JPH0198135A - Optical pickup device - Google Patents

Abstract

PURPOSE:To prevent undesirable reflection at a radiation surface of a semiconductor laser chip by providing an anti-reflection coating on the radiation surface of the laser chip except a radiative point. CONSTITUTION:The anti-reflection coating 10 is provided on the part of the radiation surface N excluding the radiative point O of the semiconductor laser chip 8 and an end surface 9a of a chip fitting base 9. By this constitution, when return beams P'2(0) and P'0(+1) are made incident upon the radiation surface N of the chip 8, the beams are absorbed in majority by the coating 10, so that the amt. of their returning to a light path X is extremely minimized. Consequently, a return beam P'0(0) is reflected by the radiation surface N and diffracted by a diffraction grating 2, and the its beam in the direction of P'2 is only interfered with the beam P'2, thus reducing drastically the fluctuation of an output signal of a photosensor.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an optical pickup device used for an optical disc (a disc on which information is recorded in an optically readable manner). In particular, it applies to semiconductor lasers constituting optical pickup devices.

(b) Conventional technology As an optical pickup device, one having the structure shown in FIG. 6 is known (for example,
Refer to No. 42 and in the figure, the laser light emitted from the semiconductor laser (top) is diffracted by the diffraction grating (2) and becomes three beams Po (main beam), Pl, and P2 (auxiliary beam). Splitter (one with the same or different ratio of transmitted light and reflected light) (3), objective lens (
4) and enters the disk' (D). Disc (D
) is reflected light P'O%P'l,p
/ t, and returns to the original optical path in the opposite direction (auxiliary beam P1
．． P2 is not perpendicular to the disk but is incident at a slight angle, but since this angle is extremely small, the reflected light P't
, p l t can be considered to essentially return to the original optical path)
, and reaches the beam splitter (3) via the objective lens (4). The beam (P) that has been completely reflected by the beam splitter (3)
'o% P's, P'do) reaches the photosensor (process) via a concave lens (5) and a cylindrical lens (6). The photosensor (, L) is composed of a sensor (7c) that receives the reflected main beam P'o, a sensor (7a) that receives the reflected auxiliary beam P's, and a sensor (7b) that receives the reflected auxiliary beam p l 2. .

It is already known that an information signal and a focus error signal can be obtained from the sensor (7c), and that a radial error signal can be obtained as the output difference between the sensors (7a) and (7b).

Now, FIG. 7 shows changes in the radial error signal (RE) in a conventional pickup device. The DC component of the radial error signal (RE) fluctuates in response to one rotation of the disk, and this fluctuation becomes larger as the surface runout of the disk increases. When performing special disc playback such as searching for songs recorded on a disc, the permissible variation range of the DC component (RE-DC) of radial error No. 7 is as follows: The amplitude is REp-
If p, it is necessary to satisfy A/REp-p≦0.2. Conventional pickup devices do not necessarily satisfy the above conditions.

The reason for the fluctuation in the DC component of the radial error signal mentioned above is that the perpendicularity of the optical axis of the beam output from the pickup device to the disk changes by one rotation depending on the surface runout of the disk, especially in the tangential direction of the disk. This is thought to be because the degree of interference of light that occurs between the beam necessary for signal reproduction and the unnecessary beam (stray light) changes depending on the fluctuation of the verticality. . This point will be explained in more detail below with reference to FIG.

In Figure 8, (8) shows a semiconductor laser chip, and this chip (8) is fixed to a chip mount (submount) (9) with solder or conductive adhesive. . The laser beam (Po) radiated from the radiation point (0) of the chip (8) is separated by the diffraction grating (2) into a main beam (Po) that goes straight (does not undergo diffraction) and an auxiliary beam P1 generated by diffraction. , P2 (±1st order diffracted light)
The beams (P'o, pl 1, P' 2) reflected by the disk head toward the disk, and a part of the beams (P'o, pl 1, P' 2) pass through the beam splitter (3) shown in Figure 6. ] returns to the diffraction grating (2). These beams pass through the diffraction grating (2) and head toward the laser chip (8). Among these beams, the beam CP'o follows the optical path of X.
(+1>, P' t (0)) is the laser de'y block (
8) Q of the radiation surface (N) (this surface is a mirror surface)
It is reflected at the point and returns to the original optical path (X) [The optical path (X) is not perpendicular to the reflecting surface (&!i crystal cleavage plane) (N) but has a slight angle; is extremely small, so it can be considered that the reflected light returns along its original optical path.] This reflected return light (stray light) interferes with the beam (P') that received this interference. The photosensor (7)
b), the output signal of this photosensor (7b) (
sb> causes fluctuations in the DC component (see Figure 9), 9th
In the figure, the horizontal axis θ indicates the angle that the optical axis of the objective lens makes in the tangential direction with respect to the perpendicular line to the disk surface, and one wavelength of the output signal (Sb) is about 1. It has become 3 times.

Considering the light intensity of the beam after diffraction, the light intensity ratio between the 0th-order diffracted light (light that does not undergo diffraction) and the ±1st-order diffracted light is 1:1/3 to 178, so the beam has been diffracted more than once. The light intensity level is small and there is no need to consider interference. In Figure 8, the beam P'o (+1> is the +1st-order diffracted light of the reflected light P'o from the disk, and once Since it only undergoes diffraction, it affects interference.
Beam P′2(0) is a laser beam (Po) that is formed by a diffraction grating (
2) from the bottom to the top in Figure 6.
) [i.e., the light that went straight without undergoing diffraction when passing through the diffraction grating (2) from top to bottom], so it also underwent only one diffraction and was not affected by interference. affect.

Note that the beam CP'o (-1) (beam P'
P'5(0) (beam P'100th order light)] are both beams that have undergone one diffraction, but the 8th
As shown in the figure, since it does not enter the laser chip (8), there is no interference with the beam (P'!>).
Therefore, the photosensor (7) receiving the beam (P' 1)
As shown in FIG. 9, the fluctuation in the DC component of the output signal (Sa) in a) is small.

According to the above explanation, beams P'2 (0) and P'o (+1) of the optical path (X) are connected to the emission surface (N) of the laser chip (8).
It can be seen that the reflected light caused by the fluctuation in the DC component of the radial error signal [the difference between the output signals (Sa) and (Sb) of the odometers 7a and 7b].

(c) Problems to be Solved by the Invention It is an object of the invention to prevent reflection at the radiation surface of a laser tip with a simple configuration.

(d) Means for Solving the Problems In the optical pickup device shown in FIG. 6, an antireflection film is provided on the radiation surface of the semiconductor laser chip except for the radiation point.

(e) The anti-reflection film provided on the active radiation surface suppresses undesired reflections on the radiation surface and suppresses fluctuations in the DC component of the radial error signal.

(f) Example FIG. 1 shows a semiconductor laser chip (8) and a chip mounting base (9) that constitute a semiconductor laser (1) according to the present invention. is approximately 10
An anti-reflection film (1
The anti-reflection film (lO) provided with 0) is as follows:
For example, apply black silicone resin. A low reflection or diffuse reflection film (these are also antireflection films in the present invention) may be formed by applying various adhesives, lacquer, tar, ink, etc., or by using an inorganic material or metal vapor deposition film.

The anti-reflection film (10) should be applied at least near the 0 point shown in Figure 1, but since the Q point may shift due to changes in the optical system, it is necessary to mask the area near the radiant point O. Then, an anti-reflection film is formed by vapor deposition on the entire surface of the radiation surface (N) except near the radiation point 0 and on the end surface (9a) of the chip mounting base (9a) (this end surface can also be a reflective surface). It's okay.

According to the above-described configuration, the return light P' shown in FIG.
2 (0), P'o (+1) is incident on the radiation surface (N) of the laser chip (8), most of it is absorbed by the anti-reflection film (10), and even though some is reflected, it is not diffusely reflected. Optical path (
The amount of return of X) will be reduced. Therefore, the return light P'
o(0) is reflected at the radiation surface (N), and the diffraction grating (2)
The light (first-order diffracted light of the light reflected by the radiation surface of P'o(0)) that heads in the P'2 direction after being diffracted by P'2 only causes interference with P'2, and the photo sensor (7b) The output signal of (
The fluctuation of Sb) is significantly reduced as shown in FIG.

(G) Effects of the Invention According to the present invention described above, it is possible to prevent undesired anti-1,1 on the radiation surface of the laser chip, and to suppress fluctuations in the DC component of the radial error signal. 3 to 5, the characteristics of the normal pickup device before implementing the present invention (X marks indicate the values of each sample) and the characteristics of the pickup device after implementing the present invention (○ marks) In FIG. 3, the vertical axis indicates the undulation of the radial error signal, which is defined by the following equation.

Swell-204! og RE-DC change amount/RE
The amount of p-pRE-DC change indicates the difference between the maximum and minimum values of RE-DC, and the measurement is ±400 at the outer circumference of the disk.
−〇It uses a disk that causes surface runout, and it can be seen that waviness is suppressed.

Figure 4 shows the ratio (%) of the amount of change in RE-DC to REp-p when the objective lens is allowed to move ±0.4'' in the radial direction. FIG. 5 shows the ratio of the DC component value of the radial error signal to REp-p in the standard state (the state in which the radial servo system of the pickup device is open). (%) (indicates the amount of deviation of the midpoint)
This shows that the pickup device according to the present invention has a small center point deviation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a semiconductor laser used in an optical pickup device according to the present invention, FIGS. 2 to 5 are characteristic diagrams thereof, and FIG. 2 is a diagram showing changes in the output signal of a photosensor. , 313Kl is a diagram showing the amount of waviness of the radial error signal, FIG. 4 is a diagram showing the amount of change in RE-DC during radial control, FIG. 5 is a diagram showing the center point shift, and FIG. 6 is a diagram showing the conventional 7 is a diagram showing the radial error signal of the conventional device, FIG. 8 is a diagram explaining the principle of interference, and FIG. 9 is a diagram showing the change in the output signal of the photosensor of the conventional device. FIG. (1) is a semiconductor laser, (8) is a laser chip, (9)
is the chip mounting base, (0) is the radiation point, and (N) is the radiation surface.

Claims (2)

[Claims] (1) An optical pickup device configured to irradiate a recording medium with a beam emitted from a semiconductor laser, guide the emitted light from the recording medium to a photosensor, and obtain a signal from the photosensor, the device comprising: An optical pickup device characterized in that a radiation prevention film is provided on the radiation surface of a semiconductor laser chip constituting a laser except for the radiation point. (2) The semiconductor laser has a semiconductor laser chip and a chip mount to which the semiconductor laser chip is attached, and the antireflection film forms an end surface of the chip mount that is substantially flush with the radiation surface of the semiconductor laser. The optical pickup device according to claim 1, wherein the optical pickup device is also provided for.