Linearization of wavelength sweeping lasers for the construction of 4-D FMCW LiDAR images of slow-moving objects using baseband beat note signals.

A FMCW LiDAR system of both the distributed feedback laser and external cavity laser is established in baseband beat notes, rather than up-conversion to an intermediate frequency to exclude flicker noise. Meanwhile, utilizing fast-scanning MEMS mirrors, high-quality real-time (1 fps) 4-D images of the slow-moving object (10 mm/s) can be directly constructed at the baseband with a central frequency as low as 100 kHz and a small Doppler shift. The proposed LiDAR architecture based on such a low-frequency baseband significantly improves the optical power budget on the transmitter side and eliminates the costly high-speed sampling circuits on the receiver side.

High-entropy chaos generation using semiconductor lasers subject to intensity-modulated optical injection for certified physical random number generation.

This study investigates high-entropy chaos generation using a semiconductor laser subject to intensity-modulated optical injection for certified physical random number generation. Chaos with a continuous spectral profile that is not only widely distributed but also broadly flattened over a bandwidth of 33 GHz is generated. The former suggests that the chaos can be sampled at a high rate while keeping sufficient un-correlation between data samples, and the latter indicates that the chaos possesses high entropy, both of which enhance the generation rate of physical random numbers with guaranteed unpredictability. A minimum entropy value of 2.19 bits/sample is obtained without any post-processing and by excluding the contribution from measurement noise, suggesting that, to the least extent, the chaotic source can be used as a 2-bit physical random number generator at a rate of 160 Gbits/s.

On channel estimation schemes for APD-based DDM-OFDM-PONs under sub-Nyquist sampling.

Implementing preprocessing in a delay-division multiplexing (DDM) orthogonal frequency-division multiplexing (OFDM) passive optical network (PON) requires a priori knowledge of channel responses, which need to be estimated under the constraint of sub-Nyquist analog-to-digital sampling. The localized approach allocates subcarriers in different frequency zones to training symbols in different time slots for channel estimation without spectral overlap. Unfortunately, the localized scheme is susceptible to inaccurate estimation when using an avalanche photodiode (APD), due to variations in APD saturation associated with different training symbols. Instead of localizing all subcarriers of a training symbol in a single frequency zone, we propose distributing training subcarriers through various frequency zones. This distributed scheme would prevent spectral overlap and also reduce the degree of variation in APD saturation, thereby improving the accuracy of channel estimation. Alternatively, we propose an orthogonal scheme in which each training symbol uses all of the subcarriers simultaneously. The orthogonality specified among consecutive training symbols should make it possible to estimate the channel response with low computational complexity. We conducted experiments to compare various schemes used for channel estimation in a 25-Gbps APD-based OFDM-PON. Our results revealed that the orthogonal scheme achieved the best results, and the localized scheme provided the worst channel estimates. We demonstrate the application of the orthogonal scheme in a penalty-free DDM system at 1/32 of the Nyquist rate, which provided a loss budget of 28 dB after fiber transmission over a distance of 25 km.

DFT/IDFT-free receiving scheme for spread-OFDM signals employing low-sampling-rate ADCs.

This paper presents a DFT/IDFT-free receiving scheme for spread-OFDM signals. Leveraging sub-Nyquist sampling and proper sampling delay, the proposed scheme enables each user to receive the requested data without the need for DFT and IDFT; thus, the complexity at receiver can be greatly reduced. Nonetheless, DC component is altered in an AC coupling system, such that severe waveform distortion is caused when the process of DFT/IDFT is omitted. Thus, a DC-zeroing algorithm is proposed to guarantee constant DC after sub-Nyquist sampling, thereby eliminating such distortion. To experimentally verify the concept of proposed scheme, a 27.15-Gbit/s optical spread-OFDM signal was transmitted over fiber and received by the DFT/IDFT-free scheme with sub-Nyquist sampling. More users will reduce the required sampling rate at receiver; for the case of 16 users, the required sampling rate for the 27.15-Gbit/s signal is as low as 1 GSample/s. The experimental results show that error-free transmission was achievable, and the penalty due to lowering sampling rate (i.e., increasing the number of users) is insignificant.

Frequency- and time-domain nonlinear distortion compensation in high-speed OFDM-IMDD LR-PON with high loss budget.

This study compared two nonlinear distortion compensation techniques, SSII cancellation (in the frequency domain) and Volterra filtering (in the time domain), in a >50-Gbps/λ OFDM-IMDD LR-PON. Experiment results for SNR, BER, and data rate (based on a bit-loading algorithm) revealed that the performance of frequency-domain SSII cancellation is unaffected by power fading; however, it depends heavily on the precision of the mathematical model. Conversely, although time-domain Volterra filtering is affected by the faded waveform, adaptive-weighting provides flexibility in dealing with mixed nonlinear distortion, particularly that associated with the interplay between fiber dispersion and fiber nonlinearity. 4-channel WDM-OFDM and 3rd-order Volterra filtering were used to demonstrate the feasibility of the proposed scheme in a 200-Gbps IMDD system. Based on 10-GHz EAM and PIN, we achieved 200-Gbps transmission over a distance of 60 km with a loss budget of >30 dB, while providing support for 128 ONUs at >1.6 Gbps/ONU without the need for an inline amplifier or pre-amplifier.

Spectrally efficient polarization multiplexed direct-detection OFDM system without frequency gap.

We experimentally demonstrate a spectrally efficient direct-detection orthogonal frequency-division multiplexing (DD-OFDM) system. In addition to polarization-division multiplexing, removing the frequency gap further improves the spectral efficiency of the OFDM system. The frequency gap between a reference carrier and OFDM subcarriers avoids subcarrier-to-subcarrier beating interference (SSBI) in traditional DD-OFDM systems. Without dynamic polarization control, the resulting interference after square-law direct detection in the proposed gap-less system is polarization-dependent and composed of linear inter-carrier interference (ICI) and nonlinear SSBI. Thus, this work proposes an iterative multiple-input multiple-output detection scheme to remove the mixed polarization-dependent interference. Compared to the previous scheme, which only removes ICI, the proposed scheme can further eliminate SSBI to achieve the improvement of ∼ 7 dB in signal-to-noise ratio. Without the need for polarization control, we successfully utilize 7-GHz bandwidth to transmit a 39.5-Gbps polarization multiplexed OFDM signal over 100 km.

Optical single-sideband OFDM transmission based on a two-segment EAM.

This paper presents a novel optical single-sideband (SSB) OFDM modulation scheme using a two-segment electro-absorption modulator (EAM). Differences in the chirp characteristics of two segments of the EAM make it possible to design driving signals capable of suppressing one of the optical sidebands, such that the optical OFDM signal does not suffer from frequency-selective power fading following dispersive fiber transmission. Our experiment results demonstrate optical OFDM transmissions at 13.5-Gbps over a 0 ∼ 200-km IM/DD system without the need for dispersion compensation and distance-dependent bit- and power-loading.

Improved technique for the characterization of micro-ring resonator using low coherence measurement.

Low-coherence interferometric measurement has been used to investigate optical waveguide devices with high accuracy. By utilizing an incoherent light source, one can generate separate interferogram features for each optical path. The distance between adjacent features of a ring resonator is related to ring length. With small ring radius, the interferogram spectrum exhibits severe cross-interference between adjacent features that hinders one to analyze the optical path individually. We propose a novel technique to overcome the light-source bandwidth limitation by signal-processing technique, which allows one to characterize small radius micro-ring resonator. This technique has been applied to both numerical simulations and experimental data with significant improvement of the extracted ring parameters. The improvements allow one to better understand the wavelength dependency properties of small radius micro-ring resonators.

High spectral efficient W-band optical/wireless system employing single-sideband single-carrier modulation.

With broader available bandwidth, W-band wireless transmission has attracted a lot of interests for future Giga-bit communication. In this article, we experimentally demonstrate W-band radio-over-fiber (RoF) system employing single-sideband single-carrier (SSB-SC) modulation with lower peak-to-average-power ratio (PAPR) than orthogonal frequency division multiplex (OFDM). To overcome the inter-symbol interference (ISI) of the penalty from uneven frequency response and SSB-SC modulation, frequency domain equalizer (FDE) and decision feedback equalizer (DFE) are implemented. We discuss the maximum available bandwidth of different modulation formats between SSB-SC and OFDM signals at the BER below forward error correction (FEC) threshold (3.8 × 10(-3)). Up to 50-Gbps 32-QAM SSB-SC signals with spectral efficiency of 5 bit/s/Hz can be achieved.

100-GHz DD-OFDM-RoF system over 150-km fiber transmission employing pilot-aided phase noise suppression and bit-loading algorithm.

This study extended the transmission distance of a 100-GHz DD-OFDM-RoF system through the reduction of chromatic dispersion-induced phase noise. The implementation of a pilot-aided phase noise suppression (PPNS) scheme enabled the transmission of distance-insensitive 16.97-Gbps QPSK OFDM over 0~150-km fiber and 2-m air transmission via a DFB laser with linewidth of 1~10-MHz. We applied a bit-loading algorithm in conjunction with PPNS to maximize spectral efficiency, resulting in a 93% improvement in the data rate from 11.53 to 22.27 Gbps at a fiber transmission of 150 km.

EAM-based high-speed 100-km OFDM transmission featuring tolerant modulator operation enabled using SSII cancellation.

In this study, a technique was developed to compensate for nonlinear distortion through cancelling subcarrier-to-subcarrier intermixing interference (SSII) in an electroabsorption modulator (EAM)-based orthogonal frequency-division multiplexing (OFDM) transmission system. The nonlinear distortion to be compensated for is induced by both EAM nonlinearity and fiber dispersion. Because an OFDM signal features an inherently high peak-to-average power ratio, a trade-off exists between the optical modulation index (OMI) and modulator nonlinearity. Therefore, the nonlinear distortion limits the operational tolerance of the bias voltage and the driving power to a small region. After applying the proposed SSII cancellation, the OMI of an OFDM signal was increased yielding only a small increment of nonlinear distortion, and the tolerance region of the operational conditions was also increased. By employing the proposed scheme, this study successfully demonstrates 50-Gbps OFDM transmission over 100-km dispersion-uncompensated single-mode fiber based on a single 10-GHz EAM.

SSII cancellation in an EAM-based OFDM-IMDD transmission system employing a novel dynamic chirp model.

We develop a novel subcarrier-to-subcarrier intermixing interference (SSII) cancellation technique to estimate and eliminate SSII. For the first time, the SSII cancellation technique is experimentally demonstrated in an electro-absorption modulator- (EAM-) based intensity-modulation-direct-detection (IMDD) multi-band OFDM transmission system. Since the characteristics of SSII are seriously affected by the chirp parameter, a simple constant chirp model, we found, cannot effectively remove the SSII. Therefore, assuming that the chirp parameter linearly depends on the optical power, a novel dynamic chirp model is developed to obtain better estimation and cancellation of SSII. Compared with 23.6% SSII cancellation by the constant chirp model, our experimental results show that incorporating the dynamic chirp model into the SSII cancellation technique can achieve up to 74.4% SSII cancellation and 2.8-dB sensitivity improvement in a 32.25-Gbps OFDM system over 100-km uncompensated standard single-mode fiber.

Design and demonstration of a colorless WDM-OFDMA PON system architecture achieving symmetric 20-Gb/s transmissions with residual interference compensation.

In this paper, we propose a two-tiered colorless WDM-OFDMA PON system architecture that draws strengths from each individual WDM and OFDM PON systems. Specifically, the two-tiered architecture enables a colorless transceiver to be shared by a group of ONUs, resulting in drastic reduction of the system cost. For achieving colorlessness via reusing downstream light sources, we discover the residual power of downstream signal unexpectedly springs back after transmissions, causing severe interference to the upstream signal, and thus limiting the data rate of the upstream signal. We devise a method of adopting a common dispersion compensation module at OLT to reduce the residual power over all wavelengths. Experimental results show that, with an improvement of upstream signal's SNR up to 10 dB, the system successfully achieves 20-Gb/s bidirectional OFDM-signal transmissions on the same wavelength over a 20-km SMF.

High spectral efficient W-band OFDM-RoF system with direct-detection by two cascaded single-drive MZMs.

W-band wireless transmission has attracted a lot of interest due to its wider available bandwidth (i.e. 75-110 GHz). In this article, we propose a direct-detection orthogonal frequency division multiplexing radio over fiber (OFDM-RoF) system via two cascaded single-drive MZMs at center frequency of 103 GHz. We discuss maximum bandwidth of different modulation formats under forward error correction (FEC) threshold (3.8 x 10(-3)). Up to 40-Gbps 16-QAM OFDM signals is achieved over 25-km fiber and 2-m wireless transmission. To overcome the penalty from uneven frequency response, bit-loading algorithm is applied to discuss data rate and spectral efficiency with signal bandwidth from 5 to 10 GHz. With 10-GHz bandwidth, 46.4-Gb/s data rate and 4.64-bit/s/Hz spectral efficiency was achieved. To achieve 40-Gbps data rate, the required bandwidth of OFDM signal with bit-loading is 2 GHz less than that without bit-loading.

Photonic chirped radio-frequency generator with ultra-fast sweeping rate and ultra-wide sweeping range.

A high-performance photonic sweeping-frequency (chirped) radio-frequency (RF) generator has been demonstrated. By use of a novel wavelength sweeping distributed-feedback (DFB) laser, which is operated based on the linewidth enhancement effect, a fixed wavelength narrow-linewidth DFB laser, and a wideband (dc to 50 GHz) photodiode module for the hetero-dyne beating RF signal generation, a very clear chirped RF waveform can be captured by a fast real-time scope. A very-high frequency sweeping rate (10.3 GHz/µs) with an ultra-wide RF frequency sweeping range (~40 GHz) have been demonstrated. The high-repeatability (~97%) in sweeping frequency has been verified by analyzing tens of repetitive chirped waveforms.

20-Gbps WDM-PON transmissions employing weak-resonant-cavity FPLD with OFDM and SC-FDE modulation formats.

Using a colorless weak-resonant-cavity (WRC) FPLD injected by a centralized light source, we have experimentally demonstrated a superior performance of 20-Gbps uplink transmission in a WDM-PON. Even though the typical modulation bandwidth of a WRC-FPLD is only ~1.25 GHz, using spectrally-efficient 32-QAM OFDM or SC-FDE modulation, 20-Gbps uplink signals can achieve the FEC limit after 25-km dispersion-uncompensated single-mode fiber transmission. Because of the advantage of lower PAPR, the SC-FDE signals outperform the OFDM signals with the fixed 32-QAM format in the proposed system; moreover, SC-FDE scheme can be another promising candidate for uplinks in WDM-PONs, for its simplification to ONUs. The signal at the mode of 1560.7 nm shows similar quality with the signal at the modes of 1545.3 nm and 1574.7 nm, the WRC-FPLD, accordingly, has wide injection wavelength range from at least 1545.3 nm to 1574.7 nm. With the mode spacing of 0.55 nm, consequently, we have demonstrated the applicability of the colorless WRC-FPLD on supporting up to 36 channels in the WDM-PON.

Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems.

This work theoretically studies the transmission performance of a DML-based OFDM system by small-signal approximation, and the model considers both the transient and adiabatic chirps. The dispersion-induced distortion is modeled as subcarrier-to-subcarrier intermixing interference (SSII), and the theoretical SSII agrees with the distortion obtained from large-signal simulation statistically and deterministically. The analysis shows that the presence of the adiabatic chirp will ease power fading or even provide gain, but will increase the SSII to deteriorate OFDM signals after dispersive transmission. Furthermore, this work also proposes a novel iterative equalization to eliminate the SSII. From the simulation, the distortion could be effectively mitigated by the proposed equalization such that the maximum transmission distance of the DML-based OFDM signal is significantly improved. For instance, the transmission distance of a 30-Gbps DML-based OFDM signal can be extended from 10 km to more than 100 km. Besides, since the dispersion-induced distortion could be effectively mitigated by the equalization, negative power penalties are observed at some distances due to chirp-induced power gain.

PMD tolerant direct-detection polarization division multiplexed OFDM systems with MIMO processing.

This work proposes a novel direct-detection polarization division multiplexed OFDM scheme without the need of dynamic polarization control at a polarization-diverse receiver, and the proposed scheme is robust against polarization mode dispersion. Setting the frequency difference between two polarization-orthogonal reference carriers as one subcarrier spacing, possible signal fading can be avoided, and the corresponding interference from adjacent subcarriers is eliminated by a novel MIMO algorithm. The penalty caused by high channel matrix condition number can be decreased by inserting empty tones among subcarriers, and the polarization-dependent OSNR penalty at the BER of 10⁻³ is <3.6 dB with an empty tone inserted every 8 subcarriers. Moreover, the numerical results demonstrate the 16 × 10³-ps/nm chromatic dispersion and the 300-ps differential group delay will not induce additional penalty.

2 × 2 MIMO radio-over-fiber system at 60 GHz employing frequency domain equalization.

This work experimentally demonstrates the efficacy of the 2 × 2 multiple-input multiple-output (MIMO) technique for capacity improvement of a 60-GHz radio-over-fiber (RoF) system employing single-carrier modulation format. We employ frequency domain equalization (FDE) to estimate the channel response, including frequency response of the 60 GHz RoF system and the MIMO wireless channel. Using FDE and MIMO techniques, we experimentally demonstrate the doubling the of wireless data capacity of a 60 GHz RoF system to 27.15 Gb/s using 16-QAM modulation format, with transmission over 25 km of standard single-mode fiber and 3 m wireless distance.

Direct-detection polarization division multiplexed orthogonal frequency-division multiplexing transmission systems without polarization tracking.

We experimentally demonstrate a direct-detection polarization division multiplexed (PDM) orthogonal frequency-division multiplexing (OFDM) scheme without dynamic polarization tracking. Simply using a polarization-diverse receiver, the proposed multiple-input multiple-output assisted system can achieve bit-error rate of 10(-) over all possible received states of polarization. Moreover, 50 Gbps PDM-OFDM transmission over 100 km single-mode fiber is successfully demonstrated without dispersion-induced penalty.