Integrated Interferometric Sensors

Laser interferometry is a well-developed technique for displacement measurement as it has high resolution and utilizes a noncontact method as described in Section 4.3. In most interferometers, a half mirror is used for making sensing and reference beams from one laser beam and for superimposing these beams after their travels through interferometer arms. The bulky interferometer is used only in expensive and extremely large pieces of equipment (e.g., stepper, electron beam drawer).

Compact optical interferometric sensors are advantageous for installing in many pieces of industrial equipment because of its small mass and size, its stability, the fact that it requires no alignment adjustment, and its low cost. Although there is great demand for integrated displacement sensors, the practical displacement sensor is still poor in signal amplitude, contrast, and dynamic range of the measurable distance.

The approach to building integrated interferometers has been based on the miniaturization of two-beam interferometers using the waveguide on a plane surface [Suhara et al., 1995]. The optical feedback technique has been studied as a compact interferometric sensor [Merlo and Donati, 1997; Kato et al., 1995]. The principle involves mixing between the back-reflected sensing beam from a moving sample and the optical field inside the laser diode. This technique gives a very simple optical configuration as only one optical arm is needed. The dynamic range of the measurement, however, is limited by the mode-hopping of the laser diode and the lack of stable operation.

Recently, a compact interferometer based on standing-wave detection using a thin-film photodiode has been developed [Sasaki et al., 1999b; Mi et al., 2001]. The key device is an ultra-thin-film photodiode.

The optical configuration is as simple as that of the optical feedback technique. When a coherent light beam is normally incident on a reflection mirror, the standing wave is generated in front of the reflection mirror. The use of an ultra-thin-film photodiode to detect the standing wave eliminates the need for a beam splitter and an optical arm for reference, whereas the usual two-beam interferometer requires two arms. The interferometer is essentially stable, with the potential for integration and suitability for use as a small sensor. In this section, interferometers based on standing-wave detection are described for developing a compact interferometric displacement sensor.

A schematic diagram of the proposed interferometer based on standing-wave detection using a thin-film photodiode is shown in Figure 4.33. The interferometer consists of a laser, an isolator, and a newly developed thin-film photodiode. The reflection mirror is on the moving object. The active layer of the ultra-thin-film photodiode, which absorbs photons, is designed to transmit most of the incident light beam. In our design, the absorption ratio to incident light power is estimated to be less than 1%. The almost incident light beam transmits through the photodiode before being absorbed and travels to the mirror on the moving object and returns to the thin-film photodiode. The incoming and reflection light beams superimpose each other and produce a standing wave that penetrates the active layer. The period of the standing wave is equal to λ/(2n)(λ is the wavelength of the incident light, n is the refractive index of the media). Because a node is generated on the reflection mirror, the intensity profile of the standing wave is spatially fixed to the reflection mirror. If the active layer has the appropriate thickness, which is thinner than the period of the standing wave of λ/2n, the intensity profile of the standing wave can be resolved. When the photodiode is located at the node of the standing wave, the signal decreases, and, when the photodiode is at the antinode, the signal increases. The relative position between the thin-film photodiode and the mirror determines the photodiode signal. This interference signal gives the displace-ment between the thin-film photodiode and the moving sample using the standing wave as the standard scale. In this interferometer, neither a reference mirror nor a beam splitter is necessary. The wavefront of the incoming laser beam is the reference. Because a laser beam with a large diameter can be used inside

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the sensor system, beam expansion due to diffraction is small. Measurement of the long distance up to the coherent length of the light source will be possible.

The active layer thickness is designed to be 40 nm in order to obtain enough spatial resolution to the standing wave [Sasaki et al., 1999b]. The absorbed power of the incident light is estimated to be about 0.4%. Inside such a thin Si film it is difficult to create a p–n junction in the thickness direction, which is the structure of the conventional photodiode. In the thin-film photodiode, the p–n junction is designed in the lateral direction in the Si film. The depletion region grows laterally in the Si film.

Figure 4.34 shows the fabrication sequence. The initial Si-on insulator wafer is prepared by direct wafer bonding between Si and quartz. The Si layer is thinned to 60 nm. The comb-shaped windows are FIGURE 4.33 (a) Interferometer using an ultra-thin-film photodiode; (b) principle of standing-wave detection.

FIGURE 4.34 Fabrication sequence of thin film photodetector.

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opened by lithography, through which As and B ions are implanted to make a p–n junction. After annealing and SiO2 deposition, the contact hole and Al electrode are formed. Finally, the phase shifter may be fabricated by etching the deposited SiO2 layer. The phase shifter can be used for obtaining the phase-shifted sinusoidal signals.

Figure 4.35 shows the fabricated thin-film photodiode with a 4-µm pitch comb shape. The p⁺ and n⁺ regions have a comb shape. The depletion region grows laterally between the p⁺ and n⁺ regions. This design is to lengthen the depleted region and to gather as many photocarriers as possible. The estimated thickness of the Si active layer is 35∼40 nm. The overall transmission rate reaches 70% in power, including the reflections at the interfaces between Si and SiO2, and between SiO2 and air. Figure 4.36 shows the fabricated photodiode array on the substrate. (In this case, the photodiode area is smaller than that shown in Figure 4.35 and a whole substrate is shown.) As shown in this figure, the sensor is transparent.

The sensitivity of the thin-film photodiode is about 0.75 mA/W and about three orders smaller than that of the usual bulk Si photodiode. The measured series resistance and equivalent capacitance of the photodiode are 100 KΩ and 15 pF, respectively. Due to the small cross-sectional area of the thin-film photodiode, the series resistance is large, whereas the parallel capacitance is small. The photodiode itself is considered to have a response speed of up to 600 KHz.

Figure 4.37(a) shows a schematic diagram for the sensor package combined with a can-type laser diode. The displacement sensor using a thin-film photodiode is packaged by stacking the thin-film FIGURE 4.35 Fabricated thin-film photodiodes.

FIGURE 4.36 Si-on insulator substrate with four arrayed small photodiodes.

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photodiode and isolator onto the collimation lens. All the optical elements are fixed within a metal housing. Figure 4.37(b) shows the fingertip-sized sensor package.

The experimental setup is the same as that shown in Figure 4.33(a). The red (632.8 nm, 7 mW) He–Ne laser is used as the light source and a fabricated photodiode having a 40-µm pitch of p⁺ and n⁺ regions is used as the detector. The reflection mirror is placed ∼45 mm away from the thin-film photodiode and moved by a piezoactuator. The signal period agrees well with λ/2. A signal amplitude of over 1 µA is obtained. The contrast reaches a maximum level of 77%. At a normal incidence on the thin-film photodiode and the mirror, the Fabry–Perot effect occurs due to multiple reflections. When the thin-film photodiode is slightly slanted (∼0.5 mrad) toward the wavefront of the incident laser beam, the interfer-ence signal becomes sinusoidal. Figure 4.38 shows two interferinterfer-ence signals obtained from the dual thin-film photodiode combined with a phase shifter. The phase shift agrees well with the designed value of π/2. Using this phase relation, the moving direction of the mirror can be determined.

Figure 4.39 shows the interference signals obtained from the packaged sensor, in which a laser diode (655.2 nm, 10 mW) is used as the light source and a fabricated photodiode having a 4-µm pitch of p⁺ and n⁺ regions is used as the detector. The contrast is around 25%. It results mainly from the inaccurate alignment between the thin-film photodiode and the laser beam.

FIGURE 4.37 (a) Schematic diagram of a sensor package; (b) view of a packaged sensor.

FIGURE 4.38 Two interference signals obtained from a dual thin-film photodiode combined with a phase shifter.

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The sensors are suitable for integration due to their simple optical setup. The principle of the inter-ferometer is based on standing-wave detection. This technique is compatible with other recently devel-oped techniques. The ultra-thin-film photodiode makes it possible to use optical standing waves in metrological applications. The sensor package is constructed using a can-type laser diode, an isolator (combination of a polarizer and a wave plate), and an ultra-thin-film photodiode. The size is as small as a fingertip.

4.5 Conclusions

In the first part of this chapter, the general principles of the optical sensors used in mechatronics were given. For industrial applications, simple and reliable techniques are preferable. The principle of position sensors and its application were described. The tracking and focusing sensors of optical disks serve as examples. Triangulation is a simple method but is widely applicable to several sensors used in industry.

Triangulation is commonly used for distance measurement in digital cameras. On the other hand, optical encoders are often used for measuring linear displacements and rotational angles in industrial mechanical systems. The principles of the three kinds of optical encoders were given. Furthermore, the principles of the optical interferometers were described from the point of view of displacement measurements. The highest precision is generally obtained with the optical interferometer, although it is very sensitive to environmental conditions.

In the latter part of the chapter advanced optical sensors were introduced. New versions of the position sensor, optical encoders, and optical interferometers were explained. In the new optical position sensors, the transmission structures have been fabricated by Si micromachining. The transmission-type position sensors are placed in tandem to measure displacements at multiple positions and are applied to the straightness measurement of the table translation in machine tools. The proposed optical encoder has also been fabricated by Si micromachining, in which some components for the sensor are integrated.

The interpolation of the encoder signal has been improved by suppressing the harmonic noise of the encoder output. A large working distance between the scale and index gratings is a useful property of the grating-image-type encoder. Integration of the grating-image-type encoder has also been demon-strated. In addition, a new optical interferometer is also explained for a precise displacement measure-ment. The interferometer consists of an ultra-thin-film photodetector, by which the intensity distribution of the standing wave is monitored. Due to its simple optical configuration (i.e., single optical arm instead of the conventional two arms of the interferometer), the proposed interferometer is compact enough to be installed in mechanical systems. An integrated displacement sensor using the new interferometry has been reported.

FIGURE 4.39 Interference signal obtained from a packaged sensor.

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References

Hane, K. and Grover, C. P., Magnified grating images used in displacement sensing, Appl. Opt., 26, 2355–2359, 1987a.

Hane, K., and Grover, C. P., Imaging with rectangular transmission gratings, J. Opt. Soc. Am., A4, 706–711, 1987b.

Hane, K., Endo, T., Ito,Y., and Sasaki, M., A compact optical encoder with micromachined photodetector, J. Opt. A: Pure Appl. Opt., 3, 191–195, 2001.

Hane, K., Endo, T., Ishimori, M., Ito, Y., and Sasaki, M., Integration of grating-image-type encoder using Si micromachining, Sensors Actuators A, 97–98, 139–146, 2002.

Kato, J., Kikuchi, N., Yamaguchi, I., and Ozono, S., Optical feedback displacement sensor using a laser diode and its performance improvement, Meas. Sci. Technol., 6, 45–52, 1995.

Merlo, S. and Donati, S., Reconstruction of displacement waveforms with a single-channel laser-diode feedback interferometer, IEEE J. Quantum Electron., 33, 527–531, 1997.

Mi, X., Sasaki, M., Hirano, T., and Hane, K., Interferometers based on standing wave detection using thin film photodiodes, Trans. IEE Jpn., 121E, 489–495, 2001.

Ohashi, T., Hirano, T., Ieki, A., Matsui, K., Nashiki, M., Sasaki, M., and Hane, K., Optical encoder having pitch-modulated photodiode array, Trans. IEE Jpn., 119E, 86–93, 1999.

Sasaki, M., Takebe, H., and Hane, K., Transmission-type position sensors for the straightness measure-ment of a large structure, J. Micromech. Microeng., 9, 429–433, 1999a.

Sasaki, M., Mi, X., and Hane, K., Standing wave detection and interferometer application using a photodiode thinner than optical wavelength, Appl. Phys. Lett., 75, 2008–2010, 1999b.

Suhara, T., Taniguchi, T., Uemukai, M., Nishihara, H., Hirata, T., Iio, S., and Suehiro, M., Monolithic iterated-optic position/displacement sensor using waveguide gratings and QW-DFB laser, IEEE Photo. Technol. Lett., 7, 1195–1197, 1995.

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Optical Fiber Basics • Basics of DOFS • DOFS Performance Parameters

Distributed optical-fiber sensing (DOFS) offers an extra dimension for the monitoring and diagnosis of large structures: the use of a one-dimensional, passive, dielectric measurement medium, flexible enough to be installed, with minimum intrusion conveniently and (if necessary) retrospectively, on extended structures such as dams, bridges, oil wells, aircraft, spacecraft, industrial pressure vessels and boilers, power generation and chemical plants, and mining installations and equipment is attractive as a means for offering the measurement of, for example, temperature and strain distributions. It can offer both a continuous monitor for early detection of anomalous (perhaps potentially destructive) conditions so that corrective action may be taken and for improving the detailed understanding of behavior (especially under extreme conditions) for use in the next generation of design. The use of these techniques promises significant improvements in structural integrity over a broad range of industries.

DOFS comprises both quasi-distributed systems, which allow for the determination of a measurand field at specific, predetermined positions along the fiber, and fully distributed systems, with a capability for measurement at any point along the length of the fiber. Primary among measurands of industrial interest are temperature and strain/pressure, and several systems, both fully and quasi-distributed, have been reported for such measurements. These include commercially available systems for temperature measurement.

This review of DOFS involves an examination of the subject’s origins and primary motivations, a description of the principles upon which it based and by means of which it has evolved, and a detailed look at the particular way in which it has developed. This will be followed by a review of current activity and application areas, closing with some projections as to possible future technical and commercial progressions.

DOFS is a particular development within the more general field of optoelectronics, a subject that effectively began with the invention of the laser in 1960. Moreover, the evolution of DOFS tracks quite Alan Rogers

University of Surrey Guildford, Surrey, U.K.

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