JPH04163732A - Optical head - Google Patents

Abstract

PURPOSE:To enable the obtaining of a correct demodulation signal using a voltage value about half in signal level as much as an amplitude of a reproduction signal by providing a variable type opening stop in an optical path of an objective lens at an optical head which has an optically reproducing function to satisfy a second optical reproduction reference suited to the reproduction of an optical information recording medium disk. CONSTITUTION:A variable type opening stop Iv is provided in an optical path of an objective lens Lo at an optical head which has an optically reproducing function to satisfy a second optical reproduction reference suited to the reproduction of an optical information recording medium disk higher in recording density than the optical information recording medium disk to be reproduced according to the first optical reproduction reference. Then, the opening stop Iv is adjusted in the use of the optical information recording medium disk to be reproduced according to the first optical reproduction reference and in the use of the disk according to the second optical reproduction reference so that light with a diameter thereof required for the respective disks forms an image on a signal plane thereby enabling the obtaining of a correct demodulation signal using a voltage value about a half in signal level as much as an amplitude of a reproduction signal.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an optical head.

[Conventional technology]

When recording information to be recorded and reproduced as digital data on an information recording medium, the digital signal based on the information to be recorded and reproduced is modulated using a specific modulation method selected from among various modulation methods. This is done even when the information recording medium is an optical disc. For example, in an optical disc known as a Compafoot Disc (CD), , digital data obtained by modulation using a modulation method (EFM) that expands 8 data bits to 14 channel bits is recorded and reproduced, and a CD employing the above-mentioned EFM. , the shortest interval between bits T w
Bit length (pulse length or pulse width) corresponding to in 3
Digital data is configured by a bit string of modulation signals of nine different bit lengths set between a modulation signal of T and a modulation signal of 11T corresponding to the longest interval between bits TmaX. . The above-mentioned CD recording/reproducing method uses digital data consisting of modulation signals of nine different bit lengths set between a modulation signal with the shortest pulse length of 3T and a modulation signal with the longest pulse length of 11T. When recording on an optical disc (CD) as an array of pits having a specific shape and size, and when reproducing (reading) recorded information recorded on the CD as an array of bits, a specific wavelength (780 nm) is used.
m) with a specific numerical aperture NA (
This is done by condensing the light using a condensing lens (objective lens) with NA=0.45) and projecting it onto the CD. The beam waist portion of the condensed reproduction light is placed on the signal surface of the CD, that is, the signal surface of the CD is placed on the focusing surface of the condensing lens. A reproduction light is projected onto an array of pits provided on the signal surface of the CD, and in this state, the reproduction light obtained from the signal surface of the CD is photoelectrically converted by a photodetector to obtain a reproduction signal. The above-described reproduced signal, that is, the reproduced signal obtained when a spot of light having a predetermined diameter is focused on the signal surface of a CD placed at the focal plane of a condensing lens. The shortest interval between bits is determined by applying it to a comparator that is supplied with a signal level voltage value of approximately 1/2 of the amplitude of the reproduced signal as a comparator reference voltage (threshold voltage) and performing a demodulation operation. A modulation signal with a bit length (pulse length or pulse width) of 3T corresponding to T win and a bit length of 11T corresponding to the longest interval between bits T vxax
It is possible to demodulate digital data made up of bit strings of modulated signals with nine different bit lengths set between the modulated signal and the modulated signal.

[Problem to be solved by the invention]

By the way, 1. For example, in order to faithfully demodulate digital data recorded in a state that satisfies the conditions specified in the CD standard, it is necessary to use a specific wavelength (7
80 nm) with a specific numerical aperture NA (NA=
The light is focused on the signal surface of the CD using a condensing lens of 0.45),
A state in which the beam waist portion of the reproduced light focused by the condensing lens is located on the signal surface of the CD, that is, C
A spot of light having a predetermined diameter is placed on the array of pits provided on the signal surface of the CD as described above so that the signal surface of the CD is located at the focal plane of the condensing lens. The reproduced signal obtained from the photodetector in a state where the light is focused is applied to a comparator which is supplied with a voltage value of a signal level of about 172 times the amplitude of the reproduced signal as a comparator reference voltage (threshold voltage). For example, if the wavelength of the reproduction light used in the reproduction device or the numerical aperture of the condensing lens differs from the value specified in the CD standard, or the reproduction If the voltage value of the comparator reference voltage used when demodulating the signal deviates from the voltage value of approximately 1/2 of the amplitude of the reproduced signal by more than the allowable range, the difference between the bits recorded on the CD By a bit string with modulation signals of nine different bit lengths set between a modulation signal of bit length 3T corresponding to the shortest interval T in and a modulation signal of bit length LIT corresponding to the longest interval T wax between bits. The configured digital data is not demodulated correctly, and for example, a modulated signal with a bit length of 3T, which corresponds to the shortest interval between bits recorded on a CD, becomes a modulated signal with a bit length other than 3 bits. Something that happens is that it gets demodulated. Therefore, in a CD playback device, for example, the bit length 3 corresponding to the shortest interval T win between bits recorded on the CD.
The modulation signal of T is bit! The CD standard stipulates that a demodulated signal having the same bit length as the modulated signal recorded on the CD can be obtained by demodulating, such as being demodulated as a 13T demodulated signal. A reproduction light having a specific wavelength (780 nm) is focused on the signal surface of the CD by a condensing lens with a specific numerical aperture NA (NA=0.45), and the bits provided on the signal surface of the CD are A reproduced signal is obtained while a light spot with a predetermined diameter is focused on the array, and the voltage value at a signal level of approximately 1/2 of the amplitude of the reproduced signal is the comparator reference voltage (threshold voltage). The demodulation operation is performed by giving a reproduced signal to a comparator that is supplied as a converter. Now, taking the case of a CD as an example, in order for a modulated signal recorded on an optical disc, in which information is recorded as an array of bits formed on the signal surface of the optical disc, to be correctly demodulated in a playback device, As is clear from the descriptions made so far, the diameter of the spot of the reproduction light to be used when reproducing the information signal recorded on the optical disk is determined in correspondence with the shape and dimensions of the bit. It is necessary that the voltage is a specified value, and that the comparator reference voltage (threshold voltage) used when demodulating the reproduced signal has a voltage value of a signal level of approximately 1/2 of the amplitude of the reproduced signal. It will be done. Therefore, from multiple optical discs with different recording densities,
An optical disk playback device capable of correctly reproducing and demodulating the information signals recorded on the disk requires one or both of a light source for playback light in an optical head and a condensing lens to be used in the configuration of the playback device. As a result, it is necessary to prepare optical heads with different configurations for each, that is, a reproduction light source that can emit light of different wavelengths, and a condensing lens with a different numerical aperture. It is. Next, in order to make the above points clearer, the following will be explained in detail. First, when the recording density is increased while the rotational speed of the optical disc is kept constant, the shape and dimensions of the bits formed on the signal surface of the optical disc become smaller as the recording density increases. In an optical head that irradiates the signal surface with reproduction light and obtains a reproduction signal by utilizing the diffraction phenomenon of the reproduction light by the bit, a predetermined width is set corresponding to the width of the bit on the optical disk being played. The signal surface of the optical disc is irradiated with a spot of reproduction light having a diameter having a relationship of . In addition, the arrangement of bits formed on the signal surface of an optical disc is different even when recording and reproduction are performed using digital data modulated by the same modulation method. The recording density differs depending on the level of recording density, and even in the case of optical discs in which optical discs with different recording densities are used by being driven and rotated at different rotational speeds. The bits in the higher density optical disc are narrower than the bits in the lower recording density optical disc. As such, the arrangement of bits formed on the signal surface of an optical disc differs depending on the recording density of the optical disc. If the bits on the optical disk have different shapes and dimensions and the bits are read using a spot of light of the same wavelength and the same diameter, it is natural that the diffraction of light at the bit will occur. Since the states are completely different, it is clear that the reproduced signals outputted from the photodetector will be different from each other. By the way, in order to solve the above-mentioned problem that the state of light diffraction caused by the bit changes as the relationship between the width of the bit and the diameter of the light spot used to read the bit changes, it is necessary to It is also conceivable to change the cross-sectional area of the reproduced light on the signal surface of the optical disk by changing the convergence state, but even if the cross-sectional area of the reproduced light on the signal surface of the optical disk is changed by the solution described above, Since the reproduction light irradiated onto the signal surface of the optical disk in this manner is not a cross section of the beam waist portion, the wavefront of the reproduction light on the signal surface of the optical disk is disordered. Once the diameter of the reproduction light is set to a predetermined diameter, it is no longer possible to reproduce the correct reproduction signal from the optical disc. Therefore, it is impossible to correctly reproduce the information signals recorded on multiple optical discs with different recording densities. For an optical head capable of reproducing data, it is necessary to have a light source for reproducing light and/or a condensing lens, each of which has a separate configuration, but as mentioned above, the recording density It is not desirable to have to prepare optical heads with different configurations for a plurality of different optical disks. By the way, in an optical head that irradiates the signal surface with reproduction light and obtains a reproduction signal by utilizing the diffraction phenomenon of the reproduction light by the bits, as mentioned above, the bits on the optical disc that are being reproduced are Unless the signal surface of the optical disk is irradiated with a spot of reproduction light having a diameter in a predetermined relationship corresponding to the width of 1
It goes without saying that a correct demodulated signal may or may not be obtained even if a signal level voltage value of /2 is used. ) cannot be easily used as an optical head. To explain the above point, let's take an example of a plurality of optical discs each having different recording densities. ■As mentioned above, a CD that uses light with a wavelength of 780 nm as the reproduction light and is reproduced using an optical head equipped with a condensing lens with a numerical aperture of 0.45;
As an optical disc other than the above-mentioned CDs, the wavelength is 670 nm.
An optical disk that uses light with a wavelength of 670 nm as the reproduction light and that is reproduced using an optical head equipped with a condensing lens with a Sekiguchi number of 0.6. An optical disc that is reproduced using an optical head equipped with a condensing lens with a numerical aperture of 0.55, and (1) a light with a wavelength of 670 nm used for reproduction light and a condensing lens with a numerical aperture of 0.5. An optical disk that is reproduced using an optical head equipped with a lens, and an optical head that uses light with a wavelength of 670 nm as the reproduction light and is equipped with a condensing lens with a numerical aperture of 0.45. (1) An optical disc that uses light with a wavelength of 670 nm as the reproduction light and that is reproduced using an optical head that is equipped with a condensing lens with a numerical aperture of 0.4. When considering the case where there was a problem, we investigated the eye pattern of the playback signal obtained when a CD was played using the optical head for each of the different optical disks mentioned above.
An eye pattern as shown in the figure was obtained. Figure 4 shows the eye pattern of the reproduced signal obtained when a CD is reproduced by an optical head for CD reproduction, and (a) to (e) in Figure 5 are the eye patterns of the CDs shown in ■ to ■ above. This is an eye pattern of a reproduced signal obtained when reproduced by an optical head for reproducing an optical disc. Regarding the reproduced signal shown in FIG.
The demodulation operation of the reproduced signal is performed by a comparator to which a voltage value of a signal level around 1/2 of the amplitude of the reproduced signal at the S-8 line position in the figure is supplied as a comparator reference voltage (threshold voltage). In this case, the bit length 3 corresponding to the shortest interval T win between bits recorded on the CD
Digital data consisting of a bit string of modulation signals of nine different bit lengths set between a modulation signal of T and a modulation signal of bit length LIT corresponding to the longest interval between bits T-aX is correct. Furthermore, demodulation can be performed in a stable state. In addition, (,) to (e) in Fig. 5 exemplify the eye patterns of the reproduced signals obtained when a CD is reproduced by the optical head used for reproducing the optical discs described above. Among the reproduced signals shown in FIG. 5, the reproduced signals shown in (d) to (e) in FIG.
), the voltage value of the signal level at the A-A line position in Ce) is used as the comparator reference voltage (threshold voltage) to demodulate the reproduced signal. The shortest interval between bits T win
A digital signal consisting of a bit string of modulation signals with nine different bit lengths set between a modulation signal with a bit length of 3 corresponding to , and a modulation signal with a bit length of 11T corresponding to the longest interval Tmax between bits. Although the data can be correctly demodulated in a relatively stable state,
Regarding the reproduced signal whose eye pattern is illustrated in C), the reproduced signal is demodulated by a comparator to which a voltage value of a signal level around 172 of the amplitude of the reproduced signal is supplied as the comparator reference voltage (threshold voltage). Even if the action is performed, C
A modulation signal with a bit length of 3T corresponding to the shortest interval Twin between bits recorded in D and the longest interval T between bits.
It is not possible to correctly demodulate digital data in a stable state consisting of a bit string of modulation signals of nine different bit lengths set between max and the corresponding modulation signal of bit length 11T. Regarding the reproduced signal whose eye patterns are illustrated in (a) to (c) of Fig. 5, a modulation signal with a bit length of 3 corresponding to the shortest interval T win between bits recorded on a CD, and a The comparator reference voltage (threshold voltage ) is the voltage at the A-A line position in (a) to (c) of FIG. However, as is clear from the reproduced signal waveforms shown in (a) to (C) in FIG.
Since it is not possible to obtain a stable reproduction signal at the A-A line position in the center, in this case, the optical head used to reproduce recorded information on multiple optical disks with different recording densities must be The optical head used for optical discs with a standard that has the minimum diameter of the spot of the reproduction light projected onto the signal surface of the optical disc in the focused state can also be used when playing back the optical disc that is being played by the playback device. This means that they cannot be used for both purposes. [Means for Solving Problem 11] The present invention reproduces an optical information recording medium disc having a higher recording density than an optical information recording medium disc that is reproduced according to the first optical reproduction standard. an optical head comprising a variable aperture diaphragm in the optical path of an objective lens in an optical head having an optical reproduction function capable of satisfying a second optical reproduction criterion suitable for satisfies a second optical reproduction standard suitable for reproducing an optical information recording medium disk having a higher recording density than an optical information recording medium disk that is adapted to be reproduced according to the optical reproduction standard of To provide an optical head using a variable focal length lens as an objective lens in the optical head having an optical reproducing function.

[Effect]

a second optical reproduction standard suitable for reproducing an optical information recording medium disk having a higher recording density than the optical information recording medium disk adapted to be reproduced according to the first optical reproduction standard; A variable aperture diaphragm is provided in the optical path of the objective lens in an optical head that is equipped with an optical reproduction function that satisfies the first optical reproduction standard. The aperture diaphragm is adjusted when the information recording medium disc is used and when the optical information recording medium disc is used for reproduction according to the second optical reproduction standard, and the respective optical information recording media are reproduced. Ensure that light of the required diameter of the disk is focused on the signal plane. Further, a second optical information recording medium disk suitable for reproducing an optical information recording medium disk having a higher recording density than the optical information recording medium disk adapted to be reproduced according to the first optical reproduction standard is provided. The focus of a variable focal length lens used as an objective lens in an optical head having an optical reproduction function that satisfies the reproduction standard is reproduced in accordance with a first optical reproduction standard. Each optical information recording medium disk is changed between when the optical information recording medium disk is used and when the optical information recording medium disk is used which is adapted to be reproduced according to the second optical reproduction standard. so that light of the required diameter is focused on the signal plane. As a result, when demodulating the reproduced signal output from any optical head, a correct demodulated signal can be obtained by using a voltage value with a signal level of approximately 1/2 of the amplitude of the reproduced signal as the comparator reference voltage (threshold voltage). This allows it to be used as an optical head.

【Example】

Hereinafter, specific details of the optical head of the present invention will be explained in detail with reference to the accompanying drawings. 1 to 3 are side views showing the configuration of essential parts of the optical head of the present invention. In the figure, D is an optical information recording medium disk (optical disk), P is a pit, BS is a beam splitter, and LD is a light source (for example, a laser diode). PD is a photodetector, Lc is a collimator lens, LS is a cylindrical lens, L is a plano-convex lens, LO is an objective lens, Loz is a variable focal length objective lens, Iv is a variable aperture diaphragm, Lof and Lob are variable focal lengths. This is a lens that makes up the objective lens of the mold. In Figure 1, the laser beam emitted from the light source LD is
The light is made into parallel light by the collimator lens Lc and then enters the beam splitter BS. The light Iv reflected by the Beekes splitter BS passes through the variable aperture diaphragm Iv and enters the objective lens LO.
Also, the reflected light from the pit P on the signal surface of the optical disk D is transmitted through the objective lens LO → variable aperture diaphragm Iv → beam splitter BS → plano-convex lens L → cylindrical lens Ls → photodetector FD The light is incident on the photodetector through the path of , where it is photoelectrically converted and a reproduced signal is obtained. Now, in the optical head shown in FIG. 1, the light of wavelength λ emitted from the light source LD is transmitted from the light source LD to the collimator lens Lc to the beam splitter BS to the variable aperture stop I.
The light passes through the path of v→objective lens Lo→signal surface of the optical disc D, is focused on the objective lens LO, and is imaged on the signal surface of the optical disc D, producing a minute spot of light. The diameter is determined by the wavelength of the light emitted from the light source LD being λ, and the numerical aperture of the objective lens Lo being NA.
Then, the size changes in proportion to λ/NA. In the optical head of the present invention shown in FIGS. 1 and 2, the numerical aperture N of the objective lens Lo is controlled by the variable aperture diaphragm Iv.
By effectively changing A, the diameter of the light spot imaged on the signal surface of the optical disc D can be set to a predetermined value determined for each optical disc system. The embodiment of the optical head shown in FIG.
This shows a configuration example in which a variable aperture stop Iv is provided in the optical path between the optical disk D and the objective lens Lo.In addition, in the embodiment of the optical head shown in FIG. A configuration example in which a variable aperture stop Iv is provided in the optical path is shown. In implementing the present invention, a state in which the variable aperture diaphragm Iv described above is inserted into a predetermined optical path and a state in which the variable aperture diaphragm Iv inserted in the predetermined optical path described above is removed are changed. It may also be implemented in a configuration mode that allows. In addition, when configuring an optical head that can be used for multiple optical disk systems, it is possible to image a light spot with the diameter required for each optical disk system on the signal surface of the optical disk. It is sufficient to prepare a variable aperture diaphragm having such a configuration. Next, the embodiment of the optical head of the present invention shown in FIG. 3 uses a variable focal length objective lens Loz as the objective lens in the optical head. f2 in (b) of the figure is the front lens group L that constitutes the variable focal length objective lens Loz.
This is the composite focal length of the lens of and the rear group lens LOb. Note that a variable focal length objective lens L is used as an objective lens. As the zoom lens 2, zoom lenses of various conventionally known configurations can be used. In the optical head of the embodiment shown in FIG. 3, there are two cases in which the focal length of the variable focal length objective lens Loz is set to fl as shown in FIG. b)
Since the effective numerical aperture of the variable focal length objective lens Loz changes depending on the setting of f2 as shown in FIG.
By variably adjusting the focal length of the optical disc D, the diameter of the light spot imaged on the signal surface of the optical disc D can be set to the value required for each optical disc system.

Effects of the Invention As is clear from the above detailed explanation, the optical head of the present invention has a higher level of performance than an optical information recording medium disc that is reproduced according to the first optical reproduction standard. A variable type optical head is provided in the optical path of the objective lens in the optical head, which is equipped with an optical reproduction function that satisfies the second optical reproduction standard suitable for reproducing optical information recording medium discs with high recording density. An aperture diaphragm is provided so that when the optical information recording medium disc is used, the disc is reproduced according to a first optical reproduction standard, and the disc is reproduced according to a second optical reproduction standard. When using the optical information recording medium disks, the aperture diaphragm is adjusted so that light having a diameter required by each optical information recording medium disk is imaged on the signal surface, and when the first optical can satisfy a second optical reproduction standard suitable for reproducing an optical information recording medium disk having a higher recording density than an optical information recording medium disk that is adapted to be reproduced according to the standard reproduction standard. An optical information recording medium disk configured to reproduce the focus of a variable focal length lens used as an objective lens in an optical head having an optical reproduction function in accordance with a first optical reproduction standard. and the diameter required for each optical information storage medium disk, which is changed between the use of the optical information storage medium disk and the use of the optical information storage medium disk adapted to be played according to the second optical playback standard. By making the light of As the voltage (threshold voltage), it is possible to obtain a correct demodulated signal by using a voltage value at a signal level of about 172 times the amplitude of the reproduced signal, and it is easy to use the optical head as well, as described above. The conventional problems are well solved.

[Brief explanation of the drawing]

1 to 3 are side views showing the configuration of essential parts of the optical head of the present invention, and FIGS. 4 and 5 are diagrams showing eye patterns of reproduced signals for explaining the problems. BS...beam splitter, LD...light source, PD...
...Photodetector, Lc...Collimator lens, Ls
...Cylindrical lens, L...Plano-convex lens, L
o...

Claims (1)

[Claims] 1. Suitable for reproducing an optical information recording medium disk having a higher recording density than an optical information recording medium disk adapted to be reproduced according to the first optical reproduction standard. Second
An optical head 2 comprising a variable aperture diaphragm in the optical path of an objective lens in an optical head equipped with an optical reproduction function that satisfies the optical reproduction standards of The optical head 3 according to claim 1, which uses the optical head 3 for reproducing an optical information recording medium disk having a higher recording density than the optical information recording medium disk that is adapted to be reproduced according to the first optical reproduction standard. suitable second
An optical head 4 that uses a variable focal length lens as an objective lens in an optical head equipped with an optical reproduction function that satisfies the optical reproduction standards of The optical head 5 according to claim 3, which uses an aperture stop that can be taken in and out of the optical path.