characteristics of the degenerate frequency comb.

[0033] [Number]

[0034] Next, this processing pipeline was applied to experimental data. Figures 4A to 4D show PSF measurements at a circular depth of approximately 250 μm using a degenerate frequency comb for order parameters in the range of ±10, acquired similarly to the simulated data in Figures 3(A) to 3(D). Figures 4A and 4B show the PSF using the full band and the red spectral band / blue spectral band, respectively. PSF walk-off was extracted by A-line cross-correlation. Note that in practice, PSF wrapping at the edges of the circular depth range must be considered. At these boundaries, the true PSF shift is given by the measured shift ΔL' and the blue band circular depth range (because fsr r >fsr b Therefore, ΔL = ΔL' ± cΔτ b The order calculated from the measured PSF shift in Figure 4B was confirmed to match the set order of each measurement (set order of -10 to 10). Using this solved order parameter, the chirp (spread) of the full spectral data was corrected as described in equation (1) to generate the dechirp PSF (Figure 4C). The recovered PSF width before and after correction is shown in Figure 4D. Due to PSF asymmetry, the root mean square is used.

number

[0035] To test the reconstruction of absolute depth information from a sample, a stack of nine US one-cent coins was imaged (Figure 5A). Circular wrap images and absolute topographic images are shown in Figures 5B and 5C, respectively. It should be noted that the absolute images, while covering a much larger depth range, retain the same height resolution (given in axial resolution) as the conventional circular wrap image depth range. The measured height error is thought to stem from a low measured SNR, and research into SNR requirements and noise reduction strategies in absolute delay-resolution CR-OCT is ongoing and beyond the scope of this study.

[0036] In the second experiment, this technique was used to provide feedback on the absolute positioning of surgical instruments relative to the tissue surface (Figure 6). Figure 6 shows an image illustrating the resolution of the absolute position of a surgical instrument using the proposed CR-OCT system. A surgical forceps was placed on a tissue (chicken skin) sample. The image shows the EN-FACE intensity projection (top), a circular depth map (middle), and a recovered absolute depth map (bottom), where the color / shading scales represent depth (see the scales on the right in the middle and bottom panels). The absolute topographic image (bottom), rather than the conventional circularly wrapped CR topographic image (middle), allows for the measurement of the instrument's proximity to the tissue. This absolute depth mapping technique enables applications such as surgical guidance with high depth perception, and real-time stereoscopic vision will be beneficial in visualizing complex organs within the surgical field during procedures.

[0037] Finally, a brief comment on the magnitude of etalon variance required to perform absolute distance measurement. Let's discuss this. Returning to the dual FSR technique as a simplified example, by setting ΔL=δZ for k=1 in equation (1) (where δZ is the Fourier transform limiting axial resolution of each measurement), we can determine the minimum FSR difference required to produce a detectable shift, Δ min This can be estimated and obtained as follows.

[0038]

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[0039] During the ceremony,

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[0040]

number

[0041] In the formula, k max =l c / (2ΔL k=l ) and l c ΔL is the coherence length. k=l ΔL for k=1, i.e., the difference in the circular depth range. Note that in the degenerate frequency comb technique, this limitation can be overcome by shifting the spectral window closer to the center frequency, thereby reducing the difference FSR between measurements.

[0042] In this study, we numerically and experimentally explained and verified that absolute delay position can be recovered without significantly affecting measurement compression using a degenerate frequency comb constructed from a dispersed Fabry-Perot etalon. The current research focuses on the principle and primary implementation, and therefore has several constraints. Firstly, uncoated Si etalons (Si wafers) are used, and multiple etalons are cascaded for the resulting low finesse. Custom-designed coated silicon (or other dispersed material) etalons, offering higher finesse values, would improve system performance, including coherence length and measurement signal-to-noise ratio. The processing approach uses simple red / blue subband analysis and PSF shift measurement based on cross-correlation. This could potentially be enhanced to better utilize continuous chirpping occurring across spectral bands. In this context, constructing an absolute delay resolution algorithm that minimizes the processing penalty compared to conventional CR-OCT is crucial and is the subject of ongoing research.

[0043] In various embodiments, the above procedure can be carried out using several different types of electromagnetic radiation sources, and in some examples, the procedure can be modified depending on the type of radiation source used.

[0044] <Dual FSR> In some embodiments, the source may be modified to generate a frequency comb having two different FSRs, as in the “dual FSR” approach disclosed above. In this approach, two A-lines are collected for each data point, one A-line is collected for each of the two different FSRs, and the A-line data corresponding to the two FSRs are combined to determine the absolute depth.

[0045] <Degenerate Frequency Com> In embodiments disclosed herein, absolute depth can be determined using a source that generates a degenerate frequency comb. In addition to the above embodiments for creating a degenerate (chirp) source using a dispersive etalon, other embodiments can create a degenerate frequency comb using a prism or a programmable laser. In various embodiments, the above technique of generating a degenerate frequency comb using an SPML laser can also be achieved using other types of laser sources such as PCML, FDML, or typical external cavity lasers. Furthermore, the degenerate frequency comb created by any particular technique does not need to be continuous.

[0046] <Modulation / Tuning / Shifting of Frequency Comb Wires> In some embodiments, a source can be used that generates a step frequency comb with a known FSR, in which the frequency comb line is shifted, tuned, and / or modulated in the spectral domain by a known amount (Δf) using a frequency shifter, phase modulator, etalon angle adjustment means, or other suitable means. For example, in some embodiments, a frequency shifter or phase modulator can be placed at the laser output in front of the interferometer. In other embodiments, etalon angle adjustment may be performed inside the laser cavity by angle-adjusting the spectral filter itself that generates the frequency comb (or by changing the refractive index or other means). Similar to the dual FSR approach described above, two A lines (depth signals) are detected, which are obtained with and without comb line shift / tuning / modulation.

[0047] The comb shift in the spectral domain yields a τ-dependent phase shift Δφ between the measured interference fringe signals (depth signals). Figure 7 shows an example of a continuous sweep interference fringe with two different delays τ1 and τ2 in the time and optical frequency domains, and the frequency domain trace shows the frequency comb shift between the two delays. The measured phase shift Δφ can be seen in the stepped interference fringe in Figure 7 and is used to solve for absolute depth. The phase difference at the baseband edge is given by the following equation.

[0048]

number

[0049] The absolute depth is determined as follows:

[0050]

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[0051] In general, the frequency shift Δf must be small enough to avoid the π-phase ambiguity of the measured phase shift at the maximum target depth.

number

[0052] In the formula, N k is the number of degrees within the coherence length.

[0053] <Step laser, plus sweeping laser> In some embodiments, absolute depth information from a sample can be obtained using a combination of a stepped laser source and a swept laser source. A stepped laser with a known FSR (laser 1, Figure 8) may be used to detect the A-line (depth signal) from the sample. As described above, this generates depth ambiguity for a circular area (Δτ=1 / FSR, LB=c / FSR / 2) in the A-line collected using laser 1. To supplement this data, a continuous swept laser with a narrow linewidth (long coherence length) (laser 2, Figure 8) can be used to obtain a second A-line (depth signal) from the same sample (e.g., cornea and / or lens, upper right, as shown in Figure 8). By comparing the relative depth (order) of the obtained depth signals with the absolute depth obtained from the stepped laser (laser 1) versus the swept laser (laser 2), the absolute depth of the data obtained using the frequency comb of laser 1 can be determined.

[0054] Generally, the axial resolution of laser 2 should be half or better than the circular range (LB) of laser 1. Furthermore, to maximize the sweep speed of laser 2, its optical bandwidth must be kept to a minimum; ideally, the A-line rates of laser 1 and laser 2 should be the same or very similar. Finally, the coherence length of laser 2 should be the same as or greater than that of laser 1.

[0055] Figure 9 shows an interferometer system that can be used with various embodiments of the present invention. Figure 9 illustrates a Mach-Zehnder interferometer that can be given using a free-space optical system (Figure 9, panel A) or a fiber optic configuration (Figure 9, panel B). Other types of interferometers (e.g., Michelson interferometers) can also be applied. The light source LS in either panel A or panel B of Figure 9 may be an SPML laser with a degenerate frequency comb as shown in Figure 2 or another type of laser (e.g., PCML, FDML, or conventional external cavity laser), a light source in which the lines of a step frequency comb are shifted, tuned, or modulated, a dual FSR light source, or a light source in which a step frequency comb is combined with a sweep laser.

[0056] The beam B9 emitted from the LS is directed to the interferometer input, where it is split into two paths of approximately equal length using a beam splitter (BS3). B10 is directed to the sample S. Then, the backscattered light from the object of interest is directed to the interferometer output (B11). In the reference arm, beam B12 is directed to the phase modulator (PM). The beam after the PM (i.e., beam B13) is directed to the interferometer output by BS4. The output beam B14 is then coupled with beam B11 and interferes with it. The output beam B14 is then detected by detector D (e.g., a photodiode). Alternatively, a fiber-based interferometer, as shown in Figure 2B, allows for immediate balanced detection with a π phase shift between output beams B14 and B15. The detected signal is digitized at a sampling rate fs using a data acquisition and processing system (which may include a data acquisition board or a real-time oscilloscope (DAQ)). Multiple wavelength sweeps (A1, A2, ..., An) may be acquired to form a two-dimensional or three-dimensional image.

[0057] Referring to Figure 10, an example 1000 of a system for resolving absolute depth in CR-OCT (e.g., a data acquisition and processing system) is shown according to some embodiments of the disclosed subject matter. As shown in Figure 10, a computing device 1010 can receive interference data from an optical interferometer system 1000. In some embodiments, the computing device 1010 can perform at least part of the absolute depth resolving system 1004 to determine the absolute depth based on the interference data received from the optical interferometer system 1000. Additionally or alternatively, in some embodiments, the computing device 1010 can transmit information about the interference data received from the optical interferometer system 1000 to a server 1020 via a communication network 1006, and the communication network 1006 can perform at least part of the absolute depth resolving system 1004 to determine the absolute depth based on the interference data. In some such embodiments, the server 1020 can return to the computing device 1010 (and / or any other suitable computing device) information indicating the output of the system 1004 for resolving absolute depth, such as absolute depth information. This information may be transmitted to and / or presented to a user (e.g., a researcher, operator, clinician, etc.) and / or stored (e.g., as part of a research database or medical records related to the subject).

[0058] In some embodiments, the computing device 1010 and / or server 1020 can be any suitable combination of computing devices or apparatus, such as a desktop computer, laptop computer, smartphone, tablet computer, wearable computer, server computer, or virtual machine running on a physical computing device. As described herein, the system 1004 for resolving absolute depth can present information about interference data and / or absolute depth information to a user (e.g., a researcher and / or physician).

[0059] In some embodiments, the optical interferometer system 1000 may include an electromagnetic radiation source 1002, which may be any radiation source suitable for optical interferometry such as CR-OCT. In other embodiments, the electromagnetic radiation source 1002 may be located near a computing device 1010. For example, the electromagnetic radiation source 1002 may be combined with the computing device 1010 (for example, the computing device 1010 may be configured as part of a device for acquiring and / or storing optical interferometry information). In other embodiments, the electromagnetic radiation source 1002 may be connected to the computing device 1010 by a cable or a direct wireless link. Additionally or alternatively, in some embodiments, the electromagnetic radiation source 1002 may be located near and / or away from the computing device 1010 and can transmit information to the computing device 1010 (and / or server 1020) via a communication network (e.g., communication network 1006).

[0060] In some embodiments, the communication network 1006 may be any suitable communication network or a combination of communication networks. For example, the communication network 1006 may be a Wi-Fi® network (one or more wireless routers, one or more This may include switches, peer-to-peer networks (e.g., Bluetooth® networks), cellular networks (e.g., 3G networks, 4G networks, etc., compliant with any suitable standard such as CDMA, GSM, LTE, LTE ADVANCED, WiMAX, etc.), wired networks, etc. In some embodiments, the communication network 1006 may be a local area network, a wide area network, a public network (e.g., the Internet), a private or semi-private network (e.g., an intranet of a company or university), any other suitable type of network, or any combination of any suitable networks. The communication links shown in Figure 10 may each be any suitable communication link or combination of communication links, such as a wired link, fiber optic link, Wi-Fi® link, Bluetooth® link, cellular link, etc.

[0061] Figure 11 shows an example of hardware 1100 that can be used to realize computing devices 1010 and servers 1020 according to some embodiments of the disclosed subject matter. As shown in Figure 11, in some embodiments, computing device 1010 may comprise a processor 1102, a display 1104, one or more inputs 1106, one or more communication systems 1108, and / or memory 1110. In some embodiments, the processor 1102 may be any suitable hardware processor or combination of processors, such as a central processing unit or graphics processing unit. In some embodiments, the display 1104 may comprise any suitable display device, such as a computer monitor, a touchscreen, or a television. In some embodiments, the inputs 1106 may comprise any suitable input devices and / or sensors that can be used to receive user input, such as a keyboard, mouse, touchscreen, or microphone.

[0062] In some embodiments, the communication system 1108 may include any suitable hardware, firmware, and / or software for communicating 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. In more specific examples, 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, and the like.

[0063] In some embodiments, memory 1110 may include one or more suitable storage devices that can be used to store instructions, values, etc., which can be used, for example, for the processor 1102 to present content via the display 1104 or to communicate with the server 1020 via a (multiple) communication system 1108, etc. Memory 1110 may include any suitable volatile memory, non-volatile memory, storage, 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, a computer program for controlling the operation of the computing device 1010 may be encoded in memory 1110. In such embodiments, the processor 1102 can execute at least a portion of the computer program to present content (e.g., images, user interfaces, graphics, tables, etc.), receive content from the server 1020, send information to the server 1020, etc.

[0064] In some embodiments, the server 1020 includes a processor 1112 and a display 11 14. The system may include one or more inputs 1116, one or more communication systems 1118, and / or memory 1120. In some embodiments, the processor 1112 may be any suitable hardware processor or combination of processors, such as a central processing unit or graphics processing unit. In some embodiments, the display 1114 may include any suitable display device, such as a computer monitor, touchscreen, or television. In some embodiments, the 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, or microphone.

[0065] In some embodiments, the communication system 1118 may include any suitable hardware, firmware, and / or software for communicating 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. In more specific examples, 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, and the like.

[0066] In some embodiments, memory 1120 may include one or more suitable storage devices that can be used to store instructions, values, etc., which can be used, for example, for the processor 1112 to present content via the display 1114 or to communicate with one or more computing devices 1010. Memory 1120 may include any suitable volatile memory, non-volatile memory, storage, 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, a server program for controlling the operation of server 1020 may be encoded in memory 1120. In such embodiments, the processor 1112 may execute at least a portion of the server program to transmit information and / or content (e.g., results of organizational identification and / or classification, 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 computers, laptops, tablet computers, smartphones, etc.), etc.

[0067] In some embodiments, instructions for performing the functions and / or processes described herein can be stored using any suitable computer-readable medium. For example, in some embodiments, the computer-readable medium may be temporary or non-temporary. For example, non-temporary computer-readable medium may include magnetic media (hard disks, floppy disks, etc.), optical media (compact disks, digital video disks, Blu-ray Discs®, etc.), semiconductor media (RAM, flash memory, electrically programmable read-only memory (EPROM), electrically processable programmable read-only memory (EEPROM), etc.), any suitable medium that is not temporary or lacks similarity of persistence during transmission, and / or any suitable tangible medium. As another example, temporary computer-readable medium may include signals in a network, in a wire, in a conductor, in an optical fiber, in a circuit, or in any suitable temporary medium that lacks similarity of persistence during transmission, and / or may include any suitable intangible medium.

[0068] In some embodiments, the optical signal is detected by a photodiode. It should be noted that any optional electronic conversion device, including but not limited to photodetectors, photodiodes, line scanning and two-dimensional cameras, and photodiode arrays, can be used to perform this detection function.

[0069] It should be noted that the term “mechanism” as used herein may encompass hardware, software, firmware, or any appropriate combination thereof.

[0070] Figure 12 shows an example 1200 of a process for resolving absolute depth in CR-OCT according to several embodiments of the disclosed subject matter. As shown in Figure 12, in 1202, process 1200 can generate radiation to irradiate a sample located at the optical path depth, and an electromagnetic radiation source can provide radiation to the sample to facilitate the determination of the optical path depth in the sample. In 1204, process 1200 can send a first portion of radiation to the reference arm of the interferometer. In 1206, process 1200 can send a second portion of radiation to the sample arm of the interferometer. In 1208, process 1200 can interrogate the sample with the radiation sent to the sample arm to generate backscattered radiation. In 1210, process 1200 can collect the backscattered radiation from the sample. In 1212, process 1200 can generate interference fringes between the collected backscattered radiation and the radiation sent to the reference arm. Finally, in step 1214, process 1200 can calculate the optical path depth of the sample from the received interference fringes.

[0071] The steps described above in the process shown in Figure 12 can be executed or performed in any order or sequence, not limited to the order and sequence shown. Furthermore, some of the steps described above in the process shown in Figure 12 can be executed or performed substantially simultaneously or in parallel, as necessary, to reduce waiting and processing times.

[0072] Therefore, although the present invention has been described in relation to specific embodiments and examples, the present invention is not necessarily limited thereto, and many other embodiments, examples, uses, improvements, and developments of improvements from embodiments, examples, and uses are intended to be included in the claims.

Claims

1. An electromagnetic radiation source comprising a step frequency comb and a modulator for generating radiation to irradiate a sample located in the optical path depth, The electromagnetic radiation source is configured to provide the radiation to the sample in order to facilitate the determination of the optical path depth within the sample. The modulator modulates the frequency comb line from the step frequency comb by modulating by a specific amount to generate a change in frequency, and includes an electromagnetic radiation source. A reference arm to which the first portion of the radiation is fed, The sample arm from which the second portion of the radiation is delivered, A first optical subsystem coupled to the sample arm for examining the sample using the radiation sent to the sample arm and for collecting backscattered radiation from the sample, and An interferometer comprising a reference arm and a second optical subsystem coupled to the first optical subsystem for generating interference fringes between the collected backscattered radiation and the radiation sent to the reference arm, A data acquisition and processing system configured to communicate with the interferometer and calculate the optical path depth of the sample from the received interference fringes, Includes, The electromagnetic radiation source generates a first frequency comb having a first free spectral range and a second frequency comb having a second free spectral range different from the first free spectral range. The aforementioned data collection and processing system is Using the first frequency comb, a first interference dataset is obtained. Using the second frequency comb described above, a second interference dataset is obtained. Determine the phase shift between the first interference dataset and the second interference dataset. An apparatus configured to determine the optical path depth of the sample based on the phase shift.

2. The apparatus according to claim 1, wherein the modulator includes at least one of a frequency shifter, a phase modulator, or an etalon angle adjustment means.

3. The apparatus according to claim 1 or 2, wherein the electromagnetic radiation source comprises a stretched pulse active mode-locked laser.

4. The apparatus according to claim 1 or 2, wherein the electromagnetic radiation source comprises a dispersed Fabry-Perot etalon filter.

5. The apparatus according to claim 4, wherein the dispersed Fabry-Perot etalon filter comprises a silicon wafer.

6. The apparatus according to claim 1 or 2, wherein the first optical subsystem includes an optical circulator circuit that guides the radiation to the sample and the backscattered radiation from the sample to an optical waveguide.

7. The apparatus according to claim 1 or 2, wherein the reference arm comprises an active phase modulator for performing complex demodulation of the interference fringes.

8. The apparatus according to claim 1 or 2, wherein the data acquisition and processing system is further configured to calculate a plurality of optical path delays of the sample corresponding to a plurality of optical path depths within the sample when calculating the optical path depth of the sample.

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