Experimental demonstration of pre-electronic dispersion compensation in IM/DD systems using an iterative algorithm

Abstract

To combat chromatic dispersion (CD) in intensity modulation and direct detection (IM/DD) systems, three chirp-free demonstrations are experimentally performed with an iterative pre-electronic dispersion compensation (pre-EDC) algorithm at the transmitter end, for 28 GBaud non-return-to-zero on-off keying (NRZ-OOK), 56 GBaud NRZ-OOK and 28 GBaud four-level pulse-amplitude-modulation (PAM-4) signals, over distances of 100 km, 50 km and 40 km of single mode fiber (SMF), respectively. The iterative pre-EDC algorithm is based on the Gerchberg-Saxton (GS) algorithm, which treats the unconstrained phase at the direct detection receiver as a degree of freedom. At the receiver side, only a linear fractionally-spaced (T/2) post-feed-forward equalizer (post-FFE) is employed to combat the residual inter-symbol interference (ISI). Experimental results show that the aforementioned three demonstrations can approach the forward error correction (FEC) bit error rate (BER) threshold of 3.8 × 10−3 with (15 pre-EDC iterations and 5-tap post-FFE), (30 pre-EDC iterations and 15-tap post-FFE), and (10 pre-EDC iterations and 25-tap post-FFE), respectively. The results indicate the applicability of the pre-EDC algorithm in high-capacity IM/DD systems for transmission distances below 100 km of SMF.

Gerchberg-Saxton Based FIR Filter for Electronic Dispersion Compensation in IM/DD Transmission Part II: Experimental Demonstration and Analysis

“…In this section, we extend our previous work [4], [9] through obtaining a closed-form non-recursive expression of the GS filter H m GS (f ) after m iterations. The following variable substitutions are introduced, x = cos(θ) and y = sin 2 (θ), resulting in H 1 GS (f ) = x + y after the first iteration.…”

“…It is possible to obtain a closed-form analytical expression for the optimized tap weights through the iterative application of the CD frequency domain small-signal (SS) transfer function to the GS algorithm [4], [9]. This method does not include the DER as a tuning parameter in the offline optimization.…”

Gerchberg-Saxton Based FIR Filter for Electronic Dispersion Compensation in IM/DD Transmission Part I: Theory and Simulation

“…7(b) at digital ER = 10 dB. The results indicate that within the range of extinction ratios where the GS algorithm offers BER improvement (digital ER between 1 dB to 5 dB) [16], both approaches produce nearly identical FIR taps. The taps can be expressed analytically in a closed form expression for the first few iterations.…”

“…The ultimate goal would be to design a CD compensation scheme that is based on an offline optimization method, which is agnostic to both modulation format and pattern-dependent effects, while primarily depending on the fiber parameters, digital extinction ratio (ER) and the sampling interval. The term digital ER follows the definition in [16]. The third approach, attempts to achieved this goal through utilizing a single real-valued FIR (SR-FIR) filter as depicted in Fig.…”

“…The iterative algorithm fundamentally linearizes the IM/DD channel effect including square-law detection. This algorithm was experimentally demonstrated to deliver 56 Gb/s over 50 km in [16], modified to reduce the implementation complexity and speed up convergence in [17], [18], [20], and implemented at the receiver as a data-aided decision directed equalizer, enabling 100 Gb/s over 100 km of single mode fiber (SMF) [19], [20]. A 56 Gb/s PAM4 was transmitted over 80 km of SMF with only 5 iterations of the modified GS algorithm and a FIR noise shaping filter [23].…”