64 Gbps Si photonic crystal slow light modulator by electro-optic phase matching

Abstract

We demonstrate 64 Gbps operation in a compact Si photonic crystal optical modulator that employs meander line electrodes and compensate for the phase mismatch between slow light and RF signals. Although low dispersion slow light increases the modulation efficiency, maintaining a sufficiently wide working spectrum, the phase mismatch becomes a limiting factor on the operation speed even when the phase shifter length is as short as 200 μm. Meander line electrodes broke this limit and enhanced the cutoff frequency by up to 31 and 38 GHz using 50 Ω and 20 Ω termination resistors, respectively. This allowed to use a group index of slow light higher than 20, and greatly improved the quality of the modulation characteristics at 25 and 32 Gbps. Clear open eye was observed even at 40-64 Gbps.

Taking silicon photonics modulators to a higher performance level: state-of-the-art and a review of new technologies

“…The change of free carrier concentration by the movement of charge carriers into or out of a waveguide results in phase modulation of the optical signal. 21,49,61 Phase modulation is translated into intensity modulation by embedding the phase modulator into MZIs, [15][16][17]77,79,108,109 RRs, 18,32,33,62,110,111,119 Bragg reflectors, 88 Michelson interferometers, 8,121 photonic crystal cavities, 98,[112][113][114] and Fabry-Perot cavities. 100 The three prominent schemes to introduce change in the free carrier concentration are (a) the injection of minority carriers 7,37,61-67 by forward biasing a PIN junction; (b) the accumulation of majority carriers 12,68,69,69,70 of opposing polarity across an insulating section in a waveguide; (c) the depletion of majority carriers 9,13,14,16,22,32,33,71,71-87 from a PN junction by reversely biasing it.…”

“…104 Depletion of electrons and holes by reversely biasing a PN junction built in or around it is the most widely adopted scheme to implement the plasma dispersion-based phase modulator. [8][9][10][11][13][14][15][16][17][18][19]22,32,33,71,[71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120][121] Depletion-based modulators were the first all-silicon modulator to reach speeds of 40 106 and 50 Gb∕s.…”

“…A recent implementation demonstrates a 64-Gb∕s MZM, in which the carrier depletion-based phase shifter is implemented through a 200-μm-long photonic crystal slow light structure. 114 The modulator used a meandered RF electrode terminated with a 20 Ω resistor to provide a bandwidth of 38 GHz by preventing the velocity mismatch of the electrical and optical signals. The modulator required a voltage swing of 5.5 V to provide an energy consumption of 21 pJ∕bit and provided 4.8 dB ER.…”

“…The latter effect is more pronounced for the slow-light modulators. 113,114 The traveling wave (TW) is the most common driving scheme for carrier depletion plasma dispersion MZMs. 2,4,25,210,212 In this scheme, a single driver drives the entire electrode of the phase shifter.…”

Silicon photonics platforms for optical communication systems, outlook on future developments

“…A compact modulator with photonic crystal waveguides was also reported, in which the phase shifter length was 200 µm. This had a wider operating wavelength range compared to that of the ringresonator type modulator [52,53]. Monolithic integration of a driving circuit on a BiCMOS platform with a MZI modulator operating at 74 Gbit/s has been reported [54].…”