Method for suppressing stimulated brillouin scattering effect by using cascaded long period fiber gratings

Abstract

The application discloses a method for suppressing stimulated Brillouin scattering effect by using a cascaded long-period fiber grating. The transmission spectrum of the cascaded long-period fiber grating has a series of comb-shaped resonance peaks, two adjacent resonance peaks are regarded as a passband and two stopbands, the passband bandwidth and resonance wavelength are changed by adjusting the parameters of the cascaded long-period fiber grating, the passband and stopband formed by the adjacent resonance peaks of the transmission spectrum correspond to signal light and Stokes light generated by the stimulated Brillouin scattering effect respectively, so that the non-working side lobe laser is coupled from the core mode to the cladding mode while the signal light passes through. The performance stability of the cascaded long-period fiber grating is improved by using a water ring flow-based desensitization packaging technology, so that the output power level and beam quality of the fiber laser are improved, and the possibility of laser damage is reduced. The application has the characteristics of low cost, high flexibility and strong applicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of high-power fiber lasers, and particularly relates to a method for suppressing stimulated Brillouin scattering effect by using cascaded long-period fiber gratings. BACKGROUND

[0002] With the continuous increase of the power of fiber lasers, the nonlinear effects generated in the lasers, such as stimulated Brillouin scattering effect, seriously restrict the development of high-power fiber lasers. The stimulated Brillouin scattering effect can generate a Stokes shift of about 10 GHz near the signal light, forming a non-working side lobe spectral component. When the output power of the fiber laser exceeds the threshold of the stimulated Brillouin scattering effect, the generated backward Stokes light will absorb the power of the signal light, and the high-power backward Stokes light will pose a great threat to the core devices of the laser, which can easily lead to the damage of the laser.

[0003] At present, the stimulated Brillouin scattering effect is usually suppressed by using an inclined Bragg fiber grating. The short-wave region of the transmission spectrum of the grating has many extremely narrow cladding mode resonance peaks, which can be used to suppress the stimulated Brillouin scattering effect. However, the back light generated by the inclined Bragg fiber grating can easily cause damage to the core devices of the laser, and also introduces an insertion loss that cannot be eliminated. Therefore, a method is needed to suppress the stimulated Brillouin scattering effect in the fiber laser with high safety and low insertion loss, so as to improve the output power level and beam quality of the fiber laser. SUMMARY

[0004] The purpose of the present application is to provide a method for suppressing the stimulated Brillouin scattering effect in a fiber laser by using cascaded long-period fiber gratings, so as to improve the output power and beam quality of the fiber laser and reduce the possibility of damage to the laser. The method has the advantages of low cost, high flexibility and strong applicability.

[0005] The technical solution for achieving the purpose of the present application is as follows: a method for suppressing stimulated Brillouin scattering effect by using cascaded long-period fiber gratings, characterized in that:

[0006] Step 1: using the combination of an ultraviolet laser and a cylindrical lens to point-by-point write a cascaded long-period fiber grating:

[0007] The cylindrical lens is used to compress the laser emitted by the ultraviolet laser, and the cylindrical lens is moved point by point by controlling the stepping motor with the period of the long-period fiber grating as the step size, and the exposure time is controlled by using a light shutter, so as to realize the writing of the long-period fiber grating; the period of the long-period fiber grating is Λ, and the length is d; the cascaded long-period fiber grating is obtained by sequentially writing two long-period fiber gratings with the same period; the distance between the two long-period fiber gratings is L; the passband bandwidth and resonance wavelength of the transmission spectrum of the cascaded long-period fiber grating are adjusted to change the parameters, so as to achieve the purpose of suppressing the stimulated Brillouin scattering effect.

[0008] Step 2, the sensitively reduced packaging technology based on water circulation is used to package the written cascade long period fiber grating.

[0009] Step 3, the packaged cascade long period fiber grating is connected to the fiber laser to suppress the stimulated Brillouin scattering effect, and there are two connection methods.

[0010] The first method is connected between the seed source and the amplification stage, the backward Stokes light generated by the stimulated Brillouin scattering effect of the seed source signal light is filtered out, the signal light power of the amplification stage is improved, the spectral composition is purified, and the possibility of damage to the fiber laser is reduced.

[0011] The second method is connected between the amplification stage and the output end, the stimulated Brillouin scattering effect generated in the amplification stage is suppressed, and the output power level and beam quality of the fiber laser are improved.

[0012] Compared with the prior art, the present application has the following advantages:

[0013] (1) In the prior art (such as using an inclined Bragg fiber grating to suppress the stimulated Brillouin scattering effect), the inclined Bragg fiber grating is used to couple the forward transmission core mode into the backward transmission cladding mode and the forward transmission core mode, and the backward transmission cladding mode is returned to the laser, which is easy to cause damage to the core device of the laser. The cascade long period fiber grating couples the forward transmission core mode to the forward transmission cladding mode, and there is no problem of returning light, which greatly improves the safety of the fiber laser system.

[0014] (2) In the prior art (such as using an inclined Bragg fiber grating to suppress the stimulated Brillouin scattering effect), the introduction of the inclined Bragg fiber grating will bring an insertion loss that cannot be eliminated, and the cascade long period fiber grating almost does not have insertion loss. Under the same conditions, the fiber laser system using the cascade long period fiber grating has higher output power.

[0015] (3) The present application first proposes an accurate expression of the distance L between the two-stage long period fiber gratings of the cascade long period fiber grating with respect to the wavelength distance w between the peak of the signal light near the resonance peak and the trough in the long wave direction, which greatly improves the efficiency of designing the cascade long period fiber grating.

[0016] (4) The cascade long period fiber grating is fully compatible with the fiber laser, so the stimulated Brillouin scattering effect suppression method based on the cascade long period fiber grating can be applied to high-power fiber lasers of different structures, and can also be applied to the communication field and the sensing field, and has a wide range of applications. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A schematic diagram of a cascaded long-period fiber grating structure capable of inhibiting stimulated Brillouin scattering effect in a fiber laser.

[0018] Figure 2 A series of data of the distance L between the two-stage long-period fiber gratings of the cascaded long-period fiber grating obtained by simulation and measurement of the present application and the wavelength distance w between the peak of the resonance peak near the signal light and the trough in the long-wave direction of the function curve of L with respect to w obtained by fitting.

[0019] Figure 3 A schematic diagram of a desensitization packaging structure based on water circulation according to the present application.

[0020] Figure 4 A transmission spectrum of a cascaded long-period fiber grating capable of inhibiting stimulated Brillouin scattering effect in a fiber laser, wherein the left graph is a cascaded long-period fiber grating transmission spectrum diagram, and the right graph is a partial enlarged view of the left graph. DETAILED DESCRIPTION

[0021] The present application will be further described below in conjunction with the drawings and specific embodiments. It should be understood that the specific embodiment methods described herein are only used to illustrate and explain the present application, and are not used to limit the embodiments of the present application.

[0022] In conjunction with Figures 1-2 A method for inhibiting stimulated Brillouin scattering effect by using a cascaded long-period fiber grating, the steps are as follows:

[0023] Step 1, using a combination of an ultraviolet laser and a cylindrical lens to point-by-point write a cascaded long-period fiber grating:

[0024] The laser emitted by the ultraviolet laser is compressed by using the cylindrical lens, and the cylindrical lens is moved point by point by a step motor with the period of the long-period fiber grating as the step length, and the exposure time is controlled by using a light shutter, so as to realize the writing of the long-period fiber grating; the period of the long-period fiber grating is Λ, and the length is d; a cascaded long-period fiber grating is obtained by sequentially writing two long-period fiber gratings with the same period; the distance between the two long-period fiber gratings is L; adjusting the parameters of the cascaded long-period fiber grating changes the passband bandwidth and resonance wavelength of the transmission spectrum, so as to achieve the purpose of inhibiting stimulated Brillouin scattering effect.

[0025] The cascaded long-period fiber grating transmission spectrum has a series of comb-shaped resonance peaks, two adjacent resonance peaks are regarded as a passband and two stopbands, the passband bandwidth and resonance wavelength are changed by adjusting the parameters of the cascaded long-period fiber grating, so that the passband and stopband formed by the adjacent resonance peaks of the grating correspond to the signal light and the Stokes light generated by the stimulated Brillouin scattering respectively, the non-working side lobe laser is coupled from the core mode to the cladding mode, the stimulated Brillouin scattering effect is suppressed while the signal light is passed, so that the output power and beam quality of the fiber laser are improved.

[0026] The cascaded long-period fiber grating is composed of two long-period fiber gratings with the same structure which are sequentially inscribed, and the first long-period fiber grating and the second long-period fiber grating are sequentially arranged from front to back; when the signal light enters the first long-period fiber grating, part of the core mode is coupled into the cladding and propagates along the fiber cladding; when passing through the second long-period fiber grating, the cladding mode is coupled back to the core to become the core mode, and meets the core mode which does not couple through the first long-period fiber grating to occur Mach-Zehnder interference, forming periodic interference fringes; this interference is expressed as:

[0027]

[0028] Wherein, I represents the light intensity of the interference fringes, I co represents the light intensity of the core mode, I cl represents the light intensity of the cladding mode, and is the phase difference between the core mode and the cladding mode.

[0029] The transmission spectrum of the cascaded long-period fiber grating is simulated according to the formula, the resonance peak closest to the center wavelength of the signal light is selected, the wavelength distance w between the peak of the resonance peak and the valley in the long-wave direction of the resonance peak is calculated, a series of data of the distance L between the two long-period fiber gratings and w is obtained, and the function expression of L about w is obtained by using the least square method fitting as follows:

[0030] w=0.2749L -0.9908 , L∈[1,7] (2).

[0031] It is verified that the determination coefficient R 2 of the fitting result of the above formula is 1, the residual sum of squares SSE is 1.7755e -6 , and the root mean square error RMSE is 4.7110e -4 , so it is considered that the formula is the accurate function expression of L and w.

[0032] The cascaded long-period fiber grating is designed based on a transmission grating, i.e., a long-period fiber grating, the core mode of which is coupled to a co-propagating cladding mode, thereby avoiding the problem of back reflection of a reflection grating, reducing the possibility of damage to the fiber laser, and ensuring the safety of the fiber laser; meanwhile, the insertion loss of the long-period fiber grating is lower than that of the reflection grating, thereby reducing the loss of signal light in the suppression process and improving the output power level of the fiber laser.

[0033] Step 2: The written cascaded long-period fiber grating is packaged by using a water circulation-based desensitization packaging technology.

[0034] In combination with Figure 3 , the water circulation-based desensitization packaging technology is used to fix the long-period fiber grating at both ends in a glass sleeve by using epoxy resin, and the glass sleeve is fixed in a packaging structure, so as to reduce the influence of axial stress and bending on the long-period fiber grating; the packaging shell is provided with a water inlet and a water outlet, cold water (20℃) is connected to reduce the temperature, and the influence of temperature on the long-period fiber grating is reduced, so as to ensure the stability of the long-period fiber grating.

[0035] Step 3: The packaged cascaded long-period fiber grating is connected to the fiber laser to suppress the stimulated Brillouin scattering effect, and there are two connection methods as follows:

[0036] The first method is to connect between the seed source and the amplification stage, filter the backward Stokes light generated by the stimulated Brillouin scattering effect excited by the seed source signal light, improve the signal light power and purify the spectral composition of the amplification stage, and avoid the back light into the fiber laser, thereby reducing the possibility of damage to the fiber laser.

[0037] The second method is to connect between the amplification stage and the output end, suppress the stimulated Brillouin scattering effect generated in the amplification stage, and improve the output power level and beam quality of the fiber laser.

[0038] The specific design implementation is as follows:

[0039] The center wavelength of the output signal light of the fiber laser system is 1080 nm, and the center wavelength of the Stokes light excited by the stimulated Brillouin scattering effect is 1080.06 nm. According to this, the cascade long-period fiber grating is designed by setting the grating period Λ to be 520 μm, the length of a single grating d to be 26 mm, and the refractive index modulation depth to be 0.00018. The range of the distance L between two cascade long-period fiber gratings is determined according to the formula (2) obtained by fitting, and the distance L between two cascade long-period fiber gratings is selected to be 4 m by comprehensively considering the requirement that the transmittance of the cascade long-period fiber grating at the center wavelength of the signal light is as small as possible. The transmission spectrum of the cascade long-period fiber grating obtained by writing according to the above design parameters is shown in Figure 4 The center wavelength of the resonance peak selected for suppressing the stimulated Brillouin scattering effect is 1079.99 nm, the long-wave direction valley wavelength is 1080.06 nm, which corresponds to the center wavelength of the Stokes light, and the 3dB bandwidth is 0.07 nm, so that the signal light is transmitted while the stimulated Brillouin scattering effect is suppressed.

Claims

1. A method for suppressing the effect of stimulated Brillouin scattering by using a cascaded long period fiber grating, characterized in that, The steps are as follows: Step 1, using a combination of ultraviolet laser and cylindrical lens to write long period fiber grating point by point: The laser emitted by the ultraviolet laser is compressed by the cylindrical lens, and the cylindrical lens is moved point by point by controlling the stepping motor to take the period of the long period fiber grating as the step, and the exposure time is controlled by the light shutter, so as to realize the writing of the long period fiber grating; The period of the long period fiber grating is Λ, and the length is d. Two long period fiber gratings are obtained by sequentially writing two long period fiber gratings with the same structure. The distance between the two long period fiber gratings is L; Adjusting the parameters of the cascaded long period fiber grating changes the passband bandwidth and resonance wavelength of the transmission spectrum, so as to achieve the purpose of suppressing stimulated Brillouin scattering effect; Step 2, based on the water ring flow-based sensitivity reduction packaging technology to package the written cascaded long period fiber grating; Step 3, connect the packaged cascaded long period fiber grating to the fiber laser to suppress the stimulated Brillouin scattering effect, which has the following two methods: The first kind is connected between the seed source and the amplification stage. By filtering the backward Stokes light generated by the stimulated Brillouin scattering effect excited by the seed source signal light, the signal light power of the amplification stage is improved, the spectral composition is purified, and the possibility of damage to the fiber laser is reduced. The second kind is connected between the amplification stage and the output end. By suppressing the stimulated Brillouin scattering effect generated in the amplification stage, the output power level and beam quality of the fiber laser are improved. The transmission spectrum of the cascaded long period fiber grating has a series of comb-shaped resonance peaks. Two adjacent resonance peaks are regarded as a passband and two stopbands. By adjusting the parameters of the cascaded long period fiber grating to change the passband bandwidth and resonance wavelength, the passband and stopband formed by the adjacent resonance peaks of the grating are respectively corresponding to the signal light and the Stokes light generated by the stimulated Brillouin scattering, so as to realize the coupling of the non-working side lobe laser from the core mode to the cladding mode, and to suppress the stimulated Brillouin scattering effect while passing through the signal light, so as to improve the output power and beam quality of the fiber laser. The cascaded long period fiber grating is composed of two long period fiber gratings with the same structure written in sequence, and the first long period fiber grating and the second long period fiber grating are arranged in sequence from front to back; When the signal light enters the first long period fiber grating, part of the core mode is coupled into the cladding and propagates along the fiber cladding; When passing through the second long period fiber grating, the cladding mode is coupled back to the core to become the core mode, and meets the core mode that has not been coupled through the first long period fiber grating to form a periodic interference fringe; This interference is expressed as: where I represents the light intensity of the interference fringes, I co represents the light intensity of the core mode, cl represents the light intensity of the cladding mode, is the phase difference between the core mode and the cladding mode; According to formula (1), the transmission spectrum of the cascaded long period fiber grating is simulated, the resonance peak closest to the center wavelength of the signal light is selected, the wavelength distance w between the peak and the valley in the long wave direction of the resonance peak is calculated, a series of data of the distance L between the two long period fiber gratings and w is obtained, and the function expression of L about w is obtained by using the least square method as follows: w = 0.2749L -0.9908 L e [1, 7] (2).

2. The method of claim 1, wherein the cascaded long period fiber grating is configured to suppress the stimulated Brillouin scattering effect. Based on the water ring flow-based sensitivity reduction packaging technology, the specific steps are as follows: In view of the characteristic that the long-period fiber grating is sensitive to environmental factors, epoxy resin is used to fix the two ends of the long-period fiber grating in a glass sleeve, and the glass sleeve is fixed in a packaging structure, so that the influence of axial stress and bending on the long-period fiber grating is reduced; the packaging shell is provided with a water inlet and a water outlet, cold water is connected for cooling, the influence of temperature on the long-period fiber grating is reduced, and the stability of the long-period fiber grating is ensured.

3. The method of claim 2, wherein the cascaded long period fiber grating is configured to suppress the stimulated Brillouin scattering effect. The environmental factors include axial stress, bending and temperature.

4. The method of claim 2, wherein the cascaded long period fiber grating is configured to suppress the stimulated Brillouin scattering effect. The temperature of the connected cold water is 20 DEG C.

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