A heterodyne detection architecture for optical fiber sensors

Abstract

The present invention relates to the field of heterodyne detection technology, and discloses a heterodyne detection architecture of an optical fiber sensor, comprising: a modulation unit, a beam splitting unit, a frequency shift unit, a sensing unit, and a sampling unit. Continuous light emitted by a light source is modulated into a first pulse by the modulation unit, and then the first pulse is injected into the beam splitting unit, which equally divides the first pulse into a second pulse and a third pulse. Then, the frequency shift unit applies different frequency shift amounts to the second pulse and the third pulse, respectively, to obtain a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse. Then, the fourth pulse and the fifth pulse are transmitted to the sensing unit, interfered with the fourth pulse and the fifth pulse, and a pulse group sequence containing multiple groups of three pulses is output, so that the sensing information contained in the pulse group sequence can be obtained through the sampling unit.

Description

Technical Field

[0001] The present invention relates to the technical field of heterodyne detection, and in particular to a heterodyne detection architecture for an optical fiber sensor. Background Art

[0002] Heterodyne detection is an interferometric fiber optic sensor detection method characterized by a wide operating frequency bandwidth and a large dynamic range. Its basic principle is to use a frequency shifter to cause two light beams to produce different frequency shifts. The two frequency-shifted light beams interfere within the fiber optic sensor, generating a carrier signal with a frequency equal to the difference between the two frequency shifts. The sensor signal is then demodulated using an electrical frequency discriminator or optical methods.

[0003] A common heterodyne detection architecture exists, which primarily consists of a heterodyne optical path, a sensor, and a driver. Its operating principle is as follows: light from a light source is split into two paths by coupler C1, and the two paths are frequency-shifted by acousto-optic modulators AOM1 and AOM2, respectively. The main frequency of AOM AOM1 is f1, and the main frequency of AOM2 is f2, and f1 is not equal to f2. Both AOMs AOM1 and AOM2 are driven by a modulated signal from a radio frequency signal source RF, after passing through a power amplifier, and function as both frequency shifters and intensity modulators. As the two light waves are frequency-shifted, they are also modulated from continuous light into pulsed light. One pulse is coupled to the other after being delayed by a delay fiber, and the output from coupler C2 is an optical pulse pair. The optical pulse pair is then injected into a sensor composed of an unbalanced interferometer. The difference in length between the two arms of the unbalanced interferometer is half the length of the delay optical fiber. Therefore, the output from the unbalanced interferometer is a light pulse group consisting of three light pulses. The middle pulse of the light pulse group is the interference pulse, which modulates the sensing signal. The interference pulse is then digitally sampled to demodulate the sensor signal.

[0004] Although the heterodyne detection architecture described above can generate optical heterodyne signals relatively conveniently, the acousto-optic modulator (AOM) has a main frequency drift. When two AOMs are used to frequency-shift two light beams in parallel, the AOM's main frequency drift will cause the heterodyne frequency to drift, resulting in a significant increase in the system's low-frequency noise. Summary of the Invention

[0005] The purpose of the present invention is to provide a fiber optic sensor heterodyne detection architecture to solve the following technical problems:

[0006] How to reduce the noise of optical signals during modulation and demodulation.

[0007] The purpose of the present invention can be achieved through the following technical solutions:

[0008] A heterodyne detection architecture for an optical fiber sensor, comprising:

[0009] a modulation unit, configured to modulate the input continuous light into a first pulse;

[0010] a beam splitting unit connected to the output end of the modulation unit, configured to equally split the first pulse into a second pulse and a third pulse;

[0011] a frequency shift unit connected to the output end of the beam splitting unit, configured to perform a frequency shift of a preset magnitude on the second pulse and the third pulse, and output a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse;

[0012] a sensing unit connected to the output end of the frequency shift unit, configured to interfere with the fourth pulse and the fifth pulse and output a pulse group sequence comprising a plurality of three-pulse groups; the three-pulse groups comprising the fourth pulse, the interference light pulse, and the fifth pulse arranged in time sequence;

[0013] The sampling unit is connected to the output end of the sensing unit and is used to receive the pulse group sequence and demodulate the sensing signal contained in the interference light pulse.

[0014] As a further solution of the present invention: the beam splitting unit includes a coupler C1, an unbalanced interferometer and a Faraday rotator FRM;

[0015] The coupler C1 is connected to the modulation unit and the unbalanced interferometer, and both arm terminals of the unbalanced interferometer are connected to the Faraday rotator FRM.

[0016] As a further solution of the present invention: the sensing unit includes a coupler C2, the unbalanced interferometer and a Faraday rotator FRM;

[0017] The coupler C2 is connected to the modulation unit and the unbalanced interferometer, and both arm terminals of the unbalanced interferometer are connected to the Faraday rotator FRM.

[0018] As a further solution of the present invention: the modulation unit includes an acousto-optic modulator AOM1 and a first driving module connected to the acousto-optic modulator AOM1, and the acousto-optic modulator AOM1 is driven by a first modulation signal emitted by the first driving module.

[0019] As a further solution of the present invention, the frequency shift unit includes an acousto-optic modulator (AOM2) and a second driving module connected to the acousto-optic modulator (AOM2). The acousto-optic modulator (AOM2) is driven by a second modulation signal emitted by the second driving module, and the acousto-optic modulator (AOM2) performs a frequency shift of a preset magnitude on the second pulse and the third pulse.

[0020] As a further solution of the present invention: the second driving module includes a radio frequency module RF and a power amplifier AMP, the radio frequency module RF sends the second modulation signal, and the second modulation signal is amplified by the power amplifier AMP to drive the acousto-optic modulator AOM2.

[0021] As a further solution of the present invention: the sampling unit includes a photoelectric sensor PD, an A / D converter and a modem.

[0022] As a further solution of the present invention: it also includes a laser, which is connected to the input end of the modulation unit and is used to output the continuous light.

[0023] Beneficial effects of the present invention:

[0024] (1) The continuous light emitted by the light source is modulated into a first pulse by a modulation unit, and then the first pulse is injected into a beam splitting unit, which divides the first pulse into a second pulse and a third pulse. Then, the frequency shifting unit applies different frequency shifts to the second pulse and the third pulse respectively, thereby obtaining a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse. Then, the fourth pulse and the fifth pulse are transmitted to the sensing unit, and the fourth pulse and the fifth pulse are interfered with, and a pulse group sequence containing multiple groups of three pulses is output, wherein the fourth pulse and the fifth pulse of each three-pulse group are the first and last light pulses and do not interfere with each other, and the middle pulse is the interference light pulse. After receiving the middle pulse group sequence containing the interference light pulse, the sampling unit demodulates the sensing signal contained in the interference light pulse, thereby obtaining the sensing information contained in the interference light pulse. In this way, the processes of beam splitting, frequency shifting, and interference of the light signal are separated, and the frequency shifting unit is used alone to perform a preset frequency shift on the second pulse and the third pulse, which can effectively suppress the generation of noise.

[0025] (2) The present invention adopts two acousto-optic modulators (AOMs) arranged in series, and the beam splitting unit and the sensing unit adopt a Michelson interferometer structure connected to a Faraday rotator. These measures ensure the polarization consistency of the two paths of light and effectively suppress the polarization signal fading of the interference light. The present invention uses a single AOM frequency shift to generate a heterodyne carrier signal, which can suppress the low-frequency noise generated by the AOM main frequency drift. The two acousto-optic modulators (AOMs) in the architecture modulate the transmitted light successively. The two chopping waves improve the extinction ratio of the optical pulse and can suppress the finite extinction ratio crosstalk noise in the optical fiber sensor array; the beam splitting unit and the sensing unit together form a balanced interferometer with equal arm difference, which can effectively suppress the phase noise introduced by the frequency fluctuation of the light source and obtain good system noise performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] Figure 1It is a structural diagram of a common heterodyne detection architecture;

[0028] Figure 2 It is a schematic diagram of the structure of the synchronized optical reference heterodyne architecture;

[0029] Figure 3 It is a structural diagram of the heterodyne detection architecture of the optical fiber sensor in the present invention;

[0030] Figure 4 This is the background phase noise diagram of the ordinary heterodyne detection architecture and the synchronized optical reference heterodyne detection architecture;

[0031] Figure 5 This is a background phase noise diagram of the heterodyne detection architecture of the optical fiber sensor in the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] like Figure 1 As shown in FIG, an existing common heterodyne detection architecture of optical heterodyne detection is shown, which mainly includes three parts: heterodyne optical path, sensor and driver.

[0034] Its working principle is as follows: the light from the light source is divided into two paths by the coupler C1, and the two paths of light are frequency-shifted by the acousto-optic modulators AOM1 and AOM2 respectively. The main frequency of the acousto-optic modulator AOM1 is f1, and the main frequency of the acousto-optic modulator AOM2 is f2, and f1 is not equal to f2.

[0035] Both AOM1 and AOM2 are driven by a modulated signal from an RF signal source (RF) via a power amplifier, and function as both frequency shifters and intensity modulators. The two light waves are frequency-shifted and modulated from continuous light into pulsed light. One pulse is delayed by a delay fiber and coupled with the other pulse, resulting in a light pulse pair output from coupler C2. This light pulse pair is then injected into a sensor consisting of an unbalanced interferometer. The difference in length between the two arms of the unbalanced interferometer is half the length of the delay fiber, resulting in a three-pulse group output from the unbalanced interferometer. The middle pulse of the group is the interference pulse, which modulates the sensing signal. The interference pulse is then digitally sampled to demodulate the sensor's sensing signal.

[0036] Although the above-mentioned common heterodyne detection architecture can generate optical heterodyne signals relatively conveniently, it still has some shortcomings:

[0037] 1) To couple the two optical pulses, a time-delay fiber is introduced, disrupting the optical path symmetry of the two light paths, resulting in inconsistent polarization states and polarization fading of the interference light. Using polarization-maintaining fiber to create the time-delay fiber to maintain the polarization consistency of the two light paths would significantly increase system costs in large-scale arrays, hindering engineering applications.

[0038] 2) The acousto-optic modulator (AOM) has a main frequency drift. Using two AOMs to shift the frequency of two light beams in parallel will cause the main frequency drift of the AOM to drift the heterodyne frequency, resulting in a significant increase in the low-frequency noise of the system.

[0039] In order to suppress the low-frequency noise introduced by the frequency drift of the acousto-optic modulator (AOM), an optical synchronous reference signal is used to participate in the heterodyne demodulation, which is called the synchronized optical reference heterodyne architecture.

[0040] like Figure 2 As shown, the continuous light emitted by the light source is split into two beams by coupler C1. Both beams are modulated into pulses by an acousto-optic modulator (AOM) and subjected to separate frequency shifts. Couplers C2 and C3 are used to extract a portion of the optical power from the two modulated pulses, which then interfere with each other and serve as synchronization reference light. The remaining pulses are then combined via a time-delayed optical fiber to generate a pulse pair that is injected into the fiber optic sensor.

[0041] The synchronous reference optical pulse and the interfering optical pulse returned from the sensor are simultaneously digitally sampled and demodulated. Using the synchronous reference optical pulse as the reference signal for demodulation eliminates low-frequency noise introduced by AOM main frequency drift.

[0042] Using synchronous reference light for heterodyne demodulation can achieve better system low-frequency phase noise, but this solution still has shortcomings:

[0043] 1) The system requires the use of fully polarization-maintaining devices to address the signal's polarization fading problem, which greatly increases the system hardware cost;

[0044] 2) The system has a complex structure, and the introduction of synchronous reference light requires loss of light source power, additional detectors, and digital sampling channels, which also greatly increases the hardware cost when configuring a large array.

[0045] 3) The system uses synchronous reference light to participate in demodulation, which increases the amount of calculation and makes it difficult to achieve dynamic demodulation of the sensor.

[0046] In view of the above problems, the present invention proposes a heterodyne detection architecture for optical fiber sensors, such as Figure 3 As shown, including:

[0047] a modulation unit, configured to modulate the input continuous light into a first pulse;

[0048] a beam splitting unit connected to the output end of the modulation unit, and configured to equally split the first pulse into a second pulse and a third pulse;

[0049] a frequency shift unit connected to the output end of the beam splitting unit, configured to perform a frequency shift of a preset magnitude on the second pulse and the third pulse, and output a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse;

[0050] a sensing unit connected to the output end of the frequency shift unit, configured to interfere with the fourth pulse and the fifth pulse, and output a pulse group sequence comprising a plurality of three-pulse groups; the three-pulse groups comprising the fourth pulse, the interference light pulse, and the fifth pulse arranged in time sequence;

[0051] The sampling unit is connected to the output end of the sensing unit and is used to receive the pulse group sequence and demodulate the sensing signal contained in the interference light pulse.

[0052] The continuous light emitted by the light source is modulated into a first pulse by a modulation unit, and then the first pulse is injected into a beam splitting unit, which equally splits the first pulse into a second pulse and a third pulse. The frequency shifting unit then applies different frequency shifts to the second pulse and the third pulse, respectively, to obtain a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse. The fourth pulse and the fifth pulse are then transmitted to a sensing unit, which interferes with the fourth pulse and the fifth pulse, and outputs a pulse group sequence containing multiple groups of three pulses, wherein the fourth and fifth pulses of each three-pulse group are the first and last light pulses and do not interfere with each other, and the middle pulse is the interference light pulse. After receiving the middle pulse group sequence containing the interference light pulse, the sampling unit demodulates the sensor information contained in the interference light pulse to obtain the sensor information contained in the interference light pulse. In this way, the processes of beam splitting, frequency shifting, and interference of the optical signal are separated, and the frequency shifting unit is used alone to perform a preset frequency shift on the second pulse and the third pulse, which can effectively suppress the generation of noise.

[0053] Further, such as Figure 3 As shown, the beam splitting unit includes a coupler C1, an unbalanced interferometer, and a Faraday rotator FRM;

[0054] The coupler C1 is connected to the modulation unit and the unbalanced interferometer. Both arm terminals of the unbalanced interferometer are connected to a Faraday rotator FRM.

[0055] The sensing unit includes a coupler C2, an unbalanced interferometer, and a Faraday rotator FRM;

[0056] The coupler C2 is connected to the modulation unit and the unbalanced interferometer. Both arm terminals of the unbalanced interferometer are connected to a Faraday rotator FRM.

[0057] The Faraday rotators connected to the two arms of the unbalanced interferometer are used to keep the polarization states of the two beams consistent and suppress the polarization signal fading of the interference light. Each light pulse of the first pulse becomes a pulse pair when it is output from the coupler C1 after passing through the beam splitting unit.

[0058] The modulation unit includes an acousto-optic modulator AOM1 and a first driving module connected to the acousto-optic modulator AOM1. The acousto-optic modulator AOM1 is driven by a first modulation signal sent by the first driving module.

[0059] The frequency shift unit includes an acousto-optic modulator (AOM2) and a second driving module connected to the acousto-optic modulator (AOM2). The acousto-optic modulator (AOM2) is driven by a second modulation signal emitted by the second driving module, and the acousto-optic modulator (AOM2) performs a frequency shift of a preset magnitude on the second pulse and the third pulse.

[0060] The second driving module includes a radio frequency module RF and a power amplifier AMP. The radio frequency module RF sends a second modulation signal, and the second modulation signal is amplified by the power amplifier AMP to drive the acousto-optic modulator AOM2.

[0061] The fourth and fifth pulses obtained by applying different frequency shifts to each optical pulse in the second and third pulses by the acousto-optic modulator AOM2 are sent to the sensing unit. The unbalanced interferometer in the sensing unit and the unbalanced interferometer in the beam splitting module have the same arm difference. The delay caused by the arm difference causes interference between the two pulse pairs returning from the two arms of the detector.

[0062] The sampling unit includes a photoelectric sensor PD, an A / D converter and a modem.

[0063] The present invention may further include a laser, which is connected to the input end of the modulation unit and is used to output continuous light.

[0064] like Figure 4 As shown in the figure, the upper part is the background phase noise diagram of the existing ordinary heterodyne detection architecture, and the lower part is the background phase noise diagram using the synchronous optical reference heterodyne detection architecture. It can be clearly seen that the background phase noise of the synchronous optical reference heterodyne architecture has been greatly improved compared with the ordinary heterodyne detection architecture, but the noise level is still high at frequencies below 1kHz.

[0065] And as Figure 5 As shown, it is a schematic diagram of the background phase noise of the heterodyne detection architecture of the present invention. It can also obtain a flat background phase noise below 1kHz, and has great application potential in the field of low-frequency detection.

[0066] This is because the present invention employs two AOMs arranged in series, and the beam splitting unit and sensing unit employ a Michelson interferometer structure connected to a Faraday rotator. These measures ensure polarization consistency between the two beams and effectively suppress polarization signal fading in the interfering light. The present invention employs a single AOM frequency shift to generate a heterodyne carrier signal, which can suppress low-frequency noise generated by AOM main frequency drift. The two AOMs in the architecture sequentially modulate the transmitted light, and the two chopping steps increase the extinction ratio of the optical pulse, suppressing finite extinction ratio crosstalk noise in the fiber optic sensor array. The beam splitting unit and sensing unit together form a balanced interferometer with equal arm difference, which effectively suppresses phase noise introduced by light source frequency fluctuations, resulting in good system noise performance.

[0067] The working principle of the present invention is as follows: the continuous light emitted by the light source is modulated into a first pulse by a modulation unit, and then the first pulse is injected into a beam splitting unit, which equally divides the first pulse into a second pulse and a third pulse. Then, the frequency shift unit applies different frequency shift amounts to the second pulse and the third pulse respectively, to obtain a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse; then, the fourth pulse and the fifth pulse are transmitted to the sensing unit, and the fourth pulse and the fifth pulse are interfered with, and a pulse group sequence containing multiple groups of three pulses is output, wherein the fourth pulse and the fifth pulse of each three-pulse group are the first and last light pulses and do not interfere, and the middle pulse is the interference light pulse. After receiving the middle pulse group sequence containing the interference light pulse, the sampling unit demodulates the sensor contained in the interference light pulse to obtain the sensor information contained in the interference light pulse.

[0068] In the description of the present invention, it should be understood that the terms "upper", "lower", "left", "right" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, and a specific direction structure and operation, and therefore, cannot be understood as limiting the present invention. In addition, "first" and "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0069] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0070] The above is a detailed description of an embodiment of the present invention. However, the content described is only a preferred embodiment of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A heterodyne detection architecture for an optical fiber sensor, characterized in that: include: a modulation unit, configured to modulate the input continuous light into a first pulse; a beam splitting unit connected to the output end of the modulation unit, configured to equally split the first pulse into a second pulse and a third pulse; a frequency shift unit connected to the output end of the beam splitting unit, configured to perform a frequency shift of a preset magnitude on the second pulse and the third pulse, and output a fourth pulse corresponding to the second pulse and a fifth pulse corresponding to the third pulse; a sensing unit connected to the output end of the frequency shift unit, configured to interfere with the fourth pulse and the fifth pulse and output a pulse group sequence comprising a plurality of three-pulse groups; The three-pulse group includes a fourth pulse, an interference light pulse, and a fifth pulse arranged in time sequence; a sampling unit connected to the output end of the sensing unit, and configured to receive the pulse group sequence and demodulate the sensing signal contained in the interference light pulse; The beam splitting unit includes a coupler C1 and an unbalanced interferometer and a Faraday rotator FRM; The coupler C1 is connected to the modulation unit and the unbalanced interferometer, and both arm terminals of the unbalanced interferometer are connected to the Faraday rotator FRM; The sensing unit includes a coupler C2, the unbalanced interferometer and a Faraday rotator FRM; The coupler C2 is connected to the modulation unit and the unbalanced interferometer, and both arm terminals of the unbalanced interferometer are connected to the Faraday rotator FRM; The frequency shift unit includes an acousto-optic modulator (AOM2) and a second driving module connected to the acousto-optic modulator (AOM2). The acousto-optic modulator (AOM2) is driven by a second modulation signal emitted by the second driving module, and the acousto-optic modulator (AOM2) performs a frequency shift of a preset magnitude on the second pulse and the third pulse.

2. The optical fiber sensor heterodyne detection architecture according to claim 1, characterized in that: The modulation unit includes an acousto-optic modulator (AOM1) and a first driving module connected to the acousto-optic modulator (AOM1). The acousto-optic modulator (AOM1) is driven by a first modulation signal sent by the first driving module.

3. The optical fiber sensor heterodyne detection architecture according to claim 1, characterized in that: The second driving module includes a radio frequency module RF and a power amplifier AMP. The radio frequency module RF sends the second modulation signal, and the second modulation signal is amplified by the power amplifier AMP to drive the acousto-optic modulator AOM2.

4. The optical fiber sensor heterodyne detection architecture according to claim 1, characterized in that: The sampling unit includes a photoelectric sensor PD, an A / D converter and a modem.

5. The optical fiber sensor heterodyne detection architecture according to claim 1, characterized in that: It also includes a laser, which is connected to the input end of the modulation unit and is used to output the continuous light.

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