Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield

Abstract

Conventional monolithically integrated 90° downconverter suffers from hardware-induced non-linear constellation distortion, which gets worse far away from the central wavelength or when fabrication errors are taken into account. To overcome these problems, a 120° monolithically integrated downconverter with full compensation of hardware non-idealities has been proposed. It is numerically demonstrated that, in a realistic scenario exposed to the combined effects of fabrication tolerances and limited ADC resolution, this approach exhibits a significantly better signal dynamic range and a remarkable improvement of fabrication yield.

Colorless monolithically integrated 120° downconverter

“…A realistic photodiode imbalance responsivity of 5% has also been included in simulations. A more detailed description of the integrated downconverter including relevant physical dimensions can be found in [7]. Figure 2(b) shows the resultant wavelength dependence of the maximum value of CMRR from Eq.…”

“…However, the specification of the OIF for the CMRR will be fulfilled only in half of the C-band under reasonable fabrication errors (Case II). Figure 3 shows the proposed colorless calibrated 120° optical downconverter based on a 2x3 MMI coupler monolithically integrated with three photodiodes followed by their respective TIAs [7,10]. Output electrical signals are then linearly combined with a calibrated analog circuit prior to be digitized in two ADCs and digitally processed.…”

“…However, a wideband high CMRR will require stringent fabrication tolerance requirements (resulting in high cost and low fabrication yield) to reduce hardware impairments (i.e. amplitude imbalances existing in phase diversity network or photodiode responsivity mismatch) [6,7].…”

“…This type of 120° downconverter has been reported several times for optical communications by using 3x3 fiber couplers [11,12]. The authors have recently proposed a monolithic integrated downconverter, based on a 2x3 multimode interference coupler (MMI), with a simple linear calibration strategy to fully correct receiver impairments [7,10]. Our proposal, compared to the balanced 90° downconverter (based on a 2x4 MMI), not only showed the same noise-induced penalty under ideal hardware, but exhibited a higher signal dynamic range, wider operating bandwidth and greater tolerance to fabrication errors for a single-channel reception.…”

“…Our proposal, compared to the balanced 90° downconverter (based on a 2x4 MMI), not only showed the same noise-induced penalty under ideal hardware, but exhibited a higher signal dynamic range, wider operating bandwidth and greater tolerance to fabrication errors for a single-channel reception. In this paper we compare the performance of colorless reception of 56 Gbps QAM WDM channels (enabling 112 Gbps under dual polarization), for two different types of monolithically integrated downconverters: the conventional 90°, based in a 2x4 MMI and differential transimpedance amplifiers, and the 120°, based on a 2x3 MMI [7,10] with calibrated analog in-phase and quadrature (IQ) recovery. Simulation results show that hardware imbalances arising from typical fabrication errors reduce the CMRR and increase the interference from coincident channels in a much more limiting way for the monolithic integrated conventional 90° downconverter than for the calibrated 120° downconverter.…”