JPH0716075B2 - Semiconductor laser chip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a semiconductor laser chip.

(B) Conventional Technology At present, semiconductor laser chips are used as a light source of an optical pickup device used for an optical disc (a disc on which information is optically readable recorded).

An optical pickup device having a structure shown in FIG. 6 is known (see, for example, Japanese Utility Model Laid-Open No. 58-75342). In the figure, the laser light emitted from the semiconductor laser ( 1 ) is diffracted by the diffraction grating (2) into three beams P ₀ (main beam), P ₁ and P ₂ (auxiliary beams),
A beam splitter (having the same ratio of transmitted light and reflected light or different ratio) (3) and an objective lens (4) enter the disk (D). The beam reflected by the disk (D) is reflected light P ′ ₀ , P ′ ₁ ,
Becomes P _'2, return the original optical path back (auxiliary beams P _1, P
₂ is not perpendicular to the disk and is incident at a slight angle, but since this angle is extremely small, the reflected light P ′ ₁ ,
It can be considered that P ′ ₂ substantially returns to the original optical path), and reaches the beam splitter (3) through the objective lens (4). The beam (P ' ₀ , which is reflected by the beam splitter (3),
P _'1, P' ₂₎ leads to the concave lens (5), the photosensor through a cylindrical lens (6) (7). Photosensor (7) receives a reflected main beam P _'0 sensor (7
c), the formed of a 'sensor for receiving a ₁ (7a) and the reflected auxiliary beams P' reflected auxiliary beams P sensor for receiving the ₂ (7b). Then, the information signal and the focus error signal are obtained from the sensor (7c), and the sensors (7a) and (7b)
The radial error signal can be obtained as the output difference of
Already known.

Now, FIG. 7 shows the change of the radial error signal (RE) in the conventional pickup device. The DC component of the radial error signal (RE) fluctuates corresponding to one rotation of the disk, and this fluctuation becomes larger as the surface wobble increases. When performing a special disc playback such as searching for a song recorded on the disc, the allowable fluctuation range of the direct current component (RE / DC) of the radial error signal is A, the fluctuation amplitude of the direct current component, and the amplitude of the radial error signal. Is defined as REp-p, it is necessary to satisfy A / REp-p ≦ 0.2. The conventional pickup device does not always satisfy the above conditions.

The cause of the fluctuation of the direct current component of the radial error signal is that the verticality of the optical axis of the beam output from the pickup device with respect to the disk, especially in the tangential direction of the disk, makes one revolution depending on the surface wobble of the disk. It is considered that the degree of interference of light generated between the beam required for signal reproduction and the unnecessary beam (stray light) changes in accordance with the change in the verticality.
Hereinafter, this point will be described in more detail with reference to FIG.

In FIG. 8, (8) shows a semiconductor laser chip, and this chip (8) is fixed to a chip mount (submount) (9) by brazing or a conductive adhesive. The laser beam (P ₀ ) emitted from the emission point (0) of the chip (8) is directly moved by the diffraction grating (2) to the main beam (P ₀ ) which is not directly diffracted and the auxiliary beam generated by the diffraction. Beams P ₁ and P ₂ (± 1st order diffracted light) are split and directed toward the disc. The beams (P ′ ₀ , P ′ ₁ , P ′ ₂ ) reflected by the disk are partially diffracted by the diffraction grating (2) [a part of which is transmitted through the beam splitter (3) shown in FIG. 6]. Return to. These beams pass through the diffraction grating (2) and travel toward the laser chip (8). Among these beams, the beams [P ′ ₀ (+1), P ′ ₂ that follow the optical path of X
(0)] is reflected at point Q of the emitting surface (N) of the laser chip (8) (this surface is a mirror surface) and returns to the original optical path (X) [optical path (X) is the emitting surface. (Cleavage plane of crystal)
Although it is not perpendicular to (N) but has a slight angle, it may be considered that the reflected light returns to the original optical path because this angle is extremely small]. This reflected return light (stray light) causes interference with the beam (P ′ ₂ ), and the beam (P ′ ₂ ) that has received the interference is directed to the photosensor (7 b), so the output of this photosensor (7 b). Fluctuations occur in the DC component of the signal (Sb) (see FIG. 9). In FIG. 9, the horizontal axis θ represents the angle formed by the optical axis of the objective lens in the tangential direction with respect to the vertical line with respect to the disk surface, and one wavelength of the output signal (Sb) is about 1.3. It is a degree.

Here, considering the light quantity of the beam after diffraction, the light quantity ratio between the 0th order diffracted light (light not diffracted) and the ± 1st order diffracted light is
Since it is 1: 1/3 to 1/8, the light level that is diffracted more than once is small and it is not necessary to consider interference. 8th
In the figure, the beam P ′ ₀ (+1) is the + 1st order diffracted light of the reflected light P ′ ₀ from the disk, and since it is only diffracted once, it affects the interference. Beam P _'2
(0), the laser beam (P ₀₎ is a diffraction grating (2) the first-order diffracted light (P ₂₎ disc due to reflected light occurring when passing from the bottom to the top at the sixth FIG (P _'2 ) Is the 0th order light (that is, light that goes straight without being diffracted when passing through the diffraction grating (2) from the top to the bottom). Affect.

The beam [P ′ ₀ (−1) (beam P ′ ₀
Primary light), P ′ ₁ (0) (0th order light of beam P ′ ₁ )]
Are beams that have been diffracted once, but since they do not enter the laser chip (8) as shown in FIG. 8, there is no interference with the beam (P ' ₁ ). Therefore, the fluctuation of the DC component of the output signal (Sa) of the photo sensor (7a) which receives the beam (P ' ₁ ) is small as shown in FIG.

According to the above description, the beams P ′ ₂ (0) and P ′ ₀ (+1) on the optical path (X) are emitted from the emission surface (N) of the laser chip (8).
It can be seen that the reflected light reflected by causes the fluctuation of the DC component of the radial error signal [difference between the output signals (Sa) and (Sb) of the photosensors (7a) and (7b)].

(C) Problems to be Solved by the Invention The present invention aims to prevent reflection on the emitting surface of the laser chip with a simple structure.

(D) Means for Solving the Problems A constitutional feature of the semiconductor laser chip of the present invention is that a film made of a low light reflecting material is provided on the emitting surface and the film and the laser light emitting point on the emitting surface are provided. It has been separated by 50 μm or more.

(E) Action Undesired reflection on the radiation surface is suppressed by the film provided on the radiation surface, and fluctuations in the DC component of the radial error signal are suppressed.

(F) Example FIG. 1 shows an example of the present invention, (11) is a stem made of Cu, (12) is a heat sink made of Si single crystal,
The heat sink is thermally and electrically fixed to one main surface of the stem. (13) is a semiconductor laser chip,
The chip consists of p-type Ga _1-X A on one main surface of p-type GaAs substrate (14).
l _X As (0 <X <1) first glad layer (15), non _- doped Ga _1-Y Al _Y As (0 ≦ Y <X) active layer (16), n-type Ga _1-X Al Second Glad layer made of _X As (17)
And a cap layer (18) made of n-type GaAs are sequentially laminated, and the end surface which is the radiation surface N is formed by cleavage, and a protective film (19) made of SiO ₂ or the like is formed on the cleavage surface. ing. The substrate and the layers (15) to (18)
Layer thickness of 90μm, 2μm, 0.1μm, 1.5μm, 2μ respectively
m.

In such a chip, the other main surface of the substrate (14) and the cap layer (1
8) By applying a forward bias between the ohmic first and second electrodes (20) and (21) respectively formed on the surface, laser light is excited from the active layer (16) and output from the emission point 0. Will be done. Further, such a chip is thermally and electrically fixed to the heat sink (12) on the first electrode (20) side by an Au alloy or the like. Furthermore, the radiation point N is 0
From the position 50 μm (l in the figure) away from the heat sink (1
A film (22) made of a low light-reflecting material such as black silicone resin is formed over 2). In addition, such a film (2
2) may be formed by applying various adhesives, lacquer, tar, black ink, etc., or by vapor depositing or printing an inorganic material or metal. The film (22) is not limited to the above-mentioned material, and may be any film that does not transmit laser light and has light reflectivity of half or less as compared with the cleavage surface of the chip (13). More preferably, a black material capable of absorbing laser light is preferable.

In addition, in this embodiment, the distance l between the radiation point 0 and the film (22) is set to 50.
However, if the distance is set shorter than this, there is a concern that the radiation point (0) may be covered with the film (22) due to the drooping of the film (22). Further, the distance between the intersection (Q) with the emission surface (N) of the return light passing through the optical path of X and the emission point (0) is usually 50 μm to 100.
Since it is at the position of μm, the radiation point 0 on the radiation surface N and the film (2
It is not necessary to set the distance l to 2) to 50 μm or less.

According to the above configuration, the return light P'shown in FIG.
_{When 2} (0) and P ′ ₀ (+1) are incident on the emission surface (N) of the laser chip (8), most of them are absorbed by the antireflection film (10) and some of them are reflected but diffusely reflected, Optical path (X)
The amount of return is extremely small. Therefore, the return light P '
₂ (0) is reflected by the radiation surface (N), and the diffraction grating (2)
At after being diffracted P _'2 light toward the direction [P' _{0 (0)} 1-order diffracted light of the reflected light by the emitting surface of] is, only interferes with P _'2, photo sensors (7b) The fluctuation of the output signal (Sb) will be greatly reduced as shown in FIG.

(G) Effect of the Invention According to the present invention described above, it is possible to prevent undesired reflection on the emitting surface of the laser chip and suppress fluctuations in the DC component of the radial error signal. 3 to 5, the characteristics of the pickup device using the chip having no film according to the present invention (values of each sample are indicated by X marks) and the characteristics of the pickup device using the chip of the present invention (O marks). Display the value of each sample with.) In FIG. 3, the vertical axis represents the waviness of the radial error signal, which is defined by the following equation.

Swell = 20log RE / DC change amount / REp-p RE / DC change amount indicates the difference between the maximum and minimum values of RE / DC. The measurement was performed using a disk that causes a surface runout of ± 400 μm on the outer circumference of the disk, and it can be seen that waviness is suppressed.

Fig. 4 shows the ratio (%) of the amount of change in RE / DC to REp-p when the objective lens is moved ± 0.4 mm in the radial direction.
And shows that the smaller this variation is, the more advantageous it is for special reproduction. FIG. 5 shows REp-p of the value of the DC component of the radial error signal in the standard state (the state where the radial servo system of the pickup device is opened).
The ratio (%) (indicating the amount of deviation of the midpoint) is shown, which shows that the pickup device using the semiconductor laser chip according to the present invention has a small deviation of the midpoint.

[Brief description of drawings]

FIG. 1 is a sectional view showing a semiconductor laser chip of the present invention, FIGS. 2 to 5 are characteristic diagrams thereof, FIG. 2 is a diagram showing changes in an output signal of a photosensor, and FIG. 3 is a radial error. FIG. 4 is a diagram showing the amount of signal swell, FIG. 4 is a diagram showing the amount of change in RE / DC during radial control, FIG. 5 is a diagram showing midpoint shift, and FIG. 6 is a diagram showing a conventional pickup device. FIG. 7 is a diagram showing a radial error signal of a conventional device, FIG. 8 is a diagram for explaining the principle of interference, and FIG. 9 is a diagram showing changes in output signals of a photosensor of the conventional device. (13) ... Semiconductor laser chip, (22) ... Film, (0) ... Radiating point, (N) ... Radiating surface.

Claims (2)

[Claims] 1. A film made of a low-light-reflecting material is provided on the emitting surface, and the film and the laser light emitting point on the emitting surface are 50.
The laser light emitted from the laser light emitting point is separated from the disk by a distance of at least .mu.m, so that the film prevents the light reflected from the disk from being reflected again. Semiconductor laser chip for optical pickup. 2. A semiconductor laser chip according to claim 1, wherein the film is made of a black material.