Continuously tunable true-time delay lines based on a one-dimensional grating waveguide for beam steering in phased array antennas.

In this paper, we propose integrated one-dimensional (1D) grating waveguide-based true-time delay (TTD) lines on a silicon-on-insulator (SOI) platform. Through optimizing the structure of the proposed waveguide, a time delay of 77.23 ps/mm can be readily achieved in a wavelength tuning range from 1540.20 nm to 1558.97 nm. Compared to conventional photonic crystal waveguide-based TTDs, the proposed waveguide occupies 70% less chip surface and has much lower propagation loss when compared with 2D photonic crystal devices. Therefore, a larger time delay can be achieved on-chip. To facilitate low loss coupling from strip waveguides to 1D grating waveguides and vice versa, a novel step taper is designed and shows a coupling efficiency of over 78%. Based on the 1D grating waveguide, a 1×4 beam steering module is designed and simulated. A wide beam steering angle from -67.84° to 67.84° for the X-band four-element phased array antenna with an array pitch size of 1.25 cm is obtained.

Broadband tunable filter based on the loop of multimode Bragg grating.

A broadband tunable silicon filter has been demonstrated on silicon-on-insulator platform. The device is based on the loop of multimode anti-symmetric waveguide Bragg grating. A wide bandwidth tunability about 1.455 THz (0.117-1.572 THz) is achieved. The device, functions like a ring, can realize the bandwidth tunable of the drop port and the through port. And, its feature has simultaneous wavelength tuning and no free space ranges limitation. A high out-of-band contrast of 30 dB is achieved with a bandwidth of 1.572 THz (Δλ = 13 nm). The out-of-band contrast is 18 dB at the minimum bandwidth 0.117 THz (Δλ = 1.0 nm).

Graphene-based nonvolatile terahertz switch with asymmetric electrodes.

We propose a nonvolatile terahertz (THz) switch which is able to perform the switching with transient stimulus. The device utilizes graphene as its floating-gate layer, which changes the transmissivity of THz signal by trapping the tunneling charges. The conventional top-down electrode configuration is replaced by a left-right electrode configuration, so THz signals could transmit through this device with the transmissivity being controlled by voltage pulses. The two electrodes are made of metals with different work functions. The resultant asymmetrical energy band structure ensures that both electrical programming and erasing are viable. With the aid of localized surface plasmon resonances in graphene ribbon arrays, the modulation depth is 89% provided that the Femi level of graphene is tuned between 0 and 0.2 eV by proper voltage pulses.

Silicon lateral-apodized add-drop filter for on-chip optical interconnection.

A lateral-apodized add-drop filter is demonstrated in a multimode asymmetric waveguide Bragg grating. This design utilizes two individual superposed gratings with the same sidewall corrugation depth. The strong side lobes of the grating filter are efficiently suppressed by mapping the target apodization profile into lateral shifts between the periods of the two gratings. Compared with other apodized technology, this device is easier to be realized. Experimental results show that the side-lobes suppression ratio can reach 18.5 dB, and a bandwidth of 9.5 nm is achieved by a large corrugation width of 150 nm. The insertion loss at the drop port is only 0.8 dB, and the extinction ratio is up to 24 dB at the through port.

Bandwidth and wavelength tunable optical passband filter based on silicon multiple microring resonators.

An ultra-compact silicon bandpass filter with wide bandwidth tunability is proposed and experimentally demonstrated. The filter architecture is based on a multiple micro-ring resonator-cascaded structure. A wide bandwidth tunability (from 75 to 300 GHz) can be achieved by controlling the resonant frequency of the microring resonators when a good shape factor (0.24-0.44) is held. The filter has a wide free spectral range (about 1.2 THz). The center wavelength can be tuned over several nanometers linearly. The footprint is only 0.053 mm2.

Integrated high responsivity photodetectors based on graphene/glass hybrid waveguide.

We propose and fabricate an integrated graphene/glass hybrid photodetector (PD) with high responsivity and broad spectral bandwidth. The glass straight waveguide enables high absorption of the evanescent light of transverse magnetic (TM) mode propagating parallel to the single layer of graphene. It is based on the mechanism of light-induced change in conductance. As a result, a responsivity as high as 0.72 A/W at a low bias voltage of -0.1 V for a wide wavelength range from 1510 to 1630 nm is experimentally obtained. The proposed graphene/glass hybrid PD could find important applications in graphene-based photonic integrated circuits.

Silicon three-mode (de)multiplexer based on cascaded asymmetric Y junctions.

A three-mode (de)multiplexer based on two cascaded asymmetric Y junctions is proposed and experimentally demonstrated on a silicon-on-insulator platform for mode-division multiplexing applications. Within a bandwidth from 1537 to 1566 nm, the best demultiplexing crosstalk of the fabricated device, composed of a three-mode multiplexer, a multimode straight waveguide, and a three-mode demultiplexer, is up to -31.5 dB, while in the worst case it is -9.7 dB. The measured maximum insertion loss is about 5.7 dB at a wavelength of 1550 nm. The mode crosstalk and insertion loss can be further improved by high-quality fabrication processes.

Fano-resonance-based ultra-high-resolution ratio-metric wavelength monitor on silicon.

An integrated ultra-high-resolution ratio-metric wavelength monitor (RMWM) with compact size based on slope tunable Fano resonance is demonstrated on silicon. The Fano resonance is generated by adding an asymmetric microring inside and coupling with the outer ring to produce a nonlinear phase shift. The slope tunability is achieved by controlling the microheaters to adjust the phase condition. Two asymmetric embedded microring resonators (AEMR) are functioned as edge filters and designed to achieve an "X-type" spectral response in a particular wavelength range. An ultra-high resolution of 0.8 pm in a 0.47 nm wide wavelength range is experimentally demonstrated. This device could be applied in on-chip high-sensitivity wavelength monitoring sensing.

Temperature-fluctuation-sensitive accumulative effect of the phase measurement errors in low-coherence interferometry in characterizing arrayed waveguide gratings.

We investigate the accumulative effect of the phase measurement errors in characterizing optical multipath components by low-coherence interferometry. The accumulative effect is caused by the fluctuation of the environment temperature, which leads to the variation of the refractive index of the device under test. The resulting phase measurement errors accumulate with the increasing of the phase difference between the two interferometer arms. Our experiments were carried out to demonstrate that the accumulative effect is still obvious even though the thermo-optical coefficient of the device under test is quite small. Shortening the measurement time to reduce the fluctuation of the environment temperature can effectively restrain the accumulative effect. The experiments show that when the scanning speed increases to 4.8 mm/s, the slope of the phase measurement errors decreases to 5.52×10(-8), which means the accumulative effect can be ignored.

On-chip microwave signal generation based on a silicon microring modulator.

A photonic-assisted microwave signal generator based on a silicon microring modulator is demonstrated. The microring cavity incorporates an embedded PN junction that enables a microwave signal to modulate the lightwave circling inside. The DC component of the modulated light is trapped in the cavity, while the high-order sideband components are able to exit the cavity and then generate microwave signals at new frequencies in a photodetector. In our proof-of-concept experiment, a 10 GHz microwave signal is converted to a 20 GHz signal in the optical domain with an electrical harmonic suppression ratio of 22 dB. An analytic model is also established to explain the operation mechanism, which agrees well with the measured data.