JP6645952B2 - Optical fiber amplifier - Google Patents

Description

  The present invention relates to an optical fiber amplifier used in mode division multiplex optical transmission.

  Mode division is one of the technologies to dramatically increase the transmission capacity of the optical transmission system in response to the explosion of traffic transmitted in the trunk optical transmission system due to the increase in speed and capacity of communication services. Research on multiplex optical transmission technology has been rapidly advancing in recent years. In a long-distance mode multiplexed optical transmission system, a plurality of signal lights of different linearly polarized (LP) modes are multiplexed and modulated, and separated at the output end for each mode. For this reason, the long-distance mode multiplexing optical transmission system requires a multi-mode optical fiber amplifier that amplifies a plurality of different modes of signal light with a single amplification fiber, and this research and development has been advanced (Non-patent document). 1).

  When there is a gain difference between modes in a multimode optical fiber amplifier, in an optical transmission system in which an optical fiber amplifier is applied to multi-repeater optical fiber transmission, an optical power difference between modes occurs each time optical relay is performed, and as a result, this accumulates. At the receiving end, a difference in optical signal-to-noise ratio (optical SNR) occurs between modes, and transmission characteristics differ between modes. For this reason, it is important to reduce the gain difference between modes in a multimode optical fiber amplifier.

  The gain of the multimode optical fiber amplifier is related to the overlap between the distribution of the excited amplification ions (eg, erbium ions) and the distribution of the signal light intensity. The gain is likely to increase as the overlap increases. In order to reduce the above-described gain difference between modes, it is necessary to reduce the overlap between the excited erbium ion distribution and the signal light intensity distribution between the signal light modes.

  For example, in Non-Patent Document 2, while the core diameter is made slightly larger to make the number of propagation modes of the waveguide larger than the required number of modes, the light intensity distribution of all signal light modes (less than the number of propagation modes) is reduced by erbium ion A double-clad erbium-doped multimode fiber designed to be sufficiently confined in a core doped with is used. In this erbium-doped multimode fiber, by uniformly exciting the erbium ions in the core using cladding excitation, the signal light of each mode having a different signal light intensity distribution overlaps the excitation erbium ion distribution almost equally. As a result, according to the technique of Non-Patent Document 2, it is shown that the gain of each signal light mode can be made substantially equal and the gain difference between modes can be reduced.

E. Ip et al., "88 × 3 × 112-Gb / s WDM Transmission over 50 km of Three-Mode Fiber with Inline Few-Mode Fiber Amplifier", in The 37th European Conference and Exhibition on Optical Communication (ECOC 2011) Technical Digest, PDP, Th.13.C.2. N. Fontaine et al., "Multi-mode Optical Fiber Amplifier Supporting over 10 Spatial Modes", in Optical Fiber Communication Conference (OFC), 2016, PDP, Th5A.4.

  However, Non-Patent Document 2 has the following two problems. First, since the number of propagation modes of the core is larger than the number of signal light modes, there is a problem that an unnecessary higher-order mode is excited by mode coupling while the signal light propagates through the erbium-doped multimode fiber. . Since the excited higher-order mode light is unnecessary light, the amplification gain of the signal light is reduced as much as the higher-order mode light is amplified, thereby deteriorating the gain efficiency of the optical amplifier, and When the higher-order mode light than necessary is combined with the signal light, it also has an adverse effect as noise. Second, the reliability of the excitation light source becomes a problem. The pumping light source used when the erbium-doped multimode fiber is clad-excited is a multimode semiconductor laser. However, since it has been developed mainly for fiber laser applications, there is a problem in securing communication reliability.

  SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is possible to suppress deterioration of gain efficiency and noise characteristics in an optical fiber amplifier while ensuring high communication reliability. The purpose is to:

The optical fiber amplifier according to the present invention, the pump light source that generates the pump light of the fundamental mode, a conversion unit that converts the mode of the pump light generated by the pump light source from the fundamental mode to a higher-order mode, A multiplexing section for multiplexing the higher-order mode pump light and the input signal light; an erbium-doped amplification optical fiber for multiplexing the multiplexed light that is multiplexed by the multiplexing section and amplifying the multiplexed light; A demultiplexing unit for demultiplexing the output light emitted from the optical fiber into a higher-order mode pump light and an amplified signal light. is the same as that of the optical mode, the converting unit converts the excitation light in the fundamental mode to higher modes mode LP _{0 m} to m of the amplification optical fiber capable of transmitting mode LP _lm is maximized.

As described above, according to the present invention, the fundamental mode generated from the excitation light source excitation light, m modes LP _lm of transmittable linear polarization in the amplification optical fiber is mode LP _{0 m} to a maximum Since the conversion to the higher-order mode is performed by the conversion unit, an excellent effect that deterioration of gain efficiency and noise characteristics in the optical fiber amplifier can be suppressed in a state where high communication reliability is secured is obtained.

FIG. 1 is a configuration diagram showing a configuration of an optical fiber amplifier according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing the configuration of the amplification optical fiber 104. FIG. 3 is a distribution diagram showing a light intensity distribution of each signal light mode propagating through the amplification optical fiber 104 whose signal light allowable mode is the 6-LP mode. FIG. 4 is an explanatory diagram for explaining the light intensity distribution in the odd mode and the even mode orthogonal to each of the LP ₁₁ , LP ₂₁ , LP ₃₁ , and LP ₁₂ modes. FIG. 5 is a characteristic diagram showing a gain spectrum of the optical fiber amplifier according to the embodiment in which the amplifying optical fiber 104 is constituted by the ring core erbium-doped multimode fiber. Figure 6 is an optical fiber amplifier configured to amplify optical fiber 104 than the ring core erbium doped multimode fiber is a characteristic diagram showing a gain spectrum in the case where the mode of the excitation light was set to LP _01. FIG. 7 is a characteristic diagram showing a gain spectrum of the optical fiber amplifier according to the embodiment in which the amplifying optical fiber 104 is composed of a circular core erbium-doped multimode fiber. Figure 8 is a characteristic diagram showing a gain spectrum in the case where the mode of the excitation light in the optical fiber amplifier configured to amplify optical fiber 104 from the erbium doped multimode fiber of circular core was LP _01.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration of an optical fiber amplifier according to an embodiment of the present invention. This optical fiber amplifier includes an excitation light source 101, a conversion unit 102, a multiplexing unit 103, an amplification optical fiber 104, and a demultiplexing unit 105.

  The pump light source 101 generates pump light of a fundamental mode. The converter 102 converts the mode of the excitation light generated from the excitation light source 101 from the fundamental mode to a higher-order mode. The multiplexing unit 103 multiplexes the higher-order mode pump light converted by the conversion unit 102 and the input signal light. The multiplexing unit 103 multiplexes the signal light and the pump light input via the optical isolator 106.

  The amplification optical fiber 104 receives the multiplexed light multiplexed by the multiplexing unit 103, optically amplifies the multiplexed light, and outputs the optically amplified multiplexed light. The amplification optical fiber 104 is doped with erbium. The amplification optical fiber 104 is a ring core erbium-doped multimode fiber.

The amplification optical fiber 104 includes a ring core 201, a cladding 202 formed outside the ring core 201, and an inner core 203 formed inside the ring core 201, as shown in the cross-sectional view of FIG. For example, the refractive index of the cladding glass is equal to the refractive index of the inner core glass and smaller than that of the ring core. The inner radius of the ring core 201 is 2.5 μm, the outer radius is 7 μm, and the relative refractive index difference is 1%. In the amplification optical fiber 104 having this configuration, the 6-LP mode of LP ₀₁ , LP ₁₁ , LP ₂₁ , LP ₀₂ , LP ₃₁ , and LP ₁₂ is an allowable mode (transmittable mode) at the signal light wavelength (1550 nm band). Becomes

The amplifying optical fiber 104 is one in which the mode _LPlm of the linearly polarized wave that can be transmitted is the same as the mode of the signal light. In the embodiment, the signal light is obtained by multiplexing the _{LP 01, LP 11, LP 21} , LP 02, LP 31, LP 12. In the LP _lm , as is well known, 1 is a half (integer) of the number of nodes dividing the light intensity distribution formed in the cross-sectional direction of the amplification optical fiber 104. In addition, m is the number (integer) of the maximum value of the light intensity distribution in the cross section direction of the amplification optical fiber 104.

In other words, in the embodiment, the 6-LP mode signal light is to be amplified, and the mode of the signal light and the allowable signal light mode _LPlm of the amplification optical fiber 104 are the same. The mode in which m of the allowance mode LP _lm in the amplification optical fiber 104 is maximum is a LP ₀₂ or LP _12. In this case, the amplification optical fiber 104, the higher order signal light mode than LP ₁₂ mode is not excited, the deterioration of pumping efficiency and noise characteristics can be suppressed.

  The demultiplexing unit 105 demultiplexes the outgoing light emitted from the amplification optical fiber 104 into higher-order mode excitation light and amplified signal light. The signal light split by the splitter 105 is output via the optical isolator 107. Each of the optical isolators 106 and 107 includes, for example, a magneto-optical element and two birefringent elements disposed so as to sandwich the magneto-optical element, and transmits light in only one direction.

In the above-described configuration, the conversion unit 102 converts the fundamental mode pumping light (LP ₀₁ ) generated by the pumping light source 101 into the mode LP _0m in which m of any one of the modes LP _lm of the multiplexed signal light is maximum. Convert to higher order mode. If the above, the mode in which m is maximum is LP ₀₂ or LP _12, the LP ₀₂ to be l = 0 Among them, the high-order mode to be employed in the conversion unit 102.

Here, FIG. 3 shows the light intensity distribution of each signal light mode propagating through the amplification optical fiber 104 whose signal light allowable mode is the 6-LP mode. For each of the modes LP ₁₁ , LP ₂₁ , LP ₃₁ , and LP ₁₂ , orthogonal odd modes and even modes that are orthogonal to each other are displayed. The light intensity distribution shown in FIG. 3 indicates that the darker the black, the lower the intensity, and the whiter, the higher the intensity.

Of the six LP modes, for 4 modes LP _01, LP _11, LP _21, LP _31, is not less one intensity maximum of the radial light intensity distribution in the fiber cross-section is the one ring-shaped . Further, as l increases, the spread of the ring-shaped light intensity distribution increases. On the other hand, for the two modes LP ₀₂ and LP ₁₂ , there are two maximum values in the radial direction, and the light intensity distribution in the cross section of the fiber has two ring shapes.

  As described above, when the number of rings in each mode light intensity distribution of the signal light is different, the light intensity distribution of the pump light mode is more advantageous in the mode having a large number of rings. This is because the gain of each signal light mode depends on the overlap of the excited erbium ion distribution generated by the erbium added in the ring core being excited by the pump light and the light intensity distribution of the signal light (the larger the overlap, the greater the overlap). This is because the pumping light mode having a larger number of rings in the light intensity distribution can more uniformly excite erbium ions in the ring core with respect to the light intensity distribution of each signal light mode.

The light intensity distribution in the odd mode and the even mode orthogonal to each other in the LP ₁₁ , LP ₂₁ , LP ₃₁ , and LP ₁₂ modes is as shown in FIG. As shown in (a) of FIG. 4, the mode LP _11, the number of nodes which separate the light intensity distribution formed by the cross-sectional direction of the amplification optical fiber 104 is 2. Therefore, in this case, 1 = 2 × (１ ／) = 1. Further, as shown in FIG. 4 (b), the mode LP _21, the number of nodes which separate the light intensity distribution formed by the cross-sectional direction of the amplification optical fiber 104 is four. Therefore, in this case, 1 = 4 × (１ ／) = 2. Further, as shown in (c) of FIG. 4, the mode LP _31, the number of nodes which separate the light intensity distribution formed by the cross-sectional direction of the amplification optical fiber 104 is 6. Therefore, in this case, 1 = 6 × (１ ／) = 3.

As shown in FIG. 4D, the mode LP ₁₂ has two nodes for dividing the light intensity distribution formed in the cross-sectional direction of the amplification optical fiber 104, and the mode LP ₁₂ has a cross-section of the amplification optical fiber 104. There are two numbers where the light intensity distribution has a maximum value in the direction. Therefore, in this case, 1 = 2 × (１ ／) = 1 and m = 2.

Here, since the light intensity distribution is rotated due to bending or the like, the excitation light mode is better in the mode where l = 0, which is independent of the azimuth angle. Also, since the pumping light wavelength is generally shorter than the signal light wavelength, the number of pumping light modes is larger than the number of signal light modes, but the highest-order mode in which l = 0 among the signal light modes is LP. _{In the 0μ} mode, the LP _{0μ + 1} mode is not always allowable as the pumping light mode, so that the LP _0μ mode is most suitable as the pumping light mode.

Thus, in the optical fiber amplifier of the embodiment using an erbium-doped fiber of 6-LP mode to the amplification optical fiber 104, so that the LP ₀₂ is most suitable as excitation light mode.

Here, the excitation light source 101 is, for example, a semiconductor laser for generating light of 980nm band fundamental mode (LP ₀₁ mode). For example, commonly used in the erbium-doped fiber amplifier for amplifying only the fundamental mode (LP ₀₁ mode), a semiconductor laser is suitable. This semiconductor laser has high communication reliability, and by using this semiconductor laser, a highly reliable multi-mode optical amplifier can be provided.

The multiplexing unit 103 is composed of a dichroic mirror that multiplexes the 980 nm band excitation light and the 1550 nm band signal light. Conversion unit 102, and a phase plate that converts the excitation light LP ₀₁ mode 980nm band to LP ₀₂ mode. This phase plate may be incorporated in the multiplexing unit 103, and the multiplexing unit 103 and the conversion unit 102 may be integrated. The splitter 105 splits the amplified light emitted from the amplification optical fiber 104 into 980 nm band excitation light and 1550 nm band signal light.

FIG. 5 shows a gain spectrum of the optical fiber amplifier according to the embodiment in which the amplification optical fiber 104 is constituted by the above-described ring core erbium-doped multimode fiber. As shown in FIG. 5, the maximum inter-mode gain difference when excited by LP ₀₂ mode excitation light was 1 dB. On the other hand, as the reference data in Figure 6 shows the gain spectrum of the optical fiber amplifier when the mode of the excitation light was set to LP _01. As shown in FIG. 6, when excited by LP ₀₁ mode excitation light, the gain difference between the maximum mode is greater than when excited by LP ₀₂ mode excitation light was 5 dB. As is clear from the comparison between FIG. 5 and FIG. 6, it is understood that the gain difference between the modes can be reduced by using the _LP02 pumping light rather than by using the _LP01 mode pumping light.

As Likewise reference data, if the mode of the excitation light was set to LP _11, the gain difference between modes became 4dB. If the mode of the excitation light was set to LP _21, the gain difference between modes became 3.5 dB. If the mode of the excitation light was set to LP _31, the gain difference between modes became 3.4 dB. If the mode of the excitation light was set to LP _12, the gain difference between modes became 4.5 dB. From these results, the optical fiber amplifier of the embodiment using an erbium-doped fiber of 6-LP mode to the amplification optical fiber 104, the mode of the excitation light by a LP _02, minimize the gain difference between modes It was shown that it could be done.

  In the above description, the case where the amplifying optical fiber 104 is composed of the ring core erbium-doped multimode fiber of the 6-LP mode has been described as an example, but the present invention is not limited to this. The same applies to an optical fiber having an arbitrary number of modes. The same applies to the case of an erbium-doped multimode fiber having a circular core.

  Hereinafter, a case where the amplification optical fiber 104 is formed from a 6-LP mode circular core erbium-doped multimode fiber will be described. The erbium-doped multimode fiber having a circular core has a core radius of 10 μm and a relative refractive index difference of 1%.

FIG. 7 shows a gain spectrum of the optical fiber amplifier according to the embodiment in which the amplification optical fiber 104 is formed of the circular core erbium-doped multimode fiber. As shown in FIG. 7, the maximum inter-mode gain difference when pumped by the _LP02 mode pump light was 2 dB.

On the other hand, as the reference data in FIG. 8 shows the gain spectrum of the optical fiber amplifier when the mode of the excitation light was set to LP _01. As shown in FIG. 8, when excited by LP ₀₁ mode excitation light, the gain difference between the maximum mode is greater than when excited by LP ₀₂ mode excitation light was 7 dB. As is clear from the comparison between FIG. 7 and FIG. 8, even in the case of the circular core, it is found that the gain difference between the modes can be reduced by using the _LP02 pumping light rather than by using the _LP01 mode pumping light.

Similarly, as reference data in the case of a circular core, if the mode of the excitation light was set to LP _11, the gain difference between modes became 6.5 dB. If the mode of the excitation light was set to LP _21, the gain difference between modes became 6 dB. If the mode of the excitation light was set to LP _31, the gain difference between modes became 3.5 dB. If the mode of the excitation light was set to LP _12, the gain difference between modes became 5 dB. From these results, in the case of a circular core, the mode of the excitation light by the LP _02, have been shown to be capable of minimizing the gain difference between modes.

As described above, according to the present invention, the mode LP _{0m in} which m of the linearly-polarized mode LP _lm in which the pumping light of the fundamental mode generated by the pumping light source can be transmitted through the amplification optical fiber is maximized. Since the conversion to the higher-order mode is performed by the conversion unit, it is possible to suppress the deterioration of the gain efficiency and noise characteristics of the optical fiber amplifier in a state where the high communication reliability is secured.

  Note that the present invention is not limited to the above-described embodiments, and many modifications and combinations can be made by those having ordinary knowledge in the art without departing from the technical concept of the present invention. That is clear.

  Reference numeral 101 denotes an excitation light source, 102 denotes a conversion unit, 103 denotes a multiplexing unit, 104 denotes an amplification optical fiber, 105 denotes a demultiplexing unit, 106 denotes an optical isolator, and 107 denotes an optical isolator.

Claims (1)

An excitation light source that generates an excitation light in a fundamental mode;
A conversion unit that converts a mode of the excitation light generated from the excitation light source from a fundamental mode to a higher-order mode,
A multiplexing unit that multiplexes the higher-order mode pump light converted by the conversion unit and the input signal light,
The multiplexing unit amplifies the multiplexed light by combining the multiplexed light, and amplifies the erbium-doped optical fiber.
A demultiplexing unit that demultiplexes outgoing light emitted from the amplification optical fiber into higher-order mode pumping light and the amplified signal light.
The amplification optical fiber, the mode of the linearly polarized wave that can be transmitted is the same as the mode of the signal light,
The conversion unit, an amplification optical fiber amplifier, characterized in that said amplifying optical fiber converts the excitation light in the fundamental mode to higher modes mode LP _{0 m} to m of transmittable mode LP _lm is maximized.