JPH05109107A - Optical head - Google Patents

Abstract

PURPOSE:To perform the formation of a stable ultra resolution spot with the small off-set of a focus signal and a track signal and to make constant the power of a semiconductor laser diode even when the optical axis dislocation exists. CONSTITUTION:Out of the light fluxes emitted from a semiconductor laser diode 4 to a moving optical system 2, the light flux near the optical axis of an objective lens 9 is reflected by a reflection mirror 20. The reflection light quantity is light-received by two-divided light-receiving elements 21A and 21B, an optical axis dislocation signal S3 or a sum signal S4 is obtained, and the optical axis dislocation off-set of the focus signal and the track signal is corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for recording / reproducing / erasing information on an optical disk, and more particularly to detection of optical axis deviation in a separate type optical head and offset correction of focus signal and track signal due to optical axis deviation. And to make the output of the semiconductor laser in the optical head constant.

[0002]

2. Description of the Related Art Optical discs are used as large-capacity recording devices for documents and image files, and as a means for further increasing the capacity, high-density recording of optical discs by the super-resolution method is proposed (for example, Japanese Unexamined Patent Application Publication No. Hei 2). -206036 gazette).

FIG. 6 is a diagram for explaining the principle of the above-mentioned super-resolution system, in which a laser beam emitted from a semiconductor laser (not shown) is converted into a parallel light beam by a coupling lens.
In the cross section of the collimated beam CB shown in FIG. 6, by providing the light shielding plate 7 that reduces the light intensity near the center,
The super-resolution spot S1 as shown in (2) is generated to reduce the conventional normal spot S2.

By the way, recently, in order to realize high-speed access to an optical disk, an actuator-separated optical head is realized in which an actuator for focusing and tracking is separated and moved for access to fix an optical system. Has been done.

FIG. 7 is a block diagram of a conventional separation type super-resolution optical head in which the super-resolution system shown in FIG. 6 and an actuator separation type are combined.

The operation of the fixed optical system 3 will be described below.
The laser light emitted from the semiconductor laser (LD) 4 inside is
A parallel light flux is formed by the coupling lens 5. Also,
The intensity distribution of the light beam emitted from the semiconductor laser 4 at this time is elliptical, but is shaped by the beam shaping prism 6 into a beam having a circular intensity distribution.

Further, the light near the center of the collimated beam CB is blocked by the light blocking plate 7 for obtaining the super-resolution spot S1 described in FIG. Then, the light reflected by the beam splitter 16 is reflected by the deflecting prism 8 of the moving optical system 2 and is narrowed down by the objective lens 9, and then a minute super-resolution spot S1 is formed on the optical disc 1 and information of Record
Playback / erasure is performed.

The reflected light from the above-mentioned optical disk 1 passes through the objective lens 9 and the deflection prism 8 again, and passes through the fixed optical system 3
Return to inside. That is, the light that has passed through the beam splitter 16 is
Polarization beam splitter through λ / 2 plate 10 and detection lens 11.
Incident on 12. One of the lights split by the polarization beam splitter 12 passes through a cylindrical lens 13 and enters a focus (focus) detection light receiving element 14, and the other light is
The light is incident on the track detection light receiving element 15 (composed of two-divided light receiving elements 15a and 15b). Reference numeral 17 is a condenser lens, and 18 is a front light receiving element for monitoring the forward emission output of the semiconductor laser 4.

Here, the track signal is obtained by the well-known far-field method, the focus signal is obtained by the astigmatism method, and the information signal of the optical disc 1 is obtained by the differential signal of both the light receiving elements 14 and 15.

FIG. 8 is a diagram for explaining the principle of track signal detection. When detection of the tracking position is correct (1)
And (2) shows an incorrect case, (a) shows the track detection light receiving element 15 side, and (b) shows the optical disk 1 side state.
That is, as shown in FIG. 8A, when the laser spot S focused by the objective lens 9 is applied to the center position of the track T _i of the optical disc 1, the laser spot S is bilaterally symmetrical with respect to the track T _i . Is diffracted as shown in FIG. As a result, the track detection light receiving element in the fixed optical system 3
On each of the two 15-divided photodetectors 15a and 15b, a left-right symmetrical diffraction pattern PT is formed as shown in the figure. Therefore, two two
The outputs A and B of the divided light receiving elements 15a and 15b become equal to each other, and the track signal T _S which is a correct tracking position is detected.

However, if the irradiation position of the laser spot S deviates from the center position of the track T _{i to} the left or right as shown in FIG. 8 (2), the diffraction pattern PT becomes asymmetrical and the two An output difference occurs between the outputs A and B of the two-divided photodetectors 15a and 15b, and a track signal at an incorrect tracking position is detected. Based on this, control is performed so that the difference between the outputs A and B is eliminated.

9 and 10 are diagrams for explaining the principle of detection of a focus signal. FIG. 9 shows the optical path states of the optical disc 1 and the objective lens 9, and FIG. 10 shows the focus detection light reception. The states of the laser spots S of the element 14 (hereinafter, referred to as 4-division light receiving elements 14a to 14d) are shown.

FIG. 9 (1) shows an optical path state at the time of focusing. At that time, the shape of the laser spot S on the four-divided light receiving elements 14a to 14d is divided into four equal parts as shown in FIG. 10 (1). It becomes a round shape. The focus signal F _{S in} this case is F _S = (a
+ C)-(b + d), and the value of the focus signal F _S at the time of focusing is 0.

Next, FIG. 9 (2) shows the case where the distance between the optical disk 1 and the objective lens 9 becomes short, and the shape of the laser spot S on the four-division light receiving elements 14a to 14d at that time is as shown in FIG.
As shown in 10 (2), it becomes an elliptical shape that bulges in the 14b and 14d directions. In this case, the focus signal F _S becomes negative.

Next, FIG. 9 (3) shows the case where the distance between the optical disk 1 and the objective lens 9 is increased, contrary to FIG. 9 (2), and the laser on the four-division light receiving elements 14a-14d at that time. The shape of the spot S is an elliptical shape that bulges in the directions of 14a and 14c as shown in FIG. 10 (3). In this case, the focus signal F _S becomes positive.

Focusing is performed in accordance with the shape of the laser spot S on the four-divided photodetectors 14a to 14d, that is, the focus signal F _S , and control is performed so that F _S = 0.

[0017]

The above-mentioned separation type super-resolution optical head shown in FIG. 7 has the following problems.

(1) Generation of Track Signal Offset FIG. 11 is a diagram for explaining the cause of the optical axis deviation. As shown in FIG. 11, the rail 19 which is the moving axis of the moving optical system 2 and the fixed optical system 3 are shown. When the moving optical system 2 changes vertically due to the fact that the light beam traveling from the moving optical system 2 to the moving optical system 2 is not parallel and tilted, or dust adheres to the rail 19 or rattles at the time of seeking, an optical axis shift occurs. ..

That is, when the moving optical system 2 is on the inner circumference side (left side in the drawing) of the optical disk 1, the reflected light from the optical disk 1 becomes a chain line C, and when it is on the outer circumference side (right side in the drawing), it is reflected. The light becomes a dotted line D, and the reflected light flux shifts up and down.

This means that the light beam position on the track detection light receiving element 15 shown in FIG. 8 described above shifts as shown in FIG. 8 (2), and the track signal T _S due to the optical axis shift.
Offset occurs. FIG. 12 shows each state of the track signal, and FIG. 12 (1) shows the track signal T _S when there is no optical axis deviation.
12 (2) shows a track signal T _S ″ having an offset in the case where the optical axis is deviated.

(2) Occurrence of Focus Signal Offset When the optical axis shift occurs in the same manner as the cause of (1) occurrence of track signal offset described above, an offset also occurs in the focus signal.

FIG. 13 is a diagram for explaining the focus signal offset generation. (1) shows the optical path condition between the focus detection light receiving element 14 and the detection lens 11 shown in FIG.
Further, (2) shows the states of the laser spots S of the four-division light receiving elements 14a to 14d, respectively.

It is assumed that the reflected light c from the optical disk 1 has an optical axis shift of d. Assuming that the combined focal length of the detection lens 11 and the cylindrical lens 13 is f, and the distance from the focus P to the four-division light receiving elements 14a to 14d is Δ, the optical axis shift d on the four-division light receiving elements 14a to 14d. The movement amount D of the laser spot S due to is expressed by the following equation from FIG.

[0024]

## EQU1 ## D = (Δ / f) d In this case, the four-division light receiving element is used even when focusing is performed.
Since the laser spots S on 14a to 14d shift corresponding to the optical axis shift d, the value of the focus signal (focus point P) becomes positive and an offset occurs. This pattern is shown on the right side of FIG. 13 (2), and the laser spot S is deviated and detected on the side of the four-division light receiving element 14c.

(3) Change in laser spot shape As described with reference to FIGS. 6 and 7, the light intensity near the center in the cross section of the collimated beam CB in which the light shielding plate 7 for super resolution is installed in the fixed optical system 3 Because it is decreasing
When the moving optical system 2 is displaced relative to the fixed optical system 3 due to the occurrence of the solid track signal offset in (1), the collimated beam luminous flux is displaced with respect to the objective lens 9. The shielded portion of the collimated beam passing through the objective lens is displaced due to the optical axis shift. Therefore, the size of the super-resolution spot narrowed down by the objective lens 9 changes due to the optical axis shift. In addition, FIG.
Since the light intensity of the side rope shown in (3) also changes, the amount of crosstalk from this side rope changes.

As a result, there is a first problem that information cannot be recorded, reproduced, or erased with high accuracy. Also,
Conventionally, the output of the semiconductor laser was controlled by monitoring the backward output, but the output of the semiconductor laser was controlled, but there was a second problem in that the output could not be controlled to a constant level because the semiconductor laser had a return.

In view of the above points, the present invention provides a stable super-resolution spot in a super-resolution separation type optical head with little focus signal offset and track signal offset even if there is an optical axis shift. The first purpose is to form. A second object of the present invention is to directly monitor the amount of forward laser light from the semiconductor laser and to keep the output of the semiconductor laser constant, in addition to forming the super-resolution spot in the optical head.

[0028]

In order to achieve the first object of the present invention, a light flux emitted from a laser light source arranged in a fixed optical system is collimated by a coupling lens,
Emit a light beam to the moving optical system via the beam splitter,
In a separate type optical head for condensing a light beam by an objective lens arranged in the moving optical system to record / reproduce / erase information on an optical disk, the laser light source of the fixed optical system emits the light to the moving optical system. For detecting an optical axis shift signal including a reflecting mirror installed to reflect a light beam in the vicinity of the optical axis of the objective lens among the light beams, and a light receiving element divided into at least two for receiving the amount of reflected light from the reflecting mirror. And a corrector for correcting the optical axis deviation offset of the focus signal and the track signal based on the optical axis deviation signal from the detector.

In order to achieve the second object, a detector for directly detecting the light quantity of the semiconductor laser from the forward light quantity reflected by the reflecting mirror, and the output of the semiconductor laser based on the light quantity information from the detector And a control circuit for keeping the above constant.

[0030]

According to the first means of the present invention, a light-shielding reflecting mirror for creating super-resolution is installed near the optical axis of the objective lens of the moving optical system in the separation type optical head. Even if it occurs, the light flux near the optical axis of the objective lens can always be blocked. As a result, the optical axis deviation offset of the focus signal and the track signal is corrected based on the optical axis deviation signal from the reflecting mirror, and the size and shape of the super-resolution spot become almost constant regardless of the optical axis deviation. High quality information recording / playback / erasure becomes possible. Further, according to the second means of the present invention, the light quantity of the semiconductor laser in the optical head can be directly monitored by the reflecting mirror, and the output of the semiconductor laser can be kept stable.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. Figure 1
FIG. 3 is a configuration diagram of a separation type optical head in one embodiment of the present invention. As shown in FIG. 1, a reflecting mirror 20 is arranged between the deflecting prism 8 and the objective lens 9 in the moving optical system 2, and the amount of light reflected from this reflecting mirror is received to receive the optical axis deviation signal S3 or the optical axis deviation sum. It is characterized in that at least two light receiving elements 21A and 21B for detecting the signal S4 are provided. That is,
The reflecting mirror 20 is arranged in the moving optical system 2 instead of the light shielding plate 7 in the fixed optical system 3 in FIG. 6, and contributes to more stable super-resolution creation.

As shown in FIG. 1, the reflecting mirror 20 is located in the vicinity of the optical axis of the objective lens 9 and at a position where the speed of light in the vicinity of the optical axis of the objective lens can always be blocked even if the optical axis shifts (radial direction described later). (Along R). And the reflector 20
Shows the tangential direction (Tangenti
al: T direction) is smaller than the effective diameter of the objective lens 9, and the radial direction (Radial: R direction) is larger than the effective diameter of the objective lens 9. In the reflecting mirror 20 in place of the light shielding plate 7 shown in FIG. 6, the hatched portion shields the laser light.

In each portion other than the above shown in FIG.
The same parts as those are given the same numbers and their explanations are omitted.

Next, the operation of FIG. 1 will be described. Similar to FIG. 7, the laser light from the semiconductor laser (LD) 4 in the fixed optical system 3 is made into a parallel light flux by the coupling lens 5 and is made by the beam shaping prism 6. Beam shaping. Beam splitter 16
The light flux that has passed through is received by the condenser lens 17, the front emission light output of the semiconductor laser (LD) 4 is monitored by the front light receiving element 18, and the LD front emission output is made constant by the LD drive circuit (not shown). Control.

On the other hand, the light beam reflected by the beam splitter 16 goes to the moving optical system 2 and is reflected by the deflection prism 8. A two-divided light receiving element 21A, 21B for reflecting the light beam at the center of the light beam by a reflecting mirror 20 installed between the deflecting prism 8 and the objective lens 9 and detecting the optical axis deviation signal S3 or the optical axis deviation sum signal S4. Is received. Further, the light flux not reflected by the reflecting mirror 20 is condensed by the objective lens 9, and the optical disc 1
The super-resolution spot S1 described in detail in FIG. 6 is formed on the top.

This super-resolution spot S1 is a spot having a small diameter in the tangential direction (T) of the optical disc 1, and recording / reproducing / erasing of smaller information bits is possible, thus increasing the capacity of the optical disc. Can be measured. The operation of the reflected light from the optical disk 1 is similar to that of FIG. 7 described above, and the focus detection light receiving element 14
The focus signal F _S and the track signal T _S are respectively detected by the track detection light receiving element 15.

Although it has been described that the light reflected by the reflecting mirror 20 is incident on the two-divided light receiving elements 21A and 21B, the two-divided light receiving elements 21A and 21B are in a state where there is no optical axis shift or a moving optical system. The position of the two-divided light-receiving elements 21A and 21B is adjusted so that the difference signal of the two-divided light-receiving elements becomes 0 when there is no track offset at the time of assembly in 2.

Therefore, when the optical axis shift occurs as described with reference to FIG. 11, the subtraction circuit 22 generates a negative optical axis shift signal S3 in accordance with the direction amount of the optical axis shift of the difference signal. .. Since the magnitude of this optical axis shift signal varies depending on the front emission light output of the semiconductor laser (LD) 4, it is preferable to generate the sum signal S4 of the two-divided light receiving elements 21A and 21B in the summing circuit 23. Let Then, by controlling the gain of the amplifier (not shown) for the difference signal so that the sum signal S4 is always constant, it is possible to obtain the optical axis deviation signal S3 that does not depend on the optical output.

FIG. 2 shows a functional circuit diagram of the present invention for removing the offset due to the optical axis shift of the track signal and the focus signal using the optical axis shift signal. In FIG. 2, 24, 25
R _f times the optical axis deviation signal S3 to a predetermined level, R _t
The amplifiers 26 and 27 are subtraction circuits for the optical axis deviation signal S3 multiplied by R _f and R _t , the focus signal F _S , and the track signal T _S , and the focus signals are amplified by these amplification and subtraction circuits. And a compensator for compensating the optical axis shift offset of the track signal.

That is, by amplifying the optical axis shift signal S3 to R _f times and R _t times by the amplifiers 24 and 25, and subtracting the focus signal F _S and the track signal T _S from the subtraction circuits 26 and 27, respectively. A focus signal F _S ′ and a track signal T _S ′ whose optical axis deviation has been corrected are obtained, and information recording / reproduction /
Erase is performed.

Since the optical axis shift at the time of seek is determined for each drive, it is not always necessary to detect the correction signal. That is, the seek operation is performed by previously seeking once and storing the optical axis deviation correction signal in a memory (not shown), and then correcting the focus signal and the track signal by the correction signal read from this memory. It is easy to perform the operation after carrying out the above-mentioned procedure and checking whether the correction amount is appropriate or not.

By the way, in the above-mentioned optical disc,
In order to reduce the spot formed on the optical disc in order to increase the information recording density, the super-resolution method shown in FIG. 6 was used.

Recording / reproduction of information by this spot
Although three operations of erasing are performed, the output of the semiconductor laser (LD) 4 is different in each of them. In order for these three operations to be performed stably, it is necessary to keep the output of the semiconductor laser constant during that time.

For this reason, generally, the backward emission light of the semiconductor laser is monitored and controlled, but the semiconductor laser has a return and cannot be controlled to be constant.

The embodiment of the present invention has been made in view of such a point, and FIG. 3 shows a configuration diagram when it is applied to an optical head which is not a separation type.

In FIG. 3, 20 is a coupling lens 5.
Is used for the super-resolution generation and the front light amount detection of the semiconductor laser (LD) 4 described in FIG. Reference numeral 28 is a detector for detecting the front light amount of the LD light amount, and 29 is a control circuit for controlling the light emission light amount of the LD 4 based on the detection output of the detector 28.

Reference numeral 30 is a detection lens, 31 is an information detection system, and other parts are the beam splitter explained in FIG.
16, the objective lens 9 is provided, and the optical disc 1 is irradiated with a super-resolution spot.

The operation will now be described. A semiconductor laser (L
D) The laser light from 4 is converted into a parallel light flux by the coupling lens 5, and the super-resolution spot (see FIG.
The laser light on which the detailed description will be made with reference to FIG. 5) is used to record / reproduce / erase information on the optical disc 1 through the beam splitter 16 and the objective lens 9.

In this embodiment, the detector 28 detects the laser light amount of the forward laser light reflected by the reflecting mirror 20, the detection output is input to the control circuit 29, and the output of the semiconductor laser 4 is kept constant. Controlled to be.

FIG. 4 is a graph showing the intensity distribution (I is intensity) shielded by the reflecting mirror 20 of FIG. 3, and FIG. 5 is a graph showing the super-resolution intensity distribution based on FIG. The broken line is the intensity distribution when light is not shielded.

As a result, the spot formed on the optical disk 1 has a size of D '/ D, and the recording density is expected to improve. However, the larger the light-shielding width, the smaller the spot that can be obtained, but the output on the optical disc 1 also decreases, and the side rope also increases.・ Not suitable for erasing. Therefore, the width for blocking light is set to about 20% of the luminous flux. With this, the spot diameter is 4/5.
The recording density increases by 20%.

[0052]

As described above, in the optical head of the present invention, since the separation type optical head is provided with the light-shielding reflecting mirror for creating super-resolution near the objective lens of the moving optical system,
Even if the optical axis is deviated, the light flux near the optical axis of the objective lens can always be blocked. As a result, the size and shape of the super-resolution spot become substantially constant regardless of the optical axis shift. Further, since the change is reduced at least as compared with the conventional arrangement of the light shielding plate in the fixed optical system, high quality information recording / reproducing / erasing becomes possible.

Further, since the optical axis shift can be detected by the reflected light from the light-shielding reflecting mirror and the focus signal and the track signal can be corrected by this optical axis shift detection signal, the spot can be focused and tracked at the correct position at all times. It enables high-quality information recording / playback / erasure.

Further, since the light-shielding mirror is used instead of the conventional light-shielding plate, a separate optical component such as a beam split for detecting the optical axis deviation is not required, and the cost is reduced.

Further, since the light-shielding reflecting mirror is made larger than the effective diameter of the objective lens in the radial direction, even if the optical axis is deviated, the entire light flux in that direction can be reflected by the light receiving element.

Since the optical axis deviation detection signal is stored in the memory, it is possible to check the optical axis deviation amount.
It is possible to preliminarily seek and store the correction signal, and perform a seek test with the stored correction signal to check whether the correction amount is appropriate. Then, the correction signal can be further corrected to obtain an optimum correction signal, and higher quality servo and information recording / reproducing / erasing can be performed.

Further, the optical head of the present invention uses the above-mentioned light-shielding reflecting mirror to record / reproduce / record information by a super-resolution spot.
In addition to high-density recording by erasing, the front laser light amount of the semiconductor laser can be directly monitored with a light-shielding reflecting mirror, so the output control of the semiconductor laser can be kept more stable than in the conventional rear laser light amount monitor. You can

[0058]

[Brief description of drawings]

FIG. 1 is a configuration diagram of a separation type optical head according to an embodiment of the present invention.

FIG. 2 is a functional circuit diagram for removing an offset due to an optical axis shift of a track signal and a focus signal using the optical axis shift signal.

FIG. 3 is a configuration diagram of an optical head according to another embodiment of the present invention.

FIG. 4 is a graph showing an intensity distribution shielded by the reflecting mirror of FIG.

5 is a graph showing a super-resolution intensity distribution based on FIG.

FIG. 6 is a diagram for explaining the principle of a super-resolution method using a reflecting mirror of the present invention and a conventional reflecting plate.

FIG. 7 is a configuration diagram of a conventional separation type super-resolution optical head.

FIG. 8 is a diagram illustrating a principle of detecting the track signal of FIG. 7.

9 and 10 are diagrams illustrating the principle of detecting the focus signal of FIG. 7.

FIG. 11 is a diagram illustrating a cause of the optical axis shift of FIG. 7.

FIG. 12 is a diagram showing each state of a track signal.

FIG. 13 is a diagram illustrating focus signal offset generation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical disk, 2 ... Moving optical system, 3 ... Fixed optical system, 4 ... Semiconductor laser (LD), 5 ... Coupling lens, 6 ... Beam shaping prism, 7 ... Shading plate, 8
… Deflection prism, 9… Objective lens, 10… λ / 2 plate,
11, 30… Detection lens, 12… Deflection beam splitter,
13 ... Cylindrical lens, 14 ... Focus detection light receiving element, 14a, 14b, 14c, 14d ... 4-division light receiving element,
15 ... Track detection light receiving element, 15a, 15b ... Two-divided light receiving element, 16 ... Beam splitter, 17 ... Condensing lens, 18
… Front light receiving element, 19… Rail, 20… Reflector, 21
A, 21B ... Two-division light receiving element, 22, 26, 27 ... Subtraction circuit,
23 ... Summing circuit, 24, 25 ... Amplifier, 28 ... Detector,
29 ... Control circuit, 31 ... Information detection system, S ... Laser spot, S1 ... Super-resolution spot, S2 ... Normal spot,
S3 ... Optical axis deviation signal, S4 ... Optical axis deviation sum signal, P
T ... Diffraction pattern, F _S ... Focus signal, T ... Tangential direction, R ... Radial direction, T _S ... Track signal, T _i ... Track, F _S ′ ... Corrected focus signal, T _S ′ ... Corrected Track signal, c ...
Reflected light, f ... Composite focal length, P ... Focus, d ... Optical axis deviation distance, Δ ... Distance from P to 14a to 14d, D ... Laser spot movement amount.

Claims (4)

[Claims] 1. A light flux emitted from a laser light source disposed in a fixed optical system is collimated by a coupling lens,
Emit a light beam to the moving optical system via the beam splitter,
In a separate type optical head for recording / reproducing / erasing information on / from an optical disc by condensing a light beam with an objective lens arranged in the moving optical system, a laser light source of the fixed optical system emits the light to the moving optical system. For detecting an optical axis shift signal including a reflecting mirror installed to reflect a light beam in the vicinity of the optical axis of the objective lens among the light beams, and a light receiving element divided into at least two for receiving the amount of reflected light from the reflecting mirror. And a corrector for correcting the optical axis deviation offset of the focus signal and the track signal based on the optical axis deviation signal from the detector. 2. The separation type optical head according to claim 1, wherein the tangential direction of the reflecting mirror is set smaller than the effective diameter of the objective lens, and the radial direction is set larger than the effective diameter of the objective lens. 3. The separated optical head according to claim 1, wherein the optical axis deviation of the track signal or the focus signal is corrected by the optical axis deviation correction signal stored in advance in the memory. 4. A detector having a detector for directly detecting the light amount of the semiconductor laser from the forward light amount reflected by the reflecting mirror, and a control circuit for keeping the output of the semiconductor laser constant based on the light amount information from the detector. An optical head characterized by that.