JPS63223710A - Photodetector for optical pickup - Google Patents

Abstract

PURPOSE:To permit provision of a photodetector for an optical pickup having relatively simple constitution by providing plural branch light guides which are branched from a multimode trunk light guide and are different in width sizes from each other and photoelectric converters which detect the light propagating in the respective branch light guides. CONSTITUTION:The spot of the reflected light entering the end face of the trunk light guide 10 increases its size when there is a focusing error. The incident position of the reflected light spot on the end face of the trunk light guide 10 deviates from the center when there is a tracking error. The modes excited in the light guide 10 vary with the incident forms thereof and are guided to the branch light guides 11-13 having such sizes which vary from each other so as to correspond to said modes. The incident form of the reflected light, i.e., the presence or absence of the focusing error or tracking error can be detected according to from which of the photoelectric converters 41-43 the detection signal is generated. The photodetecting optical system is thereby simplified.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention A multimode backbone optical waveguide through which reflected light from an optical disk is guided, at least two branched optical waveguides branching from this and having mutually different width dimensions, and propagation through each branched optical waveguide. It is equipped with a photoelectric conversion element that detects the light. When a focusing error occurs, the reflected light spot incident on the end face of the main optical waveguide becomes larger. Also, tracking
When an error occurs, the incident position of the reflected light spot on the end face of the main optical waveguide is shifted from the center. The mode excited in the main optical waveguide differs depending on these incident forms, and is guided to the branch optical waveguide corresponding to that mode. Therefore, depending on which photoelectric conversion element the detection signal comes from, the incident mode of one reflected light, that is, the focusing error.

The presence or absence of tracking errors can be detected.

BACKGROUND OF THE INVENTION The present invention relates to a light receiving device in an optical pickup for reading or writing recorded information from an optical disk (including a magneto-optical disk), and in particular, it relates to a light receiving device for an optical pickup for reading recorded information from an optical disk (including a magneto-optical disk). The present invention relates to a light receiving device that can detect errors.

The optical system for detecting focusing errors, tracking φu, ra, etc. in the light receiving system of conventional optical pickups is relatively complicated because it includes a single astigmatism optical system and a divided light receiving element. .

SUMMARY OF THE INVENTION An object of the present invention is to provide a light receiving device for an optical pickup having a relatively simple configuration.

Focusing light on the recording surface of an optical disc 1. In an optical pickup having a light projection system for projecting light and a light receiving system for receiving reflected light or transmitted light from an optical disk and outputting a recorded information reading signal, focusing error and tracking error signal, the light receiving device according to the present invention is included in the above light receiving system. The light-receiving device according to the present invention includes a multimode backbone optical waveguide, into which reflected light or transmitted light from an optical disk enters, and a multimode backbone optical waveguide having an a-shaped end face and a width capable of holding a plurality of optical modes. At least two branched optical waveguides that are connected to the other end so as to extend almost on the extension thereof and that have a smaller number of retained modes than the main optical waveguide, where the branching angle between them is sufficiently small and the modes that are retained. The branch optical waveguide is characterized by having a width dimension such that the orders of the optical waveguides are different from each other, and a photoelectric conversion element for detecting light propagating through each branched optical waveguide.

When a focusing error occurs, the reflected light spot incident on the end face of the main optical waveguide becomes larger. Furthermore, when a tracking error occurs, the incident position of the reflected light spot on the end face of the main optical waveguide shifts from the center. The mode excited in the main optical waveguide differs depending on these incident forms, and is guided to the branch optical waveguide corresponding to that mode. 1. Therefore, depending on which photoelectric conversion element generates the detection signal, it is possible to detect the incident form of one reflected light, that is, the presence or absence of focusing error and tracking error.

Since the above-mentioned main optical waveguide, one-branch optical waveguide and, if necessary, a photoelectric conversion element can be integrated on one substrate, it is possible to reduce the size and weight. The configuration is also simple. Since it is only necessary to couple the reflected light or transmitted light from the previous disk to the main optical waveguide, the light receiving optical system can also be simplified.

DESCRIPTION OF THE EMBODIMENTS First, a basic optical mode separation/combination element will be explained, and then the configuration of the optical pickup and the principle of detection of tracking errors and focusing errors will be explained.

(1) Single mode three-branch optical waveguide (1,1) Structure and manufacturing method First, a single mode: three-branch optical waveguide will be explained.

FIG. 1 perspectively shows an example of a single mode three-branch optical waveguide element. A SiO2 buffer layer 2 is formed on the St substrate 1'', and a glass optical waveguide layer 3 is further formed on this buffer layer 2. Ridge-shaped optical waveguides 10.11 to 13 are formed in the glass optical waveguide layer 3. The core optical waveguide IO is a multimode optical waveguide that can guide light in three modes of 0 and 1° second order. This optical waveguide 10
Three single-mode branch optical waveguides 11, 12, and 13, each having a different width, are branched from one end of the waveguide. The optical waveguide 11 has the widest width, and the optical waveguide 13 has the narrowest width.

The branching angle θ(
(see Figure 2) is sufficiently small. The fact that the branching angle θ is sufficiently small means that the light passes through the branched optical waveguide 11, 12 or 13.
This means that the change in the spacing between the branched optical waveguides when propagating over a small distance along the path is negligible relative to the distance traveled. The single mode branching optical waveguides 11 to 13 are
When the distance between them becomes sufficiently large, they become parallel to each other. The portion from the point where the branched optical waveguides 11 to 13 separate from the main optical waveguide 10 to the point where they become parallel to each other will be referred to as a branched portion.

FIG. 2 shows an example of the dimensions of these optical waveguides 10 to 3, such as one width interval. For example, the branching angle θ is 1
150.1/100.1/200.1/400 (ra
d), and the length of the branch part for these branch angles θ is 1.025, 2.05, 4.10, respectively.
．． It becomes 8.20 (mm).

Such an optical waveguide element can be manufactured, for example, as follows.

FIG. 3 shows the manufacturing process, in which the optical waveguide 12
The cross section of the section is shown.

A (100) n-type St with a resistivity of 8 to 12 Ω·1 is used as the substrate 1 (Fig. 3 (A)), and this St is heated to a temperature of 110 Ω.
Wet thermal oxidation at 0℃, 1.5μm thick S L
An O2 buffer layer 2 is produced (FIG. 3(B)). A glass optical waveguide layer 3 is formed thereon by sputtering Corning 7059 glass to a thickness of 1 μm (FIG. 3(C)).

Furthermore, a photoresist 4 (A z
-L350 J) was uniformly spin-coated at 90°C.
(Figure 3 (D)). When the resist 4 is exposed to light onto the optical waveguide pattern to be formed and then developed, the resist 4 in the portion that is to become the optical waveguide is removed (see Fig. 3 (E).
)) (Photolithography).

When Al 5 is sputtered on top of this (Fig. 3 (F)) and the resist 4 is melted, Al 5 of the optical waveguide pattern remains (
Figure 3 (G)) (lift-off).

Then, the portion of the glass layer 3 other than the A-shaped pattern 5 is dry-etched by, for example, CHF3 gas to a thickness of 0.1 am ((+1) in FIG. 3). Finally, the Al pattern 5 is removed by wet etching. Etching solution is available on HP
O; HNO3; CH3COOH-12: 1:1 may be used.

The thickness of the glass layer in parts such as the optical waveguide 12 is 1 μm, and the thickness in other parts is 0.9 μm.

(1, 2) Operation of mode separation and synthesis In the branch portion of the three-branch optical waveguide device as described above,
When the three single mode optical waveguides H, 12, and 13 are sufficiently close to each other, the light waves propagating through each optical waveguide are influenced by each other. Therefore, we consider the nine-branch portion as one optical waveguide, and perform an analysis using the local normal mode within it.

First, a model of a rib-shaped channel optical waveguide as shown in FIG. 4 is considered, and changes in the equivalent refractive index due to branching and field distribution are investigated using the one-equivalent refractive index method. This model is equivalent to the device shown in FIG. 1 with the branched portions cut off. The rib refers to a portion that forms part of the optical waveguide 11, 12, 13 and protrudes above the surrounding area.

In FIG. 4, first consider the confinement of the light wave mode field in the longitudinal direction (X direction) of the optical waveguide. It can be considered that there are two types of slab optical waveguides, I and (2), depending on the presence or absence of ribs.

The TE mode will be considered with the propagation direction of the light wave as the Z axis and the respective coordinate axes as shown in FIG.

Let n be the equivalent refractive index of the optical waveguide layer (the part of the optical waveguide I or ◯), and let n be the equivalent refractive index of the air layer above it and the SiO2 buffer layer below it, respectively. The upper surface of the optical waveguide layer is represented by x-0, and the thickness of the optical waveguide layer t or t2) is represented by t.

(2) The Y-direction component E of the electric field in each of these layers is expressed as follows.

E-Acxp(kxlx)
(-oo<x<0) I Ey2-B cos (k, x-φ)
(0<x<t)E −Cexp(−kx3(x−t)
) (t<x<+oo) where k −in
2 to 2-βml (i-1,2,3)xi
1.

ko ~ 2π/λo (2) n,: Equivalent refractive index of the i-th layer β: Phase constant λ0 in the Z direction Two free space wavelengths k - Wave number in the X direction of the i-th layer x1° x-0 and x- Since the electric and magnetic fields must be continuous at each boundary of t, a characteristic equation for determining the second-order wave number is obtained by eliminating the terms related to amplitude.

, t2-myr-tan (k, ■/kx2) +
tan (kx3/kx2) The equipolarized refractive index of the part where the layer thickness is t'' t is n.The equipolarized refractive index of the part of f'f'l'1-1 is set as n.If we do this as eff2 The model shown in FIG. 4 can be considered as a seven-layer slab optical waveguide as shown in FIG.

Next, based on the 7-layer model shown in Figure 5,
We consider the confinement of the light wave mode field and find the final equipolarized refractive index.

The width of the part corresponding to 1g and 12.11 of the optical waveguide is d
, d , d , and the optical waveguide spacing is h. 7
Consider M mode.

Letting the X-direction component of the magnetic field in each optical waveguide be H, these are given by the following equation.

H=Acosφt exp (kt y) (−country<y
<O) Hx2-mAcos(k2y+φ1) (0<y<d2) Hx3-B"cxp fk3[y-(d2+h)])
10B-(3Xl) [-3(y-d2)](d <y
<d2+h) H,4-c cos Ik4 [y (d2+d4+
h)]-φ2) (d 10h<y<d2+d4+h)
H-D eXlik5 [y-(d2+d4+2h)]
l+I)-eXp (-,, [y-(d2+d4
+h)]1(d2+d4+h<y<d2+d4+2h)
H=E cos (k [y −(d2+d4+d6
+2h)]-φ3)x6 B (d2+d4+2h<y<d2+d4+d6+2h)H
−Ecosφ exp [− to 7[y −(d2−+−
c+4+de +2h) 11x7 g (d2+d4+d6+2h<y<+oo) where k
-1n"k2-βri (i -1,2,-57)
k o ” 2π/λ (
5) Since the electric and magnetic fields must be continuous at each boundary, by eliminating the terms related to amplitude, 1
Obtain the equation that determines the next wave number.

Φ1Φ2Φ3-Φ3Φ4Φ8exp(-2k3h)-Φ
1Φ5Φ7 exp (-2k 5h )-Φ4Φ5Φ
6exp (-2 (ks 10 to 5) hl -0 but Φ -kn (nk+n2k) + Cn 2 kl k 3- (nt
ns k2)”ko tan k2 d2Φ
-k n (n k +n2k) + [n
k k −(n n k)2 t
ankdΦ −k n (n k +n2k)
+ [ns ks kr (n5n7kB)]
tan ked.

Φ −k n (−n k +n2k )+
(n k k + (n n k)
2 tankdΦ −k n (n k−n2
k) + [n k k + (n n k )
"] tank d857 57 [188 Φ-kn (nk+n"k) + [-n4 k3ks + (n3ns k4)]
tan k4d4Φ7""k4n4(n3ks -n
s ka )+ [n k k +(n
n k) 2 tankdΦ −k n
(n k−r+2k)+[n k k +(n
n k ) 21 tank d:'. In nl
” n3- n5- n7- neff12
4 6 erf2 The thickness of the optical waveguide part with ribs is tl-1 μm, and the thickness of the part without ribs is t2.
-0.9 μm.

The width of the three single mode optical waveguides 13.12.11 is d
2-2 μm, d4-2.5 μm, d6-3 μm (slightly different from what is shown in Figure 2), solve Equation (6), and calculate the equipolarized refractive index β of each optical waveguide with respect to the waveguide spacing. /k
FIG. 6 shows o.

From this graph, it can be seen that as the distance between the optical waveguides increases, the equipolarized refractive index approaches a constant value.

The first term of equation (8) is the three single mode optical waveguides 11
．． This is the product of characteristic equations when 12.13 are each independent (the interval is sufficiently large). Namely.

If the spacing between the optical waveguides is sufficiently large, the second term and subsequent terms in equation (6) can be ignored and the three optical waveguides can be considered to be in a single state.

FIG. 7 shows how the equipolarized refractive index changes with respect to the optical waveguide width d of one optical waveguide that is in an independent state where they do not influence each other. However, tl-1μm, t2-0.
It was set to 9 μm.

In FIG. 7, the points where the solid line ■ intersects each dispersion curve of equipolarized refractive index represent the propagation constant (equal polarized refractive index) of each mode in the multimode optical waveguide 10.

The point where the broken lines ■, ■, ■ intersect is the single mode optical waveguide 11
~13 propagation constants, respectively. Furthermore, arrows indicate the behavior of the waveguide mode in the three-branch optical waveguide element shown in FIG.

By the way, in the two-branch portion, the spacing between the optical waveguides changes, and no independent mode in the original strict sense can exist. However, in the case of ideal branching in which the two-branching angle is sufficiently small and there is no loss of optical power in the first branching portion, and the wavefront of the mode is approximately perpendicular to the Z-axis, each mode may be considered to be independent.

Therefore, if we assume that the first branch angle is sufficiently small,
FIG. 6 shows changes in the propagation constants of three independent local association normal Φ modes in the bifurcation section. Further, the equipolarized refractive indices of these three modes are independent without intersecting with each other.

As can be seen from FIG. 7, the zero-order mode is coupled to the optical waveguide 11 having the largest waveguide width among the three single mode optical waveguides 11 to 13, and the same goes for the rest.

The primary and secondary modes are respectively coupled to an optical waveguide 12° having an intermediate width and an optical waveguide 13 having a minimum width.

FIG. 8 shows how the field distribution changes. FIG. 8(A) shows how the zero-order mode propagating through the optical waveguide ■0 advances to the optical waveguide 11, and FIG. 8(B) shows the change in field distribution when the first-order mode advances to the optical waveguide 12. Figure 8 (
C) shows changes in the field distribution of the secondary mode.

The above explanation also applies to light traveling in the opposite direction. That is, it propagates through the optical waveguide 11 and the optical waveguide 10
The light that has entered the optical waveguide 12 and 13 becomes the zero-order mode, and the light that is propagating through the optical waveguides 12 and 13 and proceeds to the optical waveguide 10 becomes the first-order and second-order modes, respectively.

(2) Multimode Y-branch optical waveguide Next, the multimode Y-branch optical waveguide will be explained. An example of the configuration of a multimode Y-branch optical waveguide is shown in FIG.

Two branch optical waveguides 21 and 22 are branched from the main optical waveguide 20. One optical waveguide 21 is a multimode optical waveguide, and extends straight from the main optical waveguide 20, but for convenience of explanation, this will also be called a branch optical waveguide.

The other optical waveguide 22 is a single mode optical waveguide and is separated from the optical waveguide 21 by a very small angle. In FIGS. 10 to 12, the factors relating to these optical waveguides 20, 21, 22 are referred to in the following description.
They are simply indicated by a, b, and c, respectively.

In this embodiment, the main optical waveguide 20 can propagate four modes of 0, 1.2, and 3rd order, one of these four modes branches off to the branch optical waveguide 22, and the rest The three modes propagate through the optical waveguide 21.

Such a Y branch portion can also be analyzed using the local normal mode described above.

Although the behavior of the light wave mode can be understood by determining the propagation constant of the local-normal conflict mode of the five-layer slab optical waveguide, the details of the analysis will be omitted and only the results will be described below.

Three cases are possible depending on the widths of the optical waveguides 21 and 22. If the mode guided to the single mode optical waveguide 22 is the third mode in the multimode optical waveguide 20 before splitting into two (case 1), if it is the second mode (case 2), if it is the first mode mode (case 3).

Figure 1O shows case 1, Figure 10 (A) shows the change in the equipolarized refractive index with respect to the change in the normalized optical waveguide spacing, and Figure 10 (B) shows the equipolarized refractive index before and after branching. The values are plotted on the dispersion curve and the behavior of the waveguide mode is shown by arrows. Figure 10 (C) shows the branch optical waveguide 22.
It shows the field distribution of the modes (shown in (e)).

As an example of the normalized optical waveguide width of each branch optical waveguide in Case 1, the optical waveguide 21(b) has a width of 14.2. The optical waveguide 22(e) has a length of 1.58 te. Figure 1O (B) l:
Here, the intersection of the solid line a and each dispersion curve is the equipolarized refractive index in the optical waveguide 20 for each mode. Among these points, the 0.1 and 2nd order points move to a point corresponding to the width of the optical waveguide 21, as shown by the intersection with the broken line. Only the third-order mode point moves to a point corresponding to the width of the optical waveguide 22, as shown by the intersection with the broken line C. Since the branching angle is based on the condition that it is sufficiently small as described above, the equipolarized refractive indices of the four modes are independent without crossing each other. It can be seen that only the 3rd order mode branches off to the optical waveguide 22 and the other 0.1 and 2nd order modes propagate through the optical waveguide 22.

FIG. 11 shows, for case 2, the change in the equipolarized refractive index, the behavior of the propagation mode, and the field distribution of the mode proceeding to the optical waveguide z2. An example of the normalized optical waveguide width of the optical waveguides 21 and 22 is 14.2. 3.1
[It is 1. Among the 0th to 3rd order modes propagating through the optical waveguide 20, the 2nd order mode is split into the optical waveguide 22, and the remaining 0
, 1st and 3rd order modes propagate through the optical waveguide 21.

FIG. 12 shows case 3. The normalized optical waveguide widths of the optical waveguides 21 and 22 are respectively 11.1.
It is 4.74. The first-order mode branches into the optical waveguide 22 (
7, the remaining 0, 2 and 3 modes propagate through the optical waveguide 21.

By appropriately selecting the width of the one-branch optical waveguide on the dispersion curve of equipolarized refractive index as described in 1 above, any higher-order mode can be selectively extracted from the multimode optical waveguide.

By applying the following concept, as shown in FIG. 13, single mode optical waveguides 31, 32, 33, and 34 each having an appropriate width are sequentially branched from the main optical waveguide 20. ], Q propagating through the optical waveguide 20,
1.2.3rd mode can be separated from each other.

In FIGS. 9 and 13, the light propagating through the single mode branch optical waveguides 22.31 to 34 and entering the main optical waveguide 20 passes through the optical waveguide 20 in the mode set in the propagating branch optical waveguide. It is easy to understand that it is transmitted.

(3) Summary When considering the characteristics of waveguide branching, the following two methods can be considered. One is an analysis using local-normal mode, which solves the characteristic equation and considers changes in the equipolarized refractive index for each cross section of the optical waveguide.The other is an analysis using the local-normal mode. This is a method of considering changes in the equipolarized refractive index using a dispersion curve of the polarized refractive index. In the method of considering using a dispersion curve, as shown in Figure 14,
The equipolarized refractive index at each normalized optical waveguide width of the main optical waveguide before branching and each branched optical waveguide after branching is plotted.

When the branching angle is sufficiently small compared to the wavelength, the change in the 2-equivalent refractive index is slow and continuous. Therefore, the equipolarized refractive index of each mode does not cross during the change,
The refractive index changes to an equipolarized refractive index of the normalized width of each branch waveguide. This situation is indicated by an arrow. 1 before and after branching
There is no change in the order of the equivalent refractive index. According to this method of considering the characteristics of an optical waveguide branch using a dispersion curve, by determining the width of the optical waveguide before and after the two-branch, it is possible to know which mode before the two-branch is separated into which branch optical waveguide. .

In the above example, a ridge-type optical waveguide using a glass material was described, but an optical waveguide can be manufactured using any optical material, and the type of optical waveguide is a buried type (thermal diffusion, ion exchange, etc.).

It can be applied to all types including ion implantation, light/electron beam irradiation, etc.), loaded type, and voltage induced type.

(4) Optical pickup FIG. 15 is a diagram showing an example of an optical pickup constructed using the above-mentioned optical mode separation/synthesis element. There is.

A light beam emitted from the semiconductor laser 40 is deflected by a beam splitter 44 and focused by a lens system 45 onto the recording surface of an optical disk (not shown). As is well known, bits 51 representing information are formed on this recording surface. The reflected light from the optical disk is passed through the lens system 45.
The light passes through the beam and splitter 44 and enters the main optical waveguide 10 formed on the substrate 1 from its end face.

Branch optical waveguides 11 to 13 branched from the main optical waveguide 10
terminates halfway on the board 1, and at this termination part,
Photoelectric conversion elements 41 to 43 are provided, respectively, for detecting the light propagating through these branched optical waveguides 11 to 13 and converting it into an electrical signal. The photoelectric conversion elements 41 to 43 are, for example, a-3i (amorphous silicon).
, CdS, and CdTe can be used. In this case, these materials may be deposited on the terminal ends of the branched optical waveguides 11 to 13.

A photoφ diode or phototransistor using a pn junction can also be used. A hole may be formed at the end position of the optical waveguide 11-13, and a photo diode etc. may be inserted and bonded there, or if the substrate 1 is SL, a pn
It is also possible to form the joint integrally. In some cases, a circuit is connected that compares and discriminates the output signal of each photoelectric conversion element 41 to 43 with a predetermined level. In any case, output signal processing of the photoelectric conversion elements 41 to 48 may be performed according to appropriate logic.

Fig. te shows another example of optical start-up. Here, the semiconductor laser 40 is fixed on the substrate 1.

Also, a light projecting lens system 46 that focuses the emitted light from the semiconductor laser 40 onto the optical disk, and a light receiving lens system 47 that focuses the reflected light from the optical disk onto the end face of the main optical waveguide 10.
and is provided.

FIG. 17 shows the principle of focusing error detection. When the focus is correct as shown in FIG. 17(B), that is, when focusing is performed correctly, the spot diameter is focused by the lens 45 or 47 to approximately the same width as the main optical waveguide 10. If the central part of the core optical waveguide 10 is excited, it will mainly be 0.
The light of the next mode is excited in the main optical waveguide 10. This 0
Since the next light proceeds to the branched optical waveguide 11 as described above, the largest electrical signal is output from the photoelectric conversion element 41. This allows it to be detected that focusing is being performed correctly. Further, the output signal of the photoelectric conversion element 41 becomes a read signal of the optical disc 50.

When focusing is not performed correctly because the optical disk 50 is far away as shown in FIG. 17(A) or close as shown in FIG. The diameter is the basic optical waveguide 10
Since the second mode is mainly excited in the main optical waveguide lO, a detection signal is obtained from the photoelectric conversion element 43. This shows that focusing is not performed correctly.

FIG. 18 shows the principle of tracking error detection. When the projected light spot is correctly positioned on the pit 51 (track), the reflected light excites the zero-order mode in the main optical waveguide 10, as shown in FIG. 18(B), and the photoelectric conversion element A read signal is obtained from 41. This is the same as the case where focusing is performed correctly as shown in FIG. 17(B). If the projected light spot deviates from the track, it will appear in Figure 18 (^) (C
), the side part of the input end face of the main optical waveguide lO is strongly excited.

As a result, mainly the primary light is coupled to the main optical waveguide 10 and mode-separated into the two-branch optical waveguide 12, so that a large electrical signal can be obtained from the photoelectric conversion element 42. This detects that a tracking error has occurred.

It goes without saying that the optical waveguide fabricated on the substrate 1 may be a Y-branch optical waveguide as shown in FIG. Furthermore, the ends of the single-branch optical waveguides 11 to 13 are extended to the end surface of the substrate 1 as shown in FIG. 1, and the light emitted from this end surface is detected by a photoelectric conversion element placed outside the substrate 1. You can also do it.

[Brief explanation of the drawing]

1 to 8 are for explaining a single mode three-branch optical waveguide. FIG. 1 is a perspective view showing a jli-mode three-branch optical waveguide device. FIG. 2 is a plan view showing its dimensions. FIG. 3 (Δ) to (+) are cross-sectional views showing the Someme manufacturing process. FIG. 4 is a sectional view used to explain the operation. FIG. 5 is a diagram showing a model of a seven-layer slab optical waveguide. FIG. 6 is a graph showing changes in equipolarized refractive index with respect to normalized optical waveguide spacing. FIG. 7 is a graph showing the dispersion curve of equipolarized refractive index, which shows the behavior of the propagation mode. In FIG. 8, (A) to (C) are diagrams showing field distributions of branching modes, respectively. 9 to 13 are for explaining a multimode Y-branch optical waveguide. Figure 9 is a plan view. In FIGS. 10 to 12, the branching modes are 3.
2. This figure is for explaining the operation in the first-order case. In each figure, (A) is a graph showing the change in the equipolarized refractive index with respect to the normalized optical waveguide spacing, and (B) is a graph showing the dispersion curve. Graph showing the behavior of the propagation mode; (C) is a diagram showing the field distribution of the mode guided to the branched optical waveguide. FIG. 13 is a plan view showing another example of a multimode Y-branch optical waveguide. FIG. 14 is a graph showing a dispersion curve of equipolar refractive index, and is used to explain the summary. 15 to 18 show embodiments of the present invention. FIG. 15 is a perspective view showing an example of the configuration of an optical pickup. FIG. 16 is a perspective view showing another example of the configuration of the optical pickup. FIGS. 17(^) to (C) are diagrams showing the principle of focusing error detection. FIGS. 18A to 18C are diagrams showing the principle of tracking error detection. lO: Fundamental optical waveguide. 11, 12.13... Branch optical waveguide. 41, 42.43...Photoelectric conversion element. 45, 48.47... Lens system. that's all

Claims (2)

[Claims] (1) A light projection system that focuses and projects light onto the recording surface of an optical disc, and a light projection system that receives reflected light or transmitted light from the optical disc;
In an optical pickup having a light receiving system that outputs a recorded information reading signal, a focusing error signal, and a tracking error signal, the light receiving device included in the light receiving system includes one end surface on which reflected light or transmitted light from the optical disc enters. a multimode backbone optical waveguide having a width capable of holding multiple optical modes, and connected to the other end of the backbone optical waveguide so as to extend almost on its extension, and having a smaller number of modes than the backbone optical waveguide; , at least two branched optical waveguides having width dimensions such that the branching angle between them is sufficiently small and the orders of the modes to be held are different from each other, and the light propagating through each branched optical waveguide. A light receiving device in an optical pickup, comprising a photoelectric conversion element for detection. (2) The light receiving device in the optical pickup according to claim (1), wherein the branched optical waveguides are single mode optical waveguides having different width dimensions.