We propose a technique for multiplexing fibre Bragg grating (FBG) sensors with the same Bragg wavelength. We have already developed a technique for the synthesis of an optical coherence function, in which we can select one signal among various reflections along an optical path in an interferometer. By applying the technique, the reflection spectrum of each FBG in arrayed FBGs can selectively be obtained, even if the array is composed of FBGs with the same Bragg wavelength. In this paper, we first describe the principle of the proposed system, and then show the experimental results.

We have proposed and constructed a system to multiplex fiber Bragg grating (FBG) strain sensors with same reflection wavelength, adopting the synthesis of optical coherence function technique(SOCF), which has realized dynamic strain measurement. The ability to control the coherence peak position of SOCF allows random access in the spatial domain. In this paper, the function of random access strain measurement has been realized by switching the coherence peak position at every 113G, synchronizing with FBG spectrum measurement, to use the sampling rate to the full. In addition, we have proposed and confirmed the novel method for improving FBG interval by simply modulating the LD intensity.

Adopting a technique named synthesis of optical coherence function (SOCF), we have recently published a system of multiplexed fibre optic sensors with fibre Bragg gratings (FBGs) which may have similar Bragg wavelengths to each other (Hotate et al 2004 Meas. Sci. Technol. 15 148–53, Hotate and Enyama 2003 Proc. 16th Int. Conf. Optical Fiber Sensors (Nara, Japan) pp 522–5). In that system, the coherence function is synthesized into a series of periodical peaks that can be scanned along the fibre. By using one of the peaks as a measurement window, and sweeping it along a string of FBGs, the FBGs can be spatially resolved. To avoid the error signal from coherence peaks other than the designated measurement window, the string of FBGs has to be arranged within one period of the coherence peaks, i.e., the spatial measurement range of the system is limited to one period of the coherence function. In this paper, we propose a novel scheme that uses multiple coherence peaks to detect multiple FBGs. The coherence peaks and FBGs are arranged with the vernier effect, which means the period of the coherence function is slightly different from the interval of the FBGs, so at any moment, only one FBG can meet a coherence peak. In this way, the limitation to the measurement range is solved.

A novel optical system for a multiple-beam scanning electron microscope (SEM) is proposed. In the case of multiple-beam SEM, multiple secondary-electron beams passing through the column are inherently blurred because of the large energy spread and broad angular distribution of secondary electrons. To avoid cross-talk between the multiple secondary-electron beams, the optical system is designed such that it is divided into two independent parts: one for primary-beam illumination and one for secondary-electron detection. As the key components for the secondary-electron detection, a scan-cancelling deflector, and accelerating electric field were applied. To demonstrate the proposed optical system, a prototype column with four beams was developed. This column enables four SEM images to be separately but simultaneously acquired with more than 99% of the generated secondary electrons. This result demonstrates that high-speed imaging with the proposed multiple-beam SEM is possible in the near future.

Annular illumination is effective in enhancing the depth of focus for scanning electron microscopes (SEMs). However, owing to high side lobes of the point-spread function (PSF), annular illumination results in poor image sharpness. The conventional deconvolution method, which converts the PSF to a delta function, can improve image sharpness, but results in artifacts due to noise amplification. In this paper, we propose an image processing method that can reduce the deterioration of image sharpness. With this method, the PSF under annular illumination is converted to that under standard illumination. Through simulations, we verified that the image sharpness of SEM images under annular illumination with the proposed method can be improved without noise amplification.