FIGURE 6.110 Shift in peak loss wavelength as a function of the applied strain.
TABLE 6.28 Strain Sensitivity of Long-Period Gratings Written
in Four Different Types of Fibers
|
Strain sensitivity |
Type of fiber |
(nm %ε–1) |
A — Standard dispersion-shifted fiber (DSF) |
–7.27 |
B — Standard 1550 nm communication fiber |
4.73 |
C — Converntional 980 nm single-mode fiber |
4.29 |
D — Conventional 1060 nm single-mode fiber |
15.21 |
|
|
Note: The values correspond to the shift in the highest order resonance wavelength.
Temperature Sensitivity of Long-Period Gratings
Gratings written in different fibers were also tested for their cross-sensitivity to temperature [22]. The temperature coefficients of wavelength shift for different fibers are shown in Table 6.29. The temperature sensitivity of a fiber Bragg grating is 0.014 nm °C–1. Hence, the temperature sensitivity of a long-period grating is typically an order of magnitude higher than that of a Bragg grating. This large cross-sensitivity to ambient temperature can degrade the strain sensing performance of the system unless the output signal is adequately compensated. Multiparameter sensing using long-period gratings has been proposed to obtain precise strain measurements in environments with temperature fluctuations [21].
In summary, long-period grating sensors are highly versatile. These sensors can easily be used in conjunction with simple and inexpensive detection techniques. Experimental results prove that these methods can be used effectively without sacrificing the enhanced resolution of the sensors. Long-period grating sensors are insensitive to the input polarization and do not require coherent optical sources. The cross-sensitivity to temperature is a major concern while using these gratings for strain measurements.
© 1999 by CRC Press LLC
FIGURE 6.111 The shift induced by strain in a grating written in fiber C.
FIGURE 6.112 Plot of the change in transmitted intensity as a function of strain, for three different trials.
© 1999 by CRC Press LLC
Table 6.29 Temperature Sensitivity of Long-Period Gratings Written
in Four Different Types of Fibers
|
Temperature sensitivity |
Type of fiber |
(nm °C–1) |
A — Standard dispersion-shifted fiber (DSF) |
0.062 |
B — Standard 1550 nm communication fiber |
0.058 |
C — Converntional 980 nm single-mode fiber |
0.154 |
D — Conventional 1060 nm single-mode fiber |
0.111 |
|
|
Note: The values correspond to the shift in the highest order resonance wavelength.
Comparison of Sensing Schemes
Based on the above results, the interferometric sensors have a high sensitivity and bandwidth, but are limited by the nonlinearity in their output signals. Conversely, intrinsic sensors are susceptible to ambient temperature changes while the grating-based sensors are simpler to multiplex. Each may be used in specific applications.
Conclusion
We have investigated the performance of four different interferometric and grating-based sensors. This analysis was based on the sensor head fabrication and cost, signal processing, cross-sensitivity to temperature, resolution, and operating range. The relative merits and demerits of the various sensing schemes were also discussed.
References
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6.12 Optical Beam Deflection Sensing
Grover C. Wetsel
Measurements of the intensity of the light reflected and transmitted by a sample have been sources of information concerning the structure of matter for over a century. In recent decades, it has been found that measurement of the position of an optical beam that has scattered from a sample is an important and versatile means of characterizing materials and the motion of devices. Surely, the availability of a well-collimated beam from a laser has been crucial in the development of techniques and applications of optical beam deflection (OBD) sensing; however, the development and ready availability of various types of position sensing detectors (PSDs) have also been important factors. Optical beam deflection may be caused, for example, by propagation of a laser beam through a refractive-index gradient or by reflection from a displaced surface. A PSD provides an electronic signal that is a function of the laser beam position on the detector.
In this section, applications of optical beam deflection sensing are reviewed, the theories of operation of the three most common types of OBD sensors are developed, and typical operational characteristics of the devices are presented. The advantages and disadvantages of the various PSDs are also discussed.
© 1999 by CRC Press LLC