JPH10188290A - Device and method for reproducing optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a regenerative signal at a sufficient S/N ratio from an optical recording medium high density recorded by a minute pit and a minute magnetic domain, etc. SOLUTION: In the reproducing device of the optical recording medium, a control means sends a control signal (c) to a laser modulation part to modulate a wavelength of regenerative light at a fixed period. A reflection light quantity (d) from the recording pit (b) of the optical recording medium is changed depending on a wavelength modulation period. Although the regenerative signal (e) detected by a photodetector contains a random noise, a detection circuit outputs the regenerative signal (g) removing the noise by sampling only the signal component of the double period of the control signal. The regenerative signal is obtained from the high density recording medium at the excellent S/N ratio. When a magneto-optical disk is reproduced, the regenerative signal is sensitively phase detected using the reference signal of the frequency of the control signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproducing apparatus and a reproducing method for reproducing information recorded on an optical recording medium, and more particularly, to a recording apparatus in which recorded information is recorded at high density as minute pits or minute magnetic domains. The present invention relates to a reproducing apparatus for a high-density recording medium capable of improving the S / N of a reproduction signal obtained from an optical recording medium and a reproducing method thereof.

[0002]

2. Description of the Related Art Optical recording media are widely used today as mass storage media. When reproducing information recorded on such a recording medium, a laser beam for reproduction is irradiated onto the optical recording medium, and reflected light and transmitted light thereof are detected. For example, CD
In an optical disc such as an optical disc or a CD-ROM, information is reproduced by irradiating the optical disc with a reproduction laser beam having a constant intensity and detecting a reflected light intensity in accordance with the presence or absence of a pit formed on the optical disc. In addition, a magneto-optical disk (M
In O), a reproduction laser beam having a certain polarization plane is irradiated, and the magnetization information recorded on the magneto-optical disk is reproduced by detecting the rotation angle of the polarization plane of the reflected light.

[0003]

Incidentally, recently, in order to record data such as image information, it is desired to further increase the capacity of an optical recording medium. In order to increase the capacity of the optical recording medium, a method of increasing the recording density by miniaturizing and forming recording magnetic domains and pits has been adopted. In such a method, the area occupied by 1-bit information on the optical recording medium is reduced, and the recording medium can be downsized. However, when reproducing the high-density optical recording medium as described above, the amount of reflected light from the pits and magnetic domains is reduced, so that the reproduction signal level at the time of information reproduction is reduced. On the other hand, electrical components of the reproducing apparatus, such as a photodetector and an amplifier, generate noise with their operation. For this reason, when reproducing information from an optical recording medium of high density, the S /
The N ratio decreases.

As a method of improving the S / N ratio of the reproduction signal, it is conceivable to increase the reproduction laser beam output to increase the reproduction signal. However, in this method, there is a possibility that the recorded information may be destroyed by the laser light focused on the recording area at a high energy density, so that there is a limit to an increase in the output of the reproducing laser light.

An object of the present invention is to provide an optical recording medium on which high-density recording is performed by using minute pits or minute magnetic domains, etc.
An object of the present invention is to provide an optical recording medium reproducing apparatus and a reproducing method capable of obtaining a reproduction signal at a ratio.

[0006]

According to a first aspect of the present invention, there is provided a light source for irradiating a reproduction light to an optical recording medium, and the reproduction light is reflected or transmitted from the optical recording medium. In a reproducing apparatus for an optical recording medium that reproduces a reproduction signal based on the wavelength, the wavelength modulation means for modulating the wavelength of the reproduction light, and the wavelength modulation means such that the wavelength of the reproduction light is modulated at a constant period. An apparatus for reproducing an optical recording medium, comprising: control means for controlling; and detection means for detecting the reproduction signal based on the constant modulation period.

In the reproducing apparatus for an optical recording medium according to the first aspect of the present invention, the wavelength modulating means receives a control signal from the control means at the time of reproducing the recorded information and changes the wavelength of the reproducing light from the light source to a predetermined wavelength λ _0. Is modulated within a range of a constant wavelength width Δλ centered at. The pits of an optical disk such as a CD are generally formed such that the depth satisfies a predetermined interference condition with the wavelength λ _{0 of the} reproduction light. Therefore, the wavelength λ of the reproduction light
Is modulated in the range of λ ₀ ± Δλ to irradiate the optical disk, the return light from the pit changes depending on the modulated wavelength, and the detected reproduction signal changes accordingly (see FIG. 4). If the modulation of the wavelength is repeated at a constant period (modulation frequency) while the reproduction light is irradiating the portion where the pits are present, the reflected light contains the control signal modulation frequency × 2 signal components. Become. Therefore, by detecting the detection signal from the reflected light at the modulation frequency × 2, only the component based on the reproduction signal from the reflected light can be separated and detected from other noises. As a result, a good S / N reproduction signal can be obtained even if the pits are minute due to the high density.

Also, in a magneto-optical disk for reproducing recorded information using the magneto-optical effect, when the magneto-optical disk is irradiated with the wavelength of the reproduced light while being modulated, the wavelength is modulated to the return light from the magneto-optical disk. Influence appears. The magneto-optical disk has a different rotation angle of the plane of polarization of reflected light depending on the magnetization direction recorded by the magneto-optical effect of the recording film.
The magnitude of the magneto-optical effect changes depending on the wavelength of incident light (see FIG. 8). Therefore, while performing wavelength modulation of the reproduction light at a specific frequency by the wavelength modulation means,
By sampling the signal component of the specific modulation frequency from the reproduced signal by the detection means, the recorded information can be separated from noise and extracted.

The "reproducing apparatus" in the present specification means any apparatus having a function for reproducing an optical recording medium, and only a reproducing apparatus for reproducing a read-only recording medium such as a CD. However, the concept also includes a device used for recording / reproducing a recording medium capable of recording / reproducing, such as a CD-R or MO.

According to a second aspect of the present invention, an optical recording medium for irradiating an optical recording medium with reproduction light and reproducing a reproduction signal based on reflected light or transmitted light of the reproduction light from the optical recording medium. In the reproducing method, while irradiating the optical recording medium while modulating the wavelength of the reproduction light at a constant frequency, to obtain a reproduction signal by detecting reflected light or transmitted light of the reproduction light from the optical recording medium, A method for reproducing an optical recording medium is provided, wherein the reproduction signal is detected based on the constant frequency.

The method of reproducing an optical recording medium according to the present invention utilizes the fact that the return light from the reproduction light applied to a CD or MO changes depending on the wavelength of the reproduction light as described above. By performing detection such as phase-sensitive detection using the wavelength modulation frequency (or a multiple thereof) of the reproduction light, random noise can be removed from the reproduction signal and only recorded information can be obtained.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and examples of a reproducing apparatus and a reproducing method for an optical recording medium according to the present invention will be specifically described with reference to the drawings.

[First Embodiment] First, for example, a read-only optical disk in which information such as a CD is recorded according to the presence or absence of uneven phase pits, a write-once optical disk in which a hole is formed in accordance with recorded information, and the like are provided. The case of reproduction will be described with reference to FIG. FIG. 1 shows a configuration of an optical disk reproducing apparatus using the optical disk 1. The optical disc playback device is
An optical head 2 having an optical system for reproducing data from the optical disk 1, an actuator system (not shown) for driving the optical head 2 to follow an access track, and a signal from the optical disk 1 And a signal processing system 3 for detecting and decoding.

The optical head 2 condenses the laser modulation section 20 and the light emitted from the laser modulation section 20 on the optical disk 1, and further detects the return light from the optical disk 1 by a photodetector 27.
a and 27b, and optical detectors 27a and 27b. The optical system is a collimator lens 21, a polarizing beam splitter (hereinafter referred to as PBS) 2
2, 1/4 wavelength plate 23, objective lens 24, condenser lens 2
5. The knife edge prism 26 is provided.

The laser modulator 20 of the optical head 2 has a PLL.
A laser drive control signal is supplied from a circuit 34, and a control signal for optical modulation is supplied from a detection circuit 33.
The laser modulator 20 uses a laser diode (not shown) as a light source. In order to modulate the wavelength of the generated laser light, the laser modulating unit 20 includes, for example, a pair of external resonance mirrors and a dielectric material (variable in dielectric constant by an electric field applied between the resonators). (Not shown). In this case, the laser resonance wavelength may be modulated by changing the position of the external resonance mirror, or the electric field between the resonators may be changed to modulate the laser resonance wavelength. As a means for changing the position of the mirror, for example, there is a method in which a voice coil motor is attached to the back surface of the reflection surface of the mirror and the mirror is periodically moved along the optical axis direction according to a control signal from a detection circuit. The laser modulator 20 changes the laser light over the wavelength range Δλ (10 nm or more) with the movement of the mirror and emits the laser light to the optical disc 1. As another configuration for changing the laser light, the current supplied to the laser diode may be changed according to a control signal, or a wavelength-variable dye laser or the like may be used as a light source.

The signal processing system 3 includes photodetectors 27a and 27b
I / V converter 31a for converting the output current from
31b, an adder 32a that performs addition and subtraction based on outputs from the I / V converters 31a and 31b, a subtractor 32b, a detection circuit 33 that removes noise included in a reproduction signal of the adder 32a, and a synchronization circuit. It has a PLL circuit 34 for generating a signal and a decoder 35 for decoding data from the output of the detection circuit 33.

As shown in FIG. 2, the detection circuit 33 includes an oscillator 33a, a BPF 33b, a detection circuit main body 33d, and an LPF.
33c. The oscillator 33a generates a control signal for modulating the wavelength of the laser light with a cycle of the frequency ω, and outputs the control signal to the laser modulator 20. The BPF 33b is a laser modulator 2
A signal component between the bandwidths 2ω-Δf and 2ω + Δf is extracted with the modulation center 2ω twice the control signal output to 0 as the center of the band. The LPF 33c smoothes the signal by integrating the output from the BPF 33b and outputs it.

A method for reproducing information from an optical disk by using the circuits shown in FIGS. 1 and 2 will be described with reference to FIGS. FIG. 3 is a timing chart for explaining the operation of the optical disk reproducing apparatus. FIG.
6 is a graph schematically showing a change in the amount of reflected light.
It is assumed that data “0”, “1”, “1”, “1”, “0” and “1” as shown in FIG. That is, pits (recesses) corresponding to the length of data "1" are formed on the optical disc 1 (FIG. 3B). The laser light modulator 20 includes:
A control signal for modulating the wavelength of the laser light is supplied from the detection circuit 33 to the laser light modulation unit 20. FIG. 3C shows the signal waveform of the control signal. The laser modulator 20 modulates the laser light in a wavelength range of λ ₀ ± Δλ centered on the wavelength λ ₀ (zero position of the control signal) according to the amplitude of the control signal. The wavelength-modulated laser light emitted from the laser modulator 20 is collimated by a collimator 21 and is converted into a PBS 22 and a １ ／ wavelength plate 2.
The light is condensed on the optical disc 1 by the objective lens 24 after passing through the optical disc 3. The reflected light from the optical disk 1 is directed toward the condenser lens 25 by the PBS 22 and is split into two directions by the knife edge prism 26. The split light is guided to photodetectors 27a and 27b, respectively.

The amount of light reflected from the optical disk 1 detected by the photodetectors 27a and 27b is the largest from the mirror corresponding to data "0", and the amount of incident light and pits corresponding to the data "1" in the pit corresponding to data "1". Since the reflected light from the bottom surface interferes and the light amount is weakened, the reflected light amount is smaller than that of the mirror portion. In addition, the reflected light amount has an effect on the pit portion because the wavelength of the laser light is modulated. The effect reflects the relationship between the laser light wavelength λ and the reflected light amount I as shown in FIG. When the laser light changes in the range of the wavelength Δλ around the fundamental wavelength λ ₀ , the reflected light amount tends to be the smallest at the wavelength λ ₀ , and tends to increase when the laser light deviates from the wavelength λ ₀ . From the relationship between the wavelength of the laser light and the amount of reflected light in FIG. 4, it can be seen that the reflected light amount of the modulated laser light is the lowest because the wavelength is λ ₀ at the zero cross point of the control signal. As a result, wavelength laser beam is _{λ 0 ~λ 0 + Δλ / 2~λ} 0 ~λ 0 from the laser modulation unit 20 at a frequency ω of the control signal
When it changes from -Δλ / 2 to λ ₀ , the amount of light reflected from the pit portion (the position corresponding to the data “1”) fluctuates at a frequency 2ω which is twice the control signal as shown in FIG.
The control signal uses the output from the oscillator 33a in FIG. Since the wavelength modulation of the wavelength range Δλ must be performed at least once within the time for reproducing one bit of the optical disk, a sufficiently high frequency is selected as the oscillation frequency ω of the oscillator 33a in consideration of the linear velocity of the optical disk. It is. The light reflected from the optical disc is photoelectrically converted by the photodetector 27a in FIG. Further, the output from the photodetector 27a is converted into a voltage signal by the I / V converter 31a. Next, the adder 32a adds the output signal from the I / V converter 31a to the I / V converter 31a.
A reproduction signal is obtained by adding the output signals from the V converter 31b.

By the way, as the recording density increases, the 1-bit area occupancy of the optical disk 1 decreases, so that the reproduction signal level decreases. In order to obtain the reproduction signal, the photodetector, I / V By passing the signal through electrical components such as converters and adders, the electrical signal is affected by electrical random noise during signal processing. Therefore, the random noise is superimposed on the reproduced signal as shown in FIG. When reproducing high-density recording, the signal-to-noise ratio (hereinafter, referred to as S / N ratio) of the reproduced signal is inevitably reduced due to a decrease in the level of the reproduced signal and an increase in random noise. The reproduced signal having the lowered S / N ratio is sent to the detection circuit 33.

The detection circuit 33 converts the output signal from the adder 32a into a band-pass filter (hereinafter referred to as a BPF) 3.
3b. The BPF 33b is a filter that passes only a modulation signal component generated by the influence of the wavelength-modulated laser light. The center frequency of the BPF 33b is 2ω. BP
The F33b separates the signal component based on the recording information from the noise component and sends the signal from which the random noise component has been removed to the LPF 33c as shown in FIG. LPF33
c detects the supplied signal as shown in FIG.
Smoothing. The detection circuit 33 removes noise components included in the output signal of the adder 32a and outputs the signal to the decoder 35. The reproducing apparatus of the present invention can significantly improve the S / N ratio of the reproduced signal by being configured as described above.

[Second Embodiment] Next, as an optical recording medium,
A reproducing method using an overwritable magneto-optical disk such as an MO will be described with reference to FIG. FIG. 5 shows an optical disk (M
O) This is the configuration of the playback device. An optical disk reproducing apparatus includes an optical head 2 having an optical system for reproducing data from a magneto-optical disk 10 as in the first embodiment, and an actuator system for driving the optical head 2 to follow an access track. (Not shown) and a signal processing system 3 for detecting and decoding a signal from the magneto-optical disk 10.

The optical head 2 includes a laser modulator 20 and light emitted from the laser modulator 20,
And an optical system for supplying return light (transmitted light) from the magneto-optical disk 10 to the photodetectors 27a and 27b, and photodetectors 27a and 27b. The optical system includes a collimator lens 21, a first polarizing beam splitter (hereinafter, referred to as PBS) 22, an objective lens 24, a half-wave plate 221, a second PBS 222, a detection lens 223, and a detection lens 223.
24. The laser modulation control unit 20 of the optical head 2 is supplied with a laser drive control signal from the PLL circuit 34,
Further, a control signal for modulating the wavelength of the laser light is supplied from the detection circuit 33. The laser modulator 20 is configured in the same manner as in the first embodiment, and modulates a laser beam having a fundamental wavelength λ ₀ over a wavelength range Δλ and emits the modulated laser beam to the magneto-optical disk 10.

The signal processing system 3 includes photodetectors 27a and 27b
I / V converter 31a for converting the output current from
31b, an adder 32a that performs addition and subtraction based on outputs from the I / V converters 31a and 31b, a subtractor 32b, a detection circuit 33 that removes noise included in a reproduced signal of the subtractor 32b, and a detection circuit. A decoder 35 for decoding data from the output of the circuit 33, a clock extracting circuit 36 for extracting a clock from the output of the adder 32a, and a P for generating a synchronization signal based on the output from the clock extracting circuit 36.
And an LL circuit 34. The PLL circuit 34 supplies an output signal (clock) having the same phase based on the clock extraction signal to the decoder 35 and a laser driving circuit (not shown) in the laser modulation unit 20, respectively.

As shown in FIG. 6, the detection circuit 33 includes an oscillator 33a, a BPF 33b, a phase shift circuit 33d, a phase discrimination detection circuit (hereinafter, referred to as a PSD circuit) 33e, and a phase comparison circuit 33f. It has the same configuration as the amplifier. The oscillator 33a generates a control signal having a frequency ω for modulating the laser light and outputs the control signal to the laser modulator 20. The BPF 33b passes only a signal waveform having a bandwidth of 1 MHz or more and a center frequency ω. The signal that has passed through the BPF 33b is input to the PSD circuit 33e. The PSD circuit 33e performs phase discrimination based on the reference signal (frequency ω) supplied from the phase shift circuit 33d and outputs the same to a low-pass filter (LPF) 33f. Low-pass filter 33
At f, the signal subjected to phase discrimination is integrated. Detection circuit 33
Outputs the reproduced signal from which the noise has been removed to the decoder 35.

A method of reproducing information from the magneto-optical disk 10 using the circuits shown in FIGS. 5 and 6 will be described with reference to FIGS. 7 and 8. FIG. 7 is a timing chart for explaining the operation of the MO reproducing apparatus, and FIG. 8 is a graph showing a change in the magneto-optical effect accompanying a change in the wavelength of the laser beam applied to the magneto-optical disk 10. The data “0”, “1”, “1”, “1”, “0” and “1” as shown in FIG.
Are respectively recorded in the magnetization directions indicated by arrows as shown in FIG. 7B. At the time of reproduction, the PLL circuit 34 supplies a clock to the laser modulator 20. The laser modulator 20 generates laser light at the timing of this clock. A control signal for modulating the wavelength of the laser beam as shown in FIG. 7C is supplied from the detection circuit 33 to the laser modulator 20. The laser modulator 20 modulates the wavelength of the laser light within a range of λ ₀ ± Δλ centered on the wavelength λ ₀ and emits the laser light toward the magneto-optical disk 1. The emitted modulated laser light is collimated by a collimator 21 and applied to the magneto-optical disk 10 via an objective lens 24. The return light from the magneto-optical disk 10 is sent to the PBS 22 via the objective lens 24. Return light is PBS
22 and directed toward the half-wave plate 221 by the PB
In S222, the image is divided into two directions. The split light is guided to photodetectors 27a and 27b, respectively.

When perpendicularly magnetized in the same direction as the incident light depending on the information recorded on the magneto-optical disk 10,
The return light (transmitted light) becomes clockwise polarized light (Faraday rotation angle: -θF) due to the magneto-optical effect. Angle: + θF). The reproduction signal from the magneto-optical disk 10 changes according to these polarization angles. Here, it is known that the Faraday effect and the Kerr effect change according to the wavelength λ of the laser beam applied to the magneto-optical recording material of the magneto-optical disk 10. For example, magnetic garnet film formed on a glass (Bi _x Dy _3-x F
The magnitude of the Faraday effect with respect to the wavelength of the laser beam incident on e ₄ Al ₁ O ₁₂ ) has a relationship as shown in FIG.
Represent respectively the Faraday rotation angle of the transmitted light .theta.F, the ellipticity eta _F in FIG. 8. As shown in FIG. 8, by modulating the reproduction light in the wavelength region where the Faraday effect changes with respect to the wavelength, the magnitude of the reproduction signal also changes.

By utilizing the wavelength dependence of the magneto-optical effect shown in FIG. 8, the reproduction light is adjusted to a wavelength ± Δλ centered on the fundamental wavelength λ _0.
7D, the Faraday rotation angle fluctuates around the basic rotation angles -θF and + θF corresponding to the basic wavelength λ ₀ , as shown in FIG. 7D. This variation depends on the control signal shown in FIG. 7C, that is, the modulation wavelength. FIG. 7E shows a signal waveform obtained by the photodetectors 27a and 27b. Although this signal waveform depends on the Faraday rotation angle shown in FIG. 7D, it is affected by a decrease in the reproduction signal level as the recording magnetic domain becomes minute and electrical random noise due to signal processing. Therefore, noise is superimposed.

However, in the reproducing apparatus of the present invention, FIG.
The phase sensitive detection is performed by the detection circuit 33 shown in FIG. 7 using the control signal having the cycle shown in FIG. 7C as a reference signal, so that components not synchronized with the phase of the reference signal are removed. Therefore, a signal waveform from which the random noise component is removed as shown in FIG. Therefore, a reproduction signal can be obtained with excellent S / N even from the magneto-optical disk 10 on which high-density recording is performed by the minute magnetic domain.

Although the present invention has been described with reference to the embodiment, the present invention is not limited to the above-described embodiment. In the above embodiment, the magneto-optical effect that appears on the transmitted light by irradiating a modulated laser beam to a high-density recorded magneto-optical disk, that is, a case where reproduction is performed using Faraday rotation has been described. It can also be used in the case of reproducing using the magneto-optical effect appearing in the reflected light from the magneto-optical disk, that is, Kerr rotation.

Also, in the above embodiments, CDs and MOs have been described as examples of optical recording media. However, the reproducing apparatus and the reproducing method of the present invention are not limited to them, and
The present invention can be applied to various optical recording media such as a write-once optical recording medium such as R, an optical recording medium such as a read-only DVD and an MD, and a phase change recording medium.

[0032]

According to the reproducing apparatus for an optical recording medium of the present invention, the optical disk is irradiated with the reproduction light modulated by the wavelength modulation means by the control signal from the control means, and the detection means is detected from the reproduction signal from the return light. By detecting the signal component corresponding to the control signal using the control signal, the recording information can be separated and extracted from the noise. Therefore, the reproducing apparatus of the present invention can reproduce a high-density recording medium on which fine pits and magnetic domains are formed with good S / N without increasing the reproducing intensity of laser light.

According to the optical recording medium reproducing method of the present invention,
By modulating the wavelength of the reproduction light at a predetermined period and detecting the reproduction signal at the modulated period or a multiple thereof, random noise can be significantly reduced from the reproduction signal from minute pits or magnetic domains. Therefore, the reproducing method of the present invention can remarkably improve the S / N of the reproduced signal of the optical disk recorded at high density.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a first embodiment of an optical disk reproducing device according to the present invention.

FIG. 2 is a circuit diagram showing a schematic configuration of a detection circuit in the optical disk reproducing device of FIG.

FIG. 3 is a timing chart for explaining the operation of the optical disc reproducing apparatus shown in FIG.

4 is a graph showing the wavelength dependence of the amount of reflected light of an optical disk applied to the optical disk reproducing device of FIG.

FIG. 5 is a schematic configuration diagram of a second embodiment of an optical disk (MO) reproducing apparatus according to the present invention.

FIG. 6 is a circuit diagram showing a schematic configuration of a detection circuit of the optical disk (MO) reproducing apparatus shown in FIG.

FIG. 7 is a timing chart for explaining the operation of the optical disk (MO) reproducing apparatus shown in FIG.

8 is a graph showing a change in a magneto-optical effect with respect to a wavelength of incident light on a magnetic garnet film of a magneto-optical disk used in the optical disk (MO) reproducing apparatus of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical disk 2 optical header 3 signal processing unit 10 magneto-optical disk 20 laser modulation unit 33 detection circuit 33 a oscillator 34 PLL circuit 35 decoder

Claims (5)

[Claims] 1. An optical recording medium reproducing apparatus comprising: a light source for irradiating an optical recording medium with reproduction light; and reproducing a reproduction signal based on reflected light or transmitted light of the reproduction light from the optical recording medium. Wavelength modulating means for modulating the wavelength of the reproduction light, control means for controlling the wavelength modulation means so that the wavelength of the reproduction light is modulated at a constant period, and A reproducing apparatus for an optical recording medium, comprising: a detecting means for detecting based on a modulation period. 2. The apparatus according to claim 1, wherein the light source is a laser diode, and the wavelength modulating means is an external resonant mirror whose mirror position is variable or a dielectric whose dielectric constant is variable by an electric field applied between the resonators. 2. An apparatus for reproducing an optical recording medium according to claim 1. 3. The device according to claim 1, wherein the detection means is a filter that passes only a signal having a period twice as long as the predetermined modulation period or a lock-in amplifier that uses the predetermined modulation period as a reference signal. An optical recording medium reproducing apparatus according to claim 1. 4. A method for irradiating an optical recording medium with reproduction light and reproducing a reproduction signal based on reflected light or transmitted light of the reproduction light from the optical recording medium, wherein: Irradiating the optical recording medium while modulating the wavelength at a constant frequency, obtaining a reproduction signal by detecting reflected light or transmitted light of the reproduction light from the optical recording medium, and obtaining the reproduction signal based on the constant frequency. A method for reproducing an optical recording medium, comprising detecting. 5. The reproducing method for an optical recording medium according to claim 4, wherein a reproduction signal is obtained by using a change in reflected light intensity or a magneto-optical effect.