Abstract
Optical modulators for optical interconnects require a small size, small voltage, high speed and wide working spectrum. For this purpose, we developed Si slow-light Mach-Zehnder modulators via a 180 nm complementary metal-oxide-semiconductor process. We employed 200 μm lattice-shifted photonic crystal waveguides with interleaved p-n junctions as phase shifters. The group index spectrum of slow light was almost flat at ng ≈ 20 but exhibited ±10% fluctuation over a wavelength bandwidth of 20 nm. The cutoff frequency measured in this bandwidth ranged from 15 to 20 GHz; thus, clear open eyes were observed in the 25 Gbps modulation. However, the fluctuation in ng was reflected in the extinction ratio and bit-error rate. For a stable error-free operation, a 1 dB margin is necessary in the extinction ratio. In addition, we constructed a device with varied values of ng and confirmed that the extinction ratio at this speed was enhanced by larger ng up to 60. However, this larger ng reduced the cutoff frequency because of increased phase mismatch between slow light and radio frequency signals. Therefore, ng available for 25 Gbps modulation is limited to up to 40 for the current device design.
Highlights
The Si photonics platform enables the fabrication of dense photonic integrated circuits and reduces their production cost, thanks to the strong optical confinement of high-index-contrast Si devices and the utilization of the matured complementary metal-oxide-semiconductor (CMOS) process, respectively.It is well suited for the development of components and subsystems for optical interconnects in data centers and high-performance computers
Optical modulators are key [1,2,3,4,5,6] and a wide working spectrum and wide working temperature range are required in optical interconnects, in addition to small size, low loss, low drive voltage and high speed
We developed lattice-shifted photonic crystal waveguide (PCW) (LSPCWs) that generate low-dispersion (LD) slow light, showing an almost constant ng over the wide wavelength range of 10–20 nm for transverse-electric (TE) polarization [10]
Summary
The Si photonics platform enables the fabrication of dense photonic integrated circuits and reduces their production cost, thanks to the strong optical confinement of high-index-contrast Si devices and the utilization of the matured complementary metal-oxide-semiconductor (CMOS) process, respectively It is well suited for the development of components and subsystems for optical interconnects in data centers and high-performance computers. MZ circuits typically need a long phase shifter length of several millimeters to obtain a sufficient extinction ratio (ER) [1,2,3,4,5,6], resulting in high power consumption To overcome this constraint, we have developed photonic crystal waveguide (PCW) MZ modulators. We measured the frequency response, ER, and bit-error rate (BER) in detail and evaluated the relation with ng , and revealed the condition that satisfies requirements for optical interconnects
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