Methods and devices for imaging and ranging through optical fibers

Abstract

Methods, device and systems for producing images using optical fibers are described that utilize synthetic wavelength imaging. An example method for determining one or more characteristics of an object includes providing a multiwavelength laser illumination to a multi-mode optical fiber, where the illumination includes distinct first and second wavelengths. At least a portion of the laser illumination after interactions with the object is received. The detector is also illuminated by reference beams that include the first and the second wavelengths. Based on signals detected by the detector, amplitude or phase values associated with a synthetic wavelength are determined which enable determination of the object characteristics. The synthetic wavelength is inversely proportional to a difference between the two wavelengths, which are selected such that optical path variations due to propagation of the laser illumination through the optical fiber are less than a fraction of the synthetic wavelength.

Description

METHODS AND DEVICES FOR IMAGING AND RANGING THROUGH OPTICALFIBERSCROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to the provisional application with serial number 63 / 668,077 titled “METHODS AND DEVICES FOR IMAGING AND RANGING THROUGH OPTICAL FIBERS,” filed July 5, 2024. The entire contents of the above noted provisional application are incorporated by reference as part of the disclosure of this document.TECHNICAL FIELD

[0002] The technology described in this patent document relates to optical systems and methods that use optical fibers for imaging and measurements.BACKGROUND

[0003] Optical fibers have been used for delivery of optical signals to enable a variety of different communications, imaging, sensing and other applications. It is desirable to improve imaging and detection capabilities of optical systems that rely on optical fibers for illumination and / or imaging.SUMMARY

[0004] The disclosed technology can be implemented in various embodiments to improve imaging of various objects and determination of various object characteristics that use optical fiber as a means of light delivery and / or light collection. In example embodiments, among other features and benefits, the disclosed methods and systems utilize synthetic wavelength imaging to enable three-dimensional imaging through optical fibers, which can be used, for example, for imaging regions in the human body through micro-endoscopes.

[0005] One example method for determining one or more characteristics of an object includes providing a multiwavelength laser illumination to a first end of a multi-mode optical fiber, where the multiwavelength laser illumination includes a first wavelength and a second wavelength, wherein the first wavelength is distinct from the second wavelength. The multiwavelength laser illumination propagates through the multi-mode optical fiber and exits a second end of the multimode optical fiber to illuminate the object. At a detector, at least a portion of the multiwavelength laser illumination after interactions with the object is received. The detector is also illuminated by reference beams including at least a first reference beam having the first wavelength, and a secondreference beam having the second wavelength. Based on signals detected by the detector, amplitude or phase values associated with a synthetic wavelength are determined. The synthetic wavelength is inversely proportional to a difference between at least the first and the second wavelengths, and the first and the second wavelengths are selected such that optical path variations due to propagation of the multi wavelength laser illumination through the multi-mode optical fiber are less than a fraction of the synthetic wavelength. One or more characteristics of the object are determined based on the determined amplitude or phase values.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 illustrates a procedure and associated diagrams for making measurements using synthetic wavelength imaging.

[0007] FIG. 2 illustrates an example set up for a single-shot synthetic field capture through a multi-mode fiber in accordance with an example embodiment.

[0008] FIG. 3 illustrates generating a phase front of a synthetic field based on the configuration of FIG. 2 in accordance with an example embodiment.

[0009] FIG. 4 illustrates a double-pass configuration that can be used to obtain information about a sample based on synthetic wave analyses in accordance with an example embodiment.

[0010] FIG. 5 illustrates a plot of synthetic wavelength versus optical wavelength difference that represents a tuning of optical carrier frequencies in accordance with some example embodiments.

[0011] FIG. 6 illustrates a set of synthetic phase front maps corresponding to six synthetic wavelength values in accordance with some example embodiments.

[0012] FIG. 7 illustrates optical pathlength variations in an example multi-mode optical fiber based on inter-modal dispersion.

[0013] FIG. 8 illustrates a block diagram of a device that can be used to implement certain aspects of the disclosed technology.

[0014] FIG. 9 illustrates a set of operations that can be carried out to determine one or more characteristics of an object in accordance with an example embodiment.DETAILED DESCRIPTION

[0015] Optical fibers have been used for delivery of optical signals to enable a variety of different communications, imaging, sensing and other applications. One non-limiting exampleapplication of optical fibers includes medical imaging. The small cross section of optical fibers makes them an attractive candidate for medical imaging applications like key-hole access with endoscopy, e.g., in neurology. In procedures such as brain surgery, the small cross section helps minimize the risk of tissue agitation near the imaging region.

[0016] Early uses of fibers in medical imaging included single-core fibers as light probes for fluorescence spectroscopy and fiber bundles for performing fluorescence imaging deep inside tissue. Small core fiber bundles are often used for medical imaging through keyhole incisions as their small cross-sectional area minimizes potential damage to surrounding tissues. Additionally, these fiber bundles are flexible, making endoscopic imaging and light delivery easier than other methods. However, these imaging practices through fiber bundles currently are largely limited to planar imaging. There is no widely adopted method to perform depth-sensitive or volumetric imaging through an optical fiber for medical or other applications.

[0017] While there are broadly adopted methods for three-dimensional (3D) imaging, such as coherence tomography and optical coherence tomography, these technologies mostly weigh a trade-off between penetration depth through tissue and lateral or depth resolution. However, if 3D images can be easily captured while using endoscopic tools, medical doctors would be able to pinpoint regions and probe for abnormal tissues in-situ. This potential benefit to 3D imaging is enhanced with smaller cross-sectional areas of probes, such as with fiber endoscopes, as it would be possible to probe regions of the human body outside of arteries or the digestive tract using minuscule keyhole incisions.

[0018] Multi-mode fibers (MMFs) permit multiple transverse optical modes in transmission. These modes have the capability to transfer phase information separate from one another, with the caveat that these signals are mixed-together due to scattering from the fiber. Notably, these modes become encoded during transmission resulting in a scrambling of transmitted information. Under coherent illumination, the interference of these different modes results in the formation of a speckle field that is not suitable for optical imaging and ranging applications, mainly because of the completely scrambled phase front in a speckle pattern. Methods have been developed to isolate these signals from one another through observation and correction of the fiber’s transmission matrix, which completely describes the wavefront changes of light propagating through a fiber. After the transmission matrix has been captured, the phase front travelling through the fiber canbe post-, or pre-corrected, e.g., via phase conjugation. However, the transmission matrix changes with respect to the orientation, bending, or light path through the fiber, meaning that it will change with slight bends or changes in the fiber path length. But these effects are nearly unavoidable when fibers are deployed in the medical field for probing and imaging, making this method inadequately suited for endoscopic measurements. Additionally, these techniques can involve expensive and complicated detection and correction equipment and techniques.

[0019] Synthetic wavelength imaging (SWI) presents a unique solution to 3D imaging through optical fibers, such as for regions in the human body through micro-endoscopes. Synthetic wavelengths can be computationally generated using optical field measurements conducted by relatively simple detectors to digitally construct a synthetic wavefront robust to scattering on the order of optical wavelengths. This feature of synthetic wavelengths comes from its larger size of the synthetic wavelength compared to optical wavelengths. As an optical fiber exhibits its own unique form of optical wavefront scrambling, synthetic wavefronts can be utilized to produce results that are insensitive to bends, movements and / or individual characteristics of the optical fiber.

[0020] PCT Publication No. W002024 / 112903 describes the general concept of SWI which exploits the spectral diversity of multiple measurements at multiple wavelengths to image objects with optically rough surfaces. It should be noted that the methodology described in PCT Publication No. W002024 / 112903 uses single-shot or multishift off-axis techniques to capture the synthetic filed, but it is understood other techniques such as phase shifting, temporal heterodyning techniques and other can also be used to capture the synthetic field.

[0021] The basic principle of SWI in the context of measurements from a rough surface is summarized as follows: An optically rough surface is illuminated with coherent light (at wavelength Xi), and the complex field E (Xi) scattered off the object’s surface is measured using an optical interferometer. This field exhibits strong wavefront aberrations due to speckle. Optical pathlength information (i.e., the shape of the object) cannot be recovered from E (Xi). Eventually, the static object is again illuminated with a slightly different wavelength X2, and a new field E (X2) is obtained. Assuming both illumination sources originate from the exact same location (e.g., the same fiber tip), the fields E (Xi) and E (X2) are subject to the same microscopic and macroscopic optical pathlength variations. After calculating the difference between their phasemaps,(Xi) - (X2), the phase aberrations imparted by the microscopic pathlength variations cancel each other out, and the phasemap difference only contains the macroscopic pathlength variations on the order of a “Synthetic Wavelength,” which can be represented as:A =^ hi T (i)

[0022] If Xi and X2 are spaced close enough, the resulting “synthetic field,” E (A), does not exhibit speckle artifacts and can be processed like a normal time of flight (ToF) camera image or optical interferogram. This means the object depth, z, can be calculated by:

[0023] One possible way to calculate the synthetic field E (A) is by computational mixing of E (Xi) and E (X2):

[0024] In Equation (3), E* (X2) denotes the complex conjugate of E (X2). The above technique can be performed using two sequentially captured camera images to reconstruct the object's depth maps: one would capture the first field and then repeat the measurement using a different wavelength to obtain the second field. Alternatively, this can be done by conducting a single shot measurement and leveraging the specific procedures described in PCT Publication No. W002024 / 112903 to obtain images, and explained in connection with FIG. 1 in the present patent document.

[0025] As shown in panel (a) of FIG. 1, the laser beams at two different wavelengths, Xi and X2, are coupled together, forming the object beam that illuminates the object. Panel (a) illustrates that the area around the head and shoulders of the object are illuminated. In panel (b), each of the two laser beams (having wavelengths Xi and X2) form a corresponding reference beam that directly illuminates the detector (e.g., CCD camera chip) at a particular angle. In the depicted example, one reference beam (e.g., Xi) encloses an angle with the horizontal x-axis, while the other reference beam (e.g., X2) encloses an angle with the vertical y-axis of the detector. The camera lens imagesthe scene onto the detector (camera chip).

[0026] When the object beam is scattered off the object surface, it forms a speckle field at each wavelength: E (Xi) and E (X2). These two speckle fields are incident on the detector and interfere with the reference beams. The speckle field at Xi produces a static interference pattern with the Xi- reference beam (in this example, vertical fringes), while the speckle field at X2 produces a static interference pattern with the ' / ^-reference beam (in this example, horizontal fringes). The camera image, I (x, y), shows speckles that are overlayed by crossed fringes. Panel (c) shows the camera image, and a zoomed-in section that exhibits a cross-hatched pattern. For the sake of completeness, it should be mentioned that interference also happens between the fields at Xi and X2. These interferences result in temporally oscillating fringes which oscillate much faster than the integration time of our camera and hence are not further discussed here.

[0027] After acquiring the Image, I (x, y), in single-shot, the complex speckle fields (phase and amplitude) E (Xi) and E (X2) required to form the synthetic field E (A) = E (k\) E* (X2) are retrieved via computational demodulation in the Fourier domain. Panel (d) in FIG. 1 illustrates the two-dimensional (2D) Fourier transform of the captured image, F [ I x,y) ], where five spectral regions with high intensity can be distinguished, which are denoted as (A) through (D). The central region (A) represents the DC component. The horizontal regions (left (B) and right (C)) are centered around the spatial carrier frequencies of the vertical fringes, which appear due to the interferences between the Xi-reference beam and the object field E (Xi). Similarly, the vertical regions (up (D) and down (E)) are centered around the carrier frequencies for the horizontal fringes produced by the reference and object field at X2.

[0028] The complex field, E (Xi), is retrieved by, for example, finding the carrier frequency, / 1, for the respective spectrum in the Fourier domain (box around (C) in panel (d)), then shifting the Fourier spectrum to set the evaluated carrier frequency, / 1, as new center frequency. Then the spectrum is filtered (e g., with a Hanning window or Gaussian kernel) so that only the frequency band around the new center frequency remains. The resulting filtered spectrum can be denoted by Fhor [ I (x, y) ]. Next, an inverse Fourier transform (IFT) of Fhor [ I (x, y) ] eventually delivers the phase (Xi) and amplitude \E (Xi)| of the complex field E (Xi) (see panel (e) in FIG. 1):

[0029] The complex field E (X2) is retrieved from the same image in as similar fashion as above, using the vertically arranged regions in the Fourier spectrum. Eventually, the synthetic field E (A) is formed via Equation (3), the synthetic phase (A) is extracted, and the depth map of the object is calculated via Equation (2).

[0030] FIG. 2 illustrates an example set up for a single-shot synthetic field capture through a multi-mode fiber in accordance with an example embodiment. Some components in FIG. 2 are similar to those in FIG. 1, including the reference beams (Xi and X2) that illuminate the detector directly. The camera includes a sensor (e.g., a CMOS sensor), an imaging lens, and an aperture. The reference beams each generate an interference fringe pattern, as described in connection with FIG. 1. The illumination is provided by a fiber tip (“Point Source Illuminator” in FIG. 2, approximating a self-illuminating point source). For example, light from two tunable lasers operating at distinct carrier wavelengths, Xi and X2, can be guided into the fiber (illumination source). It should be noted that while FIG. 2 provides a simplified diagram to illustrate the general concept, a more detailed configuration can include the object at the position of the point source, which is illuminated by the two wavelengths. For example, the multi wav elength illumination can be provided directly to the object or can be provided by the fiber itself. Additional explanation is provided in connection with FIG. 4.

[0031] In the setup of FIG. 2, the test fiber (e.g., an MMF with a core diameter of 62.5pm and a length of 51.0 cm) is placed after the point source. The exiting light from the MMF is collimated by an infinity-corrected microscope objective. The collimated light is imaged onto the sensor using an imaging lens. In this example configuration, reference beams, located in the imaging lens plane, are mounted perpendicular to one another with respect to the optical axis. Thus, interferograms are created on the surface of the sensor between each reference beam and their respective carrier wavelength from the image of the exiting fiber tip.

[0032] The results are shown in FIG. 3, which illustrates the generation of the phase front of the synthetic field. These results are obtained using a similar procedure described in connection with FIG. 1. It can be seen that the phase front of the synthetic field, < / > (A), appears planar when exiting the fiber, while the two optical phase fronts, <f> (Xi) and < / > (X2), show extensive speckle artifacts. In this implementation, a phase offset correction was applied to mitigate residual low- frequency phase offset.

[0033] While the example setup in FIG. 2 establishes the basic principles of using SWI in conjunction with a test fiber where the received beam passes through the fiber to reach the detector (single-pass configuration), these principles are applicable to imaging and / or ranging configurations, where the light beam passes through the MMF, reaches an object, is reflected / scattered from the object, and traverses back through the MMF before reaching the detector (double-pass applications, e.g., an endoscope). FIG. 4 illustrates an example of such a double-pass configuration. In the example configuration of FIG. 4, multiwavelength light from a light source is collimated by a lens, is reflected from a beamsplitter, and focused by collimating / focusing lens onto a first end of a fiber. Light, after propagation through the fiber, illuminates the sample through a second end of the fiber. The return light is collected by the fiber, reaches the collimating lens, and passes through the beamsplitter before reaching the detector (depicted as imaging optics and sensor). Using the disclosed techniques based on synthetic wave analyses, imaging, ranging and / or volumetric information about the sample can be obtained. It should be noted that while in the example configuration of FIG. 4, the second end of the fiber is not in direct contact with the sample, in some examples, the second end of the fiber can be in direct contact with the sample.

[0034] Based on the disclosed SWI techniques, the synthetic wavelength, A, which primarily depends on the difference between Xi and X2, can be chosen within a wide range of values. For example, FIG. 5 illustrates an example range of possible synthetic wavelengths with a mean value of 850 nm, based on the tuning range of the optical carrier frequencies available for illumination. This plot shows the synthetic wavelength generated with two lasers ±AX / 2 away from the mean wavelength ( = 850 nm). The dotted line shows accessible synthetic wavelengths given the tuning range and spectral linewidth of the example lasers used. The dotted line coincides with the ideal case over a particular range of AX, and is thus not readily distinguishable from the ideal curve, other than via the two filled circles.

[0035] Additionally, the use of synthetic wavelength provides considerable flexibility for selection of the two reference wavelengths. Notably, the same synthetic wavelength, A, can be realized for different carrier wavelength pairs, Xi and X2, across the spectrum. For example, the two wavelengths can be selected in UV, IR, visible or other band(s). In this way, the desired illumination wavelengths can be selected to allow achieving a particular image contrast and / orselected based on reflectance or transmission properties of a target object. In some embodiments, after the selection of an initial pair of illumination wavelengths, the selection can be changed as part of a fine-tuning process, or to provide illumination at a different band as may be needed. Furthermore, because A» i and 2, the influence of scattering, which is disruptive at optical wavelengths, is significantly reduced.

[0036] One application of the disclosed technology relates to measuring the distance to an object and / or depth or surface variations of an object (collectively “ranging” applications) that can be obtained via, for example, time of flight (ToF) sensing. As the measured depth in ranging applications is directly proportional to the synthetic wavelength, A, (see, e.g., Equation (2)), it is beneficial to test the robustness of the approach at different synthetic wavelengths. In general, longer synthetic wavelengths are more robust to phase changes from optical scattering. However, this comes at the expense of the ability to resolve fine structures. As part of an experiment, we tested synthetic wavelengths that ranged from 0.4 mm up to 62.3 mm associated with an MMF. A selection of captured synthetic phase maps is shown in FIG. 6. The smooth phase front captured in the synthetic field indicates that the MMF behaves comparable to a single-mode fiber at the synthetic wavelength. For synthetic wavelengths less than about 1 mm, we observe synthetic speckle in the synthetic phase front for this particular fiber, indicating that the MMF-induced optical path length difference became too large to be corrected.

[0037] One aspect associated with using synthetic wavelengths for 3D imaging is when the two carrier (or reference) beams have a larger wavelength separation, their speckle patterns begin to decorrelate. One symptom of this spectral decorrelation is an appearance of speckle-like artifacts in the synthetic phase-front. The appearance of this synthetic speckle is related to the Rayleigh quarter wave criterion shown in Equation (5).Tmax < A / 4 (5)

[0038] The phase front from the synthetic wave is resolvable so long as the maximum wavefront error, max, is less than one quarter the length of the synthetic wavelength, A. This sets a lower limit for depth resolution when imaging synthetic waves through scattering environments. The criteria in the context of a scattering medium, such as an MMF, can be stated as requiring the optical pathlength variations within the fiber (or scattering media) to be less than or equal to A / 4, for a double-pass configuration. This value becomes A / 2 for a single-pass configuration. Ingeneral, this criterion ensures that the beams corresponding to largest and the shortest optical pathlengths destructively interfere, and / or an interference pattern at full contrast is achieved.

[0039] FIG. 7 illustrates the optical pathlength variations in an example multi-mode optical fiber based on inter-modal dispersion. MMFs permit multiple transverse optical modes in transmission. The multi-mode optical fiber can be treated as a thick scattering medium with some unique properties, such as mode-mixing and inter-modal dispersion. As light transmits through the MMF, most of its energy will transmit as one of the fibers guided modes. A fraction of this energy, however, will either be lost to dissipation into the cladding of the fiber or will change to a different guiding mode through mode-mixing. The level at which the energy will switch modes or dissipate is dependent on the change in refractive index along the fiber and across its core, which can change when bends or strain is introduced on the fiber. Alongside mode-mixing, coherent light rays will travel through the fiber with slightly changing distances dependent on their entrance angle, resulting in a temporal spread of the rays as they exit the fiber. This is known as inter-modal dispersion. This effect arises from light rays propagating at different speeds through the fiber due to shorter or longer path lengths. The fastest a light ray can travel through a step-index or uniform refractive index MMF is directly through the center, parallel to the surface normal of the entrance tip. In contrast, the longest path for a light ray to take through MMFs is at the acceptance angle of the fiber, which is constrained by the fiber’s numerical aperture NA. The change in path length through a MMF is shown in FIG. 7.

[0040] The previously discussed criteria that sets the optical pathlength variations to be within a fraction of the synthetic wavelength already includes the effects of refractive index and the length of the optical fiber which are incorporated as part of the optical pathlength calculus. As one example, for a graded index (GRIN) multi-mode fiber with optical mode number 1 (OM1) of about 1 meter long, the synthetic wavelength should be greater than or equal to 3 mm when carrier beams with a center wavelength of 850 nm are used. This criterion provides a rough guideline for the shortest wavelength which remains coherent through group velocity delay from the multi-mode fiber. This value can be tuned based on, for example, analysis of images or detections signals as may be needed.

[0041] The disclosed embodiments can be implemented using different multi-mode optical fibers, including, for example, step-index or graded index fibers. Additionally, while a singlemulti-mode fiber has been used to facilitate the description of the disclosed technology, in some embodiments, multiple optical fibers, such as multiple multi-mode fibers arranged as a collection of fibers in a fiber bundle, and / or a multi-core fiber can be used. Notably, a multi-core fiber acts similar to a large bundle of multi-mode fibers. Each core in the multi-core fiber transmits light independently from another core. The number of modes in each core is reduced due to size, but optical phase scrambling still occurs in each core due to inter-modal dispersion. In such a configuration, each core transmits one synthetic field value from one object point and imitates a camera “pixel” that allows an extended area of the object to be imaged. This concept is applicable to other collections of multi-mode fibers, where a plurality of spatially-separated fibers provide imaging and / or ranging capabilities.

[0042] The disclosed techniques for a single MMF allows for imaging of an extended area of the object by using a collection of fibers as described above, and / or by moving the fiber or the object to scan the area of interest. An important aspect of the disclosed embodiments is their immunity to movement or bends in the optical fiber. Notably, the synthetic field is robust to changes in the fiber transmission function from movements or changes in the fiber light path due the relatively large value of the synthetic wavelength compared to perturbations caused by the fiber movement or bends in the optical fiber.

[0043] As noted earlier, the disclosed technology may be implemented as part an imaging device, such as an endoscope, for obtaining information medical imaging and ranging information. In one example embodiment, an MMF can be used as part of a medical imaging device to deliver a multi wavelength illumination to a tissue, receive reflected / scattered illumination from the target tissue, and deliver the received light to a detector. The detected signals can then be processed, using similar operations discussed in connection with FIGS. 1 and 2, and the imaging / ranging / volumetric information can be extracted based on synthetic wavelength computations. It should be noted that the use of fibers or fiber bundles with cores smaller than the synthetic wavelength can further enhance the spatial resolution of said endoscopic measurements: If the fiber tip is placed close to the object under test, the object surface is located in the near field of the synthetic wave, meaning that the spatial resolution is not limited by the synthetic wavelength anymore. This procedure can be seen as an equivalent to “scanning near-field optical microscopy” in the synthetic wavelength domain.

[0044] Another feature of the disclosed embodiments is that the detector does not need to be a complicated or expensive device, and it suffices to include the basic functionality of detecting signals in the particular range of illumination that is received thereon. For example, the detector can include a CMOS detector.

[0045] It should be noted that in example configurations of the disclosed technology that the fiber tip is used to receive the light from the object (e.g., endoscopic configurations), there is no need for imaging optics at the object side of the fiber, thus improving the “endoscope tip” by making is less bulky and easier to insert. If inclusion of a lens is needed, a fiber lens can be used or the lens (or even a lens array or hologram in case of multi-core fibers) can be printed on the fiber tip.

[0046] It should be noted that the use of synthetic wavelengths can be extended to include more than two wavelengths. For example, in some example embodiments, three illumination wavelengths can used along with three reference beams that are positioned to illuminate the detector directly. For instance, the three reference beams can be separated by 60° around the aperture (instead of 90° as was illustrated for 2 wavelengths). In this scenario, the transform domain signal can include another pair of maxima that can be isolated and demodulated. As a result, three optical fields at three different optical wavelengths are obtained that are subsequently combined to two different synthetic fields at two different synthetic wavelengths. This allows performing multi -frequency phase unwrapping in only one shot. This scenario can be generalized to more than three wavelengths.

[0047] FIG. 9 illustrates a set of operations that can be carried out to determine one or more characteristics of an object in accordance with an example embodiment. At 902, a multiwavelength laser illumination is provided to a first end of a multi-mode optical fiber, wherein the multi wavelength laser illumination comprises a first wavelength and a second wavelength, wherein the first wavelength is distinct from the second wavelength. At 904, the multiwavelength laser illumination is allowed to propagate through the multi-mode optical fiber and exit a second end of the multi-mode optical fiber to illuminate the object. At 906, at least a portion of the multiwavelength laser illumination after interactions with the object is received at a detector, wherein the detector is also illuminated by reference beams including at least a first reference beam having the first wavelength, and a second reference beam having the second wavelength. At 908,based on signals detected by the detector, amplitude or phase values associated with a synthetic wavelength are determined, wherein the synthetic wavelength is inversely proportional to a difference between at least the first and the second wavelengths, and the first and the second wavelengths are selected such that optical path variations due to propagation of the multi wavelength laser illumination through the multi-mode optical fiber are less than a fraction of the synthetic wavelength. At 910, one or more characteristics of the object are determined based the determined amplitude or phase values.

[0048] In one example embodiment, the multi wavelength laser illumination after the interactions with the object is received by the second end of the multi-mode optical fiber, traverses through the multi-mode optical fiber, and exits the first end of the multi-mode optical fiber before reaching the detector. In another example embodiment, the first wavelength is represented by Xi, and the second wavelength is represented by X2, and the synthetic wavelength, A, is determined according to Equation (1).

[0049] According to another example embodiment, the first and the second wavelengths are selected such that the synthetic wavelength is at least one order of magnitude greater than each of the first and the second wavelengths. In another example embodiment, the first and the second wavelengths are selected such that the optical path variations due to propagation of the multi wavelength laser illumination through the multi-mode optical fiber is less than or equal to one-half of the synthetic wavelength. In yet another example embodiment, the first and the second wavelengths are selected such that the optical path variations due to propagation of the multi wavelength light through the multi-mode optical fiber is less than or equal to one-fourth of the synthetic wavelength. In still another example embodiment, the first and the second wavelengths are selected (a) to produce a particular image contrast or (b) based on reflectance or transmission properties of the object.

[0050] In one example embodiment, the first and the second wavelengths are selected such that a difference between a largest and a smallest optical pathlength through the multi-mode optical fiber allows destructive interference of light that is received at the detector, or an interference pattern at full contrast is achieved. In still another example embodiment, the above noted method includes analyzing information obtained from the signals detected by the detector, and modifying one or both of the first or the second wavelengths to produce a modified multiwavelength laserillumination. In yet another example embodiment, the method further includes analyzing information obtained from signals detected by the detector in response to the modified multiwavelength laser illumination, and further modifying one or both of the first or the second wavelengths to produce an improved measurement result.

[0051] According to one example embodiment, the multi wavelength laser illumination has a wavelength range in one of an ultraviolet range, an infrared range, or a visible range. In another example embodiment, the multi-mode optical fiber is one of a graded-index or a step-index optical fiber. In yet another example embodiment, the multi-mode optical fiber is part of a collection of multi-mode optical fibers for light delivery to the object. In still another example embodiment, the multi-mode optical fiber is part of a multi -core fiber bundle. In one example embodiment, determining the one or more characteristics of the obj ect includes determining a depth of the obj ect. In another example embodiment, the above noted method includes laterally moving the object or the multi-mode optical fiber to enable producing an image of the object. In still another example embodiment, the first and the second wavelengths are selected to produce a synthetic wavelength that is larger than a dimension of perturbations caused by movement of the multi-mode optical fiber or by one or more bents in the multi-mode optical fiber.

[0052] Another aspect of the disclosed embodiments relates to an optical system that includes a multi-mode optical fiber, and one or more illumination sources configured to produce a multiwavelength laser illumination to a first end of the multi-mode optical fiber. The multiwavelength laser illumination comprises a first wavelength and a second wavelength and the first wavelength is distinct from the second wavelength, and the multi-mode optical fiber configured to allow the multiwavelength laser illumination to propagate therethrough and exit a second end thereof for illumination of an object. The optical system also includes a detector positioned to receive at least a portion of the multi wavelength laser illumination after interactions with the object, wherein the detector is further configured to directly receive reference beams including at least a first reference beam having the first wavelength, and a second reference beam having the second wavelength. The optical system additionally includes a processor and a memory with instructions stored thereon, wherein the instructions upon execution by the processor configure the processor to: determine amplitude or phase values associated with a synthetic wavelength, wherein the synthetic wavelength is inversely proportional to a difference between atleast the first and the second wavelengths, and the first and the second wavelengths are selected such that optical path variations due to propagation of the multiwavelength laser illumination through the multi-mode optical fiber is less than a fraction of the synthetic wavelength, and to determine one or more object characteristics based the determined amplitude or phase values.

[0053] In one example embodiment, the multi-mode optical fiber is positioned to receive, at the second end thereof, light corresponding to the multiwavelength laser illumination after interactions with the object, and to allow the received light to traverse therethrough and exit the first end before reaching the detector. In another example embodiment, the optical system includes one or more lenses positioned to direct light corresponding to the multiwavelength laser illumination after interactions with the object to the detector. In yet another example embodiment, the multi-mode optical fiber is part of an endoscope configured to deliver the multiwavelength laser illumination to the object, and receive light corresponding to the multiwavelength laser illumination after interactions with the object. In still another example embodiment, the optical system is configured to accommodate features recited in claims 3-17.

[0054] It is understood that the various disclosed embodiments may be implemented individually, or collectively, in devices comprised of various optical components, electronics hardware and / or software modules and components. These devices, for example, may comprise a processor, a memory unit, an interface that are communicatively connected to each other, and may range from desktop and / or laptop computers, to mobile devices and the like. FIG. 8 illustrates a block diagram of a device 800 that can be used to implement certain aspects of the disclosed technology. For example, the device of FIG. 8 can be used to receive, process, store, provide for display and / or transmit various data and signals associated with disclosed image sensors, and / or to control the operation of light sources disclosed herein. The device 800 comprises at least one processor 804 and / or controller, at least one memory 802 unit that is in communication with the processor 804, and at least one communication unit 806 that enables the exchange of data and information, directly or indirectly, through the communication link 808 with other entities, devices, databases and networks. The communication unit 806 may provide wired and / or wireless communication capabilities in accordance with one or more communication protocols, and therefore it may comprise the proper transmitter / receiver, antennas, circuitry and ports, as well as the encoding / decoding capabilities that may be necessary for proper transmission and / or receptionof data and other information. The exemplary device 800 may be integrated as part of larger component (e g., a server, a computer, tablet, smart phone, etc.) that can be used for performing various computations, methods or algorithms disclosed herein. For example, the processor may be configured to receive electrical signals or information from the disclosed sensors (e.g., CMOS sensors), and to process the received information to produce images or other information of interest.

[0055] The processor(s) 804 may include central processing units (CPUs) to control the overall operation of, for example, the host computer. In certain embodiments, the processor(s) 804 accomplish this by executing software or firmware stored in memory 802. The processor(s) 804 may be, or may include, one or more programmable general -purpose or special -purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), graphics processing units (GPUs), or the like, or a combination of such devices.

[0056] The memory 802 can be or can include the main memory of a computer system. The memory 802 represents any suitable form of random access memory (RAM), read-only memory (ROM), flash memory, or the like, or a combination of such devices. In use, the memory 802 may contain, among other things, a set of machine instructions which, when executed by processor 804, causes the processor 804 to perform operations to implement certain aspects of the presently disclosed technology.

[0057] While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0058] Similarly, while operations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

[0059] Various information and data processing operations described herein may be implemented in one embodiment by a computer program product, embodied in a computer- readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Therefore, the computer-readable media that is described in the present application comprises non-transitory storage media. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

[0060] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A method for determining one or more characteristics of an object, the method comprising: providing a multiwavelength laser illumination to a first end of a multi-mode optical fiber, wherein the multiwavelength laser illumination comprises a first wavelength and a second wavelength, wherein the first wavelength is distinct from the second wavelength; allowing the multiwavelength laser illumination to propagate through the multi-mode optical fiber and exit a second end of the multi-mode optical fiber to illuminate the object; receiving, at a detector, at least a portion of the multiwavelength laser illumination after interactions with the object, wherein the detector is also illuminated by reference beams including at least a first reference beam having the first wavelength, and a second reference beam having the second wavelength; based on signals detected by the detector, determining amplitude or phase values associated with a synthetic wavelength, wherein the synthetic wavelength is inversely proportional to a difference between at least the first and the second wavelengths, and the first and the second wavelengths are selected such that optical path variations due to propagation of the multiwavelength laser illumination through the multi-mode optical fiber are less than a fraction of the synthetic wavelength; and determining the one or more characteristics of the object based on the determined amplitude or phase values.

2. The method of claim 1, wherein the multi wavelength laser illumination after the interactions with the object is received by the second end of the multi-mode optical fiber, traverses through the multi-mode optical fiber, and exits the first end of the multi-mode optical fiber before reaching the detector.

3. The method of claim 1, wherein the first wavelength is represented by Xi, and the second wavelength is represented by X2, and the synthetic wavelength, A, is determined according to:

4. The method of claim 1, wherein the first and the second wavelengths are selected such that the synthetic wavelength is at least one order of magnitude greater than each of the first and the second wavelengths.

5. The method of claim 1, wherein the first and the second wavelengths are selected such that the optical path variations due to propagation of the multiwavelength laser illumination through the multi-mode optical fiber is less than or equal to one-half of the synthetic wavelength.

6. The method of claim 5, wherein the first and the second wavelengths are selected such that the optical path variations due to propagation of the multiwavelength light through the multimode optical fiber is less than or equal to one-fourth of the synthetic wavelength.

7. The method of claim 1, wherein the first and the second wavelengths are selected (a) to produce a particular image contrast or (b) based on reflectance or transmission properties of tire object.

8. The method of claim 1, wherein the first and the second wavelengths are selected such that a difference between a largest and a smallest optical pathlength through the multi-mode optical fiber allows destructive interference of light that is received at the detector, or an interference pattern at full contrast is achieved.

9. Tire method of claim 1, comprising analyzing information obtained from the signals detected by the detector, and modifying one or both of the first or the second wavelengths to produce a modified multiwavelength laser illumination.

10. The method of claim 9, further comprising analyzing information obtained from signals detected by the detector in response to the modified multiwavelength laser illumination, and further modifying one or both of the first or the second wavelengths to produce an improved measurement result.

11. The method of claim 1, wherein the multiwavelength laser illumination has a wavelength range in one of: an ultraviolet range, an infrared range, or a visible range.

12. Tire method of claim 1, wherein the multi-mode optical fiber is one of a graded-index or a stepindex optical fiber.

13. The method of claim 1, wherein the multi-mode optical fiber is part of a collection of multi-mode optical fibers for light delivery to the object.

14. The method of claim 1, wherein the multi-mode optical fiber is part of a multi-core fiber bundle.

15. The method of claim 1, wherein determining the one or more characteristics of the object includes determining a depth of the object.

16. The method of claim 1 , comprising laterally moving the object or the multi-mode optical fiber to enable producing an image of the object.

17. The method of claim 1, wherein the first and the second wavelengths are selected to produce a synthetic wavelength that is larger than a dimension of perturbations caused by movement of the multi-mode optical fiber or by one or more bents in the multi-mode optical fiber.

18. An optical system, comprising: a multi-mode optical fiber; one or more illumination sources configured to produce a multiwavelength laser illumination to a first end of the multi-mode optical fiber, wherein the multiwavelength laser illumination comprises a first wavelength and a second wavelength and the first wavelength is distinct from the second wavelength, the multi-mode optical fiber configured to allow the multi wavelength laser illumination to propagate therethrough and exit a second end thereof for illumination of an object;a detector positioned to receive at least a portion of the multiwavelength laser illumination after interactions with the object, the detector further configured to directly receive reference beams including at least a first reference beam having the first wavelength, and a second reference beam having the second wavelength; and a processor and a memory with instructions stored thereon, wherein the instructions upon execution by the processor configure the processor to: determine amplitude or phase values associated with a synthetic wavelength, wherein the synthetic wavelength is inversely proportional to a difference between at least the first and the second wavelengths, and the first and the second wavelengths are selected such that optical path variations due to propagation of the multiwavelength laser illumination through the multi-mode optical fiber is less than a fraction of the synthetic wavelength; and determine one or more object characteristics based the determined amplitude or phase values.

19. The optical system of claim 18, wherein the multi-mode optical fiber is positioned to receive, at the second end thereof, light corresponding to the multiwavelength laser illumination after interactions with the object, and to allow the received light to traverse therethrough and exit the first end before reaching the detector.

20. The optical system of claim 18, comprising one or more lenses positioned to direct light corresponding to the multi wavelength laser illumination after interactions with the object to the detector.

21. The optical system of claim 18, wherein the multi-mode optical fiber is part of an endoscope configured to deliver the multiwavelength laser illumination to the object, and receive light corresponding to the multiwavelength laser illumination after interactions with the object.

22. The optical system of claim 18, configured to accommodate features recited in claims 3-17.

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