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Microjetted lenslet triplet fibers
1 February
1996
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
123 ( 1996
) 492-496
Microjetted lenslet triplet fibers W.R. Cox ‘, T. Chen ‘, D. Ussery a, D.J. Hayes a, J.A. Tatum b, D.L. MacFarlane
‘,b
aMicroFab Technologies Inc.. 1104 Summit Avenue, Piano, TX 75074, USA
’ The University ofTexas at Dallas, Erik Jonsson School of Engineering and Computer Science, and Centerfor Applied Optics, Richardson. TX 75083, USA Received 25 May 1995; revised version received 14 August 1995
Abstract
technology is used to precisely place lenslets on the tips of multimode optical fibers with cladding diameters of 140 These 1.1 times diffraction limited lenslets approximately double the numerical aperture, or acceptance angle, as well as the coupling efficiency of the fibers. Microjet
pm.
Microlenses have previously been fabricated on the ends of optical fibers in several ways [ l-101. Often a heat source is used to melt the glass tips into a rounded lens. For example, several studies used either a hydrogen torch or an arc welder to provide the intense heat [ 1,_5,6].Typical lens radii of curvature reported varied between 3 and 17 pm for the arc welder and between 50 and 125 pm for the torch. More precise control has been gained using a CO, laser, lately in conjunction with a microlathe to spin the fiber [4,101. Room temperature processes include photolithographic techniques [ 2,3,7,9] and HF acid etching [ 81. Photoresist itself has been shaped into lenslets at the tips of fibers [2,7,9], and a photolithographic mask process has allowed glass lenslets to be formed [ 31. HF acid etching has the advantage of obtaining dimensions measured in nanometers, but has the disadvantage of compromising the material’s integrity through cracks in the glass. This variety of techniques has successfully produced a variety of lens shapes, ranging fromconical, to hemicylindrical, to hemispherical. Many of these studies have been motivated by the telecommunica’ Author to whom correspondence 0030-40 I8 /96/$12.00 0 1996 SSDIOOOO-4018(95)00614-1
should be addressed.
Elsevier Science B.V. All rights reserved
tions application of coupling a diode laser source into a single mode fiber; most of the studies obtained 3-9 dB factors of improvement in light transfer. Moreover, the lens curvature reduces the amount of light reflected back into the laser diode and significantly improves the noise properties of the laser source [ 51. We have reported recently on the use of microjet, or ink jet, technology to manufacture micro-optic components [ 1I 1. To date lenslets and lenslet arrays have been made by this technique with diameters ranging from 30-1200 pm, and focal lengths ranging from 3% 2400 p,m. Optical materials jetted include optical adhesives, thermoplastics, and resins. The jetting of fluorescent dye doped materials and sol gels have also been demonstrated. Plano-convex lenslet shapes have been square, spherical, cylindrical and elliptical. In addition, short, multimode, hemicylindrical waveguides have also been printed, and these have been used to splice connections between two fibers. In this letter we present our results of printing microlenses directly onto the tips of fibers. The droplet on demand microjet system used in this work consisted of a micromachined piezoelectric ceramic printhead contained within a heating shell.
W.R. Cm et al. / O/>tics Communications
Fig. I. Microphotographs of fiber tips with different microjetted tiber diameter is 140 km.
microlenses.
This printhead is connected to a stainless steel heated reservoir. Thus the high index of refraction ( nd = 1.7) melt mount (Cargille 24170) was kept in a liquid, molten phase at approximately 145°C until jetting. The optical fibers used were 100 micron diameter core germano-silica Corning fibers with a 140 micron diameter cladding. Individual fibers were mounted in a V-block fixture attached to a motorized x-y micropositioning stage. A computer controlled the driving voltage applied to the print head, as well as any movement of the x-y stage. The 50 pm diameter orifice printhead was visually aligned to the 140 p,rn diameter fiber; observing the impact of the first droplets allowed us to fine tune the fiber position. The outer edge of the fiber effectively contained the spread of the deposited liquid,
123 (1996) 492496
A quantitative
sense of scale may be gained by noting that the
and acted to center the lens on the fiber tip. Varying the number of 60 pm diameter droplets varied the amount of deposited material, and therefore, the shape and radius of curvature of the lens. In Fig. la-d are shown microphotographs of fiber tips with four microlenses of different sizes. In Fig. la is shown a 60 km radius of curvature lenslet made with 1 droplet deposited. In Fig. lb is shown a 70 km radius of curvature lenslet made with two droplets. In Fig. lc is shown a 75 Frn radius of curvature lenslet made with four droplets. In Fig. Id is shown a 80 pm radius of curvature lenslet made with 7 droplets. The approximate volumes of material in these lenslets are 100, 200, 400 and 700 picoliters, respectively. The approximate focal lengths of these lendets are 85, 100, 105 and 115 Km respec-
494
W. R. Cm et al. /Optics
Communica fions 123 (I 996) 492496
Fig. 2. Polar plot of power versus angle ofemission for a bare cleaved
fiber. tively; these values agree well with those measured from high quality lenslets made from this material by this technique on glass microscope slides. To characterize the change in numerical aperture of a lenslet tipped fiber we coupled HeNe laser light into the bare end of the fiber using a 20 X microscope objective. The lenslet tipped output end of the fiber defined an axis of rotation. A detector with a 25 p,rn slit rotated around this axis in steps of 0.10 degrees ( 1.7 mrad) . A computer controlled the stepping motor movement and the lock-in amplifier detection. Typically several runs were made for each lenslet tipped fiber, and these runs were averaged together to smooth out the speckle pattern. In Fig. 2 is a polar plot of measured power versus angle of emission from a bare, cleaved fiber. The approximate full width cone angle of diffraction is 15 degrees. In Fig. 3a is a plot of the emission from the lenslet tipped fiber shown in Fig. la. The approximate full width cone angle of diffraction shown here is 30 degrees. In Fig. 3b is a plot of the emission from the lenslet tipped fiber shown in Fig. 1b. The approximate full width cone angle of diffraction shown here is 30 degrees. In Fig. 3c is a plot of the emission from the lenslet tipped fiber shown in Fig. Id. The approximate full width cone angle of diffraction shown here is 35 degrees.
We verified that the light collection capability of the system was increased by the lenslets by measuring coupling efficiency. The maximum power of a plane wave coupled into a lenslet tipped fiber end was compared to that of the plane wave coupled into a bare, cleaved fiber end. The total output power which excited the other end of the fiber was measured for three different lenslet tipped fibers, with five repetitions each. These powers were compared with a cleaved fiber with no lenslet. On average, we measured an increase in coupling by a factor of 2.5-3, a range which is theoretically reasonable for the experimental configuration [ 12,131. Although these references refer mainly to coupling in single mode fibers, general mode distribution overlap formulae are given which may be applied here. This improvement in coupling indicates to us that the lenslets are of high quality, and that the increase in numerical aperture is not due to scattering in the lenslets. Another measure of lenslet quality can come from measuring the minimal spot patterns of a plane wave focussed by the lenslet. In this experimental set-up we propagate a plane wave through a lenslet, and image the focal pattern using a magnifying microscope objective. A typical such point spread distribution function is shown in Fig. 4. The 90 pm radius lenslet used here was deposited on a planar glass substrate and had a focal length of 340 p,m. Note in Fig. 4, the presence of diffraction ripples, and the small triangular pedestal caused by scatter. By comparing the full width at half maximum of the measured curve with the Airy pattern expected by diffraction theory [ 141 we can consider the figure of merit called “times diffraction limit”: ) / ( I .6 I34Afi. Here a is the radius ( ~J~HM,,,, of the lenslet, A is the wavelength of the light used in the experiment andfis the focal length of the lens. The factor of 1.6 134 comes from the half maximum of the Airy function. The lenslet pattern shown in Fig. 4 is 1.1 times diffraction limited, and is therefore of high quality. We have described above a new method for placing a micro-optic lens at the end of an optical fiber. Lenslet tipped fibers have applications in remote viewing, light collection and coupling from optical sources into fibers. Microjetting is a precise micro-optic fabrication technique which may be used to place other materials on the tips of fibers which may be relevant for other applications. Fluorescent dye doped resins, for example, may have application as remotely excited micro-optic
(ai
Angle (degrees) Angie ,(degrees)
-3
(4
1
Angle (degrees)
Fig. 3. Polar plots of power versus angle of emission for fibers tipped with lenslets made with 100 picoliter (a), 200 picoliter (b) and 700 picoliter (c ) volumes of material.
I
I
I
2io.aa, .-5 Lo SO.6 .-E D x Go.4 z 3 go.2 -
0 0
I 5
A 10 15 20 Scanned Distance (microns)
I 25
1 30
Fig. 4. Point spread distribution function for a 90 micron radius lenslet with a 340 p,rn focal length.
496
W.R. Co.x et al. /Optics
Communications
light sources in displays [ 151. Other active materials may have application in fiber-based sensors.
References
] I ] D. Kato, J. Appl. Phys. 44 (1973) 2756. 12 1LG. Cohen and M.V. Schneider, Appl. Optics 13 (1974) 89. 13)E. Weidel. Optics Comm. 12 ( 1974) 93. 14 ] U.C. Paek and A.L. Weaver, Appl. Optics 14 ( 1975) 294. 1S1H. Kuwahara, M. Sasaki and N. Tokoyo, Appl. Optics 19 (1980) 2578.
123 (1996) 492-496
16 I J-l. Yamada, Y. Murakami, J.-I. Sakai and T. Kimura, IEEE J. QuantumElectron. QE-16 (1980) 1067. [ 71 P.D. Bear, Appl. Optics 19 ( 1980) 2906. [ 81 G. Eisenstein and D. Vitello, Appl. Optics 21 ( 1982) 3470. [ 91 KS. Lee and F.S. Barnes, Appl. Optics 24 (1985) 3134. ] IO] H.M. Presby, A.F. Benner and CA. Edwards, Appl. Optics 29 ( 1990) 2692. [ 111 D.L. MacFarlane, V. Narayan, J.A. Tatum, W.R. Cox. T. Chen and D.J. Hayes, IEEE Photonics Technology Letters 6 ( 1994) 1112. [ 121 W.B. Joyce and B.C. DeLoach, Appl. Optics 23 (1984) 4187. [ 131 D. Marcuse, Bell System Technical Journal 56 ( 1977) 703. [ 14) M. Born and E. Wolf, Principles of Optics, sixth edition, ( Pergamon Press, Oxford, 1984). [IS] D.L. MacFarlane, Appl. Optics 33 ( 1994) 7453.
1996
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
123 ( 1996
) 492-496
Microjetted lenslet triplet fibers W.R. Cox ‘, T. Chen ‘, D. Ussery a, D.J. Hayes a, J.A. Tatum b, D.L. MacFarlane
‘,b
aMicroFab Technologies Inc.. 1104 Summit Avenue, Piano, TX 75074, USA
’ The University ofTexas at Dallas, Erik Jonsson School of Engineering and Computer Science, and Centerfor Applied Optics, Richardson. TX 75083, USA Received 25 May 1995; revised version received 14 August 1995
Abstract
technology is used to precisely place lenslets on the tips of multimode optical fibers with cladding diameters of 140 These 1.1 times diffraction limited lenslets approximately double the numerical aperture, or acceptance angle, as well as the coupling efficiency of the fibers. Microjet
pm.
Microlenses have previously been fabricated on the ends of optical fibers in several ways [ l-101. Often a heat source is used to melt the glass tips into a rounded lens. For example, several studies used either a hydrogen torch or an arc welder to provide the intense heat [ 1,_5,6].Typical lens radii of curvature reported varied between 3 and 17 pm for the arc welder and between 50 and 125 pm for the torch. More precise control has been gained using a CO, laser, lately in conjunction with a microlathe to spin the fiber [4,101. Room temperature processes include photolithographic techniques [ 2,3,7,9] and HF acid etching [ 81. Photoresist itself has been shaped into lenslets at the tips of fibers [2,7,9], and a photolithographic mask process has allowed glass lenslets to be formed [ 31. HF acid etching has the advantage of obtaining dimensions measured in nanometers, but has the disadvantage of compromising the material’s integrity through cracks in the glass. This variety of techniques has successfully produced a variety of lens shapes, ranging fromconical, to hemicylindrical, to hemispherical. Many of these studies have been motivated by the telecommunica’ Author to whom correspondence 0030-40 I8 /96/$12.00 0 1996 SSDIOOOO-4018(95)00614-1
should be addressed.
Elsevier Science B.V. All rights reserved
tions application of coupling a diode laser source into a single mode fiber; most of the studies obtained 3-9 dB factors of improvement in light transfer. Moreover, the lens curvature reduces the amount of light reflected back into the laser diode and significantly improves the noise properties of the laser source [ 51. We have reported recently on the use of microjet, or ink jet, technology to manufacture micro-optic components [ 1I 1. To date lenslets and lenslet arrays have been made by this technique with diameters ranging from 30-1200 pm, and focal lengths ranging from 3% 2400 p,m. Optical materials jetted include optical adhesives, thermoplastics, and resins. The jetting of fluorescent dye doped materials and sol gels have also been demonstrated. Plano-convex lenslet shapes have been square, spherical, cylindrical and elliptical. In addition, short, multimode, hemicylindrical waveguides have also been printed, and these have been used to splice connections between two fibers. In this letter we present our results of printing microlenses directly onto the tips of fibers. The droplet on demand microjet system used in this work consisted of a micromachined piezoelectric ceramic printhead contained within a heating shell.
W.R. Cm et al. / O/>tics Communications
Fig. I. Microphotographs of fiber tips with different microjetted tiber diameter is 140 km.
microlenses.
This printhead is connected to a stainless steel heated reservoir. Thus the high index of refraction ( nd = 1.7) melt mount (Cargille 24170) was kept in a liquid, molten phase at approximately 145°C until jetting. The optical fibers used were 100 micron diameter core germano-silica Corning fibers with a 140 micron diameter cladding. Individual fibers were mounted in a V-block fixture attached to a motorized x-y micropositioning stage. A computer controlled the driving voltage applied to the print head, as well as any movement of the x-y stage. The 50 pm diameter orifice printhead was visually aligned to the 140 p,rn diameter fiber; observing the impact of the first droplets allowed us to fine tune the fiber position. The outer edge of the fiber effectively contained the spread of the deposited liquid,
123 (1996) 492496
A quantitative
sense of scale may be gained by noting that the
and acted to center the lens on the fiber tip. Varying the number of 60 pm diameter droplets varied the amount of deposited material, and therefore, the shape and radius of curvature of the lens. In Fig. la-d are shown microphotographs of fiber tips with four microlenses of different sizes. In Fig. la is shown a 60 km radius of curvature lenslet made with 1 droplet deposited. In Fig. lb is shown a 70 km radius of curvature lenslet made with two droplets. In Fig. lc is shown a 75 Frn radius of curvature lenslet made with four droplets. In Fig. Id is shown a 80 pm radius of curvature lenslet made with 7 droplets. The approximate volumes of material in these lenslets are 100, 200, 400 and 700 picoliters, respectively. The approximate focal lengths of these lendets are 85, 100, 105 and 115 Km respec-
494
W. R. Cm et al. /Optics
Communica fions 123 (I 996) 492496
Fig. 2. Polar plot of power versus angle ofemission for a bare cleaved
fiber. tively; these values agree well with those measured from high quality lenslets made from this material by this technique on glass microscope slides. To characterize the change in numerical aperture of a lenslet tipped fiber we coupled HeNe laser light into the bare end of the fiber using a 20 X microscope objective. The lenslet tipped output end of the fiber defined an axis of rotation. A detector with a 25 p,rn slit rotated around this axis in steps of 0.10 degrees ( 1.7 mrad) . A computer controlled the stepping motor movement and the lock-in amplifier detection. Typically several runs were made for each lenslet tipped fiber, and these runs were averaged together to smooth out the speckle pattern. In Fig. 2 is a polar plot of measured power versus angle of emission from a bare, cleaved fiber. The approximate full width cone angle of diffraction is 15 degrees. In Fig. 3a is a plot of the emission from the lenslet tipped fiber shown in Fig. la. The approximate full width cone angle of diffraction shown here is 30 degrees. In Fig. 3b is a plot of the emission from the lenslet tipped fiber shown in Fig. 1b. The approximate full width cone angle of diffraction shown here is 30 degrees. In Fig. 3c is a plot of the emission from the lenslet tipped fiber shown in Fig. Id. The approximate full width cone angle of diffraction shown here is 35 degrees.
We verified that the light collection capability of the system was increased by the lenslets by measuring coupling efficiency. The maximum power of a plane wave coupled into a lenslet tipped fiber end was compared to that of the plane wave coupled into a bare, cleaved fiber end. The total output power which excited the other end of the fiber was measured for three different lenslet tipped fibers, with five repetitions each. These powers were compared with a cleaved fiber with no lenslet. On average, we measured an increase in coupling by a factor of 2.5-3, a range which is theoretically reasonable for the experimental configuration [ 12,131. Although these references refer mainly to coupling in single mode fibers, general mode distribution overlap formulae are given which may be applied here. This improvement in coupling indicates to us that the lenslets are of high quality, and that the increase in numerical aperture is not due to scattering in the lenslets. Another measure of lenslet quality can come from measuring the minimal spot patterns of a plane wave focussed by the lenslet. In this experimental set-up we propagate a plane wave through a lenslet, and image the focal pattern using a magnifying microscope objective. A typical such point spread distribution function is shown in Fig. 4. The 90 pm radius lenslet used here was deposited on a planar glass substrate and had a focal length of 340 p,m. Note in Fig. 4, the presence of diffraction ripples, and the small triangular pedestal caused by scatter. By comparing the full width at half maximum of the measured curve with the Airy pattern expected by diffraction theory [ 141 we can consider the figure of merit called “times diffraction limit”: ) / ( I .6 I34Afi. Here a is the radius ( ~J~HM,,,, of the lenslet, A is the wavelength of the light used in the experiment andfis the focal length of the lens. The factor of 1.6 134 comes from the half maximum of the Airy function. The lenslet pattern shown in Fig. 4 is 1.1 times diffraction limited, and is therefore of high quality. We have described above a new method for placing a micro-optic lens at the end of an optical fiber. Lenslet tipped fibers have applications in remote viewing, light collection and coupling from optical sources into fibers. Microjetting is a precise micro-optic fabrication technique which may be used to place other materials on the tips of fibers which may be relevant for other applications. Fluorescent dye doped resins, for example, may have application as remotely excited micro-optic
(ai
Angle (degrees) Angie ,(degrees)
-3
(4
1
Angle (degrees)
Fig. 3. Polar plots of power versus angle of emission for fibers tipped with lenslets made with 100 picoliter (a), 200 picoliter (b) and 700 picoliter (c ) volumes of material.
I
I
I
2io.aa, .-5 Lo SO.6 .-E D x Go.4 z 3 go.2 -
0 0
I 5
A 10 15 20 Scanned Distance (microns)
I 25
1 30
Fig. 4. Point spread distribution function for a 90 micron radius lenslet with a 340 p,rn focal length.
496
W.R. Co.x et al. /Optics
Communications
light sources in displays [ 151. Other active materials may have application in fiber-based sensors.
References
] I ] D. Kato, J. Appl. Phys. 44 (1973) 2756. 12 1LG. Cohen and M.V. Schneider, Appl. Optics 13 (1974) 89. 13)E. Weidel. Optics Comm. 12 ( 1974) 93. 14 ] U.C. Paek and A.L. Weaver, Appl. Optics 14 ( 1975) 294. 1S1H. Kuwahara, M. Sasaki and N. Tokoyo, Appl. Optics 19 (1980) 2578.
123 (1996) 492-496
16 I J-l. Yamada, Y. Murakami, J.-I. Sakai and T. Kimura, IEEE J. QuantumElectron. QE-16 (1980) 1067. [ 71 P.D. Bear, Appl. Optics 19 ( 1980) 2906. [ 81 G. Eisenstein and D. Vitello, Appl. Optics 21 ( 1982) 3470. [ 91 KS. Lee and F.S. Barnes, Appl. Optics 24 (1985) 3134. ] IO] H.M. Presby, A.F. Benner and CA. Edwards, Appl. Optics 29 ( 1990) 2692. [ 111 D.L. MacFarlane, V. Narayan, J.A. Tatum, W.R. Cox. T. Chen and D.J. Hayes, IEEE Photonics Technology Letters 6 ( 1994) 1112. [ 121 W.B. Joyce and B.C. DeLoach, Appl. Optics 23 (1984) 4187. [ 131 D. Marcuse, Bell System Technical Journal 56 ( 1977) 703. [ 14) M. Born and E. Wolf, Principles of Optics, sixth edition, ( Pergamon Press, Oxford, 1984). [IS] D.L. MacFarlane, Appl. Optics 33 ( 1994) 7453.