JP2878292B2 - Optical head - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head that reproduces information by condensing a light beam toward an optical memory element.

[Conventional technology]

2. Description of the Related Art In recent years, a magneto-optical memory element has been in the spotlight as an optical memory element capable of rewriting information. The magneto-optical memory element includes, for example, a magneto-optical card and a magneto-optical disk. Among them, the magneto-optical disk has a disk-shaped substrate and a magnetic recording film between the substrates. As shown in FIG. 5 (a), information is reproduced from the magneto-optical disk by a substrate 15a constituting the magneto-optical disk 15.
The linearly polarized laser beam 17 is condensed and incident on the lower magnetic recording film 15b perpendicularly by the objective lens 18, and the plane of polarization of the reflected light is rotated according to the magnetization direction of the magnetic recording film 15b. Using the phenomenon (Kerr effect), the change in rotation is detected by transmitting the light through an analyzer (not shown) and then detecting the change in light amount with a photodetector.

By the way, glass or plastic is used as the material of the substrate 15a of the magneto-optical disk. Among them, plastics such as acrylic and polycarbonate are being studied from the viewpoint of productivity and ease of handling.

[Problems to be solved by the invention]

However, one of the drawbacks of these plastics is that birefringence tends to occur more easily than glass (particularly polycarbonate), and the optical properties are inferior. In particular, in plastic substrates obtained by injection molding, birefringence is likely to occur,
The linearly polarized light used in the reproduction of information becomes elliptically polarized light, greatly reducing the quality of a reproduced signal. That is,
When a signal is actually read out from a magneto-optical disk, the laser light is normally focused on the magnetic recording film 15b through the substrate 15a by a lens having an NA (numerical aperture) of 0.5 to 0.6. If it has refraction, the linearly polarized light changes to elliptically polarized light while passing through the substrate 15a. When the polarization becomes elliptically polarized, the degree of modulation of the signal decreases, and when the degree of modulation decreases, noise in the signal increases, resulting in a reduction in S / N.

There are two types of birefringence of the substrate: an in-plane direction and a vertical direction. Among them, birefringence in the in-plane direction means that there is a difference in the refractive index in the in-plane direction of the substrate different from each other. Also,
The birefringence in the vertical direction means that there is a difference between the refractive index in the direction perpendicular to the substrate and the refractive index in the in-plane direction. Birefringence in the in-plane direction similarly affects polarization at any position in a plane perpendicular to the optical axis of a light beam, but birefringence in the vertical direction has a different effect on polarization depending on the position in the light beam. I have. That is, at the center of the light beam, the laser beam is perpendicularly incident on the substrate, so that it is affected only by in-plane birefringence,
While the influence of the vertical birefringence is hardly affected, the light outside the light beam is obliquely incident on the substrate, so it is affected not only by the in-plane birefringence but also by the vertical birefringence. Will be. When the direction of the incident polarized light is parallel or perpendicular to the incident plane, the linearly polarized light can reach the recording film as linearly polarized light even if the light is incident obliquely.

The polycarbonate substrate obtained by injection molding has a birefringence in the in-plane direction of the order of 10 ^-6 , whereas the birefringence in the vertical direction is of the order of 10 ^-4 , and the birefringence in the vertical direction is more in-plane. It is customary to be two orders of magnitude larger than that in the direction. Therefore, polarization change due to birefringence of the substrate is mainly due to birefringence in the vertical direction.

Here, the main surface direction perpendicular to the substrate and Z ₁ axis, taking the X ₁ axis in a direction perpendicular to each other to, Y ₁ axis, respectively, to each refractive index of n _x, n _y, and n _z. Further, the angle of the laser light entrance surface and the X ₁ axis alpha, when the refraction angle at the substrate theta, refractive index n _s of the incident plane and perpendicular is expressed by the first equation below.

Further, the refractive index n _{p in the in-} plane direction of incidence is represented by the following second equation.

When made incident linearly polarized light is polarized in the direction of the X ₁ axis using the above equation, as shown in FIG. (B). That is, as described above, the linearly polarized light incident on the substrate 15a via the objective lens 18 remains as it is on the X ₁ and Y ₁ axes, but is inclined by 45 ° from the X ₁ and Y ₁ axes. The ellipticity increases toward the outside in the same direction. This reduces the quality of the reproduced signal.

[Means for solving the problem]

In order to solve the above problem, an optical head according to the present invention is an optical head that condenses a light beam toward an optical memory element and reproduces information. 1 / 2λ to rotate the polarization direction of the part
A polarization direction rotating means such as a plate is interposed in a portion other than a portion where the direction of the incident polarized light is parallel or perpendicular to the incident surface in an outer portion of the light beam at a position before focusing on the optical memory element. It is characterized by becoming.

(Operation)

According to the above configuration, it is possible to partially rotate the polarization direction of the light beam of the light beam focused toward the optical memory element for information reproduction. In addition, a polarization direction rotating means such as a 1 / 2λ plate may be used in a portion other than the portion where the direction of the incident polarized light is parallel or perpendicular to the incident surface in the outer portion of the light beam at the position before focusing on the optical memory element. The characteristic that linear polarized light can reach the optical recording film as linear polarized light with almost no influence of birefringence can be used, and the linear polarized light used in information reproduction is partially elliptically polarized. Can be prevented.

As a result, a decrease in the degree of modulation of the signal is avoided, noise in the signal is suppressed, S / N is increased, and the quality of the reproduced signal can be improved. Therefore, sufficient reliability can be ensured even when an optical memory element using a plastic substrate or the like obtained by injection molding that easily causes birefringence is used. Also, instead of simply placing a light blocking plate,
Since the 1 / 2λ plate is interposed, the amount of light transmitted therethrough does not change, so that the amount of light beam during recording does not decrease.

〔Example〕

One embodiment of the present invention will be described below with reference to FIGS. 1 to 4.

In the optical head according to the present invention, as shown in FIG. 1, a laser beam emitted from a laser diode 1 serving as a light source is collimated by a collimator lens 2 for converting the laser beam into a parallel beam, and reflected by an optical path of 90 degrees.ハ ー フ Having a half mirror 3 for changing the direction and a plurality of 1 / 2λ plates 4a
Magneto-optical disk (optical memory element) sequentially passing through a / 2λ plate group 4 and a condenser lens 5 for condensing laser light
The light is condensed and irradiated onto the magneto-optical recording film 6b. The magneto-optical disk 6 has a transparent substrate 6a and a magneto-optical recording film 6b.
And a polycarbonate substrate is used as the transparent substrate 6a, for example. The laser light reflected by the magneto-optical disk 6 passes through the condenser lens 5, the 1 / 2λ plate group 4, and the half mirror 3, and then branches the optical path for servo signal detection and information signal detection. A half mirror 7 passes through an analyzer 8 provided with a polarization axis set at 45 ° on the optical path for detecting the information signal, and then a photodetector constituting a magneto-optical signal detection unit based on a differential detection method. 9 and 10, respectively.

As shown in FIG. 2, the 1 / 2λ plate group 4 is used to change the polarization direction of a part of the light beam c of the laser beam to a direction in which the influence of birefringence by the transparent substrate 6a is small. c
It is composed of four square 1 / 2λ plates 4a. Specifically, each 1 / 2λ plate 4a is a portion outside the light beam c (a portion obliquely incident on the transparent substrate 6a), and the direction of the incident polarized light is parallel to the incident surface. Alternatively, it is disposed at a portion other than the vertical portion, that is, at a portion which is located at approximately 45 ° with respect to the X axis and the Y axis shown in the drawing. Then, each 1 / 2λ plate 4a changes the polarization direction of the light passing therethrough (direction A in the figure) in a direction parallel to the incident surface (direction B in the figure), that is, by 45 °. And as a means for that,
The 1 / 2λ plate 4a is + 22.5 ° or −2 with respect to the polarization direction of the light flux c.
It is arranged with a set angle of 2.5 °. Here, 1 / 2λ plate
4a has a function of rotating the polarization direction of light by twice the set angle.

In addition, the shape of the 1 / 2λ plate 4a is not limited to the above square shape,
As shown in FIG. 3, a circular shape may be used, and the specific shape, number, and the like are not limited.

According to the above configuration, even in a portion where the direction of the polarized light in the light beam c is not parallel (or perpendicular) to the incident surface, the polarization direction of each of the 1 / 2λ plates 4 is parallel to the incident surface. In this case, even if the incident light has an azimuth of parallel (or perpendicular) to the plane of incidence, the linearly polarized light is affected by the birefringence even if the light is obliquely incident. Thus, it is possible to utilize the characteristic that linearly polarized light can reach the magneto-optical recording film while receiving substantially no polarized light, and it is possible to prevent linearly polarized light used in information reproduction from becoming partially elliptically polarized. Just 1/2
By interposing the λ plate 4a, circular polarization is prevented as described above, and at the same time, the signal amount is reduced. That is, although there is no difference in the signal amount (light amount) between the light before passing through the 1 / 2λ plate 4a and the light after passing through, the light passing through the 1 / 2λ plate 4a. Will be reduced. However, there is no difference in the signal amplitude between the case where the 1 / 2λ plate 4a is interposed and the case where the 1 / 2λ plate 4a is not interposed. Will do. Then, as the degree of modulation increases, the S / N ratio increases, and the quality of the signal improves.

As a result, sufficient reliability can be ensured even when a magneto-optical disk using a plastic substrate or the like obtained by injection molding in which birefringence easily occurs is used. Also,
By interposing the 1 / 2λ plate 4a instead of the light-shielding plate, the amount of light transmitted therethrough does not change, so that the amount of light beam does not decrease during recording.

Here, the relationship between the interposed area of the 1 / 2λ plate 4a and the signal amount s and the modulation degree m was as shown in FIG.
In this figure, the horizontal axis represents the ratio a / r between the distance a and the radius r of the light beam c shown in FIG. 2, and the vertical axis represents the signal amount and the modulation factor when not passing through the 1 / 2λ plate 4a. 1 indicates the signal amount and the modulation factor when the 1 / 2λ plate 4a is interposed. However, the NA (numerical aperture) of the lens used in this test was 0.6,
A polycarbonate substrate is used as the substrate 6a. The polycarbonate substrate has a refractive index n _{0 in} the X-axis direction of 1.585, an in-plane birefringence of 5 × 10 ⁻⁶ , and a vertical birefringence of 5 × 1.
0 ^-4 , the transmittance of the half mirror is set to 0.03.
As is apparent from this figure, in the installation configuration shown in FIG. 2, the modulation degree is highest when a / r is 0.3.

〔The invention's effect〕

As described above, the optical head according to the present invention includes, on the light beam in the light beam, a polarization direction rotating means such as a 1 / 2λ plate for rotating the polarization direction of a part of the light beam before focusing on the optical memory element. In the outer part of the light beam at the position of, the direction of the incident polarized light is interposed in a part other than the part parallel or perpendicular to the incident surface.

Thereby, the polarization direction of the light beam of the light beam focused toward the optical memory element for information reproduction is partially rotated, and the rotation of the polarization direction of the partial light beam improves the quality of the reproduced signal. be able to. That is, it is possible to prevent the linearly polarized light used in the reproduction of information from becoming partially elliptically polarized.

Therefore, a decrease in the modulation factor of the signal is avoided, noise in the signal is suppressed, S / N is increased, and the quality of the reproduced signal can be improved. As a result, sufficient reliability can be ensured even when an optical memory element using a plastic substrate or the like obtained by injection molding in which birefringence easily occurs is used. Further, since a 1 / 2λ plate is interposed instead of a light-shielding plate, the amount of light transmitted therethrough does not change, so that the effect that the amount of light beam does not decrease at the time of recording is also exhibited.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 to FIG. 4 show an embodiment of the present invention, FIG. 1 is a schematic configuration diagram of an optical head, and FIG.
FIG. 3 is an explanatory diagram showing an arrangement of a 2λ plate group, FIG. 3 is an explanatory diagram showing another example of an arrangement of a 1 / 2λ plate group, and FIG. 4 is a diagram showing changes in the signal amount and modulation degree with respect to a / r change. FIG. 5 shows a conventional example, and FIG. 5 (a) is an explanatory view showing how light condensed by an objective lens is incident on a magneto-optical disk, and FIG. 5 (b). FIG. 4 is an explanatory diagram showing a state in which a part of a light beam is converted into elliptically polarized light. 1 is a laser diode, 2 is a collimator lens, 3.7
Is a half mirror, 4 is a 1 / 2λ plate group (polarization direction rotating means), 4a is a 1 / 2λ plate (polarization direction rotating means), 5 is a condenser lens, 6 is a magneto-optical disk (optical memory element), and 6a is transparent. A substrate, 6b is a magneto-optical recording film, 8 is an analyzer, and 9 and 10 are photodetectors.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kenji Ota 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Junji 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72) Inventor Tetsuya Inui 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-60-214433 (JP, A) Real-life 63-157818 (JP, U)

Claims (3)

(57) [Claims] 1. An optical head for reproducing information by condensing a light beam toward an optical memory element, comprising: a polarization direction rotating means for rotating a polarization direction of a part of the light beam on the light beam of the light beam. An optical head characterized in that an azimuth of incident polarized light is interposed in a portion other than a portion parallel or perpendicular to an incident surface in an outer portion of a light beam at a position before focusing on an optical memory element. . 2. An optical head for reproducing information by condensing a light beam toward an optical memory element, comprising: a 1 / 2λ plate for rotating a polarization direction of a part of the light beam on the light beam of the light beam. An optical head characterized in that an azimuth of incident polarized light is interposed in a portion other than a portion parallel or perpendicular to an incident surface in an outer portion of a light beam at a position before focusing on an optical memory element. . 3. An optical head for reproducing information by condensing a light beam toward an optical memory element, wherein a part of the light beam has a polarization direction on a light beam of the light beam which is less affected by birefringence. The 1 / 2λ plate to be rotated in the outer part of the light beam at the position before focusing on the optical memory element, and the direction of the incident polarized light is interposed in a part other than the part parallel or perpendicular to the incident surface. An optical head, comprising: