OPTICAL SYSTEM FOR HOLOGRAPHIC STORAGE AND DESIGN METHOD FOR FRESNEL LENS AND META LENS THEREOF

Abstract

An optical system for holographic storage includes a reference light path, a signal light path, a servo light path and a reproduction light path. The reference light path and the signal light path contain a first Fourier lens and a second Fourier lens for transmitting reference light and signal light carrying data information, and adjusting the incident position and angle of the reference light and the signal light on a storage medium. The optical system includes a reference light objective lens for converging the reference light, a third Fourier lens for performing Fourier transformation on a signal light field, and a fourth Fourier lens for performing Fourier transformation on a reproduced signal light field to read the data information. The first Fourier lens, the second Fourier lens, the third Fourier lens, the fourth Fourier lens and the reference light objective lens is respectively a Fresnel lens or a meta lens.

Claims

1. An optical system for holographic storage, comprising a reference light path, a signal light path, a servo light path and a reproduction light path, wherein the reference light path and the signal light path both contains a first Fourier lens and a second Fourier lens for transmitting a reference light and a signal light carrying data information, and adjusting an incident position and angle of the reference light and the signal light on a storage medium, wherein the optical system further comprises: a reference light objective lens for converging the reference light; a third Fourier lens for performing Fourier transformation on a signal light field; and a fourth Fourier lens for performing Fourier transformation on a reproduced signal light field to read the data information, and wherein the first Fourier lens, the second Fourier lens, the third Fourier lens, the fourth Fourier lens and the reference light objective lens is respectively composed of a Fresnel lens or a meta lens.

2. The optical system according to claim 1, further comprising a servo light calibration lens which is configured for calibrating a servo light spot, the calibration lens being a Fresnel lens or a meta lens.

3. The optical system according to claim 1, further comprising a servo light objective lens which is configured for focusing a servo light, the servo light objective lens being a Fresnel lens or a meta lens.

4. The optical system according to claim 1, further comprising a magnification lens for matching pixel sizes of a spatial light modulator and an image sensor, the magnification lens being a Fresnel lens or a meta lens.

5. The optical system according to claim 1, wherein the reference light path and the signal light path further comprise a beam expanding collimating lens group for performing beam expanding on the reference light and the signal light, and the beam expanding collimating lens group is composed of a Fresnel lens or a meta lens.

6. The optical system according to claim 1, further comprising an astigmatic cylindrical lens for detecting defocusing condition of a servo light using astigmatism, the astigmatic cylindrical lens being a Fresnel lens or a meta lens.

7. The optical system according to claim 1, wherein after passing through the reference light objective lens, the reference light changes in direction and converges to the direction of the signal light at a certain angle.

8. The holographic optical storage light path system according to claim 1, wherein after passing through the third Fourier lens, the signal light changes in direction and converges to the direction of the reference light at a certain angle.

9. A design method for a Fresnel lens and a meta lens used in the optical system for holographic storage according to claim 1, comprising the following steps: S1. optimizing optical design of the optical system for holographic storage to obtain a lens or lens group meeting optical performance requirements; S2. extracting a phase distribution accumulated after parallel lights pass through the lens or lens group obtained in step S1; and S3. designing the Fresnel lens or the meta lens according to the phase distribution obtained in step S2.

10. The design method according to claim 9, wherein the step S3 includes: dividing the phase distribution by m•2π and taking the remainder to obtain a compressed phase distribution, wherein m is 5-50; designing the Fresnel lens according to the obtained compressed phase distribution and a formula $h=\frac{\phi }{2\pi }\cdot \frac{\lambda }{\left(n-n_0\right)},$ wherein n is a refractive index of material of the Fresnel lens, n.sub.0 is a refractive index of an environmental medium, φ is the phase modulation distribution of the Fresnel lens for incident parallel light beams, λ is wavelength of the incident light, and h is a thickness of any point on the refractive surface of an annular band relative to the lowest point thereof; and/or designing nanometer antennae and a layout thereof according to the phase distribution to obtain the meta lens.

11. A design method for a Fresnel lens and a meta lens used in the optical system for holographic storage according to claim 7, comprising the following steps: R1. optimizing optical design of the reference light objective lens or the third Fourier lens by setting deflection phase modulation at an appropriate position behind the reference light objective lens or the third Fourier lens to deflect the reference light or the signal light into a required direction, and thus obtaining a lens or a lens group meeting optical performance requirements; R2. extracting a lens phase distribution accumulated when parallel lights passthrough the lens or the lens group obtained in step R1 and propagates to a plane where a deflection phase is located, and superimposing the deflection phase distribution by the deflection phase modulation in step R1 on the lens phase distribution to obtain a deflection lens phase distribution; and R3. designing the Fresnel lens or the meta lens according to the deflection lens phase distribution obtained in step R2.

13. The design method according to claim 11, wherein the step R3 includes: dividing the deflection lens phase distribution by m•2π and taking the remainder to obtain a compressed deflection lens phase distribution, wherein m is 5-50; designing the Fresnel lens according to the obtained compressed deflection lens phase distribution and a formula $h=\frac{\phi }{2\pi }\cdot \frac{\lambda }{\left(n-n_0\right)},$ wherein n is a refractive index of material for making the Fresnel lens, n.sub.0 is a refractive index of an environmental medium, φ is the phase modulation distribution of the Fresnel lens for incident parallel light beams, λ is a wavelength of incident light, and h is a thickness of any point on the refractive surface of an annular band relative to the lowest point thereof; and/or designing nanometer antennae and a layout thereof according to the deflection lens phase distribution to obtain the meta lens.

14. A design method for a Fresnel lens and a meta lens used in the optical system for holographic storage according to claim 8, comprising the following steps: R1. optimizing optical design of the reference light objective lens or the third Fourier lens by setting deflection phase modulation at an appropriate position behind the reference light objective lens or the third Fourier lens to deflect the reference light or the signal light into a required direction, and thus obtaining a lens or a lens group meeting optical performance requirements; R2. extracting a lens phase distribution accumulated when parallel lights passthrough the lens or the lens group obtained in step R1 and propagates to a plane where a deflection phase is located, and superimposing the deflection phase distribution by the deflection phase modulation in step R1 on the lens phase distribution to obtain a deflection lens phase distribution; and R3. designing the Fresnel lens or the meta lens according to the deflection lens phase distribution obtained in step R2.

15. The design method according to claim 13, wherein the step R3 includes: dividing the deflection lens phase distribution by m•2π and taking the remainder to obtain a compressed deflection lens phase distribution, wherein m is 5-50; designing the Fresnel lens according to the obtained compressed deflection lens phase distribution and a formula $h=\frac{\phi }{2\pi }\cdot \frac{\lambda }{\left(n-n_0\right)},$ wherein n is a refractive index of material for making the Fresnel lens, n.sub.0 is a refractive index of an environmental medium, φ is the phase modulation distribution of the Fresnel lens for incident parallel light beams, λ is a wavelength of incident light, and h is a thickness of any point on the refractive surface of an annular band relative to the lowest point thereof; and/or designing nanometer antennae and a layout thereof according to the deflection lens phase distribution to obtain the meta lens.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a schematic diagram showing a meta lens or a Fresnel lens in place of a common optical lens group.

[0042] FIG. 2 is a schematic diagram showing a Fresnel lens or a meta lens as a first Fourier lens and a second Fourier lens.

[0043] FIG. 3 is a schematic diagram showing a Fresnel lens or a meta lens as a third Fourier lens.

[0044] FIG. 4 is a schematic diagram showing a Fresnel lens or a meta lens as the third Fourier lens and a reference light objective lens.

[0045] FIG. 5 is another schematic diagram showing a Fresnel lens or a meta lens as the third Fourier lens and the reference light objective lens.

[0046] FIG. 6 is another schematic diagram showing a Fresnel lens or a meta lens as the third Fourier lens and the reference light objective lens.

[0047] FIG. 7 is a schematic diagram of an optical system for holographic storage containing a Fresnel lens or a meta lens.

[0048] FIG. 8 is another schematic diagram of an optical system for holographic storage containing a Fresnel lens or a meta lens.

[0049] FIG. 9 is a schematic diagram showing a Fresnel lens or a meta lens as a Fourier lens for on-axis hologram recording.

[0050] FIG. 10 is a schematic diagram of a design method for a Fresnel lens or a meta lens in the optical system of holographic storage.

[0051] FIG. 11 is a schematic diagram of a design method for a deflection Fresnel lens or a deflection meta lens in the optical system for holographic storage.

DETAILED DESCRIPTION

[0052] An optical system for holographic storage containing a Fresnel lens or a meta lens is provided according to at least one embodiment. Referring to FIG. 7, the optical system includes a reference light path 1, a signal light path 2, a servo light path 3 and a reproduction light path 4. The reference light path 1 and the signal light path 2 both contains a first Fourier lens 10 and a second Fourier lens 20 for transmitting reference light 1 and signal light 2 carrying data information and adjusting the incident position and angle of the reference light 1 and the signal light 2 on a storage medium. In the present embodiment, the reference light path 1 and the signal light path 2 partially overlap, the first Fourier lens 10 and the second Fourier lens 20 are disposed at the overlapping position, that is, the reference light path 1 and the signal light path 2 share the first Fourier lens 10 and the second Fourier lens 20.

[0053] The optical system further includes a reference light objective lens 50 for converging the reference light, a third Fourier lens 30 for performing Fourier transformation on a signal light field, a fourth Fourier lens 40 for performing Fourier transformation on a reproduced signal light field to read the data information, a magnification lens 70 for matching pixel sizes of a spatial light modulator and an image sensor, a servo light objective lens 50 for focusing a servo light, a servo light calibration lens 60 for calibrating a servo light spot, an astigmatic cylindrical lens 80 for detecting defocusing condition of the servo light by astigmatism, a beam expanding collimating lens group 90 for performing beam expanding on the reference light and the signal light, and a spatial light modulator 100 for loading the data information onto the signal light.

[0054] The first Fourier lens 10 and the second Fourier lens 20 are composed of a Fresnel lens or a meta lens according to at least one embodiment. As shown in FIG. 2, the reference light and the signal light are Fourier-transformed by the first Fourier lens 10 and then converge on the focal plane thereof, and after filtering, the reference light and the signal light are Fourier-transformed by the second Fourier lens 20 and transferred to the focal plane thereof. With such configuration, the size of a hologram thus can be controlled by filtering of a Fourier transform system formed by the first and second Fourier lenses.

[0055] As shown in FIG. 3, the third Fourier lens 30 is composed of a Fresnel lens or a meta lens according to at least one embodiment. As shown in FIG. 4, both the third Fourier lens 30 and the reference light objective lens 50 consist of a Fresnel lens or a meta lens according to at least one embodiment. The signal light is Fourier-transformed by the third Fourier lens 30 and then converges to the focal plane thereof which is located inside the storage medium. After passing through the reference light objective lens 50, the reference light converges to the front focal point of the reference light objective lens 50 in the direction of the original optical axis, and continues to propagate to form a divergent spherical wave. The signal light then interferes with the reference light inside the storage medium to form holograms by exposure.

[0056] Referring to FIG. 1, The Fourier lens has the function requirement of transmitting the data information completely, while the reference light objective lens requires a higher numerical aperture and a larger working distance, so its aperture is larger. Meanwhile, since it is necessary to eliminate the influence of aberration, the Fourier lens and the reference light objective lens are generally a lens group consisting of a plurality of lenses, which are large in volume and mass. However, it can considerably reduce the lens volume and mass when designed as the Fresnel lens or the meta lens. On the one hand, reduction of volume and mass facilitates miniaturization of the entire optical system, on the other hand, it is advantageous to compensate for the expansion or contraction and deviation of the optical disk by controlling the movement of the lens through a servo system. The expansion or contraction of the holographic optical disk includes volume contraction due to polymerization of photopolymer monomers into macromolecules after writing in data and a volume change due to a temperature change, such volume change may cause a change to holographic grating. The deviation of the optical disk includes translational deviation in the direction of three axes and angular deviation around the three axes. In order to compensate for the effects of the volume change and the deviation of the holographic optical disk to fully reproduce the stored data information, compensation may be performed by controlling the movement or rotation of the lens, such as the second Fourier lens or other lenses, on the light path. Moreover, in order to facilitate control and achieve fast response, the volume and mass of these lenses should be reduced as much as possible, accordingly, replacing a conventional lens or lens group with the Fresnel lens and the meta lens is an important or even the unique method.

[0057] According to one preferable embodiment, the servo light calibration lens 60 is a Fresnel lens or a meta lens.

[0058] The servo light calibration lens 60 is configured for adjusting the position of a convergence light spot of the servo light. In the calibration process, the servo light calibration lens 60 needs to be frequently moved to complete the focusing of the servo light. The Fresnel lens or the meta lens in place of the common calibration lens thus can greatly reduce the volume and mass of the lens, which is more conducive to rapid movement of the servo light calibration lens 60.

[0059] According to one preferable embodiment, the servo light objective lens 50 is a Fresnel lens or a meta lens. As shown in FIG. 7, the servo light objective lens 50 and the reference light objective lens are the same lens.

[0060] The configuration of the servo light objective lens 50 being a Fresnel lens or a meta lens simplifies the volume of the lens, reduces the mass of the lens, facilitates miniaturization and integration of the system, and facilitates rapid movement of the servo light objective lens.

[0061] According to one preferable embodiment, the magnification lens 70 is a Fresnel lens or a meta lens.

[0062] According to one preferable embodiment, the beam expanding collimating lens group is composed of a Fresnel lens or a meta lens, which reduces the volume and facilitates miniaturization and integration of the system.

[0063] According to one preferable embodiment, the astigmatic cylindrical lens 80 is a Fresnel lens or a meta lens.

[0064] FIG. 5 provides an embodiment that the third Fourier lens 30 and the reference light objective lens 50 are both composed of a Fresnel lens or a meta lens. By designing the surface structure of the Fresnel lens or the meta lens, after passing through the reference light objective lens 50, the reference light converges to the front focal point of the reference light objective lens 50 in the direction of the original optical axis; and after the signal light passes through the third Fourier lens 30, the signal light changes in direction and converges to the direction of the reference light at a certain angle.

[0065] FIG. 6 provides another embodiment that the third Fourier lens 30 and the reference light objective lens 50 are composed of a Fresnel lens or a meta lens. By designing the surface structure of the Fresnel lens or the meta lens, after passing through the third Fourier lens 30, the signal light converges to the front focal point of the reference light objective lens 50 in the direction of the original optical axis, and after the reference light passes through the reference light objective lens 50, the reference light changes in direction and converges to the direction of the signal light at a certain angle.

[0066] The Fresnel lens or the meta lens may also be applicable to an on-axis optical system for holographic storage. As shown in FIG. 9, the reference light and the signal light have the same symmetry axis. The third Fourier lens and the reference light objective lens are the same lens.

[0067] The Fresnel lens or the meta lens may also be applicable to other off-axis optical system for holographic storage. As shown in FIG. 8, the reference light and the signal light are not coaxial, and the servo light is not coaxial with the reference light and the signal light, either.

[0068] FIG. 9 provides a design method for a Fresnel lens and a meta lens applied in the above optical system for holographic storage, which includes the following steps: [0069] S1. optimizing the optical design of the optical system for holographic storage to obtain a lens or a lens group meeting the optical performance requirements; [0070] S2. extracting a phase distribution accumulated after parallel lights pass through the lens or lens group obtained in step S1; and [0071] S3. designing the Fresnel lens or the meta lens according to the phase distribution obtained in step S2.

[0072] For the Fresnel lens, the step S3 specifically includes dividing the phase distribution by m.Math.2π and taking the remainder to obtain a compressed phase distribution. The larger the value of m, the less the order of the Fresnel lens (i.e. the less the number of the annular band), the wider the annular band, and the greater the thickness of the annular band; the smaller the value of m, the greater the order of the Fresnel lens (i.e. the greater the number of the annular band), the narrower the annular band, and the smaller the thickness of the annular band. However, when the value of m is too large or too small, it is difficult to manufacture the Fresnel lens. The value of m in this embodiment is 5-50. Designing the Fresnel lens according to the obtained compressed phase distribution and the formula

[00003]$h=\frac{\phi }{2\pi }\cdot \frac{\lambda }{\left(n-n_0\right)},$

wherein n is the refractive index of the material of the Fresnel lens, n.sub.0 is the refractive index of an environmental medium, φis the phase modulation distribution of the Fresnel lens for incident parallel light beams, λ is the wavelength of incident light, and h is the thickness of any point on the refractive surface of the annular band relative to the lowest point thereof.

[0073] For the meta lens, the step S3 specifically includes designing the nanometer antennae and a layout thereof according to the phase distribution to obtain the meta lens. The meta lens of the present invention achieves phase regulation and control by plasmon resonance or dielectric resonance. The light is coupled to an electromagnetic wave propagating back and forth along the surface or inside of the nanometer antenna so as to regulate and control the transmitted or reflected light by an oscillating mode on the nanometer antenna. The optical nanometer antennae of the meta lens may be nano-structures such as holes, slits or protrusions. The optical meta lens can regulate and control the amplitude, phase, polarization and transmission spectrum of light in the sub-wavelength range through the interaction of the nanostructure unit with light.

[0074] FIG. 10 provides another design method for a Fresnel lens and a meta lens applied in the optical system for holographic storage includes the following steps: [0075] R1. optimizing the optical design of a reference light objective lens or a third Fourier lens, and in the optimization process, setting deflection phase modulation at an appropriate position behind the reference light objective lens or the third Fourier lens so as to deflect light into a required direction, thereby obtaining a lens or a lens group meeting the optical performance requirements; [0076] R2. extracting a lens phase distribution accumulated when parallel lights pass through the lens or lens group obtained in step R1 and propagates to a plane where a deflection phase is located, and superimposing the deflection phase distribution by the deflection phase modulation obtained in step R1 on the lens phase distribution to obtain a deflection lens phase distribution; and [0077] R3. designing the Fresnel lens or the meta lens according to the deflection lens phase distribution obtained in step R2.

[0078] For a deflection Fresnel lens, the step R3 specifically includes dividing the deflection lens phase distribution by m.Math.2π and taking the remainder to obtain a compressed deflection lens phase distribution. The larger the value of m, the less the order of the Fresnel lens (i.e. the less the number of the annular band), the wider the annular band, and the greater the thickness of the annular band; the smaller the value of m, the greater the order of the Fresnel lens (i.e. the greater the number of the annular band), the narrower the annular band, and the smaller the thickness of the annular band. However, when the value of m is too large or too small, it is difficult to manufacture the Fresnel lens. The value of m in this embodiment is 5-50. The Fresnel lens is designed according to the obtained compressed deflection lens phase distribution and the formula

[00004]$h=\frac{\phi }{2\pi }\cdot \frac{\lambda }{\left(n-n_0\right)},$

wherein n is the refractive index of the material for making the Fresnel lens, n0 is the refractive index of an environmental medium, φis the phase modulation distribution of the Fresnel lens for incident parallel light beams, λ is the wavelength of incident light, and h is the thickness of any point on the refractive surface of the annular band relative to the lowest point thereof.

[0079] For a deflection meta lens, the step R3 specifically includes designing nanometer antennae and a layout thereof according to the deflection lens phase distribution to obtain the meta lens. The meta lens of the present invention achieves phase regulation and control by plasmon resonance or dielectric resonance. The light is coupled to an electromagnetic wave propagating back and forth along the surface or inside of the nanometer antenna to regulate and control the transmitted or reflected light by an oscillating mode on the nanometer antenna. The optical nanometer antennae constituting the meta lens may be nano-structures such as holes, slits or protrusions. The optical meta lens can regulate and control the amplitude, phase, polarization and transmission spectrum of light in the sub-wavelength range through the interaction of the nanostructure unit with light.