Abstract

A reflective-type photonic displacement sensor has been proposed and realized by taking advantage of a compact optical sensing head that incorporates a micro-optic beam shaper in conjunction with a rotary scale. The miniature beam shaper, which includes a pair of aspheric lenses, plays the role of optimally focusing a light beam emitted by a VCSEL source onto a rotary scale by utilizing efficient collimating optics. The focused beam is selectively reflected by a periodic grating pattern relevant to the scale; the beam then arrives at the photodetector (PD) receiver. Hence, an arbitrary displacement, encoded by the scale, could readily translate into an output signal available from the receiver. The proposed sensor was thoroughly designed through ray tracing based simulations and then analyzed in terms of the alignment tolerance for the VCSEL and code scale. The slim beam shaper was cost effectively constructed using plastic injection molding, and it was precisely integrated with the VCSEL and PD in a passive alignment manner, in order to complete the optical sensing head. In order to construct the displacement sensor, a code-wheel type scale containing alternate patterns of high- and low-reflection, was integrated with the optical head. The sensor was primarily characterized with respect to the evolution of generated beams for single-mode (SM) and multi-mode (MM) VCSELs, taking into consideration that the modulation depth of the output signal is elevated with decreasing focused beam size. For an embodied displacement sensor based on an SM VCSEL, leading to a focused beam spot of ~30 μm, a well-defined output with a modulation depth of 7% was obtained in response to the displacement of the rotary scale engraved with a grating of 10-μm pitch. The linear and angular resolutions were accordingly estimated to be better than 5 μm and 0.02°, respectively.

© 2014 Optical Society of America

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  1. K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
    [CrossRef]
  2. A. Yacoot, N. Cross, “Measurement of picometre non-linearity in an optical grating encoder using x-ray interferometry,” Meas. Sci. Technol. 14(1), 148–152 (2003).
    [CrossRef]
  3. N. Rigoni, R. Lugones, A. Lutenberg, and J. Lipovetzky, “Design of a customized CMOS active pixel sensor for a non-diffractive beam optical encoder,” in Proc. 6th Argentine School of Micro-Nanoelectronics, Technology and Applications, 84–88 (2011).
  4. L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).
  5. W. Yanyong, D. Fang, S. Jian, and X. Lishuan, “ANFIS parallel hybrid modeling method for optical encoder calibration,” 2012 24th Chinese Control and Decision Conf. (CCDC), 1591–1596 (2012).
  6. N. Johnson, J. Mohan K, E. Janson K, and J. Jose, “Optimization of incremental optical encoder pulse processing,” International Multi-Conf. on Automation, Computing, Communication, Control and Compressed Sensing (iMac4s), 769–773 (2013).
  7. L. Liang, Q. Wan, L. Qi, J. He, Y. Du, and X. Lu, “The design of composite optical encoder,” The Ninth International Conference on Electronic Measurement & Instruments 2009, 642–645 (2009).
    [CrossRef]
  8. K. Engelhardt, P. Seitz, “Absolute, high-resolution optical position encoder,” Appl. Opt. 35(1), 201–208 (1996).
    [CrossRef] [PubMed]
  9. H. Miyajima, E. Yamamoto, K. Yanagisawa, “Optical micro encoder using a twin-beam VCSEL with integrated microlenses,” Transducers ’97: Proceedings of the 11th International Conf. on Solid-State Sensors and Actuators, 1233–1235 (1997).
    [CrossRef]
  10. H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).
  11. H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
    [CrossRef]
  12. J. Akedo, H. Machida, H. Kobayashi, Y. Shirai, H. Ema, “Point source diffraction and its use in an encoder,” Appl. Opt. 27(22), 4777–4781 (1988).
    [CrossRef] [PubMed]
  13. P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
    [CrossRef]
  14. A. Lutenberg, F. Perez-Quintián, “Optical encoder based on a nondiffractive beam III,” Appl. Opt. 48(27), 5015–5024 (2009).
    [CrossRef] [PubMed]
  15. N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
    [CrossRef]
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    [CrossRef]
  17. K. Engelhardt, P. Seitz, “High-resolution optical position encoder with large mounting tolerances,” Appl. Opt. 36(13), 2912–2916 (1997).
    [CrossRef] [PubMed]
  18. S. Wekhande, V. Agarwal, “High-resolution absolute position Vernier shaft encoder suitable for high-performance PMSM servo drives,” IEEE Trans. Instrum. Meas. 55(1), 357–364 (2006).
    [CrossRef]
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    [CrossRef]
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    [CrossRef] [PubMed]
  21. Avago Technologies, URL http://www.avagotech.com/pages/home/ .
  22. A. Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567 (2011).
    [CrossRef]

2011 (1)

A. Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567 (2011).
[CrossRef]

2009 (1)

2008 (1)

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

2006 (1)

S. Wekhande, V. Agarwal, “High-resolution absolute position Vernier shaft encoder suitable for high-performance PMSM servo drives,” IEEE Trans. Instrum. Meas. 55(1), 357–364 (2006).
[CrossRef]

2003 (1)

A. Yacoot, N. Cross, “Measurement of picometre non-linearity in an optical grating encoder using x-ray interferometry,” Meas. Sci. Technol. 14(1), 148–152 (2003).
[CrossRef]

2001 (1)

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

2000 (2)

L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).

D. Crespo, J. Alonso, E. Bernabeu, “Reflection optical encoders as three-grating moiré systems,” Appl. Opt. 39(22), 3805–3813 (2000).
[CrossRef] [PubMed]

1997 (1)

1996 (2)

K. Engelhardt, P. Seitz, “Absolute, high-resolution optical position encoder,” Appl. Opt. 35(1), 201–208 (1996).
[CrossRef] [PubMed]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

1994 (1)

J. R. R. Mayer, “High-resolution of rotary encoder analog quadrature signals,” IEEE Trans. Instrum. Meas. 43(3), 494–498 (1994).
[CrossRef]

1992 (1)

N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
[CrossRef]

1988 (2)

J. Akedo, H. Machida, H. Kobayashi, Y. Shirai, H. Ema, “Point source diffraction and its use in an encoder,” Appl. Opt. 27(22), 4777–4781 (1988).
[CrossRef] [PubMed]

P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
[CrossRef]

Agarwal, V.

S. Wekhande, V. Agarwal, “High-resolution absolute position Vernier shaft encoder suitable for high-performance PMSM servo drives,” IEEE Trans. Instrum. Meas. 55(1), 357–364 (2006).
[CrossRef]

Akedo, J.

Alonso, J.

Aubert, P.

P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
[CrossRef]

Bernabeu, E.

Crespo, D.

Cross, N.

A. Yacoot, N. Cross, “Measurement of picometre non-linearity in an optical grating encoder using x-ray interferometry,” Meas. Sci. Technol. 14(1), 148–152 (2003).
[CrossRef]

Dong, L. L.

L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).

Ema, H.

Endo, T.

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

Engelhardt, K.

Hagiwara, N.

N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
[CrossRef]

Hane, K.

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

Hashimoto, S.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Ito, M.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Ito, Y.

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

Ko, J.-P.

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

Kobayashi, H.

Komazaki, I.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Larsson, A.

A. Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567 (2011).
[CrossRef]

Lee, J. M.

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

Lutenberg, A.

Machida, H.

Mayer, J. R. R.

J. R. R. Mayer, “High-resolution of rotary encoder analog quadrature signals,” IEEE Trans. Instrum. Meas. 43(3), 494–498 (1994).
[CrossRef]

Miyajima, H.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, K. Yanagisawa, “Optical micro encoder using a twin-beam VCSEL with integrated microlenses,” Transducers ’97: Proceedings of the 11th International Conf. on Solid-State Sensors and Actuators, 1233–1235 (1997).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Murase, H.

N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
[CrossRef]

Nat, P.

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

Oguey, H. J.

P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
[CrossRef]

Perez-Quintián, F.

Sasaki, M.

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

Seitz, P.

Shinohara, S.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Shirai, Y.

Suzuki, Y.

N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
[CrossRef]

Vuilleumier, R.

P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
[CrossRef]

Wan, Q. H.

L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).

Wekhande, S.

S. Wekhande, V. Agarwal, “High-resolution absolute position Vernier shaft encoder suitable for high-performance PMSM servo drives,” IEEE Trans. Instrum. Meas. 55(1), 357–364 (2006).
[CrossRef]

Xiong, J. W.

L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).

Yacoot, A.

A. Yacoot, N. Cross, “Measurement of picometre non-linearity in an optical grating encoder using x-ray interferometry,” Meas. Sci. Technol. 14(1), 148–152 (2003).
[CrossRef]

Yamamoto, E.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

H. Miyajima, E. Yamamoto, K. Yanagisawa, “Optical micro encoder using a twin-beam VCSEL with integrated microlenses,” Transducers ’97: Proceedings of the 11th International Conf. on Solid-State Sensors and Actuators, 1233–1235 (1997).
[CrossRef]

Yanagisawa, K.

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

H. Miyajima, E. Yamamoto, K. Yanagisawa, “Optical micro encoder using a twin-beam VCSEL with integrated microlenses,” Transducers ’97: Proceedings of the 11th International Conf. on Solid-State Sensors and Actuators, 1233–1235 (1997).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Yun, J.

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

Appl. Opt. (5)

IEEE J. Sel. Top. Quantum Electron. (1)

A. Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567 (2011).
[CrossRef]

IEEE J. Solid-State Circuits (1)

P. Aubert, H. J. Oguey, R. Vuilleumier, “Monolithic optical position encoder with on-chip photodiodes,” IEEE J. Solid-State Circuits 23(2), 465–473 (1988).
[CrossRef]

IEEE Trans. Instrum. Meas. (4)

N. Hagiwara, Y. Suzuki, H. Murase, “A method of improving the resolution and accuracy of rotary encoders using a code compensation technique,” IEEE Trans. Instrum. Meas. 41(1), 98–101 (1992).
[CrossRef]

J. R. R. Mayer, “High-resolution of rotary encoder analog quadrature signals,” IEEE Trans. Instrum. Meas. 43(3), 494–498 (1994).
[CrossRef]

S. Wekhande, V. Agarwal, “High-resolution absolute position Vernier shaft encoder suitable for high-performance PMSM servo drives,” IEEE Trans. Instrum. Meas. 55(1), 357–364 (2006).
[CrossRef]

J. Yun, J.-P. Ko, J. M. Lee, P. Nat, “An inexpensive and accurate absolute position sensor for driving assistance,” IEEE Trans. Instrum. Meas. 57(4), 864–873 (2008).
[CrossRef]

J. Opt. A, Pure Appl. Opt. (1)

K. Hane, T. Endo, Y. Ito, M. Sasaki, “A compact optical encoder with micromachined photodetector,” J. Opt. A, Pure Appl. Opt. 3(3), 191–195 (2001).
[CrossRef]

Meas. Sci. Technol. (1)

A. Yacoot, N. Cross, “Measurement of picometre non-linearity in an optical grating encoder using x-ray interferometry,” Meas. Sci. Technol. 14(1), 148–152 (2003).
[CrossRef]

Optics and Precision Engineering (1)

L. L. Dong, J. W. Xiong, Q. H. Wan, “Development of photoelectric rotary encoders,” Optics and Precision Engineering 8(2), 198–202 (2000).

Sens. Actuators A Phys. (1)

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using a vertical-cavity surface-emitting laser,” Sens. Actuators A Phys. 57(2), 127–135 (1996).
[CrossRef]

Other (7)

N. Rigoni, R. Lugones, A. Lutenberg, and J. Lipovetzky, “Design of a customized CMOS active pixel sensor for a non-diffractive beam optical encoder,” in Proc. 6th Argentine School of Micro-Nanoelectronics, Technology and Applications, 84–88 (2011).

W. Yanyong, D. Fang, S. Jian, and X. Lishuan, “ANFIS parallel hybrid modeling method for optical encoder calibration,” 2012 24th Chinese Control and Decision Conf. (CCDC), 1591–1596 (2012).

N. Johnson, J. Mohan K, E. Janson K, and J. Jose, “Optimization of incremental optical encoder pulse processing,” International Multi-Conf. on Automation, Computing, Communication, Control and Compressed Sensing (iMac4s), 769–773 (2013).

L. Liang, Q. Wan, L. Qi, J. He, Y. Du, and X. Lu, “The design of composite optical encoder,” The Ninth International Conference on Electronic Measurement & Instruments 2009, 642–645 (2009).
[CrossRef]

H. Miyajima, E. Yamamoto, K. Yanagisawa, “Optical micro encoder using a twin-beam VCSEL with integrated microlenses,” Transducers ’97: Proceedings of the 11th International Conf. on Solid-State Sensors and Actuators, 1233–1235 (1997).
[CrossRef]

H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, “Optical micro encoder using surface-emitting laser,” in Proc. IEEE Micro Electro Mechanical Systems, 412–417 (1996).

Avago Technologies, URL http://www.avagotech.com/pages/home/ .

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Figures (11)

Fig. 1
Fig. 1

Proposed reflective-type displacement sensor incorporating a micro-optic beam shaper.

Fig. 2
Fig. 2

Operation of the proposed displacement sensor (a) Propagation of light beams (b) PD output depending on the size of the focused beam spot.

Fig. 3
Fig. 3

Design results for the proposed reflective-type displacement sensor.

Fig. 4
Fig. 4

Observed beam profiles for the SM and MM VCSELs.

Fig. 5
Fig. 5

(a) Optical response of the sensor as a function of the displacement for different focused beams (b) Modulation depth with the focused beam diameter for the SM and MM VCSELs.

Fig. 6
Fig. 6

(a) Alignment tolerance for the SM VCSEL (b) Alignment tolerance for the code scale.

Fig. 7
Fig. 7

Passive assembly procedure for the optical sensing head.

Fig. 8
Fig. 8

(a) Completed optical sensing head with the beam shaper aligned with the VCSEL and PD (b) Micro-optic beam shaper consisting of the collimating and focusing lenses.

Fig. 9
Fig. 9

Test setup for the reflective-type displacement sensor based on a rotary scale resembling a code wheel.

Fig. 10
Fig. 10

(a) Evolution of the beam tailored by the focusing lens (b) Irradiance profile of the focused beam.

Fig. 11
Fig. 11

Measured transfer characteristics of the reflective displacement sensor exploiting an SM and MM VCSEL.

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