180-Gb/s (90-GBd QPSK) single carrier transmitter using a Polymer I/Q modulator

We demonstrate high-speed (90-GBd) modulation of a polymer I/Q modulator achieving single-polarization quadtrature phase shift keyed (QPSK) data rates of 180 Gb/s. Introduction In-phase/quadrature (I/Q) optical modulators are key components for today’s coherent optical transmission systems. Increasing the modulation rate and moving to more complex modulation formats is necessary to achieve higher capacity on a single optical carrier. Today, commercial I/Q modulators typically have modulation bandwidths ranging from 25 to 40 GHz with LiNbO3 technology being most widely deployed. Small improvements in the modulation bandwidth of LiNBO3 modulators has been achieved since 40 Gb/s modulators first appeared the late 1990’s 1 . However, owing to their benign frequency roll-off, LiNbO3 modulators have been shown to operate at symbol rates as high as 80-125 GBd in quadrature phase shift keying (QPSK), 16-ary quadrature amplitude modulation (16QAM) and 64QAM experiments 2,3,4 . Several other materials and technologies such as InP, GaAs, Silicon, polymer and plasmonic modulators are currently being explored to provide smaller form factor, lower power consumption and potential integration with other optical and electrical components 5 . In terms of higher modulation bandwidths, discrete single modulators in InP (electroabsorption) 6,7 , InP on silicon 8 and polymer 9 have demonstrated bandwidths above 60 GHz. For I/Q modulators, the nested Mach Zehnder modulator (MZM) is the most prevalent structure. In material systems other than LiNbO3, the highest symbol rates reported are 72 Gbaud using a plasmonic I/Q modulator 10 and 56 Gbaud using Silicon 11 . Fortunately, coherent reception techniques using powerful digital signal processing enables modulation above the specified 3-dB modulator bandwidth although performance is accompanied by larger implementation penalties. Here, we report high speed modulation up to 90 GBd of a packaged polymer I/Q modulator achieving data rates of 180 Gb/s using QPSK in a single polarization. To the best of our knowledge this is the highest symbol rate achieved to date for an I/Q modulator technology other than LiNbO3. 180-Gb/s Polymer I/Q modulator based Transmitter Figure 1 shows the schematic of the 90-Gbaud QPSK transmitter set-up. The modulator is a commercially available polymer modulator from BRphotonics packaged in a small form factor package (39.7 x 11.75 x 7.71 mm) with fiber pigtails and GPPO high speed RF connectors as shown in Fig. 1 a). The chip footprint is 18.9 x 3 mm. The optical insertion loss is 8 dB, and the extinction ratio is 18 and 20 dB for the respective I and Q sub-MZMs. The modulator is driven by two 90-Gb/s signals that are derived by multiplexing two 45 Gb/s PRBS 2 15 -1 sequences in a 2:1 multiplexing IC that is fabricated using double-heterojunction bipolar transistor (DHBT) technology with a measured peak fT and fmax of 400 and 370 GHz respectively 12 . The 45-Gb/s signals are delaydecorrelated before multiplexing and the 90 Fig. 1: 90-GBd (180-Gb/s) single polarization QPSK transmitter based on polymer I/Q modulator. Insets: a) photograph of InP DHBT 2:1 multiplexer and packaged I/Q modulator. b) Optical 90 GBd QPSK eye before optical equalizer (OF). 90-Gb/s Delay 215-1 PRBS Gen 45 Gb/s