10-Gb/s SCM fiber system using optical SSB modulation

Abstract

Abstract-A 10-Gb/s subcarrier multiplexing long-haul optical system is reported. 4 2.5 Gb/s data streams are combined into one wavelength, which occupies a 20-GHz optical bandwidth. Optical single sideband is used to increase bandwidth efficiency and reduce dispersion penalty. The receiver sensitivity is calculated using a simplified receiver model with an optical preamplifier. The measured results agree well with the analytical prediction.Index Terms-Modulation, optical communication, optical modulation, optical signal processing.I N ORDER to use the optical bandwidth provided by optical fibers more efficiently, new transmission technologies have been developed rapidly in recent years, such as time division multiplexing (TDM), wavelength division multiplexing (WDM), and their combinations. Apart from noise accumulation, high-speed TDM optical systems suffer from chromatic dispersion, nonlinear crosstalk, and polarization-mode dispersion (PMD). Optical systems with data rates of 10 Gb/s and higher require precise dispersion compensation, and careful link engineering. On the other hand, WDM technology uses lower data rates at each wavelength. However, due to the limitations in the wavelength stability of semiconductor lasers and selectivity of optical filters, the minimum channel spacing is 50 GHz in current commercial WDM systems. Narrower channel spacing requires better wavelength stability of laser sources and better wavelength selectivity of optical filters.Optical subcarrier multiplexing (SCM) is a scheme where multiple signals are multiplexed in the RF domain and transmitted by a single wavelength. A significant advantage of SCM is that microwave devices are more mature than optical devices: the stability of a microwave oscillator and the frequency selectivity of a microwave filter are much better than their optical counterpart. A popular application of SCM technology in fiber optic systems is analog cable television (CATV) distribution [1], [2]. Because of the simple and low-cost implementation, SCM has also been proposed to transmit multichannel digital-optical signals using direct detection [3], [4] for local area optical networks.The basic configuration of an SCM/WDM optical system is shown in Fig. 1. In this example, -independent high-speed digital signals are mixed by -different microwave carrier frequencies . These are combined and optically modulated onto an optical carrier. -wavelengths are then multiplexed together in an optical WDM configuration. At the receiver, an optical demultiplexer separates the wavelengths for individual optical detectors. Then, RF coherent detection is used at the SCM level to separate the digital signal channels. Channel add/drop is also possible in both wavelength level and SCM level. While this SCM/WDM is in fact an ultradense WDM system, sophisticated microwave and RF technology enables the channel spacing to be comparable to the baseband, which is otherwise not feasible by using optical technology. Compared to conventional high-speed TDM systems, SCM is less sensitiv...

Highly Linear Single Sideband Transmitter for Radio-Over-Fiber Systems

“…Similarly to [4], the transmitter output optical signal is described by the optical carrier and sine terms, whose Taylor series do not include any even order terms. The beating of second order terms with fundamental tones during direct detection is a major contribution for IMD3 [2]; therefore, the proposed transmitter reduces signal distortion when compared to common SSB quadrature modulation [5].…”

“…However, both methods still include second order terms, and rely on the wavelength stability of a sharp filter and optical source, and the latter requires a noncommercial complex interferometric structure to couple In this letter, we propose the combination of a highly linear optical CS SSB signal with an optical carrier (CS+C), which theoretically does not generate even order terms at its output. The transmitter linearity is characterized and compared to the conventional technique of [5] by means of carrier to interference ratio (CIR) measurements, as a function of the optical signal carrier suppression ratio (CSR), and accumulated chromatic dispersion. The proposed transmitter may be implemented using a commercially available quad-MZM [6].…”

Subcarrier multiplexing for high-speed optical transmission

“…At the receiver, the optical carrier beats with the subcarriers at the photodiode, down-converting the optical subcarrier into the RF domain. The generated photocurrent at the receiver is (5) where is the system transmission and coupling loss, is the photodiode responsivity, is the gain of the optical preamplifier, is the average photocurrent, is the average power of the optical signal reaching the preamplified optical receiver, and is the normalized modulation index. Obviously, the useful photocurrent signal for the th channel is .…”

“…OSSB modulation was achieved using a balanced dual-electrode electrooptic modulator. This 10-Gb/s composite optical signal was transmitted over 150-km equivalent standard single-mode fiber (SMF) without any dispersion compensation [5]. The combination of SCM and WDM may provide a more flexible platform for high-speed optical transport networks with high optical bandwidth efficiency and high dispersion tolerance.…”

The effects of drive level errors on optical SSB system performance

“…One of the advantages of SCM is the microwave devices used in SCM are more stable than their optical counterpart [2]. However, transmission in conventional SCM systems is limited due to the optical sidebands experiencing different amounts of phase shift as a result of fiber chromatic dispersion (CD).…”