Quadrature phase shift interferometer (QPSI) decoder and method of decoding

An arrangement and method for reliably finding the wave form extrema of interference signals produced by a quadrature phase shift interferometer (QPSI) takes advantage of the quadrature property of I and Q signals. The zero-crossing points in the I and Q signals are determined. Peak detection is performed for peaks and valleys in the Q signal in close proximity to the zero-crossing points in the I signal, and for peaks and valleys in the I signal in close proximity to the zero-crossing points in the Q signal. These represent the maximum and minimum points of the I and Q signals. From these points, intensity envelopes are created and QPSI phase wrapping is performed to determine the phase angle and ultimately, out-of-plane displacement may be determined.

FIELD OF THE INVENTION

The present invention relates to characterizing the topography of a surface, more particularly, to arrangements and methods for finding the waveform extrema of interference signals produced by a quadrature phase shift interferometer (QPSI). The location and value of these extrema are required in order to create accurate intensity envelopes, which are then used by a QPSI phase unwrapping algorithm.

BACKGROUND OF THE INVENTION

A form of an information storage and retrieval device is a hard disk drive (hereinafter “disk drive”). A disk drive is conventionally used for information storage and retrieval with computers, data recorders, redundant arrays of independent disks (RAIDs), multi-media recorders, and the like. A disk drive comprises one or more disk media.

Each disk medium comprises a substrate upon which materials are deposited to provide a magnetically sensitive surface. In forming a disk medium, a substrate is ground or polished, conventionally by chemical-mechanical or mechanical polishing, to provide a substantially planar surface. Layers of materials are substantially uniformly deposited on this substantially planar surface to provide magnetic properties for writing to and reading from the disk media.

However, defects, such as pits, voids, particles, bumps and scratches, among others, may arise on a disk medium surface. These defects affect the surface topography of the disk medium. These defects need to be detected and characterized. A number of different types of apparatus can be used to measure the surface topography. These include a Candela profilometer, a quadrature phase shift interferometer, or a laser doppler vibrometer, for example.

In particular, a quadrature phase shift interferometer is designed to provide an optical, non-contact testing method for inspecting a disk medium surface, or other ultra-smooth surface. Defects are detected and characterized by out-of-plane displacement. The interferometer described is able to measure out-of-plane displacements with nanometer resolution with frequency responses in a range of DC to hundreds of megahertz depending on detector rise time.

Phase angle calculation is an intermediate goal of a QPSI decoding algorithm. The ultimate goal of the QPSI is to measure out-of-plane displacement, which can be related to the phase angle through fundamental and well-known constants. One of the known methods used to resolve phase angle is the maximum/minimum intensity method. To employ this method, an accurate intensity envelope of the signal is required. A simple method to detect the intensity envelope uses the location of peaks and valleys within the signal. Curves are fitted through these peaks and valleys to create an intensity envelope. There are several peak/valley detection methods. The most commonly used method is the peak detector method. This method is based on an algorithm that fits a quadratic polynomial to sequential groups of data points from the signals produced by the detector. The peak detector method searches for a zero crossing in the first derivative of the signal in conjunction with the condition that the signal satisfies a threshold criterion at the location of the derivative zero crossing. However, referring now toFIG. 1, the method is easily confounded by false peaks and valleys that are present in the detected signals. InFIG. 1, a typical set of I and Q signals are depicted with modulated amplitude and corresponding ideal intensity envelopes.

According to conventional peak detection methods, the points a, b and c inFIG. 1can be mistakenly identified by the peak detector as maximum and minimum intensity points, although they are not true maximum or minimum intensity points. Since the signal envelope will not be correctly identified, due to these false maximum and minimum intensity points, a significant decoding error will result.

SUMMARY OF THE INVENTION

There is a need for a decoding algorithm for a quadrature phase shift interferometer (QPSI) that identifies the true maximum and minimum intensity points of the interference signals produced by the QPSI so that the location and value of these extrema may be employed to create accurate intensity envelopes, which are then used by the QPSI phase unwrapping algorithm.

This and other needs are met by embodiments of the present invention which provide a method of determining extrema of interference signals produced by QPSI, comprising the steps of obtaining I and Q signals from the QPSI, determining zero-crossing points in the I signal, and peak detecting for peaks and valleys in the Q signal in close proximity to the zero-crossing points in the I signal, to thereby determine maximum and minimum points of the Q signal. The zero-crossing points are determined in the Q signal, and peak detecting is performed for peaks and valleys in the I signal in close proximity to the zero-crossing points in the Q signal, to thereby determine maximum and minimum points of the I signal.

The present invention takes advantage of the quadrature property of the I and Q signals, such that if one signal is at a peak or valley, the corresponding signal must be at a zero-crossing, and vice versa. With the true extrema located in the I and Q signals, accurate intensity envelopes are created. From the accurate intensity envelopes that are created, the QPSI phase unwrapping algorithm may then accurately perform a decoding.

The earlier stated needs are met by other embodiments of the invention that provide an arrangement for determining intensity envelopes of interference signals, comprising a QPSI that generates interference signals I and Q. A processor is configured to determine true maximum and minimum points from the interference signals I and Q to create intensity envelopes from the true maximum and minimum points.

The earlier stated needs are met by still other embodiments of the invention that provide a system for determining extrema of interference signals produced by a QPSI. The system comprises the QPSI that generates I and Q signals, and means for determining extrema of the I and Q signals based on zero-crossing points in the I and Q signals and detected peaks and valleys in the I and Q signals in close proximity to the zero-crossing points.

The foregoing and other features, aspects and advantages of the present invention will become more apparent in the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses and solves problems related to determining the extrema of interference signals produced by a quadrature phase shift interferometer (QPSI) in the dynamic measurement of hard disk static surface topography. In particular, the present invention obtains the I and Q signals from the QPSI and determines the zero-crossing points in each of the respective signals. Peak detection is performed for the peaks and valleys in the Q signal in close proximity to the zero-crossing points of the I signal to accurately determine the maximum and minimum points of the Q signal. Similarly, peak detection is employed to determine the peaks and valleys in the I signal in close proximity to the zero-crossing points in the Q signal, to thereby determine the maximum and minimum points of the I signal. From the accurately determined maximum and minimum points of the I and Q signals, intensity envelopes are formed, such as by curve fitting. From the intensity envelopes, a QPSI phase unwrapping may be performed and a phase angle is determined based on the intensity envelopes. Out-of-plane displacement of a recording media disk can be calculated based upon the determined phase angle.

For purposes of explanation, an exemplary embodiment of a quadrature phase shift interferometer (QPSI) will be described with respect toFIG. 4. However, the invention may be applied to other examples of QPSI arrangements, as will be recognized by one of ordinary skill in the art. The QPSI arrangement ofFIG. 4provides I and Q signals, such as the exemplary signals depicted inFIG. 1.

FIG. 4is an optical layout of an exemplary portion of an embodiment of an interferometer system50in accordance with one or more aspects of the present invention. As will be understood, interferometer system50, or more particularly quadrature phase shift interferometer45, uses two polarization processes to create two independent interference signals, which are phase shifted with respect to one another. The presence of two independent signals in phase quadrature facilitates unwrapping of phase.

With continuing reference toFIG. 4, laser or laser beam source20is configured to provide a laser or other optical energy beam21. Laser20may be configured to provide a linearly polarized laser beam. For example, a Helium-Neon (He—Ne) laser may be used, though it should be understood that the present invention may be used with known lasers of other wavelengths. Laser beam21is a linearly polarized laser beam. Laser beam21is provided to variable ratio beam splitter49.

Variable ratio beam splitter49comprises a polarizing beam splitter (PBS)25and half-wave plate (HWP)22. Notably, half-wave plate22is configured to rotate. By rotating half-wave plate22, relative intensity or amplitude of reference beam26and object beam27may be adjusted. Half-wave plate22is used to rotate the direction of polarization of laser beam21with respect to polarizing beam splitter25. In other words, direction of orientation is adjusted such that polarizing beam splitter receives components of s-polarization and p-polarization. Laser beam21is provided to half-wave plate22and then to polarizing beam splitter25. Polarizing beam splitter25splits laser beam21into a reference beam26and an object or measurement beam27according to s-polarization and p-polarization components. An aspect of the present invention is to balance intensity of reference beam26and object beam27. Alternatively, half-wave plate22may be removed and direction of polarization controlled by rotation of laser20.

Reference beam26and object beam27are polarized beams with polarizations perpendicular or orthogonal to one another. Thus, reference beam26may comprise only the s-polarized component of laser beam21and object beam27may comprise only the p-polarized component. Notably, reference beam26and object beam27are interchangeable.

Reference beam26is provided to quarter-wave plate (QWP)28and then to mirror31. Reference beam26enters a passive side of quarter-wave plate28. Reference beam26is reflected off an optically reflective surface of mirror31to provide reflected reference beam30, as shown with a dashed line. For purposes of clarity, beams post-reflection and pre-recombination are shown with a dashed line.

Reflected reference beam30is provided to quarter-wave plate28. Quarter-wave plate28, as well as quarter wave-plate29, are used to reduce power loss due to subsequent combination of reflected reference beam30and reflected object beam31, respectively. Reference beam26immediately prior to passing through quarter-wave plate28comprises only linear polarization, namely s-polarization, components. After passing through quarter-wave plate28, reference beam26linear polarization components are converted to circular polarization components. Reflected reference beam30immediately prior to passing through quarter-wave plate28comprises only circular polarization components. After passing through quarter-wave plate28, reflected reference beam30circular polarization components are converted to linear polarization, namely p-polarization, components, and thus reflected reference beam with p-polarization components passes straight through polarizing beam splitter25for providing a portion of combinative beam33.

Object beam27is provided to a passive side of quarter-wave plate29and then to lens46. Lens46is used to reduce spot size of object beam27for imaging of surface32of disk medium10. Spot size determines resolution for inspection purposes, and thus a smaller spot size allows smaller defects to be resolved. Focused object beam27from lens46leaves interferometer system50and then is reflected from surface32to re-enter interferometer system50back to lens46, where it is reset to approximately the same spot size prior to focusing. Disk10is a moving, such as rotating, target. From lens46, reflected object beam31is provided to quarter-wave plate29. Object beam27immediately prior to passing through quarter-wave plate29comprises only p-polarization components. After passing through quarter-wave plate29, object beam27comprises only circular polarization components. Reflected object beam31immediately prior to passing through quarter-wave plate29comprises only circular polarization components. After passing through quarter-wave plate29, reflected object beam31comprises only s-polarization components, and thus as reflected object beam31enters from a side opposite to that of original entry to polarizing beam splitter25, it is orthogonal diverted by polarizing beam splitter25in a direction opposite to that of reference beam26for providing a portion of combinative beam33.

Notably, the difference in optical path length48and optical path length47is less than laser beam coherence length. Furthermore, it should be understood that surface defects on surface32causes displacement in optical path length48. For example, depending on reference level, a depression lengthens optical path length48, both with respect to object beam27and reflected object beam31, while a bump shortens optical path length48. Maximum allowed displacement is limited by focus depth of lens46. Optical path length48is modulated by surface32, if surface32is moving. Optical path length48is modulated by out-of-plane, or more particularly out-of-reference plane, movement of surface32.

Reflected reference beam30and reflected object beam31are combined by polarizing beam splitter25to provide combinative beam33. Combinative beam33comprises a reflected reference beam portion and a reflective object beam portion, as respective polarization directions of these portions are orthogonal. In other words, the reflected reference beam portion and the reflective object beam portion in combinative beam33do not interfere with one another.

One of output beam23or24is provided to a quarter-wave plate. In the embodiment shown inFIG. 4, output beam23is provided to quarter-wave plate35. Quarter-wave plate35introduces a phase shift between reflected reference and reflected object beam portions or components of output beam23. Quarter-wave plate35may be adjustable. Thus, for example, quarter-wave plate35could be adjusted, as needed, to introduce a target phase shift, for example approximately 90 degrees, between reflected reference and reflected object beam components of output beam23. Because two waves phase shifted with respect to one another are used, unwrapping of phase is facilitated. Such a phase shift is used for providing a quadrature output, as stated above. However, if outputs were viewed only in parallel, then quarter-wave plate35may be omitted. Notably, reflected reference and reflected object beam components of output beam23, or output beam24for that matter, are still orthogonally polarized with respect to one another.

Polarizer36receives phase-shifted output beam23and assembles its reflected reference and reflected object beam components along a predetermined direction, for example approximately 45 degrees, to the vertical and horizontal axes of polarization of such components to provide assembled beam38. As mentioned above HWP22is used to balance the beams, but if such beams were out of balance, a predetermined direction or angle may be selected or adjusted to enhance contrast of the interference. So, if reflected object and reference beam components are out of balance, then another angle may be selected to enhance the contrast by equalizing contributions of each such component in assembly of assembled beam38. Assembled beam38may have interference between assembled reflected reference and reflected object beam components in response to displacement in optical path length48caused by surface defects or other surface inconsistencies, or from a nominal surface condition depending on reference plane selection, as mentioned above.

Polarizer37receives output beam24and assembles its reflected reference and reflected object beam components along a predetermined direction, for example approximately 45 degrees, to the vertical and horizontal axes of polarization of such components to provide assembled beam39. Assembled beam39may have interference between assembled reflected reference and reflected object beam components in response to displacement in optical path length48caused by surface defects or other surface inconsistencies, or from a nominal surface condition, as mentioned above.

Assuming surface defects exist and are detected, reflected reference and reflected object beam components interfere in assembled beams38and39to produce moving fringes representing modulation of optical path length48. Such moving fringes, which are temporal variation in light intensity, may be observed in both output beams38and39in parallel. Alternatively, such moving fringes may be observed in both assembled beams38and39in parallel and in phase quadrature.

Assembled beams38and39are provided to optical detectors40and41, respectively. Optical detectors40and41may be photodiode detectors. Detectors40and41operate at a speed sufficient to capture fringes from assembled beams38and39and deliver respective voltages proportional to temporal light intensity change as signals43and44, respectively, for subsequent digital signal processing by information processing system (IPS)42.

Referring toFIG. 5, there is shown a block diagram of an exemplary portion of an embodiment of an information processing system42configured to receive light intensity voltage signals43and44in accordance with one or more aspects of the present invention. Information processing system comprises processor53, memory54, input/output interface55and display device56. Information processing system42may be a programmed personal computer or a digital oscilloscope or other known device for processing signals of the form of signals43and44.

The QPSI design is based on the principle of using the polarization property of the light to create two independent interference signals I and Q which are phase shifted by 90° with respect to one another. These two signals are represented by:
I=Io+Ir+2√{square root over (IoIr)}·cos(φ)  (0)
Q=Qo+Qr+2√{square root over (QoQr)}·cos(φ)
where, Ioand Ir, Qoand Qrare the intensities of the object and reference means, respectively. The φ symbol is the phase angle between the object and reference beam. In these equations, all parameters are unknown, except for I and Q, which are detected by the photo detectors. The phase angle φ is the unknown that needs to be resolved, because it carries the object displacement (out-of-plane) information. Signals43and44, as mentioned above, represent temporal interference fringes formed in response to temporal phase difference, φ, between reflected reference beam30and reflected object beams31. Temporal phase difference or phase, φ, is a function of object displacement, d, namely displacement caused by disk medium surface32out-of-plane motion. This relationship may be expressed as,
φ=(4πd)/λ  (1)
where wavelength, λ, is the wavelength of laser beam21. Notice that if displacement, d, equals 0, then phase φ equals 0, or in other words disk medium surface32is flat, which may be taken as a reference location. However, it is not necessary to take the flat or unaffected portion of a disk media surface32as a reference location or plane. Accordingly, it should be understood that displacement, d, is a value depending on a reference location. Thus, displacement d is actually a change in displacement, Δd, with respect to such a reference location. Likewise, phase, φ, is actually a change in phase, Δφ, due to change in displacement.

where Imaxand Iminare the maximum and minimum intensities of the I beam, namely assembled beam38, and where Qmaxand Qminare the maximum and minimum intensities of the Q beam, namely assembled beam39.

Therefore, we can solve the equation (0) or (2), provided that we can find Imaxand Iminand Qmaxand Qminrespectively. The approach described here provides a reliable method for finding the values of the true maxima and minima of the interference signals. This method is described in details in the following paragraphs.

Because phase angle is used as the argument for a sine and a cosine function as in Equations (2A) and (2B) [collectively “Equations (2)”], phase wrapping occurs. In other words, phase wraps around to the same value for every 2π increase or decrease. To obtain the actual phase in Equation (1), phase from Equations (2) must be unwrapped. However, because phase φ directly resolved from Equations (2) yield the principal value of phase, the first step of unwrapping phase is to calculate the phase angle and extend it into a 0 to 2π phase range. To calculate phase angle in a 0 to 2π phase range, phase is calculated according to rules or boundary conditions of Equations (4A) and (4B) for phase angle of assembled beam38,
φ=cos−1[(I−Ia)/Ib] forQ−Qa≦0  (4A)
φ=2π−cos−1[(I−Ia)/Ib] forQ−Qa>0  (4B)
and Equations (5A), (5B) and (5C) for phase angle of assembled beam39,
φ=sin−1[(Qa−Q)/Qb] forI−Ia≧0 andQ−Qa≦0  (5A)
φ=π−sin−1[(Qa−Q)/Qb] forI−Ia<0  (5B)
φ=2π+sin−1[(Qa−Q)/Qb] forI−Ia≧0 andQ−Qa>0  (5C).

The present invention provides a reliable method for finding the values of true maxima and minima of the interference signals. The method is based on the quadrature property of I and Q signals, which is that if one signal is at a peak or valley, the corresponding signal must be at a zero crossing, and vice versa. It is assumed in this embodiment that the I and Q signals are AC coupled to the digitizer such that their base lines will be close to the value zero. The equations of 2A and 2B have a graphical interpretation shown inFIG. 2. From this figure, it is clear to see that equations 3A–3D result.

FIG. 3re-plots the signals fromFIG. 1such that Q is shown as a function of I. This plot consists of either circles or ellipses, depending upon the amplitude ratio of the signals I and Q. An x-y axis is located at the point I=0, Q=0. All of the true extreme of the Q signal are located on the y-axis inFIG. 3, and all of the true extreme of the I signal are located on the x-axis.

FIG. 6is a flow chart of an exemplary method for determining the out-of-plane displacement in accordance with embodiments of the present invention. This method is performed by the processor53ofFIG. 5based upon the signals from the QPSI detector.

InFIG. 6, the method starts in step70with the obtaining of the I and Q signals from the QPSI detector, such as that shown inFIG. 4. The processor53determines zero-crossing points in one of the signals in step72. In the exemplary embodiment described inFIG. 6, the zero-crossing points are determined in the I signal in this step. The peaks and valleys in the Q signal are then detected in step74. These peaks and valleys are those that are determined to be in close proximity to the zero-crossing points in the I signal detected in step72.

In step76the zero-crossing points in the Q signal are determined. In step78the peaks and valleys in the I signal in close proximity to the zero-crossing point to the Q signal are detected by peak detection.

From the detected peaks and valleys in the I and Q signals that are located in close proximity to the zero-crossing points of the other respective signals, processor53determines the true maximum and minimum points of the I and Q signals respectively, in step80. From these true maximum and minimum plants of the I and Q signals, intensity envelopes are formed in step82by the processor53. The forming of the intensity envelopes includes curve fitting to respectively link: the maximum points of the Q signal; the minimum points of the Q signal; the maximum points of the I signal; and the minimum points of the I signal. The curve fitting may be performed by selecting a curve fitting method based on decoding error analysis. In certain embodiments of the invention, a second order polynomial curve fit to a segment of every three detected extrema points provides satisfactory results. Following the determination of the true maximum and minimum points of the I and Q signals, equations 3A–3D may be resolved.

Hence, the process continues to step84in which the phase angle is determined by the processor53. Finally, in step86, out-of-plane displacement is determined from the previously determined phase angle.

The above-described methodology is exemplary only, as the order of determining the peaks and valleys in the I and Q signals may be switched. In other words, steps72,74may be performed after steps76,78.

With the present invention, the quadrature property of I and Q signals is utilized to determine the points of true maximum and minimum intensity of the interference signals and to create accurate intensity envelopes of the interference signals. The invention provides a scheme for detecting the maximum and the minimum value and location accurately and may be employed with any number of different QPSI arrangements.