Patent Application
20080117521
The present invention is directed, in general, to refractive optics, and more particularly, to a liquid lens and methods of using a liquid lens.
This section introduces aspects that may be helpful to facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light. The statements of this section are not to be understood as admissions about what is in the prior art or what is not in the prior art.
There are many optical applications that use refractive optics (e.g., lenses). Refractive optics using liquid lenses provide the opportunity to tune a lens easier, and sometimes to a greater extent, than possible for flexible polymeric or mechanically adjustable lenses. In optical apparatuses ranging from telescopes to micro-electro-mechanical systems (MEMS), it is often important to make an apparatus that is as compact as possible. Unfortunately, some conventional liquid lenses have a small refractive index contrast, which translates into a substantially longer than desired focal length. This in turn, necessitates using a large amount of space for the optical components of the apparatus, thereby limiting the extent to which the apparatus can be miniaturized.
Embodiments of the invention address these deficiencies by providing an apparatus that features a liquid lens with a shorter focal length than hitherto possible.
To address one or more of the above-discussed deficiencies, one embodiment is an apparatus. The apparatus comprises a substrate with a top surface and a liquid lens on the top surface. A clear retaining fluid surrounds the lens. One of the retaining fluid and liquid lens comprises a nonpolar liquid, and the other of the retaining fluid and liquid lens comprises a polar liquid. The nonpolar liquid includes one or more cyclic saturated organic compounds.
Another embodiment is a method of use that comprises transmitting an optical signal using the above-described liquid lens. Transmitting includes directing the optical signal towards the liquid lens, the liquid lens being located a top surface of a substrate and surrounded by the above-described clear retaining fluid. Transmitting also includes refracting the optical signal at an interface between the liquid lens and retaining fluid and passing the refracted optical signal onto a receiving surface.
The invention is best understood from the following detailed description, when read with the accompanying FIGUREs. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Embodiments of the invention benefit from the recognition that the refractive index contrast, that is the difference, between a liquid lens and a surrounding retaining fluid can be increased by using a nonpolar fluid that comprises cyclic saturated organic compounds. In particular, one of a retaining fluid surrounding a liquid lens, or the liquid lens itself, comprises a nonpolar fluid that includes a cyclic saturated organic compound. The other of the retaining fluid and liquid lens comprises a polar liquid. While cyclic saturated organic compounds have been considered for ultraviolet lithography applications, their beneficial use in refractive liquid lens optics has not previously been recognized.
One embodiment of the invention is an apparatus. In some cases, the apparatus can be a tunable light-processing device. In tunable devices, the direction of light passed through the liquid lens, e.g., to focus the light, can be adjusted by applying a voltage to the liquid lens or to the retaining fluid to change the shape of the liquids lens. Example devices include MEMS devices that are incorporated into image projectors, televisions, and computer or cell-phone displays. In other cases, however, the apparatus can be a passive light-processing device. In passive a light-processing device, the direction of light passing through the liquid lens is not altered by applying a voltage to change the shape of the lens.
The term clear retaining fluid as used herein refers to a retaining fluid 120 that is substantially transparent to a light 135 (e.g., a U.V. or visible light configured as an optical signal to communicate information) configured to pass through the lens 115 for the purposes of altering the direction of the light 135. E.g., in some cases, at least about 80%, and more preferably at least about 90%, of the light 135, at the wavelength of interest, is transmitted through a 1 cm pathlength of retaining fluid.
The term polar liquid 130 as used herein refers to a liquid having a dielectric constant of about 20 or greater (e.g., water and acetone have dielectric constants of about 80 and 21, respectively). The term non-polar liquid 125 as used herein refers to a liquid that has a dielectric constant of less than about 20.
The refractive index (nnp) of the non-polar liquid 125 and refractive index (np) of the polar liquid 130 are substantially different from each other. E.g., in some preferred embodiments, the difference (Δn=np−nnp) is greater than about 0.15, and more preferably, greater than about 0.25, at the wavelength of light 135 passed through the lens 115 (e.g., 589 nm or other visible or U.V. wavelengths). In other cases a ratio of nnp to np ranges from about 1.13:1 to 1.2:1.
In addition to increasing the refractive index contrast with the liquid lens 115, the retaining fluid 120 surrounding the liquid lens 115 advantageously protects the liquid lens 115 from evaporation. The retaining fluid 120 can also deter the undesired movement of the liquid lens 115 due to, e.g., movement or vibration of the apparatus 100.
It is desirable for the liquid lens 115 and retaining fluid 120 to form a refractive index contrast interface 140 between these two structures 115, 120. A sharp interface is facilitated by selecting nonpolar liquids 125 and polar liquids 130 that are substantially immiscible in each other. E.g., in some preferred embodiments, the volume fraction solubility of the nonpolar liquid 125 in the polar liquid 130 is about 1 percent or less and more preferably, about 0.1 percent or less at the operating temperature range of the apparatus 100.
A nonpolar liquid 125 that comprises, and sometimes is, a cyclic saturated organic compound has several advantageous. Cyclic saturated organic compounds having a high density (e.g., a density of about 0.95 gm/cm3 or greater) also have a high refractive index (e.g., about 1.5 or higher at visible wavelengths). Consequently, there is a large refractive index difference compared to polar liquids or lower density non-polar alkanes. Cyclic saturated organic compounds are also immiscible in polar liquids, which is conducive to forming a sharp interface 140 lens 115 and fluid 120. Cyclic saturated organic compounds are more transparent to a broad range of U.V. and visible wavelengths of light 135, as compared to e.g., unsaturated acyclic or cyclic organic compound.
It is desirable for the cyclic saturated organic compound to be free of any conjugation of pi-bonds so as to minimize the absorption of the light 135 passed through the lens 115. Preferred embodiments of the cyclic saturated organic compound include a polycyclic cycloalkane. The polycyclic cycloalkane comprises at least two saturated hydrocarbon rings joined together with common atoms (e.g., ortho-fused rings). Some preferred polycyclic cycloalkanes have a refractive index ranging from about 1.5 to 1.6 (e.g., at about 589 nm or other visible wavelengths) Examples include ortho-fused and ortho- and peri-fused saturated hydrocarbons rings having 2 to 6 rings, such as decalin (I), perhydrofluorene (II), or perhydrophenanthrene (III):
Polycyclic cycloalkanes having a large number of rings (e.g., 4 or more rings) are desirable because such compound tend to have a higher refractive index than polycyclic cycloalkanes with a lesser number of rings, but still remain transparent at visible or U.V. wavelengths. Examples include perhydropyrene (IV), perhydrotetracene (V), perhydronapthoanthracene (VI), and perhydronapthotetracene (VII) and adamentane (VIII):
It can be advantageous for the nonpolar liquid 125 to include more than one cyclic saturated organic compound, or other organic compounds, to increase the refractive index contrast (e.g., increase Δn). E.g., the nonpolar liquid 125 can comprise one or more first cyclic saturated organic compounds that is a liquid in its pure form at the operating temperature range of the apparatus (e.g., about 0 to 50° C., and in some cases about 20° C.), plus a one or more second cyclic saturated organic compounds that is a solid in its pure form, but is soluble in the first cyclic saturated organic compound. Example first cyclic saturated organic compounds include two- or three-ring polycyclic cycloalkanes such as decalin (I), perhydrofluorene (II), or perhydrophenanthrene (III). Example second cyclic saturated organic compounds include four-ring or larger polycyclic cycloalkanes such as perhydropyrene (IV), perhydrotetracene (V), perhydronapthoanthracene (VI), and perhydronapthotetracene (VII) or adamentane (VIII).
In addition to increasing the refractive index of the nonpolar liquid 125 the second cyclic saturated organic compound can be added to lower the melting point of the nonpolar liquid 125. This may be useful when it is desirable for the nonpolar liquid 125, configured as either the liquid lens 115 or retaining fluid 120, to become solidified by lowering the temperature of the apparatus 100. E.g., after tuning the shape of the liquid lens 115, the nonpolar liquid 125 is solidified. Other methods to solidify liquid lens are discussed in U.S. Pat. No. 6,936,196 which is incorporated by reference herein in their entirety.
The use of electrically conductive polar liquids 130 is desirable in embodiments where the liquid lens 115 or retaining fluid 120 is configured to be tunable by applying a voltage to the polar liquid 130 to change the shape of the lens 115. Example polar liquids 130 include molten salts or aqueous or organic solutions of salts, such as described in U.S. Pat. Nos. 6,538,823; 6,891,682; and the above-mentioned U.S. Pat. No. 6,936,196 patent, all of which are incorporated by reference herein in their entirety. Some preferred embodiments of the polar liquid 130 have an index of refraction ranging from about 1.3 to 1.4 (e.g., at about 589 nm or other visible wavelengths) Other preferred embodiments in include room temperature molten salts like 1-ethyl-3-methylimidazolium tetrafluoroborate.
In some cases to facilitate the focusing of light 135, the liquid lens 115 is preferably configured as a droplet disposed on the substrate's surface 110. In other instances, however, the liquid lens 115 can be configured to have other shapes, e.g., an ellipsoidal or planar shape, if desired.
A focal length (f) 145 of the lens 115 can be changed by changing its shape. E.g., the shape of a liquid lens 115 configured as a droplet, such as shown in
The focal length 145 of the liquid lens 115 also depends upon the radius (r) 155 of the lens 115 and the refractive index contrast (e.g., Δn) between the lens 115 and the retaining fluid 120. The focal length 145 is given by the equation:
f=r/Δn
where r is the surface curvature of the lens 115 in meters (see e.g., the U.S. Pat. No. 6,538,823 patent). It follows therefore, that a ratio of the focal length 145 to the radius 155 of the liquid lens 115 is inversely related to Δn (e.g., f/r=1/Δn). Therefore the focal length 145 can be decreased by increasing Δn.
Consider embodiments of the apparatus 100 where the liquid lens 115 has a radius of about 100 microns. The liquid lens 115 is a polar liquid 130 having a refractive index of about 1.33, and the retaining fluid 120 is a non-polar liquid having a refractive index of about 1.5 to 1.6 (e.g., Δn=0.27 to 0.17). In such embodiments, the focal length 145 ranges from about 370 to 580 microns. That is, the ratio of focal length 145 to the radius 155 ranges from about 3.7:1 to 5.8:1. This is substantially shorter than a focal length 145 of about 1000 microns, obtained for a liquid lens 115 of the same curvature, but surrounded by a retaining fluid 120 having a refractive index of about 1.43 (e.g., Δn=0.1).
In some cases, the insulating layer 205 can include an opening 215 to allow the liquid 110 to contact a biasing electrode 220 that is in contact with the liquid lens 115. As shown in
In some preferred embodiments one of the liquid lens 115 and the retaining fluid 120 is electrically conductive and is disposed over a surface 225 of the insulating layer 205, and the other of the liquid lens 115 or the retaining fluid 120 is not electrically conductive. E.g., in some cases, the liquid lens 115 comprises an electrically conductive polar liquid 130 (e.g., a molten salt or aqueous or organic solvent having salts dissolved therein), and the retaining fluid 120 is a non-conducting non-polar liquid 125 (e.g., a cyclic saturated organic compound such as decalin or perhydrofluorene). In other cases, liquid lens 115 is a non-conducting non-polar liquid 125 and the retaining fluid 120 is a conducting polar liquid 130.
The plurality of electrodes 210 are configured to adjust the shape of the liquid lens 115 (e.g., a lateral position 230 or a contact angle 150 of the liquid lens 115 relative to the insulating layer's surface 225) when a voltage (V) is applied between the liquid lens 115 (e.g., via biasing electrode 220) and one or more of the electrodes 210.
In some embodiments, it is desirable for the liquid lens 115, the insulating layer 205, the substrate 105 and the electrodes 210 to be transparent with respect to the light 135 to be passed through the lens 115. E.g., the transparent liquid lens 115 can comprise water or molten salt, the transparent insulating layer 205 can comprise a polyimide, the transparent conductive substrate 105 can comprise glass, silicon dioxide, quartz, sapphire, diamond or other transparent solid materials, and the transparent electrodes 210 can comprise indium tin oxide.
In some cases, the insulating layer's surface 225 is covered with a coating of low-surface-energy material 240. The coating 240 serves to adjust the contact angle 150 of the liquid lens 115 to a predefined value (e.g., from about 80 to 100 degrees in some embodiments). Adjusting the contact angle 150 advantageously modifies the refractive properties (e.g., focal length) of the liquid lens 115. The term low-surface-energy material, as used herein, refers to a material having a surface energy of about 22 dyne/cm (about 22×10−5 N/cm) or less. Those of ordinary skill in the art would be familiar with the methods to measure the surface energy of materials. In some instances, the coating 240 comprises a fluorinated polymer like polytetrafluoroethylene or other highly fluorinated hydrocarbon, or an alkylsilane like polydimethylsilane. In some instances, the insulating layer 205 and low surface energy coating 240 comprise a single material, such as Cytop® (Asahi Glass Company, Limited Corp. Tokyo, Japan), a fluoropolymer that is both an electrical insulator and a low-surface-energy material.
The liquid mirror 305 can be configured to alter the optical signal 340 in any number of ways familiar to those skilled in the art. E.g., the liquid lens 305 can alter the direction of the optical signal 340 by focusing or diffusing the signal 340. When the liquid lens 305 is configured as a tunable liquid lens, the shape or position of the lens 305 can be adjusted to improve the optical coupling between various components of the apparatus 300.
As further illustrated in
Another aspect of the invention is a method of use that comprises transmitting an optical signal using a liquid lens. Any of the embodiments of the liquid lenses described in the context of
As illustrated in
In some cases, the optical signal comprises parallel beams of light 135 that are directed to the retaining fluid 120 and the liquid lens 110. The optical signal 135 can be refracted though the fluid 120 and lens 115 and thereby be focused or concentrated at a focal point 160. In such cases the liquid lens 115 is referred to as a concentrating lens. E.g., the concentrating liquid lens 115 comprises a material (e.g., a polar liquid 130) having a lower index of refraction than the surrounding retaining fluid 120 (e.g., a non-polar liquid 125). In such cases the receiving surface 170 at the focal point 160 of the lens, and can be part of an optical signal receiver e.g., a photodetector. In some embodiments, the focal length 145 between the liquid lens 115 and the receiving surface 170 ranges from about 3.7 to 5.9 times the radius 155 of the liquid lens 115.
In other cases, the optical signal 135, can be refracted through the lens 115 and fluid 120 and thereby be diverged into substantially parallel beams of light 135. In such cases the liquid lens 115 is referred to as a diverging lens. E.g., the diverging liquid lens 115 liquid lens 115 comprises a material (e.g., a non-polar liquid 125) having a higher index of refraction than the surrounding retaining fluid 120 (e.g., a polar liquid 130). The receiving surface 170, is such cases may be another lens or mirror located in the path of the parallel beams of light 135, which then focuses or reflect the optical signal 135 to another component of the apparatus 100.
In some preferred embodiments, transmitting the optical signal further includes tuning the liquid lens by changing the shape of the lens. For instance, as illustrated in
In some cases tuning includes increasing a focal length 145 of the liquid lens 115 by applying a voltage (V) to the liquid lens 115. E.g., when the voltage is applied, the contact angle 150 of the liquid lens 115 decreases, thereby increasing the focal length. In other cases, tuning includes decreasing a focal length 145 of the liquid lens 115 by removing a voltage (V) applied to liquid lens 115. A non-conductive non-polar liquid lens 115 could be similarly tuned by apply a voltage between a retaining fluid 120 comprising an electrically conductive polar liquid, and the plurality of electrodes 210.
Tuning the liquid lens is not limited to tuning a liquid droplet, however. E.g., the apparatus 400 shown in
An example tunable liquid lens 115 at different stages of use is illustrated in
Although the present invention has been described in detail, those of ordinary skill in the art should understand that they can make various changes, substitutions and alterations herein without departing from the scope of the invention.
