W1J.4.pdf OFC 2018 © OSA 2018 Single Photodiode-per-Polarization Receiver for 400G Systems Bill Corcoran 1,2 , Benjamin Foo 1,3 , Arthur J. Lowery 1,2 1 Electro-Photonics Laboratory, Dept. Electrical and Computer Systems Engineering, Monash University, Clayton, VIC 3800, Australia 2 Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Australia 3 Now with: Photonics Laboratory, Microtechnology and Nanoscience (MC2), Chalmers University of Technology, 412 58 Gothenburg, Sweden bill.corcoran@monash.edu Abstract: We present a simplified heterodyne receiver using one single ended photodiode per polarization for polarization multiplexed coherent signals. We demonstrate this receiver for the reception of PM-16QAM over field-installed metro-area fibers at distances up to 306-km. OCIS codes: (000.0000) General; (000.0000) General [8-pt. type] For codes, see http://www.osapublishing.org/submit/ocis/ 1. Introduction Systems using single-ended photodiodes at the receiver-side have recently regained interest for application in short- haul metropolitan-area and data center interconnect networks [1,2], in an attempt to simplify receiver architecture while allowing for high spectral efficiency. Several high-capacity demonstrations have used a residual optical carrier to allow QAM signaling and dispersion compensation, while overcoming signal-signal beat interference (SSBI) through various digital signal processing approaches (DSP) [1-4]. However, these techniques require significant modification to transmitter- or receiver-side DSP, and often increase computational complexity, which may lead to penalties in power consumption, cost and processing latency. An alternative technique is to suppress SSBI by increasing the optical carrier-to-signal power ratio (CSPR). If the carrier is transmitted with the signal, this can lead to a large sensitivity or required OSNR penalty. However, if the optical carrier is provided at the receiver by a local oscillator (LO) laser, the receiver-side CSPR can be large without incurring performance penalties. LOs are commonly used in intradyne coherent reception, though the required precision optical hybrids and balanced photodiode pairs have hindered adoption in cost-sensitive applications. Here we use a simplified heterodyne detection front-end, requiring only a standard 3dB optical coupler and a single photodiode. This arrangement significantly reduces the complexity of the optical componentry, requires a single ADC per polarization, and uses the same DSP blocks as an intradyne system. We demonstrate transmission of PM-16QAM at 60-62 Gbd over installed metro-area fiber up to 306 km. Moreover, back-to-back measurements indicate greater sensitivity than state-of-the-art single photodiode techniques [1,2]. This shows that complicated SSBI cancellation DSP can be traded for an off-the-shelf laser and fiber couplers for single photodiode coherent reception. 2. Concept and Experiment Heterodyne reception systems have been used in optical systems for decades (e.g. [5]), and recently to simplify coherent receivers [6]. Here we propose using a strong LO to further simplify the receiver by removing the need for path-length-matched optical couplers (as opposed to [6]). For each received polarization, a signal and LO are passed into the input ports of a 3dB coupler, with one output port connected to a single photodiode, as shown in Fig. 1. Fig. 1: Basic architecture of (left) intradyne and (middle) single photodiode heterodyne receivers. The insets compare the received power spectral densities (PSDs) of the optical and electrical signals. (right) Receiver DSP flow for the intradyne and heterodyne systems. These are identical, excepting the frequency shift stage at the start of the heterodyne Rx DSP (red). If a Nyquist-shaped signal is received, the LO and signal need to be detuned by just over half the signal's symbol rate. The resultant detected signal is then mixed down by this frequency detuning to base-band, which allows standard intradyne DSP blocks to be used to extract the signal data (Fig. 1, right). In comparison to intradyne systems, this system requires a quarter of the number of photodiodes, half the number of ADCs, and does not require precise path lengths in either the optical or electrical domains. However, the electrical bandwidths of the photodiode and ADC need to be doubled, as is the case with other single-photodiode systems [1-4].