JPH05258322A - Method and device for compensating tracking error offset - Google Patents

Abstract

PURPOSE:To prevent the axis deviation of a tracking beam in an optical system track by generating a prescribed laser beam wavelength and the laser beam wavelength of a prescribed second hamonics and obtaining a tracking error signal from the diffracted light beams of both wavelengths. CONSTITUTION:The beams of two wavelengths generated by a second harmonics generation crystal element 710 are reflected by the surface of a recording medium 707 and polarized by a polarizing beam splitter 704. Then two wavelength components lambda1, lambda2 are separated by a separation means 717 and detected by photodetectors 714, 716. The tracking error signals with respect to respective two wavelength components are generated by differential amplifiers 713, 715. Although these tracking signals TE1, TE2 have offsets, the offsets are canceled by adjusting the gains of subtracters 712, 713, 715 in a state where an tracking error is not present and setting the output TE3 of the subtracter 712 to zero. Thus, the track axis deviation of an information recording and reproducing device is corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking error offset compensation method and apparatus.

[0002]

In the field of optical data storage technology, information is stored in the form of pits or marks on the surface of an optical memory. Laser light is then used to read that information. The stored information is read by scanning the surface of the memory (recording medium) with a laser beam and modulating the diffracted light intensity. Grooves are provided on the surface of the memory to control the tracking of the laser beam. Current technology uses monochromatic beams for reading and writing.

The structure of a typical optical head at present is shown in FIG. The Gaussian light beam generated by the laser diode 1 passes through the collimator lens 2, then the polarization beam splitter 4, the quarter wave plate 5 and the objective lens 6, and is focused on the surface of the memory 7. Memory 7
The laser beam reflected by the surface of 1 passes through the objective lens 6 and the quarter-wave plate 5 and is directed to the photodetector 3 by the polarization beam splitter 4. The intensity of light incident on the surface of the detector 3 depends on the surface reflection characteristics of the memory 7.
The detector 3 has two detectors, the output of one detector is at one input end of the subtracter 8 and the adder 9, and the output of the other detector is the other of the subtracter 8 and the adder 9. Is input to the input terminal of. The subtracter 8 outputs a tracking error signal, and the adder 9 outputs an information signal.

In this optical system, as shown in FIG. 10, the groove 22 is cut into the reflective base material of the memory 7, and the pitch p _{t of the} groove 22 is 1.6 μm. Information is written in the form of pits or marks 21 in the flats between the grooves 22, which are used to control the tracking of the focused laser beam 20.

The beam spot converges on the center of the track. Tracking errors occur when light is no longer symmetrically reflected from the disk surface. This situation occurs when the laser spot is shifted from the center of the groove 22. The asymmetry of the profile of the diffracted light is detected by dividing the photodetector 3 in half in parallel with the groove 22 of the disc and determining the difference signal between both sides. Since the groove 22 has a period p _t , the tracking error signal has the same period as that. In a first-order approximation, the tracking error signal can be expressed by equation (1). TE = A × sin (x / _pt ) + B (1) where A is the maximum amplitude, _pt is the track pitch, B is the offset, and x is the tracking error.

[0006]

By the way, when the above-mentioned conventional optical head tracks the reflecting surface, the objective lens moves to keep the laser beam at the center of the track. As a result, the lens is displaced from the center of the laser beam during tracking, which introduces an error in the tracking error signal in the form of an offset B or a curve shift. The shift supplies a spurious value to the tracking control actuator, and the tracking beam is displaced from the center of the track. The conventional optical head has the offset B caused by the deviation of the axis of the optical system from the axis of the laser beam as described above, and since the offset B is not compensated for, the tracking becomes inaccurate. .. It is an object of the present invention to provide a method and apparatus for correcting tracking error offset, and at the same time increase the information recording density of a memory device.

[0007]

A tracking error offset compensating method according to the present invention for achieving the above object comprises a step of generating a laser beam having a predetermined frequency, and the laser beam being incident on a second harmonic generating element. Then, a step of generating a second harmonic of the laser light and outputting the laser light and the second harmonic, the laser light and the second harmonic being incident on a recording medium, It is characterized by including a step of obtaining diffracted light and a step of separating the diffracted light of the two wavelengths to obtain an information signal from one and obtaining a tracking error signal from both diffracted light.
A tracking error offset compensating apparatus according to the present invention for achieving the above object is a mechanism for generating a laser beam having a predetermined frequency, and a mechanism for detecting the diffracted light from the recording medium by making the laser beam incident on the recording medium. An optical head provided with, an element arranged at a predetermined position of the optical head, for generating a second harmonic of the laser light of the predetermined frequency,

Diffracted light from the recording medium of the laser light of the predetermined frequency and the second harmonic generated from the element is detected at the same time, and information is written in the recording medium or information is recorded from the recording medium. A dual wavelength detection system having a mechanism for reading out and a mechanism for obtaining a tracking error signal from the diffracted light is provided. The two-wavelength detection system uses a means for separating the diffracted light corresponding to the laser light and the diffracted light corresponding to the second harmonic, and a tracking error signal from the information contained in one diffracted light and the other diffracted light It is more preferable to have a means for outputting.
Further, the means for outputting the tracking error signal has a pair of differential amplifiers with variable gain, and the one differential amplifier generates a first tracking error signal from the diffracted light corresponding to the laser light. , The other differential amplifier is
Generating a second tracking error signal from the diffracted light corresponding to the second harmonic,

When the tracking error does not actually exist, the gains of the pair of differential amplifiers are adjusted so as to equalize the first and second tracking error signals, and the offset is removed. Is also possible.

Furthermore, if the shorter wavelength of the laser light and the second harmonic is used to write information to or read information from the recording medium, the information recording density of the memory device is increased. Can be increased.

[0011]

The method and apparatus of the present invention are configured to compensate for the effects of optical head tracking error offsets. This compensation is achieved by incorporating a second harmonic generating crystal (element) in the optical head. A dichroic beam, that is, a beam having two wavelengths is generated by the second harmonic generating element. The beam is reflected by the surface of the memory device and the two wavelength components are separated and detected. The tracking error signal for each of the two wavelength components is generated by, for example, a differential amplifier. At this stage, the tracking error signals of these two wavelength components have an offset, but in the absence of tracking error,
The offsets can be canceled by adjusting the outputs of these differential amplifiers to zero by adjusting the gains of the differential amplifiers.

If the shorter wavelength of the two wavelength components is used for writing information to the recording medium or reading information from the recording medium, a laser beam having a small spot can be used for recording and reading information. Therefore, even if the information recording density of the memory device is increased, the recording / reproducing can be performed, and the recording / reproducing with higher density can be performed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A tracking error offset compensating apparatus according to an embodiment of the present invention will be described with reference to FIGS. 1, 2 and 3.

The laser beam generated by the laser diode 701 passes through the collimator lens 702 and is incident on the second harmonic generation crystal element (SHG) 710.
The laser beam of wavelength λ1 incident on the SHG 710 has wavelengths λ1 and λ2 (λ2 = λ1) as shown in FIG.
/ 2) laser beam. The second beam of wavelength λ2 is generated by the nonlinear characteristics of the crystal. This two-color laser beam is polarized beam splitter 704, 1/4
The light enters the recording medium 707 through the wave plate 705 and the objective lens 706.

The laser beam reflected by the recording medium 707 passes through the objective lens 706 and the quarter-wave plate 705, enters the polarization beam splitter 704, and is deflected in the direction in which the photodetector is arranged. The two-color laser beam transmitted through the polarization beam splitter 704 is separated into wavelength components by a separating means 717, for example, a dichroic prism, a dichroic mirror, an optical filter or a diffraction grating. The wavelength λ1 component enters the photodetector 716, and the wavelength λ2 component enters the photodetector 714.

The short wavelength components incident on the detector 714 are detected by different parts of the detector 714, and the respective outputs are supplied to the adder 711 to generate an information signal. The signals detected by different parts of the detector 714 are simultaneously supplied to the subtractor (differential amplifier) 713 to generate the tracking error signal TE2.

The long-wavelength components incident on the detector 716 are detected by different parts of the detector, and the respective detection signals are supplied to the subtractor 715, and the tracking error signal TE.
1 is generated. Tracking error signals TE1 and TE2
Is supplied to the subtractor 712 to generate the tracking error signal TE3.

As expected from using a dual wavelength signal, two different sized spots are formed,
Both are diffracted from the disk surface. The tracking error signals for the two beams are expressed by equations (2) and (3). TE1 = A (λ1) × sin (x / _pt ) + B (λ1) (2) TE2 = A (λ2) × sin (x / _pt ) + B (λ2) (3)

TE1 is output from the differential amplifier 715,
TE2 is output from the differential amplifier 713. The differential amplifiers 713 and 715 amplify the tracking error signal, and the amplification factor (gain) of the amplifier 715 is α and the amplification factor of the amplifier 713 is −β. A differential amplifier 712 outputs these two outputs.
By using the linear combination, the offset error can be compensated.
This can be generally expressed by the following equation (4). α × TE1-β × TE2 = (α × A (λ1) -β × A (λ2)) sin (x / p t) + αB (λ1) -β × B (λ2) ... (4)

Since the conversion efficiency of the SHG crystal is low, the wavelength λ
The intensity of the second beam is lower than that of the original beam generated by the laser diode. Therefore, generally, the gain β of the wavelength λ2 must be larger than the gain α of the beam of the wavelength λ1. In the expression (4), if αB (λ1) −β × B (λ2) = 0, that is, αB (λ1) = β × B (λ2) (5), the offset can be removed (compensated). This is because in the equation (4), when x = 0, that is, when there is no tracking error, α × TE1 = β × TE2 (6)

In other words, this means that it can be achieved by making the output values of the differential amplifiers 713 and 715 equal. For that purpose, the gains of the differential amplifiers 713 and 715 may be made variable so that the equation (6) is satisfied. Alternatively, such a gain may be obtained in advance and set to that value.

By the way, this compensation method brings another advantage that the information density of the device, that is, the density of the output of the adder 711 can be increased by introducing a short wavelength. FIG. 3 shows a schematic diagram of a focused light beam on the surface of a memory device. As is clear from the figure, the size of the short wavelength spot 723 is smaller than the size of the original laser beam spot 720. This is because the diameter SD of the light spot converged on the disc is calculated using the following approximate expression (7). SD = λ / NA (7) Here, NA is the numerical aperture of the lens, which is usually 0.5. In FIG. 3, reference numeral 722 indicates a track groove, and 721 indicates a pit or mark. Next, two simulations were performed using the device of the present invention. The laser wavelength was changed for each simulation.

The results of the first simulation will be described with reference to FIGS. 4 and 5. The following parameters were adopted in this simulation. Laser wavelength λ1 is 0.83μ
m, and λ2 is ½ of λ1. The normalized depth of the groove 722 is λ1 / 8. The depth value of the groove 722 is an important value because a groove having a vertical wall surface generates a proper tracking error signal.
Twice the groove depth, λ 1/4, produces a zero tracking error signal in the first approximation, and a comprehensive list of this system parameter is given in Table 1 below. Table _{1 p t 1.6 μm λ1 0.830 μm} SD1 1.66μm λ2 0.415 μm SD2 0.83μm NA 0.5 ND 4.66mm

4 and 5 show the effect of lens shift on the tracking error signal. Both beams deviate from their original values as the lens shifts. In the figure, TE1 and TE2 in the above equations (2) and (3) are shown, and TE3 is a signal whose offset is compensated. That is, from the equations (4) and (5), TE3 = (α × A (λ1) −β × A (λ2)) sin (x / _pt ) (8). If the normalized groove depth for the wavelength λ2 beam is λ2 / 4, A (λ in equations (3) and (8)
2) becomes zero.

The output of the detector in figure, -p _t / 4 (-
0.4) have been shown for + p _t /4(+0.4) to the tracking error, has a curve TE1 is large gradient of the wavelength λ1 beam, the wavelength λ2 beams curve TE2 is parallel to the tracking error axis .. As the lens shift increases, the shifts of both signals TE1, TE2 change similarly. When the shift amounts of both signals are subtracted based on the equation (5), the obtained shift amount of the curve TE3 becomes negligibly small, the offset is compensated, and the spurious signal previously applied to the tracking servo is canceled. ..

FIG. 6 shows the compensated signal (C) and the uncompensated signal (UC) as a function of lens shift. The values of the signal (C) and the signal (UC) are shown in FIG. 4 (d). This FIG. 6 shows that the sensitivity of the optical head can be increased with the compensated signal of the present invention, which exhibits small changes in response to lens shift.

The second simulation result will be described with reference to FIGS. 7 and 8. This second simulation was performed in the same manner as the first simulation, but only the laser wavelength was 0.76 μm, which was different. The normalized groove depth is likewise λ1 / 8. Table 2 shows the parameters used for this simulation. Table _{2 p t 1.6 μm λ1 0.760 μm} SD1 1.52μm λ2 0.380 μm SD2 0.76μm NA 0.5 ND 4.66mm

FIGS. 7 and 8 show the results of the simulation, and show the same characteristics as those of the first simulation. It can be seen that the shift level of the signal of wavelength λ2 is higher than in the first simulation, but the shift ratio for the curves of λ1 and λ2 is constant, and the offset is canceled again. Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Various modifications can be implemented.

[0029]

According to the present invention, a beam having two wavelengths is used, and the two wavelength components are separated to generate tracking error signals corresponding to the respective wavelength components. Since the offset included is adjusted and removed so that the tracking error signal becomes zero in the absence of tracking, more accurate tracking can be achieved.

Furthermore, since the shorter wavelength of the two wavelength components is used for writing information to the recording medium or reading information from the recording medium, a laser beam having a small spot can be used for recording and reading the information. .. Therefore,
Even if the information recording density of the memory device is increased, recording / reproduction is possible, and higher density recording / reproduction is possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a tracking error offset compensating apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing input / output beam characteristics of the SHG used in the above embodiment.

FIG. 3 is a view showing a state of a converged beam spot on a disk surface.

FIG. 4 shows the response of the device to various lens shifts in the device.

FIG. 5 shows the response of the device to various lens shifts in the device.

FIG. 6 shows compensated / decompensated tracking error offset as a function of lens shift.

FIG. 7 shows another response of the device to various lens shifts in the device.

FIG. 8 shows another response of the device to various lens shifts in the device.

FIG. 9 is a schematic configuration diagram of a conventional optical head.

FIG. 10 is a diagram showing a state of a converged beam spot on a disk when the above conventional optical head is used.

[Explanation of symbols]

 701 ... Laser diode 702 ... Collimator lens 704 ... Polarization beam splitter 705 ... Quarter wave plate 706 ... Objective lens 707 ... Recording medium 710 ... Second harmonic generation crystal element (SHG) 711 ... Adder 712, 713, 715 ... Subtractor (differential amplifier) 714, 716 ... Photodetector 717 ... Separating means

Claims (6)

[Claims] 1. A step of generating a laser beam having a predetermined frequency, the laser beam being incident on a second harmonic generating element to generate a second harmonic of the laser beam, the laser beam and the second harmonic wave. A step of outputting a harmonic, and the laser light and the second harmonic are incident on a recording medium,
Tracking, comprising: obtaining diffracted light of two wavelengths; and separating the diffracted light of the two wavelengths to obtain an information signal from one and obtaining a tracking error signal from both diffracted light. Error offset compensation method. 2. The one having the shorter wavelength of the laser light and the second harmonic is used for writing information to a recording medium or reading information from the recording medium. The tracking error offset compensation method described. 3. A mechanism for generating laser light having a predetermined frequency,
An optical head having a mechanism for detecting the diffracted light from the recording medium by making the laser light incident on the recording medium, and a second harmonic of the laser light having a predetermined frequency arranged at a predetermined position of the optical head. An element for generating, a laser beam having a predetermined frequency, and the second harmonic generated by the element, simultaneously detecting diffracted light from the recording medium, and writing information to the recording medium or recording medium. And a mechanism for reading information from the device and a mechanism for obtaining a tracking error signal from the diffracted light,
A tracking error offset compensating apparatus having: 4. The dual wavelength detection system comprises means for separating the diffracted light corresponding to the laser light and the diffracted light corresponding to the second harmonic, and the information contained in one diffracted light and the other diffracted light. The tracking error offset compensating apparatus according to claim 3, further comprising: a unit that outputs a tracking error signal. 5. The means for outputting the tracking error signal includes a pair of differential amplifiers having variable gains, wherein the one differential amplifier converts a diffracted light corresponding to the laser light into a first tracking error. A signal is generated, and the other differential amplifier generates a second tracking error signal from the diffracted light corresponding to the second harmonic, and when the tracking error does not actually exist, the first and second differential amplifiers are generated. 5. The tracking error offset compensating apparatus according to claim 4, wherein the tracking error offset compensating device is configured to adjust the gains of the pair of differential amplifiers so as to equalize the tracking error signals of 1. 6. The method according to claim 3, wherein one of the laser beam and the second harmonic having a shorter wavelength is used for writing information to a recording medium or reading information from the recording medium. The tracking error offset compensator described.