Fausto Meli

Fundamental Limits of Electronic Signal Processing in Direct-Detection Optical Communications

Electronic signal processing is becoming very attractive to overcome various impairments that affect optical communications, and electronic dispersion compensation (EDC) represents a typical application in the currently designed systems. However, the inherent limits in performance achievable by electronically processing the signal at the output of a nonlinear photodetector have not received the attention they deserve. In this paper, we investigate the information-theoretic limits of electronic signal processing in transmission systems employing direct photodetection and two possible modulation formats: 1 on-off keying (OOK) with nonreturn-to-zero pulses; and 2 optical duobinary modulation (ODBM). The analysis is based on the computation of the information rate, i.e., the maximum achievable data transfer rate, and accounts for the modulation format as well as relevant parameters of the transmission scheme. In particular, we investigate the impact of sampling rate, uncompensated chromatic dispersion (CD), and quantization resolution of the electrical signal at the output of a direct photodetector. For OOK systems, the obtained results show that the optical signal-to-noise ratio penalty entailed by EDC can be limited to about 2 dB at most values of CD of interest in current applications. Moreover, ODBM systems at high values of CD can almost perform as OOK systems at zero CD. For all the considered modulation formats, the obtained results show that the received electrical signal can be sampled at a rate of two samples per bit interval and quantized with a precision of 3 bits per sample to practically achieve the ultimate performance limits.

All-Raman transmission of 192 25-GHz spaced WDM channels at 10.66 Gb/s over 30 x 22 dB of TW-RS fiber

Using all Raman amplification with 10 dB of copropagating distributed gain, we demonstrate 25-GHz spacing transmission of 2.04 Tb/s (192 /spl times/ 10.66 Gb/s) over 30 /spl times/ 22 dB spans of TW-RS fiber. We show that all Raman amplification with substantial co-propagating distributed gain and very low launched signal power per channel into the transmission fiber is the key technology of this feasibility experiment.

New Raman pump module for reducing pump-signal four-wave-mixing interaction in co-pumped distributed Raman amplifiers

We propose a new Raman pump module that effectively reduces four-wave-mixing (FWM) nonlinear interaction among longitudinal pump modes and wavelength-division-multiplexed (WDM) signals in co-pumped distributed Raman amplifiers based on nonzero dispersion-shifted fibers (NZ-DSFs). Theory and experiments confirm that the proposed pump module structure, based on polarization- and wavelength-multiplexed Fabry-Perot lasers, allows distributed amplification with high co-propagating on-off Raman gain, providing a key technology for all-Raman-based dense-WDM ultralong-haul transmission.

23 dBm output power Er/Yb co-doped fiber amplifier for WDM signals in the 1575-1605 nm wavelength region

We report, for the first time to our knowledge, theoretical and experimental results concerning a broadband and gain-flattened Er,Yb-codoped double-cladding-fiber amplifier, which provides up to 23-dBm output power in the wavelength region 1575 nm to 1605 nm.

EDFA with continuous amplification of C and L bands for submarine applications

Erbium Doped Fiber Amplifiers (EDFA) are a key element for optical communications and several EDFA architectures were already proposed. This paper presents the theoretical and experimental results of the development of a silica based Erbium Doped Fiber Amplifier (EDFA) composed of three Erbium Doped Fiber (EDF) stages assembled in a serial architecture with two Gain Flattening Filters (GFF) between these stages. It was obtained 70 nm of amplified bandwidth in this architecture by using only two pump lasers operating below 225 mW. From these results, the proposed amplifier achieved an output power of 18.8 dBm by using dual pump architecture with 974 nm and 976 nm pump lasers. The high output power and wide amplification bandwidth from the resulting EDFA enables the transmission of up to 228 DWDM channels for more than 7000 km and is suitable for submarine applications.

State-Complexity Reduction in MLSD Receivers for Optical Communications With Direct Photodetection

We investigate the impact of state-complexity reduction on the performance of maximum likelihood sequence detection (MLSD) receivers for direct-photodetection long-haul optical communication systems affected by uncompensated chromatic dispersion (CD). We directly compare two possible approaches: (i) detection through a simple ldquobrute-forcerdquo state-complexity reduction strategy and (ii) a more structured reduced-state sequence detection (RSSD) strategy. The performance of both state-complexity reduction techniques is evaluated considering two realistic optical transmission schemes, based on on-off keying (OOK) and optical duobinary modulation (ODBM), respectively. The detection algorithms are characterized considering the impact of the timing offset, the quantization scheme, and the amount of uncompensated CD. As one would expect, for a given number of states in MLSD receivers, the schemes based on RSSD exhibit better performance with respect to those based on simple brute-force state-complexity reduction. However, we show that MLSD schemes based on the use of brute-force state-complexity reduction are characterized by a better complexity/performance trade-off for low/medium CD values, whereas RSSD is the best choice for high CD values.

Optical switching technologies and their applications

In this paper system applications, geometry, physical effects and materials of optical switching devices are reviewed. Main system scenarios are presented and some key features such as size, loss, speed, scalability and granularity are highlighted. Two principal categories of optical switches are considered, i.e. guided-wave switches and free-space switches. In the first category some sub- classes have been identified according to their geometrical configuration, principle of operation and, then physical mechanism and materials. As for the geometry, the most frequently used configurations are briefly described together with their advantages and disadvantages. Different physical effects suitable to obtain the index change, which the switching function is based on, are also described with reference to the material substrates. Switches based on semiconductor optical amplifier gate are also analyzed. In the free-space category the main sub-classes are represented by the opto-mechanical devices and micro-opto-electro- mechanical systems switches. The last technology combines the free-space interconnecting with the integration capability on a single silica chip. The main advantages such as the ability to scaling up to large switch fabric and some issues such as packaging and reliability are analyzed. Finally, devices based on polarization change, acousto- optics interaction, total internal reflection and holography are illustrated.

Narrow Linewidth and Compact External-Cavity Lasers for Coherent Optical Communications

In this chapter, the activities related to external-cavity laser development executed in CPqD will be presented in detail. The target application is high-speed and high-order modulation formats optical systems for telecom. Using cavity design techniques, a narrow linewidth and high output power laser suitable for manufacturing is presented. It is presented the operation principle of the tunable mirror which is used as channel selector for the external cavity. Experimental results of a fixed-wavelength prototype are presented, showing optical output power above 16 dBm, Side-mode Suppression Ratio (SMSR) of 60 dB, and linewidth around 75 kHz. The cavity shows good stability for long-term high-temperature storage.

Fundamental limits of signal processing in optical communications

We discuss the use and the interpretation of the information rate as a performance evaluation tool for optical communication systems. In particular, we obtain insights on the impact of the receiver structure on the ultimate achievable performance limits.

25-GHz spacing all-Raman transmission of 192 WDM channels at 10.66 Gb/s over 30/spl times/22 dB of TW-RS fiber

Using all Raman amplification with 10 dB of co-propagating distributed gain we demonstrate 25 GHz spacing transmission of 1.92 Tb/s (192/spl times/10.66 Gb/s) over 30/spl times/22 dB spans of TW-RS fiber.