Device for inhibiting influence of light polarization state on stimulated Raman signal

Abstract

The invention relates to a device for inhibiting the influence of a light polarization state on a stimulated Raman signal, which is characterized in that a passive depolarizer device is added to a pump light branch of a stimulated Raman gas detection system to reduce the degree of polarization of pump light, so that the influence of the light polarization state change caused by the light path state change on a stimulated Raman gain signal is reduced. According to the device for inhibiting the influence of the light polarization state on the stimulated Raman gain signal, a passive depolarizer device is constructed by adopting a passive optical device and a single-mode optical fiber, and the passive depolarizer device comprises a first polarization controller, a first polarization beam splitter, a single-mode delay optical fiber, a second polarization controller and a second polarization beam splitter. According to the invention, the environmental adaptability of the gas sensing system based on the stimulated Raman effect in a real measurement scene is improved, and the practicability of the gas sensor is accelerated. The polarization degree of the pump light is reduced, meanwhile, the system cost is not obviously increased, and compared with an optical fiber scrambler and a passive depolarizer achieved through polarization maintaining optical fibers, the passive depolarizer has the obvious cost advantage.

Description

Technical Field

[0001] This invention relates to a device for suppressing the influence of optical polarization state on stimulated Raman signals, belonging to the field of optical gas detection technology. Background Technology

[0002] Stimulated Raman (SRA) gas sensing systems offer advantages in detecting gases that lack infrared absorption characteristics, making them suitable for trace detection of such gases. However, the amplitude of the Raman signal is significantly affected by the relative polarization states of the pump and Stokes beams. In practical applications, changes in the state of the sensing optical path and temperature cannot guarantee that the polarization states of the two beams will remain unchanged. Traditional methods of manually adjusting polarization controllers in the laboratory to achieve optimal signal performance are impractical. Therefore, it is necessary to reduce the polarization degree of the pump beam to suppress the influence of the light polarization state on the Raman signal. Summary of the Invention

[0003] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a device for suppressing the influence of optical polarization state on stimulated Raman signals, reducing the influence of optical polarization state changes on stimulated Raman gain signals caused by changes in optical path state, reducing the cost of gas sensing systems based on stimulated Raman effect, and accelerating the practical application of such gas sensors.

[0004] The technical solution of the present invention is: a device for suppressing the influence of optical polarization state on stimulated Raman signal, comprising a Stokes laser, a detection probe, and a pump laser, a passive depolarizer, an optical amplifier, a first wavelength division multiplexer, a gas detection chamber, a second wavelength division multiplexer, a photodetector, and a modulation and demodulation module arranged sequentially along the optical path. Driven by the modulation and demodulation module, the pump laser emits pump light, and the Stokes laser emits Stokes light. The pump light passes through a passive depolarizer to reduce its polarization degree, then is amplified by an optical amplifier, and finally combined with the Stokes light through a first wavelength division multiplexer. The combined light enters the gas detection chamber, interacts with the gas to be measured through a detection probe, and the light output from the gas detection chamber is filtered out by a second wavelength division multiplexer to remove the Stokes light containing gas concentration information. The Stokes light is finally converted into an electrical signal by a photodetector and enters the modulation and demodulation module for signal demodulation. The modulation and demodulation module outputs a drive signal to control the pump laser and the Stokes laser.

[0005] Furthermore, both the pump light and the Stokes light are single-frequency continuous light, which interact with the gas to be tested in the gas detection chamber to generate a stimulated Raman scattering process. Part of the pump light is converted into Stokes light, which is a Raman gain signal related to the gas concentration.

[0006] Furthermore, both the pump laser and the Stokes laser are butterfly-packaged semiconductor distributed feedback lasers with a pigtail. The laser linewidth is approximately 5 MHz, and the frequency difference between the pump laser and the Stokes laser is equal to the rotational Raman transition frequency of hydrogen. The wavelength of the pump laser is selected as 15.638 nm, and the wavelength of the Stokes laser is selected as 1620.657 nm. The frequency difference between the two corresponds to the rotational Raman transition frequency of hydrogen molecule S0(0).

[0007] Furthermore, the optical amplifier is an erbium-doped fiber amplifier with an output optical power of 500mW.

[0008] Furthermore, the wavelengths of the transmission port and the reflection port of the first wavelength division multiplexer correspond to Stokes light and pump light, respectively. The two beams are input from the corresponding ports and then combined and output from the common port.

[0009] Furthermore, the passive depolarizer device includes a first polarization controller, a first polarization beam splitter, a single-mode delay fiber, a second polarization controller, and a second polarization beam splitter. The input light first passes through the first polarization controller and enters the input port of the first polarization beam splitter, where it is split into two perpendicularly polarized beams that are output from the two ports. One beam enters the first input port of the second polarization beam splitter, and the other beam passes through the single-mode delay fiber and then through the second polarization controller to enter the second input port of the second polarization beam splitter. The two beams are then combined and output from the output port of the second polarization beam splitter.

[0010] Furthermore, the first polarization controller and the second polarization controller are adjusted to make the optical power of the two paths that ultimately enter the input port of the second polarization beamsplitter equal: First, the first polarization controller is adjusted while simultaneously monitoring the optical power of the two output ports of the first polarization beamsplitter to make the two optical powers the same; then, the branch containing the single-mode delay fiber is connected, and the second polarization controller is adjusted while monitoring the optical power of the output port of the second polarization beamsplitter until the optical power is maximized, after which the second polarization controller is fixed; then, the two branches are connected to the two input ports of the second polarization beamsplitter respectively, and the optical power of the output port of the second polarization beamsplitter is measured respectively, and the first polarization controller is finely adjusted multiple times until the optical power of the output port of the second polarization beamsplitter is equal when the two branches are connected respectively.

[0011] Furthermore, the first polarization controller is a squeeze-type fiber polarization controller, used to adjust the polarization direction of the linearly polarized light output from the pump laser when it enters the input port of the first polarization beam splitter; the input port of the first polarization beam splitter is a single-mode fiber, and the output port is a polarization-maintaining fiber, both of which operate on the slow axis, used to split the input light into two beams of light with perpendicular polarization directions, which are output from the two output ports respectively; the operating bandwidth of the first polarization beam splitter covers the wavelength of the pump light.

[0012] Furthermore, the single-mode delay fiber is located on one of the branches between the first polarization beam splitter and the second polarization beam splitter. The fiber length is greater than the coherence length of the input light and is used to delay the light in this polarization branch, making the phase relationship between the two branches random. The length of the single-mode delay fiber is selected to be 250 meters.

[0013] Furthermore, the second polarization controller is located after the single-mode delay fiber and is used to adjust the polarization direction of the light after passing through the single-mode delay fiber so that it is aligned with the slow axis direction of the second input port of the second polarization beam splitter.

[0014] Furthermore, the second polarization beamsplitter is of the same model as the first polarization beamsplitter, but with the opposite light transmission direction. It is actually used as a polarization beam combiner, and the original output port becomes an input port.

[0015] The advantages of this invention compared to the prior art are: (1) This invention proposes a device to suppress the influence of optical polarization state on stimulated Raman gain signal, which reduces the influence of optical polarization state change on stimulated Raman gain signal caused by optical path state change, improves the environmental adaptability of gas sensing system based on stimulated Raman effect in real measurement scenarios, and accelerates the practical application of this type of gas sensor.

[0016] (2) This invention proposes a device to suppress the influence of optical polarization state on stimulated Raman gain signal. The device uses passive devices and single-mode optical fiber to construct a passive depolarizer, which reduces the polarization degree of pump light without significantly increasing the system cost. Compared with optical fiber polarization scramblers and passive depolarizers implemented using polarization-maintaining optical fibers, it has a significant cost advantage. Attached Figure Description

[0017] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 This is a block diagram of the experimental system for the method of suppressing the influence of optical polarization state on stimulated Raman gain signal according to the present invention; Figure 2 This is a schematic diagram of the device for suppressing the influence of optical polarization state on stimulated Raman gain signal according to the present invention. Detailed Implementation

[0018] To better understand the above technical solutions, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of the present invention, rather than limitations on the technical solutions of the present invention. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0019] The following description, in conjunction with the accompanying drawings, provides a more detailed explanation of an embodiment of the present invention for a device that suppresses the influence of optical polarization state on stimulated Raman signals. Specifically, the device may include: a Stokes laser 2, a detection probe 7, and, arranged sequentially along the optical path, a pump laser 1, a passive depolarizer 3, an optical amplifier 4, a first wavelength division multiplexer 5, a gas detection chamber 6, a second wavelength division multiplexer 8, a photodetector 9, and a modulation / demodulation module 10.

[0020] This invention proposes a device for suppressing the influence of optical polarization state on stimulated Raman gain signal, such as... Figure 1 As shown, driven by the modulation and demodulation module 10, the pump laser 1 emits pump light, and the Stokes laser 2 emits Stokes light. A passive depolarizer 3 is added to the pump light branch to reduce the polarization degree of the pump light. After passing through this device, the pump light is amplified by the optical amplifier 4, and then combined with the Stokes light by the first wavelength division multiplexer 5. The combined light enters the gas detection chamber 6 and interacts with the gas to be measured by the detection probe 7. The light output from the gas detection chamber is filtered out by the second wavelength division multiplexer 8 to remove the Stokes light carrying gas concentration information. Finally, the Stokes light is converted into an electrical signal by the photodetector 9 and enters the modulation and demodulation module 10 for signal demodulation.

[0021] Both the pump light and the Stokes light are single-frequency continuous light. They interact with the gas to be tested in the gas detection chamber 6 to generate stimulated Raman scattering. Part of the pump light is converted into Stokes light, which is a Raman gain signal related to the gas concentration.

[0022] The pump light's polarization degree is reduced after passing through the passive depolarizer device, thus suppressing the influence of the light polarization state on the Raman gain signal amplitude during the stimulated Raman scattering process.

[0023] Hydrogen molecules are typical perfectly symmetrical molecules with no dipole moment change and are generally considered to have no infrared activity. In recent years, researchers have found several hydrogen absorption peaks in the infrared spectrum, but their absorption intensity is several orders of magnitude lower than that of other gases. Therefore, gas sensing systems based on the stimulated Raman effect are very suitable for hydrogen detection. The following example uses hydrogen detection as a specific illustration.

[0024] Specifically, both the pump laser 1 and the Stokes laser 2 are butterfly-packaged semiconductor distributed feedback (DFB) lasers with a pigtail. The laser linewidth is approximately 5 MHz, and the frequency difference between the pump light and the Stokes light must be equal to the rotational Raman transition frequency of hydrogen.

[0025] As a preferred option, the wavelength of pump laser 1 is selected as 1532.638nm, and the wavelength of Stokes laser 2 is selected as 1620.657nm. The frequency difference between the two corresponds to the rotational Raman transition frequency of hydrogen molecule S0(0).

[0026] Furthermore, the bandwidth of the optical fiber and optical devices in the passive depolarizer device 3 covers the pump light wavelength.

[0027] Optionally, the optical amplifier 4 is an erbium-doped fiber amplifier with an output optical power of 500mW.

[0028] In one possible implementation, the wavelengths of the transmission port and the reflection port of the first wavelength division multiplexer 5 correspond to Stokes light and pump light, respectively, and the two beams are input from the corresponding ports and then combined and output from the common port.

[0029] In one possible implementation, the gas detection chamber 6 is a sealed box with an inlet and an outlet, and can be made of metal or other easily processed materials. The detection probe 7 is placed in the center of the box, suspended in the middle, with optical fibers at both ends passing through pre-drilled holes in the box. The box is filled with the gas to be tested, enclosing the detection probe.

[0030] In one possible implementation, the detection probe 7 achieves mode field overlap between the pump light and the Stokes light and interacts with the gas to be measured. The detection probe can be made of hollow fiber or micro / nano fiber.

[0031] Preferably, the detection probe 7 is made of micro-nano fiber. Compared with hollow fiber, micro-nano fiber can limit the mode field diameter of light to a smaller size and the power density of pump light to a higher size, thereby improving the Raman gain signal amplitude.

[0032] Furthermore, the second wavelength division multiplexer 8 is the same model as the first wavelength division multiplexer 5, and the light output from the detection probe is input from the common port, while the Stokes light is filtered out and output from the transmission port.

[0033] In one possible implementation, the photodetector 9 converts the filtered Stokes light into an electrical signal, which is then input to the modulation and demodulation module 10 for signal demodulation. The modulation and demodulation module internally runs a lock-in amplifier algorithm to extract hydrogen concentration information from the signal.

[0034] like Figure 2The aforementioned passive depolarizer device 3 is a key component for reducing the polarization degree of the pump light, including a first polarization controller 31, a first polarization beam splitter 32, a single-mode delay fiber 33, a second polarization controller 34, and a second polarization beam splitter 35.

[0035] The input light first passes through the first polarization controller 31 and enters the input port of the first polarization beam splitter 32, where it is split into two perpendicularly polarized beams that are output from the two ports. One beam directly enters the first input port of the second polarization beam splitter 35, while the other beam passes through a single-mode delay fiber 33 and then through the second polarization controller 34 to enter the second input port of the second polarization beam splitter 35. The two beams are then combined and output from the output port of the second polarization beam splitter 35.

[0036] Furthermore, the first polarization controller 31 is a small squeeze-type fiber polarization controller, which is used to adjust the polarization direction of the linearly polarized light output from the pump laser 1 when it enters the input port of the first polarization beam splitter 32.

[0037] In one possible implementation, the first polarization beam splitter 32 has a single-mode fiber input port and a polarization-maintaining fiber output port, both operating on the slow axis. Its function is to split the input light into two beams of light with perpendicular polarization directions, which are output from the two output ports respectively.

[0038] Preferably, the operating bandwidth of the first polarization beam splitter 32 covers the pump light wavelength.

[0039] In one possible implementation, the single-mode delay fiber 33 is located on one of the branches between the first and second polarization beam splitters. The fiber length is much greater than the coherence length of the input light, and its function is to delay the light in this polarization branch, thereby making the phase relationship between the two branches random.

[0040] Preferably, the length of the single-mode delay fiber 33 is selected to be 250 meters, which is much greater than the coherence length of the pump light of about 40 meters.

[0041] In one possible implementation, the second polarization controller 34 is located after the single-mode delay fiber 33 and its function is to adjust the polarization direction of the light after passing through the single-mode delay fiber 33 so that it is aligned with the slow axis direction of the second input port of the second polarization beam splitter 35.

[0042] Furthermore, the second polarization beam splitter 35 is the same model as the first polarization beam splitter 32, but with opposite light transmission directions. It is actually used as a polarization beam combiner, and the original output port becomes an input port.

[0043] Furthermore, the first polarization controller 31 and the second polarization controller 34 are adjusted so that the optical power of the two paths that ultimately enter the input port of the second polarization beam splitter 35 is equal. Due to the presence of the single-mode delay fiber 33, the polarization degree of the output light obtained after combining the two orthogonally polarized, phase-independent beams is close to 0.

[0044] Specifically, firstly, the first polarization controller 31 is adjusted while simultaneously monitoring the optical power at the two output ports of the first polarization beamsplitter 32 to ensure that the two optical powers are equal. Then, the branch containing the single-mode delay fiber 33 is connected, and while monitoring the optical power at the output port of the second polarization beamsplitter 35, the second polarization controller 34 is adjusted until the optical power is maximized and then fixed. Next, the two branches are connected to the two input ports of the second polarization beamsplitter 35 respectively, and the optical power at the output port of the second polarization beamsplitter 35 is measured respectively. The first polarization controller 31 is fine-tuned multiple times until the optical power at the output port of the second polarization beamsplitter 35 is equal when the two branches are connected respectively.

[0045] This invention proposes a device to suppress the influence of optical polarization state on stimulated Raman gain signal. A passive depolarizer is added to the pump light branch of a stimulated Raman gas detection system to reduce the pump light polarization degree, thereby mitigating the impact of optical polarization state changes on the stimulated Raman gain signal caused by changes in the optical path state. The passive depolarizer device is constructed using passive optical components and single-mode fiber, offering significant cost advantages while reducing pump light polarization degree.

[0046] It should be noted that in this paper, relational terms such as first and second are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0047] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

[0048] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A device for suppressing the influence of optical polarization state on stimulated Raman signals, characterized in that, It includes a Stokes laser (2), a detection probe (7), and pump laser (1), passive depolarizer (3), optical amplifier (4), first wavelength division multiplexer (5), gas detection chamber (6), second wavelength division multiplexer (8), photodetector (9), and modulation / demodulation module (10) arranged sequentially along the optical path. Driven by the modulation and demodulation module (10), the pump laser (1) emits pump light and the Stokes laser (2) emits Stokes light. The pump light is reduced in polarization by a passive depolarizer (3), then amplified by an optical amplifier (4), and then combined with the Stokes light by a first wavelength division multiplexer (5). The combined light enters the gas detection chamber (6) and interacts with the gas to be tested through a detection probe (7). The light output from the gas detection chamber (6) is filtered out by a second wavelength division multiplexer (8) to remove the Stokes light carrying gas concentration information. The Stokes light is finally converted into an electrical signal by a photodetector (9) and enters the modulation and demodulation module (10) for signal demodulation. The modulation and demodulation module (10) outputs a driving signal to control the pump laser (1) and the Stokes laser (2).

2. The device for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, Both the pump light and the Stokes light are single-frequency continuous light. They interact with the gas to be tested in the gas detection chamber (6) to generate stimulated Raman scattering. Part of the pump light is converted into Stokes light, which is the Raman gain signal related to the gas concentration.

3. The device for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, Both the pump laser (1) and the Stokes laser (2) are butterfly-packaged semiconductor distributed feedback lasers with a pigtail. The laser linewidth is about 5MHz. The frequency difference between the pump laser and the Stokes laser is equal to the rotational Raman transition frequency of hydrogen. The wavelength of the pump laser (1) is 15 (32).638nm, and the wavelength of the Stokes laser (2) is 1620.657nm. The frequency difference between the two corresponds to the rotational Raman transition frequency of hydrogen molecule S0 (0).

4. The device for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The optical amplifier (4) is an erbium-doped fiber amplifier with an output optical power of 500mW.

5. The device for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The wavelengths of the transmission port and the reflection port of the first wavelength division multiplexer (5) correspond to Stokes light and pump light, respectively. The two beams are input from the corresponding ports and then output from the common port.

6. The apparatus for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The passive depolarizer device (3) includes a first polarization controller (31), a first polarization beam splitter (32), a single-mode delay fiber (33), a second polarization controller (34), and a second polarization beam splitter (35). The input light first passes through the first polarization controller (31) and enters the input port of the first polarization beam splitter (32), splitting into two perpendicularly polarized beams that are output from the two ports. One beam enters the first input port of the second polarization beam splitter (35), and the other beam passes through the single-mode delay fiber (33) and then through the second polarization controller (34) to enter the second input port of the second polarization beam splitter (35). The two beams are combined and output from the output port of the second polarization beam splitter (35).

7. The apparatus for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, Adjust the first polarization controller (31) and the second polarization controller (34) to make the optical power of the two paths that finally enter the input port of the second polarization beam splitter (35) equal: First, adjust the first polarization controller (31) and monitor the optical power of the two output ports of the first polarization beam splitter (32) to make the two optical powers the same; then connect the branch where the single-mode delay fiber (33) is located, monitor the optical power of the output port of the second polarization beam splitter (35) and adjust the second polarization controller (34) to make the optical power maximum and then fix the second polarization controller (34); then connect the two branches to the two input ports of the second polarization beam splitter (35) respectively, measure the optical power of the output port of the second polarization beam splitter (35) respectively, and fine-tune the first polarization controller (31) multiple times until the optical power of the output port of the second polarization beam splitter (35) is equal when the two branches are connected respectively.

8. The apparatus for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The first polarization controller (31) is a squeezed fiber polarization controller, used to adjust the polarization direction of the linearly polarized light output from the pump laser (1) when it enters the input port of the first polarization beam splitter (32); the input port of the first polarization beam splitter (32) is a single-mode fiber, and the output port is a polarization-maintaining fiber, both of which operate on the slow axis, used to split the input light into two beams of light with perpendicular polarization directions, which are output from the two output ports respectively; the operating bandwidth of the first polarization beam splitter (32) covers the wavelength of the pump light.

9. The device for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The single-mode delay fiber (33) is located on one of the branches between the first polarization beam splitter (31) and the second polarization beam splitter (32). The fiber length is greater than the coherence length of the input light and is used to delay the light in the polarization branch so that the phase relationship between the two branches becomes random. The length of the single-mode delay fiber (33) is selected to be 250 meters.

10. The apparatus for suppressing the influence of optical polarization state on stimulated Raman signals according to claim 1, characterized in that, The second polarization controller (34) is located after the single-mode delay fiber (33) and is used to adjust the polarization direction of the light after passing through the single-mode delay fiber (33) so that it is aligned with the slow axis direction of the second input port of the second polarization beam splitter (35). The second polarization beam splitter (35) is the same model as the first polarization beam splitter (32) but the light transmission direction is opposite. It is actually used as a polarization beam combiner, and the original output port becomes the input port.

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