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We propose an analytical model to investigate the intrinsic frequency chirp with the method of 4-level pulse-amplitude-modulation (PAM-4) optical modulation by driving a single dual-drive silicon optical modulator. With the analytical model, we calculate the chirp parameters of this PAM-4 generation numerically. The intensity and phase of output signal changes with the PAM levels’ switching, which results in the frequency chirp. We find that the modulator operating at different quadrature points (Q- and Q+) will have different frequency chirp behavior. The Q- modulator has mainly negative chirp parameters while Q+ modulator has mainly positive chirp parameters. We characterize the eye diagrams and BER of the optical PAM-4 signals generated by the Q- and Q+ modulator at the modulation rates of 25 Gbaud and 32 Gbaud, respectively, for both back-to-back and 2 km transmission in single mode fiber. The BER results show the Q- modulator has a small power penalty attribute to the positive fiber dispersion, which is suited for the short-distance transmission in data center application.

We propose and demonstrate a four-port mode-selective optical router on a silicon-on-insulator platform. The passive routing property ensures that the router consumes no power to establish the optical links. For each port, input signals with different modes are selectively routed to the target ports through the pre-designed architecture. In general, the device intrinsically supports broadcasting of multiplexed signals from one port to the other three ports through mode division multiplexing. In some applications, the input signal from one port would only be sent to another port as in reconfigurable optical routers. The prototype is constructed by mode multiplexers/de-multiplexers and single-mode interconnect waveguides between them. The insertion losses for all optical links are lower than 8.0 dB, and the largest optical crosstalk values are lower than -18.7 dB and -22.0 dB for the broadcasting and port-to-port routing modes, respectively, at the wavelength range of 1525-1565 nm. In order to verify the routing functionality, a 40-Gbps bidirectional data transmission experiment is performed. The device offers a promising building block for passive routing by utilizing the dimension of the modes.

We present a silicon thermo-optic 2☓2 four-mode optical switch optimized for optical space switching plus local optical mode switching. Four asymmetric directional couplers are utilized for mode multiplexing and de-multiplexing. Sixteen 2☓2 single-mode optical switches based on balanced thermally tunable Mach-Zehnder interferometers are exploited for switching function. The measured insertion losses are 8.0~12.2 dB and the optical signal-to-noise ratios are larger than 11.2 dB in the wavelength range of 1525~1565 nm. The optical links in “all-bar” and “all-cross” states exhibit less than 2.0 dB and 1.4 dB power penalties respectively below 10−9 bit error rates for 40 Gbps data transmission.

The optical switches for single-mode operation cannot be directly utilized in optical communication and interconnect systems adopting mode-division multiplexing. In this paper, three general architectures for on-chip optical space and mode switching are proposed, which are optimized for optical space switching, optical space switching plus local optical mode switching, and global optical mode switching, respectively. A silicon thermo-optic 2×2 four-mode optical switch is demonstrated. The minimum and maximum optical link insertion losses are 16.0 and 20.9 dB (including ∼6 dB coupling loss), respectively, in the wavelength range of 1525–1565 nm, while the optical signal-to-noise ratios of the optical links are larger than 15.3 dB. The optical power penalty at a bit error rate of 10−9 varies from 1.0 to 5.6 dB for 40 Gbps data transmission through different optical links. This work provides a systematic solution to on-chip information switching for different physical and mode channels.

We propose a 2 × 2 multimode optical switch, which is composed of two mode de-multiplexers, n 2 × 2 single-mode optical switches where n is the number of the supported spatial modes, and two mode multiplexers. As a proof of concept, asymmetric directional couplers are employed to construct the mode multiplexers and de-multiplexers, balanced Mach-Zehnder interferometer is utilized to construct the 2 × 2 single-mode optical switches. The fabricated silicon 2 × 2 multimode optical switch has a broad optical bandwidth and can support four spatial modes. The link-crosstalk for all four modes is smaller than −18.8 dB. The inter-mode crosstalk for the same optical link is less than −22.1 dB. 40 Gbps data transmission is performed for all spatial modes and all optical links. The power penalties for the error-free switching (BER

We optimize the linearity performance of silicon carrier-depletion Mach-Zehnder optical modulator through controlling the doping concentration. The optical field distribution in the waveguide is a Gaussian-like distribution. As the doping concentration increases, the dynamic depletion width of the PN junction under the same modulation signal will decrease, and the integration width of the overlap between the Gaussian-like optical field distribution and the depletion region will become smaller. Therefore the modulated signal has less nonlinear components. Our simulation results proved this analysis. We also fabricated different devices with different doping concentrations. By adopting a ten times doping concentration, the spurious free dynamic range (SFDR) for third-order intermodulation distortion (TID) increases from 109.2 dB.Hzsup2/3/supto 113.7 dB.Hzsup2/3/supand the SFDR for second harmonic distortion (SHD) increases from 87.6 dB.Hzsup1/2/supto 97.5 dB.Hzsup1/2/supat a driving frequency of 2 GHz. When the driving frequency is 20 GHz, the SFDRs for TID and SHD distortions are 110.3 dB.Hzsup2/3/supand 96 dB.Hzsup1/2/sup, respectively.

We report the implementation of the XOR and XNOR logical operations using an electro-optic circuit, which is fabricated by CMOS-compatible process in the silicon-on-insulator (SOI) platform. The circuit consists of two cascaded add-drop microring resonators (MRRs), which are modulated through electric-field-induced carrier depletion in reverse biased pn junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs caused by fabrication errors is compensated by thermal tuning. Simultaneous bitwise XOR and XNOR operations of the two electrical modulating signals at the speed of 12.5 Gb/s are demonstrated. And 20 Gb/s XOR operation at one output port of the circuit is achieved. We explain the phenomena that one half of the resonance regions of the device are much more sensitive to the round-trip phase shift in the ring waveguides than the other half resonance regions. Characteristic graphs with logarithmic phase coordinate are proposed to analyze the sensitivity of the demonstrated circuit, as well as several typical integrated optical structures. It is found that our circuit with arbitrary chosen parameters has similar sensitivity to MRRs under the critical coupling.

We demonstrate a carrier-depletion Mach-Zehnder silicon optical modulator, which is compatible with CMOS fabrication process and works well at a low driving voltage. This is achieved by the optimization of the coplanar waveguide electrode to reduce the electrical signal transmission loss. At the same time, the velocity and impedance matching are both considered. The 12.5 Gbit/s data transmission experiment of the fabricated device with a 2-mm-long phase shifter is performed. The driving voltages with the swing amplitudes of 1 V and 2 V and the reverse bias voltages of 0.5 V and 0.8 V are applied to the device, respectively. The corresponding extinction ratios are 7.67 and 12.79 dB.

We experimentally demonstrate a spatially non-blocking five-port optical router, which is based on microring resonators tuned through the thermo-optic effect. The characteristics of the microring-resonator-based switching element are investigated to achieve balanced performances in its two output ports. The optical router is fabricated on the SOI platform using standard CMOS processing. The effective footprint of the device is about 440×660 μm2. The microring resonators have 3-dB bandwidths of larger than 0.31 nm (38 GHz), and extinction ratios of better than 21 dB for through ports and 16 dB for drop ports. Finally, 12.5 Gbps high-speed signal transmission experiments verify the routing functionality of the optical router.

We propose and demonstrate a directed OR/NOR and AND/NAND logic circuit consisting of two parallel microring resonators (MRRs). We use two electrical signals representing the two operands of the logical operation to modulate the two MRRs through the thermo-optic effect, respectively. The final operation results are represented by the output optical signals. Both OR/NOR and AND/NAND operations at 10 kbps are demonstrated.