JPH0636335A - Playback device - Google Patents

Abstract

(57) [Summary] [Purpose] To improve S / N. [Structure] The second harmonic laser light emitted from the SHG laser 4 is condensed on the optical disc 1, and in the light receiving section 5,
The reflected light is received and converted into an RF signal. Then, in the HPF 6, the low frequency component of the output of the light receiving unit 5 is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproducing apparatus suitable for reproducing information recorded on an optical recording medium such as an optical disk or a magneto-optical disk.

[0002]

2. Description of the Related Art In a conventional optical disk device, for example, a semiconductor laser irradiates an optical disk with laser light, and the reflected light is received and converted into an RF signal (photoelectric conversion). Then, this RF signal is demodulated, and the information recorded on the optical disc is reproduced.

The semiconductor laser used in such a device has a short resonator length, and multimode oscillation is performed in order to prevent interference (interference noise) due to returning light (stray light) from the optical disk. Therefore, the wavelength distribution is wide and laser noise in a far wider band than the frequency band of the RF signal is generated. However, the level of the laser noise is set to be sufficiently smaller than the level of the RF signal in advance (FIG. 9), whereby a sufficient S / N is obtained.

By the way, with the recent increase in recording density of information on optical discs, in order to reproduce information recorded at high density, the wavelength of laser light generated from a semiconductor laser has been shortened. There is.

Current technology has wavelengths of about 750 to 780.
A semiconductor laser that emits red laser light of about nm has been realized, but a single semiconductor laser that emits light having a shorter wavelength, such as green light (laser light), is used. It was difficult to make.

Therefore, as shown in FIG. 10, a YAG (Yttrium Aluminum) as a laser medium is provided inside a resonator 44 composed of mirrors 44a and 44b arranged to face each other.
Garnet) crystal 42 and S as a nonlinear optical crystal element
An HG (Second Harmonic Generation) crystal 43 is arranged, and a YAG crystal 42 is provided via a mirror 44a.
In addition, a light source (hereinafter referred to as an SHG laser) in which a semiconductor laser 41 is arranged so as to emit a high-power laser beam has been developed.

In this SHG laser, when the laser light generated from the semiconductor laser 41 passes through the mirror 44a and enters the YAG crystal 42, the YAG crystal 42
Are excited to generate fundamental wave laser light such as red laser light. This fundamental wave laser light passes through the SHG crystal 43, is reflected by the mirror 44b, and is again reflected by the SHG crystal 4.
3, transmitted through the YAG crystal 42 and reflected by the mirror 44a.

Therefore, the fundamental wave laser light reciprocates between the mirrors 44a and 44b, whereby the resonator 44 resonates and the fundamental wave laser light generated by the YAG crystal 42 is strengthened. At the same time, in the SHG crystal 43, when the fundamental wave laser light passes therethrough, its second harmonic laser light is generated and passes through the mirror 44b.

As described above, in the SHG laser shown in FIG. 10, the second harmonic laser light having a half wavelength of the wavelength of the red laser light as the fundamental laser light generated from the YAG crystal 42. For example, green laser light is generated.

[0010]

By the way, this SHG
Since the semiconductor laser 41 that generates high-power laser light is used for the laser as described above, the level of the laser noise is much higher than that shown in FIG.

Since the resonator 44 has a factor corresponding to the selectivity Q of the resonance circuit in the electric circuit, the resonator 44 acts as an LPF (low-pass filter) for the laser noise from the semiconductor laser 41. To work.

Therefore, when the optical disk device is constructed by using the SHG laser, there is a problem that a large level of laser noise concentrated in the low frequency region as shown in FIG. 11 occurs and the S / N is deteriorated.

Therefore, there is a method of using a gas laser instead of the semiconductor laser 41, but this method has a problem that the apparatus becomes large and the cost becomes high.

Further, the interference noise due to the return light (stray light) from the optical disk is also concentrated in the low range, which occurs when the coherence length of the laser light is long. Since the relationship between the coherence length L _C and the half-width B of the wavelength of light is L _C = C / (2πB) C: speed of light, the coherence length L _C becomes longer as the half-width B becomes narrower. Become.

The green laser light from the SHG laser has a relatively narrow half-width B, and therefore interference noise is generated which concentrates in the low frequency band, which further deteriorates the S / N.

The present invention has been made in view of such a situation, and is to improve the S / N of the apparatus.

[0017]

A reproducing apparatus according to a first aspect of the present invention uses a fundamental wave laser light generated when a laser light is incident on a laser medium such as a YAG crystal 42 to generate an SHG crystal 43 or the like. SHG as a light emitting means for emitting a second harmonic laser beam, such as a second harmonic laser beam, generated by a resonator 44 having a nonlinear optical crystal element
A laser 4, an objective lens 2 as a focusing means for focusing the harmonic laser light emitted from the SHG laser 4 on an optical recording medium such as the optical disc 1, and a second harmonic laser from the optical disc. Light receiving section 5 as a light receiving means for receiving reflected light or transmitted light and converting it into an RF signal.
And an HPF (high-pass filter) 6 as a filter unit that suppresses the low-frequency component of the output of the light receiving unit 5.

According to a second aspect of the present invention, there is provided a reproduction apparatus including an HPF6.
Is, for example, a resistor R and a capacitor C, or a resistor R ₁
To R _N and capacitors C _{1 to} C _N.

According to a third aspect of the present invention, there is provided a reproduction apparatus including an HPF6.
Is an active filter.

A reproducing apparatus according to a fourth aspect is characterized by further comprising a direct current reproducing circuit 11 as a restoring means for restoring the output of the HPF 6 and outputting a restored signal.

In the reproducing apparatus according to the fifth aspect, the DC reproducing circuit 11 includes an arithmetic unit 21 as an adding means for adding the output of the HPF 6 and the frequency band component lacking in the output of the HPF 6, and the arithmetic unit 21. Comparator 22 as a comparison means for comparing the output with a predetermined level and outputting a restoration signal.
And a LPF 23 (a resistor R ₂₁ and a capacitor C ₂₁ ) as a detection means for detecting a frequency band component lacking in the output of the HPF 6 from the restored signal output from the comparator 22.

According to a sixth aspect of the present invention, in the direct current regeneration circuit 11, the HPF6 filters the output of the HPF6 and a comparator 31 as a comparing means for comparing the output of the HPF6 with a predetermined level. LPF33 (HPF34 and calculator 35) as complementary characteristic filter means having complementary characteristics of, and LPF33 (HPF34 and calculator 3)
5) and an output of the HPF 6, and an arithmetic unit 36 as a generating means for generating a restoration signal.

[0023]

In the reproducing apparatus having the above structure, the laser light is Y
Harmonic laser light such as second harmonic laser light generated by a resonator 44 having a non-linear optical crystal element such as an SHG crystal 43 from fundamental wave laser light generated by being incident on a laser medium such as an AG crystal 42. Is emitted and the harmonic laser light is condensed on an optical recording medium such as the optical disc 1.
Then, the reflected light is received and converted into an RF signal, and the low frequency component of the RF signal is suppressed. Therefore, noise (laser noise) concentrated in the low frequency range is suppressed, so that the S / N can be improved.

Further, when the output of the HPF 6 is restored, the S / N can be further improved.

[0025]

1 is a block diagram showing the configuration of an embodiment of an optical disk device to which a reproducing apparatus of the present invention is applied. S
As described above, the HG laser 4 is configured as shown in FIG. 10, and has a second wavelength having a half of the wavelength of the red laser light as the fundamental laser light generated from the YAG crystal 42. A green laser beam is emitted as a harmonic laser beam. The green laser light from the SHG laser 4 passes through the prism 3 and is supplied to the objective lens 2. Objective lens 2
Collects the green laser light from the SHG laser 4 that has passed through the prism 3 onto the optical disc 1.

The prism 3 supplies the light reflected from the optical disk 1 to the light receiving section 5. The light receiving unit 5 receives the reflected light from the optical disc 1 supplied via the prism 3 and outputs an electric signal corresponding to the received light amount, that is, an RF signal.
As shown in FIG. 2, the HPF 6 is a filter having a frequency characteristic as shown in FIG. 3, in which one end of the capacitor C and one end of the resistor R are connected and the other end of the resistor R is grounded. The low-frequency component (laser noise and interference noise) of the RF signal from is suppressed and supplied to the data extraction circuit 7. The data extraction circuit 7 detects a synchronization signal from the RF signal in which low-frequency components (laser noise and interference noise) are suppressed by the HPF 6, and demodulates the RF signal.

In the optical disc device having such a structure, the SHG laser 4 emits a green laser beam having a wavelength half the wavelength of the red laser beam generated from the YAG crystal 42 and causes the prism 3 to pass through. It is transmitted and supplied to the objective lens 2. In the objective lens 2, the green laser light from the SHG laser 4 that has passed through the prism 3 is condensed on the optical disc 1, and the reflected light is reflected by the prism 3 and supplied to the light receiving unit 5. The light receiving section 5 receives the reflected light from the optical disc 1 supplied through the prism 3, and outputs an electric signal corresponding to the received light amount, that is, an RF signal.

In the HPF 6, the low frequency component of the RF signal from the light receiving section 5 is suppressed and supplied to the data extracting circuit 7. As a result, FIG. 3 or FIG. 1 included in the RF signal is included.
The laser noise concentrated in the low range as shown in 1 is almost removed, the interference noise is also suppressed, and the RF signal with improved S / N is supplied to the data extraction circuit 7.

Here, the cutoff frequency of the HPF 6 is set to 2
When set to 0 to 100 kHz, S / N of RF signal
It has been confirmed by an experiment that the value can be improved by about 2 to 2.5 dB.

The RF signal output from the HPF 6 is demodulated while the data extracting circuit 7 detects the synchronization signal.

The HPF 6 can also be constructed by inserting a coil (not shown) in series with the resistor R of the filter shown in FIG. Furthermore, HPF6
With the configuration shown in FIG. 2 as a basic filter, the capacitor C ₁
And a basic filter composed of the resistor R ₁ , a basic filter composed of the capacitor C ₂ and the resistor R ₂ , ..., A basic filter composed of the capacitor C _N and the resistor R _N may be connected in series one after another ( (Fig. 4).

Further, the HPF 6 is not a so-called passive filter composed only of passive elements such as resistors, capacitors, or coils, but a capacitor C _{11 is provided at} the input terminal of an operational amplifier OP as an active element as shown in FIG. 5, for example. One end of the resistor R ₁₁ is connected to its output terminal, and the other end of the resistor R ₁₁ is connected to the operational amplifier OP.
A so-called active filter configured by connecting to the connection point of the capacitor C ₁₁ and the capacitor C ₁₁ may be used.

Next, FIG. 6 is a block diagram showing the configuration of a second embodiment of an optical disc apparatus to which the reproducing apparatus of the present invention is applied. This reproducing apparatus has a structure in which a DC reproducing circuit 11 is connected between the HPF 6 and the data extracting circuit 7 of the embodiment shown in FIG.

By the way, if the pits formed on the optical disk 1, that is, the modulation code is not DC-free, or if the HPF 6
When the cutoff frequency is set to a high value, the HPF 6 may significantly suppress the low-frequency signal component of the RF signal, as well as the laser noise or the interference noise.

Therefore, in the DC regeneration circuit 11, the HPF6
The low-frequency signal component of the RF signal suppressed by is compensated.

The DC restorer circuit 11 compares for example arithmetic units 21 for adding the outputs of the LPF23 of HPF6 as shown in FIG. 7, the output of the arithmetic unit 21 with a predetermined voltage V _TH, the calculator 21 If the output is less than a predetermined voltage V _TH of the outputs a voltage V _L, when the output of the arithmetic unit 21 is greater than a predetermined voltage V _TH, the comparator 22 outputs a voltage V _H, and the output of the comparator 22 The LPF 23 having a complementary characteristic (complementary characteristic) of the HPF 6 for filtering the.

The complementary characteristics (complementary characteristics) of HPF6
Is 1-P, where P (ω) is the characteristic of HPF6.
It means (ω).

The output of the HPF 6 is input to the comparator 22 via the calculator 21. In the comparator 22, the calculator 2
Compare the first output with a predetermined voltage V _TH is, when the output of the arithmetic unit 21 is lower than a predetermined voltage V _TH, the voltage V _L, restoring signal subjected to the compensation of the signal components of low frequency RF signals ( When the output of the calculator 21 is larger than a predetermined voltage V _TH , the voltage V _H is restored by compensating for the low-frequency signal component of the RF signal. It is output as a signal (a signal corresponding to a pit formed on the optical disc 1).

The restoration signal output from the comparator 22 is input to the LPF 23, which connects one end of the resistor R ₂₁ and one end of the capacitor C ₂₁ and grounds the other end of the capacitor C ₂₁ , and is filtered there.

Since the LPF 23 has the complementary characteristic of the HPF 6 as described above, the low-frequency signal component of the RF signal suppressed by the HPF 6 is output from the LPF 23.

The low-frequency signal component of the RF signal suppressed by the HPF 6 output from the LPF 23 is supplied to the calculator 21, where it is added to the output of the HPF 6 and output to the comparator 22.

As described above, the low-frequency signal component of the RF signal suppressed by the HPF 6 is compensated.

The RF signal (restored signal) in which the low-frequency signal component has been compensated by the DC reproducing circuit 11 is input to the data extracting circuit 7, where the synchronizing signal is detected and
Demodulated.

Further, the DC regenerating circuit 11 is constructed in a so-called feedback type as shown in FIG.
It can be configured as a so-called feed-forward type shown in.

That is, the DC regeneration circuit 11 compares the output of the HPF 6 with a predetermined voltage V _TH, and when the output of the HPF 6 is less than or equal to the predetermined voltage V _TH , outputs the voltage V _L and the output of the HPF 6 is predetermined. is greater than the voltage V _TH, the comparator 31 outputs a voltage V _H, to filter the output of the comparator 31, the delay circuit 32, and delay circuit 32 for delaying the LPF 33, the output of HPF6 with complement properties of HPF6 It can be configured by an arithmetic unit 36 that adds the output and the output of the LPF 33.

In the DC regenerating circuit 11 thus constructed, the comparator 31 compares the output of the HPF 6 with the predetermined voltage V _TH . When the output of the HPF 6 is less than the predetermined voltage V _TH , the voltage V _L Is output to the LPF 33, and when the output of the HPF 6 is larger than the predetermined voltage V _TH , the voltage V _H is the LPF 3
3 is output.

The LPF 33 is composed of an HPF 34 and an arithmetic unit 35 having the same characteristics as the HPF 6,
The output of the comparator 31 is filtered by the HPF 34,
In the calculator 35, the difference between the output of the comparator 31 and the HPF 34 is calculated. Therefore, the arithmetic unit 3
5 (LPF33) outputs the output of the comparator 31 as HP
It is the one filtered by the filter having the complementary characteristic of F34, that is, the output of the comparator 31 is filtered by the filter having the complementary characteristic of HPF6.

That is, the LPF 33 outputs a low-frequency signal component of the RF signal suppressed by the HPF 6.

On the other hand, in the delay circuit 32, the output of the HPF 6 is delayed by the time required for the processing in the comparator 31 and the LPF 33 and output to the calculator 36. In the arithmetic unit 36, the output of the HPF 6 from the delay circuit 32,
The low-frequency signal component of the RF signal suppressed by the HPF 6 from the LPF 33 is added, and a restored signal in which the low-frequency signal component of the RF signal suppressed by the HPF 6 is compensated is output.

Although the case where the present invention is applied to the optical disc device has been described above, the reproducing device of the present invention records not only on the optical disc device but also on a recording medium such as a magneto-optical disc device or an optical card reading device. It can be applied to any device that optically reproduces the stored information.

[0051]

As described above, according to the reproducing apparatus of the present invention, the harmonic generated by the resonator having the nonlinear optical crystal element is generated from the fundamental wave laser light generated by the laser light incident on the laser medium. Wave laser light is emitted and the harmonic laser light is condensed on the optical recording medium. Then, the reflected light or the transmitted light is received and converted into an RF signal, and the low frequency component of the RF signal is suppressed. Therefore, noise (laser noise) concentrated in the low frequency range is suppressed, so that the S / N can be improved.

Further, according to the reproducing apparatus of the present invention, the output of the filter means is restored, so that S /
N can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of an optical disc device to which a reproducing device of the present invention is applied.

FIG. 2 is a circuit diagram showing more details of the HPF 6 of the embodiment shown in FIG.

FIG. 3 is a diagram for explaining the action of the HPF 6 of the embodiment of FIG.

FIG. 4 is a circuit diagram showing more details of the HPF 6 of the embodiment of FIG.

5 is a circuit diagram showing more details of the HPF 6 of the embodiment of FIG.

FIG. 6 is a block diagram showing the configuration of a second embodiment of an optical disc device to which the reproducing device of the present invention is applied.

FIG. 7 is a circuit diagram showing more details of the DC regeneration circuit 11 of the embodiment of FIG.

FIG. 8 is a circuit diagram showing more details of the DC regeneration circuit 11 of the embodiment of FIG.

FIG. 9 is a diagram showing the level of an RF signal and the level of laser noise obtained by a conventional semiconductor laser.

FIG. 10 is a block diagram showing a configuration of an example of a conventional SHG laser.

11 is a diagram showing the level of an RF signal and the level of laser noise obtained by the SHG laser of FIG.

[Explanation of symbols]

 1 Optical Disc 2 Objective Lens 3 Prism 4 SHG Laser 5 Light Receiving Section 6 HPF 7 Data Extraction Circuit 11 DC Reproducing Circuit 21 Calculator 22 Comparator 23 LPF 31 Comparator 32 Delay Circuit 33 LPF 34 HPF 35, 36 Calculator 41 Semiconductor Laser 42 YAG (Yttrium Aluminum Garnet) crystal 43 SHG (Second Harmonic Generation) crystal 44 Resonator 44a, 44b Mirror

Claims (6)

[Claims] 1. A light emitting means for emitting a harmonic laser light generated by a resonator having a nonlinear optical crystal element from a fundamental wave laser light generated when laser light is incident on a laser medium; Condensing means for condensing the emitted harmonic laser light on the optical recording medium, and the reflected light or transmitted light of the harmonic laser light from the optical recording medium is received and converted into an RF signal. A reproducing apparatus comprising: a light receiving unit; and a filter unit for suppressing a low frequency component of an output of the light receiving unit. 2. The reproducing apparatus according to claim 1, wherein the filter means is composed of only passive elements. 3. The reproducing apparatus according to claim 1, wherein the filter means is an active filter. 4. The reproducing apparatus according to claim 1, further comprising a restoring unit that restores an output of the filter unit and outputs a restored signal. 5. The restoring means compares an output of the filter means with an adder means for adding a frequency band component lacking in the output of the filter means, and an output of the adder means with a predetermined level. And comparing means for outputting the restored signal, and detecting means for detecting a frequency band component lacking in the output of the filter means from the restored signal output by the comparing means. The playback device described. 6. The restoring means comprises: comparing means for comparing the output of the filter means with a predetermined level; and complementary characteristic filter means for filtering the output of the comparing means and having complementary characteristics of the filter means. The reproducing apparatus according to claim 4, further comprising: a generating unit that generates the restored signal from an output of the complementary characteristic filter unit and an output of the filter unit.