US-12624971-B2 - Optical measurement system

US12624971B2US 12624971 B2US12624971 B2US 12624971B2US-12624971-B2

Abstract

An optical measurement system in which a coherent light source generates a light signal, and a launch stage receives the light signal from the light source and generates a test signal. The test signal is launched along an optical path. The launch stage includes a first IQ modulator for controlling the test signal. The optical measurement system further includes a local oscillator stage configured to generate a local oscillator signal, wherein local oscillator stage has a second IQ modulator for controlling the local oscillator signal. A detector stage of the system is configured to receive the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, and interfere the local oscillator signal with the scattered signal. The optical measurement system is implemented with one or more photonic integrated circuits.

Inventors

Stuart J. Russell

Assignees

Sintela Limited

Dates

Publication Date: 20260512
Application Date: 20231215
Priority Date: 20221221

Claims (16)

1 . An optical measurement system comprising: a coherent light source configured to generate a light signal; a launch stage configured to receive the light signal from the light source and generate a test signal and launch the test signal along an optical path, wherein the launch stage includes a first IQ modulator for controlling the test signal; a local oscillator stage configured to generate a local oscillator signal, wherein local oscillator stage comprises a second IQ modulator for controlling the local oscillator signal; and a detector stage configured to receive the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, and interfere the local oscillator signal with the scattered signal, wherein the optical measurement system is implemented with one or more photonic integrated circuits.
2 . The optical measurement system according to claim 1 , further comprising a polarising beam splitter configured to split the light signal received by the launch stage into a first polarisation state and a second polarisation state, and wherein the first IQ modulator includes a pair of IQ modulators for controlling the light signal in the first polarisation state and the second polarisation state, respectively.
3 . The optical measurement system according to claim 1 , wherein the first IQ modulator and/or the second IQ modulator respectively comprises a first branch and a second branch with a phase delay introduced between the first branch and the second branch, and wherein each of the first branch and the second branch comprises a respective amplitude and phase modulator.
4 . The optical measurement system according to claim 1 , further comprising a reference interferometer connected to receive a portion of the light signal from the light source, wherein the reference interferometer is configured to interfere a first portion of the received light signal with a second, delayed portion of the received light signal.
5 . The optical measurement system according to claim 4 , further configured to determine a phase noise of the light source based on the interference of the first portion and the second portion of the received light signal in the reference interferometer.
6 . The optical measurement system according to claim 1 , wherein the detector stage comprises a polarising beam splitter arranged to split a first one of the local oscillator signal and the scattered signal into a first polarisation state and a second polarisation state, and wherein the detector stage is configured to interfere a second one of the local oscillator signal and the scattered signal with each of the first and second polarisation states.
7 . The optical measurement system according to claim 1 , wherein the detector stage comprises a plurality of input channels, each input channel configured to receive a respective scattered signal, and wherein the detector stage is configured to separately interfere the local oscillator signal with each one of the respective scattered signals.
8 . The optical measurement system according to claim 1 , further comprising a controller configured to control the first IQ modulator and the second IQ modulator.
9 . The optical measurement system according to claim 8 , wherein the controller is configured to control the first IQ modulator to vary a frequency of the test signal over time, and/or the controller is configured to control the second IQ modulator to vary a frequency of the local oscillator over time.
10 . The optical measurement system according to claim 8 , wherein: the controller is configured to control a frequency of the test signal by driving the first IQ modulator as a single sideband modulator; and/or the controller is configured to control a frequency of the local oscillator by driving the second IQ modulator as a single sideband modulator.
11 . The optical measurement system according to claim 8 , wherein: the controller comprises two or more selectable measurement modes, and a memory arranged to store, for each of the two or more measurement modes, a respective set of control parameters for the first IQ modulator and the second IQ modulator; and the controller is configured to control the first IQ modulator and the second IQ modulator in accordance with the set of control parameters associated with a selected one of the two or more measurement modes.
12 . The optical measurement system according to claim 1 , wherein the coherent light source is implemented on a separate photonic integrated circuit from the launch stage, local oscillator stage and the detector stage.
13 . The optical measurement system according to claim 9 , wherein: the controller is configured to control a frequency of the test signal by driving the first IQ modulator as a single sideband modulator; and/or the controller is configured to control a frequency of the local oscillator by driving the second IQ modulator as a single sideband modulator.
14 . The optical measurement system according to claim 13 , wherein: the controller comprises two or more selectable measurement modes, and a memory arranged to store, for each of the two or more measurement modes, a respective set of control parameters for the first IQ modulator and the second IQ modulator; and the controller is configured to control the first IQ modulator and the second IQ modulator in accordance with the set of control parameters associated with a selected one of the two or more measurement modes.
15 . The optical measurement system according to claim 9 , wherein: the controller comprises two or more selectable measurement modes, and a memory arranged to store, for each of the two or more measurement modes, a respective set of control parameters for the first IQ modulator and the second IQ modulator; and the controller is configured to control the first IQ modulator and the second IQ modulator in accordance with the set of control parameters associated with a selected one of the two or more measurement modes.
16 . The optical measurement system according to claim 10 , wherein: the controller comprises two or more selectable measurement modes, and a memory arranged to store, for each of the two or more measurement modes, a respective set of control parameters for the first IQ modulator and the second IQ modulator; and the controller is configured to control the first IQ modulator and the second IQ modulator in accordance with the set of control parameters associated with a selected one of the two or more measurement modes.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2023/086159, filed Dec. 15, 2023, which claims priority to Great Britain Patent Application No. 2219449.2, filed Dec. 21, 2022, the contents of which are hereby incorporated by reference in their respective entireties. TECHNICAL FIELD The invention relates to an optical measurement system, for performing measurements on an optical path such as an optical fibre. BACKGROUND TO THE INVENTION There exist various techniques for measuring properties of an optical fibre by interrogating the optical fiber with a transmitted optical signal. These techniques make use of different scattering mechanisms within the optical fibre. Distributed Acoustic Sensing Distributed Acoustic Sensing (DAS) is an established technology with several commercial systems available. In these systems, a pulse or pulses of laser light are launched into a length of optical fibre and the light that is scattered within the fibre is analysed in order to derive the nature of the acoustic environment, i.e. any physical vibrations, of the fibre transducer. In particular, these systems typically make a measurement of the acoustic strain environment of an optical fibre transducer using an optical time domain reflectometer (OTDR) approach. This gives a differential strain measurement as a function of position along the optical fibre. As an optical fibre is manufactured it is cooled or quenched from a high temperature as it is drawn. This process leads to the presence of small variations in the density of the optical fibre. These tiny variations in density equate to variations in the effective refractive index of the fibre. These discontinuities lead to scattering of laser light passing through the optical fibre, particularly by Rayleigh scattering. The amplitude of the scattering follows a Rayleigh distribution, but the phase angle of the scattering is uniformly distributed around a unit circle, i.e. −π≤Φ≤π where Φ is the phase angle. Rayleigh scattering is an elastic scattering mechanism, such that the frequency of the scattered light is the same as the pulse of laser light used to probe the optical fibre. However, since the probe pulse is highly coherent, the scattered light interferes with itself as it scatters along the optical fibre. The bandwidth required to detect and fully utilise the Rayleigh scatter for DAS sensing is inversely proportional to the pulse duration. For typical applications, this bandwidth is of the order of 30 MHz to 200 MHZ. Distributed Strain and Temperature Sensing Distributed Strain and Temperature Sensing (DSTS) is a technique that enables simultaneous measurement of temperature and strain in an optical fibre, using a laser pulse which is transmitted along the optical fibre. The scattering mechanism of interest for DSTS is Brillouin scattering. Brillouin scattering is an inelastic scattering process which is due to photons interacting with and scattering from acoustic band phonons created by the lattice vibrations of the fibre material. The photons may be scattered to a lower energy state (e.g. with the emission of an optical phonon) which is termed Stokes scattering, or the photons may scatter to a higher energy state (e.g. by absorbing energy from a phonon) which is termed anti-Stokes scattering. The shift in signal frequency caused by Brillouin frequency is typically at around 10 to 11 GHz. The amplitudes of the Stokes and Anti-Stokes components for Brillouin scattering are approximately equal, at least in the spontaneous domain. At higher launch powers, Brillouin scattering can be stimulated whereby the Stokes component is amplified relative to the Anti-Stokes. Since this scattering is related to lattice vibrations, both the Stokes shift and the spontaneous amplitude of the Stokes and Anti-Stokes emission are related to the strain experienced by the fibre and the absolute temperature of the fibre. Signals related to temperature and strain of the optical fibre can be separated by measuring the frequency and amplitude of the Anti-Stokes emission. SUMMARY OF THE INVENTION At its most general, the present invention provides an optical measurement system which can be used to perform various different types of optical measurements such as DAS and/or DSTS on an optical path (e.g. an optical fibre). In other words, a single optical setup can be used to perform different types of optical measurements. This is achieved by providing an optical measurement system with a light source, a launch stage configured to receive a light signal from the light source and generate a test signal, and a local oscillator stage for generating a local oscillator signal, each of the launch stage and the local oscillator stage comprising a respective IQ modulator. The IQ modulators in the launch stage and the local oscillator stage provide a high level of control over the test signal and l