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(a) CMRR as a function of frequency for various P/N skew levels and P/N power imbalance values. The normalized one-sided 32-Gbaud PM-QPSK optical spectrum is also overlaid. (b) Effective CMRR value based on different combinations of P/N skew values and P/N power imbalance values. The effective CMRR is extracted from (a) at around 8 to 10 GHz frequency content.
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In addition to linear compensation of fiber channel impairments, coherent receivers also provide colorless selection of any desired data channel within multitude of incident wavelengths, without the need of a channel selecting filter. In this paper, we investigate the design requirements for colorless reception using a coherent balanced receiver, c...
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Citations
... where N tot is the total input-referred noise density at the input of the TIA. and S 0 is the signal power occurring by the beat between signal power and local oscillator at the input of the TIA. S 0 can be written as [4,5]: ...
Digital coherent transmission features a very large transmission bandwidth and has played a main role in core optical transmission networks. With the progress of semiconductor technologies, practical coherent transceivers with rates over 100 Gbaud are becoming feasible. With such advances, the transceiver components must have lower power consumption and lower costs, and it becomes important to know how each component contributes to the overall transmission performance. Here, to decompose the effects of noise factors in high-baud-rate DP-16QAM transmissions, we used the theoretical relationship between the bit error rate (BER) and noise-to-signal ratio (NSR) and performed linear analyses. The NSR could be decomposed into individual noise contributions according to dependences on the inverse signal and local photocurrents. The obtained parameters were shown to be useful for predicting required optical signal-to-noise ratio (ROSNR) characteristics.
... The optical signal was mixed with the LO in an integrated coherent receiver (ICR) and converted to an analog signal. The subcarriers can be selected by LO wavelength tuning without using optical filters [43]. Then the analog signal was digitized by a real-time oscilloscope (RTO, Keysight UXR0594AP) with a 256 GSa/s sampling rate. ...
p>It is foreseeable that the 100 Gb/s/λ and beyond passive optical network (PON) will be required in future optical access networks to meet the explosive growth of data traffic. The coherent optical systems could be a promising solution for the future beyond 100G PON. Coherent PON using digital subcarrier multiplexing (DSCM) can provide flexible bandwidth allocation to a large number of access subscribers by dividing subcarriers of the DSCM signal into time slots for time-and-frequency division multiple access. When the optical network unit is allocated a new subcarrier, digital signal processing (DSP) should converge fast in the allocated time slot to ensure a low handoff latency for real-time bandwidth allocation. However, the traditional coherent DSP is hard to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific training sequence (TS) structure and propose data-aided DSP to achieve fast convergence for coherent PON. The feasibility of the proposed scheme is experimentally verified in an 8 Gbaud/SC×8 SCs 400 Gb/s-net-rate coherent PON using DSCM with 16 quadrature amplitude modulation. The experimental results show that fast convergence is jointly realized by the proposed TS structure and data-aided DSP using a 416-symbol TS with a 52 ns duration. The receiver sensitivity at the 20% soft-decision forward error correction limit is approximately −27 dBm and an optical power budget of about 35.5 dB is achieved with a booster amplifier.</p
... The optical signal was mixed with the LO in an integrated coherent receiver (ICR) for coherent detection. The channel selection can be achieved by tuning the wavelength of the LO without using the optical filter [39]. After the coherent detection, the optical signal was converted to an analog signal and digitized by a real-time oscilloscope (RTO, Keysight UXR0594AP) with 256GSa/s sampling rate. ...
p>Optical access networks have been evolving to meet the explosive growth of data traffic. It is foreseeable that the 100Gb/s/λ and beyond passive optical network (PON) will be required in future optical access networks. Coherent optical communication is a promising solution for the future beyond 100G PON. However, the traditional digital signal processing (DSP) for coherent optical communication is difficult to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific preamble structure and propose a burst-mode DSP to achieve fast convergence for the coherent PON. For verifying the feasibility of the proposed scheme, point-to-multi-point (P2MP) coherent PON is experimentally built up based on digital subcarrier multiplexing (DSCM), which is a kind of frequency division multiple access. When the allocated frequency of the optical network unit is suddenly changed, the DSP should converge fast to ensure a low handoff latency. In P2MP coherent PON, the proposed specific preamble structure and burst-mode DSP jointly implement the fast convergence using a short preamble with only 416 symbols. The experimental results show that the 8-Gbaud/SC×8-SCs 400Gb/s-net-rate coherent PON in burst-mode detection achieves the receiver sensitivity of approximately −27dBm at the 20% soft-decision forward error correction limit and approximately 35.5dB optical power budget with an optical pre-amplifier.</p
... The optical signal was mixed with the LO in an integrated coherent receiver (ICR) for coherent detection. The channel selection can be achieved by tuning the wavelength of the LO without using the optical filter [39]. After the coherent detection, the optical signal was converted to an analog signal and digitized by a real-time oscilloscope (RTO, Keysight UXR0594AP) with 256GSa/s sampling rate. ...
p>Optical access networks have been evolving to meet the explosive growth of data traffic. It is foreseeable that the 100Gb/s/λ and beyond passive optical network (PON) will be required in future optical access networks. Coherent optical communication is a promising solution for the future beyond 100G PON. However, the traditional digital signal processing (DSP) for coherent optical communication is difficult to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific preamble structure and propose a burst-mode DSP to achieve fast convergence for the coherent PON. For verifying the feasibility of the proposed scheme, point-to-multi-point (P2MP) coherent PON is experimentally built up based on digital subcarrier multiplexing (DSCM), which is a kind of frequency division multiple access. When the allocated frequency of the optical network unit is suddenly changed, the DSP should converge fast to ensure a low handoff latency. In P2MP coherent PON, the proposed specific preamble structure and burst-mode DSP jointly implement the fast convergence using a short preamble with only 416 symbols. The experimental results show that the 8-Gbaud/SC×8-SCs 400Gb/s-net-rate coherent PON in burst-mode detection achieves the receiver sensitivity of approximately −27dBm at the 20% soft-decision forward error correction limit and approximately 35.5dB optical power budget with an optical pre-amplifier.</p
... As for the homodyne detection scheme, only one quadrature component X or P needs to be measured, so basis choice is an indispensable step, and imperfect basis choice will cause security problems [27]. In the heterodyne detection scheme, both quadrature components X and P need to be measured, which is similar to phase diversity reception in classical optical communication [28,29]. In the CV-QKD scheme using heterodyne detection, we usually choose the optical phase shifter to help measure the P quadrature. ...
The optical phase shifter that constantly rotates the local oscillator phase is a necessity in continuous-variable quantum key distribution systems with heterodyne detection. In previous experimental implementations, the optical phase shifter is generally regarded as an ideal passive optical device that perfectly rotates the phase of the electromagnetic wave of $90^\circ$ 90 ∘ . However, the optical phase shifter in practice introduces imperfections, mainly the measurement angular error, which inevitably deteriorates the security of the practical systems. Here, we will give a concrete interpretation of measurement angular error in practical systems and the corresponding entanglement-based description. Subsequently, from the parameter estimation, we deduce the overestimated excess noise and the underestimated transmittance, which lead to a reduction in the final secret key rate. Simultaneously, we propose an estimation method of the measurement angular error. Next, the practical security analysis is provided in detail, and the effect of the measurement angular error and its corresponding compensation scheme are demonstrated. We conclude that measurement angular error severely degrades the security, but the proposed calibration and compensation method can significantly help improve the performance of the practical CV-QKD systems.
... For future 1.6 Tb/s modules, it is possible that a symbol rate beyond 200 Gbaud will be implemented. Commercial coherent transceivers utilize balanced photodetectors (BPDs) with high single-port rejection ratio (SPRR) to mitigate the signal-signal beat interference (SSBI) due to the square-law detection process [8,9]. The SPRR depends on the balance (in both amplitude and skew) of the optical hybrid, RF circuity and also the similarity of the two single-ended photodetectors (PDs) of a BPD in responsivities, polarization dependencies and frequency responses [10]. ...
Commercial coherent receivers utilize balanced photodetectors (PDs) with high single-port rejection ratio (SPRR) to mitigate the signal-signal beat interference (SSBI) due to the square-law detection process. As the symbol rates of coherent transponders are increased to 100 Gbaud and beyond, maintaining a high SPRR in a cost-effective manner becomes more and more challenging. One potential approach for solving this problem is to leverage the concept of single-ended coherent receiver (SER) where single-ended PDs are used instead of the balanced PDs. In this case, the resulting SSBI should be mitigated in the digital domain. In this paper, we show that SSBI can be effectively mitigated using various low-complexity techniques, such as the direct filed reconstruction (DFR), clipped iterative SSBI cancellation (CIC) and gradient decent (GD). In addition, we present a self-calibration technique for SERs which can be extended for characterizing the optical-to-electrical (O/E) response of a conventional balanced coherent receiver (BR). Using the developed techniques, we then experimentally demonstrate a 90 Gbaud probabilistically constellation shaped 64-QAM (PCS-64QAM) transmission using a SER, achieving a net data rate of 882 Gb/s over 100 km of standard single mode fiber (SSMF). The sensitivity penalty compared to the BR is below 0.5 dB. We expect that when the symbol rate is increased further, a SER can potentially outperform a BR, especially when applied to cost-sensitive commercial pluggable coherent transceivers
... As for the homodyne detection scheme, only one quadrature component X or P needs to be measured, so basis choice is an indispensable step, and imperfect basis choice will cause security problems [18]. In the heterodyne detection scheme, both quadrature components X and P need to be measured, which is similar to phase diversity reception in classical optical communication [21,29]. In the CV-QKD scheme using heterodyne detection, we usually choose the optical phase shifter to help measure the P quadrature. ...
The optical phase shifter that constantly rotates the local oscillator phase is a necessity in continuous-variable quantum key distribution systems with heterodyne detection. In previous experimental implementations, the optical phase shifter is generally regarded as an ideal passive optical device that perfectly rotates the phase of the electromagnetic wave of $90^\circ$. However, the optical phase shifter in practice introduces imperfections, mainly the measurement angular error, which inevitably deteriorates the security of the practical systems. Here, we will give a concrete interpretation of measurement angular error in practical systems and the corresponding entanglement-based description. Subsequently, from the parameter estimation, we deduce the overestimated excess noise and the underestimated transmittance, which lead to a reduction in the final secret key rate. Simultaneously, we propose an estimation method of the measurement angular error. Next, the practical security analysis is provided in detail, and the effect of the measurement angular error and its corresponding compensation scheme are demonstrated. We conclude that measurement angular error severely degrades the security, but the proposed calibration and compensation method can significantly help improve the performance of the practical CV-QKD systems.
... Under this condition there is no spectral component of the classical signal falling within the reception band of the receiver, which is defined by the LO. However, in case of imperfect detector balancing, common mode noise will arise due to direct detection terms [51]. We have therefore investigated the dependence of crosstalk as a function of the balancing quality, which can be expressed through the CMRR [52]. ...
A coherent transmission methodology for a continuous-variable quantum key distribution (CV-QKD) system based on quantum-heterodyne measurement through a coherent intradyne receiver is experimentally demonstrated in the framework of 5G mobile fronthaul links. Continuous optical carrier synchronization is obtained through training information, which is multiplexing to the quantum signal as pilot tone in both, frequency and polarization. Spectral tailoring by means of optical carrier suppression and single-sideband modulation is adopted to simultaneously mitigate crosstalk into the quantum channel and self-interference for the pilot tone, thus allowing for a high signal-to-noise ratio for this training signal. Frequency offset correction and optical phase estimation for the free-running local oscillator of the receiver is accurately performed and guarantees low-noise quantum signal reception at high symbol rates of 250 MHz and 500 MHz with additional Nyquist pulse shaping. A low excess noise in the order of 0.1% to 0.5% of shot-noise units is obtained for fiber-based transmission over a fronthaul link reach of 13.2 km. Moreover, co-existence with 11 carrier-grade classical signals is experimentally investigated. Joint signal transmission in the C-band of both, quantum signal and classical signals, is successfully demonstrated. Secure-key rates of 18 and 10 Mb/s are obtained under strict security assumptions, where Eve has control of the receiver noise, for a dark and a lit fiber link, respectively. Moreover, rates of 85 and 72 Mb/s are resulting for a trusted receiver scenario. These secure-key rates are well addressing the requirements for time-shared CV-QKD system in densified 5G radio access networks with cloud-based processing.
... A filterless receiver can be based on coherent detection by tuning LO wavelength to the desired signal. Colorless coherent detection can be instrumental in improving receiver sensitivity thanks to the removal of the filter loss and lower costs/size [30,31], however, the system performance can be degraded by receiver saturation at WDM reception. For current colorless/directionless/contentionless (CDC) ROADMs using coherent transmission, TFs such as MEMS-based ones are commonly used for maximizing repeater spacing (transparent transmission distance) and the number of WDM channels. ...
... This originates from power imbalance and skew mismatch between the two inputs in each of the four BPDs. If they are assumed to be Gaussian random processes with zero mean, the signal quality can be analyzed according to a known receiver design guideline [30]. The signal-to-noise ratio (SNR) at the output of the colorless receiver is expressed using the signal photocurrent ( ) as ...
... To prove the capability of colorless detection, we examined the receiver performance through the analytical formulas derived in Section III-A. Table II summarizes the parameters used in the analytical calculation; they are based on published reports [30,31]. To simulate the colorless degradation, we assumed CMRR = − 19 dB and = 0.1 as the front-end specifications. ...
We investigate a large-scale and fast optical circuit switch system that uses digital coherent technologies. The achievable port count is expanded by introducing colorless detection which eliminates channel selecting filters at receivers. Wavelength selection is performed using a shared local oscillator (LO) bank, which is configured by combining a multi-wavelength optical source with Silicon-photonic tunable filters (TFs). Microsecond switching times are realized with the thermo-optic effect available with the Silicon-photonic TF. Design optimization of the proposed system handles complex level of freedom, i.e., available wavelength number, space switch port count, optical amplifier location and gain, and device loss. In addition, colorless coherent detection induces penalties, which further complicates switch scale assessment. In this paper, we develop a simulator to clarify how those parameters affect switch performance; the switch throughput is maximized with consideration of the degradation stemming from colorless detection. The design performance accurately matches results measured in various system scenarios. A dual-carrier 256-Gb/s DP-QPSK experiment successfully demonstrates a 1,856 × 1,856 optical switch system with switching time under 3.52 μs. The resulting total throughput is 441 Tb/s assuming 7%-overhead HD-FEC. To the best of our knowledge, this is the first demonstration of 400-Tbps class large bandwidth optical switches that use TFs as wavelength selective devices for coherent detection.
... The expansion of video streaming services and mobile terminals has continuously promoted the significant increase of data traffic, and it has accelerated the development of highspeed and large-capacity transmission. To cope with this demand, the digital coherent transmission technology using spectrally-efficient modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) has been widely adopted for not only longhaul networks but also metropolitan area networks [1]- [4]. High port density which drives the miniaturization of optical components and low power dissipation is indispensable for cost-sensitive metro and data center interconnect applications, and the downsizing of a coherent receiver is inevitable for pluggable transceivers with a smaller footprint and lower power dissipation, like a 100 Gb/s form-factor pluggable (CFP/CFP2) transceiver. ...
We present InP-based photodetectors monolithically integrated with a 90° hybrid toward over 400Gb/s coherent transmission systems. To attain a wide 3-dB bandwidth of more than 40GHz for 400Gb/s dual-polarization (DP)-16-ary quadrature amplitude modulation (16QAM) and 600Gb/s DP-64QAM through 64GBaud operation, A p-i-n photodiode structure consisting of a GaInAs thin absorption and low doping n-typed InP buffer layers was introduced to overcome the trade-off between short carrier transit time and low parasitic capacitance. Additionally, this InP buffer layer contributes to the reduction of propagation loss in the 90° hybrid waveguide, that is, this approach allows a high responsivity as well as wide 3-dB bandwidth operation. The coherent receiver module for the C-band (1530nm - 1570nm) operation indicated the wide 3-dB bandwidth of more than 40GHz and the high receiver responsivity of more than 0.070A/W (Chip responsivity within the C-band: 0.130A/W) thanks to photodetectors with this photodiode design. To expand the usable wavelengths in wavelength-division multiplexing toward large-capacity optical transmission, the photodetector integrated with the 90° hybrid optimized for the L-band (1565nm - 1612nm) operation was also fabricated, and exhibited the high responsivity of more than 0.120A/W over the L-band. Finally, the InP-based monolithically integrated photonic device consisting of eight-channel p-i-n photodiodes, two 90° hybrids and a beam splitter was realized for the miniaturization of modules and afforded the reduction of the total footprint by 70% in a module compared to photodetectors with the 90° hybrid and four-channel p-i-n photodiodes.
