CN115774365A - Demodulation system and demodulation method for monitoring high-speed fiber bragg grating - Google Patents

Abstract

The invention discloses a demodulation system and a demodulation method for monitoring a high-speed fiber grating, wherein the demodulation system comprises two optical waveguides, one of the optical waveguides is provided with a semiconductor optical amplifier A and a thermo-optical phase shifter, the other optical waveguide is provided with a semiconductor optical amplifier B and a double-spiral waveguide, one end of the two optical waveguides, which is close to the semiconductor optical amplifier, is connected with a 1 x 2 optical coupler, and the combined beam of the 1 x 2 optical coupler is coupled and connected with a fiber grating sensor through an external transmission unit; the other ends of the two optical waveguides are output to an electro-optic Bragg deflection modulator, and the output of the electro-optic Bragg deflection modulator is connected with a photodiode; the electric signal output by the photodiode is sequentially connected with the transimpedance amplifier, the data processor and the network communication module, and the signal reading unit is connected with the network communication module to read the signal output by the network communication module. The adjusting system is stable and reliable, small in size, low in power consumption and independent in packaging.

Description

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

technical field

The invention relates to high-speed optical fiber grating signal demodulation, in particular to a demodulation system and method for high-speed optical fiber grating monitoring based on an electro-optical Bragg deflection modulator, and belongs to the technical field of single-chip photoelectric integrated sensing.

Background technique

Advanced infrastructure systems are integral to the social, political and economic well-being of a nation. All aspects of infrastructure systems affect the quality of buildings and structures, the air we breathe and the water we drink, our access to energy (such as electricity, oil and gas), communications, intermodal transportation systems, and waste disposal. Because these systems are so pervasive, complex, and pervasive to our lives, they need to be better protected and function smarter. In recent years, fiber Bragg grating (FBG) sensor technology has been recognized as a new type of minimally invasive sensing element for structural health monitoring (SHM), which can be used in aviation, aerospace systems, naval and maritime, civil buildings , Petrochemical industry. The low production cost, availability of high-quality FBG demodulation systems, and practical sensor embedding and packaging techniques make FBG sensors widely popularized in real-time monitoring of infrastructure systems.

Fiber Bragg Grating (FBG) sensors can be easily cast, embedded, or surface-mounted on structures due to their lightweight, micron-sized sensors and immunity to electromagnetic interference. In addition, it is possible to distribute multiple sensors on a single fiber optic bundle. Fiber Bragg grating-based acoustic emission technology is a new and advanced acoustic emission sensing technology for structural health, in which the sensor element is mounted to the structure without interference using EMI-insensitive wiring without the need for pre-processing equipment superior. 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.

At present, the existing relatively mature fiber Bragg grating demodulation method is optoelectronic integrated arrayed waveguide grating demodulation technology, which has the characteristics of small size, high precision and fast demodulation speed. Waveguide couplers, arrayed waveguide gratings, and photodetectors (or photodiodes) are designed and packaged in an integrated manner, which has the advantages of compact structure, low cost, good stability and reliability, etc. However, this technology relies on large-capacity light Detector arrays and complex data acquisition systems. In addition, the team of Professor Shridar Krishnaswamy of Northwestern University developed an adaptive FBG sensor demodulation technology based on two-wave mixing interferometry, which can provide enhanced acoustic emission sensitivity for reliable crack detection. Monolithic integration with active optoelectronic components in indium phosphide (InP) optical chips has enabled miniaturization, but the complexity of the technology has limited its commercial application.

In optoelectronic chip fabrication, InP-based quaternary alloys play a role in the critical 1.1-1.6 μm spectral window for fiber optic systems. InGaAsP quaternary alloys offer considerable flexibility in engineered bandgap and refractive index ranges realized on high-quality, low-defect-density InP substrates. The ability to include various optical waveguide devices with different direct bandgaps on the same substrate has led to a powerful class of InP integrated photonic circuits with passive components such as optical beam splitters, filters, multiplexers, and beam combiners) and active components such as optical amplifiers, lasers, modulators, and photodetectors. Integrated loop technology improves loop-level performance by eliminating assembly complexity and variability. As the technology matures, more and more component-level interconnections are made at the wafer scale, which enables continued improvements in loop functionality, performance, and reliability while reducing loop size, power consumption, and cost.

Contents of the invention

In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to propose a demodulation system and a mediation method for high-speed fiber grating monitoring based on electro-optic deflection modulators. The mediation system is stable and reliable, small in size, low in power consumption, and packaged independent.

Technical scheme of the present invention is realized like this:

A demodulation system for high-speed fiber grating monitoring, including two optical waveguides, one of which is provided with a semiconductor optical amplifier A and a thermo-optic phase shifter, and the other optical waveguide is provided with a semiconductor optical amplifier B and a dual Spiral waveguide, two optical waveguides close to the end of the semiconductor optical amplifier are connected with 1×2 optical coupler, the 1×2 optical coupler is combined and then coupled to the external transmission unit through the first waveguide, and the external transmission unit is coupled with the fiber grating sensor ; The other ends of the two optical waveguides are output to the electro-optical Bragg deflection modulator, and the output of the electro-optic Bragg deflection modulator is connected to a photodiode to convert the optical signal output by the electro-optic Bragg deflection modulator into an electrical signal; the electrical signal output by the photodiode is connected to the transimpedance in turn The amplifier, the data processor and the network communication module, the signal reading unit is connected with the network communication module to read the signal output by the network communication module.

Furthermore, interleaved comb electrodes are arranged 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.

Further, the transimpedance amplifier, data processor and network communication module are integrated on the same PCB printed board module.

Furthermore, two optical waveguides, 1×2 optical couplers, two semiconductor optical amplifiers, thermo-optic phase shifters, double helical waveguides, electro-optic Bragg deflection modulators, and photodiodes together constitute a fiber channel and are integrated in the phosphating On the indium-based substrate, each fiber channel is used to demodulate a fiber grating sensor on one fiber, and the fiber channel and the indium-phosphide-based substrate together constitute an optoelectronic chip.

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 the transimpedance amplifier on the same PCB printed board module; all optoelectronic The diodes are arranged in an array to form a photodiode array.

A high-speed fiber grating demodulation method, which first obtains the aforementioned demodulation system for high-speed fiber grating monitoring, and then performs mediation; the specific mediation process is as follows,

The wide-spectrum light emitted from the two semiconductor optical amplifiers is coupled to the fiber grating sensor through the 1×2 optical coupler, and the narrow-band light reflected by the fiber grating sensor is divided into two by the 1×2 optical coupler and passed through the two semiconductor optical amplifier One-way amplification, the signal beam and the pump beam are respectively formed in the two optical waveguides, the thermo-optic phase shifter on the optical waveguide corresponding to the pump beam is used to fine-tune the phase difference of the Mach-Zehnder structure, and the optical waveguide corresponding to the signal beam is set The double helical waveguide is used to introduce a fixed phase difference; the signal beam and the pump beam enter the electro-optic Bragg deflection modulator at the Bragg angle ± θ _B , and the voltage V ₁ applied to the comb electrode makes the optical waveguide below the electrode refracted The rate is disturbed, so a Bragg grating is formed in the optical waveguide. This grating changes the transmission direction of the pump beam, and the light is diffracted along the direction that is twice the Bragg angle 2θ _B with the pump beam. It can 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, and the data processor measures the change of the electrical signal and transmits it through the network communication module to determine The wavelength shift of the light reflected by the FBG realizes the demodulation of the FBG sensor.

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

1. The present invention utilizes the 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 requires high photorefractive effects, which is beneficial to Reduce costs and increase stability.

2. The present invention monolithically integrates the passive and active photoelectric elements of all sensor systems in the micro-semiconductor InP integrated optical chip, which can realize miniaturized packaging. The integrated demodulation system has the characteristics of compact structure, integrated packaging, high integration, high system stability and reliability.

3. The present invention manufactures a plurality of photon-integrated unbalanced M-Z structures on the same InP base substrate and matches the printed board module of the corresponding reading signal, which can realize multi-channel synchronous measurement, and the volume basically does not increase.

The invention is a multi-channel high-speed fiber grating demodulation system based on an electro-optical Bragg deflection modulator and an indium phosphide (InP) photoelectric chip, which realizes small size, low power consumption operation and independent packaging, and can effectively demodulate the strain, High-frequency dynamic signals from fiber Bragg grating (FBG) sensors sensitive to vibration and acoustic emission. The invention helps to promote the development of optical fiber ultrasonic detection, optical fiber acoustic emission detection and other structural health monitoring markets.

Description of drawings

Fig. 1 is a schematic diagram of a demodulation system for high-speed fiber grating monitoring of the present invention.

Fig. 2 is a schematic diagram of the structure of the electro-optical Bragg deflection modulator of the present invention.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

Referring to Fig. 1, a kind of demodulation system for high-speed fiber grating monitoring of the present invention comprises two optical waveguides 9, on which optical waveguide is provided with semiconductor optical amplifier (SOA) A2 and thermo-optic phase shifter (TOM) 4, The other optical waveguide 9 is provided with a semiconductor optical amplifier B3 and a double helical waveguide 5. The two optical waveguides are connected to the 1×2 optical coupler 1 near the end of the semiconductor optical amplifier. One waveguide is coupled to the external transmission unit, and the external transmission unit is coupled to the fiber grating sensor; the other ends of the two waveguides are output to the electro-optical Bragg deflection modulator 6, and the output of the electro-optic Bragg deflection modulator 6 is connected to the photodiode 11 to convert the electro-optic Bragg deflection modulator The output optical signal is converted into an electrical signal; the electrical signal output by the photodiode 11 is connected to a transimpedance amplifier (TIA), a data processor, and a network communication module in turn, and the signal reading unit is connected with the network communication module to read the output of the network communication module. Signal.

Interleaved comb electrodes 7 are arranged on the electro-optical Bragg deflection modulator 6, and 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.

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

The present invention is a high-speed fiber grating demodulation system based on an electro-optical Bragg deflection modulator and an indium phosphide (InP) photoelectric chip. The technical solution includes: 1) a fiber Bragg grating (FBG) sensor sensitive to strain, vibration and acoustic emission ; 2) Adaptive spectrum demodulation technology for FBG sensor dynamic signal by using electro-optical Bragg deflection modulator; and 3) Monolithic integration of passive and active optoelectronic components of all sensor systems in a micro-semiconductor InP integrated optical chip. The invention helps to promote the development of the field of optical fiber Bragg grating sensing and demodulation, and further expands the application of the optical fiber Bragg grating sensing technology in navy, maritime affairs and aerospace.

The working principle and related technologies of the optoelectronic chip mediation system of the present invention are introduced below.

As a schematic representation, FIG. 1 only shows a high-speed fiber grating demodulation system composed of two fiber channels, and three or more fiber channels have a similar structure. This is a non-equilibrium Mach-Zehnder (MZ) structure for InP-based photonic integration. For the unbalanced MZ structure with single photon integration, two semiconductor optical amplifiers are deployed on the two arms of the unbalanced MZ structure respectively, and the broadband light emitted from the two semiconductor optical amplifiers is coupled to the FBG through a 1×2 MMI optical coupler Sensor (central wavelength λ _B ). The narrow-band light reflected by the FBG is split into two by a 1×2 MMI optical coupler, amplified by two semiconductor optical amplifiers in one pass, and forms a signal beam and a pump beam in the optical waveguide respectively. A thermo-optic phase shifter is set on the pump optical waveguide, which is used to fine-tune the phase difference of the MZ structure by applying a voltage V ₀ , and a double helical waveguide is set on the signal optical waveguide to introduce a fixed phase difference. The signal beam and the pump beam (the amplitudes of which are E _s0 and E _p0 respectively) enter the X cross-channel waveguide modulator at the Bragg angle ±θ _B. This X cross-channel waveguide modulator is a Bragg diffraction electro-optic modulator , combined with interlaced comb electrodes. A voltage _V1 applied to the comb electrode perturbs the refractive index beneath the electrode, thereby forming an effective grating in the optical waveguide, which is a Bragg grating (grating pitch Λ). This grating changes the transmission direction of the pump beam, and the light is diffracted along the direction that is twice the Bragg angle (2θ _B ) with the pump beam. overlap and interfere with each other. An integrated PIN photodiode array then detects the light that interferes with the two. The printed board module measures the change in photocurrent of the diode and is used to determine the wavelength shift of the light reflected from the fiber grating. A printed board module that manufactures multiple photon-integrated unbalanced MZ structures on the same InP-based substrate and matches the corresponding read signals can realize multi-channel simultaneous measurement.

The Bragg angle θ _B entering the X-cross channel waveguide modulator is determined by:

In the formula, Λ is the grating spacing in the modulator, λ _B is the central wavelength of the FBG, n _eff = β/k is the effective refractive index of the waveguide (β, k represent the waveguide propagation constant and the vacuum wavenumber respectively), and the above formula needs to satisfy the following conditions: 2πλ _B l>>Λ ² (l is the grating thickness). Small changes in λ _B will cause the angle between the pump beam and the grating to be different from the Bragg angle, but diffraction can still occur within the range of Δθ _B = 2Λ/l, but the efficiency is reduced (not more than 50%). The diffracted light intensity of the pump beam is related to the applied electric field intensity _E1 , expressed as:

E₁是施加电场强度(可通过V₁调节)，I_dp是的衍射泵浦光强，I_p0是没有施加电场时的透射光强，n_eff是波导有效折射率，r_eff是电光系数。In the formula,

E ₁ is the applied electric field intensity (can be adjusted by V ₁ ), I _dp is 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, and r _eff is the electro-optical coefficient.

The spectral shift Δλ _B (t) of the signal and pump beams effectively translates into a relative phase shift between the beams (due to propagation through the unbalanced optical path) and is given by

In the formula, ΔL is the physical length difference of the unbalanced optical path optical waveguide. It is the interference between the diffracted pump light complex amplitude E _dp and the transmitted signal beam E _s that demodulates the spectral shift Δλ _B (t). Under the limit of small dynamic phase shift (which is usually valid for dynamic strain induced by acoustic emission or shock), the interference signal at the photodiode can be written as

It can be clearly seen from formula (4) that the maximum signal appears when kl is close to π/4, that is to say, it is at the quadrature point at this time. This condition is easily achieved by applying a voltage V ₁ to the comb electrodes and adjusting the Bragg diffractive EO modulator parameters appropriately.

In photonics, building-block integration processes with control over the fundamental properties of light (amplitude, phase, and polarization) can support a wide range of functions. With a good waveguide structure, interconnections can be realized and passive components such as couplers, filters and demultiplexers can be made. With optical amplifiers (SOAs), phase modulators, and polarization converters for controlling the amplitude, phase, and polarization of light, this technology can support a wide range of functions. Usually InP optoelectronic chip preparation process includes: epitaxial growth, waveguide etching, surface planarization and metal interconnection.

InP optoelectronic chips are fabricated using common integration techniques, that is, using shallowly etched waveguides (for low-loss interconnects and high-efficiency amplifiers), deep-etched waveguides (for small bend radii and high-efficiency phase shifter parts) to build basic active components. source 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 1.25 μm, so these waveguides are called passive waveguides. Active waveguides have bulk Q1.55, multiple quantum well (MQW) or quantum dot (QD) layers within the waveguide layer. This active layer will either absorb or amplify photons with a wavelength of approximately 1.55 microns (depending on the applied current). A highly doped InGaAs contact layer on top of the waveguide allows efficient current injection in the active element (SOA) and is also used to operate the reverse-biased phase-shift element. The basic unit constituting the InP optoelectronic chip of the present invention includes passive devices and active devices, mainly including:

1) Waveguide. The bending radius of the shallow etched waveguide is 500 μm, and the bending radius of the deeply etched waveguide can be reduced to about 10 μm. Including passive components such as curved waveguides, MMI couplers, and double helix waveguides.

2) Optical amplifiers, optical detectors

The structure of optical amplifier and optical detector is very similar, the difference lies in the depth of waveguide etching and the direction of voltage applied by PN junction. The photodetector needs to work in the reverse bias state.

3) Thermo-optic phase shifter (TOM). Thermo-optic phase shifters are passive transparent waveguide sections whose phase shift can be altered by heating them with heater electrodes on top of the waveguide, with modulation speeds in the millisecond range.

4) Bragg diffraction electro-optic modulator: This is an X-cross channel waveguide modulator combined with interleaved comb electrodes. A voltage applied to the comb electrode periodically perturbs the index of refraction beneath the electrode, thereby forming an effective grating within the waveguide, which is a Bragg grating. This grating changes the direction of propagation of the incident beam (see Figure 2).

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention without limitation. Although the applicant has described the present invention in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technology of the present invention Any modification or equivalent replacement of the technical solution without departing from the spirit and scope of the technical solution shall be covered by the scope of the claims of the present invention.

Claims (7)

1. A demodulation system for monitoring high-speed fiber gratings is characterized in that: the optical fiber grating sensor comprises two optical waveguides, wherein a semiconductor optical amplifier A and a thermo-optical phase shifter are arranged on one optical waveguide, a semiconductor optical amplifier B and a double-spiral waveguide are arranged on the other optical waveguide, one ends of the two optical waveguides, which are close to the semiconductor optical amplifier, are connected with a 1 x 2 optical coupler, the 1 x 2 optical coupler is combined and then is connected with an external transmission unit through a first waveguide coupling, and the external transmission unit is connected with a fiber grating sensor in a coupling way; the other ends of the two optical waveguides are output to an electro-optic Bragg deflection modulator, and the output of the electro-optic Bragg deflection modulator is connected with a photodiode so as to convert an optical signal output by the electro-optic Bragg deflection modulator into an electric signal; the electric signal output by the photodiode is sequentially connected with the trans-impedance amplifier, the data processor and the network communication module, and the signal reading unit is connected with the network communication module to read the signal output by the network communication module.

2. The demodulation system for monitoring high-speed fiber gratings according to claim 1, wherein: the electro-optical Bragg deflection modulator is provided with comb-shaped electrodes which are interlaced, and a voltage is applied to the comb-shaped electrodesV ₁ The refractive index of the optical waveguide beneath the electrode is perturbed to form a bragg grating within the optical waveguide.

3. The demodulation system for monitoring high-speed fiber gratings according to claim 1, wherein: the trans-impedance amplifier, the data processor and the network communication module are integrated on the same PCB module.

4. The demodulation system for monitoring high-speed fiber gratings according to claim 3, wherein: the two optical waveguides, the 1 x 2 optical coupler, the two semiconductor optical amplifiers, the thermo-optical phase shifter, the double-helix waveguide, the electro-optic Bragg deflection modulator and the photodiode jointly form an optical fiber channel and are integrated on the indium phosphide substrate, each optical fiber channel is used for demodulating an optical fiber grating sensor on one optical fiber, and the optical fiber channel and the indium phosphide substrate form an optoelectronic chip together.

5. The demodulation system for monitoring high-speed fiber gratings according to claim 4, wherein: at least two optical fiber channels are integrated on an indium phosphide substrate of the photoelectric chip, and the output of a photodiode of each optical fiber channel is connected with a transimpedance amplifier on the same PCB (printed circuit board) module; all the photodiodes are arranged in an array to form a photodiode array.

6. The demodulation system for monitoring high-speed fiber gratings according to claim 1, wherein: and the external transmission unit is coupled and connected with the fiber bragg grating sensor through an optical adapter.

7. A high-speed fiber grating demodulation method is characterized in that: firstly, obtaining the demodulation system for monitoring the high-speed fiber bragg grating as claimed in claim 2, and then carrying out demodulation; the specific mediation process is as follows,

wide-spectrum light emitted from the two semiconductor optical amplifiers is coupled to the fiber grating sensor through the 1 x 2 optical coupler, narrow-band light reflected by the fiber grating sensor is divided into two parts through the 1 x 2 optical coupler and amplified in a single pass through the two semiconductor optical amplifiers, a signal light beam and a pumping light beam are respectively formed in the two optical waveguides, a thermo-optical phase shifter on the optical waveguide corresponding to the pumping light beam is used for finely adjusting the phase difference of a Mach-Zehnder structure, and a double-spiral waveguide arranged on the optical waveguide corresponding to the signal light beam is used for introducing a fixed phase difference; the signal beam and the pump beam are at a Bragg angle ±)θ _B Entering an electro-optic Bragg deflection modulator, voltages applied to comb-shaped electrodesV ₁ The refractive index of the optical waveguide under the electrode is disturbed, so that a Bragg grating is formed in the optical waveguide, which changes the transmission direction of the pump beam, the light being directed along a Bragg angle 2 of 2 times the pump beamθ _B The diffracted light of the pump beam can just enter the optical waveguide corresponding to the signal beam and coincide with the transmitted signal beam to interfere with each other; the photodiode detects the light rays which are interfered with each other and converts the light rays into electric signals, the data processor measures the change of the electric signals and transmits the electric signals through the network communication module, and therefore the wavelength shift of the reflected light of the fiber bragg grating is determined, and the demodulation of the fiber bragg grating sensor is achieved.

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