A coherent detection system

Abstract

This invention provides a coherent detection system, characterized in that it includes: a mixing module and a balanced detection module; wherein, the mixing module is used to couple the signal light with the local oscillator light; the balanced detection module is used to eliminate the DC component of the signal and improve the signal-to-noise ratio of the signal. Through this design, the signal-to-noise ratio of the echo signal can be improved.

Description

Technical Field

[0001] This invention relates to the field of detection technology, and more specifically, to a coherent detection system. Background Technology

[0002] Coherent detection involves mixing the signal light and the local oscillator light, outputting the difference frequency component, which is absorbed by the detector's receiving surface to generate a photocurrent. This difference frequency component retains the amplitude, frequency, and phase information of the signal light, achieving holographic detection of the signal light. Compared to direct detection, it has advantages such as strong detection capability, high conversion gain, high signal-to-noise ratio, and strong anti-interference capability, and is widely used in coherent optical communication, remote sensing, lidar velocities, and ranging.

[0003] In coherent detection devices, the linearly polarized light generated by the laser is often split into two paths. One path is used as the emitted light and passes through the target to generate the signal light. The other path is used as the local oscillator light and coherently interacts with the signal light. The coherent light is converted into an electrical signal by the detector receiving surface and used for the analysis and measurement of parameters such as speed and distance.

[0004] The key role of an optical mixer in a coherent detection system is to coherently mix two signal beams with similar frequencies, constant phase difference, and the same polarization direction with the local oscillator wavefront. Then, the intermediate frequency signal is processed by subsequent photoelectric balance detectors and signal recovery processing circuits. It can be seen that an optical mixer is essentially a modem.

[0005] A photoelectric balanced detector typically consists of two identical photodiodes (usually semiconductor photodiodes PIN) and a subtractor. The PIN converts the received optical signal into an electrical signal and subtracts the two, outputting a current signal representing the difference. Theoretically, the two photodiodes have identical operating characteristics such as photoelectric conversion efficiency, response bandwidth, response time, and noise. However, because photodiodes have different conversion efficiencies for different wavelengths of light, this limits the sensitivity of the coherent detection system. Furthermore, the noise of the photodiode directly affects the sensitivity of the optical receiver. Therefore, the performance of the photoelectric balanced detector has a significant impact on the coherent detection system.

[0006] In laser long-range ranging, imaging, velocimetry, and harsh environments such as underwater and foggy conditions, the echo signal amplitude decreases significantly due to medium attenuation, and the pulse waveform is distorted and broadened due to medium refraction and scattering. However, the amplitude and frequency band characteristics of noise signals such as shot noise, dark noise, and thermal noise remain basically unchanged, so the target echo is submerged in noise, resulting in a low signal-to-noise ratio. Therefore, there is an urgent need for a coherent detection system that can improve the signal-to-noise ratio of the echo signal. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of the prior art by providing a coherent detection system to solve the problem of low signal-to-noise ratio of echo signals in related technologies.

[0008] To achieve the above objectives, the technical solutions adopted in the embodiments of the present invention are as follows:

[0009] This invention provides a coherent detection system, characterized in that it includes:

[0010] Mixer module and balance detection module;

[0011] The mixing module is used to couple the signal light with the local oscillator light.

[0012] The balanced detection module is used to eliminate the DC component of the signal and improve the signal-to-noise ratio.

[0013] Optionally, the coherent detection system further includes a delay accumulation module, which is used to delay the original signal for a certain period of time before adding it to the original signal.

[0014] Optionally, the delay time is less than 1 / 4 of the original signal period.

[0015] Optionally, the balance detection module includes a first input, a second input, and a first output.

[0016] Optionally, the phase difference between the AC quantity of the first input and the AC quantity of the second input is 180°.

[0017] Optionally, the first input and the second input are operated on to obtain a third output, and the first output and the third output are operated on to obtain a fifth output.

[0018] Optionally, the balance detection module includes a third input, a fourth input, and a second output.

[0019] Optionally, the phase difference between the AC quantity of the third input and the AC quantity of the fourth input is 180°.

[0020] Optionally, the third input and the fourth input are operated to obtain the fourth output, and the second output and the fourth output are operated to obtain the sixth output.

[0021] Optionally, the output of the mixer module is a four-channel output with a phase difference of 90°.

[0022] The beneficial effects of this invention are: This invention provides a coherent detection system, characterized in that it comprises:

[0023] Mixer module and balance detection module;

[0024] The mixing module is used to couple the signal light with the local oscillator light.

[0025] The balanced detection module is used to eliminate the DC component of the signal and improve the signal-to-noise ratio. This design can improve the signal-to-noise ratio of the echo signal. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of a coherent detection system provided in an embodiment of this application;

[0028] Figure 2 A schematic diagram of a balanced detection module in a coherent detection system provided in an embodiment of this application;

[0029] Figure 3 A schematic diagram of a 90° frequency mixer provided for an embodiment of this application;

[0030] Figure 4 A schematic diagram of a dual-balanced detection module in a coherent detection system provided in an embodiment of this application;

[0031] Figure 5 A schematic diagram of a delay accumulation module provided in an embodiment of this application;

[0032] Figure 6 A schematic diagram of a delay accumulation module provided in an embodiment of this application;

[0033] Figure 7 This is a schematic diagram of a detection system provided in an embodiment of this application. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0035] Figure 1 A schematic diagram of a coherent detection system provided in this application embodiment; including a light source system for providing light that meets detection requirements and outputting a modulated carrier optical signal; a beam splitter for splitting the light into two paths, the first path serving as a local optical input coupler and the second path as signal light for detection; and a circulator, such as... Figure 1 As shown, light entering from channel 1 can only enter through channel 2, and light entering from channel 2 can only exit from channel 3; the transceiver system is as follows. Figure 1 As shown, a coupler is used to receive the echo signal from a target at a distance L. The coupler is used to mix the intrinsic light and the signal light and output them in a 50:50 ratio. A balanced detector is used to detect coherent optical signals and convert them into electrical signals. The processing unit has functions such as amplifying, storing, calculating, transmitting data, and outputting detection results for the electrical signals.

[0036] Figure 2 This is a schematic diagram of a balanced detection module in a coherent detection system provided in an embodiment of this application. Figure 2 The balance detection module shown includes detection photodiodes 201 and 202, which, after... Figure 1 The coupler shown splits the optical signal into two optical signals, I0 and I1, with a phase difference of 180 degrees. 180 The photodiodes 201 and 202 detect I0 and I1 respectively. 180 The input current i1 of the high-speed transimpedance amplifier (TIA) is the current I0 on the probe photodiode 201 and the current I on the probe photodiode 202. 180 .

[0037] Let I0 = A + cos(wt + q) (1)

[0038] I 180 =A + cos(wt + q + 180°) (2)

[0039] i1 represents I0 and I 180 The DC component is eliminated by subtraction, and the output photocurrent i1 contains no DC current. Its amplitude is twice that of the photocurrent generated by a single photodiode. The voltage signal U1 is generated by the transimpedance amplifier (TIA) 204. The output voltage generated by the TIA is proportional to the difference in photocurrent between the two detection photodiodes, which is also proportional to the difference in the light input signal.

[0040] The balanced detector also has two high-speed output monitoring outputs, M1+ and M1-, used to monitor the optical input power level of each photodiode. I0 and I 180 The signals pass through resistors and then through operational amplifiers 203 and 205 to form outputs M1+ and M1-. These outputs are low-frequency outputs and cannot be used to measure the RF modulation of signals.

[0041] Figure 3 A schematic diagram of a 90° mixer provided as an embodiment of this application; as shown Figure 3 Due to the unique operating environment of the 90° optical mixer shown, the 90° optical mixer with a bulk optical structure is widely used in inter-satellite optical communication, and the technology is mature. Its structure is as follows: Figure 3As shown. The basic working principle of the 90° optical mixer is as follows: After the signal light and the local oscillator light pass through the polarizer, they enter the 90° optical mixer from the two ports as linearly polarized light. Number 1 represents a quarter-wave plate with the fast axis direction making an angle of 45° with the polarization direction of the incident local oscillator light. After passing through the first wave plate, the local oscillator light becomes circularly polarized light. Numbers 2 and 4 represent polarizing beam splitters (PBS) that allow only the P-light component to be transmitted and only the S-light component to be reflected, thus separating the P-light and S-light. Number 3 represents a half-wave plate with the fast axis direction making an angle of -22.5° with the polarization direction of the P(S) light. After passing through the second polarizing beam splitter, the linearly polarized light passes through the third wave plate, and the polarization direction makes an angle of 45° with the P-light. Finally, after passing through the fourth polarizing beam splitter, four output lights with relative phase differences of 0°, 90°, 180° and 270° are obtained. It can be seen that the essence of the 90° optical mixer is to use PBS to separate the P-light and S-light, and then use a waveplate with an appropriate deflection angle to rotate the phase of the beam polarization state and generate a 90° delay, so that the relative phase difference of the four output beams is 0°, 180°, 90° and 270°.

[0042] Figure 4 This is a schematic diagram of a dual-balanced detection module in a coherent detection system provided in an embodiment of this application. Figure 4 As shown, 401 and Figure 2 The principle of balanced detection shown is similar and will not be elaborated here. I0 and I 180 The light is directed onto two photodiodes, and the output photocurrent i1 contains no DC current and its amplitude is twice that of the photocurrent generated by a single photodiode. This photocurrent is then amplified by a transimpedance amplifier (TIA) to form the first output voltage signal U1. 402 and Figure 2 The principle of balanced detection shown is similar, except that the phases of the two photocurrents are 90° and 270°, respectively. 90 with I 270 The light is directed onto two photodiodes, and the output photocurrent i2 contains no DC current and its amplitude is twice that of the photocurrent generated by a single photodiode. This photocurrent is then amplified by a transimpedance amplifier (TIA) to form a second output voltage signal U2. In circuit 403, the first input M1+ and the second input M1- have the same DC current but a 180° phase difference between their AC currents and are capable of responding to low-frequency signals. After passing through a subtractor, M1+ and M1- retain only the AC current to form the third output M1, assuming that the intermediate frequency (IF) is now a high-frequency signal.

[0043] Similarly, in 404, the third input M2+ and the fourth input M2- have the same DC quantity, the AC quantity has a phase difference of 180°, and can respond to low frequency signals; after M2+ and M2- pass through the subtractor, only the AC quantity is retained to form the fourth output M2, assuming that the intermediate frequency is a high frequency signal at this time.

[0044] Assuming the intermediate frequency (IF) is a high-frequency signal, then the fifth output voltage U′1 in 403 is U1 - M1, and the sixth output voltage U′2 in 404 is U2 - M2. Therefore, the fifth output U′1 and the sixth output U′2, relative to the first output U1 and the second output U2, can suppress or eliminate part or all of the low-frequency interference signals or noise.

[0045] Assuming the intermediate frequency is a low-frequency signal, then U′1=U1+M1, U′2=U2+M2. Therefore, U′1 and U′2 can improve signal utilization and increase signal-to-noise ratio compared to U1 and U2.

[0046] Figure 5 This is a schematic diagram of a delay accumulation module provided in an embodiment of this application. Figure 5 The delay accumulation module shown consists of at least one delay adder. In the delay adder, assuming the original signal is s0(t), it is divided into two equal paths. One path is delayed by a time τ, where τ should be less than 1 / 4 of the period (e.g., if the period is 1 second, the delay should be less than 0.25 seconds). The two signals are then combined through the adder to form a new signal s1(t). The delay adder can reduce noise and improve the signal-to-noise ratio. Its effect is as follows... Figure 6 As shown.

[0047] Figure 6 This is a schematic diagram of a delay accumulation module provided in an embodiment of this application. Figure 6 The original signal is s0(t), which is divided into two equal paths. One path is delayed by time τ, where τ is less than 1 / 4 of the period (e.g., if the period is 1 second, the delay should be less than 0.25 seconds). The two signals are then combined using an adder to form a new signal s1(t). Figure 6 It can be seen that the signal-to-noise ratio of the newly synthesized signal s1(t) is improved compared with that of the original signal s0(t).

[0048] Figure 7 This is a schematic diagram of a detection system provided in an embodiment of this application. Figure 7 As shown, the signal light E s If the light field returned via the channel has an arbitrary polarization state, and its amplitude is decomposed into two polarization states, x and y:

[0049]

[0050] Meanwhile, intrinsic light E L Polarization beam splitting modulation can also be divided into:

[0051]

[0052] In the mixer module, E sCoherent coupling is performed with intrinsic light of the same polarization direction, and the optical frequency and phase are modulated into four outputs with a phase difference of 90°. In this embodiment, for illustrative purposes, 0°, 90°, 180°, and 270° are used as the four outputs; I0, I... 90 I 180 I 270 This indicates that I0 and I are obviously 180 The DC components are equal, while the AC components have a 180° phase difference; similarly, I 90 and I 270 The DC sections are equal, while the AC sections are 180° apart.

[0053] Send the four optical signals to, for example Figure 4 The balance detection module shown obtains U′1 and U′2, and then passes U′1 and U′2 respectively through... Figure 5 The delay accumulation module shown is illustrated. The ADC samples the two signals separately. Since the two signals differ only by 90° in phase, forming I / Q signals, the digital processing module can synthesize the two signals into a complex signal and synchronously demodulate the phase. The complex signal has a higher signal-to-noise ratio than the single-channel signal. (The text then repeats itself, so the translation stops.) Figure 7 The coherent detection system shown can obtain signals with a higher signal-to-noise ratio. Figure 7 The 90° mixer module shown uses 0°, 90°, 180°, and 270° as four outputs for illustrative purposes. In practical applications, any four-phase output with a 90° phase difference can be used, such as 30°, 120°, 210°, and 300°. The principle and effect are similar to 0°, 90°, 180°, and 270°, so they will not be elaborated further here.

[0054] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0055] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application. It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need further definition and explanation in subsequent figures. The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A coherent detection system, characterized in that, include: The system includes a mixing module and a balanced detection module; wherein the mixing module is used to couple the signal light with the local oscillator light; the balanced detection module is used to eliminate the DC component of the signal and improve the signal-to-noise ratio; the balanced detection module includes: a first input electrical signal, a second input electrical signal, and a first output voltage signal; The circuit of the balance detection module includes: a first photodiode, a second photodiode, a first operational amplifier, a first resistor, a second operational amplifier, a second resistor, and a first transimpedance amplifier. The cathode of the first photodiode is connected to one end of the first resistor, and the two ends of the first resistor are respectively connected to the two input terminals of the first operational amplifier. The anode of the second photodiode is connected to one end of the second resistor, and the two ends of the second resistor are respectively connected to the two input terminals of the second operational amplifier. The anode of the first photodiode and the cathode of the second photodiode are respectively connected to the input terminal of the first transimpedance amplifier. I0 and I 180 Two optical signals with a phase difference of 180° are used. I0 illuminates the first photodiode, passes through the first resistor, and then passes through the first operational amplifier to obtain the first input electrical signal. I 180 The light is shone onto the second photodiode, passes through the second resistor, and then through the second operational amplifier to obtain the second input electrical signal; I0 irradiates the first photodiode and I 180 The output current illuminating the second photodiode is used to obtain a first output voltage signal through the first transimpedance amplifier. The first input electrical signal and the second input electrical signal are used to obtain a third output signal. The first output voltage signal and the third output signal are used to obtain a fifth output electrical signal.

2. The coherent detection system according to claim 1, characterized in that, The coherent detection system also includes a delay accumulation module, which is used to delay the original signal for a certain period of time before adding it to the original signal.

3. The coherent detection system according to claim 2, characterized in that, The delay time is less than 1 / 4 of the original signal period.

4. The coherent detection system according to claim 1, characterized in that, The phase difference between the AC quantity of the first input electrical signal and the AC quantity of the second input electrical signal is 180°.

5. The coherent detection system according to claim 1, characterized in that, The balance detection module includes: a third input electrical signal, a fourth input electrical signal, and a second output voltage signal; The circuit of the balanced detection module includes: a third photodiode, a fourth photodiode, a third operational amplifier, a third resistor, a fourth operational amplifier, a fourth resistor, and a second transimpedance amplifier. The cathode of the third photodiode is connected to one end of the third resistor, and the two ends of the third resistor are respectively connected to the two input terminals of the third operational amplifier. The anode of the fourth photodiode is connected to one end of the fourth resistor, and the two ends of the fourth resistor are respectively connected to the two input terminals of the fourth operational amplifier. The anode of the third photodiode and the cathode of the fourth photodiode are respectively connected to the input terminal of the second transimpedance amplifier. I 90 and I 270 For two optical signals with a phase difference of 180°, I 90 The light is shone onto the third photodiode, passes through the third resistor, and then passes through the third operational amplifier to obtain the third input electrical signal; I 270 The light shines onto the fourth photodiode, passes through the fourth resistor, and then through the fourth operational amplifier to obtain the fourth input electrical signal; I 90 Irradiation on the third photodiode and I 270 The output current illuminating the fourth photodiode is used to obtain a second output voltage signal through the second transimpedance amplifier. The third input electrical signal and the fourth input electrical signal are used to obtain a fourth output electrical signal. The second output voltage signal and the fourth output electrical signal are used to obtain a sixth output electrical signal.

6. The coherent detection system according to claim 1, characterized in that, The output of the mixer module is a four-channel output with a phase difference of 90°.

Images