JPH0497207A - Photodetector - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Attachment demarcation The present invention relates to a photodetection device, and more particularly, to a photodetection device.

The present invention relates to an integrated optical pickup for magneto-optical information recording and reproducing devices such as magneto-optical disk drives, magneto-optical cards, and magneto-optical tapes.

Figure 5 shows a conventional optical pickup using an optical integrated device. In the figure, 31 is a Si substrate, 32 is a buffer layer, 33 is an optical waveguide, 34 is a semiconductor laser, and 35 is a grating. , 36 is a focusing grating coupler (FGC
), 37 is an optical disk (optical information recording medium), and 38 is a photodetector (PD).

A waveguide layer 33 is laminated on a buffer layer 32 of a Si substrate 31. The semiconductor laser 34 is coupled to the end face of the Si substrate 31, and laser light is guided to the waveguide layer 33. The guided light passes through the grating 35, is diffracted by the condensing grating coupler 36, propagates through the air, and reaches the optical information recording medium 37. The light reflected by the optical information recording medium 37 enters the condensing grating coupler 36 again, becomes a guided light, is diffracted by the grating 35, is split into two beams, and is received by the photodetectors 38-1 to 38. . The signals from the photodetector group 38 are extracted as a reproduction signal (S), a focus error signal (FO), and a tracking error signal (TR) by calculations as shown in the figure. At this time, FO=(S311-□+8311-4) (S38-
2+ 838-3) TR: (S3Il-, +53.-4
) (S38-1”5311-2)S=53s-i
+S38-2 +838-3 +3□8-4. Note that S3B-L indicates the output signal of the photodetector 38, and +83. ．． The same applies to 2 to 538-4.

In the focus error signal FO, when the optical information recording medium 37 moves away from the pickup, FO<O, and when it approaches, FO>O.

In the optical integrated pickup shown in FIG. 5, the areas between photodetectors 38-1 and 38-2 and between 38-3 and 38-4 are dead sections that do not function as photodetectors.

Normally, this distance is about 5 to 10 μm, and cannot be made very small due to crosstalk between detectors. In focus error signal detection, light beams are focused between photodetectors 38-1 and 38-2 and between photodetectors 38-3 and 38-4 during focusing.

Therefore, if the focused beam diameter is smaller than the distance between the photodetectors, when the focus shifts before and after the focused point, F○
The value of is O, and no focus error signal can be obtained. This development is referred to as "no sensitivity to the focus error signal." If the diameter of the condensed beam is increased to be approximately the same as the spacing between the photodetectors, the condition of "no sensitivity" will disappear. However, the amount of change in the focus error signal when it deviates from the in-focus state is small. In the conventional example, in order to obtain a sufficient focus error signal, the grating 35 and the photodetector 38 are
It is necessary to increase the distance between the photodetectors 38- and 38-2, and between the photodetectors 38-3 and 38□4. The former method increases the optical pickup,
The benefits of optical integration and miniaturization are lost. Moreover, as mentioned above, the latter method has restrictions, and the interval cannot be narrowed unnecessarily. Furthermore, since this optical pickup has poor light usage efficiency, the signal level is low, which inevitably increases the load on the signal amplification circuit.

Purpose The present invention was made in view of the above-mentioned circumstances.
One photodetector has the function of a conventional photodetector, and the other photodetector has a signal multiplication effect, so essentially the gap between the detectors is not a problem. For,
Not only does this increase the degree of freedom in designing the entire optical system, but it also completely eliminates the problem of crosstalk, reducing the burden on the process.
The purpose of this invention is to provide a photodetection device that can reduce the burden on a signal amplification circuit added at a subsequent stage.

In order to achieve the above object, the present invention irradiates an optical information recording medium with light emitted from a laser light source, and forms reflected light from the optical information recording medium on the upper part of the substrate. In a photodetecting device that reads information recorded on the optical information recording medium by guiding the guided light into an optical waveguide and detecting the guided light with a photodetector, a first a concave portion, and a second portion provided at a lower portion of the first four portions and formed so as to have a length that is non-uniform with respect to the optical axis direction of the guided light. 1 photodetector, a second recess formed in a part of the guide wavepath and provided behind the first recess in the optical path direction of the incident light, and located below the second recess. a second photodetector provided at the rear of the first photodetector with respect to the optical path direction of the incident light so as to absorb the light not absorbed by the first photodetector; The second photodetector is characterized by utilizing an avalanche multiplication effect. Hereinafter, the present invention will be explained based on examples.

FIGS. 1(a) and 1(b) are configuration diagrams for explaining one embodiment of a photodetecting device according to the present invention, where FIG. 1(a) is a plan view and FIG. 1(b) is a It is an A' sectional view. In the figure, 1 is a waveguide substrate, 2 is a first photodetector, 3 is a second photodetector, 4 is an optical axis direction of incident light, 5 is a waveguide layer, 6 is a buffer layer, 7 is an n type layer, 8 is P type layer, 9 is P-(π) layer, 10
is a P+ substrate, 11.12 is a P+ layer, and 13 is a P+ layer.

A waveguide layer 5, a buffer layer 6, and the like are formed on the waveguide substrate 1. The first photodetector 2 has a pin structure,
The second photodetector 3 is also a so-called avalanche diode. The avalanche diode 3, which is the second photodetector, is composed of an n-type layer 7, a P-type layer 8, a P'' (π) layer 9, a P+ substrate 1o, and an n+ layer 11, and protects the n-type layer 7. It's ringing.

The pin type photodiode 2, which is the first photodetector, is
It consists of a p-(π) layer 9, a P+ substrate 1o, and an n-soil layer 12, and similarly to the second photodetector, a P layer 13 is configured as a channel stopper.

In the configuration shown in FIG. 1, the photodetector 2 has a non-uniform length with respect to the optical axis direction of the incident light 4.

The amount of absorbed light changes depending on the incident position of the incident light, and as a result, the output of the photodetector 2 changes. Therefore,
If the incident position of the incident light is made to correspond to the focus shift, this output becomes the focus shift as it is. Therefore, a photodetector having this shape is shown in FIG. 5 as a photodetector bear 38-□.

38-2 and 38. -,. 31. By using this instead of , a total of four photodetectors can be replaced with two photodetectors. This not only reduces the number of photodetectors, but also addresses the problems caused by building two photodetectors next to each other, such as crosstalk and dead zones (these two are contradictory factors). ) will be solved at the same time.

However, with such a shape, some light is emitted backwards without being absorbed, which reduces the amount of light that can be used as a so-called signal.
The problem of causing a decrease in the N ratio remains. This is because avalanche multiplication (avalanche multiplication: av
This can be solved by locating a photodetector 3 that has alanche multiplication effect. When using the avalanche multiplication effect, although the linearity of the relationship between the amount of incident light and the output is broken, it is possible to amplify a weak signal light and extract a large signal, so if a part of the incident light is Even if it is lost, it is possible to obtain a sufficiently large output, which is very advantageous. Note that there is no need to make the gap between the photodetectors 2 and 3 particularly small).

Next, a design example of the shape of the first photodetector will be explained based on FIG. 2.

In FIG. 2, it is assumed that the photodetector is formed in a region surrounded by y=Q and y=f(x) as shown by diagonal lines. Also, the incident light beam has a width D and an intensity.

, and the light is incident at the position x=x, ~X=Xo+D, the amount of light absorption is expressed by the following equation.

(However, α is an absorption coefficient that represents the degree to which the guided light is absorbed by silicon through the thin buffer layer at the detector. Note that this absorption coefficient is relative to energy.) Now, Assuming that the output of the photodetector is linear with respect to the absorbed energy, the amount of absorption described above has a linear relationship with the output of the detector. To make the output of the detector have a linear relationship with the position (X,) of the incident light, f(x) = C-1og
(kx) (k and C are constants) (2) It is sufficient to set α, and in this case, equation (1) becomes. By choosing C appropriately, the amount can be varied from almost O to nearly 1.00%. Let t be the maximum value of the optical axis deflection (the range that requires a linear output) and correspond to this range.

For example, a = 6 x 10-3 μm-1, t = 50 μm, D
= 10 μm and C is chosen as 3/α (=500 μm), then k = 0.318 μm-” is calculated, and at this time, in the range of x = (3, 14, 53, 14) (μm) The absorption amount can be changed from 87% to 8%.
Based on these numbers, the sensor shape is calculated as shown in Figure 3. In FIG. 3, the shaded part is the sensor part.

Of course, the shape of the sensor can also be extended, for example, as shown in Figure 4, where the sensor length is long along the optical path of the incident light. Rather, it is preferable to use this shape so that it does not exceed the range of the sensor even if the focus is significantly shifted for some reason. Also, a contact portion can be made in this extended portion.

Also, if output linearity is not required, set 1.
It goes without saying that it is possible to create various variations, such as using straight lines for curved parts or creating approximate shapes with polygonal lines depending on the mask design and process.

Effects As is clear from the above description, according to the present invention, a photodetector having a non-uniform length in the optical axis direction of incident light;
By using this photodetector as a pair with a photodetector with avalanche multiplication effect located at the rear of the photodetector when viewed from the light incident side, crosstalk, generation of dead zone, and reduction in S/N ratio can be avoided. Can solve problems.

[Brief explanation of the drawing]

FIG. 1 is a block diagram for explaining an embodiment of a photodetector according to the present invention, FIGS. 2 to 4 are diagrams showing design examples of the shape of the first photodetector, and FIG. , is a diagram showing a conventional optical pickup using an optical integrated device. DESCRIPTION OF SYMBOLS 1... Waveguide substrate, 2... First photodetector, 3...
- Second photodetector, 4... Optical axis direction of incident light, 5...
- Waveguide layer, 6... Buffer layer, 7... N-type layer, 8
...P type layer, 9...P-(π) layer, 10...P daughter substrate, 11.12...n ten layer, 13...P middle layer. Figure 1 Riko Patent Office Co., Ltd. (one idiot) Figure 2

Claims (1)

[Claims] 1. Light emitted from a laser light source is irradiated onto an optical information recording medium, the reflected light from the optical information recording medium is guided to an optical waveguide formed on the top of the substrate, and the guided light is detected by a photodetector. In the photodetecting device that reads information recorded on the optical information recording medium by: a first recess formed in a part of the optical waveguide; a first photodetector which is formed by changing the length of the guided light in the optical axis direction so as to have a non-uniform length with respect to the optical axis direction; and a first photodetector formed in a part of the optical waveguide. a second recess provided behind the first recess in the optical path direction of the incident light; and a second recess provided below the second recess, which is absorbed by the first photodetector. and a second photodetector provided behind the first photodetector with respect to the optical path direction of the incident light so as to absorb the light that did not exist, and the second photodetector absorbs the avalanche multiplication effect. A photodetection device characterized by using.