Apparatus for imaging an object using low coherence optical radiation are fairly well known. Such apparatus comprise a low coherence light source optically coupled with an optical interferometer and a photodetector, which is connected with a data processing and displaying unit. The optical interferometer is typically designed either as a Michelson optical fiber interferometer or as a Mach-Zender optical fiber interferometer. Regardless of the specific design used, an optical interferometer typically comprises one or two beam splitters, a measuring arm and a reference arm. The measuring arm includes, as a rule a measuring probe, which is most often an optical fiber probe, and is designed to deliver low coherence optical radiation to the object, the reference arm having a reference mirror at its end. Longitudinal scanning of the object is performed either by connecting the reference mirror with an element that provides mechanical movement of the reference mirror, or by fixing the position of the reference mirror and performing longitudinal scanning with the aid of a piezoelectric scanning element.
A virtue of optical interferometers applied for studying objects with the use of low coherence optical radiation is a potential for acquisition of images of turbid media with high spatial resolution as well as noninvasive diagnostics in medical studies and non-destructive control in diagnostics of various equipment.
Prior improvements of apparatus for imaging an object using low coherence optical radiation are aimed, generally, at enhancing the resolution of the apparatus, as known, or ensuring efficient use of optical source power by providing optimal signal-to-noise ratio.
The measuring probe incorporated into the measuring arm performs the function of delivering low coherence optical radiation to the object. Known measuring probes are designed typically as an optical fiber probe comprising an optical fiber positioned in such a way, that low coherence optical radiation can pass from its proximal end to its distal end, and an optical system which focuses the low coherence optical radiation on the object. The optical system includes at least one lens component with positive focal power. The measuring probe includes also a system for transverse scanning of the low coherence optical radiation. The measuring probe typically has an elongated body with a throughhole extending therethrough, wherein an optical fiber extends. The transverse scanning system includes an actuator, which may be a piezoelectric element, stepper motor, electromagnetic system or electrostatic system.
Improvements of measuring probes known in the art, which are incorporated into apparatus for imaging an object using low coherence optical radiation are aimed at imaging thin vessels, and at optimizing the probe design for obtaining a maximum amplitude of optical beam deviation with limited size of the body of the optical fiber probe.
Prior apparatus for imaging an object using low coherence optical radiation execute intrinsically the same method for obtaining an image of an object. According to this method, the low coherence optical radiation is directed simultaneously towards the object and along the reference optical path. The optical radiation is directed therewith towards the object through an optical system, which focuses the low coherence optical radiation onto the object, the optical radiation being transversely scanned over a surface approximately orthogonal to the direction of propagation of the optical radiation. Then the optical radiation having returned from the object is combined with the optical radiation, which passed through the reference optical path. The optical radiation, which is a result of the combining, is used to visualize the intensity of the optical radiation having returned from the object. In addition, longitudinal scanning of the object is performed by varying the difference in optical path lengths for the low coherence optical radiation directed towards the object and directed along the reference optical path. The difference in the optical path lengths is varied by at least several tens of wavelengths of optical radiation in compliance with a predetermined rule.
Another apparatus executes the above imaging method and comprises a low coherence optical light source optically coupled with an interferometer and at least one photodetector, the output of the photodetector being connected with a data processing and displaying unit. The interferometer includes a beam splitter optically coupled with a measuring arm and a reference arm, the measuring arm being provided with a delivering device for low coherence optical radiation designed as an optical fiber probe.
The delivering device for low coherence optical radiation is designed as an optical fiber probe. The optical fiber probe comprises an optical fiber, which is positioned allowing for the low coherence optical radiation to pass from the proximal end of the optical fiber probe to its distal end, an optical system, and a system for transverse scanning of the low coherence optical radiation. The optical system is optically coupled with the optical fiber and is used for focusing the low coherence optical radiation onto the object. The optical system comprises at least a first lens component with positive focal power. The optical fiber is incorporated into the transverse scanning system, which is arranged capable of moving the end face of the distal part of the optical fiber over the transverse scanning surface in a direction approximately perpendicular to the own axis of the optical fiber.
A drawback of the prior method, as well as of the apparatus described above for executing this method, of delivering low coherence optical radiation towards the object, as well as of any other prior technique for imaging an object using low coherence optical radiation, is that the acquired image of a flat object looks bent. This occurs due to peculiarity of imaging with the use of an interference signal, which results from combining optical radiation coming back from the object with that of the reference path. It is known that the interference signal occurs when optical path lengths for the low coherence optical radiation directed towards the object, and of the reference path are equal. However, the propagation time for the low coherence optical radiation from points, having different off axis positions in a flat transverse scanning surface, to corresponding conjugate points in the image plane is not the same. Therefore, while the optical path length for the low coherence optical radiation propagating along the reference arm is constant, the optical path length for the low coherence optical radiation directed towards the object is not constant when transverse scanning is performed. That results in a curvature of the acquired images. The later can be seen in FIG. 19, showing an example of an image acquired by the prior method, and in FIGS. 8, 9 and 10 which demonstrate prior image construction.
FIG. 8 illustrates construction of an image by the known method in prior art apparatus for a flat transverse scanning surface 28 in a case when an optical system 29 is designed as a single lens component 30 with positive focal power. Line 31 seen in the drawing corresponds to a point locus, to which the optical path length for the low coherence optical radiation passing to the object 11, has the same value from corresponding conjugate points disposed at various off axis positions in a flat transverse scanning surface 28.
FIG. 9 and FIG. 10 illustrate image construction by the known method in prior apparatus for a flat transverse scanning surface 28 in cases when the optical system 29 is designed as two lens components 32, 33 with positive focal power, the lens components being placed from each other at a distance that is respectively, either greater or smaller than the confocal distance. FIG. 9 shows also a line 34 and FIG. 10 shows a line 35, that correspond each to a point locus, to which the optical path length for the low coherence optical radiation passing to the object 11, has the same value from corresponding conjugate points disposed at various off axis positions in a flat transverse scanning surface 28. It is evident from the drawings that lines 31, 34, 35 have a curvature. In addition, in the case when the transverse scanning surface has a curvature, for instance, when the optical fiber in the optical fiber probe serves as a flexible cantilever, there occurs an additional aberration that also contributes to the curvature of the image being constructed. Another disadvantage of prior technique, is that the focusing position of the low coherence optical radiation is fixed, whereas the position of the coherence gate varies during longitudinal scanning. The later limits the transverse resolution of the method and apparatus based thereon, especially for a large scanning depth. This is due to a strong diffraction divergence of sharply focused radiation and, consequently, small depth resolution. For instance, the depth resolution for a focused Gaussian beam is d=πΦ2/4λ, where Φ is the beam waist diameter, λ is the wavelength, and π=3.1416. Accordingly, for typical parameters of Φ=0.005 mm, λ=1300 nm, the depth resolution is as small as 0.015 mm (15 μm).
To ensure a high transverse resolution for a large depth of longitudinal scanning, known apparatus perform synchronous scanning of the focal waist position, i.e., of the focusing position of the optical radiation, by moving one of the lenses of the optical system, and of the position of the coherence gate by altering the relative optical lengths of the interferometer arms. This approach is referred to as optical coherence microscopy (OCM). All known embodiments of OCM perform these two scans (of the focusing position and coherence gate position) by means of two independent synchronously operating devices. The synchronization of these devices is an independent and fairly complicated engineering task, which becomes even more complicated as the speed of the image input increases.