Optical sensing system based on linear spectrum analysis

Abstract

The invention discloses an optical sensing system based on linear spectrum analysis. Comprising a feedback processor, a first digital-to-analog converter, a low-speed laser driver, an adjustable laser, a second digital-to-analog converter, a high-speed laser driver, a first coupler, a second coupler, a first optical delay line, a first optical mixer, a first photoelectric detector array, an optical sensor, a third coupler, a second optical delay line and a second optical mixer. A second photoelectric detector array; and a signal analysis processor. According to the invention, an optical instantaneous frequency and intensity measurement method is applied to a spectral measurement analysis system, an optical sensing system with low cost and high performance is formed, and the system is higher in speed, more accurate and more linear.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical sensing technology, and particularly relates to an optical sensing system based on linear spectrum analysis. BACKGROUND

[0002] For spectrum analysis, the prior art has the following schemes:

[0003] (1) Spectrometer (using optical dispersion elements to slice the spectrum and using a photodetector PD to detect the intensity of each); the spectrometer can measure one frequency band at a time, has a wide working range, but has a large volume, a high price, a low measurement speed, limited precision, and limited signal-to-noise ratio (the power of the spectrum to be analyzed cannot be too low).

[0004] (2) High-precision tunable laser + PD (by changing the phase, gain region or filter of the laser cavity, thereby changing the wavelength of the laser); this scheme has a high laser output power and can provide a higher signal-to-noise ratio, but the high-precision laser is expensive, the frequency adjustment speed is low (to ensure accurate frequency adjustment), and the precision is relatively limited.

[0005] (3) Low-cost laser + fast frequency chirp + photodetector; this scheme is low in cost, can provide a higher signal-to-noise ratio, has a fast measurement speed, and has high precision, but cannot directly obtain the frequency-intensity spectrum, can only obtain the time-intensity spectrum, and then indirectly obtains the frequency-intensity spectrum by using the linear mapping of time and frequency; therefore, the frequency nonlinearity is large (mainly caused by the nonlinear mapping of time and intensity), especially when measuring at a high speed, the nonlinearity distortion is large, resulting in reduced measurement accuracy.

[0006] (4) Laser + high-speed modulator sweep frequency; this scheme is fast, has high precision, and has linear frequency, but is high in cost, has high-frequency devices, and has large loss. SUMMARY

[0007] The main purpose of the present application is to overcome the shortcomings and deficiencies of the prior art, and to propose an optical sensing system based on linear spectrum analysis, which applies the optical instantaneous frequency measurement method to the spectrum measurement and analysis system to form a low-cost but high-performance optical sensing system that is faster, more accurate and more linear.

[0008] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0009] An optical sensing system based on linear spectrum analysis, comprising a feedback processor, a first digital-to-analog converter, a low-speed laser driver, an adjustable laser, a second digital-to-analog converter, a high-speed laser driver, a first coupler, a second coupler, a first optical delay line, a first optical mixer, a first photodetector array, an optical sensor, a third coupler, a second optical delay line, a second optical mixer, a second photodetector array, and a signal analysis processor;

[0010] The feedback processor is connected to the first digital-to-analog converter and the second digital-to-analog converter respectively.

[0011] The first digital-to-analog converter is connected to the low-speed laser driver, and the low-speed laser driver is connected to one control port of the adjustable laser; the second digital-to-analog converter is connected to the high-speed laser driver, and the high-speed laser driver is connected to another control port of the adjustable laser; the adjustable laser is further connected to the first coupler.

[0012] The output of the first coupler is divided into two paths, one of which is connected to the second coupler; the output of the second coupler is divided into two paths, one of which is connected to the first optical mixer, and the other is connected to the first optical mixer after passing through the first optical delay line; the first optical mixer is connected to the first photodetector array, and the first photodetector array is connected to the feedback processor.

[0013] The other output of the first coupler passes through the optical sensor, and the optical sensor is connected to the third coupler; the output of the third coupler is divided into two paths, one of which is connected to the second optical mixer, and the other is connected to the second optical mixer after passing through the second optical delay line; the second optical mixer is connected to the second photodetector array, and the second photodetector array is connected to the signal analysis processor; the signal analysis processor is connected to the feedback processor.

[0014] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0015] 1. The present application applies the laser instantaneous frequency measurement method to the existing frequency-modulated continuous wave sensing system, which not only retains the low cost and simple system of the traditional method, but also eliminates the low measurement accuracy and large nonlinear error of the traditional method.

[0016] 2. The method uses the sending end to perform adaptive or real-time control loop correction of slow and large chirp nonlinearities; the receiving end performs super-accurate instantaneous frequency and intensity measurement again, and uses an accurate intensity-frequency spectrum to replace the intensity-time spectrum in the traditional method, thereby completely eliminating nonlinear distortion.

[0017] 3. The measurement structure is more accurate. Traditional methods are limited by the larger nonlinear errors caused by high-speed measurement, and generally avoid high-speed measurement or use more expensive drivers. Since the nonlinear effect is eliminated, the present invention can achieve higher speed, lower cost, ultra-high linearity, and ultra-high precision measurement, while providing large-scale sensor network solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural diagram of the system of the present invention;

[0019] Figure 2 This is a schematic diagram of the principle of the optical mixer and its corresponding laser instantaneous frequency measurement algorithm in the present invention;

[0020] Figure 3 Schematic diagram of the structure principle of the optical mixer in the present invention;

[0021] Figure 4 is a wavelength division and space division multiplexing system in an embodiment;

[0022] Explanation of the accompanying figures: 1-feedback processor; 2-first digital-to-analog converter; 3-low-speed laser driver; 4-tunable laser; 5-second digital-to-analog converter; 6-high-speed laser driver; 7-first coupler; 8-second coupler; 9-first optical delay line; 10-first optical mixer; 11-first photodetector array; 12-optical sensor; 13-third coupler; 14-second optical delay line; 15-second optical mixer; 16-second photodetector array; 17-signal analysis processor. DETAILED DESCRIPTION

[0023] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0024] Example

[0025] like Figure 1 As shown, the present invention is an optical sensing system based on linear spectrum analysis, including a feedback processor 1, a first digital-to-analog converter 2, a low-speed laser driver 3, an adjustable laser 4, a second digital-to-analog converter 5, a high-speed laser driver 6, a first coupler 7, a second coupler 8, a first optical delay line 9, a first optical mixer 10, a first photodetector array 11, an optical sensor 12, a third coupler 13, a second optical delay line 14, a second optical mixer 15, a second photodetector array 16 and a signal analysis processor 17.

[0026] like Figure 1 As shown, the feedback processor is connected to the first digital-to-analog converter and the second digital-to-analog converter via port c and port d respectively;

[0027] The first digital-to-analog converter is connected to a low-speed laser driver, and the low-speed laser driver is connected to a control port a of the adjustable laser; the second digital-to-analog converter is connected to a high-speed laser driver, and the high-speed laser driver is connected to a control port b of the adjustable laser; and an output port c of the adjustable laser is connected to the first coupler.

[0028] The output of the first coupler is divided into two paths, one of which is connected to the second coupler; the output of the second coupler is divided into two paths, one of which is connected to the first optical mixer, and the other is connected to the first optical mixer after passing through the first optical extension line; the first optical mixer is connected to the first photodetector array, and the first photodetector array is connected to the array input port a of the feedback processor.

[0029] The other output of the first coupler passes through an optical sensor, and the spectrum of the sensor changes with the change of the sensing parameter (deformation or movement); the optical sensor is connected to the third coupler, and the optical signal carrying the sensing information passing through the sensor enters the third coupler; the output of the third coupler is divided into two paths, one of which is connected to the second optical mixer, and the other is connected to the second optical mixer after passing through the second optical extension line; the second optical mixer is connected to the second photodetector array, and the second photodetector array is connected to the signal analysis processor; the output port d of the signal analysis processor is connected to the input port b of the feedback processor, and the output port e of the feedback processor is connected to the input port c of the signal analysis processor.

[0030] The output port of the first optical mixer is an array port with multiple outputs, and the number of outputs is different according to different mixer structures, which is 2 to multiple; the number of the first photodetector array corresponds to the number of the output port of the first optical mixer; similarly, the output port of the second optical mixer is an array port with multiple outputs, and the number of outputs is different according to different mixer structures, which is 2 to multiple; the number of the second photodetector array corresponds to the number of the output port of the second optical mixer.

[0031] In the embodiment, as shown in Figure 2 and Figure 3 The first optical mixer and the second optical mixer include multiple types, and three main types are given in the embodiment, i.e., a 180-degree optical mixer (including two outputs, such as a 2x2 coupler or a multimode interferometer (MMI)), a 120-degree optical mixer (including three outputs, such as a 3x3 coupler or a multimode interferometer (MMI)), and a 90-degree optical mixer (including four outputs, such as a 4x4 multimode interferometer (MMI)); the laser instantaneous frequency and intensity measurement algorithm of the first photodetector array, the second photodetector array, and the signal analysis processor corresponds to the type of the optical mixer.

[0032] In the embodiment, the tunable laser can be a tunable external cavity laser (ECL), a distributed feedback laser (DFB), a feedback Bragg grating laser (DBR), a vertical cavity surface emitting laser (VCSEL), or other lasers that can achieve the same function.

[0033] In the embodiment, the first and second optical delay lines are fixed-length optical fibers or integrated waveguides; the length of the first optical delay line for the transmitter feedback control is relatively fixed after being adjusted to a suitable value according to the specific situation of the chirp signal; and the second optical delay line for the receiver can be appropriately increased or decreased according to the specific measurement situation (such as the signal-to-noise ratio of the received signal and the frequency resolution of the measurement).

[0034] In the embodiment, the optical sensor can be a fiber sensor (such as a fiber Bragg grating (FBG)), an integrated optical sensor (such as a micro ring resonator (MRR)), or other laser sensors (such as a free-space optical sensor).

[0035] The optical sensing system in the embodiment specifically includes the following steps when in operation:

[0036] The feedback processor sends a digital direct current bias signal from port c, which is converted into an analog direct current voltage output by the first digital-to-analog converter. The output voltage is amplified by the low-speed laser driver and used to drive the direct current bias port a of the tunable laser, so that the laser generates a stable laser signal and outputs it from the output port c.

[0037] At the same time, the feedback processor sends a digital frequency chirp signal from port d, which is converted into an analog high-frequency voltage output by the second digital-to-analog converter. The output voltage is amplified by the high-speed laser driver and used to drive the high-speed port b of the tunable laser, so that the laser superimposes a high-speed chirp frequency modulation output on the basis of the stable laser signal output.

[0038] In the embodiment, the chirped laser signal output by the tunable laser is divided into two signals after passing through the first coupler 7, one of which is output into the optoelectronic feedback control loop. The optoelectronic feedback control loop includes:

[0039] The first coupler output signal enters the second coupler and is divided into two signals, one of which is directly output into the first optical mixer, and the other signal is delayed by the first optical delay line and then enters the first optical mixer.

[0040] The output of the first optical mixer is received by a first photodetector array, realizing conversion of the optical signal into an electrical signal, the output of the first photodetector array being an electrical analog signal, which is finally sampled by an array input port a of the feedback processor and converted into a digital signal, and meanwhile the feedback processor calculates the nonlinearity of the chirp signal, generates a corrected feedback control signal for the nonlinearity distortion and updates the output of an output port d.

[0041] In the embodiment, the other signal output by the first coupler 7 enters an optical sensing branch, which comprises:

[0042] The signal output by the first coupler first passes through an optical sensor 12, and the optical signal carrying the sensing information after passing through the optical sensor is divided into two signals after entering a third coupler, one of which directly enters a second optical mixer, and the other passes through a second optical delay line and then enters the second optical mixer;

[0043] The output of the second optical mixer is received by a second photodetector array, realizing conversion of the optical signal into an electrical signal, the output of the second photodetector array being an electrical analog signal, which is finally sampled by an array input port a of the signal analysis processor and converted into a digital signal, and meanwhile the signal analysis processor calculates the instantaneous frequency and intensity information of the laser and completes calibration in combination with the reference light information provided by the output port e of the feedback processor without passing through the optical sensor, thereby obtaining an accurate linear spectrum corresponding to the optical sensor;

[0044] When the measurement of one wavelength is completed, the signal analysis processor outputs a wavelength adjustment instruction to the input port b of the feedback processor, and the feedback processor updates the direct current bias voltage of the output port c upon receiving the wavelength adjustment instruction, thereby adjusting the laser to the next measurement wavelength.

[0045] In the embodiment, the laser instantaneous frequency and intensity measurement algorithm specifically comprises the following steps:

[0046] S1. The laser instantaneous frequency and intensity information provided by the feedback processor without passing through the optical sensor is used to first generate reference light information for the signal analysis processor, and meanwhile the nonlinearity existing in the laser frequency chirp is calculated and feedback control is performed on the chirp signal, so as to compensate for most of the nonlinearity in the laser frequency chirp.

[0047] S2. The optical sensor is used to sense external signals.

[0048] S3. The chirped light source passes through the optical sensor and carries the external signals.

[0049] S4. The signal passing through the optical sensor is collected by the information analysis processor.

[0050] S5, according to the type of optical mixer, the obtained signal is used to generate the required complex signal through Hilbert transform or direct vector synthesis; for a 180-degree optical mixer, the obtained real signal is directly subjected to Hilbert transform; for a 120-degree optical mixer, a direct vector synthesis method is adopted, which is represented as:

[0051] IQ(t) = I1 x e j0 + I2 x e j2π / 3 + I3 x e j4π / 3

[0052] wherein I1, I2 and I3 are the photocurrents of the first, second and third photodetectors respectively;

[0053] For a 90-degree optical mixer, a direct vector synthesis method is adopted, which is represented as:

[0054] IQ(t) = I1 + j * I2

[0055] S6, the phase information of the obtained complex signal is subjected to unwrap calculation to obtain the laser instantaneous frequency estimation value; and the intensity information of the obtained complex signal is the laser instantaneous intensity estimation value.

[0056] The unwrap calculation mainly changes the 0-2π periodic signal into a continuous signal, and the specific operation is that when a large phase jump occurs, an integer multiple of 2π phase shift is added to make the phase change continuously. unwrap is a function in MATLAB and Python.

[0057] In addition, for the present embodiment, in order to cope with a large-scale sensing system, based on a large-range adjustable laser combined with a small-range fast chirp and using a method of collecting and separating or using super-dense wavelength division multiplexing and space division multiplexing (such as multiple optical fibers / waveguides, different waveguide modes, or different directions in free space), the available spatial dimension and frequency range are increased, and the number of sensors is increased, so as to realize a large-scale sensing system, such as Figure 4 as shown.

[0058] It should be further noted that in the present specification, terms such as "comprise", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such a process, method, article or device. Without more limitations, the element defined by the statement "comprising a" does not exclude the presence of another identical element in the process, method, article or device comprising the element.

[0059] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An optical sensing system based on linear spectrum analysis, characterized in that: The device comprises a feedback processor, a first digital-to-analog converter, a low-speed laser driver, a tunable laser, a second digital-to-analog converter, a high-speed laser driver, a first coupler, a second coupler, a first optical delay line, a first optical mixer, a first photodetector array, an optical sensor, a third coupler, a second optical delay line, a second optical mixer, a second photodetector array, and a signal analysis processor; The feedback processor is connected to the first digital-to-analog converter and the second digital-to-analog converter respectively; The first digital-to-analog converter is connected to a low-speed laser driver, which is connected to one control port of the tunable laser. The second digital-to-analog converter is connected to a high-speed laser driver, which is connected to another control port of the tunable laser. The tunable laser is also connected to a first coupler. The output of the first coupler is divided into two paths, one of which is connected to the second coupler; the output of the second coupler is divided into two paths, one of which is connected to the first optical mixer, and the other is connected to the first optical mixer after passing through the first optical extension line. The first optical mixer is connected to the first photodetector array, and the first photodetector array is connected to the feedback processor; The other output of the first coupler passes through the optical sensor, and the optical sensor is connected to the third coupler; the output of the third coupler is divided into two paths, one path is connected to the second optical mixer, and the other path passes through the second optical extension line and is connected to the second optical mixer, the second optical mixer is connected to the second photodetector array, the second photodetector array is connected to the signal analysis processor, and the signal analysis processor is connected to the feedback processor.

2. The optical sensing system based on linear spectrum analysis according to claim 1, characterized in that: The optical sensing system includes the following steps: The feedback processor sends a digital DC bias signal from port c, which is converted into an analog DC voltage output by a first digital-to-analog converter. The output voltage is amplified by a low-speed laser driver and used to drive the DC bias port of the adjustable laser, so that the laser generates a stable laser signal and outputs it from the output port. At the same time, the feedback processor sends a digital frequency chirp signal from port d, which is converted into an analog high-frequency voltage output by a second digital-to-analog converter. The output voltage is amplified by a high-speed laser driver and used to drive the high-speed port of the tunable laser, so that the laser superimposes a high-speed chirp frequency modulation output on the basis of a stable laser signal output.

3. The optical sensing system based on linear spectrum analysis according to claim 2, characterized in that: The chirped laser signal output by the tunable laser is divided into two signals after passing through the first coupler. One of the signals enters the optoelectronic feedback control loop, which includes: The output signal of the first coupler enters the second coupler and is divided into two signals, one of which directly enters the first optical mixer; at the same time, the other signal output by the second coupler is delayed by the first optical delay line and then enters the first optical mixer; The output of the first optical mixer is received by the first photodetector array to realize the conversion of optical signals into electrical signals. The output of the first photodetector array is an electrical analog signal, which is finally sampled and converted into a digital signal by the feedback processor. At the same time, the feedback processor calculates the nonlinearity of the chirp signal, generates a corrected feedback control signal for the nonlinear distortion, and updates the output of the output port d.

4. The optical sensing system based on linear spectrum analysis according to claim 3, characterized in that: The other signal output by the first coupler enters the optical sensing branch, which includes: The output signal of the first coupler first passes through the optical sensor. The optical signal carrying the sensing information enters the third coupler and is split into two signals. One signal directly enters the second optical mixer. At the same time, the other signal output by the third coupler is delayed by the second optical delay line before entering the second optical mixer. The output of the second optical mixer is received by the second photodetector array to realize the conversion of the optical signal into an electrical signal. The second photodetector array outputs an electrical analog signal, which is input into the signal analysis processor, sampled by the array input port of the signal analysis processor and converted into a digital signal. At the same time, the signal analysis processor calculates the instantaneous frequency and intensity information of the laser and combines it with the reference light information provided by the feedback processor that has not passed through the optical sensor to complete the calibration, thereby obtaining an accurate linear spectrum response of the optical sensor; After completing the measurement of one wavelength, the signal analysis processor sends a wavelength adjustment instruction to the feedback processor. The feedback processor receives the wavelength adjustment instruction and updates the DC bias voltage of the output port c to adjust the laser to the next measurement band.

5. The optical sensing system based on linear spectrum analysis according to claim 4, characterized in that: The output port of the first optical mixer is a plurality of output array ports, the number of which varies according to different mixer structures; The number of the first photodetector array corresponds to the number of output ports of the first optical mixer; The output port of the second optical mixer is a plurality of output array ports, the number of which varies according to different mixer structures; The number of the second photodetector arrays corresponds to the number of output ports of the second optical mixer.

6. The optical sensing system based on linear spectrum analysis according to claim 5, characterized in that: The first optical mixer and the second optical mixer include multiple types, not limited to the three types of 180-degree optical mixers, 120-degree optical mixers, and 90-degree optical mixers. The laser instantaneous frequency and intensity measurement algorithms used by the first photodetector array, the second photodetector array, and the signal analysis processor correspond to the types of optical mixers.

7. The optical sensing system based on linear spectrum analysis according to claim 1, characterized in that: The tunable laser is not limited to a tunable external cavity laser (ECL), a distributed feedback laser (DFB), a feedback Bragg grating laser (DBR), a vertical cavity surface emitting laser (VCSEL), or other lasers that can achieve the same function.

8. The optical sensing system based on linear spectrum analysis according to claim 1, characterized in that: The first optical delay line and the second optical delay line are optical fibers or integrated waveguides of fixed length; the length of the first optical delay line used for feedback control at the transmitting end is relatively fixed after being adjusted to an appropriate value according to the specific situation of the chirp signal; the second optical delay line used for the receiving end can be adjusted according to the specific measurement situation.

9. The optical sensing system based on linear spectrum analysis according to claim 1, characterized in that: The optical sensor is not limited to a fiber optic sensor, an integrated optical sensor, or other laser sensors.

10. The optical sensing system based on linear spectrum analysis according to claim 6, characterized in that: The laser instantaneous frequency and intensity measurement algorithm includes the following steps: S1. Using the instantaneous frequency and intensity information of the laser provided by the feedback processor before passing through the optical sensor, reference light information is first generated for use by the signal analysis processor. The nonlinearity in the laser frequency chirp is calculated and the chirp signal is fed back to compensate for most of the nonlinearity in the laser frequency chirp. S2, using optical sensors to sense external signals; S3, the chirped light source passes through the optical sensor and carries the external signal; S4, collecting signals passing through the optical sensor through an information analysis processor; S5. Depending on the type of optical mixer, the obtained signal is subjected to Hilbert transform or direct vector synthesis to generate the required complex signal. For a 180-degree optical mixer, the obtained real signal is directly subjected to Hilbert transform. For a 120-degree optical mixer, direct vector synthesis is used, which is expressed as: IQ(t)=I1×e j0 +I2×e j2π / 3 +I3×e j4π / 3 Among them, I1, I2, and I3 are the photocurrents of the first, second, and third photodetectors, respectively; For a 90-degree optical mixer, direct vector synthesis is used, which is expressed as: IQ(t)=I1+j*I2 S6. The phase information of the obtained complex signal is unwrap calculated to obtain the estimated value of the instantaneous frequency of the laser; The intensity information of the obtained complex signal is the estimated value of the instantaneous intensity of the laser.

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