Photonic integration leverages design kits of silicon photonic (SiP) components to achieve high yield and reduced costs using the mature complementary metal oxide-semiconductor (CMOS) manufacturing process. A key element of integrated transceivers is SiP modulators, which have received considerable attention during the last decade. SiP Mach–Zehnder modulators (MZMs) and microring modulators (MRMs) have thus been demonstrated with high-speed operation and high stability in the former case, and low energy per bit with an ultracompact footprint in the latter case [1–4]. Despite these attractive properties, the narrow optical bandwidth of MRMs makes them difficult to stabilize, while the long phase shifter MZMs are energy-hungry. Bragg grating modulators, through careful optimization of their structure, offer the potential to improve this performance trade-off, i.e., offering a compact solution with reduced stabilization requirements [5–7].

Analysis of Bragg grating modulators shows that operating the device in the reflection is more efficient than using it in transmission [8]. More specifically, when operated in reflection, the grating strength can be lower for a given optical modulation amplitude, thereby introducing less time delay and improving modulation speed [8]. However, operation in reflection is typically characterized by higher loss due to the lack of on-chip circulators. Therefore, the signal from Bragg grating modulators operated in reflection is typically retrieved by either an external circulator, adding complexity and cost, or by an on-chip splitter, e.g.,  a Y-junction that introduces an additional loss of 6 dB.

In Ref. [9], we proposed using asymmetric Bragg gratings to achieve modulation in reflection with low on-chip loss and without the need for circulators. The mode conversion introduced by the asymmetric Bragg grating allows one to direct the reflected signal to a drop port with low loss. In our initial work, we used an asymmetric Y-branch to filter the reflected mode, and phase modulation was achieved by a standard lateral p-n junction [10]. In this work, to optimize the phase modulation, we introduce a novel design of an asymmetric Bragg grating modulator loaded with a combination of lateral and interleaved (C-LI) p-n junctions. We experimentally show that the C-LI p-n junction increases the phase modulation by 67%, compared to the lateral p-n junction, with design features that respect the requirements of standard fabrication processes. We further propose an asymmetric directional coupler, instead of an asymmetric Y-branch, to reduce insertion loss and cross talk. This optimized design, with a phase shifter length of 290 μm, demonstrates modulation speeds up to 55 Gb/s, with an estimated power consumption of 226 fJ/bit.

Finally, note that a taper, shown in Fig. 1(c), is placed between the directional coupler multimode output and the multimode waveguide with the asymmetric Bragg grating in order to insure mode-matching. Simulations showed that a taper length of at least 10 μm was required to guarantee low loss for both modes. We chose a 20 μm taper length.

The asymmetric Bragg grating consists of a waveguide whose sidewall gratings are placed with a π phase shift, as shown in Fig. 1(d). This π phase shift suppresses the Bragg reflection of the fundamental TE0 mode but allows contradirectional coupling between the two TE0 and TE1 modes if the phase-matching condition is satisfied. The asymmetric grating is designed with a 50% duty cycle, lateral corrugations of 500 nm on each side, and a pitch of Λ=325nm in a ridge waveguide with an 800 nm width.

Compared to the asymmetric Y-branch reported in Ref. [10], the directional coupler has a broader spectral response, lower loss over the C-band, and significantly less cross talk. The asymmetric Y-branch was characterized by an operating bandwidth of 15 nm, an insertion loss of 1.5 dB, and a cross talk of 7 dB, while the directional coupler improves these parameters to 35 nm, 1 dB, and 30 dB, respectively.

We presented a mode-conversion-based SiP modulator that takes advantage of lateral and interleaved p-n junctions simultaneously to enhance phase modulation. The modulator gains a 67% improvement in the phase modulation when loaded by the C-LI p-n junction compared to the lateral one. We employed an asymmetric Bragg grating waveguide to convert the incident TE0 mode to a reflected TE1 mode in order to direct the modulated signal to a drop port by using an asymmetric directional coupler. The whole structure therefore acts similarly to an on-chip circulator and avoids the usually high insertion loss of Bragg grating modulators. The experimentally demonstrated modulator had a phase shifter length of 290 μm, an insertion loss of 2 dB, and power consumption of 226 fJ/bit. The modulator operated up to 45 Gb/s (55 Gb/s) with BER below the 7% (20%) FEC threshold, showing a great potential for high-speed, low-cost, power-efficient transmissions.

Acknowledgment. The authors thank CMC Microsystems for the fabrication subsidy and MPW service.