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Experimental Investigation of Laser Linewidth Tolerance of 32-GBaud DP-256QAM Optical Coherent System
... Several studies have been done on phase noise tolerance of US-QAM (< 64QAM) [12,13]. Previously, we reported on experiments on the phase noise robustness of US-256QAM signals and revealed that laser linewidth (0.1~550 kHz) only slightly affected the signal quality [14]. A 48-GBd US-256QAM transmission was also demonstrated using a ~100-kHz laser [15]. ...
We numerically and experimentally investigate the laser phase noise tolerance of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM, PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information rate (IR). We confirm that a sufficient shaping gain is observed with narrow linewidth lasers, whereas degradation of the shaping gain is clearly observed when large phase noise and high order modulation formats are assumed. In our experiments, we compare polarization-division-multiplexed (PDM) 16-GBd PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted digital phase locked loop. Results reveal that PS-1024QAM achieves high performance with the 0.1 kHz-laser or > 5% pilot ratio, whereas US-256QAM outperforms PS-1024QAM when lasers with 40-kHz linewidth and < 5% pilot ratio are used. We also evaluate the pilot ratio dependency of the required optical signal-to-noise ratio at the forward error correction limit and the achievable information rate. Additionally, we compare the performance of two types of CPR updating schemes: updating phase estimation at only the pilot symbol or at all symbols.
... Several studies have been done on phase noise tolerance of US-QAM (< 64QAM) [12,13]. Previously, we reported on experiments on the phase noise robustness of US-256QAM signals and revealed that laser linewidth (0.1~550 kHz) only slightly affected the signal quality [14]. A 48-GBd US-256QAM transmission was also demonstrated using a ~100-kHz laser [15]. ...
We numerically and experimentally investigate the laser phase noise tolerance of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM, PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information rate (IR). We confirm that a sufficient shaping gain is observed with narrow linewidth lasers, whereas degradation of the shaping gain is clearly observed when large phase noise and high order modulation formats are assumed. In our experiments, we compare polarization-division-multiplexed (PDM) 16-GBd PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted digital phase locked loop. Results reveal that PS-1024QAM achieves high performance with the 0.1 kHz-laser or > 5% pilot ratio, whereas US-256QAM outperforms PS-1024QAM when lasers with 40-kHz linewidth and < 5% pilot ratio are used. We also evaluate the pilot ratio dependency of the required optical signal-to-noise ratio at the forward error correction limit and the achievable information rate. Additionally, we compare the performance of two types of CPR updating schemes: updating phase estimation at only the pilot symbol or at all symbols.
Ultra-wideband (UWB) wavelength division multiplexed (WDM) transmission using high-order modulation formats is one of the key techniques to expand the transmission capacity per optical fiber. For UWB systems, the nonlinear interaction caused by inter-band stimulated Raman scattering (SRS) must be considered. Therefore, we have proposed and demonstrated a scheme to optimize the fiber input powers for UWB transmission systems considering the signal power transition caused by the inter-band SRS. We demonstrated a single-mode capacity of 150.3 Tb/s using the proposed power optimization scheme with 13.6-THz UWB in the
S
-,
C
-, and
L
-bands over 40-km transmission. Spectral efficiency of 11.05 b/s/Hz was achieved with 272-channel 50-GHz spaced WDM signals of 45-GBaud polarization division multiplexed 128 quadrature amplitude modulation.
High-capacity carrier signals with high-order modulation are being considered for data center interconnection (DCI) applications as they can reduce the cost per bit by decreasing the number of devices and increasing fiber capacity. However, signal quality degradation due to device imperfections affecting the frequency response has limited the capacity per carrier of PDM-256QAM signals to under 400 Gbps. Here, it is possible to use a calibration method for dealing with device imperfections wherein a fixed equalizer separately calibrates the frequency response of the transmitter and receiver. Our previous study showed that PDM-256QAM signals at up to 48 GBaud could be transmitted 100 km, while it used an experimental setup with total 3-dB analog bandwidth of 13 GHz and 20 GHz for transmitter and receiver, respectively. A micro-integrable tunable laser assembly (μITLA) with a linewidth with a typical linewidth of 20 kHz, maximum under 100 kHz. Actual data center interconnection (DCI) applications, however, are expected to use dense wavelength division multiplexing (WDM), so in this study, we examined transmission of 10-WDM 48-GBaud PDM-256QAM signals in a 50-GHz grid and evaluated the WDM penalties. The resulting spectral efficiency (SE) of 600-Gbps/carrier WDM transmissions was as high as 12.02 bit/s/Hz.
We demonstrate 20 Gsymbol/s, 256 QAM polarization multiplexed (pol-mux) 320 Gbit/s coherent transmission. By employing an LD-based injection locking circuit, we achieved low noise optical carrier-phase locking between the LO and the data signal. Furthermore, frequency domain equalization and digital back-propagation enabled us to realize precise compensation for transmitted waveform distortions. As a result, a 320 Gbit/s data was successfully transmitted over 160 km with a potential spectral efficiency of 10.9 bit/s/Hz. This is the highest symbol rate yet achieved in a pol-mux 256 QAM coherent transmission. In addition, we also describe a pol-mux 256 QAM transmission at a symbol rate of 10 Gsymbol/s.
We present a statistical inference based multi-pilot aided CPE algorithm and analyze its performance via simulations. We experimentally verify LDPC coded back-to-back performance using 10 GBd DP-64QAM and DP-256QAM modulation, with transmitter and receiver linewidths of 100 kHz.
The probability density function and impact of equalization-enhanced phase noise (EEPN) is analytically investigated and simulated for 100 Gb/s coherent systems using electronic dispersion compensation. EEPN impairment induces both phase noise and amplitude noise with the former twice as much as the latter. The effects of transmitter phase noise on EEPN are negligible for links with residual dispersion in excess of 700 ps/nm. Optimal linear equalizer in the presence of EEPN is derived but show only marginal performance improvement, indicating that EEPN is difficult to mitigate using simple DSP techniques. In addition, the effects of EEPN on carrier recovery techniques and corresponding cycle slip probabilities are studied.
We develop a systematic method for characterizing semiconductor-laser phase noise, using a low-speed offline digital coherent receiver. The field spectrum, the FM-noise spectrum, and the phase-error variance measured with such a receiver can completely describe phase-noise characteristics of lasers under test. The sampling rate of the digital coherent receiver should be much higher than the phase-fluctuation speed. However, 1 GS/s is large enough for most of the single-mode semiconductor lasers. In addition to such phase-noise characterization, interpolating the taken data at 1.25 GS/s to form a data stream at 10 GS/s, we can predict the bit-error rate (BER) performance of multi-level modulated optical signals at 10 Gsymbol/s. The BER degradation due to the phase noise is well explained by the result of the phase-noise measurements.
In coherent optical systems employing electronic digital signal processing, the fiber chromatic dispersion can be gracefully compensated in electronic domain without resorting to optical techniques. Unlike optical dispersion compensator, the electronic equalizer enhances the impairments from the laser phase noise. This equalization-enhanced phase noise (EEPN) imposes a tighter constraint on the receive laser phase noise for transmission systems with high symbol rate and large electronically-compensated chromatic dispersion.
Record symbol-rate PDM-256QAM single-carrier transmission of 100km is achieved using commercially available laser and modulator with only applying compensation for linear frequency response of transceiver and receiver; device nonlinearity compensation is not used.
Signal-carrier PM-256QAM generation at record-breaking line rate of 256 Gb/s and its point-to-point transmission over 20-km SMF are experimentally demonstrated, which is enabled by using a fast-converging polarization-tracking equalizer with coordinated CMA and DD-LMS algorithm.
A thorough analysis of equalization-enhanced phase noise (EEPN) and its impact on the coherent optical system is presented. We show with a time-domain analysis that EEPN is caused due to the interference of multiple delayed versions of the dispersed signal, generated by intermixing of the received dispersed signal, and the noise side bands of the local oscillator (LO) in the photodetectors. We derive statistical properties such as the mean, variance, and error vector magnitude of the received signal influenced with EEPN. We show that in coherent optical systems utilizing electronic dispersion compensation, this noise corresponds to multipath fading in wireless communication systems. Closed-form expressions of necessary LO linewidth and/or mitigation bandwidth for a general system configuration and specified OSNR penalty are given. The expressions for system design parameters, validated with system simulations, show that higher order modulation formats, such as 16-quadrature amplitude modulation and beyond, put stringent demands on the LO linewidth unless a mitigation technique is used.
We successfully applied layered decoding algorithm in decoding LDPC convolutional codes designed for applications in high speed optical transmission systems. A relatively short code was FPGA-emulated with a Q factor of 5.7dB at BER of 10-15.