Servo tracking system utilizing phase-sensitive detection of reflectance variations

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FIELD OF THE INVENTION

The invention relates generally to the field of target tracking. In particular, the invention relates to apparatus and methods for reflectance-based servo tracking for image stabilization and precision target tracking.

BACKGROUND OF THE INVENTION

Active servo tracking systems are used in numerous military, industrial, and medical applications. In operation, active servo tracking systems utilize information about a target's motion to correct the physical position of an object to be stabilized in a target frame of reference. The information about the target's motion may be obtained by numerous techniques such as direct position measurements, position correlation, and velocity sensing.

Direct position measurement techniques for obtaining information about a target's motion typically utilize position sensitive detectors, such as quadrant detectors, to detect a "hot spot" associated with the target. Position correlation techniques for obtaining information about a target's motion compare previously stored images of the target to the current image at predetermined time intervals. The resulting image overlap or correlation function is utilized to determine the target displacement.

Velocity sensing techniques for obtaining information about a target's motion typically utilize a signal proportional to the rate of displacement in the frequency domain. The signal is then integrated to give target position information. There are numerous velocity sensing techniques known in the art such as coherent laser-based Doppler and speckle methods.

U.S. Pat. No. 4,856,891 describes an eye fundus tracking system that utilizes active servo tracking and correlation. The system includes a laser source that projects a tracking strip of coherent light on the fundus and optics for producing an image of reflected light from the tracking strip onto a detecting element. The system also includes a means for scanning the intensity profile of the image strip and electronics for analyzing the scanned intensity profile and for providing correction signals which direct the optical path of both the tracking laser beam and a diagnostic laser beam to a fixed position on the fundus. The system, however, is relatively complex to implement.

Numerous applications, such as ophthalmologic and other micro-surgical procedures, require high-speed positioning with accuracy in the cellular dimension range. In addition, it is desirable for such tracking systems to utilize low-power incoherent tracking beams.

SUMMARY OF THE INVENTION

It is a principal object of this invention to provide apparatus and methods for tracking a feature on a target surface and continually providing analog corrections to tracking mirrors in real time by utilizing a low-power incoherent tracking beam to detect the movements of a reference feature on the target and confocal reflectometry to monitor the reflection from the tracking beam's current position. It is another object of this invention to utilize small, periodic, transverse oscillations in the tracking beam and phase sensitive detection of the reflectance variations to generate error signals which are utilized to compensate the target displacement.

It is another object of this invention to provide independent steering of a tracking beam and a therapeutic beam by a balanced "scan and de-scan" technique. It is another object of this invention to provide a high-speed fundus tracking system that utilizes confocal reflectometry for retinal photocoagulation.

Accordingly, the present invention features a tracking system for tracking a reference feature on a target surface. The tracking system includes a dithering device positioned in an optical path of a tracking beam. The tracking beam may be formed from a light emitting diode or from numerous other low-power coherent or incoherent light sources.

The dithering device dithers the tracking beam in a first and a second direction with an oscillatory motion having a first and a second phase, respectively. The first and second phases of oscillatory motion may be orthogonal to each other. The dithering device may comprise a pair of orthogonally mounted galvanometers operatively connected to reflectors.

The tracking system also includes a tracking device for controlling the position of a therapeutic beam relative to a target and for controlling the position of the tracking beam relative to a reference feature. The reference feature may be associated with an eye or may be a retro-reflecting material. The tracking device includes a first input for accepting a first direction control signal and a second input for accepting a second direction control signal. The first and second direction control signals cause the tracking device to move the therapeutic beam in the first and second directions, respectively. The tracking velocity of the tracking device may be proportional to the product of a dither frequency of the dithering device and a spatial dimension of the reference feature.

The tracking system also includes a reflectometer positioned in an optical path of a reflected tracking beam. The reflectometer provides an output signal with a phase corresponding to the phase of the reflected tracking beam. The reflectometer may be a confocal reflectometer.

The tracking system also includes a signal processor for comparing the phase of the reflectometer output signal to the phases of the oscillatory motion in the first and second directions. The signal processor generates the first and second direction control signals which are coupled to the first and second inputs of the tracking device, respectively. The first and second direction control signals cause the therapeutic beam to track relative to the reference feature.

The present invention also features an eye tracking system for tracking a reference feature associated with an eye. The eye tracking system includes a first pair of reflectors. The first reflector is positioned in an optical path of an incident and reflected tracking beam. The second reflector may be a beamsplitter that passes a coagulating beam in transmission and reflects the tracking beam in reflection. The first pair of reflectors controls the position of the tracking beam. The tracking beam may be formed from a light emitting diode or from numerous other low-power incoherent light sources.

The eye tracking system also includes a pair of dither drivers operatively connected to the first pair of reflectors. The dither drivers dither the first reflector in a first direction and the second reflector in a second direction with an oscillatory motion having a first and a second phase, respectively. The first and second phases may be orthogonal. The pair of dither drivers may be orthogonally mounted galvanometers operatively connected to the first pair of reflectors.

The eye tracking system also includes a second pair of reflectors for positioning the tracking beam onto a reference feature in an eye and for positioning the coagulating beam onto a target in the eye. The eye tracking system also includes a pair of tracking drivers for controlling the position of the second pair of reflectors. The pair of tracking drivers is operatively connected to the second pair of reflectors and comprises a first input for accepting a first direction control signal and a second input for accepting a second direction control signal. The first and second direction control signals cause the pair of tracking drivers to move the second pair of reflectors in the first and the second direction, respectively. A tracking velocity of the pair of tracking drivers is proportional to the product of a dither frequency of the pair of dither drivers and a spatial dimension of a reference feature.

The eye tracking system also includes a reflectometer positioned in the optical path of the reflected tracking beam. The reflectometer, which may be a confocal reflectometer, provides an output signal with a phase corresponding to a phase of the reflected tracking beam.

The eye tracking system also includes a signal processor for comparing the phase of the reflectometer output signal to the phases of the oscillatory motion in the first and second directions. The signal processor generates the first and the second direction control signals which are coupled to the first and second inputs of the tracking driver, respectively. The first and second direction control signals cause the coagulating beam to track relative to the reference feature.

The eye tracking system may include a shutter for blanking the coagulating beam so that a surgeon can precisely control when the coagulating beam is delivered to the target. The eye tracking system may also include an offset signal generator operatively coupled to the dither driver and to the tracking driver for displacing the coagulating beam with respect to the tracking beam a predetermined distance. When a "scan" signal is input to the tracking driver to reposition the therapeutic beam, an offsetting "de-scan" signal is input to the dither driver. Such an offset signal generator will significantly increase the speed at which the coagulating beam can be translated from one target to another target.

The present invention also features a method of tracking that includes directing a tracking beam to a reference feature. The tracking beam is dithered in a first and a second direction with an oscillatory motion having a first and a second phase, respectively. A reflector is positioned in an optical path of a therapeutic beam. The reflector may also be positioned in an optical path of the tracking beam.

The phase of a reflected tracking beam reflected from the reference feature is measured. The phase of the reflected tracking beam is compared to the first and the second phase of the oscillatory motion. The method also includes repositioning the reflector a distance related to the comparison of the phase of the reflected tracking beam and the first and the second phases of the oscillatory motion where the distance causes the therapeutic beam to track a displacement of the reference feature. In addition, the method may include displacing the therapeutic beam relative to the tracking beam a predetermined distance. The displacement will increase the speed at which the coagulating beam can be translated from one target to another target.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is described with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a tracking system which embodies the invention.

FIG. 2 is a functional block diagram of a signal processor utilized in the tracking systems which embody the invention.

FIG. 3 is a schematic diagram of the optics for an eye tracking system which embodies the invention.

FIGS. 4A-C illustrates the operation of the signal processor.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a tracking system 10 which embodies the invention. The tracking system 10 tracks a target 12 relative to a reference feature 14. The reflectivity of the reference feature 14 is different from the reflectivity of an adjacent background area 16 at the wavelength of a tracking beam 18. The reference feature 14 may be any approximately axisymmetric feature of appropriate size and reflectivity contrast.

The reference feature 14 may be associated with an eye or may be a retro-reflecting material. The reference feature 14 may be photocoagulation eye lesions which are useful for retinal laser surgical application. Photocoagulation lesions are commonly used for marking a physical reference position on the retina. However, many retinal features have a high enough reflectivity contrast with the background area 16 to be suitable as reference features.

The tracking beam 18 locks onto the reference feature 14 by inducing small, periodic, transverse oscillations or dithers in the tracking beam. The tracking beam 18 may be any low-power light beam that detects movement of the reference feature 14. The tracking beam 18 may be formed from a light emitting diode or from numerous other low-power incoherent light sources. Typically, the reference feature 14 is locked onto by the tracking beam in two dimensions with a circular dither.

The tracking system 10 includes a dithering device 20 positioned in an optical path 22 of the tracking beam 18. The dithering device 20 may comprise a pair of orthogonally mounted galvanometers scanner-driven mirrors (not shown). Galvanometers with low armature inertia can be used to achieve a high-speed tracking response.

The dithering device 20 dithers the tracking beam 18 in a first 19 and a second direction 21 with an oscillatory motion having a dither frequency with a first and a second phase, respectively. The dithering device 20 produces a circular dither at the reference feature 12 when the oscillatory motions, in the first and second direction, have identical amplitudes and have a phase difference of 90 degrees.

The tracking system 10 also includes a tracking device 24 for controlling the position of a therapeutic beam 26 relative to the target 12 and for controlling the position of the tracking beam 18 relative to the reference feature 14. The therapeutic beam 26 is typically a high-power coagulating beam. A blanking element 28 may be positioned in an optical path 30 of the therapeutic beam 26 for controlling when the therapeutic beam 26 is delivered to the target 12.

The tracking device 24 includes a first input 32 for accepting a first direction control signal, and a second input 34 for accepting a second direction control signal. The first and second direction control signals cause the tracking device 24 to move the therapeutic beam 26 in the first and the second direction, respectively. The tracking velocity of the tracking device 24 may be proportional to the product of the dither frequency and a spatial dimension of the reference feature 12.

The tracking system 10 also includes a reflectometer 36 positioned in an optical path 38 of a reflected tracking beam 40. The reflectometer 36 provides an output signal with a phase corresponding to a phase of the reflected tracking beam 40. The reflectometer 36 may be a confocal reflectometer. When the tracking beam 18 traverses a region of changing reflectance (not shown), a corresponding variation in the output signal of the reflectometer 36 occurs. The reflectometer output signal varies synchronously (when appropriately corrected for phase shifts) with the oscillatory motion caused by the dither driver 20.

The tracking system 10 also includes a signal processor 42 for comparing the phase