Patent Publication Number: US-9411155-B2

Title: Optical zoom probe

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2012-0109260, filed on Sep. 28, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments relate to optical zoom probes, and more particularly, to optical zoom probes in which scanning may be performed both in a near distance and in the nearest distance. 
     2. Description of the Related Art 
     In the field of medical imaging, demands for performing tomography using information regarding the surface of a tissue (human body or skin) are increasing. In particular, because most types of cancers are generated in lower parts of epithelial cells and are propagated into dermal cells in which blood vessels exist, when cancer can be detected at an early stage, damage caused by cancer can be remarkably reduced. With respect to existing imaging techniques, such as magnetic resonance imaging (MRI), x-ray computed tomography (CT), and ultrasonic waves, although tomography can be performed on an inside of a tissue by penetrating the skin, early detection of cancer which involves a tumor having a small size cannot be performed due to low resolution. Conversely, because light is used in performing an optical coherence tomography (OCT) technique that has been recently introduced, unlike in the existing art, the depth of penetration into the skin is low, i.e., about 2 to 3 mm, and resolution is high, about 10 times better than the resolution of ultrasonic waves. Thus, the OCT technique is expected to be useful in diagnosing early stages of cancer which involve a tumor having the size of about 50 to 100 μm. However, because the resolution of the OCT technique is lower than that of a microscope, the OCT technique cannot replace biopsy and histology that are actually used for making conclusive determinations relating to cancer diagnoses. 
     Recently, some OCT researchers have conducted investigations relating to performing diagnosis of cancer inside a tissue in real-time by combining tomography characteristics of OCT and high-resolution surface photographing using a confocal microscope, for example, and without performing biopsy. However, an objective lens of the microscope requires an optical system with a high numerical aperture (NA) so as to realize a high resolution in a horizontal direction, whereas OCT requires an optical system which has relatively uniform spot sizes in a depth direction, i.e., having a large depth of focus (DOF) with a low NA. 
     SUMMARY 
     Provided are optical zoom probes in which high-resolution scanning may be performed while a focus is moved in the nearest distance mode and in a near distance mode. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments. 
     According to an aspect of one or more exemplary embodiments, an optical zoom probe includes: an aperture adjuster which adjusts an aperture through which light which is transmitted by a light transmitter propagates; and a focus adjuster which focuses light that propagates through the aperture and which includes a first liquid lens and a second liquid lens, for each of which respective curvatures are independently controllable so as to adjust a respective focal length. 
     The optical zoom probe may further include a third lens which has a positive power and which is arranged between the focus adjuster and an object. 
     The third lens may include an aspheric lens. 
     A transparent medium may be interposed between the first liquid lens and the second liquid lens, and the first liquid lens, the transparent medium, and the second liquid lens may be formed as a single body. 
     In a near scanning mode, each of the first and second liquid lenses may be caused to have a concave lens surface. 
     In a nearest scanning mode, at least one of the first and second liquid lenses may be caused to have a convex lens surface. 
     In the nearest scanning mode, one of the first and second liquid lenses that is closer to a scanning object may be caused to have a convex lens surface. 
     A protrusion variation quantity of each of the first and second liquid lenses may be equal to or less than 400 μm. 
     At least one of the first and second liquid lenses may further include a transparent film which has a curved surface, and in a near scanning mode, the curved surface of the transparent film may act as a lens surface, and in a nearest scanning mode, the curved surface of the transparent film may not act as the lens surface. 
     Each of the first and second liquid lenses may form a lens surface in conjunction with a surface of a fluid and may adjust the respective focal length by adjusting a respective shape of the lens surface by using a flow of the fluid. 
     The first and second liquid lenses may be configured such that the fluid flows in a first direction with respect to the first liquid lens, which first direction is opposite to a second direction with respect to the second liquid lens. 
     The flow of the fluid may occur according to an electrowetting principle. 
     At least one of the first and second liquid lenses may include: a first fluid that is transparent; a second fluid that does not mix with the first fluid and is transparent; a chamber having an internal space in which the first fluid and the second fluid are accommodated; a first surface that is a boundary surface between the first fluid and the second fluid and forms the lens surface; a second surface that is a boundary surface between the first fluid and the second fluid and induces a variation in a curvature of the lens surface; a first intermediate plate that is disposed in the chamber and has a first through hole which has a diameter that corresponds to the curvature of the lens surface and a second through hole that forms a passage of the second fluid; and an electrode which forms an electric field that causes a variation in a position of the second surface. 
     The first fluid may include a polar liquid, and the second fluid may include one from among a gas and a non-polar liquid. 
     The fluid may be caused to flow by using pressure. 
     At least one of the aperture adjuster and the focus adjuster may include a cover glass, and the cover glass may have a gradient which is less than or equal to about 12 degrees with respect to an optical axis. 
     The cover glass may be disposed on the aperture adjuster. 
     The cover glass may be disposed on at least one of an input terminal of the focus adjuster and an output terminal of the focus adjuster. 
     The optical zoom probe may further include at least one of: a first lens unit which collimates light which is transmitted by the light transmitter to be transmitted to the aperture adjuster; and a second lens unit which is disposed between the aperture adjuster and the focus adjuster. 
     The aperture adjuster may include a liquid iris diaphragm for which an aperture size is adjustable by using a microelectrofluidic method. 
     The aperture adjuster may include: a chamber forming a space in which a fluid flows; a first fluid and a second fluid that are disposed in the chamber and which do not mix with each other, wherein one of the first fluid and the second fluid is formed of a transparent material, and an other one of the first fluid and the second fluid is formed of one of a light-shielding material and a light-absorbing material; and an electrode unit which is disposed at an inside of the chamber and in which at least one electrode to which a voltage is applied, which voltage form an electric field in the chamber, is arranged, and wherein the aperture through which light propagates is adjustable based on a variation in a position of an interface between the first fluid and the second fluid based on the electric field. 
     One of the first fluid and the second fluid may include one of a liquid metal and a polar liquid, and an other one of the first fluid and the second fluid may include one of a gas and a non-polar liquid. 
     The light transmitter may include an optical fiber. 
     The optical fiber may have at least one characteristic from among a first characteristic of a gradient which is less than or equal to about 12 degrees with respect to an optical axis on an end of the optical fiber and a second characteristic of being anti-reflection coated. 
     According to another aspect of one or more exemplary embodiments, an image diagnosis system includes: a light source; the above-described optical zoom probe that irradiates light emitted from the light source onto an object; and a detector which detects an image of the object by using light reflected from the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a conceptual view which illustrates a relationship between horizontal resolution and a depth of focus (DOF); 
         FIG. 2  is a view which schematically illustrates an overall optical configuration of an optical zoom probe, according to an exemplary embodiment; 
         FIG. 3  illustrates an example of an aperture adjuster that may be used in the optical zoom probe illustrated in  FIG. 2 ; 
         FIG. 4  illustrates another example of an aperture adjuster that may be used in the optical zoom probe of  FIG. 2 ; 
         FIG. 5  illustrates an example of a liquid lens that may be used as a first liquid lens and/or a second liquid lens of a focus adjuster of the optical zoom probe of  FIG. 2 ; 
         FIG. 6  illustrates another example of a liquid lens that may be used in a first liquid lens and/or a second liquid lens of the focus adjuster of the optical zoom probe of  FIG. 2 ; 
         FIG. 7  illustrates an example in which a focus adjuster is configured to include first and second liquid lenses by combining liquid lenses of  FIG. 5  with each other symmetrically; 
         FIG. 8  illustrates another example of a liquid lens that may be used as a first liquid lens or a second liquid lens of the focus adjuster of the optical zoom probe of  FIG. 2 ; 
         FIGS. 9A and 9B  illustrate an operating state of the optical zoom probe of  FIG. 2  when an object, for example, a tissue is scanned from a surface of the tissue to a relatively shallow depth; 
         FIG. 10  illustrates an operating state of the optical zoom probe of  FIG. 2  when scanning is performed with a long focal length; 
         FIG. 11  is a view which schematically illustrates an overall optical configuration of an optical zoom probe according to another exemplary embodiment; 
         FIG. 12  is a schematic view of an image diagnosis system using the optical zoom probe, according to an exemplary embodiment; and 
         FIGS. 13, 14, and 15  are schematic views of various optical systems that may be used in adjusting the length of a path of reference light when an optical coherence microscopy (OCM) mode and an optical coherence tomography (OCT) mode are used, according to one or more exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     Hereinafter, the present inventive concept will be described in detail by explaining exemplary embodiments with reference to the attached drawings. Like reference numerals in the drawings denote like elements. The sizes of elements in the drawings may be exaggerated for clarity and convenience. 
       FIG. 1  is a conceptual view which illustrates a relationship between horizontal resolution and a depth of focus (DOF). 
     A Gaussian beam is not a point, but instead has a beam waist which has a finite size Δx when being focused, and Δx is determined based on an aperture D and a focal length f by using Equation 1: 
     
       
         
           
             
               
                 
                   
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     Δx is related to horizontal resolution. In particular, as Δx decreases, horizontal resolution increases. As shown in Equation 1, because Δx is proportional to f/D and a numerical aperture (NA) of a focusing lens FE is proportional to D/f, an optical system which has a relatively large value of NA is needed so that Δx decreases, in order to obtain high horizontal resolution. 
     A depth of focus (DOF) is determined based on a beam diameter of √{square root over (2)}Δx by using Equation 2: 
     
       
         
           
             
               
                 
                   DOF 
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                       π 
                       
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                       2 
                     
                   
                 
               
               
                 
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     The DOF is a range within which the sizes of beam spots are relatively uniform in a depth direction. When image information which is based on a depth, such as, for example, an image of a tissue of the human body, is captured using tomography, an optical system is required to have a relatively large value of DOF, i.e., in order to have a correspondingly small value of NA. 
     In this way, horizontal resolution and the DOF have a trade-off relationship. 
     An optical zoom probe according to an exemplary embodiment may realize a horizontal resolution and a DOF that are required in order to scan an object with high resolution in a nearest distance mode and in a near distance mode. In particular, the “nearest distance” refers to a case for which a distance between a last lens of the optical zoom probe and an object, for example, the surface of a tissue, is equal to or less than about 2 mm, and the “near distance” refers to a case for which the distance between the last lens of the optical zoom probe to the object, for example, the surface of the tissue, is equal to or less than about 30 mm. 
       FIG. 2  is a view which schematically illustrates an overall optical configuration of an optical zoom probe  10 , according to an exemplary embodiment. 
     Referring to  FIG. 2 , the optical zoom probe  10  includes an aperture adjuster  50  that adjusts an aperture through which light which is transmitted through a light transmitter  20  propagates, and a focus adjuster  70  which includes a first liquid lens  71  and a second liquid lens  75 , for each of which respective curvatures are independently adjustable. The optical zoom probe  10  may further include a third lens  80  which has a positive power in order to optimize a focus between the focus adjuster  70  and an object, for example, a tissue to be inspected. The third lens  80  may be, for example, an aspheric lens. The optical zoom probe  10  may further include at least one of a first lens unit  30  that collimates light transmitted by the light transmitter  20  to be transmitted to the aperture adjuster  50  and a second lens unit  60  that is disposed between the aperture adjuster  50  and the focus adjuster  70 .  FIG. 2  and the remaining drawings illustrate exemplarily that the optical zoom probe  10  includes both of the first lens unit  30  and the second lens unit  60 . Hereinafter, the optical zoom probe  10  shall be described based on an optical system of  FIG. 2 , aspects of the present inventive concept are not limited thereto, and various modifications and equivalent embodiments thereof are possible. 
     The light transmitter  20  may further include an optical fiber  21  and a scanner  23  that is disposed on an end of the optical fiber  21 . The scanner  23  is an actuator that induces deformation of the optical fiber  21  in order to change a path of light, such as, for example, a piezoelectric actuator or a cantilever which uses a piezoelectric body or a shape memory alloy. In addition, the scanner  23  may be formed using one or more of various materials in any of various ways. 
     The optical fiber  21  may have a gradient which is less than or equal to about 12 degrees with respect to an optical axis on its end, or may be anti-reflection coated on its end, or may have two composite characteristics such that noise which is caused by reflective light on the end of the optical fiber  21  to be scanned may be removed. 
     The first lens unit  30  collimates light which is transmitted by the light transmitter  20  such that light incident on the aperture adjuster  50  may be parallel light or approximately parallel light. The first lens unit  30  may include at least one of lenses  31  and  35 .  FIG. 2  and the following drawings illustrate exemplarily that the first lens unit  30  includes a single lens  31  and a doublet lens  35  that is spaced apart from the single lens  31  by a predetermined distance. This is just for illustrative purposes, and a lens configuration of the first lens unit  30  may be modified in various ways. 
     The aperture adjuster  50  adjusts the size of a light beam which is incident on the focus adjuster  70  in order to change the NA of the focus adjuster  70 . For example, in an optical coherence microscopy (OCM) mode in which uniform high resolution is required in a section of about 2 mm in a depth direction in the relatively nearest scanning distance of about 2 mm or less, the aperture adjuster  50  increases the size of the light beam which is incident on the focus adjuster  70  so as to obtain a relatively high NA. In addition, in an optical coherence tomography (OCT) mode in which uniform spot sizes are required in a section of about 2 mm in the depth direction in the near scanning distance of between about 2 mm and 30 mm, the aperture adjuster  50  decreases the size of the light beam which is incident on the focus adjuster  70  so as to obtain a relatively low NA. 
     The aperture adjuster  50  may be, for example, a liquid iris diaphragm for which an aperture size is adjustable by using a microelectrofluidic method. In addition, the aperture adjuster  50  may be, for example, an iris diaphragm for which an aperture size is mechanically adjustable, or may be a liquid iris diaphragm for which an aperture size is adjustable by using a hydraulic pressure generated by a pump. 
     The aperture adjuster  50  may be an aperture adjuster  101  or  102  as illustrated in  FIG. 3 or 4 , for example. 
       FIG. 3  illustrates an example of the aperture adjuster  101  that may be used in the optical zoom probe  10  illustrated in  FIG. 2 . 
     Referring to  FIG. 3 , the aperture adjuster  101  may be configured so that a fluid may flow according to an electrowetting principle, and the size of an aperture A through which light propagates may be adjusted according to the flow of the fluid. The aperture adjuster  101  includes a chamber that forms a space in which the fluid flows, a first fluid F 1  and a second fluid F 2  that are accommodated in the chamber and that do not mix with each other, wherein one of the first fluid F 1  and the second fluid F 2  is formed of a transparent material and the other one is formed of one of a light-shielding material and a light-absorbing material, and an electrode unit which is disposed at an inside of the chamber and in which at least one electrode to which a voltage is applied, which voltage forms an electric field in the chamber, is arranged. The aperture A through which light propagates is adjusted based on a variation in the position of an interface between the first fluid F 1  and the second fluid F 2  based on the electric field. 
     For example, an area of the chamber includes a first channel C 1  and a second channel C 2  that is disposed above the first channel C 1  and is connected to the first channel C 1 . The range of the aperture A may be determined based on a variation in the position of the interface between the first fluid F 1  and the second fluid F 2  that occurs in each of the first channel C 1  and the second channel C 2 . The first channel C 1  may be formed by a first substrate  110 , a second substrate  150  that is spaced apart from the first substrate  110  by a predetermined distance, and a first spacer  130  that forms an internal space between the first substrate  110  and the second substrate  150 . In this case, a first through hole TH 1  may be formed in the center of the second substrate  150 , and a second through hole TH 2  may be formed on the periphery of the second substrate  150 . In addition, the second channel C 2  may be formed by the second substrate  150 , a third substrate  190  that is spaced apart from the second substrate  150  by a predetermined distance, and a second spacer  170  that forms an internal space between the second substrate  150  and the third substrate  190 . 
     One of the first fluid F 1  and the second fluid F 2  may be one of a liquid metal and a polar liquid, and the other one thereof may be one of a gas and a non-polar liquid. 
     The electrode unit may include a first electrode unit  120  that is formed on the first substrate  110  and includes at least one electrode E which is coated with an insulating material I, and a second electrode unit  180  that is formed on the third substrate  190  and includes at least one electrode E which is coated with the insulating material I. 
     The first electrode unit  120  may include a plurality of electrodes which are configured to digitally control the aperture A. 
     A ground electrode unit R may be disposed to maintain contact with a polar fluid in one or more portions inside the chamber. For example, the ground electrode unit R may be disposed to maintain contact with the polar first fluid F 1 . To this end, as illustrated in  FIG. 3 , the ground electrode unit R may be disposed on the first substrate  110 , and the position of the ground electrode unit R may be changed. 
     At least one electrode E that is a constituent of each of the first electrode unit  120  and the second electrode unit  180  may be formed of a transparent conductive material. For example, at least one electrode E may be formed of a metal oxide, such as indium tin oxide (ITO) or indium zinc oxide (IZO); a metal nanoparticle dispersion thin layer, such as gold (Au) or silver (Ag); a carbon nanostructure, such as carbon nanotube (CNT) or graphene; or a conductive polymer, such as poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), or poly(3-hexylthiophene) (P3HT). 
     Because the ground electrode unit R does not require transparency in relation of a position, the ground electrode unit R may be formed of a metal thin layer, such as Au, Ag, aluminum (Al), chromium (Cr), or titanium (Ti). 
     An electrowetting phenomenon refers to the case in which, when a voltage is applied to an electrolyte droplet on an electrode which is coated with an insulating material, an angle of contact between the electrode and the electrolyte droplet is varied. In particular, the contact angle varies based on each interfacial tension in a three-phase contact line (TCL) in which the fluid, the droplet, and the insulating material contact one another. When the electrowetting phenomenon is used, the flow of the fluid may be quickly and efficiently controlled by using a low voltage, and in addition, the fluid may be transferred and controlled. 
     When an appropriate voltage is applied to one electrode E of the first electrode unit  120 , an electromechanical force may act in the TCL on an activated driving electrode, i.e., in a contact line where the first fluid F 1 , the second fluid F 2 , and the insulating material I contact one another, and the first fluid F 1  may move toward the center of the second substrate  150  via the first channel C 1  so that the aperture A may be reduced. In addition, when an appropriate voltage is applied to the second electrode unit  180 , the first fluid F 1  may move toward the center of the second substrate  150  via the second channel C 2 , and the TCL of the first channel C 1  may be pushed out toward the edge so that the aperture A may be enlarged. When the first electrode unit  120  includes a plurality of electrodes E, the size of the aperture A may be digitally controlled by varying the activated electrode. 
     Referring back to  FIG. 2 , the aperture adjuster  50  may include a cover glass  51  that is disposed on at least one of an input terminal and an output terminal of the aperture adjuster  50 . In  FIG. 2 , the aperture adjuster  50  includes the cover glass  51  that is disposed on the input terminal of the aperture adjuster  50 . Reference numeral  55  of  FIG. 2  represents a portion of the aperture adjuster  50  where the size of the aperture A is adjusted. 
     When the size of the aperture A is adjusted by the fluid that flows according to the electrowetting principle, as illustrated in  FIGS. 3 and 4 , the first substrate  110  or the third substrate  190  of the aperture adjuster  101  may be used as a cover glass, or the aperture adjuster  101  may include an additional cover glass. 
       FIG. 4  illustrates another example of the aperture adjuster  102  that may be used in the optical zoom probe  10  of  FIG. 2 . 
     The aperture adjuster  102  of  FIG. 4  is different from the aperture adjuster  101  of  FIG. 3  in that the aperture adjuster  102  of  FIG. 4  further includes a third electrode unit  320  and a fourth electrode unit  380 , each including at least one electrode E which is coated with the insulating material I, on both surfaces of the second substrate  150 . The third electrode unit  320  may be used for increasing a driving force which is generated in the first channel C 1  in addition to the driving force which is generated by the first electrode unit  120 , and the fourth electrode unit  380  may be used for increasing a driving force which is generated in the second channel C 2  in addition to the driving force which is generated by the second electrode unit  180 . The number of electrodes that constitute the third electrode unit  320  and the fourth electrode unit  380  is not limited to the number of electrodes illustrated in  FIG. 4  and may be modified in various ways. In addition, although the third electrode unit  320  and the fourth electrode unit  380  are disposed on both surfaces of the second substrate  150 , this is just for illustrative purposes, and the third electrode unit  320  or the fourth electrode unit  380  may be disposed only one surface of the second substrate  150 . 
     Referring back to  FIG. 2 , the second lens unit  60  is used to transmit light that propagates through the aperture adjuster  50  to the focus adjuster  70 , and may include at least one lens. 
     The focus adjuster  70  may include the first and second liquid lenses  71  for each of which respective curvatures are independently controllable so as to focus on light that propagates through the aperture of the aperture adjuster  50  and to adjust a respective focal length. 
     A transparent medium  73  may be interposed between the first liquid lens  71  and the second liquid lens  75 . The first liquid lens  71 , the transparent medium  73 , and the second liquid lens  75  may be formed as a single body in a state where the transparent medium  73  is interposed between the first liquid lens  71  and the second liquid lens and  75 . In this case, the first and second liquid lenses  71  and  75  may be moved in opposite directions when a focus-adjusting operation is performed in a state where one transparent medium  73  is interposed between the first liquid lens  71  and the second liquid lens  75 , so as to optimize the focus adjuster  70  in a lengthwise direction. In this regard, the protrusion variation quantity of the first and second liquid lenses  71  and  75  may not be greater than 400 μm so as to minimize a distance between the first and second liquid lenses  71  and  75  when the curvatures of the first and second liquid lenses  71  and  75  are adjusted. 
     Each of the first and second liquid lenses  71  and  75  may be disposed to form a respective lens surface in conjunction with a surface of the fluid and to adjust the corresponding focal length by adjusting the shape of the lens surface by using the flow of the fluid. 
     When x,y scanning is performed in a predetermined range in a depth direction in the nearest distance mode between a last lens (lens  80  in  FIG. 2 ) of the optical zoom probe  10  and an object, for example, a tissue, at a nearest distance of about 2 mm or less, that is, in the OCM mode, at least one of the first and second liquid lenses  71  and  75  may be driven or caused to have a convex lens surface, as will be described below with reference to  FIGS. 9A and 9B . In this case, when scanning is performed at a nearest distance, that is, in the OCM mode, a liquid lens of the first and second liquid lenses  71  and  75  that is closer to the tissue, i.e., the second liquid lens  75 , may be driven to have the convex lens surface. 
     In addition, when x,y scanning is performed in a predetermined range in the depth direction in the near distance mode between the last lens (lens  80  in  FIG. 2 ) of the optical zoom probe  10  and the object, for example, the tissue, for example, at a near distance of about 30 mm or less, that is, in the OCT mode, the first and second liquid lenses  71  and  75  may be driven or caused to have convex lens surfaces, as will be described below with reference to  FIG. 10 . 
     The first and second liquid lenses  71  and  75  may be disposed so that the flow of the fluid, for example, may occur according to the electrowetting principle. 
       FIG. 5  illustrates an example of a liquid lens  201  that may be used as the first liquid lens  71  and/or the second liquid lens  75  of the focus adjuster  70  of the optical zoom probe  10  of  FIG. 2 . 
     Referring to  FIG. 5 , a first fluid TF 1  that is transparent and polar and a second fluid TF 2  that does not mix with the first fluid TF 1  are accommodated in the chamber of the liquid lens  201 . A boundary surface between the first fluid TF 1  and the second fluid TF 2  includes a first surface LS that forms a lens surface and a second surface IS that induces a variation in the curvature of the lens surface. In addition, an electrode unit which forms an electric field that causes a variation in the position of the second surface IS is formed in the chamber. A first intermediate plate  250 , which has a first through hole TH 1  that forms a diameter of a lens which corresponds to the lens surface and a second through hole TH 2  that forms a path of the second fluid TF 2 , is disposed in the chamber so that the boundary surface between the first fluid TF 1  and the second fluid TF 2  may form the first surface LS that forms the lens surface and the second surface IS that induces a variation in the curvature of the lens surface. 
     A lower substrate  210  and an upper substrate  290  may be respectively disposed below and above the first intermediate plate  250 , and a spacer unit may be formed between the lower substrate  210  and the first intermediate plate  250  and between the first intermediate plate  250  and the upper substrate  290  so as to form an internal space in the chamber. The spacer unit may include a first spacer  230  which is arranged between the lower substrate  210  and the first intermediate plate  250  and a second spacer  270  which is arranged between the first intermediate plate  250  and the upper substrate  290 . 
     Each of the lower substrate  210 , the first intermediate plate  250 , and the upper substrate  290  may be formed of a transparent material. 
     The first fluid TF 1  and the second fluid TF 2  may be transparent fluids having different refractive indices. In this regard, the first fluid TF 1  may be a polar liquid, and the second fluid TF 2  may be one of a gas and a non-polar liquid. 
     The electrode unit includes a first electrode unit  220  that is formed on a top surface of the lower substrate  210  and includes at least one electrode E for which a surface thereof is coated with the insulating material I, and a second electrode unit  280  that is formed on a bottom surface of the first intermediate plate  250  and includes at least one electrode E for which a surface thereof is coated with the insulating material I, as illustrated in  FIG. 6 . In an exemplary embodiment, only one of the first electrode unit  220  and the second electrode unit  280  may be disposed. 
     In addition, the electrode unit may further include a ground electrode R that contacts the first fluid TF 1 . Although the ground electrode R is illustrated as being disposed on the first substrate  210  in each of  FIG. 5  and  FIG. 6 , the ground electrode R may be located in a position where the ground electrode R may contact the first fluid TF 1  in a state where a voltage is not applied. The ground electrode R may be optional, and when the ground electrode R is disposed, a driving voltage may be reduced. 
     At least one electrode E that constitutes the first electrode unit  220  and the second electrode unit  280  may be formed of a transparent conductive material. For example, at least one electrode E may be formed of metal oxide, such as ITO or IZO, a dispersion thin film of metal nanoparticle such as Au or Ag, a carbon nanostructure, such as carbon nanotube (CNT) or grapheme, or a conductive polymer, such as poly(3,4-ethylenedioxythiophene)(PEDOT), polypyrrole (PPy), or poly(3-hexylthiophene) (P3HT). The ground electrode R may be formed of any one or more of the above-described transparent conductive materials, and when the ground electrode R does not require transparency based on its position, the ground electrode R may be formed of a thin film of metal such as Au, Ag, Al, Cr, or Ti. 
     In the liquid lens  201 , the pressure that is exerted onto the second surface IS varies due to electrowetting driving and thus, the curvature of the first surface LS that is the lens surface is adjusted based on the pressure. When no voltage is applied to the electrode E or when the magnitude of the applied voltage is decreased, the second surface IS may be moved toward the center of the liquid lens  201 , and the first surface LS that is the lens surface may become more convex. If the magnitude of the applied voltage is increased, the second surface IS may be moved toward both sides of the liquid lens  201 , and the curvature of the first surface LS is decreased, and when the magnitude of the applied voltage is at a maximum, the first surface LS may have a concave curvature. 
     In  FIG. 5 , each of the first electrode unit  220  and the second electrode unit  280  includes one electrode E, and the position of the second surface IS varies by adjusting the magnitude of the voltage which is applied to the electrode E. 
     In particular, either or both of the first electrode unit  220  and the second electrode unit  280  may include a plurality of electrodes E which are coated with the insulating material I. In this regard, a voltage is applied to at least one of the electrodes E that constitute the first electrode unit  220  and the second electrode unit  280  so that the curvature of the first surface LS that is the lens surface may be digitally controlled. More particularly, when an appropriate voltage is applied to one electrode which is selected from the electrodes E, an electromechanical force is exerted on a TCL of the activated driving electrode, i.e., on a line where the second surface IS that is a boundary surface between the first fluid F 1  and the second fluid F 2  and the insulating material contact each other, and the position of the second surface IS is formed, and the curvature of the first surface LS may be determined based on the position of the second surface IS. When an appropriate voltage is applied to the electrode E that is disposed in the innermost portion of the electrode unit, the second surface IS may be moved toward the center of the liquid lens  201  to the utmost so that the curvature of the first surface LS may be increased. In addition, when an appropriate voltage is applied to the electrode E that is disposed in the outermost portion of the electrode unit, the second surface IS may be moved to both sides of the liquid lens  201  to the utmost, and the curvature of the second surface LS may be increased or concave. 
       FIG. 6  illustrates another example of a liquid lens  202  that may be used in the first liquid lens  71  and/or the second liquid lens  75  of the focus adjuster  70  of the optical zoom probe  10  of  FIG. 2 . 
     Referring to  FIG. 6 , the liquid lens  202  may include a transparent film  203  which has a lens shape and which is disposed between liquids so that a lens surface for which shape control is not necessary in the z-axis direction due to a long focal length of about 30 mm or a lens surface for which shape deformation is minimized may be the transparent film  203 . The liquid lens  202  of  FIG. 6  is different from the liquid lens  201  of  FIG. 5  in that the liquid lens  202  of  FIG. 6  further includes the transparent film  203  which has a curved surface. The curved surface of the transparent film  203  may be concave. 
     In this case, for example, in a near scanning mode, the first fluid TF 1  and the second fluid TF 2  are moved so that the first surface LS corresponds to the curved surface of the transparent film  203 . Thus, the curved surface of the transparent film  203  may serve as a concave lens surface. In the nearest scanning mode, the first surface LS is positioned above the transparent film  203  so that the curved surface of the transparent film  203  may not serve as the lens surface. A through hole  203   a  may be formed in the transparent film  203  so that the first fluid TF 1  and/or the second fluid TF 2  may be moved. In  FIG. 6 , the first fluid TF 1  is positioned on the transparent film  203  so that a convex boundary surface between the first fluid TF 1  and the second fluid TF 2  serves as the lens surface. If the first fluid TF 1   203  is discharged to a lower part of the transparent film  203 , the concave curved surface of the transparent film  203  serves as a concave lens surface. 
     Even in this case, in the liquid lens  202 , the pressure that is exerted onto the second surface IS varies due to electrowetting driving. Thus, the curvature of the first surface LS that is the lens surface is adjustable based on the pressure. 
     In  FIGS. 5 and 6 , each of the first electrode unit  220  and the second electrode unit  280  includes one electrode E, and the position of the second surface IS varies by adjusting the magnitude of the voltage applied to the electrode E. Each of the first electrode unit  220  and the second electrode unit  280  may include a plurality of electrodes E that are coated with the insulating material I. In this case, a voltage is applied to at least one of the electrodes E that constitute the first electrode unit  220  and the second electrode unit  280  so that the curvature of the first surface LS that is the lens surface may be digitally controlled. 
       FIG. 7  illustrates an example in which the focus adjuster  70  is configured to include the first and second liquid lenses  71  and  75  by combining the liquid lenses  201  of  FIG. 5  with each other symmetrically. The focus adjuster  70 , including the first and second liquid lenses  71  and  75 , may have a structure in which the liquid lenses  202  of  FIG. 6  are combined with each other symmetrically. In this case, the curvatures of the first liquid lens  71  and the second liquid lens  75  may be adjusted while being independently controlled. 
     In this case, the transparent medium  73  that is interposed between the first liquid lens  71  and the second liquid lens  75  may correspond to the lower substrate  210 , and an additional transparent medium may be further interposed between the first liquid lens  71  and the second liquid lens  75 . In  FIG. 7 , two lower substrates  210  are combined with each other by combining a pair of liquid lenses. The focus adjuster  70  may be configured so that only one lower substrate  210  may be disposed between the first liquid lens  71  and the second liquid lens  75 . 
     Referring back to  FIG. 2 , the focus adjuster  70  may include cover glasses  77  and  79  that are disposed on at least one of an input terminal and an output terminal of the focus adjuster  70 .  FIG. 2  illustrates exemplarily that the cover glasses  77  and  79  are disposed on the input terminal and the output terminal of the focus adjuster  70 , respectively. 
     When the focus adjuster  70  is configured to include the first and second liquid lenses  71  and  75  by combining two liquid lenses with each other symmetrically, as illustrated in  FIG. 7 , upper substrates  290  that are disposed in upper and lower portions of the focus adjuster  70  may be used as cover glasses, or additional cover glasses may be provided. 
     As described above, the first and second liquid lenses  71  and  75  of the focus adjuster  70  are disposed in such a way that the flow of the fluid occurs according to the electrowetting principle. However, aspects of the present inventive concept are not limited thereto. At least one of the first and second liquid lenses  71  and  75  may be a liquid lens  205  in which the flow of the fluid is caused by using pressure, as illustrated in  FIG. 8 . 
       FIG. 8  illustrates another example of the liquid lens  205  that may be used as the first liquid lens  71  and/or the second liquid lens  75  of the focus adjuster  70  of the optical zoom probe  10  of  FIG. 2 . 
     Referring to  FIG. 8 , the liquid lens  205  may be configured in such a way that the flow of the fluid is caused by using pressure so as to vary the curvature of the lens surface. The liquid lens  205  includes a transparent fluid TF 3  that is disposed in an internal space  380  of the chamber. The internal space  380  of the chamber is formed by a substrate  310  and a frame  330  which is formed on the substrate  310 . The internal space  380  of the chamber may include a fluid chamber  382 , a fluid path  384 , and a lens chamber  386 . A membrane  350  may be disposed on the frame  330 , and an actuator  370  may be disposed on the membrane  350  that corresponds to an upper portion of the fluid chamber  382 . One surface of the membrane  350  that corresponds to the upper portion of the lens chamber  386  may be a lens surface  350   a.    
     The membrane  350  may be formed of a transparent and elastic material, such as, for example, silicon elastomer. In addition, polydimethylsiloxane (PDMS), which has high durability and flexibility, may be used in forming the membrane  350 . 
     The actuator  370  may be used in applying pressure to the transparent fluid TF 3  and may be a general actuator that uses any one or more of various methods. For example, the actuator  370  may be a general polymer actuator which is formed of an electro active polymer (EAP) having a very thin thickness and low power consumption, such as, for example, a relaxor ferroelectric polymer actuator formed of copolymer, such as P(VDF-TrFE_CFE) or P(VDF-TrFE-CTFE). The actuator  370  is electrostrictively constrained as a voltage is applied to the actuator  370 , so that the actuator  370  applies pressure to the adjacent transparent fluid TF 3 . 
     The transparent fluid TF 3  may be silicon oil, for example. 
     When pressure is applied to the transparent fluid TF 3  in the fluid chamber  382  as the actuator  370  is driven, the transparent fluid TF 3  moves to the lens chamber  386  along the fluid path  384  so that the shape of the lens surface  350   a  varies. 
     The liquid lens that may be used as the first liquid lens  71  or the second liquid lens  75  of the focus adjuster  70  of the optical zoom probe  10  of  FIG. 2  may have a different structure from the above-described structure; for example, the liquid lens may include a liquid crystal lens that adjusts a focal length by forming an electric field gradient on a liquid crystal and by inducing a refractive index gradient caused by the electric field gradient. 
     In the optical zoom probe  10  according to the exemplary embodiment as described above, the focal length varies based on respective adjustments of the curvatures of the first and second liquid lenses  71  and  75 , and a resolution of the optical zoom probe  10  may be adjusted by adjusting the size of the aperture. 
       FIGS. 9A and 9B  and  FIG. 10  illustrate a depth scanning method which may be performed by using the optical zoom probe  10  of  FIG. 2 . As illustrated in  FIGS. 9A and 9B  and  FIG. 10 , in the optical zoom probe  10  according to an exemplary embodiment, even though a depth of an object varies, scanning may be performed while maintaining a horizontal resolution of the optical zoom probe  10 . 
       FIGS. 9A and 9B  illustrate an operating state of the optical zoom probe  10  of  FIG. 2  when an object, for example, a tissue of a body part, is scanned from a surface of the tissue to a relatively shallow depth, and  FIG. 10  illustrates an operating state of the optical zoom probe  19  of  FIG. 2  when scanning is performed with a long focal length. 
     As illustrated in  FIG. 9A , when the size of the aperture of the aperture adjuster  50  is set to be appropriate and lens surfaces of the first liquid lens  71  and the second liquid lens  75  are convex curved surfaces, light is focused on the surface of an object, for example, a tissue of a body part, approximately in the nearest distance. In this case, a distance between the last lens  80  of the optical zoom probe  10  and the tissue may be less than or equal to about 2 mm, for example. In this case, when the path of light is changed by inducing deformation of the optical fiber  21  due to the scanner  23  of the light transmitter  20 , the surface of the tissue may be scanned in a predetermined range of the x,y-plane. 
     As illustrated in  FIG. 9B , when the size of the aperture of the aperture adjuster  50  is larger than in  FIG. 9A , the lens surface of the first liquid lens  71  is slightly concave and the lens surface of the second liquid lens  75  that is relatively close to the tissue is convex, a horizontal resolution of light spots may be maintained, and the light spots are formed to a predetermined depth from the surface of the tissue. For example, the light spots may be formed to a depth of about 2 mm from the surface of the tissue. Even in this case, when the light spots are formed to a predetermined depth and a path of light is changed by inducing deformation of the optical fiber  21  due to the scanner  23  of the light transmitter  20 , an inner side of the tissue may be scanned to a predetermined depth of the x,y-plane. 
     In this way, the optical zoom probe  10  according to the present exemplary embodiment may operate so that an OCM mode in which high-resolution scanning is performed in the nearest distance mode (i.e., a distance between the last lens and the surface of the tissue is less than or equal to about 2 mm) may be implemented. In particular, the tissue may be optically scanned with uniform resolution in a three-dimensional (3D) section of about 2 mm in a depth direction at a nearest distance which is less than or equal to about 2 mm. 
     As illustrated in  FIG. 10 , when the size of the aperture of the aperture adjuster  50  is smaller than in  FIGS. 9A and 9B  and the lens surfaces of the first liquid lens  71  and the second liquid lens  75  are concave, light may be focused in a relatively far distance from the last lens, for example, in a range of about 30 mm. In this case, a position in which the light spots are focused may vary based on an adjustment of the size of the aperture of the aperture adjuster  50  and respective curvatures of the concave curved surfaces of the first and second liquid lenses  71  and  75 . Even in this case, when a path of light is changed by inducing deformation of the optical fiber  21  due to the scanner  23  of the light transmitter  20 , scanning may be performed by changing a horizontal position in which the light spots are formed in a predetermined range of the x,y-plane. 
     As illustrated in  FIG. 10 , the optical zoom probe  10  according to the present exemplary embodiment may operate so that an OCT mode in which scanning may be performed in the near distance mode (i.e., a distance between the last lens and the surface of the tissue is less than or equal to about 30 mm), may be implemented. In particular, the tissue may be optically scanned with uniform resolution in a 3D section of about 2 mm in the depth direction at a near distance which is less than or equal to about 30 mm. 
     In this way, when the size of the aperture of the aperture adjuster  50  is adjusted, and simultaneously the directions of the curvatures and the curvatures of the lens surfaces of the first liquid lens  71  and the second liquid lens  72  are adjusted in order to adjust the focal length of the focus adjuster  70 , the tissue may be optically scanned to a predetermined depth while maintaining a horizontal resolution as a high resolution. 
     When scanning is performed in the nearest distance mode, the second liquid lens  75  that is relatively close to the object may form a convex curved surface, and when scanning is performed in the near distance mode, both of the first and second liquid lenses  71  and  75  may form concave curved surfaces. However, aspects of the present inventive concept are not limited thereto, and various modifications and equivalent exemplary embodiments may be possible. 
     In the optical zoom probe  10  according to the present exemplary embodiment, an element which has a plane which is perpendicular to the path of light may be anti-reflection coated, and/or may have a predetermined gradient with respect to the optical axis so as to eliminate noise caused by reflective light. 
       FIG. 11  is a view which schematically illustrates an overall optical configuration of an optical zoom probe, according to another exemplary embodiment. 
     For example, the cover glass  51  of the aperture adjuster  50  and/or the cover glasses  77  and  79  of the focus adjuster  70  may be arranged to be inclined at predetermined angles θ 1  and/or θ 2 , as illustrated in  FIG. 11 .  FIG. 11  illustrates exemplarily that both of the aperture adjuster  50  and the focus adjuster  70  are arranged to be inclined; however, in another exemplary embodiment, only one of the aperture adjuster  50  and the focus adjuster  70  may be arranged to be inclined. 
     In this regard, an inclination angle may be less than or equal to about 12 degrees. In particular, the cover glass  51  of the aperture adjuster  50  and/or the cover glasses  77  and  79  of the focus adjuster  70  may be arranged to have a gradient which is less than or equal to about 12 degrees with respect to an optical axis, for example, a gradient of about 4 degrees and a gradient of about 12 degrees with respect to the optical axis. For example, the cover glass  51  of the aperture adjuster  50  and/or the cover glasses  77  and  79  of the focus adjuster  70  may be arranged to have a gradient of about 8 degrees with respect to the optical axis. 
       FIG. 12  is a schematic view of an image diagnosis system  3000  which uses the optical zoom probe, according to an exemplary embodiment. 
     Referring to  FIG. 12 , the image diagnosis system  3000  includes a light source  1220 , the optical zoom probe  10  that scans light which is emitted from the light source toward an object S, for example, a tissue to be inspected, and a detector  1230  that detects an image of the object by using light which is reflected from the object. 
     An optical zoom probe may be the optical zoom probe  10  according to the exemplary embodiment described above, and may properly adjust the size of the aperture, the focal length, and the like based on inspection purposes. The detector may include an image sensor, such as a charge-coupled device (CCD) that senses the image of the object. 
     In particular, the image diagnosis system  3000  may further include a beam splitter that splits the path of the light which is irradiated onto the object S from the light source  1220  and the path of the light which is reflected from the object S, and an image signal processor that processes signals sensed by the detector into image signals and displays the image signals. 
     The image diagnosis system  3000  may be configured to cause an interference between light reflected from the object and reference light by scanning the tissue to be inspected, by using the optical zoom probe  10 , and to detect signal light. To this end, the optical zoom probe  10  may further include an optical system that splits light which is emitted from the light source  1220  that is the same as a light source from which the light is irradiated onto the object S so as to use one split light as the light irradiated onto the object S, to use another split light as the reference light, and that causes an interference between the light reflected from the object S and the reference light. 
     In this case, when the object S is scanned while moving the focal length in the nearest distance and in the near distance, in particular, when the conversion of the OCM mode and the OCT mode occurs, the length of the path of the light irradiated onto the object S varies, so that the length of the path of the reference light should be changed accordingly. 
       FIGS. 13, 14, and 15   15  are schematic views of various optical systems  500 ,  600 , and  700  that may be used in adjusting the length of a path of reference light when an OCM mode and an OCT mode are used. 
       FIGS. 13 and 14  illustrate examples in which the reference light proceeds with optical fibers having different lengths according to the OCM or OCT mode by using couplers  510  and  610  of the respective optical systems  500  and  600 . 
     Referring to  FIG. 13 , the coupler  510  is configured such that one end of the coupler  510  is connected to one optical fiber  520  and the other end thereof is connected to two optical fibers  530  and  540  which have different respective lengths. Collimating lenses  535  and  545  may be respectively disposed on input/output terminals of the optical fibers  530  and  540  so as to collimate the reference light which is respectively output from the optical fibers  530  and  540  and to focus the reference light which is reflected by a reflection mirror  560  to be respectively input to the optical fibers  530  and  540 . A shutter  550  may be disposed between the optical fibers  530  and  540  and the reflection mirror  560 . 
     For example, when the required length of the path of the reference light is relatively large, the shutter  550  may operate to pass the reference light which is output from the optical fiber  530 , which has a relatively large length, through the shutter  550 , and not to pass the reference light which is output from the optical fiber  540 , which has a relatively small length, through the shutter  550 . The reference light that is output from the optical fiber  530  and passes through the shutter  550  is reflected by the reflection mirror  560 , passes through the shutter  550 , and is input to the optical fiber  530 . 
     Further, when the required length of the path of the reference light is relatively small, the shutter  550  may operate to pass the reference light which is output from the optical fiber  540 , which has a relatively small length, through the shutter  550 , and not to pass the reference light which is output from the optical fiber  530 , which has a relatively large length, through the shutter  550 . The reference light that is output from the optical fiber  540  and passes through the shutter  550  is reflected by the reflection mirror  560 , passes through the shutter  550 , and is input to the optical fiber  540 . 
     Referring to  FIG. 14 , the coupler  610  is configured such that one end of the coupler  610  is connected to one optical fiber  620  and the other end thereof is connected to two optical fibers  630  and  640  which have different respective lengths. Input/output terminals  630   a  and  640   a  of the optical fibers  630  and  640  may be reflection coated using a reflection material, such as gold (Au) or silver (Ag), such that the reference lights which are output from the optical fibers  630  and  640  may be respective reflected on the input/output terminals  630   a  and  640   a  of the optical fibers  630  and  640  without any change, and the reflected reference light may proceed in opposite directions. Due to an external force which is generated when a voltage is applied to the coupler  610 , light that proceeds via two optical fibers  630  and  640  having different lengths may be selectively cut. 
       FIG. 15  illustrates an example in which reference light from one-end optical fiber  720  proceeds via one of optical fibers  730  and  740 , which have different respective lengths based on modes, by using an optical switch  710  of the optical system  700 . Input/output terminals  730   a  and  740   a  of the optical fibers  730  and  740  may be reflection coated using a reflection material, such as gold (Au) or silver (Ag), such that the reference lights output from the optical fibers  730  and  740  may be reflected without any change, and the reflected reference light may proceed in opposite directions. 
     As described above, according to the one or more of the above exemplary embodiments, a focus adjuster includes a pair of liquid lenses that are independently controlled, and an aperture adjuster adjusts the size of an aperture so that an optical zoom probe in which high-resolution scanning may be performed while a focus is moved in the nearest distance and/or in a near distance, may be implemented. 
     It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments.