We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated, low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based communication technologies because the well-developed InP-based optoelectronic technologies are already expected to enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers (SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize compact, cost-effective, and portable THz spectrometer.