Optical signal converter and conversion method

Abstract

Methods and apparatus for converting an optical signal into an electrical signal. The method comprises receiving the optical signal S302, splitting the optical signal into a first signal and a second

Description

Technical Field Embodiments of the present disclosure relate to methods and apparatus for converting optical signals, and in particular relate to converting optical signals into electrical signals. Background Analog-to-Digital Converters (ADCs) and may be used in communication technologies and optical communication systems to convert analog signals into a digital form in order to be read and processed by a microcontroller. Digital-to-Analog Converters (DACs) may similarly be used in optical communication systems to convert digital signals into an analog form. Such optical communication systems may include signal receivers and transmitters, which may in turn form wider optical fibre transmission systems. High-speed complementary metal-oxide-semiconductor (CMOS) ADCs and DACs may reach noise floors of approximately 5 effective number of bits (ENoB) for ultra-high-speed sampling rates, for example approximately 100 GS / s. The ENoB rate for an ADC / DAC converter may provide an accurate evaluation of converter performance. The capacity of optical fibre transmission systems is limited by the ENoB rates provided by ADCs. For example, increased capacity demands in optical fibre transmission systems may require higher-order modulation in order to spread the cost and power consumption of optoelectronic components across multiple bits. However, higher-order modulation may impose challenging requirements on high-speed optoelectronic transceiver components. For example, the performance of ADCs and DACs at higher order modulations may be limited by their vertical resolution. ADCs and DACs may experience more fibre nonlinearity at higher order modulations, increasing the ENoB rates required at such modulations. Accordingly, highspeed converters may limit the development of next-generation systems above 6 bit / s / Hz / pol at symbol rates beyond 100 Gbaud for Nyquist shaped channels. It may be understood that in a wider system, in the absence of fibre nonlinearity, the upper limit on the available signal-to-noise ratio is bounded by transceiver subsystems. As optical fibre transmission systems are experiencing increasing capacity demands, there may be an escalating need to improve ENoB rates for ADCs and DACs. For example, conventional ADC / DAC technology may be pushed to deliver 64-Quadratude Amplitude Modulation (QAM), with higher-order formats requiring adding approximately 1 effective bit for each extra bit-per-symbol in order to achieve the same implementation penalty. This push may not meet the needs of increasing capacity demands due to the saturation of improvements to ultra-high-speed pure CMOS converters. Such CMOS converters are also unlikely to meet the requirements of higher-order constellations, which are being considered both independently and in combination with shaping techniques such as probabilistic and / or geometric shaping techniques. Converter performance (for example, ENoB) may be dominated by clock jitter at higher frequencies. For example, clock jitter may include aperture jitter. Aperture jitter may produce a decrease in performance, resulting from the measurement time resolution of an ADC / DAC. That is, aperture jitter may result from an inability of an ADC / DAC to sample at precisely defined time instants. The signal-to-noise-plus-distortion ratio (SNDR) is related to aperture jitter through Equation 1 below: SNDR = —20 log10 2nftj Equation 1 where f is the input analog frequency and tj is the aperture jitter of the ADC. Equation 1 assumes that the ADC / DAC has infinite resolution, with SNDR being measured in decibels. ENoB may then be calculated using Equation 2 below: SNDR - 1.76 ENoB =---——-- 6.02 Equation 2 where ENoB is measured in bits. Accordingly, it may be understood that the theoretical analog bandwidth is halved for each extra effective bit. If the required ENoB is reduced by one bit, the theoretical analog bandwidth may be doubled; this may allow for an increase in the bit rate which may reduce the cost and energy consumption per bit. Figure 1 depicts a conventional signal converter 100, including a coherent receiver front end The example converter 100 comprises multiple ADCs 112. Coherent transceivers and receivers may allow for parallel processing, which may provide additional benefits such as improved performance at a sustainable cost and energy consumption over single optical head receivers. Present examples may focus on coherent receivers, however the associated teachings may be applicable to coherent transmitters for example, by functionally reverting the presented schemes and apparatus. As depicted in Figure 1, converter 100 may comprise an Optical Amplifier (OA) 102. The OA 102 may act as a pre-amplifier to the converter 100, in order to amplify the signal before processing. Converter 100 may further comprise a Polarization Beam Splitter (PBS) 104, which may split an incoming signal into two polarizations. Each polarization may be processed separately using separate hybrid couplers 108. Each branch of the split incoming signal may be mixed with a Local Oscillator (LO) 106, for example a LO laser. Accordingly, an ADC 112 and Digital Signal Processor (DSP) 114 may be used to retrieve the amplitude, phase, and polarization of the transmitted signal as shown in Figure 1. Balanced detection may also be used to retrieve the amplitude, phase, and polarization of the transmitted signal. The LO 106 signal may also be split using a PBS 104 with each LO polarization sent to the hybrid coupler 108 which received the corresponding incoming signal polarization. As shown in Figure 1, the hybrid coupler may be a 90 degree coupler. Figure 1 depicts optical devices using dashed outer lines, and electronic devices using dotted lines. Accordingly, optical signals may be processed by the OA 102 and PBS 104 and may be provided by LO 106. Such optical signals may be converted into electrical signals, and transmitted to the balanced photodetectors 116. These electrical signals may then be processed by a Transimpedence Amplifier (TIA) 110, ADC 112, and DSP 114 as shown in Figure 1. Accordingly, conventional metro and long-haul systems comprising signal converters as depicted in Figure 1 maximize throughput by simultaneously using quadrature-amplitude modulation (QAM) and polarization multiplexing (PM), with spectral efficiency increasing with constellation cardinality but limited by available vertical resolution in DACs and ADCs. Figure 2 depicts a 64-QAM diagram and associated probabilistically shaped distribution for a conventional signal converter such as that of Figure 1. In particular, Figure 2A is a diagram of a high-order 64-QAM constellation including multiple quantised energies. It can be seen that a nonconstant envelope signal leads to a notable peak-to-average-power ratio (PAPR), which in turn decreases the effective dynamic range of ADCs and thus the achievable signal-to-noise ratio (SNR) at the receiver. In order to mitigate this, the gain of the TIAs 110 in a coherent receiver (for example, placed in front of the ADCs 112 as shown in Figure 1) may be constantly adapted to strike a balance between quantization noise and signal clipping noise. For example, TIA gain may be adapted on a ps-to-ms scale. Furthermore, a high PAPR may enhance Kerr induced signal distortion, and may reduce system reach. To supress such nonlinear distortion, digital nonlinear compensation techniques such as Digital Backpropagation (DBP) may be used. For example, DBP may provide a 0.1-2 dB suppression of nonlinear distortion. However, such optical systems may still have performance limited by the ENoB of the system receiver. Conventional methods to maximise signal-to-noise ratios revolve solely around digital signal processing techniques. For example, constellation shaping techniques, such as probabilistic and geometric shaping, may be used to improve throughput by tailoring high-order constellations to a nonlinear fibre channel. Constellation shaping techniques may also be used to serve as a platform for rate adaptation. Probabilistic shaping is characterised by modifying the probability of occurrence of constellation symbols, for example by prioritizing those from the inner rings of Figure 2A. Figure 2B depicts a probabilistically shaped distribution associated with the 64-QAM constellation of Figure 2A, in which the inner rings are prioritized. In the specific example of Figure 2B, the entropy rate is 4 bits per symbol. The outer symbols in Figure 2B, which have reduced probabilities, are more susceptible to noise and thus have a higher SNR. Such probabilistic shaping may be done in a sacrificial manner, with sporadic symbols in the outer rings used to sink nonlinear distortion away from the main payload. For probabilistically shaped constellations, the high-order shaped constellations may be more susceptible to SNR penalties at reduced ENoB. Summary It is an aim of the present disclosure to provide a method for converting an optical signal into an electrical signal and a signal converter which at least partially address one or more of the challenges discussed above. It is a further aim of the present disclosure to provide a method for converting an optical signal into an electrical signal and a signal converter with a lower ENoB than conventional converters, in order to allow for a wider analog bandwidth. Accordingly, it is an aim of the present disclosure to provide methods and apparatus for converting optical signals with an improved resolution, for example by 0.5 bits. In particular, it is an aim of the present disclosure to provide improved receiver sensitivity and / or the performance of digital nonlinear compensation techniques such as DBP by providing a signal converter with improved ENoB. The present disclosure may use optical signal and optoelectrical processing rather than conventional electronic processing, in order to deliver broadband signal processing with higher energy efficiency and ultra-low-noise. An embodiment of the present disclosure provides a method for converting an optical signal into an electrical signal. The method comprises receiving the optical signal, splitting the optical signal into a first signal and a second signal, and measuring intensity information of the first signal. The method further comprises processing the first signal using an intensity modulator to generate an inverse intensity waveform, combining the inverse intensity waveform and the second signal to generate a constant intensity waveform, converting the constant intensity waveform into a digital constant intensity signal using an Analog-to-Digital Converter (ADC), combining the intensity information and the digital constant intensity signal, and generating the electrical signal using a Digital Signal Processor (DSP). A further embodiment of the present disclosure provides signal converter for converting an optical signal into an electrical signal comprising an intensity modulator, an Analog-to-Digital Converter (ADC), and a Digital Signal Processor (DSP). The converter is configured to receive the optical signal, split the optical signal into a first signal and a second signal, and measure the intensity information of the first signal. The converter is further configured to process the first signal using the intensity modulator to generate an inverse intensity waveform, combine the inverse intensity waveform and the second signal to generate a constant intensity waveform, convert the constant intensity waveform into a digital constant intensity waveform using the ADC, combine the intensity information and the digital constant intensity signal, and generate the electrical signal using the DSP. Brief Description of the Drawings For a better understanding of the present disclosure, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings in which: Figure 1 is a schematic diagram of a conventional coherent receiver; Figure 2A and Figure 2B (collectively referred to as Figure 2) are figurative illustrations of a 64-QAM signal; Figure 3 is a flowchart showing a method in accordance with embodiments; Figure 4A and Figure 4B (collectively referred to as Figure 4) are schematic diagrams of a signal converter in accordance with embodiments; Figure 5A and Figure 5B (collectively referred to as Figure 5) are schematic diagrams of a further signal converter in accordance with embodiments; Figure 6A and Figure 6B (collectively referred to as Figure 6) are figurative illustrations of a processed 64-QAM signal in accordance with embodiments; Figure 7 is a schematic diagram of a transmitter and receiver in accordance with embodiments; Figure 8A and Figure 8B (collectively referred to as Figure 8) depict graphs of Q-factors of converters in accordance with embodiments; and Figure 9A and Figure 9B (collectively referred to as Figure 9) depict graphs of Q-factors of further converters in accordance with embodiments. Detailed Description Embodiments herein provide methods and signal converters for converting an optical signal into an electrical signal. Converters of the present embodiments may be implemented, for example, in a signal receiver. Such a receiver may in turn form a part of a wider optical fibre transmission system. Embodiments herein may improve the performance of receivers such as single optical head coherent receivers through Optical Intensity Reshaping (OIR). OIR may include reshaping the intensity envelope of an incoming optical signal and removing intensity information such that the PAPR associated with the signal is reduced. Figure 3 depicts a method S300 in accordance with embodiments. As shown in Figure 3, method S300 may include receiving an optical signal S302 and splitting the optical signal into a first signal and a second signal S304. Splitting the optical signal into a first signal and a second signal S304 may comprise, for example, splitting the received signal into the first signal and the second signal using a 10:90 ratio. Method S300 may further include measuring intensity information of the first signal S306, and processing the first signal using an intensity modulator to generate an inverse intensity waveform S308. The intensity modulator may apply a reciprocal function to the first signal. That is, the present embodiments may use an intensity modulator driven by a reciprocal intensity waveform (e.g. a multiplicative inverse where x is the first signal). This reciprocal intensity waveform may be obtained from an optical tap, for example using direct detection (DD). Signal inversion may be performed using a log amplifier or an anti-log amplifier. Alternatively or additionally, signal inversion may be obtained using digital signal processing in-between a state-of-the-art ADC-DAC pair. Accordingly, present embodiments may be used in combination with digital signal processing techniques to provide additional gain. As shown in Figure 3, the method S300 may further include combining the inverse intensity waveform and the second signal to generate a constant intensity waveform S310. Preferably, the inverse intensity waveform and the second signal are combined such that the constant intensity waveform has a PAPR equal to or approximately equal to 1. The method S300 may further include converting the constant intensity waveform into a digital constant intensity signal using an ADC S312, combining the intensity information and the digital constant intensity signal S314, and generating the electrical signal using a DSP S316. The method may further comprise outputting the generated electrical signal. Examples of suitable signal converters 400 for performing the method S300 of Figure 3 are shown schematically in Figure 4. Either of the signal converters 400 shown in Figure 4A or Figure 4B may be implemented, for example, in a signal receiver. Such a receiver may in turn form a part of a wider optical fibre transmission system. As shown in Figure 4A, signal converter 400 may comprise an ADC 412 and a DSP 414. The signal converter may be configured to convert the constant intensity waveform into a digital constant intensity waveform using the ADC 412. The signal converter 400 may be configured to generate the electrical signal using the DSP 414. The signal converter 400 may be configured to receive an optical signal, split the optical signal into a first signal and a second signal, and measure the intensity information of the first signal. The signal converter 400 may be further configured to process the first signal using the intensity modulator to generate an inverse intensity waveform, combine the inverse intensity waveform and the second signal to generate a constant intensity waveform, convert the constant intensity waveform into a digital constant intensity waveform using the ADC, combine the intensity information and the digital constant intensity signal, and generate the electrical signal using the DSP. As shown in Figure 4B, the signal converter 400 may comprise a receiver 401, and outputter 405. The receiver 401 may be configured to receive an optical signal S302. The outputter 405 may be configured to output a generated electrical signal. The signal converter 400 may additionally comprise a splitter 403. The splitter 403 may be configured to split the optical signal into a first signal and a second signal S304. The splitter 403 may be configured to split the received signal into the first signal and the second signal using a 10:90 ratio. The signal converter 400 may further comprise a processor 407. The processor 407 may comprise a combiner 409, a converter 411, a digital generator 413, and an intensity modulator 415. The processor 407 may be configured to measure intensity information of the first signal S306. The intensity modulator 415 may be configured to process the first signal to generate an inverse intensity waveform S308. In some embodiments, the intensity modulator 415 may be an intensity inverter 518 as discussed below. In some embodiments, the intensity modulator 415 may be driven by a reciprocal intensity waveform. The combiner 409 may be configured to combine the inverse intensity waveform and the second signal to generate a constant intensity waveform S310. In specific embodiments, the combiner 409 may be an interferometer (for example, interferometer 520 of Figure 5, as discussed below). This interferometer 520 may also be an example of an intensity modulator 415. The converter 411 may be configured to convert the constant intensity waveform into a digital constant intensity signal S312. In some embodiments, the converter 411 may comprise an ADC. The combiner 409 may be additionally configured to combine the intensity information and the digital constant intensity signal S314. In some embodiments, the signal converter 400 may comprise an analog combiner and a digital combiner, wherein the analog combiner is configured to combine the inverse intensity waveform and the second signal to generate a constant intensity waveform S310 and the digital combiner is configured to combine the intensity information and the digital constant intensity signal S314. The digital generator 413 may be configured to generate an electrical signal using the digital output of the combiner 411. Figure 5 depicts a signal converter 500 in accordance with a further embodiment. As depicted in Figure 5, converter 500 may comprise an OA 502. The OA 502 may act as a pre-amplifier to the converter 500, in order to amplify the signal before processing. The signal from OA 502 may then pass to the splitter 516. Splitter 516 may be configured to split the optical signal from the OA 502 and / or the received signal into a first signal and a second signal. For example, the splitter 516 may be configured to split the received signal into the first signal and the second signal using a 10:90 ratio. In such an example, 90% of the split signal would be transmitted to PBS 504, forming the second signal. 10% of the split signal would be transmitted to the photodetector (PD) 522, forming the first signal. The signal converter 500 may comprise a PD 522 for measuring intensity information of the first signal. In some embodiments, the signal converter 500 may additionally comprise a TIA 510 configured to remove noise from the first signal for further processing by the converter. In specific embodiments, the signal converter 500 may additionally comprise a further ADC 512 to convert the first signal into an electrical signal. After the intensity information of the first signal has been measured, the first signal may be passed to signal inverter 518, as shown in Figure 5. Signal inverter 518 may be configured to process the first signal to generate an inverse intensity waveform. The signal inverter 518 may also receive a signal from an optical tap (not shown) in order to process the first signal. This upper branch of Figure 5 accordingly provides a signal processing branch for the first signal, in order to generate an inverse intensity waveform. The inverse intensity waveform may then be fed into one or more interferometers 520, and the DSP 514. The first signal may be processed by a photodiode (not shown) before or after being processed by the signal inverter 518 in order to generate an optical inverse intensity waveform to be fed to the one or more interferometers 520. In specific embodiments, the converter may comprise a first PBS 504 which may split the second signal into two polarizations. Each polarization may be processed separately using an interferometer 520. The interferometer 520 may be a Mach-Zehnder interferometer. Each interferometer 520 may be fed the inverse intensity waveform from the inverter 518 and a polarized signal from the first PBS 504. The interferometer 520 may then be configured to combine the inverse intensity waveform and the polarized signal to generate a constant intensity waveform. Alternatively, the interferometer 520 may be fed the inverse intensity waveform and the second signal and may be configured to combine the inverse intensity waveform and the second signal to generate a constant intensity waveform. Accordingly, the second signal may be processed in a single branch or may be polarized and processed in multiple branches. The constant intensity waveform may be processed by a hybrid coupler 508, as shown in Figure 5. The hybrid coupler may additionally be fed a signal from an LO 506. The LO 506 signal may also be split using a second PBS 528 with each LO polarization sent to the hybrid coupler 508 which received the corresponding incoming signal polarization, for example in embodiments where the second signal is split into polarized signals by the first PBS 504. As shown in Figure 5, the hybrid coupler 508 may be a 90 degree hybrid coupler. Accordingly, the hybrid coupler 508 may be configured to generate signals with a relative phase shift of 90 degrees (l / Q signals). For embodiments where the second signal has already been polarized as shown in Figure 5, each hybrid coupler may generate a set of quadrature signals or l / Q signals for each polarized signal such that at least four split signals (XI, XQ, Yl, and YQ) are generated. Each split signal may then be converted into an electrical signal using a balanced photodetector 516 and a TIA 510 and further into a digital signal using an ADC 512 as shown in Figure 5. This conversion process may additionally comprise processing each signal, for example the TIAs 510 may serve to deal with residual fluctuations of the intensity in each quadrature component. The digital signals may then be fed into a DSP 514, along with the intensity information. The DSP 514 may then be configured to combine the intensity information with each digital signal, and construct an electrical signal using each digital signal. For embodiments such as that of Figure 5, the signal converter may be configured to reconstruct the optical signal (before being split into first signal and second signal) by passing the quantization levels from both stages of processing (that is, the intensity information from the inverter 518 and the digital constant intensity signal from the ADC 512) to the receiver DSP circuit. By passing the digitized intensity signal to the DSP, the detected field may be reconstructed before the other DSP blocks. In some embodiments, the received signal may be processed as an analog signal, and then converted into a digital signal. Alternatively, in some embodiments the received signal may be converted into a digital signal before processing. Accordingly, in some embodiments the method of signal processing may comprise converting the first signal into a digital first signal using a first photodetector and first ADC. The first ADC may also be referred to as a preinverter ADC or a pre-modulator ADC. Some embodiments may further comprise processing the digital first signal using an intensity modulator to generate a digital inverse intensity waveform, converting the digital inverse intensity waveform to analog using a DAC which may form a part of a signal inverter 518, , and combining the inverse intensity waveform and the analog second signal to generate the analog constant intensity signal. Figure 5 presents an embodiment comprising digital inversion processing, wherein the first signal is converted into a digital first signal before processing. Figure 5 depicts optical devices using dashed outer lines, and electronic devices using dotted lines. However, alternative systems using analog signal processing are envisaged in which an analog signal, given by converting the first signal using a first photodetector, is passed through the inverter 518 to generate an analog inverse intensity waveform. Such alternative systems may not need a first ADC / pre-inverter ADC, and / or may not need a DAC forming a part of the signal inverter 518. Figure 5A presents an embodiment in which OIR is implemented. Figure 5B presents a modified version of the embodiment of Figure 5A, in which Optical Polarization Splitting (OPS) is also implemented. Accordingly, the embodiment of Figure 5B operates in the same manner as that of Figure 5A with each polarization of the received signal being processed separately. As shown in Figure 5B, a system implementing OPS may comprise a third PBS 530 configured to split the received optical signal into polarized signals before the generation of the inverse intensity waveform. Each polarized signal is then processed in the same manner as the signal processing described with respect to Figure 5 above, and combined at the DSP 514 to generate an electrical signal. This results in the generation of multiple first polarized signals. Accordingly, in some embodiments the method for converting an optical signal into an electrical signal may comprise splitting the received optical signal into separate polarized signals using a PBS, splitting each polarized signal into a first polarized signal and a second polarized signal, measuring the intensity information of each first polarized signal, processing each first polarized signal using the intensity modulator to generate a corresponding first polarized inverse intensity waveform, combining each first polarized inverse intensity waveform and each corresponding second polarized signal to generate corresponding polarized constant intensity waveforms, converting each polarized constant intensity waveform into a digital polarized constant intensity signal using the ADC, combining each intensity information and each digital polarized constant intensity signal, and generating the electrical signal using a DSP. In this way, the use of OPS may allow for each polarization to be separately processed. Independent processing of each polarization may provide higher fidelity in measurements, for example amplitude or intensity information. Figure 6 depicts a signal constellation associated with the signal converter of Figure 5. Figure 6A depicts a signal constellation after applying OIR with a signal converter as shown in Figure 5A to a 64-QAM using a pre-inverter ADC with ENoB = 7 bits to discretize the associated optical tap direct-detection (DD) signal. Figure 6A is idealised, such that laser phase noise, carrier frequency offset and / or polarization mode dispersion are assumed to be absent. By comparing Figure 6A with Figure 2A, it can be seen that the outer symbol transitions have been partially supressed through the application of OIR. However, the constellation of Figure 6A has not collapsed into a ring (in other words, the constellation does not have a constant amplitude). This may result from the DD in the optical tap branch being polarization insensitive. To achieve further intensity stripping, OPS may be applied using a signal converter as shown in Figure 5B such that each polarization of the received optical signal is processed independently. An idealised case of a 64-QAM signal processed in this manner is shown in Figure 6B. From Figure 6B, it can be seen that the constellation collapses into a ring of constant amplitude, with minor deviations that may result from the limited ENoB (of 7 bits) of the preinverter ADC in the optical tap path. Figure 7 depicts an embodiment of a Wavelength-Division Multiplexing (WDM) optical transmission system comprising a transmitter 724 and a receiver 726. The WDM system shown in Figure 7 may comprise a WDM system with 5 channels, each channel modulated with 28Gbaud dual-polarization 1024-QAM, spaced at 29GHz. However, it will be appreciated that other system configurations are envisaged. In order to model OIR signal conversion as detailed above, timing jitter may be introduced to the system by taking the derivate of the received signal, multiplying it with the target timing jitter, and adding it back to the received signal. This may account for ENoB frequency dependency such as sampling jitter. For the specific embodiment shown in Figure 7, the following parameters may be used: ENoBDC = 4 bits (for example, to represent hardware bit architecture), and RMS jitter = 50 fs. The ADC as shown in Figure 7 may include a 10th order Gaussian filter with a bandwidth of half the symbol rate. Alternatively, the ADC of Figure 7 may include a multiple of this order in the case of super-channel detection. Similarly, each BPD 516 in the present embodiments may have a bandwidth limitation following a 10th order Gaussian filter with a bandwidth of half the symbol rate (or alternatively a multiple in the case of superchannel detection). Each PD 522 in the pre-inverter ADC path of the present embodiments may comprise an RC-filter with a bandwidth of 0.2-times the symbol rate; this may reduce fast power fluctuations introduced by the transmission optical fibre (between transmitter 724 and receiver 726 in Figure 7) channel which may significantly increase ENoB requirements at higher frequencies beyond the symbol rate. As shown in Figure 7, transmitter 724 may be an optical transmitter and may comprise 5 channels (Tx-1024QAM 28GBd WDM#01 to Tx-1024QAM 28GBd WDM#05) each of which generates a set of quadrature or QAM signals. Each QAM signal may be fed into a corresponding IQ modulator (IQ Mod &Pol-Mux) which may convert each signal from an electrical signal to an optical signal and then transmitted across a fibre such as a single mode fibre (SMF). The SMF may for example be up to 20km. In some embodiments, the SMF may include an amplifier such as an erbium-doped fibre amplifier (EDFA) to act as an optical amplifier or booster. The transmitted signal may then be received by receiver 726. Receiver 726 may be a coherent receiver and may comprise an optical field conditioning unit and electrical field re-construction unit. These optical field conditioning unit and electrical field re-construction units correspond to the signal converter 500 of previous embodiments, and may comprise any of the constituent elements of signal converter 500 as discussed above. Receiver 726 may additionally comprise digitisation processing or DSP blocks such as a digital back propagation (DBP) block and / or a chromatic dispersion compensation block (CD comp) which may reduce signal distortion and fibre nonlinearity introduced to the signal during transmission. The DBP block may provide improved performance in present embodiments, due to the higher fidelity provided by the OIR and / or OPS of the present embodiments. Additional DSP blocks that may be present in receiver 726 include a coarse time synchronization block (Coarse Time Synchr), a fine time synchronization and channel estimation block (Fine time synchr.& channel estimat.), and / or a channel matrix inversion and Frequency Domain Equalization (FDE) block. Receiver 716 may also comprise a Q-block to measure the receiver 726 Q-factor, in order to obtain and monitor a performance metric for the receiver. In embodiments such as that of Figure 7, WDM channel demultiplexing may be implemented using an ideal brick filter with a bandwidth equal to NCH times the channel spacing, where NCH is the number of WDM channels to be simultaneously detected. Embodiments as shown in Figure 5A and Figure 5B may also include further elements to address carrier frequency offset and laser phase noise such as those depicted in Figure 7. To recover signalled information post-transmission a DSP chain may be used as shown in Figure 7. In specific embodiments, for time synchronization and channel estimation, a preamble consisting of constant amplitude zero autocorrelation (CAZAC) sequences may be transmitted together with the payload data. For example, a transmission may comprise 216 symbols, with a 211 CAZAC preamble. Root raised cosine filters with a roll-off factor of 0.01 may be used for pulse shaping. The in-phase and quadrature components of each signal may drive an optical field of an ideal laser through an optical IQ modulator, and the optical signals may then be fed into a standard SMF such as an SMF-28 at 1550nm. An amplifier such as an EDFA may compensate for fibre attenuation; in specific embodiments, an EDFA with a 3dB noise figure may be used. After homodyne detection, baseband electrical signals may be sampled at 2 samples per symbol. Forward propagation simulation for such an embodiment may be implemented using a split-step Fourier method (SSFM) with a step bounded by the local error method below 10'5, after considering polarization mode dispersion of 0.01 ps / ^km. For the embodiment depicted in Figure 7, coherently received signals may be compensated for chromatic dispersion in the frequency domain (for example, for linear compensation). Furthermore, polarization mode dispersion (PMD) may be compensated for using training-symbol-based channel estimation and equalization as shown in Figure 7. Coarse time synchronization may be performed using the Schmidl and Cox autocorrelation metric. Subsequently, fine-time synchronization and channel impulse response (CIR) estimation may be performed by cross-correlating with the training CAZAC sequences. The 2x2 CIR estimations may then be converted into the frequency domain. The Multiple-Input Multiple-Output (MIMO) frequency domain equalizer may be calculated by inverting the channel matrix. The Q-factor for each received signal may be estimated through error counting and / or through the mean and standard deviation of the received symbols when fewer than 100 errors occur. For embodiments such as Figure 7, the total line-rate may be 2.8 Tb / s, and a net-data-rate may be 2.2 Tb / s given 20% Forward Error Correction (FEC) and 3% training sequences overheads. For embodiments where nonlinear digital compensation is implemented, DBP may be implemented by launching coherently received signals into a virtual fibre with characteristics of opposite-sign values to those in the transmission channel. For example, DBP may be implemented using SSFM with fixed step size. Figure 8 depicts estimated potential gains associated with the embodiments of Figure 5 and Figure 7, in particular the Q-factor of the embodiments as a function of ADC ENoB in a back-to-back configuration loaded with chromatic dispersion and spontaneous emission noise. For the estimations of Figure 8, all ADCs used in the simulations have the same ENoB, jitter, and bandwidth limitations including the ADC following the DD optical tap. Figure 8 presents results for a central channel in a super-channel configuration, where Optical Signal to Noise Ratio (OSNR) is set to 50 dB and dispersion is set to 17ps / nm. Figure 8A shows the Q-factor as a function of ENoB for conventional methods, methods with OIR as shown in Figure 5A, and methods with both OIR and OPS as shown in Figure 5B. Figure 8B shows the Q-factor as a function of sampling jitter for conventional methods, methods with OIR as shown in Figure 5A, and methods with both OIR and OPS as shown in Figure 5B. In Figure 8A, it can be seen that overall Q-factor improves with ENoB at a pace of 6dB per extra bit. It should be noted that that the SNR of an ideal N-bit ADC is 6.02N + 1.76, measuring in dB. However, as ENoB increases, the Q-factor of an ideal N-bit ADC saturates as the link OSNR limits the overall performance. OIR as implemented in present embodiments may achieve a Q-factor improvement of 6dB without OPS and 7.5dB with OPS (for an ENoB of approximately 4-5 bits). In Figure 8B, it can be seen that the Q-factor improvement achieved by the use of OIR (with or without OPS) is most significant for an RMS jitter larger than 100fs. In embodiments, an improvement of approximately 6dB equates to approximately 1 extra resolution bits. Figure 9 depicts further estimated potential gains associated with the embodiments of Figure 5 and Figure 7, in particular the Q-factor of the embodiments with transmission over 6 spans of 20km as a function of the launching power for ENoB = 4 bits. Figure 9 specifically considers short spans to operate in a regime limited by nonlinear distortion as it may that the required energy per bit is minimized or reduced for span lengths between 35km at 6 bits / s / Hz to 20km at 12 bits / s / Hz. For embodiments with a total WDM bandwidth of 145 GHz, the center WDM channel may experience Kerr nonlinear interactions with the neighbouring channels in optical fibres. Figure 9 demonstrates how the extra ENoB at the coherent receiver improves the performance of the DBP. Full-band DBP may be implemented following a SSFM method implementation with a 100m step. Forward propagation considered SSFM but with a step bounded by the local error method below 10’5, and no polarization mode dispersion. Full-band DBP is implemented after simultaneous demultiplexing and detecting a WDM super-channel (with a bandwidth limitation equal to N-channels multiplied by the channel spacing); for the specific embodiments simulated in Figure 9, this equates to 145GHz for 5-channels. Accordingly, Figure 9 focuses on performance gains delivered by the ENoB enhancement of present embodiments. Figure 9A shows the Q-factor for methods comprising OIR as a function of launch power both with and without OPS. Figure 9A additionally considers results both with and without DBP. Figure 9B shows the Q-factor for methods comprising OIR as a function of ENoB both with and without OPS. From Figure 9A, it can be seen that for non-DBP cases, a gain of approximately 4dB is achievable for OIR embodiments at optimum launch power. These Q-factor improvements, although significant, are smaller than those in Figure 8. This may be because the linear OSNR has been decreased given the noise figure of the optical link, and / or because of the nonlinear distortion which may set the optimum launch power at around -5dBm. However, when applying DBP in combination with OIR it can be seen from Figure 9 that a significant mitigation of the nonlinear noise translates into additional gains. At optimum launch power, a gain of 6.3dB may be observed for embodiments comprising OIR and OPS (such as that of Figure 5B). Figure 9B depicts Q-factor as a function of ENoB for an optimal launch power of 3dBm when considering DBP. It can be seen from Figure 9B that the performance gains are improved for ENoB between 3.5 and 4.5 bits, when compared to ENoB >5 bits. From Figure 8 and Figure 9, it can be seen that OIR leads to performance enhancements in comparison to conventional systems. It will be understood that OIR only requires one additional photodetector and ADC in comparison to conventional systems (which may be doubled for embodiments implementing OIR and OPS) which minimises the cost and installation difficulty associated with present embodiments. OPS may also be easily integrated into existing coherent receiver architecture using existing components. These characteristics make embodiments comprising OIR and OPS suitable for practical implementation with performance gains potentially compatible with the complexity of changing and adding to the standard optical coherent receiver front-end. Accordingly, investigations conducted in both the linear and nonlinear power regimes, assuming a realistic ADC / DAC model with bandwidth limitations including jitter, demonstrate the present embodiments to provide a notable performance increase. Similar results are obtained with investigations assessing performance when applying nonlinear digital compensation. Assuming single channel transmission over a linear channel, the Q-factor results showed improvements of as much as 7dB for embodiments implementing OIR, for ADCs with ENoB of 4 bits and an RMS jitter of 50fs. These gains approach and surpass those that would be obtained by adding one extra effective bit to the ADC array in a standard coherent receiver. For WDM transmission, gains of approximately 4dB were observed in embodiments comprising OIR and OPS. When performing full-band DBP, additional gains were observed for the scenarios investigated reaching 6.3dB for embodiments employing OIR. These results show that embodiments of the present invention may enhance effective vertical resolution, and may alleviate requirements in terms of ENoB of ADCs. In particular, embodiments comprising OIR and OPS may have potential for practical implementation requiring only one extra photoreceiver and ADC over conventional systems. Present embodiments improve the effective vertical resolution of coherent optical receivers through OIR. Present techniques exploit the benefits of linearity, broadband, and low noise of optical passive operations and optoelectronic mixers. The additional ENoB boost offered by present embodiments may be used to keep operating at the same bandwidth while improving SNR using the additional ENoB. Alternatively or additionally, the additional ENoB offered by present embodiments may be used to select an ADC set with a lower ENoB but larger electronic bandwidth. Increasing the bandwidth may provide an effective way of increasing the bit rate associated with the system and sharing the cost and power consumption of the system optoelectronic components across multiple bits. In specific applications, present embodiments may find application in optical line cards, in pluggable optical transceivers for optical transmission systems in point-to-point links, and in optical networks. References in the present disclosure to “one embodiment”, “an embodiment” and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It should be understood that, although the terms “first”, “second” and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. The terms “connect”, “connects”, “connecting” and / or “connected” used herein cover the direct and / or indirect connection between two elements. The present disclosure includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof. Various modifications and adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. For the avoidance of doubt, the scope of the disclosure is defined by the claims.

Claims

1. A method for converting an optical signal into an electrical signal comprising: receiving the optical signal;splitting the optical signal into a first signal and a second signal;measuring intensity information of the first signal;processing the first signal using an intensity modulator to generate an inverse intensity waveform;combining the inverse intensity waveform and the second signal to generate a constant intensity waveform;converting the constant intensity waveform into a digital constant intensity signal using an Analog-to-Digital Converter, ADC;combining the intensity information and the digital constant intensity signal; and generating the electrical signal using a Digital Signal Processor, DSP.

2. A method as claimed in Claim 1, wherein the method further comprises: converting the first signal into a digital first signal using a first ADC; processing the digital first signal using an intensity modulator to generate an analog inverse intensity waveform;combining the analog inverse intensity waveform and the second signal to generate the constant intensity signal.

3. A method as claimed in any of Claims 1 and 2, wherein the intensity modulator applies a reciprocal function to the first signal.

4. A method as claimed in any of Claims 1 to 3, wherein the method comprises splitting the received optical signal into the first signal and the second signal using a 10:90 ratio.

5. A method as claimed in any of Claims 1 to 4, wherein the method further comprises: splitting the received optical signal into separate polarized signals using a polarization beam splitter, PBS;splitting each polarized signal into a first polarized signal and a second polarized signal;measuring the intensity information of each first polarized signal;processing each first polarized signal using the intensity modulation to generate a corresponding first polarized inverse intensity waveform;combining each first polarized inverse intensity waveform and each corresponding second polarized signal to generate corresponding polarized constant intensity waveforms;converting each polarized constant intensity waveform into a digital polarized constant intensity signal using the ADC; andcombining each intensity information and each digital polarized constant intensity signal; andgenerating the electrical signal using a Digital Signal Processor, DSP.

6. A signal converter for converting an optical signal into an electrical signal comprising an intensity modulator, an Analog-to-Digital Converter, ADC, and a Digital Signal Processor, DSP, wherein the converter is configured to:receive the optical signal;split the optical signal into a first signal and a second signal;measure the intensity information of the first signal;process the first signal using the intensity modulator to generate an inverse intensity waveform;combine the inverse intensity waveform and the second signal using the intensity modulator to generate a constant intensity waveform;convert the constant intensity waveform into a digital constant intensity waveform using the ADC;combine the intensity information and the digital constant intensity signal; and generate the electrical signal using the DSP.

7. A signal converter as claimed in Claim 6, wherein the converter comprises a first ADC and a second ADC, and wherein the converter is further configured to:convert the first signal into a digital first signal using the first ADC;process the digital first signal using the intensity modulator to generate an analog inverse intensity waveform;combine the analog inverse intensity waveform and the second signal to generate the analog constant intensity signal.

8. A signal converter as claimed in any of Claims 6 and 7, wherein the intensity modulator is configured to apply a reciprocal function to the first signal.

9. A signal converter as claimed in any of Claims 6 to 8, wherein the signal converter is configured to split the received optical signal into the first signal and the second signal using a 10:90 ratio.

10. A signal converter as claimed in any of Claims 6 to 9, wherein the signal converter further comprises a polarization beam splitter, PBS, and wherein the signal converter is further configured to:split the received optical signal into separate polarized signals using the PBS;split each polarized signal into a first polarized signal and a second polarized signal;measure the intensity information of each first polarized signal;process each first polarized signal using the intensity modulator to generate a corresponding first polarized inverse intensity waveform;combine each first polarized inverse intensity waveform and each corresponding second polarized signal to generate corresponding polarized constant intensity waveforms;convert each polarized constant intensity waveform into a digital polarized constant intensity signal using the ADC; andcombine each intensity information and each digital polarized constant intensity signal; andgenerate the electrical signal using the DSP.

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