Receiver Modeling for Optically Amplified Communication Systems

Polarization-multiplexing quadrature phase shift keying (PM-QPSK) has become a key ingredient in the design of 100 and 200-Gb/s dense wavelength-division multiplexed (DWDM) networks. The performance of the system using these formats varies according to the shape of the pulses employed by the optical carrier: non-return to zero (NRZ), return to zero (RZ) or carrier-suppressed return to zero (CSRZ). In this paper, we analyse the tolerance of PM-QPSK to linear and nonlinear optical impairments, including amplified spontaneous emission (ASE) noise, crosstalk, distortion by optical filtering, chromatic dispersion (CD), polarization mode dispersion (PMD) and fibre Kerr nonlinearities. The simulation results show that RZ formats with a low-duty cycle have a higher tolerance to CD, PMD and intrachannel nonlinearities due to the reduction of pulse-to-pulse interaction. Conversely, the PM–NRZ-QPSK format has a higher spectral efficiency (SE) than other options and shows better tolerance to filtering distortion and CD in filterless networks.

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.

A simple method to evaluate the performance of transparent optical networks (TON) that takes into account the incoherent homodyne crosstalk, signal distortion, inter-symbol interference (ISI), amplified spontaneous emission (ASE) noise, and arbitrary optical and electrical filters is proposed. This method combines the effect of the incoherent homodyne crosstalk with an exhaustive Gaussian approximation for the influence of ASE noise, signal distortion, and ISI. It is shown that the incoherent homodyne crosstalk penalty depends on the ASE noise accumulated in the TON, ISI, and extinction ratio. The proposed method estimates are compared with experimental results published by other authors. Good agreement is observed for one, two, and four incoherent homodyne crosstalk signals, revealing that the proposed method provides accurate estimates even when the methods based on a Gaussian approximation for the incoherent homodyne crosstalk fail. The proposed method is used to assess the performance of a TON composed by optical add-drop multiplexers based on arrayed waveguide gratings. Results reveal that incoherent homodyne crosstalk, ASE noise, and ISI impair the TON performance and, therefore, none can be neglected in the TON design.

The exponential growth of high-speed broadband services, an increase in the number of end-users, high bandwidth, and massive data rate demand innovative and emerging points to a multipoint network that satisfies the need for next-generation passive optical access network (NG-PON). Next-generation passive optical access network stage 2 (NG-PON2)-based time and wavelength division multiplexing passive optical network (TWDM-PON) are mostly considered as the most promising optical access candidate due to its high-speed services, low power consumption, back-to-back compatibility, and cost-effectiveness. In this paper, the eight wavelengths-based bidirectional 80/20 Gbps TWDM-PON system using return to zero (RZ), non-return to zero (NRZ), and carrier suppressed return to zero (CSRZ) data modulation formats have been proposed and analyzed by varying input power, transmission distance, and bit rate. Further, the system is evaluated in terms of minimum bit error rate (BER), quality factor (Q factor), received optical power (dBm), eye diagrams, and eye height. It has been observed that the most appropriate modulation format for downstream transmission is NRZ while for upstream transmission, the most suitable modulation formats are RZ and NRZ.

Simultaneous all-optical multi-channel RZ and CSRZ to NRZ format conversion

The article investigates the effect of four-wave mixing (FWM), amplified spontaneous emission (ASE) noise, and wavelength converter noise on the performance of WDM all-optical networks. Analysis for the evaluation of probability of error has been carried out (a) when only thermal and shot noise are present; (b) when thermal, shot, FWM, and ASE noise are present; and (c) when thermal, shot, FWM, ASE, and wavelength converter noise are present. Numerical results are presented in the graphical forms for the practical values of parameters.

We propose and experimentally demonstrate a wavelength-division multiplexed (WDM) optical stealth transmission system carried by amplified spontaneous emission (ASE) noise. The stealth signal is hidden in both time and frequency domains by using ASE noise as the signal carrier. Each WDM channel uses part of the ASE spectrum, which provides more flexibility to apply stealth transmission in a public network and adds another layer of security to the stealth channel. Multi-channel transmission also increases the overall channel capacity, which is the major limitation of the single stealth channel transmission based on ASE noise. The relations between spectral bandwidth and coherence length of ASE carrier have been theoretically analyzed and experimentally investigated.

Frequency offset modeling in presence of ASE noise and corresponding low-complexity solution for discrete spectrum modulated nonlinear frequency division multiplexing system

For a coaxial single-photon lidar system, amplified spontaneous emission (ASE) noise from the fiber amplifier is inevitable. The ASE backscattering from specular reflection annihilates the far-field weak signal, resulting in low signal-to-noise ratio, short measurement distance, and even misidentification. We propose a method for calibrating and mitigating ASE noise in all-fiber coaxial aerosol lidar and demonstrate the method for a lidar system with different single-photon detectors (SPDs). The accuracy of the coaxial aerosol lidar is comparable to that of the biaxial one. We conducted an experiment using three different detectors, namely, InGaAs/InP SPD, up-conversion SPD, and superconducting nanowire SPD in the same coaxial lidar system. Compared with the biaxial system, the three different detectors we used have achieved more than 90% ASE noise suppression, the measured visibility percent errors of InGaAs/InP SPD data, up-conversion SPD data, and superconducting nanowire SPD data all within 20%, and the percent error within 10% are 99.47%, 100%, and 95.12%, respectively. Moreover, time-sharing optical switching allowed to obtain background noise with high accuracy.

Performance evaluation of path-averaged soliton pulses in loss-managed 10-Gbps soliton transmission link over a long haul

General Atomics Electromagnetics (GA-EMS) has developed a free space optical laser communication terminal (LCT) for space applications. The system operates at 1550 nm and utilizes on-off keying to support a data rate of up to 5 Gbps. The system architecture is expandable regarding total output power and can support links from various orbits up to and including GEO-GEO as predicted from the amplifier testing and link budget analysis. The amplifier is based off of a TRL9 system originally used by GA-EMS for airborne applications that has been redesigned for space applications and is currently TRL6 based on a TVAC test conducted in 2018. The system utilizes a software defined modulation scheme that can change between non-return to zero (NRZ) and return to zero (RZ) to support various cross link distances by transitioning between RZ and NRZ. While the current LCT uses OOK, the architecture can support multiple modulation schemes including DPSK, which was tested as part of the 2018 T-VAC test. The LCT uses a novel acquisition scheme which is introduced here that enables rapid acquisition for systems even when the bus level pointing accuracy is in excess of 350urad. This results in a bus agnostic LCT architecture that can be used on multiple mission without necessitating extensive redesign and qualification. GA-EMS plans to launch two of these terminals in cubesats to host an on orbit demonstration of crosslinks between the two terminals and downlinks to a ground station.

광통신 시스템에서는 제한된 대역폭에서 최상의 성능을 얻기 위해 다양한 신호형태가 사용되고 있다. 그중에서도 CSRZ (carrier-suppressed return-to-zero) 신호는 스펙트럼 효율이 높으면서도 색분산에 대해 비교적 강인한 특성을 지니고 있어 많이 사용되고 있다. 우리는 이러한 CSRZ 신호의 중요성을 파악하고, 이전의 연구에서 CSRZ 신호의 클럭 (clock) 성분을 이용하여 색분산을 감시하는 방법을 제시한 바가 있다. 하지만, 클럭 성분을 이용하여 색분산을 감시하는 방법은 송수신단의 대역폭에 의해 그 영향을 받을 수 있다. 따라서, 본 논문에서는 CSRZ 신호에서 송수신단의 대역폭이 클럭 성분을 이용한 색분산 감시법에 미치는 영향을 분석하고, 송수신단의 대역폭이 변하더라도 그 성능이 유지될 수 있는 강인한 클럭 추출법을 제시한다. In optical fiber communications, several newly-developed signal formats are used to obtain the best performance within limited spectral bandwidth. CSRZ (carrier-suppressed return-to-zero) format is one of the new signal formats, which has better spectral efficiency and better robustness to dispersion than RZ (return-to-zero) format. Thus it is widely used for demonstrating high-speed optical communication systems. In an earlier research, we proposed a clock-extraction method of CSRZ signal to monitor chromatic dispersion. However, the clock-frequency component extracted by the clock-extraction method can be affected by the bandwidth of a transmitter or a receiver. Therefore, in this paper, we investigate the effect of bandwidth on the chromatic dispersion monitoring of CSRZ signal based on clock-frequency component. As a result, we propose a couple of robust clock-extraction methods to monitor chromatic dispersion in CSRZ signal.

We propose and demonstrate an in-band optical signal-to-noise ratio (OSNR) monitoring technique using the output power ratio (PR) of the nonlinear optical loop mirror (NOLM) theoretically and experimentally. Operation point (OP) is investigated to be the input average power which corresponds to the first maximum of the power ratio curve. The monitoring performances using PR are proved to be better than those just using one output port of NOLM for on-off keying (OOK) and differential phase-shift-keying (DPSK) signals with different duty cycles: non-return-to-zero (NRZ), carrier-suppressed return-to zero (CSRZ), return-to-zero (RZ) with 50% and 33% duty cycle (RZ 50 and RZ 33) at 40 Gb/s. For 80 Gb/s RZ-differential quadrature phase-shift keying (DQPSK) signal, we obtained 12.16 dB maximum power variation and 13-39.8 dB monitoring range experimentally. The power ratio scheme can improve the contrast ratio by 7.05 dB and the monitoring range by 13.8 dB compared with that just using the transmission port. The tolerance to residual chromatic dispersion of the monitoring performance is investigated to be less than 1.7 ps/nm. The experimental validations are consistent with the theoretical analyses. The proposed scheme can be used in ultra-speed transmission systems.

This paper presents the first unified wavelength division multiplexing (WDM) transmission model for systems incorporating cascaded optical amplifiers and a realistic demultiplexing (DEMUX) characteristic with periodic transfer function. The bit error ratio (BER) is evaluated accounting in rigorous form for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the bandwidth of the electrical receiver noise filter, the gain tilt and effective noise figure of optical amplifiers (with as well as without optical ASE noise filtering), channel crosstalk, signal extinction ratio and a one-or two-stage DEMUX implementation. The model is compared to the Gaussian receiver model in realistic design cases providing important information as to the validity of the Gaussian model. Practical design results indicate the link budget dependence on the DEMUX design and the ASE noise filtering.

This paper reports the construction of an experimental setup to attenuate the ASE (Amplified Spontaneous Emission) noise around 1548.4 nm. A Sagnac Interferometer (SI) which uses high-birefringence fiber (Hi-Bi fiber) at 8, 22 and 110 cm is used as band pass filter for attenuating the ASE noise. Temperature variations are made to tune in the filter SI. A test signal containing noise ASE is taken from an erbium-doped optical fiber and a fiber Bragg grating (FBG), therefore the 1548.4 nm signal surrounded for ASE is obtained. This test signal is introduced into the SI filter and the transmitted power of the interferometer is measured, the transmitted signal of SI has periodic variations with maximums and minimums, the SI is tuned in with temperature to bring the maximum transmitted power at the same wavelength of the signal 1548.4 nm. We found experimentally that is possible to attenuate the ASE noise contribution around 1548.4 nm by comparing the input test signal versus the output signal at the output port of SI. In addition, we found the displacement of the transmittance with respect to temperature (Δλ/ΔT) and the period of the transmitted power (Δλ).