Development of a Double-Gauss Lens Based Setup for ... - MDPI

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Development of a Double-Gauss Lens Based Setup for Optoacoustic Applications Hojong Choi 1 , Jae-Myung Ryu 2, * and Jung-Yeol Yeom 3, * 1 2 3

*

Department of Medical IT Convergence Engineering, Kumoh National Institute of Technology, Gumi 39253, Korea; [email protected] Department of Optical System Engineering, Kumoh National Institute of Technology, Gumi 39253, Korea School of Biomedical Engineering, Korea University, Seoul 02841, Korea Correspondence: [email protected] (J.-M.R.); [email protected] (J.-Y.Y.); Tel.: +82-54-478-7778 (J.-M.R.); +82-2-3290-5662 (J.-Y.Y.)

Academic Editors: Xiaoning Jiang and Chao Zhang Received: 19 November 2016; Accepted: 10 February 2017; Published: 3 March 2017

Abstract: In optoacoustic (photoacoustic) systems, different echo signal intensities such as amplitudes, center frequencies, and bandwidths need to be compensated by utilizing variable gain or time-gain compensation amplifiers. However, such electronic components can increase system complexities and signal noise levels. In this paper, we introduce a double-Gauss lens to generate a large field of view with uniform light intensity due to the low chromatic aberrations of the lens, thus obtaining uniform echo signal intensities across the field of view of the optoacoustic system. In order to validate the uniformity of the echo signal intensities in the system, an in-house transducer was placed at various positions above a tissue sample and echo signals were measured and compared with each other. The custom designed double-Gauss lens demonstrated negligible light intensity variation (±1.5%) across the illumination field of view (~2 cm diameter). When the transducer was used to measure echo signal from an eye of a bigeye tuna within a range of ±1 cm, the peak-to-peak amplitude, center frequency, and their −6 dB bandwidth variations were less than 2 mV, 1 MHz, and 6%, respectively. The custom designed double-Gauss lens can provide uniform light beam across a wide area while generating insignificant echo signal variations, and thus can lower the burden of the receiving electronics or signal processing in the optoacoustic system. Keywords: optoacoustic applications; double-Gauss lens; transducer

1. Introduction Ultrasound is widely used in a variety of the applications such as skin, intravascular, and small animal imaging; nondestructive testing; and sound navigation and ranging, because ultrasound machines can provide safe, real-time information through non-ionizing radiation-based methods [1–4]. In contrast to X-ray and optical imaging modalities, ultrasound is able to achieve higher spatial resolution of a target deep within tissue but it often suffers from low contrast due to similar acoustic properties of soft tissues [5–7]. In other words, when array ultrasound transducers are used in a system, the echo signal quality of a target—e.g., a blood vessel or a tissue—is also affected by the speckle patterns and grating lobes, thus providing a low quality of the contrast ratio of the targets [8,9]. Recently, optoacoustic systems have been introduced because the low contrast of traditional ultrasound methods are unable to provide high enough resolution for soft tissues such as blood vessel and hemoglobin [10,11]. The optoacoustic systems can achieve a high optical contrast at ultrasound penetration depth and spatial resolution which are essential characteristics to differentiate soft tissue properties [12,13]. This is possible because, in the optoacoustic systems, the light is utilized as the transmitting source and a transducer as the receiving source to detect the acoustic signal [10]. Sensors 2017, 17, 496; doi:10.3390/s17030496

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However, the penetration depth is relatively lower compared to ultrasound-only systems because the light is typically diffused in the surface of the tissue and signal intensity of the echo signal in the optoacoustic systems Sensors 2017, 17, 496 are compromised in the process [14]. In order to increase the light intensity, 2 of 15 the pulse repetition rate of the control signal for transmitted light has to be greater than that in the However, the penetration depth is relatively of lower compared to ultrasound-only systems because ultrasound-only method [15]. The configuration transmission components and optimization of the the light is typically diffused in the surface of the tissue and signal intensity of the echo signal in the light beam intensities are currently one of the technical issues to address in optoacoustic systems. optoacoustic systems are compromised in the process [14]. In order to increase the light intensity, the In optoacoustic systems, lenses have been used to diverge or focus the lights. A concave lens pulse repetition rate of the control signal for transmitted light has to be greater than that in the usuallyultrasound-only diverges a beam in order increase the field of the light,components and the light isoptimization directly transmitted method [15]. to The configuration of transmission and of the to a target such that a transducer has to be located perpendicularly to the target [16]. A conical lens can light beam intensities are currently one of the technical issues to address in optoacoustic systems. twist and focus a light beam to a certain location such that a transducer has to be located in the same In optoacoustic systems, lenses have been used to diverge or focus the lights. A concave lens usually a beamsystems, in order to increase theoffield the light, and the is to directly transmitted line [17,18]. Indiverges optoacoustic these types the of lenses are used in light order visualize the target to a target such a transducer has to be in located perpendicularly tothe the transducers target [16]. A may conical lens with transducers. In that order to scan the target a large field of view, translate can position twist andto focus a lightThe beamecho to a signals certain location such thataa certain transducer to be located the from one another. obtained from areahas would not be in uniform same line [17,18]. In optoacoustic systems, these types of the lenses are used in order to visualize the due to the non-uniform beam intensity as formed by the lens. When the received echo signals have target with transducers. In order to scan the target in a large field of view, the transducers may varying amplitudes, variable-gain amplifier or time-gain compensation amplifier has to be used to translate from one position to another. The echo signals obtained from a certain area would not be equalize the echo signal amplitudes and form the target information [19]. In this study, we introduce uniform due to the non-uniform beam intensity as formed by the lens. When the received echo signals a double-Gauss lens that can manipulate light beam from a source, thus providing a large field of have varying amplitudes, variable-gainaamplifier or time-gain compensation amplifier has to be used view with uniform light intensity [20,21]. To the best of our knowledge, we were the first to implement to equalize the echo signal amplitudes and form the target information [19]. In this study, we optoacoustic applications using a double-Gauss lens.a light beam from a source, thus providing a large introduce a double-Gauss lens that can manipulate field of view of with light intensity [20,21]. To theasbest of ourin knowledge, we werehigh-intensity the first to The efficiency theuniform lens basically needs to be as high possible order to irradiate implement optoacoustic applications using a double-Gauss lens. light on a sample or an object [22]. Therefore, one needs to design a high numerical aperture (NA) The efficiency of the lens basically be leads as high in order to irradiate high-[22]. lens that satisfies this requirement. A highneeds NA to lens toasa possible high optical efficiency of lens intensity light on a sample or an object [22]. Therefore, one needs to design a high numerical aperture Generally, the lens design can be easier if the F-number is large and the field of the view of the lens is (NA) lens that satisfies this requirement. A high NA lens leads to a high optical efficiency of lens [22]. small [23]. Additionally, the lens size would be unnecessarily large if its focal length is too long [23]. Generally, the lens design can be easier if the F-number is large and the field of the view of the lens ◦ Therefore, the lenses are preferably designed using a standard lens having an angle of view of 20–25 . is small [23]. Additionally, the lens size would be unnecessarily large if its focal length is too long Meanwhile, these type lenses been designed developed as aa standard floating type or digital single [23]. Therefore, theof lenses arehave preferably using lens having an angle of lens view reflex of (DSLR)-type where twohave lensbeen groups shouldasmove [24].type Recently, compact system 20–25°.systems, Meanwhile, thesemore type than of lenses developed a floating or digital single lens camerareflex (CSC) systems without mirrormore box in order reduce the weight and[24]. sizeRecently, of the camera have (DSLR)-type systems,a where than two to lens groups should move compact system camera (CSC) systems without a mirror box in order to reduce the weight and size of the been released [25]. However, they cannot use contrast auto focus (AF) systems such that the speed of camera haveisbeen [25].toHowever, they cannot usesystems contrast auto (AF)to systems such the CSC systems low released compared that of the DSLR-type [26].focus In order resolve thisthat issue, the speed of the CSC systems is low compared to that of the DSLR-type systems [26]. In order entire lenses have to be designed to utilize AF using only one lens; we call it an inner focus-typetolens. resolve this issue, entire lenses have to be designed to utilize AF using only one lens; we call it an Therefore, we need to design an inner focus-type standard lens with a high f-number (F/#), in which inner focus-type lens. Therefore, we need to design an inner focus-type standard lens with a high fonly a single lens moves. number (F/#), in which only a single lens moves. 1

14

H2 H1 8

13

9.60

MM

Figure 1. Optical layout of of thethe conventional lens. Figure 1. Optical layout conventionaldouble-Gauss double-Gauss lens.

It is more challenging to design entire lenses in which a single lens moves to provide AF than a

It is more challenging to design entire lenses in which a single lens moves to provide AF than a lens where more than two groups of lenses move to provide the same function [27]. In order to resolve lens where more than two groups of lenses move to provide the same function [27]. In order to resolve this issue, we need to begin the design from the paraxial approximation. If the image plane size is this issue, weto need begin from approximation. If theofimage plane size is similar a 35tomm film,the thedesign angle of the the viewparaxial of the lens with a focal length 50 mm is 23.4°.

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similar to a 35 mm film, the angle of the view of the lens with a focal length of 50 mm is 23.4◦ . Therefore, we need to start the design of a double-Gauss-type lens with a focal length of 50 mm as illustrated in Figure 1. This example is obtained from the optical design tool (CodeV, Synopsys, Mountain View, Santa Clara, CA, USA). 2. Materials and Methods In order to design an inner focus-type standard lens in which a single lens moves to provide AF in the double-Gauss lens, it is favorable for the lens group to be positioned in advance in order to compensate residual aberrations [28]. Therefore, the double-Gauss lens uses one lens group, followed by a single AF lens and an additional lens group as shown in Figure 1. In all, the double-Gauss lens has three lens groups. This lens type can be described by the refractive power and the ray height from the upper side using the Gaussian bracket in Equation (1) [29]

[ k 1 , − z1 , k 2 , − z2 , k 3 ] = K [ k 1 , − z1 , k 2 , − z2 , k 3 , − z3 ] = 0

(1)

where ki the refractive power of each lens group, zi is the distance between the principal planes of adjacent lens group, and K is the total refractive power of the entire lens. To solve Equation (1), variables other than z1 and z3 should have initial values. As shown in Figure 2, we can assume that the refractive power of Group 1 k1 is equal to K in Equation (1) if the total focal distance is 50 mm, even though the AF lens (Group 2) and the additional lens (Group 3) are included in the lens. Additionally, the lens should have appropriate air space between each lens group before and after the AF lens. This air space is to allow lens movement for focusing and also to compensate for variations in the image plane arising from manufacturing error of the double-Gauss lens. The first principal plane (H1) of the first lens group of the lens is shown, and the second principle plane (H2) of the first lens group is at a distance away from the image plane (surface 18), as shown in Figure 1. Therefore, the variable z1 can be equal to z2 to maintain the proper distance between the lens groups before and after the AF lens. The distance between the second principal plane (H2) and the last surface (surface 13) of the first lens group is 18.011 mm. Therefore, we need to assume that the variables z1 and z2 are equal to 20 mm. The distance z3 between the principal plane of the third lens group (Group 3) and the image plane (surface 18) is calculated to be 22.5 mm, as shown in Figure 2. The refractive power k2 of the AF lens and the refractive power k3 of the third lens group (Group 3) are unknown. Therefore, Equation (1) can be changed into Equation (2) according to these conditions. [K, −z1 , k2 , −z1 , k3 ] = K (2) [K, −z1 , k2 , −z1 , k3 , −z3 ] = 0 Using the Gaussian bracket properties, the refractive power k2 of the AF lens and the refractive power k3 of the third group in Equation (2) can be converted into Equation (3) k2 =

1 − Kz3 − 2Kz1 F − z3 − 2z1 = z1 (1 − z1 K ) z1 ( F − z1 )

k2 (−1 + z1 K ) z + 2z1 − F k3 = = 3 Kz3 z1 z3

(3)

Here, F is the effective focal length of the entire lens, and it is the reciprocal to the refractive power. Therefore, we can calculate that F = 50.0 mm, k2 = −1/48 mm−1 , and k3 = 1/36 mm−1 . If we assume that the AF lens and the lens of the third group are perfect lenses without chromatic aberrations, we can obtain the lens after the power arrangement process is completed, as shown in Figure 2. The process to determine the refractive power of each lens group is called power arrangement. In Figure 2, we can classify two different cases: infinity and m = −0.1. In these cases, the distance from the object is infinite

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and the magnification of the lens (m) is −0.1. Equation (4) can be derived if the magnification of the lens (m) is given to calculate the moving distance of the AF lens (∆z). Sensors 2017, 17, 496

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1 [−z0 , k1 , −z1 + ∆z, k2 , −z2 − ∆z, k3 ] = (4) 1 m [−zz0,,kk1,,−zz1 +∆z, z, k 2 ,z2  z, k3   k , − z − ∆z, k , − z = 0 0 1 2 2 3 m 3] 1 (4)  z0 , kbetween z, k 2object ,z2  and z, kthe 1, z1  the 3 , zfirst 3 0 where z0 represents the distance principal plane (H1) of the first where z 0 represents the distance between the object and the first principal plane (H1) of the first lens lens group. group.

Figure Opticallayout layout of design. Figure 2. 2.Optical of the theparaxial paraxial design.

If the distances between the refractive power k1, k2, and k3 of each group and the principal planes

If the distances between the refractive power k1 , k2 , and k3 of each group and the principal planes (z1, z2, and z3) are infinite, there are two unknown variables in Equation (4), which are the moving (z1 , z2 ,distance and z3 )ofare there arethe two unknown variables inand Equation whichplane are the moving theinfinite, AF lens (∆z) and distance between the object the first(4), principal of the distance of the AF lens (∆z) and the distance between the object and the first principal plane first lens group (z0). Therefore, we can obtain ∆z to be 6.16 mm in Equation (4) using a MATLAB of the first lens group (z0 ). Therefore, can obtain beand 6.16themm inlens Equation using a MATLAB program (MathWorks, Natick,we MA, USA). The ∆z AF to lens third group, (4) being aberrationfree(MathWorks, and thin lens Natick, systems,MA, mustUSA). be converted realand lenses. the lens AF lens has being to be physically program The AFtolens theAs third group, aberration-free lightweight, only must one AF and twotolenses in the third lens (Group must be used to and thin lens systems, belens converted real lenses. As the AFgroup lens has to be3)physically lightweight, only one AF lens and two lenses in the third lens group (Group 3) must be used to increase the

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moving velocity of the AF lens. In Equation (5), we can obtain the form factor of the lens (X) if the given spherical aberration is minimized. The spherical aberration SI is not zero for a single-type 496 5 of 15 lensesSensors [22]. 2017, The17, variable h represents the height of the axial ray at the lens front surface; k2 represents the refractive power of the AF lens; variable n represents the refractive index of the AF lens material increase the moving velocity of the AF lens. In Equation (5), we can obtain the form factor of the lens itself;(X) m2 ifrepresents the AF lens magnification; valuable c1 represents the curvature values the lens the given spherical aberration is minimized. The spherical aberration SI is not zero for aofsinglefront type surface, and[22]. c2 represents thehcurvature of the rear surface. lenses The variable representsvalues the height of lens the axial ray at the lens front surface; k2 represents the refractive( power of theAF lens; variable  n represents the of the AF )  2 refractive index 2 2 − 1) 1 4m32 represents 2(nmagnification; n + 2 the AF lens n n lens material itself; valuable c1 represents the curvature SI = h k · X+ Y + − Y n+ 2 curvature values n − 1 of the lens n + 2rear surface. values of the lens4front2 surface, and the n(n − 1)2c2 represents



c + c2 3  1+m XS ≡ 11 h 4 k, Y ≡ n  22 I c −c 2 1−m 2 2 14

(5)



2 2    n  2 n 2 1  n  X     Y   Y    2   n 2 n 2   n  1  n n  1   (5) If the refractive power is given when the lens thickness is not considered in Equations (5) and (6), c1  c 2 1 m 2 Y surfaces  the curvature of Xthefront and,rear c1 and c2 of the AF lens can be calculated [30]. c1  c 2 1 m 2

If the refractive power is given when the(clens k2 = c2 )(n − 1is ) not considered in Equations (5) and (6),(6) 1 + thickness the curvature of the front and rear surfaces c1 and c2 of the AF lens can be calculated [30].

k2  cthe c2 nmodule  1 design method should be used(6) For the two lenses in the third lens group, unlike 1  lens in Equation (5) two [31].lenses Usinginthis method, initial of thedesign lens can be determined as shown For the the third lens the group, the layout lens module method should be used unlike in in3. Equation (5) [31]. Using 2, this method, initial2 layout of the 3lens cannegative be determined shown in Figure As shown in Figure the addedthe Group and Group have powerasand positive Figure 3. As shown in Figure thefocusing added Group 2 andisGroup 3 haveofnegative power and positive power, respectively. Group 2, used2,for function, comprised only one lens while Group 3 power, respectively. Group used of foreach focusing is comprised of only one lens aberration while Group is comprised of two lenses. The 2,shape lens function, was determined to minimize Seidel and 3 is comprised of two lenses. The shape of each lens was determined to minimize Seidel aberration each lens material was selected to eliminate chromatic aberration [32]. The lens with completed power and each lens material was selected to eliminate chromatic aberration [32]. The lens with completed arrangement is, thus, constructed as shown in Figure 3. power arrangement is, thus, constructed as shown in Figure 3.

20 17

1

1516 13

8 H1

H1 H2

H1

2123

H2

36.00

H2

90.97

20

17 90.97

1 8 H2

13 H1

21 23

1516 H1 H2

H1

H2

6.16

20.27

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Figure 3. Optical layout ofof the lens. Figure 3. Optical layout thedesigned designeddouble-Gauss double-Gauss lens.

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Sensors 2017, 17, 496 6 of 15 Here, flint glass was used for the lens in Group 2 with negative power and crown glass was used for the lenses in Group 3 with positive power to minimize the color aberration of the entire system [33]. Here, flint glass was used for the lens in Group 2 with negative power and crown glass was used However, thelenses performance the entire lens is still low eventhe though Seidel aberration wassystem corrected. for the in Group of 3 with positive power to minimize color aberration of the entire Therefore, we should resolve the non-linear simultaneous equations about the curvature, thickness, [33]. However, the performance of the entire lens is still low even though Seidel aberration was and refractive of the material to minimize residual aberrations. process to corrected.index Therefore, welens should resolvein theorder non-linear simultaneous equations about theThe curvature, thickness, and refractive index of the lens materialthe in optimization order to minimize aberrations. The solve these complex non-linear equations is called [34].residual In Figure 3, the designed process tolens solveshould these complex non-linear equations is called the optimization [34].we In can Figure 3, the and double-Gauss be compensated for chromatic aberration such that design designed double-Gauss lenslens should be compensated for chromatic aberration such that we canlight design optimize a high-performance with minimal aberration, thus producing uniform beam and optimize a high-performance lens with minimal aberration, thus producing uniform light beam intensity in the area of interest.

intensity in the area of interest.

3. Results

3. Results

3.1. Double-Gauss Lens Fabrication

3.1. Double-Gauss Lens Fabrication

Figure 4 illustrates the interior and exterior structures of the manufactured product after the Figure 4 illustrates the interior and exterior structures of the manufactured product after the performance optimization process. Upon optimization, structure performance optimization process. Uponperforming performing performance performance optimization, thethe lenslens structure was modified, and nine lenses were utilized to construct the double-Gauss lens. The original design was modified, and nine lenses were utilized to construct the double-Gauss lens. The original design (Figure 3) was biconvex lens. However, it it was biconcavelens lens because (Figure 3)awas a biconvex lens. However, waschanged changed into into aa biconcave because thethe finalfinal lenslens be a field flattener-type to compensatefor forthe thefield field curvature curvature ofofthe lens. shouldshould be a field flattener-type lenslens to compensate thedouble-Gauss double-Gauss lens.

(a)

(b)

(c)

(d)

Figure 4. (a) Interior and (b) exterior images of our fabricated double-Gauss lens; (c) top and (d)

Figure 4. (a) Interior and (b) exterior images of our fabricated double-Gauss lens; (c) top and (d) bottom bottom view image of our manufactured double-Gauss lens. view image of our manufactured double-Gauss lens.

As shown in Figure 4, the designed double-Gauss lens was manufactured to achieve sufficient

As shown in Figure 4, the designed double-Gauss lens was manufactured to achieve sufficient performances for the modulation transfer function (MTF) [35]. In Figure 5a, the three-dimensional performances for the modulation transfer function [35].InIn Figure we 5a,provide the three-dimensional optical layout of the designed double-Gauss lens is (MTF) illustrated. addition, the expected optical layout of the designed double-Gauss lens is illustrated. In addition, we provide the expected modulation transfer function (MTF) graph to verify the resolving power performance of the lens.

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modulation transfer function (MTF) graph to verify the resolving power performance of the lens. The The image formation capability of our designed double-Gauss lens using CodeV software was image formation capability of our designed double-Gauss lens using CodeV software was confirmed confirmed as follows. Figure 5b shows the MTF graph of the optimized lens where “lp/mm” represents as follows. Figure 5b shows the MTF graph of the optimized lens where “lp/mm” represents the units the units of the spatial frequency as the number of line pairs per 1 mm. Thus, 10 lp/mm means that of the spatial frequency as the number of line pairs per 1 mm. Thus, 10 lp/mm means that there are there are 10 pairs of 0.05 mm thick black and white lines per 1 mm. “Sagittal” and “tangential” 10 pairs of 0.05 mm thick black and white lines per 1 mm. “Sagittal” and “tangential” respectively respectively imply that the lines are placed horizontally and vertically in an arbitrary cross section of imply that the lines are placed horizontally and vertically in an arbitrary cross section of the lens. As the lens. As shown in Figure 5, there is no resolution degradation with the designed double-Gauss shown in Figure 5, there is no resolution degradation with the designed double-Gauss lens system lens system up to 15 mm image height due to acceptable MTF variances within this range. up to 15 mm image height due to acceptable MTF variances within this range.

(a)

(b) Figure 5. The three-dimensional optical layout; and (b) of the designed doubleFigure 5. (a)(a) The three-dimensional optical layout; andthe (b)MTF the graph MTF graph of the designed Gauss lens. double-Gauss lens.

In this work, we used light modulation for the optoacoustic applications such that after the light In this work, we used light modulation for the optoacoustic applications such that after the light was located in the image plane of the designed double-Gauss lens, we investigated the distribution was located in the image plane of the designed double-Gauss lens, we investigated the distribution of of the light intensity in the opposite plane. Therefore, the distribution of the light intensity is expected the light intensity in the opposite plane. Therefore, the distribution of the light intensity is expected to to be uniform. be uniform. A sample and an LED were located at the left and right positions, respectively. The light was A sample and an LED were located at the left and right positions, respectively. The light was passed through the lens and irradiated on the sample. A light emitting diode (LED) light source with passed through the lens and irradiated on the sample. A light emitting diode (LED) light source with its its wavelength in the visible ranges is used as the transmit source in the developed optoacoustic wavelength in the visible ranges is used as the transmit source in the developed optoacoustic systems systems because LED lights in the visible wavelength and power range used in this study are because LED lights in the visible wavelength and power range used in this study are innocuous. innocuous. However, there may be some errors—such as the location of light and the distance However, there may be some errors—such as the location of light and the distance between the lens between the lens and the object—when this lens is utilized in optoacoustic systems. Therefore, we and the object—when this lens is utilized in optoacoustic systems. Therefore, we should verify the should verify the distribution of the LED light intensity with respect to the aberration caused by the distribution of the LED light intensity with respect to the aberration caused by the location errors and location errors and the distances between the double-Gauss lens and the object in an optoacoustic the distances between the double-Gauss lens and the object in an optoacoustic system as the AF lens in system as the AF lens in the double-Gauss lens should move to provide AF. the double-Gauss lens should move to provide AF.

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3.2. Double-Gauss Lens Performance Verification 3.2. Double-Gauss Lens Performance Verification 3.2. Double-Gauss Lensthe Performance Verification Figure 6 shows light intensity distribution profiles at distances of 60 mm using red, green, Figure 6 shows the light intensity distribution profiles at distances of 60 mm using red, green, and blue LEDs with peak wavelengths of 625, 530, and 459 nm, respectively. The full width Figure 6 shows light intensity distribution distances of 60 mm red, green, and blue LEDs with the peak wavelengths of 625, 530,profiles and 459atnm, respectively. Theusing full width halfhalf-maximum (FWHM) values of the red, green, and blue LEDs were 16, 35, and 22 nm, respectively. and blue LEDs withvalues peak wavelengths of 625, 459 nm,16, respectively. Therespectively. full width halfmaximum (FWHM) of the red, green, and530, blueand LEDs were 35, and 22 nm, The The powers of the red, green, and blue LEDs were 0.29, 0.36, and 0.60 W, respectively, because the maximum values and of theblue red, LEDs green,were and blue and 22 nm, respectively. powers of (FWHM) the red, green, 0.29,LEDs 0.36,were and 16, 0.6035,W, respectively, because The the efficiency of the LEDs is dependent on their LED wavelengths. Additionally, the intensity profiles in powers ofofthe green, and blueonLEDs werewavelengths. 0.29, 0.36, and 0.60 W, respectively, because efficiency thered, LEDs is dependent their LED Additionally, the intensity profilesthe in the horizontal and vertical planes were slightly different because the LEDs were rectangular in shape. efficiency of the LEDs is dependent on slightly their LED wavelengths. therectangular intensity profiles in the horizontal and vertical planes were different becauseAdditionally, the LEDs were in shape. the horizontal and vertical planes were slightly different because the LEDs were rectangular in shape. λpeak=625nm, λpeak=530nm, λpeak=459nm, λpeak=625nm, λpeak=530nm, λpeak=459nm,

Figure 6. The The light light intensity intensity distribution distribution profiles on the sample with the red, green, and blue LEDs. Figure 6. The light intensity distribution profiles on the sample with the red, green, and blue LEDs.

Intensiry W) W) (unit:(unit: Intensiry

The light light intensity intensity distribution distribution profiles the LEDs LEDs were were calculated calculated in in Figure Figure 7. 7. As shown in in The profiles of of the As shown The light intensity distribution profiles of theconstant LEDs were in Figureof Asred, shown in Figure 7, the light intensity distributions are and the green, Figure 7, the light intensity distributions are almost almost constant and calculated the light light intensities intensities of7.the the red, green, Figure 7, the light intensity distributions are almost constant and the light intensities of the red, green, and blue LEDs at the central area are 0.035, 0.042, and 0.065 mW, respectively. and blue LEDs at the central area are 0.035, 0.042, and 0.065 mW, respectively. and blue LEDs at the central area are 0.035, 0.042, and 0.065 mW, respectively. Horizontal(RED) Vertical(RED) Horizontal(RED) Horizontal(GREEN) Vertical(RED) Vertical(GREEN) Horizontal(GREEN) Horizontal(BLUE) Vertical(GREEN) Vertical(BLUE) Horizontal(BLUE) Vertical(BLUE)

Intensity distribution on sample 0.00008 Intensity distribution on sample 0.00008 0.00007 0.00007 0.00006

0.00006 0.00005 0.00005 0.00004 0.00004 0.00003 0.00003 0.00002

0.00002 0.00001 0.00001 0

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0

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Position0 (unit: mm) Position (unit: mm)

Figure 7. Light intensity distributions of the red, green, and blue LEDs with wavelengths of 628, 524, Figure Light intensity wavelengths of of 628, 628, 524, 524, and 4607. respectively. Figure 7.nm, Light intensity distributions distributions of of the the red, red, green, green, and and blue blue LEDs LEDs with with wavelengths and 460 nm, respectively. and 460 nm, respectively.

Figure 8 illustrates the optical layouts and light intensity distributions of a commonly obtainable Figure 88 illustrates the layouts andOptics, light intensity distributions a commonly Figure illustrates the optical 50 mm diameter single lens (67–277, Edmund Barrington, NJ, USA)of and the customobtainable designed 50 mm diameter Edmund Optics, Barrington, USA) and the custom designed Barrington, lens. The graph single showslens that(67–277, the light intensity distribution of a NJ, single lens (singlet) changes with lens. The graph shows that the light intensity distribution of a single lens (singlet) changes with position (29.8%) while that of our custom designed lens show negligible change (±1.5 %). This is position (29.8%) while that of our custom designed lens show negligible change (±1.5 %). This is

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lens. The graph shows that the light intensity distribution of a single lens (singlet) changes with position (29.8%) that of our negligible even change 1.5%). This is the because while the aberration of custom a singledesigned lens waslens not show fully corrected, for(± the one with anbecause aspherical aberration of a single lens was not fully corrected, even for the one with an aspherical surface. surface. The light were obtained under the assumption that the The light intensity intensitydistribution distributionprofiles profiles were obtained under the assumption thatdouble-Gauss the doublelens was constructed without any chromatic aberrations. Due to manufacturing errors that can manifest Gauss lens was constructed without any chromatic aberrations. Due to manufacturing errors that can themselves during experiments, light intensity distribution variability caused by chromatic aberrations manifest themselves during experiments, light intensity distribution variability caused by chromatic should be taken into in the experiment. Moreover, positioning of the AF lensofcan aberrations should beaccount taken into account in the experiment. Moreover,errors positioning errors thearise AF when mirror-less interchangeable lens modules are separable from the camera body after the power lens can arise when mirror-less interchangeable lens modules are separable from the camera body of thethe camera is camera turned off. However, the double-Gauss without a camera body, after powerbody of the body is turnedwe off.used However, we used thelens double-Gauss lens without and thus did not consider such effects. a camera body, and thus did not consider such effects.

Singlet

Double-Gauss Lens

Mask

LED

Sample

AF lens Stop

Intensity distribution on sample 0.00007

29.8% Intensiry (unit: W)

0.00006

±1.5%

0.00005

8.3%

0.00004 0.00003 0.00002

Horizontal(Singlet) Vertical(Singlet)

0.00001

Horizontal(Double-Gauss Lens) Vertical (Double-Gauss Lens)

-30

-20

0 0 -10 10 Position (unit: mm)

20

30

Figure 8. 8. Optical Opticallayout layoutand andlight lightintensity intensity distributions distributions of of the the single single lens lens and and our our designed designed lens. lens. Figure

Figure Figure 99 illustrates illustrates the the cross-sectional cross-sectional diagrams diagrams of of the the light light intensity intensity distribution distribution profiles profiles of of the the green light with a wavelength of 530 nm caused by the types of errors mentioned above. In Figure green light with a wavelength of 530 nm caused by the types of errors mentioned above. In Figure9,9, we we call call itit ‘defocus’ ‘defocus’ when when the the LED LED location location errors errors are are generated generated in in the the front front and and rear rear directions directions of of the the optical axis, ‘decenter’ when the LED location errors are generated in the top and bottom directions optical axis, ‘decenter’ when the LED location errors are generated in the top and bottom directions of of the the optical optical axis, axis, ‘shift’ ‘shift’ when when LED LED location location errors errors are are generated generated in in the the front front and and rear rear directions directions of of the the sample sample location, location, and and ‘focus ‘focus lens lens shift’ shift’ when when the the AF AF lens lens errors errors are are generated generated in in the the front front and and rear rear directions. directions. The Theresults results when when red, red, green, green, and and blue blue LEDs LEDs are are used used can can similarly similarly be be obtained obtained because because the designed double-Gauss lens in our research was thoroughly compensated for the designed double-Gauss lens in our research was thoroughly compensated forcolor coloraberration. aberration. Figure Figure 99 represents represents the the light light intensity intensity distributions distributions caused caused by by the the defocus, defocus, which which exhibits exhibits errors errors along along the optical axis. When the sample shifted 10 mm, or the defocus and the decenter moved 0.5 mm, or the optical axis. When the sample shifted 10 mm, or the defocus and the decenter moved 0.5 mm, focus lens shifted 0.2 mm, the intensity values increased from 0.0416 mW to 0.0433 mW between zero or focus lens shifted 0.2 mm, the intensity values increased from 0.0416 mW to 0.0433 mW between positions of –15ofmm and +15and mm, and these 2.1% compared to those to of those lensesof without zero positions −15 mm +15 mm, andvalues thesewere values were 2.1% compared lenses any errors. Therefore, this implies that the light intensity distribution variations caused by color without any errors. Therefore, this implies that the light intensity distribution variations caused by aberration or manufacturing error can becan neglected in our in optoacoustic systemsystem and should not affect color aberration or manufacturing error be neglected our optoacoustic and should not system performance. As the in addition to the light intensity, also affect system performance. AsFWHM, the FWHM, in addition to the light intensity, alsoremained remainedconstant, constant,the the physical quantity F/# or NA—which represents the index of the brightness of the lens—also physical quantity F/# or NA—which represents the index of the brightness of the lens—also changed changed slightly. slightly. Therefore, Therefore, the the lens lens was was well well designed designed with with uniform uniform light light intensity intensity for for optoacoustic optoacoustic systems. systems.

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Intensity distribution on sample 0.000045 0.0000445 0.000044

Intensiry (unit: W)

0.0000435

0.000043 0.0000425 0.000042 0.0000415 0.000041 0.0000405 0.00004 0 Position (unit: mm)

-5

-10

-15

Original(V) Sample shift: +10mm(H) defocus:+0.5mm(V) decenter: 0.5mm(H) focus lens shift: -0.2mm(V)

Original(H) Sample shift: -10mm(V) defocus:+0.5mm(H) defocus:-0.5mm(V) focus lens shift: -0.2mm(H) focus lens shift: +0.2mm(V)

15

10

5

Sample shift: -10mm(H) Sample shift: +10mm(V) defocus:-0.5mm(H) decenter: 0.5mm(V) focus lens shift: +0.2mm(H)

Intensity distribution on sample 0.00005 0.000045 0.00004

Intensiry (unit: W)

0.000035 0.00003 0.000025 0.00002

FWHM

0.000015

0.00001 0.000005 0 0 Position (unit: mm)

-10

-20

-30

Original(V) Sample shift: +10mm(H) defocus:+0.5mm(V) decenter: 0.5mm(H) focus lens shift: -0.2mm(V)

Original(H) Sample shift: -10mm(V) defocus:+0.5mm(H) defocus:-0.5mm(V) focus lens shift: -0.2mm(H) focus lens shift: +0.2mm(V)

30

20

10

Sample shift: -10mm(H) Sample shift: +10mm(V) defocus:-0.5mm(H) decenter: 0.5mm(V) focus lens shift: +0.2mm(H)

Intensity distribution on sample 0.00005 0.000045

Intensiry (unit: W)

0.00004 0.000035 0.00003 0.000025 0.00002 0.000015 0.00001

0.000005 0 12

13

14

Original(H) Sample shift: -10mm(V) defocus:+0.5mm(H) defocus:-0.5mm(V) focus lens shift: -0.2mm(H) focus lens shift: +0.2mm(V)

15

18 17 16 Position (unit: mm) Original(V) Sample shift: +10mm(H) defocus:+0.5mm(V) decenter: 0.5mm(H) focus lens shift: -0.2mm(V)

19

20

21

22

Sample shift: -10mm(H) Sample shift: +10mm(V) defocus:-0.5mm(H) decenter: 0.5mm(V) focus lens shift: +0.2mm(H)

Figure9.9. Intensity Intensity profile Figure profile on on sample sampleby bylens lenserror. error.

3.3. Pulse-Echo Response with Double-Gauss Lens 3.3. Pulse-Echo Response with Double-Gauss Lens Figure 10 shows the experimental setup of the developed optoacoustic system using the doubleFigure 10 shows the experimental setup of the developed optoacoustic system using the Gauss lens. The A-mode pulse-echo response is one of the typical methods to evaluate an double-Gauss lens. The A-mode pulse-echo response is one of the typical methods to evaluate optoacoustic system when numerous components are integrated into the entire system. In the an optoacoustic system when numerous components are integrated into the entire system. In the

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optoacoustic system, visible LED lights (CBT-120) whose wavelengths are between 400 nm and 700 nm optoacoustic system, visible LED lights (CBT-120) whose wavelengths are between 400 nm and 700 were used on eye tissue samples of a bigeye tuna. nm were used on eye tissue samples of a bigeye tuna. Power supply

Function generator Computer

Driver board LED Plastic holder Double-Gauss lens

Preamplifier Light

Sample

Ultrasound Transducer Echo signal

Oscilloscope

Water Tank

Figure 10. Experimental optoacoustic optoacoustic system system with with designed The sample sample was Figure 10. Experimental designed double-Gauss double-Gauss lens. lens. The was placed in a tank filled with de-ionized water and water temperature remained constant placed in a tank filled with de-ionized water and water temperature remained constant throughout throughout the experiments. experiments. the

Inc., Beaverton, OR,OR, USA) generated a 1 kHz A function functiongenerator generator(AFG3252, (AFG3252,Tektronics Tektronics Inc., Beaverton, USA) generated a 1and kHz5V and p-p pulsed signalsignal whichwhich controls the pulse a driver LuminusLuminus Devices)Devices) in order 5Vp-p pulsed controls themode pulse in mode in a board driver(DK-136M-1, board (DK-136M-1, to produce pulsedthe LED light.LED The function connected an oscilloscope (MSO2024B, in order to the produce pulsed light. Thegenerator functionwas generator wastoconnected to an oscilloscope Tektronics) synchronize to the synchronize timing pulses.the A plastic fabricated withholder polycarbonate material (MSO2024B,to Tektronics) timingholder pulses. A plastic fabricated with in a 3D printer (Fortus450mc, Stratasysis, Edina, MA, USA) was usedEdina, to firmly hold the was LED used and the polycarbonate material in a 3D printer (Fortus450mc, Stratasysis, MA, USA) to double-Gauss of diverginglens. LEDThe lightportion that converged on the designed double-Gauss firmly hold thelens. LEDThe andportion the double-Gauss of diverging LED light that converged lensthe wasdesigned focused onto the bigeye lens tunawas tissue sampleonto in a parallel direction. The focused on double-Gauss focused the bigeye tuna tissue samplelight in agenerates parallel thermally excited ultrasound waves within the sample, andultrasound these echo waves were in the turnsample, detectedand by direction. The focused light generates thermally excited within an in-house fabricated transducer with 2 mm focal distance. Signals were read out2by a 20 dB these echo waves were10 in MHz turn detected by an in-house fabricated 10 MHz transducer with mm focal voltage operational amplifier andby active pass filteramplifier with 20 dB gain and 100 MHz −3low dB distance. Signals were read out a 20Sallen-Key dB voltagelow operational and active Sallen-Key pass filter with 20 dB gain andthe 100 MHz -3 dB waveforms. bandwidth in amplify the received echo bandwidth in order to amplify received echo Theorder echo to waveforms were acquired by waveforms. The(MSO2024B, echo waveforms were digitized acquiredand by their the oscilloscope (MSO2024B, Tecktronics), the oscilloscope Tecktronics), spectrum in the frequency domain were digitized andMATLAB their spectrum the frequency domain using MATLAB program plotted using programin(MathWorks, Natick, MA,were USA) plotted in a computer. (MathWorks, USA)waveforms in a computer. Figure 11Natick, showsMA, the echo in the time and frequency domains. When red, green, Figure 11 shows the echo in the time and frequency were domains. and blue lights were used, the waveforms detected echo waveform amplitudes 29.33,When 47.74,red, andgreen, 79.25 and mV, blue lights were the detected echo were waveform amplitudes 29.33, andbandwidths 79.25 mV, respectively and used, the center frequencies 7.96, 9.15, and 8.4were MHz with47.74, −6 dB respectively and the center 7.96, 9.15, and 8.4 waveform MHz withcenter −6 dB bandwidths (percentages) of 42.56, 27.32, frequencies and 33.67%,were respectively. Each echo frequency was (percentages) of 42.56, the 27.32, and 33.67%, echo waveform centerinfrequency was obtained by averaging frequencies at therespectively. measured −Each 6 dB bandwidths as shown Figure 11b,c,f. obtained by in averaging the frequencies at the measured -6 dB bandwidths as have shown in Figure Slight shifts the measured center frequencies of the echo waveforms may been caused11b,c,f. by the Slight shifts in the measured center thethe echo waveformsand may have beenincaused by the long coaxial cables (4 m) between thefrequencies transducerof and preamplifier attenuation the layers of long coaxial cables However, (4 m) between the able transducer andspectra the preamplifier andamplitudes attenuation in the the tissue samples. we were to acquire of reasonable with thelayers setup. of theFigures tissue samples. able to acquire spectra of reasonable amplitudes withlens, the 12a and However, 13a show we thewere different setup configurations of the LED light source, setup. and transducers to verify echo signal uniformity with position. As depicted in Figure 12a, in the 12a and a13a show thewas different of the LEDsuch lightthat source, lens, was and ‘first’Figures measurement, transducer at ansetup angleconfigurations from the incident light the light transducers to verify signal uniformity with position. As depicted in Figure 12a, the ‘first’ focused on one side ofecho the sample and the reflected echo signals were detected from theinother side. measurement, transducer was at an angle the from the incident light such1that lightdirections was focused on In the ‘second’aand ‘third’ measurements, transducer was shifted cm the in both away one theposition sample and the reflected echosignal signalsvariations were detected from the other side. In the ‘second’ fromside theof first to check for any echo associated with the incident light beam. and ‘third’ measurements, the transducer wasamplitudes, shifted 1 cm in both directions from the first Figure 12b,c,d show the measured peak-to-peak center frequencies andaway −6 dB bandwidths position to check forRelative any echotosignal associated deviations with the incident beam. Figure 12b,c,d of the echo signals. each variations other, the calculated of the light peak-to-peak amplitudes, show measuredand peak-to-peak frequencies andsignals −6 dB bandwidths ofand the third echo centerthe frequencies their −6 dBamplitudes, bandwidth center variations of the echo in the second signals. Relative to each other, the calculated deviations of the peak-to-peak amplitudes, center frequencies and their −6 dB bandwidth variations of the echo signals in the second and third

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measurement data were less than 6%. These measurement data verified that the designed lens could measurement data were less than 6%. These measurement data verified that the designed lens could produce a uniform beam, thus generating uniform echo signal data accordingly. produce a uniform beam, thus generating uniform echo signal data accordingly. Red light Echo amplitude = 29.33 mV

0.02

0

Spectrum (dB)

Amplitude (V)

0.04

0.00 -0.02

-6 -12 Red light

-18

- 6dB BW = 42.56 % Center freq = 7.96 MHz

-0.04 2

3

4 5 Time (us)

6

-24

7

5

6

7 8 9 10 11 Frequency (MHz)

(a)

12

(b)

Green light Echo amplitude = 47.74 mV

0

Spectrum (dB)

Amplitude (V)

0.04 0.02 0.00 -0.02

-6 -12 -18

-0.04 2

3

4 5 Time (us)

6

-24

7

Green light - 6 dB BW = 27.32% Center freq = 9.15 MHz 5

6

7 8 9 10 Frequency (MHz)

(c)

0.00 -0.02

4 5 Time (us)

(e)

12

-6 -12

6

7

Blue light

-18

-0.04 3

11

0

Spectrum (dB)

Amplitude (V)

0.02

2

12

(d)

Blue light Echo amplitude = 79.25 mV

0.04

11

-24

- 6dB BW = 33.67% Center freq = 8.4 MHz 5

6

7 8 9 10 Frequency (MHz)

(f)

Figure 11. The waveform of the developed system. Echoamplitude amplitudeand and(b) (b) its its spectrum spectrum Figure 11. The echoecho waveform datadata of the developed system. (a)(a)Echo with red light, (c) echo amplitude and (d) its spectrum with green light, (e) echo amplitude and (f) its with red light, (c) echo amplitude and (d) its spectrum with green light, (e) echo amplitude and (f) its spectrum with blue light. spectrum with blue light.

In Figure 13, the light through the lens irradiates the samples perpendicularly and a transducer In Figure light through lens irradiates the samples perpendicularly andofa the transducer was placed13, atthe an angle from thethe normal. Measurements were taken from both sides normal was placed at an angle from the normal. Measurements were taken from both sides of the normal plane. At each position (‘fourth’ and ‘sixth’ measurements), another measurement was made after a plane. Atof each position and ‘sixth’ measurements), another measurement was made after shift 1 cm (‘fifth’ (‘fourth’ and ‘seventh’ measurements). Figure 13b–d show the measured peak-to-peak a shift of 1 cm (‘fifth’ ‘seventh’and measurements). Figure show the measured peak-to-peak amplitudes, centerand frequencies −6 dB bandwidths of 13b–d the echo signals. In this setup, relative to amplitudes, center frequencies and −6 of dBthe bandwidths of amplitudes, the echo signals. this setup,and relative to each other, the calculated variations peak-to-peak center In frequencies, their −6 eachdB other, the calculated variations of the peak-to-peak amplitudes, frequencies, and their bandwidths variations of the echo signals in the second and thirdcenter measurement data were also small—less thanvariations 2 mV, 1 MHz, 6%,signals respectively. −6 dB bandwidths of theand echo in the second and third measurement data were also These data confirmed that the developed optoacoustic system integrated with a small—less thanexperimental 2 mV, 1 MHz, and 6%, respectively. double-Gauss lens could produce uniform intensity with uniform echo signal acrosswith the These experimental data confirmed thatlight the beam developed optoacoustic system integrated illumination region. In the typical acoustic array system, the different echo signal amplitudes need to a double-Gauss lens could produce uniform light beam intensity with uniform echo signal across

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Double-Gausslens lens Double-Gauss Second Second First First Third Third cm 11cm

Peak-to-peak amplitude (mV) Peak-to-peak amplitude (mV)

Sensors 2017,17, 17,496 496 13ofof15 15 2017, 13 theSensors illumination region. In the typical acoustic array system, the different echo signal amplitudes need to be adjusted by additional variable gain amplifiers or time-gain compensation amplifiers to beadjusted adjustedby byadditional additionalvariable variablegain gainamplifiers amplifiersor or time-gaincompensation compensationamplifiers amplifiersto toimprove improve be improve the signal quality but in process also increasetime-gain the noise level of the system as a whole [36,37]. the signal signal quality quality but but in in process process also also increase increase the the noise noise level level of of the the system system as as aa whole whole [36,37]. [36,37]. the Therefore, having uniform echo signal amplitude could help to lower the system and successive signal Therefore, having having uniform uniform echo echo signal signal amplitude amplitude could could help help to to lower lower the the system system and and successive successive Therefore, processing burden. signalprocessing processingburden. burden. signal 80 80 70 70

79.6 79.6

79.25 78.16 78.16 79.25

60 60 50 50 40 40 30 30 20 20 10 10 00

48.14 48.14

47.74 47.74

28.76 28.76

29.33 29.33

28.6 28.6

First First

Third Third

Second Second

Redlight light Red Greenlight light Green Bluelight light Blue 9.15 9.15

9.15 9.15

9.05 9.05

8.34 8.34

8.40 8.40

8.39 8.39

9.0 9.0

8.0 8.0

8.04 8.04

8.01 8.01

7.96 7.96

7.5 7.5

Redlight light Red Greenlight light Green Bluelight light Blue

45 45

Bandwidth Bandwidth (%) (%)

Center Center frequencies frequencies (MHz) (MHz)

(b) (b)

10.0 10.0

8.5 8.5

47.67 47.67

-1-1 00 11 Distancefrom fromthe thefirst firstmeasurement measurement(cm) (cm) Distance

(a) (a)

9.5 9.5

Redlight light Red Greenlight light Green Bluelight light Blue

Second Second

Third First Third First 7.0 7.0 -1-1 00 11 Distancefrom fromthe thefirst firstmeasurement measurement(cm) (cm) Distance

40 40

43.63 43.63

42.56 42.56

44.79 44.79

35 35 30 30 25 25

34.87 34.87 26.88 26.88

33.67 33.67 27.32 27.32

Second Second

33.84 33.84 27.62 27.62

First First

Third Third -1-1 00 11 Distancefrom fromthe thefirst firstmeasurement measurement(cm) (cm) Distance

20 20

(c) (c)

(d) (d)

Double-Gausslens lens Double-Gauss Fourth Fourth Fifth Fifth

Sixth Sixth Seventh Seventh cm 11cm

cm 11cm

Peak-to-peak Peak-to-peak amplitude amplitude (mV) (mV)

Figure12. 12.(a) (a)Experimental Experimentalsetup setupwhen whenthe thetransducers transducerswere wereplaced placeddiagonally diagonallyin inthe theright rightside sideand and Figure Figure 12. (a) Experimental setup when the transducers were placed diagonally in the right side and (b) peak-to-peak amplitudes; (c) center frequencies; and (d) −6 dB bandwidths of the echo signals. (b) peak-to-peak amplitudes; (c) center frequencies; and (d) −6 dB bandwidths of the echo signals. (b) peak-to-peak amplitudes; (c) center frequencies; and (d) −6 dB bandwidths of the echo signals.

80 80 70 70

80.55 80.4680.55 79.54 79 79 80.46 79.54

60 60 50 50 40 40 30 30 20 20 10 10 00

47.79 47.79

49.78 49.75 47.91 49.75 47.91 49.78

29.35 29.35

29.3 28.25 28.25 29.3

(b) (b)

9.45 9.4 9.4 9.45

Redlight light Red Greenlight light Green Bluelight light Blue 9.34 9.34

9.0 9.0 8.5 8.5 8.0 8.0

8.44 8.44 8.36 8.36

8.37 8.37

8.34 8.34

8.31 8.31

8.24 8.24

8.41 8.41 8.33 8.33

7.5 7.5 7.0 7.0 00

Sixth Seventh Fourth Fifth Fifth Sixth Fourth Seventh 11 22 33 44 Measurementorder order Measurement

(c) (c)

45 45

Bandwidth Bandwidth (%) (%)

Center Center frequencies frequencies (MHz) (MHz)

10.0 10.0 9.25 9.25

30.29 30.29

Sixth Seventh Seventh Fifth Fourth Fifth Sixth Fourth 11 22 33 44 Measurementorder order Measurement

(a) (a)

9.5 9.5

Redlight light Red Greenlight light Green Blue light Blue light

39.80 39.80

40 40 35 35 30 30 25 25

35.14 35.14 34.22 34.22 27.02 27.02

Redlight light Red Greenlight light Green Bluelight light Blue 34.81 34.81

35.23 35.23 33.31 33.31

36.06 36.06

33.76 33.76

28.57 28.57

26.59 26.59 24.5 24.5 Sixth Seventh Fifth Fourth Fifth Seventh Sixth Fourth 20 20 11 22 33 44 Measurementorder order Measurement

(d) (d)

Figure13. 13.(a) (a)Experimental Experimental setupwhen whenthe thetransducers transducerswere wereplaced placedin inthe theleft left andright rightsides sidesand and (b) Figure Figure 13. (a) Experimentalsetup setup when the transducers were placed in and the left and right (b) sides peak-to-peakamplitudes; amplitudes;(c) (c)center centerfrequencies; frequencies;and and(d) (d)−6 −6dB dBbandwidths bandwidthsof ofthe theecho echosignal. signal. peak-to-peak and (b) peak-to-peak amplitudes; (c) center frequencies; and (d) −6 dB bandwidths of the echo signal.

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4. Conclusions We proposed an optoacoustic system using a double-Gauss lens with the goal of providing a large field of view with uniform light intensity. Such a configuration would reduce system burden associated with equalizing the detected echo signal amplitudes because currently developed lens configurations used in optoacoustic systems do not provide this kind of uniform beam intensity across a wide area. In order to evaluate the performance of our custom designed double-Gauss lens, we simulated the light intensity profiles across the illumination region of red, green, and blue LEDs (0.29, 0.36, and 0.60 W respectively). The intensity varied a mere 0.0416 mW to 0.0433 mW across a range of –15 mm to +15 mm. We also performed a pulse-echo test to assess the performance of the double-Gauss lens integrated optoacoustic system. With a 10 MHz transducer, the measured echo amplitudes were 29.33, 47.74, and 79.25 mV and their center frequencies were 7.96, 9.15, and 8.4 MHz, respectively for the three LEDs. Echo signals at different locations were also measured to verify the uniformity of the echo signal and the light source. Relative to the average of each measurement parameter, the echo signal peak-to-peak amplitude, center frequency, and −6 dB bandwidth variations were less than 2 mV, 1 MHz, and 6%, respectively. To conclude, these experimental data confirmed that we can use double-Gauss lens integrated optoacoustic systems for detecting tissue samples across ~2 cm distance with uniform echo signal intensity. This could help lower the burden of receiving electronics—such as the variable gain amplifier or time-gain compensation amplifier—in the system, and ease successive signal processing. Acknowledgments: This paper was supported by the Research Fund, Kumoh National Institute of Technology. Author Contributions: All authors conceived the idea together. J.R. designed the lens while H.C. and J.Y. planned and performed the experiments. All authors wrote the manuscript together and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4.

5. 6.

7. 8. 9. 10. 11.

Bom, N.; van der Steen, A.F.W.; Lancée, C.T. History and principles. In Vascular Ultrasound; Springer: Tokyo, Japan, 2003; pp. 51–65. Hoskins, P.R.; Martin, K.; Thrush, A. Diagnostic Ultrasound: Physics and Equipment; Cambridge University Press: Cambridge, UK, 2010. Blitz, J.; Simpson, G. Ultrasonic Methods of Non-Destructive Testing; Springer Science & Business Media: Berlin, Germany, 1995; Volume 2. Bernstein, J.J.; Finberg, S.L.; Houston, K.; Niles, L.C.; Chen, H.D.; Cross, L.E.; Li, K.K.; Udayakumar, E. Micromachined high frequency ferroelectric sonar transducers. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 1997, 44, 960–969. [CrossRef] Suri, J.S.; Chang, R.-F.; Molinar, F.; Fenster, A. Advances in Diagnostic and Therapeutic Ultrasound Imaging; Artech House: Norwood, MA, USA, 2008. Matte, G.M.; Van Neer, P.L.M.J.; Danilouchkine, M.G.; Huijssen, J.; Verweij, M.D.; de Jong, N. Optimization of a phased-array transducer for multiple harmonic imaging in medical applications: Frequency and topology. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2011, 58, 533–546. [CrossRef] [PubMed] Kremkau, F.W.; Forsberg, F. Sonography Principles and Instruments; Elsevier Health Sciences: Philadelphia, PA, USA, 2015. Qiu, W.; Yu, Y.; Tsang, F.K.; Sun, L. A multifunctional, reconfigurable pulse generator for high-frequency ultrasound imaging. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2012, 59, 1558–1567. [CrossRef] [PubMed] Zhou, Q.; Lam, K.H.; Zheng, H.; Qiu, W.; Shung, K.K. Piezoelectric single crystal ultrasonic transducers for biomedical applications. Prog. Mater. Sci. 2014, 66, 87–111. [CrossRef] [PubMed] Li, X.; Wei, W.; Zhou, Q.; Shung, K.K.; Chen, Z. Intravascular photoacoustic imaging at 35 and 80 MHz. J. Biomed. Opt. 2012, 17, 106005. [CrossRef] [PubMed] Qiu, W.; Yu, Y.; Chabok, H.R.; Liu, C.; Tsang, F.K.; Zhou, Q.; Shung, K.K.; Zheng, H.; Sun, L. A flexible annular-array imaging platform for micro-ultrasound. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2013, 60, 178–186. [PubMed]

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12. 13.

14.

15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

37.

15 of 15

Ye, S.; Yang, R.; Xiong, J.; Shung, K.K.; Zhou, Q.; Li, C.; Ren, Q. Label-free imaging of zebrafish larvae in vivo by photoacoustic microscopy. Biomed. Opt. Express 2012, 3, 360–365. [CrossRef] [PubMed] Yang, Y.; Li, X.; Wang, T.; Kumavor, P.D.; Aguirre, A.; Shung, K.K.; Zhou, Q.; Sanders, M.; Brewer, M.; Zhu, Q. Integrated optical coherence tomography, ultrasound and photoacoustic imaging for ovarian tissue characterization. Biomed. Opt. Express 2011, 2, 2551–2561. [CrossRef] [PubMed] Ning, B.; Kennedy, M.J.; Dixon, A.J.; Sun, N.; Cao, R.; Soetikno, B.T.; Chen, R.; Zhou, Q.; Shung, K.K.; Hossack, J.A. Simultaneous photoacoustic microscopy of microvascular anatomy, oxygen saturation, and blood flow. Opt. Lett. 2015, 40, 910–913. [CrossRef] [PubMed] Kim, J.; Lee, D.; Jung, U.; Kim, C. Photoacoustic imaging platforms for multimodal imaging. Ultrasonography 2015, 34, 88. [CrossRef] [PubMed] Li, L.; Yeh, C.; Hu, S.; Wang, L.; Soetikno, B.T.; Chen, R.; Zhou, Q.; Shung, K.K.; Maslov, K.I.; Wang, L.V. Fully motorized optical-resolution photoacoustic microscopy. Opt. Lett. 2014, 39, 2117–2120. [CrossRef] [PubMed] Wei, W.; Li, X.; Zhou, Q.; Shung, K.K.; Chen, Z. Integrated ultrasound and photoacoustic probe for co-registered intravascular imaging. J. Biomed. Opt. 2011, 16, 106001. [CrossRef] [PubMed] Yang, J.-M.; Li, C.; Chen, R.; Rao, B.; Yao, J.; Yeh, C.-H.; Danielli, A.; Maslov, K.; Zhou, Q.; Shung, K.K. Optical-resolution photoacoustic endomicroscopy in vivo. Biomed. Opt. Express 2015, 6, 918–932. [CrossRef] [PubMed] Shung, K.K. Diagnostic Ultrasound: Imaging and Blood Flow Measurements; Taylor & Francis: Boca Raton, FL, 2006. George, N.; Chi, W. Extended Depth of Field Using a Multi-Focal Length Lens with a Controlled Range of Spherical Aberration and a Centrally Obscured Aperture. Patent US7336430, 26 February 2008. Webb, S.L.; Youngers, K.J.; Steinle, M.J.; Eccher, J.A. Design of a 600-pixel-per-inch, 30-bit color scanner. Mouroulis, P.; Macdonald, J. Geometrical Optics and Optical Design; Oxford University Press: New York, NY, USA, 1997. Smith, G.H. Practical Computer-Aided Lens Design; Willmann-Bell, Incorporated: Richmond, VA, USA, 1998. Nikon Imaging Technology. Available online: http://imaging.nikon.com/technology/index.htm (accessed on 23 January 2017). Richard Butler, Primer: Why Would I Buy a Mirrorless Camera? Available online: https://www.dpreview. com/articles/3871263180/primer-why-would-i-buy-a-mirrorless-camera (accessed on 23 January 2017). Ryu, J.M.; Gang, G.M.; Lee, H.K.; Lee, K.W.; Heu, M.; Jo, J.H. Optical design and fabrication of a large telephoto zoom lens with fixed f/2.8 and light autofocus lens. J. Opt. Soc. Korea 2016, 19, 629–637. [CrossRef] Park, S.-C.; Lee, W.-S. Paraxial design method based on an analytic calculation and its application to a three-group inner-focus zoom system. J. Korean Phys.Soc. 2014, 64, 1671–1676. [CrossRef] Kidger, M.J. Intermediate Optical Design; SPIE: Bellingham, WA, USA, 2004. Herzberger, M. Modern Geometrical Optics; Interscience Publisher: New York, NY, USA, 1958. Pedrotti, F.L.; Pedrotti, L.S.; Pedrotti, L.M.; Education, P. Introduction to Optics; Prentice-Hall Englewood Cliffs: Upper Saddle River, NJ, USA, 1993; Volume 2. Park, S.C.; Shannon, R.R. Zoom lens design using lens modules. Opt. Eng. 1996, 35, 1668–1676. [CrossRef] Welford, W.T. Aberrations of Optical Systems; CRC Press: Boca Raton, FL, USA, 1986. Hartmann, P.; Jedamzik, R.; Reichel, S.; Schreder, B. Optical glass and glass ceramic historical aspects and recent developments: A schott view. Appl. Opt. 2010, 49, D157–D176. [CrossRef] Kingslake, R.; Johnson, R.B. Lens Design Fundamentals; Academic Press: Cambridge, MA, USA, 2009. Smith, W.J. Modern Optical Engineering; Tata McGraw-Hill Education: Noida, India, 1966. Hu, C.-H.; Snook, K.A.; Cao, P.-J.; Shung, K.K. High-frequency ultrasound annular array imaging. Part II: Digital beamformer design and imaging. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2006, 53, 309–316. [CrossRef] Szabo, T.L. Diagnostic Ultrasound Imaging: Inside Out; Elsevier Academic Press: London, UK, 2004. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).