A novel thin-film lithium niobate (TFLN) electro-optic modulator is proposed and demonstrated. LiNbO3-silica hybrid waveguide is adopted to maintain low optical loss for an electrode spacing as narrow as 3 µm, resulting in a low half-wave-voltage length product of only 1.7 V·cm. Capacitively loaded traveling-wave electrodes are employed to reduce the microwave loss, while a quartz substrate is used in place of a silicon substrate to achieve velocity matching. The fabricated TFLN modulator with a 5-mm-long modulation region exhibits a half-wave voltage of 3.4 V and a merely less than 2 dB roll-off in an electro-optic response up to 67 GHz.

This paper presents a wide-bandwidth back-illuminated modified uni-traveling-carrier photodiode (MUTC-PD) packaged with standard WR-5 rectangular waveguide for high-speed wireless communications. With optimized epitaxy structure and coplanar waveguide electrodes, the fabricated 4-μm-diameter PD exhibits ultra-flat frequency response and high saturation power. Integrated passive circuits including low-loss bias-tee and E-plane probe are designed to package the PD into a compact module with waveguide output. The packaged PD module has demonstrated a flat frequency response with fluctuations within ±2.75 dB over a broadband of 140-220 GHz and a high saturated output power of −7.8 dBm (166 μW) at 140 GHz. For wireless communication applications, the packaged PD is used to implement 1-m free space transmission at carrier frequencies of 150.5 and 210.5 GHz, with transmission rates of 75 and 90 Gbps, respectively.

Efficiency droop in GaN-based light emitting diodes (LEDs) under high injection current density perplexes the development of high-power solid-state lighting. Although the relevant study has lasted for about 10 years, its mechanism is still not thoroughly clear, and consequently its solution is also unsatisfactory up to now. Some emerging applications, e.g., high-speed visible light communication, requiring LED working under extremely high current density, makes the influence of efficiency droop become more serious. This paper reviews the experimental measurements on LED to explain the origins of droop in recent years, especially some new results reported after 2013. Particularly, the carrier lifetime of LED is analyzed intensively and its effects on LED droop behaviors are uncovered. Finally, possible solutions to overcome LED droop are discussed.

A back-illuminated mesa-structure InGaAs/InP modified uni-traveling-carrier photodiode (MUTC-PD) is fabricated and its frequency response is investigated. A bandwidth of 40 GHz and a saturation photocurrent up to 33 mA are demonstrated. A photocurrent-dependent equivalent circuit model is proposed to analyze the frequency response of the high power MUTC-PDs. The influences of the space-charge screening, self-induced electric field and over-shoot effects are discussed in detail based on the model. Fitted curves obtained from the simple equivalent circuit model are found to be in good agreement with the data measured under different bias voltages and photocurrents.

A novel backside-illuminated mesa-structure dual-drifting layer (DDL) uni-traveling-carrier photodiode (UTC-PD) is reported to demonstrate high-power performance at sub-THz frequencies. The DDL structure consists of a velocity overshoot layer and a velocity saturation layer, formed by inserting a 20 nm p-type cliff layer into the thick depletion region. In the overshoot layer, photo-generated electrons drift at overshoot velocity under the carefully designed electric field profile, thus resulting in a short electron transit time. The saturation layer serves as a voltage sacrificing layer to enable high bias voltage operation, which leads to alleviated load voltage swing effect, as well as improved saturation performance. Our DDL UTC-PD exhibits a 3-dB bandwidth of 106 GHz with a responsivity of 0.17 A/W under a wide bias voltage range from 4 to 8 V. The photocurrent reaches up to 28 mA, corresponding to an output power of 7.3 dBm at 105 GHz.

We propose a novel approach to laser frequency noise characterization by delayed self-heterodyne. Compared with the traditional treatment, our method applies to both long and short delay, corresponding to uncorrelated and correlated self-heterodyne. In the case of long delay, it overcomes the influence of $1/f$ noise on the intrinsic linewidth extraction from a broadened spectrum, and the results are more accurate than Voigt profile fitting. For short delayed correlated heterodyne, it eliminates artifact peaks at multiples of the reciprocal of delay time introduced by transferring measured RF phase noise to laser phase noise, thus extending the measurement range. In addition, it calibrates the frequency noise overestimation caused by a finite noise floor. This method remains valid when the delay and the coherence time are comparable. Experimental results are presented to demonstrate the effectiveness of the proposed approach in characterizing lasers with intrinsic linewidth ranging from sub-100 Hz to megahertz.