WO2019166433A1 - Receiver and method for reception an optical signal in a kramers-kronig receiver - Google Patents

Abstract

A Kramers-Kronig, KK, receiver (100") is described, which includes an input (102) to receive a modulated optical signal, a node (104) coupled to the input (102) and an optical receiver (106). The node (104) generates the Kramers-Kronig, KK, carrier in the modulated optical signal, and the optical receiver (106) to receive the modulated optical signal including the KK carrier from the node (104). The node (104) includes an optical coupler (108), the optical coupler (108) receiving the modulated optical signal and a KK beat signal (114), and a highly nonlinear fiber (116), HNLF, coupled to an output of the optical coupler (108), the HNLF (116) providing the modulated optical signal including the KK carrier. The modulated optical signal provided to the input (102) includes a superchannel and a master CW.

Description

RECEIVER AND METHOD FOR RECEPTION AN OPTICAL SIGNAL IN A

KRAMERS-KRONIG RECEIVER

Description

The present invention relates to the field of optical data transmission in all optical networks, more specifically to receiving based on controlled or inherently polarization-aligned Kramers-Kronig carrier insertion at the receiver. Embodiments concern a receiver and a method for reception of an optical signal, like a 400-Gb/s Net Rate Superchannel, in a Kramers-Kronig, KK, receiver, like a Single-Photodiode 110-GHz Kramers-Kronig Receiver.

For short reach systems, such as a data center interconnect (DCI), high capacity, simplified and cost-effective direct-detection (DD) schemes are desired. Contrary to other DD schemes, the Kramers-Kronig (KK) technique allows the reception of complex modulation formats as is described, e.g., in references [1], [2], [3] or [4], In addition, the polarization of the data signal and the polarization of the KK carrier needs to be aligned, which is achieved by co-generating the KK carrier at the transmitter, as is described, e.g., in references [2] or [4] For a high capacity single-polarization KK reception, the co-generation of the KK carrier requires an ultra-wideband transmitter.

Conventional approaches for receiving reception of an optical signal in a Kramers-Kronig receiver require the optical signal to be generated such that the KK carrier is included in the signal transmitted to the KK receiver. Thus, there an overhead upon preparing and transmitting such an optical signal including the KK carrier.

It is an object of the present invention to provide an improved receiver and an improved method for reception of an optical signal in a Kramers-Kronig receiver avoiding overhead associated with preparing and transmitting the optical signal.

This object is achieved by the subject-matter as defined in the independent claims, and favorable further developments are defined in the pending claims. Embodiments of the present invention are now described in further detail with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of a KK receiver in accordance with embodiments of the present invention,

Fig. 2 is a schematic representation of a KK receiver in accordance with a first embodiment of the present invention achieving the KK reception using an active polarization control at the KK receiver,

Fig. 3 is a schematic representation of a KK receiver in accordance with a second embodiment of the present invention achieving the KK reception without active polarization control by means of an inherent polarization alignment,

Fig. 4 is a block diagram of a KK receiver in accordance with the second embodiment of the present invention,

Fig. 5 is a block diagram of an experimental set-up including the KK receiver of Fig. 4, wherein Fig. 5(a) illustrates a local beat generation, Fig. 5(b) illustrates a bus topology showing the aggregation of sub-carriers (SCs) and the KK receiver in accordance with the second embodiment of the present invention,

Fig. 6 shows diagrams illustrating the performance of the inventive approach, wherein

Fig. 6(a) shows the optical spectra of the local beat in the aggregation stage (see the upper part of Fig 5(A) above the KK receiver), Fig. 6(b) shows the aggregation output, Fig. 6(c) shows the input to the photodiode at a CSPR of 15 dB, Fig. 6(d) shows a Q²-factor vs. CSPR plot for different polarization arrangements, Fig. 6(e) illustrates the 3*33-GBd 32QAM b2b Q2-factor performance with constellations diagrams at maximum OSNR, Fig. 6(f) shows the electrical spectra of the acquired superchannel, Fig. 6(g) illustrates the 3*28-GBd 32QAM b2b and 60-km transmission Q2-factor performance (for pre- FEC and post-FEC) with constellations diagrams after 60-km transmission, and

Fig. 7 illustrates an example of a computer system on which units or modules as well as the steps of the methods described in accordance with the inventive approach may execute Embodiments of the present invention are now described in more detail with reference to the accompanying drawings in which the same or similar elements have the same reference signs assigned.

Other than the conventional approaches mentioned above, which require the optical signal to be generated such that the KK carrier is included, in accordance with the inventive approach the KK carrier is generated locally, i.e., at the KK receiver, and the locally generated KK carrier is inserted into the optical signal received at the KK receiver. For example, when combining distributed aggregation, as is described, e.g., in references [5] or [6], and the inventive KK carrier insertion (shift) at the KK receiver, the ultra-wideband signal for KK-reception may be generated without using ultra-wideband transmitter. Thus, any overhead upon preparing and transmitting an optical signal is avoided as no KK carrier needs to be included and transmitted to the KK receiver.

General

The present invention provides a Kramers-Kronig, KK, receiver, comprising an input to receive a modulated optical signal, a node coupled to the input, the node generating the Kramers-Kronig, KK, carrier in the modulated optical signal, and an optical receiver to receive the modulated optical signal including the KK carrier from the node.

The present invention provides a method, comprising receiving a modulated optical signal at an input of a Kramers-Kronig, KK, receiver, locally generating the Kramers-Kronig, KK, carrier at the KK receiver, and inserting the locally generated KK carrier into the received modulated optical signal.

In other words, in accordance with the inventive approach, the Kramers-Kronig receiver locally generates the necessary KK carrier, wherein the KK receiver may include the input and a photodiode, PD, with a node or multiplexer, MUX, stage therebetween to add the KK carrier to the modulated optical signal received at the input. Fig. 1 is a schematic representation of a KK receiver 100 in accordance with embodiments of the present invention. The KK receiver 100 includes the input 102 to receive the modulated optical signal, the node 104, like an all-optical multiplexing (AOM) stage, coupled to the input 102 and the optical receiver 106 coupled to the node 104. First embodiment

In accordance with a first embodiment the KK reception, using, e.g., a single-polarization receiver-based KK carrier, may be achieved using an active polarization control.

In accordance with embodiments, the node comprises an optical coupler, the optical coupler receiving the modulated optical signal and an unmodulated optical signal, and the optical coupler providing the modulated optical signal including the KK carrier, and a polarization controller to control polarization of the unmodulated optical signal to match the polarization of the modulated optical signal received at the input.

In accordance with embodiments, the node comprises an optical source, like a continuous wave, CW, laser, generating the unmodulated optical signal, wherein the optical coupler comprises a wavelength-division multiplexing coupler, WDC.

In other words, in accordance with the first embodiment of the inventive approach, the KK carrier may be locally generated using a single CW laser and a polarization control. Fig. 2 is a schematic representation of a KK receiver 100’ in accordance with a first embodiment of the present invention achieving the KK reception using an active polarization control at the KK receiver. The KK receiver 100’ includes the input 102 to receive the modulated optical signal. The node includes an optical coupler 108 which is coupled to the input 102 to receive the modulated optical signal from the input 102. Further, the optical coupler 108 receives an unmodulated optical signal 1 10, e.g., from an optical source (not depicted) being part of the KK receiver 100’ or being an external optical source. The optical coupler 108 provides the modulated optical signal now including the KK carrier to the optical receiver 106. The KK receiver 100’ further includes a polarization controller 1 12 to control polarization of the unmodulated optical signal 1 10 to match the polarization of the modulated optical signal received at the input 104.

Second embodiment

Other than in the first embodiment in accordance with which the KK reception requires an active polarization control, in accordance with a second embodiment the KK reception may be achieved without polarization control by means of an inherent polarization alignment.

In accordance with embodiments, the node comprises an optical coupler, the optical coupler receiving the modulated optical signal and a KK beat signal, and a highly nonlinear fiber, HNLF, coupled to an output of the optical coupler, the HNL providing the modulated optical signal including the KK carrier.

In accordance with embodiments, the HNLF causes a cross-phase modulation between the modulated optical signal and the KK beat signal, and wherein polarization of the generated KK carrier is dictated by the state-of-polarization, SOP, of the modulated optical signal.

In accordance with embodiments, the node comprises a first optical source, like a continuous wave, CW, laser, generating a first unmodulated optical signal, a second optical source, like a continuous wave, CW, laser, generating a second unmodulated optical signal, and a further optical coupler receiving the first and second unmodulated signals and outputting the KK beat signal.

In accordance with embodiments, the frequencies of the first and second unmodulated optical signals are selected such that the generated KK carrier is located at a predefined frequency of the modulated optical data signal, for example, 1549.13 nm for the first unmodulated optical signal and 1551.34 nm for the second unmodulated optical signal.

In accordance with embodiments, the node comprises an optical amplifier, like an Erbium- doped fiber amplifier, EDFA, coupled to an output of the further optical coupler, and a wavelength selective switch, WSS, coupled between the input and the optical coupler, wherein the EDFA and the wavelength selective switch are provided to adjust the KK carrier- to-signal-power ratio, CSPR, of the modulated optical signal including the KK carrier provided to the optical receiver.

In accordance with embodiments, the first optical source comprises a first external cavity laser, ECL, the second optical source comprises a second external cavity laser, ECL, , and the further optical coupler comprises a wavelength-division multiplexing coupler, WDC.

In other words, in accordance with the second embodiment of the inventive approach, the KK carrier is locally generated using two CW lasers to obtain a KK beat signal, and the optical input signal and the KK beat signal are passed through the HNLF so that no polarization control is needed. Fig. 3 is a schematic representation of a KK receiver 100" in accordance with a second embodiment of the present invention achieving the KK reception without active polarization control by means of an inherent polarization alignment. The KK receiver 100” includes the input 102 to receive the modulated optical signal. The node includes an optical coupler 108 which is coupled to the input 102 to receive the modulated optical signal from the input 102. Further, the optical coupler 108 receives the KK beat signal 114 being, e.g., locally generated using two CW lasers (not shown) being part of the KK receiver 100’ or being external to the KK receiver 100”. The optical coupler 108 provides the modulated optical signal now including the KK carrier to the HNLF 116 which is coupled to the optical receiver 106.

Fig. 4 is a block diagram of a KK receiver 100" in accordance with the second embodiment of the present invention. In accordance with the second embodiment, the KK receiver may comprise, in addition to the embodiment of Fig. 3, an analog-to-digital converter 1 18, ADC, coupled to the optical receiver 106, like a 100-GHz single-ended photodiode, and a digital signal processor 120, DSP, coupled to the ADC 1 18, the DSP 120 upsampling 122a the signal provided by the ADC 1 18, implementing 122b the KK algorithm and providing the information included in the modulated optical signal. The KK receiver 100” includes an Erbium-doped fiber amplifier 124, EDFA, coupled between an output of a wavelength selective switch 126, WSS, which is coupled to the input 102, and the optical coupler 108. The EDFA 124 and the WSS 126 adjust the KK ca rrie r-to-s ig n a l-powe r ratio, CSPR, of the modulated optical signal including the KK carrier.

The node 104 provides the modulated optical signal now including the KK carrier 1 14 to the optical receiver 106. The node 104 forms an AOM stage which includes as the optical coupler 108 a wavelength-division multiplexing coupler (WDC) to combine the AOM input, i.e., the modulated optical signal including the KK carrier received from the EDFA 124, and the local beat, i.e., the KK beat 1 14. Between the HNLF 1 16 and an output of the node 104 a further WDC 1 17 is provided to remove the local beat signal. Between the output of the node 104 and the PD 106 a fiber Bragg grating 128, FBG, is coupled to remove or suppress the master CW in the optical signal output from the node 104. The WDC 1 17 and the FBG 128 may be combined to a single bandpass filter, removing local beat and suppress the master CW at the same time. Optionally, the optical signal output from the node 104, after having passed the FBG 128, may be amplified 130 and band-pass filtered 132 so as to remove the master M from the signal before it is applied to the optical receiver 106. In accordance with embodiments, the amplified and band-pass filtered optical signal may be applied to a coupler 134 coupling a portion of the optical signal to the optical spectrum analyzer 136, OSA. The OSA may be used to monitor the optical signal. In accordance with the second embodiment, the modulated optical signal has multiplexed thereto information at a plurality of different subcarriers SC#1 , SC#2, and SC#3, and the digital signal processor, DSP, demultiplexes 122c the signal and processes 122d each subcarrier SC#1 , SC#2, SC#3 separately, wherein processing a subcarrier includes one or more of the following processes: data-aided channel estimation, frequency domain equalization, blind phase search carrier phase recovery, pre forward error correction, pre- FEC, and post-FEC calculations, and bit-error ratio, BER, counting.

The modulated optical signal

In accordance with embodiments, the modulated optical signal is modulated with data at a plurality of subcarriers such that the modulated optical signal is free of a KK carrier.

In accordance with embodiments, the modulated optical signal is a superchannel obtained by propagating a master continuous wave, CW, along a path or bus having a plurality of optical nodes and collecting locally generated sub-carriers, SCs, from each optical node along the bus.

In accordance with embodiments, the modulated optical signal provided to the input only includes the superchannel and the master CW.

Detailed second embodiment

In accordance with embodiments, which are described with reference to Fig. 5, a distributed superchannel aggregation scheme 150 is used to demonstrate a single-photodiode reception based on inherently polarization-aligned Kramers-Kronig carrier insertion at the receiver 100” according to the second embodiment. Also the impact of polarization rotations is discussed, and a 400-Gb/s capacity is demonstrated for optimized conditions. Fig. 5 is a block diagram of an experimental set-up including the KK receiver 100” of Fig. 4. Fig. 5(a) illustrates a local beat generation, and Fig. 5(b) illustrates a bus topology showing the aggregation 150 of sub-carriers (SCs) and the KK receiver 100” in accordance with the second embodiment of the present invention.

As is depicted in Fig. 5(b) a 3x33 GBd superchannel is aggregated with a 495-Gb/s gross rate using a bus topology as it is described, e.g., in references [5] or [6], A master continuous wave (CW) propagates through the bus and collects locally generated sub- carriers (SCs) of each optical node along the bus. At the KK receiver 100”, in accordance with the second embodiment, another node 104 is used to generate the KK carrier. The polarization of the generated KK carrier is dictated by the state-of-polarization (SOP) of the incoming master CW. Thus, an automatic polarization alignment of the KK carrier and the data is achieved without the need for polarization tracking.

Each SC in the superchannel may be detected with a Q²-factor > 6.5 dB and an error-free performance of the 400 Gb/s payload has been verified, which is the highest capacity reported so far for a single-polarization single-photodiode KK reception. The performance evaluation over a 60-km standard single-mode fiber (SSMF) link is also conducted with 344- Gb/s net rate superchannel.

Fig. 5(b) depicts the experimental setup of the optical bus topology and the inventive KK receiver 100” in accordance with the second embodiment.

Each optical node 152-1 , 152-2 and 152-3 is referred to as an all-optical multiplexing (AOM) stage. A CW propagates as a master (1560.6 nm, +16 dBm) through the bus which includes, in the depicted embodiment three AOM stages 152-1 , 152-2 and 152-3. Locally generated SCs (SC#1 , SC#2, SC#3) are sequentially added to the master M from three different nodes. In accordance with other embodiments, more or less AOM stages may be included. At each AOM stage, the master is combined with a local beat which was obtained from the beating between a local CW and a modulated data sent to the AOM. For generating the modulated data, an external cavity laser (ECL, 1551.48-nm wavelength) is used as a CW source (ECL #Ref), and is modulated with a 32QAM 33 GBd single-polarization forward-error correction (FEC)-encoded signal, using a single-polarization IQ modulator driven by a two-channel 64-GS/s digital-to-analog converter (DAC, 64 GS/s, 8-bit) via two driver amplifiers. The DAC waveforms may include the insertion of training symbols for channel estimation, a root-raised cosine pulse shaping (Nyquist roll-off of 10%) and a Volterra based nonlinear pre-distortion of the DAC, the driver and the modulator. The modulated data is split by a 1 x3 coupler and the output is distributed to the optical nodes or AOMs.

The local beat may be obtained from a beating between a local CW (like a pump laser) and the original modulated data sent to the particular AOM (see Fig.5(a)). The optical powers of the local CWs ranges from +6.4 dBm to +12.5 dBm.

Each AOM stage 152-1 , 152-2 and 152-3 includes a wavelength-division multiplexing coupler (WDC) to combine the AOM input and the local beat. For a suppression of the local beat at the AOM output, HNLFs having a length of, e.g., 300 to 789 m were used. Due to the tight aggregation of the SCs, the frequencies of the local CWs in AOM- 1/2/3 are spaced as a multiple of a pre-defined frequency (Af). To avoid a SC overlap, Af may be changed depending on the symbol rate (e.g. Af =36.3 GHz may be used for the 33 GBd 32QAM).

The AOMs are separated by short SSMF spans to depict practical scenario before the SCs are sent to the inventive receiver. A receiver optical signal-to-noise ratio (OSNR) degradation is emulated by replacing the 60-km SSMF transmission link path (shown in the lower branch at the output of AOM-3 in Fig. 5(a)) with a noise-loading mechanism. For example, the noise-loading mechanism (see the upper branch at the output of AOM-3 in Fig. 5(a)) is achieved by adjusting a variable optical attenuator (VOA) in front of an Erbium- doped fiber amplifier (EDFA). During transmission, the EDFA serves as a booster amplifier at the aggregation output. A flat-top optical band-pass filter (OBPF) is used to remove amplified spontaneous emission (ASE) noise at a high frequency edge of the superchannel where the KK carrier is to be added so as to achieve the single-side band (SSB) condition as described, e.g., in reference [1], At the output of AOM-3, only the superchannel and the master CW propagates to the receiver.

In accordance with the inventive approach, contrary to AOM-1/2/3 152-1 , 152-2, 152-3, the beating in KK-AOM 104 in the KK receiver 100” involves two CWs (e.g., ECLs #4 and #5 at 1549.13 nm at +8.5 dBm and 1551.34 nm at +6 dBm, respectively), as depicted in Fig. 5(a). The frequencies of the beat signals are selected so that the generated KK carrier is located at the desired frequency of the superchannel, the superchannel being, e.g., the combination of the plurality of locally generated subcarriers, SCs. The KK-beat EDFA (see Fig. 5(A)) and the wavelength selective switch (WSS) may be used to adjust the KK carrier-to-signal- power ratio (CSPR) into the photodiode.

A 100-GHz single-ended photodiode 106 and an analog-to-digital converter 118 (e.g., ADC: 256-GS/s, 110-GHz) enables a joint detection of the superchannel. In accordance with embodiments, to accommodate the nonlinear functions in the KK algorithm, an upsampling 122a (see Fig. 4) to 440 GS/s is digitally applied before implementing 122b the KK scheme (see, e.g., reference [2]). After demultiplexing 122c the superchannel, conventional offline digital signal processing (DSP) 122d may be applied to each SC. The DSP may include data-aided channel estimation, frequency domain equalization, blind phase search carrier phase recovery, pre-FEC and post-FEC calculations, and/or bit-error ratio (BER) counting. Using the setup described with reference to Fig. 5, measurements were conducted with 33 GBd single-polarization 32QAM data. The KK carrier was placed, with a 2-GHz offset, at the high frequency edge of the superchannel. In Fig. 6(a), Fig. 6(b) and Fig. 6(c), the optical spectra for the superchannel are shown. More specifically, Fig. 6(a) shows the optical spectra of the local beat in the aggregation stage (see the upper part of Fig 5(A) above the KK receiver). The aggregation output is shown in Fig. 6(b) and the input to the photodiode is shown in Fig. 6(c) at a CSFR of 15 dB. The performance of the KK scheme is depends on the received CSFR, as is described, e.g., in references [1], [2], [3] or [4], In accordance with the second embodiment of the inventive approach, polarization of the KK carrier is inherently aligned to the received data signal and a high performance is be achieved for an optimized condition (i.e. when the polarizations are well aligned in the setup). The CSFR may change if (1) the input polarization to the KK-AOM changes leading to lower KK carrier power generation, or (2) the input polarization to the aggregation stage(s) changes leading to lower SC power generation.

To quantify the impact of polarization rotation on the inventive single-polarization KK receiver, three different test scenarios were investigated. A performance evaluation was exemplary conducted for the SC closest to the carrier since it suffers more signal-signal beating interference (SSBI) impairments, as is described in references [1] or [2],

Firstly, the input waves to the KK-AOM were polarization aligned in parallel to the KK-beat (i.e. optimum condition) using a polarization controller (as used in accordance with the first embodiment). At the maximum OSNR of 26.9 dB, the CSFR was adjusted from 7.9 dB to 24.4 dB and Fig. 6(d) shows the corresponding Q²-factor vs. CSFR plot. It is noted that the measured BER has been converted to the Q²-factor using the relation:

Q²(dB) = 20 x to0_lo[V2er/c^_1(2 x BER)].

The OSNR was reduced to 21.2 dB and the Q²-factor plot is also shown in Fig. 6(d). As may be seen, the optimum CSFR occurs at 15 dB for both cases. The degradation in dB-Q performance for the lower CSFR (nonlinear regime) is attributed to the degrading minimum- phase condition (see reference [1]) whereas the worse performance at the higher CSFR (linear regime) is attributed to limited receiver photodiode power and the ADC background noise for smaller modulated signals (lower ADC vertical resolution needed). Secondly, at the optimum CSPR, the input waves to the KK-AOM were orthogonally polarized to the KK-beat. In this scenario, a reduced KK carrier power was generated, thus yielding to a lower CSPR of 9.6 dB (due to the lower KK carrier power) shown as a black square in Fig. 6(d). The obtained dB-Q is above the soft-decision forward-error correction (SD-FEC) threshold (Q²-factor = 6.3 dB, BER = 2x1 O ²).

Finally, the master CW was made orthogonally polarized to beat-3 thereby yielding a lower conversion efficiency of SC#3 (about 6 dB lower than the case with parallel pumps). A corresponding 21.2-dB OSNR was measured. At the KK receiver, the input to KK-AOM was aligned in parallel polarization to the KK-beat and a higher CSPR of 21.1 dB (i.e. due to lower SC #3 power) was measured (shown as black triangle in Fig. 6(d)). As expected, the performance was found to be the same as in the parallel polarization arrangement at 21.2- dB OSNR (shown in the upper curve in Fig. 6(d)). Again, the dB-Q is still above the SD- FEC threshold.

Thus, a variation in data or in KK carrier polarization will either increase or decrease the optimum CSPR by 6 dB and the corresponding dB-Q may still be received without polarization tracking.

A further performance evaluation was conducted for all three 33 GBd 32QAM SCs and in this case the optimum CSPR case, was 10.5 dB. Fig. 6(e) shows a Q²-factor vs. OSNR plot and it may be seen that SC #2 shows a performance superior to the performance of the other two SCs. The reduced performance in SC #1 and SC#3 may be explained as follows:

(1) SC #3 suffers more SSBI impairments since it is closest to the KK carrier,

(2) the degradation in SC #1 is due to the roll-off of the photodiode at higher frequencies (see Fig. 6(f)). In addition, the ADC provides more background noise after 100 GHz.

Nevertheless, all three SCs are above the SD-FEC threshold. The constellations at the maximum OSNR for all SCs are shown in Fig. 6(e). The symbol rate was reduced to 28 GBd due to the photodiode roll-off and ADC background noise after 100 GHz. Using the 3x28 GBd 32QAM (344-Gb/s net rate) superchannel, b2b measurements as well as a transmission over a 60-km single-span SSMF link were conducted. The total launch power over all SCs and the master CW was set 7.2 dBm. The SC optical powers was changed with the goal of achieving, seemingly, equal dB-Q values. Fig. 6(g) shows a plot of the Q²- factor vs. the SC index for both cases. The minimum dB-Q after the 60-km transmission was 6.5 dB and after FEC (22% OH) decoding, no error occurred within the evaluated post- FEC bits corresponding to a dB-Q > 14.9dB. The constellations for all three SCs, after transmission, are shown in Fig. 6(g).

Thus, in accordance with embodiments of the inventive approach a novel single-polarization Kramers-Kronig (KK) receiver with inherent polarization-aligned KK carrier is provided, which may yield a record capacity of 400 Gb/s of a superchannel from a distributed aggregation of 3x33 GBd 32QAM sub-carriers. The inventive approach is robust to polarization changes. The robustness to polarization changes has been described above for a single-carrier having a performance better than a soft-decision forward-error correction.

Further Embodiments

A 1^st embodiment provides a Kramers-Kronig, KK, receiver, comprising

an input to receive a modulated optical signal,

a node coupled to the input, the node generating the Kramers-Kronig, KK, carrier in the modulated optical signal, and

an optical receiver to receive the modulated optical signal including the KK carrier from the node.

A 2^nd embodiment provides the KK receiver of the 1^st embodiment, wherein the node comprises:

an optical coupler, the optical coupler receiving the modulated optical signal and an unmodulated optical signal, and the optical coupler providing the modulated optical signal including the KK carrier, and

a polarization controller to control polarization of the unmodulated optical signal to match the polarization of the modulated optical signal received at the input.

A 3^rd embodiment provides the KK receiver of the 2^nd embodiment, wherein the node comprises:

an optical source, like a continuous wave, CW, laser, generating the unmodulated optical signal,

wherein the optical coupler comprises a wavelength-division multiplexing coupler,

WDC. A 4^th embodiment provides the KK receiver of the 1^st embodiment, wherein the node comprises:

an optical coupler, the optical coupler receiving the modulated optical signal and a KK beat signal, and

a highly nonlinear fiber, HNLF, coupled to an output of the optical coupler, the HNL providing the modulated optical signal including the KK carrier.

A 5^th embodiment provides the KK receiver of the 4^th embodiment, wherein the HNLF causes a cross-phase modulation between the modulated optical signal and the KK beat signal, and wherein polarization of the generated KK carrier is dictated by the state-of- polarization, SOP, of the modulated optical signal.

A 6^th embodiment provides the KK receiver of the 4^th or 5^th embodiments, wherein the node comprises:

a first optical source, like a continuous wave, CW, laser, generating a first unmodulated optical signal,

a second optical source, like a continuous wave, CW, laser, generating a second unmodulated optical signal, and

a further optical coupler receiving the first and second unmodulated signals and outputting the KK beat signal,

A 7^th embodiment provides the KK receiver of the 6^th embodiment, wherein the frequencies of the first and second unmodulated optical signals are selected such that the generated KK carrier is located at a predefined frequency of the modulated optical data signal, for example, 1549.13 nm for the first unmodulated optical signal and 1551.34 nm for the second unmodulated optical signal.

An 8^th embodiment provides the KK receiver of the 6^th or 7^th embodiments, wherein the node comprises

an optical amplifier, like an Erbium-doped fiber amplifier, EDFA, coupled to an output of the further optical coupler, and

a wavelength selective switch, WSS, coupled between the input and the optical coupler,

wherein the EDFA and the wavelength selective switch are provided to adjust the KK carrier-to-signal-power ratio, CSFR, of the modulated optical signal including the KK carrier provided to the optical receiver. A 9^th embodiment provides the KK receiver of any one of the 6^th to 8^th embodiments, wherein the first optical source comprises a first external cavity laser, ECL,

the second optical source comprises a second external cavity laser, ECL, , and wherein the further optical coupler comprises a wavelength-division multiplexing coupler, WDC.

A 10^th embodiment provides the KK receiver of any one of 1^st to 9^th embodiments, comprising:

an analog-to-digital converter, ADC, coupled to the optical receiver, like a 100-GHz single-ended photodiode, and

a digital signal processor, DSP, coupled to the ADC, the DSP upsampling the signal provided by the ADC, implementing the KK algorithm and providing the information included in the modulated optical signal.

An 1 1^th embodiment provides the KK receiver of the 9^th embodiment, wherein

the modulated optical signal has multiplexed thereto information at a plurality of different subcarriers, and

the digital signal processor, DSP, demultiplexes the signal and processes each subcarrier separately, wherein processing a subcarrier includes one or more of the following processes: data-aided channel estimation, frequency domain equalization, blind phase search carrier phase recovery, pre forward error correction, pre-FEC, and post-FEC calculations, and bit-error ratio, BER, counting.

A 12^th embodiment provides the KK receiver of any one of the 1^st to 11^th embodiments, wherein the modulated optical signal is modulated with data at a plurality of subcarriers such that the modulated optical signal is free of a KK carrier.

A 13^th embodiment provides the KK receiver of the 12^th embodiment, wherein the modulated optical signal is a superchannel obtained by propagating a master continuous wave, CW, along a path or bus having a plurality of optical nodes and collecting locally generated subcarriers, SCs, from each optical node along the bus.

A 14the embodiment provides the KK receiver of the 12^th or 13^th embodiments, wherein the modulated optical signal provided to the input only includes the superchannel and the master CW. A 15^th embodiment provides a method, comprising:

receiving a modulated optical signal at an input of a Kramers-Kronig, KK, receiver, locally generating the Kramers-Kronig, KK, carrier at the KK receiver, and inserting the locally generated KK carrier into the received modulated optical signal.

Although some aspects of the described concept have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or a device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus.

Various elements and features of the present invention may be implemented in hardware using analog and/or digital circuits, in software, through the execution of instructions by one or more general purpose or special-purpose processors, or as a combination of hardware and software. For example, embodiments of the present invention may be implemented in the environment of a computer system or another processing system. Fig. 7 illustrates an example of a computer system 400. The units or modules as well as the steps of the methods performed by these units may execute on one or more computer systems 400. The computer system 400 includes one or more processors 402, like a special purpose or a general purpose digital signal processor. The processor 402 is connected to a communication infrastructure 404, like a bus or a network. The computer system 400 includes a main memory 406, e.g., a random access memory (RAM), and a secondary memory 408, e.g., a hard disk drive and/or a removable storage drive. The secondary memory 408 may allow computer programs or other instructions to be loaded into the computer system 400. The computer system 400 may further include a communications interface 410 to allow software and data to be transferred between computer system 400 and external devices. The communication may be in the from electronic, electromagnetic, optical, or other signals capable of being handled by a communications interface. The communication may use a wire or a cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels 412.

The terms“computer program medium” and“computer readable medium” are used to generally refer to tangible storage media such as removable storage units or a hard disk installed in a hard disk drive. These computer program products are means for providing software to the computer system 400. The computer programs, also referred to as computer control logic, are stored in main memory 406 and/or secondary memory 408. Computer programs may also be received via the communications interface 410. The computer program, when executed, enables the computer system 400 to implement the present invention. In particular, the computer program, when executed, enables processor 402 to implement the processes of the present invention, such as any of the methods described herein. Accordingly, such a computer program may represent a controller of the computer system 400. Where the disclosure is implemented using software, the software may be stored in a computer program product and loaded into computer system 400 using a removable storage drive, an interface, like communications interface 410.

The implementation in hardware or in software may be performed using a digital storage medium, for example cloud storage, a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.

Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the present invention may be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier.

Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet. A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein are apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.

References

[1] A. Mecozzi et. a!.,“Kramers-Kronig coherent receiver”, Optica 3, 14(3), pp. 1220-1227 (2016).

[2] Z. Li et. a!., “Joint optimization of resampling rate and CSFR in direct-detection Kramers-Kronig receivers", Proc. ECOC 2017, W.2.D.3.

[3] X. Chen et. al.,“4x240 Gb/s dense WDM and RDM Kramers-Kronig detection with 125-km SSMF transmission”, Proc. ECOC 2017, W.2.D.4.

[4] S. T. Le et. al., “8x256Gbps virtual-carrier assisted WDM direct-detection transmission over a single span of 200km”, Proc. ECOC 2017, Th.PDP.B.1.

[5] S. Watanabe et. al., “All-optical data frequency multiplexing on single-wavelength carrier light by sequentially provided XPM in fiber”, IEEE JSTQE 18(2), pp. 577 (2012).

[6] T. Richter et. al., “Distributed 1-Tb/s all-optical aggregation capacity in 125-GHz bandwidth by frequency conversion in fiber”, Proc. ECOC 2015, PDP 2.5.

Claims

1. A Kramers-Kronig, KK, receiver (100”), comprising

an input (102) to receive a modulated optical signal,

a node (104) coupled to the input (102), the node (104) generating the Kramers- Kronig, KK, carrier in the modulated optical signal, and

an optical receiver (106) to receive the modulated optical signal including the KK carrier from the node (104),

wherein the node (104) comprises:

an optical coupler (108), the optical coupler (108) receiving the modulated optical signal and a KK beat signal (114), and

a highly nonlinear fiber (116), HNLF, coupled to an output of the optical coupler (108), the HNLF (116) providing the modulated optical signal including the KK carrier, and

wherein the modulated optical signal provided to the input (102) includes a superchannel and a master CW.

2. The KK receiver (100”) of claim 1 , wherein the HNLF (116) causes a cross-phase modulation between the modulated optical signal and the KK beat signal (114), and wherein polarization of the generated KK carrier is dictated by the state-of-polarization, SOP, of the modulated optical signal.

3. The KK receiver (100”) of claim 1 or 2, wherein the node (104) comprises:

a first optical source (ECL#4), like a continuous wave, CW, laser, generating a first unmodulated optical signal,

a second optical source (ECL#5), like a continuous wave, CW, laser, generating a second unmodulated optical signal, and

a further optical coupler receiving the first and second unmodulated signals and outputting the KK beat signal (114).

4. The KK receiver (100") of claim 3, wherein the frequencies of the first and second unmodulated optical signals are selected such that the generated KK carrier is located at a predefined frequency of the modulated optical data signal, for example, 1549.13 nm for the first unmodulated optical signal and 1551.34 nm for the second unmodulated optical signal.

5. The KK receiver (100”) of claim 3 or 4, wherein the node (104) comprises an optical amplifier (124), like an Erbium-doped fiber amplifier, EDFA, and a wavelength selective switch (126), WSS, coupled between the input (102) and the optical coupler (108),

wherein the optical amplifier 124 and the WSS 126 are provided to adjust the KK carrier-to-signal-power ratio, CSPR, of the modulated optical signal including the KK carrier provided to the optical receiver (106).

6. The KK receiver (100”) of any one of claims 3 to 5, wherein

the first optical source (ECL#4) comprises a first external cavity laser, ECL, the second optical source (ECL#5) comprises a second external cavity laser, ECL, and

wherein the further optical coupler comprises a wavelength-division multiplexing coupler, WDC.

7. The KK receiver (100”) of any one of the preceding claims, wherein

the modulated optical signal has multiplexed thereto information at a plurality of different subcarriers, and

the KK receiver (100”) is configured to demultiplex the signal and process each subcarrier (SC#1 , SC#2, SC#3) separately, wherein processing a subcarrier includes one or more of the following processes: data-aided channel estimation, frequency domain equalization, blind phase search carrier phase recovery, pre forward error correction, pre- FEC, and post-FEC calculations, and bit-error ratio, BER, counting.

8. The KK receiver (100”) of any one of the preceding claims, wherein the modulated optical signal is modulated with data at a plurality of subcarriers (SC#1 , SC#2, SC#3) such that the modulated optical signal is free of a KK carrier.

9. The KK receiver (100”) of any one of the preceding claims, wherein the superchannel obtained by propagating a master continuous wave, CW, along a path or bus having a plurality of optical nodes (152-1 , 152-2, 152-3) and collecting locally generated sub-carriers, SCs, from each optical node (152-1 , 152-2, 152-3) along the bus.

10. The KK receiver (100”) of any one of the preceding claims, comprising:

an analog-to-digital converter (118), ADC, coupled to the optical receiver (106), like a 100-GHz single-ended photodiode, and a digital signal processor (120), DSP, coupled to the ADC, the DSP (120) upsampling (122a) the signal provided by the ADC, implementing (122b) the KK algorithm and providing (122c) the information included in the modulated optical signal.

11. A method, comprising

receiving a modulated optical signal at an input (102) of a Kramers-Kronig, KK, receiver (100”),

locally generating the Kramers-Kronig, KK, carrier at the KK receiver (100”), and inserting the locally generated KK carrier into the received modulated optical signal, wherein inserting the locally generated KK carrier into the received modulated optical signal comprises:

generating a coupled signal by optically coupling the received modulated optical signal and a KK beat signal (114), and

applying the coupled signal to a highly nonlinear fiber, HNLF (116) for providing the modulated optical signal including the KK carrier, and

wherein the modulated optical signal provided to the input (102) includes a superchannel and a master CW.