10 Gbit/s 2-psk Transmission and Homodyne Coherent Detection Using Commercial Optical Components

We present a demonstration of coherent homodyne transmission using 2-PSK at 10Gbit/s, with commercial optical components, which leads to new compact integrated receivers and new transmission formats, thus opening new possibilities for future optical systems. Introduction In most telecommunication fields outside the optical scenario, transmission systems are based on coherent detection, i.e., on a receiver where a PhaseLocked Loop (PLL) tracks the frequency and phase of the received signal. The local oscillator signal is then mixed to the received signal to obtain homodyne or heterodyne detection. Coherent systems have several advantages, mainly related to: • Increase of receiver sensitivity • Compatibility with complex modulation formats, such as M-PSK, M-QAM, etc. Coherent systems were investigated in the field of optical transmission in the years around 1990 [1], mainly for sensitivity issues, but they never found commercial application firstly because of their complexity and cost, secondly, because RX sensitivity issues were solved by the introduction of EDFA’s. Today, we believe that there is again a rationale for optical coherent systems, as it has been recently addressed in several research projects [2]-[4], and as it will be outlined later. The goal of this paper is the demonstration that coherent homodyne optical systems are feasible today using advanced but commercially available optical components. New Rationale for Coherent Optical Transmission Even though sensitivity is not anymore a major issue, thanks to EDFA-preamplified receivers, coherent optical detection may have a fundamental role in future optical transmission systems. In fact: • Coherent (homodyne) detection would open the way to those modulation formats that are largely and successfully used in other telecommunication fields, such as M-PSK, M-QAM and others. All these formats allows a much higher spectral efficiency than conventional Intensity Modulation (IM), by a factor log2(M). For example, 8-PSK bandwidth is 3 times narrower than standard IMDD, given the same bit rate. Most important, this also allows the use of narrower band electronic and optoelectronic components in the TX and RX, by the same factor. • Beside amplitude, coherent detection allows also phase information recovery at the RX, a feature that opens totally new possibilities in receiver equalization for dispersion and PMD compensation. Consequently, all techniques used in equalization of RF receivers, such as multipath fading equalization, could in principle be applied to optics. • Coherent detection allows separating closely spaced DWDM channels without requiring narrow optical filters, since channel selection is obtained directly at baseband by electrical filtering. This feature is at the basis of several advanced optical networks proposals, such as the ACTS-SONATA [2], where coherent detection was basically used as a way to obtain ultra-fine (and relatively fast) DWDM tunability at the receiver (6.25 GHz channel spacing, 622 Mbit/s) Experimental Setup In Fig.1 we show the setup used to carry out our coherent 2-PSK transmission experiment at 10Gb/s. The transmitter is based on a Corning-OTI LiNbO3 10 Gbit/s phase modulator, driven by an NRZ signal whose amplitude has been set to a slightly lower value than the modulator Vπ voltage, in order to obtain a residual carrier 2-PSK modulation [1]. The receiver is based on a novel Optical PLL (SC-OPLL) concept based on Sub-Carrier modulation.  Patent Pending Laser A