Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-10-28
2004-12-21
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S121000
Reexamination Certificate
active
06834036
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical head used in an optical recording/reproducing apparatus. More particularly, the present invention relates to an optical head with high light utilization efficiency that is provided with a diffractive optical element and a light source emitting beams with a plurality of wavelengths and that can read plural types of information recording media.
BACKGROUND OF THE INVENTION
Optical heads are important components for reading signals from optical recording media such as optical disks, for example, compact disks (CDs) and digital video disks (DVDs), and optical card memories. Not only signal detection functions, but also control mechanisms, such as focus servos or tracking servos, are necessary for optical heads to read out signals from an optical recording medium.
Generally, an optical head comprises various optical components such as a light source, a photodetector, a condenser lens (an objective lens), a focusing/tracking error signal detection element, a mirror for bending an optical path, a collimator lens, and the like. A laser beam emitted from a light source is focused on an optical disk by an objective lens. The laser beam focused on the optical disk is reflected and then is detected by the photodetector. Thus, a reproduction signal is read out. In addition, the focusing/tracking error signal detection element controls focusing and tracking, thus enabling signals to be read out stably.
When a diffractive optical element is used as an optical element in the optical head instead of a refraction optical element such as a general lens, prism, or the like, the optical head can be reduced in size, in thickness, and in weight.
The diffractive optical element denotes an optical element that functions by utilizing diffraction phenomenon. The diffractive optical element is characterized in that a corrugated structure having depth equal to a wavelength order are formed on its surface periodically or quasi-periodically or its surface is formed so that refractive index or amplitude is distributed periodically or quasi-periodically. It has been known that when a period of the diffractive optical element is sufficiently large compared to a wavelength of light being incident onto the diffractive optical element, the diffraction efficiency can be increased almost to 100% by forming the diffractive optical element so as to have a cross section of a sawtooth shape.
When the period of the diffractive optical element is sufficiently large compared to the wavelength of light being incident onto the diffractive optical element, however, the diffraction efficiency of the diffractive optical element can reach 100% only with respect to a design wavelength.
FIG. 14
shows the relationship between a wavelength normalized by a design wavelength and first-order diffraction efficiency of the diffractive optical element. As can be seen from
FIG. 14
, the diffraction efficiency decreases gradually as the wavelength deviates from the design value. Therefore, when the diffractive optical element is used in an optical head in which a light source emitting beams with a plurality of wavelengths corresponding to plural types of optical disks is mounted, the diffractive optical element has been required to be designed for each wavelength optimally and to be positioned only in the optical path of a beam with the intended wavelength to increase the light utilization efficiency.
SUMMARY OF THE INVENTION
In one or more embodiments, the present invention aims to solve the aforementioned problem in the conventional technique and to provide an optical head with high light utilization efficiency that includes a diffractive optical element and a light source emitting beams with a plurality of wavelengths for reading plural types of information recording media.
A first configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately twice as long as the first wavelength; a photodetector; and at least one diffractive optical element provided in optical paths of the beams with the first and the second wavelengths. In the optical head, outgoing light from the diffractive optical element is substantially a second-order diffraction light with respect to the beam with the first wavelength and is substantially a first-order diffraction light with respect to the beam with the second wavelength.
According to the first configuration of the optical head, for example, even when the diffractive optical element is positioned in optical paths of beams with two wavelengths, high diffraction efficiency can be obtained with respect to both the wavelengths, thus obtaining an optical head with excellent optical characteristics.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element has a cross section substantially of a sawtooth shape, and with respect to the first wavelength &lgr;
1
, the second wavelength &lgr;
2
, and a refractive index n of a material of the diffractive optical element, the groove depth in the sawtooth shape is substantially in a range between 2&lgr;
1
/(n−1) and &lgr;
2
/(n−1) in the case of a transmission element, is substantially in a range between &lgr;
1
and &lgr;
2
/2n in the case of a reflection element onto which beams are incident from a substrate side, and is substantially in a range between &lgr;
1
and &lgr;
2
/2 in the Do case of a reflection element onto which beams are incident from an air side.
According to this preferable configuration, the highest diffraction efficiency of the diffractive optical element can be obtained, for example, with respect to the beams with the first and the second wavelengths.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is an objective lens for focusing beams on an information recording medium.
According to this preferable configuration, for example, the objective lens can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is a collimator lens for collimating beams emitted from the light source substantially into parallel beams.
According to this preferable configuration, for example, the collimator lens can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is a focusing/tracking error signal detection element.
According to this preferable configuration, for example, the focusing/tracking error signal detection element can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that with respect to the first wavelength &lgr;
1
, a minimum period &Lgr;
min
of the diffractive optical element satisfies a relationship of &Lgr;
min
≧10&lgr;
1
.
According to this preferable configuration, for example, the diffractive optical element with a high diffraction efficiency of at least 80% with respect to the beam with the first wavelength can be obtained.
In the optical head according to the above-mentioned first configuration, it is preferable that with respect to the first wavelength &lgr;
1
, a minimum period &Lgr;
min
of the diffractive optical element satisfies a relationship of &Lgr;
min
≧22&lgr;
1
.
According to this preferable configuration, for example, the diffractive optical element with a higher diffraction efficiency of at least 90% with respect to the beam with the first wavelength can be obtained.
In the optical head according to the above-mentioned first configuration, it is preferable that a refraction optical element having optical surfaces onto which beams emitted from the light source are incident obliquely is provided in optical paths of the beams with the first and the second wavelength
Matsuzaki Keiichi
Saimi Tetsuo
Shiono Teruhiro
Huber Paul W.
Osha & May L.L.P.
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