Arrangement for mitigating nonlinear noise of cascaded semiconductor optical amplifiers

Abstract

A SOA arrangement (100) including a first polarization-diversity SOA pair (102) and a second polarization-diversity SOA pair (104) arranged on a common optical path with a correlation reducing optical arrangement 106 The correlation reducing optical element is configured to receive a first and a second input component from the first SOA pair and to mix the first and second input components into a first and second output components. The first output component is a combination of the first and second input component from the first polarization-diversity SOA pair. The second output component is a combination of the first and second input component from the first polarization-diversity SOA pair. The SOA arrangement thereby being arranged to reduce the correlation between the nonlinear noise generated by the first SOA and the second SOA.

Description

[0001] ARRANGEMENT FOR MITIGATING NON LIN E AR NOISE OF CASCADED SEMICONDUCTOR OPTICAL AMPLIFIERS

[0002] TECHNICAL FIELD

[0003] The disclosure generally relates to Semiconductor Optical Amplifiers, SOAs, and more specifically, the disclosure relates to an SOA arrangement including a first polarization-diversity SOA pair and a second polarization-diversity SOA pair arranged on a common optical path.

[0004] BACKGROUND

[0005] Optical transmission systems are essential for facilitating long-range data communication. Optical transmission systems require effective means for signal regeneration to compensate inherent losses. Semiconductor Optical Amplifiers, SOAs, is a key technology to replace conventional fiber amplifiers as line amplifiers in optical transmission systems at low cost for example to facilitate long-haul transmission in the Long-wavelength range (L-band). Similarly to doped fiber amplifiers, the SOAs generate Amplified Spontaneous Emission (ASE) noise. SOAs exhibit a fast response time which exceeds that of fiber amplifiers by several orders of magnitude. This attribute leads to significant signal-dependent distortions due to the non-linear gain response of the SOAs and introduces additional noise into the optical transmission system.

[0006] For applications in modem coherent optical transmission systems, optical amplifiers are designed to amplify both polarization tributaries of the optical signal equally. Gain for the transverse electric, TE mode is generally higher than for the transverse magnetic, TM mode, leading to polarization dependent gain, PDG unless tensile strain is used to enhance the material gain for the TM mode. This requires very tight control of the material strain and waveguide dimensions. In addition, the differences in the wavelength dependence of the material gain for the two polarization tributaries leads to wavelength-dependent PDG. In practice, low PDG is more easily achieved by utilizing two SOA chips in a polarization diversity configuration, in which each SOA in the pair amplifies one of the two polarization tributaries. Utilizing two SOA chips instead of a single SOA chip has the additional advantage of increasing the saturation power by 3 decibels, dB.

[0007] To achieve the necessary gain within an optical line amplifier module, two or more polarization diversity SOA pairs are cascaded. Multiple amplifier modules are in turn cascaded in optical transmission systems for periodic signal regeneration over long distances. When SOAs are cascaded on a shared optical path, then noise contributions of all SOAs are added cumulatively. Due to the signal-dependent nature of the SOA nonlinear signal distortions, the associated noise contributions of subsequent SOAs are not independent. Nonlinear noise contributions of subsequent SOAs are correlated, unless specific measures are taken to prevent their coherent accumulation. Coherent accumulation of nonlinear noise leads to a larger nonlinear penalty than independent accumulation and to an increase in the required Optical Signal To Noise (OSNR) ratio. In the existing solutions, the ASE noise generated by amplifiers along the cascade leads to additional nonlinear distortions. When ASE generated by an optical amplifier enters previous or subsequent SOAs in the cascade, the saturation power of the SOAs is reduced and their nonlinear behavior introduces further nonlinear penalties. Placing optical isolators before and after an amplifier can prevent ASE generated by the amplifier from entering a previous SOA. The use of filters can further prevent forward or backward propagating out-of-band ASE from entering subsequent or previous SOAs on the same optical path. These arrangements increase the saturation power of the SOA and reduce the generation of nonlinear noise. The nonlinear penalties are hence reduced, leading to improved transmission performance. Coherent accumulation of the remaining nonlinear noise in cascaded SOAs is, however, not prevented.

[0008] Furthermore, using Optical Phase Conjugation (OPC) in the existing solutions can revert nonlinear distortions by conjugating the optical field post-amplification, theoretically canceling out nonlinear noise under ideal conditions. However, OPC does not fully address the problem of coherent accumulation of nonlinear noise across cascaded SOAs. The OPC, although theoretically effective, includes the need for identical SOAs, consistent input power levels, and unaltered signal conditions between stages. Such requirements are often impractical or undesirable in multi-stage amplifiers or in optical links. Even though OPC can mitigate nonlinear noise under non-ideal conditions, the incorporation of OPC introduces insertion losses and relies on nonlinear elements, which may limit performance and spectral range. These drawbacks have limited the practical application and widespread adoption of the OPC for both fiber nonlinearity and SOA nonlinearity compensation.

[0009] Therefore, there arises a need to address the aforementioned technical problem / drawbacks of SOAs to reduce the correlation between nonlinear noise generated by cascaded SOAs.

[0010] SUMMARY

[0011] It is an objective of the disclosure to provide a Semiconductor Optical Amplifier, SOA, arrangement including a first polarization-diversity SOA pair and a second polarization-diversity SOA pair arranged on a common optical path to reduce the correlation between nonlinear noise while avoiding one or more disadvantages of prior art approaches.

[0012] This objective is achieved by the features of the independent claims. Further, implementation forms are apparent from the dependent claims, the description, and the figures.

[0013] According to a first aspect, there is provided a Semiconductor Optical Amplifier, SOA arrangement comprising a first polarization-diversity SOA pair and a second polarization-diversity SOA pair arranged on a common optical path, wherein the SOA arrangement is characterized in that the first polarization-diversity SOA pair is configured to receive an optical input signal and split it into a first and a second input component, wherein the SOA arrangement further comprises a correlation reducing optical arrangement configured to receive the first and the second input components from the first polarizationdiversity SOA pair, mix the first and second input components into a first and a second output component, wherein the first output component is a combination of the first and second input components from the first polarization-diversity SOA pair and the second output component is a combination of the first and second input components from the first polarization-diversity SOA pair, and wherein the second polarization-diversity pair is arranged to receive the first and the second output components from the correlation reducing optical arrangement and to combine them into an output signal, the SOA arrangement thereby being arranged to reduce the correlation between nonlinear noise introduced by the first polarization-diversity SOA pair and the second polarization-diversity SOA pair.

[0014] The correlation reducing optical arrangement according to herein achieves this by redistribution of photons between the two amplifiers of the second pair relative to the first.

[0015] The SOA arrangement improves transmission reach and transmission performance. The output signals that enter each SOA in the second polarization-diversity SOA pair are altered compared to the optical input signals that entered either of the SOAs in the first polarization-diversity SOA pair, due to the redistribution by the correlation reducing optical element. This alteration effectively reduces the correlation between the nonlinear noise generated by the first polarization-diversity SOA pair and the second polarization-diversity SOA pair, which leads to lower associated impairments, and enhances the overall transmission performance of the SOA arrangement.

[0016] The correlation reducing optical arrangement according to herein comprises a correlation reducing optical element. Optionally, the correlation reducing optical is a polarization rotator. Optionally, the correlation reducing optical element is a beam splitter.

[0017] Therefore, in contradistinction to the existing solutions, a Semiconductor Optical Amplifier, SOA arrangement is configured to reduce the correlation between nonlinear noise introduced by a first polarization-diversity SOA pair and a second polarization-diversity SOA pair. It should also be noted that the teachings herein can also be applied to other technologies where semiconductor optical amplifiers are cascaded, and where signal distortions are undesirable. One example of such an application unrelated to optical transmission is LIDAR.

[0018] These and other aspects of the disclosure will be apparent from the implementation s) described below.

[0019] BRIEF DESCRIPTION OF DRAWINGS

[0020] Implementations of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0021] FIG. 1 is a block diagram of a Semiconductor Optical Amplifier, SOA, arrangement including a first polarization-diversity SOA pair and a second polarization-diversity SOA pair arranged on a common optical path in accordance with an implementation of the disclosure;

[0022] FIG. 2 illustrates a block diagram of a first implementation of the Semiconductor Optical Amplifier, SOA, arrangement comprising the first polarization-diversity SOA pair and the second polarization-diversity SOA pair arranged on the common optical path with a polarization rotator in accordance with an implementation of the disclosure;

[0023] FIG. 3 illustrates a block diagram of a second implementation of the Semiconductor Optical Amplifier, SOA, arrangement comprising the first polarization-diversity SOA pair and the second polarization-diversity SOA pair arranged on the common optical path with a beam splitter in accordance with an implementation of the disclosure;

[0024] FIG. 4 is a graphical representation of the SNR degradation relative to the SNR obtained if the noise generated by the two pairs of SOAs was completely uncorrelated, as a function of rotation angle and for different total output power of an SOA in each of the pairs, due to nonlinear noise correlation of a cascade of two polarization-diversity SOAs when an input signal is a polarization-division multiplexed, PDM signal carrying 16 Quadrature Amplitude Modulation, QAM over 21 wavelengthdivision multiplexed, WDM channels and the correlation reducing element in an SOA arrangement is a polarization rotator in accordance with an implementation of the disclosure;

[0025] FIG. 5 is a graphical representation of the SNR degradation as a function of the total output power of a Semiconductor Optical Amplifier, SOA in each of the pairs for a 16 Quadrature Amplitude Modulation, QAM input signal when the correlation reducing optical element in the SOA arrangement is a polarization rotator in accordance with an implementation of the disclosure; and

[0026] FIG. 6 is a graphical representation of the SNR degradation as a function of the total output power of a Semiconductor optical amplifier, SOA in each of the pairs for a 16 QAM input signal when the correlation reducing element in the SOA arrangement of FIG. 2 is a dispersive element instead of the polarization rotator, specifically a fibre with varying length as indicated, thereby showing simulation results.

[0027] DETAILED DESCRIPTION OF THE DRAWINGS

[0028] Implementations of the disclosure provide a Semiconductor Optical Amplifier, SOA, arrangement including a first polarizationdiversity SOA pair and a second polarization-diversity SOA pair arranged on a common optical path to reduce the correlation between nonlinear noise introduced by the first polarization-diversity SOA pair and the second polarization-diversity SOA pair. To make solutions of the disclosure more comprehensible for a person skilled in the art, the following implementations of the disclosure are described with reference to the accompanying drawings.

[0029] Terms such as "a first", "a second", "a third", and "a fourth" (if any) in the summary, claims, and foregoing accompanying drawings of the disclosure are used to distinguish between similar objects and are not necessarily used to describe a specific sequence or order. It should be understood that the terms so used are interchangeable under appropriate circumstances, so that the implementations of the disclosure described herein are, for example, capable of being implemented in sequences other than the sequences illustrated or described herein. Furthermore, the terms "include" and "have" and any variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, a method, a system, a product, or a device that includes a series of steps or units, is not necessarily limited to expressly listed steps or units but may include other steps or units that are not expressly listed or that are inherent to such process, method, product, or device.

[0030] FIG. 1 is a block diagram of a Semiconductor Optical Amplifier, SOA arrangement 100 including a first polarization-diversity SOA pair 102 and a second polarization-diversity SOA pair 104 arranged on a common optical path. The first polarizationdiversity SOA pair 102 is configured to receive an optical input signal IN and split it into a first input component 103-1 and a second input component 103-2. The input signal is a polarization-multiplexed signal which is split into two polarization tributaries, i.e. the first input component 103-1 and the second input component 103-2.

[0031] The SOA arrangement 100 also includes a correlation reducing optical arrangement 106 arranged in between the first polarization-diversity SOA pair 102 and the second polarization-diversity SOA pair 104. The correlation reducing optical arrangement 106 is configured to receive the first input component 103-1 and the second input component 103-2 from the first polarization-diversity SOA pair 102 and to mix the first input component and the second input component into a first output component 108-1 and a second output component 108-2. The first output component 108-1 is thus a combination of the first input component 103-1 and the second input component 103-2 from the first polarization-diversity SOA pair 102 and the second output component 108-2 is a combination of the first 103-1 and second input component 103-2 from the first polarization-diversity SOA pair 102. The SOA arrangement 100 is thereby arranged to reduce the correlation between nonlinear noise introduced by the first polarization-diversity SOA pair 102 and the second polarization-diversity SOA pair 104. The correlation reducing optical arrangement according to herein achieves this by redistribution of photons between the two amplifiers of the second pair relative to the first.

[0032] The correlation reducing optical arrangement 106 enable for splitting and mixing of optical paths between the two pairs of SOAs: the optical path of the first SOA in the first pair 102 is split in two, and the two parts each enter one of the SOA of the second pair 104. The same happens for the second SOA of the first pair 102. In essence, the photons exiting the first SOA 102- 1 in the first pair 102 enter the first or the second SOA 104-1,104-2 in the second pair 104 (apart from some photons being lost due to inherent losses of the components), at a fixed ratio. The same happens for the photons exiting the second SOA in the first pair. For example, the splitting ratio is 1 / 2 for both.

[0033] Preferably, the first polarization-diversity SOA pair 102 or the second polarization-diversity SOA pair 104 can be a pair of SOA chips that amplifies an optical signal which is split into its two polarization components and where each of the two SOA chips amplifies one of the two components.

[0034] The SOA arrangement 100 improves transmission reach and transmission performance. The output signals that enter the second polarization-diversity SOA pair 104 are altered compared to the optical input signals IN that entered the first polarizationdiversity SOA pair 102, due to the mixing by the correlation reducing optical arrangement 106. This alteration effectively reduces the correlation between the nonlinear noise generated by the first polarization-diversity SOA pair 102 and the second polarization-diversity SOA pair 104, which leads to a decrease in the accumulation of nonlinear noise and to lower associated impairments and enhances the overall transmission performance of the SOA arrangement 100.

[0035] The SOA arrangement 100 achieves a reduction in the correlation between nonlinear noise by effectively mixing the optical signals output by the first polarization-diversity SOA pair 102. While correlation of the nonlinear noise contributions from the first polarization-diversity SOA pair 102 and the second polarization-diversity SOA pair 104 along the common optical path may be addressed by introducing a dispersive element like an optical fiber, the introduction of the dispersive element is not always possible, desired or sufficient. The SOA arrangement 100 improves the optical transmission system performance by reducing the accumulation of the nonlinear noise and associated transmission penalties.

[0036] The correlation reducing optical arrangement 106 includes a correlation reducing optical element 110. In some embodiments, the correlation reducing optical element 110 is a polarization rotator 110 as in FIG. 2. In some embodiments, the correlation reducing optical element 110 is a beam splitter as in FIG. 3.

[0037] FIG. 2 illustrates a block diagram of some implementations of the SOA arrangement 100 comprising the first polarizationdiversity SOA pair 102 and a second polarization-diversity SOA pair 104 arranged on a common optical path in accordance with some implementations of the disclosure. The disclosure related to FIG. 2 also applies to the disclosure of FIG. 1.

[0038] As is shown, the first polarization-diversity SOA pair 102 comprises an input stage 101 configured to receive the optical input signal IN and split it into the first 103-1 and a second input component 103-2.

[0039] The input stage 101 (being an example of an optical input stage) comprises a beam splitter 101-1 arranged to split an input signal (in this example the optical input signal IN) into the first and second components (in this example the first and second input components 103-1, 103-2). The input stage 101 also comprises a polarization rotator 101-2 arranged to rotate one of the two components, in this example the second input component 103-2.

[0040] The first polarization-diversity SOA pair 102 also comprises a first SOA 102-1, SOA1 configured to receive and amplify the first input component 103-1, and a second SOA 102-2, SOA2 configured to receive and amplify the second input component 103-2. The first SOA 102-1, SOA1 and the second SOA 102-2, SOA2 are arranged in a polarization diversity configuration. The first component 103-1 and the second component 103-2 are two polarization tributaries i.e. optical signals.

[0041] The input stage 101 may also comprise mirrors 101-3 or other guiding elements for guiding the first and / or the second input components 103-1, 103-2 through the input stage 101 to the first and second SOAs 102-1, 102-2.

[0042] As is also shown, the second polarization-diversity SOA pair 104 comprises a first SOA 104-1, SOA3 configured to receive and amplify the first output component 108-1 received from the correlation reducing arrangement 106, and a second SOA 104- 2, SOA4 configured to receive and amplify the second output component 108-2 received from the correlation reducing arrangement 106. The first SOA 104-1, SOA3 and the second SOA 104-2, SOA4 are arranged in a polarization diversity configuration.

[0043] The second polarization-diversity SOA pair 104 also comprises an output stage 105 configured to receive the first 108-1 and the second (now amplified) output components 108-2 and combine them into an optical output signal OUT.

[0044] The output stage 105 (being an example of a general output stage) comprises a polarization rotator 105-2 arranged to rotate one component (such as the first output component 108-1) and a beam combiner 105-3 arranged to combine the two components (such as the two output components 108-1, 108-2) into an output signal (such as the output signal OUT). The output stage 105 may also comprise mirrors 105-1 or other guiding elements for guiding the first and / or the second output components 108-1, 108-2 through the output stage 105 from the first and second SOAs 104-1, 104-2.

[0045] As is also shown, the correlation reducing optical arrangement 106 is arranged to receive the output signal (being the first and second input components 103-1, 103-2) of the first polarization-diversity SOA pair 102 as its input signal and to provide its output signal (being the first and second output components 108-1, 108-2 prior to amplification) to the second polarizationdiversity SOA pair 104 as the input signal to the second polarization-diversity SOA pair 104.

[0046] The two input components 103-1, 103-2 from the first SOA 102-1 and the second SOA 102-2 of the first SOA pair 102 are combined into a single optical signal (to be rotated) through an input stage 107 comprised in the correlation reducing optical arrangement 106. The input stage 107 may be an input stage as discussed in relation to the first SOA pair 102, referenced 101.

[0047] The correlation reducing optical arrangement 106 includes a correlation reducing optical element 110, which in the embodiments of FIG. 2 is a polarization rotator. In some embodiments, the polarization rotator 110 is a polarization rotating element. In such embodiments, the photons from both amplifiers are first fed into the two polarization components of a short optical fiber and the redistribution is then achieved by the polarization rotator, after which the polarization components are split again into two distinct optical paths.

[0048] The optical signal to be rotated is fed or guided to the correlation reducing optical element 110, whereby it is rotated. In some embodiments the polarization rotatorl 10 performs a polarization rotation by an angle 7t / 4. Optionally, the polarization rotator 110 can be implemented as a Faraday rotator. The polarization rotatorl 10 may be implemented by a combination of / 2 plates oriented at an angle relative to each other.

[0049] The rotated signal is then split into two output components 108-1, 108-2 to be fed to the first SOA 104-1 and the second SOA 104-2 of the second SOA pair 104, the two output components 108-1, 108-2 being polarization tributaries. The rotated signal is split in an output stage 109 comprised in the correlation reducing optical arrangement 106. The output stage 109 may be an output stage as discussed in relation to the second SOA pair 104, referenced 105.

[0050] The rotated signal output by the polarization rotator 110 may enter into additional optical elements. The additional optical elements can be placed anywhere in the common optical path between the first polarization-diversity SOA pair and the second polarization-diversity SOA pair.

[0051] As is also shown in FIG. 2, in some embodiments, the correlation reducing optical arrangement 106 includes a Gain Flattening Filter 111 (as an example of an additional optical element) arranged to filter the rotated signal.

[0052] And, as is also shown in FIG. 2, in some embodiments, the correlation reducing optical arrangement 106 includes a Variable Optical Attenuator 112 (as another example of an additional optical element) arranged to attenuate the rotated signal. In some embodiments the Variable Optical Attenuator 112 is arranged after the Gain Flattening Filter 111 (as in FIG. 2). In some embodiments the Variable Optical Attenuator 112 is arranged before the Gain Flattening Filter 111.

[0053] FIG. 3 illustrates a block diagram of some implementations of the Semiconductor Optical Amplifier, SOA, arrangement 100 comprising the first polarization-diversity SOA pair 102 and the second polarization-diversity SOA pair 104 arranged on the common optical path with a correlation reducing arrangement 106. In the embodiments of this figure, the correlation reducing arrangement 106 comprises a correlation reducing optical element 110 being a beam splitter 110. The disclosure related to FIG. 3 also relates to FIG. 1. Furthermore, details discussed in relation to FIG. 2 also apply to the embodiments discussed in FIG. 3, for example as regards the first and second SOA pairs 102, 104, their internal components and the input and output stages. The mixing is achieved by splitting the first input component 103-1 into a first 103-1A and a second portion 103-1B thereof, and also splitting the second input component 103-2 into a first 103-2A and a second portion 103-2B thereof. As the portions of the two input components exit the beam splitter 110, they will be mixed and the first output component 108-1 is thus a combination of the first portion 103-1A of the first input component 103-1 and the second portion 103-2B of the second input component 103-2. Similarly, the second output component 108-2 is a combination of the second portion 103-1B of the first input component 103-1 and the first portion 103-2A of the second input component 103-2.

[0054] The correlation reducing arrangement 106 may also comprise mirrors 113, 114 or other guiding elements for guiding the first and / or the second input components 103-1, 103-2 through the correlation reducing arrangement 106 to the beam splitter 110.

[0055] The two output signals 108-1, 108-2 of the beam splitter 110 may in some embodiments enter into additional optical elements of the SOA arrangement 100. Optionally, the additional optical elements can be Gain-Flattening Filter, GFF or Variable Optical Attenuator, VOA.

[0056] In some embodiments the correlation reducing arrangement 106 further comprises a first Gain Flattening Filter 111-1 arranged to filter the first output component 108-1 and a second Gain Flattening Filter 111-2 arranged to filter the second output component 108-2.

[0057] In some embodiments the correlation reducing arrangement 106 further comprises a first Variable Optical Attenuator 112-1 arranged to attenuate the first output component 108-1 and a second Variable Optical Attenuator 112-2 arranged to attenuate the second output component 108-2.

[0058] Optionally, the additional optical elements can be placed anywhere in the optical paths between the first polarization-diversity SOA pair and the second polarization-diversity SOA pair and one or more further optical elements may thus be arranged on an optical path of the correlation reducing optical element 110, wherein at least one of the one or more further optical elements is arranged to operate on the input signal to the correlation reducing optical element 110 and / or at least one of the one or more further optical elements is arranged to operate on the output signal of the correlation reducing optical element 110. Optionally, the additional optical elements include an optical filter, a switch, an attenuator, an optical isolator, a coupler, and / or an optical fiber. The switch may be wavelength-selective. Optionally, the one or more additional optical elements include an active element.

[0059] Optionally, the SOA arrangement 100 is a multi-stage SOA arrangement 100 further including one or more further polarizationdiversity SOA pairs and one or more further correlation reducing optical elements. Optionally, the common optical path includes sub-paths.

[0060] FIG. 4 is a graphical representation of the SNR degradation relative to the SNR obtained if the noise generated by the two pairs of SOAs was completely uncorrelated, as a function of rotation angle and for different total output power of an SOA in each of the pairs, due to nonlinear noise correlation of a cascade of two polarization-diversity SOAs when an input signal is a polarization-division multiplexed, PDM signal carrying 16 Quadrature Amplitude Modulation, QAM over 21 wavelengthdivision multiplexed, WDM channels and the correlation reducing optical element in a SOA arrangement is a polarization rotator in accordance with an implementation of the disclosure. The graphical representation depicts the normalized rotation angle, theta / (7t / 2) on the X-axis and a signal to noise ratio, SNR degradation of the SOA pair on Y-axis.

[0061] In the case where the noise is completely uncorrelated in the SOA arrangement, then the noise powers add up which is shown as In the case where the noise is maximally correlated in the SOA arrangement, then the noise amplitudes add up which is shown as:

[0062] The SNR degradation is defined as the ratio between the nonlinear SNR after the second SOA pair when the nonlinear noise of the first and the second pair were uncorrelated to the actual nonlinear SNR after the second SOA pair. When the output power of each SOA in the two pairs is the same, the SNR degradation is the ratio of the accumulated noise power of the first and second SOA pair to the accumulated noise power when the noise were completely uncorrelated, which is shown as:

[0063] Correspondingly, when the nonlinear noise of the first polarization-diversity SOA pair and the second polarization-diversity SOA pair is fully correlated, the maximal SNR degradation is 3 decibels, dB, and the minimum is 0 dB and is attained when the nonlinear noise from the two SOA pairs is entirely uncorrelated.

[0064] The Variable Optical Attenuator, VOA 112 is adjusted to equalize the output power of the first and second polarizationdiversity SOA pairs 102, 104.

[0065] The graphical representation depicts different curves corresponding to various output powers. The different curves are labeled as 18.36 decibels relative to one milliwatt, dBm, 21.41 dBm, 22.62 dBm, and 23.95 dBm. Each curve indicates how the SNR degradation changes with the variation with the rotation angle theta for the respective output power. A “V” shape of each curve indicates that the SNR degradation minimizes at a value of the theta before increasing again. The “V” shape curve indicates an optimal operating region where the correlation reducing polarization rotator effectively compensates for signal degradation. The lowest point of each “V” shape curve denotes an optimal value of the theta, where the SNR degradation is minimum.

[0066] FIG. 5 is a graphical representation of the SNR degradation as a function of the total output power of a Semiconductor Optical Amplifier, SOA in each of the pairs for a 16 Quadrature Amplitude Modulation, QAM input signal when the correlation reducing optical element in the SOA arrangement is a polarization rotating element in accordance with an implementation of the disclosure. The graphical representation depicts the output power of one of the SOAs in each of the pairs on X-axis and the signal to noise ratio, SNR degradation on the Y-axis. The output power of the SOA pair ranges from 17 dBm (decibels relative to one milliwatt) to 24 dBm. The SNR degradation of the SOA pair ranges from 0 to 3 decibels, dB. The graphical representation depicts different curves corresponding to various rotation angles. The rotation angles range from 0 to 7t / 4. The different curves are labeled and ordered from the lowest rotation angle (largest SNR degradation) to the highest rotation angle (lowest SNR degradation), that is 0, 0.2 TT / 2, 0.3 TT / 2, 0.37 TT / 2, and 0.5 TT / 2.

[0067] The maximum reduction of the SNR degradation occurs at a rotation angle of pi / 4, showing that SNR degradation can be reduced by up to 2 decibels, dB.

[0068] FIG. 6 is a graphical representation of the SNR degradation as a function of the total output power of a Semiconductor Optical Amplifier, SOA in each of the pairs for a 16 QAM input signal when the correlation reducing element in the SOA arrangement of FIG. 2 is a dispersive element instead of the polarization rotator, specifically a fiber with varying length as indicated. The graphical representation depicts the output power of one of the SOAs in each of the two pairs on the X-axis and the SNR degradation of the SOA pair on Y-axis. The graphical representation depicts lines representing different lengths of fiber labeled in kilometers, km. The different lines are labeled and ordered from the lowest length of the fiber to the highest length of the fiber including 0 km, 0.25 km, 0.5 km, 1 km, 1.5 km, 7.5 km, and 20 km. A longer fiber length leads to lower noise correlation, and a higher output power results in stronger gain compression which also leads to lower noise correlation. Standard Single- Mode Fiber, SSMF is used with Dispersion, D of 16 picoseconds / kilometer / nanometer.

[0069] Although the disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

CLAIMS1. A Semiconductor Optical Amplifier, SOA arrangement (100) comprising a first polarization-diversity SOA pair (102) and a second polarization-diversity SOA pair (104) arranged on a common optical path, wherein the SOA arrangement (100) is characterized in that the first polarization-diversity SOA pair (102) is configured to: receive an optical input signal (IN) and split it into a first (103-1) and a second input component (103-2), wherein the SOA arrangement (100) further comprises: a correlation reducing optical arrangement (106) configured to: receive the first (103-1) and the second input components (103-2) from the first polarization-diversity SOA pair (102), mix the first (103-1) and second (103-2) input components into a first (108-2) and a second (108-2) output component, wherein the first output component is a combination of the first and second input components (103-1, 103-2) from the first polarization-diversity SOA pair (102) and the second output component is a combination of the first and second input components (103-1, 103-2) from the first polarization-diversity SOA pair (102), and wherein the second polarization-diversity pair (104) is arranged to: receive the first (108-2) and the second (108-2) output components from the correlation reducing optical arrangement (106) and to combine them into an output signal (OUT), the SOA arrangement (100) thereby being arranged to reduce the correlation between nonlinear noise introduced by the first polarization-diversity SOA pair (102) and the second polarization-diversity SOA pair (104).

2. The SOA arrangement (100) according to claim 1, wherein the first polarization-diversity SOA pair (102) comprises: an input stage (101) configured to receive an optical input signal (IN) and split it into a first (103-1) and a second input component (103-2), a first SOA (102-1, SOA1) configured to receive and amplify the first input component (103-1), and a second SOA (102-2, SOA2) configured to receive and amplify the second input component (103-2), wherein the first SOA (102-1, SOA1) and the second SOA (102-2, SOA2) are arranged in a polarization diversity configuration.

3. The SOA arrangement (100) according to claim 2, wherein the input stage (101) of the first polarization-diversity SOA pair (102) comprises a beam splitter (101-1) arranged to split the optical input signal (IN) into the first and second input components and a polarization rotator (101-2) arranged to rotate one of the two components.

4. The SOA arrangement (100) according to any preceding claim, wherein the second polarization-diversity SOA pair (104) comprises: a first SOA (104-1, SOA3) configured to receive and amplify the first output component (108-1), and a second SOA (104-2, SOA4) configured to receive and amplify the second output component (108-2), wherein the first SOA (104-1, SOA3) and the second SOA (104-2, SOA4) are arranged in a polarization diversity configuration, and wherein the second polarization-diversity SOA pair (104) further comprises an output stage (105) configured to receive the first (108-1) and the second amplified output components (108-2) and combine them into an optical output signal (OUT).

5. The SOA arrangement (100) according to claim 4, wherein the output stage (105) comprises a polarization rotator (105- 2) arranged to rotate one of the two output components and a beam combiner (105-3) arranged to combine the two output components into the output signal (OUT).

6. The SOA arrangement (100) according to claim 5, wherein the beam combiner (105-3) is a beam splitter.

7. The SOA arrangement according to any preceding claim, wherein the correlation reducing arrangement (106) comprises a correlation reducing optical element (110) being a polarization rotator (110).

8. The SOA arrangement according to claim 7, wherein the correlation reducing arrangement (106) further comprises: an input stage (107) arranged to combine the first and second input components (103-1, 103-2) into a combined signal to be rotated, wherein the correlation reducing optical element (110) is arranged to receive the combined signal to be rotated and provide a rotated signal, wherein the correlation reducing arrangement (106) further comprises an output stage (109) arranged to receive the rotated signal and to split the rotated signal into the first and second output components (108-1, 108-2).

9. The SOA arrangement according to claim 8, wherein the correlation reducing arrangement (106) further comprises a Gain Flattening Filter (111) arranged to filter the rotated signal.

10. The SOA arrangement according to claim 8 or 9, wherein the correlation reducing arrangement (106) further comprises a Variable Optical Attenuator (112) arranged to attenuate the rotated signal.

11. The SOA arrangement according to any of claims 1 to 6, wherein the correlation reducing arrangement (106) comprises a correlation reducing optical element (110) being a beam splitter (110), wherein the beam splitter (110) is configured to: receive the first (103-1) and the second input components (103-2) from the first polarization-diversity SOA pair (102), mix the first (103-1) and second (103-2) input components into a first (108-2) and a second (108-2) output component, by splitting the first (103-1) input component into a first (103-1A) and a second portion (103-1B), splitting the second (103-2) input component into a first (103-2A) and a second portion (103-2B), wherein the first output component (108-1) is a combination of the first portion (103-1A) of the first input component (103-1) and the second portion (103-2B) of the second input component (103-2) and the second output component (108-2) is a combination of the second portion (103-1B) of the first input component (103-1) and the first portion (103-2A) of the second input component (103-2).

12. The SOA arrangement according to claim 11, wherein the correlation reducing arrangement (106) further comprises a first Gain Flattening Filter (111-1) arranged to filter the first output component (108-1) and a second Gain Flattening Filter (111-2) arranged to filter the second output component (108-2).

13. The SOA arrangement according to claim 11 or 12, wherein the correlation reducing arrangement (106) further comprises a first Variable Optical Attenuator (112-1) arranged to attenuate the first output component (108-1) and a second Variable Optical Attenuator (112-2) arranged to attenuate the second output component (108-2).

14. The SOA arrangement (100) according to any preceding claim, wherein one or more further optical elements are arranged on an optical path of the correlation reducing optical element (110), wherein at least one of the one or more furtheroptical elements is arranged to operate on the input signal to the correlation reducing optical element (110) and / or at least one of the one or more further optical elements is arranged to operate on the output signal of the correlation reducing optical element (HO).

15. The SOA arrangement (100) according to claim 14, wherein the one or more further optical elements include an optical filter, such as a gain-flattening filter or a switch, an attenuator, an optical isolator, a coupler, and / or an optical fiber.

16. The SOA arrangement (100) according to claim 14 or 15, wherein the one or more further optical elements include an active element.

17. The SOA arrangement (100) according to any preceding claim, wherein the SOA arrangement (100) is a multi-stage SOA arrangement further comprising one or more further polarization-diversity SOA pairs and one or more further correlation reducing optical elements.

18. The SOA arrangement (100) according to any preceding claim, wherein the common optical path comprises sub-paths.

Images