A E Alekseev

A phase-sensitive optical time-domain reflectometer with dual-pulse diverse frequency probe signal

In the present communication we propose a novel approach to the realization of a phase sensitive optical time-domain reflectometer (OTDR) which is capable of a precise reconstruction of the phase signal which impacts the arbitrary point of a fiber-optic line. The method uses a dual-pulse probe signal with diverse carrier optical frequency within each half of the double pulse. The quasi-periodic intensity pattern which emerges as a result of double frequency backscattered signal interference contains the information of the external action over the fiber. The phase signal is extracted with the aid of an I/Q quadrature demodulation scheme, realized at the receiving side of the OTDR. The feasibility and limitations of the proposed scheme are theoretically proved and experimentally demonstrated.

The influence of the degree of coherence of a semiconductor laser on the statistic of the backscattered intensity in a single-mode optical fiber

The results of the study of the statistical dependence of the backscattered light intensity of a semiconductor laser in a single-mode fiber on the duration of the probing pulse and the source coherence time are presented. It is shown that, for a given light coherence time, the intensity distribution function changes with increasing pulse duration from the function close to the exponential one for the pulse duration approximately equal to the coherence time to the function close to the Gaussian one for the pulse duration exceeding the coherence time. The exponential statistics of scattered light makes it possible to obtain a higher sensitivity for the coherent reflectometer using the direct detection method.

Detection of the external acoustic action upon an optical fiber using the interferometer of scattered radiation with the aid of the phase diversity method

The interferometer of scattered radiation is employed for the detection of the external acoustic action upon an optical fiber. A method for the demodulation of the scattered field using the phase diversity procedure is proposed. The possibility of the fading regions in the demodulated signal is demonstrated, the reasons for such an effect are analyzed, and the methods for its elimination are proposed.

Contrast enhancement in an optical time-domain reflectometer via self-phase modulation compensation by chirped probe pulses

In the present paper we propose a novel method for optical time-domain reflectometer (OTDR)–reflectogram contrast enhancement via compensation of nonlinear distortions of propagating probe pulse, which arise due to the self-phase modulation (SPM) effect in optical fiber. The compensation is performed via preliminary frequency modulation (chirp) of the initial probe pulse according to the specific law. As a result the OTDR contrast at some distant predefined fiber point is fully restored to the value of non-distorted probe pulse at the beginning of the fiber line. As a result, the performance of the phase OTDR increases. The point of full SPM compensation could be shifted to any other point of the fiber line via preliminary frequency modulation index change. The feasibility of the proposed method is theoretically proved and experimentally demonstrated.

The detection of an external acoustic impact on an optical fiber using a scattered-light interferometer

The results of a study of a scattered-light interferometer for detecting external acoustic impacts on optical fiber are presented. A method of scattered field demodulation using the phase diversity technique is proposed. The demodulated signal is shown to contain regions of fading, the causes of which are analyzed.

Statistics of intensity of backscattered semiconductor laser radiation in single-mode optic fiber

We have studied the statistics of the intensity of backscattered radiation of a semiconductor laser in single-mode optic fiber as a function of the probing laser pulse duration and coherence time. It is shown that, at a given radiation source coherence time, the intensity distribution function varies, depending on the pulse duration, from nearly exponential to nearly Gaussian. The exponential statistics provides a better sensitivity for a coherent reflectometer with direct detection. Using the calculated distribution function, it is possible to qualitatively determine the degree of deterioration of the reflectometer sensitivity with respect to external factors during increasing probing pulse duration or decreasing laser coherence time. These data provide criteria for the optimum choice of a radiation source for the coherent fiber-optic reflectometer.

Fading reduction in a phase optical time-domain reflectometer with multimode sensitive fiber

In the present paper we propose a novel type of a coherent phase-sensitive optical time-domain reflectometer (OTDR) that utilizes a multimode optical fiber as a sensitive element and is capable of considerable reduction of signal fading. Elimination of OTDR signal fading consequently removes randomly occurring insensitivity of the fiber regions to an external phase action. The backscattered light field at the input of OTDR sensitive multimode optical fiber is represented by a speckle-like pattern, due to a so called modal noise phenomenon. This speckle pattern randomly changes when an optical probe pulse propagates in the fiber line. The backscattered field intensity in every single speckle changes in time statistically independently from the intensity change in every other speckle remote enough from the first one. Thus, on the output of a multimode sensitive fiber, there exist several statistically independent reflectograms, and every single reflectogram contains the same information about external action. The joint independent analysis of these reflectograms can result in reduced or complete fading elimination.

A coherent dual-frequency phase-sensitive reflectometer with amplitude modulation of probing pulses

The results of investigation of a new type of the optical-fiber coherent phase-sensitive reflectometer with two different optical frequencies of probing pulses are presented. The use of pulses with different optical frequencies allows recovering of the signal of an external action on the reflectometer optical fiber by means of phase diversity technique. The applicability of the proposed technique is justified theoretically and experimentally.

A phase-sensitive coherence reflectometer with amplitude-phase modulation of probing pulses

Results of investigation of a phase-sensitive fiber-optic coherence reflectometer with amplitude-phase modulation of probing pulses are presented. Using this method of modulation, it is possible to reconstruct the signal of external impact upon the reflectometer fiber by means of the phase diversity technique. The applicability of the proposed method is theoretically justified and experimentally verified.