Apparatus and method for minimizing magnification distortion in multi-track optical recording

A multi-beam optical recording system includes a semiconductor laser array. The laser beams are expanded via a prism beam expander to have generally circular cross-sections for optimum spot intensity on the recording medium. According to the teaching of the prior art, the average beam angle of incidence on the input prism of the beam expander is the Brewster angle. According to the present invention, means are provided for rotating the beam expander to minimize the variations in spacing between beams, these variations resulting from the magnification distortion of the prisms of the beam expander.

This invention relates generally to optical recording systems and more 
particularly, to a method for minimizing the track pitch variations due to 
magnification distortion in a multi-beam optical recording system. 
BACKGROUND OF THE INVENTION 
Optical recording/playback of information has been made possible by 
developments in the areas of lasers and thermal record media. Recent 
developments have led to mass data storage systems utilizing a plurality 
of individually modulated laser beams to record information at extremely 
high data rates. For example, in U.S. Pat. No. 4,449,212, issued on May 5, 
1984, in the name of the instant inventor, a multi-track record/playback 
apparatus is described. In the multi-track apparatus the light beam from a 
single high power laser is split into a plurality of beams which are 
individually modulated and focused onto the surface of a record medium. In 
general, systems of this type require large, high power lasers which 
require external cooling. Furthermore, in these systems a modulator is 
provided to individually modulate each beam of the multiple beams being 
used for recording. For these reasons, prior art multi-beam systems tend 
to be bulky, low in efficiency, and difficult to modulate. 
The recent introduction of semiconductor laser arrays has led to the 
development of multi-channel optical recorders/players which overcome some 
of the problems of the prior art multi-channel systems. A diode laser 
array system is generally more compact, has higher efficiency, and 
requires no external modulation. 
In U.S. Pat. No. 4,520,472, issued May 28, 1985, in the name of the instant 
inventor, an optical system is described for use in a multi-channel 
record/playback system. The optical system includes a laser diode array 
and an optical head comprising a collection objective, an anamorphic beam 
expander, a relay lens and a focusing lens having a finite conjugate. The 
optical head collects the laser beams emitted by the diode array, expands 
the beam cross-section to form circular beams and focuses the beams to 
diffraction limited spots. The relay lens images the lasing spots from the 
laser diode array in the conjugate plane of the finite conjugate focusing 
lens. 
In a multi-channel system of the Reno ('472) type, the beam expander, which 
may typically comprise two prisms, compensates for the elliptical 
character of the laser diode beams, reshaping their emission patterns into 
symmetrical beams, i.e., circular beams, so that the highest possible spot 
intensity can be obtained at the recording surface. It has been common 
practice to direct the central beam expander at an angle equal to the 
Brewster angle. 
When a beam of linearly polarized light is incident on the surface bounding 
two transparent media of different refraction indexes such that the 
incident beam vibrates in the plane of incidence, there exists a certain 
angle of incidence for which the intensity of the reflection beam is equal 
to zero. That angle .phi. is called the polarization angle, or Brewster 
angle, and is related to the indexes of refraction of the two media, 
n.sub.1 and n.sub.2, by the expression 
EQU .phi.=tan.sup.-1 (n.sub.2 /n.sub.1). 
It is noted that when the central beam of a multi-beam diode array, e.g., 
an array of nine beams, is incident on a prism beam expander at the 
Brewster angle, there is a variation in the magnification ratios which may 
be 5 to 6 percent between the least and the greatest. These variations 
manifest themselves in an optical recording system as variations in track 
spacing on the recording surface. Since the smallest beam spacing of the 
array must determine the recording track pitch, the 5 to 6 percent 
variation across the array produces inefficiency in the recording surface 
utilization. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention, there is 
disclosed an optical system comprising at least three solid state elements 
emitting respective coherent light beams having generally elliptical 
cross-sectional shapes. The solid state elements are arranged in a linear 
array having substantially equal spacings between adjacent pairs of the 
elements. The system also includes a first lens for collecting the 
respective coherent light beams emitted by the array of solid state 
elements and forming the respective coherent light beams into collimated 
light beams. The system further includes a beam expander for 
anamorphically expanding the collimated light beams to reshape the 
cross-sections of the collimated light beams such that the cross-sectional 
shape of the collimated light beams is generally circular. The beam 
expander comprises a pair of prisms, which distort the spacings between 
adjacent pairs of the beams due to unequal transit paths of the respective 
beams through the pair of prisms. Finally, the system includes means for 
minimizing variations in the spacings between adjacent pairs of the beams 
including means for adjusting the angles of incidence of the collimated 
light beams formed by the first lens on the beam expander by rotating the 
beam expander along an axis which is intermediate the pair of prisms and 
normal to the collimated light beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1a, there is shown an optical system for collecting a 
plurality of laser beams emitted by a diode array, expanding the beam 
cross-sections to form circular beams, and focusing the beams to 
diffraction limited spots. Semiconductor laser array 10 is a linear array 
which emits a plurality of light beams, typically nine, although for ease 
of understanding, the principal rays of only three beams 12, 14 and 16 
emitted from diode elements 7, 8 and 9, respectively, are shown in FIG. 
1a. The diode elements of array 10 are equally spaced from each other, 
typically 150 micrometers. 
Beams 12, 14 and 16 have a generally elliptical cross-sectional shape. They 
are collected into collimated, but not parallel, beams 18, 20 and 22, 
respectively, by a collection objective lens 24. Illustratively, 
collection objective lens 24 may be a plano-objective microscope lens. 
Collimated beams 18, 20 and 22 are incident on the incoming prism 28 of 
anamorphic beam expander 26 which comprises prisms 28 and 30. 
Illustratively, prisms 28 and 30 may be formed of high index glass, e.g., 
Schott LaSF-8, having an index of refraction of 1.788 at 830 nanometers. 
According to the teachings of the prior art, anamorphic beam expander 26 
is positioned so that the angle of incidence 27 on surface 29 of prism 28, 
which is seen more clearly in the magnified view of FIG. 1b, of a central 
beam 20 from array 10, is the Brewster angle, which, for the glass 
specified above by way or example, is 60.787 degrees. 
In the FIG. 1a embodiment, beams 12, 14 and 16 emitted from semiconductor 
laser device 10 are set at the polarization in which the electric field 
vector vibrates in the plane parallel to the sheet of the figure. 
Collection objective lens 24 is arranged in its focal position with 
respect to laser device 10 so as to render beams 18, 20 and 22 incident on 
surface 29 of prism 28 as collimated beams. With the polarization as 
specified and with beams 18, 20 and 22 incident on surface 29 of prism 28 
at substantially the Brewster angle, there is, theoretically, virtually no 
reflection from surface 29. Beams 18, 20 and 22 are refracted through 
prisms 28 and 30 such that the beams that exit from prism 30 are parallel 
to their original optical axes. 
The refraction of the beams through anamorphic beam expander 26 expands 
elliptical beams 18, 20 and 22 in a plane parallel to the sheet of the 
figure while leaving the beams unaffected in a plane perpendicular to the 
sheet. Thus, the beams 32, 34 and 36 exiting from prism 30 are generally 
circular in cross-section. Illustratively, the expansion factor of beam 
expander 26 may be three. 
Beams 32, 34 and 36 exiting beam expander 26 from prism 30 are incident on 
relay optics 38 consisting of a doublet of lenses 40 and 42. Relay doublet 
38 may possibly be a singlet if the aberrations of the singlet lens can be 
tolerated. The relay lens system 38 images the lasing points 7, 8 and 9 of 
array 10 at plane 44, which is the focal plane of lens system 38. The 
focusing lens 46, which is chosen to be a finite conjugate lens, has its 
finite conjugate plane coincident with the image plane 44. 
From plane 44, lasing spots 7, 8 and 9 are imaged as 7', 8' and 9' on 
surface 48 of record medium 50. Illustratively, record medium 50 may be of 
a type disclosed in U.S. Pat. No. 4,222,071, issued in the name of A. E. 
Bell et al. Illustratively, a Bell-type record medium may be a disk having 
a light sensitive surface upon which ablative recording by the thermal 
effects of a focused laser beam may be made. Alternatively, record medium 
50 may be a magneto-optic record disk, having on its surface 48 a magnetic 
material which causes the polarization angle of laser light to be changed 
when reflected from a recorded spot. 
The effect of the relay lens 38 is to take the exit pupil 52 of objective 
lens 24 where the beams 18, 20 and 22 emitted by laser diodes 7, 8 and 9, 
respectively, are coincident and image that exit pupil 52 into the 
entrance pupil of lens 46, i.e., completely filling lens 46 to form 
diffraction limited spots 7', 8' and 9', respectively, on surface 48. The 
magnification of the beams may be adjusted by varying the doublet design 
of relay lens 38. 
In the present example, in which the laser diodes 7, 8 and 9 are linearly 
spaced from each other by 150 micrometers, the focal length of collection 
objective lens 24 is typically 9.75 millimeters, resulting in angular 
divergences between adjacent beams 18,20 and 20,22 of 
EQU .theta.=sin.sup.-1 (0.15/9.75)=0.88.degree.. 
These angular displacements cause beams 18, 20 and 22 to enter prism 28 of 
beam expander 26 along non-parallel axes. Thus, beams 32, 34 and 36 
exiting prism 30 are unequally spaced from each other due to the unequal 
transit paths of beams 32, 34 and 36 through prisms 28 and 30. As a 
result, the diffraction limited spots 7', 8' and 9' focused on surface 48 
are unequally spaced. 
The unequal spacing of the focused images of a semiconductor laser array of 
nine diodes is shown (exaggerated) in FIG. 2. This distortion is 
quantitatively shown in the plot of FIG. 3, which illustrates the 
inter-spot spacing on a relative scale. Considering a central inter-spot 
spacing, e.g., the spacing between the fourth and fifth diodes, as the 
reference, it may be observed from FIG. 3 that the spacing between the 
first and second diodes is 1.3 percent greater, and the spacing between 
the eighth and ninth diodes is 4.5 percent less, than the reference. 
As a result of experimentation, I have found that the curve obtained by 
joining the points of the FIG. 3 plot becomes a saddle shape as the 
average input angle of beams 12, 14 and 16 onto prism 28 is reduced to an 
incident angle less than the Brewster angle, which is used in the prior 
art teachings. 
Referring again to FIG. 1a, in accordance with the principles of the 
present invention, anamorphic beam expander 26 is mounted on carriage 60 
which allows a small amount of rotation of beam expander 26 about a 
central axis 62 located between prisms 28 and 30 and normal to the sheet 
of the figure. Carriage 60 may, by way of example, have a generally 
circular peripheral surface including a plurality of radially-extending 
gear teeth 64. Lead screw 66 includes threads 68 in mesh engagement with 
gear teeth 64. It will be observed that clockwise solution of leadscrew 66 
causes rotation of carriage 60 about axis 62 in the direction indicated by 
arrow 70, thus reducing the angle of incidence 27 of beams 18, 20 and 22 
on the incoming surface 29 of prism 28 (see FIG. 1b). 
Referring to FIG. 4, there is shown a plot depicting improved inter-spot 
spacings according to the present invention. The curve represents the 
variations of inter-spot spacings over an array of nine laser diode 
elements. Whereas the prior art teaches an incident angle for the central 
beam at the Brewster angle, the present invention teaches that the curve 
of inter-spot spacings is saddle-shaped, and that the optimum angle of 
central beam incidence is less than the Brewster angle, viz., 
approximately 56.2.degree. for the present example, resulting in a maximum 
inter-spot spacing variation of 1.3 percent. 
The principles taught by the present invention may be practiced in a method 
using the apparatus illustrated in FIG. 5. The FIG. 5 apparatus uses the 
same elements as the FIG. 1a apparatus, but replaces focusing lens 46 and 
record medium 50 with apparatus for detecting and measuring the spacings 
between beams 32, 34 and 36, including detecting means, typically a 
charge-coupled device (CCD) camera 80, responsive to the radiant energy 
from beams 32, 34 and 36, and measuring means, typically an oscilloscope 
82, responsive to the output signal from camera 80. 
The oscilloscope display 84 includes a linear sequence of pulses which are 
unequally spaced when the central beam 20 is incident on surface 29 of 
prism 28 at the Brewster angle. According to the teaching of the present 
invention, rotation of leadscrew 66 such that carriage 60 is rotated in a 
direction which reduces the angle of incidence 27 of central beam 20 on 
surface 29 of prism 28 (see FIG. 1b), reduces the variations in spacing 
between adjacent beams. In the FIG. 5 embodiment, this reduction will be 
manifest on display 84 as a shifting of the relative positions of the 
pulses representing beams 32, 34 and 36 input to camera 80. According to 
the present invention, minimization of the effects of magnification 
distortion caused by beam expander 26 is achieved when the spacings 
between the outermost beams and their respective adjacent beams are equal. 
As seen in FIG. 4, when the outermost spacings (1-2 and 8-9) are equal, 
the inter-spot spacings are centered on the top of the saddle curve, and 
the spacings are minimized. Practicing the method using the apparatus of 
FIG. 5 requires the rotation of beam expander carriage 60 until the 
outermost pulses of display 84 are equally spaced from their respective 
adjacent pulses. 
While the principles of the present invention have been demonstrated with 
particular regard to the illustrated structure and method of the figures, 
it will be recognized that various departures from such illustrative 
structure and method may be undertaken in practice of the invention. As an 
example, since it is obvious that the orientation of beam expander 26 with 
respect to incoming beams 18, 20 and 22 needs to be set only once, the 
adjusting means comprising lead screw 66 and gear teeth 64 may be replaced 
by a far simpler means for adjusting the position of carriage 60. The 
scope of this invention is therefore not intended to be limited to the 
structure and method disclosed herein but should instead be gauged by the 
breadth of the claims which follow.