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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 25 — Dec. 12, 2005
  • pp: 10061–10065
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Picometer displacement sensing using the ultrahigh-order modes in a submillimeter scale optical waveguide

Fan Chen, Zhuangqi Cao, Qishun Shen, Xiaoxu Deng, Biming Duan, Wen Yuan, Minghuang Sang, and Shengqian Wang  »View Author Affiliations


Optics Express, Vol. 13, Issue 25, pp. 10061-10065 (2005)
http://dx.doi.org/10.1364/OPEX.13.010061


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Abstract

An improved scheme for displacement measurement using the ultrahigh-order guided modes in a symmetrical metal-cladding optical waveguide is proposed. Based on this idea together with the lock-in amplification technique, a sensor with a stable displacement resolution of 3.3 pm is experimentally demonstrated without any complicated servo system.

© 2005 Optical Society of America

1. Introduction

In this paper, a symmetrical metal-cladding optical waveguide (SMCOW) [11

11 . H. Lu , Z. Cao , H. Li , and Q. Shen , “ Study of ultrahigh-order modes in a symmetrical metal-cladding optical waveguide ,” Appl. Phys. Lett. 85 , 4579 ( 2004 ). [CrossRef]

] with variable submillimeter scale air gap is proposed to serve as the sensor head for displacement measurement. This waveguide can accommodate a great number of guided modes. For the ultrahigh-order modes, its mode order number m is usually greater than 1000 in a thick optical waveguide with metal claddings. Only the ultrahigh-order modes have special effects of extremely sensitive to the thickness of the guiding layer.

2. Principle

Due to this feature a prototype displacement sensor is presented as shown in Fig. 1. It is composed of three parts: the first is a glass prism precoated with a thin gold film on its base; the second a calibrated piezoelectric translator (PZT), which consists of a LiNbO3 slab sandwiched between two thick gold films; and the third an elastic gasket which separates the prism and PZT with a 200-μm-thick air gap. The whole sample is mounted on a translation stage. The gold film deposited on the base of the prism, the variable air gap, and the gold film deposited on the upper surface of the LiNbO3 slab form the SMCOW, in which the air gap serves as the guiding layer of the waveguide.

Fig. 1. Schematic of construction for sensitive probe: ε 3, ε 2 and ε 3 are dielectric constants of prism, metal film, and air layer, respectively; h 1 and h 2 are thicknesses of air gap and thin gold film deposited on prism base.

Fig. 2. A typical ATR curve is obtained with ε 1 = 1.0, ε 0 = ε 2 = -16 + i0.6@632.8 nm, ε 3 = 3.0, h 1 = 3 μm and h 2 = 43 nm, respectively.

3. Experiment and result

The experimental arrangement is shown schematically in Fig. 3. After passing through two apertures (A1, A2) and one polarizer (P), a collimated light beam from a diode laser of 650 nm is incident on the prism base with a small incident angle θ i to excite the ultrahigh-order modes which is considered to be very sensitive to the thickness of the guiding layer. Angular scan is carried out by a computer-controlled θ/2θ goniometer. The reflected light is detected by a photodiode and converted into a voltage signal. To obtain a higher signal-to-noise ratio, the weak signal is input into a lock-in amplifier (LI5640, NF Corp.) to restrain noise.

Fig. 3. Experimental arrangement with optical modulation and lock-in amplifier: 1, Reference In; 2, Signal In; 3, Lock-in Amplifier Output.

As expected, the guided modes are manifested themselves by a set of dips in ATR spectrum. After selecting a reflection dip and fixing the operation angle of the collimated laser beam just near the mid-point of the fall-off in the reflection dip, i.e., working point θ 0, as seen in Fig. 4, where the linearity and the slope are quite favorable for the measurement of the thickness variation of the air gap. An ac voltage generated by a function generator is applied on the electrodes of the PZT. Correspondingly, the air gap will change its thickness periodically due to piezoelectric effect of the LiNbO3 slab (MTI Co., USA). As the reflection dip fluctuates its peak position, the intensity of the reflected light is modulated, and a periodical change of the reflectivity with the amplitude of ΔI is shown in Fig. 4. In terms of the resolution of the reflectivity variation, displacement can then be evaluated from the piezoelectric effect of LiNbO3 slab.

Fig. 4. Intensity modulation of the reflected light induced by periodical thickness variation of the air gap in the SMCOW. The waveguide parameters are as follows: ε 1 = 1.0, ε 2 = -16 + i0.6, ε 3 = 3.0, h 1 = 108 μm, h 2 = 40 nm, and λ = 632.8 nm.

In the experiment, the optical system is adjusted so that a set of reflection dips with optimum coupling can be observed in the reflected angular spectrum. The preliminary experiment with the lock-in amplifier to achieve high resolution has been performed with the waveguide parameters ε 1 = 1.0, ε 2 = -11 + i1.0, ε 3 = 2.25, h 1 = 200 μm, h 2 = 40 nm and λ = 650 nm in which the thickness and complex dielectric constant of the thin gold film deposited on the prism base were determined by the double-wavelength method [12

12 . W. P. Chen and J. M. Chen , “ Use of surface plasma waves for determination of the thickness and optical constants of thin metallic films ,” J. Opt. Soc. Am. 71 , 189 ( 1981 ). [CrossRef]

], and the thickness of the air gap was determined by the m-line spectroscopy technique [14

14 . P. K. Tien , R. Ulrich , and R. J. Martin , “ Modes of propagating light waves in thin deposited semiconductor films ,” Appl. Phys. Lett. 14 , 291 ( 1969 ). [CrossRef]

].

Fig. 5. Output of Lock-in Amplifier as the modulation signal was applied on the PZT with voltage variation: 0.1 V, 0.5 V, 1.0 V, 0.5V and 0.1V at a frequency of 821 Hz.

An ac voltage is applied on the piezoelectric translator to modulate the thickness of air gap. Correspondingly, output signal from the photodiode is imported into the lock-in amplifier to shield the background noise and intrinsic low-frequency drift. Representative experimental result is shown in Fig. 5. It illustrates the device response as the PZT is driven by an 821 Hz sine wave with a 0.5V peak-to-peak amplitude. This corresponds to a step-style displacement variation with amplitude of 0.17Å (Δx 0 = Vd 33, for LiNbO3, piezoelectric coefficient d 33 = 34.45 pm/V). As shown in Fig. 5, the ripple structure with amplitude of 0.1 V represents the displacement resolution of 3.3 pm in terms of the signal-to-noise ratio (S/N) of 1. Several similar measurements are made in the frequency range from 985 Hz to 8930 Hz, and experimental results are tabulated in Table 1 by which the displacement measurement with picometer resolution is confirmed further. Because a periodical displacement variation with a fixed frequency (f) induces acceleration, the step-style displacement variation of 0.17 Å corresponds to an acceleration variation of 1.15 μg at a frequency of 821 Hz (a 0 = Δx 0 f 2, f is the modulating frequency).

Table 1. Outputs of Lock-in Amplifier (LI5640).

table-icon
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4. Conclusion and discussion

In conclusion, an improved scheme for displacement measurement based on intensity modulation and lock-in amplification techniques is proposed. Ultrahigh-order guided modes are used to respond to the thickness alternation of the guiding layer of the waveguide. A preliminary experiment is performed to test the possibilities of scheme. From the experimental result, we obtain a displacement resolution of 3.3 pm. By analyzing the fall-off of the dip, the 7-nm measurement range which covers the reflectivity region from 10% to 70% is obtained with the linearity less than ±4%.

Comparison between the SMCOW and SPR indicates that polarization independence of the ultrahigh-order modes is very clear [12

12 . W. P. Chen and J. M. Chen , “ Use of surface plasma waves for determination of the thickness and optical constants of thin metallic films ,” J. Opt. Soc. Am. 71 , 189 ( 1981 ). [CrossRef]

] by using a symmetrical metal-cladding optical waveguide. Furthermore, nearly all of the mode energy are concentrated in the guiding layer, which enhances the sensitivity of the sensor [13

13 . F. Chen , Z. Cao , Q. Shen , and Y. Feng , “ An optical approach to angular displacement measurement based on attenuated total reflection ,” Appl. Opt. 44 , 5393 ( 2005 ). [CrossRef] [PubMed]

].

Because the PZT and prism coupler are connected with a gold electrode and buffered by a gasket, the oscillation of PZT will induce the noise to the system. Experiment result shown in Fig. 5 indicates that the ripple structure with amplitude of 0.1 V in the output of lock-in amplifier can be found. To further enhance the displacement resolution of the sensor head, the mechanical design should be optimized. For more sensitive measurement, the surrounding temperature and pressure should be monitored synchronously. Furthermore, free-space coupling technique [15

15 . H. Li , Z. Cao , H. Lu , and Q. Shen , “ Free-space coupling of a light beam into a symmetrical metal-cladding optical waveguide ,” Appl. Phys. Lett. 83 , 2757 ( 2003 ) [CrossRef]

] can be used to reduce the dimension of the sensor.

Acknowledgments

This work is supported by National Nature Science Foundation of China under Grant 60408010 and 60237010, and the Municipal Scientific & Technological Development Project of Shanghai under grant 011661084.

References and links

1 .

G. A. Rines , “ Fiber-optic accelerometer with hydrophone applications ,” Appl. Opt. 20 , 3453 ( 1981 ). [CrossRef] [PubMed]

2 .

C. Liu , A. M. Barzilai , J. K. Reynolds , A. Partridge , T. W. Kenny , J. D. Grade , and H. K. Rockstad , “ Characterization of a High-sensitivity Micromachined Tunneling Accelerometer with Micro-g Resolution ,” IEEE J. Micorelectromech. Syst. 7 , 235 ( 1998 ). [CrossRef]

3 .

E. B. Cooper , E. R. Post , S. Griffith , J. Levitan , S. R. Mnanlis , M. A. Schmidt , and C. F. Quate , “ High-resolution micromachined interferometric accelerometer ,” Appl. Phys. Lett. 76 , 3316 ( 2000 ). [CrossRef]

4 .

F. A. Castro , S. R. M. Carneiro , O. Lisboa , and S. L. A. Carrara , “ Two-mode optical fiber accelerometer ,” Opt. Lett. 17 , 1474 ( 1992 ). [CrossRef] [PubMed]

5 .

A. S. Gerges , T. P. Newson , and D. A. Jackson , “ Practical fiber-optic-based submicro-g accelerometer free from source and environmental perturbations ,” Opt. Lett. 20 , 1155 ( 1989 ). [CrossRef]

6 .

A. Malki , P. Lecoy , N. Marty , C. Renouf , and P. Ferdinand , “ Optical fiber accelerometer based on a silicon micromachined cantilever ,” Appl. Opt. 34 , 8014 ( 1995 ). [CrossRef] [PubMed]

7 .

R. A. Soref and D. H. McMahon , “ Tilting-mirror fiber-optic accelerometer ,” Appl. Opt. 23 , 486 ( 1984 ). [CrossRef] [PubMed]

8 .

V. Milanovic , E. Bowen , M. E. Zaghloul , N.H. Tea , J. S. Suehle , B. Payne , and M. Gaitan , “ Micromachined convective accelerometers in standard integrated circuits technology ,” Appl. Phys. Lett. 76 , 508 ( 2000 ). [CrossRef]

9 .

B. Zhu , M. Owner-Petersen , and T. Licht , “ Accelerometer design based on attenuated total reflection ,” Appl. Opt. 27 , 2972 ( 1988 ). [CrossRef] [PubMed]

10 .

X. Liu , Z. Cao , Q. Shen , and S. Huang , “ Optical sensor based on Fabry-Perot resonance modes ,” Appl. Opt. 42 , 1 ( 2003 ). [CrossRef]

11 .

H. Lu , Z. Cao , H. Li , and Q. Shen , “ Study of ultrahigh-order modes in a symmetrical metal-cladding optical waveguide ,” Appl. Phys. Lett. 85 , 4579 ( 2004 ). [CrossRef]

12 .

W. P. Chen and J. M. Chen , “ Use of surface plasma waves for determination of the thickness and optical constants of thin metallic films ,” J. Opt. Soc. Am. 71 , 189 ( 1981 ). [CrossRef]

13 .

F. Chen , Z. Cao , Q. Shen , and Y. Feng , “ An optical approach to angular displacement measurement based on attenuated total reflection ,” Appl. Opt. 44 , 5393 ( 2005 ). [CrossRef] [PubMed]

14 .

P. K. Tien , R. Ulrich , and R. J. Martin , “ Modes of propagating light waves in thin deposited semiconductor films ,” Appl. Phys. Lett. 14 , 291 ( 1969 ). [CrossRef]

15 .

H. Li , Z. Cao , H. Lu , and Q. Shen , “ Free-space coupling of a light beam into a symmetrical metal-cladding optical waveguide ,” Appl. Phys. Lett. 83 , 2757 ( 2003 ) [CrossRef]

OCIS Codes
(120.4640) Instrumentation, measurement, and metrology : Optical instruments
(230.7390) Optical devices : Waveguides, planar

ToC Category:
Research Papers

Citation
Fan Chen, Zhuangqi Cao, Qishun Shen, Xiaoxu Deng, Biming Duan, Wen Yuan, Minghuang Sang, and Shengqian Wang, "Picometer displacement sensing using the ultrahigh-order modes in a submillimeter scale optical waveguide," Opt. Express 13, 10061-10065 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-25-10061


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References

  1. G. A. Rines, "Fiber-optic accelerometer with hydrophone applications," Appl. Opt. 20, 3453 (1981). [CrossRef] [PubMed]
  2. C. Liu, A. M.Barzilai, J. K. Reynolds, A. Partridge, T. W. Kenny, J. D. Grade, and H. K. Rockstad, "Characterization of a High-sensitivity Micromachined Tunneling Accelerometer with Micro-g Resolution," IEEE J. Micorelectromech. Syst. 7, 235 (1998). [CrossRef]
  3. E. B. Cooper, E. R. Post, S. Griffith, J. Levitan, S. R. Mnanlis, M. A. Schmidt, and C. F. Quate, "Highresolution micromachined interferometric accelerometer," Appl. Phys. Lett. 76, 3316 (2000). [CrossRef]
  4. F. A. Castro, S. R. M. Carneiro, O. Lisboa, and S. L. A. Carrara, "Two-mode optical fiber accelerometer," Opt. Lett. 17, 1474 (1992). [CrossRef] [PubMed]
  5. A. S. Gerges, T. P. Newson, and D. A. Jackson, "Practical fiber-optic-based submicro-g accelerometer free from source and environmental perturbations," Opt. Lett. 20, 1155 (1989). [CrossRef]
  6. A. Malki, P. Lecoy, N. Marty, C. Renouf, and P. Ferdinand, "Optical fiber accelerometer based on a silicon micromachined cantilever," Appl. Opt. 34, 8014 (1995). [CrossRef] [PubMed]
  7. R. A. Soref and D. H. McMahon, "Tilting-mirror fiber-optic accelerometer," Appl. Opt. 23, 486 (1984). [CrossRef] [PubMed]
  8. V. Milanovic, E. Bowen, M. E. Zaghloul, N.H. Tea, J. S. Suehle, B. Payne, and M. Gaitan, "Micromachined convective accelerometers in standard integrated circuits technology," Appl. Phys. Lett. 76, 508 (2000). [CrossRef]
  9. B. Zhu, M. Owner-Petersen, and T. Licht, "Accelerometer design based on attenuated total reflection," Appl. Opt. 27, 2972 (1988). [CrossRef] [PubMed]
  10. X. Liu, Z. Cao, Q. Shen, and S. Huang, "Optical sensor based on Fabry-Perot resonance modes," Appl. Opt. 42, 1 (2003). [CrossRef]
  11. H. Lu, Z. Cao, H. Li, and Q. Shen, "Study of ultrahigh-order modes in a symmetrical metal-cladding optical waveguide," Appl. Phys. Lett. 85, 4579 (2004). [CrossRef]
  12. W. P. Chen and J. M. Chen, "Use of surface plasma waves for determination of the thickness and optical constants of thin metallic films," J. Opt. Soc. Am. 71, 189 (1981). [CrossRef]
  13. F. Chen, Z. Cao, Q. Shen and Y. Feng, "An optical approach to angular displacement measurement based on attenuated total reflection," Appl. Opt. 44, 5393 (2005). [CrossRef] [PubMed]
  14. P. K. Tien, R. Ulrich, and R. J. Martin, "Modes of propagating light waves in thin deposited semiconductor films," Appl. Phys. Lett. 14, 291 (1969). [CrossRef]
  15. H. Li, Z. Cao, H. Lu, and Q. Shen, "Free-space coupling of a light beam into a symmetrical metal-cladding optical waveguide," Appl. Phys. Lett. 83, 2757 (2003) [CrossRef]

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