JP2010197450A - Liquid crystal optical element and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive liquid crystal optical element capable of enhancing light resistance without deteriorating light utilizing efficiency to a blue-purple laser to be short wavelength light and to provide an optical pickup device including the same. <P>SOLUTION: In the liquid crystal optical element 100 having a construction wherein a liquid crystal 5 is interposed between two transparent substrates 1a and 1b having organic alignment layers 3a and 3b respectively formed on the inner sides thereof, an additive 6 catching free radicals primarily generated by irradiating the liquid crystal 5 with a blue-purple laser beam or a peroxide decomposing agent for making a peroxide secondarily generated from the radicals innocent is incorporated in the liquid crystal 5 and thereby degradation reaction by the blue-purple laser beam can be made to be hardly generated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal optical element having light resistance to a blue-violet laser beam and an optical pickup device equipped with the liquid crystal optical element.

  Along with the increase in the capacity of information, while the improvement in the recording capacity of the optical disk is required, the wavelength of the laser light source of the optical pickup has been shortened. That is, in addition to the conventional CD wavelength of 780 nm and DVD wavelength of 650 nm, a blue-violet laser having a wavelength near 405 nm in the near ultraviolet wavelength region has come to be used. In this blue-violet laser, optical components mounted on the optical pickup are required to have higher light resistance than before, and it is expected that the light intensity of the blue-violet laser will increase as reading and recording speeds increase.

  As one of optical components mounted in the optical pickup device, there is a liquid crystal optical element that controls the wavefront and the amount of light of a laser beam by driving a liquid crystal. This liquid crystal optical element is configured by sandwiching a liquid crystal between two transparent substrates in which a transparent conductive film and an organic alignment film made of polyimide are sequentially formed. If the liquid crystal optical element using this organic alignment film is continuously irradiated with the blue-violet laser emitted from the laser light source, the irradiated alignment film part is changed by the photoreaction, and as a result, the alignment state of the liquid crystal molecules is disturbed. Problem arises. (This disorder of alignment of liquid crystal molecules is referred to as “deterioration” hereinafter.) The time until light is judged to be deteriorated (light resistance time) varies depending on the intensity of the laser beam, and tends to become shorter as the intensity increases.

  In order to solve the above problems, a liquid crystal optical element using liquid crystal in which unsaturated bonds (tolan bonds, ester bonds, etc.), which are factors of light absorption, are reduced is placed in the optical path between the laser light source and the objective lens. An arranged optical pickup device has been proposed (see Patent Document 1 below).

Japanese Unexamined Patent Publication No. 2003-90990 (page 2-4, FIG. 1-2)

  In general, an organic substance such as a resin undergoes a photodegradation reaction represented by the following reaction formulas (Chemical Formula 1) and (Chemical Formula 2) by light irradiation.

  In order to suppress this photodegradation reaction, a method of adding an additive of a UV absorber, a complex ligand, a light stabilizer, and a peroxide decomposer to a resin is known.

As a liquid crystal material of the liquid crystal optical element described in Patent Document 1, a liquid crystal in which unsaturated bonds other than an aromatic ring (trans bond, ester bond, etc.) are reduced is in the near ultraviolet wavelength region of laser light emitted from a laser light source. Although it is effective in terms of reducing light absorption, a general liquid crystal material used in a display liquid crystal panel cannot be used, resulting in an expensive element.

  In addition, nematic liquid crystal mainly used for the liquid crystal optical element causes a decrease in light utilization efficiency and coloring when a general additive mixed with a resin is used. Specifically, when an amount of UV stabilizer that has an effect on lightfastness is added to the liquid crystal, the light absorption spectrum of the UV stabilizer partially overlaps the spectrum of the blue-violet laser, so that the light emitted from the laser light source A part of the light is absorbed by the liquid crystal optical element, resulting in a problem that light utilization efficiency deteriorates. Further, when an amount of the complex ligand having an effect on the light resistance time is added to the liquid crystal, the liquid crystal is colored by the dissolved complex ligand. Due to this coloring, the amount of light passing through the liquid crystal optical element is reduced, resulting in a problem that the light use efficiency is lowered.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive liquid crystal optical element capable of improving light resistance without deteriorating light utilization efficiency with respect to a blue-violet laser, which is a short wavelength light, and an optical pickup device equipped with the liquid crystal optical element. Is to provide.

  In order to achieve the above object, the liquid crystal optical element and the optical pickup device of the present invention basically adopt the following configuration.

  The present invention is primarily generated by irradiating a liquid crystal with blue-violet laser light in a liquid crystal optical element that sandwiches liquid crystal between two transparent substrates having an alignment film formed on the inside and transmits laser light. The liquid crystal contains an additive for trapping free radicals or an additive for detoxifying peroxides secondarily generated from radicals.

  The additive for scavenging free radicals is preferably HALS (Hindered Amine Light Stabilizer).

  Moreover, it is preferable that the additive for detoxifying the peroxide is a phosphorus-based antioxidant.

  Preferably, the wavelength region of the laser light transmitted through the liquid crystal optical element is 400 to 800 nm.

  Moreover, an optical pickup device of the present invention is characterized by mounting the above-described liquid crystal optical element.

  According to the present invention, there is provided an inexpensive liquid crystal optical element capable of improving light resistance without deteriorating light utilization efficiency with respect to a blue-violet laser that is short wavelength light, and an optical pickup device equipped with the liquid crystal optical element. It becomes possible to do.

  According to the present invention, it is possible to transmit laser light having a wavelength region of 400 to 800 nm including blue-violet laser light to the liquid crystal optical element. Therefore, the liquid crystal optical element of the present invention and the optical pickup device equipped with the liquid crystal optical element exhibit sufficient effects even for blue-violet laser light, three-wavelength laser light that unifies all optical disc standards BD, DVD, and CD. it can.

It is sectional drawing of the liquid crystal optical element which concerns on embodiment of this invention. It is a figure which shows content of the additive mixed in the liquid crystal of the liquid crystal optical element which concerns on embodiment of this invention, and light resistance time. It is a figure which shows the optical characteristic before and behind the addition of the liquid crystal optical element which concerns on embodiment of this invention. It is a figure which shows the optical characteristic before and behind the addition of the liquid crystal optical element which concerns on embodiment of this invention. It is a figure which shows the optical characteristic before and behind the addition of the liquid crystal optical element which concerns on embodiment of this invention. It is a figure which shows the optical pick-up apparatus which concerns on embodiment of this invention.

  In the following description, the configuration and operation of the liquid crystal optical element according to the embodiment of the present invention will be described, and then the configuration and operation of the optical pickup device according to the embodiment of the present invention described above will be described in detail.

  First, the configuration of the liquid crystal optical element 100 of the present embodiment will be described. FIG. 1 shows a cross-sectional view of a liquid crystal optical element 100 according to this embodiment.

  As shown in FIG. 1, the liquid crystal optical element 100 includes a transparent substrate 1a having a transparent conductive film 2a and an organic alignment film 3a, and a transparent substrate 1b similarly having a transparent conductive film 2b and an organic alignment film 3b. The alignment films 3a and 3b are arranged so as to face each other, and the liquid crystal 5 containing the additive 6 is sandwiched through the sealant 4.

  As the liquid crystal 5 used in this liquid crystal optical element, for example, nematic liquid crystal having a refractive index anisotropy Δn of 0.15, a dielectric anisotropy Δε of about +6, and an NI point of about 100 ° C. is used. it can. This liquid crystal contains a component containing fluorine and is composed mainly of aromatic components of monophenyl, biphenyl and terphenyl. In addition, a liquid crystal material containing no unsaturated bonds other than aromatics such as a tolan bond, an ester bond, and an alkene side chain is used.

  A method for adding the additive 6 to the liquid crystal 5 will be described. An appropriate amount of powdery or liquid additive 6 is added to liquid crystal 5 and stirred for 15 minutes while warming to 90 ° C. The addition amount at this time needs to be set to such an amount that the additive 6 dissolved in the liquid crystal does not precipitate within the range of the environmental temperature of the liquid crystal optical element.

  Next, a specific composition of the additive 6 contained in the liquid crystal 5 will be described. The following formula (Formula 3) is a composition formula of HALS which is the first example of the additive 6.

  In (Chemical Formula 3), R 1 to R 8 are H or alkyl, M 1 is —O—C (═O) —CnH 2 n—C (═O) —O—, and n is an integer.

  The following formula (Formula 4) is a composition formula of a phosphorus antioxidant which is a second example of the additive 6.

  In (Chemical Formula 4), R1 to R18 are H or alkyl.

As mentioned above, the chemical formulas (Chemical Formula 1) and (Chemical Formula 2) indicate the photodegradation mechanism of the liquid crystal and the alignment film before the additive 6 is applied. When the liquid crystal or alignment film is irradiated with high energy light such as a blue-violet laser, a photodegradation reaction as shown in (Chemical Formula 1) and (Chemical Formula 2) occurs. First (
The photodegradation reaction of Chemical Formula 1) will be described. By irradiation with light, the alkyl group RH forming the liquid crystal and the alignment film is radicalized to R · (1), and the radicalized R · reacts with oxygen to become ROO · (2). This ROO · reacts with RH forming the liquid crystal and the alignment film to generate R · (3). By repeating these reactions, the RH of the liquid crystal and the alignment film continues to be decomposed and the deterioration proceeds. .

  On the other hand, when the ROOH group exists in the liquid crystal or the alignment film as in (Chemical Formula 2), ROOH is radicalized by light irradiation to generate RO (4), and RO. Is an RH that forms the liquid crystal and the alignment film. The reaction (5) and further the reactions (6) and (7) proceed, the RH of the liquid crystal and the alignment film is decomposed, and the deterioration proceeds.

  Next, the effect of the additive on the above-described photodegradation reactions (Chemical Formula 1) and (Chemical Formula 2) will be described using the following formulas (Chemical Formula 5) and (Chemical Formula 6).

The effect on (Chemical Formula 1) is shown in (Chemical Formula 5). By irradiation with light, the NH group of HALS is replaced with oxygen to become NO. This radical reacts with R · produced by the deterioration of the alignment film and liquid crystal shown in (1) of (Chemical Formula 1) to generate N-OR. This N-OR is combined with the ROO group shown in (2) of (Chemical Formula 1) to generate a stable ROOR.
By returning to O., the reaction (chemical formula 5) for suppressing the deterioration reaction (chemical formula 1) is repeated.

The effect on (Chemical 2) is shown in (Chemical 6). By irradiation with light, the phosphorus-based antioxidant P (OR) ₃ reacts with the alignment film or ROOH of the liquid crystal to change it into a stable ROH. By this reaction, the (4) reaction of (Chemical Formula 2), which is a deterioration reaction, can be suppressed.

  Next, the relationship between the content of the additive mixed in the liquid crystal and the light resistance time (lifetime) when irradiated with the blue-violet laser will be described. FIG. 2 is a diagram showing the additive content and light resistance time contained in the liquid crystal in the liquid crystal optical element of the present embodiment.

As shown in FIG. 2, the liquid crystal optical element 100 is irradiated with a blue-violet laser emitting a central light intensity of 70 mW / mm ² , and the time until the alignment of the liquid crystal is disturbed, that is, the deterioration is shown on the vertical axis as the light resistance time. The disorder of the alignment of the liquid crystal was observed with a polarizing microscope, and the point where the alignment disorder of several tens of μm occurred was defined as the deterioration point. The liquid crystal optical element has the above-described configuration, polyimide is used for the alignment film, and HALS is used as the additive contained in the liquid crystal. The external environment temperature when the blue-violet laser was irradiated was 70 ° C., and the additive weight ratio x with respect to the liquid crystal 5 was set between 0 wt% ≦ x ≦ 4 wt%.

  As shown in FIG. 2, the curve indicating the light resistance time with respect to the additive concentration was 160 hours when the additive concentration was 0 wt%. Improved up to time. At higher concentrations, there is no change in the light resistance time, and in this liquid crystal, it can be seen that the additive concentration of 1 wt% is the minimum concentration that exhibits the maximum effect.

  Such an effect on light resistance can be obtained in the same manner by using a phosphorus-based antioxidant as an additive instead of HALS.

In the above, a liquid crystal optical element using an organic alignment film made of polyimide, which has a particularly large effect, has been described as an example, but as another organic material, for example, using an acrylic organic alignment film, The same effect as described above can be obtained.

  Moreover, it can replace with the organic alignment films 3a and 3b, and the effect with respect to light resistance can be acquired even if it is a form using an inorganic material.

  Next, with reference to FIGS. 3 to 5, optical characteristics when HALS is added as an additive to the liquid crystal will be described.

  FIG. 3 shows the results of measuring the transmittance with a spectroscope by irradiating the liquid crystal optical element 100 with light in the vicinity region of blue-violet laser light of 400 to 500 nm. When the transmittance curve before the addition of the additive and the transmittance curve after the addition of the additive were compared, equivalent characteristics were obtained, and no change in optical characteristics due to the additive was observed. From the results described above, it can be said that by adding HALS to the liquid crystal, it is possible to improve the light resistance without changing the optical characteristics. In the figure, the transmittance curve is a waveform that vibrates finely up and down, which is due to interference by the layer members of the liquid crystal optical element such as electrodes. The two transmittance curves shown in the figure are not exactly the same because the vibrations are shifted from each other. However, if a comparative evaluation is performed using a line connecting adjacent maximum and minimum values, the variation tendency of the transmittance before and after the addition of the additive exhibits the same behavior and can be said to have the same characteristics.

  FIG. 4 shows the result of measuring the transmittance with a spectroscope after irradiating the liquid crystal optical element 100 with light in the vicinity of the red laser beam of 500 to 700 nm. When the transmittance curve before the addition of the additive and the transmittance curve after the addition of the additive were compared, equivalent characteristics were obtained, and no change in optical characteristics due to the additive was observed. In the figure, there are some differences in the two transmittance curves, but the difference between the two transmittance curves is generally within 0.5%, and the variation in transmittance before and after the addition of the additive. The tendency shows the same behavior and can be said to be equivalent to each other. The difference between the measured values before and after the addition of the additive is a slight difference that is difficult to confirm on the same scale as in FIG. 3, so that in FIG. ) Was expanded.

  FIG. 5 shows the result of measuring the transmittance with a spectroscope after irradiating the liquid crystal optical element 100 with light in the vicinity region of the red laser light of 700 to 800 nm. When the transmittance curve before the addition of the additive and the transmittance curve after the addition of the additive were compared, equivalent characteristics were obtained, and no change in optical characteristics due to the additive was observed. Considering the above results, it can be said that the element can be used for the above-described three types of optical disk (CD, DVD, BD) pickups. In FIG. 5, as in FIG. 4, the scale of the vertical axis (transmittance) is enlarged compared to FIG. 3 in order to clarify the difference in measured values before and after the addition of the additive.

  With respect to such optical characteristics, the same effect can be obtained even if a phosphorus-based antioxidant is used as an additive contained in the liquid crystal. Therefore, according to the liquid crystal optical element of the present embodiment, it is possible to cope with laser light having a broad wavelength range of 400 to 800 nm.

  Examples of radical scavengers include phenolic antioxidants. However, the ferrule system is easy to be colored, and there arises a problem that the light utilization efficiency is lowered. Moreover, although there exist sulfur type antioxidant etc. about a peroxide decomposition agent, a sulfur type oxide also tends to color, and the problem of reducing the utilization efficiency of light similarly arises. On the other hand, the liquid crystal optical element of the present embodiment does not cause such a problem, so that the light resistance can be improved without reducing the light utilization efficiency as described above. Moreover, in the said embodiment, although the radical scavenger and the peroxide decomposition agent are added individually in the liquid crystal, respectively, you may mix and add.

Here, a specific configuration example and operation of the optical pickup device 200 when the liquid crystal optical element is used as an aberration correction element will be described. FIG. 6 is a diagram showing a configuration example of an optical pickup device 200 to which the present invention is applied.

  As shown in FIG. 6, the optical pickup device according to this embodiment includes a laser light source 101 that oscillates a blue-violet laser, a collimator lens 102, a polarization beam splitter 103, a liquid crystal optical element 104 that functions as an aberration correction element, and the liquid crystal optical element. A driving circuit 108 for driving 104, a quarter wavelength plate 105, an objective lens 106 a, a condenser lens 106 b, and a light receiving diode 107 are included.

  In FIG. 6, the short wavelength blue-violet laser light emitted from the laser light source 101 is converted into parallel light by the collimator lens 102, passes through the polarization beam splitter 103, and then enters the liquid crystal optical element 104. When passing through the liquid crystal optical element 104, the blue-violet laser light is modulated by the liquid crystal optical element 104, and aberration correction of the blue-violet laser light is performed. Thereafter, the light passes through the quarter-wave plate 105 and is focused on the high-density optical disk 109 by the objective lens 106a. The blue-violet laser light reflected by the high-density optical disk 109 passes through the objective lens 106a and the quarter-wave plate 105 again, the optical path is changed by the polarization beam splitter 103, and the light receiving diode 107 is passed through the condenser lens 106b. It is focused on.

DESCRIPTION OF SYMBOLS 1a, 1b Transparent substrate 2a, 2b Transparent conductive film 3a, 3b Organic alignment film 4 Sealing agent 5 Liquid crystal 6 Additive 101 Laser light source 102 Collimator lens 103 Polarizing beam splitter 104 Liquid crystal optical element 105 1/4 wavelength plate 106a Objective lens 106b Collection Optical lens 107 Light-receiving diode 108 Drive circuit 109 High-density optical disk

Claims (5)

In a liquid crystal optical element that sandwiches liquid crystal between two transparent substrates having an alignment film formed inside and transmits laser light,
The liquid crystal contains an additive for trapping free radicals that are primarily generated by irradiating the liquid crystal with blue-violet laser light, or an additive for detoxifying peroxides that are secondarily generated from radicals. A liquid crystal optical element characterized by comprising:   The liquid crystal optical element according to claim 1, wherein the additive for trapping free radicals is HALS (Hindered Amine Light Stabilizer).   The liquid crystal optical element according to claim 1, wherein the additive for detoxifying the peroxide is a phosphorus-based antioxidant.   4. The liquid crystal optical element according to claim 1, wherein a wavelength region of the laser light transmitted through the liquid crystal optical element is 400 to 800 nm.   5. An optical pickup device on which the liquid crystal optical element according to claim 1 is mounted.

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