US-12618744-B2 - Optical fiber characteristics measurement apparatus and method of measuring optical fiber characteristics

Abstract

An optical fiber characteristics measurement apparatus includes a first optical splitter that splits modulated light into pump light and reference light, a second optical splitter that causes the pump light to be incident on a fiber under test from one end and extracts Brillouin scattered light generated within the fiber under test, an adjuster that changes frequency of at least one of the reference light and pump light to a plurality of frequencies, and a calculator that measures characteristics of the fiber under test based on a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each frequency, the second harmonic component having a frequency that is two times the modulation frequency of the modulated light.

Inventors

• Yuta Suzuki
• Shin-ichirou Tezuka
• Masayoshi Honma

Assignees

• YOKOGAWA ELECTRIC CORPORATION

Dates

Publication Date

20260505

Application Date

20240521

Priority Date

20230525

Claims (8)

1. 1 . An optical fiber characteristics measurement apparatus comprising: a first optical splitter configured to split a modulated light subjected to frequency modulation into a pump light and a reference light; a second optical splitter configured to cause the pump light to be incident on a fiber under test from one end and configured to extract Brillouin scattered light generated within the fiber under test; a modulator configured to change a frequency of at least one of the reference light and the pump light to a plurality of frequencies; and a circuit configured to detect a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each frequency, and measure characteristics of the fiber under test based on the detected second harmonic component, the second harmonic component being a component having a frequency that is two times a modulation frequency of the modulated light.
2. 2 . The optical fiber characteristics measurement apparatus according to claim 1 , further comprising an A/D converter configured to output, to the circuit, a signal yielded by sampling, at a predetermined sampling frequency, the signal yielded by homodyne detection of the interference light, wherein the predetermined sampling frequency is at least two times the frequency that is two times the modulation frequency of the modulated light.
3. 3 . The optical fiber characteristics measurement apparatus according to claim 2 , wherein the circuit is configured to calculate the second harmonic component by mixing the signal yielded by homodyne detection of the interference light and a detection signal having a frequency that is two times the modulation frequency of the modulated light.
4. 4 . The optical fiber characteristics measurement apparatus according to claim 3 , further comprising a light source configured to emit the modulated light and cause the modulated light to travel to the first optical splitter; and an oscillator configured to output a modulation signal having the modulation frequency of the modulated light to the light source and input the detection signal to the circuit.
5. 5 . The optical fiber characteristics measurement apparatus according to claim 1 , wherein the circuit is configured to calculate the second harmonic component by mixing the signal yielded by homodyne detection of the interference light and a detection signal having a frequency that is two times the modulation frequency of the modulated light.
6. 6 . The optical fiber characteristics measurement apparatus according to claim 3 , further comprising a light source configured to emit the modulated light and cause the modulated light to travel to the first optical splitter; and an oscillator configured to output a modulation signal having the modulation frequency of the modulated light to the light source and input the detection signal to the circuit.
7. 7 . The optical fiber characteristics measurement apparatus according to claim 1 , wherein the modulator is configured to sweep the frequency of at least one of the reference light and the pump light to a plurality of frequencies; and the circuit is configured to acquire a Brillouin gain spectrum determined based on characteristics of the fiber under test by combining the second harmonic component among frequency components of the signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each swept frequencies, wherein the circuit is configured to calculate a peak frequency of the Brillouin gain spectrum as a Brillouin frequency shift determined based on the characteristics of the fiber under test.
8. 8 . A method of measuring optical fiber characteristics, the method comprising: splitting a modulated light subjected to frequency modulation into a pump light and a reference light; causing the pump light to be incident on a fiber under test from one end; extracting Brillouin scattered light generated within the fiber under test; changing a frequency of at least one of the reference light and the pump light to a plurality of frequencies; and detecting a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each frequency, and measuring characteristics of the fiber under test based on the detected second harmonic component, the second harmonic component being a component having a frequency that is two times a modulation frequency of the modulated light.

Description

CROSS-REFERENCE TO RELATED APPLICATION The present application claims priority to Japanese Patent Application No. 2023-86474 filed on May 25, 2023, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD The present disclosure relates to an optical fiber characteristics measurement apparatus and a method of measuring optical fiber characteristics. BACKGROUND A method of measuring optical fiber characteristics by calculating the amount of frequency shift in the peak frequency of the spectrum of Brillouin scattered light generated in an optical fiber is known. See, for example, Patent Literature (PTL) 1. CITATION LIST Patent Literature PTL 1: JP 2023-22609 A SUMMARY An optical fiber characteristics measurement apparatus according to several embodiments includes a first optical splitter configured to split a modulated light subjected to frequency modulation into a pump light and a reference light, a second optical splitter configured to cause the pump light to be incident on a fiber under test from one end and configured to extract Brillouin scattered light generated within the fiber under test, an adjuster configured to change a frequency of at least one of the reference light and the pump light to a plurality of frequencies, and a calculator configured to measure characteristics of the fiber under test based on a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each frequency, the second harmonic component being a component having a frequency that is two times a modulation frequency of the modulated light. A method of measuring optical fiber characteristics according to several embodiments includes splitting a modulated light subjected to frequency modulation into a pump light and a reference light, causing the pump light to be incident on a fiber under test from one end, extracting Brillouin scattered light generated within the fiber under test, changing a frequency of at least one of the reference light and the pump light to a plurality of frequencies, and measuring characteristics of the fiber under test based on a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light at each frequency, the second harmonic component being a component having a frequency that is two times a modulation frequency of the modulated light. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a block diagram illustrating a configuration of an optical fiber characteristics measurement apparatus according to a comparative example; FIG. 2 is a flowchart illustrating a measurement method according to a comparative example; FIG. 3 is a block diagram illustrating a configuration example of an optical fiber characteristics measurement apparatus according to the present disclosure; FIG. 4 is a block diagram illustrating an example configuration of a calculation apparatus; FIG. 5 is a graph illustrating an example of the frequency spectrum of modulated light; FIG. 6 is a graph illustrating an example of the frequency spectrum of a signal yielded by homodyne detection of interference light; FIG. 7 is a flowchart illustrating example procedures of a method of measuring optical fiber characteristics according to the present disclosure; and FIG. 8 is a block diagram illustrating a configuration example of an optical fiber characteristics measurement apparatus according to another embodiment. DETAILED DESCRIPTION The homodyne BOCDR (Brillouin Optical Correlation Domain Reflectometry) method is sometimes used as a method of measuring the spectrum of Brillouin scattered light, which is used to measure optical fiber characteristics. The homodyne BOCDR method measures the spectrum of Brillouin scattered light by splitting a modulated light subjected to frequency modulation into a pump light and a reference light, causing the pump light to be incident on the fiber under test, and performing homodyne detection of interference light between the Brillouin scattered light generated by the pump light in the fiber under test and the reference light. In the homodyne BOCDR method, the amplitude modulation (AM) component in the modulated light becomes noise. The AM component can be the result of frequency modulation (FM) or may be generated by changes, due to temperature drift, in the splitting ratio of the coupler that splits the modulated light. AM noise reduces the measurement accuracy of the spectrum of Brillouin scattered light. The measurement accuracy of optical fiber characteristics based on the spectrum of Brillouin scattered light also degrades. It would be helpful to provide an optical fiber characteristics measurement apparatus and a method of measuring optical fiber characteristics that can improve the measurement accuracy of optical fiber char