# Optical Injection Locking for Generation of Tunable Low-Noise Millimeter Wave and THz Signals
## 2 Optical Injection Locking for Sub-THz and THz Signal Generation  
Optical injection locking, as mentioned before, is a technique of frequency and phase synchronization. Such a technique can be used in systems where frequency stability and synchronization of the lasers is crucial. OIL found use in many applications: as active filtering or so-called \"selective amplification\" [22; 25; 26], in communication systems for synchronization of a local oscillator (LO) [27] and in combination with OFCs for demodulation of dense wavelength division multiplexed (DWDM) signals [28]. Improvement of the laser stability employing OIL for generation of high-frequency RF signals was investigated for decades [29; 30]. OIL is a promising method for generation of sub-THz and THz signals, since modern modulation formats and high-resolution spectroscopy imposes strict requirements on noise level and frequency stability. Some recent works have already demonstrated the use of a photonic integrated circuit for the generation of sub-THz signals. In [21], the authors demonstrated the generation of sub-THz signals within the range of 85-120 GHz using two monolithically integrated distributed feedback (DFB) lasers locked to the same OFC. In [22], a pair of hybrid indium-phosphide-integrated (InP)/polymer distributed Bragg reflector (DBR) lasers were locked with an OFC and selected lines were used to generate a 300 GHz RF signal for wireless data transmission. In [31], the authors used a combination of injection locking with an optical feedback loop for the generation of low-linewidth signals below 50 GHz. Evidently, the combination of OIL and OFCs could be a promising approach for the generation of RF signals and mechanisms of injection of multiple lines, which faces strong interest from the scientific community [32; 33]. In general terms, the principle of generating THz radiation using OIL can be described with the scheme depicted in Figure 1, in which multiple tones are selected from a comb, processed optically and fed to a photoconductive antenna for THz signal generation and radiation. An optical frequency comb generator can play the role of a frequency seed for multiple slave lasers. Implementation of the OFC on-chip can be performed in many different ways: directed modulated lasers; systems based on microring resonators; and mode-locked lasers or electro-optical modulators [34; 35]. Each of the presented solutions have its advantages and disadvantages such as power consumption, footprint, width of the comb, output optical power, and a choice must be made based on the system requirements. The design of the comb source can be adjusted for the application's needs: for a flexible system where tuning accuracy is more important than spectral width, smaller spacing between comb lines can be chosen, while for systems in which it is important to cover the entire terahertz range, larger spacing and a more broadband comb can be preferable. Traditionally, in order to generate a terahertz signal from an optical comb, it is necessary to filter out a pair of optical tones and amplify them. Arrayed waveguide gratings (AWG) or ring resonators can act as filters, and SOAs can boost the signal. However, such filters usually introduce insertion losses of 3-5 dB and the use of such a scheme degrades the signal-to-noise ratio (SNR) [24]. As an alternative method, the OFC could be launched into a pair or array of the slave lasers, which play the role of active filtering or so-called \"selective amplifiers\". In this case, the term \"selective amplification\" means that each laser would be locked to one of the optical tones, amplify it, and suppress unwanted tones at the same time. The choice of slave lasers is also a complicated task, as locking itself relies on different parameters such as cavity \(Q\)-factor or linewidth enhancement factor [24]. Further, close attention needs to be paid to the locking range of the laser and its tunability.