JPH02152022A - Focus error detection system - Google Patents

Abstract

PURPOSE:To prevent an influence of the change of the optical axis of an optical system or the like by providing a phase element inclined at a prescribed angle to the optical axis and a polarization state detector on the optical axis of a reflected beam and obtaining a focus error signal in accordance with the difference between two outputs of photodetectors. CONSTITUTION:The reflected beam from an optical disk 5 is transmitted through a quarte-wave plate 55 and its polarization state is changed, and the transmitted light and the separated light of a polarizing beam splitter 56 are uniform and light intensity distributions on photodetectors 57 and 58 are uniform, and the focus error signal is zero. When the optical disk 5 is moved away from an objective lens 1, the light intensity on detecting parts 57b and 58b of photodetectors 57 and 58 is higher and the focus error signal is negative; but when the optical disk 5 approaches the objective lens 1, the light intensity on detecting parts 57a and 58a is higher and the focus error signal is positive. Thus, since the reflected beam reaches photodetectors 57 and 58 without being converged and the spot diameter is increased, an influence of the change of the optical axis or the like upon the focus error signal is reduced to detect the focus error signal of high precision.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus error detection method applied to a focus servo control device of an optical disk device.

2. Description of the Related Art Optical disk devices that write and read information using laser beams are attracting attention as an external storage device for computers.

This optical disk device incorporates a focus servo control device that detects a focus error signal and controls the focus.

In order to perform focus servo control with high precision, it is necessary that the focus error signal can be stably detected.

[Conventional technology]

FIG. 8 shows an astigmatism method, which is an example of a conventional focus error detection method.

1 is an objective lens, 2 is a condenser lens, 3 is a cylindrical lens, 4
is a four-part photodetector.

When the optical disc 5 is located at the focal position Po, the reflected beam 6 from the optical disc 5 forms a circular spot 7 at the center of the photodetector 4, as shown in FIG. 9(B).

When the optical disk 5 moves away from the objective lens 1 due to vibration or the like and becomes Pl, the reflected beam becomes an elliptical spot 8 as shown in FIG. 9(A). On the other hand, if it approaches P2,
This results in an oval spot 9 shown in FIG. 9(C).

If the outputs of the respective detection units 48 to 4d are a-d, the calculation unit 1
0 performs the calculation of (a+c)-(b+d), and from the terminal 11, a focus error signal shown by the line ■ in FIG. 1 is obtained.

The objective lens 1 is driven by this focus error signal and controlled to be in focus.

In a general optical disk device, tilting of the optical disk 5 and assembly errors of the optical head access mechanism may occur. When these occur, the reflected beam 6 will be tilted and the position of the spot 7 on the photodetector 4 will be shifted. It is difficult to eliminate this phenomenon.

[Problem to be solved by the invention]

The optical system shown in FIG. 8 has a condenser lens 2, and the diameter DI of the circular spot 7 is quite small, for example, 10 μm.

Therefore, the degree of influence of the spot position shift on the focus error signal increases, and the focus error signal is largely shifted upward, as shown by 12 in FIG. 10, for example.

In this case, the position P3 is considered to be in focus and controlled, which tends to cause defocus and errors during data loading and reading.

That is, the conventional focus error detection method has a problem in that it is vulnerable to aberrations of the optical system and changes in the optical axis.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focus error detection method that is less susceptible to changes in the optical axis of an optical system.

[Means to solve the problem]

FIG. 1 shows the principle of the focus error detection method of the present invention.

In the figure, parts corresponding to those shown in FIG. 8 are given the same reference numerals.

20 is a phase shifter made of crystal or calcite, and the optical disc 5
It is arranged so as to be inclined at a predetermined angle α with respect to the optical axis 21 within the optical path of the reflected beam 6 from the optical axis 21.

Reference numeral 22 denotes a polarization state detection element, which is provided after the phase shifter 20 in the optical path of the reflected light.

Reference numeral 23 denotes a photodetector divided into two parts, which is provided after the polarization state detection element 22 with the division position aligned with the optical axis 21 . 23a and 23b are photodetectors, respectively.

24 is an arithmetic circuit, and 25 is an output terminal.

The reflected beam 6 has a circular cross section and is linearly polarized.

The phase shifter 20 is arranged such that its optical axis makes, for example, 45 degrees to the polarization plane of the reflected beam 6.

[Effect]

If the refractive index of the retarder 20 in the optical axis direction is Nf, the refractive index in the direction perpendicular to the optical axis is NS, the geometrical optical path length of the portion that passes through the retarder 20 is d, and the wavelength of light is λ, then the phase Child 2
The phase difference δ imparted to light by transmitting 0 is expressed as δ=(Nf−Ns)·d·2π/λ.

When the phase difference δ is π/4, the transmitted light becomes circularly polarized light. In other cases, the light becomes elliptically polarized.

In FIG. 1, the X axis is in the direction of the page, and the Y axis is perpendicular to the page.

The reflected beam 6 when focused is parallel, and the geometrical optical path length passing through the phase shifter 20 is dl and constant at any position in the radial direction of the reflected beam 6 in the X-axis direction. dl is a value giving a phase difference of π/4.

As a result, as shown in FIG. 2(A), among the reflected beams 6, in the radial direction of The light beams 6-3 and 6-3 at the center are both circularly polarized lights, as indicated by numerals 30, 31, and 32.

That is, by transmitting the reflected beam 6 through the phase shifter 20, any portion in the radial direction of X@ is uniformly circularly polarized.

When the optical disk 5 moves away and reaches Pl, the reflected beam passes through the objective lens 1 and becomes a focused beam as shown at 6a.

As shown in FIG. 2(B), the geometrical optical path length for the retarder 20 is the same as d+ for the central light ray 6a-3, which is circularly polarized light 33.

The light ray 6a-1 at the end in the x1 direction becomes d2, which is shorter than d+, and becomes elliptical polarized light 34 with a long sleeve in the Y-axis direction.

Contrary to the above, the light ray 6a-2 at the end in the x2 direction becomes d3, which is longer than dl, and becomes elliptical polarized light 35 with a long sleeve in the X-axis direction.

That is, the reflected beam 6a is transmitted through the phase shifter 20, so that the center portion in the radial direction of the X-axis becomes circularly polarized light 33. The X+ side is elliptically polarized light 34 degrees long in the Y-axis direction, and the X2 side is elliptically polarized light 35 long in the X-axis direction.

When the optical disk 6 approaches and reaches Pl, the reflected beam passes through the objective lens 1 and becomes a divergent beam as shown by 6b.

As shown in FIG. 2C, the geometrical optical path length for the retarder 20 is the same as dl for the central light ray 6b-3, which is circularly polarized light 36.

The light ray 6b-3 at the end in the x1 direction becomes d4, which is better than dl, and becomes elliptically polarized light with a long sleeve in the X-axis direction.

Contrary to the above, the light ray 6b-2 at the end in the x2 direction becomes d5, which is shorter than d+, and becomes elliptically polarized light 38 whose major axis is in the Y-axis direction.

That is, the reflected beam 6b is transmitted through the phase shifter 20, so that the center portion in the radial direction of the X-axis becomes circularly polarized light 33. The X+ side is elliptically polarized light 37 degrees long in the X-axis direction, and the X2 side is elliptically polarized light 38 long in the Y-axis direction.

The polarization state detection element 22 reflects the polarization component in the X direction,
Controls the characteristic of transmitting the Y-direction polarized light component.

As for the reflected beam 6, since the entire portion is circularly polarized light, it is uniformly transmitted through the element 22, and the intensity distribution of the light on the photodetector 22 is left-right symmetrical, as shown by reference numeral 40, and there is no focus error. The signal will be zero volts.

Regarding the reflected beam 6a, due to its polarization state,
The X2 side is transmitted more, and the X2 side is hardly transmitted, and the intensity distribution of the light on the photodetector 23 is strong at the photodetector 23a and weak at the photodetector 23b, as shown by reference numeral 41.
The focus error signal becomes negative.

Regarding the reflected beam 6b, due to its polarization state,
Almost no light is transmitted on the 1 side, and more is transmitted on the X2 side, and the intensity distribution of light on the photodetector 23 is weak at the photodetector 23a and strong at the photodetector 23b, as shown by reference numeral 42, resulting in focus error. The signal becomes positive.

As a result, a focus error signal indicated by the line 3 in FIG. 3 is obtained from the terminal 25. The objective lens 1 is driven by this focus error signal and controlled to be in focus.

As can be seen from FIG. 1, no condensing lens is provided between the phase plate 20 and the photodetector 23, and each reflected beam 6
．． The diameter D2 of the spot on the photodetector 23 of 6a and 6b is, for example, about 4 m, which is about 4000 times that of the conventional case.

Here, even if the reflected beams 6 degrees 6a and 6b are tilted as in the conventional case due to the tilt of the optical disk 5 and the assembly error of the access mechanism of the optical head, and the positions of the slots 1 to 1 on the photodetector 23 are shifted, The ratio of this shift M to the spot diameter is 1/4000 of the conventional case, and the degree of influence on the focus error signal is also 1/4000 of the conventional case.

Therefore, even though the beam is in focus, there is an erroneous focus error caused by the tilt of the reflected beam 6 (V
+ (see FIG. 3) becomes extremely small, and defocus fim can also be suppressed to a very small amount.

As a result, the above-mentioned focus error detection method becomes resistant to aberrations of the optical system and changes in the optical axis, and can continuously output a highly accurate focus error signal.

Furthermore, since the photodetector 23 is installed close to the polarization state detection element 22, the optical system is compact.

[Actual flow example]

FIG. 4 shows a beam emitted from a focus error detection type semiconductor laser 50 according to an embodiment of the present invention, which is collimated by a collimating lens 51, transmitted through a polarizing beam splitter 52, reflected by a mirror 53, and then passed through an objective lens 1. The light is irradiated onto the optical disk 5 as a minute spot.

The beam 6 reflected by the optical disk 5 is transmitted through the objective lens 1. The light passes through the mirror 53 and reaches the polarizing beam splitter 52, where it is reflected and directed downward in the figure.

Reference numeral 55 denotes a quarter-wave plate as a phase plate, and its optical axis is oriented at an angle of 45 degrees with respect to the polarization plane of the reflected beam, and is tilted at an angle α with respect to a plane perpendicular to the reflected beam. It is provided at a position immediately below the polarizing beam splitter 52.

Reference numeral 56 denotes a polarization beam splitter as a polarization state detection element, which is provided immediately below the quarter-wave plate 55. The polarizing beam splitter 56 has a characteristic of transmitting elliptically polarized light whose long axis is Y@ and reflects elliptically polarized light whose long axis is X axis.

Reference numerals 57 and 58 denote two-split photodetectors, which are arranged close to and opposite to the polarizing beam splitter 56 without interposing a lens between them.

The photodetector 57 includes detection sections 57a and 57b, and receives the transmitted light from the polarization beam splitter 56. Another photodetector 5B includes detection sections 58a and 58b, and receives the reflected light from the polarization beam splitter 56.

The differential amplifier 59 performs an operation of subtracting the sum of the outputs of the detection sections 57b and 58b from the sum of the outputs of the photodetection sections 57a and 58a, and this differential signal is outputted from the terminal 60 as a focus error signal.

The reflected beam 6 (6a, 6b) is tilted 1/4
By transmitting the light through the wavelength plate 55, the polarization state can be changed in the same manner as described above.

In the focused state, as shown in FIG. 5, the transmitted light of the polarizing beam splitter 56 and the separated light are uniform, and the intensity distribution of the light on the photodetector 57.58 is as shown by 70.71. becomes uniform, and the focus error signal becomes zero.

When the optical disc 5 moves away from the objective lens 1, it becomes as shown in FIG.
The distribution shown by 72 is where the intensity of light is stronger. Regarding the photodetector 58, the distribution shown by 73 is such that the intensity of light is stronger in the detection part 58b. This causes the focus error signal to become negative.

When the optical disk 5 approaches the objective lens 1, the distribution becomes as shown in FIG. 7, and the light intensity of the light detector 57 becomes stronger at the detection part 57a, as shown by 74. Regarding the photodetector 58, the detection part 58a has a stronger light intensity.
The distribution is shown as 75. As a result, the focus error signal becomes positive.

As a result, the relationship between the focus error signal and the position of the optical disk becomes similar to that shown in FIG. 3 above.

The reflected beam reaches the photodetector 57, 58 without being focused,
The diameters D3 and D4 of the spots on each detector 57 and 58 are as large as several mm. Therefore, the ratio of the misaligned finger of one spot to the spot size due to the optical axis misalignment becomes considerably small for both photodetectors 57 and 58, and the influence on the focus error signal is small.

Therefore, it is possible to detect a focus error signal with a small amount of error, and it is possible to perform focus servo control accurately and stably.

Further, each of the photodetectors 57 and 58 is arranged close to the polarizing beam splitter 56, and the portion for detecting the focus error signal is configured in a convex configuration.

〔Effect of the invention〕

As explained above, according to the present invention, there is no need to reduce the diameter of the spot on the photodetector, and it can be made larger. Therefore, the ratio of the spot deviation amount due to optical system aberrations and changes in the optical axis to the spot diameter can be made much smaller than in the conventional case, and the influence on the focus error signal due to the above-mentioned changes in the optical axis can be reduced. It can be made into a slight scratch.

Therefore, the error portion of the focus error signal can be suppressed to a small value, and a highly accurate focus error signal can be detected.

By using this focus error signal, focus servo control can be performed with high precision, and the optical disk device can reduce errors when writing and reading data.

[Brief explanation of the drawing]

Fig. 1 is a diagram explaining the principle of the focus error detection method of the present invention, Fig. 2 is a diagram showing the polarization state of the reflected beam transmitted through the phase shifter and the intensity distribution on the photodetector, and Fig. 3 is a diagram showing the polarization state of the reflected beam transmitted through the phase shifter and the intensity distribution on the photodetector. Figure 4 is a diagram showing the relationship between position and focus error signal; Figure 4 is a diagram showing an embodiment of the focus error detection method of the present invention; Figure 5 is a diagram showing the polarization state and light intensity distribution during focusing; Figure 6 shows the polarization state and light intensity distribution when moving away. Figure 7 shows the polarization state and light intensity distribution when approaching. Figure 8 shows the conventional focus error signal detection method. Figure 9 The figure shows the shape of the spot on the photodetector when focusing, moving away, and approaching. FIG. 10 is a diagram showing the relationship between the position of the optical disc and the focus error signal. 40-42.70-75 is the distribution of light intensity, 55 is 1/
4 wavelength plates, 56 a polarizing beam splitter, 57 and 58 photodetectors, and 5 small differential amplifiers. Patent applicant Fujitsu Ltd. In the figure, 1 is an objective lens, 5 is an optical disk, 20 is a phase shifter, 22 is a polarization state detection element, 23.57.58 is a photodetector, 24 is an arithmetic circuit, 30.31 , 32, 33.36 are circularly polarized lights, 34.35.
37.38 is elliptically polarized light, Figure 3, Figure 2, Figure 4.

Claims (1)

[Claims] In the optical path of the reflected beam (6) from the recording medium (5),
The retarder (20,
55), the polarization states of the beams transmitted through the retarder are mutually arranged in the radial direction, which is the same direction as the inclination direction of the retarder, when the recording medium approaches the objective lens and when it moves away from the objective lens. A polarization state detection element (22, 56) is provided for detecting the polarization state of the beam that has passed through the retarder, and that the beam from the polarization state detection element is polarized with respect to the optical axis. Split photodetector (
23, 57, 8), and a focus error signal is obtained from the difference between the two outputs of the photodetector.