Terahertz (THz) band is an attractive candidate for future broadband (> 10 Gb/s) wireless backhaul and fronthaul. THz transmitter employing optical frequency comb can provide high quality THz carrier, and is useful to the future broadband THz communication systems based on coherent transmission technique. To realize coherent transmission, high quality carrier generation is essential and it is important to evaluate the signal quality of a THz transmitter. In this paper, we derive error vector magnitude (EVM) including optical impairments (optical amplifier noise, laser phase noise, optical crosstalk and IQ imbalance of optical modulator) of the optical frequency comb based transmitter. The calculated EVM is in good agreement with simulated one, and practical requirements for optical impairment are indicated. The analysis will be useful in the design of THz transmission systems employing an optical frequency comb.

A reduction in the intensity deviation of a nine-channel optical frequency comb block (OFCB) is demonstrated, by adopting an asymmetric differential drive method for an InP-based dual drive Mach-Zehnder modulator. The generation of a tailored OFCB with an intensity deviation of less than 0.8dB is confirmed by using the modulator.

The quantum dot optical frequency comb laser (QD-CML) is an attractive photonic device for generating a stable emission of fine multiple-wavelength peaks. In the present paper, 1.0-GHz and 10-ps-order short optical pulsation is successfully demonstrated from a hybrid mode-locked QD-CML with an ultrabroadband wavelength tuning range in the T+O band. In addition, 10-GHz high-repetition intensity-stable short optical pulse generation with a high S/N ratio is successfully demonstrated using an external-cavity QD-CML with a 10th-harmonic mode-locking technique.

We obtained flat optical frequency combs by using the FM laser operation of a fiber ring laser and external intensity modulation. Extremely wide FM spectra can be easily obtained by the moderate internal phase modulation of an FM laser. We used an external intensity modulator to extract a linearly chirped part from the FM light in order to obtain flat spectra. In our experiments, we obtained a flat optical frequency comb with a spectral bandwidth of about 0.5 THz and a power deviation of less than 1.5 dB.