JPH01256029A - Automatic focus adjusting device - Google Patents

Abstract

PURPOSE:To reduce the cost, to make the performance stable and to facilitate the adjustment by coating the reflection film of split prisms simultaneously. CONSTITUTION:A split prism 18 having a 1st reflecting face and a 2nd reflecting face forming the image of the reflected beam separately from a recording medium 7 and a photodetector 13 placed along the optical path of both the separated reflecting beams are provided to the device. Then the same reflectance is adopted for the coating of 1st and 2nd face reflecting films of the split prism 18 and coated simultaneously. Thus, the dispersion in the reflected film of the prism 18 caused when the reflected beam from the recording medium 7 onto the detector 13 is reduced optically. Thus, it is made easy to form just focusing of beam onto the recording medium 7.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical reproducing device for optically reading information recorded on a disc such as a video disc, or for optically recording and reproducing information on a disc. The present invention relates to an optical recording and reproducing apparatus, and in particular to an automatic focus adjustment apparatus in an optical system that uses reflected light from a disk to obtain servo signals and reproduction signals for applying various servos.

2. Description of the Related Art In general, in video discs and optical recording and reproducing devices, in order to record and reproduce information at high density, the tracks on the disc are formed into fine tracks with a width of, for example, 0.6 μm and a pitch of 1.6 μm. It has a spiral or concentric shape. The disc is irradiated with a minute spot of light having a diameter of 1 μm or less, and information on the disc is read from the reflected light.

In such a device, at least two servo techniques are required. One is that as the disk rotates, the disk causes surface deflection in a direction perpendicular to the rotation direction, and the optical system is designed so that the minute spot light focused to φ1 μm or less can always be irradiated onto the disk in response to the surface deflection. This servo is called a focus servo.

On the other hand, as the disk rotates, the track moves in the radial direction of the disk due to eccentricity, etc., but the servo makes the optical system follow this so that the minute spot light always irradiates on the track. The servo is called a tracking servo.

The servo signal (error signal) for performing the focus and tracking servo is obtained from the reflected light of the disk, and as a specific optical system, the optical system shown in FIGS. 5 and 6, for example, has been proposed. ing.

FIG. 6(, ) shows a front view of the optical system, and FIG. 6(0)) shows a side view of the same. In FIG. 6, reflected light from the disk 7 is imaged by a convex lens 8. In FIG. When the splitting prism 12 is tilted and placed in the image forming optical path, the first reflected light obtained from the upper surface 12a of the splitting prism and the lower surface 12b
The second reflected light obtained is separated and sent to each photodetector 13.
guided by. Here, the split prism is made of, for example, one glass plate, and when the light reflectance of the upper surface 12a is Ra, the light transmittance is Ta, and the light reflectance of the lower surface 12b is Rb, the following equation (1) is expressed. Adjust each reflectance so that the relationship is as shown.

The light transmittance is selected depending on the intensity of the first reflected light and the second reflected light.
The intensity of the reflected light becomes equal.

Ra=Ta xRb ・・・・・・・・・・・・
・・・・・・・・・・・・(1) Said No. 1. As shown in FIG. 6, the image formation position of the second reflected light is shifted from Pl to P2 in the X direction by the optical path difference caused by the splitting prism.

A photodetector 13 having a detection surface 13a to 13f divided into six parts when viewed from the light incident direction is placed approximately at the center of the solid image formation positions P1 and P2, and the detection surface of the divided photodetector 13 is 14.15 is the photodetector 1, which is wider than the surface 130.13b and whose diameters are approximately equal to each other.
3 is irradiated. If the output currents of the detection surfaces 13a to 13f of the photodetector 13 which have been decremented by 6 minutes are Ia to If,
The focus error signal FE is calculated from the second equation as follows: FE=(Ib+Id+I f )−(Ia+Ic+Ie
)−・−・(2) Or FE=Ib−Ie.

The principle by which the error signal is obtained will be described below.

FIG. 6 is a simplified diagram of FIG. 5 in order to explain only the method of obtaining the focus error signal, and the same signals are attached to the same components as in FIG. 5. 6th
In the figures, Fig. 6(a) shows that the diaphragm lens 6 and the disk surface are too close to each other than the desired distance, and Fig. 6(b) shows that the incident light is focused exactly at the desired distance, that is, on the disk surface. (hereinafter referred to as being in the focus position), and FIG. 6C shows the case where the distance is longer than the desired distance.

First, as shown in FIG. 6(a), if the diaphragm lens 6 and the disk 7 are too close to each other than the desired distance, the reflected light focused by the convex lens 8 will be focused at the imaging position P1. P2 is farther away than the photodetector 13. Therefore, in this case, the diameter of the light spot 16 of the second reflected light is smaller than the diameter of the light spot 14 of the first reflected light on the photodetector, the detection surfaces 13a, 13c of the photodetector 13, The detection surfaces 13b, 1sd, and 13f of the photodetector 13 are determined based on the amount of light received by the photodetector 13e.
The amount of light received is greater. On the contrary, as shown in FIG. 6(C), when the aperture lens 6 and the disk 7 move away from the desired distance, the diameter of the light spot 15 becomes larger than the diameter of the light spot 14, and the detection surface 13b
, 13d, and 13f, the detection surface 13a,
The amount of light received by 13c and 13e is greater.

Further, when the light spots 14 and 16 are at the focus position as shown in FIG.
The amount of light received by the detection surfaces 13b, 13d, and 1sf is equal to the amount of light received by the detection surfaces 13a, 13c, and 13e. Therefore, each light detection gg shown in equation (2)
A focus error signal FE is obtained by taking the difference between the output currents, and a four-force null servo can be realized by applying servo so that I a + I c + I e = I b + I d + I f .

In the configuration shown in FIG. 6, due to changes in environmental conditions such as temperature fluctuations and shocks, (1) the photodetector 13 is displaced in the Y and Z directions;

(2) The parallel light incident on the convex lens 8 is shifted by an angle θ as shown by the dashed line.

(3) The light source 1 (FIG. 5) is displaced in the Y and Z directions.

When the optical components such as the above are displaced or the optical axis is moved, both the optical spora) 14 and 15 move in the Y and Z directions, but if the distance 441' between the two optical spots is sufficiently larger than the displacement, (shown in equation 2)

FE=(Ib+Id+If)-(Ia+Ic
-) and Ie), they cancel each other out and have no effect. An example of the above cancellation is the double light spot 14.
．． 15 is shifted in the Z direction. For example, if both light spots are shifted in the +Z force direction, each detection surface 13a, 13d
The amount of light received by the detection surfaces 13b, 1se and 13c, 13f decreases.

Since the shapes of both light spots are exactly the same, FE=((Ib-α)-1-(Id+β)(If-γ)
)-((I e-ct )-4-(I a+β)+(I
cr))=(Ib+Id+If)-(Ie
+I a + I c ), and no FE fluctuation (change in focus position) occurs.

The same principle applies when FE=Ib-Ie.

In addition, in the noise signals N a - N f included in the output signals Ia to If of each photodetector, the receiver outputs light close to the center of the optical axis, Nb and Ne, and the receiver outputs light far from the center of the optical axis. As mentioned above, the frequency characteristics of Na, Nd, Nc, and Nf are different. However, both light spots 14 and 16 are just divided into two in terms of intensity and have exactly the same shape, so Na=N
When d, Nb=Ne, and Nc=Nf, the noises in the focus error signal FE cancel each other out, and the FE multiplied signal S becomes very good.

Problems to be Solved by the Invention However, the above configuration has the following problems. That is, as shown in FIGS. 6 and 6, the focus error signal is FE=(I b+I d+I f )−(I a+I
c+I e ) or FE = (I b - I e ). And the light reflectance I of the upper surface 12a and the lower surface 12b
Due to variations in Ra and Rh, it was necessary to adjust the two beam spots on the photodetector so that they had the same diameter and the same amount of light. Reflection film Ra of the current split prism 12,
:f(b) Since both sides are coated separately,
It had the following problems.

■ Adjust the first beam so that the two beams have the same amount of light on the detector.
If the reflectance of the surface is Ra, then a film with a reflectance Rb such that the reflectance Rb of the second surface is Ra=(1-Ra)eRb must be applied, which requires a difficult vapor deposition process.

■ Because the coatings are separate, there is large variation from lot to lot. Amount of reflected light on surface A P1. Amount of reflected light on surface B P2
Then, P1==Ra*Po, P2=(1-Pa)*Rb4P.

Here, the ratio of Pl and P2 is = P 1 /P2 = Ra / (1-Ra) eRb
becomes. Here, when Ra=20%, Rb=2s%, and the variation in each reflectance is assumed to be a factor of ±4.

K= (0,2th0.04)/(1-(0,2:to
,04)*(0,26fo,04)=0.70~1.3
6, and a focus error signal can be generated within this range of variation.

■ When the wavelength of the semiconductor laser shifts due to temperature, the dependence of the film on the wavelength differs, so when a vortex layer change occurs, the reflectance of the film changes. This change appears as an error in the focus error signal.

Due to these factors, the above configuration cannot detect 2g of light.
The adjustment to bring the two upper beams to the ideal position was complicated and required a lot of effort.

In view of the above problems, the present invention provides an automatic focus adjustment device that can simplify adjustment.

Means for Solving the Problem In order to achieve this object, the automatic focus adjustment device of the present invention narrows down the light emitted from the optical system onto the recording medium to a fine extent, and performs recording/reproduction or only reproduction. At the same time, the reflected light from the recording medium is divided into approximately two parts (the light intensity placed in the optical path up to the point of focus is divided into two), and images are formed separately at one position that is not on the same optical path and on the same plane. a split prism having a first reflective surface and a second reflective surface; and a photodetector placed approximately in the middle of the solid image formation position along the optical path of the separated both reflected lights, In an automatic focusing device in which the prism is divided into at least two parts so as to separately receive both of the reflected lights, and the width thereof is smaller than the diameter of the receiving light beam, the first surface and the first surface of the split prism are The feature is that the coating of the reflective film on the two surfaces has the same reflectance and is coated simultaneously.If the reflectance of the first surface and the second surface are respectively Ra, then the generated force nuller is Signal F E uFE=P1
-P2 = G1*Ra* to -G2*(1-Ra)*RaRaRa
Reflectance G1 of the twelfth reflective surface on Ra: Amplification degree of the first beam amplifier G2: Amplification degree of the second beam amplifier. Here, if G1 and G2 are set so that FE=o, G2=G1/(1-Ra) ・・・・・・・・・・・・
・・・・・・・・・・・・(3) Therefore, the first
This configuration has a configuration in which the amplification G2 of the beam and second beam amplifiers is set approximately in the relationship expressed by the above equation (3).

Effect: With the above-described configuration, the present invention can optically reduce variations in the reflective film of the splitting prism that occur when the reflected light from the recording medium is imaged on the detector, and direct the beam toward the disk. This makes it easier to apply just the right amount of force, and it also becomes more resistant to temperature fluctuations.

EXAMPLE An example of the present invention will be described below with reference to the drawings.

Fig. 1 is a front view and a side view of a main part of an optical system of an optical recording/reproducing device according to an embodiment of the present invention, Fig. 2 is a schematic side view of the main part, and Fig. 3 is a detection of a photodetector. Figure 3 shows a top view of the surface. In each figure, the same parts as in FIGS. 4 and 6 are given the same numbers, and their explanations will be omitted. FIG. 6 shows a circuit diagram of the main part of the first stage of the four force null servo signal detected by the same optical system.

In FIG. 1, the reflecting surfaces 18a, 1 of the split prism 18
8b is coated with the same coating. The optical system of this example using this splitting prism will be explained.

FIG. 2(a) is a schematic front view of the optical system of this embodiment, and FIG. 2(b) is a side view of the same. Furthermore, Fig. 2(a)
, (b), some parts that overlap with each other are omitted.

In FIG. 2, light emitted from a semiconductor laser 1 is incident on a polarizing beam splitter 17 via a collimating lens 2) and a beam shaping prism 16.

The laser beam incident on the polarizing beam splitter 17 is reflected and irradiated onto the disk 7 via the λ/4 plate and the objective lens 6a. The reflected light from the disk 7 passes through the same path and is totally reflected by the total reflection prism 4, and then enters the splitting prism 18 via the single lens 8 whose reflective surfaces 18a and 1sb are simultaneously coated to have the same reflectance. A part of the light incident on the splitting prism is reflected by the reflective surface 18a and the remaining light is transmitted, and the transmitted light passes through the next reflective surface 18.
b, and both form a beam spot on the photodetector 13. Note that the splitting prism 18c protrudes in the direction in which the light travels in order to adjust the optical path length.

The operation of this embodiment configured as above will be explained below.

The splitting prism in which the first reflecting surface 18a and the second reflecting surface 18b are coated simultaneously is improved in the following points.

Since the two beams are coated with the same reflectance Ra, variations in coating are relatively canceled out. If the amount of light reflected from the first surface of the tab is Pl, and the amount of light reflected from the second surface is P2, then P1=Ra*P.

P2=(1-Ra)*Ra*P.

Therefore, the ratio of Pl and P2 is ni-P1/P2 =17 (1-Ra) becomes.

Here, when Ra=20%, the variation in each reflectance is ±4%.

K = 1/(1-(0,2-[:0.04))=1.1
9 to 1.32, and the variation can be reduced to about %. Furthermore, even if the wavelength of the semiconductor laser shifts due to temperature fluctuations, the change in reflectance with respect to wavelength fluctuations changes at the same ratio. Therefore, it is possible to create an optical system that is not affected by wavelength fluctuations. Furthermore, since the light amounts of Pl and P2 are different, by providing a circuit gain such that G2=G1/(17Ra), it is possible to easily balance the light amounts of Pl and P2.

As described above, according to this embodiment, ■ Simultaneous coating reduces costs.

■ Less wavelength dependence.

■ Adjustment is easy because the variation in the reflective film is small.

It is possible to create an optical recording/reproducing device that is as follows.

Effects of the Invention As described above, the present invention can reduce costs, stabilize performance, and facilitate adjustment by simultaneously coating the splitting prism with a reflective film, and has great practical effects. .

[Brief explanation of the drawing]

FIG. 1(a) is a front view of essential parts of an automatic focus adjustment device in an optical recording/reproducing apparatus according to an embodiment of the present invention, FIG. 1(a) is a side view of the same, FIG. Figure 3 (a)
, (b) is a front view of the same detection surface, FIG. 4 (-) is a front view of the conventional automatic focus adjustment device, FIG. 4 (b) is a side view of the same, and FIGS. 6 (a) and (b). , (c) is a schematic side view. AI, A2... Amplifier circuit, A3... Differential circuit, 1... Semiconductor laser, 2...
Collimating lens, 4... Total reflection prism, 6
......λ/4 plate, 6...Objective lens, 7
...Disc, 8...Semi-convex lens, 1
2.18...Divided prism, 13...
Photodetector, 16. Beam shaping prism, 17.
...Polarizing beam splitter. Name of agent: Patent attorney Toshio Nakao and 1 other person1-
9-Children's rice 2 pieces-'! Fu' 2---Kowamate Shinde 4-11 and Kecho Buseop 4 5---84 Figure 1 -
One book #Rusosu 7--Gaisk 8--One rod convex sosu" 13--First stop' exit world IC--Beam vine rib delivery 17--Ryuzono beam again Fu.S, 7th Figure 3 Figure 4

Claims (2)

[Claims] (1) A light source, an objective lens that focuses a light beam emitted from the light source onto an information recording medium, a first beam that reflects a reflected beam from the information recording medium on a first reflecting surface, and a second beam that a splitting prism that divides into three beams, a second beam reflected by a reflective surface and a third beam transmitted by the second reflective surface, and the same reflective film is applied to the first reflective surface and the second reflective surface; a photodetector disposed along the optical path of the first beam and the second beam between the imaging positions of both beams; an amplifier for amplifying the output of the photodetector; and an amplifier for amplifying the output of the photodetector; An automatic focus adjustment device comprising: a drive device that moves the objective lens using a focus error signal obtained by differential outputs of amplifiers. (2) The automatic focus adjustment device according to claim 1, wherein the amplification degree G2 of the amplifier for the second beam is set to substantially the following relationship. G2=G1/1-Ra Reflectance of first and second reflecting surfaces: Ra Amplification degree of first beam amplifier: G1