JPH01287833A - Optical pickup and return optical noise measuring instrument - Google Patents

Abstract

PURPOSE:To minimize the distance between a laser chip and a reflecting mirror such as a disk and to miniaturize an optical system by using a grating coupler as an optical demultiplexing means. CONSTITUTION:A grating coupler 13 to have a waveguide 14, transmit a part out of an incident light and couple others to the waveguide 14 is provided, the light emitted from a semiconductor laser 1 and a reflecting light reflected by a disk or a mirror 12 are respectively independently coupled to the waveguide 14 and the light coupled at the waveguide 14 is photodetected by photodetectors 5 and 6. Output of two coupled light beams, the reflecting light from the disk is photodetected by the photodetecor 6, the signal on the disk is reproduced and on the other hand, the noise can be measured with the signal from the photodetectors 5 and 6 to respectively photodetect two light beams. Thus, the miniaturization can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Industrial Application Field The present invention relates to the improvement of optical pickups used in, for example, compact disc players, and also to the measurement of return light noise of semiconductor lasers generated in the optical pickups. This invention relates to improvement of a return light noise measuring device.

(Conventional technology) Compact disc players, which have become popular in recent years, use optical pickups to optically read information recorded on discs.
A semiconductor laser is used as a light source, and a beam emitted from the semiconductor laser is converged onto a disk via an optical system such as a collimator lens and an objective lens. Further, the reflected light from the disk is reflected laterally by a half mirror constituting the optical system, and this reflected light is received by a light receiving element, thereby converting information on the disk into an electrical signal. However, in the above-mentioned pickup, a half mirror is used as the optical path splitting means, so the light that is reflected from the disk and passes through the half mirror returns to the semiconductor laser and enters the semiconductor laser as light. cause noise.

Conventionally, as an example of a return light noise measuring device of a semiconductor laser, for example, Proceedings of the Japan Society of Applied Physics, Spring 1985 @3 volumes, P.
The device disclosed in No. 712.28a-ZH-8 is known. FIG. 5 shows this kind of return optical noise measuring device. In the figure, (1) is a semiconductor laser, and (2) is a half mirror.
1 (3) (4) are convergent lenses, (5) (6) are light receiving elements, (7) (8) are DC voltmeters, (9) are spectrum analyzers, αO is a neutral density filter, (111 is an objective lens,
G21 is a reflecting mirror, and αη is a collimating lens.

The light emitted from the semiconductor laser (1) is collimated by a collimator lens (171) and then passed through a half mirror (2).
), half of it is transmitted and the other half is reflected to the side. This internally transmitted light is attenuated by a neutral density filter and then focused by an objective lens U onto a reflecting mirror aperture z. The light thus converged beyond the reflecting mirror is reflected by the reflecting mirror a'2, and then,
The light enters the half mirror (2) again through the same optical path as when it entered. In this way, half of the light incident on the half mirror (2) passes through it and is transmitted to the semiconductor laser (1).
The remaining half is reflected to the side.

Now, when the reflected light enters the semiconductor laser (1) as described above, the emitted light and the reflected light (return light) interfere with each other within the laser chip of the semiconductor laser (1), resulting in the above-mentioned Noise occurs in the emitted light. Such noise is measured by monitoring the light reflected laterally by the half mirror (2) out of the light emitted from the semiconductor laser (1).

That is, such light is directed to the light receiving element (
6) The noise signal mentioned above is measured by observing the alternating current component of the signal from the light receiving element using a spectrum analyzer (9).

By the way, the occurrence of noise due to such return light depends on the size of the return light and the distance between the laser chip and the reflecting mirror 02 (L61
X). Among these, the magnitude of the returned light can be changed by variously changing the neutral density filter αO, and the actual value is the magnitude of the returned light and the semiconductor laser (1
) is defined by the ratio of the size of the emitted light from the Such a ratio is obtained by calculating the ratio of the output values of the voltmeters (7) and (8), and is given as one of the noise signal measurement conditions.

On the other hand, the distance between the laser chip and the reflector (Lex)
The objective lens [11 and the reflecting mirror oz are aligned in the optical axis direction (A-A
It can be changed by displacing it in the 'direction'.

bJ Problem to be Solved by the Invention By the way, in the optical pickup and return light noise measuring device described above, a half mirror (2) is used as a means for dividing light.
is used. However, when a half mirror is used in this way, the half mirror must be placed at an angle of 45° with respect to the optical axis, which increases the dimension that the half mirror occupies in the optical axis direction. This is greatly affected when the distance between the disk or the reflecting mirror (121) is reduced.In view of this, the present invention aims to provide an optical pickup and a return optical noise measuring device that can be made more compact. be.

B) Means for Solving the Problems In view of the above problems, the present invention provides a grating coupler which has a waveguide and which transmits a part of the incident light and couples the other part to the waveguide, by replacing it with the above-mentioned half mirror. The light emitted from the semiconductor laser and the reflected light reflected from the disk or mirror are independently coupled to the front ε waveguide, and the light coupled to the waveguide is received by the light receiving element. It was configured to do this.

(E) When using the device as an optical pickup, of the two lights coupled to the waveguide, the light reflected from the disk is received by the light receiving element.

This makes it possible to reproduce the signals on the disc.

On the other hand, when used as a return light noise measuring device, two light receiving elements are provided to respectively receive the two lights coupled to the waveguide, and noise can be measured using the signals from each light receiving element. Become.

(C) Example Hereinafter, an example of the returned light noise measuring device of the present invention will be described with reference to the drawings. Here, the same parts as those in FIG. 5 used in the conventional example are given the same symbol '+' and their explanation will be omitted.

That is, in FIG. 1, l is a grating coupler,
It is formed of a flat waveguide (141), two types of diffraction gratings (15a) (15b) formed on the waveguide (141), and a transparent substrate 416.
a) Among (15b), the diffraction grating (15a) guides a part of the light incident on the grating coupler 03) to the waveguide +14+ by diffraction, and the diffraction grating (15b)
acts to converge the light guided into the waveguide circle onto the light receiving elements (5) and (6). Therefore, as shown in FIG. 1, when light from the semiconductor laser (1) or the reflecting mirror q21 is incident on the grating coupler u3, some of the light passes through the grating coupler 131, but some of the light passes through the waveguide. (
141.

Now, the effects of the grating coupler will be explained using FIGS. 2 and 3. As shown in Figure 2, the light source (the diffused light from Ll is the grating coupler tG)
When the light is obliquely incident on the diffraction grating (gl
), a part of the light is coupled to the waveguide (WO) and propagated in the direction of arrow A within the waveguide (WG). On the other hand, as described above, the light reflected by the reflecting mirror □□□, etc. is coupled to the grating coupler 0) in the same manner as described above, and is directed in the direction of arrow B. Then, each light beam directed in the direction of the arrow A%B within the waveguide (WO) is focused by the diffraction grating (gz) ((lh)C, and is coupled to the sensor (Sl) on the side of the grating coupler tGl. (S
2), the light is received respectively.

The above is the light coupling operation when the diffused light is incident on the grating coupler IO+, but when the diffused light is incident on the grating coupler (Gl), the diffraction grating (g
The shape of (l) is given by the following equation when the X-y coordinate axes are defined as shown in FIG.

N-y+y x! +(y-fsinθ)1+(fcos
θ)2=mλ+f・(1) Here, N is the waveguide (WG
), f is the distance from the light source to the origin P, θ is the incident angle of the optical axis of the diffused light with respect to the grating coupler (G), λ is the wavelength of the diffused light, and m is an integer. Furthermore, the first
) for the derivation of the equation, see Matsuoka et al., ``Concentrating grating coupler using electron beam lithography burst'', IEICE Technical Report, OQ.
E85-84, P47-P48, and will not be described in detail here. When a diffraction grating (gl) is formed in the shape expressed by equation (1) above, the light propagated into the waveguide (WO) travels in parallel within the waveguide (g2X
gz) and is converged by diffraction.

By the way, in the above explanation, the grating coupler (Gl
is positioned at an angle with respect to the optical axis of the light source (LJ), but such an inclination causes the incident light to propagate in one direction within the waveguide (WG), while the reflected light passes through the waveguide (WG). In other words, when light is incident on the grating coupler (G) at an angle of 0 degrees, the light propagates in the opposite direction within the waveguide (WG). Since the pagoda is located in two opposite directions, the combined light of the incident light and the reflected light enters the sensor (SIXS2), so each sensor cannot receive only the combined light of the incident light or only the combined light of the reflected light. As described above, in this embodiment, only the combined light of the incident light or only the combined light of the reflected light must be received by the respective sensors (SsXSx). It is necessary to separate the respective coupled lights. However, since such an inclination is as small as a political change at most, the dimension that the grating coupler tGl occupies with respect to the optical axis of the light from the light source (light source) is extremely small. Grating coupler (Gl)
By tilting the grating coupler (G), reflected light from the grating coupler (G) is also prevented from entering the light source tLl.

Furthermore, as shown in FIG. 3, the spacing between the diffraction gratings (gl) formed in the grating coupler O) is narrower on the right side of the figure and wider on the left side.

This interval is because the light from the light source (Ll) is diffused light, so the angle of incidence differs depending on the incident part (the angle of incidence of the light on the right side of the figure is smaller than the angle of incidence of the light on the left side). It acts to couple the incident light (or reflected light) to the grating coupler (Gl) in a negative manner.

The above is about the shape of the diffraction grating (gl) when the light incident on the grating coupler tGl is diffused light or convergent light (in the case of reflected light), but when the light incident on the grating coupler CG+ is parallel light, If so, the first
) can be obtained by changing f→■ in the equation. That is, by transforming equation (1), the following equation can be obtained.

(2) In equation (2), if f+ω is set, mλ can be used to obtain the shape equation of the diffraction grating (g+)'. That is, the shape of the diffraction grating (gtf) when parallel light is incident on the grating coupler tGI is λ/(N-3j
-no) becomes a linear parallel grating with a period.

As mentioned above, in the embodiment shown in FIG.
The shape of a) is an arc. Furthermore, when a transparent flat plate such as the grating coupler 03 is arranged in a tilted manner in the diffused light, an aberration occurs in the light transmitted through the grating coupler 131, so a neutral density filter αO is used to compensate for this aberration.
It is necessary to arrange parallel to the grating coupler (131), make the thickness of the neutral density filter 00 equal to the thickness of the grating coupler l, and make the specifications of the collimating lens αη and the objective lens u11 the same.

In each of the above embodiments, the transparent substrate, the waveguide, and the diffraction grating have different refractive indexes of nf, ns, and ng.
Then, it is sufficient that the relationship nf)ns is satisfied, and it is preferable that ng is large.

By the way, in the above embodiment, the returning light weight is transmitted through the light receiving element (5),
(6) Calculate as (1-η)■2/(v1/η) (4), where η is the coupling efficiency of the crating coupler for Vl and V, respectively. I can do it. Note that the coupling efficiency η is uniquely determined by the incident angle of light, the thickness of the waveguide, the refractive index, and the like.

As described above, the grating coupler may be disposed between the objective lens (11J and the collimating lens (171) as shown in FIG. 4. However, in this case, the grating coupler l and neutral density filter 00 are both objective lens (111
and the collimating lens 07I, the distance (Lex) cannot be made as small as in the embodiment shown in FIG. However, in this case, since the grating coupler (13) is placed in the parallel light, there is no need to use restrictions on compensation for the effects of aberrations in the design of the optical system, and the grating coupler (131) Since the shape is linear as described above, the grating coupler can be easily manufactured.

Incidentally, the device shown in the above embodiment can be easily applied to an optical pickup. That is, in this case, information on the disc can be reproduced by the light receiving element (6), and at this time, the attenuation filter αO that defines the amount of reflected light is omitted. Furthermore, the converging lens (4) that converges the reflected light from the disk onto the light receiving element (6) can be omitted by providing a diffraction grating (15b), thereby making it possible to further downsize the optical system of the pickup. can.

(g) Effects of the Invention According to the present invention, a grating coupler is used as the light splitting means, and the dimension occupied by the grating coupler in the optical axis direction can be extremely small.
The distance between the laser chip and a reflecting mirror such as a disk (Le
x) i can be significantly reduced. For example, in actual measurements, when using the same lens system, the distance (Le In the second embodiment shown in Figure @4, it was possible to shorten the length to about 42111.

[Brief explanation of the drawing]

Figure @1 is a diagram of an optical system showing one embodiment of the present invention, Figures @2 and @3 are side views and plan views used to explain the effects of the grating coupler, and Figure 4 is another embodiment. It is a diagram of an optical system showing. (1) Semiconductor laser, (5) (6)...photodetector, q
2... Reflector, (1m (Gl... grating coupler.

Claims (3)

[Claims] (1) A light source, a grating coupler that guides a part of the light emitted from the light source to a waveguide and transmits other light, and a grating coupler that converges the transmitted light onto a recording medium and sends the reflected light back to the grating. An optical pickup comprising: an objective lens that guides the light to the coupler; and a light-receiving element that receives the light guided to the waveguide of the grating coupler among the reflected light and generates a reproduced signal. (2) The grating coupler is equipped with a diffraction grating that introduces a lens effect into the waveguide in order to guide the light guided to the waveguide out of the light reflected from the disk to the light receiving element. The optical pickup according to claim 1. (3) Return light noise that measures the noise generated in the beam emitted from the laser chip when the light emitted from the laser chip is reflected by an external mirror surface and then this reflected light is incident on the laser chip. A measuring device comprising: a grating coupler that guides part of the light emitted from the laser chip to a waveguide and transmits other light; a mirror member that vertically reflects the transmitted light; a first light-receiving element that receives the light guided to the waveguide of the grating coupler among the emitted light; and a first light receiving element that receives the light guided to the waveguide of the grating coupler among the light reflected from the mirror-like member. A returned light noise measuring device comprising a second light receiving element that receives light.