Method for suppressing self-phase modulation by using large-angle bragg tilted fiber grating

Abstract

This invention discloses a method for suppressing self-phase modulation using a large-angle Bragg tilted fiber grating. Addressing the spectral broadening of output laser light caused by self-phase modulation in fiber lasers, this invention proposes a method using a large-angle Bragg tilted fiber grating to suppress broadened spectral components. The grating's transmission spectrum exhibits a series of comb-shaped resonant peaks. By adjusting the grating parameters, the resonant wavelength and the spacing between adjacent resonant peaks are altered. For the passband and stopband formed by adjacent resonant peaks, when the center wavelength of the main lobe of the signal light corresponds to the center wavelength of the narrow passband, and the stopband band corresponds to the self-phase modulation-derived spectral components, it is possible to couple the derived spectral components generated by the self-phase modulation effect in the laser from the core mode to the forward cladding mode while the signal light passes through, thereby optimizing the output spectral linewidth of the fiber laser. Compared to existing methods, the method proposed in this invention has advantages such as low cost, high flexibility, and high stability.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers, specifically relating to a method for suppressing self-phase modulation using a large-angle Bragg tilted fiber grating. Background Technology

[0002] As the power threshold of high-power fiber lasers continues to be broken, the reduction in beam quality and the broadening of the output spectrum limit the improvement of fiber laser power. Among them, the spectral broadening caused by nonlinear effects seriously degrades the output spectral linewidth. Nonlinear effects include stimulated Raman scattering and stimulated Brillouin scattering caused by stimulated scattering, self-phase modulation effect caused by nonlinear refractive index modulation, four-wave mixing effect, and cross-phase modulation. Among them, the broadened spectral components caused by self-phase modulation effect are located on both sides of the main lobe of the signal light spectrum. When the output power is higher than the threshold for the self-phase modulation effect, spectral broadening will occur, seriously degrading the output spectral linewidth.

[0003] Currently, phase-shifted long-period fiber gratings (LPGFs) are commonly used to suppress self-phase modulation (SEM) in fiber lasers. These gratings have stopbands on both sides of the transmission spectrum passband, which can be used to suppress SEM. However, the center wavelength and suppression depth of the transmission spectrum of LGFs are significantly affected by external factors such as temperature and stress, making their use under demanding conditions. Therefore, a convenient and stable method is needed to suppress SEM in fiber lasers and optimize the output spectral linewidth. Summary of the Invention

[0004] The purpose of this invention is to provide a method for suppressing self-phase modulation effects in fiber lasers using large-angle Bragg tilted fiber gratings, thereby optimizing the output spectral linewidth of the fiber laser. This method has advantages such as easy fabrication of the grating, high stability, and high reusability.

[0005] The technical solution for achieving the objective of this invention is: a method for suppressing self-phase modulation using a large-angle Bragg tilt fiber grating, characterized in that:

[0006] Step 1: Fabricate a large-angle Bragg tilted fiber grating based on the phase mask method. The tilt angle of the phase mask relative to the fiber axis is called the first tilt angle α. In the writing system, the ultraviolet laser emits a 244nm He-Ne laser, which passes through the phase mask to form tilted interference fringes that irradiate the photosensitive fiber, forming periodic refractive index modulation, thus obtaining a large-angle Bragg tilted fiber grating. The tilt angle of the grating surface of the large-angle Bragg tilted fiber grating relative to the fiber radial direction is called the second tilt angle θ, and the second tilt angle θ is greater than 45°, thereby achieving the purpose of suppressing the self-phase modulation effect.

[0007] Step 2: Connect a large-angle Bragg tilt fiber grating to the fiber laser to suppress self-phase modulation effects. There are two connection methods:

[0008] The first method involves placing the signal after the seed source and suppressing the self-phase effect spectral components near the main lobe of the seed source signal light to provide high-quality narrow-linewidth signal light for subsequent amplification.

[0009] The second type, connected after the amplification stage, can filter out spectral components derived from the self-phase modulation effect on both sides of the main lobe while ensuring that the main lobe of the amplified signal light passes through, thereby making the output signal light purer and obtaining a higher quality output spectrum and beam quality.

[0010] Compared with the prior art, the significant advantages of this invention are:

[0011] (1) In existing technologies (such as using phase-shifted long-period fiber gratings to suppress self-phase modulation), when using fiber gratings to suppress nonlinear effects, the transmission spectrum of one grating corresponds to one center wavelength. However, the large-angle Bragg tilted fiber grating has a series of resonance peaks in its transmission spectrum. Any two adjacent resonance peaks can be regarded as a passband and two stopbands, which can be used to suppress the derived spectral components of signal light with different center wavelengths. Increasing the multiplexing rate of the grating also reduces the difficulty of matching the signal light with the center wavelength of the grating to a certain extent.

[0012] (2) In existing technologies (such as using phase-shifted long-period fiber gratings to suppress self-phase modulation), when the main lobe bandwidth of the signal light is less than the passband bandwidth of the phase-shifted long-period fiber grating, it is impossible to further reduce the passband bandwidth by increasing the period, and the broadened spectral components near the main lobe cannot be fully filtered out. However, the passband bandwidth of large-angle Bragg tilted fiber gratings can be as narrow as a few tenths of a nanometer, which can cope with narrow-linewidth signal light.

[0013] (3) In existing technologies (such as using phase-shifted long-period fiber gratings to suppress self-phase modulation), the center wavelength of the transmission spectrum and the depth of the suppression peak of phase-shifted long-period fiber gratings are greatly affected by external factors such as ambient temperature and stress, and the working state is unstable. In contrast, large-angle Bragg tilted fiber gratings are less affected by external factors and the working state is more stable. Attached Figure Description

[0014] Figure 1 This is a diagram of a large-angle Bragg fiber grating structure that can suppress the self-phase modulation effect in a fiber laser, as described in this invention.

[0015] Figure 2 This is an example of a large-angle Bragg tilted fiber grating spectrum that can suppress the self-phase modulation effect in a fiber laser.

[0016] Figure 3 A schematic diagram of a single-cavity oscillating fiber laser system for testing large-angle Bragg tilted fiber gratings.

[0017] Figure 4 The image shows the effect of large-angle Bragg tilt fiber grating in suppressing self-phase modulation. Detailed Implementation Plan

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments and methods described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the present invention.

[0019] A method for suppressing self-phase modulation using a large-angle tilted Bragg fiber grating comprises the following steps:

[0020] Step 1: Fabricate a large-angle Bragg tilted fiber grating based on the phase mask method. The tilt angle of the phase mask relative to the fiber axis is called the first tilt angle α. In the writing system, the ultraviolet laser emits a 244nm He-Ne laser, which passes through the phase mask to form tilted interference fringes that irradiate the photosensitive fiber, forming periodic refractive index modulation, thus obtaining a large-angle Bragg tilted fiber grating. The tilt angle of the grating surface of the large-angle Bragg tilted fiber grating relative to the fiber radial direction is called the second tilt angle θ, and the second tilt angle θ is greater than 45°, thereby achieving the purpose of suppressing the self-phase modulation effect.

[0021] Step 2: Connect a large-angle Bragg tilt fiber grating to the fiber laser to suppress self-phase modulation effects. There are two connection methods:

[0022] The first method involves placing the signal light after the seed source and suppressing the self-phase effect spectral components near the main lobe of the seed source signal light to provide high-quality narrow-linewidth signal light for subsequent amplification.

[0023] The second type, connected after the amplification stage, can filter out spectral components derived from the self-phase modulation effect on both sides of the main lobe while ensuring that the main lobe of the amplified signal light passes through, thereby making the output signal light purer and obtaining a higher quality output spectrum and beam quality.

[0024] The transmission spectrum of the large-angle Bragg tilted fiber grating has a series of comb-shaped resonant peaks. Each pair of adjacent resonant peaks is considered as a narrow passband and two narrow stopbands, with the two narrow stopbands symmetrically distributed on both sides of the narrow passband. By increasing the tilt angle of the interference fringes during writing, the large-angle Bragg tilted fiber grating increases the spacing and bandwidth of the comb-shaped peaks, thus allowing the narrow-bandwidth main lobe of the signal light to pass through without loss. At the same time, it couples the derived spectral components near the main lobe of the signal light caused by the self-phase modulation effect from the core mode to the forward-propagating cladding mode, thereby suppressing self-phase modulation in the fiber laser and optimizing the output spectral linewidth.

[0025] The large-angle Bragg tilted fiber grating is a periodically uniform Bragg tilted fiber grating, and its transmission spectrum has a series of comb-shaped resonance peaks. By adjusting the first tilt angle α and changing the second tilt angle θ, the i-th order resonant wavelength λ of the large-angle Bragg tilted fiber grating is obtained. cl,i As shown in formula (1):

[0026]

[0027] in, For the effective refractive index of the core mold, The effective refractive index of the i-th cladding mode is given. The vertical distance between adjacent grating surfaces is periodic Λ, determined by the indentation period of the phase mask. The spacing Δλ between adjacent resonance peaks is derived from formula (1). cl,i That is, the narrow passband bandwidth is as shown in formula (2):

[0028]

[0029] Formulas (1) and (2) illustrate that the narrow passband bandwidth of a large-angle tilted Bragg fiber grating is directly modulated by the period Λ and the second tilt angle θ. When the period Λ is constant, the larger the second tilt angle θ is, the greater the spacing Δλ between adjacent resonant peaks in the transmission spectrum. cl,i As the spacing increases, it forms a passband that allows signal light to pass through.

[0030] The reflectivity R of a large-angle Bragg tilted fiber grating is shown in formula (3):

[0031]

[0032] Where σ is the DC coupling coefficient, κ is the AC coupling coefficient, and L is the grating length. sinh(*) represents the hyperbolic sine function, and cosh(*) represents the hyperbolic cosine function.

[0033] When the center wavelength of the main lobe of the signal light is λ, the pulse width of the main lobe is Δλ, and the signal height is AdBm, the second tilt angle θ, the AC coupling coefficient κ, and the grating length L required for writing the large-angle Bragg tilted fiber grating are derived from formulas (1) to (3), as shown in formulas (4) to (7), where Λ eff The actual period of the grating is the axial distance between adjacent grating surfaces.

[0034]

[0035]

[0036]

[0037] R < 10lg(tanh)2 κL) (7)

[0038] Where tanh(*) represents the hyperbolic tangent function.

[0039] When the second tilt angle θ of the large-angle Bragg tilted fiber grating is greater than 45°, the large-angle Bragg tilted fiber grating couples the spectral components derived from the self-phase modulation effect near the main lobe of the signal light from the core mode to the forward-propagating cladding mode. This avoids the situation where the core mode is coupled to the backward-propagating cladding mode at a small angle, which could damage the core components of the laser, thus improving the safety of the experimental equipment.

[0040] Depend on Figures 1-2 It can be seen that the working principle of large-angle Bragg tilt fiber gratings suppressing self-phase modulation effect is as follows:

[0041] Figure 1 This is a schematic diagram of a large-angle Bragg tilted fiber grating structure, which can be fabricated using the phase mask method. Figure 1 The large-angle Bragg tilted fiber grating is shown. A phase mask is rotated relative to the fiber axis by a first tilt angle α. A 244nm He-Ne ultraviolet laser exposes the hydrogen-loaded photosensitive fiber through the phase mask, causing periodic modulation of the refractive index within the fiber core, forming a series of periodically arranged grating surfaces. In this invention, a large-angle Bragg tilted fiber grating is used to suppress self-phase modulation effects. The vertical distance between adjacent grating surfaces is a periodic Λ, and the grating surfaces are all tilted relative to the fiber radially by a second tilt angle θ. When the second tilt angle θ is greater than 45°, when the signal light passes through the large-angle Bragg tilted fiber grating, the nonlinear effect-derived spectral components near the main lobe of the signal light will couple from the core mode to the forward cladding mode, thereby suppressing the self-phase modulation effect in the fiber laser. Simultaneously, the large-angle Bragg tilted fiber grating avoids the situation where the core mode couples to the backward-propagating cladding mode at small angles, damaging the core components of the laser, thus improving equipment safety.

[0042] Figure 2 This is an example of the transmission spectrum of a large-angle Bragg-tilt fiber grating. The grating is a periodically uniform large-angle Bragg-tilt fiber grating, and the transmission spectrum has a series of comb-shaped resonant peaks. By adjusting the period Λ and the second tilt angle θ, the center wavelength and passband bandwidth of the transmission spectrum can be changed. Adjusting the period Λ makes the i-th order resonant wavelength λ... cl,i Corresponding to the center wavelength λ of the main lobe of the signal light; as the second tilt angle θ increases, the spacing Δλ between adjacent resonant peaks... cl,iThis will also increase, resulting in a passband through which the main lobe of the signal light can pass without loss. At this point, two adjacent resonant peaks can be considered as a narrow passband and two narrow stopbands, symmetrically distributed on either side of the narrow passband. If the main lobe band of a broadened signal light spectrum affected by self-phase modulation corresponds to the narrow passband, while the broadened spectral components correspond to the stopbands on either side, after passing through a large-angle Bragg tilted fiber grating, the main lobe spectral components of the signal light can continue to propagate in the fiber core, while the derived spectral components caused by self-phase modulation will couple from the core mode to the forward cladding mode, suppressing the self-phase modulation effect in the fiber laser and optimizing the output spectral linewidth.

[0043] The large-angle Bragg tilted fiber grating can be connected after the seed source of a fiber laser to suppress the self-phase modulation effect and provide a narrow linewidth light source for subsequent amplification; it can also be connected after the amplification mechanism to filter out the spectral components derived from the self-phase modulation effect on both sides of the main lobe of the signal light, optimize the bandwidth of the output signal light, and purify the output signal light energy.

[0044] Specific application examples are as follows:

[0045] The test system output signal light has a main lobe center wavelength of 1080nm and a main lobe bandwidth of 0.371nm. Therefore, the ultraviolet laser power is set to 60mW, the grating writing parameters are set with a period Λ of 188nm and a second tilt angle θ of 60°, and the ultraviolet spot is scanned along the fiber axis for a distance of 40mm at a speed of 0.006mm / s to prepare a large-angle Bragg tilted fiber grating.

[0046] Single-cavity oscillating fiber laser test system, such as Figure 3 A high-reflection grating, ytterbium-doped fiber, and a low-reflection grating form a resonant cavity. Pump light enters the cavity through a combiner, where it filters the desired light and amplifies the light in that wavelength band. A stripper-1 dissipates unabsorbed residual pump light in the cladding through scattering. A large-angle Bragg-tilt fiber grating is then connected after stripper-1 to filter out self-phase modulation-derived spectral components from the signal light. Because the second tilt angle of the Bragg fiber grating is greater than 45°, the derived spectral components couple from the core mode into the forward-propagating cladding mode and are scattered into the surrounding environment via stripper-2. The effect of the large-angle Bragg-tilt fiber grating in suppressing self-phase modulation in fiber lasers is as follows: Figure 4 As shown, large-angle Bragg tilt fiber gratings can effectively suppress self-phase modulation effects and optimize the output signal linewidth.

Claims

1. A method for suppressing self-phase modulation using a large-angle Bragg tilt fiber grating, characterized in that, The steps are as follows: Step 1: Fabricate a large-angle Bragg tilted fiber grating based on the phase mask method. The tilt angle of the phase mask relative to the fiber axis is called the first tilt angle α. The ultraviolet laser in the writing system emits a 244nm He-Ne laser, which passes through the phase mask to form tilted interference fringes that irradiate the photosensitive fiber, forming periodic refractive index modulation, thus obtaining a large-angle Bragg tilted fiber grating. The tilt angle of the grating surface of the large-angle Bragg tilted fiber grating relative to the fiber radial direction is called the second tilt angle θ, and the second tilt angle θ is greater than 45°, thereby achieving the purpose of suppressing the self-phase modulation effect. Step 2: Connect a large-angle Bragg tilt fiber grating to the fiber laser to suppress self-phase modulation effects. There are two connection methods: The first method involves placing the signal light after the seed source and suppressing the self-phase effect spectral components near the main lobe of the seed source signal light to provide high-quality narrow-linewidth signal light for subsequent amplification. The second type, connected after the amplification stage, can filter out spectral components derived from the self-phase modulation effect on both sides of the main lobe while ensuring that the main lobe of the amplified signal light passes through, thereby making the output signal light purer and obtaining a higher quality output spectrum and beam quality.

2. The method for suppressing self-phase modulation using a large-angle Bragg tilt fiber grating according to claim 1, characterized in that: The transmission spectrum of a large-angle Bragg tilted fiber grating has a series of comb-shaped resonant peaks. Each pair of adjacent resonant peaks is considered as a narrow passband and two narrow stopbands, with the two narrow stopbands symmetrically distributed on both sides of the narrow passband. The large-angle Bragg tilted fiber grating increases the spacing and bandwidth of the comb-shaped peaks by increasing the tilt angle of the interference fringes during writing, thereby allowing the narrow-bandwidth main lobe of the signal light to pass through without loss. At the same time, it couples the derived spectral components near the main lobe of the signal light caused by the self-phase modulation effect from the core mode to the forward-propagating cladding mode, thereby suppressing self-phase modulation in the fiber laser and optimizing the output spectral linewidth.

3. The method for suppressing self-phase modulation in a fiber laser using a large-angle Bragg tilted fiber grating according to claim 2, characterized in that: A large-angle Bragg tilted fiber grating is a periodically uniform Bragg tilted fiber grating whose transmission spectrum possesses a series of comb-shaped resonance peaks. By adjusting the first tilt angle α and changing the second tilt angle θ, the i-th order resonant wavelength λ of the large-angle Bragg tilted fiber grating is obtained. cl,i As shown in formula (1): in, For the effective refractive index of the core mold, The effective refractive index of the i-th cladding mode is given. The vertical distance between adjacent grating surfaces is periodic Λ, determined by the indentation period of the phase mask. The spacing Δλ between adjacent resonance peaks is derived from formula (1). cl,i That is, the narrow passband bandwidth is as shown in formula (2): Formulas (1) and (2) illustrate that the narrow passband bandwidth of a large-angle tilted Bragg fiber grating is directly modulated by the period Λ and the second tilt angle θ. When the period Λ is constant, the larger the second tilt angle θ is, the greater the spacing Δλ between adjacent resonant peaks in the transmission spectrum. cl,i As the spacing increases, it forms a passband that allows signal light to pass through; The reflectivity R of a large-angle Bragg tilted fiber grating is shown in formula (3): Where σ is the DC coupling coefficient, κ is the AC coupling coefficient, and L is the grating length; sinh(*) represents the hyperbolic sine function, and cosh(*) represents the hyperbolic cosine function; When the center wavelength of the main lobe of the signal light is λ, the pulse width of the main lobe is Δλ, and the signal height is AdBm, the second tilt angle θ, the AC coupling coefficient κ, and the grating length L required for writing the large-angle Bragg tilted fiber grating are derived from formulas (1) to (3), as shown in formulas (4) to (7), where Λ eff The actual period of the grating is represented by the axial distance between adjacent grating surfaces. R<10lg(fishy) 2 kL) (7) Where tanh(*) represents the hyperbolic tangent function.

4. The method for suppressing self-phase modulation using a large-angle Bragg tilt fiber grating according to claim 3, characterized in that: When the second tilt angle θ of a large-angle Bragg tilted fiber grating is greater than 45°, the large-angle Bragg tilted fiber grating couples the spectral components derived from the self-phase modulation effect near the main lobe of the signal light from the core mode to the forward-propagating cladding mode. This avoids the situation where the core mode is coupled to the backward-propagating cladding mode at small angles, which could damage the core components of the laser, thus improving the safety of the experimental equipment.

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