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CHAPTER 5 58 EXPERIMENTAL RESULTS AND CONCLUSIONS The experimental fabrication of two level binary photoresist lenses was presented in chapter four. In this chapter, the experimental performance of the binary lenses is presented. Focal length, diffraction efficiency, spot size, beam divergence, and coupling efficiency measurements are given. Two applications for these microlenses are demonstrated and conclusions given. 5.1 Focal Length The focal length of the microlenses was measured using the Foucault or knife edge test. 93 In the Foucault test, a knife edge placed behind an illuminated lens produces a characteristic shadow pattern or Foucault graph in image space. The shadow pattern allows the viewer to determine where the knife edge is with respect to focus (Figure 5-1). When the knife edge is inside focus, it will block the lower rays leaving the lens causing the shadow pattern to consist of a dark region on top and bright pattern on the bottom. The opposite shadow pattern is obtained when the knife edge is placed outside of focus. When the knife edge is exactly at focus, the shadow pattern will be all dark. Light from a 670 nm laser diode incident on a 500 fim pinhole was used to illuminate a single microlens at a time. A razor blade serving as the knife edge was placed on a x-y-z translation stage and positioned directly against the substrate. The knife edge was then translated away from the substrate until the focus point was located by observing the shadow patterns. The focal length
59 was taken as the distance the knife edge was translated from the lens and was measured to be approximately 1.5 mm. This is in close agreement to the design focal length of 1.562 mm. The accuracy of this measurement was limited to the precision of the micrometer on the translation stage, which was 10 ^m and the small thickness of the razor blade. Another limiting factor was determining the exact location of focus from the Foucault graph because of noise present from light scatter. This poor signal to noise ratio is a result of the relatively low diffraction efficiency of a binary element (i.e. 40.5%). 5.2 Diffraction Efficiency To measure diffraction efficiency, light from a 670 nm laser diode was first coupled into 4 |im diameter single-mode fiber. A microlens array was placed one focal length from the output of the fiber, and a single lens in the array was illuminated at a time. Light intensity was measured at two locations behind the lens array with a silicon photodiode. The first measurement was taken with the detector immediately behind the lens to measure the total power present in all diffraction orders. The second measurement was made two feet behind the lens to determine the power present in only the first diffraction order. The diffraction efficiency of each microlens was taken as the ratio of the power in the first diffraction order to the total power in all the orders. Table 5-1 gives the diffraction efficiencies from three experimental runs for several lenses on two different substrates. The average diffraction efficiency for the lenses on
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INFORMATION TO USERS This manuscrip
Page 5 and 6:
DIFFRACTIVE MICROLENSES FOR FIBER O
Page 7 and 8:
ACKNOWLEDGEMENTS 3 I would like to
Page 9 and 10:
5 Page 4.3 Photoresist Deposition 4
Page 11 and 12: 7 Page 4-2 Exposure test results fr
Page 13 and 14: ABSTRACT 9 The design, fabrication,
Page 15 and 16: 11 architectures. 3,4 5 8 In all of
Page 17 and 18: 13 microlenses have been presented
Page 19 and 20: 15 element acts as an interface bet
Page 21 and 22: CHAPTER 2 17 LENS PARAMETERS FOR FI
Page 23 and 24: 19 power coupling efficiency, T, be
Page 25 and 26: lens is, 21 NAflber = ^=m^. R R . 2
Page 27 and 28: 2.3 Chromatic Aberration 23 Despite
Page 29 and 30: 25 JL 0.985 0.99 0.995 .005 .015 >r
Page 31 and 32: 27 Propagation Distance (mm) Figure
Page 33 and 34: 29 spherical wave emitted from a po
Page 35 and 36: 31 zones is used, the light from ev
Page 37 and 38: 33 OPD=Jrl+f 2 3 3 The incident pla
Page 39 and 40: 3.3 Multi-level Diffractive Lenses
Page 41 and 42: 37 four level approximation. As the
Page 43 and 44: 39 a) T/N b) t(x) T/N ej2ro|> - I-*
Page 45 and 46: 41 A m = e-^sinci^j^t ij = 0 3.17 a
Page 47 and 48: 43 defining the structure of the le
Page 49 and 50: 45 where n is the index of the subs
Page 51 and 52: 47 B Q B mosiac aligned misaligned
Page 53 and 54: 49 4.3 Photoresist Deposition Shipl
Page 55 and 56: 51 Kfl A) 0 . . t .. .'i'S'l'lTtj
Page 57 and 58: 53 A. Substrate Preparation: 1. Soa
Page 59 and 60: 55 modulation is calculated using E
Page 61: Figure 4-5 Photographs from a scann
Page 65 and 66: 61 SAMPLE 1 t IMAX=40.37% SAMPLE 2
Page 67 and 68: 63 670 nm Laser diode Microlens Arr
Page 69 and 70: 65 1 Experimental data n Ideal Gaus
Page 71 and 72: 67 /OwQ\ xCX /TK X~X Fiber Figure 5
Page 73 and 74: 5.4 Conclusions 69 The design, fabr
Page 75 and 76: Appendix A: MATLAB PROGRAM
Page 77 and 78: theta=4*lammm/pi/dmm; z=0:100; dg=d
Page 79 and 80: 75 4" MASK 1" SQUARE CENTER I NOTES
Page 81 and 82: TWP67 V=2mm H=5mm, 77 TWD671 BL1 AM
Page 83 and 84: 15 J.Jahns and A. Huang, "Planar in
Page 85: "J.L. Vossen and W. Kem, Thin Film
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