Rapidly reconfigurable radio-frequency arbitrary waveforms synthesized on a CMOS photonic chip

Time-domain waveform synthesis using silicon optical delay lines

• The architecture synthesizes waveforms directly in time rather than using a highly dispersive, physically long medium, combining replica generation with fixed and tunable on-chip delay lines.The tunable delay line uses cascaded all-pass ring resonators; its delay spectrum was reported to be insensitive to random microring-radius errors from lithography.Source: Results · pp. 3–4
• The tunable delay range reached 27 ps, exceeding the 25 ps fixed delay step between adjacent channels.Thermal tuning of an individual pulse did not appreciably affect the adjacent pulse.Source: Figure 1 · p. 5, Results · pp. 5–6
• The system demonstrated almost 100% pulse-amplitude control without significantly affecting the reference pulse in the adjacent channel.Amplitude was controlled by mismatching the recombination-ring resonance from the replica-generation-ring resonance.Source: Figure 1 · p. 5, Results · p. 6

Synthesis of RF bursts

• Apodizing a 40 GHz burst improved its RF-spectrum mainlobe-sidelobe suppression ratio from 8 dB to 14 dB.The envelope was shaped to a smooth form.Source: Results · p. 6
• Demonstrated waveforms included a 30 GHz single-tone burst, a 40 GHz burst with an abrupt π phase shift, and a chirp from 30 GHz to 50 GHz.The time-domain approach produces ultrabroadband RF arbitrary waveforms.Source: Results · pp. 6–7

• Using a 10 GHz optical frequency comb source, the experiment generated a 40 GHz continuous RF waveform with four channels at preset 25 ps delay spacing, and also reports a 30 GHz continuous waveform.A 33 ps spacing was achieved through tunable delay (Figure 3b).Source: Figure 3 · p. 8, Results · p. 8
• Consecutive RF waveforms separated by ~4 ns show fast reconfiguration, while the last channel pulse’s amplitude can be controlled completely and individually, with ~3:1 modulation depth.The modulation depth is consistent with that measured using CW light.Source: Results · p. 10
• The update speed is limited by the comb repetition rate and embedded modulator design, but the same fabrication process has demonstrated 44 Gbps modulation, suggesting deep sub-nanosecond reconfiguration is possible.The passage also projects continuous RF waveforms with intensity modulation at any pulse.Source: Results · p. 11
• For a single-tone RF burst, modulators can operate at a speed reduced by the number of peaks per burst, with relaxed timing requirements; rapid delay tuning could modulate all-pass filters sharing an RF drive.Source: Results · p. 11

Discussion

• Controlling each frequency component’s optical delay bypasses the long dispersive element otherwise needed for time-domain waveform synthesis.The scheme provides discrete frequency-to-time mapping, with channel wavelength no longer tied to a particular delay.Source: Discussion · p. 11
• Discrete frequency-to-time mapping frees channel wavelength from a specific delay, but loses continuous tuning of waveform timing features.The authors state that the loss of continuous tuning can be mitigated.Source: Discussion · pp. 11–12
• The authors identify RF photonic filters, phased-array beamformers, and dispersive-link precompensation as potential applications beyond waveform generation.They also describe integrating modulators with pulse shapers as relevant to on-chip all-optical RF processing.Source: Discussion · p. 12
• An 8-channel shaper occupies 4 mm by 2.7 mm, and multiple copies could process the same input RF signal at different bands as an RF channeller.The discussion says the integrated form makes increasing channel count to reduce digitization error easier than in bulk optics.Source: Discussion · p. 12
• The time-domain AWG provides amplitude, frequency, and phase control with performance comparable to the earlier Si-photonic frequency-to-time-mapping shaper, while fast modulators add reconfiguration agility.The discussion contrasts this agility with prior bulk optical pulse shapers and silica- or silicon-nitride-based photonic integrated circuits.Source: Discussion · p. 12
• The demonstrated continuous waveforms extend AWG applications, while high update speeds could help match electronic AWG flexibility while retaining photonic bandwidth and speed advantages.The discussion connects high data-throughput applications with high updating speeds.Source: Discussion · pp. 12–13

Methods

• The preliminary chip used electron-beam lithography, a deposited 2 μm silicon-dioxide overlayer, and heaters made from titanium and aluminum.Compact microring heaters were titanium; serpentine tunable-delay-line heaters were aluminum.Source: Methods · p. 13
• For the preliminary chip, the optical comb was broadened from 2.4 nm to 6 nm using dispersion-decreasing fiber, covering more than four shaper channels.Low-noise and high-gain EDFAs boosted the comb before the fiber.Source: Methods · pp. 13–14
• The foundry chip was fabricated using 0.13 μm technology on 8-inch SOI wafers with a 220 nm silicon layer and 2 μm buried oxide.Source: Methods · p. 14
• The foundry chip’s eighth-channel drop-port transmission was 10 dB lower because of modulator insertion loss, so other input frequency components were pre-attenuated by 10 dB.A commercial pulse shaper applied the compensation across 1528–1568 nm.Source: Methods · p. 14
• The experiment used a 250 MHz frequency comb, compressed its pulses to about 400 fs, and synchronized the intensity modulator to a pattern-generator drive.Five percent of input pulse power supplied the pattern-generator clock; the setup also used post-chip amplification and optical filtering.Source: Methods · p. 14

Additional Information

• The authors declared no competing financial interests.Source: Additional Information · p. 18
• Reprints and permission information was available online from Nature.Source: Additional Information · p. 18
• Correspondence and materials requests were directed to M. Q., A. M. W., or F. G.Source: Additional Information · p. 18