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Optical wireless communication (OWC) systems have been widely studied to provide high-speed connections in indoor environments. In this paper, we propose a filter-enhanced indoor OWC system with hybrid spatial modulation (SM) and spatial multiplexing (SMUX) to further increase the data rate. The filter-enhanced SM enables extra information be transmitted via the spatial indices of OWC transmitters, and the SMUX further increases the data rate via the parallel transmission of multiple signals even under highly-similar channels. The proposed scheme is experimentally demonstrated. Up to 35 Gb/s data rate can be achieved with 2.5 GBaud/s symbol rate and four transmitters. Compared with the filter-enhanced SM scheme, the achievable data rate can be increased by 100%. Compared with the conventional SM and SMUX, the bit-error-rate (BER) is improved by over two orders-of-magnitude.

In the next-generation flexible point-to-multipoint optical networks such as optical access systems, it is desirable for one transceiver to handle various Internet-of-Things traffic streams. A symbol rate-adaptive multiband-coherent-transceiver, that can transmit and receive wavelength division multiplexing (WDM) signals arranged at the Nyquist spacing after Nyquist spectrum shaping, achieves flexibility by changing the number of subbands (SB) and the symbol rate per SB. Band-asymmetric coherent access systems with wide bandwidth optical transceivers on the optical-line-terminal side and narrowband optical transceivers on the optical-networking-unit side have been reported for uplink and downlink transmissions. We also examined the multiband-coherent-transceiver for ultra-dense WDM signals with a narrow analog-device bandwidth and narrow SB spacing to improve cost and spectral efficiency. To the best knowledge of the authors, there is no other research on a Nyquist spectrally shaped WDM-passive optical network system that applies these two functions and flexibly changes the symbol rate per SB with a SB spacing less than the Nyquist spacing.

Conventional modulation formats for single carrier transmission do not provide the flexibility to gradually scale the data rate and spectral efficiency. By using multidimensional pulse-amplitude modulation, fractional numbers of bits per symbol can be achieved, which extends the flexibility and reach of intensity modulation and direct detection systems but keeps the signal processing simple. By using pseudo-Gray coding, we are able to achieve optimum mappings for various non-integer bits-to-symbol assignments. We show a transmission experiment with data rates ranging from 56 to 168 Gb/s in fine granularity over different link lengths at a symbol rate of 56 GBd.

We demonstrate single channel 448 Gb/s (56 GBaud) and wavelength division multiplexed (WDM) 1.792-Tb/s transmission of Nyquist polarization division multiplexed 16-ary quadrature amplitude modulation (16-QAM) signal by employing heterodyne detection with single-ended photodiodes. The signal-signal beat interference induced by square-law detection of the single-ended photodiodes is overcome by the Kramers-Kronig scheme. For both single channel 160-km standard single mode fiber (SSMF) transmission and WDM 80-km SSMF transmission, the bit-error rates are below the 7% hard-decision forward error correction threshold of $4.5\times 10^{-3}$ . To the best of our knowledge, this letter is the first experiment of single carrier 400G optical transport with heterodyne detection. For WDM operation of high symbol rate heterodyne system, a record net spectral efficiency of 5.71 b/s/Hz is achieved.

We propose and experimentally validate a novel synchronous symbol rate switching method for digitally shaped optical signals, based on synchronized transmit- and receive-side interpolators. We show that a gradually performed rate adjustment in small discrete steps allows the dynamic equalizer at the receiver to successfully track signal changes. The proposed technique may be exploited, e.g., to reduce power consumption during night-time. Furthermore, the released spectrum may be used to accommodate other short-living opportunistic demands, such as in data center traffic.

We report the repetition frequency constraint of a passively mode-locked fiber laser (PMFL) for linear optical sampling based M-ary phase-shift keying (PSK) constellation characterization, based on the commonly used Viterbi–Viterbi carrier phase estimation algorithm. Referred to the error vector magnitude (EVM) value by the conventional high-speed electronic coherent detection, we put forward a benchmark of EVM $_{\rm {RMS\_{}error}}$ to evaluate the accuracy of constellation characterization by linear optical sampling. Due to the repetition frequency constraint of the PMFL, the EVM $_{\rm {RMS\_{}error}}$ will reach an ultimate value of 0.25% for the purpose of PSK constellation characterization, while it is insensitive to the signal symbol rate. The analytical results of a typical 28-GBaud QPSK signal well agree with those of the simulation and the experiment, therefore providing guidance for the repetition frequency optimization of PMFL-based linear optical sampling.

We demonstrate DAC-less generation and transmission of 256-quadrature amplitude modulation signals at a symbol rate of 32 GBd using an indium phosphide segmented Mach-Zehnder modulator with 15 active sections and dedicated BiCMOS driver arrays. The linear quantization characteristic of the segmented modulator with 4-bit resolution allows the generation of spectrally efficient modulation formats without any transmitter side signal processing. The chip power consumption of 1.64 W translates into the record-low energy per bit of 6.4 pJ/b. Back-to-back performance is evaluated and two different low-density parity check codes enable error-free transmission over 80 and 120 km with a net data rate of 320 and 240 Gb/s, respectively.

The performance of Nyquist wavelength division multiplexing (WDM) superchannel, using 16-ary quadrature amplitude modulation (16QAM), is severely degraded due to the existence of inter-channel interference and full-response equalization-enhanced in-band noise. Here, we propose a joint digital signal processing (DSP) technique incorporating digital spectral shaping and maximum likelihood sequence detection for 16QAM Nyquist WDM superchannel with a symbol rate of 28 GBd. Compared with traditional joint DSP, we can obtain 1.8-dB required-OSNR improvement at a bit error rate of $10^{\mathrm {\mathbf {-3}}}$ for back-to-back measurement. In addition, 0.9-dB peak $Q$ -factor improvement is also secured after 960-km standard single mode fiber transmission. Furthermore, using the proposed joint DSP technique, linewidth tolerance is relaxed from 80 to 230 kHz given 1-dB required-OSNR penalty. Meanwhile, reduction of block-size for carrier phase recovery is also secured, taking 100-kHz linewidth into account.

In this letter, a 10.4-Gb/s real-time baseband orthogonal frequency division multiplexing (OFDM) receiver utilizing 256/64/16-quadrature amplitude modulation (QAM) adaptive modulation format, incorporating several key function- alities, including symbol synchronization, channel equalization, as well as sampling clock frequency synchronization is suc- cessfully implemented using a commercial field programmable gate array chip and a sample rate of 5-GS/s analog-to-digital converter. The performance of the real-time baseband OFDM receiver is experimentally investigated in an intensity-modulation and direct-detection optical communication system. The off- line generated 10.4-Gb/s adaptively modulated 256/64/16QAM encoded optical OFDM signal with a high spectral efficiency of 4.84 bit/s/Hz can be successful transmitted over 20-km standard single mode fiber with a bit error rate

We present a 107-GBd coherent optical system for the generation and reception of m-ary phase-shift keyed (PSK) and quadrature-amplitude modulated (QAM) signals, including Nyquist pulse shaping. It is based on phase-stable optical time-division multiplex (OTDM) of two tributaries, which are modulated at half the target symbol rate, optical Nyquist filtering, and a broadband digital coherent receiver. The system is analyzed in back-to-back experiments with binary PSK (BPSK) modulation, quadrature-PSK (QPSK) modulation, as well as with 16-ary QAM. Compared to all-ETDM implementations, the transmitter is based on independent electro-optical modulation of two time-division multiplex tributaries and subsequent optical interleaving after modulation. Although this scheme implies increased hardware complexity, it enables lower penalties with respect to theory compared to all-ETDM experiments presented so far. This is due to the reduced hardware speed requirements. The measured p enalties at bit-error ratios of 1× 10-3 are 0.5, 1.5, and 6 dB for single-polarization BPSK, QPSK, and 16QAM, respectively. Furthermore, the employed phase-stable optical multiplexer allows use of standard receiver DSP at the broadband digital coherent receiver without need for special treatment of the individual OTDM tributaries.

Multidimensional constellations and time domain hybrid QAM both realize a non-integer ratio of bits per complex 2D symbol in order to allow for flexibility of spectral efficiency. We compare both approaches in terms of symbol error probability, asymptotic power efficiency, and bit error probability. In order to achieve 400G transmission, as an example for a non-integer ratio, an average number of 3.33 bits per complex 2D symbol was chosen, resulting in a symbol rate of 67.2 GBd including 12% FEC overhead. We show a 0.34-dB gain of multidimensional 16-QAM modulation over a hybrid 8-QAM/16-QAM scheme in terms of asymptotic power efficiency.

A commercially available digital supermode distributed Bragg reflector tunable laser is employed as a fast wavelength switching local oscillator (LO) in a dual polarization (DP) 16-quadrature amplitude modulation (16QAM) coherent burst mode receiver. A digital coherence enhancement technique is used to compensate both the Lorentzian and non-Lorentzian distributed phase noise of the tunable LO laser. It is shown that differential decoding is not sufficient to overcome the substantial bit errors caused by the LO laser phase noise. However, the coherence enhancement technique enables the reception of low symbol rate DP-16QAM bursts, with an average optical signal to noise ratio penalty of 3.5 dB observed relative to theory at the forward error correction threshold (1.5×10-2).

A pilot-aided channel equalizer (PACE) is proposed to mitigate the impairments caused by the polarization mode dispersion and laser phase noise simultaneously in reduced-guard-interval polarization-division-multiplexing coherent-optical orthogonal-frequency-division-multiplexing (RGI-PDM-CO-OFDM) transmission systems. Since PACE updates the channel state information symbol-by-symbol, it enables us to track the drifts in the optical channel. Adaptive PACE (APACE) and boosted PACE (BPACE) are then proposed to further improve the performance of PACE. Numerical simulations are carried out to compare the performance of APACE and BPACE with a conventional training symbols aided channel equalizer (TSACE) and adaptive decision-direct channel equalizer for a 108-Gb/s (56-GS/s) RGI-PDM-CO-OFDM system. It is revealed that both APACE and BPACE offer superior performances over the other two equalizers in the presence of laser phase noise, and they can tolerate lasers with a line width around β = 2000 k ( 2πβTs = 2.24 × 10 when it is normalized to the symbol rate 1/Ts). We also show that only small additional computation efforts are required for APACE and BPACE when compared with TSACE.

A 16-QAM signal is generated using subcarrier modulation with a subcarrier frequency of half the symbol rate, Nyquist pulse shaping, and a directly modulated passive feedback laser at bit rates of 14, 28, and 56 Gb/s. Using polarization multiplexing emulation, a pre-amplified direct detection receiver and digital signal processing, loss margins of 12.6 and 8 dB are achieved for a 112 Gb/s dual polarization signal at back-to-back and after 4 km transmission, respectively.

A bit-error rate (BER)-adaptive wavelength-switched optical network (WSON) employing the multiformat and multirate transmitter/receiver is experimentally demonstrated. The flexible transmitter is capable of generating binary phase-shift-keying, quadrature phase-shift-keying, eight-ary quadrature amplitude modulation, 8QAM, and 16QAM, as well as changing the symbol rate. On the other hand, the flexible receiver based on a coherent detection scheme is utilized not only to detect the optical signal with any modulation format/rate, but also to send the BER information to an OpenFlow controller via extended OpenFlow protocols. Since the measured BER is a reliable barometer of the optical path conditions, an extended OpenFlow-based control plane with the OpenFlow controller intelligently determines and assigns either appropriate modulation format/rate or a backup path in WSON in accordance with BER information.

We experimentally investigate the generation and long-haul transmission of polarization-switched quadrature phase shift keying (PS-QPSK) at an uncoded datarate of 112 Gbit/s (37.4 GBd). In what is the first demonstration of nonlinearity compensation for PS-QPSK, digital backpropagation is applied in order to evaluate performance improvements. We find that, even at this high symbol rate, the maximum transmission distance of this format can be increased by 20% by employing digital backpropagation.

A wide-range frequency offset estimation algorithm is proposed for optical coherent systems, which removes the modulated phase information from the received signal accurately using training sequence instead of traditional Mth power operation. A large frequency offset approximating to ±0.5 times the symbol rate can be traced in arbitrary M-ary phase-shift-keying (MPSK) M -ary quadrature amplitude modulation (MQAM) systems by the proposed algorithm with the low computational complexity and the negligible training overhead.

We propose and experimentally demonstrate a chromatic-dispersion (CD) monitoring method based on radio-frequency (RF) clock power ratio measurement in 38-Gb/s nonreturn-to-zero differential quadrature phase-shift keying (NRZ-DQPSK) and 57-Gb/s NRZ differential 8-level phase-shift keying (D8PSK) systems. A narrowband fiber Bragg grating (FBG) notch filter is centered at the optical carrier wavelength, such that the RF clock power decreases with CD. On the other hand, RF clock power increases with CD in the absence of the notch filter. By using the RF power ratio of the filtered and nonfiltered signals, polarization-mode-dispersion (PMD) effects on RF power ratio can be eliminated and CD measurement sensitivity can be increased. The CD monitoring range is 0 ~ 250 ps/nm in the systems with a symbol rate of 19-Gsym/s. Moreover, the proposed CD-monitoring method is not affected by received optical power.

Frequency offset estimation is a key function in carrier phase recovery to enable digital coherent reception. We propose a novel, feed-forward, dual-stage frequency offset estimator, and demonstrate its performance through both simulation and experiment. Our simulation results show that the estimation range can be as large as ± 9 GHz for a 10.7-GBaud polarization-multiplexed (PolMux) quadrature phase-shift keying (QPSK) system. In addition, the widest range of ±0.5 times the system symbol rate is achieved for the first time in an experimental 42.8-Gb/s coherent PolMux QPSK receiver.

We propose and demonstrate a 1-symbol delayed dual-channel linear optical sampling for observing optical differential phase-shift keying (DPSK) signals. As this technique is based on optical gate processing by means of interference with local short pulses, its performance allows ultrafast measurement for a symbol rate of greater than 100 Gsymbol/s. Moreover, as the new measurement apparatus employs a two-series interferometer system with a 1-symbol delay, the measured phase distribution reflects the signal quality between adjacent symbols of the optical DPSK signals. In our experiment, we successfully observe the waveform degradation caused by the coherence of the light source and the pattern effect of the phase modulator. The measurement system noise is also discussed.

Optical wavelength-division multiplexing (WDM) with channel spacing equal to the symbol rate is demonstrated. Coherent detection with subsequent digital signal processing is used for demultiplexing and demodulation. Experimental results for a 6-Gbaud binary phase-shift keying WDM transmission with a 6-GHz channel spacing achieve a Q-factor penalty of 2.8 dB compared to a single-channel transmission.

We describe a relatively simple but effective closed-loop bias control of LiNbO3 quadrature modulator (QM) for optical quaternary phase-shift-keyed (QPSK) transmission. Simulation and experimental results of the feedback control loop for maintaining near optimal operating points of the QM are described for 12.5-GSym/s optical QPSK. The control loop uses nonhigh-speed off-the-shelf components independent of the symbol rate. Similar performance of the control loop at higher symbol rates using the same components is expected

We present experimental investigations of the receiver sensitivity and dispersion tolerance of multilevel optical communication systems with a symbol rate of 40 Gbaud. Four- and eight-level communication is obtained by combining binary amplitude modulation with either binary or quadrature differential phase modulation. We experimentally compare the dispersion tolerance, and show that multilevel modulation formats offer much better dispersion tolerance compared to binary formats. By combining multilevel modulation with polarization multiplexing, bit rates up to 240 Gb/s were obtained. We demonstrate transmission over 50-km fiber span with no power penalty for a pseudorandom binary sequence length of 27-1 bits

We demonstrate coherent demodulation of optical multilevel (M-ary) phase-shift-keying (PSK) signals. Since the carrier phase is estimated accurately through digital signal processing after phase-diversity homodyne detection, the system performance is highly tolerant to the carrier phase noise. By off-line bit-error-rate measurements using distributed feedback semiconductor lasers with linewidths of 150 kHz as a transmitter and a local oscillator, it is shown that binary PSK (M=2), quadrature PSK (M=4), and eight-PSK (M=8) signals are successfully demodulated at the symbol rate of 10 Gsymbol/s

This synchronous quadrature-phase-shift keying (QPSK) receiver concept allows us to process signals in parallel after electronic demultiplexing. An automatic electronic polarization control with a control time constant in the low microsecond range is provided. It is followed by a phase-locked loop free carrier recovery. An intermediate frequency linewidth tolerance of up to 10/sup -3/ times the QPSK symbol rate is expected. Commercially available distributed feedback lasers shall, therefore, suffice.

We successfully demonstrated overwriting of differential quadrature phase-shift keying (DQPSK) on inverse return-to-zero (RZ) pulses for simple 3-bit/symbol operation at a 10-Gb/s symbol rate (30-Gb/s bit rate). We adopted cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) for inverse-RZ generation, which allows both low and high levels of RZ optical signal to have a finite pulse energy in a bit time slot. We verified a wide tolerance of 20% of the bit-slot for time slot alignment between amplitude-shift keying and differential phase-shift keying modulation in the proposed scheme. We also demonstrated wide dynamic range characteristics at the extinction ratio for both 2- and 3-bit/symbol operation, compared to the conventional scheme. The proposed scheme allows a cross-modulation penalty, due to the intensity to phase modulation, of less than 1.5 dB in 2-bit/symbol and less than 5 dB in 3-bit/symbol operation.

A generic scheme for return-to-zero (RZ) to nonreturn-to-zero (NRZ) format conversion of optical signals is analyzed. It relies on a simple delay interferometer with frequency periodicity twice as high as the input symbol rate and a subsequent optical band-pass filter. Simulation results at 40 Gbaud indicate the compatibility of the technique with a variety of advanced modulation formats. RZ-to-NRZ conversion of 40 Gb/s differential phase shift keying signals is experimentally demonstrated with 1.5 dB power penalty compared to the back-to-back measurement.

The performance of a feed-forward carrier frequency recovery algorithm for 16-quadrature amplitude modulation (QAM) optical coherent systems is investigated using the quaternary phase-shift keying (QPSK) partition scheme. The frequency estimation technique, which employs only a subset of the 16-QAM symbols with the QPSK partition, is demonstrated to be effective when applied prior to the carrier phase recovery. Frequency offset of 10-1 times the symbol rate is shown to be tolerable at a bit-error rate (BER) of 10-3 with combined linewidths symbol duration products of 10-4 .

We investigate the dependence of nonlinear penalties on symbol-rate in wavelength-division-multiplex (WDM)-transmission systems while maintaining a fixed spectral efficiency for several polarization-division-multiplexed (PDM) modulation formats. Transmission performance is found to be improved by up to 1.6 dB for lower symbol rates over the range 3.5-56 GBd for the case of WDM transmission.

We demonstrate the first experimental implementation of a 3.9-Gb/s differential binary phase-shift keying (DBPSK)-based double sideband (DSB) optical fast orthogonal frequency-division-multiplexing (FOFDM) system with a reduced subcarrier spacing equal to half the symbol rate over 300 m of multimode fiber (MMF) using intensity-modulation and direct-detection (IM/DD). The required received optical power at a bit-error rate (BER) of 10-3 was measured to be ~-14.2 dBm with a receiver sensitivity penalty of only ~0.2 dB when compared to the back-to-back case. Experimental results agree very well with the theoretical predictions.

A low complexity digital signal processing architecture for dual-polarization coherent detection is presented. It synchronizes impairments in the frequency domain and uses synergies between the filtering processing stages to minimize overall complexity. We show how chromatic dispersion, skew, carrier frequency offset, timing error, and the state of polarization can be mitigated with a single filtering stage in the frequency domain. The error estimation in the frequency domain is based on the modified Godard algorithm, which calculates the clock tone with less than two samples per symbol and small timing jitter. The synchronization in the frequency domain is independent of the modulation format and therefore a good candidate for higher order modulation formats and so the synchronization of probabilistic shaped (PS) signals based on a high cardinality rectangular QAM format. We show the synchronization of PS polarization-division multiplexed (PDM)-64QAM signals with a symbol rate of 42.7 GBd in the frequency domain. The simulative results for the influence of the FFT size and the number of FFT blocks used for the error estimation are shown. Finally, we show experimental results for four different probabilistic shapings in back-to-back and transmission over 300 km, which are synchronized with the introduced digital signal processing (DSP).