A demodulation system and modulation method for high-speed fiber grating monitoring

Abstract

The present invention discloses a demodulation system and modulation method for high-speed fiber Bragg grating (FBG) monitoring. The demodulation system comprises two optical waveguides, one of which is equipped with a semiconductor optical amplifier (A) and a thermo-optical phase shifter, and the other with a semiconductor optical amplifier (B) and a double-helix waveguide. The ends of the two waveguides closest to the semiconductor optical amplifier are connected to a 1×2 optical coupler, which combines the optical beams and then couples them to a fiber Bragg grating (FBG) sensor via an external transmission unit. The outputs of the two waveguides are connected to an electro-optical Bragg deflection modulator (EOBDM), which is then connected to a photodiode. The electrical signal output by the photodiode is sequentially connected to a transimpedance amplifier, a data processor, and a network communication module. A signal reading unit is connected to the network communication module to read the output signal. This modulation system is stable and reliable, compact, low-power, and self-contained.

Description

Technical Field

[0001] The present invention relates to high-speed fiber Bragg grating signal demodulation, and in particular to a high-speed fiber Bragg grating monitoring demodulation system and modulation method based on an electro-optical Bragg deflection modulator, belonging to the field of monolithic optoelectronic integrated sensing technology. Background Art

[0002] Advanced infrastructure systems are integral to a nation's social, political, and economic well-being. Various aspects of infrastructure systems impact the quality of buildings and structures, the air we breathe and the water we drink, our access to energy (such as electricity, oil, and natural gas), communications, multimodal transportation systems, and waste disposal. Because these systems are so pervasive, complex, and deeply intertwined with our lives, they require better protection and intelligent operation. In recent years, fiber Bragg grating (FBG) sensor technology has been recognized as a new, minimally invasive sensing element for structural health monitoring (SHM) in aviation, aerospace systems, naval and maritime, civil construction, and the petrochemical industry. FBG sensors have been widely adopted in real-time monitoring of infrastructure systems due to their low production costs, the availability of high-quality fiber Bragg grating demodulation systems, and practical sensor embedding and packaging technologies.

[0003] Fiber Bragg Grating (FBG) sensors can be easily cast, embedded, or surface-mounted on structures due to their lightweight, micron-sized sensors and resistance to electromagnetic interference. In addition, it is possible to distribute multiple sensors on a single fiber bundle. Fiber Bragg Grating-based acoustic emission technology is a new and advanced structural health acoustic emission sensing technology, in which the sensor elements do not require pre-treatment equipment and are non-interferingly mounted to the structure using EMI-insensitive wiring. Various countries have identified fiber Bragg Grating sensors as a potential technology to address some of the shortcomings and limitations of traditional PZT acoustic emission sensors. However, the cost, size, weight, and complexity of commercial FBG sensor interrogation systems remain a major burden for the implementation of commercial and military FBG condition monitoring systems worldwide.

[0004] Currently, the most mature fiber Bragg grating (FBG) demodulation method is optoelectronically integrated arrayed waveguide grating (AWG) demodulation technology, which features small size, high precision, and fast demodulation speed. This technology is primarily achieved through the integrated design and packaging of the AWG demodulation system's light source, optical waveguide coupler, AWG, and photodetector (or photodiode). It offers advantages such as compactness, low cost, and excellent stability and reliability. However, this technology relies on a large-capacity photodetector array and a complex data acquisition system. Separately, Professor Shridar Krishnaswamy's team at Northwestern University has developed an adaptive FBG sensor demodulation technology based on two-wave mixing interferometry, which can provide enhanced acoustic emission sensitivity for reliable crack detection. They achieved miniaturization by monolithically integrating all passive and active optoelectronic components of the sensor system onto an indium phosphide (InP) optical chip. However, the complexity of the technology has limited its commercial application.

[0005] In optoelectronic chip manufacturing, InP-based quaternary alloys have played a driving role in the 1.1-1.6μm spectral window, which is critical for fiber optic systems. InGaAsP quaternary alloys offer considerable flexibility in the range of engineered band gaps and refractive indexes that can be achieved on high-quality, low-defect-density InP substrates. The ability to include a variety of optical waveguide devices with different direct band gaps on the same substrate has led to a class of powerful InP integrated photonic circuits with passive components (such as optical splitters, filters, multiplexers, and combiners) and active components (such as optical amplifiers, lasers, modulators, and photodetectors). Integrated circuit technology improves circuit-level performance by eliminating assembly complexity and variability. As the technology matures, more and more component-level interconnects are performed at the wafer scale, which allows for continued improvements in circuit functionality, performance, and reliability while reducing circuit size, power consumption, and cost. Summary of the Invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention aims to provide a demodulation system and modulation method for high-speed fiber Bragg grating monitoring based on an electro-optical Bragg deflection modulator. The modulation system is stable and reliable, has a small size, low power consumption, and is independently packaged.

[0007] The technical solution of the present invention is achieved as follows:

[0008] A high-speed fiber Bragg grating (FBG) monitoring demodulation system comprises two optical waveguides, one of which is provided with a semiconductor optical amplifier A and a thermo-optical phase shifter, and the other with a semiconductor optical amplifier B and a double-helix waveguide. One end of the two optical waveguides, close to the semiconductor optical amplifier, is connected to a 1×2 optical coupler. The 1×2 optical coupler combines the optical beams and then couples them to an external transmission unit via a first waveguide. The external transmission unit is coupled to a fiber Bragg grating (FBG) sensor. The other ends of the two optical waveguides output light to an electro-optical Bragg deflection modulator (EOBDM). The EOBDM output is connected to a photodiode to convert the optical signal output by the EOBDM into an electrical signal. The electrical signal output by the photodiode is sequentially connected to a transimpedance amplifier, a data processor, and a network communication module. A signal reading unit is connected to the network communication module to read the signal output by the network communication module.

[0009] Furthermore, interlaced comb electrodes are provided on the electro-optical Bragg deflection modulator, and a voltage V1 is applied to the comb electrodes to disturb the refractive index of the optical waveguide below the electrodes, thereby forming a Bragg grating in the optical waveguide.

[0010] Furthermore, the transimpedance amplifier, data processor and network communication module are integrated on the same PCB printed circuit board module.

[0011] Furthermore, two optical waveguides, a 1×2 optical coupler, two semiconductor optical amplifiers, a thermo-optical phase shifter, a double-helix waveguide, an electro-optical Bragg deflection modulator, and a photodiode together constitute a fiber channel and are integrated on an indium phosphide-based substrate. Each fiber channel is used to demodulate a fiber grating sensor on one optical fiber. The fiber channel and the indium phosphide-based substrate together constitute an optoelectronic chip.

[0012] Furthermore, the present invention integrates at least two optical fiber channels on the indium phosphide-based substrate of the optoelectronic chip, and the output of the photodiode of each optical fiber channel is connected to a transimpedance amplifier on the same PCB printed circuit board module; all photodiodes are arranged in an array to form a photodiode array.

[0013] A high-speed fiber Bragg grating demodulation method first obtains the aforementioned high-speed fiber Bragg grating monitoring demodulation system and then performs modulation; the specific modulation process is as follows:

[0014] The broadband light emitted from the two semiconductor optical amplifiers is coupled to the fiber Bragg grating sensor via a 1×2 optical coupler. The narrowband light reflected by the fiber Bragg grating sensor is then split into two by the 1×2 optical coupler and amplified in one pass by two semiconductor optical amplifiers. The signal beam and pump beam are formed in the two optical waveguides respectively. The thermo-optical phase shifter on the optical waveguide corresponding to the pump beam is used to fine-tune the phase difference of the Mach-Zehnder structure. The double helix waveguide set on the optical waveguide corresponding to the signal beam is used to introduce a fixed phase difference. The signal beam and the pump beam are at a Bragg angle of ±θ.B Entering the electro-optical Bragg deflection modulator, the voltage V1 applied to the comb electrode disturbs the refractive index of the optical waveguide below the electrode, thus forming a Bragg grating in the optical waveguide. This grating changes the transmission direction of the pump beam, and the light travels along the Bragg angle 2θ that is twice that of the pump beam. B The diffracted light of the pump beam can just enter the optical waveguide corresponding to the signal beam, and overlap with the transmitted signal beam to interfere with each other; the photodiode detects the interfering light and converts it into an electrical signal. The data processor measures the change of the electrical signal and transmits it through the network communication module, thereby determining the wavelength shift of the fiber Bragg grating reflected light and realizing the demodulation of the fiber Bragg grating sensor.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. The present invention uses an electro-optical Bragg deflection modulator to perform adaptive spectral demodulation on the dynamic signal of the FBG sensor, avoiding the use of a scanning light source that cannot be demodulated at high speed or a dual-wave mixing interferometer that has high requirements for the photorefractive effect, which is conducive to reducing costs and improving stability.

[0017] 2. This invention monolithically integrates all passive and active optoelectronic components of the sensor system into a micro-semiconductor InP integrated optical chip, enabling miniaturized packaging. The integrated demodulation system features a compact structure, integrated packaging, high integration, and high system stability and reliability.

[0018] 3. The present invention manufactures multiple photon integrated unbalanced MZ structures on the same InP-based substrate and matches the printed circuit board module with corresponding read signals to achieve multi-channel synchronous measurement with little increase in volume.

[0019] This invention is a multi-channel, high-speed fiber Bragg grating (FBG) demodulation system based on an electro-optical Bragg deflection modulator and an indium phosphide (InP) optoelectronic chip. It achieves compact size, low power consumption, and self-contained packaging. It can effectively demodulate high-frequency dynamic signals from fiber Bragg grating (FBG) sensors sensitive to strain, vibration, and acoustic emission. This invention will help promote the development of structural health monitoring markets such as fiber-optic ultrasonic detection and fiber-optic acoustic emission detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the demodulation system for high-speed fiber Bragg grating monitoring of the present invention.

[0021] Figure 2 It is a structural schematic diagram of the electro-optical Bragg deflection modulator of the present invention. DETAILED DESCRIPTION

[0022] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] See also Figure 1 The present invention discloses a high-speed fiber Bragg grating (FBG) monitoring demodulation system, comprising two optical waveguides 9. One of the optical waveguides is provided with a semiconductor optical amplifier (SOA) A2 and a thermo-optical phase shifter (TOM) 4, and the other optical waveguide 9 is provided with a semiconductor optical amplifier B3 and a double-helix waveguide 5. The two optical waveguides are connected to a 1×2 optical coupler 1 at one end near the semiconductor optical amplifier. The 1×2 optical coupler 1 combines the optical beams and then couples them to an external transmission unit via a first waveguide. The external transmission unit is coupled to a fiber Bragg grating (FBG) sensor. The other ends of the two waveguides output light to an electro-optical Bragg deflection modulator 6. The output of the electro-optical Bragg deflection modulator 6 is connected to a photodiode 11 to convert the optical signal output by the electro-optical Bragg deflection modulator into an electrical signal. The electrical signal output by the photodiode 11 is sequentially connected to a transimpedance amplifier (TIA), a data processor, and a network communication module. A signal reading unit is connected to the network communication module to read the signal output by the network communication module.

[0024] Interlaced comb electrodes 7 are provided on the electro-optical Bragg deflection modulator 6. A voltage V1 is applied to the comb electrodes 7 to disturb the refractive index of the optical waveguide below the electrodes, thereby forming a Bragg grating in the optical waveguide.

[0025] The system includes an InP optoelectronic chip 10, a PCB printed circuit board module 8, and a fiber Bragg grating (FBG) sensor. The optoelectronic chip 10 is manufactured using InP-based general integration technology, and integrates optical waveguides 9, 1×2 MMI optical couplers 1, two semiconductor optical amplifiers (SOA) 2, 3, thermo-optical phase shifters (TOMs) 4, electro-optical Bragg deflection modulators 6, and photonic devices such as a PIN photodiode array composed of multiple photodiodes 11 on an iron-doped indium phosphide substrate. The transimpedance amplifier (TIA), data processor, and network communication module are integrated on the printed circuit board module 8 for signal readout and display. The optoelectronic chip 10 is electrically connected to the printed circuit board module 8. The 1×2 MMI optical coupler combines the beams and then couples them to an external transmission unit, such as an optical adapter or a fiber Bragg grating sensor, through a first waveguide. The fiber Bragg grating sensor is connected via an optical adapter or directly coupled to the fiber Bragg grating sensor. The system is suitable for demodulating signals and collecting sensing data of various types of fiber Bragg grating sensors, such as fiber Bragg grating dynamic strain, dynamic pressure, vibration acceleration, ultrasound and acoustic emission, and can realize multi-channel synchronous measurement.

[0026] This invention is a high-speed fiber Bragg grating (FBG) demodulation system based on an electro-optical Bragg deflection modulator and an indium phosphide (InP) optoelectronic chip. The technical solution includes: 1) a fiber Bragg grating (FBG) sensor sensitive to strain, vibration, and acoustic emission; 2) adaptive spectral demodulation of the FBG sensor's dynamic signals using the electro-optical Bragg deflection modulator; and 3) monolithic integration of all the sensor system's passive and active optoelectronic components within a miniature semiconductor InP integrated optical chip. This invention will help advance the field of fiber Bragg grating sensing and demodulation, and further expand the application of fiber Bragg grating sensing technology in naval, maritime, and aerospace applications.

[0027] The working principle and related technologies of the optoelectronic chip modulation system of the present invention are introduced below.

[0028] As a schematic representation, Figure 1 Only a high-speed fiber Bragg grating demodulation system consisting of two fiber channels is shown. The structure of three or more fiber channels is similar. This is an unbalanced Mach-Zehnder (MZ) structure of InP-based photon integration. For the unbalanced MZ structure of a single photon integration, two semiconductor optical amplifiers are respectively deployed on the two arms of the unbalanced MZ structure. The broadband light emitted from the two semiconductor optical amplifiers is coupled to the FBG sensor (with a central wavelength of λ) through a 1×2 MMI optical coupler. B The narrowband light reflected by the FBG is then split into two by a 1×2 MMI optical coupler and amplified in one pass by two semiconductor optical amplifiers, forming a signal beam and a pump beam in the optical waveguide. A thermo-optical phase shifter is set on the pump waveguide to fine-tune the phase difference of the MZ structure by applying a voltage V0. A double-helix waveguide is set on the signal waveguide to introduce a fixed phase difference. The signal beam and the pump beam (the amplitudes of the two beams are E s0 and E p0 ) in Bragg angle ±θ B Enter the X-shaped cross-channel waveguide modulator, which is a Bragg diffraction electro-optical modulator, combined with interlaced comb electrodes. The voltage V1 applied to the comb electrodes perturbs the refractive index beneath them, forming an effective Bragg grating (with a grating pitch of Λ) within the optical waveguide. This grating changes the propagation direction of the pump beam, directing the light along a path that is twice the Bragg angle (2θ) of the pump beam. B) diffracted in the direction of the pump beam. The diffracted light from the pump beam just happens to enter the signal optical waveguide, where it overlaps with the transmitted signal beam, causing interference. An integrated PIN photodiode array then detects the interfering light. The printed circuit board module measures the change in the diode's photocurrent and uses it to determine the wavelength shift of the light reflected from the fiber Bragg grating. Fabricating multiple photon-integrated unbalanced MZ structures on the same InP substrate and matching the corresponding readout signals with the printed circuit board module enables simultaneous multi-channel measurement.

[0029] Bragg angle θ entering the X-cross channel waveguide modulator B Determined by the following formula:

[0030]

[0031] Where Λ is the grating spacing in the modulator, λ B is the FBG center wavelength, n eff =β / k is the effective refractive index of the waveguide (β and k represent the waveguide propagation constant and vacuum wave number, respectively). The above equation is valid only if the following conditions are met: 2πλ B l>>Λ 2 (l is the grating thickness). B A small change in Δθ will cause the angle between the pump beam and the grating to be different from the Bragg angle, but B =2Λ / l, diffraction can still occur, but the efficiency is reduced (no more than 50%). The diffracted light intensity of the pump beam is related to the applied electric field strength E1, which is expressed as:

[0032]

[0033] Where, E1 is the applied electric field strength (adjustable by V1), I dp The diffracted pump light intensity, I p0 is the transmitted light intensity when no electric field is applied, n eff is the effective refractive index of the waveguide, r eff is the electro-optic coefficient.

[0034] The spectrum of the signal beam and the pump beam is shifted by Δλ B (t) is effectively converted to a relative phase shift between the beams (due to propagation through the unbalanced optical path) and is given by

[0035]

[0036] Where ΔL is the physical length difference of the unbalanced optical waveguide. It is the complex amplitude E of the diffracted pump light. dp and the transmitted signal beam E s The interference between them demodulates the spectral shift Δλ B(t). In the limit of small dynamic phase shifts (this limit is usually valid for dynamic strains caused by acoustic emission or shock), the interference signal at the photodiode can be written as

[0037]

[0038] It is clear from formula (4) that the maximum signal occurs when kl approaches π / 4, that is, at the quadrature point. This condition can be easily achieved by applying voltage V1 to the comb electrodes and appropriately adjusting the parameters of the Bragg diffraction electro-optic modulator.

[0039] In photonics, building-block integration processes that control the fundamental properties of light (amplitude, phase, and polarization) can support a wide range of functions. Well-designed waveguide structures enable interconnections and the fabrication of passive components such as couplers, filters, and demultiplexers. By controlling the amplitude, phase, and polarization of light using optical amplifiers (SOAs), phase modulators, and polarization converters, this technology can support a wide range of functions. The typical InP optoelectronic chip fabrication process includes epitaxial growth, waveguide etching, surface planarization, and metal interconnection.

[0040] InP optoelectronic chips are manufactured using universal integration technology, using shallowly etched waveguides (for low-loss interconnects and high-efficiency amplifiers) and deeply etched waveguides (for smaller bending radii and high-efficiency phase shifter parts) to build basic active and passive components. The 500nm thick waveguide layer consists of a quaternary layer of InGaAsP with a bandgap wavelength of 1.25μm (Q1.25). This layer is transparent to photons with wavelengths greater than 1.25μm, so these waveguides are called passive waveguides. The active waveguide has a bulk Q1.55, multiple quantum well (MQW) or quantum dot (QD) layer within the waveguide layer. The active layer will absorb or amplify photons with a wavelength of approximately 1.55 microns (depending on the applied current). The highly doped InGaAs contact layer on top of the waveguide allows efficient injection of current into the active element (SOA) and is also used to operate the reverse biased phase shift element. The basic units that constitute the InP optoelectronic chip of the present invention include passive devices and active devices, mainly including:

[0041] 1) Waveguides. The bending radius of shallow-etched waveguides is 500 μm, while that of deep-etched waveguides can be reduced to approximately 10 μm. These include passive components such as curved waveguides, MMI couplers, and double-helix waveguides.

[0042] 2) Optical amplifiers and photodetectors

[0043] The structures of optical amplifiers and photodetectors are very similar, differing in the depth of the waveguide etching and the direction of the voltage applied to the PN junction. Photodetectors must operate in a reverse biased state.

[0044] 3) Thermo-optic phase shifter (TOM). A TOM is a passive transparent waveguide section whose phase shift can be changed by heating it using a heater electrode on top of the waveguide, with modulation speeds in the millisecond range.

[0045] 4) Bragg diffraction electro-optic modulator: This is an X-channel waveguide modulator combined with interlaced comb electrodes. The voltage applied to the comb electrodes causes the refractive index below the electrodes to be periodically disturbed, thus forming an effective grating in the waveguide, which is a Bragg grating. This grating changes the propagation direction of the incident light beam (see Figure 2 ).

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention without departing from the purpose and scope of the technical solutions should be included in the scope of the claims of the present invention.

Claims

1. A high-speed fiber Bragg grating (FBG) monitoring demodulation system, characterized by: The system comprises two optical waveguides, one of which is provided with a semiconductor optical amplifier A and a thermo-optical phase shifter, and the other is provided with a semiconductor optical amplifier B and a double-helix waveguide. The ends of the two optical waveguides close to the semiconductor optical amplifier are connected to a 1×2 optical coupler, which is combined by the 1×2 optical coupler and then coupled to an external transmission unit via the first waveguide. The external transmission unit is coupled to a fiber grating sensor. The other ends of the two optical waveguides are output to an electro-optical Bragg deflection modulator, the output of which is connected to a photodiode to convert the optical signal output by the electro-optical Bragg deflection modulator into an electrical signal. The electrical signal output by the photodiode is sequentially connected to a transimpedance amplifier, a data processor and a network communication module, and a signal reading unit is connected to the network communication module to read the signal output by the network communication module.

2. A high-speed fiber Bragg grating monitoring demodulation system according to claim 1, characterized in that: The electro-optical Bragg deflection modulator is provided with interlaced comb electrodes, and a voltage is applied to the comb electrodes. V 1 causes the refractive index of the optical waveguide below the electrode to be disturbed, thereby forming a Bragg grating in the optical waveguide.

3. The high-speed fiber Bragg grating monitoring demodulation system according to claim 1, characterized in that: The transimpedance amplifier, data processor and network communication module are integrated on the same PCB printed board module.

4. A high-speed fiber Bragg grating monitoring demodulation system according to claim 3, characterized in that: Two optical waveguides, a 1×2 optical coupler, two semiconductor optical amplifiers, a thermo-optical phase shifter, a double-helix waveguide, an electro-optical Bragg deflection modulator, and a photodiode together constitute a fiber channel and are integrated on an indium phosphide-based substrate. Each fiber channel is used to demodulate a fiber Bragg grating sensor on one optical fiber. The fiber channel and the indium phosphide-based substrate together constitute an optoelectronic chip.

5. The high-speed fiber Bragg grating monitoring demodulation system according to claim 4, characterized in that: At least two optical fiber channels are integrated on the indium phosphide-based substrate of the optoelectronic chip. The output of the photodiode of each optical fiber channel is connected to a transimpedance amplifier on the same PCB printed board module; all photodiodes are arranged in an array to form a photodiode array.

6. The high-speed fiber Bragg grating monitoring demodulation system according to claim 1, characterized in that: The external transmission unit is coupled to the fiber grating sensor via an optical adapter.

7. A high-speed fiber Bragg grating demodulation method, characterized in that: First, obtain a high-speed fiber Bragg grating monitoring demodulation system as described in claim 2, and then perform mediation; the specific mediation process is as follows: The broadband light emitted from the two semiconductor optical amplifiers is coupled to the fiber Bragg grating sensor via a 1×2 optical coupler. The narrowband light reflected by the fiber Bragg grating sensor is then split into two by the 1×2 optical coupler and amplified in one pass by two semiconductor optical amplifiers. The signal beam and pump beam are formed in the two optical waveguides respectively. The thermo-optical phase shifter on the optical waveguide corresponding to the pump beam is used to fine-tune the phase difference of the Mach-Zehnder structure. The double helix waveguide set on the optical waveguide corresponding to the signal beam is used to introduce a fixed phase difference. The signal beam and the pump beam are at a Bragg angle of ± θ B Entering the electro-optical Bragg deflection modulator, the voltage applied to the comb electrodes V 1 makes the refractive index of the optical waveguide under the electrode disturbed, thus forming a Bragg grating in the optical waveguide. This grating changes the transmission direction of the pump beam, and the light propagates along the Bragg angle 2 times that of the pump beam. θ B The diffracted light of the pump beam can just enter the optical waveguide corresponding to the signal beam, and overlap with the transmitted signal beam to interfere with each other; the photodiode detects the interfering light and converts it into an electrical signal. The data processor measures the change of the electrical signal and transmits it through the network communication module, thereby determining the wavelength shift of the fiber Bragg grating reflected light and realizing the demodulation of the fiber Bragg grating sensor.

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