US-12624973-B2 - Optical fiber sensor utilizing faraday rotator mirrors and measuring system using the same

US12624973B2US 12624973 B2US12624973 B2US 12624973B2US-12624973-B2

Abstract

The present invention is the optical fiber sensor, that laser pulses from a laser source are separated into a reference path and a measurement path by the second optical coupler via the first optical coupler, the first FRM is provided at an end of the reference path, the second FRM is provided at an end of the measurement path, and the reference reflected light of the first FRM and the measurement reflected light of the second FRM are interfered at the second optical coupler and are converted into three phases. The first phase pulses are transmitted to the optical synthesis section via the first optical coupler, second phase pulses are transmitted to the optical synthesis section via the first delay section, and third phase pulses are transmitted to the optical synthesis section via the second delay section. The time division pulse train is outputted from the optical synthesis section.

Inventors

Toshiharu Ikeda
Tsutomu Yamate
Masashi Saijo
Minoru Yoshida

Assignees

HAKUSAN CORPORATION

Dates

Publication Date: 20260512
Application Date: 20231016
Priority Date: 20230131

Claims (5)

1 . An optical fiber sensor, wherein laser pulses from a laser source are separated into two phases of a reference path and a measurement path including a measuring object, which form an interferometer, by a second optical coupler via a first optical coupler; wherein a first Faraday Rotator Mirror (FRM) is provided at an end of said reference path, and a second FRM is provided at an end of said measurement path; and wherein a reference reflected light of said first FRM and a measurement reflected light of said second FRM are interfered at said second optical coupler and are converted into three phases, converted first phase pulses are transmitted to an optical synthesis section via said first optical coupler, converted second phase pulses are transmitted to said optical synthesis section via a first delay section, converted third phase pulses are transmitted to said optical synthesis section via a second delay section, and a time division pulse train is outputted from said optical synthesis section.
2 . The optical fiber sensor according to claim 1 , wherein all transmissions of said laser pulses are performed by using a single mode (SM) fiber.
3 . The optical fiber sensor according to claim 1 , wherein a delay time of said second delay section is longer than a delay time of said first delay section.
4 . A measuring system to measure a displacement of said measuring object by measuring a peak value of said time division pulse train as claimed in claim 1 .
5 . An optical fiber sensor that measuring pulses are transmitted to “n(≥2)” interferometers respectively including a measuring object, and Time Division Multiplexing (TDM) pulse trains corresponding to respective displacements of said measuring objects from said “n” interferometers are optically synthesized and are outputted, wherein “n−1” optical couplers to convert laser pulses into two phases of said measuring pulses and delaying pulses are provided; wherein a first stage optical coupler inputs laser pulses from a laser source, and a second stage to (n−1)th stage optical couplers respectively input said delaying pulses from a previous stage via a delay section; wherein n-th optical coupler transmits said delaying pulses from (n−1)th optical coupler to said n-th interferometer and inputs; wherein respective TDM pulse trains outputted from said “n” interferometers are sequentially and optically synthesized, and a multiple TDM pulse train is outputted; and wherein respective said “n” interferometers comprise a constitution that: said laser pulses are separated into two phases of a reference path and a measurement path including a measuring object, which form an interferometer, by a second optical coupler via a first optical coupler; a first Faraday Rotator Mirror (FRM) is provided at an end of said reference path and a second FRM is provided at an end of said measurement path; and a reference reflected light of said first FRM and a measurement reflected light of said second FRM are interfered at said second optical coupler and are converted into three phases, converted first phase pulses are transmitted to an optical synthesis section via said first optical coupler, converted second phase pulses are transmitted to said optical synthesis section via a first delay section, converted third phase pulses are transmitted to said optical synthesis section via a second delay section, and a time division pulse train is outputted from said optical synthesis section.

Description

BACKGROUND OF THE INVENTION 1. Technical Field of the Invention The present invention relates to an optical sensor that reduces an influence of a transmission path by converting three outputs into one pulse train of a one output with a Time Division Multiplexing (TDM) by extracting a phase information with 3×3 optical couplers in a sensor section having an interferometer instead a phase shift light-pulse system using a delay circuit, and to a measuring system using the optical fiber sensor. 2. Description of the Prior Art Based on reasons that an optical fiber sensor is capable of not having a power source for an optical sensor section and so on, the optical fiber sensor is superior for an application to a position where an electromagnetic environment is bad or to a position where a temperature environment is bad in comparison with a conventional electrical sensor. Further, it is also possible to show a superior characteristic for a long-distance sensing according to a low loss characteristic of the optical fiber. Consequently, many kinds of sensors using the optical fiber are proposed (e.g. Japanese Patent No. 5,118,004 (Patent Document 1), Japanese Patent No. 5,702,623 (Patent Document 2), Japanese Patent No. 6,002,329 (Patent Document 3), Japanese Published Unexamined Patent Application No. 2018-9896 (Patent Document 4)). In the conventional homodyne system, there is a defect that the intensity fluctuation of the reference light and the measurement light which are interference inputs and the intensity fluctuation of the light of the interference output become an error of the interference output signal. Further, in the conventional homodyne system, the linearity is the best setting that a phase relation between the reference light and the measurement light is put on an operating point being the phase difference 90 degrees. However, it is necessary to need a high machining accuracy to constantly keep the light path length of the reference light and the measurement light. It is further necessary to also keep the constant relation for the thermal expansion. In this manner, there is a difficulty on the manufacturing. Furthermore, in the conventional homodyne system, the phase difference between the reference light and the measurement light is put on 90 degrees and the maximum phase fluctuation is ±90 degrees, and it is in principle impossible to exceed these set degrees. As the technique to resolve such the problems of the conventional homodyne system, the real time displacement measuring method using the phase shift light-pulse system disclosed in Patent Documents 1 to 4, is proposed. However, since the delay line is needed in one side of the interferometer, the performance deterioration is caused by the characteristic change of the delay line according to the environmental fluctuation. SUMMARY OF THE INVENTION The present invention has been made in view of the above described problems, and an object of the present invention is to provide an optical fiber sensor being capable of measuring at a high accuracy without the external influence with a cheap configuration using 3×3 optical couplers in order to reduce the external influence of the real time displacement measuring performance using the phase shift light-pulse system, and to provide a measuring system using the optical fiber sensor. In accordance with the present invention, there is provided an optical fiber sensor, wherein laser pulses from a laser source are separated into two phases of a reference path and a measurement path including a measuring object, which form an interferometer, by a second optical coupler via a first optical coupler; wherein a first Faraday Rotator Mirror (FRM) is provided at an end of the reference path, and a second FRM is provided at an end of the measurement path; and wherein a reference reflected light of the first FRM and a measurement reflected light of the second FRM are interfered at the second optical coupler and are converted into three phases, converted first phase pulses are transmitted to an optical synthesis section via the first optical coupler, converted second phase pulses are transmitted to the optical synthesis section via a first delay section, converted third phase pulses are transmitted to the optical synthesis section via a second delay section, and a time division pulse train is outputted from the optical synthesis section. In accordance with the present invention, there is provided the optical fiber sensor, wherein laser pulses from a laser source are transmitted to a second Polarization Maintaining (PM) optical coupler by a first PM fiber via a polarizer and a first PM optical coupler; wherein the laser pulses are separated into two phases of a reference path and a measurement path including a measuring object, which form an interferometer, at the second PM optical coupler and are transmitted by a second PM optical fiber; wherein a first mirror is provided at an end of the reference path, and a secon