Focus-error detection using prism-enhanced spot size monitoring

Claim 1

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light source for providing a primary light beam to said medium; a prism positioned in an optical path of a return light beam reflected from said medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at said first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 2

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light source for providing a primary light beam to said medium; a prism positioned in an optical path of a return light beam reflected from said medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at said first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 6

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a prism having one surface for circularizing said primary beam; a quarter-wave plate for receiving said primary beam from said prism; a lens for focusing said primary beam from the quarter-wave plate onto said medium; said prism being positioned in the optical path of a return light beam reflected from the medium for reducing said return light beam in one dimension by a factor M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 7

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a prism having one surface for circularizing said primary beam; a quarter-wave plate for receiving said primary beam from said prism; a lens for focusing said primary beam from the quarter-wave plate onto said medium; said prism being positioned in the optical path of a return light beam reflected from the medium for reducing said return light beam in one dimension by a factor M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 8

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a quarter-wave plate for receiving said primary beam; a prism having one surface for circularizing the primary beam from the quarter-wave plate, a second surface from which the circularized light is reflected by total internal reflection, and a third surface which is reflective; a lens for focussing the beam reflected from said third surface onto the medium; said prism being positioned in the optical path of a return light beam reflected from the medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a polarizing beamsplitter for receiving said return light beam from said prism; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 9

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a quarter-wave plate for receiving said primary beam; a prism having one surface for circularizing the primary beam from the quarter-wave plate, a second surface from which the circularized light is reflected by total internal reflection, and a third surface which is reflective; a lens for focussing the beam reflected from said third surface onto the medium; said prism being positioned in the optical path of a return light beam reflected from the medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a polarizing beamsplitter for receiving said return light beam from said prism; a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third areas with the amount of light at the second area and generating a focus-error signal responsive thereto.

Claim 10

A method for detecting a focus-error in an optical head to which a light beam is directed for reading and/or writing data on an optical recording medium, the steps comprising: positioning a prism in an optical path of a return light beam reflected from said medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/ convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor M 2 ; providing a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return light beam; and comparing the sum of the amount of light at the first and third photosensitive areas with the amount of light at the second photosensitive area and generating a focus-error signal responsive thereto.

Claim 11

A method for detecting a focus-error in an optical head to which a light beam is directed for reading and/or writing data on an optical recording medium, the steps comprising: positioning a prism in an optical path of a return light beam reflected from said medium for reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/ convergence angle of said return light beam by a factor of M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor M 2 ; providing a photodetector located in said optical path for receiving an unobstructed return light beam from the prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return light beam; and comparing the sum of the amount of light at the first and third photosensitive areas with the amount of light at the second photosensitive area and generating a focus-error signal responsive thereto.

Claim 15

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a first prism for circularizing said primary beam by refraction; a lens for focussing said primary beam from the first prism on to the medium; a second prism; said first prism being positioned in the optical path of a return light beam reflected from the medium, the first prism having a partially polarizing beamsplitter surface for directing a fraction of a p-polarized component of said return beam and substantially all of a s-polarized component of said return light beam to the second prism; said second prism having a partially polarizing beamsplitter surface for refracting a fraction of the p-polarized component of said return light beam from the first prism, and said second prism reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the second prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third photosensitive areas with the amount of light at the second photosensitive area and generating a focus-error signal responsive thereto.

Claim 16

An apparatus for detecting a focus-error in an optical head adapted to read and/or write data on an optical recording medium, comprising: a light generation means for providing a primary beam of collimated light; a first prism for circularizing said primary beam by refraction; a lens for focussing said primary beam from the first prism on to the medium; a second prism; said first prism being positioned in the optical path of a return light beam reflected from the medium, the first prism having a partially polarizing beamsplitter surface for directing a fraction of a p-polarized component of said return beam and substantially all of a s-polarized component of said return light beam to the second prism; said second prism having a partially polarizing beamsplitter surface for refracting a fraction of the p-polarized component of said return light beam from the first prism, and said second prism reducing said return light beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle of said return light beam by a factor M in said dimension, whereby a change in said dimension of said return light beam indicative of said focus-error of said head is enhanced by a factor of M 2 ; a photodetector located in said optical path for receiving an unobstructed return light beam from the second prism, the photodetector having a first, second and third photosensitive areas located proximate one another along said dimension of said return beam; and a circuit connected to the photodetector for comparing the sum of the amount of light at the first and third photosensitive areas with the amount of light at the second photosensitive area and generating a focus-error signal responsive thereto.