JP2000339741A - Optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an S/N ratio by setting light quantity efficiency to a maximum for two wavelengths in an optical pickup device that utilizes two optical wavelengths. SOLUTION: The device is provided with a laser light source having a wavelength λ1, a laser light source having a wavelength λ2 that is longer than the wavelength λ1, an optial system, which guides the light beams generated by the light sources onto the surfaces of optical recording media 15 and 15', and a light receiving element which photoelectrically converts the reflected light beams from the surfaces of the media 15 and 15'. A step shaped phase diffraction grating element 13 is provided in the optical system above. In the element 13, the number of steps is set to N1+1 or N2+1 where N1 is a maximum integer that does not exceed a rational number N=1/((λ2/λ1)-1) and N2 is a minimum integer that exceeds N and the amount of a step difference in one step is set to λ1/(n-1) where n is a refractive index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device. More particularly, the present invention relates to an optical pickup device having a plurality of laser light sources and reproducing an optical recording medium of a plurality of standards.

[0002]

2. Description of the Related Art An optical pickup device that reads information from an optical recording medium such as a CD (compact disk) and a DVD (digital video disk) using a laser light source is widely used. Since some of these optical recording media have different thicknesses, reading optical recording media of different substrate thicknesses using the same pickup device is
This is generally difficult because the objective lens is designed to correct spherical aberration for a certain thickness.

As means for dealing with this, it is known to reproduce optical recording media having different substrate thicknesses by using two laser light sources having different wavelengths, Japanese Patent Application Laid-Open No. Hei 10-334504, Japanese Journal of Art. Applied Physics Vol. 36, Part 1, No. 1B, pp. 460-466
It is described in FIG. 1 of the page. FIG. 5 is an explanatory diagram for explaining the optical pickup device described therein. When reproducing the optical recording medium at the wavelength λ1, the optical system 1 of λ1 is used.
The emitted light beam passes through the beam splitter 3 and the diffraction element 4, and forms a reading spot on the recording surface of the optical recording medium 6 having a thickness d1 by the objective lens 5. At this time, the diffraction element 4 is configured so as not to have a wavefront conversion function. When reproducing the optical recording medium at the wavelength λ2,
The light beam emitted from the optical system 2 of λ2 passes through the beam splitter 3 and the diffractive element 4 and passes through the objective lens 5 to a thickness d2.
A reading spot is formed on the recording surface of the optical recording medium 6 '. If an objective lens having a spherical aberration of 0 when the wavelength λ1 is used is used, a wavefront transformation that can correct the spherical aberration is required when λ2 is used. Therefore, the diffraction element 4 performs the wavefront transformation on λ2. It is designed to be. The reflected light from the optical recording element 6 or 6 ′ is
Is detected and processed by a not-shown photoelectric conversion element.

FIG. 6 illustrates the diffraction element 4 described in the above patent publication. This diffraction element is a three-step staircase phase type diffraction element, and the pattern is concentric. In the case of a light beam of wavelength λ1,
If the light is incident perpendicular to the surface, it will not undergo wavefront conversion anywhere. This is because the height relationship between the steps is designed so that the phase difference received by all the steps is an integral multiple of 2π. However, in the case of a light beam having the wavelength λ2, the phase difference deviates from the above value, so that the light beam undergoes wavefront conversion. Wavelength λ2
In order to eliminate spherical aberration, it is necessary to make the wavefront conversion the same in amount and in the opposite direction as the spherical aberration generated by the substrate thickness difference d1-d2.

[0005] For example, in the case of the optical pickup device described in the above-mentioned patent publication, a step-like diffraction element of step 3 as shown in FIG. 6 is used.
A laser light source having two wavelengths of nm is mounted, and the light amount efficiency of the diffraction element is a theoretical value.
At the time of 56.7%.

[0006]

The conventional optical pickup device has the following problems. At the time of reproduction, the light beam passes through the diffraction grating twice, so that the true light amount efficiency is the square of the one-way efficiency.
0% and 31.6% at λ2. Therefore, unless the semiconductor laser for λ2 has a rated output at least about three times that of the laser light source for λ1, the equivalent S / S
No N ratio was obtained. For this reason, the optical pickup device is expensive and has been increased in size. Therefore, an object of the present invention is to provide an optical pickup device which realizes a high S / N ratio with low power by increasing the light amount efficiency of a diffraction element.

[0007]

In order to solve the above-mentioned problems, the number of steps and the amount of steps of a stepped phase diffraction grating can be determined from the quantitative relationship between two wavelengths and the physical properties of the material of the diffraction grating. Unique values. In addition, since the light amount efficiency at the time of wavefront conversion becomes a value that can be regarded as 100% in practical use, a semiconductor laser having a normal output is used, and an inexpensive optical pickup device with a high SN ratio is configured. That is, the present invention provides a laser light source having a wavelength λ1 and a longer wavelength λ2.
A laser light source, an optical system for guiding a light beam emitted from these light sources to the surface of the optical recording medium, and an optical pickup device having a light receiving element for photoelectrically converting a reflected light beam from the surface of the optical recording medium, In a part of the optical system, when the maximum integer not exceeding rational number N = 1 / ((λ2 / λ1) −1) is N1, and the minimum integer exceeding N is N2, the number of steps is N1 + 1 or N1 + 1. N2 + 1, and the step amount per step is λ1
The conventional problem is solved by an optical pickup device comprising a step-like phase diffraction grating element having a refractive index n of / (n-1). Here, the step amount is λ1 / (n
The reason for -1) is to make the light intensity efficiency of the step-like diffraction grating element approximately 100% with respect to the wavelength λ1. The reason why the number of steps is set to N1 + 1 or N2 + 1 is that the wavelength λ
This is in order to obtain the maximum light intensity efficiency with respect to 2. In this case, the step-like phase diffraction grating element reduces the spherical aberration with respect to λ2 caused by the difference between the material thickness of the optical recording medium reproduced using the wavelength λ1 and the base material thickness of the optical recording medium reproduced using the wavelength λ2. Preferably, it is shaped so as to adjust the wavefront transformation for correction.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an optical pickup device according to the present invention. The configuration of the optical system itself is almost the same as the conventional optical pickup device. The laser light source used was a wavelength λ1,
GaInP-based semiconductor laser of 655 nm / rated output 5 mW, GaAlAs-based / 785 nm-wavelength λ2
It is a semiconductor laser with a rated output of 5 mW. The optical paths of the optical system of λ1 and the optical system of λ2 are combined by the beam splitter 12, but the beam splitter film 12 is a dielectric multilayer film, which transmits 100% of the light beam of the wavelength of λ1 and has the wavelength of λ2. One that reflects a light beam by 100% was used. The light beam passes through the diffraction element 13 and is condensed on the optical recording media 15 and 15 ′ by the objective lens 14. The optical recording medium is a medium 1 having a substrate thickness of 0.6 mm at a wavelength of 655 nm.
5 was a medium 15 'having a substrate thickness of 1.2 mm at a wavelength of 785 nm. The 655 nm light beam is
As the zero-order light, and behaves as if no diffraction element exists. The 785 nm light beam is
Are diffracted as + 1st order light and wavefront transformed.

FIG. 2 is an explanatory view of a diffraction element. The diffraction element 13 is a step-like phase type diffraction element. The pattern provided for the wavefront conversion can realize an object-image relationship that minimizes the residual with respect to the spherical aberration having a substrate thickness difference of 0.6 mm. Next, the number of stages is determined by putting the numerical value of the wavelength in the above equation, and 1 / ((λ2 / λ1) −1) = 1 / (785/655)
Since 1) = 5.04, the number of steps is appropriately 6 or 7 obtained by adding 1 to 5 or 6, which is the nearest integer of 5.04. If a different number of steps is selected, the remainder of the phase difference of 2π given to the light of λ2 is biased to a part of the range from 0 to 2π, so that the diffraction efficiency of the + 1st-order light is significantly deteriorated. I do. If possible, it is better to choose a closer integer. In this embodiment, the number of steps is 6, and the refractive index of glass is 1.52. The step amount is λ1 / (n−1) = 0.655 / (1.52-1) = per step
1.26 μm. When the deviation from this amount is large,
Attention must be paid to the fact that the light intensity efficiency of the zero-order light is significantly deteriorated. FIG. 3 shows a cross-sectional view of this step shape. As shown in the figure, the shape is such that the stepped portion faces the outside of the circle.

The light intensity efficiency of the diffraction element is as follows. At a wavelength of 655 nm -3 order diffracted light -1 order diffracted light 0 order light +1 order diffracted light + 3rd order diffracted light 0% 0% 100% 0% 0% At a wavelength of 785 nm -3 order diffracted light -1 order diffracted light 0 order light +1 order diffracted light +3 Next order diffracted light 0% 0% 0% 91% 0%

As a diffractive element having a large amount of + 1st-order diffracted light and a small amount of 0th-order light, a blazed diffraction grating having a sawtooth-like cross section is widely known. Theoretically, the first-order diffraction efficiency is 100%. However, it should be noted that this cannot be applied to the optical pickup device having the above configuration. The reason is that high diffraction efficiency may be obtained at a wavelength of 785 nm, but only about 4% of 0-order light can be obtained at a wavelength of 655 nm.

The step-like diffraction grating (hologram) has a concentric annular shape. FIG. 4 is an explanatory diagram of a method for determining the shape of the concentric annular zone. As shown in (a), the objective lens 20 is designed such that a base material 22 having a thickness of 0.6 mm has no aberration when a parallel light beam enters. However, when the base material 23 having a thickness of 1.2 mm is used, spherical aberration occurs. In such a case, since there is an object point 24 and an image point 25 which have the object-image relationship closest to the aberration-free, the diffraction element 21 emits a parallel light beam from the object point 24 as shown in FIG. By providing a wavefront converting effect of converting the wavefront into a wavefront having a thickness of 1.2 mm, a substrate having a thickness of 1.2 mm is made aberration-free. When an example of the radius of the annular zone is shown in units of μm,
0.000, 231.042, 326.745, 40
0.182, 462.093, 516.639, 56
5.952, 611.302, 653.513, 69
3.159, 730.658, 766.325, 80
0.405, 833.093, 864.547, 89
4.896, 924.250, 952.700.
In this way, the concentric annular zone has a narrower pitch toward the outer side. Generally, such a diffraction grating can be easily designed using the principle of a hologram.

[0013]

The present invention has the following effects. The optical pickup device according to the present invention has the following effects. At the time of reproduction, even if the light beam passes through this diffraction element, the true light amount efficiency is 100 for both wavelengths.
Even if it was regarded as%, it was at a level with no practical problem. For this reason, the rated output of the semiconductor laser is 5
Even with the use of mW, a conventional level of S / N was obtained.
For this reason, the optical pickup device was inexpensive and small.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical pickup device of the present invention.

FIGS. 2A and 2B show a diffraction grating used in the optical pickup device of the present invention, wherein FIG.
(B) is a plan view.

FIG. 3 is an enlarged view of a diffraction grating according to the embodiment of the present invention.

4A and 4B are explanatory diagrams of a method of determining the shape of a concentric annular zone of a diffraction grating (hologram) according to the present invention, and FIG. 4A illustrates an aberration-free optical system for an optical recording medium having a first thickness. Show,
(B) shows a method of correcting aberrations using a concentric annular zone of a diffraction grating for an optical recording medium having a second thickness.

FIG. 5 is a schematic view of a conventional optical pickup device.

6A and 6B show a diffraction grating of a conventional optical pickup device, wherein FIG. 6A is a cross-sectional view taken along line BB of FIG. 6B, and FIG. 6B is a plan view.

[Explanation of symbols]

 Reference Signs List 10 λ1 optical system 11 λ2 optical system 12 Beam splitter 13 Step diffraction grating (hologram) 14 Condensing lens 15, 15 'Optical recording medium

Continuation of the front page (72) Inventor Toru Kineri F-term (reference) 5D119 AA41 AA43 BA01 DA01 DA05 EC01 EC47 FA05 JA09 JA43 JA47 JB03 1-13-1 Nihonbashi, Chuo-ku, Tokyo

Claims (2)

[Claims] 1. A laser light source having a wavelength λ1, a laser light source having a longer wavelength λ2, an optical system for guiding a light beam emitted from these light sources to an optical recording medium surface, and a reflected light beam from the optical recording medium surface In the optical pickup device having a light receiving element for performing photoelectric conversion of the following, a maximum integer not exceeding a rational number N = 1 / ((λ2 / λ1) −1) is set to N1, Is the minimum integer exceeding N2, the number of steps is N1 + 1 or N2 + 1, and the step amount per step is λ1
An optical pickup device comprising a step-like phase diffraction grating element having a refractive index n of / (n-1). 2. The method according to claim 1, wherein the step-like phase diffraction grating element has a wavelength λ.
Thickness and wavelength λ2 of the optical recording medium reproduced using
The optical pickup device according to claim 1, wherein the optical pickup device is shaped so as to adjust a wavefront transformation for correcting a spherical aberration caused by a difference in substrate thickness of an optical recording medium to be reproduced by using the optical recording medium.