JP7558515B2 - Wavelength gain compensator and optical amplification system - Google Patents

Description

This disclosure relates to a wavelength gain compensator that compensates for wavelength gain differences and an optical amplification system using the same.

In spatial multiplexing transmission using spatial multiplexing such as multi-core or multi-mode, the gain difference between spatial channels in an optical amplifier and the wavelength dependency of the gain of each spatial channel become major problems. For this reason, a gain equalizer using a spatial optical system that compensates for the wavelength gain difference for each spatial channel (core, mode) has been proposed (see, for example, Non-Patent Document 1).

R. A. Betts et al. , “Split-beam Fourier filter and its application in a gain-flattened EDFA,” OFC’95 Technical Digest, pp. 80-81, 1995.

Non-Patent Document 1 has the problem of requiring a spatial optical system for each spatial channel (core, mode), resulting in large size. Therefore, the present disclosure aims to realize compensation for wavelength gain difference using a small device.

The wavelength gain compensator of the present disclosure comprises:
three or more coupling portions that couple the first waveguide and the second waveguide;
a plurality of delay sections disposed between the coupling sections, the first waveguide and the second waveguide not being coupled to each other and causing a waveguide length difference between the first waveguide and the second waveguide;
Equipped with
The ratio of the coupling lengths in the coupling section, the waveguide length difference in the delay section, and the sum of the coupling lengths are set to match a set spectral shape.

The optical amplification system of the present disclosure comprises:
a multimode optical amplifier for amplifying a wavelength-multiplexed signal having a plurality of modes;
a mode demultiplexer that demultiplexes the amplified wavelength multiplexed signal into individual modes;
a wavelength gain compensator according to the present disclosure that compensates for a gain difference between wavelengths for each wavelength multiplexed signal demultiplexed by the mode demultiplexer;
an inter-mode gain compensator for compensating for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a mode multiplexer that converts the wavelength-multiplexed signal from the inter-modal gain compensator into a mode before demultiplexing by the mode demultiplexer and multiplexes the signal;
A multimode optical amplifier system comprising:

The optical amplification system of the present disclosure comprises:
a multi-core optical amplifier that amplifies a wavelength-multiplexed signal propagated through each core of the multi-core fiber;
a fan-out device that demultiplexes the wavelength-multiplexed signal amplified by the multi-core optical amplifier for each core of a multi-core fiber;
a wavelength gain compensator according to the present disclosure that compensates for a gain difference between wavelengths for each wavelength multiplexed signal demultiplexed by the fan-out device;
an inter-core gain compensator that compensates for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a fan-in device that multiplexes the wavelength-multiplexed signal from the inter-core gain compensator into a core before demultiplexing by the fan-out device;
The multi-core optical amplifier system includes:

The optical amplification system of the present disclosure comprises:
A fan-out device that splits wavelength-multiplexed signals propagated through each core of a multicore fiber into individual cores;
an optical amplifier that amplifies each of the wavelength multiplexed signals demultiplexed by the fan-out device;
a wavelength gain compensator according to the present disclosure that compensates for a gain difference between wavelengths for each wavelength multiplexed signal amplified by the optical amplifier;
an inter-core gain compensator that compensates for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a fan-in device that multiplexes the wavelength-multiplexed signal from the inter-core gain compensator into a core before demultiplexing by the fan-out device;
The multi-core optical amplifier system includes:

According to the present disclosure, wavelength gain difference compensation can be achieved using two waveguides, making it possible to achieve wavelength gain difference compensation in a small device.

FIG. 2 shows a schematic diagram of the configuration of gain compensation according to the present disclosure. 3 shows an example of the gain spectrum of an EDFA and a gain compensator. 1 shows an example of a schematic waveguide structure according to the present disclosure. 1 shows an example of transmission spectrum characteristics, where (a) shows a case where the coupling length ratio is _L1 : _L2 : _L3 = 1:1:4, (b) shows a case where _L1 : _L2 : _L3 = 1:2:3, and (c) shows a case where _L1 : _L2 : _L3 = 1:3:2. 1 shows an example of transmission spectrum characteristics, where (a) shows a case where _ΔLd is 20 μm, (b) shows a case where _ΔLd is 30 μm, and (c) shows a case where _ΔLd is 40 μm. 2 shows an example of a transmission spectrum characteristic. Illustrated are an example of the EDFA gain spectrum T _e (λ), the inverted spectrum −T _e (λ) of the EDFA gain spectrum, a spectrum T _e ′(λ) to be obtained by wavelength gain compensation, and a transmission spectrum T _w (λ) of a designed wavelength gain compensator. 2 shows an example of the gain spectrum of a 4LP mode EDFA. 1 shows an example of the maximum deviation for each mode, where (a) shows the LP ₀₁ mode, (b) shows the LP ₁₁ mode, (c) shows the LP ₂₁ mode, and (d) shows the LP ₀₂ mode. 1 is an example of optimal parameters for each mode for a 4LP mode EDFA. An example of the transmission spectrum obtained for each mode when the optimal parameters are used is shown. FIG. 2 shows a schematic diagram of the configuration of gain compensation according to the present disclosure. 2 shows an example of the configuration of a variable optical attenuator. FIG. 2 shows a schematic diagram of the configuration of gain compensation according to the present disclosure. FIG. 2 shows a schematic diagram of the configuration of gain compensation according to the present disclosure.

The following describes in detail the embodiments of the present disclosure with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are mutually identical.

(Example 1)
1 shows a schematic diagram of the gain compensation configuration of the present disclosure. In the case of two modes (LP01 mode, LP11 mode), each mode is demultiplexed by a mode demultiplexer (DEMUX: demultiplexer) 82 at the rear stage of a multi-mode optical amplifier (MM-EDFA: Multi-mode Erbium-Doped Fiber Amplifier) 81, and then the gain of each mode is compensated by a gain compensator 83, and then the mode multiplexer (MUX: multiplexer) 84 multiplexes the signals. Here, an embodiment example in which 2LP modes are used is shown, but the same can be considered when the number of modes is increased.

2, the gain spectrum G ₈₁ of the EDFA has a gain difference with respect to wavelength. Therefore, the gain compensator 83 of this embodiment compensates for this gain difference with respect to wavelength by using a wavelength gain compensator having a gain spectrum G _{83 that is the inverse of the gain spectrum G 81. In} this disclosure, the wavelength gain compensator is realized by a waveguide structure.

The waveguide structure of the wavelength gain compensator of the present disclosure is outlined in FIG. 3. The wavelength gain compensator has a coupling section in which the waveguide 111 and the waveguide 112 are arranged in parallel with a waveguide spacing g, and optical power is transferred from one waveguide 112 to the other waveguide 111. In this structure, the two waveguides (waveguide 111 and waveguide 112) have the same structure. Here, the parallel waveguide length required for complete transfer of optical power from one waveguide 112 to the other waveguide 111 at the coupling section is determined by the amount of electric field seeping from the waveguide core section to the cladding section.

In the present disclosure, the waveguide has N (N≧3) or more coupling parts, and has a plurality of delay parts of N−1 or less so that the delay amount propagating through the waveguide 111 and the waveguide 112 is different in the sections between the coupling parts. Here, the waveguide length difference corresponding to the delay amount between the waveguide 111 and the waveguide 112 caused by the delay parts is ΔL _d . In this embodiment, the length of the waveguide 111 in the delay part can be expressed as 2L _d and the length of the waveguide 112 in the delay part can be expressed as 2L _d + ΔL _d , and a case will be considered in which the waveguide length differences of the delay parts provided in the sections between each coupling part are equal to ΔL _d .

For example, consider a structure in which the refractive index difference Δ between the waveguides 111 and 112 is 1.0%, the waveguide width w of the waveguides 111 and 112 is 4.0 μm, the waveguide height h of the waveguides 111 and 112 is 5.0 μm, the waveguide spacing g of the waveguides 111 and 112 is 5.0 μm, _Ld shown in Fig. 3 is 3710 μm, and there are three coupling parts. The respective coupling lengths are _L1 , _L2 , and _L3 . By adjusting the combination of these coupling lengths and further adjusting the waveguide length difference _ΔLd , it is possible to adjust the shape of the transmission spectrum characteristics.

4 shows an example of the transmission spectrum characteristics when the coupling length ratio is L ₁ :L ₂ :L ₃ =1:1:4, L ₁ :L ₂ :L ₃ =1:2:3, and L ₁ :L ₂ :L ₃ =1:3:2. Here, ΔL _d =500 μm, (L ₁ +L ₂ +L ₃ )=8400 μm. For example, when performing wavelength gain compensation (for example, wavelength band 1530 nm to 1565 nm) of the gain spectrum G ₈₁ shown in FIG. 2, when L ₁ :L ₂ :L ₃ =1:1:4, it can be seen that a transmission spectrum characteristic having an oscillation close to that of the target gain spectrum G ₈₃ can be obtained, as shown within the dashed frame in FIG. 4(a).

Next, ΔL _d is adjusted to further tune the shape of the transmission spectral characteristic.
Fig. 5 shows an example of the transmission spectrum characteristics when _L1 : _L2 : _L3 = 1:1:4 and _ΔLd is 20 μm, 30 μm, and 40 μm. When _ΔLd is 30 μm, it can be seen that a spectrum shape close to the target one like the gain spectrum _G83 shown in Fig. 2 can be obtained, as shown in the dashed line frame in Fig. 5(b).

Next, the sum of the bond lengths (L ₁ +L ₂ +L ₃ ) is adjusted.
6 shows an example of the transmission spectrum characteristics when ( _L1 + _L2 + _L3 ) is 7800 μm and 8100 μm. It can be seen that it is possible to adjust the position and magnitude of the peak of the spectrum by adjusting ( _L1 + _L2 + _L3 ).

As described above, by adjusting the structural parameters of the proposed waveguide structure, it is possible to adjust the transmission spectrum characteristics according to the gain characteristics of the EDFA. Note that the number of coupling parts provided in the gain compensator 83 does not need to be three, and four or more coupling parts may be provided. In this case, it is possible to fine-tune the transmission spectrum characteristics.

(Example 2 of the embodiment)
7 shows an example of the EDFA gain spectrum T _e (λ), the inverted spectrum -T _e (λ) of the EDFA gain spectrum, the ideal spectrum T _e '(λ) to be obtained by wavelength gain compensation, and the transmission spectrum T _w (λ) of the designed wavelength gain compensator. When the wavelength λ is 1.55 μm, T _e (λ), -T _e (λ), T _e '(λ), and T _w (λ) can be expressed by the following equations.

By determining ( _L1 + _L2 + _L3 ) and _ΔLd that are close to the ideal spectrum _Te '(λ), a spectral shape close to the ideal spectrum _Te '(λ) can be obtained. Therefore, as shown in the following formula, the maximum deviation of the difference between _Tw (λ) and _Te '(λ) is determined, and _L1 + _L2 + _L3 and _ΔLd that minimize formula (2) are determined.

Assuming use in the C band (wavelength band 1530 nm to 1565 nm), and assuming that the gain spectrum of the 4LP mode EDFA shown in Fig. 8 is T _e (λ), an example of the maximum deviation of formula (2) resulting from a combination of L ₁ +L ₂ +L ₃ and ΔL _d is shown in Fig. 9. The optimal parameters in this case are shown in Fig. 10. In calculating the maximum deviation in Fig. 9, the refractive index difference Δ of the waveguide is 1.0%, the waveguide width w is 4.0 μm, the waveguide height h is 5.0 μm, and the waveguide interval g is 5.0 μm.

FIG. 11 shows an example of the transmission spectrum characteristic obtained when the optimum parameters shown in FIG. 10 are used. The dashed line shows the ideal spectrum T _e '(λ) obtained by inverting the gain spectrum shown in FIG. 8. The transmission spectrum characteristic for any of Δw=0.0 μm, +0.1 μm, and -0.1 _μm can be matched to the shape of the ideal spectrum. Therefore, it can be seen that wavelength gain compensation is possible not only for _1.55 μm for which the optimum parameters were derived, but also for the entire C band (wavelength band 1530 nm to 1565 nm) _by adjusting the coupling length ratio L ₁ :L ₂ :L ₃ , the waveguide length difference _{ΔL d} , and the sum of the coupling lengths (L 1 +L 2 +L 3 ). Therefore, the present disclosure is capable of accurately compensating for wavelength gain differences by optimizing the structural parameters for each mode.

In this specification, examples of embodiments of planar lightwave circuits using glass-based materials are described, but the materials may of course be other materials. For example, similar effects to those of the examples of embodiments described in this specification can be obtained even with planar lightwave circuits using semiconductors such as Si or InGaAsP, or organic materials such as polymers.

In addition, the wavelength band used in the embodiment described in this specification is approximately 1.5 to 1.6 μm, but it can also be a longer wavelength band such as the mid-infrared region (2 μm or more) or the visible light band.

(Example 3)
FIG. 12 shows a configuration example of an optical amplification system according to the present disclosure. FIG. 12 shows a configuration example of gain compensation for six modes (LP01, LP11a, LP11b, LP21a, LP21b, LP02 modes). A multimode EDFA 81 compatible with six modes amplifies a wavelength multiplexed signal having six modes. A mode demultiplexer 82 at the rear demultiplexes the amplified wavelength multiplexed signal for each mode and converts each mode to the LP01 mode, which is the fundamental mode. A wavelength gain compensator 31 at the rear compensates for the gain difference between wavelengths in each wavelength multiplexed signal. Here, the wavelength gain compensator 31 is the wavelength gain compensator of the present disclosure, and the transmission spectrum characteristic of the wavelength gain compensator 31 has a spectral shape that inverts the gain spectrum for each mode in the multimode EDFA 81. This allows compensation (DWG compensation) for the wavelength gain difference for each mode to be performed. The variable optical attenuator 32 at the downstream side functions as an inter-mode gain compensator that compensates for the gain difference between the wavelength multiplexed signals from the wavelength gain compensator 31, and compensates for the gain difference between the modes. The mode multiplexer 84 at the downstream side converts the signals into the six modes before being separated by the mode demultiplexer 82, and multiplexes them. The multimode EDFA 81 can be any multimode optical amplifier that can amplify multimode light.

The variable optical attenuator 32 may have any configuration capable of adjusting the optical intensity, and may be realized, for example, by Mach-Zehnder type waveguides 321 and 322 having a heater adjustment unit 323 as shown in FIG. 13. The mode splitter 82 may only have the function of splitting each mode without converting to the LP01 mode. In this case, the mode combiner 84 may only have the function of combining each mode without converting the LP01 mode to the 6 modes before splitting by the mode splitter 82.

As shown in FIG. 14, the optical amplification system of the present disclosure can be applied to a multi-core amplifier 91 instead of a multi-mode amplifier. The multi-core optical amplifier 91 is separated into individual cores by a fan-out device 92 at the rear stage thereof, and the respective gain differences are compensated for by a gain compensator 93. In the gain compensator 93, the wavelength gain compensator 41 compensates for the gain difference between wavelengths, and then the variable optical attenuator 42 compensates for the gain difference between cores. Here, the wavelength gain compensator 41 is the wavelength gain compensator of the present disclosure, and the transmission spectrum characteristic of the wavelength gain compensator 41 has a spectral shape that is an inversion of the gain spectrum for each core in the multi-core optical amplifier 91. The variable optical attenuator 42 also functions as an inter-core gain compensator that compensates for the gain difference between the wavelength multiplexed signals from the wavelength gain compensator 41. After compensation by the gain compensator 93, the signals are multiplexed into a multi-core fiber by a fan-in device 94. In the fan-in device 94, the wavelength multiplexed signal from the gain compensator 93 is multiplexed into the core before demultiplexing by the fan-out device 92.

In addition, instead of the configuration shown in FIG. 14, the optical amplification system of the present disclosure may be configured as shown in FIG. 15, in which an EDFA 43 is placed after the signal is split by a fan-out device 92 at the rear of the 4-core fiber, and each signal passes through a wavelength gain compensator (DWG compensation) 41, then a variable optical attenuator 42 for compensating for the gain difference between the cores, and then multiplexed by a fan-in device 94. Here, the transmission spectrum characteristic of the wavelength gain compensator 41 has a spectral shape that is the inverse of the gain spectrum of each EDFA 43.

As described above, the present disclosure comprises a first optical waveguide (111) and a second optical waveguide (112) having N (N≧3) or more coupling sections each adjacent to the first optical waveguide (111) and N-1 or less delay sections disposed between each coupling section, and the length of each coupling section and each delay section is set so that the optical transmission characteristics for the wavelength between the first optical waveguide (111) and the second optical waveguide (112) are the desired characteristics.

(Effects of the present disclosure)
In a wavelength gain compensation device using a parallel waveguide structure, the transmission spectrum characteristic having the inverse characteristic of the gain spectrum of an optical amplifier can be flexibly set, which enables the device to be miniaturized.

(Key Points of the Disclosure)
By using a waveguide with two waveguides with an adjusted ratio of coupling lengths, it is possible to realize wavelength gain compensation while miniaturizing the device. In particular, when the number of spatial channels increases in spatial multiplexing transmission, further miniaturization is possible through device integration.

This disclosure can be applied to the information and communications industry.

31, 41: Wavelength gain compensator 32, 42: Variable optical attenuator 323: Heater adjustment unit 43: EDFA
81: Multimode EDFA
82: Mode splitter 83, 93: Gain compensator 84: Mode multiplexer 91: Multi-core optical amplifier 92: Fan-out device 94: Fan-in device 111, 112, 321, 322: Waveguide

Claims (6)

three coupling portions for coupling the first waveguide and the second waveguide;
two delay sections disposed between the three coupling sections, respectively , where the first waveguide and the second waveguide are not coupled and a waveguide length difference is generated between the first waveguide and the second waveguide;
Equipped with
Two of the three bonds have equal bond lengths, and the bond length of the remaining one of the three bonds is different from the bond lengths of the two bonds ;
The first waveguides in each of the two delay sections have the same length, and the second waveguides in each of the two delay sections have the same length,
The waveguide length differences of the two delay sections are equal,
The ratio of the coupling lengths in the three coupling parts, the waveguide length difference, and the sum of the coupling lengths are set to match a set spectral shape.
Wavelength Gain Compensator. The first waveguide is composed of only a straight waveguide, and the second waveguide is composed of a curved waveguide.
2. The wavelength gain compensator of claim 1. The set spectral shape has a spectral shape that is an inversion of the gain spectrum of an optical amplifier.
3. A wavelength gain compensator according to claim 1 or 2 . a multimode optical amplifier for amplifying a wavelength-multiplexed signal having a plurality of modes;
a mode demultiplexer that demultiplexes the amplified wavelength multiplexed signal into individual modes;
a wavelength gain compensator according to claim 1 , which compensates for a gain difference between wavelengths for each wavelength multiplexed signal demultiplexed by the mode demultiplexer;
an inter-mode gain compensator for compensating for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a mode multiplexer that converts the wavelength-multiplexed signal from the inter-modal gain compensator into a mode before demultiplexing by the mode demultiplexer and multiplexes the signal;
An optical amplification system comprising: a multi-core optical amplifier that amplifies a wavelength-multiplexed signal propagated through each core of the multi-core fiber;
a fan-out device that demultiplexes the wavelength-multiplexed signal amplified by the multi-core optical amplifier for each core of a multi-core fiber;
a wavelength gain compensator according to claim 1 , which compensates for a gain difference between wavelengths for each wavelength multiplexed signal demultiplexed by the fan-out device;
an inter-core gain compensator that compensates for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a fan-in device that multiplexes the wavelength-multiplexed signal from the inter-core gain compensator into a core before demultiplexing by the fan-out device;
An optical amplification system comprising: A fan-out device that splits wavelength-multiplexed signals propagated through each core of a multicore fiber into individual cores;
an optical amplifier that amplifies each of the wavelength multiplexed signals demultiplexed by the fan-out device;
a wavelength gain compensator according to claim 1 , which compensates for a gain difference between wavelengths for each wavelength multiplexed signal amplified by the optical amplifier;
an inter-core gain compensator that compensates for a gain difference between the wavelength multiplexed signals from the wavelength gain compensator;
a fan-in device that multiplexes the wavelength-multiplexed signal from the inter-core gain compensator into a core before demultiplexing by the fan-out device;
An optical amplification system comprising:

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