JPS62189632A - Optical pickup device - Google Patents

Abstract

PURPOSE:To obtain the normal focus error signal despite the change of the ambient temperature by making the change of the focal distance of a coupling lens due to the change of the wavelength of the semiconductor laser light that is caused by the change of the ambient temperature coincident with the change of the distance between a semiconductor laser and the coupling lens also caused by the change of the ambient temperature. CONSTITUTION:The focal distance of a coupling lens 2 varies by the change of the ambient temperature and the distance between the light emitting point O of a semiconductor laser 1 and the primary point H of the lens 2 also varies. Here the lens designing is performed to make the change of the distance between both points O and H due to the change of the ambient temperature coincident with the change of the focal distance of the lens 2. Thus the focal distance of the lens 2 is made equal to the distance between both points O and H and the light dispersed from the laser 1 are turned into plural beams through the lens 2. Then the normal focus error signal is detected.

Description

[Detailed Description of the Invention] The captured Ranbon Kanmei relates to an optical recording/reproducing device, in which information is optically recorded on an information recording carrier by a recording pitch (-), and more specifically relates to an optical recording/reproducing device. This invention relates to a pick amplifier device.

In a fast-track optical pickup device, focus and tracking error signals are generally detected.

An example is shown in FIG. The laser beam emitted from the semiconductor laser 1 is turned into a parallel beam by a coupling lens 2, and is north-lighted onto an information recording carrier 6 via a polarization beam t1 splitter 3, a quarter-wave plate 4, and an objective lens 5. The reflected light from the information recording carrier 6 passes through an objective lens 5, has its polarization plane changed by a 1/4 wavelength plate 4, and is sent to a polarizing beam splitter 3.
is separated from the incident light. A condensing lens focusing detector 9 is installed near the focal point of the condensing lens 7 and is composed of two PIN photodiodes divided by a division gL1, and detects the difference signal between the two and generates a focus error signal. get.

Here, the focus error signal detection method will be described.

FIG. 4 shows the principle of the focus error signal detection method. The ridge line S of the tracking detector 8 has a knife effect, and the relationship between the objective lens 5 and the information recording carrier 6 is matched as shown in FIG. 4(a). Focusing detector 9 when focused
are adjusted so that the outputs of A and B are equal at the focal position of the condensing lens 7, and when the condensing point P is in front of the focusing detector 9, the output Pa of each pixel A and B is , Pb satisfies P a < P b. Conversely, if the focal point P is behind the focus detector 9,
Focus servo is performed to keep the distance between the cutting objective lens 5 and the information recording carrier 6 constant.

However, in the above-mentioned optical pickup device, as shown in FIG. However, as shown in the same figure (b), the emission of the semiconductor laser 1 is changed due to thermal expansion of the support member 1° that supports the semiconductor laser 1 and the coupling lens 2 due to a 1M environmental temperature change. The point ○ is shifted from the focal position of the coupling lens 2, and the light passing through the coupling lens 2 does not become parallel light.

As a result, there is a problem in that the focal position of the light that passes through each optical element and reaches the focusing detector shifts, causing an error in the focus error signal.

An 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 changes in the environmental temperature, the present invention can be made specifically for the semiconductor laser and the like. Explain.

In FIG. 5, (a) shows the positional relationship between the semiconductor laser 1, the coupling lens 2, and the holding member 10 when the environmental temperature is T. The distance M I- from point H is equal to the focal length of the coupling lens 2. Also, the same figure (
b) is the semiconductor laser 1 when the environmental temperature becomes T+ΔT.
This shows the positional relationship between the coupling lens 2 and the holding member 10, and the distance raL between the light emitting point ○ of the semiconductor laser 1 and the principal point H of the coupling lens 2 becomes L+ΔL due to thermal expansion of the holding member 10. . This relationship is number one. Furthermore, as shown in FIG. 2(c), the focal length of the coupling lens also changes depending on the wavelength of the semiconductor laser. Here, the slope of this straight line differs depending on the combination of lens materials in the coupling lens, that is, the lens design.

From FIG. 1(b) and FIG. 1(c), the relationship between the environmental temperature and the focal length of the coupling lens is determined, and is shown as in FIG. 1(d).

As described above, the focal length of the coupling lens 2 changes as the environmental temperature changes, and the distance between the light emitting point 0 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 light emitting point 0 of the semiconductor laser 1 and the principal point H of the coupling lens 2 due to the change in the environmental temperature is matched with the amount of change in the focal length of the coupling lens 2, If the coupling lens 2 is designed so that the straight line in Figure 1(a) matches the straight line approximated in Figure 1(d), even if the environmental temperature changes, 1,
When the focal length of the coupling lens 2 and the light emitting point 0 of the semiconductor laser 1 pass through the coupling lens 2, the light becomes parallel, and a normal focus error signal can be detected.

When the environmental temperature is T, the focal length of the coupling lens 2 is fc, the focal length of the objective lens 5 is fo, and the semiconductor laser 1
The distance between the light emitting point O 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. Further, let β be the coefficient of linear thermal expansion of the support member 10 that supports the semiconductor laser 1 and the coupling lens 2, and let F be the focus error.

The linear thermal expansion coefficient β of the support member 10 is expressed as β=1/L·(ΔL/ΔT). Therefore, it is sufficient that the amount of change Δfc in the focal length between the light emitting point O of the semiconductor laser I and the coupling lens pulling lens 2 due to the environmental temperature change is equal to the upper penetration ΔL, but a certain amount of error is allowed in this Δfc. Therefore, if this error is set to ±C, the focal ratio is the amount of change Δ
fc is Δfc=β・L・ΔT±ΔC Therefore, ±ΔC=Δfc−β・L・ΔT. Here, the focus error F is F=1/2・
(fc/fo)” ・6 Generally, the focus error F due to changes in environmental temperature is ±2.OX, expressed as ΔC.
Since it is allowable up to 10-3mm.

ΔG<0, 4 (, fc/fo) ”X 10-
It becomes 3mn. From the above.

1Δre-β-L-ΔT1 <0.4 (re-fo)2X 10-3m.

Therefore, if a coupling lens is used in which the amount of change Δfc in the focal length of the coupling lens satisfies the above equation when the environmental temperature changes by ΔT, a normal focus error signal can be detected.

This will be briefly explained using specific numerical values.

Distance L between the coupling lens 20 and the principal point H of the coupling lens 2 when the environmental temperature T = 25°C = 12.0 mw
Let it be n. Furthermore, the linear thermal expansion coefficient β of the support member 10 is 23.2.
X10-' ++n/℃. Here the environmental temperature is ΔT
= If it rises by 20℃, 1Δfc-β・L・Δ
From T1 <0.4 (fc/fo)"X l O-',
Substituting the above numerical values, 1Δfc-23, 2xLO-'x20xL21<0.
4x (12/4.5) 2xlO°3 Rearranging the above equation, the focal point of the coupling lens when the environmental temperature rises by 20°C from 1Δfc-5,57XlOli 1 <2.84X10-' (Thus, = 25°C) The amount of change in distance is 2.73X10-3m
The coupling lens is designed so that the range is from 8.14XIO-'wo to 8.14XIO-'wo.

As mentioned above, the change in the focal length of the coupling lens due to changes in environmental temperature is due to the change in the wavelength of the semiconductor laser due to changes in environmental temperature.
The wavelength of the semiconductor laser when T = 25 ° C) is 780 mm,
The environment'EA degree (T+
The wavelength of the semiconductor laser at ΔT=45°C is 785mm.
Then, from the above formula, the wavelength of the semiconductor laser is 780m.
When changing from m to 785 old 11, the change in focal length of the coupling lens is 2.73 as shown in Figure 2 (b).
X10-”n.

The coupling lens is designed within the range of 8.41×10 −3 mn.

As described above, in the present invention, the amount of change in the focal length of the coupling lens due to the change in the wavelength of the semiconductor laser light caused by the change in the environmental temperature, and the distance between the semiconductor laser and the coupling lens caused by the change in the environmental temperature are determined. By matching the amount of change.

[Brief explanation of drawings]

FIG. 1(a) is a diagram showing the relationship between environmental temperature and the distance between the principal point of the coupling lens and the light emitting point of the semiconductor laser. Figure (b) is a diagram of the relationship between environmental temperature and semiconductor laser wavelength;
Figure 2 (c) is a relationship diagram between the semiconductor laser wavelength and the focal length of the coupling lens, Figure 2 (d) is a relationship diagram between the environmental temperature and the focal length of the coupling lens, and Figure 2 (a) is a diagram of the relationship between the semiconductor laser wavelength and the focal length of the coupling lens. (b) is a diagram showing the tolerance range of the focal length of the coupling lens with respect to changes in the semiconductor laser wavelength of the present invention; FIG. A schematic configuration diagram showing the principle of a conventional optical pickup device, FIG. 4 is a diagram showing a focus error signal detection method, and FIG. Diagram showing the relationship, Figure (b) is a diagram showing the relationship among the 16-conductor laser, the coupling lens, and the luminous flux when the environmental temperature rises. 1...Semiconductor laser, 2...Coupling lens ,
3... Quarter wavelength plate, 4... Polarizing beam splinter, 5... Objective lens, 6... Information recording carrier (a)
(i)) Semiconductor laser wavelength Environmental temperature (()
((j) Fig. 1 S once in the evening (cl) Semiconductor laser wavelength (b) Fig. 2 (,1) Fig. 5

Claims (1)

[Claims] (1) The light flux emitted from the semiconductor laser is made into parallel light by a coupling lens, passed through a 1/4 wavelength plate and a polarizing beam splitter, and focused by an objective lens to form a minute spot on an information recording carrier. In an optical pickup device that records and reproduces information using reflected light, the amount of change in the focal length of the coupling lens due to a change in the wavelength of the semiconductor laser light caused by a change in environmental temperature, and the amount of change in the focal length of the coupling lens due to a change in the wavelength of the semiconductor laser light caused by a change in the environmental temperature are An optical pickup device characterized in that the amount of change in distance between the semiconductor laser and the coupling lens that occurs when the semiconductor laser and the coupling lens change substantially coincides with each other. (2) The coupling lens satisfies the inequality |Δfc-ΔT・β・L|<0.4(fc/fo)^2×10^-^3 The optical pickup device described in Section 1. Δfc: Amount of change in coupling lens focal length due to change in semiconductor laser wavelength caused by environmental temperature change ΔT: Amount of change in environmental temperature β: Linear thermal expansion coefficient of semiconductor laser and coupling lens support member L: Semiconductor laser light emitting point Distance to the principal point of the coupling lens fc: Focal length of the coupling lens fo: Focal length of the objective lens