Focus error detection using an equal path length lateral shearing interferometer

This invention provides an equal path length lateral shearing interferometer and its implementation in focus detection devices, particularly for optical data storage devices. The interferometer comprises a beamsplitter and two abutting roof prisms. Shifting the placement of a roof prism along an axis orthogonal to the direction of light propagation and to the peak of the roof creates a lateral shear between the two output beams. Rotation a roof prism about this axis produces a tilt between the two output beams. This creates an interference pattern with a rotational orientation which is a function of the focus of the beam impinging on the optical storage medium. A pattern sensor is provided to adjust the focus of the beam in an accurate and dynamic manner.

FIELD OF THE INVENTION 
This invention pertains to equal path length lateral shearing 
interferometers and to their use in aberration detection, in focus 
detection and in optical data storage devices. 
BACKGROUND OF THE INVENTION 
In recording or sensing data on an optical storage disk, accurate focus of 
the impinging light beam is critical. A real time servo system is needed 
to keep the optics in focus. A laser supplies a collimated monochromatic 
light beam which is focused by a lens system onto a rotating optical disk. 
The disk reflects the beam through a lens system to a beamsplitter which 
directs a portion of this beam to a focus detection system. A focus error 
signal is generated and electronically coupled to the focussing lens 
system. Most focus detection techniques rely on the following properties 
of the returning beam. If the disk is at the focal plane of the focussing 
lens system and the light source is collimated, the return beam will also 
be collimated. If the disk is too close to the lens system the return beam 
will be more divergent and if it is too far the return beam will be more 
convergent. 
The astigmatic focus scheme is representative of the conventional focus 
detection technology. A cylindrical lens provides different focal lengths 
for the horizontal and vertical axes of the return light beam. A quadrant 
detector placed at the average of the focal lengths senses a round pattern 
when the beam is in focus and vertically or horizontally elliptical 
patterns when the focus is too-close or too-far. A disadvantage of this 
method is the difficulty in aligning the cylindrical lens and quadrant 
detector along the light propagation axis and the two orthogonal axes, and 
in aligning the angular orientation of the detector about the light 
propagation axis. Further, this method is sensitive to environmental 
influences such as mechanical shock and temperature variations. Thermal 
shifts in the refractive index of optical components and in the optical 
path length change the focus of the astigmatic lens on the detector and 
thus degrade the focus detection accuracy. 
Numerous focus detection schemes have been proposed which are less 
sensitive to thermal shifts (see, for example, Ohara et al. in U.S. Pat. 
No. 4,724,533 and Smid et al. in U.S. Pat. No. 4,712,205). However, they 
typically function by focussing one or more beams onto a plurality of 
detectors and thus have the critical alignment requirements of the 
astigmatic system. D. K. Cohen (U.S. Pat. No. 4,604,739 and Ph.D. Thesis, 
Univ. of Arizona, 1987) disclosed a method of focus detection employing 
rotated interference patterns. The technique relies on the properties of 
lateral shearing interferometers (Malacara in Optical Shop Testing, John 
Wiley & Sons, New York, 1978, pp. 105-148). If a collimated beam of light 
impinges on a lateral shearing interferometer (LSI) an interference 
pattern is produced. If the beam is rendered diverging, as when the 
optical disk is too close to the focus, the interference pattern rotates 
in one direction and if the beam is converging the interference pattern 
rotates in the opposite direction. A quadrant detector placed in the 
interference pattern detects the rotational orientation of the 
interference fringes. With the LSI of Cohen's invention, the interference 
pattern does not change with propagation so the quadrant detector can be 
placed at any distance from the interferometer. This provides design 
flexibility and permits a compact detection device. In the directions 
orthogonal to the light propagation axis the quadrant detector must be 
centered on an interference fringe, a far less stringent requirement than 
placement at the focal point of a lens. In addition, this technique 
reduces sensitivity to the exact optical path lengths and thus to 
temperature variations. 
Cohen's patent does not address a critical aspect of the lateral shearing 
interferometer used in this application. Diode lasers are, in general, the 
light source of preference in optical storage systems. Typically, they 
have short coherence lengths of less than 1 mm. Consequently no fringes 
will be visible at the quadrant detector when the path length difference 
between the beams that form the interference pattern exceeds this 
coherence length. It is therefore an object of the present invention to 
solve this important implementation issue with the introduction of a 
compact equal path length lateral shearing interferometer. 
SUMMARY OF THE INVENTION 
The present invention is an equal path length lateral shearing 
interferometer and its implementation in focus detection devices, 
particularly for optical data storage systems. This invention provides a 
particular advantage over the prior art when the coherence length of the 
light source is short, as is the case with semiconductor diode lasers. 
The lateral shearing interferometer of the present invention comprises a 
beamsplitter and two abutting roof prisms. The beamsplitter divides the 
input beam into two portions and directs a portion to each of the prisms. 
The prisms return the two beams to the beamsplitter, which recombines them 
at the output. These optical elements can be arranged to provide equal 
path lengths for the two light beams. Shifting the placement of a roof 
prism along an axis orthogonal to the direction of light propagation and 
to the peak of the roof creates a lateral shear between the two beams. 
Rotating a roof prism about this axis produces a tilt between the two 
beams at the output. 
The interferometer of the present invention is a compact unit wherein the 
beamsplitter is a beamsplitting cube and the two roof prisms abut the cube 
on adjacent faces. To provide a tilt between the two beams, an abutting 
surface of the beamsplitter can be ground at an angle. Alternatively, the 
hypotenuse face of the roof prism can be ground to provide a tilt. For 
commercial applications this compact interferometer has advantages over 
the separately mounted and aligned components of the interferometers of 
the prior art. 
For focus error detection, an embodiment of this invention includes a 
collimated monochromatic coherent light source and a lens system to focus 
the beam onto a reflective target surface. The reflected beam is 
recollimated, typically by the same lens system, and a portion is directed 
to the lateral shearing interferometer. The collimation error of the 
return beam is a function of the focus error of the impinging beam, and 
the rotational orientation of the fringes generated by the interferometer 
is a function of the return beam collimation. A quadrant detector of four 
photocells is placed in the interference pattern to measure the rotational 
orientation and thereby measure focus error. 
In a specific form of this invention the focus detection device is employed 
in an optical data storage system. In this system the focus target is an 
optical record medium with addressable data storage locations. Electronic 
circuits are connected to the quadrant detector to analyze the 
interference pattern and to adjust the focus of the beam impinging on the 
optical record medium in an accurate and dynamic manner, and thereby 
reduce the focus error.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is the optical data storage device of the present invention with an 
equal path length lateral shearing interferometer focus error detection 
scheme. Monochromatic coherent light is generated by laser 12. 
Beamsplitter 22 transmits the generated beam 30 to focusser 20 which 
focusses it onto optical record storage medium 10. The reflected beam 32 
is recollimated by the focusser and directed by beamsplitter 22 to focus 
detector 40. The focus detector converts optical indications of focus to 
electrical signals 62 which are analyzed by circuits 60 and the focus is 
corrected via focus control signals 64. In the embodiment illustrated 
optical components, including the focus detector, are mounted on moveable 
arm 14. Alternatively, these elements can be stationary (the Fixed Optical 
Elements configuration) and radial motion of beam 30 can be provided by 
movement of focusser 20. 
Semiconductor diode lasers are the presently preferred light source because 
they can be amplitude modulated, and are reliable, low cost and compact. 
However, they produce a beam with a short coherence length of typically 1 
mm. Laser 12 can be a semiconductor laser and it can include optical 
elements to collimate the beam and to create a round cross section. The 
components of beamsplitter 22 depend on the type of optical record medium. 
For example, for magneto-optical media used in erasable systems, in which 
the data signal is contained in polarization changes, it can be a 
partially polarizing beamsplitter. In other storage systems such as 
read-only disks and amorphous-crystalline phase-change erasable disks, in 
which the data are retrieved by either intensity contrasts or phase 
shifts, 22 can be a polarizing beamsplitter and a quarter-wave retarder. 
The focus detector of the present invention can be used with any of these 
systems. Focusser 20 focusses the beam into a preferably 
diffraction-limited spot. The data density depends on this spot size and, 
to achieve maximum storage density, active control of the focus is needed. 
Focusser 20 can contain a tracking mirror and a focusing objective lens or 
other elements such as prisms and focussing mirrors. Optical record member 
10 is shown here as a rotating disk. It contains record tracks which can 
be concentric tracks or a single spiral track. Radial scanning of the 
tracks is achieved by reciprocating motion of arm 14 radially of rotating 
disk 10, or by motion of focusser 
An embodiment of the focus detector is shown in FIG. 2. Beamsplitter 22 
optically couples return beam 32 with detector elements 40. Most of the 
light is transmitted through beamsplitter 42 to data and radial tracking 
detector 44. Some of the light is deflected in beam 34 to equal path 
length lateral shearing interferometer 50. The interferometer comprises 
beamsplitter 52 and roof prisms 54 and 56. Input beam 34 is divided by LSI 
50 into beams 36 and 38, which interfere to produce a fringe pattern 
indicative of the focus on disk 10. This pattern is detected by pattern 
sensor 58 which, in this embodiment, is a rectangular array containing 
photodetectors A, B, C and D. It is drawn skewed to indicate that it is in 
a plane orthogonal to the plane of the paper. 
In the focus error detection device of the present invention, errors in 
focus are manifested in collimation changes of reflected beam 32, as shown 
in FIG. 3. Focusser 20 is shown as a focussing objective lens with focus 
in plane 21. When disk 10 is in focus, collimated source beam 30 is 
reflected to form collimated return beam 32 (FIG. 3A). When the disk is 
too-far from the focus (FIG. 3B) the return beam is converging and when 
the disk is too-close (FIG. 3C) the return beam is diverging. The distance 
of the disk from the focal plane is .DELTA.z. A decollimated beam has a 
curved wavefront (FIG. 3D) characterized by R.sub.o, the radius of 
curvature, which is positive for a diverging beam and negative for a 
converging beam. 
A lateral shearing interferometer produces an interference fringe pattern 
in which either the spacing or rotational orientation of the fringes is a 
function of R.sub.o. The LSI of the prior art is shown in side view (FIG. 
4A) and end view (FIG. 4B). Shearing plate 51 is a wedged plate of 
thickness t and wedge angle .THETA..sub.w, operated in this embodiment 
with input beam 34 at a 45.degree. angle of incidence. The input beam is 
divided into beams 36 and 38 which are recombined to form an interference 
pattern. Output beams 36 and 38 are laterally sheared by S.sub.x, which is 
a function of the angle of incidence, t, the thickness and the refractive 
index of the shearing plate. They are tilted by .THETA..sub.y, which is a 
function of .THETA..sub.w. In order to obtain interference fringes, the 
difference in the path lengths of beams 36 and 38 must be smaller than the 
coherence length of the laser. For optical glass with refractive index 
n=l.5 and minimum plate thickness t.apprxeq.2 mm, the path length 
difference is about 5 mm. This exceeds the typical 1 mm coherence length 
of the diode lasers employed in optical data storage devices. Thus the 
lateral shearing interferometer of the prior art can not be used for focus 
detection in these devices. 
The present invention provides a compact equal path length lateral shearing 
interferometer 50 (FIG. 5) which overcomes this implementation limitation. 
It comprises beamsplitter 52 and roof prisms 54 and 56. Element 52 is a 
means for dividing the amplitude of beam 34 and comprises two prisms which 
form a beamsplitting cube. Elements 54 and 56 can be the standard roof, or 
Porro, prisms known in the art, which have a triangular cross section with 
angles of 45.degree., 45.degree., and 90.degree.. This is the geometry 
illustrated in this embodiment but other angles can be employed. The roof 
prisms can be made of optical glass and can have reflective coatings on 
the surfaces which do not face beamsplitter 52. In the embodiment of FIG. 
5 the prism edges are parallel to the beamsplitter edges. Each prism can 
also be rotated about an axis normal to the hypotenuse face. 
To achieve a lateral shearing of beams 36 and 38 one of the roof prisms is 
shifted by .DELTA.L from the beamsplitter center, as shown in FIG. 5A. 
This shears the output beams by S.sub.x =2.DELTA.L. The shear of the 
output beams is a pure lateral shift, i.e. there exists a plane (the plane 
of the paper in FIG. 5A) in which the projections of beams 36 and 38 are 
parallel and non-coincident lines. 
The interferometer of the present invention further provides a means for 
tilting the output beams so that the projections of output beams 36 and 38 
in a plane orthogonal to the direction of shear are not parallel. To 
achieve this, one of the roof prisms is tilted by .THETA..sub.T, as shown 
in FIG. 5B, which is a view from the right side of FIG. 5A. This tilts the 
beams by .THETA..sub.y =2.THETA..sub.T. The tilt can be achieved by 
grinding a face of the beamsplitter which abuts a roof prism. 
In the embodiment of the interferometer with a shear but no tilt, the path 
lengths are in theory exactly equal, and in practice are negligibly 
affected by the manufacturing capabilities for optical components. In the 
embodiment with both shear and tilt, the path length difference is a 
function of the beamsplitter dimensions and of .THETA..sub.T. In the 
embodiments preferred for focus detection, this difference is orders of 
magnitude less than the 1 mm laser coherence length. Thus the equal path 
length lateral shearing interferometer of the present invention enables 
the implementation of the LSI in focus detection for optical storage 
devices. 
The spacing and slope of the interference fringes are a function of 
R.sub.o, S.sub.x, and .THETA..sub.y. When no tilt is introduced, the 
equation of a fringe is: 
##EQU1## 
where n is an integer and .lambda. is the laser wavelength. FIG. 6A shows 
this interference pattern 39, which has fringes of fixed orientation, 
along the y axis, with a spacing .DELTA.x which increases with the amount 
of defocus. However, the fringe spacing is independent of the sign of 
R.sub.o so this device can not distinguish between too-close and too-far 
nonfocus conditions. 
With the addition of a tilt in a direction orthogonal to the shear, the 
equation of a fringe is: 
##EQU2## 
Note that Eq. 2 reduces to Eq. 1 when .THETA..sub.y =0. FIGS. 6B-D show 
that in this interference pattern the fringes are lines with fixed spacing 
.DELTA. and with a slope that is a function of the defocus. Since the sign 
of R.sub.o determines the sign of the slope in Eq. 2 and therefore the 
rotational orientation of the fringes, .THETA..sub.f, an appropriate focus 
error signal can be generated. 
Quadrant detector 58 is placed in the fringe pattern as shown in FIGS. 
6B-D. Quadrants A through D are located as shown in FIG. 2. Because the 
fringe pattern is not a function of distance from the interferometer, the 
detector can be spaced at any distance, in contrast to prior art such as 
the astigmatic focus method in which the placement of the detector along 
the light propagation axis is critical. The tilt angle .THETA..sub.y 
between the output beams does, in fact, introduce a distance dependence to 
the shear. However, for focus detection this angle is so small that no 
difference is detected in the interference pattern viewed at 0, 1, and 100 
mm from the interferometer. The signals generated in quadrants A and C of 
the detector are added, as are the signals from quadrants B and D. The 
focus error signal is generated from the difference (A+C)-(B+D). The 
detector is positioned to yield a zero error signal when the disk is in 
focus (FIG. 6B). In the embodiment shown, when the disk is too far from 
the lens, more light will fall on detectors A and C and thus generate a 
positive error signal (FIG. 6C). When the disk is too close, the error 
signal will become negative (FIG. 6D). 
The fringe spacing and tilt can be tailored to a particular application. To 
illustrate the method, the optimization of the interferometer with both 
shear and tilt is demonstrated for an optical storage device. Following 
the analysis of Cohen (Ph.D. Thesis), the fringe separation .DELTA. is: 
##EQU3## 
for a typical optical data storage system the beam diameter is about 4 mm. 
For maximum signal intensity, the quadrant detector is the same size as 
the beam. So that only one fringe strikes the detector, the optimum fringe 
spacing is about 5 mm. For a diode laser wavelength of about 800 nm, this 
corresponds to .THETA..sub.y =0.01.degree. and thus the optimum tilt of 
the interferometer is .THETA..sub.T =0.005.degree.. 
The sensitivity of the detector to the degree of defocussing can be 
tailored through the amount of shear. The maximum error signal corresponds 
to a fringe rotation of 45.degree.. From Eq. 2, this occurs when: 
##EQU4## 
Rewriting R.sub.o as a function of the distance .DELTA.z of the disk from 
focus, the focal length f.sub.o of the objective lens, and the separation 
z.sub.I of the objective lens and the interferometer, Eq. 4 becomes: 
##EQU5## 
for a "lock on" range of .DELTA.z.sub.max =5 .mu.m for the focus, and 
typical optical storage device dimensions, Eq. 5 gives S.sub.x =0.3 mm and 
therefore the optimum roof prism shift is .DELTA.L=0.15 mm. 
The versatility of the interferometer of this invention is evident in the 
ability to tailor the spacing of the interference fringes and their 
sensitivity to beam collimation to a particular application. This is 
accomplished simply by changing the roof prism shift and tilt, as taught 
by Eqs. 3 and 5. The LSI of this invention provides independent control of 
these parameters. In the embodiment of FIG. 5 the shift and tilt are shown 
on separate roof prisms for clarity of illustration. The shift and the 
tilt can be on either of the roof prisms and both can be on the same one. 
If one roof prism is translatable, the amount of shear can be varied. The 
interferometer of this invention is a unitary structure with the roof 
prisms abutting the beamsplitter. In fact, for the use in a focus 
detector, as shown in FIG. 2, beamsplitter 22, beamsplitter 42, 
interferometer 50, and detectors 44 and 58 can be made a compact unitary 
structure by abutting the elements on facing surfaces, as is clear from 
the drawing. 
One of the roof prisms can be replaced by a flat reflective surface, which 
can be a reflective coating on the appropriate beamsplitting cube face. In 
this embodiment the single roof prism can provide both shear and tilt. 
Because a roof prism reflects a beam about the axis of the roof line to 
produce a linearly inverted image, when there is one flat mirror and one 
roof prism the interference pattern is formed between a beam and linearly 
inverted image of the beam. If the beam is symmetric about the roof line 
of the prism, the interference pattern is the same as with two roof 
prisms. If the beam is not symmetric, this is indicated by distortion of 
the interference pattern. 
The present invention, the compact equal path length lateral shearing 
interferometer and its application to focus detection and to optical data 
storage devices, is not limited to the foregoing descriptions of 
configurations and applications. Adaptations of the design and 
applications to other systems will be readily apparent to those skilled in 
the art.