KR20100050795A - Phase retardation device and optical pick-up apparatus having the same - Google Patents

Abstract

PURPOSE: A phase lag element and an optical pick-up comprising the same are provided to retard the phases by 1/4 of wavelength respectively for different wavelength of laser lights triggered from DVD light source. CONSTITUTION: A phase lag element(13) comprises first and second phase lag layers(17,18). The first and second phase lag layers are mutually stacked. The phase lag value of the second phase lag layer is 140~190nm and the phase lag value of the first phase lag layer is 450~1300nm. The ratio of the phase lag value of the first phase lag layer to the phase lag value of the second phase lag layer is 1:3.8~1:7.7. The ellipticity of the light coming out from the phase lag element is 0.89 or greater.

Description

Phase retardation device and optical pick-up apparatus including the same

The present invention relates to a phase delay device and an optical pickup device including the same, and more particularly, to implement a phase delay device by stacking first and second phase delay layers, wherein the phase delay value of the second phase delay layer is An object of the present invention is to provide a phase delay device capable of delaying phase delay by 1/4 of a wavelength for two laser beams having different wavelengths by optimizing the ratio of the phase delay values of the first phase delay layer.

Background Art An optical pickup apparatus including a phase delay element is used to record information on an optical recording medium such as a CD or a DVD or to reproduce information recorded on an optical recording medium.

1 is a block diagram of a conventional optical pickup device. As shown in FIG. 1, a general optical pickup device 1 includes a light source 10, a beam splitter 11, a collimating lens 12, an objective lens 14, and a photodetector ( 15), and a λ / 4 phase delay element 13 having birefringence is disposed on the optical path.

The light source 10 generates laser light used in the optical pickup device 1, and a laser having an output wavelength of about 785 nm is used as the light source 10 for CD, and an output of about 656 nm for DVD. Lasers with wavelengths are commonly used

The beam splitter 11 divides the laser light according to the polarization state, so that light of linearly polarized light polarized in one direction is transmitted as it is, but light of linearly polarized light polarized in a direction perpendicular to the one direction is refracted.

The laser light emitted from the light source 10 is incident to the collimating lens 12 through the beam splitter 11, and the collimating lens 12 focuses the laser light to generate parallel light. At this time, the laser light emitted from the light source 10 has a characteristic of linearly polarized light polarized in one direction and is converted into circularly polarized light while passing through the λ / 4 phase delay element 13. The laser light converted into circularly polarized light is collected through the objective lens 14 and irradiated onto the optical recording medium 16.

Then, the laser light is reflected by the optical recording medium 16 and then incident again to the objective lens 14, and then to the λ / 4 phase delay element 13 through the objective lens 14. In this case, the polarization state of the laser light is changed while the laser light is reflected from the optical recording medium 16. That is, the laser light reflected from the optical recording medium 16 has the characteristics of circularly polarized light, but its rotation direction is opposite to that of the laser light incident on the optical recording medium 16. Therefore, the laser light reflected by the optical recording medium 16 and converted into linearly polarized light by the λ / 4 phase delay element 13 is perpendicular to the laser light emitted from the light source 10. As a result, the beam splitter 11 does not penetrate and is refracted by the beam splitter 11 to travel toward the photodetector 15. The photodetector 15 converts the received laser light into an electrical signal and outputs the electrical signal.

In the optical pickup device 1, the polarization state of the laser light is converted by using the λ / 4 phase delay element 13, so when the phase delay value in the λ / 4 phase delay element 13 is not accurate, Since the light is not converted into the correct circularly polarized light and as a result, the power of the laser light is reduced in the course of passing through the λ / 4 phase delay element 13, the performance of the optical pickup device 1 is degraded, so that it has an accurate phase delay value. The design of the phase delay element 13 is required.

In particular, recently, an optical pickup device 1 employing both a CD light source 10 and a DVD light source 10 is being developed for simultaneous use on a CD and a DVD. In the λ / 4 phase delay element 13, Since the phase delay value of is dependent on the wavelength of the laser light, the phase delay value of λ / 4 is applied to the laser light emitted from the CD light source 10 and the DVD light source 10 using a single phase delay element 13. There is a difficulty. As a result, in the conventional optical pickup device 1, the device is configured such that the path of the laser light emitted from each light source 10 is different, and a separate λ / 4 phase delay element 13 is employed for each light source 10. There is a problem that the configuration of the bar optical pickup device 1 is complicated, the configuration of the device is large, and the manufacturing cost is increased.

The present invention is to solve the above problems, the first and second phase delay layer is laminated to implement a phase delay element, but the phase delay of the first phase delay layer with respect to the phase delay value of the second phase delay layer By optimizing the ratio of the values, it is to provide a phase delay element capable of causing phase delays corresponding to one-quarter of the wavelength with respect to laser light having different wavelengths emitted from the CD light source and the DVD light source.

 The phase delay element according to the present invention is disposed on an optical path of a laser beam for a CD and a laser beam for a DVD to convert a polarization state of the laser beams. And the ratio of the phase delay value of the first phase delay layer to the phase delay value of the second phase delay layer is 1: 3.8 to 1: 7.7.

Preferably, the first and second phase delay layers are formed of a liquid crystal polymer material having birefringence or a polymer stretched film stretched along one direction, and the thickness of each phase delay layer formed of the liquid crystal polymer material is 1 μm to 20. It is preferred that it is μ m.

The first and second phase delay layers may be adhered to each other by an adhesive.

The phase delay element may further include a light transmissive substrate, and the light transmissive substrate may be attached to the first and second phase delay layers by an adhesive.

The light transmissive substrate is preferably formed of glass or polyethylene telephthalide (PET), and a diffraction grating may be disposed on an outer surface of any one of the stacked first and second phase delay layers.

An optical pickup apparatus according to the present invention comprises: a light source for outputting a laser light for CD and a laser light for DVD; A collimating lens for converting the laser light output from the light source into parallel light; An objective lens for condensing laser light passing through the collimating lens on an optical recording medium; A photodetector that receives the laser light reflected from the optical recording medium and converts the laser light into an electrical signal; And a phase delay device having the above characteristics.

In the phase delay device according to the present invention, the first and second phase delay layers are laminated to form a phase delay element, but the CD and the phase delay value of the first phase delay layer are optimized by the phase delay value of the second phase delay layer. The laser light of linearly polarized light emitted from the DVD light source can be easily converted into the laser light of circularly polarized light having an ellipticity of 0.95 or more.

In addition, in the optical pickup device according to the present invention, since the polarization state is converted by the same phase delay element while the laser light emitted from the CD light source and the DVD light source travels along the same optical path, a separate phase delay element is employed for each light source. The configuration of the device is simpler than the conventional optical pickup device, and the manufacturing process is simplified to reduce the manufacturing cost.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. Hereinafter, the phase delay device according to the present invention will be described first, and then the optical pickup device including the phase delay layer will be described.

2 is a cross-sectional view of the phase delay element 13 according to the present invention. As shown in FIG. 2, the phase delay element 13 according to the present invention includes a first phase delay layer 17 and a second phase delay layer 18 stacked thereon. In this case, the first and second phase delay layers 17 and 18 may be stacked to be in close contact with each other as shown in FIG. 2, but may be stacked while being bonded to each other by an adhesive 19 as shown in FIG. 3. have.

Each of the phase delay layers 17 and 18 is birefringent so as to be disposed on an optical path of the laser light so as to convert the polarization state of the laser light. The phase delay layers 17 and 18 may be generally used as long as they have a normal refractive index or an abnormal refractive index according to the polarization direction of the incident laser light, but may be a polymer stretched film stretched in one direction or an orientation treatment in one direction. It is preferable to form the phase delay layer with the liquid crystal polymer material. In this case, when the phase delay layers 17 and 18 are formed of the liquid crystal polymer material, the thickness of the optical pickup device 1 may be 1 μm to 20 μm .

The phase delay element 13 according to the present invention may further include a translucent substrate 20 as shown in FIG. 4. The light transmissive substrate 20 is disposed on the outer surfaces of the first and second phase retardation layers 17 and 18 which are stacked to protect the phase delay layers 17 and 18, and is formed of a material of the light transmissive substrate 20. Glass or polyethylene terephthalate (PET) and the like can be used.

Since the light transmissive substrate 20 has isotropy unlike the first and second phase delay layers 17 and 18, the polarization state of the laser light does not change in the process of passing through the light transmissive substrate 20. The light transmissive substrate 20 may be arranged to be adhered to the first and second phase layers 17 and 18 by an adhesive 19 as shown in FIG. 5.

The phase delay element 13 according to the present invention may further include a polarization diffraction grating 21 as shown in FIG. The polarization diffraction grating 21 is disposed on the outer surface of the phase delay layers 17 and 18 of the first and second phase delay layers 17 and 18 so that light of linearly polarized light polarized in one direction is transmitted. However, the light of the linearly polarized light polarized in the direction perpendicular to the one direction is for diffraction, which is widely used to improve light utilization efficiency in the optical pickup device 1, and thus, a detailed description thereof will be omitted.

Hereinafter, the change in the polarization state of the laser light in the process of passing through the phase delay element 13 having the above configuration will be described. In this case, the polarization state of the laser light may be described by Equations 1 to 3 below.

M (θ, β) in Equation 1 is a Muller Matrix representing the light transmission characteristics of each phase delay layer 17, 18, and θ is light incident on the phase delay element 13 (hereinafter, 'incident light'). Angle between the polarization direction and the transmission optical axis of each of the phase delay layers 17 and 18, and has a value of 0 to 180 degrees, and β denotes the phase delay value of the phase delay layers 17 and 18. .

S in Equation 2 is a Stock's vector representing the polarization state of the laser light and is composed of four parameters as shown in Equation 2.

At this time, S ₀ means the total intensity of the laser light, S ₁ means the difference between the intensity of the component polarized in the horizontal direction and the intensity of the component polarized in the vertical direction, S ₂ is based on the horizontal direction The difference between the intensities of the components polarized in the + 45 ° direction and the intensities of the components polarized in the -45 ° direction. S ₃ is the intensity of the circularly polarized light component rotating in the right direction and the circularly polarized light component rotating in the left direction. Means a difference in intensity. Table 1 shows the typical polarization state of the laser light as a stock vector.

Polarization state Stokes Vector [S ₀ S ₁ S ₂ S ₃ ] Horizontal linear polarization state [1 1 0 0] ^t Vertical linear polarization state [1 -1 0 0] ^t Linear polarization angle +45 degrees [1 0 1 0] ^t Linear polarization angle -45 degrees [1 0 -1 0] ^t Right turn circularly polarized state [1 0 0 1] ^t Left turn circularly polarized state [1 0 0 -1] ^t

Hereinafter, for the convenience of description, the Stokes vector representing the polarization state of the laser light incident to the phase delay element 13 (hereinafter referred to as 'incident light') is S, and the polarization state of the laser beam passing through the phase delay element 13 is S. The Stokes vector representing S is referred to as S ', and the Mueller matrix for the first phase delay layer is referred to as M ₁ , and the Mueller matrix for the second phase delay layer is referred to as M ₂ . In this case, S 'is obtained by sequentially multiplying S by M ₁ and M ₂ as shown in Equation 3.

The present invention aims to convert linearly polarized light in one direction into circularly polarized light using the first and second phase delay layers 17 and 18. Hereinafter, linearly polarized light is converted into circularly polarized light. To optimize the conditions of the phase delay layers 17 and 18.

At this time, the degree of circular polarization of the emitted light can also be determined by the " ellipticity ε ". Hereinafter, the polarization state of the emitted light is represented using the ellipticity. In this case, the ellipticity (ε) means the ratio of the intensity of the component polarized in the vertical direction and the intensity of the component polarized in the horizontal direction, which can be obtained using the Stokes parameters described above as shown in Equation 4 below. .

When the polarization state of the laser light is perfectly circularly polarized light, the ellipticity is 1 since the intensity of the component polarized in the horizontal direction and the intensity of the component polarized in the vertical direction are the same. However, obtaining a perfect circularly polarized outgoing light having an ellipticity of 1 requires much effort. In the present invention, an ellipticity of 0.89 or more, preferably 0.95 or more, which is generally required in the art, can be obtained. a phase delay value (β ₂₎ of the delay layer 17, a phase delay value (β ₁₎ and a second phase retardation layer 18 is to optimize the. It can be seen that each of the phase delay values β ₁ and the phase delay values β ₂ are affected by the wavelength as shown in Equation 5 below.

Where x is the retardation layer number, x = 1, 2, λ ₀ is the wavelength, d is the thickness of the phase delay layer, and n0 and ne are the normal light refractive index and the ideal light refractive index.

7 and 8 show phase delay values of the first and second phase delay layers 17 and 18 which retard phase by a quarter of the wavelength with respect to the laser light of about 785 nm used for the CD and the laser light used for the DVD. The simulation results are shown.

As shown in FIGS. 7 and 8, the range of the phase delay value of the second phase delay layer 18 is 140 nm to 190 nm, and the range of the phase delay value of the first phase delay layer 17 is 450 nm to 1300 nm. It can be seen that the angular wavelength is delayed by 1/4 in.

FIG. 7 shows the first and second phase delay layers 17 and 18 in which outgoing light having an ellipticity of 0.89 or more can be obtained for a laser light of about 785 nm used for CD and a laser light of about 656 used for DVD. This is the result of simulating a combination of phase delay values.

In the simulation process, S ', which is a Stokes vector representing the polarization state of the emitted light, is changed while changing the phase delay values of the first and second phase delay layers 17 and 18, and S' is used to simulate the ellipticity of the emitted light. do. In the simulation, the phase delay value β ₁ of the first phase delay layer 17 was changed in the range of 0 to 1300 nm, and the phase delay value β ₂ of the second phase delay layer 18 was 0 to 700 nm. The range was changed.

Meanwhile, as described above, the polarization state of the emitted light is also affected by the angle θ formed between the polarization direction of the incident light and the transmission optical axis of each phase delay layer. In this regard, the simulation process simulates varying θ from 0 ° to 180 ° for each combination of phase delay values (β _1, β ₂ ) so that the ellipticity is 0.89 or more (β _1, β _2). The range of) is simulated.

That is, in the state where one phase delay value (β _1, β ₂ ) of each phase delay layer 17, 18 is substituted into the muller matrix M _1, M ₂ of each phase delay layer 17, 18. The ellipticity was simulated varying from 0 ° to 180 ° and repeated for the range of phase delay values described above.

Referring to the simulation results, as shown in FIG. 7, when the phase delay value of the second phase delay layer 18 is about 150 nm, the first phase delay layer 17 has a phase delay value of approximately 280 nm to 1300 nm. It can be seen that the ellipticity of the emitted light becomes more than 0.99 (by the combination of the optical axis angles θ of the phase delay layer). If the phase delay value of the second phase delay layer 18 is about 180 nm, and the phase delay value of the first phase delay layer 17 has a phase delay value of 300 nm to 1300 nm, the ellipticity of the emitted light is ( It can be seen that the combination of the optical axis angles θ of the phase delay layer is 0.89 or more. Therefore, in order to convert the light of linearly polarized light into the light of circularly polarized light having an ellipticity of 0.89 or more by combining two phase delay layers 17 and 18 having different phase delay values, the phase delay value of the second phase delay layer 18 is It is about 140nm ~ 190nm, it can be seen that the phase delay value of the first phase delay layer 17 should be approximately 450nm ~ 1300nm. In particular, the ratio of the phase delay value of the first phase delay layer 17 to the phase delay value of the second phase delay layer 18 in the range where the phase delay value of the second phase delay layer 18 is approximately 140 nm to 190 nm. Combinations near 1: 4.5 and 1: 6.5 are preferred.

In particular, as described above, the ratio of the phase delay values considers the influence of the angle (θ) between the polarization direction of the incident light and the transmission optical axis of each phase delay layer (17, 18). Means the condition to obtain the emitted light of 0.89 or more.

At this time, the phase delay value of each of the phase delay layers 17 and 18 depends on the thickness of the phase delay layers 17 and 18 or the refractive index of the material constituting the phase delay layer. By adjusting the thickness or the refractive index of the phase delay ratio as described above can be obtained. For example, when the first and second phase delay layers 17 and 18 are formed of a material having the same refractive index, the ratio of the thickness of the first phase delay layer 17 to the thickness of the second phase delay layer 18 is seen. In the range of FIG. 7 according to the present invention, that is, in the region where the phase delay value of the second phase delay layer is 140 nm to 190 nm, the ellipticity has a value of 0.95 or more.

FIG. 8 for the above case is a result of simulating a combination of phase delay values in the first and second phase delay layers 17 and 18 capable of obtaining output light having an ellipticity of 0.95. Similar to the case of FIG. The combination of the phase delay values in the first and second phase delay layers 17 and 18 capable of obtaining the emitted light having an ellipticity of 0.95 or more by the combination of the optical axes θ of the phase delay layers is simulated.

Referring to FIG. 8, when the phase delay value in the second phase delay layer 18 is 150 nm to 170 nm, the phase delay value in the first phase delay layer 17 is 700 nm to 800 nm in order to obtain the emitted light having an ellipticity of 0.95 or more. Or 1050 to 1150 nm. Accordingly, in order to convert the light of linearly polarized light into light having an ellipticity of 0.95 or more while combining the two phase delay layers 17 and 18 with phase delay of λ / 4, the phase delay value of the second phase delay layer 18 is 150 nm or more. The ratio of the phase delay value of the first phase delay layer 17 to the phase delay value of the second phase delay layer 18 in the region of 170 nm is approximately 1: 3.8 to 1: 5.4 and 1: 5.8 to 1: 7.7. It can be seen that In particular, the ratio of the phase delay value of the first phase delay layer 17 to the second phase delay layer 18 is preferably 1: 3.8 to 1: 5.4 in consideration of the thickness of the phase delay element.

9 is a block diagram of the optical pickup device 1 including the phase delay element 13 according to the present invention, showing the optical pickup device 1 compatible with the CD and DVD.

As shown in FIG. 9, the optical pickup device 1 according to the present invention includes a light source 10, a beam splitter 11, a collimating lens 12, an objective lens 14, and the like. It includes a photodetector 15, the phase delay element 13 according to the present invention is disposed on the optical path of the laser light.

At this time, the light source 10 should be configured to output 785nm laser light for recording data on the CD or reading data stored in the CD and 656nm laser light for recording data on the DVD or reading the stored data. It is preferable to use a semiconductor laser capable of outputting these two wavelengths so that the pickup apparatus 1 can be miniaturized. However, it is also possible to employ a separate laser outputting the laser light of the 785nm band and 656nm band, respectively, and to configure the laser light output from each laser to proceed along the same optical path.

In addition, the wavelength of the laser light output from the light source 10 is not limited to 785nm and 656nm, of course, can be adjusted in the range that can record data on the CD or DVD or read the recorded data.

The beam splitter, collimating lens 12, the objective lens 14 and the photodetector 15 included in the optical pickup device 1 according to the present invention are described in the background art of the present specification. As described above, the same reference numerals are used, and detailed description thereof will be omitted.

In the above, the use of the phase delay element 13 according to the present invention has been described with reference to the optical pickup device 1, but is arranged on the optical paths of two different laser lights like the display device to change the polarization state of the laser light. Of course, it can be applied to various fields that are required.

1 is a block diagram of a conventional optical pickup device.

2 is a block diagram of a phase delay device according to the present invention.

3 is a block diagram of a phase delay device according to the present invention.

4 is a configuration diagram of a phase delay device including a light transmissive substrate.

5 is a configuration diagram of a phase delay device including a light transmissive substrate.

6 is a configuration diagram of a phase delay device including a gray grating.

7 is a graph showing conditions under which an ellipticity of 0.89 or more can be obtained.

8 is a graph showing conditions under which an ellipticity of 0.95 or more can be obtained.

9 is a block diagram of an optical pickup apparatus according to the present invention.

<Brief description of the major symbols in the drawings>

1: optical pickup device 10: light source

11 beam splitter 12 collimating lens

13: phase delay element 14: objective lens

15 optical detector 16 optical recording medium

17: first phase delay layer 18: second phase delay layer

19 adhesive agent 20 transparent substrate

21: diffraction grating b

Claims (7)

A phase delay element for converting a polarization state of light, the phase delay element comprising a stacked first and second phase delay layers, The phase delay value of the second phase delay layer is 140nm to 190nm, and the phase delay value of the first phase delay layer is 450nm to 1300nm. The method of claim 1, The phase delay value of the second phase delay layer is 140 nm to 190 nm, and the ratio of the phase delay value of the first phase delay layer to the phase delay value of the second phase delay layer is 1: 3.8 to 1: 7.7, and the phase And an ellipticity of light emitted from the delay element is 0.89 or more. The method of claim 1, In the range where the phase delay value of the second phase delay layer is 140 nm to 190 nm, the ratio of the phase delay value of the first phase delay layer to the phase delay value of the second phase delay layer is 1: 3.8 to 1: 5.4 and 1 Any one of: 5.8 to 1: 7.7 and the ellipticity is 0.95 or more. The method of claim 1, And the first and second phase delay layers are formed of a liquid crystal polymer material having a birefringence or a stretched polymer stretched film. The method of claim 4, wherein The phase delay element, characterized in that the thickness of each phase delay layer formed of the liquid crystal polymer material is 1 ~ 20 μm . The method of claim 1, Phase delay device further comprises a light-transmitting substrate. The method of claim 1, And a diffraction grating disposed on an outer surface of any one of the stacked first and second phase delay layers.

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