CN1141601C - Continuous vari-focus Fresnel lens - Google Patents

Abstract

The present invention belongs to the optical device field, particularly to a continuous zoom Fresnel lens. The present invention is characterized in that the Fresnel lens is composed of an upper glass base board, a lower glass base board and a liquid crystal sandwich layer, wherein ITO conductive films on the internal surfaces of the base boards are etched into wave strip electrode structures. The refractivity of the liquid crystal layer is controlled by an electric field to make the phase difference of two light beams running through adjacent wave strips to a principal focus be 2 pi. Therefore, the light gathering capacity of the Fresnel lens is stronger than that of conventional Fresnel wave strip pieces. In addition, the Fresnel lens does not need polarizing filters and has high light utilization rate via the treatment of a liquid crystal arrangement technology. Because the refractivity of liquid crystals is continuously adjustable, the principal focal distance of the lens can continuously change within the range of 0.5f to 2f.

Description

Continuous vari-focus Fresnel lens

One, technical field

The invention belongs to the optical device field, relate to a kind ofly under electric field action, utilize liquid-crystal refractive-index generation continually varying characteristic, make the continuously adjustable Fresnel lens of focal length.Remedy half-wave position phase with liquid crystal layer, reach the optical energy loss minimum, realize the lens continuous vari-focus by the effective refractive index of electrically-controlled liquid crystal simultaneously.

Two, background technology

Lens are basic optical device, are seen everywhere in optical instrument and equipment.Along with the development of optical technology, more and more higher to the requirement of lens.One is the continuous variable of the focal length of lens, and another is the lens of big focal length.No matter be that is a kind of, wanting to meet the demands all to make optical system become huge and complicated.

Micro lens also is a kind of important optical device, makes lenticule with traditional Process at present and mainly contains ion exchange process, compression molding method, fotoceram method for hot forming and photoresist thermosol forming process.Because the restriction of material therefor and traditional handicraft makes the micro lens of making of these methods that a common characteristic be arranged, and is exactly that lens have only a focal length.The focal length that changes lens is long, needs could realize by the spacing between the mechanical adjustment lens with one group of lens.Use lens combination not only to increase device cost, increase equipment volume, and be difficult for realizing effectively needed focal length.Therefore, people wish to develop simple in structure, in light weight, Zoom lens cheaply.

Advantages such as in recent years, people have proposed the employing liquid crystal technology and made variable focus microlens, and liquid crystal microlens adopts the transmittance mode, have control simply, and good reliability and driving voltage are low, this device has huge potential application.Designed several lens device with liquid crystal technology at present,, seen U.S.patent 4 as line style separate mesh electrode structure, 572,616 (Feb.25,1985), protruding, concavees lens combine (Liquid-Crystal Lens-Cells with Variable FocalLength with liquid crystal, Japanese Journal of Applied Physics, Vol.24, No.8,1985 pp.L626-628), poroid electrode structure (APPLIED OPTICS/Vol.36, No.20/10 July1997, pp4772-4778).The lens of said structure all belong to the refractive liquid crystal lens, wherein the manufacturing technology more complicated of some device.We once adopted the Polymer Dispersed Liquid Crystal technology to prepare Fresnel zone diffractive optical element (CN2348405Y, 1999,11,10), by structural design, can realize having the adjustable Fresnel liquid crystal of the electric field wavestrip device that odd number half-wave zone or even number half-wave zone work to ITO on the substrate (tin indium oxide) electrode.

Three, summary of the invention

The present invention utilizes liquid crystal under the effect of changing electric field according to the Fresnel zone structure of routine, and the characteristic that refractive index changes, purpose provide the Fresnel lens of a kind of big focal length or miniature continuous vari-focus.

The structure of conventional fresnel's zone plate as shown in Figure 1, it is to adopt odd number half-wave zone or even number half-wave zone are covered up, allow to have identical or differ be that the light wave diffraction of 2 π integral multiple position phases sees through zone plate, and light wave is accumulated on the axis of wavestrip, thereby have lens function by interference.Yet this zone plate make the utilization factor of light reduce half, and its principal focal distance is unadjustable because the selection of light is covered.As can be seen from Figure 1, if do not cover odd number half-wave zone (or even number half-wave zone), but the phasic difference that allows light wave through odd number half-wave zone (or even number half-wave zone) produce the odd-multiple of π or π again, then this zone plate is a position phase zone plate, adopts liquid crystal technology can reach this purpose.

Fig. 2 is a same thickness, the phasic difference that different refractivity produces, n ₁The refractive index of first half liquid crystal in the expression liquid crystal cell, n ₂The refractive index of expression the latter half liquid crystal, Δ n is | n ₁-n ₂|, d is the thickness of liquid crystal cell.If the optical path difference that light produces after by two parts $\mathrm{\Δnd}=(1+\frac{1}{2})\mathrm{\λ},$ λ is an optical wavelength, and these two parts are regarded as two adjacent wavestrips, and then phase type liquid crystal zone plate can design.

According to above-mentioned principle and analysis, the present invention has designed wavestrip structure as shown in Figure 3.Fig. 3 is the ITO electrode etching pattern and the public electrode part of last lower glass substrate.

Structure according to Fig. 3 design, ITO electrode on one glass substrate wavestrip structure, another piece glass substrate in the form of a ring also has ring electrode, ring-type wavestrip on each substrate links together with a public straight belt electrode, and the ring-type wavestrip of ITO is identical on the upper and lower base plate.Make inside surface with this two ring electrodes face, upper and lower base plate electrode figure mirror image symmetry, and public electrode crossover not, folder one liquid crystal layer between two substrates.On two ring-type wavestrip electrodes, all scribble liquid crystal aligning layer polyimide (PI).Liquid crystal layer between two glass substrates is the twisted nematic liquid crystal material that is mixed with the chirality agent.The gap d of two substrates, just the thickness of liquid crystal layer can be chosen between 4～6 μ m.In this device, each wavestrip radius of a circle is expressed as $r_m=\sqrt{m}{r}_1,$ r ₁Be interior radius of a circle, m is that positive integer is the ordinal number of wavestrip ring.As can be seen, if apply voltage to upper and lower base plate, then have only between the symmetrical ring electrode of the mirror part to have electric field, promptly electric field is shaped as the wavestrip shape.With the electric field driven liquid crystal of wavestrip shape, then liquid crystal layer also is the wavestrip shape, and therefore, liquid crystal and Fresnel zone are in conjunction with can be used to make liquid crystal lens.

For the ease of understanding the present invention, further provide explanation with Fig. 4.Fig. 4 represents the oriented structure of liquid crystal molecule in the Fresnel zone, and Fig. 4 (a) is liquid crystal arrangement situation during added electric field not, Fig. 4 (b) for the Fresnel zone electrode when applying electric field, liquid crystal arrangement orientation situation.Wherein, n _AveThe mean refractive index of liquid crystal during for added electric field not, n _EffThe effective refractive index of liquid crystal arrangement orientation during for added electric field.In Fig. 4,1-glass substrate, 2-are the ITO electrode, and 3-is an oriented layer, the 4-liquid crystal layer.

According to the optical anisotropy characteristic of liquid crystal, make monochromatic natural light by on any light wave direction of vibration, producing identical bit phase delay behind the liquid crystal layer, then liquid crystal should be 180 ° of aligned twisted.Fig. 4 (a) is under the not alive situation, the printing opacity process of adjacent two wavestrips, and both are identical because of refractive index, and the transmission situation is identical.(b) has a situation that applies electric field among Fig. 4 in the adjacent wavestrip.Under the driven situation of liquid crystal, the liquid crystal of driven portion all is identical effective refractive index n in any direction _Eff, and do not have driven liquid crystal to be mean refractive index n _AveAdjacent two wavestrip refractive index differences, but the optical path difference that the light that sees through produces is all identical in any direction.By adjusting to effective refractive index, can control this liquid crystal position phase zone plate, so not only significantly improve the utilization factor of device, and can also change the focal length of device light.

Be the structure of obtaining this wavestrip device and the relation of focal length, Fig. 5 provides the structure of liquid crystal wavestrip device.When light was entered by the device left side and focuses on the right side, the phasic difference of adjacent wavestrip should be satisfied: $2\mathrm{\π}=\frac{2\mathrm{\π}}{\mathrm{\λ}}[n_{\mathrm{ave}}-n_{\mathrm{eff}}\left(V\right)]d+\frac{2\mathrm{\π}}{\mathrm{\λ}}b,----\left(1\right)$ Wherein $\frac{2\mathrm{\π}}{\mathrm{\λ}}[n_{\mathrm{ave}}-n_{\mathrm{eff}}\left(V\right)]d$ The phasic difference that produces during by liquid crystal layer for light, b is that light takes optical path difference b=b '-b that focal spot produces to by adjacent wave ", " be the light path that adjacent wave is taken focal spot to, O is a focus for b ', b here.Can derive by (1) formula

λ=[n _Ave-n _Eff(V)] d+b (2) is according to the principal focal distance formula of conventional wave strap: $f=\frac{{r_1}^2}{\mathrm{\λ}}$ (r ₁Be the radius of interior strange wavestrip) (3) got by last two formulas: $f_{\mathrm{LCFZ}}={r_1}^2/\mathrm{\λ}=\frac{{r_1}^2}{[n_{\mathrm{ave}}-n_{\mathrm{eff}}\left(V\right)]d+b}----\left(4\right)$ By (4) formula as can be seen, the focal distance f of Fresnel liquid crystal zone plate _LCFZHas the continuous adjustability of electric field.

Expression formula according to formula (3), if zone plate is regarded as the amplitude type zone plate, then its focal length is 40mm as calculated, consider that the inconsistent light focusing power that causes that changes owing to b in the 1st wavestrip and the 2nd wavestrip, the 2nd wavestrip and the 3rd wavestrip descends, geometry according to Fig. 5, can calculate by Pythagorean theorem,, can think to still have light gathering when the optical path difference of adjacent wavestrip is no more than λ/20.So we can obtain: $\sqrt{f^2+2{r_1}^2}-\sqrt{f^2+{r_1}^2}=\sqrt{f^2+{{3r}_1}^2}-\sqrt{f^2-{{2r}_1}^2}\±\frac{\mathrm{\λ}}{20}----\left(5\right)$ From then on the adjustable extent that can calculate this device focal length in the formula is 1cm～100cm.

Consider theoretically, if the intensity of laser is I _o, the vibration amplitude that laser produces on prime focus by each wavestrip is a (I _o=a ²), then the global vibration amplitude is 2 * 16a, its light intensity is I=(2 * 16a) ², promptly light intensity will increase by 1024 times.

Four, description of drawings

Fig. 1 is the structural drawing of conventional fresnel's zone plate.

Fig. 2 sees through the optical path difference situation that light produces, wherein n for the adjacent two-phase bit-type of the present invention liquid crystal wavestrip ₁, n ₂Refractive index for liquid crystal.

Fig. 3 is a structural representation of the present invention.

Fig. 4 is the variations in refractive index situation of the present invention's adjacent wavestrip under electric field action.

Fig. 5 is a focusing synoptic diagram of the present invention.

Fig. 6 is embodiment of the present invention observed wavestrip structure under polarizing microscope.

Fig. 7 is applying under the different voltage condition for the present invention, the optically focused characteristic that CCD is measured.

Five, embodiment

Adopt photoetching method that the ITO conducting film on two glass substrates all is etched into the same wave band structure, the inside radius of its wavestrip is 0.5mm, has 16 odd number bands, with public electrode separately wavestrip is coupled together.Coat polyimide alignment layers again at the ITO electrode surface, and the friction of coverlet direction.Glass substrate is made liquid crystal cell with the placement of etching figure mirror image symmetry, and make the frictional direction antiparallel, its public electrode is crossover not.Inject liquid crystal in liquid crystal cell, as the GR-41 nematic liquid crystal that Japanese Chisso company produces, the maximum of its refractive index changes difference n _Ave-n _EffBe 0.3.Be mixed with the agent of 0.5wt%CB15 chirality in the liquid crystal, make liquid crystal in liquid crystal cell with the substrate normal direction, promptly the incident light direction is arranged for axle is 180 ° of distortion rotations, screw axis is along the substrate surface normal direction, thickness of liquid crystal box is 6 μ m.

Apply a moderate voltage on public electrode, observed wavestrip structure as shown in Figure 6 under polarizing microscope.In photo, the odd number band is a liquid crystal aligning part under electric field action, and the even number band is a liquid crystal original alignment part.

Fig. 7 is applying the measured optically focused characteristic of usefulness CCD under the different voltage condition for this liquid crystal zone plate, and the distance of liquid crystal device and CCD is 40cm, in the adjustable extent of device focal length, limits the b in the formula (2) thus.Used light source is the He-Ne laser instrument, vertical incidence.Fig. 7 (a) is during for making alive not, and focusing situation of the present invention (b) and (c) is respectively voltage when being 3V and 4V, focusing situation of the present invention.As can be seen, under not alive situation, the He-Ne laser beam sees through liquid crystal cell and is not focused, and under the situation that applies 3V and 4V, transmitted light beam obviously narrows down, and light intensity significantly increases.At 3V, resulting light intensity difference under two kinds of situations of 4V illustrates the focal length difference of device.Therefore, the focal length of this device has the electric field adjustability.Detect the light intensity of having an X-rayed light with polarizing microscope, find that light intensity does not present polarizability.

Claims (4)

1. continuous vari-focus Fresnel lens, glass substrate by glass substrate, liquid crystal layer and another piece band transparency electrode of being with transparency electrode is formed, it is characterized in that the transparency electrode on the glass substrate is etched into Fresnel ring-type wavestrip electrode structure, upper and lower base plate electrode figure mirror image symmetry, and with a public straight belt electrode these wavestrip electrodes are coupled together, the upper and lower base plate public electrode is crossover not; With the electrode surface is the substrate inside surface, all is coated with the liquid crystal oriented layer on the two ring-type wavestrip electrodes; Liquid crystal layer between the infrabasal plate is the nematic liquid crystal material that is mixed with the chirality agent.

2. continuous vari-focus Fresnel lens according to claim 1 is characterized in that liquid crystal is 180 ° of distortion rotations and arranges, and its screw axis is along the substrate surface normal direction.

3. continuous vari-focus Fresnel lens according to claim 2 is characterized in that the maximum variation of the refractive index difference of liquid crystal is 0.3, and the thick clearance such as grade between the two substrates is 4～6 μ m.

4. continuous vari-focus Fresnel lens according to claim 1 is characterized in that with the refractive index of electric field modulating liquid crystalline substance the focal length of lens being changed continuously.

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