Reconfigurable radio-frequency arbitrary waveforms synthesized in a silicon photonic chip

Photonic methods of radio-frequency waveform generation and processing can provide performance advantages and flexibility over electronic methods due to the ultrawide bandwidth offered by the optical carriers. However, bulk optics implementations suffer from the lack of integration and slow reconfiguration speed. Here we propose an architecture of integrated photonic radio-frequency generation and processing and implement it on a silicon chip fabricated in a semiconductor manufacturing foundry. Our device can generate programmable radio-frequency bursts or continuous waveforms with only the light source, electrical drives/controls and detectors being off-chip. It modulates an individual pulse in a radio-frequency burst within 4 ns, achieving a reconfiguration speed three orders of magnitude faster than thermal tuning. The on-chip optical delay elements offer an integrated approach to accurately manipulating individual radio-frequency waveform features without constraints set by the speed and timing jitter of electronics, and should find applications ranging from high-speed wireless to defence electronics.

. The transmission spectra are taken at the common through and common drop ports of the replica generation rings and at the common through port of the replica recombination rings, respectively. (a) The through-port transmission spectra are taken before thermal resonance matching. (b) The through-port and (c) drop-port transmission spectra are taken after thermal resonance matching. The fibre-to-fibre loss of ~22 dB measured at the through ports is attributed to waveguide facet coupling loss.

Supplementary Figure 3
Supplementary Figure 3. Simulated delay and transmission spectra of two tuneable optical delay line designs using coupled-mode theory 2 . In both designs, all the microrings side couple to the bus waveguide at the same coupling gap of 100 nm (with a power coupling coefficient of κ 2 ≈ 0.6) to satisfy the over-coupling condition. The propagation loss in the microrings is set as 10 dB/cm in the simulations. (a) The delay (blue) and transmission (green) spectra of an optical delay line that consists of 41 microrings with a radius increment step of 1 nm and a median radius of 5 μm. The simulated delay ranges from 4.5 ps to 41.5 ps, with the insertion loss increasing from 0.32 dB to 3 dB. To test the robustness of the design, we next assumed a random variation of ±2 nm in the radii of each of the 41 rings, which corresponds to the estimated precision of E-beam lithography in fabricating the delay line. 1000 simulations with random radius variations were performed and overlaid with that of the original design, showing a small variation of ~4 ps in delay and of 0.3 dB in transmission (shaded areas). (b) The delay and transmission spectra of another optical delay line design where the 41 microrings have a radius increment step of 1.5 nm and a median radius of 5μm. Both spectra are overlaid with the simulation results where random radius variations of ±2 nm in the radii of each of the 41 rings are incorporated.

Supplementary Figure 4
Supplementary Figure 4. Characterization of the silicon intensity modulator (embedded in the pulse shaper) with a continuous-wave (CW) laser input at 1549.7 nm. The modulator was tested at a modulation speed of (a) 250 Mbps, (b) 1.06 Gbps, (c) 2.5 Gbps, and (d) 5 Gbps, respectively. The modulator is the same design as those reported in Ref.
[3] except that some modification is made to the traveling-wave electrode design to meet the testing requirement but at a price of a compromised modulation efficiency. In the experiment of rapidly reconfigurable AWG, we applied external electrical controls to the silicon chip using RF/DC probes, which required much more setup space than the wire bonding method. In the setup, we coupled light into and out of the chip through fibre butt coupling and the remaining setup space could only accommodate one 16-pin DC probe to reconfigure the pulse shaper and one RF probe to drive the modulator. However, driving a traveling-wave modulator typically requires using a pair of 'GSG' RF probes, one feeding the RF signal at one end of the electrodes and one terminating the RF wave at the other end. Thus using only one RF probe not only limits the number of modulators that can be incorporated into the shaper but also requires modification to the electrode design of the modulator. In our design, the electrodes were wrapped to one side of the modulator (See Fig. 4a in the main text) so that a single 'GSGSG' RF probe can feed and terminate the RF driving signal simultaneously. In addition, the thermal tuning element was also dropped in this modulator design due to lack of space for additional DC probing. As a consequence of the compromised electrode design and lack of thermal tuning element, the extinction ratio of the modulator was limited to ~3:1 and the modulation speed was limited to 5 Gbps.

Supplementary Figure 5
Supplementary Figure 5. Illustration of RF frequency synthesis by adjusting delays of some channels and disabling selected channels. The fixed delay is 25 ps between consecutive channels, corresponding to a 40 GHz fundamental RF tone. One can achieve a 33.3 ps channel spacing (30 GHz fundamental RF tone) by adjusting the 2nd and 3rd channel delays and disabling the 4th channel. Our architecture can easily disable any channel by shifting the recombination ring resonance away from that of the replica generation resonator. Experimental demonstrations are shown in Fig. 2e and Fig. 3b. Mathematically, since each channel can tune the delay (27 ps) more than the fixed channel spacing (25 ps), one can synthesize any RF waveforms as long as the highest frequency component is less than 1/(25 ps) = 40 GHz. To increase the highest frequency, one can reduce the fixed delay and increase the number of channels.