JPH03242827A - Information recording/reproducing system - Google Patents

Abstract

PURPOSE:To reduce mis-detection, and to improve reliability by generating the second higher harmonic of a phase matched state at a non-linear optical material or a non-linear optical material layer by a two-light beam interference method, and recording/reproducing information by this secondary higher harmonic wave. CONSTITUTION:The second higher harmonic of the phase matched state by the two-light beam interference method using the non-linear optical material or the non-linear optical material layer 2 is generated. Namely, by irradiating the surface of a base board 1 or the surface of the non-linear optical material 2 with two beams of incident light so that said two beams intersect each other at said surface, the secondary higher harmonic wave is generated to the intermediate direction of an angle formed by two dominant waves by secondary non- linear optical effect. Thus, each optical path is separated, and the mistake of information detection is prevented, and the improvement of the reliability is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an information recording/reproducing method useful for optical discs, optical cards, optical integrated circuits, and the like.

[Prior art] Conventionally, there have been information recording media using nonlinear optical materials.
Its function is to change the wavelength λ of the incident laser light to λ/2 (second harmonic) using a nonlinear optical material layer, and perform recording/reproduction using the second harmonic. The main focus was on improving the recording surface density by changing the wavelength.

In the method of recording/reproducing information using the second harmonic of a nonlinear optical material, the wavelength λ of the optical recording light source is set to λ/2.
Therefore, the optical recording density can be quadrupled (higher density).

■If you want to obtain a spot comparable to that of an optical recording light source, you can halve the NA of the objective lens used, so you can use a lens with a low angle of view, making it extremely inexpensive and lightweight. (Inexpensive and high-performance pickup) (However, in this case, high density cannot be achieved).

■The nonlinear optical material layer has a higher refractive index than the recording layer, and the wavelength-converted light travels from a layer with a high refractive index to a layer with a low refractive index, so when performing transmission type reading, even if there is a ridge around the pit, it will scatter. Since the influence of this is reduced, information can be read with a high C/N ratio. Because of these advantages, information recording media containing nonlinear optical materials are of great interest, although the SHG conversion efficiency is currently not very high.

[Problem to be solved by the invention] However, in the conventional method, an optical recording laser (fundamental wave:
λ) and the second harmonic generated by the nonlinear optical material (λ
/2) are on the same optical path regardless of whether the reading method is reflective or transmissive (see FIG. 1), and therefore there is a problem in that it is difficult to separate the fundamental wave and the second harmonic. In other words, since the SHG efficiency is usually less than a few percent and the second harmonic is weak, a beam splitter for the second harmonic and a mirror for total reflection of the fundamental wave must be used, making the drive expensive. Further, even if the above-mentioned second harmonic beam splitter and fundamental wave total reflection mirror are used, it may actually happen that the detector picks up the fundamental wave. Therefore, if we insist on blocking the fundamental wave, there is a fear that even the important second harmonic will be attenuated, leading to a decrease in the reliability of the detection signal.

Under these circumstances, the present invention aims to induce a second harmonic (essentially a laser beam for recording/reproducing) and a second harmonic generated in a nonlinear optical material or an information recording medium having a nonlinear optical material layer. The object of the present invention is to provide an information recording/reproducing method that separates the optical path from the fundamental wave that causes the detection, reduces false detections, and improves reliability.

[Means for Solving the Problems] As a method for achieving the above-mentioned problems, in the present invention, the second harmonic of the phase matching state (Δlk-〇) is obtained by two-beam interferometry using a nonlinear optical material or a nonlinear optical material layer. Make waves. Specifically, as shown in Figures 2 and 5, when two incident beams are irradiated so as to intersect on the substrate surface or nonlinear optical material surface, the angle between the two fundamental waves changes due to the second-order nonlinear optical effect. Second harmonic is generated in the intermediate direction (NoncollinearSHG)
It takes advantage of this fact. Here, by selecting a nonlinear optical material and changing the angle formed by the two fundamental waves, the relationship between wave number vectors (law of conservation of momentum) can be determined by Ik
＋
rnz / λ1 + l-4yrnz/2/λ Phase matching condition "" cos θ1+-L2? 2"L"-e o sθ
2λ λ −1Ll construction. . 8 θ.

λ 1″LLLSinθ + 1111sinθ2λ
λ -4πn 8ioθ3 λ n, refractive index n for two fundamental water waves, /2: refractive index for the second harmonic λ: wavelength of the fundamental wave in vacuum can be set to Δlk -1 to 3-1 + -1 to 2
-0.

That is, a state of phase matching is achieved, and the second harmonic can be generated only in the intermediate direction of the fundamental wave as described above. For example, if the fundamental wave is incident symmetrically with respect to the normal direction to the substrate surface, the second harmonic can be generated in the normal direction to the substrate surface. Therefore, by using the phase-matched second harmonic by this two-beam interferometry, it is possible to take completely separate optical paths for the fundamental wave and the second harmonic. Detection of information reproducing light becomes easier, and its reliability is further improved. Furthermore, since the second harmonic is used for recording/reproducing, high-density recording is possible.

This method can be widely applied to information recording media that use second harmonics using nonlinear optical materials.

Nonlinear optical materials that can be used in this recording/reproduction method include inorganic crystals, organic crystals, Any of crystals, polymers, polymer liquid crystals, polymer compositions, and polymer liquid crystal compositions may be used as long as they have an angle that allows phase matching.

As a recording film, it is transparent to the fundamental wave and
Any material that is not transparent (absorption region) to harmonics may be used. If a recording film with such characteristics is used, recording/reproduction can be performed using only the second harmonic without recording on the recording film using the fundamental wave after passing through the nonlinear optical material layer.

[Example] Examples of the present invention will be described below.

As shown in FIG. 6, a recording layer 3 is placed on a disc-shaped glass substrate 1.
(Copper phthalocyanine) was vacuum-deposited to produce an information recording medium. This information recording medium is made of nonlinear optical material m −N
A (meta-nitroaniline) single crystal 1
1 (thickness: about 3 mm) were fixed at a distance of about 0.5 cm. As a laser, Nd:YAG laser (wavelength 1.064μ
m) was used to cause two beams to interfere with the m-NA single crystal (however, the focus was adjusted to match the recording film). Mo and φ+
-98,4° φ2 = gt, eo can achieve 90° phase matching (φ0-90', i.e. a state in which the second harmonic can only be generated in the direction between the two fundamental waves). When we set the angle of the fundamental wave so that it actually satisfies the above angle determined by calculation, and detected the light generated from the m-NA single crystal using a monochromator, we found that only in the direction midway between the two fundamental waves. 2nd harmonic (0,5
A peak of 32 μm> was observed, confirming that the optical paths of the fundamental wave and the second harmonic were separated.

With the above settings, this information recording medium has a rotation speed of 180° rpm.
2M by modulating the laser beam under the condition of recording radius 30+n+n
Due to the recording signal of Hz, the above Nd:
Information was recorded using a YAG laser.

SEM photographs show the pit areas when recording with the fundamental wave (when the nonlinear optical material is removed in Figure 6) and when recording with the phase-matched second harmonic of the present invention. It was confirmed that the pit area according to the method of the present invention is about 174 compared to the conventional method.

[Effects of the Invention] As explained above, each optical path can be separated by the method of the present invention in which phase-matched second harmonics are generated only in the intermediate direction between two fundamental waves using two-beam interference. Therefore, errors in information detection can be prevented and reliability can be improved. Furthermore, since this recording/reproducing method uses second harmonic waves, the density of the recording surface is four times that of the conventional one, and high density can be achieved (an inexpensive pickup can be provided).

[Brief explanation of drawings]

Figure 1 is a diagram explaining the relationship between the fundamental wave and the second harmonic in an information recording medium using a nonlinear optical material layer, and Figure 2 is a two-beam interferometry method for the information recording medium shown in Figure 1. Figure 3 is a diagram explaining the generation of the second harmonic by
Figures 4(a) and 4(b), which illustrate the relationship between the wave number vectors of the fundamental wave and the second harmonic in two-beam interferometry, show the generation of the second harmonic in a phase-matched state using the method of the present invention. Figure 5 is a diagram explaining the relationship between the fundamental wave and the second harmonic in an optical information recording/reproduction method using nonlinear optical materials, and Figure 6 is a diagram explaining the relationship between the fundamental wave and the second harmonic in an optical information recording/reproducing method using a nonlinear optical material. FIG. 3 is a diagram illustrating the generation of second harmonics. l...Substrate, 2...Nonlinear optical material layer, 8...Recording layer, 4...Fundamental wave (λ), 5...Second harmonic (
λ/2), 6...Laser, 7...Beam splitter (for fundamental wave), 8...Total reflection mirror (for fundamental wave), 9...Total reflection mirror (for second harmonic wave) ), IO...
Detector, 11... nonlinear optical material.

Claims (1)

[Claims] Two-beam interferometry is a method of recording/reproducing information using a nonlinear optical material or an information recording medium having a nonlinear optical material layer in an optical disc that uses a laser beam to record and reproduce information on a recording layer. An information recording/information recording system characterized in that a phase-matched second harmonic is generated in the nonlinear optical material or the nonlinear optical material layer (Noncollinear SHG), and information is recorded/reproduced using the second harmonic. Playback method.