Electronic Dispersion Precompensation of Direct-Detected NRZ Using Analog Filtering

Abstract

We demonstrate (in real-time) electrical dispersion compensation in direct detection links using analog transmit side filtering techniques. By this means, we extend the fiber reach using a low complexity solution while avoiding digital preprocessing and digital-to-analog converters (DACs) which are commonly used nowadays. Modulation is done using an IQ Mach-Zehnder modulator (MZM) which allows straightforward compensation of the complex impulse response caused by chromatic dispersion in the fiber. A SiGe BiCMOS 5-tap analog complex finite impulse response (FIR) filter chip and/or a delay between both driving signals of the MZMs is proposed for the filter implementation. Several link experiments are conducted in C-band where transmission up to 60 km of standard single-mode fiber (SSMF) of direct detected 28 Gb/s NRZ/OOK is demonstrated. The presented technique can be used in applications where low power consumption is critical.

Real-Time 28 Gb/s NRZ over 80 km SSMF in C-band using Analog Electronic Precompensation

“…The transmission range can be extended up to 80 km below the HD-FEC limit and up to 70 km below the KP4-FEC limit. If no compensation of the CD for would be applied, transmission in C-band is limited to maximally 20 km as the created spectral notch is very close to the Nyquist frequency of 14 GHz [5]. This means that an increase in distance with a factor four is accomplished using the analog FIR filters with a limited length of 5 taps.…”

“…This signal is then filtered by two independently tunable, integrated distributed analog 5 tap FIR filters (shown in Fig. 2d) with a tap spacing of 27 ps and sufficient bandwidth to handle 28 Gbaud signals [5]. These filters are part of an analog IC implementing 4 filters, schematically shown in Fig.…”

“…Although very powerful, the DSP solutions introduce extra complexity, power and latency, which makes them less competitive compared to the coherent solutions that assume DSP as well. Therefore, it would be beneficial to perform the precompensation in the analog domain as proposed in [5] (using an IQ-MZM), leading to lower complexity, power and latency. In this paper, a single DD-MZM is used in combination with two on-chip analog finite impulse response (FIR) filters as shown in Fig.…”

“…These allow us to demonstrate real-time transmission at 1550 nm below the KP4-FEC threshold up to 70 km SSMF and below the 7%-OH HD-FEC threshold up to 80 km SSMF. As no compensation of the CD for 28 Gb/s NRZ/OOK leads to a sub-FEC transmission limit of only 20 km in C-band [5], an increase in distance with a factor four is accomplished. As a result, we cover a broad range for inter datacenter applications using direct detection without using any form of DSP (apart from the need of FEC, which can be done on the host devices) as all filtering is performed in the analog domain.…”