B G Gorshkov

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.

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

A novel configuration of a phase-sensitive optical time-domain reflectometer (OTDR) utilizing dual-pulse phase modulations of the probe signal is presented and experimentally demonstrated. The proposed modulation method enables one to perform the demodulation and reconstruction of an external perturbation signal which impacts the fiber using the phase diversity technique. The proposed phase-sensitive OTDR has some advantages in comparison with conventional solutions, which are discussed. The feasibility of a double pulse OTDR with phase modulation is demonstrated and theoretically proved.

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.

Optical picoscopes: new opportunities for biosensing and for molecular technologies

New true direct methods of sensitive real-time recording of molecular reactions on a surface and detection of bio- and chemical agents have been developed. The methods are based on measuring changes of thickness of a sensor layer due to binding reactions. A transparent plate or a gap between two surfaces of optical materials is used as the sensor layer. The methods allow employment as biochips of microscopic glass slips without deposition of any metal or dielectric films. Alternatively, direct pumping of liquid samples through the sensing gap with deposited recognition layers can be used. For label-free biosensing, different optical schemes were realized to record thickness changes due to receptor-ligand bindings with picometer-scale resolution. Biosensors named one-dimensional PicoscopeTM and AffinoscopeTM have been developed for real-time detection of several biological agents by different recognition spots or wells with specific receptors on the biochip surface. The devices have been successfully employed for detection of food pathogens, for investigation of pharmaceutical substances, for epitope mapping of different monoclonal antibodies and immunotherapy research, for monitoring of bacteriocin production, etc. Application of the devices can be as wide as that of optical microscopes as they provide standard lateral resolution and, in addition, offer much more comprehensive information with outstanding real-time resolution in depth, e.g., for measuring molecular binding kinetics, monitoring of assembling in molecular structures, etc. The Picoscope technology significantly increases power of research instruments for bio-, nano- and pico-technologies.

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.

Real-time sensitive detection of low molecular weight compounds by optical immunosensors

Highly sensitive label-free methods have been developed for detection of low molecular weight compounds. The methods are based on real-time detection of biomolecular interactions on the surface of standard microscopic cover glass slips used as affordable single-used sensor chips. The assay performance was successfully validated for detection of antibiotic chloramphenicol which is used in medicine and veterinary as well as for determination of natural toxins in real samples. The proposed methods are an attractive solution for medical monitoring of antibiotics in the organism and for toxicological control of food.

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.

Biosensors based on spectral correlation interferometry for biomedical research and diagnostics

Biosensors based on the spectral correlation interferometry (SCI) with assistance of magnetic nanoparticles (MP) have been developed for medical diagnostics. The SCI has been adapted for studies of affinity constants of MP covered by antibodies and for detection of protein molecules in human serum. The 50-nm MP employed as labels yield 100-fold amplification of the SCI signals to meet the requirements for myocardial infarction diagnostics.