JPH05332879A - Measuring method for diameter of light spot of optical head - Google Patents

Abstract

PURPOSE:To obtain a method for measuring the diameter of the spot of light of an optical head, by which the measurement becomes easy, the measuring time can be shortened, errors are made few, and general purpose utilization can be realized. CONSTITUTION:Light is emitted from an optical head, and the light spot is formed on the surface of a recording medium. The amount of jitter corresponding to the interference between codes, which is generated in response to the diameter of the light spot, is computed beforehand by numerical simulation, and a map is prepared. The waveform of the regenerated signal is actually measured from the light, which is reflected from the surface of the recording medium. The actually measured value of the interference jitter between the codes is obtained based on the waveform of the regenerated signal. The amount of the jitter is compared with the amount of the jitter on the map, and the diameter of the light spot is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head diameter measuring method for measuring the diameter of a light spot formed on a recording medium by an optical head used in an optical recording apparatus or the like.

[0002]

2. Description of the Related Art Most of the methods for measuring the light spot diameter of an optical head that have been studied so far are so-called static measuring methods in which the optical head itself is used for measurement. Among them, the most general measuring method is a method in which a laser beam focused on a recording medium is shielded by a movable shielding object and a light spot diameter is measured from a change in the amount of light transmitted at that time. As the shielding object, one having a pinhole or a straight edge is used, and the simplest of the latter is a knife edge.

In this knife edge method, the laser light is shielded by a knife edge that moves perpendicularly to the optical axis of the laser light, and at this time, the total amount of transmitted light of the laser light transmitted through the shielding member is measured, The light spot diameter is obtained from the amount of change. The total transmitted light amount P (a) detected at this time is expressed by the following equation.

[0004]

[Equation 1]

A method for obtaining the spot diameter from the temporal change (inclination) of the light quantity P (a) and a method for electrically differentiating the transmitted light signal to measure the spot diameter have been proposed and studied. .. Still another method is C
There is also a method of measuring the spot diameter using an image pickup device such as a CD.

On the other hand, a dynamic measurement method that is not a static measurement method,
In other words, the method of confirming / measuring the spot diameter with the optical head mounted on a device has not been studied much, but to give an example, a method of estimating the spot diameter using the amplitude difference of the recording / reproducing signal is used. Is being considered.

[0007]

However, the above-mentioned conventional method for measuring the light spot diameter has the following problems. The above knife edge method is very simple in principle, but since the accuracy of the moving distance of the knife edge is the most important factor in determining the measuring accuracy, a laser interferometer or the like is required to accurately measure the moving amount of the knife edge. Must be used. For this reason, it requires a great deal of effort to maintain the measurement environment. Also, the measurement time will be long due to the time required for the setting.

In the case of the method using the amplitude difference of the recording / reproducing signal, since it depends only on the signal reproduced on the time axis, it depends on the data pattern such as the sensitivity unevenness of the recording medium, the light spot control fluctuation, and the change due to the environmental temperature change. If there is no edge shift, the measurement accuracy will be very low. The same can be said for random noise generated by the device itself.

To deal with such a disturbance that occurs during recording and reproduction, the number of times of measurement is actually increased and the average thereof is calculated. However, the average spot size still exists without removal of each fluctuation component, and evaluation of the calculated value requires something like many years of experience, which lacks versatility.

The present invention has been made in view of the above points, and an object thereof is to provide a light spot diameter measuring method for an optical head, which is easy to measure, can shorten the measuring time, has few errors, and is versatile. To do.

[0011]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a method for measuring a light spot diameter of an optical head, which measures the diameter of a light spot formed on the surface of a recording medium by light emitted from the optical head. In the above, a map is created by previously calculating a jitter amount corresponding to intersymbol interference that occurs according to the diameter of the light spot by a numerical simulation, and a reproduced signal waveform is actually measured from the light reflected on the recording medium surface. Then, the actually measured value of the intersymbol coherence jitter was obtained from the reproduced signal waveform, and the actually measured jitter amount was compared with the jitter amount of the map to obtain the light spot diameter.

Further, the present invention provides a method for measuring a light spot diameter of an optical head, wherein the light emitted from the optical head measures the diameter of a light spot formed on the surface of the recording medium. The reproduced signal waveform is actually measured from the reproduced signal waveform, the measured value of the signal amplitude, the signal phase difference, and the intersymbol interference jitter is obtained from the reproduced signal waveform, and the impulse response distribution of the signal waveform is obtained from the reproduced signal waveform and the signal amplitude. The line image intensity distribution of the light spot whose distortion is corrected is obtained from the distribution and the signal phase difference, and the light spot diameter is derived from the line image intensity distribution of the light spot and the intersymbol coherence jitter.

[0013]

By using the above measuring method, the light spot diameter is not measured statically, but by analyzing the jitter amount of the recording / reproducing signal of the light emitted from the optical head incorporated in the device,
The effective light spot diameter in a dynamic state can be easily measured.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The gist of the present invention is to actually measure a reproduced signal waveform from the light reflected on an information recording surface, and obtain intersymbol interference jitter from the reproduced signal waveform. In the case of the first invention of the present application, the jitter amount corresponding to the intersymbol interference that occurs depending on the diameter of the light spot is calculated in advance by numerical simulation to create a map, and the map and the actually measured intersymbol interference are calculated. The optical spot diameter is measured by comparing the characteristic jitter. On the other hand, in the case of the second invention of the present application, the optical spot diameter is directly measured from the actually measured reproduction signal waveform and intersymbol interference jitter.

[First Invention] First, the intersymbol interference jitter used in the present invention will be described. The factors that theoretically determine the spot diameter to be focused on the surface of the recording medium are the wavelength λ of the laser light source forming the optical system and the numerical aperture NA of the objective lens for focusing the laser light. That is, the spot diameter (radius) is calculated by the following formula. 0.61 × (λ / NA) (2)

The spot diameter determined by the equation (2) is the size of the first annular zone of the light intensity distribution of the spot, which is generally the radius of what is called an Airy disk.
Then, as shown in FIG. 2, if the shortest mark length l of the recorded mark (recording magnetic domain) 83 becomes shorter than the radius of the light spot 81, intersymbol interference occurs there.
In the currently marketed optical recording devices, this occurs near the innermost circumference where the mark spacing is narrow. In the figure, reference numeral 85 is a recording land, and 87 is a tracking guide.

The influence of this intersymbol interference appears in the form of the amount of jitter on the reproduced signal and causes signal deterioration. In FIG. 3, the distance between the marks (recording magnetic domains) 83 is 1.56 μ.
m is a diagram showing the state of the light intensity I when the length of the mark 83 is 0.76 μm and the radius of the light spot is 0.876 μm. Note that the jitter amount at this time is the jitter amount =
It became ± 5.7 nsec.

The jitter due to the intersymbol interference is a value that is indispensable for designing the optical system and the detection circuit, and is usually grasped at the basic concept stage using a numerical simulation or the like. Next, an example of this numerical simulation is shown in FIG.

As shown in the figure, first, the light source intensity distribution I (intensity distribution before entering the objective lens) of the laser light of the reproducing and imaging system is Fourier-transformed to obtain a light spot line image after passing through the objective lens. Obtain the intensity distribution h (x). The light spot diameter is obtained by applying the above-mentioned formula (2) to the light spot line image intensity distribution h (x).

Next, this light spot line image intensity distribution h (x) and a random input signal g (x) are expressed by the following equation (3).
(Varies depending on whether there is a mark on the information recording surface)
By calculating the junction product (convolution) with and substituting the relationship of the light intensity distribution (knife edge energy distribution) due to the knife edge effect, the reproduced signal waveform f
(X) is calculated.

[0021]

[Equation 2]

Here, the input signal g (x) defines the repetition of the gate signal as expressed by the following equation (4).

[0023]

[Equation 3]

Although the reproduction signal waveform is obtained by using the knife edge energy distribution here, when obtaining the light spot diameter of the optical system for the magneto-optical medium, instead, the incident straight line by the polar Kerr effect is used. A change in the amount of light that results from a slight rotation of the wavefront of the polarized light may be used.

The intersymbol interference jitter is obtained by FFT (Fourier transfer analysis) of the reproduced signal waveform.

As described above, the intersymbol coherence jitter can be calculated from the light source intensity distribution I.

In the present invention, the amount of jitter corresponding to the intersymbol interference generated according to various light spot diameters is obtained in advance by the above numerical simulation and a map is prepared.

When actually determining the light spot diameter of the optical head, first, the reproduction signal waveform is measured from the reflected light of the light emitted from the optical head on the recording medium surface, and the time is calculated from the reproduction signal waveform. The total jitter (jitter of the entire apparatus including the jitter other than the intersymbol interference jitter) is obtained by an interval analyzer, and the intersymbol interference jitter is obtained from the total jitter by the following method. Then, the map is applied to the intersymbol interference jitter to obtain the spot diameter of the light spot.

Here, a method of obtaining the intersymbol interference jitter from the total jitter will be described. The measured total amount of jitter includes not only the jitter due to intersymbol interference, but also the jitter due to random noise of the entire device and the jitter not depending on the data pattern.
These must be deducted as shown in. As the method, the following can be experimentally considered.

Random noise can be extracted directly from the fluctuation of the reproduced signal due to this noise component. It is possible to measure the magnitude of noise by recording / reproducing a signal of a certain frequency, sampling a large number of amplitude values at a specific phase of the reproduced signal, and obtaining the standard deviation, and obtain the amount of jitter proportional to the amplitude. .. A time interval analyzer may be used to measure the fluctuation of the reproduced signal.

Also in the measurement of jitter that does not depend on the data pattern, the jitter amount can be calculated by measuring the fluctuation of the reproduction signal pulse length using a time interval analyzer and performing frequency analysis.

As shown in FIG. 4, when the respective jitter amounts thus obtained are excluded from the total jitter amount, a measured value of the jitter amount due to pure intersymbol interference can be obtained. Then, the actual measured value of the jitter amount is compared with the map of the jitter amount calculated by the numerical simulation to obtain the actual light spot diameter.

The light spot diameter thus obtained is not a static value, but a value in the operating state when the optical head is incorporated in the apparatus.

[Second Invention] Next, a method for directly measuring the optical spot diameter from the actually measured reproduced signal waveform and intersymbol interference jitter will be described.

FIG. 5 is a diagram showing the procedure for measuring the light spot diameter according to one embodiment of the present invention. As shown in the figure, first, a reproduced signal waveform is actually measured from the reflected light on the recording medium surface of the light emitted from the optical head, and the signal amplitude and the signal phase difference of each code are extracted. Also, the measured value of intersymbol interference jitter is obtained from this reproduced signal waveform using a time interval analyzer. At this time as well, since the total jitter amount is obtained as described above, the jitter due to random noise or the jitter not depending on the data pattern is obtained by the same method as described above, and these are subtracted to obtain the intersymbol interference jitter. Calculate the measured value.

Next, the impulse response distribution of the waveform is obtained from the reproduced signal waveform using the signal amplitude. This impulse response distribution corresponds to the knife edge energy distribution.

Then, the line image intensity distribution of the light spot is calculated from the impulse response distribution of the waveform. As the calculation method, the inverse calculation of the junction product (convolution) described in the first invention may be performed. Here, since the signal phase difference can detect the line image intensity distribution distortion caused by the coma aberration of the light spot due to the disc tilt, the line image intensity distribution is corrected by the signal phase difference to improve the measurement accuracy of the light spot. Can be made

The light spot diameter is derived from the line image intensity distribution of the light spot and the intersymbol coherence jitter.

The light spot diameter thus measured is not a static value, but a value in an operating state when the optical head is incorporated in the apparatus.

[0040]

As described above in detail, the optical spot diameter measuring method for an optical head according to the present invention has the following excellent effects. The effective light spot diameter in a dynamic state can be measured by analyzing the amount of jitter of the actual reproduction signal instead of statically measuring the light spot diameter.

The numerical simulation used as the basis of the present invention is a basic work that is indispensable in designing the optical head, and it is not necessary to specifically perform the numerical simulation in order to obtain the light spot diameter.

As compared with the knife edge method or the conventional method in which an image pickup device and a microscope are combined, it is not necessary to manage a highly accurate measurement jig or a highly accurate measurement environment, and the time required for measurement can be shortened.

Since a measuring device such as a jig does not have a precision standard, it is possible to perform a measurement with a small error with a small number of measurements.

Since the light spot diameter is obtained by using the reproduction signal, the cost can be reduced without the need for a dedicated device.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of a method for calculating a jitter amount due to intersymbol interference.

FIG. 2 is a diagram showing a relationship between a light spot 81 and a mark (recording magnetic domain) 83.

[FIG. 3] The spacing between marks (recording magnetic domains) 83 is 1.56 μ.
m is a diagram showing the state of the light intensity I when the length of the mark 83 is 0.76 μm and the radius of the light spot is 0.876 μm.

FIG. 4 is a diagram showing a method of calculating a jitter amount due to intersymbol interference from a total jitter amount.

FIG. 5 is a diagram showing a procedure for measuring a light spot diameter according to an embodiment of the second invention.

Claims (2)

[Claims] 1. A light spot diameter measuring method of an optical head for measuring a diameter of a light spot formed on a surface of a recording medium by light emitted from the optical head, in which intersymbol interference caused depending on the diameter of the light spot is measured. The corresponding jitter amount is calculated in advance by numerical simulation to create a map, the reproduced signal waveform is actually measured from the light reflected on the surface of the recording medium, and the actually measured value of intersymbol coherence jitter is obtained from the reproduced signal waveform. And the amount of jitter is compared with the amount of jitter in the map to obtain the diameter of the light spot. 2. A light spot diameter measuring method of an optical head for measuring a diameter of a light spot formed on a recording medium surface by light emitted from the optical head, wherein a reproduction signal is obtained from the light reflected on the recording medium surface. The waveform is actually measured, the measured value of the signal amplitude, the signal phase difference, and the intersymbol coherence jitter is obtained from the reproduced signal waveform, and the impulse response distribution of the signal waveform is obtained from the reproduced signal waveform and the signal amplitude. Obtaining a line image intensity distribution of a light spot whose distortion is corrected from the signal phase difference, and further deriving the light spot diameter by the line image intensity distribution of the light spot and the intersymbol coherence jitter. Spot diameter measurement method.