JPS611078A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To reduce the induced noise of a returning light by containing 1/2 wavelength plate, rotating the emitted laser light of a package at 90 deg. as a linear polarization, and forming as a linear polarization different from the polarization direction at 180 deg. from the initial when again returning to the package into a laser element. CONSTITUTION:A 1/2 wavelength plate 13 is provided between a semiconductor laser element 14 and an emitting window 12, the linear polarization light is rotated 90 deg. to a circular polarization light, and emitted as a linear polarization from the window 12. Since the laser light returned from the exterior again passes the plate 13, it becomes a linear polarization light rotated at 180 deg. in the polarization direction from the emitting time to the element 14. With this construction, returning light induction noise can be reduced in a light pickup using no light isolator, thereby improving the S/N ratio of a semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser device that has recently reached the stage of practical use as a light source for optical communications and optical information processing devices.

(Conventional structure and its problems) Semiconductor lasers have a narrow oscillation spectrum width and a long coherence length, but because the laser light has good coherence, if there is light returning into the semiconductor laser, the oscillation state becomes unstable and noise is generated. occurs.

When using a semiconductor laser to read an audio disc or a video disc, an optical isolator consisting of a quarter-wave plate and a polarizing beam splitter is inserted between the disc and the semiconductor laser to reduce the amount of returned light as much as possible. There is also a method of using only a beam splitter in order to make the optical pickup small and inexpensive. In this case, by increasing the amount of light returning into the semiconductor laser (by 1% or more), the spectral width is widened, the coherence of the laser light is worsened, and the noise level is lowered.

Figure 1(a) shows the optical systems of the above two types of optical pickups.
Shown in (b). In FIG. 5A, since the semiconductor laser light is linearly polarized light, it is converted into circularly polarized light when it passes through the quarter-wave plate 4. The laser beam passes through the focus lens 5, is reflected by the disk 6, and passes through the quarter-wave plate 4 again. At this time, the circularly polarized light returns to linearly polarized light, but its polarization direction is rotated by 90 degrees from when it was emitted from the semiconductor laser 1. In the optical pickup shown in FIG. 1(a), a beam splitter 3 whose transmittance differs depending on the polarization direction is used, and the reflected light from the disc 6 is reflected by the polarizing beam splitter 3 and sent to the photodetecting element 8 via the detector lens 7. I'm trying to guide you. According to this optical system, the quarter-wave plate 4
Although the returned light can be reduced as much as possible depending on the accuracy of the polarizing beam splitter 3, it increases the cost and is disadvantageous for downsizing the optical pickup.

In the optical pickup shown in Figure 1(b), the beam splitter (
Since the reflected light from the disk 6 is divided only by the half mirror 9, a considerable amount of the returned light enters the semiconductor laser 1. A semiconductor laser generally oscillates in a single longitudinal mode when there is no returned light, but when there is a large amount of returned light, the longitudinal modes become multiple, and the returned light induced noise becomes low. However, since there are variations from element to element, there is a possibility that the noise level in the optical system shown in FIG. 1(b) may exceed the permissible value of the system.

(Object of the Invention) The present invention provides a semiconductor laser device that can suppress return light induced noise to a lower level through optical pickup without using expensive parts such as optical isolators.

(Structure of the Invention) In order to achieve this object, the semiconductor laser device of the present invention incorporates a half-wave plate, converts the laser beam emitted from the package into linearly polarized light rotated by 90 degrees, and then re-injects the laser beam into the package. When the light returns and enters the semiconductor laser element, it is made into linearly polarized light with a polarization direction 180° different from the initial polarization direction, thereby reducing interference effects and reducing return light induced noise.

(Description of Embodiment) FIG. 2 is a cross-sectional view of the structure of a semiconductor laser device according to a first embodiment of the present invention, in which 10 is a stem body, 11 is a cap, 12 is a transparent exit window, and 13 is a half-section. Single wavelength plate, 1
4 is a semiconductor laser element, 15 is a heat sink (submount), and 16 is a lead wire.

In the figure, a half-wave plate 13 is provided between the semiconductor laser element 14 and the emission window 12, and converts linearly polarized light into 90°.
Laser light is output from the exit window 12 as linearly polarized light rotated by . When the laser beam returns from the outside, it passes through the half-wave plate 13 again, so it is 18
It becomes linearly polarized light with the polarization direction rotated by 0°, and the laser element 1
Enter within 4.

FIG. 3 is a diagram showing the return light induced noise reduction effect of a half-wave plate, in which the solid line represents the case without the half-wave plate, and the broken line represents the case with the half-wave plate. From the figure, the noise level can be lowered by about 10 dB by rotating the polarization direction by 180 degrees.

FIG. 4 shows a second embodiment of the present invention, in which a half-wave plate 17 also serves as an exit window.

(Effects of the Invention) As described above, according to the semiconductor laser device incorporating the half-wave plate of the present invention, return light induced noise can be reduced in optical pickup without using an optical isolator, and In addition, the S/N ratio of the semiconductor laser can be improved in the presence of returned light.

[Brief explanation of the drawing]

FIGS. 1(a) and (b) are diagrams showing the optical system of optical pickup, FIG. 2 is a cross-sectional view of the semiconductor laser of the present invention, and FIG. 3 is a diagram showing the noise reduction effect of the semiconductor laser of the present invention. FIG. 4 is a diagram showing a cross-sectional view of a semiconductor laser according to a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... Collimating lens, 3... Polarizing beam spherule, 4... Quarter wavelength plate, 5... Focus lens, 6.
・Disc, 7 ・Detector lens, 8 ・
... Photodetection element, 9 ... Beam splitter (half mirror), 10 ... Stem body, 11 ...
Cap, 12...Transparent exit window, 13.17...
Half wavelength plate, 14... semiconductor laser element, 15
...Heat sink (submount), 16...
Lead. Patent applicant Matsushita Electric Industrial Co., Ltd. Figure 1 (a) (b) Figure 3 Risk of return (%)

Claims (1)

[Claims] A semiconductor laser characterized in that a half-wave plate is provided inside the semiconductor laser device package or at the emission window, and converts the polarization direction of the semiconductor laser output beam from inside the package into linearly polarized light rotated by 90 degrees. Device.