All-fiber type isolated circular polarizer and preparation method thereof

Abstract

The invention discloses an all-fiber type isolated circular polarizer and a preparation method thereof. The all-fiber type isolation circular polarizer comprises three parts, namely a fiber type polarizer, a fiber type lambda / 4 wave plate and a holding fiber, the fiber type polarizer and the fiber type lambda / 4 wave plate with a certain length are connected in a manner that an axial included angle is 45 degrees, the lambda / 4 wave plate is prepared from novel high-birefringence crystal derived fibers, and the holding fiber is made of high-birefringence crystal derived fibers. The optical fiber type polarizer is composed of a single-polarization optical fiber, a D-type optical fiber, a 45-degree inclined grating or any optical fiber type device with a polarization effect. The all-fiber device which is manufactured based on an improved welding mode and integrates circular polarization and isolation characteristics has the advantages of high temperature resistance, ultra wide band, high performance, low cost, compactness, easiness in integration and the like; the method can be applied to the fields of optical fiber gyroscopes, AR / VR equipment, optical fiber current sensors, high-power laser, optical fiber communication, medical imaging, quantum communication, material structure analysis and measurement and the like.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing and communication technology, and more particularly to an all-optical fiber type isolated circular polarizer and a preparation method thereof. Background Art

[0002] With the development of fiber-optic sensing technology and fiber-optic communication systems, the polarization state of light has become very important to systems or components. Polarized light is mainly divided into linearly polarized light, elliptically polarized light, and circularly polarized light. Circularly polarized light plays an important role in applications such as current sensing, polarization imaging, and medical imaging. Circular polarizers are used to adjust and control the polarization state of light, so that any input non-polarized light is converted into circularly polarized light with a high degree of polarization. Traditionally, circular polarizers are mainly divided into two types: wave plate type and fiber type. Wave plate type circular polarizers are generally composed of a polarization beam splitter prism and a wave plate type λ / 4 wave plate. All-fiber circular polarizers are usually composed of fiber-type components, such as a fiber-type polarizer and a fiber-type λ / 4 wave plate.

[0003] Previously, patent authorization announcement number CN102879913A designed a rotationally adjustable circular polarizer consisting of a linear polarizer and a wave plate. This design has a certain degree of flexibility and can produce an isolation effect. However, this wave plate-type device is difficult to integrate, has high losses, and suffers from severe end-face reflections, especially a narrow isolation bandwidth. Patent authorization announcement number CN220171341U, on the other hand, prepares a composite circular polarizer that uses a structural design that combines optical fiber components with wave plate components to achieve the required polarization control function. This structure has higher integration and reliability, but there is still significant loss in the connection area between the optical fiber and the wave plate, and the fiber coupling efficiency is low. In addition, patent CN101625441B of the University of Science and Technology of China discloses a method for preparing an all-fiber circular polarizer based on melt twisting. By twisting the optical fiber in a molten state, the additional loss generated during the light source coupling from the optical fiber to the wave plate is avoided, thereby improving the device preparation accuracy. However, this method requires high control accuracy of the melting temperature and the optical fiber twisting rate, is complex to prepare, and is prone to introducing mode coupling losses. Moreover, such devices generally do not have an isolation function. In addition, in the application document with patent application number 202410555879X, Shanghai University has designed and prepared in detail an all-fiber λ / 4 wave plate. This device can have good polarization conversion characteristics and temperature sensitivity in the temperature range of -5 to 200°C, and output high-quality circularly polarized light. However, this all-fiber λ / 4 wave plate has strict requirements on the polarization state of the input light and is only suitable for converting linearly polarized light into circularly polarized light.

[0004] Therefore, existing waveplate circular polarizers generally suffer from issues such as difficulty in integration, high insertion loss, susceptibility to end-face reflections, and narrow isolation bandwidth. Composite circular polarizers suffer from low fiber coupling efficiency and limited application range. All-fiber circular polarizers require high process stability and precise control of melting temperature and torsion parameters, making their fabrication more challenging. These existing circular polarization devices struggle to balance high performance with ease of integration, limiting their development and application in the field of high-performance optical devices. Summary of the Invention

[0005] In response to the above problems, the present invention provides an all-fiber isolated circular polarizer that is easy to integrate, low-cost, low-loss, and high-performance. The device consists of a fiber-type polarizer, a fiber-type λ / 4 wave plate, and a holding fiber. It can not only convert any unpolarized natural light into high-quality circularly polarized light, but also has broadband isolation characteristics. In addition, the device is prepared using an improved fusion method, which effectively improves the optical performance and preparation reliability of the device and has good application prospects. The all-fiber isolated circular polarizer is of great significance to the application development of fiber optic gyroscopes, AR / VR devices, fiber optic current sensors, high-power lasers, fiber optic communications, medical imaging, quantum communications, and material structure analysis and measurement.

[0006] In order to achieve the above objectives, the technical solution adopted by the present invention is:

[0007] An all-fiber isolated circular polarizer, which consists of a fiber polarizer, a fiber λ / 4 wave plate and a birefringence index less than 1.0×10 -7 The fiber-optic polarizer and the fiber-optic λ / 4 wave plate are connected at an axial angle of 45°. The fiber-optic λ / 4 wave plate and the holding fiber are connected in a conventional manner. The fiber-optic polarizer can convert any unpolarized light into linearly polarized light. The fiber-optic λ / 4 wave plate is used to convert linearly polarized light into circularly polarized light. The holding fiber is used to maintain the output of circularly polarized light. The connection between different fiber-optic components can be achieved through fusion splicing, physical connection, mechanical bonding, adhesive bonding, thermal compression bonding, laser welding, ultrasonic welding, or chemical bonding.

[0008] The fiber-type polarizer is a device that achieves single-polarization transmission by guiding the transmission of one polarization mode while the other polarization mode disappears. The fiber-type polarizer can be a single-polarization fiber, a D-type fiber, a 45° tilted grating, or any fiber-type device capable of polarizing. The polarization bandwidth of the fiber-type polarizer is 0-250nm. The maintaining fiber is an optical fiber that can maintain circularly polarized light output and can be a low-birefringence single-mode fiber, an ultra-low-birefringence fiber, a spun fiber, a twisted fiber, or a circular-preserving fiber. The pitch of the spun fiber or twisted fiber is 0.2-20.0mm.

[0009] Secondly, the fiber-type λ / 4 wave plate is made of crystal-derived optical fiber whose core is rare earth ion-doped YAG or YAP crystal material. This optical fiber is a new type of high birefringence optical fiber with a birefringence index greater than 1×10 -5 .

[0010] The specific preparation steps of crystal-derived optical fiber include: Step 1, prepare high-quality rare earth ion-doped YAG crystal rods and high-purity quartz sleeves, grind and polish them to suitable sizes, and then clean them with hydrofluoric acid; Step 2, use hydrofluoric acid to clean the polished YAG crystal rods and quartz sleeves respectively until their surfaces are smooth; Step 3, place the rare earth ion-doped YAG crystal rods into pure quartz sleeves to form optical fiber preforms, and then load the prepared preforms into the fiber laser drawing machine, whose output power is 0-400W and the heating temperature zone length is about The fiber is 5-20mm thick, and the temperature of the fiber drawing tower is controlled between 1800-2400℃. The drawing furnace can also use a graphite heating drawing furnace or a resistance heating drawing furnace to accurately control the heating temperature zone; Step 4: Under negative pressure, light passes through the lens group to form a light spot and converges on the preform rod. During the drawing process, laser heating and rapid cooling cause a temperature difference between the inside and outside of the optical fiber. Therefore, residual stress and thermal gradient effect will occur inside the optical fiber, which will cause the anisotropy of the optical fiber to increase, and the birefringence effect of the optical fiber will increase significantly, and finally a high birefringence optical fiber will be prepared. The YAG in the rare earth ion doped YAG crystal rod of crystal-derived optical fiber is called yttrium aluminum garnet, and its chemical formula is Y3Al5O 12 The doped rare earth ions are one or more of terbium, cerium, erbium, thulium, and ytterbium co-doped with each other, and are suitable for any wavelength except the absorption band.

[0011] The drawn fiber core diameter is 2.0-80.0μm, the cladding diameter is 40.0-800.0μm, and the birefringence coefficient is greater than 1.0×10 -5 , with beat lengths ranging from millimeters to centimeters. The λ / 4 wave plate fabricated from crystal-derived fiber exhibits excellent thermal stability over a temperature range of -5°C to 200°C, and the polarization state of the output light is insensitive to temperature changes. This all-fiber λ / 4 wave plate can be used in applications such as fiber optic gyroscopes in unpolarized light environments, AR / VR devices, fiber optic current sensors, high-power lasers, fiber optic communications, medical imaging, quantum communications, and material structure analysis and measurement.

[0012] A method for preparing an all-fiber isolated circular polarizer, the steps of which are as follows:

[0013] (1) Determine the beat length of the crystal-derived fiber and calculate the length required to make a λ / 4 wave plate from the fiber, and cut the crystal-derived fiber to one-quarter of its beat length;

[0014] (2) connecting the fiber-type polarizer to the light source, connecting the output end of the fiber-type polarizer to the crystal-derived optical fiber, and observing the polarization state of the output light of the optical fiber;

[0015] (3) When the light source is turned on, rotate the crystal-derived optical fiber and observe the polarization state of the output light in real time until the polarization state of the output light reaches the best circular polarization state, and then fix the two together to complete the connection with an axial angle of 45°.

[0016] The all-fiber isolated circular polarizer prepared by the above operation process has excellent circular polarization effect. The output circularly polarized light has an ellipticity of 40-45°, a polarization degree of 90%-100%, an extinction ratio of less than 1.0dB, and an isolation bandwidth of 0-200nm with an isolation degree exceeding 30dB. The all-fiber isolated circular polarizer has an ellipticity variance of less than 0.5 within the polarization angle range of 0 to 360° for the input linearly polarized light. It is polarization-insensitive and insensitive to changes in the input polarization angle. It can be applied to fiber optic gyroscopes, AR / VR devices, fiber optic current sensors, high-power lasers, fiber optic communications, medical imaging, quantum communications, and material structure analysis and measurement in non-polarized light environments.

[0017] The all-fiber isolated circular polarization device prepared by the present invention integrates circular polarization and isolation functions, has the advantages of easy integration and assembly, strong anti-interference ability, stable optical performance, and low loss, and can be applied to technical fields such as optical fiber communications and optical fiber lasers under high-power backgrounds.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) The fiber-type isolated circular polarizer produced by the present invention has the characteristics of high temperature resistance, small size, easy integration and coupling, and low loss.

[0020] (2) The all-fiber isolated circular polarizer produced by the present invention can not only circularly polarize, but also achieve ultra-wideband isolation effect.

[0021] (3) The all-fiber isolated circular polarizer manufactured by the present invention has an all-fiber structure and can be directly connected to the laser system without the need for additional alignment, which can greatly improve the construction efficiency of the polarization-sensitive optical system.

[0022] (4) The all-fiber isolated circular polarizer of the present invention is easy to prepare, low in cost, simple in structure, suitable for use in harsh environments, has a wide range of applications, and has good application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of an all-fiber type isolated circular polarizer prepared based on a single polarization optical fiber according to the present invention.

[0024] Figure 2 This is a graph showing the isolation curve of the all-fiber type isolating circular polarizer tested based on single polarization optical fiber.

[0025] Figure 3 It is a schematic diagram of the all-fiber type isolated circular polarizer prepared based on D-type optical fiber of the present invention.

[0026] Figure 4 This is a schematic diagram of an all-fiber isolated circular polarizer prepared based on a 45° tilted fiber Bragg grating according to the present invention. DETAILED DESCRIPTION

[0027] The present invention will be described in more detail below with reference to the accompanying drawings and embodiments of the present invention.

[0028] Example 1

[0029] like Figure 1 As shown, this is a specific embodiment of the first all-fiber isolated circular polarizer provided by the present invention.

[0030] The all-fiber isolated circular polarization device consists of a single-polarization fiber 1, a λ / 4 wave plate 3, and a holding fiber 5. The input end of the single-polarization fiber 1 serves as the device's input. The output end of the single-polarization fiber 1 is connected to the input end of the λ / 4 wave plate 3 at a 45° axial angle via a connection point 2. The output end of the λ / 4 wave plate 3 is connected to the input end of the holding fiber 5 using a conventional connection method, including fusion splicing, physical connection, and mechanical bonding. The output end of the holding fiber 5 serves as the device's output.

[0031] The fiber-type λ / 4 wave plate is made of crystal-derived optical fiber whose core is rare earth ion-doped YAG or YAP crystal material. This optical fiber is a new type of high birefringence optical fiber with a birefringence index greater than 1×10 -5The specific preparation steps of crystal-derived optical fiber include: Step 1, prepare high-quality rare earth ion-doped YAG crystal rods and high-purity quartz sleeves, grind and polish them to suitable sizes, and then clean them with hydrofluoric acid; Step 2, use hydrofluoric acid to clean the polished YAG crystal rods and quartz sleeves respectively until their surfaces are smooth; Step 3, place the rare earth ion-doped YAG crystal rods into pure quartz sleeves to form optical fiber preforms, and then load the prepared preforms into the optical fiber laser drawing machine, whose output power is 0-400W and the heating temperature zone length is about The fiber is 5-20mm thick, and the temperature of the fiber drawing tower is controlled between 1800-2400℃. The drawing furnace can also use a graphite heating drawing furnace or a resistance heating drawing furnace to accurately control the heating temperature zone; Step 4: Under negative pressure, light passes through the lens group to form a light spot and converges on the preform rod. During the drawing process, laser heating and rapid cooling cause a temperature difference between the inside and outside of the optical fiber. Therefore, residual stress and thermal gradient effect will occur inside the optical fiber, which will cause the anisotropy of the optical fiber to increase, and the birefringence effect of the optical fiber will increase significantly, and finally a high birefringence optical fiber will be prepared. The YAG in the rare earth ion doped YAG crystal rod of crystal-derived optical fiber is called yttrium aluminum garnet, and its chemical formula is Y3Al5O 12 The doped rare earth ions are one or more of terbium, cerium, erbium, thulium, and ytterbium co-doped with each other, and are suitable for any wavelength except the absorption band.

[0032] The preparation method of the all-fiber isolated circular polarizer is as follows:

[0033] (1) Determine the beat length of the crystal-derived fiber and calculate the length required to make a λ / 4 wave plate from the fiber, and cut the crystal-derived fiber to one-quarter of its beat length;

[0034] (2) connecting the fiber-type polarizer to the light source, connecting the output end of the fiber-type polarizer to the crystal-derived optical fiber, and observing the polarization state of the output light of the optical fiber;

[0035] (3) When the light source is turned on, rotate the crystal-derived optical fiber and observe the polarization state of the output light in real time until the polarization state of the output light reaches the best circular polarization state, and then fix the two together to complete the connection with an axial angle of 45°.

[0036] The isolation test system based on unidirectional transmission was used to test the isolation of the all-fiber type isolation circular polarizer. The isolation calculation formula is shown in formula (1-1).

[0037]

[0038] in, Represents the reverse input optical power, Represents the reverse output optical power. The isolation curve of the sample is obtained by calculation, as shown in Figure 2 As shown, the all-fiber isolating circular polarizer can achieve an isolation of more than 30 dB within the wavelength range of 1550±50 nm, and has ultra-wideband isolation characteristics.

[0039] At the same time, the polarization sensitivity test experiment of the prepared all-fiber isolated circular polarizer was carried out. By changing the polarization direction of the input light, the polarization characteristics of the sample output light were analyzed, including the ellipticity (Ellipticity), extinction ratio (PER) and degree of polarization (DOP) of the output light. During the experimental test, the polarization characteristics of the sample were tested by adjusting the direction of the linearly polarized light in the range of 0° to 360°. The direction of the incident linear polarization light was changed at intervals of 40°, and the changes in the polarization state output from the all-fiber isolated circular polarizer were observed and recorded in real time, as shown in Table 1. The ellipticity of the sample output light was 40-45°, the extinction ratio was less than 1.0dB, the degree of polarization was 90-100%, and the variance of the three was less than 0.5.

[0040] In addition, the sample was tested for reverse polarization and isolation properties. Table 2 shows the test data. When the reverse input light is unpolarized natural light or linearly polarized light, the reverse output light is linear light with a loss of less than 1.2dB. When the reverse input light is right-handed or left-handed circularly polarized light, the sample's insertion loss reaches 39.3dB. This value can be used as an isolation indicator for the sample, indicating that the device has a significant isolation effect.

[0041] Table 1 Test data of all-fiber isolated circular polarizer based on single polarization fiber

[0042]

[0043]

[0044] Table 2 Reverse test data of all-fiber isolated circular polarizer prepared based on single polarization fiber

[0045]

[0046] As a result, the sample exhibits excellent polarization insensitivity, being insensitive to changes in the input polarization angle. It can output stable, high-quality circularly polarized light even when the input polarization state changes. Furthermore, it exhibits significant isolation. This all-fiber isolated circular polarizer can be applied to fiber optic gyroscopes operating in unpolarized light environments, AR / VR devices, fiber optic current sensors, high-power lasers, fiber optic communications, medical imaging, quantum communications, and material structure analysis and measurement.

[0047] Example 2

[0048] like Figure 3 As shown, this is a specific embodiment of the second all-fiber isolated circular polarizer provided by the present invention.

[0049] Table 3 Test data of all-fiber isolated circular polarizer based on D-type fiber

[0050]

[0051] The all-fiber isolated circular polarization device consists of a D-type fiber 6, a λ / 4 wave plate 3, and a holding fiber 5. The input end of the D-type fiber 6 serves as the device's input. The output end of the D-type fiber 6 is connected to the input end of the λ / 4 wave plate 3 at an axial angle of 45° via a connection point 7. The output end of the λ / 4 wave plate 3 is conventionally connected to the input end of the holding fiber 5, using a fusion splicer, physical connection, or mechanical bonding. The output end of the holding fiber 5 serves as the device's output. The polarization sensitivity of the prepared all-fiber isolated circular polarization device was tested. The direction of linearly polarized light was adjusted in 40° intervals from 0° to 360° to test the polarization characteristics of the sample. Table 3 shows the changes in the output polarization state when the polarization state of the incident light is changed.

[0052] Example 3

[0053] like Figure 4 As shown, this is a specific embodiment of the third all-fiber isolated circular polarizer provided by the present invention.

[0054] The all-fiber isolated circular polarization device consists of a 45° tilted fiber Bragg grating (FBG) 8, a λ / 4 wave plate 3, and a holding fiber 5. The input end of the tilted fiber Bragg grating 8 serves as the input end of the device. The output end of the tilted fiber Bragg grating 8 is connected to the input end of the λ / 4 wave plate 3 at an axial angle of 45° via a connection point 9, while the output end of the λ / 4 wave plate 3 is conventionally connected to the input end of the holding fiber 5. The connection method includes one of fusion splicing, physical connection, and mechanical bonding. The output end of the holding fiber 5 serves as the output end of the device. The polarization sensitivity characteristics of the prepared all-fiber isolated circular polarization device were tested. The direction of the linearly polarized light was adjusted in intervals of 40° within the range of 0° to 360° to test the polarization characteristics of the sample. The change in the output polarization state when the polarization state of the incident light was changed is shown in Table 4.

[0055] Table 4 Test data of all-fiber isolated circular polarizer based on 45° tilted fiber Bragg grating

[0056]

Claims

1. An all-fiber isolated circular polarizer, characterized in that: Including fiber-type polarizer, fiber-type λ / 4 wave plate and holding fiber, The optical fiber type polarizer is connected to the optical fiber type λ / 4 wave plate at an axial angle of 45 degrees. The optical fiber type λ / 4 wave plate is connected to the maintaining optical fiber; The optical fiber type polarizer is used to convert any unpolarized light into linearly polarized light; The optical fiber type λ / 4 wave plate is used to convert linearly polarized light into circularly polarized light; The maintaining optical fiber is used for maintaining the output of polarized light.

2. The all-fiber isolated circular polarizer according to claim 1, characterized in that: The all-fiber isolated circular polarizer can convert any unpolarized light into circularly polarized light and has an isolation function. The fiber-type isolated circular polarizer is used in fiber-optic gyroscopes, AR / VR devices, fiber-optic current sensors, high-power lasers, fiber-optic communications, medical imaging, quantum communications, and material structure analysis and measurement.

3. The all-fiber isolated circular polarizer according to claim 1, wherein: The connection method between different optical fiber components is any one of fusion splicing, physical connection, mechanical bonding, adhesive bonding, hot pressing bonding, laser welding, ultrasonic welding or chemical bonding.

4. The all-fiber isolated circular polarizer according to claim 1, wherein: The fiber-type polarizer is a device that completes single polarization transmission by guiding one polarization mode to transmit while another polarization mode disappears. The fiber-type polarizer is a single polarization fiber, a D-type fiber, or a 45° tilted grating.

5. The all-fiber isolated circular polarizer according to claim 1, wherein: The polarization bandwidth of the optical fiber polarizer is 0-250nm.

6. The all-fiber isolated circular polarizer according to claim 1, wherein: The birefringence index of the optical fiber is kept below 1.0×10 -7 .

7. The all-fiber isolated circular polarizer according to claim 1, wherein: The maintaining optical fiber is an optical fiber that can maintain circularly polarized light output, and is a low-birefringence single-mode optical fiber, an ultra-low-birefringence optical fiber, a spun optical fiber, a twisted optical fiber or a circular-preserving optical fiber, and the pitch of the spun optical fiber or the twisted optical fiber is 0.2 to 20.0 mm.

8. The all-fiber isolated circular polarizer according to claim 1, wherein: The λ / 4 wave plate is prepared by cutting an optical fiber with a certain birefringence characteristic to a quarter of the length.

9. The all-fiber isolated circular polarizer according to claim 1, wherein: The optical fiber with certain birefringence characteristics has a birefringence coefficient greater than 1.0×10 -5 The rare earth ion crystal derived optical fiber has a core diameter of 2.0-80.0 μm and a cladding diameter of 40.0-800.0 μm.

10. The all-fiber isolated circular polarizer according to claim 9, characterized in that: The rare earth ion crystal is a YAG or YAP crystal, and the doped rare earth ions in the crystal are one or more of terbium, cerium, erbium, thulium, and ytterbium co-doped with each other.

11. The all-fiber isolated circular polarizer according to claim 10, characterized in that: The optical fiber-type λ / 4 wave plate prepared by the YAG crystal-derived optical fiber has good thermal stability within the temperature range of -5°C to 200°C, and the polarization state of the output light is insensitive to temperature changes.

12. A method for preparing an all-fiber isolated circular polarizer, characterized in that: The following steps are involved: (1) Determine the λ / 4 beat length of the crystal-derived fiber and terminate the crystal derivation at one quarter of its beat length; (2) connecting the fiber-type polarizer to the light source, connecting the output end of the fiber-type polarizer to the crystal-derived optical fiber, and observing the polarization state of the output light of the optical fiber; (3) When the light source is turned on, rotate the crystal-derived wave plate fiber and observe the polarization state of the output light in real time until the polarization state of the output light reaches the best circular polarization state. Then, fix the two together to complete the connection with an axial angle of 45°.

13. The method for preparing an all-fiber isolated circular polarizer according to claim 12, wherein: The circularly polarized light output by the all-fiber isolated circular polarizer has an ellipticity of 40-45°, a polarization degree of 90%-100%, and an extinction ratio of less than 1.0dB. When the polarization angle of the input linearly polarized light varies from 0 to 360°, the variance of the ellipticity change of the circularly polarized light output by the all-fiber isolated circular polarizer is less than 0.5, and is insensitive to changes in the input polarization angle. The isolation bandwidth of the all-fiber isolated circular polarizer, which exceeds 30dB, is 0-200nm.

14. The method for preparing an all-fiber isolated circular polarizer according to claim 12, characterized in that The specific preparation steps of crystal-derived optical fiber include: Step 1, preparing high-quality rare-earth ion-doped YAG crystal rods and high-purity quartz sleeves, grinding and polishing them to the appropriate size, and then cleaning them with hydrofluoric acid; Step 2, placing the rare-earth ion-doped YAG crystal rods into pure quartz sleeves to form optical fiber preforms, and then loading the prepared preforms into a fiber laser drawing machine for drawing.

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