JPS63200328A - Objective hologram lens for optical head - Google Patents

Abstract

PURPOSE:To prevent the position shift of a focal point due to the change of the wavelength of a laser beam from being generated and to surely perform the tracking of a servo mechanism, by confronting refracting hologram lenses with specific intervals against those of condensing holograms. CONSTITUTION:In an optical head constituted of a laser diode 10, the hologram lenses 2-9, and an optical detection circuit 11, when the wavelength changes by the mode hop of the diode 10, the focal point comes off a targeted track on an optical disk 1. To prevent the shift, the specific interval is provided between the objective and the condensing holograms 4 and 2. In other words, assuming the interval between the lenses 2 and 4 as X, and that a position passing on the lens 2 is shifted by D when the wavelength lambda of a beam of light being made incident on the lens 4 at a refraction angle theta1' is changed by DELTAeta, the lenses 2 and 4 are arranged so as to satisfy an equation D=X (DELTAetasintheta1')/(eta+DELTAeta). In such a way, it is possible to prevent the position shift of the focal point from being generated, and to perform track servo and focus servo.

Description

[Detailed Description of the Invention] [Summary] When the objective lens of an optical head used in an optical information recording/reproducing device is composed of a hologram lens, when a laser diode is used as a light source, the wavelength of the light changes discontinuously due to mode hops. As a result, the focal position shift becomes discontinuous or continuous, making it impossible for the servo mechanism to track it. However, by combining a focusing hologram lens with a refractive hologram lens that faces at a specific interval, the focal position shift can be corrected. By compensating for this and making it possible for the servo mechanism to follow, it became possible to create an optical head using a holodram lens.

[Industrial application field]

The present invention relates to an optical head used in an optical information recording/reproducing device, and more particularly to an objective hologram lens for an optical head that solves a focal position shift caused by wavelength fluctuations of incident light.

For example, in a hologram lens, a plurality of fine grooves are provided at intervals of d on the surface of a glass plate, so that the light that passes through the glass plate is diffracted and interfered with.If the wavelength of the light is λ and the refraction angle is θ, then λ=dsin. The configuration is such that first-order diffracted light having a refraction angle that satisfies the relationship of θ can be obtained most efficiently.

By the way, it is being considered to create an optical head using this hologram lens and use it in an optical information recording/reproducing device such as an optical disk device, but if a laser diode is used as a light source, There are wavelength fluctuations due to temperature changes and mode hops.

As can be seen from the above equation, the refraction angle θ of the hologram lens is affected by the wavelength λ, so unless the influence of wavelength fluctuation is removed, track servo and focus servo will be difficult. In particular, since wavelength fluctuations due to mode hops are discontinuous, there is a problem that the servo mechanism cannot follow them, which is a major obstacle to practical application.

Currently, there are countermeasures against focal position fluctuations in the track direction due to wavelength fluctuations, but there is a desire for an objective hologram lens that can reduce focal position fluctuations in the focus direction and enable tracking by a servo mechanism.

[Conventional technology]

FIG. 3 is a diagram illustrating an example of a conventional technique.

This example shows a case where the objective lens of the optical head is compensated for the deviation of the focal position in the track direction due to the wavelength fluctuation of the laser diode.
The deviation in the direction indicated by -B is corrected. The hologram lens 2 has a light condensing function, and the hologram lens 3 has a refraction function.

Therefore, the hologram lens 3 refracts the incident light shown by arrows C, D, and E and sends it to the hologram lens 2, and the hologram lens 2 condenses this light onto the optical disk 1 and focuses it.

If the wavelength of the laser diode is a predetermined length, the laser diode will focus on the target track shown by ■ on the optical disc 1, as shown by the solid line, but as the wavelength becomes longer, the refraction angle will increase, so as shown by the dotted line, The focus is set in front of the optical disc 1 at the position indicated by ■.

FIG. 4 is a diagram illustrating another conventional technique.

This example shows a case where the objective lens of the optical head does not compensate for wavelength fluctuations, and in order to focus the light onto the optical disk 1 using only the hologram lens 2, which has a light focusing function, the wavelength of the laser diode is set to a predetermined wavelength. If the wavelength is longer, the incident lights F, G, and H will be focused on the target track shown by ■ on the optical disc 1, as shown by the solid line, but as the wavelength becomes longer, the refraction angle will become larger, as shown by the dotted line. The focus is set in front of the optical disc 1 and at a position indicated by a square square which is also off the target track position.

[Problem that the invention seeks to solve]

As mentioned above (conventionally, there is a method to compensate for deviations in the track direction for wavelength fluctuations, but it does not have the ability to compensate for defocus directions. Therefore, due to mode pops in the laser diode, the wavelength suddenly fluctuates). If this happens, the physical servo mechanism will not be able to track the image, resulting in an out-of-focus state.

For this reason, an optical head using a hologram lens,
The problem is that it cannot be put into practical use.

[Means for solving problems]

FIG. 1 is a diagram illustrating the present invention in detail.

In FIG. 1(a), the hologram lens 2 having a condensing function has a refraction angle that increases in the direction of arrow B with respect to the wavelength of incident light. Focusing on the optical path of the incident light that utilizes this structure, the hologram lens 2 and the hologram lens are designed to change the optical path position passing through the hologram lens 2 when the wavelength changes, and cancel the change in refraction angle due to wavelength fluctuation. 4. Determine the distance from 4.

When the hologram lenses 2 and 4 are combined at such a specific interval, the hologram lens 4 becomes a lens with a curved surface as shown in the figure.

FIG. 1(b) is a diagram illustrating a method for determining X, where X is the specific interval. As shown by the arrow shown by the solid line at wavelength λ1, the optical path consisting of the above is refracted at the position shown by ``■'' of the hologram lens 4, refracted again at the position ``■'' of the hologram lens 2, and focused at the position shown by ``■'' of the optical disc 1. When the wavelength changes to λ2 and becomes longer, the refraction angle becomes larger in the hologram lens 4 as shown by the dotted line, and the optical path is moved to the position shown in ■ of the hologram lens 2 corresponding to the size of X. changes.

The refraction angle at the position indicated by ■ of the hologram lens 2 is larger than the refraction angle at the position indicated by
■Since the angle of refraction can be calculated based on the formula, this ■
By selecting the position (2) so that the light refracted at the position indicated by (2) passes through the position (2) on the optical disc 1, it is possible to compensate for the focal shift due to wavelength fluctuation.

When the wavelength is λ1, the focal length of the hologram lens 2 is f1
Let A be the distance between the position where a line perpendicular to the hologram lens 2 passing through ■ intersects with the hologram lens 2 and the position ■, and 03 be the angle that the optical path connecting ■ and ■ forms with the perpendicular line of the hologram lens 2. /f=tanθ3ae, and since A and f are determined from the standard of the hologram lens 2, the angle θ can be calculated from the [phase] equation.

By the way, if the angle formed by the optical path connecting ■ and Φ in ■ of the hologram lens 2 with the vertical line of the hologram lens 2 is θ2, and the interval between the grooves of the interference structure in ■ of the hologram lens 2 is d, then λ, = d , 5in (θ2
+θ, ) ■.

Since λ1 and d are fixed, θ2 can be found by substituting θ obtained from the [phase] equation.

If the refraction angle of the hologram lens 4 is θ, the hologram lens 4 is curved relative to the hologram lens 2, but θl#θ2, and the difference can be ignored, so 01 can be found from the 0 formula. .

Similarly to the above, if the angle formed by the optical path connecting ■ and ■ with the vertical line of the hologram lens 2 is θ,'', and the distance between ■ and ■ is D, then (A + D) / f-tanθ,'[ Phase] Also, the angle that the optical path connecting ■ and ■ in ■ of the hologram lens 2 forms with the vertical line of the hologram lens 2 is θ2゜, and the interval between the grooves of the interference structure in ■ of the hologram lens 2 is d.
2, and if the angle formed by the optical path indicated by the dashed dotted line of the wavelength λ that passes through the point 2 at the distance from 2 in the hologram lens 4 with the vertical line of the hologram lens 2 is 04, then λ, = d, 5in θ4+θ,′) ■
λg=d, 5in (θ2°+θ,”) ■λ2=λ
, +Δλ [phase].

If the refraction angle of the hologram lens 4 at the wavelength λ2 is θ1', the hologram lens 4 is curved relative to the hologram lens 2, but the phase is θ1゛ζθ'' [phase], and the difference can be ignored. Therefore, , ◎0@ from the expression λ
Since 2 and d2 are fixed, θ1' is determined and this θ,
θI' can be found using ° as a parameter.

By the way, there is a relationship between D, θ1゛ and When the desired X is obtained from this equation 0, the light with the changed wavelength λ2 is refracted by the hologram lens 4 and then passes through the point (2).

By performing this calculation at a large number of positions on the hologram lens 4, the distance X between the hologram lenses 2 and 4 can be obtained.

This calculation is usually approximated using a computer system.

[Effect]

By arranging the hologram lens as described above, the hologram lens 4 can compensate for both the deviation in the track direction and the deviation in the focal direction with respect to fluctuations in the wavelength of the laser diode. This makes it possible to create an optical head using a hologram lens.

〔Example〕

FIG. 2 is a diagram illustrating an embodiment of the present invention.

The laser beam sent out from the laser diode 10 is refracted by the hologram lens 8, passes through the hologram lens 7 for splitting and the λ/4 plate 6, is refracted by the hologram lens 5, and enters the hologram lens 4 that constitutes the objective lens. do.

As explained in FIG. 1, the hologram lens 4 maintains a specific distance from the hologram lens 2 so as to compensate for the deviation between the track direction and the focal direction, even though the wavelength changes due to the mode hop of the laser diode 10. .

Therefore, the laser beam focused by the hologram lens 2 having a focusing effect is focused on the target track of the optical disk 1, and even if the wavelength of the laser diode 10 changes, the focus will not shift from the target track. None.

The laser beam reflected by the optical disk 1 is passed through the hologram lens 2
, 4.5 and returns, and the phase is rotated by the λ/4 plate 6. Therefore, the reflected light is separated by the hologram lens 7,
Data recorded on the optical disc 1 is detected by a photodetector circuit 11 via a hologram lens 9 .

〔Effect of the invention〕

As explained above, even if the present invention uses a hologram lens and a laser diode to create an optical head, the focus will not deviate from the target track due to changes in wavelength.
It is possible to perform track servo and focus servo, and it can be used in optical information recording and reproducing devices such as optical disc 'AM'.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the present invention in detail, FIG. 2 is a diagram explaining one embodiment of the present invention, FIG. 3 is a diagram explaining an example of the conventional technique, and FIG. 4 is a diagram explaining another conventional technique. It is a figure explaining a technique. In the figure, 1 is an optical disk, 2 to 5, 7 to 9 are hologram lenses, 6 is a λ/4 plate, 1o is a laser diode, and 11 is a photodetector circuit. A÷-→β (b> Original ep,
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Claims (1)

[Claims] In an optical head of an optical information recording/reproducing device, a first hologram lens (2) condenses incident light and focuses it on a target position on a recording medium, and the first hologram lens (2) is provided with a second hologram lens (4) that faces at a specific interval x and refracts the incident light and sends it to the first hologram lens (2); When the wavelength of the light incident on the hologram lens (4) is the first wavelength, the optical path that passes through the first hologram lens (2) and focuses on the target position of the recording medium (1) is the first hologram. When the position passing through the lens (2) and the wavelength of the incident light change to the second wavelength, it is focused on the target position of the recording medium (1) through the first hologram lens (2). The optical path is the first hologram lens (2)
D is the distance from the position where the light passes through, Δλ is the difference between the first wavelength and the second wavelength, and is the refraction angle θ_1 when the light of the second wavelength is refracted by the second hologram lens (4). objective hologram lens of an optical head, characterized in that the first and second hologram lenses are arranged so that the specific interval x satisfies the condition of D=x(Δλsinθ_1')/(λ+Δλ). .