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High-speed digital-to-analog converters (DACs) are essential in advanced optical transmission systems, utilizing multilevel modulation formats and pulse-shaping technologies. Analog bandwidth of DACs is one of the major limiting factors for the system capacity. In this paper, we present an idea to extend the analog bandwidth of DACs by utilizing a digital preprocessor, two sub-DACs, and an analog multiplexer (AMUX). The AMUX is a high-speed linear selector, and a frequency-domain interpretation of the AMUX's function has led us to devise the digital preprocessing for a seamless extension of the analog bandwidth. This configuration, which we call a digital-preprocessed analog-multiplexed DAC, provides nearly doubled analog bandwidth compared to that of each subDAC with a symmetric configuration. The idea was verified by a transmission experiment with 160-Gb/s 80-Gbaud Nyquist four-level pulse amplitude modulation. In the experiment, we used subDACs with an analog bandwidth of ∼20 GHz in combination with an AMUX fabricated with InP heterojunction bipolar transistor technology.

We propose and experimentally demonstrate modulation and Stokes space direct-detection (DD) of four orthogonal dimensions or degrees of freedom (DOF) of a single laser being the intensity on two orthogonal polarizations, the inter-polarization phase and the inter-polarization differential phase. At the transmitter, a dual-polarization in-phase quadrature-phase (DP-IQ) modulator driven by a 4-channel digital-to-analog converter is employed to imprint the transmitted data onto the aforementioned four DOF. Transmitted symbols for each polarization are drawn from multi-ring/multi-phase constellations, e.g., 2-ring/4-ary phase shift keying (PSK) or 2-ring/8PSK, resulting in a polarization division multiplexed (PDM) format with intensity, inter-polarization phase, and inter-polarization differential phase modulation (PDM-IM-IPM-IDPM). At the receiver, a novel DD front-end 1 1 A direct-detection front-end refers to a family of receivers that employ a self-beating mechanism in which the received signal beats with itself or its polarization tributaries beat with themselves. Unlike a coherent front-end, a direct-detection front-end does not incorporate a local oscillator (LO) laser.

In this paper, high-level quadrature amplitude modulation (QAM)-encoded real-time orthogonal frequency division multiplexing (OFDM) transceivers are implemented with two field programmable gate arrays and high-resolution digital-to-analog converter (DAC) and analog-to-digital converter (ADC). Some key digital signal processing (DSP) algorithms for real-time direct-detection optical OFDM (DDO-OFDM) system are presented and described in detail. To improve the effective number of bits of ADC and reduce quantization noise, the DAC operates at 5 GS/s with an oversampling factor of 2. Meanwhile, the optimal digital clipping ratio at the transmitter is also investigated by numerical simulation to optimize the performance of the real-time transmitter. The results show that the real-time measured BERs after 10-km SSMF are below the hard-decision forward error correction threshold of $3.8 \times 10^{-3}$ . For comparison, the off-line BER performance is also analyzed using off-line DSP approaches. It shows that there is a negligible power penalty between the offline and real-time processing results. To the best of our knowledge, we have achieved the highest modulation format (1024-QAM) for real-time optical OFDM systems.

A theoretical model for transmission of advanced modulation schemes [pulse amplitude modulation (PAM), carrierless amplitude-phase (CAP), and discrete multitone (DMT)] in visible light communications (VLC) link using white phosphorescent LEDs is presented. The model extends and adapts the Randel et al. , framework for advanced modulation format transmission in intensity-modulated direct-detection for the VLC channel. The main contributions are: CAP modulation, effect of transmit signal spectral shaping in PAM and CAP, analytical calculation of crest factor for CAP, and analysis of inter-symbol and cross-carrier interference due to an insufficient cyclic-prefix in DMT. Based on this model, the impact of blue filtering at the receiver is studied in the context of spectrally efficient modulation formats. The results indicate that blue filtering has a marginal effect on the performance for all the investigated modulation formats, while PAM with decision feedback equalizer shows the best performance. Without blue filtering, slightly higher data rates can be achieved, but at a price of extensive signal processing.

A packaged silicon photonic traveling wave Mach–Zehnder modulator operating at 1310 nm is presented and studied. The modulator is a series push–pull device and requires a bias applied to the common n-doped region. Effects of varying the bias voltage on the modulator's ${\rm V}_{\pi}$ , bandwidth, and on chip-insertion loss are studied, and its impact on transmission performance are experimentally investigated for PAM-4 and PAM-8 formats at a throughput of 112 Gb/s, over varying distances of 0, 2, 10, and 20 km. Residual chromatic dispersion is shown to have no impact on performance up to 20 km. The PAM RF driving waveform is generated by an 8-bit digital-to-analog converter, while direct detection is done with a PIN+TIA followed by an 8-bit analog-to-digital converter. This IM/DD system utilizes digital signal processing with transmitter spectral compensation and receiver residual equalization. The performance impact for a varying number of transmitter precompensation taps and receiver equalization taps is studied, which has a direct impact on the transceiver's power consumption. Using PAM-4, a BER below the FEC limit is obtained for either combinations of (transmitter, receiver) tap lengths: (19,19), (7,27), or (35,7), allowing flexibility in power consumption distribution.

A polarization-division-multiplexed (PDM) intensity-modulation/direct-detection (IM/DD) system enabled by a novel multiple-input and multiple-output DSP operating in the Stokes space following a DD receiver is demonstrated. Modulating the intensity of the two orthogonal polarization states of a single laser enables doubling the maximum achievable bit rate per wavelength channel, which halves the number of required laser sources in a transceiver using PDM and WDM to achieve an aggregate bit rate compared to using only WDM. Quantitatively, 224 Gb/s is experimentally transmitted over 10 km using a single 1310-nm laser and a silicon photonic intensity modulator using 56-Gbaud PDM PAM-4 with a BER of $4.1 \times 10^{-3}$ . Also, PDM enables halving the baud rate needed to achieve 112 Gb/s resulting in 20-km transmission at low BERs ( $10^{-5}\hbox{-}10^{-6}$ ), using either 56-Gbaud PAM-2 or 28-Gbaud PAM-4. These low pre-FEC BERs achieved at 112 Gb/s allow reducing the FEC overhead required compared to a single polarization system that employs twice the baud rate to achieve the same bit rate. Though the transceiver was implemented using discrete components, it can be fully integrated on a SiP chip, enabling its practical realization for short-reach optical interconnects inside datacenters. Finally, in addition to the experimental results, we perform simulations to further investigate the performance of the receiver. In particular, we studied the impact of varying the splitting ratios of the two couplers in the proposed front–end and concluded that using 67/33 couplers instead of 50/50 couplers renders the performance completely independent of the state of polarization of the received signal.

We report a field demonstration of transmission of 1.44 Tb/s over the Korean Advanced Research Network using three 341-Gb/s PDM-OFDM signals with 256-iterative-polar-modulation. The dispersion managed commercial wavelength-division multiplexed (WDM) transmission system, deployed on legacy fiber and supporting WDM transmission on a 100-GHz grid, covered the distance of 248 km between Daejeon and Seoul. Using a loop-back configuration, the three channels were transmitted over a total distance of 495 km. Successful transmission was achieved by adopting both a digital signal processing-based intersymbol-interference compensation technique and an enhanced soft-decision decoder for the coded modulation.

We propose a chromatic dispersion (CD) estimation technique by using the auto-correlation of signal power waveform for DSP based coherent systems. For single-carrier polarization-division-multiplexed (PDM) systems assuming Gaussian pulse shapes, we analytically show that the auto-correlation of signal power waveform contains a peak whose location is indicative of the accumulated CD and is insensitive to amplified spontaneous emission (ASE) noise, laser frequency offset, laser phase noise and first-order polarization-mode dispersion (PMD) and applicable to single-carrier phase-shift keyed (PSK) and quadrature-amplitude modulation (QAM) systems. As a linear scan of a preset range of CD is not needed, the number of symbols of the proposed technique is dramatically reduced. Simulation results show that to estimate the link CD for 112 Gb/s PDM-QPSK and 224 Gb/s PDM-16-QAM systems, 8192 symbols are sufficient for NRZ/RZ pulse shapes and 32768 symbols are sufficient for root-raised-cosine pulse shapes with the roll-off factor as low as 0.1 with estimation error below 175 ps/nm and 400 ps/nm, respectively.

The Nyquist wavelength-division-multiplexing technique (N-WDM) enables a solution to achieve high spectral efficiency (SE) in long-haul transmission systems. Compared to polarization-division-multiplexing (PDM) quadrature-phase-shift-keying modulation, multilevel modulation scheme such as PDM 64-ary quadrature amplitude modulation (64QAM) is much more sensitive to intrachannel noise and interchannel crosstalk caused by N-WDM. We experimentally demonstrated the generation and transmission of the 6 × 144-Gb/s N-WDM PDM-64QAM signal over 80-km single-mode fiber SMF-28 with erbium-doped-fiber-amplifier-only amplification. It achieved a net SE of 10 bits/s/Hz assuming 20% forward error correction overhead. This experiment was successfully enabled by digital-signal-processing (DSP) pre-equalization of transmitter-side impairments and DSP post-equalization of the channel and receiver-side impairments. Nyquist-band is considered in the pre-equalization to enhance the tolerance of the PDM-64QAM to the aggressive spectral shaping. The benefit from the proposed Nyquist-band pre-equalization scheme was also experimentally demonstrated.

We generate a single-carrier 21.4-Gbaud polarization-division-multiplexed (PDM) 64-ary quadrature-amplitude-modulated (QAM) signal (256.8-Gb/s line rate) using a single in-phase/quadrature (I/Q) optical modulator driven by 8-level electrical waveforms from a novel high-power digital-to-analog converter (DAC). We measure a required optical signal-to-noise ratio of 29.5 dB (0.1-nm reference bandwidth; 10-3 bit-error rate), 4.6-dB off the theoretical limit. Using ultra-large-area fiber, we achieve 400-km single-channel transmission. The DAC was also used to obtain excellent results with quadrature-phase-shift-keyed and 16-QAM signals at 21.4 Gbaud.

We present a simple 120-GHz-band millimeter-wave (MMW) modulation method that uses the bias-voltage dependence of unitraveling-carrier-photodiode output power, which we call photodiode (PD) bias modulation. We investigated the dependence of the output-power-saturation mechanisms on the bias voltage. We used a lowpass filter in the bias circuit to increase the modulation bandwidth, and the 3-dB modulation bandwidth was over 7 GHz. We demonstrated the modulation of 120-GHz MMW signals at a data rate of 10 Gb/s using PD bias modulation.

The pulse width modulation (PWM) technique has been revisited and analyzed to evaluate its merits for application to analog signal transmission in fiber-optic links. Fourier analysis of the PWM signal reveals that it can be used as a vehicle to launch an analog signal onto optical fiber when a symmetrical natural sampling process is used. The SNR of the modulated signal depends on the timing jitter of the carrier pulses and a wide-band (45 MHz) SNR of 45 dB has been obtained with a commercially available multimode laser transmitter. A linear dynamic range of over 50 dB has been experimentally demonstrated. The full fiber bandwidth can be utilized by using a very high pulse carrier frequency, while a more popular pulse frequency modulation technique provides about a 10-MHz analog signal bandwidth when 1 km of multimode fiber is used in conjunction with a short wavelength (0.87 μm) LED transmitter. Analog transmission capability was experimentally demonstrated by constructing a simple video link using common laboratory equipment. The performance of the video link supports the PWM modulation theory developed here and elsewhere. The experimental results indicated that PWM is potentially very attractive for low-cost broad-band local area network (LAN) application, including future highly interactive offices, hospitals, and automated factory floors.