Fiber specklegram sensing by means of an adaptive joint transform correlator

Although multimode fiber has a large space-bandwidth product (roughly equal to the number of modes), transmitting spatial signals by using a fiber is rather difficult. Conventional sensing schemes for fiber chemical sensors are based on the intensity and/or spectroscopic absorption or emission of fiber evanescent field of multimode fiber. However, the transmission of spatial signal has not been exploited in the field of chemical sensing. In our research, a novel chemical sensing method that employs the spatial information of a multimode fiber was investigated. When a laser beam is launched into a multimode fiber, the exit light field produces a complicated speckle pattern caused by the modal interference of the fiber. It is difficult to recover the transmitted information from the speckle field. However, the fiber speckle field can be used for fiber sensing with an inner product method. Our analysis and experiments have shown that the fiber specklegram sensor is very sensitive to chemical environment change.

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“…It can be shown that the speckle intensity distribution from a disturbed multimode fiber is given by [9] ∫∫ ∑∑∑∑ ∫∫ (4) which can be shown as…”

Section: Inner-product Fssmentioning

confidence: 99%

“…The normalized intensity inner product can be calculated by (9) From the derivation we can see that if the fiber is perturbed, the modal phase deviates. It produces a different speckle field that causes the NIP to reduce.…”

Section: Inner-product Fssmentioning

confidence: 99%

A novel fiber chemical sensor using inner-product multimode fiber speckle fields

et al. 2003

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“…The light field can be written as E1(r,t) = E1exp[i(csY-k1 .r)] i 1,...4 (2) where E, , E2 , E3 and E4 are complex amplitudes of the four waves, k1 , k2 , k3 and k4 are the wave vectors. In the case of four-wave mixing for phase conjugation, the wave vectors come in two oppositely directed pairs, that is, k2 =-k3,and k4 =-k1 (3) In this case, there will be four refraction index gratings in the photorefractive crystal, which were formed by (E1E + (E2E + E1E), (E2E)and (E,E), respectively. The refraction index grating can be represented as…”

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confidence: 99%

<title>Fiber specklegram sensing with phase-conjugation</title>

Lü

Yin

1995

SPIE Proceedings

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A multimode fiber specklegram sensor using phase-conjugate mirror is presented. This sensing technique is based on the phase compensation of phase-conjugate signal, which is very sensitive to external perturbation. The preliminary experiments on displacement measurement are also provided. . INTRODUCTIONOptical fiber sensing is a most promising technology for many industrial observation problems. The basic advantages implicit in optical fiber sensing, such as immunity from electromagnetic interference, chemical resistance, and potentially high sensitivity, make this approach attractive'. A new class of optical fiber sensor, recently proposed by Wu et a12, is based on the specklegram from multimode fiber. Since the speckle distribution is attributed to interference among those guided modes in multimode fiber, the specklegram fiber sensor is of sensitivity as high as interferomethc sensors. Based on the correlation between speckle patterns, this kind of optical fiber sensor has been used to measure displacement, vibration and temperature35. Furthermore, the specklegram fiber sensor can also be used in multichannel sensing because of the high space-bandwidth product of multimode fibers.The high space-bandwidth product of multimode fiber has also been utilized to transmit spatial information.7'8. The key point in this technique is how to reconstruct the spatial information from distortion caused by modal coupling and dispersion. Yariv7 has proposed to retrieve the image using the wave-front-reversal property of phase-conjugate mirror. Such an image transmission technique requires the same phase distribution in forward and backward paths. Therefore, in Yariv's method two identical fiber must be found before this technique can be realized. To overcome that difficulty, Dunning and Lind8 proposed a single fiber architecture, and implemented reconstruction of image transmitting through fiber. Although it is apparently not a practical method because transmitted image can only be reconstructed in the input end of fiber, they successfully demonstrated the process of phase compensation using phase-conjugate mirror. We note that, in Dunning and Lind's architecture the degradation of the image will not be compensated completely when external perturbation exists, and that deviation from original pattern can be utilized to measure that external perturbation. In this Technical Note, a fiber specklegram sensor with phase-conjugate mirror is presented. We analyzed the effect of external perturbation on the reconstructed light field, and the preliminary experimental results of submicrometer displacement sensing are also provided. . PHOTOREFRACTIVE EFFECT AND PHASE CONJUGATIONPhotorefractive effect is a phenomenon in which the local index of refraction of a medium is changed by the illumination of a beam of light with spatial variation of the intensity9. This effect is attributed to the formation of the photoinduced space-charge field and formation of index grating via the linear electro-optic effect ( the Pockles effect). In comparing...

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“…Various methods to detect specklegram changes have been tested such as phase conjugate mirror (PCM) [1] holographic detection [2] induced mode coupling, [3] joint-transform correlator (JTC) [4,5], mean-absolute speckle-intensity variation (MSV) using a CCD camera [6], spatial filtering technique [7] by butt-joining fibers with different core diameters, a reflection-type speckle interferometer combined with spatial mode filtering [8] as well as a sophisticated combination of threshold and frequency filtering based on fast Fourier transform [9].…”

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Performance evaluation of a few- and multimode fiber optic perimeter sensor with selective mode excitation

Eftimov

Miletiev

Arnaudov

et al. 2010

Photon. Lett. Poland

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Abstract-We present a study of the performance of a simple fiberoptic perimeter snsor based on selective mode excitation in few-and multimode optical fibers. The electronic unit identifies an intrusion on the basis of a pre-defined threshold and number of detected pulses per unit time. The sensor may be controlled using the keypad, a PC or a FSK modem. The statistical properties of the responses are analyzed using Weibull distributions.

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Fiber specklegram sensing by means of an adaptive joint transform correlator

Cited by 11 publications

References 9 publications

A novel fiber chemical sensor using inner-product multimode fiber speckle fields

A novel fiber chemical sensor using inner-product multimode fiber speckle fields

<title>Fiber specklegram sensing with phase-conjugation</title>

Performance evaluation of a few- and multimode fiber optic perimeter sensor with selective mode excitation

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