System and method for interlacing differing coherence length sweeps to improve OCT image quality

A system and method for sweeping electromagnetic radiation over a first coherence length and a second length over a range of wavelengths to generate an image. Electromagnetic radiation having a first coherence length is generated and swept over a range of wavelengths. Electromagnetic radiation having a second coherence length is generated and swept over a range of wavelengths. The electromagnetic radiation is splitting through a reference path and a sample path; Electromagnetic radiation returned from the reference path and the sample path is detected, wherein the detector generates output signals corresponding to the received electromagnetic radiation. In one embodiment, the output signals are processed to generate an image. The image may be interleaved with data corresponding to the electromagnetic radiation having the first coherence length and the second coherence length.

TECHNICAL FIELD

The present invention relates generally to a system and method to improve optical coherence tomography (OCT) image quality by interlacing differing coherence length sweeps from a swept-wavelength laser for improving image quality.

BACKGROUND

Speckle is a source of image quality aberration in OCT systems, wherein the laser light interferes with itself within the tissue being imaged. This results in image degradation commonly referred to as “speckle” creating dark spots (also referred to as “noise”) in the image. In an ideal setting, the only interference signal is generated from the light from the tissue with another version of the light in the other arm of the interferometer used in the OCT measurement. Unfortunately, the light can also interfere with itself as it variously reflects off the various layers of tissue as well, actually interfering within the tissue. It has often been assumed that this speckle is a natural and primarily unavoidable artifact of OCT measurement.

SUMMARY

It has been discovered that if the coherence length is roughly equivalent to the actual path length difference in the interferometer, then the actual speckle will be affected by relatively minor changes in that coherence length. If the laser itself can be adjusted to provide two or more interleaved sweeps of differing coherence length, then the data from these different sweeps can be combined or contrasted to provide additional information regarding the tissue in the area, which was obscured by the speckle.

Aspects of the present invention relate to a system and method to improve optical coherence tomography (OCT) image quality by interlacing differing coherence length sweeps from a swept-wavelength laser for improving image quality. The system and method may also improve measurements made in microscopy, optical frequency-domain reflectometry (OFDR) and light detection and ranging (LIDAR) systems, for example.

The system relates generally to performing two or more interleaved sweeps of differing coherence length and either provide both images or a single image based on the results from both sweeps. In one embodiment, data from the multiple sweeps of differing coherence lengths are combined to create a single image that more accurately reproduces the characteristics of the tissue.

One aspect of the invention relates to a system including: at least one electromagnetic radiation source operable to generate electromagnetic radiation at a first coherence length through a first range of wavelengths and at a second coherence length through a second range of wavelengths; an interferometer coupled to the at least one electromagnetic radiation source, wherein the interferometer includes a reference path and a sample path; a detector for receiving electromagnetic radiation returned from the reference path and the sample path, wherein the detector generates output signals corresponding to the received electromagnetic radiation; and a controller coupled to the detector and the at least one electromagnetic radiation source, wherein the controller selectively causes electromagnetic radiation to be generated at the first coherence length and the second coherence length and the controller processes the output signals to generate image signals from the electromagnetic radiation generated from at the first coherence length and the second coherence length.

Another aspect of the invention relates to a method for sweeping electromagnetic radiation over a first coherence length and a second length over a range of wavelengths to generate an image, the method including: generating electromagnetic radiation having a first coherence length and a second coherence length through the range of wavelengths and the electromagnetic radiation associated with the first coherence length or the second coherence length is output at a given time; splitting the electromagnetic radiation through a reference path and a sample path; detecting electromagnetic radiation returned from the reference path and the sample path, wherein the detector generates output signals corresponding to the received electromagnetic radiation; and processing the output signals to generate an image.

A number of features are described herein with respect to embodiments of the invention. It will be appreciated that features described with respect to a given embodiment also may be employed in connection with other embodiments.

The invention comprises the features described herein, including the description, the annexed drawings, and, if appended, the claims, which set forth in detail certain illustrative embodiments. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

DESCRIPTION

An exemplary swept electromagnetic radiation system10in accordance with aspects of the present invention is illustrated inFIG. 1. Referring toFIG. 1, the system10includes at least one electromagnetic radiation source12, an interferometer14(identified by dashed lines); a detector16and a controller18. Electromagnetic radiation19is directed from the at least one radiation source12to a target20to detect a physical characteristic associated with the target, as discussed herein.

The at least one electromagnetic radiation source12is operable to generate electromagnetic radiation having a first coherence length through a first range of wavelengths. In one embodiment, electromagnetic radiation source12is operable to generate electromagnetic radiation having a second coherence length (distinct from the first coherence length) over the same or a different range of wavelengths.

In one embodiment of the present invention, at least one electromagnetic radiation source12is an electronically tunable laser. The electronically tunable laser may be a semiconductor laser that is electronically tunable to discretely adjust the coherence length between at least two lengths. For example, the electronically tunable laser may be a Vernier-tuned distributed Bragg reflector (VTDBR) laser. The output wavelength of the VTDBR laser may be electronically-tuned by adjusting one or more input currents to the device. In one exemplary illustration, the device may be tuned by changing up to 5 different input currents to the device. By electronic tuning the output wavelength of the emitted electromagnetic radiation, the source12can be programmed and/or controlled by the controller18output electromagnetic radiation at a desired coherence length over a desired wavelength range. The source12may then be controlled to switch immediately to a different coherence length for the next scan. Other embodiments of the invention may comprise used of tunable lasers such as Vertical-Cavity Surface-Emitting Lasers (VCSELs) or multi-section lasers such as Super-Structure Grating DBR lasers (SSG-DBRs), Digital Supermode DBR lasers (DS-DBRs) or Y-Branch lasers.

In one embodiment, a predetermined period of time elapses during the transition of the source between the first coherence length and the second coherence length. The predetermined period of time may be any desired time. One design criterion is to minimize the period of time to shorten the sweep duration and have the period of time large enough to prevent interference between the electromagnetic radiation having different coherent lengths.

As discussed above, the electromagnetic radiation source12is operable to output electromagnetic radiation having a first coherence length and a second coherence range over a range or ranges of wavelengths. The source12may be electronically tunable to discretely output the electromagnetic radiation at the desired coherence length.

In another embodiment, illustrated inFIG. 2, the radiation source12includes two discrete sources (12A,12B) of electromagnetic radiation. Each source12A,12B is operable to generate electromagnetic radiation at fixed coherence length through a range of wavelengths. The sources12A,12B may be any source of electromagnetic radiation. The sources may be the same or different sources of electromagnetic radiation. For example, one source may be a semiconductor laser and another source may be another source (such as an external cavity diode laser or tunable solid-state laser). Any combination of sources of electromagnetic radiation is deemed to fall within the scope of the present invention.

As stated above, a predetermined period of time elapses between transition between the first source of electromagnetic radiation12A and the second source of electromagnetic radiation12B and vice versa.

The interferometer, identified generally by the dashed lines14, may be coupled to the one or more electromagnetic radiation sources12by a coupler30. Any type of interferometer may be used in accordance with aspects of the present invention. Exemplary interferometers include, for example: a Mach-Zehnder interferometer, a Michelson interferometer, a Fabry-Perot interferometer, etc. The interferometer14may include fewer components or more components than identified within the confines of the dashed line. The illustrated interferometer14is exemplary and provided to assist one skilled in the art in understanding aspects of the present invention.

Generally, the coupler30splits the received electromagnetic radiation into a reference path32and a target path34. For the reference path32, the electromagnetic radiation is output through a lens unit36and reflected off a reference38. In one embodiment, the reference38may be a mirror or any known reference reflector with partial reflectivity. For the target path, the electromagnetic radiation is output through a lens unit40and reflected off a target20. The target20may be any desirable target. Exemplary targets include, for example, human tissue, inanimate objects, etc.

Radiation returning from the reference path32and the target path34travels through the respective lens unit (e.g.,36,40) and is combined with the coupler30or another beam combining component to form signal42, as is known in the interferometry art. The coupler30may be any type of device that may be used to split or route electromagnetic radiation received (e.g., signal19) into two or more radiation paths (e.g., paths32,34) and/or join two or more received radiation paths into a single signal (e.g., signal42). For example, the coupler may be a splitter for separating and/or combining optical mediums (e.g., a fiber optic cable, etc.), a beam splitter and the like. As shown inFIG. 1, the coupler30may be a 3 dB coupler that splits the incoming light into two separate radiation paths (e.g., reference path32and target path34) of approximately equal intensity of light. The two radiation paths32and34may be recombined after the electromagnetic radiation has traversed separate radiation paths to32,34at the coupler30and/or a different coupler (not shown).

After the electromagnetic radiation has traversed one of the reference path32or the target path34, the coupler30may combine the received electromagnetic radiation into a single output signal42. In this case, the output signal is a fringe pattern of amplitude vs. time or spatial dimension that may be detected by a detector16(e.g., a photodetector, a phase detector, frequency detector, linear detector array or other suitable detector). The coupler30functions to combine the electromagnetic radiation that has traversed radiation paths32,34. One of ordinary skill in the art will readily appreciate that the output signal and the detector may vary based on the electromagnetic radiation, wavelength, frequency and/or range used in conjunction with the system.

The detector16receives electromagnetic radiation42returned from the reference path32and the target (also referred herein as sample) path34. The combined, interfered electromagnetic radiation (e.g., signal42) is received by the detector16. The detector16may be a single channel and/or a multi-channel detector. The detector16generates one or more output signals44that are based on the received electromagnetic radiation from the reference path32or target path34.

The output signals44are received by controller18. The controller18is coupled to the interferometer14, the detector16and the at least one electromagnetic radiation source12. The controller18selectively causes the source12to generate electromagnetic radiation at the first coherence length and the second coherence length.

The controller18processes the output signals to represent data corresponding to amplitude and distance information detected by the detector16. For example, a graph of reflection amplitude vs. distance may be created for each scan at each coherence length. A single scan of the electromagnetic radiation source produces a one-dimensional graph. The output signals of multiple scans may be combined to generate one or more image signals. For example, multiple scans of one-dimensional graphs are used to assemble a two-dimensional representation of data that relates to the target, as is conventional. Preferably, the controller processes the output signals to generate image signals from the electromagnetic radiation generated at the first coherence length and the second coherence length. The image signals may be interleaved image signals generated from the electromagnetic radiation generated at the first coherence length and the second coherence length. In another embodiment, the image signals are discrete image signals generated from the electromagnetic radiation generated from at the first coherence length and the second coherence length. An exemplary method100in accordance with aspects of the present invention is illustrated inFIG. 3. The exemplary method100is for sweeping electromagnetic radiation over a first coherence length and a second length over a range of wavelengths to generate an image.

At block102, the method includes generating electromagnetic radiation having a first coherence length and a second coherence length through the range of wavelengths and the electromagnetic radiation associated with the first coherence length or the second coherence length is output at a given time. As set forth both, any combination of sources may be used to generate the electromagnetic radiation having different coherence lengths. For example, two fixed sources having different coherence lengths may be used. Alternatively, one fixed source and one source having changeable coherence length may be used. In another embodiment, two sources both having changeable coherence lengths may be used.

The output from each source occurs a given time distinct from each other. For example, a predetermined period of time elapses between transition from one source to another. This could be referred to as interlacing the output sweeps of differing coherence lengths.

At block104, the electromagnetic radiation is split through a reference path and a sample path of an interferometer.

At block106, electromagnetic radiation returned from the reference path and the sample path is detected at a detector. The detector generates output signals corresponding to the received electromagnetic radiation.

At block108, the output signals are processed to generate an image. The image may be formed by combining output signals associated with the first coherence length and the second coherence length. In another embodiment, data associated with the first coherence length and the second coherence length are interleaved to generate the image. In yet another embodiment, distinct images are formed corresponding to the first coherence length and the second coherence length. The resulting output signals and/or the images may be electronically stored for any desired purpose.

Although the invention is shown and described with respect to illustrative embodiments, it is evident that equivalents and modifications will occur to those persons skilled in the art upon the reading and understanding hereof. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims if appended hereto.