Gain flattening filter, optical fiber amplifier and wavelength division multiplexing communication system

Abstract

The invention provides a gain flattening filter, an optical fiber amplifier and a wavelength division multiplexing communication system, relates to the technical field of optical communication, and is used for realizing gain flattening filtering. The gain flattening filter comprises N first optical fibers in a first fiber core mode, N second optical fibers in the first fiber core mode, M second optical fibers in the second fiber core mode and N third optical fibers in the first fiber core mode, the second optical fiber comprises a plurality of cascaded fiber bragg gratings, and the plurality of cascaded fiber bragg gratings are used for coupling at least one first fiber core mode in the N first fiber core modes to at least one second fiber core mode in the M second fiber core modes, the mode order of at least one second fiber core mode in the M second fiber core modes is higher than the mode order of at least one first fiber core mode in the N first fiber core modes; the first optical fiber, the second optical fiber and the third optical fiber are sequentially connected. According to the scheme, the gain flattening of the optical signal can be realized through the gain flattening filter with a simple structure.

Description

Technical Field

[0001] The present application relates to the field of optical communication technology, and in particular to a gain flattening filter, an optical fiber amplifier, and a wavelength division multiplexing communication system. Background Art

[0002] In optical communications, wavelength division multiplexing (WDM) systems transmit multiple wavelength signals through a single optical fiber, significantly increasing transmission capacity. For long-distance transmission, fiber amplifiers are used to amplify these wavelength signals. However, due to the gain spectrum characteristics of the ions doped in the fiber amplifiers, the gain of different channel wavelengths can be inconsistent, resulting in gain unevenness. This in turn causes power imbalance across the channels and degrades the system's signal-to-noise ratio. Therefore, flattening the gain spectrum of fiber amplifiers is a practical challenge in the application of WDM communication systems.

[0003] Existing common technology solves this problem through gain flattening filter (GFF): GFF achieves static compensation by introducing an attenuation spectrum complementary to the gain spectrum. However, the device structure of this method is generally complex, which is not conducive to the application of wavelength division multiplexing communication systems. Summary of the Invention

[0004] The present application provides a gain flattening filter, an optical fiber amplifier, and a wavelength division multiplexing communication system, which can achieve gain flattening of an optical signal through a gain flattening filter with a simple structure.

[0005] In a first aspect, the present application provides a gain-flattened filter, comprising: a first optical fiber, a second optical fiber, and a third optical fiber; the first optical fiber comprises N first core modes, the second optical fiber comprises N first core modes and M second core modes, and the third optical fiber comprises N first core modes, where M is greater than or equal to N; the second optical fiber comprises a plurality of cascaded fiber gratings, the plurality of cascaded fiber gratings being used to couple at least one first core mode of the N first core modes to at least one second core mode of the M second core modes, and the mode order of at least one second core mode of the M second core modes is higher than the mode order of at least one first core mode of the N first core modes; the first optical fiber, the second optical fiber, and the third optical fiber are connected in sequence.

[0006] In this solution, the first core modes supported by the first optical fiber and the third optical fiber are the same in number as the first core modes. The multiple cascaded fiber Bragg gratings in the second optical fiber are designed to couple the energy of at least one first core mode with a lower mode order to at least one second core mode with a higher mode order, thereby flattening the energy of the N first core modes. When the optical signal containing N first core modes and M second core modes continues to be transmitted to the third optical fiber, since the third optical fiber only supports N first core modes, the M second core modes can be filtered out. Ultimately, only the N first core modes with flattened energy can be output, thereby achieving gain flattening filtering. This solution has a simple structure and achieves gain flattening of the optical signal using only three sections of optical fiber and a plurality of cascaded fiber Bragg gratings.

[0007] In one possible implementation, multiple cascaded fiber Bragg gratings (FBGs) have different parameters, including at least one of the following: period and wavelength. In this implementation, by adjusting the parameters, the cascaded fiber Bragg gratings can couple optical signals of different wavelengths and modes transmitted in a wavelength-division multiplexing communication system, achieving gain flattening.

[0008] In another possible implementation, the coupling of the first core mode to the second core mode satisfies the following conditions:

[0009]

[0010] Formula (1) is the phase matching condition satisfied by the coupling of the first core mode to the second core mode, and formula (2) is the coupling coefficient of the first core mode to the second core mode, where m is the angular order of the first core mode, which indicates the number of oscillations of the light field along the circumferential direction, n is the radial order of the first core mode, which indicates the number of extreme points of the light field along the radial direction, p is the angular order of the second core mode, and q is the radial order of the second core mode; κ mnpq is the coupling coefficient; n eff,mn is the effective refractive index of the first core mode, n eff,pq is the effective refractive index of the second core mode; λ is the resonant wavelength of the fiber Bragg grating, Λ is the period of the fiber Bragg grating; ω0 is the center frequency of the optical signal; ε0 is the dielectric constant of vacuum; Δn(z) is the refractive index variation of the fiber Bragg grating along the axial direction; A mn is the amplitude coefficient of the transverse mode field distribution of the first core mode, A pq is the amplitude coefficient of the transverse mode field distribution of the second core mode; F mn (r) is the radial mode field distribution of the first core mode and the function of radius, F pq (r) is the radial mode field distribution of the second core mode as a function of radius; is the phase angle of the optical signal; φ mn is the initial phase angle of the first core mode in the azimuthal component, φ pq is the initial phase angle of the second core mode in the angular component. In formula (2), the coupling coefficient κ mnpq is not equal to zero, that is, the angular order m of the first core mode is equal to the angular order p of the second core mode; wherein the mode order includes the angular order and the radial order. In this scheme, the first core mode and the second core mode that are coupled, and the first core mode coupled to the second core mode can be obtained by formula (1) and formula (2).

[0011] The resonant wavelength, period and other parameters of the fiber Bragg grating required by the core mode are determined, and appropriate fiber Bragg gratings are designed accordingly.

[0012] Another possible implementation is to use a long-period fiber grating (LPFBG). LPFBGs typically have a period of hundreds of microns, making it easier to adjust the parameters of the FBGs.

[0013] In another possible implementation, the first, second, and third optical fibers are all few-mode fibers. Few-mode fibers typically support fewer modes, have larger mode spacing, and are less susceptible to crosstalk between modes, making it easier to achieve high-precision mode filtering.

[0014] In another possible implementation, the first, second, and third optical fibers have identical refractive index profiles, where the refractive index profiles include the refractive index distributions of the core and cladding regions. When the refractive index profiles of the two butted optical fibers are identical, the effective refractive index and mode field diameter of the core modes of the two fibers match, and the optical field distributions of the core and cladding can be highly overlapping. This facilitates efficient transmission of optical energy in the core mode from the first optical fiber to the second optical fiber, or from the second optical fiber to the third optical fiber, minimizing losses.

[0015] In another possible implementation, the first optical fiber, the second optical fiber, and the third optical fiber are further coaxially aligned and connected along the optical axis. The coaxial alignment between the optical fibers is conducive to reducing mode field mismatch and reducing crosstalk caused by mode field mismatch.

[0016] In a second aspect, the present application provides an optical fiber amplifier, comprising: a gain optical fiber and a gain flattening filter as described in the first aspect and any possible implementation manner of the first aspect, connected to the gain optical fiber.

[0017] In a third aspect, the present application provides a wavelength division multiplexing communication system, comprising a laser light source, a modulator, and an optical fiber amplifier connected to the modulator, such as any one of the second aspect and possible implementations of the second aspect.

[0018] The beneficial effects of the second and third aspects mentioned above refer to the corresponding description of the first aspect and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A gain flattening filter provided in this application;

[0020] Figure 2 Another gain flattening filter provided in this application. DETAILED DESCRIPTION

[0021] The terms "first", "second", and "third" in the specification and drawings of this application are used to distinguish different objects, or to distinguish different treatments of the same object, rather than to describe a specific order of objects. In addition, the terms "including" and "having" and any variations thereof mentioned in the description of this application are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include other steps or units that are not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0022] It should be noted that in the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being more preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0023] In the description of the present application, unless otherwise specified, “plurality” means two or more.

[0024] A gain flattening filter 10 is provided in the related art, such as Figure 1 As shown, it includes: a first dual-fiber pigtail 101, a first glass tube 102, a first lens 103, a second glass tube 104, a filter 105, a second lens 107, a second dual-fiber pigtail 108, a third glass tube 109, and a fourth glass tube 110.

[0025] The gain flattening filter 10 also includes an inner glass tube 106 and an outer glass tube, wherein the outer glass tube includes a first glass tube 102, a second glass tube 104, a third glass tube 109, and a fourth glass tube 110 connected in sequence. A first dual-fiber pigtail 101 is located at the input end of the gain flattening filter 10. A filter 105 is attached to the end of the inner glass tube 106, and the inner glass tube 106 and the filter 105 are nested within the outer glass tube. A first lens 103 and a second lens 107 are located on either side of the filter 105 and within the outer glass tube. A second dual-fiber pigtail 108 is located at the output end of the gain flattening filter 10. An optical signal enters the first dual-fiber pigtail 101 at the input port, is collimated by the first lens 103, and then transmitted to the filter 105. Optical powers of different wavelengths are attenuated to varying degrees by the filter 105, achieving a flat gain effect for the optical signal. The attenuated optical signal then passes through the second lens 107 and is output from the second dual-fiber pigtail 108. The gain flattening filter 10 is a 2×2 integrated device. By using dual-fiber pigtails at the input and output ports, the gain of two beams of light can be flattened, thereby improving efficiency. However, the gain flattening filter 10 is a spatial device with a relatively complex structure and cannot be applied to wavelength division multiplexing communication systems using optical fiber as the main transmission medium. In addition, for optical signal transmission containing multiple core modes, the mode field distribution of different core modes after being incident on the gain flattening filter 10 is not conducive to maintaining, affecting the transmission performance of each mode.

[0026] In response to the above technical problems, the present application provides a gain flattening filter, an optical amplifier, and a wavelength division multiplexing communication system, which can achieve gain flattening of optical signals through a gain flattening filter with a simple structure.

[0027] The embodiments provided in this application are described in detail below with reference to the accompanying drawings.

[0028] The embodiment of the present application provides a gain flattening filter 20, such as Figure 2 As shown, the optical fiber 21 includes a first optical fiber 21, a second optical fiber 22, and a third optical fiber 23. The first optical fiber 21 includes N first core modes, the second optical fiber 22 includes N first core modes and M second core modes, and the third optical fiber 23 includes N first core modes, where M is greater than or equal to N. The second optical fiber 22 includes a plurality of cascaded fiber Bragg gratings 24, which are used to couple at least one first core mode of the N first core modes to at least one second core mode of the M second core modes, and the mode order of at least one second core mode of the M second core modes is higher than the mode order of at least one first core mode of the N first core modes. The first optical fiber 21, the second optical fiber 22, and the third optical fiber 23 are connected in sequence. The mode order can be represented by a combination of angular order and radial order.

[0029] The first optical fiber 21 and the third optical fiber 23 each include N first core modes, and the mode orders of these N first core modes are different. The mode orders of the M second core modes included in the second optical fiber 22 are also different. The N first core modes in the first optical fiber 21 are transmitted to the second optical fiber 22; at least one first core mode among the N first core modes in the first optical fiber 21 is coupled to at least one second core mode among the M second core modes of the second optical fiber 22 under the action of multiple cascaded fiber gratings 24. The mode order of at least one second core mode is larger than the mode order of at least one first core mode, then at least one first core mode is a low-order mode, and at least one second core mode is a high-order mode. For example, the first core mode with higher mode energy among the N first core modes can be coupled to the M second core modes. The corresponding second core mode can also couple the N first core modes to the N second core modes among the M second core modes. The first core mode and the corresponding second core mode are coupled at the resonant wavelength of the fiber grating 24, and the energy of these N first core modes tends to be flattened. The M second core modes and the N first core modes are transmitted from the second optical fiber 22 to the third optical fiber 23. Since the third optical fiber 23 only includes the first core mode and cannot transmit the second core mode, the M second core modes can be filtered out. Ultimately, only the N first core modes with flattened energy can be output, achieving gain flattening of the N first core modes. The gain flattening filter 20 of this scheme has a simple structure. It only uses three sections of optical fiber and a plurality of cascaded fiber gratings to achieve gain flattening of the optical signal. And because it is made of optical fiber, it is suitable for wavelength division multiplexing systems with optical fiber as the main transmission medium.

[0030] In other embodiments, the multiple cascaded fiber Bragg gratings 24 in the gain flattening filter 20 have different parameters, including at least one of the following: period and wavelength. By adjusting the parameters of the multiple cascaded fiber Bragg gratings 24, the multiple cascaded fiber Bragg gratings 24 can be coupled to different wavelengths and modes of optical signals transmitted in a wavelength division multiplexing communication system, thereby achieving gain flattening.

[0031] In some other embodiments, the coupling of the first core mode to the second core mode satisfies the following conditions:

[0032]

[0033] Formula (1) is the phase matching condition satisfied by the coupling of the first core mode to the second core mode, and formula (2) is the coupling coefficient of the coupling of the first core mode to the second core mode, where m is the angular order of the first core mode, n is the radial order of the first core mode, p is the angular order of the second core mode, and q is the radial order of the second core mode; κ mnpq is the coupling coefficient; n eff,mn is the effective refractive index of the first core mode, n eff,pq is the effective refractive index of the second core mode; λ is the resonant wavelength of the fiber Bragg grating, Λ is the period of the fiber Bragg grating; ω0 is the center frequency of the optical signal; ε0 is the dielectric constant of vacuum; Δn(z) is the refractive index variation of the fiber Bragg grating along the axial direction; A mn is the amplitude coefficient of the transverse mode field distribution of the first core mode, A pq is the amplitude coefficient of the transverse mode field distribution of the second core mode; F mn (r) is the radial mode field distribution of the first core mode and the function of radius, F pq (r) is the radial mode field distribution of the second core mode as a function of radius; is the phase angle of the optical signal; φ mn is the initial phase angle of the first core mode in the azimuthal component, φ pq is the initial phase angle of the second core mode in the angular component. In formula (2), the coupling coefficient k mnpq is not equal to zero, that is, the angular order m of the first core mode is equal to the angular order p of the second core mode; the mode order includes the angular order and the radial order. In this scheme, the resonance wavelength, period and other parameters of the fiber Bragg grating 24 required for coupling the first core mode and the second core mode, and for coupling the first core mode to the second core mode can be obtained by formulas (1) and (2), and a suitable fiber Bragg grating can be designed accordingly.

[0034] For example, the optical signal is gain-amplified in the optical fiber amplifier. Due to the uneven gain spectrum of the optical fiber amplifier itself, the gain of different wavelengths and different modes is different. According to the attenuation spectrum obtained by flipping the gain spectrum and the core mode transmitted in the optical fiber, the parameters of the multiple cascaded optical fiber gratings 24 are obtained by satisfying formulas (1) and (2). The parameters of the multiple cascaded optical fiber gratings 24, such as the period or length of the multiple cascaded optical fiber gratings 24, are adjusted. Under the action of the multiple cascaded optical fiber gratings 24, the energy of the first core mode with higher gain is coupled to the second core mode. The magnitude of the coupled energy depends on the gain of the first core mode with the smallest gain in the entire gain spectrum. This process is equivalent to attenuating the energy of the first core mode with higher energy. Then, the parameters of the multiple cascaded optical fiber gratings 24 are adjusted to change the central wavelength of the attenuation spectrum, and the energy of the first core mode with higher wavelength gain is coupled to the second core mode. Since the third optical fiber 23 does not support the transmission of the second core mode, M second core modes are finally filtered out, leaving only N first core modes to continue to transmit. In this way, the gain flattening of optical signals of different wavelengths and different modes can be achieved.

[0035] In other embodiments, the fiber grating 24 is a long-period fiber grating, which generally has a period of hundreds of microns, making it easier to adjust the parameters of the fiber grating in terms of technology.

[0036] In other embodiments, the first optical fiber 21, the second optical fiber 22, and the third optical fiber 23 are all few-mode fibers. Few-mode fibers generally support fewer core modes and larger core mode spacing, so crosstalk between core modes is less likely to occur, making it easier to achieve high-precision mode filtering.

[0037] In other embodiments, the first optical fiber 21, the second optical fiber 22, and the third optical fiber 23 have the same refractive index profile, where the refractive index profile includes the refractive index distributions of the core region and the cladding region. When the refractive index profiles of the two butted optical fibers are identical, the effective refractive index and mode field diameter of the transmitted core mode match, and the optical field distributions of the core and cladding can be highly overlapped. This facilitates efficient transmission of optical energy in the core mode from the first optical fiber 21 to the second optical fiber 22, or from the second optical fiber 22 to the third optical fiber 23, thereby reducing losses.

[0038] In other embodiments, the first optical fiber 21, the second optical fiber 22, and the third optical fiber 23 are also coaxially aligned and connected along the optical axis. The coaxial alignment between the optical fibers is conducive to reducing mode field mismatch and reducing crosstalk caused by mode field mismatch.

[0039] Exemplarily, the first optical fiber 21 and the third optical fiber 23 each include four first core modes, and the four first core modes are: LP 01 LP 11 LP21 and LP 02 The second optical fiber 22 includes four first core modes and six second core modes, the four first core modes are: LP 01 LP 11 LP 21 LP 02 , the 6 second core modes are: LP 31 LP 12 LP 41 LP 22 LP 03 and LP 51 The second optical fiber 22 further includes a plurality of cascaded optical fiber gratings 24; the first optical fiber 21, the second optical fiber 22 and the third optical fiber 23 are connected in sequence.

[0040] According to formula (2), the first core mode LP is determined 01 and the second core mode LP 03 The coupling coefficient is not zero, the second core mode LP 03 The mode order relative to the first core mode LP 01 Because the mode order is large, the first core mode LP 01 is a low-order mode, with the second core mode LP 03 For high-order modes, similarly, the first core mode LP 11 and the second core mode LP 12 Between, the first core mode LP 21 and the second core mode LP 22 Between, the first core mode LP 02 and the second core mode LP 03 Coupling can also occur between them, and each fiber Bragg grating 24 produces energy attenuation for the core mode that resonates with it but does not produce energy attenuation for other core modes; according to formula (1) and the attenuation spectrum generated by the inversion of the fiber amplifier gain spectrum, the parameters of multiple cascaded fiber Bragg gratings 24 are adjusted to make the four first core modes (LP 01 LP 11 LP 21 LP 02 ) is coupled to the corresponding four second core modes (LP) in the six second core modes 03 LP 12 LP 22 LP 03), coupling occurs at the resonance wavelength λ of the multiple cascaded fiber Bragg gratings 24, realizing energy transfer from the four first core modes to the four second core modes; continuing to adjust the parameters of the multiple cascaded fiber Bragg gratings 24 to change the resonance wavelength, the four first core modes can be coupled to the four second core modes at the resonance wavelength, balancing the gains of the four first core modes; the optical signal continues to be transmitted to the third optical fiber 23, and since the third optical fiber 23 only supports the transmission of the four first core modes, there are ultimately only the four first core modes with flattened gain, i.e., LP 01 LP 11 LP 21 and LP 02 Continuing the transmission, this process realizes the gain flattening of different wavelengths and modes of the optical signal.

[0041] The embodiment of the present application further provides a fiber amplifier 30 , comprising: a gain fiber and a gain flattening filter 20 connected to the gain fiber.

[0042] The optical fiber amplifier 30 is a device used to enhance signal power in optical communication systems, compensate for transmission losses, and extend communication distances. The gain fiber is the core of the optical fiber amplifier 30 and amplifies the input optical signal through stimulated emission of radiation. Common gain fibers include, but are not limited to, erbium-doped fiber and ytterbium-doped fiber. The gain flattening filter 20 is used to compensate for the gain unevenness of the gain fiber, ensuring that the optical signal output by the optical fiber amplifier 30 is gain-flattened at different wavelengths and in different modes.

[0043] The embodiment of the present application further provides a wavelength division multiplexing communication system 40, comprising a laser light source, a modulator, and an optical fiber amplifier 30 connected to the modulator.

[0044] The laser light source generates an optical signal of a specific wavelength. The modulator adds information to the optical signal to generate a modulated optical signal for transmission. The optical fiber amplifier 30 amplifies the transmitted optical signal, compensates for loss, and flattens the gain.

[0045] The above are only specific embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or replacements within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A gain flattening filter, characterized in that: include: a first optical fiber, a second optical fiber, and a third optical fiber; The first optical fiber includes N first core modes, the second optical fiber includes N first core modes and M second core modes, and the third optical fiber includes N first core modes, where M is greater than or equal to N; The second optical fiber includes a plurality of cascaded fiber Bragg gratings, and the plurality of cascaded fiber Bragg gratings are used to couple at least one of the N first core modes to at least one of the M second core modes, and a mode order of at least one of the M second core modes is higher than a mode order of at least one of the N first core modes. The first optical fiber, the second optical fiber, and the third optical fiber are connected in sequence.

2. The gain flattening filter according to claim 1, wherein The plurality of cascaded fiber gratings have different parameters, and the parameters include at least any one of the following: period and wavelength.

3. The gain flattening filter according to claim 2, wherein: The coupling of the first core mode to the second core mode satisfies the following conditions: Formula (1) is the phase matching condition satisfied by the coupling of the first core mode to the second core mode, and formula (2) is the coupling coefficient of the first core mode to the second core mode, wherein m is the angular order of the first core mode, n is the radial order of the first core mode, p is the angular order of the second core mode, and q is the radial order of the second core mode; κ mnpq is the coupling coefficient; n eff,mn is the effective refractive index of the first core mode, n eff,pq is the effective refractive index of the second core mode; λ is the resonant wavelength of the fiber Bragg grating, Λ is the period of the fiber Bragg grating; ω0 is the center frequency of the transmitted optical signal; ε0 is the dielectric constant of vacuum; Δn(z) is the refractive index variation of the fiber Bragg grating along the axial direction; A mn is the amplitude coefficient of the transverse mode field distribution of the first core mode, A pq is the amplitude coefficient of the transverse mode field distribution of the second core mode; F mn (r) is the function of the radial mode field distribution of the first core mode and the radius, F pq (r) is a function of the radial mode field distribution of the second core mode as a function of radius; is the phase angle of the optical signal; φ mn is the initial phase angle of the first core mode in the azimuthal component, φ pq is the initial phase angle of the second core mode in the angular component. In formula (2), the coupling coefficient κ mnpq is not equal to zero, that is, the angular order m of the first core mode is equal to the angular order p of the second core mode; the mode order includes the angular order and the radial order.

4. The gain flattening filter according to any one of claims 1 to 3, characterized in that: The fiber grating is a long-period fiber grating.

5. The gain flattening filter according to any one of claims 1 to 4, characterized in that: The first optical fiber, the second optical fiber, and the third optical fiber are all few-mode optical fibers.

6. The gain flattening filter according to any one of claims 1 to 5, characterized in that: The first optical fiber, the second optical fiber, and the third optical fiber have the same refractive index profile, wherein the refractive index profile includes refractive index profiles of a core region and a cladding region.

7. The gain flattening filter according to any one of claims 1 to 6, characterized in that: The first optical fiber, the second optical fiber and the third optical fiber are also coaxially aligned and connected along the optical axis.

8. An optical fiber amplifier, characterized in that: The invention comprises a gain optical fiber and a gain flattening filter according to any one of claims 1 to 7 connected to the gain optical fiber.

9. A wavelength division multiplexing communication system, characterized in that: The optical fiber amplifier comprises a laser light source, a modulator, and the optical fiber amplifier according to claim 8 connected to the modulator.

Images