Optical OFDM Signals Research Articles

I/Q Channel Separated Baseband OFDM Optical Transmission Using Orthogonal Polarizations in IM/DD System

We have proposed baseband orthogonal frequency division multiplexing (OFDM) optical transmission technique using I/Q channel separation based on dual polarization with intensity modulation/direct detection (IM/DD). We experimentally verified performance of a proposed system in compare with intermediate frequency (IF)-upconverted OFDM system. In order to obtain better performance, adaptively modulated optical OFDM signal was used in this study. As a result, experimentally achieved maximum throughput after 50 km single mode fiber transmission was 28 Gbps with only using 4 GHz signal bandwidth. The proposed technique has relatively simple hardware structure and reduces computational complexity compared to other methods (IF upconversion or Hermitian symmetry) in IM/DD-based polarization multiplexed OFDM system.

High-Level QAM OFDM System Using DML for Low-Cost Short Reach Optical Communications

In this letter, we experimentally demonstrated a high level quadrature amplitude modulation (QAM) optical orthogonal frequency division multiplexing (OFDM) transmission system utilizing a cost-effective directly modulated laser. This experiment was enabled by intrasymbol frequency-domain averaging-based channel estimation, large fast Fourier transform size to enhance the resistance of intersymbol interference, and the discrete Fourier transform-spread technique to reduce peak-to-average power ratio. A 31.7-Gbit/s 2048QAM OFDM signal can be successfully transmitted over 20-km large effective area fiber under a 20% soft-decision forward-error-correction threshold (bit error rate of 2.4 × 10-2).

Experimental evaluation of the BER performance in optical OFDM system based on discrete Hartley transform precoding

We experimentally assess the bit error rate (BER) performance of an intensity modulation/direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) system based on discrete Hartley transform (DHT) precoding in single-mode fiber (SMF) link for 2.5 Gbit/s quadrature phase shift keying (QPSK) OFDM symbol rate. The experimental results show that for the optical OFDM system based on DHT-precoding, the receiver sensitivity at the BER of 10−4 after 100 km SMF transmission is about 1.5 dBm better than that of the original QPSK OFDM signal, and the DHT-precoded OFDM QPSK signal can achieve approximately 1.3 dB of peak-to-average power ratio (PAPR) reduction.

Timing, carrier frequency and phase recovery for OFDM and Nyquist signals using a mean modulus algorithm.

Efficient algorithms for timing, carrier frequency and phase recovery of Nyquist and OFDM signals are introduced and experimentally verified. The algorithms exploit the statistical properties of the received signals to efficiently derive the optimum sampling time, the carrier frequency offset, and the carrier phase. Among the proposed methods, the mean modulus algorithm (MMA) shows a very robust performance at reduced computational complexity. This is especially important for optical communications where data rates can exceed 100 Gbit/s per wavelength. All proposed algorithms are verified by simulations and by experiments using optical M-ary QAM Nyquist and OFDM signals with data rates up to 84 Gbit/s.

Experimental Demonstration of Nonlinearity and Phase Noise Tolerant 16-QAM OFDM W-Band (75–110 GHz) Signal Over Fiber System

We propose a nonlinearity and phase noise tolerant orthogonal frequency division multiplexing (OFDM) W-band signal over fiber system based on phase modulation and photonic heterodyne up-conversion techniques. By heterodyne mixing the phase-modulated optical OFDM signal with a free-running laser in the photodiode, the constant envelope OFDM W-band wireless signal is obtained to suppress the nonlinear impairments. Moreover, the phase noises of the beating lasers appear as additive terms to the desired signal, and could be easily filtered out without complex phase noise estimation and compensation algorithms. In our experiment, 4 Gb/s QPSK and 8 Gb/s 16-QAM constant envelope OFDM W-band signals are transmitted over 22.8 km single mode fiber and 2.3 m air distance with achieved bit-error-rate performance below the forward error correction limit.

Record-high and robust 17125 Gb/s gross-rateover 25 km SSMF transmissions of real-time dual-band optical OFDM signals directly modulated by 1 GHz RSOAs

Aggregated 17.125 Gb/s real-time end-to-end dual-band optical OFDM (OOFDM) transmissions over 25 km SSMF IMDD systems with 7 dB receiver sensitivity improvements are experimentally demonstrated, for the first time, by utilizing low-cost transceiver components such as directly modulated 1GHz RSOAs and DACs/ADCs operating at sampling speeds as low as 4GS/s. The demonstrated OOFDM transceivers have both strong adaptability and sufficiently large passband carrier frequency tunability, which enable full use of highly dynamic spectral characteristics of the transmission systems. This results in the achievements of not only excellent performance robustness to variations in system operating conditions but also significantly relaxed requirements on RSOA small-signal modulation bandwidth. It is shown that the aforementioned transmission capacity only varies by <23% over a RSOA-injected optical power variation range as large as 20dB, and that the 1 GHz RSOAs can support successful transmissions of adaptively modulated OOFDM signals having bandwidths of 8.5 GHz. By taking into account the adopted 25% cyclic prefix and a typical 7.3% FEC overhead, the demonstrated real-time OOFDM transmission systems are capable of conveying 11.6 Gb/s user data.

Demonstration of Direct Detected Optical OFDM Signals via Block-Wise Phase Switching

In this paper, we propose and demonstrate phase switching for either main carrier or subcarriers of two consecutive signal blocks to achieve fading-free double-sideband direct-detection. The proposed approach has twice of the electrical spectral efficiency (SE) of offset OFDM, and the same electrical SE as single-side band OFDM. We perform numerical simulation and verify the system robustness against polarization mode dispersion and laser phase noise. Our experimental results show that 40-Gb/s direct-detection OOFDM can be successfully delivered over 80-km SSMF with single polarization and single detector.

20-Gbps low cost WDM-OFDM-PON downstream transmission with tunable f ilter and linear APD module

This letter presents a low cost solution for wavelength division multiplexed orthogonal frequency division multiplexed passive optical network (WDM-OFDM-PON) with widely tunable optical filter and linear small form-factor pluggable (SFP) module. With 9-nm tunable range from 1551 to 1560 nm, the tunable filter can support up to 10-channel 100-GHz spacing WDM PON system. A linear avalanche photodiode (APD) based SFP+ module is designed for optical OFDM signal demodulation, which can provide better receiver performance compared with limiting APD module. Experimental results show that ~34 dB power budget can be achieved in 4×5-Gbps WDM-OFDM-PON system, which can satisfy the transmission requirements of next generation PON system.

Simple 60-GHz Radio-over-Fiber System with Transmission Distance Improvement Employing Signal-to-Signal Beat Noise Compensation

We propose a simple 60-GHz RoF system using one single-electrode Mach-Zehnder modulator (MZM). By choosing proper carrier frequencies of two MZM driving signals, fiber transmission distance can be extended up to 12 km. Compared with conventional intensity modulation with direct detection using 60-GHz MZM or electro-absorption modulator (EAM), the proposed RoF system can support longer fiber transmission distance (up to 12 km) and has less bandwidth requirement of optical orthogonal frequency-division multiplexing (OFDM) transmitter (34 GHz vs. 64 GHz). However, the generated 60-GHz OFDM signal will have beat noise after square-law photo-detection. In this paper, the beat noise is analytically analyzed. Hence, the beat noise mitigation algorithm with the beat noise re-construction technique is proposed and utilized to reduce beat-noise-induced interference and improve signal-to-noise ratio (SNR). 60-GHz 13.75-Gbps quadrature phase shift keying (QPSK) and 20.625-Gbps 8-quadrature-amplitude-modulation (QAM) OFDM signals with 3-m wireless transmission are experimentally demonstrated and transmitted over 12-km and 6-km single-mode fiber, respectively.

Simple signal-to-signal beat interference cancellation receiver based on balanced detection for a single-sideband optical OFDM signal with a reduced guard band

A simple signal-to-signal beat interference cancellation receiver based on balanced detection (ICRBD) with an interleaver, a 2×2 three-decibel optical coupler, and a balanced photodiode pair is proposed for a single-sideband optical orthogonal frequency division multiplexing (SSB-OOFDM) signal with a reduced guard band (GB). Simulation demonstration of the ICRBD for a 40 Gbit/s 16-QAM SSB-OOFDM signal with a reduced GB was achieved successfully.

Reverse polarity optical-OFDM (RPO-OFDM): dimming compatible OFDM for gigabit VLC links

Visible light communications (VLC) technology permits the exploitation of light-emitting diode (LED) luminaries for simultaneous illumination and broadband wireless communication. Optical orthogonal frequency-division multiplexing (O-OFDM) is a promising modulation technique for VLC systems, in which the real-valued O-OFDM baseband signal is used to modulate the instantaneous power of the optical carrier to achieve gigabit data rates. However, a major design challenge that limits the commercialization of VLC is how to incorporate the industry-preferred pulse-width modulation (PWM) light dimming technique while maintaining a broadband and reliable communication link. In this work, a novel signal format, reverse polarity O-OFDM (RPO-OFDM), is proposed to combine the fast O-OFDM communication signal with the relatively slow PWM dimming signal, where both signals contribute to the effective LED brightness. The advantages of using RPO-OFDM include, (1) the data rate is not limited by the frequency of the PWM signal, (2) the LED dynamic range is fully utilized to minimize the nonlinear distortion of the O-OFDM communication signal, and (3) the bit-error performance is sustained over a large fraction of the luminaire dimming range. In addition, RPO-OFDM offers a practical approach to utilize off-the-shelf LED drivers. We show results of numerical simulations to study the trade-offs between the PWM duty cycle, average electrical O-OFDM signal power, radiated optical flux as well as human perceived light.

Fabrication of a Low-Loss Fused Fiber Spatial-Mode Coupler for Few-Mode Transmission

Spatial-mode couplers (SMCs) are critical devices for mode-division-multiplexed (MDM) systems. In this letter, we report a low-loss symmetric SMC fabricated with custom-designed two-mode fiber and commercially available standard fused biconical taper station. Sophisticated hydrofluoric etching or pre-pulling on the fiber sample is not needed for fabrication. We characterize the performance of the fabricated SMC, which shows a low loss of 1.1-2 dB and high modal extinction ratio of ≥ 20 dB for a broad wavelength range from 1539 to 1555 nm. We demonstrate transmission of 69.2-Gb/s LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> /LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> dual-mode MDM coherent optical orthogonal-frequency division multiplexing signal over 4.5-km two-mode fiber using the fabricated SMC as mode multiplexer. The signal is successfully received at low (0.1/0.5 dB) optical signal-to-noise ratio penalty, and the estimated channel matrix confirms that there is very low modal crosstalk in both the SMC and the fiber.

61 Gbits/s direct-detection optical OFDM based on blockwise signal phase switching with signal-to-signal beat noise cancellation

We propose a scheme for a direct-detection optical orthogonal frequency-division multiplexing (OFDM) system based on blockwise signal-phase-switching (SPS). Experimental demonstration of 61 Gbits/s SPS direct-detection optical OFDM signal transmission over 80 km standard single-mode-fiber was successfully achieved with single polarization and a single photodetector.

Theoretical and experimental investigation of direct detection optical OFDM transmission using beat interference cancellation receiver

We theoretically and experimentally evaluate a beat interference cancellation receiver (BICR) for direct detection optical orthogonal frequency-division multiplexing (DD-OFDM) systems that improves the spectral efficiency (SE) by reducing the guard band between the optical carrier and the optical OFDM signal while mitigating the impact of signal-signal mixing interference (SSMI). Experimental results show that the bit-error-rate (BER) is improved by about three orders of magnitude compared to the conventional receiver after 320 km single-mode fiber (SMF) transmission for 10 Gb/s data with a 4-QAM modulation using reduced guard band single-sideband OFDM (RSSB-OFDM) signal with 1.67 bits/s/Hz SE.

Adaptability-Enabled Record-High and Robust Capacity-Versus-Reach Performance of Real-Time Dual-Band Optical OFDM Signals Over Various OM1/OM2 MMF Systems [Invited

Real-time dual-band optical orthogonal frequency-division multiplexing (OOFDM) transceivers incorporating digital-to-analog and analog-to-digital converters operating at sampling speeds as low as 4 GS/s and field-programmable gate-array-based digital signal processing clocked at only 100 MHz are employed to explore the maximum achievable transmission performance of electro-absorption modulated laser-based multimode fiber (MMF) systems subject to conventional optical launching. Making use of adaptive subcarrier bit and subcarrier/subband power loading, record-high 20.125 Gb/s transmission over 800 m OM2 MMF of real-time dual-band OOFDM signals is experimentally demonstrated, for the first time, with an optical power penalty as low as 1 dB. In addition, extensive experimental explorations of dual-band OOFDM capacity versus reach performance are also undertaken over the aforementioned systems consisting of different MMF types and lengths. It is shown that 20.125 Gb/s over 100 m and 19.625 Gb/s over 1000 m OOFDM transmissions are obtainable in OM2-only MMF systems and that 20 Gb/s over 100 m and 19.375 Gb/s over 1000 m OOFDM transmissions are achievable in OM1-only MMF systems. Furthermore, in various OM1 and OM2 MMF systems, minimum optical power penalties of 19 Gb/s over 1000 m transmission of dual-band OOFDM signals are practically feasible in any legacy MMF systems.

A scheme to expand the delay-bandwidth product in the resonator-based delay lines by optical OFDM technique

We propose a novel scheme to expand the inherent limit in the product of the optical delay and the transmission bandwidth in resonator-based delay lines, with the optical orthogonal frequency division multiplexing (OOFDM) technique. The optical group delay properties of a single ring resonator were theoretically studied, and double-carrier OOFDM signals were transmitted through such a device in the experiment, where the subcarrier-frequencies matched those of the resonant modes in the device. The results show that, although the delay-bandwidth product (DBP) is limited in the order of 50ps×10Gb/s for signals on each of the sub-carriers, the total DBP of the system is doubled to 2×50ps×10Gb/s due to the double-carrier transmission.

Block-wise phase switching for double-sideband direct detected optical OFDM signals

We propose phase switching for either main-carrier or subcarriers of two consecutive signal blocks to achieve fading-free double-sideband direct-detection (DD). The proposed approach has twice of the electrical spectral efficiency (SE) of offset OFDM, and the same electrical SE as single-side band (SSB) OFDM. With this scheme, 40-Gb/s DD-OOFDM is successfully received over 80-km SSMF with single polarization and single detector.

A novel optical transmission link with DHT-based constant envelope optical OFDM signal

In this paper, we have proposed a novel optical OFDM transmission link that takes advantages of discrete Hartley Transform (DHT) and constant envelope (CE) modulation, obtaining DHT-based constant envelope optical OFDM. The numerical results show that this design achieves better performance when compared with conventional O-OFDM in terms of bit error rate (BER) and peak-to-average power ratio (PAPR). The impact of phase modulation index (PMI) on both PAPR and noise tolerance is investigated. Since the scheme has simplified design, it is believed to be a cost-effective in the practical implement.

Nonlinearity mitigation for high-speed optical OFDM transmitters using digital pre-distortion

Optical orthogonal frequency-division multiplexing (OOFDM) signal is sensitive to nonlinear distortions induced by optical modulators. We propose and experimentally demonstrate a digital pre-distortion (DPD) algorithm to linearize the optical modulators including electro-absorption modulated lasers (EML) and Mach-Zehnder modulators (MZM) used in high-speed OOFDM transmitters. By using an adaptive DPD algorithm with a learning structure, the inverse transfer function of a modulator, which is based on a polynomial model, has been obtained. In the experiment, the performance improvements with and without considering the memory effects of the DPD model are illustrated. The two typical kinds of high-speed OOFDM signals with a bit rate up to 30-Gb/s have been implemented experimentally. The results show that the nonlinear distortion induced by optical modulators can be compensated by using the DPD algorithm to substantially improve the optical modulation index.

Performance evaluation of (D)APSK modulated coherent optical OFDM system

Performance of amplitude and phase shift keying (APSK) modulated coherent optical orthogonal frequency division multiplexing (CO-OFDM) with and without differential encoding is investigated. Numerical simulations based on 40Gbit/s single-channel and 5*40Gbit/s wavelength division multiplexing transmission are performed, and the impacts of amplified spontaneous emission noise, laser linewidth, chromatic dispersion, and fiber nonlinearity on the system performance are analyzed. The results show that compared with conventional 16 quadrature amplitude modulation (QAM) modulated optical OFDM signal, although 16(D)APSK modulated optical OFDM signal has a lower tolerance towards amplified spontaneous emission (ASE) noise, it has a higher tolerance towards fiber nonlinearity such as self-phase modulation (SPM) and cross-phase modulation (XPM): the optimal launch power and the corresponding Q2 factor of 16(D)APSK modulated OFDM signal are respectively 2dB and 0.5dB higher than 16QAM modulated optical OFDM signal after 640km transmission, both in single-channel and WDM CO-OFDM systems. Although the accumulated CD decreases the peak-to-average power ratio (PAPR) during transmission, 16(D)APSK modulated OFDM signal will still remain an advantage compared with 16QAM modulated OFDM signal up to 1000km single-channel transmission, meanwhile relaxing the needs for training symbols and pilot subcarriers and consequently increase the spectral efficiency.