20-km Standard Single-mode Fiber Transmission Research Articles

Hardware-efficient and accurately frequency offset compensation based on feedback structure and polar coordinates processing

Compared with off-line digital signal processing (DSP) verification, achieving the expected performance and hardware efficiency in application specific integrated circuit (ASIC) is more critical for DSP to reduce power consumption and cost. The performance of traditional frequency offset compensation algorithms based on feedforward structure is strongly dependent on the accuracy of frequency offset estimation. Therefore, frequency offset calculation and loop filtering implemented in ASIC or field programmable gate array (FPGA) usually choose high word-width fixed-point or floating-point operations, which increase the consumption of logical resources and reduce the clock frequency that hardware logic can achieve. This work proposes a frequency offset compensation scheme based on polar coordinate processing and feedback structure, and a pre-decision-based angle differential estimator is used to estimate the residual frequency offset. In the proposed feedback structure, the frequency offset estimator is realized by a simple accumulator, and the input of the accumulator is the residual frequency offset or its scaling, which will reduce the requirement for the accuracy of the residual frequency offset estimator. The offline verification results show that the performance of the proposed algorithm is close to that of the traditional algorithm, but the proposed algorithm has lower logic resource consumption and higher clock frequency in hardware implementation based on FPGA. The performance of the proposed method is evaluated in real-time through 10-Gbps data rate polarization-multiplexed quadrature phase shift keying modulation after 20-km standard single-mode fiber transmission. Compared with offline processing in MATLAB®, no hardware implementation penalty is observed.

Unequally spaced PAM-4 signaling enabled sensitivity enhancement of a simplified coherent receiver applied in a UDWDM-PON.

We experimentally demonstrate a 4 × 10 Gb/s cost-effective coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) by the use of unequally-spaced 4-level pulse-amplitude modulation (UES-PAM-4) signaling. Because of the advantages of simple architecture and low cost, the simplified coherent receiver (SCR) based on the transmitted signal diversity (TS-D) has been reported, but its receiver sensitivity is constrained by the severe noise arising in the higher level of conventional PAM-4 signals. Here, we first experimentally demonstrate the UES-PAM-4 signaling for the SCR based on the TS-D, by altering the PAM-4 level spacing and the decision threshold through the gradient descent algorithm (GDA). Consequently, we can experimentally achieve -30.1 dBm RS for single wavelength at the bit-error ratio (BER) of 3.8 × 10-3. Compared with the conventional equally-spaced PAM-4 (ES-PAM-4) signaling, 1.3 dB RS enhancement can be secured after the 20-km standard single-mode fiber (SSMF) transmission. Meanwhile, the UES-PAM-4 signaling is experimentally verified for 4 × 10 Gb/s UDWDM-PON. An average RS of -29.6 dBm and 32.6 dB power budget are obtained after the 20-km SSMF transmission. The proposed UES-PAM-4 signaling with the RS enhancement is a promising candidate for the UDWDM-PON by utilizing the existing optical distribution network (ODN).

Simple blind estimation of sampling frequency offset for real-time asynchronous DDO-OFDM system

Sampling frequency offset (SFO) will result in phase rotation, inter-channel interference, and inter-symbol interference, which can seriously degrade the transmission performance of the asynchronous direct-detection optical OFDM (DDO-OFDM). In this paper, we study a simple and overhead-free SFO blind estimation technique in a DDO-OFDM system. The key parameters in this technique are optimized by theoretical analysis, numerical simulation, and experiments. A simplified method of the SFO blind estimation is also proposed to further reduce the hardware implementation complexity. The accuracy and stability of the SFO estimation are experimentally investigated in the real-time asynchronous DDO-OFDM transmission system. The results indicate that the SFO estimation accuracy can be within ±1.5 ppm and shows high stability even at an SFO of up to 200 ppm after 20-km standard single-mode fiber transmission.

A Simple Blind Sampling Frequency Offset Estimation Scheme for Short-Reach DD-OFDM Systems

In this paper, a simple and overhead-free sampling frequency offset (SFO) blind estimation and compensation scheme is proposed and experimentally demonstrated in a short-reach direct detection orthogonal frequency division multiplexing (DD-OFDM) transmission system. After extracting subcarriers of OFDM symbols from multiple OFDM frames, the phase shift is obtained by the fourth-power algorithm, and the obtained results are averaged to perform phase estimation. It can achieve high-precision SFO estimation while ensuring low computational complexity as well as high spectral efficiency. The experimental results show that the SFO estimation accuracy using the proposed scheme can be significantly reduced to as low as ±3 ppm when SFO ranges from -1000 ppm to +1000 ppm after 20-km standard single-mode fiber (SSMF) transmission. Moreover, for the QPSK/16QAM encoded DD-OFDM systems aided by the proposed scheme, up to ±200 ppm SFO can be compensated with slight error vector magnitude (EVM) penalties after 20-km SSMF transmission. Furthermore, the results demonstrate that despite the existence of severe SFO, our proposed SFO compensation scheme can offer nearly negligible power penalties in the DD-OFDM system with optical back-to-back (OBTB) and 20-km SSMF transmission as compared to the ideal case without SFO.

Low-complexity blind polarization demultiplexing and frequency domain equalization for coherent passive optical network systems

Algorithmic and architectural options and trade-offs between performance and complexity/power dissipation are necessary to consider in the transfer of coherent technology from long-haul transmission to short-reach application. An adaptive equalization algorithm cascading polarization demultiplexing and frequency domain equalization is proposed to target passive optical network applications that feature extremely low computational complexity. Three coordinate rotation digital computer-based vector rotators are introduced to realize polarization demultiplexing without performing multiplications. A simplified gradient descent algorithm is proposed to search and trace the state of polarization. A nonbutterfly adaptive equalizer in the frequency domain for each polarization is designed to handle link chromatic dispersion and the imperfection of the transceiver frequency response. The performance of the proposed equalization scheme is experimentally verified by a 32-GBaud dual-polarization Nyquist quadrature phase-shift keying system over 20-km standard single-mode fiber transmission. With the proposed method, the number of hardened multipliers for field-programmable gate array realization is reduced by ∼85 % compared with a conventional 2 × 2 multiple-input multiple-output finite impulse response filter with the same receiver sensitivity.

Frequency-domain averaging-based hybrid channel estimation for short-reach discrete multitone transmission systems

Channel estimation is a key digital signal processing algorithm in optical discrete multitone (DMT) transmission systems. The least-square (LS)-based channel estimation is a simple method, but it is very sensitive to various noises and other interference. The estimation accuracy can be improved using the LS-based intrasymbol frequency-domain averaging (ISFA) channel estimation method. However, there exists inaccurate channel estimation for the edge data subcarriers caused by chromatic dispersion and nonsymmetric averaging windows. We propose a hybrid channel estimation method with LS-based intersymbol frequency-domain averaging and modified ISFA to improve the estimation accuracy of edge data subcarriers and the stability of the bit error rate (BER) performance. The proposed hybrid channel estimation method is experimentally investigated in a short-reach 16-ary quadrature amplitude modulation-encoded DMT transmission system. After 20-km standard single-mode fiber transmission, with the help of the proposed hybrid channel estimation and at a BER of 3.8 × 10 − 3, the receiver sensitivity in terms of the received optical power can be improved by about 1.8 dB compared to that of the conventional ISFA. Furthermore, the proposed hybrid channel estimation has better BER stability.

Experimental demonstration of wavelength-division-multiplexing passive optical network employing probabilistic shaping 4-level pulse amplitude modulation

A 5 × 25-Gbaud wavelength-division-multiplexing passive optical network (WDM-PON) employing probabilistic shaping 4-level pulse amplitude modulation (PS-PAM4) with direct detection is proposed and experimentally demonstrated. Each optical carrier at the optical line terminal (OLT) carries a 25-Gbaud PS-PAM4 signal based on Maxwell–Boltzmann distribution. By employing PS-PAM4, the receiver power sensitivity (RPS) and fiber nonlinear effect tolerance of the system can be significantly improved and enhanced compared with the unshaped one, which can provide abundant system loss budget for the legacy optical distribution networks. The experimental results show that, compared with the WDM-PON employing uniform PAM4, the superior RPS of 1.44 dB and the fiber nonlinear effect tolerance improvement of 3.07 dB over 20-km standard single-mode fiber transmission are obtained for the proposed system. In addition, the system performance by introducing a blind adaptive pre-equalization based on the joint applications of constant modulus algorithm and decision-directed least-mean-square error algorithm, which can accurately compensate for the signal high-frequency loss caused by the electro-optical components, such as digital-to-analog converter, optical modulators, and electrical drivers at the OLT, is also experimentally investigated.

Inter-Symbol Differential Detection-Enabled Sampling Frequency Offset Compensation for DDO-OFDM

Sampling frequency offset (SFO) between digital-to-analog converters and analog-to-digital converters will seriously degrade the receiver performance of the asynchronous direct-detection optical OFDM (DDO-OFDM), due to the SFO-induced inter-symbol interference, inter-channel interference, and phase rotation. In this letter, a simple and overhead-free SFO compensation scheme enabled by inter-symbol differential detection (ISDD) is proposed and experimentally demonstrated in a short-reach and low-cost directly modulated laser-based DDO-OFDM system with an optimal fast Fourier transform size. The experimental results exhibit that the SFO up to ±200 ppm can be compensated with acceptable error vector magnitude (EVM) penalties after 20-km standard single-mode fiber transmission. Moreover, the EVM performance can be further enhanced by using a decision-aided residual phase compensation method after ISDD when SFO is large.

Integrated germanium-on-silicon Franz-Keldysh vector modulator used with a Kramers-Kronig receiver.

We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz-Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800 μm×200 μm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully re-constructed with a single-ended photodiode in a recently proposed Kramers-Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.

Reflective Crosstalk Cancellation in Self-Seeded WDM PON for Mobile Fronthaul/Backhaul

Due to its large capacity and low latency, wavelength-division-multiplexed passive optical network (WDM PON) using self-seeded reflective semiconductor optical amplifier (RSOA) is an attractive, low-cost solution for mobile fronthaul/backhaul. However, multiple reflections in self-seeding cavity lead to reflective crosstalk, degrading its transmission performance. In this paper, reflective crosstalk in self-seeded RSOA under different bias conditions is experimentally characterized by its pulse response, and then novel feedforward and feedback approaches are proposed to suppress the reflective crosstalk in self-seeded WDM PON. Experimental results using a simple delay and differentiation method show significant reduction in the reflective crosstalk for self-seeded RSOA. With the proposed feedforward crosstalk cancellation, 2.5-Gb/s mobile backhaul using self-seeded RSOA achieves 1.7-dB performance improvement after transmission over 20-km single-mode fiber. In addition, low-cost bandwidth-efficient mobile fronthaul for 24 20-MHz LTE signals is demonstrated using a self-seeded RSOA with only 1.5-GHz bandwidth. With adaptive feedback crosstalk cancellation, the error vector magnitude for the received LTE signal after 20-km standard single-mode fiber transmission in the C-band is improved by 20%.

10-Gb/s Tunable External Cavity Laser Using an Asymmetric Ring Resonator Reflector

We demonstrate a 10-Gb/s tunable external cavity laser with a polymer Bragg reflector and a highly integrated semiconductor gain chip including an asymmetric ring resonator reflector. The output power of the tunable laser is increased by adjusting the coupling strengths of two multimode interferences (MMIs) of the ring resonator reflector. The output power of the tunable laser is 6 mW at gain and semiconductor optical amplifier currents of 100 mA, and at a temperature of 25°C when the cross-coupling strengths ratios of two MMI couplers are 0.36 and 0.5. The tunable laser successfully provides 10-Gb/s transmissions through 20-km standard single mode fiber (SMF). The power penalty of the tunable laser is less than 2 dB after a 20-km SMF transmission for a 16-nm tuning range. The power penalty variation is less than 2 dB during the blue-shifted wavelength tuning.