A photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum

Abstract

The present invention discloses a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum. A theoretical derivation method is proposed, and the Brillouin gain spectrum of the orbital angular momentum mode under stimulated Brillouin amplification is obtained. A photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum is designed, which is composed of a circular air hole layer distributed in a ring shape and a ring. There are five layers of circular air holes distributed in a ring shape outside the ring, and two layers of circular air holes are arranged in a regular hexagon inside the ring. All the circular air hole layers and the ring have the same center of the circle. The photonic crystal fiber amplifier of the present invention can stably and accurately synchronously amplify and long-distance transmit the orbital angular momentum mode, providing a possibility for the design of an orbital angular momentum mode laser system.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and particularly to a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum. Background Art

[0002] The generation, amplification, and transmission of high-purity orbital angular momentum modes are all the basis for realizing their applications in improving system spectral efficiency, optical communication, etc. Researchers have proposed various methods for generating orbital angular momentum (OAM) beams. The characteristics such as the purity, transmission spectrum, and amplification of OAM beams have attracted the interest of researchers. They believe that OAM beams are more suitable as information carriers than linearly polarized modes. In order to meet the application requirements of long-distance optical communication, how to achieve synchronous amplification of different orbital angular momentum modes is a problem that must be faced. There are two existing technical solutions for OAM mode amplification. One is to directly use a rare-earth doped gain medium to amplify the required orbital angular momentum mode; the other is to amplify the required orbital angular momentum mode based on the optical parametric amplification effect. Different from the traditional realization of optical amplification by directly using the energy level transition of rare-earth ions, there are no actual energy levels in the optical parametric amplification process, and its gain comes from the interaction between optical fields in the nonlinear medium. This essential difference makes the optical parametric amplification have many advantages such as large band flexibility, wide tuning range, and no amplified spontaneous emission. The Brillouin nonlinear effect is a typical third-order optical nonlinear effect. The amplification process based on the Brillouin nonlinear effect needs to meet strict phase matching conditions, so it can be applied to the amplification of high-purity and low-noise orbital angular momentum beams.

[0003] Existing OAM amplifiers include those realized by using a stimulated Brillouin amplification medium cell, erbium-doped fiber amplifiers, thulium-doped photonic crystal fiber amplifiers, and praseodymium-doped fiber amplifiers. These works have significantly improved the capacity and transmission distance of optical fiber communication systems. However, a single stimulated Brillouin amplification mechanism or photonic crystal fiber has not been fully combined and utilized to achieve OAM mode amplification. A photonic crystal fiber amplifier based on stimulated Brillouin amplification to achieve OAM mode amplification is a very promising way in realizing a large-capacity and long-distance optical fiber communication system, which will greatly improve the transmission characteristics of the optical fiber communication system. Summary of the Invention

[0004] The purpose of the present invention is to provide a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum to solve the problems existing in the existing OAM amplifiers.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] Step 1: A photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum is designed. The cross-section of the photonic crystal fiber amplifier includes a cladding and a core. It is characterized in that the photonic crystal fiber amplifier is composed of a circular air hole layer distributed in a ring shape and a ring. There are five layers of circular air holes distributed in a ring outside the ring, and two layers of circular air holes are arranged in a regular hexagon inside the ring. All the circular air hole layers and the ring have the same center of the circle.

[0007] Further, the base material of the optical fiber is Schott SF2, and the ring is filled with chalcogenide glass As2Se3.

[0008] Step 2: The pump light (topological charge number is l1) and the signal light (topological charge number is l2) carrying orbital angular momentum undergo a non-linear interaction through the excited acoustic wave field to form the stimulated Brillouin effect. The topological charge number of the orbital angular momentum carried by the excited acoustic wave field is l3 (l3 = l1 - l2). At this time, when the optical fiber is subjected to the electrostriction effect, the space-time variation and slow-varying amplitude approximation are performed on the acoustic wave equation followed by the fiber material density under steady-state conditions, and the Brillouin scattering spectrum of the Lorentz distribution satisfied by the fiber material density when the pump light and the signal light carrying orbital angular momentum undergo stimulated Brillouin scattering in the optical fiber can be obtained as follows:

[0009]

[0010] where, Ω is the acoustic wave frequency, q is the acoustic wave vector, υ = Ω / q is the acoustic wave velocity, Γ′ represents the attenuation coefficient of the acoustic wave, and r is the core radius. The maximum Brillouin gain coefficient g0 is: where, n eff is the effective refractive index, γ e is the electrostriction constant of the optical fiber, c is the speed of light in vacuum, λ is the wavelength of the incident light, and ρ is the fiber material density. The pump light and the signal light generate the stimulated Brillouin effect in the optical fiber and the signal light carrying orbital angular momentum is amplified by stimulated Brillouin. Under the action of the stimulated Brillouin scattering effect, the topological charge number carried by the orbital angular momentum mode will have a certain influence on its Brillouin gain spectrum.

[0011] Step 3: Analyze the electromagnetic field of the orbital angular momentum mode transmitted by the photonic crystal fiber amplifier in Step 1, and the effective refractive index and the electromagnetic field distribution of the OAM modes with different topological charge numbers transmitted by the optical fiber can be obtained; analyze its acoustic wave field, and the acoustic wave velocities of the OAM modes with corresponding different topological charge numbers can be obtained. Substitute the obtained effective refractive index and acoustic wave velocity into Equation (1) in Step 2, and the maximum Brillouin gain coefficient and the Brillouin scattering spectrum of each OAM mode can be obtained.

[0012] The present invention designs a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum. Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0013] It can support up to 66 OAM modes in the wavelength range of 1530nm - 1565nm, which is more suitable for large-capacity optical fiber communication systems.

[0014] High mode purity is achieved. In the wavelength range of 1530nm - 1565nm, the purity of the orbital angular momentum modes transmitted is higher than 99.4%, ensuring the stable existence of modes in the optical communication system, better signal encoding and multiplexing, and accurate transmission.

[0015] High signal gain is achieved. At a wavelength of 1550nm, a pump pulse width of 10ns, an effective fiber length of 0.4m, and pump optical energy in the range of 10 -6 J - 10 -3 J, the signal gain of the OAM mode in the fiber is as high as 1697.5dB. Description of the Drawings

[0016] Figure 1 It is a schematic cross-sectional view of a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment. In the figure: 1 - substrate material, 2 - ring material, 3 - outermost air hole outside the ring, 4 - second outermost air hole outside the ring, 5 - third outermost air hole outside the ring; 6 - fourth outermost air hole outside the ring, 7 - fifth outermost air hole outside the ring, 8 - outermost air hole inside the ring, 9 - innermost air hole inside the ring; d1 - diameter of the outermost air hole outside the ring, d2 - diameter of the second outermost air hole outside the ring, d3 - diameter of the third outermost air hole outside the ring, d4 - diameter of the fourth outermost air hole outside the ring, d5 - diameter of the fifth outermost air hole outside the ring, d6 - diameter of the outermost air hole inside the ring, d7 - diameter of the innermost air hole inside the ring, a - ring thickness, r - core radius.

[0017] Figure 2 It is a graph showing the variation of the difference in effective refractive indices of adjacent vector modes in the OAM modes supported by a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment with respect to wavelength.

[0018] Figure 3 It is a graph showing the variation of the purity of OAM modes with different topological charge numbers supported by a photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment with respect to wavelength.

[0019] Figure 4Variation relationship diagram of the confinement loss of OAM modes with different topological charge numbers supported by a photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment, as a function of wavelength.

[0020] Figures 5a - 5d Brillouin scattering spectra of OAM modes with different topological charge numbers of a photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment at wavelengths of 1530 nm, 1540 nm, 1550 nm, and 1560 nm, respectively.

[0021] Figure 6 Variation relationship diagram of the maximum Brillouin gain coefficient of the OAM mode supported by a photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum in a specific embodiment, as a function of the topological charge number at different wavelengths.

[0022] Figure 7 For a specific embodiment, when the wavelength is 1550 nm, the pump pulse width is 10 ns, the effective fiber length is 0.4 m, and the pump optical energy is in the range of 10 -6 J - 10 -3 J, the signal gain of the OAM mode in the optical fiber. Detailed implementation manners

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and through embodiments. The following embodiments are explanations of the present invention, and the present invention is not limited to the following embodiments.

[0024] Figure 1 The cross-sectional schematic diagram of a photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum according to the present invention is shown. The substrate material 1 is Schott SF2, and the ring material 2 is As2Se3. The photonic crystal fiber amplifier consists of circular air holes distributed in a ring shape and a ring. The thickness of the ring is a. There are five layers of circular air holes outside the ring distributed in a ring shape. The diameters of the circular air holes outside the ring from outside to inside are d1, d2, d3, d4, d5 in sequence. There are two layers of circular air holes inside the ring arranged in a regular hexagon. The diameters of the circular air holes inside the ring from outside to inside are d6, d7 in sequence. The core radius is r. All the circular air hole layers and the ring have the same center.

[0025] In this embodiment, the parameters of the photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum are as follows: the thickness of the ring a is taken as 1.7 μm, the diameters of the five layers of circular air holes outside the ring from outside to inside, d1, d2, d3, d4, d5, are taken as 2 μm, 1.6 μm, 1.3 μm, 1.1 μm, 0.8 μm respectively, the diameters of the two layers of circular air holes inside the ring from outside to inside, d6, d7, are taken as 2 μm, 0.8 μm respectively, and the core radius r is taken as 2 μm.

[0026] The present invention can perform theoretical calculations by using the finite element method in combination with the perfectly matched layer boundary absorption condition to obtain the effective refractive index difference, purity, confinement loss, and signal gain of the present invention.

[0027] In this embodiment, the difference in the effective refractive indices of adjacent vector modes of the photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum within the wavelength range of 1530 nm - 1565 nm is as Figure 2 shown. Observing Figure 2 it can be obtained that:

[0028] In the photonic crystal fiber amplifier of the present invention that can be used for stimulated Brillouin amplification of orbital angular momentum, although the effective refractive index difference of the high-order mode is lower than that of the low-order mode, all modes satisfy being greater than 10 -4 , and the degenerate separation of all vector modes can be realized, that is, the 66 OAM modes supported by this photonic crystal fiber amplifier can all be stably transmitted in the optical fiber. It can be seen that this photonic crystal fiber amplifier can reduce the coupling crosstalk between OAM modes, ensure the quality of OAM modes, and is beneficial to the accurate and stable transmission of signals in the optical amplification system.

[0029] In this embodiment, the purity of the OAM modes supported by the photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum within the wavelength range of 1530 nm - 1565 nm is as Figure 3 shown. Observing Figure 3 it can be obtained that:

[0030] In the photonic crystal fiber amplifier of the present invention that can be used for stimulated Brillouin amplification of orbital angular momentum, although the mode purity decreases with the increase of the wavelength, the purity of all modes is greater than 99.4%, and the purity of the low-order modes is higher. And high-quality OAM modes ensure the stable existence of modes in the optical communication system, better signal coding and multiplexing, and accurate transmission, and are also the basis for realizing optical communication, laser material processing, and nonlinear optics.

[0031] In this embodiment, the confinement loss of the OAM modes supported by the photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum within the wavelength range of 1530 nm - 1565 nm is as Figure 4 shown. Observing Figure 4 it can be obtained that:

[0032] In the photonic crystal fiber amplifier of the present invention that can be used for stimulated Brillouin amplification of orbital angular momentum, with the increase of the topological charge number, the confinement loss increases, that is, the confinement loss of the high-order mode is greater, and the confinement loss of each mode shows an upward trend with the increase of the wavelength. This is mainly because when the topological charge number and wavelength are larger, the energy has a higher leakage rate, and the confinement loss of the high-order modes all remains at 10 -6The order of dB / cm meets the actual optical fiber transmission conditions, which is beneficial to the long-distance accurate transmission of OAM modes.

[0033] The amplification of OAM modes based on stimulated Brillouin scattering can be described as the nonlinear interaction of pump light and OAM signal light with wavelengths λ1 and λ2 respectively through the excited acoustic wave field. Its optical field can be described as:

[0034] E(z, t) = E1(z, t) + E2(z, t) (2)

[0035] Among them, E1(z, t) and E2(z, t) are the pump light and the signal light respectively. When the pump light and the signal light carry orbital angular momentum, they can be expressed as:

[0036] E1(z, t) = A1(z, t)exp(i(k1 - ω1t))exp(il1θ) + c.c. (3)

[0037] E2(z, t) = A2(z, t)exp(i(k2 - ω2t))exp(il2θ) + c·c· (4)

[0038] Among them, A1 and A2 are the amplitudes of the pump light and the signal light respectively; k1 = 2π / λ1 and k2 = 2π / λ2 are the wave vectors of the pump light and the signal light respectively, ω1 and ω2 are the angular frequencies of the pump light and the signal light respectively, l1 and l2 are the topological charge numbers of the pump light and the signal light respectively, and c.c. is the complex conjugate of the previous formula.

[0039] When the signal light carries the OAM mode, the excited acoustic wave field can be described as:

[0040] ρ(z, t) = ρ0 + [ρ(z, t)exp(i(qz - Ωt))exp(il3θ) + c.c.] (5)

[0041] Among them, ρ0 is the average density of the material, q = k1 + k2 is the wave vector of the acoustic wave, Ω is the frequency of the acoustic wave, and l3 is the topological charge number of the acoustic wave. When l = 0, it means that there is no orbital angular momentum in the optical field or the acoustic wave field. According to the momentum conservation of the orbital angular momentum optical field, there is l3 = l1 - l2.

[0042] When the optical fiber is subjected to the electrostriction effect, the density of its optical fiber material follows the acoustic wave equation:

[0043]

[0044] Among them, υ = Ω / q is the acoustic wave velocity, γ e is the electrostriction constant of the optical fiber, ε is the dielectric constant of the material, and Γ′ represents the attenuation coefficient of the acoustic wave. After space-time transformation and slow-varying amplitude approximation, we get

[0045]

[0046] where r is the core radius. Under steady-state conditions, Equation (7) simplifies to:

[0047]

[0048]

[0049] Then the Brillouin frequency shift (υ B ) is The Brillouin linewidth (Γ B ) is Therefore, Equation (9) can be written as:

[0050]

[0051] Furthermore, the Brillouin scattering spectrum (Brillouin Gain Spectrum, BGS) that satisfies the Lorentz distribution is:

[0052]

[0053] The maximum Brillouin gain coefficient (g0) is:

[0054]

[0055] where c is the speed of light in a vacuum, and n eff is the effective refractive index. It can be seen that under the action of the stimulated Brillouin scattering effect, the topological charge number carried by the orbital angular momentum mode will have a certain impact on its Brillouin gain spectrum.

[0056] In this embodiment, the Brillouin scattering spectra of the OAM modes with different topological charge numbers of the photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum at wavelengths of 1530 nm, 1540 nm, 1550 nm, and 1�60 nm are as shown in Figure 5a , Figure 5b , Figure 5c , Figure 5d . Observing Figure 5a , Figure 5b , Figure 5c , Figure 5d it can be obtained that:

[0057] In the photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum according to the present invention, as the topological charge number increases, the BGS gradually shifts to the right, and the maximum Brillouin gain coefficient gradually decreases. The influence of wavelength on the maximum Brillouin gain coefficient, Brillouin frequency shift, and linewidth in the BGS is relatively small. As the topological charge number increases, the maximum Brillouin gain coefficient decreases, the Brillouin frequency shift gradually increases, and the linewidth of the Brillouin gain spectrum gradually decreases. However, the linewidth change of all modes at the same wavelength does not exceed 1 MHz. Therefore, it has a relatively stable linewidth, effectively preventing the introduction of noise and crosstalk.

[0058] In this embodiment, the maximum Brillouin gain coefficient (g0) of the photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum is as Figure 6 shown, and observing Figure 6 it can be obtained that:

[0059] In the photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum according to the present invention, the influence of wavelength on the maximum Brillouin gain coefficient in the BGS is relatively small, and the influence of the topological charge number on g is less when 1 ≤ l ≤ 9 than when 10 ≤ l ≤ 17. As the topological charge number increases, g0 decreases, but the g0 of all modes is greater than 7×10 -9 m / W, which is improved by 4 - 5 orders of magnitude, and can effectively achieve the synchronous amplification of OAM modes.

[0060] According to the theory of stimulated Brillouin scattering, the energy of the output signal after amplification satisfies Equation (13):

[0061] E out = E in exp(g0E p L eff ) (13)

[0062] wherein, E out and E in are the optical energies of the output and input signal lights respectively, and E p is the pump light energy. Under the condition of ignoring the gain loss and steady state, the gain of the signal light is exp(g0E p L eff ). It can be seen that in the photonic crystal fiber amplifier, the optical signal is exponentially amplified, and the signal gain of the optical signal is closely related to the pump light, the Brillouin gain coefficient, and the effective fiber length.

[0063] In this embodiment, the photonic crystal fiber amplifier for stimulated Brillouin amplification of orbital angular momentum has a wavelength of 1550 nm, a pump pulse width of 10 ns, an effective fiber length of 0.4 m, and a pump light energy in the range of 10 -6 J - 10 -3The signal gain of the OAM mode when within the range of J is as Figure 7 shown. Observing Figure 7 it can be obtained that:

[0064] In the photonic crystal fiber amplifier that can be used for stimulated Brillouin amplification of orbital angular momentum according to the present invention, the optical signal is exponentially amplified. As the pump light power increases, the signal gain gradually increases, and the gain of the low-order mode is larger than that of the high-order mode, that is, the low-order mode is more easily amplified. This is because the threshold of the low-order mode is low and the stimulated Brillouin effect is more easily excited. When the pump light energy is within the range of 10 -6 J - 10 -3 J, a signal gain of up to 1697.5 dB can be obtained.

[0065] The above is only one implementation mode of the present invention, not all or the only implementation mode. Any equivalent transformation of the technical solution of the present invention adopted by those of ordinary skill in the art by reading the specification of the present invention is covered by the claims of the present invention.

Claims

1. A photonic crystal fiber amplifier applicable to stimulated Brillouin amplification of orbital angular momentum, which combines stimulated Brillouin amplification with photonic crystal fiber and can realize synchronous amplification and long-distance transmission of multiple orbital angular momentum modes. It is characterized in that: The photonic crystal fiber amplifier is composed of a circular air hole layer distributed in a ring shape and a ring. There are five layers of circular air holes distributed in a ring outside the ring, and two layers of circular air holes arranged in a regular hexagon inside the ring. All the circular air hole layers and the ring have the same center of the circle. Among them, the base material of the optical fiber is Schott SF2, the ring is filled with chalcogenide glass As2Se3, the diameter of the outermost circular air hole outside the ring is the same as that of the outermost circular air hole inside the ring, and the diameter of the innermost circular air hole outside the ring is the same as that of the innermost circular air hole inside the ring.

2. A photonic crystal fiber amplifier applicable to stimulated Brillouin amplification of orbital angular momentum according to claim 1, characterized in that: The circular air holes in the same layer have the same diameter.

3. A photonic crystal fiber amplifier applicable to stimulated Brillouin amplification of orbital angular momentum according to claim 1, characterized in that: When the pump light and the signal light carry orbital angular momentum, the Brillouin scattering spectrum of the Lorentz distribution satisfied by the optical fiber material density can be obtained through the following derivation process: The OAM mode amplification based on stimulated Brillouin amplification is the nonlinear interaction of the pump light E1(z,t) carrying the OAM mode and the OAM signal light E2(z,t) through the excited acoustic wave field. When the optical fiber is subjected to the electrostriction effect, the optical fiber material density ρ follows the acoustic wave equation where υ = Ω / q is the acoustic wave velocity, q is the acoustic wave vector, Ω is the acoustic wave frequency, γ e is the electrostriction constant of the optical fiber, ε is the dielectric constant of the material, Г′ represents the attenuation coefficient of the acoustic wave, is the Laplace operator, z is the spatial coordinate along the light propagation direction, and is obtained after spatio-temporal transformation and slow-varying amplitude approximation under steady-state conditions where l3 is the topological charge number of the acoustic wave, A1 and A2 are the amplitudes of the pump light and the signal light respectively, and r is the core radius. Furthermore, the Brillouin scattering spectrum of the Lorentz distribution it satisfies is: The maximum Brillouin gain coefficient g0 is: where c is the speed of light in vacuum, and n eff is the effective refractive index, and λ is the wavelength of the incident light.

4. A photonic crystal fiber amplifier for stimulated Brillouin amplification applicable to orbital angular momentum according to claim 3, characterized in that: a. The pump light E1(z,t) and the signal light E2(z,t) carrying orbital angular momentum are expressed as: E1(z,t) = A1(z,t)exp(i(k1 - ω1t))exp(il1θ) + c.c. (6) E2(z,t) = A2(z,t)exp(i(k2 - ω2t))exp(il2θ) + c.c. (7) where k1 = 2π / λ1 and k2 = 2π / λ2 are the wave vectors of the pump light and the signal light respectively, λ1 and λ2 are the wavelengths of the pump light and the signal light respectively, ω1 and ω2 are the angular frequencies of the pump light and the signal light respectively, l1 and l2 are the topological charge numbers of the pump light and the signal light respectively, θ is the angle formed by the wave vectors of the pump light and the signal light, z is the spatial coordinate along the light propagation direction, and c.c. is to take the complex conjugate of the previous formula; b. The excited acoustic wave field is expressed as: where ρ0 is the average material density. According to the momentum conservation of the orbital angular momentum optical field, there is l3 = l1 - l2.

Images