JPH0664749B2 - Optical pickup device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical information recording / reproducing apparatus for optically recording information by recording bits recorded on an information record carrier,
More specifically, it relates to an optical pickup device.

2. Description of the Related Art Focusing and tracking error signals are generally detected in an optical pickup device. FIG. 3 shows an example thereof. The laser light emitted from the semiconductor laser 1 becomes parallel light by the coupling lens 2 and is condensed on the information recording carrier 6 via the polarization beam splitter 3, the quarter wavelength plate 4 and the objective lens 5. The reflected light from the information recording carrier 6 passes through the objective lens 5, has its plane of polarization changed by the quarter-wave plate 4, and is separated from the incident light by the polarization beam splitter 3. The separated reflected light is converted into convergent light by the condenser lens 7, and the tracking detector 8 detects a track error signal. The remaining light flux reaches the focusing detector 9. The focusing detector 9 is provided in the vicinity of the focal point of the condenser lens 7 and is composed of two PIN photodiodes or the like divided by the dividing line L1 to detect a difference signal between the two and obtain a focus error signal.

Here, the focus error signal detection method will be described. Fourth
The figure shows the principle of the focus error signal detection method. The ridge line S of the tracking detector 8 has a knife edge effect, and the relationship between the objective lens 5 and the information recording carrier 6 is in focus as shown in FIG. The focusing detector 9
When the outputs of A and B are adjusted to be equal at the focal position of the condenser lens 7 and the focal point P is in front of the focusing detector 9, the outputs P of both elements A and B are adjusted.
a and Pb are Pa <Pb. On the contrary, when the focal point P is behind the focus detector 9, the outputs of both elements A and B are Pa> Pb.
Becomes In this way, the focus state is determined by comparing the outputs of the elements A and B, and focus servo is performed to keep the distance between the objective lens 5 and the information recording carrier 6 constant.

However, in the above optical pickup device, as shown in FIG. 5A, the light emitting point O of the semiconductor laser 1 is placed at the focal position of the coupling lens 2 and the divergent light from the semiconductor laser 1 is coupled to the coupling lens. 2 is converted into parallel light, but as shown in FIG. 2B, the light emitting point O of the semiconductor laser 1 is caused by thermal expansion of the support member 10 supporting the semiconductor laser 1 and the coupling lens 2 due to environmental temperature change.
Is shifted from the focal position of the coupling lens 2, and the light passing through the coupling lens 2 is not parallel light. As a result, the light that has passed through each optical element and reached the focusing detector has a problem in that the focus position shifts, causing an error in the focus error signal.

OBJECT The object of the present invention is to solve the above-mentioned problems, and even if the distance between the light emitting point of the semiconductor laser and the objective lens changes due to the change in environmental temperature, the diverging light from the semiconductor laser is coupled to the coupling lens. The present invention provides an optical pickup device capable of forming parallel light.

Configuration The present invention will be specifically described below.

In FIG. 5, (a) shows the positional relationship between the semiconductor laser 1, the coupling lens 2 and the holding member 10 at the ambient temperature T. The emission point O of the semiconductor laser 1 and the coupling lens 2 are shown. The distance L from the principal point H is equal to the focal length of the coupling lens 2. Further, FIG. 2B shows the positional relationship between the semiconductor laser 1, the coupling lens 2 and the holding member 10 when the environmental temperature becomes T + ΔT. The emission point O of the semiconductor laser 1 and the coupling lens 2 are shown. The distance L from the principal point H becomes L + ΔL due to the thermal expansion of the holding member 10. This relationship is shown in FIG.

It is well known that the wavelength of the semiconductor laser 1 changes depending on the ambient temperature as shown in FIG. 1 (b). Further, the focal length of the coupling lens also changes depending on the wavelength of the semiconductor laser as shown in FIG. Here, the inclination of this straight line varies depending on the combination of lens materials in the coupling lens, that is, the difference in lens design.

The relationship between the environmental temperature and the focal length of the coupling lens can be obtained from FIGS. 1 (b) and (c), and is shown in FIG.

From the above, the focal length of the coupling lens 2 changes due to the change of the environmental temperature, and the distance between the emission point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 also changes. Here, if the amount of change in the distance between the emission point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 due to the change in the ambient temperature and the amount of change in the focal length of the coupling lens 2 are matched, that is, If the coupling lens 2 is designed so that the straight line of FIG. 1 (a) and the straight line of FIG. 1 (d) approximate to each other, even if the environmental temperature changes,
The focal length of the coupling lens 2 and the distance between the emission point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 become equal, and the divergent light emitted from the semiconductor laser 1 passes through the coupling lens 2 and becomes parallel light. Therefore, a normal focus error signal can be detected.

The focal length of the coupling lens 2 when the ambient temperature is T
fc, the focal length of the objective lens 5 is fo, the distance between the emission point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 is L, and the focal length of the coupling lens 2 when the environmental temperature is T + ΔT is L + ΔL And The coefficient of linear thermal expansion of the support member 100 supporting the semiconductor laser 1 and the coupling lens 2 is β, and the focus error is F.

The linear thermal expansion coefficient β of the support member 10 is represented by β = 1 / L · (ΔL / ΔT). Therefore, the amount of change ΔL in the distance between the light emitting point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 due to the environmental temperature change is ΔL = β · L · ΔT. As described above, the change amount Δfc of the focal length of the coupling lens 2 due to the change of the environmental temperature is not required to be equal to the above ΔL, but since some error is allowed in this Δfc, if this error is ± ΔC, The change amount Δfc of the focal length is Δfc = β · L · ΔT ± ΔC Therefore, ± ΔC = Δfc−β · L · ΔT. Here, the focus error F is expressed by F = 1/2 · (fo / fc) ² · ΔC. Generally focusing error F due to a change in environmental temperature, ± 0.2 × 10 ^- because it is acceptable to ^{3 mm, ΔC <0.4 (fc} / fo) 2 × 10 - a ³ mm. From the above, | Δfc-β · L · ΔT | <0.4 (fc-fo) 2 × 10 - a ³ mm. Therefore, a normal focus error signal can be detected by using a coupling lens in which the variation Δfc of the focal length of the coupling lens satisfies the above equation when the environmental temperature changes by ΔT.

Further description will be made using specific numerical values.

Focal length fc = 12.0 mm of the coupling lens 2 at the environmental temperature T = 25 ° C., focal length fo = 4.5 mm of the objective lens 5, distance between the emission point O of the semiconductor laser 1 and the principal point H of the coupling lens 2 L = 12.0mm. The linear thermal expansion coefficient beta = 23.2 × 10 of the support member 10 ^- and ⁶ mm / ° C.. Where the ambient temperature is Δ
When rises by T = 20 ℃, | Δfc- β · L · ΔT | <0.4 (fc / fo) 2 × 10 - than ^3, and substituting the numerical ^{value, | Δfc-23.2 × 10 -} 6 × 20 × 12 | <0.4 × (12 / 4.5) 2 × 10 - 3 and rearranging the above ^{equation, | Δfc-5.57 × 10 -} 3 | <2.84 × 10 - 3 thus ^{2.73 × 1 - 3 mm <Δfc} <8.41 × 10 - ³ mm, that is, as shown in FIG. 2 (a), the reference temperature (T = 25
° C.) from 20 ° C. focal length variation of 2.73 × 10 of the coupling lens when the environmental temperature rises ^- 8.14 from ³ mm × 10 ^-
Design the coupling lens so that it is within the range of ³ mm.

As described above, since the change in the focal length of the coupling lens due to the change in the ambient temperature is due to the change in the wavelength of the semiconductor laser due to the change in the ambient temperature, the wavelength of the semiconductor laser at the reference temperature (T = 25 ° C.) At 780 nm, an environmental temperature (T + ΔT = 45
Assuming that the wavelength of the semiconductor laser at ℃ is 785 nm, the change in the focal length of the coupling lens is 2.73 × 10 when the wavelength of the semiconductor laser changes from 780 nm to 785 nm, as shown in FIG. 2 (b). ^{- 3} mm from 8.41 × 10 ^{- 3} as are input in the range of mm performing lens design of the coupling lens.

Effect As described above, in the present invention, the variation of the focal length of the coupling lens due to the variation of the wavelength of the semiconductor laser light caused by the variation of the environmental temperature and the variation of the distance between the semiconductor laser and the coupling lens caused by the variation of the environmental temperature. By making the values coincide with each other, the divergent light from the semiconductor laser that has passed through the coupling lens becomes parallel light even if the environmental temperature changes, and a normal focus error signal can be obtained.

[Brief description of drawings]

FIG. 1 (a) is a relationship diagram between the environmental temperature and the distance between the principal point of the coupling lens and the semiconductor laser emission point, FIG. 1 (b) is a relationship diagram between the environmental temperature and the semiconductor laser wavelength, and FIG. ) Is a relationship diagram between the semiconductor laser wavelength and the focal length of the coupling lens, FIG. 4D is a relationship diagram between the environmental temperature and the focal length of the coupling lens, and FIG. 2A is the environmental temperature change of the present invention. Is a diagram showing the allowable range of the focal length of the coupling lens with respect to the above, FIG. 9B is a diagram showing the allowable range of the focal length of the coupling lens with respect to the change of the semiconductor laser wavelength of the present invention, and FIG. 3 is a conventional optical pickup. FIG. 4 is a schematic configuration diagram showing the principle of the device, FIG. 4 is a diagram showing a focus error signal detection method, and FIG. 5 (a) is a semiconductor laser at a reference temperature,
FIG. 3B is a diagram showing the relationship between the coupling lens and the light flux, and FIG. 7B is a diagram showing the relationship between the semiconductor laser, the coupling lens, and the light flux when the environmental temperature rises. 1 ... Semiconductor laser, 2 ... Coupling lens, 3 ...
... 1/4 wave plate, 4 ... polarized beam splitter, 5 ...
Objective lens, 6 ... Information recording carrier

Claims (2)

[Claims] 1. A light beam emitted from a semiconductor laser is converted into parallel light by a coupling lens, condensed by an objective lens through a quarter wavelength plate and a polarization beam splitter, and a minute spot is formed on an information recording carrier. Then, in the optical pickup device for recording / reproducing information by the reflected light, the change amount of the focal length of the coupling lens due to the change of the wavelength of the semiconductor laser light caused by the change of the ambient temperature and the change of the ambient temperature. An optical pickup device characterized in that the amount of change in the distance between the semiconductor laser and the coupling lens caused by the above is substantially matched. Wherein said coupling lens, the inequality | Δfc-ΔT · β · L | <0.4 (fc / fo) 2 × 10 - 3 claims, characterized by satisfying the range of claim 1 wherein Optical pickup device. Δfc: Change in focal length of coupling lens due to change in semiconductor laser wavelength caused by change in environmental temperature ΔT: Change in environmental temperature β: Linear thermal expansion coefficient of semiconductor laser and coupling lens support member L: Semiconductor laser emission point Distance from coupling lens principal point fc: Coupling lens focal length fo: Objective lens focal length