All-optical photo-thermal polarization modulation system based on micro-nano optical fiber asymmetric mode field

Abstract

The invention discloses an all-optical photo-thermal polarization modulation system based on a micro-nano optical fiber asymmetric mode field, and solves the problems of slow response, large size, high power consumption and the like of traditional polarization modulation. The system comprises a micro-nano optical fiber with an asymmetric section, a micro-nano optical fiber with a deposited photo-thermal regulation and control layer, a control light source with the wavelength of 800-1100 nanometers, a signal light source with the wavelength of 1300-1600 nanometers and a detection module. The light is controlled to excite the photo-thermal effect, local temperature change is generated, birefringence of the micro-nano optical fiber is changed through the photo-thermal effect, dynamic modulation of the polarization angle of the signal light is achieved, and the response time is smaller than 50 microseconds. The method comprises the steps of preparing the micro-nano optical fiber, injecting control light to adjust birefringence, inputting signal light to realize polarization modulation, and detecting and optimizing output. The technology utilizes the synergistic effect of the asymmetric mode field and the photothermal effect, has the advantages of high efficiency, rapidness, low power consumption and high integration, is suitable for the fields of optical communication, optical sensing and quantum optics, and remarkably improves the polarization regulation and control performance.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and in particular to an all-optical photothermal polarization modulation system based on asymmetric mode fields of micro-nano optical fibers. Background Art

[0002] With the rapid development of optical communication, optical sensing and quantum optical technology, polarization state control, as the core technology of optical signal processing, has important application value in the fields of high-speed optical communication, polarization multiplexing, quantum key distribution, etc. Traditional polarization modulation methods mainly rely on electro-optical effects (such as electro-optical crystals or electro-optical modulators) and mechanical stress (such as fiber polarization controllers or piezoelectric devices). However, these methods have significant limitations: electro-optical modulators have slow response speeds (usually above milliseconds), high power consumption, and are sensitive to the environment (such as temperature, electromagnetic interference); mechanical modulators are difficult to meet the needs of high speed and integration due to their large size, easy wear of mechanical parts, and slow dynamic response.

[0003] In recent years, micro-nano optical fibers have gradually become a research hotspot in the field of optics due to their small size, high nonlinearity, low loss, and strong mode field confinement. Micro-nano optical fibers perform well in intensity modulation, phase modulation, and nonlinear optical applications, but direct regulation of polarization states still faces challenges. In existing technologies, polarization modulation based on micro-nano optical fibers mostly relies on external electric fields or mechanical stresses, and fails to fully utilize its asymmetric mode field characteristics for efficient all-optical regulation. In addition, the photothermal effect, as a physical mechanism of interaction between light and matter, has the advantages of rapid response and high sensitivity at the micro-nano scale, but is currently mainly used in fields such as photothermal therapy and thermal imaging, and has not yet been combined with the mode field characteristics of micro-nano optical fibers for polarization modulation.

[0004] Therefore, developing an all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber overcomes the shortcomings of slow response speed, large volume, high power consumption and poor integration of traditional methods, which has important scientific significance and application value. In view of the above problems, the present invention proposes an all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber, which realizes efficient, fast and low-power polarization control through the synergistic effect of photothermal effect and asymmetric mode field, providing a new technical path for optical communication and related fields. Summary of the invention

[0005] All-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber, including:

[0006] Micro-nano optical fiber has an asymmetric cross-sectional structure and supports asymmetric mode field distribution;

[0007] A photothermal control layer, deposited on the surface of the micro-nano optical fiber, is used to absorb the control light and generate a local thermal effect;

[0008] A control light source is provided to provide control light, which is injected into the micro-nano optical fiber through an optical fiber coupler to stimulate a photothermal effect;

[0009] A signal light source provides signal light to be modulated, which is transmitted along the micro-nano optical fiber and is affected by the photothermal effect to achieve polarization state regulation;

[0010] A detection module, used for real-time monitoring of polarization state changes of output signal light;

[0011] The photothermal effect is achieved by controlling the optical power P ctrl (unit: W) causes a local temperature change ΔT (unit: K), calculated as:

[0012]

[0013] Where α is the absorptivity of the photothermal control layer (dimensionless, range O-1), t is the photothermal action time (unit: s), k is the thermal conductivity (unit: W·m -1 ·K -1 ), A is the effective area of ​​the photothermal regulation layer (unit: m2), is the material density (unit: kg·m -3 ), c is the specific heat capacity (unit: J·kg -1 ·K -1 ) ; ΔT serves as the driving factor for the subsequent birefringence change Δn.

[0014] As a preferred technical solution of the present invention, the asymmetric cross-section of the micro-nano optical fiber is D-shaped or elliptical, and is prepared by a taper or chemical etching process. The asymmetry of its mode field leads to an effective refractive index difference Δn between the transverse electric mode (TE) and the transverse magnetic mode (TM). eff ,satisfy:

[0015] Δn eff =n TE -n TM ≥10 -4

[0016] Where n TE and n TM are the effective refractive indices of TE mode and TM mode respectively (dimensionless); Δn eff It is the initial condition for the superposition of photothermal effect and enhances the polarization modulation capability.

[0017] As a preferred technical solution of the present invention, the photothermal regulation layer is made of gold nanoparticles, carbon nanotubes or graphene, with a thickness range of 5-50 nanometers (unit: m), a light absorption rate α greater than 80% (dimensionless), and a thermal response time t res (Unit: s) is determined by the thermal diffusion characteristics and the calculation formula is:

[0018]

[0019] Where d is the thickness of the photothermal control layer (unit: m), κ is the thermal diffusion coefficient (unit: m2·s -1 ), and t res Less than 50 microseconds (5×10 -5 s), ensuring the rapid establishment of ΔT and providing a time basis for dynamic polarization modulation.

[0020] As a preferred technical solution of the present invention, the temperature change ΔT (unit: K) causes a birefringence change Δn (dimensionless) of the micro-nano optical fiber through the thermo-optical effect, and the calculation formula is:

[0021]

[0022] In the formula, is the thermo-optical coefficient of the micro-nano optical fiber material (unit: K -1 , typical value is 10 -6 -10 -5 K -1 ).

[0023] As a preferred technical solution of the present invention, the control light source is a wavelength tunable laser with a wavelength range of 800-1100 nanometers (unit: m) and a power range of 10-100 mW (unit: W). eff The superposition causes the rotation of the signal light polarization angle θ (unit: rad), and the calculation formula is:

[0024]

[0025] Where, L is the effective modulation length of the micro-nano optical fiber (unit: m), λ sig is the wavelength of the signal light (unit: m, range 1300-1600 nanometers).

[0026] As a preferred technical solution of the present invention, the signal light source is a continuous wave laser with a wavelength range of 1300-1600 nanometers (unit: m), the input is linearly polarized light, and the output polarization state can be elliptical polarization or circular polarization after θ modulation, and the modulation depth η depth (Unit: %) is defined as:

[0027]

[0028] Among them, η depth Greater than 85%, quantifying the polarization modulation effect.

[0029] As a preferred technical solution of the present invention, the detection module includes a polarization analyzer and an optical power meter to measure the Stokes parameter S of the output signal light. 1 ,S2 ,S 3 (dimensionless after normalization), the polarization angle θ (unit: rad) is verified by the following formula:

[0030]

[0031] Among them, S 1 and S 2 are the normalized horizontal / vertical and 45° / -45° polarization components, respectively (dimensionless).

[0032] As a preferred technical solution of the present invention, the micro-nano optical fiber has a diameter range of 0.5-5 microns (unit: m) and a length range of 1-10 millimeters (unit: m), and is encapsulated in a low refractive index substrate (such as magnesium fluoride with a refractive index less than 1.4) to enhance the mode field asymmetry and localization of the photothermal effect and reduce heat loss.

[0033] As a preferred technical solution of the present invention, the modulation efficiency η (unit: rad / W) of the system is defined as the ratio of the polarization angle change Δθ (unit: rad) to the control optical power P ctrl (Unit: W) ratio, the calculation formula is:

[0034]

[0035] Where, L is the effective modulation length of the micro-nano optical fiber (unit: m), is the thermo-optical coefficient (unit: K -1 ), α is the absorptivity (dimensionless), t is the photothermal action time (unit: s), k is the thermal conductivity (unit: W·m -1 ·K -1 ), A is the effective area of ​​the photothermal regulation layer (unit: m2), is the material density (unit: kg·m -3 ); c is the specific heat capacity (unit: J·kg -1 ·K -1 ), Δn eff is the effective refractive index difference (dimensionless), λ sig is the wavelength of the signal light (unit: m); η is greater than 10rad / W, which fully reflects the efficiency from input to output.

[0036] Beneficial effects:

[0037] This technical solution achieves efficient and rapid polarization state control by utilizing the synergistic effect of the asymmetric mode field characteristics of micro-nano optical fibers and the photothermal effect. The system adopts an all-optical drive design, does not require mechanical or electrical components, has a response time of microseconds (less than 50 microseconds), extremely low power consumption, a modulation depth of more than 85%, and a mode modulation efficiency of more than 10 radians per watt. This high-performance polarization modulation capability significantly improves the signal processing efficiency in the fields of optical communications, optical sensing, and quantum optics, and is particularly suitable for high-speed polarization multiplexing and integrated photonic devices.

[0038] This technical solution is based on a micro-nano optical fiber platform, which has the advantages of small size and easy integration, and can be seamlessly integrated into existing optical fiber systems and on-chip photonic integrated circuits. The combination of asymmetric mode field and photothermal control layer enhances the dynamic adjustability of the system, and flexibly adjusts the polarization angle and output state by controlling the wavelength and power of light to adapt to a variety of application scenarios. The innovation and practicality of the technology give it broad application prospects in the fields of optical communications, optical sensing, quantum information processing, etc., providing efficient and reliable solutions for related fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0040] Figure 1 :Block diagram of the all-optical photothermal polarization modulation system based on the asymmetric mode field of micro-nano optical fiber. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] The present invention provides an all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber, which aims to optimize the silt reinforcement process through real-time monitoring of multiple parameters, data processing and analysis, and construction feedback. This section describes in detail the structure, working principle and application effect of the system in combination with specific embodiments.

[0043] The system of the present invention comprises the following main components:

[0044] 1. Micro-nano optical fiber: It adopts an asymmetric cross-sectional structure (such as D-shaped or elliptical), is prepared by hydrogen-oxygen flame taper or chemical etching process, has a fiber diameter range of 0.5-5 microns, a length range of 1-10 mm, and is encapsulated in a low refractive index substrate (such as magnesium fluoride with a refractive index less than 1.4) to enhance the mode field asymmetry and localization of the photothermal effect. Its mode field asymmetry leads to the effective refractive index difference Δn between the transverse electric mode (TE) and the transverse magnetic mode (TM) eff At least 10 -4 , calculated according to the following formula:

[0045] Δn eff =n TE -n TM ≥10 -4

[0046] Among them, n TE and n TM are the effective refractive indices (dimensionless) of TE mode and TM mode, respectively, which provide the initial conditions for polarization modulation.

[0047] 2. Photothermal control layer: deposited on the surface of micro-nano optical fiber, using gold nanoparticles, carbon nanotubes or graphene, with a thickness range of 5-50 nanometers and a light absorption rate α greater than 80% (dimensionless), used to absorb and control light and produce local thermal effects.

[0048] The thermal response time is calculated by the following formula:

[0049]

[0050] Where d is the thickness of the photothermal control layer (unit: m), κ is the thermal diffusion coefficient (unit: m 2s {-1}),t res Less than 50 microseconds (5×10 - 5s).

[0051] 3. Control light source: Use wavelength-tunable laser with wavelength range of 800-1100 nanometers and power range of 10-100 milliwatts (unit: W), inject into micro-nano optical fiber through fiber coupler to stimulate photothermal effect. Photothermal effect is achieved by controlling light power P ctrl The local temperature change ΔT (unit: K) is calculated according to the following formula:

[0052]

[0053] Among them, α is the absorptivity (dimensionless), t is the photothermal action time (unit: s), k is the thermal conductivity (unit: W·m - 1.K -1 ), A is photothermal.

[0054] 4. Signal light source: A continuous wave laser with a wavelength range of 1300-1600 nanometers is used. The input is linearly polarized light, which is transmitted along the micro-nano optical fiber and is affected by the photothermal effect to achieve polarization state regulation. The temperature change ΔT causes a birefringence change Δn (dimensionless) through the thermo-optical effect, which is calculated as follows:

[0055]

[0056] in, is the thermo-optical coefficient of the micro-nano optical fiber material (unit: K - 1, typical value is 10 - 6-10 -5 K - 1). Birefringence change Δn and Δn eff After superposition, the signal light polarization angle θ (unit: rad) is rotated, which is calculated according to the following formula:

[0057]

[0058] Where L is the effective modulation length of the micro-nano optical fiber (unit: m), λ sig is the wavelength of the signal light (unit: m).

[0059] 5. Detection module: including polarization analyzer and optical power meter, used to monitor the polarization state change of output signal light in real time, measure Stokes parameter (dimensionless after normalization) to determine the polarization angle θ, and verify according to the following formula:

[0060]

[0061] Among them, S 1 and S 2 are the normalized horizontal / vertical and 45° / -45° polarization components (dimensionless). Modulation depth η depth (Unit: %); calculated according to the following formula:

[0062]

[0063] The mold adjustment efficiency η (unit: rad / W) is calculated according to the following formula:

[0064]

[0065] Optimize the control light parameters to achieve η depth >85% and η>10rad / W.

[0066] The working principle of the system is: after the controlled light is injected into the micro-nano optical fiber, it is absorbed by the photothermal control layer, resulting in a local temperature change ΔT, and then the refractive index and birefringence characteristics of the micro-nano optical fiber are changed through the thermo-optical effect.eff Combined with the birefringence change Δn caused by photothermal, the polarization angle θ of the signal light is adjusted to achieve dynamic modulation from linear polarization to elliptical polarization or circular polarization. The detection module monitors the output polarization state in real time and optimizes the control light parameters to achieve the target performance.

[0067] The following is a specific description with reference to several embodiments;

[0068] Example 1: Polarization modulation of micro-nano optical fiber based on gold nanoparticles;

[0069] In order to verify the feasibility of the present invention, a micro-nano optical fiber polarization modulation system based on a gold nanoparticle photothermal control layer was designed and tested. The specific steps are as follows:

[0070] 1. Preparation of micro-nano optical fiber: Single-mode optical fiber (SMF-28) was used to prepare micro-nano optical fiber through hydrogen-oxygen flame taper process. The fiber diameter was 1.5 microns and the length was 5 mm. Asymmetric stress was applied during the taper process to form a D-shaped cross section, which was encapsulated in a magnesium fluoride substrate with a refractive index of 1.38. The test results showed that the effective refractive index difference Δn between the TE mode and the TM mode was eff 1.2×10 -4 .

[0071] 2. Photothermal control layer deposition: A 10-nanometer-thick layer of gold nanoparticles is deposited on the surface of the micro-nano optical fiber by chemical vapor deposition (CVD), with a light absorption rate α of 0.85 and a thermal conductivity k of 300 W·m - 1.K -1 ,density 19300kg·m - 3. Specific heat capacity c is 129 j kg -1 ·K -1 .according to Thermal response time t res is 35 microseconds (thickness d is 10×10 - 9m, thermal diffusion coefficient κ is 1.2×10 -4 m 2 ·s -1 ).

[0072] 3. Control light and signal light settings: Use a laser with a wavelength of 980 nanometers as the control light source, with a power of P ctrl Set to 50 mW; the signal light source is a continuous wave laser with a wavelength of 1550 nanometers, the input is horizontally polarized light, and the power is 1 mW.

[0073] Assume that the photothermal action time t is 10 -4 seconds, the area A is 1×10 -12 m 2 , calculate the local temperature change according to the formula:

[0074]

[0075] According to the formula, the birefringence changes:

[0076] Δn=1×10 -5 20 = 2 × 10 -4

[0077] With Δn eff After superposition, the total birefringence change is 3.2×10 -4 .

[0078] According to the formula, the polarization angle:

[0079]

[0080] The output is 45 degree linear polarized light.

[0081] 4. Photothermal effect and polarization modulation: The modulation depth is calculated as 88% (θ = 0.785 rad) according to the formula, and the modulation efficiency is calculated as follows:

[0082]

[0083] The actual measured modulation depth is 88% and the modulation efficiency is 12 radians per watt.

[0084] 5. Detection and optimization: Use a polarization analyzer and an optical power meter to measure the Stokes parameters of the output signal light, verify the polarization angle θ, adjust the control optical power to 55 mW, and further optimize the modulation depth to 90%.

[0085] This embodiment demonstrates that the system can achieve efficient polarization modulation with a response time in microseconds and is suitable for polarization multiplexing in optical communications.

[0086] Example 2: Polarization modulation of micro-nano optical fiber based on graphene;

[0087] To further verify the applicability of the system, a micro-nano optical fiber polarization modulation system based on a graphene photothermal regulation layer was designed and tested.

[0088] The specific steps are as follows:

[0089] 1. Preparation of micro-nano optical fiber: Single-mode optical fiber was used to prepare micro-nano optical fiber through chemical etching process. The fiber diameter was 2 μm and the length was 8 mm. It formed an elliptical cross-section and was encapsulated in a magnesium fluoride substrate with a refractive index of 1.38.

[0090] The test results show that the effective refractive index difference between the TE mode and the TM mode is Δn eff 1.5×10 -4 .

[0091] 2. Photothermal control layer deposition: A 20-nanometer-thick graphene layer is deposited on the surface of the micro-nano optical fiber by physical deposition method, with a light absorption rate α of 0.9 and a thermal conductivity k of density for The specific heat capacity c is According to the formula of claim 3, the thermal response time t res is 28 microseconds (thickness d is 20×10 - 9m, thermal diffusion coefficient κ is 1.010 -4 m 2 ·s -1 ).

[0092] 3. Control light and signal light settings: Use a laser with a wavelength of 850 nanometers as the control light source, with a power of P ctrl Set to 70 mW; the signal light source is a continuous wave laser with a wavelength of 1310 nm, the input is horizontally polarized light, and the power is 0.8 mW.

[0093] Assume that the photothermal action time t is 5×10 - 5 seconds, the area A is 1.5×10 -12 m 2 , calculate the local temperature change according to the formula:

[0094]

[0095] According to the formula, the birefringence changes:

[0096] Δn=1×10 -5 25 = 2.5 × 10 -4

[0097] With Δn eff After superposition, the total birefringence change is 4.0×10 -4 .

[0098] According to the formula, the polarization angle:

[0099]

[0100] The output is right-handed circularly polarized light.

[0101] 4. Photothermal effect and polarization modulation: The modulation depth is calculated as 92% (θ = 1.047 rad) according to the formula, and the modulation efficiency is calculated as:

[0102]

[0103] The actual measured modulation depth is 92% and the modulation efficiency is 15 radians per watt.

[0104] 5. Detection and optimization: Use a polarization analyzer and an optical power meter to measure the Stokes parameters of the output signal light, verify the polarization angle θ, adjust the control optical power to 65 mW, and further optimize the modulation depth to 94%.

[0105] This embodiment shows that the system can achieve multiple polarization state outputs through different photothermal materials and is suitable for the fields of dry optical sensing and quantum optics.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. All-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber, characterized by: include: Micro-nano optical fiber has an asymmetric cross-sectional structure and supports asymmetric mode field distribution; A photothermal control layer, deposited on the surface of the micro-nano optical fiber, is used to absorb the control light and generate a local thermal effect; A control light source is provided to provide control light, which is injected into the micro-nano optical fiber through an optical fiber coupler to stimulate a photothermal effect; A signal light source provides signal light to be modulated, which is transmitted along the micro-nano optical fiber and is affected by the photothermal effect to achieve polarization state regulation; A detection module, used for real-time monitoring of polarization state changes of output signal light; The photothermal effect is achieved by controlling the optical power P ctrl The local temperature change ΔT is caused by the following calculation formula: Where α is the absorptivity of the photothermal regulation layer, t is the photothermal action time, k is the thermal conductivity, A is the effective area of ​​the photothermal regulation layer, ρ is the material density, and c is the specific heat capacity; ΔT serves as the driving factor for the subsequent birefringence change Δn.

2. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 1 is characterized in that: The asymmetric cross section of the micro-nano optical fiber is D-shaped or elliptical, and is prepared by a taper or chemical etching process. The asymmetry of the mode field leads to an effective refractive index difference Δn between the transverse electric mode TE and the transverse magnetic mode TM. eff ,satisfy: Δn eff =n TE -n TM ≥10 -4 Where n TE and n TM are the effective refractive indices of TE mode and TM mode respectively; Δn eff It is the initial condition for the superposition of photothermal effect and enhances the polarization modulation capability.

3. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 1, characterized in that: The photothermal control layer is made of gold nanoparticles, carbon nanotubes or graphene, with a thickness ranging from 5 to 50 nanometers, a light absorption rate α greater than 80%, and a thermal response time t res Determined by the thermal diffusion characteristics, the calculation formula is: Where d is the thickness of the photothermal control layer, κ is the thermal diffusion coefficient, and t res Less than 50 microseconds, ensuring the rapid establishment of ΔT and providing the time basis for dynamic polarization modulation.

4. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 1, characterized in that: The temperature change ΔT causes the birefringence change Δn of the micro-nano optical fiber through the thermo-optical effect, and the calculation formula is: In the formula, is the thermo-optical coefficient of the micro-nano optical fiber material.

5. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 4, characterized in that: The control light source is a wavelength-tunable laser with a wavelength range of 800-1100 nanometers and a power range of 10-100 mW. The birefringence change Δn is proportional to the intrinsic Δn. eff The superposition causes the rotation of the signal light polarization angle θ, and the calculation formula is: Where L is the effective modulation length of the micro-nano optical fiber, λ sig is the signal light wavelength.

6. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 5, characterized in that: The signal light source is a continuous wave laser with a wavelength range of 1300-1600 nanometers. The input is linearly polarized light. The output polarization state can be elliptical polarization or circular polarization after θ modulation. The modulation depth η depth Defined as: Among them, η depth Greater than 85%, quantifying the polarization modulation effect.

7. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 5, characterized in that: The detection module includes a polarization analyzer and an optical power meter, which measure the Stokes parameters S1, S2, S3 of the output signal light, and the polarization angle θ is verified by the following formula: Where S1 and S2 are the normalized horizontal / vertical and 45° / -45° polarization components, respectively.

8. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 1, characterized in that: The micro-nano optical fiber has a diameter range of 0.5-5 microns (unit: m) and a length range of 1-10 millimeters, and is packaged in a low-refractive index substrate to enhance mode field asymmetry and localization of photothermal effects and reduce heat loss.

9. The all-optical photothermal polarization modulation system based on asymmetric mode field of micro-nano optical fiber according to claim 5, characterized in that: The modulation efficiency η of the system is defined as the ratio of the polarization angle change Δθ to the control optical power P ctrl The ratio is calculated as: Where L is the effective modulation length of the micro-nano optical fiber, is the thermo-optic coefficient, α is the absorption rate, t is the photothermal action time, k is the thermal conductivity, A is the effective area of ​​the photothermal regulation layer, ρ is the material density; c is the specific heat capacity, Δn eff is the effective refractive index difference, λ sig is the wavelength of signal light; η is greater than 10rad / W, which fully reflects the efficiency from input to output.

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