We present the first hybrid-integrated optical phase-lock loop (OPLL) for use in high spectral purity photonic terahertz sources. We have achieved the necessary short loop delay to lock a 1-MHz linewidth slave laser by hybrid integration of the slave laser and photodetector on a silicon motherboard with silica optical waveguides and combining this with a custom-designed low-delay electronic loop filter circuit. The laser and photodetectors are InP-based and are flip chip bonded to silicon daughter boards, which are in turn attached to the motherboard. Delay between the slave laser and photodiode was approximately 50 ps. The heterodyne between slave and master sources has a linewidth of less than 1 kHz and achieved phase noise less than -80 dBc/Hz at an offset of 10 kHz. The slave laser can be offset from the master source by 2-7 GHz, using a microwave oscillator. This integrated OPLL circuit was used with an optical comb source and an injection-locked laser comb filter to generate high spectral purity signals at frequencies up to 300 GHz with linewidths <;1 kHz and powers of about -20 dBm, while the two integrated lasers could deliver a tunable heterodyne signal at frequencies up to 1.8 THz.

We describe the operating principle, practical implementation, and experimental evaluation of a synchronous homodyne coherent optical receiver based on an optical phase-locking scheme that combines optical injection locking of the semiconductor laser local oscillator (LO) with low-speed electronic feedback to give both a large locking bandwidth and a wide tracking range. We demonstrate phase-error variance as low as 0.002 rad² in 10-GHz bandwidth for locking to a continuous-wave (CW) signal for a combined signal and LO linewidth of 1.5 MHz (full-width at half-maximum), and robust phase locking to a 10-Gb/s binary amplitude-shift-keyed (ASK) signal, enabling synchronous back-to-back demodulation of the signal with low bit error ratio (BER <; 10^{- 10}) and improved performance compared to direct detection at low optical SNR (OSNR). By locking to a low-power CW pilot carrier in the polarization orthogonal to the data signal, demodulation of binary phase-shift-keyed (BPSK) data has been achieved, with the required OSNR at BER = 10^-3 reduced by 3 dB compared to demodulation of ASK data under the same conditions. The OSNR penalty after transmission of the ASK and BPSK signals over 40 km of standard single-mode fiber was 1-2 dB at BER = 10^-3, indicating that the chromatic dispersion sensitivity of the coherent receiver is similar to that for direct detection, and verifying that the scheme for locking to the orthogonal pilot is applicable to transmission systems, provided that optical polarization tracking is employed. In addition, we demonstrate frequency-selective operation of the coherent receiver, demultiplexing and demodulating one of a pair of equal power, 10 Gb/s, ASK channels separated by 17.5 GHz, with OSNR penalty at BER = 10^{- 3} of 1.5 dB compared to single-channel operation.

We present a novel technique for optoelectronic frequency down-conversion of millimeter-wave signals generated from photomixers. The mixing element proposed here is a uni-traveling carrier (UTC) photodiode employing a traveling wave design, originally fabricated for the generation of millimeter-wave signals. For fundamental mixing at a frequency of 100 GHz, a conversion gain of was measured, representing a significant improvement on previously published results. When the device was operated as a subharmonic mixer, an additional loss of 20, 27, and 39 dB was measured for second-, third- and fourth-harmonic mixing, respectively, for the same level of RF input power (0 dBm). A nonlinear dependence of the IF signal on the optically generated signal was measured. From subharmonic mixing measurements, a flat intermediate frequency (IF) response was found over a wide range of frequencies, limited mainly by the IF electronic components.

A pilot-carrier coherent low-earth-orbit (LEO) satellite to ground (LEO-to-Ground) downlink system using an optical injection phase lock loop (OIPLL) technique is proposed and its feasibility under Doppler frequency shift conditions is demonstrated. A fiber-optic based experimental system is configured and it is demonstrated that a 10 Gbps BPSK transmission system based on the proposed configuration can successfully maintain stable frequency and phase locking status under simulated Doppler frequency shift conditions. It is demonstrated that the stable locking status is maintained over a 10.3 GHz (54 ppm) frequency offset with a maximum rate-of-change of up to 32.4 GHz/s (168 ppm/s), which is ample to meet the requirement for a coherent LEO-to-Ground downlink system. The locking capability of the experimental system for more rapidly changing Doppler frequency shift is investigated. It is shown that the OIPLL receiver remains locked for maximum rates of change of 2.6 THz/s (13 500 ppm/s) or more for peak-to-peak frequency offsets up to 2 GHz (10.7 ppm). The phase noise performance of the system is also investigated and phase noise power of less than -100 dBc/Hz at greater than 1 MHz offset frequency is achieved even if the received laser signal suffers from a simulated Doppler frequency shift with peak-to-peak frequency offset of 2.4 GHz (12.5 ppm) and maximum rate of change of 750 GHz/s (3 900 ppm/s).

Optically pumped mixing in travelling-wave uni-travelling carrier photodiodes is proposed as a novel technique for detecting millimetre-wave signals. An experimental demonstration was performed at a frequency of 100 GHz. From DC measurements, an increase in the responsivity was found at high levels of optical power. The mixing mechanism is attributed to the variation of the responsivity with the applied reverse bias and the optical input power. The maximum intermediate frequency power was found to be −35 dBm for a 4 dBm radio frequency power, while an average conversion loss of 40 dB was achieved. A wide dynamic range of more than 42 dB was measured, limited by the maximum available millimetre-wave power.

We report the generation of a 95 GHz carrier frequency by optical heterodyning of two wavelengths from adjacent channels from an arrayed waveguide grating-based multiwavelength laser. The extended cavity structure of the device provides low phase noise and narrow optical linewidth, further enhanced by the intracavity filter effect of the arrayed waveguide grating. We demonstrate that the generated RF beat note, at 95 GHz, has a $-3\mathrm{dB}$ linewidth of 250 kHz. To the best of our knowledge, this is the narrowest RF linewidth generated from a free-running dual-wavelength semiconductor laser. The device is realized as a photonic integrated circuit using active–passive integration technology, and fabricated on a multiproject wafer run, constituting a novel approach for a compact, low-cost dual-wavelength heterodyne source.